Deutsche Physikalische Gesellschaft e. V. (DPG)

WWW-Server für Frühjahrstagungen und E-Verhandlungen

Programm und Abstracts der Sitzung TT 24

LDA+DMFT results for ARPES spectra of SrVO₃

V. I. Anisimov¹, K. Held², •G. Keller³, D. E. Kondakov¹, I. Nekrasov¹, T. Pruschke⁴ und D. Vollhardt^3
1Institut für Metallphysik, Ekaterinburg GSP-170, Russland
²Max Planck Institut für Festkörperforschung, 70569 Stuttgart
³Theoretische Physik III, Universität Augsburg, 86135 Augsburg
⁴Institut für Theoretische Physik, Universität Göttingen, 37077 Göttingen

We report angle resolved photoemission spectra of SrVO₃ calculated by LDA+DMFT(QMC). Starting from LDA band structure calculations in the LMTO basis set [1], we use a projection method [2,3] to extract a Hamiltonian for the three-fold degenerate t_2g bands from the total band structure. With this reduced LDA-Hamiltonian as input to DMFT(QMC) calculations [4], we obtain (via maximum entropy) angle resolved spectral functions A(k,w) on the real axis. The spectra exhibit featureless upper and lower Hubbard bands with a quasi-particle peak which is narrowed by correlations. In view of the good agreement of our previous (not angle resolved) LDA+DMFT results [5] with high-resolution PES for SrVO₃ [6], it will be interesting to see how our predictions compare with future ARPES experiments.

[1] O. K. Andersen, Phys. Rev. B 12, 3060 (1975)

[2] N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)

[3] W. Ku et al., Phys. Rev. Lett. 89, 167204 (2002)

[4] K. Held et al., Psi-k Newsletter #56 (April 2003), p. 65-103

[5] I. Nekrasov et al., cond-mat/0211508

[6] A. Sekiyama et al., cond-mat/0206471

Charge and orbital order in Fe₃O₄

•I. Leonov^1,2, A. N Yaresko³, V. N. Antonov⁴, M. A. Korotin², V. I. Anisimov² und D. Vollhardt^1
1Theoretical Physics III, University of Augsburg, 86135 Augsburg
²Institute of Metal Physics, Ekaterinburg GSP-170, Russia
³Max Planck Institute for Physics of Complex Systems, 01187 Dresden
⁴Institute of Metal Physics, 03142 Kiev, Ukraine

The issue of charge and orbital ordering in the low-temperature monoclinic (P2/c) structure of magnetite (Fe₃O₄)[1] is investigated using LSDA+U. The ground state is found to display both charge order (CO) and orbital order (OO) [2]. The CO is described by a [001] charge density wave with a minor [00\frac12] modulation, and is incompatible with the Anderson criterion. The OO agrees with the Kugel-Khomskii theory. The system is described by two order parameters: (i) the difference between the t_2g minority occupancies of 2+ and 3+ Fe cations, and (ii) the total 3d charge difference. While the former is large (0.5), the screening of charge disproportion is so effective that the latter is rather small (0.23). This agrees well with the results of bond valence sum analysis (0.2). The LSDA+U results are also in reasonably good with measured optical spectra.

[1] J. P. Wright, J. P. Attfield and P. G. Radaelli, Phys. Rev. Let. 87, 266401 (2001)

[2] I. Leonov, A. N. Yaresko, V. N. Antonov, M. A. Korotin, V. I. Anisimov and D. Vollhardt (preprint)

DFT-Gutzwiller Calculations for ferromagnetic bcc Nickel

•Torsten Ohm¹, Stefan Weiser¹, Werner Weber¹ und Jörg Bünemann^2
1Institut für Physik, Universität Dortmund
²Fachbereich Physik, Phillips Universität Marburg

Thin films of bcc Nickel can be produced on appropriate substrates and can be studied experimentally; e.g., by photoemission spectroscopy. This has stimulated us to carry out a multi-band Gutzwiller calculation for the quasi-particle energy bands. Our single particle Hamiltonian is based on results of spin-density functional theory ground state calculations using the LAPW-Wien2k code. In the DFT calculations, some problems with the k-mesh integration had to be overcome in order to establish well converged total energy values. The spin-only magnetic moment is found to be about 20 percent smaller than in fcc Nickel. For the multi-band Gutzwiller calculations we use the DFT lattice constant and the same atomic Racah interaction parameters A,B, and C as in our previous calculations for fcc Nickel. The Gutzwiller quasi-particle bands and the Fermi surfaces are compared with the corresponding DFT results.

Quantum Monte Carlo approach to the Holstein model

•Martin Hohenadler, Hans Gerd Evertz und Wolfgang von der Linden
Institute for Theoretical and Computational Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria

Based on the canonical Lang-Firsov transformation of the Hamiltonian, we develop a novel quantum Monte Carlo algorithm for the Holstein model with one electron. Separation of the fermionic degrees of freedom by a reweighting of the probability distribution leads to a dramatic reduction in computational effort. A principle component representation of the phonon degrees of freedom allows us to sample completely uncorrelated phonon configurations. Despite a minus-sign problem, which is a consequence of the Lang-Firsov transformation, the combination of these elements enables us to perform efficient simulations for a wide range of temperature, phonon frequency and electron-phonon coupling in one to three dimensions, and on clusters large enough to avoid significant finite-size effects. The algorithm is employed to investigate the problem of small polaron formation in the Holstein model, and comparison with exact results and with existing work is made. Moreover, the new ideas presented here can also be applied to the many-electron case.

Variational cluster approach to extended Hubbard models at half filling

•Markus Aichhorn¹, Hans Gerd Evertz¹, Wolfgang von der Linden¹ und Michael Potthoff^2
1Institut für Theoretische Physik und Computational Physics, Technische Universität Graz, Petersgasse 16, A-8010 Graz, Austria
²Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany

We present an extension of the recently proposed variational cluster perturbation theory to extended Hubbard models at half filling with repulsive nearest neighbor interaction. The method takes into account short-range correlations exactly by the exact treatment of clusters of finite size via exact diagonalization, whereas long-range order beyond the size of the clusters is treated on a mean-field level. We show that for one dimension results known from Quantum Monte Carlo and Density Matrix Renormalization Group can be reproduced with very good accuracy. Moreover we apply the method to the two-dimensional extended Hubbard model on a square lattice. Different from one dimension, we find a first order phase transition between spin density wave phase and charge density wave phase down to onsite interaction U = 3t. In addition, the single-particle spectral function is calculated for both the one-dimensional and the two-dimensional system.

Influence of long-range Coulomb interaction and on-site Hubbard repulsion on the formation of d-wave Copper-pairing in high-T_c cuprates

•Dirk Manske¹, Ernst Pashitskii², Vsevolod Pentegov² und Ilya Eremin^1
1Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin
²Institute of Physics NAS of Ukraine, 46 Nauki str., Kiev, 03028, Ukraine

We develop a diagram technique for the self-consistent treatment of the long-range Coulomb interaction and on-site Hubbard repulsion in the normal and superconducting state of high-T_c cuprates. The resultant analytical expression for the screened matrix elements taking into account long-range and on-site repulsion has been derived. In particular, it accounts for processes with and without spin-flip due to an exchange of spin and charge density fluctuations. Furthermore, we derive the expressions for the normal and anomalous self-energy parts near the superconducting transition temperature T_c that takes into account the vertex corrections including crossing diagrams. The contribution of the crossing parts is taken within the ladder approximation (similar to Fluctuations Exchange approximation) where the role of Hubbard on-site interaction is replaced by the Coulomb matrix element with a spin-flip averaged over the momentum. Finally, the developed scheme allows to analyze the formation of d-wave superconductivity and its stability in presence of the long-range Coulomb repulsion within a self-consistent anisotropic Eliashberg-like approach.

Electronic structure of quasi-one-dimensional metals: Theory vs Experiment

•Holger Benthien¹, Eric Jeckelmann², Florian Gebhard¹, Michael Sing³ und Ralph Claessen^3
1Fachbereich Physik, Universität Marburg, 35032 Marburg
²Institut für Physik, Universität Mainz, 55099 Mainz
³Experimental Physik II, Universität Augsburg, 86135 Augsburg

We compare theoretical and experimental results for the electronic structure of quasi-one-dimensional correlated metals. On the one hand the one-particle Green's function is calculated numerically in the one-dimensional Hubbard model away from half filling using the dynamical density-matrix renormalization group method [1]. On the other hand the angle-resolved photoemission spectrum has been measured in the quasi-one-dimensional organic charge transfer salt TTF-TCNQ [2]. We show that the experimental spectrum at intermediate to high energy (up to about 1eV) can be explained consistently in the framework of the one-dimensional Hubbard model. In particular, our results show clear evidence for spin-charge separation in the excitation spectrum (and thus for a non-Fermi liquid) up to the energy scale of the bandwidth.

[1] E. Jeckelmann, Phys. Rev. B 66, 045114 (2002).

[2] R. Claessen et al., Phys. Rev. Lett. 88, 096402 (2002).

Thermodynamic properties of ferromagnetic mixed-spin chain systems

•Noboru Fukushima¹, Andreas Honecker¹, Stefan Wessel² und Wolfram Brenig^1
1Institut für Theoretische Physik, Technische Universität Braunschweig, Germany
²Theoretische Physik, ETH Zürich, Switzerland

Using a combination of high-temperature series expansion, exact diagonalization and quantum Monte Carlo, we perform a complementary analysis of the thermodynamic properties of one-dimensional mixed-spin systems with alternating magnetic moments. In particular, we focus on the difference between ferromagnetic (FM) and antiferromagnetic (AFM) exchange. Not only the peak structure of the specific heat is more pronounced in the FM case, but also the number of the peaks appears different. Namely, the FM case seems to have some additional energy scale(s). The origin of these differences will be discussed. In addition, the impact of interchain exchange will be also investigated.

Scaling of the conductance in interacting quantum wires

•Tilman Enss¹, Sabine Andergassen¹, Volker Meden², Walter Metzner¹, Ulrich Schollwöck³ und Kurt Schönhammer^2
1Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart
²Institut für Theoretische Physik, Universität Göttingen, D-37073 Göttingen
³Sektion Physik, Universität München, Theresienstr. 37, D-80333 München

The conductance through an interacting nanowire containing impurities and coupled to non-interacting leads is studied at zero and finite temperature. Our functional RG method starts from the microscopical model and arrives at an effective low-energy model correctly showing Luttinger liquid behavior. For a single impurity we observe one-parameter scaling of the conductance. Using several impurities, we study resonances of the conductance and temperature-dependent transport through a quantum dot.

Functional RG for Luttinger liquids with impurities

•Sabine Andergassen¹, Tilman Enss¹, Volker Meden², Walter Metzner¹, Ulrich Schollwöck³ und Kurt Schönhammer^2
1Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart
²Institut für Theoretische Physik, Universität Göttingen, D-37073 Göttingen
³Sektion Physik, Universität München, D-80333 München

Using a functional RG we compute the flow of the renormalized impurity potential for a single impurity in a Luttinger liquid over the entire energy range from the microscopic scale of a lattice-fermion model down to the low-energy limit. The non-perturbative method provides a complete real-space picture of the effective impurity potential. Further developments of the functional RG approach including 2-particle vertex renormalization considerably improve the quantitative accuracy of the results in particular at moderate interaction strenghts, which are compared to exact DMRG results. Focusing on the spectral properties of the single-particle excitations near an impurity or boundary and on the density profile, we confirm the universality of the open chain fixed point, but it turns out that very large systems are required to reach the asymptotic regime for realistic choices of the impurity and interaction parameters.

Coexistence of s-wave Superconductivity and Antiferromagnetism

•Martin Feldbacher¹, Fakher Assaad², Frederic Hébert³ und George Batrouni^3
3Institut Non-Linéaire de Nice, Université de Nice-Sophia Antipolis, France
¹Max-Planck-Institut für Festkörperforschung, Heisenbergstraß e 1, D-70569 Stuttgart
²Universität Würzburg, Institut für Theoretische Physik I, Am Hubland, 97074 Würzburg

We study the phase diagram of a new model that exhibits a first order transition between s-wave superconducting and antiferromagnetic phases. The model, a generalized Hubbard model augmented with competing spin-spin and pair-pair interactions, was investigated using the projector Quantum Monte Carlo method. Upon varying the Hubbard U from attractive to repulsive we find a first order phase transition between superconducting and antiferromagnetic states. Phys. Rev. Lett., 91:056401, 2003

Hubbard model in a magnetic field at weak coupling

•Carsten L. Knecht und P.G.J. van Dongen
Institut für Physik, Johannes Gutenberg-Universität, Staudinger Weg 7, 55099 Mainz, Germany

The phase diagram of the half-filled Hubbard model is studied at weak coupling in two spatial dimensions. A homogeneous magnetization in the z-direction and a staggered magnetization in the x-direction are assumed. We apply perturbation theory at fixed order parameter (PTFO) to this system. The results are compared with the well know Hartree-Fock solutions that usually overestimate the order parameters. This calculation is also relevant for superconductivity in the doped two-dimensional negative-U Hubbard model.

Self-energy-functional approach: New results and developments

•Michael Potthoff
Institut für Theoretische Physik und Astrophysik, Universität Würzburg

The self-energy-functional approach [1-3] is a general variational framework to construct non-perturbative and thermodynamically consistent approximations for lattice models of correlated electrons. The main idea is to shift the problem to a more simple reference system with the same (two-particle) interaction which is exactly solved for different one-particle parameters. The parameters are fixed by evaluation of a fundamental variational principle for the self-energy.

This contribution gives an overview over the recent developments in the extension of the approach to Bose systems, two-particle excitations or disordered systems and discusses efficient numerical methods to evaluate the variational principle.

[1] M.P., EPJB 32, 429 (2003)

[2] M.P., M. Aichhorn and C. Dahnken, PRL 91, 206402 (2003)

[3] M.P., EPJB in press, cond-mat/0306278

Crossover from Non-equilibrium to Equilibrium Behavior in the Time-DependentKondo Model

•Stefan Kehrein und Dmitry Lobaskin
Insitut für Physik - Elektronische Korrelationen und Magnetismus, Universität Augsburg

We investigate the equilibration of a Kondo model that is initially prepared in a non-equilibrium state towards its equilibrium behavior for large times. Such initial non-equilibrium states can e.g. be realized in quantum dot experiments with time-dependent gate voltages or in rf SQUIDS. We evaluate the spin-spin correlation function at the Toulouse point of the Kondo model exactly and show that there is a smooth crossover between non-equilibrium and equilibrium behavior as the non-equilibrium initial state decays as a function of the waiting time. In particular, we demonstrate that the decaying non-equilibrium state cannot be thought of as an equilibrium state with an effective temperature that depends on the waiting time.

Finite temperature Drude weight of strongly correlated 1d systems

•Stefan Glocke und Andreas Klümper
Universität Wuppertal, Theoretische Physik, Gauß -Straß e 20

We present results for the Drude weight D_s of the spin conductivity s of the integrable spin-1/2 XXZ chain at arbitrary temperature (s = electrical conductivity in the spinless fermion language of the model). The results are obtained by a combination of linear response theory (Kubo formula) and solution techniques of Bethe ansatz type. Unlike the case of thermal conductivity, the state of investigation of the spin conductivity of the XXZ chain is less complete if not controversial. The reason for this lies in the non-conservation of the spin (charge) current. Despite this, there is a delta-function peak at zero frequency in the dynamical conductivity s(w) = D_s·d(w)+s_reg(w). The controversies of [1],[2] and more recent treatments arise about the anisotropy (interaction) and temperature dependence of D_s. Some of the controversies are resolved by the observation of a strong variation of the energy level curvatures even within a microcanonical ensemble between ``very similar states''.

[1] B.N. Narozhny, A.J. Millis, N. Andrei: Phys. Rev. B 58, R2921 (1998)

[1] X. Zotos: Phys. Rev. Lett. 82, 1764 (1999)

General susceptibilities of highly correlated electron systems from a local point of view

•Sebastian Schmitt und Norbert Grewe
Institut für Festkörperphysik, Hochschulstraß e 6, 64289 Darmstadt

Starting from a cumulant expansion in terms of the transfer between lattice sites in the presence of large local Coulomb matrix elements, Bethe-Salpeter equations are derived. Using only local particle-hole or particle-particle irreducible vertices, as in the dynamical mean field theory, general susceptibilities can be approximated. Employing a decoupling scheme for the energy summations, a typical Stoner-form is obtained, where the local interaction part is explicitly known.

In the case of the Hubbard model the magnetic susceptibility is studied in detail.

Charge Fluctuations in the t-U-V₁-V₂ Model

•Satoshi Ejima¹, Florian Gebhard¹, Satoshi Nishimoto¹ und Yukinori Ohta^2
1Fachbereich Physik, Philipps-Universität Marburg, 35032 Marburg
²Department of Physics, Chiba University, Chiba 263-8522, Japan

Recently, charge dynamics has attracted much interest in the transition-metal oxide PrBa₂Cu₃O₇ and in the organic conductor (TYMTSF)₂. In the low-energy physics of these materials, charge fluctuations play a crucial role, due to the frustration of the long-range Coulomb interactions.

The minimal model for these systems is the one-dimensional quarter-filled Hubbard model with nearest-neighbor (V₁) and next-nearest-neighbor (V₂) Coulomb interactions. We investigate the ground state and some excited states using the (dynamical) density-matrix renormalization group technique. We present an accurate phase diagram and discuss the possible emergence of a metallic (or superconducting) phase induced by charge fluctuations. Moreover, we compare our single-chain results with calculations for double-chain systems such as PrBa₂Cu₄O₈ [1].

[1] S. Nishimoto and Y. Ohta, cond-mat/0305610

Analytical and numerical treatment of the Mott-Hubbard insulator in infinite dimensions

•Satoshi Nishimoto¹, Satoshi Ejima¹, Florian Gebhard¹, Eva Kalinowski¹, Reinhard M. Noack¹ und Eric Jeckelmann^2
1Fachbereich Physik, Philipps-Universität Marburg, 35032 Marburg
²Institut für Physik, Johannes-Gutenberg Universität Mainz, 55099 Mainz

The half-filled Hubbard model on a Bethe-lattice with infinite coordination number describes a Mott-Hubbard insulator above a critical interaction strength. We extend our previous study [1] by using the Dynamical Density-Matrix Renormalization Group method (DDMRG) [2,3] to solve the Dynamical Mean-Field equations. To this end, we analyze a single-impurity Anderson model with up to n_s = 64 levels in the bath. We refine our results in [1] which were limited to n_s = 14 in exact diagonalization. Furthermore, we extend our analytical results on the ground-state energy of the Mott-Hubbard insulator, to ninth-order in the inverse coupling strength.

[1] M.P. Eastwood, F. Gebhard, E. Kalinowski, S. Nishimoto, and R.M. Noack, Eur. Phys. J. B 35, 155 (2003).

[2] E. Jeckelmann, Phys. Rev. B 66, 045114 (2002).

[3] F. Gebhard, E. Jeckelmann, S. Mahlert, S. Nishimoto, and R.M. Noack, cond-mat/0306438.

Quartic Oscillator: Diagonalization by Continuous Unitary Transformations

•Sébastien Dusuel und Götz S. Uhrig
Zülpicherstr. 77, D-50937 Köln, Germany

The method of Continuous Unitary Transformations (CUTs) has been applied successfully to various many-body Hamiltonians. We here consider a simple model, namely a single quartic oscillator. Our aim is two-fold: First, it can be used as a pedagogical and illustrative introduction to the CUTs. Second, thanks to its simplicity, the quartic oscillator allows to gain an intuition about the CUTs. Various generators of the transformations are tested and their efficiency is compared. Various high-order truncation schemes are equally analyzed. The results are also compared to results from the functional renormalization group.

The physics of chiral helices in a metal

•Inga Fischer und Achim Rosch
Institut für Theoretische Physik, Universität zu Köln

At high pressures, MnSi exhibits non-Fermi liquid behaviour over a wide temperature and pressure range [1,2]. This may point to the existence of a genuine non-Fermi liquid phase in an extremely pure cubic system. Recent neutron scattering measurements [3] suggest the presence of partial helical order in this phase, motivating us to study the physics of chiral helices theoretically.

Helical order with a large pitch is stabilized by weak spin-orbit (Dzyaloshinskii-Moriya) interactions in the non-centrosymmetric crystal structure of MnSi. The interplay of static helical order and further spin-orbit interactions in the conduction electron bands leads to the formation of surprisingly large band-gaps in certain regions of the Brillouin zone. We discuss their experimental consequences and furthermore investigate the nature of magnetic fluctuations in MnSi.

[1

Die stark-Kopplungs-Phase des ionischen Hubbard Modells

•Salvatore Manmana^1,2, Volker Meden³, Reinhard Noack² und Kurt Schönhammer^3
1Institut f. Theoretische Physik III, Universität Stuttgart, Pfaffenwaldring 57/V, 70550 Stuttgart
²AG Vielteilchennumerik, Philipps-Universität Marburg, Renthof 6, 35032 Marburg
³Institut f. Theoretische Physik, Georg-August-Universität Göttingen, Tammannstr. 1, 37077 Göttingen

Das Phasendiagramm des ionischen Hubbard-Modells war ein in der Literatur viel diskutiertes Thema. In einer numerischen Arbeit mit Hilfe der DMRG-Methode (Dichtematrix-Renormierungsgruppe) ist es uns gelungen, ein konsistentes Bild des Phasendiagramms zu erstellen. Bei der Untersuchung der Eigenschaften der Phasen findet man überraschenderweise eine Korrelierte-Isolator-Phase mit einer divergierenden elektrischen Suszeptibilität und einem Potenzgesetzabfall in der Dichte-Dichte-Korrelationsfunktion. In diesem Poster sollen diese ungewoehnlichen Eigenschaften und die bisherigen Erklärungsversuche vorgestellt werden.

Incommensurate charge-density wave in the stripe phase of the two-dimensional Hubbard-Holstein model

•Serguei Varlamov und Götz Seibold
Institut für Physik, BTU Cottbus, PBox 101344, 03013 Cottbus, Germany

We investigate the structure of the stripe phase in the two-dimensional infinite-U Hubbard-Holstein model by means of the slave-boson technique. We find in the mean-field approximation that the incommensurate charge density waves (ICDW) appear in the region of the g vs d phase diagram where an ICDW is expected to form in the frustrated phase-separation scenario. Our main finding is that due to the competition between strong on-site correlations and CDW ordering instability the incommensurability of CDW depends from the number of carriers leading to the reduction of the incommensurability with increasing of hole doping. The influence of long-range Coulomb interaction on the structure of ICDW is discussed.

Electronic structure, magnetism, and superconductivity in pseudo-ladder compounds

•S.-L. Drechsler¹, H. Rosner¹, N.M. Plakida², A. Vladimirov³, H. Eschrig¹ und M. Knupfer^1
1IFW-Dresden, P.O. Box 270116, 01171 Dresden, Germany
²MPI-CPfS Dresden, Germany
³J. Inst. f.  Nuclear Res., Dubna, Russia

The electronic structure of ACu₂O₃ compounds [A=Ca,Mg] has been calculated within the local density approximation and compared with x-ray absorption spectroscopy and magnetic neutron scattering data [1]. Both compounds deviate markedly from the usual pds cuprate picture. Significant interlayer exchange in the stacking c-direction is found to be responsible for the missing spin gap (generic for ideal two-leg ladders) and the finite Néel-temperature. Hence, they can be modelled as weakly coupled anisotropic Heisenberg (Hubbard) bi-layers. Within a derived effective t-J-model in mean-field approximation nodeless superconductivity below 50 K is predicted for Ca_0.88Na_0.12Cu₂O₃ (optimal hole doping).

[1]V. Kiryukhin et al., PRB 64, 1444418 (2001).

Time-dependent Gutzwiller theory of magnetic excitations in the Hubbard model

•Goetz Seibold¹, Federico Becca² und Jose Lorenzana^3
2SISSA, 34014 Trieste, Italy
³Dip. di Fisica, Universita di Roma 'La Sapienza', 00185 Roma, Italy
¹Lehrstuhl Theoretische Physik, BTU Cottbus, 03013 Cottbus, Germany

We use a spin-rotational invariant Gutzwiller energy functional to compute random-phase-approximation-like (RPA) fluctuations on top of the Gutzwiller approximation (GA). Since no restrictions are imposed on the symmetry of the underlying saddle-point solution, our approach is suitable for the evaluation of the magnetic susceptibility and dynamical structure factor in strongly correlated inhomogeneous systems. We present a detailed study of the quality of our approach by comparing with exact diagonalization results and show its much higher accuracy compared to the conventional Hartree-Fock+RPA theory. In infinite dimensions, where the GA becomes exact for the Gutzwiller variational energy, we evaluate ferromagnetic and antiferromagnetic instabilities from the transverse magnetic susceptibility. The resulting phase diagram is in complete agreement with previous variational computations.

Stability of metallic stripes in the extended one-band Hubbard model

•Goetz Seibold¹ und Jose Lorenzana^2
1Lehrstuhl Theoretische Physik, BTU Cottbus, 03013 Cottbus, Germany
²Dip. di Fisica, Universita di Roma 'La Sapienza', 00185 Roma, Italy

Based on an unrestricted Gutzwiller approximation (GA) we investigate the stripe orientation and periodicity in an extended one-band Hubbard model. A negative ratio between next-nearest and nearest neighbor hopping t¢/t, as appropriate for cuprates, favors partially filled (metallic) stripes for both vertical and diagonal configurations. At around optimal doping diagonal stripes, site centered (SC) and bond centered (BC) vertical stripes become degenerate suggesting strong lateral and orientational fluctuations. We find that within the GA the resulting phase diagram is in agreement with experiment whereas it is not in the Hartree-Fock approximation due to a strong overestimation of the stripe filling. Results are in agreement with previous calculations within the three-band Hubbard model but with the role of SC and BC stripes interchanged.

Ionic Hubbard Model in High Dimension

•Krunoslav Pozgajcic und Claudius Gros
Universität des Saarlandes, Fakultät 7, Theoretische Physik, Postfach 15 11 50, D-66041 Saarbrücken

Ionic Hubbard model was proposed for the description of organic mixed-stack charge-transfer crystals and also for the description of the ferroelectric transition in perovskite materials. Theoretical efforts have mostly been directed to the one dimensional case, which has be thoroughly investigated by various methods on disposal for the one dimensional systems. Recently, the self-energy functional approximation (SFA) has been proposed (M. Potthoff, Eur. Phys. J. B 32, 429(2003)) as a unifying scheme for a few cluster methods. We will present results for the one dimensional and the higher dimensional ionic Hubbard model calculated in the SFA framework.

Self-energy effects in the two-dimensional Hubbard-Model from Functional Renormalisation

•Daniel Rohe
Max-Planck-Institut FKF, Heisenbergstr. 1, 70569 Stuttgart

We present a numerical study of the two-dimensional Hubbard Model in the normal state at low temperatures by means of a functional renormalisation group scheme. It is found that anomalous features arise in the self-energy slightly above a mean-field transition temperature T*. This offers information on possible mechanisms responsible for unusual properties of the model in the normal (non-symmetry-broken) state.

Druckeinfluss auf die magnetische Ordnung in CeNiGa₂

•Andreas Eichler¹, Ernst Bauer², Robert Hauser², Heinrich Weiss¹ und Torsten Burghardt^1
1Institut für Technische Physik, TU Braunschweig, Mendelssohnstr. 2, D-38106 Braunschweig
²Institut für Festkörperphysik, TU Wien, A-1040 Wien

Die Verbindung CeNiGa₂ ist bekannt als antiferromagnetisches Kondogitter mit einer Néel-Temperatur von ca. 4 K und relativ hoher elektronischer spezifischer Wärme von 490 mJ/molK. Messungen des elektrischen Widerstandes unter Druck hatten ergeben, dass T_N schnell abnimmt und nahe 0,4 GPa verschwindet.

Wir präsentieren hier Ergebnisse aus Messungen der magnetischen Suszeptibilität und spezifischen Wärme unter Druck. Danach sind noch schwache Kennzeichen für einen magnetischen Ordnungsübergang jenseits der 0,4 GPa zu finden, die nahe 0,5 GPa definitiv verschwinden. Dabei bildet sich sukzessive ein Verlauf von c heraus, der mit abnehmender Temperatur ungefähr logarithmisch ansteigt.

Specific heat of PrOs₄Sb₁₂ at high magnetic fields

•K. Grube¹, S. Drobnik², C. Pfleiderer², A. Faisst², R. Vollmer², H. v. Löhneysen^1,2, E.D. Bauer³ und M.B. Maple^3
3Department of Physics and Institute of Pure and Applied Physical Sciences, University of California San Diego, La Jolla, California 92093-0319
¹Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsuhe
²Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsuhe

PrOs₄Sb₁₂ is the first Pr based heavy-fermion superconductor. Detailed measurements of the low temperature specific heat [1] have helped identify the superconductivity in PrOs₄Sb₁₂ as a strong contender for quadrupolar pairing, i.e., superconductivity that is neither electron-phonon nor magnetically mediated. Here we focus on a related issue, the formation of an ordered phase at low temperatures and high magnetic fields, that appears to be connected with the Zeeman splitting of the crystal electric field ground state. We have revisited this high field phase in a detailed study of the specific heat up to 14 T at temperatures down to 0.4 K. The possible nature of the high-field phase is discussed in the context of the crystal field ground state and the evidence for several superconducting phases.

[1] R. Vollmer et al., Phys. Rev. Lett. 90 (2003) 057001.

Thermische Ausdehnung von CeCu_6-xAu_x

•K. Grube¹, P. Estrela², A. de Visser², O. Stockert³ und H. v. Löhneysen^1,4
1Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe
²Van der Waals-Zeeman Institute, Universiteit van Amsterdam, 1018XE Amsterdam
³Max-Planck-Institut für chemische Physik fester Stoffe, D-01187 Dresden
⁴Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsruhe

CeCu_6-xAu_x ordnet oberhalb von x > 0.1 antiferromagnetisch. Während der magnetische Ordnungsvektor sich drastisch zwischen x = 0.4 und 0.5 ändert, steigt die Néeltemperatur zwischen x = 0.1 und 1.0 linear an. Am Übergang zur magnetischen Ordnung zeigen die linearen thermischen Ausdehnungskoeffizeinten für x = 0.3 positive Sprünge in a und b Richtung und einen negativen Sprung in c Richtung. Für x = 0.5 wechseln die Sprünge entlang der a und b Achse ihr Vorzeichen. Dieses Verhalten scheint die drastische Änderung des Ordnungsvektors widerzuspiegeln. Mit Hilfe der elastischen Konstanten lassen sich die resultierenden uniaxialen Druckabhängigkeiten der Néeltemperatur in Dehnungsabhängigkeiten umrechnen und mit der Dotierungs- und Druckabhängigkeit der magnetischen Struktur vergleichen.

Ferromagnetic to paramagnetic quantum phase transition of single crystal CeSi_1.81 at high pressure

•S. Drotziger¹, C. Pfleiderer¹, M. Uhlarz¹, H. v. Löhneysen^1,2, D. Souptel³, W. Löser³ und G. Behr^3
1Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsuhe
²Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsuhe
³Institut of Solid State and Materials Research Dresden, Postfach 270116, D-01171 Dresden

Silicon deficient CeSi_2-x crystallizes over a large range of compositions in the orthorhombic a-GdSi₂ structure. Below a critical Si content, easy-plane ferromagnetism is observed. Being one of the rare examples of a ferromagnetic Ce compound, the nature of this ferromagnetic state has been studied intensively in the past. Here we report a high pressure study of the magnetisation of a new generation of CeSi_2-x single crystals grown with an optical floating zone furnace under ultrapure conditions. In CeSi_1.81 where T_C = 9.5 K, moderate pressures are sufficient to suppress ferromagnetism. The easy-plane ferromagnetic to paramagnetic quantum phase transition is compared with the predictions of weak-coupling spin fluctuation theory.

Investigation of the Low Ordered Moment in CeBiPt

•G. Goll¹, O. Stockert², M. Prager³, N. Stüßer⁴ und T. Takabatake^5
3Institut für Festkörperforschung, Forschungszentrum Jülich, 52425 Jülich
⁴Hahn-Meitner-Institut, 14109 Berlin
⁵Department of Quantum Matter ASDM, Hiroshima University, Higashi Hiroshima, 739-8526, Japan
¹Physikalisches Institut, Universität Karlsruhe, 76128 Karlsruhe
²Max-Planck-Institut CPfS, 01187 Dresden

The electronic properties of CeBiPt show semimetallic behavior with low charge carrier concentration n_h = 7.7 ·10¹⁷ cm^-3. Shubnikov-de Haas experiments reveal a Fermi surface consisting mainly of a small hole pocket around the G point with a volume of 1.5 ·10^-4 of the volume of the first Brillouin zone [1]. To explain these result by band-structure calculation a local character of the 4f electrons is required. The magnetic behavior supports a local character as well. The magnetic susceptibility follows a Curie-Weiss law above T = 100 K with the full moment of free Ce³⁺ ions and magnetic order was found below T_N = 1.1 K by specific-heat measurements [2]. However, previous neutron-diffraction data suggested an ordered moment well below 1 m_B. Crystal-electric fields (CEF) in the cubic crystal symmetry can only partly account for the reduced moment. Inelastic neutron scattering as well as neutron diffraction on powder were used to investigate the CEF ground state and the magnetically ordered moment. The measurements suggest additional magnetic excitations with a characteristic temperature of 10 - 20 K being responsible for the low ordered moment.

[1] G. Goll et al., Europhys. Lett. 57, 233 (2002).

[2] T. Pietrus et al., Physica B 281 & 282, 745 (2000).

Successful growth of the intermetallic compound CeCu₂(Si,Ge)₂ by high-temperature solution

•M. Deppe, H. S. Jeevan, R. Borth, C. Geibel und F. Steglich
Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dreden, Germany

Among the intermetallic compounds the system CeCu₂(Si,Ge)₂ is of high interest because of the interplay between heavy-fermion superconductivity and magnetism. For an intensive microscopic study large single crystals of high quality are necessary. Large single crystals of this compound were not available because of the incongruent formation of CeCu₂Si₂ and substantial loss of copper during the typical growing process. To overcome this difficulty the typical procedure is to use an off-stochiometric starting melt with an excess of copper. Four different ground states exist for CeCu₂Si₂ which are very sensitiv to the exact Cu/Si ratio. This makes it difficult to grow single crystals with a predetermind ground state. To overcome this problem we started the growth process with higher amount of Germanium because this reduced the variety of physical ground states to only one. Later on we reduce the Germanium content. We will discuss the optimal growth conditions for our modified Bridgman technique using copper as flux. For the physical characterisation we used resistivity and specific heat measurements. These measurements show that our crystal growth method gives reproducible results and evidence a systematic development of the magnetic state with increasing Germanium content in the concentration range x £ (0 - 0.5). The discussion will focus on the future optimization of our growth methods and the comparison with other crystal grow techniques.

Transferred hyperfine fields in the heavy fermion compounds CeCu_6-xAu_x

•Max Winkelmann, Gerda Fischer und Elmar Dormann
Physikalisches Institut, Universitaet Karlsruhe, D-76128 Karlsruhe

CeCu₆ is one of the best known heavy-fermion paramagnets, CeCu₅Au₁ is a heavy-fermion antiferromagnet with T_N = 2.3 K, and for x » 0.1 signatures of non-Fermi-liquid behaviour were reported. We present temperature dependent studies of ⁶³Cu- and ⁶⁵Cu-NMR line shifts for CeCu_6-xAu_x (x=0;0.1;1) single-crystal powder samples. These are different to the earlier[1] presented line shifts for polycrystalline powder samples. Via the correlation with the temperature dependence of the static magnetic susceptibility, Cu-site dependent Knight shift and transferred hyperfine field coupling constants are derived. Local polarization varies in sign and is larger by a factor of 10 than indicated by the earlier published[2] high-temperature average value.

[1] M. Winkelmann, et al. Eur. Phys. J. B 26, (2002) 199

[2] I. Pop, et al. Czech. J. Phys. B 24 (1974) 1398

Heavy Fermion CePd₃: Coherent State Versus Kondo Holes

•Philipp Haas, Peter Weber, Konstantin Petukhov, Oleg Ushakov, Boris Gorshunov und Martin Dressel
1. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart

The formation of the coherent heavy-fermion state in CePd₃ is studied when doped with non-magnetic La. The La atoms act as Kondo-holes. Low-energy excitations are studied by dc, microwave, THz, and infrared measurements at temperatures from 5 K to 300 K. Pure CePd₃ shows typical features for heavy-fermion systems in the conductivity spectra: A hybridization gap opens at around 1000 cm^-1; below 50 cm^-1 the optical conductivity increases. In addition, at microwave frequency a feature is detected which resembles an extremely small energy gap ( » 2 cm^-1).

Calculated de Haas-van Alphen quantities of CeMIn₅ (M = Co, Rh, and Ir) compounds

•Saad Elgazzar, Ingo Opahle, Roland Hayn und Peter M. Oppeneer
Leibniz-Institute of Solid State and Materials Research, P.O. Box 270016, 01171 Dresden

We report a critical analysis of the electronic structures and de Haas-van Alphen (dHvA) quantities of the heavy-fermion superconductors CeCoIn₅, CeRhIn₅, and CeIrIn₅. The electronic structures are investigated ab initio on the basis of full-potential band-structure calculations, adopting both the scalar and fully relativistic formulations within the framework of the local spin-density approximation (LSDA). In contrast to another recent study, in which a pronounced change of the Fermi surface due to relativistic effects and therefore the importance of relativistic interactions for the superconductivity was claimed, we find only minor relativistic modifications of the band structure in our calculations. The ab initio calculated dHvA quantities are in good agreement with experimental data for CeCoIn₅ and CeInIn₅, when we adopt the delocalized LSDA description for the Ce 4f states. For CeRhIn₅, however, a better agreement with experiment is obtained when the Ce 4f electron is treated as a localized core electron. The implications for an emerging picture of the localization behavior of the 4f electron in these materials are discussed. We furthermore compare our calculated dHvA quantities with other recent relativistic calculations and discuss the differences between them.

Transport Properties of LaTiO_3+x Films and Heterostructures

•Andreas Schmehl¹, Frank Lichtenberg¹, Darrell Schlom², Hartmut Bielefeldt¹ und Jochen Mannhart^1
1Experimentalphysik VI, Elektronische Korrelationen und Magnetismus, Institut für Physik, Universität Augsburg, Universitätsstr. 1, D-86135 Augsburg, Deutschland
²Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802-5505, USA

We report on the transport properties of LaTiO_3+d and LaTiO_3.5 films. The LaTiO_3+d samples show metallic transport and several samples exhibit a hysteretic drop of resistance during cooldown at ~ 240 K. The ferroelectric LaTiO_3.5 samples, grown in capacitor structures, have nonlinear, diode-like and hysteretic V(I)-characteristics. Two charge-controlled transport regimes are found, which can be utilized for switching the devices between two voltage states.

Eigenschaften von YbAl₂ bei tiefen Temperaturen

•T. Görlach¹, C. Pfleiderer¹ und H. v. Löhneysen^1,2
2Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe
¹Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsruhe

Die zwischenvalente Legierung YbAl₂ hat bei tiefen Temperaturen eine Reihe interessanter Eigenschaften. So wurde bei Messungen oberhalb 100 K festgestellt, dass die Magnetisierung nur schwach temperaturabhängig ist und das für zwischenvalente Verbindungen typische Verhalten zeigt [1]. Wir präsentieren Messungen der Magnetisierung, des Widerstandes und der spezifischen Wärme im Temperaturbereich zwischen 1,5 und 300 K. Der elektrische Widerstand ist bei tiefen Temperaturen (T £ 20 K) proportional zu T³ - ein auch bei anderen zwischenvalenten Verbindungen beobachtetes Verhalten. Die spezifische Wärme wird durch den Gitterbeitrag dominiert, weicht aber von der Debye-Funktion ab, was auf zusätzliche Anregungen hinweist. Die Magnetisierungsmessungen sind qualitativ konsistent mit älteren Messungen für T > 100 K und zeigen bei tieferen Temperaturen Curie-Verhalten.

[1] A. IANDELLI, A. PALENZONA, Journal of the Less-Common Metals 29 (1972), 293

Single crystal studies of a new ternary Europium pnictide EuZn₂Sb₂

•F. Weber¹, A. Cosceev¹, A. Nateprov^1,2, M. Uhlarz¹, C. Pfleiderer¹ und H. v. Löhneysen^1,3
1Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsuhe, Germany
²Institute of Applied Physics, Academei 5, MD-2028 Kishinev, Moldova
³Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe, Germany

We report the synthesis and characterisation of a new ternary Europium pnictide EuZn₂Sb₂. Single crystals were grown by a solid state reaction of high-purity starting materials in carbon glass crucibles. The crystal structure was determined using powder x-ray diffraction. Single crystals were oriented using Laue x-ray diffraction. EuZn₂Sb₂ is isostructural with the hexagonal Yb compound YbZn₂Sb₂. The bulk properties were characterised in terms of measurements of the resistivity, magnetoresistance, Hall effect, specific heat and magnetisation. Pronounced features characteristic of antiferromagnetic order with T_N = 13.3 K are observed. For field parallel to the c-axis the antiferromagnetism is suppressed above 2.7 T while it persists up to ~ 4.7 T for field in the basal plane. An analysis of the nature of the antiferromagnetic state as inferred from the bulk properties is presented.

Orbital and spin exchange in LiNiO₂

•Roland Hayn¹, Daré Anne-Marie¹ und Richard Jean-Louis^2
1L2MP, Université Aix-Marseille, Fac. St. Jerôme, 13397 Marseille Cedex 20, France
²Centre de Physique Theorique, Luminy Case 907, 13288 Marseille Cedex 9, France

We derive an effective orbital and spin exchange Hamiltonian using a perturbative approach to study the compound LiNiO₂. We show that the inclusion of the trigonal crystal field splitting at the oxygen sites leads to the appearance of antiferromagnetic exchange integrals in deviation from the Goodenough-Kanamori-Anderson rules for this 90 degree bond. That gives a microscopic foundation for the recently observed coexistence of ferromagnetic and antiferromagnetic couplings in the orbitally frustrated state of LiNiO₂.

High energy photoemission study on the Verwey transition in Fe₃O₄

•David Schrupp¹, Michael Sing^1,2, Sigemasa Suga² und Ralph Claessen^1
1Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany
²Department of Material Physics, Osaka University, Osaka 560-8531, Japan

Magnetite (Fe₃O₄) undergoes a first-order phase transition called the Verwey transition (VWT). Although known for a long time the VWT is currently a matter of intensive debate questioning fundamental issues such as the charge ordering of Fe²⁺ and Fe³⁺ ions which is widely believed to be the driving mechanism of the VWT.

We present a study of the VWT by valence band high energy photoemission. Compared to VUV PES this enhances the probing depth by at least a factor of 2 with surface effects greatly reduced. Nonetheless, we still observe subtle differences in the valence band spectra between differently prepared surfaces. While the spectra of sputtered, annealed, and post-oxidized (111) and (100) single crystal surfaces display a continuous temperature evolution through the Verwey temperature (T_V), a first order VWT is observed on surfaces obtained by fracturing a crystal at 40 K. The non-equilibrium atomic configuration of these surfaces is thus apparently rather bulk-like, whereas the annealed samples display strong surface relaxations. The spectra at T < T_V of the fractured samples are identified as those of a correlated insulator with a gap of at least 150 meV, which at T_V closes into a pseudogap situation. The absence of clear quasiparticle signatures characterizes the T > T_V phase as an incoherent metal, consistent with transport and optical data.

Metal-Insulator transitions driven by magnetic field in half-doped manganites

•Horacio Aliaga und Dieter Vollhardt
Institut fuer Physik, Universitaet Augsburg D-86135, Germany

We study the first order transition between an antiferromagnetic (AF) charge-ordered (CO) insulator to a ferromagnetic (FM) metal under external applied magnetic field. The temperature variation of the charge gap and magnetization as a function of temperature for different magnetic fields are calculated using a model that considers the competition between double exchange (DE), super exchange (SE) and electron-phonon interactions. We show a comparison to transport and magnetization experiments in related compounds.

Comparison of LSDA+U calculations and photo emission data of Fe₃O₄

•David Schrupp¹, Ivan Leonov¹, Alexander Yaresko², Shigemasa Suga³, Vladimir Anisimov⁴ und Ralph Claessen^1
2Max Planck Institute for Physics of Complex Systems, 01187 Dresden, Germany
³Department of Material Physics, Osaka University, Osaka 560-8531, Japan
⁴Institute of Metal Physics, Ekaterinburg GSP-170, Russia
¹Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany

Magnetite, undergoes a first-order phase transition, reflected by an increase of electrical resistivity by two orders of magnitude, called the Verwey transition. Although known for a long time the Verwey transition is currently a matter of intensive debate questioning fundamental issues such as the charge ordering of Fe²⁺ ions which is widely believed to be the driving mechanism of the Verwey transition.

The LSDA+U calculations based on new structure refinements in the low temperature phase of magnetite result in a charge and orbitally ordered insulator. The self-consistent solution corresponding to charge order does not satisfy the widely-accepted Anderson condition of minimum electrostatic repulsion, but agrees with photoemission spectra, taken below the Verwey transition temperature.

These spectra where obtained by using polished, sputtered, annealed and post-oxidized single crystals. The correct stoichiometry and long-range order of the thus prepared surfaces were proven by LEED, STM, and XPS. The samples were measured with high energy photoemission ((^h/_2p)w » 700 eV), which means an enhanced information depth.

Dimerization versus Orbital Moment Ordering in the Mott insulator YVO₃

•Peter Horsch¹, Giniyat Khaliullin¹ und Andrzej M. Oles^2
1Max-Planck-Institut f. Festkoerperforschung, D-70569 Stuttgart, Germany.
²Smoluchowski Institute of Physics, PL-30059 Krakow, Poland.

We investigate the magnetic and orbital ordering within the spin-orbital model [1] for cubic vanadates using exact diagonalization in combination with mean-field theory [2]. Increasing Hund's exchange J_H triggers a crossover from the valence-bond orbital states to C-type antiferromagnetic (AF) phase, while a finite spin-orbit coupling induces the staggered component of magnetization which couples to the t_2g orbital moments. Our results provide a qualitative explanation of the observed spin canting and large reduction of magnetization in the C-AF phase of YVO₃ [3]. At finite temperature an orbital instability in the C-type antiferromagnetic phase induces modulation of magnetic exchange constants even in the absence of lattice distortions. This dimerization should be distinguished from a usual Peierls instability, as it emerges at finite temperatures (and vanishes at zero temperature) due to an interplay of quantum effects and thermal fluctuations, which open the way towards dimerized orbital- and spin-correlations. The calculated spin structure factor shows a magnon splitting at [q\vec] = (0,0,\fracp2) due to the orbital dimerization, similar to the spinwave dispersions measured by Ulrich et al.

[1] G. Khaliullin, P. Horsch, A.M. Oles, Phys. Rev. Lett. 86,3879 (2001).

[2] P. Horsch, G. Khaliullin, A.M. Oles, Phys. Rev. Lett. in print (2003).

[3] C. Ulrich et al., Phys. Rev. Lett. in print (2003).

Orbital assisted metal insulator transition in VO₂

•L.H. Tjeng¹, T. Koethe¹, M.W. Haverkort¹, Z. Hu¹, A. Tanaka², S. Streltsov³, M. Korotin³, V. Anisimov³, W. Reichelt⁴, H.H. Hsieh⁵, H.-J. Lin⁵ und C.T. Chen^5
1II. Physikalisches Institut, Universität zu Köln
²ADSM, Hiroshima University, Japan
³IMP, Ekaterinburg, Russia
⁴Institut für Anorganische Chemie, Dresden
⁵NSRRC, Hsinchu, Taiwan

VO₂ is a non-magnetic oxide that undergoes a metal-to-insulator transition at 340 Kelvin. Above this temperature, VO₂ is metallic and has a rutile (TiO₂) structure (R-phase). At low temperatures, it is an insulator with a monoclinic structure (M₁-phase), in which V-V pairs are formed.

The long-standing debate about this compound concerns the nature of the metal-to-insulator transition. The issue is whether the non-magnetic insulating state would be regarded as a Peierls-insulator with the character of a band insulator (one-electron picture), or whether it should be viewed as a Mott-insulator (many-body picture).

We have used polarization dependent soft-X-ray absorbtion at the V L_2,3 edges to investigate the local electronic structure of VO₂, and have found that the V 3d orbital occupation symmetry changes dramatically across the metal-insulator transition. From a comparison with LDA and LDA+U calculations we infer that this phase transition is assisted by the orbital degrees of freedom, i.e. requiring an explanation beyond the classical Peierls mechanism.

Orbital occupation and momentum in LaTiO₃

•M.W. Haverkort¹, Z. Hu¹, H. Roth¹, T. Lorenz¹, C. de Nadai², N.B. Brookes², A. Tanaka³, H.H. Hsieh⁴, H.-J. Lin⁴, C.T. Chen⁴ und L.H. Tjeng^1
1II. Physikalisches Institut, Universität zu Köln
²ESRF, Grenoble, France
³ADSM, Hiroshima, Japan
⁴NSRRC, Hsinchu, Taiwan

LaTiO3 is an antiferromagnetic insulator with a magnetic ordering temperature of T_N » 145 K. Neutron measurements reveal a magnetic moment of 0.45 ~ 0.57 m_B, significantly less than the 1 m_B expected for a S = \frac12 system (Ti = 3d¹). One may envision that this discrepancy can be ascribed to the presence of an anti-parallel aligned orbital momentum in this quasi cubic material. However, neutron data also found an almost perfect isotropic spin wave spectrum with an extremely low spin gap, indicative for the absence of an orbital moment. An orbital liquid model is then proposed to explain these data. We have carried out detailed temperature dependent soft-x-ray absorption and circularly-polarized/spin-resolved photoemission experiments. We find that the orbital moment is practically quenched, in agreement with the neutron spin wave data. Using the results from LDA and LDA+U calculations, we infer that this quenching is caused by non-cubic crystal fields associated with the small but non-negligible non-cubic distortions in LaTiO₃. These crystal fields are much stronger than the spin-orbit interaction, and crucial for the explanation of the lineshape and the lack of temperature dependence of the spectra. It seems that these conditions do not favor the formation of an orbital liquid.

Orbital excitations in LaTiO₃ and YTiO₃ investigated by infrared and Raman spectroscopy

•A. Gössling¹, R. Rückamp¹, M. Grüninger¹, M. Cwik¹, H Roth¹, T. Lorenz¹, A. Freimuth¹, B. Keimer² und C. Ulrich^2
1II. Physikalisches Institut, Universität zu Köln
²Max-Planck-Institut FKF, Stuttgart

There is still discussion about the puzzling compound LaTiO₃ [1,2,3]. We compare LaTiO₃ with orbitally ordered YTiO₃ using IR and Raman spectroscopy. With IR we observe broad peaks at approximately 0.30 eV in both compounds, with Raman at 0.25 eV. The difference in energy is due to the simultaneous excitation of a phonon in IR spectroscopy, breaking the parity selection rule. This clearly reveals the even parity of the excitation, typical for orbital excitations. We discuss the origin of the peaks within the framework of different theoretical models. Supported by the DFG through SFB 608.

[1] B. Keimer et al., Phys. Rev. Lett. 85, 3946 (2000).

[2] G. Khaliullin et al.,

[Phys. Rev. Lett. 85, 3950 (2000). 3] M. Cwik et al., Phys. Rev. B 68, 060401 (2003).

Search for orbital excitations in YTiO₃: calculation of phonon dispersion relations

•M. Guennou¹, C. Ulrich¹, C. Frost², S. Miyasaka³, Y. Taguchi³, Y. Tokura³ und B. Keimer^1
1Max-Planck-Institut FKF, Stuttgart
²ISIS, Oxford, England
³Departement of Applied Physics, University of Tokyo, Japan

The purpose of our study was to search for elementary excitations of the orbital magnetization, the so-called orbitons, in YTiO₃. We have performed single crystal inelastic neutron scattering experiments up to high energies (350 meV) at the spectrometer MAPS at the ISIS facility in Oxford. In order to give an interpretation of the features observed at high energies, we have calculated the phonon dispersion relations and the first and higher order density of states. There is a good agreement between the calculated phonon dispersion relations and the first order phonon modes measured by neutron and Raman scattering. These results will be compared with the theoretical predictions for the orbital dispersion relations given by G. Khaliullin [1].

[1] G. Khaliullin et al., Phys.Rev.Lett. 89, 167201 (2002).

Giant phonon anomalies in the pseudo-gap phase of TiOCl

•P. Lemmens^1,2, K.-Y. Choi², G. Caimi³, L. Degiorgi^3,4, A. Seidel⁵ und F.C. Chou^5
22. Physikalisches Institut, D-52056 RWTH Aachen
³Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich
⁴Paul Scherrer Institute, CH-5232 Villigen
⁵Center for Material Science and Engineering, MIT, Cambridge, MA 02139
¹MPI for Solid State Research, D-70569 Stuttgart

We report infrared and Raman spectroscopy results of the spin-1/2 quantum magnet TiOCl. Giant anomalies are found in the temperature dependence of the phonon spectrum, which hint to unusual coupling of the electronic degrees of freedom to the lattice. These anomalies develop over a broad temperature interval, suggesting the presence of an extended fluctuation regime. This defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a dimensionality cross-over leads to a dimerized ground state with a global spin-gap of about 2D_spin »  430 K. Work supported by the DFG SPP1073, MRSEC Program of NSF under award number DMR 02-13282, NATO PST.CLG.9777766, INTAS 01-278, and the Swiss National Foundation for the Scientific Research.

Orbital-spin coupling in La_1-xSr_1+xMnO₄ observed by Raman spectroscopy

•K.-Y. Choi¹, D. Heydhausen¹, T. Sahaoui¹, P. Reutler², B. Büchner², P. Lemmens^1,3 und G. Güntherodt^1
12. Physikalisches Institut, RWTH Aachen
²Institute for Solid State Research, IFW Dresden
³MPI for Solid State Research, Stuttgart

We present an inelastic light scattering study of the layered manganites La_1-xSr_1+xMnO₄ (x = 0 and 1/8). The samples have the K₂NiF₄-type tetragonal structure (symmetry group I4/mmm ) and show a C-type antiferromagnetic ordering at T_N » 125 K for x = 0 as well as of an orbital ordering with a dominant character of the d_3z²-r² orbital. The undoped sample shows a pronounced two-magnon spectrum (2MS) in the ab plane. The 2MS undergoes a strong damping already above 1.5 T_N and evolves into quasielastic scattering consisting of two components. The latter implies the rapid suppression of short-range antiferromagnetic correlations while there appear other magnetic correlations. In addition, a symmetry-forbidden phonon mode is observed around 700 cm^-1 in the high-temperature region for in-plane polarizations. Surprisingly, new modes appear upon cooling below T_N. This is due to zone-folded modes induced by the formation of a superstructure. This provides evidence for induced orbital ordering through spin-orbital coupling in the plane. As to x = 1/8 the 2MS is totally suppressed while activated phonon modes develop which are not Raman-active for x = 0. This feature is discussed in terms of the mixture of d_x²-y² with d_3z²-r² orbitals as well as of the motion of holes. Work supported by DFG SPP1073

Orbital-induced phonon anomalies in (La_1-yPr_y)_1-xSr_xMnO₃ and LaMnO_3+d

•K.-Y. Choi¹, P. Lemmens^1,2, G. Güntherodt¹, Yu. Pashkevich³, V. Gnezdilov⁴, P. Reutler⁵, B. Büchner⁵ und A. Revcolevschi^6
4B. I. Verkin Inst. for Low Temp. Physics NASU, Ukraine
⁵Institute for Solid State Research, IFW Dresden
⁶Laboratoire de Physico-Chimie, Université Paris-Sud, France
¹2. Physikalisches Institut, RWTH Aachen
²MPI for Solid State Research, Stuttgart
³A. A. Galkin Donetsk Phystech NASU, Ukraine

We present an inelastic light scattering study of single crystalline (La_1-yPr_y)_1-xSr_xMnO₃ ( 0 £ x £ 0.14,  y = 0 and x = 1/8,  0 £ y £ 0.5) and LaMnO_3+d (0.071 £ d £ 0.125). The studied samples range from canted antiferromagnetic insulating (CAF) to ferromagnetic insulating (FMI) phase. A giant softening up to 20 -30 cm^-1 of the Mn-O breathing mode around 610 cm^-1 is observed only for the FMI samples (0.11 £ x £ 0.14 and 0.085 £ d £ 0.125) upon cooling below the Curie temperature. Moreover, Pr-doping leads to the gradual suppression of the softening. This is attributed to a coupling of the breathing mode to orbital polarons, giving evidence for their presence in the FMI phase. No obvious saturation of the softening signals the instability of an orbital polaron state. In addition, the CAF samples (0 £ x £ 0.1 and d = 0.071) exhibit pronounced multi-phonon features at 1000-1300 cm^-1. Unexpectedly, they undergo a strong softening of the peak energy and a damping as a function of temperature and doping. This odd behavior is interpreted in terms of the change of orbital forms. Work supported by DFG SPP1073

Electron Spin Resonance in charge and orbitally ordered manganites

•J. Deisenhofer¹, H.-A. Krug von Nidda¹, A. Loidl¹, T. Nakajima² und Y. Ueda^2
1EP V, Center for Electronic Correlations and Magnetism, Institute for Physics, Augsburg University, D-86135
²Material Design and Characterization Laboratory,Institute for Solid State Physics, University of Tokyo, 5-1-5

Both the paramagnetic and the magnetically ordered regime have been investigated for La_1-xSr_xMnO₃ (0 £ x £ 0.2) single crystals by ESR: For x=0.05 the analysis of the temperature dependence and the anisotropy of ESR linewidth and g-value in the orbitally ordered phase allows to determine unambiguously the orbital mixing parameter above T_N, indicating that also in pure LaMnO₃ orbital ordering is stabilized to a large extent by superexchange processes in the ferromagnetic bonds. For 0.075 £ x < 0.15 we observe anisotropic quasi-ferromagnetic resonance signals persisting far into the paramagnetic regime, indicating the existence of a Griffiths-like phase above long-range magnetic order on crossing from the canted antiferromagnetic state at x=0.075 to a purely ferromagnetic ground state at x=0.175. Moreover, we report on correlations between the ESR linewidth and resistivity at the charge-ordering transition in the layered manganites RBaMn₂O₆ (R=Y,Dy).

Magnetic and orbital correlations in single layered manganates

•D. Senff¹, O. Friedt¹, M. Ben Omar¹, Y. Sidis², P. Reutler^3,4, A. Revcolevschi⁴ und M. Braden^1
1II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln
²Laboratoire Léon Brillouin,CE-Saclay, F-91191 Gif-sur-Yvette
³IfW Dresden, Helmholtzstr. 20, D-01069 Dresden
⁴Laboratoire de Physico-Chimie de l'Etat Solide, Université Paris Sud, F-91405 Orsay Cedex

We have studied the magnetic and charge/orbital order in La_1-xSr_1+xMnO₄ by elastic and inelastic neutron scattering. The antiferromagnetic transition and the spinwave dispersion of the parent compound LaSrMnO₄ show a typical 2D behavior. There is a large anisotropy gap of 9meV, but we find additional localized magnetic scattering below this spin-wave gap whose intensities increase upon minor Sr-content enhancement. For higher Sr-concentrations, x > 0.4, the system exhibits at low temperatures charge-orbital ordering, best established for the concentration of x=0.5. In La_0.4Sr_1.6MnO₄ (x=0.6) we find incommensurate ordering most likely induced by the additional number of Mn⁴⁺-ions. The resulting orbital and magnetic structure may be interpreted in a stripe picture similar to the nickelates.

Orbital ordering in single-crystal and thin-film La_7/8Sr_1/8MnO₃

•Y. Su¹, D. Wermeille², O.H. Seeck¹, A. Fattah¹, J. Persson¹, P. Foucart¹, K. Istomin¹, D. Hupfeld¹, Th. Brueckel¹, Zhi-Hong Wang³, G. Cristiani³, H.-U. Habermeier³, F. Wu⁴ und R.L. Johnson^4
2MuCAT, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA
³Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
⁴II Institut fuer Experimentalphysik, Universitaet Hamburg, D-22761 Hamburg, Germany
¹Institut fuer Festkoerperforschung, Forschungszentrum Juelich GmbH, D-52425 Juelich, Germany

The orbital ordering (OO) is believed to be a key ingredient for the metal-to-insulator transition in the ferromagnetic regime of lightly doped manganites. By utilising the resonant X-ray scattering (RXS) technique, a novel OO was identified in the ferromagnetic insulating (FI) regime of single-crystal La_7/8Sr_1/8MnO₃. Meanwhile, Templeton scattering due to anisotropic structural effects was systematically measured at a set of forbidden Bragg reflections in the full temperature range, enabling us to quantitatively determine the atomic scattering tensor in both orbital ordered and disordered states. The complex structural phase transition and lattice modulations associated with this novel OO were also comprehensively examined by high-energy X-ray diffraction. A definite model on the OO in the FI regime will be proposed based on these new X-ray scattering results. In addition, latest results on the RXS investigations of orbital ordering and related structural modulation in thin-film La_7/8Sr_1/8MnO₃ will also be presented

Orbital ordering in manganites on the band approach.

•Dmitry Efremov¹ und Daniel Khomskii^2
1Technische Universität Dresden, Institut für Theoretische Physik, 01062 Dresden
²Universität zu Köln, 50937 Köln

We consider the orbital ordering in LaMnO₃ and similar systems, proceeding from the band picture. We show that for the realistic magnetic structure of A-type there exists a complete nesting between two e_g-bands. As a result there occurs an instability towards Q = (p,p) an excitonic insulator like state - an electron-hole pairing with the wave vector, which opens a gap in the spectrum and makes the system insulating. In the resulting state there appear an orbital ordering - orbital density wave (ODW), the type of which coincides with these existing in LaMnO₃.

Magnetic ordering in the trigonal chain compounds Ca₃CoRhO₆ and Ca₃FeRhO₆

•Udo Schwingenschlögl, Volker Eyert und Ulrich Eckern
Theoretische Physik II, Institut für Physik, Universität Augsburg, 86135 Augsburg

Low-dimensionality and frustration effects in compounds containing magnetic ions have attracted a lot of attention since long owing to the expectation of a variety of fascinating properties. Much interest has focused on systems, where the chains are arranged in a triangular lattice. Continuing recent work on the prototypical compound Ca₃Co₂O₆ [1] we present the results of augmented spherical wave (ASW) electronic structure calculations for the closely related compounds Ca₃CoRhO₆ and Ca₃FeRhO₆. In accordance with experimental data we find strong intrachain magnetic coupling of high-spin 3d-metal sites via the d-states of the interjacent low-spin rhodium sites resulting in the observed ferromagnetic and antiferromagnetic order in Ca₃CoRhO₆ and Ca₃FeRhO₆, respectively. Considerable hybridization with the O 2p states leads to polarization of the latter and the formation of extended magnetic moments, which are well localized at the high-spin sites.

[1] V. Eyert. C. Laschinger, T. Kopp, and R. Frésard, submitted to Chem. Phys. Lett.

[HOME]

[E-Verhandlungen 2004]

[Regensburg]

[Fachverband TT]

[Sitzung TT 24]

Zuletzt geändert am 27.02.2004

Fragen, Kommentare, Anregungen und Kritik