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Sep 21, 2013
09/13

by
F. C. Zhang; C. Gros; T. M. Rice; H. Shiba

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The effective Hamiltonian of strongly correlated electrons on a square lattice is replaced by a renormalised Hamiltonian and the factors that renormalise the kinetic energy of holes and the Heisenberg spin-spin coupling are calculated using a Gutzwiller approximation scheme. The accuracy of this renormalisation procedure is tested numerically and found to be qualitatively excellent. Within the scheme a resonant valence bond (RVB) wavefunction is found at half-filling to be lower in energy than...

Source: http://arxiv.org/abs/cond-mat/0311604v1

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Sep 19, 2013
09/13

by
P. Lemmens; M. Fischer; G. Güntherodt; C. Gros; P. G. J. van Dongen; M. Weiden; W. Richter; C. Geibel; F. Steglich

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The effect of in-chain and off-chain substitutions on 1D spin fluctuations in the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations and the ground state of low dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D gap-induced mode as well as a constant...

Source: http://arxiv.org/abs/cond-mat/9703060v1

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39

Sep 20, 2013
09/13

by
K. Pozgajcic; C. Gros

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We study the ionic Hubbard model at temperature T=0 within the mean-field approximation, and show that the charge gap does not close completely at the ionic-band insulator to antiferromagnetic insulator transition, contrary to previous expectations. Furthermore, we find an intermediate phase for on-site repulsions $U>U_c$ for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.

Source: http://arxiv.org/abs/cond-mat/0301376v2

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Sep 22, 2013
09/13

by
P. Lemmens; K. Y. Choi; A. Ionescu; J. Pommer; G. Guentherodt; R. Valenti; C. Gros; W. Brenig; M. Johnsson; P. Millet; F. Mila

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Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum...

Source: http://arxiv.org/abs/cond-mat/0111065v1

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49

Sep 20, 2013
09/13

by
R. Werner; C. Gros; M. Braden

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Using RPA results, mean field theory, and refined data for the polarization vectors we determine the coupling constants of the four Peierls-active phonon modes to the spin chains of CuGeO_3. We then derive the values of the coupling of the spin system to the linear ionic displacements, the bond lengths and the angles between bonds. Our values are consistent with microscopic theories and various experimental results. We discuss the applicability of static approaches to the spin-phonon coupling....

Source: http://arxiv.org/abs/cond-mat/9810038v2

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49

Sep 22, 2013
09/13

by
C. Gros; W. Wenzel; A. Fledderjohann; P. Lemmens; M. Fischer; G. Güntherodt; M. Weiden; C. Geibel; F. Steglich

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In a magnetic substance the gap in the Raman spectrum, Delta_R, is approximatively twice the value of the neutron scattering gap, Delta_S, if the the magnetic excitations (magnons) are only weakly interacting. But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in the quasi-1D Spin-Peierls compound CuGe_3. We present numerical estimates for Delta_R/Delta_S from exact diagonalization studies for finite...

Source: http://arxiv.org/abs/cond-mat/9612101v1

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67

Sep 22, 2013
09/13

by
G. Y. Chitov; C. Gros

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We study the spin-pseudospin Hamiltonian of the Ising Model in Transverse Field (IMTF) for pseudospins, coupled to the XY-spins on a triangular lattice. This model appears from analyses of the quarter-filled ladder compound $NaV_2O_5$, and pseudospins represent its charge degrees of freedom. In the molecular-field approximation we find that the model possesses two phases: charge-disordered without spin gap; and a low-temperature phase containing both the anti-ferroelectric (zigzag) charge order...

Source: http://arxiv.org/abs/cond-mat/0310494v1

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80

Sep 18, 2013
09/13

by
Kim Louis; C. Gros

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A current of magnetic moments will flow in the spin-1/2 Heisenberg chain in the presence of an external magnetic field $B$ and a temperature gradient $\Delta T$ along the chain. We show that this magnetothermal effect is strictly {\em infinite} for the integrable Heisenberg-model in one dimension. We set-up the response formalism and derive several new generalized Einstein relations for this magnetothermal effect which vanishes in the absence of an external magnetic field. We estimate the size...

Source: http://arxiv.org/abs/cond-mat/0212161v3

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Sep 18, 2013
09/13

by
C. Gros; W. Wenzel; J. Richter

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We present a spin-1/2 bilayer model for the quantum order-disorder transition which (i) can be solved by mean-field theory for bulk quantities, (ii) becomes critical at the transition, and (iii) allows to include intralayer frustration. We present numerical data (for systems with up to 240 sites) and analytical results for the critical coupling strength, ground-state energy, order parameter and for the gap. We show that the critical coupling decreases linearly with frustration.

Source: http://arxiv.org/abs/cond-mat/9510151v1

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68

Sep 23, 2013
09/13

by
C. Gros; G. Kaczor

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The human brain is autonomously active, being characterized by a self-sustained neural activity which would be present even in the absence of external sensory stimuli. Here we study the interrelation between the self-sustained activity in autonomously active recurrent neural nets and external sensory stimuli. There is no a priori semantical relation between the influx of external stimuli and the patterns generated internally by the autonomous and ongoing brain dynamics. The question then arises...

Source: http://arxiv.org/abs/0903.1979v1

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Sep 19, 2013
09/13

by
C. Gros; P. Lemmens; K. -Y. Choi; G. G"untherodt; M. Baenitz; H. H. Otto

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The study of quantum phase transitions, which are zero-temperature phase transitions between distinct states of matter, is of current interest in research since it allows for a description of low-temperature properties based on universal relations. Here we show that the crystal green dioptase Cu_6Si_6O_18 . 6H_2O, known to the ancient Roman as the gem of Venus, has a magnetic crystal structure, formed by the Cu(II) ions, which allows for a quantum phase transition between an...

Source: http://arxiv.org/abs/cond-mat/0208106v1

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1.1K

Jul 22, 2013
07/13

by
C. Gros

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An thorough introduction is given at an introductory level to the field of quantitative complex system science, with special emphasis on emergence in dynamical systems based on network topologies. Subjects treated include graph theory and small-world networks, a generic introduction to the concepts of dynamical system theory, random Boolean networks, cellular automata and self-organized criticality, the statistical modeling of Darwinian evolution, synchronization phenomena and an introduction...

Source: http://arxiv.org/abs/0807.4838v3

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Sep 21, 2013
09/13

by
I. Opahle; H. C. Kandpal; Y. Zhang; C. Gros; R. Valenti

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We investigate the effect of external pressure on the Fe magnetic moment in undoped LaOFeAs within the framework of density functional theory and show that this system is close to a magnetic instability: The Fe moment is found to drop by nearly a factor of 3 within a pressure range of $\pm$ 5 GPa around the calculated equilibrium volume. While the Fe moments show an unusually strong sensitivity to the spin arrangement (type of anti-ferromagnetic structure), the low temperature structural...

Source: http://arxiv.org/abs/0808.0834v3

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50

Sep 20, 2013
09/13

by
C. Gros; P. Lemmens; M. Vojta; R. Valenti; K. -Y. Choi; H. Kageyama; Z. Hiroi; N. V. Mushnikov; T. Goto; M. Johnsson; P. Millet

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We present a comprehensive study of the coupled tetrahedra-compound Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T_N. This energy is determined experimentally. The ordering temperature T_N has an inverse-log dependence on the coupling parameters near...

Source: http://arxiv.org/abs/cond-mat/0301546v1

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44

Sep 17, 2013
09/13

by
F. Capraro; C. Gros

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Using the density matrix renormalization group technique, we evaluate the low-energy spectrum (ground state and first excited states) of the anisotropic antiferromagnetic spin-one-half chain under magnetic fields. We study both homogeneous longitudinal and transversal fields as well as the influence of a transversal staggered field on opening of a spin-gap. We find that only a staggered transversal field opens a substantial gap.

Source: http://arxiv.org/abs/cond-mat/0207279v1

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263

Sep 19, 2013
09/13

by
H. Smolinski; C. Gros; W. Weber; U. Peuchert; G. Roth; M. Weiden; C. Geibel

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A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and...

Source: http://arxiv.org/abs/cond-mat/9801276v2

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90

Sep 18, 2013
09/13

by
G. Y. Chitov; C. Gros

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We study the 2D Ising model on a square lattice with additional non-equal diagonal next-nearest neighbor interactions. The cases of classical and quantum (transverse) models are considered. Possible phases and their locations in the space of three Ising couplings are analyzed. In particular, incommensurate phases occurring only at non-equal diagonal couplings, are predicted. We also analyze a spin-pseudospin model comprised of the quantum Ising model coupled to XY spin chains in a particular...

Source: http://arxiv.org/abs/cond-mat/0401295v2

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96

Jul 20, 2013
07/13

by
P. Lemmens; K. -Y. Choi; E. E. Kaul; Ch. Geibel; K. Becker; W. Brenig; R. Valenti; C. Gros; M. Johnsson; P. Millet; F. Mila

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Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl and Br. While the phase transition observed in the Cl system at T_o=18.2 K is consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.

Source: http://arxiv.org/abs/cond-mat/0107046v1

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Sep 21, 2013
09/13

by
K. Louis; C. Gros

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In the situation of two electro-statically coupled conductors a current in one conductor may induce a current in the other one. We will study this phenomenon, called Coulomb drag, in the Hubbard chain where the two ``conductors'' are given by fermions with different spin orientation. With the aid of a Monte Carlo (MC) approach which we presented in a recent paper we calculate the Transconductance in different variants of the Hubbard chain (with/without impurity and additional [long-ranged]...

Source: http://arxiv.org/abs/cond-mat/0407701v2

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50

Sep 22, 2013
09/13

by
Debanand Sa; C. Gros

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An effective intra- and inter-ladder charge-spin hamiltonian for the quarter-filled ladder compound $\alpha'$-NaV$_2$O$_5$ has been derived by using the standard canonical transformation method. In the derivation, it is clear that a finite inter-site Coulomb repulsion is needed to get a meaningful result otherwise the perturbation becomes ill-defined. Various limiting cases depending on the values of the model parameters have been analyzed in detail and the effective exchange couplings are...

Source: http://arxiv.org/abs/cond-mat/0004025v1

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79

Sep 20, 2013
09/13

by
T. Saha-Dasgupta; Roser Valenti; F. Capraro; C. Gros

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Following the recent discussion on the puzzling nature of the interactions in the nanotubular system Na2V3O7, we present a detailed ab-initio microscopic analysis of its electronic and magnetic properties. By means of a non-trivial downfolding study we propose an effective model in terms of tubes of nine-site rings with the geometry of a spin-diamond necklace with frustrated inter-ring interactions. We show that this model provides a quantitative account of the observed magnetic behavior.

Source: http://arxiv.org/abs/cond-mat/0410066v2

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39

Sep 22, 2013
09/13

by
J. Jensen; P. Lemmens; C. Gros

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Raman light-scattering experiments in the antiferromagnetic phase of the Cu_2Te_2O_5(Br_(1-x)Cl_x)_2 compounds are analyzed in terms of a dimerized spin model for the tetrahedral Cu-clusters. It is shown that the longitudinal magnetic excitation in the pure Br system hybridizes with a localized singlet excitation due to the presence of a Dzyaloshinskii-Moriya anisotropy term. The drastic change of the magnetic scattering intensities observed when a proportion of Br is replaced by Cl ions, is...

Source: http://arxiv.org/abs/cond-mat/0309351v1

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75

Sep 22, 2013
09/13

by
Kim Louis; C. Gros

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In this paper we develop a cluster-variant of the Stochastic Series expansion method (SCSE). For certain systems with longer-range interactions the SCSE is considerably more efficient than the standard implementation of the Stochastic Series Expansion (SSE), at low temperatures. As an application of this method we calculated the T=0-conductance for a linear chain with a (diagonal) next nearest neighbor interaction.

Source: http://arxiv.org/abs/cond-mat/0310465v1

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51

Sep 20, 2013
09/13

by
P. Lemmens; G. Guntherodt; C. Gros

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An overview of one- and two-dimensional quantum spin systems based on transition-metal oxides and halides of current interest is given, such as spin-Peierls, spin-dimer, geometrically frustrated and ladder systems. The most significant and outstanding contributions of magnetic light scattering to the understanding of these materials are discussed and compared to results of other spectroscopies and thermodynamic measurements.

Source: http://arxiv.org/abs/cond-mat/0301413v1

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57

Sep 20, 2013
09/13

by
C. Gros; W. Wenzel

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An iterative procedure for the explicit construction of the nontrivial subspace of all symmetry-adapted configurations with non-zero weight in the ground-state of the infinite-dimensional Hubbard model is developed on the basis of a symmetrized representation of the transition operators on a sequence of Bethe-Lattices of finite depth. The relation ship between these operators and the well known mapping of the infinite-dimensional Hubbard model onto an effective impurity problem coupled to a...

Source: http://arxiv.org/abs/cond-mat/9810318v1

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54

Sep 21, 2013
09/13

by
G. Y. Chitov; C. Gros

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We propose a mechanism for the observed stacking charge order in the quarter-filled ladder compound NaV2O5. Via a standard mapping of the charge degrees of freedom onto Ising spins we explain the stacking order as a result of competition between couplings of the nearest and next-nearest planes with the 4-fold degenerate super-antiferroelectric in-plane order.

Source: http://arxiv.org/abs/cond-mat/0407700v1

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106

Jul 20, 2013
07/13

by
Debanand Sa; R. Valenti; C. Gros

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We develop a generalized Ginzburg-Landau theory for second harmonic generation (SHG) in magnets by expanding the free energy in terms of the order parameter in the magnetic phase and the susceptibility tensor in the corresponding high-temperature phase. The non-zero components of the SHG susceptibility in the ordered phase are derived from the symmetries of the susceptibility tensor in the high-temperature phase and the symmetry of the order parameter. In this derivation, the dependence of the...

Source: http://arxiv.org/abs/cond-mat/9910283v1

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79

Sep 18, 2013
09/13

by
R. Valenti; T. Saha-Dasgupta; J. V. Alvarez; K. Pozgajcic; C. Gros

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We determine the electronic structure of the one-dimensional spin-1/2 Heisenberg compound $\gamma$-LiV$_2$O$_5$, which has two inequivalent vanadium ions, V(1) and V(2), via density-functional calculations. We find a relative V(1)-V(2) charge ordering of roughly $70:30$. We discuss the influence of the charge ordering on the electronic structure and the magnetic behavior. We give estimates of the basic hopping matrix elements and compare with the most studied $\alpha '$-NaV$_2$O$_5$.

Source: http://arxiv.org/abs/cond-mat/0101282v3

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112

Sep 18, 2013
09/13

by
B. Edegger; V. N. Muthukumar; C. Gros

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We review the Resonating Valence Bond (RVB) theory of high temperatur e superconductivity using Gutzwiller projected wave functions that incorporate strong correlations. After a general overview of the phenomenon of high temperature superconductivity, we discuss Anderson's RVB picture and its implementation by renormalised mean field theory (RMFT) and variational Monte Carlo (VMC) techniques. We review RMFT and VMC results with an emphasis on recent development s in extending VMC and RMFT...

Source: http://arxiv.org/abs/0707.1020v1

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31

Jul 26, 2017
07/17

by
F C Zhang, C Gros, T M Rice and H Shiba

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