Optical properties and Raman scattering of vanadium ladder compounds

We investigate electronic and optical properties of the V-based ladder compounds NaV2O5, the iso-structural CaV2O5, as well as MgV2O5, which differs from NaV2O5 and CaV2O5 in the c axis stacking. We calculate ab initio the A_g phonon modes in these compounds as a basis for the investigation of the electron-phonon and spin-phonon coupling. The phonon modes together with the dielectric tensors as a function of the corresponding ion displacements are the starting point for the calculation of the A_g Raman scattering.Comment: 4 pages, 5 figures, .bbl file with references included. Accepted for publication in Physica Script

Cluster Algorithm for a Solid-On-Solid Model with Constraints

We adapt the VMR (valleys-to-mountains reflections) algorithm, originally devised by us for simulations of SOS models, to the BCSOS model. It is the first time that a cluster algorithm is used for a model with constraints. The performance of this new algorithm is studied in detail in both phases of the model, including a finite size scaling analysis of the autocorrelations.Comment: 10 pages, 3 figures appended as ps-file

Loop algorithms for quantum simulations of fermion models on lattices

Two cluster algorithms, based on constructing and flipping loops, are presented for worldline quantum Monte Carlo simulations of fermions and are tested on the one-dimensional repulsive Hubbard model. We call these algorithms the loop-flip and loop-exchange algorithms. For these two algorithms and the standard worldline algorithm, we calculated the autocorrelation times for various physical quantities and found that the ordinary worldline algorithm, which uses only local moves, suffers from very long correlation times that makes not only the estimate of the error difficult but also the estimate of the average values themselves difficult. These difficulties are especially severe in the low-temperature, large-$U$ regime. In contrast, we find that new algorithms, when used alone or in combinations with themselves and the standard algorithm, can have significantly smaller autocorrelation times, in some cases being smaller by three orders of magnitude. The new algorithms, which use non-local moves, are discussed from the point of view of a general prescription for developing cluster algorithms. The loop-flip algorithm is also shown to be ergodic and to belong to the grand canonical ensemble. Extensions to other models and higher dimensions is briefly discussed.Comment: 36 pages, RevTex ver.

Accessing the dynamics of large many-particle systems using Stochastic Series Expansion

The Stochastic Series Expansion method (SSE) is a Quantum Monte Carlo (QMC) technique working directly in the imaginary time continuum and thus avoiding "Trotter discretization" errors. Using a non-local "operator-loop update" it allows treating large quantum mechanical systems of many thousand sites. In this paper we first give a comprehensive review on SSE and present benchmark calculations of SSE's scaling behavior with system size and inverse temperature, and compare it to the loop algorithm, whose scaling is known to be one of the best of all QMC methods. Finally we introduce a new and efficient algorithm to measure Green's functions and thus dynamical properties within SSE.Comment: 11 RevTeX pages including 7 figures and 5 table

Charge ordering in quarter-filled ladder systems coupled to the lattice

We investigate charge ordering in the presence of electron-phonon coupling for quarter-filled ladder systems by using Exact Diagonalization. As an example we consider NaV2O5 using model parameters obtained from first-principles band-structure calculations. The relevant Holstein coupling to the lattice considerably reduces the critical value of the nearest-neighbor Coulomb repulsion at which formation of the zig-zag charge-ordered state occurs, which is then accompanied by a static lattice distortion. Energy and length of a kink-like excitation on the background of the distorted lattice are calculated. Spin and charge spectra on ladders with and without static distortion are obtained, and the charge gap and the effective spin-spin exchange parameter J are extracted. J agrees well with experimental results. Analysis of the dynamical Holstein model, restricted to a small number of phonons, shows that low frequency lattice vibrations increase the charge order, accompanied by dynamically produced zig-zag lattice distortions.Comment: 11 pages, 17 figures, revised version as to appear in Phys. Rev.

Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCu2_2O3_3 and CaV2_2O5_5

We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder system, both by perturbation around the dimer limit and by quantum Monte Carlo simulations. We discuss the influence of the inter-ladder coupling on the spin gap and the dispersion, and present results for the temperature dependence of the uniform susceptibility. The latter was found to be parameterized well by a mean-field type scaling ansatz. Finally we discuss fits of experimental measurements on SrCu$_2$O$_3$ and CaV$_2$O$_5$ to our results.Comment: 7 pages, 8 figure

Thermodynamic and diamagnetic properties of weakly doped antiferromagnets

Finite-temperature properties of weakly doped antiferromagnets as modeled by the two-dimensional t-J model and relevant to underdoped cuprates are investigated by numerical studies of small model systems at low doping. Two numerical methods are used: the worldline quantum Monte Carlo method with a loop cluster algorithm and the finite-temperature Lanczos method, yielding consistent results. Thermodynamic quantities: specific heat, entropy and spin susceptibility reveal a sizeable perturbation induced by holes introduced into a magnetic insulator, as well as a pronounced temperature dependence. The diamagnetic susceptibility introduced by coupling of the magnetic field to the orbital current reveals an anomalous temperature dependence, changing character from diamagnetic to paramagnetic at intermediate temperatures.Comment: LaTeX, 10 pages, 10 figures, submitted to Phys. Rev.

Phase diagram of depleted Heisenberg model for CaV4O9

We have numerically investigated the 1/5-depleted Heisenberg square lattice representing CaV4O9 using the Quantum Monte Carlo loop algorithm. We have determined the phase diagram of the model as a function of the ratio of the two different couplings: bonds within a plaquette and dimer bonds between plaquettes. By calculating both the spin gap and the staggered magnetization we determine the range of stability of the long range ordered (LRO) phase. At isotropic coupling LRO survives the depletion. But the close vicinity of the isotropic point to the spin gap phase leads us to the conclusion that already a small frustrating next nearest neighbor interaction can drive the system into the quantum disordered phase and thus explain the spin gap behavior of CaV4O9

Effects of Nonmagnetic Impurity Doping on Spin Ladder System

Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder system are studied by the QMC method. A single nonmagnetic impurity induces a localized spin-1/2 moment accompanied by "static" and enhanced AF correlations around it. Small and finite concentration of impurities induces a remarkable change of magnetic and thermodynamic properties with gapless excitations. It also shows rather sharp but continuous crossover around the concentration of about 4%. Above the crossover concentration, all the spins are strongly coupled participating in the enhanced and rather uniform power-law decay of the antiferromagnetic correlation. Below the crossover, each impurity forms an antiferromagnetic cluster only weakly coupled each other. For random distribution of impurities, large Curie-like susceptibility accompanied with small residual entropy is obtained at low temperatures in agreement with recent experimental observation in Zn-doped $SrCu_{2}O_{3}$. Temperature dependence of AF susceptibility shows power-law-like but weaker divergence than the single chain AFH in the temperature range studied.Comment: 4 pages, LaTeX+epsf.sty, submitted to J.Phys.Soc.Jpn. New results of AF susceptibility are adde

Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2 Chains with Bond Disorder

One-dimensional Heisenberg spin 1/2 chains with random ferro- and antiferromagnetic bonds are realized in systems such as $Sr_3 CuPt_{1-x} Ir_x O_6$. We have investigated numerically the thermodynamic properties of a generic random bond model and of a realistic model of $Sr_3 CuPt_{1-x} Ir_x O_6$ by the quantum Monte Carlo loop algorithm. For the first time we demonstrate the separation into three different temperature regimes for the original Hamiltonian based on an exact treatment, especially we show that the intermediate temperature regime is well-defined and observable in both the specific heat and the magnetic susceptibility. The crossover between the regimes is indicated by peaks in the specific heat. The uniform magnetic susceptibility shows Curie-like behavior in the high-, intermediate- and low-temperature regime, with different values of the Curie constant in each regime. We show that these regimes are overlapping in the realistic model and give numerical data for the analysis of experimental tests.Comment: 7 pages, 5 eps-figures included, typeset using JPSJ.sty, accepted for publication in J. Phys. Soc. Jpn. 68, Vol. 3. (1999