We have adjusted the Density Matrix Renormalization method to handle two dimensional systems of limited width. The key ingredient for this extension is the incorporation of symmetries in the method. The advantage of our approach is that we can force certain symmetry properties to the resulting ground state wave function. Combining the results obtained for system sizes up-to 30 × 6 and finite size scaling, we derive the phase transition point and the critical exponent for the gap in the Ising model in a Transverse Field on a two dimensional square lattice.

We combine the Density Matrix Technique (DMRG) with Green Function Monte Carlo (GFMC) simulations. Both methods aim to determine the groundstate of a quantum system but have different limitations. The DMRG is most successful in 1-dimensional systems and can only be extended to 2-dimensional systems for strips of limited width. GFMC is not restricted to low dimensions but is limited by the efficiency of the sampling. This limitation is crucial when the system exhibits a so-called sign problem, which on the other hand is not a particular obstacle for the DMRG. We show how to combine the virtues of both methods by using a DMRG wavefunction as guiding wave function for the GFMC. This requires a special representation of the DMRG wavefunction to make the simulations possible within reasonable computational time. As a test case we apply the method to the 2-dimensional frustrated Heisenberg antiferromagnet. By supplementing the branching in GFMC with Stochastic Reconfiguration (SR) we get a stable simulation with a small variance also in the region where the fluctuations due to minus sign problem are maximal. The sensitivity of the results to the choice of the guiding wavefunction is extensively investigated. We analyse the model as a function of the ratio of the next-nearest to nearest neighbor coupling strength which is a measure for the frustration. In agreement with earlier calculations it is found from the DMRG wavefunction that for small ratios the system orders as a Néel type antiferromagnet and for large ratios as a columnar antiferromagnet. The spin stiffness suggests an intermediate regime without magnetic long range order. The energy curve indicates that the columnar phase is separated from the intermediate phase by a first order transition. The combination of DMRG and GFMC allows to substantiate this picture by calculating also the spin correlations in the system. We observe a pattern of the spin correlations in the intermediate regime which is in-between dimerlike and plaquette type ordering, states that have recently been suggested. It is a state with strong dimerization in one direction and weaker dimerization in the perpendicular direction and thus it lacks the the square symmetry of the plaquette state.

High-field magnetoresistance oscillations in nbis(ethylenedithio) tetrathiafulvalene &KHg(SCN) 4. The efFects of magnetic breakdown, exchange interactions, and Fermi-surface reordering

We calculate the spin stiffness of the S = 1 2 frustrated Heisenberg antiferromagnet directly from a general formula which is evaluated in the Schwinger boson mean-field approximation. Both Néel and collinear ordering are considered. For collinear ordering, we take the anisotropy of this phase into account, unlike previous approaches. For Néel ordering, a detailed study is made of the finite-size scaling behavior of the two terms that make up the spin stiffness.The exponents of the scaling with the system size of the two terms comprising the spin stiffness turn out to be identical to those of the unfrustrated case. PACS numbers : 75.10.Jm, 75.30.Kz, 75.40.Cx I. INTRODUCTION The recent interest in the frustrated Heisenberg antiferromagnets is motivated by high T c -superconductivity; the undoped compounds show long-range antiferromagnetic order, similar to the Heisenberg model. Upon doping superconductivity occurs. Adding frustration to the Heisenberg model can be thought of as to mimic the effect of hole doping.We consider the frustrated Heisenberg model on a square lattice with N = L 2 sites. It is described by the following Hamiltonian for quantum spins S j on a lattice:where nn denotes a pair (ij) of nearest-neighbor sites and nnn a pair of next-nearest-neighbor sites. The spin length is fixed; S = 1 2 . Both J 1 and J 2 are taken to be non-negative. If

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The spin stiffness ρ s of the repulsive Hubbard model that occurs in the hydrodynamic theory of antiferromagnetic spin waves is shown to be the same as the thermodynamically defined stiffness involved in twisting the order parameter. New expressions for ρ s are derived, which enable easier interpretation, and connections with superconducting weight and gauge invariance are discussed.

We analyze the coherent dynamics of optically excited alkenes in a fully correlated 3d tight-binding model with extended Hubbard interactions. The scenario that emerges is that the steric repulsive interactions are the driving force behind ultrafast cis-trans photoisomerizations. This resolves the apparent discrepancy between values for the torsional stiffness obtained from band-structure potentials and from vibrational spectra. The mechanism is illustrated in quantitative detail for ethylene and is also shown to yield a promising scenario for the coherent dynamics of molecules like retinal.

The conductivity of quasi-2D organic conductor (BEDT-TTF)$& 2H,Os have been investigated at low temperatures in high magnetic fields up to 14 T using Van der Pauw method. The sample undergoes metal-insulator transition at T -100 K with the total increase of the resistivity more than five orders of magnitude when the temperature reaches T = 4.2 K. It had been found out that the excess of the critical value by the measuring current induces irreversible changes of the sample conductivity. A wide region of temperature, 30 K 5 T 5 100 K, appears, where the resistivity p(T) acquires chaotic behavior. At T < 30K the abrupt decrease of the conductivity by two orders of magnitude or the bistability of the conductivity in a form of two p(T) branches is observed. The model explaining the metal insulator transition and above mentioned anomalies by the charge density wave formation with the well-developed domain structure is proposed.