S. El Shawish

Anomalous scaling of conductivity in integrable fermion systems

Department of Physics, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom(Dated: 2nd February 2008)We analyze the high-temperature conductivity in one-dimensional integrable models of interacting fermions:the t-V model (anisotropic Heisenberg spin chain) and the Hubbard model, at half-filling in the regime cor-responding to insulating ground state. A microcanonical Lanczos method study for finite size systems revealsanomalously large finite-size effects at low frequencies while a frequency-moment analysis indicates a finited.c. conductivity. This phenomenon also appears in a prototype integrable quantum system of impenetrableparticles, representing a strong-coupling limit of both models. In the thermodynamic limit, the two results couldconverge to a finite d.c. conductivity rather than an ideal co nductor or insulator scenario.

Finite-temperature dynamical correlations using the microcanonical ensemble and the Lanczos algorithm

We show how to generalize the zero-temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical ensemble but with the evaluation of a single expectation value. We can employ the same system sizes as for zero temperature but, whereas the statistical fluctuations present in small systems are prohibitive, the spectra of the largest system sizes are surprisingly smooth. We investigate, as a test case, the spin conductivity of the spin-1/2 anisotropic Heisenberg model and in particular we present a comparison of spectra obtained by the canonical and microcanonical ensemble methods.

Numerical study of the E⊗e Jahn-Teller polaron and bipolaron

The properties of the polaron and bipolaron are explored in the one-dimensional Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical conductivity s(v). Our numerical results suggest that the Jahn-Teller and Holstein polarons are similar. However, in the strong-coupling regime qualitative differences in s(v) between the two models are found and discussed. The influence of the electron-phonon coupling and the electrostatic repulsion on the bipolaron binding energy, bipolaron masses, and correlation functions is investigated.

Electron spin resonance in S=1/2 antiferromagnets at high temperature

We study the electron spin resonance (ESR) of low-dimensional spin systems at high temperature, and test the Kubo-Tomita theory of exchange narrowing. In finite-size systems (molecular magnets), we found a double-peak resonance which strongly differs from the usual Lorentzian. For infinite systems, we have predictions for the linewidth and lineshape as a function of the anisotropy strength. For this, we have used an interpolation between a non-perturbative calculation of the memory function at short times (exact diagonalization) and the hydrodynamic spin-diffusion at long times. We show that the Dzyaloshinskii-Moriya anisotropies generally induce a much larger linewidth than the exchange anisotropies in two dimensions, contrary to the one-dimensional case.

Electron spin resonance of Sr Cu 2 ( B O 3 ) 2 at high magnetic fields

We calculate the electron spin resonance (ESR) spectra of the quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of magnetic field B. Using the standard Lanczos method, we solve a Shastry-Sutherland Hamiltonian with additional Dzyaloshinsky-Moriya (DM) terms which are crucial to explain different qualitative aspects of the ESR spectra. In particular, a nearest-neighbor DM interaction with a non-zero D_z component is required to explain the low frequency ESR lines for B || c. This suggests that crystal symmetry is lowered at low temperatures due to a structural phase transition.

Thermal entanglement of qubit pairs on the Shastry-Sutherland lattice

Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia andJ. Stefan Institute, Ljubljana, Slovenia(Dated: 2 April 2007)We show that temperature and magnetic field properties of the entanglement between spins on the two-dimensional Shastry-Sutherland lattice can be qualitatively described by analytical results for a qubit tetramer.Exact diagonalization of clusters with up to 20 sites reveals that the regime of fully entangled neighboring pairscoincides with the regime of finite spin gap in the spectrum. A dditionally, the results for the regime of vanishingspin gap are discussed and related to the Heisenberg limit of the model.

Spin-polaron excitations in a doped Shastry-Sutherland model

Using variational algorithm on an infinite Shastry-Sutherland (SS) lattice we show that the introduction of a static nonmagnetic impurity into a dimerized ground state leads to a formation of a small, localized spin polaron surrounding the impurity site. Due to a particular symmetry of the SS lattice, the polaron is extremely anisotropic with a short spatial extent. The presence of nonmagnetic impurities leads to a formation of pronounced in-gap peaks in the dynamical spin structure factor, which we attribute to the spin-doublet excitations of a single unpaired spin

Electron Spin Resonance of SrCu2(BO3)2 at High Magnetic Field

S=1∕2

Electron spin resonance ofSrCu2(BO3)2at high magnetic fields

surrounded by triplet fluctuations. Our results are relevant for the description of

Itinerant Ferromagnetism for Mixed Valence Systems

\mathrm{Sr}{\mathrm{Cu}}_{2}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}