Hirokazu Tsunetsugu

Thermodynamics and spin gap of the Heisenberg ladder calculated by the look-ahead Lanczos algorithm

We have developed an improved version of the quantum transfer-matrix algorithm. The extreme eigenvalues and eigenvectors of the transfer matrix are calculated by the recently developed look-ahead Lanczos algorithm for non-Hermitian matrices with higher efficiency and accuracy than by the power method. We have applied this method to the antiferromagnetic Heisenberg ladder. The temperature dependence of the susceptibility, specific heat, correlation length, and nuclear spin relaxation rate 1/

Pairing and excitation spectrum in doped t-J ladders.

{mathit{T}}_{1}

Properties of lightly doped t-J two-leg ladders.

are calculated. Our results support the existence of a spin gap of about 0.5J.

SINGLET STRIPE PHASES IN THE PLANAR T-J MODEL

Exact diagonalization studies for a doped t-J ladder (or double chain) show hole pairing in the ground state. The excitation spectrum separates into a limited number of quasiparticles which carry charge

Properties of lightly doped {ital t}-{ital J} two-leg ladders

+|e|

Phase diagram of the one-dimensional Kondo-lattice model.

and spin

Dynamic Exponent of t- J and t- J- W Model

{1 over 2}

Effect of the three-site hopping term on the t-J model.

and a triplet mode. At half-filling the former vanish but the latter evolves continuously into the triplet band of the spin liquid. At low doping the quasiparticles form a dilute Fermi gas with a strong attraction but simultaneously the Fermi wavevector, as would be measured in photoemission, is large.

The d-wave resonance valence bond state

We have numerically investigated the doped {ital t}-{ital J} ladder using exact diagonalization. We have studied both the limit of strong interchain coupling and isotropic coupling. The ladder scales to the Luther-Emery liquid regime in the strong interchain coupling limit. In this strong coupling limit there is a simple picture of the excitation spectrum that can be continued to explain the behavior at isotropic coupling. At {ital J}=0 we have indications of a ferromagnetic ground state. At a large {ital J}/{ital t} the ladder is phase separated into holes and a Heisenberg ladder. At intermediate coupling the ground state shows hole pairing with a modified {ital d}-wave symmetry. The excitation spectrum separates into a limited number of quasiparticles which carry charge +{parallel}{ital e}{parallel} and spin 1/2 and a triplet magnon mode. At half filling the former vanish but the latter evolves continuously into the magnon band of the spin liquid. At low doping the quasiparticles form a dilute Fermi gas with a strong attraction but simultaneously the Fermi wave vector, as would be measured in photoemission, is large. The dynamical structure factors are calculated and are found to be very similar to calculations on two-dimensional clusters. {copyright} {ital 1996 Themorexa0» American Physical Society.}«xa0less

Thermodynamics of Doped Kondo Insulator in One Dimension - Finite-Temperature DMRG Study -

The dimensional crossover of coupled t-J ladders to a planar model is examined using a mean-field approach with coupling constants determined by numerical diagonalization. Having a finite spin gap, uncoupled ladders belong to the Luther-Emery class in one-dimensional fermion systems, leading to a different crossover from coupling chains. For a wide region around J/tensuremath{gtrsim}1 the hole-hole correlations in a single ladder are found to be predominantly charge-density-wave type, but an attraction between hole pairs on adjacent ladders leads to a stripe phase. A quantum-mechanical melting of the hole lines at J/tensuremath{lesssim}1 leads to a Bose condensate of hole pairs, i.e., a superconducting phase.