Shaojin Qin

Phase diagram of a one-dimensional spin-orbital model

We study a one-dimensional spin-orbital model using both analytical and numerical methods. Renormalization group calculations are performed in the vicinity of a special integrable point in the phase diagram with SU(4) symmetry. These indicate the existence of a gapless phase in an extended region of the phase diagram, missed in previous studies. This phase is SU(4) invariant at low energies apart from the presence of different velocities for spin and orbital degrees of freedom. The phase transition into a gapped dimerized phase is in a generalized Kosterlitz-Thouless universality class. The phase diagram of this model is sketched using the density matrix renormalization group technique.

Strongly reduced gap in the zigzag spin chain with a ferromagnetic interchain coupling

We study a spin-(1/2) Heisenberg zigzag-spin-chain model near decoupled two chains. Taking into account a symmetry breaking perturbation, we discuss the existence of an energy gap in the ferromagnetic interchain coupling as well as the antiferromagnetic one. In the ferromagnetic model, a marginally relevant fixed line reduces the gap strongly, so that the correlation length becomes an astronomical length scale even in order one coupling. This result agrees with density-matrix renormalization group results.

Zigzag spin chains with antiferromagnetic-ferromagnetic interactions: Transfer-matrix renormalization group study

Properties of the zigzag spin chains with various nearest-neighbor and next-nearest-neighbor interactions are studied by making use of the transfer-matrix renormalization group method. Thermodynamic quantities of the systems (temperature dependence of the susceptibility and the specific heat), as well as the field dependence of the magnetization are analyzed numerically with a high accuracy in the thermodynamic limit. The results have been compared with the recent experimental data on Rb(2)Cu(2)Mo(3)O(12).

EDGE STATES IN OPEN ANTIFERROMAGNETIC HEISENBERG CHAINS

We report our results in investigating edge effects of open antiferromagnetic Heisenberg spin chains with spin magnitudes S = 1/2,1,3/2,2 using the density-matrix renormalization-group method initiated by White. For integer spin chains, we find that edge states with spin magnitude S-edge=S/2 exist, in agreement with the valence-bond-solid model picture. For half-integer spin chains, we find that no edge states exist for the S=1/2 spin chain, but an edge state exists in the S=3/2 spin chain with a S-edge=1/2, in agreement with a previous conjecture by Ng. The properties of the edge state in the S=3/2 chain is studied in detail. Strong finite-size effects associated with spin dimerization in half-integer spin chains are also discussed.

Heisenberg spin-1 chain in a staggered magnetic field: A density-matrix-renormalization-group study

Using the density-matrix-renormalization-group technique, we calculate numerically the low-energy excitation spectrum and magnetization curve of the spin-1 antiferromagnetic chain in a staggered magnetic field, which is expected to describe the physics of the R2BaNiO5 (R not equal Y) family below the Neel temperature of the magnetic rare-earth (Rf sublattice. These results are valid in the entire range of the staggered field, and agree with those given by the nonlinear sigma model study for small fields, but differ from the latter for large fields. They are consistent with the available experimental data. The correlation functions for this model are also calculated. The transverse correlations display the anticipated exponential decay with-shorter correlation length, while the: longitudinal correlations show explicitly the induced staggered magnetization. [S0163-1829(99)04021-7].

Logarithmic corrections in quantum impurity problems

The effect of a bulk marginal operator on boundary critical phenomena in two space-time dimensions is considered. The particular case of an open S = ½ antiferromagnetic Heisenberg chain, corresponding to a Wess-Zumino-Witten nonlinear model, is solved. In this case, the required renormalization group coefficient is associated with a novel operator product expansion in which three operators approach the same point. Resulting logarithmic corrections occurring in finite-size calculations and nuclear magnetic resonance experiments are discussed.

Staggered dimer order and criticality in an S=1/2 quantum spin ladder system with four-spin exchange

We study an S= quantum-spin ladder system with four-spin cyclic exchange, using the density matrix renormalization group (DMRG) and exact diagonalization methods. Recently, the phase transition and its universality class in this system have been studied. However, controversies remain on whether the phase transition is second-order type or another type and on the nature of critical phenomena. In addition, there are several arguments concerning where the transition point is. Analyzing DMRG data, we use an approach to determine the ordered phase which appears after the phase transition. We find that the edge state appears under the open boundary condition by investigating excitation energies of states with higher magnetizations. We also estimate the correlation length and discuss the critical behavior.

Transition from band insulator to Mott insulator in one dimension: Critical behavior and phase diagram

We report a systematic study of the transition from a band insulator (BI) to a Mott insulator (MI) in a one-dimensional Hubbard model at half-filling with an on-site Coulomb interaction U and an alternating periodic site potential V. We employ both the ze

Impurity in a Luttinger liquid away from half-filling: A numerical study

Conformal field theory gives quite detailed predictions for the low-energy spectrum and scaling exponents of a massless Luttinger liquid at generic filling in the presence of an impurity. While these predictions were verified for half-filled systems, there was until now no analysis away from this particular filling. Here, we fill in this gap by numerically investigating a quarter-filled system using the density-matrix renormalization-group technique. Our results confirm conformal-field-theory predictions, and suggest that they are indeed valid for arbitrary fillings.

Field-induced gap in the spin-12antiferromagnetic Heisenberg chain: A density-matrix renormalization-group study

We study the spin-½ antiferromagnetic Heisenberg chain in both uniform and (perpendicular) staggered magnetic fields using the density-matrix renormalization-group method. This model has been shown earlier to describe the physics of the copper benzoate materials in magnetic field. In the present work, we extend the study to more general case for a systematic investigation of the field-induced gap and related properties of the spin-½ antiferromagnetic Heisenberg chain. In particular, we explore the high magnetic field regime where interesting behaviors in the field-induced gap, magnetization, and spin correlation functions are found. Careful examination of the low energy properties and magnetization reveals interesting competing effects of the staggered and uniform fields. The incommensurate behavior in the spin correlation functions is demonstrated and discussed in detail. The present work reproduces earlier results in good agreement with experimental data on copper benzoate and predicts new interesting field-induced features at very high magnetic field.