Anders W. Sandvik

Bond-order-wave phase and quantum phase transitions in the one-dimensional extended Hubbard model

We use a stochastic series-expansion quantum Monte Carlo method to study the phase diagram of the one-dimensional extended Hubbard model at half-filling for small to intermediate values of the on-site U and nearest-neighbor V repulsions. We confirm the existence of a novel, long-range-ordered bond-order-wave (BOW) phase recently predicted by Nakamura [J. Phys. Soc. Jpn. 68, 3123 (1999)] in a small region of the parameter space between the familiar charge-density-wave (CDW) state for

Computational Studies of Quantum Spin Systems

V\ensuremath{\gtrsim}U/2

Evidence for deconfined quantum criticality in a two-dimensional Heisenberg model with four-spin interactions.

and the state with dominant spin-density-wave (SDW) fluctuations for

Variational quantum Monte Carlo simulations with tensor-network states

V\ensuremath{\lesssim}U/2.

Order-disorder transition in a two-layer quantum antiferromagnet.

We discuss the nature of the transitions among these states and evaluate some of the critical exponents. Further, we determine accurately the position of the multicritical point,

Critical temperature and the transition from quantum to classical order parameter fluctuations in the three-dimensional Heisenberg antiferromagnet

{(U}_{m}{,V}_{m})=(4.7\ifmmode\pm\else\textpm\fi{}0.1,2.51\ifmmode\pm\else\textpm\fi{}0.04)

Classical percolation transition in the diluted two-dimensionalS=12Heisenberg antiferromagnet

(in energy units where the hopping integral is normalized to unity), above which the two continuous SDW-BOW-CDW transitions are replaced by one discontinuous (first-order) direct SDW-CDW transition. We also discuss the evolution of the CDW and BOW states upon hole doping. We find that in both cases the ground state is a Luther-Emery liquid, i.e., the spin gap remains but the charge gap existing at half-filling is immediately closed upon doping. The charge and bond-order correlations decay with distance r as

Striped phase in a quantum XY model with ring exchange.

{r}^{\ensuremath{-}{K}_{\ensuremath{\rho}}},

Beam test results from a prototype for the delphi microvertex detector

where

Multichain Mean-Field Theory of Quasi-One-Dimensional Quantum Spin Systems

{K}_{\ensuremath{\rho}}