Fermi-Edge Singularity with Backscattering in the Luttinger-Liquid Model

We study the response of the interacting electron gas in one dimension on the local external potential. In the low frequency limit the power-law singularities are essentially modifyed by backscattering effects which, in the case of zero forward scattering, result in the universal critical exponent depending only on the Luttinger-liquid interactions. The results obtained may be used to describe singular responses of the 1D boson chains.Comment: 11 pages in LaTex, UBCTP-NP-93-00

Worm Algorithm for Problems of Quantum and Classical Statistics

This is a chapter of the multi-author book "Understanding Quantum Phase Transitions," edited by Lincoln Carr and published by Taylor and Francis. In this chapter, we give a general introduction to the worm algorithm and present important results highlighting the power of the approachComment: 27 pages, 15 figures, chapter in a boo

Supercurrent Stability in a Quasi-1D Weakly Interacting Bose Gas

We discuss a possibility of observing superfluid phenomena in a quasi-1D weakly interacting Bose gas at finite temperatures. The weakness of interaction in combination with generic properties of 1D liquids can result in a situation when relaxational time of supercurrent is essentially larger than the time of experimental observation, and the behavior of the system is indistinguishable from that of a genuine superfluid.Comment: Revtex, 4 pages, no figures; Submitted to Phys. Rev. A (Brief Reports

Criticality in Trapped Atomic Systems

We discuss generic limits posed by the trap in atomic systems on the accurate determination of critical parameters for second-order phase transitions, from which we deduce optimal protocols to extract them. We show that under current experimental conditions the in-situ density profiles are barely suitable for an accurate study of critical points in the strongly correlated regime. Contrary to recent claims, the proper analysis of time-of-fight images yields critical parameters accurately.Comment: 4 pages, 3 figures; added reference

Comment on ``One-Dimensional Disordered Bosonic Hubbard Model: A Density-Matrix Renormalization Group Study"

We present the phase diagram of the system obtained by continuous-time worldline Monte Carlo simulations, and demonstrate that the actual phase diagram is in sharp contrast with that found in Phys. Rev. Lett., 76 (1996) 2937.Comment: 1 page, LaTex, 1 figur

Effective Hamiltonian in the Problem of a "Central Spin" Coupled to a Spin Environment

We consider here the problem of a "giant spin", with spin quantum number S>>1, interacting with a set of microscopic spins. Interactions between the microscopic spins are ignored. This model describes the low-energy properties of magnetic grains or magnetic macromolecules interacting with a surrounding spin environment, such as nuclear spins. We describe a general method for truncating the model to another one, valid at low energies, in which a two-level system interacts with the environmental spins, and higher energy terms are absorbed into a new set of couplings. This is done using an instanton technique. We then verify the accuracy of this technique, by comparing the results for the low energy effective Hamiltonian, with results derived for the original giant spin, coupled to a microscopic spin, using exact diagonalisation techniques.Comment: 15 pages, Latex, with 9 ps figure