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Impurities near an Antiferromagnetic-Singlet Quantum Critical Point
Heavy fermion systems, and other strongly correlated electron materials,
often exhibit a competition between antiferromagnetic (AF) and singlet ground
states. Using exact Quantum Monte Carlo (QMC) simulations, we examine the
effect of impurities in the vicinity of such AF- singlet quantum critical
points, through an appropriately defined impurity susceptibility, .
Our key finding is a connection, within a single calculational framework,
between AF domains induced on the singlet side of the transition, and the
behavior of the nuclear magnetic resonance (NMR) relaxation rate . We
show that local NMR measurements provide a diagnostic for the location of the
QCP which agrees remarkably well with the vanishing of the AF order parameter
and large values of . We connect our results with experiments on
Cd-doped CeCoIn
Disordered two-dimensional superconductors: roles of temperature and interaction strength
We have considered the half-filled disordered attractive Hubbard model on a
square lattice, in which the on-site attraction is switched off on a fraction
of sites, while keeping a finite on the remaining ones. Through Quantum
Monte Carlo (QMC) simulations for several values of and , and for system
sizes ranging from to , we have calculated the
configurational averages of the equal-time pair structure factor , and,
for a more restricted set of variables, the helicity modulus, , as
functions of temperature. Two finite-size scaling {\it ansatze} for have
been used, one for zero-temperature and the other for finite temperatures. We
have found that the system sustains superconductivity in the ground state up to
a critical impurity concentration, , which increases with , at least up
to U=4 (in units of the hopping energy). Also, the normalized zero-temperature
gap as a function of shows a maximum near , for . Analyses of the helicity modulus and of the pair structure factor
led to the determination of the critical temperature as a function of , for
4 and 6: they also show maxima near , with the highest
increasing with in this range. We argue that, overall, the observed
behavior results from both the breakdown of CDW-superconductivity degeneracy
and the fact that free sites tend to "push" electrons towards attractive sites,
the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe
Spin-3 Chromium Bose-Einstein Condensates
We analyze the physics of spin-3 Bose-Einstein condensates, and in particular
the new physics expected in on-going experiments with condensates of Chromium
atoms. We first discuss the ground-state properties, which, depending on still
unknown Chromium parameters, and for low magnetic fields can present various
types of phases. We also discuss the spinor-dynamics in Chromium spinor
condensates, which present significant qualitative differences when compared to
other spinor condensates. In particular, dipole-induced spin relaxation may
lead for low magnetic fields to transfer of spin into angular momentum similar
to the well-known Einstein-de Haas effect. Additionally, a rapid large
transference of population between distant magnetic states becomes also
possible.Comment: 4 pages, 3 eps figures. Error in the previous version correcte
Measuring von Neumann entanglement entropies without wave functions
We present a method to measure the von Neumann entanglement entropy of ground
states of quantum many-body systems which does not require access to the system
wave function. The technique is based on a direct thermodynamic study of
entanglement Hamiltonians, whose functional form is available from field
theoretical insights. The method is applicable to classical simulations such as
quantum Monte Carlo methods, and to experiments that allow for thermodynamic
measurements such as the density of states, accessible via quantum quenches. We
benchmark our technique on critical quantum spin chains, and apply it to
several two-dimensional quantum magnets, where we are able to unambiguously
determine the onset of area law in the entanglement entropy, the number of
Goldstone bosons, and to check a recent conjecture on geometric entanglement
contribution at critical points described by strongly coupled field theories
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