2,868 research outputs found
Itinerant-localized dual character of a strongly-correlated superfluid Bose gas in an optical lattice
We investigate a strongly-correlated Bose gas in an optical lattice.
Extending the standard-basis operator method developed by Haley and Erdos to a
boson Hubbard model, we calculate excitation spectra in the superfluid phase,
as well as in the Mott insulating phase, at T=0. In the Mott phase, the
excitation spectrum has a finite energy gap, reflecting the localized character
of atoms. In the superfluid phase, the excitation spectrum is shown to have an
itinerant-localized dual structure, where the gapless Bogoliubov mode (which
describes the itinerant character of superfluid atoms) and a band with a finite
energy gap coexist. We also show that the rf-tunneling current measurement
would give a useful information about the duality of a strongly-correlated
superfluid Bose gas near the superfluid-insulator transition.Comment: 10 pages, 4 figure
Probing Phases and Quantum Criticality using Deviations from the Local Fluctuation-Dissipation Theorem
Introduction Cold atomic gases in optical lattices are emerging as excellent
laboratories for testing models of strongly interacting particles in condensed
matter physics. Currently, one of the major open questions is how to obtain the
finite temperature phase diagram of a given quantum Hamiltonian directly from
experiments. Previous work in this direction required quantum Monte Carlo
simulations to directly model the experimental situation in order to extract
quantitative information, clearly defeating the purpose of an optical lattice
emulator. Here we propose a new method that utilizes deviations from a local
fluctuation dissipation theorem to construct a finite temperature phase
diagram, for the first time, from local observables accessible by in situ
experimental observations. Our approach extends the utility of the
fluctuation-dissipation theorem from thermometry to the identification of
quantum phases, associated energy scales and the quantum critical region. We
test our ideas using state-of-the-art large-scale quantum Monte Carlo
simulations of the two-dimensional Bose Hubbard model.Comment: 7 pages; 4 figures; also see supplementary material of 7 pages with 3
figure
A Universal Interacting Crossover Regime in Two-Dimensional Quantum Dots
Interacting electrons in quantum dots with large Thouless number in the
three classical random matrix symmetry classes are well-understood. When a
specific type of spin-orbit coupling known to be dominant in two dimensional
semiconductor quantum dots is introduced, we show that a new interacting
quantum critical crossover energy scale emerges and low-energy quasiparticles
generically have a decay width proportional to their energy. The low-energy
physics of this system is an example of a universal interacting crossover
regime.Comment: 4 pages, 1 figur
Boson Core Compressibility
Strongly interacting atoms trapped in optical lattices can be used to explore
phase diagrams of Hubbard models. Spatial inhomogeneity due to trapping
typically obscures distinguishing observables. We propose that measures using
boson double occupancy avoid trapping effects to reveal key correlation
functions. We define a boson core compressibility and core superfluid stiffness
in terms of double occupancy. We use quantum Monte Carlo on the Bose-Hubbard
model to empirically show that these quantities intrinsically eliminate edge
effects to reveal correlations near the trap center. The boson core
compressibility offers a generally applicable tool that can be used to
experimentally map out phase transitions between compressible and
incompressible states.Comment: 11 pages, 11 figure
Width of the longitudinal magnon in the vicinity of the O(3) quantum critical point
We consider a three-dimensional quantum antiferromagnet in the vicinity of a
quantum critical point separating the magnetically ordered and the magnetically
disordered phases. A specific example is TlCuCl where the quantum phase
transition can be driven by hydrostatic pressure and/or by external magnetic
field. As expected two transverse and one longitudinal magnetic excitation have
been observed in the pressure driven magnetically ordered phase. According to
the experimental data, the longitudinal magnon has a substantial width, which
has not been understood and has remained a puzzle. In the present work, we
explain the mechanism for the width, calculate the width and relate value of
the width with parameters of the Bose condensate of magnons observed in the
same compound. The method of an effective quantum field theory is employed in
the work.Comment: 6 pages, 3 figure
Using Superconducting Qubit Circuits to Engineer Exotic Lattice Systems
We propose an architecture based on superconducting qubits and resonators for
the implementation of a variety of exotic lattice systems, such as spin and
Hubbard models in higher or fractal dimensions and higher-genus topologies.
Spin systems are realized naturally using qubits, while superconducting
resonators can be used for the realization of Bose-Hubbard models. Fundamental
requirements for these designs, such as controllable interactions between
arbitrary qubit pairs, have recently been implemented in the laboratory,
rendering our proposals feasible with current technology.Comment: 7 pages (two-column), 3 figure
Nonlinear conductance of long quantum wires at a conductance plateau transition: Where does the voltage drop?
We calculate the linear and nonlinear conductance of spinless fermions in
clean, long quantum wires where short-ranged interactions lead locally to
equilibration. Close to the quantum phase transition where the conductance
jumps from zero to one conductance quantum, the conductance obtains an
universal form governed by the ratios of temperature, bias voltage and gate
voltage. Asymptotic analytic results are compared to solutions of a Boltzmann
equation which includes the effects of three-particle scattering. Surprisingly,
we find that for long wires the voltage predominantly drops close to one end of
the quantum wire due to a thermoelectric effect.Comment: 4+ pages, 3 figures plus supplementary material (2 pages, 1 figure);
minor changes, references correcte
Quantum criticality near the Stoner transition in a two-dot with spin-orbit coupling
We study a system of two tunnel-coupled quantum dots, with the first dot
containing interacting electrons (described by the Universal Hamiltonian) not
subject to spin-orbit coupling, whereas the second contains non-interacting
electrons subject to spin-orbit coupling. We focus on describing the behavior
of the system near the Stoner transition. Close to the critical point quantum
fluctuations become important and the system enters a quantum critical regime.
The large- approximation allows us to calculate physical quantitites
reliably even in this strongly fluctuating regime. In particular, we find a
scaling function to describe the crossover of the quasiparticle decay rate
between the renormalized Fermi liquid regime and the quantum critical regime.Comment: 19 pages, 5 figure
Scanning the Parameter Space of Holographic Superconductors
We study various physical quantities associated with holographic s-wave
superconductors as functions of the scaling dimensions of the dual condensates.
A bulk scalar field with negative mass squared , satisfying the
Breitenlohner-Freedman stability bound and the unitarity bound, and allowed to
vary in unit intervals, were considered. We observe that all the physical
quantities investigated are sensitive to the scaling dimensions of the dual
condensates. For all the , the characteristic lengths diverge at the
critical temperature in agreement with the Ginzburg-Landau theory. The
Ginzburg-Landau parameter, obtained from these length scales indicates that the
holographic superconductors can be type I or type II depending on the charge
and the scaling dimensions of the dual condensates. For a fixed charge, there
exists a critical scaling dimension, above which a holographic superconductor
is type I, below which it becomes a type II.Comment: 24 pages 47 figure
Metamagnetic Quantum Criticality Revealed by 17O-NMR in the Itinerant Metamagnet Sr3Ru2O7
We have investigated the spin dynamics in the bilayered perovskite Sr3Ru2O7
as a function of magnetic field and temperature using 17O-NMR. This system sits
close to a metamagnetic quantum critical point (MMQCP) for the field
perpendicular to the ruthenium oxide planes. We confirm Fermi-liquid behavior
at low temperatures except for a narrow field region close to the MMQCP. The
nuclear spin-lattice relaxation rate divided by temperature 1/T1T is enhanced
on approaching the metamagnetic critical field of 7.9 T and at the critical
field 1/T1T continues to increase and does not show Fermi- liquid behavior down
to 0.3 K. The temperature dependence of T1T in this region suggests the
critical temperature Theta to be 0 K, which is a strong evidence that the spin
dynamics possesses a quantum critical character. Comparison between uniform
susceptibility and 1/T1T reveals that antiferromagnetic fluctuations instead of
two-dimensional ferromagnetic fluctuations dominate the spin fluctuation
spectrum at the critical field, which is unexpected for itinerant
metamagnetism.Comment: 5 pages, 4 figures, Accepted by Phys. Rev. Let
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