5,990 research outputs found
Optical Bragg, atom Bragg and cavity QED detections of quantum phases and excitation spectra of ultracold atoms in bipartite and frustrated optical lattices
Ultracold atoms loaded on optical lattices can provide unprecedented
experimental systems for the quantum simulations and manipulations of many
quantum phases and quantum phase transitions between these phases. However, so
far, how to detect these quantum phases and phase transitions effectively
remains an outstanding challenge. In this paper, we will develop a systematic
and unified theory of using the optical Bragg scattering, atomic Bragg
scattering or cavity QED to detect the ground state and the excitation spectrum
of many quantum phases of interacting bosons loaded in bipartite and frustrated
optical lattices.
We show that the two photon Raman transition processes in the three detection
methods not only couple to the density order parameter, but also the {\sl
valence bond order} parameter due to the hopping of the bosons on the lattice.
This valence bond order coupling is very sensitive to any superfluid order or
any Valence bond (VB) order in the quantum phases to be probed. These quantum
phases include not only the well known superfluid and Mott insulating phases,
but also other important phases such as various kinds of charge density waves
(CDW), valence bond solids (VBS), CDW-VBS phases with both CDW and VBS orders
unique to frustrated lattices, and also various kinds of supersolids.
The physical measurable quantities of the three experiments are the light
scattering cross sections, the atom scattered clouds and the cavity leaking
photons respectively. We analyze respectively the experimental conditions of
the three detection methods to probe these various quantum phases and their
corresponding excitation spectra. We also address the effects of a finite
temperature and a harmonic trap.Comment: REVTEX4-1, 32 pages, 16.eps figures, to Appear in Annals of Physic
Abelian bosonization approach to quantum impurity problems
Using Abelian Bosonization, we develop a simple and powerful method to
calculate the correlation functions of the two channel Kondo model and its
variants. The method can also be used to identify all the possible boundary
fixed points and their maximum symmetry, to calculate straightforwardly the
finite size spectra, to demonstrate the physical picture at the boundary
explicitly. Comparisons with Non-Abelian Bosonization method are made. Some
fixed points corresponding to 4 pieces of bulk fermions coupled to s=1/2
impurity are listed.Comment: 12 pages, REVTEX, 1 Table, no figures. To appear in Phys. Rev. Letts.
July 21, 199
Measurement of the magnetic octupole susceptibility of PrV2Al20
In the electromagnetic multipole expansion, magnetic octupoles are the
subsequent order of magnetic multipoles allowed in centrosymmetric systems,
following the more commonly observed magnetic dipoles. As order parameters in
condensed matter systems, magnetic octupoles have been experimentally elusive.
In particular, the lack of simple external fields that directly couple to them
makes their experimental detection challenging. Here, we demonstrate a
methodology for probing the magnetic octupole susceptibility using a product of
magnetic field and shear strain to couple to the
octupolar fluctuations, while using an adiabatic elastocaloric effect to probe
the response to this composite effective field. We observe a Curie-Weiss
behavior in the obtained octupolar susceptibility of \ce{PrV2Al20} up to
temperatures approximately forty times the putative octupole ordering
temperature. Our results demonstrate the presence of magnetic octupole
fluctuations in the particular material system, and more broadly highlight how
anisotropic strain can be combined with magnetic fields to formulate a
versatile probe to observe otherwise elusive emergent `hidden' electronic
orders.Comment: 7 pages, 3 figure
Fermionic random transverse-field Ising spin chain
The interplay of spin and charge fluctuations in the random transverse-field
Ising spin chain on the fermionic space is investigated. The finite chemical
potential, which controls the charge fluctuations, leads to the appearance of
the quantum critical region in the phase diagram where the magnetic
correlations are quenched by nonmagnetic sites. Regions of nonmonotonous
temperature dependence of spin-spin correlation length appear at nonzero .
The results on the one-fermion density of states of the model are discussed.Comment: 10 pages, 2 figures included, citation correcte
Anderson localization of electron states in graphene in different types of disorder
Anderson localization of electron states on graphene lattice with diagonal
and off-diagonal (OD) disorder in the absence of magnetic field is investigated
by using the standard finite-size scaling analysis. In the presence of diagonal
disorder all states are localized as predicted by the scaling theory for
two-dimensional systems. In the case of OD disorder, the states at the Dirac
point (E=0) are shown to be delocalized due to the specific chiral symmetry,
although other states () are still localized. In OD disorder the
conductance at E=0 in an rectangular system at the thermodynamical
limit is calculated with the transfer-matrix technique for various values of
ratio and different types of distribution functions of the OD elements
. It is found that if all the 's are positive the conductance
is independent of as restricted by 2 delocalized channels at E=0. If the
distribution function includes the sign randomness of elements , the
conductivity, rather than the conductance, becomes independent. The
calculated value of the conductivity is around , in consistence
with the experiments.Comment: 24 pages, 12 figure
Bose Glass in Large N Commensurate Dirty Boson Model
The large N commensurate dirty boson model, in both the weakly and strongly
commensurate cases, is considered via a perturbative renormalization group
treatment. In the weakly commensurate case, there exists a fixed line under RG
flow, with varying amounts of disorder along the line. Including 1/N
corrections causes the system to flow to strong disorder, indicating that the
model does not have a phase transition perturbatively connected to the Mott
Insulator-Superfluid (MI-SF) transition. I discuss the qualitative effects of
instantons on the low energy density of excitations. In the strongly
commensurate case, a fixed point found previously is considered and results are
obtained for higher moments of the correlation functions. To lowest order,
correlation functions have a log-normal distribution. Finally, I prove two
interesting theorems for large N vector models with disorder, relevant to the
problem of replica symmetry breaking and frustration in such systems.Comment: 16 pages, 7 figure
Can Short-Range Interactions Mediate a Bose Metal Phase in 2D?
We show here based on a 1-loop scaling analysis that short-range interactions
are strongly irrelevant perturbations near the insulator-superconductor (IST)
quantum critical point. The lack of any proof that short-range interactions
mediate physics which is present only in strong coupling leads us to conclude
that short-range interactions are strictly irrelevant near the IST quantum
critical point. Hence, we argue that no new physics, such as the formation of a
uniform Bose metal phase can arise from an interplay between on-site and
nearest-neighbour interactions.Comment: 3 pages, 1 .eps file. SUbmitted to Phys. Rev.
The effects of weak disorders on Quantum Hall critical points
We study the consequences of random mass, random scalar potential and random
vector potential on the line of clean fixed points between integer/fractional
quantum Hall states and an insulator. This line of fixed points was first
identified in a clean Dirac fermion system with both Chern-Simon coupling and
Coulomb interaction in Phys. Rev. Lett. {\bf 80}, 5409 (1998). By performing a
Renormalization Group analysis in 1/N (N is the No. of species of Dirac
fermions) and the variances of three disorders , we find that is irrelevant along this line, both
and are marginal. With the presence of all the three
disorders, the pure fixed line is unstable. Setting Chern-Simon interaction to
zero, we find one non-trivial line of fixed points in plane
with dynamic exponent z=1 and continuously changing , it is stable against
small in a small range of the line ,
therefore it may be relevant to integer quantum Hall transition. Setting
, we find a fixed plane with z=1, the part of this plane with
is stable against small , therefore it may be relevant to
fractional quantum Hall transition.Comment: 16 pages, 19 figure
Quantum critical points with the Coulomb interaction and the dynamical exponent: when and why z=1
A general scenario that leads to Coulomb quantum criticality with the
dynamical critical exponent z=1 is proposed. I point out that the long-range
Coulomb interaction and quenched disorder have competing effects on z, and that
the balance between the two may lead to charged quantum critical points at
which z=1 exactly. This is illustrated with the calculation for the Josephson
junction array Hamiltonian in dimensions D=3-\epsilon. Precisely in D=3,
however, the above simple result breaks down, and z>1. Relation to other
theoretical studies is discussed.Comment: RevTex, 4 pages, 1 ps figur
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