16,291 research outputs found
Valence bond solid order near impurities in two-dimensional quantum antiferromagnets
Recent scanning tunnelling microscopy (STM) experiments on underdoped
cuprates have displayed modulations in the local electronic density of states
which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a
paradigm of the pinning of such bond-centered ordering in strongly correlated
systems, we present the theory of valence bond solid (VBS) correlations near a
single impurity in a square lattice antiferromagnet. The antiferromagnet is
assumed to be in the vicinity of a quantum transition from a magnetically
ordered Neel state to a spin-gap state with long-range VBS order. We identify
two distinct classes of impurities: i) local modulation in the exchange
constants, and ii) a missing or additional spin, for which the impurity
perturbation is represented by an uncompensated Berry phase. The `boundary'
critical theory for these classes is developed: in the second class we find a
`VBS pinwheel' around the impurity, accompanied by a suppression in the VBS
susceptibility. Implications for numerical studies of quantum antiferromagnets
and for STM experiments on the cuprates are noted.Comment: 41 pages, 6 figures; (v2) Minor changes in terminology, added
reference
Generic Bell inequalities for multipartite arbitrary dimensional systems
We present generic Bell inequalities for multipartite multi-dimensional
systems. The inequalities that any local realistic theories must obey are
violated by quantum mechanics for even-dimensional multipartite systems. A
large set of variants are shown to naturally emerge from the generic Bell
inequalities. We discuss particular variants of Bell inequalities, that are
violated for all the systems including odd-dimensional systems.Comment: Accepted in Phys. Rev. Let
Domain walls of high-density QCD
We show that in very dense quark matter there must exist metastable domain
walls where the axial U(1) phase of the color-superconducting condensate
changes by 2pi. The decay rate of the domain walls is exponentially suppressed
and we compute it semiclassically. We give an estimate of the critical chemical
potential above which our analysis is under theoretical control.Comment: 4 pages; Eq. (16) corrected, 2 new references added, published
versio
Electroweak phase transition in the MSSM with four generations
By assuming the existence of the sequential fourth generation to the minimal
supersymmetric standard model (MSSM), we study the possibility of a strongly
first-order electroweak phase transition. We find that there is a parameter
region of the MSSM where the electroweak phase transition is strongly first
order. In that parameter region, the mass of the lighter scalar Higgs boson is
calculated to be above the experimental lower bound, and the scalar quarks of
the third and the fourth generations are heavier than the corresponding quarks.Comment: 12 pages, 2 tables, 2 figure
Critical dynamics, duality, and the exact dynamic exponent in extreme type II superconductors
The critical dynamics of superconductors is studied using renormalization
group and duality arguments. We show that in extreme type II superconductors
the dynamic critical exponent is given exactly by . This result does not
rely on the widely used models of critical dynamics. Instead, it is shown that
follows from the duality between the extreme type II superconductor and
a model with a critically fluctuating gauge field. Our result is in agreement
with Monte Carlo simulations.Comment: 7 pages, no figures; version accepted for publication in PR
Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO/SrTiO Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
LaNiO (LNO) is an intriguing member of the rare-earth nickelates in
exhibiting a metal-insulator transition for a critical film thickness of about
4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such
thin films also show a transition to a metallic state in superlattices with
SrTiO (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to
better understand this transition, we have studied a strained LNO/STO
superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an
(LaAlO)(SrAlTaO) substrate using soft x-ray
standing-wave-excited angle-resolved photoemission (SWARPES), together with
soft- and hard- x-ray photoemission measurements of core levels and
densities-of-states valence spectra. The experimental results are compared with
state-of-the-art density functional theory (DFT) calculations of band
structures and densities of states. Using core-level rocking curves and x-ray
optical modeling to assess the position of the standing wave, SWARPES
measurements are carried out for various incidence angles and used to determine
interface-specific changes in momentum-resolved electronic structure. We
further show that the momentum-resolved behavior of the Ni 3d eg and t2g states
near the Fermi level, as well as those at the bottom of the valence bands, is
very similar to recently published SWARPES results for a related
LaSrMnO/SrTiO superlattice that was studied using the
same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which
further validates this experimental approach and our conclusions. Our
conclusions are also supported in several ways by comparison to DFT
calculations for the parent materials and the superlattice, including
layer-resolved density-of-states results
QCD-like Theories at Finite Baryon and Isospin Density
We use 2-color QCD as a model to study the effects of simultaneous presence
of chemical potentials for isospin charge, , and for baryon number,
. We determine the phase diagrams for 2 and 4 flavor theories using the
method of effective chiral Lagrangians at low densities and weak coupling
perturbation theory at high densities. We determine the values of various
condensates and densities as well as the spectrum of excitations as functions
of and . A similar analysis of QCD with quarks in the adjoint
representation is also presented. Our results can be of relevance for lattice
simulations of these theories. We predict a phase of inhomogeneous condensation
(Fulde-Ferrel-Larkin-Ovchinnikov phase) in the 2 colour 2 flavor theory, while
we do not expect it the 4 flavor case or in other realizations of QCD with a
positive measure.Comment: 17 pages, 14 figure
Pion Propagation near the QCD Chiral Phase Transition
We point out that, in analogy with spin waves in antiferromagnets, all
parameters describing the real-time propagation of soft pions at temperatures
below the QCD chiral phase transition can be expressed in terms of static
correlators. This allows, in principle, the determination of the soft pion
dispersion relation on the lattice. Using scaling and universality arguments,
we determine the critical behavior of the parameters of pion propagation. We
predict that when the critical temperature is approached from below, the pole
mass of the pion drops despite the growth of the pion screening mass. This fact
is attributed to the decrease of the pion velocity near the phase transition.Comment: 8 pages (single column), RevTeX; added references, version to be
published in PR
Real-time pion propagation in finite-temperature QCD
We argue that in QCD near the chiral limit, at all temperatures below the
chiral phase transition, the dispersion relation of soft pions can be expressed
entirely in terms of three temperature-dependent quantities: the pion screening
mass, a pion decay constant, and the axial isospin susceptibility. The
definitions of these quantities are given in terms of equal-time (static)
correlation functions. Thus, all three quantities can be determined directly by
lattice methods. The precise meaning of the Gell-Mann--Oakes--Renner relation
at finite temperature is given.Comment: 25 pages, 2 figures; v2: discussion on the region of applicability
expanded, to be published in PR
Infrared Hall conductivity of NaCoO
We report infrared Hall conductivity of
NaCoO thin films determined from Faraday rotation angle
measurements. exhibits two types of hole
conduction, Drude and incoherent carriers. The coherent Drude carrier shows a
large renormalized mass and Fermi liquid-like behavior of Hall scattering rate,
. The spectral weight is suppressed and disappears at T
= 120K. The incoherent carrier response is centered at mid-IR frequency and
shifts to lower energy with increasing T. Infrared Hall constant is positive
and almost independent of temperature in sharp contrast with the dc-Hall
constant.Comment: 5 Pages, 5 Figures. Author list corrected in metadata only, paper is
unchange
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