1,454 research outputs found
Phase diagrams and universality classes of random antiferromagnetic spin ladders
The random antiferromagnetic two-leg and zigzag spin-1/2 ladders are
investigated using the real space renormalization group scheme and their
complete phase diagrams are determined. We demonstrate that the first system
belongs to the same universality class of the dimerized random spin-1/2 chain.
The zigzag ladder, on the other hand, is in a random singlet phase at weak
frustration and disorder. Otherwise, we give additional evidence that it
belongs to the universality class of the random antiferromagnetic and
ferromagnetic quantum spin chains, although the universal fixed point found in
the latter system is never realized. We find, however, a new universal fixed
point at intermediate disorder.Comment: 10 pages, 10 figure
A Holographic Fractional Topological Insulator
We give a holographic realization of the recently proposed low energy
effective action describing a fractional topological insulator. In particular
we verify that the surface of this hypothetical material supports a fractional
quantum Hall current corresponding to half that of a Laughlin state.Comment: 4 pages, 2 figure
An improved model of vector mesons in holographic QCD
We analyze the sector of dimension-three vector meson operators in the "hard
wall" model of holographic QCD, including the vector and axial currents, dual
to gauge fields in the bulk, and the tensor operator
, dual to a two-form field satisfying a complex
self-duality condition. The model includes the effect of chiral symmetry
breaking on vector mesons, that involves a coupling between the dual gauge
field and the two-form field. We compute the leading logarithmic terms in the
operator product expansion of two-point functions and the leading
non-perturbative contribution to the tensor-vector correlator. The result is
consistent with the operator product expansion of QCD. We also study the
spectrum of vector mesons numerically.Comment: 19 page
Control of fluorescence in quantum emitter and metallic nanoshell hybrids for medical applications
We study the light emission from quantum emitter and double metallic
nanoshell hybrid systems. Quantum emitters act as local sources which transmit
their light efficiently due to a double nanoshell near field. The double
nanoshell consists a dielectric core and two outer nanoshells
Valence-bond theory of highly disordered quantum antiferromagnets
We present a large-N variational approach to describe the magnetism of
insulating doped semiconductors based on a disorder-generalization of the
resonating-valence-bond theory for quantum antiferromagnets. This method
captures all the qualitative and even quantitative predictions of the
strong-disorder renormalization group approach over the entire experimentally
relevant temperature range. Finally, by mapping the problem on a hard-sphere
fluid, we could provide an essentially exact analytic solution without any
adjustable parameters.Comment: 5 pages, 3 eps figure
Adaptive density matrix renormalization group for disordered systems
FAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOWe propose a simple modification of the density matrix renormalization-group (DMRG) method in order to tackle strongly disordered quantum spin chains. Our proposal, akin to the idea of the adaptive time-dependent DMRG, enables us to reach larger system sizes in the strong disorder limit by avoiding most of the metastable configurations, which hinder the performance of the standard DMRG method. We benchmark our adaptive method by revisiting the random antiferromagnetic XXZ spin-1/2 chain for which we compute the random-singlet ground-state average spin-spin correlation functions and von Neumann entanglement entropy. We then apply our method to the bilinear-biquadratic random antiferromagnetic spin-1 chain tuned to the antiferromagnet and gapless highly symmetric SU(3) point. We find the new result that the mean correlation function decays algebraically with the same universal exponent phi = 2 as the spin-1/2 chain. We then perform numerical and analytical strong-disorder renormalization-group calculations, which confirm this finding and generalize it for any highly symmetric SU(N) random-singlet state.981919FAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informaçãoSem informaçãoSem informaçãoAgências de fomento estrangeiras apoiaram essa pesquisa, mais informações acesse artig
Excited electronic states from a variational approach based on symmetry-projected Hartree--Fock configurations
Recent work from our research group has demonstrated that symmetry-projected
Hartree--Fock (HF) methods provide a compact representation of molecular ground
state wavefunctions based on a superposition of non-orthogonal Slater
determinants. The symmetry-projected ansatz can account for static correlations
in a computationally efficient way. Here we present a variational extension of
this methodology applicable to excited states of the same symmetry as the
ground state. Benchmark calculations on the C dimer with a modest basis
set, which allows comparison with full configuration interaction results,
indicate that this extension provides a high quality description of the
low-lying spectrum for the entire dissociation profile. We apply the same
methodology to obtain the full low-lying vertical excitation spectrum of
formaldehyde, in good agreement with available theoretical and experimental
data, as well as to a challenging model insertion pathway for BeH.
The variational excited state methodology developed in this work has two
remarkable traits: it is fully black-box and will be applicable to fairly large
systems thanks to its mean-field computational cost
Multi-component symmetry-projected approach for molecular ground state correlations
The symmetry-projected Hartree--Fock ansatz for the electronic structure
problem can efficiently account for static correlation in molecules, yet it is
often unable to describe dynamic correlation in a balanced manner. Here, we
consider a multi-component, systematically-improvable approach, that accounts
for all ground state correlations. Our approach is based on linear combinations
of symmetry-projected configurations built out of a set of non-orthogonal,
variationally optimized determinants. The resulting wavefunction preserves the
symmetries of the original Hamiltonian even though it is written as a
superposition of deformed (broken-symmetry) determinants. We show how short
expansions of this kind can provide a very accurate description of the
electronic structure of simple chemical systems such as the nitrogen and the
water molecules, along the entire dissociation profile. In addition, we apply
this multi-component symmetry-projected approach to provide an accurate
interconversion profile among the peroxo and bis(-oxo) forms of
[CuO], comparable to other state-of-the-art quantum chemical
methods
Alternative large Nc baryons and holography
In gauge theories in the limit of a large number Nc of colors, baryons are
usually described as heavy solitonic objects with mass of order Nc. We discuss
an alternative large Nc description both directly in the field theory as well
as using holography. In this alternative large Nc limit at least some of the
baryons behave like mesons, that is they stay light even at large Nc and their
interactions vanish in that limit. For Nc=3 these alternative large Nc baryons
are equivalent to the standard baryons. In the holographic description it is
manifest that the Regge slopes of mesons and alternative baryons are
degenerate.Comment: 15 pages, 1 figure, references adde
- …