7,141 research outputs found
Quantum Transports in Two-Dimensions with Long Range Hopping: Shielding, Localization and the Extended Isolated State
We investigate the effects of disorder and shielding on quantum transports in
a two dimensional system with all-to-all long range hopping. In the weak
disorder, cooperative shielding manifests itself as perfect conducting channels
identical to those of the short range model, as if the long range hopping does
not exist. With increasing disorder, the average and fluctuation of conductance
are larger than those in the short range model, since the shielding is
effectively broken and therefore long range hopping starts to take effect. Over
several orders of disorder strength (until times of nearest
hopping), although the wavefunctions are not fully extended, they are also
robustly prevented from being completely localized into a single site. Each
wavefunction has several localization centers around the whole sample, thus
leading to a fractal dimension remarkably smaller than 2 and also remarkably
larger than 0, exhibiting a hybrid feature of localization and delocalization.
The size scaling shows that for sufficiently large size and disorder strength,
the conductance tends to saturate to a fixed value with the scaling function
, which is also a marginal phase between the typical metal
() and insulating phase (). The all-to-all coupling expels
one isolated but extended state far out of the band, whose transport is
extremely robust against disorder due to absence of backscattering. The bond
current picture of this isolated state shows a quantum version of short circuit
through long hopping.Comment: 15 pages, 8 figure
Ground-state properties via machine learning quantum constraints
Ground-state properties are central to our understanding of quantum many-body
systems. At first glance, it seems natural and essential to obtain the ground
state before analyzing its properties; however, its exponentially large Hilbert
space has made such studies costly, if not prohibitive, on sufficiently large
system sizes. Here, we propose an alternative strategy based upon the
expectation values of an ensemble of operators and the elusive yet vital
quantum constraints between them, where the search for ground-state properties
simply equates to simple, classical constrained minimization. These quantum
constraints are generally obtainable via machine learning on a large number of
sample quantum many-body states systematically consistent with physical
presumptions. We showcase our perspective on 1D fermion chains and spin chains
for applicability, effectiveness, and several unique advantages, especially for
strongly correlated systems, thermodynamic-limit systems, property designs,
etc.Comment: 6 pages, 4 figure
Correlation function of flavored fermion in holographic QCD
By using the gauge-gravity duality, we investigate the correlation function
of flavored fermion in the model as top-down
approaches of holographic QCD for . The bulk spinor, as the source of
the flavored fermion in QCD, is identified to the worldvolume fermion on the
flavor -branes and the standard form of its action can be
therefore obtained by the T-duality rules in string theory. Keeping this in
hand, we afterwards generalize the prescription for two-point correlation
function in AdS/CFT dictionary into general D-brane backgrounds and apply it to
the case of , i.e. the D4/D8 and D3/D7 approach respectively.
Resultantly, our numerical calculation with the bubble background always
displays discrete peaks in the correlation functions which imply the bound
states created by the flavored fermions as the confinement in QCD. With the
black brane background, the onshell condition illustrated by the correlation
function covers basically the dispersion curves of fermion obtained by the hard
thermal loop approximation in the hot medium. In this sense, we conclude
remarkably that our top-down approach in this work could reveal the fundamental
properties of QCD both in the confined and deconfined phase.Comment: 41 pages; 11 figures; 1 table; fix Figure 7 and Figure 1
Orbital Magnetization under an Electric Field and Orbital Magnetoelectric Polarizabilty for a Bilayer Chern System
In the the real space formalism of orbital magnetization (OM) for a Chern
insulator without an external electric field, it is correct to average the
local OM either over the bulk region or over the whole sample. However for a
layered Chern insulator in an external electric field, which is directly
related to the nontrivial nature of orbital magnetoelectric coupling, the role
of boundaries remains ambiguous in this formalism. Based on a bilayer model
with an adjustable Chern number at half filling, we numerically investigate the
OM with the above two different average types under a nonzero perpendicular
electric field. The result shows that in this case, the nonzero Chern number
gives rise to a gauge shift of the OM with the bulk region average, while this
gauge shift is absent for the OM with the whole sample average. This indicates
that only the whole sample average is reliable to calculate the OM under a
nonzero electric field for Chern insulators. On this basis, the orbital
magnetoelectric polarizablity (OMP) and the Chern-Simons orbital
magnetoelectric polarizablity (CSOMP) with the whole sample average are
studied. We also present the relationship between the OMP (CSOMP) and the
response of Berry curvature to the electric field. The stronger the response of
Berry curvature to electric field, the stronger is the OMP (CSOMP). Besides
clarify the calculation methods, our result also provides an effective method
to enhance OMP and CSOMP of materials.Comment: 11 pages, 11 figure
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