11,446 research outputs found
Increasing d-wave superconductivity by on site repulsion
We study by Variational Monte Carlo an extended Hubbard model away from half
filled band density which contains two competing nearest-neighbor interactions:
a superexchange favoring d-wave superconductivity and a repulsion
opposing against it. We find that the on-site repulsion effectively
enhances the strength of meanwhile suppressing that of , thus favoring
superconductivity. This result shows that attractions which do not involve
charge fluctuations are very well equipped against strong electron-electron
repulsion so much to get advantage from it.Comment: 4 pages, 3 figure
Current induced magnetization reversal on the surface of a topological insulator
We study dynamics of the magnetization coupled to the surface Dirac fermions
of a three di- mensional topological insulator. By solving the
Landau-Lifshitz-Gilbert equation in the presence of charge current, we find
current induced magnetization dynamics and discuss the possibility of mag-
netization reversal. The torque from the current injection depends on the
transmission probability through the ferromagnet and shows nontrivial
dependence on the exchange coupling. The mag- netization dynamics is a direct
manifestation of the inverse spin-galvanic effect and hence another ferromagnet
is unnecessary to induce spin transfer torque in contrast to the conventional
setup.Comment: 4 pages, 4 figure
Universal quantum computation with temporal-mode bilayer square lattices
We propose an experimental design for universal continuous-variable quantum
computation that incorporates recent innovations in linear-optics-based
continuous-variable cluster state generation and cubic-phase gate
teleportation. The first ingredient is a protocol for generating the
bilayer-square-lattice cluster state (a universal resource state) with temporal
modes of light. With this state, measurement-based implementation of Gaussian
unitary gates requires only homodyne detection. Second, we describe a
measurement device that implements an adaptive cubic-phase gate, up to a random
phase-space displacement. It requires a two-step sequence of homodyne
measurements and consumes a (non-Gaussian) cubic-phase state.Comment: (v2) 14 pages, 5 figures, consistent with published version; (v1) 13
pages, 5 figure
Mott Transitions and d-wave Superconductivity in Half-Filled-Band Hubbard Model on Square Lattice with Geometric Frustration
Mechanisms of Mott transitions and dx2-y2-wave superconductivity (SC) are
studied in the half-filled-band Hubbard model on square lattices with a
diagonal hopping term (t'), using an optimization (or correlated) variational
Monte Carlo method. In the trial wave functions, a doublon-holon binding effect
is introduced in addition to the onsite Gutzwiller projection. We mainly treat
a d-wave singlet state and a projected Fermi sea. In both wave functions,
first-order Mott transitions without direct relevance to magnetic orders take
place at U=Uc approximately of the bandwidth for arbitrary t'/t. These
transitions originate in the binding or unbinding of a doublon to a holon.
d-wave SC appears in a narrow range immediately below Uc. The robust d-wave
superconducting correlation are necessarily accompanied by enhanced
antiferromagnetic correlation; the strength of SC becomes weak, as t'/t
increases.Comment: 18 pages, 30 figure
Crossover of superconducting properties and kinetic-energy gain in two-dimensional Hubbard model
Superconductivity in the Hubbard model on a square lattice near half filling
is studied using an optimization (or correlated) variational Monte Carlo
method. Second-order processes of the strong-coupling expansion are considered
in the wave functions beyond the Gutzwiller factor. Superconductivity of
d_x^2-y^2-wave is widely stable, and exhibits a crossover around U=U_co\sim 12t
from a BCS type to a new type. For U\gsim U_co (U\lsim U_co), the energy gain
in the superconducting state is derived from the kinetic (potential) energy.
Condensation energy is large and \propto exp(-t/J) [tiny] on the strong [weak]
coupling side of U_co. Cuprates belong to the strong-coupling regime.Comment: 4 pages, 6 figure
Modeling UV and X-Ray Emission in a Post-CME Current Sheet
A post-CME current sheet (CS) is a common feature developed behind an
erupting flux rope in CME models. Observationally, white light observations
have recorded many occurrences of a thin ray appearing behind a CME eruption
that closely resembles a post-CME CS in its spatial correspondence and
morphology. UV and X-ray observations further strengthen this interpretation by
the observations of high temperature emission at locations consistent with
model predictions. The next question then becomes whether the properties inside
a post-CME CS predicted by a model agree with observed properties. In this
work, we assume that the post-CME CS is a consequence of Petschek-like
reconnection and that the observed ray-like structure is bounded by a pair of
slow mode shocks developed from the reconnection site. We perform
time-dependent ionization calculations and model the UV line emission. We find
that such a model is consistent with SOHO/UVCS observations of the post-CME CS.
The change of Fe XVIII emission in one event implies an inflow speed of ~10
km/s and a corresponding reconnection rate of M_A ~ 0.01. We calculate the
expected X-ray emission for comparison with X-ray observations by Hinode/XRT,
as well as the ionic charge states as would be measured in-situ at 1 AU. We
find that the predicted count rate for Hinode/XRT agree with what was observed
in a post-CME CS on April 9, 2008, and the predicted ionic charge states are
consistent with high ionization states commonly measured in the interplanetary
CMEs. The model results depend strongly on the physical parameters in the
ambient corona, namely the coronal magnetic field, the electron density and
temperature during the CME event. It is crucial to obtain these ambient coronal
parameters and as many facets of the CS properties as possible by observational
means so that the post-CME current sheet models can be scrutinized more
effectively
A method for measuring the contact area in instrumented indentation testing by tip scanning probe microscopy imaging
The determination of the contact area is a key step to derive mechanical
properties such as hardness or an elastic modulus by instrumented indentation
testing. Two families of procedures are dedicated to extracting this area: on
the one hand, post mortem measurements that require residual imprint imaging,
and on the other hand, direct methods that only rely on the load vs. the
penetration depth curve. With the development of built-in scanning probe
microscopy imaging capabilities such as atomic force microscopy and indentation
tip scanning probe microscopy, last generation indentation devices have made
systematic residual imprint imaging much faster and more reliable. In this
paper, a new post mortem method is introduced and further compared to three
existing classical direct methods by means of a numerical and experimental
benchmark covering a large range of materials. It is shown that the new method
systematically leads to lower error levels regardless of the type of material.
Pros and cons of the new method vs. direct methods are also discussed,
demonstrating its efficiency in easily extracting mechanical properties with an
enhanced confidence
Effects of Long-Range Correlations on Nonmagnetic Mott Transitions in Hubbard model on Square Lattice
The mechanism of Mott transition in the Hubbard model on the square lattice
is studied without explicit introduction of magnetic and superconducting
correlations, using a variational Monte Carlo method. In the trial wave
functions, we consider various types of binding factors between a
doubly-occupied site (doublon, D) and an empty site (holon, H), like a
long-range type as well as a conventional nearest-neighbor type, and add
independent long-range D-D (H-H) factors. It is found that a wide choice of D-H
binding factor leads to Mott transitions at critical values near the band
width. We renew the D-H binding picture of Mott transitions by introducing two
characteristic length scales, the D-H binding length l_{DH} and the minimum D-D
distance l_{DD}, which we appropriately estimate. A Mott transition takes place
at l_{DH}=l_{DD}. In the metallic regime (l_{DH}>l_{DD}), the domains of D-H
pairs overlap with one another, thereby doublons and holons can move
independently by exchanging the partners one after another. In contrast, the
D-D factors give only a minor contribution to the Mott transition.Comment: 13 pages, 18 figures, submitted to J. Phys. Soc. Jp
A-VIP: Anonymous Verification and Inference of Positions in Vehicular Networks
MiniconferenceInternational audienceKnowledge of the location of vehicles and tracking of the routes they follow are a requirement for a number of applications, including e-tolling and liability attribution in case of accidents. However, public disclosure of the identity and position of drivers jeopardizes user privacy, and securing the tracking through asymmetric cryptography may have an exceedingly high computational cost. Additionally, there is currently no way an authority can verify the correctness of the position information provided by a potentially misbehaving car. In this paper, we address all of the issues above by introducing A-VIP, a lightweight framework for privacy preserving and tracking of vehicles. A-VIP leverages anonymous position beacons from vehicles, and the cooperation of nearby cars collecting and reporting the beacons they hear. Such information allows an authority to verify the locations announced by vehicles, or to infer the actual ones if needed. We assess the effectiveness of A-VIP through both realistic simulation and testbed implementation results, analyzing also its resilience to adversarial attacks
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