16,969 research outputs found
Low Frequency Quantum Transport in a Three-probe Mesoscopic Conductor
The low frequency quantum transport properties of a three-probe mesoscopic
conductor are studied using B\"uttiker's AC transport formalism. The static
transmission coefficients and emittance matrix of the system were computed by
explicitly evaluating the various partial density of states (PDOS). We have
investigated the finite size effect of the scattering volume on the global
PDOS. By increasing the scattering volume we observed a gradual improvement in
the agreement of the total DOS as computed externally or locally. Our numerical
data permits a particular fitting form of the finite size effect.Comment: 13 pages, LaTeX, submitted to Phys. Rev.
Revisiting f(R) gravity models that reproduce CDM expansion
We reconstruct an gravity model that gives rise to the particular
CDM background evolution of the universe. We find well-defined,
real-valued analytical forms for the model to describe the universe both
in the early epoch from the radiation to matter dominated eras and the late
time acceleration period. We further examine the viability of the derived
model and find that it is viable to describe the evolution of the
universe in the past and there does not exist the future singularity in the
Lagrangian.Comment: 7 pages, 2 figures, revised version, accepted for publication in PR
Weakly nonlinear quantum transport: an exactly solvable model
We have studied the weakly non-linear quantum transport properties of a
two-dimensional quantum wire which can be solved exactly. The non-linear
transport coefficients have been calculated and interesting physical properties
revealed. In particular we found that as the incoming electron energy
approaches a resonant point given by energy , where the transport is
characterized by a complete reflection, the second order non-linear conductance
changes its sign. This has interesting implications to the current-voltage
characteristics. We have also investigated the establishment of the gauge
invariance condition. We found that for systems with a finite scattering
region, correction terms to the theoretical formalism are needed to preserve
the gauge invariance. These corrections were derived analytically for this
model.Comment: 15 pages, LaTeX, submitted to Phys. Rev.
High Thermoelectric Figure of Merit by Resonant Dopant in Half-Heusler Alloys
Half-Heusler alloys have been one of the benchmark high temperature
thermoelectric materials owing to their thermal stability and promising figure
of merit ZT. Simonson et al. early showed that small amounts of vanadium doped
in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change
with the increased density of states near the Fermi level. We herein report a
systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta)
as prospective resonant dopants in enhancing the ZT of n-type half-Heusler
alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was found to increase
the Seebeck coefficient in the temperature range 300-1000 K, consistent with a
resonant doping scheme. In contrast, Nb and Ta act as normal n-type dopants, as
evident by the systematic decrease in electrical resistivity and Seebeck
coefficient. The combination of enhanced Seebeck coefficient due to the
presence of V resonant states and the reduced thermal conductivity has led to a
state-of-the-art ZT of 1.3 near 850 K in n-type
(Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 alloys.Comment: Submitted to AIP Advance
Scales of Mass Generation for Quarks, Leptons and Majorana Neutrinos
We study 2 --> n inelastic fermion-(anti)fermion scattering into multiple
longitudinal weak gauge bosons and derive universal upper bounds on the scales
of fermion mass generation by imposing unitarity of the S-matrix. We place new
upper limits on the scales of fermion mass generation, independent of the
electroweak symmetry breaking scale. We find that the strongest 2 --> n limits
fall in a narrow range, 3-170 TeV (with n=2-24), depending on the observed
fermion masses.Comment: Phys. Rev. Lett.(in press), minor rewordin
Spin resolved Hall effect driven by spin-orbit coupling
Spin and electric Hall currents are calculated numerically in a
two-dimensional mesoscopic system with Rashba and Dresselhaus spin-orbit
coupling by means of the Landauer-Buttiker formalism. It is found that both
electric and spin Hall currents circulate when two spin-orbit couplings
coexist, while the electric Hall conductance vanishes if either one is absent.
The electric and spin Hall conductances are suppressed in strong disorder, but
survive in weak disorder. Physically it can be understood that the spinomotive
transverse "force" generated by spin-orbit coupling is responsible for the
formation of the spin Hall current and the lack of transverse reflection
symmetry is the origin of the electric Hall current.Comment: 4 pages, 5 figure
Entanglement and quantum phase transitions
We examine several well known quantum spin models and categorize behavior of
pairwise entanglement at quantum phase transitions. A unified picture on the
connection between the entanglement and quantum phase transition is given.Comment: 4 pages, 3 figure
Optical Nondestructive Controlled-NOT Gate without Using Entangled Photons
We present and experimentally demonstrate a novel optical nondestructive
controlled-NOT gate without using entangled ancilla. With much fewer
measurements compared with quantum process tomography, we get a good estimation
of the gate fidelity. The result shows a great improvement compared with
previous experiments. Moreover, we also show that quantum parallelism is
achieved in our gate and the performance of the gate can not be reproduced by
local operations and classical communications.Comment: 5 pages, 3 figures, Slight changes have been made, Journal-ref adde
- …