1,951 research outputs found
Exploration of The Duality Between Generalized Geometry and Extraordinary Magnetoresistance
We outline the duality between the extraordinary magnetoresistance (EMR),
observed in semiconductor-metal hybrids, and non-symmetric gravity coupled to a
diffusive gauge field. The corresponding gravity theory may be
interpreted as the generalized complex geometry of the semi-direct product of
the symmetric metric and the antisymmetric Kalb-Ramond field:
(). We construct the four dimensional covariant
field theory and compute the resulting equations of motion. The equations
encode the most general form of EMR within a well defined variational
principle, for specific lower dimensional embedded geometric scenarios. Our
formalism also reveals the emergence of additional diffusive pseudo currents
for a completely dynamic field theory of EMR. The proposed equations of motion
now include terms that induce geometrical deformations in the device geometry
in order to optimize the EMR. This bottom-up dual description between EMR and
generalized geometry/gravity lends itself to a deeper insight into the EMR
effect with the promise of potentially new physical phenomena and properties.Comment: 13 pages and 6 figures. Revised/edited for clarity and purpose.
Several references added. Updated title based on suggestions and comments
received. Version accepted for publication in Phys.Rev.
Lateral transition metal dichalcogenide heterostructures for high efficiency thermoelectric devices
Increasing demands for renewable sources of energy has been a major driving
force for developing efficient thermoelectric materials. Two-dimensional (2D)
transition-metal dichalcogenides (TMDC) have emerged as promising candidates
for thermoelectric applications due to their large effective mass and low
thermal conductivity. In this article, we study the thermoelectric performance
of lateral TMDC heterostructures within a multiscale quantum transport
framework. Both -type and -type lateral heterostructures are considered
for all possible combinations of semiconducting TMDCs: MoS, MoSe,
WS, and WSe. The band alignment between these materials is found to
play a crucial in enhancing the thermoelectric figure-of-merit () and power
factor far beyond those of pristine TMDCs. In particular, we show that the
room-temperature value of -type WS with WSe triangular
inclusions, is five times larger than the pristine WS monolayer. -type
MoSe with WSe inclusions is also shown to have a room-temperature
value about two times larger than the pristine MoSe monolayer. The peak
power factor values calculated here, are the highest reported amongst gapped 2D
monolayers at room temperature. Hence, 2D lateral TMDC heterostructures open
new avenues to develop ultra-efficient, planar thermoelectric devices
Tuning spatial entanglement in interacting few-electron quantum dots
Confined geometries such as semiconductor quantum dots are promising
candidates for fabricating quantum computing devices. When several quantum dots
are in proximity, spatial correlation between electrons in the system becomes
significant. In this article, we develop a fully variational action integral
formulation for calculating accurate few-electron wavefunctions in
configuration space, irrespective of potential geometry. To evaluate the
Coulomb integrals with high accuracy, a novel numerical integration method
using multiple Gauss quadratures is proposed. Using this approach, we
investigate the confinement of two electrons in double quantum dots, and
evaluate the spatial entanglement. We investigate the dependence of spatial
entanglement on various geometrical parameters. We derive the two-particle
wavefunctions in the asymptotic limit of the separation distance between
quantum dots, and obtain universal saturation values for the spatial
entanglement. Resonances in the entanglement values due to avoided
level-crossings of states are observed. We also demonstrate the formation of
electron clusters, and show that the entanglement value is a good indicator for
the formation of such clusters. Further, we show that a precise tuning of the
entanglement values is feasible with applied external electric fields
Finite size scaling for quantum criticality using the finite-element method
Finite size scaling for the Schr\"{o}dinger equation is a systematic approach
to calculate the quantum critical parameters for a given Hamiltonian. This
approach has been shown to give very accurate results for critical parameters
by using a systematic expansion with global basis-type functions. Recently, the
finite element method was shown to be a powerful numerical method for ab initio
electronic structure calculations with a variable real-space resolution. In
this work, we demonstrate how to obtain quantum critical parameters by
combining the finite element method (FEM) with finite size scaling (FSS) using
different ab initio approximations and exact formulations. The critical
parameters could be atomic nuclear charges, internuclear distances, electron
density, disorder, lattice structure, and external fields for stability of
atomic, molecular systems and quantum phase transitions of extended systems. To
illustrate the effectiveness of this approach we provide detailed calculations
of applying FEM to approximate solutions for the two-electron atom with varying
nuclear charge; these include Hartree-Fock, density functional theory under the
local density approximation, and an "exact"' formulation using FEM. We then use
the FSS approach to determine its critical nuclear charge for stability; here,
the size of the system is related to the number of elements used in the
calculations. Results prove to be in good agreement with previous Slater-basis
set calculations and demonstrate that it is possible to combine finite size
scaling with the finite-element method by using ab initio calculations to
obtain quantum critical parameters. The combined approach provides a promising
first-principles approach to describe quantum phase transitions for materials
and extended systems.Comment: 15 pages, 19 figures, revision based on suggestions by referee,
accepted in Phys. Rev.
Exploration of Near-Horizon CFT Duality and in Conformal Weyl Gravity
We compute near horizon black hole entropy via the N\"other current method
within the conformal Weyl gravity paradigm for vacuum and non-vacuum
spacetimes. We do this, in the vacuum case, for the near horizon near extremal
Kerr metric and for the non-vacuum case we couple the conformal Weyl gravity
field equations to a near horizon (linear) gauge potential and analyze
the respective found solutions. We highlight the non-universality of black hole
entropy between black hole solutions of varying symmetries, yet their
congruence with Wald's entropy formula for the respective gravity theory.
Finally, we implement an construction to compute the full
asymptotic symmetry group of one of the non-vacuum conformal Weyl black holes.
We do this by performing a Robinson-Wilczek two dimensional reduction, thus
enabling the construction of an effective quantum theory of the remaining field
content. The effective stress energy tensor generates an asymptotic Virasoro
algebra, to -wave approximation, whose center in conjunction with their
proper regularized lowest Virasoro eigen-mode is implemented to compute black
hole entropy via the statistical Cardy formula. We additionally implement
quantum holomorphic fluxes (of the dual CFT) in the near horizon to compute the
Hawking temperature of the respective black hole spacetime. We conclude with a
discussion and outlook for future work.Comment: 20 pages, no figure
Infrared Nonlinear Optics
Contains report on one research project.U.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010
Quantum response of weakly chaotic systems
Chaotic systems, that have a small Lyapunov exponent, do not obey the common
random matrix theory predictions within a wide "weak quantum chaos" regime.
This leads to a novel prediction for the rate of heating for cold atoms in
optical billiards with vibrating walls. The Hamiltonian matrix of the driven
system does not look like one from a Gaussian ensemble, but rather it is very
sparse. This sparsity can be characterized by parameters and that
reflect the percentage of large elements, and their connectivity respectively.
For we use a resistor network calculation that has direct relation to the
semi-linear response characteristics of the system.Comment: 7 pages, 5 figures, expanded improved versio
Center-of-Mass Properties of the Exciton in Quantum Wells
We present high-quality numerical calculations of the exciton center-of-mass
dispersion for GaAs/AlGaAs quantum wells of widths in the range 2-20 nm. The
k.p-coupling of the heavy- and light-hole bands is fully taken into account. An
optimized center-of-mass transformation enhances numerical convergence. We
derive an easy-to-use semi-analytical expression for the exciton groundstate
mass from an ansatz for the exciton wavefunction at finite momentum. It is
checked against the numerical results and found to give very good results. We
also show multiband calculations of the exciton groundstate dispersion using a
finite-differences scheme in real space, which can be applied to rather general
heterostructures.Comment: 19 pages, 12 figures included, to be published in Phys. Rev.
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