7,453 research outputs found
Optical response of graphene under intense terahertz fields
Optical responses of graphene in the presence of intense circularly and
linearly polarized terahertz fields are investigated based on the Floquet
theory. We examine the energy spectrum and density of states. It is found that
gaps open in the quasi-energy spectrum due to the single-photon/multi-photon
resonances. These quasi-energy gaps are pronounced at small momentum, but
decrease dramatically with the increase of momentum and finally tend to be
closed when the momentum is large enough. Due to the contribution from the
states at large momentum, the gaps in the density of states are effectively
closed, in contrast to the prediction in the previous work by Oka and Aoki
[Phys. Rev. B {\bf 79}, 081406(R) (2009)]. We also investigate the optical
conductivity for different field strengths and Fermi energies, and show the
main features of the dynamical Franz-Keldysh effect in graphene. It is
discovered that the optical conductivity exhibits a multi-step-like structure
due to the sideband-modulated optical transition. It is also shown that dips
appear at frequencies being the integer numbers of the applied terahertz field
frequency in the case of low Fermi energy, originating from the quasi-energy
gaps at small momentums. Moreover, under a circularly polarized terahertz
field, we predict peaks in the middle of the "steps" and peaks induced by the
contribution from the states around zero momentum in the optical conductivity.Comment: 15 pages, 10 figure
Excitation Induced Dephasing in Semiconductor Quantum Dots
A quantum kinetic theory is used to compute excitation induced dephasing in
semiconductor quantum dots due to the Coulomb interaction with a continuum of
states, such as a quantum well or a wetting layer. It is shown that a frequency
dependent broadening together with nonlinear resonance shifts are needed for a
microscopic explanation of the excitation induced dephasing in such a system,
and that excitation induced dephasing for a quantum-dot excitonic resonance is
different from quantum-well and bulk excitons.Comment: 6 pages, 4 figures. Extensively revised text, two figures change
Opportunities and limitations of transition voltage spectroscopy: a theoretical analysis
In molecular charge transport, transition voltage spectroscopy (TVS) holds
the promise that molecular energy levels can be explored at bias voltages lower
than required for resonant tunneling. We investigate the theoretical basis of
this novel tool, using a generic model. In particular, we study the length
dependence of the conducting frontier orbital and of the 'transition voltage'
as a function of length. We show that this dependence is influenced by the
amount of screening of the electrons in the molecule, which determines the
voltage drop to be located at the contacts or across the entire molecule. We
observe that the transition voltage depends significantly on the length, but
that the ratio between the transition voltage and the conducting frontier
orbital is approximately constant only in strongly screening (conjugated)
molecules. Uncertainty about the screening within a molecule thus limits the
predictive power of TVS. We furthermore argue that the relative length
independence of the transition voltage for non-conjugated chains is due to
strong localization of the frontier orbitals on the end groups ensuring binding
of the rods to the metallic contacts. Finally, we investigate the
characteristics of TVS in asymmetric molecular junctions. If a single level
dominates the transport properties, TVS can provide a good estimate for both
the level position and the degree of junction asymmetry. If more levels are
involved the applicability of TVS becomes limited.Comment: 8 pages, 12 figure
Scalable measures of magic for quantum computers
Non-stabilizerness or magic characterizes the amount of non-Clifford
operations needed to prepare quantum states. It is a crucial resource for
quantum computing and a necessary condition for quantum advantage. However,
quantifying magic beyond a few qubits has been a major challenge. Here, we
introduce Bell magic to efficiently measure magic with a sampling cost that is
independent of the number of qubits. Our method uses Bell measurements over two
copies of a state, which we implement in experiment together with a cost-free
error mitigation scheme. We show the transition of classically simulable
stabilizer states into intractable quantum states on the IonQ quantum computer.
For applications, we efficiently distinguish stabilizer and non-stabilizer
states with low measurement cost even in the presence of experimental noise.
Further, we propose a variational quantum algorithm to maximize the Bell magic
of quantum states via the shift-rule. Our algorithm can be free of barren
plateaus even for highly expressible variational circuits. Finally, we
experimentally demonstrate a Bell measurement protocol for the stabilizer
R\'enyi entropy as well as the Wallach-Meyer entanglement measure. Our results
pave the way to understand the non-classical power of quantum computers,
quantum simulators and quantum many-body systems.Comment: 23 pages, 10 figure
Thermoelectric and thermal rectification properties of quantum dot junctions
The electrical conductance, thermal conductance, thermal power and figure of
merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator
matrix connected with metallic electrodes are theoretically investigated in the
Coulomb blockade regime. The multilevel Anderson model is used to simulate the
multiple QDs junction system. The charge and heat currents in the sequential
tunneling process are calculated by the Keldysh Green function technique. In
the linear response regime the ZT values are still very impressive in the small
tunneling rates case, although the effect of electron Coulomb interaction on ZT
is significant. In the nonlinear response regime, we have demonstrated that the
thermal rectification behavior can be observed for the coupled QDs system,
where the very strong asymmetrical coupling between the dots and electrodes,
large energy level separation between dots and strong interdot Coulomb
interactions are required.Comment: 8 page and 14 figure
Quasi-equilibrium optical nonlinearities in spin-polarized GaAs
Semiconductor Bloch equations, which microscopically describe the dynamics of
a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in
two limits: the coherent and the quasi-equilibrium regime. These equations have
been recently extended to include the spin degree of freedom, and used to
explain spin dynamics in the coherent regime. In the quasi-equilibrium limit,
one solves the Bethe-Salpeter equation in a two-band model to describe how
optical absorption is affected by Coulomb interactions within a
spin-unpolarized plasma of arbitrary density. In this work, we modified the
solution of the Bethe-Salpeter equation to include spin-polarization and light
holes in a three-band model, which allowed us to account for spin-polarized
versions of many-body effects in absorption. The calculated absorption
reproduced the spin-dependent, density-dependent and spectral trends observed
in bulk GaAs at room temperature, in a recent pump-probe experiment with
circularly polarized light. Hence our results may be useful in the microscopic
modelling of density-dependent optical nonlinearities in spin-polarized
semiconductors.Comment: 7 pages, 6 figure
Double non-equivalent chain structure on vicinal Si(557)-Au surface
We study electronic and topographic properties of the vicinal Si(557)-Au
surface using scanning tunneling microscopy and reflection of high energy
electron diffraction technique. STM data reveal double wire structures along
terraces. Moreover behavior of the voltage dependent STM tip - surface distance
is different in different chains. While the one chain shows oscillations of the
distance which are sensitive to the sign of the voltage bias, the oscillations
in the other chain remain unchanged with respect to the positive/negative
biases. This suggests that one wire has metallic character while the other one
- semiconducting. The experimental results are supplemented by theoretical
calculations within tight binding model suggesting that the observed chains are
made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.
I-V curves of Fe/MgO (001) single- and double-barrier tunnel junctions
In this work, we calculate with ab initio methods the current-voltage
characteristics for ideal single- and double-barrier Fe/MgO (001) magnetic
tunnel junctions. The current is calculated in the phase-coherent limit by
using the recently developed SMEAGOL code, combining the nonequilibrium Green
function formalism with density-functional theory. In general we find that
double-barrier junctions display a larger magnetoresistance, which decays with
bias at a slower pace than their single-barrier counterparts. This is explained
in terms of enhanced spin filtering from the middle Fe layer sandwiched in
between the two MgO barriers. In addition, for double-barrier tunnel junctions,
we find a well defined peak in the magnetoresistance at a voltage of V=0.1 V.
This is the signature of resonant tunneling across a majority quantum well
state. Our findings are discussed in relation to recent experiments
Phonon Hall Effect in Four-Terminal Junctions
Using an exact nonequilibrium Green's function formulism, the phonon Hall
effect for paramagnetic dielectrics is studied in a four-terminal device
setting. The temperature difference in the transverse direction of the heat
current is calculated for two-dimensional models with the magnetic field
perpendicular to the plane. We find a surprising result that the square lattice
does not have the phonon Hall effect while a honeycomb lattice has. This can be
explained by symmetry. The temperature difference changes sign if the magnetic
field is sufficiently large.Comment: 4 pages, 5 figure
Effective Hamiltonian for Excitons with Spin Degrees of Freedom
Starting from the conventional electron-hole Hamiltonian , we
derive an effective Hamiltonian for excitons with
spin degrees of freedom. The Hamiltonian describes optical processes close to
the exciton resonance for the case of weak excitation. We show that
straightforward bosonization of does not give the correct form
of , which we obtain by a projection onto the subspace
spanned by the excitons. The resulting relaxation and renormalization
terms generate an interaction between excitons with opposite spin. Moreover,
exciton-exciton repulsive interaction is greatly reduced by the
renormalization. The agreement of the present theory with the experiment
supports the validity of the description of a fermionic system by bosonic
fields in two dimensions.Comment: 12 pages, no figures, RevTe
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