5,480 research outputs found
Optimal entanglement generation in cavity QED with dissipation
We investigate a two-level atom coupled to a cavity with a strong classical
driving field in a dissipative environment and find an analytical expression of
the time evolution density matrix for the system. The analytical density
operator is then used to study the entanglement between the atom and cavity by
considering the competing process between the atom-field interactions and the
field-environment interactions. It is shown that there is an optimal
interaction time for generating atom-cavity entanglement.Comment: 9 pages, 7 figure
Strong stability of Nash equilibria in load balancing games
We study strong stability of Nash equilibria in the load balancing games of m (m >= 2) identical servers, in which every job chooses one of the m servers and each job wishes to minimize its cost, given by the
workload of the server it chooses.
A Nash equilibrium (NE) is a strategy profile that is resilient to unilateral deviations. Finding an NE in such a game is simple. However, an NE assignment is not stable against coordinated deviations of several jobs, while a strong Nash equilibrium (SNE) is. We study how well an
NE approximates an SNE.
Given any job assignment in a load balancing game, the improvement ratio (IR) of a deviation of a job is defined as the ratio between the pre-and post-deviation costs. An NE is said to be a ρ-approximate SNE (ρ >= 1) if there is no coalition of jobs such that each job of the coalition
will have an IR more than ρ from coordinated deviations of the coalition.
While it is already known that NEs are the same as SNEs in the 2-server load balancing game, we prove that, in the m-server load balancing game for any given m >= 3, any NE is a (5=4)-approximate SNE, which together with the lower bound already established in the literature implies that the approximation bound is tight. This closes the final gap in the literature on the study of approximation of general NEs to SNEs in the load balancing games. To establish our upper bound, we apply with novelty a powerful graph-theoretic tool
Robustness of Quantum Spin Hall Effect in an External Magnetic Field
The edge states in the quantum spin Hall effect are expected to be protected
by time reversal symmetry. The experimental observation of the quantized
conductance was reported in the InAs/GaSb quantum well {[}Du et al,
arXiv:1306.1925{]}, up to a large magnetic field, which raises a question on
the robustness of the edge states in the quantum spin Hall effect under time
reversal symmetry breaking. Here we present a theoretical calculation on
topological invariants for the Benevig-Hughes-Zhang model in an external
magnetic field, and find that the quantum spin Hall effect retains robust up to
a large magnetic field. The critical value of the magnetic field breaking the
quantum spin Hall effect is dominantly determined by the band gap at the
point instead of the indirect band gap between the conduction and
valence bands. This illustrates that the quantum spin Hall effect could persist
even under time reversal symmetry breaking.Comment: 9 pages, 5 figures, to appear in Phys. Rev.
Linear magnetoconductivity in an intrinsic topological Weyl semimetal
Searching for the signature of the violation of chiral charge conservation in
solids has inspired a growing passion on the magneto-transport in topological
semimetals. One of the open questions is how the conductivity depends on
magnetic fields in a semimetal phase when the Fermi energy crosses the Weyl
nodes. Here, we study both the longitudinal and transverse magnetoconductivity
of a topological Weyl semimetal near the Weyl nodes with the help of a two-node
model that includes all the topological semimetal properties. In the semimetal
phase, the Fermi energy crosses only the 0th Landau bands in magnetic fields.
For a finite potential range of impurities, it is found that both the
longitudinal and transverse magnetoconductivity are positive and linear at the
Weyl nodes, leading to an anisotropic and negative magnetoresistivity. The
longitudinal magnetoconductivity depends on the potential range of impurities.
The longitudinal conductivity remains finite at zero field, even though the
density of states vanishes at the Weyl nodes. This work establishes a relation
between the linear magnetoconductivity and the intrinsic topological Weyl
semimetal phase.Comment: An extended version accepted by New. J. Phys. with 15 pages and 3
figure
High-field magnetoconductivity of topological semimetals with short-range potential
Weyl semimetals are three-dimensional topological states of matter, in a
sense that they host paired monopoles and antimonopoles of Berry curvature in
momentum space, leading to the chiral anomaly. The chiral anomaly has long been
believed to give a positive magnetoconductivity or negative magnetoresistivity
in strong and parallel fields. However, several recent experiments on both Weyl
and Dirac topological semimetals show a negative magnetoconductivity in high
fields. Here, we study the magnetoconductivity of Weyl and Dirac semimetals in
the presence of short-range scattering potentials. In a strong magnetic field
applied along the direction that connects two Weyl nodes, we find that the
conductivity along the field direction is determined by the Fermi velocity,
instead of by the Landau degeneracy. We identify three scenarios in which the
high-field magnetoconductivity is negative. Our findings show that the
high-field positive magnetoconductivity may not be a compelling signature of
the chiral anomaly and will be helpful for interpreting the inconsistency in
the recent experiments and earlier theories.Comment: An extended version accepted by Phys. Rev. B, with 11 pages and 4
figure
Edge states and integer quantum Hall effect in topological insulator thin films
The integer quantum Hall effect is a topological state of quantum matter in
two dimensions, and has recently been observed in three-dimensional topological
insulator thin films. Here we study the Landau levels and edge states of
surface Dirac fermions in topological insulators under strong magnetic field.
We examine the formation of the quantum plateaux of the Hall conductance and
find two different patterns, in one pattern the filling number covers all
integers while only odd integers in the other. We focus on the quantum plateau
closest to zero energy and demonstrate the breakdown of the quantum spin Hall
effect resulting from structure inversion asymmetry. The phase diagrams of the
quantum Hall states are presented as functions of magnetic field, gate voltage
and chemical potential. This work establishes an intuitive picture of the edge
states to understand the integer quantum Hall effect for Dirac electrons in
topological insulator thin films.Comment: 10 pages, 5 figure
Appearance of the universal value of the zero-bias conductance in a Weyl semimetal-superconductor junction
We study the differential conductance of a time-reversal symmetric Weyl
semimetal-superconductor (N-S) junction with an s-wave superconducting state.
We find that there exists an extended regime where the zero-bias differential
conductance acquires the universal value per unit channel,
independent of the pairing and chemical potentials on each side of the
junction, due to a perfect cancellation of Andreev and normal reflection
contributions. This universal conductance can be attributed to the interplay of
the unique spin/orbital-momentum locking and s-wave pairing that couples Weyl
nodes of the same chirality. We expect that the universal conductance can serve
as a robust and distinct signature for time-reversal symmetric Weyl fermions,
and be observed in the recently discovered time-reversal symmetric Weyl
semimetals.Comment: 12 pages, 4 figure
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