8,171 research outputs found
Transport through a quantum spin Hall quantum dot
Quantum spin Hall insulators, recently realized in HgTe/(Hg,Cd)Te quantum
wells, support topologically protected, linearly dispersing edge states with
spin-momentum locking. A local magnetic exchange field can open a gap for the
edge states. A quantum-dot structure consisting of two such magnetic tunneling
barriers is proposed and the charge transport through this device is analyzed.
The effects of a finite bias voltage beyond linear response, of a gate voltage,
and of the charging energy in the quantum dot are studied within a combination
of Green-function and master-equation approaches. Among other results, a
partial recurrence of non-interacting behavior is found for strong
interactions, and the possibility of controlling the edge magnetization by a
locally applied gate voltage is proposed.Comment: 12 pages, 7 figure
Time-convolutionless master equation for quantum dots: Perturbative expansion to arbitrary order
The master equation describing the non-equilibrium dynamics of a quantum dot
coupled to metallic leads is considered. Employing a superoperator approach, we
derive an exact time-convolutionless master equation for the probabilities of
dot states, i.e., a time-convolutionless Pauli master equation. The generator
of this master equation is derived order by order in the hybridization between
dot and leads. Although the generator turns out to be closely related to the
T-matrix expressions for the transition rates, which are plagued by
divergences, in the time-convolutionless generator all divergences cancel order
by order. The time-convolutionless and T-matrix master equations are contrasted
to the Nakajima-Zwanzig version. The absence of divergences in the
Nakajima-Zwanzig master equation due to the nonexistence of secular reducible
contributions becomes rather transparent in our approach, which explicitly
projects out these contributions. We also show that the time-convolutionless
generator contains the generator of the Nakajima-Zwanzig master equation in the
Markov approximation plus corrections, which we make explicit. Furthermore, it
is shown that the stationary solutions of the time-convolutionless and the
Nakajima-Zwanzig master equations are identical. However, this identity neither
extends to perturbative expansions truncated at finite order nor to dynamical
solutions. We discuss the conditions under which the Nakajima-Zwanzig-Markov
master equation nevertheless yields good results.Comment: 13 pages + appendice
A model of the effect of collisions on QCD plasma instabilities
We study the effect of including a BGK collisional kernel on the collective
modes of a QCD plasma which has a hard-particle distribution function which is
anisotropic in momentum space. We calculate dispersion relations for both the
stable and unstable modes and show that the addition of hard particle
collisions slows the rate of growth of QCD plasma unstable modes. We also show
that for any anisotropy there is an upper limit on the collisional frequency
beyond which no instabilities exist. Estimating a realistic value for the
collisional frequency for alpha_s ~ 0.2 - 0.4 we find that for the
large-anisotropy case which is relevant for the initial state of matter
generated by free streaming in heavy-ion collisions that the collisional
frequency is below this critical value.Comment: 15 pages, 12 figure
Kapitza-Dirac effect in the relativistic regime
A relativistic description of the Kapitza-Dirac effect in the so-called Bragg
regime with two and three interacting photons is presented by investigating
both numerical and perturbative solutions of the Dirac equation in momentum
space. We demonstrate that spin-flips can be observed in the two-photon and the
three-photon Kapitza-Dirac effect for certain parameters. During the
interaction with the laser field the electron's spin is rotated, and we give
explicit expressions for the rotation axis and the rotation angle. The
off-resonant Kapitza-Dirac effect, that is, when the Bragg condition is not
exactly fulfilled, is described by a generalized Rabi theory. We also analyze
the in-field quantum dynamics as obtained from the numerical solution of the
Dirac equation.Comment: minor correction
- …