3,156 research outputs found
Symmetry of `molecular' configurations of interacting electrons in a quantum dot in strong magnetic fields
A molecular description for magic-number configurations of interacting
electrons in a quantum dot in high magnetic fields developed by one of the
authors has been elaborated for four, five and six electron dots. For four
electrons, the magic spin-singlet states are found to alternate between two
different resonating valence bond (RVB)-like states. For the five-electron
spin-polarized case, the molecular description is shown to work for the known
phenomenon of magic-number sequences that correspond to both the N-fold
symmetric ring configuration and a -fold symmetric one with a center
electron. A six-electron dot is shown here to have an additional feature in
which inclusion of quantum mechanical mixing between classical configurations,
which are deformed and degenerate, restores the N-fold symmetry and reproduces
the ground-state energy accurately.Comment: 4 pages, to be published in Physisca
Onset of superconductivity in a voltage-biased NSN microbridge
We study the stability of the normal state in a mesoscopic NSN junction
biased by a constant voltage V with respect to the formation of the
superconducting order. Using the linearized time-dependent Ginzburg-Landau
equation, we obtain the temperature dependence of the instability line,
V_{inst}(T), where nucleation of superconductivity takes place. For
sufficiently low biases, a stationary symmetric superconducting state emerges
below the instability line. For higher biases, the normal phase is destroyed by
the formation of a non-stationary bimodal state with two superconducting nuclei
localized near the opposite terminals. The low-temperature and large-voltage
behavior of the instability line is highly sensitive to the details of the
inelastic relaxation mechanism in the wire. Therefore, experimental studies of
V_{inst}(T) in NSN junctions may be used as an effective tool to access
parameters of the inelastic relaxation in the normal state.Comment: 5 pages, 2 figure
New Dirac points and multiple Landau level crossings in biased trilayer graphene
Recently a new high-mobility Dirac material, trilayer graphene, was realized
experimentally. The band structure of ABA-stacked trilayer graphene consists of
a monolayer-like and a bilayer-like pairs of bands. Here we study electronic
properties of ABA-stacked trilayer graphene biased by a perpendicular electric
field. We find that the combination of the bias and trigonal warping gives rise
to a set of new Dirac points: in each valley, seven species of Dirac fermions
with small masses of order of a few meV emerge. The positions and masses of the
emergent Dirac fermions are tunable by bias, and one group of Dirac fermions
becomes massless at a certain bias value. Therefore, in contrast to bilayer
graphene, the conductivity at the neutrality point is expected to show
non-monotonic behavior, becoming of the order of a few e^2/h when some Dirac
masses vanish. Further, we analyze the evolution of Landau level spectrum as a
function of bias. Emergence of new Dirac points in the band structure
translates into new three-fold-degenerate groups of Landau levels. This leads
to an anomalous quantum Hall effect, in which some quantum Hall steps have a
height of 3e^2/h. At an intermediate bias, the degeneracies of all Landau
levels get lifted, and in this regime all quantum Hall plateaus are spaced by
e^2/h. Finally, we show that the pattern of Landau level crossings is very
sensitive to certain band structure parameters, and can therefore provide a
useful tool for determining their precise values.Comment: 11 pages, 6 figures; v2: expanded introduction, new references added,
a few typos correcte
Spin-Blockade in Single and Double Quantum Dots in Magnetic Fields: a Correlation Effect
The total spin of correlated electrons in a quantum dot changes with magnetic
field and this effect is generally linked to the change in the total angular
momentum from one magic number to another, which can be understood in terms of
an `electron molecule' picture for strong fields. Here we propose to exploit
this fact to realize a spin blockade, i.e., electrons are prohibited to tunnel
at specific values of the magnetic field. The spin-blockade regions have been
obtained by calculating both the ground and excited states. In double dots the
spin-blockade condition is found to be less stringent than in single dots.Comment: 4pages, to be published in Phys. Rev. B (Rapid Communication
Overscreened Kondo fixed point in S=1 spin liquid
We propose a possible realization of the overscreened Kondo impurity problem
by a magnetic s=1/2 impurity embedded in a two-dimensional S=1 U(1) spin liquid
with a Fermi surface. This problem contains an interesting interplay between
non-Fermi-liquid behavior induced by a U(1) gauge field coupled to fermions and
a non-Fermi-liquid fixed point in the overscreened Kondo problem. Using a
large-N expansion together with an expansion in the dynamical exponent of the
gauge field, we find that the coupling to the gauge field leads to weak but
observable changes in the physical properties of the system at the overscreened
Kondo fixed point. We discuss the extrapolation of this result to a physical
case and argue that the realization of overscreened Kondo physics could lead to
observations of effects due to gauge fields.Comment: 10 pages, 5 figure
Quantum quenches in the many-body localized phase
Many-body localized (MBL) systems are characterized by the absence of
transport and thermalization, and therefore cannot be described by conventional
statistical mechanics. In this paper, using analytic arguments and numerical
simulations, we study the behaviour of local observables in an isolated MBL
system following a quantum quench. For the case of a global quench, we find
that the local observables reach stationary, highly non-thermal values at long
times as a result of slow dephasing characteristic of the MBL phase. These
stationary values retain the local memory of the initial state due to the
existence of local integrals of motion in the MBL phase. The temporal
fluctuations around stationary values exhibit universal power-law decay in
time, with an exponent set by the localization length and the diagonal entropy
of the initial state. Such a power-law decay holds for any local observable and
is related to the logarithmic in time growth of entanglement in the MBL phase.
This behaviour distinguishes the MBL phase from both the Anderson insulator
(where no stationary state is reached), and from the ergodic phase (where
relaxation is expected to be exponential). For the case of a local quench, we
also find a power-law approach of local observables to their stationary values
when the system is prepared in a mixed state. Quench protocols considered in
this paper can be naturally implemented in systems of ultra cold atoms in
disordered optical lattices, and the behaviour of local observables provides a
direct experimental signature of many-body localization.Comment: 11 pages, 4 figure
Currents in a many-particle parabolic quantum dot under a strong magnetic field
Currents in a few-electron parabolic quantum dot placed into a perpendicular
magnetic field are considered. We show that traditional ways of investigating
the Wigner crystallization by studying the charge density correlation function
can be supplemented by the examination of the density-current correlator.
However, care must be exercised when constructing the correct projection of the
multi-dimensional wave function space. The interplay between the magnetic field
and Euler-liquid-like behavior of the electron liquid gives rise to persistent
and local currents in quantum dots. We demonstrate these phenomena by collating
a quasi-classical theory valid in high magnetic fields and an exact numerical
solution of the many-body problem.Comment: Uses RevTeX4, figures included in the tex
The Application of Pulsating Resonance Fuel Burning during Steel- Teeming Ladles Drying and Heating Processes
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