923 research outputs found
Yu-Shiba-Rusinov states in phase-biased S-QD-S junctions
We study the effects of a phase difference on Yu-Shiba-Rusinov (YSR) states
in a spinful Coulomb-blockaded quantum dot contacted by a superconducting loop.
In the limit where charging energy is larger than the superconducting gap, we
determine the subgap excitation spectrum, the corresponding supercurrent, and
the differential conductance as measured by a normal-metal tunnel probe. In
absence of a phase difference only one linear combination of the superconductor
lead electrons couples to the spin, which gives a single YSR state. With finite
phase difference, however, it is effectively a two-channel scattering problem
and therefore an additional state emerges from the gap edge. The energy of the
phase-dependent YSR states depend on the gate voltage and one state can cross
zero energy twice inside the valley with odd occupancy. These crossings are
shifted by the phase difference towards the charge degeneracy points,
corresponding to larger exchange couplings. Moreover, the zero-energy crossings
give rise to resonant peaks in the differential conductance with magnitude
. Finally, we demonstrate that the quantum fluctuations of the dot spin
do not alter qualitatively any of the results.Comment: 13 pages, 7 figure
Significant g-factor values of a two-electron ground state in quantum dots with spin-orbit coupling
The magnetization of semiconductor quantum dots in the presence of spin-orbit
coupling and interactions is investigated numerically. When the dot is occupied
by two electrons we find that a level crossing between the two lowest many-body
eigenstates may occur as a function of the spin-orbit coupling strength. This
level crossing is accompanied by a non-vanishing magnetization of the
ground-state. Using first order perturbation theory as well as exact numerical
diagonalization of small clusters we show that the tendency of interactions to
cause Stoner-like instability is enhanced by the SO coupling. The resulting
g-factor can have a significant value, and thus may influence g-factor
measurements. Finally we propose an experimental method by which the predicted
phenomenon can be observed.Comment: 7+ pages, 7 figure
PT-symmetric interpretation of double-scaling
The conventional double-scaling limit of an O(N)-symmetric quartic quantum
field theory is inconsistent because the critical coupling constant is
negative. Thus, at the critical coupling the Lagrangian defines a quantum
theory with an upside-down potential whose energy appears to be unbounded
below. Worse yet, the integral representation of the partition function of the
theory does not exist. It is shown that one can avoid these difficulties if one
replaces the original theory by its PT-symmetric analog. For a zero-dimensional
O(N)-symmetric quartic vector model the partition function of the PT-symmetric
analog is calculated explicitly in the double-scaling limit.Comment: 11 pages, 2 figure
Shapiro steps in Josephson junctions with alternating critical current density
We treat theoretically Shapiro steps in tunnel Josephson junctions with
spatially alternating critical current density. Explicit analytical formulas
for the width of the first integer (normal) and half-integer (anomalous)
Shapiro steps are derived for short junctions. We develop coarse-graining
approach, which describes Shapiro steps in the voltage-current curves of the
asymmetric grain boundaries in YBCO thin films and different
superconductor-ferromagnet-superconductor Josephson-type heterostructures.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
High-field vortices in Josephson junctions with alternating critical current density
We study long Josephson junctions with the critical current density
alternating along the junction. New equilibrium states, which we call the field
synchronized or FS states, are shown to exist if the applied field is from
narrow intervals centered around equidistant series of resonant fields, .
The values of are much higher than the flux penetration field, . The
flux per period of the alternating critical current density, , is fixed
for each of the FS states. In the -th FS state the value of is
equal to an integer amount of flux quanta, . Two types of
single Josephson vortices carrying fluxes or/and can exist
in the FS states. Specific stepwise resonances in the current-voltage
characteristics are caused by periodic motion of these vortices between the
edges of the junction.Comment: 4 pages, 5 figure
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