151 research outputs found
Lifshitz transition in the double-core vortex in 3He-B
We study the spectrum of fermion states localized within the vortex core of a
weak-coupling p-wave superfluid. The low energy spectrum consists of two
anomalous branches that cross the Fermi level as functions of the impact
parameter, and generate large density of states at the locations of the half
cores of the vortex. Fermi liquid interactions significantly change the vortex
structure, which leads to Lifshitz transition in the effective Fermi surface of
the vortex core fermions. We apply the results to revise the interpretation of
an experiment on rotational dynamics of vortices in superfluid 3He-B.Comment: 11 pages, 6 figures, updated to the published versio
Transport through superconductor/magnetic dot/superconductor structures
The coupling of two s-wave superconductors through a small magnetic dot is
discussed. Assuming that the dot charging energy is small compared to the
superconducting gap, , and that the moment of the dot is
classical, we develop a simple theory of transport through the dot. The
presence of the magnetic dot will position Andreev bound states within the
superconducting gap at energies tunable with the magnetic properties of the
dot. Studying the Josephson coupling it is shown that the constructed junction
can be tuned from a "0" to a ""-junction via a degenerate two-level state
either by changing the magnetic moment of the dot or by changing temperature.
Furthermore, it is shown that details of the magnetic dot can be extracted from
the sub-harmonic structure in the current-voltage characteristics of the
junction.Comment: 5 pages, 4 figures, paper presented at the conference SDP 2001 in
Tokyo on June 2
Nonlinear magnetic field dependence of the conductance in d-wave NIS tunnel junctions
The ab-plane NIS-tunnelling conductance in d-wave superconductors shows a
zero-bias conductance peak which is predicted to split in a magnetic field. In
a pure d-wave superconductor the splitting is linear for fields small on the
scale of the thermodynamic critical field. The field dependence is shown to be
nonlinear, even at low fields, in the vicinity of a surface phase transition
into a local time-reversal symmetry breaking state. The field evolution of the
conductance is sensitive to temperature, doping, and the symmetry of the
sub-dominant pairing channel.Comment: 4 pages, 4 figure
Landau Ginzburg theory of the d-wave Josephson junction
This letter discusses the Landau Ginzburg theory of a Josephson junction
composed of on one side a pure d-wave superconductor oriented with the
axis normal to the junction and on the other side either s-wave or d-wave
oriented with normal to the junction. We use simple symmetry arguments
to show that the Josephson current as a function of the phase must have the
form . In principle vanishes
for a perfect junction of this type, but anisotropy effects, either due to a-b
axis asymmetry or junction imperfections can easily cause to be
quite large even in a high quality junction. If is sufficiently
small and is negative local time reversal symmetry breaking will appear.
Arbitrary values of the flux would then be pinned to corners between such
junctions and occasionally on junction faces, which is consistent with
experiments by Kirtley et al
Phase Crystals
Superconductivity owes its properties to the phase of the electron pair
condensate that breaks the symmetry. In the most traditional ground
state, the phase is uniform and rigid. The normal state can be unstable towards
special inhomogeneous superconducting states: the Abrikosov vortex state, and
the Fulde-Ferrell-Larkin-Ovchinnikov state. Here we show that the phase-uniform
superconducting state can go into a fundamentally different and more ordered
non-uniform ground state, that we denote as a phase crystal. The new state
breaks translational invariance through formation of a spatially periodic
modulation of the phase, manifested by unusual superflow patterns and
circulating currents, that also break time-reversal symmetry. We list the
general conditions needed for realization of phase crystals. Using microscopic
theory we then derive an analytic expression for the superfluid density tensor
for the case of a non-uniform environment in a semi-infinite superconductor. We
demonstrate how the surface quasiparticle states enter the superfluid density
and identify phase crystallization as the main player in several previous
numerical observations in unconventional superconductors, and predict existence
of a similar phenomenon in superconductor-ferromagnetic structures. This
analytic approach provides a new unifying aspect for the exploration of
boundary-induced quasiparticles and collective excitations in superconductors.
More generally, we trace the origin of phase crystallization to non-local
properties of the gradient energy, which implies existence of similar
pattern-forming instabilities in many other contexts.Comment: 8 pages, 4 figure
Effects of quasiparticle tunneling in a circuit-QED realization of a strongly driven two-level system
We experimentally and theoretically study the frequency shift of a driven
cavity coupled to a superconducting charge qubit. In addition to previous
studies, we here also consider drive strengths large enough to energetically
allow for quasiparticle creation. Quasiparticle tunneling leads to the
inclusion of more than two charge states in the dynamics. To explain the
observed effects, we develop a master equation for the microwave dressed charge
states, including quasiparticle tunneling. A bimodal behavior of the frequency
shift as a function of gate voltage can be used for sensitive charge detection.
However, at weak drives the charge sensitivity is significantly reduced by
non-equilibrium quasiparticles, which induce transitions to a non-sensitive
state. Unexpectedly, at high enough drives, quasiparticle tunneling enables a
very fast relaxation channel to the sensitive state. In this regime, the charge
sensitivity is thus robust against externally injected quasiparticles and the
desired dynamics prevail over a broad range of temperatures. We find very good
agreement between theory and experiment over a wide range of drive strengths
and temperatures.Comment: 25 pages, 7 figure
Andreev Bound States at the Interface of Antiferromagnets and d-wave Superconductors
We set up a simple transfer matrix formalism to study the existence of bound
states at interfaces and in junctions between antiferromagnets and d-wave
superconductors. The well-studied zero energy mode at the {110} interface
between an insulator and a d-wave superconductor is spin split when the
insulator is an antiferromagnet. This has as a consequence that any competing
interface induced superconducting order parameter that breaks the time reversal
symmetry needs to exceed a critical value before a charge current is induced
along the interface.Comment: 4 pages, 3 figure
Spin-precession-assisted supercurrent in a superconducting quantum point contact coupled to a single-molecule magnet
The supercurrent of a quantum point contact coupled to a nanomagnet strongly
depends on the dynamics of the nanomagnet's spin. We employ a fully microscopic
model to calculate the transport properties of a junction coupled to a spin
whose dynamics is modeled as Larmor precession brought about by an external
magnetic field and find that the dynamics affects the charge and spin currents
by inducing transitions between the continuum states below the superconducting
gap edge and the Andreev levels. This redistribution of the quasiparticles
leads to a non-equilibrium population of the Andreev levels and an enhancement
of the supercurrent which is visible as a modified current-phase relation as
well as a non-monotonous critical current as function of temperature. The
non-monotonous behavior is accompanied by a corresponding change in
spin-transfer torques acting on the precessing spin and leads to the
possibility of using temperature as a means to tune the back-action on the
spin.Comment: 11 pages, 5 figure
Dynamic parity recovery in a strongly driven Cooper-pair box
We study a superconducting charge qubit coupled to an intensive
electromagnetic field and probe changes in the resonance frequency of the
formed dressed states. At large driving strengths, exceeding the qubit
energy-level splitting, this reveals the well known Landau-Zener-Stuckelberg
(LZS) interference structure of a longitudinally driven two-level system. For
even stronger drives we observe a significant change in the LZS pattern and
contrast. We attribute this to photon-assisted quasiparticle tunneling in the
qubit. This results in the recovery of the qubit parity, eliminating effects of
quasiparticle poisoning and leads to an enhanced interferometric response. The
interference pattern becomes robust to quasiparticle poisoning and has a good
potential for accurate charge sensing.Comment: 5 pages, 4 figure
- …