86 research outputs found
Evidence for Triplet Superconductivity in a Superconductor-Ferromagnet Spin Valve
We have studied the dependence of the superconducting (SC) transition
temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers
in the spin valve system CoO_x/Fe1/Cu/Fe2/Pb. We find that this dependence is
nonmonotonic when passing from the parallel to the antiparallel case and
reveals a distinct minimum near the orthogonal configuration. The analysis of
the data in the framework of the SC triplet spin valve theory gives direct
evidence for the long-range triplet superconductivity arising due to
noncollinearity of the two magnetizations.Comment: 5 pages (including 4 EPS figures). Version 2: final version as
published in PR
Decoherence due to nodal quasiparticles in d-wave qubits
We study the Josephson junction between two d-wave superconductors, which is
discussed as an implementation of a qubit. We propose an approach that allows
to calculate the decoherence time due to an intrinsic dissipative process:
quantum tunneling between the two minima of the double-well potential excites
nodal quasiparticles which lead to incoherent damping of quantum oscillations.
The decoherence is weakest in the mirror junction, where the contribution of
nodal quasiparticles corresponds to the superohmic dissipation and becomes
small at small tunnel splitting of the energy level in the double-well
potential. For available experimental data, we estimate the quality factor.Comment: 5 pages, 3 EPS figures; the style file jetpl.cls is included. Version
2: minor correction
Density of states in d-wave superconductors of finite size
We consider the effect of the finite size in the ab-plane on the surface
density of states (DoS) in clean d-wave superconductors. In the bulk, the DoS
is gapless along the nodal directions, while the presence of a surface leads to
formation of another type of the low-energy states, the midgap states with zero
energy. We demonstrate that finiteness of the superconductor in one of
dimensions provides the energy gap for all directions of quasiparticle motion
except for \theta=45 degrees (\theta is the angle between the trajectory and
the surface normal); then the angle-averaged DoS behaves linearly at small
energies. This result is valid unless the crystal is 0- or 45-oriented (\alpha
\ne 0 or 45 degrees, where \alpha is the angle between the a-axis and the
surface normal). In the special case of \alpha=0, the spectrum is gapped for
all trajectories \theta; the angle-averaged DoS is also gapped. In the special
case of \alpha=45, the spectrum is gapless for all trajectories \theta; the
angle-averaged DoS is then large at low energies. In all the cases, the
angle-resolved DoS consists of energy bands that are formed similarly to the
Kronig-Penney model. The analytical results are confirmed by a self-consistent
numerical calculation.Comment: 9 pages (including 5 EPS figures), REVTeX
Peculiarities of the density of states in SN junctions
We study the density of states (DoS) in a normal-metallic (N) film
contacted by a bulk superconductor (S). We assume that the system is diffusive
and the SN interface is transparent. In the limit of thin N layer (compared to
the coherence length), we analytically find three different types of the DoS
peculiarity at energy equal to the bulk superconducting order parameter
. (i) In the absence of the inverse proximity effect, the peculiarity
has the check-mark form with as long as the thickness of the
N layer is smaller than a critical value. (ii) When the inverse proximity
effect comes into play, the check-mark is immediately elevated so that
. (iii) Upon further increasing of the inverse proximity
effect, gradually evolves to the vertical peculiarity (with an
infinite-derivative inflection point at ). This crossover is
controlled by a materials-matching parameter which depends on the relative
degree of disorder in the S and N materials.Comment: 20 pages, 8 figures. Version 2: updated Introduction and Conclusions,
added Discussion section (on experimental relevance) and references. Final
version as published in Annals of Physic
Asymmetric higher-harmonic SQUID as a Josephson diode
We theoretically investigate asymmetric two-junction SQUIDs with different
current-phase relations in the two Josephson junctions, involving higher
Josephson harmonics. Our main focus is on the "minimal model" with one junction
in the SQUID loop possessing the sinusoidal current-phase relation and the
other one featuring additional second harmonic. The current-voltage
characteristic (CVC) turns out to be asymmetric, . The
asymmetry is due to the presence of the second harmonic and depends on the
magnetic flux through the interferometer loop, vanishing only at special values
of the flux such as integer or half-integer in the units of the flux quantum.
The system thus demonstrates the flux-tunable Josephson diode effect (JDE), the
simplest manifestations of which is the direction dependence of the critical
current. We analyze asymmetry of the overall shape both in the absence
and in the presence of external ac irradiation. In the voltage-source case of
external signal, the CVC demonstrates the Shapiro spikes. The integer spikes
are asymmetric (manifestation of the JDE) while the half-integer spikes remain
symmetric. In the current-source case, the CVC demonstrates the Shapiro steps.
The JDE manifests itself in asymmetry of the overall CVC shape, including
integer and half-integer steps.Comment: 18 pages, 5 figures. Version 2: extended introduction, added
references. Final version as published in PR
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