132 research outputs found
High-frequency oscillations in low-dimensional conductors and semiconductor superlattices induced by current in stack direction
A narrow energy band of the electronic spectrum in some direction in
low-dimensional crystals may lead to a negative differential conductance and
N-shaped I-V curve that results in an instability of the uniform stationary
state. A well-known stable solution for such a system is a state with electric
field domain. We have found a uniform stable solution in the region of negative
differential conductance. This solution describes uniform high-frequency
voltage oscillations. Frequency of the oscillation is determined by antenna
properties of the system. The results are applicable also to semiconductor
superlattices.Comment: 8 pages, 3 figure
Charge imbalance and Josephson effects in superconductor-normal metal mesoscopic structures
We consider a Josephson junction the superconducting electrodes of
which are in contact with normal metal reservoirs ( means a barrier). For
temperatures near we calculate an effective critical current and the resistance of the system at the currents and . It is found that the charge imbalance,
which arises due to injection of quasiparticles from the reservoirs into
the wire, affects essentially the characteristics of the structure. The
effective critical current is always larger than the critical
current in the absence of the normal reservoirs and increases with
decreasing the ratio of the length of the wire to the charge imbalance
relaxation length . It is shown that a series of peaks arises on the
characteristics due to excitation of the Carlson-Goldman collective
modes. We find the position of Shapiro steps which deviates from that given by
the Josephson relation.Comment: 12 pages, 4 figures; accepted for publication in Phys. Rev.
Electrodynamics of Josephson vortex lattice in high-temperature superconductors
We studied response of the Josephson vortex lattice in layered
superconductors to the high-frequency c-axis electric field. We found a simple
relation connecting the dynamic dielectric constant with the perturbation of
the superconducting phase, induced by oscillating electric field. Numerically
solving equations for the oscillating phases, we computed the frequency
dependences of the loss function at different magnetic fields, including
regions of both dilute and dense Josephson vortex lattices. The overall
behavior is mainly determined by the c-axis and in-plane dissipation
parameters, which is inversely proportional to the anisotropy. The cases of
weak and strong dissipation are realized in
and underdoped correspondingly. The main feature of the response is the
Josephson-plasma-resonance peak. In the weak-dissipation case additional
satellites appear in the dilute regime mostly in the higher-frequency region
due to excitation of the plasma modes with the wave vectors set by the lattice
structure. In the dense-lattice limit the plasma peak moves to higher frequency
and its intensity rapidly decreases, in agreement with experiment and
analytical theory. Behavior of the loss function at low frequencies is well
described by the phenomenological theory of vortex oscillations. In the case of
very strong in-plane dissipation an additional peak in the loss function
appears below the plasma frequency. Such peak has been observed experimentally
in underdoped . It is caused by frequency
dependence of in-plane contribution to losses rather then a definite mode of
phase oscillations.Comment: 10 pages, 7 figures, to be published in Phys.Rev.B, supplementary
animations of oscillating local electric field can be found at
http://mti.msd.anl.gov/homepages/koshelev/projects/JPRinJVL/Nz2vc0_32vab6_0Anim.ht
Intrinsic Josephson Effect and Violation of the Josephson Relation in Layered Superconductors
Equations describing the resistive state of a layered superconductor with
anisotropic pairing are derived. The similarity with a stack of Josephson
junctions is found at small voltages only, when current density in the
direction perpendicular to the layers can be interpreted as a sum of the
Josephson superconducting, the Ohmic dissipative and the interference currents.
In the spatially uniform state differential conductivity at higher voltages
becomes negative. Nonuniformity of the current distribution generates the
branch imbalance and violates the Josephson relation between frequency and
voltage.Comment: 11 pages, no figures, revtex, to be published in Phys. Rev. Let
On Effect of Equilibrium Fluctuations on Superfluid Density in Layered Superconductors
We calculate suppression of inter- and intralayer superconducting currents
due to equilibrium phase fluctuations and find that, in contrast to a recent
prediction, the effect of thermal fluctuations cannot account for linear
temperature dependence of the superfluid density in high-Tc superconductors at
low temperatures. Quantum fluctuations are found to dominate over thermal
fluctuations at low temperatures due to hardening of their spectrum caused by
the Josephson plasma resonance. Near Tc sizeable thermal fluctuations are found
to suppress the critical current in the stack direction stronger, than in the
direction along the layers. Fluctuations of quasiparticle branch imbalance make
the spectral density of voltage fluctuations at small frequencies non zero, in
contrast to what may be expected from a naive interpretation of Nyquist
formula.Comment: 5 pages, LaTeX, RevTeX, Submitted to PR
Dissipationless BCS Dynamics with Large Branch Imbalance
In many situations a BCS-type superconductor will develop an imbalance
between the populations of the holelike and electronlike spectral branches.
This imbalance suppresses the gap. It has been noted by Gal'perin et al. [Sov.
Phys. JETP 54, 1126 (1981)] that at large imbalance, when the gap is
substantially suppressed, an instability develops. The analytic treatment of
the system beyond the instability point is complicated by the fact that the
Boltzmann approach breaks down. We study the short-time behavior following the
instability, in the collisionless regime, using methods developed by Yuzbashyan
et al. [J. Phys. A 38, 7831 (2005); Phys. Rev. B 72, 220503(R) (2005)].Comment: 12 pages, 3 figure
Nonequilibrium effects in tunnel Josephson junctions
We study nonequilibrium effects in current transport through voltage biased
tunnel junction with long diffusive superconducting leads at low applied
voltage, , and finite temperatures. Due to a small value of the
Josephson frequency, the quasiparticle spectrum adiabatically follows the time
evolution of the superconducting phase difference, which results in the
formation of oscillating bound states in the vicinity of the tunnel junction
(Andreev band). The quasiparticles trapped by the Andreev band generate higher
even harmonics of the Josephson ac current, and also, in the presence of
inelastic scattering, a non-equilibrium dc current, which may considerably
exceed the dc quasiparticle current given by the tunnel model. The distribution
of travelling quasiparticles also deviates from the equilibrium due to the
spectrum oscillations, which results in an additional contribution to the dc
current, proportional to .Comment: 11 pages, 7 figures, to be published in Phys. Rev.
Subgap current in superconducting tunnel junctions with diffusive electrodes
We calculate the subgap current in planar superconducting tunnel junctions
with thin-film diffusive leads. It is found that the subharmonic gap structure
of the tunnel current scales with an effective tunneling transparency which may
exceed the junction transparency by up to two orders of magnitude depending on
the junction geometry and the ratio between the coherence length and the
elastic scattering length. These results provide an alternative explanation of
anomalously high values of the subgap current in tunnelling experiments often
ascribed to imperfection of the insulating layer. We also discuss the effect of
finite lifetime of quasiparticles as the possible origin of additional
enhancement of multiparticle tunnel currents.Comment: 4 pages, 4 figures, to be published in Phys. Rev.
Low-energy quasiparticle states at superconductor-CDW interfaces
Quasiparticle bound states are found theoretically on transparent interfaces
of d-wave superconductors (dSC) with charge density wave solids (CDW), as well
as s-wave superconductors (sSC) with d-density waves (DDW). These bound states
represent a combined effect of Andreev reflection from the superconducting side
and an unconventional quasiparticle Q-reflection from the density wave solid.
If the order parameter for a density wave state is much less than the Fermi
energy, bound states with almost zero energy take place for an arbitrary
orientation of symmetric interfaces. For larger values of the order parameter,
dispersionless zero-energy states are found only on (110) interfaces. Two
dispersive energy branches of subgap quasiparticle states are obtained for
(100) symmetric interfaces. Andreev low-energy bound states, taking place in
junctions with CDW or DDW interlayers, result in anomalous junction properties,
in particular, the low-temperature behavior of the Josephson critical current.Comment: 6 pages, 2 figure
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