2,198 research outputs found
Subgap states in dirty superconductors and their effect on dephasing in Josephson qubits
We present a theory of the subgap tails of the density of states in a
diffusive superconductor containing magnetic impurities. We show that the
subgap tails have two contributions: one arising from mesoscopic gap
fluctuations, previously discussed by Lamacraft and Simons, and the other
associated to the long-wave fluctuations of the concentration of magnetic
impurities. We study the latter both in small superconducting grains and in
bulk systems [], and establish the dimensionless parameter that
controls which of the two contributions dominates the subgap tails. We observe
that these contributions are related to each other by dimensional reduction. We
apply the theory to estimate the effects of a weak concentration of magnetic
impurities [] on the phase coherence of Josephson
qubits. We find that at these typical concentrations, magnetic impurities are
relevant for the dephasing in large qubits, designed around a
scale, where they limit the quality factor to be .Comment: 13 pages, 1 figur
Nonequilibrium transport via spin-induced sub-gap states in superconductor/quantum dot/normal metal cotunnel junctions
We study low-temperature transport through a Coulomb blockaded quantum dot
(QD) contacted by a normal (N), and a superconducting (S) electrode. Within an
effective cotunneling model the conduction electron self energy is calculated
to leading order in the cotunneling amplitudes and subsequently resummed to
obtain the nonequilibrium T-matrix, from which we obtain the nonlinear
cotunneling conductance. For even occupied dots the system can be conceived as
an effective S/N-cotunnel junction with subgap transport mediated by Andreev
reflections. The net spin of an odd occupied dot, however, leads to the
formation of sub-gap resonances inside the superconducting gap which gives rise
to a characteristic peak-dip structure in the differential conductance, as
observed in recent experiments.Comment: 13 pages, 13 figures (new version contains reformulations and
corrections of typos etc
Iron based superconductors: magnetism, superconductivity and electronic structure
Angle resolved photoemission spectroscopy (ARPES) reveals the features of the
electronic structure of quasi-two-dimensional crystals, which are crucial for
the formation of spin and charge ordering and determine the mechanisms of
electron-electron interaction, including the superconducting pairing. The newly
discovered iron based superconductors (FeSC) promise interesting physics that
stems, on one hand, from a coexistence of superconductivity and magnetism and,
on the other hand, from complex multi-band electronic structure. In this review
I want to give a simple introduction to the FeSC physics, and to advocate an
opinion that all the complexity of FeSC properties is encapsulated in their
electronic structure. For many compounds, this structure was determined in
numerous ARPES experiments and agrees reasonably well with the results of band
structure calculations. Nevertheless, the existing small differences may help
to understand the mechanisms of the magnetic ordering and superconducting
pairing in FeSC.Comment: Invited Revie
Optical signature of sub-gap absorption in the superconducting state of Ba(Fe,Co)2As2
The optical conductivity of Ba(FeCo)As shows a
clear signature of the superconducting gap, but a simple -wave description
fails in accounting for the low frequency response. This task is achieved by
introducing an extra Drude peak in the superconducting state representing
sub-gap absorption, other than thermally broken pairs. This extra peak and the
coexisting -wave response respect the total sum rule indicating a common
origin for the carriers. We discuss the possible origins for this absorption as
(i) quasiparticles due to pair-breaking from interband impurity scattering in a
two band gap symmetry model, which includes (ii) the possible
existence of impurity levels within an isotropic gap model; or (iii) an
indication that one of the bands is highly anisotropic.Comment: 5 pages, 4 figure
Local electronic structures on the superconducting interface
Motivated by the recent discovery of superconductivity on the heterointerface
, we theoretically investigate its local electronic
structures near an impurity considering the influence of Rashba-type spin-orbit
interaction (RSOI) originated in the lack of inversion symmetry. We find that
local density of states near an impurity exhibits the in-gap resonance peaks
due to the quasiparticle scattering on the Fermi surface with the reversal sign
of the pairing gap caused by the mixed singlet and RSOI-induced triplet
superconducting state. We also analyze the evolutions of density of states and
local density of states with the weight of triplet pairing component determined
by the strength of RSOI, which will be widely observed in thin films of
superconductors with surface or interface-induced RSOI, or various
noncentrosymmetric superconductors in terms of point contact tunneling and
scanning tunneling microscopy, and thus reveal an admixture of the spin singlet
and RSOI-induced triplet superconducting states.Comment: Phys. Rev. B 81, 144504 (2010)
Rashba split surface states in BiTeBr
Within density functional theory, we study bulk band structure and surface
states of BiTeBr. We consider both ordered and disordered phases which differ
in atomic order in the Te-Br sublattice. On the basis of relativistic ab-initio
calculations, we show that the ordered BiTeBr is energetically preferable as
compared with the disordered one. We demonstrate that both Te- and
Br-terminated surfaces of the ordered BiTeBr hold surface states with a giant
spin-orbit splitting. The Te-terminated surface-state spin splitting has the
Rashba-type behavior with the coupling parameter \alpha_R ~ 2 eV\AA.Comment: 8 pages, 7 figure
Majorana fermions emerging from magnetic nanoparticles on a superconductor without spin-orbit coupling
There exists a variety of proposals to transform a conventional s-wave
superconductor into a topological superconductor, supporting Majorana fermion
mid-gap states. A necessary ingredient of these proposals is strong spin-orbit
coupling. Here we propose an alternative system consisting of a one-dimensional
chain of magnetic nanoparticles on a superconducting substrate. No spin-orbit
coupling in the superconductor is needed. We calculate the topological quantum
number of a chain of finite length, including the competing effects of disorder
in the orientation of the magnetic moments and in the hopping energies, to
identify the transition into the topologically nontrivial state (with Majorana
fermions at the end points of the chain).Comment: 7 pages, 5 figure
Calculation of Field Characteristics in Periodic Nanostructures from Composite Elements with Activated Plasmon Modes
Modeling of the electromagnetic field of the optical frequency in periodic structures composed of parallel conducting nanorods as well as composite spherical nanoparticles with an excitonogenic envelope in hexagonal 2d-lattices - in the quasistatic approximation and the FDTD method. The qualitative agreement of these approaches is shown when calculating field characteristics in lattices of cylindrical elements
Surface impedance of superconductors with magnetic impurities
Motivated by the problem of the residual surface resistance of the
superconducting radio-frequency (SRF) cavities, we develop a microscopic theory
of the surface impedance of s-wave superconductors with magnetic impurities. We
analytically calculate the current response function and surface impedance for
a sample with spatially uniform distribution of impurities, treating magnetic
impurities in the framework of the Shiba theory. The obtained general
expressions hold in a wide range of parameter values, such as temperature,
frequency, mean free path, and exchange coupling strength. This generality, on
the one hand, allows for direct numerical implementation of our results to
describe experimental systems (SRF cavities, superconducting qubits) under
various practically relevant conditions. On the other hand, explicit analytical
expressions can be obtained in a number of limiting cases, which makes possible
further theoretical investigation of certain regimes. As a feature of key
relevance to SRF cavities, we show that in the regime of "gapless
superconductivity" the surface resistance exhibits saturation at zero
temperature. Our theory thus explicitly demonstrates that magnetic impurities,
presumably contained in the oxide surface layer of the SRF cavities, provide a
microscopic mechanism for the residual resistance.Comment: 9 pages, 3 figs; v2: published versio
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