33 research outputs found
Theory of the spin-galvanic effect and the anomalous phase-shift in superconductors and Josephson junctions with intrinsic spin-orbit coupling
Due to the spin-orbit coupling (SOC) an electric current flowing in a normal
metal or semiconductor can induce a bulk magnetic moment. This effect is known
as the Edelstein (EE) or magneto-electric effect. Similarly, in a bulk
superconductor a phase gradient may create a finite spin density. The inverse
effect, also known as the spin-galvanic effect, corresponds to the creation of
a supercurrent by an equilibrium spin polarization. Here, by exploiting the
analogy between a linear-in-momentum SOC and a background SU(2) gauge field, we
develop a quasiclassical transport theory to deal with magneto-electric effects
in superconducting structures. For bulk superconductors this approach allows us
to easily reproduce and generalize a number of previously known results. For
Josephson junctions we establish a direct connection between the inverse EE and
the appearance of an anomalous phase-shift in the current-phase
relation. In particular we show that is proportional to the
equilibrium spin-current in the weak link. We also argue that our results are
valid generically, beyond the particular case of linear-in-momentum SOC. The
magneto-electric effects discussed in this study may find applications in the
emerging field of coherent spintronics with superconductors.Comment: v1: article version of the preprints arXiv:1408.4533 and
arXiv:1409.4563 in letter format, with far more results and details. v2: some
typos and mistakes corrected, new presentation of the derivation at all
temperature in the ballistic regime (section VI), including a new fig.2 to
illustrate this section. v3: accepted version, with extra reference
Ballistic Josephson junctions in the presence of generic spin dependent fields
Ballistic Josephson junctions are studied in the presence of a spin-splitting
field and spin-orbit coupling. A generic expression for the quasi-classical
Green's function is obtained and with its help we analyze several aspects of
the proximity effect between a spin-textured normal metal (N) and singlet
superconductors (S). In particular, we show that the density of states may show
a zero-energy peak which is a generic consequence of the spin-dependent
couplings in heterostructures. In addition we also obtain the spin current and
the induced magnetic moment in a SNS structure and discuss possible coherent
manipulation of the magnetization which results from the coupling between the
superconducting phase and the spin degree of freedom. Our theory predicts a
spin accumulation at the S/N interfaces, and transverse spin currents flowing
perpendicular to the junction interfaces. Some of these findings can be
understood in the light of a non-Abelian electrostatics.Comment: published versio
Spectral Properties and Quantum Phase Transitions in Superconducting Junctions with a Ferromagnetic Link
We study theoretically the spectral and transport properties of a
superconducting wire with a magnetic defect. We start by modelling the system
as a one dimensional magnetic Josephson junction and derive the equation
determining the full subgap spectrum in terms of the normal-state transfer
matrix for arbitrary length and exchange field of the magnetic region. We
demonstrate that the quantum phase transition predicted for a short-range
magnetic impurity, and associated with a change of the total spin of the
system, also occurs in junctions of finite length. Specifically, we find that
the total spin changes discontinuously by integer jumps when bounds states
cross the Fermi level. The spin can be calculated by using a generalization of
Friedel sum rule for the superconducting state, which we also derive. With
these tools, we analyze the subgap spectrum of a junction with the length of
the magnetic region smaller than the superconducting coherence length and
demonstrate how phase transitions also manifest as change of the sign of the
supercurrent.Comment: 7 pages, 3 figure
Current-induced spin polarization at metallic surfaces from first-principles
We present the results of first-principles calculations based on density
functional theory estimating the magnitude of the current-induced spin
polarization (CISP) at the surfaces of the transition metals with fcc and
bcc crystal structures. We predict that the largest surface CISP occurs for W
and Ta, whereas CISP is considerably weaker for Pt and Au surfaces. We then
discuss how CISP emerges over a length scale equal to few atomic layers as
opposed to the spin accumulation characteristic of the SHE, which is related to
the materials' spin diffusion length. Finally, using our estimates for the CISP
magnitude, we suggest that the spin density appearing near W surfaces in
experiments is mostly due to CISP, whereas that at Pt surfaces stems from the
Hall effect
Sodium: a charge-transfer insulator at high pressures
By means of first-principles methods we analyze the optical response of
transparent dense sodium as a function of applied pressure. We discover an
unusual kind of charge-transfer exciton that proceeds from the interstitial
distribution of valence electrons repelled away from the ionic cores by the
Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum
shows a strong anisotropy with light polarization that just at pressures above
the metal-insulator transition manifests as sodium being optically transparent
in one direction but reflective in the other. This result provides a key
information about the crystal structure of transparent sodium, a new
unconventional inorganic electride.Comment: revtex4, 5+8 page
Theory of magnetic response in finite two-dimensional superconductors
We present a theory of magnetic response in a finite-size two-dimensional
superconductors with Rashba spin-orbit coupling. The interplay between the
latter and an in-plane Zeeman field leads on the one hand to an out-of-plane
spin polarization which accumulates at the edges of the sample over the
superconducting coherence length, and on the other hand, to circulating
supercurrents decaying away from the edge over a macroscopic scale. In a long
finite stripe of width W both, the spin polarization and the currents,
contribute to the total magnetic moment induced at the stripe ends. These two
contributions scale with W and W2 respectively, such that for sufficiently
large samples it can be detected by current magnetometry techniques.Comment: 6 pages, 3 figures; typos correcte
Dielectric screening in two-dimensional insulators: Implications for excitonic and impurity states in graphane
For atomic thin layer insulating materials we provide an exact analytic form
of the two-dimensional screened potential. In contrast to three-dimensional
systems where the macroscopic screening can be described by a static dielectric
constant in 2D systems the macroscopic screening is non local (q-dependent)
showing a logarithmic divergence for small distances and reaching the
unscreened Coulomb potential for large distances. The cross-over of these two
regimes is dictated by 2D layer polarizability that can be easily computed by
standard first-principles techniques. The present results have strong
implications for describing gap-impurity levels and also exciton binding
energies. The simple model derived here captures the main physical effects and
reproduces well, for the case of graphane, the full many-body GW plus
Bethe-Salpeter calculations. As an additional outcome we show that the impurity
hole-doping in graphane leads to strongly localized states, what hampers
applications in electronic devices. In spite of the inefficient and nonlocal
two-dimensional macroscopic screening we demonstrate that a simple
approach is capable to describe the electronic and
transport properties of confined 2D systems.Comment: 17 pages, 3 figure