84 research outputs found
The Berry curvature of the Bogoliubov quasiparticle Bloch states in the unconventional superconductor SrRuO
We will extend the concept of electron band Berry curvatures to
superconducting materials. We show that this can be defined for the
Bogoliubov-de Gennes equation describing the superconducting state in a
periodic crystal. In addition, the concept is exploited to understand the
driving mechanism for the optical Kerr effect in time reversal symmetry
breaking superconductors. Finally, we establish a sum rule analogue to the
normal state Hall sum rule making quantitative contact between the imaginary
part of the optical conductivity and the Berry curvature. The general theory
will be applied and tested against the drosophila of the p-wave paired
materials SrRuO
Optical Gyrotropy and the Nonlocal Hall Effect in Chiral Charge Ordered TiSe
It has been suggested that materials which break spatial inversion symmetry,
but not time reversal symmetry, will be optically gyrotropic and display a
nonlocal Hall effect. The associated optical rotary power and the suggested
possibility of inducing a Kerr effect in such materials, in turn are central to
recent discussions about the nature of the pseudogap phases of various cuprate
high-temperature superconductors. In this letter, we show that optical
gyrotropy and the nonlocal Hall effect provide a sensitive probe of broken
inversion symmetry in -TiSe. This material was recently found to
possess a chiral charge ordered phase at low temperatures, in which inversion
symmetry is spontaneously broken, while time reversal symmetry remains unbroken
throughout its phase diagram. We estimate the magnitude of the resulting
gyrotropic constant and optical rotary power and suggest that -TiSe may
be employed as a model material in the interpretation of recent Kerr effect
measurements in cuprate superconductors.Comment: 5 pages, 3 figure
Polar Kerr effect from a time-reversal symmetry breaking unidirectional charge density wave
We analyze the Hall conductivity of a charge ordered
state with momentum and calculate the intrinsic
contribution to the Kerr angle using the fully reconstructed
tight-binding band structure for layered cuprates beyond the low energy hot
spots model and particle hole symmetry. We show that such a unidirectional
charge density wave (CDW), which breaks time reversal symmetry as recently put
forward by Wang and Chubukov [Phys. Rev. B {\bf 90}, 035149 (2014)], leads to a
nonzero polar Kerr effect as observed experimentally. In addition, we model a
fluctuating CDW via a large quasiparticle damping of the order of the CDW gap
and discuss possible implications for the pseudogap phase. We can qualitatively
reproduce previous measurements of underdoped cuprates but making quantitative
connections to experiments is hampered by the sensitivity of the polar Kerr
effect with respect to the complex refractive index .Comment: 6 pages, 4 figure
Colossal Spin Hall Effect in Ultrathin Metallic Films
We predict spin Hall angles up to 80% for ultrathin noble metal films with
substitutional Bi impurities. The colossal spin Hall effect is caused by
enhancement of the spin Hall conductivity in reduced sample dimension and a
strong reduction of the charge conductivity by resonant impurity scattering.
These findings can be exploited to create materials with high efficiency of
charge to spin current conversion by strain engineering.Comment: 4 pages, 5 figure
A random wave model for the Aharonov-Bohm effect
We study an ensemble of random waves subject to the Aharonov-Bohm effect. The
introduction of a point with a magnetic flux of arbitrary strength into a
random wave ensemble gives a family of wavefunctions whose distribution of
vortices (complex zeros) are responsible for the topological phase associated
with the Aharonov-Bohm effect. Analytical expressions are found for the vortex
number and topological charge densities as functions of distance from the flux
point. Comparison is made with the distribution of vortices in the isotropic
random wave model. The results indicate that as the flux approaches
half-integer values, a vortex with the same sign as the fractional part of the
flux is attracted to the flux point, merging with it at half-integer flux.
Other features of the Aharonov-Bohm vortex distribution are also explored.Comment: 16 pages, 5 figure
The Kerr rotation in the unconventional superconductor SrRuO
The interpretation of Kerr rotation measurements in the superconducting phase
of SrRuO is a controversial topic. Both intrinsic and extrinsic
mechanisms have been proposed, and it has been argued that the intrinsic
response vanishes by symmetry. We focus on the intrinsic contribution and
clarify several conflicting results in the literature. On the basis of symmetry
considerations and detailed calculations we show that the intrinsic Kerr signal
is not forbidden in a general multi- band system but has a rich structure in
the near infrared regime. We distinguish different optical transitions
determined by the superconducting gap (far infrared) and the inter orbital
coupling of the normal state (near infrared). We argue that the low frequency
transitions do not contribute to the Hall conductivity while only the
inter-orbital transitions in the near infrared regime contribute. Finally, we
discuss the difficulties to connect the calculations for the optical Hall
conductivity to the experimental measurement of the Kerr angle. We will compare
different approximations which might lead to conflicting results.Comment: 9 pages, 8 figures, 1 tabl
Temperature dependence of spin diffusion length and spin Hall angle in Au and Pt
We have studied the spin transport and the spin Hall effect as a function of
temperature for platinum (Pt) and gold (Au) in lateral spin valve structures.
First, by using the spin absorption technique, we extract the spin diffusion
length of Pt and Au. Secondly, using the same devices, we have measured the
spin Hall conductivity and analyzed its evolution with temperature to identify
the dominant scattering mechanisms behind the spin Hall effect. This analysis
confirms that the intrinsic mechanism dominates in Pt whereas extrinsic effects
are more relevant in Au. Moreover, we identify and quantify the phonon-induced
skew scattering. We show that this contribution to skew scattering becomes
relevant in metals such as Au, with a low residual resistivity.Comment: 13 pages, 5 figure
Spin accumulation in metallic thin films induced by electronic impurity scattering
In order to explore the spin accumulation, evaluating the spin galvanic and
spin Hall effect, we utilize the semi-classical Boltzmann equation based on
input from the relativistic Korringa-Kohn-Rostoker Green's function method,
within the density functional theory. We calculate the spin accumulation
including multiple contributions, especially skew-scattering (scattering-in
term) and compare this to three different approximations, which include the
isotropic and anisotropic relaxation time approximation. For heavy metals, with
strong intrinsic spin-orbit coupling, we find that almost all the effects are
captured within the anisotropic relaxation time approximation. On the other
hand, in light metals the contributions from the vertex corrections
(scattering-in term) are comparable to the induced effect in anisotropic
relaxation time approximation. We put a particular focus on the influence of
the atomic character of the substitutional impurities on the spin accumulation
as well as the dependence on the impurity position. As impurities will break
space inversion symmetry of the thin film, this will give rise to both
symmetric and antisymmetric contributions to the spin accumulation. In general,
we find the impurities at the surface generate the largest efficiency of
charge-to-spin conversion in case of the spin accumulation. Comparing our
results to existing experimental findings for Pt we find a good agreement
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