105 research outputs found
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
On calculating the Berry curvature of Bloch electrons using the KKR method
We propose and implement a particularly effective method for calculating the
Berry curvature arising from adiabatic evolution of Bloch states in wave vector
k space. The method exploits a unique feature of the Korringa-Kohn-Rostoker
(KKR) approach to solve the Schr\"odinger or Dirac equations. Namely, it is
based on the observation that in the KKR method k enters the calculation via
the structure constants which depend only on the geometry of the lattice but
not the crystal potential. For both the Abelian and non-Abelian Berry curvature
we derive an analytic formula whose evaluation does not require any numerical
differentiation with respect to k. We present explicit calculations for Al, Cu,
Au, and Pt bulk crystals.Comment: 13 pages, 5 figure
Gauge freedom for degenerate Bloch states
In nonmagnetic crystals with inversion symmetry the electronic bands are
twofold degenerate. As a consequence, any orthonormalized linear combination
of the two corresponding eigenfunctions can represent the electron wave
function. A priori it is not obvious which superposition, gauge, should be
chosen to calculate a quantity which is not gauge invariant within a certain
approximation. Here we consider gauge options appropriate under particular
physical conditions
The Fermi Surface Effect on Magnetic Interlayer Coupling
The oscillating magnetic interlayer coupling of Fe over spacer layers
consisting of CuPd alloys is investigated by first principles
density functional theory. The amplitude, period and phase of the coupling, as
well as the disorder-induced decay, are analyzed in detail and the consistency
to the Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory is discussed. For the first
time an effect of the Fermi surface nesting strength on the amplitude is
established from first principles calculations. An unexpected variation of the
phase and disorder-induced decay is obtained and the results are discussed in
terms of asymptotics
Spin-Orbit Coupling and Symmetry of the Order Parameter in Strontium Ruthenate
Determination of the orbital symmetry of a state in spin triplet
SrRuO superconductor is a challenge of considerable importance. Most of
the experiments show that the chiral state of the type
is realized and remains stable on lowering the temperature. Here we have
studied the stability of various superconducting states of SrRuO in the
presence of spin-orbit coupling.
Numerically we found that the chiral state is never the minimum energy. Alone
among the five states studied it has and is
therefore not affected to linear order in the coupling parameter . We
found that stability of the chiral state requires spin dependent pairing
interactions. This imposes strong constraint on the pairing mechanism.Comment: 4 pages, 4 figure
First-principles calculations of the Berry curvature of Bloch states for charge and spin transport of electrons
Recent progress in wave packet dynamics based on the insight of Berry
pertaining to adiabatic evolution of quantum systems has led to the need for a
new property of a Bloch state, the Berry curvature, to be calculated from
first principles. We report here on the response to this challenge by the ab
initio community during the past decade. First we give a tutorial introduction
of the conceptual developments we mentioned above. Then we describe four
methodologies which have been developed for first-principle calculations of
the Berry curvature. Finally, to illustrate the significance of the new
developments, we report some results of calculations of interesting physical
properties such as the anomalous and spin Hall conductivity as well as the
anomalous Nernst conductivity and discuss the influence of the Berry curvature
on the de Haas–van Alphen oscillation
Preparation, physical characterization and catalytic properties of unsupported Pt–Rh catalyst
Rh was deposited on a parent platinum black catalyst by an underpotential deposition method. Mean particle size and bulk composition of this Rh–Pt sample was determined by TEM and EDS. No individual Rh grains could be observed, but Rh was present in the near-surface regions, according to energy-filtered TEM images. The surface-sensitive cyclic voltammetry indicated 15–20% Rh on the surface. XPS, in turn, detected ∼2–2.5% Rh in the information depth. The Rh–Pt catalyst was tested in methylcyclopentane hydrogenative ring-opening reaction between 468 and 603 K and 8 to 64 kPa H2 pressure (with 1.3 kPa MCP). The parent Pt black as well as a Rh black catalyst was also studied for comparison. MCP produced ring opening and hydrogenolysis products. The ring-opening products (ROP) consisted of 2- and 3-methylpentane (2MP and 3MP) as well as hexane (nH). These were the main products, together with some fragments and unsaturated hydrocarbons. The amount of the latter class increased at higher temperatures. The selectivities of ROP, fragments, and benzene over Rh–Pt catalyst as a function of temperature were between the values observed on Pt and Rh. The hydrogen pressure dependence of selectivities on Rh–Pt was more similar to that observed on Pt. Four subsequent treatments with O2 and H2 up to T = 673 K were applied on the bimetallic catalyst, followed by XPS and catalytic runs, respectively. These treatments promoted structural rearrangement, with XPS detecting less Rh in the near surface region, partly as oxidized Rh after O2 treatment. The catalytic behavior became more Pt-like on these structural and composition changes. We concluded that adding a relatively small amount of Rh to Pt creates bimetallic active sites with properties different from those of its components, behaving as a true bimetallic catalyst
Preparation, physical characterization and catalytic properties of unsupported Pt–Rh catalyst
Rh was deposited on a parent platinum black catalyst by an underpotential deposition method. Mean particle size and bulk composition of this Rh–Pt sample was determined by TEM and EDS. No individual Rh grains could be observed, but Rh was present in the near-surface regions, according to energy-filtered TEM images. The surface-sensitive cyclic voltammetry indicated 15–20% Rh on the surface. XPS, in turn, detected ∼2–2.5% Rh in the information depth. The Rh–Pt catalyst was tested in methylcyclopentane hydrogenative ring-opening reaction between 468 and 603 K and 8 to 64 kPa H2 pressure (with 1.3 kPa MCP). The parent Pt black as well as a Rh black catalyst was also studied for comparison. MCP produced ring opening and hydrogenolysis products. The ring-opening products (ROP) consisted of 2- and 3-methylpentane (2MP and 3MP) as well as hexane (nH). These were the main products, together with some fragments and unsaturated hydrocarbons. The amount of the latter class increased at higher temperatures. The selectivities of ROP, fragments, and benzene over Rh–Pt catalyst as a function of temperature were between the values observed on Pt and Rh. The hydrogen pressure dependence of selectivities on Rh–Pt was more similar to that observed on Pt. Four subsequent treatments with O2 and H2 up to T = 673 K were applied on the bimetallic catalyst, followed by XPS and catalytic runs, respectively. These treatments promoted structural rearrangement, with XPS detecting less Rh in the near surface region, partly as oxidized Rh after O2 treatment. The catalytic behavior became more Pt-like on these structural and composition changes. We concluded that adding a relatively small amount of Rh to Pt creates bimetallic active sites with properties different from those of its components, behaving as a true bimetallic catalyst
Interlayer Coupling and p-wave Pairing in Strontium Ruthenate
On the basis of a three orbital model and an effective attractive interaction
between electrons we investigate the possible superconducting states, with
and -wave internal symmetry, of SrRuO. For an orbital dependent
interaction which acts between in plane and out of plane nearest neighbour
Ruthenium atoms we find a state for which the gap in the quasi-particle spectra
has a line node on the and sheets of the Fermi Surface, but
it is complex with no nodes on the -sheet. We show that this state is
consistent with all the available experimental data. In particular, we present
the results of our calculations of the specific heat and penetration depth as
functions of the temperature.Comment: 4 pages, 5 figure
Magnetic field induced rotation of the d-vector in the spin triplet superconductor SrRuO
In zero magnetic field the superconductor SrRuO is believed to have a
chiral spin triplet pairing state in which the gap function d-vector is aligned
along the crystal c-axis. Using a phenomenological but orbital specific
description of the spin dependent electron-electron attraction and a realistic
quantitative account of the electronic structure in the normal state we analyze
the orientation of the spin triplet Cooper pair d-vector in response to an
external c-axis magnetic field. We show that for suitable values of the model
parameters a c-axis field of only 20 mT is able to cause a reorientation phase
transition of the d-vector from along to the plane, in agreement with
recent experiments.Comment: 6 pages, 7 figures. Submitted Phys Rev
- …