1,073 research outputs found
Spin-orbit driven Peierls transition and possible exotic superconductivity in CsWO
We study \textit{ab initio} a pyrochlore compound, CsWO, which
exhibits a yet unexplained metal-insulator transition. We find that (1) the
reported low- structure is likely inaccurate and the correct structure has a
twice larger cell; (2) the insulating phase is not of a Mott or dimer-singlet
nature, but a rare example of a 3D Peierls transition, with a simultaneous
condensation of three density waves; (3) spin-orbit interaction plays a crucial
role, forming well-nested bands. The high- (HT) phase, if stabilized, could
harbor a unique superconducting state that breaks the time
reversal symmetry, but is not chiral. This state was predicted in 1999, but
never observed. We speculate about possible ways to stabilize the HT phase
while keeping the conditions for superconductivity
Pressure effects on crystal and electronic structure of bismuth tellurohalides
We study the possibility of pressure-induced transitions from a normal
semiconductor to a topological insulator (TI) in bismuth tellurohalides using
density functional theory and tight-binding method. In BiTeI this transition is
realized through the formation of an intermediate phase, a Weyl semimetal, that
leads to modification of surface state dispersions. In the topologically
trivial phase, the surface states exhibit a Bychkov-Rashba type dispersion. The
Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the
Weyl semimetal--TI transition occurs, the surface electronic structure is
characterized by gapless states with linear dispersion. The peculiarities of
the surface states modification under pressure depend on the band-bending
effect. We have also calculated the frequencies of Raman active modes for BiTeI
in the proposed high-pressure crystal phases in order to compare them with
available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological
phase transition does not occur. In BiTeBr, the crystal structure changes with
pressure but the phase remains a trivial one. However, the transition appears
to be possible if the low-pressure crystal structure is retained. In BiTeCl
under pressure, the topological phase does not appear up to 18 GPa due to a
relatively large band gap width in this compound
Ab initio lattice dynamics and electron-phonon coupling of Bi(111)
We present a comprehensive ab initio study of structural, electronic, lattice
dynamical and electron-phonon coupling properties of the Bi(111) surface within
density functional perturbation theory. Relativistic corrections due to
spin-orbit coupling are consistently taken into account. As calculations are
carried out in a periodic slab geometry, special attention is given to the
convergence with respect to the slab thickness. Although the electronic
structure of Bi(111) thin films varies significantly with thickness, we found
that the lattice dynamics of Bi(111) is quite robust and appears converged
already for slabs as thin as 6 bilayers. Changes of interatomic couplings are
confined mostly to the first two bilayers, resulting in super-bulk modes with
frequencies higher than the optic bulk spectrum, and in an enhanced density of
states at lower frequencies for atoms in the first bilayer. Electronic states
of the surface band related to the outer part of the hole Fermi surfaces
exhibit a moderate electron-phonon coupling of about 0.45, which is larger than
the coupling constant of bulk Bi. States at the inner part of the hole surface
as well as those forming the electron pocket close to the zone center show much
increased couplings due to transitions into bulk projected states near
Gamma_bar. For these cases, the state dependent Eliashberg functions exhibit
pronounced peaks at low energy and strongly deviate in shape from a Debye-like
spectrum, indicating that an extraction of the coupling strength from measured
electronic self-energies based on this simple model is likely to fail.Comment: 30 pages, 11 figure
Electron–phonon coupling and superconductivity in a 2D Tl–Pb compound on Si(111)
Electron–phonon interaction in a single-layer Tl–Pb compound on Si(111) is investigated within the density-functional theory and linear-response approach in the mixed-basis pseudopotential representation. It is found that phonon-induced scattering of electrons at the Fermi level is primarily determined by surface electronic states responsible for bonding at the interface and by low-energy, predominantly shear-vertical vibrations of adatoms. The contribution of substrate-localized vibrations involved in the electron–phonon scattering turns out to be small. We have also estimated the superconducting transition temperature Tc by solving the linearized gap equation of the Eliashberg theory. An analysis of phonon-mediated transitions for a number of electronic states in the Tl–Pb surface bands showed that the strength of the coupling varies with the binding energy, increasing as it approaches the Fermi level, and significantly depends on the surface band to which the state belongs
Pressure effects on crystal and electronic structure of bismuth tellurohalides
Westudy the possibility of pressure-induced transitions from a normal semiconductor to a topological insulator (TI) in bismuth tellurohalides using density functional theory and tight-binding method. In BiTeI this transition is realized through the formation of an intermediate phase, a Weyl semimetal, that leads to modification of surface state dispersions. In the topologically trivial phase, the
surface states exhibit a Bychkov–Rashba type dispersion. The Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the Weyl semimetal–TI transition occurs, the surface electronic structure is characterized by gapless states with linear dispersion. The peculiarities of the surface states modification under pressure depend on the band-bending effect.Wehave also calculated the frequencies of Raman active modes for BiTeI in the proposed high-pressure crystal phases in order to compare them with available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological phase transition does not occur. In BiTeBr, the crystal structure changes with pressure but the phase remains a trivial one. However, the transition appears to be possible if the low-pressure crystal structure is retained. In BiTeCl under pressure, the topological phase does not appear up to 18 GPa due to a relatively large band gap width in this compound
Pressure-induced topological phases of KNa2Bi
We report an ab initio study of the effect of hydrostatic pressure and uniaxial strain on electronic properties of KNa2Bi, a cubic bialkali bismuthide. It is found that this zero-gap semimetal with an inverted band structure at the Brillouin zone center can be driven into various topological phases under proper external pressure. We show that upon hydrostatic compression KNa2Bi turns into a trivial semiconductor with a conical Dirac-type dispersion of electronic bands at the point of the topological transition while the breaking of cubic symmetry by applying a uniaxial strain converts the compound into a topological insulator or into a three-dimensional Dirac semimetal with nontrivial surface Fermi arcs depending on the sign of strain. The calculated phonon dispersions show that KNa2Bi is dynamically stable both in the cubic structure (at any considered pressures) and in the tetragonal phase (under uniaxial strain)
Gender-specific association of adiponectin as a predictor of progression of chronic kidney disease: The Mild to Moderate Kidney Disease Study
Progressive renal vascular sclerosis is a key feature of chronic kidney disease (CKD). Adiponectin, an adipokine with potent anti-inflammatory and antiatherosclerotic properties, is associated with insulin resistance, type II diabetes and cardiovascular disease. In this study, we evaluated the predictive value of adiponectin for the progression of CKD in patients enrolled in the Mild to Moderate Kidney Disease Study. The primary end point was defined as a doubling of the baseline serum creatinine and/or terminal renal failure in 177 patients who completed a prospective follow-up of 7 years. Patients who reached a progression endpoint (n=65) were significantly older, had higher baseline serum creatinine, proteinuria and adiponectin concentrations and more components of the metabolic syndrome. A gender-stratified Cox model revealed adiponectin in men as a significant predictor of progression after adjustment for age, glomerular filtration rate, and proteinuria. Male patients with adiponectin levels above their ROC analysis-derived optimal cutoff of 4μg/ml had a significantly faster progression than patients below this point. This prospective long-term study in patients with CKD indicates high adiponectin as a novel independent predictor of disease progression in men but not in women. Our observation may be relevant for other conditions of progressive vascular sclerosis and diabetic nephropathy
Avalanche Photo-Detection for High Data Rate Applications
Avalanche photo detection is commonly used in applications which require
single photon sensitivity. We examine the limits of using avalanche photo
diodes (APD) for characterising photon statistics at high data rates. To
identify the regime of linear APD operation we employ a ps-pulsed diode laser
with variable repetition rates between 0.5MHz and 80MHz. We modify the mean
optical power of the coherent pulses by applying different levels of
well-calibrated attenuation. The linearity at high repetition rates is limited
by the APD dead time and a non-linear response arises at higher photon-numbers
due to multiphoton events. Assuming Poissonian input light statistics we
ascertain the effective mean photon-number of the incident light with high
accuracy. Time multiplexed detectors (TMD) allow to accomplish photon- number
resolution by photon chopping. This detection setup extends the linear response
function to higher photon-numbers and statistical methods may be used to
compensate for non-linearity. We investigated this effect, compare it to the
single APD case and show the validity of the convolution treatment in the TMD
data analysis.Comment: 16 pages, 5 figure
On the Puzzle of Odd-Frequency Superconductivity
Since the first theoretical proposal by Berezinskii, an odd-frequency
superconductivity has encountered the fundamental problems on its thermodynamic
stability and rigidity of a homogenous state accompanied by unphysical Meissner
effect. Recently, Solenov {\it et al}. [Phys. Rev. B {\bf 79} (2009) 132502.]
have asserted that the path-integral formulation gets rid of the difficulties
leading to a stable homogenous phase with an ordinary Meissner effect. Here, we
show that it is crucial to choose the appropriate saddle-point solution that
minimizes the effective free energy, which was assumed {\it implicitly} in the
work by Solenov and co-workers. We exhibit the path-integral framework for the
odd-frequency superconductivity with general type of pairings, including an
argument on the retarded functions via the analytic continuation to the real
axis.Comment: 6 pages, in JPSJ forma
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