655 research outputs found

### Spin-density wave Fermi surface reconstruction in underdoped YBa2Cu3O6+x

We consider the reconstruction expected for the Fermi surface of underdoped
YBa2Cu3O6+x in the case of a collinear spin-density wave with a characteristic
vector Q=(pi[1+/-2 delta],pi), assuming an incommensurability delta~0.06
similar to that found in recent neutron scattering experiments. A Fermi surface
possibly consistent with the multiple observed quantum oscillation frequencies
is obtained. From the low band masses expected using this model as compared
with experiment, a uniform enhancement of the quasiparticle effective mass over
the Fermi surface by a factor of ~7 is indicated. Further predictions of the
Fermi surface topology are made, which may potentially be tested by experiment
to indicate the relevance of this model to underdoped YBa2Cu3O6+x.Comment:

### Superconductivity

Ripon Professorship Lecture for 1953 delivered at IACS in 195

### Dirac nodal pockets in the antiferromagnetic parent phase of FeAs superconductors

We show that previously measured small Fermi surface pockets within the
antiferromagnetic phase of SrFe2As2 and BaFe2As2 are consistent with a Dirac
dispersion modulated by interlayer hopping, giving rise to a Dirac point in
k-space and a cusp in the magnetic field angle-dependent magnetic quantum
oscillation frequencies. These findings support the existence of a nodal
spin-density wave in these materials, which could play an important role in
protecting the metallic state against localization effects. The speed of the
Dirac fermions in SrFe2As2 and BaFe2As2 is found to be 14-20 times slower than
in graphene, suggesting that the pnictides provide a laboratory for exploring
the effects of strongly interacting Dirac fermions.Comment: 4 page

### An analytical treatment of in-plane magnetotransport in the Falicov-Sievert model

We derive an analytical expression which allows efficient computation of the
effect of all the Fermi surface trajectories induced by a combination of Bragg
scattering and magnetic breakdown on the in-plane components of the resistivity
tensor. The particular network of coupled orbits which we consider was first
formulated by Falicov and Sievert, who studied the problem numerically. Our
approach, based upon a method used previously to derive an analytical solution
for interlayer transport, allows us to show that the conductivity tensor can be
written as a sum of a matrix representing the effect of total magnetic
breakdown and one representing a combination of complex electronic
trajectories, and we find a compact expression for the in-plane components of
the resistivity tensor that can be evaluated straightforwardly.Comment: 4 pages, 4 figure

### Magnetization in two-dimensional electron gas in a perpendicular magnetic field: the roles of edge states and spin-orbit coupling

We study the de Haas--van Alphen (dHvA) oscillations in the magnetization of
a two-dimensional electron gas (2DEG) under the influence of the edge states
and/or the Rashba spin-orbit interaction (SOI). The boundaries of the systems
lift partially the degeneracies of Landau levels (LL's) and the resulting edge
states lead to the changes of both the center and the amplitude of the
sawtoothlike magnetization oscillation. The SOI mixes the spin-up and spin-down
states of neighboring LL's into two unequally spaced energy branches. The
inclusion of SOI changes the well-defined sawtooth pattern of the dHvA
oscillations in the magnetization. The weaker the magnetic field is, the larger
is the change of the dHvA oscillations due to the edge effect and/or the
spin-orbit coupling. Some theoretical results are compared with the
experimental data.Comment: 9 pages, 9 figure

### Shubnikov-de Haas oscillations in SrTiO3\LaAlO3 interface

Quantum magnetic oscillations in SrTiO3/\LaAlO3 interface are observed. The
evolution of their frequency and amplitude at various gate voltages and
temperatures is studied. The data are consistent with the Shubnikov de-Haas
theory. The Hall resistivity rho exhibits nonlinearity at low magnetic field.
It is fitted assuming multiple carrier contributions. The comparison between
the mobile carrier density inferred from the Hall data and the oscillation
frequency suggests multiple valley and spin degeneracy. The small amplitude of
the oscillations is discussed in the framework of the multiple band scenario

### Quantum oscillations in topological superconductor candidate Cu$_{0.25}$Bi$_2$Se$_3$

Quantum oscillations are generally studied to resolve the electronic
structure of topological insulators. In Cu$_{0.25}$Bi$_2$Se$_3$, the prime
candidate of topological superconductors, quantum oscillations are still not
observed in magnetotransport measurement. However, using torque magnetometry,
quantum oscillations (the de Hass - van Alphen effect) were observed in
Cu$_{0.25}$Bi$_2$Se$_3$ . The doping of Cu in Bi$_2$Se$_3$ increases the
carrier density and the effective mass without increasing the scattering rate
or decreasing the mean free path. In addition, the Fermi velocity remains the
same in Cu$_{0.25}$Bi$_2$Se$_3$ as that in Bi$_2$Se$_3$. Our results imply that
the insertion of Cu does not change the band structure of Bi$_2$Se$_3$.Comment: 5 pages, 4 figure

### Thermodynamic properties of Pb determined from pressure-dependent critical-field measurements

We have carried out extensive low-temperature (1.5 to 10 K) measurements of
the critical field, $H_c$, for the element Pb up to a pressure of $P=1.2$ GPa.
From this data the electronic entropy, specific heat, thermal expansion
coefficient and compressibility is calculated as a function of temperature,
pressure and magnetic field. The zero-field data is consistent with direct
thermodynamic measurements and the $P$-dependence of $T_c$ and specific heat
coefficient, $\gamma(T,P)$ allows the determination of the $P$-dependence of
the pairing interaction.Comment: 5 pages, 6 figures, in press Phys. Rev.

### Fermi Surface of Cr$_{1-x}$V$_x$ across the Quantum Critical Point

We have measured de Haas-van Alphen oscillations of Cr$_{1-x}$V$_x$, $0 \le x
\le 0.05$, at high fields for samples on both sides of the quantum critical
point at $x_c=0.035$. For all samples we observe only those oscillations
associated with a single small hole band with magnetic breakdown orbits of the
reconstructed Fermi surface evident for $x<x_c$. The absence of oscillations
from Fermi surface sheets most responsible for the spin density wave (SDW) in
Cr for $x>x_c$ is further evidence for strong fluctuation scattering of these
charge carriers well into the paramagnetic regime. We find no significant mass
enhancement of the carriers in the single observed band at any $x$. An
anomalous field dependence of the dHvA signal for our $x=0.035$ crystal at
particular orientations of the magnetic field is identified as due to magnetic
breakdown that we speculate results from a field induced SDW transition at high
fields.Comment: 8 pages with 7 figure

### Interpretation of the angular dependence of the de Haas-van Alphen effect in MgB_2

We present detailed results for the amplitude and field dependence of the de
Haas-van Alphen (dHvA) signal arising from the electron-like $\pi$ sheet of
Fermi surface in MgB_2. Our data and analysis show that the dip in dHvA
amplitude when the field is close to the basal plane is caused by a beat
between two very similar dHvA frequencies and not a spin-zero effect as
previously assumed. Our results imply that the Stoner enhancement factors in
MgB_2 are small on both the Sigma and Pi sheets.Comment: 4 pages with figures. Submitted to PR

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