12 research outputs found
de Haas-van Alphen effect investigations of the electronic structure of pure and aluminum-doped MgB_2
Understanding the superconducting properties of MgB_2 is based strongly on
knowledge of its electronic structure. In this paper we review experimental
measurements of the Fermi surface parameters of pure and Al-doped MgB_2 using
the de Haas-van Alphen (dHvA) effect. In general, the measurements are in
excellent agreement with the theoretical predictions of the electronic
structure, including the strength of the electron-phonon coupling on each Fermi
surface sheet. For the Al doped samples, we are able to measure how the band
structure changes with doping and again these are in excellent agreement with
calculations based on the virtual crystal approximation. We also review work on
the dHvA effect in the superconducting state.Comment: Contribution to the special issue of Physica C "Superconductivity in
MgB2: Physics and Applications" (10 Pages with figures
Advances in single crystal growth and annealing treatment of electron-doped HTSC
High quality electron-doped HTSC single crystals of and have been
successfully grown by the container-free traveling solvent floating zone
technique. The optimally doped and crystals have transition temperatures
of \,K and \,K, respectively, with a transition width of less than
\,K. We found a strong dependence of the optimal growth parameters on the Ce
content . We discuss the optimization of the post-growth annealing treatment
of the samples, the doping extension of the superconducting dome for both
compounds as well as the role of excess oxygen. The absolute oxygen content of
the as-grown crystals is determined from thermogravimetric experiments and is
found to be . This oxygen surplus is nearly completely removed by a
post-growth annealing treatment. The reduction process is reversible as
demonstrated by magnetization measurements. In as-grown samples the excess
oxygen resides on the apical site O(3). This apical oxygen has nearly no doping
effect, but rather influences the evolution of superconductivity by inducing
additional disorder in the CuO layers. The very high crystal quality of
is particularly manifest in magnetic quantum
oscillations observed on several samples at different doping levels. They
provide a unique opportunity of studying the Fermi surface and its dependence
on the carrier concentration in the bulk of the crystals.Comment: 19 pages, 7 figures, submitted to Eur. Phys. J.
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the
MgB2-like system Li_{1-x}BC is explored using first principles calculations. It
is found that the high temperature superconductivity of such systems results
from extremely strong coupling to only ~2% of the phonon modes. Novel
characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and
greater compared to a mean of for other modes, (2) a precipitous
Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the
frequency itself, indicating impending breakdown of linear electron-phonon
theory. This behavior in borne out by recent inelastic x-ray scattering studies
of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4
changed. Comparison with new data include
Fermi Surfaces of Diborides: MgB2 and ZrB2
We provide a comparison of accurate full potential band calculations of the
Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen
date of Yelland et al. and Tanaka et al., respectively. The discrepancies in
areas in MgB2 can be removed by a shift of sigma-bands downward with respect to
pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged
electron-phonon coupling constants lambda(sigma)=1.3 (both orbits),
lambda(pi)=0.5. The required band shifts, which we interpret as an exchange
attraction for sigma states beyond local density band theory, reduces the
number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of
superconductivity in MgB2 a somewhat closer call than previously recognized,
and increases the likelihood that additional holes can lead to an increased Tc.Comment: 7 pages including 4 figure
A Theory for the High-T_c Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing
A theory of highly correlated layered superconducting materials isapplied for
the cuprates. Differently from an independent-electron approximation, their
low-energy excitations are approached in terms of auxiliary particles
representing combinations of atomic-like electron configurations, where the
introduction of a Lagrange Bose field enables treating them as bosons or
fermions. The energy spectrum of this field accounts for the tendency of
hole-doped cuprates to form stripe-like inhomogeneities. Consequently, it
induces a different analytical behavior for auxiliary particles corresponding
to "antinodal" and "nodal" electrons, enabling the existence of different
pairing temperatures at T^* and T_c. This theory correctly describes the
observed phase diagram of the cuprates, including the non-Fermi-liquid to FL
crossover in the normal state, the existence of Fermi arcs below T^* and of a
"marginal-FL" critical behavior above it. The qualitative anomalous behavior of
numerous physical quantities is accounted for, including kink- and
waterfall-like spectral features, the drop in the scattering rates below T^*
and more radically below T_c, and an effective increase in the density of
carriers with T and \omega, reflected in transport, optical and other
properties. Also is explained the correspondence between T_c, the
resonance-mode energy, and the "nodal gap".Comment: 28 pages, 7 figure
Spin dynamics in Sr3 Ru2 O7 near the metamagnetic transition by inelastic neutron scattering
The bilayered ruthenate Sr3 Ru2 O7 has a metamagnetic quantum critical point. This is the first example of a material with a quantum-critical end-point. This work addresses the problem of the metamagnetic criticality in this system through the measurement of spin fluctuations by inelastic neutron scattering. Our results show that incommensurate antiferromagnetic fluctuations are present in a wide range of applied fields. We have also observed how ferromagnetic fluctuations develop at the metamagnetic field. Finally, the intensity of the two types of fluctuations is followed as a function of the energy transfer between the neutron and the spins. © 2007 Elsevier B.V. All rights reserved