919 research outputs found
Disorder, Metal-Insulator crossover and Phase diagram in high-Tc cuprates
We have studied the influence of disorder induced by electron irradiation on
the normal state resistivities of optimally and underdoped YBa2CuOx
single crystals, using pulsed magnetic fields up to 60T to completely restore
the normal state. We evidence that point defect disorder induces low T upturns
of rho(T) which saturate in some cases at low T in large applied fields as
would be expected for a Kondo-like magnetic response. Moreover the magnitude of
the upturns is related to the residual resistivity, that is to the
concentration of defects and/or their nanoscale morphology. These upturns are
found quantitatively identical to those reported in lower Tc cuprates, which
establishes the importance of disorder in these supposedly pure compounds. We
therefore propose a realistic phase diagram of the cuprates, including
disorder, in which the superconducting state might reach the antiferromagnetic
phase in the clean limit.Comment: version 2 with minor change
Total suppression of superconductivity by high magnetic fields in YBa2 Cu3O6.6
We have studied in fields up to 60T the variation of the transverse
magnetoresistance (MR) of underdoped YBCO6.6 crystals either pure or with Tc
reduced down to 3.5K by electron irradiation. We evidence that the normal state
MR is restored above a threshold field H'c(T), which is found to vanish at
T'c>>Tc. In the pure YBCO6.6 sample a 50 Tesla field is already required to
completely suppress the superconducting fluctuations at Tc. While disorder does
not depress the pseudogap temperature, it reduces drastically the phase
coherence established at Tc and weakly H'c(0), T'c and the onset Tnu of the
Nernst signal which are more characteristic of the 2D local pairing.Comment: 4 pages, 4 figure
Competing types of quantum oscillations in the 2D organic conductor (BEDT-TTF)8Hg4Cl12(C6H5Cl)2
Interlayer magnetoconductance of the quasi-two dimensional organic metal
(BEDT-TTF)8Hg4Cl12(C6H5Cl)2 has been investigated in pulsed magnetic fields
extending up to 36 T and in the temperature range from 1.6 to 15 K. A complex
oscillatory spectrum, built on linear combinations of three basic frequencies
only is observed. These basic frequencies arise from the compensated closed
hole and electron orbits and from the two orbits located in between. The field
and temperature dependencies of the amplitude of the various oscillation series
are studied within the framework of the coupled orbits model of Falicov and
Stachowiak. This analysis reveals that these series result from the
contribution of either conventional Shubnikov-de Haas effect (SdH) or quantum
interference (QI), both of them being induced by magnetic breakthrough.
Nevertheless, discrepancies between experimental and calculated parameters
indicate that these phenomena alone cannot account for all of the data. Due to
its low effective mass, one of the QI oscillation series - which corresponds to
the whole first Brillouin zone area - is clearly observed up to 13 K.Comment: 8 pages, 8 figures. To be published in Phys. Rev.
Fermi-surface reconstruction and two-carrier model for the Hall effect in YBa2Cu4O8
Pulsed field measurements of the Hall resistivity and magnetoresistance of
underdoped YBa2Cu4O8 are analyzed self-consistently using a simple model based
on coexisting electron and hole carriers. The resultant mobilities and Hall
numbers are found to vary markedly with temperature. The conductivity of the
hole carriers drops by one order of magnitude below 30 K, explaining the
absence of quantum oscillations from these particular pockets. Meanwhile the
Hall coefficient of the electron carriers becomes strongly negative below 50 K.
The overall quality of the fits not only provides strong evidence for
Fermi-surface reconstruction in Y-based cuprates, it also strongly constrains
the type of reconstruction that might be occurring.Comment: 5 pages, 4 figures, updated after publication in Physical Review B
(Rapid Communication
Phenomenology of the normal state in-plane transport properties of high- cuprates
In this article, I review progress towards an understanding of the normal
state (in-plane) transport properties of high- cuprates in the light of
recent developments in both spectroscopic and transport measurement techniques.
Against a backdrop of mounting evidence for anisotropic single-particle
lifetimes in cuprate superconductors, new results have emerged that advocate
similar momentum dependence in the transport decay rate ({\bf k}). In
addition, enhancement of the energy scale (up to the bare bandwidth) over which
spectroscopic information on the quasiparticle response can be obtained has led
to the discovery of new, unforeseen features that surprisingly, may have a
significant bearing on the transport properties at the dc limit. With these two
key developments in mind, I consider here whether all the ingredients necessary
for a complete phenomenological description of the anomalous normal state
transport properties of high- cuprates are now in place.Comment: 31 pages, 10 figure
Physics of the Merging Clusters Cygnus A, A3667, and A2065
We present ASCA gas temperature maps of the nearby merging galaxy clusters
Cygnus A, A3667, and A2065. Cygnus A appears to have a particularly simple
merger geometry that allows an estimate of the subcluster collision velocity
from the observed temperature variations. We estimate it to be ~2000 km/s.
Interestingly, this is similar to the free-fall velocity that the two Cygnus A
subclusters should have achieved at the observed separation, suggesting that
merger has been effective in dissipating the kinetic energy of gas halos into
thermal energy, without channeling its major fraction elsewhere (e.g., into
turbulence). In A3667, we may be observing a spatial lag between the shock
front seen in the X-ray image and the corresponding rise of the electron
temperature. A lag of the order of hundreds of kiloparsecs is possible due to
the combination of thermal conduction and a finite electron-ion equilibration
time. Forthcoming better spatial resolution data will allow a direct
measurement of these phenomena using such lags. A2065 has gas density peaks
coincident with two central galaxies. A merger with the collision velocity
estimated from the temperature map should have swept away such peaks if the
subcluster total mass distributions had flat cores in the centers. The fact
that the peaks have survived (or quickly reemerged) suggests that the
gravitational potential also is strongly peaked. Finally, the observed specific
entropy variations in A3667 and Cygnus A indicate that energy injection from a
single major merger may be of the order of the full thermal energy of the gas.
We hope that these order of magnitude estimates will encourage further work on
hydrodynamic simulations, as well as more quantitative representation of the
simulation results.Comment: Corrected the Cyg-A figure (errors shown were 1-sigma not 90%); text
unchanged. ApJ in press. Latex, 5 pages, 3 figures (2 color), uses
emulateapj.st
Transport in Ultraclean YBaCuO: neither Unitary nor Born Impurity Scattering
The thermal conductivity of ultraclean YBaCuO was measured at
very low temperature in magnetic fields up to 13 T. The temperature and field
dependence of the electronic heat conductivity show that two widespread
assumptions of transport theory applied to unconventional superconductors fail
for clean cuprates: impurity scattering cannot be treated in the usual unitary
limit (nor indeed in the Born limit), and scattering of quasiparticles off
vortices cannot be neglected. Our study also sheds light on the long-standing
puzzle of a sudden onset of a "plateau" in the thermal conductivity of Bi-2212
versus field.Comment: 5 pages, 3 figures, submitted to Physical Review Letter
Fermi Surface of the Electron-doped Cuprate Superconductor Nd_{2-x}Ce_xCuO_{4} Probed by High-Field Magnetotransport
We report on the study of the Fermi surface of the electron-doped cuprate
superconductor NdCeCuO by measuring the interlayer
magnetoresistance as a function of the strength and orientation of the applied
magnetic field. We performed experiments in both steady and pulsed magnetic
fields on high-quality single crystals with Ce concentrations of to
0.17. In the overdoped regime of we found both semiclassical
angle-dependent magnetoresistance oscillations (AMRO) and Shubnikov-de Haas
(SdH) oscillations. The combined AMRO and SdH data clearly show that the
appearance of fast SdH oscillations in strongly overdoped samples is caused by
magnetic breakdown. This observation provides clear evidence for a
reconstructed multiply-connected Fermi surface up to the very end of the
overdoped regime at . The strength of the superlattice potential
responsible for the reconstructed Fermi surface is found to decrease with
increasing doping level and likely vanishes at the same carrier concentration
as superconductivity, suggesting a close relation between translational
symmetry breaking and superconducting pairing. A detailed analysis of the
high-resolution SdH data allowed us to determine the effective cyclotron mass
and Dingle temperature, as well as to estimate the magnetic breakdown field in
the overdoped regime.Comment: 23 pages, 8 figure
Hall, Seebeck, and Nernst Coefficients of Underdoped HgBa2CuO4+d: Fermi-Surface Reconstruction in an Archetypal Cuprate Superconductor
Charge density-wave order has been observed in cuprate superconductors whose
crystal structure breaks the square symmetry of the CuO2 planes, such as
orthorhombic YBa2Cu3Oy (YBCO), but not so far in cuprates that preserve that
symmetry, such as tetragonal HgBa2CuO4+d (Hg1201). We have measured the Hall
(R_H), Seebeck (S), and Nernst coefficients of underdoped Hg1201 in magnetic
fields large enough to suppress superconductivity. The high-field R_H(T) and
S(T) are found to drop with decreasing temperature and become negative, as also
observed in YBCO at comparable doping. In YBCO, the negative R_H and S are
signatures of a small electron pocket caused by Fermi-surface reconstruction,
attributed to charge density-wave modulations observed in the same range of
doping and temperature. We deduce that a similar Fermi-surface reconstruction
takes place in Hg1201, evidence that density-wave order exists in this
material. A striking similarity is also found in the normal-state Nernst
coefficient, further supporting this interpretation. Given the model nature of
Hg1201, Fermi-surface reconstruction appears to be common to all hole-doped
cuprates, suggesting that density-wave order is a fundamental property of these
materials
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