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Добавлено: Пт авг 19, 2005 12:53
вроде очень интересно

Laser ARPES, the sudden approximation, and quasiparticle-like peaks in Bi2Sr2CaCu2O8+delta
Authors: J. D. Koralek (1 and 2), J.F. Douglas (1), N.C. Plumb (1), Z. Sun (1 and 3), A. Fedorov (3), M. Murnane (1 and 2), H. Kapteyn (1 and 2), S. Cundiff (2), Y. Aiura (4), K. Oka (4), H. Eisaki (4), D.S. Dessau (1 and 2) ((1) Department of Physics, University of Colorado, Boulder, CO, USA, (2) JILA, University of Colorado and NIST, Boulder, CO, USA, (3) Advanced Light Source, LBNL, Berkeley, CA, USA, (4) AIST Tsukuba Central 2, Tsukuba, Ibaraki, Japan)
Comments: 15 pages, 3 figures
Subj-class: Superconductivity; Strongly Correlated Electrons

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi2Sr2CaCu2O8+delta. The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy - the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink and associated weight transfer are robust to a possible breakdown of the sudden approximation.
Full-text    http://xxx.lanl.gov/pdf/cond-mat/0508404

Добавлено: Ср авг 31, 2005 14:40
Macroscopic Einstein-Podolsky-Rosen pairs in superconducting circuits
Authors: L.F. Wei, Yu-xi Liu, Markus J. Storcz, F. Nori


Добавлено: Пт сен 02, 2005 15:49
Magnetic evidence for hot superconductivity in multi-walled carbon nanotubes
Authors: Guo-meng Zhao, Pieder Beeli
Comments: 16 pages, 17 figures
Subj-class: Superconductivity; Strongly Correlated Electrons

We report magnetic measurements up to 1200 K on three different multi-walled carbon nanotube mat samples using Quantum Design vibrating sample magnetometers with an ultra low-field option. Three different samples prepared from arc discharge or chemical vapor deposition contain magnetic impurities ranging from about 100 ppm to about 1.5%. Our extensive magnetic data consistently show two superconducting transitions, one at temperatures between 550 K and 720 K, and another at about 1200 K. The first transition temperature T_cJ, which coincides with the transition temperature seen in the resistance data, depends very strongly on the magnetic field, as expected from the onset of intergrain Josephson coupling. The strong field dependence of T_cJ also excludes magnetic contaminants as the origin of the first transition. A large paramagnetic susceptibility (12% of 1/4\pi) is observed at 1140 K for the most pure sample in a field of 0.03 Oe and this susceptibility decreases rapidly with increasing field. On the other hand, the most impure sample shows a large diamagnetic susceptibility (3% of -1/4\pi) at 1000 K. These data can {\em only} be explained by a scenario where paramagnetic susceptibility-due to circulating currents around vortices (paramagnetic Meissner effect)-competes with diamagnetic susceptibility-due to diamagnetic shielding currents. The present results provide compelling evidence for superconductivity well above room temperature in multi-walled carbon nanotubes.

Добавлено: Пт сен 02, 2005 15:50
                  A. S. Alexandrov
Loughborough University, Loughborough LE11 3TU, United Kingdom

The seminal work by Bardeen, Cooper and Schrieffer (BCS)1 extended further by Eliashberg2 to the intermediate coupling regime solved one of the major scientific problems of Condensed Matter Physics in the last century. While the BCS theory provides a qualitatively correct description of some novel superconductors like magnesium diborade and doped fullerenes, if the phonon dressing of carriers (i.e. polaron formation) is properly taken into account, hightemperature superconductivity (HTS) of cuprates represents a challenge to the conventional theory. Here I discuss a multi-polaron approach to the problem based on our extension of the BCS theory to the strong-coupling regime.3 Attractive electron correlations, prerequisite to any HTS, are caused by an almost unretarted electron-phonon (e-ph) interaction sufficient to overcome the direct Coulomb repulsion in this regime. Low energy physics is that of small polarons and bipolarons (real-space electron (hole) pairs dressed by phonons). They are itinerant quasiparticles existing in the Bloch states at temperatures below the characteristic phonon frequency. Since there is almost no retardation (i.e. no Tolmachev-Morel-Anderson logarithm) reducing the Coulomb repulsion, e-ph interactions should be relatively strong to overcome the direct Coulomb repulsion, so carriers must be polaronic to form pairs in novel superconductors. I identify the Fr¨ohlich electron-phonon interaction as the most essential for pairing in superconducting cuprates. Many experimental observations have been satisfactorily understood in the framework of the bipolaron theory3 providing evidence for a novel state of electronic matter in layered cuprates, which is a charged Bose-liquid of small mobile bipolarons. Here the band structure and essential interactions in oxide superconductors are discussed in section 1, and the ?Fr¨ohlich-Coulomb? model of HTS is introduced in section 2, including discussions of single-polaron (2.1, 2.2, 2.3, 2.4, 2.5) and multipolaron (2.4, 2.6, 3.1, 3.2) problems, low-energy structures (3.3), and the phase diagram of cuprates (3.4). ?Individual? versus Cooper pairing (3.5), normal state properties (section 4), in particular in-plane resistivity, the Hall effect, magnetic susceptibility and the Lorenz number (4.1), the Nernst effect (4.2), diamagnetism (4.3), spin and charge pseudogaps, and c-axis transport (4.4) are also discussed. I present a parameterfree evaluation of Tc (5.1), and an explanation of isotope effects (5.2), specific heat anomaly (5.3), upper critical fields (5.4), symmetries and space modulations of the order parameter (5.5), and a model of overdoped cuprates as mixtures of mobile bipolarons and degenerate lattice polarons (section 6).


Добавлено: Ср сен 07, 2005 10:39
Fourier transform spectroscopy of d-wave quasiparticles in the presence of atomic scale pairing disorder
Tamara S. Nunner, Wei Chen, Brian M. Andersen, Ashot Melikyan, and P. J. Hirschfeld
Department of Physics, University of Florida, Gainesville, FL 32611, USA
(Dated: September 7, 2005)
The local density of states power spectrum of optimally doped Bi2Sr2CaCu2O8+x (BSCCO) has
been interpreted in terms of quasiparticle interference peaks corresponding to an ?octet? of scattering
wave vectors connecting k-points where the density of states is maximal. Until now, theoretical
treatments have not been able to reproduce the experimentally observed weights and widths of these
?octet? peaks; in particular, the predominance of the dispersing ?q1? peak parallel to the Cu-O bond
directions has remained a mystery. In addition, such theories predict ?background? features which
are not observed experimentally. Here, we show that most of the discrepancies can be resolved when a
realistic model for the out-of-plane disorder in BSCCO is used. Weak extended potential scatterers,
which are assumed to represent cation disorder, suppress large-momentum features and broaden
the low-energy ?q7?-peaks, whereas scattering at order parameter variations, possibly caused by
a dopant-modulated pair interaction around interstitial oxygens, strongly enhances the dispersing



Добавлено: Сб сен 17, 2005 8:42
Magneto-Transport Properties of the Rutheno-Cuprate RuSr_2GdCu_2O_8
Authors: J.E. McCrone, J.R. Cooper, J.L. Tallon
Comments: LaTex2e, 5 pages, 3 figures. To be published in JLTP: proceedings of MOS'99
Subj-class: Superconductivity

RuSr_2GdCu_2O_8 (Ru-1212) is a triple perovskite containing both CuO_2 and RuO_2 layers. It has attracted a great deal of interest recently because it displays electronic ferromagnetism and superconductivity coexisting on a microscopic scale, with T_Curie of 135K and T_c up to 50K.
We report magnetisation and magnetoresistance (MR) data that exhibit effects due to the interaction between the ferromagnetic ruthenium moments and the conduction electrons. The MR is negative at temperatures above T_Curie, and is proportional to the square of the applied field well above this temperature. Below T_Curie the MR displays a positive peak at fields of around 15 kOe, but at high fields it becomes negative again, and approximately linear. We analyse the high temperature data in terms of spin-disorder scattering theory and extract a value for the exchange interaction between the carriers in the CuO_2 planes and the Ru spins.
Full-text http://arxiv.org/abs/cond-mat/9909263

Добавлено: Пн окт 10, 2005 22:25
Lev P.Gor?kov,a Gregory B.Teitel?baumb∗
aNNHMFL, Florida State University, 1800 E P.Dirac Dr., Tallahassee FL 32310, USA
bE.K.Zavoiskii Institute for Technical Physics of the RAS, Sibirskii Trakt 10/7,
Kazan 420029, RUSSIA
We conclude from the analysis of the experimental NMR data for electron-doped
cuprates that the Coulomb effects caused by doping lead to dynamical spatial phase separation
that contributes to the nuclear spin relaxation. Remarkable, the "infinite-layer" Sr0.9La0.1CuO2
reveals unexpected electron-hole symmetry. Its 63Cu nuclear spin relaxation rate is the sum of a
constant and the temperature dependent dissipation components, moreover, the latter turns out to
be identical to the 1/63T1(T)-behavior in the stoichiometric hole-type compound YBa2Cu4O8.
Connection to fluctuations of a magnetic sub-phase is discussed.

Добавлено: Чт окт 20, 2005 22:45
Persistence of Strong Electron Coupling to a Narrow Boson Spectrum in Overdoped BiSrCaCuO (Bi2212) Tunneling Data
Authors: J.F.Zasadzinski, L.Ozyuzer, L.Coffey, K.E.Gray, D.G.Hinks, C.Kendziora
Categories: cond-mat.supr-con cond-mat.str-el
Comments: RevTex4. 5 pages. 3 figures. Submitted to Physical Review Letters
Subj-class: Superconductivity; Strongly Correlated Electrons

A d-wave, Eliashberg analysis of break junction and STM tunneling spectra on BiSrCaCuO (Bi2212) reveals that a spectral dip feature is directly linked to strong electronic coupling to a narrow boson spectrum, evidenced by a large peak in the boson spectral weight. The tunneling dip feature remains robust in the overdoped regime of Bi2212 with bulk Tc values of 56 K-62 K. This is contrary to recent optical conductivity measurements of the self-energy that suggest the narrow boson spectrum disappears in overdoped Bi2212 and therefore cannot be essential for the pairing mechanism. The discrepancy is resolved by considering the way each technique probes the electron self-energy, in particular, the unique sensitivity of tunneling to the off-diagonal or pairing part of the self-energy.
Full-text: http://xxx.lanl.gov/pdf/cond-mat/0510057

Добавлено: Пн ноя 14, 2005 21:02
Pairing States of Superfluid 3He in Uniaxially Anisotropic Aerogel
Authors: Kazushi Aoyama, Ryusuke Ikeda
Comments: 4 pages, 4 figures
Subj-class: Disordered Systems and Neural Networks; Superconductivity

Stable pairing states of superfluid 3He in aerogel are examined in the case with a global anisotropy created by a uniaxial stress for the aerogel. Irrespective of the types of uniaxial stress, the stability region of a (deformed) ABM pairing state with point gap-nodes becomes wider in the presence of a global uniaxial anisotropy. In uniaxially stretched aerogels, the pure polar pairing state with a horizontal line node is predicted to appear, as a 3d superfluid phase, over a measurable width just below the superfluid transition curve T_c(P). A possible relevance of the present results to 3He in globally isotropic aerogels is also discussed.

Добавлено: Чт ноя 24, 2005 20:10
High Frequency Behavior of the Infrared Conductivity of Cuprates
M. R. Norman1 and A. V. Chubukov2
1Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 and
2Department of Physics, University of Wisconsin, Madison, WI 53706
(Dated: November 24, 2005)
We analyze recent infrared conductivity data in the normal state of the cuprates. We find that
the high frequency behavior, which has been suggested as evidence for quantum critical scaling,
is generally characteristic of electrons interacting with a broad spectrum of bosons. From explicit
calculations, we find a frequency exponent for the modulus of the conductivity, and a phase angle,
in good agreement with experiment. The data indicate an upper cut-off of the boson spectrum of
order 300 meV. This implies that the bosons are of electronic origin rather than phonons.

http://xxx.lanl.gov/PS_cache/cond-mat/p ... 511584.pdf

Добавлено: Пн дек 12, 2005 20:10
Comparison of the Electronic Structures of Two Non-cuprate Layered Transition Metal Oxide Superconductors
Authors: E. R. Ylvisaker, K.-W. Lee, W. E. Pickett (UC Davis)
Comments: 5 pages, 3 embedded figures (Proceedings in third Hiroshima international workshop)
Subj-class: Superconductivity; Strongly Correlated Electrons

Comparison is made of the electronic structure of the little-studied layered transition metal oxide LiNbO$_2$ with that of Na$_x$CoO$_2$, which has attracted tremendous interest since superconductivity was discovered in its hydrate. Although the active transition metal $d$ states are quite different due to different crystal fields and band filling, both systems show a strong change of electronic structure with changes in the distance between the transition metal ion layer and the oxygen layers. The niobate is unusual in having a large second-neighbor hopping amplitude, and a nearest neighbor hopping amplitude that is sensitive to the Nb-O separation. Li$_x$NbO$_2$ also presents the attractive simplicity of a single band triangular lattice system with variable carrier concentration that is superconducting.
Full-text:  http://lanl.arxiv.org/PS_cache/cond-mat ... 512211.pdf

Добавлено: Пн дек 26, 2005 23:47
"S-triplet" order in superconductors without inversion symmetry: Li$_2$Pd$_3$B and Li$_2$Pt$_3$B

Authors: H. Q. Yuan, D. F. Agterberg, N. Hayashi, P. Badica, D. Vandervelde, K. Togano, M. Sigrist, M. B. Salamon
Comments: 4 pages, 3 figures
Subj-class: Superconductivity

We investigate the order parameter of noncentrosymmetric superconductors Li$_2$Pd$_3$B and Li$_2$Pt$_3$B via the behavior of the penetration depth $\lambda(T)$. Broken inversion symmetry admits antisymmetric spin-orbit coupling that in turn admixes spin-singlet and spin-triplet pairing. The triplet contribution is weak in Li$_2$Pd$_3$B, a BCS-like superconductor with an anisotropic gap. With increased spin-orbit coupling the spin-triplet component dominates in Li$_2$Pt$_3$B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of $\lambda(T)$. The experimental data are in quantitative agreement with theory. Our findings demonstrate that unconventional superconducting properties (e.g., line nodes) can originate from a conventional phonon pairing mechanism without the need for other (electronic) coupling mechanisms.

Добавлено: Сб фев 25, 2006 0:15
Universal optimal hole-doping concentration in single-layer high-temperature cuprate superconductors
Authors: T. Honma, P. H. Hor
Comments: 7 pages, 4 figures, submitted to Europhysics Letters
Subj-class: Superconductivity; Strongly Correlated Electrons

We argue that in cuprate physics there are two types, hole content per CuO2 plane Ppl and the corresponding hole content per unit volume P3D, of hole-doping concentrations for addressing physical properties that are two-dimensional (2D) and three-dimensional (3D) in nature, respectively. We find that superconducting transition temperature Tc varies systematically with P3D$ as a superconducting dome with a universal optimal hole-doping concentration P3Dopt. = 1.6 x 10^21 cm-3 for single-layer high-Tc superconductors. We suggest that P3Dopt. determines the upper bound of the electronic energy of underdoped single-layer high-Tc cuprates.

Добавлено: Сб фев 25, 2006 0:57
Doping dependence of the coupling of electrons to bosonic modes in the single-layer high-temperature Bi2Sr2CuO6 superconductor
Authors: W. Meevasana, N.J.C. Ingle, D.H. Lu, J.R. Shi, F. Baumberger, K.M. Shen, W.S. Lee, T. Cuk, H. Eisaki, T.P. Devereaux, N. Nagaosa, J. Zaanen, Z.-X. Shen
Comments: 4 pages, 3 figures
Subj-class: Superconductivity

A recent highlight in the study of high-Tc superconductors is the observation of band renormalization / self-energy effects on the quasiparticles. This is seen in the form of kinks in the quasiparticle dispersions as measured by photoemission and interpreted as signatures of collective bosonic modes coupling to the electrons. Here we compare for the first time the self-energies in an optimally doped and strongly overdoped, non-superconducting single-layer Bi-cuprate (Bi2Sr2CuO6). Besides the appearance of a strong overall weakening, we also find that weight of the self-energy in the overdoped system shifts to higher energies. We present evidence that this is related to a change in the coupling to c-axis phonons due to the rapid change of the c-axis screening in this doping range.

Добавлено: Вт мар 07, 2006 0:36
Electrostatic Modulation of the Superfluid Density in a Ultrathin La2-xSrxCuO4 Film
Authors: A. Rufenacht, J.-P. Locquet, J. Fompeyrine, D. Caimi, P. Martinoli
Comments: 4 pages, 3 figures
Subj-class: Superconductivity

By capacitively charging an underdoped ultrathin La2-xSrxCuO4 film with an electric field applied across a gate insulator with a high dielectric constant, relative changes of the areal superfluid density n_{s\Box}$ of unprecedented strength were observed in measurements of the film kinetic inductance. Although $n_{s\Box}$ appears to be substantially reduced by disorder, the data provide, for the first time on the same sample, direct compelling evidence for the Uemura relation $T_{c} \propto n_{s\Box}(T=0)$ in the underdoped regime of copper-oxide superconductors.