Сверхпроводимость

Модераторы: mike@in-russia, varlash

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Номер сообщения:#46   morozov » Вт июн 27, 2006 15:31

From Cooper-pairs to resonating bipolarons

Authors: Julius Ranninger
Comments: 21 pages, 8 figures
Subj-class: Superconductivity; Strongly Correlated Electrons
Journal-ref: Proc. Int. School of Physics "E. Fermi", Course CLXI, Eds. G. Iadonisi, J.Ranninger, and G. De Filippis, IOS Press Amsterdam (2006) pp 327-347

After a short review of the different physics in the weak and strong coupling regime, resulting in amplitude fluctuation controlled Cooper pair superconductivity and phase fluctuation controlled superfluidity of tightly bound local polaron pairs, we suggest a scenario for the cross-over regime between those two. It is based on the idea of resonating Bipolarons inside a Fermi sea of uncorrelated electrons, similar to the Feshbach resonance in Fermionic gases in optical traps. Due to the competition between amplitude and phase correlations such systems in the cross-over regime can exhibit a superconductor to insulator transition upon varying the strength of that resonance.

Full-text: http://xxx.lanl.gov/pdf/cond-mat/0606656
С уважением, Морозов Валерий Борисович

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Номер сообщения:#47   morozov » Ср июн 28, 2006 16:59

ОБ ЭЛЕКТРОННОМ МЕХАНИЗМЕ СВЕРХПРОВОДИМОСТИ


Б.Т. Гейликман

Загрузить .pdf файл (r662d.pdf, 1006Kb)
http://www.ufn.ru/ufn66/ufn66_2/Russian/r662d.pdf
С уважением, Морозов Валерий Борисович

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Номер сообщения:#48   morozov » Чт июн 29, 2006 14:40

Evolution of superconductivity in Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$: Upper critical field measurements
Authors: C.R. Rotundu, H. Tsujii, P. Kumar, B. Andraka, H. Sugawara, Y. Aoki, H. Sato
Subj-class: Superconductivity; Strongly Correlated Electrons

Measurements of the upper critical field $H_{c2}$ near $T_c$ of Pr$_{1- x}$La$_{x}$Os$_{4}$Sb$_{12}$ were performed by specific heat. A positive curvature in $H_{c2}$ versus $T$ was observed in samples and concentrations exhibiting two superconducting transitions. These results argue against this curvature being due to two-band superconductivity. The critical field slope - $dH_{c2}/dT$ suggests the existence of a crossover concentration $x_{cr} \approx 0.25$, below which there is a rapid suppression of effective electron mass with La-alloying. This crossover concentration was previously detected in the measurement of the discontinuity of $C/T$ at $T_c$.
http://xxx.lanl.gov/pdf/cond-mat/0606730
С уважением, Морозов Валерий Борисович

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Номер сообщения:#49   morozov » Чт июн 29, 2006 15:56

Unconventional pairing in bipolaronic theories

J.P.Hague a P.E.Kornilovitch b A.S.Alexandrov a J.H.Samson a
aDepartment of Physics, Loughborough University, Loughborough, LE11 3TU, UK
bHewlett-Packard Company, 1000 NE Circle Blvd, Corvallis, Oregon 97330, USA
Abstract
Various mechanisms have been put forward for cuprate superconductivity, which fit largely into two camps: spin-
fluctuation and electron-phonon (el-ph) mechanisms. However, in spite of a large eort, electron-phonon interactions
are not fully understood away from clearly defined limits. To this end, we use a numerically exact algorithm to simulate
the binding of bipolarons. We present the results of a continuous-time quantum Monte-Carlo (CTQMC) algorithm
on a tight-binding lattice, for bipolarons with arbitrary interaction range in the presence of strong coulomb repulsion.
The algorithm is suciently ecient that we can discuss properties of bipolarons with various pairing symmetries.
We investigate the eective mass and binding energies of singlet and triplet real-space bipolarons for the first time,
and discuss the extensions necessary to investigate d-symmetric pairs.
http://xxx.lanl.gov/pdf/cond-mat/0606719
С уважением, Морозов Валерий Борисович

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Номер сообщения:#50   morozov » Вс июл 09, 2006 14:36

On the pseudogap and doping-dependent magnetic properties of La2-xSrxCu1-yZnyO4

Authors: R. S. Islam, J. R. Cooper, J. W. Loram, S. H. Naqib
Comments: Submitted to Physica C (Proceedings of the M2S-HTSC-VIII Conference)
Subj-class: Superconductivity; Strongly Correlated Electrons

The effects of planar hole content, p (= x), on the uniform (q = 0) magnetic susceptibility, c(T), of La2-xSrxCu1-yZnyO4 were investigated over a wide range of Sr (x) and Zn (y) contents. A strongly p-dependent Zn-induced magnetic behavior was observed. The apparent Zn-induced magnetic moment is larger in underdoped La2-xSrxCu1-yZnyO4 and it decreases quite sharply around p ~ 0.19. It does not change much for further overdoping. This indicates a possible role of the pseudogap on the Zn induced magnetic behavior, as there is growing evidence that pseudogap vanishes quite abruptly at p ~ 0.19.

Full-text: http://xxx.lanl.gov/pdf/cond-mat/0607153
С уважением, Морозов Валерий Борисович

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Номер сообщения:#51   morozov » Вт июл 18, 2006 21:22

Optical determination of the superconducting energy gap in electron-doped Pr1.85Ce0.15CuO4

C. C. Homes,1, ∗ R. P. S. M. Lobo,2 P. Fournier,3 A. Zimmers,4 and R. L. Greene4
1Condensed Matter Physics & Materials Science Department,
Brookhaven National Laboratory, Upton, New York 11973, USA
2Laboratoire de Physique du Solide (UPR 5 CNRS) ESPCI, 10 Rue Vauquelin 75231 Paris, France
3D´epartement de Physique, Universit´e de Sherbrooke, Sherbrooke, Qu´ebec, J1K 2R1 Canada.
4Center for Superconductivity Research, Department of Physics,
University of Maryland, College Park, MD 20742, USA
(Dated: July 17, 2006)
The optical properties of single crystal Pr1.85Ce0.15CuO4 have been measured over a wide frequency
range above and below the critical temperature (Tc ≃ 20 K). In the normal state the coherent
part of the conductivity is described by the Drude model, from which the scattering rate just above
Tc is determined to be 1/τ ≃ 80 cm−1. The condition that ¯h/τ ≈ 2kBT near Tc appears to be
a general result in many of the cuprate superconductors. Below Tc the formation of a superconducting
energy gap is clearly visible in the reflectance, from which the gap maximum is estimated
to be 0 ≃ 35 cm−1 (4.3 meV). The ability to observe the superconducting energy gap in the
optical properties favors the nonmonotonic over the monotonic description of the d-wave gap. The
penetration depth for T ≪ Tc is λ ≃ 2000 °A, which when taken with the estimated value for the dc
conductivity just above Tc of σdc ≃ 35×103
−1cm−1 places this material on the general scaling line
for the cuprates defined by 1/λ2 ∝ σdc(T ≃ Tc) · Tc. This result is consistent with the observation
that 1/τ ≈ 20, which implies that the material is not in the clean limit.
http://xxx.lanl.gov/pdf/cond-mat/0607377
С уважением, Морозов Валерий Борисович

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Номер сообщения:#52   morozov » Чт июл 27, 2006 2:03

Doping dependent optical properties of
Bi2Sr2CaCu2O8+


J Hwang1, T Timusk1,2 and G D Gu3
1Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S
4M1, Canada
2The Canadian Institute of Advanced Research, Toronto, Ontario M5G 1Z8, Canada
3Department of Physics, Brookhaven National Laboratory, Upton, NY 11973-5000,
USA
E-mail: hwangjs@mcmaster.ca

Abstract. We report on the ab-plane reflectance of the high temperature
superconductor Bi2Sr2CaCu2O8+ (Bi-2212). Samples spanning the doping range from
under doped with Tc = 67 K(UD), to optimally doped with Tc =96 K (OPT), to
over doped with Tc =60 K (OD) were measured from room temperature down to the
superconducting state. The measured reflectance data were analyzed to extract the
optical conductivity and the real and imaginary parts of the free carrier optical selfenergy.
We get an estimate of the dc resistivity from the low frequency extrapolation
of the optical conductivity and the superfluid density from the imaginary part of the
optical conductivity. The conductivity sum rule can be related to the changes of the
kinetic energy of the system. When this system becomes a superconductor, the kinetic
energy increases in the underdoped samples and decreases in overdoped ones. The
optical self-energy, obtained from the extended Drude model, is dominated by two
channels of interaction, a sharp mode and a broad background. The amplitude of the
mode is strongly doping and temperature dependent whereas the background decreases
weakly with doping and is nearly temperature independent.

http://xxx.lanl.gov/abs/cond-mat/0607653
С уважением, Морозов Валерий Борисович

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Номер сообщения:#53   morozov » Вс сен 24, 2006 15:22

Pressure dependence of superconductivity in doped two-leg ladder cuprates

Jihong Qin
Department of Physics, Beijing University of Science and Technology, Beijing 100083, China
Ting Chen and Shiping Feng
Department of Physics, Beijing Normal University, Beijing 100875, China
Within the kinetic energy driven superconducting mechanism, the eect of the pressure on superconductivity
in doped two-leg ladder cuprates is studied. It is shown that the superconducting
transition temperature in doped two-leg ladder cuprate superconductors increases with increasing
pressure in the underpressure regime, and reaches a maximum in the optimal pressure, then decreases
in the overpressure regime. This domed shape of the pressure dependence of the superconducting
transition temperature is similar to that of the pressure dependence of the longitudinal part of the
superconducting gap parameter, indicating that superconductivity in doped two-leg ladder cuprates
is mainly produced by the development of the pairing correlation along legs.
http://xxx.lanl.gov/PS_cache/cond-mat/p ... 609499.pdf
С уважением, Морозов Валерий Борисович

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Номер сообщения:#54   morozov » Вт сен 26, 2006 13:31

Asymmetric tunneling, Andreev reflection and dynamic conductance spectra in strongly correlated metals

Authors: V.R. Shaginyan, K.G. Popov
Comments: 8 pages, 7 figures
Subj-class: Strongly Correlated Electrons; Superconductivity

Landau Fermi liquid theory predicts that the differential conductivity between metallic point and metal is a symmetric function of voltage bias V. This symmetry holds if the particle-hole symmetry is preserved. We show that the situation can be different when one of the two metals is a strongly correlated one whose electronic system can be represented by a heavy fermion liquid. When the heavy fermion liquid undergoes fermion condensation quantum phase transition, the particle-hole symmetry is violated making both the differential tunneling conductivity and dynamic conductance asymmetric as a function of applied voltage. This asymmetry can be observed when the strongly correlated metal is either normal or superconducting. We show that at small values of $V the asymmetric part of the dynamic conductance is a linear function of V and inversely proportional to the maximum value of the gap and does not depend on temperature provided that metal is superconducting, when it becomes normal the asymmetric part diminishes at elevated temperatures.

Full-text
http://xxx.lanl.gov/PS_cache/cond-mat/p ... 609579.pdf
С уважением, Морозов Валерий Борисович

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Номер сообщения:#55   morozov » Пт сен 29, 2006 14:39

Competing impurities and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) revisited. The role of the Dzyaloshinskii-Moriya and XY anisotropies

Authors: L. Adamska, M. B. Silva Neto, C. Morais Smith
Comments: 13 pages, 10 figures
Subj-class: Strongly Correlated Electrons; Superconductivity

We study the order-from-disorder transition and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) within the framework of a long-wavelength nonlinear sigma model that properly incorporates the Dzyaloshinskii-Moriya and XY anisotropies. Doping with nonmagnetic impurities, such as Zn, is considered according to classical percolation theory, whereas the effect of Sr, which introduces charge carriers into the CuO(2) planes, is described as a dipolar frustration of the antiferromagnetic order. We calculate several magnetic, thermodynamic, and spectral properties of the system, such as the antiferromagnetic order parameter, the Neel temperature, the spin-stiffness, and the anisotropy gaps, as well as their evolution with both Zn and Sr doping. We explain the nonmonotonic and reentrant behavior experimentally observed for T_N by Hucker et al. in Phys. Rev. B 59, R725 (1999), as resulting from the reduction, due to the nonmagnetic impurities, of the dipolar frustration induced by the charge carriers (order-from-disorder). Furthermore, we find a similar nonmonotonic and reentrant behavior for all the other observables studied. Most remarkably, our results show that while for x=2% and z=0 the Dzyaloshinskii-Moriya gap \Delta_{DM}=0, for z=15% it is approximately \Delta_{DM} = 7.5 cm^(-1). The later is larger than the lowest low-frequency cutoff for Raman spectroscopy (~ 5 cm^(-1)), and could thus be observed in one-magnon Raman scattering.

Full-text: http://xxx.lanl.gov/PS_cache/cond-mat/p ... 609653.pdf
С уважением, Морозов Валерий Борисович

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Номер сообщения:#56   morozov » Вт окт 03, 2006 22:00

Real and marginal isotope effects in cuprate superconductors

Authors: A. R. Bishop, A. Bussmann-Holder, O. V. Dolgov, A. Furrer, H. Kamimura, H. Keller, R. Khasanov, R. K. Kremer, D. Manske, K. A. Mueller, A. Simon
Comments: 7 pages, 4 figures
Subj-class: Superconductivity

We critically review recent and earlier results on isotope effects in cuprate superconductors and emphasize that the sample preparation and the isotope exchange and back exchange are crucial in understanding and interpreting the data. Only extremely careful preparation techniques yield reliable results and permit differentiation between real isotope effects and marginal ones. The former are substantial and highlight the lattice vibrational importance in cuprate superconductors.

Full-text: http://xxx.lanl.gov/ftp/cond-mat/papers ... 610036.pdf
С уважением, Морозов Валерий Борисович

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Номер сообщения:#57   morozov » Вс дек 17, 2006 13:15

R. Beck,1, Y. Dagan,1, 2 A. Milner,1, ? G. Leibovitch,1 A. Gerber,1 R. G. Mints,1 and G. Deutscher1
1School of Physics and Astronomy, Raymond and Beverly Sackler
Faculty of Exact Science, Tel-Aviv University, 69978 Tel-Aviv, Israel.
2Center for Superconductivity Research, Department of Physics,
University of Maryland, College Park, Maryland 20742, USA.
(Dated: May 5, 2005)
We studied the tunneling density of states in YBCO films under strong currents flowing along
node directions. The currents were induced by fields of up to 32.4T parallel to the film surface and
perpendicular to the CuO2 planes. We observed a remarkable change in the tunneling conductance
at high fields where the gap-like feature shifts discontinuously from 15meV to a lower bias of 11meV,
becoming more pronounced as the field increases. The eect takes place in increasing fields around
9T and the transition back to the initial state occurs around 5T in decreasing fields.
С уважением, Морозов Валерий Борисович

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Номер сообщения:#58   morozov » Чт дек 21, 2006 2:24

Einstein and the Early Theory of Superconductivity, 1919-1922

Authors: Tilman Sauer
Comments: 71 pp., 10 figs
Subj-class: History of Physics

Einstein's early thoughts about superconductivity are discussed as a case study of how theoretical physics reacts to experimental findings that are incompatible with established theoretical notions. One such notion that is discussed is the model of electric conductivity implied by Drude's electron theory of metals, and the derivation of the Wiedemann-Franz law within this framework. After summarizing the experimental knowledge on superconductivity around 1920, the topic is then discussed both on a phenomenological level in terms of implications of Maxwell's equations for the case of infinite conductivity, and on a microscopic level in terms of suggested models for superconductive charge transport. Analyzing Einstein's manuscripts and correspondence as well as his own 1922 paper on the subject, it is shown that Einstein had a sustained interest in superconductivity and was well informed about the phenomenon. It is argued that his appointment as special professor in Leiden in 1920 was motivated to a considerable extent by his perception as a leading theoretician of quantum theory and condensed matter physics and the hope that he would contribute to the theoretical direction of the experiments done at Kamerlingh Onnes' cryogenic laboratory. Einstein tried to lived up to these expectations by proposing at least three experiments on the phenomenon, one of which was carried out twice in Leiden. Compared to other theoretical proposals at the time, the prominent role of quantum concepts was characteristic of Einstein's understanding of the phenomenon. The paper concludes with comments on Einstein's epistemological reflections on the problem.

Full-text: http://xxx.lanl.gov/PS_cache/physics/pd ... 612159.pdf
С уважением, Морозов Валерий Борисович

Danemon
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Номер сообщения:#59   Danemon » Пт янв 12, 2007 11:42

1. В сверхпроводнике все заряженные частицы движутся с одинаковой скоростью?

2.а Как будут взаимодействовать два параллельных проводника с током?

2.б Как будет вести себя электрон, движущийся в начальный момент параллельно проводнику с током?

2.в Как будут вести себя два электрона, если в начальный момент времени они движутся параллельно с одинаковой скоростью и в одном направлении?

Спасибо.

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Номер сообщения:#60   morozov » Сб янв 13, 2007 20:37

"1. В сверхпроводнике все заряженные частицы движутся с одинаковой скоростью? "

нет. Там строго говая и частичек нет.

"2.а Как будут взаимодействовать два параллельных проводника с током? "

Как и при Ампере. В точности также.

"2.б Как будет вести себя электрон, движущийся в начальный момент параллельно проводнику с током? "

Согласно правилу соответствующей руки.

"2.в Как будут вести себя два электрона, если в начальный момент электрическоевремени они движутся параллельно с одинаковой скоростью и в одном направлении? "

сложным образом, кроме магнитного поля электрическое
С уважением, Морозов Валерий Борисович

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