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Добавлено: Вт мар 07, 2006 0:48
Measurements of the doping effects on the in-plane paraconductivity in cuprate superconductors
Authors: Felix Vidal, Manuel V. Ramallo, Gonzalo Ferro, Jose Antonio Veira
Comments: 7 pages and 3 figures (LaTeX and .eps). Contribution to the NATO Advanced Research Workshop on New Challenges in Superconductivity: Experimental Advances and Emerging Theories (University of Miami Workshop on Unconventional Superconductivity)
Subj-class: Superconductivity
Journal-ref: in "New Challenges in Superconductivity: Experimental Advances and Emerging Theories", pp.85-90, edited by J.Ashkenazi et al. (Springer, New York, 2005)

We will summarize here some of our measurements of the superconducting fluctuations effects on the in-plane electrical resistivity (the so-called in-plane paraconductivity) in La_{2-x}Sr_xCuO_4 thin films with different Sr content. Our results suggest that these superconducting fluctuations effects are not related to the opening of a pseudogap in the normal-state of underdoped compounds.

Добавлено: Ср мар 22, 2006 0:22
Design for a Room Temperature Superconductor
Authors: Warren E. Pickett
Comments: 7 pages, 4 figures. Submitted to V. Ginzburg commemorative volume
Subj-class: Superconductivity; Materials Science

The vision of ``room temperature superconductivity'' has appeared intermittently but prominently in the literature since 1964, when W. A. Little and V. L. Ginzburg began working on the `problem of high temperature superconductivity' around the same time. Since that time the prospects for room temperature superconductivity have varied from gloom (around 1980) to glee (the years immediately after the discovery of HTS), to wait-and-see (the current feeling). Recent discoveries have clarified old issues, making it possible to construct the blueprint for a viable room temperature superconductor.
Full-text: http://xxx.lanl.gov/abs/cond-mat/0603482

Добавлено: Пт мар 24, 2006 16:51
Incommensurate lattice distortion in the high temperature tetragonal phase of La$_{2-x}$(Sr,Ba)$_{x}$CuO$_{4}$

Authors: Shuichi Wakimoto, Hiroyuki Kimura, Masaki Fujita, Kazuyoshi Yamada, Yukio Noda, Gen Shirane, Genda Gu, Hyunkyung Kim, Robert J. Birgeneau
Comments: 6 pages, 6 figures
Subj-class: Superconductivity

We report incommensurate diffuse (ICD) scattering appearing in the high-temperature-tetragonal (HTT) phase of La$_{2-x}$(Sr,Ba)$_{x}$CuO$_{4}$ with $0.07 \leq x \leq 0.20$ observed by the neutron diffraction technique. For all compositions, a sharp superlattice peak of the low-temperature-orthorhombic (LTO) structure is replaced by a pair of ICD peaks with the modulation vector parallel to the CuO$_6$ octahedral tilting direction, that is, the diagonal Cu-Cu direction of the CuO$_2$ plane, above the LTO-HTT transition temperature $T_s$. The temperature dependences of the incommensurability $\delta$ for all samples scale approximately as $T/T_s$, while those of the integrated intensity of the ICD peaks scale as $(T-T_s)^{-1}$. These observations together with absence of ICD peaks in the non-superconducting $x=0.05$ sample evince a universal incommensurate lattice instability of hole-doped 214 cuprates in the superconducting regime.

Добавлено: Вт мар 28, 2006 12:47
Temperature Dependence of the Conductivity Sum Rule in the Normal State due to Inelastic Scattering
Authors: L. Benfatto, J.P.Carbotte, F. Marsiglio
Comments: 11 pages, 9 figures
Subj-class: Strongly Correlated Electrons; Superconductivity

We examine the temperature dependence of the optical sum rule in the normal state due to interactions. To be concrete we adopt a weak coupling approach which uses an electron-boson exchange model to describe inelastic scattering of the electrons with a boson, in the Migdal approximation. While a number of recent works attribute the temperature dependence in the normal state to that which arises in a Sommerfeld expansion, we show that in a wide parameter regime this contribution can be quite small. Instead, most of the temperature dependence arises from the zeroth order term in the `expansion', through the temperature dependence of the spectral function, and the interaction parameters contained therein. For low boson frequencies this circumstance causes a linear T-dependence in the sum rule. We develop some analytical expressions and understanding of the temperature dependence.

Добавлено: Ср мар 29, 2006 12:03
The Origin of Anomalous Low-Temperature Downturns in the Thermal Conductivity of Cuprates
Authors: M. F. Smith, Johnpierre Paglione, M. B. Walker, Louis Taillefer
Comments: 8 pages, 5 figures
Subj-class: Superconductivity
Journal-ref: Phys. Rev. B, 71, 014506 (2005)
DOI: 10.1103/PhysRevB.71.014506

We show that the anomalous decrease in the thermal conductivity of cuprates below 300 mK, as has been observed recently in several cuprate materials including Pr$_{2-x}$Ce$_x$CuO$_{7-\delta}$ in the field-induced normal state, is due to the thermal decoupling of phonons and electrons in the sample. Upon lowering the temperature, the phonon-electron heat transfer rate decreases and, as a result, a heat current bottleneck develops between the phonons, which can in some cases be primarily responsible for heating the sample, and the electrons. The contribution that the electrons make to the total low-$T$ heat current is thus limited by the phonon-electron heat transfer rate, and falls rapidly with decreasing temperature, resulting in the apparent low-$T$ downturn of the thermal conductivity. We obtain the temperature and magnetic field dependence of the low-$T$ thermal conductivity in the presence of phonon-electron thermal decoupling and find good agreement with the data in both the normal and superconducting states.

Добавлено: Ср мар 29, 2006 12:14
In-plane optical spectral weight transfer in optimally doped Bi2Sr2Ca2Cu3O10

F. Carbone, A. B. Kuzmenko, H. J. A. Molegraaf, E. van Heumen, E. Giannini and D. van der Marel
DPMC, University of Geneva, 24,
Quai Ernest-Ansermet, Geneva 4, Switzerland
(Dated: March 29, 2006)
We examine the redistribution of the in-plane optical spectral weight in the normal and superconducting
state in tri-layer Bi2Sr2Ca2Cu3O10 (Bi2223) near optimal doping (Tc = 110 K) on a
single crystal via infrared reflectivity and spectroscopic ellipsometry. We report the temperature
dependence of the low-frequency integrated spectral weight W(
c) for dierent values of the cuto
c. Two dierent model-independent analyses consistently show that for
c = 1 eV, which
is below the charge transfer gap, W(
c) increases below Tc, implying the lowering of the kinetic
energy of the holes. This is opposite to the BCS scenario, but it follows the same trend observed in
the bi-layer compound Bi2Sr2CaCu2O8 (Bi2212). The size of this eect is larger in Bi2223 than in
Bi2212, approximately scaling with the critical temperature. In the normal state, the temperature
dependence of W(
c) is close to T2 up to 300 K.


Добавлено: Вт апр 04, 2006 11:43
Scaling properties of the optical conductivity
of Bi-based cuprates

D. van der Marel ∗, F. Carbone, A. B. Kuzmenko, E. Giannini
D´epartement de Physique de la Mati`ere Condens´ee, Universit´e de Gen`eve, Quai
Ernerst-Ansermet 24, Gen`eve 4, Suisse
We present novel infrared optical conductivity data on the three layer high Tc
superconductor Bi2Sr2Ca2Cu3O10 at optimal doping. We extend the analysis of an
earlier publication, providing a universal scaling function (!, T) = T−1g(!/T) for
the optical conductivity. In the present manuscript we obtain a good scaling collapse
of the experimental curves on the g(!/T) over a wide range of values of !/T (at
least in range 0 to 10), if we assume that g(!/T) is superimposed on a non-universal
background which is temperature independent. We obtain the same result, if in our
analysis we allow this background to have a T2 temperature dependent correction.
The most striking property of g(!/T) is, that it corresponds to a scattering rate
which varies linearly as a function of temperature, but which is independent of the


Добавлено: Вт апр 04, 2006 11:46
d-wave Checkerboard Order in Cuprates
Authors: Kangjun Seo, Han-Dong Chen, Jiangping Hu
Subj-class: Superconductivity

When superconductivity is suppressed through phase fluctuation rather than pair-breaking, orderings in both the particle-particle and the particle-hole channels can be naturally induced. We show that the mechanism leads to a d-wave ordering in the particle-hole channel, dubbed as d-wave checkerboard order, in underdoped cuprates. Even a weak d-wave checkerboard order can effectively suppress the coherence peak in the single-particle spectrum and leave the nodal direction almost unaffected. The d-wave checkerboard order also generates a Fermi arc with little dispersion around nodal points, consistent with results from ARPES experiments. Therefore, we suggest that the phase fluctuations in d-wave superconductors can naturally explain the nodal-antinodal dichotomy observed in ARPES and STM experiments.
Full-text: http://xxx.lanl.gov/abs/cond-mat/0604061

Добавлено: Ср апр 05, 2006 16:27
High Pressure E?ects
James S. Schilling
Department of Physics, Washington University
CB 1105, One Brookings Dr., St. Louis, Missouri 63130
March 28, 2006
Experiments under hydrostatic and uniaxial pressure serve not only as a
guide in the synthesis of materials with superior superconducting properties
but also allow a quantitative test of theoretical models. In this chapter the
pressure dependence of the superconducting properties of elemental, binary,
and multi-atom superconductors are explored, with an emphasis on those ex-
hibiting relatively high values of the transition temperature Tc. In contrast to
the vast majority of superconductors, where Tc decreases under pressure, in
the cuprate oxides Tc normally increases. Uniaxial pressure studies give evi-
dence that this increase arises mainly from the reduction in the area A of the
CuO2 planes (Tc / A


Добавлено: Пн апр 10, 2006 10:14
Rotational Symmetry Breaking in Sodium Doped Cuprates
Authors: Yan Chen, T. M. Rice, F. C. Zhang
Comments: 10 pages, 4 figures
Subj-class: Superconductivity; Strongly Correlated Electrons

For reasonable parameters a hole bound to a Na^{+} acceptor in Ca_{2-x}Na_{x}CuO_{2}Cl_{2} has a doubly degenerate ground state whose components can be represented as states with even (odd) reflection symmetry around the x(y) -axes. The conductance pattern for one state is anisotropic as the tip of a tunneling microscope scans above the Cu-O-Cu bonds along the x(y)-axes. This anisotropy is pronounced at lower voltages but is reduced at higher voltages. Qualitative agreement with recent experiments leads us to propose this effect as an explanation of the broken local rotational symmetry.

Добавлено: Вт апр 11, 2006 16:38
What is a Cooper pair ?
Authors: G. Ortiz, J. Dukelsky
Comments: Proccedings of the International Workshop on Condensed Matter Theories held in Japan (Sept. 2005)
Subj-class: Superconductivity

Recently, the nature of Cooper pairs in the BCS-BEC crossover has regained attention due to the observation of a large fraction of preformed fermion pairs on the BCS side of the Feshbach resonance in ultracold atomic Fermi gases. While several theoretical explanations were proposed, the interpretations are still controversial. The root of the controversy is understanding what represents a Cooper pair in a correlated Fermi system. This paper discusses these issues at the most elementary level.

Добавлено: Вт апр 11, 2006 21:28
MgB2 tunnel junctions with native or thermal oxide barriers
Authors: R. K. Singh, R. Gandikota, J. Kim, N. Newman, J. M. Rowell
Comments: 9 pages, 3 figures
Subj-class: Superconductivity

MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers therefore survive the deposition of the second electrode at 300oC, even over junction areas of ~1 mm2. Studies of such junctions, and those of the type MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers grown on MgB2 exhibit a wide range of barrier heights and widths.
Full-text: http://xxx.lanl.gov/pdf/cond-mat/0604229

Добавлено: Вт апр 11, 2006 23:28
Photoinduced Suppression of Superconductivity in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$
Authors: Subhasis Ghosh, Niladri Sarkar
Subj-class: Superconductivity; Strongly Correlated Electrons

Superconductivity in high temperature superconductors is achieved by introducing charge carriers into cuprate insulators containing CuO$_2$ planes. Perturbation in these CuO$_2$ planes suppresses superconductivity by strongly affecting the electronic environment. Here we have use photoinduced electronic modification of Cu ion in CuO$_2$ plane to investigate the effect of local perturbation in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$. This method has been used to suppress superconducting transition temperature T$_c$. We show that our results on photoinduced suppression of superconductivity are consistent with a scenario based on pinning of fluctuating stripes in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$.
Full-text:  http://xxx.lanl.gov/pdf/cond-mat/0604218

Добавлено: Сб апр 15, 2006 14:09
Angle-dependence of the Hall eect in HgBa2CaCu2O6 thin films
H. Richter1, I. Puica1, W. Lang1, M. Peruzzi2, J. H. Durrell2,3, H. Sturm2, J. D. Pedarnig2, D. B¨auerle2
1Institut f¨ur Materialphysik der Universit¨at Wien, Boltzmanngasse 5, A-1090 Wien, Austria
2Institut f¨ur Angewandte Physik, Johannes-Kepler-Universit¨at Linz, A-4040 Linz, Austria
3Department of Materials Science and Metallurgy, University of Cambridge,
Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
Superconducting compounds of the family Hg-Ba-Ca-Cu-O have been the subject of intense study
since the current record-holder for the highest critical temperature of a superconductor belongs to
this class of materials. Thin films of the compound with two adjacent copper-oxide layers and a
critical temperature of about 120 K were prepared by a two-step process that consists of the pulsedlaser
deposition of precursor films and the subsequent annealing in mercury-vapor atmosphere. Like
some other high-temperature superconductors, Hg-Ba-Ca-Cu-O exhibits a specific anomaly of the
Hall eect, a double-sign change of the Hall coecient close to the superconducting transition. We
have investigated this phenomenon by measurements of the Hall eect at dierent angles between
the magnetic field direction and the crystallographic c-axis. The results concerning the upper part
of the transition, where the first sign change occurs, are discussed in terms of the renormalized
fluctuation model for the Hall conductivity, adapted through the field rescaling procedure in order
to take into account the arbitrary orientation of the magnetic field.

Добавлено: Пн апр 17, 2006 11:44
Observation of double superconducting transition in Y0.55Pr0.45Ba2Cu3O7-б polycrystalline sample by Hall effect measurement

M. S. da Luz, C. A. M. dos Santos*, and A. J. S. Machado
Grupo de Supercondutividade, Departamento de Engenharia de Materiais,
FAENQUIL, 12.600-970, Lorena-SP, Brazil
This work reports longitudinal (RXX) and transverse (RXY) resistance as
a function of the temperature measured at low magnetic fields for a
Y0.55Pr0.45Ba2Cu3O7-δ polycrystalline sample. It is observed nonzero transverse
voltage at zero external magnetic below superconducting transition and above
no such voltage was detected. Comparing RXX (T) and RXY (T) curves it was
possible to observe a correlation between Hall resistance and double resistive
superconducting transition. Magneto-resistance RXX(H) measurements showed
that the dissipation is dominated by weak coupling between superconducting
clusters. I-V and R(T) curves of both components suggest that the positive
Hall voltage at superconducting state must be related to quasiparticle currents.
Keywords: double superconducting transition, granular superconductors, Hall
* Corresponding author. Fax: +55-12-553-3006
E-mail address: cams@demar.faenquil.br