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Nr. 2 - September 2005

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Warning : this section contains news from outside MaNEP. Most are reproduced here without critical editing by MaNEP. Thus claims made by authors or journalists are not necessarily approved by MaNEP and should be read critically. In some cases, relevant information and comments to a given information is added at the end as “MaNEP's comment”.

Titles
Researcher Anatoly Larkin died
French physicists make new carbon-based superconductor
Room-Tc in multi-walled carbon nanotubes ?
The first layered silver oxide superconductor ?
Electrodynamics of high-Tc superconductors
Superconductivity in doped copper oxides : article in Nature
Magnetic field induced superconductivity : article in Science
2 publications related to the LNS at ETHZ and PSI



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Researcher Anatoly Larkin died
Anatoly Larkin
Anatoly Larkin, a University of Minnesota researcher whose work led to advances in the understanding of theoretical physics, died on Thursday August 11th. He was 72.
The university said in a news release that Larkin died unexpectedly in Aspen (Colorado), where he was attending a workshop. He had joined the University of Minnesota faculty in 1995 after a career at several institutions in his native Russia. Larkin was best known for research into condensed matter theory, particularly superconductivity (...). The school said his research also was key to the study of one-dimensional systems and clusters that are used in the fields of nanoscience and nanotechnology. Larkin was considered a founding father of the renowned Russian school of theoretical physics. Many of his former students hold leading academic positions at universities and institutes both in Russia and the Western world. Larkin published more than 50 papers during his 10 years in Minnesota, as well as the 600-page "Theory of Fluctuations in Superconductors."
(Press Release)

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French physicists make new carbon-based superconductor
From physicsweb.org
Bulk quantities of a new carbon-based superconductor have been made by physicists in France. Known as CaC6, the material contains "foreign" calcium atoms inserted between hexagonal two-dimensional sheets of graphite. The technique for making the material was devised by Nicolas Emery of the University Henri Poincaré in Nancy and colleagues at the University of Paris 6 & 7. The material becomes a superconductor when cooled to below 11.5 K.

[Find out more here]

MaNEP's comment : Superconductivity in this compound was first reported by Tom Weller and co-workers at University College London (UCL) and Cambridge University who discovered that CaC6 becomes a superconductor when cooled to below 11.5 Kelvin. Dr. S. Saxena from Cambridge Unversity shall give an invited talk on this topic at the SWM in Les Diablerets next week.

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Room-Tc in multi-walled carbon nanotubes ?
From arxiv.org
Guo-meng Zhao and Pieder Beeli are researchers at the Department of Physics and Astronomy, California State University, Los Angeles. They 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 arcdischarge 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.'

[Find out more here]

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The first layered silver oxide superconductor ?
From arxiv.org
Two Japanese researchers report :
'
(...) zero-resistivity transition and the details of magnetic transition of a layered silver oxide Ag5Pb2O6 single crystal, which make definitive evidence of superconductivity in this compound. In the AC susceptibility of a mono-crystal, we observed large supercooling, as well as positive peaks in the real part of the susceptibility indicating the reversibility of magnetic process. These observations reveal that Ag5Pb2O6 is probably the first oxide that shows type-I superconductivity. Evaluation of the superconducting parameters not only gives confirming evidence of type-I superconductivity, but also indicates that it is a dirty-limit superconductor. We also analyze supercooling to determine the upper limit of the Ginzburg-Landau parameter. '

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Electrodynamics of high-Tc superconductors
From Aps Journals
This long article (59 pages) by D. N. Basov and T. Timusk gives a full review of a major category of properties of high-Tc superconductors. It also extensively quotes the work of MaNEP member Prof. Dirk van der Marel.
Abstract
Recent studies of the electromagnetic response of high-Tc superconductors using terahertz, infrared, and optical spectroscopies are reviewed. In combination these experimental techniques provide a comprehensive picture of the low-energy excitations and charge dynamics in this class of materials. These results are discussed with an emphasis on conceptual issues, including evolution of the electronic spectral weight in doped Mott-Hubbard insulators, the d-wave superconducting energy gap and the normal-state pseudogap, anisotropic superfluid response, electronic phase segregation, emergence of coherent electronic state as a function of both temperature and doping, the vortex state, and the energetics of the superconducting transition. Because the theoretical understanding of these issues is still evolving the review is focused on the analysis of the universal trends that are emerging out of a large body of work carried on by many research teams. Where possible data generated by infrared/optical techniques are compared with the data from other spectroscopic and transport methods.

[
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Superconductivity in doped copper oxides - article in Nature
From the review Nature - 436, 829-832 (11 August 2005)
Negative lattice expansion from the superconductivity−antiferromagnetism
crossover in ruthenium copper oxides

A. C. Mclaughlin, F. Sher and J. P. Attfield
Abstract
The mechanism of high-transition-temperature (high-Tc) superconductivity in doped copper oxides is an enduring problem. Antiferromagnetism is established as the competing order, but the relationship between the two states in the intervening 'pseudogap' regime has become a central puzzle. The role of the crystal lattice, which is important in conventional superconductors, also remains unclear. Here we report an anomalous increase of the distance between copper oxide planes on cooling, which results in negative thermal volume expansion, for layered ruthenium copper oxides that have been doped to the boundary of antiferromagnetism and superconductivity. We propose that a crossover between these states is driven by spin ordering in the ruthenium oxide layers, revealing a novel mechanism for negative lattice expansion in solids. The differences in volume and lattice strain between the distinct superconducting and antiferromagnetic states can account for the phase segregation phenomena found extensively in low-doped copper oxides, and show that Cooper pair formation is coupled to the lattice. Unusually large variations of resistivity with magnetic field are found in these ruthenium copper oxides at low temperatures through coupling between the ordered Ru and Cu spins.

[Find out more here]
[also read the article on physorg.com]

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Magnetic field induced superconductivity - article in Science
From Physicsweb.org -
Strong magnetic fields normally destroy the superconducting properties of materials. However, physicists in France have now discovered a metal that becomes a superconductor in the presence of an extremely strong field (
Science 309 1343). The new work is the latest breakthrough in the growing field of quantum criticality.

[
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MaNEP's comment : Field induced superconductivity was first observed by Meul et al. in the group of Ø. Fischer at UNIGE in 1984 in the Chevrel phase compound Eu1-xSnxMo6S8 (PRL 53, 497 (1984)). The mechanism behind this striking effect was shown to be the Jaccarino-Peter effect. In the new work quoted above another mechanism is considered.

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2 publications related to the LNS at ETHZ and PSI
Forwarded by MaNEP Forum member Dr Joel Mesot, PSI

Quasi-elastic scattering, random fields and phonon-coupling effects in PbMg1/3Nb2/3O3
J. Phys.: Condens. Matter 17 (2005) 4343-4359
S N Gvasaliya (picture)1, B Roessli, R A Cowley, PHuber and S G Lushnikov.
1 Laboratory for Neutron Scattering ETHZ and Paul Scherrer Institut
Abstract
The low energy part of the vibration spectrum in PbMg1/3Nb2/3O3 (PMN) relaxor ferroelectric has been studied by means of neutron scattering above and below the Burns temperature, Td. The transverse acoustic and the lowest transverse optic phonons are strongly coupled andwe have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasielastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below Td. The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature.

[Find out more here]

Magnetic Inversion Symmetry Breaking and Ferroelectricity in TbMnO3
PRL 95, 087206 (2005)
M. Kenzelmann (picture) 1, A. B. Harris, S. Jonas, C. Broholm, J. Schefer 1, S. B. Kim, C. L. Zhang, S.-W. Cheong, O. P. Vajk and J.W. Lynn
1 Laboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institute
Abstract
TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature TN < 41 K is accompanied by ferroelectric order for T <28 K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.


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