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Nr. 2 - September 2005

Editor : Anne Rougemont
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You will find below a selection of recent top publications by MaNEP members. Please, keep us informed of the publications you would like to appear in the next MaNEP E-newsletters. A complete list of our scientific publications can be found each year in MaNEP's annual report and on MaNEP's website.

Physical Review B (3)
Nature Materials (1)
Journal of Physics : Condensed Matter (1)
arXiv.org (1)

Physical Review B
Specific heat and magnetization of a Zr B12 single crystal :
characterization of a type-II/1 superconductor
Phys. Rev. B 72, 024548 (2005)
Prof. Alain Junod by Y. Wang, R. Lortz, Y. Paderno, V. Filippov, S. Abe, U. Tutsch and
A. Junod / DPMC University of Geneva (picture)

We measured the specific heat, the magnetization, and the magnetoresistance of a single crystal of ZrB12, which is superconducting below Tc~=6 K. The specific heat in zero field shows a BCS-type superconducting transition. The normal- to superconducting-state transition changes from first order (with a latent heat) to second order (without latent heat) with increasing magnetic field, indicating that the pure compound is a low-kappa, type-II/1 superconductor in the classification of Auer and Ullmaier [Phys. Rev. B 7, 136 (1973)]. This behavior is confirmed by magnetization measurements. The H-T phase diagram based on specific-heat and magnetization data yields Hc2(0)=550 G for the bulk upper critical field, whereas the critical field defined by vanishing resistance is a surface critical field Hc3(0)~1000 G.

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Specific heat, magnetic susceptibility, resistivity
and thermal expansion of the superconductor Zr B12

Phys. Rev. B 72, 024547 (2005)
by R. Lortz, Y. Wang, S. Abe, C. Meingast, Yu. B. Paderno, V. Filippov and A. Junod

In an attempt to clarify conflicting published data, we report new measurements of specific heat, resistivity, magnetic susceptibility, and thermal expansivity up to room temperature for the 6 K superconductor ZrB12, using well-characterized single crystals with a residual resistivity ratio >9. The specific heat gives the bulk result 2Delta(0)/kBTc=3.7 for the superconducting gap ratio, and excludes multiple gaps and d-wave symmetry for the Cooper pairs. The Sommerfeld constant gamman=0.34 mJ K–2 gat–1 and the magnetic susceptibility chi=–2.1×10–5 indicate a low density of states at the Fermi level. The Debye temperature thetaD is in the range 1000–1200 K near zero and room temperature, but decreases by a factor of ~2 at ~35 K. The specific heat and resistivity curves are inverted to yield approximations of the phonon density of states F(omega) and the spectral electron-phonon scattering function alpha<sub>tr</sub><sup>2</sup>F(omega) respectively. Both unveil a 15 meV mode, attributed to Zr vibrations in oversized B cages, which gives rise to electron-phonon coupling. The thermal expansivity further shows that this mode is anharmonic, while the vanishingly small discontinuity at Tc establishes that the cell volume is nearly optimal with respect to Tc.

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Optical conductivity of CeMIn5 (M=Co, Rh, Ir)
Phys. Rev. B 72, 045119 (2005)
Prof. van der Marel
by F. P. Mena, D. van der Marel / DPMC - University of Geneva (picture) and J. L. Sarrao

The optical properties of the heavy fermion family CeMIn5 (M=Co, Rh, Ir) have been determined in the photon energy range from 2 meV to 4.5 eV using the combination of near normal incidence reflectivity (2 meV–0.8 eV) and ellipsometry (0.8–4.5 eV). In all compounds signatures of the formation of a coherent state are found. However, as a result of the competition with antiferromagnetism, the energy region where the coherent state occurs is much lower in the compound that orders magnetically.

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Nature Materials
Three-dimensionality of field-induced magnetism in a high-temperature superconductor
Advance online publication - 14 August 2005
H. Ronnowby B. Lake, K. Lefmann, N. B. Christensen, G. Aeppli, D. F. Mcmorrow, H. M. Ronnow / PSI (picture), P. Vorderwisch, P. Smeibidl, N. Mangkorntong, T. Sasagawa, M. Nohara and H. Tagaki.

Many physical properties of high-temperature superconductors are two-dimensional phenomena derived fromtheir square-planar CuO2 building blocks. This is especially true of the magnetism from the copper ions. As mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the parent insulators, where each copper spin is antiparallel to its nearest neighbours1, evolves into a fluctuating state where the spins show tendencies towards magnetic order of a longer periodicity. For certain charge-carrier densities, quantum fluctuations are sufficiently suppressed to yield static long-period order2–6, and external magnetic fields also induce such order7–12.Here we show that, in contrast to the chemically controlled order in superconducting samples, the field-induced order in these same samples is actually three-dimensional, implying significant magnetic linkage between the CuO2 planes. The results are important because they show that there are three-dimensional magnetic couplings that survive into the superconducting state, and coexist with the crucial inter-layer couplings responsible for three-dimensional superconductivity. Both types of coupling will straighten the vortex lines, implying that we have finally established a direct link between technical superconductivity, which requires zero electrical resistance in an applied magnetic field and depends on vortex dynamics, and the underlying antiferromagnetism of the cuprates.

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Journal of Physics : Condensed Matter
Evolution of the specific-heat anomaly of the high-temperature superconductor YBa2Cu3O7
under the influence of doping through application of pressure up to 10 GPa

J. Phys.: Condens. Matter 17, 4135-4145 (2005)
by Rolf Lortz / DPMC-Geneva (picture), Alain Junod , Didier Jaccard, Yuxing Wang, Christoph Meingast, Takahiko Masui and Setsuko Tajima

The evolution of the specific-heat anomaly in the overdoped range of a single crystal of the high-temperature superconductor YBa2Cu3O7 has been studied under the influence of pressure up to 10 GPa, using AC calorimetry in a Bridgman-type pressure cell. We show that the specific-heat jump as well as the bulk Tc are reduced with increasing pressure in accordance with a simple charge-transfer model. This new method enables us through pressure-induced charge transfer to study the doping dependence of the superconducting transition, as well as the evolution of the superconducting condensation energy on a single stoichiometric sample without adding atomic disorder.

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Resolution of the shadow band enigma in Bi2Sr2CaCu2O{8+delta}
Helmuth Bergerby A. Mans, I. Santoso, Y. Huang, W. Siu, S. Tavaddod , V. Arpiainen, M. Lindroos, H. Berger / EPFL (picture), V. N. Strocov, M. Shi, L. Patthey, M. S. Golden, Van der Waals-Zeeman.

By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free Bi2Sr2CaCu2O{8+delta} with structural information from LEED and ab initio, one-step photoemission simulations, we prove that the shadow Fermi surface in these systems has structural origin, being due to orthorhombic distortions from tetragonal symmetry. Consequently, one of the longest standing open issues in the fermiology of these widely studied systems finally meets its resolution.

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