Large Broadening of the Superconducting Transition by Fluctuations in a 3D Metal at High Magnetic Fields: The MgB2_{2} case

It is shown that the transition to the low temperature superconducting state in a 3D metal at high magnetic field is smeared dramatically by thermal fluctuation of the superconducting order parameter. The resulting superconducting-to-normal crossover occurs in a vortex liquid state which is extended well below the mean-field $H_{c2}$. Application to MgB$_{2}$ yields good quantitative agreement with recently reported data of dHvA oscillation in the superconducting state

Fermi-surface topology of the iron pnictide LaFe2_2P2_2

We report on a comprehensive de Haas--van Alphen (dHvA) study of the iron pnictide LaFe$_2$P$_2$. Our extensive density-functional band-structure calculations can well explain the measured angular-dependent dHvA frequencies. As salient feature, we observe only one quasi-two-dimensional Fermi-surface sheet, i.e., a hole-like Fermi-surface cylinder around $\Gamma$, essential for $s_\pm$ pairing, is missing. In spite of considerable mass enhancements due to many-body effects, LaFe$_2$P$_2$ shows no superconductivity. This is likely caused by the absence of any nesting between electron and hole bands.Comment: 5 pages, 4 figure

Spin-zero anomaly in the magnetic quantum oscillations of a two-dimensional metal

We report on an anomalous behavior of the spin-splitting zeros in the de Haas-van Alphen (dHvA) signal of a quasi-two-dimensional organic superconductor. The zeros as well as the angular dependence of the amplitude of the second harmonic deviate remarkably from the standard Lifshitz-Kosevich (LK) prediction. In contrast, the angular dependence of the fundamental dHvA amplitude as well as the spin-splitting zeros of the Shubnikov-de Haas signal follow the LK theory. We can explain this behavior by small chemical-potential oscillations and find a very good agreement between theory and experiment. A detailed wave-shape analysis of the dHvA signal corroborates the existence of an oscillating chemical potential

Fermi surface of a system with strong valence fluctuations : evidence for a noninteger count of valence electrons in EuIr2Si2

We present de Haas-van Alphen (dHvA) measurements on an Eu-based valence-fluctuating system. EuIr2Si2 exhibits a temperature-dependent, noninteger europium valence with Eu2.8+ at low temperatures. The comparison of experimental results from our magnetic-torque experiments in fields up to 32 T and density functional theory band-structure calculations with localized 4f electrons shows that the best agreement is reached for a Fermi surface based on a valence of Eu2.8+. The calculated quantum-oscillation frequencies for Eu3+ instead cannot explain all the experimentally observed frequencies. The effective masses, derived from the temperature dependence of the dHvA oscillation amplitudes, show not only a significant enhancement with masses up to 11me (me being the free electron mass), but also a magnetic-field dependence of the heaviest mass. We attribute the formation of these heavy masses to strong correlation effects resulting from valence fluctuations of 4f electrons being strongly hybridized with conduction electrons. The increase of the heavy masses with magnetic field likely results from the onset of the expected field-induced valence crossover that enhances these valence fluctuations but does not alter the Fermi-surface topology in the field range studied

Fermi surface of the superconductor BaIr

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Broadening of the superconducting transition by fluctuations in three-dimensional metals at high magnetic fields

The Bragg-chain model of the two-dimensional (2D) vortex state at high magnetic field [V. N. Zhuravlev and T. Maniv, Phys. Rev. B 60, 4277 (1999)] is extended to an array of coupled superconducting (SC) layers. Application to MgB2 and YNi2B2C yields good quantitative agreement with high-field magnetization measurements, indicating that the smeared transitions observed in these materials are, at least in great part, due to SC fluctuations. Similar to the situation in a 2D system, the melting of the vortex lattice in strongly coupled SC layers is predicted to occur well below the mean field H-c2.This article is published as Maniv, T., V. Zhuravlev, J. Wosnitza, O. Ignatchik, B. Bergk, and P. C. Canfield. "Broadening of the superconducting transition by fluctuations in three-dimensional metals at high magnetic fields." Physical Review B 73, no. 13 (2006): 134521. DOI: 10.1103/PhysRevB.73.134521. Copyright 2006 American Physical Society. Posted with permission

Superconducting phase diagram of Rh<SUB>17</SUB>S<SUB>15</SUB>

We report on measurements of the magnetization up to 7 T, of the specific heat and electrical resistivity in fields up to 14 T, and of the magnetic susceptibility in fields up to 20 T of a polycrystalline sample of Rh<SUB>17</SUB>S<SUB>15</SUB>. Our data allow us to complement the superconducting phase diagram. The existence of narrow 4d-band states (and thus of strong electronic correlations that seem not to provide magnetic correlations) is supported by the moderately high electronic contribution to the specific heat of about 107 mJ/molK<SUP>2</SUP>, favoring the existence of a strong superconducting interaction. This fact, and the remarkably high upper critical field (exceeding the simple Pauli limit by a factor of two), give evidence of the uncommon nature of the superconductivity in Rh<SUB>17</SUB>S<SUB>15</SUB>

Electronic band structure of the borocarbide superconductor LuNi2B2C

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Competing exchange interactions in Co-doped ZnO: Departure from the superexchange picture

International audienceWe report the results of a comprehensive study of the exchange interactions in Co-doped ZnO using inelastic neutron scattering, electron paramagnetic resonance, and magnetic property measurements. In particular, we observe an unprecedentedly strong spatial anisotropy of the two nearest-neighbor exchanges, J (1) = −25.6 ± 0.3 K and J (2) = −8.5 ± 0.4 K, along with the distant-neighbor J values of ferromagnetic sign. We argue that the superexchange mechanism alone cannot account for the obtained data and we suggest that an additional mechanism leading to a strong ferromagnetic spin coupling is responsible for these findings. We also discuss the origin of this ferromagnetic mechanism