Morré, E., S. Grigera, E. Osquiguil, G. Nieva, and F. de la Cruz. "Angular restrictions on the glass transition and on the c-axis correlated liquid phase in twinned YBa2Cu3O7-?" Physics Letters A 233, no. 1-2 (1997): 130–134.
Abstract: We have measured the angular dependence of the vortex velocity correlation temperature Tth(H) in twinned YBa2Cu3O7-? single crystals. The data show that the c-axis correlated vortex liquid phase exists up to angles much larger than expected within a single vortex pinning picture. Despite this, the second-order solid-liquid glass transition is shown to be restricted only to external magnetic fields applied within a few degrees from the c-axis direction.
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Sereni, J. G., O. Trovarelli, G. Schmerber, and J. P. Kappler. "Comparative study of thermal and magnetic properties of CeTyX4-y ferromagnets." Physical Review B – Condensed Matter and Materials Physics 56, no. 9 (1997): 5380–5386.
Abstract: Low-temperature thermal (specific heat, C-P) and magnetic [ac susceptibility, chi(ac), and isothermal magnetization M(B)] measurements on CeTyX4-y ferromagnets (with T = Ni, Cu, Pd, Ag, Au, and X = Ga, Al, with 0.3 < y < 0.7) are reported. With the exception of T = Ni, all compounds with X = Ga show a typical second-order ferromagnetic transition. In the case of T = Ni, magnetic correlations set in at T approximate to 3T(C), (T-C being the Curie temperature) and the C-P(T) and chi(ac)(T) maxima (at 3.3 and 4.2 K, respectively) do not coincide, while the M(B) curves reveal an antiferromagnetic character at lower temperatures. In these systems, the magnetic entropy of the ordered phase is larger than 85% of Rln2, in coincidence with the low values of the Sommerfeld coefficient gamma less than or equal to 10 mJ/mol K-2. For T = Ag and X = Al, a strong diamagnetic signal was observed at T-s = 0.63 K in chi(ac)(T), with the characteristics of a superconducting component. The thermodynamical properties of these compounds confirm the lack of hybridization when the Ce atom is in a ferromagnetic ground state.
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Trovarelli, O., M. Weiden, R. MullerReisener, M. GomezBerisso, P. Gegenwart, M. Deppe, C. Geibel, J. G. Sereni, and F. Steglich. "Evolution of magnetism and superconductivity in CeCu2(Si1-xGex)(2)." Physical Review B – Condensed Matter and Materials Physics 56, no. 2 (1997): 678–685.
Abstract: We have determined the magnetic phase diagram of CeCu2(Si1-xGex)(2) using specific heat, susceptibility, and resistivity measurements and additional thermal expansion and muon spin resonance experiments. The system evolves continuously from the heavy-fermion state at x = 0 to an antiferromagnetically (AF) ordered state at x = 1, though the magnetic structure undergoes significant modifications. The results strongly suggest that the AF ordering emerges from the A phase of pure CeCu2Si2. The phase diagram can be divided into three regions: Low Ge content (x less than or equal to 0.2) leads to an enhancement of the A phase of pure CeCu2Si2 and to a moderate decrease of the Kondo temperature. Despite this increase of the magnetic character, we could observe superconductivity up to x = 0.1. At large Ge content (x > 0.5) the behavior resembles that of pure CeCu2Ge2. The intermediate region is characterized by the appearance of a second magnetic transition below T-N, which seems to be of first-order type. The appearance of this transition leads to a well-defined critical point at x = 0.3 and suggests the possibility of another critical point at x = 0.5.
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Guimpel, J., B. Maiorov, E. Osquiguil, G. Nieva, and F. Pardo. "Interrelation between persistent photoconductivity and oxygen order in GdBa2Cu3Ox thin films." Physical Review B – Condensed Matter and Materials Physics 56, no. 7 (1997): 3552–3555.
Abstract: We study the metastable states induced by photoexcitation, oxygen disorder, and both effects combined in superconducting GdBa2Cu3Ox thin films. The states are identified by the temperature dependence of the resistivity. The relaxation dynamics is characterized through the time evolution of the conductivity. The results show that photoexcitation and oxygen disorder behave as noninteracting and do not cancel each other out. However, in both cases the relaxation dynamics is related to oxygen movement .
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Bauer, E., R. Hauser, L. Keller, P. Fischer, O. Trovarelli, J. G. Sereni, J. J. Rieger, and G. R. Stewart. "Onset of magnetic order in YbCu5-xAlx." Physical Review B 56, no. 2 (1997): 711–718.
Abstract: Spectroscopic, magnetic, thermal, and transport measurements under pressure are presented which were performed on YbCu5-xAlx intermetallics. The substitution of Cu by Al in YbCu5 drives the system from an almost divalent behavior of the Yb ion in YbCu5 to a trivalent state in YbCu3Al2. A compensation between the normal thermal expansion and a valence-driven contraction was found for YbCu4Al. Antiferromagnetic order sets in for x>1.5, where an increasing Al content causes both a growing transition temperature and rising saturation moments reaching 2 K and about 2.1 mu(B), respectively, for x=2. At the critical concentration (x approximate to 1.5) the possibility of a non-Fermi-liquid state is analyzed together with its tuning by pressure.
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Pardo, F., A. P. Mackenzie, F. delaCruz, and J. Guimpel. "Effect of the reversibility region on the low-temperature vortex structure imaged by Bitter magnetic decoration." PHYSICAL REVIEW B 55, no. 21 (1997): 14610–14613.
Abstract: We have combined magnetic measurements and imaging of the vortex lattice at low fields in Tl2Ba2CuO6-delta single crystals with different oxygen content. Those with high critical temperature T-c > 80 K have a reversible region while those with low T-c < 30 K have an irreversible magnetization in the whole range of temperatures, up to T-c. The presence of the reversible region allows the formation of ordered vortex structures, while amorphous structures are obtained when no reversible region is present below T-c. The results are discussed in terms of vortex nucleation processes in the individual pinning regime.
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Weiden, M., R. Hauptmann, W. Richter, C. Geibel, P. Hellmann, M. Koppen, F. Steglich, M. Fischer, P. Lemmens, G. Guntherodt et al. "Magnetic phase diagram of CuGe1-xSixO3." PHYSICAL REVIEW B 55, no. 22 (1997): 15067–15075.
Abstract: The effect of Si doping on the magnetic properties of the spin-Peierls (SP) system CuGeO3 was found to differ strongly between polycrystals (PC's) and single crystals (SC's). In SC's, the SP state is suppressed much mon strongly, whereas the existence region of the antiferromagnetic (AF) state is enhanced. We investigated the origin of this difference by means of magnetic susceptibility, specific heat, thermal expansion, Raman scattering, elastic neutron scattering, and x-ray measurements on CuGe1-xSixO3 samples prepared under different conditions. The partial oxygen pressure and the temperature during the synthesis were found to have a profound influence on the magnetic properties: preparation under reduced oxygen pressure leads to a stabilization of the AF state, whereas heating above the melting point results in a strong decrease of T-SP in Si-doped samples. Therefore, both the AF stabilization and the TSP reduction observed in SC's are not an intrinsic effect of Si doping PC samples, which can be prepared at lower temperatures and more oxidizing conditions, reflect much better the intrinsic properties of CuGe1-xSixO3. We were able to prepare PC samples up to 50 at. % Si and found a continuous decrease of the one-dimensional character of the magnetic properties without pronounced changes in the structure.
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Righi, E. F., S. A. Grigera, G. Nieva, D. Lopez, and F. delaCruz. "Finite vortex correlation in the c direction in YBa2Cu3O7-delta above the first-order melting transition." PHYSICAL REVIEW B 55, no. 21 (1997): 14156–14159.
Abstract: The in- and out-of-plane resistivities of untwinned YBa2Cu3O7-delta show that at the melting temperature of the vortex lattice T-m(H), the c-axis vortex correlation length has a finite value, demonstrating that the first-order transition is a true melting and not a decoupling transition. The vortex liquid just above T-m(H) has a correlation length in the field direction on the order of a micron. The in-plane I-V characteristics in the liquid state show non-Ohmic behavior, providing evidence that thermal fluctuations dominate the response even at the first-order transition.
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Righi, E. F., S. A. Grigera, D. Lopez, G. Nieva, F. delaCruz, L. Civale, G. Pasquini, and P. Levy. "Enhancement of c-axis vortex correlation by twin boundaries and columnar defects in YBa2Cu3O7-delta." PHYSICAL REVIEW B 55, no. 9 (1997): 5663–5665.
Abstract: We report on transport measurements using the flux transformer configuration in a twinned single crystal of YBa2Cu3O7-delta with columnar defects. The vortex liquid shows velocity correlation along the c axis (the direction of the applied magnetic field and the columnar defects) at temperatures that correspond to an uncorrelated liquid in unirradiated samples of the same thickness. These results and previous work about the effect of twin boundaries on vortex velocity correlation show that phase coherence in the field direction is not an intrinsic property of vortex liquids but one induced by extended defects.
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Pasquini, G., P. Levy, L. Civale, G. Nieva, and H. Lanza. "Linear response in the AC susceptibility of vortices in YBa2Cu3O7 crystals with columnar defects." Physica C: Superconductivity 274, no. 1-2 (1997): 165–172.
Abstract: We explore the AC magnetic susceptibility of YBa2Cu3O7 single crystals with columnar defects. For vortices parallel to the defects the linear response is mainly determined by the Campbell penetration depth with a field independent Labusch parameter indicating that vortices are individually pinned. The small dissipation in this limit is likely to arise from thermal fluctuations of vortex segments between pairs of metastable states (two-level systems) in the Bose glass phase. Tilting vortices away from the tracks results in a reduction of the Bose glass temperature and an increase of the dissipation due to the reduction of the pinning forces. We discuss the onset of nonlinear response as the AC field increases.
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