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Goffman, Marcelo F. "Vórtices en superconductores laminares de alta temperatura crítica." Ph.D. thesis, , 1997.
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Sereni, J. G., C. Geibel, M. G. Berisso, P. Hellmann, O. Trovarelli, and F. Steglich. "Scaling of the ''C-p proportional to TlnT'' dependence in Ce systems." Physica B – Condensed Matter 230-232 (1997): 580–582.
Abstract: We analyze the T ln T dependence of the specific heat (C-p) in several Ce binaries and ternaries. We find that these systems can be directly compared by using a scaling law of the type: C-p/t = -D log t + ET0, where t = T/T-0, with the scaling temperature T-0 similar to T-K, D = 7.2 J/(mol K) and 0 < E < 0.14 J/(mol K-2). This general function describes those systems within the 0.005 < t < 0.5 range and accounts for nearly 1/2 of the expected entropy of the Ce-doublet.
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Trovarelli, O., J. G. Sereni, and J. P. Kappler. "Role of Ce sublattices in the thermal and magnetic properties of Ce7X3 (X=Ni, Ru, Rh, Ir, Pd, and Pt) compounds." Journal of Low Temperature Physics 108, no. 1-2 (1997): 53–85.
Abstract: The role of the Ce sublattices in the thermal and magnetic properties of the Ce7X3 (X = Ni, Ru, Rh, Ir, Pd, and Pt) family of compounds is studied by means of ac and dc magnetic susceptibility, mngnetization and mainly specific-heat experiments in applied magnetic field. The experimental data show that in these compounds there is coexistence of magnetic order, heavy-fermion and intermediate-valence behavior, which is interpreted in terms of the contribution of the three different sublattices present in the crystalline structure of Th7Fe3-type (denoted by 1Ce(I), 3Ce(II) and 3Ce(III)). From the available volume of the Ce-III atoms in their crystallographic environment it is found that sublattice Ce-III has an intermediate-valence behavior, whereas from entropic considerations sublattices Ce-I and Ce-II are identified as responsible for the magnetic order or heavy-fermion behavior, depending on the Ce-ligand electronic structure. This systematics evidences a clear correlation between the thermal and magnetic properties of these compounds and the position of the respective Ce-ligands in the periodic table, through the particular sensitivity of Ce to the environmental conditions.
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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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Luzuriaga, J. "Measurements in the laminar and turbulent regime of superfluid4He by means of an oscillating sphere." Journal of Low Temperature Physics 108, no. 3-4 (1997): 267–277.
Abstract: The translational oscillations of a sphere in liquid helium have been measured as a way of studying superfluid turbulence. Experiments were carried out in the laminar flow regime for reference purposes, and good agreement found between measured and calculated quantities. In the turbulent region, the dissipation is found to be proportional to the square of the velocity of the sphere, as found previously by other workers. For high vibration amplitudes there is an increase in the hydrodynamic mass. This seems to scale with the superfluid fraction in a way that strongly suggests that the superfluid component plays an important role in the turbulent 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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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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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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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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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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