Braun, E., A. Bürger, G. L. Nieva, B. Roden, and J. G. Sereni. "Unexpected magnetic behaviour in a singlet crystal field ground state induced by Tb 4f-instability." Zeitschrift für Physik B Condensed Matter 70, no. 2 (1988): 187–192.
Abstract: The magnetic susceptibility (chi) of Th1-x Tb x alloys (with 5<x<55a/o Tb) in the range of 1 K<T<350K and the specific heat (forx=1.5a/o Tb) for 0.5<T<10 K were measured. A maximum in chi(T) was observed at a temperature which depends on concentration. The crystal field level scheme that fits the specific heat measurements shows two non-magnetic singlets as lowest levels. Such ambiguity is discussed within the framework of the 4f-Tb instability. The results are in agreement with the matrix superconductiveT c decrease. The contribution of the interconfigurational mixing on chi and the specific heat is also discussed.
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Braun, E., A. Bürger, B. Roden, and J. G. Sereni. "Spin glass-like behaviour of the singlet ground state system: ThTb." Solid State Communications 66, no. 11 (1988): 1141–1144.
Abstract: We have studied the temperature and field dependence of the magnetic susceptibility, X, on the single-ground-state Th1?nTbn system, in the 0.2 < n < 0.6 concentration range. The characteristic maximum of a spin glass transition was found at a temperature Tg, which increases linearly with the impurity concentration. As expected, the maximum of X is reduced by the increment of the applied magnetic field. The spin glass order parameter, as well as its critical exponent were evaluated by using the Sherrington-Kirkpatrick theory.
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Braun, E., G. Jackel, B. Roden, J. G. Sereni, and D. Wohlleben. "Comparative study of the specific heat of superconducting and non-superconducting YBa2Cu3O7-x." Zeitschrift für Physik B Condensed Matter 72, no. 2 (1988): 169–173.
Abstract: We have measured the specific heat, the electrical resistance and the magnetic susceptibility of YBa2Cu3O7−x in a superconducting (sc) and in a non-superconducting (nsc) version. The latter was obtained by inducing a small loss of oxygen. In the sc sample we find a jump of the specific heat with ΔC=3.6 J mole−1 K−1. Below the maximum nearT c the specific heat of the sc version drops too fast for an electronic effect alone: there is a cross-over from excess to deficiency already at 0.9T c . The specific heat of the nsc version shows a change of slope in the temperature range between 70 and 80 K, which indicates the existence of a second specific heat anomaly, which apparently exists independently of that due to the onset of superconductivity and explains at least partially the premature crossover.
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Causa, M. T., C. Fainstein, G. Nieva, R. S\'anchez, L. B. Steren, M. Tovar, R. Zysler, D. C. Vier, S. Schultz, S. B. Oseroff et al. "Crystal-field interaction in the GdxEu1-xBa2Cu3O7-? superconductors." Physical Review B 38 (1988): 257–261.
Abstract: We report here experimental data on the crystal-field interaction of gadolinium ions with their environment in GdxEu1-xBa2Cu3O7-?. The ESR spectrum and a Schottky anomaly in the specific heat of dilute samples (x?1) indicate the existence of a crystal-field splitting of the 8S7/2 ground state of Gd3+ ions of about 1.5 K. We discuss the parametrization of the measured splitting in terms of an effective Hamiltonian, and the microscopic origin of the interaction within the framework of a superposition model. Since the single-ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd3+ moments in GdBa2Cu3O7-? (TN?2.24 K), we analyze the effects of the crystal-field interaction on the magnetic transition.
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Causa, M. T., S. M. Dutrus, C. Fainstein, G. Nieva, R. Sanchez, L. B. Steren, M. Tovar, R. Zysler, S. B. Oseroff, D. C. Vier et al. "Magnetic ordering in dilute GdxEu1−xBa2Cu3O7−δ superconductors." Physica C: Superconductivity 153 (1988): 188–189.
Abstract: We have measured the specific heat of GdxEu1−xBa2Cu3O7−δ (0 ≤ × ≤ 1). The data show a λ-type anomaly at high concentrations and a broad Schottky type anomaly at intermediate and low concentrations. We compare this behavior with theoretical predictions for a 2-dimensional Ising system.
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Causa, M. T., C. Fainstein, G. Nieva, R. Sánchez, L. B. Steren, M. Tovar, R. Zysler, D. C. Vier, S. Schultz, S. B. Oseroff et al. "Crystal Field Interaction and Magnetic Order in GdBa2Cu3O7." In Progress in High Temperature Superconductivity, edited by J. L. Heiras. Vol. 5. N.J.: World Scientific, 1988.
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Civale L., and E. N. Martinez. "Linear temperature dependence of the resistivity in the new superconductors." Physical Review B 38, no. 1 (1988): 928–930.
Abstract: We present an elementary proof that, irrespective of dimensionality, the electrical resistivity of a simple conductor should be linear with temperature for T>2PFS / kB??D if PF?h? / a, where PF is the Fermi momentum, s the speed of sound, a the lattice constant, and ?D the Debye temperature.
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Durán, C., P. Esquinazi, C. Fainstein, and M. Núñez Regueiro. "Anomalies in the internal friction and sound velocity in YBa2Cu3O7-x and EuBa2Cu3O7-x superconductors." Solid State Communications 65, no. 9 (1988): 957–961.
Abstract: Internal friction Q-1 and Young Modulus sound velocity vE measurements at 1 kHz in YBa2Cu3O7?x and EuBa2Cu3O7?x polycrystalline samples reveal anomalies at the superconducting transition and at not, vert, similar37 K. The anomaly in vE at not, vert, similar90 K is accompanied by an absorption peak and is in agreement with published ultrasonic measurements, suggesting a lattice deformation at the superconducting transition.
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Esquinazi, P., and C. Durán. "Magnetization and pinning measurements with the vibrating Reed on high-Tc ceramics." Physica C: Superconductivity 153-155, no. Part 3 (1988): 1499–1500.
Abstract: By means of the vibrating reed technique we have measured the lower critical field and the elastic coupling of vortices to pinning centers in La-Sr-Cu-O and Eu-Cu-O ceramics.our results indicate that the diamagnetism probed by the reed and the pinning scaling behaviour depend on the amount of oxygen in the La-based ceramic. In the Eu-based sample the lower critical field has a highly anomalous temperature dependence.
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Esquinazi, P., C. Durán, C. Fainstein, and M. Núñez Regueiro. "Evidence of low-energy tunneling excitations in the high-Tc superconductor YBa2Cu3O7-x." Physical Review B 37, no. 1 (1988): 545–547.
Abstract: We have measured the internal friction Q-1 and the Young's-modulus sound velocity vE at 1 kHz for YBa2Cu3O7-x. Below 20 K the relative change in the sound velocity and the temperature dependence of the internal friction perfectly resemble the behavior obtained for Suprasil W. Below 1 K we observe a logarithmic temperature dependence for the sound velocity and between 1 and 5 K a well-defined plateau in the internal friction is obtained. We find quantitative agreement with the predictions of the standard tunneling model which is widely used to explain the low-temperature properties of amorphous materials. The implications of the existence of tunneling systems in these materials are discussed.
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