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Castello, D., M. Jaime, and M. Núñez Regueiro. "Thermal conductivity of high temperature superconductor TlBaCaCuO." Solid State Communications 79, no. 11 (1991): 967–970.
Abstract: We have measured the thermal conductivity of ceramic samples of TlBaCaCuO between 2 and 250 K. At high temperatures it follows a constant behaviour down to the superconducting transition temperature where a sudden increase of the thermal conductivity occurs, attributed to reduced phonon scattering by condensating electrons. At low temperatures a T2 law is observed, confirming the assumption that it is a common feature in most of the high temperature superconducting oxides. Phonon scattering against tunneling systems analogous to those found in amorphous solids can explain this behaviour.
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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., 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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Izbizky, M. A., M. Núñez Regueiro, P. Esquinazi, C. Durán, and C. Fainstein. "Phonon transport in superconducting EuBa2Cu3O7-x." Physics Letters A 129, no. 1 (1988): 71–75.
Abstract: We have measured the thermal conductivity, Young modulus sound velocity and internal friction of a polycrystalline sample of the ceramic superconductor EuBa2Cu3O7-x. The low temperature data can be quantitavely understood assuming the interaction of phonons with tunneling systems.
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Izbizky, M. A., M. Núñez Regueiro, P. Esquinazi, and C. Fainstein. "Thermal conductivity and superconductivity in EuBa2Cu3O7-x." Physical Review B 38, no. 13 (1988): 9220–9223.
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Núñez Regueiro, M., D. Castello, M. A. Izbizky, D. Esparza, and C. D'Ovidio. "Anomalous thermal conductivity of La2-xSrxCuO4: Possibility of a polaronic glass." Physical Review B 36, no. 16 (1987): 8813–8815.
Abstract: e present measurements of the thermal conductivity of La2CuO4 and La1.85Sr0.15CuO4. The results on the pure sample show that lattice conduction is predominant with a T1.8 dependence at low temperatures and a strong T-independent scattering above 30 K. We observe a similar behavior for the superconductor but with a T1.4 law at low temperature and a small electronic contribution in the normal state. These results together with the behavior of the specific heat and sound attenuation are consistent with the existence of a two-level tunneling system for an amorphous state.
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Núñez Regueiro, M., P. Esquinazi, M. A. Izbizky, C. Durán, D. Castello, and J. Luzuriaga. "Tunneling systems in high temperature superconductors." Annales de Physique 13, no. 5 (1988): 401–406.
Abstract: We have measured the thermal conductivity, sound attenuation and relative variation of sound velocity of compounds of the La-Sr-Cu-O and RBa2Cu3O7 families, where R is a rare-earth. We have found that all these properties can be quantitatively correlated within a tunneling system model similar to that used in amorphous materials. This interpretation would naturally explain the anomalous linear term of the low temperature specific heat observed in these materials.
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Núñez Regueiro, M., P. Esquinazi, M. A. Izbizky, C. Duran, D. Castello, J. Luzuriaga, and G. Nieva. "Evidence of tunneling systems through thermal and acoustic measurements in ceramic materials." Physica C: Superconductivity and its applications 153-155, no. PART 2 (1988): 1016–1017.
Abstract: We have neasured the sound velocity, internal friction, thermal conductivity and specific heat of La-based and (Y/Eu)- based ceramics. The low temperature data can be understood in terms of the Tunneling Model assuming the existence of Tunneling Systems (TS). From these measured properties we obtain values for the parameter C=P?2/?v2, where P is the density of states of TS, ? the coupling constant between phonons and TS, ? the mass density and v the sound velocity close to those obtained for amorphous and “orientational” disordered crystalline materials. Our data would indicate that the TS are intrinsic defects independent of the sample porosity.
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Núñez Regueiro, M., M. A. Izbizky, and P. Esquinazi. "Thermal conductivity and “disorder” in high temperature superconductors." Solid State Communications 67, no. 4 (1988): 401–404.
Abstract: We analyze the controversial thermal conductivity results on high temperature superconductors by scaling the data with material parameters to compare them to other materials with similar behaviour. We show that they have the same general features observed in amorphous and disordered crystalline materials. Possible origins of the disorder are discussed.
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Núñez Regueiro, M., D. Castello, and M. Jaime. "Thermal conductivity of high temperature superconductors." Physica B: Condensed Matter 169, no. 1-4 (1991): 631–632.
Abstract: We have measured the thermal conductivity of Tl1Ba2Cu3O9 and of the related compounds CuO and La2CuO4. For the superconducting sample we observe the characteristic T2 behaviour at low temperatures, that does not appear in the non-superconducting samples. The superconducting transition and the antiferromagnetic ordering are detected by our measurements.
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