Carmen Almasan

Pressure study of Tc and Josephson absorption in YBa2Cu3O7−λ and Tl-Ca-Ba-Cu-O superconductors

Abstract The effect of pressure on high- T c superconductors was studied by Josephson absorption as a new and very accurate method for determination of the critical temperature. The uncertainty in T c was less than 0.06 K. We propose our method as a new standard of T c measurements. The pressure derivative d T c /d p equals 1.06 K GPa -1 for YBa 2 Cu 3 O 7−λ and 2.20 K GPa -1 for Tl-Ca-Ba-Cu-O ceramics. Absence of thermal hysteresis was observed. The change of microwave absorption under hydrostatic pressure was also investigated.

Transport critical current and magnetization measurements of melt‐processed YBa2Cu3O7−x

We report magnetic field dependence of the transport critical current and dc magnetic susceptibility measurements on YBa2Cu3O7−x superconductors formed by melt‐solid reactions at 950 °C between Ba‐Cu‐O (or Tb‐Ba‐Cu‐O) and solid nonstoichiometric Y‐Ba‐Cu‐oxide. Four‐probe dc critical current measurements at 77, 64, and 4.2 K show strong depression of the critical current density with increasing magnetic field in agreement with a model of weakly linked superconducting regions. Diamagnetic shielding and Meissner flux expulsion measurements in the temperature range 10–300 K show about one third volume fraction of perfect superconductivity. Both shielding and flux expulsion were observed to be approximately temperature independent below 60 K indicating strong coupling between the grains throughout the entire volume below this temperature.

Experimental realizations of mean-field Ising and Heisenberg spin-glasses

Abstract In the Ising mean-field model of a spin-glass it is predicted that the onset of strong irreversibility and the susceptibility cusp coincide and that the critical exponent of the order parameter is unity. Both of these behaviors have been observed in our measurements on the alloy Fe65Mn21Cr14, near 280 K. In contrast, recently published measurements on Fe69Mn26Cr5 exibited two distinct changes of magnetic behavior below the transition temperature as predicted in the Heisenberg mean-field model. In view of these observations, we believe that these two materials represent the extreme cases of mean-field theory.

Magnetic behavior of Ho1Ba2(Cu1−xFex)3O7−y

We have carried out dc magnetization (x=0 and 0.05) and 57Fe Mossbauer spectroscopy (x=0.05) measurements of Ho1Ba2(Cu1−x Fex)3O7−y , between room temperature (RT) and 4.2 K. Based on x‐ray diffraction results all samples are single phase, and either orthorhombic (x=0) or tegragonal (x=0.05). Magnetization measurements determined the onset Tc to be either 90 K (orthorhombic) or 48 K (tetragonal). Mossbauer spectra between RT and 20 K are nearly identical, each consisting of three superposed quadrupole doublets. As the temperature is lowered below about 10 K the Mossbauer spectra exhibit paramagnetic relaxation. The relaxation time increases by about two orders of magnitude as the temperature decreases from 10 to 4.2 K. No magnetic ordering of the Fe moments was observed.

Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

Abstract The low-temperature (5 K T T N . The dc susceptibility, χ( T ), did not exhibit a simple Curie-Weiss dependence. Above T N , atemperature independent component χ 0 was observed, i.e., χ( T ) = χ 0 + C / (itT + θ. T N was systematically influenced by the lattice parameter, a , decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ B obtained from neutron scattering was lower than the ≈ 1 μ B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction ( J 1 ) and ferromagnetic second-nearest-neighbors interaction ( J 2 ) indicate that | J 2 / J 1 |≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.

Period doubling and magnetic chaos in superconducting YBa2Cu3O7

Abstract The time dependence of the field-cooled cool down and field-cooled warm up remnant magnetic moments, M(t), were studied as functions of temperature. For T 0 in the “chaotic” temperature region and α

Magnetic behavior of Y‐Ba‐Cu‐O superconductors prepared under different oxygen anneals

In our study of three YBa2 Cu3 oxides prepared with different oxygen anneals, we observed that the temperature derivative of the magnetization or susceptibility χ(T) of these high‐Tc superconductors has a maximum at approximately the transition temperature, Tc . This temperature as estimated by the peak value of (∂χ/∂T)H , ranged between 55 and 65 K. The most oxygen‐poor sample showed the lowest Tc and the most oxygen‐rich sample showed the highest Tc . In the subcritical region, a novel irreversibility is observed in the field‐cooled warm‐up and cool‐down behaviors.

A NOTE ON FLUX TRAPPING AND SUPERCONDUCTIVE GLASS STATE IN HIGH-Tc SUPERCONDUCTORS

We have shown that the power law behavior of H versus T*, attributed to a de Almeida-Thouless criticality in a high temperature superconductor, may be due to shielding effects and critical currents. We calculate a hitherto unrecognized scaling of T* with J0 and specimen size. Verification of these predictions will favor the conventional Type II model over the glass model for these systems.

Magnetic Behavior of YBaCuO

Some of the most intriguing behavior of a superconducting medium pertain to its magnetic properties. An ideal plasma is just as highly conducting as a superconductor, but magnetically these are totally different systems. Indeed the statement, that it may be more accurate to regard a superconductor as a perfect diamagnet than a perfect conductor, has been attributed to John Bardeen. This pivotal role of magnetism makes it essential to understand the magnetic properties of superconductors.