Bulk Modulus of the YBaCuO Oxide Superconductor Obtained from Magnetic Measurements
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BULK
MODULUS OF THE YBaCuO
OXIDE SUPERCONDUCTOR MEASUREMENTS
OBTAINED FROM
MAGNETIC
BOKHIMI Instituto de Fisica UNAM, a. p. 20-364, 01000 Mexico D. F., Mexico ABSTRACT Starting from the fact that the YBa Cu O oxide superconductor system has a constant magnetic susceptibility at hIgh temperatures for each oxygen deficiency x, and assuming that the charge carriers have a freeelectron-like behavior, the observed magnetic susceptibility of the samples at room temperature can be associated to the Pauli paramagnetismus and to the Landau diamagnetism of the charge carriers. This result allows one to calculate the charge carriers density from the magnetic susceptibility in the sample. If besides one assume that by applying an external pressure on the sample, there is a change of the volume of the unit cell, but not in the number of the charge carriers in it, then, it is possible to calculate the Bulk modulus of the YBaCuO system at room temperature starting from these magnetic measurements. The obtained results are of the same order of magnitude to those reported in the literature and obtained from thermodynamic and mechanical measurements. INTRODUCTION The behavior of oxide superconductors under external pressure has played an important role in the development of systems with higher transition temperatures to the superconducting state 11-3]. Although till now it does not exist any theory that relate these changes with the origin of the superconductivity in these materials. It is also well known that in the YBaCuO oxide superconductor, by applying an external pressure to the sample, the transition temperature changes, increasing it at higher pressures [4-6]. Therefore, it is important to measure the parameter that are related to the response of the system to an external pressure. From them, the Bulk modulus, supplies information about the variation of the volume of the sample under pressure, and also it can be related to the repulsion between atoms due to the overlap of their wave functions. Because many of the oxide samples are porous, and particularly those of the YBaCuO oxide superconductor, by the measurement of the Bulk modulus, it must be distinguished between the changes of the volume between grains and the volume of the unit cell, originated by the applied external pressure. That means that the results will depend on the technique employed to measure the Bulk modulus. In the YBaCuO system this parameter has been measured by sound velocity [7-9] and by hydrostatic pressure [10]. In this work it is reported a different method to obtain it, starting from magnetic measurements at room temperature. Similar results can be obtained if one use for the calculation of the Bulk modulus the charge carriers density obtained by any other technique. EXPERIMENTAL Sample preparation Powders of Y2 0,? BaC0 3 , and CuO in the molar ratio Y:Ba:Cu=1:2:3 were well mixed, calcined in oxygen at 800 C for 24 hours and sintered also 24 hours in oxygen at 920 C. In order to obtain samples with different oxygen deficiencies, portions of
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