An ultrasonic study of superconducting and non-superconducting GdBa₂Cu₃O_7-x

Abstract

Detailed measurements are presented of ultrasonic wave velocity and attenuation in the orthorhombic, superconducting and the tetragonal, non-superconducting forms of GdBa2Cu30,-,. Theorthorhombicstate datadisplay asharpelastictransition at about 240 K which is absent from that of the tetragonal state; however, x-ray diffraction measurements show no evidence of a structural phase transition. The lattice thermal expansion is found to be fairly isotropic. The elastic response in the superconducting state is found to be dominated by conventional anharmonic and anelastic phenomena: no effects that can be specifically attributed to the formation of the superconducting state have been detected. The ultrasonic velocity and attenuation data are explained by the presence in the tetragonal state of one and in the orthorhombic state of two thermally activated anelastic relaxation mechanisms which are attributed to various oxygen vacancy hopping processes.

Effect of Ba-site substitution by Sr on ultrasonic velocity and electron–phonon coupling constant of DyBa2−xSrxCu3O7−δ superconductors

“…The  D for superconducting DyBa 2 Cu 3 O 7−␦ was 397 K and is comparable to acoustic Debye temperature values reported for other REBa 2 Cu 3 O 7−␦ superconductors [16,17,28]. The increase in absolute longitudinal and shear velocities at 80 K with x may be related to the decrease of cell volume as a result of substitution of smaller Sr 2+ (ionic radius: 1.25 Å) for Ba 2+ (ionic radius: 1.42 Å).…”

“…Table 3. RE123 materials. Ultrasonic velocity measurements in oxygenated and oxygen depleted RE123 samples consistently demonstrated that step-like anomalies above 200 K disappeared after oxygen reduction confirming the oxygen related character of the anomaly [16,17,28,42]. Nevertheless, previous ultrasonic report on differently annealed Gd1113 samples which is isostructural to RE123, indicates that oxygen content is not the key factor influencing the anomaly [43].…”

“…3(b)) and DyBa 1.4 Sr 0.6 Cu 3 O 7−␦ (Fig. 3(c)) are generally similar with aver- age grains sizes between 4 m and 6 m. The correction for porosity was achieved by comparing the measured sample density to the non-porous sample density derived from X-ray diffraction data [16]. The ideal sound velocity was calculated from the measured velocity using the relation suggested in Ref.…”

“…As a comparison to the measured temperature dependent sound velocity curves, one of several anharmonicity models [31,36,37] can be plotted together with the velocity curves to illustrate the extent of deviation of the velocity behavior from the calculated curve at high temperatures [16,38]. The simple model proposed by Lakkad [31] which uses a phenomenological model of anharmonic oscillator together with the Debye lattice vibration spectrum was chosen in this work.…”

“…The ultrasonic method, which is sensitive to lattice instabilities and phase transitions, was found to be very useful to probe bulk property changes of ceramic materials such as glasses [8][9][10][11], magnetoresistance materials [12][13][14][15], and also high-temperature superconductors [16-24 and references therein]. In the latter, step-like velocity anomalies above 200 K indicating sudden or unexpected elastic hardening taking place in superconducting samples have been observed in several RE123 [16,17], RE1113 [20] and Bi-based samples [18,21]. The step-like anomalies were found to be oxygen related but its relationship to superconductivity is still not clear [23].…”