X.N. Ying

Charge disproportionation in La1-xCaxFeO3−δ (x = 0.4 and 0.5) investigated by mechanical spectroscopy

The nature of charge disproportionation in orthorhombic lanthanum orthoferrites La1-xCaxFeO3-δ (x = 0.4 and 0.5) has been investigated by kilohertz mechanical spectroscopy. Contrast to rhombohedral La1-xSrxFeO3-δ (x = 0.5), no elastic softening was observed in La1-xCaxFeO3-δ above charge disproportionation. This different behavior of elastic modulus in La1-xCaxFeO3-δ and La1-xSrxFeO3-δ unveils the implicit orbital effect. Below charge disproportionation, an internal friction peak around 140 K was observed in La1-xCaxFeO3-δ, which is similar to that in La1-xSrxFeO3-δ. This peak is ascribed to an elastic manifestation of charge freezing and can be used as an indirect indication of CD in doped LaFeO3.

The effect of strain on the low-temperature internal friction of Y(Ba1-xSrx)2Cu3O7-δ

The internal friction of partially Sr-substituted Y(Ba1-xSrx)2Cu3O7-? (Y123) ceramics was measured by the vibrating-reed method from liquid nitrogen temperature to room temperature at kilohertz frequencies. The intensity of the internal friction peak, which appears around 220?K, decreases upon Sr doping while the peak position shows no systematic change. Our results do not support the previous oxygen order-disorder explanation; we present another approach relating this peak to the crossover in charge-carrier dynamics, and its evolution with Sr doping is discussed in terms of release of internal strain and electron-phonon coupling.

The mechanical spectra of glycerol measured by a composite reed vibration method

We extend an experimental procedure to measure the complex Young’s modulus of glycerol from liquid to glassy state, a composite reed vibration method. In this report, a rigorous analysis is obtained to subtract the mechanical spectrum of deposited materials from a composite system consisting of the substrate reed and of materials deposited on it. The dynamic glass transition of glycerol was measured by the mechanical spectra method at two frequencies in the same heating route. The consistency with the mechanical spectrum available in the literature shows the composite reed vibration method to be reliable and effective.

A low-temperature internal friction study of Y124 superconductors

A ceramic sample of Y Ba2Cu4O8 (Y124) was synthesized at atmospheric pressure with the addition of NaNO3 as an enhancer. The internal friction was measured from liquid nitrogen temperature to room temperature at kilohertz frequency using the vibrating reed method. Instead of a broad internal friction peak around 220 K observed in Y Ba2Cu3O7−δ (Y123) a small step in the internal friction spectrum was observed around 200 K with increase of temperature in Y124. This internal friction behaviour is similar to that of Sr-substituted Y Ba0.8Sr1.2Cu3O7−δ. We expect that the internal friction step in Y124 originates from charge carrier crossover. The small size of the change in internal friction around 200 K in Y124 is due to suppression of the lattice response compared with that in undoped Y123.

Effect of zinc substitution on internal friction in YBCO superconductors

The temperature spectra of internal friction for Zn doped ceramic YBa2(Cu1-xZnx)3O6+ are presented with zinc content x varying from 0 to 0.1. Two thermally activated peaks (P1, P2) near the superconducting transition temperature Tc were found to show different behaviours upon zinc doping: the peak at 110 K (P1) decreases rapidly with increasing of x while another one at 120 K (P2) has no significant change. The relaxation mechanism of P1 is interpreted in terms of jumps of apical oxygen atoms between off-centred positions produced by the Jahn-Teller effect. The effect of zinc substitution on the depression of superconductivity is also discussed.

The mechanical relaxation study of polycrystalline MgC1−xNi3

The mechanical relaxation spectra of a superconducting and a nonsuperconducting MgC-{1-x}Ni-3 samples were measured from liquid nitrogen temperature to room temperature at frequency of kilohertz. There are two internal friction peaks (at 300K labeled as P-1 and 125K as P-2) for the superconducting sample. For the nonsuperconducting one, the position of P-1 shifts to 250K, while P-2 is almost completely depressed. It is found that the peak position of P-2 shifts towards higher temperature under higher measuring frequency. The calculated activation energy is 0.13eV. We propose an explanation relating P-2 to the carbon atom jumping among the off-center positions. And further we expect that the behaviors of carbon atoms maybe correspond to the normal state crossovers around 150K and 50K observed by many other experiments.

Normal-state anomalous behaviours studied by the internal friction of YBa2Cu3O7

The internal friction of Ca partially substituted Y1−xCaxBa2Cu3O7−δ ceramics was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. There are two thermally activated relaxation peaks (called P1 and P2 at 95 K and 120 K, respectively). The intensity of P1 almost remains unchanged with Ca substitution, while that of P2 decreases. Another internal friction peak appears around 220 K (called P3). With the increase of Ca content, the intensity of P3 decreases and the peak position shifts toward low temperature. We also have observed that Zn substitution affects P3 much less and Fe substitution seems to result in another contribution to the internal friction around 250 K. We expect that the P3 peak originates from a charge-carrier crossover and possibly has some relationship with the occurrence of the dynamic stripe at low temperature.

Charge ordering transition in (La, Pr)1/3Sr2/3FeO3−δ with different oxygen concentration investigated by mechanical spectroscopy

We present the results of the kilohertz mechanical spectrum of oxygen-varied La 1/3 Sr 2/3 FeO 3− δ and Pr 1/3 Sr 2/3 FeO 3− δ . On cooling, a substantial modulus softening above the charge ordering transition temperature T CO , and an abrupt stiffness and step-like decrease of the internal friction below T CO were observed in oxygen stoichiometric La 1/3 Sr 2/3 FeO 3− δ . No step-like change of the mechanical spectrum was observed in charge-ordered Pr 1/3 Sr 2/3 FeO 3− δ , which shows the absence of dynamic Jahn-Teller distortions in Pr 1/3 Sr 2/3 FeO 3− δ . Another modulus softening below was observed in charge-ordered (La, Pr) 1/3 Sr 2/3 FeO 3− δ , which was used to discriminate the charge ordering transition from the charge disproportionation transition in perovskite iron oxides.

The normal-state anomalous behaviours studied by internal friction of partially Sr-substituted Y123 in the underdoped range

The internal friction of partially Sr-substituted Y(Ba1?xSrx)2Cu3O7?? (x = 0.1, 0.2, and 0.5) ceramics with different oxygen content was measured using the vibrating-reed method from liquid nitrogen temperature to room temperature at kilohertz frequencies. A broad mechanical-energy dissipation peak and a sharp one were observed around 220 K. The position of the broad internal-friction peak does not show a systematic change with the oxygen content, and its intensity decreases after depleting a great amount of oxygen. For the sample with high oxygen concentration, another broad peak of lower intensity appeared around 150 K, which might relate to a microstructure change. At the same time, we have not found any anomalous behaviour of internal friction between 90 and 200 K for the sample with lower oxygen content, where the opening of a pseudogap for underdoped YBa2Cu3O7?? has been observed in many other experiments. Our results provide more information about the normal-state properties of high-temperature superconductors.

Effect of zinc doping on the microstructure in YBCO

Abstract XRD experiment was conducted on zinc substituted YBCO ceramics in the temperature range 90–270K. In undoped or lightly doped samples, the abrupt change of lattice parameter at several temperatures and the large average temperature derivative of lattice parameter, originating from both thermal expansion and subtle changes of lattice parameter, give evidences for the onset and development of CuO 5 tilting upon cooling. Such titling was suppressed by zinc doping and this effect is discussed in favor of the strong and expanded nature of the disturbance on electronic states created by Zn that is essential for tilting mechanism of CuO 5 pyramids.