Y. C. Liao

Magnetic field dependence of low-temperature specific heat of the spinel oxide superconductor LiTi2O4

Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi_2O_4 has been elaborately investigated. In the normal state, the obtained electronic coefficient of specific heat gamma_n = 19.15 mJ/mol K2, the Debye temperature is 657 K and some other parameters are compared with those reported earlier. The superconducting transition at Tc ~ 11.4 K is very sharp (DeltaTc ~ 0.3 K) and the estimated delataC/gamma_nTc is ~ 1.78. In the superconducting state, the best fit of data leads to the electronic specific heat C_es/Gamma_nTc = 9.87 exp (-1.58 Tc/T) without field and gamma(H) ~ H^0.95 with fields. In addition, Hc2(0) ~ 11.7 T, Hc(0) ~ 0.32 T, xi_GL(0) ~ 5.5 nm, lambda_GL(0) ~ 160 nm, and Hc1(0) ~ 26 mT are estimated from Werthamer-Helfand-Hohenberg (WHH) theory or other relevant relations. All results from the present study indicate that LiTi2O4 can be well described by a typical type-II, BCS-like, moderate coupling, and fully gapped superconductor in the dirty limit. It is further suggested that LiTi2O4 is a moderately electron-electron correlated system.

The Preparation Effect of Li1+xTi2O4 and Its Aging Effect

We have successfully prepared the spinel Li1+xTi2O4, which is a type II superconductor with Tc∼12K, by solid-state reaction process. Owing to the volatility of lithium, excess lithium precursor up to molar fraction 0.15 was added during the synthesis to result in single phase Li1+xTi2O4 without any impurity phase in high resolution XRD pattern. The normal state temperature dependence of resistivity shows an initial semiconductor-like behavior, but turn into metallic-like at about 200K. Magnetization measurements show bulk superconductivity in the fresh samples. The sample is unstable to the exposure of air and humidity. It is observed that the resistive transition quickly diminishes with the exposure to air. However, the magnetic transition remains as sharp, only with the decrease in its superconducting volume fraction, even when the resistive transition completely disappears. An anomalous metal-insulator transition in the aged sample is observed at the same temperature as the superconducting transition temperature of the fresh sample.

Effect of different nanoscale RE2BaCuO5 additions on the formation of compositional fluctuation in Sm-Ba-Cu-O superconducting bulk materials

This study presents the effect of different nanoscale RE211 additions—Y 2 BaCuO 5 (nmY211), Sm 2 BaCuO 5 (nmSm211), and Nd 4 Ba 2 Cu 2 O 10 (nmNd422)—on the nano-scale compositional fluctuation and associated pinning mechanism of the melt-textured growth (MTG) Sm–Ba–Cu–O [SmBCO, of which the composition is Sm123 (Sm 1 Ba 2 Cu 3 O y ) + 25 wt% Sm211 (Sm 2 BaCuO 5 )] superconducting bulk materials. The superconductivity and microstructure results indicated that in samples with the addition of these nano-sized particles, nanoscale compositional fluctuations form during the peritectic transformation of melt-growth process, which cause T c variation on a nanoscale and result in the formation of δ T c pinning centers at high magnetic field. The compositional fluctuation regions (δ T c pinning centers) are altered by the difference in peritectic temperature, the solubility in the liquid phase, and the ion radius. The direct current transport R - T properties elucidate the change of flux pinning behavior. In addition, the different influence on microstructure and superconductivity between the two methods: mixing rare-earth elements in nanoscale RE211 or in the homogeneous mixed precursor powders [e.g., (Nd, Eu, Gd)–Ba–Cu–O (NEG)] is also discussed.

Elastic stiffness of single-crystalline FeSe measured by picosecond ultrasonics

We report investigations on the elasticity of superconducting FeSe using picosecond ultrasonic technique. The tetragonal (001) FeSe thin film, deposited on a processed SrTiO3 substrate by the pulsed laser deposition, exhibits distinct c-axis preferred orientation and single-crystalline features as a result of the x-ray diffraction. The high-quality crystallinity thus enables quantitative examinations of anisotropic stiffness coefficients (C33) of FeSe, correlating to the interatomic interaction in the simplest iron-based superconductor. Our experiment indicates a room-temperature C33 of 40.9 ± 0.4 GPa and material stiffening of 4.3% at low temperatures, which can be explained by the weakening of anharmonic phonon–phonon interactions.

Concentration effect on high field pinning and transport behaviors in nano-Y2BaCuO5 doped Sm–Ba–Cu–O superconductor

Recent studies indicate that the addition of nanoscale RE2BaCuO5 (nmRE211) (RE=Y, Sm, and Nd) in melt-texture-grown Sm–Ba–Cu–O bulk material can enhance the critical current density (Jc), especially in high magnetic fields. It is known that the δTc pinning is the reason for high field peak effect in Jc‐H curves. The Jc‐H curves exhibit enhancement compared with those of undoped samples. The analysis of first derivations of R‐T curves found two peaks in dR∕dT curves representing two characteristic temperatures in high magnetic fields, where the lower temperature is attributed to the weak superconducting phase. The relative strength of the two peaks varies with the concentration of nmY211 additions. This argument is consistent with microstructural observations and pinning analysis results.

The Transport Properties of Li(1+x)Ti2O4 in High Magnetic Field

The spinel oxide superconductor LiTi2O4 shows intricate transport properties in both normal state and superconducting state. The Hall effect measurements exhibit an abrupt change in carrier density near T=170 K where the temperature derivative of resistivity changes sign from negative to positive on cooling. In addition, the sample shows significant positive magnetoresistance below 40 K, e.g., ΔR(H)/R(H=0)∼3% at T=14 K and H=16 T. A most interesting feature is the magnetic field effect on the resistivity near the superconducting state. In addition to the field-induced broadening of R(T), there exhibits an anomalous upturn in R(T) near the superconducting regime when magnetic field is above 3T. The temperature T1 at which R(T) shows upturn linearly correlates with magnetic field H and can be expressed as T1/TC(H=0)=1−0.04H for 3T<H<12T. The superconductivity is completely suppressed at field above 14T and the low temperature resistance behaves semiconductor-like.