Jingqin Shen

Enhanced thermopower in an intergrowth cobalt oxide Li0.48Na0.35CoO2

We report the measurements of thermopower, electrical resistivity and thermal conductivity in a complex cobalt oxide Li0.48Na0.35CoO2, whose crystal structure can be viewed as an intergrowth of the O3 phase of LixCoO2 and the P2 phase of NayCoO2 along the c-axis. The compound shows a large room-temperature thermopower of ~180??V?K?1, which is substantially higher than those of LixCoO2 and NayCoO2. The figure of merit for the polycrystalline sample increases rapidly with increasing temperature, and achieves nearly 10?4?K?1 at 300?K, suggesting that the LixNayCoO2 system is a promising candidate for thermoelectric applications.

The effect of a charge-density wave transition on the transport properties of 2H-NbSe2

Single crystals of two-dimensional 2H-NbSe2, which undergoes a superconducting transition at Tc = 7.2 K and a charge-density wave (CDW) transition at TCDW = 30 K, were synthesized. Measurements of the resistivity, Hall coefficient and magnetoresistance (MR) versus temperature were performed on NbSe2 crystals with different resistance residual ratios (RRR = R(300 K)/R(8 K)), chosen from different batches. The superconducting transition temperature hardly changes with the RRR, while the MR and Hall coefficient are strongly dependent on the RRR values of the samples. Moreover, the temperature and field dependence of the MR violate Kohlers rule, indicating that the scattering times of the charge carriers are no longer isotropic. A steep decrease of the Hall coefficient RH below 50 K is only observed for the high quality sample; this can be interpreted in terms of a drastic increase of the mean free path for the electron-type charge carriers. The effect of the CDW transition on the Hall coefficient is discussed using a two-band model and a sharp change in the scattering rate on an electron-like orbit below TCDW is suggested.

Study of high temperature resistivity and thermal stability of superconductor MgB2

Abstract The single phase and polycrystalline MgB 2 superconductor was synthesized by the Mg diffusion method and its temperature dependence of resistivity was studied from T c up to 460 K. The normal state temperature dependence of electrical resistivity is linear above 270 K and can be fitted well by the Bloch–Gruneisen expression with the Debye temperature θ D =1200 K for the temperature range from 40 to 450 K. There is an unexpected abrupt jump in ρ ( T ) curve near 450 K. Thermal-gravity and differential thermal-analysis (TG–DTA) indicated this jump of ρ ( T ) could be caused by the oxidation of few Mg impurities in air atmosphere.

The magnetoresistance of the quasi-one-dimensional conductor NbSe3

The temperature dependence of the resistivity and magnetoresistance (MR) of quasi-one-dimensional NbSe3 was studied. The sharp increase of the positive MR in the lower charge-density wave (CDW) phase is consistent with previous reports, and the violation of Kohlers rule is obvious. We found that the MR data can be fitted very well with a modified two-band model, and the temperature dependence of the resulting parameters was discussed. The MR, the effect of magnetic field on the CDW gap, and the thermoelectric power of NbSe3 can be coherently understood within this model.

THERMAL STABILITY AND TRANSPORT PROPERTIES OF Na0.495CoO2 SINGLE CRYSTALS

Single crystals of Na0.495CoO2 were obtained through a flux method followed by de-intercalation of sodium. The resistivity (ρ) and thermal stability were studied for the de-intercalated samples exposed to humid air. It was found that the de-intercalated Na0.495CoO2 crystal is vulnerable to water in the air, and a hydration process in which H2O molecules fill oxygen vacancies in CoO2 layers instead of being intercalated between CoO2 layers is suggested to be responsible for the remarkable change in resistivity induced by water absorption. The Na0.495CoO2 sample shows a weak localization of carriers just below 200 K and Co3+–Co4+ charge ordering at about 40 K, lower than the corresponding charge ordering transition temperature reported in Na0.5CoO2. The lower charge ordering transition temperature might be caused by the slight deviation of Na content (x) from 0.5.

MAGNETORESISTANCE AND HALL EFFECT OF TWO-DIMENSIONAL 2H-NbSe2

Single crystals of two-dimensional 2H-NbSe2, which undergo superconducting transition at Tc=7.1K and charge-density wave (CDW) transition at TCDW=30K, were synthesized. The resistance residual ratio (RRR=R(300K)/R(8K)) of the samples from different batches varies from 10 to 30, but the superconducting properties hardly change with RRR. The magneto-resistance (MR)is generally small and its temperature and field dependence of MR violate the Kohlers rule that holds for many conventional metals, indicating that the scattering times of charge carriers are no longer isotropic. The steep decrease of Hall coefficient RH below 50K can be interpreted by the drastic increase of the mean-free-path of electron type charge carrier. The influence of CDW transition on the resistivity and Hall effect is discussed in a two-band model.