Unusual T_c variation with hole concentration in Bi_2Sr_{2-x}La_xCuO_{6+\delta}

We have investigated the $T_c$ variation with the hole concentration $p$ in the La-doped Bi 2201 system, Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$. It is found that the Bi 2201 system does not follow the systematics in $T_c$ and $p$ observed in other high-$T_c$ cuprate superconductors (HTSC's). The $T_c$ vs $p$ characteristics are quite similar to what observed in Zn-doped HTSC's. An exceptionally large residual resistivity component in the inplane resistivity indicates that strong potential scatterers of charge carriers reside in CuO$_2$ planes and are responsible for the unusual $T_c$ variation with $p$, as in the Zn-doped systems. However, contrary to the Zn-doped HTSC's, the strong scatter in the Bi 2201 system is possibly a vacancy in the Cu site.Comment: RevTeX, 3 figures, to be published in the Physical Review

Quasiparticle Excitation in the Superconducting Pyrochlore Cd2Re2O7 Probed by Muon Spin Rotation

The quasiparticle excitations in the mixed state of Cd2Re2O7 have been studied by means of muon spin rotation/relaxation. The temperature dependence of the magnetic penetration depth (lambda) is consistent with a nearly isotropic superconducting order parameter, although a slight discrepancy which is dependent on the details in the analysis may be present. This is also supported by the relatively weak field dependence of lambda.Comment: 12 pages, 2 figures, to appear in J. Phys. Soc. Jp

Cost-effectiveness of different monitoring strategies in a screening and treatment programme for hepatitis B in The Gambia

Background: Clinical management of chronic hepatitis B virus (HBV) infection is complex and access to antiviral treatment remains limited in sub-Saharan Africa. International guidelines recommend monitoring at least annually for disease progression among HBV-infected people not meeting treatment criteria at initial diagnosis. This study aimed to assess the impact and cost-effectiveness of alternative strategies for monitoring. Methods: We used a mathematical model of HBV transmission and natural history, calibrated to all available West African data, to project the population-level health impact, costs and cost-effectiveness of different monitoring strategies for HBV-infected individuals not initially eligible for antiviral treatment. We assumed that these patients were found in the year 2020 in a hypothetical community-based screening programme in The Gambia. Monitoring frequencies were varied between every 5 and every 1 year and targeted different age groups. Results: The currently recommended annual monitoring frequency was likely to be not cost-effective in comparison with other strategies in this setting. 5-yearly monitoring in 15-45-year olds, at US$338 per disability-adjusted life year averted, had the highest probability of being the most effective cost-effective monitoring strategy. Conclusions: Monitoring less frequently than once a year is a cost-effective strategy in a community-based HBV screening and treatment programme in The Gambia, with the optimal strategy depending on the cost-effectiveness threshold. Efficiencies may be gained by prioritising the 15-45-year age group for more intensive monitoring

Spin Dynamics of the Magnetoresistive Pyrochlore Tl_2Mn_2O_7

Neutron scattering has been used to study the magnetic order and spin dynamics of the colossal magnetoresistive pyrochlore Tl_2Mn_2O_7. On cooling from the paramagnetic state, magnetic correlations develop and appear to diverge at T_C (123 K). In the ferromagnetic phase well defined spin waves are observed, with a gapless ($\Delta <0.04$ meV) dispersion relation E=Dq^{2} as expected for an ideal isotropic ferromagnet. As T approaches T_C from low T, the spin waves renormalize, but no significant central diffusive component to the fluctuation spectrum is observed in stark contrast to the La$_{1-x}$(Ca,Ba,Sr)$_x$MnO$_3$ system. These results argue strongly that the mechanism responsible for the magnetoresistive effect has a different origin in these two classes of materials.Comment: 4 pages (RevTex), 4 figures (encapsulated postscript), to be published in Phys. Rev. Let

Electronic transport and optical properties of proton-implanted amorphous 2CdO Á GeO 2 ®lms

Abstract Films of amorphous 2CdO Á GeO 2 with the band gap of 3.4 eV were prepared by rf sputtering. Protons were implanted into the ®lms at doses 2 Â 10 14 ±2 Â 10 16 cm À2 . On going from 2 Â 10 14 to 2 Â 10 16 cm À2 , dc conductivity at 300 K increased from $10 À9 to$ 10 1 S cm À1 and its activation energy fell from $1 eV to almost zero (degenerate state). This result indicates that the Fermi level of this amorphous material may be controlled by proton implantation. The sign of Hall and Seebeck coecients were negative, showing n-type electrical conduction and no pn sign anomaly in Hall voltages. The Hall mobility was of the order of 10 cm 2 V À1 s À1 (even at carrier concentration of$ 3 Â 10 19 cm À3 ), which is larger by several orders of magnitude than that of existing amorphous semiconductors. No degradation of visible transparency was observed in all implanted samples. The optical conductivity may be described by the classical Drude formula with a single relaxation time, 2X7 Â 10 À15 s. X-ray radial distribution function revealed that the local structure around Cd 2 (coordination number; $6) and Ge 4 ($ 4) in the amorphous state is close to that of crystalline Cd 2 GeO 4 and the amorphous state has a distribution of Cd±O±Cd bond angles. We assume that the electronic transport properties of the present material primarily originate from the extended conduction bands composed of Cd 5s orbitals.

Correlated local distortions of the TlO layers in Tl2_2Ba2_2CuOy_{y}: An x-ray absorption study

We have used the XAFS (x-ray-absorption fine structure) technique to investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic Tl-2201 with a superconducting transition temperature T$_c$=60 K. Our results clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as given by the diffraction measurements, while the Tl and O(3) atoms are more disordered than suggested by the average crystal structure. The Tl-Tl distance at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at 3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33 \AA, significantly shorter than the distance calculated with both the Tl and O(3) atoms at their ideal $4e$ sites ( $x=y=$0 or $\frac{1}{2}$). A model based on these results shows that the Tl atom is displaced along the directions from its ideal site by about 0.11 \AA; the displacements of neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$ site by about 0.53 \AA{ } roughly along the directions. A comparison of the Tl L$_{III}$-edge XAFS spectra from three samples, with T$_c$=60 K, 76 K, and 89 K, shows that the O environment around the Tl atom is sensitive to T$_c$ while the Tl local displacement is insensitive to T$_c$ and the structural symmetry. These conclusions are compared with other experimental results and the implications for charge transfer and superconductivity are discussed. This paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.

Large Thermoelectric Power Factor in TiS2 Crystal with Nearly Stoichiometric Composition

A TiS$_{2}$ crystal with a layered structure was found to have a large thermoelectric power factor.The in-plane power factor $S^{2}/ \rho$ at 300 K is 37.1~$\mu$W/K$^{2}$cm with resistivity ($\rho$) of 1.7 m$\Omega$cm and thermopower ($S$) of -251~$\mu$V/K, and this value is comparable to that of the best thermoelectric material, Bi$_{2}$Te$_{3}$ alloy. The electrical resistivity shows both metallic and highly anisotropic behaviors, suggesting that the electronic structure of this TiS$_{2}$ crystal has a quasi-two-dimensional nature. The large thermoelectric response can be ascribed to the large density of state just above the Fermi energy and inter-valley scattering. In spite of the large power factor, the figure of merit, $ZT$ of TiS$_{2}$ is 0.16 at 300 K, because of relatively large thermal conductivity, 68~mW/Kcm. However, most of this value comes from reducible lattice contribution. Thus, $ZT$ can be improved by reducing lattice thermal conductivity, e.g., by introducing a rattling unit into the inter-layer sites.Comment: 11 pages, 4 figures, to be published in Physical Review

Magnetic anisotropy, first-order-like metamagnetic transitions and large negative magnetoresistance in the single crystal of Gd2_{2}PdSi3_3

Electrical resistivity ($\rho$), magnetoresistance (MR), magnetization, thermopower and Hall effect measurements on the single crystal Gd$_{2}$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure are reported. The well-defined minimum in $\rho$ at a temperature above N\'eel temperature (T$_N$= 21 K) and large negative MR below $\sim$ 3T$_N$, reported earlier for the polycrystals, are reproducible even in single crystals. Such features are generally uncharacteristic of Gd alloys. In addition, we also found interesting features in other data, e.g., two-step first-order-like metamagnetic transitions for the magnetic field along [0001] direction. The alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.Comment: RevTeX, 5 pages, 6 encapsulated postscript figures; scheduled to be published in Phy. Rev. B (01 November 1999, B1