arXiv subjects

Yayu Wang

Publications and source records attributed to Yayu Wang.

113 records · Page 7Linked to original sources

arXiv2003

Chemical Instability of the Cobalt Oxyhydrate Superconductor under Ambient Conditions

The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 angstroms implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 angstroms at nominally constant chemical doping.

cond-mat.supr-con↗

arXiv2002

The high-field phase diagram of the cuprates derived from the Nernst effect

Measurements of the Nernst signal in the vortex-liquid state of the cuprates to high fields (33 T) reveal that vorticity extends to very high fields even close to the zero-field critical temperature $T_{c0}$. In overdoped $\rm La_{2-x}Sr_xCuO_4$ (LSCO) we show that the upper critical field $H_{c2}(T)$ curve does not end at $T_{c0}$, but at a much higher temperature. These results imply that $T_{c0}$ corresponds to a loss in phase rigidity rather than a vanishing of the pairing amplitude. An intermediate field $H^*(T)\ll H_{c2}(T)$ is shown to be the field scale for the flux-flow resistivity.

cond-mat.supr-con↗

arXiv2001

Particle-hole symmetry in the antiferromagnetic state of the cuprates

In the layered cuprate perovskites, the occurence of high-temperature superconductivity seems deeply related to the unusual nature of the hole excitations. The limiting case of a very small number of holes diffusing in the antiferromagnetic (AF) background may provide important insights into this problem. We have investigated the transport properties in a series of crystals of $\rm YBa_2Cu_3O_y$, and found that the temperature dependences of the Hall coefficient $R_H$ and thermopower $S$ change abruptly as soon as the AF phase boundary is crossed. In the AF state at low temperatures $T$, both $R_H$ and $S$ are unexpectedly suppressed to nearly zero over a broad interval of $T$. We argue that this suppression arises from near-exact symmetry in the particle-hole currents. From the trends in $R_H$ and $S$, we infer that the symmetry is increasingly robust as the hole density $x$ becomes very small ($x\simeq 0.01$). We discuss implications for electronic properties both within the AF state and outside.

cond-mat.str-el↗

arXiv2001

The onset of the vortex-like Nernst signal above Tc in La_{2-x}Sr_xCuO_4 and Bi_2Sr_{2-y}La_yCuO_6

The diffusion of vortices down a thermal gradient produces a Josephson signal which is detected as the vortex Nernst effect. In a recent report, Xu et al., Nature 406, 486 (2000), an enhanced Nernst signal identified with vortex-like excitations was observed in a series of La_{2-x}Sr_xCuO_4 (LSCO) crystals at temperatures 50-100 K above T_c. To pin down the onset temperature T_ν of the vortex-like signal in the lightly doped regime (0.03 < x < 0.07), we have re-analyzed in detail the carrier contribution to the Nernst signal. By supplementing new Nernst measurements with thermopower and Hall-angle data, we isolate the off-diagonal Peltier conductivity α_{xy} and show that its profile provides an objective determination of T_ν. With the new results, we revise the phase diagram for the fluctuation regime in LSCO to accomodate the lightly doped regime. In the cuprate Bi_2Sr_{2-y}La_yCuO_6, we find that the carrier contribution is virtually negligible for y in the range 0.4-0.6. The evidence for an extended temperature interval with vortex-like excitations is even stronger in this system. Finally, we discuss how T_ν relates to the pseudogap temperature T* and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.

cond-mat.str-el↗

arXiv2001

Low Temperature Fabrication of MgB2

We report the fabrication of MgB2 with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 C. Mg deficiencies of the type Mg1-xB2 were tested. Tc was found to decrease by about 1K at large x, though the amount of non-stoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25-30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB2 with bulk properties in in-situ thin films at temperatures of 600 C or less.

cond-mat.supr-con↗