Thermal Conductivity of Pr_{1.3-x}La_{0.7}Ce_xCuO_4 Single Crystals and Signatures of Stripes in an Electron-Doped Cuprate

It was recently demonstrated that the anisotropic phonon heat transport behavior is a good probe of the stripe formation in La_{2-x}Sr_xCuO_4 (LSCO) [X. F. Sun {\it et al.}, Phys. Rev. B {\bf 67}, 104503 (2003)]. Using this probe, we examined an electron-doped cuprate Pr_{1.3-x}La_{0.7}Ce_xCuO_4 (PLCCO) and found that essentially the same features as those in LSCO are observed. Moreover, the in-plane resistivity \rho_{ab} of lightly-doped PLCCO shows metallic behavior (d\rho_{ab}/dT > 0) in the N\'eel ordered state with a mobility comparable to that in LSCO. It is discussed that these peculiar properties in common with LSCO signify the existence of stripes in electron-doped cuprates.Comment: 4 pages, 4 figures, revised version accepted for publication in Phys. Rev. Let

Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2

We have performed low-temperature specific heat $C$ and thermal conductivity $\kappa$ measurements on the Ni-pnictide superconductors BaNi$_2$As$_2$ ($T_\mathrm{c}$=0.7 K and SrNi$_2$P$_2$ ($T_\mathrm{c}$=1.4 K). The temperature dependences $C(T)$ and $\kappa(T)$ of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual $\kappa$ for BaNi$_2$As$_2$ rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba$_{0.6}$La$_{0.4}$Ni$_2$As$_2$ shows a mild suppression of $T_\mathrm{c}$ and $H_\mathrm{c2}$ relative to BaNi$_2$As$_2$.Comment: 5 pages, 3 figures, to be published in J. Phys.: Conf. Se

Electronic inhomogeneity and competing phases in electron-doped superconducting Pr0.88LaCe0.12CuO4

We use neutron scattering to demonstrate that electron-doped superconducting Pr0.88LaCe0.12CuO4 in the underdoped regime is electronically phase separated in the ground state, showing the coexistence of a superconducting phase with a three-dimensional antiferromagnetically ordered phase and a quasi-two-dimensional spin density wave modulation. The Neel temperature of both antiferromagnetic phases decreases linearly with increasing superconducting transition temperature (Tc) and vanishes when optimal superconductivity is achieved. These results indicate that the electron-doped copper oxides are close to a quantum critical point, where the delicate energetic balance between different competing states leads to microscopic heterogeneity.Comment: 14 pages, 4 figures, accepted to Phys. Rev. B as a rapid communicatio

Tension term, interchange symmetry, and the analogy of energy and tension laws of the AdS soliton solution

In this paper, we reconsider the energy and tension laws of the Ricci flat black hole by taking the contribution of the tension term into account. After this considering and inspired by the interchange symmetry between the Ricci flat black hole and the AdS soliton solution which arises from the double analytic continuation of the time and compact spatial direction, we find out the analogy of the energy and tension laws of the AdS soliton solution. Moreover, we also investigate the energy and tension laws of the boosted Ricci flat black hole, and discuss the boosted AdS soliton solution. However, although there is the same interchange symmetry between the boosted Ricci flat black hole and boosted AdS soliton, the analogy of laws of the boosted AdS soliton solution may be of no sense for the existence of the closed timelike curves and conical singularity. In spite of that, the conserved charges such as the energy and momentum of the boosted AdS soliton are well-defined, and an interesting result is that its energy is lower than that of the static AdS soliton. On the other hand, note that although the laws obtained above are the same as those of the asymptotically flat case, the underlying deduced contents are different. Thus, our results could also be considered as a simple generalization to the asymptotically AdS case. Moreover, during the calculation, we find that there may be a new way to define the gravitational tension which can come from the quasi-local stress tensor of the counter-term method.Comment: V4: 15 pages, no figure, version to appear in JHE

A note on Hawking radiation via complex path analysis

As long as we neglect backreaction, the Hawking temperature of a given black hole would not depend upon the parameters of the particle species we are considering. In the semiclassical complex path analysis approach of Hawking radiation, this has been verified by taking scalar and Dirac spinors separately for different stationary spacetime metrics. Here we show, in a coordinate independent way that, for an arbitrary spacetime with any number of dimensions, the equations of motion for a Dirac spinor, a vector, spin-$2$ and spin-$\frac{3}{2}$ fields reduce to Klein-Gordon equations in the WKB semiclassical limit. We then obtain, under some suitable assumptions, the complex solutions of those resulting scalar equations across the Killing horizon of a stationary spacetime to get a coordinate independent expression for the emission probability identical for all particle species. Finally we consider some explicit examples to demonstrate the validity of that expression.Comment: 12 pages, v2; manuscript divided into sections, many discussions and references adde

"Spin-Flop" Transition and Anisotropic Magnetoresistance in Pr_{1.3-x}La_{0.7}Ce_{x}CuO_{4}: Unexpectedly Strong Spin-Charge Coupling in Electron-Doped Cuprates

We use transport and neutron-scattering measurements to show that a magnetic-field-induced transition from noncollinear to collinear spin arrangement in adjacent CuO_{2} planes of lightly electron-doped Pr_{1.3-x}La_{0.7}Ce_{x}CuO_{4} (x=0.01) crystals affects significantly both the in-plane and out-of-plane resistivity. In the high-field collinear state, the magnetoresistance (MR) does not saturate, but exhibits an intriguing four-fold-symmetric angular dependence, oscillating from being positive at B//[100] to being negative at B//[110]. The observed MR of more than 30% at low temperatures induced by a modest modification of the spin structure indicates an unexpectedly strong spin-charge coupling in electron-doped cuprates.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let

Naked singularity resolution in cylindrical collapse

In this paper, we study the gravitational collapse of null dust in the cylindrically symmetric spacetime. The naked singularity necessarily forms at the symmetry axis. We consider the situation in which null dust is emitted again from the naked singularity formed by the collapsed null dust and investigate the back-reaction by this emission for the naked singularity. We show a very peculiar but physically important case in which the same amount of null dust as that of the collapsed one is emitted from the naked singularity as soon as the ingoing null dust hits the symmetry axis and forms the naked singularity. In this case, although this naked singularity satisfies the strong curvature condition by Kr\'{o}lak (limiting focusing condition), geodesics which hit the singularity can be extended uniquely across the singularity. Therefore we may say that the collapsing null dust passes through the singularity formed by itself and then leaves for infinity. Finally the singularity completely disappears and the flat spacetime remains.Comment: 17 pages, no figur