Imaging ionospheric inhomogeneities using spaceborne synthetic aperture radar

We present a technique and results of 2-D imaging of Faraday rotation and total electron content using spaceborne L band polarimetric synthetic aperture radar (PolSAR). The results are obtained by processing PolSAR data collected using the Phased Array type L-band Synthetic Aperture Radar (PALSAR) on board the Advanced Land Observation Satellite. Distinguished ionospheric inhomogeneities are captured in 2-D images from space with relatively high resolutions of hundreds of meters to a couple of kilometers in auroral-, middle-, and low-latitude regions. The observed phenomena include aurora-associated ionospheric enhancement arcs, the middle-latitude trough, traveling ionospheric disturbances, and plasma bubbles, as well as ionospheric irregularities. These demonstrate a new capability of spaceborne synthetic aperture radar that will not only provide measurements to correction of ionospheric effects in Earth science imagery but also significantly benefit ionospheric studies

Vortex Phase Diagram of Layered Superconductor Cu0.03TaS2 for H || c

The magnetization and anisotropic electrical transport properties have been measured in high quality Cu0.03TaS2 single crystal. A pronounced peak effect has been observed, indicating that the high quality and homogeneity are vital to peak effect. A kink has been observed in the magnetic field H dependence of the in-plane resistivity {\rho}ab for H || c, which corresponds to a transition from activated to diffusive behavior of vortex liquid phase. In the diffusive regime of the vortex liquid phase, the in-plane resistivity {\rho}ab shows {\rho}ab $\propto$ H0.3 relation, which does not follow the Bardeen-Stephen law for free flux flow. Finally, a simplified vortex phase diagram of Cu0.03TaS2 for H || c is given.Comment: 28 pages, 9 figure

Statistics of Lyapunov exponent in one-dimensional layered systems

Localization of acoustic waves in a one dimensional water duct containing many randomly distributed air filled blocks is studied. Both the Lyapunov exponent and its variance are computed. Their statistical properties are also explored extensively. The results reveal that in this system the single parameter scaling is generally inadequate no matter whether the frequency we consider is located in a pass band or in a band gap. This contradicts the earlier observations in an optical case. We compare the results with two optical cases and give a possible explanation of the origin of the different behaviors.Comment: 6 pages revtex file, 6 eps figure

Anomalies of upper critical field in the spinel superconductor LiTi2_2O4−δ_{4-\delta}

High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi$_2$O$_{4-\delta}$ films with various oxygen content. We find that the upper critical field $B_\mathrm{c2}$ gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an astonishing isotropic $B_\mathrm{c2}$ up to $\sim$ 26 Tesla is observed in oxygen-rich sample, which is doubled compared to the anoxic sample and breaks the Pauli limit. Such anomalies of $B_\mathrm{c2}$ were rarely reported in other three dimensional superconductors. Combined with all the anomalies, three dimensional spin-orbit interaction induced by tiny oxygen impurities is naturally proposed to account for the remarkable enhancement of $B_\mathrm{c2}$ in oxygen-rich LiTi$_2$O$_{4-\delta}$ films. Such mechanism could be general and therefore provides ideas for optimizing practical superconductors with higher $B_\mathrm{c2}$

Field Driven Quantum Criticality in the Spinel Magnet ZnCr2_2Se4_4

We report detailed dc and ac magnetic susceptibilities, specific heat, and thermal conductivity measurements on the frustrated magnet ZnCr$_2$Se$_4$. At low temperatures, with increasing magnetic field, this spinel material goes through a series of spin state transitions from the helix spin state to the spiral spin state and then to the fully polarized state. Our results indicate a direct quantum phase transition from the spiral spin state to the fully polarized state. As the system approaches the quantum criticality, we find strong quantum fluctuations of the spins with the behaviors such as an unconventional $T^2$-dependent specific heat and temperature independent mean free path for the thermal transport. We complete the full phase diagram of ZnCr$_2$Se$_4$ under the external magnetic field and propose the possibility of frustrated quantum criticality with extended densities of critical modes to account for the unusual low-energy excitations in the vicinity of the criticality. Our results reveal that ZnCr$_2$Se$_4$ is a rare example of 3D magnet exhibiting a field-driven quantum criticality with unconventional properties.Comment: 6 pages, 4 figures + supplementary: 2 pages, 1 figure; accepted for publication in Phys. Rev. Let

Incommensurate spin correlations in highly oxidized cobaltates La2−x_{2-x}Srx_{x}CoO4_{4}

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2-xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2-xSrxCoO4.Comment: http://www.nature.com/articles/srep25117

Anomalous thermoelectric effects of ZrTe5_{5} in and beyond the quantum limit

Thermoelectric effects are more sensitive and promising probes to topological properties of emergent materials, but much less addressed compared to other physical properties. Zirconium pentatelluride (ZrTe$_{5}$) has inspired active investigations recently because of its multiple topological nature. We study the thermoelectric effects of ZrTe$_{5}$ in a magnetic field and find several anomalous behaviors. The Nernst response has a steplike profile near zero field when the charge carriers are electrons only, suggesting the anomalous Nernst effect arising from a nontrivial profile of Berry curvature. Both the thermopower and Nernst signal exhibit exotic peaks in the strong-field quantum limit. At higher magnetic fields, the Nernst signal has a sign reversal at a critical field where the thermopower approaches to zero. We propose that these anomalous behaviors can be attributed to the Landau index inversion, which is resulted from the competition of the $\sqrt{B}$ dependence of the Dirac-type Landau bands and linear-$B$ dependence of the Zeeman energy ($B$ is the magnetic field). Our understanding to the anomalous thermoelectric properties in ZrTe$_{5}$ opens a new avenue for exploring Dirac physics in topological materials.Comment: 6 pages, 4 figure

Remarks on the Scalar Graviton Decoupling and Consistency of Horava Gravity

Recently Horava proposed a renormalizable gravity theory with higher derivatives by abandoning the Lorenz invariance in UV. But there have been confusions regarding the extra scalar graviton mode and the consistency of the Horava model. I reconsider these problems and show that, in the Minkowski vacuum background, the scalar graviton mode can be consistency decoupled from the usual tensor graviton modes by imposing the (local) Hamiltonian as well as the momentum constraints.Comment: Some clarifications regarding the projectable case added, Typos corrected, Comments (Footnote No.9, Note Added) added, References updated, Accepted in CQ