X. G. Qiu

Optical spectroscopy of the Weyl semimetal TaAs

B. Xu, ∗ Y. M. Dai, ∗ L. X. Zhao, K. Wang, R. Yang, W. Zhang, J. Y. Liu, H. Xiao, 3 G. F. Chen, 4 A. J. Taylor, D. A. Yarotski, R. P. Prasankumar, † and X. G. Qiu 4, ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China Collaborative Innovation Center of Quantum Matter, Beijing 100190, China Associate Directorate for Chemistry, Life and Earth Sciences, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA (Dated: October 5, 2015)

Electron-doped superconducting (La,Ce)2CuO4 thin films grown by dc magnetron sputtering and their transport properties

Electron-doped La2?xCexCuO4 (LCCO) thin films were successfully prepared on (100) SrTiO3 substrates by the dc magnetron sputtering method. The optimal-doped films show a highly c-axis oriented single -type structure, and the zero resistance temperature TC0 is 25?K. In the normal state, the nearly quadratic temperature dependence of the resistivity was found and attributed to the Landau?Fermi liquid behaviour due to electron?electron scattering. At the same time, the negative sign of the Hall coefficient for such films well above TC obviously confirms the electron-type nature. The optimal conditions for the growth of single-phase -type LCCO thin films are also discussed in detail.

Exchange bias effect and enhanced magnetoresistance in La0.67Sr0.33MnO3∕SrTiO3 superlattices

The magnetization and electrical transport in the superlattices consisting of ferromagnetic La0.67Sr0.33MnO3 and nonmagnetic insulating SrTiO3 layers have been investigated. A significant displacement of the hysteresis loop along the field axis is observed when the sample is field-cooled through the blocking temperature TB. The strength of displacement, termed as exchange field HE, is found to exponentially decay with temperature. The magnetoresistance in field-cooling process is obviously enhanced compared to that in zero-field-cooling process. The existence of the disordered spin state at the interface is suggested to be the origin of such phenomena.

Far infrared optical properties of the pyrochlore spin ice compound Dy2Ti2O7

Near normal incidence far infrared reflectivity spectra of [111] dysprosium titanate (Dy2Ti2O7) single crystal have been measured at different temperatures. Seven phonon modes (eight at low temperature) are identified at frequencies below 1000 cm−1. Optical conductivity spectra are obtained by fitting all the reflectivity spectra with the factorized form of the dielectric function. Both the Born effective charges and the static optical permittivity are found to increase with decreasing temperature. Moreover, phonon linewidth narrowing and a phonon mode shift with decreasing temperature are also observed, which may result from enhanced charge localization. The redshift of several low frequency modes is attributed to the spin–phonon coupling. All observed optical properties can be explained within the framework of the nearest neighbour ferromagnetic (FM) spin ice model.

Magnetic properties of the self-doped yttrium manganites YMn1+xO3

Magnetic properties of the hexagonal compounds of YMn1+xO3 (0≤x≤0.15) have been studied by measuring the dc magnetization, ac magnetization, magnetic hysteresis, and relaxation of the magnetization. All the results give evidence of a spin glass (SG) state at low temperatures in samples with x≥0.08. This is attributed to the competition between the antiferromagnetic (AFM) matrix and the ferromagnetic (FM) phase induced by the excess Mn. Moreover, careful investigations on the thermoremanent magnetization (TRM) of the sample with x = 0.10 indicate that this SG state is an ideal state with nearly infinite equilibration time and that it shows almost perfect full ageing.

Heteroepitaxial multilayer of YBa2Cu3O7 and PrBa2Cu3O7 on SrTiO3 and LaAlO3 substrates by sputtering

We have successfully fabricated epitaxially grown YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PBCO) multilayer thin films on SrTiO3 and LaAlO3 substrates by dc/rf magnetron sputtering. The thicknesses of YBCO and PBCO varied from 1 to 8 unit cells. Satellite peaks in x‐ray diffraction patterns clearly indicate the formation of periodic modulation structures of different wavelengths. At a certain thickness of the YBCO layer, the zero resistance transition temperature Tc0 decreased with the increase of the PBCO layer thickness. In contrast, Tc0 increased with the increase of the YBCO layer thickness at a constant PBCO layer thickness.

Crossover behaviors in magnetoresistance oscillations for Nb thin film with rectangular arrays of antidots

Superconducting Nb thin films with rectangular arrays of submicron antidots (holes) have been systemically investigated by transport measurements. A series of crossover behaviors is found in magnetoresistance oscillations, corresponding to three different superconducting states: the wire network-like state, the interstitial vortex state and the single-loop–like state. These states are identified by the field intervals and hysteretic effect. The crossover fields between them are found to be both temperature and geometry dependent. Furthermore, in dense arrays, the saturation number is distinctly larger than the theoretical calculation for a single insulating inclusion. Our results indicate that in the process of magnetic field penetration into nanostructured superconductors, the order parameters are strongly modulated and finally localized near the edges, resulting in changes of oscillation modes.

Transport properties of electron-doped La 2 − x Ce x CuO 4 cuprate thin films

Electron-doped La2-xCexCuO4 (x=0.06-0.20) thin films with two orientations (100) and (103) were successfully grown and their transport properties were investigated. It is found that the temperature dependence of the in-plane resistivity rho(ab) evolves from semiconducting behavior for underdoped films to two-dimensional Fermi liquid behavior for optimally and overdoped ones, while the c-axis resistivity rho(c) shows nonmetallic behavior except for heavily overdoped films. The doping-dependent anisotropy factor rho(c)/rho(ab) is found to vary from 60 to 1500 at 50 K, and the largest anisotropy value related to the highest T-C is obtained at the optimal doping (x=0.10).

Optical conductivity of Ba0.6K0.4Fe2As2: The effect of in-plane and out-of-plane doping in the superconducting gap

We measured the in-plane optical conductivity of a nearly optimally doped (T-c = 39.1K) single crystal of Ba0.6K0.4Fe2As2. Upon entering the superconducting state the optical conductivity vanishes below similar to 20meV, indicating a fully gapped system. A quantitative modeling of the optical response of this material requires two different isotropic gaps, the larger of which dominates the London penetration depth. The temperature dependence of these gaps indicate a strong interband interaction, but no impurity-scattering-induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped BaFe1.84Co0.08As2 and strongly supports an s +/- gap symmetry for both compounds. Copyright (C) EPLA, 2013

Tuning asymmetry parameter of Fano resonance of spoof surface plasmons by modes coupling

We report a kind of subwavelength, compound hole arrays formed by two nested sub-lattices used to modulate the Fano resonance of spoof surface plasmons (SSPs). Experiments complemented with numerical simulations show that the asymmetry parameter (q) of the Fano line shape can be tuned artificially and continuously: the q value experiences a sign reversal and varies linearly with the variation of hole size. The coupling between different SSP modes of sub-lattices is suggested to be responsible for the tunability of Fano resonance in the compound hole arrays.