Lag time and parameter mismatches in synchronization of unidirectionally coupled chaotic external cavity semiconductor lasers

We report an analysis of synchronization between two unidirectionally coupled chaotic external cavity master/slave semiconductor lasers with two characteristic delay times, where the delay time in the coupling is different from the delay time in the coupled systems themselves. We demonstrate for the first time that parameter mismatches in photon decay rates for the master and slave lasers can explain the experimental observation that the lag time is equal to the coupling delay time.Comment: LaTex, 5 pages, submitted to PRE(R

On possible superconductivity in the doped ladder compound La_(1-x)Sr_xCuO_2.5

LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be doped systematically by Sr substitution. Motivated by the recent synthesis of single crystals, we investigate theoretically the possibility of superconductivity in this compound. We use a model of spin fluctuation-mediated superconductivity, where the pairing potential is strongly peaked at \pi in the ladder direction. We solve the coupled gap equations on the bonding and antibonding ladder bands to find superconducting solutions across the range of doping, and discuss their relevance to the real material.Comment: RevTex, 4 pages, 7 figure

Hole-doping induced ferromagnetism in 2D materials

Two-dimensional (2D) ferromagnetic materials are considered as promising candidates for the future generations of spintronic devices. Yet, 2D materials with intrinsic ferromagnetism are scarce. High-throughput first-principles simulations are performed in order to screen 2D materials that present a non-magnetic to a ferromagnetic transition upon hole doping. A global evolutionary search is subsequently performed, in order to identify alternative possible atomic structures of the eligible candidates, and 122 materials exhibiting a hole-doping induced ferromagnetism are identified. Their energetic and dynamic stability, as well as their magnetic properties under hole doping are investigated systematically. Half of these 2D materials are metal halides, followed by chalcogenides, oxides and nitrides, some of them having predicted Curie temperatures above 300 K. The exchange interactions responsible for the ferromagnetic order in these 2D materials are also discussed. This work not only provides theoretical insights into hole-doped 2D ferromagnetic materials, but also enriches the family of 2D magnetic materials for possible spintronic applications

Gigantic anisotropic uniaxial pressure effect on superconductivity within the CuO2 plane of La1.64Eu0.2Sr0.16CuO4 - strain control of stripe criticality

The effect of uniaxial pressure on superconductivity was examined for a high-Tc cuprate La1.64Eu0.2Sr0.16CuO4, which is located at the boundary between the superconducting and stripe phases. We found remarkably large anisotropy of the uniaxial pressure effect not only between the in-plane and out-of-plane pressures but also within the CuO2-plane. When the pressure is applied along the tetragonal [110] direction, we found the largest pressure effect ever observed in cuprates, dTc/dP - 2.5 K/kbar, while the change of Tc was not appreciable when applied along [100]. This substantial in-plane anisotropy is attributed to an intimate link between the symmetry of the one-dimensional stripes and that of the strain produced within the CuO2 plane.Comment: 4 pages including 3 figure

Structural phase control of (La1.48_{1.48}Nd0.40_{0.40}Sr0.12_{0.12})CuO4_4 thin films by epitaxial growth technique

Epitaxial growth of (La$_{1.48}$Nd$_{0.40}$Sr$_{0.12}$)CuO$_4$ thin films was studied by pulsed-laser deposition technique on three different substrates, SrTiO$_3$ (100), LaSrAlO$_4$ (001), and YAlO$_3$ (001). The (Nd,Sr,Ce)$_2$CuO$_4$-type structure appears at the initial growth stage on SrTiO$_3$ (100) when the film is deposited under the growth conditions optimized for (La,Sr)$_2$CuO$_4$. This (Nd,Sr,Ce)$_2$CuO$_4$-type structure can be eliminated by increasing the substrate temperature and the laser repetition frequency. Films on LaSrAlO$_4$ (001) maintain a La$_2$CuO$_4$-type structure as bulk samples, but those on YAlO$_3$ (001) show phase separation into La$_2$CuO$_4$- and Nd$_2$CuO$_4$-type structures. Such complicated results are explained in terms of the competition between lattice misfit and thermodynamic conditions. Interestingly the films with La$_2$CuO$_4$-type structure prepared on SrTiO$_3$ and LaSrAlO$_4$ show different surface structures and transport properties. The results indicate the possibility of controlling charge stripes of (La$_{1.48}$Nd$_{0.40}$Sr$_{0.12}$)CuO$_4$ as was demonstrated in (La,Ba)$_2$CuO$_4$ thin films by Sato et al. (Phys. Rev. B {\bf 62}, R799 (2000)).Comment: 5 pages, 6 EPS figure, accepted for publication in Phys. Rev.

Ultrafast loss of lattice coherence in the light-induced structural phase transition of V2_2O3_3

In solids, the response of the lattice to photo-excitation is often described by the inertial evolution on an impulsively modified potential energy surface which leads to coherent motion. However, it remains unknown if vibrational coherence is sustained through a phase transition, during which coupling between modes can be strong and may lead to rapid loss of coherence. Here we use coherent phonon spectroscopy to track lattice coherence in the structural phase transition of V$_2$O$_3$. In both the low and high symmetry phases unique coherent phonon modes are generated at low fluence. However, coherence is lost when driving between the low and high symmetry phases. Our results suggest non-inertial dynamics dominate during phase transition due to disorder and multi-mode coupling.Comment: 4 figures and supplementary informatio

Effects of epitaxial strain on the growth mechanism of YBa2Cu3O7-x thin films in [YBa2Cu3O7-x / PrBa2Cu3O7-x] superlattices

We report on the growth mechanism of YBa2Cu3O7-x (YBCO). Our study is based on the analysis of ultrathin, YBa2Cu3O7-x layers in c-axis oriented YBa2Cu3O7-x / PrBa2Cu3O7-x superlattices. We have found that the release of epitaxial strain in very thin YBCO layers triggers a change in the dimensionality of the growth mode. Ultrathin, epitaxially strained, YBCO layers with thickness below 3 unit cells grow in a block by block two dimensional mode coherent over large lateral distances. Meanwhile, when thickness increases, and the strain relaxes, layer growth turns into three dimensional, resulting in rougher layers and interfaces.Comment: 10 pages + 9 figures, accepted in Phys. Rev.