DSpace Collection:https://dspace.lboro.ac.uk/2134/16802017-08-21T15:51:50Z2017-08-21T15:51:50ZMagnetic field tunable vortex diode made of YBaCuO Josephson junction asymmetrical arraysChesca, BorisJohn, DanielPollett, RichardGaifullin, MaratCox, JonathanMellor, ChristopherSavel'ev, Sergeyhttps://dspace.lboro.ac.uk/2134/261492017-08-18T15:13:14Z2017-01-01T00:00:00ZTitle: Magnetic field tunable vortex diode made of YBaCuO Josephson junction asymmetrical arrays
Authors: Chesca, Boris; John, Daniel; Pollett, Richard; Gaifullin, Marat; Cox, Jonathan; Mellor, Christopher; Savel'ev, Sergey
Editors: Bader, S
Abstract: Several Josephson ratchets designed as asymmetrically structured parallel-series arrays of Josephson junctions made of YBa2Cu3O7−δ have been fabricated. From the current-voltage characteristics measured for various values of applied magnetic field, B, in the temperature range of 10–89 K, we demonstrate that the devices work as magnetic field-tunable highly reversible vortex diodes. Thus, at 89 K, the ratchet efficiency η could be reversed from +60% to −60% with a change in B as small as 3 μT. By decreasing the operation temperature, η improves up to −95% at 10 K while the dynamics in the B-tunability degrades. The ratchet designs we propose here can be used to control unidirectional vortex flow vortices in superconducting devices as well as building integrated nano-magnetic sensors. Numerical simulations qualitatively confirm our experimental findings and also provide insight into the related and more general problem of the control of the transport of nano/quantum objects in thin films.
Description: This paper is closed access until 8th August 2018.2017-01-01T00:00:00ZComparative study of the structural and optical properties of epitaxial CuFeO2 and CuFe1 - xGaxO2 delafossite thin films grown by pulsed laser deposition methodsWheatley, R.A.Rojas, S.Oppolzer, C.Joshi, ToyanathBorisov, PavelLederman, DavidCabrera, A.L.https://dspace.lboro.ac.uk/2134/258192017-08-04T14:11:20Z2017-01-01T00:00:00ZTitle: Comparative study of the structural and optical properties of epitaxial CuFeO2 and CuFe1 - xGaxO2 delafossite thin films grown by pulsed laser deposition methods
Authors: Wheatley, R.A.; Rojas, S.; Oppolzer, C.; Joshi, Toyanath; Borisov, Pavel; Lederman, David; Cabrera, A.L.
Abstract: Three samples of epitaxial delafossite CuFeO2 and CuFe1 − xGaxO2 films were grown using Pulsed Laser Deposition techniques in high vacuum. The sample thicknesses were estimated to be 21 nm, 75 nm for the CuFeO2 films and ~ 37 nm for the composite sample containing gallium. The estimated gallium fraction of substituted ferric atoms was x = 0.25 for the composite sample. We present the study of the fundamental band gap(s) for each sample via observation of their respective optical absorption properties in the NIR-VIS region using transmittance and diffuse reflection spectroscopy. Predominant absorption edges measured at 1.1 eV and 2.1 eV from transmittance spectra were observed for the CuFeO2 samples. The sample of CuFe1 − xGaxO2 showed a measurable shift to 1.5 eV of the lower band-gap and a strong absorption edge located at 2.3 eV attributed to direct band to band transitions. This study also found evidence of changes between apparent absorption edges between transmittance and diffuse reflectance spectroscopies of each sample and it may be resultant from absorption channels via surface states.
Description: This paper is closed access until 5th February 2018.2017-01-01T00:00:00ZAnomalous in-plane magnetoresistance of electron-doped cuprate La2−xCexCuO4±δYu, H.S.He, G.Jia, YanliZhang, X.Yuan, J.Zhu, BeiyiKusmartseva, Anna F.Kusmartsev, F.V.Jin, K.https://dspace.lboro.ac.uk/2134/252992017-06-08T12:55:33Z2017-01-01T00:00:00ZTitle: Anomalous in-plane magnetoresistance of electron-doped cuprate La2−xCexCuO4±δ
Authors: Yu, H.S.; He, G.; Jia, Yanli; Zhang, X.; Yuan, J.; Zhu, Beiyi; Kusmartseva, Anna F.; Kusmartsev, F.V.; Jin, K.
Abstract: We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2−xCexCuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14T. A crossover from negative to positive magnetoresistance occurs between the doping level x = 0.07 and 0.08. Above x = 0.08, the positive magnetoresistance effect appears, and is almost
indiscernible at x = 0.15. By tuning the oxygen content, the as-grown samples show negative magnetoresistance effect, whereas the optimally annealed ones display positive magnetoresistance effect at the doping level x = 0.15. Intriguingly, a linear-field dependence of in-plane magnetoresistance is observed at the underdoping level x = 0.06, the optimal doping level x = 0.1 and slightly overdoping level x = 0.11. These anomalies of in-plane magnetoresistance may be related to the intrinsic inhomogeneity in the cuprates, which is discussed in the framework of network model.
Description: This paper is closed access until 19th May 2018.2017-01-01T00:00:00ZBogoliubov-Born-Green-Kirkwood-Yvon chain and kinetic equations for the level dynamics in an externally perturbed quantum systemQureshi, Mumnuna A.Zhong, JohnnyBetouras, Joseph J.Zagoskin, Alexandre M.https://dspace.lboro.ac.uk/2134/252572017-06-05T14:14:56Z2017-01-01T00:00:00ZTitle: Bogoliubov-Born-Green-Kirkwood-Yvon chain and kinetic equations for the level dynamics in an externally perturbed quantum system
Authors: Qureshi, Mumnuna A.; Zhong, Johnny; Betouras, Joseph J.; Zagoskin, Alexandre M.
Abstract: Theoretical description and simulation of large quantum coherent systems out of equilibrium remains a daunting task. Here we are developing a new approach to it based on the Pechukas-Yukawa formalism, which is especially convenient in case of an adiabatically slow external perturbation. In this formalism the dynamics of energy levels in an externally perturbed quantum system as a function of the perturbation parameter is mapped on that of a fictitious one-dimensional classical gas of particles with cubic repulsion. Equilibrium statistical mechanics of this Pechukas gas allows to reproduce the random matrix theory of energy levels. In the present work, we develop the nonequilibrium statistical mechanics of the Pechukas gas, starting with the derivation of the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) chain of equations for the appropriate generalized distribution functions. Sets of approximate kinetic equations can be consistently obtained by breaking this chain at a particular point (i.e. approximating all higher-order distribution functions by the products of the lower-order ones). When complemented by the equations for the level occupation numbers and inter-level transition amplitudes, they allow to describe the nonequilibrium evolution of the quantum state of the system, which can describe better a large quantum coherent system than the currently used approaches. In particular, we find that corrections to the factorized approximation of the distribution function scale as 1/N, where N is the number of the "Pechukas gas particles" (i.e. energy levels in the system).
Description: This paper is in closed access.2017-01-01T00:00:00Z