Electron transport nonlocality in monolayer graphene modified with hydrogen silsesquioxane polymerization

A number of practical and fundamental applications of graphene requires modification of some of its properties. In this paper we study the effect of polymerization of a hydrogen silsesquioxane film on top of monolayer graphene with the intent to increase the strength of the spin-orbit interaction. The measured nonlocal resistances R-NL were found to be up to 700 Omega, significantly exceeding the expected contribution from conventional Ohmic currents. The RNL dependence on the channel length resembles exponential decay with a characteristic length of lambda similar or equal to 500 nm that is close to the spin-relaxation length in graphene reported elsewhere. The sensitivity of the measured effect to the electron-beam exposure was shown to decrease with an increased level of the surface contamination. However, no modulation of the effect is observed when an in-plane magnetic field is applied. This implies that a spin Hall model fails to explain the observed phenomenon and an alternative interpretation is required.</p

Spin injection and spin accumulation in permalloy-copper mesoscopic spin valves

We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as non-magnetic metal. Our multi-terminal geometry allows us to experimentally distinguish different magneto resistance signals, being 1) the spin valve effect, 2) the anomalous magneto resistance (AMR) effect and 3) Hall effects. We find that the AMR contribution of the Py contacts can be much bigger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a 'conventional' measurement geometry. However, these 'contact' magneto resistance signals can be used to monitor the magnetization reversal process, making it possible to determine the magnetic switching fields of the Py contacts of the spin valve device. In a 'non local' spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T=4.2 K as well as at room temperature. We obtain spin diffusion lengths in copper of 1 micrometer and 350 nm at T=4.2 K and room temperature respectively.Comment: 8 pages (incl. figures), 7 figures, RevTex, conferenc

The controllable pi - SQUID

We have fabricated and studied a new kind of DC SQUID in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and $\pi$ in the temperature range of 0.1 - 4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.Comment: 3 Figures, submitted to Applied Physics Letter

Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids

Magnons in ferromagnets behave as a viscous fluid over a length scale, the momentum-relaxation length, below which momentum-conserving scattering processes dominate. We show theoretically that in this hydrodynamic regime viscous effects lead to a sign change in the magnon chemical potential, which can be detected as a sign change in the nonlocal resistance measured in spin transport experiments. This sign change is observable when the injector-detector distance becomes comparable to the momentum-relaxation length. Taking into account momentum- and spin-relaxation processes, we consider the quasiconservation laws for momentum and spin in a magnon fluid. The resulting equations are solved for nonlocal spin transport devices in which spin is injected and detected via metallic leads. Because of the finite viscosity we also find a backflow of magnons close to the injector lead. Our work shows that nonlocal magnon spin transport devices are an attractive platform to develop and study magnon-fluid dynamics

Transport Gap in Suspended Bilayer Graphene at Zero Magnetic Field

We report a change of three orders of magnitudes in the resistance of a suspended bilayer graphene flake which varies from a few k$\Omega$s in the high carrier density regime to several M$\Omega$s around the charge neutrality point (CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of appearing quantum Hall plateaus at filling factor $\nu=2$ followed by $\nu=1$ suggests that the observed gap is caused by the symmetry breaking of the lowest Landau level. Investigation of the gap in a tilted magnetic field indicates that the resistance at the CNP shows a weak linear decrease for increasing total magnetic field. Those observations are in agreement with a spontaneous valley splitting at zero magnetic field followed by splitting of the spins originating from different valleys with increasing magnetic field. Both, the transport gap and $B$ field response point toward spin polarized layer antiferromagnetic state as a ground state in the bilayer graphene sample. The observed non-trivial dependence of the gap value on the normal component of $B$ suggests possible exchange mechanisms in the system.Comment: 8 pages, 5 figure

Proximity and Josephson effects in superconductor - two dimensional electron gas planar junctions

The DC Josephson effect is theoretically studied in a planar junction in which a two dimensional electron gas (2DEG) infinite in lateral directions is in contact with two superconducting electrodes placed on top of the 2DEG. An energy gap in the excitation spectrum is created in the 2DEG due to the proximity effect. It is shown that under certain conditions, the region of the 2DEG underneath the superconductors is analogous to a superconducting region with an order parameter $\varepsilon_g\exp(i\phi)$\/, where $\varepsilon_g~(\varepsilon_g<\Delta)$\/ depends on the interface transmittance and the Fermi velocity mismatch between the superconductors and the 2DEG.Comment: 9 pages REVTeX, 5 figures available on reques

Dynamic Data Selection for Neural Machine Translation

Intelligent selection of training data has proven a successful technique to simultaneously increase training efficiency and translation performance for phrase-based machine translation (PBMT). With the recent increase in popularity of neural machine translation (NMT), we explore in this paper to what extent and how NMT can also benefit from data selection. While state-of-the-art data selection (Axelrod et al., 2011) consistently performs well for PBMT, we show that gains are substantially lower for NMT. Next, we introduce dynamic data selection for NMT, a method in which we vary the selected subset of training data between different training epochs. Our experiments show that the best results are achieved when applying a technique we call gradual fine-tuning, with improvements up to +2.6 BLEU over the original data selection approach and up to +3.1 BLEU over a general baseline.Comment: Accepted at EMNLP201