Session T6: Focus Session: Graphene Devices - Spin, Charge, and Superconductivity

Spin Relaxation and Spin Transport in Graphene

In this talk we are going to present our theoretical investigations on spin dynamics of graphene under various conditions based on a fully microscopic kinetic-spin-Bloch-equation approach [1]. We manage to nail down the solo spin relaxation mechanism of graphene in measurements from two leading groups, one in US and one in the Netherland. Many novel effects of the electron-electron Coulomb interaction on spin relaxation in graphene are addressed. Our theory can have nice agreement with experimental data.\\[4pt] [1] M. W. Wu, J. H. Jiang, and M. Q. Weng, ``Spin dynamics in semiconductors,'' Phys. Rep. \textbf{493}, 61 (2010).   

Spin and charge transport in high-mobility suspended graphene

Recent developments in graphene device fabrication techniques have made it possible to study the intrinsic properties of graphene by removing the substrate and making it suspended. Here we report electronic spin and charge transport measurements in high-mobility (over 70 000 cm$^{2}$/Vs) suspended graphene devices. To achieve this high quality we apply a large DC current density to heat the graphene flake, removing contaminations. We show that using the current annealing technique it is possible to produce ballistic nanoconstrictions, where quantized conductance at zero magnetic field is observed [N. Tombros et al., Nat. Phys. 7, 697 (2011)]. Studying the evolution of the position of the conductance plateaus in magnetic field we determine the width of the constriction. Spin transport measurements using ferromagnetic electrodes were also performed in our suspended graphene devices [M.H.D. Guimar\~{a}es et al., in preparation]. Analyzing non-local Hanle precession measurements we extract the spin relaxation time and the spin diffusion constant as a function of the charge carrier density. Combining our measurements with computer simulations we show that the measured spin relaxation times are limited by the non-cleaned regions of the device.   

Conductance Quantization in Graphene Nanoconstrictions

We present measurements of conductance quantization in a narrow Graphene constriction, of approximate width 200nm. Graphene is exfoliated on top of a Silicon Dioxide, and is not suspended. In high mobility samples ($>$10000cm$^{2}$V$^{-1}$s$^{-1})$ , we observe pinch-off at the Diract point, with a resistance at 4.2K of $\sim $ 40k$\Omega $. As a function of gate voltage at zero magnetic field, the conductance displays a few plateaus with the quantized value close to G$_{0}$=2e$^{2}$/h, indicating valley degeneracy splitting. At high carrier density ($>$5x10$^{12}$/cm$^{2})$ in a weak magnetic field, conductance exhibits ~strong beating in the Shubnikov-de Haas oscillations, which is also attributed to the valley splitting, analogous the Rashba interaction beats observed in the Shubnikov-de Haas oscillations in semiconducting quantum wells. In the Quantum Hall regime, the conductance of the constriction has quantized values nG$_{0}$, ..,. In comparison, measurements in the leads of the constriction display normal graphene behavior without the valley splitting.   

Chiral orbital current and anomalous magnetic moment in gapped graphene

We present a low-energy effective theory to describe chiral orbital current and anomalous magnetic moment in graphene monolayer and multilayers with band gap. We show that an electronic state of general Bloch system may intrinsically contain a quantum mechanical current circulation due to interband matrix elements. In gapped graphene, the current circulation is opposite between different valleys (K,K'), and the corresponding magnetic moment accounts for valley splitting of Landau levels. In gapped bilayer and ABC-stacked multilayer graphenes, in particular, the valley-dependent magnetic moment causes a huge paramagnetism at low energy, and a full valley polarization is possible up to relatively high electron density. The formulation also applies to gapped surface states of three-dimensional topological insulator, where the chiral orbital current is related to the magneto-electric response of the system.   

Quantum coherence versus dephasing effects in the giant spin Hall current and nonlocal voltage in magnetotransport through multiterminal graphene bridges

Motivated by the recent experiments [D. A. Abanin {\em et al.}, Science {\bf 323}, 328 (2011)] probing magnetotransport near the Dirac point in six-terminal graphene bridges from low to room temperature, we develop a nonequilibrium Green function (NEGF) theory of this phenomena. In the quantum-coherent regime, we find giant spin Hall (SH) conductance in four-terminal bridges, where the SH current is pure only at the Dirac point (DP), as well as the nonlocal voltage at a remote location in six-terminal bridges where the direct and inverse SH effect operate at the same time. The momentum-relaxing dephasing reduces their values at the DP by two orders of magnitude while washing out features away from the DP. Our theory is based on the linearized version of the Meir-Wingreen formula applied to multiterminal devices where dephasing is introduced through the self-energy within the active region of the bridge and currents and voltages are connected via generalized matrix of conductance coefficients.   

Magneto-transport studies of hydrogenated graphene

We study the magnetoresistance of hydrogenated graphene devices on a SiO$_{2}$ substrate. A large negative magnetoresistance of up to 30{\%} in a field of 2.5T is observed at low temperatures and at the film's charge neutrality point without any sign of saturation. A detailed analysis of the gate voltage dependence demonstrates a suppression of the large, negative magnetoresistance, which appears to be driven by a crossover from strong localization at low carrier concentrations to weak localization at higher carrier concentrations. Evidence of electron-hole symmetry breaking is found in the magnetic field traces at low temperature.   

In-plane magnetotransport in gapped bilayer graphene

The tunability of the band gap in bilayer graphene using a perpendicular electric field makes this material a promising candidate for future carbon electronics.\footnote{J. B. Oostinga et al., \emph{ Nat. Mat.}, \textbf{7}, 151 (2007) Y. Zhang et al., \emph{Nature}, \textbf{459}, 820 (2009)} Recent studies show that the residual conductivity at low temperature in the gapped state with zero total carrier density is a result of hopping transport.\footnote{T. Taychatanapat and P. Jarillo-Herrero, \emph{Phys. Rev. Lett.}, \textbf{105}, 166601, (2010)} We have studied the transport in this regime as a function of an in-plane magnetic field. We find a strikingly strong positive magnetoresistance that leads to a increase of the resistance by an order of magnitude at 10 Teslas in-plane magnetic field compared to the value at 0 T. The temperature dependence of the resistance is well described by variable range hopping transport for all magnetic field values, and suggests that the hopping range is strongly dependent on the in-plane magnetic field.   

Direct measurements of the current-phase relation in graphene Josephson junctions

The current-phase relation (CPR) of a Josephson junction can provide key information about the microscopic processes and symmetries that influence the supercurrent. In this talk, we present CPR results on Josephson junctions containing single-layer graphene as a weak link. The measurements are based on a phase-sensitive SQUID technique in which we determine the supercurrent amplitude and phase as a function of both temperature and electrostatic doping (gate voltage). We present CPR measurements of narrow junctions (5 - 12 $\mu$m) in the diffusive regime spanning the temperature range of 25 - 800 mK. We compare these data with previous CPR measurements on wide junctions in the temperature range of 800 - 900 mK.   

Anomalous supercurrent switching in graphene under proximity e

We report a study of hysteretic current-voltage characteristics in superconductor-graphene-superconductor (SGS) junctions. The stochastic nature of the phase slips is characterized by measuring the distribution of the switching currents. We find that in SGS junctions the dispersion of the switching current scales with temperature as $\sigma_I\propto T^{\alpha_G}$ with $\alpha_G\approx 1/3$. This observation is in sharp contrast with the known Josephson junction behavior where $\sigma_I\propto T^{\alpha_J}$ with $\alpha_J=2/3$. We propose an explanation using a modified version of Kurkijarvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.   

Gate Tuning of Different Phase-Particle Escape Regimes in Graphene-Based Josephson Junctions

Graphene-based Josephson junctions (GJJs) provide a unique system to investigate superconducting proximity effect with in-situ tunable Josephson coupling strength. While the phase-coherent behaviors of a GJJ under a magnetic field and microwave irradiation have been observed previously\footnote{H. B. Heersche et al., Nature 446, 56 (2007); D. Jeong et al. Phys. Rev. B 83, 094503 (2011).}, we investigated the stochastic switching behavior of the supercurrent in this system. Here, we present the observation of the three different escaping regimes for a phase particle from a washboard potential of the GJJ; macroscopic quantum tunneling (MQT), thermal activation (TA), and phase diffusion (PD).\footnote{G.-H. Lee et al., Phys. Rev. Lett. 107, 146605 (2011).} The crossover temperature ($T^*_{MQT}$) between the classical to quantum regime can be controlled by the gate voltage, implying that discrete energy levels of a phase particle are also gate-tunable. Moreover, direct observation of energy level quantization (ELQ) by microwave spectroscopy shows the consistent gate dependence of $T^*_{MQT}$. A new class of hybrid quantum devices such as a gate-tunable phase qubit is potentially realized by utilizing the MQT and ELQ behavior of the GJJs.   

Tunable resistance anomaly in graphene-superconductor hybrid structure

Junctions between superconductors and normal metals often exhibit a resistance anomaly: near the superconducting transition temperature, the resistance increases above the normal-state value. The magnitude of the excess resistance varies over a wide range, decreases in a magnetic field and can depend on the driving current and the history of thermal cycling. Several physical mechanisms have been proposed to explain the origin of the excess resistance, including the fluctuation-induced resistive state, nonequilibrium quasiparticle charge imbalance around NS boundary or phase-slip centers. Here we present an electronic transport study of superconductor-graphene hybrid structures that show a large resistance anomaly which survives even in presence of a relatively large magnetic field. We will show that, by changing the graphene resistance, we can tune the magnitude and position of the resistance peak and will examine the applicability of existing models to explain our results.