H. Goto

Inter-Layer Screening Length to Electric Field in Thin Graphite Film

Electric conduction in thin graphite film was tuned by two gate electrodes to clarify how the gate electric field induces electric carriers in thin graphite. The graphite was sandwiched between two gate electrodes arranged in a top and bottom gate configuration. A scan of the top gate voltage generates a resistance peak in ambiploar response. The ambipolar peak is shifted by the bottom gate voltage, where the shift rate depends on the graphite thickness. The thickness-dependent peak shift was clarified in terms of the inter-layer screening length to the electric field in the double-gated graphite film. The screening length of 1.2 nm was experimentally obtained.

Introducing Nonuniform Strain to Graphene Using Dielectric Nanopillars

A method for inducing nonuniform strain in graphene films is developed. Pillars made of a dielectric material (electron beam resist) are placed between graphene and the substrate, and graphene sections between pillars are attached to the substrate. The strength and spatial pattern of the strain can be controlled by the size and separation of the pillars. Application of strain is confirmed by Raman spectroscopy as well as from scanning electron microscopy (SEM) images. From SEM images, the maximum stretch of the graphene film reaches about 20%. This technique can be applied to the formation of band gaps in graphene.

Gate control of spin transport in multilayer graphene

We experimentally studied the gate voltage dependence of spin transport in multilayer graphene (MLG) using the nonlocal spin detection technique. We found that the spin signal is a monotonically decreasing linear function of the resistance of MLG, which is characteristic of the intermediate interfacial transparency between the MLG and the ferromagnetic electrodes (Co). The linear relation indicates a large spin relaxation length significantly exceeding 8μm. This shows the superiority of MLG for the utilization of the graphite-based spintronic devices.

Coulomb Blockade Oscillations in Narrow Corrugated Graphite Ribbons

We report Coulomb blockade oscillations in an atomically thin graphite ribbon fabricated by the micromechanical cleavage technique. Aperiodic current oscillations as a function of the gate voltage indicate the formation of multiple Coulomb islands inside the thin graphite ribbon. We conclude that the Coulomb islands originate from puddles of electrons and holes caused by the inhomogeneous interface between the ribbon and the substrate.

Effect of current annealing on electronic properties of multilayer graphene

While ideal graphene has high mobility due to the relativistic nature of carriers, it is known that the carrier transport in actual graphene samples is dominated by the influence of scattering from charged impurities, which almost conceals the intrinsic splendid properties of this novel material. The common techniques to improve the graphene mobility include the annealing in hydrogen atmosphere and the local annealing by imposing a large biasing current. Although annealing is quite important technique for the experimental study of graphene, detailed evaluation of the annealing effect is lacking at present. In this paper, we study the effect of the current annealing in multilayer graphene devices quantitatively by investigating the change in the mobility and the carrier density at the charge neutrality point. We find that the current annealing sometimes causes degradation of the transport properties.

Introducing Nonuniform Strain to Graphene: Toward Strain Engineering

1 Institute of Physics and Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba, Tennodai, Tsukuba, Ibaraki 305-8571, Japan Phone and fax : +81-29-853-4345, E-mail: tomori@lt.px.tsukuba.ac.jp 2 CREST-JST, Kawaguchi, Saitama 332-0012, Japan 3 MANA, NIMS, Namiki, Tsukuba 305-0047, Japan 4 Nanotechnology Innovation Center, NIMS, Tsukuba, Ibaraki 305-0047, Japan

Inverse spin valve effect in multilayer graphene device

We report the gate-voltage dependence of the spin transport in multilayer graphene (MLG) studied experimentally by the local measurement. The sample consists of a Ni/MLG/Ni junction, where the thickness of the MLG is 9 nm and the spacing of two Ni electrodes is 300 nm. At zero gate voltage, we observed the normal spin valve effect, in which the resistance for the antiparallel alignment of magnetization in ferromagnetic electrodes is larger than that for the parallel alignment. By applying a large gate voltage, on the other hand, the spin valve effect is reversed: the resistance for the antiparallel alignment becomes smaller than that for the parallel alignment. The result is qualitatively interpreted as a quantum interference effect, indicating that the mean free path and the spin relaxation length of the MLG are longer than the electrode spacing (300 nm).

Thickness-dependent Resistance Change of Dual-gated Thin Graphite Films

We present fundamental researches on thin graphite film, with the goal of realizing future nanometer scale electronic applications. Because thin graphite films are by nature nanometer scale materials with remarkable electrical conductions, they are expected to be an important element in nano-carbon electronics. For a control of the conduction of the thin graphite channel, gating effect must be fully clarified. Our starting materials are thin layers (thickness 1-10 nm) of graphite films pealed off from bulk graphite on SiO2/doped-Si substrate. The thin film is connected to two or multiple metallic electrodes. In general, conduction of the graphite can be changed in gate voltage applied to the doped-Si substrate. In this configuration, the gate electric field can be applied from the substrate side (back-gate configuration). Observed resistance in the gate-voltage change shows ambipolar behavior based on clear carrier polarity change. We attached a front gate, which was directly formed on the surface of the graphite film. We deposit an Al electrode on the graphite film (Fig. 1). The graphite channel and the Al electrode are naturally insulated by exposed in air. Then the Al electrode can be used as a front gate. The front gate also changes the conduction of the thin graphite film. A scan of the top gate voltage (Vtg) generates a resistance peak in the ambiploar response. In relatively thicker film (~7 nm), the back gate voltage (Vbg) shifts the ambipolar peak depending only slightly (Fig. 2). On the other hand, the shift is clear in thinner film (~1 nm) (Fig. 2). The thickness-dependent peak shift is clarified in terms of the inter-layer screening length to the electric field in the dual-gated graphite film. We assume that the gate-induced carriers decay exponentially from both surfaces, and that the conductivity in each layer increases proportionally to the induced carrier density. The ambipolar peak corresponds to the situation that the carriers induced by the back gate is maximally ejected by the top gate. Then the condition for the ambipolar resistance peak in Vtg scan is obtained as a function of Vbg, , and the graphite film thickness d. Then, we estimated a screening length of 1.2 nm [1]. In films thicker than 2 nm, ambipolar resistance peak decreases at large Vbg. This is because the carriers induced by the back gate cannot be ejected completely Fig.1 Optical microscope image of a thin graphite film with source-drain electrodes and a Al top gate on SiO2/Si substrate. Extended Abstracts of the 2008 International Conference on Solid State Devices and Materials, Tsukuba, 2008,

Observation of gate-controlled superconducting proximity effect in microfabricated thin graphite films

We investigated the influence of the oxygen plasma etching on the electron transport in thin graphite films. The semimetallic temperature dependence of zero-bias resistance was observed for samples microfabricated with both Al mask and resist mask, but the possible damage by e-beam irradiation was observed in films with Al mask. In thin graphite films microfabricated by O2 plasma with resist mask, the proximity-induced superconductivity was observed and the critical supercurrent and temperature strongly depend on the gate voltage.

Magnetic Response of a Mesoscopic Superconducting Disk Surrounded by a Normal Metal

Magnetic response of a superconductor/normal‐metal (S/N) concentric disk is studied by use of a ballistic Hall magnetometer, which enables us to investigate the mutual proximity effects in a single and a micrometer‐sized sample. The core of the sample is a type‐I superconductor whose diameter is comparable to the coherence length and the magnetic penetration depth. The core and the surround are prepared by an improved double‐angle evaporation method to realize their metallic contacts. At T = 1.3 K, the pair breaking effect on the S has been predominant. We have observed the smaller diamagnetic susceptibility, the larger critical field, and the less stable vortex states in the S/N sample than in only S sample. These results are attributed to the suppression of the surface superconductivity, which effectively decreases the diameter of the superconductor.