Response of a mesoscopic superconducting disk to perpendicular magnetic fields is studied by using the multiple-small-tunnel-junction method, in which transport properties of several small tunnel junctions attached to the disk are measured simultaneously. This allows us for the first experimental distinction between the giant vortex states and multivortex states. Moreover, we experimentally find magnetic field induced rearrangement and combination of vortices. The experimental results are well reproduced in numerical results based on the nonlinear Ginzburg-Landau theory.The appearance of vortices in various quantum systems, such as superconductors, superfluids and BoseEinstein condensates, is an intriguing phenomenon in nature. A conventional quantum vortex is singly quantized, having a core where the value of the order parameter decreases to zero, while its phase changes by 2π when encircling the core. Recently, an important breakthrough was established by the observation of doubly quantized vortex lines in superfluid 3 He-A[1]. For superconductors expectations are even more spectacular. In macroscopic type-II superconductors a triangular lattice of single flux quanta is formed, whereas two kinds of fundamentally new vortex states have theoretically been predicted in mesoscopic superconductors where the sample size approaches the size of Cooper-pairs [2,3,4,5]; (i) multivortex states (MVSs) with a unique spatial arrangement of singly quantized vortices, and (ii) multiply quantized or giant vortex states (GVSs) with a single core in the center [6,7].Although several experimental techniques have been developed for observing these novel states [7,8,9,10,11,12,13], none of them has been able to make a clear distinction between MVSs and GVSs. In this Letter, we present the first experimental evidence for the existence of GVSs and MVSs in a circular disk, and demonstrate magnetic-field induced MVS-GVS and MVS-MVS transitions. Our results are in good agreement with the theoretical prediction based on the nonlinear GinzburgLandau (G-L) theory.Here we used the multiple-small-tunnel-junction (MSTJ) method, in which several small tunnel junctions with high tunnel resistance are attached to a mesoscopic superconductor to simultaneously detect small changes in the local density of states (LDOS) under the junctions [14,15]. Since the LDOS depends on the local supercurrent density, the MSTJ method gives us information on the distribution of the supercurrent, which reflects the detailed vortex structure inside the disk. Figure 1 shows a schematic drawing and a scanning electron microscopy (SEM) image of the sample. Four normal-metal (Cu) leads are connected to the periphery of the superconducting Al disk through highly resistive small tunnel junctions, A, B, C, and D. The sample is designed to be symmetrical with respect to the central axis SS ′ . The angles AOD and BOC are 120 and 32 degrees, respectively. Although junctions A and D and junctions B and C ideally have the same area and tunnel resistance, small differences actu...

Electron transport in bilayer graphene placed under a perpendicular electric field is revealed experimentally. Steep increase of the resistance is observed under high electric field; however, the resistance does not diverge even at low temperatures. The observed temperature dependence of the conductance consists of two contributions: the thermally activated (TA) conduction and the variable range hopping (VRH) conduction. We find that for the measured electric field range (0-1.3 V/nm) the mobility gap extracted from the TA behavior agrees well with the theoretical prediction for the band gap opening in bilayer graphene, although the VRH conduction deteriorates the insulating state more seriously in bilayer graphene with smaller mobility. These results show that the improvement of the mobility is crucial for the successful operation of the bilayer graphene field effect transistor.

We developed a simple and novel method to fabricate complementary-like logic inverters based on ambipolar graphene field-effect transistors (FETs). We found that the top gate stacks (with both the metal and oxide layers) can be simply prepared with only one-step deposition process and show high capacitive efficiency. By employing such a top gate as the operating terminal, the operating bias can be lowered within 2 V. In addition, the complementary p- and n-type FET pairs can be also simply fulfilled through potential superposition effect from the drain bias. The inverters can be operated, with up to 4-7 voltage gains, in both the first and third quadrants due to the ambipolarity of graphene FETs. For the first time, a match between the input and output voltages is achieved in graphene logic devices, indicating the potential in direct cascading of multiple devices for future nanoelectronic applications.

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.Comment: Appl. Phys. Express, in pres