Layered transition metal dichalcogenides (TMDs) are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single 2D layer of NbSe2 by means of low temperature scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3x3 CDW order in NbSe2 remains intact in 2D. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of  = 4 meV at the Fermi energy, which is accessible via STS due to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing new insight into CDW formation and superconductivity in this model strongly-correlated system.

Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy) to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene.

Both transport τtr and elastic τe scattering times are experimentally determined from the carrier density dependence of the magnetoconductance of monolayer and bilayer graphene. Both times and their dependences on carrier density are found to be very different in the monolayer and the bilayer. However, their ratio τtr/τe is found to be close to 1.8 in both systems and nearly independent of the carrier density. These measurements give insight on the nature (neutral or charged) and range of the scatterers. Comparison with theoretical predictions suggests that the main scattering mechanism in our samples is due to strong (resonant) scatterers of a range shorter than the Fermi wavelength; likely candidates being vacancies, voids, ad-atoms or short-range ripples.PACS numbers: 63.22. Np, 73.23.Hk, 73.21.Ac Since the discovery of the fascinating electronic properties of graphene [1] due to its electronic spectrum with linear dispersion and a perfect electron-hole symmetry at the Fermi level [2], the nature of defects has been shown to play an essential role in determining the carrier density (n c ) dependence of the conductance. The wavevector and energy dependences of the impurity potential are known to determine the characteristic scattering times of the carriers. It is important to distinguish the transport time τ tr , which governs the current relaxation and enters the Drude conductivity (σ), from the elastic scattering time τ e , which is the lifetime of a plane wave state [3]. Since τ tr and τ e involve different angular integrals of the differential cross section, they differ as soon as the Fourier components of the potential depend on the wavevector q. A large ratio τ tr /τ e indicates that scattering is predominantly in the forward direction, so that transport is not affected much by this type of scattering. This is the case in 2D electron gases (2DEG) confined to GaAs/GaAlAs heterojunctions with the scattering potential produced by remote charged Si donors [4], where τ tr /τ e is found to be larger than 10.The nature of the main scattering mechanism limiting the carrier mobility in graphene is still subject to controversy. It has indeed been shown [5][6][7] that "white noise" (q independent) scattering leads to a weak (logarithmic) dependence of σ(n c ), in contradiction with experiments which typically find a linear increase. In contrast, scattering on charged impurities originates from a q dependent screened Coulomb potential described in the Thomas Fermi approximation [8][9][10]. This leads to a linear σ(n c ) both for a monolayer (ML) and a moderately doped bilayer (BL). Recent experiments performed to probe this question measured the change in σ upon immersion of graphene samples in high-K dielectric media. Their conclusions differ [11]. Alternate explanations involve resonant scattering centers with a large energy mismatch with the Fermi energy of carriers [7,12].In order to gain insight into the scattering mechanism in graphene, we have extracted τ e and τ tr from magnetotransport in monolayer and b...

We have tuned in situ the proximity effect in a single graphene layer coupled to two Pt/Ta superconducting electrodes. An annealing current through the device changed the transmission coefficient of the electrode/ graphene interface, increasing the probability of multiple Andreev reflections. Repeated annealing steps improved the contact sufficiently for a Josephson current to be induced in graphene.

We fabricate a vertical thin-film barristor device consisting of highly doped silicon (gate), 300 nm SiO 2 (gate dielectric), monolayer graphene, pentacene, and a gold top electrode. We show that the current across the device is modulated by the Fermi energy level of graphene, tuned with an external gate voltage. We interpret the device current within the thermionic emission theory, showing a modulation of the energy barrier between graphene and pentacene as large as 300meV.

We investigate conductance fluctuations as a function of carrier density n and magnetic field in diffusive mesoscopic samples made from monolayer and bilayer graphene. We show that the fluctuations' correlation energy and field, which are functions of the diffusion coefficient, have fundamentally different variations with n, illustrating the contrast between massive and massless carriers. The field dependent fluctuations are nearly independent of n, but the n-dependent fluctuations are not universal and are largest at the charge neutrality point. We also measure the second order conductance fluctuations (mesoscopic rectification). Its field asymmetry, due to electron-electron interaction, decays with conductance, as predicted for diffusive systems. PACS numbers:Reproducible conductance fluctuations (CF) are one of the most striking signature of phase coherent transport [1]. The conductance of a mesoscopic sample results from interference between all wave packets traversing the sample. This interference pattern is sensitive to variations in disorder configuration, Fermi energy or magnetic flux, leading to reproducible CF as one of these parameters is changed. In diffusive or chaotic systems the CF amplitude has been shown to be universal [1][2][3] and ergodic, i.e. independent of the mechanism of phase randomization (magnetic field, Fermi energy, configuration of impurities for diffusive systems, sample shape for ballistic systems). The CF amplitude is of the order of e 2 /h, with a coefficient which only depends on the symmetry class of the mesoscopic system. The typical correlation energies E ϕ and fields B ϕ of the fluctuations depend upon the typical time τ int and area A int over which interference occur: E ϕ =h/τ int and B ϕ = Φ 0 /A int , with Φ 0 = h/e [2]. CF have been extensively investigated in metallic and semiconducting systems [1,3]. The recently discovered graphene [4] provides a unique system in which the Fermi energy and diffusion constant can be tuned at will, over a broad carrier density range extending from hole to electron metallic conduction. Theoretical simulations of CF in graphene suggest a possible enhancement of the fluctuation amplitude with respect to standard mesoscopic samples, depending on the strength or nature of disorder (intervalley scattering) [5][6][7]. On the experimental side CF have been reported by several groups [8][9][10][11][12][13]. But to our knowledge the present work is the first complete investigation of their correlations and amplitudes as a function of Fermi energy and magnetic field, for both monolayer (ML) and bilayer (BL) graphene. The importance of the comparison lies in the fact that whereas ML and BL have similar resistivities and thus mean free paths (see Fig. 3), the (massless and massive) carriers have different velocities because of the different dispersion relations in these two materials. Thus the diffusion constants and therefore correlation energies and fields will have different carrier density dependences, providing a powerful test of the applica...

Charge transfer at the interface between dissimilar materials is at the heart of electronics and photovoltaics. Here we study the molecular orientation, electronic structure, and local charge transfer at the interface region of C deposited on graphene, with and without supporting substrates such as hexagonal boron nitride. We employ ab initio density functional theory with van der Waals interactions and experimentally characterize interface devices using high-resolution transmission electron microscopy and electronic transport. Charge transfer between C and the graphene is found to be sensitive to the nature of the underlying supporting substrate and to the crystallinity and local orientation of the C. Even at room temperature, C molecules interfaced to graphene are orientationally locked into position. High electron and hole mobilities are preserved in graphene with crystalline C overlayers, which has ramifications for organic high-mobility field-effect devices.

A scheme is proposed to electrically measure the spin-momentum coupling in the topological insulator surface state by injection of spin polarized electrons from silicon. As a first approach, devices were fabricated consisting of thin (<100nm) exfoliated crystals of Bi 2 Se 3 on n-type silicon with independent electrical contacts to silicon and Bi 2 Se 3 . Analysis of the temperature dependence of thermionic emission in reverse bias indicates a barrier height of 0.34 eV at the Si-Bi 2 Se 3 interface. This robust Schottky barrier opens the possibility of novel device designs based on sub-band gap internal photoemission from Bi 2 Se 3 into Si.The most remarkable feature of the three-dimensional strong topological insulators (TIs) is the existence of a metallic surface state within the bulk bandgap with chiral charge carriers exhibiting perfect spin-momentum coupling. The chiral TI surface state has been proposed as the basis of spintronics and quantum computing devices. 1,2 Although the spin helicity of the TI surface has been experimentally measured by spin-angle resolved photoemission spectroscopy (spin-ARPES) 3-5 , no electrical transport experiment has yet shown clear evidence of it for several reasons. Since spin and momentum are perfectly coupled, non local measurement of a spin current in the absence of a charge current 6,7 is precluded. Perhaps even more importantly, spin precession induced by a weak perpendicular magnetic field is also eliminated by momentum scattering in the diffusive regime. This is particularly problematic since evidence of spin precession and dephasing are used to unambiguously identify spin transport in both inorganic semiconductors 8 and metals 9 .Direct measurement of the spin Hall effect by injection of spin from a ferromagnetic (FM) contact is also problematic since the desired signal will be difficult to distinguish from the ordinary Hall effect due to stray fields from the nearby FM electrode.More sophisticated measurement geometries are unlikely to solve the problem. For instance, if the FM magnetization is oriented by an in-plane magnetic field at an angle with the charge current, anisotropic magnetoresistance results. This gives rise to a planar Hall effect 10-12 which is difficult to distinguish from a signal due to the spin-momentum coupling in the TI 13 . The situation would not be different if the current flows perpendicular to the interface between TI and the ferromagnet, since the strong spin orbit interaction of the TI may induce an anisotropic tunneling magnetoresistance signal, as has been observed in devices consisting of a tunneling barrier between a ferromagnet and a non-magnetic layer. 14,15 Here, we propose a class of transport experiments a) Condensed Matter and Materials division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA (present address) to confirm the spin-momentum coupling in TI surface states which circumvents these problems by injecting spin polarized electrons from a long-distance silicon transport channel into the topological ...