We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier-assisted thermoelectric technologies for efficient solar energy harvesting.

The development of spintronics devices relies on efficient generation of spin-polarized currents and their electric-field-controlled manipulation. While observation of exceptionally long spin relaxation lengths makes graphene an intriguing material for spintronics studies, electric field modulation of spin currents is almost impossible due to negligible intrinsic spin-orbit coupling of graphene. In this work, we create an artificial interface between monolayer graphene and few-layer semiconducting tungsten disulphide. In these devices, we observe that graphene acquires spin-orbit coupling up to 17 meV, three orders of magnitude higher than its intrinsic value, without modifying the structure of the graphene. The proximity spin-orbit coupling leads to the spin Hall effect even at room temperature, and opens the door to spin field effect transistors. We show that intrinsic defects in tungsten disulphide play an important role in this proximity effect and that graphene can act as a probe to detect defects in semiconducting surfaces.

The physics of Dirac fermions in condensed-matter systems has received extraordinary attention following the discoveries of two new types of quantum Hall effect in single-layer and bilayer graphene [1][2][3] . The electronic structure of trilayer graphene (TLG) has been predicted to consist of both massless single-layer-graphene-like and massive bilayer-graphene-like Dirac subbands [4][5][6][7] , which should result in new types of mesoscopic and quantum Hall phenomena. However, the low mobility exhibited by TLG devices on conventional substrates has led to few experimental studies 8,9 . Here we investigate electronic transport in high-mobility (>100,000 cm Bernal-or ABA-stacked TLG (Fig. 1b) is an intriguing material to study Dirac physics and the quantum Hall effect (QHE) because of its unique band structure, which, in the simplest approximation, consists of massless single-layer-graphene (SLG)-like and massive bilayer graphene (BLG)-like subbands at low energy (Fig. 1c;. The energies of the Landau levels (LLs) for massless charge carriers depend on the square root of the magnetic field √ B (refs 1, 2,11-13), whereas for massive charge carriers they depend linearly on B (refs 3,11,12,14). Therefore, the LLs from these two different subbands in TLG should cross at some finite fields, resulting in accidental LL degeneracies at the crossing points. However, one of the main challenges so far to observe the QHE in TLG has been its low mobility on SiO 2 substrates 8,9 . To overcome this problem, we use hexagonal boron nitride (hBN) single crystals 15 as local substrates, which have been shown to reduce carrier scattering in graphene devices 16 (See Methods and Supplementary Information for fabrication). Substrate-supported devices also enable us to reach higher carrier density than suspended samples 17 , which is necessary for the observation of the LL crossings. Figure 1e,f shows the resistivity and conductivity of a TLG device at zero magnetic field. The resistivity at the Dirac peak exhibits a strong temperature dependence, which in SLG is a strong indication of high device quality 18,19 . In addition, we also observe a doublepeak structure at low temperatures (Fig. 1e). This double-peak structure is probably due to the band overlap that occurs in TLG when all SWMcC parameters are included in the tight-binding calculation of its band structure, as we show below. The field-effect mobility of this device reaches 110,000 cm 2 V −1 s −1 at 300 mK at densities as high as 6 × 10 11 cm −2 . This mobility value is two orders of magnitude higher than previously reported values for supported TLG (refs 8,9) and comparable to suspended samples 17,19 . The low disorder and high mobility enable us to probe LL crossings of Dirac fermions through the measurement of Shubnikov-de Haas oscillations (SdHOs). Figure 2a shows longitudinal resistivity ρ xx as a function of 1/B, for a carrier density n = −4.4 × 10 12 cm −2 . At low B (below ∼1 T), there are a number of oscillations characterized by broad minima separated by relatively n...

We study the electronic transport properties of dual-gated bilayer graphene devices. We focus on the regime of low temperatures and high electric displacement fields, where we observe a clear exponential dependence of the resistance as a function of displacement field and density, accompanied by a strong non-linear behavior in the transport characteristics. The effective transport gap is typically two orders of magnitude smaller than the optical band gaps reported by infrared spectroscopy studies. Detailed temperature dependence measurements shed light on the different transport mechanisms in different temperature regimes.PACS numbers: 72.80. Vp, 73.20.Hb, 73.22.Pr The ability to electrostatically tune and deplete the charge density in two-dimensional electron gases enables the fabrication of basic mesoscopic devices, such as quantum point contacts or quantum dots, which enhance our understanding of electronic transport in nanostructures [1]. Creating such electrically tunable nanostructures in monolayer graphene, a novel two-dimensional system [2], is far more challenging due to its gapless nature. In this respect, Bernal-stacked bilayer graphene (BLG) is an interesting material, because of the possibility of opening a band gap by breaking the symmetry between the top and bottom graphene sheets [3][4][5].The low-energy band structure of free-standing BLG is gapless but in the presence of an on-site energy difference between the bottom and top layers a band gap develops. Different methods have been employed to induce a band gap including molecular doping, coupling to the substrate, and electric displacement field generated by gate electrodes [6][7][8][9][10][11][12]. However, the low-temperature (≤ 100 K) transport characteristics of dual-gated BLG devices do not exhibit the strong suppression of conductance expected given the large band gaps (up to 250 meV) measured by infrared spectroscopy [9][10][11][12]. In addition, only very weak non-linearities were found in the current versus source-drain voltage (I-V SD ) characteristics [9], in contrast with the strong non-linear behavior of typical semiconducting devices. A more complete study is needed to address the transport characteristics of gapped BLG devices as well as the role played by disorder.In this letter, we report on the electronic transport properties of dual-gated (back-gated (BG) and topgated (TG)) BLG devices. We focus on the regime of large transverse electric displacement fields, 0.8 V/nm < |D| < 2.5 V/nm, over 3 times larger than in previous low-temperature experiments [9]. Upon the application of a large displacement field, we observe an exponential dependence of the device resistance on |D| and density, which is accompanied by a strong non-linear behavior in the I-V SD characteristics. However, the size of the effective transport gap is on the order of a few meV, two orders of magnitude smaller than the optical band gaps at the same D [11,12], suggesting a strong role played by disorder. Temperature dependent measurements in the 300 mK to 10...

We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron 13 C nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time, T1, first decreases with parallel magnetic field then goes through a minimum in a field of 1.4 T. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which suppresses hyperfine mediated relaxation and enhances relaxation due to soft phonons. The inhomogeneous dephasing time, T * 2 , is consistent with previous data on hyperfine coupling strength in 13 C nanotubes.Few-electron double quantum dots have enabled the coherent manipulation and detection of individual and coupled electron spin states required to form qubits [1,2,3,4]. Although recent protocols mitigate decoherence due to hyperfine coupling in GaAs-based devices [5,6], an attractive alternative is to base spin qubits on group IV elements, which primarily comprise isotopes free of nuclear spins. Progress in this direction includes double quantum dots in Si/SiGe 2DEGs [7], P donors in Si [8], Ge/Si nanowires [9], and carbon nanotubes [10]. Recent advances in nanotube double dots include observation of singlet-triplet physics [11] and Pauli blockade [12]. Developing these systems as spin qubits depends crucially on understanding their modes of relaxation and dephasing.This Letter reports measurements of relaxation and dephasing times in a two-electron nanotube double quantum dot grown from isotopically enriched (99%) 13 C methane. Measurements use fast pulses applied to electrostatic gates combined with charge sensing measurements in the Pauli blockade regime, including spin and isospin quantum states. The relaxation time of these states, T 1 , initially decreases with parallel field and has a minimum in a field of 1.4 T. We interpret these results within the context of the recently observed [13] spinorbit interaction in carbon nanotubes [14,15]. We also measure a relatively short two-electron inhomogeneous dephasing time, T * 2 ∼ 3 ns, which presumably arises from hyperfine coupling. The implied hyperfine coupling strength is consistent with values measured recently by transport [16]. In contrast, the long T 1 does not show signatures of hyperfine coupling.The double dot studied here is based on a single-walled carbon nanotube grown by chemical vapor deposition using 99%13 CH 4 feedstock [17,18]. After deposition of two pairs of Pd contacts [ Fig. 1(a), red], the device is coated with a 30 nm functionalized Al 2 O 3 top-gate oxide using atomic layer deposition [19,20]. Aluminum top-gates (blue, yellow, and gray) define a double dot between contacts 1 and 2 and a single dot between contacts 3 and 4, capacitively coupled [orange wire in Fig. 1(a)] to the double dot to allow charge sensing [9,21]. The small bandgap (∼ 25 meV) nanotube is operated in the electron regime. Direct current and standard lock-in measurements are carried out in a dilution refrigerator (electron temperature ∼ 100 mK). Electron occupancies (N L , N R ) of the dou...

Electrons in a periodic lattice can propagate without scattering for macroscopic distances despite the presence of the nonuniform Coulomb potential due to the nuclei 1 . Such ballistic motion of electrons allows the use of a transverse magnetic field to focus electrons 2 . This phenomenon, known as transverse magnetic focusing (TMF), has been used to study the Fermi surface of metals 3 and semiconductor heterostructures 4 , as well as to investigate Andreev reflection 3 and spin-orbit interaction 5 , and to detect composite fermions 6,7 . Here we report on the experimental observation of TMF in high-mobility mono-, biand tri-layer graphene devices. The ability to tune the graphene carrier density enables us to investigate TMF continuously from the hole to the electron regime and analyse the resulting focusing fan. Moreover, by applying a transverse electric field to tri-layer graphene, we use TMF as a ballistic electron spectroscopy method to investigate controlled changes in the electronic structure of a material. Finally, we demonstrate that TMF survives in graphene up to 300 K, by far the highest temperature reported for any system, opening the door to new room-temperature applications based on electron-optics.

We investigate electronic transport in dual-gated twisted bilayer graphene. Despite the subnanometer proximity between the layers, we identify independent contributions to the magnetoresistance from the graphene Landau level spectrum of each layer. We demonstrate that the filling factor of each layer can be independently controlled via the dual gates, which we use to induce Landau level crossings between the layers. By analyzing the gate dependence of the Landau level crossings, we characterize the finite inter-layer screening and extract the capacitance between the atomically-spaced layers. At zero filling factor, we observe magnetic and displacement field dependent insulating states, which indicate the presence of counter-propagating edge states with inter-layer coupling.

Using a scanning electron microscope, we make real-time movies of gold nanowires during the process of electromigration. We confirm the importance of using a small series resistance when employing electromigration to make controlled nanometer-scale gaps suitable for molecular-electronics studies. We are also able to estimate the effective temperature experienced by molecular adsorbates on the nanowire during the electromigration process.