Search

Inducing and controlling spin-orbit coupling (SOC) in graphene is key to create topological states of matter, and for the realization of spintronic devices. Placing graphene onto a transition metal dichalcogenide is currently the most successful strategy to achieve this goal, but there is no consensus as to the nature and the magnitude of the induced SOC. Here, we show that the presence of backscattering in graphene-on-WSe2 heterostructures can be used to probe SOC and to determine its strength quantitatively, by imaging quasiparticle interference with a scanning tunneling microscope. A detailed theoretical analysis of the Fourier transform of quasiparticle interference images reveals that the induced SOC consists of a valley-Zeeman (λvZ ≈ 2 meV) and a Rashba (λR ≈ 15 meV) term, one order of magnitude larger than what theory predicts, but in excellent agreement with earlier transport experiments. The validity of our analysis is confirmed by measurements on a 30 degree twist angle heterostructure that exhibits no backscattering, as expected from symmetry considerations. Our results demonstrate a viable strategy to determine SOC quantitatively by imaging quasiparticle interference. Graphene has many intriguing electronic properties. One of note is the absence of backscattering of electrons confined to a single valley. Spin-orbit interactions can allow backscattering, and here, Sun et al. use this spin-orbit coupling dependence of backscattering to measure the strength of the spin-orbit interaction in a graphene/tungsten selenide heterostructure. [ABSTRACT FROM AUTHOR]

Spin-orbit interaction (SOI) that is gate-tunable over a broad range is essential to exploiting novel spin phenomena. Achieving this regime has remained elusive because of the weakness of the underlying relativistic coupling and lack of its tunability in solids. Here we outline a general strategy that enables exceptionally high tunability of SOI through creating a which-layer spin-orbit field inhomogeneity in graphene multilayers. An external transverse electric field is applied to shift carriers between the layers with strong and weak SOI. Because graphene layers are separated by subnanometer scales, exceptionally high tunability of SOI can be achieved through a minute carrier displacement. A detailed analysis of the experimentally relevant case of bilayer graphene on a semiconducting transition metal dichalchogenide substrate is presented. In this system, a complete tunability of SOI amounting to its ON/OFF switching can be achieved. New opportunities for spin control are exemplified with electrically driven spin resonance and topological phases with different quantized intrinsic valley Hall conductivities. [ABSTRACT FROM AUTHOR]

We introduce a new scheme to realizesuspended, multiterminal graphenestructures that can be current annealed successfully to obtain uniform,very high quality devices. A key aspect is that the bulky metalliccontacts are not connected directly to the part of graphene probedby transport measurements, but only through etched constriction, whichprevents the contacts from acting invasively. The device high qualityand uniformity is demonstrated by a reproducibly narrow (δn∼ 109cm–2) resistancepeak around charge neutrality, by carrier mobility values exceeding106cm2V–1s–1, by the observation of integer quantum Hall plateaus starting at30 mT and of symmetry broken states at about 200 mT, and by the occurrenceof a negative multiterminal resistance directly proving the occurrenceof ballistic transport. As these multiterminal devices enable measurementsthat cannot be done in a simpler two-terminal configuration, we anticipatethat their use in future studies of graphene-based systems will beparticularly relevant. [ABSTRACT FROM AUTHOR]

To minimize parasitic doping effects caused by uncontrolled material adsorption, graphene is often investigated under vacuum. Here we report an entirely unexpected phenomenon occurring in vacuum systems, namely strong n-doping of graphene due to chemical species generated by common ion high-vacuum gauges. The effect—reversible upon exposing graphene to air—is significant, as doping rates can largely exceed 1012 cm−2 h−1, depending on pressure and the relative position of the gauge and the graphene device. It is important to be aware of this phenomenon, as its basic manifestation can be mistakenly interpreted as vacuum-induced desorption of p-dopants. [ABSTRACT FROM AUTHOR]

Suspended graphene nanoribbons formed during current annealing of suspended graphene flakes have been investigated experimentally. Transport measurements show the opening of a transport gap around charge neutrality due to the formation of "Coulomb islands", coexisting with quantum Hall conductance plateaus appearing at moderate values of the magnetic field B. Upon increasing B, the transport gap is rapidly suppressed, and is taken over by a much larger energy gap due to electronic correlations. Our observations show that suspended nanoribbons allow the investigation of phenomena that could not so far be accessed in ribbons on SiO2 substrates. [ABSTRACT FROM AUTHOR]

We present a comparative study of high carrier density transport in mono-, bi-, and trilayer graphene using electric double-layer transistors to continuously tune the carrier density up to values exceeding 1014 cm-2. Whereas in monolayer the conductivity saturates, in bi- and trilayer filling of the higher-energy bands is observed to cause a nonmonotonic behavior of the conductivity and a large increase in the quantum capacitance. These systematic trends not only show how the intrinsic high-density transport properties of graphene can be accessed by field effect, but also demonstrate the robustness of ion-gated graphene, which is crucial for possible future applications. [ABSTRACT FROM AUTHOR]

The edges of graphene-based systems possess unusual electronic properties, originating from the non-trivial topological structure associated with the pseudospinorial character of the electron wavefunctions. These properties, which have no analogue for electrons described by the Schrödinger equation in conventional systems, have led to the prediction of many striking phenomena, such as gate-tunable ferromagnetism and valley-selective transport. In most cases, however, the predicted phenomena are not expected to survive the strong structural and chemical disorder that unavoidably affects the edges of real graphene devices. Here, we present a theoretical investigation of the intrinsic low-energy states at the edges of electrostatically gapped bilayer graphene, and find that the contribution of edge modes to the linear conductance of realistic devices remains sizable even for highly imperfect edges. This contribution may dominate over that of the bulk for sufficiently clean devices, such as those based on suspended bilayer graphene samples. Our results illustrate the robustness of those phenomena whose origin is rooted in the topology of the electronic band structure, even in the absence of specific protection mechanisms. [ABSTRACT FROM AUTHOR]

The article focuses on several studies that explored trilayer graphene behavior concluding on the importance of layers where they are stacked and the thickness to determine electronic properties of the stack. It references three studies published in "Nature Physics" in 2011 that classified graphene layers with physical two-dimensional (2D) systems. Also discussed are the ABC trilayers, the ABA-stacked trilayers and the Bernal-stacked multilayers in even numbers than can be used as insulators.

The article reviews two articles about a study showing that the fractional quantum Hall effect (FQHE) can also be observed in graphene by X. Du, K. I. Bolotin and colleagues.

We discuss transport measurements through graphene Andreev interferometers exhibiting reentrance of the superconducting proximity effect. We observe that at high gate voltage (VBG) the energy dependence of the Andreev conductance oscillations exhibits a scaling in agreement with theoretical expectations, which breaks down at low VBG, when the Fermi energy approaches the charge neutrality point. The phenomenon is a manifestation of single particle dephasing that increasingly limits the propagation of superconducting correlations away from the superconductor-graphene interface. Our work addresses the interplay between microscopic decoherence and superconductivity, and shows that graphene provides a useful experimental platform to investigate unexplored regimes and phenomena in the superconducting proximity effect. [ABSTRACT FROM AUTHOR]

We report transport measurements through graphene on SrTiO3 substrates as a function of magnetic field B, carrier density n, and temperature T. The large dielectric constant of SrTiO3 very effectively screens long-range electron-electron interactions and potential fluctuations, making Dirac electrons in graphene virtually noninteracting. The absence of interactions results in an unexpected behavior of the longitudinal resistance in the N = 0 Landau level and in a large suppression of the transport gap in nanoribbons. The "bulk" transport properties of graphene at B = 0 T, on the contrary, are completely unaffected by the substrate dielectric constant. [ABSTRACT FROM AUTHOR]

Suspended Bernal-stacked graphene multilayers up to an unexpectedly large thickness exhibit a broken-symmetry ground state whose origin remains to be understood. We show that a finite-temperature second-order phase transition occurs in multilayers whose critical temperature (Tc) increases from 12 kelvins (K) in bilayers to 100 K in heptalayers. A comparison of the data with a phenomenological model inspired by a mean-field approach suggests that the transition is associated with the appearance of a selfconsistent valley- and spin-dependent staggered potential that changes sign from one layer to the next, appearing at Tc and increasing upon cooling. The systematic evolution with thickness of several measured quantities imposes constraints on any microscopic theory aiming to analyze the nature of electronic correlations in this system. [ABSTRACT FROM AUTHOR]

We investigate low-temperature magneto-transportin recently developed,high-quality multiterminal suspended bilayer graphene devices, enablingthe independent measurement of the longitudinal and transverse resistance.We observe clear signatures of the fractional quantum Hall effectwith different states that are either fully developed, and exhibita clear plateau in the transverse resistance with a concomitant dipin longitudinal resistance or incipient, and exhibit only a longitudinalresistance minimum. All observed states scale as a function of fillingfactor ν, as expected. An unprecedented even-denominator fractionalstate is observed at ν = −1/2 on the hole side, exhibitinga clear plateau in Rxyquantized at the expected value of 2h/e2with a precision of ∼0.5%. Many of our observations,together with a recent electronic compressibility measurement performedin graphene bilayers on hexagonal boron-nitride (hBN) substrates,are consistent with a recent theory that accounts for the effect ofthe degeneracy between the N= 0 and N= 1 Landau levels in the fractional quantum Hall effect and predictsthe occurrence of a Moore-Read type ν = −1/2 state. Owingto the experimental flexibility of bilayer graphene, which has a gate-dependentband structure, can be easily accessed by scanning probes, and canbe contacted with materials such as superconductors, our findingsoffer new possibilities to explore the microscopic nature of even-denominatorfractional quantum Hall effect. [ABSTRACT FROM AUTHOR]