46 research outputs found
Ballistic transport in graphene antidot lattices
Graphene samples can have a very high carrier mobility if influences from the
substrate and the environment are minimized. Embedding a graphene sheet into a
heterostructure with hexagonal boron nitride (hBN) on both sides was shown to
be a particularly efficient way of achieving a high bulk mobility.
Nanopatterning graphene can add extra damage and drastically reduce sample
mobility by edge disorder. Preparing etched graphene nanostructures on top of
an hBN substrate instead of SiO2 is no remedy, as transport characteristics are
still dominated by edge roughness. Here we show that etching fully encapsulated
graphene on the nanoscale is more gentle and the high mobility can be
preserved. To this end, we prepared graphene antidot lattices where we observe
magnetotransport features stemming from ballistic transport. Due to the short
lattice period in our samples we can also explore the boundary between the
classical and the quantum transport regime
BandleitfĂ€higkeit im HofstadterâSchmetterling
Die erlaubten Energiewerte von Elektronen in Kristallen liegen in BĂ€ndern, die auch den Stromtransport in leitfĂ€higen Materialien bestimmen. Setzt man Kristalle einem Magnetfeld aus, so zerfallen die BĂ€nder in ein fraktales Spektrum, den Hofstadter-Schmetterling. Wir konnten in einem kĂŒnstlichen Kristallpotential in einlagigem Graphen zeigen, dass sich diese Struktur durch Oszillationen der BandleitfĂ€higkeit nachweisen lĂ€sst. Diese Oszillationen sind noch bei Temperaturen sichtbar, bei denen der innere Aufbau des Hofstadter-Schmetterlings schon nicht mehr zu erkennen ist
Terahertz Electric Field Driven Electric Currents and Ratchet Effects
Terahertz field induced photocurrents in graphene were studied experimentally and by microscopic modeling. Currents were generated by cw and pulsed laser radiation in large area as well as small-size exfoliated graphene samples. We review general symmetry considerations leading to photocurrents depending on linear and circular polarized radiation and then present a number of situations where photocurrents were detected. Starting with the photon drag effect under oblique incidence, we proceed to the photogalvanic effect enhancement in the reststrahlen band of SiC and edge-generated currents in graphene. Ratchet effects were considered for in-plane magnetic fields and a structure inversion asymmetry as well as for graphene with non-symmetric patterned top gates. Lastly, we demonstrate that graphene can be used as a fast, broadband detector of terahertz radiation
Counterintuitive gate dependence of weak antilocalization in bilayer graphene/WSe heterostructures
Strong gate control of proximity-induced spin-orbit coupling was recently
predicted in bilayer graphene/transition metal dichalcogenides (BLG/TMDC)
heterostructures, as charge carriers can easily be shifted between the two
graphene layers, and only one of them is in close contact to the TMDC. The
presence of spin-orbit coupling can be probed by weak antilocalization (WAL) in
low field magnetotransport measurements. When the spin-orbit splitting in such
a heterostructure increases with the out of plane electric displacement field
, one intuitively expects a concomitant increase of WAL visibility. Our
experiments show that this is not the case. Instead, we observe a maximum of
WAL visibility around . This counterintuitive behaviour originates in
the intricate dependence of WAL in graphene on symmetric and antisymmetric spin
lifetimes, caused by the valley-Zeeman and Rashba terms, respectively. Our
observations are confirmed by calculating spin precession and spin lifetimes
from an model Hamiltonian of BLG/TMDC.Comment: Accepted by Phys Rev
Signal enhancement in amperometric peroxide detection by using graphene materials with low number of defects
Two-dimensional carbon nanomaterials ranging from single-layer graphene to defective structures such as chemically reduced graphene oxide were studied with respect to their use in electrodes and sensors. Their electrochemical properties and utility in terms of fabrication of sensing devices are compared. Specifically, the electrodes have been applied to reductive amperometric determination of hydrogen peroxide. Low-defect graphene (SG) was obtained through mechanical exfoliation of natural graphite, while higher-defect graphenes were produced by chemical vapor deposition (CVDG) and by chemical oxidation of graphite and subsequent reduction (rGO). The carbonaceous materials were mainly characterized by Raman microscopy. They were applied as electrode material and the electrochemical behavior was investigated by chronocoulometry, cyclic voltammetry, electrochemical impedance spectroscopy and amperometry and compared to a carbon disc electrode. It is shown that the quality of the graphene has an enormous impact on the amperometric performance. The use of carbon materials with many defects (like rGO) does not result in a significant improvement in signal compared to a plain carbon disc electrode. The sensitivity is 173 mA center dot M-1 center dot cm(-2) in case of using CVDG which is about 50 times better than that of a plain carbon disc electrode and about 7 times better than that of rGO. The limit of detection for hydrogen peroxide is 15.1 mu M (at a working potential of -0.3 V vs SCE) for CVDG. It is concluded that the application of two-dimensional carbon nanomaterials offers large perspectives in amperometric detection systems due to electrocatalytic effects that result in highly sensitive detection
Interplay of boundary states of graphene nanoribbons with a Kondo impurity
We present and discuss the methodology for modeling 4f photoemission spectra, 4f photoelectron diffraction (PED) patterns, and magnetic dichroism effects for rare-earth-based materials. Using PED and magnetic dichroism in photoemission, we explore the electronic and magnetic properties of the near-surface region of the valence-fluctuating material EuIr2Si2. For the Eu-terminated surface, we found that the topmost Eu layer is divalent and exhibits a ferromagnetic order below 10 K. The valency of the next Eu layer, that is the fifth atomic layer, is about 2.8 at low temperature that is close to the valency in the bulk. The properties of the Si-terminated surface are drastically different. The first subsurface Eu layer (fourth atomic layer below the surface) behaves divalently and orders ferromagnetically below 48 K. Experimental data indicate, however, that there is an admixture of trivalent Eu in this layer, resulting in its valency of about 2.1. The next deeper lying Eu layer (eighth atomic layer below the surface) behaves mixed valently, but the estimated valency of 2.4 is notably lower than the value in the bulk. The presented approach and obtained results create a background for further studies of exotic surface properties of 4f-based materials, and allow us to derive information related to valency and magnetism of individual rare-earth layers in a rather extended area near the surface
Spin field-effect transistor action via tunable polarization of the spin injection in a Co/MgO/graphene contact
We fabricated a non-local spin valve device with Co-MgO injector/detector tunnel contacts on a graphene spin channel. In this device, the spin polarization of the injector contact can be tuned by both the injector current bias and the gate voltage. The spin polarization can be turned off and even inverted. This behavior enables a spin transistor where the signal is switched off by turning off the spin injection using the field-effect. We propose a model based on a gate-dependent shift of the minimum in the graphene density of states with respect to the tunneling density of states of cobalt, which can explain the observed bias and gate dependence. Published by AIP Publishing
Magnetotransport in heterostructures of transition metal dichalcogenides and graphene
We use a van der Waals pickup technique to fabricate different heterostructures containing WSe2(WS2) and graphene. The heterostructures were structured by plasma etching, contacted by one-dimensional edge contacts, and a top gate was deposited. For graphene/WSe2/SiO2 samples we observe mobilities of similar to 12 000 cm(2) V-1 s(-1). Magnetic-field-dependent resistance measurements on these samples show a peak in the conductivity at low magnetic fields. This dip is attributed to the weak antilocalization (WAL) effect, stemming from spin-orbit coupling. Samples where graphene is encapsulated between WSe2(WS2) and hexagonal boron nitride show a much higher mobility of up to similar to 120 000 cm(2) V-1 s(-1). However, in these samples noWAL peak can be observed. We attribute this to a transition from the diffusive to the quasiballistic regime. At low magnetic fields a resistance peak appears, which we ascribe to a size effect due to boundary scattering. Shubnikov-de Haas oscillations in fully encapsulated samples show all integer filling factors due to complete lifting of the spin and valley degeneracies