50 research outputs found
Transport Spectroscopy of Sublattice-Resolved Resonant Scattering in Hydrogen-Doped Bilayer Graphene
We report the experimental observation of sublattice-resolved resonant scattering in bilayer graphene by performing simultaneous cryogenic atomic hydrogen doping and electron transport measurements in an ultrahigh vacuum. This allows us to monitor the hydrogen adsorption on the different sublattices of bilayer graphene without atomic-scale microscopy. Specifically, we detect two distinct resonant scattering peaks in the gate-dependent resistance, which evolve as a function of the atomic hydrogen dosage. Theoretical calculations show that one of the peaks originates from resonant scattering by hydrogen adatoms on the a sublattice (dimer site) while the other originates from hydrogen adatoms on the beta sublattice (nondimer site), thereby enabling a method for characterizing the relative sublattice occupancy via transport measurements. Utilizing this new capability, we investigate the adsorption and thermal desorption of hydrogen adatoms via controlled annealing and conclude that hydrogen adsorption on the beta sublattice is energetically favored. Through site-selective desorption from the alpha sublattice, we realize hydrogen doping with adatoms primarily on a single sublattice, which is highly desired for generating ferromagnetism
Probing Tunneling Spin Injection into Graphene via Bias Dependence
The bias dependence of spin injection in graphene lateral spin valves is
systematically studied to determine the factors affecting the tunneling spin
injection efficiency. Three types of junctions are investigated, including MgO
and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC
bias current applied to the injector electrode induces a strong nonlinear bias
dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers.
Furthermore, this signal reverses its sign at a negative DC bias for both kinds
of tunnel barriers. The analysis of the bias dependence for injector electrodes
with a wide range of contact resistances suggests that the sign reversal
correlates with bias voltage rather than current. We consider different
mechanisms for nonlinear bias dependence and conclude that the energy-dependent
spin-polarized electronic structure of the ferromagnetic electrodes, rather
than the electrical field-induced spin drift effect or spin filtering effect of
the tunnel barrier, is the most likely explanation of the experimental
observations.Comment: 10 pages, 6 figure
Topological Dirac Semimetal Na3Bi Films in the Ultrathin Limit via Alternating Layer Molecular Beam Epitaxy
Ultrathin films of Na3Bi on insulating substrates are desired for opening a
bulk band gap and generating the quantum spin Hall effect from a topological
Dirac semimetal, though continuous films in the few nanometer regime have been
difficult to realize. Here, we utilize alternating layer molecular beam epitaxy
(MBE) to achieve uniform and continuous single crystal films of Na3Bi(0001) on
insulating Al2O3(0001) substrates and demonstrate electrical transport on films
with 3.8 nm thickness (4 unit cells). The high material quality is confirmed
through in situ reflection high-energy electron diffraction (RHEED), scanning
tunneling microscopy (STM), x-ray diffraction (XRD), and x-ray photoelectron
spectroscopy (XPS). In addition, these films are employed as seed layers for
subsequent growth by codeposition, leading to atomic layer-by-layer growth as
indicated by RHEED intensity oscillations. These material advances facilitate
the pursuit of quantum phenomena in thin films of Dirac semimetals.Comment: 11 pages, 5 figure