Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires, and graphene. Recently, a new paradi
gm has emerged with the advent of symmetry-protected surface states on the boundary of topological insulators, enabling the creation of electronic systems with novel properties. For example, time reversal symmetry (TRS) endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, locking the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations of the one-dimensional boundary states of a two-dimensional topological insulator are also possible, but have yet to be observed in the leading candidate materials. Here, we demonstrate experimentally that charge neutral monolayer graphene displays a new type of quantum spin Hall (QSH) effect, previously thought to exist only in TRS topological insulators, when it is subjected to a very large magnetic field angled with respect to the graphene plane. Unlike in the TRS case, the QSH presented here is protected by a spin-rotation symmetry that emerges as electron spins in a half-filled Landau level are polarized by the large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic (CAF) state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture.
Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of a van der Waals heterostructure composed of a monolayer graphene flake
coupled to a rotationally-aligned hexagonal boron nitride substrate. The spatially-varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moire superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.
We investigate the electronic structure of carbon nanotubes functionalized by adsorbates anchored with single C-C covalent bonds. We find that, despite the particular adsorbate, a spin moment with a universal value of 1.0 $mu_B$ per molecule is induc
ed at low coverage. Therefore, we propose a mechanism of bonding-induced magnetism at the carbon surface. The adsorption of a single molecule creates a dispersionless defect state at the Fermi energy, which is mainly localized in the carbon wall and presents a small contribution from the adsorbate. This universal spin moment is fairly independent of the coverage as long as all the molecules occupy the same graphenic sublattice. The magnetic coupling between adsorbates is also studied and reveals a key dependence on the graphenic sublattice adsorption site.
In this article we present a neutron diffraction in-situ study of the thermal evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2 O5+{delta}. Neutron thermodiffractograms and magnetic susceptibility measurements are reported i
n the temperature range 20 K <= T <= 570 K, as well as high resolution neutron diffraction experiments at selected temperatures. Starting from the as-synthesized samples with {delta} ~ 0.5, we show that the room temperature phases remain stable up to 550 K, where they start loosing oxygen and transform to a vacancy-disordered 112 structure with tetragonal symmetry. Our results also show how the so-called 122 structure can be stabilized at high temperature (around 450 K) in a sample in which the addition of Sr at the Ba site had suppressed its formation. In addition, we present the structural and magnetic properties of the resulting samples with a new oxygen content {delta} ~ 0.25 in the temperature range 20 K <= T <= 300 K.
We demonstrate anisotropic etching of single-layer graphene by thermally-activated nickel nanoparticles. Using this technique, we obtain sub-10nm nanoribbons and other graphene nanostructures with edges aligned along a single crystallographic directi
on. We observe a new catalytic channeling behavior, whereby etched cuts do not intersect, resulting in continuously connected geometries. Raman spectroscopy and electronic measurements show that the quality of the graphene is resilient under the etching conditions, indicating that this method may serve as a powerful technique to produce graphene nanocircuits with well-defined crystallographic edges.
Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus pro
jectile velocity displays an effective threshold velocity of ~0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is ~2.4 at velocities slightly above the threshold (v~0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be stationary and extremely local.
