We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS2-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS2-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be ∼ 0.2ps −1 and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a'valley-Zeeman' SOC from first-principles calculations. The interplay between these two SOC's can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS2 system provides a possible platform for these novel edge states.

High lattice thermal conductivity has been the bottleneck for further improvement of Broader ContextHow to scavenge the vast amount of waste heat has increasingly become a major concern.Possibly, thermoelectrics can provide an economical and environmentally friendly way to achieve this. High thermoelectric figure-of-merit (ZT) materials are preferred for efficient performance. Historically, PbTe and skutterudites have been considered the candidates for medium temperature applications owing to their good ZT values. However, their uses are limited owing to either toxicity or low thermal stability. In the same temperature range, half-Heuslers are chemically non-toxic and thermally stable compared with the PbTe and skutterudites but the peak ZT of p-type half-Heuslers has remained around 0.5 for quite a long time till our recent work that improved the ZT to 0.8. In this work, we achieved ZT about 1 for p-type half-Heusler, which makes it practically useful for medium to high temperature power generation applications, such as waste heat recovery in car exhaust system. Specifically, the ZT enhancement mainly comes from the reduction of thermal conductivity, which arises partly from the enhanced alloy scattering due to larger differences in atomic mass and size of Hf and Ti than Hf and Zr and partly from enhanced boundary scattering due to various nanostructures.3

As a generic property, all substances transfer heat through microscopic collisions of constituent particles . A solid conducts heat through both transverse and longitudinal acoustic phonons, but a liquid employs only longitudinal vibrations. As a result, a solid is usually thermally more conductive than a liquid. In canonical viewpoints, such a difference also serves as the dynamic signature distinguishing a solid from a liquid. Here, we report liquid-like thermal conduction observed in the crystalline AgCrSe. The transverse acoustic phonons are completely suppressed by the ultrafast dynamic disorder while the longitudinal acoustic phonons are strongly scattered but survive, and are thus responsible for the intrinsically ultralow thermal conductivity. This scenario is applicable to a wide variety of layered compounds with heavy intercalants in the van der Waals gaps, manifesting a broad implication on suppressing thermal conduction. These microscopic insights might reshape the fundamental understanding on thermal transport properties of matter and open up a general opportunity to optimize performances of thermoelectrics.

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The rotational and vibrational transitions of a hydrogen molecule weakly adsorbed on the Au(110) surface at 10 K were detected by inelastic electron tunneling spectroscopy with a scanning tunneling microscope. The energies of the j=0 to j=2 rotational transition for para-H(2) and HD indicate that the molecule behaves as a three-dimensional rigid rotor trapped within the tunnel junction. An increase in the bond length of H(2) was precisely measured from the downshift in the rotational energy as the tip-substrate distance decreases.

Electron-diffraction and high-resolution lattice images reveal superstructure stripes with wave vectors of (1/3, 0, 1/3) and (-1/3, 0, 1/3), which are associated with the ordered arrangement of F(-). Charge density distribution suggests that these stripes manifest themselves electronically as F(-)-Cr(3+)-F(-) zigzag chains, driven by the anisotropic charge interaction of F(-) anions.

Abstract:A major obstacle to using SQUIDs as qubits is flux noise. We propose that the heretofore mysterious spins producing flux noise could be O 2 molecules adsorbed on the surface. Using density functional theory calculations, we find that an O 2 molecule adsorbed on an α-alumina surface has a magnetic moment of ~1.8 µ B . When the spin is oriented perpendicular to the axis of the O-O bond, the barrier to spin rotations is about 10 mK. Monte Carlo simulations of ferromagnetically coupled, anisotropic XY spins on a square lattice find 1/f magnetization noise, consistent with flux noise in Al SQUIDs.PACS numbers: 85.25. Dq, 74.25.Ha, 73.50.Td, 71.15 The microscopic origin of these spins remains unclear. Choi et al. [20] proposed that they are electrons in localized states at the metal-insulator interface, though spins have also been found on the surface of the dielectric, aluminum oxide, without a metal present [11]. Density functional theory (DFT) calculations [21] on sapphire (α-Al 2 O 3 ), emulating the oxide layer that typically forms on surfaces of SQUIDs, indicate that thermodynamically stable charged vacancies are unlikely to be the source of flux noise because of the large energy differences associated with spin reorientation, though these energy differences decrease as the charge decreases. Lee et al. [21] used DFT to suggest that ambient molecules, such as OH, adsorbed on the surface could be the culprits, though the energy differences between different spin orientations is hundreds of degrees Kelvin, making thermal spin fluctuations unlikely.Since SQUIDs are exposed to the atmosphere, we propose that the primary source of spins producing flux noise is O 2 molecules adsorbed on the surface. The free O 2 molecule has a spin triplet electronic configuration with a magnetic moment of 2.0 µ B [22] and is 3 strongly paramagnetic in its liquid phase. O 2 molecules absorbed on metal or oxide surfaces can form ordered lattices and exhibit exotic magnetic properties [23]. A natural question is whether they retain a large magnetic moment on the surface of metal oxides as well as on the surface of dielectric materials used to encapsulate SQUIDs [24]. If they do retain a large moment, it is important to know the associated magnetic anisotropy energies (MAEs) that are the energy barriers for spin reorientation and hence key to understanding thermal fluctuations. Due to the weak spin orbit coupling of oxygen, the MAEs of these systems are small, making them difficult to investigate theoretically and experimentally.In this Letter, using systematic DFT calculations, we report that O 2 molecules with a surface density of 1.08 × 10 18 m -2 have a large magnetic moment, 1.8 µ B /molecule, on an α-Al 2 O 3 (0001) surface. These spin moments are weakly coupled and can reorient almost freely in a plane perpendicular to the O-O bond, with an energy barrier at the level of a few mK. Our Monte Carlo simulations on ferromagnetically coupled, anisotropic XY spins on a 2D square lattice suggest that they indeed produce 1/f magn...