Andrew C. Potter

Discovery of sodium in the atmosphere of Mercury

The spectrum of Mercury at the Fraunhofer sodium D lines shows strong emission features that are attributed to resonant scattering of sunlight from sodium vapor in the atmosphere of the planet. The total column abundance of sodium was estimated to be 8.1 x 1011 atoms per square centimeter, which corresponds to a surface density at the subsolar point of about 1.5 x 105 atoms per cubic centimeter. The most abundant atmospheric species found by the Mariner 10 mission to Mercury was helium, with a surface density of 4.5 x 103 atoms per cubic centimeter. It now appears that sodium vapor is a major constituent of Mercurys atmosphere.

Quantum oscillations from surface Fermi arcs in Weyl and Dirac semimetals

In a magnetic field, electrons in metals repeatedly traverse closed magnetic orbits around the Fermi surface. The resulting oscillations in the density of states enable powerful experimental techniques for measuring a metals Fermi surface structure. On the other hand, the surface states of Weyl semimetals consist of disjoint, open Fermi arcs raising the question of whether they can be observed by standard quantum oscillatory techniques. Here, we find that the open Fermi arcs participate in unusual closed magnetic orbits by traversing the bulk of the sample to connect opposite surfaces. These orbits have anomalous features that are impossible for conventional surface states, and result in quantum oscillations that contain observable signatures of the topological character of the bulk Weyl semimetal. We also apply our predictions to the compounds Cd3As2 and Na3Bi that were recently proposed to be three-dimensional Dirac (doubled Weyl) semimetals, and propose experimental signatures of their possible Fermi arc states.

Landau quantization and quasiparticle interference in the three-dimensional Dirac semimetal Cd3As2

Condensed-matter systems provide a rich setting to realize Dirac and Majorana fermionic excitations as well as the possibility to manipulate them for potential applications. It has recently been proposed that chiral, massless particles known as Weyl fermions can emerge in certain bulk materials or in topological insulator multilayers and give rise to unusual transport properties, such as charge pumping driven by a chiral anomaly. A pair of Weyl fermions protected by crystalline symmetry effectively forming a massless Dirac fermion has been predicted to appear as low-energy excitations in a number of materials termed three-dimensional Dirac semimetals. Here we report scanning tunnelling microscopy measurements at sub-kelvin temperatures and high magnetic fields on the II-V semiconductor Cd3As2. We probe this system down to atomic length scales, and show that defects mostly influence the valence band, consistent with the observation of ultrahigh-mobility carriers in the conduction band. By combining Landau level spectroscopy and quasiparticle interference, we distinguish a large spin-splitting of the conduction band in a magnetic field and its extended Dirac-like dispersion above the expected regime. A model band structure consistent with our experimental findings suggests that for a magnetic field applied along the axis of the Dirac points, Weyl fermions are the low-energy excitations in Cd3As2.

Transport evidence for Fermi-arc-mediated chirality transfer in the Dirac semimetal Cd3As2

The dispersion of charge carriers in a metal is distinctly different from that of free electrons owing to their interactions with the crystal lattice. These interactions may lead to quasiparticles mimicking the massless relativistic dynamics of high-energy particle physics, and they can twist the quantum phase of electrons into topologically non-trivial knots—producing protected surface states with anomalous electromagnetic properties. These effects intertwine in materials known as Weyl semimetals, and in their crystal-symmetry-protected analogues, Dirac semimetals. The latter show a linear electronic dispersion in three dimensions described by two copies of the Weyl equation (a theoretical description of massless relativistic fermions). At the surface of a crystal, the broken translational symmetry creates topological surface states, so-called Fermi arcs, which have no counterparts in high-energy physics or conventional condensed matter systems. Here we present Shubnikov–de Haas oscillations in focused-ion-beam-prepared microstructures of Cd3As2 that are consistent with the theoretically predicted ‘Weyl orbits’, a kind of cyclotron motion that weaves together Fermi-arc and chiral bulk states. In contrast to conventional cyclotron orbits, this motion is driven by the transfer of chirality from one Weyl node to another, rather than momentum transfer of the Lorentz force. Our observations provide evidence for direct access to the topological properties of charge in a transport experiment, a first step towards their potential application.

Classification of Interacting Electronic Topological Insulators in Three Dimensions

Interacting and Topological Topological insulators (TIs), which have a bandgap and a robust conducting surface state protected by time-reversal symmetry, are typically materials with weak electron-electron interaction and are well-described by band theory. A major experimental goal has been to observe such symmetry-protected topological (SPT) phases in interacting systems. Wang et al. (p. 629) used a theoretical approach to classify SPT phases of interacting fermions in three dimensions and found six other phases in addition to the noninteracting ones. The results lay the groundwork for future microscopic models and inform the experimental search for such materials. Six symmetry-protected topological phases that have no counterpart in noninteracting systems are identified. A fundamental open problem in condensed-matter physics is how the dichotomy between conventional and topological band insulators is modified in the presence of strong electron interactions. We show that there are six interacting electronic topological insulators that have no noninteracting counterpart. Combined with the previously known band insulators, these produce a total of eight topologically distinct phases. Two of the six interacting topological insulators can be described as Mott insulators in which the electron spins form spin analogs of the topological band insulator. The remaining phases are obtained as combinations of these two “topological paramagnets” and the topological band insulator. We prove that these eight phases form a complete list of all possible interacting topological insulators and discuss their experimental signatures.

Midinfrared spectra of Mercury

Observations of Mercury in the mid infrared (8–12.5 μm region) obtained over a variety of longitudes using the Fourier Transform Spectrometer (FTS) on the McMath-Pierce 1.5 m solar telescope at Kitt Peak reveal low-contrast spectra, except in cases where atmospheric noise or difficulties in guiding the telescope caused spurious signals. Although there are features that are suggestive of emissivity maxima (Christiansen features), their proximity to the telluric absorption band, and the noise in the spectra, preclude their unambiguous interpretation. We see little evidence of reststrahlen bands in our data; however, there are strong indications of transparency features. A shallow emissivity minimum occurs at 12 μm in the spectra centered at 80°, 256°, and 266° Hermean longitudes. A minimum occurs at 12.5 μm in the spectra centered at 15°, and a doublet minimum, with one trough at 12.2 μm and a second trough at 12.4–12.6 μm, is seen in the spectra centered at 229°. These features indicate the presence of a fine powder on the Mercurian surface, and their low spectral contrast suggests a significant percentage of agglutinitic material. On the basis of the position of the transparency features, we conclude that our spectra are indicative of intermediate, mafic, and ultramafic rock types. Further specificity is not warranted by the data.

Anomalous supercurrent from Majorana states in topological insulator Josephson junctions

We propose a Josephson junction setup based on a topological insulator (TI) thin film to detect Majorana states that exploits the unique helical and extended nature of the TI surface state. When the magnetic flux through the junction is close to an integer number of flux quanta, Majorana states, present on both surfaces of the film, give rise to a narrow peak-dip structure in the current-phase relation by hybridizing at the edge of the junction. Remarkably, the maximal Majorana-state contribution to Josephson current takes a (nearly) universal value, approximately equal to the supercurrent capacity of a single quantum channel. These features provide a characteristic signature of Majorana states based entirely on supercurrent.

Thermoelectric transport signatures of Dirac composite fermions in the half-filled Landau level

The half filled Landau level is expected to be approximately particle-hole symmetric, which requires an extension of the Halperin-Lee-Read (HLR) theory of the compressible state observed at this filling. Recent work indicates that, when particle-hole symmetry is preserved, the composite Fermions experience a quantized

Magnetic torque anomaly in the quantum limit of Weyl semimetals

pi

Recycling of Ions in Mercury’s Magnetosphere

-Berry phase upon winding around the composite Fermi-surface, analogous to Dirac fermions at the surface of a 3D topological insulator. In contrast, the effective low energy theory of the composite fermion liquid originally proposed by HLR lacks particle-hole symmetry and has vanishing Berry phase. In this paper, we explain how thermoelectric transport measurements can be used to test the Dirac nature of the composite Fermions by quantitatively extracting this Berry phase. First we point out that longitudinal thermopower (Seebeck effect) is non-vanishing due to the unusual nature of particle hole symmetry in this context and is not sensitive to the Berry phase. In contrast, we find that off-diagonal thermopower (Nernst effect) is directly related to the topological structure of the composite Fermi surface, vanishing for zero Berry phase and taking its maximal value for