Weyl Semimetal Path to Valley Filtering in Graphene.

We propose a device in which a sheet of graphene is coupled to a Weyl semimetal, allowing for the physical access to the study of tunneling from two- to three-dimensional massless Dirac fermions. Because of the reconstructed band structure, we find that this device acts as a robust valley filter for electrons in the graphene sheet. We show that, by appropriate alignment, the Weyl semimetal draws away current in one of the two graphene valleys, while allowing current in the other to pass unimpeded. In contrast to other proposed valley filters, the mechanism of our proposed device occurs in the bulk of the graphene sheet, obviating the need for carefully shaped edges or dimensions.

Helical Edge States and Quantum Phase Transitions in Tetralayer Graphene.

Helical conductors with spin-momentum locking are promising platforms for Majorana fermions. Here we report observation of two topologically distinct phases supporting helical edge states in charge neutral Bernal-stacked tetralayer graphene in Hall bar and Corbino geometries. As the magnetic field B_{⊥} and out-of-plane displacement field D are varied, we observe a phase diagram consisting of an insulating phase and two metallic phases, with 0, 1, and 2 helical edge states, respectively. These phases are accounted for by a theoretical model that relates their conductance to spin-polarization plateaus. Transitions between them arise from a competition among interlayer hopping, electrostatic and exchange interaction energies. Our work highlights the complex competing symmetries and the rich quantum phases in few-layer graphene.

Quantum zigzag transition in ion chains.

A string of trapped ions at zero temperature exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or the interparticle distance. The transition is driven by transverse, short wavelength vibrational modes. We argue that this is a quantum phase transition, which can be experimentally realized and probed. Indeed, by means of a mapping to the Ising model in a transverse field, we estimate the quantum critical point in terms of the system parameters, and find a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality. These results can be extended to describe the transverse instability of ultracold polar molecules in a one-dimensional optical lattice.

Large violation of the Wiedemann-Franz law in Luttinger liquids.

We show that in weakly disordered Luttinger liquids close to a commensurate filling the ratio of thermal conductivity kappa and electrical conductivity sigma can deviate strongly from the Wiedemann-Franz law valid for Fermi liquids scattering from impurities. In the regime where the umklapp scattering rate Gamma(U) is much larger than the impurity scattering rate Gamma(imp), the Lorenz number L = kappa/(sigmaT) rapidly changes from very large values L approximately Gamma(U)/Gamma(imp) >> 1 at the commensurate point to very small values L approximately Gamma(imp)/Gamma(U) << 1 for a slightly doped system. This surprising behavior is a consequence of approximate symmetries existing even in the presence of strong umklapp scattering.

Experimental estimates of dephasing time in molecular magnets.

Muon spin relaxation measurements in isotropic molecular magnets (MM) with a spin value S ranging from 7/2 to 27/2 are used to determine the magnitude and origin of dephasing time tau(phi) of molecular magnets. It is found that tau(phi) approximately 10 nsec with no S or ligand dependence. This indicates a nuclear origin for the stochastic field. Since tau(phi) is a property of the environment, we argue that it is a number common to similar types of MM. Therefore, tau(phi) is shorter than the Zener and tunneling times of anisotropic MM such as Fe(8) or Mn(4) for standard laboratory sweep rates. Our findings call for a stochastic Landau-Zener theory in this particular case.