Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics

Composite two-particle sources (1906.07712v1)

Michael Moskalets, Janne Kotilahti, Pablo Burset, Christian Flindt

2019-06-18

Multi-particle sources constitute an interesting new paradigm following the recent development of on-demand single-electron sources. Versatile devices can be designed using several single-electron sources, possibly of different types, coupled to the same quantum circuit. However, if combined \textit{non-locally} to avoid cross-talk, the resulting architecture becomes very sensitive to electronic decoherence. To circumvent this problem, we here analyse two-particle sources that operate with several single-electron (or hole) emitters attached in series to the same electronic waveguide. We demonstrate how such a device can emit exactly two electrons without exciting unwanted electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic regime, perfect two-electron emission can be achieved by driving two quantum dot levels across the Fermi level of the external reservoir. If a single-electron source is combined with a source of holes, the emitted particles can annihilate each other in a process which is governed by the overlap of their wave functions. Importantly, the degree of annihilation can be controlled by tuning the emission times, and the overlap can be determined by measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel experiment, the wave functions overlap close to the emitters and not after propagating to the beam splitter, making the shot noise reduction less susceptible to electronic decoherence.

Directional shift current in mirror-symmetric BCN (1906.07627v1)

J. Ibañez-Azpiroz, I. Souza, F. de Juan

2019-06-18

We present a first-principles theoretical study of the shift current in a noncentrosymmetric polytype of graphitic BCN, and find that the photoconductivity exhibits two distinctive features at the band edge. First, it ranks among the largest bulk nonlinear responses reported to date, with the peak value occurring in an energy range suitable for optical manipulation. Secondly, it is strongly anisotropic, due to the vanishing of particular tensor components not foretold by phenomenological symmetry arguments; this is a consequence of dipole selection rules imposed by mirror symmetry, which imply that the relative parities between valence and conduction bands are key for determining the directionality of the band-edge response. Our work identifies graphitic BCN as a promising candidate for next-generation photovoltaics, and opens up a broad framework for future studies.

Nonlinear chiral refrigerators (1904.04506v2)

David Sanchez, Rafael Sanchez, Rosa Lopez, Bjorn Sothmann

2019-04-09

We investigate a mesoscopic refrigerator based on chiral quantum Hall edge channels. We discuss a three-terminal cooling device in which charge transport occurs between a pair of voltage-biased terminals only. The third terminal, which is to be cooled, is set as a voltage probe with vanishing particle flux. This largely prevents the generation of direct Joule heating which ensures a high coefficient of performance. Cooling operation is based on energy-dependent quantum transmissions. The latter are implemented with the aid of two tunable scattering resonances (quantum dots). To find the optimal performance point and the largest temperature difference created with our refrigerator, it is crucial to address the nonlinear regime of transport, accounting for electron-electron interaction effects. Our numerical simulations show that the maximal cooling power can be tuned with the quantum dot couplings and energy levels. Further, we provide analytical expressions within a weakly nonlinear scattering-matrix formalism which allow us to discuss the conditions for optimal cooling in terms of generalized thermopowers. Our results are important for the assessment of chiral conductors as promising candidates for efficient quantum refrigerators with low dissipation.

Field Emission Characterization of MoS2 Nanoflowers (1906.07577v1)

Filippo Giubileo, Alessandro Grillo, Maurizio Passacantando, Francesca Urban, Laura Iemmo, Giuseppe Luongo, Aniello Pelella, Melanie Loveridge, Luca Lozzi, Antonio Di Bartolomeo

2019-06-18

Nanostructured materials have wide potential applicability as field emitters due to their high aspect ratio. We hydrothermally synthesized MoS2 nanoflowers on copper foil and characterized their field emission properties, by applying a tip-anode configuration in which a tungsten tip with curvature radius down to 30-100nm has been used as the anode to measure local properties from small areas down to 1-100um2. We demonstrate that MoS2 nanoflowers can be competitive with other well-established field emitters. Indeed, we show that a stable field emission current can be measured with a turn-on field as low as 12 V um-1 and a field enhancement factor up to 880 at 600nm cathode-anode separation distance.

Curved spacetime theory of inhomogeneous Weyl materials (1906.07540v1)

Long Liang, Teemu Ojanen

2019-06-18

We show how the universal low-energy properties of Weyl semimetals with spatially varying time-reversal (TR) or inversion (I) symmetry breaking are described in terms of chiral fermions experiencing curved-\emph{spacetime} geometry and synthetic gauge fields. By employing Clifford representations and Schrieffer-Wolff transformations, we present a systematic derivation of an effective curved-space Weyl theory with rich geometric and gauge structure. To illustrate the utility of the formalism, we give a concrete prescription of how to fabricate nontrivial curved spacetimes and event horizons in topological insulators with magnetic textures. Our theory can also account for strain-induced effects, providing a powerful unified framework for studying and designing inhomogeneous Weyl materials.

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