Mesoscale And Nanoscale Physics
Superfluid weight and Berezinskii-Kosterlitz-Thouless transition temperature of twisted bilayer graphene (1906.06313v1)
Aleksi Julku, Teemu Peltonen, Long Liang, Tero Heikkilä, Päivi Törmä
2019-06-14
We study superconductivity of twisted bilayer graphene with local and non-local attractive interactions. We obtain the superfluid weight and Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for microscopic tight-binding and low-energy continuum models. We predict qualitative differences between local and non-local interaction schemes which could be distinguished experimentally. In the flat band limit where the pair potential exceeds the band width we show that the superfluid weight and BKT temperature are determined by multiband processes and quantum geometry of the band.
Engineering electron-phonon coupling of quantum defects to a semi-confocal acoustic resonator (1906.06309v1)
Huiyao Chen, Noah F. Opondo, Boyang Jiang, Evan R. MacQuarrie, Raphaël S. Daveau, Sunil A. Bhave, Gregory D. Fuchs
2019-06-14
Diamond-based microelectromechanical systems (MEMS) enable direct coupling between the quantum states of nitrogen-vacancy (NV) centers and the phonon modes of a mechanical resonator. One example, diamond high-overtone bulk acoustic resonators (HBARs), feature an integrated piezoelectric transducer and support high-quality factor resonance modes into the GHz frequency range. The acoustic modes allow mechanical manipulation of deeply embedded NV centers with long spin and orbital coherence times. Unfortunately, the spin-phonon coupling rate is limited by the large resonator size,
m, and thus strongly-coupled NV electron-phonon interactions remain out of reach in current diamond BAR devices. Here, we report the design and fabrication of a semi-confocal HBAR (SCHBAR) device on diamond (silicon carbide) with
(
). The semi-confocal geometry confines the phonon mode laterally below 10~
m. This drastic reduction in modal volume enhances defect center electron-phonon coupling. For the native NV centers inside the diamond device, we demonstrate mechanically driven spin transitions and show a high strain-driving efficiency with a Rabi frequency of
~MHz/V
, which is comparable to a typical microwave antenna at the same microwave power.
Thermoelectric properties of gapped bilayer graphene (1810.02280v3)
Dominik Suszalski, Grzegorz Rut, Adam Rycerz
2018-10-04
Unlike in conventional semiconductors, both the chemical potential and the band gap in bilayer graphene (BLG) can be tuned via application of external electric field. Among numerous device implications, this property also designates BLG as a candidate for high-performance thermoelectric material. In this theoretical study we have calculated the Seebeck coefficients for abrupt interface separating weakly- and heavily-doped areas in BLG, and for a more realistic rectangular sample of mesoscopic size, contacted by two electrodes. For a given band gap (
) and temperature (
) the maximal Seebeck coefficient is close to the Goldsmid-Sharp value
, the deviations can be approximated by the asymptotic expression
, with the electron charge
, the Boltzmann constant
, and
. Surprisingly, the effects of trigonal warping term in the BLG low-energy Hamiltonian are clearly visible at few-Kelvin temperatures, for all accessible values of
meV. We also show that thermoelectric figure of merit is noticeably enhanced (
) when a rigid substrate suppresses out-of-plane vibrations, reducing the contribution from
phonons to the thermal conductivity.
Tunable phonon induced steady state coherence in a double quantum dot (1906.06271v1)
Archak Purkayastha, Giacomo Guarnieri, Mark T. Mitchison, Radim Filip, John Goold
2019-06-14
Charge qubits can be created and manipulated in solid-state double-quantum-dot (DQD) platforms. Typically, these systems are strongly affected by quantum noise stemming from coupling to substrate phonons. This is usually assumed to lead to decoherence towards steady states that are diagonal in the energy eigenbasis. In this letter we show, to the contrary, that due to the presence of phonons the equilibrium steady state of the DQD charge qubit may be engineered to display coherence in the energy eigenbasis. The magnitude of the coherence can be controlled by tuning the DQD parameters and regimes of high purity maybe found. In addition, we show that the steady-state coherence can be used to drive an auxiliary cavity mode coupled to the DQD.
Electron pairing in nanostructures driven by an oscillating field (1905.11942v2)
O. V. Kibis
2019-05-28
It is shown theoretically that the confinement of an electron at a repulsive potential can exist in nanostructures subjected to a strong high-frequency electromagnetic field. As a result of the confinement, the metastable bound electron state of the repulsive potential appears. This effect can lead, particularly, to electron pairing in nanostructures containing conduction electrons with different effective masses.
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