797 research outputs found
From semiconductors to superconductors: a simple model for pseudogaps
We consider a two dimensional semiconductor with a local attraction among the
carriers. We study the ground state of this system as a function of the
semiconductor gap. We find a direct transition from a superconducting to an
insulating phase for no doping at a critical value, the single particle
excitations being always gapped. For finite doping we find a smooth crossover.
We calculate the critical temperature due to both the particle excitations and
the Berezinkii-Kosterlitz-Thouless transition.Comment: 14 pages. Accepted for publication on Eur. Phys. Jour.
Tunable spin-polaron state in a singly clamped semiconducting carbon nanotube
We consider a semiconducting carbon nanotube (CNT) laying on a ferromagnetic
insulating sub-strate with one end depassing the substrate and suspended over a
metallic gate. We assume that the polarised substrate induces an exchange
interaction acting as a local magnetic field for the electrons in the
non-suspended CNT side. Generalizing the approach of I. Snyman and Yu.V.
Nazarov [Phys. Rev. Lett. 108, 076805 (2012)] we show that one can generate
electrostatically a tun-able spin-polarized polaronic state localized at the
bending end of the CNT. We argue that at low temperatures manipulation and
detection of the localised quantum spin state is possible
Detection of ultrafast oscillations in Superconducting Point-Contacts by means of Supercurrent Measurements
We present a microscopic calculation of the nondissipative current through a
superconducting quantum point contact coupled to a mechanical oscillator. Using
the non-equilibrium Keldysh Green function approach, we determine the
current-phase relation. The latter shows that at certain phases, the current is
sharply suppressed. These dips in the current-phase relation provide
information about the oscillating frequency and coupling strength of the
mechanical oscillator. We also present an effective two-level model from which
we obtain analytical expressions describing the position and width of the dips.
Our findings are of relevance for nanomechanical resonators based on
superconducting materials.Comment: 8 pages, 5 figures. Published in Phys. Rev.
Electro-Mechanical Transition in Quantum dots
The strong coupling between electronic transport in a single-level quantum
dot and a capacitively coupled nano-mechanical oscillator may lead to a
transition towards a mechanically-bistable and blocked-current state. Its
observation is at reach in carbon-nanotube state-of-art experiments. In a
recent publication [Phys. Rev. Lett. 115, 206802 (2015)] we have shown that
this transition is characterized by pronounced signatures on the oscillator
mechanical properties: the susceptibility, the displacement fluctuation
spectrum and the ring-down time. These properties are extracted from transport
measurements, however the relation between the mechanical quantities and the
electronic signal is not always straightforward. Moreover the dependence of the
same quantities on temperature, bias or gate voltage, and external dissipation
has not been studied. The purpose of this paper is to fill this gap and provide
a detailed description of the transition. Specifically we find: (i) The
relation between the current-noise and the displacement spectrum. (ii) The
peculiar behavior of the gate-voltage dependence of these spectra at the
transition. (iii) The robustness of the transition towards the effect of
external fluctuations and dissipation
Steps and facets at the surface of soft crystals
We consider the shape of crystals which are soft in the sense that their
elastic modulus is small compared to their surface tension , more
precisely where is the lattice spacing. We show that
their surface steps penetrate inside the crystal as edge dislocations. As a
consequence, these steps are broad with a small energy which we calculate. We
also calculate the elastic interaction between steps a distance apart,
which is a repulsion. We finally calculate the roughening temperatures
of successive facets in order to compare with the remarkable shapes of
lyotropic crystals recently observed by P. Pieranski et al. Good agreement is
found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.
Charge noise at Cooper-pair resonances
We analyze the charge dynamics of a superconducting single-electron
transistor (SSET) in the regime where charge transport occurs via Cooper-pair
resonances. Using an approximate description of the system Hamiltonian, in
terms of a series of resonant doublets, we derive a Born-Markov master equation
describing the dynamics of the SSET. The average current displays sharp peaks
at the Cooper-pair resonances and we find that the charge noise spectrum has a
characteristic structure which consists of a series of asymmetric triplets of
peaks. The strongest feature in the charge noise spectrum is the triplet of
peaks centered at zero frequency which has a peak spacing equal to the level
separation within the doublets and is similar to the triplet in the spectrum of
a driven, damped, two-level system. We also explore the back-action that the
SSET charge noise would have on an oscillator coupled to the island charge,
measurement of which provides a way of probing the charge noise spectrum.Comment: 14 pages, 7 figure
TeMA: A Tensorial Memetic Algorithm for Many-Objective Parallel Disassembly Sequence Planning in Product Refurbishment
The refurbishment market is rich in opportunities—the global refurbished smartphones market alone will be $38.9 billion by 2025. Refurbishing a product involves disassembling it to test the key parts and replacing those that are defective or worn. This restores the product to like-new conditions, so that it can be put on the market again at a lower price. Making this process quick and efficient is crucial. This paper presents a novel formulation of parallel disassembly problem that maximizes the degree of parallelism, the level of ergonomics, and how the workers' workload is balanced, while minimizing the disassembly time and the number of times the product has to be rotated. The problem is solved using the Tensorial Memetic Algorithm (TeMA), a novel two-stage many-objective (MaO) algorithm, which encodes parallel disassembly plans by using third-order tensors. TeMA first splits the objectives into primary and secondary on the basis of a decision-maker's preferences, and then finds Pareto-optimal compromises (seeds) of the primary objectives. In the second stage, TeMA performs a fine-grained local search that explores the objective space regions around the seeds, to improve the secondary objectives. TeMA was tested on two real-world refurbishment processes involving a smartphone and a washing machine. The experiments showed that, on average, TeMA is statistically more accurate than various efficient MaO algorithms in the decision-maker's area of preference
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