FORUM DQMP

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Day / Time
Date(s) - 24/11/2015
13 h 00 min - 14 h 00 min

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Tuesday, November 24th 2015 – 13h00
Auditoire Stückelberg
Ecole de physique
Coffee and tea will be available from
12h50 at the entrance of the Auditoire

 

Thermal properties and stability of superconductors for magnet applications
Marco Bonura (group of Prof. Senatore)
Reliability is one of the major requirements that all devices must meet, superconducting magnets
included. Superconductivity exists within a phase volume bounded by three parameters: current
density, magnetic field, and temperature. Among these parameters, the temperature is the least
tractable one.
Due to thermal disturbances, the magnetic energy stored in the magnet can be converted into heat in
very few seconds. This process, referred as a “quench”, can lead to permanent damages of the
magnet. Understanding the physics behind a quench is essential for determining the stability margins
as well as for defining the best protection strategies of superconducting magnets.
In this talk, I will present recent studies about the thermal conduction and stability properties of MgB2
and REBCO technical superconductors. The thermal conductivity has been investigated in previously
unexplored magnetic field ranges. This has allowed us to propose solutions to optimize the conductors
and to study the heat diffusion process during a quench in conditions expected in “next generation”
magnets.
Electrostatically induced superconductivity in transition metal
dichalcogenides: from bulk-WS2 to mono-layer MoS2
Davide Costanzo (group of Prof. Morpurgo)
Transition metal dichalcogenides (TMD) constitute a class of layered materials characterized by strong
in-plane bonding and weak out-of-plane interactions. This feature enables their exfoliation into twodimensional
crystals with thickness going down to the single unit cell.
In this work we investigate low-temperature transport in ionic-liquid gated transistors based on bulk
WS2 and few-layers thick MoS2 (both semiconducting TMDs), exploiting the large gate capacitance to
induce very high electron density (~1014 cm-2). In WS2 we observe for the first time a superconducting
transition with a fully developed zero-resistance state, demonstrating that superconductivity (SC) is not
confined to Mo-based TMD [1]. Analysis of our results indicate a granular character of the SC –likely
due to local detaching of the frozen liquid- and provide evidence for a 2D Berezinskii-Kosterlitz-
Thouless mechanism, as expected considering the short screening length at this carrier density. In
MoS2 we observe a SC transition for layers of all thicknesses, down to individual monolayers, which
represents the first observation of SC in atomically thin van der Waals materials. We compare the
superconducting transitions in layers of different thicknesses (from 1 ML to 6 ML) and observe a drastic
weakening of the SC in monolayers as compared to all other thicker multilayers crystals, which
manifests itself in a significantly lower critical field and temperature. We discuss possible reasons, such
as the decrease of the system thickness below the electrostatic screening length, or that for
monolayers, but not for thicker multi-layers, electrons are accumulated at the K/K’ point.
[1] J. T. Ye et al, Science, 338, 1193, (2012)
Forum Committee : L. Foini, C. Lichtensteiger, N. Ubrig (18.11.2015)

 

 

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