253 research outputs found
Quantum Phase Transitions and Vortex Dynamics in Superconducting Networks
Josephson junction arrays are ideal model systems where a variety of
phenomena, phase transitions, frustration effects, vortex dynamics, chaos, to
mention a few of them, can be studied in a controlled way. In this review we
focus on the quantum dynamical properties of low capacitance Josephson junction
arrays. The two characteristic energy scales in these systems are the Josephson
energy, associated to the tunneling of Cooper pairs between neighboring
islands, and the charging energy, which is the energy cost to add an extra
electron charge to a neutral island. The phenomena described in this review
stem from the competition between single electron effects with the Josephson
effect. One example is the (quantum) Superconductor-Insulator phase transition
which occurs by varying the ratio between the coupling constants and/or by
means of external magnetic/electric fields. We will describe how the phase
diagram depends on the various control paramters and the transport properties
close to the quantum critical point. The relevant topological excitations on
the superconducting side of the phase diagram are vortices. In low capacitance
junction arrays vortices behave as massive underdamped particles that can
exhibit quantum behaviour. We will report on the various experiments and
theoretical treatments on quantum vortex dynamics.Comment: To be published in Physics Reports. Better quality figures can be
obtained upon reques
Investigating laser induced phase engineering in MoS2 transistors
Phase engineering of MoS2 transistors has recently been demonstrated and has
led to record low contact resistances. The phase patterning of MoS2 flakes with
laser radiation has also been realized via spectroscopic methods, which invites
the potential of controlling the metallic and semiconducting phases of MoS2
transistors by simple light exposure. Nevertheless, the fabrication and
demonstration of laser patterned MoS2 devices starting from the metallic
polymorph has not been demonstrated yet. Here, we study the effects of laser
radiation on 1T/1T'-MoS2 transistors with the prospect of driving an in-situ
phase transition to the 2H-polymorph through light exposure. We find that
although the Raman peaks of 2H-MoS2 become more prominent and the ones from the
1T/1T' phase fade after the laser exposure, the semiconducting properties of
the laser patterned devices are not fully restored and the laser treatment
ultimately leads to degradation of the transport channel
Static capacitive pressure sensing using a single graphene drum
To realize nanomechanical graphene-based pressure and gas sensors, it is
beneficial to have a method to electrically readout the static displacement of
a suspended graphene membrane. Capacitive readout, typical in
micro-electro-mechanical systems (MEMS), gets increasingly challenging as one
starts shrinking the dimensions of these devices, since the expected
responsivity of such devices is below 0.1 aF/Pa. To overcome the challenges of
detecting small capacitance changes, we design an electrical readout device
fabricated on top of an insulating quartz substrate, maximizing the
contribution of the suspended membrane to the total capacitance of the device.
The capacitance of the drum is further increased by reducing the gap size to
110 nm. Using external pressure load, we demonstrate successful detection of
capacitance changes of a single graphene drum down to 50 aF, and pressure
differences down to 25 mbar
Amplitude calibration of 2D mechanical resonators by nonlinear optical transduction
Contactless characterization of mechanical resonances using Fabry-Perot
interferometry is a powerful tool to study the mechanical and dynamical
properties of atomically thin membranes. However, amplitude calibration is
often not performed, or only possible by making assumptions on the device
parameters such as its mass or the temperature. In this work, we demonstrate a
calibration technique that directly measures the oscillation amplitude by
detecting higher harmonics that arise from nonlinearities in the optical
transduction. Employing this technique, we calibrate the resonance amplitude of
two-dimensional nanomechanical resonators, without requiring knowledge of their
mechanical properties, actuation force, geometric distances or the laser
intensity
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