54 research outputs found
Strong dopant dependence of electric transport in ion-gated MoS2
We report modifications of the temperature-dependent transport properties of
thin flakes via field-driven ion intercalation in an electric
double layer transistor. We find that intercalation with ions
induces the onset of an inhomogeneous superconducting state. Intercalation with
leads instead to a disorder-induced incipient metal-to-insulator
transition. These findings suggest that similar ionic species can provide
access to different electronic phases in the same material.Comment: 5 pages, 3 figure
Anomalous screening of an electrostatic field at the surface of niobium nitride
The interaction between an electric field and the electric charges in a
material is described by electrostatic screening, which in metallic systems is
commonly thought to be confined within a distance of the order of the
Thomas-Fermi length. The validity of this picture, which holds for surface
charges up to , has been recently questioned by
several experimental results when dealing with larger surface charges, such as
those routinely achieved via the ionic gating technique. Whether these results
can be accounted for in a purely electrostatic picture is still debated. In
this work, we tackle this issue by calculating the spatial dependence of the
charge carrier density in thin slabs of niobium nitride via an ab initio
density functional theory approach in the field-effect transistor
configuration. We find that perturbations induced by surface charges are mainly screened within the first layer, while
those induced by larger surface charges can
penetrate over multiple atomic layers, in reasonable agreement with the
available experimental data. Furthermore, we show that a significant
contribution to the screening of large fields is associated not only to the
accumulation layer of the induced charge carriers at the surface, but also to
the polarization of the pre-existing charge density of the undoped system.Comment: 8 pages, 4 figure
Towards the insulator-to-metal transition at the surface of ion-gated nanocrystalline diamond films
Hole doping can control the conductivity of diamond either through boron
substitution, or carrier accumulation in a field-effect transistor. In this
work, we combine the two methods to investigate the insulator-to-metal
transition at the surface of nanocrystalline diamond films. The finite boron
doping strongly increases the maximum hole density which can be induced
electrostatically with respect to intrinsic diamond. The ionic gate pushes the
conductivity of the film surface away from the variable-range hopping regime
and into the quantum critical regime. However, the combination of the strong
intrinsic surface disorder due to a non-negligible surface roughness, and the
introduction of extra scattering centers by the ionic gate, prevents the
surface accumulation layer to reach the metallic regime.Comment: 5 pages, 4 figure
Mapping multi-valley Lifshitz transitions induced by field-effect doping in strained MoS2 nanolayers
Gate-induced superconductivity at the surface of nanolayers of semiconducting
transition metal dichalcogenides (TMDs) has attracted a lot of attention in
recent years, thanks to the sizeable transition temperature, robustness against
in-plane magnetic fields beyond the Pauli limit, and hints to a
non-conventional nature of the pairing. A key information necessary to unveil
its microscopic origin is the geometry of the Fermi surface hosting the Cooper
pairs as a function of field-effect doping, which is dictated by the filling of
the inequivalent valleys at the K/K and Q/Q points of the
Brillouin Zone. Here, we achieve this by combining Density Functional Theory
calculations of the bandstructure with transport measurements on ion-gated
2H-MoS nanolayers. We show that, when the number of layers and the amount
of strain are set to their experimental values, the Fermi level crosses the
bottom of the high-energy valleys at Q/Q at doping levels where
characteristic kinks in the transconductance are experimentally detected. We
also develop a simple 2D model which is able to quantitatively describe the
broadening of the kinks observed upon increasing temperature. We demonstrate
that this combined approach can be employed to map the dependence of the Fermi
surface of TMD nanolayers on field-effect doping, detect Lifshitz transitions,
and provide a method to determine the amount of strain and spin-orbit splitting
between sub-bands from electric transport measurements in real devices.Comment: 8 pages, 4 figure
Two-dimensional hole transport in ion-gated diamond surfaces: A brief review
Electrically-conducting diamond is a promising candidate for next-generation electronic, thermal and electrochemical applications. One of the major obstacles towards its exploitation is the strong degradation that some of its key physical properties - such as the carrier mobility and the superconducting transition temperature - undergo upon the introduction of disorder. This makes the two-dimensional hole gas induced at its surface by electric field-effect doping particularly interesting from both a fundamental and an applied perspective, since it strongly reduces the amount of extrinsic disorder with respect to the standard boron substitution. In this short review, we summarize the main results achieved so far in controlling the electric transport properties of different field-effect doped diamond surfaces via the ionic gating technique. We analyze how ionic gating can tune their conductivity, carrier density and mobility, and drive the different surfaces across the insulator-to-metal transition. We review their strongly orientation-dependent magnetotransport properties, with a particular focus on the gate-tunable spin-orbit coupling shown by the (100) surface. Finally, we discuss the possibility of field-induced superconductivity in the (110) and (111) surfaces as predicted by density functional theory calculations
Spectroscopic studies of the superconducting gap in the 12442 family of iron-based compounds
The iron-based compounds of the so-called 12442 family are very peculiar in
various respects. They originate from the intergrowth of 122 and 1111 building
blocks, display a large in-plane vs. out-of-plane anisotropy, possess double
layers of FeAs separated by insulating layers, and are generally very similar
to double-layer cuprates. Moreover, they are stoichiometric superconductors
because of an intrinsic hole doping. Establishing their superconducting
properties, and in particular the symmetry of the order parameter, is thus
particularly relevant in order to understand to what extent these compounds can
be considered as the iron-based counterpart of cuprates. In this work we review
the results of various techniques from the current literature and compare them
with ours, obtained in Rb-12442 by combining point-contact Andreev-reflection
spectroscopy and coplanar waveguide resonator measurements of the superfluid
density. It turns out that the compound possesses at least two gaps, one of
which is certainly nodal. The compatibility of this result with the
theoretically allowed gap structures, as well as with the other results in
literature, is discussed in detail.Comment: 16 pages, 12 figure
Multi-Valley Superconductivity In Ion-Gated MoS2 Layers
Layers of transition metal dichalcogenides (TMDs) combine the enhanced
effects of correlations associated with the two-dimensional limit with
electrostatic control over their phase transitions by means of an electric
field. Several semiconducting TMDs, such as MoS, develop superconductivity
(SC) at their surface when doped with an electrostatic field, but the mechanism
is still debated. It is often assumed that Cooper pairs reside only in the two
electron pockets at the K/K' points of the Brillouin Zone. However,
experimental and theoretical results suggest that a multi-valley Fermi surface
(FS) is associated with the SC state, involving 6 electron pockets at the Q/Q'
points. Here, we perform low-temperature transport measurements in ion-gated
MoS flakes. We show that a fully multi-valley FS is associated with the SC
onset. The Q/Q' valleys fill for dopingcm, and the
SC transition does not appear until the Fermi level crosses both spin-orbit
split sub-bands Q and Q. The SC state is associated with the FS
connectivity and promoted by a Lifshitz transition due to the simultaneous
population of multiple electron pockets. This FS topology will serve as a
guideline in the quest for new superconductors.Comment: 12 pages, 7 figure
A model for critical current effects in point-contact Andreev-reflection spectroscopy
It is well known that point-contact Andreev reflection spectroscopy provides
reliable measurements of the energy gap(s) in a superconductor when the contact
is in the ballistic or nearly-ballistic regime. However, especially when the
mean free path of the material under study is small, obtaining ballistic
contacts can be a major challenge. One of the signatures of a Maxwell
contribution to the contact resistance is the presence of "dips" in the
differential conductance, associated to the sudden appearance of a Maxwell
term, in turn due to the attainment of the critical current of the material in
the contact region. Here we show that, using a proper model for the of
the material under study, it is possible to fit the experimental curves
(without the need of normalization) obtaining the correct values of the gap
amplitudes even in the presence of such dips, as well as the temperature
dependence of the critical current in the contact. We present a test of the
procedure in the case of Andreev-reflection spectra in
MgAlB single crystals.Comment: 7 pages, 5 figure
- …