59 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
Ionic gating in metallic superconductors: A brief review
Ionic gating is a very popular tool to investigate and control the electric charge transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, and has been proven to be capable of tuning even the properties of metallic systems. In this short review, I summarize the main results which have been achieved so far in controlling the superconducting (SC) properties of thin films of conventional metallic superconductors by means of the ionic gating technique. I discuss how the gate-induced charge doping, despite being confined to a thin surface layer by electrostatic screening, results in a long-range "bulk" modulation of the SC properties by the coherent nature of the SC condensate, as evidenced by the observation of suppressions in the critical temperature of films much thicker than the electrostatic screening length, and by the pronounced thickness-dependence of their magnitude. I review how this behavior can be modelled in terms of proximity effect between the charge-doped surface layer and the unperturbed bulk with different degrees of approximation, and how first-principles calculations have been employed to determine the origin of an anomalous increase in the electrostatic screening length at ultrahigh electric fields, thus fully confirming the validity of the proximity effect model. Finally, I discuss a general framework - based on the combination of ab-initio Density Functional Theory and the Migdal-Eliashberg theory of superconductivity - by which the properties of any gated thin film of a conventional metallic superconductor can be determined purely from first principles
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
A Short Review on Nanostructured Carbon Containing Biopolymer Derived Composites for Tissue Engineering Applications
The development of new scaffolds and materials for tissue engineering is a wide and open realm of material science. Among solutions, the use of biopolymers represents a particularly interesting area of study due to their great chemical complexity that enables creation of specific molecular architectures. However, biopolymers do not exhibit the properties required for direct application in tissue repair—such as mechanical and electrical properties—but they do show very attractive chemical functionalities which are difficult to produce through in vitro synthesis. The combination of biopolymers with nanostructured carbon fillers could represent a robust solution to enhance composite properties, producing composites with new and unique features, particularly relating to electronic conduction. In this paper, we provide a review of the field of carbonaceous nanostructure-containing biopolymer composites, limiting our investigation to tissue-engineering applications, and providing a complete overview of the recent and most outstanding achievements
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
Anomalous Metallic Phase in Molybdenum Disulphide Induced via Gate-Driven Organic Ion Intercalation
Transition metal dichalcogenides exhibit rich phase diagrams dominated by the interplay of superconductivity and charge density waves, which often result in anomalies in the electric transport properties. Here, we employ the ionic gating technique to realize a tunable, non-volatile organic ion intercalation in bulk single crystals of molybdenum disulphide (MoS2). We demonstrate that this gate-driven organic ion intercalation induces a strong electron doping in the system without changing the pristine 2H crystal symmetry and triggers the emergence of a re-entrant insulator-to-metal transition. We show that the gate-induced metallic state exhibits clear anomalies in the temperature dependence of the resistivity with a natural explanation as signatures of the development of a charge-density wave phase which was previously observed in alkali-intercalated MoS2. The relatively large temperature at which the anomalies are observed (∼150 K), combined with the absence of any sign of doping-induced superconductivity down to ∼3 K, suggests that the two phases might be competing with each other to determine the electronic ground state of electron-doped MoS2
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
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