72 research outputs found
Real-Axis Solution of Eliashberg Equations in Various Order-Parameter Symmetries and Tunneling Conductance of Optimally-Doped HTSC
In the present work we calculate the theoretical tunneling conductance curves
of SIN junctions involving high-Tc superconductors, for different possible
symmetries of the order parameter (s, d, s+id, s+d, anisotropic s and extended
s). To do so, we solve the real-axis Eliashberg equations in the case of an
half-filled infinite band. We show that some of the peculiar characteristics of
HTSC tunneling curves (dip and hump at eV > Delta, broadening of the gap peak,
zero bias and so on) can be explained in the framework of the Migdal-Eliashberg
theory. The theoretical dI/dV curves calculated for the different symmetries at
T=4 K are then compared to various experimental tunneling data obtained in
optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit
the experimental data, the scattering by non-magnetic impurities has to be
taken into account, thus limiting the sensitivity of this procedure in
determining the exact gap symmetry of these materials. Finally, the effect of
the temperature on the theoretical tunneling conductance is also discussed and
the curves obtained at T=2 K are compared to those given by the analytical
continuation of the imaginary-axis solution.Comment: 6 pages, 3 figures, Proceedings of SATT10 Conference, to be published
in Int. J. Mod. Phys.
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
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
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
Strong band-filling-dependence of the scattering lifetime in gated MoS2 nanolayers induced by the opening of intervalley scattering channels
Gated molybdenum disulphide (MoS2) exhibits a rich phase diagram upon
increasing electron doping, including a superconducting phase, a polaronic
reconstruction of the bandstructure, and structural transitions away from the
2H polytype. The average time between two charge-carrier scattering events -
the scattering lifetime - is a key parameter to describe charge transport and
obtain physical insight in the behavior of such a complex system. In this work,
we combine the solution of the Boltzmann transport equation (based on ab-initio
density functional theory calculations of the electronic bandstructure) with
the experimental results concerning the charge-carrier mobility, in order to
determine the scattering lifetime in gated MoS2 nanolayers as a function of
electron doping and temperature. From these dependencies, we assess the major
sources of charge-carrier scattering upon increasing band filling, and discover
two narrow ranges of electron doping where the scattering lifetime is strongly
suppressed. We indentify the opening of additional intervalley scattering
channels connecting the simultaneously-filled K/K' and Q/Q' valleys in the
Brillouin zone as the source of these reductions, which are triggered by the
two Lifshitz transitions induced by the filling of the high-energy Q/Q' valleys
upon increasing electron doping.Comment: 10 pages, 6 figure
Cluster charge-density-wave glass in hydrogen-intercalated TiSe
The topotactic intercalation of transition-metal dichalcogenides with atomic
or molecular ions acts as an efficient knob to tune the electronic ground state
of the host compound. A representative material in this sense is
1-TiSe, where the electric-field-controlled intercalations of lithium
or hydrogen trigger superconductivity coexisting with the charge-density wave
phase. Here, we use the nuclear magnetic moments of the intercalants in
hydrogen-intercalated 1-TiSe as local probes for nuclear magnetic
resonance experiments. We argue that fluctuating mesoscopic-sized domains
nucleate already at temperatures higher than the bulk critical temperature to
the charge-density wave phase and display cluster-glass-like dynamics in the
MHz range tracked by the H nuclear moments. Additionally, we observe a
well-defined independent dynamical process at lower temperatures that we
associate with the intrinsic properties of the charge-density wave state. In
particular, we ascribe the low-temperature phenomenology to the collective
phason-like motion of the charge-density wave being hindered by structural
defects and chemical impurities and resulting in a localized oscillating
motion.Comment: 9 pages, 4 figure
Ambipolar suppression of superconductivity by ionic gating in optimally-doped BaFe2(As,P)2 ultrathin films
Superconductivity (SC) in the Ba-122 family of iron-based compounds can be
controlled by aliovalent or isovalent substitutions, applied external pressure,
and strain, the combined effects of which are sometimes studied within the same
sample. Most often, the result is limited to a shift of the SC dome to
different doping values. In a few cases, the maximum SC transition at optimal
doping can also be enhanced. In this work, we study the combination of charge
doping together with isovalent P substitution and strain by performing ionic
gating experiments on BaFe(AsP) ultrathin films. We
show that the polarization of the ionic gate induces modulations to the
normal-state transport properties that can be mainly ascribed to surface charge
doping. We demonstrate that ionic gating can only shift the system away from
the optimal conditions, as the SC transition temperature is suppressed by both
electron and hole doping. We also observe a broadening of the resistive
transition, which suggests that the SC order parameter is modulated
nonhomogeneously across the film thickness, in contrast with earlier reports on
charge-doped standard BCS superconductors and cuprates.Comment: 10 pages, 5 figure
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