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 $\sim 10^{13}\,\mathrm{cm^{-2}}$, 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 $\lesssim 10^{14}\,\mathrm{cm^{-2}}$ are mainly screened within the first layer, while those induced by larger surface charges $\sim 10^{15}\,\mathrm{cm^{-2}}$ 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$_2$, 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$_2$ flakes. We show that a fully multi-valley FS is associated with the SC onset. The Q/Q' valleys fill for doping$\gtrsim2\cdot10^{13}$cm$^{-2}$, and the SC transition does not appear until the Fermi level crosses both spin-orbit split sub-bands Q$_1$ and Q$_2$. 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 $R(I)$ 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 Mg$_{0.75}$Al$_{0.25}$B$_2$ 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 TiSe2_{2}

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$T$-TiSe$_{2}$, 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$T$-TiSe$_{2}$ 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 $^{1}$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$_2$(As$_{0.8}$P$_{0.2}$)$_2$ 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