Anomalous screening of an electrostatic field at the surface of niobium nitride

Abstract

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 ∼ 10 13 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 10 14 cm −2 are mainly screened within the first layer, while those induced by larger surface charges ∼ 10 15 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.

Towards the insulator-to-metal transition at the surface of ion-gated nanocrystalline diamond films

“…This can not be simply attributed to the presence of grain boundaries, since the bulk conductivity is firmly in the disordered metal regime. However, the gate-induced AL is confined within few atomic layers [20,[39][40][41][42]] from a surface that features a much larger roughness 30 nm, and local "valleys" between the grains (up to ∼ 100 nm deep, see Fig.1a) may also lead to local percolative transport through the lighly-doped bulk. Therefore, disorder may be strongly amplified at the surface were the AL is confined, shifting the metallic edge of its MIT to much larger doping values with respect to the bulk.…”

Theoretical Explanation of Electric Field‐Induced Superconductive Critical Temperature Shifts in Indium Thin Films

“…The induced charge in the material is treated assuming that the additional carriers spread uniformly inside the surface layer of thickness , which is justified for not too high values of the applied electric field. Indeed, in this case, Thomas–Fermi approximation is still valid and the thickness of the perturbed surface layer is taken to be of the order of the Thomas–Fermi screening length (i.e., weak‐field limit). As a matter of fact, in a field‐effect device, it is possible to modulate the carrier density per unit surface , , by tuning the polarization of the gate electrode.…”

“…However, it has been shown that, in the weak-field limit, d s can be taken to be equal to the Thomas-Fermi screening length d TF . [39] The electron-phonon coupling constants are defined as…”

“…In the case of metals, the thickness of the surface layer () is not trivially identifiable, especially for high magnitudes of the applied electric field. However, it has been shown that, in the weak‐field limit, can be taken to be equal to the Thomas–Fermi screening length …”

Migdal-Eliashberg theory of multi-band high-temperature superconductivity in field-effect-doped hydrogenated (111) diamond

“…This was done by performing DFT calculations in the proper field-effect geometry with Quantum ESPRESSO [36][37][38] as described in Ref. 38 in the case of graphene: This is a very versatile approach, which has been employed to reliably calculate the properties of many different materials from first principles, including zirconium nitride chloride 39 , arsenene and phosphorene 40 , transition-metal dichalcogenides [40][41][42][43] , and niobium nitride 44 . For field-effect doped (111) diamond, all the computational details can be found in Ref.…”