Over 50 years ago, Anderson and Blount discussed symmetry-allowed polar distortions in metals, spawning the idea that a material might be simultaneously metallic and ferroelectric. While many studies have ever since considered such or similar situations, actual ferroelectricity—that is, the existence of a switchable intrinsic electric polarization—has not yet been attained in a metal, and is in fact generally deemed incompatible with the screening by mobile conduction charges. Here we refute this common wisdom and show, by means of first-principles simulations, that native metallicity and ferroelectricity coexist in the layered perovskite Bi₅Ti₅O₁₇. We show that, despite being a metal, Bi₅Ti₅O₁₇ can sustain a sizable potential drop along the polar direction, as needed to reverse its polarization by an external bias. We also reveal striking behaviours, as the self-screening mechanism at work in thin Bi₅Ti₅O₁₇ layers, emerging from the interplay between polar distortions and carriers in this compound.

The electronic structure of ε-Ga₂O₃ thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that can be attributed to photoelectron recoil. The photoemission experiments indicate that the energy separation between the valence band and the Fermi level is about 4.4 eV, a valence band maximum at the Γ point and an effective mass of the highest lying bands of – 4.2 free electron masses. The value of the bandgap compares well with that obtained by optical experiments and with that obtained by calculations performed using a hybrid density-functional, which also reproduce well the dispersion and density of states.

Understanding the nature of charge carriers at the LaAlO₃/SrTiO₃ interface is one of the major open issues in the full comprehension of the charge confinement phenomenon in oxide heterostructures. Here, we investigate thermopower to study the electronic structure in LaAlO₃/SrTiO₃ at low temperature as a function of gate field. In particular, under large negative gate voltage, corresponding to the strongly depleted charge density regime, thermopower displays high negative values of the order of 10⁴–10⁵μVK⁻¹, oscillating at regular intervals as a function of the gate voltage. The huge thermopower magnitude can be attributed to the phonon-drag contribution, while the oscillations map the progressive depletion and the Fermi level descent across a dense array of localized states lying at the bottom of the Ti 3d conduction band. This study provides direct evidence of a localized Anderson tail in the two-dimensional electron liquid at the LaAlO₃/SrTiO₃ interface.

We evaluate theoretically the thermoelectric efficiency of the layered perovskite ${\mathrm{Sr}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ via calculations of the electronic structure and transport coefficients within density-functional theory and Bloch-Boltzmann relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$, computed with energy-, chemical potential–, and temperature-dependent relaxation times, has one component increasing monotonically from around 0.4 at room temperature to 2.4 at 1250 K at an optimal carrier density of around $2\times {10}^{20}{\mathrm{cm}}^{-3}$, while the other components are small. The Seebeck coefficient is about 250 to 300 $\mu \mathrm{V}$/K at optimal doping and reaches 800 $\mu \mathrm{V}$/K at lower doping. We provide a python code implementing various approximations to the energy-dependent relaxation-time transport, which can be used to address different systems with an appropriate choice of material parameters.

Using advanced first-principles calculations we predict that the nonpolar SrTiO${}_{3}$/SrZrO${}_{3}$ (001) interface, designed as either thin SrZrO${}_{3}$ film deposited on SrTiO${}_{3}$ or short-period (SrTiO${}_{3}$)${}_{m}$/(SrZrO${}_{3}$)${}_{n}$ superlattice, hosts a two-dimensionally confined electron gas. Mobile electron charge due to native impurities, field effect, or modulation doping remains tightly trapped at the interface. Key ingredients for this occurrence are (a) the peculiar chemistry of $3d$ orbitals and (b) the large band offset at the titanate-zirconate interface.