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Abstract Migdal-Eliashberg theory is one of the state-of-the-art methods for describing conventional superconductors from first principles. However, widely used implementations assume a constant density of states around the Fermi level, which hinders a proper description of materials with distinct features in its vicinity. Here, we present an implementation of the Migdal-Eliashberg theory within the EPW code that considers the full electronic structure and accommodates scattering processes beyond the Fermi surface. To significantly reduce computational costs, we introduce a non-uniform sampling scheme along the imaginary axis. We demonstrate the power of our implementation by applying it to the sodalite-like clathrates YH₆and CaH₆, and to the covalently-bonded H₃S and D₃S. Furthermore, we investigate the effect of maximizing the density of states at the Fermi level in doped H₃S and BaSiH₈within the full-bandwidth treatment compared to the constant-density-of-states approximation. Our findings highlight the importance of this advanced treatment in such complex materials. 

Abstract Anharmonicity and local disorder (polymorphism) are ubiquitous in perovskite physics, inducing various phenomena observed in scattering and spectroscopy experiments. Several of these phenomena still lack interpretation from first principles since, hitherto, no approach is available to account for anharmonicity and disorder in electron–phonon couplings. Here, relying on the special displacement method, we develop a unified treatment of both and demonstrate that electron–phonon coupling is strongly influenced when we employ polymorphous perovskite networks. We uncover that polymorphism in halide perovskites leads to vibrational dynamics far from the ideal noninteracting phonon picture and drives the gradual change in their band gap around phase transition temperatures. We also clarify that combined band gap corrections arising from disorder, spin-orbit coupling, exchange–correlation functionals of high accuracy, and electron–phonon coupling are all essential. Our findings agree with experiments, suggesting that polymorphism is the key to address pending questions on perovskites’ technological applications. 

Abstract Ising superconductivity, observed in NbSe₂and similar materials, has generated tremendous interest. Recently, attention was called to the possible role that spin fluctuations (SF) play in this phenomenon, in addition to the dominant electron–phonon coupling (EPC); the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations. However, these hypotheses have not been put to a quantitative test. In this paper, we report first principle calculations of the EPC and also estimate coupling with SF, including full momentum dependence. We find that: (1) EPC is strongly anisotropic, largely coming from the$$K-{K}^{{\prime} }$$ $K-K^{\prime }$scattering, and therefore excludes triplet symmetry even as an excited state; (2) superconductivity is substantially weakened by SF, but anisotropy remains as above; and, (3) we do find the possibility of a Leggett mode, not in a singlet–triplet but in ans₊₊–s_±channel. 

Abstract The lattice thermal conductivity (κ_ph) of metals and semimetals is limited by phonon‐phonon scattering at high temperatures and by electron‐phonon scattering at low temperatures or in some systems with weak phonon‐phonon scattering. Following the demonstration of a phonon band engineering approach to achieve an unusually high κ_phin semiconducting cubic‐boron arsenide (c‐BAs), recent theories have predicted ultrahigh κ_phof the semimetal tantalum nitride in the θ‐phase (θ‐TaN) with hexagonal tungsten carbide (WC) structure due to the combination of a small electron density of states near the Fermi level and a large phonon band gap, which suppress electron‐phonon and three‐phonon scattering, respectively. Here, measurements on the thermal and electrical transport properties of polycrystalline θ‐TaN converted from the ε phase via high‐pressure synthesis are reported. The measured thermal conductivity of the θ‐TaN samples shows weak temperature dependence above 200 K and reaches up to 90 Wm⁻¹K⁻¹, one order of magnitude higher than values reported for polycrystalline ε‐TaN and δ‐TaN thin films. These results agree with theoretical calculations that account for phonon scattering by 100 nm‐level grains and suggest κ_phincrease above the 249 Wm⁻¹K⁻¹value predicted for single‐crystal WC when the grain size of θ‐TaN is increased above 400 nm. 

Titanium dioxide (TiO₂) is a wide-gap semiconductor with numerous applications in photocatalysis, photovoltaics, and neuromorphic computing. The unique functional properties of this material critically depend on its ability to transport charge in the form of polarons, namely narrow electron wavepackets accompanied by local distortions of the crystal lattice. It is currently well established that the most important polymorphs of TiO₂, the rutile and anatase phases, harbor small electron polarons and small hole polarons, respectively. However, whether additional polaronic species exist in TiO₂, and under which conditions, remain open questions. Here, we provide definitive answers to these questions by exploring the rich landscape of polaron quasiparticles in TiO₂via recently developed ab initio techniques. In addition to the already known small polarons, we identify three species, namely a large hole polaron in rutile, a large quasi-two-dimensional electron polaron in anatase, and a large exciton polaron in anatase. These findings complete the puzzle on the polaron physics of TiO₂and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials. 

The properties of excitons, or correlated electron–hole pairs, are of paramount importance to optoelectronic applications of materials. A central component of exciton physics is the electron–hole interaction, which is commonly treated as screened solely by electrons within a material. However, nuclear motion can screen this Coulomb interaction as well, with several recent studies developing model approaches for approximating the phonon screening of excitonic properties. While these model approaches tend to improve agreement with experiment, they rely on several approximations that restrict their applicability to a wide range of materials, and thus far they have neglected the effect of finite temperatures. Here, we develop a fully first-principles, parameter-free approach to compute the temperature-dependent effects of phonon screening within the ab initio $\mathit{GW}$-Bethe–Salpeter equation framework. We recover previously proposed models of phonon screening as well-defined limits of our general framework, and discuss their validity by comparing them against our first-principles results. We develop an efficient computational workflow and apply it to a diverse set of semiconductors, specifically AlN, CdS, GaN, MgO, and ${\text{SrTiO}}_3$. We demonstrate under different physical scenarios how excitons may be screened by multiple polar optical or acoustic phonons, how their binding energies can exhibit strong temperature dependence, and the ultrafast timescales on which they dissociate into free electron–hole pairs.