Can disorder alone destroy the eg' hole pockets of NaxCoO2? A Wannier function based first-principles method for disordered systems

We investigate from first principles the proposed destruction of the controversial eg' pockets in the Fermi surface of NaxCoO2 due to Na disorder, by calculating its k-dependent configurational averaged spectral function . To this end, a Wannier function based method is developed that treats the effects of disorder beyond the mean field. Remarkable spectral broadenings of order ~eV are found for the oxygen orbitals, possibly explaining their absence in the experiments. Contrary to the current lore, however, the eg' pockets remain almost perfectly coherent. The developed method is expected to generate exciting opportunities in the study of the countless functional materials that owe their important electronic properties to disordered dopants

Unfolding first-principles band structures

A general method is presented to unfold band structures of first-principles super-cell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angular resolved photoemission spectroscopy experiments. With negligible computational expense via the use of Wannier functions, this simple method has great practical value in the studies of a wide range of materials containing impurities, vacancies, lattice distortions, or spontaneous long-range orders.Comment: 4 pages, 3 figure

Enhanced superconductivity due to forward scattering in FeSe thin films on SrTiO3 substrates

We study the consequences of an electron-phonon ($e$-$ph$) interaction that is strongly peaked in the forward scattering ($\mathbf{q} = 0$) direction in a two-dimensional superconductor using Migdal-Eliashberg theory. We find that strong forward scattering results in an enhanced $T_c$ that is linearly proportional to the strength of the dimensionless $e$-$ph$ coupling constant $\lambda_m$ in the weak coupling limit. This interaction also produces distinct replica bands in the single-particle spectral function, similar to those observed in recent angle-resolved photoemission experiments on FeSe monolayers on SrTiO$_3$ and BaTiO$_3$ substrates. By comparing our model to photoemission experiments, we infer an $e$-$ph$ coupling strength that can provide a significant portion of the observed high $T_c$ in these systems.Comment: Main text 5 pages, 4 figures; and Supplementary Informatio

What is the valence of Mn in Ga1−x_{1-x}Mnx_xN?

We investigate the current debate on the Mn valence in Ga$_{1-x}$Mn$_x$N, a diluted magnetic semiconductor (DMSs) with a potentially high Curie temperature. From a first-principles Wannier-function analysis, we unambiguously find the Mn valence to be close to $2+$ ($d^5$), but in a mixed spin configuration with average magnetic moments of 4$\mu_B$. By integrating out high-energy degrees of freedom differently, we further derive for the first time from first-principles two low-energy pictures that reflect the intrinsic dual nature of the doped holes in the DMS: 1) an effective $d^4$ picture ideal for local physics, and 2) an effective $d^5$ picture suitable for extended properties. In the latter, our results further reveal a few novel physical effects, and pave the way for future realistic studies of magnetism. Our study not only resolves one of the outstanding key controversies of the field, but also exemplifies the general need for multiple effective descriptions to account for the rich low-energy physics in many-body systems in general.Comment: 4 figure

Static and dynamical magnetic properties of the extended Kitaev-Heisenberg model with spin vacancies

Motivated by the potential to suppress the antiferromagnetic long-range order in favor of the long-sought-after Kitaev quantum spin liquid state, we study the effect of spin vacancies in the extended Kitaev-Heisenberg model. In particular, we focus on a realistic model obtained from fitting inelastic neutron scattering on $\alpha$-RuCl$_3$. We observe that the long-range zigzag magnetic ordered state only survives when the doping concentration is smaller than 5\%. Upon further increasing the spin vacancy concentration, the ground state becomes a short-range ordered state at low temperatures. Compared with experiments, our classical solution over-stabilizes the zigzag correlation in the presence of spin vacancies. Our theoretical results provide guidance toward interpreting inelastic neutron scattering experiments on magnetically diluted Kitaev candidate materials.Comment: 9 figure

Breaking Rayleigh's law with spatially correlated disorder to control phonon transport

Controlling thermal transport in insulators and semiconductors is crucial for many technological fields such as thermoelectrics and thermal insulation, for which a low thermal conductivity ($\kappa$) is desirable. A major obstacle for realizing low $\kappa$ materials is Rayleigh's law, which implies that acoustic phonons, which carry most of the heat, are insensitive to scattering by point defects at low energy. We demonstrate, with large scale simulations on tens of millions of atoms, that isotropic long-range spatial correlations in the defect distribution can dramatically reduce phonon lifetimes of important low-frequency heat-carrying modes, leading to a large reduction of $\kappa$ -- potentially an order of magnitude at room temperature. We propose a general and quantitative framework for controlling thermal transport in complex functional materials through structural spatial correlations, and we establish the optimal functional form of spatial correlations that minimize $\kappa$. We end by briefly discussing experimental realizations of various correlated structures