First principles vs second principles: Role of charge self-consistency in strongly correlated systems

First principles approaches have been successful in solving many-body Hamiltonians for real materials to an extent when correlations are weak or moderate. As the electronic correlations become stronger often embedding methods based on first principles approaches are used to better treat the correlations by solving a suitably chosen many-body Hamiltonian with a higher level theory. Such combined methods are often referred to as second principles approaches. At such level of the theory the self energy, i.e. the functional that embodies the stronger electronic correlations, is either a function of energy or momentum or both. The success of such theories is commonly measured by the quality of the self energy functional. However, self-consistency in the self-energy should, in principle, also change the real space charge distribution in a correlated material and be able to modify the electronic eigenfunctions, which is often undermined in second principles approaches. Here we study the impact of charge self-consistency within two example cases: TiSe$_{2}$, a three-dimensional charge-density-wave candidate material, and CrBr$_{3}$, a two-dimensional ferromagnet, and show how real space charge re-distribution due to correlation effects taken into account within a first principles Greens function based many-body perturbative approach is key in driving qualitative changes to the final electronic structure of these materials.

Electronic Structure of Chromium Trihalides beyond Density Functional Theory

We explore the electronic band structure of free standing monolayers of chromium trihalides, CrXtextsubscript{3}{, X= Cl, Br, I}, within an advanced emph{ab-initio} theoretical approach based in the use of Greens function functionals. We compare the local density approximation with the quasi-particle self-consistent emph{GW} approximation (QSemph{GW}) and its self-consistent extension (QS$Gwidehat{W}$) by solving the particle-hole ladder Bethe-Salpeter equations to improve the effective interaction emph{W}. We show that at all levels of theory, the valence band consistently changes shape in the sequence Cl{textrightarrow}Br{textrightarrow}I, and the valence band maximum shifts from the M point to the $Gamma$ point. However, the details of the transition, the one-particle bandgap, and the eigenfunctions change considerably going up the ladder to higher levels of theory. The eigenfunctions become more directional, and at the M point there is a strong anisotropy in the effective mass. Also the dynamic and momentum dependent self energy shows that QS$Gwidehat{W}$ adds to the localization of the systems in comparison to the QSemph{GW} thereby leading to a narrower band and reduced amount of halogens in the valence band manifold.

Disentangling the role of bond lengths and orbital symmetries in controlling $T_c$ in YBa$_2$Cu$_3$O$_7$

Optimally doped YBCO (YBa$_{2}$Cu$_{3}$O$_{7}$) has a high critical temperature, at 92 K. It is largely believed that Cooper pairs form in YBCO and other cuprates because of spin fluctuations, the issue and the detailed mechanism is far from settled. In the present work, we employ a state-of-the-art emph{ab initio} ability to compute both the low and high energy spin fluctuations in optimally doped YBCO. We benchmark our results against recent inelastic neutron scattering and resonant inelastic X-ray scattering measurements. Further, we use strain as an external parameter to modulate the spin fluctuations and superconductivity. We disentangle the roles of Barium-apical Oxygen hybridization, the interlayer coupling and orbital symmetries by applying an idealized strain, and also a strain with a fully relaxed structure. We show that shortening the distance between Cu layers is conducive for enhanced Fermi surface nesting, that increases spin fluctuations and drives up $T_{c}$. However, when the structure is fully relaxed electrons flow to the d$_{z^2}$ orbital as a consequence of a shortened Ba-O bond which is detrimental for superconductivity

Electronic structure correspondence of singlet-triplet scale separation in strained Sr2RuO4

At a temperature of roughly 1,K, ce{Sr2RuO4} undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state is not even after 25 years of study. More recently it has been found that critical temperatures can be enhanced by application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an `instability approach and seek for divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show that in the unstrained compound the singlet and triplet instabilities of the normal Fermi liquid phase are closely spaced. Under uniaxial strain electrons residing on all orbitals contributing to the Fermiology become more coherent while the electrons of Ru-$d_{xy}$ character become heavier and electrons of Ru-$d_{xz,yz}$ characters become lighter. In the process, Im,$chi(mathbf{q},omega)$ increases rapidly around the incommensurate vector $mathbf{q}{=}(0.3,0.3,0)2pi/a$ while it gets suppressed at all other commensurate vectors, in particular at $q{=}0$, which is essential for spin-triplet superconductivity. Thus the triplet superconducting instability remains the lagging instability of the system and the singlet instability enhances under strain, leading to a large energy-scale separation between these competing instabilities. At large strain an instability to a spin density wave overtakes the superconducting one. The analysis relies on a high-fidelity, emph{ab initio} description of the one-particle properties and two-particle susceptibilities, based on the Quasiparticle Self-Consistent emph{GW} approximation augmented by Dynamical Mean Field theory. This approach is described and its high fidelity confirmed by comparing to observed one- and two-particle properties.

Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe

Bulk FeSe superconducts inside a nematic phase, that sets in through an orthorhombic distortion of the high temperature tetragonal phase. Bulk non-alloy tetragonal superconducting FeSe does not exist as yet. This raises the question whether nematicity is fundamental to superconductivity. We employ an advanced ab-initio ability and show that bulk tetragonal FeSe can, in principle, superconduct at almost the same Tc as the orthorhombic phase had that been the ground state. Further, we perform rigorous benchmarking of our theoretical spin susceptibilities against experimentally observed data over all energies and relevant momentum direction. We show that susceptibilities computed in both the tetragonal and orthorhombic phases already have the correct momentum structure at all energies, but not the desired intensity. The enhanced nematicity that simulates the correct spin fluctuation intensity can only lead to a maximum 10-15% increment in the superconducting Tc . Our results suggest while nematicity may be intrinsic property of the bulk FeSe, is not the primary force driving the superconducting pairing.

Controlling T$_c$ through band structure and correlation engineering in collapsed and uncollapsed phases of iron arsenides

Recent observations of selective emergence (suppression) of superconductivity in the uncollapsed (collapsed) tetragonal phase of LaFe$_2$As$_2$ has rekindled interest in understanding what features of the band structure control the superconducting T$_c$. We show that the proximity of the narrow Fe-d$_{xy}$ state to the Fermi energy emerges as the primary factor. In the uncollapsed phase this state is at the Fermi energy, and is most strongly correlated and source of enhanced scattering in both single and two particle channels. The resulting intense and broad low energy spin fluctuations suppress magnetic ordering and simultaneously provide glue for Cooper pair formation. In the collapsed tetragonal phase, the d$_{xy}$ state is driven far below the Fermi energy, which suppresses the low-energy scattering and blocks superconductivity. A similar source of broad spin excitation appears in uncollapsed and collapsed phases of CaFe$_{2}$As$_{2}$. This suggests controlling coherence provides a way to engineer T$_c$ in unconventional superconductors primarily mediated through spin fluctuations.

Electron-Phonon-Driven Three-Dimensional Metallicity in an Insulating Cuprate

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron-electron and the electron-phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron-phonon coupling to specific fully-symmetric optical modes during the build-up of a three-dimensional metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully-symmetric lattice modes can find extensive application in a plethora of correlated solids.

Interplay between band structure and Hunds correlation to increase T$_{c}$ in FeSe

FeSe is classed as a Hunds metal, with a multiplicity of $d$ bands near the Fermi level. Correlations in Hunds metals mostly originate from the exchange parameter emph{J}, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with $d_{xy}$ the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hunds systems. By applying a recently developed high-fidelity emph{ab initio} theory, we show explicitly the connections between correlations in $d_{xy}$ and the superconducting critical temperature $T_{c}$. Starting from the emph{ab initio} results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls $T_{c}$. We show small excursions in $J$ can cause colossal changes in $T_{c}$. Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO$_{3}$ substrate (M-FeSe/STO). The twin conditions of proximity of the $d_{xy}$ state to the Fermi energy, and the strength of $J$ emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls $T_{c}$. Using constrained RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance $J$. We explain why M-FeSe/STO has a high $T_{c}$, whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls $T_{c}$ in bulk, layers, and interfaces of Hunds metals by hole pocket and electron screening cloud engineering.

Questaal: a package of electronic structure methods based on the linear muffin-tin orbital technique

This paper summarises the theory and functionality behind Questaal, an open-source suite of codes for calculating the electronic structure and related properties of materials from first principles. The formalism of the linearised muffin-tin orbital (LMTO) method is revisited in detail and developed further by the introduction of short-ranged tight-binding basis functions for full-potential calculations. The LMTO method is presented in both Greens function and wave function formulations for bulk and layered systems. The suites full-potential LMTO code uses a sophisticated basis and augmentation method that allows an efficient and precise solution to the band problem at different levels of theory, most importantly density functional theory, LDA+U, quasi-particle self-consistent GW and combinations of these with dynamical mean field theory. This paper details the technical and theoretical bases of these methods, their implementation in Questaal, and provides an overview of the codes design and capabilities.

Possible phonon-induced electronic bi-stability in VO$_2$ for ultrafast memory at room temperature

VO$_{2}$ is a model material system which exhibits a metal to insulator transition at 67$^circ$C. This holds potential for future ultrafast switching in memory devices, but typically requires a purely electronic process to avoid the slow lattice response. The role of lattice vibrations is thus important, but it is not well understood and it has been a long-standing source of controversy. We use a combination of ultrafast spectroscopy and ab initio quantum calculations to unveil the mechanism responsible for the transition. We identify an atypical Peierls vibrational mode which acts as a trigger for the transition. This rules out the long standing paradigm of a purely electronic Mott transition in VO$_{2}$; however, we found a new electron-phonon pathway for a purely reversible electronic transition in a true bi-stable fashion under specific conditions. This transition is very atypical, as it involves purely charge-like excitations and requires only small nuclear displacement. Our findings will prompt the design of future ultrafast electro-resistive non-volatile memory devices.