The recently discovered nickelate superconductors appear, at first glance, to be even more complicated multi-orbital systems than cuprates. To identify the simplest model describing the nickelates, ...we analyse the multi-orbital system and find that it is instead the nickelates which can be described by a one-band Hubbard model, albeit with an additional electron reservoir and only around the superconducting regime. Our calculations of the critical temperature TC are in good agreement with experiment, and show that optimal doping is slightly below 20% Sr-doping. Even more promising than 3d nickelates are 4d palladates.
We describe the hybridization-expansion continuous-time quantum Monte Carlo code package “w2dynamics”, developed in Wien and Würzburg. We discuss the main features of this multi-orbital quantum ...impurity solver for the Anderson impurity model, dynamical mean field theory as well as its coupling to density functional theory. The w2dynamics package allows for calculating one- and two-particle quantities; it includes worm and further novel sampling schemes. Details about its download, installation, functioning and the relevant parameters are provided.
Program title: w2dynamics
Program Files doi:http://dx.doi.org/10.17632/rjs7kmjm2b.1
Licensing provisions: GNU General Public License v3
Programming language: Python, Fortran 90, and C++11
Required dependencies: cmake (≥2.8.5), MPI, LAPACK, FFTW3, Python (≥2.4)
Optional dependencies: NFFT, pip, numpy (≥1.4), scipy (≥0.10), h5py, mpi4py, configobj
Nature of problem: Numerically unbiased solutions of one- and two-particle propagators for quantum impurity models at finite temperature. Approximate solutions for general lattice models with strong electronic correlation.
Solution method: Continuous-time quantum Monte Carlo in the hybridization expansion, including worm sampling, for the impurity problem. Dynamical mean field theory solver for the lattice problem.
Victory, i.e.vienna computational tool depository, is a collection of numerical tools for solving the parquet equations for the Hubbard model and similar many body problems. In the current release, ...we focus on the single-band 2D Hubbard model, based on which generalizations to non-local interactions, multi orbitals and other lattices are straightforward. The parquet formalism is a self-consistent theory at both the single- and two-particle levels, and can thus describe individual fermions as well as their collective behavior on equal footing. This is essential for the understanding of various emergent phases and their transitions in many-body systems, in particular for cases in which a single-particle description fails. Our implementation of victory is in modern Fortran and it fully respects the structure of various vertex functions in both momentum and Matsubara frequency space. We found the latter to be crucial for the convergence of the parquet equations, as well as for the correct determination of various physical observables. In this release, we thoroughly explain the program structure and the controlled approximations to efficiently solve the parquet equations, i.e. the two-level kernel approximation and the high-frequency regulation.
Program Title: Victory
Program Files doi:http://dx.doi.org/10.17632/ym5kscj9sz.1
Licensing provisions: GPLv3
Programming language: Fortran 90
Nature of problem: The parquet equations require the knowledge of the fully irreducible vertex from which all one- and two-particle vertex and Green’s functions are calculated. The underlying two-particle vertex functions are large memory objects that depend on three momenta with periodic boundary conditions and three frequencies with open ones. The coupled diagrams of the parquet equations extend the frequency dependence of the reducible vertex functions to a larger frequency space where, a priori, no information is available.
Solution method: The reducible vertex functions are found to possess the simplest structure among all two-particle vertex functions and can be approximated by functions with a reduced number of arguments, i.e. the kernel functions. The open boundary issue of the vertex functions in Matsubara-frequency space is then solved as follows: we solve the parquet equations with the reducible vertex functions whenever it is possible, otherwise we supplement these by the kernel functions.
We study the influence of the pulse energy and fluence on the thermalization of photodoped Mott insulators. If the Mott gap is smaller than the width of the Hubbard bands, the kinetic energy of ...individual carriers can be large enough to produce additional doublon-hole pairs via a process analogous to impact ionization. The thermalization dynamics, which involves an adjustment of the doublon and hole densities, thus changes as a function of the energy of the photo-doped carriers and exhibits two time scales: a fast relaxation related to the impact ionization of high-energy carriers and a slower time scale associated with higher-order scattering processes. The slow dynamics depends more strongly on the gap size and the photodoping concentration.
High-temperature unconventional superconductivity quite generically emerges from doping a strongly correlated parent compound, often (close to) an antiferromagnetic insulator. The recently developed ...dynamical vertex approximation is a state-of-the-art technique that has quantitatively predicted the superconducting dome of nickelates. Here, we apply it to study the effect of pressure in the infinite-layer nickelate Sr
Pr
NiO
. We reproduce the increase of the critical temperature (T
) under pressure found in experiment up to 12 GPa. According to our results, T
can be further increased with higher pressures. Even without Sr-doping the parent compound, PrNiO
, will become a high-temperature superconductor thanks to a strongly enhanced self-doping of the Ni
orbital under pressure. With a maximal T
of 100 K around 100 GPa, nickelate superconductors can reach that of the best cuprates.
The parquet formalism and Hedin's GWγ approach are unified into a single theory of vertex corrections, corresponding to an exact reformulation of the parquet equations in terms of boson exchange. The ...method has no drawbacks compared to previous parquet solvers but has the significant advantage that the vertex functions decay quickly with frequencies and with respect to distances in real space. These properties coincide with the respective separation of the length and energy scales of the two-particle correlations into long/short-ranged and high/low-energetic.
Vertex functions are a crucial ingredient of several forefront many-body algorithms in condensed matter physics. However, the full treatment of their frequency and momentum dependence severely ...restricts numerical calculations. A significant advancement requires an efficient treatment of the high-frequency asymptotic behavior of the vertex functions. In this work, we first provide a detailed diagrammatic analysis of the high-frequency structures and their physical interpretation. Based on these insights, we propose a parametrization scheme, which captures the whole high-frequency domain for arbitrary values of the Coulomb interaction and electronic density, and we discuss the details of its algorithmic implementation in many-body solvers based on parquet equations as well as functional renormalization group schemes. Finally, we assess its validity by comparing our results for a single impurity Anderson model with exact diagonalization calculations. The proposed parametrization is pivotal for the algorithmic development of all quantum many-body methods based on vertex functions arising from both local and nonlocal static microscopic interactions as well as effective dynamic interactions which uniformly approach a static value for large frequencies. In this way, our present technique can substantially improve vertex-based diagrammatic approaches including spatial correlations beyond dynamical mean-field theory.
Superconductivity has entered the nickel age marked by enormous experimental and theoretical efforts. Notwithstanding, synthesizing nickelate superconductors remains extremely challenging, not least ...due to incomplete oxygen reduction and topotactic hydrogen. Here, we present density-functional theory calculations for nickelate superconductors with additional topotactic hydrogen or oxygen, namely La1−xSrxNiO2Hδ and LaNiO2+δ. We identify a phonon mode as a possible indication for topotactic hydrogen and discuss the charge redistribution patterns around oxygen and hydrogen impurities.
Ranking bit patterns—finding the index of a given pattern in an ordered sequence—is a major bottleneck in scaling up numerical quantum many-body calculations, as fermionic and hard-core bosonic ...states translate naturally to bit patterns. Traditionally, ranking is done by bisectioning search, which has poor cache performance on modern machines. We instead propose to use tries (prefix trees), thereby achieving a two- to tenfold speedup in numerical experiments with only moderate memory overhead. For the important problem of ranking permutations, the corresponding tries can be compressed. These compressed “staggered” lookups allow for a considerable speedup while retaining the memory requirements of prior algorithms based on the combinatorial number system.
Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some ...functionalities. Here we present a minimal setup for such a 2DEG—the SrTiO3(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising "semiheavy" band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.
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