462 research outputs found
iQIST : An open source continuous-time quantum Monte Carlo impurity solver toolkit
Quantum impurity solvers have a broad range of applications in theoretical studies of strongly correlated electron systems. Especially, they play a key role in dynamical mean-field theory calculations of correlated lattice models and realistic materials. Therefore, the development and implementation of efficient quantum impurity solvers is an important task. In this paper, we present an open source interacting quantum impurity solver toolkit (dubbed iQIST). This package contains several highly optimized quantum impurity solvers which are based on the hybridization expansion continuous-time quantum Monte Carlo algorithm, as well as some essential pre- and post-processing tools. We first introduce the basic principle of continuous-time quantum Monte Carlo algorithm and then discuss the implementation details and optimization strategies. The software framework, major features, and installation procedure for iQIST are also explained. Finally, several simple tutorials are presented in order to demonstrate the usage and power of iQIST
TRIQS/CTHYB: A Continuous-Time Quantum Monte Carlo Hybridization Expansion Solver for Quantum Impurity Problems
We present TRIQS/CTHYB, a state-of-the art open-source implementation of the
continuous-time hybridisation expansion quantum impurity solver of the TRIQS
package. This code is mainly designed to be used with the TRIQS library in
order to solve the self-consistent quantum impurity problem in a multi-orbital
dynamical mean field theory approach to strongly-correlated electrons, in
particular in the context of realistic calculations. It is implemented in C++
for efficiency and is provided with a high-level Python interface. The code is
ships with a new partitioning algorithm that divides the local Hilbert space
without any user knowledge of the symmetries and quantum numbers of the
Hamiltonian. Furthermore, we implement higher-order configuration moves and
show that such moves are necessary to ensure ergodicity of the Monte Carlo in
common Hamiltonians even without symmetry-breaking.Comment: 19 pages, this is a companion article to that describing the TRIQS
librar
Multiplet effects in orbital and spin ordering phenomena: A hybridization-expansion quantum impurity solver study
Orbital and spin ordering phenomena in strongly correlated systems are
commonly studied using the local-density approximation + dynamical mean-field
theory approach. Typically, however, such simulations are restricted to
simplified models (density-density Coulomb interactions, high symmetry
couplings and few-band models). In this work we implement an efficient general
hybridization-expansion continuous-time quantum Monte Carlo impurity solver
(Krylov approach) which allows us to investigate orbital and spin ordering in a
more realistic setting, including interactions that are often neglected (e.g.,
spin-flip and pair-hopping terms), enlarged basis sets (full d versus eg),
low-symmetry distortions, and reaching the very low-temperature (experimental)
regime. We use this solver to study ordering phenomena in a selection of
exemplary low-symmetry transition-metal oxides: LaMnO3 and rare-earth
manganites as well as the perovskites CaVO3 and YTiO3. We show that spin-flip
and pair hopping terms do not affect the Kugel-Khomskii orbital-order melting
transition in rare-earth manganites, or the suppression of orbital fluctuations
driven by crystal field and Coulomb repulsion. For the Mott insulator YTiO3 we
find a ferromagnetic transition temperature 50 K, in remarkably good agreement
with experiments. For LaMnO3 we show that the classical t2g-spin approximation,
commonly adopted for studying manganites, yields indeed an occupied eg orbital
in very good agreement with that obtained for the full d 5-orbital Hubbard
model, while the spin-spin e_g-t_{2g} correlation function calculated from the
full d model is 0.74, very close to the value expected for aligned eg and t2g
spins; the eg spectral function matrix is also well reproduced. Finally, we
show that the t2g screening reduces the eg-eg Coulomb repulsion by about 10%Comment: 9 pages, 5 figure
Dynamical Mean-Field Theory within the Full-Potential Methods: Electronic structure of Ce-115 materials
We implemented the charge self-consistent combination of Density Functional
Theory and Dynamical Mean Field Theory (DMFT) in two full-potential methods,
the Augmented Plane Wave and the Linear Muffin-Tin Orbital methods. We
categorize the commonly used projection methods in terms of the causality of
the resulting DMFT equations and the amount of partial spectral weight
retained. The detailed flow of the Dynamical Mean Field algorithm is described,
including the computation of response functions such as transport coefficients.
We discuss the implementation of the impurity solvers based on hybridization
expansion and an analytic continuation method for self-energy. We also derive
the formalism for the bold continuous time quantum Monte Carlo method. We test
our method on a classic problem in strongly correlated physics, the
isostructural transition in Ce metal. We apply our method to the class of heavy
fermion materials CeIrIn_5, CeCoIn_5 and CeRhIn_5 and show that the Ce 4f
electrons are more localized in CeRhIn_5 than in the other two, a result
corroborated by experiment. We show that CeIrIn_5 is the most itinerant and has
a very anisotropic hybridization, pointing mostly towards the out-of-plane In
atoms. In CeRhIn_5 we stabilized the antiferromagnetic DMFT solution below 3K,
in close agreement with the experimental N\'eel temperature.Comment: The implementation of Bold-CTQMC added and some test of the method
adde
Multiconfiguration time-dependent Hartree impurity solver for nonequilibrium dynamical mean-field theory
Nonequilibrium dynamical mean-field theory (DMFT) solves correlated lattice
models by obtaining their local correlation functions from an effective model
consisting of a single impurity in a self-consistently determined bath. The
recently developed mapping of this impurity problem from the Keldysh time
contour onto a time-dependent single-impurity Anderson model (SIAM) [C. Gramsch
et al., Phys. Rev. B 88, 235106 (2013)] allows one to use wave function-based
methods in the context of nonequilibrium DMFT. Within this mapping, long times
in the DMFT simulation become accessible by an increasing number of bath
orbitals, which requires efficient representations of the time-dependent SIAM
wave function. These can be achieved by the multiconfiguration time-dependent
Hartree (MCTDH) method and its multi-layer extensions. We find that MCTDH
outperforms exact diagonalization for large baths in which the latter approach
is still within reach and allows for the calculation of SIAMs beyond the system
size accessible by exact diagonalization. Moreover, we illustrate the
computation of the self-consistent two-time impurity Green's function within
the MCTDH second quantization representation.Comment: 12 pages, 8 figure
Quantum Monte Carlo Impurity Solver for Cluster DMFT and Electronic Structure Calculations in Adjustable Base
We generalized the recently introduced new impurity solver based on the
diagrammatic expansion around the atomic limit and Quantum Monte Carlo
summation of the diagrams. We present generalization to the cluster of
impurities, which is at the heart of the cluster Dynamical Mean-Field methods,
and to realistic multiplet structure of a correlated atom, which will allow a
high precision study of actinide and lanthanide based compounds with the
combination of the Dynamical Mean-Field theory and band structure methods. The
approach is applied to both, the two dimensional Hubbard and t-J model within
Cellular Dynamical Mean Field method. The efficient implementation of the new
algorithm, which we describe in detail, allows us to study coherence of the
system at low temperature from the underdoped to overdoped regime. We show that
the point of maximal superconducting transition temperature coincides with the
point of maximum scattering rate although this optimal doped point appears at
different electron densities in the two models. The power of the method is
further demonstrated on the example of the Kondo volume collapse transition in
Cerium. The valence histogram of the DMFT solution is presented showing the
importance of the multiplet splitting of the atomic states.Comment: 12 pages, 4 figure
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