Sergei Iskakov

Updated core libraries of the ALPS project

Abstract The open source ALPS (Algorithms and Libraries for Physics Simulations) project provides a collection of physics libraries and applications, with a focus on simulations of lattice models and strongly correlated systems. The libraries provide a convenient set of well-documented and reusable components for developing condensed matter physics simulation code, and the applications strive to make commonly used and proven computational algorithms available to a non-expert community. In this paper we present an updated and refactored version of the core ALPS libraries geared at the computational physics software development community, rewritten with focus on documentation, ease of installation, and software maintainability. Program summary Program Title: ALPS Core libraries Program Files doi: http://dx.doi.org/10.17632/fckj5d7wtr.1 Programming language: C++ Licensing provisions: GNU GPLv3 Nature of problem: Need for modern, lightweight, tested and documented libraries covering the basic requirements of rapid development of efficient physics simulation codes, especially for modeling strongly correlated electron systems. Solution method: We present a C++ open source computational library that provides a convenient set of components for developing parallel physics simulation code. The library features a short development cycle and up-to-date user documentation. External routines/libraries: CMake , MPI , Boost , HDF5 .

Diagrammatic Monte Carlo for dual fermions

We introduce a numerical algorithm to stochastically sample the dual fermion perturbation series around the dynamical mean field theory, generating all topologies of two-particle interaction vertices. We show results in the weak and strong coupling regime of the half-filled Hubbard model in two dimensions, illustrating that the method converges quickly where dynamical mean field theory is a good approximation, and show that corrections are large in the strong correlation regime at intermediate interaction. The fast convergence of dual corrections to dynamical mean field results illustrates the power of the approach and opens a practical avenue towards the systematic inclusion of non-local correlations in correlated materials simulations. An analysis of the frequency scale shows that only low-frequency propagators contribute substantially to the diagrams, putting the inclusion of higher order vertices within reach.

Exact diagonalization solver for extended dynamical mean-field theory

We present an efficient exact diagonalization scheme for the extended dynamical mean-field theory and apply it to the extended Hubbard model on the square lattice with nonlocal charge-charge interactions. Our solver reproduces the phase diagram of this approximation with good accuracy. Details on the numerical treatment of the large Hilbert space of the auxiliary Holstein-Anderson impurity problem are provided. Benchmarks with a numerically exact strong-coupling continuous-time quantum-Monte Carlo solver show better convergence behavior of the exact diagonalization in the deep insulator. Special attention is given to possible effects due to the discretization of the bosonic bath. We discuss the quality of real axis spectra and address the question of screening in the Mott insulator within extended dynamical mean-field theory.

Valence fluctuations and empty-state resonance for Fe adatom on a surface

We report on the formation of the high-energy empty-state resonance in the electronic spectrum of the iron adatom on the Pt(111) surface. By using the combination of the first-principles methods and the finite-temperature exact diagonalization approach, we show that the resonance is the result of the valence fluctuations between atomic configurations of the impurity. Our theoretical finding is fully confirmed by the results of the scanning tunneling microscopy measurements [M.F. Crommie et al., Phys. Rev. B 48, 2851 (1993)]. In contrast to the previous theoretical results obtained by using local spin density approximation, the paramagnetic state of the impurity in the experiment is naturally reproduced within our approach. This opens a new way for interpretation of STM data collected earlier for metallic surface nanosystems with iron impurities.

Spin-Unrestricted Self-Energy Embedding Theory

We present a new theoretical approach, unrestricted self-energy embedding theory (USEET), that is a Greens function embedding theory used to study problems in which an open, embedded system exchanges electrons with the environment. USEET has a high potential to be used in studies of strongly correlated systems with an odd number of electrons and open shell systems such as transition metal complexes important in inorganic chemistry. In this paper, we show that USEET results agree very well with common quantum chemistry methods while avoiding typical bottlenecks present in these methods.

Correlation effects in insulating surface nanostructures

WestudytheroleofstaticanddynamicalCoulombcorrelationeffectsontheelectronicandmagneticproperties of individual Mn, Fe, and Co adatoms deposited on the CuN surface. For these purposes, we construct a realistic Andersonmodel,solveitbyusingthefinite-temperatureexactdiagonalizationmethod,andcomparethecalculated one-particle spectral functions with the LDA + U densities of states. In contrast to Mn/CuN and Fe/CuN, the cobaltsystemtendstoformtheelectronicexcitationsattheFermilevel.Basedonthecalculatedmagneticresponse functions, transverse relaxation times for the magnetic moments of impurity orbitals are estimated. To study the effect of the dynamical correlations on the exchange interaction in nanoclusters, we solve the two-impurity Anderson model for the Mn dimer on the CuN surface. It is found that the experimental exchange interaction can be well reproduced by employing U = 3 eV, which is two times smaller than the value used in static mean-field LDA + U calculations. This suggests an important role of dynamical correlations in the interaction between adatoms on a surface. To estimate the correlated exchange interaction in the general case we derive a simple and transparent analytical expression demonstrating that the renormalization of the electronic spectrum due to dynamical correlations leads to a rescaling of the magnetic interactions compared to density functional results.

Orbital-selective conductivity of Co adatom on the Pt(111) surface

We propose an orbital-selective model for the transport and magnetic properties of the individual Co impurity deposited on the Pt(111). Using the combination of the Anderson-type Hamiltonian and the Kubos linear response theory we show that the magnetization and dI/dV spectrum of Co adatom are originated from the 3d states of the different symmetry. A textbook expression for the spin-dependent differential conductance provides a natural connection between magnetic and transport properties of Co/Pt(111). We found that it is possible to detect and to manipulate the different 3d states of the Co adatom by tuning the spin polarization of the tip and tip-impurity distance in STM experiments.