| 1. |
- Ardizzone, I., et al.
(författare)
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Optical properties of LaNi O3 films tuned from compressive to tensile strain
- 2020
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9969 .- 2469-9950. ; 102:15
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Tidskriftsartikel (refereegranskat)abstract
- Materials with strong electronic correlations host remarkable - and technologically relevant - phenomena such as magnetism, superconductivity, and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counterintuitive effect is due to a change of orientation of the oxygen octahedra. The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.
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| 2. |
- Bittner, Nikolaj, et al.
(författare)
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Coupled charge and spin dynamics in a photo-excited Mott insulator
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:23
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Tidskriftsartikel (refereegranskat)abstract
- Using a nonequilibrium implementation of the extended dynamical mean-field theory (EDMFT) we simulate the relaxation after photoexcitation in a strongly correlated electron system with antiferromagnetic spin interactions. We consider the t−J model and focus on the interplay between the charge and spin dynamics in different excitation and doping regimes. The appearance of string states after a weak photoexcitation manifests itself in a nontrivial scaling of the relaxation time with the exchange coupling and leads to a correlated oscillatory evolution of the kinetic energy and spin-spin correlation function. A strong excitation of the system, on the other hand, suppresses the spin correlations and results in a relaxation that is controlled by hole scattering. We discuss the possibility of detecting string states in optical and cold-atom experiments.
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| 3. |
- Dong, Xinyang, et al.
(författare)
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Excitations and spectra from equilibrium real-time Green?s functions
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:12
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Tidskriftsartikel (refereegranskat)abstract
- The real-time contour formalism for Green's functions provides time-dependent information of quantum many-body systems. In practice, the long-time simulation of systems with a wide range of energy scales is challenging due to both the storage requirements of the discretized Green's function and the computational cost of solving the Dyson equation. In this paper, we apply a real-time discretization based on a piecewise high-order orthogonal-polynomial expansion to address these issues. We present a superconvergent algorithm for solving the real-time equilibrium Dyson equation using the Legendre spectral method and the recursive algorithm for Legendre convolution. We show that the compact high-order discretization in combination with our Dyson solver enables long-time simulations using far fewer discretization points than needed in conventional multistep methods. As a proof of concept, we compute the molecular spectral functions of H2, LiH, He2, and C6H4O2 using self-consistent second-order perturbation theory and compare the results with standard quantum chemistry methods as well as the auxiliary second-order Green's function perturbation theory method.
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| 4. |
- Dong, Xinyang, et al.
(författare)
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Legendre-spectral Dyson equation solver with super-exponential convergence
- 2020
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Ingår i: Journal of Chemical Physics. - Lancaster : AIP Publishing. - 1089-7690 .- 0021-9606. ; 152:13
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Tidskriftsartikel (refereegranskat)abstract
- Quantum many-body systems in thermal equilibrium can be described by the imaginary time Green's function formalism. However, the treatment of large molecular or solid ab initio problems with a fully realistic Hamiltonian in large basis sets is hampered by the storage of the Green's function and the precision of the solution of the Dyson equation. We present a Legendre-spectral algorithm for solving the Dyson equation that addresses both of these issues. By formulating the algorithm in Legendre coefficient space, our method inherits the known faster-than-exponential convergence of the Green's function's Legendre series expansion. In this basis, the fast recursive method for Legendre polynomial convolution enables us to develop a Dyson equation solver with quadratic scaling. We present benchmarks of the algorithm by computing the dissociation energy of the helium dimer He-2 within dressed second-order perturbation theory. For this system, the application of the Legendre spectral algorithm allows us to achieve an energy accuracy of 10(-9)E(h) with only a few hundred expansion coefficients.
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| 5. |
- Golež, Denis, et al.
(författare)
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Nonequilibrium GW+EDMFT : Antiscreening and inverted populations from nonlocal correlations
- 2017
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 118:24
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Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of screening in photo-doped Mott insulators with long-ranged interactions using a nonequilibrium implementation of the GW plus extended dynamical mean field theory (GW+EDMFT) formalism. Our study demonstrates that the complex interplay of the injected carriers with bosonic degrees of freedom (charge fluctuations) can result in long-lived transient states with properties that are distinctly different from those of thermal equilibrium states. Systems with strong nonlocal interactions are found to exhibit a self-sustained population inversion of the doublons and holes. This population inversion leads to low-energy antiscreening which can be detected in time-resolved electron-energy loss spectra.
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| 6. |
- Granath, Mats, 1972, et al.
(författare)
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Discretized thermal Green’s functions
- 2012
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Ingår i: Annalen der Physik. - : Wiley - VCH Verlag GmbH. - 0003-3804 .- 1521-3889. ; 524:3-4, s. 147-152
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Tidskriftsartikel (refereegranskat)abstract
- We present a spectral weight conserving formalism for Fermionic thermal Green's functions that are discretized in imaginary time t and thus periodic in imaginary (Matsubara) frequency i pi n. The formalism requires a generalization of the Dyson equation G (G0, S) and the Baym-Kadanoff-Luttinger-Ward functional for the free energy beta O = G (G). A conformal transformation is used to analytically continue the periodized Matsubara Green's function to real frequencies in a way that conserves the discontinuity at t = 0 of the corresponding real-time Green's function. This allows numerical Green's function calculations of very high precision and it appears to give a well controlled convergent approximation in the t discretization. The formalism is tested on dynamical mean field theory calculations of the paramagnetic Hubbard model.
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| 7. |
- Granath, Mats, 1972, et al.
(författare)
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Distributional exact diagonalization formalism for quantum impurity models
- 2012
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 2469-9950 .- 2469-9969. ; 86:11
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Tidskriftsartikel (refereegranskat)abstract
- We develop a method for calculating the self-energy of a quantum impurity coupled to a continuous bath by stochastically generating a distribution of finite Anderson models that are solved by exact diagonalization, using the noninteracting local spectral function as a probability distribution for the sampling. The method enables calculation of the full analytic self-energy and single-particle Green's function in the complex frequency plane, without analytic continuation, and can be used for both finite and zero temperature at arbitrary fillings. Results are in good agreement with imaginary frequency data from continuous-time quantum Monte Carlo calculations for the single-impurity Anderson model and the two-orbital Hubbard model within dynamical mean-field theory (DMFT) as well as real frequency data for self-energy of the single-band Hubbard model within DMFT using the numerical renormalization group. The method should be applicable to a wide range of quantum impurity models and particularly useful when high-precision real frequency results are sought.
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| 8. |
- Hügel, Dario, et al.
(författare)
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Anisotropic Harper-Hofstadter-Mott model : Competition between condensation and magnetic fields
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:5
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Tidskriftsartikel (refereegranskat)abstract
- We derive the reciprocal cluster mean-field method to study the strongly-interacting bosonic Harper-Hofstadter-Mott model. The system exhibits a rich phase diagram featuring band insulating, striped superfluid, and supersolid phases. Furthermore, for finite hopping anisotropy we observe gapless uncondensed liquid phases at integer fillings, which are analyzed by exact diagonalization. The liquid phases at fillings 1 and 3 exhibit the same band fillings as the fermionic integer quantum Hall effect, while the phase at filling 2 is CT-symmetric with zero charge response. We discuss how these phases become gapped on a quasi-one-dimensional cylinder, leading to a quantized Hall response, which we characterize by introducing a suitable measure for non-trivial many-body topological properties. Incompressible metastable states at fractional filling are also observed, indicating competing fractional quantum Hall phases. The combination of reciprocal cluster mean-field and exact diagonalization yields a promising method to analyze the properties of bosonic lattice systems with non-trivial unit cells in the thermodynamic limit.
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| 9. |
- Hügel, Dario, et al.
(författare)
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Bosonic self-energy functional theory
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 94:19
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Tidskriftsartikel (refereegranskat)abstract
- We derive the self-energy functional theory for bosonic lattice systems with broken U(1) symmetry by parametrizing the bosonic Baym-Kadanoff effective action in terms of one- and two-point self-energies. The formalism goes beyond other approximate methods such as the pseudoparticle variational cluster approximation, the cluster composite boson mapping, and the Bogoliubov+U theory. It simplifies to bosonic dynamical-mean field theory when constraining to local fields, whereas when neglecting kinetic contributions of non-condensed bosons it reduces to the static mean-field approximation. To benchmark the theory we study the Bose-Hubbard model on the two- and three-dimensional cubic lattice, comparing with exact results from path integral quantum Monte Carlo. We also study the frustrated square lattice with next-nearest neighbor hopping, which is beyond the reach of Monte Carlo simulations. A reference system comprising a single bosonic state, corresponding to three variational parameters, is sufficient to quantitatively describe phase-boundaries, and thermodynamical observables, while qualitatively capturing the spectral functions, as well as the enhancement of kinetic fluctuations in the frustrated case. On the basis of these findings we propose self-energy functional theory as the omnibus framework for treating bosonic lattice models, in particular, in cases where path integral quantum Monte Carlo methods suffer from severe sign problems (e.g. in the presence of non-trivial gauge fields or frustration). Self-energy functional theory enables the construction of diagrammatically sound approximations that are quantitatively precise and controlled in the number of optimization parameters, but nevertheless remain computable by modest means.
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| 10. |
- Hügel, Dario, et al.
(författare)
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Self-energy functional theory with symmetry breaking for disordered lattice bosons
- 2018
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Ingår i: Quantum Science and Technology. - : IOP Publishing. - 2058-9565. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- We extend the self-energy functional theory to the case of interacting lattice bosons in the presence of symmetry breaking and quenched disorder. The self-energy functional we derive depends only on the self-energies of the disorder-averaged propagators, allowing for the construction of general non-perturbative approximations. Using a simple single-site reference system with only three variational parameters, we are able to reproduce numerically exact quantum Monte Carlo (QMC) results on local observables of the Bose–Hubbard model with box disorder with high accuracy. At strong interactions, the phase boundaries are reproduced qualitatively but shifted with respect to the ones observed with QMC due to the extremely low condensate fraction in the superfluid phase. Deep in the strongly-disordered weakly-interacting regime, the simple reference system employed is insufficient and no stationary solutions can be found within its restricted variational subspace. By systematically analyzing thermodynamical observables and the spectral function, we find that the strongly interacting Bose glass is characterized by different regimes, depending on which local occupations are activated as a function of the disorder strength. We find that the particles delocalize into isolated superfluid lakes over a strongly localized background around maximally-occupied sites whenever these sites are particularly rare. Our results indicate that the transition from the Bose glass to the superfluid phase around unit filling at strong interactions is driven by the percolation of superfluid lakes which form around doubly occupied sites.
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| 11. |
- Kaye, Jason, et al.
(författare)
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A fast time domain solver for the equilibrium Dyson equation
- 2023
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Ingår i: Advances in Computational Mathematics. - : Springer. - 1019-7168 .- 1572-9044. ; 49:4
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Tidskriftsartikel (refereegranskat)abstract
- We consider the numerical solution of the real-time equilibrium Dyson equation, which is used in calculations of the dynamical properties of quantum many-body systems. We show that this equation can be written as a system of coupled, nonlinear, convolutional Volterra integro-differential equations, for which the kernel depends self-consistently on the solution. As is typical in the numerical solution of Volterra-type equations, the computational bottleneck is the quadratic-scaling cost of history integration. However, the structure of the nonlinear Volterra integral operator precludes the use of standard fast algorithms. We propose a quasilinear-scaling FFT-based algorithm which respects the structure of the nonlinear integral operator. The resulting method can reach large propagation times and is thus well-suited to explore quantum many-body phenomena at low energy scales. We demonstrate the solver with two standard model systems: the Bethe graph and the Sachdev-Ye-Kitaev model.
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| 12. |
- Kaye, Jason, et al.
(författare)
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libdlr : Efficient imaginary time calculations using the discrete Lehmann representation
- 2022
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Ingår i: Computer Physics Communications. - : Elsevier. - 0010-4655 .- 1879-2944. ; 280
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Tidskriftsartikel (refereegranskat)abstract
- We introduce libdlr, a library implementing the recently introduced discrete Lehmann representation (DLR) of imaginary time Green's functions. The DLR basis consists of a collection of exponentials chosen by the interpolative decomposition to ensure stable and efficient recovery of Green's functions from imaginary time or Matsubara frequency samples. The library provides subroutines to build the DLR basis and grids, and to carry out various standard operations. The simplicity of the DLR makes it straightforward to incorporate into existing codes as a replacement for less efficient representations of imaginary time Green's functions, and libdlr is intended to facilitate this process. libdlr is written in Fortran, provides a C header interface, and contains a Python module pydlr. We also introduce a stand-alone Julia implementation, Lehmann.jl. Program summary Program Title: libdlr CPC Library link to program files: https://doi .org /10 .17632 /56z594pzsj .1 Developer's repository link: https://github .com /jasonkaye /libdlr Licensing provisions: Apache-2.0 Programming language: Fortran, C, Python, Julia Nature of problem: Discretization and compression of functions (Green's functions and self-energies) with an imaginary time variable. Solution method: Explicit basis functions and discretization points obtained by low rank compression of the analytical continuation kernel.
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| 13. |
- Kugler, Fabian B., et al.
(författare)
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Strongly Correlated Materials from a Numerical Renormalization Group Perspective : How the Fermi-Liquid State of Sr2RuO4 Emerges
- 2020
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Ingår i: Physical Review Letters. - New York : American Physical Society. - 0031-9007 .- 1079-7114. ; 124:1
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Tidskriftsartikel (refereegranskat)abstract
- The crossover from fluctuating atomic constituents to a collective state as one lowers temperature or energy is at the heart of the dynamical mean-field theory description of the solid state. We demonstrate that the numerical renormalization group is a viable tool to monitor this crossover in a real-materials setting. The renormalization group flow from high to arbitrarily small energy scales clearly reveals the emergence of the Fermi-liquid state of Sr2RuO4. We find a two-stage screening process, where orbital fluctuations are screened at much higher energies than spin fluctuations, and Fermi-liquid behavior, concomitant with spin coherence, below a temperature of 25 K. By computing real-frequency correlation functions, we directly observe this spin-orbital scale separation and show that the van Hove singularity drives strong orbital differentiation. We extract quasiparticle interaction parameters from the low-energy spectrum and find an effective attraction in the spin-triplet sector.
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| 14. |
- Käser, Stefan, et al.
(författare)
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Interorbital singlet pairing in Sr2RuO4 : A Hund's superconductor
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:15
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Tidskriftsartikel (refereegranskat)abstract
- We study the superconducting gap function of Sr2RuO4. By solving the linearized Eliashberg equation with a correlated pairing vertex extracted from a dynamical mean-field calculation we identify the dominant pairing channels. An analysis of the candidate gap functions in orbital and quasiparticle band basis reveals that an interorbital singlet pairing of even parity is in agreement with experimental observations. It reconciles in particular the occurrence of a two-component order parameter with the presence of line nodes of quasiparticles along the c axis in the superconducting phase. The strong angular dependence of the gap along the Fermi surface is in stark contrast to its quasilocality when expressed in the orbital basis. We identify local interorbital spin correlations as the driving force for the pairing and thus reveal the continuation of Hund's physics into the superconducting phase.
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| 15. |
- Lanata, Nicola, et al.
(författare)
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Efficient implementation of the Gutzwiller variational method
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121. ; 85:3
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Tidskriftsartikel (refereegranskat)abstract
- We present a self-consistent numerical approach to solve the Gutzwiller variational problem for general multi-band models with arbitrary on-site interaction. The proposed method generalizes and improves the procedure derived by Deng et al., Phys. Rev. B. 79 075114 (2009), overcoming the restriction to density-density interaction without increasing the complexity of the computational algorithm. Our approach drastically reduces the problem of the high-dimensional Gutzwiller mini- mization by mapping it to a minimization only in the variational density matrix, in the spirit of the Levy and Lieb formulation of DFT. For fixed density the Gutzwiller renormalization matrix is deter- mined as a fixpoint of a proper functional, whose evaluation only requires ground-state calculations of matrices defined in the Gutzwiller variational space. Furthermore, the proposed method is able to account for the symmetries of the variational function in a controlled way, reducing the number of variational parameters. After a detailed description of the method we present calculations for multi-band Hubbard models with full (rotationally invariant) Hund’s rule on-site interaction. Our analysis shows that the numerical algorithm is very efficient, stable and easy to implement. For these reasons this method is particularly suitable for first principle studies – e.g., in combination with DFT – of many complex real materials, where the full intra-atomic interaction is important to obtain correct results.
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| 16. |
- Lanata, N., et al.
(författare)
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Orbital selectivity in Hund's metals: The iron chalcogenides
- 2013
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:4
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Tidskriftsartikel (refereegranskat)abstract
- We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion U and Hund’s rule coupling J. The evolution of the quasi particle weight Z as a function of the interaction terms reveals a clear crossover at U ≃ 2.5 eV. In the weak coupling limit Z decreases for all correlated d orbitals as a function of U and beyond the crossover coupling they become weakly dependent on U while strongly depend on J. A marked orbital dependence of the Z’s emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of U. This two-stage reduction of the quasi particle coherence due to the combined effect of Hubbard U and the Hund’s J, suggests that the iron-based superconductors can be referred to as Hund’s correlated metals.
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| 17. |
- Lanata, N., et al.
(författare)
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Principle of Maximum Entanglement Entropy and Local Physics of Strongly Correlated Materials
- 2014
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 113:3
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Tidskriftsartikel (refereegranskat)abstract
- We argue that, because of quantum entanglement, the local physics of strongly correlated materials at zero temperature is described in a very good approximation by a simple generalized Gibbs distribution, which depends on a relatively small number of local quantum thermodynamical potentials. We demonstrate that our statement is exact in certain limits and present numerical calculations of the iron compounds FeSe and FeTe and of the elemental cerium by employing the Gutzwiller approximation that strongly support our theory in general.
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| 18. |
- Lanata, N., et al.
(författare)
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Time-dependent and steady-state Gutzwiller approach for nonequilibrium transport in nanostructures
- 2012
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:11
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Tidskriftsartikel (refereegranskat)abstract
- We extend the time-dependent Gutzwiller variational approach, recently introduced by Schiro and Fabrizio [Phys. Rev. Lett. 105, 076401 (2010)], to impurity problems. Furthermore, we derive a consistent theory for the steady state, and show its equivalence with the previously introduced nonequilibrium steady-state extension of the Gutzwiller approach. The method is shown to be able to capture dissipation in the leads, so that a steady state is reached after a sufficiently long relaxation time. The time-dependent method is applied to the single-orbital Anderson impurity model at half filling, modeling a quantum dot coupled to two leads. In these exploratory calculations, the Gutzwiller projector is limited to act only on the impurity. The strengths and the limitations of this approximation are assessed via comparison with state-of-the-art continuous-time quantum Monte Carlo results. Finally, we discuss how the method can be systematically improved by extending the region of action of the Gutzwiller projector.
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| 19. |
- Li, Jiajun, et al.
(författare)
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Theory of photoinduced ultrafast switching to a spin-orbital ordered hidden phase
- 2018
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Photo-induced hidden phases are often observed in materials with intertwined orders. Understanding the formation of these non-thermal phases is challenging and requires a resolution of the cooperative interplay between different orders on the ultra-short timescale. In this work, we demonstrate that non-equilibrium photo-excitations can induce a state with spin-orbital orders entirely different from the equilibrium state in the three-quarter-filled two-band Hubbard model. We identify a general mechanism governing the transition to the hidden state, which relies on a non-thermal partial melting of the intertwined orders mediated by photoinduced charge excitations in the presence of strong spin-orbital exchange interactions. Our study theoretically confirms the crucial role played by orbital degrees of freedom in the light-induced dynamics of strongly correlated materials and it shows that the switching to hidden states can be controlled already on the fs timescale of the electron dynamics.
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| 20. |
- Ligges, Manuel, et al.
(författare)
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Ultrafast doublon dynamics in photo-excited 1T-TaS2
- 2018
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 120:16
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Tidskriftsartikel (refereegranskat)abstract
- Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.
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| 21. |
- Sayyad, Sharareh, et al.
(författare)
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Comparative study of nonequilibrium insulator-to-metal transitions in electron-phonon systems
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 99:4
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Tidskriftsartikel (refereegranskat)abstract
- We study equilibrium and nonequilibrium properties of electron-phonon systems described by the Hubbard-Holstein model using dynamical mean-field theory. In equilibrium, we benchmark the results for impurity solvers based on the one-crossing approximation and slave-rotor approximation against non-perturbative numerical renormalization group reference data. We also examine how well the low-energy properties of the electron-boson coupled systems can be reproduced by an effective static electron-electron interaction. The one-crossing and slave-rotor approximations are then used to simulate insulator-to-metal transitions induced by a sudden switch-on of the electron-phonon interaction. The slave-rotor results suggest the existence of a critical electron-phonon coupling above which the system is transiently trapped in a nonthermal metallic state with coherent quasiparticles. The same quench protocol in the one-crossing approximation results in a bad metallic state.
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| 22. |
- Schüler, Michael, et al.
(författare)
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NESSi : The Non-Equilibrium Systems Simulation package
- 2020
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Ingår i: Computer Physics Communications. - Amsterdam : Elsevier. - 0010-4655 .- 1879-2944. ; 257
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Tidskriftsartikel (refereegranskat)abstract
- The nonequilibrium dynamics of correlated many-particle systems is of interest in connection with pump–probe experiments on molecular systems and solids, as well as theoretical investigations of transport properties and relaxation processes. Nonequilibrium Green’s functions are a powerful tool to study interaction effects in quantum many-particle systems out of equilibrium, and to extract physically relevant information for the interpretation of experiments. We present the open-source software package NESSi (The Non-Equilibrium Systems Simulation package) which allows to perform many-body dynamics simulations based on Green’s functions on the L-shaped Kadanoff–Baym contour. NESSi contains the library libcntr which implements tools for basic operations on these nonequilibrium Green’s functions, for constructing Feynman diagrams, and for the solution of integral and integro-differential equations involving contour Green’s functions. The library employs a discretization of the Kadanoff–Baym contour into time points and a high-order implementation of integration routines. The total integrated error scales up to , which is important since the numerical effort increases at least cubically with the simulation time. A distributed-memory parallelization over reciprocal space allows large-scale simulations of lattice systems. We provide a collection of example programs ranging from dynamics in simple two-level systems to problems relevant in contemporary condensed matter physics, including Hubbard clusters and Hubbard or Holstein lattice models. The libcntr library is the basis of a follow-up software package for nonequilibrium dynamical mean-field theory calculations based on strong-coupling perturbative impurity solvers.
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| 23. |
- Strand, Hugo, 1983-, et al.
(författare)
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Beyond the Hubbard bands in strongly correlated lattice bosons
- 2015
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 92:6
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Tidskriftsartikel (refereegranskat)abstract
- We investigate features in the single-particle spectral function beyond the Hubbard bands in the strongly correlated normal phase of the Bose-Hubbard model. There are two distinct classes of additional peaks generated by the bosonic statistics. The first type is thermally activated Hubbard “sidebands”, with the same physical origin as the zero-temperature Hubbard bands, but generated by excitations from thermally activated local occupation number states. The second class are two-particle fluctuation resonances driven by the lattice dynamics. In the unity filling Mott insulator, this takes the form of a localized triplon combined with a dispersing holon. Both types of resonances also manifest themselves in the structure factor and the interaction modulation spectra obtained from nonequilibrium bosonic dynamical mean-field theory calculations. Our findings explain experimental lattice modulation and Bragg spectroscopy results, and they predict a strong temperature dependence of the first sideband, thereby opening the door to precise thermometry of strongly correlated lattice bosons.
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| 24. |
- Strand, Hugo, 1983-
(författare)
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Correlated materials : models and methods
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis encompass a series of studies on methods and models for electron systems with local interactions, relevant for correlated materials. The first study focus on the canonical model for local correlation, the Hubbard model. Using dynamical mean field theory, the critical properties of the finite temperature end point of the metal insulator transition are determined. The issue of computing real frequency spectral functions is also addressed through the development of the novel method, distributional exact diagonalization. Next topic is the multiband Gutzwiller variational method, for which an efficient solver is presented, applicable to realistic delectron models when accounting for lattice symmetries. The solver is applied to the iron based superconductors FeSe and FeTe, where the Hund’s coupling is found to drive orbital differentiation in the correlated parent state. A central issue is how to model the local Coulomb interaction. Imposing rotational invariance on the complete set of d-states results in the Slater-Condon interaction, to be compared with the simpler Kanamori interaction, that is shown to be a Laporte-Platt degenerate point of the former. The derivation of a minimalistic form for the Kanamori interaction in terms o fdensity-density, total spin, and total quasi-spin operators enables an exact parametrization of the Slater-Condon interaction in terms of the Kanamori parameters.The additional interactions contained in the Slater-Condon form are identified as higher order multipole scattering, and the parametrization enables a direct study of the effect of these interaction processes. The multipole scattering is found to drive charge disproportionation and valence-skipping for a subset of multipole active d-band fillings, and raises the question whether such multipole effects are manifested in real materials.
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| 25. |
- Strand, Hugo, 1983-, et al.
(författare)
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Hund's coupling driven photocarrier relaxation in the two-band Mott insulator
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:16
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Tidskriftsartikel (refereegranskat)abstract
- We study the relaxation dynamics of photocarriers in the paramagnetic Mott insulating phase of the half-filled two-band Hubbard model. Using nonequilibrium dynamical mean-field theory, we excite charge carriers across the Mott gap by a short hopping modulation, and simulate the evolution of the photodoped population within the Hubbard bands. We observe an ultrafast charge-carrier relaxation driven by the emission of local spin excitations with an inverse relaxation time proportional to the Hund's coupling. The photodoping generates additional side-bands in the spectral function, and for strong Hund's coupling, the photodoped population also splits into several resonances. The dynamics of the local many-body states reveals two effects, thermal blocking and kinetic freezing, which manifest themselves when the Hund's coupling becomes of the order of the temperature or the bandwidth, respectively. These effects, which are absent in the single-band Hubbard model, should be relevant for the interpretation of experiments on correlated materials with multiple active orbitals. In particular, the features revealed in the nonequilibrium energy distribution of the photocarriers are experimentally accessible, and provide information on the role of the Hund's coupling in these materials.
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| 26. |
- Strand, Hugo, 1983-, et al.
(författare)
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Magnetic response of Sr2RuO4 : quasi-local spin fluctuations due to Hund’s coupling
- 2019
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - New York : American Physical Society. - 1098-0121 .- 1550-235X. ; 100:12
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Tidskriftsartikel (refereegranskat)abstract
- We study the magnetic susceptibility in the normal state of Sr2RuO4 using dynamical mean-field theory including dynamical vertex corrections. Besides the well known incommensurate response, our calculations yield quasi-local spin fluctuations which are broad in momentum and centered around the Gamma-point, in agreement with recent inelastic neutron scattering experiments [Steffens et al., Phys. Rev. Lett. 122, 047004 (2019)]. We show that these quasi-local fluctuations are controlled by the Hund’s coupling and account for the dominant contribution to the momentum-integrated response. While all orbitals contribute equally to the incommensurate response, the enhanced Gamma-point response originates from the planar xy orbital.
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| 27. |
- Strand, Hugo, 1983-, et al.
(författare)
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Nonequilibrium Dynamical Mean-Field Theory for Bosonic Lattice Models
- 2015
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Ingår i: Physical Review X. - : American Physical Society. - 2160-3308. ; 5:1
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Tidskriftsartikel (refereegranskat)abstract
- We develop the nonequilibrium extension of bosonic dynamical mean-field theory and a Nambu real-time strong-coupling perturbative impurity solver. In contrast to Gutzwiller mean-field theory and strong-coupling perturbative approaches, nonequilibrium bosonic dynamical mean-field theory captures not only dynamical transitions but also damping and thermalization effects at finite temperature. We apply the formalism to quenches in the Bose-Hubbard model, starting from both the normal and the Bose-condensed phases. Depending on the parameter regime, one observes qualitatively different dynamical properties, such as rapid thermalization, trapping in metastable superfluid or normal states, as well as long-lived or strongly damped amplitude oscillations. We summarize our results in nonequilibrium “phase diagrams” that map out the different dynamical regimes.
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| 28. |
- Strand, Hugo, 1983, et al.
(författare)
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The Dynamical Mean Field Theory phase space extension and critical properties of the finite temperature Mott transition
- 2011
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Ingår i: Phys. Rev. B. - : American Physical Society. - 1098-0121. ; 83:20
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Tidskriftsartikel (refereegranskat)abstract
- We consider the finite temperature metal-insulator transition in the half filled paramagnetic Hubbard model on the infinite dimensional Bethe lattice. A new method for calculating the Dynamical Mean Field Theory fixpoint surface in the phase diagram is presented and shown to be free from the convergence problems of standard forward recursion. The fixpoint equation is then analyzed using dynamical systems methods. On the fixpoint surface the eigenspectra of its Jacobian is used to characterize the hysteresis boundaries of the first order transition line and its second order critical end point. The critical point is shown to be a cusp catastrophe in the parameter space, opening a pitchfork bifurcation along the first order transition line, while the hysteresis boundaries are shown to be saddle-node bifurcations of two merging fixpoints. Using Landau theory the properties of the critical end point is determined and related to the critical eigenmode of the Jacobian. Our findings provide new insights into basic properties of this intensively studied transition.
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| 29. |
- Strand, Hugo, 1983
(författare)
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Valence-skipping and negative-U in the d-band from repulsive local Coulomb interaction
- 2014
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 2469-9950 .- 2469-9969. ; 90:15
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Tidskriftsartikel (refereegranskat)abstract
- We show that repulsive local Coulomb interaction alone can drive valence-skipping charge disproportionation in the degenerate d-band, resulting in effective negative-U. This effect is shown to originate from anisotropic orbital-multipole scattering, and it occurs only for d(1), d(4), d(6), and d(9) fillings (and their immediate surroundings). Explicit boundaries for valence-skipping are derived, and the paramagnetic phase diagram for d(4) and d(6) is calculated. We also establish that the valence-skipping metal is very different, in terms of its local valence distribution, compared to the atomiclike Hund's metal. These findings explain why transition-metal compounds with the aforementioned d-band fillings are more prone to valence-skipping charge order and anomalous superconductivity.
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| 30. |
- Suzuki, H., et al.
(författare)
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Distinct spin and orbital dynamics in Sr2RuO4
- 2023
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- The unconventional superconductor Sr2RuO4 has long served as a benchmark for theories of correlated-electron materials. The determination of the superconducting pairing mechanism requires detailed experimental information on collective bosonic excitations as potential mediators of Cooper pairing. We have used Ru L3-edge resonant inelastic x-ray scattering to obtain comprehensive maps of the electronic excitations of Sr2RuO4 over the entire Brillouin zone. We observe multiple branches of dispersive spin and orbital excitations associated with distinctly different energy scales. The spin and orbital dynamical response functions calculated within the dynamical mean-field theory are in excellent agreement with the experimental data. Our results highlight the Hund metal nature of Sr2RuO4 and provide key information for the understanding of its unconventional superconductivity.
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| 31. |
- Tamai, Anna, et al.
(författare)
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High-Resolution Photoemission on Sr2RuO4 Reveals Correlation-Enhanced Effective Spin-Orbit Coupling and Dominantly Local Self-Energies
- 2019
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Ingår i: Physical Review X. - New York : American Physical Society. - 2160-3308. ; 9:2
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Tidskriftsartikel (refereegranskat)abstract
- We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about 2, due to electronic correlations. The self-energies for the β and γ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies and provides strong evidence for an electronic origin of the observed nonlinear frequency dependence of the self-energies, leading to “kinks” in the quasiparticle dispersion of Sr2RuO4.
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| 32. |
- Van Loon, Erik G.C.P., et al.
(författare)
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Dual Bethe-Salpeter equation for the multiorbital lattice susceptibility within dynamical mean-field theory
- 2024
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:15
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Tidskriftsartikel (refereegranskat)abstract
- Dynamical mean -field theory describes the impact of strong local correlation effects in many -electron systems. While the single -particle spectral function is directly obtained within the formalism, two -particle susceptibilities can also be obtained by solving the Bethe-Salpeter equation. The solution requires handling infinite matrices in Matsubara frequency space. This is commonly treated using a finite frequency cutoff, resulting in slow linear convergence. A decomposition of the two -particle response in local and nonlocal contributions enables a reformulation of the Bethe-Salpeter equation inspired by the dual boson formalism. The reformulation has a drastically improved cubic convergence with respect to the frequency cutoff, considerably facilitating the calculation of susceptibilities in multi -orbital systems. This improved convergence arises from the fact that local contributions can be measured in the impurity solver. The dual Bethe-Salpeter equation uses the fully reducible vertex which is free from vertex divergences. We benchmark the approach on several systems including the spin susceptibility of strontium ruthenate Sr 2 RuO 4 , a strongly correlated Hund's metal with three active orbitals.
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| 33. |
- van Loon, Erik, et al.
(författare)
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Larmor precession in strongly correlated itinerant electron systems
- 2023
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Many-electron systems undergo a collective Larmor precession in the presence of a magnetic field. In a paramagnetic metal, the resulting spin wave provides insight into the correlation effects generated by the electron-electron interaction. Here, we use dynamical mean-field theory to investigate the collective Larmor precession in the strongly correlated regime, where dynamical correlation effects such as quasiparticle lifetimes and non-quasiparticle states are essential. We study the spin excitation spectrum, which includes a dispersive Larmor mode as well as electron-hole excitations that lead to Stoner damping. We also extract the momentum-resolved damping of slow spin waves. The accurate theoretical description of these phenomena relies on the Ward identity, which guarantees a precise cancellation of self-energy and vertex corrections at long wavelengths. Our findings pave the way towards a better understanding of spin wave damping in correlated materials.
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| 34. |
- Werner, Philipp, et al.
(författare)
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Enhanced pairing susceptibility in a photo-doped two-orbital Hubbard model
- 2018
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - New York : American Physical Society. - 1098-0121 .- 1550-235X. ; 97:16
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Tidskriftsartikel (refereegranskat)abstract
- Local spin fluctuations provide the glue for orbital-singlet spin-triplet pairing in the doped Mott insulating regime of multiorbital Hubbard models. At large Hubbard repulsion U, the pairing susceptibility is nevertheless tiny because the pairing interaction cannot overcome the suppression of charge fluctuations. Using nonequilibrium dynamical mean field simulations of the two-orbital Hubbard model, we show that out of equilibrium the pairing susceptibility in this large-U regime can be strongly enhanced by creating a photoinduced population of the relevant charge states. This enhancement is supported by the long lifetime of photodoped charge carriers and a built-in cooling mechanism in multiorbital Hubbard systems.
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| 35. |
- Werner, Philipp, et al.
(författare)
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Ultrafast switching of composite order in A(3)C(60)
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:19
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Tidskriftsartikel (refereegranskat)abstract
- We study the controlled manipulation of the Jahn-Teller metal state of fulleride compounds using nonequilibrium dynamical mean-field theory. This anomalous metallic state is a spontaneous orbital-selective Mott phase, which is characterized by one metallic and two insulating orbitals. Using protocols based on transiently reduced hopping amplitudes or periodic electric fields, we demonstrate the possibility to switch orbitals between Mott insulating and metallic on a subpicosecond time scale, and to rotate the order parameter between three equivalent states that can be distinguished by their anisotropic conductance. The Jahn-Teller metal phase of alkali-doped fullerides thus provides a platform for ultrafast persistent memory devices.
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