| 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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