| 1. |
- Nilsson, F., et al.
(författare)
-
Dynamically screened Coulomb interaction in the parent compounds of hole-doped cuprates : Trends and exceptions
- 2019
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : The American Physical Society. - 1098-0121 .- 1550-235X. ; 99:7
-
Tidskriftsartikel (refereegranskat)abstract
- Although cuprate high-temperature superconductors were discovered already in 1986 the origin of the pairing mechanism remains elusive. While the doped compounds are superconducting with high transition temperatures T-c, the undoped compounds are insulating due to the strong effective Coulomb interaction between the Cu 3d holes. We investigate the dependence of the maximum superconducting transition temperature T-cmax on the on-site effective Coulomb interaction U using the constrained random-phase approximation. We focus on the commonly used one-band model of the cuprates, including only the antibonding combination of the Cu d(x2-y2) and O p(x) and p(y) orbitals and find a screening-dependent trend between the static value of U and T-cmax for the parent compounds of a large number of hole-doped cuprates. Our results suggest that superconductivity may be favored by a large on-site Coulomb repulsion. We analyze both the trend in the static value of U and its frequency dependence in detail and, by comparing our results to other works, speculate on the mechanisms behind the trend.
|
|
| 2. |
- Nilsson, F., et al.
(författare)
-
Effects of dynamical screening on the BCS-BEC crossover in double bilayer graphene : Density functional theory for exciton bilayers
- 2021
-
Ingår i: Physical Review Materials. - 2475-9953. ; 5:5
-
Tidskriftsartikel (refereegranskat)abstract
- We derive a gap equation for bilayer excitonic systems based on density functional theory and benchmark our results against quantum Monte Carlo simulations and recent experiments on double bilayer graphene. The gap equation has a mean-field form but includes a consistent treatment of dynamical screening. We show that the gap survives at much higher densities than previously thought from mean-field estimates which gives strong indications that the double-bilayer graphene systems at zero magnetic field can be used as model systems to investigate the BCS-BEC crossover. Furthermore, we show that Josephson-like transfer of pairs can be substantial for small band gaps and densities.
|
|
| 3. |
- Nilsson, F., et al.
(författare)
-
Electronic structure of strongly correlated materials : From one-particle to many-body theory
- 2017
-
Ingår i: Materials Research Express. - : IOP Publishing. - 2053-1591. ; 4:3
-
Tidskriftsartikel (refereegranskat)abstract
- One of the great challenges of modern condensed matter theory is to develop reliable and practical methods for describing the electronic structure of strongly correlated materials fully from first principles. It has been known for a long time that the widely used Kohn-Sham density functional theory within the local density approximation (LDA) often fails for these systems. This paper describes the theoretical development of including electron correlations beyond the LDA with emphasis on ab initio methods, starting from the highly successful LDA+U and the many-body Green's function-based GW method to the state-of-the-art combination of dynamical mean-field theory and GW. Apart from reviewing the development so far we also present some previously unpublished results.
|
|
| 4. |
- Nilsson, F., et al.
(författare)
-
Multitier self-consistent GW+EDMFT
- 2017
-
Ingår i: Physical Review Materials. - 2475-9953. ; 1:4
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss a parameter-free and computationally efficient ab initio simulation approach for moderately and strongly correlated materials, the multitier self-consistent GW+EDMFT method. This scheme treats different degrees of freedom, such as high-energy and low-energy bands, or local and nonlocal interactions, within appropriate levels of approximation, and provides a fully self-consistent description of correlation and screening effects in the solid. The ab initio input is provided by a one-shot G0W0 calculation, while the strong-correlation effects originating from narrow bands near the Fermi level are captured by a combined GW plus extended dynamical mean-field (EDMFT) treatment. We present the formalism and technical details of our implementation and discuss some general properties of the effective EDMFT impurity action. In particular, we show that the retarded impurity interactions can have noncausal features, while the physical observables, such as the screened interactions of the lattice system, remain causal. As a first application, we present ab initio simulation results for SrMoO3, which demonstrate the existence of prominent plasmon satellites in the spectral function not obtainable within LDA+DMFT, and provide further support for our recent reinterpretation of the satellite features in the related cubic perovskite SrVO3. We then turn to stretched sodium as a model system to explore the performance of the multitier self-consistent GW+EDMFT method in situations with different degrees of correlation. While the results for the physical lattice spacing a0 show that the scheme is not very accurate for electron-gas-like systems, because nonlocal corrections beyond GW are important, it does provide physically correct results in the intermediate correlation regime, and a Mott transition around a lattice spacing of 1.5a0. Remarkably, even though the Wannier functions in the stretched compound are less localized, and hence the bare interaction parameters are reduced, the self-consistently computed impurity interactions show the physically expected trend of an increasing interaction strength with increasing lattice spacing.
|
|
| 5. |
- Sjöstrand, T. J., et al.
(författare)
-
Position representation of effective electron-electron interactions in solids
- 2019
-
Ingår i: Physical Review B. - 2469-9950. ; 99:19
-
Tidskriftsartikel (refereegranskat)abstract
- An essential ingredient in many model Hamiltonians, such as the Hubbard model, is the effective electron-electron interaction U, which enters as matrix elements in some localized basis. These matrix elements provide the necessary information in the model, but the localized basis is incomplete for describing U. We present a systematic scheme for computing the manifestly basis-independent dynamical interaction in position representation, U(r,r′;ω), and its Fourier transform to time domain, U(r,r′;τ). These functions can serve as an unbiased tool for the construction of model Hamiltonians. For illustration we apply the scheme within the constrained random-phase approximation to the cuprate parent compounds La2CuO4 and HgBa2CuO4 within the commonly used one- and three-band models, and to nonsuperconducting SrVO3 within the t2g model. Our method is used to investigate the shape and strength of screening channels in the compounds. We show that the O2px,y-Cu3dx2-y2 screening gives rise to regions with strong attractive static interaction in the minimal (one-band) model in both cuprates. On the other hand, in the minimal (t2g) model of SrVO3 only regions with a minute attractive interaction are found. The temporal interaction exhibits generic damped oscillations in all compounds, and its time integral is shown to be the potential caused by inserting a frozen point charge at τ=0. When studying the latter within the three-band model for the cuprates, short time intervals are found to produce a negative potential.
|
|
| 6. |
- Sottile, F., et al.
(författare)
-
Macroscopic and microscopic components of exchange-correlation interactions
- 2003
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 68:20, s. 205112-205122
-
Tidskriftsartikel (refereegranskat)abstract
- We consider two commonly used approaches for the ab initio calculation of optical-absorption spectra, namely, many-body perturbation theory based on Green’s functions and time-dependent density-functional theory (TDDFT). The former leads to the two-particle Bethe-Salpeter equation that contains a screened electron-hole interaction. We approximate this interaction in various ways, and discuss in particular the results obtained for a local contact potential. This, in fact, allows us to straightforwardly make the link to the TDDFT approach, and to discuss the exchange-correlation kernel fxc that corresponds to the contact exciton. Our main results, illustrated in the examples of bulk silicon, GaAs, argon, and LiF, are the following. (i) The simple contact exciton model, used on top of an ab initio calculated band structure, yields reasonable absorption spectra. (ii) Qualitatively extremely different fxc can be derived approximatively from the same Bethe-Salpeter equation. These kernels can however yield very similar spectra. (iii) A static fxc, both with or without a long-range component, can create transitions in the quasiparticle gap. To the best of our knowledge, this is the first time that TDDFT has been shown to be able to reproduce bound excitons.
|
|
| 7. |
- Aryasetiawan, F., et al.
(författare)
-
Calculations of Hubbard U from first-principles
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : The American Physical Society. - 1098-0121 .- 1550-235X. ; 74:12, s. Article number 125106-
-
Tidskriftsartikel (refereegranskat)abstract
- The Hubbard U of the 3d transition metal series as well as SrVO3, YTiO3, Ce, and Gd has been estimated using a recently proposed scheme based on the random-phase approximation. The values obtained are generally in good accord with the values often used in model calculations but for some cases the estimated values are somewhat smaller than those used in the literature. We have also calculated the frequency-dependent U for some of the materials. The strong frequency dependence of U in some of the cases considered in this paper suggests that the static value of U may not be the most appropriate one to use in model calculations. We have also made comparison with the constrained local density approximation (LDA) method and found some discrepancies in a number of cases. We emphasize that our scheme and the constrained local density approximation LDA method theoretically ought to give similar results and the discrepancies may be attributed to technical difficulties in performing calculations based on currently implemented constrained LDA schemes.
|
|
| 8. |
- Aryasetiawan, F.
(författare)
-
Continuity equation for the many-electron spectral function
- 2023
-
Ingår i: Physical Review B. - 2469-9950. ; 108:11
-
Tidskriftsartikel (refereegranskat)abstract
- Starting from the recently proposed dynamical exchange-correlation field framework, the equation of motion of the diagonal part of the many-electron Green function is derived, from which the spectral function can be obtained. The resulting equation of motion takes the form of the continuity equation of charge and current densities in electrodynamics with a source. An unknown quantity in this equation is the divergence of the temporal current density, corresponding to the kinetic energy. A procedure à la Kohn-Sham scheme is then proposed, in which the difference between the kinetic potential of the interacting system and the noninteracting Kohn-Sham system is shifted into the exchange-correlation field. The task of finding a good approximation for the exchange-correlation field should be greatly simplified since only the diagonal part is needed. A formal solution to the continuity equation provides an explicit expression for calculating the spectral function, given an approximate exchange-correlation field.
|
|
| 9. |
- Aryasetiawan, F., et al.
(författare)
-
Energy loss spectra and plasmon dispersions in alkali metals : Negative plasmon dispersion in Cs
- 1994
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 73:12, s. 1679-1682
-
Tidskriftsartikel (refereegranskat)abstract
- We present ab initio calculations of the energy loss spectra and plasmon dispersions of the alkali metals Na, K, Rb, and Cs within the random phase approximation, including band structure effects and core electrons as well as local field effects, due to inhomogeneity in the induced charge density. Band structure effects are found to cause a negative dispersion in Cs.
|
|
| 10. |
|
|
