| 1. |
- Backes, Steffen, et al.
(författare)
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Nonlocal correlation effects in fermionic many-body systems : Overcoming the noncausality problem
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 105:24
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by the intriguing physics of quasi-two-dimensional fermionic systems, such as high-temperature superconducting oxides, layered transition metal chalcogenides, or surface or interface systems, the development of many-body computational methods geared at including both local and nonlocal electronic correlations has become a rapidly evolving field. It has been realized, however, that the success of such methods can be hampered by the emergence of noncausal features in the effective or observable quantities involved. Here, we present an approach wherein local many-body techniques such as dynamical mean-field theory (DMFT) are extended to nonlocal correlations and interactions, which preserves causality and has a physically intuitive interpretation. Our strategy has implications for the general class of DMFT-inspired many-body methods and can be adapted to cluster, dual boson, or dual fermion techniques with minimal effort.
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| 2. |
- Bacq-Labreuil, Benjamin, et al.
(författare)
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Oxygen vacancies at the origin of pinned moments in oxide interfaces : The example of tetragonal CuO/SrTiO3
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 106:23
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Tidskriftsartikel (refereegranskat)abstract
- Obtaining an accurate theoretical description of the emergent phenomena in oxide heterostructures is a major challenge. Recently, intriguing paramagnetic spin and pinned orbital moments have been discovered by x-ray magnetic circular dichroism measurements at the Cu L2,3 edge of a tetragonal CuO/SrTiO3 heterostructure. Using first-principles calculations, we propose a scenario that explains both types of moments, based on the formation of oxygen vacancies in the TiO2 interface layer. We show the emergence of a paramagnetic two-dimensional electron gas hosted in the interface CuO layer. It is invisible at the Ti L2,3 edge since the valence of the Ti atoms remains unchanged. Strong structural distortions breaking both the local and global fourfold rotation C4 symmetries at the interface lead to the in-plane pinning of the Cu orbital moment close to the vacancy. Our results, and in particular the pinning of the orbital moment, may have implications for other systems, especially monoxide/dioxide interfaces with similar metal-oxygen bond length and weak spin-orbit coupling.
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| 3. |
- Boust, James, et al.
(författare)
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Combining semilocal exchange with dynamical mean-field theory : Electronic structure and optical response of rare-earth sesquioxides
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 105:8
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Tidskriftsartikel (refereegranskat)abstract
- In rare-earth semiconductors, wide ligand p and rare-earth 5d bands coexist with localized, partially filled 4f shells. A simultaneous description for both extended and localized states represents a significant challenge for first-principles theories. Here, we combine an ab initio dynamical mean-field theory approach to strong local correlations with a perturbative application of the semilocal modified Becke-Johnson exchange potential to correct the semiconducting gap. We apply this method to calculate the electronic structure and optical response of the light rare-earth sesquioxides R2O3 (R=La,Ce,Pr,andNd). Our calculations correctly capture a nontrivial evolution of the optical gap in R2O3 due to a progressive lowering of the 4f states along the series and their multiplet structure. 2p-4f hybridization is found to induce a substantial upward shift for the occupied 4f states occurring within the p-d gap, thus reducing the magnitude of the optical gap. We show that a characteristic plateau observed in the optical conductivity in the Pr and Nd sesquioxides right above their absorption edge is a fingerprint of 4f states located within the p-d gap.
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| 4. |
- Bramberger, Max, et al.
(författare)
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Formation of CuO2 sublattices by suppression of interlattice correlations in tetragonal CuO
- 2023
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Ingår i: SciPost Physics. - 2542-4653. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the tetragonal phase of the binary transition metal oxide CuO (t-CuO) within the context of cellular dynamical mean-field theory. Due to its strong antiferromagnetic correlations and simple structure, analysing the physics of t-CuO is of high interest as it may pave the way towards a more complete understanding of high-temperature superconductivity in hole-doped antiferromagnets. In this work we give a formal justification for the weak-coupling assumption that has previously been made for the interconnected sublattices within a single layer of t-CuO by studying the non-local self-energies of the system. We compute momentum-resolved spectral functions using a Matrix Product State (MPS)-based impurity solver directly on the real axis, which does not require any numerically ill-conditioned analytic continuation. The agreement with photoemission spectroscopy indicates that a single-band Hubbard model is sufficient to capture the material’s low energy physics. We perform calculations on a range of different temperatures, finding two magnetic regimes, for which we identify the driving mechanism behind their respective insulating state. Finally, we show that in the hole-doped regime the sublattice structure of t-CuO has interesting consequences on the symmetry of the superconducting state.
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| 5. |
- Delange, Pascal, et al.
(författare)
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Large effects of subtle electronic correlations on the energetics of vacancies in alpha-Fe
- 2016
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 94:10, s. 100102-
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Tidskriftsartikel (refereegranskat)abstract
- We study the effect of electronic Coulomb correlations on the vacancy formation energy in paramagnetic alpha-Fe within ab initio dynamical mean-field theory. The calculated value for the formation energy is substantially lower than in standard density-functional calculations and in excellent agreement with experiment. The reduction is caused by an enhancement of electronic correlations at the nearest neighbors of the vacancy. This effect is explained by subtle changes in the corresponding spectral function of the d electrons. The local lattice relaxations around the vacancy are substantially increased by many-body effects.
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| 6. |
- Galler, Anna, et al.
(författare)
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Correlated electronic structure and optical response of rare-earth based semiconductors
- 2021
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Ingår i: Physical Review B. - 2469-9950. ; 103:24
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Tidskriftsartikel (refereegranskat)abstract
- The coexistence of Mott localized f states with wide conduction and valence bands in f-electron semiconductors results, quite generically, in a complex optical response with the nature of the absorption edge difficult to resolve both experimentally and theoretically. Here, we combine a dynamical mean-field theory approach to localized 4f shells with an improved description of band gaps by a semilocal exchange-correlation potential to calculate the optical properties of the light rare-earth fluorosulfides LnSF (Ln=Pr, Nd, Sm, Gd) from first principles. In agreement with experiment, we find the absorption edge in SmSF to stem from S-3p to Sm-4f transitions, while the Gd compound behaves as an ordinary p-d gap semiconductor. In the unexplored PrSF and NdSF systems we predict a rather unique occurrence of strongly hybridized 4f-5d states at the bottom of the conduction band. The nature of the absorption edge results in a characteristic anisotropy of the optical conductivity in each system, which may be used as a fingerprint of the relative energetic positions of different states.
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| 7. |
- Galler, Anna, et al.
(författare)
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Intrinsically weak magnetic anisotropy of cerium in potential hard-magnetic intermetallics
- 2021
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Ingår i: npj Quantum Materials. - : Springer Science and Business Media LLC. - 2397-4648. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Cerium-based intermetallics are currently attracting much interest as a possible alternative to existing high-performance magnets containing scarce heavy rare-earth elements. However, the intrinsic magnetic properties of Ce in these systems are poorly understood due to the difficulty of a quantitative description of the Kondo effect, a many-body phenomenon where conduction electrons screen out the Ce-4f moment. Here, we show that the Ce-4f shell in Ce–Fe intermetallics is partially Kondo screened. The Kondo scale is dramatically enhanced by nitrogen interstitials suppressing the Ce-4f contribution to the magnetic anisotropy, in striking contrast to the effect of nitrogenation in isostructural intermetallics containing other rare-earth elements. We determine the full temperature dependence of the Ce-4f single-ion anisotropy and show that even unscreened Ce-4f moments contribute little to the room-temperature intrinsic magnetic hardness. Our study thus establishes fundamental constraints on the potential of cerium-based permanent magnet intermetallics.
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| 8. |
- Grånäs, Oscar, 1979-
(författare)
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Theoretical Studies of Magnetism and Electron Correlation in Solids
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work presents new development and applications of ab-initio simulation tools for material science. Focus lies on materials with strong electronic correlation and strong spin-orbit coupling. Improvements on methods for solving the impurity problem in LDA+DMFT is presented, as well as a reliant method for charge self-consistency in a LMTO based electronic structure code. A new adaptive scheme for Brillouin zone integration is developed, where we show a strong reduction of numerical noise compared to standard techniques. A reformulation of the standard LDA+U method aiming to reduce the number of free parameters is introduced. Fast and realistic reduction of the number of free parameters provides the possibility of high throughput calculations and enabled us to study a large number of compounds. An analysis method for polarization in terms of coupled multipoles, and their corresponding energy contributions is developed and applied. This led to the formulation of Katt's rules, a set of rules complementary to Hund's rules. Katt's rules applies for occupying the orbitals of an electronic shell with strong spin-orbit coupling. The analysis is also used to investigate the unconventional Uranium based superconductors URu2Si2, UPt3, UPd2Al3 and UNi2Al3, as well as the high temperature superconductor LaOFeAs. We also investigate the non-magnetic delta-phase of Plutonium, providing insight to the electronic structure and the branching ratios of 4d to 5f transitions seen in photo emission spectra.The influence of surface reconstruction on the magneto crystalline anisotropy is investigated in multilayer Fe/ZnSe, showing that Fe deposited on an unreconstructed interface strongly reduces the uniaxial component of the MAE. We provide a detailed understanding of the magnetic properties of Fe2P, opening possible routes for enhancing the MAE in this system. A general route to strong MAE in nano-laminates is presented, we apply this to propose a candidate with extremely strong anisotropy energy density, 5Fe/2W1-xReX for x=[0.6-0.8].
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| 9. |
- Jana, Somnath, et al.
(författare)
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Doping induced site-selective Mott insulating phase in LaFeO3
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Tailoring transport properties of strongly correlated electron systems in a controlled fashion counts among the dreams of materials scientists. In copper oxides, vary- ing the carrier concentration is a tool to obtain high- temperature superconducting phases. In manganites, dop- ing results in exotic physics such as insulator-metal tran- sitions (IMT), colossal magnetoresistance (CMR), orbital- or charge-ordered (CO) or charge-disproportionate (CD) states. In most oxides, antiferromagnetic order and CD phase is asssociated with insulating behavior. Here we re- port the realization of a unique physical state that can be induced by Mo doping in LaFeO3: the resulting metallic state is a site-selective Mott insulator where itinerant elec- trons evolving on low-energy Mo states coexist with local- ized carriers on the Fe sites. In addition, a local breathing- type lattice distortion induces charge disproportionation on the latter, without destroying the antiferromagnetic order. A state, combining antiferromangetism, metallic- ity and CD phenomena is rather rare in oxides and have utmost significance for future antiferromagnetic memory devices.
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| 10. |
- Jeong, Jaehong, et al.
(författare)
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Magnetization Density Distribution of Sr_{2}IrO_{4} : Deviation from a Local j_{eff}=1/2 Picture
- 2020
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Ingår i: Physical Review Letters. - 1079-7114. ; 125:9, s. 97202-97202
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Tidskriftsartikel (refereegranskat)abstract
- 5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr_{2}IrO_{4} is particularly in the spotlight because of the so-called j_{eff}=1/2 state consisting of a quantum superposition of the three local t_{2g} orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state. Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_{eff}=1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.
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