arXiv subjects

Viktor Christiansson

Publications and source records attributed to Viktor Christiansson.

18 recordsLinked to original sources

arXiv2026

Ultrafast dynamics of excitons in black phosphorus

Excitons are key quasiparticles determining the optical properties of solids. As such, they can be utilized to coherently control the electronic structure of materials using optical femtosecond pulses. Identifying the decoherence mechanism during the early non-equilibrium dynamics is crucial to achieve light-induced band-structure engineering in semiconductors. Here, we generate excitons in the direct band gap semiconductor black phosphorus with a resonant mid-infrared photoexcitation. Using time- and angle-resolved photoemission spectroscopy, we track their complex ultrafast dynamics on the few-picosecond time scale. We develop a quantum-kinetic theoretical framework to model the decoherence of excitons into dark excitons via phonon scattering. By combining simulation and experiment, we quantify key parameters describing the early dynamics of the excitons. Our work highlights phonon-mediated intravalley scattering as a fundamental limitation for coherent exciton phenomena in single-valley semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

Beyond the conventional Emery model: crucial role of long-range hopping for cuprate superconductivity

The Emery model is the quintessential model for cuprate superconductors. In his eponymous paper, Emery only considered the next-nearest-neighbor oxygen-copper hopping. Later, also the relevance of nearest- and next-nearest oxygen-oxygen hoppings has been pointed out. Using dynamical vertex approximation, we find a superconducting dome consistent with cuprates. However, long-range hoppings beyond the three conventional hopping parameters are necessary for the quantitatively correct phase diagram and for a proper d-wave order parameter.

cond-mat.str-el↗

arXiv2026

Plasmon assisted superconductivity in LiTi$_2$O$_4$

We combine $GW$ plus extended dynamical mean field theory ($GW$+EDMFT) with the density functional theory for superconductors (SCDFT) framework to study the electronic properties of LiTi$_2$O$_4$. Excellent agreement with experiment is obtained for the density of states, mass enhancement, Sommerfeld coefficient and superconducting $T_c$, if the dynamical nature of the screened Coulomb interaction is taken into account. Our results show that the coupling to collective charge fluctuations (plasmons) plays an important role in the pairing mechanism and explains the remarkably high $T_c$ of this moderately correlated spinel compound.

cond-mat.supr-con↗

arXiv2025

Substrate and cation engineering for optimizing superconductivity in infinite-layer nickelates

In a recent experiment [Nature 642, 58 (2025)], a new record for the superconducting critical temperature $T_c$ among infinite-layer nickelates has been reported in doped SmNiO$_2$. Here, we use the cutting-edge dynamical vertex approximation (D$Γ$A), and qualitatively as well as quantitatively reproduce the $T_c$ vs. doping dome for this compound. Encouraged by this, we go further and identify a path towards realizing even higher $T_c$'s by changing the cation along the line Nd$\rightarrow$Sm$\rightarrow$Y$\rightarrow$Lu with matching substrates. The successively smaller cation radius allows for smaller lattice constants of the substrate. This in turn increases the in-plane hopping and thus eventually $T_c$.

cond-mat.supr-con↗

arXiv2024

Self-consistent $GW$+Extended Dynamical Mean Field Theory for semiconductors and insulators

Theoretical studies of semiconductors and band insulators are usually based on variants of the $GW$ method without full self-consistency, like single-shot $G^0W^0$ or quasiparticle self-consistent $GW$. Fully self-consistent $GW$ provides a poor description of the gap size and electronic structure due to the lack of vertex corrections. While it is hard to predict at which order corrections can be neglected, local vertex corrections to all orders can be consistently included by combining $GW$ with extended dynamical mean field theory (EDMFT). Here, we show that \textit{ab initio} multitier $GW$+EDMFT calculations, which achieve full self-consistency in a suitably defined low-energy space, provide an accurate description of semiconductors and band insulators, comparable to the established methods which are typically used for this class of materials. Our results demonstrate that the range of applicability of $GW$+EDMFT extends to weakly correlated systems, and they imply that despite the weak correlations, local vertex corrections are an important ingredient in the diagrammatic treatment of semiconductors and band insulators.

cond-mat.str-el↗

arXiv2024

Photo-induced insulator-metal transition in paramagnetic (V$_{1-x}$Cr$_{x}$)$_2$O$_3$

Pump-probe experiments with femtosecond time resolution allow to disentangle the electronic dynamics from the lattice response and thus provide valuable insights into the non-equilibrium behavior of correlated materials. In Cr-doped V$_2$O$_3$, a multi-orbital Mott-Hubbard material which has been intensively investigated for decades, time-resolved experiments reported a photo-induced insulator-metal transition leading to a transient metal state with nonthermal properties. Here, we combine non-equilibrium dynamical mean-field theory with realistic first principles modeling to simulate the ultrafast response of this material to a laser excitation. Our calculations reproduce the insulating initial state, with orbital occupations in agreement with experiment, and reveal an ultrafast pump-induced gap filling associated with a charge reshuffling between the $e_g^π$ and $a_{1g}$ orbitals. However, in contrast to the related compound VO$_2$, the electronic system thermalizes within a few tens of femtoseconds and we find no evidence for the existence of a metastable nonthermal metal. This suggests that the reported nonthermal behavior in the experiments may be associated with the mismatch between the electronic and lattice temperatures.

cond-mat.str-el↗

arXiv2024

Origin of multiple Lifshitz transitions in the Weyl semi-metal RhSi

It is known from density functional theory (DFT) calculations that RhSi has a multifold degenerate Dirac point at the Fermi energy, with the dominant states in the low-energy region displaying mostly Rh $d$ character. Using DFT+U, we calculate the band structure by considering an effective local interaction on the Rh $d$ states, with a realistic effective Hubbard $U_\textrm{eff}=2.5$ eV derived from a constrained random-phase approximation calculation, and find the emergence of a double hump structure close to the Fermi energy.By further deriving a low-energy tight-binding model from our first-principles results, we show that the double hump is a direct consequence of a competition between the Rh $d$-Rh $d$ and Rh $d$-Si $p$ interactions, which differ in their momentum dependence. As a consequence, through an artificial tuning of the energy level of the Si $p$ orbitals this hump structure can be suppressed due to the effectively reduced Rh $d$ -Si $p$ interaction.This peculiar low-energy electronic structure additionally results in that a small hole/electron doping ($\sim$ 0.1 $\%$) can tune the Fermi surface topology, going from closed to open Fermi surfaces, which has dramatic consequences for the thermal transport.

cond-mat.str-el↗

arXiv2024

Internal consistency of multi-tier $GW$+EDMFT

The multi-tier $GW$+EDMFT scheme is an ab-initio method for calculating the electronic structure of correlated materials. While the approach is free from ad-hoc parameters, it requires a selection of appropriate energy windows for describing low-energy and strongly correlated physics. In this study, we test the consistency of the multi-tier description by considering different low-energy windows for a series of cubic SrXO$_3$ (X=V,Cr,Mn) perovskites. Specifically, we compare the 3-orbital $t_{2g}$ model, the 5-orbital $t_{2g}+e_g$ model, the 12-orbital $t_{2g}+O_p$ model, and (in the case of SrVO$_3$) the 14-orbital $t_{2g}+e_g+O_p$ model and compare the results to available photoemission and X-ray absorption measurements. The multi-tier method yields consistent results for the $t_{2g}$ and $t_{2g}+e_g$ low-energy windows, while the models with $O_p$ states produce stronger correlation effects and mostly agree well with experiment, especially in the unoccupied part of the spectrum. We also discuss the consistency between the fermionic and bosonic spectral functions and the physical origin of satellite features, and present momentum-resolved charge susceptibilities.

cond-mat.str-el↗

arXiv2023

Correlated electronic structure of La$_3$Ni$_2$O$_7$ under pressure

Recently, superconductivity with a $T_c$ up to 78 K has been reported in bulk samples of the bilayer nickelate La$_3$Ni$_2$O$_7$ at pressures above 14 GPa. Important theoretical tasks are the formulation of relevant low-energy models and the clarification of the normal state properties. Here, we study the correlated electronic structure of the high-pressure phase in a four-orbital low-energy subspace using different many-body approaches: $GW$, dynamical mean field theory (DMFT), extended DMFT (EDMFT) and $GW$+EDMFT, with realistic frequency-dependent interaction parameters. The nonlocal correlation and screening effects captured by $GW$+EDMFT result in an instability towards the formation of charge stripes, with the $3d_{z^2}$ as the main active orbital. We also comment on the potential relevance of the rare-earth self-doping pocket, since hole doping suppresses the ordering tendency.

cond-mat.str-el↗

arXiv2023

Nature of the photo-induced metallic state in monoclinic VO$_2$

The metal-insulator transition of VO$_2$, which in equilibrium is associated with a structural phase transition, has been intensively studied for decades. In particular, it is challenging to disentangle the role of Mott physics from dimerization effects in the insulating phase. Femtosecond time-resolved experiments showed that optical excitations can induce a transient metallic state in the dimerized phase, which is distinct from the known equilibrium phases. In this study, we combine non-equilibrium cluster dynamical mean-field theory with realistic first principles modeling to clarify the nature of this laser-induced metallic state. We show that the doublon-holon production by laser pulses with polarization along the V-V dimers and the subsequent inter-orbital reshuffling of the photo-carriers leads to a population of orbital-mixed states and the filling of the gap. The photo-induced metal state is qualitatively similar to a hot electronic state in the dimerized structure, and does not involve a collapse of the Mott gap.

cond-mat.str-el↗

arXiv2023

Quaternary borocarbides: a testbed for DFT for superconductors

Using ab-initio density functional theory for superconductors (SCDFT), we systematically study the quaternary borocarbides $RM_2$B$_2$C. Treating the retarded (frequency-dependent) interaction $W(ω)$ within the random-phase approximation (RPA), we find good agreement with experiments for the calculated superconducting critical temperature $T_c$ in the nonmagnetic Ni- and Pd-based compounds. We argue that the problem of accurately placing the $f$-bands within DFT, and possibly the lack of an explicit magnetic pair-breaking mechanism, explain the difficulties of SCDFT in reproducing $T_c$ in members of the magnetic $R$Ni$_2$B$_2$C series ($R$ rare-earth with partially filled $4f$). While the calculated $T_c$ is overestimated, SCDFT qualitatively captures the experimentally observed trend along the rare-earth series, which indicates that the electron-phonon couplings and dynamically screened interactions have a significant effect on $T_c$.

cond-mat.supr-con↗

arXiv2023

Berry Curvature Signatures in Chiroptical Excitonic Transitions

The topology of the electronic band structure of solids can be described by its Berry curvature distribution across the Brillouin zone. We theoretically introduce and experimentally demonstrate a general methodology based on the measurement of energy- and momentum-resolved optical transition rates, allowing to reveal signatures of Berry curvature texture in reciprocal space. By performing time- and angle-resolved photoemission spectroscopy of atomically thin WSe$_2$ using polarization-modulated excitations, we demonstrate that excitons become an asset in extracting the quantum geometrical properties of solids. We also investigate the resilience of our measurement protocol against ultrafast scattering processes following direct chiroptical transitions.

cond-mat.mtrl-sci↗

arXiv2023

Correlated electronic structure of Pb$_{10-x}$Cu$_x$(PO4)$_6$O

Recently, above-room temperature superconductivity was reported in the Cu doped lead apatite Pb$_{10-x}$Cu$_x$(PO4)$_6$O, dubbed LK-99. By relaxing the structure with Cu substitution, we derive a four-band low-energy model with two 3/4 filled bands of predominantly Cu $d_{xz}$ and $d_{yz}$ character and two filled O $p_x$ and $p_y$ bands. This model is further downfolded to a two-band Cu-$d_{xz/yz}$ model. Using {\it ab-initio} derived interaction parameters, we perform dynamical mean field theory calculations to determine the correlated electronic structure in the normal state. These calculations yield a Mott insulator at $x=1$ and a strongly correlated non-Fermi liquid metal upon doping. The very large interaction versus bandwidth ratio $U/W\approx 30$-$50$ and the local moment paramagnetic behavior in the relevant filling regime are hard to reconcile with diamagnetism and high-temperature superconductivity. Hence, our calculations suggest that this behavior should come from a component with a different stoichiometry.

cond-mat.str-el↗

arXiv2022

$GW$+EDMFT investigation of Pr$_{1-x}$Sr$_x$NiO$_2$ under pressure

Motivated by the recent experimental observation of a large pressure effect on $T_c$ in Pr$_{1-x}$Sr$_x$NiO$_2$, we study the electronic properties of this compound as a function of pressure for $x=0$ and $0.2$ doping using self-consistent $GW$+EDMFT. Our numerical results demonstrate a non-trivial interplay between chemical doping and physical pressure, and small but systematic changes in the orbital occupations, local level energies, and interaction parameters with increasing pressure. The proper treatment of correlation effects, beyond density function theory, is shown to play an important role in revealing these trends. While the pressure dependent changes in the electronic structure of the undoped compound suggest a more single-band-like behavior in the high-pressure regime, a qualitatively different behavior is found in the doped system. We also point out that the fluctuations in the orbital occupations and spin states are not consistent with a single-band picture, and that at least a two-band model is necessary to reproduce the full result. This multi-orbital nature manifests itself most clearly in the doped compound.

cond-mat.str-el↗

arXiv2022

Self-screening corrections beyond the random-phase approximation: Applications to band gaps of semiconductors

The self-screening error in the random-phase approximation (RPA) and the $GW$ approximation (GWA) is a well-known issue and has received attention in recent years with several methods for a correction being proposed. We here apply two of these, a self-screening and a so-called "self-polarization" correction scheme, to model calculations to examine their applicability. We also apply an explicit self-screening correction to \textit{ab-initio} calculations of real materials. We find indications for the self-polarization scheme to be the more appropriate choice of correction for localized states, and additionally we observe that it suffers from causality violations in the strongly correlated regime. The self-screening correction used in this work on the other hand significantly improves the description in more delocalized states. It provides a notable reduction in the remaining GWA error when calculating the band gaps of several semiconductors, indicating a physical explanation for a part of the remaining discrepancy in one-shot $GW$ compared to experiment, while leaving the localized semicore $d$ states mostly unaffected.

cond-mat.str-el↗

arXiv2022

Superconductivity in black phosphorus and the role of dynamical screening

Simple cubic phosphorus exhibits superconductivity with a maximum $T_c$ of up to 12 K under pressure. The pressure dependence of $T_c$ cannot be consistently explained with a simple electron-phonon mechanism, which has stimulated investigations into the role of electronic correlations and plasmonic contributions. Here, we solve the gap equation of density functional theory for superconductors using different electron-electron and electron-phonon contributions to the kernel. We find that the phonon contribution alone yields an overestimation of $T_c$, while the addition of the static electronic contribution results in an underestimation. Taking into account the full frequency dependence of the screened interaction, the one-shot $GW$ approximation predicts $T_c$ values in good agreement with the experiments in the pressure range appropriate for the cubic phase. We also explore the use of quasi-particle bands in the calculation of the electronic and phononic kernels, and show that this modification significantly improves $T_c$ in the high-pressure region.

cond-mat.supr-con↗

arXiv2021

Fully {\it ab-initio} electronic structure of Ca$_{2}$RuO$_{4}$

The reliable {\it ab-initio} description of strongly correlated materials is a long-sought capability in condensed matter physics. The $GW$+EDMFT method is a promising scheme, which provides a self-consistent description of correlations and screening, and does not require user-provided parameters. In order to test the reliability of this approach we apply it to the experimentally well characterized perovskite compound Ca$_2$RuO$_4$, in which a temperature-dependent structural deformation drives a paramagnetic metal-insulator transition. Our results demonstrate that the nonlocal polarization and self-energy components introduced by $GW$ are essential for setting the correct balance between interactions and bandwidths, and that the $GW$+EDMFT scheme produces remarkably accurate predictions of the electronic properties of this strongly correlated material.

cond-mat.str-el↗

arXiv2020

Normal state of Nd$_{1-x}$Sr$_x$NiO$_2$ from self-consistent $GW$+EDMFT

The recent discovery of superconductivity in hole-doped NdNiO$_2$ thin films has captivated the condensed matter physics community. Such compounds with a formal Ni$^+$ valence have been theoretically proposed as possible analogues of the cuprates, and the exploration of their electronic structure and pairing mechanism may provide important insights into the phenomenon of unconventional superconductivity. At the modeling level, there are however fundamental issues that need to be resolved. While it is generally agreed that the low-energy properties of cuprates can to a large extent be captured by a single-band model, there has been a controversy in the recent literature about the importance of a multi-band description of the nickelates. The origin of this controversy is that studies based entirely on density functional theory (DFT) calculations miss important correlation and multi-orbital effects induced by Hund coupling, while model calculations or simulations based on the combination of DFT and (extended) dynamical mean field theory ((E)DMFT) involve ad-hoc parameters and double counting corrections that substantially affect the results. Here we use a multi-site extension of the recently developed $GW$+EDMFT method, which is free of adjustable parameters, to self-consistently compute the interaction parameters and electronic structure of hole-doped NdNiO$_2$. This full ab-initio simulation demonstrates the importance of a multi-orbital description, even for the undoped compound, and produces results for the resistivity and Hall conductance in qualitative agreement with experiment.

cond-mat.str-el↗