arXiv subjects

Juraj Krsnik

Publications and source records attributed to Juraj Krsnik.

8 recordsLinked to original sources

arXiv2026

Unconventional plasmon dynamics due to strong correlations in Sr$_2$RuO$_4$

Plasmon modes, their dispersion, and the onset of damping when approaching the electron-hole continuum are well understood when electron correlations are weak. However, we know little about how this picture is modified and what additional features emerge in strongly correlated materials. Here, we present a fully ab initio approach to plasmon excitations that combines density functional theory with dynamical mean-field theory, and we use it to reconcile controversial electron energy-loss spectroscopy results in Sr$_2$RuO$_4$. In particular, we show that electronic correlations reproduce the plasmon dispersion, while generating a large intrinsic width already below the electron-hole continuum. An additional high-energy peak reflecting transitions between incoherent features and a sharp increase of the plasmon's energy-momentum dispersion, akin to waterfalls in photoemission spectroscopy, are identified as genuine correlation effects.

cond-mat.str-el↗

arXiv2026

Polaron formation as the vertex function problem: From Dyck's paths to self-energy Feynman diagrams

We present an iterative method for generating the complete set of self-energy Feynman diagrams at arbitrary order for the single-polaron problem with arbitrary linear coupling to the lattice. The approach combines a combinatorial representation of noncrossing diagrams, based on Dyck paths associated with Stieltjes-Rogers polynomials, with the constraints of the Ward-Takahashi identity to systematically incorporate vertex corrections. This construction yields a one-to-one correspondence between terms in the expansion based on Stieltjes-Rogers polynomials and diagrammatic contributions, and provides, through a sequence of simple steps, a closed, algorithmic framework for generating all diagrams of a given order, together with their relative weights. The method enables efficient, unbiased evaluation of diagrammatic series and improves the convergence of diagrammatic Monte Carlo by eliminating the need for stochastic weighting between different topologies. We further outline how the construction can be generalized to finite-density electron systems.

cond-mat.str-el↗

arXiv2025

Entanglement in the pseudogap regime of cuprate superconductors

We find a strongly enhanced entanglement within the pseudogap regime of the Hubbard model. This entanglement is estimated from the quantum Fisher information and, avoiding the ill-conditioned analytical continuation, the quantum variance. Both are lower bounds for the actual entanglement that can be calculated from the (antiferromagnetic) susceptibility, obtained here with the dynamical vertex approximation. Our results qualitatively agree with experimental neutron scattering experiments for various cuprates. Theory predicts a $\ln(1/T)$ divergence of the entanglement for low temperatures $T$, which is however cut-off by the onset of superconductivity.

cond-mat.str-el↗

arXiv2025

Analytical expression for $π$-ton vertex contributions to the optical conductivity

Vertex corrections from the transversal particle-hole channel, so-called $π$-tons, are generic in models for strongly correlated electron systems and can lead to a displaced Drude peak (DDP). Here, we derive the analytical expression for these $π$-tons, and how they affect the optical conductivity as a function of correlation length $ξ$, fermion lifetime $τ$, temperature $T$, and coupling strength to spin or charge fluctuations $g$. In particular, for $T\rightarrow T_c$, the critical temperature for antiferromagnetic or charge ordering, the dc vertex correction is algebraic $σ_{VERT}^{dc}\propto ξ\sim (T-T_c)^{-ν}$ in one dimension and logarithmic $σ_{VERT}^{dc}\propto \lnξ\sim ν\ln (T-T_c)$ in two dimensions. Here, $ν$ is the critical exponent for the correlation length. If we have the exponential scaling $ξ\sim e^{1/T}$ of an ideal two-dimensional system, the DDP becomes more pronounced with increasing $T$ but fades away at low temperatures where only a broadening of the Drude peak remains, as it is observed experimentally, with the dc resistivity exhibiting a linear $T$ dependence at low temperatures. Further, we find the maximum of the DPP to be given by the inverse lifetime: $ω_{DDP} \sim 1/τ$. These characteristic dependencies can guide experiments to evidence $π$-tons in actual materials.

cond-mat.str-el↗

arXiv2024

Closing in on possible scenarios for infinite-layer nickelates: comparison of dynamical mean-field theory with angular-resolved photoemission spectroscopy

Conflicting theoretical scenarios for infinite-layer nickelate superconductors have been hotly debated, particularly regarding whether {only} a single Ni-3$d_{x^2-y^2}$ band is relevant at low energies besides electron pockets or whether multi-orbital physics including Ni-3$d_{z^2}$ is instead essential. The first scenario has emerged from density-functional theory plus dynamical mean-field theory (DFT+DMFT) calculations. Comparing the previous DFT+DMFT spectra to recent angular-resolved photoemission spectroscopy (ARPES) experiments, we find excellent agreement for both the Fermi surface and the strongly renormalized quasi-particle bands, supporting the first scenario. Our key findings further suggest that the "waterfalls" observed in ARPES might emerge from the quasi-particle--to--Hubbard-band crossover, and that additional spectral weight close to the $A$-pocket {likely} originates from the Ni-3$d_{xy}$ orbital.

cond-mat.supr-con↗

arXiv2024

Local correlations necessitate waterfalls as a connection between quasiparticle band and developing Hubbard bands

Waterfalls are anomalies in the angle-resolved photoemission spectrum where the energy-momentum dispersion is almost vertical, and the spectrum strongly smeared out. These anomalies are observed at relatively high energies, among others, in superconducting cuprates and nickelates. The prevalent understanding is that they originate from the coupling to some boson, with spin fluctuations and phonons being the usual suspects. Here, we show that waterfalls occur naturally in the process where a Hubbard band develops and splits off from the quasiparticle band. Our results for the Hubbard model with $\textit{ab initio}$ determined parameters well agree with waterfalls in cuprates and nickelates, providing a natural explanation for these spectral anomalies observed in correlated materials.

cond-mat.str-el↗

arXiv2024

Superconductivity in 2D systems enhanced by nonadiabatic phonon-production effects

We investigate the dynamical effects of electron-phonon coupling (EPC) on the superconducting properties of two-dimensional (2D) systems, calculating the Eliashberg function in terms of dynamically renormalized phonons. By studying different approximations for the phonon self-energy, we identify the important role of charge fluctuations in shaping the superconductivity properties, not only through the renormalization of phonon frequencies and damping rates but also through structural changes in the phonon spectral function. With the dynamical effects treated consistently, we argue that a part of the phonon spectral weight necessarily shifts to low frequencies due to the coupling to the 2D gapless plasmon. Furthermore, we find that the EPC leads to excess phonon spectral weight as well - i.e., phonon production - which generally tends to enhance the transition temperature. Our calculations point out that the influence of phonon production becomes greater as the density and the effective mass of electrons increase.

cond-mat.supr-con↗

arXiv2021

Acoustic-pressure-assisted engineering of aluminium foams

Foaming metals modulates their physical properties, enabling attractive applications where lightweight, low thermal conductivity or acoustic isolation are desirable. Adjusting the size of the bubbles in the foams is particularly relevant for targeted applications. Here we provide a method with a detailed theoretical understanding how to tune the size of the bubbles in aluminium melts in-situ via acoustic pressure. Our description is in full agreement with the high-rate three-dimensional X-Ray radioscopy of the bubble formation. We complement our study with the intriguing results on the effect of foaming on electrical resistivity, Seebeck coefficient and thermal conductivity from cryogenic to room temperature. Compared to bulk materials the investigated foam shows an enhancement in the thermoelectric figure of merit. These results herald promising application of foaming in thermoelectrics materials and devices for thermal energy conversion.

cond-mat.mtrl-sci↗