arXiv subjects

Maria Daghofer

Publications and source records attributed to Maria Daghofer.

At least 19 recordsLinked to original sources

arXiv2026

Electronic correlations driving Chirality-Induced Spin Selectivity

We explicitly account for electron-electron interactions when modeling low-dimensional helical organic molecules. We show that competition between various hopping channels, together with interaction-induced double- and superexchange mechanisms, can stabilize non-collinear helical magnetic order. The resulting single-electron bands exhibit partial spin polarization, a manifestation of $p$-wave magnetism. Using density-matrix renormalization group, cluster perturbation theory, and Monte Carlo methods, we find that even vanishingly small spin-orbit coupling triggers strong spin selectivity at temperatures significantly above the spin-orbit scale. While strong correlations are essential for this mechanism, long-range spin ordering is not required. We thus propose non-collinear spin correlations driven by Coulomb interactions as an explanation of chirality-induced spin selectivity and discuss connections to experiments.

cond-mat.str-el↗

arXiv2026

Electronic excitations in the Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$

SrCu2(BO3)2 (SCBO) is a paradigmatic realization of the Shastry-Sutherland model, hosting geometrically frustrated spin dimers and a variety of quantum magnetic phases and phenomena. Although its magnetic properties have been extensively studied, the high-energy electronic excitations that determine the crystal-field environment and Cu-O hybridization have remained largely unexplored. Here we combine Cu L3-edge resonant inelastic x-ray scattering (RIXS), broadband optical spectroscopy, and electronic-structure calculations to determine the relevant local and interband excitation energy scales in SCBO. RIXS resolves a well-defined manifold of localized Cu2+ d-d excitations between 1.8 and 2.4 eV, whose energies and polarization dependence are well reproduced by multireference quantum-chemistry calculations. In contrast, optical spectroscopy identifies charge-transfer excitations with an absorption onset near 1.2-1.6 eV and a broader higher-energy structure around 4.5 eV, which are qualitatively captured by DFT+U calculations. Taken together, these results define the characteristic energy scales of d-d and CT excitations, offering quantitative benchmarks for computational frameworks and providing essential input for refining superexchange-based magnetic models of this prototypical frustrated quantum antiferromagnet.

cond-mat.str-el↗

arXiv2025

Robustness of the flux-free sector of the Kitaev honeycomb against environment

The Kitaev honeycomb model (KHM) consists of spin-$1/2$ particles on a honeycomb lattice with direction-dependent Ising-like interactions. It can alternatively be described in terms of non-interacting Majorana fermions, can be solved exactly, and has a quantum spin-liquid ground state. Open boundaries then host Majorana zero modes (MZMs) that are robust against some types of disorder. We analyze the fate of the MZMs when they couple to an environment via a Lindblad master equation. By computing the time evolution of the density matrix, we find that when decoherence occurs, the steady state is mostly the maximally mixed state. Among the few exceptions is a parameter regime that realizes the superconducting Kitaev chain model with periodic boundary conditions. We consistently observe a quantum Zeno effect in the density matrix as well as in the entropy and fidelity, while it is not found in the energy gap of some gapped spin liquids. We thus present a comprehensive overview over MZMs coupled to a spin bath that is relevant to proposals to detect MZMs of Kitaev layers on surfaces using scanning tunneling microscopy (STM).

cond-mat.str-el↗

arXiv2025

Altermagnetic polarons: the fate of alter magnetic band splittings at strong coupling

While a spin-dependent band splitting is one of the characteristic features of altermagnets, the conventional band picture itself breaks down in the many altermagnets that are correlated Mott materials. We employ two numerical many-body methods, the self-consistent Born approximation and variational cluster approach, to explore this strongly correlated regime and investigate hole motion in Mott altermagnets. Our results reveal that spin-dependent spectral-weight transfer is the dominant signature of Mott altermagnetism. This pronounced spin-momentum locking of the quasiparticle spectral weight arises from the formation of altermagnetic polarons, whose dynamics are governed by the interplay between free hole motion and the coupling of the hole to magnon excitations in the altermagnet. We demonstrate this effect by calculating ARPES spectra for three canonical altermagnetic systems: the checkerboard $J$-$J'$ model, a variant describing the transition-metal--ion sites of the inverse Lieb lattice, and the Kugel-Khomskii spin-orbital altermagnet based on cubic vanadates RVO$_3$ (R=La, Pr, Nd, Y).

cond-mat.str-el↗

arXiv2025

Robustness of topological edge states in alternating spin chains against environment

Both the Haldane spin-$1$ chain and dimerized chains of spin-$1/2$ exhibit topologically protected edge states that are robust against specific perturbations. Recently, such spin chains have been specifically assembled on surfaces and we investigate here the robustness of these edge states against coupling to the surface. Since no physical system can be considered perfectly isolated, it is crucial to examine whether topological robustness is maintained in the presence of environmental coupling. We apply exact diagonalization to a Lindblad master equation that couples an alternating Heisenberg spin chain based on spins $1/2$ to a surface via various jump operators. The robustness of topological states is assessed via the time evolution of quantities such as the ground-state degeneracy, correlation function, entropy, and magnetization of edge states. We investigate chains built from dimers with antiferromagnetic and ferromagnetic intra-dimer coupling, which resemble Su-Schrieffer-Heeger and the Haldane models, resp., and assess the impact of $z$-axis anisotropy and longer-ranged couplings. Generally, we find that signatures of topological properties are more robust in Su-Schrieffer-Heeger-like chains than in Haldane-like chains.

cond-mat.str-el↗

arXiv2025

Charge and magnetic orders in a two-band model with long-range interactions for infinite-layer nickelates NdNiO$_2$

We present an effective two-band model for infinite-layer nickelates NdNiO$_2$ that consisting of a $d$ band centered at Ni site and an interstitial $s$-like band centered at Nd site. To the large extent of the wave functions, we find intersite Coulomb interactions to be substantial. We then use the variational cluster approach together with mean-field theory to investigate magnetic and charge ordering. While tendencies towards charge modulation are found, they are weak and might be due to finite-size effects. Magnetic order is determined mostly by the filling of the $d$ band and hardly affected by including longer-ranged interactions. For a $d$-band density consistent with density-functional theory, magnetic ordering vanishes once quantum fluctuations are included to a sufficient spatial extent. Apart from self-doping, $d$ and $s$ bands remain largely uncoupled despite the presence of inter-orbital Coulomb interaction and (small) inter-orbital hopping.

cond-mat.str-el↗

arXiv2024

Fluctuation induced piezomagnetism in local moment altermagnets

It was recently discovered that, depending on their symmetries, collinear antiferromagnets may break spin degeneracy in momentum space, even in absence of spin-orbit coupling. Such systems, dubbed altermagnets, have electronic bands with a spin-momentum texture set mainly by the combined crystal-magnetic symmetry. This discovery motivates the question which novel physical properties derive from altermagnetic order. Here we show that one consequence of altermagnetic order is a fluctuation-driven piezomagnetic response. Using two Heisenberg models of d-wave altermagnets, a checkerboard one and one for rutiles, we determine the fluctuation induced piezomagnetic coefficients considering temperature induced transversal spin fluctuations. We establish in addition that magnetic fluctuations induce an anisotropic thermal spin conductivity.

cond-mat.str-el↗

arXiv2024

Promising regimes for the observation of topological degeneracy in spin chains

Both the Haldane spin chain and a topologically dimerized chain feature topologically protected edge states that are expected to be robust against some kind of noise. To elucidate whether it might be feasible to create such edge states in dimerized chains in a controlled manner in solid states environments, e.g. as spin chains on surfaces, as has already been successfully achieved with the Haldane chain, we investigate their robustness with respect to long-range coupling, anisotropies and finite chain length. The theoretical investigation is based on an alternating Heisenberg spin chain with spin-1/2, which is investigated using exact diagonalization. We find that dimerized chains and Haldane chains have robustness against long-range coupling and anisotropies. In particular, dimerized spin chains are significantly more robust than Haldane chains

cond-mat.str-el↗

arXiv2023

Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens

We have investigated the 3d orbital excitations in CaCuO2 (CCO), Nd2CuO4 (NCO), and La2CuO4 (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the dxy orbital clearly disperse, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. We show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.

cond-mat.str-el↗

arXiv2023

From linear to circular polarized light -- Floquet engineering in Kitaev-Heisenberg materials with Lissajous figures

This paper discusses Floquet engineering with arbitrary polarization in $\alpha$-RuCl$_3$. We describe the influence of arbitrary polarization and the limiting cases of linear and circular polarization. The corresponding model is derived via perturbation theory up to fourth-order. Starting from linear and circular polarization we bridge the gap between those two limiting cases. We then we study more complex Lissajous figures and general trends arising for them.

cond-mat.str-el↗

arXiv2022

Hubbard subbands and superconductivity in the infinite-layer nickelate

An effective two-dimensional two-band model for infinite-layer nickelates consists of bands obtained from $d_{x^2-y^2}$ and $s$--like orbitals. We investigate whether it could be mapped onto a single-band Hubbard model and the filling of Hubbard bands. We find that both one-band physics and a Kondo-lattice regime emerge from the same two-orbital model, depending on the strength of electronic correlations and the filling of the itinerant $s$-band. Next we investigate one-particle excitations by changing the screening. First, for weak screening the strong correlations push electrons out of the $s$-band so that the undoped nickelate is similar to a cuprate. Second, for strong screening the $s$ and $d_{x^2-y^2}$ bands are both partly filled and weakly couple. Particularly in this latter regime mapping to a one-band model gives significant spectral weight transfer between the Hubbard subbands. Finally we point out that the superconducting phases may have either $d$-wave or $s$-wave symmetry.

cond-mat.str-el↗

arXiv2022

Screening in a two-band model for superconducting infinite-layer nickelate

Starting from an effective two-dimensional two-band model for infinite layered nickelates, consisting of bands obtained from $d$ and $s$--like orbitals, we investigate to which extend it can be mapped onto a single-band Hubbard model. We identify screening of the more itinerant $s$-like band as an important driver. In absence of screening one strongly-correlated band gives an antiferromagnetic ground state. For weak screening, the strong correlations push electrons out of the $s$-band so that the undoped nickelate remains a Mott insulator with half filled $d$ orbitals. This regime markedly differs from the observations in high-$T_c$ cuprates and pairing with $s$-wave symmetry would rather be expected in the superconducting state. In contrast, for strong screening, the $s$ and $d_{x^2-y^2}$ bands are both partly filled and couple only weakly, so that one approximately finds a self-doped $d$ band as well as tendencies towards $d$-wave pairing. Particularly in the regime of strong screening mapping to a one-band model gives interesting spectral weight transfers when a second $s$ band is also partly filled. We thus find that both one-band physics and a Kondo-lattice--like regime emerge from the same two-orbital model, depending on the strength of electronic correlations and the size of the $s$-band pocket.

cond-mat.str-el↗

arXiv2022

Experimentally accessible scheme for a fractional Chern insulator in Rydberg atoms

We present a setup with Rydberg atoms for the realization of a bosonic fractional Chern insulator in artificial matter. The suggested setup relies on Rydberg atoms arranged in a honeycomb lattice, where excitations hop through the lattice by dipolar exchange interactions, and can be interpreted as hard-core bosons. The quantum many-body Hamiltonian is studied within exact diagonalization and DMRG. We identify experimentally accessible parameters where all signatures indicate the appearance of a fractional state with the same topological properties as the $\nu=1/2$ bosonic Laughlin state. We demonstrate an adiabatic ramping procedure, which allows for the preparation of the topological state in a finite system, and demonstrate an experimentally accessible smoking gun signature for the fractional excitations.

quant-ph↗

arXiv2021

Comparing the influence of Floquet dynamics in various Kitaev-Heisenberg materials

In this paper we examine the possibility of Floquet engineering in the three candidate Kitaev materials $\mathrm{Na}_2\mathrm{IrO}_3$, $\alpha$-$\mathrm{Li}_2\mathrm{IrO}_3$ and $\alpha$-$\mathrm{RuCl}_3$. We derive an effective Floquet Hamiltonian and give an approximation for heating processes arising from doublon holon propagation.This suggests that compounds with stronger Hund's-rule coupling are less prone to heating. We then investigate the impact of light frequency and amplitude on magnetic interaction terms up to third-nearest-neighbor and find that third-neighbor Heisenberg coupling $J_3$ is very susceptible to tuning by circularly polarized light. Finally, we discuss uses of linear polarized light in selectively tuning single bond directions.

cond-mat.str-el↗

arXiv2021

Character of Doped Holes in Nd$_{1-x}$Sr$_x$NiO$_2$

We investigate charge distribution in the recently discovered high-$T_c$ superconductors, layered nickelates. With increasing value of charge-transfer energy we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the $d-p$ Coulomb interaction $U_{dp}$ plays a role and makes Zhang-Rice singlets less favorable, while the amplitude of local triplets is enhanced. By investigating the effective two-band model with orbitals of $x^2-y^2$ and $s$ symmetries we show that antiferromagnetic interactions dominate for electron doping. The screened interactions for the $s$ band suggest the importance of rare-earth atoms in superconducting nickelates.

cond-mat.str-el↗

arXiv2021

Magnetic phases for two $t_{2g}$ holes with spin-orbit coupling and crystal field

We investigate two holes in the the $t_{2g}$ levels of a square-lattice Mott insulator with strong spin-orbit coupling. Exact diagonalization of a spin-orbital model valid at strong onsite interactions, but arbitrary spin-orbit coupling and crystal field is complemented by an effective triplon model (valid for strong spin-orbit coupling) and by a semiclassical variant of the model. We provide the magnetic phase diagram depending on crystal field and spin-orbit coupling, which largely agrees for the semiclassical and quantum models, as well as excitation spectra characterizing the various phases.

cond-mat.str-el↗

arXiv2021

Interplay between Zhang-Rice singlets and high-spin states in a model for doped NiO$_2$ planes

Superconductivity found in doped NdNiO$_2$ is puzzling as two local symmetries of doped NiO$_2$ layers compete, with presumably far-reaching implications for the involved mechanism: a cuprate-like regime with Zhang-Rice singlets {\cblue is replaced by local triplet states at realistic values of charge-transfer energy, which would suggest a rather different superconductivity scenario from high-$T_c$ cuprates}. We address this competition by investigating Ni$_4$O$_8$ clusters with periodic boundary conditions in the parameter range relevant for the superconducting nickelates. With increasing value of charge-transfer energy we observe upon hole doping the expected crossover from the cuprate regime dominated by Zhang-Rice singlets to the local triplet states. We find that smaller charge-transfer energy $\Delta$ is able to drive this change of the ground state character when realistic values for nickel-oxygen repulsion $U_{dp}$ are taken into account. For large values of the charge-transfer energy, oxygen orbitals are less important than in superconducting cuprates as their spectral weight is found only at rather high excitation energies. However, a second Ni($3d$) orbital can easily become relevant, with either the $xy$ or the $3z^2-r^2$ orbitals contributing in addition to the $x^2-y^2$ orbital {\cblue to the formation of triplet states. In addition,} our result that $U_{dp}$ (acting between Ni and O) favors onsite triplets implies that correlation effects beyond purely onsite interactions should be taken into account when obtaining effective two-band models.

cond-mat.str-el↗

arXiv2021

Suppression of effective spin-orbit coupling by thermal fluctuations in spin-orbit coupled antiferromagnets

We apply the finite-temperature variational cluster approach to a strongly correlated and spin-orbit coupled model for four electrons (i.e. two holes) in the $t_{2g}$ subshell. We focus on parameters suitable for antiferromagnetic Mott insulators, in particular Ca$_2$RuO$_4$, and identify a crossover from the low-temperature regime, where spin-orbit coupling is essential, to the high-temperature regime where it leaves few signatures. The crossover is seen in one-particle spectra, where $xz$ and $yz$ spectra are almost one dimensional (as expected for weak spin-orbit coupling) at high temperature. At lower temperature, where spin-orbit coupling mixes all three orbitals, they become more two dimensional. However, stronger effects are seen in two-particle observables like the weight in states with definite onsite angular momentum. We thus identify the enigmatic intermediate-temperature 'orbital-order phase transition', which has been reported in various X-ray diffraction and absorption experiments at $T\approx 260\;K$, as the signature of the onset of spin-orbital correlations.

cond-mat.str-el↗