arXiv subjects

Krzysztof Wohlfeld

Publications and source records attributed to Krzysztof Wohlfeld.

At least 19 recordsLinked to original sources

arXiv2026

Resonantly Enhanced Multiphonon Scattering and Local Orbital-Phonon Coupling in Bulk and Thin Flakes of 2D Single Crystals of Antiferromagnetic (NixFe1-x)2P2S6

Orbital degrees of freedom play a pivotal role in shaping the physical properties of two-dimensional (2D) van der Waals magnetic systems, strongly influencing electron-phonon coupling and intermediate-state dynamics. In this study, we present a comprehensive temperature and thickness-dependent Raman investigation of the single crystals of 2D van der Waals antiferromagnetic series (NixFe1-x)2P2S6. Utilizing Raman spectroscopy, we explore the interplay between local orbital excitations and lattice vibrations, observing higher-order phonon modes extending up to the fourth order. We observe an anomalously high and weakly temperature-dependent intensity ratio of these higher-order modes relative to low-energy first-order phonons. Theoretical cluster calculations and Franck-Condon modelling reveal that these features originate from resonantly enhanced multiphonon scattering mediated by localized intermediate Ni 3d8 multiplet excitations. The local orbital occupancy of these intermediate states couples strongly to lattice coordinates, providing a selective enhancement mechanism for specific phonon channels. Notably, these higher-order features are absent in the end-member Fe2P2S6. This distinction reflects the charge-transfer character of Ni2P2S6, which places optical transitions in resonance with local multiplet states, contrasted with the Mott-Hubbard insulator regime in Fe-rich analogues. Our findings clarify the microscopic origin of high-frequency Raman scattering in transition metal thiophosphates and underscore the central role of local orbital-phonon interactions in 2D correlated magnets.

cond-mat.other↗

arXiv2026

Spacetime Dynamics of Altermagnetic Magnons

Distinguishing altermagnetism from conventional ferromagnetism and antiferromagnetism typically relies on momentum-space probes. Here, we show that the real-space spreading of a localized spin excitation provides a distinctive dynamical fingerprint of altermagnetic order. Using linear spin-wave theory on a two-dimensional checkerboard lattice with nearest-neighbor and next-nearest-neighbor couplings J_1 and J_2, we demonstrate that finite J_2 produces direction-dependent magnon group velocities and splits the two magnon branches. The resulting propagation remains closer to that of an antiferromagnet than a ferromagnet, retaining an approximately circular outer wavefront, while the direction-dependent magnon velocities produce a pronounced cross-like spatial structure inside this front. We further show that changing the sign of J_2 to enter the unfrustrated regime (J_2<0) increases the characteristic propagation velocities and interchanges the diagonal directions of enhanced propagation, corresponding to a pi/2 rotation of the anisotropic spatial pattern. These results establish spacetime dynamics as a complementary probe of altermagnetic magnons, providing a route to identifying unconventional magnetic order through real-space propagation patterns in solid-state and synthetic quantum systems.

cond-mat.str-el↗

arXiv2026

Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO2 and insulating CaCuO2

Infinite-layer nickelates are among the most promising cuprate-akin superconductors, although relevant differences from copper oxides have been reported. Here, we present momentum- and polarization-resolved RIXS measurements on chemically undoped, superconducting PrNiO2, and compare its magnetic and orbital excitations with those of the reference infinite layer cuprate CaCuO2. In PrNiO2, the in-plane magnetic exchange integrals are smaller than in CaCuO2, whereas the out-of-plane values are similar, indicating that both materials support a three-dimensional antiferromagnetic order. Orbital excitations, associated to the transitions within 3d states of the metal, are well reproduced within a single-ion model and display similar characteristics, except for the Ni-dxy peak which, besides lying at significantly lower energy, shows an opposite dispersion to that of Cu-dxy. This is interpreted as a consequence of orbital superexchange coupling between nearest neighbor sites, which drives the orbiton propagation. Our observations demonstrate that infinite layer cuprates and nickelates share most of the spin and orbital properties, despite their markedly different charge-transfer energy Delta.

cond-mat.supr-con↗

arXiv2026

Microscopic Emergence of Ancilla Lattice Physics in the Emery Model

We derive a controlled low-energy effective theory for the three-band Emery model on the Lieb lattice near (n=2) filling. Starting from the positive charge-transfer regime, a sequence of Schrieffer--Wolff transformations rigorously establishes a direct microscopic mapping onto an ancillary lattice structure. For realistic cuprate-like parameters, this framework naturally reproduces the conventional Fermi-liquid (FL) phase at a filling shifted by one electron per site. Our analysis identifies the precise microscopic conditions required to access the exotic fractionalized Fermi-liquid (FL$^*$) phase. We show that realizing FL$^*$ requires nonstandard cuprate parameter regimes together with additional interactions that stabilize a quantum spin liquid in the background ancilla layer. These results establish a microscopic foundation for ancilla physics in multiorbital materials and clarify the conditions for realizing FL$^*$. We discuss the possibility to replicate ancilla model physics in Lieb lattice cold atom simulations.

cond-mat.str-el↗

arXiv2026

Evidence for Many-Body States in NiPS$_3$ Revealed by Angle-Resolved Photoelectron Spectroscopy

We present $μ$-ARPES spectra of the Mott-insulating van der Waals antiferromagnet NiPS$_3$. Signatures of strong correlations -- such as the onset of atomic or atomic-ligand multiplets and spin-orbit-entangled exciton have been observed in this material by various two-particle spectroscopies, but not previously in photoemission. Our measurements reveal a weakly dispersive feature at the valence-band edge that is absent in DFT+$U$ calculations and remains unchanged across the Néel transition. After critically examining and ruling out alternative interpretations, we show that an exact diagonalization of a NiS$_6$ cluster yields low-energy final-state configurations of mixed multiplet $d^7$ and $d^8\underline{L}$ character, whose energy differences are consistent with the observed additional feature. This implies that ARPES directly accesses local Ni-S multiplet physics in NiPS$_3$, revealing a many-body structure beyond mean-field theory. Our results confirm that NiPS$_3$ is an excellent model platform in which strong correlations, reduced dimensionality, and covalent metal-ligand bonding jointly shape both two- and single-particle spectroscopies, underscoring the need for a genuinely quantum many-body description of two-dimensional quantum materials.

cond-mat.str-el↗

arXiv2026

Probing the band structure of the strongly correlated antiferromagnet NiPS3 across its phase transition

NiPS3 is an exfoliable van-der-Waals intralayer antiferromagnet with zigzag-type spin arrangement. It is distinct from other TMPS3 (TM: transition metal) materials by optical excitations into a strongly correlated state that is tied to the magnetic properties. However, the related, fundamental band structure across the antiferromagnetic phase transition has not been probed yet. Here, we use angular-resolved photoelectron spectroscopy with μm resolution in combination with DFT+U calculations for that purpose. We identify a characteristic band shift across TN. It is attributed to bands of mixed Ni and S character related to the superexchange interaction of Ni 3t2g orbitals. Moreover, we find a structure above the valence band maximum with little angular dispersion that could not be reproduced by the calculations. The discrepancy suggests the influence of many-body interactions beyond the DFT+U approximations in striking contrast to the results on MnPS3 and FePS3, where these calculations were sufficient for an adequate description.

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

Anomalous Eigenstates of a Doped Hole in the Ising Antiferromagnet

The problem of a mobile hole doped into an antiferromagnet Mott insulator is believed to underly the rich physics of several paradigmatic strongly correlated electron systems, ranging from heavy fermions to high-Tc superconductivity. Arguably the simplest incarnation of this problem corresponds to a doped Ising antiferromagnet, a problem widely considered essentially solved since almost 60 years by a popular yet approximate mapping to a single-particle problem on the Bethe lattice. Here we show that, despite its deceptive simplicity, the local spectrum of a single hole in a classical Ising-Néel state contains a series of anomalous, long-lived states that go beyond the well-known ladder-like spectrum with excited energies spaced as $J^{2/3} t^{1/3}$. The anomalous states we find through exact diagonalization and within the self-avoiding path approximation have excitation energies scaling approximately linear with $J$ and lead to a series of avoided crossings with the more pronounced ladder spectrum. By also computing different local, rotational spectra we explain the origin of the anomalous states as rooted in an approximate emergent local $C_3$ symmetry of the problem. From their direct spectral signatures we further conclude that these states lead to anomalously slow thermalization behavior -- hence representing a new type of quantum many-body scar state, potentially related to many-body scars predicted in lattice gauge theories.

cond-mat.str-el↗

arXiv2025

Neodymium ions as charge reservoir in NdNiO$_2$: from lack of long range order to electron-doping-induced antiferromagnetism

We study magnetism in the electron-doped infinite-layer nickelate NdNiO$_2$. We perform an unrestricted Hartree-Fock calculation for a tight-binding model which contains both nickel and neodymium orbitals. We reproduce the self-doping effect, which is the escape of charge onto the neodymium bands. By fixing all free parameters to realistic values we find that undoped NdNiO$_2$ lies right {\it outside} the antiferromagnetic (AFM) region of the phase diagram. This is consistent with experiments, which find no long-range order in the ground state of NdNiO$_2$, yet see short-range AFM correlations and broad magnetic excitations. We also find that the self-doping effect leads to a dramatic increase in the stability of the AFM solution upon electron doping -- a behavior that is strikingly different from what is, for instance, observed in the cuprates. Finally, for smaller charge transfer energies than suggested for NdNiO$_2$, the self-doping effect may be quite strong and stabilise various stripe configurations already on the mean-field level.

cond-mat.str-el↗

arXiv2025

Exploring d-Wave Magnetism in Cuprates from Oxygen Moments

The antiferromagnetic parent phase of high-T$_c$ cuprates has been established as a Néel state of copper moments, but early work pointed out the important role of ligand oxygen orbitals. Using the three-orbital Emery model, we explore how, and under which conditions, doping-induced antiferromagnetic ordering of weak magnetic moments on the oxygen sites can lead to unconventional d-wave magnetism with spin-split electronic bands. The mechanism for forming such altermagnetic (AM) states in cuprates does not rely on a lowering of the crystal symmetry but rather on interaction-induced formation of magnetic moments on directional oxygen orbitals within the crystallographic unit cell. Therefore, we obtain two different types of AM, namely a (0,0)-AM and a ($π$,$π$)-AM. We explore different regimes and challenges for realizing oxygen AM supported by Hartree-Fock calculations and complementary exact diagonalization of small clusters. While the region of interacting parameters needed to realize these states may be difficult to achieve in known high-T$_c$ cuprates, we propose a scenario to realize AM induced by oxygen magnetic moments in a cuprate-based candidate compound using density functional theory and discuss experimental implications.

cond-mat.str-el↗

arXiv2025

Nature of Spinons in 1D Spin Chains

We provide an intuitive understanding of the collective low-energy spin excitation of the one-dimensional spin-1/2 antiferromagnetic Heisenberg chain, known as the spinon. To this end, we demonstrate how a single spinon can be excited by adding one extra spin to the ground state. This procedure accurately reproduces all key features of the spinon's dispersion. These follow from the vanishing norm of the excited state which is triggered by the ground state entanglement. Next, we show that the spinon dispersion can be approximately reproduced if we replace the true ground state with the simplest valence-bond solid. This proves that the spinon of the one-dimensional Heisenberg model can be understood as a single spin flowing through a valence-bond solid.

cond-mat.str-el↗

arXiv2025

Magnetic polarons due to spin-length fluctuations in $\boldsymbol{d}^\mathbf{4}$ spin-orbit Mott systems

Mott insulators based on $4d$ and $5d$ transition-metal ions, where spin-orbit interaction plays a key role, can exhibit various forms of unusual magnetism. A particular example is the antiferromagnet Ca$_2$RuO$_4$ containing $d^4$ Ru$^{4+}$ ions. Here the spin-orbit interaction stabilizes the non-magnetic $J=0$ singlet ionic ground state, which gets dynamically mixed - via exchange interactions - with low-energy $J=1$ ionic excitations. Thanks to a sufficient strength of the exchange, these excitations condense and a long-range order emerges. The resulting ordered moments are soft and prone to fluctuations of their effective length. The corresponding amplitude mode appears as a prominent magnetic excitation and complements the conventional magnons involving rotations of the moments. Motivated by this peculiar kind of magnetic order and the specific spectrum of magnetic excitations, we study their influence on the propagation of doped carriers. To this end, we construct a microscopic model including both $d^4$ and $d^5$ degrees of freedom and address the propagation of an injected electron by employing self-consistent Born approximation. We find that the electron shows a combination of both free and a polaronic type of motion, where the mobile carrier strongly interacts with an accompanying cloud of magnetic excitations. Remarkably, in the latter case it is the exotic excitation - the amplitude mode - that is found to dominate over the contribution of magnons. Our soft-spin situation thus largely contrasts with spin polarons widely discussed in the context of doped Heisenberg-like magnets based on rigid spin moments.

cond-mat.str-el↗

arXiv2024

Topology of chalcogen chains

We investigate the topological properties of the helical atomic chains occurring in elemental selenium and tellurium. We postulate a realistic model that includes spin-orbit interaction and show this to be topologically non-trivial, with a topological invariant protected by a crystalline symmetry. We describe the end-states, which are orbitally polarized, with an orbital density modulation strongly peaked at the edge. Furthermore, we propose a simplified model that decomposes into three orbital chains, allowing us to define a topological invariant protected by a crystalline symmetry. We contrast this result with recent observations made for the orbital Su-Schrieffer-Heeger model containing a $p$-orbital zigzag chain.

cond-mat.mtrl-sci↗

arXiv2024

The fate of the spin polaron in the 1D antiferromagnets

The stability of the spin polaron quasiparticle, well established in studies of a single hole in the 2D antiferromagnets, is investigated in the 1D antiferromagnets using a t-J model. We perform an exact slave fermion transformation to the holon-magnon basis, and diagonalize numerically the resulting model in the presence of a single hole. We demonstrate that the spin polaron collapses - and the spin-charge separation takes over - due to the specific role played by the magnon-magnon interactions and the magnon hard-core constraint in the 1D t-J model. Moreover, we prove that the spin polaron is stable for any strength of the magnon-magnon interaction other than the unique value found in a 1D antiferromagnet with the continuous symmetry of the spin interactions. Fine-tuning to this unique value is extremely unlikely to occur in quasi-1D antiferromagnets, therefore the spin polaron is the stable quasiparticle of realistic 1D materials. Our results lead to a new interpretation of the ARPES spectra of quasi-1D antiferromagnets in the spin polaron language.

cond-mat.str-el↗

arXiv2024

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

Rare collapse of fermionic quasiparticles upon coupling to local bosons

We study the stability of the fermionic quasiparticle in a fermion-boson model on a Bethe lattice, with fermions interacting with local bosons via a polaronic-type coupling. We solve the problem by mapping it onto a non-interacting chain with site-dependent potential. We show that, despite a finite number of bosonic excitations costing zero energy, among the many analyzed cases, the occurrence of a complete collapse of the quasiparticle is rare. The quasiparticle disappearance becomes easier with an increase in: (i) the total number of bosons with zero energy, and (ii) the relative strength of the coupling between bosons and fermions. The postulated model can, among other things, be applied to study systems in which fermions are introduced into antiferromagnetic (or antiferro-orbital) domains surrounded by ferromagnetic (or ferro-orbital) ordered states. This might take place in the overdoped cuprates or upon doping manganese or vanadium oxides. Finally, we show how this model leads to an in-depth understanding of the onset of quasiparticles in the 1D and 2D $t$-$J^z$ model.

cond-mat.str-el↗

arXiv2023

Long-range order in the XY model on the honeycomb lattice

Using the reflection positivity method we provide the rigorous proof of the existence of long range magnetic order for the XY model on the honeycomb lattice for large spins $S\ge 2$. This stays in contrast with the result obtained using the {\it same} method but on the square lattice -- which gives a stable long-range order for spins $S\ge 1$. We suggest that the difference between these two cases stems from the enhanced quantum spin fluctuations on the honeycomb lattice. Using linear spin-wave theory we show that the enhanced fluctuations are due to the overall much higher kinetic energy of the spin waves on the honeycomb lattice (with Dirac points) than on the square lattice (with good nesting properties).

cond-mat.str-el↗

arXiv2023

Unravelling the Nature of Spin Excitations Disentangled from Charge Contributions in a Doped Cuprate Superconductor

The nature of the spin excitations in superconducting cuprates is a key question toward a unified understanding of the cuprate physics from long-range antiferromagnetism to superconductivity. The intense spin excitations up to the over-doped regime revealed by resonant inelastic X-ray scattering bring new insights as well as questions like how to understand their persistence or their relation to the collective excitations in ordered magnets (magnons). Here, we study the evolution of the spin excitations upon hole-doping the superconducting cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ by disentangling the spin from the charge excitations in the experimental cross section. We compare our experimental results against density matrix renormalization group calculations for a $t$-$J$-like model on a square lattice. Our results unambiguously confirm the persistence of the spin excitations, which are closely connected to the persistence of short-range magnetic correlations up to high doping. This suggests that the spin excitations in hole-doped cuprates are related to magnons -- albeit short-ranged.

cond-mat.supr-con↗