arXiv subjects

Harald O. Jeschke

Publications and source records attributed to Harald O. Jeschke.

At least 19 recordsLinked to original sources

arXiv2026

Dimensional reduction by singlet blockade in the distorted kagome magnet YCa$_3$(CrO)$_3$(BO$_3$)$_4$

The distorted kagome borate YCa$_3$(CrO)$_3$(BO$_3$)$_4$ has a Curie-Weiss temperature of -140 K and shows no magnetic order down to $65$ mK. We determine its Heisenberg Hamiltonian by density-functional energy mapping, obtaining twenty-four symmetry-inequivalent Cr-Cr exchange paths dominated by $J_1=116$ K, and measure the susceptibility, specific heat and ac susceptibility to $50$ mK. The Hamiltonian does not by itself account for the absence of order: simulated classically it orders at $10$ K, a factor of $154$ above the bound. We trace the gap to the site symmetry of the $P6_3$ structure, which divides the exchange network in two: $J_1$ binds two thirds of the Cr into dimers, while the remaining third forms uniform spin-$3/2$ chains along $c$ through a weaker $J_2=33$ K. The chains share no direct bond, and each of the fourteen paths leaving a chain terminates on a spin locked into a dimer singlet of gap $Δ=J_1$. This singlet blockade demotes the interchain coupling to second order; summed over paths with their relative Néel parity it falls to a few tens of millikelvin, against $9.8$ K at first order, and what survives is frustrated on the kagome lattice the chains themselves form. Treating the dimers exactly and the chains by exact diagonalization accounts, without free parameters, for the position of the magnetic specific-heat maximum and the entropy recovered by $100$ K, but not for the susceptibility, which additionally requires a broad distribution of exchange couplings that we attribute to Ca/Y site mixing. That disorder also impacts the thermodynamics at lowest temperatures: below $0.7$ K the magnetic specific heat follows a field-robust $T^2$ law, which excludes one-dimensional chains with gapless excitations, Goldstone modes and spinons alike and implies a linear density of predominantly non-magnetic states, matched by the singlet-triplet gaps the disorder generates.

cond-mat.str-el↗

arXiv2026

From dimensional reduction to tetramerization in mixed ferro-antiferro breathing pyrochlores

We study the spin-1/2 nearest-neighbor Heisenberg model on the breathing pyrochlore lattice in the mixed ferro-antiferromagnetic regime, where one tetrahedral sublattice is antiferromagnetic and the other ferromagnetic. The classical ground-state manifold is then that of the nearest-neighbor face-centered-cubic (fcc) antiferromagnet built from composite tetrahedral moments. Classical Monte Carlo simulations and the self-consistent Gaussian approximation show that thermal order-by-disorder lifts its subextensive degeneracy and selects the collinear Type-I state with wave vector $X=(1,0,0)$ through a strongly first-order transition. Pseudofermion functional renormalization group calculations for $S=1/2$ and $S=1$ reveal a nonmagnetic window adjacent to the decoupled antiferromagnetic-tetrahedron limit, beyond which $X$ order reappears. Density matrix renormalization group calculations show that this window is not featureless: the antiferromagnetic tetrahedra develop nearly ideal tetramer correlations, with four bonds carrying $\langle\mathbf{S}_i\cdot\mathbf{S}_j\rangle\simeq-1/2$ and the two opposite bonds $\simeq+1/4$, while the structure factor retains broad maxima at $X$. A third-order effective Hamiltonian in the tetrahedral-singlet manifold explains this: reversing the sign of the inter-tetrahedron coupling converts Tsunetsugu's dimer selection into a uniform tetramer selection, as confirmed by exact diagonalization. A dynamic high-temperature expansion traces how local tetrahedral excitations give way to low-energy $X$-centered spectral weight. Finally, we place density-functional parameters for eight structures of spin-3/2 breathing chromium thiospinels in the classical phase diagram. The mapping reproduces the $\mathbf{k}=(1,0,0)$ order of CuInCr$_4$S$_8$, accounts for the absence of order in LiGaCr$_4$S$_8$, and predicts Type-I order for LiInCr$_4$S$_8$.

cond-mat.str-el↗

arXiv2026

Universality of the $1/9$ Magnetization Plateau and Quantum-Disordered States in the Kagome Family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$)

The microscopic origin of the low-field $1/9$ magnetization plateau in spin-$1/2$ kagome antiferromagnets remains unresolved. Here, we show that chemical pressure reshapes the hierarchy of fractional magnetization plateaus in the titanium-based kagome family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$). High-field magnetization measurements up to 60 T reveal a robust $1/9$ plateau-like phase in the expanded $\mathrm{Cs_8RbK_3Ti_{12}F_{48}}$ and $\mathrm{Cs_8LiK_3Ti_{12}F_{48}}$ compounds, despite the absence of the conventionally more robust $1/3$ plateau. In contrast, compressed $\mathrm{Cs_8LiNa_3Ti_{12}F_{48}}$ exhibits neither the $1/9$ plateau-like phase nor a quantum-disordered ground state. Specific-heat measurements and first-principles calculations show that lattice expansion preserves a frustrated, fully connected kagome exchange network and gapless quantum-disordered ground states, whereas compression reorganizes the exchange network into weakly coupled quasi-one-dimensional subsystems and induces successive magnetic transitions. These results demonstrate that the $1/9$ and $1/3$ plateaus need not share a common microscopic origin and suggest that the $1/9$ plateau may represent a more universal feature of frustrated spin-$1/2$ kagome magnetism.

cond-mat.str-el↗

arXiv2026

Crystallography-driven molecularization of a two-dimensional spin-$3/2$ magnet

Large-spin two-dimensional magnets are generally expected to develop conventional long-range order once the dominant exchange scale becomes appreciable. The layered spin-$3/2$ maple-leaf compound Na$_2$Mn$_3$O$_7$ defies this expectation: despite sizable antiferromagnetic interactions and no evident disorder, it exhibits no magnetic ordering and displays two well-separated thermodynamic crossover scales. We show that this behavior originates from a crystallography-driven molecularization of the magnetic degrees of freedom. The low-symmetry structure partitions the Mn sublattice into inequivalent exchange pathways, generating a pronounced hierarchy that nearly isolates antiferromagnetic hexagons. Magnetic correlations therefore develop in two stages: first within individual hexagons at a scale set by the dominant exchange, and only at much lower temperatures do frustrated inter-hexagon couplings attempt to establish coherence across the lattice. While isolated hexagons reproduce the two-step thermodynamic structure, the experimentally relevant temperature scales emerge only once the hexagons are embedded in the frustrated two-dimensional network. The resulting quantum ground state is magnetically disordered, characterized by strong intra-hexagon correlations and rapidly decaying inter-hexagon correlations. These results identify crystallographic inequivalence as a materials-level mechanism for stabilizing molecularized and quantum-disordered states even in large-spin two-dimensional magnets.

cond-mat.str-el↗

arXiv2026

Structure, Composition, and High-Field Superconductivity in Metal-Rich $\mathrmη$-Carbide-Type Compounds

$\mathrmη$-Carbide-type compounds have recently emerged as a diverse class of materials in the study of superconductivity. These phases contribute to a growing family of metal-rich quantum materials that exhibit unusual superconducting properties emerging from complex metallic bonding. Several members of the $\mathrmη$-carbide-type phases have been found to be bulk superconductors -- such as Nb$_4$Rh$_2$C$_{1-δ}$, Ta$_4$Rh$_2$C$_{1-δ}$, Ti$_4$Ir$_2$O$_{1-δ}$, and Ti$_4$Co$_2$O$_{1-δ}$ -- with transition temperatures up to $T_{\rm c} \approx$ 10 K and upper critical fields as high as $μ_0 H_{\rm c2}(0) \approx$ 30 T. Whereas the transition temperatures may fall within the range typical for intermetallic superconductors, the pronounced violation of the weak-coupling Pauli limit in many of these crystallographically high-symmetry materials is noteworthy. Here, we review recent progress on superconducting $\mathrmη$-carbide-type phases, emphasizing how crystal symmetry, synthetic challenges, transition-metal composition, and electronic structure govern their superconducting properties. Furthermore, we outline open questions and future directions, including the possible discovery of new $\mathrmη$-carbide-type materials.

cond-mat.supr-con↗

arXiv2026

Realization of the Ruby Lattice Antiferromagnet in Layered Transition-Metal Fluorides

The antiferromagnet on the ruby lattice is expected to host a range of exotic emergent phenomena, yet its material realization has remained elusive. Here we show that the layered transition metal fluorides CsBaFe$_3$F$_{12}$ and CsBaCr$_3$F$_{12}$ with Fe$^{3+}$ and Cr$^{3+}$ ions realize only slightly distorted ruby lattice geometries with spin moments $S=5/2$ and $S=3/2$, respectively. Their microscopic Hamiltonians, calculated with DFT energy mapping, are dominated by short-ranged antiferromagnetic interactions within the ruby layers. Classical Monte Carlo simulations reveal strong frustration in both compounds, with local Néel correlations on the hexagonal plaquettes and distinct long-range ordering tendencies governed by weaker triangular links. For CsBaFe$_3$F$_{12}$, the calculated thermodynamic behaviour is consistent with the experimentally reported magnetic ordering scale. For CsBaCr$_3$F$_{12}$, classical Monte Carlo and Luttinger-Tisza analysis reveal competing low-energy ordering wave vectors, strong finite-size sensitivity, and a tendency toward incommensurate order. Overall, our results establish these fluorides as experimentally accessible ruby-lattice antiferromagnets and provide quantitative predictions for future neutron-scattering studies.

cond-mat.str-el↗

arXiv2026

In-Plane Ferromagnetism and Critical Dynamics in Alkali-Deficient K$_{1-x}$CrTe$_2$ (with $x \approx$ 0.3) Single Crystals

Layered chromium tellurides are model systems for studying low-dimensional magnetism in van der Waals materials. We report the synthesis and characterization of K$_{1-x}$CrTe$_2$ single crystals ($x \approx 0.3$), which crystallize in the $Cm$ space group with trigonal prismatic K$^+$ coordination, unlike the octahedral environments of more stoichiometric ACrX$_2$ compounds. Magnetization measurements show a sharp ferromagnetic transition at $T_{\rm C}=117$ K and in-plane magnetic anisotropy, supported by density functional theory. Neutron diffraction reveals ferromagnetic alignment of Cr spins within and between layers. This contrasts with the out-of-plane A-type antiferromagnetism in LiCrTe$_2$ and NaCrTe$_2$, but resembles CrTe$_2$. These differences likely arise from changes in interlayer spacing, Cr oxidation state, or stacking. AC susceptibility and $μ$SR indicate short-range order above $T_{\rm C}$ and dynamic behavior below. Overall, K$_{1-x}$CrTe$_2$ provides a tunable platform for studying spin orientation and dimensionality in two-dimensional magnets.

cond-mat.mtrl-sci↗

arXiv2026

Correlation-Driven Spin Reorientation via Competing Anisotropy Channels in CrPS4

We identify a correlation-driven mechanism for the temperature-induced spin reorientation in the quasi-one-dimensional van der Waals antiferromagnet CrPS4. Magnetic pair distribution function (mPDF) analysis resolves the local spin direction and shows that ferromagnetic intrachain correlations persist far above TN. Combining these correlations with a DFT-derived spin Hamiltonian reveals competing single-ion and exchange-anisotropy channels, with single-ion anisotropy remaining local while exchange anisotropy is renormalized as intersite correlations decay. This differential renormalization rotates the effective easy axis and captures the ordered-state canting. Above TN, the continued rotation beyond the model prediction delineates the limits of the dominant-chain approximation. These results establish mPDF-derived correlations as direct inputs to microscopic Hamiltonians and show how low-dimensional correlations can control magnetic anisotropy.

cond-mat.str-el↗

arXiv2026

Origin of pressure-induced anomalies in the nodal-line ferrimagnet Mn$_3$Si$_2$Te$_6$

A pressure-induced insulator-to-metal transition (IMT) has recently been discovered in the nodal-line ferrimagnet Mn$_3$Si$_2$Te$_6$. The electronic phase transition is accompanied by anomalies in the magnetic ordering temperature and the anomalous Hall conductivity, which peak at or near the critical pressure of the IMT. We perform density functional theory (DFT) calculations as a function of pressure to establish the connection between the IMT and the magnetic anomalies in Mn$_3$Si$_2$Te$_6$. We extract Heisenberg Hamiltonians as a function of pressure based on our DFT calculations. Our classical Monte Carlo simulations for these Hamiltonians yield ordering temperatures and magnetic ordering patterns, in agreement with the experimental data. Although we can accurately explain the evolution of magnetism with pressure, it seems that the anomalous Hall conductivity in Mn$_3$Si$_2$Te$_6$ can only be accounted for by extrinsic contributions or moderate electron doping of the samples in the experiment.

cond-mat.str-el↗

arXiv2026

Crystal structure, magnetic and resonant properties of decorated spin kagome system (CsCl)Cu$_5$As$_2$O$_{10}$

We report the synthesis and investigation of the crystal structure and physical properties of the averievite-like arsenate, $(\mathrm{CsCl})\mathrm{Cu}_5\mathrm{As}_2\mathrm{O}_{10}$. Just above room temperature, this compound undergoes a structural phase transition from the high-temperature trigonal $P\bar{3}m1$ $(a=b=a_0$ and $c=c_0)$ to the low-temperature monoclinic $I2/a$ phase $(a \approx \sqrt{3}a_0$, $b=a_0$, $c \approx 2c_0$ and $β\approx 90.6^\circ)$. According to thermodynamic and nuclear magnetic resonance measurements, it experiences a phase transition into a canted antiferromagnetic state at $T_N = 21~\mathrm{K}$. The density functional theory calculations place the energy scale of kagome exchange interaction parameter in $(\mathrm{CsCl})\mathrm{Cu}_5\mathrm{As}_2\mathrm{O}_{10}$ in between those in V- and P-analogs.

cond-mat.str-el↗

arXiv2026

Semi-Dirac spin liquids and frustrated quantum magnetism on the trellis lattice

Geometrical frustration in quantum magnets provides a fertile setting for unconventional phases of matter, including quantum spin liquids (QSLs). The trellis lattice, with its complex site arrangements and edge-sharing triangular motifs, presents a promising platform for such physics. In this work, we undertake a comprehensive classification of all fully symmetric QSLs on the trellis lattice using the projective symmetry group approach within the Abrikosov-fermion representation. We find 7 U(1) and 25 $Z_2$ short-ranged Ansätze and analyze the phase diagram in the mean-field parameter space, uncovering both gapped and Dirac QSLs as well as a semi-Dirac spin liquid that emerges at the level of projective symmetry group classification and mean-field band structure, in which the spinon dispersion is linear along one momentum direction but quadratic along the orthogonal one. We demonstrate that such dispersions can occur only at high-symmetry points in the Brillouin zone with $C_{2v}$ little groups and analyze their characteristic correlation signatures. Moreover, by optimizing over all symmetry-allowed mean-field states, we map out a phase diagram -- featuring six distinct phases -- of the nearest-neighbor Heisenberg Hamiltonian on the trellis lattice. Among these, we find four quasi-one-dimensional QSL phases, one dimer phase, and one Dirac QSL phase. Going beyond mean field, we also assess equal-time and dynamical spin structure factors of these phases using density-matrix renormalization group and Keldysh pseudofermion functional renormalization group calculations and compare qualitative momentum-space features of these spectra with those obtained at the mean-field level. Finally, we identify four cuprate and vanadate compounds as promising experimental realizations and provide spectroscopic predictions, based on first-principles Hamiltonians, as a guide for neutron-scattering studies.

cond-mat.str-el↗

arXiv2025

From localized 4f electrons to anisotropic exchange interactions in ferromagnetic CeRh6Ge4

CeRh6Ge4 is a cerium-based ferromagnetic material exhibiting a quantum critical behavior under pressure. We derive effective exchange interactions, using the framework of density functional theory combined with dynamical mean-field theory. Our results reveal that the nearest-neighbor ferromagnetic interaction along the c axis is isotropic in spin space, leading to a formation of spin chains. On the other hand, the inter-chain coupling is highly anisotropic: The in-plane moment weakly interacts ferromagnetically in the a--b plane to stabilize the ferromagnetic state, whereas the z-component couples antiferromagnetically, contributing to its destabilization. The magnetic anisotropy of the interchain interactions as well as of the local 4f wavefunctions characterizes the magnetic properties underlying the ferromagnetic transition and the quantum critical behavior in CeRh6Ge4.

cond-mat.str-el↗

arXiv2025

Tensor network analysis of the maple-leaf antiferromagnet spangolite

Spangolite (Cu$_6$Al(SO$_4$)(OH)$_{12}$Cl$\cdot$3H$_2$O) is a hydroxy-hydrated copper sulfate mineral with a one-seventh depleted triangular lattice of Cu$^{2+}$ ions in each layer. Experimental measurements revealed a non-magnetic ground state at $T \sim 8\, \text{K}$ with magnetic properties dominated by dimerization. We propose a spatially anisotropic Heisenberg model for the Cu$^{2+}$ spin-$1/2$ degrees of freedom on this geometrically frustrated and effectively two-dimensional maple-leaf lattice, featuring five symmetry inequivalent couplings with ferromagnetic bonds on hexagons and antiferromagnetic triangular bonds. The validity of the proposed Hamiltonian is demonstrated by state-of-the-art tensor network calculations, which can assess both the nature of the ground state as well as low-temperature thermodynamics, including the effects of a magnetic field. We provide theoretical support for a picture of a non-trivially correlated dimer ground state, which accounts for the appreciable reduction of the magnetic moment at high temperatures observed in experiment, thereby resolving a long-standing puzzle. We predict the static spin structure factor as well as the emergence of magnetisation plateaus at high values of an external magnetic field, explore the nature of the quantum states in them, and study their melting with increasing temperature.

cond-mat.str-el↗

arXiv2025

Robustness of Majorana modes to potential disorder in Fe chains on a superconducting Rashba alloy

Majorana modes offer great potential for fault-tolerant quantum computation due to their topological protection. However, for superconductor-semiconductor nanowire hybrids, intrinsic disorder makes the unambiguous detection of Majorana modes difficult. Here, we construct 1D spin chains from individual Fe atoms on the Rashba surface alloy BiAg2/Ag(111) with proximity-induced superconductivity from a Nb(110) substrate. While the Fe chains exhibit perfect crystalline order, we observe nano-scale potential disorder of the BiAg2/Ag(111)/Nb(110) heterostructure by scanning tunneling microscopy. However, this does not prevent the emergence of zero-energy modes at both ends of the Fe chains, in agreement with tight-binding calculations showing that they are only found in the topologically non-trivial regime of the phase diagram. These Majorana modes are indeed robust against potential disorder.

cond-mat.supr-con↗

arXiv2025

Discovery of the Type-II Superconductor Ta$_4$Rh$_2$C$_{1-δ}$ with a High Upper Critical Field

We report on the discovery of superconductivity in the previously unknown compound Ta$_4$Rh$_2$C$_{1-δ}$. Ta$_4$Rh$_2$C$_{1-δ}$ crystallizes in the $η$-carbide structure type, in the cubic space group $Fd\bar{3}m$ (No.227) with a unit cell parameter of $a = $ 11.7947 Å. Temperature-dependent magnetic susceptibility, resistivity, and specific heat capacity measurements reveal that Ta$_4$Rh$_2$C$_{1-δ}$ is a type-II bulk superconductor with a critical temperature of $T_{\rm c}$ = 6.4 K, and a normalized specific heat jump $ΔC/γT_{\rm c}$ = 1.56. Notably, we find Ta$_4$Rh$_2$C$_{1-δ}$ has a high upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ = 17.4 T, which is exceeding the BCS weak coupling Pauli limit of $μ_0 H_{\rm Pauli}$ = 11.9 T.

cond-mat.supr-con↗

arXiv2025

Field-induced spin liquid in the decorated square-kagome antiferromagnet nabokoite KCu$_7$TeO$_4$(SO$_4$)$_5$Cl

Quantum antiferromagnets based on the square-kagome lattice are proving to be a fertile platform for realizing nontrivial phenomena in frustrated magnetism. Recently, several decorated square-kagome compounds of the nabokoite family have been synthesized, allowing for experimental exploration of model Hamiltonians. Here, we carry out a theoretical analysis of KCu$_7$TeO$_4$(SO$_4$)$_5$Cl nabokoite using a Heisenberg Hamiltonian derived from density functional theory energy mapping. We employ classical Monte Carlo simulations to explain the two transitions experimentally observed in the low-temperature magnetization curve. Interestingly, the intermediate-field phase is also found in a purely two-dimensional model and is described by a spin liquid featuring subextensive degeneracy with a ferrimagnetic component. We show that this phase can be approximated by a checkerboard lattice in a magnetic field. Finally, we assess the effects of quantum fluctuations in zero fields using the pseudo-Majorana functional renormalization group method.

cond-mat.str-el↗

arXiv2025

Near room-temperature ferromagnetism from double-exchange in the van der Waals material CrGeTe$_3$: evidence from optical conductivity under pressure

The unexpected discovery of intrinsic ferromagnetism in layered van der Waals materials has sparked interest in both their fundamental properties and their potential for novel applications. Recent studies suggest near room-temperature ferromagnetism in the pressurized van der Waals crystal CrGeTe$_3$. We perform a comprehensive experimental and theoretical investigation of magnetism and electronic correlations in CrGeTe$_3$, combining broad-frequency reflectivity measurements with density functional theory and dynamical mean-field theory calculations. Our experimental optical conductivity spectra trace the signatures of developing ferromagnetic order and of the insulator-to-metal transition (IMT) as a function of temperature and hydrostatic pressure. With increasing pressure, we observe the emergence of a mid-infrared feature in the optical conductivity, indicating the development of strong orbital-selective correlations in the high-pressure ferromagnetic phase. We find a distinct relationship between the plasma frequency and Curie temperature of CrGeTe$_3$, which strongly suggests that a double-exchange mechanism is responsible for the observed near room-temperature ferromagnetism. Our results clearly demonstrate the existence of a charge-transfer gap in the metallic phase, ruling out its previously conjectured collapse under pressure.

cond-mat.str-el↗

arXiv2025

Spin-Peierls transition in the frustrated spinels ZnCr2O4 and MgCr2O4

The chromium spinels MgCr2O4 and ZnCr2O4 are prime examples of the highly frustrated pyrochlore lattice antiferromagnet. Experiment has carefully established that both materials, upon cooling, distort to lower symmetry and order magnetically. We study the nature of this process by a combination of density-functional-theory based energy mapping and classical Monte Carlo simulations. We first computationally establish precise Heisenberg Hamiltonian parameters for the high temperature cubic and the low temperature tetragonal and orthorhombic structures of both spinels. We then investigate the respective ordering temperatures of high symmetry and low symmetry structures. We carefully compare our results with experimental facts and find that our simulations are remarkably consistent with a type of spin-Peierls mechanism, adapted to three dimensions, where the structural distortion is mediated by a magnetic energy gain due to a lower degree of frustration.

cond-mat.str-el↗