arXiv subjects

Clemens Ritter

Publications and source records attributed to Clemens Ritter.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical magnetism in the disordered cubic lattice material $\gamma$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$

$\gamma$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$ realizes a disordered simple-cubic spin-$1/2$ lattice in which Co$^{2+}$ ions randomly occupy one third of the sites, placing the system close to the site-percolation threshold for magnetic order. Specific-heat, susceptibility, neutron spin-echo, and muon spin-rotation measurements reveal a broad thermodynamic crossover, short-range magnetic correlations, and persistent fast spin dynamics down to at least 0.1~K, with no evidence for static order or conventional spin-glass freezing. Monte Carlo simulations yield a broad distribution of orphan spins, finite clusters, and an infinite network. The calculated orphan-spin fraction ($\approx 8.8\%$) agrees well with the weakly correlated spin fraction inferred from magnetization ($\approx 8.2\%$). Exact diagonalization of a diluted $S = 1/2$ Heisenberg model captures the broad magnetic specific-heat anomaly and supports the coexistence of weakly and strongly correlated spin environments. These results support a picture in which spin-$1/2$ quantum fluctuations, together with dilution and proximity to the percolation threshold, can support a disorder-driven dynamical state with short-range correlations in three dimensions, distinct from both classical spin glasses and geometrically frustrated quantum spin liquids.

cond-mat.str-el↗

arXiv2025

The interplay between Jahn-Teller distortions and structural degrees of freedom on pseudocubic states in manganite perovskites

The average structure of the solid solution LaMn$_{1-x}$Ga$_x$O$_3$ (LMGO) has been investigated from a symmetry-motivated approach utilizing synchrotron x-ray and neutron powder diffraction techniques. We show experimentally that a trilinear coupling term ($\Gamma_5^+$M$_2^+$M$_3^+$) between shear strain, octahedral rotation, and the $C$-type orbital ordering mode is responsible for driving the orthorhombic to pseudocubic phase transition occurring in the composition range 0.5 $<$ $x$ $<$ 0.6. Our Monte Carlo simulations elucidate the macroscopic origin of this coupling to shear strain, and point to its importance with respect to controlling the orbital order-disorder transitions. We find that the emergence of the pseudocubic state can be rationalized by considering the competition between this trilinear term and a linear-quadratic term of the out-of-phase octahedral tilting with strain ($\Gamma_5^+$(R$_5^-$)$^2$). Illustrating the general nature of these results, we construct a simple function that captures the change in Landau free energy at the order-disorder transition, in parameters that are trivial to relate to the concentration of Jahn--Teller active species, temperature, tolerance factor and unit cell strain, for a broad range of manganite perovskites. Our results point to the fact that far from the pseudocubic state being a symptom of orbital disorder, it is in many cases more correctly to view it as a cause. The results have a broad impact on the study of orbital ordering physics in the perovskite materials and on chemical and physical control parameters through which to tune the richness of the intertwined physical properties.

cond-mat.str-el↗

arXiv2024

Proximity to an orbital order with charge disorder state in optimally-doped \textit{RE}\textsubscript{5/8}Ca\textsubscript{3/8}MnO\textsubscript{3} perovskites

The evolution of charge and orbital ordering phenomena in optimally-doped \textit{RE}\textsubscript{5/8}Ca\textsubscript{3/8}MnO\textsubscript{3} (RECMO, \textit{RE} $=$ rare-earth) manganite perovskites has been investigated through average structure synchrotron x-ray and neutron powder diffraction techniques. We demonstrate the intricate relationship between the \textit{B}O\textsubscript{6} octahedral rotation magnitude and lattice strain distortions acting in this series and how they tune macroscopic signatures describing ordering behavior. Through careful symmetry-motivated crystallographic analysis, we show that for the range of RECMO compositions which famously contain maxima in the colossal magnetoresistance (CMR) response, their lattice strain states are in close proximity to that associated with a novel orbital order:charge disordered state we have recently unveiled in the quadruple manganite perovskites Na\textsubscript{1-\textit{x}}Ca\textsubscript{\textit{x}}Mn\textsubscript{7}O\textsubscript{12}. We establish that this order is the primary state which competes with the ferromagnetic metallic state which ultimately leads to phase coexistence and the emergence of CMR. Our results lend themselves to aiding a further understanding of how particular chemical complexities can control charge and orbital ordering phenomena, and also the general properties of manganite perovskites and other related systems \textit{via} strain effects.

cond-mat.str-el↗

arXiv2023

Geometrical frustration and incommensurate magnetic order in Na$_3$RuO$_4$ with two triangular motifs

Incommensurate magnetic order in the spin-3/2 antiferromagnet Na$_3$RuO$_4$ is uncovered by neutron diffraction combined with ab initio calculations. The crystal structure of Na$_3$RuO$_4$ contains two triangular motifs on different length scales. The magnetic Ru$^{5+}$ ions form a lozenge (diamond) configuration, with tetramers composed of two isosceles triangles. These tetramers are further arranged in layers, such that an effective triangular lattice is formed. The tetramers are nearly antiferromagnetic but frustration between them leads to an incommensurately modulated magnetic structure described by the propagation vector $\vec{k}=(0.242(1), 0, 0.313(1))$. We show that the long-range Ru-O-O-Ru couplings between the tetramers play a major role in Na$_3$RuO$_4$ and suggest an effective description in terms of the spatially anisotropic triangular lattice if the tetramers are treated as single sites.

cond-mat.str-el↗

arXiv2022

One-Dimensional Quantum Magnetism in the S = 1/2 Mo(V) system, KMoOP2O7

We present a comprehensive experimental and ab-initio study of the $S=1/2$ Mo$^{5+}$ system, KMoOP$_2$O$_7$, and show that it realizes the $S = 1/2$ Heisenberg chain antiferromagnet model. Powder neutron diffraction reveals that KMoOP$_2$O$_7$ forms a magnetic network comprised of pairs of Mo$^{5+}$ chains within its monoclinic $P2_1/n$ structure. Antiferromagnetic interactions within the Mo$^{5+}$ chains are identified through magnetometry measurements and confirmed by analysis of the magnetic specific heat. The latter reveals a broad feature centred on $T_\textrm{N} = 0.54$ K, which we ascribe to the onset of long-range antiferromagnetic order. No magnetic Bragg scattering is observed in powder neutron diffraction data collected at 0.05 K, however, which is consistent with a strongly suppressed ordered moment with an upper limit $μ_\textrm{ord} < 0.15 μ_\textrm{B}$. The one-dimensional character of the magnetic correlations in KMoOP$_2$O$_7$ is verified through analysis of inelastic neutron scattering data, resulting in a model with $J_\textrm{1} \approx 34$ K and $J_\textrm{2} \approx -2$ K for the intrachain and interchain exchange interactions, respectively. The origin of these experimental findings are addressed through density-functional theory calculations.

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arXiv2022

Incommensurate magnetic modulation in K-rich cryptomelane, K$_x$Mn$_8$O$_{16}$ ($x\approx1.45$)

Cryptomelane is a hollandite-like material consisting of K$^+$ cations in an $\alpha$-MnO$_2$ tunnel-like crystallographic motif. A sample with stoichiometry K$_{1.448(3)}$Mn$_8$O$_{16}$ has been synthesised and its magnetic properties investigated using variable-temperature magnetic susceptibility, heat capacity, and neutron powder diffraction. Three distinct transitions at $T_1=184$\,K, $T_2=54.5$\,K, and $T_3=24$\,K are observed. At $T_1$ there is a subtle tetragonal$\rightarrow$monoclinic transition associated with Mn$^{3+}$/Mn$^{4+}$ ordering, and a set of non-magnetic superstructure peaks emerge; these could not be indexed definitively and are indicative of an ordering that is incommensurate with the unit cell. Magnetic Bragg peaks emerge below $T_2=54.5$\,K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual-$\vec{k}_\mathrm{mag}$ structure with a ferromagnetic $|\vec{k}_\mathrm{mag}|=0$ component and an incommensurate $\vec{k}_\mathrm{mag}$ parallel to the $\alpha$-MnO$_2$ tunnels [$|\vec{k}_\mathrm{mag}|=0.36902(15)$], with the latter most likely to be helical. The period of oscillation of the helical component is in line with predictions based on a Heisenberg spin Hamiltonian [Mandal \textit{et al}. Phys. Rev. B 90, 104420 (2014)]. Below $T_3=24$\,K, there is a magnetic transition, which gives rise to a different set of magnetic Bragg peaks indicative of a highly complex magnetic structure.

cond-mat.mtrl-sci↗

arXiv2022

Observation of a possible diluted ferromagnetism above room temperature in cobalt-substituted LaTa(O,N)3-d

Since 2000, the intensive effort in materials research to develop a diluted magnetic semiconductor exhibiting high-temperature (HT) ferromagnetism above room temperature was not successful. Here, the possible first bulk diluted HT-ferromagnetic non-metallic materials, based on the perovskite-type oxynitrides LaTa1-xCox(O,N)3-d (x = 0.01, 0.03, 0.05) are realized. The Curie temperature of the synthesized powders exceeds 600 K and the sample magnetizations are large enough to be directly attracted by permanent magnets. Cobalt clusters as a possible source for the observed HT-ferromagnetism can be excluded, since all applied characterization methods verify phase purity. Applied conventional and element-specific magnetometry imply ferromagnetic intermediate spin (IS) Co3+ which is included in a ferromagnetic host matrix. This indicates a complex magnetic interplay between the existing crystal structure, the observed anionic vacancies, and the introduced cobalt ions. These results lay the foundation for the experimental investigation and design of further diluted HT-ferromagnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2022

Nematic state of the FeSe superconductor

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives up to temperatures above 90 K where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 angström. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations.

cond-mat.supr-con↗

arXiv2022

Investigation of metamagnetism and crystal-field splitting in pseudo-hexagonal CeRh$_3$Si$_2$

CeRh$_3$Si$_2$ has been reported to exhibit metamagnetic transitions below 5~K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) reveal a $J_z$\,=\,1/2 groundstate for Ce when considering the crystallographic $a$ direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78\,meV for the $J$\,=\,5/2 multiplet. The neutron diffraction study in zero field reveals that on cooling from the paramagnetic state, the system first orders at $T_{\text{N}_1}=4.7$\,K in a longitudinal spin density wave with ordered Ce moments along the $b$-axis (i.e. the [0 1 0] crystal direction) and an incommensurate propagation vector $\textbf{k}=(0,0.43,0$). Below the lower-temperature transition $T_{\text{N}_2}=4.48$\,K, the propagation vector locks to the commensurate value $\textbf{k}=(0,0.5,0)$, with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field $H\parallel b$-axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at $H=3$\,kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at $H = 7$\,kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behaviour is also observed for the $H\parallel$ [0 1 1] crystal direction.

cond-mat.str-el↗

arXiv2021

MoP$_3$SiO$_{11}$: a $4d^3$ honeycomb antiferromagnet with disconnected octahedra

We report the crystal structure and magnetic behavior of the $4d^3$ spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group ($R\bar 3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$ ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic order with the propagation vector $\mathbf k=0$ observed below $T_N=6.8$ K is a combined effect of the nearest-neighbor in-plane exchange coupling $J\simeq 2.6$ K, easy-plane single-ion anisotropy $D\simeq 2.2 $ K, and a weak interlayer coupling $J_c\simeq 0.8$ K. The 12% reduction in the ordered magnetic moment of the Mo$^{3+}$ ions and the magnon gap of $Δ\simeq 7$ K induced by the single-ion anisotropy further illustrate the impact of spin-orbit coupling on the magnetism. Our analysis puts forward single-ion anisotropy as an important ingredient of $4d^3$ honeycomb antiferromagnets despite their nominally quenched orbital moment.

cond-mat.str-el↗

arXiv2021

Magnetic structure determination of rare-earth based, high moment, atomic laminates; potential parent materials for 2D magnets

We report muon spin rotation ($μ$SR) and neutron diffraction on the rare-earth based magnets (Mo$_{2/3}$RE$_{1/3}$)$_2$AlC, also predicted as parent materials for 2D derivatives, where RE = Nd, Gd (only ($μ$SR), Tb, Dy, Ho and Er. By crossing information between the two techniques, we determine the magnetic moment ($m$), structure, and dynamic properties of all compounds. We find that only for RE = Nd and Gd the moments are frozen on a microsecond time scale. Out of these two, the most promising compound for a potential 2D high ($m$) magnet is the Gd variant, since the parent crystals are pristine with $m = 6.5 \pm 0.5 μ_B$, Néel temperature of $29 \pm 1$ K, and the magnetic anisotropy between in and out of plane coupling is smaller than $10^{-8}$. This result suggests that magnetic ordering in the Gd variant is dominated by in-plane magnetic interactions and should therefore remain stable if exfoliated into 2D sheets.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic frustration in a van der Waals metal CeSiI

The realization of magnetic frustration in a metallic van der Waals (vdW) coupled material has been sought as a promising platform to explore novel phenomena both in bulk matter and in exfoliated devices. However, a suitable material platform has been lacking so far. Here, we demonstrate that CeSiI hosts itinerant electrons coexisting with exotic magnetism. In CeSiI, the magnetic cerium atoms form a triangular bilayer structure sandwiched by van der Waals stacked iodine layers. From resistivity and magnetometry measurements, we confirm the coexistence of itinerant electrons with magnetism with dominant antiferromagnetic exchange between the strongly Ising-like Ce moments below 7 K. Neutron diffraction directly confirms magnetic order with an incommensurate propagation vector k ~ (0.28, 0, 0.19) at 1.6 K, which points to the importance of further neighbor magnetic interactions in this system. The presence of a two-step magnetic-field-induced phase transition along c axis further suggests magnetic frustration in the ground state. Our findings provide a novel material platform hosting a coexistence of itinerant electron and frustrated magnetism in a vdW system, where exotic phenomena arising from rich interplay between spin, charge and lattice in low dimension can be explored.

cond-mat.str-el↗

arXiv2020

Ferrimagnetic 120$^\circ$ magnetic structure in Cu2OSO4

We report magnetic properties of a 3d$^9$ (Cu$^{2+}$) magnetic insulator Cu2OSO4 measured on both powder and single crystal. The magnetic atoms of this compound form layers, whose geometry can be described either as a system of chains coupled through dimers or as a Kagomé lattice where every 3rd spin is replaced by a dimer. Specific heat and DC-susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with a $S=1/2$ degree of freedom per Cu$^{2+}$, and so is the effective moment extracted from DC-susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to a $\sim120$ degree spin structure in which ferromagnetic intra-dimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that \sample \ represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

cond-mat.str-el↗

arXiv2020

Establishing magneto-structural relationships in the solid solutions of the skyrmion hosting family of materials: GaV$_4$S$_{8-y}$Se$_{y}$

The GaV$_4$S$_{8-y}$Se$_y$ $(y = 0$ to $8)$ family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the $F\bar{4}3m$ space group at ambient temperature. However, in contrast to the end members GaV$_4$S$_8$ and GaV$_4$Se$_8$, that undergo a structural transition to the $R3m$ space group at 42 and 41 K respectively, the solid solutions $(y = 1$ to $7)$ retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV$_4$S$_8$) and 18 K (GaV$_4$Se$_8$), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV$_4$S$_8$ shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the $y = 1$ to $7$ materials can be explained by their structural properties.

cond-mat.str-el↗

arXiv2019

A quantum liquid of magnetic octupoles on the pyrochlore lattice

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores - a state described by a lattice gauge theory of quantum electrodynamics$^{2-4}$. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders$^{5-6}$. Here we show that the correlated ground state of a Ce$^{3+}$-based pyrochlore, Ce$_2$Sn$_2$O$_7$, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with the formation of a fluid-like state of matter, but the intensity distribution is weighted to larger scattering vectors, which indicates that the correlated degrees of freedom have a more complex magnetization density than that typical of magnetic dipoles in a spin liquid. The temperature evolution of the bulk properties in the correlated regime below 1 Kelvin is well reproduced using a model of dipole-octupole doublets on a pyrochlore lattice$^{7-8}$. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies Ce$_2$Sn$_2$O$_7$ as a unique example of a material where frustrated multipoles form a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

cond-mat.str-el↗

arXiv2019

Octupolar vs Néel Order in Cubic 5$d^2$ double perovskites

We report time-of-flight neutron spectroscopic and diffraction studies of the 5$d^2$ cubic double pervoskite magnets, Ba$_2$MOsO$_6$ ($M$ = Zn, Mg, Ca). These cubic materials are all described by antiferromagnetically-coupled 5$d^2$ Os$^{6+}$ ions decorating a face-centred cubic (FCC) lattice. They all exhibit thermodynamic anomalies consistent with phase transitions at a temperature $T^*$, and exhibit a gapped magnetic excitation spectrum with spectral weight concentrated at wavevectors typical of type I antiferromagnetic orders. While muon spin resonance experiments show clear evidence for time reversal symmetry breaking, no corresponding magnetic Bragg scattering is observed at low temperatures. These results, consistent with low temperature octupolar or quadrupolar order, are discussed in the context of other 5$d^2$ DP magnets, and theories for $d^2$ ions on a FCC lattice which predict exotic orders driven by multipolar interactions.

cond-mat.str-el↗

arXiv2019

High-pressure optical floating-zone growth of Li2FeSiO4 single crystals

We report the growth of mm-sized Pmnb-Li2FeSiO4 single crystals by means of the optical floating-zone method at high argon pressure and describe the conditions required for a stable growth process. The crystal structure is determined and refined by single-crystal X-ray diffraction. The lattice constants amount to a = 6.27837(3) A, b = 10.62901(6) A and c = 5.03099(3) A at 100 K. In addition, we present high-resolution neutron powder diffraction data that suggest that the slight Li-Fe site exchange seems to be intrinsic to this material. High quality of the crystal is confirmed by very sharp anomalies in the static magnetic susceptibility and in the specific heat associated with the onset of long-range antiferromagnetic order at TN = 17.0(5) K and pronounced magnetic anisotropy for the three crystallographic axes. Furthermore, magnetic susceptibility excludes the presence of sizable amounts of magnetic impurity phases.

cond-mat.str-el↗

arXiv2018

Stripe order and magnetic anisotropy in the $S=1$ antiferromagnet BaMoP$_2$O$_8$

Magnetic behavior of BaMoP$_2$O$_8$ with the spatially anisotropic triangular arrangement of the $S=1$ Mo$^{4+}$ ions is explored using thermodynamic measurements, neutron diffraction, and density-functional band-structure calculations. A broad maximum in the magnetic susceptibility around 46\,K is followed by the stripe antiferromagnetic order with the propagation vector ${\mathbf k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ formed below $T_N\simeq 21$ K. This stripe phase is triggered by a pronounced one-dimensionality of the spin lattice, where one of the in-plane couplings, $J_2\simeq 4.6$ meV, is much stronger than its $J_1\simeq 0.4$ meV counterpart, and stabilized by the weak easy-axis anisotropy. The ordered moment of 1.42(9) $μ_B$ at 1.5 K is significantly lower than the spin-only moment of 2 $μ_B$ due to a combined effect of quantum fluctuations and spin-orbit coupling.

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