arXiv subjects

Stephan Rosenkranz

Publications and source records attributed to Stephan Rosenkranz.

At least 19 recordsLinked to original sources

arXiv2026

Spin correlations, low-energy scales, and anisotropy scaling in kagome frustrated magnets

Neutron scattering is central to identifying quantum states of magnetic materials. In the search for quantum spin liquids, broad spectral features of inelastic spectra have been cited as evidence for spinon excitations, but can also arise from magnon excitations excitations in the presence of quenched disorder and strong magnon interactions. We develop a new approach to this problem, based on the adiabatic continuity in the $XXZ$ Heisenberg model on geometrically frustrating (GF) lattices as a function of the model's anisotropy. Using this approach, we identify universal features and energies of finite-temperature spin correlators. Focusing on the kagome lattice, we show that the low-energy spin spectral function contains robust, momentum-independent peaks with frequencies: $ω_1 \approx 3.4 T^*$ and $ω_2 \approx 6.3 T^*$, where the ``hidden energy scale'' $T^*$ is the characteristic scale of a low-temperature peak in the heat capacity, at which many GF magnets also display spin-glass freezing. We show that the spectral features at low energies $ω\lesssim T^*$ arise from single-magnon scattering and identify the magnetizations of the respective excitations. We explore the evolution of the spectral features with temperature and discuss extensions to other GF lattices. Our results provide a sharp spectroscopic criterion for interpreting neutron scattering in kagome and other GF quantum magnets.

cond-mat.str-el↗

arXiv2026

Evolution of charge-density-wave soft phonon modes in $\mathrm{Pd}_x\mathrm{ErTe}_3$

We investigated the lattice dynamics of quasi-two-dimensional Pd-intercalated $\mathrm{ErTe}_3$ in relation to its charge-density-wave (CDW) transitions by means of x-ray diffuse and meV-resolution inelastic x-ray scattering. In pristine $\mathrm{ErTe}_3$, CDW order develops at orthogonal in-plane wave vectors $\boldsymbol{\mathrm{q}}_{1}^{c} = (0, 0, 0.29)$ (the $c\text{-}\mathrm{CDW}$) and $\boldsymbol{\mathrm{q}}_{2}^{a} = (0.31, 0, 0)$ (the $a\text{-}\mathrm{CDW}$), with transition temperatures $T_{1}^{c} = 270$ K and $T_{2}^{a} = 160$ K, respectively. Remarkably, we observe diffuse x-ray scattering already near the higher transition temperature $T_{1}^{c}$ along $a\text{-}\mathrm{CDW}$ but at a slightly different wave vector $\boldsymbol{\mathrm{q}}_{1}^{a} = (0.29, 0, 0)$. Inelastic x-ray scattering for $\mathrm{Pd}_{0.01}\mathrm{ErTe}_3$ shows that a partial phonon softening at $\boldsymbol{\mathrm{q}}_{1}^{a}$, underscoring the strong competition between ordering tendencies along the nearly equivalent in-plane axes of the orthorhombic lattice. For intercalation levels $x \geq 0.02$, the $a\text{-}\mathrm{CDW}$ state is suppressed. Nevertheless, a similar correlation between phonon softening and diffuse scattering persists along the $[100]$ direction, again observed at $\boldsymbol{\mathrm{q}}_{1}^{a} = (0.29, 0, 0)$ and $T_{1}^{c}$. These findings suggest that the $a\text{-}\mathrm{CDW}$ is fully suppressed for $x \geq 0.02$, and that the residual diffuse scattering at $\boldsymbol{\mathrm{q}}_{1}^{a}$ originates from the partial phonon softening associated with the $c\text{-}\mathrm{CDW}$, reflected by the near equality of the absolute size of $\boldsymbol{\mathrm{q}}_{1}^{c}$ and $\boldsymbol{\mathrm{q}}_{1}^{a}$.

cond-mat.str-el↗

arXiv2026

Model-free Analysis of Scattering and Imaging Data with Escort-Weighted Shannon Entropy and Divergence Matrices

We demonstrate a model-free data analysis framework that leverages escort-weighted Shannon Entropy and several divergence matrices to detect phase transitions in scattering and imaging datasets. By establishing a connection between physical entropy and informational entropy, this approach provides a sensitive method for identifying phase transitions without an explicit physical model or order parameter. We further show that pairwise divergence matrices, including Kullback-Leibler divergence, Jeffrey Divergence, Jensen-Shannon Divergence and antisymmetric Kullback-Leibler divergence, provide more comprehensive measures of statistical changes than scalar entropy alone. Our approach successfully detects the onset of both long- and short-range order in neutron and X-ray scattering data, as well as a non-trivial phase transition in magnetic skyrmion lattices observed through Lorentz-transition electron microscopy. These results establish a framework for automated, model-free analysis of experimental data with broad applications in materials science and condensed matter physics.

cond-mat.mtrl-sci↗

arXiv2025

4D-QENS Analysis of Correlated Ionic Conduction in SrCl$_2$

Methods of elucidating the mechanisms of fast-ion conduction in solid-state materials are pivotal for advancements in energy technologies such as batteries, fuel cells, sensors, and supercapacitors. In this study, we examine the ionic conduction pathways in single crystal SrCl$_2$, which is a fast-ion conductor above 900~K, using four-dimensional Quasi-Elastic Neutron Scattering (4D-QENS). We explore both coherent and incoherent neutron scattering at temperatures above the transition temperature into the superionic phase to explore the correlated motion of hopping anions. Refinements of the incoherent QENS yield residence times and jump probabilities between lattice sites in good agreement with previous studies, confirming that ionic hopping along nearest-neighbor directions is the most probable conduction pathway. However, the coherent QENS reveals evidence of de Gennes narrowing, indicating the importance of ionic correlations in the conduction mechanism. This highlights the need for improvements both in the theory of ionic transport in fluorite compounds and the modeling of coherent 4D-QENS in single crystals.

cond-mat.mtrl-sci↗

arXiv2025

Evolution from Topological Dirac Metal to Flat-band-Induced Antiferromagnet in Layered KxNi4S2 (0<=x<=1)

Condensed matter systems with coexisting Dirac cones and flat bands, and a switchable control between them within a single system, are desirable but remarkably uncommon. Here we report a layered quantum material system, KxNi4S2 (0 <= x <= 1), that simultaneously hosts both characteristics without involving typical Kagome/honeycomb lattices. Enabled by a topochemical K-deintercalation process, the Fermi surface can be fine-tuned continuously over a wide range of energies. Consequently, a non-magnetic Dirac-metal state with a topological nontrivial Z2 index of 1;(000), supported by first-principles calculations and high mobility up to 1471 cm2V-1s-1, is observed on the K-rich x = 1 side, whereas a flat-band induced antiferromagnetic state with TN up to 10.1 K emerges as K-content approaches 0. The KxNi4S2 system offers a versatile platform for exploring emerging phenomena and underscores a viable pathway for in-situ control of quantum materials dominated by Dirac cones, flat bands, and their interplay.

cond-mat.mtrl-sci↗

arXiv2025

Lattice-charge coupling in a trilayer nickelate with intertwined density wave order

Intertwined charge and spin correlations are ubiquitous in a wide range of transition metal oxides and are often perceived as intimately related to unconventional superconductivity. Theoretically envisioned as driven by strong electronic correlations, the intertwined order is usually found to be strongly coupled to the lattice as signaled by pronounced phonon softening. Recently, both charge/spin density waves (CDW/SDW) and superconductivity have been discovered in several Ruddlesden-Popper (RP) nickelates, in particular trilayer nickelates RE4Ni3O10 (RE=Pr, La). The nature of the intertwined order and the role of lattice-charge coupling are at the heart of the debate about these materials. Using inelastic X-ray scattering, we mapped the phonon dispersions in RE4Ni3O10 and found no evidence of phonon softening near the CDW wavevector over a wide temperature range. Calculations of the electronic susceptibility revealed a peak at the observed SDW ordering vector but not at the CDW wavevector. The absence of phonon softening is in sharp contrast to that in canonical oxide materials, notably cuprates. Our experimental and theoretical findings highlight the crucial role of the spin degree of freedom and establish a foundation for understanding the interplay between superconductivity and density-wave transitions in RP nickelate superconductors and beyond.

cond-mat.str-el↗

arXiv2025

Charge-density-wave quantum critical point under pressure in 2$H$-TaSe$_2$

Suppressing of an ordered state that competes with superconductivity is one route to enhance superconducting transition temperatures. Whereas the effect of suppressing magnetic states is still not fully understood, materials featuring charge-density waves and superconductivity offer a clearer scenario as both states can be associated with electron-phonon coupling. Metallic transition-metal dichalcogenides are prime examples for such intertwined electron-phonon-driven phases, yet, various compounds do not show the expected interrelation or feature additional mechanisms which makes an unambiguous interpretation difficult. Here, we report high-pressure X-ray diffraction and inelastic X-ray scattering measurements of the prototypical transition-metal dichalcogenide 2$H$-TaSe$_2$ and determine the evolution of the charge-density-wave state and its lattice dynamics up to and beyond its suppression at the critical pressure $p_c = 19.9(1)\,\rm{GPa}$ and at low temperatures. The high quality of our data allows the full refinement of the commensurate charge-density-wave superstructure at low pressure and we find the quantum critical point of the charge-density-wave to be in close vicinity to the reported maximum superconducting transition temperature $T_{sc} = 8.2\,\rm{K}$. $Ab-initio$ calculations corroborate that 2$H$-TaSe$_2$ is a reference example of order-suppressed enhanced superconductivity and can serve as a textbook case to investigate superconductivity near a charge-density-wave quantum critical point.

cond-mat.str-el↗

arXiv2025

In-plane anisotropy of charge density wave fluctuations in 1$T$-TiSe$_2$

We report measurements of anisotropic triple-$q$ charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1$T$-TiSe$_2$ over a large volume of reciprocal space with X-ray diffuse scattering. Above the transition temperature, $T_{\text{CDW}}$, the out-of-plane diffuse scattering is characterized by rod-like structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. In addition, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations. At $T_{\text{CDW}}$, long range coherence is established in both the in-plane and out-of-plane directions, consistent with the large observed value of the CDW gap compared to $T_{\text{CDW}}$, and the predicted presence of a hierarchy of energy scales.

cond-mat.str-el↗

arXiv2024

Effect of disorder on the strain-tuned charge density wave multicriticality in Pd$_x$ErTe$_3$

We explore, through a combination of x-ray diffraction and elastoresistivity measurements, the effect of disorder on the strain-tuned charge density wave and associated multicriticality in Pd$_x$ErTe$_3$ (x = 0, 0.01, 0.02 and 0.026). We focus particularly on the behavior near the strain-tuned bicritical point that occurs in pristine ErTe$_3$ (x=0). Our study reveals that while Pd intercalation somewhat broadens the signatures of the CDW phase transitions, the line of first-order transitions at which the CDW reorients as a function of applied strain persists in the presence of disorder and still seemingly terminates at a critical point. The critical point occurs at a lower temperature and a lower strain compared to pristine ErTe$_3$. Similarly, the nematic elastoresistance of Pd$_x$ErTe$_3$, though suppressed in magnitude and broadened relative to that of ErTe$_3$, has a markedly more symmetric response around the critical point. These observations point to disorder driving a reduction in the system's electronic orthorhombicity even while the material remains irrevocably orthorhombic due to the presence of a glide plane in the crystal structure. Disorder, it would appear, reinforces the emergence of a "pseudo-tetragonal" electronic response in this fundamentally orthorhombic material.

cond-mat.str-el↗

arXiv2024

Non-equilibrium dynamics in geometrically frustrated spin glass Bi$_2$Fe$_3$GaO$_9$ with a Cairo lattice

We have explored the relaxation process of the spin glass phase in Bi$_2$Fe$_3$GaO$_9$, a geometrically frustrated magnet with a unique structure consisting of pentagonal building blocks known as the Cairo lattice. Using dc magnetization measurements on single crystals, we estimate the relaxation time across various temperatures and fields. Our results indicate that the relaxation time of Bi$_2$Fe$_3$GaO$_9$ follows the Arrhenius law as a function of temperature but remains relatively constant as the applied magnetic field varies. Further, through a carefully designed protocol, we observe significant rejuvenation and memory effects. Remarkably, the memory effects in Bi$_2$Fe$_3$GaO$_9$ are found to be more similar to those observed in classical spin glasses rather than in spin jamming systems, both in terms of the magnitude of the memory effect and the value of the exponent that describes the relaxation behavior. Our findings suggest that Bi$_2$Fe$_3$GaO$_9$ provides an excellent platform for investigating the time-dependent evolution of underlying magnetism using neutron scattering, paving the way for future research.

cond-mat.dis-nn↗

arXiv2024

Structural and magnetic properties of CoTeMoO$_6$ revisited

We have conducted a comprehensive investigation into the magnetic properties of the chiral multiferroic material CoTeMoO$_6$. In contrast with the previous claim of canted antiferromagnetic order with ferromagnetic components, our investigation reveals an antiferromagnetic ground state with compensated moments, providing an interesting platform for exploring exotic material properties. Through careful measurements of magnetization under a series of applied field, we demonstrate that there exist two sequential field-induced magnetic transitions in CoTeMoO$_6$, with one occurring at $H_{c1}$=460 Oe along the a-axis, and the other at $H_{c2}$=1.16 T with the field along the b-axis. The values of $H_{c1}$ and $H_{c2}$ exhibit strong angular dependence and diverge with different rates as the applied field is rotated 90 degrees within the ab plane. This reflects the distinct nature of these transitions, which is further supported by the different critical behavior of $H_{c1}$ and $H_{c2}$, characterized by the values of $γ$,in the function of $H_c=H_0\times(1-\frac{T}{T_c})^n$. Furthermore, we have demonstrated that there exist structural and magnetic twin domains in CoTeMoO$_6$ that strongly affect the experimental measurement of their macroscopic properties. Intriguingly, these twin domains can be related to the orthorhombicity/chirality of the crystal structure with the space group $P2_1 2_1 2$. We further explored the magnetic and structural domains with uniaxial pressure and polarized light microscopy. Our results suggest that CoTeMoO$_6$ could be used as a unique platform for investigating the intriguing physics involving intertwined degrees of freedom. The tunability of the underlying domain distribution and its strong anisotropy could also be useful for developing functional devices and applications.

cond-mat.mtrl-sci↗

arXiv2024

Structural Properties of Plastically Deformed SrTiO3 and KTaO3

Dislocation engineering has the potential to open new avenues toward the exploration and modification of the properties of quantum materials. Strontium titanate (SrTiO3, STO) and potassium tantalate (KTaO3, KTO) are incipient ferroelectrics that show metallization and superconductivity at extremely low charge carrier concentrations, and have been the subject of resurgent interest. These materials also exhibit remarkable ambient-temperature ductility, and thus represent exceptional platforms for studies of the effects of deformation-induced dislocation structures on electronic properties. Recent work on plastically deformed STO revealed an enhancement of the superconducting transition temperature and the emergence of local ferroelectricity and magnetism near self-organized dislocation walls. Here we present a comprehensive structural analysis of plastically deformed STO and KTO, employing specially designed strain cells, diffuse neutron and x-ray scattering, Raman scattering, and nuclear magnetic resonance (NMR). Diffuse scattering and NMR provide insight into the dislocation configurations and densities and their dependence on strain. As in the prior work on STO, Raman scattering reveals evidence for local ferroelectric order near dislocation walls in plastically deformed KTO. Our findings provide valuable information about the self-organized defect structures in both materials, and they position KTO as a second model system with which to explore the associated emergent physics.

cond-mat.mtrl-sci↗

arXiv2024

Diffuse Scattering from Correlated Electron Systems

The role of inhomegeneity in determining the properties of correlated electron systems is poorly understood because of the dearth of structural probes of disorder at the nanoscale. Advances in both neutron and x-ray scattering instrumentation now allow comprehensive measurements of diffuse scattering in single crystals over large volumes of reciprocal space, enabling structural correlations to be characterized over a range of length scales from 10~Å to 200~Å or more. When combined with new analysis tools, such as three-dimensional difference pair-distribution functions, these advanced capabilities have produced novel insights into the interplay of structural fluctuations and electronic properties in a broad range of correlated electron materials. This review describes recent investigations that have demonstrated the importance of understanding structural inhomogeneity pertaining to phenomena as diverse as superconductivity, charge-density-wave modulations, metal-insulator transitions, and multipolar interactions.

cond-mat.str-el↗

arXiv2024

Competition between metal bonding and strain in tetragonal V$_{1-x}$M$_x$O$_2$ (M = Nb, Mo)

Though the effects of metal dopants on the electrostructural transition of rutile VO$_2$ have been studied for many decades, there is still no consensus explanation for the observed trends. A major challenge has been to separate the impact of a dopant's size from other factors such as its electronic configuration, which stems from the difficulty in directly probing the local bonding environment around a dopant atom. This work addresses the special case of larger dopant ions by combining X-ray total scattering experiments on V$_{0.83}$Mo$_{0.17}$O$_2$ and V$_{0.89}$Nb$_{0.11}$O$_2$ single crystals with multiple Monte Carlo method models to simulate local size effects in the high-temperature tetragonal phase (R). We find that sufficiently long apical metal-oxygen bonds (M$-$O$_{ap}$) induce a strain field in the neighboring chains that locally resembles the metal-metal dimer formation present in the low-temperature distorted structure of VO$_2$ (M1). The dimer mode in the M1 structure is antisymmetric along M$-$O$_{ap}$, however, while the strain-induced pseudodimer motif is symmetric. The implied direct competition between motifs is verified experimentally. This finding provides a new mechanistic parameter toward understanding the phase transition. More generally, the work highlights how local strain fields around dopants can lead to complex distortions that are ordinarily attributed to electronic origins.

cond-mat.str-el↗

arXiv2024

Fragile 3D Order in V$_{1-x}$Mo$_x$O$_2$

The metal-to-insulator transition (MIT) in rutile VO$_2$ has proven uniquely difficult to characterize because of the complex interplay between electron correlations and atomic structure. Here we report the discovery of the sudden collapse of three-dimensional order in the low-temperature phase of V$_{1-x}$Mo$_x$O$_2$ at $x=0.17$ and the emergence of a novel frustrated two-dimensional order at $x=0.19$, with only a slight change in electronic properties. Single crystal diffuse x-ray scattering reveals that this transition from the 3D M1 phase to a 2D variant of the M2 phase results in long-range structural correlations along symmetry-equivalent (11L) planes of the tetragonal rutile structure, yet extremely short-range correlations transverse to these planes. These findings suggest that this two-dimensionality results from a novel form of geometric frustration that is essentially structural in origin.

cond-mat.str-el↗

arXiv2024

Bragg glass signatures in Pd$_x$ErTe$_3$ with X-ray diffraction Temperature Clustering (X-TEC)

The Bragg glass phase is a nearly perfect crystal with glassy features predicted to occur in vortex lattices and charge density wave systems in the presence of disorder. Detecting it has been challenging despite its sharp theoretical definition in terms of diverging correlation lengths. Here, we present evidence supporting a Bragg glass phase in the systematically disordered charge density wave material PdxErTe3. We do this using comprehensive x-ray data and a machine learning analysis tool called X-ray temperature clustering, or X-TEC. We establish a diverging correlation length in samples with moderate intercalation over a wide temperature range. To enable this analysis, we introduced a high-throughput measure of inverse correlation length that we call peak spread. The detection of Bragg glass order and the resulting phase diagram advance our understanding of the complex interplay between disorder and fluctuations significantly. Moreover, the use of our analysis technique to target fluctuations through a high-throughput measure of peak spread can revolutionize the study of fluctuations in scattering experiments.

cond-mat.str-el↗

arXiv2023

Frustration-Mediated Crossover from Long-Range to Short-Range Magnetic Ordering in $\mathbf{Y_{1-x}Lu_xBaCo_4O_7} $

We present a comprehensive magnetic and structural phase diagram for $\mathrm{Y_{1-x}Lu_xBaCo_4O_7}$, established through neutron diffraction and magnetization measurements. Our results outline the evolution of various nuclear structures, encompassing both orthorhombic and monoclinic symmetries, in response to changes in composition and temperature. The phase transition temperature of the orthorhombic phase, $T_{s1}$, decreases with increasing Lu content from 310 K for x = 0.0 to 110 K for x = 1.0. In Lu-rich compositions (0.7 \le x \le 1.0), first-order structural transitions are observed with coexisting and competing orthorhombic Pbn$2_1$ and metastable monoclinic Cc phases. Composition- and temperature-dependent refinements of the magnetic structure reveal an antiferromagnetic arrangement of Co spin pairs, below the magnetic transition temperatures of the Y-rich compositions, in the ab plane within both the triangular and Kagomé layers. A gradual suppression of long-range magnetic order is observed with increasing the Lu content, accompanied by the development of short-range magnetic correlations present in all the samples.

cond-mat.str-el↗

arXiv2023

Multiple lattice instabilities and complex ground state in Cs$_2$AgBiBr$_6$

Metal halides perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with Cs$_2$AgBiBr$_6$ one of the main contenders among lead-free systems. Cs$_2$AgBiBr$_6$ crystallizes in a nominally double-perovskite structure, but exhibits a soft lattice with large atomic fluctuations characteristic of MHPs. While crucial to understand electron-phonon and phonon-phonon couplings, the spatio-temporal correlations of these fluctuations remain largely unknown. Here, we reveal these correlations using comprehensive neutron and x-ray scattering measurements on Cs$_2$AgBiBr$_6$ single-crystals, complemented with first-principles simulations augmented with machine-learned neural-network potentials. We report the discovery of an unexpected complex modulated ground state structure containing several hundred atoms, arising from a soft-phonon instability of the low-temperature tetragonal phase. Further, our experiments and simulations both reveal extensive correlated 2D fluctuations of Br octahedra at finite temperature, arising from soft anharmonic optic phonons, reflecting very shallow potential wells. These results provide new insights into the atomic structure and fluctuations in MHPs, critical to understand and control their thermal and optoelectronic properties.

cond-mat.mtrl-sci↗