arXiv subjects

Pascal Puphal

Publications and source records attributed to Pascal Puphal.

At least 19 recordsLinked to original sources

arXiv2026

Ab Initio Investigation of Pressure Effects in the Spin-Liquid Candidate Y-Kapellasite

Motivated by recent experiments showing pressure-induced suppression of magnetic order and the emergence of a dynamical ground state in the anisotropic kagome antiferromagnet Y-kapellasite Y3Cu9(OH)19Cl8, we perform ab initio density functional theory (DFT)calculations to investigate the evolution of magnetic exchange interactions under hydrostatic pressure. We show that pressure efficiently tunes the magnetic Hamiltonian by altering the CuOCu bond geometry, thereby driving the system towards a spin-liquid regime. This evolution is governed by a nonlinear dependence of the dominant exchange coupling on the CuOCu bond angle. We further examine the influence of hydrogen positions and find that both the OH bond length and the hydrogen out-of-plane angle strongly affect the magnetic interactions. Our results provide a microscopic explanation for the experimentally observed pressure-induced enhancement of frustration and highlight the key role of hydrogen geometry.

cond-mat.str-el↗

arXiv2026

Unveiling orbital landscapes in strongly correlated bulk nickelates with $s$-NIXS

We leverage $s$-orbital non-resonant inelastic X-ray scattering ($s$-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3$d$ electron filling) from Ni$^{3+}$ (3$d^7$) to Ni$^{1+}$ (3$d^9$). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-spin orbital configuration of trivalent LaNiO$_3$, the $d_{x^2-y^2}$ configuration of monovalent LaNiO$_2$, and resolve the effective $e_g$ crystal field splitting in the distorted octahedral environment of divalent La$_2$NiO$_4$. This work illustrates the potential of $s$-NIXS to study the ground state and excited states of strongly correlated materials without needing complex theoretical analysis of spectroscopic data.

cond-mat.str-el↗

arXiv2026

Imaging stripe dynamics in trilayer nickelate La$_4$Ni$_3$O$_{10}$

Since the discovery of high-temperature superconductivity in nickelate superconductors, it is an open question how closely the superconducting state resembles that of cuprate superconductors. One salient feature of the phase diagram of the high-temperature cuprate superconductors is stripe order. Despite their prevalence, real-space imaging has been limited to the charge sector. Here we use spin-polarised scanning tunnelling microscopy to visualize the local magnetic and charge distribution emerging due to a stripe order in the trilayer nickelate La$_4$Ni$_3$O$_{10}$. The stripe order exhibits a four unit cell periodicity, closely resembling that seen in cuprates, and opens a near-complete $\sim66\mathrm{meV}$ gap at the Fermi level. Crucially, discrete phase slips can be triggered by tunneling electrons above a $\sim 20\mathrm{meV}$ threshold, allowing imaging of stripe dynamics at the atomic scale. These results highlight the importance of correlation physics driving stripe-like orders in lanthanum nickelates with striking similarities to the cuprates.

cond-mat.str-el↗

arXiv2026

Gapped 1/9 Magnetization Plateau in the Anisotropic Kagome Antiferromagnet Y-kapellasite

Fractional magnetization plateaus provide a sensitive probe of many-body spin states in frustrated quantum magnets, yet their microscopic origin in kagome antiferromagnets remains unresolved. This is particularly true of the mysterious $1/9$ plateau, which is predicted by theory but infrequently observed in experiment. Here, we investigate this problem in the $S = 1/2$ anisotropic kagome antiferromagnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, using pulsed-field magnetization measurements on single crystals and high-field $^{35}$Cl NMR. We identify a hierarchy of field-induced fractional features, including $1/3$ and $1/9$ plateaus, as well as a weaker low-field feature. Analysis of the NMR spectra and the magnetic susceptibility across the $1/9$ plateau demonstrate that it is accompanied by an ordered local spin configuration, a strong suppression of low-energy spin fluctuations and activated behavior, consistent with a gapped fractional state. These features differ from those in the only other material YCu$_3$(OH)$_6$Br$_2$[Br$_{1-y}$(OH)$_y$] in which this plateau is observed, implying a surprising robustness of the $1/9$ state to the details of the underlying magnetism.

cond-mat.str-el↗

arXiv2026

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

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

cond-mat.str-el↗

arXiv2026

Topochemical Fluorination of La$_2$NiO$_{4+\delta}$ Single Crystals

Topochemical fluorination offers a low--temperature route for modifying the anion chemistry and electronic ground states of layered transition-metal oxides, providing access to metastable phases and functionalities that are not able to be achieved through conventional solid--state synthesis. Despite extensive work on polycrystalline samples and thin films, topochemical fluorination of bulk single crystals has not been studied, limiting insights into intrinsic structure property relationships. Here, we investigate the topochemical fluorination of optical float zone grown (OFZ) La$_2$NiO$_{4+\delta}$ single crystals using polymer-based PTFE, PVDF and inorganic CuF$_{2}$ fluorination agents and compare it to our topochemical pathways of reduction of LaNiO$_{3-x}$. By systematically investigating direct and indirect contact reaction pathways, we can understand fluorination mechanisms, quantify the degree of fluorine incorporation, and evaluate the resulting structural and magnetic modifications in a detail that was not possible in powder and thin films. Powder and single--crystal X-ray diffraction reveal that fluorination proceeds without destroying the Ruddlesden--Popper framework, while inducing lattice parameter changes consistent with anion intercalation in the bulk and ion exchange on the surface. This even induces a clear superstructure, which was not reported before and extends the understanding of anion insertion reactions beyond what is known on stage ordering in nickelates. Energy-dispersive X--ray spectroscopy confirms strong fluorine incorporation on the surface and reduced homogeneity in the bulk. Magnetic susceptibility measurements demonstrate a change in antiferromagnetic ordering upon fluorination.

cond-mat.str-el↗

arXiv2026

Anisotropic magnetoelastic coupling in the honeycomb magnet Na$_3$Co$_2$SbO$_6$

We present magnetization and dilatometry measurements on the honeycomb cobaltate Na$_3$Co$_2$SbO$_6$ and map out its detailed field-temperature phase diagram down to sub-Kelvin temperatures. Our data for in-plane magnetic fields show a strongly anisotropic $c^{*}$-axis lattice response, which is dominated by the variation of Co--O--Co bond angles according to \textit{ab initio} calculations. At $T = 0.4$~K, the magnetization $M(B)$ exhibits step-like features that are also highly anisotropic. In the case of $B \parallel b$, a small hysteresis observed around the second field-induced magnetic transition ($B_{c2}$) indicates its first-order character, whereas divergence of the magnetic Gr\"uneisen parameter at $B_{c2}$ is suppressed upon cooling and signals the absence of quantum critical behavior upon entering the field-polarized state. None of our thermodynamic measurements provide evidence for a field-induced quantum spin liquid state near or above $B_{c2}$.

cond-mat.str-el↗

arXiv2025

Magnetism of the alternating monolayer-trilayer phase of La$_3$Ni$_2$O$_7$

Understanding the magnetic ground state of Ruddlesden-Popper nickelates is crucial, as these materials exhibit superconductivity under high pressure and host competing electronic orders that may play a key role in the pairing mechanism. In this work, we investigate the magnetic properties of the alternating monolayer-trilayer phase of La$_3$Ni$_2$O$_7$ (1313-La$_3$Ni$_2$O$_7$) using muon-spin rotation/relaxation ($\mu$SR) under both ambient and hydrostatic pressure conditions. The monolayer-trilayer phase develops incommensurate magnetic order below approximately 150 K, with a mean ordering temperature of $T_{SDW} \simeq 123$ K and a transition width of $\Delta T_{SDW} \simeq 15$ K. The abrupt onset of the internal magnetic field indicates a first-order-like transition. Hydrostatic pressure ($p$) suppresses the magnetic ordering temperature at a rate of $dT_{SDW}/d p \simeq -3.9$ K/GPa, demonstrating a progressive destabilization of the ordered state. By comparison with the bilayer 2222-La$_3$Ni$_2$O$_7$ and the trilayer 3333-La$_4$Ni$_3$O$_{10}$ systems, and within a unified phenomenological framework, systematic trends are identified linking the pressure dependence of $T_{SDW}$, the (in)commensurability of the magnetic order, and the character of the magnetic transition. These trends consistently indicate a gradual reduction of electronic correlation strength from the bilayer to the monolayer-trilayer and trilayer nickelates. This hierarchy suggests that the higher superconducting transition temperature observed in the 2222 phase may be closely connected to its more strongly correlated electronic nature. These results position the alternating monolayer-trilayer 1313-La$_3$Ni$_2$O$_7$ as an intermediate member linking the magnetic behavior of the bilayer 2222-La$_3$Ni$_2$O$_7$ and the trilayer 3333-La$_4$Ni$_3$O$_{10}$ Ruddlesden-Popper compounds.

cond-mat.supr-con↗

arXiv2025

Comparative Raman study of Ruddlesden-Popper nickelates and the monolayer-trilayer polymorph

Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including bilayer (BL) La$_3$Ni$_2$O$_7$, trilayer (TL) La$_4$Ni$_3$O$_{10}$, and structural polymorphs composed of monolayer-bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL lattice architecture.

cond-mat.str-el↗

arXiv2025

Terahertz light driven coherent excitation of a zone-folded Raman-active phonon mode in the Spin-Ladder System $\alpha'$-NaV$_2$O$_5$

We investigate the out-of-equilibrium lattice dynamics in the spin-ladder system $\alpha'$-NaV$_2$O$_5$ using intense terahertz (THz) pump and near-infrared (NIR) probe spectroscopy. When quasi-single-cycle THz pulses interact with $\alpha'$-NaV$_2$O$_5$ in its low-temperature, dimerized charge-ordered phase, they induce coherent oscillations in the time domain at the zone-folded Raman-active phonon frequency of 1.85 THz. By combining pump-probe measurements with lattice dynamics modeling based on equation-of-motion approach, we propose that these oscillations arise from a nonlinear coupling between Raman-active and infrared (IR)-active phonon modes, with the latter being resonantly excited by the THz pulses. In contrast, excitation with NIR femtosecond laser pulses does not produce measurable vibrational dynamics, highlighting the unique potential of THz-driven, nonlinear light-matter interactions for the coherent and selective control of structural dynamics in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Spinon mediation of witness spin dynamics in herbertsmithite

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such 'impurity' atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

cond-mat.str-el↗

arXiv2025

Lattice dynamics of the infinite-layer nickelate LaNiO$_2$

Infinite-layer (IL) nickelates have rapidly emerged as a new class of superconductors. However, due to the technical challenges of their topotactic synthesis, they have so far been realized primarily as thin films or polycrystalline powder samples, limiting comprehensive investigations of fundamental physical properties such as the lattice dynamics. Here, we present a time-of-flight inelastic neutron scattering study on a sample composed of a large number of co-aligned bulk crystals of the IL nickelate LaNiO$_2$. We observe several dispersive phonon branches, which are in good agreement with lattice dynamical calculations based on density-functional perturbation theory. In addition, we compare the characteristics of selected LaNiO$_2$ phonon modes to those of isostructural cuprate superconductors. Our findings provide a reference point for future experimental and theoretical efforts aimed at understanding the interplay between lattice dynamics and electronic properties in IL nickelates.

cond-mat.str-el↗

arXiv2025

Exploring Low-Dimensional Magnetism in Cobalt Vanadates, ${A}$CoV$_{2}$O$_{7}$~(${A}$~=~Ca, Sr) : Crystal Growth and Magnetic Properties of Effective Spin-1/2 Zigzag Chains

We report the successful growth of high-quality single crystals of \ACVO, a quasi-one-dimensional zigzag chain compound containing Co$^{2+}$ ions, using the optical floating zone method. The crystal growth was stabilized under high-pressure argon-oxygen gas with slow growth rates, overcoming challenges associated with the incongruent melting behavior of this material. X-ray diffraction confirms the zigzag arrangement of Co$^{2+}$ ions, forming a quasi-one-dimensional chain structure. Magnetic susceptibility and heat capacity measurements reveal an antiferromagnetic phase transition at the N\'eel temperature ($T_{\text{N}} \sim 3.5$ K) and negative Curie-Weiss temperatures, indicative of dominant antiferromagnetic interactions. The distorted CoO$_6$ octahedral geometry and strong spin-orbit coupling suggest that Co$^{2+}$ ions likely exhibit an effective $J = 1/2 $ Kramers doublet state. The results presented here demonstrate the potential of \ACVO\ as a platform for investigating low-dimensional magnetism and quantum magnetic phenomena. These insights shed light on the role of the ${A}$-site ion in tuning the magnetic interactions, which will foster future research into the field-induced behavior in these cobalt vanadates.

cond-mat.str-el↗

arXiv2025

Direct Evidence of Metal-Ligand Redox in Li-ion Battery Positive Electrodes

Describing Li-ion battery positive electrodes in terms of distinct transition metal or oxygen redox regimes can lead to confusion in understanding metal-ligand hybridisation, oxygen dimerisation, and degradation. There is a pressing need to study the electronic structure of these materials and determine the role each cation and anion plays in charge compensation. Here, we employ transition metal L-edge X-ray Resonance Photoemission Spectroscopy in conjunction with Single Impurity Anderson models, Self-consistent Real Space Multiple Scattering spectral simulations, and Dynamical Mean-Field theory calculations to directly evaluate the redox mechanisms in (de-)lithiated battery electrodes. This approach reconciles the redox description of two canonical cathodes -- LiMn$_{0.6}$Fe$_{0.4}$PO$_{4}$ and LiNiO$_{2}$ -- in terms of varying degrees of charge transfer using the established Zaanen-Sawatzky-Allen framework, common to condensed matter physics. In LiMn$_{0.6}$Fe$_{0.4}$PO$_{4}$, the absence of charge transfer means capacity arises due to the depopulation of metal $\textit{3d}$ states, i.e. conventional metal redox. Whereas, in LiNiO$_{2}$, charge transfer dominates and redox occurs through the formation and elimination of ligand hole states. This work clarifies the role of oxygen in Ni-rich system and provides a framework to explain how capacity can be extracted from oxygen-dominated states in highly covalent systems without needing to invoke dimerisation.

cond-mat.mtrl-sci↗

arXiv2025

Exerting chemical pressure on the kagome lattice as frustration control in the kapellasite family $A$Cu$_3$(OH)$_{6+x}$(Cl,Br)$_{3-x}$

The kapellasite family $A$Cu$_3$(OH)$_{6+x}$(Cl,Br)$_{3-x}$ forms a series of compounds, wherein the chemical pressure realized by the $A-$site cation tunes the spin exchange in the frustrated distorted kagome lattice. Via hydrothermal synthesis we have grown single crystals of the whole series for $A=$ rare-earths and found a clear structural transition to a superstructure variant at a specific chemical pressure level exerted by $A$~=~Dy. Phases with crystal structures in the vicinity of this superstructure transition realize a distorted kagome lattice of the Cu$^{2+}$ cations with characteristic features of a spin liquid. Here, subtle structural disorder as well as controlled chemical pressure can stabilize the spin liquid phase. We study the crystal structure and the magnetic ground state of these phases via single crystal x-ray diffraction, magnetic susceptibility, specific heat and muon spin spectroscopy measurements.

cond-mat.str-el↗

arXiv2025

Anatomy of anomalous Hall effect due to magnetic fluctuations

The anomalous Hall {\color{black} e}ffect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic field dependence of the Hall response. We show that discerning whether the AHE is an intrinsic property of the field free system becomes intricate in the presence of strong magnetic fluctuations. {\color{black}As a study case,} we use the Weyl semimetal PrAlGe, where TRSB can be toggled via a ferromagnetic transition, providing a transparent view of the AHE's topological origin. Through a combination of thermodynamic, transport and muon spin relaxation measurements, we contrast the behaviour below the ferromagnetic transition temperature to that of strong magnetic fluctuations above. Our results {\color{black}on PrAlGe provide general insights into the} interpretation of anomalous Hall signals in systems where TRSB is debated, such as families of Kagome metals or certain transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2025

Emergence of a Fluctuating Ground State in Y-kapellasite under Pressure

Y-kapellasite (Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$), which hosts an original anisotropic kagome sublattice, is a promising candidate for studying elusive and complex correlated physics. It exhibits a theoretically predicted in-plane $(1/3, 1/3)$ magnetic order [1] but its magnetic interaction values place it close to a phase boundary to a spin liquid state [2]. Our $\mu$SR measurements under hydrostatic pressure demonstrate the complete suppression of static magnetism in favor of a fully dynamical ground state at $2.3$~GPa. Complementary high-pressure x-ray and optical phonon measurements reveal a gradual reduction of the kagome anisotropy, enhancing magnetic frustration without structural transitions. Our results establish Y-kapellasite as a rare clean kagome model in which long-range order is suppressed by pressure-tuned frustration, the first fingerprint for the realization of a quantum spin liquid without strong disorder.

cond-mat.str-el↗

arXiv2024

Chemo-Structural Disorder in the kagom\'e spin $S$ = 1/2 systems ZnCu$_3$(OH)$_6$Cl$_2$ and YCu$_3$(OH)$_{6}$Br$_{2}$[Br$_x$(OH)$_{1-x}$]

By single crystal diffraction we characterize the chemo-structural disorder introduced by Zn-Cu site mixing in the kagom\'e spin $S$-1/2 systems herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$ and YCu$_3$(OH)$_{6}$Br$_{2}$[Br$_x$(OH)$_{1-x}$]. For an untwinned single crystal of herbertsmithite of composition Zn$_{0.95(1)}$Cu$_{2.99(3)}$O$_{5.9(1)}$H$_{5.8(1)}$Cl$_2$ we find substitution by Cu of the Zn atoms in the layers separating the kagom\'e layers as well as substantial Zn substitution for Cu in the kagom\'e layers. In YCu$_3$(OH)$_{6}$Br$_{2}$[Br$_x$(OH)$_{1-x}$] site mixing disorder is present for intermediate $x$. Analogous to the Cl homologous system in crystals with $x = 1/3$ disorder is absent and a low-temperature structural transition emerges driven by strong magneto-phonon coupling as a release of frustration. Apart from this structural anomaly we find the physical properties of these crystals unchanged compared to intermediate $x$ and closely resembling the Cl homologue where long-range magnetic order was observed.

cond-mat.str-el↗