arXiv subjects

Matthieu Le Tacon

Publications and source records attributed to Matthieu Le Tacon.

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

Interfacial charge-transfer in 3d/5d complex oxide heterostructures

Interfacial charge transfer (ICT) provides a powerful route to engineer electronic phases in correlated oxide heterostructures, yet predictive design principles remain elusive. Here, we systematically investigate superlattices composed of the 5d spin-orbit coupled semimetal SrIrO3 and a series of correlated 3d perovskites (LaMnO3, LaFeO3, LaCoO3, and NdNiO3), thereby establishing a quantitative framework for ICT across 3d/5d interfaces. Combining element-specific x-ray absorption spectroscopy with spatially resolved electron energy loss spectroscopy, a homogeneous electron transfer from the 5d to the 3d layers is directly quantified, reaching up to 0.35 e per unit cell in the cobaltate superlattice. We show that the magnitude of ICT scales linearly with the difference in electronegativity between the transition-metal oxide layers, identifying electronegativity-driven band alignment as the dominant mechanism for ICT. Beyond interfacial doping, we find that strong 3d-5d hybridization induces a complete low-spin to high-spin conversion in the cobaltate layers, demonstrating interface-controlled spin-state engineering without chemical substitution. These results establish electronegativity mismatch as a predictive design parameter for correlated oxide interfaces and provide a materials platform for tailoring band filling, orbital hierarchy, and spin configurations in quantum oxide heterostructures, paving the way towards advanced oxide electronics and next-generation information technologies.

cond-mat.mtrl-sci↗

arXiv2026

Competing magnetic order in EuPd$_3$Si$_2$

Single crystals of EuPd$_3$Si$_2$ were grown using a high-temperature EuPd-flux method. The material was structurally and chemically characterized by single-crystal x-ray diffraction, powder x-ray diffraction, Laue method and energy-dispersive x-ray spectroscopy. The structural analysis confirmed the orthorhombic crystal structure (space group $Imma$) but revealed differences in the lattice parameters and bond distances in comparison to previous work by Sharma et al.. The composition is close to the ideal 1:3:2 stoichiometry with an occupation of 7 % of the Si sites by Pd. The heat capacity, electrical resistivity, and magnetic susceptibility show two magnetic transitions indicating magnetic ordering below $T_{\rm N1}= 61\,\rm K$ and a spin reorientation at $T_{\rm N2}= 40\,\rm K$. The orthorhombic material shows magnetic anisotropy with field applied along the three main symmetry axes, which is summarized in the temperature-field phase diagrams. The susceptibility data hint to an alignment of the magnetic moments along $[100]$ between $T_{\rm N1}$ and $T_{\rm N2}$. Below $T_{\rm N2}$ the magnetic structure changes to an arrangement with moments canted away from $[100]$. In contrast to published work by Sharma et al., the single crystals investigated in this study are suggested to show AFM order below $T_{\rm N1}$ instead of ferromagnetism that sets in at higher $T_{\rm C1}=78\,\rm K$ which might originate from certain differences in the structure, composition or defects that have an impact on the dominant coupling constants of the RKKY interaction.

cond-mat.str-el↗

arXiv2026

Orbital Ordering in the Charge Density Wave Phases of BaNi$_2$(As$_{1-x}$P$_x$)$_2$

We use resonant X-ray scattering at the nickel L$_{2,3}$ edges to investigate the interplay between orbital degrees of freedom and charge density waves (CDW) in the superconductor BaNi$_2$(As$_{1-x}$P$_x$)$_2$. Both the incommensurate and commensurate CDWs in this system exhibit strong resonant enhancement with distinct energy and polarization dependencies, indicative of orbital ordering. Azimuthal-angle-dependent measurements reveal a lowering of the local Ni site symmetry, consistent with monoclinic or lower point group symmetry. The scattering signatures of both CDWs are dominated by contributions from Ni $d_{xz,yz}$ orbitals, with similar orbital character despite their distinct wave vectors. These findings point to a shared orbital-driven formation mechanism and provide new insight into the symmetry breaking and orbital/nematic fluctuations in the high-temperature regime of the superconductor BaNi$_2$(As$_{1-x}$P$_x$)$_2$.

cond-mat.supr-con↗

arXiv2026

Enhanced Charge-Density-Wave Order and Suppressed Superconductivity in Intercalated Bulk $\mathrm{Nb}{\mathrm{Se}}_{2}$

The electronic ground states of transition-metal dichalcogenides are strongly shaped by reduced dimensionality, yet the properties of atomically thin layers remain difficult to probe due to their small size and environmental sensitivity. Here we demonstrate that controlled electrochemical intercalation of organic cations provides a robust bulk platform for accessing monolayer-like physics in NbSe$_2$. Intercalation of tetrapropylammonium and tetrabutylammonium expands the interlayer spacing by nearly a factor of two, electronically decoupling the NbSe$_2$ layers while simultaneously introducing well-defined charge doping. Using a combination of Raman spectroscopy, scanning tunneling microscopy, X-ray diffraction, and photoemission, we uncover a pronounced enhancement of the charge-density-wave transition temperature to $\sim 130$ K together with a strong suppression of superconductivity, reproducing the phase diagram observed in exfoliated monolayers. The enhanced charge-density-wave order and reduced $T_c$ arise from the combined effects of dimensionality reduction and electron injection, and are accompanied by distinct dip-hump anomalies in the tunneling spectra suggestive of collective mode excitations. Our results establish molecular intercalation as a powerful and scalable route for engineering competing orders in layered quantum materials.

cond-mat.supr-con↗

arXiv2025

Strain-induced structural change and nearly-commensurate diffuse scattering in the model high-temperature superconductor HgBa$_2$CuO$_{4+δ}$

We investigate the strain response of underdoped HgBa$_2$CuO$_{4+δ}$ (Hg1201), by synchrotron X-ray diffraction and corresponding simulations of thermal diffuse scattering. The compression in the crystallographic $a$ direction leads to relatively small expansion in the $b$ and $c$ directions, with Poisson ratios $ν_{ba}$=0.16 and $ν_{ca}$=0.11, respectively. However, the Cu-O distance in the $c$ direction exhibits a notable 0.9% increase at 1.1% $a$-axis compression. We further find strain-induced diffuse scattering which corresponds to a new type of two-dimensional charge correlation. Interestingly, this signal is insensitive to the onset of superconductivity and instead corresponds to a short-range, nearly commensurate modulation with a wave vector close to (0.5, 0, 0) and a correlation length of approximately four unit cells. It closely resembles the charge order theoretically predicted in the phase diagram of the spin-liquid model with resonating valence bonds on a square lattice.

cond-mat.supr-con↗

arXiv2025

Soft Mode Origin of Charge Ordering in Superconducting Kagome CsV$_3$Sb$_5$

Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV$_3$Sb$_5$ (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV$_3$Sb$_5$. Guided by structure factor analysis, we identify a soft phonon mode along the reciprocal M-L direction, with the strongest effect at the L point, where the elastic scattering intensity also grows most rapidly upon cooling. First-principles calculations incorporating lattice anharmonicity and electron-phonon coupling reproduce these observations and establish a soft-mode instability at the L point as the driving mechanism of CDW formation. Despite the weakly first-order character of the transition, our results unambiguously demonstrate that the CDW in CsV$_3$Sb$_5$ originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals.

cond-mat.supr-con↗

arXiv2025

Unveiling Excitonic Insulator Signatures in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$

The high-temperature phase of Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, a near-zero-gap semiconductor ($E_G$ = 0), is a promising candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator around $E_G$, we investigated Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, the more semi-metallic Ta$_\mathrm{2}$(Ni,Co)Se$_\mathrm{5}$, and semiconducting Ta$_\mathrm{2}$NiS$_\mathrm{5}$ using high-resolution single-crystal x-ray diffraction and near-edge x-ray absorption fine structure (NEXAFS). Our findings reveal a second-order structural phase transition from orthorhombic (space group: $Cmcm$) to monoclinic (space group: $C2/c$) in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and Ta$_\mathrm{2}$(Ni,Co)Se$_\mathrm{5}$, but no transition in Ta$_\mathrm{2}$NiS$_\mathrm{5}$ down to 2 K. This transition breaks two mirror symmetries, enabling and enhancing the hybridization of Ta, Ni, and Se atoms, shortening bond lengths, and strengthening orbital interactions. NEXAFS data confirm stronger hybridization, significant changes in excitonic binding energies, and a key alteration in orbital character, suggesting an excitonic insulating state in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.

cond-mat.mtrl-sci↗

arXiv2025

Charge density waves and soft phonon evolution in the superconductor BaNi$_2$(As$_{1-x}$P$_x$)$_2$

The superconductor BaNi$_2$As$_2$ exhibits a soft-phonon-driven, incommensurate charge density wave (I-CDW) which is accompanied by a small orthorhombic structural phase transition. Upon further cooling, BaNi$_2$As$_2$ undergoes a first-order structural transition to a triclinic phase in which a commensurate CDW (C-CDW) appears. The relationship and interplay between the I-CDW, C-CDW and structural phase transitions has remained elusive. To investigate this issue, we present a complementary study of thermal diffuse X-Ray scattering and inelastic X-Ray scattering for phosphorus substituted BaNi$_2$(As$_{1-x}$P$_x$)$_2$ $(x\lessapprox0.12)$ and down to 2.2 K. We show that most of the diffuse scattering signal can be well described by first-principles lattice dynamics calculations. Furthermore, we find that although phosphorus substitution rapidly suppresses the structural transition temperatures, the temperature dependence of the correlation length of the I-CDW fluctuations and the formation of Bragg-like superstructure peaks associated with long-range ordering of this order depends only weakly on the substitution level. Finally, we present the absence of signatures of the I-CDW to C-CDW or triclinic transition in the lattice dynamics, indicating that these instabilities are not (soft) phonon driven.

cond-mat.supr-con↗

arXiv2025

Intertwined Orders in a Quantum-Entangled Metal

Entanglement underpins quantum information processing and computing, yet its experimental quantification in complex, many-body condensed matter systems remains a considerable challenge. Here, we reveal a highly entangled electronic phase proximate to a quantum metal-insulator transition, identified by resonant inelastic x-ray scattering interferometry. This approach reveals that entanglement across atomic sites generates characteristic interference patterns, which our model accurately reproduces, enabling extraction of a full entanglement spectrum and resolution of the underlying quantum states. Our analysis of the pyrochlore iridate Nd2Ir2O7 demonstrates that the system undergoes pronounced quantum fluctuations in its spin, orbital and charge degrees of freedom, even in the presence of a long-range 'all-in-all-out' antiferromagnetic order. Importantly, the observed entanglement signatures facilitate the coexistence of multiple exotic symmetry-breaking orders. Complementary investigations using Raman spectroscopy corroborate the presence of these hidden orders and their emergent excitations. In particular, we observe a two-magnon-bound state below the lowest single-magnon excitation energy, which, together with split phonon modes, provides strong evidence for cubic symmetry-breaking orders of magnetic origin juxtaposed with the all-in-all-out order. Our work thus establishes a direct link between quantum entanglement and emergent unconventional orders, opening new avenues for investigating quantum materials.

cond-mat.str-el↗

arXiv2025

Robust Charge-Density Wave Correlations in Optimally-Doped YBa2Cu3Oy

Charge-density wave (CDW) order is a key property of high-Tc cuprates, but its boundaries in the phase diagram and potential connections to other phases remain controversial. We report nuclear magnetic resonance (NMR) measurements in the prototypical cuprate YBa2Cu3Oy demonstrating that short-range static CDW order remains robust at optimal doping (p=0.165), exhibiting a strength and temperature dependence in the normal state similar to those observed at p=0.11 in the underdoped regime. For an overdoped sample with p=0.184, we detect no static CDW down to T=Tc, though weak CDW order plausibly emerges below Tc. More broadly, we argue that both quenched disorder and competition with superconductivity influence the apparent boundary of the CDW phase, likely causing an underestimation of its intrinsic extent in doping. These findings challenge the view that the CDW phase boundary lies below p*=0.19, widely regarded as the critical doping where the pseudogap phase ends in YBa2Cu3Oy.

cond-mat.supr-con↗

arXiv2025

Robust spin-orbit coupling in semi-metallic SrIrO3 under hydrostatic pressure

The semi-metallic behavior of the perovskite iridate SrIrO3 shifts the end-member of the strongly spin-orbit (SO) coupled Ruddlesden-Popper series Srn+1IrnO3n+1 away from the Mott insulating regime and the half-filled pseudospin Jeff=1/2 ground state well-established in the layered iridates (n = 1 and 2). To investigate the robustness of the SO coupled ground state of SrIrO3, X-ray absorption spectroscopy was carried out at the Ir L2,3 edges under hydrostatic pressure up to 50 GPa at room temperature. The effective SO coupling was deduced from the branching ratio of the Ir L2 and L3 white lines. With increasing pressure, the branching ratio decreases, and the Ir L2,3 peak positions shift to higher energies. The number of 5d holes remains constant indicating that the spectral weight redistribution and peak shifts arise from orbital mixing between t2g and eg related states. The expectation value of the angular part of the SO operator decreases by about 15% at 50 GPa. This reduction, which is very similar to that observed in the layered iridates, is well explained by an increase of the octahedral crystal field due to the shortening of the Ir-O bond-length under compression. Consistent with theoretical predictions, the orbital mixing and decrease as the crystal field increases. However, the effective SO coupling remains robust against pressure and does not indicate a covalency-driven breakdown within the investigated pressure range.

cond-mat.str-el↗

arXiv2024

Evidence for reduced periodic lattice distortion within the Sb-terminated surface layer of the kagome metal CsV$_3$Sb$_5$

The discovery of the kagome metal CsV$_3$Sb$_5$ sparked broad interest, due to the coexistence of a charge density wave (CDW) phase and possible unconventional superconductivity in the material. In this study, we use low-energy electron diffraction (LEED) with a $μ$m-sized electron beam to explore the periodic lattice distortion at the antimony-terminated surface in the CDW phase. We recorded high-quality backscattering diffraction patterns in ultrahigh vacuum from multiple cleaved samples. Unexpectedly, we did not find superstructure reflexes at intensity levels predicted from dynamical LEED calculations for the reported $2 \times 2 \times 2$ bulk structure. Our results suggest that in CsV$_3$Sb$_5$ the periodic lattice distortion accompanying the CDW is less pronounced at Sb-terminated surfaces than in the bulk.

cond-mat.str-el↗

arXiv2024

Anomalous phonon Grüneisen parameters in semiconductor Ta$_2$NiS$_5$

Strain tuning is a powerful experimental method in probing correlated electron systems. Here we study the strain response of the lattice dynamics and electronic structure in semiconductor Ta$_2$NiS$_5$ by polarization-resolved Raman spectroscopy. We observe an increase of the size of the direct semiconducting band gap. Although the majority of the optical phonons show only marginal dependence to applied strain, the frequency of the two B$_{2g}$ phonon modes, which have quadrupolar symmetry and already anomalously soften on cooling under zero strain, increases significantly with tensile strain along the $a$ axis. The corresponding Grüneisen parameters are unusually large in magnitude and negative in sign. These effects are well captured by first-principles density functional theory calculations and indicate close proximity of Ta$_2$NiS$_5$ to a structural instability, similar to that encountered in excitonic insulator candidate Ta$_2$NiSe$_5$.

cond-mat.str-el↗

arXiv2024

Using strain to uncover the interplay between two- and three-dimensional charge density waves in high-temperature superconducting YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$

Uniaxial pressure provides an efficient approach to control the competition between charge density waves (CDWs) and superconductivity in underdoped YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$. It can enhance the correlation volume of ubiquitous short-range 2D CDW correlations, and induces a long-range 3D CDW otherwise only accessible at large magnetic fields. Here, we use x-ray diffraction to study the strain and doping evolution of these CDWs. No signatures of discommensurations nor pair density waves are observed in the investigated strain-temperature parameter space, but direct evidence for a form of competition between 2D and 3D CDWs is uncovered. We show that the interplay between the 3D CDW, the 2D CDWs and superconductivity is qualitatively well described by including strain effects in simulations of a nonlinear sigma model of competing superconducting and CDW orders. From a broader perspective, our results underscore the potential of strain tuning as a powerful tool for probing and manipulating competing orders in quantum materials.

cond-mat.supr-con↗

arXiv2024

The high-pressure phase diagram of BaNi$_2$As$_2$: unconventional charge-density-waves and structural phase transitions

Structural phase transitions accompanied by incommensurate and commensurate charge density wave (CDW) modulations of unconventional nature have been reported in BaNi$_2$As$_2$, a nonmagnetic cousin of the parent compound of Fe-based superconductors, BaFe$_2$As$_2$. The strong dependence of the structural and CDW transitions of BaNi$_2$As$_2$ on isoelectronic substitutions alongside original dynamical lattice effects suggests strong tunability of the electronic phase of the system through structural effects. Here, we present a comprehensive synchrotron x-ray diffraction and first-principles calculation study of the evolution of the crystal structure and lattice instabilities of BaNi$_2$As$_2$ as a function of temperature and hydrostatic pressure (up to 12 GPa). We report a cascade of pressure-induced structural phase transitions and electronic instabilities up to 10 GPa, above which all CDW superstructures disappear. We reveal that the stable high-pressure phase consists of planar Ni zigzag chains, from which the surrounding As atoms have been pushed away. This yields a strong reduction of the interlayer As-As distance (along the original c axis), akin to what is observed in the collapsed tetragonal structure of other pnictides, albeit here with a monoclinic structure. The discovery of polymorphs in the pressure-temperature phase diagram of BaNi$_2$As$_2$ emphasizes the importance of the relative Ni-Ni and Ni-As bond lengths in controlling the electronic ground state of this compound and increases our understanding of viable electronic phases under extreme conditions.

cond-mat.supr-con↗

arXiv2023

Origin of chirality in transition-metal dichalcogenides

Chirality is a ubiquitous phenomenon in which a symmetry between left- and right-handed objects is broken, examples in nature ranging from subatomic particles and molecules to living organisms. In particle physics, the weak force is responsible for the symmetry breaking and parity violation in beta decay, but in condensed matter systems interactions that lead to chirality remain poorly understood. Here, we unravel the mechanism of chiral charge density wave formation in the transition-metal dichalcogenide 1T-TiSe2. Using representation analysis, we show that charge density modulations and ionic displacements, which transform as a continuous scalar field and a vector field on a discrete lattice, respectively, follow different irreducible representations of the space group, despite the fact that they propagate with the same wave-vectors and are strongly coupled to each other. This charge-lattice symmetry frustration is resolved by further breaking of all symmetries not common to both sectors through induced lattice distortions, thus leading to chirality. Our theory is verified using Raman spectroscopy and inelastic x-ray scattering, which reveal that all but translation symmetries are broken at a level not resolved by state-of-the-art diffraction techniques.

cond-mat.str-el↗

arXiv2023

Giant non-volatile electric field control of proximity induced magnetism in the spin-orbit semimetal SrIrO3

With its potential for drastically reduced operation power of information processing devices, electric field control of magnetism has generated huge research interest. Recently, novel perspectives offered by the inherently large spin-orbit coupling of 5d transition metals have emerged. Here, we demonstrate non-volatile electrical control of the proximity induced magnetism in SrIrO3 based back-gated heterostructures. We report up to a 700 % variation of the anomalous Hall conductivity σ_AHE and Hall angle θ_AHE as function of the applied gate voltage Vg. In contrast, the Curie temperature TC = 100K and magnetic anisotropy of the system remain essentially unaffected by Vg indicating a robust ferromagnetic state in SrIrO3 which strongly hints to gating-induced changes of the anomalous Berry curvature. The electric-field induced ferroelectric-like state of SrTiO3 enables non-volatile switching behavior of σ_AHE and θ_AHE below 60 K. The large tunability of this system, opens new avenues towards efficient electric-field manipulation of magnetism.

cond-mat.mtrl-sci↗