arXiv subjects

S. Petit

Publications and source records attributed to S. Petit.

At least 19 recordsLinked to original sources

arXiv2026

Signature of spin liquid state in a frustrated 3D antiferromagnet

Frustrated pyrochlore lattices in transition-metal oxides provide an ideal platform for realizing exotic quantum states, including spin liquids with unconventional low-energy excitations arising from the macroscopic ground-state degeneracy of corner-sharing tetrahedral networks. Here, we report the synthesis and comprehensive characterization of ZnCrGaO$4$, a frustrated three-dimensional pyrochlore-like magnet in which intrinsic cation ordering gives rise to unavoidable atomic-site disorder. A Curie--Weiss analysis of the high-temperature magnetic susceptibility yields a large negative Curie--Weiss temperature, $\theta{\mathrm{CW}} \approx -205$ K, indicating dominant antiferromagnetic exchange interactions ($J/k_{\mathrm{B}} \sim 55$ K) between Cr$^{3+}$ ($S = 3/2$) moments. Despite the presence of strong antiferromagnetic interactions, no signature of long-range magnetic ordering is observed down to 125 mK, as evidenced by specific-heat and ac-susceptibility measurements. Furthermore, the absence of bifurcation between zero-field-cooled and field-cooled dc magnetic susceptibilities measured at 0.01 T indicates the absence of spin freezing, which is further supported by the frequency-independent ac susceptibility down to 250 mK. The presence of broad maxima in the magnetic specific heat and ac susceptibility at low temperatures suggests the development of short-range spin correlations within a dynamic magnetic state. In addition, the low-temperature specific heat follows a power-law behavior below 1 K, indicating the presence of unconventional low-energy excitations and algebraic spin correlations. These results provide compelling evidence for a dynamic correlated ground state in ZnCrGaO$_4$, establishing it as a promising platform for exploring highly frustrated $S > 1/2$ three-dimensional quantum magnets and potential spin-liquid behavior.

cond-mat.str-el↗

arXiv2026

Vortex order in magnetic frustrated GeNi$_2$O$_4$ and GeCo$_2$O$_4$ spinels

In the search for new spin textures based on singular magnetic objects like Bloch-points or vortices, spinel compounds emerge as an interesting playground due to the interplay between magnetic anisotropy and complex interactions that extend well beyond first neighbors on a pyrochlore lattice. Based on an exploration of the exchange interaction phase diagrams of members of the Ge$B_2$O$_4$ family with $B$=Co and Ni, we show, using simultaneous modeling of inelastic neutron scattering measurements and single-crystal neutron diffraction data, that a 2-$k$ magnetic structure may be stabilized in these compounds. This leads to a short period spin vortex crystal, a variant induced by the magnetic anisotropy of the 3-$k$ Bloch-point structure predicted for isotropic spins. Our study rationalizes the formation of these multi-$k$ spin textures in frustrated antiferromagnets, as well as their anisotropy-dependent evolution.

cond-mat.str-el↗

arXiv2026

Quantum spin liquid on a 3D bipartite lattice of spin trimers stabilized by enhanced effective anisotropy

Quantum spin liquids (QSLs) represent highly entangled states of matter in which frustration-induced quantum fluctuations suppress any symmetry-breaking phase transition down to absolute zero, giving rise to fractionalized excitations and emergent gauge fields. Theoretically, bond anisotropy can stabilize QSLs even on bipartite lattices, as exemplified by the Kitaev honeycomb model; however, no material has so far been established to realize such a state as its true ground state. Here we identify the three-dimensional spin-trimer magnet KBa$_3$Ca$_4$Cu$_3$V$_7$O$_{28}$ as a promising candidate for a bipartite quantum spin liquid persisting to the lowest temperatures. Strongly coupled Cu$^{2+}$ trimers form effective pseudospin-1/2 degrees of freedom upon cooling, which in turn constitute a three-dimensional bipartite network. Bulk thermodynamic measurements, neutron scattering, $\mu$SR, and NMR detect no spin freezing or symmetry-breaking phase transition down to 20 mK, but instead reveal a gapless dynamical ground state with algebraic spin autocorrelations. Complementary Monte Carlo and exact-diagonalization calculations show that this state is stabilized by a strong enhancement of effective anisotropy: a weak microscopic Cu-Cu exchange anisotropy of approximately 15 percent is generically amplified at the trimer level, producing effective pseudospin-pseudospin interaction anisotropies of 60 to 100 percent. Our results establish trimer-based networks as a promising platform for realizing anisotropy-stabilized quantum entangled states, even in three-dimensional bipartite systems with only weak microscopic anisotropy.

cond-mat.str-el↗

arXiv2026

Exotic magnetism and persistent short-range spin correlations in a frustrated honeycomb lattice antiferromagnet

Two-dimensional high-spin bipartite honeycomb networks, where anisotropy, competing exchange interactions, and spin fluctuations interplay, provide an alternative platform to test theoretical models that distinguish between classical and quantum magnetism in the context of emergent many-body phenomena and exotic excitations. Here, we report the crystal structure, magnetization, specific heat, and inelastic neutron scattering measurements of the $S = 5/2$ distorted honeycomb magnet $\mathrm{CaZn_2Fe(PO_4)_3}$. Magnetization measurements reveal dominant antiferromagnetic interactions between the $\mathrm{Fe^{3+}}$ ($S = 5/2$) moments. The development and field evolution of a dip in the magnetic susceptibility under an external magnetic field indicate an unconventional field-induced transition, further supported by anomalies observed in magnetization isotherms. Zero-field specific heat measurements show an antiferromagnetic transition at $T_N \approx 1.67 \mathrm{K}$, which evolves under applied magnetic field, suggesting stabilization of a field-induced spin-canted state. Thermodynamic measurements reveal short-range spin correlations above the transition temperature. Inelastic neutron scattering results further corroborate antiferromagnetic ordering, consistent with specific heat data. Spin-wave calculations indicate competing exchange interactions that introduce magnetic frustration, along with weak Ising-like anisotropy. The interplay of competing interactions and anisotropy gives rise to exotic field-induced behavior and places the system in close proximity to a mean-field tricritical point in the $J_2/J_1$--$J_3/J_1$ phase diagram, opening a route to unconventional states in high-spin frustrated honeycomb magnets.

cond-mat.str-el↗

arXiv2026

Proximate Spin Liquid Ground State Arising from Competing Stripy and 120$^{\circ}$ Spin Correlations in the Triangular Quantum Antiferromagnet ErMgGaO$_4$

ErMgGaO$_4$ is a quantum antiferromagnet wherein the pseudospin-1/2 degrees of freedom of Er$^{3+}$ decorate two-dimensional triangular planes separated by disordered non-magnetic bilayers of Mg$^{2+}$ and Ga$^{3+}$. Unlike its sister compound, YbMgGaO$_4$, our powder ErMgGaO$_4$ sample shows a clear spin glass transition near $T_g \sim 2.5$~K, about 1/6 of its Curie-Weiss temperature. We have carried out new inelastic neutron scattering measurements on these powder ErMgGaO$_4$ samples. At high energies, we observed crystalline electric field (CEF) transitions within the $J=15/2$ multiplet of Er$^{3+}$, but with the first excited CEF level sufficiently low in energy ($\sim$ 3~meV) so as to allow the possibility that virtual CEF transitions influence the exchange couplings. At E=0, we observe diffuse elastic scattering which is analysed using Warren lineshapes appropriate for two dimensional correlations. This reveals dominant 2D stripy correlations below $T_g$, coexisting with 2D 120$^\circ$-type correlations that persist above $T_g$. At low temperatures, the low energy inelastic component of the scattering shows a continuum with bandwidth of $\sim$ 0.8~meV. This dynamic magnetic spectral weight can be modeled at all $Q$, energies, and temperatures as the sum of high energy and low energy damped harmonic oscillators (DHO), with the high energy DHO defining the bandwidth of $\sim$ 0.8~meV. We use linear spin wave theory to model this inelastic scattering and to estimate its spin Hamiltonian parameters in terms of a $J_1-J_2-\Delta$ model on the triangular lattice. This gives a good description of the low lying spectral weight for ErMgGaO4, and allows us to place it on the theoretical $J_1-J_2-\Delta$ phase diagram with $\frac{J_1}{J_2}=0.13 \pm 0.03$ and $\Delta=0.4 \pm 0.1$, which is close to the expected quantum phase boundary between the spin liquid and the stripy ordered phases.

cond-mat.str-el↗

arXiv2026

About the origin of the magnetic ground state of Tb$_{2}$Ir$_{2}$O$_{7}$

Magnetic-rare-earth pyrochlore iridates exhibit a rich variety of unconventional phases, driven by the complex interactions within and between the rare-earth and the iridium sublattices. In this study, we investigate the peculiar magnetic state of Tb$_{2}$Ir$_{2}$O$_{7}$, where a component of the Tb$^{3+}$ moment orders perpendicular to its local Ising anisotropy axis. By means of neutron diffraction and inelastic neutron scattering down to dilution temperatures, complemented by specific heat measurements, we show that this intriguing magnetic state is fully established at 1.5 K and we characterize its excitation spectrum across a broad range of energies. Our calculations reveal that bilinear interactions between Tb$^{3+}$ ions subjected to the Ir molecular field capture several key features of the experiments, but need to be supplemented to fully reproduce the observed behavior.

cond-mat.str-el↗

arXiv2025

Spin liquid state in a three-dimensional pyrochlore-like frustrated magnet

The three-dimensional frustrated spin lattice in MgCrGaO4, where Cr3+ ions occupy a pyrochlore-like network, exemplifies a quantum magnet with competing interactions, macroscopic degeneracy, and exotic low-energy excitations. Using thermodynamic, electron spin resonance (ESR), muon spin relaxation (muSR), and inelastic neutron scattering (INS) techniques, we observe no magnetic order or spin freezing down to 57 mK, despite a sizable exchange interaction (J= 58 K) between Cr3+ (S=3/2) moments and inherent site disorder. Below the characteristic exchange energy scale, all experimental probes detect the emergence of antiferromagnetic short-range spin correlations, corroborated by magnetic diffuse scattering in the wave vector dependence of low-energy magnetic excitations centered on Q = 1.5 A^-1 in inelastic neutron scattering experiments. The low-temperature specific heat follows a near-quadratic dependence without a gap, consistent with algebraic spin correlations. These results establish MgCrGaO4 as a rare three-dimensional classical spin liquid featuring a highly degenerate ground-state manifold and gapless excitations, offering a strong impetus for the experimental realization of spin liquids in higher-dimensional frustrated quantum magnets.

cond-mat.str-el↗

arXiv2025

From hierarchical triangular spin liquid to multi-$q$ spin texture in spinel GeFe$_2$O$_4$

Combining macroscopic measurements, neutron scattering and modeling, we identify in the GeFe$_2$O$_4$ spinel a correlated paramagnetic state resulting from the predominance of third-neighbor antiferromagnetic interactions. These interactions materialize 4 isolated families of triangular planes with 120$^{\circ}$ spins emerging from the underlying pyrochlore lattice. At lower temperatures, a phase transition occurs from this hierarchical spin liquid to a non-coplanar spin texture that is characterized by 6 propagating vectors. This unusual multi-$q$ order is triggered by the presence of weaker interactions up to the sixth neighbors. The system is remarkably successful in coupling the different triangular planes while maintaining their two-dimensional 120$^{\circ}$ order. Our study highlights the hierarchy of interactions involved in GeFe$_2$O$_4$, which is singular among spinel compounds since first-neighbor interactions are only a small fraction of the dominant third neighbor ones.

cond-mat.str-el↗

arXiv2022

The role of damping rate amplitude in the synchronization of two coupled oscillators

We investigate the synchronization phenomenon between two Spin-transfer Torque Nano-Oscillators (STNOs) of different frequencies in two pillar systems under vortex configuration detunings or driving frequencies. The oscillators' mutual synchronization occurs through magnetic dipolar interaction. Our micromagnetic simulations show that an amplitude fluctuation referred to as $\Gamma_p$ has a significant impact on determining the synchronization frequency. The evolution of frequency and amplitude fluctuation rate in two different oscillator sizes versus external perpendicular field are compared and discussed. Our results reveal that the oscillator with lower $\Gamma_p$, referred to as the "Leader" oscillator, leads the synchronization process. As such, the "follower" oscillator adjusts its frequency as to that of the "Leader", thus achieving synchronization. We believe that taking $\Gamma_p$ into consideration can help in controlling synchronization frequencies in future building blocks of any network multi-array spintronics' devices.

cond-mat.other↗

arXiv2022

From spin liquid to magnetic ordering in the anisotropic kagome Y-Kapellasite Y3Cu9(OH)19Cl8: a single crystal study

Y3Cu9(OH)19Cl8 realizes an original anisotropic kagome model hosting a rich magnetic phase diagram [M. Hering et al, npj Computational Materials 8, 1 (2022)]. We present an improved synthesis of large phase-pure single crystals via an external gradient method. These crystals were investigated in details by susceptibility, specific heat, thermal expansion, neutron scattering and local muSR and NMR techniques. At variance with polycristalline samples, the study of single crystals gives evidence for subtle structural instabilities at 33K and 13K which preserve the global symmetry of the system and thus the magnetic model. At 2.1K the compound shows a magnetic transition to a coplanar (1/3,1/3) long range order as predicted theoretically. However our analysis of the spin wave excitations yields magnetic interactions which locate the compound closer to the phase boundary to a classical jammed spin liquid phase. Enhanced quantum fluctuations at this boundary may be responsible for the strongly reduced ordered moment of the Cu2+, estimated to be 0.075muB from muSR.

cond-mat.str-el↗

arXiv2022

ETpathfinder: a cryogenic testbed for interferometric gravitational-wave detectors

The third-generation of gravitational wave observatories, such as the Einstein Telescope (ET) and Cosmic Explorer (CE), aim for an improvement in sensitivity of at least a factor of ten over a wide frequency range compared to the current advanced detectors. In order to inform the design of the third-generation detectors and to develop and qualify their subsystems, dedicated test facilities are required. ETpathfinder prototype uses full interferometer configurations and aims to provide a high sensitivity facility in a similar environment as ET. Along with the interferometry at 1550 nm and silicon test masses, ETpathfinder will focus on cryogenic technologies, lasers and optics at 2090 nm and advanced quantum-noise reduction schemes. This paper analyses the underpinning noise contributions and combines them into full noise budgets of the two initially targeted configurations: 1) operating with 1550 nm laser light and at a temperature of 18 K and 2) operating at 2090 nm wavelength and a temperature of 123 K.

astro-ph.IM↗

arXiv2021

Spin dynamics and unconventional Coulomb phase in Nd$_2$Zr$_2$O$_7$

We investigate the temperature dependence of the spin dynamics in the pyrochlore magnet Nd$_2$Zr$_2$O$_7$ by neutron scattering experiments. At low temperature, this material undergoes a transition towards an "all in - all out" antiferromagnetic phase and the spin dynamics encompass a dispersion-less mode, characterized by a dynamical spin ice structure factor. Unexpectedly, this mode is found to survive above $T_{\rm N} \approx 300$ mK. Concomitantly, elastic correlations of the spin ice type develop. These are the signatures of a peculiar correlated paramagnetic phase which can be considered as a new example of Coulomb phase. Our observations near $T_{\rm N}$ do not reproduce the signatures expected for a Higgs transition, but show reminiscent features of the "all in - all out" order superimposed on a Coulomb phase.

cond-mat.str-el↗

arXiv2021

Signature of a randomness-driven spin-liquid state in a frustrated magnet

Collective behaviour of electrons, frustration induced quantum fluctuations and entanglement in quantum materials underlie some of the emergent quantum phenomena with exotic quasi-particle excitations that are highly relevant for technological applications. Herein, we present our thermodynamic and muon spin relaxation measurements, complemented by ab initio density functional theory and exact diagonalization results, on the recently synthesized frustrated antiferromagnet Li4CuTeO6, in which Cu2+ ions (S = 1/2) constitute disordered spin chains and ladders along the crystallographic [101] direction with weak random inter-chain couplings. Our thermodynamic experiments detect neither long-range magnetic ordering nor spin freezing down to 45 mK despite the presence of strong antiferromagnetic interaction between Cu2+ moments leading to a large effective Curie-Weiss temperature of -154 K. Muon spin relaxation results are consistent with thermodynamic results. The temperature and magnetic field scaling of magnetization and specific heat reveal a data collapse pointing towards the presence of random-singlets within a disorder-driven correlated and dynamic ground-state in this frustrated antiferromagnet.

cond-mat.str-el↗

arXiv2021

Field-temperature phase diagram of the enigmatic Nd$_2$(Zr$_{1-x}$Ti$_x$)$_2$O$_7$ pyrochlore magnets

By combining neutron scattering and magnetization measurements down to 80 mK, we determine the $(H,T)$ phase diagram of the Nd$_2$(Zr$_{1-x}$Ti$_x$)$_2$O$_7$ pyrochlore magnet compounds. In those samples, Zr is partially substituted by Ti, hence tuning the exchange parameters and testing the robustness of the various phases. In all samples, the ground state remains "all in / all out", while the field induces phase transitions towards new states characterized by "2 in - 2 out" or "1 out - 3 in / 1 in - 3 out" configurations. These transitions manifest as metamagnetic singularities in the magnetization vs field measurements. Strikingly, it is found that moderate substitution reinforces the stability of the "all in / all out" phase: the Néel temperature, the metamagnetic fields along with the ordered magnetic moment are higher in substituted samples with $x <$ 10\%.

cond-mat.str-el↗

arXiv2021

Kitaev interactions in the Co honeycomb antiferromagnets Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$

Co$^{2+}$ ions in an octahedral crystal field, stabilise a j$_{eff}$ = 1/2 ground state with an orbital degree of freedom and have been recently put forward for realising Kitaev interactions, a prediction we have tested by investigating spin dynamics in two cobalt honeycomb lattice compounds, Na$_2$Co$_2$TeO$_6$ and Na$_3$Co$_2$SbO$_6$, using inelastic neutron scattering. We used linear spin wave theory to show that the magnetic spectra can be reproduced with a spin Hamiltonian including a dominant Kitaev nearest-neighbour interaction, weaker Heisenberg interactions up to the third neighbour and bond-dependent off-diagonal exchange interactions. Beyond the Kitaev interaction that alone would induce a quantum spin liquid state, the presence of these additional couplings is responsible for the zigzag-type long-range magnetic ordering observed at low temperature in both compounds. These results provide evidence for the realization of Kitaev-type coupling in cobalt-based materials, despite hosting a weaker spin-orbit coupling than their 4d and 5d counterparts.

cond-mat.str-el↗

arXiv2021

From one- to two-magnon excitations in the S=3/2 magnet $β$-CaCr$_2$O$_4$

We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2 magnet $β$-CaCr$_2$O$_4$ (T$_N$=21 K). The low-energy fluctuations, in the ordered state, resemble large-S linear spin-waves from the incommensurate ground state. However, at higher energy transfers, these semi-classical and harmonic dynamics are replaced by an energy and momentum broadened continuum of excitations. Applying kinematic constraints required for energy and momentum conservation, sum rules of neutron scattering, and comparison against exact diagonalization calculations, we show that the dynamics at high-energy transfers resemble low-S one-dimensional quantum fluctuations. $β$-CaCr$_2$O$_4$ represents an example of a magnet at the border between classical Néel and quantum phases, displaying dual characteristics.

cond-mat.str-el↗

arXiv2020

High-order parametric generation of coherent XUV radiation

We observe a new regime of coherent XUV radiation generation in noble gases induced by femtosecond pulses at very high intensities. This XUV emission has both a reduced divergence and spectral width as compared to high-order harmonic generation (HHG). It is not emitted at a moderate intensity of the driving pulses where only high-order harmonics are generated. At high driving intensities, the additional XUV comb appears near all harmonic orders and even exceeds the HHG signal on the axis. The peaks are observed in several gases and their frequencies do not depend on the driving intensity or gas pressure. We analyze the divergence, spectral width and spectral shift of this XUV emission. We show that these specific features are well explained by high-order parametric generation (HPG) involving multiphoton absorption and combined emission of an idler THz radiation and an XUV beam with remarkably smooth spatial and spectral characteristics.

physics.optics↗

arXiv2020

Dimer Physics in the Frustrated Cairo Pentagonal Antiferromagnet Bi2Fe4O9

The research field of magnetic frustration is dominated by triangle-based lattices but exotic phenomena can also be observed in pentagonal networks. A peculiar noncollinear magnetic order is indeed known to be stabilized in Bi2Fe4O9 materializing a Cairo pentagonal lattice. We present the spin wave excitations in the magnetically ordered state, obtained by inelastic neutron scattering. They reveal an unconventional excited state related to local precession of pairs of spins. The magnetic excitations are then modeled to determine the superexchange interactions for which the frustration is indeed at the origin of the spin arrangement. This analysis unveils a hierarchy in the interactions, leading to a paramagnetic state (close to the Néel temperature) constituted of strongly coupled dimers separated by much less correlated spins. This produces two types of response to an applied magnetic field associated with the two nonequivalent Fe sites, as observed in the magnetization distributions obtained using polarized neutrons.

cond-mat.str-el↗