Interplay Between Magnetic Frustration and Quantum Criticality in the Unconventional Ladder Antiferromagnet C9H18N2CuBr4

Quantum fluctuation in frustrated magnets and quantum criticality at the transition between different quantum phases of matter are two of the cornerstones in condensed matter physics. Here we demonstrate the nontrivial interplay between them in the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4. Employing the high-resolution neutron spectroscopy, we unambiguously identify a weakly first-order hydrostatic pressure-driven quantum phase transition, which arises from fluctuations enhanced by the frustrating interlayer coupling. An exotic pressure-induced quantum disordered state is evidenced by the broad spectral linewidth observed near the phase transition. Interestingly, we find that the gapped transverse excitations in the Neel-ordered phase at ambient pressure cannot be described by the conventional S=1 magnons, i.e., the spin wave quanta, associated with explicit symmetry breaking, and thus the three-dimensional magnetic order ought to emerge in an unconventional way. We further apply the quantum Fisher information to show the presence of bipartite entanglement at criticality at least up to 1.1 K in the same material.Comment: 10 pages and 6 figures. We call for theoretical understanding of the nontrivial interplay observed in this materia

Gradual pressure-induced enhancement of magnon excitations in CeCoSi

CeCoSi is an intermetallic antiferromagnet with a very unusual temperature-pressure phase diagram: at ambient pressure it orders below TN = 8.8 K, while application of hydrostatic pressure induces a new magnetically ordered phase with exceptionally high transition temperature of ∼40 K at 1.5 GPa. We studied the magnetic properties and the pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic neutron scattering (INS) and heat capacity measurements. At ambient pressure CeCoSi orders into a simple commensurate AFM structure with a reduced ordered moment of only mCe = 0.37(6)μB. Specific heat and low-energy INS indicate a significant gap in the low-energy magnon excitation spectrum in the antiferromagnetic phase, with the CEF excitations located above 10 meV. Hydrostatic pressure gradually shifts the energy of the magnon band towards higher energies and the temperature dependence of the magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the CEF excitations are also drastically modified under pressure.Fil: Nikitin, S. E.. Max Planck Institute for Chemical Physics of Solids; Alemania. Technische Universität Dresden; AlemaniaFil: Franco, Diego Gaspar. Max Planck Institute for Chemical Physics of Solids; Alemania. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Kwon, J.. Max Planck Institute for Chemical Physics of Solids; AlemaniaFil: Bewley, R.. Harwell Campus. STFC Rutherford Appleton Laboratory. ISIS Facility; Reino UnidoFil: Podlesnyak, Andrey. Oak Ridge National Laboratory. Neutron Scattering Division; Estados UnidosFil: Hoser, A.. Helmholtz-Zentrum Berlin für Materialien und Energie; AlemaniaFil: Koza, M. M.. Institute Laue Langevin; FranciaFil: Geibel, C.. Max Planck Institute for Chemical Physics of Solids; AlemaniaFil: Stockert, O.. Max Planck Institute for Chemical Physics of Solids; Alemani

Cobalt-Based Pyroxenes: A New Playground for Kitaev Physics and Ising Model Realization

Recent advances in the study of cobaltites have unveiled their potential as a promising platform for realizing Kitaev physics in honeycomb systems and the Ising model in weakly coupled chain materials. In this manuscript, we explore the magnetic properties of pyroxene SrCoGe$_2$O$_6$ using a combination of neutron scattering, {\it ab initio} methods, and linear spin-wave theory. Through careful examination of inelastic neutron scattering powder spectra, we propose a modified Kitaev model to accurately describe the twisted chains of edge-sharing octahedra surrounding Co$^{2+}$ ions. The extended Kitaev-Heisenberg model, including a significant anisotropic bond-dependent exchange term with $K/|J|=0.96$, is identified as the key descriptor of the magnetic interactions in SrCoGe$_2$O$_6$. Furthermore, our heat capacity measurements reveal an effect of an external magnetic field (approximately 13~T) which shifts the system from a fragile antiferromagnetic ordering with $T_{\mathrm{N}}=9$~K to a field-induced state. We argue that pyroxenes, particularly those modified by substituting Ge with Si and its less extended $p$ orbitals, emerge as a novel platform for the Kitaev model. This opens up possibilities for advancing our understanding of Kitaev physics.Comment: 11 + 2 pages, 7 + 3 figure

Pressure-induced unconventional quantum phase transition with fractionalization in the coupled ladder antiferromagnet C9H18N2CuBr4

We present a comprehensive study of the effect of hydrostatic pressure on the magnetic structure and spin dynamics in the spin-1/2 coupled ladder compound C$_9$H$_{18}$N$_2$CuBr$_4$. The applied pressure is demonstrated as a parameter to effectively tune the exchange interactions in the spin Hamiltonian without inducing a structural transition. The single-crystal heat capacity and neutron diffraction measurements reveal that the N$\rm \acute{e}$el ordered state breaks down at and above a critical pressure $P_{\rm c}$$\sim$1.0 GPa through a continuous quantum phase transition. The thorough analysis of the critical exponents indicates that such transition with a large anomalous exponent $\eta$ into a quantum-disordered state cannot be described by the classic Landau's paradigm. Using inelastic neutron scattering and quantum Monte Carlo methods, the high-pressure regime is proposed as a $Z_2$ quantum spin liquid phase in terms of characteristic fully gapped vison-like and fractionalized excitations in distinct scattering channels.Comment: 6 pages and 4 figures in the main text. The Supplementary Material is available upon the reques

Magnetic Excitations in Metalloporphyrins by Inelastic Neutron Scattering: Determination of Zero-Field Splittings in Iron, Manganese, and Chromium Complexes

Zero field splitting (ZFS) parameters of several nondeuterated metalloporphyrins [M­(TPP)­Cl] and [Mn­(TPP)] (H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are the following: <i>D</i> = 6.33(8) cm^–1 for [Fe­(TPP)­Cl], −2.24(3) cm^–1 for [Mn­(TPP)­Cl], 0.79(2) cm^–1 for [Mn­(TPP)], and |<i>D</i>|= 0.234(12) cm^–1 for [Cr­(TPP)­Cl]. The work shows that compounds with magnetic excitations below ∼30 cm^–1 could be determined using nondeuterated samples

Inelastic neutron scattering study of the anisotropic S=1spin chain [Ni(HF2)(3−Clpyridine)4]BF4

[Ni(HF$_2$)(3-Clpyridine)$_4$]BF$_4$ (NBCT) is a one-dimensional, $S = 1$ spin chain material that shows no magnetic neutron Bragg peaks down temperatures of 0.1 K. Previous work identified NBCT to be in the Haldane phase and near a quantum phase transition as a function of $D/J$ to the large-$D$ quantum paramagnet phase (QPM), where $D$ is the axial single-ion anisotropy and $J$ is the intrachain superexchange. Herein, inelastic neutron scattering results are presented on partially deuterated, $^{11}$B enriched NBCT polycrystalline samples in zero magnetic field and down to temperatures of 0.3 K. Comparison to density matrix renormalization group calculations yields $D/J = 1.51$ and a significant rhombic single-ion anisotropy $E$ ($E/D \approx 0.03$, $E/J \approx 0.05$). These $D$, $J$, and $E$ values place NBCT in the large-$D$ QPM phase but precipitously near a quantum phase transition to a long-range ordered phase

Magnetic Transitions in Iron Porphyrin Halides by Inelastic Neutron Scattering and Ab Initio Studies of Zero-Field Splittings

Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe­(TPP)­X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are <i>D</i> = 4.49(9) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­F], and <i>D</i> = 8.8(2) cm^–1, <i>E</i> = 0.1(2) cm^–1 and <i>D</i> = 13.4(6) cm^–1, <i>E</i> = 0.3(6) cm^–1 for monoclinic polycrystalline [Fe­(TPP)­Br] and [Fe­(TPP)­I], respectively. Along with our recent report of the ZFS value of <i>D</i> = 6.33(8) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe­(TPP)­X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting <i>D</i> of the ⁶A₁ ground state. <i>D</i> was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe­(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of <i>D</i> from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the <i>S</i> = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆^3– complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of <i>D</i> and decrease of the π- and σ-antibonding energies <i>e</i>_λ^X (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. <i>D</i> parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental <i>D</i> and spectroscopic transitions for high-spin Fe­(III) complexes are proposed

Probing spin waves in Co3O4 nanoparticles for magnonics applications

The magnetic properties of spinel nanoparticles can be controlled by synthesizing particles of a specific shape and size. The synthesized nanorods, nanodots and cubic nanoparticles have different crystal planes selectively exposed on the surface. The surface effects on the static magnetic properties are well documented, while their influence on spin waves dispersion is still being debated. Our ability to manipulate spin waves using surface and defect engineering in magnetic nanoparticles is the key to designing magnonic devices. We synthesized cubic and spherical nanoparticles of a classical antiferromagnetic material Co3O4 to study the shape and size effects on their static and dynamic magnetic proprieties. Using a combination of experimental methods, we probed the magnetic and crystal structures of our samples and directly measured spin wave dispersions using inelastic neutron scattering. We found a weak, but unquestionable, increase in exchange interactions for the cubic nanoparticles as compared to spherical nanoparticle and bulk powder reference samples. Interestingly, the exchange interactions in spherical nanoparticles have bulk-like properties, despite a ferromagnetic contribution from canted surface spins