The compound [Formula: see text] exhibits a charge order (CO) state at [Formula: see text] and [Formula: see text], which recalls the CO state with a decrease in the temperature of the superconducting transition, [Formula: see text], observed in all cuprates at this doping value. Local excitations of lattice and magnetic origins measured in the two-dimensional metallic state of [Formula: see text] reveal the existence of bipolarons of size 4a resulting from structural and antiferromagnetic pairings of hole-rich orbital polarons of size 2a. They are intertwined with hole-poor domains in a disordered state at [Formula: see text] which become ordered on a chessboard organized in a 3D-order state of ferromagnetically paired polarons at [Formula: see text]. Applied to the [Formula: see text] planes of the cuprates of the "214" family, this model produces stripes of bipolarons intertwined with stripes of antiferromagnetically arranged spins, hole-poor, both of size 4a, leading to a spin density wave with a wave vector [Formula: see text], a charge density wave with [Formula: see text], the Yamada laws [Formula: see text] and [Formula: see text] and a decrease of [Formula: see text] at x=1/8. This work invokes relevance of a bipolaronic origin of high [Formula: see text] superconductivity, in which bipolarons of size 4a can play a major role.

SrCu 2 (BO 3 ) 2 is a frustrated quantum magnet which realizes the Shastry–Sutherland model. The system exhibits a sequence of pressure-induced phases, including a long-range antiferromagnetically ordered state. We have performed inelastic neutron scattering experiments on a single crystal at 4.2 (2) GPa, in the antiferromagnetic phase. The observed dispersive spin excitations were modeled using linear-spin-wave theory to extract exchange parameters. Our results are consistent with previously reported parameters at higher pressures and confirm the existence of a Goldstone mode as well as underlining the importance of the interlayer coupling. These results complement previous high-pressure studies and provide further input for theoretical models of SrCu 2 (BO 3 ) 2 under extreme conditions.

We investigate lattice dynamics in LaCoO_3 using inelastic neutron and x-ray scattering over T = 2650 K, spanning the spin-state crossover at T_1≈ 100 K and the insulator–metal transition at T_2≈ 550 K. Comparison with quasi-harmonic ab-initio lattice-dynamical calculations helps reveal anomalous softening of a ≈ 10 meV oxygen phonon, confined to the temperature interval T_1≤ T ≤ T_2 and localized in momentum space at q_SSO = ( 1/2,1/2,1/2)_c. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co^3+ states in LaCoO_3, linking spin-state fluctuations to anomalous phonon renormalization.

SrCu2(BO3)2 is a frustrated quantum magnet which realizes the Shastry-Sutherland model. The system exhibits a sequence of pressure-induced phases, including a long-range antiferromagnetically ordered state. We have performed inelastic neutron scattering experiments on a single crystal at 4.2 (2) GPa, in the antiferromagnetic phase. The observed dispersive spin excitations were modeled using linear-spin-wave theory to extract exchange parameters. Our results are consistent with previously reported parameters at higher pressures and confirm the existence of a Goldstone mode as well as underlining the importance of the interlayer coupling. These results complement previous high-pressure studies and provide further input for theoretical models of SrCu2(BO3)2 under extreme conditions.

The compound La1-xSrxMnO3 exhibits a charge order (CO) state at x approximate to 1/8 and TT(co)which become ordered on a chessboard organized in a 3D-order state of ferromagnetically paired polarons at T

Altermagnetic order gives rise to momentum-dependent spin splitting of electronic and magnonic excitations even in the absence of a net magnetization. Here, we investigate the magnetic field dependence of the spin-wave spectrum of altermagnetic α-MnTe using inelastic neutron scattering and linear spin-wave theory. An in-plane magnetic field continuously reorients the Néel vector by overcoming the weak crystalline anisotropy, while remaining small compared with the dominant exchange scale. We find that this reorientation leaves the magnon energies and line widths essentially unchanged, but strongly modifies the measured spectral intensity through the transverse-momentum projection. Our results demonstrate a clear separation between the soft orientational degree of freedom of the antiferromagnetic order and the robust exchange-dominated chiral magnon spectrum. This combination establishes α-MnTe as a platform for reconfigurable magnon coupling, in which external fields tune how excitations interact with polarized probes without substantially altering their frequency or coherence.

Inelastic neutron scattering experiments with a large set of comounted Na2IrO3 crystals reveal the low-energy magnon dispersion in this candidate material for Kitaev physics. The magnon gap amounts to 1.7(1) meV and can be interpreted similarly to the sister compound alpha-RuCl3 to stem from the zone boundaries in the antiferromagnetic zigzag structure. The neutron experiments find no evidence for low-energy excitations with ferromagnetic character, which contrasts with the findings in alpha-RuCl3. Our results are consistent with a recently proposed microscopic model that involves an antiferromagnetic Heisenberg nearest-neighbor exchange in Na2IrO3 in contrast to the ferromagnetic one considered for alpha-RuCl3. Although the magnetic response shows the signatures of bond-directional anisotropy in both materials the different relative signs of Kitaev and Heisenberg interaction result in different deviations from the initial Kitaev model. Low-energy ferromagnetic fluctuations cannot be considered as a fingerprint of ferromagnetic Kitaev interaction.

Superconducting domes, ubiquitous across a variety of quantum materials, are often understood as a window in which pairing is favored, opened by the fluctuations of competing orders. Yet, the understanding of how such a window closes is missing. Here, we show that inelastic neutron scattering, by quantifying a length scale associated with the dipoles correlation, ℓ0, addresses this issue. We find that, within the experimental precision, the end of the superconducting dome coincides with the end of a highly polarizable state (in which ℓ0 is longer than the interatomic distance). Thus, the superconducting dome is driven by the competition between the increase in the density of states and the inevitable collapse of the quantum paraelectric phase. This is compatible with a crucial role played by the soft ferroelectric mode in driving superconductivity. Such a scenario may also be at work in other quantum paraelectric materials, either bulk or at interfaces.

Inelastic neutron scattering measurements on the hexagonal Zn67Mg33S semiconductor alloy reveal a bimodal pattern of the optical modes across the Brillouin zone, confirmed by first-principles simulations. Such modes are sensitive to the local fluctuations in the composition inherent to random Zn/Mg alloying, distinguishing homo from hetero environments of a given bond (1-bond/2-mode), as is formalized for cubic alloys by the percolation model. The latter model thus emerges as a generic framework for systematizing the optical modes of semiconductor alloys in various crystal structures.

Cubic pseudo-unary A1-xBx high-entropy metallic alloys and pseudo-binary A1-xBxC disordered semiconductor alloys set a benchmark to explore how physical properties are impacted by disorder. Through its diversity, the lattice dynamics offers a unique playground to assign the relevant length scales at which operate various kinds of disorders induced by alloying. (i) In high-entropy metallic alloys, the overdamping of the bond-collective (multi-bond→1-mode) acoustic modes at short wavelength originates from force-constant fluctuations. (ii) In semiconductor alloys, the lattice mismatch splits, at any wavelength, the bond-specific (1-bond→1-mode) optical modes in duos distinguishing "same" from "alien" environments, as explained by the percolation model. Zn1-xMgxS is ideal to test both univocal assignments. Its force-constant disorder is small, reducing the cause for overdamping of the acoustic modes. Its local strain is inverted, the lighter substituent being the larger one and forming the longer bond. This forecasts a dramatic inversion of the mode-duos. Further, its wurtzite structure enables (iii) to test whether/how the percolation model for the mode-duos transfers under lowering the crystal symmetry from cubic to hexagonal. The triple acoustic-(i)/optical-(ii-iii) test on Zn1-xMgxS, combining inelastic neutron scattering with first-principles simulations, is positive. This highlights a few key points behind the lattice dynamics of atomic alloys.

Superconducting domes, ubiquitous across a variety of quantum materials, are often understood as a window favorite for pairing opened by the fluctuations of competing orders. Yet, a quantitative understanding of how such a window closes is missing. Here, we show that inelastic neutron scattering, by quantifying a length scale associated with polar fluctuations, $\ell_0$, addresses this issue. We find that the superconducting dome of strontium titanate definitely ends when $\ell_0$ vanishes. Moreover, the product of $\ell_0$ and the Fermi wavevector peaks close to the maximum critical temperature. Thus, this superconducting dome stems from the competition between the increase of the density of states and the unavoidable collapse of the quantum paraelectric phase, both induced by doping. The successful quantitative account of both the peak and the end of the superconducting dome implies a central role in the pairing mechanism played by the soft ferro-electric mode and its hybridisation with the acoustic branch. Such a scenario may also be at work in other quantum paraelectric materials, either bulk or interfaces.

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of 580∘C using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group Pm−3n, Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a=5.510(5)Å at T=85 K. The hydrogen content determined by hot extraction was H/Ta=1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps—first, ΔH/Ta=0.2 was desorbed at around −70∘C, and then the rest of the hydrogen was desorbed between 100∘C and 390∘C. The A15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A15−TaH1.23(5) demonstrated that hydrogen atoms occupy the 24k and 16i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24k and an increase of the 16i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A15−TaH1.23(5) at 72, 135, 145, and 166meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24k and 16i sites, respectively. No superconductivity was found in A15−TaH1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K. Published by the American Physical Society 2024

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of 580 degrees C using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group Pm-3n, Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a = 5.510(5) & Aring; at T = 85 K. The hydrogen content determined by hot extraction was H/Ta = 1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps-first, AH/Ta = 0.2 was desorbed at around -70 degrees C, and then the rest of the hydrogen was desorbed between 100 degrees C and 390 degrees C. The A15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A15-TaH1.23(5) demonstrated that hydrogen atoms occupy the 24k and 16i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24k and an increase of the 16i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A15-TaH1.23(5) at 72, 135, 145, and 166 meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24k and 16i sites, respectively. No superconductivity was found in A15-TaH1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K.

Understanding spin and lattice excitations in a metallic magnetic ordered system forms the basis to unveil the magnetic and lattice exchange couplings and their interactions with itinerant electrons. Kagome lattice antiferromagnet FeGe is interesting because it displays a rare charge density wave (CDW) deep inside the antiferromagnetic ordered phase that interacts with the magnetic order. We use neutron scattering to study the evolution of spin and lattice excitations across the CDW transition T_{CDW} in FeGe. While spin excitations below ∼100  meV can be well described by spin waves of a spin-1 Heisenberg Hamiltonian, spin excitations at higher energies are centered around the Brillouin zone boundary and extend up to ∼180  meV consistent with quasiparticle excitations across spin-polarized electron-hole Fermi surfaces. Furthermore, c-axis spin wave dispersion and Fe-Ge optical phonon modes show a clear hardening below T_{CDW} due to spin-charge-lattice coupling but with no evidence of a phonon Kohn anomaly. By comparing our experimental results with density functional theory calculations in absolute units, we conclude that FeGe is a Hund's metal in the intermediate correlated regime where magnetism has contributions from both itinerant and localized electrons arising from spin polarized electronic bands near the Fermi level.

In metal-organic-framework (MOF) perovskites, both magnetic and ferroelectric orderings can be readily realized by compounding spin and charge degrees of freedom. The hydrogen bonds that bridge the magnetic framework and organic molecules have long been thought of as a key in generating multiferroic properties. However, the underlying physical mechanisms remain unclear. Here, we combine neutron diffraction, quasielastic and inelastic neutron scattering, and THz spectroscopy techniques to thoroughly investigate the dynamical properties of the multiferroic MOF candidate [CH$_3$NH$_3$][Co(HCOO)$_3$] through its multiple phase transitions. The wide range of energy resolutions reachable by these techniques enables us to scrutinize the coupling between the molecules and the framework throughout the phase transitions and interrogate a possible magnetoelectric coupling. Our results also reveal a structural change around 220 K which may be associated with the activation of a nodding donkey mode of the methylammonium molecule due to the ordering of the CH$_3$ groups. Upon the occurrence of the modulated phase transition around 130 K, the methylammonium molecules undergo a freezing of its reorientational motions which is concomitant with a change of the lattice parameters and anomalies of collective lattice vibrations. No significant change has been however observed in the lattice dynamics around the magnetic ordering, which therefore indicates the absence of a substantial magneto-electric coupling in zero-field.

Cubic yttria-stabilized zirconia has long been a ceramic material of interest for its many uses in thermal-based applications. Its very low and weakly temperature-dependent thermal conductivity has been ascribed to the large oxygen vacancies content, which introduces disorder and strongly scatters phonons. Still, despite many experimental works in the literature, phonon dynamics has not been fully understood yet, with several points to be clarified, such as the apparent absence of optic modes throughout the Brillouin zone. In this paper, we present findings on the phonon dispersions of this material, showing experimental evidence of low-lying optical branches throughout the Brillouin zone, which reduce the pure acoustic regime for some branches. Furthermore, the observed energy dependence of the intrinsic acoustic phonon linewidths clearly suggests the existence of competing Mie and Rayleigh scattering mechanisms. Our findings allow to uncover a different phonon dynamics scenario in this material and point to a deeper understanding of heat transport in yttria-stabilized zirconia, based on two different, concomitant mechanisms, generated by the large vacancy content.

There are few inelastic neutron scattering (INS) reports on the superconducting single crystals of FeAs-1111 system, even though it was first discovered in 2008, due to the extreme difficulty in large single crystal growth. In this paper, we have studied the low-energy spin excitations in the optimally electron-doped CaFe$_{0.88}$Co$_{0.12}$AsF single crystals with $T_\mathrm{c}$ = 21 K by INS. The resonance energy of the superconducting spin resonant mode with $E_\mathrm{r}$ = 12 meV amounts to 6.6 $k_\mathrm{B}$$T_\mathrm{c}$, which constitutes the largest $E_\mathrm{r}$/$k_\mathrm{B}$$T_\mathrm{c}$ ratio among iron-based superconductors reported to date. The large ratio implies a strong coupling between conduction electrons and magnetic excitations in CaFe$_{0.88}$Co$_{0.12}$AsF. The resonance possesses a magnonlike upward dispersion along transverse direction due to the anisotropy of spin-spin correlation length within $ab$ plane in the normal-state, which points to a spin fluctuation mediated sign-reversed ${s}\mathbf\pm$ wave pairing in CaFe$_{0.88}$Co$_{0.12}$AsF.

IN8 is a high-flux three-axis thermal neutron spectrometer designed to measure inelastic neutron scattering on single crystals in a wide energy and momentum transfer range. It is the highest thermal flux spectrometer worldwide with the monochromatic flux at the sample up to 109 neutrons/cm2/s where users can perform demanding experiments not possible anywhere else. In recent years, the IN8 spectrometer has undergone several major upgrades that have further extended the capabilities of the instrument to the limits achievable with the standard setup. The first part of the upgrade was centred on the replacement of the monochromator assembly, now consisting of four double focusing reflecting crystal faces with enhanced luminosity. The ensuing upgrade was to replace the entire secondary spectrometer. As the result, the new spectrometer, called Thermes, profits from a compact design and it is well shielded against ambient experimental background. All the incorporated modifications have given the instrument an unprecedented and unique luminosity combined with an optimal signal-to-noise level and a remarkable configuration flexibility for a broad range of experimental requirements.

Understanding the high-pressure lattice dynamics is crucial to modulate the thermal transport in thermoelectric materials beyond the ambient environment. Herein, using molecular dynamics simulations in combination with an accurate machine-learning interatomic potential, we find the well-known double-peak feature of the transverse-optical (TO) mode in PbTe gradually vanishes when pressure is enhanced. An anomalous nonmonotonic pressure dependence of the frequency of the transverse-acoustic phonon in PbTe is computationally reproduced. The longitudinal-acoustic, longitudinal-optical, and TO phonons harden as expected when pressure increases. The theoretical results are compared with inelastic neutron scattering experimental data. We have also calculated the pressure-dependent lattice thermal conductivity and revealed the phonon transport mechanisms.

In this paper, we focus on the thermal transport properties of antiferromagnetic spin chains cuprates. The chain magnetic excitations, the spinons, partake in heat transport at low temperature, but spinon heat transport decays well below room temperature, possibly because of a coupling with phonons. By means of inelastic neutron scattering, we thoroughly study the lattice dynamics of spin chain compounds Sr2CuO3, Ca2CuO3, along with double spin-chain compounds SrCuO2. We come to the conclusion that there are no obvious anomalies in the phonon dispersions, which suggests a weak spinon-phonon coupling regime.