Opening remarks

• Gediminas Simutis (PSI - Paul Scherrer Institut)

Room: Auditorium (WHGA)

Frustrated pyrochlore lattice in FeV$_{2}$O$_{4}$ under uniaxial pressure

• Jana Pásztorová (Forschungszentrum Jülich, JCNS)

Room: Auditorium (WHGA) Multiferroics have been at the forefront of condensed-matter research for several decades due to their importance for fundamental understanding of the coupling between magnetic and electric properties, as well as their potential for the development of energy-efficient data storage technologies [1,2]. Magnetic spinels such as iron vanadate, FeV$_{2}$O$_{4}$, in which V ions occupy a geometrically frustrated pyrochlore lattice of corner-sharing tetrahedra, form a highly versatile class of systems for studying exotic phenomena such as frustration and/or electron itinerancy. Synchrotron x-ray scattering studies [3,4] have revealed several structural phase transitions in FeV$_{2}$O$_{4}$. Two of these transitions coincide with the onset of long-range magnetic order: a collinear ferrimagnetic phase below T$_{N1}$=110K and a non-collinear ferrimagnetic structure below T$_{N2}$=60K [5,6], where the V spins adopt a 2-in-2-out configuration. Furthermore, this material is remarkable for the strong effect of applied hydrostatic pressure on both the ferromagnetic coupling and the electrical resistivity [7]. The spin dynamics at ambient pressure were previously investigated on SEQUOIA [8] and HB-3 [9], revealing anomalous spin-wave broadening in the temperature range between T$_{N1}$ and T$_{N2}$. The sequence of structural, electric, and magnetic transitions in FeV$_{2}$O$_{4}$ highlights the strong coupling between the crystal lattice and the spin-orbital degrees of freedom of both Fe and V magnetic ions. One focus of our work is the effect of uniaxial pressure on structure and dynamics of FeV$_{2}$O$_{4}$. By deforming the crystal environment, uniaxial strain may lift the magnetic frustration of the pyrochlore lattice. In our first experiment, we monitored the order parameter of the (111)$_c$ magnetic reflection on the EIGER triple-axis spectrometer together with a recently developed in-situ uniaxial pressure device (UPD) [10]. Pressure was applied along [110] crystallographic direction with forces up to 180 N ($\sim$ 0.1 GPa). Even under such a modest force load, both magnetic ordering temperatures change – both magnetic ordering temperatures shift: T$_{N1}$ increases while T$_{N2}$ decreases. This behavior indicates that breaking the bond-length symmetry of the pyrochlore tetrahedra suppresses the 2-in-2-out ordering temperature of the V sublattice. These neutron results were confirmed with measurements of the elastocaloric effect on a FeV$_{2}$O$_{4}$ single crystal along the same direction. In a subsequent neutron experiment on the IN8 triple-axis spectrometer, we observed a narrowing of magnetic excitations near 4.5 meV at T=80K under an applied force of 70 N ($\sim$ 0.07 GPa), together with a significant reduction in intensity and a change in the critical exponent of the (200)$_c$ reflection, which contains a magnetic contribution solely from the V sublattice. Ongoing data analysis, combined with density functional theory (DFT) calculations, aims to further clarify the complex interplay between lattice distortion, frustration, and spin dynamics in this material. [1] M. Bibes et al., *Nature Materials* **7**, 425–426 (2008) [2] S. Manipatruni et al., *Nature* **565**, (2019):35-42 [3] Y. Nii et al., *Phys. Rev. B* **86**, 125142 (2012) [4] T. Katsufuji et al., *J. Phys. Soc. Jpn.* **77**, 053708 (2008) [5] G. J. MacDougall et al., *Phys. Rev. B* **86**, 060414(R) (2012) [6] Q. Zhang et al., *Phys. Rev. B* **85**, 054405 (2012) [7] A. Kismarahardja et al., *Phys. Rev. Lett.* **106**, 056602 (2011) [8] G. J. MacDougall et al., *Phys. Rev. B* **89**, 224404 (2014) [9] Q. Zhang et al., *Phys. Rev. B* **89**, 224416 (2014) [10] G. Simutis et al., *Rev. Sci. Instrum.* **94**, 013906 (2023)

Two Plaquette-Singlet Phases and Emergent SO(5) Deconfined Quantum Criticality in SrCu$_2$(BO$_3$)$_2$

• Bruce Normand

Room: Auditorium (WHGA) The deconfined quantum critical point (DQCP) has become a central open concept in the physics of quantum matter. The theoretical proposal of a DQCP at the plaquette-singlet--to--antiferromagnet (PS--AFM) transition in the Shastry-Sutherland model was followed by experimental evidence for a minimal DQCP scenario induced by an applied magnetic field in SrCu$_2$(BO$_3$)$_2$. However, the nature of the PS phase in SrCu$_2$(BO$_3$)$_2$ remains unresolved, and with it the identification of the possible DQCP. Here we perform detailed high-pressure $^{11}$B NMR studies in the PS phase between 1.8 and 2.7 GPa to reveal the presence of both the full-plaquette (FP) and empty-plaquette (EP) phases of SrCu$_2$(BO$_3$)$_2$, which coexist at a first-order, pressure-driven transition with a volume-fraction effect. Finding the field-driven EP--AFM transition complements our previous observations of the FP--AFM transition, although the temperature-dependence of the spin-lattice relaxation rate around the EP--AFM transition, $1/T_1 \propto T^{0.6}$, implies an anomalous scaling exponent $\eta \approx 0.6$ that is different from the FP--AFM value of $\eta \approx 0.2$, indicating that the critical fluctuations are governed by DQCPs of different universality classes. A conventional field-driven PS--AFM transition should be a Bose-Einstein condensation of plaquette triplons with no anomalous features, and hence we deduce that the role of the field is to suppress the EP/FP and AFM order parameters that arise from additional interactions in SrCu$_2$(BO$_3$)$_2$ (meaning beyond the Shastry-Sutherland model), thereby revealing the critical properties of the underlying proximate DQCP. A DQCP is characterized by exotic and fractional excitations that emerge during the complete rearrangement of spin correlations, and the PS--AFM DQCP can be formulated as an O(4) field theory with monopole defects. The EP-FP phase-coexistence we discover implies line-like domain walls with spinon excitations, whose combination with monopoles requires that the DQCP symmetry is raised to SO(5). Hence our results take an important step towards a complete understanding of deconfined quantum criticality in SrCu$_2$(BO$_3$)$_2$ and challenge theory to provide a definitive determination of the anomalous scaling exponents for each universality class.

Pressure Tuned Magnetism in the 3D Frustrated Antiferromagnet K₂Ni₂(SO₄)₃

• S. S. Islam (PSI Center for Neutron and Muon Sciences CNM, 5232 Villigen PSI, Switzerland)

Room: Auditorium (WHGA) K₂Ni₂(SO₄)₃ is a three dimensional frustrated antiferromagnet in which Ni²⁺(S = 1) ions occupy two interpenetrating trillium sublattices that together form a unique tetra-trillium network. Recent thermodynamic, inelastic neutron scattering, and muon spin relaxation (μSR) studies show that, at ambient pressure and zero magnetic field, K₂Ni₂(SO₄)₃ hosts a highly dynamic, correlated ground state with features reminiscent of a quantum spin liquid (QSL), coexisting with a very small static ordered component — behavior that places the system close to a quantum critical regime [1-3] To investigate how this dynamic state evolves when magnetic interactions are tuned, we performed detailed μSR experiments on K₂Ni₂(SO₄)₃ under hydrostatic pressure up to 23 kbar. At low pressures (below about 8.6 kbar), the ZF-μSR spectra measured at millikelvin temperatures show predominantly dynamic relaxation, consistent with persistent spin fluctuations. At and above approximately 9.5 kbar, pronounced oscillations appear in the ZF μSR time spectra, providing clear evidence for the development of static internal magnetic fields and indicating that pressure drives the system toward a more static, magnetically ordered regime. The evolution of the internal field strength with pressure and temperature highlights a systematic suppression of spin fluctuations and increasing stabilization of static correlations under compression. These results demonstrate that hydrostatic pressure is a powerful tuning parameter in K₂Ni₂(SO₄)₃, transforming the magnetic ground state from a dynamic, frustration dominated regime toward static magnetism. This behavior underscores the sensitivity of three dimensional frustrated spin networks to small perturbations and establishes pressure as a promising route for exploring competing quantum phases and proximate QSL behavior in 3D frustrated magnets. [1] I. Živković et al., Phys. Rev. Lett **127**, 157204 (2021). [2] M. G. Gonzalez et al., Nat. Commun. **15**, 7191 (2024). [3] W. Yao et al., Phys. Rev. Lett **131**, 146701 (2023).

New structures and exotic properties of simple molecular systems under extreme conditions: using neutrons to explore planetary interiors

• Livia E. Bove (1)IMPMC, CNRS-UMR 7590, Université P&M Curie, 75252 Paris, France 2) Physics Department, Università di Roma La Sapienza, piazzale Aldo Moro 5, 00196, Roma, Italy 3)LQM, Physics Department, Ecole Politecnique Federale Lausanne, Lausanne, Suisse)

Room: Auditorium (WHGA) Simple molecular systems—such as water, methane, ammonia, hydrogen, and their mixtures—play a central role across disciplines, from energy storage technologies to condensed matter and planetary physics [1]. They are abundant on Earth, widespread in planetary bodies throughout our solar system, [2] and have even been detected on newly discovered water-rich exoplanets, representing not only a vast natural resource but also a unique testing ground for fundamental science. Thanks to their simple stoichiometry and electronic struc-ture, these systems serve as ideal model compounds for unraveling the physicochemical behavior of more complex molecular materials. When subjected to the extreme pressure–temperature conditions characteri-stic of planetary interiors, they exhibit remarkably rich phase diagrams, anomalous dynamical, thermal, and transport properties, enhanced solubility, as well as emergent phenomena such as superionicity, plasticity, and quantum entanglment. In this talk, I will present our recent experimental insights into the structure and dynamics of simple molecu-lar systems under extreme conditions, obtained through neutron, X-ray, and light scattering techniques [3–17]. I will also highlight their implications for planetary modeling and potential energy applications. ![enter image description here][1] [1] W.L. Mao et al., Physics Today 60, 42 (2007). [2] Leigh N. Fletcher et al., Planetary and Space Science, 191, 105030 (2020). [3] S. Klotz, L. E. Bove et al. Nat. Mat. 8, 405 (2009). [4] L. E. Bove, R. Gaal, et al., PNAS 112, 8216 (2015). [5] S. Klotz, L.E. Bove, et al., Sci. Rep. 6, 32040 (2016). [6] U. L. Ranieri et al., Nature Com., 8, 1076 (2017). [7] S. Schaack et al., PNAS, 10.1073/pnas.1904911116 (2019). [8] U. L. Ranieri, et al., Nature Com. 12: 195 (2021). [9] S. Schaack et al., PNAS, 10.1073/pnas.1904911116 (2019). [10] H. Zhang, et al., The Journal of Physical Chemistry Letters 14(9) 2301 (2023) [11] U. Ranieri et al., PNAS 120 52 (2023) [12] S. Di Cataldo et al., Phys. Rev. Letters 133, 236101 (2024) [13] M. Rescigno et al., Nature 640 (8059), 662–667 (2025). [11] L. Andriambariarijaona, et al. Physical Review B, 111(21), 214109 (2025). [12] L. Monacelli, et al. Physical Review B DOI: 10.1103/1cgl-mklx (2025). [13] S. Berni et al. Comm. Chemistry (accepted), arXiv:2508.09771 (2025). [14] L. Renaud et al. PNAS (accepted), arXiv:2508.09771 (2025). Acknowledgements: This work was supported by the Swiss National Science Fund under Grant 200021149487, the French National Research Program under Grant ANR- 23-CE30-0034 EXOTIC-ICE and the Franco-Japonais program Sakura 53273PD [1]: https://indico.psi.ch/event/17608/images/5265-Picture1.jpg

Gas/liquid interfaces at high pressure: Neutron Imaging, Molecular Dynamics

• Ondrej Vopicka (Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 166 28 Prague 6, Czechia)

Room: Auditorium (WHGA) The exposure of liquids to pressurized gases induces gas adsorption at the interface, its diffusion into the bulk, and liquid swelling. Neutron imaging of systems from deuterated liquids (hydrocarbons, water) and protium-containing gases (H$_2$, CH$_4$, C$_2$H$_6$) at up to 100 bar and near-ambient temperatures provides interface shape, liquid level, and concentration distribution over time. Interfacial tension, apparent molar volume, diffusivity, and solubility of the gas in the liquid are thus used to calibrate the molecular dynamics model. The contributions are summarized as: $i$) the technical improvements that led us to the $2^\text{nd}$ generation of the neutron imaging process [1-3], $ii$) lessons learned for the neutron imaging and molecular interpretation of three-phase systems (gas, two liquids) [4], and $iii$) simulation-based predictions of physico-chemical properties at industrially relevant temperatures and pressures [5] beyond the reach of the neutron imaging setup. REFERENCES [1] J. Šercl et. al, J. Radioanal. Nucl. Chem., 334 (2025) 8921–8928. https://doi.org/10.1007/s10967-025-10561-w. [2] O. Vopička et. al, Springer Nature Proceedings, 2026, pp. 1–7. https://doi.org/10.1007/978-3-032-15003-5_6. [3] J. Lee et. al, Sci. Rep., 15 (2025) 25835. https://doi.org/10.1038/s41598-025-09425-w. [4] M. Melčák et. al, Adv. Mater. Interfaces, (2026) e00786. https://doi.org/10.1002/admi.202500786. [5] M. Melčák et. al, Sci. Rep., 15 (2025) 1284. https://doi.org/10.1038/s41598-024-85093-6. ACKNOWLEDGEMENT Authors acknowledge the financial support obtained from Czech Science Foundation (GACR) and Swiss National Science Foundation (SNSF), GACR number 23-04741K, SNSF number 200021E_213197. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. Experiments were conducted at NEUTRA thermal neutron imaging beamline at the Paul Scherrer Institute.

High-pressure research at the Department of Earth and Planetary Sciences (ETHZ)

• Christian Liebske (ETH Zurich)

Room: Auditorium (WHGA) The Department of Earth and Planetary Sciences at ETHZ has a nearly 50-year long history in experimental high-pressure projects. Several research groups operate a wide range of instruments, from gas- and solid-media pressure devices to diamond anvil cell systems, covering conditions from hundreds of MPa to hundreds of GPa, while heating to several thousands of Kelvin. This presentation provides a brief overview of the experimental facilities available across the department and highlights selected research questions, experimental approaches, and key findings, with current work focusing on the structure and chemical evolution of planetary bodies such as Earth, Mars, and the Moon.

High-pressure total scattering using neutrons

• Nick Funnell (UKRI - ISIS Neutron and Muon Facility)

Room: Auditorium (WHGA) The majority of neutron total scattering experiments have been performed under ambient or variable-temperature conditions whereas comparatively little work has been carried out under high pressure. This is partly a consequence of the difficulties involved in accurately removing non-sample scattering contributions when using devices such as the Paris-Edinburgh press, which are required to deliver multi-GPa pressures. The PEARL instrument at the ISIS Neutron and Muon Facility has a demonstrable track record in performing these measurements with non-crystalline samples where strain broadening is not an issue [1-3]. However there has been little work performed on crystalline materials as these usually require hydrostatic compression, achieved by the inclusion of a pressure-transmitting medium (PTM). The PTM can further complicate the analysis of pair distribution functions. This talk will focus on the use of the unique gas-loading capability [4] at ISIS where argon is used as an 'invisible' PTM. Science examples will include the pressure-induced order-disorder transition in ammonium bromide [5] and the nature of correlated displacements in barium titanate under hydrostatic pressure [6]. [1] H Playford, M Tucker and C Bull, *J. Appl. Crystallogr.*, 2017, **50**, 87-95 [2] S Klotz *et al, J. Phys.: Condens. Matter*, 2005, **17**, S967-S974 [3] P Salmon *et al, J. Phys.: Condens. Matter*, 2012, **24**, 415102 [4] S Klotz *et al, High Pressure Res.*, 2013, **33(1)**, 214-220 [5] N Funnell , C Bull, S Hull and C Ridley, *J. Phys.: Condens. Matter*, 2022, **34**, 325401 [6] A Herlihy *et al, Phys. Rev. B*, 2022, **105**, 094114

In situ mechanical uniaxial pressure devices for SINQ and SμS facilities at PSI

• Tina Arh (PSI - Paul Scherrer Institut)

Room: Auditorium (WHGA) Uniaxial pressure is emerging as a powerful experimental tuning parameter for quantum materials that can be employed with various experimental techniques. In recent years, the sample environment capabilities of PSI’s neutron facility SINQ have been enriched by the adoption of the 200 N *in situ* uniaxial pressure device [1]. Its mechanical transmission rod allows the user to accurately measure the applied force using a load cell and keep it constant through the temperature changes with the control feedback loop. In addition to the compressive stress, it allows for the application of tension to the samples, as well as the measurement of true zero force by mechanical decoupling of the force rod from the pressure cell. The device was already successfully tested in user program, both for diffraction and inelastic neutron scattering studies, which will be briefly presented in the talk. To offer comparable uniaxial capabilities to the users of PSI’s muon spin spectroscopy (μSR) facility SμS, we designed and built an *in situ* 2000 N press with a uniaxial pressure cell that will be ready for the first tests on the μSR beamline in 2026. The press has all the advantages of mechanical force transmission, while the pressure cell is designed for simple sample mounting, allowing for different sizes and thicknesses of the samples, permanently mounted strain gauges, pluggable temperature sensor and the ACS coils, and a reusable sample holder that can be immobilized until the experiment to protect the samples. I will present the current status of the commissioning of the device, focusing on the force calibration and the integration of strain gauge readings into the control software. [1] Simutis *et al.*, Rev. Sci. Instrum. **94**, 013906 (2023)

Time-domain Brillouin scattering for 3D-imaging of texture, phase transitions, elastic and plastic properties of solids compressed in a diamond anvil cell

• Andreas Zerr (LSPM-CNRS)

Room: Auditorium (WHGA) Time-domain Brillouin scattering (TDBS) technique uses ultrafast laser pulses to generate coherent acoustic pulses (CAPs) and monitor their propagation through transparent samples. Local velocity of a CAP along depth of a sample (*V*_A, where A represents longitudinal (L) or transversal (T) sound velocity) is extracted from oscillating intensity of the probe light scattered by the CAP and interfering with that reflected by stationary interfaces [1]. Frequency of the oscillation, *f*_B, is given by the well-known equation used in classical Brillouin light scattering in the backscattering geometry: *f*_B=2 *n* *V*_A/ λ, where λ is the wavelength in vacuum of the probe laser beam and *n* the refractive index of the examined material at this wavelength. Spatial distribution of the *f*_B values can be extracted with micron and submicron resolution in lateral and axial directions, respectively. The latter capability of the TDBS technique permits measurement of single crystal elastic moduli, *C*_ij(*P*), of any transparent solid compressed in a diamond anvil cell (DAC) to Mbar pressures [2] including polycrystalline cubic phases not available at ambient conditions, e.g. high pressure phases [3, 4]. Similarly, shear modulus of an amorphous or of an isotropic polycrystalline solid, *G*( *P*), can be determined if its bulk modulus, *B*( *P*), is known [5]. In the case of elastically anisotropic solids, the TDBS technique provides 3D-images of texture of the polycrystalline samples including orientation of individual crystallites in space [6]. Finally, it permits 3D-monitoring of progress of phase transitions or chemical reactions to pressures accessible using the DAC technique [7]. Combination of the TDBS technique with high-resolution X-ray diffraction in a DAC provides a straight-forward way to an assumption-free high-pressure scale. A novel secondary pressure scale based on the f_B(*P*) dependence of the mixture of methanol and ethanol in the volume ratio 4:1 was already proposed and applied to measure pressure dependence of yield strength, σ_y(*P*), of this amorphous solid [5]. References: 1. Kuriakose M. et al., *Ultrasonics* **69**, 259-267 (2016) 2. Zhang X. et al., *J. Geophys. Res.-Solid Earth* **128**, e2022JB026311 (2023) 3. Raetz S. et al., *Phys. Rev. B* **99**, 224102 (2019) 4. Xu F. et al., *Appl. Phys. Lett.* **125**, 164102 (2024) 5. Hong C. et al., *Phys. Rev. B* **111**, 184110 (2025) 6. Sandeep S. et al., *J. Appl. Phys.* **130**, 053104 (2021) 7. Kuriakose M. et al., *New J. Phys.* **19**, 053026 (2017)

Recent advances on the SNAP diffractometer

• Christopher Ridley (Oak Ridge National Laboratory)

Room: Auditorium (WHGA) The SNAP diffractometer at the Spallation Neutron Source (Oak Ridge, TN) is unique in providing routine access to a diamond anvil cell (DAC)-based neutron diffraction program. These capabilities underpin SNAP’s broader micro-diffraction program, routinely handling sample volumes ranging from 0.03 to 0.3 mm3. This enables neutron diffraction studies of materials across a wide range of extreme conditions and provides a useful point of reference for planned diffraction capabilities at the European Spallation Source (ESS). This contribution presents an overview of recent developments on SNAP, including current operational DAC capabilities and details of how the program has been implemented. Topics include data reduction and correction strategies, an assessment of the current limitations, recent initiatives aimed at broadening access for under-served research communities (including experiments at extreme temperatures or with isotopically challenging samples), and an outlook on future developments and challenges for high-pressure neutron diffraction.

Pressure induced spin liquid state in the anisotropic kagome Y-kapellasite Y3Cu9(OH)19Cl8

• Dipranjan Chatterjee (University Of Oxford)

Room: Auditorium (WHGA) Y-kapellasite (Y3Cu9(OH)19Cl8) materializes an anisotropic kagome model with 3 different nearest neighbor interactions, yielding a rich phase diagram [1]. Besides two long range ordered phases, this phase diagram features a large spin liquid area, which encompasses the isotropic kagome model. Noticeably the large difference in the Y and Cu radii prevents inter-site mixing and the anisotropic kagome planes are free from magnetic defects. We present a detailed investigation of large, phase pure, single crystals of this compound by neutron scattering, and local μSR and NMR techniques [2]. At variance with polycrystalline samples, the study of single crystals gives evidence for subtle structural instabilities at 33 and 13 K and a bulk magnetic transition at 2.1 K, well below the antiferromagnetic 100 K Weiss temperature. The structural instabilities involve the localization of one interlayer proton and, importantly, preserve the kagome planes. At 2.1 K the compound shows a magnetic transition to the coplanar (1/3,1/3) long-range order as predicted theoretically. However, our analysis of the spin-wave excitations yields an estimate of magnetic interactions, which locate the compound closer to the phase boundary to the spin-liquid phase than expected from ab initio calculations. Enhanced quantum fluctuations at this boundary may be responsible for the reduced ordered moment of the Cu2+ and hint at a strong effect of external perturbations. **Indeed, in recent μSR experiment under pressure, we could establish that the fragile long range order is suppressed in favor of a fluctuating ground state with a moderate 23 kbar applied pressure [3]. This finding is rationalized by new high pressure diffraction results showing a tendency towards a more isotropic lattice in the same range of applied pressures [3].** [1] M.Hering et al , npj Comput Mater 8, 10 (2022) [2] D.Chatterjee et al , Phys. Rev. B 107, 125156 (2023) [3] D.Chatterjee et al , arXiv:2502.09733 (2025)

Decoupling of static and dynamic charge correlations revealed by uniaxial strain in a cuprate superconductor

• Leonardo Martinelli (University of Zurich)

Room: Auditorium (WHGA) The physics of charge order in high-temperature superconducting cuprates is still largely unexplained. While the formation of one-dimensional charge modulations is a common feature of the two-dimensional Hubbard model, no calculation yet succeeds in reproducing even the basic properties of charge order. Recent experiments revealed the presence of strong quantum fluctuations associated to the charge order parameter [1]. Their doping and temperature dependence suggests the closeness to a quantum critical point (QCP). The energy of such collective modes also links them to the strange metal phase, associating this QCP with the pseudogap endpoint at doping p∼0.19 [2]. Indeed, the scattering provided by charge fluctuations could explain the quasi-isotropic scattering rate at the base of the strange-metal behavior. The study of the interplay between charge order, fluctuations and phonons has therefore the potential to shed light on the complex normal state of cuprates. We used high-resolution Resonant Inelastic X-ray Scattering (RIXS) to investigate the electronic and phonon excitations in the prototypical stripe-ordered cuprate La2-xSrxCuO4. We employed a strain device developed in our group to apply a small, in-plane uniaxial strain along the copper-oxygen direction, pinning charge order to the perpendicular direction [3]. This allowed us to investigate the properties of the associated quantum fluctuations, of phonon softening and of the electron phonon coupling in a detwinned striped state. Our measurements highlight a clear connection between quantum charge fluctuations and bond-stretching phonons. At the same time, we reveal a non-trivial relationship between the charge order parameter and its quantum fluctuations, which display a different symmetry [4].

Development of Muon Spin Relaxation Measurement under High Pressure at J-PARC and Application to Study of Organic Magnets

• Wataru Higemoto (Advanced Science Research Center, Japan Atomic Energy Agency)

Room: Auditorium (WHGA) The muon is a useful elementary particle for probing materials. By using accelerator, spin polarized muons can be implanted into a sample, and depolarization of muon spin is detected at the outside by observing muon decay positron or electron. This technique is known as muon spin rotation, relaxation and resonance (µSR). For µSR measurements, high pressure is also a crucial experimental condition and is often used to investigate a material property. However, at the pulsed muon facility, the beam size is larger than the other continuous ones, and only a few examples of high pressure µSR apparatus are found at the pulsed muon facility. We have been developing the high pressure µSR technique at the Muon Experimental Facility (MUSE), Japan Proton Accelerator Research Complex (J-PARC). The intense pulsed muons at J-PARC are useful because we can measure long-time muon spin relaxation or synchronize with external pulse conditions. In my presentation, I will report on the current status of our pulsed µSR measurements under high pressure, up to 1.5 GPa, at the J-PARC MUSE facility. Additionally, we will present the recent application of high-pressure µSR to study the organic magnet λ-(BEDSe-TTF)₂GaCl₄ under pressures up to 1.2 GPa at J-PARC. [1] S.Saito, W. Higemoto et al., JPS-CP, in press

Pressure- and strain-enhanced superconducting and normal-state electronic response of kagome LaRu$_{3}$Si$_{2}$

• Zurab Guguchia (PSI - Paul Scherrer Institut)

Room: Auditorium (WHGA) The interplay between superconductivity and charge or spin order is a key focus in condensed matter physics, with kagome lattice systems providing unique insights. We discovered that the kagome superconductor LaRu$_{3}$Si$_{2}$ ($T_{c}$ = 6.5 K) exhibits a characteristic kagome band structure and a hierarchy of charge-order transitions at 400 K and 80 K, as well as an additional electronic and magnetic transition at 35 K [1,2]. Furthermore, using magnetotransport and X-ray diffraction under pressures up to 40 GPa, we find $T_{c}$ peaks at 9 K (2 GPa), remains stable up to 12 GPa, and decreases to 2 K at 40 GPa, forming a dome-shaped phase diagram [3]. Similarly, the resistivity anomaly at 35 K and magnetoresistance exhibit dome-like pressure dependence. Above 12 GPa, charge order transitions from long-range to short-range, correlating with $T_{c}$ suppression, indicating superconductivity is strongly tied to the charge-ordered state [3]. Notably, $T_{c}$ is maximized when charge order and normal-state electronic responses are optimized. This conclusion is further supported by our uniaxial stress experiments, which reveal an enhancement of both the superconducting transition temperature $T_{c}$ and magnetoresistance [4]. These results offer fresh insights into the relationship between superconductivity and charge order, paving the way for theoretical advancements. [1] I. Plokhikh et. al., and Z. Guguchia, Communications Physics 7, 182 (2024). [2] C. Mielke III, et. al., and Z. Guguchia, Advanced Materials 37(40), 2503065 (2025). [3] K. Ma et. al., and Z. Guguchia, Nature Communications 16, 6149 (2025). [4] P. Král et. al. and Z. Guguchia, in preparation.

Neutron and x-rays scattering on quantum magnets at high pressures and low temperatures

• Björn Wehinger (European Synchrotron Radiation Facility)

Room: Auditorium (WHGA) Application of external pressure acts as clean tuning parameter of inter-atomic distances and bond angles and therefor allows for precise control of magnetic interactions in condensed matter. Extreme conditions can thus be used to stabilize novel quantum phases and drive systems close to quantum criticality where new and exciting phenomena are expected. I will show how recent advances in neutron and x-ray scattering allow to access fingerprints of quantum correlations and present novel results on quantum magnets at the extreme.

Competing Quantum Orders in the Bulk Layered Heterostructure 6R-TaS2

• Petr Král (PSI Center for Neutron and Muon Sciences)

Room: Auditorium (WHGA) Hydrostatic pressure provides a clean and continuous route to tune competing electronic phases in quantum materials, offering unique insights into the relationship between multiple emergent quantum phenomena. This approach is particularly powerful in layered systems, where pressure directly modifies interlayer coupling and electronic structure without introducing disorder. In this context, the transition metal dichalcogenide 6R-TaS$_2$ stands out as a natural platform for studying the interplay of charge density wave (CDW) order, superconductivity, and transport anomalies – recently, a hidden order in the intermediate temperature range ($T^*≈35$ K) has been reported, evidenced by strong magnetoresistance and nonlinear Hall effect (NHE). Using μSR, magnetotransport, and hydrostatic pressure techniques, we identify a nodal superconducting state with low superfluid density at ambient pressure, with no spontaneous magnetic order detected below $T^*$. This suggests that the NHE is primarily associated with band-structure effects rather than time-reversal symmetry breaking. Under pressures up to 2 GPa, the superfluid density rises markedly in correlation with the superconducting transition temperature, the nodal pairing shifts to a nodeless state, and the CDW onset is reduced by half. Notably, NHE is fully suppressed, and magnetoresistance drops by 50% within just 0.2 GPa, highlighting the fragility of the hidden order. These results reveal an unconventional superconducting pairing in 6R-TaS$_2$, competing with both CDW and hidden orders through weakened interlayer coupling and competition for the same electronic states. With a multifaceted approach, we establish a comprehensive phase diagram that reveals the intricate interplay and competition between the intertwined quantum orders in 6R-TaS$_2$. Ref. V. Sazgari et al., arXiv:2503.13944v1

The role of neutrons in understanding the behaviour of metal oxides at extreme conditions

• Craig Bull (ISIS Neutron and Muon Facility)

Room: Auditorium (WHGA) Pearl is the dedicated high-pressure diffraction instrument at the ISIS Neutron and Muon Source. Using a series of examples from the user programme the current and developing capabilities of the instrument will be represented. In particular, we will demonstrate the pressure range and quality of different data were able to measure, We will also showcase the technology we are using to provide access to a wide range of temprau8res as well. Provided below are some of the science areas which will be used to highlight the instrument and technique capabilities. The behaviour of the crystallography structure of more simple perovskites such as LaCoO3 will be presented [1]. Highlighting changes in the polyhedral compression mechanism and indications as to the electronic configuration which can be extracted from such structural studies. We will then showcase the behaviour of BaTiO3 as a function of pressure and temperature demonstrating the nature of the phase transitions which and the role in which neutron diffraction adds a unique insight into this material and even allowing estimation of the polarisation of the distorted material. High pressure also provides the capability to prepare materials in previously inaccessible structural forms which can be recoverable to ambient pressure. One such recent study are metastable phases of FePO4 which is a chemical analogue of orthorhombic (CrVO4-type, phase-II) SiO2. By preparing the metastable phase II of FePO4 and recovering back to ambient pressure were able to infer the high-pressure behaviour of the equivalent crystalline phase of SiO2 normally only observed as an amorphous solid by direct compression of the orthorhombic phase II of FePO4. [3] Finally, we will show how with the application of pressure and temperature it is possible to prepare new perovskite related materials aby taking the structure out of its stability field and preparing a series of perovskites which can be recovered back to ambient conditions. The example presented will be a series of perovskites in the solid solution SeCo1-xMnO3 [4]. Neutron diffraction data will be presented on the structural behaviour of the solution and correlated to magnetic properties. [1] M. Capone et al, High-Pressure Neutron Diffraction Study of LaCoO3, Physica status solidi a, (2019), 216, 1800736 [2] C. L. Bull et al, Comprehensive determination of the high-pressure structural behaviour of BaTiO3, Materials Advances, (2021), 2, 6094 [3] C.L. bull et al, The distortion of two FePO4 polymorphs with high pressure, Materials Advances, (2021), 2, 5096 [4] C. J. Ridley et al, Structure and physical properties of SeCo1−xMnxO3, J. Physics: Condensed Matter (2019), 31, 3195402

High-Pressure Synthesis of Multifunctional Spinels in the Element System Ge-Ga-Cr-O-N

• Marcus Schwarz (Freiberg High Pressure Research Centre / Institute for Inorganic Chemistry, TU Bergakademie Freiberg)

Room: Auditorium (WHGA) The use of nitride semiconductors such as AlN, GaN and InN for (opto)electronic applications has become paramount in recent years. High-pressure-derived nitrides with direct band gap, e.g. γ-Ge3N4 with spinel-type structure, are currently being explored. Similarly to alredy known ambient- and high pressure forms of the lighter group-4 nitride, Si3N4 and its solid solutions with Al and O, e.g. Si3-xAlxOxN4-x, γ-Ge3N4 could be modified by exchanging [Ge-N]+ pairs with [Ga-O]+ or even [Cr-O]+ pairs to create novel solid solutions with spinel-type structure. Other oxynitrides containing Ga and/or Ge are already known to show promising photocatalytic properties for water-splitting. Moreover, the introduction of a magnetic ion into the spinel structure could result in novel materials with frustrated magnetism. In this contribution, we present results on the synthesis and first characterization of spinel compounds in the quintary element system Ge-Ga-Cr-O-N and its ternary and quarternerary subsystems.

Comparative High-Pressure Study on Rare-Earth Fluorite-Type Oxides with Increasing Configurational Entropy

• Pablo Botella Vives (University of Valencia)

Room: Auditorium (WHGA) Rare-earth fluorite-type oxides provide an ideal platform to investigate the role of configurational entropy on structural stability under extreme conditions. In this talk, I will present a comparative high-pressure study of a series of multicationic rare-earth oxides with increasing configurational entropy, ranging from binary systems ((CePr)O₂−δ) to ternary, quaternary ((CePrLaNd)O₂−δ), and higher-order multicationic compositions. High-pressure synchrotron X-ray diffraction and Raman spectroscopy experiments up to ~30 GPa reveal that all compositions retain the cubic fluorite structure over a broad pressure range, despite pronounced lattice disorder and chemical complexity. A reproducible compression anomaly is observed between ~9 and 16 GPa, characterized by a plateau in the volume–pressure relation and changes in vibrational modes, which is attributed to internal lattice rearrangements and bond-angle distortions rather than a crystallographic phase transition. With increasing configurational entropy, the fluorite lattice exhibits enhanced resilience against pressure-induced transformations. However, in the intermediate-entropy systems, signatures of pressure-induced amorphization emerge at higher pressures, manifested as reversible broad background contributions in diffraction patterns, highlighting the delicate balance between entropy stabilization and local structural frustration. By extending the study to quaternary and higher-order compositions, this work provides new insights into how entropy, cation size mismatch, and lattice distortion collectively govern the high-pressure response of fluorite-type rare-earth oxides. These results contribute to a broader understanding of entropy-stabilized oxides under extreme conditions and their potential robustness in high-pressure environments.

Towards neutron diffraction of high-pressure hydrides

• Mads Hansen (University of Cambridge)

Room: Auditorium (WHGA) Hydrogen is an elusive atom and detecting it with X-ray diffraction techniques is nearly impossible, especially when it is incorporated in heavy atom matrices, as in many inorganic materials. In high pressure research, using neutrons to probe hydrogen positions is challenging as neutron sources have low flux, relative to synchrotrons, and pressure generation requires small samples. At the same time, there is active interest in structural hydrogen in several fields, including solid-state physics. Recent theoretical work in superconducting hydrides has focused on systems that are stable at lower pressures, achievable in the SNAP diamond anvil cell. This includes systems such as metallic borohydrides like K-doped CaB2H4 as well as ternary hydrides like Mg2IrH6. Several compounds are predicted to have a stability field well within what is possible to probe using neutron diffraction. In this talk we will show the feasibility of measuring hydride structures at high pressures using diamond anvil cells at the time-of-flight diffractometer SNAP, using MgH2 as a model system. We will discuss the use of hydrogen donors and laser heating as a route to in-situ synthesis of the target materials and address the challenges that come with it. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.

Equilibrium p-T phase diagram of ZnO

• Vladimir Solozhenko (Centre National de la Recherche Scientifique (CNRS))

Room: Auditorium (WHGA) Melting and solid-state phase transitions of wurtzite (w-ZnO) and rocksalt (rs-ZnO) polymorphs of zinc oxide have been studied at pressures to 8 GPa and temperatures to 2500 K using in situ synchrotron X-ray diffraction, electrical resistivity measurements, and quenching experiments. The equilibrium p-T phase diagram of zinc oxide has been constructed based on experimental data and thermodynamic analysis. Calculations of phase equilibria have been performed using models of phenomenological thermodynamics with interaction parameters derived from our experimental data on ZnO melting at high pressures and high temperatures. The proposed phase diagram represents thermodynamic equilibria between crystalline phases and liquid, not influenced by kinetic phenomena, and explains all thermodynamic aspects of ZnO polymorphism.

Closing Remarks Room: Auditorium (WHGA)