Name: High Pressure Workshop
Start: 2023-11-22T09:00:00+01:00
End: 2023-11-22T19:00:00+01:00
Location: PSI Villigen

09:00

Registration and Coffee

1

Opening remarks

Speaker: Gediminas Simutis (PSI - Paul Scherrer Institut)

Morning Session: Invited talks

2

Squeezing charge stripe order unidirectionally

This talk will present resonant and non-resonant x-ray diffraction experiments upon uniaxial pressure application to cuprate superconductors. Special attention is given to the symmetry properties of charge stripe ordering.

Speaker: Johan Chang
3

Probing material properties with neutrons under pressure

Quantum magnets are physical realisations of many-body quantum systems which may host interesting phenomena such as entangled states or spin-nematic states and quantum phase transitions. There exists a number of experimental knobs for controlling the state of such system: Temperature, magnetic field, chemical doping and pressure. Of all these, the latter is the cleanest way of manipulating exchange paths in a system and therefore offers the possibility to dramatically manipulate the ground state. Inelastic neutron scattering is one of the most powerful tools to probe the finger print of non-ordered quantum entangled states: The spin dynamics. Therefore, in combination, pressure and inelastic neutron scattering are a super tool in experimental quantum magnetism. Using the archetypic quantum magnet, SrCu2(BO3)2, we present a number of high-pressure inelastic neutron scattering studies. SrCu2(BO3)2 is the realisation [1] of what is known as the Shastry-Sutherland lattice [2] consisting of a network of spin dimers with exchange interaction J inside the dimer and J’ between the dimers. For low ratios of J’/J, a product of dimer singlets is the ground state. Upon increasing J’/J, a singlet plaquette phase is encountered and finally an ordered antiferromagnetic state is established [3]. The phase diagram of SrCu2(BO3)2 resembles the predicted one remarkably well with phase transitions around 1.8 GPa and 3.0 GPa to enter the plaquette and antiferromagnetic phases respectively [4]. We performed inelastic neutron scattering experiments with high pressures to investigate the nature of the predicted phases and in this way contribute with a piece in the puzzle for understanding many-body quantum physics.

[1] Kageyama et al., Phys. Rev. Lett. 82, 3168-3171 (1999)
[2] B. S. Shastry and B. Sutherland, Physica 108B, 1069-1070 (1981)
[3] P. Corboz and F. Mila, Phys. Rev. B 87, 115144 (2013)
[4] M. E. Zayed et al., Nature Physics 13, 962-966 (2017)

Speaker: Ellen Fogh (EPFL - EPF Lausanne)
4

Revealing the KDP soft-mode coupling mechanism with infrared spectroscopy under pressure

Potassium dihydrogen phosphate, KH$_2$PO$_4$ (KDP), is a classic, broadly used ferroelectric material. It is a model system of an order-disorder material, with a Curie temperature $T_C$ of 123 K. Above this temperature, it is a tetragonal paraelectric. Below, it becomes orthorhombic. In the 1940s, Slater wrote an order-disorder theory to describe rather well the physics of KDP [1]. However, his theory failed to describe why the polarization doesn’t change below the ordering temperature, and why $T_C$ increases when hydrogen is replaced by deuterium. Therefore, it was understood that phonons must also play a role, through coupling to the proton which tunnels in a double well potential [2]. How exactly this happens remained unclear for a long time [3].

In our work, which spanned more than a decade and took place across two continents, we measured the far-infrared reflectivity of KDP up to 2 GPa in its ferroelectric and paraelectric phases. We identified an infrared mode that couples the hydrogen network to the lattice modes, to create the ferroelectric polarization.

[1] J. C. Slater, Theory of the Transition in KH$_2$PO$_4$, TheJournal of Chemical Physics 9, 16 (1941).
[2] J. Pirenne, On the ferroelectricity of KH$_2$PO$_4$ and KD$_2$PO$_4$ crystals, Physica 15, 1019 (1949).
[3] P. Simon and F. Gervais, Phase-transition mechanism in RbH$_2$PO$_4$-type ferroelectrics, Phys. Rev. B 32, 468 (1985).

Speaker: Ana Akrap (University of Fribourg)
5

Tuning and understanding correlated quantum phases of layered materials

The classification and deep understanding of phases of quantum matter is a necessary premise for utilizing quantum materials in all areas of modern and future electronics in a controlled and optimal way. In this respect, layered systems with highly anisotropic electronic properties have been found to be potential hosts for rich, unconventional and tunable exotic quantum states. Prominent classes of layered materials are cuprates, transition metal dichalcogenides (TMDs) and kagome-lattice systems.

In this talk, I will provide brief overview of systems, from different material classes, with novel electronic and magnetic properties, where the application of temperature, magnetic field, hydrostatic pressure, and uniaxial strain lead to large and unexpected effects. These include the topological kagome magnet TbMn$_6$Sn$_6$ [1] (where we show that the topological electronic properties tied to the spin-polarized Dirac dispersion is promoted only by true static out-of-plane ferrimagnetic order and is washed out by the slow commensurate magnetic fluctuations), the topological kagome metals AV$_3$Sb$_5$ (A=K,Rb) [2-4] (where we found intertwining of a TRSB charge ordered state with tunable unconventional superconductivity), the cuprate system La$_2-x$Ba$_x$CuO$_4$ [5] (where an extremely low uniaxial stress of 0.1 GPa induces a dramatic rise in the onset of 3D superconductivity), and superconducting TMDs 2H-Nb$X_2$ (X=Se,S) [6] (where a strong strain/hydrostatic pressure effect on the superfluid density and its unconventional scaling with the critical temperature were observed). I will discuss these results using a combination of muon-spin rotation under pressure/strain/field, magnetization, transport, and diffraction techniques.

[1] Mielke et. al., and Guguchia, Communications Physics 5, 107 (2022).
[2] Mielke et. al., and Guguchia, Nature 602, 245-250 (2022).
[3] Guguchia et. al., Nature Communications 14, 153 (2023).
[4] Guguchia et. al., NPJ Quantum Materials 8, 41 (2023).
[5] Guguchia et. al., Physical Review Letters 125, 097005 (2020).
[6] Rohr et. al., and Guguchia, Science Advances 5(11), eaav8465 (2019).

Speaker: Zurab Guguchia (PSI - Paul Scherrer Institut)

12:00

Lunch

Afternoon Session 1: Contributed Talks

6

THz control of quantum materials under pressure

Quantum materials exhibit rich phase diagrams, strongly sensitive to external parameters, which include intriguing properties such as magnetic and ferroelectric order, electronic correlations, superconductivity, and spin and charge order. These macroscopic properties arise from the complex interactions between electronic, structural, spin and orbital degrees of freedom. While key in defining the unique response of quantum materials, the complexity of these couplings and interactions poses a tremendous challenge for the physical understanding, theoretical modeling and technological device applications of these systems.
One approach that has proven successful in decoupling the effect of different degrees of freedom is to perform time-resolved measurements, which yield the out-of-equilibrium response of different components of the system following an ultrafast perturbation. Of particular interest is photoexcitation by a terahertz pulse, where the low photon energy ensures that the out-of-equilibrium sample remains closer to its electronic ground state than when e.g. an optical pump is used. Another approach to address the complexity of quantum materials is to reduce the available parameter space by choosing one external parameter, such as pressure, which can be continuously controlled (contrary to doping) while preserving thermal equilibrium (contrary to temperature). Pressure has been used extensively to draw phase diagrams in equilibrium but only to some extent with ultrafast measurements.
Combining terahertz spectroscopy or terahertz photoexcitation with pressure poses significant technological challenges, which has led to slow progress in this field despite its great potential. I will discuss our preliminary results in mapping out the pressure and temperature dependence of the THz response of Cr-doped V$_2$O$_3$, a canonical Mott insulator.

Speaker: Elsa Abreu (ETH Zürich)
7

Pressure tuning and Evolution of Structural, Magnetic and Electronic Properties in TMPX3 van-der-Waals Compounds

Control of dimensionality in condensed matter continues to reveal novel quantum phenomena and effects. Transition metal phosphorous trichalcogenides TM P X 3 (TM = Mn, Fe, Ni, V, etc., X = S, Se) have proven to be ideal examples where structural, magnetic and electronic properties evolve into novel states when their dimensionality is tuned with pressure. At ambient pressure, they are two-dimensional van-der-Waals antiferromagnets with strongly correlated physics. Our recent experimental studies [1-4] have shown dimensionality crossover related pressure-induced insulator-to-metal transitions and novel magnetic phases in FePS3. To elucidate the relationship between structural transitions, magnetism and electronic properties, we also performed a random structure search using first-principles calculations at high pressures and DFT+U studies [5]. We experimentally explored the coexistence of the low- and intermediate-pressure phases, and we predict a novel high-pressure phase with distinctive dimensionality and possible options for interpreting the origins of metallicity.
We will also present our most recent single-crystal high-pressure synchrotron X-ray study on crystalline structures and the high-pressure neutron scattering study on magnetic structures of FePSe3, a similar compound to FePS3 with reported high-pressure induced superconductivity occurring at 2.5 K and 9.0 GPa [6]. The new work performed at the DIAMOND light source and Institut Laue Langevin finally provides clear crystallographic assignments related to phases which emerge with the application of pressure.

References:
[1] C. R. S. Haines, et al., Phys. Rev. Lett., 121, 266801 (2018).
[2] M. J. Coak, et al., J. Phys. Condens. Matter, 32, 124003 (2020).
[3] M. J. Coak, et al., Phys. Rev. X, 11, 011024 (2021).
[4] D. M. Jarvis, et al., Phys. Rev. B, 107, 54106 (2023).
[5] S. Deng, et al., SciPost Phys. 15, 020 (2023).
[6] Y. Wang, et al., Nat. Commun. 9, 1914 (2018).

Speaker: Shiyu Deng (University of Cambridge)
8

Giant pressure-enhancement of multiferroicity in CuBr$_2$

Type-II multiferroic materials, in which ferroelectric polarization is induced by inversion-nonsymmetric magnetic order, promise new and highly efficient multifunctional applications based on the mutual control of magnetic and electric properties. Although this phenomenon has to date been limited to low temperatures, we have found a giant pressure-dependence of the multiferroic critical temperature in CuBr$_2$, specifically from 73.5 K at ambient pressure to 162 K at 4.5 GPa. Not only is this to our knowledge the highest value yet reported for a nonoxide type-II multiferroic but its growth also shows no sign of saturating, and the dielectric loss remains small, at these pressures. We establish the structure under pressure and demonstrate a 60% increase in the two-magnon Raman energy scale up to 3.6 GPa. First-principles structural and magnetic energy calculations provide a quantitative explanation in terms of dramatically pressure-enhanced interactions between CuBr$_2$ chains. These large, pressure-tuned magnetic interactions motivate structural control in cuprous halides as a route to applied high-temperature multiferroicity.

Speaker: Dr Bruce Normand (LTC, SCD, Paul Scherrer Institute)
9

Mobile interfaces in tubes revealed for three-phase systems containing pressurized methane, p-xylene, and water using neutron imaging

Interfacial tensions for systems containing model compounds for the freeze out from natural gas can be measured at high pressures by observing the interface shapes in tubes. Mobile interfaces in opaque tubes positioned parallel to gravity are easy to prepare and neutron imaging can provide related system properties (composition, density, etc.). We have observed the phase interfaces in the titanium tubes for the pressurized systems consisting of perdeuterated p-xylene (p-C8D10) layered over water (10.88 mol.% of H2O in D2O), and exposed to pressurized methane (CH4, 1.0 to 101 bar) at 7.0 to 30.0 °C. The shape of the meniscus through the central plane of the axially symmetric interface follows the Young-Laplace equation

The tomographic reconstruction of the meniscus shape were based on the assumption of axial symmetry and derived from the single radiographies (pixel size 20.3 micrometer) using the onion-peeling algorithm [2]. While the shape of the reconstructed meniscus is crucial for the calculation of the interfacial and surface tensions, the swelling and composition of the phases provides information on the density change. Constraints determining the sensitivity and uncertainty of the method will be discussed.

References
1 Dasch, C. J. Applied optics 31, 1146-1152, doi:10.1364/AO.31.001146 (1992).
2 Vopička, O., Durďáková, T.-M., Číhal, P., Boillat, P. & Trtik, P. Scientific Reports 13, 136, doi:10.1038/s41598-022-27142-6 (2023).

Acknowledgement
Authors acknowledge the financial support obtained from GACR and SNSF, project 23-04741K. This work is based on experiments performed at the NEUTRA thermal neutron imaging beamline (proposal 20200129), Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland.

Speaker: Ondrej Vopicka (Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 166 28 Prague 6, Czechia)
10

Resolving spin reorientation of CrCl3 induced by high-pressure with MuSR and neutron diffraction

Researches on two-dimensional (2D) materials have attracted tremendous attention both from fundamental and applied sciences since accelerated by the discovery of graphene. Among a large number of 2D materials, chromium trihalides CrX3 (X = Cl, Br, I) van der Waals (vdW) magnets have also raised a large interest due to the existence of many magnetic subtleties that cannot be explained by their magnetic and/or structural transitions.
Numerous studies were performed on CrI3, but only a few have been reported so far on its analogue CrCl3. The 2D vdW CrCl3 compound is stabilized under a rhombohedral symmetry, consisting of 2D Cr layers arranged in a honeycomb web fashion and surrounded by octahedrally coordinated Cl, with weak vdW inter-layer coupling. This makes CrCl3 an ideal system to study under external stimuli such as pressure or magnetic field, where new intriguing states can be unveiled. Expectantly, studies of CrCl3 under high pressure and room temperature have been reported. [1] However, its spin dynamics at low-temperature and high-pressure regimes remain unexplored. Motivated by the variability of the spin degree of freedom and spin dynamics under such conditions, we performed muon spin rotation (MuSR) and neutron powder diffractions (NPD) on ambient and hydrostatically pressured CrCl3 up to 23 kbar down to 2 K. [2,3]
In this study, by incorporating the two techniques and high-pressure, we resolved a suppression of the magnetic ground state and a stronger relaxation rate by MuSR. Within the magnetically ordered states, a spin reorientation was also observed by NPD at high pressure. A linear extrapolation points toward the suppression of magnetism at about pc= 30 kbar indicating the possible existence of a critical point at pc. [3]
[1] Ahmad, A., et al. Nanoscale 12.45 (2020): 22935-22944.
[2] Forslund, O., et al. arXiv:2111.06246 (2021).
[3] Ge, Y., et al., in preparation.

Speaker: Yuqing Ge (Chalmers University of Technology)

14:30

Coffee

Afternoon Session 2: Contributed talks

11

Expanding high-pressure neutron instrumentation capabilities

The demand for high-pressure equipment has doubled over the last decade at the Institut Laue-Langevin. To cope with this demand and ensure successful experiments, we have enhanced pressure generators and expanded our suite of pressure devices.

First, we have significantly improved the 1 GPa liquid pressure generators with:
- a comprehensive revamp of the automation program improving the reliability,
- a modern user interface easing control and maintenance,
additional sensors and controls enhancing safety,
- programmable pressure ramps controlled with greater precision,
- remote control and data archiving capabilities.
These enhancements will also be extended to 1 GPa helium gas pressure generators.

We have also developed a non-magnetic Ø6 mm sample bore double-wall pressure cell accommodating pressures up to 1 GPa with liquid or gas pressure transmitting media. Compared to other cells, it reaches higher pressures and features improved neutron transmission and signal-to-noise ratio. Our processes also now adhere to European Certification standards for answering new safety regulations. Today, we prepare a 2 GPa clamp incorporating in-situ temperature and pressure measurements via Ruby fluorescence [1, 2].

As for cells tailored for NSE and SANS experiments, they have demonstrated exceptional qualities, including ultra-low neutron background and high neutron transmission. While the 50 MPa and 300 MPa versions have proven high reliability, the 500 MPa variant has encountered issues and we actively seek advices and know-how to develop a optimal design.

References:
[1] P. Naumov, R. Gupta, M. Bartkowiak et al., Optical Setup for a Piston-Cylinder Pressure Cell: A Two-Volume Approach. Phys. Rev. Applied 17 (2022) 024065
[2] R. Khasanov, M. Elender and S. Klotz. The use of LEDs as a light source for fluorescence pressure measurements, High Pressure Research 43 (2023) 192

Speaker: Julien GONTHIER (Institut Laue Langevin)
12

Study of Background Noise in High-Pressure Neutron Scattering Experiments

Advanced high-pressure neutron scattering experiments demand a high neutron flux and precise phase space at small sample volumes, while maintaining a high signal-to-noise ratio. This work is dedicated to a comprehensive evaluation of background noise in high-pressure neutron scattering experiments, employing simulations and benchmark experiments. McStas 3.2 with the Union component is used to simulate the sources of background noise and its effects on high-pressure experiments. Validation experiments are conducted at the CAMEA (Cold Neutron Triple-Axis Spectrometer) at SINQ (Swiss Spallation Neutron Source), utilizing Ho2Ti2O7 powder samples placed in a 5 mm diameter × 19 mm height container. The container is then housed within a CuBe clamp cell, which is subsequently placed in orange cryostats. Simulations and tests are compared to understand the sources of background noise and assess its impact on high-pressure experiments. Furthermore, the potential solutions to reduce background noise from pressure cells are discussed.

Speaker: Zhanwen Ma (PSI - Paul Scherrer Institut)
13

Magnetic properties of strained (010)-DyFeO3 thin films and single crystals

Strain in antiferromagnetic orthoferrite thin films is predicted to significantly change magnetic properties and result in a polar response up to room temperature. Orthorhombic DyFeO₃ is of particular interest since the Fe-spins undergo a spin-reorientation with transition temperatures depending strongly on the Dy-Fe interaction and a magnetic field induced ferroelectric phase below the Dy ordering temperature of 4K. To gain an understand of the magnetic properties of highly strained, coherently grown (010)-oriented DyFeO₃ thin films we studied the pressure dependence of a DyFeO₃ single crystal at the thermal triple-axis spectrometer EIGER, SINQ. The scattering experiments were conducted in the (0kl) scattering plane in the temperature range between 1.5 and 100K. For the pressure dependent measurements, a helium gas pressure cell has been used with a max reachable pressure of 5 kbar to study the spin reorientation transition (T_SR) at 40K under uniaxial pressure conditions. In the temperature regime of interest, the He-pressure cell has the advantage that the pressure medium is still liquid and the applied pressure therefore truly isostatic. For the single crystal we measure an increase in T_SR with a rate of 1K/500bar. As a rule of thumb, a lattice mismatch between film and substrate of 1% corresponds roughly to a chemical pressure of 10kbar. Hence a T_SR=70K equals a measured film lattice change of approximately 3%. Overall, pressure depedent magnetic data for a DyFeO₃ single crystal are in broad agreement with values obtained for highly strained thin films.

Speaker: Christof Schneider (PSI - Paul Scherrer Institut)
14

Complex Order Parameter Tuning in Superconducting La1.88Sr0.12CuO4

High-temperature superconducting cuprates are a model system to examine the relationship between intertwined quantum phases. The competition has, however, been difficult to tune with external stimuli without inducing superconducting vortices by a magnetic field at the same time. In our study, we show that $c$-axis strain couples directly to the phase competition between charge stripe order and superconductivity in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). To track the evolution of charge order upon application of strain at different temperatures, dopings, and magnetic fields, x-ray diffraction measurements were performed at DESY. We show, that compressive $c$-axis pressure enhances stripe order only within the superconducting state. The strain furthermore diminishes the magnetic field enhancement of stripe order. It thus provides a fruitful approach to study the interplay between superconductivity and charge order in the cuprates.

Speaker: Julia Küspert (University of Zurich)
15

Pressure tuning of quantum matter: A local-probe perspective

The demanding experimental conditions required to access the quantum critical behavior of many materials (including high magnetic fields, high pressures, and ultra-low temperatures), make their microscopic investigation often problematic. Over the years, techniques such as the nuclear magnetic resonance and muon-spin rotation/relaxation have emerged as complementary, well suited (and often unrivalled) methods up to the challenge.

Here, we focus on the effects of high pressure on strongly-correlated quantum matter, as observed from the local-probe perspective of nuclei (NMR) and muons (μSR). Pressure tuning is used to establish phase diagrams, induce phase transitions, and identify critical points. By means of selected examples, comprising low-dimensional magnets [1,2], unconventional superconductors [3], and heavy-electron systems [4], we show how the delicate balance between competing ground states, reflecting close-lying energy scales, is modified by pressure. We conclude with recent developments in uniaxial strain experiments, where the breaking of rotational lattice symmetry can provide access to the underlying symmetries.

Despite the many challenges involved with high-pressure experiments, once the associated technical difficulties are overcome, even conventional materials are shown to exhibit extraordinary properties, as demonstrated by the record-breaking T_c superconducting hydrides [5,6].

References
1. N. Barbero et al., Phys. Rev. B 93, 054425 (2016).
2. G. Simutis et al., Phys. Rev. B 98, 104421 (2018).
3. T. Shiroka et al., Nature Commun. 8, 156 (2017).
4. G. Lamura et al., Phys. Rev. B 101, 054410 (2020).
5. A. P. Drozdov et al. Nature 525, 73 (2015).
6. D. V. Semenok et al., J. Phys. Chem. Lett. 9, 1920 (2018).

Speaker: Toni Shiroka (PSI - Paul Scherrer Institut)

Posters + Beers + Wine + Apero

Poster session with drinks and food

High Pressure Workshop

Auditorium (WHGA)

PSI Villigen