Quantum materials are fascinating because they allow us to investigate fundamental questions, such as the behavior of a large assembly of interacting quantum objects, within laboratory settings. These materials also hold the potential for major technological advances, which might revolutionize our modern technology. However, due to the inherent complexity of the real materials, understanding how different types of order with potential functionality emerge and how to probe and control these states is the key challenge in the field.
In this seminar, I will discuss how my group is exploiting the strong coupling of electronic and lattice degrees of freedom in quantum materials to unravel the “hidden secrets” of correlated and topological quantum material using high stresses and strains. Using the example cases of frustrated and topological magnetic systems, I will show how these challenging experiments with bespoke apparatuses allow us to explore regions of the phase space that have remained inaccessible by chemical means. The results of these experiments under elastic tuning serve as a benchmark for theoretical modelling of the effects of correlation and frustration and hold the promise to answer central questions in the field. Throughout the talk, I will also highlight how we are continuously pushing the limits of the state-of-the-art technology under extreme conditions in order to address new questions in the field of correlated metals and insulators.

The recent discovery of high-temperature superconductivity in bulk crystals of nickelate La₃Ni₂O₇ [1] with T_c ~ 80 K is a truly remarkable achievement. On one hand, it is the culmination of a year-long process of careful material design, driven by advanced material synthesis and guided by theoretical predictions [2,3]. On the other hand, it has once again demonstrated the unparalleled power of hydrostatic pressures to induce, tune, and study unconventional electronic phases in quantum materials.
In this talk, I will discuss some experimental opportunities and technical challenges of high-pressure work, including some of our own recent high-pressure developments. Moreover, I will present our recent experiments into the superconducting and the normal state properties of La₃Ni₂O₇ and related systems, including high-pressure structural, transport, thermodynamic, and spectroscopic probes, as well as recent theoretical models [4]. Through comparison with the cuprates and the iron-pnictides, I will highlight the similarities and differences, and I provide an outlook into future research directions in this quickly emerging new field.
[1] Sun et al., Nature 621 (2023) 493
[2] Anisomov et al., Phys. Rev. B 59 (1999) 7901
[3] Lee and Pickett, Phys. Rev. B 70 (2004) 165109
[4] Puphal, PR, et al., arXiv:2312.07341, arXiv:2312.07341

Topological superconductors have attracted considerable attention due to the anticipated emergence of Majorana zero modes, a unique type of quasiparticle. These modes are particularly intriguing for their potential application in the development of fault-tolerant topological qubits for quantum computing. While the existence of these quasiparticles remains elusive, the search for topological superconductors led us through a fascinating journey. We have encountered a variety of highly unusual superconducting states, including nematic superconductivity in the doped topological insulator Bi₂Se₃ and the layered superconductor NbSe₂, nodal superconducting phases induced by high magnetic fields, interfacial superconductivity between FeTe (a non-superconducting parent compound of iron-based superconductors) and the topological insulators Bi₂Te₃ and Sb₂Te₃, and even cases of unconventional superconductivity coexisting with magnetism at the interface with the altermagnet MnTe. In this study we present our exploration of this diverse "zoo" of unusual superconducting phases in these materials.

Finding and understanding novel electronic materials is an exciting area where high-pressure studies can contribute in multiple ways. This includes tuning, crystal structures, competing ground states, and interactions between electrons. In this seminar, I will focus on key insight into how to optimize strong-coupling modes for superconductivity. I will present studies of hydride superconductors, where high-energy phonons couple to hydrogen orbitals in structures stabilised above 1 Mbar. In addition, I will present studies of transition metal dichalcogenides where soft charge-density-wave modes promote a dome of unconventional superconductivity around a quantum critical point.
Record superconductivity has been discovered in H₃S, LaH₁₀, YH₉, and other hydride compounds [1–3]. However, experimental studies of superconductivity in hydrides remain an enormous challenge. Our results provide the first independent confirmation of superconductivity in H₃S [4] where we identify clean-limit superconductivity and demonstrate the effects of thermal fluctuations. In addition, we discovered the new superconductor La₄H₂₃ and discuss structural instabilities [5].
In TiSe₂, we observe a dome of superconductivity around the quantum critical point of the charge-density-wave order [6]. We show that superconductivity only emerges once a Lifshitz transition releases electron and hole pockets connected by the wave vector of the charge order. This suggests s^± superconductivity mediated by CDW fluctuations analogue to some of the iron pnictide superconductors.
[1] P. Kong et al., Nature Communications 12 (2021) 5075
[2] M. Somayazulu et al., Phys. Rev. Lett. 122 (2019) 027001
[3] A. P. Drozdov et al., Nature 525 (2015) 73
[4] I. Osmond et al., Phys. Rev. B 105 (2022) L220502
[5] S. Cross et al., Phys. Rev. B 109 (2024) L020503
[6] R. D. H. Hinlopen et al., Science Advances accepted (2024)

In the recent years two-dimensional van der Waals materials are at the forefront of the research in condensed matter physics and material science. On the one hand the magic angle bi-layer graphene has set a new trend in unconventional superconductivity. On the other hand the presence of long range magnetic order in two- dimensional van der Waals materials has completely opened a new avenue for the investigation of magnetism in true 2D-systems. In my talk I will present, explorative research of these emerging two dimensional quantum materials. Highlighting the aspects of the intimate relation between long range magnetic order, topology and superconductivity in these systems. Later, I will discuss detailed anisotropic magnetic properties aiming to understand the ground states of selected compounds.
[1] S. Selter et al., Phys. Rev. Mater. 5 (2021) 073401
[2] Y. Shemerliuk et al., Electron. Mater. 2 (2021) 284
[3] G. Shipunov et al., J. Phys. Chem. Lett. 12 (2021) 6730
[4] G. Shipunov et al., Phys. Rev. Mater. 4 (2020) 124202
[5] A. Kuibarov et al., Nature 626 (2024) 294

Angle resolve photoelectron spectroscopy (ARPES) is a very powerful approach to clarify the electronic structures of conductive materials. However, the sample rotation was usually employed to cover the wide (k_x,k_y) region. Then the selection rules changes and often the probed regions on the surface changed also. The low detection efficiency of the conventional ARPES also induced the surface radiation damages, spoiling the quality of the obtained results.
To solve these problems, photoelectron momentum microscope (PMM) was developed in early 2010th. When I met with this approach in the Kirschner’s Lab in Max-Planck-Institute for Microstructure Physics, Halle in 2013, I recognized its high potential. By the high extraction voltage between the sample surface and the PEEM type objective lens up 15 or 20 kV, the whole photoelectrons emitted into 2 π steradian can be simultaneously recorded by the 2D detector in the case of Ek up to ~70 eV. Then the ARPES efficiency becomes orders of magnitude higher by PMM than the conventional ARPES. In addition, effective spin detection is feasible by use of 2D spin filter at low electron kinetic energy (10.25 and 11.5 eV) with FoM of around 100. Then the spin detection efficiency becomes around 5 million times higher than the Fe-O VLEED single channel detection.
Since we have installed the PMM at UVSOR BL6U + BL7U we can now perform E_B(k_x,k_y,k_z) PMM not only at 68° incidence but also normal incidence for the 1st time in the world. The selection rule dependence of the E_B(k_x,k_y,k_z) is now measurable and theoretically anlyzable. Within this year, we will install a Au/Ir(001) spin filter and start SP-PMM in UVSOR, Okazaki, Japan. The sub-μm resolution without any sample movement and radiation damage will provide us useful and reliable information for device development. We will soon start measurements of magnetized samples as well as samples under electric field and uniaxial strain.
On the other hand, non-conductive materials should be studied by other method. Soft X-ray resonant inelastic scattering (SX-RIXS) combined with theoretical analyses can provide rich information on electronic states. Operando measurement is feasible under the magnetic field, electric field as well as under a uniaxial strain. Since this technique is bulk sensitive, the focusing of the soft X-ray smaller than 1 μm facilitates the reliablemeasurement. By combining these techniques and spin-STM/STS, we would like to study electronic states of operand device in the μm scale to develop new devices for the fruitful future of human beings.
[1] Photoelectron Spectroscopy: Bulk & Surface Electron Structures. Springer Series in Surface Sciences 72 (2021) by S.Suga, A.Sekiyama and C.Tusche

New emergent states of matter arise when large numbers of particles (e.g. electrons in a solid) interact with each other. One of the most fascinating and famous examples is superconductivity, that has many applications such as Maglev trains or MRI scanners. Before emergent states can unfold their great potential for future technological applications, our understanding of those states needs to be enhanced. However, they are extremely difficult to predict theoretically because it means solving the many-body problem. Hence, experiments play a key role and drive many of the breakthrough discoveries in the field. In this talk, I show how my group develops and uses bespoke state-of-the-art techniques to find and explore such states experimentally. We measure thermal, electronic and magnetic properties of bulk samples under extreme experimental conditions of very low temperatures, high pressures and high magnetic fields. One of our research directions is based on the recent discovery and investigation of the highly unconventional superconductor CeRh₂As₂ that has two superconducting states and an unusual ordered state above the critical temperature. It serves as a platform to study the interplay of ordered states in the novel class of locally non-centrosymmetric materials potentially hosting odd-parity topological superconductivity. This is just one example of the exotic phenomena we unveil in unconventional metals, magnets, and superconductors.

Quantum materials, whose macroscopic response is determined either by topology or strong electronic correlations are at the cutting edge of contemporary condensed matter physics research. The interplay between topology and electronic correlations imparts robustness and order to these materials, offering the potential for groundbreaking technological advancements. Experiments conducted under extreme conditions, such as high magnetic fields are crucial for the determination and understanding of the unique properties of quasi-particles and the material's Fermi surface.
In my seminar, I will present the beauty of quantum oscillatory phenomena observed in nodal-line semimetals (NLSMs). Specifically, I will delve into the Fermi surface analysis of ZrSiS, an exemplary NLSM, as determined by quantum oscillation measurements. This analysis provides a comprehensive understanding of the material’s electronic ground state, suggesting the presence of electronic correlations. Additionally, I will highlight recent findings on the tunability of these layered materials under uniaxial strain and devices based on nanostructures. I will conclude with an overview of ongoing projects and future research goals.

The search for materials for energy-efficient and eco-friendly refrigeration technologies remains a significant challenge in replacing conventional vapor compression systems. Barocaloric refrigeration is based on the adiabatic temperature and isothermal entropy changes of materials under external hydrostatic pressure. Spin CrossOver (SCO) compounds have recently been pointed out as promising candidates, exhibiting substantial barocaloric effects, particularly at low hydrostatic pressures (<1.2GPa) [1].
In SCO compounds, the central metal ion transitions between low spin (LS, favored by low temperature and high pressure) and a high spin (HS, favored by high temperature and low pressure) state, resulting in significant changes in spin entropy due to inter- and intramolecular vibrations [2-4]. However, understanding the microscopic mechanisms driving the HS-LS transition and its impact on physical properties remains a key point of research.
Our investigation focuses on Fe(Pm-Bia)2(NCS)2, where Pm-Bia = (N-(2’-pyridylmethylene)-4 amino bi-phenyl) SCO compound that crystallizes into two different polymorphs exhibiting SCO transitions with significantly different behavior. By employing DSC, magnetization, and powder and single-crystal X-ray diffraction measurements as a function of temperature and pressure, we investigate the structural changes induced by spin transitions in both polymorphs [5]. Our findings reveal marked differences in transition nature between the orthorhombic and monoclinic polymorphs.
We explore the role of hydrogen bonding and π- π interactions in influencing observed behaviors, providing insights into underlying mechanisms. The monoclinic polymorph, governed by π – π interactions, exhibits temperature-induced spin transitions over a wide range, also pressure-induced transitions to LS due to the presence of π - π interactions, however, the emergence of H-bonds at higher pressures hinder the completion of the transition to the LS. On the other hand, the orthorhombic polymorph undergoes swift temperature-induced transitions facilitated by H-bonding, and with applying pressure the H-bond hiders the transition to the LS due to limited volume availability.
Our study delves into the cooperativity of the two polymorphs, quantified through a thermodynamic model. We propose looking for candidates for barocaloric applications with robust intermolecular interactions, and large volume space so that pressure-induced transitions to the LS state are not hindered by space limitations.
Our research sheds light on the intricate interplay between molecular-scale phenomena and macroscopic properties in SCO compounds, covering the way for the development of novel materials or efficient refrigeration technologies based on SCO as a barocaloric material.
[1] K. G. Sandeman, APL Mater. 4 (2016) 111102
[2] S. P. Vallone et al., Adv. Mater. 31 (2019) 1807334
[3] P. Gütlich et al., Beilstein J. Org. Chem. 9 (2013) 342
[4] M. Buron-Le Cointe et al., Phys. Rev. B 85 (2012) 064114
[5] H. Shahed et al., Acta Crystallogr. B 79 (2023) 354

More than 35 years after the discovery of non-conventional superconductivity in layered cuprates, a consistent picture of their special behavior is still missing despite the efforts deployed. Experimental observations and theoretical hypotheses have been accumulating over the years, with scant advances in terms of clear-cut simplifications. The reason for that is in the deep entwining of charge, spin and lattice degrees of freedom that govern cuprates. ResonantX-ray scattering (elastic and, mostly, inelastic, ie RIXS) is however providing observations helpful for the simplification of the picture. In fact, by RIXS the energy scale of orbital [1] and spin excitations [2] were definitely assessed, in parent compounds and in superconductors. More recently, RIXS revealed elusive charge order and associated fluctuations strongly mixed with some lattice modes [3]. Moreover, RIXS has eventually been used to detect the opening of the superconducting gap and of the pseudogap in YBCO [4]. And superconducting infinite-layer (IL) nickleates, the closest analogues of cuprates, are being studied with RIXS very successfully [5]. I will provide an overview of the recent results on cuprates and IL nickelates obtained by our group, with an outlook to the new opportunities available at XFELs.
[1] L. Martinelli, K. Wohlfeld, J. Pelliciari, R. Arpaia, N. B. Brookes, D. Di Castro, G. Ghiringhelli, Phys. Rev. Lett. 132 (2024) 066004
[2] L. Martinelli, D. Betto, K. Kummer, R. Arpaia, L. Braicovich, D. Di Castro, G. Ghiringhelli, Phys. Rev. X 12 (2022) 021041
[3] R. Arpaia, L. Martinelli, M. M. Sala, S. Caprara, A. Nag, N. B.Brookes, G. Ghiringhelli, Nat. Commun. 14 (2023) 7198
[4] G. Merzoni, L. Martinelli, L. Braicovich, N. B. Brookes, F. Lombardi, R.Arpaia, G. Ghiringhelli, Phys. Rev. B 109 (2024) 184506
[5] F. Rosa, L. Martinelli, G. Krieger, L. Braicovich, N.B. Brookes, G. Merzoni, M. Moretti Sala, R. Arpaia, D. Preziosi, M. Salluzzo, M. Fidrysiak, G. Ghiringhelli, unpublished

The recent discovery of multiple superconducting phases in UTe₂ boosted research on correlated-electron systems [1,2,3,4]. The proximity to a ferromagnetic quantum phase transition was initially proposed as a driving force to triplet-pairing superconductivity [1], and this heavy-fermion paramagnet was rapidly identified as a reference compound to study the interplay between magnetism and unconventional superconductivity with multiple degrees of freedom.
I will present series of electrical-resistivity experiments we have performed on UTe₂ at the LNCMI in the last few years [2,5,6,7]. We have combined pulsed magnetic fields with sub-kelvin temperatures (dilution or 3He fridges) or high pressures (Bridgeman cell)
Superconducting phases were found to be induced by a magnetic field in the vicinity of metamagnetic transitions. Indirect evidences for enhanced magnetic fluctuations at pressure and field- induced transitions were extracted from Fermi-liquid fits to the data, and 2D and 3D electronic phase diagrams were built.
I will also present investigations by neutron scattering of the magnetic properties of UTe₂ [8,9,10]. We have observed quasi-two-dimensional antiferromagnetic fluctuations, but no ferromagnetic fluctuations as initially speculated, at ambient pressure. A gapping of these fluctuations develops in the superconducting phase and is a signature of the interplay between magnetism and superconductivity. More recently, we found that UTe₂ becomes antiferromagnetically ordered under pressure. This indicates that superconductivity in UTe₂ develops in the vicinity of a long-range antiferromagnetic phase, which differs from the initial proposition of a nearby ferromagnetic phase.
[1] Ran et al., Science 365, 684 (2019)
[2] Knebel et al., J. Phys. Soc. Jpn. 88, 063707 (2019)
[3] Braithwaite et al., Commun. Phys. 2, 147 (2019)
[4] Ran et al., Nat. Phys. 15, 1250–1254 (2019)
[5] Knafo et al., Commun. Phys. 4, 40 (2021)
[6] Valiska et al., Phys. Rev. B 104, 214507 (2021)
[7] Thebault et al, arXiv:2403.20277
[8] Knafo et al., Phys. Rev. B 104, L100409 (2021)
[9] Raymond et al., J. Phys. Soc. Jpn. 90, 113706 (2021)
[10] Knafo et al., arXiv:2311.05455

The study of thin films has been a cornerstone of experimental research at reduced dimensions. Furthermore, the interest in 2D materials and heterostructures has grown rapidly in the last two decades since the discovery of graphene, and has really exploded after highly correlated electronic phases and superconductivity were found in twisted bilayer graphene. However, there exists a whole other class of materials, namely oxides, where precise control over stoichiometry, interfaces and thickness can be achieved at the nanoscale using a variety of growth techniques. These oxides exhibit nearly all flavors of physical phases from magnetic to ferroelectric to superconducting to even exotic multiferroic ground states. I will take this opportunity to focus on recent developments in oxide growth enabling the separation of the grown thin film from the growth substrate, resulting in freestanding oxide membranes. These membranes have allowed for unprecedented access to avenues in experiments, with novel interfaces, which were previously inaccessible due to constraints of the film being bound on the substrate. I will talk about our studies in previously unexplored territories for oxides, through which I hope to convey how these developments in oxides promise a fertile ground for remarkable discoveries in material science and physics.
[1] H. Wang, V. Harbola, Y. J. Wu, P. A. van Aken, J. Mannhart, arXiv:2403.08736
[2] V. Harbola, Y. J. Wu, F. V. Hensling, H. Wang, P. A. van Aken, J. Mannhart
Advanced Functional Materials 34 (2024) 2306289
[3] V. Harbola, Y. J. Wu, H. Wang, S. Smink, S. C. Parks, P. A. van Aken, J. Mannhart
Advanced Materials 35 (2023) 2210989

Iridium-based 5d transition metal oxides have been focused on realizing novel superconductivity or topological phases with an interplay between large spin-orbit coupling (0.3~0.4 eV) and short-range electron-electron interaction (~0.5 eV) [1, 2, 3]. Among these materials, SrIrO₃ (001) films have been reported to show a dimensionality-controlled metal-insulator transition (MIT) at a critical thickness of 4 unit cells. Monolayer SrIrO₃ (001) exhibits a band structure similar to that of Sr₂IrO₄, which has been proposed as a parent material for unconventional superconductivity [4]. Furthermore, SrIrO₃ films with (111) orientation have been predicted to exhibit a topological crystalline insulator [5]. We report on the epitaxial growth of SrIrO₃ films on SrTiO₃ (111) substrates revealing a twinned perovskite-like superstructure with a periodicity of 3 unit cells (uc) and the observation of magnetoresistance and anomalous Hall effect (AHE). The interfaces between the 3uc thick stacks are formed by face-sharing octahedra with a larger Ir-Ir lattice spacing than within the stacks where the octahedra share corners. X-ray circular magnetic dichroism and magneto-optic Kerr effect microscope confirm a ferromagnetic state in agreement with predictions from density-functional theory calculations. In addition, these calculations indicate the presence of Weyl points which might additionally contribute to the AHE. To study this, we performed hard X-ray momentum microscopy of the valence band at beamline P22 of DESY. Although the data is not yet conclusive, we discuss the microscopic origin of AHE in relation to topological effects, drawing insights from theoretical and experimental results in our films. This discussion paves the way for exploring topological phases.
References
[1] Y. K. Kim, Science 345, 187 (2014).
[2] S. J. Moon et al., Phys. Rev. Lett. 101, 226402 (2008).
[3] Z. Tian et al., Nat. Phys. 12, 134 (2016).
[4] P. Schütz et al. Phys. Rev. Lett. 119, 256404 (2017).
[5] P. M. Gunnink, R. L. Bouwmeester and A. Brinkman, J. Phys: Condens. Matter 33, 085601 (2021).

Symmetry and Topology in Advanced Electronic Materials Electronic properties of materials are encoded in the symmetry of their atomic or molecular lattice. Examples are ferroelectrics that break space inversion symmetry or ferromagnets that break time reversal symmetry. In recent years it became evident that novel properties can emerge at the nanoscale. In addition to symmetry, also topology, such as vorticity, determines the properties of these materials on the nanoscale.
This talk will highlight two searches for such textured materials:
1. Polar magnets RE₂Cu₂O₅ exhibiting complex magnetic phase diagrams and
2. Transition metal di-chalcogenides showing superconductivity at high magnetic fields.
Topology thus offers new electronic device opportunities.

Controlling magnetic exchange in quantum materials with low dimensional interactions is a promising route to address novel quantum many body effects in proximity to quantum criticality and opens the perspective for clean and tenable quantum simulators [1]. Within this seminar I will discuss recent results on the prototypical kagomé superconductor LaRu₃Si₂ where charge orders persist above room temperature [2]. I will show how pressure acts as clean tuning parameter for the underlying correlations with intriguing changes on the diffuse scattering.
References:
[1] Wehinger et al., Phys. Rev. Lett. 121, 117201
[2] I. Plokhikh et al., arXiv:2309.09255v1 (2023).

In the modern technological landscape, the seamless functioning of everyday technologies hinges on the advancements in magnetic materials. This seminar highlights the pivotal role played by magnetic materials, with the primary focus on R₂IrSi₃-type intermetallic compounds (R = Gd - Ho) and some novel Heusler alloys. The research investigates previously unexplored R₂IrSi₃-type compounds, unveiling their distinct crystallographic structures as well as the influence of minute atomic vacancies on their magnetic properties. The magnetic behaviors of these compounds, including complex magnetic transitions and ground state degeneracy, are thoroughly analyzed. Experimental studies on Gd₂Ir_0.97Si_2.97 and Tb₂Ir_0.95Si_2.95 reveal ground state degeneracy along with multiple magnetic transitions in both the materials. The role of atomic vacancies on the magnetic properties of these two compounds could be understood by studying the physical properties of stoichiometric Dy₂IrSi₃, revealing the vacancy-induced short range ferromagnetic ordering at high temperatures in these compounds. Dy₂IrSi₃ also exhibits a rather large value of adiabatic temperature change close to its Néel temperature (~6.6 K). The other member of the series, Ho₂IrSi₃, has been synthesized in single-crystalline form and an enigmatic broad hump in the heat capacity data signifies a considerable crystalline electric field splitting of 4f-level. Our investigation on some selected members of different itinerant moment Heusler systems, viz. full-, half- and inverse Heusler alloys also divulge some very interesting features. For example, the inverse Heusler alloy, Fe₂RuGe, defies theoretical predictions with a high-temperature magnetic ordering, that we have attributed to rather large Bader charge transfer. RuMnGa, a supposedly non-magnetic compound, had earlier been reported to exhibit a ferromagnetic ground state, contrary to theoretical expectations. Through our detailed study, we have shown that the magnetic ordering in this system is induced by minute presence of off-stoichiometry in composition. Rh₂FeAl demonstrates high-temperature magnetic ordering and Griffith's phase-like behavior, a first among Heusler compounds. The seminar presents a short glimpse in the very interesting aspects of correlations of crystal structure as well as atomic disorders and vacancies in relation to the magnetic ground states of the concerned materials and calls for further exploration. It also sheds light on the rich tapestry of magnetic materials, offering new insights and avenues for future research and innovation.