The magnetic phases in materials that host high-temperature superconductivity are often as exciting as the superconductivity itself. Our recent study of the Ni-doped CaKFe4As4 is a good example – we used muon spin rotation spectroscopy to determine the magnetic structure and found that it is the exotic hedgehog-type spin-vortex crystal structure shown on the left of the figure.

Obtaining the magnetic structure using muons is not an easy task. To achieve it, we used ab-initio calculations to determine the stopping position of the muons in the sample. We could then calculate the magnetic fields at the muon sites arising from different hypothetical structures. Combined with accurate muSR measurements on a single crystal shown on the right of the figure, we found that only one structure is consistent with the observation – indeed that one of the hedgehog-type spin-vortex crystal.

To find out more, see our paper at Phys. Rev. B 102, 094504

One of the dreams of the great P. W. Anderson was to use a spin liquid as a substrate for superconductivity. Starting with a highly entangled quantum state (In his original proposal, the resonating valence bond state) and adding charge carriers would give you a superconductor or at least a highly-correlated metallic state. Unfortunately, usually some form of localisation wins and the dreams are shattered.

There is a recent exception – a hyperkagome iridate Na4Ir3O8 is an insulating spin liquid, but one can also create a Na3Ir3O8 compound, which is a hole-doped system and it conducts! The big mystery is what happens to the original spin-liquid magnetism?

We used NMR to access the intrinsic magnetic properties of this doped spin liquid and found that the susceptibility is governed by the semimetal electronic structure. The dynamical properties were studied by measuring the relaxation of the nuclear spins. In  Na3Ir3O8, there are two sites for the 23Na nucleus – One has two Ir-triangles connected to it, whereas the other one has only one full triangle and the remaining Ir atoms come from three different triangles. This local structure is crucial – it allows us to contrast the dynamical fluctuations in two different environments.

By comparing the relaxation of the two nuclei, we found that the antiferromagnetic fluctuations persist even in the semimetal compound.

To find out more, see our preprint: https://arxiv.org/abs/2007.01633

The kagome magnet Co3Sn2S2 has recently attracted a lot of attention as a promising Weyl semimetal candidate. One of the interesting observations has been that the magnitude of the Anomalous Hall Effect is constant with temperature below 90 K and then gradually decreases until the magnetic order disappears at around 170 K.

In our recent muon study, we found that this temperature range, in fact, corresponds to the presence of two competing magnetic orders: At low temperatures, there is a ferromagnetic ground state but an in-plane antiferromagnetic state appears at temperatures above 90 K, eventually attaining a volume fraction of 80% around 170 K, before reaching a non-magnetic state. The behavior of the anomalous Hall conductivity is found to be related to the volume fraction of the ferromagnetic state.

This study is interesting for two reasons. First, it establishes a link between the topological properties, such as Berry curvature and magnetism as well as provides the evidence of tuning the Berry curvature through magnetic phase competition. The second reason is no less exciting – In this study, we combined detailed DFT calculations which obtained the muon stopping site with the local field calculations at the muon site to obtain a qualitative picture and a physical model of the two competing phases. This points to the direction of muSR progressively becoming a more quantitative technique.

Find out more at: Nature Communications 11, 559 (2020)

The recent discovery of pressure-induced superconductivity in the iron-based spin ladders has added a new flavor to the study of iron-based superconductivity. On one hand, they have a quasi-one-dimensional structure and on the other – they are insulating at ambient conditions. Hence they appear to be more like cuprates than the other iron-based superconductors. Moreover, since these compounds do not require doping, they open an avenue to study the interplay of superconductivity and magnetism without the introduction of any disorder.

The exact nature of the magnetic ground states and their evolution has been a focus of intense studies and recently, through the use of multiple techniques we have answered some of the issues relating to the magnetic and crystal structure of these compounds.

In the case of the sulfur-end compound BaFe2S3, we have resolved the discrepancies of the previously reported anomalous jumps and have provided a consistent picture of the evolution of the magnetism with pressure.
Find out more at Phys. Rev. B 98, 180402(R)

More recently, for the selenium-end compound, BaFe2Se3, a detailed phase diagram shown above was determined. In contrast to the sulfur-end compound, there exists an abrupt structural transition. Nevertheless, the modifications of magnetic ordering temperature and ordered moment size remain smooth throughout the whole pressure range. Even more strikingly, the exotic block magnetism (antiferromagnetically coupled ferromagnetic clusters) remains robust across the transition and is stable in the whole studied pressure range.
Find out more at Phys. Rev. B 100, 214511

The discovery of the iron-based superconductors a decade ago has put a new fire into the research of unconventional superconductivity. Even the simplest compound of the family – FeSe – has opened up a canvas of new scientific discussions.

Recently, we tackled a few problems that were open in this emblematic compound. There are different magnetic and superconducting phases present in the material and the questions on how they compete and transform from one phase into another have been puzzling the community for a long time. Muon spin rotation under high pressures turned out to be the key experimental technique to answer them – it offers the independent measurement of the volume fraction and magnetic moment as a function of a control parameter.

First, we could find a tricritical point in the pressure-temperature phase diagram, where the magnetic ordering phase transition at very high pressures switches from the second-order to the first order.
Find more at Phys. Rev. B 97, 224510.

Second, when looking at the version with the substitution of sulfur for selenium, we found an emergence of a magnetically ordered phase, similar to the case of the pure system but shifted to lower pressures. We could study the complex interplay between the superconductivity and the magnetism, but what we found to be the most surprising was the discovery of an extended dome of long-range magnetic order that spans a pressure range between previously reported separated magnetic phases.
Find more at Phys. Rev. Lett. 123, 147001

I had a chance to visit the International Conference for Strongly Correlated Electron Systems in Okayama. It featured a really broad spectrum of scientific discussion both at the oral presentations as well as in the lively poster sessions. Throughout the week I had a number of beneficial discussions about systems with strong spin-orbit coupling, Mott transitions, and frustrated magnets. Perhaps the highlight to me was the prominence of the heat transport measurements as a new window to look at exotic excitations in the strongly correlated electron materials and it will be interesting to see where it goes in a few years.

Known since ancient times, elemental Bismuth is a brittle metal, among the most well-characterized materials available. At ambient pressure it becomes superconducting at extremely low temperature of 0.5 mK which makes it extremely difficult to study.

The situation changes radically under pressure. Upon pressure-induced structural transition, the superconducting temperature rises to several Kelvin. Our recent muon work investigates these superconducting phases. At the intermediate pressures, the superconducting phase expels the applied magnetic field showing very clear Meissner effect, whereas the high-pressure phase allows penetration of magnetic field and forms vortices throughout the sample, which can be picked up by the implanted muons. These observations are textbook-like example of the different behavior of the Type-I and Type-II superconductors and the transformation between the two.

Find more on the two different phases in the papers:
Bi-II phase: Phys. Rev. B 99, 174506
Bi-III phase: Phys. Rev. B 98, 140504(R)

I gave a popular science talk at the LPS about high-pressure science. It was a great opportunity to introduce the topic to a broad audience and to discuss possible experiments. Thanks to Manon Marchand for organizing the seminar as well as for the neat poster design.

I have been awarded a Mobility Fellowship by the SNF to study new aspects of frustrated magnets as well as to learn the NMR technique. This has allowed me to move to the Laboratoire de Physique des Solides (LPS), which is a joint institute of CNRS and Paris-Sud university. I have joined the Spectroscopy of Quantum Matter group, where I am currently looking at some interesting iridates.

With the European spallation source slowly becoming a reality, the Nordic countries are working on building a fresh and energetic neutron user community. As part of that effort, they have been running annual fall schools to introduce master and PhD students to the potential of neutron scattering. This year I joined to give a presentation about using neutrons to learn about the dynamics of solid state systems. It is an excellent initiative and I hope it keeps running for many years to come.