Correlated electron systems often host a range of phases that have comparable energy scales and can compete for the electrons or coexist with them. Archetypical examples are the electron and spin density wave states that appear in the vicinity of superconductivity in the La-based cuprate superconductors.
Nevertheless, the exact nature of these states as well as the coupling between electronic and spin instabilities has remained elusive. To attack this question, we applied uniaxial pressure to a high temperature superconductor La1.88Sr0.12CuO4. In our tour-de-force experiment we combined a newly designed pressure cell, state of the art neutron ray simulations and unique focusing capabilities of the ThALES instrument at the ILL neutron source to extract information from tiny samples.
As seen in the figure above, we found that uniaxial pressure repopulates the domains, with only one domain surviving. This effectively excludes all potential multi-q states, settles the uniaxial stripe phase as the ground state for the La-based superconductors and demonstrates the coupling between spin and charge order in these systems.
Physical properties of strongly correlated electron materials ultimately depend on the orbital overlaps of the electronic wavefunctions. One of the most direct methods to alter this overlap in a precise manner is uniaxial strain and over the last year we have been working on multiple uniaxial strain experiments.
In summer, we ran the first measurements with a new type of in-situ cell optimized for low angle scattering experiments, such as high-energy X-ray diffraction or small angle neutron scattering (SANS). You can read a highlight on the UZH website about our measurements at PETRA-III synchrotron.
Having learned a lot about the performance, we have spent the last few months improving it and adding new features. It is now packed again and ready to be shipped back to Hamburg for more experiments!
After a long period of interacting with researchers using online-only methods, the meeting in Innsbruck was a great opportunity for live interaction with fellow physicists. On one hand, it was a pleasure to present our results on doped spin liquids and listen to a number of interesting talks in the condensed matter sessions. However, the most important benefits of in-person meetings are the discussions that take place between the official sessions. I had a number of excellent conversations during coffee breaks about ongoing projects, technical developments and new ideas in the field.
Most of the time we report on studies, where our subjects are materials made in the lab. It can also be rewarding to be a subject yourself. One such study recently came out, where I had a pleasure to climb under a watchful eyes and instruments of the researchers at the ETH:
Triangular Lattice Heisenberg Antiferromagnets are a testbed for quantum magnetism. Initially proposed as quantum spin liquid candidates, we now know that the triangular lattice orders at zero temperature. In real materials, however this transition is shifted to finite temperature due to interlayer exchange interaction. We have recently learned that the quantum fluctuations persist deep into the ordered phase.
By combining NMR and muon spin techniques, we have found that there is a broad region in temperature that hosts persistent magnetic fluctuations. Moreover, in all of the compounds of the chromate family, these fluctuations appear universal, when scaled with the transition temperature of the system. This is suggestive of a scenario with a crossover from 2D to 3D correlations, preceded by a typical 2D regime that is intrinsic to the triangular lattice.
The recent results and future plans to study quantum matter has been recognized by two European grants. It is a great honor to receive both the Marie Curie COFUND as well as Alexander von Humboldt fellowships. On the 1st of May I have already started the MSCA grant as a PSI Fellow, meanwhile the AvH fellowship will allow pursuing a new line of research together with the University of Augsburg in a near future.
Our NMR study on doped spin liquid has now been published.
Quantum Spin Liquids are among the most exotic quantum states, with long-range quantum entanglement but no magnetic order. We have now shown that the magnetic correlations of the spin liquid survive even when an iridate spin liquid changes from an insulator to a semimetal.
Doping a spin liquid has been a goal ever since Anderson proposed it as a substrate for superconductivity, but until recently all attempts have led to localized electrons. The emergence of new Iridium-based spin liquids enabled crystals that can be seen as doped quantum spin liquids, but due to small samples, impurities and strong neutron absorption the magnetic properties have remained elusive. Here we solved the problem by using nuclei as probes for local magnetism. Using NMR we obtained intrinsic static and dynamic properties and found that the electronic bands are substantially modified by correlations. By contrasting the response of two different nuclei, we could pinpoint these correlations as survivals from the parent quantum spin liquid state.
At the start of the year, I moved to the Paul Scherrer Institute with the intention to develop and put to use various high-pressure setups to control the quantum matter. The key is that by applying high-pressure, we can modify the orbital overlaps between the magnetic ions, which in turn enables to tune the magnetic ground states, their excitations and observe novel phenomena.
What makes PSI special is that there are three types of probes on site – neutrons, muons and X-rays. Combining these techniques can often allow pinpointing the physical processes underlying the novel quantum phenomena. Moreover, I am collaborating with Swedish researchers at Chalmers University to bring the advances of the instrumentation to the upcoming European Spallation Source.
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.
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.