Low dimensional systems are among the prime places to look for interesting emergent magnetic behavior in quantum systems. We have recently been studying a metal-organic coordination compound – (C5H9NH3)2CuBr4, or Cu-CPA for short, where magnetic properties are dictated by the one dimensional ladders of S=1/2 copper ions. Surprisingly, we have found that in fact there are two distinct ladders and not just one as was previously assumed. To learn more about the structure (and a cool isotope effect), have a look at the preprint: https://arxiv.org/abs/2404.08274
This fall, on the 22nd of November we will be hosting a workshop of all-things-pressure at PSI. The workshop is aimed to foster collaboration of researchers in Switzerland and the surrounding region using high-pressure to study various fields of physics, chemistry and material science. The talks will cover physics accessed by extreme conditions as well as the development of techniques and devices that enables them. Find out more here and hope to see you at PSI!
The various ground states in cuprate superconductors are finely balanced, with only a small energy differences separating them. It was recently found that using uniaxial pressure the balance can be tilted away from stripe order and towards superconductivity in the archetypical cuprate Ba-doped La2CuO4. We used our new uniaxial pressure device on a hard X-ray beamline at DESY to uncover how the different quantum phases are controlled by modifying the underlying crystal structure between orthorhombic and tetragonal phases. Find out more in our preprint: https://arxiv.org/abs/2302.07015
Our new uniaxial device has been successfully commissioned and some very interesting results have already come out of it. For more details on the design and performance have a look at our preprint:
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!
The apparatus in the beamline at DESY on the left and all packed and airborne on the right.
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.
Zero field muon depolarization rate (left axis) suggests a broad fluctuating regime, even in the state where the system is magnetically ordered. The ordering transition is tracked by the asymmetry loss in a transverse field muon experiment (right axis).
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.
Gediminas Simutis 