Date added: 2026-06-10
The 45th Seminar of Advanced Materials Center
We warmly invite you to the 45th seminar of the Advanced Materials Center, which will take place on June 19th, 2026 (Friday) at 1:15 p.m. in NE 309 (building 42, WETI B).
Prof. John Singleton z Los Alamos National Laboratory and Oxford University, USA/GB), will present a talk entitled "Magnetic quantum oscillations due to a spinon Fermi surface in a wide band-gap Kagome-Mott insulator".
Afterwards, Prof. Eteri Svanidze, Max Planck Institute for Chemical Physics of Solids, Dresden, Germany, will present a talk entitled "Imperfections as a measure of exotic superconductivity?".
After the seminar, we traditionally invite you for pizza!
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Abstracts:
1. "Magnetic quantum oscillations due to a spinon Fermi surface in a wide band-gap Kagome-Mott insulator". I describe recent measurements of the Kagome-lattice Mott insulator YCu3(OH)6Br2[Br1−y(OH)y] (YCOB) in magnetic fields of up to 75 T. Our pulsed-field magnetization experiments strongly suggest that YCOB is a quantum spin liquid. In such a system, antiferromagnetic order is suppressed by geometrical frustration and quantum fluctuations. Under these conditions, spin–charge separation of electrons can produce charge-neutral spinons, fermions that possess spin but no charge. Using ultrasensitive torque magnetometry, de Haas-van Alphen oscillations are observed, giving strong evidence for both the spinons and an effective gauge field which allows the coupling of the applied magnetic field to these charge-neutral particles. Further constraints are placed on the spinon Fermi surface using high-field susceptibility, capacitance and thermal experiments on YCOB single crystals. A theoretical model of spinon band structure that includes Dirac nodes near a 1/9 magnetization plateau produces quantitative predictions consistent with the observed oscillations. Finally I shall review very recent measurements on other spin-liquid candidates that point to the oscillations as possible universal behaviour in such systems.
2. "Imperfections as a measure of exotic superconductivity?". The extreme fascination with uranium-based superconductors has once again come to light as a result of the many peculiar facets of UTe2. In an effort to unify the chemical and physical understanding of this peculiar material, our previous work highlighted the minute crystallographic imperfections that drive differences in the superconducting state – from complete suppression to the emergence of one or even two superconducting transitions – whose origin lies in two types of defects. The more important role is played by uranium deficiencies, which can completely suppress superconductivity of UTe2 with an abundance of only 4%. A subtler imperfection arises due to local variations of translation symmetry, which amount to only 0.01%, splitting one superconducting transition into two.
In a related unconventional superconductor UBe13, the sensitivity to imperfections is even higher. This cage-like structure hosts uranium atoms in the center of each cage, surrounded by 24 beryllium atoms. In our recent study, we find that removal of just a few beryllium atoms in one unit cell drastically changes the critical temperature by nearly 30%. The volume of the lattice stays the same, which signals that this change is miniscule, even when comparing to systems with strong structure-property relationships. We also consider what happens in another uranium-based unconventional superconductor UPt3 – both in bulk and on the micro-scale. Overall, the extreme sensitivity to imperfections in uranium-based systems likely originates in the flexibility of the uranium oxidation state, which is easily affected by the small changes to the crystallographic environment of uranium. Currently, only a few uranium-based superconductors are known, with nearly half of them exhibiting various degrees of crystallographic defects. This talk, I will compare the effects of various chemical features to the resultant superconductivity and related phenomena.