Talk
Physics Colloquium - Electrons behaving badly: metallic properties at high magnetic fields in wide band-gap insulators
When
· 4:00 PM MDT
Shown in the venue’s time zone (America/Denver), not yours.
Where
Boulder, CO
Tickets
No on-sale date recorded
This event came from a public calendar feed. Calendar feeds publish when a show happens, not when its tickets go on sale — so this is genuinely unknown rather than zero, and rather than “already on sale”.
What the source said
Presented by: John Singleton, National High Magnetic Field Laboratory, Los Alamos National Laboratory Abstract: From an early age, we are taught that metals are good conductors of electricity and heat but that insulators are not. At High School we learn that metals contain vast numbers of charged electrons that are free to move and carry heat and current, whereas insulators do not. At University, we find out that electrons are fermions, and perhaps comprehend Fermi-Dirac statistics, leading to the well-known definition that “a metal is a solid with a Fermi surface”. The Fermi surface is the constant-energy surface which at zero temperature separates the occupied electron states from the empty in momentum space; if we know the size and shape of a metal’s Fermi surface, we understand how its free electrons behave and hence can account for almost all of its electrical, thermal and magnetic properties. Over the past decade, this comforting picture has been upset by experiments on various materials at high magnetic fields and low temperatures. Though these substances are electrical insulators, they exhibit the de Haasvan Alphen effect - an oscillatory phenomenon in magnetic field that is taken as smoking-gun evidence for a Fermi surface; i.e., it is usually seen only in metals. After a brief review of the above phenomena, I shall concentrate on measurements of the Kagomelattice 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 their companion particles, bosonic chargons; the latter cause the coupling of the applied magnetic field to the charge-neutral spinons. Further constraints are placed on the spinon Fermi surface using high-field susceptometry, 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 such as Y-kapellasite, that point to the oscillations as possible universal behaviour in such insulators. Biography: John Singleton is a condensed-matter physicist and electrical engineer at the National High Magnetic Field Laboratory Pulsed-Field Facility (PFF), visiting professor at Oxford University and fellow of both the American Physical Society and the Institute of Physics. He has authored or co-authored over 500 papers in scientific journals, along with a popular undergraduate text book. The first member of his family to go to university, he was educated at Oxford University (BA, MA, D.Phil.); his D.Phil. involved semiconductor research at the industrial company GEC. Before coming to the PFF, he worked at the University of Nijmegen High-Field Magnet Laboratory, and was the equivalent of a full professor at Oxford University, where he founded a large research group that continues to this day. Other interests include unusual antennas (involving radio waves emitted by accelerated polarization currents that travel faster than the speed of light, mimicking some astronomical objects), playing the pipe organ, theology, caving (led the expedition that first bottomed the then seventh deepest cave in the world), youth work, mountains, free-range chickens and bread making. Host: Minhyea Lee