Seminar By John Singleton, Pulsed Field Facility, Los Alamos

Title: Electrons behaving badly: experimental techniques applied to electrical insulators and neutral fermions

Series: Condensed Matter Sciences Seminar

Host: Ali Bangura, CMS - DC Field

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 College, 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 of well-behaved electrons 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 Haas-van Alphen effect - an oscillatory phenomenon in magnetic field that is smoking-gun evidence for a Fermi surface; i.e., it is usually seen only in metals. Equally striking are the low-temperature heat capacities of these materials, which look as though they come from large concentrations of free fermions. Mobile fermions are present, but are unable to conduct electricity.

In this talk I will describe data on two of these insulating compounds in magnetic fields of up to 75 Tesla. A slew of experimental techniques – heat capacity, magnetization, resistivity, torque magnetometry, radiofrequency skin-depth and capacitance - reveal that the insulators are actually full of electrons behaving badly- pretending to be something else. In YbB12 an ansatz due to Varma that invokes neutral fermions appears able to account for virtually all of the data, including the slow death of the neutral fermions as the field approaches 75 T. In the other insulator, YCOB (YCu3(OH)6Br2[Br1−y(OH)y]), the electrons undergo spin–charge separation, which results in them behaving like three separate particles: (i) the spinon, which carries the spin of the electron; (ii) the orbiton, which carries its orbital location; and (iii) the holon, which carries its charge. In YCOB, the holons show up in capacitance and the spinons form a liquid of mobile, neutral fermions that account for the de Haas-van Alphen data. An important lesson from all of this is that multiple experimental techniques are needed to pin down the phenomena observed.