PHYSICS PHD DISSERTATION DEFENSE: Alexander Hristov

Ph.D. Candidate:  Alexander T. Hristov

Research Advisor:  Ian R. Fisher

Date: Friday, July 6, 2018
Time: 10:00 AM
Location: Physics and Astrophysics Building, Room 102/103

Title: The Response of Electronic Nematic Order to Dynamical Strain: A Study of the Iron Pnictide Superconductors

Abstract:

Over the last decade, there has been increasing evidence that electronic nematic states, which break discrete rotational symmetry in crystalline solids, and their associated critical points are important constituents in the phase diagrams of several widely studied families of superconductors. Electronic nematic states are evident in the iron-based superconductors, and there are mounting indications of their presence in the cuprate superconductors, as well as in the 115 family of heavy fermion superconductors.
This work discusses new techniques to investigate signatures of electronic nematicity, and more broadly addresses how mechanical deformation can be used to control and study materials. We begin with the development and characterization of an improved technique for measuring the resistance response of materials to mechanical deformation (strain), using AC strain at frequencies between 5mHZ and 50kHz. This elastoresistivity technique (measuring sample resistance changes due to strain) is supplemented by measurements of the elastocaloric effect (measuring sample heating due to strain) to the study of the iron-pnictide Ba(Fe0.975Co0.025)2As2, in which we demonstrate and explain elastocaloric and elastoresistive responses that both track the specific heat of the material. Finally, we study the frequency dependence of the nematic susceptibility in samples from the Ba(Fe1-xCox)2As2 family of iron pnictides for signatures of critical slowing down, particularly that associated with disorder-induced random strains and realizations of the random-field Ising model, and for signatures of strain tuned domain motion.