Quantum Materials Seminar

semester, 2019

Wednesday, January 30th 2019
The seminar has been cancelled.
Faculty Host: Martin Greven

Wednesday, April 3rd 2019
Speaker: Graeme Luke, McMaster University
Subject: Exotic Probes and Extreme Conditions Reveal New States of Quantum Matter

Condensed matter systems provide an exciting laboratory for observing new states of quantum matter via emergence, where the collective behavior of electrons results in quasi-particles with fractional statistics, spin-charge separation, magnetic monopoles and Majorana fermions (particles that are their own anti-particles). I will describe how we design and synthesize new quantum materials that can host these exotic new states of matter and then use a variety of experimental techniques including muon spin relaxation and neutron scattering to probe their properties.

Faculty Host: Martin Greven

Monday, May 6th 2019
Speaker: Nihat Berker, Kadir Has University, Turkey
Subject: Chiral Spin Glasses, Chaos, and Spin-Glass Lower-Critical Dimension from Continuously Variable Dimensional Realizations

The numerically exact renormalization-group studies of spin glasses on hierarchical lattices continues to yield surprising new results. In this talk, we shall concentrate on two such results: (1) An entirely new type of spin-glass, resulting from only competing chiral interactions (in the absence of ferromagnetic-antiferromagnetic competition). (2) A lower lower-critical spin-glass dimension, resulting from the invention of continuously variable spatial dimensionality in hierarchical lattices. The chiral clock spin-glass model with q = 5 states, with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations, is studied in d = 3 spatial dimensions by renormalization-group theory.[1] The global phase diagram is calculated in temperature, antiferromagnetic bond concentration p, random chirality strength, and right-chirality concentration c. The system has a ferromagnetic phase, a multitude of different chiral phases, a chiral spin-glass phase, and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil’s staircases, where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase, bordered by fragments of regular and temperature-inverted devil’s staircases, are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos. By quenched-randomly mixing local units of different spatial dimensionalities, we have studied Ising spinglass systems on hierarchical lattices continuously in dimensionalities 1 <=d >=3.[2] The global phase diagram in temperature, antiferromagnetic bond concentration, and spatial dimensionality is calculated. We find that, as dimension is lowered, the spin-glass phase disappears to zero temperature at the lower-critical dimension dc = 2.431. Our system being a physically realizable system, this sets an upper limit to the lower-critical dimension in general for the Ising spin-glass phase. As dimension is lowered towards dc, the spin-glass critical temperature continuously goes to zero, but the spin-glass chaos fully subsists to the brink of the disappearance of the spin-glass phase. The Lyapunov exponent, measuring the strength of chaos, is thus largely unaffected by the approach to dc and shows a discontinuity to zero at dc.
[1] T. Çağlar and A.N. Berker, Phys. Rev. E 96, 032103, 1-6 (2017).
[2] B. Atalay and A.N. Berker, Phys. Rev. E 98, 042125, 1-5 (2018).

Wednesday, May 8th 2019
Speaker: Turab Lookman
Subject:  Vortex structures in ferroelectric nanoparticles

There has been much interest in topological defects of spontaneous polarization as templates for unique physical phenomena and in the design of electronic devices. Experimental investigations of the complex topologies of polarization have been limited to surface phenomena, which has restricted the probing of the dynamic volumetric domain morphology in operando. I will discuss the behavior of three-dimensional vortices formed due to competing interactions involving ferroelectric domains observed by Bragg coherent diffractive imaging. I will show results for a single BaTiO3 nanoparticle of size ~100 nm in a composite polymer/ferroelectric capacitor, and discuss the structural phase transitions under the influence of an external electric field, including a mobile vortex core exhibiting a reversible hysteretic transformation path and changes in toroidal moment. Results and extensions to Barium Hexaferrite, as well as some recent results on observations of skyrmions in YIG, will be pointed out. Time permitting, I will also discuss the observation of very large magnetostrictive coefficients in nanowires.

Faculty Host: Martin Greven

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