Condensed Matter Seminar
Our condensed matter seminars are held on Tuesdays at 3:30pm in Chemistry-Physics Building, Room 179, unless otherwise noted below. A number of the department colloquium may also be of interest.
Spin-Orbit Tuned Ground States in Single-Crystal Iridates
The iridates have become fertile ground for studies of new physics driven by spin-orbit coupling (SOC) that is comparable to the on-site Coulomb and other relevant interactions. This unique circumstance creates a delicate balance between interactions that drives complex magnetic and dielectric behaviors and exotic states seldom or never seen in other materials. A profound manifestation of this competition is the novel Jeff = 1/2 Mott state that was observed in the layered iridates with tetravalent Ir4+(5d5) ions. On the other hand, very little attention has been drawn to iridates having pentavalent Ir5+(5d4) ions, primarily because the strong SOC limit is expected to impose a nonmagnetic singlet ground state (Jeff = 0). In this talk, we review the underlying physical properties of the iridates including perovskites, honeycomb lattices and double perovskites with pentavalent Ir5+ ions, and report results of our recent studies that emphasize spin-orbit-tuned ground states stabilized by chemical doping, application of pressure and magnetic field. In addition, we address the urgent question that the Jeff states may not survive in the presence of strong non-cubic crystal fields and/or exchange interactions.
In situ X-ray Studies of Functional Oxides for Energy Systems
Crystal growth and physical properties of Iron - based superconductors
A Multi-Probe STM Quest for Quantum Transport at 2D Materials Boundaries
Generating femtosecond second-harmonic pulses from ultrathin Archimedean nanospirals
Nanoscale probing of electromechanical responses by scanning probe microscopy: from piezoresponse to electrochemical strain
Electromechanical responses (the mechanical displacement induced by an applied electric field, and vice versa) are ubiquitous in nature. One of the most typical examples is converse piezoelectric response in ferroelectric and multiferroic materials. Another example is electrochemical strain induced by ionic motion, e.g., in Li-ion batteries and solid oxide fuel cells. Here, the recent scanning probe microscopy studies of those electromechanical responses in a variety of material systems are presented. First, the piezoresponse force microscopy (PFM) study on the origin of polarization fatigue in epitaxial ferroelectric Pb(Zr,Ti)O3 capacitors will be shown [1]. In this study, PFM allows to visualize ferroelectric domain nucleation and growth during the fatigue process at the nanoscale time and length scales. It reveals that the evolution of domain wall pinning process is the primary origin of polarization fatigue, which has been a long-standing important problem in ferroelectrics. Second, the electrochemical strain microscopy (ESM) study on the nonlinear electromechanical responses in Ag-ion based ionic conductive glasses will be presented. ESM has recently emerged as a powerful tool to probe ionic transports and electrochemical phenomena at the nanoscale in many material systems [2]. In this study, interesting anti-
correlation between the first and second harmonic ESM responses are observed, and its possible origins are discussed.
[1] S. M. Yang et al., Adv. Funct. Mater. 22, 2310 (2012).
[2] N. Balke et al., Nat. Nanotechnol. 5, 749 (2010).
Crystal growth and exotic magnetic transitions in rare earth orthoferrites RFeO3"
If time allows the speaker will also discuss
Exchange bias in Co/Ca2Ru0.98Fe0.02O4 heterostructure
"Quantum phase transitions and disorder: Griffiths singularities, infinite randomness, and smearing"
Dynamics of Electrons in Structured Graphene in a Magnetic Field
Graphene is the most two-dimensional platform currently available
as a host for an electron gas, and offers promise to make observable a variety of
effects in a perpendicular magnetic field. For example, recent advances in aligning
graphene on a boron-nitride substrate have led to the creation of high-quality
Moire patterns with large unit cells. Similar large-cell superlattices can also be
created in twisted bilayers. In a perpendicular magnetic field, near zero energy
the periodicity has little effect on the spectrum, but with increasing energy the
spectrum evolves into the much-anticipated Hofstadter butterfly. The crossover
between these behaviors is controlled by a saddle point in the zero-field spectrum.
We demonstrate through a semiclassical analysis how the quantization of orbits changes
as the saddle point is crossed, allowing the richness of the Hofstadter spectrum to emerge
above it, and discuss some possible experimental consequences. We then consider
graphene systems in much higher magnetic fields -- the quantum Hall regime -- where
transport is controlled by edge states. For undoped graphene these edge states
may have a helical nature. We discuss what happens when an "internal edge" is
created in bilayer graphene using a split gate geometry, where a surprisingly rich
internal structure emerges with a number of possible states. Such a geometry admits
transport probes which potentially reveal different aspects of the internal structure, and we
discuss our expectations for how the state of this internal edge can be reflected in
such measurements.