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.

In situ X-ray Studies of Functional Oxides for Energy Systems

Functional oxides and their interfaces offer new opportunities to overcome the 
current limits of energy storage and conversion systems, e.g., water dissociation 
and formation. However, the understanding of complex oxide interfaces and 
their electrochemical properties is far from complete, particularly with regard to 
electronic and ionic dynamics occurring in aqueous solutions or ionic liquids under 
applied electric fields. To elucidate the physical and electrocatalytic properties 
of oxide surfaces and interfaces, it is necessary to build a model system and to 
employ in situ experimental tools to detect and analyze the complex time-dependent 
phenomena. In this talk, I will introduce recent in situ synchrotron studies [1,2] 
conducted at Argonne National Laboratory that combine structural, spectroscopic, 
and electrochemical characterization on model systems, e.g., epitaxial perovskite 
or layered oxide thin films. With this methodology, we can determine both the 
reactivity and stability of active sites on complex oxide surfaces during water 
dissociation and formation. This approach offers much needed insight into the 
electrocatalytic properties of oxide interfaces and provides new strategies for the 
creation of new stable and active energy materials designed at the atomic level. 
 
[1] S. H. Chang et al., ACS Nano 8, 1584 (2014). 
[2] S. H. Chang et al., Nature Commun. 5, 4191 (2014).
Date:
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Location:
CP179
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Generating femtosecond second-harmonic pulses from ultrathin Archimedean nanospirals

The explosive growth in the development of plasmonic devices for sensors, switches, catalysis 
and optical data links is driving the evolution of increasingly sophisticated nanostructures for 
these applications. Arrays of ultrathin Archimedean nanospirals exhibit linear and nonlinear 
optical properties associated with near-field plasmon resonances within the nanostructures. The 
enhanced electric fields at these resonances enable efficient second-harmonic generation (SHG)
because individual nanospirals have no symmetry axis. We observe efficient SHG from arrays 
of lithographically fabricated, sub-wavelength-dimension nanospirals, initiated by transform-
limited 15 fs pulses at a wavelength of 800 nm. The nanospiral arrays respond selectively to 
incident linear and circular polarization states and exhibit conversion among polarization states. 
I will conclude by suggesting some ways in which the asymmetry and two-dimensional chirality 
of the nanospirals may lead to interesting applications in metamaterial devices.
 
Date:
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Location:
CP179
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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).

Date:
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CP179
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"Quantum phase transitions and disorder: Griffiths singularities, infinite randomness, and smearing"

Phase transitions are fascinating phenomena in nature with consequences
ranging from the large scale structure of the universe to exotic quantum phases
at low temperatures. Many realistic systems contain impurities, defects and
other forms of quenched disorder. This talk explores the consequences of such
randomness on the properties of phase transitions.
At zero-temperature quantum phase transitions, randomness can have
particularly peculiar and strong effects. Often, rare strong disorder fluctuations
and the rare spatial regions that support them dominate the physics close to the
transition. They give rise to strong singularities in the free energy, the so-called
quantum-Griffiths singularities, In some systems such as metallic magnets, the
effects of rare fluctuations can be even stronger, leading to a destruction of the
phase transition by smearing. These general results are illustrated using experiments in transition
metal alloys and heavy fermion systems.
Date:
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Location:
CP 179
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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.

Date:
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Location:
CP179
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Ground states of the effective spin-1/2 XY pyrochlores: "Quantum Spin Ice" and "Order By Disorder"

A knowledge of the spin-wave excitations in magnetic materials allows a robust determination of exchange parameters in suitable model Hamiltonians. We have used time-of-flight inelastic neutron scattering to measure the excitation spectra from field-polarized states of exotic frustrated magnets. We have taken this approach with two pyrochlores, Er2Ti2O7 and Yb2Ti2O7, whose magnetic properties have until this point been somewhat puzzling. Our strategy is to apply a magnetic field to push the systems to be in as classical a phase as possible, and then extract the exchange parameters using linear spin wave theory. Then, ramping the field down in the model, we reveal new information about the zero-field states of these magnets. The ground states of the frustrated spin-1/2 XY pyrochlores Er2Ti2O7 and Yb2Ti2O7 are thus revealed to be realizations of "quantum order by disorder" and "quantum spin ice", respectively. I will explain what these terms mean and show how the models lead to predictions, one of which we have confirmed (a small spin wave gap in Er2Ti2O7 opened by quantum fluctuations) and another which is still debated (does Yb2Ti2O7 support emergent electrodynamics?).

Date:
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CP179
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“Molecules” in solids and novel states close to Mott transition.

Usually, when considering Mott transitions, one treats the system as homogeneous – either with localized, or with itinerant electrons. However, such localized – itinerant crossover can occur not nesessarily homogeneously in the whole sample, but there may appear in a solid small clusters –“molecules” (dimers, trimers, heptamers etc) in which electrons already can be treated as weakly-correlated, whereas the hopping between such clusters may still be small, so that the whole material may still be insulating. In this talk I will discuss the conditions for such phenomenon (in particular, low-dimensionality and magnetic frustrations usually facilitate creation of such clusters), present several examples, and consider some special properties of such molecular clusters, such as e.g. the orbital-selective Peierls transition with partial quenching of magnetic moments.

Date:
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CP179
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