Session C19: DMP/GMAG Awards Session

George E. Valley, Jr. Prize Talk: Quantum Frustrated Magnetism and its Expression in the Ground State Selection of Pyrochlore Magnets

In the search for novel quantum states of matter, such as highly entangled Quantum Spin Liquids, ``geometrically frustrated'' magnetic lattices are essential for suppressing conventional magnetic order. In three dimensions, the pyrochlore lattice is the canonical frustrated geometry. Magnetic materials with pyrochlore structures have the potential to realize unusual phases such as ``quantum spin ice'', which is predicted to host emergent magnetic monopoles, electrons, and photons as its fundamental excitations. Even in pyrochlores that form long range ordered phases, this often occurs through unusual routes such as ``order by disorder'', in which the fluctuation spectrum dictates the preferred ordered state. The rare earth-based pyrochlore series $R_2$Ti$_2$O$_7$ provides a fascinating variety of magnetic ground states. I will introduce the general anisotropic interaction Hamiltonian that has been successfully used to describe several materials in this series. Using inelastic neutron scattering, the relevant anisotropic interaction strengths can be extracted quantitatively. I will discuss this approach, and its application to two rare earth pyrochlore materials, Er$_2$Ti$_2$O$_7$ and Yb$_2$Ti$_2$O$_7$, whose ground state properties have long been enigmatic. From these studies, Er$_2$Ti$_2$O$_7$ and Yb$_2$Ti$_2$O$_7$ have been suggested to be realizations of ``quantum order by disorder'' and ``quantum spin ice'', respectively.   

Synthesis as the heart of New Materials Physics

Humanity needs to find the materials that will ease is growing needs for reliable, renewable, clean, energy and/or will allow for greater insight into the mysteries of collective and, in some cases, emergent states. The design, discovery and growth of novel materials is heart of the research effort that will, hopefully address these needs. In this talk I will present a broad overview of New Materials Physics and describe how a practitioner can go from staring at the periodic table to deciding what ``the next growth will be''. I will present and discuss the three basic motivations for making a growth: wanting a specific compound; wanting a specific ground state; searching for known and unknown unknowns. Materials discussed will span superconductors, quasicrystals, heavy fermions, fragile magnets, topological electronic systems, local moment magnets and a few lost puppies. The goal of this talk is to inspire and entertain, any resemblance to persons living or dead is coincidental.   

Mixed Dimensional Van der Waals Heterostructures for Opto-Electronics.

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into ``all-2D'' van der Waals heterostructures, this concept is not limited to 2D materials alone. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions. In the first part of this talk I will present our work on emerging mixed-dimensional (2D $+$ nD, where n is 0, 1 or 3) heterostructure devices performed at Northwestern University. I will present two distinct examples of gate-tunable p-n heterojunctions 1. Single layer n-type MoS$_{\mathrm{2\thinspace }}$(2D) combined with p-type semiconducting single walled carbon nanotubes (1D) and 2. Single layer MoS$_{\mathrm{2}}$ combined with 0D molecular semiconductor, pentacene. I will present the unique electrical properties, underlying charge transport mechanisms and photocurrent responses in both the above systems using a variety of scanning probe microscopy techniques as well as computational analysis. This work shows that van der Waals interactions are robust across different dimensionalities of materials and can allow fabrication of semiconductor devices with unique geometries and properties unforeseen in bulk semiconductors. Finally, I will briefly discuss our recent work from Caltech on near-unity absorption in atomically-thin photovoltaic devices.   

Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials which show response functions quantized in terms of fundamental physical constants. It has been predicted that this manifests as Faraday and Kerr rotations quantized in units of the fine structure constant $\alpha=e^2/2 \epsilon_0 hc$. We use a charge-transfer-doping preparation to lower the chemical potential of Bi$_2$Se$_3$ films into the bulk gap and as low as $\sim$ 30 meV above the Dirac point, and then probe their low-energy electrodynamic response with high-precision time-domain terahertz polarimetry. As a function of field, a crossover from semi-classical cyclotron resonance to a quantum regime was observed. In this regime, although the DC transport is still semi-classical, we observed quantized Faraday and Kerr rotations. A non-trivial Berry's phase offset to these values is consistent with half-integer quantized conductance on each surface and therefore provides evidence for the long-sought axion electrodynamics and topological magnetoelectric effect. Among other aspects, the unique time structure used in these measurements allow us a direct measure of the fine structure constant based on a topological invariant of a solid-state system. I will also discuss how optics can detect quantized Hall conductance without involving the edge states. Main reference: arXiv:1603.04317.   

David Adler Lectureship Award Talk: III-V Semiconductor Nanowires on Silicon for Future Devices.

Bottom-up grown nanowires are very attractive materials for direct integration of III-V semiconductors on silicon thus opening up new possibilities for the design and fabrication of nanoscale devices for electronic, optoelectronic as well as quantum information applications. Template-Assisted Selective Epitaxy (TASE) allows the well-defined and monolithic integration of complex III-V nanostructures and devices on silicon. Achieving atomically abrupt heterointerfaces, high crystal quality and control of dimension down to 1D nanowires enabled the demonstration of FETs and tunnel devices based on In(Ga)As and GaSb. Furthermore, the strong influence of strain on nanowires as well as results on quantum transport studies of InAs nanowires with well-defined geometry will be presented.