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Wednesday, January 17th 2018

1:25 pm:

Our understanding of crystal growth fundamentals and processes are advanced when the synergy between mathematical models and novel experiments is exploited. We present recent examples of how modeling and experiments together have enabled the identification of fundamental mechanisms important during the growth of bulk crystals from the melt.

We first discuss how microgravity experiments, carried out via sounding rockets, motivated a reexamination of classical theories for foreign particle engulfment during crystal growth. Via the development and application of rigorous numerical models, we were able, for the first time in over a decade of research on this system, to quantitatively describe data on the engulfment of SiC (silicon carbide) particles during the growth of crystalline silicon. Moreover, model results identified previously unascertained mechanisms responsible for the behavior of this system and, via this insight, provided insight for analytical derivation of a new scaling law for the dependence of critical engulfment velocity on particle size.

We finish with an overview of exciting, new research that employs neutron imaging to directly “see,” in operando, the bulk growth of scintillator crystals during a gradient-freeze process. We argue that the synergies of “seeing” via both models and neutron imaging will improve our fundamental understanding and provide for a closed-loop approach for optimizing the growth of large, single crystals from the melt.

This research was supported in part by NASA NNX10AR70G, DOE/NNSA DE-NA0002514, DOE/NNSA/DNN R&D (LBNL subcontract AC0205CH11231); no official endorsement should be inferred.

Wednesday, January 24th 2018

1:30 pm:

Recent progress in synthesizing insulators with a new type of dominant spin-exchange interaction, known as the Kitaev interaction, has opened new possibilities for experimentally realizing spin liquid compounds. Among the distinctive features of these spin liquids is the possibility that they can harbour protected gapless boundary modes which carry spin but not charge. This possibility raises a challenge of how best to detect these chargeless boundary modes. I will discuss two possibilities — Raman scattering and the heat capacity — as well as what such measurements can reveal about the bulk phase.

Wednesday, January 31st 2018

1:30 pm:

Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the magnetic-field driven topological phase transition. Specifically, we map out the phase diagram of the topological phase in the space of Zeeman energy and chemical potential, and investigate the apparent closing and re-opening of the superconducting gap. We investigate how the topological superconducting phase would manifest in finite size systems, by electrostatically splitting the wire into segments of varied length. By chaining up several segments of a nanowire, we are realizing a quantum simulator of the Kitaev chain with tunable on-site energies and couplings between the sites, a step towards quantum simulation with semiconductor nanostructures.

Wednesday, February 7th 2018

1:30 pm:

The generation of an effective attraction between electrons out of the bare Coulomb repulsion is a long sought-after goal of the condensed matter community. In this talk I will introduce a pairing mechanism between holes in the dilute limit of doped frustrated Mott insulators. We will see that magnons provide a strong glue in the infinitely repulsive limit of the triangular lattice Hubbard model. The strongly attractive hole-magnon interaction is a manifestation of a “counter-Nagaoka” theorem: the single-hole kinetic energy is minimized for an antiferromagnetically ordered state. We will demonstrate that the resulting hole-magnon attraction is strong enough to bind a second hole and to form a hole-hole-magnon three-body bound state. Remarkably, the binding energy of this “composite Cooper pair” is rather strong, while its effective mass still has a moderate value, giving rise to relatively high transition temperature for superconductivity in the dilute limit. I will discuss a few interesting consequences of this new mechanism for unconventional superconductivity.

Work done in collaboration with Shangshun Zhang (University of Tennessee) and Wei Zhu (Los Alamos National Laboratory).

Wednesday, February 14th 2018

1:25 pm:

I will discuss new experimental results and insights into the physics of cuprate high-temperature superconductors, providing an overarching framework for understanding these materials.

Motivated by transport measurements, I will consider an inhomogeneous Mott-like (de)localization model wherein exactly one hole per copper-oxygen unit is gradually delocalized with increasing doping and temperature. The model comprehensively captures pivotal unconventional experimental results, including the temperature and doping dependence of the pseudogap phenomenon, the strange-metal linear temperature dependence of the planar resistivity, and the doping dependence of the superfluid density. The simple model greatly demystifies the cuprate phase diagram, and points to a local superconducting pairing mechanism involving the (de)localized hole. The spatial inhomogeneity of the localization gap is thus expected to cause a distribution of superconducting gaps as well, leading to superconducting percolation. Accordingly, for several representative cuprates the superconducting diamagnetism, nonlinear conductivity, and paraconductivity exhibit an unusual temperature dependence above Tc that is captured by a simple percolation model. The results show that that intrinsic, universal gap inhomogeneity is highly relevant to understanding the properties of the cuprates.

Wednesday, February 21st 2018

1:25 pm:

In normal metals, the electron's mean free path is much larger than its wavelength, allowing a semiclassical treatment of transport. Conversely, whenever scattering is so strong that the mean free path becomes comparable to the electron's wavelength, the concept of a quasiparticle becomes ill defined, and a new theoretical framework is needed. I will introduce a family of lattice models for interacting electrons that can be solved exactly in the limit of a large number of interacting electron flavors and/or phonon modes. Depending on details, these models exhibit either "resistivity saturation" at high temperatures to a value of the order of the quantum of resistance, or "bad metallic behavior" where the resistivity grows without bound with increasing temperature. Translationally invariant higher-dimensional generalizations of the Sachdev-Ye-Kitaev model can capture a variety of phenomena arising purely from electron-electron interactions, including local criticality, non-Fermi liquid, and marginal Fermi liquid behavior. I will describe the implications of these results for the problem of non-quasiparticle transport at large, local quantum criticality, and fundamental bounds on dissipation rates in quantum systems.

Wednesday, February 28th 2018

1:25 pm:

Wednesday, March 7th 2018

1:30 pm:

Wednesday, March 21st 2018

1:25 pm:

Photons carry spin angular momentum when circularly or elliptically polarized. During light-matter interaction, transfer of angular momentum induces optical torque. Here, we demonstrate the measurement of the spin angular momentum of photons propagating in a silicon waveguide and the use of optical torque to actuate rotational motion of an optomechanical device. We show that the sign and magnitude of the optical torque are determined by the photon polarization states that are synthesized on the chip. Our study reveals the mechanical effect of photon’s polarization degree of freedom and demonstrates its control in integrated photonic devices.

Tuesday, March 27th 2018

3:35 pm:

Waves with a hyperbolic dispersion relation are exotic yet surprisingly widespread phenomena that occur in anisotropic media with internal resonances. Such media have been investigated in numerous fields, ranging from condensed matter physics to plasma physics to optics to fluid dynamics and geophysics. Hyperbolic waves can be found in magnetic materials, in both usual and topological insulators, in superconductors, as well as in our oceans, beaches, atmosphere, and space. The characteristic lengths and frequencies of such waves vary vastly, from atomic to cosmic. However, they all exhibit certain common attributes, such as strict directionality, diverging density of states, and anomalous reflection. This talk will contain a primer on hyperbolic materials, a recipe for the death ray, and a report on our nano-optics studies of hyperbolic phonon-polaritons in new quasi-2D materials such as graphene and hexagonal boron nitride.

References:

1. L. V. Brown et al, “Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures,” Nano Lett. 18, 1628 (2018).

2. A. J. Giles et al., "Imaging of Anomalous Internal Reflections of Hyperbolic Phonon-Polaritons in Hexagonal Boron Nitride," Nano Lett. 16, 3858 (2016).

3. S. Dai et al., “Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material,” Nature Comms 6, 6963 (2015).

4. S. Dai et al., “Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride”, Science 343, 1125 (2014).

Wednesday, March 28th 2018

2:30 pm:

Design, discovery, growth and characterization of novel materials is at the heart of New Materials Physics. One of the key steps is deciding what materials to study or try to grow. In this talk I will try to enunciate and elaborate the motivations for making/studying specific compounds. Many examples from current research will be touched upon and discussed. Humor of all types will be used to lighten the load and make the time fly by.

Wednesday, April 4th 2018

1:25 pm:

Non-Fermi liquids are exotic metallic states which do not support well defined quasiparticles. Due to strong quantum fluctuations and the presence of extensive gapless modes near the Fermi surface, it has been difficult to understand universal low-energy properties of non-Fermi liquids. In this talk I will discuss recent progress made on field theories for non-Fermi liquids. Based on a dimensional regularization scheme which tunes the co-dimension of Fermi surface, critical exponents that control scaling behaviors of physical observables can be computed in controlled ways. The systematic expansion provides important insight into strongly interacting non-Fermi liquids. This allows us to find the non-perturbative solution for the strange metal realized at the antiferromagnetic quantum critical point in 2+1 dimensions, and predict the exact critical exponents that can be experimentally tested in layered systems.

Wednesday, April 11th 2018

1:25 pm:

Wednesday, April 18th 2018

1:25 pm:

The iron pnictides represent a new family of unconventional superconductors in which superconductivity appears in close proximity to a magnetically ordered phase. In this talk I will review the magnetic order of the iron pnictides and discuss recent discoveries in the magnetic phase diagram, focusing on the reorientation of magnetic moments observed to occur in hole-doped compounds. Considering the impact of magnetic fluctuations on the phase diagram I will argue that even the modest spin-orbit coupling observed in iron pnictides has important consequences and cannot be neglected.

Wednesday, April 25th 2018

1:15 pm:

Wednesday, May 9th 2018

1:30 pm:

Tuesday, May 15th 2018

11:00 am:

The recent discovery of topological semimetals, which possess distinct electron-band crossing with non-trivial topological characteristics, has stimulated intense research interest. By extending the notion of symmetry-protected band crossing into one of the simplest magnetic groups, namely by including the symmetry of time-reversal followed by space-inversion, we predict the existence of topological magnon-band crossing in three-dimensional (3D) antiferromagnets. The crossing takes on the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a “spin-web” compound, we theoretically demonstrate the presence of Dirac magnons over a wide parameter range using the linear spin-wave approximation, and obtain the corresponding topological surface states [1].

Inelastic neutron scattering experiments have then been carried out to detect the bulk magnon-band crossing in a single-crystal sample. The highly interconnected nature of the spin lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with spin-wave calculations. The predicted topological band crossing is confirmed [2].

[1] K. Li et al., PRL 119, 247202 (2017).

[2] W. Yao et al., arXiv:1711.00632.

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