Spring 2017

Rajiv Singh (UC Davis): Kagome Spin-Liquids and Nuclear Relation in Herbertsmithites

The Kagome Lattice Heisenberg Model is one of the simplest realistic spin models with a quantum spin-liquid ground state. We discuss the current status of our understanding of this well-studied model. The precise nature of the spin-liquid state and the existence of a spin-gap in the model still remain in dispute. We also discuss experimental studies of Herbertsmithite material Kagome-antiferromagnet ZnCu_3(OH)_6Cl_2. We focus on NMR measurements by Imai and collaborators, who have presented strong evidence for a spin-gap in the excitation spectra. Through a Numerical Linked Cluster (NLC) calculation of the frequency moments, we show that despite the existence of substitutional disorder in these materials, the high temperature nuclear relaxation rates are well described by the Heisenberg model.

Rajiv Singh (UC Davis): From Conjectures to Paradigms

The 2016 Nobel Prize in Physics to Kosterlitz, Thouless and Haldane honors a new set of ideas and theoretical formalism that has gradually become the mainstay of modern condensed matter thinking. Quantum Field Theory, Topology and the Renormalization Group, lie at the heart of present day theory of condensed matter. The Kosterlitz-Thouless theory of phase transitions and Haldane's conjecture on the spin dependence of spectrum of spin-chains were two of the most influential works in bringing these ideas together. These along with the Thouless-Kohmoto-Nightingale-Den Nijs-(TKNN) work on topological invariants of band structures were duly recognized by the Nobel committee. This talk will discuss how these theories defied existing paradigms and no-go theorems. And, how they continue to play a huge role in how we address quantum phases and phase transitions today. This Nobel Prize potentially sets the stage for many more prizes in the field of Topological Matter.

Magnetic Nanostructures: A Playground for Fundamental Physics

Speaker(s): 
Ran Cheng
Dates: 
Thursday, March 2, 2017 - 4:30pm to 5:30pm

Nature becomes amazingly different from what we perceive with our eyes when zoomed in to the nanometer scale, where atomic spins interact and form diverse magnetic configurations. Besides holding great technological promise, magnetic nanostructures have also enabled a vibrant playground for fundamental physics—a thriving field known as spintronics. In this talk, I will introduce selected recent progress in spintronics that has reshaped our understanding of transport phenomena occurring at the microscopic scale. Special attention will be paid to antiferromagnetic...

Kagome Spin Liquids and Nuclear Relaxation Rates in Herbertsmithites

Speaker(s): 
Rajiv Singh
Dates: 
Tuesday, February 28, 2017 - 4:00pm to 5:00pm

The Kagome Lattice Heisenberg Model is one of the simplest realistic spin models with a quantum spin-liquid ground state. We discuss the current status of our understanding of this well-studied model. The precise nature of the spin-liquid state and the existence of a spin-gap in the model still remain in dispute. We also discuss experimental studies of Herbertsmithite material Kagome-antiferromagnet ZnCu_3(OH)_6Cl_2. We focus on NMR measurements by Imai and collaborators, who have presented strong evidence for a spin-gap in the excitation spectra. Through a Numerical...

Control of Light-Matter Interaction in 2D Atomic Crystals

Speaker(s): 
Vinod Menon
Dates: 
Thursday, April 6, 2017 - 3:30pm to 4:30pm

Two-dimensional (2D) atomic crystals have emerged as a very attractive class of optoelectronic material due to the unprecedented strength in its interaction with light. In this talk I will discuss approaches to enhance the strength of this interaction even further using microcavities, and metamaterials. Specifically I will discuss enhancement of spontaneous emission [1], formation of strongly coupled exciton-photon quasiparticles (microcavity polaritons) [2], valley polarized polaritons and enhancement of nonlinear optical response from 2D transition metal...

Using Ultrafast Laser Pulses and Computational Modeling to Understand Nonproportionality in Spectroscopic Gamma-Ray Detectors

Speaker(s): 
Richard Williams
Dates: 
Thursday, March 23, 2017 - 12:00pm to 1:30pm

Under gamma-ray or charged-particle excitation, scintillation light yield is a complicated function of carrier diffusion and cooling in the track along with kinetic rate terms depending on local excitation density. Up to 1021 electron-hole pairs/cm3 are produced in an initial track radius of about 3 nm. Extracting the fundamental rate constants directly from such conditions would require solving the diffusion and cooling problems in complex track structures first. Laser interband photon density response and time-resolved pump-probe studies are surrogate experiments that...

Nanotechnology: Small Things Matter

Speaker(s): 
Chad Mirkin
Dates: 
Thursday, February 2, 2017 - 4:30pm to 6:00pm

Nanotechnology is an interdisciplinary field focused on studying and manipulating ultraminiaturized structures, ones with at least one dimension 10,000 times smaller than the diameter of a human hair. This field has the potential to transform almost every aspect of our lives for the better – from enabling our cell phones and computers to run faster to generating our power more efficiently to making our tennis racquets and golf clubs lighter and more durable to making our medicines and drugs more efficacious. How can such small materials lead to such big advances in...

In Silico Searches for Efficient Renewable Energy Catalysts Through Chemical Compound Space

Speaker(s): 
John Keith
Dates: 
Friday, February 3, 2017 - 11:30am to 12:30pm

This talk will provide an overview of our group’s work using both standard and atypical high-performance computational chemistry modeling to elucidate atomic scale reaction mechanisms of catalytic reactions. I will introduce our toolkit of in silico methods for accurately modeling solvating environments and realistic nanoscale architectures. I will then present how these methods can be used for predictive insights into chemical and material design. The talk will then summarize our progress in unraveling reaction mechanisms for 1) electrochemical CO2 reduction with...

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