# Upcoming Seminars

UMass Boston Physics Colloquia

Talks are on Thursdays, 1:00 pm, at S(cience)-3-126, unless stated otherwise

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**Thursday, Sep 22, 1:00pm**

**Dmitri Voronine
Institute for Quantum Science and Engineering, Texas A&M **

__Quantum Biology: From Photosynthesis to Bioinspired Devices__

Quantum mechanics and thermodynamics have deep connections which govern the behavior of laser and photocell quantum heat engines (QHEs) [1]. Same principles can be used to understand biological systems, for example, the light harvesting efficiency in photosynthesis and to mimic Nature designs to build more efficient devices. Natural photosynthetic complexes such as FMO antennas and reaction centers from photosynthetic bacteria and plants can be described as QHEs that operate under the natural conditions of incoherent excitation by sunlight [2]. Quantum effects such as noise-induced quantum coherence play a role in enhancing energy and charge transfer efficiencies and hold promise for improving the design of light-harvesting devices. Structural and dynamical disorder is also important for understanding their quantum behavior.

References:

1. Scully, M. O.; Chapin, K. R.; Dorfman, K. E.; Kim, M. B.; and Svidzinsky, A. A. "Quantum heat engine power can be increased by noise-induced coherence" PNAS 108, 15097 (2011).

2. Dorfman, K. E.; Voronine, D. V.; Mukamel, S. and Scully, M. O. "Photosynthetic reaction center as a quantum heat engine" PNAS 110, 2746 (2013).

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**Thursday, Sep 29, 1:00pm**

**Xiaoqin E Li
U Texas at Austin **

__Fundamental Excitations in Solids __

Fundamental excitations such as plasmons, excitons, and magnons determine both the equilibrium and non-equilibrium properties of solids such as metals, semiconductors, and magnetic materials. We use various laser spectroscopy methods to probe their properties. In this talk, I will discuss two examples addressing the coupling between these quasiparticles. How can we probe effective magnon and phonon temperatures separately? And what does it tell us about coupling between these quasiparticles when they are driven out of equilibrium? What happens when a single exciton in a semiconductor quantum dot (a two-level quantum resonance) is coupled to a localized surface plasmon mode in a metallic nanoparticle (a classical resonance)?

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**Thursday, Oct 6, 1:00pm**

**NO SEMINAR **

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**Thursday, Oct 13, 1:00pm**

**Sameer Iyer
Brown U **

__Global Steady Prandtl Expansions over a Moving Boundary __

We will outline a recent result which proves that steady, incompressible Navier-Stokes flows posed on the quadrant [1, ∞) × [0, ∞) with moving boundary can be decomposed into Euler and Prandtl boundary layer flows, globally in the tangential variable.

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**Thursday, Oct 20, 1:00pm**

**Clarissa Zimmerman Cooley
MGH A.A. Martinos Center for Biomedical Imaging **

__Developing new systems for neuroimaging: Portable MRI and Magnetic Particle Detection __

This talk will describe the development of two new imaging system designs, from back-of-envelope to functioning prototypes. First the development of a low-cost, portable MRI scanner for human brain imaging, and second, a new method for functional brain imaging with Magnetic Particle Imaging (MPI).

As the premiere modality for brain imaging, there is strong motivation to develop low-cost, portable MRI scanners available for unconventional locations (e.g. ICUs, doctor offices, ambulances, ERs, rural clinics). It is infeasible to scale down conventional MRI scanners, which rely on large superconducting magnets and high-power linear gradient fields for image encoding. Instead, we co-designed new imaging hardware and image reconstruction algorithms starting from a permanent magnet Halbach array to produce a magnetic field with an intrinsic image encoding field. The magnet will rotate around the patient’s head to perform generalized projection imaging with the built-in field variation of the magnet. Careful modeling of the system is conducted to enable generalized image reconstruction based on an image encoding matrix.

Magnetic Particle Imaging (MPI) is a young but promising technology, and an altogether separate imaging modality from MRI, that relies on the non-linear magnetic response of iron-oxide nanoparticles to a magnetic drive field. Our goal is to detect activation-induced Cerebral Blood Volume (CBV) changes by monitoring the local iron oxide concentration. This CBV-contrast source is well-proven in animal and human fMRI studies. However, while MRI uses secondary effects on the signal relaxation times, MPI directly detects the injected nanoparticle’s magnetization - which potentially provides a 120-fold increase in contrast-to-noise ratio (CNR) of neuronal activation. The preliminary design of a Magnetic Particle Detection system will be presented.

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**Thursday, Oct 27, 1:00pm**

**Walter Strauss
Brown U **

__Steady Water Waves __

The mathematical study of water waves began with the derivation of the basic mathematical equations of fluids by Euler in 1752. Later, water waves (with a free boundary at the air interface) played a central role in the work of Poisson, Cauchy, Stokes, Levi-Civita and many others. It remains a very active area to the present day.

I will consider classical 2D traveling water waves with vorticity. By means of local and global bifurcation theory using topological degree, we now know that there exist many such waves. They are exact smooth solutions of the Euler equations with the physical boundary conditions. I will exhibit some numerical computations of such waves. Many fundamental problems remain open. For instance, how steep can such a wave be?

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**Thursday, Nov 3, 1:00pm**

**Ilija Zeljkovic
Boston College **

__Interplay of Dirac Fermions and Structural Deformations in Topological Crystalline Insulators __

Topological crystalline insulators (TCIs) are a new class of topological materials which harbor massless Dirac surface states (SS). Theory postulates that these SS are protected by crystalline symmetries, and that SS electrons can acquire a mass if these symmetries are broken. Moreover, this unique crystalline protection has led to a series of intriguing predictions of strain-generated phenomena, such as the appearance of pseudo-magnetic fields and topological phase transitions. In this talk, I will present our recent scanning tunneling microscopy (STM) investigations of two TCI systems: single crystals of Pb1-xSnxSe and strained thin films of SnTe. Simultaneous imaging of the atomic and electronic structures in TCI single crystals reveals that a fraction of the Dirac electrons acquire mass due to a surface distortion that breaks a crystalline mirror symmetry. Furthermore, we discover that even in the absence of any symmetry breaking, nanoscale strain in TCI heteroepitaxial thin films can induce spatially dependent changes in the SS dispersion associated with the momentum-space shift of the Dirac nodes. Our experiments provide the first direct visualization of the effects of strain on the SS band structure in any topological material and suggest a novel pathway for manipulation of Dirac electrons via structural deformations.

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**Thursday, Nov 10, 1:00pm**

**NO SEMINAR **

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**Thursday, Nov 17, 1:00pm**

**Panayotis Kevrekidis**

__Multi-Component Nonlinear Waves in Optics and Atomic Condensates: Theory, Computations and Experiments __

Multi-Component Nonlinear Waves in Optics and Atomic Condensates: Theory, Computations and Experiments Motivated by work in nonlinear optics, as well as more recently in Bose-Einstein condensate mixtures, we will explore a series of nonlinear states that arise in such systems. We will start from a single structure, the so-called dark-bright solitary wave, and then expand our considerations to multiple such waves, their spectral properties, nonlinear interactions and experimental observations. A twist will be to consider the dark solitons of the one component as effective potentials that will trap the bright waves of the second component, an approach that will also prove useful in characterizing the bifurcations and instabilities of the system. Beating so-called dark-dark soliton variants of such states will also be touched upon. Generalizations of all these notions in higher dimensions and, so-called, vortex-bright solitons will also be offered and challenges for future work will be discussed.

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**Thursday, Nov 24, 1:00pm**

**THANKSGIVING, NO SEMINAR **

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**Thursday, Dec 1, 1:00pm**

**Kirill Korolev
Boston U **

__To grow or not to grow: From microtubules to cancer __

Many biological systems either grow or stay dormant. I will argue that the transition from dormancy to growth contains a lot of information about the system dynamics and provide two specific examples: one in polymer networks and one in cancer. For microtubules, I will show that auto-catalytic nucleation is necessary to assemble large microtubule complexes. Our model predicts an explosive transition from dormancy to growth, a striking phenomenon that we confirmed by a biochemical perturbation. These findings suggest a new paradigm for the robust assembly of functional polymer networks. For cancer, I will argue that somatic evolution is akin a tug-of-war between a few strong mutations that promote cancers and many weak mutations that hold it back. Our theory predicts that majority of genetic lesions fail to become clinical cancers and explains paradoxical observations in genomic and clinical data. Recent experiments confirm many of our predictions and suggest that genetic load presents a possible target for an anticancer therapy.

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**Thursday, Dec 8, 1:00pm**

**David C. Kaspar
Brown U **

__Kinetics of shock clustering: theorems, problems, and connections to integrability __

TBA

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