Colloquium schedule. 

Refreshments at 3:30, Talk 4:00 - 5:00 p.m., in CII 3051, unless otherwise noted.

2018

Mar
28
2018
“Topological Phases of Quantum Matter as Novel Platforms for Fundamental Science and Applications”

I will discuss how topological phases arise in quantum matter through spin-orbit coupling effects in the presence of protections provided by time-reversal, crystalline and particle-hole symmetries, and highlight our recent work aimed at predicting new classes of topological insulators (TIs), topological crystalline insulators, Weyl semi-metals, and quantum spin Hall insulators. [1-7] Surfaces of three-dimensional (3D) topological materials and edges of two-dimensional (2D) topological materials support novel electronic states. For example, the surface of a 3D TI supports gapless or metallic states, which are robust against disorder and non-magnetic impurities, and in which the directions of momentum and spin are locked with each other. Similarly, in 2D TIs, also called quantum spin Hall insulators, the 1D topological edge states are not allowed to scatter since the only available backscattering channel is forbidden by constraints of time-reversal symmetry. The special symmetry protected electronic states in topological materials hold the exciting promise of providing revolutionary new platforms for exploring fundamental science questions, including novel spin textures and exotic superconductors, and for the realization of multifunctional topological devices for thermoelectric, spintronics, information processing and other applications.  Work supported by the U. S. Department of Energy.

[1] Bansil, Lin and Das, Reviews of Modern Physics 88, 021004 (2016).
[2] Xu et al., Science Advances 3, e1603266 (2017).
[3] Vargas et al., Science Advances 3, e1601741 (2017).
[4] Hafiz et al., Science Advances 3, e1700971 (2017).
[5] Chang et al., Physical Review Letters 119,156401 (2017).
[6] Okada et al., Physical Review Letters 119, 086801 (2017).
[7] Chang et al., Physical Review Letters (2017).
 

Short bio: Bansil is a University Distinguished Professor in physics at Northeastern University (NU). He served at the US Department of Energy managing the flagship Theoretical Condensed Matter Physics program (2008-10). He is an academic editor of the international Journal of Physics and Chemistry of Solids (1994-), the founding director of NU’s Advanced Scientific Computation Center (1999-), and serves on various international editorial boards and commissions. He has authored/co-authored over 370 technical articles and 18 volumes of conference proceedings covering a wide range of topics in theoretical condensed matter and materials physics, and a major book on X-Ray Compton Scattering (Oxford University Press, Oxford, 2004). Bansil is a 2017 Highly Cited Researcher (Web of Science/Clarivate Analytics). 

Low Center for Industrial Innovation (CII) 3051 4:00 pm

Mar
7
2018
CANCELLED: “The U.S. Electron Ion Collider: In Pursuit of Understanding the Glue that Binds Us All.”

Quantum Chromodynamics (QCD) is no doubt the correct theory of strong interactions within the Standard Model of Physics. However, despite decades of significant theoretical progress made based on a broad range of experimental observations, some of the most intellectually compelling questions remain unanswered. For example, how do quark, gluons and their interactions, collectively result in the observed fundamental properties of hadrons such as the spin and mass? What is the QCD/partonic origin of the nucleon-nucleon forces in nuclei? How does nuclear environment impact the parton’s momentum distribution in confined nucleons? Does the gluon density in nucleons and nuclei saturate at extremely high energy, and form a universal form of gluonic matter? Such questions can now be addressed quantitatively through a polarized high-energy high-luminsity Electron Ion Collider (EIC), now possible with the advances in accelerator technology. Theoretical frame work is now mature enough to put the resulting measurements in to a comprehensive picture within QCD that could directly address the compelling questions in QCD.  The EIC was recommended in the 2015 US Longe Range Plan prepared by the US Nuclear Science Advisory Committee (NSAC) as the highest priority facility for new construction in the US. I will summarize its science case, and present the status
and plans for its realization.

Low Center for Industrial Innovation (CII) 3051 4:00 pm

Feb
21
2018
“Bulk-Boundary Correspondence: What is the role of boundary conditions?”

In a periodic solid, electrons can only occupy certain bands of allowed energies. This fact, together with the Fermi-Dirac statistics of electrons, explains the sharp difference between metals and insulators and seems to leave no room for any kind of ``in-between” material.  However, we know now that not all band structures are born equal: There are subtle but robust differences captured by topological invariants, integer numbers computed from the Berry curvature of Bloch functions. Topologically non-trivial insulators are precisely ``in-between” materials, because they display both a metallic surface and an insulating bulk. The idea that a topologically non-trivial bulk dictates a metallic surface is dubbed the bulk-boundary correspondence, and there are myriads of heuristic arguments and numerical experiments for open boundary conditions that support it. Nonetheless, our understanding of this conjecture is arguably shallow: What exactly is the mechanism by which a purely bulk property, completely reliant on translation symmetry for its existence, forces robust edge states? In this talk I will introduce a new tool of band structure theory, a generalization of Bloch’s theorem for arbitrary boundary conditions,* and an associated algorithm for solving exactly tight binding models subjected to arbitrary boundary conditions on two parallel hyperplanes. The generalized Bloch theorem yields a sharp description of the possible wave functions of tight-binding models, showing that power-law modes and perfectly localized modes can coexist with the usual oscillating or exponentially decaying modes. One finds by simple trial and error that boundary conditions can break the classifying symmetries of the topological system badly without affecting the edge modes: it is apparent that from the point of view of the metallic surface there are relevant and irrelevant directions in boundary space and these directions are not simply determined by bulk symmetries.

*Abhijeet Alase, Emilio Cobanera, Gerardo Ortiz, and Lorenza Viola, Generalization of Bloch's theorem for arbitrary boundary conditions: Theory, Phys. Rev. B 96, 195133.
 

Low Center for Industrial Innovation (CII) 3051 4:00 pm

Feb
14
2018
“Valleytronic multi-particle bound states in an atomically-thin”

Since the isolation of graphene in 2004, a monolayer of covalently-bonded carbon atoms, the field of two-dimensional (2D) materials has been rapidly expanding to other atomic layers, presenting a plethora of fascinating new discoveries. In particular, 2D hexagonal transition metal dichalcogenides (TMDCs), a class of direct gap semiconductors, have been attracting much recent attention due to their unique optoelectronic properties.  In this talk, I will discuss our recent studies on high-quality monolayer tungsten diselenide (1L-WSe2), a 2D TMDC, whose excitonic excitations exhibit intriguing valleytronic properties. We found that with efficient removal of disorder and phonon, the exciton, composed of an electron and a hole in 1L-WSe2, can radiate not only as the 1s ground state, but also as the excited 2s Rydberg state. Interestingly, the 2s exciton shows superior valleytronic properties compared to 1s, providing key information on the fundamental valley scattering processes in TMDCs.1 In a strong magnetic field up to 31 Tesla, we further observe excitonic emissions due to the 3s and 4s states; at lower energies, complexes due to bound states of more than three particles emerge. These interesting optical features provide a rich variety of quasi-particle states for manipulating the valley degree of freedom in monolayer TMDCs. 


Reference:
1. Chen, S.-Y. et al. Superior Valley Polarization and Coherence of 2s Excitons in Monolayer WSe2. Phys. Rev. Lett. 120, 46402 (2018).
 

Low Center for Industrial Innovation (CII) 3051 4:00 pm

Jan
31
2018
William Cunningham, Northeastern University
Low Center for Industrial Innovation (CII) 3051 4:00 pm
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Jan
24
2018
"FAILURES, DYNAMICS, EVOLUTION AND CONTROL IN THE GLOBAL RISK NETWORK "

Risks that threaten modern societies form an intricately interconnected network, so it is important to understand how risk activations in distinct domains influence each other. We study the global risks network defined by World Economic Forum experts. Risks are modeled as Cascading Alternating Renewal Processes (CARP) with variable intensities driven by hidden values of exogenous and endogenous failure probabilities. We use maximum likelihood evaluation to find the optimal model parameters based on the expert assessments and historical status of each risk. This approach enables us to analyze risks that are particularly difficult to quantify, such as geo-political or social risks in addition to more quantitative risks such as economic, technological and natural.

In the talk, we describe model dynamics and discuss how to use the model to provide quantitative means for measuring interdependence and materialization of risks in the network. We also talk about limits of the predictability of the system parameters from historical data and model ability to recover hidden variable. We also describe how the network evolved recently by comparing steady state which would be reached if the risks were left unabated at different time points. We also analyze the model resilience and optimal control. Our findings elucidate the identity of risks most detrimental to system stability at various points in time. The model provides quantitative means for measuring the adverse effects of risk interdependence and the materialization of risks in the global risk network.

BIO OF THE PRESENTER:  Dr. Boleslaw K. Szymanski is the Claire and Roland Schmitt Distinguished Professor and the Director of the ARL Social and Cognitive Networks Academic Research Center at the Rensselaer Polytechnic Institute and the Rensselaer Network Science and Technology (NeST) Center. He received his Ph.D. in Computer Science from Institute of Informatics of National Academy of Science in Warsaw, Poland, in 1976. He published over 300 scientific articles, is a foreign member of the National Academy of Science in Poland and an IEEE Fellow and was a National Lecturer for the ACM. In 2009, he received the Wilkes Medal of British Computer Society and in 2003, William H. Wiley 1866 Distinguished Faculty Award from RPI. His current research interests focus on computer networks and technology-based social networks.

Low Center for Industrial Innovation (CII) 3051 4:00 pm

Jan
17
2018
Low Center for Industrial Innovation (CII) 3051 4:00 pm

2017

Oct
4
2017
Andrew Cupo: “Nanostructures and Phonon Anharmonicity in Atomically-Thin Black Phosphorus.” Alaa Moussawi: “Cascading Overload Failures in Power Grids: Analysis and Mitigation”

Andrew Cupo: “Nanostructures and Phonon Anharmonicity in Atomically-Thin Black Phosphorus.”

Abstract: Atomically-thin black phosphorus has been of interest recently [1] due to its high carrier mobility [2] and band gap which remains direct independent of the number of layers [3]. Using first-principles density functional theory (DFT) calculations we have investigated nanoribbons [4], nanopores [4], antidot lattices [5], and phonon anharmonicity in black phosphorus. We showed that the few-nm wide armchair and zigzag nanoribbons fabricated by collaborators have similar electronic properties as their single-layer counterparts. Furthermore, we rationalized the asymmetric opening of nanopores in black phosphorus under uniform irradiation by showing that the energy barrier for removing atoms from the edge is anisotropic in phosphorene. In addition, we explored the electronic properties of phosphorene antidot lattices. We demonstrated a tunable band gap due to quantum confinement with deviations from the general trend attributed to self-passivating edge morphologies. The spatial distribution of the band gap is bimodal with higher band gap atoms emanating from the zigzag nanoconstrictions, which reflects the material anisotropy. Lastly, we carried out ab initio molecular dynamics simulations in combination with the power spectrum method to show that phosphorene’s phonon frequencies decrease with increasing temperature. This accounts for the observed temperature dependence of the phonon frequencies from Raman spectroscopy [6].

[1] Quantum Confinement in Black Phosphorus-Based Nanostructures, A. Cupo and V. Meunier, Journal of Physics: Condensed Matter, 29 (28), 2017
[2] Achieving Ultrahigh Carrier Mobility in Two-Dimensional Hole Gas of Black Phosphorus, G. Long et al., Nano Letters, 16 (12), pp 7768-7773, 2016
[3] Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene, L. Li et al., Nature Nanotechnology, 12, pp 21-25, 2017
[4] Controlled Sculpture of Black Phosphorus Nanoribbons, P. M. Das*, G. Danda*, A. Cupo* et al., ACS Nano, 10 (6), pp 5687-5695, 2016
[5] Periodic Arrays of Phosphorene Nanopores as Antidot Lattices with Tunable Properties, A. Cupo*, P. M. Das* et al., ACS Nano, 11 (7), pp 7494-7507, 2017
[6] Temperature Evolution of Phonon Properties in Few-Layer Black Phosphorus, A. Łapińska et al., The Journal of Physical Chemistry C, 120 (9), pp 5265-5270, 2016

Alaa Moussawi: “Cascading Overload Failures in Power Grids: Analysis and Mitigation”

Abstract: Cascading overload failures (blackouts) are a common and catastrophic vulnerability of spatially-embedded distributed flow networks that are poorly understood. The efficiencies that locally connected networks afford us come at an also high cost. With increasing energy demands taxing old infrastructures, power grids are currently operating at a critical phase where the capacity of these systems is approaching load demands. This highlights the importance of understanding the dynamics of power systems so that they can most effectively be utilized at this critical phase without major failure. Mitigation techniques will be presented, and their effectiveness under varying constraints will be investigated. A simple strategy for approximating the severity of multi-node failures will be presented. Finally, it will be shown that such networks exhibit a phase transition at a given capacity threshold. Moreover, we show that cascade size distributions measured in this region exhibit a power-law decay.
 

Andrew Cupo and Alaa Moussawi, Physics, Applied Physics and Astronomy, Rensselaer
Low Center for Industrial Innovation (CII), Room 3051 4:00 pm

Apr
19
2017
"Classical vs. Quantum Decoherence and the Quantum- Classical Path Integral"

The path integral formulation of time-dependent quantum mechanics provides the ideal framework for rigorous quantum-classical or quantum-semiclassical treatments, as the spatially localized, trajectory-like nature of the quantum paths circumvents the need for mean-field-type assumptions. However, the number of system paths grows exponentially with the number of propagation steps. In addition, each path of the quantum system generally gives rise to a distinct classical solvent trajectory. This exponential proliferation of trajectories with propagation time is the quantum-classical manifestation of time nonlocality, familiar from influence functional approaches. A real-time quantum-classical path integral (QCPI) methodology has been developed. The starting point is the identification of two components in the effects induced on a quantum system by a polyatomic environment. The first, “classical decoherence mechanism” dominates completely at high temperature/low-frequency solvents and/or when the system-environment interaction is weak. Within the QCPI framework, the memory associated with classical decoherence is removable. A second, nonlocal in time, “quantum decoherence process” is also operative at low temperatures, although the contribution of the classical decoherence mechanism continues to play the most prominent role. The classical decoherence is analogous to the treatment of light absorption via an oscillating dipole, while quantum decoherence is primarily associated with spontaneous emission, whose description requires quantization of the radiation field. The QCPI methodology takes advantage of the memory-free nature of system-independent solvent trajectories to account for all classical decoherence effects on the dynamics of the quantum system in an inexpensive fashion. Inclusion of the residual quantum decoherence is accomplished via phase factors in the path integral expression, which is amenable to large time steps and iterative decompositions. The methodology can be used to perform an all-atom simulation of nonadiabatic processes in condensed phase environments with unprecedented accuracy. Applications to charge transfer reactions in solution will be discussed.

CII 3051 4:00 pm

Feb
22
2017
"Atomically thin and two-dimensional materials: Science and Applications"

In the past decade, there has been enormous progress in materials science and engineering at the very limits of quantum confinement. Ground-breaking discoveries and innovations have resulted from a range of atomically-thin systems such as carbon nanotubes, graphene, 2D transition metal dichalcogenides and other layered materials. These developments in ultrathin matter at the very quantum limit of stability have spearheaded an explosive growth in new science and technology.  This talk will attempt to outline the contributions of our research group in this exciting new field, including our attempts to develop and manipulate new types of atomically-thin materials, exploring the behaviour of charge, photons and phonons in them, and utilizing their unique properties to develop applications in the nanoelectronics, optoelectronics, sensing, detection, actuation, energy, and other areas. Through these discussions, I will try to motivate how quantum matter can potentially transform several important applications and enable them to operate at ultra-high and unprecedented performances.

Bio: Prof. Kar received his BSc degree in Physics (Honours) from Presidency College, Kolkata, in 1995, and obtained his MS (1998) and PhD (2004) degrees in Physics from the Indian Institute of Science, India. He worked as a postdoc at the Universitaet Karlsruhe, Germany, and Rensselaer Polytechnic Institute, USA (with Prof. PM Ajayan). He also held a research assistant professor position at RPI with Prof. Saroj Nayak, before joining Northeastern University, USA, as an assistant professor of physics, in 2010. Prof. Kar has published about 65 papers in peer-reviewed journals, including in Nature Nanotechnology, Nature Materials, Nature Photonics, Nature Communications, and Science Advances. Prof. Kar has presented over 45 invited talks worldwide, and has 6 patent applications. He currently serves as an Editorial Board member of Scientific Reports, and has served on several US and international grant application review panels. Prof. Kar enthusiastically promotes the science and technology of 2D materials and systems by regularly organizing conferences and symposia at top international physics and materials science congresses venues.

CII 3051 4:00 pm

Feb
8
2017
"Understanding light-matter interactions at the single-molecule level."

Metal nanoparticles sustain a collective oscillation of their free electrons, called a localized surface plasmon resonance (LSPR), when excited by an electromagnetic wave. When this incident wave is resonant with the LSPR frequency, the field intensity is strongly increased in the near field of the nanoantenna. Plasmonics thus provides a unique setting for the manipulation of light via the confinement of the electromagnetic field to regions well below the diffraction limit. This has opened up a wide range of applications based on extreme light concentration, including nanophotonic lasers and amplifiers optical metamaterials, biochemical sensing and antennas transmitting and receiving light signals at the nanoscale. However, many difficulties remain in experimentally measuring the shape, size, and enhanced field properties of the localized electromagnetic modes in the vicinity of the nano-particles due to the limitations of optical microscopy. In this seminar, I will discuss how we can unravel the coupling of light to a nano-antenna through single-molecule fluorescence imaging. This technique is a powerful tool to optically study structures beyond the diffraction limit by localizing isolated fluorophores and fitting the emission profile to the microscope point-spread function. By using the random motion of single dye molecules in solution to stochastically scan the surface, and by assessing emission intensity, wavelength, and density of emitters as a function of position, we gain new insight into the properties of these systems and pave the way for the development of better plasmonic devices.

CII 3051 4:00 pm
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Jan
25
2017
CII 3051 4:00 pm
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