Refreshments at 3:30, Talk 4:00 - 5:00 p.m., in CII 3051, unless otherwise noted.
Refreshments at 3:30, Talk 4:00 - 5:00 p.m., in CII 3051, unless otherwise noted.
Modern scientific computing has certain limitations when it comes to calculations with many-body quantum systems. A possible solution to this problem is the idea of quantum computing, or quantum simulation, suggested by Richard Feynman. Here, the actual computer used to calculate is itself a quantum system, overcoming the limitations from regular computers. In this talk I will discuss mathematical methods, specifically the tensor renormalization group, which allow one to understand and possibly create a particular kind of quantum simulator using very cold atoms trapped in a lattice made of laser light. I will discuss this for two physically relevant models and mention possible future directions the research could take.
Nonlinear optical processes, such as second harmonic generation (SHG) are an important component of modern technology such as imaging and high speed data communication. However, traditional materials have a small response which limits the efficiency of devices. The inherently small size of two dimensional (2D) materials makes them attractive options to seamlessly interface with existing technology. In addition, 2D materials have been shown to exhibit extraordinary SHG response. In this talk we will characterize the SHG response of 2D materials using first principles density functional theory plus Bethe-Salpeter equation (BSE) calculations. We examine how strain, alloying, and excitons affect the response. In addition, theoretical structures are studied to gain insight into ways to enhance the SHG response.
The question of the identity of dark matter remains one of the most important outstanding puzzles in modern physics. Weakly Interacting Massive Particles (WIMPs) have long been the frontrunner dark matter candidate, with the supersymmetric neutralino serving as the canonical WIMP. In this talk, I’ll discuss recent results relevant to the search for dark matter, supersymmetric and otherwise, and highlight the spectrum of theoretical and phenomenological approaches to its study. As I’ll demonstrate, even canonical WIMPs may reveal themselves in surprising ways!
Abstract: Increasingly powerful computers and better theoretical insights continue to improve the predictive power of numerical simulations, from atmospheric dynamics to jet engines to lattice quantum chromodynamics. As our resolution power advances, we can study ever larger systems at fine grained detail, exposing multiple scales of physics. However, this comes at a superlinear cost, as with extra detail comes the phenomena of critical slowing down. One class of algorithms tackles critical slowing down, literally at all scales: multigrid algorithms. In this talk I will discuss the anatomy of multigrid from the scale-invariant Laplace equation to the multiscale dynamics in lattice QCD.
Thermoelectrics are semiconductor materials that have a strong interaction between the flows of heat and electricity. They are used in solid state refrigerators, or in converting heat flow to electrical power. We review their history and device applications. We define their “figure of merit” which determines which materials are the best thermoelectrics. Then we discuss the electrical properties that make the figure of merit large. We prove the Mahan-Sofo theorem which defines the best material. This theorem is used to guide the quest for new or revised materials. We also discuss their properties at low temperatures.
"In past several years, materials called halide perovskites have been reshaping researchers’ understanding on designing and developing high-performance optoelectronic/electro-optical semiconductors. In this talk, I will present our recent observations and understanding in the field of halide perovskite materials. Several topics will be covered: epitaxy of the thin film crystals, dimensionality and strain engineering, and hidden carrier dynamics."
Andrew Cupo: “Nanostructures and Phonon Anharmonicity in Atomically-Thin Black Phosphorus.”
Abstract: Atomically-thin black phosphorus has been of interest recently  due to its high carrier mobility  and band gap which remains direct independent of the number of layers . Using first-principles density functional theory (DFT) calculations we have investigated nanoribbons , nanopores , antidot lattices , 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 .
 Quantum Confinement in Black Phosphorus-Based Nanostructures, A. Cupo and V. Meunier, Journal of Physics: Condensed Matter, 29 (28), 2017
 Achieving Ultrahigh Carrier Mobility in Two-Dimensional Hole Gas of Black Phosphorus, G. Long et al., Nano Letters, 16 (12), pp 7768-7773, 2016
 Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene, L. Li et al., Nature Nanotechnology, 12, pp 21-25, 2017
 Controlled Sculpture of Black Phosphorus Nanoribbons, P. M. Das*, G. Danda*, A. Cupo* et al., ACS Nano, 10 (6), pp 5687-5695, 2016
 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
 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.
Via nanophotonics, one can tailor the laws of physics (as far as light is concerned) almost at will. This way, a variety of novel physical phenomena can be enabled and observed. Some examples in topology and light-matter interaction will be presented. Nanophotonics can also enable many new potential applications; examples in energy conversion and lighting will be presented.
“The primary goal of a dividing cell is to accurately and equally segregate its genome into two new daughter cells. In eukaryotes, this process is carried out by a self-organized structure called the mitotic spindle. It has long been appreciated that mechanical forces must be applied to chromosomes. At the same time, the network of microtubules that do not directly interact with kinetochores must be able to sustain large forces to maintain spindle integrity and allow for efficient remodeling and repair of the spindle. The goal of the Forth lab at RPI is to directly measure the forces generated within microtubule networks by ensembles of key mitotic proteins, including motor proteins that push microtubules apart and non-motor proteins that act as sources of viscous drag. New paradigms of mechanical regulation, such as length-dependent force generation and protein clustering by asymmetric friction, will help advance models of force generation across the spindle.”
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.
We exploit strong light matter interaction in graphene for graphene based optoelectronic devices. We use both scanning photocurrent microscopy and optical pump terahertz (THz) probe spectroscopy to reveal hot carrier behavior in graphene. This hot carrier behavior is crucial to understand the effect of optical excitation on graphene and can potentially lead to efficient solar energy conversion and ultrafast optoelectronic devices. We also exploit the strong light matter interaction in THz regime to make graphene based THz modulator.
Transitional metal dichalcogenide (TMD) is a new class of 2D semiconductors which become direct bandgap semiconductor at a single layer limit. MX2 exhibits intriguing excitonic physics as well as strong absorption. We combine optical spectroscopy and scanning tunneling microscopy to determine the extraordinarily large exciton binding energy of MoSe2. This giant exciton binding energy presents a challenge for efficient carrier separation in solar cell applications. We demonstrate that, by using MoS2/WS2 heterostructure, we can achieve a type-II band alignment and realize extremely fast carrier separation.
Advances in Power, Performance and Area scaling of semiconductor devices have been mostly driven by aggressive scaling of device dimensions. This approach has allowed a dramatic increase in integration density, power efficiency, and performance. However, atomic limits have been reached on some of the key device elements, giving rise to the need to look for new optimization pathways. The exploration of novel materials as well as emerging architectures becomes paramount for continuing performance and power advances. In this talk, I will discuss the use of atomistic simulations to optimize materials and devices for future semiconductor technologies.
The group 14 graphane analogues represent a unique class of covalently modifiable 2D materials, as there have been many recent exciting predictions of the existence of 2D quantum spin Hall behavior at room temperature in these materials. Here, we will describe our recent efforts in the creation and properties of hydrogen and organic-terminated group IV graphane analogues, from the topochemical deintercalation of precursor Zintl phases in order to create these topological phases. First, through the synthesis and characterization of a wide array of ligand terminated germanane analogues we have established experimental limits to which the electronic structure can be manipulated via surface chemistry. Second, we will discuss our recent efforts on the synthesis and properties of ligand-terminated Sn-contain graphane analogues to create systems that span from trivial insulators to 2D topological phases depending on the surface functionalization group. Finally, we will describe our emerging discoveries on the existence of topological insulating and magnetic phenomena in Sn-containing layered Zintl phase materials.
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.
"The reduction in price of solar modules coupled with policies geared towards the use of renewable sources for electricity generation has resulted in a dramatic increase in the deployment of photovoltaic panels, at residential, commercial and utility levels. Crystalline and multi-crystalline silicon panels are still the option of lowest cost. However, where thin-film modules were only considered for “niche” applications that required low weight and flexibility, the increase in efficiency achieved in the past few years are turning them a more mainstream alternative. A general discussion of the metrology needs, and the tools available for the production of thin-film photovoltaic materials and characterization of the modules, before and after deployment, will be presented."
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.
The three pillars in semiconductor device technologies are (1) the p-n diode, (2) the MOSFET and (3) the Bipolar Junction Transistor (BJT). They have enabled the unprecedented growth in the information technology that we see today. Here, we will describe our efforts to fabricate and characterize these three benchmark devices in two-dimensional (2D) materials, including graphene and transition metal dichalcogenide semiconductors (TMDs).
Although graphene is gapless, we will describe device concepts based on graphene p-n junctions that can lead to steep subthreshold slope devices. Critical to realizing such devices is the demonstration of relativistic Klein tunneling, a property of chiral carriers in graphene that arises from the Dirac equation. We will describe the fabrication and characterization of graphene p-n junctions and discuss the unique tunneling properties of these junctions.
Using TMD materials, we have fabricated a single device that can reconfigure into p-n, MOSFET, and BJT devices. These devices allow us to provide fundamental linkages between material properties and device performance not possible by fabricating these devices individually. We will describe our method of fabrication and discuss both the electrical and optical properties of these devices. Finally, we will describe compact logic circuits that can be realized from the single reconfigurable device.
The LUX collaboration has recently released its 1-year WIMP search result. LUX remains at the forefront of the search for this dark matter candidate particle in the 10 GeV and higher range, the Weakly Interacting Massive Particle, a potential explanation for a "missing" 25% of the mass-energy of the Universe. Plans for LUX’s 10-ton-scale, Generation-2 successor LZ, plus the NEST quantum chemistry simulation model of WIMP-Xe interactions, will be discussed. Lastly, progress on Generation-3 R&D in collaboration with RPI with superheated Xe will be highlighted from a 100-g-scale prototype at UAlbany (BubXe) focused on also informing existing G-2 experiments like LZ and XENON.
The performance of organic electronic devices such as OLEDs and organic photovoltaic (OPV) cells depends critically on the morphology of the grown layers of which they are comprised. For example, the emission zone of and OLED should be amorphous, yet alignment of the emitting species relative to the substrate plane can result in increased optical outcoupling. Further, OPV cells benefit from a nanocrystalline morphology to allow for rapid and efficient exciton diffusion and charge extraction. In our laboratory we have developed several growth technologies that have demonstrated the ability to precisely control the organic film morphology from the molecular to the macroscopic levels. In this talk I will focus on two growth processes that have consistently exhibited the most precise control of morphology, and in some cases device performance. These methods are the ultrahigh vacuum process of organic molecular beam deposition (OMBD) and organic vapor phase deposition (OVPD). OMBD can controllably deposit monolayer thick films in ultrapure environments. On the other hand, OVPD is based on volatilizing small molecule organics into a hot, inert carrier gas. The molecules are then transported to a cooled substrate where they are physisorbed. OVPD is now finding use in the manufacture of organic electronic devices due to its extraordinary ability to exquisitely control film morphology across large substrate areas while allowing for very rapid film deposition as required for high throughput device fabrication. Both methods of growth will be compared to the widely used and well-known process of high vacuum thermal evaporation.
The field of nano-photonics and in particular, photonic-crystal, has become one of the most influential and wide-ranging realms of contemporary electro-magnetism and optics. In this talk, I will review two recent advances in random and periodic nanostructures, respectively. The first is the creation of the darkest artificial material on earth, having a world-record absorptance of 99.97%1,2. The second is the striking discovery of super Planckian thermal radiation in a 3D metallic photonic-crystal at elevated temperatures 3,4.
1. Zu-Po Yang, Lijie Ci, James A. Bur, Shawn-Yu Lin, and P.M. Ajayan “A vertically aligned carbon nanotube array: the darkest manmade material”, Nano Letters 8, 446 (2008).
2. Zu-Po Yang, Mei-Li Hsieh, James A. Bur, Lijie Ci, Leonard M. Hanssen, Boris Wilthan, P.M. Ajayan and Shawn-Yu Lin, “Experimental observation of extremely weak optical scattering from an interlocking carbon nanotube array”, Applied Optics 50, 1850 (2011).
3. J.G. Fleming, S. Y. Lin, I. El-Kady, R. Biswas and K.M. Ho, “All-metallic 3D photonic crystals with a large photonic band-gap” Nature 417, 52-55 (2002).
4. Mei-Li Hsieh, Shawn-Yu Lin, J. Bur and R. Shenoi, “Experimental observation of anomalous thermal radiation: quasi-equilibrium limit”, Nanotechnology 26, 234002 (2015).
A year ago, the Laser Interferometer Gravitational-Wave Observatory (LIGO) recorded a signal generated by the collision of two black holes 1.3 billion light years away. Minute vibrations of space were all that remained of one of the most powerful events in the universe. Remarkably, we can listen to this signal simply by amplifying it and playing it through speakers. The sound tells the story, never before witnessed, of what happens when black holes collide. We’ll learn what gravitational waves are, how LIGO detected them, and what discoveries might come next from this new way of exploring the universe.
The heliosphere is an extremely rich laboratory for the study of plasma turbulence. However, its exploration is restricted by the limitations associated with remote observation and direct measurements through satellites. In this talk, I will discuss a different approach to the examination of heliospherically-relevant turbulence using a laboratory-based plasma experiment. With plasma gun sources on the wind tunnel configuration of the Swarthmore Spheromak Experiment (SSX), magnetically dynamic plasma can be produced which mimic certain aspects of astrophysical MHD turbulence. I will explain the conditions through which this type of plasma can be created and then give an overview of the myriad of statistical techniques we use to understand this plasma’s turbulence characteristics. Using these metrics, we can compare the laboratory plasma to various regions of the heliosphere, including the solar wind and the magnetosheath. By utilizing the controlled conditions of the plasma in the laboratory, we can complement results from space physics analyses to generate a more comprehensive picture of heliospheric turbulence. I will conclude with the progress of a new experiment in development at Bryn Mawr College designed to better address the pressing turbulence questions.
Two-dimensional (2D) atomic crystals have emerged as a very attractive class of photonic 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, formation of strongly coupled exciton-photon quasiparticles and enhanced nonlinear optical response from 2D transition metal dichalcogenides (TMD) embedded in such structures. Potential applications of such structures with controlled exciton-photon interaction and the use of unique valley properties in these TMDs will also be addressed. Finally, I will also briefly discuss our recent work on room temperature single photon emission from hexagonal boron nitride and the prospects of developing robust quantum emitters using them.
The electronic structure of bilayer graphene under pressure develops very interesting features with an enhancement of the trigonal warping and a splitting of the parabolic touching bands at K into four Dirac cones, one at K and three along the T symmetry lines. As pressure is increased, these cones separate in reciprocal space and in energy, breaking the electron-hole symmetry. Due to their separation in energy, their opposite Berry curvature can be observed in valley Hall effect experiments and in the structure of the Landau levels. Based on the electronic structure obtained by Density Functional Theory, we developed a low energy Hamiltonian that allows us to predict the effects of pressure on measurable quantities such as the Hall conductivity and the Landau levels of the system
The microscopic electronic dielectric response function is a fundamental physical quantity that captures the many-electron correlation effect. Although it is non-local by definition, a local representation in real space can provide insightful understanding of its chemical nature and to improve the computational efficiency of first principles excited state methods. In a recent work [Phys. Rev. B 92, 241107(R), 2015], we have developed a local representation of the electronic dielectric response function, based on a spatial partition of the dielectric response into contributions from each occupied Wannier function using a generalized density functional perturbation theory. We show that the locality of the bare response function is determined by the locality of three quantities: Wannier functions of the occupied manifold, the density matrix, and the Hamiltonian matrix. In systems with a gap, the bare dielectric response is exponentially localized, which supports the physical picture of the dielectric response function as a collection of interacting local response that can be captured by a tight-binding model.
Several applications of the local response theory will be discussed: a) local descriptors in linear response, e.g., bond polarizability and bond screening constant, b) dielectric band structure interpolation using a local basis set, and c) direct formulation and evaluation of molecular polarizability in condensed phase beyond the dipole approximation.
This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.
The assembly of objects into materials and the properties of those materials are dependent on their size and a comparison of their interactions to thermal excitations. Materials that are dominated by behaviors at atomic length scales are often either hard solids or soft liquids. However, there are many materials with structures in the nanometer to micron length scale with interaction energies comparable to thermal fluctuations. These materials are “squishy” with properties similar to both solids and liquids. In this talk, I will describe interesting and unique physics of squishy materials through examples from polymers and active matter.
Long polymers are able to change their configuration significantly through bond rotations with minimal changes in energy. The entropic effects that result often determine the response to external forcing (like fluid flows or electric fields). I will describe our work to efficiently incorporate these effects into computational models and to use those models to understand how to manipulate the dynamics of the polymers.
Active matter is a class of materials that is continually pushed out of equilibrium from every point within the material instead of from the outside. Because they are always out of equilibrium, their dynamics and assembly are not limited by equilibrium statistical mechanics. Other techniques must be used to understand them. I will describe our work to understand the roles of fluid flows in determining the structure and dynamics within active matter.
CUORE-0 is a cryogenic detector that uses an array of tellurium dioxide bolometers to search for neutrinoless double-beta decay of 130Te. The detector consists of 52 TeO2 crystal bolometers held in a ultra-pure copper frame and it was assembled using the new low-background techniques developed for CUORE. Using bolometers operated at ~ 10mK provides excellent energy resolution (< 0.2% FWHM) at the neutrinoless double-beta decay Q-value. CUORE-0 is located at the Laboratori Nazionali del Gran Sasso in Italy and has been taking data since March 2013. I will present the experiment and its neutrinoless double-beta decay search results with a 9.8 kgyr exposure of 130Te. I will also discuss the prospects of CUORE, which has a 130Te mass 19 times greater than that of CUORE-0. CUORE is in the final stages of the construction and scheduled to begin data-taking in 2016.