About
Physics explores the natural world. Advances in physics lead to new technologies. Students in physics acquire mathematical skills and ways of thinking that support success in many careers. Many of our physics graduates have gone on to earn graduate degrees in physics. Many others have succeeded in law, medicine, engineering, business and finance.
The Department of Physics has state-of-the-art laboratory equipment for introductory, intermediate and advanced experiments in mechanics, electromagnetism, optics, lasers, electronics, quantum and nuclear physics. A weekly colloquium brings physicists from all over the world to present and discuss their research.
Colloquia: Spring 2015
March 10
Mark Edelman (Yeshiva University / New York University)
Title: Systems with Power-law Memory and Fractional Attractors
Abstract:
Systems with power-law memory
appear in many areas of science (physics, biology, psychology, …), engineering
applications (material with memory),and they are used in control. We
investigated a relatively simple but still general model of such systems. In
the case of discrete systems this model is equivalent to a difference equation
with coefficients which are fractional/integer Eulerian numbers. In the
continuous limit this equivalence leads to the equivalence of differential
equations with Grunvald-Lentnikov fractional derivatives and integral Volterra equations
of the second kind. This analysis allows to derive new properties of fractional
Eulerian numbers.
We investigated behavior of nonlinear discrete systems with power-law memory in
the case of harmonic (standard map) and quadratic (logistic map)nonlinearities.
In addition to sinks and chaotic attractors which appear in regular dynamics
systems with memory demonstrate new type of attractors– cascade of bifurcations
type trajectories (CBTT). In CBTT a trajectory first converges to a fixed
point, then this fixed point abruptly turns into a period-2 sink, then to
period-4, and so on without any changes in map parameters.
Bifurcation diagrams of systems with power-low memory depend on two parameters:
the nonlinearity parameter and the memory parameter (exponent in a power-law).
This may allow additional control of the corresponding biological,
psychological, social systems, in which bifurcations can be caused by
manipulating a memory parameter.
March 17
Alexandre Almedia (Universidade de São Paulo)
Title: Simulations of liquid bridge trapped in different geometries.
A small amount of liquid trapped between two solid surfaces due to capillary forces is called a liquid bridge. Liquid bridges can be found in different situations, for example, in the liquid phase sintering process, in atomic force microscopy in an environment with humidity, and also in the course of certain lung diseases. We are studying liquid bridges in these three situations using Monte Carlo or molecular dynamic simulations. We explore the effect of the solid-liquid contact angle and the distance between of the solid surfaces on the stability of the bridge. We compare the profile the of the bridge and in the force applied from the bridge on the solid surfaces, with the prediction of the classical theory based on the minimization of the surface energy. We find that the classical theory gives a good approximation even at nanoscales when the bridge consists of a few thousand molecules.
March 10, 17, 24
April 14, 28
May 5
Colloquia: Fall 2014
November 4 Pouyan Ghaemi
November 18 Anatoly Frenkel
December 2 Ignacio Pascual
December 9 Lev Ostrovsky
Past Colloquia
January 28^{th}
Alexander Khanikaev, Physics Department, Queens College (Emil)
Photonic topological insulators: from theory to practical realization
The past three decades have witnessed the discovery of the Quantum Hall Effect (QHE), Quantum Spin Hall Effect (QSHE) and Topological Insulators (TIs) and transformed our views on the quantum states of matter. These exotic states are characterized by insulating behavior in the bulk and the presence of the edge states contributing to charge or spin currents which persist even when the edge is distorted or contains impurities. In the last few years, a number of research groups have realized that the same "robust" conducting edge states can be implemented in photonic systems. An early theoretical prediction [1, 2] and experimental demonstration [3] of the topologically protected light transport opened a new direction in photonics. In this talk I will review development of this field with focus on photonic topological insulators with preserved time-reversal symmetry that we have recently proposed to implement with the use of bianisotropic metamaterials [4]. I will present new designs of photonic topological insulators based on waveguide geometries that can be readily implemented at microwave frequencies and will discuss perspectives for applications. I will show that photonic topological insulators offer an unprecedented platform for controlling light: deliberately created distribution of the bianisotropy, playing the role of the effective magnetic field, allows routing of photons along arbitrary pathways without significant loss or backscattering [5].
[1] F. Haldane and S. Raghu, Phys. Rev. Lett. 100, 13904 (2008).
[2] Z. Wang, Y. D. Chong, J. D. Joannopoulos, and M. Soljačić, Phys. Rev. Lett. 100, 013905 (2008).
[3] Z. Wang, Y. Chong, J. D. Joannopoulos, and M. Soljacic, Nature 461, 772 (2009).
[4] A. B. Khanikaev, S. H. Mousavi, W.-K. Tse, et al.,Nature Mater. 12, 233 (2013).
[5] A. B. Khanikaev, Nature Photon. 7, 941 (2013).
February 4^{th}
Edward Belbruno, Princeton University / Innovative Orbital Design (Marian)
Structure of Singularities in Black Holes and the Big Bang
A new approach is described on studying the dynamical structure of the gravitational singularity in the big bang. This is accomplished, in part, by a McGehee regularization map. Current work by the speaker and BingKan Xue is discussed which addresses realistic physical modeling. A surprising condition is derived, necessary for resolution of the big bang and extending solutions through it. This methodology, in part, was applied in an earlier work with Frans Pretorius for Schwarzschild black holes.
February 11^{th}
Emanuel Lazar, University of Pennsylvania (Fredy)
Dynamical Cell Complexes: Evolution, Universality, and Statistics
Many natural structures are cellular in nature -- soap foams, biological tissue, and polycrystalline metals are but a few examples that we frequently encounter in everyday life. In many of these systems, energetic factors force the geometry and topology of these structures to evolve in a continuous manner that drives the system towards more stable configurations. We use computer simulations to study how mean curvature flow shapes cell structures in two and three dimensions and consider how this can be measured in a statistical manner. This research lightly touches on discrete geometric flows, combinatorial polyhedra and their symmetries, and the quantification of topological features of large cellular systems.
February 18^{th}
Ivan Saika-Voivod, Memorial University of Newfoundland (Sergey)
Can one melt a crystal by cooling at constant pressure?
February 25^{th}
Gabrielle Long, X-ray Science Division, Argonne National Laboratory
(Anatoly)
A metallic glass that grows from the melt like a crystal
When a molten material is cooled, it typically grows into orderly crystls. But if the cooling rate is too fast for the entire melt to crystallize, the remaining material ends up in a non-crystalline state known as a glass. This talk is about the discovery and characterization of a unique metallic glass that, during rapid cooling, forms a solid by means of nucleation followed by growth normal to a moving interface between the solid and melt, with partitioning of the chemical elements. We were able to show experimentally that this is not a polycrystalline composite with nanometer-sized grains, and conclude that this may be a new kind of structure: an atomically ordered, isotropic, non-crystalline solid, possessing no long-range translational symmetry. This novel structure-isotropic with infinite rotational symmetry and no translational symmetry-is considered theoretically possible, but has never before been observed.
March 11^{th}
Marija Vucelja, Rockefeller (Mark)
Non-equilibrium statistical physics, population genetics and evolution
I will present a glimpse into the fascinating world of biological complexity from the perspective of theoretical physics. Currently the fields of evolution and population genetics are undergoing a renaissance, with the abundance of accessible sequencing data. In many cases the existing theories are unable to explain the experimental findings. The least understood aspects of evolution are intrinsically quantitative and statistical and we are missing a suitable theoretical description. It is not clear what sets the time scales of evolution, whether for antibiotic resistance, emergence of new animal species, or the diversification of life. I will try to convey that physicists are invaluable in framing such pertinent questions. The emerging picture of genetic evolution is that of a strongly interacting stochastic system with large numbers of components far from equilibrium. In this colloquium I plan to focus on the dynamics of evolution. I will discuss evolutionary dynamics on several levels. First on the microscopic level - an evolving population over its history explores a small part of the whole genomics sequence space. Next I will coarse-grain and review evolutionary dynamics on the phenotype level. I will also discuss the importance of spatial structures and temporal fluctuations. Along the way I will point out similarities with physical phenomena in condensed matter physics, polymer physics, spin-glasses and turbulence.
March 18^{th}
Sultan Catto, Baruch College (Amish)
The Search for Higher Symmetries in Nature
Symmetry is a wide-reaching concept that has been used in a variety of ways in physics. Originally it was used mainly to describe the arrangement of atoms in molecules and crystals (geometric symmetries). Over the course of the past century it has been considerably extended, covering some of the most fundamental ideas in physics. This talk will center on the role played by the new symmetry principles and their consequences.
March 25^{th}
Dr. Philip Kim, Dept. of Physics, Columbia University (Mady)
April 1^{st}
Austen Angel, Arizona State University (Sergey)
With participation of Martin Goldstein, Yeshiva University
Transformations in supercooled liquids
April 8^{th}
Dr. Osgood, Dept. of Applied Physics and Electrical Engineering, Columbia University (Mady)
What dimensionality does to crystals: The new 2D crystals
Two-dimensional crystals present new physical phenomena and materials properties. These ideas have excited a wide range of pure and applied physicists. In our talk we will illustrate these intriguing properties by examples from our research in graphene and the metal dichalcogenides, as well as others in the field and show how dimensionality affects both the structural and electronics properties of these materials. Typically these crystals are prepared by either exfoliation or CVD growth. Our analysis is based on either high-energy/momentum resolution probes or via ultrafast time-resolved photoemission. In one example, for MoS_{2} there is an evolution in band structure with layer number; that is, there is an indirect-to-direct bandgap transition in going from few-layer to monolayer MoS_{2} crystals due to changes in quantum confinement as the number of layer decreases. In addition it has strong strong spin-orbit-coupling-induced split valence bands due to broken inversion symmetry, which makes it interesting for spin-physics exploration. One of the consequences of this evolution is a decrease in dispersion of the valence band at in monolayer MoS_{2}, thus leading to a dramatic increase in the hole effective mass.
April 29^{th}
Xavier Leoncini, Centre de Physique Théorique, Aix-Marseille University (Mark)
Self-regularisation in systems with long-range interactions
Dynamics of many-body long-range interacting systems is investigated, using the XY-Hamiltonian mean-field model as a case study. We show that regular trajectories, associated with invariant tori of the single-particle dynamics emerge as the number of particles is increased. Moreover, the construction of stationary solutions as well as studies of the maximal Lyapunov exponent of the systems show the same trend towards
integrability. This feature provides a dynamical interpretation of the emergence of long-lasting out-of-equilibrium regimes observed generically
in long-range systems. Extension beyond the mean-field system is considered and display similar features. At the end of the talk I will consider the influence of the topology, and show that some state with infinite susceptibility can emerge.
April 9
Paul Brumer, Department of Chemistry, University of Toronto
Coherence, Decoherence, and Incoherence in Natural Light Harvesting Systems
Abstract:
A number of 2D Photon Echo experiments have shown the presence of long-lived coherences in light harvesting systems, such as FMO and PC645. Such studies have led to conjectures about the role of quantum coherences in biology, leading to arguments in favor of "quantum biology." However, experiments of this kind involve excitation with coherent laser sources, whereas nature irradiates with essentially incoherent sunlight/moonlight. We discuss the differing responses of molecular systems to coherent vs. incoherent excitation in both open and closed quantum systems, demonstrating that the experimentally observed coherences, although revealing features of the system Hamiltonian and of the system-bath interactions, do not argue for quantum coherent evolution in nature.
April 23
Luat Vuong
Queens College, CUNY
Demonstrations of Photo-induced Magnetism in Metallic Nanocolloids Uusing Sunlight and Fridge Magnets
Abstract:
The focus of this talk is on nonlinear plasmonic vortex dynamics, which are far from understood and lead to appreciable photo-induced magnetic fields in metallic nanostructures.
We have recently experimentally, analytically and numerically demonstrated the nonlinear photo-induced plasmon-assisted magnetic response that occurs with 80-nm gold particles in aqueous solution. The anomalously large magnetic response-theoretically considered too small to observe at room temperature- was observed using light from a solar simulator and small (micro-to-milli-Tesla) magnetic fields. I will explain why the effect is observable using disperse nanocolloidal liquids and present our theoretical model of an increased and anisotropic electrical conductivity, which yields modified absorption spectra in agreement with our experimental results.
This work, which is the first nano-demonstration of old physics, improves our fundamental understanding of surface charges in nanostructures and aids the development of broad-band photonics metamaterials, new polarization-encoded imaging methods, photocatalytic materials, photovoltaic devices, and sensors.
April 30
Bala Sundaram
University of Massachusetts, Boston
Structural and Dynamical Aspects of Networks: Some New Results
Abstract:
The talk addresses two aspects of our work where I will first discuss a new mechanism for generating networks with a wide variety of degree distributions. The idea is variation of the well-studied preferential attachment scheme in which the degree of each node is used to determine its evolving connectivity. Though modifications to this base protocol, involving features other than connectivity have been considered, schemes based on preferential attachment in any form require substantial information about the network. We propose instead a parsimonious protocol based only on a single statistical feature which results from the reasonable assumption that the effect of various attributes, which determine the affinity of each node to other nodes, is multiplicative. This composite attribute or fitness is then used in forming the complex network. It is shown that, by varying a single statistical parameter, we can recover all known degree distributions. In the case of power-law networks, the exponents exhibit a range consistent with that seen in real-world networks and the network exhibits other attributes seen in data. In the last part of the talk, a variety of applications will be discussed including the issue of robustness and centrality, as well as pattern formation and dynamics on complex networks.
Read about our past colloquia (PDF).
If you have any questions about physics at Yeshiva College, please contact Professor Zypman at zypman@yu.edu or 212.960.5400, ext.104.