Physics Lecture

The Physics Department and Center for Theoretical Physics will host a seminar Thursday, October 25 at 12:00 pm in Namm Room 823.  Faculty and students are welcome.

Title: Odd surface waves in two-dimensional incompressible fluids

Dr. Sriram Ganeshan
City College of CUNY
New York, NY, USA

Abstract:

In everyday fluids, the viscosity is the measure of resistance to the fluid flow and has a dissipative character. Avron, Seiler, and Zograf showed that viscosity of a quantum Hall (QH) fluid at zero temperature is non-dissipative. This non-dissipative viscosity (also known as ‘odd’ or ‘Hall’ viscosity) is the antisymmetric component of the total viscosity tensor and can be non-zero for parity violating fluids. I will discuss free surface dynamics of a two-dimensional incompressible fluid with the odd viscosity (not quite quantum Hall hydro). For the case of incompressible fluids, the odd viscosity manifests itself through the free surface (no stress) boundary conditions. We first find the free surface wave solutions of hydrodynamics in the linear approximation and study the dispersion of such waves. As expected, the surface waves are chiral.  In the limit of vanishing shear viscosity and gravity, we derive effective nonlinear Hamiltonian equations for the surface dynamics, generalizing the linear solutions to the weakly nonlinear case.  In a small surface angle approximation, the equation of motion results in a new class of non-linear chiral dynamics which we dub as chiral Burgers equation. I will briefly discuss how this program can be extended to the free surface of quantum Hall hydrodynamics.

Physics Seminar: Exciton complexes in quasi-2D crystals in the configuration space approach

Presenter:

Prof. Igor V. Bondarev
North Carolina Central University
Durham, NC, USA

Abstract:

I will present a universal method for binding energy evaluation of the lowest energy neutral and charged exciton complexes (biexciton, trion) in spatially confined semiconductor nanostructures [1]. The method, originally pioneered by Landau, Gor’kov, Pitaevski, Holstein, and Herring in their studies of molecular binding and magnetism, was first utilized for quasi-1D nanostructures to evaluate biexciton and trion binding energies in small-diameter carbon nanotubes [2]. I will show that the method can also be used for the electron-hole complexes of indirect excitons in quasi-2D semiconductor systems such as coupled quantum wells and bilayer van der Waals bound transition metal dichalcogenide heterostructures. The method works in the effective configuration space of two relative electron-hole motion coordinates in two non-interacting spatially confined excitons. Biexciton and trion bound states form due to the under barrier tunneling of the electron-hole system between the equivalent configurations in the configuration space. Tunneling rate controls the binding strength and can be turned into the binding energy by means of an appropriate variational procedure. Exciton complexes in coupled quantum wells and bilayer van der Waals bound transition metal dichalcogenide heterostructures are of great interest for nonlinear optics and spin-optronics applications [3-5].

[1] I.V.Bondarev, Modern Physics Letters B 30, 1630006 (2016).
[2] I.V. Bondarev, Phys. Rev. B 90, 245430 (2014); ibid. 83, 153409 (2011).
[3] G.J.Schinner, J.Repp, E.Schubert, et al., Phys. Rev. Lett. 110, 127403 (2013).
[4] M.M.Fogler, L.V.Butov, and K.S.Novoselov, Nature Commun. 5, 4555 (2014).
[5] J.S.Ross, P.Rivera, J.Schaibley, et al., Nano Lett. 17, 638 (2017).

Physics Seminar: Quantum materials: insights from near field nano-optics

Presented by
Prof. Dmitri Basov
Columbia University
New York, NY, USA

Abstract
In 1944 Hans Bethe reported on “the diffraction of electromagnetic radiation by a hole small compared with the wave-length” [Physical Review 66, 163 (1944)]. This seminal paper was among the early precursors to a new and vibrant area of research: near field nano-optics. I will discuss recent nano-optical experiments on quantum materials including: transition metal oxides undergoing the insulator to metal transition and graphene. Central to the nano-optical exploration of quantum materials is the notion of polaritons: hybrid light matter modes that are omnipresent in polarizable media. Infrared nano-optics allows one to directly image polaritonic standing waves [Science 343, 1125 (2014), Nature Materials 14, 1217 (2015)] yielding rich insights into the electronic phenomena of the host material supporting polaritons [Science 354, 195 (2016)]. I will give a progress report on the search for the role of the Berry phase in the properties of graphene via transient polaritonic imaging [Nature Photonics 10, 244 (2016)]. In a parallel development, we harnessed near field optics to uncover the elusive electronic and magnetic phases that occur only at the nano-scale in the vicinity of the insulator to metal transition in correlated oxides [Nature Physics 13, 80 (2017) and Nature Materials 15, 956 (2016)].

Physics Seminar: Spin transport by a supercurrent in a room-temperature magnon Bose-Einstein condensate

Speaker: Dr. Oleksandr Serha
University of Kaiserslautern
Kaiserslautern, Rhineland-Palatinate, Germany

Abstract:
With the fast growth in the volume of information being processed, researchers are charged with the task of finding new ways for fast and energy efficient computing. An extraordinary challenge is the use of macroscopic quantum phenomena such as magnon Bose-Einstein condensates (BEC) for the information transfer and processing.

Here, I present experimental evidence for the excitation of a supercurrent—the transport of angular momentum driven by a phase gradient in the wave function of a magnon BEC. In our experiments, the magnon BEC was formed at room temperature by a para­met­rically populated magnon gas in a single-crystal ferrimagnetic film of yttrium iron garnet (Y3Fe5O12, YIG). The temporal evolutions of the magnon density was studied by wave­vector-, frequen­cy-, time- and space-resolved Brillouin light scattering spectros­copy. It has been found that local heating of the YIG film by focused laser light creates a spatially varying phase shift imprinted into the BEC wavefunction and, thus, propels the outflow of condensed magnons from the heated area. This outflow does not alter the dynamics of a non-coherent gaseous magnon phase but decreases the density of the freely evolving magnon BEC in the heated area. Moreover, it creates a solitary magnon wave, which pro­pa­gates many hundreds of BEC’s wavelengths through the “cold” magnon condensate.

The Physics Department and the Center for Theoretical Physics presents Top Quark and Higgs Boson at the LHC

Speaker: Miguel C. N. Fiolhais, BMCC CUNY

Abstract: The recent discovery of a new scalar particle, compatible with the SM Higgs boson, by the ATLAS and CMS collaborations of the Large Hadron Collider at CERN brought us the most important missing piece of the Standard Model. With the LHC operating at a center-of-mass energy of 13 TeV, this is an exciting epoch for particle physics as the next decade will feature an unprecedented quest for new physics beyond the SM. In this talk, the role of precision measurements of the top quark and Higgs boson properties in the search for new anomalous physics contributions will be explored. In particular, the measurement of the W boson polarization in top quark decays and the production of a top quark pair in association with a Higgs boson will be presented in detail.

Physics Seminar presents
Gravitational Wave Observations and the Physics of Neutron Stars

Guest Speaker: Simone Dall’Osso of SUNY Stony Brook

The first direct detection of gravitational waves (GW) from a binary black hole made by Advanced LIGO has opened the era of GW astronomy. Sources for the current detectors are catastrophic events involving neutron stars (NS) and black holes (BH), isolated or in binaries, in which huge amounts of energy are released in very small regions and short timescales. Besides GWs, bright electromagnetic (EM) transients, as well as copious emission of neutrinos and ultrarelativistic cosmic rays are expected in association to such events, making them ideal targets for multi-messenger observations. Because GW interact so weakly with the environment, they are unique probes of the physical processes occurring in the extremely dense and turbulent regions where NS/BHs form and/or collide. Because photons are easily emitted (and detected) from the surrounding regions, on the other hand, EM transients are the counterparts to GW sources that we can more effectively reveal and study. I will focus in particular on a class of highly magnetized, millisecond spinning NS, that could form both in the core-collapse of massive stars and in binary NS mergers. Such NS have been proposed as possible sources of the brightest EM transients (gamma-ray bursts, super-luminous supernovae), and as progenitors of a galactic population of peculiar X-ray pulsars (magnetars). I will present a mechanism, the so-called “spinfip” instability, by which newly born, highly magnetized, millisecond spinning NS can also produce powerful and distinctive GW signals carrying pristine information about the physics of their interiors and the equation of state (EOS) of matter at supra-nuclear density. The EM emission expected in association with these GW signals is particularly bright and carries its own signatures of the millisecond spinning NS: this makes these sources ideally suited for multimessenger studies.

Physics Seminar: 2D materials in the ultraclean limit: basic science and applications

NEW YORK CITY COLLEGE OF TECHNOLOGY
Physics Department
Center for Theoretical Physics

2D materials in the ultraclean limit: basic science and applications

Presented by: Prof. James Hone

Columbia University
New York, NY, USA

Thursday, March 02 at 1:00 PM
Namm, Room 823

Abstract

Two-dimensional materials offer a wide range of outstanding properties but are highly sensitive to disorder from the environment. We have developed a ‘van der Waals transfer’ technique to encapsulate graphene within crystalline h-BN with nearly perfect interfaces, and an ‘edge contact’ technique to achieve electrical contact to the encapsulated channel. Using these and related techniques, we can study 2D materials and applications in the ultraclean limit. This talk will summarize recent results in four areas: [1] Demonstration of negative refraction at graphene p-n junctions; [2] Tunable interactions and evidence for exciton condensation in double-layer heterostructures of bilayer graphene; [3] graphene light emitters; and [4] approaching the intrinsic photoluminescence linewidth in 2D semiconductors.

Light refreshments will be served.

Symposium on Amazing Stories: Inspiration, Learning, and Adventure in Science Fiction

Symposium on Amazing Stories

View Program

“By ‘scientifiction’ I mean the Jules Verne, H. G. Wells and Edgar Allan Poe type
of story—a charming romance intermingled with scientific fact and prophetic
vision … Not only do these amazing tales make tremendously interesting
reading—they are always instructive.”
-Hugo Gernsback, 1926.

When the widely recognized “Father of Science Fiction,” Hugo Gernsback first coined the term that captured the essence of the genre we now call science fiction (SF), he envisioned SF as a new form of literature that inspired with prophecy, taught with scientific and technical facts, and engaged with adventure. These traits unique to SF have launched many of its readers on trajectories into the STEAM (Science, Technology, Engineering, Arts, and Mathematics) fields.

Join us for a one-day symposium exploring SF as a medium for engaging imagination, a means for exploring STEM/STEAM fields, and an instrument for discovering interdisciplinary connections, and also celebrating the new City Tech Science Fiction Collection held in the Archives and Special Collections of the Ursula C. Schwerin Library.

Contact Jason Ellis at JEllis@citytech.cuny.edu for more information.

Organizing Committee: Jason Ellis (Chair), Aaron Barlow, Jill Belli, and Mary Nilles.

Hosted by the School of Arts and Sciences at the New York City College of Technology, CUNY.

Physics Seminar: Negative refractive index for graphene and surface Plasmon instability for hybrid structures

Presented by: Distinguished Professor Godfrey Gumbs

Femtosecond and subfemtosecond time scales typically rule electron dynamics at metal surfaces. Recent advances in experimental techniques allow the experimental study of such dynamics. In this talk, we shall analyze electron dynamics at surfaces and nanostructures with emphasis on screening times, spin dependence of charge transfer of adsorbates and smaller system sizes. We will discuss the effect of energy gaps on possible “Veselago lenses” for completely flat graphene sheets. We will also discuss how Plasmon instabilities may be exploited for tunable radiation generation which may be employed in detectors.

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