In the 1970s, it was argued by Bekenstein that the laws of thermodynamics need to be modified in the presence of black holes. Notably, he argued that one must associate an entropy that is proportional to the area of the horizon of the black hole. Hawking, using a semiclassical computation associated a temperature, called the Hawking temperature, to blackholes. A microscopic/statistical mechanical understanding of the origin of this entropy was lacking until the mid 1990s when Strominger and Vafa carried out the first such computation. In this talk, we will discuss their result and subsequent progress over the last 20 years. The microscopic counting turns out to be of an interdisciplinary nature involving string theory, statistical physics, number theory, combinatorics and theoretical computer science.
Melting of the vortex lattice in a Type II superconductor: A story from images
In a Type II superconductor, magnetic flux lines interact among themselves to form a periodic structure, the "vortex lattice", which mimics a soft periodic solid. This system has long been used as a versatile model system to study the interplay between interaction and random pinning. Here, I will present real space images of the vortex lattice as it melts with temperature or magnetic field, acquired using the low temperature, high field scanning tunneling microscope built in our laboratory. These images reveal that the presence of a random pinning potential fundamentally alters the order-disorder transition, and gives rise to a variety of metastable states that can be accessed through thermomagnetic cycling.
1) Anand Kamlapure, Garima Saraswat, Somesh Chandra Ganguli, Vivas Bagwe, Pratap Raychaudhuri, and Subash P. Pai, Review of Scientific Instruments 84, 123905 (2013).
2) Somesh Chandra Ganguli, Harkirat Singh, Indranil Roy, Vivas Bagwe, Dibyendu Bala, Arumugam Thamizhavel, and Pratap Raychaudhuri Phys. Rev. B 93, 144503 (2016).
3) Somesh Chandra Ganguli, Harkirat Singh, Rini Ganguly, Vivas Bagwe, Arumugam Thamizhavel and Pratap Raychaudhuri, J. Phys.: Condens. Matter 28, 165701 (2016).
4) Somesh Chandra Ganguli, Harkirat Singh, Garima Saraswat, Rini Ganguly, Vivas Bagwe, Parasharam Shirage, Arumugam Thamizhavel &Pratap Raychaudhuri Scientific Reports 5, 10613 (2015).
New prospects for room temperature superconductivity: Molecular solids under high pressure:
Wigner and Huntington surmised, in 1935, that insulating molecular solid H2 will become a metal at a pressure ~25 GPa. However, H2 and other molecular solids have resisted becoming a metal even at a few 100 GPa. Instead they undergo covalent bond reorganization and several structural changes with pressure. In this background, in a recent experiment, molecular solid H2S has become a superconductor with a Tc ~205K, under a pressure of ~200 GPa. We present a theory where covalent bond reorganization and a resulting resonating valence bond state causes superconductivity and offers new hope for room temperature superconductivity. Does the old and wise nature realize this very high temperature superconductivity somewhere ?
The Messiah of Masses---- An Answer That Becomes Questions
Starting at a level where one does not need to know any particle physics, the experimental and theoretical issues that led to the conjecture on, and discovery of, the Higgs boson will be briefly reviewed. It will be then emphasized that, rather than closing our book of accounts on elementary particles, the Higgs boson leads us to new questions, to which we have no answers yet. Thus we are currently facing fresh challenges related to fundamental physics.
The discovery of the Higgs boson at the CERN Large Hadron Collider(LHC) has opened up the door for the next phase of investigation into the concept of spontaneous breaking of gauge symmetries. The Higgs boson has a unique property, not shared by other states of the Standard Model(SM), namely that it interacts with itself. The measurement of the self couplings of the Higgs boson is essential for the reconstruction of the Higgs potential, and thereby confirm the idea of spontaneous breaking of the gauge symmetry of the SM. I shall discuss the issue of the measurement of trilinear couplings of the Higgs boson in the SM as well as in the Minimal Supersymmetric Standard Model(MSSM) in detail. In the case of the MSSM we shall take into account all the current experimental constraints on the parameters of the model in order to assess the possibility of the measurement of the trilinear Higgs couplings at a high energy electron positron collider.
Investigating Cosmic string theories with Liquid Crystal Experiments
Liquid crystals provide a very convenient system where topological defects, such as strings, can be experimentally studied in a variety of physical conditions. We will discuss how the observations of string formation in a liquid crystal system can be used to test theories of cosmic string formation in the early universe. Main focus of these investigations is on various universal aspects of defect formation with which one can establish rigorous quantitative correspondence between these condensed matter experiments and elementary particle physics models of the early universe.
Revisiting the theory of melting of three-sublattice order
In condensed matter systems with competing interactions, the low temperature behaviour often exhibits interesting patterns of ordering. One example is three-sublattice ordering of scalar degrees of freedom in a variety of quasi-two dimensional systems with triangular lattice symmetry. Examples include three-sublattice ordered monolayers of various adsorbates on graphite-like substrates, and three-sublattice ordered low temperature states in various frustrated easy-axis magnets. Standard Landau-Ginzburg theory treatments of the temperature-driven melting of three-sublattice order predict a two-step melting transition in such magnets in zero easy-axis field (or at half-filling for the monolayers), and a three-state Potts transition in a nonzero field. In this talk, we revisit these old and classical results, and show that such two-step melting is associated with an unusual thermodynamic signature in the easy-axis susceptibility. Additionally, we argue that further neighbour interactions can drive the system to a multicritical point that represents a new universality class of two-dimensional melting.
Levitated Optomechanics: Phonon cooling, bistability and lasing
In this talk I will first introduce the subject of cavity optomechanics, which involves the interaction of electromagnetic modes with mechanical oscillators. Subsequently, I will describe the emerging field of levitated optomechanics. Theoretical results on the topic will be reported from our group at RIT, in the context of ongoing experiments in various groups, especially that of our collaborator A. N. Vamivakas at the University of Rochester. Phenomena such as cooling, bistability and phonon lasing will be considered.
Presence of Matter in the Universe-An Unresolved Puzzle
In this general talk we review the great puzzle of presence of matter and the amount of it in the Universe. In a perfectly symmetric universe matter and antimatter created at the Big Bang should completely disappear later leaving only radiation. The resolution to the puzzle needs among other conditions, violation of a symmetry called CP. In this talk we review great experimental strides taken to prove CP is violated. Nevertheless the amount of matter present is still an unsolved problem. What future experimental strides are planned that may resolve this issue are discussed.
Exploiting Shape-sensitive Interactions in Colloidal Suspensions - From Directed Self-assembly to Structural Glasses
Colloidal suspensions comprise of micrometer-sized particles that remain suspended in a fluid by Brownian motion. Their large size, typically a micron, allows for the investigation of dynamics at the single-particle level using relatively simple tabletop experimental techniques. This feature combined with the tunability of particle shape and interactions makes them promising candidates to address a plethora of problems in statistical mechanics and condensed matter. Apart from their role as model systems, periodic arrangements of colloidal particles have typical lattice spacing comparable to the wavelength of visible light. Thus, colloidal crystals Bragg-scatter visible light and this feature is exploited in the design of photonic band-gap and optoelectronic devices.
In my talk, I will describe recent results from our group that exploit the shape-sensitive nature of attractive depletion interactions to address issues in colloidal self-assembly and structural glasses [1-5].
1. Chandan K Mishra, A K Sood and Rajesh Ganapathy, Proc. Natl. Acad. Sci. U.S.A. 113, 12094 (2016)
2. Shreyas Gokhale, A K Sood and Rajesh Ganapathy, Advances in Physics 65, 363 (2016)
3. Chandan K Mishra and Rajesh Ganapathy, Phys. Rev. Lett. 114, 198302 (2015)
4. Chandan K Mishra, Hima K Nagamanasa, Rajesh Ganapathy, A K Sood and Shreyas Gokhale, Proc. Natl. Acad. Sci. U.S.A. 111, 15362 (2014)
5. Chandan K Mishra, Amritha Rangarajan and Rajesh Ganapathy, Phys. Rev. Lett. 110, 188301 (2013)