SYMPHY 2020

Abstract:The Nobel-winning discovery of gravitational waves (GW) from the mergers of binary black holes by the LIGO-Virgo collaboration has opened a new window to the universe. It is expected that ground-based detectors such as advanced LIGO, Virgo, Kagra, and in near future, Indian LIGO will make hundreds of such detections in the next few years. Upcoming and proposed detectors, including LISA and DECIGO will detect even more sources at different orbital frequencies. These detections very soon will allow us to study properties of merging black hole binaries as a population and interpret the astrophysical implications. But, how does nature make these extraordinary systems? I will review various astrophysical formation channels for merging binary black holes. I will also discuss a few detectable properties that can shed light on the formation channel of specific mergers.

Abstract: The standard model of particle physics has been remarkably successful in describing almost all the experimental observations related to the basic building blocks of matter and fundamental interactions among them. However, there are a number glaring examples which one cannot explain within this theoretical framework. These include dark matter, matter-antimatter asymmetry and neutrino mass. While we have several proposals on the table, none of them has found any taker with the experiments at the Large Hadron Collider of CERN or elsewhere. This talk is an experimenter’s perspective on the current status and what the future holds for us in this noble pursuit for the next fundamental layer of physics.

Abstract:The discovery of the Higgs boson is a long desired landmark of the Standard Model of Elementary Particle Physics. But it suggests the presence of cosmological vacuum energy of the order of 10^2GeV, which is completely contradictory to observations. “Running coupling constants” is an elegant feature of the gauge theories. It means the couplings of various fundamental forces are energy dependent and can unify at a very high energy scale. Again, the grand unified Higgs sector signals the presence of large vacuum energy. The very long life 13 billion years of our present Universe and its enormous scale 10^26m suggest that vacuum energy dominated near the unified scale of 10^14 GeV – so called inflationary Universe – but this energy must have also dissipated – so called graceful exit from Inflation. In an apparently different phenomenon, direct observations of the expanding Universe suggest we have vacuum energy of scale 3 × 10^−11 GeV. All the ingredients needed to understand the fundamental forces in a unified framework seem to be in place yet the numbers have a mismatch by enormous orders of magnitude. Either our paradigm of Quantum Field Theory or of General Theory of Relativity, or both, could be in the need of serious revision. We hope for hints towards resolution from future high energy colliders and from improved precision in cosmology and strong gravity experiments.

Abstract: Magic sand, a hydrophobic toy granular material, is widely used in popular science instructions because of its nonintuitive mechanical properties. A detailed study of the failure of an underwater column of magic sand shows that these properties can be traced to a single phenomenon: the system self-generates a cohesive *skin* that encapsulates the material inside. The skin, consisting of pinned air-water-grain interfaces, shows multiscale mechanical properties: they range from contact-line dynamics in the intragrain roughness scale, to plastic flow at the grain scale, all the way to sample-scale mechanical responses.

Abstract: Active matter is a collection of particles that operate out of equilibrium by converting external energy into motion. This simple definition of a new class of matter not only raises fundamental questions for physicists but also has the potential to uncover laws governing various natural and biological processes. For instance, the collective behavior of living organisms like birds, fish, etc., to cellular and subcellular processes like cell division and vesicular transport, fall under the category of active matter. In this talk, I will introduce two simple table-top experiments which are capable of imitating various complex features of active systems. The first system is a collection of millimeters-long granules placed on a vertically vibrating surface which faithfully mimics phenomena of flocking and trapping. The second system consists of a synthetic assembly of cytoskeletal biopolymers and molecular motors in the form of an active nematic liquid crystal. Here I will discuss how the defect dynamics and elastic properties can be tuned as a function of activity. These results open exciting opportunities in creating a new class of tunable materials that are active and perform tasks which are otherwise forbidden in conventional materials.

Abstract: The creation of self-assembled uniform monolayers of colloids at fluid interfaces and transforming them on to solid surfaces is not only crucial to fundamental science but also of utmost importance for soft nanotechnology. Here, we demonstrate a facile strategy which exploits the interface assisted self-assembly to obtain two-dimensional layers of soft colloids. Solvent-swollen submicrometer-sized hydrogel particles (microgels) with delicate hydrophobic - hydrophilic balance and sensitive to environmental conditions such as temperature/pH are used as soft colloids. The surface activity of the microgel particles, the interparticle interactions, and the kinetics of evaporation are tuned to generate a variety of novel two-dimensional structures, including monolayer coffee-rings, loosely packed uniform layers, and interconnected cell-like patterns with extraordinary richness and diversity. A smooth order-disorder transition in the self-assembled monolayers of soft colloids is identified by constructing Euclidean Voronoi diagrams. The viscoelastic nature of the soft colloid laden interface probed by using multiple particle tracking passive microrheology and its dependence on the crosslinking-density of microgels will be presented. In addition, the suppression of the “coffee-ring effect” for hard colloids in presence of microgel particles and the origin of the depletion zones observed in evaporative self-assembly will be discussed in detail.

Abstract:There is a great interest in understanding and finding new Topological Insulators and has become important due to many uses of it in spintronic applications in future. Topological Transitions also are interesting to enhancing the thermoelectric applications of materials. High pressure provides an apt tool to understand the effect of strain in tuning the electronic topological properties of the materials. High pressure simulates the effect of chemical doping in achieving this in real materials. Electronic topological transitions are very well seen by angle resolved photo emission spectroscopy (ARPES). But to perform this the samples need to be freshly cleaved inside a ultrahigh vacuum and the surface is cleaned using Argon sputtering. The electronic band structure should be measured before the sample relaxes. In high pressure since the sample need to be bathed in a hydrostatic medium for applying the pressure, these procedures are near impossible at the present. Hence we need to find alternative methods to identify the topological transitions. Hence in this talk we will demonstrate that use of a combination of Raman spectroscopy, X-ray diffraction and DFT calculations could combined to identify the topological transitions under high pressure conditions. Raman spectroscopy is an inelastic light scattering measurements, which measure the vibrational properties of the molecules and lattice. Here there is a large interaction between electron and phonon (lattice vibration) and it is called the electron phonon coupling. These would affect the lifetime of the phonons and this manifests in the Raman spectroscopy as the anomalous changes in phonon frequency and phonon linewidth (which is directly influenced by lifetime of the phonon). X-ray diffraction helps us to know if these transformations in the Raman spectra are linked with any structural changes. If there are no structural transitions, these can be easily concluded originating due to electronic origin. In order to confirm that this is due to topological transitions, one can do DFT calculations under pressure. This clearly gives the band overlaps in the electronic band structure and parity. Based on this one can easily find the Topological Transition under pressure. The talk will show using examples how this is achieved. The pressure induced topological quantum phase transitions (TQPT) in TlBiS2 and 1T-TiTe2 [1,2] are seen using this approach. We show the evidence of two isostructural electronic transitions in TlBiSe2, deduced from the unusual electron-phonon coupling (A1g and Eg phonons) at ~0.5 GPa and ~1.8 GPa observed in the Raman scattering measurements. Our first principles density functional theory based electronic band structure, topological invariant Z2 and mirror Chern number 𝑛𝑀 calculations reveal that the phonon anomalies at ~0.5 GPa and ~1.8 GPa are related to the topological insulator and topological crystalline insulator (TCI) transitions, respectively. Both high pressure Raman and powder synchrotron XRD confirm a reversible first order structural phase transition of the rhombohedral phase above 4 GPa. We see that the effect of high pressure on transition metal chalcogenide 1T-TiTe2, a prominent layered 2D system. Here, we have explored the topologically non-trivial and trivial quantum phase transitions at ∼2 GPa and ∼4 GPa with evidence of the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes. Between ~4 GPa and ∼8 GPa, a transformation from an anisotropic 2D layer to a quasi-3D crystal network is noticed, which occurs due to increased interlayer Te-Te interactions (bridging) by the charge density overlap. In addition, we observed a reversible first-order structural phase transition from a trigonal (P3̅m1) to monoclinic (C2/m) phase above 8 GPa. In order to show that all electronic transitions deduced from Raman and X-ray cannot be deduced to be TQPT, we have taken another example. We have investigated Raman scattering and synchrotron XRD measurements on InTe compound [3]. The pressure induced a semiconductor-to-metal transition in InTe is deduced from the phonon anomalies of A1g and Eg modes along with the decrease and anomaly in Raman intensities at ~3.6 GPa. In distinct pressure regime, the presence of strong anharmonic phonon-phonon interactions and electron-phonon interactions are noticed from phonons’ peculiar behaviour. Our Raman scattering experiments up to ∼19 GPa reveals the pressure induced structural transitions (B37 → B1 → B2) in InTe. References [1] V. Rajaji, Raagya Arora, Saurav Ch. Sarma, B. Joseph, Umesh V. Wagmare, Sebastian C. Peter, and Chandrabhas Narayana “Phonon signatures of multiple topological quantum phase transitions in compressed TlBiS2: A combined Experimental and theoretical study“ Phys. Rev. B 99, 184109 (2019). [2] V. Rajaji, Utpal Dutta, P. C. Sreeparvathy, Saurav Ch.Sarma, Y. A. Sorb, B. Joseph, Subodha Sahoo, Sebastian C. Peter, V. Kanchana, and Chandrabhas Narayana, “Structural, vibrational, and electrical properties of 1T-TiTe2 under hydrostatic pressure: Experiments and theory” Phys. Rev. B 97,085107 (2018). [3] V. Rajaji, Koushik Pal, Saurav Ch.Sarma, B. Joseph, Sebastian C. Peter, Umesh V. Waghmare, and Chandrabhas Narayana, “Pressure induced band inversion, electronic and structural phase transitions in InTe: A combined Experimental and theoretical study” Phys. Rev. B 97, 155158 (2018).

Abstract: Quantum materials are materials that exhibit exotic electronic properties that are manifested due to reduced dimensionality, quantum confinement, topology of wave functions etc. Materials such as graphene, topological insulators, Weyl semimetals, spin-liquids etc. belong to this category and is widely investigated by condensed matter physicist and materials scientists in the past couple of decades. Among these, topological insulators are materials characterized by an insulating bulk and gapless metallic states on the sample surface. Electrical transport in three dimensional topological insulators occurs through spin-momentum locked topological surface states that enclose an insulating bulk. In the presence of a magnetic field, surface states get quantized into Landau levels giving rise to chiral edge states that are naturally spin-polarized due to spin momentum locking. Robust access to topological surface states has presented itself as a formidable challenge due to inevitable bulk doping that mires the effects arising from the topological surface states. In this lecture, I will demonstrate that surface states that are highly amenable to detection and control using electrostatic gating1-7 if one uses bulk-insulating topological insulators. Here, we have also fabricated electrostatically defined n-p-n junctions8 of bulk insulating topological insulator BiSbTe1.25Se1.75. I will also demonstrate how these fabrication technologies can be extended to get diverse devises such as edge-contacted topological insulator FETs and topological insulator/superconductor hetero-interfaces9,10 opening up the possibility for further understanding of this new materials class. *in collaboration with: A Banerjee, R Ganesan, BR Sekhar and Diptiman Sen References [1] Applied Physics Letters 109 (2016) 232408 [2] Applied Physics Letters 110 (2017) 162102 [3] Journal of Physics: Condensed Matter 29 (2017) 185001 [4] Nanoscale 9 (2017) 6755 [5] Scientific Reports 7 (2017) 4567 [6] Applied Physics Letters 113 (2018) 072105 [7] Physical Review B 98 (2018) 155423 [8] Nanoscale 11 (2019) 5317 [9] ACS Nano 12, 12 (2018) 12665 [10] Nano Lett.19 (2019) 1625

Abstract: World of quantum matter continues to challenge us with new experimental results and situations of increasing complexity and variety. During these happy encounters, contemplation and deep insights result in models. These mathematical models are caricatures of reality. They help us navigate the vast Hilbert space, without getting lost and discover new worlds. We will illustrate the art of model building using some models, including Majumdar-Ghosh model.