Condensed Matter, Statistical and Nonlinear Physics Events

IISER Pune

Upper bounds on superfluid stiffness and superconducting Tc: Applications to twisted bilayer graphene, monolayer FeSe on SrTiO3 and cold atoms

We discuss fundamental limitations on the superconducting critical temperature Tc. In many novel superconductors phase fluctuations determine Tc , rather than the collapse of the pairing amplitude. We derive rigorous upper bounds on the superfluid phase stiffness for multiband systems, valid in any dimension. This in turn leads to an upper bound on Tc in two dimensions, which holds irrespective of pairing mechanism, interaction strength, or order-parameter symmetry. Our bound is particularly useful for the strongly correlated regime of low-density and narrow-band systems, where mean-field theory fails. For a simple parabolic band in 2D with Fermi energy $E_F$ , we find that $k_B T_c ≤ E_F/8$, an exact result that has direct implications for the 2D BCS-BEC crossover in ultracold Fermi gases. Applying ourmultiband bound to magic-angle twisted bilayer graphene, we find that band structure results constrain the maximum Tc to be close to the experimentally observed value. Finally, we discuss the question of deriving rigorous upper bounds on Tc in 3D.

Gate-Tunable Flatbands in Graphene Moiré Superlattices

Periodic moiré patterns in the length scale of a few tens of nanometers can give rise to moiré mini Brillouin zones whose zone corners are at energy ranges accessible by conventional field effects in gated transistor devices. Recent experiments have shown resistance peaks as a function of carrier doping indicative of Mott-like phases in twisted bilayer graphene at the first magic twist angle [1, 2] and in ABC trilayer graphene (TLG) nearly aligned with hexagonal boron nitride (BN) [3] when the Fermi energy is brought near the superlattice flat bands (SFB). The possibility of tailoring narrow flatbands in systems with such remarkable simplicity in composition as graphene consisting of only carbon atoms makes it an attractive pathway to engineer artificial materials. Here, we study the flatbands that can be engineered in twisted multilayer graphene. In the twisted bilayer graphene (tBG), the flatband minima angles are found to grow linearly with interlayer coupling and decrease with Fermi velocity [4]. In tBG, the fltabands emerge as a function of twist angle, vertical pressure, and interlayer potential differences between the layers. Interestingly, in twisted double bilayer graphene (tDBG) the bandwidth is generally flatter than in tBG by roughly up to a factor of 2 in the same parameter space of twist angle and interlayer coupling, making it in principle simpler to tailor narrow bandwidth flat bands [5]. A related system where flattening of the low-energy bands is facilitated by the presence of a vertical electric field is the ABC trilayer graphene (TG) on hexagonal boron nitride (hBN), TLG/BN [6], where Coulomb effects can lead to correlated gapped phases even without a specific twist angle. We show the narrow bandwidths of ~10 meV are achievable for a continuous range of twist angles θ ≤ 0.6^{ㅇ} with moderate interlayer potential differences of ~50 meV make the TLG/BN systems a promising platform for the study of electric-field tunable devices. The gate-tunable narrow SFB in graphene moiré superlattices become non-trivial topological bands when they are isolated through avoided crossing of the bands in k-space that can impact the character of the ground-state Hall conductivity depending on the specific configuration of the ground states.

References:

[1]. Y. Cao, V. Fatemi, A. Demir, S. Fang, S. L. Tomarken, J. Y. Luo, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, E. Kaxiras, R. C. Ashoori, and P. Jarillo-Herrero, Nature, 556, 80-84 (2018).
[2] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero, Nature, 556, 43-50 (2018).

[6] Ch. Bheema Lingam, Guorui Chen, Yuanbo Zhang, Feng Wang, and Jeil Jung, Phys. Rev Lett, 122, 016401 (2019).

Past Events

Exploiting symmetries of states and operators to study quantum correlations such as entanglement and discord

While symmetry considerations have played a central role in many areas of physics, this has not been generally true in the new field of quantum information (computation, cryptography, and teleportation). We will discuss states and operators of two (or more) qubits (spin-1/2), and higher dimensional "qudits," in terms of the symmetry groups and algebras pertaining to them. The study of the time evolution of such systems can gain much from the insights and techniques provided by such considerations, especially of sub-groups relevant to particular Hamiltonians. Geometrical pictures that generalize the familiar Bloch sphere of a single spin/qubit are one product of such studies, useful both for calculations and experimental measurements.

An Introduction to out of time ordered correlators

Quantum physics and the butterfly effect

The butterfly effect is a metaphor for the extreme sensitivity of nonlinear chaotic systems found generically when there is more than 1-degree of freedom, examples abound from the gravitational three-body problem to the weather. Chaos and quantum physics have had an uneasy relationship which has implications in explorations of the no-man's land of classical-quantum boundaries and goes to the heart of the foundations of statistical physics. The talk will introduce surprising connections of classical chaos to quantum entanglement and implications therein to many-body localization and thermalization of isolated systems. Recent proposals of measuring quantum chaos, such as out-of-time-ordered correlators (OTOC) and scrambling will be discussed briefly as they bring us to the latest avatar of the butterfly effect which seems to have reached black holes, now conjectured to be nature's most chaotic and fastest scramblers.

Floquet analysis of the dynamics of current flow in a periodically driven hard-core bosonic system

Periodically driven classical and quantum systems are ubiquitous in different areas of physics. Recently, these kinds of driven systems have got its prominence in the condensed-matter community because of its possibilities in engineering new phases of matter having exotic quantum properties. These systems are theoretically studied under Floquet formalism. The most important element of the Floquet formalism is to derive an effective time-independent Hamiltonian, also known as the Floquet Hamiltonian, which gives effectively the same dynamics as that of the original time-dependent Hamiltonian. Except for a very few cases, one has to employ a perturbation scheme to obtain the Floquet Hamiltonian approximately. A chain of hard-core bosons (HCBs), subject to a periodic Dirac delta-function kick in the staggered on-site potential is one such exceptional case. This talk will start with a brief discussion about the Floquet formalism. Then the chain of HCBs system will be introduced. Finally, our recent results related to the current flow through the system and the work done on the system to maintain the current flow will be discussed.

Theory of Bilayer Graphene - Some Twists

Monolayer graphene and bilayer graphene behave differently. Certain
magic angle twists between two layers spring new surprises. I will
present a theory,
a twist on existing theories. Weak electron tunneling between two layers
amplify the quiescent electron correlations present in a single layer
graphene and creates rich physics. An interlayer spin-charge decoupling
and consequences are predicted.

Room Temperature Superconductors: Ephemeral, Elusive to Stable Ones

Ephemeral, elusive and unstable ambient temperature Superconductivity have been reported in the past. I will summarize some important instances, Ogg's observation (1946) of persistent current signals in metal-ammonia solution ( ~ 190 K) to recent ones (~ 300 K)from Thapa, Pandey and collaborators at IISc. Is room temperature superconductivity allowed by theory? - given real world constraints from solid state chemistry and quantum chemistry. My finding is that electron correlation mechanism allows ambient Tc superconductivity; however, it is accompanied by several competing and entangling orders. Control of competing orders and unusual non-equilibrium phenomena at nanoscales are new challenges one faces in this game. Experiments and theory have to go hand in hand. I will end with some optimistic note.

Visualizing topological Fermi-arcs and Weyl node connectivity in the ferromagnetic Weyl semimetal Co3Sn2S2

Topological “Fermi-arc” states exist on certain surfaces of Weyl semimetals. They connect the surface projection of the bulk Weyl nodes. However, the actual connectivity among them remains ambiguous in the presence of multiple pairs of surface projected Weyl nodes. In this presentation, I will discuss the classification of the ferromagnetic Co3Sn2S2 as a magnetic Weyl semimetal as well as its time reversal broken origin using Fourier-transform scanning tunneling microscopy/spectroscopy. By studying three different surface terminations we show that both the Fermi-arc connectivity as well as the Fermi-arc contour in Co3Sn2S2 vary with the different surface potential. While on the Sn surface we find intra-Brillouin zone connectivity, on the Co surface the Fermi-arcs connect Weyl nodes across the Brillouin zone edge. On the S termination the Fermi–arcs hybridize with non-topological bulk and thus their connectivity remain obscured.

Effects of viscoelasticity in fluids and living systems

In this seminar, I will talk about the effects of viscoelasticity on passive fluid flows and active living system. It is well known that viscoelasticity affects the fluid flows significantly, in the turbulent flow they lead to dissipation reduction.
In the first part of my talk, I will discuss the effects of polymer additives in two-dimensional homogeneous, isotropic turbulence and this work focuses on studying the statistical properties using direct numerical simulations. The kinetics of the polymers is introduced using constitutive equations for viscoelastic fluids with finitely extensible non-linear elastic dumbbells with Peterlin’s closure (FENE-P). Our study reveals that the polymers have a significant effect on multiple scales of the turbulent flow affecting some of the physical quantities and their intermittent properties.
In the second part, I will discuss the mechanical regulation of shape deformation by matrix viscoelasticity in breast tissues. The shape change is one of the phenomena seen in cancerous tissue and the physical mechanism that allows this process to take place has not been clear. The synthetic extracellular matrix (ECM) are typically almost purely elastic. In contrast, the physiological ECM in various tissues, such as brain, liver, adipose tissue, and coagulated bone marrow, etc. are all viscoelastic. Most of the studies to date have focussed largely on elastic properties of ECM. Recently synthetic ECMs have been developed which closely mimic the natural viscoelastic ECMs. In this study, we are further looking into the role of such mechanical properties in inducing the malignant phenotype in normal mammary epithelium MCF10A cell line. Based on these experimental findings we have proposed a mathematical model to capture the qualitative results. This is the first mechanical model to capture this epithelial to mesenchymal transition by changing the viscoelastic properties of ECM.

Parton paradigm for the quantum Hall effect

The fractional quantum Hall effect (FQHE) forms a paradigm in our understanding of strongly correlated systems. FQHE in the lowest Landau level (LLL) is understood in a unified manner in terms of composite fermions, which are bound states of electrons and vortices. The strongest states in the LLL are understood as integer quantum Hall states of composite fermions and the compressible 1/2 state as a Fermi liquid of composite fermions. For the FQHE in the second LL, such a unified description does not exist: experimentally observed states are described by different physical mechanisms. In this talk, I will discuss our first steps towards a unified understanding of states in the second LL using the ``parton" theory. I will elucidate in detail our recent work on the parton construction of wave functions to describe many of the FQH states observed in the second LL.

Spins, Brains and Markets: Transitions between frustration and balance in adaptive systems

Structural balance in social networks, viz., the idea that agents adapt central idea of social psychology from the time it was introduced by Fritz 1950s. It is closely allied to the concept of frustration in spin systems,triangular lattice in 1950 and later more extensively by others in the spin networks), we have a studied a process where interaction strengths dynamics is inspired by Hebb’s principle, originally proposed in the which may apply more broadly to a large class of systems, e.g., in generegulation networks where the co-expression of genes has been suggested to result fluctuations, the time required to converge to the balanced state exhibits suggest that fluctuations can prevent a system from attaining a balanced important implications for biological and social networks. More transition from balance to frustration during the onset of major systemic network dynamics indicative of such pan-sector economic collapses.

Quantum photons from dipole coupled two-level atomic system

We demonstrate that light quanta of well-defined characteristics can be generated in a coupled system of three two-level atoms. The quantum nature of light is controlled by the entanglement structure, discord, and monogamy of the system, which leads to sub- and superradiant behavior, as well as sub-Poissonian statistics, at lower temperatures. Two distinct phases with different entanglement characteristics are observed with uniform radiation in one case and the other displaying highly focused and anisotropic radiation in the far-field regime. At higher temperatures, radiance witness is found to exhibit sub- and superradiant behavior of radiation intensity in the absence of entanglement albeit with non-zero quantum discord. This establishes the physical manifestation of quantum discord. It is also observed that the radiation intensity can be a precise estimator of the inter-atomic distance of a coupled system of two-level atomic systems. Our investigation shows, for the first time, the three body correlation in the form of a ‘monogamy score’ controlling the sub- and superradiant nature of radiation intensity.

Reference: 1. M. K. Parit, S. Ahmed, S. Singh, P. A. Lakshmi, P. K. Panigrahi, “Correlated photons of desired characteristics from a dipole coupled three-atom system”, OSA Continuum 2(8), 2293-2307 (2019).

Run-and-tumble particles on a 1D lattice

We study run-and-tumble particles (RTPs) on a 1D lattice, where each lattice site cannot hold more than one particle. Each RTP carries a spin which points in the positive or negative direction, and hops on the lattice at unit rate in the direction of the spin. The spin itself flips at a rate D_r.
I will show that the steady-state of the model determined using the independent interval approximation shows excellent agreement with simulations for D_r>1. I will also also derive the hydrodynamics in this picture, and show that there are strong non-equilibrium effects, like the violation of the Einstein relation. I will also briefly describe a coalescence picture for D_r << 1, and time permitting, describe the hydrodynamics in this limit.

Probing the physical basis of living systems, working towards a physics of life

Feynman, in his Lectures on Physics, gives a succinct description of physics: "Physicists always have a habit of taking the simplest example of any phenomenon and calling it “physics,” leaving the more complicated examples to become the concern of other fields — say of applied mathematics, electrical engineering, chemistry, or crystallography [or biology].”. What then, is the physics of life? And, given the complexity, where do we start?
In this talk, I will try to make a case for studying the physical basis for metabolism, the veritable engine of life. I will go through one of the very few quantitative "laws" in biology -- the so called Kleiber's law -- relating metabolic activity with organismal body mass. The data spans roughly 20 orders of magnitude ranging from a single mitochondrion to the largest mammals in a power-law relationship. After detailing some theoretical frameworks to understand this data and also their shortcomings, I will briefly describe our own proposal and efforts in this direction.

Halide perovskites: A new class of semiconductors with emergent functional properties

Halide (hybrid) perovskites (HaP) have emerged as a new class of semiconductors that truly encompass all the desired physical properties for building optoelectronic and quantum devices such as large tunable band-gaps, large absorption coefficients, long diffusion lengths, low effective mass, good mobility and long radiative lifetimes. In addition, HaPs are solution processed or low-temperature vapor grown semiconductors and are made from earth abundant materials thus making them technologically relevant in terms of cost/performance. As a result, proof-of-concept high efficiency optoelectronic devices such as photovoltaics and LEDs have been fabricated. In fact, photovoltaic efficiencies have sky rocketed to 23% merely in the past five years and are nearly on-par with mono-crystalline Si based solar cells. Such unprecedented progress has attracted tremendous interest among researchers to investigate the structure-function relationship and understand as to what makes Halide hybrid perovskites special?
In my talk, I will attempt to answer some of the key questions and in doing so share the results from our work on HaPs over the past four years in understanding structure induced properties of HaPs. I will also highlight fundamental bottlenecks that exist going forward which present opportunities to create platforms to understand the interplay between light, fields and structure on the properties of perovskite-based materials.

Heterostructures and Heterointerfaces for Nanoelectronics and Photovoltaics

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures,^{1, 2} this concept is not limited to 2D materials alone. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions.^{3} In the first part of this talk I will present our work on emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. I will present two distinct examples of gate-tunable p-n heterojunctions.^{4-6}I will show that when a single layer n-type molybdenum disulfide (MoS_{2}) (2D) is combined with p-type semiconducting single walled carbon nanotubes (1D), the resulting p-n junction is gate-tunable and shows a tunable diode behavior with rectification as a function of gate voltage and a unique anti-ambipolar transfer behavior.^{4} The same concept when extended to p-type organic small molecule semiconductor (pentacene) (0D) and n-type 2D MoS_{2} leads to a tunable p-n junction with a photovoltaic effect and an asymmetric anti-ambipolar transfer response.^{6} I will present the underlying charge transport and photocurrent responses in both the above systems using a variety of scanning probe microscopy techniques as well as computational methods. Finally, I will show that the anti-ambipolar field effect observed in the above systems can be generalized to other semiconducting heterojunction systems and extended over large areas with practical applications in wireless communication circuits.^{5}

The second part of talk will discuss my more recent work on photovoltaic devices from 2D semiconductors such as transition metal dichalcogenides (TMDCs). High efficiency inorganic photovoltaic materials (e.g., Si, GaAs and GaInP) can achieve maximum above-bandgap absorption as well as carrier-selective charge collection at the cell operating point. I will show experimental demonstration of light confinement in ultrathin (< 15 nm) Van der Waals semiconductors (MoS_{2}, WS_{2} and WSe_{2}) leading to nearly perfect absorption.^{7} I will further present the fabrication and performance of our, broadband absorbing, heterostructure photovoltaic devices using sub-15 nm TMDCs as the active layers, with record high quantum efficiencies.^{7, 8} I will then present ongoing work on addressing the key remaining challenges^{9} for application of 2D materials and their heterostructures in high efficiency photovoltaics which entails engineering of interfaces and open-circuit voltage as well as on going work on novel band-bending heterojunctions as well as probing of buried metal/semiconductor interfaces^{10} with sub 50 nm resolutions. I will conclude by giving a broad perspective of future work on 2D materials from fundamental science to applications.

References:

1. Grigorieva, I. V.; Geim, A. K. Nature 2013,499, 419-425.

2. Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. ACS Nano2014, 8, 1102-20.

3. Jariwala, D.; Marks, T. J.; Hersam, M. C. Nat. Mater. 2017, 16, 170-181.

4. Jariwala, D.; Sangwan, V. K.; et al. Proc. Nat. Acad. Sci. USA 2013, 110, 18076–18080.

5. Jariwala, D.; Sangwan, V. K.; et al. Nano Lett. 2015, 15, 416-421.

6. Jariwala, D.; Howell, S. L.; et al. Nano Lett. 2016, 16, 497–503.

7. Jariwala, D.; Davoyan, A. R.; et al. Nano Lett. 2016,16, 5482-5487.

There has been tremendous interest in recent years to discover, explore and demonstrate unique properties and applications of two-dimensional (2D) materials. This got started with the spectacular discovery of graphene and the outstanding properties that graphene presented. This talk will focus on the status of this field, with emphasis on the materials science of 2D atomic layers and their hybrid structures. Several aspects that include synthesis, characterization, and manipulation will be discussed with the objective of achieving functional structures and applications based on 2D atomic layers. The concept of artificially stacked van der Waals solids, atomically thin planar heterojunctions, 2D alloys, and 2D layers based 3D constructs will be described using a number of examples consisting of graphene and other 2D layer compositions. The talk will explore the emerging landscape of 2D materials systems that include graphene, boron-nitrogen-carbon systems, and a large number of transition metal dichalcogenide compositions.

Mesoscopic entanglement with Bose-Einstein condensates

It is an open fundamental question how the classical appearance of our environment arises from the underlying quantum many-body theory. Experiments explore this question through the creation
of superposition states involving ever larger numbers of constituents.
Atomic Bose-Einstein condensates (BECs) are a promising platform, due to their typically very well defined many-body state. We present proposals how entire clouds of cold atoms can be brought into mesoscopically entangled (or Schroedinger's cat) states in position space, implying that the cloud as a whole will be in a superposition of two different places at once. The first avenue presented involves highly excited Rydberg atoms. Due to their extreme interactions, these atoms are prone to the generation of entangled states. This entanglement can then be transferred to ultra-cold ground state atoms using the technique of Rydberg dressing, which can give rise to spatial cat states using dressed dipole-dipole interactions [1] or Rydberg phase imprinting [2]. However mesoscopically entangled states can also arise among cold ground state atoms alone, such as in binary collision of bright solitons. We show that these collisions exhibit intricate many-body quantum behavior, invalidating mean field theory [3,4]. After collision the two solitons find themselves in a superposition state of various constituent atom numbers,positions and velocities, in which all these quantities are entangled with those of the collision partner. As the solitons appear to traverse the quantum-classical boundary back and forth during their scattering process,they emerge as natural probe of mesoscopic quantum coherence and decoherence phenomena.
[1] “Entangling distant atom clouds through Rydberg dressing”,
S. Möbius, M. Genkin, A. Eisfeld, S. Wüster and J. M. Rost, PRA 87, 051602(R) (2013).
[2] “Phase-Imprinting of Bose-Einstein Condensates with Rydberg Impurities”,
R. Mukherjee, C. Ates, W. Li and S. Wüster, PRL 115, 040401 (2015).
[3] "Condensate soliton collisions beyond mean-field theory"
S. Choudhury, A. Sreedharan, R. Mukherjee, A. Streltsov and S. Wüster (2019), (in preparation).
[4] "Solitons explore the quantum classical boundary"
A. Sreedharan, S. Choudhury, R. Mukherjee, A. Streltsov and S. Wüster, http://arxiv.org/abs/1904.06552 (2019)

Understanding disorder-induced phases in dipolar spin ice

Spin liquids, despite their apparently featureless ground states, are exotic magnetic states which host fractionalised excitations and emergent gauge fields. Interestingly, quenched disorder can nucleate defects with unusual properties and thus reveal the hidden collective excitations of such states. In this talk, I will explain how disorder affects the physics of a prototypical frustrated magnet, dipolar spin ice, both at high and low temperatures and in fact leads to a new phase at low temperature. This "topological spin glass” phase shows signatures of both spin liquidity and glassiness. I will also describe a new cluster algorithm that allows us to study the continuous phase transition to the topological spin glass and reliably extract the critical temperature and exponents.

Percolation of zero mode wavefunctions in diluted graphene

The problem of bipartite random hopping on diluted bipartite lattices ( e.g diluted graphene) has interesting spectral properties near zero energy, including a nonzero density of zero modes and an unusually long crossover to the modified Gade-Wegner form of the density of states expected from universality arguments. Here we exploit an interesting connection to maximum matchings of the same lattice to study the generic morphology of the maximally localised zero modes of the system, finding evidence for a percolation like transition as a function of vacancy concentration.

Is Contextuality Sufficient for Quantum Computation? Bound States for Qudit Magic State Distillation

Is there a feature of quantum mechanics responsible for the power of quantum computation? What are the minimal conditions that must be met to physically construct a large-scale fault tolerant quantum computer? Magic state distillation (MSD) is a promising approach to fault-tolerant quantum computation that provides a natural framework to answer both these questions. In particular, by applying MSD to qudits, Howard et al. identified contextuality – an abstract generalization of non-locality – as a necessary resource for quantum computation. But is contextuality also sufficient? Here, we show that, for any MSD routine based on a finite stabilizer code; there exist bound states that exhibit contextuality but are use- less for magic state distillation. Our result implies that contextuality is not a sufficient condition for quantum computing, at least for finite MSD protocols.

Out of Time Ordered Quantum Dissipation

Recently the connection of Out of Time Ordered Correlators (OTOCs) to scrambling, quantum chaos, thermalisation, etc has been realised. With the increasing interest in OTOCs, it will be useful to develop a systematic prescription to compute them. In this talk, I will discuss techniques of effective theory to compute OTOCs in open quantum systems. In particular, I will focus on the problem of a quantum Brownian particle interacting with a dissipative bath composed of two sets of harmonic oscillators. Besides the linear coupling between the particle and each set of oscillators, we add a small 3-body interaction, between the particle and the oscillators. This cubic coupling induces non-linear dissipation and noise terms in the effective theory of the particle. For appropriate choice of distributions of bath oscillators, all the bath correlators decay exponentially fast at late times. Hence after the decay of the bath correlators has set in, one can write down a suitable 1-PI effective action of the particle which is local in time. The correlators computed from the microscopic theory evolve the same way as the correlators computed from this 1-PI effective action. If the bath has microscopic time-reversal invariance and thermality, it imposes constraints on the 1-PI effective action. These constraint relations between the effective couplings of 1-PI action are OTO generalizations of the well-known Onsager-Casimir reciprocal relations and fluctuation-dissipation relations. Combining these relations, the non-Gaussianity of the thermal noise gets related to the thermal jitter in the damping constant of the Brownian particle.
Reference: arXiv-1811:01513

(not) Designed in California - inclusive, diverse, community led innovation with PV and Digital Technologies

In this talk we will outline our experience of working with and for "emergent user" communities in India and Africa to develop future technologies. As well as explaining our methods and rationale, we will provide case studies of their application to PV and digital devices deployed in Rwanda and India. We hope this talk will interest anyone who wants to learn more about how to involve "everday" community members in research and innovation; and, those who are interested in uses of PV technologies in diverse contexts.

Tuning Magnetic Anisotropy in Nanostructures for Biomedical and Electromagnetic Applications

Magnetic nanoparticles have been building blocks in applications ranging from high density recording to spintronics and nanomedicine [1]. Magnetic anisotropies in nanoparticles arising from surfaces, shapes, and interfaces in hybrid structures are important in determining the functional response in various applications. In this talk I will first introduce the basic aspects of anisotropy and discuss resonant radio-frequency (RF) transverse susceptibility, which we have used extensively, as a powerful method to probe the effective anisotropy in magnetic materials. The tuning of anisotropy has a direct impact on the performance of functional magnetic nanoparticles in biomedical applications such as contrast enhancement in magnetic resonance imaging and magnetic hyperthermia for cancer therapy. I will focus on the role of tuning surface and interfacial anisotropy with a goal to enhance specific absorption rate or heating efficiency. Strategies going beyond simple spherical structures to include exchange coupled core-shell nanoparticles, nanowires, and nanotubes, can be exploited to increase heating efficiency in magnetic hyperthermia [2], [3]. In addition to biomedical applications, composites of anisotropic nanoparticles dispersed in polymers pave the way to a range of electrically and magnetically tunable materials for RF and microwave device applications [4]. This lecture will combine insights into fundamental physics of magnetic nanostructures along with recent research advances in their application to nanomedicine and electromagnetic devices.

A New Superconducting Mechanism in the Market

One of the fascinating facts about superconductivity is that despite more than 100 years of extensive research and 7 Nobel prizes, we are far behind our main goals. The race continues to both achieving room temperature superconductivity as well as to obtain a theoretical understanding of the mechanism. How can two electrons attract each other when they experience a repulsive Coulomb interaction? By now we know two mechanisms for attractive interaction between the same-charge fermions: Meson mediates an attraction between protons. Phonon mediates attractive potential between electrons with superconductivity. In this talk, I will present a new mechanism of attractive potential between electrons, forming superconductivity. In many intermetallics and heavy-fermion compounds, atoms can possess fractional valency due to valence fluctuation between conduction and localized electrons (such as f-orbitals or flat bands). In such localized f-orbitals, two f-electrons with opposite spins cannot be occupied on the same orbit since they have strong Coulomb repulsion. This means, in the field theory language, a singly occupied f-electron site is attached with an unoccupied f-state (which is a holon gauge field) whose job is to repel another f-electron. However, the unoccupied f-site can be occupied by a conduction electron since the presence of valence fluctuation channel allows mutation between the f- and conduction electrons. We show that the doubly occupied state with f- and conduction electrons condensates like a Cooper pair. I will present this theory along with detailed comparison with recent experimental surprises of conventional superconductivity in heavy-fermion materials where decades old studies predicted unconventional superconductivity.

Quantum Spin Chains and Topology

In this set of lectures I will introduce the simplest quantum many body problems, quantum spin chains. The physics that spin chains host is diverse and profound and covers a large number of exciting topics in contemporary theoretical physics, including the powerful role of topology in statistical mechanics. The lectures will explain some of the discoveries that were recognized in the 2016 physics Nobel prize.
Following topics will be discussed during the lectures:

Classical Ising model

Quantum classical mapping

Transverse field Ising model

XY Chain

Topological Terms

Spin Coherent Path Integrals

Heisenberg Chain and Haldane's theta term

Quantum critical points

Quantum Spin Chains and Topology

In this set of lectures I will introduce the simplest quantum many body problems, quantum spin chains. The physics that spin chains host is diverse and profound and covers a large number of exciting topics in contemporary theoretical physics, including the powerful role of topology in statistical mechanics. The lectures will explain some of the discoveries that were recognized in the 2016 physics Nobel prize.
Following topics will be discussed during the lectures:

Classical Ising model

Quantum classical mapping

Transverse field Ising model

XY Chain

Topological Terms

Spin Coherent Path Integrals

Heisenberg Chain and Haldane's theta term

Quantum critical points

Quantum Spin Chains and Topology

In this set of lectures I will introduce the simplest quantum many body problems, quantum spin chains. The physics that spin chains host is diverse and profound and covers a large number of exciting topics in contemporary theoretical physics, including the powerful role of topology in statistical mechanics. The lectures will explain some of the discoveries that were recognized in the 2016 physics Nobel prize.
Following topics will be discussed during the lectures:

Classical Ising model

Quantum classical mapping

Transverse field Ising model

XY Chain

Topological Terms

Spin Coherent Path Integrals

Heisenberg Chain and Haldane's theta term

Quantum critical points

Quantum spin liquids in rare earth pyrochlores: quantum spin ice and beyond

Rare earth pyrochlores present many possible candidates for hosting Quantum spin liquids (QSL). In this talk, starting with the most well known QSL proposed for these materials-- the Quantum Spin Ice, I will discuss the effect of an external electric field to probe the QSL and drive transition. I will end with a discussion of possible alternatives candidate QSLs beyond Quantum Spin Ice and in particular, discuss the properties of a fermionic QSL and its competition with magnetic phases in context of rare earth pyrochlores.

Capturing electrons in motion at the nanometer-femtosecond scale

The flow of electrons in materials drives much of technology today. The ability to directly image these charge dynamics promises to deepen our understanding of the fundamental processes involved and impact future technology. Today, I will discuss our recent work in time resolved photoemission electron microscopy, which allows making movies of electron dynamics in materials with nanometer, femtosecond scale resolution. We will discuss the imaging the motion of electrons across semiconductor heterojunctions1, separating photoexcited electrons on an ultrafast timescale2, and probing the ultrafast dynamics of carriers at nanoscale trap sites in perovskites3. Time permitting, I will talk about the unique scientific environment on the island resort of Okinawa, where these experiments were done.
References:
[1] Man, et al., Nature Nanotech. 2017, 12, 36
[2] Wong, et al., Science Adv. 2018, 4, eaat9722
[3] Winchester, et al., in review

Solid-Solid Interfacial Engineering to Enable High Energy All-Solid-State Batteries

Enabling high-voltage oxide cathodes is a persistent challenge for all-solid-state batteries based on sulfides solid electrolytes (SSEs). Although most of them have the ionic conductivity comparable to or higher than that of liquid electrolytes, their electrochemical performance remains unsatisfactory when compared to the liquid electrolyte, mainly due to a high cathodic charge transfer resistance. In this work, we succeeded to engineer a stable interface between the electrolyte Li6PS5Cl (LPSCl) and the cathode LiNi0.85Co0.1Al0.05O2 (NCA) after figuring out their interfacial problems. Both the chemical reactions between NCA and LPSCl, and the electrochemical decomposition of LPSCl increase the cathode charge transfer. These interfacial reactions are differentiated and their products were probed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and cryo-STEM. The reaction driving force is analyzed by the first-principles calculation. Both experimental and computational results demonstrate that the thermodynamic instability between NCA and LPSCl can be significantly reduced by LiNbO3 (LNO) coating. We also reveal that in situ passivation takes place when the LPSCl is electrochemically decomposed during the first charge. This self-limiting interfacial reaction along with LNO coating helps to construct a stable interphase and enables to achieve a long-life high-energy all solid-state battery. Same observation was also noticed for all solid state Na ion battery where coating Li4Ti5O12 coating drastically improves the electrochemical performance.

Doughnuts, Soccer Balls and Topological Materials: Schrodinger to Einstein and Dirac

The revolution started by the discovery of topological insulators a few years ago has turned out to be the proverbial tip of the much larger iceberg of exotic phases of quantum matter driven by spin-orbit coupling effects.[1] Consideration of electronic states protected by time-reversal, crystalline and particle-hole symmetries has led to the prediction of many novel materials, which can support Weyl, Dirac, Majorana and other even more exotic fermions. I will discuss how common notions of topology in geometry get carried into materials physics and our recent work aimed at predicting new topological materials. I will also comment on potential of topological materials as next generation platforms for thermoelectric, spintronics, information processing and other applications, and some recent breakthroughs toward first-principles modeling of correlated materials such as the cuprate high-temperature superconductors [2].
[1] A Bansil, H Lin and T Das, Reviews of Modern Physics 88, 021004 (2016)
[2] J W Furness et al., Nature Communications Physics 1, 11 (2018)

Transport in ballistic junctions of topological materials

Over the past decade, the role of topology in shaping the behavior of low-energy quasiparticles of a quantum system has been actively studied. Several well known materias such as graphene, various topological insulators, and Weyl semimetals hosts such quasiparticles. In this talk I am going to address the signature of topology in transport of ballistic junctions of these materials and discuss possible experimental realization of these junctions.

Extended moment formation in monolayer TMDCs

Employing the first-principles calculations with onsite Coulomb interaction, the electronic structure of monolayer WS2 doped substitutionally with 3d transition metals is investigated. While neither W vacancies nor strain induces spin polarization, we demonstrate an unprecedented tendency to extended moment formation under doping. The extended magnetic moments are characterized by dopant specific spin density patterns with rich structural features involving the nearest neighbor W and S atoms.

Ab-initio Calculations of the Thermoelectric Properties of MXenes

First-principles calculations are employed to predict the thermoelectric properties of the 2D materials Ti2CO2, Zr2CO2, and Hf2CO2 in order to evaluate the role of the metal atom. Flat conduction bands are found to promote the thermopower in the case of n-doping. The lattice thermal conductivity is demonstrated to grow along the series Ti-Zr-Hf in the temperature range 300-700 K, resulting in the highest figure of merit in the case of Ti2CO2.

Figures of Sound and Threads of Silence

The quiescent parts of wave patterns created on vibrating drums are
intriguing and beautiful. Similar patterns appear when we propagate
a microwave pulse in a cavity, propagate water waves in different
arrangements of circular scatterers, sound waves in solid blocks, antidot
lattices mimicking Lorentz gas, electronic transport through ballistic
microscopic semiconductor structures, quantum corrals, quantum well
billiards, and of course in simple quantum billiards. Classification of
the ``sound figures" of certain non-separable billiards leads to
successful counting of the nodal domains. Several statistical measures
will be shown to unfold the geometrical features of the nodal lines.
Probability distributions of the amplitudes of the eigenfunctions and
their intensity moments reveal non-analytic features in the neighbourhood
of quiescent or silent or dark regions. In conclusion, Inverse problems
will be mentioned where a useful connection is being estalished with
neuroimaging.

Patterned Flows: from Thin Films to Turbulence

Many fluid dynamic problems in nature and industry are nonlinear and give rise to spatio-temporal patterns. Indeed, these patterns contribute to the beauty and fascination of the subject; they also impact transport rates and other macroscopic physical properties. In the course of this talk, we will encounter two very different kinds of patterns: (a) interfacial patterns that arise from the instability of thin liquid films, and (b) vortical flow structures that characterise the small-scales of turbulent flows. In the first part of the talk, I shall revisit the oldest paradigm of pattern formation, first propounded by Rayleigh, which asserts that the length-scale of an interfacial pattern is determined by the fastest growing linear instability mode. By way of a model problem, I will demonstrate a clear counter-example to this paradigm and shed light on the role of nonlinearity in patterning fluid interfaces. The second half of the talk will focus on the transport of model elastic filaments (bead-spring chains) by a turbulent flow. Here, we shall see that contrary to our expectation of uniform mixing, the filaments actually show a preferential sampling of vortical regions in the flow. Some exciting new questions raised by these results will also be discussed.

Scales and scalings in fully developed turbulence

Fully developed turbulence is supposed to be an extremely complicated problem which defies understanding. Yet everyday thousands of aircrafts fly through turbulent conditions without encountering any difficulty. This lecture will try to explain how inspite of the fact that there is no problem in real life , turbulence as set up by Kolmogorov is still considered a problem worth studying.

The Quantum Anomalous Hall Effect in Magnetic Topological Insulator Heterostructures

When time-reversal symmetry is broken in a three-dimensional topological insulator, the twodimensional helical surface Dirac states are replaced by one-dimensional chiral edge states. This is most readily achieved in magnetically-doped tetradymite semiconductor thin films and heterostructures grown by molecular beam epitaxy [1]. Analogous to the well-known quantum Hall edge states, these dissipationless ‘quantum anomalous Hall’ edge states are characterized by a vanishing longitudinal conductance (σxx = 0) and a quantized Hall conductance (σxy = h/e2 ). In strong contrast to the quantum Hall effect, however, the quantum anomalous Hall effect does not require Landau levels: it occurs even in the absence of an external magnetic field and can be observed in highly disordered samples (Drude mobility ~ 200 cm2 /V.s). We provide an overview of the development of this phenomenon, from its conception in theory [2] to its experimental observation [3], with examples from our recent work that probes the interplay between the magnetization and the electronic transport in magnetically-doped topological insulators [4-6]. Finally, we show how epitaxially engineered heterostructures that incorporate different magnetic doping may be used to realize a novel ‘axion insulator’ state [7].
This work is supported by ONR, ARO MURI, and NSF-MIP.
1. N. Samarth, Nature Materials 16, 1068-1076 (2017).
2. R. Yu, W. Zhang, H. J. Zhang, S. C. Zhang, X. Dai, and Z. Fang, Science 329, 61 (2010).
3. C. Z. Chang, et al., Science 340, 167 (2013).
4. A. Kandala, A. Richardella, S. Kempinger, C. X. Liu, and N. Samarth, Nat. Commun. 6, 7434 (2015).
5. E. O. Lachman, et al., Sci. Adv. 1, e1500740 (2015).
6. M. H. Liu, W. D. Wang, A. R. Richardella, A. Kandala, J. Li, A. Yazdani, N. Samarth, and N. P. Ong, Sci. Adv. 2, e1600167 (2016).