420 research outputs found
Some Unconventional Phases And Phase Transitions In Condensed Matter : Spin-Nematics, Spin-Liquids, Deconfined Critical Points And Graphene NIS Junctions
No full textCondensed matter physics provides us with an opportunity to explore a large variety of systems with diverse properties. Central to the understanding of these systems is a characterization of the nature of their ground states and low energy excitation. Often, such systems show various forms of emergent properties that are absent in the microscopic level. Identification of such emergent phases of condensed matter form an important avenue of research in the field. In this thesis example of such phases and their associated phase transitions have been studied.
The work presented here may be broadly divided into two themes: construction of the theoretical framework for understanding materials already studied experimentally, and, trying to provide new theoretical avenues which may be relevant for understanding future experiments. In these studies we shall explore some unconventional phases and phase transitions that may occur in condensed matter systems. A comprehensive understanding of the properties of such unconventional phases and phase transitions is important in the context of the large array of experimentally studied materials that regularly defy conventional wisdom in more than one way. The thesis consists of two distinct parts. In the first part we study three problems in frustrated magnets. The second part consists of studies of the tunnelling spectroscopy of metal-insulator-superconductor junctions in graphene.
Studies in frustrated magnets have opened up the possibility of existence of a whole range of phases beyond the already known magnetically ordered ones. Some of these new phases, like the spin nematic or the valence bond solid, display some other conventional order themselves. Others, like the much sort after spin liquid phases displays a whole new kind of order that cannot be captured through the celebrated Landau’s classification of phases on the basis of symmetry breaking and associated order parameters. The phase transitions in these systems are also equally interesting and lead to intriguing possibilities that demand new modes of analysis. In this part of the thesis we shall study the different properties of three magnets with spin-1/2, 1 and 3/2 respectively.
We start by providing an introduction to frustrated spin systems in Chapter [1]. The origin of antiferromagnetic interactions in Mott insulators is discussed and the concept of frustration of magnetic interaction is explained. We also point out the causes that may destroy magnetic order in spin systems, particularly the role of quantum fluctuations in presence or absence of magnetic frustration. This is followed with a brief outline of various magnetically ordered and disordered ground states with particular emphasis on the description of the later. We also give a brief outline of various properties of such phases and associated quantum phase transitions particularly noting the influences of quantum interferences encoded in the Berry phase terms. A brief description of the finite temperature properties is also provided. We end an outline of various experimentally relevant compounds that requires comprehensive understanding, some of which have been addressed in this thesis.
In Chapter [2] we study the properties of a spin-nematic state in context of the recently discovered spin-1 Mott insulator Nickel Gallium Sulphide (NiGa2S4). This isotropic triangular lattice compound shows no spin ordering till low temperatures. We propose that it may have a particular type of spin-nematic ground state and explain the experimentally observed properties of the compound on the basis of our proposal. Starting from a two band Hubbard model description, relevant for the compound, we derive the Bilinear Biquadratic spin Hamiltonian. We then show, within mean field theory, that this Hamiltonian describes a transition from the spiral state to a ferro-nematic state as a function of the ratio of bilinear and biquadratic couplings. We also study the possible effects of small pinning disorder andmagnetic field and suggest experiments that can possibly distinguish the proposed nematic state from others.
In Chapter [3] we explore the effects of the magneto-elastic coupling in the spin-3/2 B-site chromite spinel Cadmium Chromite (CdCr2O4). In this compound the spins form a pyrochlore lattice. Nearest neighbour spins interact antiferromagnetically. Due to frustration the system does not order at low temperatures and instead goes into a classical spin liquid state. Such a cooperative paramagnet is very susceptible to external perturbations which may relieve their frustration. In CdCr2O4, at lower temperatures the magnetic frustration is relieved by distorting the lattice through a first order magnetoelastic transition. Thus the compound presents a case where the relevant perturbation to the frustrated spin interactions is provided by spin-phonon coupling. An effect of such perturbations on a cooperative paramagnet is of general interest and all aspects of this are not understood presently. We take the initial step of characterizing the spin-phonon interaction in detail. Based on recent sound velocity experiments, we construct a microscopic theory for the sound velocity renormalization due to the spin-phonon coupling and explain the recent experimental data obtained by S. Zherlitsyn et al. using our theory we can explain the dependence of the sound velocity on temperature as well as magnetic field. We also construct a Landau theory to explain (qualitatively) the behaviour of sound velocity across the magneto-structural transition. Further, we discuss the effects due to the small Dzyaloshinskii-Moriya interaction that may be present in these compounds.
In Chapter [4] we study the possibility of a direct second order quantum phase transition from spiral to dimer phase in two dimensional antiferromagnets. Such transitions between phases with incompatible symmetries are forbidden within conventional Landau Ginzburg-Wilson paradigm of critical phenomena. Early works showed that when the spiral is destroyed by long wavelength fluctuations a fractionalized Z2 spin liquid is obtained. In this work we show an alternative way–the quantum destruction of the spiral magnet. We argue that, when the defects of the spiral phase proliferate and condense, their associated Berry phase automatically leads to dimerization. We apply our theory to study concrete lattice models where such transitions may be observed. This transition is an example of a Landau forbidden deconfined quantum phase transition. The proposed critical theory is naturally written in terms of fractional degrees of freedom which emerge right at the critical point. These fractional particles interact with each other through emergent gauge fields and are deconfined right at the critical point (but are confined in either of the two adjoining phases). We argue, based on existing results, that the monopoles of the gauge field are dangerously irrelevant right at the critical point rendering the later noncompact. The critical point is characterized by an emergent global U (1) conservation law that is absent in the microscopic model, a typical feature of a deconfined quantum critical point. The resultant field theory belongs to the class of anisotropic NCCP3 class which may be studied numerically in future to understand its critical properties.
In modern condensed matter physics the emergence of new and novel phases of matter have often been associated with the presence of strong correlations. Indeed, strongly correlated systems seem to harbour in them the potential to realize some of the most unconventional and exotic emergent phases of matter. However in graphene, which is a single layer of graphite, the emergence of novel properties, as present experiments suggest, is due to its unique band structure and not a fallout of intricate correlation effects. Band structure studies of graphene suggest that the material is a zero gap semiconductor with the low energy excitations resembling massless Dirac quasi-particles. The consequence of this is immediate and interesting. It has lead to the possibility of exploring the physics of relativistic fermions in two spatial dimensions and much of this has been studied with great vigour in the last five years.
In our studies, presented in Chapter [5], we explore one of the many consequence of this emergent Dirac structure of the low energy quasi-particles, namely the properties of metal-insulator-superconductor junctions of graphene. The twin effect of Klein tunneling of Dirac fermions (and associated transmission resonances) and Andreev reflection (both specular and retro) sets them aside from their conventional counterparts. The graphene normal metal-insulator-superconductor (NIS) junctions show strikingly different properties like oscillations in the sub-gap tunneling conductance as a function of both barrier strength and width. We make a detailed study of this for arbitrary barrier strengths and widths with and without Fermi-surface mismatch between the normal and the superconducting sides. The amplitude of these oscillations are maximum for aligned Fermi surface and vanishes for large Fermi surface mismatch. We provide an understanding for this unconventional behaviour of graphene NIS junctions. We also suggest experimental tests for our theory. Such experimental verification will reveal one more remarkable emergent property in a condensed matter system
Colloquium: Nonequilibrium dynamics of closed interacting quantum systems
No full textThis Colloquium gives an overview of recent theoretical and experimental progress in the area of nonequilibrium dynamics of isolated quantum systems. There is particularly a focus on quantum quenches: the temporal evolution following a sudden or slow change of the coupling constants of the system Hamiltonian. Several aspects of the slow dynamics in driven systems are discussed and the universality of such dynamics in gapless systems with specific focus on dynamics near continuous quantum phase transitions is emphasized. Recent progress on understanding thermalization in closed systems through the eigenstate thermalization hypothesis is also reviewed and relaxation in integrable systems is discussed. Finally key experiments probing quantum dynamics in cold atom systems are overviewed and put into the context of our current theoretical understanding. © 2011 American Physical Society
Competing order and non-Landau-Ginzburg-Wilson criticality in (Bose) Mott transitions
No full textThis paper reviews a recent non-Landau-Ginzburg-Wilson (LGW) approach to superfluid to Mott insulator transitions in two dimensional bosonic lattice systems, using a dual vortex field theory [L. Balents, L. Bartosch, A. Burkov, S. Sachdev and K. Sengupta, Phys. Rev. B 71 (2005), 144508; Phys. Rev. B 71 (2005), 144509]. The physical interpretation of conventional LGW theory of quantum criticality is re-examined and similarities and differences with the vortex picture are discussed. The "unification" of various competing (insulating) orders, and the coincidence of these orders with the Mott transition are readily understood formulation. Some aspects of the recent theory of "deconfined" quantum criticality, which are to an extent subsumed approach, are discussed. A pedagogical presentation of the "nuts and bolts" of boson-vortex duality at the hamiltonian level is included, tailored to a condensed matter audience
Pricing Derivatives: The Financial Concepts Underlying the Mathematics of Pricing Derivatives
No full textA fresh, fundamentals-based approach for accurate derivative pricing
Pricing Derivatives presents a specialized approach to accurately pricing derivatives by stressing the conceptual foundations underlying the mathematics. Noted mathematics professor and investing consultant Ambar Sengupta provides a sound understanding of the essential topics of derivative pricing and outlines methodologies for arriving at exact pricing formulas based on the fundamental relationship between price and probability.
Short, to-the-point chapters present original ideas and approaches for pricing derivative products, supplying professional money managers and institutional investors with the foundation they need to: Integrate both the theoretical and mathematical foundations of pricing derivatives Establish optimal prices in terms of the no-arbitrage principle Derive model-independent pricing formulas for options, futures, forwards, and other key derivatives
Experience has shown that derivative traders must focus on conceptual, as opposed to trading, issues if they are to improve trading accuracy and profitability. Pricing Derivatives presents conceptually sound approaches for pricing derivatives and shows how to use them to compute specific pricing formulas.
Pricing Derivatives unveils a fundamentally clear-cut approach to accurate derivative pricing. Based upon author Ambar Sengupta\u27s years of consulting experience working with derivatives traders to hone their trading performance, it steers around the mechanics of popular financial models to focus on the conceptual foundations and underlying mathematics of pricing derivatives as well as other financial instruments.
Exploring the relationshipbetween price and probability, Pricing Derivatives demonstrates methods for determining model-independent pricing formulas and applying them to specific market models for more distinct and applicable pricing formulas.https://repository.lsu.edu/facultybooks/1593/thumbnail.jp
Assessment of dopaminergic neuron degeneration in a C. elegans model of Parkinson's disease
No full textTransgenic Caenorhabditis elegans that expresses the full-length wild-type human α-synuclein in dopaminergic neurons provides a well-established Parkinson's disease (PD) nematode model. Here, we present a detailed protocol to monitor and dissect the molecular underpinnings of age-associated neurodegeneration using this PD nematode model. This protocol includes preparation of nematode growth media and bacterial food sources, as well as procedures for nematode growth, synchronization, and treatment. We then describe procedures to assess dopaminergic neuronal death in vivo using fluorescence imaging. For complete details on the use and execution of this protocol, please refer to SenGupta et al. (2021). © 2022 The Author(s
Financial Management of Globalization of Developing Countries
No full texthuman development, economic growth, globalization, inequality, poverty
Free vibration analysis of composite right angle triangular plate using a shear flexible element
No full text© 2003 SAGE Publications A high precision triangular plate bending element proposed by the second author of this paper has been upgraded for the free vibration analysis of laminated composite right angle triangular plates. The effect of shear deformation has been incorporated. An efficient mass lumping scheme with rotary inertia has been recommended. Numerical examples of composite triangular plates having different thickness ratios, side ratios, fibre-orientations, number of layers and boundary conditions have been solved by this element. For validation of the present formulation and element few results on isotropic and orthotropic plates have been compared with those obtained from literatures. The results on composite plates have been presented as new results. S. Haldar, D. Sengupta, and A. H. Sheik
Dataset to accompany "Deposition of brown carbon onto snow: changes of snow optical and radiative properties" by Beres et al., 2020
No full textThis dataset, organized in an Excel spreadsheet, accompanies:
Beres, N. D., Sengupta, D., Samburova, V., Khlystov, A. Y., and Moosmüller, H.: Deposition of brown carbon onto snow: changes in snow optical and radiative properties, Atmos. Chem. Phys., 20, 6095–6114, https://doi.org/10.5194/acp-20-6095-2020, 2020.
Each tab of the spreadsheet represents data presented in Tables and Figures of the manuscript, which allows for the replication of the figure or for use in calculations presented throughout the manuscript.
Any questions or comments should be forwarded to the corresponding author
MICROWAVE SPECTRUM AND MOLECULAR STRUCTURE OF CIS-1,2-DIFLUOROCYCLOPROPANE
No full text S.K. Sengupta and V.W. Laurie, Paper TGS, 32nd Symposium on Molecular Spectroscopy, Columbus, Ohio (1977).Author Institution: Department of Chemistry, Oberlin College; Department of Chemistry, Rensselaer Polytechnic Institute; Department of Chemistry, Temple UniversityThe microwave spectrum of a carbon-13 isotope of cis-1,2-difluorocyclopropane,, has been assigned in natural abundance. The normal and deuterium isotopic rotational constants reported were combined with the carbon-13 data to partially determine the molecular structure. The parameters are and . These structural parameters will be compared to the trans isomer of 1,2-difluorocyclopropane as well as to related geometrical pairs of fluorinated three-membered rings. The results will also be discussed in terms of recent theoretical studies of these ring systems
“Eyes on the Street”: Estimating Natural Surveillance Along Amsterdam’s City Streets Using Street-Level Imagery
No full textNeighborhood safety and its perception are important determinants of citizens’ health and well-being. Contemporary urban design guidelines often advocate urban forms that encourage natural surveillance or “eyes on the street” to promote community safety. However, assessing a neighborhood’s level of natural surveillance is challenging due to its subjective nature and a lack of relevant data. We propose a method for measuring natural surveillance at scale by employing a combination of street-level imagery and computer vision techniques. We detect windows on building facades and calculate sightlines from the street level and surrounding buildings across forty neighborhoods in Amsterdam, the Netherlands. By correlating our measurements with the city’s Safety Index, we also validate how our method can be used as an estimator of neighborhood safety. We show how perceived safety varies with window level and building distance from the street, and we find a non-linear relationship between natural surveillance and (perceived) safety.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Human-Centred Artificial IntelligenceInternet of Thing
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