Open Seminar 2015
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Govind Dayal Singh Y9109067 Metamaterials for Infra-red Multi-spectral Absorbers 18th March, 2015 (Wednesday) 3:30 pm FB-382 Electromagnetic absorbers have wide applicability and need to be impedance matched with free space to reduce the reflectance from their surface. Here we discuss the design and fabrication of metamaterials that show multi-band perfect absorbances at infra-red frequencies. The key a highly absorbing medium is to choose resonant structures that simultaneously have matched electrical and magnetic resonances. An array of metallic (conducting) particles separated from a conducting ground plane by a dielectric spacer layer constitutes the resonant structure. Proper choice of the particle size and layer thickness can result in perfect absorption that is reasonably independent of the excitation angle and polarization. Use of multi-layered stacks of particles that can also yield multi-band absorption. While most metamaterial designs utilize the fundamental mode in a sub-wavelength sized resonator, highly localized higher order modes can also be utilized for multi-band perfect absorption. Multi-band metamaterial absorbers based on fundamental as well as higher order resonances for Infra-red frequencies have been designed, fabricated and characterized. The metamaterial absorbers have multiple absorption bands across the MWIR and LWIR bands with peak absorbances exceeding 90%. Metamaterial absorbers, with broadband absorption at the mid-infrared frequencies and high transmittance at visible frequencies, have been fabricated using a semiconductor Indium Tin Oxide (ITO) film as the ground plane.The metamaterials were fabricated by simple, cost-effective laser micromachining techniques, shadow mask deposition and oblique angle deposition technique. A strategy to tune or switch the metamaterial absorption by incorporated a phase change material such as VO2 in the metamaterial has been implemented and results in a thermochromic metamaterial. |
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Pranati Rath Y9109077 Large Scale Anisotropy in Cosmic Microwave Background Radiation and its Dependence on the Primordial Power Spectrum 20th March, 2015 (Friday) 11:30 am FB-382 The Cosmological principle says that the universe at large distance scale is assumed to be statistically homogeneous and isotropic. The Cosmic Microwave Background Radiation (CMBR) is considered to be one of the best experimental evidence supporting this principle. However, there exists considerable evidence in cosmological data which suggests violation of this principle. In this talk, I will revisit two of these anomalies observed in the CMBR temperature data provided by WMAP and recently by PLANCK team. These includes the alignment of the quadrupole (l=2) and octopole (l=3) and the hemispherical power asymmetry. To explain the low-l alignment, I will discuss some anisotropic inflationary models within the framework of the Big Bang cosmology. In these models, the anisotropy decays very quickly during the inflationary phase of expansion. The resulting direction dependent power spectrum in this anisotropic background leads to violation of isotropy and hence explain the alignment of the low l CMBR modes. I will also discuss an inhomogeneous power spectrum model in order to explain the hemispherical power asymmetry, observed in the CMBR data. The hemispherical asymmetry can be parametrized in terms of the dipole modulation model. Alternatively, I will show that, an anisotropic dipolar imaginary primordial power spectrum, which is possible within the framework of noncommutative space-times, also provides a good description of the observed dipole modulation in CMBR data. |
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Tutul Biswas 10109877 Wave packet dynamics and phonon scattering in spin-orbit coupled fermionic systems. 9th March, 2015 (Monday) 11 am FB-382 |
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Subhash Mahapatra 10109069 Applications of the Gauge-Gravity duality : Superconductivity, holographic Optics and Entanglement Thermodynamics. 10th February, 2015 (Tuesday) 11 am FB-382 A remarkable connection, in the context of string theory, has emerged in last few years between gravity and the strongly coupled field theories. This connection, which is generally called as the AdS/CFT correspondence or the gauge/gravity duality, maps a quantum theory of gravity in (d +1) dimension Anti de sitter (AdS) spacetime to d dimensional conformal field theory (CFT) - living at the boundary of AdS spacetime. An important feature of this duality is that in certain approximation, the strongly coupled limit of field theory corresponds to the weakly coupled limit of the gravity theory. This strong-weak nature of the correspondence has provided a unique approach to address some questions in strongly coupled condensed matter systems which otherwise would be intractable. In this thesis, we discuss applications of the AdS/CFT correspondence, on three main directions: superconductivity, optics and entanglement thermodynamics. We generalize the minimal models of holographic superconductors in a gauge invariant way and explore the rich phase structure of the boundary theory. Then, we study the electromagnetic response functions and entanglement entropy of these generalized holographic superconductors for various space-time geometries. |
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Uday Bhanu Paramanik Y8109071 Magnetism and superconductivity of Eu-based "122" pnictides. 6th February, 2015 (Friday) 11 am FB-382 The discovery of high-temperature superconductivity in Iron pinctides has created enormous interest leading to the finding of a wide range of layered pnictide compounds which exhibit a rich variety of magnetic and transport phenomena. Among the '122' pnictides, EuFe2As2 got particular attention as this system demonstrates interesting interplay between superconductivity and local moment magnetism of Eu moments. We have investigated the effect of Iridium doping on the magnetic and superconducting properties of EuFe2As2. The optimal Iridium doping suppresses the Fe antiferromagnetic order and induces superconductivity below 22 K. The Eu moments order magnetically at ~18K with strong ferromagnetic (FM) component. The competition between the superconducting and the magnetic state leads to the reentrance of superconductivity. While the superconducting properties of the Fe-As based materials have been studied extensively, very recently, unconventional superconductivity has been discovered in the vicinity of antiferromagnetic order in a class of CrAs based compounds. We present the synthesis and physical properties of a new compound EuCr2As2 in the '122' pnictide family which exhibits antiferromagntic ordering of Cr itinerant moments in addition to the ferromagnetic order of Eu local moment. This behavior is reminiscent of the parent compound of Fe based superconductor EuFe2As2 but with a slightly different magnetic structure. Furthermore, since the valence fluctuations in classical heavy-fermion systems are considered to be responsible for unconventional superconductivity, we discuss the important aspect of the evolution of magnetism upon Ge-doping in the valence fluctuating heavy fermion compound EuNi2P2. The rare-earth ion configuration changes from valence fluctuating (VF) state in EuNi2P2 to nearly integral valence Eu2+ (4f7) in EuNi2PGe which orders antiferromagnetically below TN = 12 K. No superconductivity was observed in the Ge-doped EuNi2P2 down to 2K. |
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Bahadur Singh Y9209061 Electronic structure and spin texture of non-trivial surface states of topological insulators 28th Jan. 2015 (Wednesday) 4:0 pm FB-382 A new paradigm for classifying condensed matter systems by topology of bulk band structure has spurred the discovery of several new states of matter having exotic properties. In particular, topological insulators are such new states of matter that are distinct from conventional insulators. These materials support spin-polarized conducting states at the boundaries/surfaces while remaining insulating in the bulk. The surface states often exhibit Dirac-cone energy dispersion with helical or chiral spin-texture and are protected by time reversal symmetry. The topological protection ensures the backscattering free transport at the boundaries of topological insulators. The recent developments in the field show that these nontrivial states not only offer potential applications in quantum computing and spintronics, but also provide platforms for realizing novel quantum phenomena such as Weyl semimetals, and Majorana fermions in a condensed matter system. In this work, we present an analysis of topological surface state properties of several selected materials and predict new materials or thin films of materials that realize the quantum spin Hall state, Dirac semimetal, Weyl semimetal, or Rasbha effect, using the ab-initio density functional theory framework and k.p theory. Through systematic analysis of bulk and surface electronic structures, we show that thallium based ternary III-V-VI2 series of compounds, TlMQ2 (where M= Bi or Sb and Q= S, Se, or Te), contain both topological and normal insulators and therefore, it is possible to study the topological phase transition (TPT) by tuning the lattice parameters as well as spin-orbit coupling (SOC). The electronic structure and spin-texture analysis of topological surface states show that the surface states form an unusual “planer metal” that is essentially half of an ordinary two dimensional (2D) conductor and carry nontrivial π Berry phase. Furthermore, we discuss possible TPT in Ge(BixSb1-x)2Te4 thin films as a function of layer thickness and Bi concentration x. The systematic examination of band topologies suggests that thin films of Ge(BixSb1-x)2Te4 are viable candidates for 2D topological insulators, which would undergo a 2D-TPT as a function of x. Finally, our analysis shows that the inversion asymmetric compound, Sb2Se2Te, harbors both a novel nontrivial band topology and giant Rasbha-type spin splitting in its native form driven by strong SOC. The Rasbha splitting in the bulk bands of Sb2Se2Te is the largest that has been found to date and attributed to large polarization field (electric field), which arises from the broken inversion symmetry in the system. |