arXiv subjects

R. Rajaraman

Publications and source records attributed to R. Rajaraman.

16 recordsLinked to original sources

arXiv2020

Porous Structured Au Colloids: Insights in to Morphology, Optical and Antimicrobial Activity

Porous structured Au colloids have been prepared from bulk nanoporous Au by means of an element dissociation method. The microscopic techniques (scanning electron microscope and atomic force microscope), UV-Vis spectroscopy and zone diffusion method have been employed to study their morphology, optical property and anti-microbial activity against Gram-positive and Gram-negative bacterial strains, respectively. It is shown that the porous structured Au colloidal suspension exhibit excellent optical and antimicrobial properties. The noticeable features present in the optical studies are two Plasmon resonance peaks at 477 and 546 nm with overlapping of these peaks at a wavelength of 520 nm. The morphology studies by SEM and AFM indicate that the Au colloids are rod shaped with an assembly of skeletal pore and ligament structure. The analysis of the antimicrobial activity testing reveals that the porous structured Au colloids have greater inhibitory effect against the tested Gram-positive and Gram-negative bacterial strains.

cond-mat.mtrl-sci↗

arXiv2016

Effect of Sb substitution on the Topological Surface States in Bi_{2}Se_{3} single crystals: a magneto-transport study

Magneto-transport measurements have been carried out on Bi2-xSbxSe3 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals at 4.2 K temperature in the magnetic field range of -15 T to 15 T. Shubnikov-de Haas (SdH) oscillations of 2D nature were observed in samples with Sb concentration upto x = 0.3. The analyses of SdH oscillations observed in magneto-resistance data using Lifshitz-Kosevich equation reveal a systematic decrease in the Fermi surface area with Sb substitution. The Berry phase obtained from the Landau Level fan diagram suggests the occurrence of 2D oscillations arising from a Topological Surface State (TSS) for Sb concentrations of x = 0, 0.05 and 0.1; while 2D oscillation seen at higher Sb concentration is attributed to surface 2D electron gas consequent to downward band bending.

cond-mat.mtrl-sci↗

arXiv2014

Role of Se vacancies on Shubnikov de Haas oscillations in Bi2Se3: a combined magneto-resistance and positron annihilation study

Magneto resistance measurements coupled with positron lifetime measurements, to characterize the vacancy type defects, have been carried out on the topological insulator (TI) system Bi2Se3, of varying Se/Bi ratio. Pronounced Shubnikov de Haas (SdH) oscillations are seen in nominal Bi2Se3.1 crystals for measurements performed in magnetic fields up to 15 T in the 4 K to 10 K temperature range, with field applied perpendicular to the (001) plane of the crystal. The quantum oscillations, characteristic of 2D electronic structure, are seen only in the crystals that have a lower concentration of Se vacancies, as inferred from positron annihilation spectroscopy.

cond-mat.mtrl-sci↗

arXiv2011

Dislocation Driven Chromium Precipitation in Fe-9Cr Binary Alloy: A Positron Lifetime Study

The influence of initial heat treatment on anomalous Cr precipitation within high temperature solubility region in Fe-9Cr alloy has been investigated using positron lifetime studies. Air-quenched samples with pre-existing dislocations exhibited a distinct annealing stage in positron lifetime between 800 and 1100 K corresponding to Cr-precipitation. During this stage, Transmission Electron Microscopy showed fine precipitates of average size 4 nm, dispersed throughout the sample and from EDS analysis they are found to be Cr-enriched. The existence of dislocations is found to be responsible for Cr precipitation.

cond-mat.mtrl-sci↗

arXiv2001

CP_N Solitons in Quantum Hall Systems

We will present here an elementary pedagogical introduction to $CP_N$ solitons in quantum Hall systems. We begin with a brief introduction to both $CP_N$ models and to quantum Hall (QH) physics. Then we focus on spin and layer-spin degrees of freedom in QH systems and point out that these are in fact $CP_N$ fields for N=1 and N=3. Excitations in these degrees of freedom will be shown to be topologically non-trivial soliton solutions of the corresponding $CP_N$ field equations. This is followed by a brief summary of our own recent work in this area, done with Sankalpa Ghosh. [ Invited Plenary Lecture at the International Conference on Geometry, Integrability and Nonlinearity in Condensed Matter & Soft Condensed Matter Physics held at Bansko, Bulgaria, July 15-20, 2001. ]

cond-mat↗

arXiv2001

Fractional Charge

The origin and quantum status of Fractional Charge in polyacetylne and field theory are reviewed, along with reminiscences of collaboration with John Bell on the subject.

cond-mat.mes-hall↗

arXiv2000

Effective attraction between like-charged colloids in a 2D plasma

The existence of attractions between like-charged colloids immersed in ionic solution have been discovered in recent experiments. This phenomenon contradicts the predictions of DLVO theory and indicates a failure of mean field theory. We study a toy model based on a two dimensional one-component plasma, which is exactly soluble at one particular coupling constant. We show that colloidal interaction results from a competition between ion-ion repulsion and longer ranged ion-void attraction.

cond-mat.soft↗

arXiv2000

Quantum Hall Solitons with Intertwined Spin and Pseudospin at $ν= \ 1$

In this paper we study in detail different types of topological solitons which are possible in bilayer quantum Hall systems at filling fraction $ν=1$ when spin degrees of freedom are included. Starting from a microscopic Hamiltonian we derive an effective energy functional for studying such excitations. The gauge invariance and $CP^{3}$ character of this energy fuctional and their consequences are examined. Then we identify permissible classes of finite energy solutions which are topologically non-trivial. We also numerically evaulate a representative solution in which a pseudospin (layer degrees of freedom) bimeron in a given spin component is intertwined with spin-skyrmions in each layer, and and discuss whether it is energetically favoured as the lowest lying excitation in such system with some numerical results.

cond-mat.mes-hall↗

arXiv1999

Broken Symmetry Ground States in ν=2 Bilayer Quantum Hall Systems

We report on a study of a bilayer two-dimensional electron gas at Landau level filling factor $ν=2$. The system exhibits both magnetic and spontaneous interlayer phase coherence broken symmetries. We propose a 3-parameter Slater determinant variational wavefunction which describes the ground state over the full range of bias potential ($Δ_V$) Zeeman coupling ($Δ_z$) and interlayer tunneling amplitude ($Δ_t$) strengths. Broken symmetry states occur inside a volume in this three-dimensional phase diagram near the $Δ_z^2=Δ_V^2+Δ_t^2$ surface. We have obtained analytic results for the intersections of the phase diagram with the $Δ_t=0$, $Δ_z=0$, and $Δ_V=0$, planes and show that the differential capacitance of the bilayer system is singular at the phase boundary.

cond-mat.mes-hall↗

arXiv1998

Bimerons in Double Layer Quantum Hall Systems

In this paper we discuss bimeron pseudo spin textures for double layer quantum hall systems with filling factor $ν=1$. Bimerons are excitations corresponding to bound pairs of merons and anti-merons. Bimeron solutions have already been studied at great length by other groups by minimising the microsopic Hamiltonian between microscopic trial wavefunctions. Here we calculate them by numerically solving coupled nonlinear partial differential equations arising from extremisation of the effective action for pseudospin textures. We also calculate the different contributions to the energy of our bimerons, coming from pseudospin stiffness, capacitance and coulomb interactions between the merons. Apart from augmenting earlier results, this allows us to check how good an approximation it is to think of the bimeron as a pair of rigid objects (merons) with logarithmically growing energy, and with electric charge ${1 \over 2}$. Our differential equation approach also allows us to study the dependence of the spin texture as a function of the distance between merons, and the inter layer distance. Lastly, the technical problem of solving coupled nonlinear partial differential equations, subject to the special boundary conditions of bimerons is interesting in its own right.

cond-mat.mes-hall↗

arXiv1997

Meron Pseudospin Solutions in Quantum Hall Systems

In this paper we report calculations of some pseudospin textures for bilayer quantum hall systems with filling factor $ ν=1$. The textures we study are isolated single meron solutions. Meron solutions have already been studied at great length by others by minimising the microscopic Hamiltonian between microscopic trial wavefunctions. Our approach is somewhat different. We calculate them by numerically solving the nonlinear integro -differential equations arising from extremisation of the effective action for pseudospin textures. Our results can be viewed as augmenting earlier results and providing a basis for comparison.Our differential equation approach also allows us to dilineate the impact of different physical effects like the pseudospin stiffness and the capacitance energy on the meron solution.

cond-mat.mes-hall↗

arXiv1997

Generalised Chern-Simons Theory of Composite Fermions in Bilayer Hall Systems

We present a field theory of Jain's composite fermion model as generalised to the bilayer quantum Hall systems. We define operators which create composite fermions and write the Hamiltonian exactly in terms of these operators. This is seen to be a complexified version of the familiar Chern Simons theory. In the mean-field approximation, the composite fermions feel a modified effective magnetic field exactly as happens in usual Chern Simons theories, and plateaus are predicted at the same values of filling factors as Lopez and Fradkin and Halperin . But unlike normal Chern Simons theories, we obtain all features of the first-quantised wavefunctions including its phase, modulus and correct gaussian factors at the mean field level. The familiar Jain relations for monolayers and the Halperin wavefunction for bilayers come out as special cases.

cond-mat.mes-hall↗

arXiv1996

Umklapp Processes for Electrons and Their Renormalisation Group Flow

We study Umklapp couplings and their renormalisation group flow for electrons in a two dimensional lattice. It is shown that the effective low energy Hamiltonian involves not only forward scattering, but also considerable nonforward umklapp scattering, when the electrons fill a substantial fraction of the valence band. The behaviour of these couplings under renormalisation group is studied. It is shown that they remain marginal. We conclude with the possible consequences of this on properties of such electronic systems and their Fermi liquid theory.

cond-mat↗

arXiv1996

A Field Theory for the Read Operator

We introduce a new field theory for studying quantum Hall systems. The quantum field is a modified version of the bosonic operator introduced by Read. In contrast to Read's original work we do {\em not} work in the lowest Landau level alone, and this leads to a much simpler formalism. We identify an appropriate canonical conjugate field, and write a Hamiltonian that governs the exact dynamics of our bosonic field operators. We describe a Lagrangian formalism, derive the equations of motion for the fields and present a family of mean-field solutions. Finally, we show that these mean field solutions are precisely the Laughlin states. We do not, in this work, address the treatment of fluctuations.

cond-mat↗

arXiv1994

Landau Level Mixing and Solenoidal Terms in Lowest Landau Level Currents

We calculate the lowest Landau level (LLL) current by working in the full Hilbert space of a two dimensional electron system in a magnetic field and keeping all the non-vanishing terms in the high field limit. The answer a) is not represented by a simple LLL operator and b) differs from the current operator, recently derived by Martinez and Stone in a field theoretic LLL formalism, by solenoidal terms. Though that is consistent with the inevitable ambiguities of their Noether construction, we argue that the correct answer cannot arise naturally in the LLL formalism.

cond-mat↗

arXiv1993

Chiral Vertex Operators in Off-Conformal Theory: The Sine-Gordon Example

We study chiral vertex operators in the sine-Gordon [SG] theory, viewed as an off-conformal system. We find that these operators, which would have been primary fields in the conformal limit, have interesting and, in some ways, unexpected properties in the SG model. Some of them continue to have scale- invariant dynamics even in the presence of the non-conformal cosine interaction. For instance, it is shown that the Mandelstam operator for the bosonic representation of the Fermi field does {\it not} develop a mass term in the SG theory, contrary to what the real Fermi field in the massive Thirring model is expected to do. It is also shown that in the presence of the non-conformal interactions, some vertex operators have unique Lorentz spins, while others do not.

hep-th↗