arXiv subjects

Chiranjib Mitra

Publications and source records attributed to Chiranjib Mitra.

At least 37 records · Page 2Linked to original sources

arXiv2020

S-band electron spin resonance spectroscopy using a short-circuited coplanar waveguide resonator

In this work, we study the development of a coplanar waveguide (CPW) resonator and its use in an electron spin resonance (ESR) spectrometer. The CPW resonator is designed to operate in S-band. It has a short circuit configuration which leads to miniaturization. It is so constructed such that it has a characteristic impedance of 50 ohms. Detailed electromagnetic simulation with a particular emphasis on the excitation of the structure has been performed for this resonator owing to its uniplanar nature. The design parameters and the electromagnetic field distribution are obtained from the simulation. The resonator is fabricated using optical lithography with a rapid prototyping technique. The characteristic response of the resonator is measured by coupling it to a Vector Network Analyzer (VNA). The ESR absorption spectrum of free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) is captured by using this resonator in reflection geometry. The microwave magnetic field distribution at the sample position is investigated.The measured g-factor value is found to be consistent with that reported in the literature. The quality factor of this resonator is found to be low and this makes it suitable for use in a Pulsed ESR spectrometer.

physics.app-ph↗

arXiv2020

Development of planar microstrip resonators for electron spin resonance spectroscopy

This work focuses on the development of planar microwave resonators which are to be used in electron spin resonance spectroscopic studies. Two half wavelength microstrip resonators of different geometrical shapes, namely straight ribbon and omega, are fabricated on commercially available copper clad microwave laminates. Both resonators have a characteristic impedance of 50 Ω. We have performed electromagnetic field simulations for the two microstrip resonators and have extracted practical design parameters which were used for fabrication. The effect of the geometry of the resonators on the quasi-transverse electromagnetic (quasi-TEM) modes of the resonators is noted from simulation results. The fabrication is done using optical lithography technique in which laser printed photomasks are used. This rapid prototyping technique allows us to fabricate resonators in a few hours with accuracy up to 6 mils. The resonators are characterized using a Vector Network Analyzer. The fabricated resonators are used to standardize a home built low-temperature continuous wave electron spin resonance (CW-ESR) spectrometer which operates in S-band, by capturing the absorption spectrum of the free radical DPPH, at both room temperature and 77 K. The measured value of g-factor using our resonators is consistent with the values reported in literature. The designed half wavelength planar resonators will be eventually used in setting up a pulsed electron spin resonance spectrometer by suitably modifying the CW-ESR spectrometer.

physics.app-ph↗

arXiv2020

Magnetocaloric effect as a signature of quantum level-crossing for a spin-gapped system

Recent research dealing with magnetocaloric effect (MCE) study of antiferromagnetic (AFM) low dimensional spin systems have revealed a number of fascinating ground-state crossover characteristics upon application of external magnetic field. Herein, through MCE investigation we have explored field-induced quantum level-crossing characteristics of one such spin system: NH4CuPO4.H2O (NCP), an AFM spin 1/2 dimer. Experimental magnetization and specific heat data are presented and the data have been employed to evaluate entropy, magnetic energy and magnetocaloric properties. We witness a sign change in magnetic Grueneisen parameter across the level-crossing field B_C. An adiabatic cooling is observed at low temperature by tracing the isentropic curves in temperature-magnetic field plane. Energy-level crossover characteristics in NCP interpreted through MCE analysis are well consistent with the observations made from magnetization and specific heat data.

cond-mat.str-el↗

arXiv2019

Pulsed laser deposition of highly c-axis oriented thin films of BSTS topological insulator

We report the growth of highly c-axis oriented topological insulator (TI) BiSbTe1.5Se1.5 (BSTS) thin films by pulsed laser deposition (PLD) technique. The various growth parameters such as substrate temperature, Argon pressure in the deposition chamber and target to substrate distance are tuned to obtain the optimized conditions essential for stoichiometric and bulk insulating TI thin films. These films are highly c-axis oriented and exhibit all the four Raman modes characteristic to the R-3m space group. The quality of the deposited thin films is investigated using X-ray diffraction for crystallinity, Raman spectroscopy for lattice dynamics, morphological studies using scanning electron microscope and compositional analysis using Energy dispersive X-ray spectroscopy. Resistance vs temperature measurements confirm bulk insulating nature of the prepared thin films and magnetoresistance data exhibits the phenomena of weak antilocalization with a large phase coherence length.

cond-mat.mtrl-sci↗

arXiv2019

Signature of Quantum Entanglement in NH4CuPO4.H2O

Entangled solid state systems have gained a great deal of attention due to their fruitful applications in modern quantum technologies. Herein, detection of entanglement content from experimental magnetic susceptibility and specific heat data is reported for NH4CuPO4.H2O in its solid state crystalline form. NH4CuPO4.H2O is a prototype of Heisenberg spin 1/2 dimer system. Temperature dependent magnetic susceptibility and specific data are fitted to an isolated dimer model and the exchange coupling constant is determined. Field dependent magnetization isotherms taken at different temperatures are plotted in a three dimensional plot. Subsequently, entanglement is detected both from susceptibility and specific heat through two different entanglement measures; entanglement witness and entanglement of formation. The temperature evolution of entanglement is studied and the critical temperature is determined up to which entanglement exists. Temperature dependent nature of entanglement extracted from susceptibility and specific heat shows good consistency with each other. Moreover, the field dependent entanglement is also investigated.

cond-mat.str-el↗

arXiv2018

Investigation of Thermodynamic Properties of Cu(NH3)4SO4.H2O, a Heisenberg Spin Chain Compound

Detailed experimental investigations of thermal and magnetic properties are presented for Cu(NH3)4SO4.H2O, an ideal uniform Heisenberg spin half chain compound. A comparison of these properties with relevant spin models is also presented. The temperature dependent magnetic susceptibility and specific heat data has been compared with the exact solution for uniform Heisenberg chain model derived by means of Bethe ansatz technique. Field dependent isothermal magnetization curves are simulated by Quantum Monte Carlo technique and compared with the corresponding experimental ones. Specific heat as a function of magnetic field (up to 7T) and temperature (down to 2K) is reported. Subsequently, the data are compared with the corresponding theoretical curves for the infinite Heisenberg spin half chain model with J=6K. Moreover, internal energy and entropy are calculated by analyzing the experimental specific heat data. Magnetic field and temperature dependent behavior of entropy and internal energy are in good agreement with the theoretical predictions.

cond-mat.str-el↗

arXiv2017

Linear magnetoresistance and surface to bulk coupling in topological insulator thin films

We explore the temperature dependent magnetoresistance of bulk insulating topological insulator thin films. Thin films of Bi2Se2Te and BiSbTeSe1.6 were grown using Pulsed Laser Deposition technique and subjected to transport measurements. Magnetotransport measurements indicate a non-saturating linear magnetoresistance (LMR) behavior at high magnetic field values. We present a careful analysis to explain the origin of LMR taking into consideration all the existing models of LMR. Here we consider that the bulk insulating states and the metallic surface states constitute two parallel conduction channels. Invoking this, we were able to explain linear magnetoresistance behavior as a competition between these parallel channels. We observe that the crossover field, where LMR sets in, decreases with increasing temperature. We propose that this cross over field can be used phenomenologically to estimate the strength of surface to bulk coupling.

cond-mat.str-el↗

arXiv2016

Bulk Saturable absorption in Topological Insulator thin Films

We present nonlinear optical absorption properties of pulsed laser deposited thin films of topological insulator (TI), Bi2Se3 on quartz substrate, using open aperture Z - scan technique. The saturable intensity of as-deposited thin films has been found remarkably improved by an order of magnitude compared to the values reported earlier in the literature. Past results from the literature are inconclusive in establishing whether the saturable absorption is coming from surface states or the bulk. Specifically designed experiments with magnetically doped TI samples allow us to attribute the saturable absorption characteristic of TI to the bulk states. Detailed experimental procedures and possible explanation of observed results have been discussed.

cond-mat.str-el↗

arXiv2016

Topological delocalization and tuning of surface channel separation in Bi2Se2Te Topological Insulator Thin films

The surface states of a 3D topological insulator (TI) exhibit topological protection against backscattering. However, the contribution of bulk electrons to the transport data is an impediment to the topological protection of surface. We report the tuning of the chemical potential in the bulk of Bi2Se2Te TI thin films, pinning it near the center of the bulk band gap, thereby suppressing the bulk carriers. The temperature dependent resistance of these films shows activated behavior down to 50K, followed by a metallic transition at lower temperatures, a hallmark of the robustness of TI surface states. Manifestation of topological protection and surface dominated transport is explained by 2D weak antilocalization phenomenon. We further explore the effect of surface to bulk coupling in TI in this work, which is captured by the number of effective conducting surface channels that participate in the transport. The presence of a single conducting channel indicates a strong surface to bulk coupling which is detrimental to purely topological transport. We demonstrate the decoupling of topological surface states on opposite surfaces of thin films, thereby suppressing the bulk transport. Our findings provide a deeper understanding of surface to bulk coupling along with topological transport behavior and their respective tunability.

cond-mat.str-el↗

arXiv2016

Tuning Chemical Potential in the Dirac Cone by Compositional Engineering

We report the successful formation of bulk insulating ternary topological insulators candidate Bi2Se2Te (BST) by pulsed laser deposition technique. The films were deposited with sequential ablation of separate Bi2Se3 (BS) and Bi2Te3 (BT) targets. From the X-ray diffraction analysis and temperature dependent resistivity we were able to conclude that the as grown thin films have ordered chalcogen layers and the chemical potential in these thin films lie in the bulk gap. To realize entirely topological transport for any device applications it is essential to tune the chemical potential in the bulk gap of the Dirac cone. Magnetotransport data exhibits pronounced two dimensional weak-antilocalization behavior (WAL) at low temperatures. BS and BT thin films do not exhibit topological transport as the chemical potential does not lie entirely in the bulk gap. It was found that BST thin films grown with this cost effective and simple yet elegant technique using double target can be used to deposit quaternary TI thin films, thereby tuning the chemical potential at will in the gap.

cond-mat.mes-hall↗

arXiv2015

Surface Optical and Bulk Acoustic Phonons in the Topological Insulator, Bi2Se2Te

We explore the phonon dynamics of thin films of the topological insulator material Bi2Se2Te using ultrafast pump-probe spectroscopy. The time resolved differential reflectivity of the films exhibit fast and slow oscillations. We have given a careful analysis of variation of phonon frequency as a function of film thickness attributing this to existence of standing acoustic modes. However, no variation in the frequency of the optical phonon modes was found with film thickness. This indicates that the optical phonons intrinsically belong to the surface of the topological insulators. The fact that the acoustic phonons can be tuned by changing the film thickness has tremendous potential for room temperature low power spintronic devices and in topological quantum computation.

cond-mat.mes-hall↗

arXiv2014

Weak-antilocalization and Surface Dominated Transport in Topological Insulator Bi2Se2Te

We explore the phase coherence of thin films of the topological insulator material Bi2Se2Te grown through pulsed laser deposition (PLD) technique. The films were characterised using various techniques for phase and composition. The films were found to be of good quality. We carried out extensive magneto-transport studies of these films and found that they exhibit two dimensional weak antilocalization behaviour. A careful analysis revealed a relatively high phase coherence length (58nm at 1.78K) for a PLD grown film. Since PLD is an inexpensive technique, with the possibility to integrate with other materials, one can make devices which can be extremely useful for low power spintronics and topological quantum computation.

cond-mat.mes-hall↗

arXiv2014

Experimental Evidences of Quantum Phase Transition in a Spin Cluster Compound

Experimental realization of magnetic field induced quantum phase transition (QPT) is reported for NH4CuPO4.H2O, a two spin cluster material with isotropic Heisenberg interaction. Experimental magnetization and specific heat data have been collected as a function of temperature and magnetic field. Experimental data have been analyzed in terms of Heisenberg dimer model. Two quantum complementary observables representing local and non-local properties of the spins are constructed using the experimental data and a clear evidence of QPT is observed through partial quantum information sharing when the magnetic field is swept through a particular value. Signature of QPT is also observed when specific heat is measured as a function of magnetic field at low temperature. Furthermore, using the experimental specific heat data, magnetic energy values are calculated and their variations are captured as a function of magnetic field and temperature.

cond-mat.str-el↗

arXiv2012

Quantification of Entanglement from Magnetic Susceptibility for a Heisenberg Spin 1/2 system

Temperature and magnetic field dependent magnetization and extraction of entanglement from the experimental data is reported for dichloro(thiazole)copper(II), a Heisenberg spin chain system. The magnetic susceptibility vs. temperature plot exhibits signature of infinite spin chain. Isothermal magnetization measurements (as a function of magnetic field) were performed at various temperatures below the antiferromagnetic (AFM) ordering, where the AFM correlations persist significantly. These magnetization curves are fitted to the theoretically generated Bonner-Fisher model. Magnetic susceptibility can be used as a macroscopic witness of entanglement; using which entanglement is extracted from the experimentally measured magnetic data.

cond-mat.str-el↗

arXiv2012

Comparative Study of Magnetic Behaviour in Three Classic Molecular Magnets

We have studied the magnetic properties of three phenalenyl based organic neutral radicals. The first one is a Heisenberg chain antiferromagnet with one unpaired spin per molecule; second one is a diamagnetic, exhibiting a diamagnetic to paramagnetic phase transition at high temperature; the third one comprises of free neutral radicals and shows paramagnetic behaviour. Temperature dependent magnetic susceptibility measurements and isothermal magnetization measurements (as a function of magnetic field) were performed on all the three systems. In the case of the antiferromagnetic system, temperature dependent susceptibility and magnetization isotherms were fitted to the Bonner Fisher model. In the case of second system the diamagnetic to paramagnetic phase transition is investigated by performing isothermal magnetization measurements in the two different phases. The diamagnetic to paramagnetic phase transition seems to be of first order in nature.

cond-mat.str-el↗

arXiv2012

Magnetization study to probe the effect of N2+ irradiation on Fe/Co bilayers

Fe/Co bilayers were made using ion beam sputtering and then were subjected to nitrogen-ion irradiation to explore the influence of irradiation on their magnetic properties. Magnetization studies of Fe/Co bilayers along with their irradiated counterparts were then performed. The magnetization and coercivity of the as-deposited sample exhibited a two-fold uniaxial symmetry while the behavior for high temperature irradiated sample depicts a small shift in the two fold behavior. The magnetization data measured with the magnetic field perpendicular to the plane of the film exhibited a hysteresis in both the case of the irradiated sample as well as as-deposited sample. A small increase in coercivity for the irradiated sample was observed which is due to various reasons including formation of FeCo solid solution due to ion irradiation at the interface.

cond-mat.str-el↗

arXiv2011

Experimental detection of quantum information sharing and their quantification in quantum spin systems

We study the macroscopic entanglement properties of a low dimensional quantum spin system by investigating its magnetic properties at low temperatures and high magnetic fields. The tempera- ture and magnetic field dependence of entanglement from the susceptibility and magnetization data comparing the experimental extraction with theoretical estimates are given. Extraction of entan- glement has been made possible through the macroscopic witness operators magnetic susceptibility. The protocol followed in doing so has been outlined in some recent work. Various plots of entan- glement comparing the experimental extraction with theoretical estimates are given. Quantitative comparison between concurrence and entanglement witness is given for both the theoretical and experimental results. Theory and experiments match over a wide range of temperature and field. The spin system studied is a chain, which exhibits dimerisation and yields fascinating entanglement properties when the temperature and magnetic field is varied. These spin systems exhibit quantum phase transition (QPT) at low temperatures, when the magnetic field is swept through a critical value. We show explicitly for the first time, using tools used in quantum information processing (QIP), that quantum phase transition (QPT) can be captured experimentally using canonically conjugate observables. Macroscopically, quantum complementarity relation clearly delineates entangled states from separable states across the QPT. We have estimated the partial information sharing in this system from our magnetization and susceptibility data. The complementarity relation has been experimentally verified to hold in this system

quant-ph↗

arXiv2009

Multipartite entangled magnon states as quantum communication channels

We explicate conditions under which, the two magnon state becomes highly entangled and is useful for several quantum communication protocols. This state, which is experimentally realizable in quantum dots using Heisenberg exchange interaction, is found to be suitable for carrying out deterministic teleportation of an arbitrary two qubit composite system. Further, conditions for which the channel capacity reaches "Holevo bound", allowing maximal amount of classical information to be encoded, are derived. Later, an explicit protocol for the splitting and sharing of a two qubit entangled state among two parties, using this state as an entangled resource, is demonstrated.

quant-ph↗