arXiv subjects

Ranjan Chaudhury

Publications and source records attributed to Ranjan Chaudhury.

At least 19 recordsLinked to original sources

arXiv2022

Some aspects of Microscopic Mechanism of Superconductivity in conventional and non-conventional systems

A broad review of theoretical research work involving different types of microscopic mechanism in various classes of superconductors, carried out in our research group over a decade or so, is presented. These mechanisms include both conventional as well as exotic ones. Special emphasis is placed on the possible applications to the experimental situations. Moreover, comparison of our works with various theoretical proposals made by various other researchers, with regard to high temperature superconductivity in particular, is made. The crucial importance and special significance of our results are highlighted.

cond-mat.supr-con↗

arXiv2022

A Theoretical Analysis of Superconducting Pairing in Correlated Metallic Systems

We have introduced a Coulomb correlated normal state to study the superconducting pairing with a Fermi liquid like normal phase background in a 3 dimensional system. The role of the Coulomb correlation has been actively incorporated by means of Gutzwiller projection scheme, in the presence of phonon mediated attractive electron-electron interaction throughout the entire regime of its applicability. We variationally modulate the Coulomb correlation strength from the weak to the strong regime in our calculational procedure. The main highlight of our results is the appearance of a '2-gap-like' structure in the superconducting phase, arising out of the two-body interacting matrix elements, calculated in the presence of the active Coulomb correlation. We have made use of the 'two-square well model' potential to evaluate the superconducting pairing gaps at zero temperature. Our Calculational scheme has been applied to a simple cubic lattice system for a physical realization. All the consequences are elaborated and discussed.

cond-mat.supr-con↗

arXiv2020

Interplay of Pairing Correlation and Coulomb Correlation in Boson Exchange Superconductors

A theoretical methodology for exploring the conventional Bardeen-Cooper-Schrieffer (BCS) pairing instability for superconductivity from a correlated normal phase for all possible degrees of many-body correlation, has been developed. The Gutzwiller projection scheme with a correlation parameter was made use of in generating the BCS pairing state. A variational scheme was thereafter implemented, leading to a self-consistent equation for superconducting gap function. This equation shows explicit dependence of the gap function on the many body correlation parameter. This `pairing-gap' and the corresponding self-consistent gap equation in zero correlation limit, becomes identical in nature with those of the pure (1-well) BCS formalism, as expected and the Coulomb correlation affects the pairing significantly with the strength of correlation. The detailed consequences are being presented here.

cond-mat.supr-con↗

arXiv2020

Study of effective coupling between charge degrees of freedom in low dimensional hole-doped quantum antiferromagnets

Expressions for generalized charge stiffness constant at zero temperature are derived corresponding to low dimensional hole doped quantum antiferromagnets, describable by the t-J-like models, with a view to understanding fermionic pairing possibilities and charge couplings in the itinerant antiferromagnetic systems. A detailed comparison between spin and charge correlations and couplings are presented in both strong and weak coupling limits. The result highlights that the charge and spin couplings show very similar behaviour in the over-doped region in both the dimensions, whereas they show a completely different trend in the lower doping regimes. A qualitative equivalence of generalized charge stiffness constant with the effective Drude weight and Coulomb interaction is established based on the comparison with other theoretical and experimental results. The fall in charge stiffness with increase in doping then implies reduction in the magnitude of effective Coulomb repulsion between the mobile carriers. This leads to an enhanced possibility of fermionic pairing with increase in doping in the possible presence of some other attraction producing mechanism from a source outside the t-J-like models. Moreover, under certain conditions in the weakly correlated phase, the t-J-like models themselves are able to produce attractive interaction for pairing.

cond-mat.str-el↗

arXiv2018

Theoretical investigation of the feasibility of electronic mechanism for superconducting pairing in overdoped cuprates with Fermi liquid like and Marginal Fermi liquid- like descriptions for the normal phase

Cooper's formalism for fermionic pairing has been revisited considering upto 3rd neighbour hopping terms, firstly with a Fermi liquid like background on a square lattice keeping in mind the overdopped cuprates. Then the whole scheme is repeated with a Marginal Fermi liquid- like background, taking into account the self energy correction of the mobile electrons to include a more realitic density of states in the calculation. Detailed comparison of our theoretical results with those from experiments strongly supports the Marginal Fermi liquid- like character of the normal phase with exciton mediated superconducting pairing in the concerned materials, in the lightly overdoped phase.

cond-mat.supr-con↗

arXiv2017

Effective interaction in a non-Fermi liquid conductor and spin correlations in under-doped cuprates

The effective interaction between the itinerant spin degrees of freedom in the paramagnetic phases of hole doped quantum Heisenberg antiferromagnets is investigated theoretically, based on the single-band t-J model on 1D lattice, at zero temperature. The effective spin-spin interaction for this model in the strong correlation limit, is studied in terms of the generalized spin stiffness constant as a function of doping concentration. The plot of this generalized spin stiffness constant against doping shows a very high value of stiffness in the vicinity of zero doping and a very sharp fall with increase in doping concentration, signifying the rapid decay of original coupling of semi-localized spins in the system. Quite interestingly, this plot also shows a maximum occurring at a finite value of doping, which strongly suggests the tendency of the itinerant spins to couple again in the unconventional paramagnetic phase. As the doping is further increased, this new coupling is also suppressed and the spin response becomes analogous to almost Pauli-like. The last two predictions of ours are quite novel and may be directly tested by independent experiments and computational techniques in future. Our results in general receive good support from other theoretical works and experimental results extracted from the chains of YBa$_2$Cu$_3$O$_{6+x}$.

cond-mat.str-el↗

arXiv2016

Calculation of generalized spin stiffness constant of strongly correlated doped quantum antiferromagnet on two-dimensional lattice and it's application to effective exchange constant for semi-itinerant systems

The generalized spin stiffness constant for a doped quantum antiferromagnet has been investigated both analytically and numerically as a function of doping concentration at zero temperature, based on the strongly correlated t-J model on two-dimensional square lattice. The nature of the theoretical dependence of the stiffness constant on doping shows a striking similarity with that of the effective exchange constant, obtained from the combination of other theoretical and experimental techniques in the low doping region. This correspondence once again establishes that spin stiffness can very well play the role of an effective exchange constant even in the strongly correlated semi-itinerant systems. Our theoretical plot of the stiffness constant against doping concentration in the whole doping region exhibits the various characteristic features like a possible crossover in the higher doping regions and persistence of short range ordering even for very high doping with the complete vanishing of spin stiffness occurring only close to 100% doping. Our results receive very good support from various other theoretical approaches and also brings out a few limitations of some of them. Our detailed analysis highlights the crucial importance of the study of spin stiffness for the proper understanding of magnetic correlations in a semi-itinerant magnetic system described by the strongly correlated t-J model. Moreover, our basic formalism can also be utilized for determination of the effective exchange constant and magnetic correlations for itinerant magnetic systems, in general in a novel way.

cond-mat.str-el↗

arXiv2016

Semi-phenomenological analysis of neutron scattering results for quasi-two dimensional quantum anti-ferromagnet

The available results from the inelastic neutron scattering experiment performed on the quasi-two dimensional spin $\frac{1}{2}$ anti-ferromagnetic material $La_2 Cu O_4$ have been analysed theoretically. The formalism of ours is based on a semi-classical like treatment involving a model of an ideal gas of mobile vortices and anti-vortices built on the background of the N$\acute{e}$el state, using the bipartite classical spin configuration corresponding to an XY- anisotropic Heisenberg anti-ferromagnet on a square lattice. The results for the integrated intensities for our spin $\frac{1}{2}$ model corresponding to different temperatures, show occurrence of vigorous unphysical oscillations, when convoluted with a realistic spectral window function. These results indicate failure of the conventional semi-classical theoretical model of ideal vortex/anti-vortex gas arising in the Berezinskii-Kosterlitz-Thouless theory for the low spin magnetic systems. A full fledged quantum mechanical formalism and calculations seem crucial for the understanding of topological excitations in such low spin systems. Furthermore, a severe disagreement is found to occur at finite values of energy transfer between the integrated intensities obtained theoretically from the conventional formalism and those obtained experimentally. This further suggests strongly that the full quantum treatment should also incorporate the interaction between the fragile-magnons and the topological excitations. This is quite plausible in view of the recent work establishing such a process in XXZ quantum ferromagnet on 2D lattice. The high spin XXZ quasi-two dimensional antiferromagnet like $MnPS_3$ however follows the conventional theory quite well

cond-mat.str-el↗

arXiv2016

Investigation of fermionic pairing on two- dimensional tight binding lattice under phonon and electronic mechanisms within a simple Fermi liquid like scenario and beyond- possible consequences for superconductivity in overdoped cuprates

Cooper's one pair problem is investigated for a 2D lattice in the background of both passive and active Fermi sea in a weakly correlated environment. Boson exchange mechanisms involving excitons as well as phonons are invoked for pairing in the s- wave channel. The important quantities calculated are pairing energy and coherence length as functions of bosonic energy, attracted coupling constant and band filling factor. Comparison of our theoretical results with those from experiments on overdoped cuprate superconductors and other type of theoretical calculations show electronic mechanism to be the more likely mechanism of pairing in the overdoped phase.

cond-mat.supr-con↗

arXiv2015

The connection between vortex-like topological excitations and conventional excitations in quantum ferromagnetic spin systems on two dimensional lattice and their stability

We present a scheme for the construction of quantum states of vortex like topological excitations corresponding to spin- 1/2 strongly XY anisotropic nearest neighbor Heisenberg Ferromagnet on two dimensional lattice. The procedure involving Pauli spin basis states is carried out corresponding to both infinite dilute limit and finite density limit of vortex/anti-vortex. It is found that the corresponding quantum mechanical states representing charge 1 quantum vortices/ anti-vortices can be expressed as linear combinations of single magnon states, composite multi-magnon states and the ground state. Detailed calculations show that these states are quantum mechanically stable states of the Hamiltonian only when the system size exceeds certain threshold value. Our analysis indicates that the interactions between different magnon modes can very well generate these topological excitations. Possible applications of our calculations to real magnetic systems are also discussed. Magnetic measurements probing spin dynamics may be undertaken to verify the existence of the threshold size for the stability of vortices.

cond-mat.str-el↗

arXiv2014

Investigation of fermionic pairing on tight binding lattice for low dimensional systems

Cooper's original one pair problem in continuum is revisited here corresponding to a lattice of tight binding nature, with an aim to investigate superconductivity in low dimensional systems. An electronic type of boson mediated attraction is considered for the pairing mechanism with the non trivial energy dependence of the electronic density of states taken into account in the calculation in a rigorous way. Some of the very important electronic and optical properties of a class of one dimensional organic conductors are used for the development of the formalism and calculation. The result of our calculations show that fermionic pair formation is indeed possible with some constraints. Similarities emerge in the physical properties of the electron pair formed from Cooper's treatment and ours excepting the striking difference appearing in the form of occurrence of a maximum allowed band filling for pairing and of an upper bound of the pairing energy found in our approach.

cond-mat.supr-con↗

arXiv2013

Synthesis of the Conventional Phenomenological Theories of Superconductivity with Marginal Fermi Liquid Model

In this work we have done phenomenology based model calculations for some of the thermodynamic and electrodynamic properties of the strongly correlated superconductors of Cuprate type. The method involves the application of the theoretical result for electronic specific heat in the normal phase from Marginal Fermi Liquid theory to the Gorter-Casimir two fluid model to derive the temperature dependence of the critical magnetic field corresponding to a type-I system, using the standard variational technique. We also applied this modified two fluid scheme to the London theory and obtained an expression for the temperature dependence of the magnetic field penetration depth in the superconducting phase. Our results are in fairly good agreement with other theoretical results based on different approaches, as well as with the experimental results.

cond-mat.supr-con↗

arXiv2013

Can Excitonic Mechanism Contribute Significantly to Superconducting Pairing in Quasi 1-D Systems?

We have revisited Cooper's one pair problem of calculating the binding energy for two electrons, experiencing an attractive interaction near the Fermi surface, in case of quasi one dimensional lattice system. Though it is a generalized formalism, we have chosen two materials viz., (TMTSF)2ClO4 and (TMTSF)2AsF6 as examples. Some of their electronic and optical features are used for calculations and validity this formalism is checked too for these materials. It is generally believed that spin density fluctuation mechanism is very strong candidate for the occurrence of superconductivity in these quasi one dimensional materials. We have attempted to invoke another type of electronic mechanism viz., charge transfer Exciton mechanism to explore if some substantial contribution for attractive coupling can be generated. This mechanism may be plausible in these types of organic materials where ionicity and co-valency co-exist. Making use of the experimental results for optical absorption spectra corresponding to above materials, we extracted the relevant parameters for incorporating in the binding energy calculation. Our calculation shows that Fermionic pair formation is possible for all the three cases viz., (1) less than half filled, (2) half filled and (3) more than half filled.

cond-mat.supr-con↗

arXiv2012

Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets

A theoretical study has been carried out to analyse the available results from the inelastic neutron scattering experiment performed on a quasi-two dimensional spin-1/2 ferromagnetic material $K_2CuF_4$. Our formalism is based on a conventional semi-classical like treatment involving a model of an ideal gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg ferromagnet on a square lattice. The results for dynamical structure functions for our model corresponding to spin-1/2, show occurrence of negative values in a large range of energy transfer even encompassing the experimental range, when convoluted with a realistic spectral window function. This result indicates failure of the conventional theoretical framework to be applicable to the experimental situation corresponding to low spin systems. A full quantum formalism seems essential for treating such systems.

cond-mat.str-el↗

arXiv2010

Physical realization and possible identification of topological excitations in quantum Heisenberg anti-ferromagnet on a two dimensional lattice

Physical spin configurations corresponding to topological excitations, expected to be present in the XY limit of a quantum spin 1/2 Heisenberg anti-ferromagnet, are probed on a two dimensional square lattice . Quantum vortices (anti-vortices) are constructed in terms of coherent staggered spin field components, as limiting case of meronic (anti-meronic) configurations . The crucial role of the associated Wess-Zumino-like (WZ-like) term is highlighted in our procedure . The time evolution equation of coherent spin fields used in this analysis is obtained by applying variational principle on the quantum Euclidean action corresponding to the Heisenberg anti-ferromagnet on lattice . It is shown that the WZ-like term can distinguish between vortices and anti-vortices only in a charge sector with odd topological charges. Our formalism is distinctly different from the conventional approach for the construction of quantum vortices (anti-vortices) .

cond-mat.stat-mech↗

arXiv2009

A Possible Theoretical Model For Studying Superconductivity In Fe-based Systems

A theoretical approach with a microscopic model is proposed for the observed "high temperature superconductivity" in the Iron-based compounds. The above scheme takes into account two important aspects viz. (i) superconducting transition close to magnetic ordering and (ii) the layered structure. From the calculation of the superconducting transition temperature, it is shown that in the Fe-based superconductors the magnetic mechanism for superconductivity, operating through the effective attractive Coulomb interaction within the framework of the Fermi Liquid theory, is highly plausible.

cond-mat.supr-con↗

arXiv2008

Physical realization and identification of topological excitations in quantum Heisenberg ferromagnet on lattice

Physical spin configurations corresponding to topological excitations expected to be present in the XY limit of a purely quantum spin 1/2 Heisenberg ferromagnet, are probed on a two dimensional square lattice. Quantum vortices (anti-vortices) are constructed in terms of coherent spin field components as limiting case of meronic (anti-meronic) configurations. The crucial role of the associated Wess-Zumino term is highlighted in our procedure. It is shown that this term can identify a large class of vortices (anti-vortices). In particular the excitations having odd topological charges form this class and also exihibit a self-similar pattern regarding the internal charge distribution. This manifestation of different behaviour of the odd and the even topological sectors is very prominent in the strongly quantum regime but fades away as we go to higher spins. Our formalism is distinctly different from the conventional approach for the construction of quantum vortices (anti-vortices).

cond-mat.stat-mech↗

arXiv2007

Physical realization and possible identification of topological excitations in quantum Heisenberg ferromagnet on lattice

Physical configurations corresponding to topological excitations present in the XY limit of a quantum spin 1/2 Heisenberg ferromagnet, are investigated on a two dimensional square lattice. Quantum vortices(anti-vortices) are constructed in terms of terms of coherent spin field components and the crucial role of the associated Wess-Zumino term is highlighted. It is shown that this term can identify a large class of vortices(anti-vortices). In particular the excitations with odd topological charge belonging to this class, are found to exhibit a self-similar pattern regarding the internal charge distribution. Our formalism is distinctly different from the coventional approach for the construction of quantum vortices(anti-vortices).

cond-mat.stat-mech↗