arXiv subjects

Debanand Sa

Publications and source records attributed to Debanand Sa.

13 recordsLinked to original sources

arXiv2026

Hubbard model at U=$\infty$: Role of single and two-boson fluctuations

We have developed a semi-analytical framework formulated in the canonical fermion representation to investigate strongly correlated electron systems. We consider the U=$\infty$ Hubbard model and used the equation of motion method to calculate the fermion self-energy which has two parts: single and two-boson exchange processes. The emergent bosons here are self-generated local charge and spin-density fluctuations which become strongly time-dependent due to extreme correlations. The computed boson spectral density is a diffusive damped mode with a long tail. The electron self-energy at $d=\infty$ is computed self-consistently. The corresponding fermionic spectral density displays a pronounced coherence peak at $\omega=0$, while its frequency derivative develops a two-peak structure at finite $\omega$. The resistivity shows a linear temperature dependence over a broad range, crossing over to coherent Fermi-liquid behavior at extremely low temperatures.

cond-mat.str-el↗

arXiv2017

Chiral magnetic effect and Maxwell-Chern-Simons electrodynamics in Weyl semimetals

The Weyl semimetal, due to a non-zero energy difference in the pair of Weyl nodes shows chiral magnetic effect(CME). This leads to a flow of dissipationless electric current along an applied magnetic field. Such a chiral magnetic effect in Weyl semimetals has been studied using the laws of classical electrodynamics. It has been shown that the CME in a such a semimetal changes the properties namely, frequency dependent skin depth, capacitive transport, plasma frequency etc. in an unconventional way as compared to the conventional metals. In the low frequency regime, the properties are controlled by a natural length scale due to CME called the chiral magnetic length. Further, unlike the conventional metals, the plasma frequency in this case is shown to be strongly magnetic field-dependent. Since the plasma frequency lies below the optical frequency, the Weyl semimetals will look transparent. Such new and novel observations might help in exploiting these class of materials in potential applications which could completely change the future technology.

cond-mat.str-el↗

arXiv2015

Thermoelectric transport in the topological phase due to the coexistence of superconductivity and spin-density-wave

We study the thermoelectric transport in two dimensional topological system which has coexistence of superconductivity(SC) and spin-density wave(SDW). The SC is presumed to be of $d_{x^2-y^2}+(p_x + i p_y) $ type whereas the SDW order parameter is of $BCS$ symmetry. The Hamiltonian describing such a coexistence phase is shown to have topological phase in addition to the conventional one. The transport properties in such topological system have two distinct contributions: (i) the surface/edge and (ii) the bulk. The competition between the surface/edge versus the bulk transport is analyzed in different parameter regimes and the possibility of enhancing the figure of merit is discussed.

cond-mat.supr-con↗

arXiv2015

Topological phase in a $d_{x^2-y^2}+(p+ip)$ superconductor in presence of spin-density-wave

We consider a mean-field Hamiltonian for a $d_{x^2-y^2}+(p+ip)$ superconductor(SC) in presence of spin-density-wave(SDW) order. This is due to the fact that the non-commutativity of any two orders produces the third one. The energy spectrum of such a Hamiltonian is shown to be gapped and it yields a topological phase in addition to the conventional one. A phase diagram characterizing different topological phases is construted. The Chern numbers and hence the nature of the topological phases are determined. The edge state spectrum and the possibility of whether the vortex state harbouring the zero modes are discussed.

cond-mat.supr-con↗

arXiv2014

Topological phases due to the coexistence of superconductivity and spin-density wave: Application to high Tc superconductors in the underdoped regime

We consider the coexistence of superconductivity(SC) and spin-density wave(SDW). The SC is presumed to be of $d_{x^2-y^2}+id_{xy} (d_1 + i d_2) $ type whereas the SDW order parameter is of $BCS$/$d_{x y} $ symmetry. The Hamiltonian having such a structure is shown to have topological coexistence phases in addition to the conventional one. It is shown that the amplitudes of both the order parameters determine the nature of topological phases. A phase diagram characterizing different topological phases with their Chern numbers are obtained. The experimental realization of such topological phases with reference to high temperature superconductors in the extreme underdoped regime are discussed.

cond-mat.supr-con↗

arXiv2004

Is Li$_2$Pd$_3$B a self-doped hole superconductor ?

We propose that the electrons responsible for superconductivity in Li2Pd3B come from the palladium 4d-electrons. So, its electronic properties are likely to be dominated by strong electronic correlations. The basic unit in this material are Pd$_6$B octahedra which share vertices to form a 3-dimensional network. Due to the highly distorted nature of the Pd$_6$B octahedron, one far stretched Pd atom per octahedra becomes almost inactive for electronic conduction. Thus, the material escapes the fate of becoming a half- filled insulating Mott antiferromagnet by hiding extra charges at these inactive Pd sites and becomes a self-doped correlated metal. We propose a 3-dimensional single band t-J model which could be the correct minimal model for this material.

cond-mat.supr-con↗

arXiv2003

Superconductivity in Na$_x$CoO$_2$$\cdot$yH$_2$O : Is Spin-Charge Separation Protecting a d$_1$+id$_2$ State ?

Superconductivity in Na$_x$CoO$_2$$\cdot$yH$_2$O is likely to be a p or d-wave; however, experiments are unable to pinpoint the symmetry. A simple estimate of pair breaking effects from an unavoidable `Na$^+$ vacancy disorder' in an ordered Na$^+$ lattice, at an optimal $x_{\rm opt} \approx 0.30$ is shown to destroy a Fermi liquid based p or d-wave superconductivity. However, a robustness of superconducting and normal states, seen in experiments is pointed out and argued to imply presence of a `quantum protectorate', possibly a `spin-charge decoupling' that protects a d$_1$+id$_2$ and not a p-state. A calculation of Knight shift and ${1\over T_1}$ in the framework of RVB mean field theory and a fit to the data of Kobayashi ${\it et al.}$ [9] is made.

cond-mat.supr-con↗

arXiv2003

Fermi surface topology and ferromagnetic superconductivity in UGe$_2$

We consider a Stoner ferromagnet in presence of a quasi-one dimensional Fermi surface in its spin majority band. Assuming a twin $δ$-function peaked density of state due to the low dimensionality, we computed the single particle self-energy. There appears an additional divergence in the self-energy and hence in the effective mass inside the ferromagnetic phase (besides the standard logarithmic divergence at the Stoner critical point). Since such an effect is purely due to density of states, it might correspond to a first order phase transition making the ferromagnetic phase into two distinct phases. This result is in qualitative agreement with the recent specific heat capacity measurement. We also discuss its relevance to the superconducting state in UGe$_2$.

cond-mat.str-el↗

arXiv2002

Hidden Quantum Critical Point in a Ferromagnetic Superconductor

We consider a coexistence phase of both Ferromagnetism and superconductivity and solve the self-consistent mean-field equations at zero temperature. The superconducting gap is shown to vanish at the Stoner point whereas the magnetization doesn't. This indicates that the para-Ferro quantum critical point becomes a hidden critical point. The effective mass in such a phase gets enhanced whereas the spin wave stiffness is reduced as compared to the pure FM phase. The spin wave stiffness remains finite even at the para-Ferro quantum critical point.

cond-mat.supr-con↗

arXiv2002

Pairbreaking Without Magnetic Impurities in Disordered Superconductors

We study analytically the effects of inhomogeneous pairing interactions in short coherence length superconductors, using a spatially varying Bogoliubov-deGennes model. Within the Born approximation, it reproduces all of the standard Abrikosov-Gor'kov pairbreaking and gaplessness effects, even in the absence of actual magnetic impurities. For pairing disorder on a single site, the T-matrix gives rise to bound states within the BCS gap. Our results are compared with recent scanning tunneling microscopy measurements on Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ with Zn or Ni impurities.

cond-mat.supr-con↗

arXiv2000

Effective Charge and Spin Hamiltonian for the Quarter-Filled Ladder Compound $α'$-NaV$_2$O$_5$

An effective intra- and inter-ladder charge-spin hamiltonian for the quarter-filled ladder compound $α'$-NaV$_2$O$_5$ has been derived by using the standard canonical transformation method. In the derivation, it is clear that a finite inter-site Coulomb repulsion is needed to get a meaningful result otherwise the perturbation becomes ill-defined. Various limiting cases depending on the values of the model parameters have been analyzed in detail and the effective exchange couplings are estimated. We find that the effective intra-ladder exchange may become ferromagnetic for the case of zig-zag charge ordering in a purely electronic model. We estimate the magnitude of the effective inter-rung Coulomb repulsion in a ladder and find it to be about one-order of magnitude too small in order to stabilize charge-ordering.

cond-mat↗

arXiv1999

A Generalized Ginzburg-Landau Approach to Second Harmonic Generation

We develop a generalized Ginzburg-Landau theory for second harmonic generation (SHG) in magnets by expanding the free energy in terms of the order parameter in the magnetic phase and the susceptibility tensor in the corresponding high-temperature phase. The non-zero components of the SHG susceptibility in the ordered phase are derived from the symmetries of the susceptibility tensor in the high-temperature phase and the symmetry of the order parameter. In this derivation, the dependence of the SHG susceptibility on the order parameter follows naturally, and therefore its nonreciprocal optical properties. We examine this phenomenology for the magnetoelectric compound Cr$_2$O$_3$ as well as for the ferroelectromagnet YMnO$_3$.

cond-mat↗

arXiv1994

Effect of Single Particle Hopping and out of Plane Magnetic Impurity on Coupled Planar Superconductors

It is shown that the single particle band motion along the $c$ axis is harmful for superconductivity. Variation of $T_c$ with $c$ axis hopping parameter is shown for both the conventional planar superconductors and for interlayer pair tunneling mechanism of Wheatley Hsu and Anderson(WHA).Effect of out of plane magnetic impurity substitution is shown to suppres $T_c$ more for conventional superconductors whereas there is very sharp decrease of $T_c$ in the WHA mechanism at larger concentrations.

cond-mat↗