arXiv subjects

S. Gayen

Publications and source records attributed to S. Gayen.

6 recordsLinked to original sources

arXiv2026

Quasi-Two-Dimensional Quantum Antiferromagnetism in the Distorted Honeycomb Compound KCuIn(PO4)2

We investigate the electronic structure and magnetic properties of the distorted honeycomb lattice compound KCuInP2O8 through a combination of experimental measurements, first principles calculations and quantum monte carlo simulations. Density functional theory calculations within the GGA+U framework establishes KCuInP2O8 as an indirect gap insulator with Cu2+ local moments and finite magnetocrystalline anisotropy arising from spin orbit coupling. A microscopic evaluation of magnetic exchange interactions using the magnetic force theorem reveals a pronounced hierarchy of couplings, with the next nearest neighbor interaction dominating over the nearest neighbor exchange, while interlayer couplings remain negligible. This exchange hierarchy naturally maps the system onto weakly coupled antiferromagnetic spin chains embedded in a distorted honeycomb lattice. Motivated by the ab initio estimated exchange interactions, we construct an effective spin half Hamiltonian and investigate its magnetic response using large scale quantum Monte Carlo simulations. The calculated temperature dependent susceptibility and field dependent magnetization quantitatively reproduce the experimental behavior and capture key signatures of low dimensional quantum magnetism, including a broad susceptibility maximum and a field induced saturation at low temperatures. Our results establish KCuInP2O8 as a quasi-two-dimensional quantum antiferromagnet composed of coupled spin chains, providing a consistent theoretical framework that links electronic structure, exchange interactions, and collective magnetic behavior.

cond-mat.str-el↗

arXiv2017

Possible multigap Type-I superconductivity in the layered Boride RuB$_2$

The structure of the layered transition-metal Borides $A$B$_2$ ($A =$ Os, Ru) is built up by alternating $T$ and B layers with the B layers forming a puckered honeycomb. Here we report superconducting properties of RuB$_2$ with a $T_c \approx 1.5$K using measurements of the magnetic susceptibility versus temperature $T$, magnetization $M$ versus magnetic field $H$, resistivity versus $T$, and heat capacity versus $T$ at various $H$. We observe a reduced heat capacity anomaly at $T_c$ given by $ΔC/γT_c \approx 1.1$ suggesting multi-gap superconductivity. Strong support for this is obtained by the successful fitting of the electronic specific heat data to a two-gap model with gap values $Δ_1/k_BT_c \approx 1.88$ and $Δ_2/k_BT_c \approx 1.13$. Additionally, $M$ versus $H$ measurements reveal a behaviour consistent with Type-I superconductivity. This is confirmed by estimates of the Ginzburg-Landau parameter $κ\approx 0.1$--$0.66$. These results strongly suggest multi-gap Type-I superconductivity in RuB$_2$. We also calculate the band structure and obtain the Fermi surface for RuB$_2$. The Fermi surface consists of one quasi-two-dimensional sheet and two nested ellipsoidal sheets very similar to OsB$_2$. An additional small $4^{\rm th}$ sheet is also found for RuB$_2$. RuB$_2$ could thus be a rare example of a multi-gap Type-I superconductor.

cond-mat.supr-con↗

arXiv2007

Raman spectra of unfilled and filled carbon nanotubes: Theory

The Raman spectra of two G-bands and a radial breathing mode (RBM) of unfilled and filled single-wall semiconducting and metallic carbon nanotubes have been investigated theoretically, in the presence of electron-phonon and phonon-phonon interactions. Excitation of low frequency optical plasmons in the metallic nanotube is responsible for the peak known as the Breit-Wigner-Fano (BWF) line shape in the G-band Raman spectra. In a filled nanotube there is an additional peak due to excitation of the phonon of the filling atom or molecule. Positions, shapes and relative strengths of these Raman peaks depend on the phonon frequencies of the nanotube and that of the filling atoms, and strengths and forms of the plasmon-phonon and phonon-phonon interactions. For example, filling atoms with phonon frequency close to the RBM frequency of the nanotube may broaden and lower the RBM Raman peak to such an extent that it may become barely visible. Hybridization between the G-bands and the filling atom phonon is also strong when these two frequencies are close to each other and it has important effects on the G-band and the BWF line shapes. When the phonon frequency of the filling atom is far from the RBM and G-band frequencies, it gives rise to a separate peak with modest effects on the RBM and G-band spectra. Raman spectra of semiconducting unfilled and filled nanotubes have similar behavior as those of metallic nanotubes except that normally they have Lorentzian line shapes and do not show a BWF line shape. However, if a semiconducting nanotube is filled with donor atoms, it is predicted that the BWF type line shape may be observed near the RBM, or the G-band or the filling atom Raman peak.

cond-mat.other↗

arXiv2005

Theory of the tangential G-band feature in the Raman spectra of metallic carbon nanotubes

The tangential G-band in the Raman spectra of a metallic single-wall carbon nanotube shows two peaks: a higher frequency component having the Lorentzian shape and a lower-frequency component of lower intensity with a Breit-Wigner-Fano (BWF)-type lineshape. This interesting feature has been analyzed on the basis of phonon-plasmon coupling in a nanotube. It is shown that while the gapless semi-acoustic plasmon cannot account for the observed spectra as claimed by other investigators, the low-lying optical plasmon corresponding to the tangential motion of the electrons on the nanotube surface can explain the observed features. In particular, this theory can explain occurrence of both the Lorentzian and BWF lineshapes in the G-band Raman spectra of metallic single-wall carbon nanotubes. Furthermore, the theory shows that the BWF peak moves to higher frequency, has a lower intensity and a lower half width at higher diameters of the nanotube. All these features are in agreement with experimental observations.

cond-mat.other↗

arXiv2005

Single hole doped strongly correlated ladder with a static impurity

We consider a strongly correlated ladder with diagonal hopping and exchange interactions described by $t-J$ type hamiltonian. We study the dynamics of a single hole in this model in the presence of a static non-magnetic (or magnetic) impurity. In the case of a non-magnetic (NM) impurity we solve the problem analytically both in the triplet (S=1) and singlet (S=0) sectors. In the triplet sector the hole doesn't form any bound state with the impurity. However, in the singlet sector the hole forms bound states of different symmetries with increasing $J/t$ values. Binding energies of those impurity-hole bound states are compared with the binding energy of a pair of holes in absence of any impurity. In the case of magnetic impurity the analytical eigenvalue equations are solved for a large (50 X 2) lattice. In this case also, with increasing $J/t$ values, impurity-hole bound states of different symmetries are obtained. Binding of the hole with the impurity is favoured for the case of a ferromagnetic (FM) impurity than in the case of antiferromagnetic (AFM) impurity. However binding energy is found to be maximum for the NM impurity. Comparison of binding energies and various impurity-hole correlation functions indicates a pair breaking mechanism by NM impurity.

cond-mat.str-el↗

arXiv2004

Plasmon exchange model for superconductivity in Carbon nanotubes

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zigzag nanotube can become superconducting at around 15 K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature $T_c$ less than 1K. To explain the observed higher critical temperature we explo re the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature $T_c$ in a nanotube. We find that $T_c$ is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons an d the radius of the nanotube. Our theoretical results can explain the observed $T_c$ in a nanotube.

cond-mat.supr-con↗