arXiv subjects

Raman Sharma

Publications and source records attributed to Raman Sharma.

14 recordsLinked to original sources

arXiv2025

On the T-linear resistivity of cuprates: theory

By partitioning the electronic system of the optimally doped cuprates in two electronic components: (1) mobile electrons on oxygen sub-lattice; and (2) localized spins on copper sub-lattice, and considering the scattering of mobile electrons (on oxygen sub-lattice) via generation of paramagnons in the localized sub-system (copper spins), we ask what should be the electron-paramagnon coupling matrix element $M_q$ so that T-linear resistivity results. This 'reverse engineering approach' leads to $|M_q|^2 \sim \frac{1}{q^2+\xi(T)^{-2}}$. We comment how can such exotic coupling emerge in 2D systems where short range magnetic fluctuations resides. In other words, the role of quantum criticality is found to be crucial. And the T-linear behaviour of resistivity demands that the magnetic correlation length scales as $\xi(T)\propto\frac{1}{T}$, which seems to be a reasonable assumption in the quantum critical regime of cuprates (that is, near optimal doping where T-linear resistivity is observed).

cond-mat.str-el↗

arXiv2023

Janus $β$-PdXY (X/Y = S, Se, Te) Materials with high Anisotropic Thermoelectric Performance

Two-dimensional (2D) materials have garnered considerable attention as an emerging thermoelectric (TE) material owing to their unique density of state (DOS) near the Fermi level. We investigate the TE performance of Janus $β$-PdXY (X/Y=S, Se, Te) monolayer materials as a function of carrier concentration and mid-temperature range (300 to 800 K) by combining density functional theory (DFT) and semi-classical Boltzmann transport theory. The phonon dispersion spectra and AIMD simulations confirm their thermal and dynamical stability. The transport calculation results reveal the highly anisotropic TE performance for both n and p-type Janus $β$-PdXY monolayers. Meanwhile, the coexistence of low phonon group velocity and converged scattering rate leads to lower lattice thermal conductivity (K_l) of 0.80 W/m K, 0.94 W/m K, and 0.77 W/m K along y-direction for these Janus materials. While the high TE power factor is attributed to the high Seebeck coefficient (S) and electrical conductivity, which is due to the degenerate top valance bands of these Janus monolayers. The combination of lower K_l and high-power factor at 300K (800 K) leads to an optimal figure of merit (ZT) as 0.68 (2.21), 0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe monolayers. To capture rational electron transport properties, the effects of acoustic phonon scattering ($τ$_ac), impurity scattering ($τ$_imp), and polarized phonon scattering ($τ$_polar) are included in the temperature-dependent electron relaxation time. These findings indicated that the Janus $β$-PdXY monolayers are promising candidates for TE conversion devices.

cond-mat.mtrl-sci↗

arXiv2022

3D intrinsic shapes of quiescent galaxies in observations and simulations

We study the intrinsic 3D shapes of quiescent galaxies over the last half of cosmic history based on their axial ratio distribution. To this end, we construct a sample of unprecedented size, exploiting multi-wavelength $u$-to-$K_s$ photometry from the deep wide area surveys KiDS+VIKING paired with high-quality $i$-band imaging from HSC-SSP. Dependencies of the shapes on mass, redshift, photometric bulge prominence and environment are considered. For comparison, the intrinsic shapes of quenched galaxies in the IllustrisTNG simulations are analyzed and contrasted to their formation history. We find that over the full $0 10^{11}\ M_{\odot}$, with the effect being most pronounced at lower redshifts. In TNG, the most massive galaxies feature the highest ex-situ stellar mass fractions, pointing to violent relaxation via mergers as the mechanism responsible for their 3D shape transformation. Larger differences between observed and simulated shapes are found at low to intermediate masses. At any mass, the most spheroidal quiescent galaxies in TNG feature the highest bulge mass fractions, and conversely observed quiescent galaxies with the highest bulge-to-total ratios are found to be intrinsically the roundest. Finally, we detect an environmental influence on galaxy shape, at least at the highest masses, such that at fixed mass and redshift quiescent galaxies tend to be rounder in denser environments.

astro-ph.GA↗

arXiv2022

Cool outflows in MaNGA: a systematic study and comparison to the warm phase

This paper investigates the neutral gas phase of galactic winds via the Na I D$λλ5890,5895$Å feature within $z \sim 0.04$ MaNGA galaxies, and directly compares their incidence and strength to the ionized winds detected within the same parent sample. We find evidence for neutral outflows in 127 galaxies ($\sim 5$ per cent of the analysed line-emitting sample). Na I D winds are preferentially seen in galaxies with dustier central regions and both wind phases are more often found in systems with elevated SFR surface densities, especially when there has been a recent upturn in the star formation activity according to the SFR$_{5Myr}$/SFR$_{800Myr}$ parameter. We find the ionized outflow kinematics to be in line with what we measure in the neutral phase. This demonstrates that, despite their small contributions to the total outflow mass budget, there is value to collecting empirical measurements of the ionized wind phase to provide information on the bulk motion in the outflow. Depending on dust corrections applied to the ionized gas diagnostics, the neutral phase has $\sim 1.2 - 1.8$ dex higher mass outflow rates ($\dot{M}_{out}$), on average, compared to the ionized phase. We quantify scaling relations between $\dot{M}_{out}$ and the strengths of the physical wind drivers (SFR, $L_{AGN}$). Using a radial-azimuthal stacking method, and by considering inclination dependencies, we find results consistent with biconical outflows orthogonal to the disk plane. Our work complements other multi-phase outflow studies in the literature which consider smaller samples, more extreme objects, or proceed via stacking of larger samples.

astro-ph.GA↗

arXiv2021

Density-Driven Resistance Response in $MnS_{2}$: Theory

A colossal insulator-to-metal transition in high-spin pyrite phase of $MnS_{2}$ has been experimentally observed \cite{colomns2}. There are two possibilities behind this colossal insulator-to-metal transition: (1) migration of $Mn$ electrons to unoccupied $S^{2-}_{2}$ antibonding states under pressure which leads to conducting ligand states and hence metallic transition, and (2) possibility of band crossing transition. We have analyzed this experimental obervation theoretically using a toy statistical model and found that the transition is due to the migration of electrons from the transition metal ions to the ligand sites (i.e. the possibility (1)). The calculated resistivity compares well with the experimental data within the fitting parameters of the model.

cond-mat.str-el↗

arXiv2020

Optical Anisotropy in Tellurene and its Janus Allotropes -- A first principle Study

Here, we present first principle study of structural, electronic, and optical properties of pristine and janus tellurene allotropes using density functional theory (DFT). The $α$, $β$, $δ$, and $η$ allotrope of pristine tellurene exhibit indirect band gap while $γ$ and $σ$ allotropes are metallic. The bandgap shows tunability in janus tellurene compared to pristine tellurene. A metallic to semiconductor transition occurs in janus $γ$, $σ$ and its allotropes. Dirac cone-like interesting feature has been observed for pristine $σ$ tellurene which remains preserved with the opening in energy gap for janus allotrope. In optical properties, imaginary part dielectric function and electron energy loss spectra show a blue shift in janus tellurene as compared to pristine form. Static dielectric constant is tunable in janus tellurene. All allotropes of tellurene are optically active in UV-Vis region with optical anisotropy in all three directions. We believe that our findings will provide useful pointers in the experimental fabrication of devices based on janus tellurene.

cond-mat.mtrl-sci↗

arXiv2019

Electronic Structure and Carrier Mobilities of Twisted Graphene Helix

We have investigated the effect of twisting on electronic band structure, effective mass and carrier mobilities of three prototypes of AGNRs (N=6, 7 & 8) using Density functional theory combined with Deformation potential theory and Effective mass approximation. It is shown that the influence of twisted modes electron-phonon interaction is smaller than stretching modes, nonetheless, twisting has a profound effect on effective mass and mobilities. Similar to earlier reported conclusion in which the ideal N=3n wide HAGNR potentially exhibit an electron mobility comparable to intrinsic graphene, we also found that the ideal N=3n+2 HAGNRs hole mobility reside more closely to intrinsic graphene which could be increased further through Fluorine passivation. Thus, the control of the ribbon width along with passivation and extent of deformation are of paramount importance for determining the n-type or p-type of ribbons. Also, because of strong response to torsional strain, the N=8 F passivated AGNRs are the most appropriate for mechanical and high frequency switching. Our results suggest that twisting a ribbon can be considered as a good alternative way for controlled manipulation of the band structure and carrier mobilities for applications in mechanical switching devices.

cond-mat.mes-hall↗

arXiv2019

A theory of resistivity in Kondo lattice materials: the memory function approach

We theoretically analyse D.C. resistivity($\rho$) in the Kondo-lattice model using the powerful memory function approach. The complete temperature evolution of $\rho$ is investigated using the W\"{o}lfle-G\"{o}tze expansion of the memory function. The resistivity in this model originates due to spin-flip magnetic scattering of conduction $s$-electron off the quasi-localized $d$ or $f$ electron spins. We find the famous resistivity upturn at lower temperature regime ($k_B T<<\mu_d$), where $\mu_d$ is the effective chemical potential of $d$-electrons. In the high temperature regime $(\mu_d<<k_B T)$ we discover that $\rho \propto T^{\frac{3}{2}}$. The worked out theory is quantitatively compared with experimental data and reasonably good agreement is found.

cond-mat.str-el↗

arXiv2019

Twisted Helical shaped Graphene Nano-Ribbons: Role of Symmetries and Passivation

The Hydrogen and Fluorine planar armchairs graphene nanoribbons (H and F AGNRs), subjected to twist deformation within fixed periodic boundary conditions, eventually morph to a helical conformations are investigated at few tractable points. Unlike structural properties, no effect of symmetries on mechanical properties is observed, though passivation does have a significant effect on mechanical as well as on electrical properties. Hookes law for severely twisted AGNRs indicates the high elasticity of H-AGNRs whereas the F-AGNRs shows plasticity after threshold torsional strain. Torsional stress($E_θ$) is approximated from the variation in total energy($ΔE$) with square of torsional strain($θ^4 Σ^4$). Further, the effect of passivation on the electronic properties of helical conformations with different torsional strain is decisive in metal-to-semimetal and semimetal-to-metal transition. The band gap response of narrow GNRs N=6, 7 and 8, within a fixed cell under sever twisting arranged itself in two group as (i) monotonously increasing for q=0,2 and (ii) decreasing for q=1, here q=mod(N,3) in effective strain space ($θ^2 Σ^2$). This trend has also been observed for Fluorine passivated AGNRs, though band gap of N=7 F-AGNRs drops from 0.95eV to 0.05eV at extreme torsional strain forming Dirac cone at K allows dissipation less transport for longer wavelength electrons.

cond-mat.mtrl-sci↗

arXiv2018

Resistivity of a $2d$ quantum critical metal

We calculate resistivity in the paramagnetic phase just above the curie temperature in a $2d$ ferromagnetic metal. The required dynamical susceptibility in the formalism of resistivity is calculated within the Random Phase Approximation(RPA). The mechanism of resistivity is magnetic scattering in which $s$-band electrons are scattered off the magnetic spin fluctuations of d-band electrons. We use the $s$-$d$ Hamiltonian formalism. We find that near the quantum critical point the resistivity in $2d$ scales as $T^{\frac{4}{3}}$, whereas in $3d$ it scales as $T^{\frac{5}{3}}$. In contrast to it, resistivity due to phonon scattering is given by $T^5$ in low temperature limit as is well known. Our RPA result agrees with the Self-Consistence Renormalisation(SCR) theory result.

cond-mat.str-el↗

arXiv2018

Quantum oscillations of the Stoner susceptibility: theory

Oscillatory effects in magnetic susceptibility of free electrons in a strong magnetic field is well known phenomenon and is well captured by Lifshitz-Kosevich formula. In this paper we point out similar oscillatory effects in Stoner susceptibility which makes the system to oscillate between paramagnetic phase and ferromagnetic phase alternatively as a function of external magnetic field strength. This effect can happen in a material which is tuned near to its magnetic instability. We suggest an experimental set-up to observe this effect. We also suggest that our result can be exploited to control a quantum critical system around its quantum critical point to study its thermodynamical or transport properties.

cond-mat.str-el↗

arXiv2015

Mesoscopic Quantum Superposition of Generalized Cat State: A Diffraction limit

The orthogonality of cat and displaced cat states, underlying Heisenberg limited measurement in quantum metrology, is studied in the limit of large number of states. The mesoscopic superposition of the generalized cat state is correlated with the corresponding state overlap function, controlled by the sub-Planck structures arising from phase space interference. The asymptotic expression of this overlap function is evaluated and the validity of large phase space support and distinguishability of the constituent states, in which context the asymptotic limit is achieved, are discussed in detail. For large number of coherent states, uniformly located on a circle, the overlap function significantly matches with the diffraction pattern for a circular ring source with uniform angular strength. This is in accordance with the van Cittert-Zernike theorem, where the overlap function, similar to the mutual coherence function, matches with a diffraction pattern. The physical situation under consideration is delineated in phase space by utilizing Husimi-Q function.

quant-ph↗

arXiv2012

Diffraction limit of the sub-Planck structures

The orthogonality of cat and displaced cat states, underlying Heisenberg limited measurement in quantum metrology, is studied in the limit of large number of states. The asymptotic expression for the corresponding state overlap function, controlled by the sub-Planck structures arising from phase space interference, is obtained exactly. The validity of large phase space support, in which context the asymptotic limit is achieved, is discussed in detail. For large number of coherent states, uniformly located on a circle, it identically matches with the diffraction pattern for a circular ring with uniform angular source strength. This is in accordance with the van Cittert-Zernike theorem, where the overlap function, similar to the mutual coherence function matches with a diffraction pattern.

quant-ph↗

arXiv2010

Diffraction limit of the sub-Planck structures

The orthogonality of cat and displaced cat states, underlying Heisenberg limited measurement in quantum metrology, is studied in the limit of large number of states. The asymptotic expression for the corresponding state overlap function, controlled by the sub-Planck structures arising from phase space interference, is obtained exactly. For large number of coherent states, uniformly located on a circle, it identically matches with the diffraction pattern for a circular ring with uniform angular source strength. This is in accordence with the van Cittert-Zernike theorem, where the overlap function, similar to the mutual coherence function matches with a diffraction pattern. Interestingly, the convergence to asymptotic limit is quite rapid, reminiscent of the convergence of superposed incoherent sources.

quant-ph↗