Rishabh Shukla

Bio: Rishabh Shukla is an academic researcher from Indian Institute of Technology Delhi. The author has contributed to research in topics: Rietveld refinement & Magnetization. The author has an hindex of 9, co-authored 27 publications receiving 170 citations.

Anomalous magnetic and spin glass behavior in Nb-substituted LaCo 1 − x Nb x O 3

Abstract: We report the structural, magnetic, transport, and electronic properties of Nb-substituted ${\mathrm{LaCo}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ ($x=0--0.2$). The Rietveld refinement of x-ray diffraction data demonstrate structural phase transitions from rhombohedral to orthorhombic and further to monoclinic with increasing the Nb concentration up to $x\ensuremath{\ge}0.2$. Interestingly, we observed dramatic changes in the magnetization ($M$) with increasing the Nb concentration, as the $M$ sharply increases below 10 K even at 2.5% substitution. Furthermore, ac susceptibility data show the spin glass behavior in $x=0.1$ sample. We find that the density of states near the Fermi level decreases and the activation energy increases, which results in the decreasing conductivity with higher Nb concentration. A significant shift in the peak position of ${A}_{2g}$ phonon mode has been observed using Raman spectroscopy, which indicates the change in the coupling due to the structural distortion with Nb substitution. The core-level photoemission study confirms that the Nb is present in $5+$ valence state. Our study reveals that the nonmagnetic ${\mathrm{Nb}}^{5+}$ (${d}^{0}$) substitution converts ${\mathrm{Co}}^{3+}$ ions to ${\mathrm{Co}}^{2+}$ and stabilizes both in the high-spin state. Our results suggest that structural and spin-state transitions as well as the difference in the ionic radii between ${\mathrm{Nb}}^{5+}$ and ${\mathrm{Co}}^{3+}$ are playing an important role in tuning the physical properties.

Spin Dynamics and Unconventional Magnetism in Insulating La(1–2x)Sr2xCo(1–x)NbxO3

Abstract: We study the structural, magnetic, transport, and electronic properties of LaCoO3 with Sr/Nb cosubstitution, i.e., La(1–2x)Sr2xCo(1–x)NbxO3 using X-ray and neutron diffraction, dc- and ac-magnetiza...

Intermediate-spin state and properties of LaCoO_3

Abstract: The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

Metal Insulator Transition

Abstract: Metal Insulator transition(MIT) is characterized by the conductivity which will be zero in the insulator phase. In this term paper, we focus on Mott insulator, and a simple theoritical way to describe this MIT is the Hubbard Model .Finally, we will look at the experiments of Mott insulator transition .