M. Bharath | Searcharxiv

arXiv2024

Coupling of magnetism and transport properties to the lattice degrees of freedom in NdBaCo$_2$O$_{5+δ}$ ($δ\sim 0.65$)

We have studied the origin of zero volume expansion below the Curie temperature (Tc), variable range hopping (VRH) behaviour using structural, magnetic, transport and thermal studies on the oxygen deficient double perovskite NdBaCo$_2$O$_{5+δ}$ ($δ\sim 0.65$). The valence state of Co ions and the possible properties exhibited by such compound were studied using electronic structure calculations for $δ= 0.75$. Careful investigation of structure shows that the compound stabilizes in tetragonal structure (P4/mmm) having $2a_p \times 2a_p \times 2a_p$ (222) superstructure, where $a_p$ is the cubic perovskite lattice parameter. The compound exhibits a minimum in resistivity, ferromagnetic and ferrimagnetic transitions around 375 K, 120 K ($T_c$) and 60 K, respectively with signature of Griffiths phase above $T_c$. Our detailed structural analysis suggests signature of the onset of the above magnetic transitions at temperatures well above its stabilisation at long range level thereby leading to VRH behaviour. The observed zero thermal expansion in volume below Tc appears to be due to competing magnetic interactions within and between the magnetic sublattices. Our electronic structure calculations show (a) the importance of electron-electron correlation in Nd $4f$ and Co $3d$ states (b) Co ions stabilize in intermediate spin (IS) state, having oxidation state less than +3 (c) half metallicity. Our results show the possibility of coupling between magnetism and ferroelectricity. We believe that our results especially on the valence state of the Co ion, zero thermal expansion in volume, short range magnetic orderings and the connection between different degrees of freedom will be helpful in clearing the ambiguities existing in literature on the nature of magnetism and thereby aiding in designing new functionalities.

cond-mat.str-el↗

arXiv2022

Evidence of Lattice Strain as a Precursor to Superconductivity in BaPb$_{0.75}$Bi$_{0.25}$O$_3$

In this work, we have investigated the precursor effects to superconductivity in BaPb$_{0.75}$Bi$_{0.25}$O$_3$ using temperature dependent resistivity, x-ray diffraction technique and photoemission spectroscopy. The present compound exhibits superconductivity around 11 K ($T_C$). The synthesis procedure adopted is much simpler as compared to the procedure available in the literature. In the temperature range (10 K-25 K) i.e. above $T_C$, our results show an increase in both the orthorhombic and tetragonal strain. The well screened features observed in Bi and Pb 4$f_{7/2}$ core levels are indicative of the metallic nature of the sample. The compound exhibits finite intensity at the Fermi level at 300 K and this intensity decreases with decrease in temperature and develops into a pseudogap; the energy dependence of the spectral density of states suggests disordered metallic state. Furthermore, our band structure calculations reveal that the structural transition upon Pb doping results in the closing of the band gap at the Fermi level.

cond-mat.supr-con↗