arXiv subjects

Shivani Bhardwaj

Publications and source records attributed to Shivani Bhardwaj.

7 recordsLinked to original sources

arXiv2025

Hund's coupling driven nature of magnetism in negative charge transfer material, $\mathrm{SrCoO_3}$

In this work, we investigate the microscopic origin of magnetism in $\mathrm{SrCoO_3}$ by incorporating electronic correlations within the dynamical mean-field theory (DMFT) framework. We note a remarkable agreement of the calculated magnetic observables ( saturation magnetization $\sim$2.4 $μ_B$; magnetic transition temperature, $T_c$$\sim$350 K) with the experimental results. The system exhibits Hund's coupling-induced strong quasiparticle mass enhancements of upto $m^*/m$ $\sim$7 for Co 3$d$ states, with the largest renormalization occurring in the majority spin $t_{2g}$ orbitals, marking the onset of orbital-selectivity. Our results reveal a Stoner-$like$ collapse of exchange splitting that drives the loss of long-range ferromagnetic order at $T_c$. The breakdown of Fermi-liquid behavior down to $T$$\sim$100 K suggests a suppressed coherence scale. Local magnetic moment originates from a mixed-spin configuration formed through dynamical fluctuation between intermediate-spin and high-spin states. Large charge fluctuations ($\langle$$Δ$N$^2$$\rangle$$\sim$0.6) together with heavy quasiparticles establish the correlation effects regime, governed predominantly by Hund's physics in $\mathrm{SrCoO_3}$.

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arXiv2025

Valence band-satellite, temperature dependent magnetic and spectral study of α-Fe

We investigate the influence of correlations and plasmonic excitation on valence band-satellite of $α$-Fe, along with magnetic and spectral properties as function of temperature. Coulomb interaction parameters are obtained by systematically employing various schemes in constrained random phase approximation (cRPA). This study identifies the presence of valence band satellite in Fe at $\sim$6 eV binding energy supported by (i) substantial incoherent spectral weight in the valence band spectra obtained from Density Functional Theory plus Dynamical Mean Field Theory (DFT+DMFT) and (ii) plasmonic excitations in the frequency range $\sim$6-8 eV suggested by $G_0W_0$ calculations. We note presence of significant contribution of temperature-dependent Pauli-spin susceptibility indicating competing degree of itinerancy. $e_g$ state shows a strong temperature driven non-Fermi-liquid behavior emerging near $T_c$. Our results reveal a high-temperature orbital-selective loss of coherence eventually leads to a orbital selective collapse of magnetization at $T_c$, suggesting a ferromagnetic phase characterized by strong correlation- and temperature- dependent spectral features.

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arXiv2025

Spin-dependent orbital selectivity and partial Kondo-screening in magnetically ordered Hund's metal

Hund's metallicity in 3$d$ transition metal oxides constitutes a rare class of compounds, since they have been long understood considering the dominance of Hubbard $U$. $\mathrm{LiV_2O_4}$\& $\mathrm{Sr_2CoO_4}$ belong to this rare class of metals; among them, $\mathrm{LiV_2O_4}$ has been the subject of extensive investigations for its unconventional heavy-fermion behavior, while studies on $\mathrm{Sr_2CoO_4}$ remain limited despite its anomalous ferromagnetic ground state. In this study, we report an unusual spin-orbital selective localization in $\mathrm{Sr_2CoO_4}$ leading to a sharp Kondo resonance at $\sim$70 K in the spin-$up$ channel of orbitals of $t_{2g}$ symmetry using a combination of Density functional theory and Dynamical mean field theory (DFT+DMFT) calculations. Correspondingly, an appreciable reduction in the magnetization below $T$=100 K further suggests partial Kondo screening of local moments active at low temperatures, explaining its effective spin magnetization state and upturn in its resistivity observed in experimental reports. We note a significant effect of Hund's induced spin-orbital selective incoherence in dictating the temperature evolution of its macroscopic observables e.g. spin-spin correlation function and effective local moment. Our results reveal a potentially distinct/new form of spin-dependent selectivity induced via Hund's coupling in addition to the conventional orbital-selectivity in the Hund's metals, as a plausible key mechanism in stabilizing their long-range magnetic order.

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arXiv2025

Signatures of Hund$'s$ metal physics in single-layered 3d transition metal oxide, $\mathrm{Sr_2CoO_4}$

With density functional theory plus dynamical mean-field theory, we study the influence of Hund's coupling on the nature of electronic correlations in $\mathrm{Sr_2CoO_4}$. Our results suggest strong signatures of Hund's metal physics in this compound. The Co 3$d$ states show large orbital differentiation in the degree of correlations and mass enhancement. The imaginary-time correlation functions suggest the presence of spin-orbital separation and large local charge fluctuations in the system. Breakdown of the Fermi-liquid picture is observed at the lowest calculated temperature for various strengths of Hund's coupling, suggesting the Fermi-liquid coherence scale lower than $\sim$100 K. Interestingly, a sudden emergence of a gapped state is noted for $e_g$ orbitals in its spectral density of states at $\sim$200 K in the vicinity of Fermi-level. Among the Co 3$d$ states 3$d_{z^2}$ and 3$d_{x^2-y^2}$ foster enlarged correlations. This study conclusively identifies $\mathrm{Sr_2CoO_4}$ as the first single-layered 3$d$ transition metal oxide to be classified as Hund's metal.

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arXiv2024

Enhanced Pauli spin response, failure of Stoner \& spin fluctuation models, and presence of 6 $eV$ plasmonic excitations in Ni metal

We revisit the electronic structure of Ni, using the density functional theory (DFT) and dynamical mean-field theory (DMFT) for the theoretical description of its electronic structure properties along with finite-temperature magnetism. Our study provides a comprehensive account of electronic and magnetic properties with the same set of Coulomb interaction parameters, $U$($J$)=5.78(1.1) $eV$ calculated using first-principles approach. The nature of theoretical magnetization curves obtained from DFT \& DFT+DMFT as well as the experimental curve show deviation from the standard models of magnetism, $viz$ Stoner and spin fluctuation model. The temperature dependent DFT approach is found to well describe the finite-temperature M(T) of Ni below critical temperature ($T$ $\leq$ 631 K). The study finds significant Pauli-spin susceptibility contribution to paramagnetic spin susceptibility. Excluding the Pauli-spin response yields a linear Curie-Weiss dependence of the inverse paramagnetic susceptibility at higher temperatures. Also, the presence of mixed valence electronic configuration (3$d^8$, 3$d^9$ and 3$d^7$) is noted. The competing degrees of both the itinerant and localized moment picture of 3$d$ states are found to dictate the finite-temperature magnetization of the system. Furthermore, the quasiparticle scattering rate is found to exhibit strong deviation from $T^2$ behavior in temperature leading to the breakdown of conventional Fermi-liquid theory. In addition to the 6 $eV$ feature, our calculated electronic excitation spectrum confirms the satellite feature extending $\sim$10 $eV$ binding energy, being consistent with experimental observation. Interestingly, our $G_0W_0$ results find the presence of plasmonic excitation contribution to the intensity of famous 6 $eV$ satellite along with the electronic correlation effects,paving way for its reinterpretation.

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arXiv2024

Observation of correlation induced metal to half-metal phase transition and large orbital moment in $\mathrm{Sr_2CoO_4}$

We present a detailed mean-field study to address the fundamental discrepancy in the ground state magnetization of $\mathrm{Sr_{2}CoO_{4}}$ (SCO). In contrast to the ferromagnetic metallic ground state obtained from density functional theory (DFT), DFT+$U$ gives three ferromagnetic solutions converging to integer moment values (1, 2 \& 3 $μ_B$/f.u) over a range of $U$. Interestingly, two of the solutions are found to exhibit half-metallicity with correspondingly $S$=1/2 and S=3/2 spin states. The half-metallic ferromagnetic state with $S$=3/2 is found to be the ground state solution for SCO. Co atoms show a large deviation from the formal +4 oxidation state indicating the presence of strong covalency effects. Our results suggest a plausible metal to half-metal phase transition around $U$($J$)=4.4(1.16) eV. The Fermi surface study shows gradual collapse in states leading to half-metallicity suggesting \textit{\textbf{k}}-dependence of effective $U$ around the critical region. Surprisingly, in the presence of spin-orbit coupling (SOC), unexpectedly large orbital moment ($L_{z}$=0.6) is noted in SCO putting it among the class of 3$d$ based transition metal compounds exhibiting pronounced orbital magnetization. The calculations give large magnetocrystalline anisotropy energy (MAE) of $\sim$48 meV. Large values of orbital magnetic moment contribution and MAE, in the presence of strong correlation effects, provide a better interpretation of experimental magnetization observed in SCO.

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arXiv2023

Lattice dynamics related properties of Nickel: A comparative DFT and DFT+U study

The simultaneous influence of electronic correlations and magnetic ordering on the theoretical estimation of phonons and related properties of Ni is investigated. The work includes a comparative DFT and DFT+U study, where on-site Coulomb interaction parameter for 3$d$ electrons, $U$($U_{full}$)= 0.516 eV obatined from constarined random phase approximation (cRPA) calculations, is considered for DFT+U calculations. The analysis of phonon frequency estimates along high symmetric k-directions and sampled full-BZ (Brillouin zone) using Frozen phonon displacement method suggests the importance of both on-site Coulomb correlations and magnetism to account for the experimental frequencies. Further, prominent role of both the aspects is observed in the derived thermodynamic properties - Free-energy, specific heat \& entropy, within quasi-harmonic approximation (QHA) specially at high temperatures. The temperature dependent evaluation of thermal expansion coefficient($α$) and phonon density of states is performed together with the equilibrium elastic constants. The results obtained for Ni, suggest the significance of electronic energy correction due to both on-site Coulomb correlations and magnetic phase incorporation, to account for realistic description of experimental findings. This study realizes the inevitable role of correlation effects in studying the phononic properties of a correlated transition metal, hence directing a way to explore various other correlated electron systems for their lattice dynamics.

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