Tilak Das

The generic perovskite-like A BO ₃ stoichiometry allows for tuning electronic correlations in condensed matter physics, for example A MnO ₃ manganites. Manganites are particularly exotic due to partial occupancies of Mn(3 d ) orbitals, typically composed of Mn ³⁺ (3 d ⁴ ) or Mn ⁴⁺ (3 d ³ ) magnetic cations and size dependence of non-magnetic cations, A = Ca, Sr, La, Y, and Ba. The Coulomb repulsion of 3 d -electrons, is perturbed via chemical strain imposed by varying A -site radii and deformations of Mn–O polyhedrons in different crystal symmetries—orthorhombic vs hexagonal. Here, we disentangle the role of Mn-site’s electronic correlations from structural symmetry, spin-orbital ordering, [MnO _n ] ^{m −} polyhedral deformations, and on-site Coulomb correction (Hubbard on-site U ) using density functional theory (DFT) calculations. The ad hoc U ( U _eff ) results in identical structural descriptors before and after U correction vs the A -site radii. Overall, relative magnetic ground state stability Δ E is monotonously increasing with the A -site radii in the orthorhombic Mn ³⁺ manganites (towards ferro-magnetic), but decreasing (towards G-type anti-ferro order) in the less distorted class Mn ⁴⁺ manganites. The unexplained origin of the ad hoc but correct relative magnetic ground state of these manganites in experimental structures with intrinsic lattice distortions is found to be associated with cancellations of self-interaction errors and underestimations of orbital localization. We note that the trend of electronic band gaps in the Mn ³⁺ and Mn ⁴⁺ manganites are random in the DFT + U _eff approach. An optimal on-site Hubbard U that is fast and reasonably accurate for predicting the structure and properties for manganites is proposed. The on-site U correction on the Mn(3 d ) and/or O(2 p ) orbitals against the screened hybrid functional HSE06 by comparing with the self-consistent Hubbard U i.e. U _self is established, showing the limitations of screened exchange. The Γ -point Raman mode analysis (i.e. spin-phonon coupling) across various magnetic and non-magnetic ground states, we finally rationalize the accuracy of U _self in A MnO ₃ oxides.

A first-principles approach to Magnetostrictronics in the case of bulk L1 ₀ -MnPt. Mn and Pt atoms are marked with deep purple and grey solid balls.

The quantum-confinement (QC) phenomena in quantum-size quasi-2D solids of transition-metal dichalcogenides (TMDs) are viable to investigate from optoelectronic and photocatalytic perspectives. Here, we have investigated the QC effect using first-principles calculations within density functional theory (DFT) and crystal structure modelling on six hexagonal TMDs, exfoliated from bulk stochiometric 2H-MX2 (M = Mo, W and X = S, Se, Te). Local and non-local DFT functional are applied to get geometries of these bulk TMDs and better geometry was obtained from the later one. Pertaining to the inter-layer, intra-layer and spin-orbital coupling (SOC) energies correction on better geometries, our present study rationalizes the QC effect bridging the known experimental evidences of wires (1D) dots (3D) quantum-size solids. Local and non-local DFT functional is benchmarked with present and previous theoretical data. The cancelation of errors on inter-planer van der Waals spacing (dv, underestimated) and intra-layer MX2 thickness (dt, overestimated) was noticed. SOC bands splitting vs. Fermi level (EF = 0.0 eV), perturbed the nature of optical transitions on-set (direct or indirect) in absolute band gaps (Eg) opening in bulk vs. dt and dv thickness. The layer dependent Eg and its deviation (∆Eg) in ultra-thin quantum-size model vs. the bulk phase, is closely associated with SOC effects. S-/Se-TMDs switches from direct to indirect Eg in the monolayer to bi-layers, but the similar multi-layered Te-TMDs are still likely to be direct gap, Eg – unfold the possible atomistic role of inter- and intra-layers interactions on optical transitions. Quantification of QC effect is done based on the estimated slopes (C) ≈ 150-300 eV-Å2 in wells and aligned excellent to the known trend for wires and dots, C ≈ 380-400 and 600-620 eV-Å2, respectively. The microscopic origin of QC effect from the k-space resolved bands dispersions allows to conclude lowest quasi-2D thickness of 3-4 nm for TMDs.

We have identified intrinsic high-spin paramagnetic defects in MoS ₂ nanocrystals and measured their spin dynamics, which have not been reported so far.

The origin of magnetocrystalline anisotropic energy (MAE) guided by spin–orbit coupling in the L1₀-FePt alloy was analyzed. Correlation of MAE with the calculated magnetoelastic constants in the single and polycrystal model was determined by means of first-principles calculations using density functional theory (DFT). More specifically, a systematic analysis of the large MAE and magnetostriction coefficients (λ’s) were done by means of post-processing of eigenvalues (orbital energies) and eigenfunctions (orbital occupancies). Our numerical analysis includes the convolution of the projected wave-function (density of states) of each orbital of the Fe and Pt sub-lattices into their orbital energies, which in principle should be valid for any solid regardless of the strength of the MAE. The determined anisotropic magnetostriction coefficients are of the λ ∼ 10 − 4 –10^–3 order for several crystallographic directions; which is in accord to the known experimental data. However, the polycrystalline model based on the uniform stress approximation leads to a decrease of the λ by two orders in the overall magnetostrictive behavior ( λ ∼ 10 − 5 ). Such a large difference is not unexpected as the magnetostriction for polycrystalline FePt thin films were recorded as very low, thus validating the robustness of the current theoretical proposition of analyzing MAE.

A microscopic control over the origin and dynamics of localised spin centres in lower dimensional solids turns out to be a key factor for next generation spintronics and quantum technologies. With the help of low temperature electron paramagnetic resonance (EPR) measurements, supported by the first-principles calculations within density functional theory (DFT) formulation, we found the origin of different high-spin paramagnetic intrinsic charge-centres, Mo3+(4d3) and Mo2+(4d4) present in the nano-crystalline sulfur deficit hexagonal molybdenum disulfide (2H-MoS_(2-x)), against the established notion of spin-1/2 , Mo5+ centres. A critical strain generated in the nano-structured 2H-MoS_(2-x) was found to be very crucial for spin-localization in this layered material. Indeed, computationally effective proposition of the PBE+U exchange-correlations within DFT including D3-dispersion corrections found to be more viable than expensive higher rung of exchange-correlation functionals, explored earlier. It is also found that the oxygen vacancy of the reduced oxide phase, embedded in 2H-MoS_(2-x) host lattice, has the longest relaxation times. Moreover, the temperature dependence of spin-lattice relaxation measurements reveals a direct process for interstitial spin centres and a Raman process for both sulfur and oxygen vacancy sites. We expect such observation would be a valuable pillar for better understanding of the next generation quantum technologies and device applications.