arXiv subjects

Padmanabhan Balasubramanian

Publications and source records attributed to Padmanabhan Balasubramanian.

6 recordsLinked to original sources

arXiv2024

Fast Bipartitioned Hybrid Adder Utilizing Carry Select and Carry Lookahead Logic

We present a novel fast bipartitioned hybrid adder (FBHA) that utilizes carry-select and carry-lookahead logic. The proposed FBHA is an accurate adder with a significant part and a less significant part joined together by a carry signal. In an N-bit FBHA, the K-bit less significant part is realized using carry-lookahead adder logic, and the (N-K)-bit significant part is realized using carry-select adder logic. The 32-bit addition was considered as an example operation for this work. Many 32-bit adders ranging from the slow ripple carry adder to the fast parallel-prefix Kogge-Stone adder and the proposed adder were synthesized using a 28-nm CMOS standard cell library and their design metrics were compared. A well-optimized FBHA achieved significant optimizations in design metrics compared to its high-speed adder counterparts and some examples are mentioned as follows: (a) 19.8% reduction in delay compared to a carry-lookahead adder; (b) 19.8% reduction in delay, 24.4% reduction in area, and 19.4% reduction in power compared to a carry-select adder; (c) 45.6% reduction in delay, and 13.5% reduction in power compared to a conditional sum adder; and (d) 46.5% reduction in area, and 29.3% reduction in power compared to the Kogge-Stone adder.

cs.AR↗

arXiv2022

Origins of multi-sublattice magnetism and superexchange interactions in double-double perovskite CaMnCrSbO6

We have deployed density functional theory, Wannier function analysis and mean-field calculations to investigate the double-double perovskite compound CaMnCrSbO_{6}. The crystallographically non-equivalent Mn atoms in the unit cell have tetrahedral and planar oxygen coordinations (labelled as Mn(1) and Mn(2)), while the Cr atom is in the centre of distorted oxygen octahedron. While the bulk magnetization and neutron diffraction suggest a simpler ferrimagnetic order (T_C=49 K) between Mn2+ and Cr3+ spins, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The electronic structure calculations yield a ferrimagnetic insulating ground state even in absence of Hubbard U which persists for a wide range of U. The Mn(1)-O-Mn(2) (out of plane and in-plane), Mn(1)-O-Cr and Mn(2)-O-Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr-O-O-Cr super-superexchange is found to be ferromagnetic. The Mn(2)-O-Cr superexchange is weaker than the Mn(1)-O-Cr superexchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strength J_AFM and the weaker ferromagnetic J_FM. The relative strengths of JAFM for the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr-O-O-Cr super-superexchange strength (J_SS), which is derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattice for Mn2+ spins and a single magnetic sublattice for Cr3+ spins yields a much improved T_C, while a simple two magnetic sublattice model yields a much higher T_C.

cond-mat.mtrl-sci↗

arXiv2021

Complex interplay of magnetic ordering and spin-lattice coupling in orthochromite Nd$_{0.5}$Dy$_{0.5}$CrO$_{3}$

The mixed rare-earth orthochromite Nd$_{0.5}$Dy$_{0.5}$CrO$_{3}$ has a Néel temperature ($T_\mathrm{N}$) of ${\sim}$ 175\,K, resulting in the G-type antiferromagnetic ordering of Cr$^{3+}$ spins. The inverse susceptibility shows a deviation from Curie-Weiss law at 230\,K, with a large effective paramagnetic moment of 8.8\,$μ_{\mathrm{B}}$. The ZFC-FC magnetization bifurcate just above $T_\mathrm{N}$ and show a distinct signature of spin reorientation near 60\,K. Neutron diffraction show that below $T_\mathrm{N}$, the Cr$^{3+}$ spins align in $Γ_{2}$ representation as ($F_{x}$, $G_{z}$). Below 60\,K, due to spin reorientation, the magnetic structure is in $Γ_{1}$ ($G_{y}$) configuration. The neutron diffraction does not show any signature of rare-earth ordering even at 1.5\,K. First principles density functional theory calculations within GGA+U and GGA+U+SO approximations reveal that the G-type antiferromagnetic order is the ground state magnetic structure of Cr sublattice and the spin-reorientation of Cr$^{3+}$ spins can happen in the absence of 3d-4f interactions unlike in the case of orthoferrites. The specific heat shows a `$λ$' anomaly at $T_\mathrm{N}$, while at low temperature two distinct Schottky anomalies are observed; a Schottky peak at 2\,K and an additional step-like feature above 10\,K. Above $T_\mathrm{N}$, the magnetic transition is preceded by structural anomalies as seen in our x-ray diffraction and Raman measurements. The deviation of structural parameters near Néel temperature is smaller. The phonon frequencies show deviation from the standard anharmonic behaviour: first near 250\,K, due to magneto-volume effects while the second deviation occurs near 200\,K due to spin-phonon coupling.

cond-mat.str-el↗

arXiv2021

Coexisting magnetic structures and spin-reorientation in Er$_{0.5}$Dy$_{0.5}$FeO$_{3}$: Bulk magnetization, neutron scattering, specific heat, and \emph{Ab-initio} studies

The complex magnetic structures, spin-reorientation and associated exchange interactions have been investigate in Er$_{0.5}$Dy$_{0.5}$FeO$_3$ using bulk magnetization, neutron diffraction, specific heat measurements and density functional theory calculations. The Fe$^{3+}$ spins order as G-type antiferromagnet structure depicted by $Γ_{4}$($G_{x}$,$A_{y}$,$F_{z}$) irreducible representation below 700K, similar to its end compounds. The bulk magnetization data indicate occurrence of the spin-reorientation and rare-earth magnetic ordering below $\sim$75 K and 10 K, respectively. The neutron diffraction studies confirm an "incomplete" $Γ_{4}$${\rightarrow}$ $Γ_{2}$($F_{x}$,$C_{y}$,$G_{z}$) spin-reorientation initiated $\leq$75 K. Although, the relative volume fraction of the two magnetic structures varies with decreasing temperature, both co-exist even at 1.5 K. At 8 K, Er$^{3+}$/Dy$^{3+}$ moments order as $c_{y}^R$ arrangement develop, which gradually increases in intensity with decreasing temperature. At 2 K, magnetic structure associated with $c_{z}^R$ arrangement of Er$^{3+}$/Dy$^{3+}$ moments also appears. At 1.5 K the magnetic structure of Fe$^{3+}$ spins is represented by a combination of $Γ_{2}$+$Γ_{4}$+$Γ_{1}$, while the rare earth moments coexists as $c_{y}^R$ and $c_{z}^R$ corresponding to $Γ_{2}$ and $Γ_{1}$ representation, respectively. The observed Schottky anomaly at 2.5 K suggests that the "rare-earth ordering" is induced by polarization due to Fe$^{3+}$ spins. The Er$^{3+}$-Fe$^{3+}$ and Er$^{3+}$-Dy$^{3+}$ exchange interactions, obtained from first principle calculations, primarily cause the complicated spin-reorientation and $c_{y}^R$ rare-earth ordering, respectively, while the dipolar interactions between rare-earth moments, result in the $c_{z}^R$ type rare-earth ordering at 2 K.

cond-mat.str-el↗

arXiv2020

Successive spin reorientation and rare earth ordering in Nd$_{0.5}$Dy$_{0.5}$FeO$_{3}$: Experimental and $Ab$-$initio$ investigations

In present study, the magnetic structure and spin reorientation of mixed doped orthoferrite Nd$_{0.5}$Dy$_{0.5}$FeO$_3$ have been investigated. Similar to both parent compounds (NdFeO$_3$ and DyFeO$_3$), the magnetic structure of Fe$^{3+}$ belongs to $Γ_{4}$ irreducible representation (G$_{x}$, F$_{z}$) at room temperature. The experimental measurements confirmed the spin reorientation where magnetic structure of Fe$^{3+}$ changes from $Γ_{4}$ to $Γ_{2}$(F$_{x}$, G$_{z}$) between 75 and 20 \,K while maintaining G-type configuration. Such a gradual spin reorientation is unusual since the large single ion anisotropy of Dy$^{3+}$ ions causes an abrupt $Γ_{4}$${\rightarrow}$ $Γ_{1}$(G$_{y}$) spin reorientation in DyFeO$_3$. Between 20 and 10 \,K, the Fe$^{3+}$ magnetic structure is represented by $Γ_{2}$ (F$_{x}$, G$_{z}$). Unexpectedly, magnetic structure of Fe$^{3+}$ with $Γ_{4}$ representation re-emerges below 10\,K which also coincides with the development of rare-earth (Nd$^{3+}$/Dy$^{3+}$) magnetic ordering having C$_{y}$ configuration with magnetic moment of 1.8 $μ_{B}$. The absence of any signature of second order phase transition in the specific heat confirms the role of $R$(Nd$^{3+}$/Dy$^{3+}$)-Fe$^{3+}$ exchange interaction in the observed "rare-earth ordering" unlike DyFeO$_3$ where Dy$^{3+}$ ordering takes place independently to the magnetic ordering of Fe$^{3+}$ magnetic structure. Our (DFT+U+SO) calculations show that the C-type arrangement of rare-earth ions (Nd$^{3+}$/Dy$^{3+}$) with $Γ_{2}$ configuration for Fe$^{3+}$ moments is the ground state whereas $Γ_{4}$ phase is energetically very close. Nd-Fe and Nd-Dy exchange interactions, estimated from DFT, are observed to have significant roles in the rare earth ordering and Fe spin reorientation corroborating our experimental results.

cond-mat.str-el↗

arXiv2017

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies Δ of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the atomic values of 8.00 and 3.00 respectively thereby indicating the covalent nature of the system. The O 1s edge absorption spectra reveal a band gap of 0.9 eV which is comparable to the value obtained from first principle calculations for U-J >= 2 eV. The density of states clearly reveal a strong p-d type charge transfer character of the system, with band gap proportional to average charge transfer energy of Ni2+ and Mn4+ ions.

cond-mat.str-el↗