arXiv subjects

Puru Jena

Publications and source records attributed to Puru Jena.

11 recordsLinked to original sources

arXiv2026

Symmetry Breaking by Interfacial Dead Layers: Observation of Forbidden Self-Induced Spin-Orbit Torque in Symmetric Ferromagnets

Conventionally, spin-orbit torques (SOTs) in ferromagnets require heavy-metal layers or engineered structural asymmetry to break inversion symmetry. In this work, we report the observation of robust, self-generated SOTs in a nominally symmetric, heavy-metal-free MgO/NiFe/MgO trilayer - a geometry where such torques are theoretically forbidden. By combining harmonic Hall measurements with SQUID magnetometry and X-ray photoelectron spectroscopy, we identify the symmetry-breaking origin: a 1.8 nm magnetic dead layer at the bottom interface. Crucially, we demonstrate a quantitative agreement between our data and the drift-diffusion theory predicted by Kim and Lee, yielding a theoretically extracted dead-layer thickness (1.2 nm) which matches structural characterization. Furthermore, density-functional calculations confirm that NiFe possesses sufficient intrinsic spin Hall conductivity to support the observed spin currents. These results reframe the parasitic dead layer as a functional spintronic component, establishing a universal, all-ferromagnetic route to SOTs in standard magnetic heterostructures.

cond-mat.mes-hall↗

arXiv2026

Harnessing Native Chromium Oxidation for Giant Orbital Torque and Field-Free Magnetization Switching in NiFe/Cr Bilayers

Orbital currents offer charge-to-spin conversion beyond the efficiency limit of conventional heavy-metal Spin Hall sources. However, harnessing them has so far required either thick orbital-Hall materials or additional heavy-metal conversion layers. Here, we show that the native oxide of chromium, typically regarded as parasitic, transforms a simple NiFe\Cr bilayer into a self-contained dual-channel orbital-current source without the need for any conversion layer. First-principles calculations predict a nearly threefold enhancement of the orbital Hall conductivity upon surface oxygenation, driven by Cr(3d)-O(2p) hybridization. Experimentally, naturally oxidized NiFe\Cr heterostructures exhibit a giant damping-like torque efficiency of $3.9 \times 10^{6}$ $\Omega^{-1}$ m$^{-1}$, exceeding Pt (Ta) by one (two) orders of magnitude. The torque depicts a non-monotonic Cr-thickness dependence which cannot be explained by a conventional model. We have developed a drift-diffusion model with an oxidation-gated interfacial source which quantitatively reproduces the data, revealing that the Cr-CrO$_x$ interface generates orbital currents over an order of magnitude stronger than the bulk orbital Hall channel with an orbital transport length of $\approx 4$ nm. The enhanced torque enables field-free magnetization switching at $1.58 \times 10^{11}$ A m$^{-2}$, outperforming heavy-metal and CuO$_x$ benchmarks. These results establish native oxidation as a scalable strategy for realizing efficient orbital-torque devices.

cond-mat.mes-hall↗

arXiv2024

Investigating Quantum Spin-Textures Using Universal MJ Hamiltonians

This work introduces a pair of novel universal MJ spintronic models that precisely mirror the complex spin textures observed in spintronic materials. The spin-orbit coupled (SOC) Hamiltonians H_MJ1 and H_MJ2 reveal a range of novel and intriguing spin phenomena by modulating spin-orbit interactions. The Hamiltonian H_MJ1 reshapes the existing paradigm, providing a more robust and versatile framework than the Hamiltonian H_RD, with the potential to catalyze new advancements in the study of quantum materials. H_MJ1 encapsulates two distinct spin textures: a unidirectional, momentum-independent persistent spin texture (PST), and a bidirectional (partial) PST. In contrast Hamiltonian H_MJ2 portrays a spiral spin texture, drawing a conceptual link to the cosmological process of expansion and contraction, mirrored within a two-dimensional quantum framework. We also explore the fundamental aspects of earlier analytical models that underpin the construction of the present MJ spintronic model. The physical interpretations of these models are illustrated graphically, and the emerging spin phenomena resulting from complex SOC are elucidated using a simple vector model.

cond-mat.mtrl-sci↗

arXiv2023

(110) Facet of MgTe Zinc Blende Semiconductor: A Holy Grail for Modern Spintronics

Unlike, momentum-dependent Rashba spin-splitting, materials exhibiting intrinsic momentum-independent unidirectional spin polarization also known as persistent spin texture (PST) in the full Brillouin zone are scarce. In this work, a list of characteristic electronic properties for identifying an ideal PST material is provided based on earlier analytical models, and a new semiconductor, the MgTe(110) facet is proposed which satisfies all these conditions and exhibits PST in the full Brillouin zone. The atomic arrangement in this particular facet exhibits three basic symmetries found in nature: rotation, reflection, and translation. Using the method of invariance, an effective Hamiltonian is constructed which reproduces the results obtained using the density functional theory. Further, mono/few layers of MgTe (110) facets of the zinc-blende structure are proposed for a ferromagnet-free non-ballistic spin-field effect transistor (s-FET) that combines both the spin-Hall effect and inverse spin-Hall effect, thus harmonizing spintronics with conventional electronics. Although only quantum well structures have been experimentally studied for nonballistic s-FET under the stringent condition of equal Rashba and Dresselhaus strength, PST originating intrinsically in the proposed 2D structures makes them an ideal alternate.

physics.app-ph↗

arXiv2020

Built-in Electric-Field-Control of Magnetic Coupling in van der Waals semiconductors

Electrical control of magnetism in a two-dimensional (2D) semiconductor is of great interest for emerging nanoscale low-dissipation spintronic devices. Here, we propose a general approach of tuning magnetic coupling and anisotropy of a van der Waals (vdW) 2D magnetic semiconductor via a built-in electric field generated by the adsorption of superatomic ions. Using first-principles calculations, we predict a significant enhancement of ferromagnetic (FM) coupling and a great change of magnetic anisotropy in 2D semiconductors when they are sandwiched between superatomic cations and anions. The magnetic coupling is directly affected by the built-in electric field, which lifts the energy levels of mediated ligands' orbitals and enhances the super-exchange interactions. These findings will be of interest for ionic gating controlled ferromagnets and magnetoelectronics based on vdW 2D semiconductors.

cond-mat.mtrl-sci↗

arXiv2020

U-carbon: metallic and magnetic

We report the discovery of a pristine crystalline 3D carbon that is magnetic, electrically conductive and stable under ambient conditions. This carbon material, which has remained elusive for decades, is synthesized by using the chemical vapor deposition (CVD) technique with a particular organic molecular precursor 3,3-dimethyl-1-butene (C6H12). An exhaustive computational search of the potential energy surface reveals its unique sp2-sp3 hybrid bonding topology. Synergistic studies involving a large number of experimental techniques and multi-scale first-principles calculations reveal the origin of its novel properties due to the special arrangement of sp2 carbon atoms in lattice. The discovery of this U-carbon, named such because of its unusual structure and properties, can open a new chapter in carbon science.

cond-mat.mtrl-sci↗

arXiv2018

Dissociation dynamics of 3- and 4-nitrotoluene radical cations: Coherently driven C$-$NO$_2$ bond homolysis

Monosubstituted nitrotoluenes serve as important model compounds for nitroaromatic energetic molecules such as trinitrotoluene. This work investigates the ultrafast nuclear dynamics of 3- and 4-nitrotoluene radical cations using femtosecond pump-probe measurements and the results of density functional theory calculations. Strong-field adiabatic ionization of 3- and 4-nitrotoluene using 1500 nm, 18 fs pulses produces radical cations in the ground electronic state with distinct coherent vibrational excitations. In both nitrotoluene isomers, a one-photon excitation with the probe pulse results in \ce{NO2} loss to form \ce{C7H7+}, which exhibits out-of-phase oscillations in yield with the parent molecular ion. The oscillations in 4-nitrotoluene with a period of 470 fs are attributed to the torsional motion of the \ce{NO2} group based on theoretical results showing that the dominant relaxation pathway in 4-nitrotoluene radical cation involves the rotation of the \ce{NO2} group away from the planar geometry. The distinctly faster oscillation period of 216 fs in 3-nitrotoluene is attributed to an in-plane bending motion of the \ce{NO2} and \ce{CH3} moieties based on analysis of the normal modes. These results demonstrate that coherent nuclear motions determine the probability of \ce{C-NO2} homolysis in the nitrotoluene radical cations upon optical excitation within several hundred femtoseconds of the initial ionization event.

physics.chem-ph↗

arXiv2016

Quantum anomalous Hall effect in ferromagnetic transition metal halides

The quantum anomalous Hall (QAH) effect is a novel topological spintronic phenomenon arising from inherent magnetization and spin-orbit coupling. Various theoretical and experimental efforts have been devoted in search of robust intrinsic QAH insulators. However, up to now, it has only been observed in Cr or V doped (Bi,Sb)2Te3 film in experiments with very low working temperature. Based on the successful synthesis of transition metal halides, we use first-principles calculations to predict that RuI3 monolayer is an intrinsic ferromagnetic QAH insulator with a topologically nontrivial global band gap of 11 meV. This topologically nontrivial band gap at the Fermi level is due to its crystal symmetry, thus the QAH effect is robust. Its Curie temperature, estimated to be ~360 K using Monte-Carlo simulation, is above room temperature and higher than most of two-dimensional ferromagnetic thin films. We also discuss the manipulation of its exchange energy and nontrivial band gap by applying in-plane strain. Our work adds a new experimentally feasible member to the QAH insulator family, which is expected to have broad applications in nanoelectronics and spintronics.

physics.comp-ph↗

arXiv2011

Theoretical study of C60 as catalyst for dehydrogenation in LiBH4

Complex light metal hydrides possess many properties which make them attractive as a storage medium for hydrogen, but typically, catalysts are required to lower the hydrogen desorption temperature and to facilitate hydrogen uptake in the form of a reversible reaction. The overwhelming focus in the search for catalysing agents has been on compounds containing titanium, but the precise mechanism of their actions remains somewhat obscure. A recent experiment has now shown that fullerenes (C$_{60}$) can also act as catalyst for both hydrogen uptake and release in lithium borohydride (LiBH$_4$). In an effort to understand the involved mechanism, we have employed density functional theory to carry out a detailed study of the interaction between this complex metal hydride and the carbon nanomaterial. Considering a stepwise reduction of the hydrogen content in LiBH$_4$, we find that the presence of C$_{60}$ can lead to a substantial reduction of the involved H-removal energies. This effect is explained as a consequence of the interaction between the BH$_x^-$ complex and the C$_{60}$ entity.

cond-mat.mtrl-sci↗

arXiv2011

Borane Derivatives: A New Class as Super and Hyperhalogens

Super and hyperhalogens are a class of highly electronegative species whose electron affinities far exceed those of halogen atoms and are important to chemical industry as oxidizing agents, bio-catalysts, and building blocks of salts. Using Wade-Mingos rule well known for describing the stability of closo-boranes (BnHn2-) and state of the art theoretical method we show that a new class of super and hyperhalogens, guided by this rule, can be formed by tailoring the size and composition of borane derivatives. Unlike conventional superhalogens which have a metal atom at the core surrounded by halogen atoms, the superhalogens formed using the Wade-Mingos rule do not have to have either halogen or metal atoms. We demonstrate this by using B12H13 and its isoelectronic cluster, CB11H12 as examples. We also show that while conventional superhalogens containing alkali atoms require at least two halogen atoms, only one borane-like moiety is sufficient to render M(B12H12) (M=Li, Na, K, Rb, Cs) clusters superhalogen properties. In addition, hyperhalogens can be formed by using the above superhalogens as building blocks. Examples include M(B12H13)2 and M(C-B11H12)2 (M=Li - Cs). This finding opens the door to an untapped source of superhalogens and weakly coordinating anions with potential applications.

physics.chem-ph↗

arXiv2009

Novel electronic and magnetic properties of BN sheet decorated with hydrogen and fluorine

First principles calculations based on density functional theory reveal some unusual properties of BN sheet functionalized with hydrogen and fluorine. These properties differ from those of similarly functionalized graphene even though both share the same honeycomb structure. (1) Unlike graphene which undergoes a metal to insulator transition when fully hydrogenated, the band gap of the BN sheet significantly narrows when fully saturated with hydrogen. Furthermore, the band gap of the BN sheet can be tuned from 4.7 eV to 0.6 eV and the system can be a direct or an indirect semiconductor or even a half-metal depending upon surface coverage. (2) Unlike graphene, BN sheet has hetero-atomic composition, when co-decorated with H and F, it can lead to anisotropic structures with rich electronic and magnetic properties. (3) Unlike graphene, BN sheets can be made ferromagnetic, antiferromagnetic, or magnetically degenerate depending upon how the surface is functionalized. (4) The stability of magnetic coupling of functionalized BN sheet can be further modulated by applying external strain. Our study highlights the potential of functionalized BN sheets for novel applications.

cond-mat.mtrl-sci↗