arXiv subjects

Ramesh C. Budhani

Publications and source records attributed to Ramesh C. Budhani.

12 recordsLinked to original sources

arXiv2026

Origin of High-Temperature Antiferromagnetic Order in a van der Waals Material

While Van der Waals (vdW) itinerant antiferromagnets with high Neel temperatures (TN) are highly desirable for spintronics, they remain relatively scarce. Here, we unravel the physical origin of the unusually high TN (= 250 K) in the newly identified vdW compound (Fe0.65Co0.35)4GeTe2. The substitution of Co in Fe4GeTe2 induces layer-selective Fe-Co ordering and stabilizes a robust antiferromagnetic (AFM) state primarily driven by Co moments. The AFM order is further strengthened by enhanced electronic correlations of quasi-localized Co 3d-states at the Fermi level, giving rise to an itinerant-localized duality of the 3d electrons. This interplay generates strong magnetic correlations well above TN and stabilizes low-temperature spin canting with a possible nontrivial Berry curvature. Our results establish (Fe0.65Co0.35)4GeTe2 as a rare material bridging fundamental magnetic interactions with potential applications in AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2026

Light Polarization Sensitive Transistor Action in the van der Waals ferroelectric 4H-SnS2

Van der Waals (vdW) ferroelectric semiconductors provide a unique platform for exploring the interplay between spontaneous polarization, electronic transport, ion migration, and photocarrier generation at the nanoscale. Here, we establish a room-temperature ferroelectric state in SnS2 and its sensitivity to structural polytype by directly contrasting the centrosymmetric 2H-phase with the polar 4H-phase. Raman spectroscopy distinguishes the 2H and 4H polymorphs of SnS2 through their characteristic phonon fingerprints. Piezoresponse force microscopy confirms room-temperature ferroelectricity in the 4H phase, while the 2H phase exhibits no ferroelectric response. The built-in polarization in a three-terminal transistor device of the 4H-SnS2 is modulated significantly by electrostatic gating and on exposure to linear and circularly polarized light. This device reveals polarization-controlled output characteristics with distinct gate-voltage induced hysteretic response and thermally activated carrier transport, confirming p-type semiconducting behavior strongly coupled to ferroelectric polarization. The photoresponse likewise exhibits polarization-assisted carrier separation, sublinear power-law scaling, a nonmonotonic temperature response correlated with the characteristic Raman modes, ferroelectric hysteresis, and a pronounced dependence on the circular and linear polarization states of the incident laser beam. These results establish 4H-SnS2 as a promising material system for polarization-driven electronic and optoelectronic technologies, including nonvolatile memory and photoferroelectric functionalities.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic Order in Pulsed Laser Deposited (Fe,Ni)5GeTe2 Films

We report the successful growth of highly textured thin films of (Fe,Ni)5GeTe2 two-dimensional ferromagnet on c-plane sapphire using pulsed laser deposition. Structural characterization via X-ray diffraction confirms preferential orientation along the (000l) direction, indicative of a high crystallographic texture. These films of van der Waals (vdW) type interplanar bonding exhibit robust ferromagnetism with a Curie temperature reaching ~ 498 K. Electrical transport measurements reveal a clear anomalous Hall effect, with an anomalous Hall conductivity and Hall angle (%) of ~ 20 Ohm-1cm-1 and ~ 0.90, respectively. Furthermore, the magnetoresistance displays a pronounced dependence on film thickness, highlighting the tunability of spin-dependent transport in these vdW ferromagnetic thin films.

cond-mat.mtrl-sci↗

arXiv2026

Selenization of V$_2$O$_5$/WO$_3$ Bilayers for Tuned Optoelectronic Response of WSe$_2$ Films

Scalable and controlled doping of two-dimensional transition metal dichalcogenides is essential for tuning their electronic and optoelectronic properties. In this work, we demonstrate a robust approach for substitution of vanadium in tungsten diselenide (WSe$_2$) via the selenization of pre-deposited V$_2$O$_5$/WO$_3$ thin films. By adjusting the thickness of the vanadium oxide layer, the V concentration in W$_{1-x}$V$_x$Se$_2$ is systematically varied. Electrical measurements on field-effect transistors reveal a substantial enhancement in hole conduction, with drain current increasing by nearly three orders of magnitude compared to undoped WSe$_2$. Temperature-dependent electrical resistivity indicates a clear insulator-to-metal transition with increasing V content, likely due to band structure modifications. Concurrently, the photoconductive gain decreases, suggesting enhanced recombination and charge screening effects. These results establish vanadium doping via selenization of V$_2$O$_5$/WO$_3$ films as a scalable strategy for modulating the transport and photoresponse of WSe$_2$, offering promising implications for wafer-scale optoelectronic device integration.

cond-mat.mes-hall↗

arXiv2026

Photoferroelectric Coupling and Polarization-Controlled Interfacial Band Modulation in van der Waal Compound CuInP2S6

Understanding how optical excitation couples with polarization and interfacial electrostatics in van der Waals (vdW) ferroelectrics (FEs) is essential for the development of light-programmable nanoelectronic and optoelectronic devices. Here, we present direct nanoscale evidence of photoferroionic coupling in the vdW FE semiconductor CuInP2S6 (CIPS), where optical excitation jointly modulates electronic band bending, FE switching, and Cu+ ionic relaxation. The use of correlated Kelvin probe force microscopy, piezoresponse force microscopy, and conductive atomic force microscopy under above-bandgap illumination reveals illumination-induced enhancement of surface work function, persistent surface photovoltage, reduced coercive field, and positive imprint shifts. These effects arise from synergistic photocarrier redistribution and slow Cu+ migration that reshape interfacial depletion widths and internal electric fields. Illumination-assisted barrier lowering further enhances carrier injection and produces sweep-rate-dependent ferroionic transport hysteresis. Our results establish photoferroionic coupling as the governing mechanism for light-controlled band modulation and polarization stability in CIPS, providing a nanoscale framework for designing light-addressable FE memories, optoelectronic switches, and neuromorphic devices based on layered ferroionic materials.

cond-mat.mtrl-sci↗

arXiv2026

Cr3+ spin dynamics under the octahedral crystal field in van der Waals antiferromagnets

The magnetic moment in van der Waals (vdW) materials containing 3d transition metals originates from unpaired d-electron spins and their interaction with surrounding ligands. The interplay between exchange interactions and magnetic anisotropy stabilizes long-range ordering of such moments. The compound CuCrP2S6 (CCPS) presents an interesting class of vdW solids where the coupling of Cr3+ moments and ordering of Cu1+ ions give rise to a multiferroic ground state. Here we investigate the spin dynamics of Cr3+ ions in CCPS through magnetization and broadband as well as single sub-THz magnetic resonance measurements. The orbital moment of Cr3+ is quenched under the octahedral crystal field of surrounding chalcogen ions, resulting in negligible magnetic anisotropy-a feature common to Cr-based vdW antiferromagnets (AFM). Resonance spectra over a wide frequency-field-temperature range reveal quasi-2D AFM dynamics governed mainly by isotropic Cr-Cr exchange interactions, which determine the magnetic order, spin reorientation, and damping. Sub-THz resonance spectra also uncover a field-induced ferromagnetic polarization, highlighting the universal role of Cr-Cr exchange in layered Cr compounds. Moreover, persistent magnetic correlations far above the Néel temperature (TN ~ 32 K) points to short-range magnetic order in CCPS and motivates future studies of a possible interplay between AFM and antiferroelectric orders. These results establish CCPS as an exemplary system for exploring 2D magnetism and electric-field-tunable spintronic functionalities in layered multiferroics.

cond-mat.mtrl-sci↗

arXiv2025

Quenching the Non-Collinear Spin Order in High-Tc Layered Ferromagnet Fe5GeTe2

The realization of long-range spin order in two-dimensions (2D) has catapulted the search for layered materials with magnetic ordering above room temperature. These efforts aim to understand and enhance the spin spin interactions in 2D. An emergent class of such magnets is the layered FeNGeTe2 (N = 3, 4, and 5). Here, we investigate the magnetic states over a wide field temperature phase space in the high-Tc ferromagnet Fe5GeTe2 using magnetization, ferromagnetic resonance (FMR), and magneto-transport measurements. Our findings reveal a magnetic phase transition from a collinear to a complex non-collinear magnetic order near the temperature T* = 160 K, below which magnetic susceptibility is reduced, FMR linewidth broadened, and anomalous Hall resistivity suppressed. Such non-collinearity results from the competition between magnetocrystalline anisotropy and Dzyaloshinskii Moriya interaction arising from the unusual Fe1 ordering in two possible split sites. Our study focuses on the strategy to quench the non-collinear spin order. Substituting 40% Ni in Fe5GeTe2 is found to be one such quenching strategy. This provides deeper insights into the magnetism of a high-Tc layered-ferromagnet, offering opportunities to develop 2D magnet-based devices.

cond-mat.mtrl-sci↗

arXiv2023

Planar Hall effect and Anisotropic Magnetoresistance in Thin Films of Chiral Antiferromagnet Mn3Sn

Antiferromagnetic Weyl semimetals with spin chirality offer excellent platforms to address the Berry phase physics, which manifests prominently in several of their electro-optical and electro-magnetic responses including as a large anomalous Hall effect (AHE) and spin Hall conductivity. Here, we report measurements of magneto-transport in c-axis textured Mn3Sn thin films grown on the [111] plane of single crystal MgO. At room temperature, these films display a weak uncompensated magnetic moment of \approx 0.12 \micro_{B}/f.u. in the basal plane and a longitudinal resistivity (ρ_{xx}) close to \approx 3.8 \microΩ.m. A residual resistivity ration (ρ_{xx} (300 K)/ρ_{xx} (2 K)) of \approx 3.92 further indicates the high quality of the films. While at 300 K a weak AHE together with field-linear Hall resistivity (ρ_{xy}) is observed in magnetic fields (H) applied perpendicular to the Kagome planes, the temperature (T) dependence of ρ_{xy} shows prominent signatures of three magnetic phases in the temperature regime of 2 to 300 K. The ρ_{xy} also derives a non-trivial topological contribution (\r{ho}THE \approx 1nΩ.m) in the spin glass phase which appears at T \geq 100 K. Our measurements of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) over a wide H-T phase space reveal the hitherto unseen effects in the three magnetic phases of Mn3Sn. While the AMR and PHE are negative in the inverse triangular spin phase (250 K \geq T \geq TN), the helical phase (100 \geq T \geq 250 K) is devoid of anisotropic in-plane resistivity, and the spin glass phase shows a sign reversal of AMR with the increasing magnetic field. The origin of this sign change in AMR/PHE is attributed to the emergence of topologically protected spin textures like skyrmions where the fictitious effective magnetic field is estimated to be \approx 4.4 tesla.

cond-mat.mtrl-sci↗

arXiv2023

Superfluid response of two-dimensional filamentary superconductors

Different classes of low-dimensional superconducting systems exhibit an inhomogeneous filamentary superconducting condensate whose macroscopic coherence still needs to be fully investigated and understood. Here we present a thorough analysis of the superfluid response of a prototypical filamentary superconductor embedded in a {two-dimensional} metallic matrix. By mapping the system into an exactly solvable random impedance network, we show how the dissipative (reactive) response of the system non-trivially depends on both the macroscopic and microscopic characteristics of the metallic (superconducting) fraction. We compare our calculations with resonant-microwave transport measurements performed on LaAlO$_3$/SrTiO$_3$ heterostructures over an extended range of temperatures and carrier densities finding that the filamentary character of superconductivity accounts for unusual peculiar features of the experimental data.

cond-mat.supr-con↗

arXiv2022

Magneto-transport and magnetic textures in Ho/FeCoGd/β-W multilayers

The enhancement of interfacial Dzyaloshinskii-Moriya Interaction (DMI) in magnetic multilayers results in the stabilization of topological spin textures like chiral domain walls and skyrmions. Here we report on the evaluation of interface-driven magnetic interactions in a uniquely designed multilayer where each magnetic layer of two AFM coupled sublattices of 3d and 4f moments is sandwiched between the layers of β-tungsten and holmium whose spin Hall angles are large but opposite in sign. The atomic and magnetic periodicity of these multilayers is established by polarized neutron reflectivity measurements and the presence of a labyrinth domain spin texture of zero remanence with x-ray photoelectron microscopy. Measurements of the Hall resistivity (ρ_{xy}(T, H)) together with static magnetization (M(T,H)) over a broad range of temperature (T) and magnetic field (H) indicate impending compensation between 3d and 4f sublattices at T>350 K. These multilayers are characterized by a small (0.04 %) but positive magnetoresistance indicative of interface enhance scattering and a large (40 nΩ.m) and negative anomalous ρ_{xy}(T,H) which results from a parallel alignment of 4f moments with the external magnetic field. No distinct scaling is seen between ρ_{xy}(T,H), ρ_{xx}(T, H) and M(T,H) at temperatures above 200K where the magnetization develops out-of-plane anisotropy. The field scans of ρ_{xy} at T>200K show a distinct cusp in the vicinity of magnetic saturation. These Hall data have been analyzed in the framework of a model where a distinct topological contribution to ρ_{xy} rides over the anomalous Hall resistivities of the 3d and 4f magnetic sublattices. It is suggested that this apparent topological effect results from an interfacial DMI and dominates ρ_{xy}(T,H) in the temperature regime where the 3d and 4f lattices are nearly compensated.

cond-mat.mtrl-sci↗

arXiv2022

Light and microwave driven spin pumping across FeGaB-BiSb interface

3-D topological insulators (TI) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet (FM) Fe_{78}Ga_{13}B_{9} (FeGaB) and 3-D TI Bi_{85}Sb_{15} (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. DC magnetization measurements establish the soft magnetic character of FeGaB films, which remains unaltered in the heterostructures of FeGaB-BiSb. Broadband ferromagnetic resonance (FMR) studies reveal enhanced damping of precessing magnetization and large value of spin mixing conductance (5.03 x 10^{19} m^{-2}) as the spin angular momentum leaks into the TI layer. Magnetic field controlled bipolar dc voltage generated across the TI layer by inverse spin Hall effect is analyzed to extract the values of spin Hall angle and spin diffusion length of BiSb. The spin pumping parameters derived from the measurements of the femtosecond light-pulse-induced terahertz emission are consistent with the result of FMR. Kubo-Bastin formula and tight-binding model calculations shed light on the thickness-dependent spin-Hall conductivity of the TI films, with predictions that are in remarkable agreement with the experimental data. Our results suggest that room temperature deposited amorphous and polycrystalline heterostructures provide a promising platform for creating novel spin orbit torque devices.

cond-mat.mtrl-sci↗

arXiv2022

Resonant Precession of Magnetization and Precession -- Induced DC voltages in FeGaB Thin Films

Measurements of frequency dependent ferromagnetic resonance (FMR) and spin pumping driven dc voltage (V_{dc}) are reported for amorphous films of Fe_{78}Ga_{13}B_{9} (FeGaB) alloy to address the phenomenon of self-induced inverse spin Hall effect (ISHE) in plain films of metallic ferromagnets. The V_{dc} signal, which is antisymmetric on field reversal, comprises of symmetric and asymmetric Lorentzians centered around the resonance field. Dominant role of thin film size effects is seen in setting the magnitude of static magnetization, V_{dc} and dynamics of magnetization precession in thinner films (\leq 8 nm). The film thickness dependence of magnetization parameters indicates the presence of a magnetically disordered region at the film-substrate interface, which may promote preferential flow of spins generated by the precessing magnetization towards the substrate. However, the V_{dc} signal also draws contributions from rectification effects of a \approx 0.4 \% anisotropic magnetoresistance and a large (\approx 54 nΩ.m) anomalous Hall resistivity (AHR) of these films which ride over the effect of spin-orbit coupling driven spin-to-charge conversion near the film-substrate interface. We have addressed these data in the framework of the existing theories of electrodynamics of a ferromagnetic film subjected to radio-frequency field in a coplanar waveguide geometry. Our estimation of the self-induced ISHE for the sample with 54 nΩ.m AHR shows that it may contribute significantly (\approx 90\%) to the measured symmetric voltage. This study is expected to be very useful for fully understanding the spin pumping induced dc voltages in metallic ferromagnets with disordered interfaces and large anomalous Hall effect.

cond-mat.mtrl-sci↗