arXiv subjects

Sumedh Rathi

Publications and source records attributed to Sumedh Rathi.

5 recordsLinked to original sources

arXiv2026

Revealing Intrinsic Anisotropy of Collective Magnetic Excitations in Twinned Crystals of a Kitaev-Heisenberg Quantum Magnet

Quantum magnets with competing interactions often emerge from delicate balances among microscopic parameters, making it essential to disentangle intrinsic spin dynamics from extrinsic disorder effects. Here, we introduce a multimodal optical approach combining magneto-infrared spectroscopy with domain-resolved micro-Raman spectroscopy at high magnetic fields to reconstruct the intrinsic magnetic excitation spectrum of twinned crystals of the Kitaev-Heisenberg quantum magnet Na$_3$Co$_2$SbO$_6$. Far-infrared spectroscopy reveals multiple field-tunable magnetic excitations, but the intrinsic response is obscured by replica features arising from twin domains. By correlating magneto-infrared and domain-resolved Raman spectra, we isolate the single-domain magnon response and uncover a pronounced twofold in-plane magnon anisotropy. This anisotropy far exceeds that expected from the measured in-plane g-factor anisotropy and is instead dominated by anisotropic bond-dependent exchange interactions. By unifying high-field, high-resolution and spatially selective optical probes, our work establishes a broadly applicable framework for revealing intrinsic spin dynamics and constraining the spin Hamiltonian in multidomain quantum magnets.

cond-mat.mtrl-sci↗

arXiv2026

Observation and Control of Spontaneous Magnon Emission from Spin Ensembles in 2D Hexagonal Boron Nitride

Hybrid systems consisting of color centers and magnetic materials provide an appealing solid-state platform for advancing the burgeoning quantum technological revolution. Exploring novel coupling mechanisms between optically active spin defects and quantum degrees of freedom is directly relevant in this context. Here, we report observation and control of spontaneous magnon emission from boron-vacancy centers in 2D hexagonal boron nitride (hBN), an unconventional qubit-magnon dipole coupling channel that dominates in the near-zero temperature limit. The spontaneous magnon emission process starts to be overshadowed by thermal magnon effect as temperature increases, reflecting the crossover from an emission-dominated, effectively cold magnon reservoir to a thermally occupied spin bath where absorption and stimulated processes restore balance. By increasing the spin defect density, we further present that spontaneous magnon emission into a common spin bath could help establish quantum correlations in dense hBN spin ensembles. Our results are quantitatively captured by detailed theoretical modeling, bringing insights into understanding qubit-magnon coupling, correlated spin dynamics, and many-body physics of color centers in the quantum regime.

cond-mat.mes-hall↗

arXiv2025

Sharp spectroscopic fingerprints of disorder in an incompressible magnetic state

Disorder significantly impacts the electronic properties of conducting quantum materials by inducing electron localization and thus altering the local density of states and electric transport. In insulating quantum magnetic materials, the effects of disorder are less understood and can drastically impact fluctuating spin states like quantum spin liquids. In the absence of transport tools, disorder is typically characterized using chemical methods or by semi-classical modeling of spin dynamics. This requires high magnetic fields that may not always be accessible. Here, we show that magnetization plateaus -- incompressible states found in many quantum magnets -- provide an exquisite platform to uncover small amounts of disorder, regardless of the origin of the plateau. Using optical magneto-spectroscopy on the Ising-Heisenberg triangular-lattice antiferromagnet K$_2$Co(SeO$_3$)$_2$ exhibiting a 1/3 magnetization plateau, we identify sharp spectroscopic lines, the fine structure of which serves as a hallmark signature of disorder. Through analytical and numerical modeling, we show that these fingerprints not only enable us to quantify minute amounts of disorder but also reveal its nature -- as dilute vacancies. Remarkably, this model explains all details of the thermomagnetic response of our system, including the existence of multiple plateaus. Our findings provide a new approach to identifying disorder in quantum magnets.

cond-mat.mtrl-sci↗

arXiv2025

Visualizing Field-free Deterministic Magnetic Switching of all-van der Waals Spin-Orbit Torque System Using Spin Ensembles in Hexagonal Boron Nitride

Recently, optically active spin defects embedded in van der Waals (vdW) crystals have emerged as a transformative quantum sensing platform to explore cutting-edge materials science and quantum physics. Taking advantage of excellent solid-state integrability, this new class of spin defects can be arranged in controllable nanoscale proximity of target materials in vdW heterostructures, showing great promise for improving spatial resolution and field sensitivity of current sensing technologies. Building on this state-of-the-art measurement platform, here we report hexagonal boron nitride-based quantum imaging of field-free deterministic magnetic switching of room-temperature two-dimensional magnet Fe3GaTe2 in an all-vdW spin-orbit torque (SOT) system. By visualizing SOT-driven variations of nanoscale Fe3GaTe2 magnetic stray field profile under different conditions, we have revealed how the observed magnetic switching evolves from deterministic to indeterministic behavior due to the interplay between out-of-plane spins, in-plane spins and Joule heating. This understanding, which is otherwise difficult to access by conventional transport measurements, offers valuable insights on material design, testing, and evaluation of next-generation vdW spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Spin wavepackets in the Kagome ferromagnet Fe$_3$Sn$_2$: propagation and precursors

The propagation of spin waves in magnetically ordered systems has emerged as a potential means to shuttle quantum information over large distances. Conventionally, the arrival time of a spin wavepacket at a distance, $d$, is assumed to be determined by its group velocity, $v_g$. He we report time-resolved optical measurements of wavepacket propagation in the Kagome ferromagnet Fe$_3$Sn$_2$ that demonstrate the arrival of spin information at times significantly less than $d/v_g$. We show that this spin wave "precursor" originates from the interaction of light with the unusual spectrum of magnetostatic modes in Fe$_3$Sn$_2$. Related effects may have far-reaching consequences toward realizing long-range, ultrafast spin wave transport in both ferromagnetic and antiferromagnetic systems.

cond-mat.mtrl-sci↗