arXiv subjects

Sidhanta Sahu

Publications and source records attributed to Sidhanta Sahu.

5 recordsLinked to original sources

arXiv2026

Long-lived relaxation channel and exciton-phonon coupling in Ta2NiSe5 via non-degenerate pump-probe spectroscopy

An excitonic insulator represents a quantum phase in which spontaneous condensation of excitons leads to novel many-body phenomena. Ta$_2$NSi$_5$ (TNSe), a layered narrow-gap semiconductor, has emerged as a model platform to probe these correlated excitonic phases and their underlying dynamics below 327 K. In this work, we investigate the nonequilibrium dynamics of TNSe using temperature-dependent, non-degenerate optical pump-probe spectroscopy with a 3.14 eV pump and a 1.57 eV probe, extending the accessible pump-probe delay window up to 500 ps. In addition to the well-established sub-picosecond relaxation channel ($\sim$ 0.7- 0.9 ps) associated with carrier cooling and recombination, accompanied by exciton reformation, we uncover a much slower recovery process with a decay time of $\sim$ 280-600~ps, significantly longer than previously reported. We attribute this unusually prolonged recovery to enhanced scattering between excitons and nonequilibrium phonons, which delays the re-establishment of equilibrium excitonic correlations. On top of this bi-exponential background, we observe two coherent phonon modes at 1.0 and 2.9 THz with distinctly different coupling behaviors. The 1.0 THz mode exhibits an order-parameter-like temperature dependence, consistent with strong coupling to the excitonic condensate in TNSe. In contrast, the 2.9 THz mode does not exhibit any discernible coupling to the excitonic order parameter, and appears to arise from anharmonic lattice dynamics associated with the structural phase transition. Together, these results elucidate the hierarchy of relaxation pathways in TNSe and highlight the importance of extending the temporal detection window in pump-probe measurements to fully capture long-lived exciton-phonon dynamics.

cond-mat.str-el↗

arXiv2026

Ultrafast Critical Slowing of Spin Dynamics and Emergent Nonequilibrium Fano Interference in Fe3GeTe2

Fe$_3$GeTe$_2$ is a prototypical metallic van der Waals ferromagnet with itinerant magnetism and a highly tunable Curie temperature, yet how electronic excitations couple to spin and lattice degrees of freedom across its magnetic transition remains largely unexplored. Here, we use two-color pump-probe reflectivity to investigate the coupled electronic, spin, and lattice dynamics. The time-resolved reflectivity exhibits a tri-exponential relaxation, in which the intermediate component shows an anomaly near the Curie temperature due to enhanced interlayer spin-lattice interactions, while the slowest component displays pronounced critical slowing down with an exponent of ~ 0.3, revealing non-universal relaxation dynamics associated with intralayer spin correlations. Furthermore, we observe an emergent nonequilibrium A1g phonon Fano asymmetry that is suppressed in the ferromagnetic phase but anomalously enhanced in the paramagnetic regime, driven by thermally activated anharmonic decay pathways that bridge the kinematic gap to a hot electronic continuum. The pronounced enhancement of the acoustic strain pulse amplitude near T$_c$ further evidences robust magnetoelastic coupling. Overall, our results reveal how magnetic order governs the interplay among critical spin dynamics, electronic continuum excitations, and lattice response in metallic van der Waals ferromagnets

cond-mat.mtrl-sci↗

arXiv2025

Terahertz Time-Domain Spectroscopy and Density Functional Theory Analysis of Low-Frequency Vibrational Modes of a Benzoxazolium-Coumarin Donor-pi-Acceptor Chromophore

To elucidate low-frequency vibrational modes, we investigate a benzoxazolium--coumarin (BCO+) donor-pi-acceptor derivative using transmission terahertz time-domain spectroscopy (THz-TDS). The retrieved complex refractive index reveals distinct modes at 0.62, 0.85, 1.30, 1.81, and 2.07 THz. Gas-phase density functional theory (DFT) agrees well with these features and enables assignment of specific intramolecular motions. Together, THz-TDS and DFT identify the characteristic low-frequency modes of BCO+ and suggest their connection to intramolecular charge transfer-relevant nuclear motions, highlighting that THz-TDS can serve as a sensitive probe of vibrational signatures in donor-pi-acceptor systems.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast Dynamics of Spin-Orbit Entangled Excitons Coupled to Magnetic Ordering in van der Waals Antiferromagnet NiPS3

Spin-orbit entangled excitons (SOEE) in two-dimensional (2D) antiferromagnets provide direct access to explore unconventional many body interactions in correlated electron systems. In this work, we carry out a detailed investigation using non-degenerate isotropic and anisotropic pump-probe reflection spectroscopy to probe the ultrafast dynamics of SOEE and their coupling to spin fluctuations in NiPS3. Transient reflectivity data reveals acoustic phonon oscillations at ~ 27 GHz, along with two distinct relaxation timescales: fast (1-9 ps) and slower components (1-4 ns) associated with SOEE coherence and spin reordering, respectively. Both timescales exhibit pronounced temperature dependence near the exciton dissociation (TED = 120 K) and Neel (TN = 155 K) temperatures. The SOEE coherence shortens from ~ 8-9 ps at T < TED to ~ 3 ps at T > TED with a finite tail persisting beyond TN. The spin reordering time grows near 120 K, and shows critical slowing down around TN. Pump fluence studies further corroborate their spin origin. Our findings uncover the direct interplay between the excitonic and spin degrees of freedom across ultrafast and longer timescales, offering new opportunities to probe and engineer emergent many-body interactions in 2D antiferromagnets.

cond-mat.other↗

arXiv2025

Ultrafast dynamics of carriers, coherent acoustic phonons and strain pulses in BiSbTe1.5Se1.5 topological insulator thin films

We Investigate the ultrafast carrier, coherent acoustic phonons (CAPs), and acoustic strain pulse dynamics in topological insulator BiSbTe1.5Se1.5 (BSTS) thin films of varying thickness using degenerate pump-probe reflection spectroscopy. Here, Sapphire has been chosen as the main substrate due to its maximum acoustic reflectivity at the BSTS-sapphire interface compared to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. For the films with thickness more than twice the penetration depth, the transient reflectivity data predominantly exhibits travelling acoustic strain pulses (TASP) on the top of single-exponential electronic decay (~ 2 ps). In contrast, films with thickness less than penetration depth are dominated by CAPs and a bi exponential electronic background with decay times of ~ 2 ps and ~ 260-380 ps. The observed TASP dynamics are well-described by a theoretical acoustic strain model. Further, to elucidate the underlying physical mechanisms governing the behavior of photo-excited carriers, CAPs, and strain pulses, we performed carrier density and temperature-dependent (7-294 K) studies on BSTS films with thicknesses of 22 nm and 192 nm. In the 22 nm film, the both fast and slow decay processes increase with carrier density at room temperature but decrease with temperature at a carrier density of 1.7*10^{19} cm^{-3}. A detailed analysis suggests that the faster decay arises from electron-phonon scattering and carrier diffusion, while the slower decay likely results from defect-assisted and phonon-assisted recombination. Furthermore, increasing the sample temperature leads to anharmonic decay induced softening of ~ 14 % in the phonon frequency and an anomalous ~ 48 % decrease in the phonon damping parameter due to reduced Dirac surface electron and acoustic phonon scattering.

cond-mat.mtrl-sci↗