arXiv subjects

Anjan Kumar N M

Publications and source records attributed to Anjan Kumar N M.

3 recordsLinked to original sources

arXiv2026

Dynamics of Charge-Density-Wave puddles in 2$H$-NbSe$_2$

Electronic phases in quantum materials are often governed by nanoscale inhomogeneity, where local order develops within spatially confined regions or puddles. A prominent example is an incommensurate charge-density-wave (I-CDW) that comprises locally commensurate domains. In 2$H$-NbSe$_2$, such an I-CDW state persists alongside lattice anharmonicity and superconductivity, raising fundamental questions about the dynamical stabilization of CDW order in puddles. Here, we probe the puddle-dynamics in 2$H$-NbSe$_2$. Raman scattering reveals a strong Fano-coupling between the interlayer shear vibration and the CDW amplitude mode. Time-resolved reflectivity measurement shows a low-frequency ~0.15 THz coherent overdamped oscillation onsetting within the CDW regime at ~17 K, pointing towards a so far unexplored transition. This we identify as a Fano-coupled phonon-CDW hybrid emerging from the collective dynamics of CDW puddles. We establish a framework for capturing intertwinned glassy dynamics with an overdamped oscillatory model. Such dynamics in 2H-NbSe2 highlight how lattice pinning and electronic correlations in layered materials affect the CDW order, which is crucial for the design of novel Van der Waals devices.

cond-mat.str-el↗

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↗

arXiv2024

Probing of magnetic dimensional crossover in CrSiTe$_{3}$ through picosecond strain pulses

Elucidating the emergence of long-range magnetic ordering from its precursor short-range magnetic ordering (SRMO) in two-dimensional van der Waals materials holds profound implications for fundamental research and technological advancements. However, directly observing the intricate stages of this magnetic dimensional crossover (MDC) remains a significant experimental challenge. While magneto-elastic coupling offers a promising avenue, detecting the minute lattice response to SRMO proves challenging. Recent investigations utilizing second harmonic generation have unveiled a two-step MDC in a van der Waals ferromagnetic insulator. However, an unambiguous detection of MDC through the time-resolved techniques remains elusive. To meet this goal, we have executed an alternative approach by employing picosecond acoustic strain pulses generated by femtosecond lasers to probe the various stages of MDC through the magneto-elastic coupling for the first time. By analyzing the shape of the strain pulse in both the time and frequency domains as a function of temperature, we clearly demonstrate the detection of the subtle influence of spin fluctuations on the lattice. Additionally, the ultrafast carrier dynamics also show signatures of MDC. Our measurements pave the way towards characterizing magnetic materials in time-resolved experiments that are crucial in designing a new generation of spin-based optoelectronic devices.

cond-mat.mtrl-sci↗