arXiv subjects

Sk Kalimuddin

Publications and source records attributed to Sk Kalimuddin.

13 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

Spin-reorientation as a switch for electronic topology in van der Waals ferromagnets

The interplay between spin reorientation and topological electronic structure in two-dimensional (2D) van der Waals (vdW) ferromagnets is central to understanding how magnetic anisotropy shapes charge transport. Although spin-reorientation transitions (SRTs) are common in 2D metallic ferromagnets, their impact on electronic-topology-driven thermodynamic and transport properties remains largely unexplored. Here we investigate this issue in Fe$_4$GeTe$_2$ (F4GT), a room-temperature quasi-2D vdW ferromagnet, using temperature-dependent magnetization, specific heat, magnetotransport, and thermoelectric measurements. Magnetization and specific heat establish a reorientation of the magnetic easy axis near $T_{\mathrm{SRT}} \sim 100$~K, in addition to ferromagnetic ordering at $T_C \sim 270$~K. Across the SRT, the Seebeck coefficient and anisotropic magnetoresistance show clear anomalies, indicating Fermi-surface reconstruction. The magnetoresistance exhibits a two-step field dependence: a low-field enhancement near the SRT associated with scattering from canted spins and evolving domains, followed by a higher-field negative response as spin fluctuations are suppressed. The simultaneous sign change of the ordinary Hall coefficient $R_0$ and the sharp anomaly in the anomalous Hall resistivity $\rho^{A}_{yx}$ further point to a temperature-driven modification of the underlying band topology. Analysis of the anomalous Hall conductivity $\sigma^{A}_{xy}$ and the scaling of $\rho^{A}_{yx}$ shows that the Berry-curvature-driven anomalous Hall response below $T_{\mathrm{SRT}}$ is strongly modified above the transition. Our results identify spin reorientation as an internal control parameter for switching between distinct topological transport regimes in a 2D vdW ferromagnet, providing a symmetry-controlled route to engineer spin-polarized electronic states and domain-texture-driven functionalities.

cond-mat.str-el↗

arXiv2025

Emergence of a hidden-order phase well below the charge density wave transition in a topological Weyl semimetal (TaSe$_4$)$_2$I

The emergence of a charge density wave (CDW) in a Weyl semimetal -- a correlated topological phase, is exceptionally rare in condensed matter systems. In this context, the quasi-one-dimensional type-III Weyl semimetal (TaSe$_4$)$_2$I undergoes a CDW transition at $T_{\mathrm{CDW}} \approx 263$~K, providing an exceptional platform to investigate correlated topological CDW states. Here, we uncover an additional hidden-order phase transition at $T^* \sim 100$ K, well below the CDW onset, using low-frequency resistance noise spectroscopy, electrical transport, and thermoelectric measurements. This transition is characterized by a sharp enhancement in the noise exponent ($\alpha$) and variance of resistance fluctuations. Analysis of higher-order statistics of resistance fluctuations reveals the correlated dynamics underlying the transition. A pronounced anomaly in the Seebeck coefficient near $T^*$ further suggests a Fermi surface reconstruction. First-principles calculations reveal a structural distortion from the high-symmetry $I422$ phase to a low-symmetry $C2$ phase, via an intermediate $I4$ symmetry. This leads to renormalization of the electronic structure near the Fermi level and opening of a bandgap in the hidden-order phase. These findings demonstrate a previously unidentified correlated phase transition in the topological CDW-Weyl semimetal (TaSe$_4$)$_2$I, enriching the phase diagram of this material and establishing it as an ideal platform for studying intertwined electronic and structural orders.

cond-mat.str-el↗

arXiv2025

Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$

Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition to a low temperature gapped phase. Here we present experimental signatures of this room temperature phase transition from a high-temperature $\beta$-phase with a surface state to a gapped $\alpha$-phase hosting hinge states. Using real-space current mapping and resistance fluctuation spectroscopy, we identify signatures of a displacive topological phase transition mediated by a first-order thermodynamic structural change. Near the emergence of $\beta$-phase, we observe pronounced telegraphic noise, indicating fluctuating phase domains with topological surface states. The spatially resolved current map reveals electron transport via the gapless surface states in the $\beta$-phase, which vanishes upon transitioning to the $\alpha$-phase with localized conduction channels (or hinge modes). Our experimental results, supported by first principles estimates and effective theory of a topological displacive phase transition, establish Bi$_4$I$_4$ as a candidate material showing intricate interplay of classical thermodynamic phase transitions with topological quantum phenomena.

cond-mat.str-el↗

arXiv2025

Direction-Dependent Conduction Polarity in Altermagnetic CrSb

CrSb has recently gained immense attention as an altermagnetic candidate. This work reports on the experimental observation of direction-dependent conduction polarity (DDCP) in altermagnetic CrSb through Hall and Seebeck thermopower measurements. Conduction is dominated by holes along the c-axis and by electrons in the ab-plane of the hexagonal crystal of CrSb. Density functional theory (DFT) calculations indicate that DDCP in CrSb arises from a multicarrier mechanism, where electrons and holes living in distinct bands dominate conduction along different crystallographic directions. Furthermore, DFT predicts that DDCP exists within a narrow energy window near the Fermi level and is sensitive to small doping levels. This prediction is experimentally validated by the loss of DDCP in hole-doped Cr$_{0.98}$V$_{0.02}$Sb. These findings highlight the potential for tunable electronic behavior in CrSb, offering promising avenues for applications in devices that require both p-type and n-type functionalities within a single material.

cond-mat.mtrl-sci↗

arXiv2024

Barrier height inhomogeneity and origin of 1/f-noise in topological insulator-based photo-detector

Topological insulators (TIs) with symmetry-protected surface states, offer exciting opportunities for next-generation photonic and optoelectronic device applications. The heterojunctions of TIs and semiconductors (e.g. Si, Ge) have been observed to excellent photo-responsive characteristics. However, the realization of high-frequency operations in these heterojunctions can be hindered by unwanted 1/f (or Flicker) noise and phase noise. Therefore, an in-depth understanding of 1/f noise figures becomes paramount for the effective utilization of such materials.Here we report optoelectronic response and 1/f noise characteristics of a p-n diode fabricated using topological insulator, Bi2Se3 and silicon for potential photo-detector. Through meticulous temperature-dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements, we ascertain crucial parameters like barrier height, ideality factor, and reverse saturation current of the photodetector. The low-frequency 1/f conductance noise spectra suggest a significant presence of trap states influencing the optoelectronic transport properties. The forward noise characteristics exhibit typical 1/f features, having a uni-slope across four decades of frequency, suggesting a homogeneous distribution of barrier height. The spectral and photocurrent-dependent responses show the power law behavior of noise level on photon flux. The hybrid heterojunction demonstrates excellent photo-response and reasonably low noise level, promising signatures for the room-temperature visible photodetector applications.

cond-mat.str-el↗

arXiv2023

Anomalous Hall effect induced by Berry curvature in topological nodal-line van der Waals ferromagnet Fe$_4$GeTe$_2$

The exploration of nontrivial transport phenomena associated with the interplay between magnetic order and spin-orbit coupling (SOC), particularly in van der Waals (vdW) systems has gained a resurgence of interest due to their easy exfoliation, ideal for two-dimensional (2D) spintronics. We report the near room temperature quasi-2D ferromagnet, Fe$_4$GeTe$_2$ from the iron-based vdW family (Fe$_n$GeTe$_2$, $n$=3,4,5), exhibiting a large anomalous Hall conductivity (AHC), $\sigma^A_{xy}$ $\sim$ 490 $\Omega^{-1}\textrm{cm}^{-1}$ at 2 K. The near quadratic behavior of anomalous Hall resistivity ($\rho^{A}_{xy}$) with the longitudinal resistivity ($\rho_{xx}$) suggests that a dominant AHC contribution is coming from an intrinsic Berry curvature (BC) mechanism. Concomitantly, the electronic structure calculations reveal a large BC arising from SOC induced gaped nodal lines around the Fermi level, governing such large AHC property. Moreover, we also report an exceptionally large anomalous Hall angle ($\simeq$ 10.6\%) and Hall factor ($\simeq$ 0.22 V$^{-1}$) values which so far, are the largest in compared to those for other members in this vdW family.

cond-mat.str-el↗

arXiv2023

Exceptionally Slow, Long Range, and Non-Gaussian Critical Fluctuations Dominate the Charge Density Wave Transition

$(TaSe_4)_2I$ is a well-studied quasi-one-dimensional compound long-known to have a charge-density wave (CDW) transition around 263 K. We argue that the critical fluctuations of the pinned CDW order parameter near the transition can be inferred from the resistance noise on account of their coupling to the dissipative normal carriers. Remarkably, the critical fluctuations of the CDW order parameter are slow enough to survive the thermodynamic limit and dominate the low-frequency resistance noise. The noise variance and relaxation time show rapid growth (critical opalescence and critical slowing down) within a temperature window of $ \varepsilon \approx \pm 0.1$, where $\varepsilon$ is the reduced temperature. This is very wide but consistent with the Ginzburg criterion. We further show that this resistance noise can be quantitatively used to extract the associated critical exponents. Below $|\varepsilon | \lesssim 0.02$, we observe a crossover from mean-field to a fluctuation-dominated regime with the critical exponents taking anomalously low values. The distribution of fluctuations in the critical transition region is skewed and strongly non-Gaussian. This non-Gaussianity is interpreted as the breakdown of the validity of the central limit theorem as the diverging coherence volume becomes comparable to the macroscopic sample size. The large magnitude critical fluctuations observed over an extended temperature range, as well as the crossover from the mean-field to the fluctuation-dominated regime highlight the role of the quasi-one dimensional character in controlling the phase transition.

cond-mat.str-el↗

arXiv2023

Raman signatures of lattice dynamics across inversion symmetry breaking phase transition in quasi-1D compound, (TaSe$_4$)$_3$I

Structural phase transition can occur due to complex mechanisms other than simple dynamical instability, especially when the parent and daughter structure is of low dimension. This article reports such an inversion symmetry-breaking structural phase transition in a quasi-1D compound (TaSe$_4$)$_3$I at T$_S\sim$ 141~K studied by Raman spectroscopy. Our investigation of collective lattice dynamics reveals three additional Raman active modes in the low-temperature non-centrosymmetric structure. Two vibrational modes become Raman active due to the absence of an inversion center, while the third mode is a soft phonon mode resulting from the vibration of Ta atoms along the \{-Ta-Ta-\} chains. Furthermore, the most intense Raman mode display Fano-shaped asymmetry, inferred as the signature of strong electron-phonon coupling. The group theory and symmetry analysis of Raman spectra confirm the displacive-first-order nature of the structural transition. Therefore, our results establish (TaSe$_4)_3$I as a model system with broken inversion symmetry and strong electron-phonon coupling in the quasi-1D regime.

cond-mat.str-el↗

arXiv2023

Centrosymmetric-noncentrosymmetric Structural Phase Transition in Quasi one-dimensional compound, (TaSe$_4$)$_3$I

(TaSe$_4$)$_3$I, a compound belonging to the family of quasi-one-dimensional transition-metal tetrachalcogenides, has drawn significant attention due to a recent report on possible coexistence of two antagonistic phenomena, superconductivity and magnetism below 2.5~K (Bera et. al, arXiv:2111.14525). Here, we report a structural phase transition of the trimerized phase at temperature, $T~\simeq$~145~K using Raman scattering, specific heat, and electrical transport measurements. The temperature-dependent single-crystal X-ray diffraction experiments establish the phase transition from a high-temperature centrosymmetric to a low-temperature non-centrosymmetric structure, belonging to the same tetragonal crystal family. The first-principle calculation finds the aforementioned inversion symmetry-breaking structural transition to be driven by the hybridization energy gain due to the off-centric movement of the Ta atoms, which wins over the elastic energy loss.

cond-mat.str-el↗

arXiv2022

Enhanced coercivity and emergence of spin cluster glass state in 2D ferromagnetic material Fe3GeTe2

Two-dimensional (2D) van der Waals (vdW) magnetic materials with high coercivity and high $T_\text{C}$ are desired for spintronics and memory storage applications. Fe$_3$GeTe$_2$ (F3GT) is one such 2D vdW ferromagnet with a reasonably high $T_\text{C}$, but with a very low coercive field, $H_\text{c}$ ($\lesssim$100~Oe). Some of the common techniques of enhancing $H_\text{c}$ are by introducing pinning centers, defects, stress, doping, etc. They involve the risk of undesirable alteration of other important magnetic properties. Here we propose a very easy, robust, and highly effective method of phase engineering by altering the sample growth conditions to greatly enhance the intrinsic coercivity (7-10 times) of the sample, without compromising its fundamental magnetic properties ($T_\text{C}\simeq$210K). The phase-engineered sample (F3GT-2) comprises of parent F3GT phase with a small percentage of randomly embedded clusters of a coplanar FeTe (FT) phase. The FT phase serves as both mosaic pinning centers between grains of F3GT above its antiferromagnetic transition temperature ($T_\text{C1}\sim$70~K) and also as anti-phase domains below $T_\text{C1}$. As a result, the grain boundary disorder and metastable nature are greatly augmented, leading to highly enhanced coercivity, cluster spin glass, and meta-magnetic behavior. The enhanced coercivity ($\simeq$1~kOe) makes F3GT-2 much more useful for memory storage applications and is likely to elucidate a new route to tune useful magnetic properties. Moreover, this method is much more convenient than hetero-structure and other cumbersome techniques.

cond-mat.str-el↗

arXiv2022

Revisiting the magnetic ordering through anisotropic magnetic entropy change in quasi-two-dimensional metallic ferromagnet, Fe$_4$GeTe$_2$

We have investigated the nature of ferromagnetic order and phase transitions in two dimensional (2D) van der Waals (vdW) layered material, Fe$_4$GeTe$_2$ through measurements of magnetization, magneto-caloric Effect (MCE), and heat capacity. Fe$_4$GeTe$_2$ hosts a complex magnetic phase with two distinct transitions: paramagnetic to ferromagnetic at around $T_\text{C}$ $\sim$ 266 K and another spin reorientation transition (SRT) at around $T_\text{SRT}$ $\sim $ 100 K. The magnetization measurements shows a prominent thermal hysteresis in proximity to $T_\text{SRT}$ at $H\parallel c$, which implies the first-order nature of SRT. Reasonable MCE has been observed around both transition temperatures ( at around $T_\text{C}$, -$\Delta$S$_M^\text{max}$ = 1.95 and 1.99 J.Kg$^{-1}$K$^{-1}$ and at around $T_\text{SRT}$, -$\Delta$S$_M^\text{max}$= 3.9 and 2.4 J.Kg$^{-1}$K$^{-1}$ along $H\parallel ab$ and $H\parallel c$ respectively) at 50 kOe magnetic field change. The above results reveal higher MCE value at $T_\text{SRT}$ compared to the values of MCE at $T_\text{C}$. The scaling analysis of MCE at $T_\text{C}$, shows that the rescaled $\Delta$S$_M (T, H)$ follow a universal curve confirming the second-order character of the ferromagnetic transition. The same scaling analysis of MCE breaks down at $T_\text{SRT}$ suggesting that SRT is not a second order phase transition. The exponent $n$ from field dependence of magnetic entropy change presents a maximum of $|n|>2$ confirming the first-order nature of SRT.

cond-mat.str-el↗

arXiv2022

Nonlinear coherent light-matter interaction in 2D MoSe$_2$ nanoflakes for all-optical switching and logic applications

We report a strong nonlinear optical response of 2D MoSe$_2$ nanoflakes (NFs) through spatial self-phase modulation (SSPM) and cross-phase modulation (XPM) induced by nonlocal coherent light-matter interactions. The coherent interaction of light and MoSe$_2$ NFs creates the SSPM of laser beams, forming concentric diffraction rings. The nonlinear refractive index ($n_2$) and the third-order broadband nonlinear optical susceptibility ($\chi^{(3)}$) of MoSe$_2$ NFs are determined from the self diffraction pattern at different exciting wavelengths of 405, 532, and 671 nm with varying the laser intensity. The evolution and deformation of diffraction ring patterns are observed and analyzed by the `wind-chime' model and thermal effect. By taking advantage of the reverse saturated absorption of 2D SnS$_2$ NFs compared to MoSe$_2$, an all-optical diode has been designed with MoSe$_2$/SnS$_2$ hybrid structure to demonstrate the nonreciprocal light propagation. Also a few other optical devices based on MoSe$_2$ and other semiconducting materials such as Bi$_2$Se$_3$, CuPc, and graphene have been investigated. The all-optical logic gates and all-optical information conversion have been demonstrated through the XPM technique using two laser beams. The proposed optical scheme based on MoSe$_2$ NFs has been demonstrated as a potential candidate for all-optical nonlinear photonic devices such as all-optical diodes and all-optical switches.

physics.optics↗