arXiv subjects

Surajit Saha

Publications and source records attributed to Surajit Saha.

At least 19 recordsLinked to original sources

arXiv2026

Ferroelastic exciton splitting in hybrid perovskite nanowalls

Hybrid metal-halide perovskites are soft semiconductors in which electronic excitations are strongly influenced by lattice distortions and structural phase transitions. An important open question is whether ferroelastic symmetry breaking merely broadens optical resonances or instead modifies excitonic states through exciton-lattice coupling. Here, we address this question using highly aligned MAPbI3 nanowalls fabricated by glancing-angle deposition, enabling symmetry-selective coupling between ferroelastic texture, structural anisotropy, and a well-defined optical axis. Combining temperature-dependent photoluminescence, X-ray diffraction and polarization-resolved ultrafast transient absorption spectroscopy, we observe a polarization-selective excitonic splitting in the orthorhombic phase at 5 K, characterized by orthogonal optical selection rules and a 45 meV energy separation. Near 160 K, where orthorhombic and tetragonal phases coexist, a lower-energy lattice-coupled excitation emerges 58 meV below the centre of the anisotropically split excitonic structure, consistent with coupling between excitonic and lattice-dressed states. At higher temperatures, these excitations progressively acquire lattice-dressed character accompanied by reduced optical anisotropy. A symmetry-guided effective Hamiltonian captures the evolution from anisotropically split excitons to coupled excitonic and lattice-dressed states across the structural transition. Our results show that ferroelastic texture and phase coexistence can modify exciton-lattice coupling, providing a route to symmetry-selective optical responses in soft polar semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Random walk with multiple memory channels: a new paradigm

A new class of one-dimensional, discrete time random walk model with memory, termed "Random walk with $n$ memory channels" (RW$n$MC) is proposed. In this model the information of $n$ ($n\in \mathbb{Z}$) previous steps from the walker's entire history are needed to decide future step. Exact calculation of the mean and variance of position of the RW2MC ($n=2$) has been done which shows that it can lead to asymptotic diffusive and superdiffusive behavior in different parameter regimes. A connection between RW$n$MC and P\'olya type urn model evolving by $n$ drawings has also been reported. This connection for the RW2MC is discussed in detail which suggests the applicability of RW$2$MC in many population dynamics model with multiple competing species.

cond-mat.stat-mech↗

arXiv2025

Emergent Quantum Walk Dynamics from Classical Interacting Particles

The dynamics of a discrete-time quantum walk (DTQW) can be realized within a purely classical interacting particle system composed of some boxes and a large but finite number of balls, and can, in principle, be implemented in a tabletop experimental setting. The distribution of the balls evolves under stochastic, occupation-dependent update rules at each lattice site, producing quantum-walk dynamics without invoking a wavefunction. The update parameters are fixed by the parameters of coin and shift operations of the DTQW. This framework naturally yields a generalized active spin model and provides a minimal lattice-based microscopic understanding of the emergence of quantum-like dynamics in active matter systems. This interdisciplinary approach connects the classical models to the broad range of applications where DTQWs are successfully employed.

quant-ph↗

arXiv2025

Evidence of spin-phonon-charge coupling in quasi-one-dimensional Ising spin chain system $\alpha$-CoV$_2$O$_6$

The quasi-one-dimensional Ising spin chain system $\alpha$-CoV$_2$O$_6$ is considered to exhibit fascinating magnetic properties at lower temperatures. We comprehensively study magnetic properties and lattice dynamics using a combination of x-ray diffraction (XRD), DC magnetization, specific heat, temperature-dependent XRD and Raman scattering measurements, along with the theoretical charge density map calculations. The DC magnetization and specific heat confirm the antiferromagnetic (AFM) long-range order (LRO) at Neel temperature T$_N$ = 15 K as well as the short-range ordering (SRO) below 100 K. Temperature-dependent Raman measurement has been performed in the temperature range of 5 K to 180 K, revealing spin-phonon coupling (SPC) below 100 K, which is well above the T$_N$ and is attributed to SRO. The SPC is calculated for different vibrational modes. The temperature-dependent XRD supports the key finding of magnetoelastic coupling at T$_N$, which is strongly correlated to the magnetoelectric phenomena. The renormalization of Raman modes and lattice anomalies near T$_N$ illustrate spin-lattice coupling via magnetoelastic and spin-phonon interactions leading to the interplay between spin, charge, and phonon degrees of freedom in $\alpha$-CoV$_2$O$_6$. The presence of spin-charge coupling is confirmed by the theoretical charge density maps, which show that the formation of electric dipoles between the Co and O atoms in the AFM state results from the p-d hybridization.

cond-mat.mtrl-sci↗

arXiv2025

Electron-phonon interactions and tensor analysis in topological insulator bismuth telluride using angle resolved polarized Raman spectroscopy

We report on the angle-resolved polarized Raman spectroscopy and estimation of the Raman tensor elements using both classical and quantum treatments to analyse the polarized Raman spectra of single crystal Bismuth Telluride. The observed polar patterns and systematic variations in the relative intensities of four characteristic Raman active modes indicate a higher differential polarizability along the c-axis, accompanied by anisotropic photon-phonon interactions. This interplay of electron-photon-phonon interactions is crucial for understanding the lattice dynamics of Bismuth Telluride, which underpin its thermoelectric performance and topological properties.

cond-mat.mtrl-sci↗

arXiv2024

Defect-mediated electron-phonon coupling in halide double perovskite

Optically active defects often play a crucial role in governing the light emission as well as the electronic properties of materials. Moreover, defect-mediated states in the mid-gap region can trap electrons, thus opening a path for the recombination of electrons and holes in lower energy states that may require phonons in the process. Considering this, we have probed electron-phonon interaction in halide perovskite systems with the introduction of defects and investigated the thermal effect on this interaction. Here, we report Raman spectroscopy study of the thermal evolution of electron-phonon coupling, which is tunable with the crystal growth conditions, in the halide perovskite systems Cs2AgInCl6 and Cs2NaInCl6. The signature of electron-phonon coupling is observed as a Fano anomaly in the lowest frequency phonon mode (51 cm-1) which evolves with temperature. In addition, we observe a broad band in the photoluminescence (PL) measurements for the defect-mediated systems, which is otherwise absent in defect-free halide perovskite. The simultaneous observation of the Fano anomaly in the Raman spectrum and the emergence of the PL band suggests the defect-mediated mid-gap states and the consequent existence of electron-phonon coupling in the double perovskite.

cond-mat.mtrl-sci↗

arXiv2024

On interactive anisotropic walks in two dimensions generated from a three state opinion dynamics model

A system of interacting walkers is considered in a two-dimensional hypothetical space, where the dynamics of each walker are governed by the opinion states of the agents of a fully connected three-state opinion dynamics model. Such walks, studied in different models of statistical physics, are usually considered in one-dimensional virtual spaces. Here, the mapping is done in such a way that the walk is directed along the Y-axis while it can move either way along the X-axis. The walk shows that there are three distinct regions as the noise parameter, responsible for driving a continuous phase transition in the model, is varied. In absence of any noise, the scaling properties and the form of the distribution along either axis do not follow any conventional form. For any finite noise below the critical point the bivariate distribution of the displacements is found to be a modified biased Gaussian function while above it, only the marginal distribution along one direction is Gaussian. The marginal probability distributions can be extracted and the scaling forms of different quantities, showing power law behaviour, are obtained. The directed nature of the walk is reflected in the marginal distributions as well as in the exponents.

cond-mat.stat-mech↗

arXiv2024

Ferrimagnetic hexagonal Mn$_2$CuGe Heusler alloy with a low-temperature spin-glass state

An extensive experimental investigation on the structural, static magnetic, and non-equilibrium dynamical properties of polycrystalline Mn$_2$CuGe Heusler alloy using powder X-ray diffraction, DC magnetization, magnetic relaxation, magnetic memory effect, and specific heat measurements is presented. Structural studies reveal that the alloy crystallizes in a mixed hexagonal crystal structure (space groups P3c1 (no. 158) and P6$_3$/mmc (no. 194)) with lattice parameters a = b = 7.18(4) $\mathring{A}$ and c = 13.12(4) $\mathring{A}$ for the majority phase. The DC magnetization analysis reveals a paramagnetic to ferrimagnetic phase transition around T$_C$ $\approx$ 682 K with a compensation of magnetization at $\approx$ 250 K, and a spin-glass transition around T$_P$ $\approx$ 25.6 K. The N\'eel theory of ferrimagnets supports the ferrimagnetic nature of the studied alloy and the estimated T$_C$ ($\approx$ 687 K) from this theory is consistent with that obtained from the DC magnetization data. A detailed study of non-equilibrium spin dynamics via magnetic relaxation and memory effect experiments shows the evolution of the system through a number of intermediate states and striking magnetic memory effect. Furthermore, heat capacity measurements suggest a large electronic contribution to the specific heat capacity suggesting strong spin fluctuations, due to competing magnetic interactions. All the observations render a spin-glass behavior in Mn$_2$CuGe, attributed to the magnetic frustration possibly arising out of the competing ferromagnetic and antiferromagnetic interactions.

cond-mat.mtrl-sci↗

arXiv2024

Signatures of novel magnon-phonon coupling in frustrated double perovskite square lattices

Low-dimensional frustrated magnetic square networks feature a variety of unconventional phases with novel emergent excitations. Often these excitations are intertwined and manifest into intriguing phenomena, an area that has remained largely unexplored in square-lattice systems, especially, double perovskites (A2BB'O6). In this study, we explore these interactions between the fundamental excitations such as phonons and magnons in square-lattice Sr2CuTeO6, Sr2CuWO6, and Ba2CuWO6 isostructural double perovskites that exhibit both short-ranged (TS) as well as long-ranged Neel antiferromagnetic (TN) transitions. Our Raman measurements at variable temperatures reveal an intriguing broad peak (identified as 2-magnon (2M)) surviving beyond TS for W-based compositions contrary to the Te-based system, suggesting a key role of diamagnetic B'-site cation on their magnetism. The thermal response of 2M intriguingly shows signatures of correlation with phonons and control over their anharmonicity, depicting magnon-phonon interaction. Further, a few phonons exhibit anomalies across the magnetic transitions implying the presence of spin-phonon coupling. In particular, the phonon modes at ~ 194 cm-1 of Sr2CuTeO6 and ~ 168 cm-1 of Sr2CuWO6, that show a strong correlation with the 2M, exhibit the strongest spin-phonon coupling suggesting their roles in mediating magnon-phonon interactions in these systems.

cond-mat.mtrl-sci↗

arXiv2023

Emergence of a spin-liquid-like phase in quantum spin-ladder Ba2CuTeO6 with chemical substitution

Stabilization of the quantum spin liquids is vital to realize applications in spintronics and quantum computing. The unique magnetic structure of Ba2CuTeO6 comprising of coupled spin-ladders with finite inter-ladder coupling brings the system close to the quantum critical point. This opens up possibilities to stabilize unconventional magnetic phases by tailoring the intra- and inter-ladder exchange couplings. Here, we demonstrate a spin-liquid-like phase in Ba2CuTeO6 using the method of chemical substitution. We choose non-magnetic La3+ cation to substitute the Ba2+ in Ba2CuTeO6 and present signature fingerprints such as deprived magnetic transition, non-dispersive AC susceptibility, magnetic field-independent heat capacity, and broad Raman continuum supporting the emergence of a spin-liquid-like phase. We believe that an increased magnetic frustration and spin-fractionalization upon chemical substitution play a crucial role in driving such a state. In addition, temperature and magnetic field-dependent phonon response indicate the presence of magnetostriction (spin-lattice coupling) in La-doped Ba2CuTeO6, a notable property of spin-liquids.

cond-mat.str-el↗

arXiv2023

Tailoring the defects and electronic band structure in WS2/h-BN heterostructure

The 2D semiconducting transition metal dichalcogenides (e.g., WS2) host strong coupling between various degrees of freedom leading to potential applications in next-generation device applications including optoelectronics. Such applications are strongly influenced by defects which can control both the optical and electronic properties of the material. We demonstrate the possibility to tailor the defect-related electronic states and the lattice dynamics properties of WS2 in their heterostructures with h-BN which host a strong interlayer coupling between the charge carriers in the WS2 layer and the phonons of h-BN. This coupling is observed to induce modifications to the interlayer phonons (manifested by their modified Raman-activity) and to the charge carrier mobilities in the WS2 layer (which results in creation of mid-gap energy states associated with many-body quasiparticle states). Our study also includes a detailed characterization of the defects through Raman measurements revealing an A_1g-type nature with differential resonance behavior for the modes that are related to defect scattering with respect to the other normal phonon modes of WS2.

cond-mat.mes-hall↗

arXiv2023

Spin-phonon coupling suppressing the structural transition in perovskite-like oxide

Multifunctional properties in quantum systems require the interaction between different degrees of freedom. As such, spin-phonon coupling emerges as an ideal mechanism to tune multiferroicity, magnetism, and magnetoelectric response. In this letter, we demonstrate and explain, based on theoretical and experimental analyses, an unusual manifestation of spin-phonon coupling, i.e., prevention of a ferroelastic structural transition, and locking of high-temperature R-3m phase in a magnetically frustrated perovskite-like oxide Ba2NiTeO6. We present Ba2NiTeO6 as a prototype example among its family where long-range antiferromagnetic structure couples with a low-frequency Eg mode (at 55 cm-1) that exhibits a large anharmonicity. Our findings establish that spin-phonon coupling clearly suppresses the phonon anharmonicity preventing the structural phase transition from the R-3m to the C2/m phase in Ba2NiTeO6.

cond-mat.mtrl-sci↗

arXiv2023

Evidence of Charge-Phonon coupling in Van der Waals materials Ni1-xZnxPS3

NiPS3 is a Van der Waals antiferromagnet that has been found to display spin-charge and spin-phonon coupling in its antiferromagnetically ordered state below TN = 155 K. Here, we study high-quality crystals of site-diluted Ni1-xZnxPS3 (0 < x < 0.2) using temperature-dependent specific heat and Raman spectroscopy probes. The site dilution suppresses the antiferromagnetic ordering in accordance with the mean-field prediction. In NiPS3, we show that the phonon mode P2 (176 cm-1) associated with Ni vibrations show a distinct asymmetry due to the Fano resonance, which persists only in the paramagnetic phase, disappearing below T_N = 155 K. This was further supported by temperature-dependent Raman data on an 8% Zn-doped crystal (T_N = 135 K) where Fano resonance similarly van in the magnetically ordered phase. This is contrary to the behaviour of the Raman mode P9 (570 cm-1), which shows a Fano resonance at low temperatures below T_N due to its coupling with the two-magnon continuum. We show that the Fano resonance of P2 arises from its coupling with an electronic continuum that weakens considerably upon cooling to low temperatures. In the doped crystals, the Fano coupling is found to enhance with Zn-doping. These observations suggest the presence of strong electron-phonon coupling in the paramagnetic phase of NiPS3 due to charge density fluctuations associated with the negative charge transfer state of Ni.

cond-mat.str-el↗

arXiv2023

Tuning the magnetic properties in MPS3 (M = Mn, Fe, and Ni) by proximity-induced Dzyaloshinskii Moriya interactions

Tailoring the quantum many-body interactions in layered materials through appropriate heterostructure engineering can result in emergent properties that are absent in the constituent materials thus promising potential future applications. In this article, we have demonstrated controlling the otherwise robust magnetic properties of transition metal phosphorus trisulphides (Mn/Fe/NiPS3) in their heterostructures with Weyl semimetallic MoTe2 which can be attributed to the Dzyaloshinskii Moriya (DM) interactions at the interface of the two different layered materials. While the DM interaction is known to scale with the strength of the spin-orbit coupling (SOC), we also demonstrate here that the effect of DM interaction strongly varies with the spin orientation/dimensionality of the magnetic layer and the low-energy electronic density of state of the spin-orbit coupled layer. The observations are further supported by a series of experiments on heterostructures with a variety of substrates/underlayers hosting variable SOC and electronic density of states.

cond-mat.mes-hall↗

arXiv2023

Topological phase transition in MoTe$_2$: A Review

Transition metal dichalcogenides (TMDs) are a branch of two-dimensional materials which in addition to having an easy-to-exfoliate layered structure, also host semiconducting, metallic, superconducting, and topological properties in various polymorphs with potential applications. MoTe$_2$ is an example of such a TMD, which shows semiconducting (in 2H phase), metallic (in 1T' phase), topological Weyl semimetallic and superconducting behavior (in Td phase). Consequently, an extensive amount of research has been done on MoTe$_2$, particularly on the topological phase transition between the metallic-type 1T' phase and the topological Td phase. This phase transition has been reviewed and its association with the crystal structure, charge transport, and electronic band structure is elaborately discussed. Also, the effect of various stimuli like reduced dimensionality, pressure, charge doping, and chemical substitution, which affect the structural transition as well as the superconducting transition temperatures is reviewed; thereby, suggesting certain correlations between the apparently unrelated structural and superconducting phase transitions. The review also brings out some open questions which are likely to interest the community to address the physics associated with the phase transition and its potential applications.

cond-mat.supr-con↗

arXiv2022

High-yield exfoliation of MoS2 nanosheets by a novel spray technique and the importance of soaking and surfactants

Liquid-phase exfoliation of two-dimensional materials is very attractive for large-scale applications. Although used extensively, isolating MoS2 layers (<10) with high efficiency is reported to be extremely difficult. Further, the importance of soaking has not yet been studied, and the surfactants' role in stabilizing MoS2 nanosheets is poorly understood1. Herein, we report a novel approach to exfoliating large quantities of MoS2 via high-pressure (HP) liquid-phase exfoliation (LPE) in deionized (DI) water. 4 to 7 layers of MoS2 nanosheets were obtained from 60 days-soaked samples and they were found to be stable in solvents for periods of up to six months. Studies on the effect of three surfactants, namely sodium dodecyl benzenesulfonate (SDBS), sodium cholate (SC), and tetra-butyl ammonium bromide (TBAB), indicate that exfoliation of MoS2 nanosheets in SDBS is highly efficient than the other two surfactants. The estimated yield reaches up to 7.25%, with a nanosheet concentration of 1.45 mg/ml, which is one of the highest ever reported. Our studies also suggest that the nanosheets' concentration and the lateral size depend on exfoliation cycles, applied pressure and surfactant concentration. Hydrogen evolution reaction (HER) and ion-transport study show that the nanosheets prepared by our method are stable in an acidic medium and free from surfactants. A high hydrogen evolution rate of 30.13 mmol g-1 h-1 was estimated under ambient laboratory conditions.

cond-mat.mes-hall↗

arXiv2022

Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: A Raman and first-principles study

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. With this note, we have investigated the structural transition of perovskite-type trigonal Ba2ZnTeO6, across Tc ~ 150 K, which is also accompanied by a para- to ferroelastic transition. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, here we report new intriguing observations associated with the phase transition in Ba2ZnTeO6 elucidating the lattice dynamics across the Tc. We have observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile in concomitant with the damping of the soft mode near Tc, shedding light on the lattice dynamics during the phase transition. While most of the phonon bands split below Tc confirming the phase transition, we have observed thermal hysteretic behavior of phonon modes that signifies the first-order nature of the transition and presence of coexisting phases, which are corroborated by our temperature-dependent x-ray diffraction and specific heat measurements. Further, an evidence of the concomitant structural transition appears in the form of huge softening in the thermal response of the soft phonon mode at ~ 31 cm-1 which is remarkable compared to the hitherto known behavior of soft modes in well-known ferroelectrics. This is further corroborated by our phonon calculations that show an unstable Eg-mode in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) which later condenses into the C2/m low-symmetry phase.

cond-mat.mtrl-sci↗

arXiv2022

Photosensitive SrMnO3

In recent years, photosensitive materials have been in huge demand because of their fascinating ability to convert absorbed photon energy to generate strain and henceforth tuning the physical properties. In this report we detect the photosensitive activity of SrMnO3. Using the power dependent and temperature dependent Raman study with different laser sources having wavelengths across the optical band gap of SrMnO3, we divulge the photosensitive character of SrMnO3 thin films. Upon laser light illumination, Raman modes soften and softening further increases with increase in laser power. Similar kind of mode variation is observed with increasing temperature at fixed laser power. XAS in presence of laser illumination, reveals the change in crystal field splitting associated with mode softening.

cond-mat.mtrl-sci↗