arXiv subjects

Sourabh Singh

Publications and source records attributed to Sourabh Singh.

13 recordsLinked to original sources

arXiv2026

Caustic-Driven Fluidic Microlenses for Enhanced Nonlinear and High-Energy-Density Physics

We demonstrate that caustic microlensing occurring in a liquid jet efficiently drives linear, nonlinear, and high-energy-density phenomena. In the linear regime, caustics provide localized focusing, distinct from external high-NA optics. In the nonlinear regime, they enhance the input field at the liquid-air interface and boost surface-sensitive processes. In the high-energy-density domain, caustic-driven localized laser absorption generates gigapascal shocks using microjoule femtosecond pulses, with scalability up to repetition rates of 0.2 MHz. Caustic-driven fluidic microlensing offers opportunities for surface nonlinear optics, ultrafast science, and high-energy-density physics.

physics.optics↗

arXiv2023

Nano-Patterned Magnetic Edges in CrGeTe3 for Quasi 1-D Spintronic Devices

The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Interior and edge magnetization in thin exfoliated CrGeTe3 films

CrGeTe3 (CGT) is a semiconducting vdW ferromagnet shown to possess magnetism down to a two-layer thick sample. Although CGT is one of the leading candidates for spintronics devices, a comprehensive analysis of CGT thickness dependent magnetization is currently lacking. In this work, we employ scanning SQUID-on-tip (SOT) microscopy to resolve the magnetic properties of exfoliated CGT flakes at 4.2 K. Combining transport measurements of CGT/NbSe2 samples with SOT images, we present the magnetic texture and hysteretic magnetism of CGT, thereby matching the global behavior of CGT to the domain structure extracted from local SOT magnetic imaging. Using this method, we provide a thickness dependent magnetization state diagram of bare CGT films. No zero-field magnetic memory was found for films thicker than 10 nm and hard ferromagnetism was found below that critical thickness. Using scanning SOT microscopy, we identify a unique edge magnetism, contrasting the results attained in the CGT interior.

cond-mat.mes-hall↗

arXiv2019

Pulsed laser deposition of highly c-axis oriented thin films of BSTS topological insulator

We report the growth of highly c-axis oriented topological insulator (TI) BiSbTe1.5Se1.5 (BSTS) thin films by pulsed laser deposition (PLD) technique. The various growth parameters such as substrate temperature, Argon pressure in the deposition chamber and target to substrate distance are tuned to obtain the optimized conditions essential for stoichiometric and bulk insulating TI thin films. These films are highly c-axis oriented and exhibit all the four Raman modes characteristic to the R-3m space group. The quality of the deposited thin films is investigated using X-ray diffraction for crystallinity, Raman spectroscopy for lattice dynamics, morphological studies using scanning electron microscope and compositional analysis using Energy dispersive X-ray spectroscopy. Resistance vs temperature measurements confirm bulk insulating nature of the prepared thin films and magnetoresistance data exhibits the phenomena of weak antilocalization with a large phase coherence length.

cond-mat.mtrl-sci↗

arXiv2019

Predicting Young's Modulus of Glasses with Sparse Datasets using Machine Learning

Machine learning (ML) methods are becoming popular tools for the prediction and design of novel materials. In particular, neural network (NN) is a promising ML method, which can be used to identify hidden trends in the data. However, these methods rely on large datasets and often exhibit overfitting when used with sparse dataset. Further, assessing the uncertainty in predictions for a new dataset or an extrapolation of the present dataset is challenging. Herein, using Gaussian process regression (GPR), we predict Young's modulus for silicate glasses having sparse dataset. We show that GPR significantly outperforms NN for sparse dataset, while ensuring no overfitting. Further, thanks to the nonparametric nature, GPR provides quantitative bounds for the reliability of predictions while extrapolating. Overall, GPR presents an advanced ML methodology for accelerating the development of novel functional materials such as glasses.

cond-mat.mtrl-sci↗

arXiv2019

Quantum Light on Demand

We demonstrate that light quanta of well defined characteristics can be generated in a coupled two-level system of three atoms. The quantum nature of light is controlled by the entanglement structure, discord, and monogamy of the system which leads to sub and superradiant behavior as well as sub-Poissonian statistics, at lower temperatures. Two distinct phases with different entanglement characteristics are observed with uniform radiation in one case and the other displaying highly focused and anisotropic radiation in far field regime. At higher temperatures, sub and superradiant light is found to persist in the absence of entanglement but with non-zero quantum discord, showing bunching of photons. It is shown that the radiation intensity can be a precise estimator of the inter-atomic distance of coupled two-level atomic systems. Our investigation shows for the first time, the three body correlation in the form of `monogamy score' controlling sub and superradiant nature of radiation intensity.

quant-ph↗

arXiv2017

Linear magnetoresistance and surface to bulk coupling in topological insulator thin films

We explore the temperature dependent magnetoresistance of bulk insulating topological insulator thin films. Thin films of Bi2Se2Te and BiSbTeSe1.6 were grown using Pulsed Laser Deposition technique and subjected to transport measurements. Magnetotransport measurements indicate a non-saturating linear magnetoresistance (LMR) behavior at high magnetic field values. We present a careful analysis to explain the origin of LMR taking into consideration all the existing models of LMR. Here we consider that the bulk insulating states and the metallic surface states constitute two parallel conduction channels. Invoking this, we were able to explain linear magnetoresistance behavior as a competition between these parallel channels. We observe that the crossover field, where LMR sets in, decreases with increasing temperature. We propose that this cross over field can be used phenomenologically to estimate the strength of surface to bulk coupling.

cond-mat.str-el↗

arXiv2016

Bulk Saturable absorption in Topological Insulator thin Films

We present nonlinear optical absorption properties of pulsed laser deposited thin films of topological insulator (TI), Bi2Se3 on quartz substrate, using open aperture Z - scan technique. The saturable intensity of as-deposited thin films has been found remarkably improved by an order of magnitude compared to the values reported earlier in the literature. Past results from the literature are inconclusive in establishing whether the saturable absorption is coming from surface states or the bulk. Specifically designed experiments with magnetically doped TI samples allow us to attribute the saturable absorption characteristic of TI to the bulk states. Detailed experimental procedures and possible explanation of observed results have been discussed.

cond-mat.str-el↗

arXiv2016

Topological delocalization and tuning of surface channel separation in Bi2Se2Te Topological Insulator Thin films

The surface states of a 3D topological insulator (TI) exhibit topological protection against backscattering. However, the contribution of bulk electrons to the transport data is an impediment to the topological protection of surface. We report the tuning of the chemical potential in the bulk of Bi2Se2Te TI thin films, pinning it near the center of the bulk band gap, thereby suppressing the bulk carriers. The temperature dependent resistance of these films shows activated behavior down to 50K, followed by a metallic transition at lower temperatures, a hallmark of the robustness of TI surface states. Manifestation of topological protection and surface dominated transport is explained by 2D weak antilocalization phenomenon. We further explore the effect of surface to bulk coupling in TI in this work, which is captured by the number of effective conducting surface channels that participate in the transport. The presence of a single conducting channel indicates a strong surface to bulk coupling which is detrimental to purely topological transport. We demonstrate the decoupling of topological surface states on opposite surfaces of thin films, thereby suppressing the bulk transport. Our findings provide a deeper understanding of surface to bulk coupling along with topological transport behavior and their respective tunability.

cond-mat.str-el↗

arXiv2016

Parallel and series conduction model in Topological Insulators

In the past few years there has been a surge in the material science engineering in order to synthesize bulk insulating and surface metallic Topological Insulating (TI) materials. This quest is not only theoretically important but also promising from the novel application perspective. The dependence of temperature on resistance (R-T) of a particular sample reveals a plethora of information about the electronic properties especially in a unique sample like TI where there are two components comprising of an insulating bulk and metallic surface states. Depending on the amount of intrinsic doping during the sample formation, the bulk can either couple or remain decoupled with the surface. The former leads to a metallic R-T profile whereas the latter is captured by an insulating R-T behavior. These two behaviors can be represented by series and parallel resistor models respectively. In this work we study the R-T behavior in the framework of resistor models capturing the essential features of our sample.

cond-mat.str-el↗

arXiv2016

Tuning Chemical Potential in the Dirac Cone by Compositional Engineering

We report the successful formation of bulk insulating ternary topological insulators candidate Bi2Se2Te (BST) by pulsed laser deposition technique. The films were deposited with sequential ablation of separate Bi2Se3 (BS) and Bi2Te3 (BT) targets. From the X-ray diffraction analysis and temperature dependent resistivity we were able to conclude that the as grown thin films have ordered chalcogen layers and the chemical potential in these thin films lie in the bulk gap. To realize entirely topological transport for any device applications it is essential to tune the chemical potential in the bulk gap of the Dirac cone. Magnetotransport data exhibits pronounced two dimensional weak-antilocalization behavior (WAL) at low temperatures. BS and BT thin films do not exhibit topological transport as the chemical potential does not lie entirely in the bulk gap. It was found that BST thin films grown with this cost effective and simple yet elegant technique using double target can be used to deposit quaternary TI thin films, thereby tuning the chemical potential at will in the gap.

cond-mat.mes-hall↗

arXiv2015

Surface Optical and Bulk Acoustic Phonons in the Topological Insulator, Bi2Se2Te

We explore the phonon dynamics of thin films of the topological insulator material Bi2Se2Te using ultrafast pump-probe spectroscopy. The time resolved differential reflectivity of the films exhibit fast and slow oscillations. We have given a careful analysis of variation of phonon frequency as a function of film thickness attributing this to existence of standing acoustic modes. However, no variation in the frequency of the optical phonon modes was found with film thickness. This indicates that the optical phonons intrinsically belong to the surface of the topological insulators. The fact that the acoustic phonons can be tuned by changing the film thickness has tremendous potential for room temperature low power spintronic devices and in topological quantum computation.

cond-mat.mes-hall↗

arXiv2014

Weak-antilocalization and Surface Dominated Transport in Topological Insulator Bi2Se2Te

We explore the phase coherence of thin films of the topological insulator material Bi2Se2Te grown through pulsed laser deposition (PLD) technique. The films were characterised using various techniques for phase and composition. The films were found to be of good quality. We carried out extensive magneto-transport studies of these films and found that they exhibit two dimensional weak antilocalization behaviour. A careful analysis revealed a relatively high phase coherence length (58nm at 1.78K) for a PLD grown film. Since PLD is an inexpensive technique, with the possibility to integrate with other materials, one can make devices which can be extremely useful for low power spintronics and topological quantum computation.

cond-mat.mes-hall↗