Showing 1–50 of 60 results for author: Alam, S

Stabilizing Magnetic Bubble Domains in Epitaxial 2D Magnet/Topological Insulator Heterostructures through Interfacial Interactions

Authors: Thow Min Jerald Cham, Mowen Zhao, Wenyi Zhou, Andrew Koerner, Dang-Khoa Le, Ziling Li, Lukas Powalla, Derek Bergner, Eklavya Thareja, Camelia Selcu, Sadikul Alam, Sebastian Wintz, Markus Weigand, Jinwoo Hwang, Jacob Gayles, Roland Kawakami, Yunqiu Kelly Luo

Abstract: Epitaxial heterostructures of two-dimensional van der Waals magnets and topological insulators offer a powerful platform for probing interfacial spin interactions that govern magnetic textures in low-dimensional quantum systems, while simultaneously enabling highly efficient, atomically thin spin-orbit-torque memory and computing architectures. Despite this promise, the fundamental role of these i… ▽ More Epitaxial heterostructures of two-dimensional van der Waals magnets and topological insulators offer a powerful platform for probing interfacial spin interactions that govern magnetic textures in low-dimensional quantum systems, while simultaneously enabling highly efficient, atomically thin spin-orbit-torque memory and computing architectures. Despite this promise, the fundamental role of these interfacial interactions in determining magnetic domain-phase stability remain largely uncharted. Here, we perform scanning transmission X-ray microscopy to image nanoscale magnetic textures in epitaxial Fe3GeTe2 Bi2Te3 heterostructures, enabled by a thermal-release-tape dry transfer process onto X-ray transparent silicon-rich nitride membranes. Under zero-field-cooled conditions, we observe robust bubble domain phases from 75 to 165 K, and across different number of folds of the multilayer Fe3GeTe2 Bi2Te3 heterostructures. This is in stark contrast with exfoliated single-crystal Fe3GeTe2 flakes, where ZFC stripe domains are observed for flakes thicker than 20 nm and no domains have been reported for thin flakes less than 15 nm. First-principles calculations and micromagnetic simulations reveal that interfacial coupling to Bi2Te3 modifies the magnetic anisotropy and introduces interfacial Dzyaloshinskii-Moriya interaction, shifting the magnetic phase space towards bubble-domain stabilization without field-cooling. Together, our results offer a new strategy for phase-selective control of magnetic domains through interfacial engineering. △ Less

Submitted 25 March, 2026; originally announced March 2026.

Degradation-Aware and Machine Learning-Driven Uncertainty Quantification in Crystal Plasticity Finite Element: Texture-Driven Plasticity in 316L Stainless Steel

Authors: Dinesh Kumar, Eralp Demir, Julio Spadotto, Kazuma Kobayashi, Syed Bahauddin Alam, Brian Connolly, Ed Pickering, Paul Wilcox, David Knowles, Mahmoud Mostafavi

Abstract: The mechanical properties and long-term structural reliability of crystalline materials are strongly influenced by microstructural features such as grain size, morphology, and crystallographic texture. These characteristics not only determine the initial mechanical behavior but also govern the progression of degradation mechanisms, such as strain localization, fatigue damage, and microcrack initia… ▽ More The mechanical properties and long-term structural reliability of crystalline materials are strongly influenced by microstructural features such as grain size, morphology, and crystallographic texture. These characteristics not only determine the initial mechanical behavior but also govern the progression of degradation mechanisms, such as strain localization, fatigue damage, and microcrack initiation under service conditions. Variability in these microstructural attributes, introduced during manufacturing or evolving through in-service degradation, leads to uncertainty in material performance. Therefore, understanding and quantifying microstructure-sensitive plastic deformation is critical for assessing degradation risk in high-value mechanical systems. This study presents a first-of-its-kind machine learning-driven framework that couples high-fidelity crystal plasticity finite element (CPFE) simulations with data-driven surrogate modeling to accelerate degradation-aware uncertainty quantification in welded structural alloys. Specifically, the impact of crystallographic texture variability in 316L stainless steel weldments, characterized via high-throughput electron backscatter diffraction (EBSD), is examined through CPFE simulations on calibrated representative volume elements (RVEs). A polynomial chaos expansion-based surrogate model is then trained to efficiently emulate the CPFE response using only 200 simulations, reducing computational cost by several orders of magnitude compared to conventional Monte Carlo analysis. The surrogate enables rapid quantification of uncertainty in stress-strain behavior and identifies texture components such as Cube and Goss as key drivers of degradation-relevant plastic response. △ Less

Submitted 24 May, 2025; originally announced May 2025.

AI-driven Uncertainty Quantification & Multi-Physics Approach to Evaluate Cladding Materials in a Microreactor

Authors: Alex Foutch, Kazuma Kobayashi, Ayodeji Alajo, Dinesh Kumar, Syed Bahauddin Alam

Abstract: The pursuit of enhanced nuclear safety has spurred the development of accident-tolerant cladding (ATC) materials for light water reactors (LWRs). This study investigates the potential of repurposing these ATCs in advanced reactor designs, aiming to expedite material development and reduce costs. The research employs a multi-physics approach, encompassing neutronics, heat transfer, thermodynamics,… ▽ More The pursuit of enhanced nuclear safety has spurred the development of accident-tolerant cladding (ATC) materials for light water reactors (LWRs). This study investigates the potential of repurposing these ATCs in advanced reactor designs, aiming to expedite material development and reduce costs. The research employs a multi-physics approach, encompassing neutronics, heat transfer, thermodynamics, and structural mechanics, to evaluate four candidate materials (Haynes 230, Zircaloy-4, FeCrAl, and SiC-SiC) within the context of a high-temperature, sodium-cooled microreactor, exemplified by the Kilopower design. While neutronic simulations revealed negligible power profile variations among the materials, finite element analyses highlighted the superior thermal stability of SiC-SiC and the favorable stress resistance of Haynes 230. The high-temperature environment significantly impacted material performance, particularly for Zircaloy-4 and FeCrAl, while SiC-SiC's inherent properties limited its ability to withstand stress loads. Additionally, AI-driven uncertainty quantification and sensitivity analysis were conducted to assess the influence of material property variations on maximum hoop stress. The findings underscore the need for further research into high-temperature material properties to facilitate broader applicability of existing materials to advanced reactors. Haynes 230 is identified as the most promising candidate based on the evaluated criteria. △ Less

Submitted 18 March, 2025; originally announced March 2025.

Anisotropic suppression of the phononic thermal conductivity by magnetic field in SmAlSi

Authors: Mujeeb Ahmad, Md Shahin Alam, Xiaohan Yao, Fazel Tafti, Marcin Matusiak

Abstract: We report the thermal and electrical conductivity data for the magnetic Weyl semimetal SmAlSi measured in a magnetic field (B) with two different orientations. In one case, B was applied perpendicular to the heat or charge current, in the other they were parallel. For both configurations, the magnetic field affects the magnetic structure identically as B is always parallel to the equivalent tetrag… ▽ More We report the thermal and electrical conductivity data for the magnetic Weyl semimetal SmAlSi measured in a magnetic field (B) with two different orientations. In one case, B was applied perpendicular to the heat or charge current, in the other they were parallel. For both configurations, the magnetic field affects the magnetic structure identically as B is always parallel to the equivalent tetragonal axis. Our results indicate that phonon heat transport in response to the magnetic field exhibits strong anisotropy at low temperature: it appears to be independent of B in the perpendicular configuration but is strongly suppressed in the parallel configuration. Understanding this unusual behaviour can lead to designing better materials for thermoelectricity or directional heat switches. △ Less

Submitted 23 January, 2025; originally announced January 2025.

Comments: 18 pages, 5 figures

Harnessing Ferro-Valleytricity in Penta-Layer Rhombohedral Graphene for Memory and Compute

Authors: Md Mazharul Islam, Shamiul Alam, Md Rahatul Islam Udoy, Md Shafayat Hossain, Kathleen E Hamilton, Ahmedullah Aziz

Abstract: Two-dimensional materials with multiple degrees of freedom, including spin, valleys, and orbitals, open up an exciting avenue for engineering multifunctional devices. Beyond spintronics, these degrees of freedom can lead to novel quantum effects such as valley-dependent Hall effects and orbital magnetism, which could revolutionize next-generation electronics. However, achieving independent control… ▽ More Two-dimensional materials with multiple degrees of freedom, including spin, valleys, and orbitals, open up an exciting avenue for engineering multifunctional devices. Beyond spintronics, these degrees of freedom can lead to novel quantum effects such as valley-dependent Hall effects and orbital magnetism, which could revolutionize next-generation electronics. However, achieving independent control over valley polarization and orbital magnetism has been a challenge due to the need for large electric fields. A recent breakthrough involving penta-layer rhombohedral graphene has demonstrated the ability to individually manipulate anomalous Hall signals and orbital magnetic hysteresis, forming what is known as a valley-magnetic quartet. Here, we leverage the electrically tunable Ferro-valleytricity of penta-layer rhombohedral graphene to develop non-volatile memory and in-memory computation applications. We propose an architecture for a dense, scalable, and selector-less non-volatile memory array that harnesses the electrically tunable ferro-valleytricity. In our designed array architecture, non-destructive read and write operations are conducted by sensing the valley state through two different pairs of terminals, allowing for independent optimization of read/write peripheral circuits. The power consumption of our PRG-based array is remarkably low, with only ~ 6 nW required per write operation and ~ 2.3 nW per read operation per cell. This consumption is orders of magnitude lower than that of the majority of state-of-the-art cryogenic memories. Additionally, we engineer in-memory computation by implementing majority logic operations within our proposed non-volatile memory array without modifying the peripheral circuitry. Our framework presents a promising pathway toward achieving ultra-dense cryogenic memory and in-memory computation capabilities. △ Less

Submitted 2 August, 2024; originally announced August 2024.

Robust Quantum Sensing with Multiparameter Decorrelation

Authors: Shah Saad Alam, Victor E. Colussi, John Drew Wilson, Jarrod T. Reilly, Michael A. Perlin, Murray J. Holland

Abstract: The performance of a quantum sensor is fundamentally limited by noise. This noise is particularly damaging when it becomes correlated with the readout of a target signal, caused by fluctuations of the sensor's operating parameters. These uncertainties limit sensitivity in a way that can be understood with multiparameter estimation theory. We develop a new approach, adaptable to any quantum platfor… ▽ More The performance of a quantum sensor is fundamentally limited by noise. This noise is particularly damaging when it becomes correlated with the readout of a target signal, caused by fluctuations of the sensor's operating parameters. These uncertainties limit sensitivity in a way that can be understood with multiparameter estimation theory. We develop a new approach, adaptable to any quantum platform, for designing robust sensing protocols that leverages multiparameter estimation theory and machine learning to decorrelate a target signal from fluctuating off-target (``nuisance'') parameters. Central to our approach is the identification of information-theoretic goals that guide a machine learning agent through an otherwise intractably large space of potential sensing protocols. As an illustrative example, we apply our approach to a reconfigurable optical lattice to design an accelerometer whose sensitivity is decorrelated from lattice depth noise. We demonstrate the effect of decorrelation on outcomes and Bayesian inferencing through statistical analysis in parameter space, and discuss implications for future applications in quantum metrology and computing. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 10 figures, 16 pages

Quantum transport properties of the topological Dirac Semimetal $α$-Sn

Authors: Md Shahin Alam, Alexandr Kazakov, Mujeeb Ahmad, Rajibul Islam, Fei Xue, Marcin Matusiak

Abstract: We report measurements of the electrical resistivity ($ρ$) and thermoelectric power (S) in a thin film of strained single-crystalline $α$-Sn grown by molecular beam epitaxy on an insulating substrate. The temperature (T) dependence of the resistivity of $α$-Sn can be divided into two regions:below T* $\approx$ 135 K $ρ$(T) shows a metallic-like behaviour, while above this temperature an increasing… ▽ More We report measurements of the electrical resistivity ($ρ$) and thermoelectric power (S) in a thin film of strained single-crystalline $α$-Sn grown by molecular beam epitaxy on an insulating substrate. The temperature (T) dependence of the resistivity of $α$-Sn can be divided into two regions:below T* $\approx$ 135 K $ρ$(T) shows a metallic-like behaviour, while above this temperature an increasing contribution from thermally excited holes to electrical transport is observed. However, it is still dominated by highly mobile electrons, resulting in a negative sign of the Seebeck coefficient above T = 47 K. In the presence of the magnetic field (B) applied along an electric field or thermal gradient, we note a negative magnetoresistance or a negative slope of S(B), respectively. The theoretical prediction for the former (calculated using density functional theory) agrees well with the experiment. However, these characteristics quickly disappear when the magnetic field is deviated from an orientation parallel to the electrical field or the thermal gradient. We indicate that the behaviour of the electrical resistivity and thermoelectric power can be explained in terms of the chiral current arising from the topologically non-trivial electronic structure of $α$-Sn. Its decay at high temperature is a consequence of the decreasing ratio between the intervalley Weyl relaxation time to the Drude scattering time. △ Less

Submitted 31 May, 2024; v1 submitted 29 February, 2024; originally announced March 2024.

Determining Strain Components in a Diamond Waveguide from Zero-Field ODMR Spectra of NV$^{-}$ Center Ensembles

Authors: M. Sahnawaz Alam, Federico Gorrini, Michał Gawełczyk, Daniel Wigger, Giulio Coccia, Yanzhao Guo, Sajedeh Shahbazi, Vibhav Bharadwaj, Alexander Kubanek, Roberta Ramponi, Paul E. Barclay, Anthony J. Bennett, John P. Hadden, Angelo Bifone, Shane M. Eaton, Paweł Machnikowski

Abstract: The negatively charged nitrogen-vacancy (NV$^{-}$) center in diamond has shown great potential in nanoscale sensing and quantum information processing due to its rich spin physics. An efficient coupling with light, providing strong luminescence, is crucial for realizing these applications. Laser-written waveguides in diamond promote NV$^{-}$ creation and improve their coupling to light but, at the… ▽ More The negatively charged nitrogen-vacancy (NV$^{-}$) center in diamond has shown great potential in nanoscale sensing and quantum information processing due to its rich spin physics. An efficient coupling with light, providing strong luminescence, is crucial for realizing these applications. Laser-written waveguides in diamond promote NV$^{-}$ creation and improve their coupling to light but, at the same time, induce strain in the crystal. The induced strain contributes to light guiding but also affects the energy levels of NV$^{-}$ centers. We probe NV$^{-}$ spin states experimentally with the commonly used continuous-wave zero-field optically detected magnetic resonance (ODMR). In our waveguides, the ODMR spectra are shifted, split, and consistently asymmetric, which we attribute to the impact of local strain. To understand these features, we model ensemble ODMR signals in the presence of strain. By fitting the model results to the experimentally collected ODMR data, we determine the strain tensor components at different positions, thus determining the strain profile across the waveguide. This shows that zero-field ODMR spectroscopy can be used as a strain imaging tool. The resulting strain within the waveguide is dominated by a compressive axial component transverse to the waveguide structure, with a smaller contribution from vertical and shear strain components. △ Less

Submitted 12 August, 2024; v1 submitted 9 February, 2024; originally announced February 2024.

Comments: 12 pages, 7 figures + Supplementary Information (6 pages, 3 figures)

Effect of Dispersion and Different Carrier Transitions on Absorption Characteristics of GaAs

Authors: Subarna Alam, Mohammad Jahangir Alam

Abstract: One of the basic optoelectronic characteristics is the absorption of any optoelectronic device or material. We present the characteristics of pure GaAs and the deviation of ideal spectra. Due to crystal defects, vacancies, impurities, etc., energy states exist in the semiconductor other than the allowed bands (conduction and valence bands). In this research, the effect of transitions between these… ▽ More One of the basic optoelectronic characteristics is the absorption of any optoelectronic device or material. We present the characteristics of pure GaAs and the deviation of ideal spectra. Due to crystal defects, vacancies, impurities, etc., energy states exist in the semiconductor other than the allowed bands (conduction and valence bands). In this research, the effect of transitions between these states on absorption spectra and how the absorption coefficients change with the density and location of those states are analyzed numerically. Among the cases of 0.07 eV, 0.12 eV, and 0.17 eV with the density of 2%, 5%, 10%, 20%, and 30%, the minimum deviation occurs for the states located 0.07 eV below the conduction band and when the energy states have the minimum density of 2% and the deviation increases for the cases of 0.12 eV and 0.17 eV with higher densities. △ Less

Submitted 9 December, 2023; originally announced January 2024.

Comments: 9th IEEE International Women in Engineering (WIE) Conference on Electrical and Computer Engineering 2023 (IEEE WIECON-ECE 2023)

Understanding Phonon Thermal Transport in Twisted Bilayer Graphene

Authors: Shahid Ahmed, Shadab Alam, Ankit Jain

Abstract: The phonon thermal transport properties of twisted bi-layer graphene are investigated using lattice dynamics and the Boltzmann transport equation. The thermal conductivities of 13.2 and 21.8 twisted configurations are 56% and 36% lower than the untwisted configuration which has a room temperature thermal conductivity of 2260 W/m-K. The reason for this lower thermal conductivity is unraveled from p… ▽ More The phonon thermal transport properties of twisted bi-layer graphene are investigated using lattice dynamics and the Boltzmann transport equation. The thermal conductivities of 13.2 and 21.8 twisted configurations are 56% and 36% lower than the untwisted configuration which has a room temperature thermal conductivity of 2260 W/m-K. The reason for this lower thermal conductivity is unraveled from phonon mode-level analysis made possible through the untwisting of layers. Due to a large commensurate unitcell of twisted configuration, the Brillouin zone is folded but this folding has no bearing on the phonon scattering phase space. The major impact of twisting is felt by flexural phonons with out-of-plane vibrations via the change in strength of flexural interatomic interactions and since these flexural phonons carry majority of the heat in bilayer graphene (65% at room temperature), the thermal conductivity is sensitive to layer twisting. Our study suggests that twisting will affect thermal transport only for those materials that have a major contribution from flexural phonon modes. △ Less

Submitted 6 November, 2023; originally announced November 2023.

Quantized Non-Volatile Nanomagnetic Synapse based Autoencoder for Efficient Unsupervised Network Anomaly Detection

Authors: Muhammad Sabbir Alam, Walid Al Misba, Jayasimha Atulasimha

Abstract: In the autoencoder based anomaly detection paradigm, implementing the autoencoder in edge devices capable of learning in real-time is exceedingly challenging due to limited hardware, energy, and computational resources. We show that these limitations can be addressed by designing an autoencoder with low-resolution non-volatile memory-based synapses and employing an effective quantized neural netwo… ▽ More In the autoencoder based anomaly detection paradigm, implementing the autoencoder in edge devices capable of learning in real-time is exceedingly challenging due to limited hardware, energy, and computational resources. We show that these limitations can be addressed by designing an autoencoder with low-resolution non-volatile memory-based synapses and employing an effective quantized neural network learning algorithm. We propose a ferromagnetic racetrack with engineered notches hosting a magnetic domain wall (DW) as the autoencoder synapses, where limited state (5-state) synaptic weights are manipulated by spin orbit torque (SOT) current pulses. The performance of anomaly detection of the proposed autoencoder model is evaluated on the NSL-KDD dataset. Limited resolution and DW device stochasticity aware training of the autoencoder is performed, which yields comparable anomaly detection performance to the autoencoder having floating-point precision weights. While the limited number of quantized states and the inherent stochastic nature of DW synaptic weights in nanoscale devices are known to negatively impact the performance, our hardware-aware training algorithm is shown to leverage these imperfect device characteristics to generate an improvement in anomaly detection accuracy (90.98%) compared to accuracy obtained with floating-point trained weights. Furthermore, our DW-based approach demonstrates a remarkable reduction of at least three orders of magnitude in weight updates during training compared to the floating-point approach, implying substantial energy savings for our method. This work could stimulate the development of extremely energy efficient non-volatile multi-state synapse-based processors that can perform real-time training and inference on the edge with unsupervised data. △ Less

Submitted 11 September, 2023; originally announced September 2023.

Reimagining Sense Amplifiers: Harnessing Phase Transition Materials for Current and Voltage Sensing

Authors: Md Mazharul Islam, Shamiul Alam, Mohammad Adnan Jahangir, Garrett S. Rose, Suman Datta, Vijaykrishnan Narayanan, Sumeet Kumar Gupta, Ahmedullah Aziz

Abstract: Energy-efficient sense amplifier (SA) circuits are essential for reliable detection of stored memory states in emerging memory systems. In this work, we present four novel sense amplifier (SA) topologies based on phase transition material (PTM) tailored for non-volatile memory applications. We utilize the abrupt switching and volatile hysteretic characteristics of PTMs which enables efficient and… ▽ More Energy-efficient sense amplifier (SA) circuits are essential for reliable detection of stored memory states in emerging memory systems. In this work, we present four novel sense amplifier (SA) topologies based on phase transition material (PTM) tailored for non-volatile memory applications. We utilize the abrupt switching and volatile hysteretic characteristics of PTMs which enables efficient and fast sensing operation in our proposed SA topologies. We provide comprehensive details of their functionality and assess how process variations impact their performance metrics. Our proposed sense amplifier topologies manifest notable performance enhancement. We achieve a ~67% reduction in sensing delay and a ~80% decrease in sensing power for current sensing. For voltage sensing, we achieve a ~75% reduction in sensing delay and a ~33% decrease in sensing power. Moreover, the proposed SA topologies exhibit improved variation robustness compared to conventional SAs. We also scrutinize the dependence of transistor mirroring window and PTM transition voltages on several device parameters to determine the optimum operating conditions and stance of tunability for each of the proposed SA topologies. △ Less

Submitted 30 August, 2023; originally announced August 2023.

A Deep Dive into the Design Space of a Dynamically Reconfigurable Cryogenic Spiking Neuron

Authors: Md Mazharul Islam, Shamiul Alam, Catherine D Schuman, Md Shafayat Hossain, Ahmedullah Aziz

Abstract: Spiking neural network offers the most bio-realistic approach to mimic the parallelism and compactness of the human brain. A spiking neuron is the central component of an SNN which generates information-encoded spikes. We present a comprehensive design space analysis of the superconducting memristor (SM)-based electrically reconfigurable cryogenic neuron. A superconducting nanowire (SNW) connected… ▽ More Spiking neural network offers the most bio-realistic approach to mimic the parallelism and compactness of the human brain. A spiking neuron is the central component of an SNN which generates information-encoded spikes. We present a comprehensive design space analysis of the superconducting memristor (SM)-based electrically reconfigurable cryogenic neuron. A superconducting nanowire (SNW) connected in parallel with an SM function as a dual-frequency oscillator and two of these oscillators can be coupled to design a dynamically tunable spiking neuron. The same neuron topology was previously proposed where a fixed resistance was used in parallel with the SNW. Replacing the fixed resistance with the SM provides an additional tuning knob with four distinct combinations of SM resistances, which improves the reconfigurability by up to ~70%. Utilizing an external bias current (Ibias), the spike frequency can be modulated up to ~3.5 times. Two distinct spike amplitudes (~1V and ~1.8 V) are also achieved. Here, we perform a systematic sensitivity analysis and show that the reconfigurability can be further tuned by choosing a higher input current strength. By performing a 500-point Monte Carlo variation analysis, we find that the spike amplitude is more variation robust than spike frequency and the variation robustness can be further improved by choosing a higher Ibias. Our study provides valuable insights for further exploration of materials and circuit level modification of the neuron that will be useful for system-level incorporation of the neuron circuit △ Less

Submitted 30 August, 2023; originally announced August 2023.

Fabrication and Characterization of Graphene-Barium Titanate-Graphene layered capacitors by spin coating at low processing temperatures

Authors: M. S. Habib, S. F. U. Farhad, N. I. Tanvir, M. S. Alam, M. N. A. Bitu, M. S. Islam, S. Islam, N. Khatun, M. S Hossain

Abstract: Barium titanate, BaTiO3 (BT), materials have been synthesized by two different routes: one ball-mill-derived (BMD) nanopowder and another precursor-derived (PCD) BT synthesis method were used separately to fabricate BT thin films on stainless steel (SS) and quartz substrates by spin coating. Then thin films from both synthesis routes were characterized by Ultraviolet-Visible-Near Infrared (UV-Vis-… ▽ More Barium titanate, BaTiO3 (BT), materials have been synthesized by two different routes: one ball-mill-derived (BMD) nanopowder and another precursor-derived (PCD) BT synthesis method were used separately to fabricate BT thin films on stainless steel (SS) and quartz substrates by spin coating. Then thin films from both synthesis routes were characterized by Ultraviolet-Visible-Near Infrared (UV-Vis-NIR) Spectroscopy, Field-Emission Scanning Electron Microscopy (FE-SEM), X-ray Diffractometry (XRD), Raman Spectroscopy, and Four-point collinear probe; all carried out at room temperature. Our studies revealed that the PCD synthesis process did not produce the BT phase even under the 900^0C air-annealing condition. In contrast, a homogeneous BT thin film has been formed from the BMD-BT nanopowder. The optical band gap of BMD-BT thin films was found in the 3.10 - 3.28 eV range. Finally, a Graphene-Barium Titanate-Graphene (G-BT-G) structure was fabricated on a SS substrate by spin coating at processing temperatures below 100^0C and characterized by two different pieces of equipment: a Potentiostat/Galvanostat (PG-STAT) and a Precision Impedance Analyzer (PIA). The G-BT-G structure exhibited a capacitance of 8 nF and 7.15 nF, a highest dielectric constant of 800 and 790, and a low dielectric loss of 4.5 and 5, investigated by PG-STAT and PIA equipment, respectively. △ Less

Submitted 17 June, 2023; originally announced June 2023.

Comments: 25 pages, 11 Figures

Cryogenic Reconfigurable Logic with Superconducting Heater Cryotron: Enhancing Area Efficiency and Enabling Camouflaged Processors

Authors: Shamiul Alam, Dana S. Rampini, Bakhrom G. Oripov, Adam N. McCaughan, Ahmedullah Aziz

Abstract: Superconducting electronics are among the most promising alternatives to conventional CMOS technology thanks to the ultra-fast speed and ultra-high energy efficiency of the superconducting devices. Having a cryogenic control processor is also a crucial requirement for scaling the existing quantum computers up to thousands of qubits. Despite showing outstanding speed and energy efficiency, Josephso… ▽ More Superconducting electronics are among the most promising alternatives to conventional CMOS technology thanks to the ultra-fast speed and ultra-high energy efficiency of the superconducting devices. Having a cryogenic control processor is also a crucial requirement for scaling the existing quantum computers up to thousands of qubits. Despite showing outstanding speed and energy efficiency, Josephson junction-based circuits suffer from several challenges such as flux trapping leading to limited scalability, difficulty in driving high impedances, and so on. Three-terminal cryotron devices have been proposed to solve these issues which can drive high impedances (>100 kΩ) and are free from any flux trapping issue. In this work, we develop a reconfigurable logic circuit using a heater cryotron (hTron). In conventional approaches, the number of devices to perform a logic operation typically increases with the number of inputs. However, here, we demonstrate a single hTron device-based logic circuit that can be reconfigured to perform 1-input copy and NOT, 2-input AND and OR, and 3-input majority logic operations by choosing suitable biasing conditions. Consequently, we can perform any processing task with a much smaller number of devices. Also, since we can perform different logic operations with the same circuit (same layout), we can develop a camouflaged system where all the logic gates will have the same layout. Therefore, this proposed circuit will ensure enhanced hardware security against reverse engineering attacks. △ Less

Submitted 27 September, 2023; v1 submitted 16 June, 2023; originally announced June 2023.

Comments: 13 pages, 6 figures

Renormalization of shell model of turbulence

Authors: Mahendra K. Verma, Shadab Alam

Abstract: Renormalization enables a systematic scale-by-scale analysis of multiscale systems. In this paper, we employ \textit{renormalization group} (RG) to the shell model of turbulence and show that the RG equation is satisfied by $ |u_n|^2 =K_\mathrm{Ko} ε^{2/3} k_n^{-2/3}$ and $ ν_n = ν_* \sqrt{K_\mathrm{Ko}} ε^{1/3} k_n^{-4/3}$, where $k_n, u_n $ are the wavenumber and velocity of shell $ n $;… ▽ More Renormalization enables a systematic scale-by-scale analysis of multiscale systems. In this paper, we employ \textit{renormalization group} (RG) to the shell model of turbulence and show that the RG equation is satisfied by $ |u_n|^2 =K_\mathrm{Ko} ε^{2/3} k_n^{-2/3}$ and $ ν_n = ν_* \sqrt{K_\mathrm{Ko}} ε^{1/3} k_n^{-4/3}$, where $k_n, u_n $ are the wavenumber and velocity of shell $ n $; $ν_*, K_\mathrm{Ko}$ are RG and Kolmogorov's constants; and $ ε$ is the energy dissipation rate. We find that $ν_* \approx 0.5$ and $K_\mathrm{Ko} \approx 1.7$, consistent with earlier RG works on Navier-Stokes equation. We verify the theoretical predictions using numerical simulations. △ Less

Submitted 3 June, 2023; originally announced June 2023.

Comments: To appear in Phys. Rev. E

Optical pumping of electronic quantum Hall states with vortex light

Authors: Deric Session, Mahmoud Jalali Mehrabad, Nikil Paithankar, Tobias Grass, Christian J. Eckhardt, Bin Cao, Daniel Gustavo Suárez Forero, Kevin Li, Mohammad S. Alam, Kenji Watanabe, Takashi Taniguchi, Glenn S. Solomon, Nathan Schine, Jay Sau, Roman Sordan, Mohammad Hafezi

Abstract: A fundamental requirement for quantum technologies is the ability to coherently control the interaction between electrons and photons. However, in many scenarios involving the interaction between light and matter, the exchange of linear or angular momentum between electrons and photons is not feasible, a condition known as the dipole-approximation limit. An example of a case beyond this limit that… ▽ More A fundamental requirement for quantum technologies is the ability to coherently control the interaction between electrons and photons. However, in many scenarios involving the interaction between light and matter, the exchange of linear or angular momentum between electrons and photons is not feasible, a condition known as the dipole-approximation limit. An example of a case beyond this limit that has remained experimentally elusive is when the interplay between chiral electrons and vortex light is considered, where the orbital angular momentum of light can be transferred to electrons. Here, we present a novel mechanism for such an orbital angular momentum transfer from optical vortex beams to electronic quantum Hall states. Specifically, we identify a robust contribution to the radial photocurrent, in an annular graphene sample within the quantum Hall regime, that depends on the vorticity of light. This phenomenon can be interpreted as an optical pumping scheme, where the angular momentum of photons is transferred to electrons, generating a radial current, and the current direction is determined by the vorticity of the light. Our findings offer fundamental insights into the optical probing and manipulation of quantum coherence, with wide-ranging implications for advancing quantum coherent optoelectronics. △ Less

Submitted 27 October, 2023; v1 submitted 6 June, 2023; originally announced June 2023.

Journal ref: Nature Photonics, 19, 156 (2025)

Gravitational anomaly in the ferrimagnetic topological Weyl semimetal NdAlSi

Authors: Pardeep Kumar Tanwar, Mujeeb Ahmad, Md Shahin Alam, Xiaohan Yao, Fazel Tafti, Marcin Matusiak

Abstract: Quantum anomalies are the breakdowns of classical conservation laws that occur in quantum-field theory description of a physical system. They appear in relativistic field theories of chiral fermions and are expected to lead to anomalous transport properties in Weyl semimetals. This includes a chiral anomaly, which is a violation of the chiral current conservation that takes place when a Weyl semim… ▽ More Quantum anomalies are the breakdowns of classical conservation laws that occur in quantum-field theory description of a physical system. They appear in relativistic field theories of chiral fermions and are expected to lead to anomalous transport properties in Weyl semimetals. This includes a chiral anomaly, which is a violation of the chiral current conservation that takes place when a Weyl semimetal is subjected to parallel electric and magnetic fields. A charge pumping between Weyl points of opposite chirality causes the chiral magnetic effect that has been extensively studied with electrical transport. On the other hand, if the thermal gradient, instead of the electrical field, is applied along the magnetic field, then as a consequence of the gravitational (also called the thermal chiral) anomaly an energy pumping occurs within a pair of Weyl cones. As a result, this is expected to generate anomalous heat current contributing to the thermal conductivity. We report an increase of both the magneto-electric and magneto-thermal conductivities in quasi-classical regime of the magnetic Weyl semimetal NdAlSi. Our work also shows that the anomalous electric and heat currents, which occur due to the chiral magnetic effect and gravitational anomalies respectively, are still linked by a 170 years old relation called the Wiedemann-Franz law. △ Less

Submitted 14 November, 2023; v1 submitted 8 May, 2023; originally announced May 2023.

Comments: 26 pages, 8 figures

Journal ref: Phys. Rev. B 108, L161106 (2023)

Preparing quantum many-body scar states on quantum computers

Authors: Erik J. Gustafson, Andy C. Y. Li, Abid Khan, Joonho Kim, Doga Murat Kurkcuoglu, M. Sohaib Alam, Peter P. Orth, Armin Rahmani, Thomas Iadecola

Abstract: Quantum many-body scar states are highly excited eigenstates of many-body systems that exhibit atypical entanglement and correlation properties relative to typical eigenstates at the same energy density. Scar states also give rise to infinitely long-lived coherent dynamics when the system is prepared in a special initial state having finite overlap with them. Many models with exact scar states hav… ▽ More Quantum many-body scar states are highly excited eigenstates of many-body systems that exhibit atypical entanglement and correlation properties relative to typical eigenstates at the same energy density. Scar states also give rise to infinitely long-lived coherent dynamics when the system is prepared in a special initial state having finite overlap with them. Many models with exact scar states have been constructed, but the fate of scarred eigenstates and dynamics when these models are perturbed is difficult to study with classical computational techniques. In this work, we propose state preparation protocols that enable the use of quantum computers to study this question. We present protocols both for individual scar states in a particular model, as well as superpositions of them that give rise to coherent dynamics. For superpositions of scar states, we present both a system-size-linear depth unitary and a finite-depth nonunitary state preparation protocol, the latter of which uses measurement and postselection to reduce the circuit depth. For individual scarred eigenstates, we formulate an exact state preparation approach based on matrix product states that yields quasipolynomial-depth circuits, as well as a variational approach with a polynomial-depth ansatz circuit. We also provide proof of principle state-preparation demonstrations on superconducting quantum hardware. △ Less

Submitted 2 November, 2023; v1 submitted 19 January, 2023; originally announced January 2023.

Comments: 20 Pages, 15 Figures, 2 Tables. V2: corrected typos

Report number: FERMILAB-PUB-22-904-SQMS

Journal ref: Quantum 7, 1171 (2023)

Sign change of the anomalous Hall effect and the anomalous Nernst effect in Weyl semimetal CeAlSi

Authors: Md Shahin Alam, Amar Fakhredine, Mujeeb Ahmed, P. K. Tanwar, Hung-Yu Yang, Fazel Tafti, Giuseppe Cuono, Rajibul Islam, Bahadur Singh, Artem Lynnyk, Carmine Autieri, Marcin Matusiak

Abstract: We report the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) data for the non-collinear Weyl semimetal CeAlSi. The anomalous Hall conductivity (σ_ij^A) was measured for two different orientations of the magnetic field (B), namely σ_yz^A for B II a and σ_xy^A for B II c, where a and c denote the crystallographic axes. We find that σ_xy^A and σ_yz^A are of opposite sign and both a… ▽ More We report the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) data for the non-collinear Weyl semimetal CeAlSi. The anomalous Hall conductivity (σ_ij^A) was measured for two different orientations of the magnetic field (B), namely σ_yz^A for B II a and σ_xy^A for B II c, where a and c denote the crystallographic axes. We find that σ_xy^A and σ_yz^A are of opposite sign and both are large below the Curie temperature (T_C). In the paramagnetic phase, σ_xy^A raises even more and goes through a maximum at T ~ 170 K, whereas the absolute value of σ_yz^A decreases with increasing temperature. The origin of the sign difference between σ_xy^A and σ_yz^A was attributed to the reconstruction of the band structure under the variation of the spin orientation. Further, in a system where humps in the AHE are present and scalar spin chirality is zero, we show that the k-space topology plays an important role to determine the transport properties at both low and high temperatures. We also observed the anomalous contribution in the Nernst conductivity (α_xy^A) measured for B II c. α_xy^A/T turns out to be sizeable in the magnetic phase and above T_C slowly decreases with temperature. We were able to recreate the temperature dependences of σ_xy^A and α_xy^A/T in the paramagnetic phase using a single band toy-model assuming a non-zero Berry curvature in the vicinity of the Weyl node. A decisive factor appears to be a small energy distance between the Fermi level and a Weyl point. △ Less

Submitted 28 February, 2023; v1 submitted 18 October, 2022; originally announced October 2022.

Comments: 37 pages, 11 figures

Journal ref: Phys. Rev. B 107, 085102 (2023)

Compact Model of a Topological Transistor

Authors: Md Mazharul Islam, Shamiul Alam, Md Shafayat Hossain, Ahmedullah Aziz

Abstract: The precession of a ferromagnet leads to the injection of spin current and heat into an adjacent non-magnetic material. Besides, spin-orbit entanglement causes an additional charge current injection. Such a device has been recently proposed where a quantum-spin hall insulator (QSHI) in proximity to a ferromagnetic insulator (FI) and superconductor (SC) leads to the pumping of charge, spin, and hea… ▽ More The precession of a ferromagnet leads to the injection of spin current and heat into an adjacent non-magnetic material. Besides, spin-orbit entanglement causes an additional charge current injection. Such a device has been recently proposed where a quantum-spin hall insulator (QSHI) in proximity to a ferromagnetic insulator (FI) and superconductor (SC) leads to the pumping of charge, spin, and heat. Here we build a circuit-compatible Verilog-A-based compact model for the QSHI-FI-SC device capable of generating two topologically robust modes enabling the device operation. Our model also captures the dependence on the ferromagnetic precision, drain voltage, and temperature with an excellent (> 99%) accuracy. △ Less

Submitted 7 October, 2022; originally announced October 2022.

Interpreting convolutional neural networks' low dimensional approximation to quantum spin systems

Authors: Yilong Ju, Shah Saad Alam, Jonathan Minoff, Fabio Anselmi, Han Pu, Ankit Patel

Abstract: Convolutional neural networks (CNNs) have been employed along with Variational Monte Carlo methods for finding the ground state of quantum many-body spin systems with great success. In order to do so, however, a CNN with only linearly many variational parameters has to circumvent the ``curse of dimensionality'' and successfully approximate a wavefunction on an exponentially large Hilbert space. In… ▽ More Convolutional neural networks (CNNs) have been employed along with Variational Monte Carlo methods for finding the ground state of quantum many-body spin systems with great success. In order to do so, however, a CNN with only linearly many variational parameters has to circumvent the ``curse of dimensionality'' and successfully approximate a wavefunction on an exponentially large Hilbert space. In our work, we provide a theoretical and experimental analysis of how the CNN optimizes learning for spin systems, and investigate the CNN's low dimensional approximation. We first quantify the role played by physical symmetries of the underlying spin system during training. We incorporate our insights into a new training algorithm and demonstrate its improved efficiency, accuracy and robustness. We then further investigate the CNN's ability to approximate wavefunctions by looking at the entanglement spectrum captured by the size of the convolutional filter. Our insights reveal the CNN to be an ansatz fundamentally centered around the occurrence statistics of $K$-motifs of the input strings. We use this motivation to provide the shallow CNN ansatz with a unifying theoretical interpretation in terms of other well-known statistical and physical ansatzes such as the maximum entropy (MaxEnt) and entangled plaquette correlator product states (EP-CPS). Using regression analysis, we find further relationships between the CNN's approximations of the different motifs' expectation values. Our results allow us to gain a comprehensive, improved understanding of how CNNs successfully approximate quantum spin Hamiltonians and to use that understanding to improve CNN performance. △ Less

Submitted 2 October, 2022; originally announced October 2022.

Comments: 27 pages, 6 figures. Yilong Ju and Shah Saad Alam contributed equally to this work

Spherical and Rod-shaped Gold Nanoparticles for Surface Enhanced Raman Spectroscopy

Authors: Md. Shaha Alam, Syed Farid Uddin Farhad, Nazmul Islam Tanvir, Md. Nur Amin Bitu, Mohammad Moniruzzaman, Mahmuda Hakim, Md Aftab Ali Shaikh

Abstract: Raman Spectroscopy offers an in-situ, rapid, and non-destructive characterization tool for chemical analysis of diverse samples with no or minimal preparation. However, due to the inherent weak signal of conventional Raman spectroscopy, surface plasmon resonance features of noble metal nanoparticles have been utilized to conduct Surface Enhanced Raman Spectroscopy (SERS) in detecting trace label c… ▽ More Raman Spectroscopy offers an in-situ, rapid, and non-destructive characterization tool for chemical analysis of diverse samples with no or minimal preparation. However, due to the inherent weak signal of conventional Raman spectroscopy, surface plasmon resonance features of noble metal nanoparticles have been utilized to conduct Surface Enhanced Raman Spectroscopy (SERS) in detecting trace label contaminants in foods and foodstuffs. In this effort, we synthesized gold nanoparticles (AuNPs) by reduction of chloroauric acid (HAuCl4) with sodium citrate dehydrate. We prepared different sizes of AuNPs at a fixed temperature (100 oC) but with varying pHs of 4 and 8. The as-synthesized AuNPs were characterized by UV-Vis spectroscopy, dynamic light scattering (DLS), and Field Emission Scanning Electron Microscopy (FE-SEM). FE-SEM micrographs revealed spherical AuNPs with an average diameter of approx. 55 nm and rod-shaped AuNPs with an average length of approx. 170 nm for sample synthesis at pH 8 and 4, respectively. The effectiveness of the as-prepared AuNPs for SERS is tested by detecting Rhodamine 6G diluted at a trace level. This study suggests that plasmonic nanoparticles coupled with SERS have great potential for broad applications in detecting other trace amounts of hazardous chemicals in foods and foodstuffs. △ Less

Submitted 1 October, 2022; v1 submitted 19 September, 2022; originally announced September 2022.

Comments: 4 pages, 5 figures

Generalized Bose-Fermi mapping and strong coupling ansatz wavefunction for one dimensional strongly interacting spinor quantum gases

Authors: Li Yang, Shah Saad Alam, Han Pu

Abstract: Quantum many-body systems in one dimension (1D) exhibit some peculiar properties. In this article, we review some of our work on strongly interacting 1D spinor quantum gas. First, we discuss a generalized Bose-Fermi mapping that maps the charge degrees of freedom to a spinless Fermi gas and the spin degrees of freedom to a spin chain model. This also maps the strongly interacting system into a wea… ▽ More Quantum many-body systems in one dimension (1D) exhibit some peculiar properties. In this article, we review some of our work on strongly interacting 1D spinor quantum gas. First, we discuss a generalized Bose-Fermi mapping that maps the charge degrees of freedom to a spinless Fermi gas and the spin degrees of freedom to a spin chain model. This also maps the strongly interacting system into a weakly interacting one, which is amenable for perturbative calculations. Next, based on this mapping, we construct an ansatz wavefunction for the strongly interacting system, using which many physical quantities can be conveniently calculated. We showcase the usage of this ansatz wavefunction by considering the collective excitations and quench dynamics of a harmonically trapped system. △ Less

Submitted 1 August, 2022; originally announced August 2022.

Comments: 23 pages, 4 figures

Severe violation of the Wiedemann-Franz law in quantum oscillations of NbP

Authors: Pardeep Kumar Tanwar, Md Shahin Alam, Mujeeb Ahmad, Dariusz Kaczorowski, Marcin Matusiak

Abstract: The thermal conductivity (k) of the Weyl semimetal NbP was studied with the thermal gradient and magnetic field applied parallel to [0 0 1] direction. At low temperatures k(B) exhibits large quantum oscillations with frequencies matching two of several determined from the Shubnikov - de Haas effect measured on the same sample with analogous electrical current and magnetic field orientation. Both f… ▽ More The thermal conductivity (k) of the Weyl semimetal NbP was studied with the thermal gradient and magnetic field applied parallel to [0 0 1] direction. At low temperatures k(B) exhibits large quantum oscillations with frequencies matching two of several determined from the Shubnikov - de Haas effect measured on the same sample with analogous electrical current and magnetic field orientation. Both frequencies found in k(B) originate from the electron pocket enclosing a pair of Weyl nodes. The amplitude of the oscillatory component of the thermal conductivity turns out to be two orders of magnitude larger than the corresponding value calculated from the electrical conductivity using the Wiedemann - Franz law. Analysis of possible sources of this discrepancy indicates the chiral zero sound effect as a potential cause of its appearance. △ Less

Submitted 24 May, 2022; originally announced May 2022.

Comments: 26 pages, 9 figures

Journal ref: Phys. Rev. B 106, L041106 (2022)

Impact of reaction temperatures on the particle size of V2O5 synthesized by facile hydrothermal technique and their auspicious photocatalytic performance in dye degradation

Authors: M. A. Jalil, M. N. I. Khan, S. Mandal, F. -U. -Z. Chowdhury, M. M. Hossain, D. Jana, M. S. Alam, M. M. Uddin

Abstract: In this study, a complete study of the effect of hydrothermal reaction temperatures on the synthesis and physical properties of V2O5 using the green facile mild hydrothermal method has been performed with six different temperatures 100 °C to 200 °C, in the step of 20 °C. . The XRD pattern confirm the stable orthorhombic crystal structure of the synthesized samples at all reaction temperatures. The… ▽ More In this study, a complete study of the effect of hydrothermal reaction temperatures on the synthesis and physical properties of V2O5 using the green facile mild hydrothermal method has been performed with six different temperatures 100 °C to 200 °C, in the step of 20 °C. . The XRD pattern confirm the stable orthorhombic crystal structure of the synthesized samples at all reaction temperatures. The SEM and TEM images demonstrate the particle-like morphology, and these characterizations affirmed that the particles size became larger with the increase of reaction temperatures. The FTIR analysis is employed to study the functional groups, and the obtained results are consistent with the XRD analysis. The bandgap has been estimated at various reaction temperatures using UV-vis diffuse reflectance spectra (UV-DRS) and was found to be varied 2.09 eV to 2.15 eV that are suitable range to absorb a significant amount of visible light. The photocatalysis of methylene blue (MB) with synthesized samples has been accomplished to investigate photocatalytic efficiency. The pure V2O5 synthesized at lower reaction temperature (100 °C) possess a lower bandgap and, accordingly, higher photocatalytic efficiency. △ Less

Submitted 9 May, 2022; originally announced May 2022.

Comments: 19 pages

Fermionic approach to variational quantum simulation of Kitaev spin models

Authors: Ammar Jahin, Andy C. Y. Li, Thomas Iadecola, Peter P. Orth, Gabriel N. Perdue, Alexandru Macridin, M. Sohaib Alam, Norm M. Tubman

Abstract: We use the variational quantum eigensolver (VQE) to simulate Kitaev spin models with and without integrability breaking perturbations, focusing in particular on the honeycomb and square-octagon lattices. These models are well known for being exactly solvable in a certain parameter regime via a mapping to free fermions. We use classical simulations to explore a novel variational ansatz that takes a… ▽ More We use the variational quantum eigensolver (VQE) to simulate Kitaev spin models with and without integrability breaking perturbations, focusing in particular on the honeycomb and square-octagon lattices. These models are well known for being exactly solvable in a certain parameter regime via a mapping to free fermions. We use classical simulations to explore a novel variational ansatz that takes advantage of this fermionic representation and is capable of expressing the exact ground state in the solvable limit. We also demonstrate that this ansatz can be extended beyond this limit to provide excellent accuracy when compared to other VQE approaches. In certain cases, this fermionic representation is advantageous because it reduces by a factor of two the number of qubits required to perform the simulation. We also comment on the implications of our results for simulating non-Abelian anyons on quantum computers. △ Less

Submitted 11 April, 2022; originally announced April 2022.

Report number: FERMILAB-PUB-22-083-SQMS

Journal ref: Phys. Rev. A 106, 022434 (2022)

Two-stroke Quantum Measurement Heat Engine

Authors: M. Sahnawaz Alam, B. Prasanna Venkatesh

Abstract: We propose and analyze the theoretical model for a two-stroke quantum heat engine with one of the heat baths replaced by a non-selective quantum measurement. We show that the engine's invariant reference state depends on whether the cycle is monitored or unmonitored via diagnostic measurements to determine the engine's work output. We explore in detail the average work output and fluctuations of t… ▽ More We propose and analyze the theoretical model for a two-stroke quantum heat engine with one of the heat baths replaced by a non-selective quantum measurement. We show that the engine's invariant reference state depends on whether the cycle is monitored or unmonitored via diagnostic measurements to determine the engine's work output. We explore in detail the average work output and fluctuations of the proposed heat engine for the monitored and unmonitored cases. We also identify unitary work strokes for which the invariant states can support coherences in the energy basis leading to differing predictions for the average energy change during the unitary work strokes and the average work from the standard two-projective measurement approach. △ Less

Submitted 17 January, 2022; originally announced January 2022.

Comments: 10 pages, 7 figures, Comments welcome

Skyrmion based energy efficient straintronic physical reservoir computing

Authors: Md Mahadi Rajib, Walid Al Misba, Md. Fahim F. Chowdhury, Muhammad Sabbir Alam, Jayasimha Atulasimha

Abstract: Physical Reservoir Computing (PRC) is an unconventional computing paradigm, which exploits nonlinear dynamics of reservoir blocks to perform recognition and classification tasks. Here we show with simulations that patterned thin films hosting several skyrmions, particularly one, two, four and nine skyrmions, can implement energy efficient reservoir computing. This reservoir computing (RC) block is… ▽ More Physical Reservoir Computing (PRC) is an unconventional computing paradigm, which exploits nonlinear dynamics of reservoir blocks to perform recognition and classification tasks. Here we show with simulations that patterned thin films hosting several skyrmions, particularly one, two, four and nine skyrmions, can implement energy efficient reservoir computing. This reservoir computing (RC) block is based on nonlinear breathing dynamics of skyrmions, which are coupled to each other through dipole interaction and spin waves, in response to a voltage generated strain. This nonlinear and coupled magnetization dynamics is exploited to perform temporal pattern recognition. Two performance metrics, namely Short-Term Memory (STM) and Parity Check (PC) capacity are studied to demonstrate the potential of such skyrmion based PRC in addition to showing it can classify sine and square waves with 100% accuracy. Furthermore, our study demonstrates that nonlinear magnetization dynamics and interaction through spin waves and dipole coupling have a strong influence on STM and PC capacity, thus explaining the role of physical interactions in a dynamical system on its ability to perform Reservoir Computing (RC). △ Less

Submitted 13 April, 2022; v1 submitted 27 December, 2021; originally announced December 2021.

Temperature driven spin-zero effect in TaAs$_2$

Authors: Md Shahin Alam, P. K. Tanwar, Krzysztof Dybko, Ashutosh S. Wadge, Przemyslaw Iwanowski, Andrzej Wisniewski, Marcin Matusiak

Abstract: The electrical and thermo-electrical transport effects of the TaAs$_2$ semimetal were measured in a magnetic field applied along [-2 0 1] direction. The resulting field dependences of the resistivity as well as the Hall, Seebeck and Nernst coefficient below T ~ 100 K can be satisfactory described within the two-band model consisting of the electron and hole pockets. At low temperature all the meas… ▽ More The electrical and thermo-electrical transport effects of the TaAs$_2$ semimetal were measured in a magnetic field applied along [-2 0 1] direction. The resulting field dependences of the resistivity as well as the Hall, Seebeck and Nernst coefficient below T ~ 100 K can be satisfactory described within the two-band model consisting of the electron and hole pockets. At low temperature all the measured effects exhibit significant contribution from quantum oscillations. The fast Fourier transform (FFT) of the oscillatory Nernst signal shows two fundamental frequencies, Fa = 105 T and Fb = 221 T, and the second harmonic of the latter (F2b = 442 T). The ratio between FFT amplitudes of Fb and F2b changes with temperature in an unusual way, indicating that we observe the spin-zero effect caused by temperature change. This is likely related to substantial temperature dependence of the Lande g-factor, which in turn can result from non-parabolic energy dispersion or temperature evolution of the spin-orbit coupling. △ Less

Submitted 24 May, 2022; v1 submitted 16 December, 2021; originally announced December 2021.

Comments: 30 pages, 14 figures

Journal ref: J. Phys. Chem. Solids, 170, 110939 (2022)

Pressure dependent magnetic properties on bulk CrBr3 single crystals

Authors: Rubyann Olmos, Shamsul Alam, Po Hao Chang, Kinjal Gandha, Ikenna C. Nlebedim, Andrew Cole, Fazel Tafti, Rajendra Zope, Srinivasa R. Singamaneni

Abstract: The van der Waals class of materials offer an approach to two-dimensional magnetism as their spin fluctuations can be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin-lattice coupling and long-range magnetic ordering can be modified with pressure in van der Waals materials. In this work, the magnetic properties of quasi two-dimensional CrBr3 are reported applying hy… ▽ More The van der Waals class of materials offer an approach to two-dimensional magnetism as their spin fluctuations can be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin-lattice coupling and long-range magnetic ordering can be modified with pressure in van der Waals materials. In this work, the magnetic properties of quasi two-dimensional CrBr3 are reported applying hydrostatic pressure. The application of pressure (0 - 0.8 GPa) shows a 72 % decrease in saturation magnetization with small decrease in the Curie temperature from 33 to 29 K. Density functional theory calculations with pressure up to 1 GPa show a reduction in volume and interplanar distance as pressure increases. To further understand magnetic properties with applied pressure, the magnetocrystalline anisotropy energy (MAE) and exchange coupling parameter (J) are calculated. There is minimal decrease in MAE and the first nearest neighbor interaction (J1) (U = 2.7 eV and J = 0.7 eV), shows an increase in J1 with respect to pressure. Overall, CrBr3 displays ferromagnetic interlayer coupling and the calculated exchange coupling and MAE parameters match well with the observations from the experimental work. △ Less

Submitted 30 November, 2021; originally announced December 2021.

Chiral Phase Change Nanomaterials

Authors: Joshua A. Burrow, Md Shah Alam, Evan M. Smith, Riad Yahiaoui, Ryan Laing, Piyush J. Shah, Thomas A. Searles, Shivashankar Vangala, Joshua R. Hendrickson, Andrew Sarangan, Imad Agha

Abstract: Chiral nanostructures offer the ability to respond to the vector nature of a light beam at the nanoscale. While naturally chiral materials offer a path towards scalability, engineered structures offer a path to wavelength tunability through geometric manipulation. Neither approach, however, allows for temporal control of chirality. Therefore, in the best of all worlds, it is crucial to realize chi… ▽ More Chiral nanostructures offer the ability to respond to the vector nature of a light beam at the nanoscale. While naturally chiral materials offer a path towards scalability, engineered structures offer a path to wavelength tunability through geometric manipulation. Neither approach, however, allows for temporal control of chirality. Therefore, in the best of all worlds, it is crucial to realize chiral materials that possess the quality of scalability, tailored wavelength response, and dynamic control at high speeds. Here, a new class of intrinsically chiral phase change nanomaterials (PCNMs) is proposed and explored, based on a scalable bottom-up fabrication technique with a high degree of control in three dimensions. Angular resolved Mueller Matrix and spectroscopic ellipsometry are performed to characterize the optical birefringence and dichroism, and a numerical model is provided to explain the origin of optical activity. This work achieves the critical goal of demonstrating high-speed dynamic switching of chirality over 50,000 cycles via the underlying PCNM. △ Less

Submitted 18 November, 2021; originally announced November 2021.

Comments: 21 pages, 10 page supplement, 16 figures

Dynamical Fermionization in One Dimensional Spinor Gases

Authors: Shah Saad Alam, Timothy Skaras, Li Yang, Han Pu

Abstract: Dynamical fermionization refers to the phenomenon in Tonks-Girardeau (TG) gases where, upon release from harmonic confinement, the gas's momentum density profile evolves asymptotically to that of an ideal Fermi gas in the initial trap. This phenomenon has been demonstrated theoretically in hardcore and anyonic TG gases, and recently experimentally observed in a strongly interacting Bose gas. We ex… ▽ More Dynamical fermionization refers to the phenomenon in Tonks-Girardeau (TG) gases where, upon release from harmonic confinement, the gas's momentum density profile evolves asymptotically to that of an ideal Fermi gas in the initial trap. This phenomenon has been demonstrated theoretically in hardcore and anyonic TG gases, and recently experimentally observed in a strongly interacting Bose gas. We extend this study to a one dimensional (1D) spinor gas of arbitrary spin in the strongly interacting regime, and analytically prove that the total momentum distribution after the harmonic trap is turned off approaches that of a spinless ideal Fermi gas, while the asymptotic momentum distribution of each spin component takes the same shape of the initial real space density profile of that spin component. Our work demonstrates the rich physics arising from the interplay between the spin and the charge degrees of freedom in a spinor system. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Comments: 5 pages, 2 figures

Journal ref: Phys. Rev. Lett. 127, 023002 (2021)

Comparison of regularized SCAN functional with SCAN functional with and without self-interaction for a wide-array of properties

Authors: Yoh Yamamoto, Alan Salcedo, Carlos M. Diaz, Md Shamsul Alam, Tunna Baruah, Rajendra R. Zope

Abstract: The Strongly Constrained and Appropriately Normed (SCAN) functional is a non-empirical meta-generalized-gradient approximation (meta-GGA) functional that satisfies all the known constraints that a meta-GGA functional can, but it also exhibits a great degree of sensitivity to numerical grids. Its numerical complexities are amplified when used in Perdew-Zunger (PZ) self-interaction correction (SIC)… ▽ More The Strongly Constrained and Appropriately Normed (SCAN) functional is a non-empirical meta-generalized-gradient approximation (meta-GGA) functional that satisfies all the known constraints that a meta-GGA functional can, but it also exhibits a great degree of sensitivity to numerical grids. Its numerical complexities are amplified when used in Perdew-Zunger (PZ) self-interaction correction (SIC) which requires evaluating energies and potentials using orbital densities that vary far more rapidly than spin densities. Recent regularization of the SCAN functional (rSCAN) simplifies numerical complexities of SCAN at the expense of violation of some exact constraints. To develop a good understanding of the performance of rSCAN and the effect of loss of an exact constraint at the limit of slowly varying density, we have compared its performance against SCAN for vibrational frequencies, infra-red and Raman intensities of water clusters, electric dipole moments, spin magnetic moments of a few molecular magnets, weak interaction energies of dimers, barrier heights of reactions, and atomization energies for benchmark sets of molecules. Likewise, we examined the performance of SIC-rSCAN using the PZ-SIC method by studying atomic total energies, ionization potentials and electron affinities, molecular atomization energies, barrier heights, and dissociation and reaction energies. We find that rSCAN requires a much less dense numerical grid and gives very similar results as SCAN for all properties examined with the exception of atomization energies which are somewhat worse in rSCAN. On the other hand, SIC-rSCAN gives marginally better performance than SIC-SCAN for almost all properties studied in this work. △ Less

Submitted 28 April, 2020; originally announced April 2020.

Comments: 50 pages, 5 figures

Lattice model parameters for ultracold nonreactive molecules: chaotic scattering and its limitations

Authors: Michael L. Wall, Rick Mukherjee, Shah Saad Alam, Nirav P. Mehta, Kaden R. A. Hazzard

Abstract: We calculate the parameters of the recently-derived many-channel Hubbard model that is predicted to describe ultracold nonreactive molecules in an optical lattice, going beyond the approximations used in Doçaj \textit{et al.}~[Phys. Rev. Lett. \textbf{116}, 135301 (2016)]. Although those approximations are expected to capture the qualitative structure of the model parameters, finer details and qua… ▽ More We calculate the parameters of the recently-derived many-channel Hubbard model that is predicted to describe ultracold nonreactive molecules in an optical lattice, going beyond the approximations used in Doçaj \textit{et al.}~[Phys. Rev. Lett. \textbf{116}, 135301 (2016)]. Although those approximations are expected to capture the qualitative structure of the model parameters, finer details and quantitative values are less certain. To set expectations for experiments, whose results depend on the model parameters, we describe the approximations' regime of validity and the likelihood that experiments will be in this regime, discuss the impact that the failure of these approximations would have on the predicted model, and develop theories going beyond these approximations. Not only is it necessary to know the model parameters in order to describe experiments, but the connection that we elucidate between these parameters and the underlying assumptions that are used to derive them will allow molecule experiments to probe new physics. For example, transition state theory, which is used across chemistry and chemical physics, plays a key role in our determination of lattice parameters, thus connecting its physical assumptions to highly accurate experimental investigation. △ Less

Submitted 20 December, 2016; originally announced December 2016.

Comments: 23 pages, 6 figures

Journal ref: Phys. Rev. A 95, 043636 (2017)

Microscopic derivation of multi-channel Hubbard models for ultracold nonreactive molecules in an optical lattice

Authors: Michael L. Wall, Nirav P. Mehta, Rick Mukherjee, Shah Saad Alam, Kaden R. A. Hazzard

Abstract: Recent experimental advances in the cooling and manipulation of bialkali dimer molecules have enabled the production of gases of ultracold molecules that are not chemically reactive. It has been presumed in the literature that in the absence of an electric field the low-energy scattering of such nonreactive molecules (NRMs) will be similar to atoms, in which a single $s$-wave scattering length gov… ▽ More Recent experimental advances in the cooling and manipulation of bialkali dimer molecules have enabled the production of gases of ultracold molecules that are not chemically reactive. It has been presumed in the literature that in the absence of an electric field the low-energy scattering of such nonreactive molecules (NRMs) will be similar to atoms, in which a single $s$-wave scattering length governs the collisional physics. However, in Ref. [1], it was argued that the short-range collisional physics of NRMs is much more complex than for atoms, and that this leads to a many-body description in terms of a multi-channel Hubbard model. In this work, we show that this multi-channel Hubbard model description of NRMs in an optical lattice is robust against the approximations employed in Ref. [1] to estimate its parameters. We do so via an exact, albeit formal, derivation of a multi-channel resonance model for two NRMs from an ab initio description of the molecules in terms of their constituent atoms. We discuss the regularization of this two-body multi-channel resonance model in the presence of a harmonic trap, and how its solutions form the basis for the many-body model of Ref. [1]. We also generalize the derivation of the effective lattice model to include multiple internal states (e.g., rotational or hyperfine). We end with an outlook to future research. △ Less

Submitted 20 December, 2016; originally announced December 2016.

Comments: 19 pages, 4 figures

Journal ref: Phys. Rev. A 95, 043635 (2017)

Room temperature dielectric and magnetic properties of Gd and Ti co-doped BiFeO$_{3}$ ceramics

Authors: M. A. Basith, O. Kurni, M. S. Alam, B. L. Sinha, Bashir Ahmmad

Abstract: Room temperature dielectric and magnetic properties of BiFeO$_3$ samples, co-doped with magnetic Gd and non-magnetic Ti in place of Bi and Fe, respectively, were reported. The nominal compositions of Bi$_{0.9}$Gd$_{0.1}$Fe$_{1-x}$Ti$_x$O$_3$ (x = 0.00-0.25) ceramics were synthesized by conventional solid state reaction technique. X-ray diffraction patterns revealed that the substitution of Fe by T… ▽ More Room temperature dielectric and magnetic properties of BiFeO$_3$ samples, co-doped with magnetic Gd and non-magnetic Ti in place of Bi and Fe, respectively, were reported. The nominal compositions of Bi$_{0.9}$Gd$_{0.1}$Fe$_{1-x}$Ti$_x$O$_3$ (x = 0.00-0.25) ceramics were synthesized by conventional solid state reaction technique. X-ray diffraction patterns revealed that the substitution of Fe by Ti induces a phase transition from rhombohedral to orthorhombic at x $>$ 0.20. Morphological studies demonstrated that the average grain size was reduced from $\sim {~}$1.5 $μm$ to $\sim {~}$200 $nm$ with the increase in Ti content. Due to Ti substitution, the dielectric constant was stable over a wide range of high frequencies (30 kHz to 20 MHz) by suppressing the dispersion at low frequencies. The dielectric properties of the compounds are associated with their improved morphologies and reduced leakage current densities probably due to the lower concentration of oxygen vacancies in the compositions. Magnetic properties of Bi$_{0.9}$Gd$_{0.1}$Fe$_{1-x}$Ti$_x$O$_3$ (x = 0.00-0.25) ceramics measured at room temperature were enhanced with Ti substitution up to 20 $\%$ compared to that of pure BiFeO$_3$ and Ti undoped Bi$_{0.9}$Gd$_{0.1}$FeO$_3$ samples. The enhanced magnetic properties might be attributed to the substitution induced suppression of spiral spin structure of BiFeO$_3$. An asymmetric shifts both in the field and magnetization axes of magnetization versus magnetic field (M-H) curves was observed. This indicates the presence of exchange bias effect in these compounds notably at room temperature. △ Less

Submitted 27 July, 2015; originally announced July 2015.

Comments: 8 pages, 5 figures

Journal ref: Journal of Applied Physics 115, 024102 (2014

Hybrid CMOS-MQCA Logic Architectures using Multi-Layer Spintronic Devices

Authors: Jayita Das, Syed M. Alam, Srinath Rajaram, Sanjukta Bhanja

Abstract: We present a novel hybrid CMOS-MQCA architecture using multi-layer Spintronic devices as computing elements. A feasibility study is presented with 22nm CMOS where new approaches for spin transfer torque induced clocking and read-out scheme for variability-tolerance are introduced. A first-of-its-kind Spintronic device model enables circuit simulation using existing CAD infrastructure. Approximatel… ▽ More We present a novel hybrid CMOS-MQCA architecture using multi-layer Spintronic devices as computing elements. A feasibility study is presented with 22nm CMOS where new approaches for spin transfer torque induced clocking and read-out scheme for variability-tolerance are introduced. A first-of-its-kind Spintronic device model enables circuit simulation using existing CAD infrastructure. Approximately 70% reduction in energy consumption is observed when compared against conventional field-induced clocking scheme. △ Less

Submitted 17 March, 2012; v1 submitted 19 February, 2011; originally announced February 2011.

Comments: The paper has been withdrawn

Clear Experimental Signature of Charge-Orbital density wave in Nd$_{1-x}$Ca$_{1+x}$MnO$_{4}$:Heat Capacity and Magnetization study

Authors: Sher Alam, A. T. M. N. Islam, T. Nagai, M. Xu, Javed Ahmad, Y. Matsui, I. Tanaka

Abstract: Single Crystals of Nd$_{1-x}$Ca$_{1+x}$MnO$_{4}$ have been prepared by the traveling floating-zone method, and possible evidence of a charge -orbital density wave in this material presented earlier [PRB68,092405 (2003)] using High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In the current note we present direct evidence of charge-orbital ordering in this material using… ▽ More Single Crystals of Nd$_{1-x}$Ca$_{1+x}$MnO$_{4}$ have been prepared by the traveling floating-zone method, and possible evidence of a charge -orbital density wave in this material presented earlier [PRB68,092405 (2003)] using High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In the current note we present direct evidence of charge-orbital ordering in this material using heat capacity measurements. Our heat capacity measurements indicate a clear transition consistent with prior observation. We find two main transitions, one at temperature $T_{_H}^{HC}=310-314$ K, and other in the vicinity of $T_{_A}^{HC}=143$ K. In addition, we may also conclude that there is a strong electron-phonon coupling in this material. In order to further study and confirm theseanomalies we have performed dc magnetization measurements. The dc magnetic measurements confirm these two transitions. Again,we find two main transitions, one at temperature $T_{_H}^{M}=318-323$ K, and other at around $T_{_A}^{M}=164$ K. △ Less

Submitted 22 December, 2006; originally announced December 2006.

Comments: 24 pages, 17 figures

Journal ref: Phys. Rev. B75, 205134 (2007)

Optical phonons in new ordered perovskite Sr2Cu(Re0.69Ca0.31) Oy system observed by infrared reflectance spectroscopy

Authors: J. Ahmad, M. Shizuya, M. Isobe, S. Alam, H. Uwe

Abstract: We report infrared reflectivity spectra for a new correlated cupric oxide system Sr2Cu(Re0.69Ca0.31)Oy with y ~ 0.6 at several temperatures ranging between 8 and 380 K. The reflectivity spectrum at 300 K comprises of several optical phonons. A couple of residual bands located around 315 and 653 cm-1 exhibit exceptionally large intensity as compared to the other ones. The overall reflectivity spe… ▽ More We report infrared reflectivity spectra for a new correlated cupric oxide system Sr2Cu(Re0.69Ca0.31)Oy with y ~ 0.6 at several temperatures ranging between 8 and 380 K. The reflectivity spectrum at 300 K comprises of several optical phonons. A couple of residual bands located around 315 and 653 cm-1 exhibit exceptionally large intensity as compared to the other ones. The overall reflectivity spectrum lifts up slightly with increasing temperature. The energy and damping factor of transverse-optical phonons are determined by fitting the imaginary dielectric constant by Lorentz oscillator model and discussed as a function of temperature in terms of lattice anharmonicity. △ Less

Submitted 18 February, 2006; originally announced February 2006.

Comments: 9 pages, 3 figures, presented at ISS2005, to appear in Physica C

Heat Capacity study of $β$-FeSi$_2$ single crystals

Authors: Sher Alam, T. Nagai, Y. Matsui

Abstract: Heat Capacity of needle-like [length=5mm, diameter=1 mm] $β$-FeSi$_{2}$ single crystal, grown by chemical vapor transport has been measured. Two anomalies are found, a broad deviation centered around 160 K and a clear deviation at a temperature of 255 K approximately. We have attempted to relate these to the anomalies previously reported in the case of the resistivity data. The Transient Thermoe… ▽ More Heat Capacity of needle-like [length=5mm, diameter=1 mm] $β$-FeSi$_{2}$ single crystal, grown by chemical vapor transport has been measured. Two anomalies are found, a broad deviation centered around 160 K and a clear deviation at a temperature of 255 K approximately. We have attempted to relate these to the anomalies previously reported in the case of the resistivity data. The Transient Thermoelectric Effect [TTE] results lead us to the inference that the system under goes from single carrier system to at least two carrier system at 220 K-our heat capacity results seem to provide further independent evidence for this transition in this system. △ Less

Submitted 31 January, 2006; originally announced January 2006.

Comments: 8 pages, 5 figures

Journal ref: Phys. Lett. A, 353, (2006) 516-518

Clear Experimental Signature of Charge-Orbital density wave in Nd$_{1-x}$Ca$_{1+x}$MnO$_{4}$

Authors: Sher Alam, A. T. M. N. Islam, T. Nagai, M. Xu, Y. Matsui, I. Tanaka

Abstract: Single Crystals of Nd$_{1-x}$Ca$_{1+x}$MnO$_{4}$ have been prepared by the travelling floating-zone method, and possible evidence of a charge -orbital density wave in this material presented earlier [PRB68,092405 (2003)] using High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In the current note we present direct evidence of charge-orbital ordering in this material using… ▽ More Single Crystals of Nd$_{1-x}$Ca$_{1+x}$MnO$_{4}$ have been prepared by the travelling floating-zone method, and possible evidence of a charge -orbital density wave in this material presented earlier [PRB68,092405 (2003)] using High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In the current note we present direct evidence of charge-orbital ordering in this material using heat capacity measurements. Our heat capacity measurements indicate a clear transition consistent with prior observation. We find two main transitions, one at temperature $T_{_H}=310-314$ K, and other at $T_{_A}=143$ K. In addition, we may also conclude that there is a strong electron-phonon coupling in this material. △ Less

Submitted 29 September, 2005; originally announced September 2005.

Comments: 7 pages, 8 figures

Journal ref: Phys. Rev. B75, 205134 (2007)

Local Electronic Structure of PbVO3, a New Member of PbTiO3 Family, studied by XANES/ELNES

Authors: Sher Alam, Alexei A. Belik, Y. Matsui

Abstract: Recently an interesting multi-ferroic system PbVO3 [Chem. Mater. 2004] was successfully prepared using a high-pressure and high-temperature technique. The crystallographic features were reported. In this note we concentrate on the theoretical XANES spectra by considering the K-edge of Vanadium. The tetragonality [c/a=1.229 at 300 K] of PbVO3 is the largest in the PbTiO3 family of compounds. Thus… ▽ More Recently an interesting multi-ferroic system PbVO3 [Chem. Mater. 2004] was successfully prepared using a high-pressure and high-temperature technique. The crystallographic features were reported. In this note we concentrate on the theoretical XANES spectra by considering the K-edge of Vanadium. The tetragonality [c/a=1.229 at 300 K] of PbVO3 is the largest in the PbTiO3 family of compounds. Thus one is led naturally to examine the effect of the change of tetragonality and the axial oxygen position on the electronic structure [i.e. XANES spectrum]. We study this effect in two ways. At a given temperature we vary the tetragonality and the axial oxygen position and quantify it in terms of XANES difference spectrum. Secondly, we compute the XANES spectra at three different temperatures, 90 K, 300 K, and 530 K and quantify the change in terms of the difference spectrum. We note that in this compound the tetragonality increases almost monotonically with temperature from 12 K to 570 K without transition to the cubic phase under ambient pressure. A key objective of the current investigation is to gain an understanding of various absorption features in the vicinity of K-edge of V, in terms of valence, local site symmetry, local coordination geometry, local bond distances, charge transfer, and local projected density of states. We consider both the polarized and the unpolarized XANES spectra. In short we have performed a local electronic study, which nicely complements the crystallographic features reported recently in PbVO3. △ Less

Submitted 31 March, 2005; originally announced March 2005.

Comments: 16 pages, 7 pdf figures

X-ray Absorption Near Edge Structure of FePt nanoclusters

Authors: Sher Alam, J. Ahmed, Y. Matsui

Abstract: X-ray Absorption Near Edge Structure [XANES] of FePt nanoclusters has been studied using a full multiple scattering, self-consistent field [SCF], real-space Green`s function approach realized via the powerful ab initio FEFF8 code. One purpose of our study is to determine the sensitivity of Pt L3 edge with respect to the size and shape of the FePt nanoclusters. We also give the results of the cal… ▽ More X-ray Absorption Near Edge Structure [XANES] of FePt nanoclusters has been studied using a full multiple scattering, self-consistent field [SCF], real-space Green`s function approach realized via the powerful ab initio FEFF8 code. One purpose of our study is to determine the sensitivity of Pt L3 edge with respect to the size and shape of the FePt nanoclusters. We also give the results of the calculations with respect to the Fe L3 edge. Calculations are made with and without core-hole for two main reasons, to check and cross-check the FEFF code and also since in some cases it is known such as Pt clusters that better results are obtained without the core-hole. This is mainly because the screening electron will occupy empty d or f states and correspondingly reduce the white line intensity. △ Less

Submitted 24 February, 2004; originally announced February 2004.

Comments: TeX version 3.14159, LaTex2e, 6 pages with 6 figures

Journal ref: Applied Surface Science, Volume 241, Issues 1-2, 28 February 2005, Pages 199-204

Quantum Group Based Theory for Antiferromagnetism and Superconductivity: Proof and Further Evidence

Authors: Sher Alam, S. M. Mamun, T. Yanagisawa, M. O. Rahman, J. A. S. Termizi

Abstract: Previously one of us presented a conjecture [APF-4 Proceedings] to model antiferromagnetism and high temperature superconductivity and their 'unification' by quantum group symmetry rather than the corresponding classical symmetry in view of the critique by Baskaran and Anderson of Zhang's classical SO(5) model. This conjecture was further sharpened, experimental evidence and the important role o… ▽ More Previously one of us presented a conjecture [APF-4 Proceedings] to model antiferromagnetism and high temperature superconductivity and their 'unification' by quantum group symmetry rather than the corresponding classical symmetry in view of the critique by Baskaran and Anderson of Zhang's classical SO(5) model. This conjecture was further sharpened, experimental evidence and the important role of 1-d systems [stripes] was emphasized and moreover the relationship between quantum groups and strings via WZWN models were given in [Phys. Lett A272, (2000)]. In this brief note we give and discuss mathematical proof of this conjecture, which completes an important part of this idea, since previously an explicit simple mathematical proof was lacking. Moreover an independent calculation [IC/99/2] which constructs the generators forming SO(5) algebra not only supports our previous conjecture but provides a check on our calculations. It is important to note that in terms of physics that the arbitariness [freedom] of the d-wave factor g$^{2}$(k) is tied to quantum group symmetry whereas in order to recover classical SO(5) one must set it to unity in an adhoc manner. We intuitively expect that this freedom may be related to psuedogap behaviour in cuprates. △ Less

Submitted 5 November, 2002; originally announced November 2002.

Comments: 9 pages revtex. 15th International Symposium on Superconductivity [ISS] 2002 [PCP-3/ISS 2002], Yokohama, Japan

Journal ref: Physica C 392-396 (2003) 149-152

Temperature Dependent Polarized XANES Spectra for Zn-doped LSCO system

Authors: Sher Alam, A. T. M. N Islam, I. Tanaka, P. Badica, H. Oyanagi, H. Kawanaka, M. O. Rahman, T. Yanagisawa

Abstract: The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective], spin-charge separation, stripes [static and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in these materials must be consistently understood. Zn-doped LSCO single crystal were grown by… ▽ More The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective], spin-charge separation, stripes [static and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in these materials must be consistently understood. Zn-doped LSCO single crystal were grown by TSFZ technique. Temperature dependent Polarized XANES [near edge local structure] spectra were measured at the BL13-B1 [Photon Factory] in the Flourescence mode from 10 K to 300 K. Since both stripes and nonmagnetic Zn impurities substituted for Cu give rise to inhomogeneous charge and spin distribution it is interesting to understand the interplay of Zn impurities and stripes. To understand these points we have used Zn-doping and some of the results obtained are as follows: The spectra show a strong dependence with respect to the polarization angle, $θ$, as is evident at any temperature by comparing the spectra where the electric field vector is parallel with ab-plane to the one where it is parallel to the c-axis. By using the XANES [temperature] difference spectra we have determined T* [experimentally we find, T* $\approx$ 160-170 K] for this sample. The XANES difference spectra shows that the changes in XANES features are larger in the ab-plane than the c-axis, this trend is expected since zinc is doped in the ab-plane at the copper site. Our study also complements the results in literature namely that zinc doping does not affect the c-axis transport. △ Less

Submitted 8 February, 2002; originally announced February 2002.

Comments: To appear in Physica C [ISS2001 Special Issue], related talk presented at ISS2001 as PC-16, 10 pages revtex and 7 pages of figures (pdf)

Journal ref: Physica C 378-381(2002)78-83

Quantum Groups, Strings and HTSC materials II

Authors: Sher Alam, M. O. Rahman, T. Yanagisawa, H. Oyanagi

Abstract: Previously we have indicated the relationship between quantum groups [Phys. Lett A272, (2000)] and strings via WZWN models in the context of applications to cuprates and related materials.The connection between quantum groups and strings is one way of seeing the validity of our previous conjecture [i.e. that a theory for cuprates may be constructed on the basis of quantum groups]. The cuprates s… ▽ More Previously we have indicated the relationship between quantum groups [Phys. Lett A272, (2000)] and strings via WZWN models in the context of applications to cuprates and related materials.The connection between quantum groups and strings is one way of seeing the validity of our previous conjecture [i.e. that a theory for cuprates may be constructed on the basis of quantum groups]. The cuprates seems to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective] must be understood. The Hubbard model captures some of the behaviour of the phase transitions in these materials. On the other hand the phases such as stripes in these materials bear relationship to quantum group or string-like solutions. One thus expects that the relevant solutions of Hubbard model may thus be written in terms of stringy solutions. In short this approach may lead to the non-perturbative formualtion of Hubbard and other condensed matter Hamiltonians. The question arises that how a 1-d based symmetry such as quantum groups can be relevant in describing a 3-d [spatial dimensions] system such as cuprates. The answer lies in the key observation that strings which are 1-d objects can be used to describe physics in $d$ dimensions. For example gravity [which is a 3-d [spatial] plus time] phenomenon can be understood in terms of 1-d strings. Thus we expect that 1-d quantum group object induces physics in 2-d and 3-d which may be relevant to the cuprates. We present support for our contention using [numerical] variational Monte-Carlo [MC] applied to 2d d-p model. We also briefly discuss others ways to formulate a string picture for cuprates, namely by exploiting connection between gauge theories and strings and t'Hooft picture of quark confinement. △ Less

Submitted 1 October, 2001; originally announced October 2001.

Comments: 9 pages revtex and three figures, Presented in the 14th International Symposium on Superconductivity [ISS] 2001, Kobe, Japan

SET based experiments for HTSC materials: II

Authors: Sher Alam, M. O. Rahman, T. Yanagisawa, H. Oyanagi

Abstract: The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective], spin-charge separation, stripes [static and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in these materials must be consistently understood. In this note we further discuss our origi… ▽ More The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective], spin-charge separation, stripes [static and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in these materials must be consistently understood. In this note we further discuss our original suggestion of using Single Electron Tunneling Transistor [SET] based experiments to understand the role of charge dynamics in these systems. Assuming that SET operates as an efficient charge detection system we can expect to understand the underlying physics of charge transport and charge fluctuations in these materials for a range of doping. Experiments such as these can be classed in a general sense as mesoscopic and nano characterization of cuprates and related materials. In principle such experiments can show if electron is fractionalized in cuprates as indicated by ARPES data. In contrast to flux trapping experiments SET based experiments are more direct in providing evidence about spin-charge separation. In addition a detailed picture of nano charge dynamics in cuprates may be obtained. △ Less

Submitted 5 August, 2001; originally announced August 2001.

Comments: 10 pages revtex plus four figures; ICMAT 2001 Conference Symposium P: P10-06

Journal ref: Supercond. Sci. Technol. 15 (2002) 1330-1334

EXAFS study of Tl$_{0.75}$Cu$_{.25}$Ba$_2$Ca$_3$Cu$_4$ O$_y$ and Cu$_{0.68}$C$_{.32}$Ba$_2$Ca$_3$Cu$_4$ O$_y$ superconductors [bulk] at 300 K

Authors: Sher Alam, H. Oyanagi, S. B. Mohamed, H. Ihara, A. Iyo, H. Kito, P. Badica, M. O. Rahman, T. Yanagisawa

Abstract: The results of EXAFS measurements at 300 K for the superconducting compounds Tl$_{0.75}$Cu$_{0.25}$Ba$_{2}$Ca$_{3}$Cu$_4$O$_{y}$ [Tl-1234], TlBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{y}$ [Tl-1212], and CuBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{y}$ [Cu-1234]. are reported. We have measured the EXAFS spectrum for Tl$_{0.75}$Cu$_{0.25}$Ba$_{2}$Ca$_{3}$Cu$_4$O$_{y}$ in the range 10K-300K, however here we limit our discussio… ▽ More The results of EXAFS measurements at 300 K for the superconducting compounds Tl$_{0.75}$Cu$_{0.25}$Ba$_{2}$Ca$_{3}$Cu$_4$O$_{y}$ [Tl-1234], TlBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{y}$ [Tl-1212], and CuBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{y}$ [Cu-1234]. are reported. We have measured the EXAFS spectrum for Tl$_{0.75}$Cu$_{0.25}$Ba$_{2}$Ca$_{3}$Cu$_4$O$_{y}$ in the range 10K-300K, however here we limit our discussion to the spectrum at 300 K. This material is prepared under high pressure [3.5 GPa] from precursors with small carbon concentrations and exhibits a T$_c$ of $~127$ K. We have also performed ``aging'' study by looking at XRD for this material after approximately one year. The XRD results at 300 K are ``unchanged''. It is of interest to compare the EXAFS spectrum of this compound with the corresponding compound Cu-1234. Remarks on the choice of appropriate EXAFS standard for this and related compounds are also given. Based on our data analysis we quantify disorder in these systems. By using the Cu-O in-plane distance we give values for the microstrain parameter, which can be related to the charge ordering transition. △ Less

Submitted 5 August, 2001; originally announced August 2001.

Comments: 8 pages revtex and 10 pages of figures

Parameters for Systems Exhibiting Local Lattice Distortions, Charge and Spin Ordering

Authors: Sher Alam, T. Yanagisawa, H. Oyanagi

Abstract: Keeping in mind the experimental results that indicate local lattice distortions, charge and spin orderings, we have developed a phenomenological approach which allows us to describe the electronic phase diagram of cuprates and related systems in terms of few parameters.In the present work we consider a third-order parameter theory which characterize charge, spin and superconductivity orderings.… ▽ More Keeping in mind the experimental results that indicate local lattice distortions, charge and spin orderings, we have developed a phenomenological approach which allows us to describe the electronic phase diagram of cuprates and related systems in terms of few parameters.In the present work we consider a third-order parameter theory which characterize charge, spin and superconductivity orderings. We are thus led to a theory of three scalar fields. By coupling these scalars to gauge fields we are naturally led to string-like solutions, which we interpret as stripes. This ties nicely with our quantum group conjecture that 1d systems play an important role in the physics of cuprates and related materials. We show that this simple approach can give rough values for two-order parameters which can be naively be interpreted as charge and spin orderings. We also report our attempt to understand how local lattice distortions are involved and what role they play in terms of these two order parameters. △ Less

Submitted 30 June, 2001; originally announced July 2001.

Comments: 8 pages revtex, Published in AIP Conference Proceedings 554: Int. Sym. on Physics in Local Lattice Distortions, July 23-26, 2000