arXiv subjects

Mehdi Hosseini

Publications and source records attributed to Mehdi Hosseini.

15 recordsLinked to original sources

arXiv2024

Enhancing Critical Current Density in Thin Superconductor Layers by Moiré Pinning Centers

One important factor affecting the critical current density in type-II superconductors is the formation of artificial pinning centers. Hence, the engineering of pinning centers in superconducting systems has garnered considerable attention. In this study, the effect of moiré patterned pinning centers on the critical current density of superconducting tapes is investigated. The Langevin equation is solved by taking into account the prominent forces within the superconductor medium, using the appropriate boundary conditions for vortices. The vortices dynamics are investigated by performing molecular dynamics simulations, which are used to calculate the corresponding critical current densities. Results show a significant enhancement in the critical current density at particular angles of the relative rotation of the primary lattices. It is also revealed that for stronger pinning forces, the calculated critical current densities are higher in the moiré lattices compared to the primary lattices of pinning centers.

cond-mat.supr-con↗

arXiv2020

Resilient In-Season Crop Type Classification in Multispectral Satellite Observations using Growth Stage Normalization

Crop type classification using satellite observations is an important tool for providing insights about planted area and enabling estimates of crop condition and yield, especially within the growing season when uncertainties around these quantities are highest. As the climate changes and extreme weather events become more frequent, these methods must be resilient to changes in domain shifts that may occur, for example, due to shifts in planting timelines. In this work, we present an approach for within-season crop type classification using moderate spatial resolution (30 m) satellite data that addresses domain shift related to planting timelines by normalizing inputs by crop growth stage. We use a neural network leveraging both convolutional and recurrent layers to predict if a pixel contains corn, soybeans, or another crop or land cover type. We evaluated this method for the 2019 growing season in the midwestern US, during which planting was delayed by as much as 1-2 months due to extreme weather that caused record flooding. We show that our approach using growth stage-normalized time series outperforms fixed-date time series, and achieves overall classification accuracy of 85.4% prior to harvest (September-November) and 82.8% by mid-season (July-September).

cs.LG↗

arXiv2020

Rapid Response Crop Maps in Data Sparse Regions

Spatial information on cropland distribution, often called cropland or crop maps, are critical inputs for a wide range of agriculture and food security analyses and decisions. However, high-resolution cropland maps are not readily available for most countries, especially in regions dominated by smallholder farming (e.g., sub-Saharan Africa). These maps are especially critical in times of crisis when decision makers need to rapidly design and enact agriculture-related policies and mitigation strategies, including providing humanitarian assistance, dispersing targeted aid, or boosting productivity for farmers. A major challenge for developing crop maps is that many regions do not have readily accessible ground truth data on croplands necessary for training and validating predictive models, and field campaigns are not feasible for collecting labels for rapid response. We present a method for rapid mapping of croplands in regions where little to no ground data is available. We present results for this method in Togo, where we delivered a high-resolution (10 m) cropland map in under 10 days to facilitate rapid response to the COVID-19 pandemic by the Togolese government. This demonstrated a successful transition of machine learning applications research to operational rapid response in a real humanitarian crisis. All maps, data, and code are publicly available to enable future research and operational systems in data-sparse regions.

cs.CV↗

arXiv2020

The effect of Joule heating on the terahertz radiation in the superconductor media

Moving vortex lattice in type-II superconductors can result in the radiation of electromagnetic waves in the range of terahertz frequency. The vortex flow dynamics in superconductors follow the London equation. In this paper, the effects of the superconductor coherence length and the London penetration depth on the radiated power is investigated by solving the London equation in the presence of vortices. The results show that by decreasing the coherence length, the radiation power will significantly increase. Also, it is possible to obtain more radiated power with a sharp peak by increasing the penetration depth. Further investigation of the effect of Joule heating reveals that radiation power is almost independent of temperature when we are far from critical temperature. But on the other hand, near the critical temperature, the radiation power is strongly dependent on the temperature variation. Also, by increasing the bias current, the radiation power will augment, and the frequency of radiated peaks will change accordingly.

cond-mat.supr-con↗

arXiv2020

The effect of the second nearest neighbor interaction on the population transfer in a four-particle Landau-Zener system

Population transfer in quantum systems has always been an interesting area in physics since the introduction of quantum mechanics. In this paper, transition probabilities for a coupled system consisting of four two - level particles are studied by solving Landau - Zener Hamiltonian. The effects of the first and second nearest neighbors interactions are investigated. Presented results indicate that the second nearest neighbors interactions will decrease the transition probability when the coupling strength for each neighborhood has the same sign. The fast sweep effect on transition probability is also studied here.

quant-ph↗

arXiv2020

Current density distribution in resistive fault current limiters and its effect on device stability

The increase of current uniformity along of a resistive type superconductor fault current limiter (R-SFCL) in the design of this type of limiters is well perceived as an important issue. The non-uniform distribution of current in R-SFCL only increases the current in some superconducting regions, as a result, in the fault conditions, only certain parts of the superconductor undergo a phase change that increases the heat pressure in those areas and causes the breakdown and destruction of the device. In this paper, the current density distributions in common patterns used in R-SFCs constructions have been simulated and investigated. To this end, an effective model is proposed for R-SFCL to achieve the highest uniformity of current and harmonic phase change over superconductors compared to other patterns. The simulation results in the Ansys Maxwell Software advocate the appropriate and satisfying performance of the proposed model.

physics.app-ph↗

arXiv2019

Optimization of terahertz absorption in periodic quantum well structures

In this work, the linear absorption spectra for periodic arrays of GaAs-GaAsAl quantum wells with different thicknesses, inside electromagnetic wave has been studied. The eigen energies and eigen functions are calculated by solving the Schrödinger equation numerically. The absorption spectra are obtained using the density matrix approach and the effects of quantum well parameters have been studied. Results show that for a wide range of parameters the absorption peaks lie in the terahertz region. Furthermore, it is possible to adjust the frequency of absorption peak in the terahertz range by changing the width and height of the wells or array numbers that could be used in terahertz devices.

cond-mat.mes-hall↗

arXiv2019

Full quantum analysis of complete population transfer using frequency boost

In this paper, we have proposed and demonstrated a new method of atomic population transfer. Transition dynamic of a two-level system is studied in a full quantum description of the Jaynes-Cummings model. Solving the time-dependent Schrödinger equation, we have investigated the transition probabilities numerically and analytically by using a sudden boost of the laser frequency. The results show that complete population transfer can be achieved by adjusting the time of the frequency boost.

physics.atom-ph↗

arXiv2019

Investigation of robust population transfer using quadratically chirped laser interacting with two-level system

We have proposed and demonstrated a fast and robust method of population transfer between two quantum states using a quadratically chirped laser source. Incorporating the Jaynes-Cummings in a full quantum description of the interaction, and numerically solving the time-dependent Schrödinger equation, transition probabilities have been obtained and the condition of the adiabatic passage is investigated. In this scheme, a laser source has been swept quadratically in time for arbitrarily engineering the transition probabilities. The results show that complete and robust population transfer could be selectively achieved by appropriate adjusting of the laser chirping parameter, the center frequency and the coupling strength which the time of the complete transition could be drastically decreased compared to linearly traditional chirped laser. Furthermore, another feature of using the quadratically chirped laser is the stimulation of intermediate transitions under the nonadiabatic passage.

physics.atom-ph↗

arXiv2019

Stimulated Raman adiabatic passage: effect of system parameters on population transfer

The Stimulated Raman Adiabatic Passage (STIRAP) procedure is a robust and complete population transfer method which have various application in chemistry and atomic physics. Here, we study the effects of one-photon detuning, transition time, pulse width, and pulse delay parameters on the population transfer via STIRAP and b-STIRAP techniques. Moreover, the impact of the field amplitude which decreases the population transfer has been analyzed and it is shown that in b-STIRAP, the robustness of the complete transition can be improved by decreasing the field amplitude in the single-photon resonance condition.

physics.atom-ph↗

arXiv2018

Robust population transfer by finite chirping method in a two-level system

Considering a two-level quantum system, we have proposed and represented a new approach for robust population transfer. In this scheme, the laser frequency has been swept in a finite time interval which simplifies the experimental limitations of the population transfer process. It is shown that using the Jaynes-Cummings model and engineering the coupling strength, the frequency sweeping range and its time interval, it is possible to achieve a robust, stable and full population transfer.

physics.atom-ph↗

arXiv2011

Phase diagram of the strongly correlated Kane-Mele-Hubbard model

We explore the phase diagram of the strongly correlated Hubbard model with intrinsic spin orbit coupling on the honeycomb lattice. We obtain the low energy effective model describing the spin degree of freedom. We study the resulting model within the Schwinger boson and Schwinger fermion approaches. The Schwinger boson approach gives the boundary between the spin liquid phase and the magnetically ordered phases, Neel order and incommensurate Neel order. We find that increasing the strength of the spin orbit coupling, narrows the width of the spin liquid region. The Schwinger fermion approach sheds further light on the nature of the spin liquid phase. We obtain three different candidates for the spin liquid phase within the mean field approximation which are gapless spin liquid, topological Mott insulator, and the chiral spin liquid phases. We argue that the gauge fluctuations and the instanton effect may suppress the first two spin liquids, while the chiral spin liquid is stable against gauge fluctuations due to its nontrivial topology.

cond-mat.str-el↗

arXiv2011

Effect of Substrate Thickness on Responsivity of Free-Membrane Bolometric Detectors

The effect of substrate thickness and its Debye temperature on the bolometric response of the freestanding-membrane type of superconductor transition-edge detectors is investigated. The investigation is based on the calculation of the variation of the specific heat per unit volume and the heat conductivity using the finite-size scaling method for different Debye temperatures and micron size thickness of substrate. We also calculated the heat capacity and time constant for various materials with Debye temperatures in the range of 600-1400K. The calculated results are compared to previously reported response values obtained theoretically and experimentally for the thermal-based infrared detectors. The IR response versus substrate thickness of these types of devices for different modulation frequencies is also presented. This inspected response at low thicknesses of substrate shows anomalistic behavior with respect to the previously reported results of response analyses. Here, we also obtained the optimized substrate thickness and Debye temperatures for maximum responsivity of these types of devices.

cond-mat.supr-con↗

arXiv2011

Compound Hertzian Chain Model for Copper-Carbon Nanocomposites' Absorption Spectrum

The infrared range optical absorption mechanism of Carbon-Copper composite thin layer coated on the Diamond-Like Carbon (DLC) buffer layer has been investigated. By consideration of weak interactions between copper nanoparticles in their network, optical absorption is modeled using their coherent dipole behavior induced by the electromagnetic radiation. The copper nanoparticles in the bulk of carbon are assumed as a chain of plasmonic dipoles, which have coupling resonance. Considering nearest neighbor interactions for this metallic nanoparticles, surface plasmon resonance frequency (ω\neg0) and coupled plasmon resonance frequency (ω\neg1) have been computed. The damping rate versus wavelength is derived which leads to the derivation of the optical absorption spectrum in the term of ω\neg0 and ω\neg1. The dependency of the absorption peaks to the particle-size and the particle mean spacing is also investigated. The absorption spectrum is measured for different Cu-C thin films with various Cu particle size and spacing. The experimental results of absorption are compared with the obtained analytical ones.

physics.optics↗

arXiv2011

Tunable Bandgap Opening in the Proposed Structure of Silicon Doped Graphene

A specific structure of doped graphene with substituted silicon impurity is introduced and ab. initio density-functional approach is applied for energy band structure calculation of proposed structure. Using the band structure calculation for different silicon sites in the host graphene, the effect of silicon concentration and unit cell geometry on the bandgap of the proposed structure is also investigated. Chemically silicon doped graphene results in an energy gap as large as 2eV according to DFT calculations. As we will show, in contrast to previous bandgap engineering methods, such structure has significant advantages including wide gap tuning capability and its negligible dependency on lattice geometry.

cond-mat.mes-hall↗