arXiv subjects

Hamidreza Simchi

Publications and source records attributed to Hamidreza Simchi.

At least 19 recordsLinked to original sources

arXiv2025

Two-Dimensional Materials-Based Josephson Junctions

We consider a two-dimensional monolayer MoS2-based Josephson junction which is composed by an intermediate semiconductor flake and the semi-infinite topological and non-topological superconductor leads and study its quantum transport properties by using the tight-binding non-equilibrium Green function method. By introducing a simple tight-binding model, it is shown that, when the absolute value of chemical potential is much smaller than the superconductor paring potential, the Majorana zero modes, whose Chern number is two, are formed in the topological leads. Also, we show that, in Josephson junction with ordinary superconductor leads, the Josephson current has sinusoidal behavior (due to forming the Andreev bound states (ABS)), when the absolute value of energy of carriers (and the chemical potential) is much smaller (greater) than the superconductor pairing potential. Of course, for Josephson junction with topological superconductor leads, it is shown that the ABS are not formed and in consequence the related Josephson current is zero. Therefore, one can consider the two-dimensional monolayer MoS2-based Josephson junction as a two-state switch which is in open-state (due to ABS) when the chemical potential is greater than 0.8 eV and is in close-state (due to Majorana) when the chemical potential is less than < 0.8 or is equal to zero, i.e., ABS cannot mimic the Majorana state, as zero-bias conductance.

cond-mat.supr-con↗

arXiv2023

Topos Many-Node Theory: Roots, Foundations and Predictions

Assuming that the first creatures of creation create a network, it is shown that how the network can be mapped to a topos discrete quantum manifold which is equipped with both the discrete calculus and the Alexandrov's algebra. We assign a locale to each nodes of the space-time network and show that in general, invariance under Lorentz transformations is no longer true. it is shown that the cosmological constant is non-zero and is proportional to the second power of the Hubble radius. By considering a population (set), it is shown that the entropy of the space-time network is quantized and increases as the network grows. In consequence the inflation of the world is expected phenomenon. Also, we show that how world inflation can be described based on the concept of truth object and truth value belong to the topos theory. It is shown that the quanta of vibrations, called netons, can be attributed to the vibration of space-time network, and it is expected that they will be observed in the future experiments related to cosmic background radiation. Finally, it is shown that the root of noncommutative geometry is in attributing the locale to the nodes of the space-time network instead of a point. This theory, which includes the roots, foundations and predictions, is called the many-node theory.

gr-qc↗

arXiv2022

General Formulation of Topos Many-Node Theory

We consider the created entities (events) in the first moments of universe creation. It is assumed that there exists a causal energetic relationship between all events (nodes) such that all nodes are placed on a world line and each node occupies a region (instead of a point) in space-time, called locale, in mathematical terms. The set of locale nodes form a topos many-node system. Using some basic assumptions, we introduce two kinds of Hamiltonians. By attributing a general structural Hamiltonian to the system, it is shown that the system has an optimized critical dimension with a probable Raman and infrared spectrums. Also, we consider a general nonstructural Hamiltonian which includes a set of commutative self-adjoint operators and an interaction terms due to the spin, charge, or other kinds of probable degrees of freedoms for each $n^{th}$-optimized graph. For finding the state-space, truth values and quantity valued objects of the many-node system, a general procedure is introduced. The set of these values is a classical snapshot of the $n^{th}$-optimized graph which forms its kinematic. We show that the dynamic of the system can be explained by defining a combined map between the $n^{th}$- state-space belongs to the $n^{th}$-graph and the $({n+1)}^{th}$-state-space belong to $({n+1)}^{th}$-graph. Finally, by providing an interpretation of the general formulation of many-node theory, we discuss and explain how one can use the data of the cosmic background radiations and cosmic rays for finding a detailed model of both general structural and nonstructural introduced Hamiltonian. Here, time is no more than the change in truth value during comparison between $n^{th}$ and $({n+1)}^{th}$-graph.

gr-qc↗

arXiv2021

Statistical Representation of Spacetime

It is assumed that the spacetime is composed by events and can be explained by partially ordered set (causal set). The parent events born two kinds of children. Some children have a causal relation with their parents and other kinds have not. It is assumed that evolution of the population is only happen by the causal children. The assumed population can be modeled by finite (infinite) dimension Leslie matrix. In both finite and infinite cases, it is shown that the stationary state of the population always exists and the matrix has positive eigenvalues. By finding the relation between the statistical information of the population and the stationary state, a probability matrix and a Shannon-like entropy is defined. It is shown that the change in entropy is always quantized and positive and in consequence, the world is inflating. We show that the vacuum energy can be attributed to the necessary done work for preserving the causal relation between the parents and the children (cohesive energy). By assuming that the sum of cohesive energy and kinetic energy of the denumerable causal spacetime is equal to the heat, which flows across a causal horizon, we find the relation between energy-momentum tensor and discrete Ricci tensor which can be called the Einstein state equation. Finally, it is shown that the constant of proportionality $\eta$ between the entropy and the area is proportional to $\frac{k_{B}}{l_{p}^{2}}$ at Planck scale which is in good agreement with the Hawking's result.

gr-qc↗

arXiv2021

Causality, Uncertainty Principle, and Quantum Spacetime Manifold in Planck Scale

In causal set theory, there are three ambiguous concepts that this article tries to provide a solution to resolve these ambiguities. These three ambiguities in Planck's scale are: the causal relationship between events, the position of the uncertainty principle, and the kinematic. Assuming the interaction between events, a new definition of the causal relationship is presented. Using the principle of superposition, more than one world line are attributed to two events that are interacting with each other to cover the uncertainty principle. Using these achievements, it is shown that kinematics has no place in the Planck dimension and that quantum spacetime manifold should be used instead.

quant-ph↗

arXiv2021

Raman spectra and infrared intensities of graphene-like clusters in compared to epitaxial graphene on Si

There are several growing methods for graphene. In this study, the growth of graphene-like clusters on the SiC wafers is done by annealing the wafers in a vacuum evaporation system equipped with a heating source accessory. For evaluating the quality of the growth method, the Raman spectra and infrared intensities of graphene-like clusters are studied theoretically and experimentally. For doing the theoretical study, three types of graphene clusters are considered and their Raman spectrum and infrared intensities are found using the Hartree-Fock method. The results show that the geometry of the cluster, and in consequence the geometry-dependent high (low) non-uniformity of charge distribution on the cluster surfaces causes the high (low) infrared intensities. The experimental spectrums are measured and compared with the theoretical ones. An agreement was seen between the experimental and theoretical Raman spectrum when the wave number is less than 1700 Cm-1. It is shown that more accurate temperature control and higher vacuum level of the chamber are essential for using the physical evaporation method for growing the single-layer graphene on the SiC substrate.

cond-mat.mes-hall↗

arXiv2020

Interface Dark Excitons at Sharp Lateral Two-Dimensional Heterostructures

We study the dark excitons at the interface of sharp lateral heterostructure of two-dimensional transition metal dichalcogenides. By introducing a low-energy effective Hamiltonian model, we find the energy dispersion relation of exciton and show how it depends on the onsite energy of composed materials and their spin-orbit coupling strengths. It is shown that the effect of geometrical structure of interface, as a deformation gauge field (pseudo-spin-orbit coupling), should be considered in calculating the binding energy of exciton. By discretization of real-space version of the dispersion relation on a triangular lattice, we show that the binding energy of exciton depends on its distance from the interface line. For exciton near the interface the binding energy is equal to 0.36 eV, while for the exciton enough far from the interface it is equal to 0.26 eV. Also, it has been shown that for zigzag interface the binding energy increases by 0.34 meV in comparison with the armchair interface due to the pseudo-spin-orbit interaction (gauge filed). The results can be used for designing the two-dimensional lateral heterostructure based optoelectronic devices and improving their characteristics.

cond-mat.mes-hall↗

arXiv2019

The Concept of Time: A Grand Unified Reaction Platform

The universe is things which change and called events. The events are matter and field. A boundary divides a system to things and environment. The things belong to the environment have no significant effect on the things belong to the system. The physical observables are the variations of things and it is always assumed that the conscious thing is placed in environment because the science cannot explain consciousness. There is not only an obligated minimum boundary between things (space) but also between past and future (present). The gravitational field has significant effect on these obligated minimums, especially at Planck scale. By using the above concept we introduce a grand unified reaction platform for categorizing the current physical paradigms and possible future explanation of the universe as a whole.

physics.gen-ph↗

arXiv2019

A simple Tight-Binding Approach to Topological Superconductivity in Monolayer MoS2

Monolayer molybdenum disulfide (MoS2) has a honeycomb crystal structure. We consider the triangular sublattice of molybdenum atoms and introduce a simple tight-binding Hamiltonian for studying the phase transition and topological superconductivity in MoS2 under uniaxial strain. It is shown that spin-singlet p+ip wave phase is a topological superconducting phase which possesses nonzero Chern numbers. When the chemical potential is bigger (smaller) than the spin-orbit coupling (SOC) strength the Chern number is equal to four (two) and otherwise it is equal to zero. Also, it is shown that, when the superconductivity gap is smaller and the chemical potential is bigger than the SOC strength, the zero energy Majorana states exist. Finally, we show the topological superconducting phase is preserved under uniaxial strain.

cond-mat.mes-hall↗

arXiv2018

Trivial and Non-trivial Superconductivity in dsDNA

A double-stranded DNA (dsDNA) is modeled by two coupled one-dimensional Kitaev's chain and the topological superconductivity is studied. It is shown that the zero energy mode exists under some specific conditions. The wave function of zero mode is calculated and it is shown that the Majorana quasi-particles exist on the ends of each strand. By calculating the winding number, we show that the topological phase transition can happen if the hopping integral between two strands is very smaller than the pairing potential between the Cooper pairs. It means that the dsDNA behaves as a trivial superconductor, commonly, but single-stranded DNA (or two coupled ssDNA with very small hopping between them) may behave as a non-trivial superconductor. Finally, we suggest an experimental setup for probable detection of Majorana quasi-particle in DNA.

cond-mat.supr-con↗

arXiv2017

Tunable free energy and heat flux between two-dimensional materials

We study the free energy across a stratified media made by graphene (G) and/or molybdenum disulfide (MoS2). The flux depends not only on the number of G/MoS2 layers but also on the priority of graphene layer respect to MoS2 layer in the media. The rule is; the thinner layer should be the nearest neighborhood of leftmost and rightmost sides of the media for getting the highest energy flux. The free energy can be tuned by applying an external gate voltage to MoS2 layers due to the tunable property of the dielectric constant of MoS2 by the gate voltage. Also we show in the silicon/MoS2/silicon three-body configuration, the photon heat tunneling is amplified significantly due to the increasing the number of the coupled modes. Due to the amplifying effect, this mechanism could be exploited to improving detection ability in the near infrared detection systems.

cond-mat.mes-hall↗

arXiv2016

Graphene-based three-body amplification of photon heat tunneling

We consider a three slabs configuration including two non-doped single layer graphene on insulating silicon dioxide (G/SiO2) substrates and one non-doped suspended single layer graphene (SG). The suspended layer is placed between two G/SiO2 layers. Without SG layer, the heat flux has maximum at Plasmon frequency supported by the G/SiO2 slabs. In three slabs configuration, the photon heat tunneling is amplified between two G/SiO2 layers significantly, only for specific range of vacuum gap between SG layer and G/SiO2 layers and Plasmon frequency, due to the coupling of modes between each G/SiO2 layer and SG layer. Since, the SG layer is a single atomic layer, the photon heat tunneling assisted by this configuration does not depend on the thickness of middle layer and in consequence, it can enable novel applications for nanoscale thermal management.

cond-mat.mes-hall↗

arXiv2015

Computational Materials: III-V Semiconductor Clusters

Here, we review the progress in our understanding of the structure, and electronic spectra of InAs, GaAs, InSb and CdSe clusters. First principles approaches based on Hartree-Fock and Density Functional Theory have become central to such studies and a brief overview of them is given.

cond-mat.mes-hall↗

arXiv2015

Controllable spin-polarization using external electric field and spin-inversion in a silicene quantum dot

It is shown that, for appropriate values of electron energy, the silicene dot can work as a controllable spin polarizer. The spin polarizer can polarize the spin of transmitted electrons from nearly pure down to nearly pure up by changing the strength of an external electric field. Also, for spin polarized incoming electrons, the silicene quantum dot can invert the spin of electrons and can works as a spin inverter or a spin NOT gate. In addition, we investigate the effects of exchange field, induced by ferromagnetic substrate, on electron conductance and show that the silicene quantum dot can act as a nearly perfect spin-filter in the presence of exchange field.

cond-mat.mes-hall↗

arXiv2015

Prefect spin filtering and spin inverting using a MoS2 quantum dot

The spin-dependent conductance and spin-polarization of a MoS2 quantum dot are studied in presence (absent) of an external electric field perpendicular to the molybdenum plane. It is shown that in absence of the electric field, the deformation of a MoS2 ribbon structure causes spin-splitting in the dot and non-prefect spin filtering is seen. Therefore, the technique could be used for designing spin-dependent devices of MoS2. Also, we show that the device could behave as a prefect spin filter and spin inverter under applying an external electric.

cond-mat.mes-hall↗

arXiv2014

Tunable spin transport using zigzag MoS2 nanoribbon

We consider a zigzag nanoribbon of MoS2 and study its spin-dependent conductance and spin-polarization in presence (absent) of an external electric field. The field is not only perpendicular to the molybdenum plane (called vertical field Ez) but also to the transport direction of electrons (called transverse field Et). It is shown that, while in the absent of the external electric field, two bands of seven bands are spin split in the valence band but no spin splitting is seen at point . Under applying the electric field we show, the ribbon behaves as a prefect spin up (down) filter if and only if the both components of the electrical field are present. Finally it is shown that, by changing the strength of , the ribbon acts as a spin inverter for fixed values of Ez .

cond-mat.mes-hall↗

arXiv2014

Spin transport and polarization properties of manganese-doped dual-guanine molecule

We study the spin transport and polarization properties of manganese-doped dual-guanine molecules connected to graphene leads using non-equilibrium Green's function method. It is shown that a manganese doped dual-guanine molecule is a biological semiconductor and behaves as a prefect spin filter. We show that this semiconductor can behave as a spin switch when the Rashba spin-orbit interaction is considered. In addition, it is shown that, a large conductance is observed due to the Fano-Kondo-Rashba resonance effect.

cond-mat.mes-hall↗

arXiv2013

Proximity-induced superconductivity effect in a double-stranded DNA

We study the proximity-induced superconductivity effect in a double-stranded DNA by solving the Bogoliubov-de Gennes equations and taking into account the effect of thermal fluctuations of the twist angle between neighboring base pairs. We show that the electron conductance is spin-dependent and the conductance of spin up (down) increases (decreases) due to the spin-orbit coupling. It is found that, for T<100K, the band gap energy is temperature-independent and it decreases due to the SOC. In addition, by solving the Bogoliubov-de Gennes equations and local gap parameter equation self-consistently, we find the critical temperature at which transition to superconductivity can take place.

cond-mat.mes-hall↗