arXiv subjects

Hamidreza Ramezani

Publications and source records attributed to Hamidreza Ramezani.

At least 19 recordsLinked to original sources

arXiv2025

Two-dimensional M2X2 family with emerging semiconducting, semimetallic, and magnetic properties

The exploration for novel two-dimensional (2D) materials with diverse electronic characteristics has attracted growing interest in recent years. Using density functional theory (DFT) calculations, we have predicted a new family of 2D transition-metal (TM) based compounds under the nomenclature M_2X_2 (where M represents TMs, and X denotes chalcogen elements like S, Se, and Te). Our investigation delves into the examination of the formation energies, dynamical/thermal stabilities, mechanical properties, electronic structures, and magnetic properties of various systems within this family. Through our computational analyses, we have discovered a total of 35 thermodynamically and dynamically stable M_2X_2 monolayer materials that exhibit remarkable diversity in terms of their electronic and magnetic properties. Our findings will pave the way for the experimental realization of various M_2X_2 structures in the near future. In particular, among the predicted compounds, M_2X_2(M=Zn, Cd; X=S, Se, Te) are a direct band-gap semiconductor with band gaps between 0.9 to 2.6 eV (1.3 to 3.7 eV) by DFT+PBE (hybrid functional HSE) calculations. M_2X_2(M=Ti, Zr, Hf, Tc, Re) are zero-gap semiconductor (semimetals) in standard DFT+PBE calculation. Inclusion of spin-orbit coupling leads to a gap opening of 0.1 eV. Notably, our analysis has also unveiled the magnetic nature of certain materials, such as Mn_2X_2(X=S, Se), Fe_2X_2(X=Se, Te), and Ti_2Te_2. The prediction of semiconducting (magnetic) M_2X_2 materials not only offers valuable insights into the underlying electronic properties (magnetism) of 2D systems but also positions these materials as promising candidates for the development of advanced electronic (spintronic) devices.

cond-mat.mtrl-sci↗

arXiv2023

Non-Hermitian Floquet-Free Analytically Solvable Time Dependant Systems

The non-Hermitian models, which are symmetric under parity (P) and time-reversal (T) operators, are the cornerstone for the fabrication of new ultra-sensitive optoelectronic devices. However, providing the gain in such systems usually demands precise contorol of nonlinear processes, limiting their application. In this paper, to bypass this obstacle, we introduce a class of time-dependent non-Hermitian Hamiltonians (not necessarily Floquet) that can describe a two-level system with temporally modulated on-site potential and couplings. We show that implementing an appropriate non-Unitary gauge transformation converts the original system to an effective one with a balanced gain and loss. This will allow us to derive the evolution of states analytically. Our proposed class of Hamiltonians can be employed in different platforms such as electronic circuits, acoustics, and photonics to design structures with hidden PT-symmetry potentially without imaginary onsite amplification and absorption mechanism to obtain an exceptional point.

quant-ph↗

arXiv2022

Anomalous relocation of topological states

I show that a single embedded non-Hermitian defect in a one-dimensional topological system at certain degrees of non-Hermiticity can remove the topological mode from the edge and restore it inside the lattice at the same place where the non-Hermitian defect is placed. I relate this unexpected phenomenon to the wave matching condition and continuity of the wave function at different sites in the lattice. These findings pave the way for controlling the position of topological states at will.

quant-ph↗

arXiv2022

Observation of Robust Zero Energy Extended States

Topological edge states arise at the interface of two topologically-distinct structures and have two distinct features: they are localized and robust against symmetry protecting disorder. On the other hand, conventional transport in one dimension is associated with extended states, which typically do not have topological robustness. In this paper, using lossy coupled resonators in one dimension, we demonstrate both theoretically and experimentally the existence of robust states residing in the bulk. We show that they are unusually robust against disorders in coupling between adjacent sites and losses. Our work paves the way to a new form of robust transport that is not limited to boundary phenomena and can be accessed more easily from far field.

cond-mat.mes-hall↗

arXiv2022

Coexistence of extended and localized states in one-dimensional non-Hermitian Anderson model

In one-dimensional Hermitian tight-binding models, mobility edges separating extended and localized states can appear in the presence of properly engineered quasi-periodical potentials and coupling constants. On the other hand, mobility edges don't exist in a one-dimensional Anderson lattice since localization occurs whenever a diagonal disorder through random numbers is introduced. Here, we consider a nonreciprocal non-Hermitian lattice and show that the coexistence of extended and localized states appears with or without diagonal disorder in the topologically nontrivial region. We discuss that the mobility edges appear basically due to the boundary condition sensitivity of the nonreciprocal non-Hermitian lattice.

cond-mat.dis-nn↗

arXiv2022

Exceptional Point Generated Robust Asymmetric High-Order Harmonics

We propose a metallic-silicon system with a complex optical potential modulated along the length of the waveguide for a robust higher harmonic generation. For right moving fields when the strength of non-Hermiticity becomes equal to the real part of the optical potential, the dynamical equations associated with modal field amplitudes in our proposed system are described by a Jordan form Hamiltonian. This ultimately will allow for a unidirectional higher frequency generation which always has a maximum value for a specific length of the waveguide irrespective of the geometrical imperfections in the design of the waveguide. Furthermore, the phase of the generated higher harmonic mode becomes independent of the coupling between the fundamental frequency and higher harmonic one. Unlike other proposed spatiotemporal modulated systems when the system has a Jordan form Hamiltonian, the fundamental mode remains reciprocal while the harmonic generation is non-reciprocal. Consequently, while the proposed device cannot be used as an optical isolator it can be used for many other devices such as laser cavities, interferometry, and holographic processes.

physics.optics↗

arXiv2020

Spectral Singularities with Directional Sensitivity

We propose a class of spectral singularities that are sensitive to the direction of excitation and are arising in nonlinear systems with broken parity symmetry. These spectral singularities are sensitive to the direction of the incident beam and result in diverging transmission and reflection for the left (right) incident while the transmission and reflection of the right (left) side of the system remain finite. For pedagogical reason first, we review the scattering formalism in nonlinear systems using an abstract delta-function model. Then using a parity symmetry broken nonlinear system consists of two delta functions, one linear and the other nonlinear, we show the existence of our proposed spectral singularities. Then, we use an experimentally feasible realistic model based on coupled disk resonators and we identify the spectral singularity with directional sensitivity (SSDS). Our proposed SSDS might have applications in designing nonlinear sensors. Furthermore, it can provide a solution for the hole burning problem in pumped laser cavities.

physics.optics↗

arXiv2020

Diffraction-free beam propagation at the exceptional point of non-Hermitian Glauber Fock lattices

We construct localized beams that are located at the edge of a non-Hermitian Glauber Fock (NGF) lattice made of coupled waveguides and can propagate for a long distance without almost no diffraction. Specifically, we calculate the closed-form of the eigenstates at the exceptional point of a semi-infinite NGF lattice composed of waveguides which are coupled in a unidirectional manner. We use the closed-form solution to construct the non-diffracting beams in finite NGF lattices. We provide a numerical approach to find other lattices that are capable of supporting non-diffracting beams at an exceptional point.

physics.optics↗

arXiv2020

Non-diffracting states at exceptional points

We propose to use exceptional points (EPs) to construct diffraction-free beam propagation and localized power oscillation in lattices. Specifically, here we propose two systems to utilize EPs for diffraction-free beam propagation, one in synthetic gauge lattices and the other one, in unidirectionally coupled resonators where each resonator individually is capable of creating orbital angular momentum beams (OAM). In the second system, we introduce the concept of robust and tunable OAM beam propagation in discrete lattices. We show that one can create robust OAM beams in an arbitrary number of sites of a photonic lattice. Furthermore, we report power oscillation at the EP of a non-Hermitian lattice. Our study widens the study and application of EPs in different photonic systems including the OAM beams and their associated dynamics in discrete lattices.

physics.optics↗

arXiv2020

Interplay of quantum phase transition and flat band in hybrid lattices

We investigate the interplay of superradiant phase transition (SPT) and energy band physics in an extended Dicke-Hubbard lattice whose unit cell consists of a Dicke model coupled to an atomless cavity. We found in such a periodic lattice the critical point that occurs in a single Dicke model becomes a critical region that is periodically changing with the wavenumber $k$. In the weak-coupling normal phase of the system we observed a flat band and its corresponding localization that can be controlled by the ground-state SPT. Our work builds the connection between flat band physics and SPT, which may fundamentally broaden the regimes of many-body theory and quantum optics.

quant-ph↗

arXiv2020

Robust localized zero-energy modes from locally embedded PT-symmetric defects

We demonstrate the creation of robust localized zero-energy states that are induced into topologically trivial systems by insertion of a PT-symmetric defect with local gain and loss. A pair of robust localized states induced by the defect turns into zero-energy modes when the gain-loss contrast exceeds a threshold, at which the defect states encounter an exceptional point. Our approach can be used to obtain robust lasing or perfectly absorbing modes in any part of the system.

physics.optics↗

arXiv2020

Fast and efficient deterministic quantum state transfer between two remote mechanical resonators

The main challenge in deterministic quantum state transfer in long-distance quantum communications is the transmission losses in the communication channel. To overcome this limitation, here we use the adiabatic theorem and find a lossless evolution path between two remote mechanical modes. By adiabatic variation of the effective coupling strengths between the two nodes and the intermediate optical channel modes, we engineer a transmission path for the quantum state transfer that is decoupled from the decaying fiber modes. Using our proposed method we show that one obtains a quantum state transfer with high efficiency. Furthermore, to bypass the slow nature of the adiabatic process and its sensitivity to the mechanical damping and noise as well as the strength of the driving pulses, we develop the shortcut to adiabatic passage protocol for our proposed quantum state transfer. Our results show that the shortcut to adiabaticity provides an efficient and fast quantum state transfer even for small values of the coupling strength. We show that the performance of our protocol for long-distance quantum communications remains efficient for transferring the quantum states between two remote mechanical resonators being hundred meters apart.

quant-ph↗

arXiv2019

Eigenstates Transition Without Undergoing an Adiabatic Process

We introduce a class of non-Hermitian Hamiltonians that offers a dynamical approach to short-cut to adiabaticity (DASA). In particular, in our proposed 2 * 2 Hamiltonians, one eigenvalue is absolutely real and the other one is complex. This specific form of the eigenvalues helps us to exponentially decay the population in an undesired eigenfunction or amplify the population in the desired state while keeping the probability amplitude in the other eigenfunction conserved. This provides us with a powerful method to have a diabatic process with the same outcome as its corresponding adiabatic process. In contrast to standard shortcuts to adiabaticity, our Hamiltonians have a much simpler form with a lower thermodynamic cost. Furthermore, we show that DASA can be extended to higher dimensions using the parameters associated with our 2 * 2 Hamiltonians. Our proposed Hamiltonians not only have application in DASA but also can be used for tunable mode selection and filtering in acoustics, electronics, and optics.

quant-ph↗

arXiv2018

Robust Exceptional Points in Disordered Systems

We construct a theory to introduce the concept of topologically robust exceptional points (EP). Starting from an ordered system with $N$ elements, we find the necessary condition to have the highest order exceptional point, namely $N^\text{th}$ order EP. Using symmetry considerations, we show an EP associated with an order system is very sensitive to the disorder. Specifically, if the EP associated with the ordered system occurs at the fixed degree of non-Hermiticity $γ_{EP}$, the disordered system will not have EP at the same $γ_{PT}$ which puts an obstacle in front of the observation and applications of EPs. To overcome this challenge, by incorporating an asymmetric coupling we propose a disordered system that has a robust EP which is extended all over the space. While our approach can be easily realized in electronic circuits and acoustics, we propose a simple experimentally feasible photonic system to realize our robust EP. Our results will open a new direction to search for topologically robust extended states (as opposed to topological localized states) and find considerable applications in direct observation of EPs, realizing topological sensors and designing robust devices for metrology.

physics.optics↗

arXiv2017

The effect of electric field on multiple exciton generation in lead chalcogenide nanocrystals

Unique properties of lead chalcogenides have enabled multiple exciton generation (MEG) in their nanocrystals that can be beneficial in enhancing the efficiency of the third generation solar cells. Although the intrinsic electric field plays an imperative role in a solar cell, its effect on the multiple exciton generation (MEG) has been overlooked, so far. Using EOM-CCSD as a many-body approach, we show that any electric field can affect the absorptivity spectra of the lead chalcogenide nanocrystals (Pb4Te4, Pb4Se4, and Pb4S4). The same electric field, however, has insignificant effects on the MEG quantum probabilities and the thresholds in these nanocrystals. Furthermore, simulations show that Pb4Te4, among the aforementioned nanocrystals, has the lowest MEG threshold and the strongest absorptivity peak that is located in the multi-excitation window, irrespective of the field strength, making it the most suitable candidate for MEG applications. Simulations also demonstrate that an electric field affects the MEG characteristics in the Pb4Te4 nanocrystal, in general, less than it perturbs MEG characteristics in Pb4Se4 and Pb4S4 nanocrystals. Our results can have a great impact in designing optoelectronic devices whose performance can be significantly influenced by MEG.

physics.atm-clus↗

arXiv2017

Non-Hermiticity Induced Flat Band

We demonstrate the emergence of an entire flat band embedded in dispersive bands at the exceptional point of a PT symmetric photonic lattice. For this to occur, the gain and loss parameter effectively alters the size of the partial flat band windows and band gap of the photonic lattice simultaneously. The mode associated with the entire flat band is robust against changes in the system size and survives even at the edge of the lattice. Our proposal offers a route for controllable localization of light in non-Hermitian systems and a technique for measuring non-Hermiticity via localization.

physics.optics↗

arXiv2017

The Effect of non-Hermiticity on Adiabatic Elimination

We investigate the influence of non-Hermiticity on the adiabatic elimination in coupled waveguides. We show that adiabatic elimination is not affected when the system is in parity-time symmetric phase. However, in the broken phase the eliminated waveguide loses its darkness namely its amplitude starts increasing, which means adiabatic elimination does not hold in the broken phase. Our results can advance the control of the dynamics in coupled laser cavities, and help the design of controllable absorbers.

physics.optics↗

arXiv2017

Unidirectional Localization of Photons

Artificial defects embedded in periodic structures are important foundation for creating localized states with vast range of applications in condensed matter physics, photonics and acoustics. In photonics, localized states are extensively used to confine and manipulate photons. Up to now, all the proposed localized states are reciprocal and restricted by time reversal symmetry. Consequently, localization is bidirectional and photons at the allowed passband in the otherwise forbidden stop band are confined irrespective of the direction of incident beam. In this report, by embedding a single defect in a one-dimensional spatiotemporally modulated photonic lattice, we demonstrate that it is possible to have localization of photon only in one direction. In a spatiotemporally modulated photonics lattice, a time dependent potential generates an effective magnetic biasing, which breaks the reciprocity. Moreover, in such moving lattices the dispersion relation obtains a shift depending on the direction of effective magnetic biasing. A static defect synthesized in a temporally modulated lattice will generate a spatial localization of light in the bandgap. However, due to the shift of the bandgap the localization occurs in different frequencies depending on the direction of incident field. We envisage that this phenomenon might has impact not only in photonics but also other areas of physics and engineering such as condensed matter and acoustics, opens the doors for designing new types of devices such as non-reciprocal traps, sensors, unidirectional tunable filters, and might result in unconventional transports such as unidirectional lasing. Despite its applications, our proposal, namely a defect sate in a driven system, can be considered as a pedagogical example of Floquet problem with analytical solution.

physics.optics↗