arXiv subjects

Reza Asgari

Publications and source records attributed to Reza Asgari.

At least 73 records · Page 4Linked to original sources

arXiv2020

Low-loss two-dimensional plasmon modes in antimonene

The effects of spin-orbit (SOC) and electron-phonon coupling on the collective excitation of doped monolayer Sb$_2$ are investigated using density functional and many-body perturbation theories. The spin-orbit coupling is exclusively important for the monolayer Sb$_2$ and it leads to the reconstruction of the electronic band structure. In particular, plasmon modes of monolayer Sb$_2$ are quite sensitive to the SOC and are characterized by very low damping rates owing to small electron-phonon scatterings. Our results show plasmons in antimonene are significantly less damped compared to monolayer graphene when plasmon energies are $\hbar ω> 0.2$ eV due to smaller plasmon-phonon coupling in the former material.

cond-mat.mtrl-sci↗

arXiv2019

Dirac fermion and superconductivity in two-dimensional transition-metal MOH (M= Zr, Hf )

Discovery of the new two-dimensional (2D) Dirac semimetals incorporating both superconductivity and the topological band structure has provided a novel platform for realizing the intriguing applications of Dirac fermions and Majorana quasiparticles, ranging from high-speed quantum devices at the nanoscale to topological quantum computations. In this work, utilizing first-principles calculations and symmetry analysis, we introduce MOH (M= Zr, Hf) as a new topological superconductor with Dirac points close to a Fermi level which are connected with nearly flat edge states as a striking feature of topological semimetals. Our calculations show that ZrOH as a 2D topological semimetal can exhibit superconductivity and is a novel platform for studying the interplay between superconductivity and Dirac states in low-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2019

Hall viscosity for optical phonons

We generalize the notion of dissipationless, topological Hall viscosity tensor to optical phonons in thin film Weyl semimetals. By using the strained Porphyrin thin film Weyl semimetal as a model example, we show how optical phonons can couple to Weyl electrons as chiral pseudo gauge fields. These chiral vector fields lead to a novel dissipationless two-rank viscosity tensor in the effective dynamics of optical phonons whose origin is the chiral anomaly. We also compute the contribution to this two rank Hall viscosity tensor due to the presence of an external magnetic field, whose origin is the conventional Hall response of Weyl electrons. Finally, the phonon dispersion relations of the system at the long-wavelength limit with and without an electromagnetic field are calculated showing a measurable shift in the Raman response of the system. Our results can be investigated by Raman scattering or infrared spectroscopy by attenuated total reflectance experiments.

cond-mat.mes-hall↗

arXiv2019

Spin transfer torque and exchange coupling in Josephson junctions with ferromagnetic superconductor reservoirs

In this paper, the spin transfer torque (STT) and the exchange coupling of the Josephson junctions containing interesting cases of diffusive/ballistic-triplet/singlet ferromagnetic superconductor materials together with diffusive Josephson junction of the form S$_{1}$/F$_{1}$/I$_1$/N/I$_2$/F$_2$ with I being insulating barrier are investigated. Using the Nazarov quantum circuit theory, it is found that for the diffusive FS$_1$/N/FS$_2$ structure the only emerged torque in normal direction to the plane of the exchange fields of F$_1$ and F$_2$ results in antiparallel/parallel or vice versa parallel/antiparallel transition of favorable exchange coupling depending on the considered parameters of the system, including the nonmagnetic spacer thickness, the superconducting phase difference, the length and the exchange field of the ferromagnets. Furthermore, the analyze of the width of the transitions, the phase difference interval in which an interlayer length-induced antiparallel/parallel transition can be occurred, is performed. For instance, as the exchange field or the temperature increases, the interval of phase difference gets larger. On the other hand, ballistic Josephson junction containing the triplet ferromagnetic superconductor reservoirs solving the 16$\times$16 Bogoliubov-de-Gennes equation is studied. It is found that although the exchange fields of the FS are laid in the z and y direction, the STT interestingly exists in all three directions of x, y and z. This exciting finding suggests that the favorable equilibrium configuration concerning the least exchange coupling occurs in the relative exchange field direction different from 0 or $π$. To the best of our knowledge it is for the first time that the occurrence of the in-plane STT is reported. Moreover, the occurrence of the beat like behavior with two oscillation period for the out-of-plane STT is interestingly acquired.

cond-mat.mes-hall↗

arXiv2018

Composite quasiparticles in strongly-correlated dipolar Fermi liquids

Strong particle-plasmon interaction in electronic systems can lead to composite \emph{hole-plasmon} excitations. We investigate the emergence of similar composite quasiparticles in ultracold dipolar Fermi liquids originating from the long-range dipole-dipole interaction. We use the $G_0W$ technique with an effective interaction obtained from the static structure factor to calculate the quasiparticle properties and single-particle spectral function. We first demonstrate that within this formalism a very good agreement with the quantum Monte Carlo results could be achieved over a wide range of coupling strengths for the renormalization constant and effective mass. The composite \textit{quasiparticle-zero sound} excitations which are undamped at long wavelengths emerge at intermediate and strong couplings in the spectral function and should be detectable through the radio frequency spectroscopy of nonreactive polar molecules at high densities.

cond-mat.quant-gas↗

arXiv2018

Optical absorption properties of few-layer phosphorene

We investigate the optical absorption and transmission of few-layer phosphorene in the framework of ab initio density functional simulations and many-body perturbation theory at the level of random phase approximation. In bilayer phosphorene, the optical transition of the valence band to the conduction band appears along the armchair direction at about 0.72 eV, while it is absent along the zigzag direction. This phenomenon is consistent with experimental observations. The angle-resolved optical absorption in few-layer phosphorene shows that it is transparent when illuminated by near grazing incidence of light. Also, there is a general trend of an increase in the absorption by increasing the number of layers. Our results show that the bilayer phosphorene exhibits greater absorbance compared to that of bilayer graphene in the ultraviolet region. Moreover, the maximal peak in the calculated absorption of bilayer MoS2 is in the visible region, while bilayer graphene and phosphorene are transparent. Besides, the collective electronic excitations of few-layer phosphorene are explored. An optical mode (in-phase mode) that follows a low-energy \sqrt q dependence for all structures, and another which is a damped acoustic mode (out-of-phase mode) with linear dispersion for multilayer phosphorene are obtained. The anisotropy of the band structure of few-layer phosphorene along the armchair and zigzag directions is manifested in the collective plasmon excitations.

cond-mat.mes-hall↗

arXiv2018

Phosphorene as a nanoelectromechanical material

Based on density functional simulations combined with the Landauer transport theory, the mechanical strain impacts on the chemical bonds of phosphorene and their effects on the electronic properties are studied. Moreover, the effect of the tensile strain along the zigzag direction on the charge transport properties of a two-terminal phosphorene device is evaluated. Enhancement of the intraplanar interactions, in particular between the next-nearest neighbors in strained phosphorene, is found to be essential in the band-structure evolution. The charge transport analyzing shows that phosphorene has a strong piezoconductance sensitivity, which makes this material highly desirable for high-pressure nanoelectromechanical applications. The piezoconductance gauge factor increases by strain from 46 in 5% tension to 220 in 12% tension, which is comparable to state-of-the-art silicon strain sensors. The transmission pathways monitor the current flowing in terms of the chemical bonds and hopping, however, the transport mostly arises from the charge transferring through the chemical bonds. The strong anisotropy in the transport properties along zigzag and armchair directions is observed.

cond-mat.mtrl-sci↗

arXiv2018

Fulde-Ferrell-Larkin-Ovchinnikov pairing states of a polarized dipolar Fermi gas trapped in a one-dimensional optical lattice

We study the interplay between the long- and short-range interaction of a one-dimensional optical lattice system of two-component dipolar fermions by using the density matrix renormalization group method. The atomic density profile, pairing-pairing correlation function, and the compressibility are calculated in the ground state, from which we identify the parameter region of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state, half-metal (HM) state, FFLO-HM state, and the normal polarized state, and thus the phase diagram in the coordinates of the long- and short-range interaction strength. The effect of the long-range dipolar interaction on the FFLO state is discussed in details. We find that the long-range part of the dipole-dipole interaction does not sweep away the FFLO superconducting region that is driven by the short-range interaction in the Hubbard model, and thus the FFLO state survives in the wide parameter space of the long-range interaction, polarization and the filling.

cond-mat.quant-gas↗

arXiv2018

Effect of boron and phosphorus codoping on the electronic and optical properties of graphitic carbon nitride monolayers: First-principle simulations

We study the effect of boron (B) and Phosphorous (P) co-doping on electronic and optical properties of graphitic carbon nitride (g-C$_3$N$_4$ or GCN) monolayer using density functional simulations. The energy band structure indicates that the incorporation of B and P into a hexagonal lattice of GCN reduces the energy band gap from $3.1$ for pristine GCN to $1.9$ eV, thus extending light absorption toward the visible region. Moreover, on the basis of calculating absorption spectra and dielectric function, the co-doped system exhibits an improved absorption intensity in the visible region and more electronic transitions, which named $π^*$ electronic transitions that occurred and were prohibited in the pristine GCN. These transitions can be attributed to charge redistribution upon doping, caused by distorted configurable B/P co-doped GCN confirmed by both electron density and Mulliken charge population. Therefore, B/P co-doped GCN is expected to be an auspicious candidate to be used as a promising photoelectrode in Photoelectrochemical water splitting reactions leading to efficient solar H$_2$ production.

cond-mat.mtrl-sci↗

arXiv2018

Charge transport in doped zigzag phosphorene nanoribbons

The effects of lattice distortion and chemical disorder on charge transport properties of two-terminal zigzag phosphorene nanoribbons (zPNRs), which shows resonant tunneling behavior under an electrical applied bias, are studied. Our comprehensive study is based on an {\it ab~initio} quantum transport calculations on the basis of the Landauer theory. We use nitrogen and silicon substitutional dopant atoms, and employ different physical quantities such as $I-V$ curve, voltage drop behavior, transmission spectrum, transmission pathway, and atomic current, to explore the transport mechanism of zPNR devices under a bias voltage. The calculated transmission pathways show the transition from a ballistic transport regime to a diffusive and in some particular cases to localized transport regimes. Current flowing via the chemical bonds and hopping are monitored, however, the conductance originates mainly from a charge traveling through the chemical bonds in the vicinity of the zigzag edges. Our results show that, in the doped systems, the device conductance decreases and negative differential resistance characteristic becomes weak or eliminates. Besides, the conductance in a pure zPNR system is almost independent of the ribbon width.

cond-mat.mes-hall↗

arXiv2018

Magnetotransport of a 2DEG with anisotropic Rashba interaction at the LaAlO$_3$/SrTiO$_3$ interface

We investigate the magnetotransport properties of a two-dimensional electron gas with anisotropic k-cubic Rashba interaction at the $\rm{LaAlO_3}$/$\rm{SrTiO_3}$ interface. The Landau levels and density of states of the system as well as the magnetotransport coefficients are evaluated. A somehow anomalous beating pattern in low magnetic field regime is found both in the density profile and magnetoresistivity. We discuss the impact of electron density, Landau level broadening and Rashba spin-orbit constant on the appearance of the beatings in low magnetic fields and find that at low electron concentrations and not very strong spin-orbit interactions the beatings smooth out. On the other hand, as the magnetic field increases, the Zeeman term becomes the dominant splitting mechanism leading to the spin-split peaks in SdH oscillations. We also show that the observation of the beatings in low magnetic fields needs a system with rather higher carrier concentration so that the beatings persist up to sufficiently large fields where the oscillations are not smoothed out by Landau level broadening. The quantum Hall plateaus are evaluated and we show the Chern number with both even and odd values is replaced by the odd numbers when two subband energies are close with spin degenerate energy levels. Along with the numerical evaluation of the magnetotransport properties, a perturbative calculation is also performed which can be used in the case of low densities and not very large filling factors.

cond-mat.mtrl-sci↗

arXiv2018

Effects of low-dimensional material channels on energy consumption of Nano-devices

It is commonly believed that the significant energy saving advantages are belonged to the logic circuits which operate at low temperature as less enegy is needed for cooling them to the treshold temperature after operation. Also, nanoscale thermal management, efficient energy usage in nanoscale and especially thermal optimization are the most challenging issues, while dealing with the new generation of transistors as the miniaturizing unlimitedly the silicon channels of the transistors has resulted in an increase in the energy consumption of computers and the leakage currents. In this paper, the non-Fourier thermal attitudes of well-known two-dimensional crystalline materials of graphene, blue phosphorene, germanene, silicene and MoS$_2$ as the silicon channels replacements are studied by using the phonon Monte-Carlo method. We show that graphene and blue phosphorene have the least maximum temperature, representer of the reliability of the transistors, among the all five investigated nano-channels during the Monte-Cralo simulation. The established hotspots of these two materials are always cooler, not reaching the temperature threshold level, and they lose the heat much faster as the heat generation zone is switched off. The obtained results considered along with the electrical disadvantages of the graphene layer, suggests the blue phosphorene as the more thermally appropriate and optimal choice for the silicon channel replacement in new designed field effect transistors. That is to say that the limit of the energy and economic cost of the producing the advanced blue phosphorene chips meets the value of the product for the computing enterprise.

cond-mat.mes-hall↗

arXiv2018

Plasmonic physics of 2D crystalline materials

Collective modes of doped two-dimensional crystalline materials, namely graphene, MoS$_2$ and phosphorene, both monolayer and bilayer structures, are explored using the density functional theory simulations together with the random phase approximation. The many-body dielectric functions of the materials are calculated using an {\it ab initio} based model involving material-realistic physical properties. Having calculated the electron energy-loss, we calculate the collective modes of each material considering the in-phase and out-of-phase modes for bilayer structures. Furthermore, owing to many band structures and intreband transitions, we also find high-energy excitations in the systems. We explain that the material-specific dielectric function considering the polarizability of the crystalline material such as MoS$_2$ are needed to obtain realistic plasmon dispersions. For each material studied here, we find different collective modes and describe their physical origins.

cond-mat.mes-hall↗

arXiv2017

Strongly anisotropic RKKY interaction in monolayer black phosphorus

We theoretically study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in two-dimensional black phosphorus, phosphorene. The RKKY interaction enhances significantly for the low levels of hole doping owing to the nearly valence flat band. Remarkably, for the hole-doped phosphorene, the highest RKKY interaction occurs when two impurities are located along the zigzag direction and it tends to a minimum value with changing the direction from the zigzag to the armchair direction. We show that the interaction is highly anisotropic and the magnetic ground-state of two magnetic adatoms can be tuned by changing the rotational configuration of impurities. Owing to the anisotropic band dispersion, the oscillatory behavior with respect to the angle of the rotation is well-described by $\sin(2k_{\rm F}R)$, where the Fermi wavelength $k_{\rm F}$ changes in different directions. We also find that the tail of the RKKY oscillations falls off as $1/R^2$ at large distances.

cond-mat.mes-hall↗

arXiv2017

Superconducting critical temperature of hole doped blue phosphorene

We theoretically explore the superconducting critical temperature of hole doped blue phosphorene. Implementing the density functional theory calculations, we show that for the hole doped blue phosphorene, the isotropic superconducting state is induced owing to the quite strong electron-phonon coupling. The theory is based on the Migdal-Eliashberg formalism and the critical temperature is obtained through set-of-equations, self-consistency. In addition, we include a vertex correction diagram to the Migdal-Eliashberg formalism. The inclusion of the vertex correction beyond the Migdal-Eliashberg formalism changes the $T_c$ about $\pm20$K, depending on the level of the doping. Our accurate numerical results show that the superconducting critical temperature is still quite high, even in the cases that the vertex correction is implemented.

cond-mat.supr-con↗

arXiv2017

Quantum capacitance of double-layer graphene

We study the ground-state properties of a double layer graphene system with the Coulomb interlayer electron-electron interaction modeled within the random phase approximation. We first obtain an expression of the quantum capacitance of a two layer system. In addition, we calculate the many-body exchange-correlation energy and quantum capacitance of the hybrid double layer graphene system at zero-temperature. We show an enhancement of the majority density layer thermodynamic density-of-states owing to an increasing interlayer interaction between two layers near the Dirac point. The quantum capacitance near the neutrality point behaves like square root of the total density, $α\sqrt{n}$, where the coefficient $α$ decreases by increasing the charge density imbalance between two layers. Furthermore, we show that the quantum capacitance changes linearly by the gate voltage. Our results can be verified by current experiments.

cond-mat.mtrl-sci↗

arXiv2017

Plasmon modes of bilayer molybdenum disulfide: A density functional study

We explore the collective electronic excitations of bilayer molybdenum disulfide (MoS$_2$) using the density functional theory together with the random phase approximation. The many-body dielectric function and electron energy-loss spectra are calculated using an {\it ab initio} based model involving material-realistic physical properties. The electron energy-loss function of bilayer MoS$_2$ system is found to be sensitive to either electron or hole doping and it is owing to the fact that the Kohn-Sham band dispersions are not symmetric for energies above and below the zero Fermi level. Three plasmon modes are predicted. A damped high-energy mode, one optical mode (in-phase mode) for which the plasmon dispersion exhibits $\sqrt q $ in the long wavelength limit originating from low-energy electron scattering and finally a highly damped acoustic mode (out-of-phase mode).

cond-mat.mtrl-sci↗

arXiv2017

Plasmons at the LaAlO$_3$/SrTiO$_3$ interface and Graphene-LaAlO$_3$/SrTiO$_3$ double layer

We study plasmon modes of the two-dimensional electron gas residing at the interface of band insulators $\rm{LaAlO_3}$ and $\rm{SrTiO_3}$ (LAO/STO) and the plasmon excitations of graphene-LAO/STO double layer as well. Considering the electron-electron interaction within random phase approximation, we calculate the plasmon dispersions of both systems numerically and in the long wavelength limit analytical expressions for collective modes are found. One optical mode and two (three) acoustic modes are predicted for the LAO/STO (graphene-LAO/STO) system where only the uppermost acoustic mode of both systems can emerge above the electron-hole continuum depending on the characteristics of each system. In the case of LAO/STO interface, thanks to the spatial separation between $\rm{t_{2g}}$ orbitals, the upper acoustic mode might be undamped at the long wavelength limit depending on the exact value of the dielectric constant of $\rm{SrTiO_3}$. Same as other double layer systems, the interlayer distance for the graphene-LAO/STO system plays a crucial role in damping the upper acoustic mode. Faster damping of all plasmon modes of the present double layer system in comparison with the ones with conventional 2DEG instead of $\rm{t_{2g}}$ electron gas is also found due to heavier effective masses of the gas and also stronger interlayer Coulomb interaction.

cond-mat.mes-hall↗