Quantum Green Functions Research Articles

Intramolecular rate-constant calculations based on the correlation function using temperature dependent quantum Green's functions.

A theoretical method for calculating rate constants for internal conversion (IC), intersystem crossing (ISC) and radiative (R) electronic transitions is presented. The employed method uses temperature-dependent quantum Green's functions, which give the opportunity to consider almost any nth-order polynomial perturbation operator and the influence of external electromagnetic fields on the rate constants. The rate constants of the IC, ISC and R processes are calculated for two important indocyanine molecules namely indocyanine green (ICG) and heptamethine cyanine (IR808) at the Franck-Condon level using the temperature-dependent quantum Green's function approach. Calculations at the time-dependent density functional theory level with the MN15 functional show that ICG and IR808 have only one triplet state below the S1 state. The main deactivation channel of the S1 state is the IC process with a large (kIC(S1 → S0)) rate constant of ∼109-1011 s-1. The estimated quantum yield of fluorescence (φfl) is ∼0.001-0.24 for the two studied molecules, which agrees rather well with experimental values. Thus, the present approach enables calculations of the three kinds of rate constants and the quantum yield of fluorescence using the same computational methodology.

X-ray cavity quantum optics with inner-shell transitions

In the last decade, X-ray quantum optics has emerged as a new research field, driven by significant advancements in X-ray sources such as new generation synchrotron radiations and X-ray free electron lasers, as well as improvements in X-ray methodologies and sample fabrication. A very successful physical platform is the X-ray planar thin-film cavity, also known as the X-ray cavity QED setup, which represents a significant branch of X-ray quantum optics. So far, most X-ray cavity quantum optical studies are based on the Mössbauer nuclear resonances. However, the scope of the applications is limited by the few available nuclear isotope candidates and the lack of general applicability. Recently, X-ray cavity quantum control in atomic inner-shell transitions has been realized in experiments where the cavity effects simultaneously modify the transition energy and the core-hole lifetime. These pioneer works suggest that the X-ray cavity quantum optics with inner-shell transitions will become a new promising platform. Actually, the core-hole state is the fundamental concept in a variety of modern X-ray spectroscopic techniques. Therefore, integrating X-ray quantum optics with X-ray spectroscopies could lead to potential applications in core-level spectroscopies communities.<br>In this review, we introduce the experimental systems for the X-ray cavity quantum optics with inner-shell transitions, including the cavity structure, sample fabrications, and experimental methods. We explain that X-ray thin-film cavity samples require high flux, high energy resolution, small beam divergence, and precise angular control, necessitating synchrotron radiations. The grazing reflectivity and fluorescence measurements are shown in Fig.1, along with a brief introduction to resonant inelastic X-ray scattering. We also describe the theoretical simulation tools, including the classical Parratt's algorithm, semi-classical matrix formalism, quantum optical theory based on the Jaynes-Cummings model, and the quantum Green's function method. We discuss the similarities and characteristics of the electronic inner-shell transition compared to the nuclear resonance. Based on the observables, such as reflectivity and fluorescence spectra, we introduce several recent works, including cavity-induced energy shift, Fano interference, and core-hole lifetime control. In conclusion, we summarize the review and discuss several future directions. In particular, designing new cavity structures is essential to addressing current debates on the cavity effects with inner-shell transitions and discovering new quantum optical phenomena. Integrating modern X-ray spectroscopies with X-ray cavity quantum optics is a promising research area that could lead to valuable applications. Furthermore, X-ray free-electron lasers, which offer much higher pulse intensity and much shorter pulse duration, will advance X-ray cavity quantum optics studies from linear to multiphoton and nonlinear regimes.

DC and RF Performance of an N-channel Monolayer Black Phosphorus Nanoribbon Transistor

Two-dimensional black phosphorus is a relatively new discovery. There are numerous studies on black phosphorus two-dimensional transistors that focus on analog and RF performance. However, the RF performance of black phosphorus nanoribbon transistors is yet to be explored. We use a four-band tight binding Hamiltonian in conjunction with a non-equilibrium Green's function quantum transport simulator to investigate both the DC and RF performance of a monolayer black phosphorus nanoribbon transistor. We found that electron intra-band tunneling is responsible for current flow in the off-state, while in the on-state, the electrons flow over the top of the channel barrier potential. With a VDD of 0.4 volt and a gate length of 5 nm, our black phosphorus nanoribbon transistor has DC performance metrics of 510 µA/µm on-state current, 105 on/off current ratio, and 65 mV/dec inverse subthreshold slope. The device's RF performance characteristics are as follows: cut-off (unity current gain) frequency of 772.84 GHz, maximum oscillation (unity power gain) frequency of 1.15 THz, and open circuit voltage gain of 26.7 dB with transistor operating in the on-state. The RF performance of the device is found to be significantly impacted by the source and drain contact resistances. With source and drain resistances set to zero, the cut-off frequency increases to 995.23 GHz and the unity power gain frequency increases to 4.16 THz. The device shows unconditional stability above 893 GHz and it is conditionally stable below this frequency.

Structured transverse modes governed by maximum entropy principle.

Based on the birefringent effect of the gain medium, a diode-pumped Nd-doped vanadate laser with nearly hemispherical cavity is exploited to emulate the quantum Green functions of two-dimensional commensurate harmonic oscillators. By matching the theoretical calculations to the far-field patterns of lasing modes, the resonant transverse frequencies can be accurately determined up to extremely high orders. The Shannon entropy is further employed to calculate the spatial entanglement of the quantum Green function as a function the transverse frequency. From the resonant transverse frequencies, all lasing modes are confirmed to be in excellent agreement with the maximum entropy states. This discovery implies that the formation of lasing modes is relevant to the coupling interaction between the pump source and the laser cavity.

Quantum membrane phases in synthetic lattices of cold molecules or Rydberg atoms

We calculate properties of dipolar interacting ultracold molecules or Rydberg atoms in a semisynthetic three-dimensional configuration---one synthetic dimension plus a two-dimensional real-space optical lattice or periodic microtrap array---using the stochastic Green's function quantum Monte Carlo method. Through a calculation of thermodynamic quantities and appropriate correlation functions, along with their finite-size scalings, we show that there is a second-order transition to a low-temperature phase in which two-dimensional ``sheets'' form in the synthetic dimension of internal rotational or electronic states of the molecules or Rydberg atoms, respectively. Simulations for different values of the interaction $V$, which acts between atoms or molecules that are adjacent both in real and synthetic space, allow us to compute a phase diagram. We find a finite-temperature transition at sufficiently large $V$ as well as a quantum phase transition---a critical value ${V}_{c}$ below which the transition temperature vanishes.

Thermoelectric power factor of nanocomposite materials from two-dimensional quantum transport simulations

Nanocomposites are promising candidates for the next generation of thermoelectric materials since they exhibit extremely low thermal conductivities as a result of phonon scattering on the boundaries of the various material phases. The nanoinclusions, however, should not degrade the thermoelectric power factor, and ideally should increase it, so that benefits to the ZT figure of merit can be achieved. In this work we employ the nonequilibrium Green's function quantum transport method to calculate the electronic and thermoelectric coefficients of materials embedded with nanoinclusions. For computational effectiveness we consider two-dimensional nanoribbon geometries, however, the method includes the details of geometry, electron-phonon interactions, quantization, tunneling, and the ballistic to diffusive nature of transport, all combined in a unified approach. This makes it a convenient and accurate way to understand electronic and thermoelectric transport in nanomaterials, beyond semiclassical approximations, and beyond approximations that deal with the complexities of the geometry. We show that the presence of nanoinclusions within a matrix material offers opportunities for only weak energy filtering, significantly lower in comparison to superlattices, and thus only moderate power factor improvements. However, we describe how such nanocomposites can be optimized to limit degradation in the thermoelectric power factor and elaborate on the conditions that achieve the aforementioned mild improvements. Importantly, we show that under certain conditions, the power factor is independent of the density of nanoinclusions, meaning that materials with large nanoinclusion densities which provide very low thermal conductivities can also retain large power factors and result in large ZT figures of merit.

Photoelectron spectra and biological activity of cinnamic acid derivatives revisited

The electronic structures of several derivatives of cinnamic acid have been studied by UV photoelectron spectroscopy (UPS) and Green's function quantum chemical calculations. The spectra reveal the presence of dimers in the gas phase for p-coumaric and ferulic acids. The electronic structure analysis has been related to the biological properties of these compounds through the analysis of some structure-activity relationships (SAR).

Characterizing classical periodic orbits from quantum Green's functions in two-dimensional integrable systems: Harmonic oscillators and quantum billiards.

A general method is developed to characterize the family of classical periodic orbits from the quantum Green's function for the two-dimensional (2D) integrable systems. A decomposing formula related to the beta function is derived to link the quantum Green's function with the individual classical periodic orbits. The practicality of the developed formula is demonstrated by numerically analyzing the 2D commensurate harmonic oscillators and integrable quantum billiards. Numerical analyses reveal that the emergence of the classical features in quantum Green's functions principally comes from the superposition of the degenerate states for 2D harmonic oscillators. On the other hand, the damping factor in quantum Green's functions plays a critical role to display the classical features in mesoscopic regime for integrable quantum billiards, where the physical function of the damping factor is to lead to the coherent superposition of the nearly degenerate eigenstates.

Quantum Green's function in N-dimensional space with spherically piecewise continuous potentials

Quantum Green's function in N-dimensional space with spherically piecewise continuous potentials

High-Current Tunneling FETs With (110) Orientation and a Channel Heterojunction

We propose InAs/GaSb ultrathin-body tunneling field-effect transistors (TFETs) using confinement in the ( $1\bar {1}0$ ) plane and transport in the [110] direction to increase the tunneling probability by reducing the tunnel barrier energy and hole effective mass. To reduce the OFF-state leakage current, we add an InAs/In1– n Al n As1– n Sb n heterojunction to the channel, which increases the valence band barrier. The heterojunction also increases the tunneling probability and ON-current by reducing the tunneling distance through the p-n junction and introducing a resonant state. A fully atomistic non-equilibrium Green function quantum transport approach in NEMO5 is used to explore the design space. While choosing $10^{-3}$ A/m OFF-current ( $I_{\mathrm{\scriptscriptstyle OFF}}$ ) and a 0.3 V power supply, we simulate 270 A/m ON-current ( $I_{\mathrm{\scriptscriptstyle ON}}$ ) for a 30-nm gate length and 170 A/m for a 15-nm gate length $(L_{g})$ , while a conventional 15-nm $L_{g_{_{_{}}}}$ GaSb/InAs TFET under (001) confinement shows only 24 A/m $I_{\mathrm{\scriptscriptstyle ON}}$ .

Quantum mechanical modeling the emission pattern and polarization of nanoscale light emitting diodes.

Understanding of the electroluminescence (EL) mechanism in optoelectronic devices is imperative for further optimization of their efficiency and effectiveness. Here, a quantum mechanical approach is formulated for modeling the EL processes in nanoscale light emitting diodes (LED). Based on non-equilibrium Green's function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra of the nanowire device under different bias voltages are obtained and, more importantly, the radiation pattern and polarization of optical emission can be determined using the current approach. This work is an important step forward towards atomistic quantum mechanical modeling of the electrically induced optical response in nanoscale systems.

Comparison Analogy Between Properties of Hypernucleus and Supernucleus with Properties of the Elementary Particles and Resonances Electroproduction by Spin Shock Waves

It is showed generalization of discovering of the simple fact of proportionality between all elementary particles masses and 24 resonance masses in one side and nuclei weights in another side with some constant coefficient to all-known elementary particles and resonances masses. For resonances that linear dependence led to elementary particles type when electric charge of ions is defined by the same charge of muons. This is done within the frame of the “eight spin shock wave” model, which is based on an accurate solution of the Maxwell equations for a dust-like medium of charged particles in the flat space. Parameters of the group, such as Euler’s angles and Lorentz boosts, are all localized. In this way, they become functions of space coordinates and time. As a result, it is found that the Lorentz boost’s argument satisfies the linear wave equation. For small angles, the “eight spin” model is similar to the fundamental O(3) model. It is stated some analogy between properties of hypernucleus and supernucleus with properties of elementary particles and resonances electroproduction by spin shock waves (SSW) in beam ions. In particularly theoretically for all hypernucleus it very may be possible that Λ interacts with two NN from the external level of nucleus core. The ions list for resonances begin by (n, p) nucleus, on the first hypernucleus decay channel – on D2. For He4 in the τ – atom mτM-1 = 0.475 is near 0.41 is seen through the lepton universality as atom on the outer shell of which, instead of electron – a heavy lepton – tauon etc. In this approach neutral Higgs`s boson with it mass 125 GeV connected with nucleus from the island of stability, so as neutral Z boson – to U – 238. On this set of quarks distribution their dilaton currents conservation is confirmed. Theoretically nuclei weights are defined by according to dilaton quark – lepton X – structure identified with jumps features of the quantum Green function. Whereas the definition of a value of the ratio between the masses of particles and nuclei requires taking into account the contribution of SSW C-number components of classical interact.

A Green's function quantum average atom model

A quantum average atom model is reformulated using Green's functions. This allows integrals along the real energy axis to be deformed into the complex plane. The advantage being that sharp features such as resonances and bound states are broadened by a Lorentzian with a half-width chosen for numerical convenience. An implementation of this method therefore avoids numerically challenging resonance tracking and the search for weakly bound states, without changing the physical content or results of the model. A straightforward implementation results in up to a factor of 5 speed-up relative to an optimized orbital based code.

Competing phases, phase separation, and coexistence in the extended one-dimensional bosonic Hubbard model

We study the phase diagram of the one-dimensional bosonic Hubbard model with contact $(U)$ and near neighbor $(V)$ interactions focusing on the gapped Haldane insulating (HI) phase which is characterized by an exotic nonlocal order parameter. The parameter regime $(U,V$, and $\ensuremath{\mu})$ where this phase exists and how it competes with other phases, such as the supersolid (SS) phase, is incompletely understood. We use the stochastic Green function quantum Monte Carlo algorithm as well as the density matrix renormalization group to map out the phase diagram. Our main conclusions are that the HI exists only at $\ensuremath{\rho}=1$, the SS phase exists for a very wide range of parameters (including commensurate fillings), and displays power law decay in the one-body Green function. In addition, we show that at fixed integer density, the system exhibits phase separation in the $(U,V)$ plane.

Explicit Quantum Green's Functions on a Piecewise Continuous Symmetrical Spherical Potential

This work provides new results which are related to the calculation of Green's function for time-independent Schrodinger equation in three-dimensional space. Particularly, we have focused on computing the quantum Green's function for two kinds of spherical potentials. In the first case, we assumed that the potential is a piecewise continuous potential V ( r ) which means to be equal to a constant V 0 inside the sphere (radius a ) and it is equal to zero outside the sphere. For this potential, to derive the Green function, we have used continuity of the solution and discontinuity of its first derivative at r = a (at the edge). We have derived the solution in two cases: E > V 0 and E 0 . For the second kind of potential, we have assumed that the potential V(r) is equal to a negative constant –V 0 inside the sphere, and it is equal to zero outside it. Also, we used the continuity of its solution and discontinuity of its derivative at the edge ( r = a ) to obtain the associated Green's function which shows the discrete spectra of the Hamiltonian when — V 0 E 0.

A Renormalizable Theory of Quantum Gravity: Renormalization Proof of the Gauge Theory of Volume Preserving Diffeomorphisms

Inertial and gravitational mass or energy-momentum need not be the same for virtual quantum states. Separating their roles naturally leads to the gauge theory of volume-preserving diffeomorphisms of an inner four-dimensional space. The gauge-fixed action and the path integral measure occurring in the generating functional for the quantum Green functions of the theory are shown to obey a BRST-type symmetry. The related Zinn-Justin-type equation restricting the corresponding quantum effective action is established. This equation limits the infinite parts of the quantum effective action to have the same form as the gauge-fixed Lagrangian of the theory proving its spacetime renormalizability. The inner space integrals occurring in the quantum effective action which are divergent due to the gauge group's infinite volume are shown to be regularizable in a way consistent with the symmetries of the theory demonstrating as a byproduct that viable quantum gauge field theories are not limited to finite-dimensional compact gauge groups as is commonly assumed.

The equation of state for hydrogen at high densities

We use a two-fluid model combining the quantum Green's function technique for the electrons and a classical HNC description for the ions to calculate the high-density equation of state of hydrogen. This approach allows us to describe fully ionized plasmas of any electron degeneracy and any ionic coupling strength which are important for the modeling of a variety of astrophysical objects and inertial confinement fusion targets. We have also performed density functional molecular dynamics simulations (DFT-MD) and show that the data obtained agree with our approach in the high-density limit. Good agreement is also found between DFT-MD and quantum Monte Carlo simulations.

Unified decoupling scheme for exchange and anisotropy contributions and temperature-dependent spectral properties of anisotropic spin systems

We compute the temperature-dependent spin-wave spectrum and the magnetization for a spin system using the unified decoupling procedure for the high-order Green's functions for the exchange coupling and anisotropy, both in the classical and quantum case. Our approach allows us to establish a clear crossover between quantum-mechanical and classical methods by developing the classical analog of the quantum Green's function technique. The results are compared with the classical spectral density method and numerical modeling based on the stochastic Landau-Lifshitz equation and the Monte Carlo technique. As far as the critical temperature is concerned, there is a full agreement between the classical Green's functions technique and the classical spectral density method. However, the former method turns out to be more straightforward and more convenient than the latter because it avoids any \emph{a priori} assumptions about the system's spectral density. The temperature-dependent exchange stiffness as a function of magnetization is investigated within different approaches.

Discrete Random Dopant Fluctuation Impact on Nanoscale Dopant-Segregated Schottky-Barrier Nanowires

The impact of discrete random dopant fluctuations on 10-nm-long high-performance Schottky-barrier (SB) dopant-segregated (DS) nanowire MOSFETs is investigated through nonequilibrium Green's function quantum simulations. Using DS, nanoscale SB-FETs could outperform standard doped source and drain FETs in terms of ION/IOFF. By reintroducing dopants in SB, however, variability problems are unavoidable and will be strong, at least for thin-width SB-FETs, because of the dopant influence on the SB profile on which the tunneling current is quite sensitive.

Biaxial strain effect of spin dependent tunneling in MgO magnetic tunnel junctions

We study the effect of strain on magnetic tunnel junctions (MTJ) induced by a diamond like carbon (DLC) film. The junction resistance as well as the tunnel magnetoresistance (TMR) reduces with the DLC film. Non-equilibrium Green's function quantum transport calculations show that the application of biaxial strain increases the conductance for both the parallel and anti-parallel configurations. However, the conductance for the minority channel and for the anti-parallel configuration is significantly more sensitive to strain, which drastically increases transmission through a MgO tunnel barrier, therefore, the TMR ratio decreases with biaxial strain.