Static Field Limit Research Articles

Non-cancellation of the parity anomaly in the strong-field regime of QED2+1

Quantum fluctuations lead to an anomalous violation of parity symmetry in quantum electrodynamics for an even number of spatial dimensions. While the leading parity-odd electric current vanishes in vacuum, we uncover a non-cancellation of the anomaly for strong electric fields with distinct macroscopic signatures. We perform real-time lattice simulations with fully dynamical gauge fields and Wilson fermions in 2+1 space-time dimensions. In the static field limit, relevant at early times, we solve the problem analytically. Our results point out the fundamental role of quantum anomalies for strong-field phenomena, relevant for a wide range of condensed matter and high-energy applications, but also for the next generation of gauge theory quantum simulators.

Static field limit of excitation probabilities in laser-atom interactions

We consider the interaction of atomic hydrogen, in its ground state, with an electromagnetic pulse whose duration is fixed in terms of the number of optical cycles. We study the probability of excitation of the atom in the static field limit i.e. for field frequencies going to zero. Despite the fact that the well-known Born–Fock adiabatic theorem is valid only for a system whose energy spectrum is discrete, we show that it is still possible to use this theorem to derive, in the low frequency limit, an analytical formula which gives the probability of transition to any excited state of the atom as a function of the field intensity, the carrier envelope phase and the number of optical cycles within the pulse. The results for the probability of excitation to low-lying excited states, obtained with this formula, agree with those we get by solving the time-dependent Schrödinger equation. The domain of validity is discussed in detail.

Change in optoelectronic properties of ExBox+4 on functionalization and guest encapsulation.

The impact of functionalization as well as guest encapsulation on the optoelectronic properties of the ExBox+4 moiety has been investigated with the aid of density functional theory (DFT) based calculations. To this end, three functionalized variants of the ExBox+4 moiety have been modelled in silico, viz. NH2-ExBox+4-F, CN-ExBox+4-NH2 and NO2-ExBox+4-NH2, whereas coronene (Cor), B-doped coronene (BCor), N-doped coronene (NCor), tetrathiafulvalene (TTF), biphenyl (BiPh), tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) molecules have been employed as guests. The results indicate that as a result of functionalization, the ExBox+4 moiety can exhibit tangible non-linear optical (NLO) response properties as vindicated by the first static hyperpolarizability (β). As a result of guest encapsulation, significant variation in the optoelectronic properties of the chosen hosts takes place. BCor/TCNQ/NCor molecules enhance the NLO activity of the hosts while TTF/TCNE suppresses the same. In particular, NCor@host moieties demonstrate very high β values at the static field limit. Time dependent DFT results suggest that in general, the ability of the guest in tuning the transition energy associated with the pivotal electronic transitions of the host plays the dominant role in the observed variation in β. In addition, BCor/NCor@host moieties demonstrate broadband optical absorption capability thereby elucidating the possibility of deriving multi-purpose optoelectronic features such as NLO activity as well as utility in photovoltaic systems. Thermochemical results suggest that all the guest@host systems are stable at 298.15 K. Non-covalent and electrostatic binding forces stabilize the studied systems.

From the UV to the static-field limit: Rates and scaling laws for intense-field ionization of helium

We present high-accuracy calculations of ionization rates of helium at UV (195 nm) wavelengths. The numerical results are obtained from full-dimensional numerical integration of the two-electron time-dependent Schrödinger equation. Comparison is made between ionization rates at 195 nm with previously obtained data at 390 nm and 780 nm. In addition, we have obtained quantitatively accurate solutions of the full-dimensional time-dependent Schrödinger equation for static-field ionization of helium. We compare our numerically integrated rates with those of time-independent calculations, and obtain good agreement over a wide range of intensities.

From the UV to the static-field limit: rates and scaling laws of intense-field ionization of helium

We present high-accuracy calculations of ionization rates of helium at UV (195 nm) wavelengths. The data are obtained from full-dimensionality integrations of the helium-laser time-dependent Schrödinger equation. Comparison is made with our previously obtained data at 390 nm and 780 nm. We show that scaling laws introduced by Parker et al extend unmodified from the near-infrared limit into the UV limit. Static-field ionization rates of helium are also obtained, again from time-dependent full-dimensionality integrations of the helium Schrödinger equation. We compare the static-field ionization results with those of Scrinzi et al and Themelis et al, who also treat the full-dimensional helium atom, but with time-independent methods. Good agreement is obtained.

Low-frequency–high-intensity limit of the Keldysh-Faisal-Reiss theory

When a frequency of the circularly polarized laser field approaches zero the above threshold ionization rate should approach the well-known static-field limit of tunneling ionization. In the high-intensity limit of the laser field the Keldysh-Faisal-Reiss (KFR) theory is expected to be valid. For the ground state of a hydrogen atom we study various forms of the KFR theory when both conditions: $\ensuremath{\omega}⪡1\phantom{\rule{0.3em}{0ex}}\mathrm{a}.\mathrm{u}.$ and $\ensuremath{\gamma}⪡1\phantom{\rule{0.3em}{0ex}}(\ensuremath{\omega}$ is the frequency and $\ensuremath{\gamma}$ the Keldysh parameter) are satisfied. For the circularly polarized laser field ionization rate in the Keldysh theory [which utilizes the length gauge $(\stackrel{P\vec}{d}∙\stackrel{P\vec}{E})$ form of the matrix element] is calculated analytically. We show numerically that if the WKB Coulomb correction in the final state of the ionized electron is included, the Keldysh theory gives the correct result in the tunneling domain. In the barrier-suppression regime the Keldysh theory without this correction gives ionization rates close to the exact static-field results. The Reiss theory [which utilizes the velocity gauge $(\stackrel{P\vec}{p}∙\stackrel{P\vec}{A})$ form of the matrix element] leads to too small ionization rates in the limit $\ensuremath{\omega}\ensuremath{\rightarrow}0$, $\ensuremath{\gamma}\ensuremath{\rightarrow}0$.

Isotropic second-order nonlinear optical susceptibilities

The second-order nonlinear optical susceptibility, in the electric dipole approximation, is only nonvanishing for materials that are noncentrosymmetric. Should the medium be isotropic, then only a chiral system, such as an optically active liquid, satisfies this symmetry requirement. We derive the quantum-mechanical form of the isotropic component of the sum- and difference-frequency susceptibility and discuss its unusual spectral properties. We show that any coherent second-order nonlinear optical process in a system of randomly oriented molecules requires the medium to be chiral, and the incident frequencies to be different and nonzero. Furthermore, a minimum of two nondegenerate excited molecular states are needed for the isotropic part of the susceptibility to be nonvanishing. The rotationally invariant susceptibility is zero in the static field limit and shows exceptionally sensitive resonance and dephasing effects that are particular to chiral centers.

Simple finite field method for calculation of static and dynamic vibrational hyperpolarizabilities: Curvature contributions

In the static field limit, the vibrational hyperpolarizability consists of two contributions due to: (1) the shift in the equilibrium geometry (known as nuclear relaxation), and (2) the change in the shape of the potential energy surface (known as curvature). Simple finite field methods have previously been developed for evaluating these static field contributions and also for determining the effect of nuclear relaxation on dynamic vibrational hyperpolarizabilities in the infinite frequency approximation. In this paper the finite field approach is extended to include, within the infinite frequency approximation, the effect of curvature on the major dynamic nonlinear optical processes.

A coupled-cluster based effective Hamiltonian method for dynamic electric polarizabilities

A coupled-cluster based approach for calculating dynamic polarizabilities is described. In this procedure, the polarizability is calculated by a strategy that is formally equivalent to a sum over states corresponding to the diagonal representation of a similarity transformed Hamiltonian operator. However, the explicit evaluation of excited state wave functions and energies is avoided. The present treatment is closely related to the equation of motion coupled-cluster approximation for excited states and offers an accurate approximation to the second derivative of the energy with respect to an applied electric field; the two approaches are equivalent in the limit that the spectrum of states corresponding to the effective Hamiltonian is exact within the basis set. Terms contributing to the second derivative, but neglected in the proposed approach are shown to be insignificant for a representative set of small molecules. The method is applied to calculate the polarizability of benzene at the wavelength of the sodium D line as well as in the static field limit, using a basis set of 192 contracted Gaussian functions.

Resonances in low frequency ionization by periodic electric fields

The behaviour of a one-dimensional system perturbed by a low frequency, periodic electric field is examined in the limit as the field frequency, Omega , tends to zero, that is the static field limit. In particular the authors obtain estimates of the widths of each member of the infinite set of resonances between any finite value of Omega and 0. In order to obtain this estimate they derive a new analytic approximation of the two-state equation of motion. Their analysis shows why recent experiments on the ionization of excited hydrogen atoms by low frequency fields failed to observe any resonances.

Effect of Electric Fields on the Excess Electric Current of the Maki Process above the Superconducting Transition Point

The nonlinear electric field dependence of the excess conductivity of the Maki process in dirty superconductors is recalculated microscopically above the transition point. The present result exhibits a quite different behavior E-213 ln(E213ja) compared with E-213 of Maki's previous calculation in the strong static field limit of films where a is proportional to the depairing parameter a. § I. Introduction

Effect of electric fields on the excess electric current of the Maki process above the superconducting transition point

Correct expression of the excess conductivity of the Maki process is given under finite electric fields E, showing a quite different behavior E −2 3 ln ( E 2 3 a ) compared with E −2 3 of Maki's original in the strong static field limit of films, where a ∝ the depairing parameter.