Incoherent Transitions Research Articles

Magneto-optical effect induced by spin chirality of the itinerant ferromagnetNd2Mo2O7

It is demonstrated both theoretically and experimentally that the spin chirality associated with a noncoplanar spin configuration produces a magneto-optical effect. Numerical study of the two-band Hubbard model on a triangle cluster shows that the optical Hall conductivity $\sigma_{xy}(\omega)$ is proportional to the spin chirality. The detailed comparative experiments on pyrochlore-type molybdates $R_2$Mo$_2$O$_7$ with $R=$Nd (Ising-like moments) and $R=$Gd (Heisenberg-like ones) clearly distinguishes the two mechanisms, i.e., spin chirality and spin-orbit interactions. It is concluded that for $R$=Nd, $\sigma_{xy}(\omega)$ is dominated by the spin chirality for the dc ($\omega=0$) and the $d \to d$ incoherent intraband optical transitions between Mo atoms.

PROBING TRANSIENT MOLECULAR STRUCTURES IN PHOTOCHEMICAL PROCESSES USING LASER-INITIATED TIME-RESOLVED X-RAY ABSORPTION SPECTROSCOPY

Molecular structures during chemical processes are crucial for predicting molecular reactivity and reaction mechanisms. Using a laser pulse as an internal clock for starting fundamental chemical processes, molecular structural dynamics can be characterized by coherent vibrational motions and by incoherent transitions between different intermediate states. Recent developments in pulsed X-ray facilities allow structural determination of discrete excited states and reaction intermediates using laser-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS). Moreover, femtosecond X-ray sources have begun making significant contributions in monitoring coherent molecular motions. This review summarizes recent developments in the field, including technical and scientific challenges as well as several examples involving excited state molecular structure and electronic configuration determinations. Future applications of this technique with high time resolution will enable visualization of fundamental chemical events in many systems and further our understanding in photochemistry.

Generation of compressed sub-poisson Bose condensate by an atom laser

The dynamics of Bose-condensate generation by a cw atom laser with simultaneous stimulated evaporative cooling in a magnetic trap was analyzed using a quantum-mechanical master equation. The model of the atom laser includes irreversible processes of incoherent trap mode pumping and spontaneous atomic transitions due to the interaction of the atomic ensemble with heat reservoirs. The inelastic atomic collisions in the trap and the continual coherent Bose-condensate output coupling from the trap were considered. At certain values of parameters, the Bose condensate created in this laser scheme occurs in a compressed sub-Poisson state. For large Bose condensates with a mean number of atoms ∼106, the Fano factor may be as high as ≅0.5. The influence of spontaneous transitions from the excited trap modes on the statistics of Bose condensate was analyzed.

Three-pulse photon echoes for model reactive systems

A theoretical description of the three-pulse photon echo peak shift for model reaction systems is presented. An electronic two-state system with a finite upper-state lifetime and a three-state system in which electronic transitions can occur are considered. A probabilistic argument is employed to incorporate the incoherent transitions. New pathways describing the transition of electronic population are introduced and the nuclear propagator in the electronic population state is written by a convolution integral between those of the nonreactive two-state system weighted by some factors for the electronic transition. The response functions are given by multitime correlation functions and are analyzed by the cumulant expansion method. Some numerical calculations are presented and the influence of incoherent reactions on the peak shift is discussed. Comparison with experimental data confirms the existence of the effects predicted here.

Semiclassical study of electronically nonadiabatic dynamics in the condensed-phase: Spin-boson problem with Debye spectral density

The linearized semiclassical initial value representation (LSC-IVR) [H. Wang, X. Sun and W. H. Miller, J. Chem. Phys. 108, 9726 (1998)] is used to study the nonadiabatic dynamics of the spin-boson problem, a system of two electronic states linearly coupled to an infinite bath of harmonic oscillators. The spectral density of the bath is chosen to be of the Debye form, which is often used to model the solution environment of a charge transfer reaction. The simulation provides a rather complete understanding of the electronically nonadiabatic dynamics in a broad parameter space, including coherent to incoherent transitions along all three axes (the T-axis, the η-axis, and the ωc-axis) in the complete phase diagram and the determination of rate constants in several physically interesting regimes. Approximate analytic theories are used to compare with the simulation results, and good agreement is found in the appropriate physical limits.

Magnetic blocking temperatures of magnetite calculated with a three‐dimensional micromagnetic model

We present an analysis of thermal stability of magnetic remanence in fine grains of magnetite (grain size d = 15–120 nm). In order to model incoherent transitions between single‐domain (SD) and pseudo‐single‐domain (PSD) magnetization configurations, we employ a three‐dimensional constrained minimization method proposed by Enkin and Williams [1994]. Using this approach, one can track in detail the transition from one local energy minimum state into another by constraining the magnetization vectors of appropriate cells in a discrete model. For each particle, we obtain the energy barriers EB(T) from 25° to 578°C. Magnetic blocking temperatures (TB) are calculated by integrating EB(T) for two extreme cooling schedules representing laboratory and geological timescales. The computed blocking temperatures for laboratory timescales are in excellent agreement with the experimentally determined blocking temperatures for magnetite by Dunlop [1973b]. The results of our computations are summarized as relaxation time versus blocking temperature curves, which deviate from the curves of Pullaiah et al. [1975] for particles with grain sizes in the SD‐PSD transition region. A consequence of the dependence of TB on timescale is that some PSD size particles are blocked in vortex states on geologic timescales but are blocked in the SD state on laboratory timescales. Paleointensity determinations with the Thellier method on such samples can therefore underestimate the paleofield. The superparamagnetic to SD threshold size dS is determined as 50 nm for cubic grains, whereas a small aspect ratio of q=1.1 is sufficient to depress dS to 27 nm. SD particles of magnetite with small shape anisotropy and cubic grains with 58 nm≤d≤72 nm are reliable carriers of paleomagnetic information.

A microscopic model for size effect influence in unmixing f.c.c. A1−xBx alloys: Mean field approximation and Monte-Carlo simulations

A microscopic model taking account of elastic interactions is used to discuss the phase diagram of unmixing binary alloys. From mean field approximation, it is shown that elastic interactions, as a perturbation of the usual first neighbour Ising Hamiltonian, displace the top of the incoherent miscibility gap. In some cases, the order of the transition is changed. Monte-Carlo simulations confirm the trends predicted by mean field methods. This model is also used to sketch the discussion of coherent equilibrium in the case of a simple microstructure. This method is applied to the AlZn system. The displacement of both incoherent and coherent transitions lines are given.

A one-dimensional model for incoherent phase changes

We present here a simplified version of the model of incoherent solid-solid transitions with mass diffusion developed by Gurtin and Cermelli in [3]. An incoherent phase change is always associated with some kind of defect production at the interface: we consider here a one-dimensional continuum, so that the resulting equations allow study to be made of the influence of volume (vacancy) production on the evolution of the system.

On the kinematics of incoherent phase transitions

Incoherent phase transitions are far more difficult to treat than their coherent counterparts. The interface, which appears as a single surface in the deformed configuration, is represented in its undeformed state by a separate surface in each phase. This leads to a rich but detailed kinematics, one in which defects such as vacancies and dislocations are generated by the moving interface. We introduce an incoherency tensor that measures the stretching and twisting of one phase relative to the other. We show that incoherency is completely characterized by the incoherency tensor, the vacancy production, and the slip between phases.

The dynamics of solid-solid phase transitions 2. Incoherent interfaces

Incoherent phase transitions are more difficult to treat than their coherent counterparts. The interface, which appears as a single surface in the deformed configuration, is represented in its undeformed state by a separate surface in each phase. This leads to a rich but detailed kinematics, one in which defects such as vacancies and dislocations are generated by the moving interface. In this paper we develop a complete theory of incoherent phase transitions in the presence of deformation and mass transport, with phase interface structured by energy and stress. The final results are a complete set of interface conditions for an evolving incoherent interface.

Three-level Atom in a Squeezed Vacuum II

Abstract The resonance fluorescence of a three-level system is studied in each of its three possible configurations—ladder, lambda and vee—when it also interacts with broad band squeezed vacua and the applied fields are resonant. We obtain general expressions which apply for any configuration and do not make use of the secular approximation. Detailed results are presented which depend upon the degree of squeezing and the squeezing phase, in addition to the intensities of the applied fields. It is shown that asymmetric spectra may be obtained in certain circumstances. These are interpreted in terms of coherent and incoherent transitions induced by the squeezed fields. We also take the opportunity to assess the accuracy of the secular approximation.

Nonlinear response of a periodically driven damped two-state system.

We study the nonlinear response of a periodically driven dissipative two-state system. The exact formal solution in the form of a series in the number of system transitions is derived. The series is summed in analytic form in the parameter region where incoherent transitions prevail. We also present the exact solution for the time evolution of the system in the entire region of temperatures and driving strength for a special value of the Ohmic viscosity. The destruction of quantum coherence by incoherent processes which are induced by the fluctuating and the driving forces is discussed on the basis of these solutions.

Photon bunching in the fluorescence from single molecules: A probe for intersystem crossing

Fluorescence photons emitted by a single molecule trapped in a solid at low temperature are correlated in time. A simple theory of the correlation pattern is presented, including in the same formalism photon antibunching from coherent Rabi oscillations and photon bunching due to incoherent quantum transitions between electronic levels. The correlation method is applied to single pentacene molecules in a para-terphenyl crystal. A clear photon bunching allows us to determine the ISC rates for each molecule. We attribute the scatter of our results for the transition rate between excited singlet and triplet states, and the difference with the average value taken from the literature, to molecular distortions induced by crystal defects in our small sample. We conclude that the correlation method associated to single molecule spectroscopy has a great potential to study dynamical processes on intermediate time scales in condensed matter.

Quantum chemistry, anomalous dimensions, and the breakdown of Fermi liquid theory in strongly correlated systems

We formulate a local picture of strongly correlated systems as a Feynman sum over atomic configurations. The hopping amplitudes between these atomic configurations are identified as the renormalization group charges, which describe the local physics at different energy scales. For a metallic system away from half-filling, the fixed point local Hamiltonian is a generalized Anderson impurity model in the mixed valence regime. There are three types of fixed points: a coherent Fermi liquid (FL) and two classes of self-similar (scale invariant) phases which we denote incoherent metallic states (IMS). When the transitions between the atomic configurations proceed coherently at low energies, the system is a Fermi liquid. Incoherent transitions between the low energy atomic configurations characterize the incoherent metallic states. The initial conditions for the renormalization group flow are determined by the physics at rather high energy scales. This is the domain of local quantum chemistry. We use simple quantum chemistry estimates to specify the basin of attraction of the IMS fixed points.

Descriptions of low energy misfit dislocation structures using the parabolic interaction potential

The need for and development of the parabolic models as a power series approximation of the sinusoidal Frenkel-Kontorowa model, used to study low energy misfit dislocation (MD) structures in epicrystals, is briefly discussed. Its application to monolayers on thick substrates, thickening epilayers, superlattices and structural ledges is outlined with special emphasis on critical misfit and thickness in coherent to incoherent transitions. The “nucleation” of an MD by climb from the free surface is calculated. The critical misfits obtained from (a) the continuum (approximation) and exact solutions, using the parabolic model, and (b) the continuum solutions of the sinusoidal and parabolic models are compared and the differences between the various approaches assessed.

Theory of Multi‐Phonon Transitions beyond the Condon Approximation

AbstractFor light interstitials in deformable host‐materials (“small‐polaron”‐like systems) the theory of multi‐phonon transitions is developed beyond the so‐called Condon approximation, i.e., the effect of phonon fluctuations on the defect transfer‐integrals J is explicitly taken into account. In particular the rate of incoherent defect transitions is evaluated up to second order in J and the temperature dependence of the multi‐phonon processes is discussed in the regime above half of the Debye temperature.

The incoherent phase transitions of hydrogen and deuterium in niobium

The incoherent phase transitions of H and D in Nb have been studied by X-ray scattering. For all investigations Nb was loaded with hydrogen in situ in order to prevent precipitation of the new phase, before starting the experiment, which irreversibly changes the crystal lattice and gives misleading results. The incoherent phase boundary was determined from the variation of the lattice parameter with hydrogen concentration. The lattice parameter varies linearly with concentration at least up to 0.34 H/Nb. The thermal expansion coefficient increases with hydrogen concentration and shows an anomalous behaviour just above the alpha - alpha ' phase transition. For the alpha - alpha ' transition of H and D in Nb the critical temperature is Tc=171+or-1 degrees C and the critical concentration cc=0.31+or-0.01 H/Nb. The triple point temperature of the alpha + beta to alpha + alpha ' transition is 88 degrees C for H and 99 degrees C for D in Nb.

Theory of non-steady kinetic phenomena for defectons in crystals

A microscopical theory is developed which describes the non-steady kinetic phenomena (e.g. conduction and diffusion) for defectons in a crystal. The principal features of the phenomena, the effects of the lattice deformation and the many-level nature of the defecton potential wells are discussed and expressions for the conductivity and diffusion coefficient for free and bound defectons are obtained. The role of the coherent and incoherent underbarrier and overbarrier transitions is explained. The quasiclassical case of non-steady diffusion and Debye-type and resonance type dielectric losses are compared with classical results and the effects of strong external or random internal fields are noted.

Elastic Scattering of 78-Mev Pions from Copper

The angular distributions of 78\ifmmode\pm\else\textpm\fi{}5 Mev ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\pi}}^{+}$ mesons scattered elastically from copper have been measured. The experimental results are compared with the distributions found from a range of exact optical model phase shift calculations using a complex attractive square well potential for $r<{R}_{0}$ (where ${R}_{0}=1.4{A}^{\frac{1}{3}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm) and the Coulomb potential for $r>{R}_{0}$. The effect of adding to the potential a term depending upon the gradient of the nuclear density, and thereby introducing a discontinuity in the matching of logarithmic derivatives at $r={R}_{0}$, is also explored. The optical model as well as several modified Born approximation calculations show reasonable agreement with the experimental curve in the forward direction and in the region of the first diffraction maximum. At backward angles the calculated curves show pronounced maxima and minima which do not appear experimentally, perhaps because incoherent transitions at these angles mask the true coherent distribution in the experimental measurement.

The Scattering of Slow Neutrons byO2Molecules

The scattering of slow neutrons by ${\mathrm{O}}_{2}$ is due to nuclear interaction as well as magnetic forces arising from two magnetically active electrons. The molecular (nuclear) scattering is subdivided into elastic (coherent and incoherent) transitions and inelastic (mostly hyperelastic) transitions. The magnetic scattering is purely incoherent and essentially elastic. Numerical values are obtained for the integral cross sections of these various scattering processes in dependence upon the wavelength. The case of very long neutron wavelength is of greatest physical interest, since only then can a sizable magnetic effect be expected. The analysis requires careful consideration of the thermal motion of the ${\mathrm{O}}_{2}$ molecules, which sometimes influences the order of magnitude of the result. More accurate experiments can be expected to lead to an independent measurement measurement of the distribution of the magnetically active shell of valence electrons.