Coherent control of laser-induced breakdown

We demonstrate coherent control of laser-induced optical breakdown in Ar and Xe with a femtosecond time-scale pulse train. By using a genetic algorithm to set the relative phases of seven optical sidebands that span two octaves of bandwidth, we enhance or suppress the probability of breakdown, vary the onset time of the spark, and to some extent, vary the position of the spark and the timing of the laser-produced shock wave.     

Coherent soft X-ray magnetic scattering

X-ray sources are exceptional tools for studying the structure of matter down to atomic-length scales, but across a wide range of fields there are samples that have remained notoriously difficult to study, such as airborne particles, particles in solution, membrane proteins, and macromolecular complexes. The advancement of imaging techniques to address these elusive samples has been a big motivation for constructing new X-ray light sources. Fourth-generation light sources, commonly called X-ray free-electron lasers (XFELs) [1 Madey, J. 1971. Journal of Applied Physics, 42: 1906–1913. [Crossref], [Web of Science ®] , [Google Scholar]], represent a huge step forward, with upwards of nine orders of magnitude increase in peak brightness for hard X-rays. As of 2012, four such XFELs are already productively operational (in Germany [2 Ackermann, W. 2007. Nature Photonics, 1: 336–342. [Crossref], [Web of Science ®] , [Google Scholar]], the U.S. [3 Emma, P. 2010. Nature Photonics, 4: 641–647. [Crossref], [Web of Science ®] , [Google Scholar]], Japan [4 Ishikawa, T. 2012. Nature Photonics, 6: 540–544. [Crossref], [Web of Science ®] , [Google Scholar]], and Italy [5 Allaria, E. 2012. Nature Photonics, 6: 699–704. [Crossref], [Web of Science ®] , [Google Scholar]]), with at least five more planned for the next 10 years. These lasers produce femtosecond pulses of extremely intense coherent radiation through the positive feedback between a co-moving electron bunch and the radiation it emits when they traverse an extended undulator. This process creates remarkably strong, tunable probes that will undoubtedly change the way we examine nanoscale structure and dynamics.     

Coherent spin-current oscillations in transverse magnetic fields

We address the coherence of the dynamics of spin-currents with components transverse to an external magnetic field for the spin-1/2 Heisenberg chain. We study current autocorrelations at finite temperatures and the real-time dynamics of currents at zero temperature. Besides a coherent Larmor oscillation, we find an additional collective oscillation at higher frequencies, emerging as a coherent many-magnon effect at low temperatures. Using numerical and analytical methods, we analyze the oscillation frequency and decay time of this coherent current-mode versus temperature and magnetic field.     

Pseudopotential approach to magnetic energy bandstructure and magnetic breakdown in metals

The orthogonalized plane wave method of energy bandstructure calculation is generalized to the case of a metal under the influence of an external de magnetic field, with the magnetic translational symmetry taken into account fully. The magnetic field-dependent effective lattice potential or pseudopotential derived from it is interpreted as a “magnetic” pseudopotential and shown to depend only weakly on the magnetic field strength so that, to a good approximation, it can be replaced by an ordinary pseudopotential, and treated as a perturbation in the calculation of magnetic energy bands and galvanomagnetic properties in nearly-free-electron metals. Physical properties connected with the phenomenon of magnetic breakdown, in particular the Landau level broadening, which were previously shown by Pippard, Zak and others to be proportional to an unspecified pseudopotential, are reformulated in terms of the magnetic pseudopotential. The convergence of the method is also discussed.     

Observation of magnetic breakdown in double quantum wells

We report on our studies of magnetic breakdown (MB) in coupled GaAs/Al_0.3Ga_0.7As double quantum wells (DQWs) subject to crossed magnetic fields. MB is a failure of semiclassical theory that occurs when a magnetic field causes electrons to tunnel across a gap ink-space from one Fermi surface (FS) branch to another. We study MB in a two-branch FS created by subjecting a DQW to an in-plane magnetic field (B_∥). The principal effect ofB_∥is a distortion in the dispersion curve of the system, yielding a FS consisting of two components, a lens-shaped inner orbit and an hour-glass-shaped outer orbit. The perpendicular field (B_⊥) causes Landau level formation and Shubnikov–de Haas (SdH) oscillations for each branch of the FS. At higher perpendicular fields MB occurs and electrons tunnel throughk-space from one FS orbit to the other. MB is observed by noting which peaks are present in the Fourier power spectrum of the magnetoresistance versus 1/B_⊥at constantB_∥. We observe MB in two DQW samples over a range ofB_∥.     

Quantum localization in one-dimensional quasi-random systems and magnetic breakdown

The magnetic breakdown in metals is shown to cause the appearance of a new class of one-dimensional quasi-random “incommensurable” systems where the electrons are localized due to quantum interference effects. At this time both absolute localization and phase transition of “metal- dielectric” type can be realized.     

Effect of a transverse magnetic field on breakdown time

Results are given for discharges in air and nitrogen near atmospheric pressure; the strength of the transverse magnetic field varies from 0 to 270 kOe. The results are explained in terms of the equivalent-pressure theory.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 69–74, May, 1973.     

Coherent population trapping spectra in presence of ac magnetic fields

Experimental and theoretical investigations are reported on the effects induced by an alternating magnetic field on coherent population trapping resonances. We show that the ac magnetic field produces sidebands of these resonances in such a way that the spectrum observed is similar to those observed via the FM spectroscopic technique. Because of the very narrow linewidth of the resonances, sidebands are resolved even for ac field frequencies as low as a fraction of a kHz. The theoretical model developed, which takes into account a very simple atomic structure, fits the experimental data quite well.     

Giant magnetothermopower oscillations in indium: Evidence for magnetic breakdown

Large amplitude, low frequency oscillations in the transverse magnetothermopower of indium have been observed below 2.5 K for magnetic fields between 10 kG and 20 kG directed along either [010], [$\text{1}$01], or [1$\text{1}$0] crystallographic axes. The oscillations are attributed to magnetic breakdown.     

Coherent backscattering in nematic liquid crystals in an external magnetic field

We consider multiple light scattering in a nematic liquid crystal. Using the Monte Carlo method, we calculate for the first time the effect of a magnetic field on the shape of the peak of coherent backscattering taking into account the long-range action of fluctuations of the orientational order and anisotropy of the scattering length. For a small number of initial and final scattering events, we take into account the ordinary mode of light, which is weakly scattered in a nematic liquid crystal (NLC), whereas a strongly scattered extraordinary mode is taken into account for all scattering events. For simplicity, we use a single-constant approximation of the NLC elastic moduli. We show that the angular shape of the peak of coherent backscattering remains nearly unchanged, whereas the magnetic field and the scattering phase function vary by several orders of magnitude.     

Coherent inelastic light scattering from a microwave-excited array of magnetic particles

Inelastic light scattering from an array of Permalloy particles driven by a microwave magnetic field is shown to be a coherent phenomenon in which the scattered radiation is observed only at diffraction angles corresponding to the reciprocal lattice of the array. The results are explained in terms of the phase coherence of the inelastically scattered light by each of the particles.     

Coherent population trapping states with cold atoms in a magnetic field

Using potassium atoms cooled with a MOT, ground-state hyperfine coherent population trapped (CPT) states were prepared in a magnetic (B) field, and the behavior of CPT states was experimentally studied. We carefully measured the preparation of the CPT state as a function of time and the CPT signal as a function of laser power. The experimental CPT signal linewidth was approximately proportional to the square root of laser intensity in the range of parameters studied, and limits of this relation were explored theoretically.     

Ambipolar drift and breakdown of oscillistor effect in strong magnetic fields

The drift mechanism of helical instability suppression (oscillistor) in electron-hole plasma Ge was, for the first time, investigated. It was found that in strong magnetic fields ($\text{γ}{}_{\mathrm{i=e,h}}\text{= μ}{}_{\mathrm{i}}\text{H/c}\text{a}\text{?}\text{1}$) due to anisotropy of mobility (μ_i ≠ μ_i⊥) the ambipolar drift of helical disturbances of density along the electric field occurs even in quasineutral plasma (n = p), provided μ_e ≠ μ_h. To corroborate the drift mechanism of oscillistor effect breakdown in strong fields the experiments were carried out under the conditions of uniaxial crystal deformation oriented towards 〈111〉 (H, E  〈111〉) resulting in the intervalley redistribution of electrons and additional anisotropy of mobility. Variation in the ambipolar drift velocity connected with the deformation influences essentially the conditions of the helical instability breakdown in strong fields.     

Formation of local space-charge densities in gas breakdown in magnetic fields

Experimental and theoretical data on the dependence of the delay time of breakdown on the longitudinal and transverse magnetic field strength are given. The theoretical data correlate with the analogous experimental dependence with weak irradiation of the discharge interval. Equivalent-pressure theory gives satisfactory agreement with experiment in the breakdown of atmospheric-pressure gases in a transverse magnetic field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 36–40, May, 1988.     

Electron neutral collision frequency from breakdown measurements in crossed electric and magnetic fields

A simplified method of determining the electron-neutral collision frequency at unit pressurev ₀ is presented in a number of gases (hydrogen, argon and air) in crossed electric and magnetic fields utilising the sparking voltage data and equivalent pressure concept. The values ofv ₀ for lowH/p are found to agree with those reported by other authors. But at highH/p, the estimated values ofv ₀ are appreciably higher for all the three gases. The increase in the values ofv ₀ possibly occurs due to the change of the electron energy distribution at higherH/p.     

Enhancement of the electric field threshold for microwave breakdown by crossed magnetic field

The enhancement of breakdown threshold is of benefit to the high power microwave transmission. We propose a magnetic field in the transverse direction to the electric field to enhance the breakdown threshold. A theory of electric field threshold with crossed magnetic field for short pulse is developed, and verified by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. The result shows that the crossed magnetic field can enhance the breakdown threshold significantly.