Small Coherence Length Research Articles

Beam wander of random electromagnetic Gaussian-shell model vortex beams propagating through a Kolmogorov turbulence

Beam wander of random electromagnetic Gaussian-Shell model (EGSM) vortex beams propagating through atmospheric turbulence is investigated. We develop the expression for beam wander of random EGSM vortex beams in theory. The effects of topological charge, turbulence strength, initial spatially coherent length, transverse scale, and wavelength on beam wander are illustrated numerically. The numerical results show that vortex beams with both positive and negative topological charges have the same beam wander, decreasing the coherent length and decreasing the transverse scale, or increasing the topological charge, can decrease the beam wander. In free-space optical (FSO) communication, we can choose beams with smaller coherent length, smaller wavelength, and larger topological charge to reduce beam wander.

Thermodynamic observation of a vortex melting transition in the Fe-based superconductor Ba0.5K0.5Fe2As2

In cuprate high-temperature superconductors the small coherence lengths and high transition termperatures result in strong thermal fluctuations, which render the superconducting transition in applied magnetic fields into a wide continuous crossover. A state with zero resistance is found only below the vortex melting transition, which occurs well below the onset of superconducting correlations. Here we investigate the vortex phase diagram of the novel Fe-based superconductor in form of a high-quality single crystal of Ba0.5K0.5Fe2As2, using three different experimental probes (specific heat, thermal expansion and magnetization). We find clear thermodynamic signatures of a vortex melting transition, which shows that the thermal fluctuations in applied magnetic fields also have a considerable impact on the superconducting properties of iron-based superconductors.

Propagation properties of partially coherent higher-order cosh-Gaussian beam in non-Kolmogorov turbulence

Based on the extended Huygens–Fresnel principle and second-order moments of the Wigner distribution function (WDF), the analytical formulation for the average intensity, the root-mean-square (rms) beam width and M2-factor of partially coherent higher-order cosh-Gaussian (ChG) beams propagating in non-Kolmogorov turbulence are derived. The influences of the beam parameters and the spectrum parameters associated with the non-Kolmogorov turbulence on the propagation properties of partially coherent higher-order ChG beams in non-Kolmogorov turbulence are numerically investigated. Numerical results reveal that partially coherent higher-order ChG beams with higher beam order, larger decentered parameter and smaller coherence length are less affected by the turbulence. Furthermore, it is found that the beams will be more affected by the turbulence with smaller inner scale, larger outer scale or larger structure constant. In addition, numerical results reveal that the propagation properties of beams in non-Kolmogorov turbulence are largely dependent on the generalized exponent α, the structure constant, the inner and outer scale, which is much different from that in the case of Kolmogorov turbulence. This research may be useful for the practical applications of partially coherent higher-order ChG beams in connection with optical communications and the remote sensing.

Bragg-Angle Diffraction in Slant Gratings Fabricated by Single-Beam Interference Lithography

A single-beam interference-lithography scheme is demonstrated for the fabrication of large-area slant gratings, which requires exposure of the photoresist thin film spin-coated on a glass plate with polished side-walls to a single laser beam in the ultraviolet and requires small coherence length of the laser. No additional beam splitting scheme and no adjustments for laser-beam overlapping and for optical path-length balancing are needed. Bragg-angle diffractions are observed as strong optical extinction that is tunable with changing the angle of incidence. This device is important for the design of efficient filters, beam splitters, and photonic devices.

Structural properties of ZnO films grown by picosecond pulsed-laser deposition

Zinc oxide thin films have been grown on c-cut (0001) and r-cut (1 1¯ 0 2) sapphire substrates by pulsed-laser deposition using a Nd:YAG laser operating at 355nm in picosecond regime (pulse duration: 42ps). The composition and the structural properties of the films have been investigated by scanning electron microscopy, Rutherford backscattering spectroscopy and X-ray diffraction according to different substrate temperatures. The RBS spectra show a Zn/O ratio close to 1.1 with a constant in-depth oxygen concentration. The XRD diagrams in Bragg–Brentano geometry display a preferred orientation depending on the used substrate. The large width of XRD peaks is indicative of a small coherence length. In addition, according to the pole figures recorded in asymmetric configuration, epitaxial relationships between substrate and film are evidenced. An increase in the substrate temperature leads to a film crystalline quality improvement. The results are discussed regarding the well-known properties of ZnO films obtained by nanosecond and femtosecond PLD.

Anisotropic Superconductivity and Vortex Dynamics in Magnetically Coupled F/S and F/S/F Hybrids

Magnetically coupled superconductor–ferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and scanning tunneling microscopy images of vortex structures in superconductor–ferromagnet hybrids reveal rich superconducting phase diagrams. Focusing on a particular combination of a ferromagnet with a well-ordered periodic magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above the domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels.

New Fe-based superconductors: properties relevant for applications

Less than two years after the discovery of high temperature superconductivity inoxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers(1111, 122, 11, 111) are available. They share several characteristics with cupratesuperconductors that compromise easy applications, such as the layered structure,the small coherence length and unconventional pairing. On the other hand, theFe-based superconductors have metallic parent compounds and their electronicanisotropy is generally smaller and does not strongly depend on the level of doping,and the supposed order parameter symmetry is s-wave, thus in principle not sodetrimental to current transmission across grain boundaries. From the applicationpoint of view, the main efforts are still devoted to investigate the superconductingproperties, to distinguish intrinsic from extrinsic behaviors and to compare thedifferent families in order to identify which one is the fittest for the quest forbetter and more practical superconductors. The 1111 family shows the highestTc,huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitionsreminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropicwith sharper resistive transitions as in low temperature superconductors, but with about half theTc of the 1111 compounds. An overview of the main superconducting propertiesrelevant to applications will be presented. Upper critical field, electronic anisotropyparameter, and intragranular and intergranular critical current density will bediscussed and compared, where possible, across the Fe-based superconductor families.

AVERAGE INTENSITY AND SPREADING OF PARTIALLY COHERENT STANDARD AND ELEGANT LAGUERRE-GAUSSIAN BEAMS IN TURBULENT ATMOSPHERE

Analytical expressions for the average intensity, mean- squared beam width and angular spread of partially coherent standard and elegant Laguerre-Gaussian (LG) beams propagating in turbulent atmosphere are derived. The properties of the average intensity, spreading and directionality of partially coherent standard and elegant LG beams in turbulent atmosphere are studied numerically and comparatively. It is found that the beam parameters and structure constant of turbulence together determine the properties of the beams in turbulent atmosphere. Partially coherent standard and elegant LG beams with smaller coherence length, larger beam orders and longer wavelength are less afiected by the turbulence. A partially coherent elegant LG beam is less afiected by turbulence than a partially coherent standard LG beam under the same condition. Furthermore, it is found that there exist equivalent partially coherent standard and elegant LG beams, equivalent fully coherent standard and elegant LG beams, equivalent Gaussian Schell-model beams that may have the same directionality as a fully coherent Gaussian beam both in free space and in turbulent atmosphere. Our results will be useful in long distance free-space optical communications.

Vortex matching effect in engineered thin films of NbN

We report robust vortex matching effects in antidot arrays fabricated on thin films of NbN. The near absence of hysteresis between field sweep directions indicates a negligible residual pinning in the host thin films. Owing to the very small coherence length of NbN thin films (ξ<5 nm), the observations suggest the possibility of probing physics of vortices at true nanometer length scales in suitably fabricated structures.

Realization of extremely broadband quantum-dot superluminescent light-emitting diodes by rapid thermal-annealing process

The first demonstration, to our knowledge, of the creation of ultrabroadband superluminescent light-emitting diodes using multiple quantum-dot layer structure by rapid thermal-annealing process is reported. The device exhibits a 3 dB emission bandwidth of 146 nm centered at 984 nm with cw output power as high as 15 mW at room temperature corresponding to an extremely small coherence length of 6.6 microm.

Multichannel optical modulator for a laser diode array

The possibility of the development of a multichannel electrooptical modulator of laser radiation with a large diffraction divergence and a small coherence length is studied experimentally and its design is described.

Application of the vector Monte-Carlo method in polarisation optical coherence tomography

The vector Monte-Carlo method is developed and applied to polarisation optical coherence tomography. The basic principles of simulation of the propagation of polarised electromagnetic radiation with a small coherence length are considered under conditions of multiple scattering. The results of numerical simulations for Rayleigh scattering well agree with the Milne solution generalised to the case of an electromagnetic field and with theoretical calculations in the diffusion approximation.

The Markov–Einstein coherence length—a new meaning for the Taylor length in turbulence

Small scale velocity statistics are measured in different turbulent flows with Reynolds numbers up to 10 6. The stochastic features of the turbulent signals are investigated by means of Markov processes for the velocity increments evolving along the cascade. The analysis of the validity of the Markovian properties yields to a new small scale coherence length. This coherence length can be seen as analogue to the mean free path length of a Brownian motion as pointed out in [A. Einstein, Ann. Phys. 17 (1905) 549]. For length scales larger than this coherence length the complexity of turbulence can be treated as a Markov process. We present experimental evidence that this Markov–Einstein coherence length scales with Re 1 / 2 and show that it is closely related to the Taylor micro-scale and not to the Kolmogorov dissipation length.

QPOs during magnetar flares are not driven by mechanical normal modes of the crust

Abstract QPOs have been observed during three powerful magnetar flares, from SGR 0526−66, SGR 1806−20 and SGR 1900+14. These QPOs have been commonly interpreted as being driven by the mechanical modes of the magnetar's solid crust which are excited during the flare. Here we show that this interpretation is in sharp contradiction with the conventional magnetar model. Firstly, we show that a magnetar crustal mode decays on the time-scale of at most 1 s due to the emission of Alfvén waves into the neutron star interior. A possible modification is then to assume that the QPOs are associated with the magnetars' global modes. However, we argue that at the frequencies of the observed QPOs, the neutron star core is likely to support a continuum of magnetohydrodynamic normal modes. We demonstrate this on a completely solvable toy model which captures the essential physics of the system. We then show that the frequency of the global mode of the whole star is likely to have a significant imaginary component, and its amplitude is likely to decay on a short time-scale. This is not observed. Thus we conclude that either (i) the origin of the QPO is in the magnetar's magnetosphere, or (ii) the magnetic field has a special configuration: either it is expelled from the magenta's core prior to the flares, or its poloidal component has very small coherence length.

Analysis of High‐Tc Superconductor Compounds on the Nanometre Scale

AbstractFor Abstract see ChemInform Abstract in Full Text.

Superconducting Fluctuating Diamagnetism Versus Precursor Diamagnetism in Heterogeneous Superconductors

The fluctuation-induced diamagnetism (FD) above the transition temperature Tc is studied through the isothermal field-dependence of the diamagnetic magnetization Mdia. While for T ≈ Tc and H→0 one has –Mdia ∝ (H)1/2,on increasing field an upturn occurs and \(|M_{{\rm dia}} |\) begins to decrease, in correspondence to field-induced quenching of the fluctuating pairs. For BCS superconductors (SC) as MgB2 the upturn field Hup is in the range 100–800 Oe, while in optimally doped high-temperature SC(HTcSC) Hup is expected in the range of several Tesla, because of the small coherence length. At variance, in non-homogeneous HTcSC (under- or overdoped) or in the presence of impurities causing diffuse transition, strong increase of Mdia and Hup reduced by order of magnitudes are observed. The reasons of these behaviors are discussed in the report. In particular the difference between the FD in heterogeneous HTcSC (related to SF) and the precursor diamagnetism for diffuse transitions such as in Al-doped MgB2 (unrelated to SF) is emphasized and it is shown how the temperature dependence of Hup discriminates between the two effects.

Boundary layers and emitted excitations in nonlinear Schrödinger superflow past a disk

The stability and dynamics of nonlinear Schrödinger superflows past a two-dimensional disk are investigated using a specially adapted pseudo-spectral method based on mapped Chebychev polynomials. This efficient numerical method allows the imposition of both Dirichlet and Neumann boundary conditions at the disk border. Small coherence length boundary-layer approximations to stationary solutions are obtained analytically. Newton branch-following is used to compute the complete bifurcation diagram of stationary solutions. The dependence of the critical Mach number on the coherence length is characterized. Above the critical Mach number, at coherence length larger than fifteen times the diameter of the disk, rarefaction pulses are dynamically nucleated, replacing the vortices that are nucleated at small coherence length.

Distinctions Between Classes of Superconductors Involve Their Degree of Structural Connectivity

A distinction is made between extreme types, such as high T c cuprates and soft-metal superconductors concerning the extended, relatively exchange frustration-free covalent ring structures (eight-membered) in the former. Phenomenology indicates for cuprates that bond order (BO) effects within a given number of doped bonds can create real space organization of pairs around O super-exchange centers (trijugate position) within a mobile pair kernel. Indications for this postulated new quantum chemical principle have now been found in related electronic crystal behavior by STM. This situation creates small coherence lengths (∼4 atoms) and unusually high pair numbers. For optimal doping, all holes are converted into pairs and these pairs show trends to geometric checkerboard patterns due to characteristic charge-lattice lock-ins, such as configurations with configurations with hop = 0.25 or hop = 0.16. Period of BO and the degree of isolation of the planes determine T c. Doping curves reflect BO events and are roughly predictable from structural data alone. It is suggested that a variety of related materials with large member rings (e.g. C60) follow the relevant BO phenomenology. By contrast, soft metal superconductors (e.g. Pb) are based on 3D close packed structures with small triangular rings in which exchange interactions are frustrated and small. “Normal” BCS isotope effects suggest the operation of phonons in pair formation over extended coherence lengths (>100 atoms). Very small pair numbers are created through phonons.

Analysis of high‐ T c superconductor compounds on the nanometre scale

Due to the small coherence lengths of the high-Tc compounds, effective pinning sites are defects or particles of nanometer size according to ξ3. Integral magnetic measurements of the magnetization as a function of temperature in large applied magnetic fields (up to 7 T) have revealed that practically all high-Tc compounds exhibit spatial inhomogeneities, which can be caused by either oxygen deficiency (YBCO), solid solutions of Nd/Ba (NdBCO and light rare earth 123-type compounds), intergrowths (Bi-based superconductors) or chemical doping by pair-breaking dopants like Zn, Pr, etc. Such local variations of the superconducting properties should be visible in low-temperature scanning tunneling microscopy experiments, and their effects on flux pinning could be studied in a direct way. Various irradiation experiments by neutrons, protons, and heavy-ions have enabled the artificial introduction of effective pinning sites into the high-Tc samples, thus creating many different observations in the integral magnetic data. Furthermore, several sub-structure formations are found in several multi-light rare earth 123-superconductor samples by means of AFM investigations, which may play an important role for the considerably improved critical current densities in these materials. From all these observations, we construct a pinning diagram explaining many features observed in high-Tc samples. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles study of magnetization relaxation enhancement and spin transfer in thin magnetic films

The interface-induced magnetization damping of thin ferromagnetic films in contact with normal-metal layers is calculated from first principles for clean and disordered Fe/Au and Co/Cu interfaces. Interference effects arising from coherent scattering turn out to be very small, consistent with a very small magnetic coherence length. Because the mixing conductances which govern the spin transfer are to a good approximation real valued, the spin pumping can be described by an increased Gilbert damping factor but an unmodified gyromagnetic ratio. The results also confirm that the spin-current induced magnetization torque is an interface effect.