The wavelength dependence of gold nanorod‐mediated optical breakdown during infrared ultrashort pulses

This paper investigates the wavelength dependence of the threshold of gold nanorod‐mediated optical breakdown during picosecond and femtosecond near infrared optical pulses. It was found that the wavelength dependence in the picosecond regime is governed solely by the changes of a nanorod's optical properties. On the other hand, the optical breakdown threshold during femtosecond pulse exposure falls within one of two regimes. When the ratio of the maximum electric field from the outside to the inside of the nanorod is less then 7 (the absorption regime) the seed electrons are initiated by photo‐thermal emission, and the wavelength dependence in the threshold of optical breakdown is the result of optical properties of the nanoparticle. When the ratio is greater than 7 (the near‐field regime) more seed electrons are initiated by multiphoton ionization, and the wavelength dependence of the threshold of optical breakdown results from a combination of nanorod's optical properties and transitions in the order of multiphoton ionization. The findings of this study can guide the design of nanoparticle based optical breakdown applications. This analysis also deepens the understanding of nanoparticle‐mediated laser induced breakdown for picosecond and femtosecond pulses at near infrared wavelengths.

     

Observation of stimulated Raman scattering in polar tetragonal crystals of barium antimony tartrate trihydrate,Ba[Sb2((+)C4H2O6)2]·3H2O

The non‐centrosymmetric polar tetragonal (P 4₁) barium antimony tartrate trihydrate, Ba[Sb₂((+)C₄H₂O₆)₂]·3H₂O, was found to be an attractive novel semi‐organic crystal manifesting numerous χ ⁽²⁾‐ and χ ⁽³⁾‐nonlinear optical interactions. In particular, with picosecond single‐ and dual‐wavelength pumping SHG and THG via cascaded parametric four‐wave processes were observed. High‐order Stokes and anti‐Stokes lasing related to two SRS‐promoting vibration modes of the crystal, with ω_SRS1 ≈ 575 cm^?1 and ω_SRS2 ≈ 2940 cm^?1, takes place. Basing on a spontaneous Raman investigation an assignment of the two SRS‐active vibration modes is discussed.

     

Relic photon temperature versus redshift and the cosmic neutrino background

Presuming that CMB photons are described by the deconfining phase of an SU(2) Yang‐Mills theory with the critical temperature for the deconfining‐preconfining phase transition matching the present CMB temperature K (SU(2)_CMB), we investigate how CMB temperature T connects with the cosmological scale factor a in a Friedmann‐Lemaître‐Robertson‐Walker Universe. Owing to a violation of conformal scaling at late times, the tension between the (instantaneous) redshift of reionisation from CMB observation () and quasar spectra () is repealed. Also, we find that the redshift of CMB decoupling moves from to which questions ΛCDM cosmology at high redshifts. Adapting this model to the conventional physics of three flavours of massless cosmic neutrinos, we demonstrate inconsistency with the value N_eff ～ 3.36 extracted from Planck data. Interactions between cosmic neutrinos and the CMB implies a common temperature T of (no longer separately conserved) CMB and neutrino fluids. N_eff ～ 3.36 then entails a universal, temperature induced cosmic neutrino mass with . Our above results on z_re and z_dec, derived from SU(2)_CMB alone, are essentially unaffected when including such a neutrino sector.

     

Tunable pinning effects produced by non‐uniform antidot arrays in YBCO thin films

Uniform, graded and spaced arrays of 3 μm triangular antidots in pulsed laser deposited YBa₂Cu₃O₇ (YBCO) superconducting thin films are compared by examining the improvements in the critical current density they produced. The comparison is made to establish the role of their lithographically defined (non‐)uniformity and the effectiveness to control and/or enhance the critical current density. It is found that almost all types of non‐uniform arrays, including graded ones enhance over the broad applied magnetic field and temperature range due to the modified critical state. Whereas uniform arrays of antidots either reduce or produce no effect on compared to the original (as‐deposited) thin films.

     

Beryllium Silicate,Be2SiO4 (phenakite) – a Novel Trigonal SRS‐Active Crystal

In single crystals of the beryllium silicate Be₂SiO₄ with trigonal symmetry , known also as the mineral phenakite, χ⁽³⁾‐nonlinear lasing by stimulated Raman scattering (SRS) is investigated. All observed Stokes and anti‐Stokes lasing components are identified and ascribed to a single SRS‐promoting vibration mode with ω_SRS ≈876 cm⁻¹. With picosecond single‐wavelength pumping at one micrometer the generation of an octave‐spanning Stokes and anti‐Stokes comb is observed.     

Quantum states and vortex patterns in nanosuperconductors

The quanum levels and corresponding vortex states in nanoscale superconductors are investigated within generalized Bogolubov‐de Gennes theory. For symmetric (square‐shaped) samples thermodynamically stable vortex phases form symmetry‐consistent patterns and no transition to conventional Abrikosov‐like vortex patterns occurs till T=0K for sizes not exceeding 25 nm. For vorticity a giant vortex is stabilized at temperatures in the vicinity of , which transforms into a giant antivortex and four normal vortices with lowering the temperature. On the other hand, the vortex pattern for vorticity corresponds to an antivortex and four normal vortices in the whole temperature domain.     

Optical Switch Based on Dressed Intracavity Dark States

We present a scheme to realize two‐direction optical switch by a single‐mode optical cavity containing some four‐level atoms. The high switching efficiency can be obtained through low photon loss and large third‐order nonlinear susceptibility of this N‐type atomic system in cavity. Without the microwave source, it can be reduced to a Λ‐type atomic system where a coupling laser is used to realize single intracavity electromagnetically induced transparency (EIT). Namely, the probe field can be transmitted almost totally at resonance. Thus a two‐direction optical switch is operated and the state for forward (backward) direction is set as “open” (“closed”). When microwave source is introduced, dressed splitting of intracavity dark state happens. The probe field is reflected almost completely at resonance and the state of the optical switch at forward and backward directions (transmitted and reflected channels) is shifted as “closed” and “open”, respectively. Moreover, this scheme is much advantageous to realize splitting of intracavity dark state because weak microwave field () induces the coupling between intracavity dark state and one sublevel of ground state. While a strong pump laser () which couples the intracavity dark state with an excited level is applied to realize this splitting in ref. [Phys. Rev. A 85 013814 (2012)].     

Ultra‐Fast Control of Magnetic Relaxation in a Periodically Driven Hubbard Model

Motivated by cold atom and ultra‐fast pump‐probe experiments we study the melting of long‐range antiferromagnetic order of a perfect Néel state in a periodically driven repulsive Hubbard model. The dynamics is calculated for a Bethe lattice in infinite dimensions with non‐equilibrium dynamical mean‐field theory. In the absence of driving melting proceeds differently depending on the quench of the interactions to hopping ratio from the atomic limit. For decay occurs due to mobile charge‐excitations transferring energy to the spin sector, while for it is governed by the dynamics of residual quasi‐particles. Here we explore the rich effects that strong periodic driving has on this relaxation process spanning three frequency ω regimes: (i) high‐frequency , (ii) resonant with integer l, and (iii) in‐gap away from resonance. In case (i) we can quickly switch the decay from quasi‐particle to charge‐excitation mechanism through the suppression of ν₀. For (ii) the interaction can be engineered, even allowing an effective regime to be reached, giving the reverse switch from a charge‐excitation to quasi‐particle decay mechanism. For (iii) the exchange interaction can be controlled with little effect on the decay. By combining these regimes we show how periodic driving could be a potential pathway for controlling magnetism in antiferromagnetic materials. Finally, our numerical results demonstrate the accuracy and applicability of matrix product state techniques to the Hamiltonian DMFT impurity problem subjected to strong periodic driving.     

Quantum Transport in Presence of Bound States – Noise Power

The impact of bound states in Landauer‐Büttiker scattering approach to non‐equilibrium quantum transport is investigated. We show that the noise power at frequency ν is sensitive to all bound states with energies ω_b satisfying . We derive the exact expression of the bound state contribution and compare it to the one produced by the scattering states alone. The theoretical and experimental consequences of this result are discussed.     

Layered MoS2 grown on c ‐sapphire by pulsed laser deposition

Layered growth of molybdenum disulphide (MoS₂) was successfully achieved by pulsed laser deposition (PLD) method on c ‐plane sapphire substrate. Growth of monolayer to a few monolayer MoS₂, dependent on the pulsed number of excimer laser in PLD is demonstrated, indicating the promising controllability of layer growth. Among the samples with various pulse number deposition, the frequency difference (A_1g–E¹_2g) in Raman analysis of the 70 pulse sample is estimated as 20.11 cm^–1, suggesting a monolayer MoS₂ was obtained. Two‐dimensional (2D) layer growth of MoS₂ is confirmed by the streaky reflection high energy electron diffraction (RHEED) patterns during growth and the cross‐sectional view of transmission electron microscopy (TEM). The in‐plane relationship, (0006) sapphire//(0002)MoS₂and sapphire//MoS₂ is determined. The results imply that PLD is suitable for layered MoS₂ growth. Additionally, the oxide states of Mo 3d core level spectra of PLD grown MoS₂, analysed by X‐ray photoelectron spectroscopy (XPS), can be effectively reduced by adopting a post sulfurization process. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

X‐ray and neutron scattering on disordered nanosize clusters: a case study of lead‐zirconate‐titanate solid solutions

Defects and frequently used defect models of solids are reviewed. Signatures for identifying the disorder from x‐ray and neutron scattering data are given. To give illustrative examples how technologically important defects contribute to x‐ray and neutron scattering numerical method able to treat non‐periodical solids possessing several simultaneous defect types is given for simulating scattering in nanosize disordered clusters. The approach takes particle size, shape, and defects into account and isolates element specific signals. As a case study a statistical approximation model for lead‐zirconate titanate [Pb(Zr_xTi)O₃, PZT] is introduced. PZT is a material possessing several defect types, including substitutional, displacement and surface defects. Spatial composition variation is taken into account by introducing a model in which the edge lengths of each cell depend on the distribution of Zr and Ti ions in the cluster. Spatially varying edge lengths and angles is referred to as microstrain. The model is applied to compute the scattering from ellipsoid shaped PZT clusters and to simulate the structural changes as a function of average composition. Two‐phase co‐existence range, the so called morphotropic phase boundary composition is given correctly. The composition at which the rhombohedral and tetragonal cells are equally abundant was . Selected x‐ray and neutron Bragg reflection intensities and line shapes were simulated. Examples of the effect of size and shape of the scattering clusters on diffraction patterns are given and the particle dimensions, computed through Scherrer equation, are compared with the exact cluster dimensions. Scattering from two types of 180° domains in spherical particles, one type assigned to Ti‐rich PZT and the second to the MPB and Zr‐rich PZT, is computed. We show how the method can be used for modelling polarization reversal.

     

Theory of the Robin quantum wall in a linear potential. II. Thermodynamic properties

A theoretical analysis of the thermodynamic properties of the Robin wall characterized by the extrapolation length Λ in the electric field that pushes the particle to the surface is presented both in the canonical and two grand canonical representations and in the whole range of the Robin distance with the emphasis on its negative values which for the voltage‐free configuration support negative‐energy bound state. For the canonical ensemble, the heat capacity at exhibits a nonmonotonic behavior as a function of the temperature T with its pronounced maximum unrestrictedly increasing for the decreasing fields as and its location being proportional to . For the Fermi‐Dirac distribution, the specific heat per particle is a nonmonotonic function of the temperature too with the conspicuous extremum being preceded on the T axis by the plateau whose magnitude at the vanishing is defined as , with N being a number of the particles. The maximum of is the largest for and, similar to the canonical ensemble, grows to infinity as the field goes to zero. For the Bose‐Einstein ensemble, a formation of the sharp asymmetric feature on the ‐T dependence with the increase of N is shown to be more prominent at the lower voltages. This cusp‐like dependence of the heat capacity on the temperature, which for the infinite number of bosons transforms into the discontinuity of , is an indication of the phase transition to the condensate state. Some other physical characteristics such as the critical temperature and ground‐level population of the Bose‐Einstein condensate are calculated and analyzed as a function of the field and extrapolation length. Qualitative and quantitative explanation of these physical phenomena is based on the variation of the energy spectrum by the electric field.     

London penetration depth and thermal fluctuations in the sulphur hydride 203 K superconductor

Recently, compressed H₂S has been shown to become superconducting at 203 K under a pressure of 155 GPa. One might expect fluctuations to dominate at such temperatures. Using the magnetisation critical current, we determine the ground‐state London penetration depth, λ₀=189 nm, and the superconducting energy gap, Δ₀=27.8 meV, and find these parameters are similar to those of cuprate superconductors. We also determine the fluctuation temperature scale, K, which shows that, unlike the cuprates, of the hydride is not limited by fluctuations. This is due to its three dimensionality and suggests the search for better superconductors should refocus on three‐dimensional systems where the inevitable thermal fluctuations are less likely to reduce the observed .

     

Mechanical detection of ultraslow,Debye‐like Li‐ion motions in LiAlO single crystals

Single crystalline LiAlO is known as a very poor ion conductor. Thus, in its crystalline form it unequivocally disqualifies itself from being a powerful solid electrolyte in modern energy storage systems. On the other hand, its interesting crystal structure proves beneficial to sharpen our understanding of Li ion dynamics in solids which in return might influence application‐oriented research. LiAlO allows us to apply and test techniques that are sensitive to extremely slow Li ion dynamics. This helps us clarifying their diffusion behaviour from a fundamental point of view. Here, we combined two techniques to follow Li ion translational hopping in LiAlO that can be described by the same physical formalism: dynamic mechanical relaxation and electrical relaxation, i.e., ionic conductivity measurements. Via both methods we were able to track the same transport mechanism in LiAlO. Moreover, this enabled us to directly probe extremely slow Li exchange rates at temperatures slightly above 430 K. The results were compared with recent insights from nuclear magnetic resonance spectroscopy. Altogether, an Arrhenius‐type Li diffusion process with an activation energy of ca. 1.12 eV was revealed over a large dynamic range covering 10 orders of magnitude, i.e., spanning a dynamic range from the nano‐second time scale down to the second time scale.

     

Tens of GHz Tantalum pentoxide‐based micro‐ring all‐optical modulator for Si photonics

A tantalum pentoxide‐based (Ta₂O₅‐based) micro‐ring all‐optical modulator was fabricated. The refractive index inside the micro‐ring cavity was modified using the Kerr effect by injecting a pumped pulse. The transmittance of the ring resonator was controlled to achieve all‐optical modulation at the wavelength of the injected probe. When 12 GHz pulses with a peak power of 1.2 W were coupled in the ring cavity, the transmission spectrum of the Ta₂O₅ resonator was red‐shifted by 0.04 nm because of the Kerr effect. The relationship between the modulation depth and gap of the Ta₂O₅ directional coupler is discussed. An optimized gap of 1100 nm was obtained, and a maximum buildup factor of 11.7 with 84% modulation depth was achieved. The nonlinear refractive index of Ta₂O₅ at 1.55 μm was estimated as 3.4 × 10^?14 cm²/W based on the Kerr effect, which is almost an order of magnitude higher than that of Si₃N₄. All results indicate that Ta₂O₅ has potential for use in nonlinear waveguide applications with modulation speeds as high as tens of GHz.

     

Mössbauer spectroscopy evidence of intrinsic non‐stoichiometry in iron telluride single crystals

The FeTe parent compound for iron‐superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room‐temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of over‐stoichiometric iron or impurity phases. Low‐temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine‐split sextets, although no other foreign phases different from Fe_1.05Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over‐stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub‐lattice, and, as a result, four non‐equivalent groups of iron atoms are formed by their local environment. The resulting four‐group model consistently describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of FeSeTe_x compounds, which contain excess iron atoms.

     

On decoherence in quantum gravity

It was previously argued that the phenomenon of quantum gravitational decoherence described by the Wheeler‐DeWitt equation is responsible for the emergence of the arrow of time. Here we show that the characteristic spatio‐temporal scales of quantum gravitational decoherence are typically logarithmically larger than a characteristic curvature radius of the background space‐time. This largeness is a direct consequence of the fact that gravity is a non‐renormalizable theory, and the corresponding effective field theory is nearly decoupled from matter degrees of freedom in the physical limit . Therefore, as such, quantum gravitational decoherence is too ineffective to guarantee the emergence of the arrow of time and the “quantum‐to‐classical” transition to happen at scales of physical interest. We argue that the emergence of the arrow of time is directly related to the nature and properties of physical observer.

     

Transient dynamics of a one‐dimensional Holstein polaron under the influence of an external electric field

Following the Dirac‐Frenkel time‐dependent variational principle, transient dynamics of a one‐dimensional Holstein polaron with diagonal and off‐diagonal exciton‐phonon coupling in an external electric field is studied by employing the multi‐D₂ Ansatz, also known as a superposition of the usual Davydov D₂ trial states. Resultant polaron dynamics has significantly enhanced accuracy, and is in perfect agreement with that derived from the hierarchy equations of motion method. Starting from an initial broad wave packet, the exciton undergoes typical Bloch oscillations. Adding weak exciton‐phonon coupling leads to a broadened exciton wave packet and a reduced current amplitude. Using a narrow wave packet as the initial state, the bare exciton oscillates in a symmetric breathing mode, but the symmetry is easily broken by weak coupling to phonons, resulting in a non‐zero exciton current. For both scenarios, temporal periodicity is unchanged by exciton‐phonon coupling. In particular, at variance with the case of an infinite linear chain, no steady state is found in a finite‐sized ring within the anti‐adiabatic regime. For strong diagonal coupling, the multi‐ Anstaz is found to be highly accurate, and the phonon confinement gives rise to exciton localization and decay of the Bloch oscillations.

     

THz and above THz electron or hole oscillations in DNA dimers and trimers

A non conventional source or receiver of THz and above THz electromagnetic radiation is proposed. Specifically, electron or hole oscillations in DNA dimers (two interacting DNA base‐pairs or monomers) are predicted, with frequency in the range 0.25–100 THz (period 10–4000 fs) i.e. potentially absorbing or emitting electromagnetic radiation mainly in the mid‐ and far‐infrared with wavelengths ≈ 3–1200 μm. The efficiency of charge transfer between the two monomers which make up the dimer is described with the maximum transfer percentage p  and the pure maximum transfer rate . For dimers made of identical monomers , but for dimers made of different monomers . The investigation is extended to DNA trimers (three interacting DNA base‐pairs or monomers). For trimers made of identical monomers the carrier oscillates periodically with 0.5–33 THz ( 30–2000 fs); for 0 times crosswise purines , for 1 or 2 times crosswise purines . For trimers made of different monomers the carrier movement may be non periodic. Generally, increasing the number of monomers above three, the system becomes more complex and periodicity is lost; even for the simplest tetramer the carrier movement is not periodic.