Dynamic fluctuations and static speckle in critical X-ray scattering from SrTiO3

We report a study of critical x-ray scattering from SrTiO3 near the antiferrodistortive structural phase transition at T(C) approximately 105 K. A line shape analysis of the thermal diffuse scattering results in the most precise experimental determination to date of the critical exponent gamma. The microscopic mechanism behind the anomalous "central peak" critical scattering component is clarified here by the first-ever observation of a static coherent diffraction pattern (speckle pattern) within the anomalous critical scattering of SrTiO3. This observation allows us to directly attribute the origins of the central peak to Bragg diffraction from remnant static disorder above T(C).     

Dynamics of Spin I=3/2 under Spin-Locking Conditions in an Ordered Environment

We have derived approximate analytic solutions to the master equation describing the evolution of the spin I=3/2 density operator in the presence of a radio-frequency (RF) field and both static and fluctuating quadrupolar interactions. Spectra resulting from Fourier transformation of the evolutions of the on-resonance spin-locked magnetization into the various coherences display two satellite pairs and, in some cases, a central line. The central line is generally trimodal, consisting of a narrow component related to a slowly relaxing mode and two broad components pertaining to two faster relaxing modes. The rates of the fast modes are sensitive to slow molecular motion. Neither the amplitude nor the width of the narrow component is affected by the magnitude of the static coupling, whereas the corresponding features of the broad components depend in a rather complicated manner on the spin-lock field strength and static quadrupolar interaction. Under certain experimental conditions, the dependencies of the amplitudes on the dynamics are seen to vanish and the relaxation rates reduce to relatively simple expressions. One of the promising emerging features is the fact that the evolutions into the selectively detected quadrupolar spin polarization order and the rank-two double-quantum coherence do not exhibit a slowly relaxing mode and are particularly sensitive to slow molecular motion. Furthermore, these coherences can only be excited in the presence of a static coupling and this makes it possible to discern nuclei in anisotropic from those in isotropic environment. The feasibility of the spin-lock pulse sequences with limited RF power and a nonvanishing average electric field gradient has been demonstrated through experiments on sodium in a dense lyotropic DNA liquid crystal.     

Dynamics of spin I=3/2 under spin-locking conditions in an ordered environment

We have derived approximate analytic solutions to the master equation describing the evolution of the spin I=3/2 density operator in the presence of a radio-frequency (RF) field and both static and fluctuating quadrupolar interactions. Spectra resulting from Fourier transformation of the evolutions of the on-resonance spin-locked magnetization into the various coherences display two satellite pairs and, in some cases, a central line. The central line is generally trimodal, consisting of a narrow component related to a slowly relaxing mode and two broad components pertaining to two faster relaxing modes. The rates of the fast modes are sensitive to slow molecular motion. Neither the amplitude nor the width of the narrow component is affected by the magnitude of the static coupling, whereas the corresponding features of the broad components depend in a rather complicated manner on the spin-lock field strength and static quadrupolar interaction. Under certain experimental conditions, the dependencies of the amplitudes on the dynamics are seen to vanish and the relaxation rates reduce to relatively simple expressions. One of the promising emerging features is the fact that the evolutions into the selectively detected quadrupolar spin polarization order and the rank-two double-quantum coherence do not exhibit a slowly relaxing mode and are particularly sensitive to slow molecular motion. Furthermore, these coherences can only be excited in the presence of a static coupling and this makes it possible to discern nuclei in anisotropic from those in isotropic environment. The feasibility of the spin-lock pulse sequences with limited RF power and a nonvanishing average electric field gradient has been demonstrated through experiments on sodium in a dense lyotropic DNA liquid crystal.     

Coherent control of phonons probed by time-resolved x-ray diffraction

Time-resolved x-ray diffraction with picosecond temporal resolution is used to probe the product state of a coherent control experiment in which a single acoustic mode in a bulk semiconductor is driven to large amplitude or canceled out. It is demonstrated that by shaping ultrafast acoustic pulses one can coherently control the x-ray diffraction efficiency of a crystal on the time scale of a vibrational period, with application to coherent switching of x-ray beams.     

Time-resolved X-Ray diffraction from coherent phonons during a laser-induced phase transition

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory coupled to analytic solutions for the laser-induced strain profile. The results are consistent with a 12 ps thermal electron-acoustic phonon coupling time together with an instantaneous component from the deformation-potential interaction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coherent atomic motion.     

Visualization of multi-scale turbulent structure in lobed mixing jet using wavelets

The two-dimensional orthogonal wavelet transform was applied to the LIF image of lobed mixing jet for identifying the multi-scale turbulent structures. The digital imaging slice photographs atz /D=1.0 and 1.5 withRe=3000 were respectively decomposed into seven image components with different broad scales. These image components provided the visualized information on the multi-scale structures in a lobed mixing turbulent jet. The cores and edges of the vortices and the coherent structures at different resolutions or scales can be easily extracted. It was found that the scale range of the coherent structure becomes narrow along the downstream direction. The size of the intermediate- and small-scale structures does not vary significantly with downstream distance.     

Optical study of charge dynamics in CaCo_2As_2

We present an infrared spectroscopy study of charge dynamics in CaCo_2As_2 single crystal. In this material, the optical conductivity can be described by two Drude components with different scattering rates(1/τ): a broad incoherent background and a narrow Drude component. By monitoring the temperature dependence, we find that only the narrow Drude component is temperature-dependent and determines the transport properties. Especially a Fermi liquid behavior of carriers is revealed by the T~2 behavior in the dc resistivity ρ_n and scattering rate 1/τ_n, indicating a coherent nature of quasiparticles in the narrow Drude subsystem.     

Structural and magnetic dynamics of a laser induced phase transition in FeRh

We use time-resolved x-ray diffraction and magneto-optical Kerr effect to study the laser-induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (τ(1)~30 ps). This is significantly faster than the magnetic response (τ(2)~60 ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases coexist on short time scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.     

Probing impulsive strain propagation with X-ray pulses

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes.     

Nanoscale imaging of buried structures with elemental specificity using resonant x-ray diffraction microscopy

We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of approximately 15 nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M5 edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens.     

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultrathin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this nonergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.     

Bulk dislocation core dissociation probed by coherent x rays in silicon

We report on a new approach to probe bulk dislocations by using coherent x-ray diffraction. Coherent x rays are particularly suited for bulk dislocation studies because lattice phase shifts in condensed matter induce typical diffraction patterns which strongly depend on the fine structure of the dislocation cores. The strength of the method is demonstrated by performing coherent diffraction of a single dislocation loop in silicon. A dissociation of a bulk dislocation is measured and proves to be unusually large compared to surface dislocation dissociations. This work opens a route for the study of dislocation cores in the bulk in a static or dynamical regime, and under various external constraints.     

Equilibrium and nonequilibrium dynamics of the sub-Ohmic spin-boson model

Employing the nonperturbative numerical renormalization group method, we study the dynamics of the spin-boson model, which describes a two-level system coupled to a bosonic bath with a spectral density J(omega) proportional to omega(s). We show that, in contrast with the case of Ohmic damping, the delocalized phase of the sub-Ohmic model cannot be characterized by a single energy scale only, due to the presence of a nontrivial quantum phase transition. In the strongly sub-Ohmic regime, s<1, weakly damped coherent oscillations on short time scales are possible even in the localized phase--this is of crucial relevance, e.g., for qubits subject to electromagnetic noise.     

Signature of a type-a glass transition and intrinsic confinement effects in a binary glass-forming system

We study dynamically highly asymmetric binary mixtures comprised of small methyl tetrahydrofuran (MTHF) molecules and polystyrene. Combined use of dielectric spectroscopy, ^{2}H nuclear magnetic resonance, incoherent quasielastic neutron scattering, and depolarized dynamic light scattering allows us to selectively probe the dynamics of the components in a broad dynamic range. It turns out that the mixtures exhibit two glass transitions in a wide concentration range although being fully miscible on a macroscopic scale. In between both glass transition temperatures, the dynamics of the small molecules show strong confinement effects, e.g., a crossover from Vogel-Fulcher to Arrhenius behavior of the time constants. Moreover, the dynamical behavior of small molecules close to the slow matrix is consistent with mode coupling theory predictions for a type-A glass transition, which was expected from recent theoretical and simulation studies in comparable systems.     

Coherent x-ray diffraction imaging with nanofocused illumination

Coherent x-ray diffraction imaging is an x-ray microscopy technique with the potential of reaching spatial resolutions well beyond the diffraction limits of x-ray microscopes based on optics. However, the available coherent dose at modern x-ray sources is limited, setting practical bounds on the spatial resolution of the technique. By focusing the available coherent flux onto the sample, the spatial resolution can be improved for radiation-hard specimens. A small gold particle (size <100 nm) was illuminated with a hard x-ray nanobeam (E=15.25 keV, beam dimensions approximately 100 x 100 nm2) and is reconstructed from its coherent diffraction pattern. A resolution of about 5 nm is achieved in 600 s exposure time.     

Femtosecond X-Ray fluorescence

Using few-cycle-driven coherent laser harmonics, K-shell vacancies have been created in light elements, such as boron (E(B) = 188 eV) and carbon (E(B) = 284 eV), on a time scale of a few femtoseconds for the first time. The capability of detecting x-ray fluorescence excited by few-femtosecond radiation with an accuracy of the order of 1 eV paves the way for probing the evolution of the microscopic environment of selected atoms in chemical and biochemical reactions on previously inaccessible time scales (<100 fs) by tracing the temporal evolution of the "chemical shift" of peaks associated with inner-shell electronic transitions in time-resolved x-ray fluorescence and photoelectron spectra.     

Consequences of a Cosmological Phase Transition at the TeV Scale

A finite vacuum energy density implies the existence of a UV scale for gravitational modes. This gives a phenomenological scale to the dynamical equations governing the cosmological expansion that must satisfy constraints consistent with quantum measurability and spatial flatness. Examination of these constraints for the observed dark energy density establishes a time interval from the transition to the present, suggesting major modifications from the thermal equations of state far from Planck density scales. The assumption that a phase transition initiates the radiation dominated epoch is shown under several scenarios to be able to produce fluctuations to the CMB of the order observed. Quantum measurability constraints (eg. uncertainly relations) define cosmological scales bounded by luminal expansion rates. It is shown that the dark energy can consistently be interpreted as being due to the vacuum energy of collective gravitational modes which manifest as the zero-point motions of coherent Planck scale mass units prior to the UV scale onset of gravitational quantum de-coherence for the cosmology. A cosmological model with multiple scales, one of which replaces an apparent cosmological “constant”, is shown to reproduce standard cosmology during intermediate times, while making the exploration of the early and late time cosmology more accessible. Talk presented at the 2006 biennial conference of the International Association for Relativistic Dynamics, June 12–14, University of Connecticut (Storrs).     

The discrete evolution of the structure of hydrogenated palladium-based alloys

The discrete character of the diffraction-angle dependence of displacements in the position of the components of diffraction peaks of hydrogenated Pd-Mo and Pd-Ta alloys is established using precise x-ray diffractometry. Such behavior is interpreted in terms of discontinuous transitions between long-lived metastable states under the assumption of a multivalley structure of the thermodynamic potential in k space. This assumption makes it possible to explain the fact that the components of diffraction reflections were narrow over the storage time (which was as long as several thousand hours).     

Chemical Vapor Deposition of Two-Dimensional PbS Nanoplates for Photodetection

Non-layered two-dimensional(2D) lead sulfide(PbS) has attracted growing interest recently due to its direct narrow bandgap(0.4 eV) and broad spectral detection from visible to mid-IR region, which lead to remarkable electronic and optoelectronic properties promising for real applications. We report the chemical vapor deposition growth of highly crystalline 2D PbS crystals on mica substrates. The high quality and uniformity of 2D PbS nanoplates are confirmed by atomic force microscopy, x-ray powder diffraction, transmission electron microscopy and x-ray photoelectron spectroscopy. The morphology and lateral size are controllable by different growth temperatures. Photodetectors made from 2D PbS nanoplates reveal good stability, high photoresponsivity, and fast response time, which indicates their promising applications for ultrathin optoelectronics.     

Nanoscale imaging with resonant coherent X rays: extension of multiple-wavelength anomalous diffraction to nonperiodic structures

The methodology of multiple-wavelength anomalous diffraction, widely used for macromolecular structure determination, is extended to the imaging of nonperiodic nanostructures. We demonstrate the solution of the phase problem by a combination of two resonantly recorded coherent scattering patterns at the carbon K edge (285 eV). Our approach merges iterative phase retrieval and x-ray holography approaches, yielding unique and rapid reconstructions. The element, chemical, and magnetic state specificity of our method further renders it widely applicable to a broad range of nanostructures, providing a spatial resolution that is limited, in principle, by wavelength only.