Search

The development of laser wakefield accelerators (LWFA) over the past several years has led to an interest in very compact sources of X-ray radiation – such as “table-top” free electron lasers. However, the use of conventional undulators using permanent magnets also implies system sizes which are large. In this work, we assess the possibilities for the use of novel mini-undulators in conjunction with a LWFA so that the dimensions of the undulator become comparable with the acceleration distances for LWFA experiments (i.e., centimeters). The use of a prototype undulator using laser machining of permanent magnets for this application is described and the emission characteristics and limitations of such a system are determined. Preliminary electron propagation and X-ray emission measurements are taken with a LWFA electron beam at the University of Michigan.

Recently, IceCube found evidence for a diffuse signal of astrophysical neutrinos in an energy range of $60\,\mathrm{TeV}$ to the $\mathrm{PeV}$-scale. The origin of those events, being a key to understanding the origin of cosmic rays, is still an unsolved question. So far, analyses have not succeeded to resolve the diffuse signal into point-like sources. Searches including a maximum-likelihood-ratio test, based on the reconstructed directions and energies of the detected down- and up-going neutrino candidates, were also performed on IceCube data leading to the exclusion of bright point sources. In this paper, we present two methods to search for faint neutrino point sources in three years of IceCube data, taken between 2008 and 2011. The first method is an autocorrelation test, applied separately to the northern and southern sky. The second method is a multipole analysis, which expands the measured data in the northern hemisphere into spherical harmonics and uses the resulting expansion coefficients to separate signal from background. With both methods, the results are consistent with the background expectation with a slightly more sparse spatial distribution, corresponding to an underfluctuation. Depending on the assumed number of sources, the resulting upper limit on the flux per source in the northern hemisphere for an $E^{-2}$ energy spectrum ranges from $1.5 \cdot 10^{-8}\,\mathrm{GeV}/(\mathrm{cm}^2 \mathrm{s})$, in the case of one assumed source, to $4 \cdot 10^{-10} \,\mathrm{GeV}/(\mathrm{cm}^2 \mathrm{s})$, in the case of $3500$ assumed sources., Comment: 35 pages, 16 figures, 1 table

The thesis of Chapter 7 is that the language of freedom and liberty in the seventeenth century had markedly different connotations from its modern usage and that this has significant implications for a radical language of liberty/freedom in the period. Rather than being seen as antithetical, liberty and authority were seen as complementary. Civil freedom was commonly identified with having a standing law to live by. In the religious sense, liberty was widely taken to imply submission to the will of a God of millennial purpose and providential power. The polarisation of liberty and authority in traditional accounts of the puritan revolution, the reduction of religious liberty to an element within a greater struggle over constitutional forms and the identification of religious liberty with individual and corporate rights are called in question.

The importance of the Norman Yoke, consequent upon the Norman Conquest, in the thinking of Gerrard Winstanley has been widely acknowledged, following Christopher Hill’s seminal essay on that subject. It was focal in the expropriation and exploitation of the labouring English, rendered landless by the Conquest. Winstanley’s strategies for overcoming the Yoke have commonly been seen as non violent. Chapter 4 argues that his thinking on violence both in the overthrow of the Conquest and in the achievement of a new social order and its defence was much more ambiguous than this would suggest.

In earlier work, a time-dependent, ionization dynamic model of a cluster of xenon atoms was constructed [2] , [3] in an effort to determine conditions under which the X-ray line amplification data that was observed experimentally at wavelengths between 2.71 and 2.88 A [1] could be replicated. Model calculations showed that, at laser intensities greater than 1019 W/cm2, the outermost N-shell electrons of xenon would be stripped away by tunnel ionization in less than a femtosecond. They also showed that L-shell electrons within the resulting cluster of Ni-like ions could be photoionized at a sufficient rate as to generate population inversions between these hole states and the states they radiatively decayed into. These inversions only lasted for several femtoseconds, and they were generated early in time when the cluster was being rapidly heated and the cluster's density was rapidly evolving, but was still high. They were seen to depend on the heating and expansion dynamics of the cluster, which had not been modeled in detail in this early work. In this paper, molecular dynamics calculations are described in which the rapidly evolving temperatures and ion densities of an intensely laser-heated cluster are calculated for different peak laser intensities and for two different sized xenon nano-clusters. This data is then used as an input to the ionization dynamic calculations in order to determine the influence of cluster size and of peak laser intensity on the gain coefficient calculations. In these calculations, inner-shell photoionization rates are shaped by the temperature and density dependence of the bremsstrahlung emissions under the assumption that these emissions drive the photoionizations. This shaping produces calculated gain coefficients that agree well with the measured ones.

The last several years have witnessed a surge of activity involving the interaction of clusters with intense ultrashort pulse lasers. The interest in laser–cluster interaction has not been only of academic interest, but also because of the wide variety of potential applications. Clusters can be used as a compact source of X-rays, incoherent as well as coherent, and of fast ions capable of driving a fusion reaction in deuterium plasmas. In one set of xenon cluster experiments, in particular, amplification of ∼2.8 A X-rays has been observed [ 28 ]. X-ray amplification in cluster media is a phenomenon of critical importance and may lead to applications such as EUV lithography, EUV and X-ray microscopy, X-ray tomography, and variety of applications in biology and material sciences. However, while amplification of ∼2.8 A X-rays has been documented in experiments, the mechanism for producing it remains to be fully understood. In this talk, a xenon model of laser–cluster interaction dynamics is presented to shed light on the processes responsible for amplification. The focus of this research is on the feasibility of creating population inversions and gain in some of the inner-shell hole state transitions within the M-shell of highly ionized xenon. The model couples a molecular dynamics (MD) treatment of the explosively-driven, non-Maxwellian cluster expansion to a comprehensive multiphoton-radiative ionization dynamic (ID) model including single- and double-hole state production within the Co- and Fe-like ionization stages of xenon. The hole-state dynamics is self-consistently coupled to a detailed valence-state collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunneling ionization rates that confirm an initial condition assumption that Ni-like ground states can be created almost instantaneously, on the order of a femtosecond or less, i.e., at laser intensities larger than 1019 W/cm2, all of the N-shell, n = 4 electrons are striped from a xenon atom in less than a femtosecond. Because of the abundance of these ground states, large numbers of n = 2, inner-shell hole states and large population inversions can be created when the Ni-like ground states are photo- or collisionally ionized. Once the M-shell is entered, tunneling ionization slows down as does collisional ionization due to the fall in ion density as the cluster expands. Moreover, as the cluster density goes down, our combined MD and ID calculations show that so do the calculated population inversions. Thus, our calculations do not support the initial experimental data interpretations in which the measured gains have been associated with double holes in more highly ionized stages of xenon (Xe32+, Xe34+, Xe35+, and Xe37+), which our calculations suggest would require laser intensities in excess of 1.5 × 1020 W/cm2, for a 248 nm, ∼250 fs laser pulse focused in a gas of xenon clusters. At laser intensities used in the experiment, such ionization stages would be reached, but only later in time when cluster densities have fallen by several orders of magnitude from their initial values to values where pumping rates are too low and gains cannot be generated.

Using a novel SQUID-based torsional oscillator (TO) technique to achieve increased sensitivity and dynamic range, we studied TO’s containing solid 4He. Below ∼250 mK, the TO resonance frequency f increases and its dissipation D passes through a maximum as first reported by Kim and Chan. To achieve unbiased analysis of such 4He rotational dynamics, we implemented a new approach based upon the generalized rotational susceptibility $\chi_{{}^{4}\mathrm{He}}^{ - 1}(\omega,T)$ . Upon cooling, we found that equilibration times within f(T) and D(T) exhibit a complex synchronized ultraslow evolution toward equilibrium indicative of glassy freezing of crystal disorder conformations which strongly influence the rotational dynamics. We explored a more specific $\chi_{{}^{4}\mathrm{He}}^{ -1}(\omega,\tau(T))$ with τ(T) representing a relaxation rate for inertially active microscopic excitations. In such models, the characteristic temperature T ∗ at which df/dT and D pass simultaneously through a maximum occurs when the TO angular frequency ω and the relaxation rate are matched: ωτ(T ∗)=1. Then, by introducing the free inertial decay (FID) technique to solid 4He TO studies, we carried out a comprehensive map of f(T,V) and D(T,V) where V is the maximum TO rim velocity. These data indicated that the same microscopic excitations controlling the TO motions are generated independently by thermal and mechanical stimulation of the crystal. Moreover, a measure for their relaxation times τ(T,V) diverges smoothly everywhere without exhibiting a critical temperature or velocity, as expected in ωτ=1 models. Finally, following the observations of Day and Beamish, we showed that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the 4He shear modulus. Together, these observations imply that ultra-slow equilibration of crystal disorder conformations controls the rotational dynamics and, for any given disorder conformation, the anomalous rotational responses of solid 4He are associated with generation of the same microscopic excitations as those produced by direct shear strain.

We use spectroscopic imaging scanning tunneling microscopy (SI-STM) to visualize the spatial symmetries of the electronic states that occur at the pseudogap energy scale in underdoped cuprates. We find evidence for the local intra-unit-cell electronic nematicity—by which we mean the disordered breaking of C4v symmetry within each CuO2 unit cell [1] . We also find that the coexisting incommensurate (smectic) electronic modulations couple to the intra-unit-cell nematicity through their 2 π topological defects [2] .

Superconducting excitations—Bogoliubov quasiparticles—are the quantum mechanical mixture of negatively charged electron (−e) and positively charged hole ( + e). Depending on the applied voltage bias in scanning tunneling microscope (STM) one can sample the particle and hole content of such a superconducting excitation. Recent STM experiments offer a unique insight into the inner workings of the superconducting state of superconductors. We propose a new observable quantity for STM studies that is the manifestation of the particle–hole dualism of the quasiparticles. We call it a Bogoliubov angle. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We propose that this quantity can be measured locally by comparing the ratio of tunneling currents at positive and negative biases locally. This Bogoliubov angle allows one to measure directly the energy and position dependent particle–hole admixture and therefore visualize robustness of superconducting state locally. It may also allow one to measure the particle–hole admixture of excitations in normal state above critical temperature and thus may be used to measure superconducting correlations in pseudogap state.

A tunable source of synchrotron radiation based on the interaction of a high intensity laser with small Xe clusters is studied. The synchrotron radiation yield for clusters with initial radii of 20 and 50 A using a KrF laser (248 nm) with a pulse duration of 83.3 fs and laser intensity varying from 1017 to 1020 W cm−2 was examined theoretically. Clusters irradiated by an intense femtosecond laser pulse produce x-rays more efficiently than solids or gases, because the electrons ionized by and oscillating in the electromagnetic field of the laser absorb more energy and radiate together. This paper calculates the conversion efficiency for a ~100 fs KrF laser pulse around 1019 W cm−2 on Xe clusters with nm sizes to be a few percent.

Since its discovery in 1986, understanding both the ground state and excited states of high temperature superconductivity (HTSC) has been one of the main challenges in modern condensed matter physics. One remarkable observation in HTSC cuprates is that the superconductivity appears as the localized electrons of a Mott insulator become itinerant due to carrier doping. Momentum sensitive probes like Angle Resolved Photo-Emission Spectroscopy (ARPES) and Inelastic Neutron Scattering (INS) have played important roles in mapping the momentum-space properties of the cuprate electronic structure. However, considering that cuprate superconductivity develops from atomically localized electrons and shows nanoscale disorder, a purely momentum dependent description is far from sufficient. Here, we describe novel spectroscopic imaging scanning tunneling microscopy (SI-STM) studies, which can probe the real-space electronic structure at the nano-scale, along with momentum-space electronic structure. We discuss a series of recent results from SI-STM studies of Bi 2 Sr 2 CaCu 2 O 8+ x and Ca 2- x Na x CuO 2 Cl 2 .

High-temperature superconductivity in the cuprates emerges when the localized electrons of a Mott insulator become mobile due to carrier doping. Understanding both the electronic ground state and the excited states of these systems are key challenges in physics today. Angle-resolved photoemission spectroscopy (ARPES) and inelastic neutron-scattering (INS) studies have been remarkably successful in mapping the momentum-space characteristics of the cuprate electronic structure. However, since cuprate superconductivity develops from atomically localized electrons and exhibits nanoscale disorder, a pure momentum-space description is unlikely to be sufficient. Instead, simultaneous information on electronic structure at the nanoscale in real space, and throughout momentum space, is required. Here, we describe a combination of novel spectroscopic imaging scanning tunneling microscopy (SI-STM) techniques that we have developed to achieve these apparently contradictory aims, along with the outcome of a series of SI-STM studies of the electronic structure of Bi2Sr2CaCu2O8+x.

Dark matter which is bound in the Galactic halo might self-annihilate and produce a flux of stable final state particles, e.g. high energy neutrinos. These neutrinos can be detected with IceCube, a cubic-kilometer sized Cherenkov detector. Given IceCube’s large field of view, a characteristic anisotropy of the additional neutrino flux is expected. In this paper we describe a multipole method to search for such a large-scale anisotropy in IceCube data. This method uses the expansion coefficients of a multipole expansion of neutrino arrival directions and incorporates signal-specific weights for each expansion coefficient. We apply the technique to a high-purity muon neutrino sample from the Northern Hemisphere. The final result is compatible with the null-hypothesis. As no signal was observed, we present limits on the self-annihilation cross-section averaged over the relative velocity distribution (Formula Presented.) down to ([Formula Presented.) for a dark matter particle mass of 700–1,000 GeV and direct annihilation into (Formula Presented.). The resulting exclusion limits come close to exclusion limits from γ-ray experiments, that focus on the outer Galactic halo, for high dark matter masses of a few TeV and hard annihilation channels., 0, SCOPUS: ar.j, info:eu-repo/semantics/published

X-rays emitted from high-density plasmas created by femtosecond laser pulses in the sub-keV to MeV energy regions are of great interest as diagnostics particularly for ICF plasmas. The incoherent X-rays radiated have a high-brightness due to the small size and short lifetime of the high energy density plasma. These radiation sources with short pulse durations are expected to provide high temporal resolution for x-ray absorption and other applications such as x-ray photoelectron spectroscopy used to measure dynamic processes in laser excited materials. We describe our theoretical model which couples time-dependent hot electron production with a non-equilibrium aluminum collisional-radiative model. The numerical simulations provide the optimum parameters (laser intensity and target thickness) for maximizing the efficiency of converting 40-fs of incident energy from HERCULES laser into K α and K-shell-line aluminum emission. Predictions as a diagnostic tool relevant to future experiments will be presented.

Atomic physics model of aluminum has been incorporated into a 2D3V PIC code to study the interaction of an intense laser with thin solid targets. Each ionization stage of Al contains one ground and one lumped excited state, for which atomic physics processes such as optical field and collisional ionization, excitation, de-excitation and radiative decay describe the population density. Radiation emitted during the laser-target interaction is computed by accounting for both bound-bound transitions and Bremsstrahlung radiation. Using 2D PIC simulations for laser pulses with intensity 3×1020 W/cm2, duration 40 fs, spot size 5 µm and energy 1 J interacting with ultrathin (0.2 µm) Al foil, we demonstrate that the radiation signature of laser-produced plasma can be used as a complementary tool to other diagnostic techniques used in laser-plasma interactions.

We have previoslly reported a 1.36-nm DOS modulation in the CuO chains on the surface of cold-cleaved, atomically-resolved YBCO 1,2 . We have recently completed new experiments in which we obtained detailed spectroscopic data on the same crystal plane, thereby confirming and extending our earlier work. Here we present data showing twin boundaries and steps, and focusing on the effects of these structures on the CuO chain DOS modulations.

We report on low temperature scanning tunneling microscopy (STM) studies of the electronic structure of vortex cores in Bi 2Sr 2CaCu 2O (8+delta). At the vortex core center, an enhanced density of states is observed at energies near Omega = +/-7 meV. Spectroscopic imaging at these energies reveals an exponential decay of these "core states" with a decay length of 22+/-3 A. The fourfold symmetry sometimes predicted for d-wave vortices is not seen in spectroscopic vortex images. A locally nodeless order parameter induced by the magnetic field may be consistent with these measurements.

Ultrafast laser pulses with complex envelopes (amplitude and frequency modulated) are used to excite an optically dense column of rubidium vapor. Pulse reshaping, stimulated emission dynamics, and residual electronic excitation in the Rb vapor are all shown to depend strongly on the laser pulse shape. Pulses that produce adiabatic passage in the optically thin limit exhibit more complex behavior in optically thick samples, including an unexpected dependence on the sign of the frequency sweep. Numerical solutions of the Maxwell-Bloch equations are shown to account for our results. [S0031-9007(99)09123-1]

We describe the design and development of a scanning tunneling microscope (STM) which can operate at temperatures down to 240 mK and in magnetic fields up to 7 T with high spatial and energy resolution. The compact and rigid STM head is mounted directly on a low vibration, single shot, 3He refrigerator. This refrigerator can be operated at its base temperature continuously for several days before the 3He needs to be recondensed. The system is equipped with a sample transport manipulator from room temperature, and a cleavage device at low temperature, so that the cryogenic ultrahigh vacuum condition inside the cryostat can be utilized. A superconducting magnet provides a magnetic field of up to 7 T at the sample along the STM tip direction. Test results have shown that, at the base temperature, this instrument has better than 0.5 pm z-direction resolution in imaging mode, and better than 20 μV energy resolution in spectroscopy mode.

The IceCube project has transformed one cubic kilometer of deep natural Antarctic ice into a Cherenkov detector. Muon neutrinos are detected and their direction inferred by mapping the light produced by the secondary muon track inside the volume instrumented with photomultipliers. Reconstructing the muon track from the observed light is challenging due to noise, light scattering in the ice medium, and the possibility of simultaneously having multiple muons inside the detector, resulting from the large flux of cosmic ray muons. This manuscript describes work on two problems: (1) the track reconstruction problem, in which, given a set of observations, the goal is to recover the track of a muon; and (2) the coincident event problem, which is to determine how many muons are active in the detector during a time window. Rather than solving these problems by developing more complex physical models that are applied at later stages of the analysis, our approach is to augment the detectors early reconstruction with data filters and robust statistical techniques. These can be implemented at the level of on-line reconstruction and, therefore, improve all subsequent reconstructions. Using the metric of median angular resolution, a standard metric for track reconstruction, we improve the accuracy in the initial reconstruction direction by 13%. We also present improvements in measuring the number of muons in coincident events: we can accurately determine the number of muons 98% of the time., Comment: 14 pages, 4 figures, submitted to Nuclear Instruments and Methods in Physics Research Section A

Generation of heavy ion beams from solid targets irradiated by intense short laser pulses is studied. The interest is driven by the ion beam's potential for many groundbreaking applications in scientific, technological, and medical areas [1,2]. Numerical modeling using relativistic implicit Particle-in-Cell (PIC) code has been performed to study mechanisms of heavy ion acceleration. The study aims to model the efficient production of 100s of MeV heavy ions on the Titan laser at LLNL. We explore the conventional target normal sheath acceleration (TNSA), as well as more advanced mechanisms such as the radiation pressure acceleration (RPA). We studied the energy scaling from ultrathin foils made of low-, mid- and high-z material and determined that the kinetic energy of ions increases with ion mass, while the charge-to-mass ratio gradually decreases from ~0.5 for light ions (carbon) to ~0.3 for heavy ions (gold).

Experiments at the HERCULES laser facility, originally reported by C. Zulick, et al in Applied Physics Letters (2013), have produced neutron beams with energies up to 16:8(+/- 0:3) MeV using Li-7(3)(d,n)(4)Be-8 reactions. These efficient deuteron reactions required the selective acceleration of deuterons through the introduction of a deuterated plastic or cryogenically frozen D2O layer on the surface of a thin film target. It was shown that a optimized frozen D2O layer, formed in situ, yielded the highest efficiency deuteron acceleration with deuterons constituting over 99% of the accelerated light ions. The deuteron signal was optimized with respect to the delay between the heavy water deposition and laser pulse arrival, as well as the temperature of the target. A total conversion efficiency of laser energy to neutron energy of 1(+/- 0:5) x 10(-5) was obtained. The simulated neutron signal was found to be in reasonable agreement with the experimental spectra. The scattering of neutrons through shielding and target materials was investigated with MCNPX and determined to have a small effect on the observed neutron energies.

We present the first results in the search for relativistic magnetic monopoles with the IceCube detector, a subsurface neutrino telescope located in the South Polar ice cap containing a volume of 1 km$^{3}$. This analysis searches data taken on the partially completed detector during 2007 when roughly 0.2 km$^{3}$ of ice was instrumented. The lack of candidate events leads to an upper limit on the flux of relativistic magnetic monopoles of $\Phi_{\mathrm{90%C.L.}}\sim 3\e{-18}\fluxunits$ for $\beta\geq0.8$. This is a factor of 4 improvement over the previous best experimental flux limits up to a Lorentz boost $\gamma$ below $10^{7}$. This result is then interpreted for a wide range of mass and kinetic energy values., Comment: 11 pages, 11 figures. v2 is minor text edits, no changes to result

The X-ray emission from an ultrashort-pulse laser incident on a solid aluminum and a layered aluminum /silicon slab is investigated with a non-LTE radiation-hydrodynamics model coupled to a Helmholtz wave equation for P-wave polarization. A fraction of the absorbed energy is expended in the production of fast electrons, which are transported and deposited in the cold material. These electrons create K-shell vacancies that produce characteristic Kα line radiation. A time history of the emission spectra, including the Kα lines and continuum radiation, is presented for a laser intensity of 1017 W/cm2.

Short pulse laser interactions at intensities of 2×10(21) W cm(-2) with ultrahigh contrast (10(-15)) on submicrometer silicon nitride foils were studied experimentally by using linear and circular polarizations at normal incidence. It was observed that, as the target decreases in thickness, electron heating by the laser begins to occur for circular polarization leading to target normal sheath acceleration of contaminant ions, while at thicker targets no acceleration or electron heating is observed. For linear polarization, all targets showed exponential energy spreads with similar electron temperatures. Particle-in-cell simulations demonstrate that the heating is due to the rapid deformation of the target that occurs early in the interaction. These experiments demonstrate that finite spot size effects can severely restrict the regime suitable for radiation pressure acceleration.

The detection of acoustic signals from ultra-high energy neutrino interactions is a promising method to measure the tiny flux of cosmogenic neutrinos expected on Earth. The energy threshold for this process depends strongly on the absolute noise level in the target material. The South Pole Acoustic Test Setup (SPATS), deployed in the upper part of four boreholes of the IceCube Neutrino Observatory, has monitored the noise in Antarctic ice at the geographic South Pole for more than two years down to 500 m depth. The noise is very stable and Gaussian distributed. Lacking an in-situ calibration up to now, laboratory measurements have been used to estimate the absolute noise level in the 10 to 50 kHz frequency range to be smaller than 20 mPa. Using a threshold trigger, sensors of the South Pole Acoustic Test Setup registered acoustic pulse-like events in the IceCube detector volume and its vicinity. Acoustic signals from refreezing IceCube holes and from anthropogenic sources have been used to localize acoustic events. Monte Carlo simulations of sound propagating from the established sources to the SPATS sensors have allowed to check corresponding model expectations. An upper limit on the neutrino flux at energies $E_��> 10^{11}$ GeV is derived from acoustic data taken over eight months., 17 pages, 14 figures

The last several years have witnessed an explosion of activity involving the interaction of clusters with intense ultrashort pulse lasers. The interest in laser-cluster interaction has not been only of academic interest, but also because of the wide variety of potential applications. Clusters can be used as a compact source of X-rays, incoherent as well as coherent radiation, and fast ions capable of driving a fusion reaction in deuterium plasma. Some of the applications include EUV lithography, EUV and X-ray microscopy, X-ray tomography and a variety of applications in biology and material sciences. In this talk a model of laser-cluster interaction dynamics is presented that describes the process of amplification from the incident laser pulse to the final gain calculations. The focus of this research is on the feasibility of creating population inversions and gain in some of the inner shell hole state transitions in highly ionized Xe. The model couples a molecular dynamics treatment of the explosively-driven cluster expansion to a comprehensive multiphoton-radiative ionization model including single- and double-hole state production within the Co-and Fe-like ionization stages of Xe. The hole state dynamics is self-consistently coupled to the valence-state collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunneling ionization rates that confirm the initial condition assumption that Ni-like ground states are created almost instantaneously, the creation of which is needed to support the interpretations of the measured x-ray data. With the use of tunneling ionization rates, all of the N-shell, n=4 electrons are striped from a xenon atom in less than a femtosecond at laser intensities larger than 1019W/cm2. Thus, our calculations do not support the initial experimental data interpretations in which the measured gains have been associated with double holes in more highly ionized stages of xenon (Xe32+, Xe34+, Xe35+, and Xe37+).

The new generation of ultrashort pulse lasers offers a novel way of producing high-energy (1–100 MeV) neutrons from table-top devices. Ultrashort lasers of sufficiently high intensity (1018–1021 W/cm2) are capable of producing fast ions with energies in the megaelectronvolt (MeV) range from thin (∼1–10 µm) planar foils, which drive nuclear fusion reactions and generate a copious amount of neutrons. The Naval Research Laboratory (NRL) in collaboration with the Center for Ultrafast Optical Science (CUOS) at the University of Michigan have been performing basic and applied research on the development of laser-based non-thermal neutron sources. The program's focus is on production of collimated neutron beams with energy in the 10's of MeV at moderate peak fluence (∼106–109 neutrons/ster per shot). We study the neutron production from protons/deuterons impinging on low-Z material and compare the neutron yield with that from traditional sources such as d-d and d-t. Recent simulation results and experimental data on both light ion acceleration and ion-induced nuclear fusion reactions driven by high-intensity lasers will be presented [1,2]. The mechanism of the neutron production process will be discussed with emphasis on the underlying fundamental physics and numerical implementation, as well as technological issues and challenges associated with the neutron production. An introduction to the current state of the art will be given and possible applications will be discussed.

By using temporal pulse shaping of high-contrast, short pulse laser interactions with solid density targets at intensities of 2 x 10(21) W cm(-2) at a 45 degrees incident angle, we show that it is possible to reproducibly generate quasimonoenergetic proton and ion energy spectra. The presence of a short pulse prepulse 33 ps prior to the main pulse produced proton spectra with an energy spread between 25% and 60% (Delta E/E) with energy of several MeV, with light ions becoming quasimonoenergetic for 50 nm targets. When the prepulse was removed, the energy spectra was broad. Numerical simulations suggest that expansion of the rear-side contaminant layer allowed for density conditions that prevented the protons from being screened from the sheath field, thus providing a low energy cutoff in the observed spectra normal to the target surface.

Scanning tunneling microscopy is used to image the additional quasi-particle states generated by quantized vortices in the high critical temperature superconductor Bi2Sr2CaCu2O8+δ. They exhibit a copper-oxygen bond–oriented “checkerboard” pattern, with four unit cell (4a0) periodicity and a ∼30 angstrom decay length. These electronic modulations may be related to the magnetic field–induced, 8a0periodic, spin density modulations with decay length of ∼70 angstroms recently discovered in La1.84Sr0.16CuO4. The proposed explanation is a spin density wave localized surrounding each vortex core. General theoretical principles predict that, in the cuprates, a localized spin modulation of wavelength λ should be associated with a corresponding electronic modulation of wavelength λ/2, in good agreement with our observations.

In the high-transition-temperature (high-Tc) superconductors the pseudogap phase becomes predominant when the density of doped holes is reduced1. Within this phase it has been unclear which electronic symmetries (if any) are broken, what the identity of any associated order parameter might be, and which microscopic electronic degrees of freedom are active. Here we report the determination of a quantitative order parameter representing intra-unit-cell nematicity: the breaking of rotational symmetry by the electronic structure within CuO2 unit cell. We analyze spectroscopic-imaging scanning tunneling microscope images of the intra-unit-cell states in underdoped Bi2Sr2CaCu2O8+{\delta} and, using two independent evaluation techniques, find evidence for electronic nematicity of the states close to the pseudogap energy. Moreover, we demonstrate directly that these phenomena arise from electronic differences at the two oxygen sites within each unit cell. If the characteristics of the pseudogap seen here and by other techniques all have the same microscopic origin, this phase involves weak magnetic states at the O sites that break 90o -rotational symmetry within every CuO2 unit cell., Comment: See the Nature website for the published version. High-resolution version of figures, supplementary information and supplementary movies are available at http://eunahkim.ccmr.cornell.edu/KimGroup/highlights.html

We present results from initial large diameter wire array experiments at the refurbished Z generator. 65mm on 32.5mm nested stainless steel wire arrays were fielded at two different total masses (2.5mg & 5mg). Each array was 20mm tall and had an inner array half the mass of the outer. The experiments were designed to evaluate trends in Fe K-shell power and yield and re-establish previous output, and large diameter tungsten wire arrays are further planned to study total x-ray output.

The intriguing idea that strongly interacting electrons can generate spatially inhomogeneous electronic liquid crystalline phases is over a decade old, but these systems still represent an unexplored frontier of condensed matter physics. One reason is that visualization of the many-body quantum states generated by the strong interactions, and of the resulting electronic phases, has not been achieved. Soft condensed matter physics was transformed by microscopies that allowed imaging of real-space structures and patterns. A candidate technique for obtaining equivalent data in the purely electronic systems is Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM). The core challenge is to detect the tenuous but 'heavy' k-space components of the many-body electronic state simultaneously with its r-space constituents. Sr3Ru2O7 provides a particularly exciting opportunity to address these issues. It possesses (i) a very strongly renormalized 'heavy' d-electron Fermi liquid and (ii) exhibits a field-induced transition to an electronic liquid crystalline phase. Finally, as a layered compound, it can be cleaved to present an excellent surface for SI-STM., Comment: Some errors fixed

Superconducting excitations --Bogoliubov quasiparticles -- are the quantum mechanical mixture of negatively charged electron (-e) and positively charged hole (+e). Depending on the applied voltage bias in STM one can sample the particle and hole content of such a superconducting excitation. Recent Scanning Tunneling Microscope (STM) experiments offer a unique insight into the inner workings of the superconducting state of superconductors. We propose a new observable quantity for STM studies that is the manifestation of the particle-hole dualism of the quasiparticles. We call it a {\em Bogoliubov angle}. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We argue that this quantity can be measured locally by comparing the ratio of tunneling currents at positive and negative biases. This Bogoliubov angle allows one to measure directly the energy and position dependent particle-hole admixture and therefore visualize robustness of superconducting state locally. It may also allow one to measure the particle-hole admixture of excitations in normal state above critical temperature and thus may be used to measure superconducting correlations in pseudogap state., 16 pages, latex file, 9 eps figures

Summary form only given. A variety of wire array experiments in the last few years at the 20 MA Z accelerator have been performed to assess the impact of initial load mass, initial load diameter, and variations of the nested array configuration on the K-shell output. Nominally, optimized configurations have been identified, with optimization determined by the highest K-shell output with the fastest rising, narrowest X-ray pulse. Experimental and computational work has suggested that other load configurations may continue to improve the output. For example, experiments with W arrays have found that closure of the radial anode-cathode (RAK) gap may be reducing the radiated output achievable and wire number studies with single arravs have indicated that higher wire number loads could further increase output. Additionally, the impact of wire orientation on the large (> 1 mm) interwire gap nested loads has not been assessed. In this paper, the results of experiments performed to evaluate these configuration changes are presented. For stainless steel wire arrays (K-shell emission 5.6-7.7 keV), increasing the wire number on the nested arrays resulted in increased K-shell yield and K-shell power. Increasing the RAK gap, however, from 6 mm to 10 mm did not impact the K-shell emission, although changes were observed in the soft X-ray emission. The orientation of the wires on the inner and outer arrays of Cu wire arrays was not observed to impact the radiated output, although calculations suggest that the effect of wire orientation will be overwhelmed by magnetic field asymmetries induced by the wire location relative to the openings in the return current can. Stagnated plasma conditions from these experiments are discussed and compared with the conditions observed from previous experiments. Computational results for these configurations are also presented.

We report on the study of atomically sharp superconducting tips for scanning tunneling microscopy and spectroscopy. The results clearly show vacuum tunneling of superconducting quasiparticles from atomically sharp tips. Observed deviations of the energy gap of the superconducting tip from its bulk value are attributed to the proximity effect. We show that a combination of a superconducting tip and an atomic resolution scanning tunneling microscope provides a means of achieving very high resolution local spectroscopy. We also discuss how this combination paves the way for a number of important applications.

Imaging of the surface of a cleaved Bi2Sr2CaCu2O8+δ (BSCCO) single crystal with a scanning tunneling microscope reveals a series of repeating terraces, whose separations are then used to identify the atomic planes which are exposed. On each of the exposed planes, the incommensurate modulation is also clearly resolved with atomic resolution. The measured separations between the terraces lead to the deduction that any atomic layer can be exposed by mechanical cleavage of BSCCO. We, therefore, suggest that the identity of atomic planes, and the direction of tunneling, should always be taken into consideration when interpreting tunneling spectra obtained on such cleaved BSCCO crystals.

We present studies of the electronic structure of La2-xBaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x=1/8. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x=1/8. Thus, the non-superconducting, "striped" state at x=1/8 is consistent with a phase incoherent d-wave superconductor whose Cooper pairs form spin/charge ordered structures instead of becoming superconducting., This is the author's version of the work

Over the last several years, the Z Accelerator has been engaged in research on near-Planckian x-ray sources for inertial confinement fusion and on K-shell emitting sources for radiation-material interaction studies. These radiating z pinches exhibit complex dynamics that have been, and continue to be, studied over a wide range of configurations. In this paper, the progress to date for the production of tens to hundreds of kilojoules of K-shell emission from 8 keV to 3 keV will be presented. Nested wire arrays and multi-shell gas puffs have been employed to help mitigate implosion instabilities and asymmetries to produce x-ray powers of tens of terawatts with emitted x-ray risetimes of a few nanoseconds. Spectroscopy and modeling of these pinches are providing insight into the role of temperature and density gradients and other plasma phenomena in the production of the radiation. Future directions will also be discussed.

Recent scanning tunneling microscopy (STM) experimentally observed strong gap inhomogeneity in Bi2Sr2CaCu2O(8+delta) (BSCCO). We argue that disorder in the pair potential underlies the gap inhomogeneity, and investigate its role in the Fourier-transformed inelastic tunneling spectra as revealed in the STM. We find that the random pair potential induces unique q-space patterns in the local density of states (LDOS) of a d-wave superconductor. We consider the effects of electron coupling to various bosonic modes and find the pattern of LDOS modulation due to coupling to the B(1g) phonon mode to be consistent with the one observed in the inelastic electron tunnneling STM experiment in BSCCO. These results suggest strong electron-lattice coupling as an essential part of the superconducting state in high-Tc materials.

We analyze the effects of the electronic coupling to bosonic modes in a d-wave superconductor. The role of the scattering due to boson on the momentum transfer between electronic states in the Brilloine zone is addressed. We consider specific examples of $B_{1g}$ phonon, breathing mode phonon and spin resonance at $(\pi,\pi)$. The Fourier spectrum of the energy derivative local density of states (LDOS) is calculated. To properly calibrate the effects of different modes we fix the quasipartilce renormalization at specific momentum points. It is found that the $B_{1g}$ mode with highly anisotropic momentum-dependent coupling matrix element gives rise to well definded features in the Fourier spectrum, at the energy of mode plus gap, with a momentum transfer along the Cu-O bond direction of cuprates. This result is in a striking contrast to the cases of the coupling to other modes and also to the case of no mode coupling. The origin of this difference is explored in detail. A comparison with the recent STM experiments is briefly discussed., Comment: 9 pages, 4 eps figures included

The doping dependence of nanoscale electronic structure in superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ is studied by scanning tunneling microscopy. At all dopings, the low energy density-of-states modulations are analyzed according to a simple model of quasiparticle interference and found to be consistent with Fermi-arc superconductivity. The superconducting coherence peaks, ubiquitous in near-optimal tunneling spectra, are destroyed with strong underdoping and a new spectral type appears. Exclusively in regions exhibiting this new spectrum, we find local ``checkerboard'' charge ordering of high energy states, with a wave vector of $\stackrel{\ensuremath{\rightarrow}}{Q}=(\ifmmode\pm\else\textpm\fi{}2\ensuremath{\pi}/4.5{a}_{0},0)$; $(0,\ifmmode\pm\else\textpm\fi{}2\ensuremath{\pi}/4.5{a}_{0})\ifmmode\pm\else\textpm\fi{}15%$. Surprisingly, this spatial ordering of high energy states coexists harmoniously with the low energy Bogoliubov quasiparticle states.

The controlled formation in an underdense plasma of stable multi-PW relativistic micrometer-scale channels, which conduct a confined power at 248 nm exceeding 10{sup 4} critical powers and establish a peak channel intensity of {approx}10{sup 23} W/cm{sup 2}, can be achieved with the use of an appropriate gradient in the electron density in the initial launching phase of the confined propagation. This mode of channel formation optimizes both the power compression and the stability by smoothing the transition from the incident spatial profile to that associated with the lowest channel eigenmode, the dynamically robust structure that governs the confined propagation. A chief outcome is the ability to stably conduct coherent energy at fluences greater than 10{sup 9} J/cm{sup 2}.

Summary form only given. Rapid heating is required to ionize moderate atomic number plasmas into the K-shell. When a plasma shell is imploded, this heating occurs when the implosion kinetic energy is thermalized. However, if a plasma shell is imploded onto a plasma cylinder, rapid heating of the cylinder occurs by strong shock wave generation and compressive heating. In this talk, we investigate two kinds of shell-on-cylinder implosions. In one case, an Al plasma is imploded onto an Fe plasma. In a second case, Fe-on-Fe implosions are investigated to benchmark the calculations to some stainless steel data that was obtained in nested wire array experiments conducted on the Z accelerator at Sandia National Laboratories. The replacement of iron by an equal mass of Al plasma in the imploding shell should have a small impact on the heating of and X-ray emissions from the inner Fe core plasma, but it adds the possibility of generating kilovolt emissions from the imploding Al. In the Al-on-Fe calculations, we first determine the implosion kinetic energies that are needed to maximize the production of Fe K-shell emission, and then evaluate the capability of the Z and ZR generators to meet these requirements. How L-shell emissions impact the ionization dynamics of Fe is investigated along with the problem of optimizing the collateral production of super-kilovolt X-rays from the K-shell of Al.

We present scanning tunneling spectroscopy measurements of the CuO chain plane in YBa2Cu3O6+x, showing a similar to25 meV gap in the local density of states (LDOS) filled by numerous intragap resonances: intense peaks in LDOS spectra associated with one-d

It has been observed over the years that the energy coupled to the load in many zpinch experiments is larger than can be accounted for by the sum of the j×B work and classical Ohmic heating. Moreover, this energy enhancement appears to be a function of the generator design, increasing as the risetime of the current is increased. In recent experiments on the Saturn generator, for example, which was operated at current risetimes in excess of 160 ns, observed energy enhancements were factors of 2 to 4 times the energy input expected from J×B work alone. When Saturn operates with risetimes of less than 90 ns, much smaller energy enhancements over the J×B energy are seen. In the past, it was conjectured that some form of anomalous resistivity was needed to account for the extra energy input, while recently, a new idea was proposed based on the buildup of internally generated tubes of magnetic flux energy. [1,2] It was hypothesized that the growth of the Rayleigh-Taylor instability at the surface of the z-pinch plasma would generate bubbles of magnetic flux-tube energy that deposit their energy in the plasma at a current-to-the-third-power rate. While 0-D modeling of the Saturn experiments shows that an anomalously high load resistance can input the required energy needed to match the x-ray data, an alternate mechanism than magnetic flux-tubes exists for anomalous heating that is based on the production of micro-instabilities at the pinch surface. Both this and the flux-tube model are phenomenological and require guidance from experiments to be implemented. Several issues that arise from these enhanced energy coupling mechanisms are discussed in this paper.

Scanning tunneling microscopy is used to image the additional quasiparticle states generated by quantized vortices in the high-Tc superconductor Bi2Sr2CaCu2O8+d. They exhibit a Cu-O bond oriented 'checkerboard' pattern, with four unit cell (4a0) periodicity and a ~30 angstrom decay length. These electronic modulations may be related to the magnetic field-induced, 8a0 periodic, spin density modulations of decay length ~70 angstroms recently discovered in La1.84Sr0.16CuO4. The proposed explanation is a spin density wave localized surrounding each vortex core. General theoretical principles predict that, in the cuprates, a localized spin modulation of wavelength L should be associated with a corresponding electronic modulation of wavelength L/2, in good agreement with our observations., Comment: 10 pages, 3 figures

In conventional superconductors, magnetic interactions and magnetic impurity atoms are destructive to superconductivity. By contrast, in some unconventional systems, e.g. superfluid 3He and superconducting UGe2, superconductivity or superfluidity is actually mediated by magnetic interactions. A magnetic mechanism has also been proposed for high temperature superconductivity (HTSC) in which an electron magnetically polarizes its environment resulting in an attractive pairing-interaction for oppositely polarized spins. Since a magnetic impurity atom would apparently not disrupt such a pairing-interaction, it has also been proposed that the weaker influences on HTSC of magnetic Ni impurity atoms compared to those of non-magnetic Zn are evidence for a magnetic mechanism. Here we use scanning tunneling microscopy (STM) to determine directly the influence of individual Ni atoms on the electronic structure of Bi2Sr2CaCu2O8+d. Two local d-wave impurity-states are observed at each Ni. Analysis of their energies surprisingly reveals that the primary quasiparticle scattering effects of Ni atoms are due to non-magnetic interactions. Nonetheless, we also demonstrate that a magnetic moment coexists with unimpaired superconductivity at each Ni site. We discuss the implications of these phenomena, and those at Zn, for the pairing-mechanism., Comment: 13 pages, 4 figures

Although their crystal structures are complex, all high temperature superconductors contain some crystal planes consisting of only Cu and O atoms in a square lattice. Superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substitution of a single impurity atom at a Cu site creates a simple but powerful perturbation to these interactions. Detailed knowledge of the effects of such an impurity atom on the superconducting order parameter and on the quasi-particle local density of states (LDOS) could allow competing theories of high temperature superconductivity (HTSC) to be tested at the atomic scale. The fundamental implications of results from numerous bulk measurements on samples doped with impurity atoms could also be clarified with such data. Here we describe scanning tunneling microscopy studies of the effects of individual Zn impurity atoms located at the Cu site in the high-Tc superconductor Bi2Sr2CaCu2O8+delta. Tunneling spectroscopy shows intense quasi-particle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within about 1.5 nm. Imaging of the quasi-particle LDOS at these sites reveals the long sought four-fold symmetric "quasi-particle cloud" aligned with the d-wave gap nodes. Several unexpected phenomena, which can shed new light on the atomic-scale response of HTSC to a probe impurity atom, are also observed., Comment: 18 pages, 4 figures