Search Results (121 total)

Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.

The mean-lifetimes, τ, of various medium-spin excited states in ¹⁰³Pd and ^106,107Cd have been deduced using the Recoil Distance Doppler Shift technique and the Differential Decay Curve Method. In ¹⁰⁶Cd, the mean-lifetimes of the I^π=12⁺ state at E_x=5418 keV and the I^π=11^- state at E_x=4324 keV have been deduced as 11.4(17)ps and 8.2(7)ps, respectively. The associated β₂ deformation within the axially-symmetric deformed rotor model for these states are 0.14(1) and 0.14(1), respectively. The β₂ deformation of 0.14(1) for the I^π=12⁺ state in ¹⁰⁶Cd compares with a predicted β₂ value from total Routhian surface (TRS) calculations of 0.17. In addition, the mean-lifetimes of the yrast I^π=15 / 2^- states in ¹⁰³Pd (at E_x=1262 keV) and ¹⁰⁷Cd (at E_x=1360 keV) have been deduced to be 31.2(44)ps and 31.4(17)ps, respectively, corresponding to β₂ values of 0.16(1) and 0.12(1) assuming axial symmetry. Agreement with TRS calculations are good for ¹⁰³Pd but deviate for that predicted for ¹⁰⁷Cd.

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.

The combined data of two Coulomb excitation experiments has verified the purely electromagnetic population of the K^π=4⁺,6⁺,8^-, and 16⁺ rotational bands in ¹⁷⁸Hf via 2≤ν≤14 K-forbidden transitions, quantifying the breakdown of the K-selection rule with increasing spin in the low-K bands. The γ-, 4⁺, and 6⁺ bands were extended, and four new states in a rotational band were tentatively assigned to a previously known K^π=0⁺ band. The quasiparticle structure of the 6⁺ (t_{1 / 2}=77 ns) and 8^- (t_{1 / 2}=4 s) isomer bands were evaluated, showing that the gyromagnetic ratios of the 6⁺ isomer band are consistent with a pure π7 / 2⁺[404],π5 / 2⁺[402] structure. The 8^- isomer band at 1147 keV and the second 8^- band at 1479 keV, thought to be predominantly ν7 / 2^-[514],ν9 / 2⁺[624] and π9 / 2^-[514],π7 / 2⁺[404], respectively, are mixed to a degree approaching the strong-mixing limit. Based on measured 〈K^π=16⁺‖E2‖K^π=0⁺〉 matrix elements, it was shown that heavy-ion bombardment could depopulate the 16⁺ isomer at the ~1% level, although no states were found that would mediate photodeexcitation of the isomer via low-energy x-ray absorption.

Single crystal x-ray diffraction (XRD), high-resolution electron holography and density functional calculations (DFT) are employed to investigate single-crystalline BaTiO₃ in the noncentrosymmetric tetragonal phase. From XRD and DFT the structure parameters, the electron density and corresponding properties, such as atomic charges and the dipole moment are determined. For this purpose the maximum-entropy method was utilized to get accurate electron densities in the case of XRD, whereas all-electron calculations were performed in the framework of DFT. A comparison of experimental results and density functional calculations yield a rather good agreement. The electron density distributions are used to determine the “natural” unit cell corresponding to the neutral boundary cells of the whole crystal and its dipole moment, providing the boundary conditions necessary for calculating the electrostatic potential within the unit cell through the Poisson equation. The electrostatic potential was then utilized to perform electron scattering simulations within the framework of the Multislice formalism, resembling unique features of experimentally recorded electron holograms. It is shown that the phase wedge in the scattered wave, which is due to the polarization field within the specimen, is essential for the image reconstruction. This essential feature has not been included in simulations before.

The nature of 0⁺ excitations, especially in transitional and deformed nuclei, has attracted new attention. Following a recent experiment studying ¹⁵⁸Gd, we investigated a large group of nuclei in the rare-earth region with the (p,t) pickup reaction using the Q3D magnetic spectrograph at the University of Munich MP tandem accelerator laboratory. Outgoing tritons were recorded at various lab angles, and their angular distributions are compared to those calculated using the distorted-wave Born approximation. Using the unique shape of the L=0 angular distribution, more than double the number of 0⁺ states than were previously known are identified. The distribution of 0⁺ energies and cross sections is discussed in terms of collective and noncollective degrees of freedom, and the density of low-lying 0⁺ states is discussed as a corroboration of a characteristic feature of phase transition regions. The degree of level mixing, as extracted from Brody distribution fits to the energy spacings of adjacent 0⁺ levels, is also explored.

Recently, a set of nine nonmagic nuclei with anomalous values of the B(E2) ratio B_4/2≡B(E2;4₁⁺→2₁⁺)/B(E2;2₁⁺→0₁⁺) < 1 were identified. Such values are outside the range allowed by current collective models. In the present work, the B(E2;4₁⁺→2₁⁺) values for two of these nuclei, ⁹⁸Ru and ¹⁸⁰Pt, were re-measured to determine if the current literature values for these nuclei are correct. ⁹⁸Ru was studied in a ²⁷Al(⁹⁸Ru,⁹⁸Ru^*) Coulomb excitation experiment in inverse kinematics, while the lifetime of the 4₁⁺ state in ¹⁸⁰Pt was measured in a ¹²²Sn(⁶²Ni, 4n)¹⁸⁰Pt recoil distance method (RDM) experiment. For both nuclei, the remeasured B_4/2 values are well above 1, removing the deviations from collective models.

Lifetimes of prolate intruder states in ¹⁸⁶Pb and oblate intruder states in ¹⁹⁴Po have been determined by employing, for the first time, the recoil-decay tagging technique in recoil distance Doppler-shift lifetime measurements. In addition, lifetime measurements of prolate states in ¹⁸⁸Pb up to the 8⁺ state were carried out using the recoil-gating method. The B(E2) values have been deduced from which deformation parameters |β₂|=0.29(5) and |β₂|=0.17(3) for the prolate and the oblate bands, respectively, have been extracted. The results also shed new light on the mixing between different shapes.

Excited states in the deformed nucleus ¹⁶⁸Er have been studied with high-energy resolution, in the (p,t) reaction, with the Munich Q3D spectrograph. A number of 25 excited 0⁺ states (four tentative) and 63  2⁺ states have been assigned up to 4.0 MeV excitation energy. This unusually rich characterization of the 0⁺ and 2⁺ states in a deformed nucleus, close to a complete level scheme, offers a unique opportunity to check, in detail, models of nuclear structure that incorporate many excitation modes. A comparison of the experimental data is made with two such models: the quasiparticle-phonon model (QPM), and the projected shell model (PSM). The PSM wave functions appear to contain fewer correlations than those of the QPM and than required by the data.

The first case of a Bose-Fermi critical point symmetry, E(5/4), representing the coupling of a j=3/2 fermion to an E(5) core, was recently proposed. Since ¹³⁴Ba has been found to be an empirical manifestation of E(5), we carried out a β-decay experiment to study levels in ¹³⁵Ba, where the last neutron can occupy the 2d_3/2 orbit, as the first test of E(5/4). The comparison shows significant areas of agreement as well as significant discrepancies. Comparison of interacting boson-fermion approximation and shell model calculations with the data are also presented.

Lifetime measurements of yrast levels in ¹⁶²Yb and ¹⁶⁶Hf were performed using the recoil distance Doppler-shift method in coincidence mode. Excited states in ¹⁶²Yb and ¹⁶⁶Hf were populated via the reactions ¹¹⁶Cd(⁵⁰Ti, 4n) and ¹²²Sn(⁴⁸Ti, 4n), respectively. The resulting B(E2) values are compared with the X(5) critical point model predictions and interacting boson approximation (IBA) model calculations. The X(5) model provides a reasonable description of the yrast B(E2) values in ¹⁶⁶Hf, whereas the IBA fails to reproduce the transition strengths from the higher spin levels. In ¹⁶²Yb, some transitions agree with the X(5) predictions while others are more consistent with the predictions of the IBA or a deformed symmetric rotor.

We study a one-dimensional lattice gas “dynamical geometry model” in which local reversible interactions of counter-rotating groups of particles on a ring can create or destroy lattice sites. We exhibit many periodic orbits and show that all other solutions have asymptotically growing lattice length in both directions of time. We explain why the length grows as sqrt[t] in all cases examined. We completely solve the dynamics for small numbers of particles with arbitrary initial conditions.

Excited states in ²⁰⁹Fr were produced following the ¹⁷⁶Yb(³⁷Cl, 4n) reaction. An excitation function was measured with data taken at 173, 179, and 185 MeV. Recoiling fusion-evaporation products were separated using the SASSYER gas-filled spectrometer. HPGe clover detectors of the YRAST Ball array at the target position and at the focal plane were used to detect prompt and delayed gamma ray decays, providing the first spectroscopic information about prompt and delayed transitions in ²⁰⁹Fr. The decay from a proposed πi_13/2 configuration isomer was observed in ²⁰⁹Fr, and its half-life was measured to be 446(14)ns. States above the isomer were also observed.

Coulomb activation of the four quasiparticle K^π=16^{+ 178}Hf isomer (t_1/2=31  y) has led to the measurement of a set of Eλ matrix elements coupling the isomer band to the ground band. The present data combined with earlier ¹⁷⁸Hf Coulomb excitation data have probed the K components in the wave functions and revealed the onset and saturation of K mixing in low-K bands, whereas the mixing is negligible in the high-K bands. The implications can be applied to other quadrupole-deformed nuclei.

Certain quantum-information tasks require entanglement of assistance, namely, a reduction of a tripartite entangled state to a bipartite entangled state via local measurements. We establish that concurrence of assistance (COA) identifies capabilities of and limitations to producing pure bipartite entangled states from pure tripartite entangled states and prove that COA is an entanglement monotone for (2×2×n)-dimensional pure states. Moreover, if the COA for the pure tripartite state is at least as large as the concurrence of the desired pure bipartite state, then the former may be transformed to the latter via local operations and classical communication, and we calculate the maximum probability for this transformation when this condition is not met.

The g factor of the 2₁⁺ state of ¹⁶⁰Er was measured by perturbed γ-γ angular correlation in a static external magnetic field of 5.82 T. The result, g(2₁⁺)=0.33(6), is discussed within the systematics of g factors of even-even isotopes for the Ba-Pt region.

We give a comprehensive analysis of the singular dynamics and of the low-energy fixed point of one-channel impurity s-d models with ferromagnetic and underscreened antiferromagnetic couplings. We use the numerical renormalization group (NRG) to perform calculations at T=0. The spectral densities of the one-electron Green’s functions and t-matrices are found to have very sharp cusps at the Fermi level (ω=0), but do not diverge. The approach of the Fermi level is governed by terms proportional to 1∕ln²(ω∕T₀) as ω→0. The scaled NRG energy levels show a slow convergence as 1∕(N+C) to their fixed point values, where N is the iteration number and C is a constant dependent on the coupling J from which the low energy scale T₀ can be deduced, with a renormalized coupling J̃(ω)=1∕ln(ω∕T₀). We calculate also the dynamical spin susceptibility, and the elastic and inelastic scattering cross sections as a function of ω. The inelastic scattering goes to zero as ω→0, as expected for a Fermi liquid, but anomalously slowly compared to the fully screened case. We obtain the asymptotic forms for the phase shifts for elastic scattering of the quasiparticles in the high-spin and low-spin channels. The asymptotic forms found for the one-electron Green’s functions and t-matrices do not simply correspond to a perturbation expansion in powers of J̃(ω).

Excited states in ¹⁶⁶Hf were populated in the β⁺/ε decay of ¹⁶⁶Ta and studied through off-beam γ-ray spectroscopy at the Yale moving tape collector. New coincidence data found no support for two previously reported excited 0⁺ states and led to a substantially revised level scheme. Similarities between the revised level scheme of ¹⁶⁶Hf and the X(5) critical point symmetry are discussed, and the extent of X(5) behavior in this mass region is explored through the W and Os isotopes. Among X(5) candidates with N>90, good agreement is observed for most energies and interband B(E2) strengths, while all exhibit similar disagreements with other key observables, in particular, yrast B(E2) values and spacing in the excited K  =  0⁺ sequence.

Low-energy excited states of ²⁰⁸Ra were investigated using the ¹⁸²W(³⁰Si, 4n) reaction at the Wright Nuclear Structure Laboratory of Yale University. Fusion evaporation recoils were selected using the gas-filled spectrometer SASSYER. Delayed γ rays, following isomeric decays, were detected at the focal plane of SASSYER with a small array of three clover Ge detectors. Transitions following a proposed J^π=8⁺ isomer were observed, and the half-life was measured.

The nucleus ⁹⁸Ru, the lightest collective Ru isotope with N>50, has been studied employing the (α,2n) reaction using YRAST Ball detectors. The new data, as well as a systematic set of limits on the intensities of unobserved transitions, reveal a structure that resembles a near harmonic vibrator spectrum up through the two-phonon triplet but show an almost complete breakdown of vibrator structure thereafter. These results are not understood theoretically and are at variance with previous interpretations of this nucleus. A rare anomaly in yrast B(E2) values is also pointed out.

We present results on the dynamical correlation functions of the particle-hole symmetric Holstein-Hubbard model at zero temperature, calculated using the dynamical mean-field theory which is solved by the numerical renormalization group method. We clarify the competing influences of the electron-electron and electron-phonon interactions particularity at the different metal to insulator transitions. The Coulomb repulsion is found to dominate the behavior in large parts of the metallic regime. By suppressing charge fluctuations, it effectively decouples electrons from phonons. The phonon propagator shows a characteristic softening near the metal to bipolaronic transition but there is very little softening on the approach to the Mott transition.

Calculations of cross sections for dissociative attachment to water through the ²B₁ resonance state are presented using the ab initio surfaces calculated previously for the energy E_R and width Γ of this resonance state as a function of nuclear geometry. The dynamics of the dissociative attachment process are treated in full dimensionality using the local complex potential model. For the H⁻+OH channel, the calculations presented here are in substantial agreement with experiment with regard to total cross section and vibrational excitation of the OH fragment. Cross sections for dissociative attachment to excited initial ro-vibrational states are presented and isotope effects are also examined.

Excited states in ²¹⁰Ra and ²⁰⁹Ra were populated using the ¹⁸⁴W(³⁰Si,xn) reaction at 148  MeV beam energy. Fusion evaporation recoils were selected using the gas-filled spectrometer, SASSYER. Prompt γ rays were detected using Compton-suppressed Ge detectors from the YRAST Ball array surrounding the target. Delayed γ rays, following isomeric decays, were detected at the focal plane of SASSYER with a smaller array of Ge detectors. The decay energy and lifetime for the 8⁺(πh_9∕2)ⁿ isomer of ²¹⁰Ra were determined; values for the yrast B(E2;8⁺→6⁺) in ²¹⁰Ra and neighboring nuclei are interpreted within the seniority scheme. This isomer was also used to select γ-rays deexciting levels above the isomeric state in ²¹⁰Ra. In addition, transitions in ²⁰⁹Ra were identified for the first time. Two high-spin isomers are suggested to exist in this isotope.