Search Results (47 total)

We present a measurement of the spin-dependent cross sections for the ³He→(e→,e^′)X reaction in the quasielastic and resonance regions at a four-momentum transfer 0.1≤Q²≤0.9  GeV². The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

Magnetostriction and thermal expansion of the spin-ladder compound piperidinium copper bromide (C₅H₁₂N)₂CuBr₄ are analyzed in detail. We find perfect agreement between experiments and the theory of a two-leg spin-ladder Hamiltonian for more than a decade in temperature and in a wide range of magnetic fields. Relating the magnetostriction along different crystallographic directions to two static spin-spin correlation functions, which we compute with quantum Monte Carlo, allows us to reconstruct the magnetoelastic couplings of (C₅H₁₂N)₂CuBr₄. We especially focus on the quantum critical behavior near the two critical magnetic fields H_c1 and H_c2, which is characterized by strong singularities rooted in the low dimensionality of the critical spin system. Extending our discussion in Lorenz [ Phys. Rev. Lett. 100, 067208 (2008)], we show explicitly that the thermal expansion near the upper critical field H_c2 is quantitatively described by a parameter-free theory of one-dimensional, nonrelativistic fermions. We also point out that there exists a singular quantum critical correction to the elastic moduli. This correction is proportional to the magnetic susceptibility χ, which diverges as χ∼1/sqrt[T] at the critical fields and thus leads to a strong softening of the crystal.

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl₃ through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.

We report a first study of the depolarization behavior of spin polarized ³He in a mixture of ³He-⁴He at a temperature below the ⁴He λ point in a deuterated tetraphenyl butadiene-doped deuterated polystyrene (dTPB-dPS) coated acrylic cell. In our experiment the measured ³He relaxation time is due to the convolution of the ³He longitudinal relaxation time, T₁, and the diffusion time constant of ³He in superfluid ⁴He since depolarization takes place on the walls. We have obtained a ³He relaxation time of ∼3000 s at a temperature around 1.9 K. We have shown that it is possible to achieve values of wall depolarization probability on the order of (1–2)×10⁻⁷ for polarized ³He in the superfluid ⁴He from a dTPB-dPS coated acrylic surface.

Non-negligible higher-order exchange terms are ubiquitous in the spin Hamiltonians of a large variety of magnetic compounds. Yet the origin of higher-order exchange has not been satisfactorily established. This question was addressed by performing inelastic neutron scattering experiments for the magnetically diluted compound KMn_0.1Zn_0.9F₃. The observed excitations can be associated with transitions between the low-lying electronic states of Mn²⁺ multimers which are well described by a spin Hamiltonian including bilinear and biquadratic exchange interactions. The bilinear exchange parameter J derived from the dimer spectra exhibits a strong temperature dependence, whereas the biquadratic exchange parameter K is independent of temperature. By minimizing the total elastic and magnetic energy of the dimer, we can express the parameter K in terms of lattice, elastic, and magnetic properties. The good agreement between the calculated and observed values of K indicates that the mechanism of exchange striction is the likely origin of the biquadratic interaction in the title compound.

Down-conversion in lanthanide doped luminescent materials is a promising route to significantly enhance the energy efficiency of silicon solar cells, plasma display panels, or mercury-free lighting tubes because it results in the emission of two photons for each absorbed higher energy photon. The Gd³⁺∕Eu³⁺ ion couple shows down-conversion of vacuum-ultraviolet light into visible light with an efficiency close to 190%. The low absorption strength of the ⁶G_7∕2 levels of Gd³⁺ (the starting point of the down-conversion process), however, prevents efficient excitation of the down-conversion process and therefore application. We have performed a high resolution luminescence spectroscopy study, using synchrotron radiation, in order to investigate the possibility to use the strong 4f→5d absorption transitions of Nd³⁺ and Tm³⁺ to sensitize the high energy ⁶G_7∕2 level of Gd³⁺ in the phosphors NaGdF₄:2%Nd³⁺ and NaGdF₄:2%Tm³⁺. Tm³⁺ appears to be an efficient sensitizer of the ⁶G_7∕2 state of Gd³⁺. It was also found that sensitization is followed by two successive energy transfer processes exciting two Tm³⁺ ions in the ³H₄ state which results in the emission of two infrared photons for one absorbed vacuum-ultraviolet photon. Nd³⁺ is not a good sensitizer of the ⁶G_7∕2 state in NaGdF₄. Instead Nd³⁺ efficiently transfers its energy by cross relaxation to the lower energy ⁶D_J states of Gd³⁺ but leaving the Nd³⁺ ion excited in the ⁴F_3∕2 state. Successive energy transfer from Gd³⁺ back to Nd³⁺ excites a second Nd³⁺ ion in the ⁴F_3∕2 state. Also, in this case, two infrared photons can be emitted for one absorbed vacuum-ultraviolet photon.

We present high-resolution measurements of the c^⋆-axis thermal expansion and magnetostriction of piperidinium copper bromide (C₅H₁₂N)₂CuBr₄. The experimental data at low temperatures are well accounted for by a two-leg spin-ladder Hamiltonian. The thermal expansion shows a complex behavior with various sign changes and approaches a 1/sqrt[T] divergence at the critical fields. All low-temperature features are semiquantitatively explained within a free-fermion model; full quantitative agreement is obtained with quantum Monte Carlo simulations.

We have measured the nuclear transparency of the A(e,e^′π⁺) process in ²H, ¹²C, ²⁷Al, ⁶³Cu, and ¹⁹⁷Au targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range Q²=1.1 to 4.7  (GeV/c)². The nuclear transparency was extracted as the super-ratio of (σ_A/σ_H) from data to a model of pion-electroproduction from nuclei without π-N final-state interactions. The Q² and atomic number dependence of the nuclear transparency both show deviations from traditional nuclear physics expectations and are consistent with calculations that include the quantum chromodynamical phenomenon of color transparency.

The cross section and decay angular distributions for the coherent ϕ-meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t=(p_γ-p_ϕ)²=-2 GeV²/c², using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a rescattering model. In a framework of vector meson dominance, the data are consistent with the total ϕ-N cross section σ_ϕN at about 10 mb. If vector meson dominance is violated, a larger σ_ϕN is possible by introducing a larger t slope for the ϕN→ϕN process than that for the γN→ϕN process. The decay angular distributions of the ϕ are consistent with helicity conservation.

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5–11 and -t=2–7  GeV² with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0±0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

We report neutron diffraction experiments on the light-induced metastable state SI in single crystals of Na₂[Fe(CN)₅NO]∙2D₂O. It is shown that the metastable state SI corresponds to a linkage isomer of the NO group, the so-called isonitrosyl configuration where the NO ligand is oxygen bound to the central iron atom.

The compound TlCuCl₃ represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Würtz [Phys. Rev. B 50, 13 515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important.

We present the first measurement of the Q² dependence of the neutron spin structure function g₂ⁿ at five kinematic points covering 0.57  (GeV/c)²≤Q²≤1.34  (GeV/c)² at x≃0.2. Though the naive quark-parton model predicts g₂=0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g₂ⁿ can be predicted using next-to-leading order fits to world data for g₁ⁿ. Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q², indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g₁ⁿ are consistent with next-to-leading order fits to world data.

Theoretical calculations and experimental crossed-beam measurements are compared for electron-impact single ionization of Pb^q+ ions for q=1–10. We compare with two main theoretical methods. First, we check against configuration-average distorted-wave calculations, which include both direct-ionization and indirect excitation-autoionization contributions. Second, for ion stages Pb⁺ through to Pb⁵⁺, we calculate the dominant excitation-autoionization channels using level-resolved distorted-wave theory to evaluate the excitation cross sections. We find that for ion stages Pb⁺, Pb²⁺, and Pb³⁺, distorted-wave theory significantly overestimates the total-ionization cross section, due to an overestimation of the direct-ionization cross section from the 5d subshell. For ion stages Pb⁴⁺ through to Pb¹⁰⁺ there is good agreement between theory and experiment. We find evidence for significant metastable fraction in the ion beam of the experiment for ion stages Pb²⁺, Pb³⁺, Pb⁴⁺, Pb⁵⁺, and Pb⁶⁺. For ion stage Pb³⁺ we find that the level-resolved distorted-wave calculation of the excitation autoionization results in a slight reduction of the configuration-average theoretical results, due to splitting of levels within the autoionizing configurations. We also investigate two semiempirical methods of calculating the direct-ionization cross sections: namely, the Lotz method and the binary encounter Bethe method. We find that both methods provide results which are significantly lower than the distorted-wave method for the 5d-subshell direct ionization of Pb⁺, Pb²⁺, and Pb³⁺. For the higher ion stages, both methods are lower than the distorted-wave direct-ionization cross-section results, trending towards the distorted-wave results as the ion stage increases.

Compton scattering from the proton was investigated at s=6.9  GeV² and t=-4.0  GeV² via polarization transfer from circularly polarized incident photons. The longitudinal and transverse components of the recoil proton polarization were measured. The results are in disagreement with a prediction of perturbative QCD based on a two-gluon exchange mechanism, but agree well with a prediction based on a reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton.

The differential cross sections for the γn→π^-p and the γp→π⁺n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50^° to 110^°. The π^- and π⁺ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing π⁺ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios [dσ/dt(γn→π^-p)/dσ/dt(γp→π⁺n)] at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.

We report on measurements of the neutron spin asymmetries A_1,2ⁿ and polarized structure functions g_1,2ⁿ at three kinematics in the deep inelastic region, with x=0.33, 0.47, and 0.60 and Q²=2.7, 3.5, and 4.8  (GeV∕c)², respectively. These measurements were performed using a 5.7  GeV longitudinally polarized electron beam and a polarized ³He target. The results for A₁ⁿ and g₁ⁿ at x=0.33 are consistent with previous world data and, at the two higher-x points, have improved the precision of the world data by about an order of magnitude. The new A₁ⁿ data show a zero crossing around x=0.47 and the value at x=0.60 is significantly positive. These results agree with a next-to-leading-order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (PQCD) assuming hadron helicity conservation. Results for A₂ⁿ and g₂ⁿ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d₂ⁿ was evaluated and the new result has improved the precision of this quantity by about a factor of 2. When combined with the world proton data, polarized quark distribution functions were extracted from the new g₁ⁿ∕F₁ⁿ values based on the quark-parton model. While results for Δu∕u agree well with predictions from various models, results for Δd∕d disagree with the leading-order PQCD prediction when hadron helicity conservation is imposed.

The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl₃, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.

Theoretical calculations and experimental crossed-beam measurements are compared for electron-impact single ionization of Bi^q+ for q=1–10. The configuration-average distorted-wave calculations include both direct ionization and indirect excitation-autoionization contributions. For Bi²⁺ it is necessary to account for levels within an excited configuration that straddle the ionization threshold. This was included via level-resolved atomic structure calculations, from which statistical partitions of the configuration-averaged cross sections could then be made. Evidence is found for ionization contributions from an excited configuration in Bi³⁺ and Bi⁵⁺. Good agreement is found between theory and experiment for all the ionization stages of Bi studied.

The generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH₄CuCl₃, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T₁≈156  K and at T₂=70  K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below T_N=1.3  K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model.

The origin of higher-order exchange interactions in localized S-state systems has been the subject of intensive investigations in the past. In particular, it has been suggested that a biquadratic exchange term may arise from the magnetoelastic energy. Here we report on the pressure and temperature dependence of the excitation spectra of magnetic Mn²⁺ dimers in CsMn_0.28Mg_0.72Br₃ probed by inelastic neutron scattering. Biquadratic exchange and a strong distance dependence of the bilinear exchange are observed. It is shown that the mechanism of local exchange striction may explain the occurrence of biquadratic exchange in accordance with the elastic properties of the compound.