Search Results (100 total)

We demonstrate both efficient control of polarizations and high tunability of high-order sum and difference frequencies generated by adding an intense parallel- and perpendicular-polarized infrared laser field. When 805 nm pulses from a Ti:sapphire laser system and 1300 nm pulses from an optical parametric amplifier (OPA) are combined with perpendicular polarizations, the sum frequencies with two or four OPA photons are generated stronger than those with one or three OPA photons. This observation directly reflects the difference in their polarizations of the generated sum frequencies. Sum frequencies absorbing up to eight OPA photons are also observed for the parallel polarizations. Our observations are successfully reproduced by the theoretical calculations with the Lewenstein model including a weighting factor.

Gamow-Teller and dipole transitions to final states in ¹³B were studied via the ¹³C(t,³He) reaction at E_t=115A MeV. In addition to the strong Gamow-Teller transition to the ¹³B ground state, a weaker Gamow-Teller transition to a state at 3.6 MeV was found. This state was assigned a spin-parity of 3/2^- by comparison with shell-model calculations using the WBP and WBT interactions which were modified to allow for mixing between nℏω and (n+2)ℏω configurations. This assignment agrees with a recent result from a lifetime measurement of excited states in ¹³B. The shell-model calculations also explained the relatively large spectroscopic strength measured for a low-lying 1/2⁺ state at 4.83 MeV in ¹³B. The cross sections for dipole transitions up to E_x(¹³B)=20 MeV excited via the ¹³C(t,³He) reaction were also compared with the shell-model calculations. The theoretical cross sections exceeded the data by a factor of about 1.8, which might indicate that the dipole excitations are “quenched”. Uncertainties in the reaction calculations complicate that interpretation.

The double-differential cross sections for the ⁴⁸Ca(p,n) and ⁴⁸Ti(n,p) reactions were measured at 300 MeV. A multipole decomposition technique was applied to the spectra to extract the Gamow-Teller (GT) components. The integrated GT strengths up to an excitation energy of 30 MeV in ⁴⁸Sc are 15.3±2.2 and 2.8±0.3 in the (p,n) and (n,p) spectra, respectively. In the (n,p) spectra additional GT strengths were found above 8 MeV where shell models within the fp shell-model space predict almost no GT strengths, suggesting that the present shell-model description of the nuclear matrix element of the two-neutrino double-β decay is incomplete.

The deuteron to proton polarization transfer coefficients K_yy^y' and K_y^y' together with the deuteron analyzing powers are measured in three coplanar configurations of the deuteron-proton breakup reaction with a 135-MeV/nucleon polarized deuteron beam at the RIKEN Accelerator Research Facility. The data are compared with theoretical predictions based on exact solutions of the three-nucleon (3N) Faddeev equations with high-precision nucleon-nucleon (NN) forces, alone or combined with two 3N force (3NF) models, the 2π-exchange Tucson-Melbourne^'(99) (TM^'99) and Urbana IX. Large 3NF effects have been found for all the measured observables. Predicted effects are supported by the data, with the exception of the vector analyzing power A_y^d. For this observable, theory based on only NN forces is sufficient to explain the data. The behavior of the breakup analyzing powers is found to be different from the corresponding observables in elastic nucleon-deuteron scattering.

We report ²⁰⁹Bi nuclear-magnetic-resonance and nuclear-quadrupole-resonance measurements on a single crystal of the Kondo insulator U₃Bi₄Ni₃. The ²⁰⁹Bi nuclear-spin-lattice relaxation rate (T₁⁻¹) shows activated behavior and is well fit by a spin gap of 220 K. The ²⁰⁹Bi Knight shift (K) exhibits a strong temperature dependence arising from 5f electrons, in which K is negative at high temperatures and increases as the temperature is lowered. Below 50 K, K shows a broad maximum and decreases slightly upon further cooling. Our data provide insight into the evolution of the hyperfine fields in a fully gapped Kondo insulator based on 5f electron hybridization.

Heat capacity, magnetic susceptibility, NMR, and resistivity of SrNi₂P₂ single crystals are presented, illustrating a purely structural transition at 325 K with no magnetism. Bulk superconductivity is found at 1.4 K. The magnitude of the transition temperature T_c, fits to the heat-capacity data, the small upper critical field H_c2=390 Oe, and Ginzburg-Landau parameter κ=2.1 suggest a conventional fully gapped superconductor. With applied pressure a second structural phase transition occurs which results in an 8% reduction in the c/a ratio of lattice parameters. We find that superconductivity persists into this high-pressure phase, although the transition temperature is monotonically suppressed with increasing pressure. Comparison of these Ni-P data as well as layered Fe-As and Ni-As superconductor indicates that reduced dimensionality can be a mechanism for increasing the transition temperature.

¹¹⁵In nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on an antiferromagnet UIn₃ with the cubic AuCu₃-type structure. The NQR frequency (ν_Q) and Knight shift (K) of ¹¹⁵In in UIn₃ have been estimated in the paramagnetic state from NMR experiments under applied field. The perpendicular component of transferred hyperfine coupling constant (A_⊥) has been deduced from scaled behavior of K to the static susceptibility (χ). Under zero field, the observation of the NQR spectrum has led to an estimated ν_Q of 11.8 MHz at 90 K. The temperature variation in the NQR relaxation rates (1/T₁) far above the Néel temperature T_N=88 K approaches a constant value, which indicates a localized nature for the 5f electrons in this system. On the other hand, in the antiferromagnetically ordered state at 4 K (well below T_N), the ¹¹⁵In-NMR spectrum has been scanned over frequencies ranging from ∼20 to ∼70 MHz under zero applied field. From the analysis of the NMR spectrum, we propose that the most plausible direction of antiferromagnetic U moments may be ⟨110⟩ among the possible orientations of ⟨100⟩, ⟨110⟩, or ⟨111⟩.

³¹P-NMR measurements have been performed on a single crystal of the neptunium-based filled skutterudite NpFe₄P₁₂. The compound undergoes a ferromagnetic phase transition at T_C=23 K. From the field-orientation dependence of the ³¹P-NMR line splitting, the angular dependence of the hyperfine interactions between Np 5f spin and ³¹P nuclear moments has been investigated. We have observed anisotropic transferred hyperfine interactions at the P sites, which lead to an estimate of the local spin density in the P 3p orbitals. It is shown that a fraction of the Np 5f spin moments is transferred mostly into the P 3p orbitals extending toward the inside of a P cage. The weak hybridization between Np 5f and P 3p orbitals suggests a localized character for Np 5f electrons in NpFe₄P₁₂. We have also measured the field and temperature dependences of the nuclear spin-lattice relaxation rate (1/T₁) in several magnetic fields between 18.5 and 78.0 kOe. The 1/T₁ data reveal that the low-energy spin fluctuations of Np 5f spin moments are strongly suppressed by applied fields over a relatively wide temperature range up to 4T_C. In this compound, a large and negative magnetoresistance has been known to occur in the same temperature range. The present NMR results demonstrate that the negative magnetoresistance comes from a reduction in the magnetic scattering from Np 5f spin moments by an applied field.

Gamow-Teller (GT) unit cross sections, σ̂_GT, are obtained at 198 and 297 MeV by measuring the double differential cross sections at 0^° for the (p,n) reaction on ⁵⁸Ni, ⁷⁰Zn, ¹¹⁴Cd, ¹¹⁸Sn, and ¹²⁰Sn. The mass dependence of σ̂_GT and the ratio of σ̂_GT to the Fermi unit cross section, σ̂_F, (R²) are also derived in the mass region of 58≤A≤120. The σ̂_GT value for ⁹⁰Zr at 297 MeV interpolated using the A-dependence obtained herein agrees with that used in a previous analysis where the GT transition strength over a wide energy region up to the continuum was discussed. However, the deduced ⁶⁴Niσ̂_GT value at 198 MeV is 20% smaller than that obtained from the analysis of a previous (n,p) measurement. The present R² values in the mass region heavier than ⁴²Ca are larger than those in the region up to ⁴²Ca and increase as a function of A.

We propose a practical and versatile technique to achieve completely field-free molecular orientation with an intense, nonresonant, two-color laser field with a slow turn on and rapid turn off. The technique is based on the combined effects of both anisotropic polarizability interaction and anisotropic hyperpolarizability interaction. Using a FCN molecule as a sample, we show that the orientation achieved adiabatically by the peak of the laser pulse can be successfully revived at the rotational period of the molecule with the same degree of orientation. The crucial importance of the sufficiently slow turn on of the laser pulse is emphasized to achieve the highest possible degree of orientation.

We report measurements of the ¹¹⁹Sn nuclear spin-echo decay rate 1/T_2G in the heavy-fermion compound USn₃. From 1/T_2G, the magnetic spin-spin correlation length ξ is found to vary as ξ∼T^-3/4 above ∼100  K, which is expected for a quantum critical regime at high temperatures. Combined with the spin-lattice relaxation rate 1/T₁, T₁T/T_2G² is found to be temperature independent in the heavy-fermion state below T^*∼30  K. This indicates that the heavy-fermion state of USn₃ is categorized in the overdamped regime with a dynamical critical exponent z=2. These observations are consistent with a spin density wave magnetic instability at the quantum critical point.

We observe high-order harmonic spectra generated from a thin atomic medium, Ar, Kr, and Xe, by intense 800-nm and 1300-nm femtosecond pulses. A clear signature of a single-atom response is observed in the harmonic spectra. Especially in the case of Ar, a Cooper minimum, reflecting the electronic structure of the atom, is observed in the harmonic spectra. We successfully extract the photorecombination cross sections of the atoms in the field-free condition with the help of an accurate recolliding electron wave packet. The present protocol paves the way for exploring ultrafast imaging of molecular dynamics with attosecond resolution.

We searched for the neutrino-less double-β decay(0νββ) of ⁴⁸Ca by using CaF₂(Eu) scintillators. Analysis of their pulse shapes was effective to reduce backgrounds. No events are observed in the Q_ββ value region for the data of 3394 kg · day. It gives a lower limit (90% confidence level) of T_1/2^0νββ>2.7×10²² year for the half-life of 0νββ of ⁴⁸Ca. Combined with our previous data for 1553 kg · day [I. Ogawa , Nucl. Phys. A730, 215 (2004)], we obtained a more stringent limit of T_1/2^0νββ>5.8×10²² year.

We demonstrate laser-field-free molecular orientation with the combination of a moderate electrostatic field and an intense nonresonant rapidly turned-off laser field, which can be shaped with the plasma shutter technique. We use OCS (carbonyl sulfide) molecules as a sample. Molecular orientation is adiabatically created in the rising part of the laser pulse, and it is found to revive at around the rotational period of an OCS molecule with the same degree of orientation as that at the peak of the laser pulse in the virtually laser-field-free condition. This accomplishment means that a new class of molecular sample has become available for various applications.

USn₃ is a heavy-fermion system with an electronic specific heat coefficient γ∼170  mJ∕K²  mol. In order to further characterize the heavy-fermion phenomena for USn₃, the Knight shift and spin-lattice relaxation time T₁ of ¹¹⁹Sn NMR have been measured. The static (specific heat) and dynamical (T₁) properties in the heavy-fermion state can be described in a quantitatively consistent way in terms of a spin-fluctuation model with two constant energy scales. However, it is necessary to introduce a T-dependent “effective” Ruderman–Kittel–Kasuya–Yosida interaction J_Q (J_Q≃a+bsqrt[T]) in order to describe the crossover from an incoherent, localized state to a coherent, heavy-fermion state. In addition, a universal scaling behavior is proposed for the crossover regime. The parameters obtained are used to predict the T dependence of the thermal expansion coefficient.

Alignment dependence of structural deformation in the production process of multiply charged CO₂ molecules in an intense femtosecond laser field has been revealed by using aligned sample molecules. Our properly ordered observations clarify that the structural change takes place in the charge state of CO₂²⁺. The bending angle decreases monotonically from the maximum value of 24° with the laser polarization parallel to the molecular axis to the minimum value of 16° with the polarization perpendicular to the molecular axis. The alignment dependence is discussed in terms of the field-induced nonadiabatic transition between the lowest two adiabatic states formed by the coupling between the electronically excited and the ground states of CO₂²⁺ ions.

We propose a strategy to achieve laser-field-free molecular orientation with the combination of an electrostatic field and an intense, nonresonant laser field with rapid turn off. The adiabatically created pendular state is effectively transferred to the rotational wave packet in the nonadiabatic regime after the rapid turn off of the laser pulse and the orientation achieved at the peak of the laser pulse is revived at the rotational period of the molecule with the same degree of orientation. The remarkable difference in molecular behavior is found between alignment and orientation, which reveals the crucial importance of the sufficiently long rising time of the laser pulse to achieve the highest possible degree of orientation. The feasibility has been examined for OCS molecules as a sample under practical experimental conditions.

Nonadiabatic alignment of rotationally cold N₂ molecules is optimally controlled by shaping femtosecond pump pulses with the feedback of degree of alignment evaluated by an ion imaging technique. The alignment is optimized by doubly peaked pulses with approximately equal intensities. A doubly peaked pulse with an appropriate interval can be regarded as a single pulse with a center trough based on the considerations from both time and frequency domains, suggesting that the effective duration of a doubly peaked pulse rather than its structure is crucial to nonadiabatic molecular alignment.

We report NMR studies on a single crystal of NpFeGa₅, which has the tetragonal HoCoGa₅(115) structure and exhibits two antiferromagnetically ordered states. In the antiferromagnetic I phase below 118  K, the observed internal field at the Ga(2) (4i) site is consistent with the ordered structure revealed by neutron diffraction measurements. Below 78  K in the antiferromagnetic II phase, two different Ga(2) sites with different electric field gradient tensors appear, which indicates an orbitally related ordering. A possible quadrupolar ordering in the antiferromagnetic II phase is discussed.

Impurity effects on the stability of a ferromagnetic-metallic state in a bicritical-state manganite, (La_0.7Pr_0.3)_0.65Ca_0.35MnO₃, on the verge of metal-insulator transition have been investigated by substituting a variety of transition-metal atoms for Mn ones. Among them, Fe doping exhibits the exceptional ability to dramatically decrease the ferromagnetic transition temperature. Systematic studies on the magnetotransport properties and x-ray diffraction for the Fe-doped crystals have revealed that charge-orbital ordering evolves down to low temperatures, which strongly suppresses the ferromagnetic-metallic state. The observed glassy magnetic and transport properties as well as diffuse phase transition can be attributed to the phase-separated state where short-range charge-orbital-ordered clusters are embedded in the ferromagnetic-metallic matrix. Such a behavior in the Fe-doped manganites form a marked contrast to the Cr-doping effects on charge-orbital-ordered manganites known as the impurity-induced collapse of charge-orbital ordering.

Charge exchange spin-dipole (SD) excitations in ⁹⁰Zr and ²⁰⁸Pb are studied by using a Skyrme Hartree-Fock + random phase approximation. The calculated spin-dipole strength distributions are compared with experimental data obtained by ⁹⁰Zr(p,n)⁹⁰Nb and ⁹⁰Zr(n,p)⁹⁰Nb reactions. The model-independent SD sum rule values of various Skyrme interactions are studied in comparison with the experimental values to determine the neutron skin thickness of ⁹⁰Zr. The pressure of the neutron matter equation of state and the nuclear matter symmetry energy are discussed in terms of the neutron skin thickness and peak energies of SD strength distributions.

The differential cross sections and vector analyzing powers for nd elastic scattering at E_n=248 MeV were measured for 10°–180° in the center-of-mass (c.m.) system. To cover the wide angular range, the experiments were performed separately by using two different setups for forward and backward angles. The data are compared with theoretical results based on Faddeev calculations with realistic nucleon-nucleon (NN) forces such as AV18, CD Bonn, and Nijmegen I and II, and their combinations with the three-nucleon forces (3NFs), such as Tucson-Melbourne 99 (TM99), Urbana IX, and the coupled-channel potential with Δ-isobar excitation. Large discrepancies are found between the experimental cross sections and theory with only 2N forces for θ_c.m.>90°. The inclusion of 3NFs brings the theoretical cross sections closer to the data but only partially explains this discrepancy. For the analyzing power, no significant improvement is found when 3NFs are included. Relativistic corrections are shown to be small for both the cross sections and the analyzing powers at this energy. For the cross sections, these effects are mostly seen in the very backward angles. Compared with the pd cross section data, quite significant differences are observed at all scattering angles that cannot be explained only by the Coulomb interaction, which is usually significant at small angles.

We report NMR studies on a single crystal of UCoGa₅, which has the same tetragonal HoCoGa₅ (115) structure as the d-wave high-T_c superconductor PuCoGa₅. The spin-lattice relaxation rate (1∕T₁) in UCoGa₅ is small compared with those of other actinide 115 compounds. This fact is consistent with a semimetallic nature of UCoGa₅ suggested by de Haas–van Alphen effect measurements and band calculations. From comparison of 1∕T₁ at crystallografically different Ga and Co sites and the static susceptibility, an existence of two bands with rather different characters is revealed.

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments have been performed on a single crystal of tetragonal NpCoGa₅, an itinerant antiferromagnet with a Néel temperature T_N=47  K. The antiferromagnetic phase is inverted to a field-induced ferromagnetic (FIF) phase with an applied field (H₀) above H_m (T→0) ∼40  kOe oriented along the c axis. NMR spectra have been measured above and below T_N with H₀‖c and a axes and have been assigned to ^69,71Ga nuclei on two crystallographically inequivalent 1c and 4i sites and to ⁵⁹Co nuclei on the 1b site. Using second-order perturbation calculations, Knight shift (K), electric field gradient (EFG), frequency (ν_Q), and asymmetry parameter (η) of the EFG are deduced for each site. These parameters for the ⁶⁹Ga(1c) and ^69,71Ga(4i) sites are confirmed by NQR measurements in zero field. The Knight shifts obtained in the paramagnetic (PM) state obey a Curie-Weiss law, which scales with the bulk susceptibility (χ). Hyperfine tensors for each site are deduced from K-χ plots with temperature as an implicit parameter. Antiferromagnetic NMR spectra in zero field were also observed, finding an internal field of ∼20  kOe at the Ga(4i) site at the lowest temperature. The ordered moment can be estimated from this to be 0.81  μ_B∕Np. The nuclear quadrupolar parameters (ν_Q and η) are found to exhibit an anomaly just below T_N in the FIF phase. T₁ and T₂ have been measured for each site. For H₀‖c, T₁∼constant behavior suggests localized 5f character for T>100  K and itinerant (1∕T₁∝T) behavior for T_N<T<∼100  K in NpCoGa₅. T₂ measurements in the case of H₀‖c clearly define a phase crossover between PM and FIF phases. A sharp anisotropy for spin fluctuations in NpCoGa₅ has also been demonstrated.

We describe the improvements of a Fresnel zone plate (FZP) beam-profile monitor, which is being developed at the KEK-ATF damping ring to measure ultralow emittance electron-beam profiles. This monitor, which is designed to have a submicrometer spatial resolution, is based on x-ray imaging optics composed of two FZPs. Various improvements were performed to the old setup. First, a new crystal monochromator was introduced to suppress the beam image drift. Second, the two FZP folders were improved in order to realize a precise beam-based alignment during x-ray imaging. Third, a fast mechanical shutter was installed to achieve a shorter time resolution, and an x-ray mask system was also installed to obstruct direct synchrotron radiation through the FZPs. These improvements could make beam-profile measurements more precise and reliable. The beam profiles with less than 50   μm horizontal beam size and less than 6   μm vertical beam size could be measured within a 1 ms time duration. Furthermore, measurements of the damping time and the coupling dependence of the ATF damping ring were successfully carried out with this upgraded FZP monitor.