Search Results (42 total)

Performing highly precise Monte-Carlo simulations of SU(2) gluodynamics, we observe for the first time Abelian dominance in the confining part of the static potential in local unitary gauges such as the F12 gauge. We also study the flux-tube profile between the quark and antiquark in these local unitary gauges, and we find a clear signal of the dual Meissner effect. The Abelian electric field is found to be squeezed into a flux tube by the monopole supercurrent. This feature is the same as that observed in the nonlocal maximally Abelian gauge. These results suggest that the Abelian confinement scenario is gauge independent. Observing the important role of spacelike monopoles in the Polyakov gauge also indicates that the monopoles defined on the lattice do not necessarily correspond to those proposed by ’t Hooft in the context of Abelian projection.

An experimental study of crossing a third order uncoupled resonance was performed in a proof of principle fixed field alternating gradient with various widths of resonance and crossing speed. We observed that part of the beam is transported to a larger amplitude when the crossing speed is relatively small. We derived an “adiabatic parameter” from a “particle trapping” model as a useful index for avoiding beam deterioration due to resonance crossing. It relates the crossing speed to the resonance width and the amount of nonlinear detuning. When the adiabatic parameter is more than 7, resonance crossing will not affect a beam.

The Raman scattering mechanism of orbital waves is investigated by resonant Raman scattering and hot luminescence in YTiO₃. The observation of hot luminescence from the charge-transferred TiO₆ indicates that two-orbital wave scattering has a larger probability than one-orbital wave scattering. The energies of the broad two-orbital wave peaks are 530–800, 1770–1920, and 2430 cm⁻¹. The energies are in good agreement with the theoretical predication.

The vacuum type of SU(2) gluodynamics is studied using Monte Carlo simulations in maximally Abelian (MA) gauge and in Landau (LA) gauge, where the dual Meissner effect is observed to work. The dual Meissner effect is characterized by the coherence and the penetration lengths. Correlations between Wilson loops and electric fields are evaluated in order to measure the penetration length in both gauges. The coherence length is shown to be fixed in the MA gauge from measurements of the monopole density around the static quark-antiquark pair. It is also shown numerically that a dimension 2 gluon operator A⁺A^-(s) and the monopole density has a strong correlation as suggested theoretically. Such a correlation is observed also between the monopole density and A²(s)=A⁺A^-(s)+A³A³(s) condensate if the remaining U(1) gauge degree of freedom is fixed to U(1) Landau gauge (U1LA). The coherence length is determined numerically also from correlations between Wilson loops and A⁺A^-(s) and A²(s) in MA+U1LA gauge. Assuming that the same physics works in the LA gauge, we determine the coherence length from correlations between Wilson loops and A²(s). Penetration lengths and coherence lengths in the two gauges are almost the same. The vacuum type of the confinement phase in both gauges is near to the border between the type 1 and the type 2 dual superconductors.

Chemical control of the spontaneous motion of a reactive oil droplet moving on a glass substrate under an aqueous phase is reported. Experimental results show that the self-motion of an oil droplet is confined on an acid-treated glass surface. The transient behavior of oil-droplet motion is also observed with a high-speed video camera. A mathematical model that incorporates the effect of the glass surface charge is built based on the experimental observation of oil-droplet motion. A numerical simulation of this mathematical model reproduced the essential features concerning confinement of oil droplet motion within a certain chemical territory and also its transient behavior. Our results may shed light on physical aspects of reactive spreading and a chemotaxis in living things.

We study QCD with two flavors of nonperturbatively improved Wilson fermions at finite temperature on the 16³8 lattice. We determine the transition temperature at lattice spacing as small as a∼0.12 fm, and study string breaking below the finite temperature transition. We find that the static potential can be fitted by a two-state ansatz, including a string state and a two-meson state. We investigate the role of Abelian monopoles at finite temperature.

Infrared reflectivity measurements of CeSb under multiextreme conditions (low temperatures, high pressures, and high magnetic fields) were performed. A pseudogap structure, which originates from the magnetic band folding effect, responsible for the large enhancement in the electrical resistivity in the single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure of 4 GPa and at temperatures of 35–50 K. The optical spectrum of the pseudogap changes to that of a metallic structure with increasing magnetic field strength and increasing temperature. This change is the result of the magnetic phase transition from the AF-1 phase to other phases as a function of the magnetic field strength and temperature. This result is an important optical observation of the formation and collapse of a pseudogap under multiextreme conditions.

The dual Meissner effect is observed without monopoles in quenched SU(2) QCD with Landau gauge fixing. Magnetic displacement currents that are time-dependent Abelian magnetic fields act as solenoidal currents squeezing Abelian electric fields. Monopoles are not always necessary for the dual Meissner effect. A mean-field calculation suggests that the dual Meissner effect through the mass generation of the Abelian electric field is related to a gluon condensate ⟨A_μ^aA_μ^a⟩≠0 of mass dimension 2.

We investigate the confining properties of the QCD vacuum with N_f=2 flavors of dynamical quarks, and compare the results with the properties of the quenched theory. We use nonperturbatively O(a) improved Wilson fermions to keep cutoff effects small. We focus on color magnetic monopoles. Among the quantities we study are the monopole density and the monopole screening length, the static potential and the profile of the color electric flux tube. We furthermore derive the low-energy effective monopole action. Marked differences between the quenched and dynamical vacuum are found.

We study the distribution of color electric flux of the three-quark system in quenched and full QCD (with N_f=2 flavors of dynamical quarks) at zero and finite temperature. To reduce ultraviolet fluctuations, the calculations are done in the Abelian projected theory fixed to the maximally Abelian gauge. In the confined phase we find clear evidence for a Y-shape flux tube surrounded and formed by the solenoidal monopole current, in accordance with the dual superconductor picture of confinement. In the deconfined, high temperature phase monopoles cease to condense, and the distribution of the color electric field becomes Coulomb-like.

Magnetism of layered cobaltites Na_xCoO₂ with x=0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation (μ⁺SR) spectroscopy together with magnetic susceptibility and specific-heat measurements, using single-crystal samples in the temperature range between 250 and 1.8 K. Zero-field-(ZF-) μ⁺SR measurements on Na_0.9CoO₂ indicate a transition from a paramagnetic to an incommensurate spin-density wave state (IC-SDW) at 19 K(=T_SDW). The anisotropic ZF-μ⁺SR spectra suggest that the oscillating moments of the IC-SDW direct along the c axis. Since Na_0.6CoO₂ is paramagnetic down to 1.8 K, the magnitude of T_SDW is found to strongly depend on x. This behavior is well explained using the Hubbard model within a mean-field approximation on two-dimensional triangular lattice in the CoO₂ plane. Also, both the appearance of the IC-SDW state by the change in x and the magnitude of the electronic specific-heat parameter of Na_0.6CoO₂ indicate that Na_xCoO₂ is unlikely to be a typical strongly correlated electron system.

Beam blowup due to a half-integer resonance was observed in the HIMAC synchrotron with a nondestructive two-dimensional beam-profile monitor. As the betatron tune approached a half-integer, the vertical beam size became larger by about 13%. The measured rms beam size is in good agreement with a space-charge-included numerical simulation.

Using muon spin spectroscopy we have found that, for both Na_xCoO₂ (0.6≤x≤0.9) and three- and four-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO₂ planes. Here we summarize those results and report an almost dome-shaped relation between the transition temperature into the low-T magnetic state and the composition x for Na_xCoO₂ and/or the high-temperature asymptotic limit of thermopower in the more complex three- and four-layer cobaltites. This behavior is explained using the Hubbard model on a two-dimensional triangular lattice in the CoO₂ plane.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We have investigated the high-pressure optical absorption of Iron disilicide, β-FeSi₂, thin films(90 nm in thickness) prepared from Si/Fe multilayers on Si (001) with template and SiO₂ capping. It is found that the experimental absorption coefficient in the range of photon energy of about 0.3 eV beyond the band gap is a few orders of magnitude larger than the first-principles calculated absorption coefficient. A possible explanation for this large absorption coefficient is the saddle-point exciton effect by the calculated band structure. No critical points with negative hydrostatic pressure coefficients such as those of Si and GaAs are observed in β-FeSi₂ near the band gap. The pressure coefficient for the direct band gap of β-FeSi₂ is determined to be 15.9 meV/GPa. This small coefficient is due to the negative deformation potential of the valence-band maximum, and the large bulk modulus of β-FeSi₂.

Beam losses due to half-integer resonance have been observed in the Heavy Ion Medical Accelerator in Chiba synchrotron, along with exciting the harmonic component of gradient field errors. During operation, while varying the defocusing quadrupole to cross a half-integer tune in the vertical space, the region of bare tunes which causes the half-integer resonance was evaluated. When the initial beam intensity was high, the bare tune where the beam loss occurred became higher. The beam loss occurred rapidly when the half-integer tune was crossed upward, but gradually when it was crossed downward. Those results mean that the half-integer resonance is affected by space-charge-induced tune shifts. This fact was verified experimentally for the first time. The results from a one-dimensional multiparticle simulation agreed with those characteristics. Finally, the beam-size growth and the change in distribution were studied by a simulation.

We have measured the nuclear spin-lattice relaxation rate at the planar O site (1/T₁)^17O  on the spin-triplet superconducting (SC) Sr₂RuO₄. It was found that the T dependence of (1/T₁)^17O  in the zero field exhibits a small peak just below T_c, followed by the T-linear behavior. The T dependence of (1/T₁)^17O  contrasts with the behavior of (1/T₁)^101Ru  of the Ru site in the SC state, and based on a comparison of (1/T₁)^17O  and (1/T₁)^101Ru , we suggest that some spin dynamics with low energy appear perpendicular to the RuO₂ plane in the triplet SC state.

An adequate interpretation of scanning tunneling microscopy (STM) images of the clean Si(001) surface is presented. We have performed both STM observations and ab initio simulations of STM images for buckled dimers at the S_A step of the clean Si(001) surface. By comparing experimental results with theoretical ones, it is revealed that STM images depend on the sample bias and the tip-sample separation. This enables us to elucidate the relationship between the corrugation in STM images and the atomic structure of buckled dimers. Moreover, to elucidate these changes, we analyze details of the spatial distributions of the π,π^* surface states and σ,σ^* Si-Si bond states in the local density of states, which contribute to STM images.

We investigated the oxygen isotope effect (IE) on the transition temperature T_c in the spin-triplet superconductor Sr₂RuO₄ (with the intrinsic T_c0=1.5 K). A clear IE shift in T_c was observed. Moreover, we found that the IE coefficient alpha exhibits an unusual variation with T_c. For lower-T_c crystals containing impurities and defects, α is positive and increases with decreasing T_c; α(T_c) is described well by the universal behavior expected theoretically. However, for crystals with T_c approaching T_c0, α deviates from the universal α(T_c) and becomes negative. We discuss its possible mechanism on the basis of existing models.

We have investigated a gap structure in the spin-triplet superconductor Sr₂RuO₄ through the measurement of the ¹⁰¹Ru nuclear spin-lattice relaxation rate ¹⁰¹(1/T₁) down to 0.09 K at zero magnetic field. In the superconducting state, 1/T₁ in a high-quality sample with T_c∼1.5 K exhibits a sharp decrease without the coherence peak, followed by a T³ behavior down to 0.15 K. This result is in marked contrast to the behavior observed below ∼0.4 K in samples with lower T_c, where T₁T is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in Sr₂RuO₄ is significantly anisotropic, consistent with line-node-like models.

A barrier bucket experiment with two dedicated barrier cavities was performed at the Brookhaven AGS. One of the barrier cavities was a magnetic alloy (MA)–loaded cavity and the other was a ferrite-loaded cavity. They generated a single sine wave with a peak voltage of 40 kV at a repetition rate of 351 kHz. A barrier rf system was established with these cavities and five bunches from the AGS booster were accumulated. A total of 3×10¹³ protons were stored without beam loss, and were successfully rebunched and accelerated. The longitudinal emittance growth was observed during accumulation by the barrier bucket, the blowup factor of which was about 3. The longitudinal mismatch between the rf bucket and the beam bunch was the main reason for the emittance growth. The potential distortions by beam loading of the ferrite cavity and the overshooting voltage of the MA cavity disturbed the smooth debunching.

We report inelastic neutron scattering measurements in the normal state of Sr₂RuO₄ that reveal the existence of significant incommensurate magnetic spin fluctuations located at q₀  =  (±0.6π/a,±0.6π/a,0). This finding confirms recent band-structure calculations that have predicted incommensurate magnetic responses related to dynamical nesting properties of its Fermi surface and points towards the possibility of a competition between p-wave spin triplet and d-wave spin singlet superconductivity in Sr₂RuO₄. We present a comparison between low energy spin fluctuations, converted in absolute units, from inelastic neutron scattering measurements and previous nuclear magnetic resonance studies.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

We have investigated depairing effects in Sr₂RuO₄, the unconventional superconductor in the layered perovskite structure. We prepared crystals of Sr₂RuO₄ with very low levels of impurity elements, and systematically controled their superconducting transition temperature T_c ranging from 1.5 to 0.6 K by adjustments of crystal growth conditions. The dependence of T_c on the residual resistivity ρ₀ in these crystals suggests that the defects are strong pair breakers, in addition to impurities. We further characterized the effects of pair breaking in this unconventional superconductor. We found that the in-plane coherence length ξ_ab(0) evaluated from H_c2(T) is inversely proportional to T_c, and decreases with increasing mean free path l; the latter behavior is opposite to that of conventional superconductors. In addition, we examined the temperature dependence of H_c2∥c which substantially deviates from the BCS theory and even from a recent theory of a p-wave superconductor.

The Fermi surface topology of the layered perovskite Sr₂RuO₄ was investigated through the observation of angle-dependent magnetoresistance oscillation under a magnetic field up to 17 T at 1.5 K. A series of angles giving the resistance peaks, observed under a constant field above 5 T, can be assigned to the Fermi wave-number vector of both the α and β branches of the calculated Fermi surface.