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T. Sato

Publications and source records attributed to T. Sato.

At least 235 records · Page 13Linked to original sources
arXiv2003

BCS-like Bogoliubov Quasiparticles in High-Tc Superconductors Observed by Angle-Resolved Photoemission Spectroscopy

We performed high-resolution angle-resolved photoemission spectroscopy on triple-layered high-Tc cuprate Bi2Sr2Ca2Cu3O10+delta. We have observed the full energy dispersion (electron and hole branches) of Bogoliubov quasiparticles and determined the coherence factors above and below EF as a function of momentum from the spectral intensity as well as from the energy dispersion based on BCS theory. The good quantitative agreement between the experiment and the theoretical prediction suggests the basic validity of BCS formalism in describing the superconducting state of cuprates.

cond-mat.supr-con↗
arXiv2003

Fermi-liquid ground state in n-type copper-oxide superconductor Pr0.91Ce0.09LaCuO4-y

We report nuclear magnetic resonance studies on the low-doped n-type copper-oxide Pr_{0.91}LaCe_{0.09}CuO_{4-y} (T_c=24 K) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed, the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and contrast with that in p-type copper-oxides with a similar doping level where high-T_c superconductivity sets in within the pseudogap phase. The data in the superconducting state are consistent with the line-nodes gap model.

cond-mat.supr-con↗
arXiv2003

Dynamical Model of Weak Pion Production Reactions

The dynamical model of pion electroproduction has been extended to investigate the weak pion production reactions. The predicted cross sections of neutrino-induced pion production reactions are in good agreement with the existing data. We show that the renormalized(dressed) axial N-$Δ$ form factor contains large dynamical pion cloud effects and this renormalization effects are crucial in getting agreement with the data. We conclude that the N-$Δ$ transitions predicted by the constituent quark model are consistent with the existing neutrino induced pion production data in the $Δ$ region.

nucl-th↗
arXiv2003

Spin waves and the origin of commensurate magnetism in Ba2CoGe2O7

The square-lattice antiferromagnet Ba2CoGe2O7 is studied by means of neutron diffraction and inelastic scattering. This material is isostructural to the well-known Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O7, but exhibits commensurate long-range Neel order at low temperatures. Measurements of the spin wave dispersion relation reveal strong in-plane anisotropy that is the likely reason for the suppression of helimagnetism.

cond-mat.str-el↗
arXiv2002

Neutrino-deuteron reactions at solar neutrino energies

In interpreting the SNO experiments, accurate estimates of the νd reaction cross sections are of great importance. In our recent work, we have improved our previous calculation by updating some of its inputs and by incorporating the results of a recent effective-field-theoretical calculation. The new cross sections are slightly (\sim 1%) larger than the previously reported values. It is reasonable to assign 1% uncertainty to the νd cross sections reported here; this error estimate does not include radiative corrections.

nucl-th↗
arXiv2002

Neutrino-deuteron reactions at solar neutrino energies

In interpreting the SNO experiments, very accurate estimates of the νd reaction cross sections are of great importance. We improve the previous estimates of our group by updating some of its inputs and by taking into account the results of a recent effective-field-theoretical calculation. The new cross sections are slightly (\sim 1%) larger than the previously reported values. We present arguments that lead to the conclusion that it is reasonable to assign 1% uncertainty to the νd cross sections reported here.

nucl-th↗
arXiv2001

Quasiparticles and Energy Scaling in Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4}$ ($\it{n}$=1-3): Angle-Resolved Photoemission Spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) has been performed on the single- to triple-layered Bi-family high-{\it T$_c$} superconductors (Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4}$, $\it{n}$=1-3). We found a sharp quasiparticle peak as well as a pseudogap at the Fermi level in the triple-layered compound. Comparison among three compounds has revealed a universal rule that the characteristic energies of superconducting and pseudogap behaviors are scaled with the maximum {\it T$_c$}.

cond-mat.supr-con↗
arXiv2001

High-Resolution Photoemission Study of MgB2

We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K. The temperature dependence (15 - 40 K) of gap value follows well the BCS form, suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.

cond-mat.str-el↗
arXiv2001

Millisecond Electron-Phonon Relaxation in Ultrathin Disordered Metal Films at Millikelvin Temperatures

We have measured directly the thermal conductance between electrons and phonons in ultra-thin Hf and Ti films at millikelvin temperatures. The experimental data indicate that electron-phonon coupling in these films is significantly suppressed by disorder. The electron cooling time $τ_ε$ follows the $T^{-4}$-dependence with a record-long value $τ_ε=25ms$ at $T=0.04K$. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise equivalent power of a detector with the area $1\mum^2$ would be $(2-3)10^{-20}W/Hz^{1/2}$, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.

cond-mat.dis-nn↗
arXiv2001

On the determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy

We study the normal state electronic excitations probed by angle resolved photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously: it is a single large hole barrel centered about (pi,pi) in both materials.

cond-mat.supr-con↗
arXiv2000

Neutrino Reactions on Deuteron

The cross sections for the ν-d and \barν-d reactions are calculated for the incident energy up to E_ν= 170 MeV, with the use of a phenomenological Lagrangian approach. We assess and improve the reliability of the employed calculational method by examining the dependence of the results on various input and approximations that go into the calculation. The main points of improvements over the existing work are: (1) use of the "modern" NN potentials; (2) use of the more accurate nucleon weak-interaction form factors; (3) monitoring the strength of a vertex that governs the exchange-current contribution, with the use of data on the related process, n+p\to d+γ. In addition to the total cross sections, we present various differential cross sections that are expected to be useful for the SNO and other experiments. In the low energy regime relevant to the solar neutrinos, the newly calculated total cross sections agree with the existing literature values. The origins of slight differences found for higher energies are discussed. The ratio between the neutral-current and charged-current reaction cross sections is found to be extremely stable against any variations in the input of our calculation.

nucl-th↗
arXiv2000

Dynamical Study of the Delta Excitation in N(e,e' pi) Reactions

The dynamical model developed in [Phys. Rev. C 54, 2660 (1996)] has been applied to investigate the pion electroproduction reactions on the nucleon. It is found that the model can describe to a very large extent the recent data of p(e,e' pi^0) reaction from Jefferson Laboratory and MIT-Bates. The extracted magnetic dipole(M1), electric dipole(E2), and Coulomb(C2) strengths of the gamma N -> Delta transition are presented. It is found that the C2/M1 ratio drops significantly with Q^2 and reaches about -13% at Q^2=4 (GeV/c)^2, while the E2/M1 ratio remains close to the value \sim -3 % at the Q^2=0 photon point. The determined M1 transition form factor drops faster than the usual dipole form factor of the proton. We also find that the non-resonant interactions can dress the gamma N -> Delta vertex to enhance strongly its strength at low Q^2, but much less at high Q^2. Predictions are presented for future experimental tests. Possible developments of the model are discussed.

nucl-th↗
arXiv2000

Search for the Decay K+ -> pi+ pi0 nu nubar

The first search for the decay K^+ -> pi^+ pi^0 nu nubar has been performed with the E787 detector at BNL. Based on zero events observed in the kinematical search region defined by 90 MeV/c < P_{pi^+} < 188 MeV/c and 135 MeV < E_{pi^0} < 180 MeV, an upper limit B(K^+ -> pi^+ pi^0 nu nubar) < 4.3 x 10^{-5} at 90% confidence level is established.

hep-ex↗
arXiv2000

Renormalization of spectral lineshape and dispersion below Tc in Bi2Sr2CaCu2O8+d

Angle-resolved photoemission (ARPES) data in the superconducting state of Bi2Sr2CaCu2O8+d show a kink in the dispersion along the zone diagonal, which is related via a Kramers-Kronig analysis to a drop in the low-energy scattering rate. As one moves towards (pi,0), this kink evolves into a spectral dip. The occurrence of these anomalies in the dispersion and lineshape throughout the zone indicate the presence of a new energy scale in the superconducting state.

cond-mat.supr-con↗
arXiv2000

Pion Production Model - Connection between Dynamics and Quark Models

We discuss the difficulties in testing the hadron models by using the N^* parameters extracted from the empirical amplitude analyses of the pi-N and gamma-N reaction data. As an alternative or perhaps a more advantageous approach, we present a Hamiltonian formulation that can relate the pion production dynamics and the constituent quark models of N^* structure. The application of the approach in investigating the Delta and N^*(S_{11}) excitations is reviewed. It is found that while the Delta excitation can be described satisfactory, the pi-N scattering in S_{11} channel can not be described by the constituent quark models based on either the one-gluon-exchange or one-meson-exchange mechanisms. A phenomenological quark-quark potential has been constructed to reproduce the S_{11} amplitude.

nucl-th↗
arXiv2000

Berry Connections and Induced Gauge Fields in Quantum Mechanics on Sphere

Quantum mechanics on sphere $S^{n}$ is studied from the viewpoint that the Berry's connection has to appear as a topological term in the effective action. Furthermore we show that this term is the Chern-Simons term of gauge variables that correspond to the extra degrees of freedom of the enlarged space.

hep-th↗