arXiv subjects

Y. Abe

Publications and source records attributed to Y. Abe.

52 records · Page 3Linked to original sources

arXiv2008

Doping dependence of phonon and quasiparticle heat transport of pure and Dy-doped Bi_2Sr_2CaCu_2O_{8+δ} single crystals

The temperature and magnetic-field (H) dependences of thermal conductivity (κ) of Bi_2Sr_2CaCu_2O_{8+δ} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+δ} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for the first time in Bi2212 system, which means Dy^{3+} ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of κat temperature above 5 K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of κ, the Dy-doped samples present an additional "dip"-like term of κ(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x \simeq 0.50, an interesting "plateau" feature shows up in the low-T κ(H) isotherms with characteristic field at 1 -- 2 T, for which we discuss the possible revlevance of magnon excitations.

cond-mat.supr-con↗

arXiv2008

Non-Markovian diffusion over a parabolic potential barrier: influence of the friction-memory function

The over-passing probability across an inverted parabolic potential barrier is investigated according to the classical and quantal generalized Langevin equations. It is shown that, in the classical case, the asymptotic value of the over-passing probability is determined by a single dominant root of the "characteristic function", and it is given by a simple expression. The expression for the over-passing probability is quite general, and details of dissipation mechanism and memory effects enter into the expression only through the dominant root of the characteristic equation.

cond-mat.stat-mech↗

arXiv2004

Optical diffraction spectroscopy of excitons in uniaxially-strained GaN films

The degenerate four-wave mixing spectroscopy of uniaxially strained GaN layers is demonstrated using colinearly polarized laser pulses. The nonlinear response of FWM signal on exciton oscillator strength enhances the sensitivity for polarized exciton, allowing for mapping out the in-plane anisotropy of the strain field. The observed high-contrast spectral polarization clearly shows fine structure splittings of excitons, which are also confirmed in the change of quantum beating periods of time.

cond-mat.other↗

arXiv2004

4D equivalence theorem and gauge symmetry on orbifold

We investigate the high-energy behavior of the scattering amplitudes in the extra dimensional gauge theory where the gauge symmetry is broken by the boundary condition. We study, in particular, the 5-dimensional SU(5) grand unified theory whose 5th-dimensional coordinate is compactified on S^1/Z_2. We pay attention to the gauge symmetry compatible with the boundary condition on the orbifold and give the BRST formalism of the 4D theory which is obtained through integration of the 5D theory along the extra dimension. We derive the 4D equivalence theorem on the basis of the Slavnov-Taylor identities. We calculate the amplitudes of the process including four massive gauge bosons in the external lines and compare them with the ones for the connected reactions where the gauge fields are replaced by the corresponding would-be NG-like fields. We explicitly confirm the equivalence theorem to hold.

hep-th↗

arXiv2004

Local Ordering in the Pseudogap State of the High-T$_c$ Superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We report atomic scale characterization of the pseudogap state in a high-T$_c$ superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The electronic states at low energies within the pseudogap exhibit spatial modulations having an energy-independent incommensurate periodicity. These patterns, which are oriented along the copper-oxygen bond directions, appear to be a consequence of an electronic ordering phenomenon--the observation of which correlates with the pseudogap in the density of electronic states. Our results provide a stringent test for various ordering scenarios in the cuprates, which have been central in the debate on the nature of the pseudogap and the complex electronic phase diagram of these compounds.

cond-mat.supr-con↗

arXiv2003

Unitarity in gauge symmetry breaking on orbifold

We study the unitarity bounds of the scattering amplitudes in the extra dimensional gauge theory where the gauge symmetry is broken by the boundary condition. The estimation of the amplitude of the diagram including four massive gauge bosons in the external lines shows that the asymptotic power behavior of the amplitude is canceled. The calculation will be done in the 5 dimensional standard model and the SU(5) grand unified theory, whose 5th dimensional coordinate is compactified on $S^1/Z_2$. The broken gauge theories through the orbifolding preserve the unitarity at high energies similarly to the broken gauge theories where the gauge bosons obtain their masses through the Higgs mechanism. We show that the contributions of the Kaluza-Klein states play a crucial role in conserving the unitarity.

hep-th↗

arXiv2002

Non-Anomalous Flavor Symmetries and $SU(6) \times SU(2)_R$ Model

We introduce the flavor symmetry ${\bf Z}_M \times {\bf Z}_N \times D_4$ into the $SU(6) \times SU(2)_R$ string-inspired model. The cyclic group ${\bf Z}_M$ and the dihedral group $D_4$ are R symmetries, while ${\bf Z}_N$ is a non-R symmetry. By imposing the anomaly-free conditions on the model, we obtain a viable solution under many phenomenological constraints coming from the particle spectra. For neutrino sector, we find a LMA-MSW solution but not SMA-MSW ones. The solution includes phenomenologically acceptable results concerning fermion masses and mixings and also concerning hierarchical energy scales including the GUT scale, the $μ$ scale and the Majorana mass scale of R-handed neutrinos.

hep-ph↗

arXiv2002

Neutrino Large Mixing in Universal Yukawa Coupling Model with Small Violation

We have analyzed the possibility that the universal Yukawa coupling (democratic mass matrix) with small violations of Dirac and Majorana neutrinos can induce the large mixing of neutrinos through the seesaw mechanism. The possibility can be achieved by the condition that the violation parameters of Majorana neutrinos are sufficiently smaller than the violation parameters of Dirac neutrinos. Allowed regions of the violation parameters producing the observed neutrino mass hierarchy and large neutrino mixing are not so restricted at present in contrast to the violation parameters for quark sector.

hep-ph↗

arXiv2001

Flavor Symmetry on Non-Commutative Compact Space and $SU(6) \times SU(2)_R$ Model

In the four-dimensional effective theory from string compactification discrete flavor symmetries arise from symmetric structure of the compactified space and generally contain both the $R$ symmetry and non-$R$ symmetry. We point out that a new type of non-Abelian flavor symmetry can also appear if the compact space is non- commutative. Introducing the dihedral group $D_4$ as such a new type of flavor symmetry together with the $R$ symmetry and non-$R$ symmetry in $SU(6) \times SU(2)_R$ model, we explain not only fermion mass hierarchies but also hierarchical energy scales including the breaking scale of the GUT-type gauge symmetry, intermediate Majorana masses of R-handed neutrinos and the scale of $μ$-term.

hep-ph↗

arXiv2000

Unusual behaviors in the transport properties of REFe$_{4}$P$_{12}$ (RE: La, Ce, Pr, and Nd)

We have investigated the resistivity ($ρ$), thermoelectric power (TEP) and Hall coefficient ($R_{H}$) on high quality single crystals of REFe$_{4}$P$_{12}$. TEP in CeFe$_{4}$P$_{12}$ is extremely large ($\sim$ 0.5mV/K at 290K) with a peak of $\sim$ 0.75mV/K at around 65K. The Hall mobility also shows a peak at $\sim$ 65K, suggesting carriers with heavy masses developed at lower temperatures related with the f-hybridized band. Both Pr- and Nd- systems exhibit an apparent increase of $ρ$ with decreasing temperature far above their magnetic transition temperatures. In the same temperature ranges, TEP exhibits unusually large absolute values of -50$μ$V/K for PrFe$_{4}$P$_{12}$ and -15$μ$V/K for NdFe$_{4}$P$_{12}$, respectively. For PrFe$_{4}$P$_{12}$, such anomalous transport properties suggest an unusual ground state, possibly related with the Quadrupolar Kondo effect.

cond-mat.str-el↗

arXiv1999

Diffusion over a saddle with a Langevin equation

The diffusion problem over a saddle is studied using a multi-dimensional Langevin equation. An analytical solution is derived for a quadratic potential and the probability to pass over the barrier deduced. A very simple solution is given for the one dimension problem and a general scheme is shown for higher dimensions.

nucl-th↗

arXiv1999

Theory of Nuclear Reactions Leading to Superheavy Elements

Dynamical reaction theory is presented for synthesis of superheavy elements. Characteristic features of formation and surviving are discussed, which combinedly determine final residue cross sections of superheavy elements. Preliminary results on Z=114 are also given.

nucl-th↗

arXiv1999

Spin Alignments in Heavy-ion Resonances

Recent particle-particle-$γ$ coincident measurements on a $^{28}\rm Si+{}^{28}Si$ resonance have suggested ''vanishing spin alignments''. New analyses for spin alignments by using a molecular model are reported. Different aspects between di-nuclear systems with prolate deformed nuclei and those with oblate deformed nuclei are clarified.

nucl-th↗

arXiv1999

Self-Dual Point and Unification

We hypothesize that the vacuum is spontaneously shifted from the self-dual point in the moduli space, at which all dimensions are compact, and that the true vacuum results from the decompactification of the four space-time dimensions and remains in the vicinity of the self-dual point for extra dimensions. On the basis of our hypotheses we explore new types of the coupling unification which are not realized in the four-dimensional effective theory but in the framework of the higher dimensional theory. In the $SU(6) \times SU(2)_R$ string model, if the SU(6) gauge group lives in the world-volume of 9-branes and the $SU(2)_R$ in the world-volume of 5-branes, or {\it vice versa}, unification solutions can be found in the vicinity of the self-dual point.

hep-ph↗

arXiv1999

Theoretical predictions of residues cross sections of superheavy elements

Dynamical reaction theories are reviewed for synthesis of superheavy elements. Characteristic features of formation and surviving are discussed with reference to possible incident channels. Theoretical predictions are presented on favorable incident channels and on optimum energies for synthesis of Z=114.

nucl-th↗

arXiv1998

Spin-Alignment and Quasi-Molecular Resonances in Heavy-Ion Collision

Fragment-fragment-$γ$ triple coincidence measurements of the $^{28}$Si~$+$~$^{28}$Si reaction at E$_{\rm c.m.}$~=~55.8 MeV, carefully chosen to populate a well known quasi-molecular resonance in $^{56}$Ni, have been performed at the VIVITRON tandem facility by using the Eurogam Phase~II $γ$-ray spectrometer. In the $^{28}$Si~$+$~$^{28}$Si reaction, the resonant behavior of the $^{28}$Si $+$ $^{28}$Si exit-channel is clearly observed by the present fragment-fragment coincidence data. The more unexpected result is the spin disalignment of the $^{28}$Si $+$ $^{28}$Si resonance. This has been demonstrated first by the measured angular distributions of the elastic 0$^{+}$, inelastic 2$^{+}$ and mutual excitation channels 2$^{+}-2^{+}$, which are dominated by a unique and pure partial wave with L = 38 ${\rm \hbar}$, and has been confirmed by measuring their particle-$γ$ angular correlations with Eurogam Phase II. The spin disalignment supports new molecular model predictions, in which the observed resonance would correspond to the ''Butterfly mode''. A discussion concerning the {\it spin alignment and spin disalignment} for different systems : $^{12}$C~$+$~$^{12}$C, $^{24}$Mg~$+$~$^{24}$Mg and $^{28}$Si~$+$~$^{28}$Si will be given.}

nucl-ex↗