Search Results (77 total)

We study B→K^(*)ℓ⁺ℓ^- decays (ℓ=e, μ) based on a data sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB e⁺e^- collider. We report the differential branching fraction, isospin asymmetry, K^* polarization, and the forward-backward asymmetry (A_FB) as functions of q²=M_ℓℓ²c². The fitted A_FB spectrum exceeds the standard model expectation by 2.7 standard deviations. The measured branching fractions are B(B→K^*ℓ⁺ℓ^-)=(10.7_-1.0^+1.1±0.9)×10^-7 and B(B→Kℓ⁺ℓ^-)=(4.8_-0.4^+0.5±0.3)×10^-7, where the first errors are statistical and the second are systematic, with the muon to electron ratios R_K^*=0.83±0.17±0.08 and R_K=1.03±0.19±0.06.

A Reply to the Comment by Beate Müller et al..

The polarized Raman-scattering spectra of nonsuperconducting α-FeTe and of As-free superconductor Fe_1.03Se_0.3Te_0.7 are measured at room temperature on single crystals. The phonon modes are assigned by combining symmetry analysis with first-principles calculations. In the parent compound α-FeTe, the A_1g mode of the Te atom and the B_1g mode of the Fe atom are observed clearly, while in superconducting Fe_1.03Se_0.3Te_0.7, only a softened Fe B_1g mode can be seen. No electron-phonon coupling feature can be distinguished in the spectra of the two samples. By contrast, the spectra of the superconducting system show a slight enhancement below 300 cm⁻¹, which may be of electronic origin.

The nature of the spin-density wave (SDW) and its relation with superconductivity are crucial issues in the newly discovered iron-pnictide superconductors. Particularly, it is unclear whether the superconducting phase and SDW are truly exclusive from each other. We here report splittings of the band structures in Sr_1-xK_xFe₂As₂ (x=0, 0.1, 0.18), and their unusual doping dependence. Our data on single crystalline samples prove that the SDW and superconductivity could coexist in iron pnictides.

We study the charmless decays B→ΛΛ̅ h, where h stands for π⁺, K⁺, K⁰,K^*+, or K^*0, using a 605  fb^-1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider. We observe B⁰→ΛΛ̅ K⁰ and B⁰→ΛΛ̅ K^*0 with branching fractions of (4.76_-0.68^+0.84(stat)±0.61(syst))×10^-6 and (2.46_-0.72^+0.87±0.34)×10^-6, respectively. The significances of these signals in the threshold-mass enhanced mass region, M_ΛΛ̅ <2.85  GeV/c², are 12.4σ and 9.3σ, respectively. We also update the branching fraction B(B⁺→ΛΛ̅ K⁺)=(3.38_-0.36^+0.41±0.41)×10^-6 with better accuracy, and report the following measurement or 90% confidence level upper limit in the threshold-mass-enhanced region: B(B⁺→ΛΛ̅ K^*+)=(2.19_-0.88^+1.13±0.33)×10^-6 with 3.7σ significance; B(B⁺→ΛΛ̅ π⁺)<0.94×10^-6. A related search for B⁰→ΛΛ̅ D̅ ⁰ yields a branching fraction B(B⁰→ΛΛ̅ D̅ ⁰)=(1.05_-0.44^+0.57±0.14)×10^-5. This may be compared with the large, ∼10^-4, branching fraction observed for B⁰→pp̅ D̅ ⁰. The M_ΛΛ̅ enhancements near threshold and related angular distributions for the observed modes are also reported.

The magnetic properties in the parent compounds are often intimately related to the microscopic mechanism of superconductivity. Here we report the first direct measurements on the electronic structure of a parent compound of the newly discovered iron-based superconductor, BaFe₂As₂, which provides a foundation for further studies. We show that the energy of the spin density wave in BaFe₂As₂ is mainly lowered through exotic exchange splitting of the band structure.

NbSe₂ is a prototypical charge-density-wave (CDW) material, whose mechanism remains mysterious so far. With angle resolved photoemission spectroscopy, we recovered the long-lost nesting condition over a large broken-honeycomb region in the Brillouin zone, which consists of six saddle band point regions with high density of states (DOS), and large regions away from Fermi surfaces with negligible DOS at the Fermi energy. We show that the major contributions to the CDW formation come from these barely occupied states rather than the saddle band points. Our findings not only resolve a long-standing puzzle, but also overthrow the conventional wisdom that CDW is dominated by regions with high DOS.

We investigate charge pumping in carbon nanotube quantum dots driven by the electric field of a surface acoustic wave. We find that, at small driving amplitudes, the pumped current reverses polarity as the conductance is tuned through a Coulomb blockade peak using a gate electrode. We study the behavior as a function of wave amplitude, frequency, and direction and develop a model in which our results can be understood as resulting from adiabatic charge redistribution between the leads and quantum dots on the nanotube.

Angle resolved photoemission spectroscopy study is reported on a high quality optimally doped Bi₂Sr_1.6La_0.4CuO_6+δ high-T_c superconductor. In the antinodal region with a maximal d-wave gap, the symbolic superconducting coherence peak, which has been widely observed in multi-CuO₂-layer cuprate superconductors, is unambiguously observed in a single-layer system. The associated peak-dip separation is just about 19 meV, which is much smaller than its counterparts in multilayered compounds, but correlates with the energy scales of spin excitations in single-layer cuprates.

We present a search for the decays B⁺→π⁺ℓ⁺ℓ^- and B⁰→π⁰ℓ⁺ℓ^-, where ℓ⁺ℓ^- is either a μ⁺μ^- or e⁺e^- pair, with a data sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB e⁺e^- collider. Signal events are reconstructed from a charged or a neutral pion candidate and a pair of oppositely charged electrons or muons. No significant signal is observed and we set an upper limit on the isospin-averaged branching fraction B(B→πℓ⁺ℓ^-)<6.2×10^-8 at the 90% confidence level.

Since 2001, the Relativistic Heavy Ion Collider has experienced electron cloud effects, some of which have limited the beam intensity. These include dynamic pressure rises (including pressure instabilities), tune shifts, a reduction of the instability threshold for bunches crossing the transition energy, and possibly incoherent emittance growth. We summarize the main observations in operation and dedicated experiments as well as countermeasures including baking, nonevaporable getter coated warm beam pipes, solenoids, bunch patterns, antigrazing rings, prepumped cold beam pipes, scrubbing, and operation with long bunches.

The electronic structure of a new charge-density-wave system or superconductor, 1T-Cu_xTiSe₂, has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case, which resolves a long-standing controversy in the system. With Cu doping, the charge-density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states, and possibly suppressed at higher doping by the strong scattering.

The mechanical behavior of single-walled carbon nanocones (SWCNCs) with 19.2° apex angles under compression was investigated in the study reported herein. The analysis was performed using molecular dynamics simulation. Two different loading directions, i.e., axial compression and in-plane compression, were applied to study their influence on the strain energy of CNCs. We derived empirical formulas for calculating the critical strains of CNCs with various top radii that were subjected to both axial and in-plane compression. The results of the simulation reveal that cones under in-plane compression show a higher energy level than those under axial compression. In the buckling and post-buckling stage, each shape change, accompanied by an abrupt release of energy in the energy-strain curve, was analyzed. The possible reasons for early plastic failure were examined, with the buckling behavior of CNCs with geometrical parameters (top radius, bottom radius, and height) involved. Our computed results show that for the fixed ratio of height/bottom radius, CNCs with smaller top radii tend to be stiffer.

Scanning tunneling spectroscopy was performed on c-axis Y_1-xCa_xBa₂Cu₃O_7-δ thin films for x=0, 0.05, 0.15, and 0.20 at 4.2 K. The measured spectra show main-gap, subgap, and satellite features which scale similarly in energy versus Ca doping, suggesting that they are associated with a single pairing energy. The data are analyzed with a multiband tunneling model which attributes the subgap features to the chain band and the satellite and main-gap features to the plane band for d_x²-y²+s pairing symmetry. These results suggest that the superconductivity in Y_1-xCa_xBa₂Cu₃O_7-δ involves multiple bands.

Spin filtering in a metal/organic-ferromagnet/metal sandwich device was investigated based on an extended SSH+Heisenberg (SSH=Su-Schrieffer-Heeger) model and the Landauer-Büttiker formula. A large spin-polarized current was predicted, which is dependent on the property of the organic ferromagnetic interlayer as well as the interfacial interactions between the metal electrode and the organic ferromagnet. The effect of thermal fluctuation of the radical spins on the current polarization is discussed. Our investigation suggests a possible application of an organic ferromagnet in a spin filter device without any external field modulation.

In the present photoemission study of a cuprate superconductor Bi_1.74Pb_0.38Sr_1.88CuO_6+δ, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. Similar behavior observed in blue bronze and the Mott insulator Ca₂CuO₂Cl₂ suggests that the origin of hopping-dominated dispersion in an overdoped cuprate might be quite complicated. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate. These results recovered significant missing pieces in our current understanding of the electronic structure of cuprates.

The quantum ballistic transport of electrons through nanowire junctions formed by putting one nanowire (with square, rectangular, and circular cross sections) on top of another wire (the two wires are not lying in the same plane) is studied using tight-binding models and the Green’s function approach. Different tight-binding models are considered to find the optimum number of tight-binding sites in the transverse plan for approximating the continuum model in a finite range of energy and also to find the minimal site number for qualitative description of the transport characteristics. Resonant dips and peaks found in the interwire and intrawire conductances can be explained by the formation of bound and quasibound states at the cross junction. How the conduction channels of the wires are coupled together in the formation of the bound and quasibound states are analyzed using the projected Green’s functions. Quasibound states unbound in both wires give rise to resonant peaks in the interwire conductance as well as resonant dips in the intrawire conductance. Quasibound states unbound in only one wire give rise to only resonant dips in the intrawire conductance without corresponding resonant peaks in the interwire conductance. Some exception in the latter case is discussed. Reduction of interwire coupling strength is shown to suppress the conductance at energy far from the subband edges. In the weak interwire coupling regime, larger conductances are found at energies close to the subband edges. A comparison of the square and rectangular wire junctions studies is made. Increase in the dimension perpendicular to a square wire junction reduces the interwire conductance and enhances the intrawire conductance. On the other hand, degeneracy of subbands due to the higher symmetry of a square cross section enhances the interwire conductance. Comparison of circular wire junctions with square wire junctions show that some conductance features found in square wire junctions disappear in circular wire junctions owing to the weaker interwire coupling in circular wire junctions, which is the result of a smaller contact area between wires. The energies and probability densities of some bound and quasibound states are also determined and analyzed in the present study.

Transport process of a charged polaron under a linearly increasing electric field in conjugated polymers is investigated by using a nonadiabatic evolution method. It is found that the behavior of a polaron depends not only upon the strength of the field but also upon the mode of the application of the field. A polaron can conserve even under a strong field if the field is applied slowly. The result is compared with that of the adiabatic approximation and it is obtained that the nonadiabatic critical time is much larger than the adiabatic value. Dependence of the applying mode of the external electric field on the polaron dissociation and the transition of its velocity from the subsonic to supersonic value are further described.

Defects in ferrite can change the local magnetic coupling from ferromagnetic to strongly antiferromagnetic. In eptiaxial magnetite (Fe₃O₄) films, defects locally altering exchange interaction, i.e., “antiphase boundaries” (APB), essentially determine magnetic and magnetotransport properties. We locally observed specific magnetization reversal events in epitaxial magnetite Fe_3−δO₄∕MgO films (δ≈0.03) by magnetic force microscopy in external magnetic fields. A dominating phenomenon is pinning of bubble domains at single APB. Pinning is a particular consequence of antiferromagnetic coupling across APB. Antiferromagnetic coupling across APB could be directly verified for a sample with an APB domain size above the resolution limit of magnetic force microscopy.

The electron spin transport in an asymmetrical magnetic superlattice (MSL), which is due to the periodic barriers with abnormal one in height, has been theoretically studied. The effects of the abnormal barrier in the MSL have been introduced by a defined asymmetry factor, which corresponds to the changeable height of the abnormal barrier. The results show that in the ballistic transport region, the spin-up electron transmission and conductance can be dramatically depressed by the enhancement of the asymmetry factor. However, the spin-down transmission and conductance are only slightly affected. It means that the spin transport in the asymmetric MSL is obviously different from that in the symmetric MSL. In addition, the value of spin polarization of the conductance can be well controlled by the asymmetry factor in a wide range of energy, and highly polarized transport is possible. Furthermore, the number of the abnormal barriers can also strongly affect the oscillation of the spin polarization.

A Reply to the Comments by G. Sheet and P. Raychaudhuri and W. K. Park and L. H. Greene.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has received renewed interest recently due to the experimental indication of its presence in CeCoIn₅, a quasi-two-dimensional (2D) d-wave superconductor. However direct evidence of the spatial variation of the superconducting order parameter, which is the hallmark of the FFLO state, does not yet exist. In this work we explore the possibility of detecting the phase structure of the order parameter directly using conductance spectroscopy through microconstrictions, which probes the phase sensitive surface Andreev bound states of d-wave superconductors. We employ the Blonder-Tinkham-Klapwijk formalism to calculate the conductance characteristics between a normal metal (N) and a 2D s- or d_x²−y²-wave superconductor in the Fulde-Ferrell state, for all barrier parameter z from the point contact limit (z=0) to the tunneling limit (z≫1). We find that the zero-bias conductance peak due to these surface Andreev bound states observed in the uniform d-wave superconductor is split and shifted in the Fulde-Ferrell state. We also clarify what weighted bulk density of states is measured by the conductance in the limit of large z.

We explore the electronic structure in the heavily overdoped regime of the single-layer cuprate superconductor Bi_1.74Pb_0.38Sr_1.88CuO_6+δ. We found that the nodal quasiparticle behavior is dominated mostly by phonons, while the antinodal quasiparticle line shape is dominated by spin fluctuations. Moreover, while long range spin fluctuations diminish at very high doping, the local magnetic fluctuations still dominate the quasiparticle dispersion, and the system exhibits a strange metal behavior in the entire overdoped regime.

It has been shown that the maintenance condition for a crystalline beam requires that there be no resonance between the crystal’s phonon frequencies, and the frequency associated with a beam moving through a lattice of N_sp periods. This resonance can be avoided provided that the phonon frequencies all are below half the lattice frequency. Here, we study in detail the phonon modes of several crystalline beams. The analytic results obtained in the smooth approximation are compared with numerical evaluations employing Fourier transform of the molecular dynamics (MD) modes. The stability of various crystalline structures is examined through systematic MD simulations based on several different lattice designs. The maintenance condition, when combined with either the simple analytic theory or the numerical evaluation of phonon modes, exhibits excellent agreement with the MD calculations of crystal stability. A confirmed maintenance condition, derived from linear-resonance criteria, is that the lattice frequency must not equal the sum of any two phonon frequencies.

We report on the effect of monochromatic microwave (MW) radiation on the weak-localization corrections to the conductivity of quasi-one-dimensional silver wires. Because of the improved electron cooling in the wires, the MW-induced dephasing is observed without a concomitant overheating of electrons over wide ranges of the MW power P_MW and frequency f. The observed dependences of the conductivity and MW-induced dephasing rate on P_MW and f are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky [Solid State Commun. 39, 619 (1981)]. Our results suggest that in the low-temperature experiments with 1D wires, saturation of the temperature dependence of the dephasing time can be caused by an MW electromagnetic noise with a sub-pW power.