arXiv subjects

Y. Li

Publications and source records attributed to Y. Li.

At least 577 records · Page 32Linked to original sources

arXiv2012

Symmetry breaking and low energy conformational fluctuations in amorphous graphene

Recently, the prospects for amorphous phases of graphene (a-G) have been explored computationally. Initial models were flat, and contained odd-member rings, while maintaining three-fold coordination and sp2 bonding. Upon relaxation, puckering occurs, and may be traced to the existence of pentagons, in analogy with the situation for fullerenes. In this work, we systematically explore the inherent structures with energy close to the flat starting structure. As expected, the planar symmetry can be broken in various ways, which we characterize for 800-atom model of a-G, always using local basis density functional techniques. The classical normal modes of various structural models are discussed, with an emphasis on imaginary modes indicating the evolution from flat to puckered. We also discuss very low energy conformational fluctuations akin to those seen previously in amorphous silicon, and reflect on the nature of the amorphous "ground state" within a network of fixed topology. For completeness, high energy modes were also computed, and are found to be associated with strained parts of the network.

cond-mat.dis-nn↗

arXiv2012

Evidence for B- -> Ds+ K- l- nubar and search for B- -> Ds*+ K- l- nubar

We report measurements of the decays B- -> Ds(*)+ K- l- nubar in a data sample containing 657x10^6 BBbar pairs collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. We observe a signal with a significance of 6 sigma for the combined Ds and Ds* modes and find the first evidence of the B- -> Ds+ K- l- nubar decay with a significance of 3.4 sigma. We measure the following branching fractions: BF(B- -> Ds+ K- l nubar) = (0.30 +/- 0.09(stat) +0.11 -0.08(syst)) x 10^-3 and BF(B- -> Ds*+ K- l- nubar) = (0.59 +/- 0.12(stat) +/- 0.15(syst)) x 10^-3 and set an upper limit BF(B- -> Ds*+ K- l- nubar) < 0.56 x 10^-3 at the 90% confidence level. We also present the first measurement of the Ds+K- invariant mass distribution in these decays, which is dominated by a prominent peak around 2.6 GeV/c^2.

hep-ex↗

arXiv2012

First observation of CP violation and improved measurement of the branching fraction and polarization of B0 -> D+ D- decays

We report the measurement of the branching fraction, the polarization, and the parameters of the time-dependent CP violation in B0 -> D*+ D*- decays using a data sample of 772 million BB pairs, collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain a branching fraction of B = (7.82 +- 0.38 +- 0.63) x10^{-4}, a CP-odd fraction of R_\perp = 0.138 +- 0.024 +- 0.006 and, additionally, a fraction of the longitudinal component in the transversity base of R_0 = 0.624 +- 0.029 +- 0.011. The measured values of the parameters of the CP violation are S_{D*+ D*-} = -0.79 +- 0.13 +- 0.03 and A_{D*+ D*-} = 0.15 +- 0.08 +- 0.04.

hep-ex↗

arXiv2012

Probability-preserving evolution in a non-Hermitian two-band model

A non-Hermitian PT-symmetric system can have full real spectrum but does not ensure probability preserving time evolution, in contrast to that of a Hermitian system. We present a non-Hermitian two-band model, which is comprised of dimerized hopping terms and staggered imaginary on-site potentials, and study the dynamics in the exact PT-symmetric phase based on the exact solution. It is shown that an initial state, which does not involve two equal-momentum-vector eigenstates in different bands, obeys perfectly probability-preserving time evolution in terms of the Dirac inner product. Beyond this constriction, the quasi-Hermitian dynamical behaviors, such as non-spreading propagation and fractional revival of a Gaussian wave packet, are also observed.

quant-ph↗

arXiv2012

Dynamics aspect of subbarrier fusion reaction in light heavy ion systems

Subbarrier fusion of the 7Li + 12C reaction is studied using an antisymmetrized molecular dynamics model (AMD) with an after burner, GEMINI. In AMD, 7Li shows an α+ t structure at its ground state and it is significantly deformed. Simulations are made near the Coulomb barrier energies, i.e., E_{cm} = 3 - 8 MeV. The total fusion cross section of the AMD + GEMINI calculations as a function of incident energy is compared to the experimental results and both are in good agreement at E_{cm} > 3 MeV. The cross section for the different residue channels of the AMD + GEMINI at E_{cm} = 5 MeV is also compared to the experimental results.

nucl-th↗

arXiv2012

On the stability of $ϕ$-uniform domains

We study two metrics, the quasihyperbolic metric and the distance ratio metric of a subdomain $G \subset {\mathbb R}^n$. In the sequel, we investigate a class of domains, so called $φ$-uniform domains, defined by the property that these two metrics are comparable with respect to a homeomorphism $φ$ from $[0,\infty)$ to itself. Finally, we discuss a number of stability properties of $φ$-uniform domains. In particular, we show that the class of $φ$-uniform domains is stable in the sense that removal of a geometric sequence of points from a $φ$-uniform domain yields a $φ_1$-uniform domain.

math.MG↗

arXiv2012

Universal optimal broadband photon cloning and entanglement creation in one dimensional atoms

We study an initially inverted three-level atom in the lambda configuration embedded in a waveguide, interacting with a propagating single-photon pulse. Depending on the temporal shape of the pulse, the system behaves either as an optimal universal cloning machine, or as a highly efficient deterministic source of maximally entangled photon pairs. This quantum transistor operates over a wide range of frequencies, and can be implemented with today's solid-state technologies.

quant-ph↗

arXiv2012

Analysis and Modeling of Two Flare Loops Observed by AIA and EIS

We analyze and model an M1.0 flare observed by SDO/AIA and Hinode/EIS to investigate how flare loops are heated and evolve subsequently. The flare is composed of two distinctive loop systems observed in EUV images. The UV 1600 Åemission at the feet of these loops exhibits a rapid rise, followed by enhanced emission in different EUV channels observed by AIA and EIS. Such behavior is indicative of impulsive energy deposit and the subsequent response in overlying coronal loops that evolve through different temperatures. Using the method we recently developed, we infer empirical heating functions from the rapid rise of the UV light curves for the two loop systems, respectively, treating them as two big loops of cross-sectional area 5\arcsec by 5\arcsec, and compute the plasma evolution in the loops using the EBTEL model (Klimchuk et al. 2008). We compute the synthetic EUV light curves, which, with the limitation of the model, reasonably agree with observed light curves obtained in multiple AIA channels and EIS lines: they show the same evolution trend and their magnitudes are comparable by within a factor of two. Furthermore, we also compare the computed mean enthalpy flow velocity with the Doppler shift measurements by EIS during the decay phase of the two loops. Our results suggest that the two different loops with different heating functions as inferred from their footpoint UV emission, combined with their different lengths as measured from imaging observations, give rise to different coronal plasma evolution patterns captured both in the model and observations.

astro-ph.SR↗

arXiv2012

Evidence for CP Violation in the Decay $D^+\rightarrow K^0_Sπ^+$

We observe evidence for CP violation in the decay $D^+\rightarrow K^0_Sπ^+$ using a data sample with an integrated luminosity of 977 fb$^{-1}$ collected by the Belle detector at the KEKB $e^+e^-$ asymmetric-energy collider. The CP asymmetry in the decay is measured to be $(-0.363\pm0.094\pm0.067)%$, which is 3.2 standard deviations away from zero, and is consistent with the expected CP violation due to the neutral kaon in the final state.

hep-ex↗

arXiv2012

Spin transfer torque devices utilizing the giant spin Hall effect of tungsten

We report a giant spin Hall effect (SHE) in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip we determine the spin Hall angle to be |θ|=0.30\pm0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices we independently determine |θ|=0.33\pm0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.

cond-mat.mtrl-sci↗

arXiv2012

Search for $B^{0}$ decays to invisible final states at Belle

We report a search for $B^{0}$ decays into invisible final states using a data sample of $657 \times 10^{6}$ $B\bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^{+}e^{-}$ collider. The signal is identified by fully reconstructing a hadronic decay of the accompanying $B$ meson and requiring no other particles in the event. No significant signal is observed, and we obtain an upper limit of $1.3 \times 10^{-4}$ at the 90% confidence level for the branching fraction of invisible $B^{0}$ decay.

hep-ex↗

arXiv2012

Three-dimensionality and orbital characters of Fermi surface in (Tl,Rb)$_y$Fe$_{2-x}$Se$_2$

We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor (Tl,Rb)$_y$Fe$_{2-x}$Se$_2$ ($T_c=32$ K). We determined the orbital characters and the $k_z$ dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4$p_z$ and Fe 3$d_{xy}$ orbitals to the small electron pocket. We argue that the emergence of Se 4$p_z$ states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.

cond-mat.supr-con↗

arXiv2012

First Measurement of phi_3 with a Model-independent Dalitz Plot Analysis of B->DK, D->KsPiPi Decay

We present the first measurement of the angle phi_3 of the Unitarity Triangle using a model-independent Dalitz plot analysis of B->DK, D->KsPiPi decays. The method uses an input measurements of the strong phase of the D->KsPiPi amplitude from the CLEO collaboration. The result is based on the full data set of 772x10^6 BBbar pairs collected by the Belle experiment at the Upsilon(4S) resonance. We obtain phi_3 = (77.3^{+15.1}_{-14.9} +- 4.1 +- 4.3)^{\circ} and the suppressed amplitude ratio r_B = 0.145 +- 0.030 +- 0.010 +- 0.011. Here the first error is statistical, the second is the experimental systematic uncertainty, and the third is the error due to the precision of the strong-phase parameters obtained by CLEO.

hep-ex↗

arXiv2012

Evidence for a Fermi liquid in the pseudogap phase of high-Tc cuprates

Cuprate high-T_c superconductors on the Mott-insulating side of "optimal doping" (with respect to the highest T_c's) exhibit enigmatic behavior in the non-superconducting state. Near optimal doping the transport and spectroscopic properties are unlike those of a Landau-Fermi liquid. For carrier concentrations below optimal doping a pseudogap removes quasi-particle spectral weight from parts of the Fermi surface, and causes a break-up of the Fermi surface into disconnected nodal and anti-nodal sectors. Here we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. Our optical measurements reveal that the dynamical relaxation rate 1/tau(omega,T) collapses on a universal function proportional to (hbar omega)^2+(1.5 pi k_B T)^2. Hints at possible Fermi liquid behavior came from the recent discovery of quantum oscillations at low temperature and high magnetic field in underdoped YBa2Cu3O6+d and YBa2Cu4O8, from the observed T^2-dependence of the DC (ω=0) resistivity for both overdoped and underdoped cuprates, and from the two-fluid analysis of nuclear magnetic resonance data. However, the direct spectroscopic determination of the energy dependence of the life-time of the excitations -provided by our measurements- has been elusive up to now. This observation defies the standard lore of non-Fermi liquid physics in high T_c cuprates on the underdoped side of the phase diagram.

cond-mat.supr-con↗

arXiv2012

First observation of exclusive $Υ(1S)$ and $Υ(2S)$ decays into light hadrons

Using samples of 102 million $Υ(1S)$ and 158 million $Υ(2S)$ events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to the three-body final states $ϕK^+ K^-$, $ωπ^+ π^-$ and $K^{\ast 0}(892) K^- π^+ $, and to the two-body Vector-Tensor ($ϕf_2'(1525)$, $ωf_2(1270)$, $ρa_2(1320)$ and $K^{\ast 0}(892) \bar{K}_2^{\ast 0}(1430) $) and Axial-vector-Pseudoscalar ($K_1(1270)^+ K^-$, $K_1(1400)^+ K^- $ and $b_1(1235)^+ π^- $) pairs. Signals are observed for the first time in the $Υ(1S) \to ϕK^+ K^-$, $ωπ^+ π^-$, $K^{\ast 0} K^- π^+$, $K^{\ast0} K_2^{\ast 0}$ and $Υ(2S) \to ϕK^+ K^-$, $K^{\ast 0} K^- π^+$ decay modes. Branching fractions are determined for all the processes, while 90% confidence level upper limits are established on the branching fractions for the modes with a statistical significance less than $3σ$. The ratios of the branching fractions of $Υ(2S)$ and $Υ(1S)$ decays into the same final state are used to test a perturbative QCD prediction for OZI suppressed bottomonium decays.

hep-ex↗

arXiv2012

Search for B -> phi pi decays

We report on a search for the charmless decays $B^{+} \toϕπ^{+}$ and $B^{0} \toϕπ^{0}$ that are strongly suppressed in the Standard Model. The analysis is based on a data sample of $657 \times 10^6$ $B \bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We find no significant signal and set upper limits of $3.3 \times 10^{-7}$ for $B^{+} \to ϕπ^{+}$ and $1.5 \times 10^{-7}$ for $B^0 \to ϕπ^0$ at the 90% confidence level.

hep-ex↗

arXiv2012

Measurements of Branching Fractions and Time-dependent CP Violating Asymmetries in $B^{0} \to D^{(*)\pm}D^{\mp}$ Decays

We report measurements of branching fractions and time-dependent CP asymmetries in $B^{0} \to D^{+}D^{-}$ and $B^{0} \to D^{*\pm}D^{\mp}$ decays using a data sample that contains $(772 \pm 11)\times 10^6 B\bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We determine the branching fractions to be $\mathcal{B}(B^{0} \to D^{+}D^{-})=(2.12 \pm 0.16 \pm 0.18)\times 10^{-4}$ and $\mathcal{B}(B^{0} \to D^{*\pm}D^{\mp}=(6.14 \pm 0.29 \pm 0.50)\times 10^{-4}$. We measure CP asymmetry parameters $\mathcal{S}_{D^{+}D^{-}} = -1.06_{-0.14}^{+0.21} \pm 0.08$ and $\mathcal{C}_{D^{+}D^{-}} = -0.43 \pm 0.16 \pm 0.05$ in $B^{0} \to D^{+}D^{-}$ and $\mathcal{A}_{D^{*}D} = +0.06 \pm 0.05 \pm 0.02$, $\mathcal{S}_{D^{*}D} = -0.78 \pm 0.15 \pm 0.05$, $\mathcal{C}_{D^{*}D} = -0.01 \pm 0.11 \pm 0.04$, $Δ\mathcal{S}_{D^{*}D} = -0.13 \pm 0.15 \pm 0.04$ and $Δ\mathcal{C}_{D^{*}D} = +0.12 \pm 0.11 \pm 0.03$ in $B^{0} \to D^{*\pm}D^{\mp}$, where the first uncertainty is statistical and the second is systematic. We exclude the conservation of CP symmetry in both decays at equal to or greater than $4σ$ significance.

hep-ex↗

arXiv2012

Search for Time-Dependent CPT Violation in Hadronic and Semileptonic B Decays

We report a new sensitive search for CPT violation, which includes improved measurements of the CPT-violating parameter z and the total decay-width difference normalized to the averaged width DeltaGamma_d/Gamma_d of the two Bd mass eigenstates. The results are based on a data sample of 535 x 10^6 BBbar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We obtain Re(z) = [+1.9 +/- 3.7 (stat) +/- 3.3 (syst)] x 10^{-2}, Im(z) = [-5.7 +/- 3.3 (stat) +/- 3.3 (syst)] x 10^{-3}, and DeltaGamma_d/Gamma_d = [-1.7 +/- 1.8 (stat) +/- 1.1 (syst)] x 10^{-2}, all of which are consistent with zero. This is the most precise single measurement of these parameters in the neutral B-meson system to date.

hep-ex↗