arXiv subjects

L. Li

Publications and source records attributed to L. Li.

At least 235 records · Page 13Linked to original sources

arXiv2014

Investigation of the dd -> 3He n π0 reaction with the FZ Jülich WASA-at-COSY facility

An exclusive measurement of the dd -> 3He n π0 reaction was carried at a beam momentum of p = 1.2 GeV/c using the WASA-at-COSY facility. For the first time data on the total cross section as well as differential distributions were obtained. The data are described with a phenomenological approach based on a combination of a quasi-free model and a partial wave expansion for three-body reaction. The total cross section is found to be σ(tot) = (2.89 +- 0.01(stat) +- 0.06(sys) +- 0.29(norm)) μb. The contribution of the quasi-free processes (with the neutron being target or beam spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of the phase space. The remaining part of the cross section can be described within a partial wave decomposition indicating the significance of p-wave contributions in the final state.

nucl-ex↗

arXiv2013

Spectroscopic factors for low-lying 16N levels and the astrophysical 15N(n,gamma)16N reaction rate

Fluorine is a key element for nucleosynthetic studies since it is extremely sensitive to the physical conditions within stars. The astrophysical site to produce fluorine is suggested to be asymptotic giant branch (AGB) stars. In these stars the 15N(n, g)16N reaction could affect the abundance of fluorine by competing with 15N(a, g)19F. The 15N(n, g)16N reaction rate depends directly on the neutron spectroscopic factors of the low-lying states in 16N. The angular distributions of the 15N(7Li, 6Li)16N reaction populating the ground state and the first three excited states in 16N are measured using a Q3D magnetic spectrograph and are used to derive the spectroscopic factors of these states based on distorted wave Born approximation (DWBA) analysis. The spectroscopic factors of these four states are extracted to be 0.96+-0.09, 0.69+-0.09, 0.84+-0.08 and 0.65+-0.08, respectively. Based on the new spectroscopic factors we derive the 15N(n,g)16N reaction rate. The accuracy and precision of the spectroscopic factors are enhanced due to the first application of high-precision magnetic spectrograph for resolving the closely-spaced 16N levels which can not be achieved in most recent measurement. The present result demonstrates that two levels corresponding to neutron transfers to the 2s1/2 orbit in 16N are not so good single-particle levels although 15N is a closed neutron-shell nucleus. This finding is contrary to the shell model expectation. The present work also provides an independent examination to shed some light on the existing discrepancies in the spectroscopic factors and the 15N(n, g)16N rate.

nucl-ex↗

arXiv2013

Measurement of the $pn \to ppπ^0π^-$ Reaction in Search for the Recently Observed Resonance Structure in $dπ^0π^0$ and $dπ^+π^-$ systems

Exclusive measurements of the quasi-free $pn \to ppπ^0π^-$ reaction have been performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region $\sqrt s$ = (2.35 - 2.46) GeV, which includes the region of the ABC effect and its associated resonance structure. No ABC effect, {\it i.e.} low-mass enhancement is found in the $π^0π^-$-invariant mass spectrum -- in agreement with the constraint from Bose statistics that the isovector pion pair can not be in relative s-wave. At the upper end of the covered energy region $t$-channel processes for Roper, $Δ(1600)$ and $ΔΔ$ excitations provide a reasonable description of the data, but at low energies the measured cross sections are much larger than predicted by such processes. Adding a resonance amplitude for the resonance at $m$=~2.37 GeV with $Γ$ =~70 MeV and $I(J^P)=~0(3^+)$ observed recently in $pn \to dπ^0π^0$ and $pn \to dπ^+π^-$ reactions leads to an agreement with the data also at low energies.

nucl-ex↗

arXiv2013

Novel magnetism of Ir5+ ions in the double perovskite Sr2YIrO6

We synthesize and study single crystals of a new double-perovskite Sr2YIrO6. Despite two strongly unfavorable conditions for magnetic order, namely, pentavalent Ir5+(5d4) ions which are anticipated to have Jeff=0 singlet ground states in the strong spin-orbit coupling (SOC) limit, and geometric frustration in a face centered cubic structure formed by the Ir5+ ions, we observe this iridate to undergo a novel magnetic transition at temperatures below 1.3 K. We provide compelling experimental and theoretical evidence that the origin of magnetism is in an unusual interplay between strong non-cubic crystal fields and intermediate-strength SOC. Sr2YIrO6 provides a rare example of the failed dominance of SOC in the iridates.

cond-mat.str-el↗

arXiv2013

Evolution of Magnetism in Single-Crystal Honeycomb Iridates

We report the successful synthesis of single-crystals of the layered iridate, (Na$_{1-x}$Li$_{x}$)$_2$IrO$_3$, $0\leq x \leq 0.9$, and a thorough study of its structural, magnetic, thermal and transport properties. The new compound allows a controlled interpolation between Na$_2$IrO$_3$ and Li$_2$IrO$_3$, while maintaing the novel quantum magnetism of the honeycomb Ir$^{4+}$ planes. The measured phase diagram demonstrates a dramatic suppression of the Néel temperature, $T_N$, at intermediate $x$ suggesting that the magnetic order in Na$_2$IrO$_3$ and Li$_2$IrO$_3$ are distinct, and that at $x\approx 0.7$, the compound is close to a magnetically disordered phase that has been sought after in Na$_2$IrO$_3$ and Li$_2$IrO$_3$. By analyzing our magnetic data with a simple theoretical model we also show that the trigonal splitting, on the Ir$^{4+}$ ions changes sign from Na$_2$IrO$_3$ and Li$_2$IrO$_3$, and the honeycomb iridates are in the strong spin-orbit coupling regime, controlled by $\jeff=1/2$ moments.

cond-mat.str-el↗

arXiv2013

Search for the eta-mesic 4He with WASA-at-COSY detector

An exclusive measurement of the excitation function for the dd->3Heppi- reaction was performed at the Cooler Synchrotron COSY-Juelich with the WASA-at-COSY detection system. The data were taken during a slow acceleration of the beam from 2.185 GeV/c to 2.400 GeV/c crossing the kinematic threshold for the eta meson production in the dd->4He-eta reaction at 2.336 GeV/c. The corresponding excess energy with respect to the 4He-eta system varied from -51.4MeV to 22MeV. The integrated luminosity in the experiment was determined using the dd->3Hen reaction. The shape of the excitation function for the dd->3Heppi- was examined. No signal of the 4He-eta bound state was observed. An upper limit for the cross-section for the bound state formation and decay in the process dd->(4He-eta)bound->3Heppi- was determined on the 90% confidence level and it varies from 20nb to 27nb for the bound state width ranging from 5MeV to 35MeV, respectively.

nucl-ex↗

arXiv2013

Search for a dark photon in the $π^0 \to e^+e^-γ$ decay

The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->ppπ^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. ε^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.

hep-ex↗

arXiv2013

Dirac vs. Weyl in topological insulators: Adler-Bell-Jackiw anomaly in transport phenomena

Dirac metals (gapless semi-conductors) are believed to turn into Weyl metals when perturbations, which break either time reversal symmetry or inversion symmetry, are employed. However, no experimental evidence has been reported for the existence of Weyl fermions in three dimensions. Applying magnetic fields near the topological phase transition from a topological insulator to a band insulator in Bi1-xSbx, we observe not only the weak anti-localization phenomenon in magnetoconductivity near zero magnetic fields (B < 0.4 T) but also its upturn above 0.4 T only for E // B. This incompatible coexistence between weak anti-localization and negative magnetoresistivity is attributed to the Adler-Bell-Jackiw anomaly (topological E B term) in the presence of weak anti-localization corrections.

cond-mat.str-el↗

arXiv2013

Experimental Quantum Computing to Solve Systems of Linear Equations

Solving linear systems of equations is ubiquitous in all areas of science and engineering. With rapidly growing data sets, such a task can be intractable for classical computers, as the best known classical algorithms require a time proportional to the number of variables N. A recently proposed quantum algorithm shows that quantum computers could solve linear systems in a time scale of order log(N), giving an exponential speedup over classical computers. Here we realize the simplest instance of this algorithm, solving 2*2 linear equations for various input vectors on a quantum computer. We use four quantum bits and four controlled logic gates to implement every subroutine required, demonstrating the working principle of this algorithm.

quant-ph↗

arXiv2013

Tuning Jeff = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridate Sr3Ir2O7

Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure, PC = 13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles θ. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7 readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results are discussed along with comparisons drawn with Sr2IrO4, a prototype of the Jeff = 1/2 insulator.

cond-mat.str-el↗

arXiv2013

Parity breaking and scaling behavior in light-matter interaction

The light-matter interaction described by Rabi model and Jaynes-Cummings (JC) model is investigated by parity breaking as well as the scaling behavior of ground-state population-inversion expectation. We show that the parity breaking leads to different scaling behaviors in the two models, where the Rabi model demonstrates scaling invariance, but the JC model behaves in cusp-like way. Our study helps further understanding rotating-wave approximation and could present more subtle physics than any other characteristic parameter for the difference between the two models. More importantly, our results could be straightforwardly applied to the understanding of quantum phase transitions in spin-boson model. Furthermore, the scaling behavior is observable using currently available techniques in light-matter interaction.

quant-ph↗

arXiv2013

Topological phase transitions driven by magnetic phase transitions in FexBi2Te3 (0 < x < 0.1) single crystals

We propose a phase diagram for FexBi2Te3 (0 < x < 0.1) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from ferromagnetic- to antiferromagnetic- gives rise to topological phase transitions, where the paramagnetic topological insulator of Bi2Te3 turns into a band insulator with ferromagnetic-cluster glassy behaviours around x ~ 0.025, and it further evolves to a topological insulator with valence-bond glassy behaviours, which spans over the region between x ~ 0.03 up to x ~ 0.1. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

cond-mat.str-el↗

arXiv2013

Superconductivity and Strong Non-Fermi-Liquid Behavior in Single-Crystal Ir3Te8

We observe superconductivity below a critical temperature TC = 1.8 K in single-crystal Ir3Te8, which also exhibits normal-state diamagnetism and a linear temperature dependence of electrical resistivity for a wide temperature interval, 20 K < T < 700 K. Single-crystal Ir3Te8 also undergoes a structural phase transition at TS = 350 K from a cubic (above TS) to a rhombohedral lattice below TS. Our first-principles electronic structure calculations reveal two bands crossing the Fermi level; despite the three-dimensional lattice, one band is quasi-two-dimensional, and is responsible for the observed diamagnetism and structure transition. The strong non-Fermi-liquid behavior characterized by the observed linearity in resistivity in such a nonmagnetic state suggests novel physics in this newly discovered superconductor.

cond-mat.supr-con↗

arXiv2012

Annealing induced colossal magnetocapacitance and colossal magnetoresistance in In-doped CdCr2S4

The correlation between colossal magnetocapacitance (CMC) and colossal magnetoresistance (CMR) in CdCr2S4 system has been revealed. The CMC is induced in polycrystalline Cd0.97In0.03Cr2S4 by annealing in cadmium vapor. At the same time, an insulator-metal transition and a concomitant CMR are observed near the Curie temperature. In contrast, after the same annealing treatment, CdCr2S4 displays a typical semiconductor behavior and does not show magnetic field dependent dielectric and electric transport properties. The simultaneous occurrence or absence of CMC and CMR effects implies that the CMC in the annealed Cd0.97In0.03Cr2S4 could be explained qualitatively by a combination of CMR and Maxwell-Wagner effect.

cond-mat.mtrl-sci↗

arXiv2012

Isospin Decomposition of the Basic Double-Pionic Fusion in the Region of the ABC Effect

Exclusive and kinematically complete high-statistics measurements of the basic double pionic fusion reactions pn -> dpi0pi0, pn -> d pi+pi- and pp -> dpi+pi0 have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the dpi0pi0 channel exhibits the ABC effect, i.e. a low-mass enhancement in the pipi-invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the dpi+pi0 channel shows a smooth behavior consistent with the conventional t-channel Delta Delta process. The dpi+pi- data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account.

nucl-ex↗

arXiv2012

New Insight into Collimated Plasmon Beam: Nondiffracting versus Linear Focusing

We worked out a new group of collimated plasmon beams by the means of in-plane diffraction with symmetric phase modulation. As the phase type changes from 1.8 to 1.0, the beam undergoes an interesting evolution from focusing to a straight line. Upon this, an intuitive diagram was proposed to elucidate the beam nature and answer the question whether they are nondiffracting or linear focusing. Based on this diagram, we further achieved a highly designable scheme to modulate the beam intensity (e.g., 'lossless' plasmon). Our finding holds remarkable generality and flexibility in beam engineering and would inspire more intriguing photonic designs.

physics.optics↗

arXiv2012

Yunnan-III models for Evolutionary population synthesis

We build the Yunnan-III evolutionary population synthesis (EPS) models by using the MESA stellar evolution code, BaSeL stellar spectra library and the initial mass functions (IMFs) of Kroupa and Salpeter, and present colours and integrated spectral energy distributions (ISEDs) of solar-metallicity stellar populations (SPs) in the range of 1Myr-15 Gyr. The main characteristic of the Yunnan-III EPS models is the usage of a set of self-consistent solar-metallicity stellar evolutionary tracks (the masses of stars are from 0.1 to 100Msun). This set of tracks is obtained by using the state-of-the-art MESA code. MESA code can evolve stellar models through thermally pulsing asymptotic giant branch (TP-AGB) phase for low- and intermediate-mass stars. By comparisons, we confirm that the inclusion of TP-AGB stars make the V-K, V-J and V-R colours of SPs redder and the infrared flux larger at ages log(t/yr)>7.6 (the differences reach the maximum at log(t/yr)~8.6, ~0.5-0.2mag for colours, ~2 times for K-band flux). The stellar evolutionary tracks, isochrones, colours and ISEDs can be obtained on request from the first author or from our website (http://www1.ynao.ac.cn/~zhangfh/). Using the isochrones, you can build your EPS models. Now the format of stellar evolutionary tracks is the same as that in the STARBURST99 code, you can put them into the STARBURST99 code and get the SP's results. Moreover, the colours involving other passbands or on other systems (for example, HST $F439W-F555W$ colour on AB system) can also be obtained on request.

astro-ph.SR↗

arXiv2012

Frustration by competing interactions in the highly-distorted double perovskites La2NaB'O6 (B' = Ru, Os)

The usual classical behaviour of S = 3/2, B-site ordered double perovskites generally results in simple, commensurate magnetic ground states. In contrast, heat capacity and neutron powder diffraction measurements for the S = 3/2 systems La2NaB'O6 (B' = Ru, Os) reveal an incommensurate magnetic ground state for La2NaRuO6 and a drastically suppressed ordered moment for La2NaOsO6. This behaviour is attributed to the large monoclinic structural distortions of these double perovskites. The distortions have the effect of weakening the nearest neighbour superexchange interactions, presumably to an energy scale that is comparable to the next nearest neighbour superexchange. The exotic ground states in these materials can then arise from a competition between these two types of antiferromagnetic interactions, providing a novel mechanism for achieving frustration in the double perovskite family.

cond-mat.str-el↗