In quantum field theory, Lorentz invariance leads to three types of fermion-Dirac, Weyl and Majorana. Although the existence of Weyl and Majorana fermions as elementary particles in high-energy ...physics is debated, all three types of fermion have been proposed to exist as low-energy, long-wavelength quasiparticle excitations in condensed-matter systems. The existence of Dirac and Weyl fermions in condensed-matter systems has been confirmed experimentally, and that of Majorana fermions is supported by various experiments. However, in condensed-matter systems, fermions in crystals are constrained by the symmetries of the 230 crystal space groups rather than by Lorentz invariance, giving rise to the possibility of finding other types of fermionic excitation that have no counterparts in high-energy physics. Here we use angle-resolved photoemission spectroscopy to demonstrate the existence of a triply degenerate point in the electronic structure of crystalline molybdenum phosphide. Quasiparticle excitations near a triply degenerate point are three-component fermions, beyond the conventional Dirac-Weyl-Majorana classification, which attributes Dirac and Weyl fermions to four- and two-fold degenerate points, respectively. We also observe pairs of Weyl points in the bulk electronic structure of the crystal that coexist with the three-component fermions. This material thus represents a platform for studying the interplay between different types of fermions. Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in condensed-matter systems.
Topological materials with exotic quantum properties are promising candidates for quantum spin electronics. Different classes of topological materials, including Weyl semimetal, topological ...superconductor, topological insulator and Axion insulator, etc., can be connected to each other via quantum phase transition. For example, it is believed that a trivial band insulator can be twisted into topological phase by increasing spin-orbital coupling or changing the parameters of crystal lattice. With the results of LDA calculation and measurement by angle-resolved photoemission spectroscopy (ARPES), we demonstrate in this work that the electronic structure of SrSn2As2 single crystal has the texture of band inversion near the critical point. The results indicate the possibility of realizing topological quantum phase transition in SrSn2As2 single crystal and obtaining different exotic quantum states.
HIAF (High Intensity heavy ion Accelerator Facility), a new facility planned in China for heavy ion related researches, consists of two ion sources, a high intensity Heavy Ion Superconducting Linac ...(HISCL), a 45 Tm Accumulation and Booster Ring (ABR-45) and a multifunction storage ring system. The key features of HIAF are unprecedented high pulse beam intensity and versatile operation mode. The HIAF project aims to expand nuclear and related researches into presently unreachable region and give scientists possibilities to conduct cutting-edge researches in these fields. The general description of the facility is given in this article with a focus on the accelerator design.
Cross-resonance (CR) gates have emerged as a promising scheme for fault-tolerant quantum computation with fixed-frequency qubits. We experimentally implement an entangling CR gate by using a ...microwave-only control in a tunable coupling superconducting circuit, where the tunable coupler provides extra degrees of freedom to verify optimal conditions for constructing a CR gate. By developing a three-qubit Hamiltonian tomography protocol, we systematically investigate the dependency of gate fidelities on spurious qubit interactions and present the first experimental approach to the evaluation of the perturbation impact arising from spectator qubits. Our results reveal that the spectator qubits lead to reductions in CR gate fidelity dependent on Z Z interactions and particular frequency detunings between spectator and gate qubits. The target spectator demonstrates a more serious impact than the control spectator under a standard echo pulse scheme, whereas the degradation of gate fidelity is observed up to 22.5% as both the spectators are present with a modest ZZ coupling to the computational qubits. Our experiments uncover an optimal CR operation regime, and the method we develop here can readily be applied to improving other kinds of two-qubit gates in large-scale quantum circuits.
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Due to the strong reactivity of alkaline metals and the easy formation of the impurity phase, the superconducting transition temperature (Tc) of alkaline metals intercalated FeSe is usually limited ...to 45 K. To avoid the formation of impurity and improve the Tc, we intercalate a more chemically inert organic ion (rather than the chemically reactive alkaline metals) into FeSe single crystal in this report. A new FeSe-based superconductor, namely (TBA)0.3FeSe, with Tc of 50 K, is synthesized by intercalating FeSe single crystal with organic ion tetrabutyl ammonium (TBA+) via an electrochemical intercalation method, which has the highest Tc among FeSe-based bulk superconductors. The structure of the organic ion intercalated product consists of the alternate stacking of monolayer FeSe and the organic molecule. The superconductivity of (TBA)0.3FeSe is confirmed by both the magnetic susceptibility and the transport measurement. It is suggested that the chemically inert organic ion should play a key role in the enhancement of Tc by avoiding the formation of impurity and disorder in FeSe plane as possible. We also suggest that the TBA+ intercalated FeSe with well defined shape and higher Tc offer a good playground for further bulk measurement investigation.
Following Dirac and Weyl semimetals, nodal line semimetals represent a new type of topological semimetal and attracted extensive interest in modern condensed matter physics. Recently, a kind of ...hybrid nodal lines have been proposed (Zhang et al 2018 Phys. Rev. B 97 125143) and it mainly come from anisotropy interaction. Based on the first-principle calculations and k p model, we predict a cubic Be2Si with topological nontrivial electronic states exhibiting fascinating type-I and type-II hybrid nodal line around Fermi level. Unexpectedly the low energy electronic states of Be2Si are isotropy in kx and ky directions and a new effective model has been proposed for the appearance of this new type hybrid nodal line. The four-fold degenerated type-I and type-II nodal points in the hybrid nodal lines along the high symmetry lines of Γ-K and Γ-X are protected by mirror reflection, time reversal and space inversion symmetry. The spin-orbit coupling effect only induces slight band gap in the hybrid nodal lines in Be2Si whose nontrivial nature remains. Moreover, the nontrivial hybrid nodal lines in Be2Si are robust to external strain. The results in present work indicate that Be2Si are promising candidates for future experimental studies of nontrivial topological semimetals and an excellent platform to study the interaction between the two types Dirac fermions.
B-free and B-containing near α titanium alloys were fabricated by induction skull melting method. Microstructures, textures, mechanical properties and fracture behaviors were investigated. ...Observations of as-casted microstructures revealed that prior β grains and α colonies were obviously refined after adding trace boron element, and widmannstatten structure of B-free alloy transformed to basket-weave structure of B-containing alloy. For as-forged alloys, prior β grains were broken and squashed, and they stretched along direction perpendicular to forging direction. TiB whiskers promoted dynamic recrystallization behavior, which led to the precipitation of more equiaxed α grains around them. Texture intensity of overall α phase obviously decreased after the addition of trace boron element, which manifested that TiB whiskers prohibited texture formation and improved microstructure homogeneity. B-containing alloy exhibited excellent combination of room temperature (RT) and high temperature (650 °C) mechanical properties. Fracture characteristics of B-free and B-containing alloys were dominant by abundant dimples and cleavage fracture of α lamellas during RT tensile processes. At 650 °C, the cleavage fracture of α lamellas was replaced by dimples, illustrating ductile fracture characteristics. Longitudinal and transverse fracture of TiB whiskers existed no matter at RT or at 650 °C for B-containing alloy. By observations of lateral surfaces, cracks and voids were easier to generate at boundaries between grains and colonies with large size for B-free alloy. For B-containing alloy, comparing with macro-zones in matrix, voids were inclined to generate around interfaces of TiB whiskers and matrix due to strain mismatch. It illustrated TiB whiskers accelerated the fracture of alloy. Voids, transverse fracture of TiB whiskers and lateral abhesion of TiB whiskers were detected at RT, while lateral abhesion of TiB whiskers was seldom discovered during 650 °C tensile process.
•Prior β grains and α colonies are obviously refined after boron addition.•TiB phase promotes dynamic recrystallization behavior.•TiB whiskers prohibit the formation of texture and improve the homogeneity of microstructure during forging process.•B-containing alloy exhibits excellent combination of RT and 650 °C tensile mechanical properties.•Strain mismatch between TiB whiskers and matrix is more easier to cause the formation of voids comparing with macro-zones in matrix.
The electronic and magnetic properties of single 3d transition-metal (TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed graphdiyne (GDY) and graphyne (GY) are systematically studied using density ...functional theory (DFT). It is found that the electronic structures of TM-GDY/GY are sensitive to the value of the on-site Coulomb energy for the TM 3d orbital. It is crucial to use DFT+U method and accurately account for the electron correlation in the calculations. By using linear response method, we are able to determine the U eff value for all TM adatom. We find that the adsorption of TM atom not only efficiently modulates the electronic structures of GDY/GY system but also introduces excellent magnetic properties, such as spin-polarized half-semiconductor. Such modulation originates from the charge transfer between TM adatom and GDY/GY sheet as well as the electron redistribution of the TM intra-atomic s, p, and d orbitals. Our results indicate that the TM adsorbed GDY/GY are excellent candidates for spintronics.
We study the process e^{+}e^{-}→Λ_{c}^{+}Λover ¯_{c}^{-} at twelve center-of-mass energies from 4.6119 to 4.9509 GeV using data samples collected by the BESIII detector at the BEPCII collider. The ...Born cross sections and effective form factors (|G_{eff}|) are determined with unprecedented precision after combining the single and double-tag methods based on the decay process Λ_{c}^{+}→pK^{-}π^{+}. Flat cross sections around 4.63 GeV are obtained and no indication of the resonant structure Y(4630), as reported by Belle, is found. In addition, no oscillatory behavior is discerned in the |G_{eff}| energy dependence of Λ_{c}^{+}, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the Λ_{c}^{+} baryon at each energy point, the moduli of electric and magnetic form factors (|G_{E}| and |G_{M}|) are extracted and separated. For the first time, the energy dependence of the form factor ratio |G_{E}/G_{M}| is observed, which can be well described by an oscillatory function.
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