Here we report a breakthrough in the fabrication of a long lifetime transmon qubit. We use tantalum films as the base superconductor. By using a dry etching process, we obtained transmon qubits with a best T1 lifetime of 503 μs. As a comparison, we also fabricated transmon qubits with other popular materials, including niobium and aluminum, under the same design and fabrication processes. After characterizing their coherence properties, we found that qubits prepared with tantalum films have the best performance. Since the dry etching process is stable and highly anisotropic, it is much more suitable for fabricating complex scalable quantum circuits, when compared to wet etching. As a result, the current breakthrough indicates that the dry etching process of tantalum film is a promising approach to fabricate medium- or large-scale superconducting quantum circuits with a much longer lifetime, meeting the requirements for building practical quantum computers.

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We report structural and physical properties of the single crystalline . The X-ray diffraction (XRD) results show that adopts the trigonal -type structure. Temperature-dependent electrical resistivity measurements indicate an insulating ground state for with activation energies of 40 meV and 0.64 meV for two distinct regions, respectively. Magnetization measurements show no apparent magnetic phase transition under 400 K. Different from other , Sr, and Ba, and , As, and Sb) compounds with the same structure, heat capacity and reveal that has a first-order transition at and the transition temperature shifts to high temperature upon increasing pressure. The emergence of plenty of new Raman modes below the transition, clearly suggests a change in symmetry accompanying the transition. The combination of the structural, transport, thermal and magnetic measurements points to an unusual origin of the transition.

We report single crystal growth of CoSi, which has recently been recognized as a new type of topological semimetal hosting fourfold and sixfold degenerate nodes. The Shubnikov-de Haas quantum oscillation (QO) is observed on our crystals. There are two frequencies originating from almost isotropic bulk electron Fermi surfaces, in accordance with band structure calculations. The effective mass, scattering rate, and QO phase difference of the two frequencies are extracted and discussed.CoSi is a long known material with FeSi-type cubic structure (B20, space group P2 1 3), of which the thermoelectric property and application have been the focus of study [1,2], it is generally regarded as a semimetal. With the development of characterizing materials by the topology of their electronic band structure, it has recently been suggested that in crystalline systems, in addition to Dirac, Weyl, and nodal line semimetals, band crossing points (nodes) with three-, six-, or eight-fold degeneracies can be stablized [3], the low energy fermionic excitations close to the nodes are called "new fermions". For CoSi and isostructural transition metal silicides RhSi, RhGe, and CoGe, the crystal structure is asymmorphic with threefold rotation symmetry and twofold screw symmetry. Theoretical studies [4,5] found that with inclusion of spin-orbit-coupling (SOC), close to the Fermi level there only exist bands containing a fourfold degenerate node at the Brillouin zone (BZ) center Γ, and a sixfold degenerate node at BZ corner R, both are chiral, thus could serve as a model system in the study of unconventional chiral fermions. Besides, the phonon spectra of these monosilicides host double Weyl points [6]. Subsequent angle resolved photoemission spectrescopy (ARPES) measurements[7-9] have confirmed the overall bulk band structure and the existence of surface Fermi arcs. However, it has been noted that for CoSi preparation of a flat surface for ARPES is at least challenging. Therefore other probes of its bulk electronic structure are needed. At the same time, in the context of searching for feature characteristic of chiral fermions, the physical properties of CoSi are also worth revisiting on high quality single crystalline samples.In this work, we report single crystal growth by the flux method, different from the Czochralski, Bridgman, or chemical vapor transport techniques previously used. The resulting crystals are of decent quality. To our knowledge, this is the first time SdH is observed in CoSi. The angular dependence of the QO frequencies reveals that they are from the bulk electron Fermi surface centered at R in BZ, agreeing with band calculation and * gli@iphy.ac.cn † jlluo@iphy.ac.cn ARPES data. Thus, it is a good starting point for further investigations.The single crystal growth starts with Co powder (99.999%, Alfa), Si powder (99.999%, Alfa), and Sn grains (99.99%), which were mixed with a molar ratio of 1:1:20, loaded into an alumina crucible, then sealed with a partial Ar pressure inside a silica capsule. The reactant was quic...