High-resolution ARPES endstation for in situ electronic structure investigations at SSRF

Constructing vacuum-ultraviolet beamlines at synchrotron radiation facilities with giga-electron volt storage ring results in serious heat load on the beamlines which can reduce their performance. To solve this problem, an APPLE-Knot undulator with eight magnet rows has been built at the Shanghai Synchrotron Radiation Facility and has achieved very good performance. However, its performance in vertical polarization mode is imperfect. Here, a new configuration of a magnet-merged APPLE-Knot undulator that has achieved a better performance is reported.

Practical design and performance of a new merged APPLE-Knot undulator.

Kagome superconductors AV 3 Sb 5 (A = K, Rb, Cs) provide a fertile playground for studying various intriguing phenomena such as non-trivial band topology, superconductivity, giant anomalous Hall e ﬀ ect, and charge density wave (CDW). Remarkably, the recent discovery of C 2 symmetric nematic phase prior to the superconducting state in AV 3 Sb 5 has drawn enormous attention, as the unusual superconductivity might inherit the symmetry of the nematic phase. Although many e ﬀ orts have been devoted to resolve the charge orders using real-space microscopy and transport measurements, the direct evidence on the rotation symmetry

Observation of electronic nematicity driven by three-dimensional charge density wave in kagome lattice KV 3 Sb 5

Users of HEPS require polarization adjustable and high-flux synchrotron radiation with energy of 100–2000 eV. To achieve 100 eV photon energy at HEPS, a 6 GeV storage ring, the problem of on-axis heat load becomes a significant challenge for users. A new type of four-row merged APPLE–Knot undulator recently proposed seems to be the most effective solution to reduce on-axis power density. However, as an undulator with such complex magnetization orientation distribution of magnetic blocks, its field integrals correction and the effect of field errors on the radiation performance are different from that of conventional EPU. Whether it can realize its field integrals correction and perform as well as in theory with field errors has become a concern. Here, a set of field integrals correction schemes is reported and field errors analysis has been done. A general method for evaluating the effect of magnetization orientation deviation of the magnetic block on radiation performance is proposed. It can be used for any insertion device containing permanent magnetic magnets if necessary.

A practical design and field errors analysis of a merged APPLE–Knot undulator for High Energy Photon Source

By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), piezoresponse force microscopy (PFM) and first-principles calculations, we have studied the low-energy band structure, atomic structure and charge polarization on the surface of a topological semimetal candidate TaNiTe5. Dirac-like surface states were observed on the (010) surface by ARPES, consistent with the first-principles calculations. On the other hand, PFM reveals a switchable ferroelectric-like polarization on the same surface. We propose that the noncentrosymmetric surface reconstruction observed by STM could be the origin of the observed ferroelectric-like state in this novel material. Our findings provide a new platform with the coexistence of ferroelectric-like surface charge distribution and novel surface states. 

Coexistence of Ferroelectriclike Polarization and Dirac-like Surface State in 
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The vacuum ultraviolet beamline BL03U with a photon energy range from 7 eV upwards has been constructed at the 3.5 GeV Shanghai Synchrotron Radiation Facility. Equipped with an APPLE-Knot undulator, this beamline is dedicated to angle-resolved photoemission spectroscopy. An energy-resolving power of higher than 4.6 × 104 has been achieved in the photon energy range 21.6–48 eV, which is almost the same as the theoretical estimation.

fayuan zhang

Papers

Performance of the BL03U beamline at SSRF.

Practical design and performance of a new merged APPLE-Knot undulator.