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Heterostructure based interface engineering has been proved an effective method for finding new superconducting systems and raising superconductivity transition temperature (T C ) 1-7 . In previous work on one unit-cell (UC) thick FeSe films on SrTiO 3 (STO) substrate, a superconducting-like energy gap as large as 20 meV 8 , was revealed by in situ scanning tunneling microscopy/spectroscopy (STM/STS). Angle resolved photoemission spectroscopy (ARPES) further revealed a nearly isotropic gap of above 15 meV, which closes at a temperature of 65 ± 5 K 9-11 . If this transition is indeed the superconducting transition, then the 1-UC FeSe represents the thinnest high T C superconductor discovered so far. However, up to date direct transport measurement of the 1-UC FeSe films has not been reported, mainly because growth of large scale 1-UC FeSe films ischallenging and the 1-UC FeSe films are too thin to survive in atmosphere. In this work, we successfully prepared 1-UC FeSe films on insulating STO substrates with non-superconducting FeTe protection layers. By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset T C above 40 K and a extremely large critical current density J C ~ 1.7×10 6 A/cm 2 at 2 K. Our work may pave the way to enhancing and tailoring superconductivity by interface engineering.The FeSe films and FeTe protection layer are grown by molecular beam epitaxy (MBE) (see Methods).

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Organic spacer cations in layered 2D (A1)2(A2)n−1BnX3n+1 (where A1 is an organic cation acting as a spacer between the perovskite layers, A2 is a monovalent cation, e.g., Cs+,CH3NH3+, CH(NH2)2+) perovskite materials improve the long‐term stability of the resulting solar cells, but hamper their power conversion efficiency due to poor carrier generation/transportation. Rational guidelines are thus required to enable the design of organic spacer cations. Herein, mixed A1 cations are employed in layered 2D perovskites to investigate the interplay between alkylamine cations and unsaturated alkylamine cations. It is revealed that alkylamine spacer cations are able to facilitate precursor assembly, which results in the orientated growth of perovskite crystals. Unsaturated alkylamine cations further lead to reduced exciton binding energy, which improves carrier pathway in the 2D perovskites. By mixing both cations, substantially improved open circuit voltage is observed in the resultant photovoltaic cells with the efficiency of 15.46%, one of the highest one based on (A1)2(A2)3Pb4I13 layered 2D perovskites. The generality of the design principle is further extended to other cation combinations.

HfTe 5 is predicted to be a promising platform for studying topological phases. Here through an electrical transport study, we present the first observation of chiral anomaly and ultrahigh mobility in HfTe 5 crystals. Negative magneto-resistivity in HfTe 5 is observed when the external magnetic and electrical fields are parallel (B//E) and quickly disappears once B deviates from the direction of E. Quantitative fitting further confirms the chiral anomaly as the underlying physics. Moreover, by analyzing the conductivity tensors of longitudinal and Hall traces, ultrahigh mobility and ultralow carrier density are revealed in HfTe 5 , which paves the way for potential electronic applications. Chiral anomaly is a quantum anomaly phenomenon that breaks the chiral symmetry and leads to the non-conservation of chiral current [1,2]. This anomaly was proposed to be observed in lattice system in 1983 [3]. Recently, the study of Weyl fermions pushes forward the realization of chiral anomaly in crystals [4][5][6]. In Dirac/Weyl semimetals, the axial current is non-conserved due to the chiral anomaly, and further leads to charge pumping effect between the Weyl nodes with opposite chirality. This anomaly effect is suggested to give rise to negative magneto-resistivity when the magnetic and electrical fields are parallel (B//E) [4]. Related experimental evidences have been intensively pursued in various condensed matter systems Here we present the first electrical transport evidence for the chiral anomaly and the ultrahigh mobility in HfTe 5 crystals.

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