Recently discovered alongside its sister compounds KV3Sb5 and RbV3Sb5, CsV3Sb5 crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV3Sb5, demonstrating bulk superconductivity in single crystals with a Tc = 2.5 K. The normal state electronic structure is studied via angleresolved photoemission spectroscopy (ARPES) and density functional theory (DFT), which categorize CsV3Sb5 as a Z2 topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (EF ), and signatures of normal state correlation effects are also suggested by a high temperature charge density wave-like instability. The implications for the formation of unconventional superconductivity in this material are discussed.

The kagome lattice of transition metal atoms provides an exciting platform to study electronic correlations in the presence of geometric frustration and nontrivial band topology, which continues to bear surprises. In this work, using spectroscopic imaging scanning tunneling microscopy, we discover a cascade of different symmetry-broken electronic states as a function of temperature in a new kagome superconductor, CsV3Sb5. At a temperature far above the superconducting transition Tc ~ 2.5 K, we reveal a tri-directional charge order with a 2a0 period that breaks the translation symmetry of the lattice. As the system is cooled down towards Tc, we observe a prominent V-shape spectral gap opening at the Fermi level and an additional breaking of the six-fold rotation symmetry, which persists through the superconducting transition. This rotation symmetry breaking is observed as the emergence of an additional 4a0 unidirectional charge order and strongly anisotropic scattering in differential conductance maps. The latter can be directly attributed to the orbital-selective renormalization of the V kagome bands. Our experiments reveal a complex landscape of electronic states that can co-exist on a kagome lattice, and provide intriguing parallels to high-Tc superconductors and twisted bilayer graphene.Quantum solids composed of atoms arranged on a lattice of corner-sharing triangles (kagome lattice) are a fascinating playground for the exploration of novel correlated and topological electronic phenomena [1][2][3][4] . Due to their intrinsic geometric frustration, kagome systems are predicted to host to a slew of exotic electronic states [5][6][7][8][9][10][11][12][13][14][15][16][17][18] , such as bond and charge ordering 7,8,10,[16][17][18] , spin liquid phases 5,15 and chiral superconductivity 9,10,17 . The majority of the experimental efforts thus far have focused on transition-metal kagome magnets, for example Co3Sn2S2 [19][20][21][22][23] FeSn 24,25 and Fe3Sn2 26,27 , in which different forms of magnetism dominate the low-temperature electronic ground state. Electronic correlations in the absence of magnetic ordering could in principle favor the emergence of new symmetry-broken electronic states, but this has been difficult to explore in many of the existing kagome materials due to a tendency towards magnetic ordering.

Hybrid halide double perovskites are a class of compounds attracting growing interest because of their richness of structure and property. Two-dimensional (2D) derivatives of hybrid double perovskites are formed by the incorporation of organic spacer cations into three-dimensional (3D) double perovskites. Here, we report a series of seven new layered double perovskite halides with propylammonium (PA), octylammonium (OCA), and 1,4-butydiammonium (BDA) cations. The general formulae of the compounds are AmM I M III X8 (single-layered Ruddlesden-Popper with m = 4 and A = PA or OCA, and single-layered Dion-Jacobson with m = 2 and A = BDA, M I = Ag, M III = In or Bi, X = Cl or Br) and PA2CsM I M III Br7 (bi-layered, with M I = Ag, M III = In or Bi). These families of compounds demonstrate great versatility, with tunable layer thickness, the ability to vary the interlayer spacing, and the ability to selectively tune the band gap by varying the M I and M III cations along with the halide anions. The band gap of the single-layered materials varies from 2.41 eV for PA4AgBiBr8 to 3.96 eV for PA4AgInCl8. Photoluminescent emission spectra of the layered double perovskites at low-temperature (100 K) are reported, and density functional theory electronic structure calculations are presented to understand the nature of the band gap evolution. The development of new structural and compositions in layered double perovskite halides enhances the understanding of structure-property relations in this important family.

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Principles that predict reactions or properties of materials define the discipline of chemistry. In this work we derive chemical rules, based on atomic distances and chemical bond character, which predict topological materials in compounds that feature the structural motif of a square-net. Using these rules we identify over 300 potential new topological materials. We show that simple chemical heuristics can be a powerful tool to 1 arXiv:2002.04611v1 [cond-mat.mtrl-sci] 11 Feb 2020 characterize topological matter. In contrast to previous database-driven materials categorization our approach allows us to identify candidates that are alloys, solid-solutions, or compounds with statistical vacancies. While previous material searches relied on density functional theory, our approach is not limited by this method and could also be used to discover magnetic and statistically-disordered topological semimetals.

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