The kagome lattice1, which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid phases, to topological matter, to intertwined orders2, 3,4,5,6,7,8 and, most rarely, to unconventional su-perconductivity6,9. Recently, charge sensitive probes have indicated that the kagome superconductors AV3Sb5 (A = K, Rb, Cs)9,10,11 exhibit unconventional chiral charge order12, 13,14,15,16,17,18,19, which is analogous to the long-sought-after quantum order in the Haldane model20 or Varma model21. However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV3Sb5. We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV3Sb5 and that the Tc/−2ab ratio (where Tc is the superconducting transition temperature and ab is the magnetic penetration depth in the kagome plane) is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.

We report the observation of a dense triangular network of one-dimensional (1D) metallic modes in a continuous and uniform monolayer of MoSe 2 grown by molecular-beam epitaxy. High-resolution transmission electron microscopy and scanning tunneling microscopy and spectroscopy studies show that these 1D modes are midgap states at inversion domain boundaries. Scanning tunneling microscopy and spectroscopy measurements further reveal intensity undulations of the metallic modes, presumably arising from the superlattice potentials due to the moiré pattern and the quantum confinement effect. A dense network of the metallic modes with a high density of states is of great potential for heterocatalysis applications. The interconnection of such midgap 1D conducting channels may also imply new transport behaviors distinct from the 2D bulk. DOI: 10.1103/PhysRevLett.113.066105 PACS numbers: 68.37.Ef, 68.55.-a, 73.20.Fz, 73.21.-b Layered transition metal dichalcogenides (TMDs), with the common formula of MX 2 (M ¼ Mo, W; X ¼ S, Se), are of great current interest for their potential electronic, optoelectronic, and catalytic applications [1][2][3]. Remarkable intrinsic properties of these two-dimensional (2D) crystals have been discovered recently by optical and transport experiments [1,[4][5][6][7][8][9][10][11][12][13][14]. At smooth edges or domain boundaries of the TMD monolayers (MLs), one-dimensional (1D) metallic states are found in the bulk gap, which have also attracted considerable interest [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. These midgap states can affect the optical, transport, and magnetic properties of the 2D materials [30][31][32]. Owing to their large density of states (DOS), the midgap metallic modes in TMDs are catalytically active for hydrodesulfurization (HDS) and hydrogen evolution reaction (HER) and are actively explored for catalytic applications [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. To improve catalytic efficiency, networks of the edge sites have been widely adopted for HDS and HER applications [21,26,29].In this Letter, we report the observation of a dense triangular network of midgap 1D metallic modes in uniform and continuous ML MoSe 2 grown by molecular-beam epitaxy (MBE). Scanning tunneling microscopy and spectroscopy (STM and STS) and high-resolution transmission electron microscopy (HRTEM) reveal that these 1D modes are associated with inversion domain boundary (IDB) defects. Moreover, we find intensity undulations of these 1D modes in STM and STS micrographs and attribute them to the superlattice potential of the moiré pattern and quantum confinement effect. The experiments of MBE growth, STM and STS, HRTEM, and density functional theory calculations are presented in the Supplemental Material, Secs. 1-3 [33].Figures 1(a) and 1(b) show two STM images of the same surface under different bias conditions. The MoSe 2 film was grown on highly oriented pyrolytic graphite (HOPG) with the nominal thickness of 1.4 MLs. The terrace-and-step morpho...

Semiconductor heterostructures are fundamental building blocks for many important device applications. The emergence of two-dimensional semiconductors opens up a new realm for creating heterostructures. As the bandgaps of transition metal dichalcogenides thin films have sensitive layer dependence, it is natural to create lateral heterojunctions (HJs) using the same materials with different thicknesses. Here we show the real space image of electronic structures across the bilayer–monolayer interface in MoSe2 and WSe2, using scanning tunnelling microscopy and spectroscopy. Most bilayer–monolayer HJs are found to have a zig-zag-orientated interface, and the band alignment of such atomically sharp HJs is of type-I with a well-defined interface mode that acts as a narrower-gap quantum wire. The ability to utilize such commonly existing thickness terraces as lateral HJs is a crucial addition to the tool set for device applications based on atomically thin transition metal dichalcogenides, with the advantage of easy and flexible implementation.

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We show theoretically that a heterostructure of monolayer FeTe1−xSex -a superconducting quantum spin Hall material -with a monolayer of FeTe -a bicollinear antiferromagnet -realizes a higher order topological superconductor phase characterized by emergent Majorana zero modes pinned to the sample corners. We provide a minimal effective model for this system, analyze the origin of higher order topology, and fully characterize the topological phase diagram. Despite the conventional s-wave pairing, we find rather surprising emergence of a novel topological nodal superconductor in the phase diagram. Featured by edge-dependent Majorana flat bands, the topological nodal phase is protected by an antiferromagnetic chiral symmetry. We also discuss the experimental feasibility, the estimation of realistic model parameters, and the robustness of the Majorana corner modes against magnetic disorder. Our work provides a new experimentally feasible high-temperature platform for both higher order topology and non-Abelian Majorana physics. arXiv:1905.10647v2 [cond-mat.supr-con]

Cuprates and iron-based superconductors are two classes of unconventional high T c superconductors based on 3d transition elements. Recently, two principles, correspondence principle and magnetic selective pairing rule, have been emerged to unify their high T c superconducting mechanisms. These principles strongly regulate electronic structures that can host high T c superconductivity. Guided by these principles, here we propose high T c superconducting candidates that are formed by cation-anion trigonal bipyramidal complexes with a d 7 filling configuration on the cation ions. Their superconducting states are expected to be dominated by the d xy ±id x 2 −y 2 pairing symmetry. * Electronic address: jphu@iphy.ac.cn 1 arXiv:1506.03904v2 [cond-mat.supr-con] 19 Jun 2015Almost three decades ago, cuprates[1], the Cu-based high T c superconductors, were discovered.Since then, understanding the superconducting mechanism behind unconventional high temperature superconductors has become a great challenge in condensed matter physics. In the past six years, new light has been shined to this decades-old problem due to the discovery of iron-based high T c superconductors[2]. The two high temperature superconductors share many common electronic properties [3]. In principle, comparing these two classes of materials, we may determine the key ingredients that are essential to the high T c superconducting mechanism. However, even if we have identified them, without a realistic prediction of new high T c superconductors, reaching a final consensus will be extremely difficult.Most recently, one of us emphasized and proposed two basic principles to unify the understanding for both high T c superconductors[4]: (1) the HDDL correspondence principle, which was first specified in ref.[5] by Hu and Ding and was generalized to include other orders later in ref.[6] by Davis and Lee: the short range magnetic exchange interactions and the Fermi surfaces act collaboratively to achieve high T c superconductivity and determine pairing symmetries; (2) the selective magnetic pairing rule: the superconductivity is only induced by the magnetic exchange couplings from the superexchange mechanism through cation-anion-cation chemical bondings but not those from direct exchange couplings resulted from the direct cation's d-d chemical bondings. These two principles provide an unified explanation why the d-wave pairing symmetry and the s-wave pairing symmetry are robust respectively in curpates and iron-based superconductors[4]. In the meanwhile, the above two principles can serve as direct guiding rules to search high T c superconductors. The two principles provide many constrains on electronic structures that can host high T c superconductivity. The detailed summary of these constraints and their microscopic origins were discussed in ref. [4]. Essentially, the two principles suggest that the electronic environment that hosts high T c superconductivity must include quasi-two dimensional bands formed dominantly by the d-orbitals through a d-p hybridization...

We predict that CaFeAs2, a newly discovered iron-based high temperature (Tc) superconductor, is a staggered intercalation compound that integrates topological quantum spin hall (QSH) and superconductivity (SC). CaFeAs2 has a structure with staggered CaAs and FeAs layers. While the FeAs layers are known to be responsible for high Tc superconductivity, we show that with spin orbital coupling each CaAs layer is a Z2 topologically nontrivial two-dimensional QSH insulator and the bulk is a 3-dimensional weak topological insulator. In the superconducting state, the edge states in the CaAs layer are natural 1D topological superconductors. The staggered intercalation of QSH and SC provides us an unique opportunity to realize and explore novel physics, such as Majorana modes and Majorana Fermions chains.

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