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Multiferroicity allows magnetism to be controlled using electric fields or vice versa, which has gained tremendous interest in both fundamental research and device applications. A reduced dimensionality of multiferroic materials is highly desired for device miniaturization, but the coexistence of ferroelectricity and magnetism at the two-dimensional limit is still debated. Here, we used a NbSe2 substrate to break both the C3 rotational and inversion symmetries in monolayer VCl3 and thus introduced exceptional in-plane ferroelectricity into a two-dimensional magnet. Scanning tunnelling spectroscopy directly visualized ferroelectric domains and manipulated their domain boundaries in monolayer VCl3, where coexisting antiferromagnetic order with canted magnetic moments was verified by vibrating sample magnetometer measurements. Our density functional theory calculations highlight the crucial role that highly directional interfacial Cl-Se interactions play in breaking the symmetries and thus in introducing in-plane ferroelectricity, which was further verified by examining an ML-VCl3/graphene sample. Our work demonstrates an approach to manipulate the ferroelectric states in monolayered magnets through van der Waals interfacial interactions.

Trends of miniaturized devices and quantum interference electronics lead to the long desire of Fano interference in single-molecule junctions, here, which is successfully demonstrated using the 2,7-di(4-pyridyl)-9,9'-spirobifluorene molecule with a long backbone group and a short side group. Experimentally, the two electrically coupled groups are found to contribute to two blurred degenerate points in the differential conductance mapping. This forms a characteristic non-centrosymmetric double-crossing feature, with distinct temperature response for each crossing. Theoretically, we describe the practical in-junction electron transmission using a new two-tunnelling-channel coupling model and obtain a working formula with a Fano term and a Breit-Wigner term. The formula is shown to provide a good fit for all the mapping data and their temperature dependence in three dimensions, identifying the Fano component. Our work thus forms a complete set of evidence of the Fano interference in a single-molecule junction induced by two-tunnelling-channel coupling transport. Density functional theory calculations are used to corroborate this new physics., Comment: 13 pages (main text), 4000 Words (main text), 5 figures

The fabrication of one-dimensional (1D) magnetic systems on solid surfaces, although of high fundamental interest, has yet to be achieved for a crossover between two-dimensional (2D) magnetic layers and their associated 1D spin chain systems. In this study, we report the fabrication of 1D single-unit-cell-width CrCl3 atomic wires and their stacked few-wire arrays on the surface of a van der Waals (vdW) superconductor NbSe2. Scanning tunneling microscopy/spectroscopy and first-principles calculations jointly revealed that the single wire shows an antiferromagnetic large-bandgap semiconducting state in an unexplored structure different from the well-known 2D CrCl3 phase. Competition among the total energies and nanostructure-substrate interfacial interactions of these two phases result in the appearance of the 1D phase. This phase was transformable to the 2D phase either prior to or after the growth for in situ or ex situ manipulations, in which the electronic interactions at the vdW interface play a nontrivial role that could regulate the dimensionality conversion and structural transformation between the 1D-2D CrCl3 phases., Comment: 4 figures

Two-dimensional electronic flat bands and their induced correlated electronic interactions have been discovered, probed, and tuned in interlayer regions of hexagonally shaped van der Waals moire superlattices. Fabrication of anisotropic one-dimensional correlated bands by moire interference of 2D, however, remains a challenge. Here, we report an experimental discovery of 1D electronic flat bands near the Fermi level in an anisotropic rectangular moire superlattice composed of in situ grown, vdW stacked two-atomic-layer thick Bi(110) well-aligned on a SnSe(001) substrate. The epitaxial lattice mismatch between the aligned Bi and SnSe zigzag atomic chains causes strong three-dimensional anisotropic atomic relaxations with associated one-dimensional out-of- and in-plane strain distributions that are expressed in electronic bands of the Bi(110) layer, which are characterized jointly by scanning probe microscopy and density functional theory. At the regions of the strongest out-of-plane shear strain, a series of 1D flat bands near the Fermi level are experimentally observed and defined in our calculations. We establish that 1D flat bands can arise in moir\'e superlattices in absence of the relative layer twist, but solely through the lattice strain. We generalize the strategy of utilizing strain in lattice mismatched rectangular hetero-bilayers for engineering correlated anisotropic electronic bands., Comment: 4 figures and 10 supplementary figures

The weak antilocalization (WAL) effect is known as a quantum correction to the classical conductivity, which never appeared in two-dimensional magnets. In this work, we reported the observation of a WAL effect in the van der Waals ferromagnet Fe5-xGeTe2 with a Curie temperature Tc ~ 270 K, which can even reach as high as ~ 120 K. The WAL effect could be well described by the Hikami-Larkin-Nagaoka and Maekawa-Fukuyama theories in the presence of strong spin-orbit coupling (SOC). Moreover, A crossover from a peak to dip behavior around 60 K in both the magnetoresistance and magnetoconductance was observed, which could be ascribed to a rare example of temperature driven Lifshitz transition as indicated by the angle-resolved photoemission spectroscopy measurements and first principles calculations. The reflective magnetic circular dichroism measurements indicate a possible spin reorientation that kills the WAL effect above 120 K. Our findings present a rare example of WAL effect in two-dimensional ferromagnet and also a magnetotransport fingerprint of the strong SOC in Fe5-xGeTe2. The results would be instructive for understanding the interaction Hamiltonian for such high Tc itinerant ferromagnetism as well as be helpful for the design of next-generation room temperature spintronic or twistronic devices., Comment: main text 32 pages + SI 7 pages;main text 8 figures + SI 5 figures and 3 tables

Dirac nodal line semimetals (DNLSs) host relativistic quasiparticles in their one-dimensional (1D) Dirac nodal line (DNL) bands that are protected by certain crystalline symmetries. Their novel low-energy fermion quasiparticle excitations and transport properties invite studies of relativistic physics in the solid state where their linearly dispersing Dirac bands cross at continuous lines with four-fold degeneracy. In materials studied up to now, the four-fold degeneracy, however, has been vulnerable to suppression by the ubiquitous spin-orbit coupling (SOC). Despite the current effort to discover 3D DNLSs that are robust to SOC by theory, positive experimental evidence is yet to emerge. In 2D DNLSs, because of the decreased total density of states as compared with their 3D counterparts, it is anticipated that their physical properties would be dominated by the electronic states defined by the DNL. It has been even more challenging, however, to discover robust 2D DNLSs against SOC because of their lowered symmetry; no such materials have yet been predicted by theory. By combining molecular beam epitaxy growth, STM, nc-AFM characterisation, with DFT calculations and space group theory analysis, here we reveal a novel class of 2D crystalline DNLSs that host the exact symmetry that protects them against SOC. The discovered quantum material is a brick phase 3-AL Bi(110), whose symmetry protection and thermal stability are imparted by the compressive vdW epitaxial growth on black phosphorus substrates. The BP substrate templates the growth of 3-AL Bi(110) nano-islands in a non-symmorphic space group structure. This crystalline symmetry protects the DNL electronic phase against SOC independent of any orbital or elemental factors. We theoretically establish that this intrinsic symmetry imparts a general, robust protection of DNL in a series of isostructural 2D quantum materials., Comment: 32 pages, 4 figures, SI included

Several intriguing electronic phenomena and electric properties were discovered in three-dimensional Dirac nodal line semimetals (3D-DNLSM), which are, however, easy to be perturbed under strong spin-orbit coupling (SOC). While two-dimensional (2D) layers are an emerging material category with many advantages, 2D-DNLSM against SOC is yet to be uncovered. Here, we report a 2D-DNLSM in odd-atomic-layer Bi (the brick phase, another Bi allotrope), whose robustness against SOC is protected by the little co-group C_2v \times Z^T_2, the unique protecting symmetry we found in 2D.Specially, (4n+2) valence electrons fill the electronic bands in the brick phase, so that the Dirac nodal line with fourfold degeneracy locates across the Fermi level. There are almost no other low energy states close to the Fermi level; this allows to feasibly observe the neat DNLSM-induced phenomena in transport measurements without being affected by other bands. In contrast, Other VA-group elements also form the brick phases, but their DNL states are mixed with the extra states around the Fermi level. This unprecedented category of layered materials allows for exploring nearly isolated 2DDNL states in 2D., Comment: Totally 25 pages including main text, methods and supporting information, 4 figures, 8 SI figures