Domain walls in interacting electronic systems can have distinct localized states, which often govern physical properties and may lead to unprecedented functionalities and novel devices. However, electronic states within domain walls themselves have not been clearly identified and understood for strongly correlated electron systems. Here, we resolve the electronic states localized on domain walls in a Mott-charge-density-wave insulator 1T-TaS2 using scanning tunneling spectroscopy. We establish that the domain wall state decomposes into two nonconducting states located at the center of domain walls and edges of domains. Theoretical calculations reveal their atomistic origin as the local reconstruction of domain walls under the strong influence of electron correlation. Our results introduce a concept for the domain wall electronic property, the walls own internal degrees of freedom, which is potentially related to the controllability of domain wall electronic properties.

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The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.

A prototypical quasi-2D metallic compound, 1T-TaS 2 has been extensively studied due to an intricate interplay between a Mottinsulating ground state and a charge-density-wave order. In the low-temperature phase, 12 out of 13 Ta 4+ 5d-electrons form molecular orbitals in hexagonal star-of-David patterns, leaving one 5d-electron with S = ½ spin free. This orphan quantum spin with a large spin-orbit interaction is expected to form a highly correlated phase of its own. And it is most likely that they will form some kind of a short-range order out of a strongly spin-orbit coupled Hilbert space. In order to investigate the low-temperature magnetic properties, we performed a series of measurements including neutron scattering and muon experiments. The obtained data clearly indicate the presence of the short-ranged phase and put the upper bound on~0.4 µ B for the size of the magnetic moment, consistent with the orphan-spin scenario.npj Quantum Materials (2017)2:42 ; doi:10.1038/s41535-017-0048-1 INTRODUCTIONThe instability of charge density waves (CDW) found in lowdimensional electron systems of layered materials has attracted enormous attention recently. 1T-TaS 2 is a prototypical quasi-2D metallic compound with a strong electron-phonon coupling responsible often for CDW instabilities. Upon cooling, it undergoes a series of first-order phase transitions to CDW, Mott and superconducting phases (see e.g., ref. 1 and references therein).1T-TaS 2 bulk crystal has a lamellar structure, with each layer composed of a triangular lattice of Ta atoms which is then sandwiched by S atoms in an octahedral TaS 6 coordination with a weak Van der Waals bonding between the layers. Above~540 K, the structure is unmodulated trigonal with P-3m1 symmetry; below which the triangular lattice exhibits a series of structural modulations.2, 3 First, an incommensurate CDW phase sets in at T 540 K. Upon further cooling, the structure changes to a nearly commensurate phase at T nCDW~3 50 K. Finally, the material turns into a Mott insulating phase with in-plane √13 × √13 superlattice distortion, which coexists with a commensurate CDW phase below T CDW~1 80 K. Superconductivity emerges in 1T-TaS 2 below~2 K by introducing disorders.4 Recent angle-resolved photoemission spectroscopy experiments suggest that the melted Mott state and the superconductivity coexist in real space 5 providing better understanding of the interplay between electron correlation, charge order, and superconductivity. Unlike other Mott insulators, the CDW superlattices play the role of localization centers in the ground state of 1T-TaS 2 .According to the current understanding, the CDW phase is composed of molecular orbitals of 13 Ta atoms, forming a hexagonal pattern of so-called David-star clusters. As proposed more than three decades ago, 6 each Ta 4+ of 1T-TaS 2 provides one 5d electron, and thus there are 13 5d-electrons per each David star. Out of these 13 electrons, 12 electrons form 6 covalent bonds