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By combining angle-resolved photoemission spectroscopy, scanning tunneling microscopy, atomic force microscope based piezoresponse force microscopy and first-principles calculations, we have studied the low-energy band structure, atomic structure, and charge polarization on the surface of a topological semimetal candidate TaNiTe_{5}. Dirac-like surface states were observed on the (010) surface by angle-resolved photoemission spectroscopy, consistent with the first-principles calculations. On the other hand, piezoresponse force microscopy reveals a switchable ferroelectriclike polarization on the same surface. We propose that the noncentrosymmetric surface relaxation observed by scanning tunneling microscopy could be the origin of the observed ferroelectriclike state in this novel material. Our findings provide a new platform with the coexistence of a ferroelectriclike surface charge distribution and novel surface states.

Nanographenes with sublattice imbalance host a net spin according to Lieb's theorem for bipartite lattices. Here, we report the on-surface synthesis of atomically precise nanographenes and their atomic-scale characterization on a gold substrate by using low-temperature noncontact atomic force microscopy and scanning tunneling spectroscopy. Our results clearly confirm individual nanographenes host a single spin of S=1/2 via the Kondo effect. In covalently linked nanographene dimers, two spins are antiferromagnetically coupled with each other as revealed by inelastic spin-flip excitation spectroscopy. The magnetic exchange interaction in dimers can be well engineered by tuning the local spin density distribution near the connection region, consistent with mean-field Hubbard model calculations. Our work clearly reveals the emergence of magnetism in nanographenes and provides an efficient way to further explore the carbon-based magnetism.

We report direct experimental evidence for a room-temperature, $\ensuremath{\sim}130\text{ }\text{ }\ensuremath{\mu}\mathrm{C}/{\mathrm{cm}}^{2}$ ferroelectric polarization from the tetragonal-like ${\mathrm{BiFeO}}_{3}$ phase. The physical origin of this remarkable enhancement of ferroelectric polarization has been investigated by a combination of x-ray absorption spectroscopy, scanning transmission electron microscopy, and first principles calculations. A large strain-induced Fe-ion displacement relative to the oxygen octahedra, combined with the contribution of Bi $6s$ lone pair electrons, is the mechanism driving the large ferroelectric polarization in this tetragonal-like phase.

We determine the atomic structure of the pseudotetragonal $T$ phase and the pseudorhombohedral $R$ phase in highly strained multiferroic ${\mathrm{BiFeO}}_{3}$ thin films by using a combination of atomic-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy. The coordination of the Fe atoms and their displacement relative to the O and Bi positions are assessed by direct imaging. These observations allow us to interpret the electronic structure data derived from electron energy-loss spectroscopy and provide evidence for the giant spontaneous polarization in strained ${\mathrm{BiFeO}}_{3}$ thin films.

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density functional theory how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by interfacial charge generated, for example, by oxygen vacancies., Comment: 3 figures

A nanoscale phase is known to coincide with colossal magnetoresistance (CMR) in manganites, but its volume fraction is believed to be too small to affect CMR. Here we provide scanning-electron-nanodiffraction images of nanoclusters as they form and evolve with temperature in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$, $x=0.45$. They are not doping inhomogeneities, and their structure is that of the bulk compound at $x=0.60$, which at low temperatures is insulating. Their volume fraction peaks at the CMR critical temperature and is estimated to be 22% at finite magnetic fields. In view of the known dependence of the nanoscale phase on magnetic fields, such a volume fraction can make a significant contribution to the CMR peak.

Recent magnetic measurements at ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}/{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ interfaces led to conflicting interpretations: a magnetic ``dead'' layer on the LCMO side or antiparallel Cu and Mn magnetic moments. Here we report results of first-principles density-functional calculations of position-dependent magnetic couplings between interlayer and intralayer Mn atoms. The couplings in the first few layers near the interface are found to be weak ferromagnetic or even antiferromagnetic. The results suggest that a ``dead'' magnetic layer 2--3 atoms thick is present, as needed to account for the observations.

The structural, electronic, and magnetic properties of mixed-valence compounds are believed to be governed by strong electron correlations. Here we report benchmark density-functional calculations in the spin-polarized generalized-gradient approximation (GGA) for the ground-state properties of doped ${\mathrm{CaMnO}}_{3}$. We find excellent agreement with all available data, while inclusion of strong correlations in the $\mathrm{GGA}+\mathrm{U}$ scheme impairs this agreement. We demonstrate that formal oxidation states reflect only orbital occupancies, not charge transfer, and resolve outstanding controversies about charge ordering.

Doping evolution of the Fermi surface topology of ${\mathrm{N}\mathrm{a}}_{x}{\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ is studied systematically. Both local density approximation (LDA) and local spin density approximation (LSDA) predict a large Fermi surface as well as small hole pockets for doping levels $x\ensuremath{\sim}0.5$. In contrast, the hole pockets are completely absent for all doping levels within $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$. More importantly, we find no violation of Luttinger's rule in this system. The measured Fermi surface of ${\mathrm{N}\mathrm{a}}_{0.7}{\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ can be explained by its half-metallic behavior and agrees with our $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$ calculations.

Doping evolution of the Fermi surface topology of Na(x)CoO(2) is studied systematically. Both local density approximation (LDA) and local spin density approximation (LSDA) predict a large Fermi surface as well as small hole pockets for doping levels x approximately 0.5. In contrast, the hole pockets are completely absent for all doping levels within LSDA+U. More importantly, we find no violation of Luttinger's rule in this system. The measured Fermi surface of Na(0.7)CoO(2) can be explained by its half-metallic behavior and agrees with our LSDA+U calculations.