Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.

The skyrmion, a vortex-like spin-swirling object, is anticipated to play a vital role in quantum magneto-transport processes such as the quantum Hall and topological Hall effects. The existence of the magnetic skyrmion crystal (SkX) state was recently verified experimentally for MnSi and Fe(0.5)Co(0.5)Si by means of small-angle neutron scattering and Lorentz transmission electron microscopy. However, to enable the application of such a SkX for spintronic function, materials problems such as a low crystallization temperature and low stability of SkX have to be overcome. Here we report the formation of SkX close to room temperature in thin-films of the helimagnet FeGe. In addition to the magnetic twin structure, we found a magnetic chirality inversion of the SkX across lattice twin boundaries. Furthermore, for thin crystal plates with thicknesses much smaller than the SkX lattice constant (as) the two-dimensional SkX is quite stable over a wide range of temperatures and magnetic fields, whereas for quasi-three-dimensional films with thicknesses over as the SkX is relatively unstable and observed only around the helical transition temperature. The room-temperature stable SkX state as promised by this study will pave a new path to designing quantum-effect devices based on the controllable skyrmion dynamics.

2Charge density wave (CDW) transitions are a frequent occurrence in transition metal chalcogenides due to their low structural dimensionality. Layered MX 2 compounds and chain-based MX 3 compounds, where M is a group 4 or 5 metal and X = S, Se, or Te, are the best known examples [1][2][3][4][5][6][7]. These transitions arise to allow electronic systems to minimize their energy by removing electronic states at the Fermi level. This is achieved by introducing a new structural periodicity at the Fermi wave vector, inducing a band gap. Superconductivity and the CDW state are two very different cooperative electronic phenomena, and yet both occur due to Fermi surface instabilities and electron-phonon coupling. A number of CDW-bearing materials are also superconducting [8][9][10][11][12][13], and the idea that superconductivity and CDW states are competing electronic states at low temperatures is one of the fundamental concepts of condensed matter physics. Surprisingly, no system has yet been reported in which the emergence of a superconducting state after a charge density wave state has been suppressed via doping has been studied in detail: a transition that implies a deep connection between the two states, i.e., that the same electrons are participating in both transitions. TiSe 2 was one of the first CDW-bearing compounds known, and is also one of the most frequently studied as the nature of its CDW transition has been controversial for decades. The CDW transition, at approximately 200 K, is to a state with a commensurate (2a,2a,2c) wavevector without an intermediate incommensurate phase [3,16,17]. The commensurate CDW wavevector and electronic structure calculations indicate that, unlike the case in most materials, the CDW in TiSe 2 is not driven by Fermi surface nesting. The normal state is presently believed to be either a semimetal or a semiconductor with a small indirect gap [3, 16, 18 -22] (Fig. 1a, inset). This results in a systematic expansion of the unit cell with Cu content in Cu x TiSe 2 , as evidenced by the lattice parameters shown in Fig. 1a. The expansion of the cell parameters is maintained up to x = 0.11. For higher Cu contents, both a and c remain unchanged from their value at x = 0.11. It can therefore be concluded that the solubility limit for Cu in TiSe 2 is x = 0.11 ± 0.01.Of particular interest is the evolution of the charge density wave with Cu doping.Electron and X-ray diffraction studies of pure TiSe 2 at low temperatures show the presence of reflections corresponding to the basic trigonal structure and also the 2a, 2c superstructure reflections associated with the CDW state [3,19]. increases with Cu content. This suggests that the Cu doping introduces carriers into the conduction band in TiSe 2 , increasing the electronic density of states and therefore the Pauli paramagnetism. This is further confirmed by specific heat measurements, described below. A drop in the susceptibility of pure TiSe 2 is seen as the temperature is lowered below the CDW transition at 200 K, consistent with th...

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.