In high T c superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO 2 planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, for three antiferromagnetic parent compounds (single-layer Bi 2 Sr 0.99 La 1.1 CuO 6+ , double-layer Nd 1.2 Ba 1.8 Cu 3 O 6 and infinite-layer CaCuO 2 ) differing by the number of apical atoms, we compare the magnetic spectra measured by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO 2 , it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature.

Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate di-electric, we have successfully tuned carrier density in FeSe0.5Te0.5 thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in FeSe0.5Te0.5. During the gating process, the (001) peak in XRD patterns stays at the same position and no new diffraction peak emerges, indicating no evident Li + ions intercalation into the FeSe0.5Te0.5. It indicates that a systematic change of electronic properties in FeSe0.5Te0.5 arises from the electrostatic doping induced by the accumulation of Li + ions at the interface between FeSe0.5Te0.5 and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which Tc is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions. PACS numbers: 74.25.F-, 74.70.Xa, 74.78.-w Tuning carrier concentration is one of the most powerful approaches in the condensed matter physics for the explorations of novel quantum phases and exotic electronic properties as well as their underlying physical mechanics [1-8]. To overcome the inherent doping limit in the material synthetic methods, field effect transistor (FET) configurations have been applied to tune material properties using gating by electric field [9]. Two types of FET, metal-insulator-semiconductor (MIS) FET and electric double layer (EDL) FET, are widely used to control the charge carrier density on the surface of materials [10, 11]. In order to change the carrier density in the bulk, the so-called ionic field-effect transistor (iFET) with gel-like electrolyte as the gate medium has been used to drive Li + ions into layered materials. This type of FET configuration can effectively modulate 1T-TaS 2 electronic properties by the tunable Li + ion intercalation [12]. However, the heavily-doped electronic states in all these FET configurations are confined at the interfaces or overlaid with electrolyte, which prevents them from being characterized by many physical measurements. On the other hand, conventional MIS-FET devices cannot provide sufficient carriers to induce novel phases, such as superconductivity, by electrostatic doping, and the liquid or gel-like electrolyte is not compatible with modern solid electronics and may react with samples when gating voltage is applied [11, 13, 14]. Recently, we have fabri-[ †] These authors contributed equally to this work.