In the classic transistor, the number of electric charge carriers--and thus the electrical conductivity--is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system's state, as successfully demonstrated in electric-field-induced ferromagnetism and superconductivity. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas-Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal-insulator-semiconductor field-effect transistors based on vanadium dioxide--a strongly correlated material with a thermally driven, first-order metal-insulator transition well above room temperature--and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal-insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.

The pseudogap state between T c and the temperature T*, below which the gap in the DOS occurs, has been the subject of a wide range of theoretical proposals-from those focused on superconducting pairing correlations without phase coherence 14,15 to those based on some form of competing electronic order or proximity to the Mott state [6][7][8][9] . Some

Besides superconductivity, copper-oxide high-temperature superconductors are susceptible to other types of ordering. We used scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of charge ordering in the high-temperature superconductor Bi2Sr2CaCu2O(8+x). Depending on the hole concentration, the charge ordering in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity. These results indicate the similarity of charge organization competing with superconductivity across different families of cuprates. We observed this charge ordering to leave a distinct electron-hole asymmetric signature (and a broad resonance centered at +20 milli-electron volts) in spectroscopic measurements, indicating that it is likely related to the organization of holes in a doped Mott insulator.

We report atomic-scale characterization of the pseudogap state in a high-Tc superconductor, Bi2Sr2CaCu2O(8+delta). The electronic states at low energies within the pseudogap exhibit spatial modulations having an energy-independent incommensurate periodicity. These patterns, which are oriented along the copper-oxygen bond directions, appear to be a consequence of an electronic ordering phenomenon, the observation of which correlates with the pseudogap in the density of electronic states. Our results provide a stringent test for various ordering scenarios in the cuprates, which have been central in the debate on the nature of the pseudogap and the complex electronic phase diagram of these compounds.

We propose that resistivity curvature mapping (RCM) based on the in-plane resistivity data is a useful way to objectively draw electronic phase diagrams of high-Tc cuprates, where various crossovers are important. In particular, the pseudogap crossover line can be conveniently determined by RCM. We show experimental phase diagrams obtained by RCM for Bi2Sr2-zLazCuO6+delta, La2-xSrxCuO4, and YBa2Cu3Oy, and demonstrate the universal nature of the pseudogap crossover. Intriguingly, the electronic crossover near optimum doping depicted by RCM appears to occur rather abruptly, suggesting that the quantum-critical regime, if it exists, must be very narrow.

Manipulating a quantum state via electrostatic gating has been of great importance for many model systems in nanoelectronics. Until now, however, controlling the electron spins or, more specifically, the magnetism of a system by electric-field tuning has proven challenging. Recently, atomically thin magnetic semiconductors have attracted significant attention due to their emerging new physical phenomena. However, many issues are yet to be resolved to convincingly demonstrate gate-controllable magnetism in these two-dimensional materials. Here, we show that, via electrostatic gating, a strong field effect can be observed in devices based on few-layered ferromagnetic semiconducting CrGeTe. At different gate doping, micro-area Kerr measurements in the studied devices demonstrate bipolar tunable magnetization loops below the Curie temperature, which is tentatively attributed to the moment rebalance in the spin-polarized band structure. Our findings of electric-field-controlled magnetism in van der Waals magnets show possibilities for potential applications in new-generation magnetic memory storage, sensors and spintronics.

In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at Tc is caused by the loss of long-range phase coherence, rather than the vanishing of the pair condensate. Here we report a series of torque magnetization measurements on single crystals of La2−xSrxCuO4 (LSCO), Bi2Sr2−yLayCuO6 (Bi 2201), Bi2Sr2CaCu2O 8+δ (Bi 2212) and optimal YBa2Cu3O7. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above Tc, we show that the diamagnetic term M d appears at an onset temperature T M onset high above Tc. We construct the phase diagram of both LSCO and Bi 2201 and show that T M onset agrees with the onset temperature of the vortex Nernst signal T ν onset . Our results provide thermodynamic evidence against a recent proposal that the high-temperature Nernst signal in LSCO arises from a quasiparticle contribution in a charge-ordered state.

The diffusion of vortices down a thermal gradient produces a Josephson signal which is detected as a vortex Nernst effect. In a recent report by Xu et al. ͓Nature 406, 486 ͑2000͔͒, an enhanced Nernst signal identified with vortex-like excitations was observed in a series of La 2Ϫx Sr x CuO 4 ͑LSCO͒ crystals at temperatures 50-100 K above T c . To pin down the onset temperature T of the vortexlike signal in the lightly doped regime (0.03рxр0.07), we have reanalyzed the carrier contribution to the Nernst signal in detail. By supplementing Nernst measurements with thermopower and Hall-angle data, we isolate the off-diagonal Peltier conductivity ␣ xy and show that its profile provides an objective determination of T . With the results, we revise the phase diagram for the fluctuation regime in LSCO to accommodate the lightly doped regime. In the cuprate Bi 2 Sr 2Ϫy La y CuO 6 , we find that the carrier contribution is virtually negligible for y in the range 0.4 -0.6. The evidence of an extended temperature interval with vortexlike excitations is even stronger in this system. Finally, we discuss how T relates to the pseudogap temperature T* and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.