Since the discovery of n-type copper oxide superconductors, the evolution of electron- and hole-bands and its relation to the superconductivity have been seen as a key factor in unveiling the mechanism of high-Tc superconductors. So far, the occurren
ce of electrons and holes in n-type copper oxides has been achieved by chemical doping, pressure, and/or deoxygenation. However, the observed electronic properties are blurred by the concomitant effects such as change of lattice structure, disorder, etc. Here, we report on successful tuning the electronic band structure of n-type Pr2-xCexCuO4 (x = 0.15) ultrathin films, via the electric double layer transistor technique. Abnormal transport properties, such as multiple sign reversals of Hall resistivity in normal and mixed states, have been revealed within an electrostatic field in range of -2 V to +2 V, as well as varying the temperature and magnetic field. In the mixed state, the intrinsic anomalous Hall conductivity invokes the contribution of both electron and hole-bands as well as the energy dependent density of states near the Fermi level. The two-band model can also describe the normal state transport properties well, whereas the carrier concentrations of electrons and holes are always enhanced or depressed simultaneously in electric fields. This is in contrast to the scenario of Fermi surface reconstruction by antiferromagnetism, where an anti-correlation between electrons and holes is commonly expected. Our findings paint the picture where Coulomb repulsion plays an important role in the evolution of the electronic states in n-type cuprate superconductors.
Transition-metal oxides offer an opportunity to explore unconventional superconductors, where the superconductivity (SC) is often interrelated with novel phenomena such as spin/charge order, fluctuations, and Fermi surface instability (1-3). LiTi2O4
(LTO) is a unique compound in that it is the only known spinel oxide superconductor. In addition to electron-phonon coupling, electron-electron and spin fluctuation contributions have been suggested as playing important roles in the microscopic mechanism for its superconductivity (4-8). However, the lack of high quality single crystals has thus far prevented systematic investigation of their transport properties (9). Here, we report a careful study of transport and tunneling spectroscopy in epitaxial LTO thin films. In the superconducting state, the energy gap was found to decrease as a quadratic function of magnetic field. In the normal state, an unusual magnetoresistance (MR) was observed where it changes from anisotropic positive to isotropic negative as the temperature is increased. A constant charge carrier concentration without any abrupt change in lattice parameters as a function of temperature suggests that the isotropic MR stems from the suppression of spin scattering/fluctuations, while the anisotropic term originates from an orbital contribution. These observations point to an important role strong correlations play in this unique superconductor.
We have fabricated Fe-B thin film composition spreads in search of possible superconducting phases following a theoretical prediction by Kolmogorov et al.^1 Co-sputtering was used to deposit spreads covering a large compositional region of the Fe-B b
inary phase diagram. A trace of superconducting phase was found in the nanocrystalline part of the spread, where the film undergoes a metal to insulator transition as a function of composition in a region with the average composition of FeB_2. The resistance drop occurs at 4K, and a diamagnetic signal has also been detected at the same temperature. The superconductivity is suppressible in the magnetic field up to 2 Tesla.
