Quantum transport of quasi–two-dimensional Dirac fermions is largely controlled by magnetic order in a layered magnet.
(1 − / (0)) with ϕ 0 and ξ(0) being the flux quantum and the GL coherent length at zero temperature, respectively, we obtained ξ(0) = 2.0 nm, which is around two times shorter than the bulk value 3 .8 Getting back to the thickness dependence of T c on in Figs. 2c and 2d, we note that the first single step superconducting transition appears at the thickness of 9.6 nm and 5.9 nm for sample A and N, respectively. The appearance of high-T C superconductivity in the rather thicker conditions offers us to consider that how the electric-field effect contributes to the high-T C superconductivity. Here, we examined the effect of electrostatic doping on the superconductivity in both ultrathin and thicker conditions by different experimental schemes in the same device set-up as sample A. At first, samples B, C on SrTiO 3 and M on MgO were etched at T = 245 K to induce the superconductivity as shown in Fig. 3a, the top panel of Fig. 3c and Fig. 3d (red line data), respectively. The thicknesses of samples B, C and M are tuned to the one/two-unit-cell (the red square in Fig. 2c), 9.4 nm (the blue triangle in Fig. 2c) and 3.7 nm (the green circle in Fig. 2d), respectively, by monitoring the leakage current under the assumption that the same etching rate with sample A holds. After the detection of the high-T C superconducting behavior, we examined the electrostatic effect with removing and then applying V G at T = 220 K (along the blue arrow in Fig. 1b; Note that no electrochemical etching occurs at this temperature as mentioned above).For sample B expectedly in the one/two-unit-cell condition, the initial insulating behavior was not recovered by removing V G from 5 V to 3 and 0 V as shown in Fig. 3b. 9 This result is a good evidence for the thickness reached close to one/two-unit-cell, because the high-T C superconductivity under V G = 0 V is consistent with the result of previous reports on FeSe/SrTiO 3 with charge transfer solely from the SrTiO 3 substrate 8 .Interestingly, the increase of T c on from 43.5 K to 46.3 K as a result of reduced V G implicates the over-doping effect by electrostatic doping. In contrast, in sample C under the thicker 9.4 nm condition, the superconductivity vanishes and the R s -T curve is back to the insulating behavior when V G is removed (blue line in middle panel of Fig. 3c).The reappearance of insulating behavior indicates the uniform etching without local ultrathin region providing a high-T C superconducting current pass. Then, application of V G = 5 V recovers the superconductivity again (blue line in bottom panel of Fig. 3c), demonstrating a reversible control of insulator-superconductor transition by electrostatic mean. These series of results in sample C suggests that the observable thickness condition for high-T c on superconductivity is widely expanded from one/two-unit-cell to above 10-unit-cells by the application of electric field. Additionally, T c on in sample M decreases with reducing the accumulated charge as shown in Fig. 3d. Therefore, we suppose that the high-T C supercondu...
We investigate magnetotransport properties in a single crystal of pyrochore-type Nd2Ir2O7. The metallic conduction is observed on the antiferromagnetic domain walls of the all-in-all-out-type Ir 5d moment ordered insulating bulk state that can be finely controlled by an external magnetic field along [111]. On the other hand, an applied field along [001] induces the bulk phase transition from insulator to semimetal as a consequence of the field-induced modification of the Nd 4f and Ir 5d moment configurations. A theoretical calculation consistently describing the experimentally observed features suggests a variety of exotic topological states as functions of electron correlation and Ir 5d moment orders, which can be finely tuned by the choice of rare-earth ion and magnetic field, respectively.
The thermoelectric effect is attracting a renewed interest as a concept for energy harvesting technologies. Nanomaterials have been considered a key to realize efficient thermoelectric conversions owing to the low dimensional charge and phonon transports. In this regard, recently emerging two-dimensional materials could be promising candidates with novel thermoelectric functionalities. Here we report that FeSe ultrathin films, a high-Tc superconductor (Tc; superconducting transition temperature), exhibit superior thermoelectric responses. With decreasing thickness d, the electrical conductivity increases accompanying the emergence of high-Tc superconductivity; unexpectedly, the Seebeck coefficient α shows a concomitant increase as a result of the appearance of two-dimensional natures. When d is reduced down to ~1 nm, the thermoelectric power factor at 50 K and room temperature reach unprecedented values as high as 13,000 and 260 μW cm−1 K−2, respectively. The large thermoelectric effect in high Tc superconductors indicates the high potential of two-dimensional layered materials towards multi-functionalization.
One perovskite oxide, ASnO3 (A = Sr, Ba), is a candidate for use as a transparent conductive oxide with high electron mobility in single crystalline form. However, the electron mobility of films grown on SrTiO3 substrates does not reach the bulk value, probably because of dislocation scattering that originates from the large lattice mismatch. This study investigates the effect of insertion of bilayer BaSnO3 / (Sr,Ba)SnO3 for buffering this large lattice mismatch between La:BaSnO3 and SrTiO3 substrate. The insertion of 200-nm-thick BaSnO3 on (Sr,Ba)SnO3 bilayer buffer structures reduces the number of dislocations and improves surface smoothness of the films after annealing as proved respectively by scanning transmission electron microscopy and atomic force microscopy. A systematic investigation of BaSnO3 buffer layer thickness dependence on Hall mobility of the electron transport in La:BaSnO3 shows that the highest obtained value of mobility is 78 cm2V−1s−1 because of its fewer dislocations. High electron mobility films based on perovskite BaSnO3 can provide a good platform for transparent-conducting-oxide electronic devices and for creation of fascinating perovskite heterostructures.
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