We have studied the surface termination of atomically flat SrTiO3 surfaces treated by chemical etching and subsequent thermal annealing, for all commercially available orientations (001), (110), and (111). Atomic force microscopy confirms that our treatment processes produce unit cell steps with flat terrace structures. We have also determined the topmost atomic layer of SrTiO3 surfaces through time-of-flight mass spectroscopy. We found that all three orientations exhibit a Ti-rich surface. Our observation opens doors for interface engineering along the [110] and [111] directions in addition to a well known [100] case, which widens the range of functional heterostructures and interfaces.

In VO 2 , the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO 2 film using conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO 2 with the M2 phase is a Mott insulator.Among the Magnéli phases of vanadium oxides, VO 2 shows an insulator-to-metal transition (IMT) at 340 K, which is remarkably close to room temperature 1 . On top of it, the tunability of the transition involves various aspects of defects 2-4 , gate voltage 5 , and photon excitation 6 . In addition, its sensitivity to lattice modifications has triggered several investigations such as doping 7 , strain 8-12 , and pressure 13 . Owing to the applicability and flexibility of the transition, VO 2 has been widely examined to uncover its transition nature.From the electronic point of view, a Mott insulator due to on-site Coulomb interaction experiences an IMT, when the Hubbard gap is reduced and is overcome by the attractive electron-hole pair energy 14,15 . At the crossover, a large number of free charge carriers form discontinuously, meeting Mott's criteria of the critical carrier density 5,14-16 . On the other hand, the IMT in VO 2 is accompanied by a structural phase transition (SPT) from a monoclinic (M1) to a rutile (R) phase. The driving force initiating the IMT remains unclear since both occur concomitantly. Therefore, two mechanisms are competitively discussed at present: (1) a Peierls instability, i.e., a dimerization of vanadium atoms, and (2) a Mott IMT. The former is supported by band-structure calculations, considering a homopolar V-V bond to split d orbitals 17 , and by DFT-LDA calculations 18 . It is further corroborated by a simultaneous observation of both SPT and IMT within the internal thermalization time of a pump-probe method 19 and the measurement of a delayed structural response time upon a pulsed excitation 20 . The latter mechanism, a) Corresponding author: jeehoon@postech.ac.kr which induces a gap by repulsive electron-electron correlation, is mainly supported by experimental evidence. In the context of a Mott IMT, examples include a diverging effective mass at the crossover 21 , an estimation of the temperature difference between SPT and IMT 22 , a metallic monoclinic phase 6,22-25 , and an IMT triggered by electron injection 5,26 . The conflicting results also have implications on the cooperativity between the two mechanisms, but the conclusive origin ...

The optimization of the superconducting properties in a bottom electrode and the quality of an insulator barrier are the first steps in the development of superconductor/insulator/superconductor tunnel junctions. Here, we study the quality of a BaTiO3 tunnel barrier deposited on a 16 nm thick GdBa2Cu3O7−δ thin film by using conductive atomic force microscopy. We find that the tunnel current is systematically reduced (for equal applied voltage) by increasing the BaTiO3 barrier thickness between 1.6 and 4 nm. The BaTiO3 layers present an energy barrier of ≈1.2 eV and an attenuation length of 0.35–0.5 nm (depending on the applied voltage). The GdBa2Cu3O7−δ electrode is totally covered by a BaTiO3 thickness above 3 nm. The presence of ferroelectricity was verified by piezoresponse force microscopy for a 4 nm thick BaTiO3 top layer. The superconducting transition temperature of the bilayers is systematically suppressed by increasing the BaTiO3 thickness. This fact can be associated with stress at the interface and a reduction of the orthorhombicity of the GdBa2Cu3O7−δ. The reduction in the orthorhombicity is expected by considering the interface mismatch and it can also be affected by reduced oxygen stoichiometry (poor oxygen diffusion across the BaTiO3 barrier).

To extend the existing AAO mask assisted pulsed laser deposition (PLD) method, which is limited to fabricating a nanoisland array in a small area, we introduced a novel method to fabricate a high density array of multiferroic BFO nanoislands in a large area (wafer scale) by utilizing a nanoporous polymer template.