Although enhanced conductivity at ferroelectric domain boundaries has been found in BiFeO$_3$ films, Pb(Zr,Ti)O$_3$ films, and hexagonal rare-earth manganite single crystals, the mechanism of the domain wall conductivity is still under debate. Using
conductive atomic force microscopy, we observe enhanced conductance at the electrically-neutral domain walls in semiconducting hexagonal ferroelectric TbMnO$_3$ thin films where the structure and polarization direction are strongly constrained along the c-axis. This result indicates that domain wall conductivity in ferroelectric rare-earth manganites is not limited to charged domain walls. We show that the observed conductivity in the TbMnO$_3$ films is governed by a single conduction mechanism, namely, the back-to-back Schottky diodes model tuned by the segregation of defects.
We present results of Scanning Tunneling Microscopy and Spectroscopy (STS) measurements on the Kondo insulator SmB$_6$. The vast majority of surface areas investigated was reconstructed but, infrequently, also patches of varying size of non-reconstru
cted, Sm- or B-terminated surfaces were found. On the smallest patches, clear indications for the hybridization gap and inter-multiplet transitions were observed. On non-reconstructed surface areas large enough for coherent co-tunneling we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure.
Strongly correlated electron systems show many exotic properties such as unconventional superconductity, quantum criticality, and Kondo insulating behavior. In addition, the Kondo insulator SmB6 has been predicted theoretically to be a 3D topological
insulator with a metallic surface state. We report here transport measurements on doped SmB6, which show that ~3% magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior, evidence that the metallic surface state is only destroyed when time reversal symmetry is broken. We find as well a quantum percolation limit of impurity concentration which transform the topological insulator into a conventional band insulator by forming impurity band. Our careful thickness dependence results show that SmB6 is the first demonstatrated perfect 3D topological insulator with virtually zero residual bulk conductivity.
The impact of oxygen vacancies on local tunneling properties across rf-sputtered MgO thin films was investigated by optical absorption spectroscopy and conducting atomic force microscopy. Adding O$_2$ to the Ar plasma during MgO growth alters the oxy
gen defect populations, leading to improved local tunneling characteristics such as a lower density of current hotspots and a lower tunnel current amplitude. We discuss a defect-based potential landscape across ultrathin MgO barriers.
We investigated the time-dependent domain wall motion of epitaxial PbZr0.2Ti0.8O3 capacitors 100 nm-thick using modified piezoresponse force microscopy (PFM). We obtained successive domain evolution images reliably by combining the PFM with switching
current measurements. We observed that domain wall speed (v) decreases with increases in domain size. We also observed that the average value of v, obtained under applied electric field (Eapp),showed creep behavior: i.e. <v> ~ exp(-E0/Eapp)^$mu$ with an exponent $mu$ of 0.9 $pm$ 0.1 and an activation field E0 of about 700 kV/cm.
We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at par
ticular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at ferroelectric/electrode interfaces.
We investigated the ferroelectric properties of strontium titanate (STO) thin films deposited on SrTiO3 (001) substrate with SrRuO3 electrodes. The STO layer was grown coherently on the SrTiO3 substrate without in-plane lattice relaxation, but its ou
t-of-plane lattice constant increased with a decrease in the oxygen pressure during deposition. Using piezoresponse force microscopy and P-V measurements, we showed that our tetragonal STO films possess room-temperature ferroelectricity. We discuss the possible origins of the observed ferroelectricity.
