We demonstrate electrical mapping of tetragonal domains and electric field-induced twin walls in SrTiO 3 as a function of temperature and gate bias utilizing the conducting LaAlO 3 =SrTiO 3 interface and low-temperature scanning electron microscopy. Conducting twin walls appear below 105 K, and new twin patterns are observed after thermal cycling through the transition or on electric field gating. The nature of the twin walls is confirmed by calculating their intersection angles for different substrate orientations. Numerous walls formed when a large side-or back-gate voltage is applied are identified as field-induced ferroelectric twin walls in the paraelectric tetragonal matrix. The walls persist after switching off the electric field and on thermal cycling below 105 K. These observations point to a new type of ferroelectric functionality in SrTiO 3 , which could be exploited together with magnetism and superconductivity in a multifunctional context. DOI: 10.1103/PhysRevLett.116.257601 The emerging field of domain boundary engineering requires interfaces with unique functionalities [1] such as in the SrTiO 3 (STO)-based heterostructures [2][3][4][5]. STO is cubic at room temperature, but undergoes a ferroelastic transition to tetragonal structure at around 105 K. It does not become spontaneously ferroelectric at low temperatures despite its huge permittivity [6,7]. Nevertheless, electric order can be induced by stress, or by electric field (E) [8][9][10] at a threshold of 1.40 kV=cm at ∼5 K.When STO is used as a substrate or gate insulator for materials such as topological insulators and superconductors [11,12], films grown on STO are assumed to be biased uniformly. However, this is not really the case as nonuniformity can arise from the tetragonal domain structure. Twin boundaries between domains (twin walls) in STO are of particular interest, as they have been suggested to become conducting and ferroelectric at low temperatures [13][14][15][16][17][18][19]. Further, it has been demonstrated that ferroelectric "stripes" can be injected in ferroelectric thin films by increasing the applied voltage [20,21], which requires the material, or at least its domain walls, to be ferroelectric. Here, we image the ferroelastic twin walls in STO, and show that their response above the threshold field is strong evidence that it is the field-induced twin walls that become ferroelectric.STO can be made conducting by doping, by oxygen vacancies, or by electronic reconstruction induced by a polar oxide LaAlO 3 (LAO) that produces a few nanometers of two-dimensional electron gas (2DEG) below the interface [22][23][24], while the bulk of the STO remains insulating. Recently, an influence of the local tetragonal domain structure on the conductivity of the 2DEG at the LAO/ STO interface was demonstrated by probing the magnetic field [25] or electric potential [26] induced by the 2DEG, rather than mapping the electronic properties directly, which is difficult with a probe such as scanning tunneling microscopy because the 2DEG is...

We have measured the electrical properties of a V2O3 thin film micro bridge at the insulator-metal transition (IMT). Discontinuous jumps to lower voltages in the current voltage characteristic (IV) followed by an approximately constant voltage progression for high currents indicate an electrical breakdown of the device. In addition, the IV curve shows hysteresis and a training effect, i.e., the subsequent IV loops are different from the first IV loop after thermal cycling. Low-temperature scanning electron microscopy (LTSEM) reveals that the electrical breakdown over the whole device is caused by the formation of electro-thermal domains (ETDs), i.e., the current and temperature redistribution in the device. On the contrary, at the nanoscale, the electrical breakdown causes the IMT of individual domains. In a numerical model we considered these domains as a network of resistors and we were able to reproduce the electro-thermal breakdown as well as the hysteresis and the training effect in the IVs.

Josephson junctions with ferromagnetic barrier can have positive or negative critical current depending on the thickness d F of the ferromagnetic layer. Accordingly, the Josephson phase in the ground state is equal to 0 (a conventional or 0 junction) or to π (π junction). When 0 and π segments are joined to form a "0-π junction", spontaneous supercurrents around the 0-π boundary can appear. Here we report on the visualization of supercurrents in superconductor-insulatorferromagnet-superconductor (SIFS) junctions by low-temperature scanning electron microscopy (LTSEM). We discuss data for rectangular 0, π, 0-π, 0-π-0 and 20 × (0-π-) junctions, disk-shaped junctions where the 0-π boundary forms a ring, and an annular junction with two 0-π boundaries.Within each 0 or π segment the critical current density is fairly homogeneous, as indicated both by measurements of the magnetic field dependence of the critical current and by LTSEM. The π parts have critical current densities j π c up to 35 A/cm 2 at T = 4.2 K, which is a record value for SIFS junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology is capable to produce Josephson devices with a unique topology of the 0-π boundary.

We have realized multifacet Josephson junctions with periodically alternating critical current density (MJJs) using superconductor-insulator-ferromagnet-superconductor heterostructures. We show that anomalous features of critical current vs. applied magnetic field, observed also for other types of MJJs, are caused by a non-uniform flux density (parallel to the barrier) resulting from screening currents in the electrodes in the presence of a (parasitic) off-plane field component.

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