The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO3) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.

Structural maintenance of chromosome (SMC) protein complexes are able to extrude DNA loops. While loop extrusion constitutes a fundamental building block of chromosomes, other factors may be equally important. Here, we show that yeast cohesin exhibits pronounced clustering on DNA, with all the hallmarks of biomolecular condensation. DNA-cohesin clusters exhibit liquid-like behavior, showing fusion of clusters, rapid fluorescence recovery after photobleaching and exchange of cohesin with the environment. Strikingly, the in vitro clustering is DNA length dependent, as cohesin forms clusters only on DNA exceeding 3 kilo–base pairs. We discuss how bridging-induced phase separation, a previously unobserved type of biological condensation, can explain the DNA-cohesin clustering through DNA-cohesin-DNA bridges. We confirm that, in yeast cells in vivo, a fraction of cohesin associates with chromatin in a manner consistent with bridging-induced phase separation. Biomolecular condensation by SMC proteins constitutes a new basic principle by which SMC complexes direct genome organization.

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In this article we introduce a, video-rate, control system that can be used with any type of scanning probe microscope, and that allows frame rates up to 200 images/ s. These electronics are capable of measuring in a fast, completely analog mode as well as in the more conventional digital mode. The latter allows measurements at low speeds and options, such as, e.g., atom manipulation, currentvoltage spectroscopy, or force-distance curves. For scanning tunneling microscope ͑STM͒ application we implemented a hybrid mode between the well-known constant-height and constant-current modes. This hybrid mode not only increases the maximum speed at which the surface can be imaged, but also improves the resolution at lower speeds. Acceptable image quality at high speeds could only be obtained by pushing the performance of each individual part of the electronics to its limit: we developed a preamplifier with a bandwidth of 600 kHz, a feedback electronics with a bandwidth of 1 MHz, a home-built bus structure for the fast data transfer, fast analog to digital converters, and low-noise drivers. Future improvements and extensions to the control electronics can be realized easily and quickly, because of its open architecture with its modular plug-in units. In the second part of this article we show our high-speed results. The ultrahigh vacuum application of these control electronics on our ͑UHV͒-STM enabled imaging speeds up to 0.3 mm/ s, while still obtaining atomic step resolution. At high frame rates, the images suffered from noticeable distortions, which we have been able to analyze by virtue of the unique access to the error ͑dZ͒ signal. The distortions have all been associated with mechanical resonances in the scan head of the UHV-STM. In order to reduce such resonance effects, we have designed and built a scan head with high resonance frequencies ͑ജ64 kHz͒, especially for the purpose of testing the fast electronics. Using this scanner we have reached video-rate imaging speeds up to 200 frames/ s ͑5 ms/ image͒, while still obtaining atomically resolved structures.

A new method for extracting quantitative data from amplitude modulation dynamic force-distance measurements is developed. The method is based on the harmonic oscillator model of vibrating atomic force microscope cantilevers, and is capable of extracting both the conservative and dissipative parts of the tip-sample interaction from a measurement of oscillation amplitude and phase as a function of distance. Numerical simulations are used to demonstrate the validity of the method. Further proof of the accuracy of this method is provided by a measurement of electrostatic forces between an AFM tip and a graphite sample.