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Register a new userDesign and Performance of a Practical Variable-Temperature Scanning Tunneling Potentiometry System
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We have constructed a scanning tunneling potentiometry system capable of simultaneously mapping the transport-related electrochemical potential of a biased sample along with its surface topography. Combining a novel sample biasing technique with a continuous current-nulling feedback scheme pushes the noise performance of the measurement to its fundamental limit - the Johnson noise of the STM tunnel junction. The resulting 130 nV voltage sensitivity allows us to spatially resolve local potentials at scales down to 2 nm, while maintaining angstrom scale STM imaging, all at scan sizes of up to 15 um. A mm-range two-dimensional coarse positioning stage and the ability to operate from liquid helium to room temperature with a fast turn-around time greatly expand the versatility of the instrument. By performing studies of several model systems, we discuss the implications of various types of surface morphology for potentiometric measurements.
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A theoretical description of scanning tunneling potentoimetry (STP) measurement is presented to address the increasing need for a basis to interpret experiments on macrscopic samples. Based on a heuristic understanding of STP provided to facilitate theoretical understanding, the total tunneling current related to the density matrix of the sample is derived within the general framework of quantum transport. The measured potentiometric voltage is determined implicitly as the voltage necessary to null the tunneling current. Explicit expressions of measured voltages are presented under certain assumptions, and limiting cases are discussed to connect to previous results. The need to go forward and formulate the theory in terms of a local density matrix is also discussed.
Under mesoscopic conditions, the transport potential on a thin film with current is theoretically expected to bear spatial variation due to quantum interference. Scanning tunneling potentiometry is the ideal tool to investigate such variation, by virtue of its high spatial resolution. We report in this {it Letter} the first detailed measurement of transport potential under mesoscopic conditions. Epitaxial graphene at a temperature of 17K was chosen as the initial system for study because the characteristic transport length scales in this material are relatively large. Tip jumping artifacts are a major possible contribution to systematic errors; and we mitigate such problems by using custom-made slender and sharp tips manufactured by focussed ion beam. In our data, we observe residual resistivity dipoles associated with topoographical defects, and local peaks and dips in the potential that are not associated with topographical defects.
We constructed a dilution-refrigerator (DR) based ultra-low temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in a UHV environment to an STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 hours. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance and applications for low temperature physics.
Scanning Superconducting QUantum Interference Device (SQUID) microscopy is a powerful tool for imaging local magnetic properties of materials and devices, but it requires a low-vibration cryogenic environment, traditionally achieved by thermal contact with a bath of liquid helium or the mixing chamber of a wet dilution refrigerator. We mount a SQUID microscope on the 3 K plate of a Bluefors cryocooler and characterize its vibration spectrum by measuring SQUID noise in a region of sharp flux gradient. By implementing passive vibration isolation, we reduce relative sensor-sample vibrations to 20 nm in-plane and 15 nm out-of-plane. A variable-temperature sample stage that is thermally isolated from the SQUID sensor enables measurement at sample temperatures from 2.8 K to 110 K. We demonstrate these advances by imaging inhomogeneous diamagnetic susceptibility and vortex pinning in optimally-doped YBCO above 90 K.
We describe the design and performance of a scanning tunneling microscope (STM) which operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in-situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated and electromagnetically shielded laboratory optimized for extremely low noise scanning probe measurements. We demonstrate the overall performance by illustrating atomic resolution and quasiparticle interference imaging and detail the vibrational noise of both the laboratory and microscope. We also determine the electron temperature via measurement of the superconducting gap of Re(0001) and illustrate magnetic field-dependent measurements of the spin excitations of individual Fe atoms on Pt(111). Finally, we demonstrate spin resolution by imaging the magnetic structure of the Fe double layer on W(110).
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