Design and performance of an ultra-high vacuum scanning tunneling microscope operating at dilution refrigerator temperatures and high magnetic fields

Abstract

We describe the construction and performance of a scanning tunneling microscope (STM) capable of taking maps of the tunneling density of states with sub-atomic spatial resolution at dilution refrigerator temperatures and high (14 T) magnetic fields. The fully ultra-high vacuum system features visual access to a two-sample microscope stage at the end of a bottom-loading dilution refrigerator, which facilitates the transfer of in situ prepared tips and samples. The two-sample stage enables location of the best area of the sample under study and extends the experiment lifetime. The successful thermal anchoring of the microscope, described in detail, is confirmed through a base temperature reading of 20 mK, along with a measured electron temperature of 250 mK. Atomically-resolved images, along with complementary vibration measurements, are presented to confirm the effectiveness of the vibration isolation scheme in this instrument. Finally, we demonstrate that the microscope is capable of the same level of performance as typical machines with more modest refrigeration by measuring spectroscopic maps at base temperature both at zero field and in an applied magnetic field.PACS numbers: 07.79.Cz,07.79.Fc Scanning tunneling microscopy (STM), since its development almost 30 years ago, has become a powerful technique in condensed matter physics, providing not only structural information about surfaces, but also spectroscopic measurements of the electronic density of states at the atomic length scale. However, most instruments operate at temperatures above 1 K, limiting access to exotic electronic phases and quantum effects expected at lower temperatures, which are studied as a matter of routine by other techniques. Generally, very little spectroscopic information about the electronic density of states is known at dilution refrigerator temperatures, usually being limited to what can be learned using either point contact spectroscopy or planar tunnel junctions. Moreover, STM can make such measurements on the atomic length scale, allowing it to probe systems, such as single spins and atomic chains, which are not directly accessible any other way.While the integration of STM with a dilution refrigerator can be conceptually reduced to simply attaching the microscope to the end of a mixing chamber in lieu of some other cryogenic refrigerator, the technical requirements for sub-Angstrom positioning of an STM tip above an atomically clean surface are often at odds with those for cooling a sample to milli-Kelvin temperatures. For example, when attaching the microscope to the refrigerator, the former would favor the use of a soft mechanical joint using springs, which would isolate vibrations, while a) Electronic mail: yazdani@princeton.edu the latter would favor the use of a rigid mechanical joint with a metal rod, which would provide a strong thermal contact. Nevertheless, a number of STM instruments have been developed that cool the sample using a dilution refrigerator 1-8 . However, among these, few feature ultra-high vacuum (UHV...

Scanning Josephson spectroscopy on the atomic scale

“…We do not observe a commensurate shift in the coherence peak energies in the single particle spectrum, which is consistent with theoretical calculations [20,21], highlighting the ability of the Josephson STM technique to probe physics inaccessible through traditional STM quasiparticle tunneling. Additionally, spatially resolved spectroscopy with a superconducting tip allows us to detect novel Andreev tunneling processes through impurity-bound Shiba states of the individual adatoms.Our measurements have been carried out using a home-built dilution refrigerator STM system, with a base temperature of 20 mK and a spectroscopic resolution that corresponds to an effective electron temperature of 250 mK [22]. For the present experiments, we used a Pb(110) single crystal that was prepared in-situ with several cycles of Ar sputtering and annealing to produce an atomically ordered flat surface.…”

Bulk crystal growth and electronic characterization of the 3D Dirac semimetal Na3Bi

“…To investigate the electronic properties of Na 3 Bi, we used a home built low-temperature STM to obtain topographic and spectroscopic information with high spatial resolution. 18,32 The Na 3 Bi crystals were mounted on a stage in the glove box and transferred to the STM systems. The STM measurements were performed at a temperature of approximately 3K.…”

Acoustic buffeting by infrasound in a low vibration facility

“…The measured performance typically exhibits nearly ideal 2nd-order transfer functions at low frequencies, but deviations in the form of excess vibration amplitude typically appear above 10-20 Hz. [5][6][7][8][9][10] There are many potential sources of such deviations, including rigid-body rolling modes of the slab, flexural modes, non-linearity in the response under test conditions, and acoustic excitation of the block motion. This latter source, referred to as acoustic buffeting or the sail-effect, is a known problem in a wide range of facilities ranging across fields such as metrology, 10 scanning electron microscopy, 11 medical imaging, 12 nanotechnology, 13 and the testing of components used in gravitational wave detection.…”

“…4 The entire system can be further isolated by supporting it on pneumatic vibration isolators, and a number of state-of-theart STM facilities go even further by placing the system atop a massive inertia slab that is itself supported on large pneumatic isolators. 5,6 One or more layers of acoustic enclosure are also employed to prevent acoustic excitation of vibrations of the instrument. This more extreme approach is especially important for ultra-low-temperature STMs.…”