We report experimental demonstration of the feasibility of reaching temperatures below 1 mK using cryogen-free technology. Our prototype system comprises an adiabatic nuclear demagnetization stage, based on hyperfineenhanced nuclear magnetic cooling, integrated with a commercial cryogenfree dilution refrigerator and 8 T superconducting magnet. Thermometry was provided by a current-sensing noise thermometer. The minimum temperature achieved at the experimental platform was 600 µK. The platform remained below 1 mK for over 24 h, indicating a total residual heat-leak into the experimental stage of 5 nW. We discuss straightforward improvements to the design of the current prototype that are expected to lead to enhanced performance. This opens the way to widening the accessibility of temperatures in the microkelvin regime, of potential importance in the application of strongly correlated electron states in nanodevices to quantum computing.
We present a scanning tunneling microscope ͑STM͒ designed to operate between 275 mK and room temperature, in magnetic fields up to 14 T and in ultrahigh vacuum ͑UHV͒. The system features a compact STM connected to an UHV compatible 3 He refrigerator fitting into a bottom loading cryostat with a superconducting magnet. In this configuration, the cryostat is sitting on top of the UHV chamber, resulting in a very short distance between the STM access and the experimental position. It further enables proper thermal anchoring of the entire STM setup, allowing millikelvin temperatures to be reached in true UHV conditions. We achieve a hold time of about 40 h at 275 mK and a turnaround time of 10 h between room and base temperature. We demonstrate atomic resolution and present tunneling spectra obtained at 275 mK on the high-T c superconductors Bi 2 Sr 2 CaCu 2 O 8ϩ␦ and YBa 2 Cu 3 O 7Ϫ␦ .
We describe the design and performance of a series of fast, precise current sensing noise thermometers. The thermometers have been fabricated with a range of resistances from 1.290 Ω down to 0.2 mΩ. This results in either a thermometer that has been optimised for speed, taking advantage of the improvements in superconducting quantum interference device (SQUID) noise and bandwidth, or a thermometer optimised for ultra-low temperature measurement, minimising the system noise temperature. With a single temperature calibration point, we show that noise thermometers can be used for accurate measurements over a wide range of temperatures below 4 K. Comparisons with a melting curve thermometer, a calibrated germanium thermometer and a pulsed platinum nuclear magnetic resonance thermometer are presented. For the 1.290 Ω resistance we measure a 1 % precision in just 100 ms, and have shown this to be independent of temperature.
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