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Register a new userDeveloping compact tuning fork thermometers for sub-mK temperatures and high magnetic fields
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The National High Magnetic Field Laboratory (NHMFL) High B/T facility at the University of Florida in Gainesville provides a unique combination of ultra-low temperatures below 1 mK and high magnetic fields up to 16 T for user experiments. To meet the growing user demand for calorimetric and thermal transport measurements, particularly on milligram-sized solid samples, we are developing scaleable thermometers based on quartz tuning fork resonators immersed in liquid $^3$He. We demonstrate successful thermometer operation at the combined extreme conditions available at our user facility, and discuss the feasibility of fast and compact thermal probes.
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We describe the development and performance of an ultra-high vacuum scanning tunneling microscope working under combined extreme conditions of ultra-low temperatures and high magnetic fields. We combined a top-loading dilution refrigerator and a standard bucket dewar with a bottom-loading superconducting magnet to achieve 4.5 days operating time, which is long enough to perform various spectroscopic-imaging measurements. To bring the effective electron temperature closer to the mixing-chamber temperature, we paid particular attention to filtering out the radio-frequency noise, as well as enhancing the thermal link between the microscope unit and the mixing chamber. We estimated the lowest effective electron temperature to be below 90 mK by measuring the superconducting-gap spectrum of aluminum. We confirmed the long-term stability of the spectroscopic-imaging measurement by visualizing superconducting vortices in the cuprate superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+delta}$.
We developed an impedance bridge that operates at cryogenic temperatures (down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed circuit board containing the device under test and thereby parallel to the magnetic field. The measured amplitude and phase of the output signal allows for the separation of the total impedance into an absolute capacitance and a resistance. Through a detailed noise characterization, we find that the best resolution is obtained when operating the HEMT amplifier at the highest gain. We obtained a resolution in the absolute capacitance of 6.4~aF$/sqrt{textrm{Hz}}$ at 77 K on a comb-drive actuator, while maintaining a small excitation amplitude of 15~$k_text{B} T/e$. We show the magnetic field functionality of our impedance bridge by measuring the quantum Hall plateaus of a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of 12.8~$k_text{B} T/e$.
This study presents a fabrication process for lithium-drifted silicon (Si(Li)) detectors that, compared to previous methods, allows for mass production at a higher yield, while providing a large sensitive area and low leakage currents at relatively high temperatures. This design, developed for the unique requirements of the General Antiparticle Spectrometer (GAPS) experiment, has an overall diameter of 10 cm, with ~9 cm of active area segmented into 8 readout strips, and an overall thickness of 2.5 mm, with $gtrsim$2.2 mm ($gtrsim$90%) sensitive thickness. An energy resolution $lesssim$4 keV full-width at half-maximum (FWHM) for 20-100 keV X-rays is required at the operating temperature ~-40C, which is far above the liquid nitrogen temperatures conventionally used to achieve fine energy resolution. High-yield production is also required for GAPS, which consists of $gtrsim$1000 detectors. Our specially-developed Si crystal and custom methods of Li evaporation, diffusion and drifting allow for a thick, large-area and uniform sensitive layer. We find that retaining a thin undrifted layer on the p-side of the detector drastically reduces the leakage current, which is a dominant component of the energy resolution at these temperatures. A guard-ring structure and optimal etching of the detector surface are also confirmed to suppress the leakage current. We report on the mass production of these detectors that is ongoing now, and demonstrate it is capable of delivering a high yield of ~90%.
Magnetism and superconductivity often compete for preeminence as a materials ground state, and in the right circumstances the fluctuating remains of magnetic order can induce superconducting pairing. The intertwining of the two on the microscopic level, independent of lattice excitations, is especially pronounced in heavy fermion compounds, rare earth cuprates, and iron pnictides. Here we point out that for a helical arrangement of localized spins, a variable magnetic pitch length provides a unique tuning process from ferromagnetic to antiferromagnetic ground state in the long and short wavelength limits, respectively. Such chemical or pressure adjustable helical order naturally provides the possibility for continuous tuning between ferromagnetically and antiferromagnetically mediated superconductivity. At the same time, phonon mediated superconductivity is suppressed because of the local ferromagnetic spin configuration. We employ synchrotron-based magnetic x-ray diffraction techniques to test these ideas in the recently discovered superconductor, MnP. This sensitive probe directly reveals a reduced-moment, helical spin order at high pressure proximate to the superconducting state, with a tightened pitch in comparison to that at ambient pressure where superconductivity is absent. The correlation between magnetic pitch length and superconducting transition temperature in the (Cr/Mn/Fe)(P/As) family suggests a strategy for using spiral magnets as interlocutors for spin fluctuation mediated superconductivity.
We present the evaluation of two different design configurations of a two-stage PrNi$_5$ continuous nuclear demagnetization refrigerator. Serial and parallel configurations of the two stages are considered, with emphasis on the attainable cooling power at sub-mK temperatures and the impact of the design choices on the operation of the refrigerator. Numerical simulations of heat transfer in the setup are used to evaluate the performance of the refrigerator as well as the technological requirements for the essential thermal links. In accord with similar findings for adiabatic demagnetization refrigerators [Shirron, emph{Cryogenics} textbf{62}, 2014], our simulations show that the performance of both configurations improves as the thermal links improve, and that the parallel configuration yields a higher cooling power than the series design for a given thermal link resistance and sample temperature.
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