Session V23: Low Temperature Instrumentation

Compact Telescoping Sample Manipulator for Ultra-High Vacuum

Scanning tunneling microscopes (STMs) are used to resolve the morphology and electronic structure of a material surface with atomic resolution. Key factors to achieve state of the art STM measurements are stability (i.e. decoupling from external noise sources), low temperatures (<10K) and ultra-high vacuum (UHV) conditions, which usually require a long sample transfer mechanism. However, such a long manipulator is a pendulum that acts as an antenna for acoustic and vibrational noise in the lab and couples the noise into the STM. Here we show the design, construction, and performance of a compact telescoping manipulator attached to an STM system. Our compact manipulator reduces noise coupling both by decreasing the oscillation amplitude and by increasing the resonant frequency and shifting it away from typical ambient noise frequencies (typically 10s of Hz) and resonances of the vibration isolation systems (typically 1 - 3 Hz). In addition, our telescoping arm would be a useful tool for any UHV precision measurement system in a confined space.   

Design and development of a low temperature, inductance based 3.5 MHz ac susceptometer.

We discuss the development of an induction based, low temperature high frequency ac susceptometer capable of measuring at frequencies up to 3.5 MHz and at temperatures between 2 K and 300 K, in applied magnetic dc fields of up to 9 T. Careful balancing of the detection coils and calibration have allowed a sample magnetic moment resolution of 5x10^-10 Am² at 1 MHz. We will discuss the design and characterization of the susceptometer, and explain the calibration process and the applicabilty of the system to exisiting cryostat designs. We also include some example measurements on the spin ice materials, iron oxide based nanoparticles and superconductors to describe functionality to illustrate functionality.   

A Modular Design for Ultra-High Vacuum Millikelvin STM Systems

We describe the unique modular design of an ultra-high vacuum (UHV) STM system, capable of reaching dilution-refrigerator temperatures (10 mK) and equipped with a vector magnetic field. The goals of our design are to separate the UHV needs of STM instrumentation from the typical non-UHV construction of a dilution fridge system, and to make the STM head module interchangeable. To accomplish this, a custom-built dilution refrigerator surrounds a cooled UHV tube chamber, which houses our STM, with specially designed electrical connections at low temperatures to connect to an interchangeable STM head module. For diagnostic purposes, the microscope can also be used at room temperature in the UHV preparation chambers that are integrated with the low-temperature UHV tube system. The microscope exchange can be done at any time without compromising cryogenics or vacuum. The system works with a 240L liquid He cryostat, with a vector magnet (9T-1T-1T), that has a 10-day hold time. Our current microscope head design features multiple contacts to perform STM on gated structures and simultaneous low-temperature transport measurements on devices. Microscope designs, instrument construction, and preliminary data will be discussed.   

Scanning probe microscopy at ultra-high magnetic fields

Up to now, low temperature scanning probe microscopy (SPM) has been limited to a magnetic field strength of around 20 T, as most designs have been based on superconductor magnets. For some experimental applications – for example fractal spectra in graphene superlattices, the room temperature quantum Hall effect and some metamagnetic transitions – higher fields are required. Static fields of more than 30 T can be generated in dedicated high-field facilities by water-cooled, resistive Bitter magnets or hybrid resistive-superconducting magnets. However, implementing SPM in a Bitter magnet is a major challenge, due to the high level of vibrational noise produced by the turbulent cooling water, in addition to the space constraints resulting from the small magnet bore.

We present a novel cryogenic scanning tunnelling microscope (STM) designed to operate inside a 38 T water-cooled Bitter magnet. The performance of the STM is demonstrated through Landau level tunnelling spectroscopy of graphite, at 4.2 K in magnetic fields up to 34 T. Additionally we show the design of a highly compact atomic force microscope (AFM) for operation at cryogenic temperatures in an extremely high magnetic field. We show preliminary imaging data on the frustrated spinel CdCr₂O₄ at up to 30 T.   

Achieving µeV resolution in scanning tunneling spectroscopy at mK temperatures in Al-Al SIS junctions

Improved energy resolution in spectroscopic measurements is one of the main motivations for conducting Scanning Probe Microscopy (SPM) experiments at temperatures of tens of mK using a dilution refrigerator (DR). However, SPM instruments at these temperatures have not always achieved an energy resolution consistent with the DR temperature due to electrical noise coupling into the tunnel junction. Here, we present Scanning tunneling spectroscopy measurements of an Al-Al Josephson junction using an Al thin film grown in situ, together with an Al probe tip. The improved performance is achieved by optimizing our SPM operating in ultra-high vacuum (UHV) inside a DR with a base temperature of 10mK and magnetic field up to 15T [1]. Home built Radio Frequency (RF) powder filters located at the mixing chamber shield the tunnel junction from room temperature RF noise. Further RF filtering at the UHV chamber room temperature feedthroughs, together with attention to grounding, lead to an order of magnitude improved resolution over previous measurements. We demonstrate an energy resolution in the low ueV range and discuss the trials and tribulations to achieve this resolution and the causes of the residual intrinsic energy broadening that is observed.

[1] Song et al., RSI 81, 121101 (2010)   

Josephson-junction-based bolometer

We recently introduced a calorimeter capable of detecting packets containing 200 8.4-GHz photons, or 1.1 zJ of energy. Now we report a record low noise equivalent power (NEP) of 50 zW/rtHz, which is reduced to 20 zW/rtHz by introducing a Josephson parametric amplifier to the readout circuitry. The lower NEP value suggests energy resolution of h×480 GHz. Sensitivity of the detector can be further increased by decreasing the heat capacity of the absorbing element. To this end, we have experimentally studied graphene as an absorber, which is promising material due to its two-dimensional structure. Our preliminary results suggest energy resolution of h×60 GHz, bringin resolution of thermal detectors close to the qubit-based detectors while maintaining superior detection bandwidth.   

Temperature sensing via photoluminescence lifetimes of Rhodamine B

Non-invasive temperature probes have use in many settings where conventional thermometers may not be suitable or as efficient. An optical temperature probe is a material whose optical properties, such as photoluminescence (PL) or PL lifetime, are known as a function of temperature. We present results of PL lifetime studies of the organic dye Rhodamine B, which is a good candidate for use in temperature probes due to its large PL emission. We have measured PL lifetimes using time correlated single photon counting (TCSPC). The lifetimes were measured from temperatures of 15 K to 330 K. The lifetimes appear to be non-monotonic: they increase with temperature to a point, then decrease again. It is uncertain what is causing this unexpected trend, and we are in the process of verifying these lifetime measurements as well as studying other possible luminescent materials such as semiconductor quantum dots for application as temperature probes.   

Single cell temperature measurement with a thin-film thermocouple array

Single cell temperature measurement is an important fundamental issue. A full picture of the temperature distribution and thermal response of a single cell under different conditions is very helpful for answering fundamental biology questions in cell thermogenesis and thermal regulation. However, a reliable method for precise cellular temperature measurement remains a technical challenge.
In this work, we developed high-performance Pd/Cr micron thin-film thermocouple (TFTC) arrays and double-stabilized system with a stability of ±5 mK for single cell temperature measurement. We have obtained frequent temperature increments of 10-60 mK and a maximum up to 200-300 mK for cultured HepG2 cells¹. Recently, we have reduced the stripe width of TFTC down to 170 nm，and fabricated 10×10 TFTC arrays on 100×100 µm² Si₃N₄ suspended platforms. We aim to realize the real-time 2D mapping of local temperature distribution for a single live cell with our nano-sized TFTC arrays and double-stabilized measurement system. The measurement results may shed some light on further investigation in determining the absolute temperature distribution of individual cultured live cells, both outside and inside of the cells, in a more precise way.
1. F. Yang, et al., Scientific Reports, 7, 1721 (1-11) (2017).   

Development of a New Dark Matter Detector that Uses Liquid He and Field Ionization

We discuss a new method of dark matter detection[1] that involves He atoms evaporated from a cold surface and their detection using field ionization. When a dark matter particle collides with a liquid He atom it produces phonons and rotons. When these excitations make it to the surface of the liquid He they evaporate a He atom, which can then be detected by ionization in a strong electric field. The >1 meV binding energy of a He atom to the surface opens the door for the detection of dark matter particles with a mass as low as 1 MeV/c². We will discuss the design of the proposed detector as well as the results of preliminary experiments on field ionization. We will also discuss the future of such experiments