Session T45: Instrumentation for Spectroscopic and Scattering Techniques

Accessible electron spin resonance instrumentation within cryostat environments; a step towards sub-Kelvin ESR

ESR has been increasingly used within the field of quantum information for coherent manipulation and readout of qubit states. These advances have brought about the growing use of unconventional and homebuilt ESR instruments, allowing for a high degree of control and customization over experimental parameters. In addition, ESR has recently been used as a probe of exotic excitations in quantum magnets, mainly focusing on studying the existence and dynamics of spinons in quantum spin chain materials. Investigating these types of systems requires sub-Kelvin temperatures, only reachable inside dilution refrigerators. Apart from a few innovative laboratories, most low-temperature ESR is limited to above few Kelvin.

As a step towards full implementation at millikelvin temperatures, we utilize this modular approach to design and assemble a simple CW spectrometer to characterize the g-factors of materials of interest. Our probe is easily implemented into an existing measurement platform (Quantum Design PPMS), facilitating reproducibility in groups with similar equipment.

    

Heat transport across thermal contraction activated linear polymer clamps

As quantum processors and cryogenic apparatus scale up in complexity, wiring experiments in-situ has become untenable. Top and side-loading wiring inserts allow for faster wiring than installing individual wires but are still time consuming. Moreover, the confined space of cryogenic systems often results in improperly torqued bolts, leading to poor thermalization, damaged components, and inconsistent performance of sensitive equipment. A novel polymer clamp is developed to provide a higher density, firmer, and more consistent clamping force for thermalization while reducing footprint. The thermal transport at 50K and 4K is characterized and compared for clamped and bolted interfaces. Thermal cycling, repeatability, and durability of clamped connections after >100 thermal cycles were also investigated.   

Low-energy single-electron detection using a large-area superconducting micro-strip

Superconducting strip single-photon detectors (SSPDs) are excellent tools not only for single-photon detection but also for single-particle detection due to their high detection efficiency, low dark counts, and low time jitter. Although the detection of a variety of particles, including electrons with keV-scale energy, has been reported so far, there have been no studies for the detection of low-energy electrons. Also, it has not been clarified how low-energy electrons interact with electrons and/or phonons in a superconductor during the detection of electrons. Here we report the detection property of a superconducting micro-strip single-electron detector (SSED) for electrons with energies below 152 eV. We show that the minimum detectable energy of electrons is about 15 eV with our SSED, which is much lower than those of ions, implying that electron-electron interaction plays a significant role. We also estimate the detection efficiency as at least 45 % when electrons impinging on the stripline possess the energy of 152 eV. SSEDs might open a wide range of applications ranging from condensed matter physics to quantum information science because of their compatibility with the cryogenic environment.   

Optical Thermometry Using Absorptions in Thulium-doped YAG

A general approach for modeling optical absorptions in rare-earth-doped crystals is applied to transitions in Tm³⁺:YAG and demonstrates their use as an optical thermometer at cryogenic temperatures. The modeling of the optical absorption, using two temperature-dependent processes – Boltzmann population shifts and homogeneous line broadening – is sufficient to generate predictions for absorption spectra that are in close agreement with observations. The model allows the temperature dependence of Tm³⁺ absorptions to be calibrated and used as an optical thermometer. Absorbed light is analyzed for three spectral ranges: 650-725 nm which includes absorption into the ³F₃ manifold, 725-850 nm which includes absorption into the ³H₄ manifold, and 1100-1300 nm which includes absorption into the ³H₅ manifold at temperatures between 10 K and 300 K. The modeling of the temperature dependence of multiple absorption bands allows for a single crystal to be used as a multi-range temperature sensor. 

    

Interpreting Enigmatic Work Function Spectra in UPS

Work function measurements using Ultraviolet Photoemission Spectroscopy (UPS) can produce complex and difficult to interpret spectra. In contrast, the theory is simple and dates to Einstein’s quantum theory of light.  By grounding a conducting sample, photoemission from the Fermi level has the kinetic energy of the light source, and the work function corresponds to the minimum observed electron energy.  Experimentally, detectors cannot measure very low energy electrons, so the sample is biased negatively to increase the onset energy to an efficient range. Unfortunately, the minimum kinetic energy can be difficult to identify in practice when the spectral onset is not simple, particularly on materials with a small density of states near the E_f.  This talk provides examples of work function measurements from photoemission onsets for clean and dirty elemental metals, oxides, and carbon nanostructures. While spectra for clean elemental metals are “textbook”, complex surfaces can exhibit intensity at energies below the true onset.  The origins and practical approaches to identify and mitigate deceptive features are discussed. 

    

Complex filter configuration for lock-in amplifiers.

Lock-in amplifiers are the workhorse of experimental physics and material research for detection of low level AC signals in the presence of high levels of noise and interfering signals. Many measurements require the mixture of DC and AC signals, for instance differential conductance and AC susceptibility. Traditional lock-in amplifiers employ a simple IIR filter which can have long measurement times to produce acceptable noise reduction. Using measurements on model systems, we will demonstrate that a combination of digital filters of a variety of types, high pass, low pass, infinite impulse response and finite impulse response can produce improved noise reduction and measurement time over the traditional IIR filters. We will present step response, equivalent noise bandwidth and autocorrelation times for a n pole IIR filter (n <=4) followed by a filter response of any finite length. Finally, we will present criteria for statistically independent samples for the various filter configurations.

    

Modified Angstrom's Method for the Measurement of Thermal Diffusivity on the Microscale within Carbon-Carbon Composite Materials

An infrared camera was used to monitor temperature evolution across the surface of carbon-carbon composite materials.  One side of the sample was periodically heated using a pulsed laser.  To avoid emissivity variations, infrared images were taken at 5 equilibrium temperatures between room temperature and 110 ^oC. A temperature calibration polynomial was determined for each pixel using those images.  A custom program was written to determine the diffiusivity values between any two, user-selected points. The thermal diffiusivity calculation requires knowing the amplitude and phase of the "temperature waves."  Due to limited data, a sine-wave fit was made to the data at each pixel to determine amplitude and phase.  The results for two composite samples will be shown that are consistent with bulk sample flash thermal diffusivity measurements made on uni-directional carbon-carbon composites.  Video of the temperature with position and time will also be shown for carbon composites with 0^o and 90⁰ unidirectional fiber orientation in alternated layers with two different sample orientations..    

Application of Optoadmittance and Cross-Correlation Noise Spectroscopy to White OLEDs

The recently developed method of optoadmittance spectroscopy applies an alternating voltage across a light emitting device and sweeps its frequency across a specified bandwidth. The current through the device and the light emitted are then concurrently measured and the phase delay and amplitude response are determined. Here, the method is applied to an organic LED (OLED) which emits white light. The OLED is composed of three layers responsible for emission of light composed of three distinct wavelengths. Optoadmittance spectroscopy is performed while using optical bandpass filters to examine only one of these wavelengths at a time. This method is then combined with cross-correlation noise spectroscopy, where the noise present in the optical signal is cross-correlated with the noise present in the electrical current. This is also done while using optical filters to select a specific wavelength range. By comparing the frequency response determined by optoadmittance spectroscopy with the power spectral density of the cross-correlated noise, we characterize relaxation processes related to the three different color components.   

Nanokelvin-Resolution Micro-Thermometry at Room Temperature

Ultrahigh-resolution thermometry is critical for sensitive bolometry for infrared (IR) and terahertz (THz) detection and imaging as well as sensitive calorimetry for probing dissipation in electronic, optoelectronic and quantum devices. In spite of recent advances in the field, achieving high-resolution measurements from microscale devices at room temperature remains an outstanding challenge. Here, we present a photonic thermometer with optical readout – called the Band-Edge Thermometer (BET) – that achieves this goal by relying on the strong, temperature-dependent optical properties of GaAs at its band edge. Specifically, using a suspended asymmetric Fabry–Pérot resonator and a narrow-linewidth probe laser we demonstrate a thermoreflectance coefficient of >30 K⁻¹, enabling thermometry with a noise floor of ~60 nK Hz^−1/2 and a resolution of <100 nK in a bandwidth of 0.1 Hz. The advances presented here are expected to enable a broad range of studies and applications in calorimetry and bolometry where miniaturized high-resolution thermometers are required.   

Measuring Vibrational Movements of Levitated Pyrolytic Graphite with a 10 GHz Microwave Cavity

Highly isolated mechanical oscillator systems with high quality-factors have profound applications in both sensing applications and quantum physics. The use of a material's diamagnetic response to achieve levitation is often overlooked as a means of creating this sort of isolated system. We present results for magnetically levitated pyrolytic graphite slabs within a microwave cavity. The motion of the pyrolytic graphite slab perturbs the resonant microwave mode which can be monitored to quantify the vibrational activity of the slab. This serves as a low-impact method of sensing the particle's movement and additionally allows for applications in cavity electro-mechanics. Acoustic measurements of a levitating 1mg slab of pyrolytic graphite in the 10 – 50 Hz frequency range utilizing a λ/4 coaxial microwave cavity with 10 GHz resonance are presented. These cavity measurements are compared to object tracking in slow motion video and results by other researchers on the rigid body dynamics of this levitated pyrolytic graphite system. [X. Chen, A. Keskekler, F. Alijani, and P. G. Steeneken, Appl. Phys. Lett. 116, 243505 (2020)]   

Nanoscale Imaging of Super-High-Frequency Microelectromechanical Resonators with Femtometer Sensitivity

The implementation of microelectromechanical system (MEMS) resonators calls for detailed microscopic understanding of the devices, such as energy dissipation channels, spurious modes, and imperfections from microfabrication. Unfortunately, such features are mostly overlooked by conventional transducer-based electrical readouts, which becomes the major bottleneck for designing efficient acoustic or cross-domain microsystems. Using transmission-mode microwave impedance microscopy (TMIM), we report the nanoscale imaging of a freestanding super-high-frequency (3 – 30 GHz) lateral overtone bulk acoustic resonator (LOBAR) with a spatial resolution of ~ 100 nm and an equivalent in-plane displacement sensitivity of ~ 10 fm/ÖHz. We have visualized acoustic mode profiles of individual overtones and quantitatively analyzed higher-order transverse spurious modes and anchor loss. Our work provides insightful guidance for designing and optimizing future MEMS resonator and contribute to broad areas from electromechanical device applications to advanced quantum information research.   

Detecting internal symmetry changes using Resonant Ultrasound Spectroscopy

Elastic constants are of great significance because they are fundamental thermodynamic susceptibilities that connect directly to thermodynamics and electronic structure, as well as to mechanical properties. Resonant Ultrasound Spectroscopy (RUS) measures non-destructively all fundamental elastic properties, by determining the natural frequencies at which a 3D object resonates when is mechanically excited (normal modes). By using an inversion scheme, it is possible to extract the entire elastic tensor of a material with extreme sensitivity in a single frequency sweep using only one, small sample. This makes RUS particularly advantageous to study physical properties of low-symmetry single crystals and textured bulk materials.

I will present recent RUS measurement examples of its applications to thermodynamic and materials science problems. I will focus on the effect of texture in elastic constants as a result of mechanical or growth processes. In materials such as Aluminum that is often considered isotropic but shows clear anisotropy when it is extruded.  Similar but much larger effect is found in additive manufactured stainless steel where the growth process leads to a large anisotropy with a softening of 40-50% along the build direction   

Measurement of Phonon Angular Momentum via the Einstein-de Haas Effect, Fiber-Optic Interferometry, and a High-Q Oscillator

We report the design and use of a fiber-optic-interferometer system to measure the predicted¹ macroscopic phonon angular momentum. An oscillating magnetic field is applied to an insulating ferromagnet attached to our single-crystal high-Q double torsional oscillator. By the Einstein-de Haas effect, oscillator displacement measurements between low temperatures and those closer to the Debye temperature allow extraction of the changing phonon angular momentum. A force change of 5 x 10^-8 N was detected between 77 K and 300 K for a 1 mm³ MgZn ferrite sample. Our oscillator, with a resonance at 1.3 kHz, has a thermal noise limit on the order of 10^-14 N/√Hz, allowing the possibility of high-accuracy detection. Competing effects are being minimized; for example, induced eddy current momentum can overwhelm the phonon effect for metallic ferromagnets, and careful temperature-dependent studies are required for force calibrations.   

Monte-Carlo Ray-Tracing Studies on Multiplexing Prismatic Analyzers for Implementation on MANTA at the High Flux Isotope Reactor

Condensed matter physics has been revolutionized by the 1956 invention of the Nobel prize-winning triple-axis spectrometer. The latest development in triple-axis spectroscopy is the use of so-called multiplexing analyzer systems, seen at several neutron scattering laboratories and most recently at the Swiss Neutron Source’s instrument CAMEA [2]. By using multiplexing analyzers, CAMEA is able to greatly improve upon the detection efficiency of a traditional triple-axis by using the combination of multiple analyzers and position-sensitive detectors to provide energy-resolved measurements. Oak Ridge National Laboratory aims to utilize this technology, along with further developments, for a next generation triple-axis spectrometer known as the Multi-Analyzer Neutron Triple Axis (MANTA). An optimal way to design a neutron scattering instrument is to use Monto-Carlo simulation programs, such as McStas. This talk will focus on in-silico optimization of the prismatic analyzer concept, to simultaneously optimize MANTA’s future hardware design and data analysis technique.