Session P4: Keithley Award Session

Nanocalorimetry: Using Si-micromachined Devices for Thermodynamic Measurements of Thin Films and Tiny Crystals

We have used Si micromachining to fabricate membrane-based calorimeters for measuring thermodynamic properties of microgram-quantity samples over a temperature range from 1.7 to 550K in magnetic fields to 8T. Prototype scaled down devices have been made which allow precise measurements of nanogram quantities. Different types of thermometers are used for different purposes and in different temperature ranges. Current development efforts are extending the temperature range to 0.3 - 800K, and we are collaborating with the national high magnetic field lab to extend the field range to 65T in pulsed magnets. These devices are particularly useful for specific heat measurements of thin film samples (100-400 nm thick) deposited directly onto the membrane through a Si micromachined evaporation mask. They have also been used for small bulk samples attached by conducting paint or In, and for powder samples dissolved in a solvent and dropped onto devices. The measurement technique used (relaxation method) is particularly suited to high fields because thermal conductance is measured in zero field and is field independent, while the relaxation time constant does not depend on thermometer calibration. The devices have been used with little modification for thermal conductivity and thermopower measurements, and are well suited to measurements of calorimetric signals such as those occurring at phase transitions or under irreversible thermal behavior. I will discuss device fabrication and thermal analysis which allow us to precisely identify heat flow in the devices and consequent limits on the absolute accuracy, as well as possible future directions for device development. I will also briefly discuss examples of measurements on several materials of current interest: 1) amorphous Si and its alloys, 2) high precision critical temperature studies of La$_{1-x}$Sr$_{x}$MnO$_{3}$ and La$_{1-x}$Ca$_{x}$MnO$_{3}$, 3) antiferromagnetic CoO nanoparticles and thin layers, 4) Fe/Cr giant magnetoresistance multilayers.   

High-Resolution Microcalorimeter Detectors for X-ray Spectroscopy

For many decades, standard wavelength- and energy-dispersive x-ray detectors have dominated experimental physics. Recently, microcalorimeter detectors of various types that count individual photons have started to make an appearance on the experimental scene. I shall describe the development of Transition Edge Sensor (TES) detectors at NIST, with particular emphasis on their use in high-resolution x-ray spectrometry. These detectors combine the broad energy range of a SiLi or Ge detector with energy resolution approaching that of diffraction-related methods. The technologies for producing ultra-cold temperatures and for fabricating superconducting electronics have made these detectors practical to use on a daily basis. By means of careful matching of absorber and energy, detectors can be built to cover energy ranges from 1 keV to 100 keV. By extrapolating from single-pixel detectors to arrays, the possibility of large detection areas, high count rates, and even imaging is starting to look realistic. While embodying some challenging technical constraints, microcalorimeter x-ray detectors will provide attractive advantages and opportunities for physicists in a number of fields.   

Angle-Resolved High Field Low Temperature Calorimetric Measurements of Low Dimensional Materials

Quasi-two-dimensional materials exhibit a rich variety of magnetic-field-induced superconducting and magnetic states. These states are highly anisotropic with respect to magnetic field orientation; in some cases, the very existence of the state is field angle dependent. To establish the phase boundaries of these high-field, low temperature, angle-dependent states, we have fabricated miniature rotatable calorimeters for measurements of specific heat and the magnetocaloric effect at temperatures ranging from 0.1K to 20K in magnetic fields up to 20, 35 or 45 tesla. The sample orientation relative to the applied field can be continuously varied at low temperature along a single axis (with a resolution of 0.02 degrees) and at room temperature along a second axis (with a resolution of 2 degrees). The sample temperature can be programatically set and regulated to better than 0.1 percent over the entire field and temperature range, allowing field sweeps at constant temperature in addition to temperature sweeps at fixed fields. In this talk, I will discuss the design, performance, and evolution of our calorimeter and recently obtained results, including the calorimetric observation of an angle-dependent magnetically enhanced FFLO superconducting state in a heavy fermion superconductor and an angle-dependent quantum fluctuation induced ``plateau state'' at 1/3 of the saturation magnetization in a quasi 2D S = 1/2 Heisenberg antiferromagnet.   

Some non-traditional approaches to thermal and thermodynamic measurements

Three non-traditional measurement methods for measurement of thermal and thermodynamic quantities are explored. Each method is not commonly used, has some astonishing advantages, and produces outstanding accuracy with little chance for error for several reasons. One reason in common is that for each method, only one quantity is measured. The methods include noise spectroscopy for the measurement of the elastic tensor of solids (and maybe other states of matter), third-harmonic measurements of thermal conductivity and specific heat, and impulse methods for obtaining the difficult-to-acquire ``ZT'' for thermoelectrics.