Overview

Superconducting Electronics and Low-Temperature Physics Laboratory  

Prof. Martin E. Huber, Proprietor Since 1991  

Introduction/Background

The Superconducting Electronics and Low-Temperature Physics Laboratory is an undergraduate research facility conducting state-of-the-art research into Superconducting Quantum Interference Devices (SQUIDs) and related devices and applications. The laboratory is supported by external research grants from various agencies. Present funding is from the National Science Foundation (NSF), the Department of Energy (DoE) and the National Institute of Standards and Technology (NIST). In the past, the laboratory has also been supported by the Research Corporation, Stanford University, and the Office of Naval Research (ONR). The grants fund primarily salary (principal investigator and undergraduate research assistants), capital equipment, and operating expenses. The principal investigator (Prof. Martin E. Huber) and undergraduate research assistants (URAs) from the Physics and Electrical Engineering Departments at UCD and the Physics Department at MSCD perform all work (there are no graduate students, postdoctoral research associates, or other research staff). The URA's are therefore an essential part of the research program.  

Superconducting electronic devices only operate very close to absolute zero and operate under fundamentally different physical principles than semiconductor electronic devices. They offer unparalleled sensitivity to electric currents and magnetic fields. The fundamental "active element" in superconductor electronics (similar to the diode or transistor in semiconductor electronics) is the Josephson tunnel junction. The most common analog device in superconductor electronics is the Superconducting Quantum Interference Device (SQUID). In most situations, high-frequency internal oscillations and positive feedback paths degrade the operating characteristics of SQUIDs. In collaboration with NIST, Prof. Huber has developed a patented technique (•intracoil damping,• US Pat. #6169397B1) to reduce the effects of these phenomena and are using the improved SQUIDs to build higher-level analog integrated circuits.  

Our superconductor electronics devices are fabricated using techniques similar to familiar semiconductor devices. They are composed of many layers of metals and insulators on a silicon wafer substrate, and are of similar sizes of common integrated circuits. We design and model the devices in our UCD laboratory, and they are fabricated in collaboration with the Boulder Laboratories of the National Institute of Standards and Technology (NIST). We then perform device testing and analysis in our UCD laboratory.  

Device characterization includes measurements of low-frequency properties such as critical current, resistance, and gain, as well as high-frequency properties such as frequency response and noise levels. The laboratory is equipped with a UNIX workstation, three Macintosh desktop computers, a dual Xeon •Wintel• computer, two liquid helium Dewars, several cryostats, two fast-fourier transform (FFT) spectrum analyzers, computerized data acquisition facilities, a 500 MHz digital oscilloscope, a 1.3 GHz vector network analyzer, an optical inspection microscope, and an ultrasonic wire-bonder. The laboratory will soon have a 3He cryostat capable of cooling samples to 300 mK.  

Our primary work is in dc SQUID Series Array Amplifiers (SSAAs). The output signal from an individual SQUID is too small to be sufficiently amplified by a room-temperature pre-amplifier without significant loss in sensitivity. Associated electronics required to regain the sensitivity are expensive and result in reduced bandwidth (frequency range). Placing large numbers of SQUIDs in series more efficiently matches the SQUID output to the room-temperature electronics input without the need for expensive electronics and with little reduction in bandwidth. Presently, our devices are sensitive to electric currents at the level of 2 pA/rt Hz or greater, and over a bandwidth from dc to 120 MHz.  

Our SSAAs are used as superconducting pre-amplifiers for a number of experiments that utilize a new form of detector called Transition Edge Sensors (TESs). These detectors are sensitive to light from the infrared to X-rays and can also be used to detect certain fundamental particles and biological molecules. We are participating in a number of world-leading collaborations, including the NIST detector group and the Cryogenic Dark Matter Search (including Stanford University, UC Berkeley, FNAL, and other institutions).  

Hands-on laboratory experience gained by undergraduates in the Superconductor Electronics Laboratory better prepares them for careers in the high-tech industry and for graduate school. There are typically six or more URAs working in the laboratory any semester. Some are in paid positions, others are participating in independent study projects or cooperative education courses, and yet others are just volunteering in the laboratory for experience to add to their resume. In any situation, the students are considered valuable members of the research group and are given as much independence as they are prepared to take. Many of the techniques used to characterize superconductor electronics devices are also used in more mainstream electronics laboratories. Measurements tend to be more sensitive to background noise and measurement techniques, giving the student useful experience in delicate measurements.  

The process in SQUID research includes many steps, and all but one are conducted on the UC Denver campus. First, the electrical characteristics of the device are modeled using various numerical techniques. Once an appropriate design is successfully modeled, the physical layout of the device for the microfabrication process is created. The device is fabricated in the clean-room facilities at NIST•this is the only step not performed on campus, since such facilities can cost over $1,000,000 in initial costs and many hundreds of thousands of dollars annually in maintenance. After fabrication, the devices are brought to the UC Denver laboratory for testing and characterization. The laboratory is equipped with all necessary cryogenics and electronics. Certain specialized apparatus and electronics are fabricated in-house, in the Physics Department•s machine shop.  

Recent research has included successful development of Intra-Coil resistors (patented) to improve performance of SQUIDs and SQUID Series Array Amplifiers (SSAAs). SSAAs are the enabling technology for superconducting transition edge sensor devices for measuring X-ray spectra of faint sources. This microcalorimetric technology is used in microanalysis, astronomy, and particle physics. We are collaborating with institutions nationally and internationally on a variety of projects. Our principle collaboration is with NIST, where the group is working on developing X-ray and optical sensors for microanalysis and astronomy. Their present emphasis is on development of large-format arrays for astronomy. Our SSAAs are used as preamplifiers for in sensors for weakly interacting massive particles (WIMPs). The sensors are so sensitive that they can measure WIMP interactions through the vibrations caused by nuclear recoil. We are also collaborating with institutions in France and the Netherlands.  

Present Work

We are developing a new application of SQUID technology, the SQUID operational amplifier. One difficulty with existing SQUID devices is that the characteristics are non-linear. The performance of non-linear devices is overly sensitive to operating point, and makes operation difficult. In particular, the need for room-temperature feedback electronics to linearize the system performance also limits the bandwidth due to cable lengths. A superconducting circuit with on-chip feedback could have both linear output and high bandwidth, and we are working on developing such a device. Erik Lucero, a Physics/Electrical Engineering double major and NIST Professional Research Experience Program fellow, is working on this project with a large degree of independence.  

We are now developing a new generation of Scanning SQUID Microsusceptometers for use in studies of nanoscale magnetic and electronic systems. Nanoscale characterization of magnetic systems is of particular interest to the magnetic recording and data storage industry. As memory density increases, the size of each data bit decreases. Our devices will be on the cutting edge of nanotechnology characterization, with sensitivities surpassing existing devices. Part of our method for attaining the next level in device sensitivity will be to build devices with smaller features, using aluminum instead of niobium as the superconductor and using electron-beam lithography rather than optical lithography when fabricating the devices. We will be submitting proposals to develop a scanning stage for use in our new 300 mK 3He refrigerator. At these temperatures, new experiments will become possible and our research will shift to include study of nanoscale structures and phenomena. In collaboration with NIST, we should be able to develop unique integrated superconducting and micromachined nanoscale devices.  

Significance in Field            

Our SSAAs are perhaps the best in the world. Few institutions have facilities able to manufacture the devices. Our devices are in high demand among institutions across the country and around the world. They are used for a variety of applications, as discussed above. Improvement in X-ray detection technology is of interest to the space science industry, which is strongly represented in the Denver metropolitan area. Furthermore, development of the SQUID operational amplifier would greatly simplify SQUID operation, particularly in multi-channel applications. This work drew great interest at recent national and international conferences. Finally, our work on developing improved SQUID susceptometers for use in scanning microscopes will lead to greater ability to characterize nano-scale magnetic samples, of interest to the magnetic recording industry, also strongly represented locally.  

Future Work            

Through the collaboration with NIST, Prof. Huber has the ability to design custom circuits for unique experiments. As his work has evolved, it has grown from investigating exploratory circuits to using highly engineered devices in specific applications. He and his students are about to take the next step in this evolution, acquiring a 3He cryostat capable of reaching temperatures roughly ten times closer to absolute zero than presently available in his laboratory. This cryostat will reach temperatures of 300 mK, at which aluminum superconducts and quantum phenomena begin to dominate thermal phenomena. Prof. Huber will apply for grants to equip this cryostat with a scanning stage, to allow scanning SQUID susceptometer measurements in Denver. Eventually, Prof. Huber will leverage this work to obtain an adiabatic demagnetization refrigerator (ADR) that will reach temperatures of 50 mK and allow even more sensitive measurements of nanoscale systems. Prof. Huber is also conducting investigative research with collaborators at NIST into the potential of magnetic microcalorimeters as replacements for the resistively-based transition edge sensor microcalorimeters now used in photon detection.

Involvement of Undergraduates

The Department of Physics at the University of Colorado at Denver does not have graduate programs, so all research is conducted entirely by Prof. Huber and undergraduate interns. Undergraduates participate through paid internships, volunteer internships, or for credit through independent-study courses. A number of Electrical Engineering students have done their senior project in this laboratory. Student interns begin with simple chip-handling skills and then move to basic screening techniques (oscilloscope operation and 4-terminal measurements). Thus, they can be involved in a meaningful way as soon as they have finished Physics II. Students move to more advanced projects as their skills and coursework develops. Students are both Physics Majors and Electrical Engineers, with students from related fields (mechanical engineering, computer science, and mathematics) considered and hired when appropriate. Over 40 students have passed through the laboratory since its inception in 1991, with an average stay of two years. Two to three new students are brought into the laboratory each semester. The longer the students are involved in the lab, of course, the greater their range and depth of experience. Students can also work full-time over the summer and interim periods.  

Students obtain experience in computer modeling, machining techniques, circuit design, and circuit evaluation. Many techniques used in superconducting device fabrication were developed in the semiconductor industry first, and there is a great deal of overlap in skills used in both fields. Therefore, the experience these students receive helps better prepare them for jobs in the high-technology sector (Exabyte, Storage Tek, Quantum, etc.). For those headed for graduate school, the experience helps prepare them for independent experimental work so that their training time in graduate school is reduced and they are ready to begin their thesis work earlier than other students. Students are also involved in writing proposals, whether at the internal level or for external grants.  

Students have helped prepare (and some have even presented) technical presentations at professional conferences. Students are encouraged to attend these conferences as finances permit. The present funding provides travel and lodging for selected interns to travel to Stanford University and work with graduate students in a collaborator•s laboratory.  

Approximately six students are active in the laboratory each semester. Students come from all backgrounds, and include female students and other students from under-represented groups in science and engineering. A high-school student preparing for local science fairs and the Westinghouse Talent search has also used the laboratory and its facilities. His experience culminated in a presentation and publication at the same national conference attended by the principal investigator and UCD students.  

Recent interns have gone on to jobs at Sun Microsystems and to graduate school (CalTech, U. of Rochester).

Here are some statements from present interns on their experiences:    

Erik A. Lucero

Applied Physics & Electrical Engineering University of Colorado at Denver

National Institute of Standards and Technology Undergraduate Fellow  

I have been working with Dr. Huber for a little over two years. I would place all of my experiences in his lab as the highest value of: knowledge, academically challenging, and quite frankly the most enjoyable times in all of my undergraduate studies. The Superconducting Quantum Interference Devices (SQUID) Laboratory has been an integral part of my studies in both Electrical Engineering and Applied Physics. I have learned firsthand, the rigor and dedication one must apply in order to succeed in the field of academic research. I have had the opportunity to collaborate with Dr. Huber and his colleagues at the National Institute of Standards and Technology (NIST) on a pioneering field of SQUID electronics, the SQUID Operational Amplifier (Op-Amp). During our research on this project I created the software for controlling the device, and conducting gain, noise, and linearity measurements. This project alone, introduced me to numerous scientific and engineering methods, hardware, measurements, design techniques, circuit fabrication, and fundamental theories that are just not available in my undergraduate courses.  

Tommy Azua

Electrical Engineering University of Colorado at Denver  

Working in the Superconducting Electronics lab has been an excellent experience for me. I have realized that being involved in research is a challenging and rewarding opportunity. This internship has allowed me to get a lot of hands on experience with electronic equipment such as oscilloscopes, signal generators, and spectrum & network analyzers. I have also gained skill and knowledge in using various electromagnetic simulation software packages. Although my major is in Electrical Engineering and not Physics, I have been given the opportunity to learn the basics of Superconducting theory through lecture sessions and I have even attended an Applied Superconductivity Conference where physicists from around the world came and shared with the rest of the Superconducting world their current and past projects. Overall, this experience has given me the chance to work in a laboratory environment, allowing me to learn skills that are not taught in the classroom such as problem solving and teamwork. Furthermore this internship has left me hungry for knowledge, giving me the desire to pursue my Master•s degree and continue being apart of research.    

Sean T. Halloran

Applied Physics & Electrical Engineering University of Colorado at Denver  

This lab is a chance for me to be involved in real-world research at a level not usually found in undergraduate labs. One huge benefit is that I am continually exposed to new technologies and methodologies before they are introduced in the classroom. When new concepts come up in lectures, I can relate them to my experience in the lab and I have found this to be an enormous aid in understanding difficult concepts.  

This is a short biography:    

Martin E. Huber received his Ph.D. (1988) and M.S. (1986) in physics from Stanford University and his B.S. (1982) in physics from the Massachusetts Institute of Technology. His doctoral thesis was titled "A 1.5 m2 Superconducting Detector for Cosmic Ray Magnetic Monopoles". After a postdoctoral fellowship at the National Institute of Standards and Technology in Boulder, CO, he joined the faculty at the University of Colorado at Denver in the Department of Physics as an Assistant Professor in 1991 and was promoted to Associate Professor in 1998. He is currently a Professor of Physics at UCD with a joint appointment to the Department of Electrical Engineering.

Dr. Huber continues to collaborate closely with colleagues at NIST in applications involving SQUIDs as sensors and amplifiers. He has been the recipient of a NSF Graduate Fellowship and a NRC Postdoctoral Research Associate Grant. He is a member of Phi Beta Kappa and Sigma Xi.   Dr. Huber has extensive experience with SQUID design, fabrication, and characterization. He is a co-inventor of a technology for damping SQUID resonances to improve SQUID operation, and has successfully applied this technology to SQUID series array amplifiers. His current research program is to develop new SQUID-based devices including a SQUID operational amplifier and SQUID susceptometer for use in scanning stages at cryogenic temperatures.