As a little girl I would play school with the neighborhood children. Ever since fourth grade I knew I wanted to be a teacher in a classroom full of eager-to-learn nine-year olds, but it wasn’t until my freshman year of college that my plans changed. In Geology for Elementary Teachers, I remember thinking, “This material is great! I need to learn more!” My hunger for a deeper understanding of how the physical world works led me to reflect on what my favorite science in high school was: Physics. Not long after, I changed my major to Physics and I was on the path to becoming a high school Physics teacher. Fast forward a decade, and I have my dream job. I get to explore the exciting world of Physics all day with 150+ adolescents and I wouldn’t change that for the world.
Today, I am feeling much like the curious, eager nine year-old I once was, asking countless questions and soaking up as many new ideas as possible. I have been able to quench some of my thirst for answers about how the world works, in the areas of lasers and superconductivity anyhow. Over the last several weeks I have had the opportunity to work as a part of the Yeh Group at Caltech that is conducting research on high-temperature superconductors. I work in a lab under the guidance of visiting Physics Professor Zhenjie Feng to deepen my understanding of Physics topics, to learn about cutting-edge research, and to gain experience doing research in a university laboratory. The following are a few of the interesting topics that I have learned more about.
A superconductor is a type of material in which current flow has zero resistance. This is astounding because current flowing along a superconducting wire can theoretically continue flowing forever. As substances are cooled, their resistance decreases due to damping of thermal excitations (think jogging through a crowd of people in downtown New York versus speeding through an empty highway). Before superconductors were discovered around the turn of the 20th century, scientists had thought that zero resistance would only occur once absolute zero was reached. However, when certain metal and ceramic substances reach a critical temperature above absolute zero (-460oF, or 0 Kelvin), the resistance drops suddenly to zero! This is an amazingly interesting quantum behavior which has allowed for technology such as extremely sensitive submarine detection, hyper-efficient power transmission, magnetic levitation, Magnetic Resonance Imaging (MRI), and particle colliders, such as the Large Hadron Collider that discovered the Higgs boson, to name a few!
High-temperature superconductors (HTSC) were first discovered in 1987, just weeks prior to the annual meeting of The American Physical Society. At this convention, now called the “Woodstock of Physics”, physicists packed into the New York City Hilton to learn about the groundbreaking research. HTSC were a huge breakthrough because they significantly lowered the cost of reaching superconductivity. Instead of requiring expensive liquid Helium to lower temperatures to a few degrees above 0 Kelvin, high-temperature superconductors use inexpensive liquid Nitrogen to lower temperatures to about 77 Kelvin, in our experiment’s case.
Since then, Physicists have worked ingeniously to create new ways of generating HTSC as well as coming up with novel practical applications. I have recently learned that there are thin films of HTSC on 1cm 2 substrates, long nano-ribbons of HTSC films, long HTSC wires and more. There are high hopes for extremely efficient power transmission using the nano-ribbons and wires.
My Lab’s Superconductor
The high-temperature superconductor my lab is working with is Calcium-doped YBCO (Y1-xCaxBa2Cu3O7- δ ) . Just as steel is alloyed to achieve superior strength and resistance to corrosion, scientists can dope a material with a combination of elements in order to achieve the desired electrical properties. As a participant, I have had the chance to see firsthand how these superconducting thin films are formed on a substrate by a process called Pulsed Laser Deposition (PLD). A (very powerful) 100mW green KrF laser is directed toward a target, also known as a source material. The laser rapidly heats the target, in this case the Calcium-doped YBCO, into a plasma. The beautiful hot pink plasma plume of YBCO expands perpendicularly to the target surface, reaches a substrate surface, and dries. It is mind-blowing that the thin film formed on the substrate is only about 200 nanometers (2×10-7m) thick with each layer of YBCO crystals being about 1.1 nanometers thick! A myriad of parameters must be carefully set in order for the YBCO thin film to properly reach and form a thin film on the substrate. For example, the substrate must undergo a slow cool-down process from 970O C (1780O F) to room temperature at a rate of 1O C per minute in order to ensure that a sufficient amount of oxygen in the chamber will get into the sample, to improve the doping level. Once the substrate film is sufficiently cooled, we can examine the unique physical and electrical properties that make it a superconductor.
Testing Our Superconductor
One method I have been fortunate to explore for examining the structural properties of the YBCO thin film is to have the film tested by X-ray diffraction (XRD). Precise positioning of all parameters is vital so that the laser hits the center of the only 200 nm thick film atop the substrate. Not only are the three dimensions of position realigned, the three angles of motion for the substrate with the thin film are each adjusted, as well as the position of the detector. Through this summer’s research I am gaining an extraordinary amount of respect for scientists who demonstrate significant patience and discipline as they ensure every parameter for their experiment is correctly set up.
My final work this summer will be examining the electrical properties of the superconductor. There are two types of tests to determine the thin film’s superconductive properties. The first is AC Susceptibility in which alternating current flows through a circular wire surrounding the substrate and film. This generates an alternating magnetic field and hence causes a flux to penetrate through the substrate and film. How the superconducting film behaves in this alternating flux is then measured.
Following AC Susceptibility, I will be learning about a second way to explore the electrical properties of the film, this time using direct current. We will be visiting the Superconducting-Quantum-Interference-Device, also known as SQUID, used for measuring the smallest of magnetic fields.
This learning experience has not just been about superconductivity and lasers. In the process, I have also had hands-on experience learning how to use ultrasonic cleaners for preparing and cleaning substrates, furnace heating units for oxidizing the substrates, lock-in amplifiers for AC Susceptibility testing, and much more. The direct application of the sciences in the laboratory is an exhilarating process and I am grateful for this opportunity. As I have learned with my own students, reading about a concept in a book cannot compare to participating in the real-life applications. I cannot wait for the Fall to share my lab experiences with my own students, to incorporate what I have learned into the classroom, and to share my ever-growing awe and love for the study of Physics!
Editor’s note: Eryn Walsh has been teaching AP, Honors, and General Physics at La Canãda High School for the past three years. She has a B.Sc. in Physics from UCLA and a M.Ed in Education from Azusa Pacific University. Like Mr. Fackrell (“How I learned to stop worrying and love graphene” in Real Science), she is part of IQIM’s Summer Research Institute, a six-week program designed to expose local physics teachers and high-school students to cutting-edge research taking place at Caltech’s research labs. She is currently working with the Yeh Group and is a member of IQIM’s Curriculum Development team, a group of teachers, students and Caltech postdocs, who are working to bring Quantum Mechanics to High Schools across the state through interactive simulations and hands-on experiments with lasers, polarizers and slit-films.