Superconducting nanowire memory cell, miniaturized technology

Siv Schwink
6/13/2017 10:00 AM

Professor Alexey Bezryadin works in the lab with graduate student Andrew Murphy, at the Loomis Laboratory of Physics in Urbana. Photo by Siv Schwink, Physics Illinois
Professor Alexey Bezryadin works in the lab with graduate student Andrew Murphy, at the Loomis Laboratory of Physics in Urbana. Photo by Siv Schwink, Physics Illinois
Developing a superconducting computer that would perform computations at high speed without heat dissipation has been the goal of several research and development initiatives since the 1950s. Such a computer would require a fraction of the energy current supercomputers consume, and would be many times faster and more powerful. Despite promising advances in this direction over the last 65 years, substantial obstacles remain, including in developing miniaturized low-dissipation memory.

Researchers at the University of Illinois at Urbana-Champaign have developed a new nanoscale memory cell that holds tremendous promise for successful integration with superconducting processors. The new technology, created by Professor of Physics Alexey Bezryadin and graduate student Andrew Murphy, in collaboration with Dmitri Averin, a professor of theoretical physics at State University of New York at Stony Brook, provides stable memory at a smaller size than other proposed memory devices.

The device comprises two superconducting nanowires, attached to two unevenly spaced electrodes that were “written” using electron-beam lithography. The nanowires and electrodes form an asymmetric, closed superconducting loop, called a nanowire ‘SQUID’ (superconducting quantum interference device). The direction of current flowing through the loop, either clockwise or counterclockwise, equates to the “0” or “1” of binary code.

The memory state is written by applying an oscillating current of a particular magnitude, at a specific magnetic field. To read the memory state the scientists ramp up the current and detect the current value at which superconductivity gets destroyed. It turns out that such destruction or critical current is different for the two memory states, “0” or “1”. The scientists tested memory stability, delaying reading of the state, and found no instances of memory loss. The team performed these experiments on two nanowire SQUIDS, made of the superconductor Mo75Ge25, using a method called molecular templating. The results are published in the June 13, 2017 New Journal of Physics (v.19, p.063015).

Superconducting nanoscale memory cell. Binary information is encoded in the direction of the electrical current in the loop. Clockwise indicates '0', counter clockwise, '1'. The superconducting electrons flow indefinitely, so memory is nonvolatile. (a) Photo of device: A superconducting strip of Mo75Ge25 (yellow) with a pair of superconducting nanowires forming a closed loop (also yellow). (b) The critical current (maximum current that can be injected without destroying superconductivity) plotted as a function of magnetic field: To set the memory state '0', a positive current is applied, targeting the shaded diamond. To set the memory to '1', a negative current is applied. To read out the state, the current is ramped, as shown by the red rhombus, and the current value at which voltage occurs is measured. This measured value, the critical current, depends on the pre-set memory; its statistical distribution is shown in (c). Images by Bezryadin and Murphey, U. of I. at Urbana-Champaign
Superconducting nanoscale memory cell. Binary information is encoded in the direction of the electrical current in the loop. Clockwise indicates '0', counter clockwise, '1'. The superconducting electrons flow indefinitely, so memory is nonvolatile. (a) Photo of device: A superconducting strip of Mo75Ge25 (yellow) with a pair of superconducting nanowires forming a closed loop (also yellow). (b) The critical current (maximum current that can be injected without destroying superconductivity) plotted as a function of magnetic field: To set the memory state '0', a positive current is applied, targeting the shaded diamond. To set the memory to '1', a negative current is applied. To read out the state, the current is ramped, as shown by the red rhombus, and the current value at which voltage occurs is measured. This measured value, the critical current, depends on the pre-set memory; its statistical distribution is shown in (c). Images by Bezryadin and Murphey, U. of I. at Urbana-Champaign
Bezryadin comments, “This is very exciting. Such superconducting memory cells can be scaled down in size to the range of few tens of nanometers, and are not subject to the same performance issues as other proposed solutions.”

Murphy adds, “Other efforts to create a scaled-down superconducting memory cell weren’t able to reach the scale we have. A superconducting memory device needs to be cheaper to manufacture than standard memory now, and it needs to be dense, small, and fast.”

Up to now, the most promising supercomputing memory devices, called ‘single-flux quanta’ devices, rely on manipulating circuits composed of Josephson junctions and inductive elements. These are in the micrometer range, and miniaturization of these devices is limited by the size of the Josephson junctions and their geometric inductances. Some of these also require ferromagnetic barriers to encode information, where Bezryadin and Murphy’s device does not require any ferromagnetic components and eliminates magnetic-field cross-talk.

 “Because the kinetic inductance increases with decreasing cross-sectional dimensions of the wire, nanowire SQUID memory elements could be reduced further, into the range of tens of nanometers,” Bezryadin continues.

The researchers argue that this device can operate with a very low dissipation of energy, if the energies of two binary states are equal or near equal. The theoretical model for such operations was developed in collaboration with Averin. The switching between the states of equal energy will be achieved either by quantum tunneling or by adiabatic processes composed of multiple jumps between the states.

In future work, Bezryadin plans to address the measurements of the switching time and to study larger arrays of the nanowire squids functioning as arrays of memory elements. They will also test superconductors with higher critical temperatures, with the goal of a memory circuit that would operate at 4 Kelvin. Rapid operations will be achieved by utilizing microwave pulses.

This research is supported by the National Science Foundation, Division of Electrical, Communications and Cyber Systems.

Recent News

  • Research

Developing a superconducting computer that would perform computations at high speed without heat dissipation has been the goal of several research and development initiatives since the 1950s. Such a computer would require a fraction of the energy current supercomputers consume, and would be many times faster and more powerful. Despite promising advances in this direction over the last 65 years, substantial obstacles remain, including in developing miniaturized low-dissipation memory.

Researchers at the University of Illinois at Urbana-Champaign have developed a new nanoscale memory cell that holds tremendous promise for successful integration with superconducting processors. The new technology, created by Professor of Physics Alexey Bezryadin and graduate student Andrew Murphy, in collaboration with Dmitri Averin, a professor of theoretical physics at State University of New York at Stony Brook, provides stable memory at a smaller size than other proposed memory devices.

  • In the Media

As NASA prepares for this evening’s launch of the NICER space astronomy mission, Emeritus Professor of Physics Fred Lamb of the University of Illinois at Urbana-Champaign, is at the Kennedy Space Center, as a member of three of the mission’s Science Working Groups. The launch from the world-famous Pad 39A is scheduled for 5:55 P.M. EST.

Lamb, who continues to hold a post-retirement research appointment at Physics Illinois, is a world-recognized expert on the U.S. ground-based missile defense system. He served as co-chair of the American Physical Society’s Study Group on Boost-Phase Intercept for National Missile Defense, which published its report in July 2003. He has been fielding questions from the media on Tuesday's successful interception of an interncontinental ballistic missile during the latest test of its ground-based intercept system, as reported by the U.S. Missile Defense Agency.

Tuesday's ground-based interceptor launched from Vandenberg Air Force Base in California just after 3:30 p.m. EST. A little more than one hour later, the Pentagon confirmed it had successfully collided with an ICBM-class target over the Pacific Ocean, which had been launched from the Ronald Reagan Ballistic Missile Defense Test Site on Kwajalein Atoll in the Marshall Islands, 4,200 miles away.

In this Q&A, Lamb briefly turns his attention away from the pending NICER launch to answer a few questions on the current status of the U.S. Ground-Based Missile Defense System.

  • Research
  • Particle Physics
  • High Energy Physics

What do you get when you revive a beautiful 20-year-old physics machine, carefully transport it 3,200 miles over land and sea to its new home, and then use it to probe strange happenings in a magnetic field? Hopefully you get new insights into the elementary particles that make up everything.

The Muon g-2 experiment, located at the U.S. Department of Energy’s (DOE) Fermi National Accelerator Laboratory, has begun its quest for those insights. This month, the 50-foot-wide superconducting electromagnet at the center of the experiment saw its first beam of muon particles from Fermilab’s accelerators, kicking off a three-year effort to measure just what happens to those particles when placed in a stunningly precise magnetic field. The answer could rewrite scientists’ picture of the universe and how it works.

  • Accolades
  • Alumni News

Congratulations to Physics Illinois alumnus M. George Craford on being presented today with the IEEE Edison Medal of the Institute of Electrical and Electronics Engineers. The medal is awarded annually in recognition of a career of meritorious achievement in electrical science, electrical engineering, or the electrical arts. The citation reads, “for a lifetime of pioneering contributions to the development and commercialization of visible LED materials and devices.”

 

Craford is best known for his invention of the first yellow light emitting diode (LED). During his career, he developed and commercialized the technologies yielding the highest-brightness yellow, amber, and red LEDs as well as world-class blue LEDs. He is a pioneer whose contributions to his field are lasting.