Robert John Soulen, Jr. - A Man of Many Passions

January 6, 2016 (PO42). Robert John Soulen, Jr. succumbed to Parkinson’s disease on November 19, 2015, after more than a decade of struggle with the ailment. Robert (henceforth called “Bob”) was born in Phoenixville, PA on July 16, 1940. He attended Rutgers University and was granted a B. A. degree in physics in 1962. He was awarded a Ph. D. degree in physics by Rutgers in 1966. Bob married Rosemarie Vosseler while in graduate school.

At Rutgers, Bob was Prof. Peter Lindenfeld’s doctoral student, but he also benefitted from strong interactions with Prof. E. A. Lynton, Prof. William McLean, and Prof. Bernie Serin. Including Lindenfeld, they were known as the Rutgers Superconductivity Group. Under their tutelage, Bob became a master at making electrical and thermal transport measurements at cryogenic temperatures. This experience led Bob to accept an offer of a position in the Cryogenics Section of the Heat Division at the National Bureau of Standards (NBS) in 1968.

At NBS, Bob immediately began efforts to build or obtain commercially a ³He-⁴He dilution refrigerator (DR) that would eventually facilitate cryogenic studies at temperatures as low as 10 millikelvin (mK). While working on his refrigerator quest, Bob collaborated with James Schooley in the development of a temperature reference device for the range 0.5 K to 7 K (ultimately to 9 K). The device contained five (later six) carefully annealed wire samples all enclosed within sensor coils; thus, the operator could monitor all of the individual transitions in one experiment. NBS registered the device as Standard Reference Material No. 767. It proved to be very popular within the cryogenics community. In 1976, a new provisional international temperature scale was created by the International Bureau of Weights and Measures (the French acronym is BIPM). The NBS SRM 767 device provided five of the eleven reference points on the scale.

Once his new refrigerator was operative, Bob developed another SRM, No.768, which could be used in the same fashion as the 767 device, but with five samples selected for the 16 mK to 200 mK range. Again, the SRM 768 offered easily observable, reproducible superconductive transitions for temperature references in a compact device.

Recognizing the significance of experimental work on noise thermometry performed in the NBS Boulder cryogenics laboratory during the late 1960s, Bob spent nearly twenty years applying the technique to the NBS low-temperature program. In this work, Bob collaborated with William Fogle and Jack Colwell, who were creating a composite temperature scale that involved the melting curve of ³He and the temperature dependence of paramagnetic salt susceptibility. The trio described their work in a pair of papers during a 1992 international temperature conference: “A new cryogenic temperature scale from 0.0063 K to 0.65 K” and “A decade of absolute noise thermometry at NIST using a resistive SQUID”.

Following the meeting in 1992, the three scientists decided to pursue an absolute temperature scale that would extend deep into the millikelvin range. Their intention was to marshal all available very-low-temperature methods into one laboratory experiment, so as to minimize the level of experimental uncertainty. The results of this effort were encouraging. They recorded all of the experimental and theoretical progress in a 102-page paper that they published in the Journal of Low Temperature Physics. The paper included a thorough discussion of their resistively biased use of the Josephson junction and their experimental comparisons of temperature as derived from the SRM 768, from the SQUID-based noise thermometer, from the 1976 provisional temperature scale, and from the ³He melting-curve results. Their work provided much of the basis for the international 2000 Provisional Low-Temperature Scale from 0.9 mK to 1 K.

Bob’s scientific reputation for excellence in his research grew throughout the cryogenics community as time went on. An adept experimenter, Bob also sought an understanding of the theoretical basis for his laboratory work. He was able in many cases to extend existing theory to new laboratory regimes. Bob received the 1976 NBS E. U. Condon Award, the 1979 Department of Commerce Gold Medal (shared with James Schooley), and the 2002 American Physical Society Joseph F. Keithley Award for Advances in Measurement Science. The Keithley citation read “For developing low-temperature noise thermometry to achieve an absolute thermometer which now defines the 2000 Provisional Low-Temperature Scale between 1 mK and 1 K to an accuracy of 0.1 % and for other significant contributions to temperature measurement over a distinguished career”.

Shortly after the discovery of high-temperature superconductivity (HTS) in 1986, Bob moved to the Naval Research Laboratory (NRL), although he still participated in the cryogenic thermometry effort at NBS/NIST in his “spare time”. Bob wanted to focus more intently on research in HTS. He brought with him his skills as a researcher and a leader. He quickly established a program focused first on vortex dynamics and the unusual behavior of the HTS materials in intense magnetic fields. Then, using his experience in noise thermometry, he turned to tunneling into magnetic and superconducting materials to gain a better understanding of the fundamentals of the superconducting state.

Although Bob spent the bulk of his career studying superconductivity, one of his major accomplishments in another area led to his most-cited scientific paper. Utilizing his knowledge of superconducting point contacts, he examined spin-polarized transport in magnetic metals. Motivated by a suggestion by Jeff Byers, a theory postdoc in his group, he performed a ground-breaking experiment that observed the transport of superconducting Cooper pairs into magnetic metals (Andreev scattering) that became a standard technique in the study of spin polarization in magnetic metals, including some novel materials called half magnets. Published in the journal Science, the paper, of which he was the lead author, has been cited more than 1000 times.

Bob and his colleagues also related their laboratory work to practical matters, including critical-current measurements, ac losses in superconducting tapes, and device characterization in the high-temperature superconducting space experiments (HTSSE).

Because of his inherent managerial ability, Bob was asked to head the NRL Material Physics Branch, with responsibility for directing research in magnetic materials, sensor materials, and materials synthesis and characterization. Administration was not Bob’s “cup of tea”, but his leadership in the multidisciplinary physics area was successful over a period of several years before he decided to return to his laboratory studies.

Bob’s calm demeanor masked his many passions, his drive, and a well-developed sense of humor. Besides physics, he enjoyed softball, fly fishing, fine wines, and good cigars. He shared his hobbies with family and friends, especially fishing and softball. His sense of humor once brought him to make a presentation while wearing hip waders during a laboratory review. The program manager had emphasized “come as you are” dress for the review, and Bob took her at her word.

For years, Bob and several colleagues enjoyed running at lunchtime. These events came to be known as “Bob runs” because the group often slowed to a walk while discussing family matters, recollections from his trips to China and Finland, details of tying fishing lures, and, of course, physics. In a single “run”, it was not unusual for the conversation to include electron-phonon interactions, woolly bears (fuzzy caterpillars), and Voltaire.

Bob is survived by his wife of 52 years, Rosemarie, by two daughters, Stephanie Harrington and Heidi Clark, and by three grandsons, Michael, Trevor, and Henry. In the early 1970s, Bob and Rosemarie lost an infant son, Robert John Soulen III.

As noted above, Bob contracted Parkinson’s disease more than a decade ago, but he refused to capitulate to the ailment even as its symptoms became debilitating. An article in the Washington Post (June 29, 2010) described Bob’s devotion to softball during the later stages of his affliction. He used a walker to approach home plate in games played in the 60-and-older Senior Montgomery County Softball League; he was a designated hitter in the lineups for both teams. Even as the Parkinson’s disease progressed to its final stages, Bob wrote two books (self-published on Amazon) about his passion for softball and fly fishing.

Bob also continued to exercise his hobby of tying fishing flies, using feathers obtained from birds. The feathers also found places on the covers of specialty cigar boxes that he decorated for friends, and as components of artistic bird montages. We will not soon forget his scientific abilities, his friendship, his wit, his passion for life, and his courage in the face of certain death. We grieve for his family, but we rejoice in the life that he led.

These notes were written by Jim Schooley, Don Gubser, Mike Osofsky, Boris Nadgorny, Bill Fogle, and Stu Wolf. We thank Keith Martin, librarian at NIST, for providing the reference to the Washington Post.