History of the Packard Group

Physics Department, University of California at Berkeley, Berkeley, CA 94720, Phone: (510) 642-1664

The Laboratory was formed when the principal investigator, Richard E. Packard, joined the faculty of the University of California, Berkeley, in 1971. At that time, Packard and his students, Gary Williams and Keith DeConde, pursued studies of interactions of ions with quantized vortex lines. Some noteworthy accomplishments included the first measurement of the trapping lifetime of positive ions on superfluid vortex lines, the discovery that ³He atoms become trapped on the core of vortices, the discovery that ³He slowed vortex motion at low temperatures and, the subsequent explanation of why lifetimes of trapped negative ions at low temperatures did not follow the predicted Boltzmann-factor dependence. The group also succeeded in making the first demonstration that free energy calculations could accurately predict the ground state critical velocity for vortex formation in superfluid ⁴He.

Knowledge and techniques acquired in these ion-vortex experiments led Packard and G.A. Williams to develop techniques to photograph the positions of quantized vortex lines in rotating superfluid ⁴He. This work required the development of a rotating dilution refrigerator. In the mid 1970s, Packard, postdoc Ed Yarmchuck, and graduate student Mickey Gordon demonstrated that quantized vortices formed regular symmetric patterns that were predictable based on free energy calculations. Time-evolved studies of the vortex motion demonstrated the existence of collective vortex modes for small numbers of vortices and provided a test of our detailed understanding of three dimensional vortex dynamics. These photographic studies provided the most direct evidence for the existence of quantized vortices.

Also in those early years, Packard conceived the idea that vortex metastability in the interior of superfluid neutron stars is responsible for sudden speed-up events seen in pulsar signals. This concept has since been developed and is now the accepted mechanism for sudden changes in pulsar timing.

In other work, the group made the first observation of the thinning of a flowing superfluid film. This effect, predicted by Kontorovitch, had previously been reported to be nonexistent in direct experiments. This initial null result inspired several new theoretical ideas. The Berkeley group's positive result, at temperatures below 0.9K, taken with other work at higher temperatures, proved that the earlier experiment had erred. The Kontorovitch effect existed as predicted.

In 1975, the Laboratory staff began to develop equipment to permit studies of superfluid ³He, a phase discovered at Cornell three years earlier. This emerging field required temperatures about one thousand times colder than ⁴He experiments. A modest nuclear adiabatic demagnetization cryostat was developed which could achieve temperatures in ³He below 0.5mK. The new machine was assembled with a total equipment cost of only $23K, and the efforts of Packard and graduate student K. DeConde. At that time, similar facilities were developed in other larger laboratories for an order of magnitude higher cost.

The earliest experiment with the new cryostat made the first measurement of the coupling energy between the orbital l-vector and an applied electric field. Packard and his student G. Swift observed and measured the magnitude of a predicted anisotropy (with respect to a magnetic field) of the dielectric constant of ³He-A. Although early calculations predicted this anisotropy to be at the level of 10^-7, a series of null measurements at Berkeley and elsewhere caused a continued downward theoretical reassessment of the effect. The Berkeley group ultimately detected the anisotropy at the level of 10^-10, a sensitivity record for capacitance measurements that has not yet been surpassed.

In a parallel research project, Packard and student J. Eisenstein investigated superfluid flow through small tubes. The techniques created for this research enabled a new determination of superfluid density in ³He-B. In addition, the experiments provided the first demonstration that a well-defined critical mass current existed. The critical current density was found to match predictions based on depairing phenomena in the superfluid. Other related experiments included one of the first quantitative demonstrations of the applicability of slip corrections in the momentum transport properties of ³He, when quasiparticle mean-free-paths become non-negligible.

By the early 1980's, the Berkeley ULT group had refined their techniques so that they could study the superfluid properties in a single micron-sized cylindrical channel. Packard and his postdoc J. Pekola developed a method to manufacture such a channel by firing a single nuclear fission fragment through thin plastic foil. Etching the single track created the flow channel of interest. The high-resolution capacitance techniques, developed earlier for the dielectric anisotropy experiments, enabled the scientists to develop displacement transducers, which could monitor the extremely small mass currents relevant for this work.

The experiments demonstrated that the depairing critical current could dominate dissipation in the small channel. It was also possible to show that, because the channel size was approaching the superfluid coherence length, there was noticeable suppression of the superfluid transition temperature in the hole. The laboratory staff also measured effects associated with the onset of dissipation and developed a theory of fluctuating phase slips, which could qualitatively describe the phenomenon.

By the mid 1980's, attention was being focused on questions concerning the possible existence of persistent currents in the ³He phases. To help probe these questions, the Berkeley ULT group created an apparatus, which could detect the angular momentum stored in superfluid flowing in an annular container. To create the mass currents, the Berkeley apparatus was mounted on a rotating cryostat in Helsinki, Finland. In subsequent experiments the Berkeley-Helsinki collaboration demonstrated the existence of persistent currents in ³He-B. These currents were also detected at Cornell University. The Berkeley-Helsinki experiments showed the existence of a well-defined critical velocity, which is believed to herald the creation of quantized vortices. With the unexpected discovery that this critical velocity abruptly changed at well-defined temperature and pressure, it became apparent that a phase transition existed in the type of vortex structure that determined the critical velocity. The experimenters were able to construct a phase diagram of the two types of vortices.

In a quite different series of experiments, the Berkeley group used fourth sound propagation in ³He in packed powders to study the effects of large magnetic fields on the superfluid state in porous media. Part of this project involved developing skills in microfabrication of Silicon. This expertise is now of central importance in the Laboratory's ongoing research.

The above-mentioned experiments on hydrodynamics in confined geometry led Packard and his students J. Davis and A. Amar, to investigate superfluidity in superfluid ³He films. They invented a new technique for ³He film-flow studies, which enabled rapid determinations of the presence of the superfluid state and a measurement of the critical current and transition temperature. Using these techniques it was possible to demonstrate that earlier reports of film superfluidity at elevated temperatures were in error. The Berkeley experiments provided a test of the Landau-Ginzburg predictions for suppression of the transition temperature in thin films. A match to the theory showed that the quasiparticle scattering at the solid substrate was diffuse in character. The magnitude of the depairing critical current density in the film was also determined

By the late 1980's, the original demagnetization cryostat was showing its age and Packard and his students, J. Davis, R. Zieve and J. Close, undertook the development of a more modern and powerful submillikelvin facility. In order to pursue studies of rotating superfluid ³He the new machine was designed to rotate at speeds up to several radians per sec. The rotating cryostat, completed in 1990, has achieved temperatures in ³He as low as 165mK and heat leaks of about 10^-9W during rotation. A second, smaller demagnetization cryostat, has replaced the original non-rotating machine. The stationary cryostat also achieves ³He temperatures below 200mK.

The first experiments using the rotating cryostat demonstrated that fluid circulation in ³He-B is quantized. This proves that ³He-B can be described by a quantum condensate wavefunction. The quantum of circulation was measured to be h/2m₃, thus giving the first direct proof of the existence of Cooper pairs within the superfluid.

In 1991 the phenomenon of superfluid vortex precession was discovered by Packard and his students (Rena Zieve, John Close and Seamus Davis) and they developed a theory explaining the phenomenon. This was the first experimental work that allowed an exact test of vortex dynamics at the single vortex level. It was also the first direct measurement of a superfluid Josephson frequency relation. Other research with the rotating cryostat led to the observation of stable circulation in ³He-A and to a new technique to study textural relaxation in ³He-A.

In work on ⁴He student Ajay Amar made the first confirmation of work in Paris that showed the existence of quantized 2p phase slips in the superflow through a submicron-sized orifice. The researchers used this observation to demonstrate the relevance of the Anderson-Josephson phase evolution equation in superfluid flow. Based on these experiments, Packard and S. Vitale developed a stochastic theory that explains the connection between superfluid dc critical currents and discrete phase slip events. This has led to a deeper understanding of the nature of the intrinsic critical velocity problem.

In 1992 the phase slip studies led to a discovery of an effect that suggests that quantum tunneling may be responsible for vortex nucleation at very low temperatures. This effect is stimulating a re-examination of the role of quantum fluctuations in vortex nucleation processes.

The phase slip phenomena led the Berkeley lab to make an in depth study of vortex nucleation. They performed experiments and developed a theory that led to the determination of a universal energy barrier that characterizes vortex nucleation. Due to this work, it is now possible to predict several aspects of intrinsic flow dissipation in superfluid ⁴He.

An outgrowth of the phase slip studies led to a project to make a superfluid rotation sensor that may have great sensitivity. Sensors using ⁴He and ³He are under development. The insight developed from this research led Packard and Stefano Vitale to the theory describing a superfluid helium gyroscope or SHe-SQUID. In 1996 Packard and students Keith Schwab and Niels Bruckner demonstrated the proof-of-principle of the AC-SHe⁴SQUID. They built the device on a Silicon wafer and used it to detect the rotation of the Earth. Graduate student Niels Bruckner extended the sensitivity of the first prototype by a factor of almost 100.

Using the techniques of micro-fabrication of small apertures and sensitive displacement transducers, Packard and his students pursued the long-sought goal of developing a superfluid Josephson weak link exhibiting a sinusoidal current-phase relation. ³He is chosen for the research because it has a large coherence length (longer than 65nm at ambient pressure P=0), which matches the aperture size that can be manufactured. After several years of studying the flow through such apertures, Packard and his collaborators began to use arrays of small apertures, hoping that small effects would be coherently amplified by the number of apertures. This turned out to be the case and in quick succession, beginning in the Spring of 1997, the team observed the following significant phenomena: quantum oscillation at the Josephson frequency, a sinusoidal current-phase relation, plasma oscillations in the phase, a metastable state, bi-stability of the weak links, and novel sources of superfluid dissipation.

A sort of Holy Grail for the group was to develop a superfluid interferometer using a pair of Josephson weak links configured like a DC-SQUID. Such a device, named a DC-SHeQUID could detect small changes in absolute rotation (i.e. serving as a superfluid gyroscope) and might also be a probe to set upper limits on proposed exotic interactions. This milestone was reached in 2001 by graduate student Ray Simmonds, postdoc Alosha Marchenkov, and collaborator Seamus Davis who, after being a graduate student and post doc in the group, joined the Berkeley faculty in 1993.

The ³He dc-SQUID is a more sensitive sensor of rotation than the ⁴He phase slip device. However, due to the complexity of using submillikelvin technology to enter the applicable temperature regime for superfluid ³He, this kind of physical probe would only be used for experiments with potentially very high impact.

The group also continued their studies of ³He superfluid films. In 1998 they succeeded in detecting surface waves, so called third sound. This phenomenon can be used as a probe to study this two dimensional Fermi superfluid.

In related film studies, postdoc Kostya Penanen and graduate student Joan Hoffman investigated an old mystery: Why is the dissipation of third sound very large in superfluid ⁴He films? Their studies indicated that the dissipation arises due to the induced motion of trapped vorticity. However, since trapped vorticity is an uncontrolled parameter the sequence of experiments did not provide definitive answers.

The group’s research became refocused in 2005 when graduate student Emile Hoskinson made the discovery that an array of nano-apertures in ⁴He exhibited the same kind of “quantum whistle” as that seen earlier in ³He. To observe the phenomenon the helium must be a few millikelvin below from the transition temperature, 2.17K. This discovery was unanticipated because in this temperature regime the apertures exhibit quantized phase slippage rather than sine-like Josephson behavior. The appearance of the whistle implies that all of the apertures are phase slipping in unison and that thermal fluctuations that eliminate periodic oscillations are suppressed. Further surprises emerged with the discovery that at even colder temperatures the apertures lose the coherence property. Although one theory has been presented to explain these observations, there is at yet no proven explanation. Graduate student Aditya Joshi is at present (late 2008) planning to empirically map out the characteristics of various aperture arrays to test models of the array coherence.

The quantum whistle in ⁴He can be used for SHeQUID devices. Hoskinson and grad student Yuki Sato demonstrated a DC-SHeQUID (used as a gyroscope), which successfully measured the Earth’s rotation rate. Since the high (compared to ³He) operating temperature of this device is achievable using a mechanical cryocooler, one could for the first time envision superfluid interferometers being practical probes of rotation and other more fundamental interactions. The group has used these interferometers to explicitly test the conceptual link between the two-fluid model of Landau and the macroscopic quantum picture developed by others. They also found ways to linearize SHeQUIDs and to make a superfluid diffraction grating device that promises even higher sensitivity to phase shifts than the original double weak link devices.

All of the results outlined above were attained by Packard working with a small group of dedicated students, of which many have gone on to productive careers in science. Some of Packard's students and postdocs include, G. Williams (professor at U.C.L.A.), K. DeConde (presently an executive in high tech industry and previously a tenured professor at Princeton), E. Yarmchuk (staff scientist at I.B.M.), G. Swift (head of the thermo-acoustics group at Los Alamos and winner of an Enrico Fermi Award), S. Garrett (professor at Penn State University and the 1993 winner of the Rolex Enterprise Award and the silver medal of the Acoustical Society of America), J. Eisenstein (professor at Cal Tech and member of the National Academy), K. Daly (staff scientist at TRW), J. Davis (professor Cornell), R. Zieve (faculty at UC Davis), A. Amar (executive position in industry), J. Steinhauer (faculty at the Technion), Emile Hoskinson (postdoctoral scholar at UC Berkeley), Yuki Sato (postdoctoral scholar at UC Berkeley), K. Schwab (faculty at Cornell), Kostya Pennanen (staff scientist at JPL), J. Close (faculty at Australia National University), A. Loshak (staff scientist Livermore Laboratory), S. Backhaus (staff scientist Los Alamos Laboratory)

A Brief History of the Berkeley Ultralow Temperature Laboratory