L. V. Levitin

Phase Diagram of the Topological Superfluid 3He Confined in a Nanoscale Slab Geometry

Confined Helium Helium-3 (3He) has superfluid phases closely related to topological insulators and topological superconductors. In geometrical confinement, 3He is expected to support exotic excitations, but its phase diagram is largely unknown, as such measurements are experimentally challenging. Levitin et al. (p. 841) confine 3He in a slab geometry of defined height and use nuclear magnetic resonance as a probe. As predicted by theory, the authors find that the confinement stabilizes some of the superfluid phases across a larger portion of the phase diagram with respect to the bulk and potentially provides a testbed for topological superfluidity. Geometrical confinement affects the stability of the superfluid phases of helium-3. The superfluid phases of helium-3 (3He) are predicted to be strongly influenced by mesoscopic confinement. However, mapping out the phase diagram in a confined geometry has been experimentally challenging. We confined a sample of 3He within a nanofluidic cavity of precisely defined geometry, cooled it, and fingerprinted the order parameter using a sensitive nuclear magnetic resonance spectrometer. The measured suppression of the p-wave order parameter arising from surface scattering was consistent with the predictions of quasi-classical theory. Controlled confinement of nanofluidic samples provides a new laboratory for the study of topological superfluids and their surface- and edge-bound excitations.

Anodically bonded submicron microfluidic chambers

We demonstrate the use of anodic bonding to fabricate cells with characteristic size as large as 7 x 10 mm(2), with height of approximately 640 nm, and without any internal support structure. The cells were fabricated from Hoya SD-2 glass and silicon wafers, each with 3 mm thickness to maintain dimensional stability under internal pressure. Bonding was carried out at 350 degrees C and 450 V with an electrode structure that excluded the electric field from the open region. We detail fabrication and characterization steps and also discuss the design of the fill line for access to the cavity.

A nuclear magnetic resonance spectrometer for operation around 1MHz with a sub-10-mK noise temperature, based on a two-stage dc superconducting quantum interference device sensor

We have developed a nuclear magnetic resonance spectrometer with a series tuned input circuit for measurements on samples at millikelvin temperatures based on an integrated two-stage superconducting quantum interference device current sensor, with an energy sensitivity e=26±1h when operated at 1.4K. To maximize the sensitivity, both the nuclear magnetic resonance spectrometer pickup coil and tuning capacitor need to be cooled, and the tank circuit parameters should be chosen to equalize the contributions from circulating current noise and voltage noise in the superconducting quantum interference device. A noise temperature TN=7±2mK was measured, at a frequency of 0.884MHz, with the circuit parameters close to optimum.

Primary current-sensing noise thermometry in the millikelvin regime.

The use of low-temperature platforms with base temperatures below 1 K is rapidly expanding, for fundamental science, sensitive instrumentation and new technologies of potentially significant commercial impact. Precise measurement of the thermodynamic temperature of these low-temperature platforms is crucial for their operation. In this paper, we describe a practical and user-friendly primary current-sensing noise thermometer (CSNT) for reliable and traceable thermometry and the dissemination of the new kelvin in this temperature regime. Design considerations of the thermometer are discussed, including the optimization of a thermometer for the temperature range to be measured, noise sources and thermalization. We show the procedure taken to make the thermometer primary and contributions to the uncertainty budget. With standard laboratory instrumentation, a relative uncertainty of 1.53% is obtainable. Initial comparison measurements between a primary CSNT and a superconducting reference device traceable to the PLTS-2000 (Provisional Low Temperature Scale of 2000) are presented between 66 and 208 mK, showing good agreement within the k=1 calculated uncertainty.

Study of Superfluid ^3He Under Nanoscale Confinement

We review recent experiments in which superfluid 3 He has been studied under highly controlled confinement in nanofluidic sample chambers. We discuss the experimental challenges and their resolution. These methods open the way to a systematic investigation of the superfluidity of 3 He films, and the surface and edge

The A-B transition in superfluid helium-3 under confinement in a thin slab geometry

The influence of confinement on the phases of superfluid helium-3 is studied using the torsional pendulum method. We focus on the transition between the A and B phases, where the A phase is stabilized by confinement and a spatially modulated stripe phase is predicted at the A–B phase boundary. Here we discuss results from superfluid helium-3 contained in a single 1.08-μm-thick nanofluidic cavity incorporated into a high-precision torsion pendulum, and map the phase diagram between 0.1 and 5.6 bar. We observe only small supercooling of the A phase, in comparison to bulk or when confined in aerogel, with evidence for a non-monotonic pressure dependence. This suggests that an intrinsic B-phase nucleation mechanism operates under confinement. Both the phase diagram and the relative superfluid fraction of the A and B phases, show that strong coupling is present at all pressures, with implications for the stability of the stripe phase.

Fabrication of microfluidic cavities using Si-to-glass anodic bonding

We demonstrate the fabrication of ∼1.08 μm deep microfluidic cavities with characteristic size as large as 7 mm × 11 mm or 11 mm diameter, using a silicon-glass anodic bonding technique that does not require posts to act as separators to define cavity height. Since the phase diagram of 3He is significantly altered under confinement, posts might act as pinning centers for phase boundaries. The previous generation of cavities relied on full wafer-bonding which is more prone to failure and requires dicing post-bonding, whereas these cavities are made by bonding a pre-cut piece of Hoya SD-2 glass to a patterned piece of silicon in which the cavity is defined by etching. Anodic bonding was carried out at 425 °C with 200 V, and we observe that pressurizing the cavity to failure (>30 bars pressure) results in glass breaking, rather than the glass-silicon bond separation. In this article, we discuss the detailed fabrication of the cavity, its edges, and details of the junction between the coin silver fill line and the silicon base of the cavity that enables a low internal-friction joint. This feature is important for mass coupling torsional oscillator experimental assays of the superfluid inertial contribution where a high quality factor (Q) improves frequency resolution. The surface preparation that yields well-characterized smooth surfaces to eliminate pinning sites, the use of transparent glass as a cover permitting optical access, low temperature capability, and attachment of pressure-capable ports for fluid access may be features that are important in other applications.

Study of Superfluid 3 He Under Nanoscale Confinement A New Approach to the Investigation of Superfluid 3 He Films

We review recent experiments in which superfluid 3 He has been studied under highly controlled confinement in nanofluidic sample chambers. We discuss the experimental challenges and their resolution. These methods open the way to a systematic investigation of the superfluidity of 3 He films, and the surface and edge

Study of Superfluid \(^3\)He Under Nanoscale Confinement

We review recent experiments in which superfluid 3 He has been studied under highly controlled confinement in nanofluidic sample chambers. We discuss the experimental challenges and their resolution. These methods open the way to a systematic investigation of the superfluidity of 3 He films, and the surface and edge