A. Casey

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.

Strongly Correlated Two Dimensional Fluid 3He

A two dimensional system of strongly correlated fermions can be realised by a fluid monolayer of3He adsorbed on an atomically flat surface. We report extensive measurements of the heat capacity, c, of3He adsorbed on graphite plated with two layers of hydrogen deuteride (HD) over a closely spaced grid of coverages in the temperature range l-80mK.The quasiparticle effective mass ratio, inferred from the linear temperature coefficient of the heat capacity, increases from near unity at low densities and appears to diverge approaching localisation. The maximum value of m*/m we are able to unambiguously extract from the data is 13 at a density 5 nm−2. At this density c/T is temperature independent only below 5mK. The behaviour of m*/m and F0a, inferred from magnetization data, are consistent with the model of almost-localised fermions. We can track the evolution of strong finite temperature deviations from Fermi liquid behaviour as the density of the film and hence m*/m are increased. At sufficiently low temperature this correction is of T2form going over to a large TlnT deviation for T > 0.05 TF*, as predicted for 2D spin fluctuations. Thus in this system ‘non-Fermi liquid’ behaviour extends to very low temperatures in the strongly correlated, large m */m, limit.

Nuclear magnetic resonance on room temperature samples in nanotesla fields using a two-stage dc superconducting quantum interference device sensor

We describe a compact system for pulsed nuclear magnetic resonance at ultralow magnetic fields on small liquid samples (∼0.14ml) at room temperature. The broadband spectrometer employs an integrated two-stage superconducting quantum interference device current sensor with a coupled energy sensitivity of 50h, in the white noise limit. Environmental noise is screened using a compact arrangement of mu-metal and a superconducting shield. Proton signals in water have been observed down to 93nT (a Larmor frequency of 4.0Hz), with a minimum linewidth of 0.16Hz measured at ∼40Hz. Two-component free induction decays were observed from oil/water mixtures between 275 and 300K.

Torsion pendulum for the study of thin 3He films

We report on the design of a torsion pendulum that can resolve the mass loading from 5×10173He atoms (equivalent to a 1000Å film) with a 0.1% resolution. The oscillator is fabricated from coin silver alloy, and the working surfaces are two highly polished coin silver discs, each with well-characterized surface roughness, that are diffusion welded together using a copper gasket. We report on the cells temperature dependent background. The cell will be used to examine the evolution of the superfluid density and transition temperature as a function of film thickness as well as the normal fluid behavior.

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.

Current-sensing noise thermometry from 4.2 K to below 1 mK using a DC SQUID preamplifier

Abstract We are using a low-Tc DC SQUID to perform current-sensing noise thermometry, by measuring the thermal noise currents in a copper resistor. The temperature is obtained from the Nyquist formula. This is a practical thermometer for use from 4.2 K to below 1 mK , with a percentage precision independent of temperature. Using a 0.34 mΩ resistor, the thermometer had an amplifier noise temperature TN of 8 μK . A precision of 1.5% was obtained in 200 s . The thermometer was in good agreement with the PLTS-2000 3 He melting curve scale down to 4.5 mK . We have cooled the thermometer successfully below 1 mK , achieving a minimum electron temperature of 300 μK .

Quantum transport in mesoscopic 3He films: experimental study of the interference of bulk and boundary scattering.

We discuss the mass transport of a degenerate Fermi liquid ^{3}He film over a rough surface, and the film momentum relaxation time, in the framework of theoretical predictions. In the mesoscopic regime, the anomalous temperature dependence of the relaxation time is explained in terms of the interference between elastic boundary scattering and inelastic quasiparticle-quasiparticle scattering within the film. We exploit a quasiclassical treatment of quantum size effects in the film in which the surface roughness, whose power spectrum is experimentally determined, is mapped into an effective disorder potential within a film of uniform thickness. Confirmation is provided by the introduction of elastic scattering centers within the film. The improved understanding of surface roughness scattering may impact on enhancing the conductivity in thin metallic films.

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.

A microkelvin cryogen-free experimental platform with integrated noise thermometry

We report experimental demonstration of the feasibility of reaching temperatures below 1?mK using cryogen-free technology. Our prototype system comprises an adiabatic nuclear demagnetization stage, based on hyperfine-enhanced nuclear magnetic cooling, integrated with a commercial cryogen-free dilution refrigerator and 8?T superconducting magnet. Thermometry was provided by a current-sensing noise thermometer. The minimum temperature achieved at the experimental platform was 600??K. The platform remained below 1?mK for over 24?h, indicating a total residual heat-leak into the experimental stage of 5?nW. We discuss straightforward improvements to the design of the current prototype that are expected to lead to enhanced performance. This opens the way to widening the accessibility of temperatures in the microkelvin regime, of potential importance in the application of strongly correlated electron states in nanodevices to quantum computing.

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.