Nuclear magnetism in silver at positive and negative absolute temperatures in the low nanokelvin range

We have investigated the susceptibility and entropy in the thermally isolated system of silver nuclei down to 0.8 nK and, at negative temperatures, up to -4.3 nK. Low-frequency SQUID-NMR techniques were employed to measure the dynamic sysceptibility. Curie-Weiss behavior was observed for the static susceptibility both at T>0 and T<0; for FTHETA we deduce -4.4±1.0 nK. Our results show directly that antiferromagnetic nuclear alignment at positive temperatures transforms into ferromagnetic orientation at T<0 in the nuclear-spin system of silver, dominated by exchange interaction.Peer reviewe

Method for Cooling Nanostructures to Microkelvin Temperatures

We propose a new scheme aimed at cooling nanostructures to microkelvin temperatures, based on the well established technique of adiabatic nuclear demagnetization: we attach each device measurement lead to an individual nuclear refrigerator, allowing efficient thermal contact to a microkelvin bath. On a prototype consisting of a parallel network of nuclear refrigerators, temperatures of $\sim 1\,$mK simultaneously on ten measurement leads have been reached upon demagnetization, thus completing the first steps toward ultracold nanostructures.Comment: 4 pages, 3 (color) figure

Observation of nuclear ferromagnetic ordering in silver at negative nanokelvin temperatures

The ferromagnetically ordered state in the nuclear spin system of silver has been reached at negative absolute temperatures by adiabatic nuclear demagnetization at entropies below 0.82 ln2. The ordering, caused by the antiferromagnetic Ruderman-Kittel interaction, was observed below -1.9 nK as a saturation of susceptibility close to -1 and as an increase of the NMR frequencies. Comparison with recent mean-field calculations by Viertiö and Oja suggests a domain configuration. The phase diagram of silver nuclei at T<0 is outlined in the magnetic field versus entropy plane.Peer reviewe

Observations on Superfluid Meniscus in Rotating 3He-B

The parabolic superfluid meniscus has been seen for states in which only (1) the normal or (2) the superfluid component of 3He-B rotates. A reduced, temperature-dependent meniscus (1) was formed at small speeds Ω≲0.21 rad /s. A deeper-than-expected meniscus (2) was observed after a rapid halt of the cryostat, when the normal fluid stops during a short relaxation time, followed by slow decay of the superfluid circulation. The depth of the meniscus (2) was explained by a reactive radial force between the rotating superfluid and the stationary normal liquid.Peer reviewe

Persistent-Current Experiments on Superfluid 3He-B and 3He-A

We have investigated persistent flow of superfluid 3He with an ac gyroscope filled with 20-μm powder. In 3He-B, currents circulate undiminished for 48 h at least; this implies a viscosity 12 orders of magnitude lower than in the normal fluid. In 3He-A, the current does not persist. The observed critical velocity in 3He-B at P12 bars there are two regimes in the B phase: For example, at 29.3 bars the ultimate critical velocities are 5.4 and 7.8 mm/s, respectively.Peer reviewe

An automated and versatile ultra-low temperature SQUID magnetometer

We present the design and construction of a SQUID-based magnetometer for operation down to temperatures T = 10 mK, while retaining the compatibility with the sample holders typically used in commercial SQUID magnetometers. The system is based on a dc-SQUID coupled to a second-order gradiometer. The sample is placed inside the plastic mixing chamber of a dilution refrigerator and is thermalized directly by the 3He flow. The movement though the pickup coils is obtained by lifting the whole dilution refrigerator insert. A home-developed software provides full automation and an easy user interface.Comment: RevTex, 10 pages, 10 eps figures. High-resolution figures available upon reques

Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs

We present nuclear spin relaxation measurements in GaAs epilayers using a new pump-probe technique in all-electrical, lateral spin-valve devices. The measured T1 times agree very well with NMR data available for T > 1 K. However, the nuclear spin relaxation rate clearly deviates from the well-established Korringa law expected in metallic samples and follows a sub-linear temperature dependence 1/T1 ~ T^0.6 for 0.1 K < T < 10 K. Further, we investigate nuclear spin inhomogeneities.Comment: 5 pages, 4 (color) figures. arXiv admin note: text overlap with arXiv:1109.633

Search for the ac Josephson effect in superfluid 3He

Experiments testing for the existence of the ac Josephson effect in superfluid 3He, analogous to phenomena observed in superconducting microbridges, have been performed. Small holes were employed as the weak link between two reservoirs filled with 3He; several different orifice geometries were tried. Simple model calculations suggest that steps in the flow characteristics should be observable with our resolution when an ac pressure modulation is applied across the weak link. We found that such effects do not exist for the parameter values used in our experiments.Peer reviewe

Energy Down Conversion between Classical Electromagnetic Fields via a Quantum Mechanical SQUID Ring

We consider the interaction of a quantum mechanical SQUID ring with a classical resonator (a parallel $LC$ tank circuit). In our model we assume that the evolution of the ring maintains its quantum mechanical nature, even though the circuit to which it is coupled is treated classically. We show that when the SQUID ring is driven by a classical monochromatic microwave source, energy can be transferred between this input and the tank circuit, even when the frequency ratio between them is very large. Essentially, these calculations deal with the coupling between a single macroscopic quantum object (the SQUID ring) and a classical circuit measurement device where due account is taken of the non-perturbative behaviour of the ring and the concomitant non-linear interaction of the ring with this device.Comment: 7 pages, 6 figure

Phase Diagram of the First-Order Vortex-Core Transition in Superfluid 3He-B

The phase diagram of quantized vortices in 3He-B has been obtained with two techniques. Our NMR measurements show that magnetization displays a pressure-dependent first-order phase change at TV(P). Our gyroscopic experiments reveal a transition curve TG(P) which marks an abrupt change in the critical velocity. Both curves show a weak magnetic field dependence, run similarly in the (P, T) plane, and seem to mark the same vortex-core transition.Peer reviewe