[en] We describe studies of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) of liquid samples at room temperature in microtesla magnetic fields. The nuclear spins are prepolarized in a strong transient field. The magnetic signals generated by the precessing spins, which range in frequency from tens of Hz to several kHz, are detected by a low-transition temperature dc SQUID (Superconducting QUantum Interference Device) coupled to an untuned, superconducting flux transformer configured as an axial gradiometer. The combination of prepolarization and frequency-independent detector sensitivity results in a high signal-to-noise ratio and high spectral resolution (∼1 Hz) even in grossly inhomogeneous magnetic fields. In the NMR experiments, the high spectral resolution enables us to detect the 10-Hz splitting of the spectrum of protons due to their scalar coupling to a 31P nucleus. Furthermore, the broadband detection scheme combined with a non-resonant field-reversal spin echo allows the simultaneous observation of signals from protons and 31P nuclei, even though their NMR resonance frequencies differ by a factor of 2.5. We extend our methodology to MRI in microtesla fields, where the high spectral resolution translates into high spatial resolution. We demonstrate two-dimensional images of a mineral oil phantom and slices of peppers, with a spatial resolution of about 1 mm. We also image an intact pepper using slice selection, again with 1-mm resolution. In further experiments we demonstrate T1-contrast imaging of a water phantom, some parts of which were doped with a paramagnetic salt to reduce the longitudinal relaxation time T1. Possible applications of this MRI technique include screening for tumors and integration with existing multichannel SQUID systems for brain imaging

No abstract available

[en] We investigate the characteristics and noise performance of rf Superconducting Quantum Interference Devices (SQUIDs) by solving the corresponding Langevin equations numerically and optimizing the model parameters with respect to noise energy. After introducing the basic concepts of the numerical simulations, we give a detailed discussion of the performance of the SQUID as a function of all relevant parameters. The best performance is obtained in the crossover region between the dispersive and dissipative regimes, characterized by an inductance parameter β(prime)_L (triple_bond) 2πLI₀/Φ₀ ∼ 1; L is the loop inductance, I₀ the critical current of the Josephson junction, and φ₀ the flux quantum. In this regime, which is not well explored by previous analytical approaches, the lowest (intrinsic) values of noise energy are a factor of about 2 above previous estimates based on analytical approaches. However, several other analytical predictions, such as the inverse proportionality of the noise energy on the tank circuit quality factor and the square of the coupling coefficient between the tank circuit and the SQUID loop, could not be well reproduced. The optimized intrinsic noise energy of the rf SQUID is superior to that of the dc SQUID at all temperatures. Although for technologically achievable parameters this advantage shrinks, particularly at low thermal fluctuation levels, we give an example for realistic parameters that leads to a noise energy comparable to that of the dc SQUID even in this regime

[en] Impurity-helium solids are porous gel-like materials held together by Van der Waals forces. They consist of impurity atoms, molecules or clusters of atoms and molecules, each surrounded by very thin layers of solid helium. Impurities studied include neon, krypton, and molecular and atomic nitrogen, hydrogen and deuterium. The pore sizes and cluster sizes in impurity-helium solids are determined by ultrasound attenuation and x-ray diffraction. The ESR technique is employed to study atomic impurities. The tunneling exchange chemical reactions D+H₂ -> HD+D and D+HD ->D₂+H are observed. Large concentrations of atomic hydrogen (∼8 x 10¹⁷ per cm³) are produced in these reactions

[en] Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. Measured was the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D₂-He solids. These measurements show that D atoms are mainly stabilized in D₂ clusters. using an x-ray method we found the size of D₂ clusters to be ∼ 90 Angstroms in diameter and the densities of D₂ molecules in the samples to be of the order 2.5 x 10²¹ cm^-3. The highest average concentration of D atoms achieved in D-D₂-He solids was ∼ 1.5 x 10¹⁸ cm^-3. The local concentrations of D atoms within D₂ clusters is found to be large (∼2 x 10¹⁹ cm^-3)

[en] We investigate rf SQUIDs (Superconducting QUantum Interference Devices), coupled to a resonant input circuit, a readout tank circuit and a preamplifier, by numerically solving the corresponding Langevin equations and optimizing model parameters with respect to noise temperature. We also give approximate analytic solutions for the noise temperature, which we reduce to parameters of the SQUID and the tank circuit in the absence of the input circuit. The analytic solutions agree with numerical simulations of the full circuit to within 10%, and are similar to expressions used to calculate the noise temperature of dc SQUIDs. The best device performance is obtained when β_L(prime) (triple_bond) 2πLI₀/Φ₀ is 0.6-0.8; L is the SQUID inductance, I₀ the junction critical current and F₀ the flux quantum. For a tuned input circuit we find an optimal noise temperature T_N,opt ∼ 3Tf/f_c, where T, f and f_c denote temperature, signal frequency and junction characteristic frequency, respectively. This value is only a factor of 2 larger than the optimal noise temperatures obtained by approximate analytic theories carried out previously in the limit β_L(prime) << 1. We study the dependence of the noise temperature on various model parameters, and give examples using realistic device parameters of the extent to which the intrinsic noise temperature can be realized experimentally

[en] The numerical result of U_kin = 9.9 E_J at Φ/Φ₀ = 1/2 should be replaced by 2.63 E_J in Sec. 5.1, paragraph 4, in Sec. 5.2, paragraphs 2 and 3, and in Appendix A, the last paragraph. Accordingly, the value of ΔU_kin = 6.2 E_J should be replaced by U_kin = 0.17 E_J in Sec. 5.2, paragraph 4. This lower value of the kinetic energy barrier does not significantly affect the conclusions of the paper. It actually improves the agreement with the experimental data in the low temperature range (Appendix B), but the authors still tend to overestimate the oscillation of the periodic pinning potential at higher temperature, as discussed in Sec. 5.2 of the paper

[en] Flow measurements in ultra-pure ⁴He through a micron-size orifice at millikelvin temperatures show the transition from thermal to quantum nucleation of nanometer-size vortices below a crossover temperature of 0.147 K. Detailed analysis, similar to that done for macroscopic quantum tunneling in superconducting junctions, yields parameters that accurately predict the mean value and the distribution width of the critical velocities at all temperatures. These observations establish the close relationship between this type of critical flow and negative ion motion in superfluid ⁴He and strongly suggest that the underlying mechanism is identical

[en] Based on the field-theoretical renormalization group theory for O(N) symmetric systems in 4 - var-epsilon dimensions the free energy of critical films confined between two parallel plates at distance L is analyzed. For temperatures above T_cbulk the universal scaling functions Θ are presented and discussed. At bulk criticality the scaling functions reduce to the universal Casimir amplitudes. Using these field-theoretical results, quantitative predictions on the specific heat of ⁴He films and films of ³He-⁴He mixtures close to the onset of superfluidity are made. Wetting of a substrate by ⁴He or ³He-⁴He mixtures close to the λ-transition is studied in detail, and thus the full scaling function Θ is probed

[en] In the presence of a homogeneous heat current, the isotropy of superfluid ⁴He is broken and the propagation of second-sound waves becomes anisotropic. The author calculates the angular dependence of the velocity and damping of second sound near T_λ within model F of Halperin, Hohenberg, and Siggia to leading order in the renormalized couplings. The critical behavior is incorporated using results of the renormalization-group theory. Second-sound waves propagating parallel to the heat current become unstable if the heat current exceeds some critical value. The theory agrees with recent experiments