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Nuclear magnetism in the second layer of 3He films on graphite has been studied as a function of density in the zero-field limit at temperatures down to 300 µK. Total surface coverages in this study ranged from 20 atoms/nm2 (the lowest coverage at which the effective exchange parameter turns ferromagnetic) through the two-phase region (ending at about 24 atoms/nm2) and into the single-phase imcommensurate solid region up to 31.2 atoms/nm2. The surface layers were studied using both pulsed and cw NMR measured with our SQUID NMR system in magnetic fields of 500 µT and lower. As the surface density continues to increase in the single-phase region, the exchange constant decreases consistently with a picture of steric hindrance impeding ferromagnetic exchange. Several different observational features scale with this inferred exchange: low-field magnetization, NMR frequency shift, and the temperature of onset of significant magnetization.

We report measurements of the magnetic susceptibility of liquid 3He at saturated vapor pressure in the normal state from the superfluid transition temperature up to 1 K. Our results are comparable to those of Ramm, et al.1 at temperatures above 200 mK, but are systematically higher as T→0. We conclude that the zero-temperature limiting susceptibility is approximately 5.9% larger than reported by Ramm, et al. Consequently, the discrepancy in the liquid's diamagnetism between values expected theoretically and our experimentally measured results is essentially eliminated.

3 He films on graphite provide a model system for 2D magnetism. At low temperatures, the thermodynamic properties of these films are dominated by exchange interactions. However more subtle interactions such as dipolar coupling and other anisotropies may eventually play a substantial role, especially in small magnetic fields. Our recent experiments use a SQUID system for both pulsed and continuous wave NMR at very low applied fields (B

We have measured the absolute thermodynamic magnetization of 3 He in all of its liquid phases while using a new calibration technique which relies on the nuclear paramagnetism over the entire temperature range below 1 K as measured by Ramm, et al. We have directly measured the temperature dependence separately of the nuclear paramagnetism via NMR and of the total magnetism via our free-surface modulation technique. We report substantial agreement with Ramm, et al. In consequence, we find that the diamagnetism of liquid 3 He is significantly smaller than expected theoretically, a condensed matter effect not seen previously. We have also remeasured the magnitude of the magnetization discontinuity at the superfluid A-B transition and find close agreement at saturated vapor pressure with NMR-deduced values from Scholz.

The difference between total thermodynamic magnetizations in the $A$ and $B$ phases of superfluid ${}^{3}\mathrm{He}$ was measured at the equilibrium $A$- $B$ transition temperature over the entire range of applicable magnetic fields at pressures from 0 to 27 bars. We present evidence that dynamic measurements (NMR) only partially account for the thermodynamic magnetization changes in the superfluid. The present experiment also measures the nontrivial strong-coupling correction to the magnetic field coupling strength in the Landau-Ginzburg free energy.

We have measured the thermodynamic magnetization in the various phases of normal and superfluid3He. The A-B discontinuity differs from that inferred from nuclear-specific (NMR) experiments. We offer reasons why this apparent disagreement can be consistent with our current picture of super-fluid3He. One consequence of this measurement is an improved set of values for the phenomenological Landau-Ginzburg free energy β-parameters.

We present data for the magnitude of the thermodynamic magnetization discontinuity at the equilibrium A-B transition in superfluid3He, ΔMAB, over its entire pressure-temperature-magnetic field phase diagram. The data was taken by measuring flux changes in a SQUID pick-up coil, which was calibrated by a novel first-principles technique. We compare our results with those of NMR experiments, and find that the two types of measurements yield different values, and this discrepancy is not the result of thermometry or field errors.

We have developed an integrated dc SQUID magnetometer with additional positive feedback (APF) for low frequency true NMR applications. The APF scheme allows direct coupled read out from the SQUID to room temperature electronics and eliminates the need for the conventional modulation scheme, thereby greatly simplifying the flux-locked loop electronics. We have configured our SQUID system for the specific needs of sensitive NMR measurements which include large bandwidth and high slew rate. We have achieved a bandwidth of 1.2 MHz and a slew rate greater than 105Φ0/s for frequencies above 10 kHz.

Conventional s-wave superconductors have order parameters with no internal degrees of freedom - they are characterized by an amplitude and a phase. All unconventional superconductors share the property of a nontrivial internal symmetry which makes their behavior more interesting and their identification more difficult. The determination of the order parameter of the longest-studied exotic superconductor, superfluid 3 He, remains inconclusive despite the existence of a Standard Model. Using superfluid 3 He as a cautionary tale, the experimental evidence for the identification of order parameters of other exotic superconductors is also examined.

Three small superconducting magnets with active and passive shields that produce confined magnetic fields in low temperature cryostats have been designed and built. The design of these magnets uses both on and off-axis field calculations which are fed back to an optimization algorithm. These design principles and construction techniques are discussed.

We have studied thin disordered metal films at low temperatures under nonequilibrium conditions. The electron temperature was raised above the lattice temperature by applying a dc bias. The Coulomb anomaly and weak localization were used as thermometers for the electrons. We confirm that at temperatures below 1 K the electron temperatures as derived from either the Coulomb anomaly or weak localization are consistent. The electron-phonon energy relaxation rate was measured to be proportional to T 3

The early studies of the superfluid phases of3He were marked by often spectacular agreements between theoretical predictions and experimental results. Nevertheless, several experiments have failed to achieve either agreement with the prevailing theory or consistency with each other — and improved experiments do not remove these discrepancies. We will discuss two recent experiments at U.S.C. which indicate that there remain fundamental questions beyond experimental uncertainties. One of these is the stability of superflow in a complex texture in the A-phase. Experimental evidence indicates that superflow may actually be stabilized by the existence of a texture. The other is the long-standing discrepancy between static (SQUID) and dynamic (NMR) measurements of the magnetization discontinuity at the A-B transition. Our recent experiments using a first principles measurement of ΔM support the continued existence of the discrepancy.

The conventional wisdom regarding the identification of the microscopic order parameter of the A phase of superfluid 3He as the axial state is questioned. Examination of current experimental evidence reveals an inability to discriminate between the alternative models of the axial and axi-planar states. The theoretical basis for the conventional wisdom is shown to disagree with experiment.

Using a sensitive differential pressure gauge, we have measured the limits to dissipationless superflow in bulk superfluid 3He-A far from stabilizing walls. We report the first quantitative measurements of the breakdown of bulk superflow in the A phase. In addition to confirming the existence of a finite critical current in bulk uniform textures, we have discovered an unexpected tortuous texture in which there is an even larger critical current. These results indicate that the current theoretical models for determining textures are incomplete.

We have measured the initial magnetic suppression of the {ital B} phase of the superfluid {sup 3}He at pressures from zero to 29 bars. This suppression, which is asymptotically quadratic in the field strength, is always significantly greater than the weak-coupling prediction, even at zero pressure. We also provide the first values of the {ital A}-phase specific-heat jump for pressures below the polycritical point. Microscopic models of {ital normal} liquid {sup 3}He do not conform to these experimental constraints. Consequently, it is possible that the conventional identification of the order parameter of superfluid {sup 3}He-{ital A} is incorrect.

Mesure de la resistance du contact thermique a travers l'interface entre PrNi 5 et 3 H e . Sur tout l'intervalle de temperatures etudiees (2-15 mK), la resistance du contact varie comme T −2 . En presence de champs magnetiques faibles (0-40 mT) la resistance du contact diminue de facon significative avec l'augmentation de l'intensite du champ. Discussion de la capacite calorifique de PrNi 5 deduite des constantes thermiques mesurees. Comparaison avec d'autres resultats

In earlier experiments on ferromagnetic 3He films, we observed a complex lineshape due in part to the dipolar field generated by polarization of the 3He nuclei. Much of the complex lineshape can be explained by the known distribution of the Grafoil platelets. However, there remained some evidence for a split NMR line at some temperatures. In our new experiments on ZYX grade exfoliated graphite where the size of individual platelets is much larger and the angular distribution is three times smaller, this splitting has become more evident over a wider range of temperatures. Now it is clear that the complex lineshape includes two peaks along with remaining orientation effects. We also find that roughly 2% of our signal comes from randomly oriented platelets. We present the details of our model for analyzing these lineshapes and the experimental results for the line splitting at several coverages in the ferromagnetic range. We discuss the possible sources of this line splitting.

What is the effect of the structural length scale on the ordering of 3He films? NMR experiments on the magnetism of second layer 3He on Grafoil in the low field limit found ferromagnetic ordering for coverages over 20 atoms/nm2. Finite temperature phase transitions are prohibited in 2D when only Heisenberg interactions are present. However ordering of a two‐dimensional magnetic film can be a result of a phase transition caused by weak anisotropy and/or dipolar interactions, or could be a less interesting manifestation of finite size effects. By replacing Grafoil with ZYX grade highly oriented graphite, we can study the magnetism of two‐dimensional films with a substantially increased structural coherence length and test the importance of finite size effects. Our new experiments find a region of coverages where the second layer 3He films become ferromagnetic at temperatures above 1 mK, with no evidence for an increased suppression of the ordering due to increasing the coherence length. We show the results ...

We provide evidence for a finite-temperature ferromagnetic transition in two dimensions as $H\ensuremath{\rightarrow}0$ in thin films of ${}^{3}\mathrm{He}$ on graphite, a model system for the study of two-dimensional magnetism. We perform pulsed and cw NMR experiments at fields of 0.03--0.48 mT on ${}^{3}\mathrm{He}$ at areal densities of $20.5--24.2\mathrm{atoms}/{\mathrm{nm}}^{2}$. At these densities, the second layer of ${}^{3}\mathrm{He}$ has a strongly ferromagnetic tendency. With decreasing temperature, we find a rapid onset of magnetization that becomes independent of the applied field at temperatures in the vicinity of 1 mK. Both the dipolar field and the NMR linewidth grow rapidly as well, which is consistent with a large (order unity) polarization of the ${}^{3}\mathrm{He}$ spins.

We provide evidence for a finite-temperature ferromagnetic transition in two dimensions as H --0 in thin films of 3He on graphite, a model system for the study of two-dimensional magnetism. We perform pulsed and cw NMR experiments at fields of 0.03-0.48 mT on 3He at areal densities of 20.5-24.2 atoms/nm(2). At these densities, the second layer of 3He has a strongly ferromagnetic tendency. With decreasing temperature, we find a rapid onset of magnetization that becomes independent of the applied field at temperatures in the vicinity of 1 mK. Both the dipolar field and the NMR linewidth grow rapidly as well, which is consistent with a large (order unity) polarization of the 3He spins.

Recent experiments at USC have shown that two-dimensional films of 3 He on graphite order ferromagnetically at finite temperatures ( T >0) for densities above 20 atoms / nm 2 . These results, obtained by measuring NMR on these films in the zero field limit, appear to contradict several studies which concluded that there is no spontaneous order by measuring magnetic susceptibility through conventional NMR in finite magnetic fields, and heat capacity in zero field. The low field limit is important in understanding these two-dimensional magnetic systems. At higher temperatures and/or fields, these films have been described in terms of the multiple spin exchange model. However, two-dimensional systems are extremely sensitive to anisotropies such as nuclear dipole interactions, even when they are several orders of magnitude less than the dominant exchange mechanism.

We use SQUID NMR to observe the magnetization of 3 He films with densities between 20 and 24 atoms / nm 2 in the zero field limit. Ferromagnetism in these nearly 2D Heisenberg exchange systems is stabilized by weak anisotropies. In the ferromagnetic phase, the NMR line becomes very broad and shifts to lower frequencies, consistent with a large dipolar field opposing a perpendicular applied field. Grafoil can be modeled by a Gaussian distribution of platelet angles centered on the normal to the Grafoil plus a randomly distributed set of platelets. Using the spin dynamics of a 2D polarized sheet with reasonable assumptions for the distributions of platelet angles, we show that the magnetization, frequency shift and line shape form a consistent picture of a highly polarized sheet.

Conflicting results between different laboratories of measurements of the phase diagram of superfluid 3 He when immersed in aerogel, even with nominally identical aerogels, suggest that details of the aerogels other than the porosity are important in determining superfluid properties. We have simulated the growth of small clusters ðNB10 5 Þ by offlattice single-particle diffusion limited aggregation (DLA). Measurements of the free-path distribution function of 3 He quasiparticles scattered by these clusters—truncated and repeated to simulate experimental conditions—show significant differences from uncorrelated scatterers at the same density, which differences may be significant for superfluid properties. r 2003 Elsevier Science B.V. All rights reserved.

We present measurements of hysteretic resistive SQUIDs (R-SQUIDs) as a function of the hysteresis parameter β L =2 πLI c / Φ 0 . Varying β L in situ from 15 to 45, we observe quasiperiodic peaks in frequency noise which decrease toward the thermal limit with increasing temperature. We have developed a model illustrating the dynamics of the two characteristic time scales: Josephson phase oscillations and catastrophic transitions in phase. From this model, an intuitive energy picture emerges wherein low thermal energy is shown to promote trapping in false minima leading to excess frequency noise. The energy model further illustrates that confusion between degenerate branch solutions results in quasiperiodic noise peaks.

We have measured the absolute magnetization of liquid 3He at saturated vapor pressure over the temperature range 0.3–500 mK. The technique compares the magnetization of the liquid to that of vacuum by passing the liquid–vacuum interface through a SQUID coil. We find that, contrary to expectations, the electronic diamagnetism of the normal liquid is approximately 6% less than the calculated atomic value. This appears to be the first measurement of the diamagnetic susceptibility of any noble gas in a condensed state.

3He films on graphite at densities above 20 atoms / nm 2 have a strong ferromagnetic tendency. Models of multiple spin exchange are believed to describe the dominant interaction. However, other much smaller interactions can significantly alter the low-temperature behavior of these systems, especially in very low magnetic fields. Our recent experiments have used a SQUID system for both pulsed and continuous wave NMR at very low applied fields (B mT ) . Our results show that the polarization of this system grows much more rapidly than is expected from the Heisenberg exchange models, even approaching saturation. Possible explanations are the influence of finite size effects and dipolar interactions.

The use of a SQUID coupled to a sample by a superconducting circuit allows NMR measurements over wide bandwidths. For our recent NMR experiments on 3He monolayers we developed a sample cell that allows for indirect cooling of the grafoil without excessive eddy currents. The details of the cell design are discussed. In addition, we have found that the 13C signal from the graphite is observable in this system. The FID time constant for this signal is comparable to the field homogeneity and the amplitude follows the Curie law over our temperature range ( 15 mK >T>0.2 mK ). Since the NMR signal in 13C is observed simultaneously with the 3He signal, this signal can act as a thermometer for the graphite substrate.

Combinations of the Landau-Ginzburg phenomenological β-parameters characterizing superfluid3He have been deduced from several thermodynamic measurements. These β-parameters impose constraints on microscopic models of normal3He, and select the structure of the microscopic order parameter in the superfluid.

The absolute thermodynamic magnetization of liquid3He has been measured from 0.5 mK–100 mK at saturated vapor pressure. The measurement uses a SQUID magnetometer and a simple displacement technique to reference the total magnetization of the liquid to an absolute vacuum level. Because the nuclear paramagnetic magnetization equilibrates on a time scale long compared to that of the core diamagnetic magnetization, we are able to separate these two signals throughout much of the normal Fermi liquid regime. The diamagnetic component dominantes the paramagnetic over the entire temperature range.

We report results from a new SQUID-based measurement of the total magnetization change of superfluid 3 He at the A-B phase transition, ΔM AB total . We used a simple first-principles calibration technique. At low fields ∼0.03 T, our data agree with past measurements, verifying the long-standing discrepancy between ΔM AB total and ΔM AB nuclear , the magnetization change which is measured by NMR. Use of a self-shielded solenoid allowed the first measurement of ΔM AB total over the full range of magnetic field. We found the magnetization discrepancy to persist through the whole phase diagram.

We have measured the A-B phase transition temperature of superfluid3He at pressures from zero to 29 bars, and in all magnetic fields up to the highest field limit of the B phase (∼0.59 Tesla). We have also devised a simple function which accurately describes the entire pressure-temperature-magnetic field phase diagram of the A-B transition. This function intrinsically satisfies all correct asymptotic behaviors in the low- and high-field limits. Using thermodynamic identities we can measure properties which previously could not be directly measured.

We are measuring the boundary between the superfluid A and B phases of 3He as a function of magnetic field and pressure. The results yield new information on Landau-Ginzburg parameters, microscopic quasiparticle scattering amplitudes, superfluid specific heats, and potentially other normal liquid properties through comparison with models of the superfluid.

A transparent replacement for the usual set of precision ratio transformers within ultralow temperature thermometry systems is described. This new device uses digital electronics to both lower costs and to provide local intelligence for greater flexibility. The device has been used in our experiments on superfluid 3He without the difficulties usually attendant with interfacing computers to ultralow temperature apparatuses.

We have searched for the pressure drops which are a necessary consequence of the dissipation attendant textural motion in 3HeA when uniform bulk superflow exceeds a critical value. We find no reliable pressure signature at a level significantly smaller than expected theoretically. We have modified our sensitive differential pressure gauge by replacing the plastic diaphragm with an extremely thin aluminum film, increasing the resolution by two orders of magnitude. Further investigations are underway.

The low-field NMR spectra of 3He boundary layers on graphite show collective modes for T

Measurements of the magnetic susceptibility ($\ensuremath{\chi}$) of quinolinium ${(\mathrm{tetracyanoquinodimethane})}_{2}$ are reported over the temperature range $0.45 \mathrm{mK}lTl40 \mathrm{mK}$ with use of low-field ESR. Down to 7 mK, it is found that $\ensuremath{\chi}\ensuremath{\propto}{T}^{\ensuremath{-}\ensuremath{\alpha}}$ with $\ensuremath{\alpha}=0.81$ characteristic of a random-exchange Heisenberg antiferromagnetic chain (REHAC). At lower temperatures, the susceptibility peaks, the ESR linewidth greatly increases, and an internal field rapidly builds up. These changes may be due to either hyperfine or dipolar interactions, signalling a crossover from one-dimensional REHAC to a three-dimensional behavior-possibly a dipolar spin-glass.

In this work we present careful measurements of the resonance lines in the A phase performed in a closed, parallel plate geometry. The linewidths are in good agreement with the recent relaxation theories, except near the transition temperature. The satellite longitudinal resonance at a frequency 1/√2 times that of the usual mode, first identified by the Orsay group, is found to depend upon geometry. Finally, the integrals of both the transverse and longitudinal resonance lines fail to satisfy the Kramers-Kronig rule for the total intensity and are too small by roughly a factor of two.

Magnetic textures have been created and studied in the A phase of superfluid 3He at several pressures, in magnetic fields from 15 to 20 mT, by intense rf magnetic pulses which invert the magnetization of the fluid. To create this new state we find that the initial large pulse must tip the magnetization by at least 140‡. In this state the fluid evolves from a condition where the NMR frequency shift Ω-γH 0is smaller than that in the unperturbed superfluid by a factor of 0.54, over a time scale of tens of seconds to a condition where the frequency shift is 0.70 times the unperturbed value. We have studied the fluid's linear response to small-angle sampling pulses as well as the nonlinear response to large-angle pulses. By using two independent NMR coils we have seen this new state move through the fluid; we find that a lower limit on its propagation velocity is around one-half the spin wave velocity. All of our results are generally consistent with the model of solitons in the order parameter proposed by Maki and co-workers.

A propagating magnetic texture has been observed in superfluid /sup 3/He-A confined to 17-..mu..m slabs using pulsed NMR. The results support a model of an expanding lattice of solitons in the order parameter. The propagation velocity approaches the spin-wave velocity as T..-->..T/sub c/, and goes to zero for T

We present experimental evidence for f-italic-wave pair interactions in superfluid /sup 3/He-A-italic. These interactions cause the frequency of the clapping mode to be significantly reduced relative to the value expected in a pure axial state, at temperatures arbitrarily close to T-italic/sub c-italic/. We find that the predicted f-italic-wave transition temperature in the absence of p-italic-wave pairing is 0.2--0.3 times the observed T-italic/sub c-italic/.

The conditions have been measured for which the texture of the order parameter of superfluid3He-A is uniform and aligned with an applied superflow. At finite superflows, a finite parallel magnetic field is required to disrupt the uniform aligned texture. The shape of the uniform texture's phase boundary in av s -H plane is similar to theoretical predictions, for which the state just outside of the phase boundary is a helical texture. However, substantial disagreements with the theory remain.

A small superconducting magnet with a large highly confined field has been designed for ultralow‐temperature cryostats. The magnet provides a central field of 3 T, with a homogeneity of 0.1% over 7.3 mm and negligible hysteresis, while the field at a radial distance of 2 cm is zero. The confinement is accomplished by using the magnet wire itself to simulate the current distribution of a perfect type I superconducting shield.

The voltage power spectrum of NbSe 3 has been studied in the upper charge-density wave (CDW) state where only one CDW exists. In the non-Ohmic regime sharp spikes appear in the spectrum. The frequency of these spikes increases monotonically with current as in the lower CDW state. We study the “fine structure” of the spectrum and find evidence for “frequency pulling” between the modes. A linear coupled-mode treatment is used to model this effect. There is also evidence of heterodyning of an intrinsic (current independent) 2MHz mode. We briefly discuss these phenomena in terms of Frohlich sliding conductivity. Difficulties with the simple two-fluid model as well as models based on solid state turbulence are pointed out.

A review of theory and experiments on flowing 3He-A is presented. We explore the question of the stability of the uniform texture against deformation in the presence of flow and magnetic fields. The texture probe used in these experiments is the attenuation of zero sound. We find a region of stability that appears to extend to greater velocities than expected. The limitations of the sound probe are discussed, and we suggest possible extensions of this work.

The low-field NMR spectra of 3He boundary layers on exfoliated graphite show collective modes for T

Production de transitions de texture par un systeme de soufflerie controlant l'ecoulement de masse. Attribution a une competition des energies magnetique, d'ecoulement et de gradient