Low Frequency Susceptibility of High-Density bcc Solid 3He in the Paramagnetic and Nuclear-Ordered States

We have measured the AC magnetic susceptibility and static magnetization of high-density bcc solid ³ He through the nuclear-ordering transition. The susceptibility in the paramagnetic state strongly depends on the frequency of the measuring field. Near the transition temperature a sharp peak in the real part of the AC susceptibility and an abrupt depression in the imaginary part are observed. The transition temperature indicated by the AC susceptibility is higher than that derived from static magnetization. We analyzed the new behavior in the susceptibility in terms of the spin relaxation between the Zeeman system and the exchange system. The relaxation time in the energy flow in the two systems is in the range of milliseconds in the paramagnetic state, and decreases drastically by two orders of magnitude in the ordered state. The relaxation time in the paramagnetic state is interpreted as due to exchange narrowing, while in the ordered state is explained to be the drift time of the spin wave limited by the size of the sample grown in the pores of the sintered silver. The ordering temperature is given as a function of molar volume in the entire range of the bcc phase.     

Concentration susceptibility of 3He-4He mixtures near the superfluid transition

Measurements of the concentration susceptibility % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabiqaaiaacaGaaeqabaWaaeaaeaaakeaacuaHhpWygaacai% abg2da9iabgkGi2kaadIfacaGGVaGaeyOaIyRaeuiLdqKaaiykamaa% BaaaleaacaWGubaabeaaaaa!3C99!\[\tilde \chi = \partial X/\partial \Delta )_T \] are reported for ³He-⁴He mixtures at saturated vapor pressure and at constant mole fraction X(³He) as a function of temperature. Here is the isotopic difference in chemical potentials. The mixtures cover the range from X = 0.60 to X = 0.677. Particular emphasis is given to the region near the lambda line for three mixtures and to the region close to the tricritical point. The method makes use of the vertical concentration gradient induced in the mixture by gravity. This difference X is measured via the dielectric constant over a height of 2 mm using a capacitance technique. The predicted peak of the susceptibility at the superfluid transition is observed and as the tricritical point is approached, this peak is progressively broadened by gravity effects. The data are compared with those from other methods and also with estimations based on calorimetric data for more dilute mixtures. The susceptibility data are transformed into results along a path at constant chemical potential . Sufficiently close to T , these can be fitted to an expression with a weakly singular term, which is consistent with the predictions on the grounds of universality. These predictions are based on the calorimetric data of more dilute mixtures. The width of this critical region is found to be consistent with theoretical estimations by Riedel, Meyer, and Behringer. The tricritical scaling scheme by these authors is tested by the new data and the resulting scaling curve is found to be in reasonable agreement with that obtained from earlier data by Goellner, Behringer, and Meyer, except in the region closest to the tricritical point. There the new data appear to be more consistent with measurements from light scattering. In addition, the concentration susceptibility for more dilute mixtures (0.05 < X < 0.4) is calculated both from calorimetric data and from saturated vapor pressure measurements and the results are found to be internally consistent.Supported by grants from the AFOSR and from the NSF.     

Magnetic Susceptibility of Liquid 3He

We report measurements of the magnetic susceptibility of liquid ³ He 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 T0. 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.     

Study of a Helium Film in 10 μm Porous Glass by Torsional Oscillator in Free Decay Mode

Dissipation and superfluid density of a thin helium film (superfluid transition at temperature T_c=0.88 K), placed in 10 m porous glass are studied as a function of oscillation amplitude or AC amplitude at fixed temperatures near superfluid transition. The measurements are performed using a high-Q torsional oscillator in the free decay mode as well as constant drive mode. Results show decreasing superfluid density with increasing velocity and non-monotonic behavior of the dissipation in the film.     

Incommensurate modulated structure in the cubic-tetragonal transition of V-Ru alloys studied by high resolution electron microscopy

The cubic-tetragonal transition of V-Ru alloys near 50 at % has been investigated by high resolution transmission electron microscopy. In the cubic to tetragonal transition process of thin foil specimens, we found an incommensurate modulated structure showing satellite spots near 1/4 0 1 1 positions in the electron diffraction patterns. This incommensurate structure is composed of many domains of the commensurate modulated structure with a pseudotetragonal cell of A=B=2(a ² + c ²)^1/2 and C=c, where a and c (1.07a) are the lattice constants of the tetragonally distorted CsCl-type structure. The modulation is interpreted in terms of the combination of dual elastic shear distortion waves nearly parallel to the 0 1 1 directions with the approximate wavelength of 22a. We propose a model for the incommensurate modulated structure.     

Structure and Magnetism of Second-Layer Solid 3He Films in the Intermediate Regime

We report a detailed investigation of the second solid layer of ³ He adsorbed on graphite in the intermediate coverage range where the magnetic susceptibility displays an evolution from antiferromagnetism to ferromagnetism. Different interpretations have been proposed for the unusual magnetic properties of this system in the narrow coverage region around =20 atoms/nm ² where the Curie temperature vanishes. Our results are consistent with the hypothesis that a first order structural phase transition occurs in this coverage range. We suggest that heat capacity data do not rule out this interpretation.     

Many-impurity effects in a weakly interacting Kondo system: Comparison of theory with experiment

The theory of the many-impurity Kondo system developed in a previous paper is coupled with a statistical model, using a phenomenological relationship between the magnetic susceptibility and the effective Kondo temperature, to obtain the magnetic susceptibility of a very dilute Kondo system well below the single-impurity Kondo temperature. We find that the magnetic susceptibility can be represented as arising from a single-impurity effect proportional to the impurity concentration c, another term proportional to c ², as well as a much smaller c ³ contribution. The coefficient of the c ² term in has an approximate Curie law behavior at sufficiently low temperatures. The detailed temperature and concentration dependence of is in very good quantitative agreement with experiment, and is the first theoretical explanation of the anomalous magnetic susceptibility for Cu-Fe below T _K. The modification of the susceptibility arises in the theory from the inhibition of the spin-compensated state by the impurity-impurity interaction. The latter renormalizes the ln T term even in the region of temperature where the system is magnetically completely disordered.Part of this work was supported by USAFOR, under contract 73-2430 during a summer visit at Yeshiva University.     

Bose fluid in restricted geometry: High-energy neutron scattering

The existence is discussed of a low temperature phase with infinite susceptibility in a Bose fluid in two dimensions and in thin films. In particular the high temperature approach to the relative phase transition is considered by a second-order theory based on the use of finite temperature Green functions. It is shown that the small momenta singularity of the momentum distribution present in the low temperature phase gives an anomalous contribution to the high-energy neutron scattering and its temperature dependence is very similar to the one expected in the case of scattering by a system with Bose-Einstein condensation and to the experimental data on bulk liquid ⁴ He. It is suggested that ⁴ He in a porous medium with an appropriate statistical distribution of pore size has an intermediate phase, characterized by an infinite susceptibility, between the high temperature normal phase and the low temperature phase with Bose-Einstein condensation.     

Metamagnetic-like transition in CeRu2Si2?

Extensive magnetization and magnetoresistivity experiments on CeRu₂Si₂ are described, with special emphasis on the metamagnetic-like transition that occurs atH*=80 kOe. By contrast to the low-field susceptibility, the differential susceptibility atH* almost diverges atT0. It is suggested that high magnetic fields (H>H*) restore a situation where the interactions between particles have collapsed. Comparison is made with other well-known examples: TiBe₂, UPt₃, TmSe, and³He.     

Electrical transport and semiconductor-semimetal transition in LaVO4

The electrical conductivity (), and thermoelectric power (S) (300 to 1200 K), magnetic susceptibility () (300 to 900 K) and DTA and TGA (300 to 1200 K) together with X-ray diffraction studies are reported. At room temperature the lattice is orthorhombic. The DTA study shows a broad peak around 1173 K indicating a possible structural phase transition of the compound. Practically no weight loss was observed in TGA from 300 to 1200 K. The electrical conductivity exhibits an exponential increase up to 855 K and between 855 and 885 K, it then jumps by a factor of 5×10⁴ and remains practically constant up to 1200 K, indicating a transition from semiconducting to the metallic state around 870 K. S values remain positive throughout the temperature range studied indicating electrons to be the main charge carrier. In the semiconducting state (300 to 855 K) the plot of S against T ^–1 is linear with a slope of 0.04 eV against the activation energy of 0.83 eV. It has been shown that conduction is due to hopping of holes from V⁴⁺ defect centres to V⁵⁺ normal sites. The defects exist due to a small oxygen deficiency. The semiconductor semi-metal transition appears due to the overlapping of vanadium 3d and oxygen 2p bands around 870 K.     

Magnetic Susceptibility of 3He nano-Clusters

Using pulsed NMR, we have measured the magnetic susceptibility versus temperature of ³ He nano-clusters embedded in a matrix of ⁴ He, following phase separation of the mixture. Measurements were made in the temperature range from 2 mK to 35 mK at three different pressures, 3.647 MPa, 3.410 MPa and 3.075 MPa, for which the samples remain all solid, undergo partial melting, or are separated as a liquid, respectively. For 3.410 MPa, for which partial melting occurs near 15 mK, measurements up to 35 mK suggest a positive Weiss , which would indicate a ferromagnetic tendency.     

Self-Organized Superfluid States in Gravity and Heat Flow

When ⁴ He is heated from above near the superfluid transition, there can appear a self-organized region, either in normal fluid or superfluid, with the temperature gradient equal to the transition temperature gradient. When it is in a superfluid state, there can be two regimes. Regime M is realized relatively far from the superfluid transition, where thermal resistance due to vortices is described in terms of the conventional Gorter–Mellink mutual friction. In regime G vortices are densely generated, where the line density in units of ^–2 is much larger than in any other previous experiments. Such a self-organized superfluid can coexist with a normal fluid or a superfluid containing only a small number of vortices in a dynamical steady state.     

NMR studies on nuclear ordering in metallic copper below 1 ΜK

The nuclear spin system of metallic copper in a poly crystalline sample consisting of thin wires has been studied using a cryostat in which a dilution refrigerator and two copper nuclear stages operate in series. The spin system was cooled to 50 nK, which is an all-time low-temperature record. The NMR absorption curves were measured in external fields from zero to B _ext = 15 mT and at entropies corresponding to polarizations up to 0.9. The line shapes reveal the effect of indirect exchange interaction corresponding to an antiferromagnetic value of _j J _ij /²\ ² ₀ = –0.42 ± 0.05. This result was obtained both from the interference of the isotopic absorption lines at 15 mT and from the shift of the second harmonic in fields below 2 mT. Thus, an antiferromagnetic transition in zero field at about 200 nK is expected. The Curie—Weiss points to a transition temperature of 150 nK; the inverse static susceptibility vs temperature curve clearly predicts antiferromagnetism. However, no evidence of such a transition was found in NMR spectra measured with the excitation transverse to the copper wires, although the entropy was reduced to about 0.45 R ln 4, with the corresponding temperatures well below 100 nK. On the other hand, in measurements during which the excitation was parallel to the wires, a clear saturation of susceptibility to a constant value could be seen as a sudden change in the apparent relaxation rate at the lowest entropies.Work supported by the Academy of Finland.On leave of absence from Centre d'Etudes Nucléaires de Saclay, Gif-sur-Yvette, France.     

Order of the PP-HFP transition in solid3He

New measurements of the melting pressure of solid³He P_m(T,B) have been made through the paramagnetic phase-to-high field phase transition. Temperatures were determined with Pt NMR and the³He melting pressure in zero field measured in a second cell. In a field of 0.405 T, we find a change in entropy through the HFP-PP transition of S/Rln2=0.13 over a temperature interval of only about 4 µK. Thus, we conclude that the transition is first-order at low fields. The entropy discontinuity decreases with field and the transition region broadens, with the transition possibly becoming second-order above a tricritical point near 0.6 T.     

Gravitational Effects on Nonlinear Heat Transport Near the Superfluid Transition in 4He

We have recently observed nonlinear heat transport within 30 nK of the superfluid transition temperature using heat flux, Q, in the range 0.1 < Q < 2 erg/(s cm ² ). While Haussmann and Dohm (HD) accurately predict the initial departure of the thermal conductivity, , from the linear response region, is greater than expected very close to T . We anticipate that the nature of the thermal conductivity's nonlinearity may depend upon Earth's gravity in the low heat flux limit (Q < 0.5 erg/(s cm ² )). Comparison of our data to similar data to be taken in a microgravity laboratory will provide an experimental determination of the effect of gravity on nonlinear heat transport near the superfluid transition. The microgravity measurements will also permit the first experimental test of theories that do not consider gravitational effects, such as those by HD.     

Electrical conduction in γ-irradiated and unirradiated zinc-iron ferrites

The electrical conductivity of -irradiated and unirradiated finely-divided spinels of composition (Zn _x ²⁺ Fe _1–x ²⁺ Fe ₂ ³⁺ )O ₄ ^2– was studied in a nitrogen atmosphere as a function of temperature. The results of both -irradiated and unirradiated ferrites with compositions x 0.79 showed that the electrical conduction occurred by fast electron exchange amongst Fe²⁺ and Fe³⁺ ions situated on octahedral sites in the spinel lattice, by a hopping mechanism. For composition x > 0.79 it was found that the transition of the charge carriers through cation vacancies is the predominant mechanism. Some breaks in the conductivity-temperature curves were found near the Curie points. Seebeck voltage measurements showed that -irradiated and unirradiated spinels behave as n-type semiconductors. The effect of -irradiation on the conductivity, activation energy, charge carriers and the conduction mechanism is discussed.     

The effect of oxygen and lead doping on electrical and optical properties of highT c GdBa2(Cu1−x Pb x )3Oδ compound superconductors

A systematic study of the electrical and infrared properties of the high critical temperature,T _c , compound superconductor GdBa₂(Cu_1–xPb_x)₃O has been performed. Resistivity and a.c. susceptibility measurements show a significant drop inT _c when copper is replaced by lead in the range 0.005 x 0.025. Infrared measurements show that the conductivity of the compound increases with the increase in lead concentration. For undoped samples, all the Restrahlen vibrations, common to these materials, are present in the infrared reflectance spectra. As the concentration of lead is increased, a shift of the phonon modes, as well as broadening of the 150 cm^–1 phonon mode, is observed. Also the Restrahlen vibration around 410 cm^–1 present in the normal material starts to disappear. This 150 mode is attributed to a change in the oxygen concentration, and is shown to be correlated with the critical temperature.     

Coupling of order parameter collective modes to spin density in3He-B

We derive a general expression for the dynamic spin susceptibility of³He-B which is valid for all magnetic fields. The coupling of real and imaginary modes by particle-hole asymmetry is taken into account. Then we calculate the contribution of the mode at frequency =2 – 1/4 ( is the effective Larmor frequency) to the transverse susceptibility. The spectral weight of this mode in magnetic resonance absorption is proportional to (/)^1/2 (–)², where and are particle-hole asymmetry parameters. From the experimental coupling strength of the real squashing mode to sound we estimate (–)²10^–4. The dynamic susceptibility satisfies the sum rules of Leggett. Finally we point out the difficulties in calculating the transverse NMR frequency of³He-B. These difficulties arise from theS _z =0 Cooper pairs and from the coupling ofJ _z =±1 modes forJ=1 andJ=2.     

Magnetic Ordering in 3He Nano-Clusters

Simultaneous measurements of magnetic susceptibility from 0.5 to 10 mK and pressure from 2.88 to 3.54 MPa have been made in ³ He nano-clusters embedded in a ⁴ He matrix, following phase separation. The susceptibility of the 3.54 MPa, all-solid sample behaves similarly to that of bulk ³ He for v=21.3 cm ³ /mole, with a Weiss constant =–250K. For the 2.88 MPa, liquid-droplet sample, =140K, indicating a ferromagnetic tendency, similar to 2-D films at some coverages. At intermediate pressures, has a peak near 1.05 mK, but without a discontinuity. For all samples, had a solid-like contribution to the lowest temperatures. Magnetic ordering in nano-clusters appears to be different than the U2D2 phase of bulk ³ He.     

Mutual friction and vortex motion near the superfluid transition

The mutual friction parametersB and B for a moving vortex are calculated near the superfluid transition. They are proportional to the kinetic coefficient associated with the order parameter and, asT , diverge as (T – T)^–1/3, in agreement with experiment. The nonlinear couplings between the order parameter and the entropym, both the reversible one and the one in the free energy, are found to be crucial in the mutual friction near the point. These couplings were neglected in a previous paper by the author. First, the reversible coupling in the dynamic equations makes the chemical potential deviation long-ranged and causes the dissipation to take place only near the vortex core. Second,B can diverge asT T only in the presence of the coupling of the formm||² in the free energy. Thus theE model of Halperin et al., where the latter coupling is absent, cannot explain the critical anomaly ofB. The helical mode of a single vertex line is also investigated and its dispersion relation is found to be quite different from that at low temperatures. This mode has the same time scale as that of the second-sound mode when the wave vectors are of the order of the inverse correlation length, thus obeying the usual dynamic scaling law. The time correlation functions of the displacement fluctuations of a vortex line are also obtained. The force acting on a moving vortex is calculated and is found to be equal to the classical Magnus force.