Pressure anomaly near the triple point on the magnetic phase diagram of bcc solid3He

The nuclear magnetic phase diagram of bcc solid ³ He has been determined by pressure measurements. We have observed a pressure jump at the transition from the high field phase to the paramagnetic phase (HP) near the triple point. This jump indicates that HP is first order.     

Frequency dependence of AC susceptibility of bcc solid3He

We have measured AC susceptibility of bcc solid³ He in a two orders of magnitude with a wider range than in the previous study. For the sample of 23.0 cm³/mol, the relaxation time between the Zeeman and exchange reservoirs in the paramagnetic state is on the order of 10 ms, while the relaxation time in the ordered state decreases down to one hundredth of the value in the paramagnetic state. This small relaxation time is related to the process of spin wave relaxation in the ordered state.     

Moment calculation of vacancy-induced spin diffusion in bcc solid3He

The self-diffusion constant in bcc solid ³ He is calculated in the temperature regime where an atom changes place with a neighboring vacancy more often than flip-flops are induced through the Pauli exchange interaction. The calculation is based on the method of moments and describes vacancies with a model of fictitious spinS=1. The scattering of phonons by vacancies impedes the quantum tunneling of vacancies: the quantitative discussion of this effect shows that its relation with the lifetime of vacancy waves is not simple.     

Isentropic Pressure of Magnetically Ordered bcc Solid 3He Near the Triple Point

We have studied the thermodynamics of magnetically ordered bcc solid ³He near the triple point, where the high-field phase (HFP), low-field phase (LFP), and paramagnetic phase (PP) coexist, by using isochoric pressure measurements along isentropes. The experiment gives the first unambiguous proof that the transition from the paramagnetic phase to the high-field phase is first order. It also shows that Δv _h/Δv _l>Δs _h/Δs _l at the triple point, where Δv _{h, l} is the molar-volume difference between the HFP (LFP) and the PP, and Δs _{h, l} the corresponding molar entropy difference.     

Dislocation Motion in bcc Solid 3He

We have measured the shear modulus of bcc solid ³ He at 827 Hz using the high- Q torsional oscillator technique. We observed a decrease in the shear modulus from the low-temperature value G₀ to about 0.4G₀ at a molar volume of 24.80 cm ³/mole and to 0.6G₀ at 24.32 cm ³/mole near the melting point. The reduction of an effective shear modulus is explained by the dislocation theory of Granato and Lücke.     

Magnetic Phase Transitions in bcc 3He

We study magnetic phase transitions in the bcc solid ³He by means of Monte Carlo simulations. We employ a classical spin model on the bcc lattice with multiple ring-exchange interactions. In the present study, we take into account up to 4-spin ring exchanges. In order to clarify the character of phase transitions we examine energy histograms generated by Monte Carlo simulations. It is shown that the phase transition between the cnaf and the paramagnetic phases is discontinuous at low magnetic fields while it is continuous at high magnetic fields. This result agrees qualitatively with experimental observations.     

Isobaric vacancy formation volume in bcc 3He

The value of vacancy volume formation are obtained in bcc crystall ³ He at the temperature 0.5-0.66 K, and at molar volume 24,20 - 24,72 cm ³ /mole range. This data are obtained by measuring the pressure dependence of self-diffusion coefficient. The volume of formation is in agreement with the values calculated from theory, under assuming of existing the wide-zone tunneling motion in bcc ³ He.     

Specific Heat Anomaly in bcc Solid 3He

We have studied the origin of the excess specific heat (anomaly) above 10 mK in bcc solid ³ He near melting pressure. We applied strong magnetic fields to the sample to see whether the anomaly arises from spin polarons due to vacancies. The specific heat is the same before and after applying magnetic fields of 10-12 T. This result possibly indicates that the anomaly arises from the origin different from vacancies. Next, in order to check whether the anomaly comes from the surface magnetism, we measured the specific heat by coating the surface of sintered silver with three layers and two layers of ⁴ He. The results showed that unexpected large heat capacity due to phase separation of solid ³ He-⁴ He surpassed and smeared the original specific heat anomaly. We are investigating the origin of the anomaly further.     

Pinning of Dislocation Motion in bcc Solid 3He

We have measured the dissipation of dislocation motion in bcc solid ³ He using the high-Q torsional oscillator technique at 1079 Hz. We observed a broad maximum in the temperature dependence of the dissipation. The maximum of the dissipation can be explained by the theory of Granato and Lucke in which the dislocation mobility depends upon the interactions of dislocations with point defects. ⁴ He impurities tend to bind to the dislocation lines at low temperatures and pin this dislocation motion. The maximum of the dissipation corresponds to the depinning of the dislocation motion. From the amplitude dependence of the depinning temperature we first obtained the activation energy of 1.03 K of the impurity ⁴ He atom trapped on the dislocations in bcc solid ³ He at a molar volume of 24.30 cm ³ /mol. The activation energy of the impurity atom in bcc ³ He was found to be larger than the value of 0.7 Kin hcp ⁴ He.     

Consistent low-field picture of bcc3He exchange

Consideration of a physically plausible potential for bcc solid³He suggests that there is significant antiferromagnetic pair exchange between next nearest neighbors and that triple exchange is less important than previously believed. Experimental consequences of such a thesis are consistent with all existing low magnetic field data.     

Pinning Mechanism of the Dislocation Lines in bcc Solid 3He

The strain amplitude dependence of the energy dissipation and the shear modulus at several temperatures are measured in bcc solid ³ He and the results are compared with the model of a pinned dislocation loop of Granato and Lücke. In spite of the pure solid, the strong amplitude dependence of the dissipation is found in bcc solid ³ He by the high-Q torsional oscillator measurements, in contrast with the result of the ultrasonic and the torsional-oscillator measurements in hcp solid ⁴ He. In pure solids, the loop length of the dislocations cannot be limited by impurities. In order to clarify a pinning mechanism, the loop length and the network length are obtained from the experimental result by using the model of Granato and Lücke. Where, the loop length is determined by the pinning centers, and the network length is limited by the dislocation network nodes. It is found that the temperature dependence of the loop length is the same as that of the network length. This result shows that the pinning centers are the network nodes of the dislocation lines in ultra pure bcc solid ³ He.     

Spin dynamics and onset of Suhl instability in bcc solid3He in the nuclear-ordered U2D2 phase

Pulsed NMR experiments have been performed on U2D2 solid³He with a single domain in high fields where the Larmor frequency was much larger than the zero-field antiferromagnetic resonance frequency. The free induction signal decayed rapidly under certain conditions. The rapid decay is attributed to the onset of the instability of the uniform precession. We propose a model for the instability due to the self-induced emission from the upper-mode magnon to the lower-mode magnon which is similar to the Suhl instability in electronic ferromagnets. A large negative frequency shift was observed during the rapid decay, which is consistently explained by the model. Under stable conditions of the spin motion outside of the instability region, we observed the tipping-angle-dependent frequency shift and multiple spin echoes, both of which agree well with Namaizawa's theory.     

Nuclear relaxation in bcc3He for large molar volumes

We report the results of measurements of longitudinal and transverse nuclear relaxation timesT ₁ andT ₂ in bcc³He at 300 mK and at a Larmor frequency of 3 MHz for molar volumes 22.6V24.9 cm³. It is found, even in this extended range of molar volumes, that the relaxation times increase withV as , with ₁=19±1 and ₂=18±1.5. This molar volume dependence has also been found in various thermodynamic data, including recent results of pressure measurements through the nuclear ordering temperature. This universality in molar volume dependence remains unexplained within the context of the present models of multiple exchange.     

3He impurity excitations in solid4He

The problem of³He mobility in dilute³He-⁴He solid mixtures has been treated both theoretically and experimentally. The Hamiltonian of the system can be reduced to the Hamiltonian of strongly interacting impurity quasiparticles corresponding to some time-averaged states. The experiments carried out on solid mixtures with³He concentration 2.17%, 0.75%, 0.25%, or 0.092% by the NMR method show that at sufficiently low (T<1.2 K) temperatures the diffusion coefficient becomes temperature independent and inversely proportional to³He concentration. These results substantiate qualitative conclusions of the theory. Analysis of the experimental data makes it possible to take into account the phonon part of the³He diffusion coefficient and to obtain, up to a constant, the³He-⁴He exchange energyJ10^–7 K. All the facts mentioned above testify to the substantially quantum nature of the³He diffusion process in dilute³He-⁴He mixtures.     

Pressure measurement during nuclear demagnetization of BCC and HCP solid3He

Direct nuclear demagnetization is used to cool high density solid³He into the magnetically ordered phases. The pressure change in the samples is detected by a capacitive strain gauge. The entropy of the samples is removed by precooling to a temperature of 0.5 mK in a field of 2.5 T. For a bcc sample with a molar volume of 20.10 cm³/mol, two magnetic transitions have been observed. A similar experiment on an hcp sample with a molar volume of 19.65 cm³/mol is being done. Preliminary data indicates no abrupt change in the pressure as observed in the bcc sample.     

Path-Integral Monte Carlo Study of Phonons in the bcc Phase of 3He

Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate the dynamic structure factor of solid ³He in the bcc phase at a finite temperature of T = 1.6 K and a molar volume of 21.5 cm³. From the single phonon dynamic structure factor, we obtain both the longitudinal and transverse phonon branches along the main crystalline directions, [001], [011] and [111]. Our results are compared with other theoretical predictions and available experimental data.