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We report the development of a millimeter-wave electron-spin-resonance (ESR) measurement system at the University of Fukui using a 3He/4He dilution refrigerator to reach temperatures below 1 K. The system operates in the frequency range of 125–130 GHz, with a homodyne detection. A nuclear-magnetic-resonance (NMR) measurement system was also developed in this system as the extension for millimeter-wave ESR/NMR double magnetic-resonance (DoMR) experiments. Several types of Fabry–Perot-type resonators (FPR) have been developed: A piezo actuator attached to an FPR enables an electric tuning of cavity frequency. A flat mirror of an FPR has been fabricated using a gold thin film aiming for DoMR. ESR signal was measured down to 0.09 K. Results of ESR measurements of an organic radical crystal and phosphorous-doped silicon are presented. The NMR signal from 1H contained in the resonator is also detected successfully as a test for DoMR.

We have developed a millimeter-wave electron-spin-resonance (ESR) measurement system using a 3He-4He dilution refrigerator for the ultralow-temperature range below 1 K. The currently available frequency range is 125–130 GHz. This system is based on a Fabry-Perot-type resonator (FPR) that is composed of two mirrors. The frequency can be changed by adjusting the distance between the mirrors using a piezoelectric actuator installed at the bottom of the resonator. A homodyne detection system with an InSb detector is built into the low-temperature section of the 3He-4He dilution refrigerator; this system provides high sensitivity. Using this system, we performed ESR measurements on a Heisenberg quantum-spin chain—copper pyrazine dinitrate, Cu(C4H4N2)(NO3)2—over the temperature range from 6.6 down to 0.25 K. The ESR lines change continuously with decreasing temperature. Our results suggest that the ESR spectrum of copper pyrazine dinitrate may be useful as a temperature sensor for the very low-temperature range.

We have developed a millimeter-wave electron spin resonance (ESR) measurement system using a 3He- 4He dilution refrigerator for the ultralow-temperature range below 1 K. The currently available frequency range is 125–130 GHz. The frequency can be changed by adjusting the distance between the mirrors using a piezoelectric actuator. Using this apparatus, we performed ESR measurements on a Heisenberg quantum-spin chain - copper pyrazine dinitrate (CuPzN for short), Cu(C 4 H 4 N 2 )(NO3)2 - over the temperature range from 6.6 down to 0.25 K. The ESR lines shift continuously with decreasing temperature. The results suggest that the ESR spectrum of CuPzN is possibly useful as a temperature sensor below 1 K.

Efficient manipulation of nuclear spins is important for utilizing them as qubits for quantum computing. In this work we report record high polarizations of ${}^{31}\mathrm{P}$ and $^{29}\mathrm{Si}$ nuclear spins in P-doped silicon in a strong magnetic field (4.6 T) and at temperatures below 1 K. We reached ${}^{31}\mathrm{P}$ nuclear polarization values exceeding $98%$ after 20 min of pumping the high-field electron spin resonance (ESR) line with a very small microwave power of 0.4 $\ensuremath{\mu}\mathrm{W}$. We evaluate that the ratio of the hyperfine-state populations increases by three orders of magnitude after 2 hours of pumping, and an extremely pure nuclear spin state can be created, with less than 0.01 ppb impurities. A negative dynamic nuclear polarization has been observed by pumping the low-field ESR line of ${}^{31}\mathrm{P}$ followed by the flip-flip cross relaxation, the transition which is fully forbidden for isolated donors. We estimate that while pumping the ESR transitions of ${}^{31}\mathrm{P}$ also the nuclei of $^{29}\mathrm{Si}$ get polarized, and polarization exceeding $60%$ has been obtained. We performed measurements of relaxation rates of flip-flop and flip-flip transitions which turned out to be nearly temperature independent. Temperature dependence of the ${}^{31}\mathrm{P}$ nuclear relaxation was studied down to 0.75 K, below which the relaxation time became too long to be measured. We found that the polarization evolution under pumping and during relaxation deviates substantially from a simple exponential function of time. We suggest that the nonexponential polarization dynamics of ${}^{31}\mathrm{P}$ donors is mediated by the orientation of $^{29}\mathrm{Si}$ nuclei, which affect the transition probabilities of the forbidden cross-relaxation processes.

We report NMR measurements of transverse relaxation rate 1/T 2 of the 31P nuclear spins in metallic Si:P (concentration of dopant P, n=18×1018 and 56×1018 cm−3) at temperatures between 45 mK and 5 K in a magnetic field of 7 T. Above 1.4 K, 1/T 2 is constant independent of temperature as well as concentration and is determined by magnetic dipolar interaction between the 31P and 29Si nuclear spins. As temperature decreases below 1.4 K, 1/T 2 increases over the dipolar-determined value by an order of magnitude and levels off around 0.6 K. The concentration dependence of 1/T 2 at low temperatures suggests that 1/T 2 below 1.4 K is determined by the coupling between the 31P nuclear spins. We understand 1/T 2 at low temperatures originates from the RKKY interaction. We explain the temperature dependence of 1/T 2 between 0.6 K and 1.4 K by the motional-narrowing expression of 1/T 2 with a temperature-dependent correlation time of the fluctuating local field due to the RKKY interaction.

Superfluid 3 He A-like and B-like phases in 98% aerogel have been studied under rotation up to 2π rad/s by using cw-NMR and Homogeneous Pressing Domain (HPD) NMR at 29 mT and 3.0 MPa. We studied the dissipation of the HPD in aerogel and found that the HPD-mode was destroyed by onset of the thermal runaway for sufficient large sample under a large field gradient. From the field dependence of the onset of thermal runaway, we estimated dissipation of the HPD in aerogel and compared with that of the bulk B-phase. When angular velocity of rotation goes beyond Ω c , vortices are introduced and pinned. We estimate roughly the excess dissipation due to vortices and compare it with that of vortices in bulk B-phase.

We have measured the temperature and magnetic-field dependences of the sound velocity for one longitudinal and two transverse waves in the low field phase (LFP) and the high field phase (HFP) of nuclear spin ordered bcc solid 3He crystals with a single magnetic domain along the melting curve. From sound velocity measurements for various crystal orientations as a function of the sound propagation direction, we determined the elastic stiffness constants, cij(T,B). In the LFP with tetragonal symmetry for the nuclear spin structure, we extracted six nuclear spin elastic stiffness constants Δcijl(T,0.06 T) from the temperature dependence of the sound velocity at 0.06 T and Δcijl(0.5 mK,B) from the magnetic-field dependence of sound velocity at 0.5 mK. In the HFP with cubic symmetry for the nuclear spin structure, we extracted three Δcijh(T,0.50 T) at 0.50 T and Δcijh(0.5 mK,B) at 0.5 mK. At the first-order magnetic phase transition from the LFP to the HFP at the lower critical field Bc1, large jumps in sound velocities were observed for various crystal directions and we extracted three \(\Delta c_{ij}^{\mathrm{total}}|_{B_{c1}}\) . Using the thermodynamic relation between Δcij and the change in the internal energy for the exchange interaction in this system, ΔUex(T,B), Δcij are related to the generalized second-order Gruneisen constants ΓijX≡∂2ln X/∂ei∂ej as Δcij(T,B)=ΓijXΔUex(T,B), where X represents some physical quantity which depends on the molar volume and ej is the j-th component of a strain tensor. In the LFP, the Δcijl(T,0.06 T) were proportional to T4, and Δcijl(0.5 mK,B) were proportional to B2. We extracted \(\Gamma_{ij}^{s^{\ell}}\) for the spin wave velocity in the LFP, sl, from Δcijl(T,0.06 T) and \(\Gamma^{1/\chi^{\ell}}_{ij}\) for the inverse susceptibility, 1/χl from Δcijl(0.5 mK,B). In the HFP, Δcijh(T,0.50 T) were proportional to T4 and Δcijh(0.5 mK,ΔB) were proportional to ΔB(≡B−Bc1). We obtained \(\Gamma _{ij}^{s^{h}}\) for the spin wave velocity in the HFP, sh, from Δcijh(T,0.50 T) and \(\Gamma^{B_{c1}}_{ij}\) for Bc1 from Δcijh(0.5 mK,ΔB). The values obtained for \(\Gamma_{ij}^{s^{\ell}}\) and \(\Gamma _{ij}^{1/\chi^{\ell}}\) were compared with the Multiple Spin Exchange model (MSE) with three parameters by using analytic expressions for sl and χl. The three-parameter MSE does not agree with the observed Δcij in the LFP.

A microwave single-photon detector was developed with highly-excited alkaline Rydberg-atoms in a cooled resonant cavity to search for dark matter axions. This detector belongs to a microwave single-photon counter, thus being free from the standard quantum limit (SQL). High sensitivity of the present detector system was demonstrated by measuring the thermal blackbody radiations in the cavity at temperatures as low as 70 mK where the sensitivity is below the SQL. The detection sensitivity of the present system is mainly limited by stray electric fields present in the detection region. Practical design of a new experimental scheme with a guiding electric field through the atomic-beam trajectory is here presented and discussed to avoid the effect of stray electric field and thus to improve the detection sensitivity.

The orientation of the order parameter in the A-like and B-like phases of superfluid 3He immersed in uniaxially compressed aerogel is reported. With the use of NMR methods, it is found that the orbital momentum of the A-and B-like phases is oriented along the deformation. In the A-like phase, a relatively narrow NMR line with an anomalously large negative frequency shift is observed. The Leggett frequency in the A-like phase, which shows the same energy gap suppression as in the B-like phase, is measured.

Frost heave phenomena have been studied in 4He on porous vycor glass, in which 4He in the pores remained supercooled fluid below the bulk melting temperature, Tm. When we cool a bulk solid at T below Tm on the vycor, the bulk solid sucks the supercooled liquid in the pores and grows. We measured the maximum frost heave pressure over bulk melting pressure, Pm, as a function of ΔT=Tm−T. When temperature was suddenly lowered, the frost heave pressure increased in time to a next equilibrium pressure and we measured the time constant and derived the frost heave rate. The frost heave rate was measured as a function of temperature and decreased very rapidly as temperature was lowered. We propose models to explain the mass transport in solid either by vacancy or by amorphous solid between bulk solid 4He and vycor. From measured temperature dependence of the rate in comparison with our model, we conclude the frost heave rate is determined by mass flow in solid 4He due to thermally-activated vacancy diffusion.

Superfluid 3He A-like and B-like phases in 98% aerogel under rotation up to 2π rad/sec have been studied by cw-NMR and Homogeneous Precessing Domain (HPD) NMR at 29 mT and 3.0 MPa. In the A-like phase, a large negative frequency shift was observed in the cw-NMR spectrum and no effect of rotation on the spectrum was found. In the B-like phase, the spin wave spectrum was observed when the sample was cooled through Tc under rotation and was affected by the counter flow due to the trapped vortices. The HPD was excited in a part of the aerogel sample but introduction of vortices into the sample by rotation or trapped vortices destroyed the HPD.

We demonstrate an efficient control of $^{29}\mathrm{Si}$ nuclear spins for specific lattice sites near $^{31}\mathrm{P}$ donors in silicon at temperatures below 1 K and in a high magnetic field of 4.6 T. Excitation of the forbidden electron-nuclear transitions leads to a pattern of well-resolved holes and peaks in the electron spin resonance (ESR) lines of $^{31}\mathrm{P}$. The pattern originates from dynamic polarization (DNP) of the $^{29}\mathrm{Si}$ nuclear spins near the donors via the solid effect. DNP of $^{29}\mathrm{Si}$ is demonstrated also with the Overhauser effect where the allowed ESR transitions are excited. In this case mostly the remote $^{29}\mathrm{Si}$ nuclei having weak interaction with the donors are polarized, which results in a single hole and a sharp peak pair in the ESR spectrum. Our work shows that the solid effect can be used for initialization of $^{29}\mathrm{Si}$ nuclear spin qubits near the donors.

We measured the sound velocity of nuclear-ordered bcc solid 3 He in the low field phase (U2D2), and the high field phase (CNAF) along the melting curve. Previously we reported a temperature dependence of the sound velocity in a single magnetic domain of U2D2 3 He at almost zero fields. We extended this measurement and obtained a field dependence in both the U2D2 and the CNAF phases to further investigate the nuclear magnetism of nuclear-ordered bcc solid 3 He by using the multiple-spin ring exchange model. The sound velocity varied as B 2 in the U2D2 phase and had a large jump at the phase transition, and then gradually increased as field increased in the CNAF phase. These results are attributed to the nuclear-spin part of the internal energy.

We have constructed an Ultra Low Temperature cryostat for Magnetic Resonance Imaging (ULT-MRI) applicable for various condensed phases of 3He at ultra low temperature, such as Nuclear Ordered Solid 3He or superfluid 3He. In contrast to conventional MRI for general use, where a pulsed-field gradient method is commonly used, we used a steady-field gradient method to avoid eddy current heating to metal parts at ultra low temperature. To avoid nonlinear response from the nuclear ordered spin system, which contradicts a principle of conventional MRI, we employed Free Induction Decay (FID) after a small tipping angle RF pulse to obtain the spectrum.

Dynamics of the 1st order phase transition between the U2D2 and the high field phases (HFP) of nuclear ordered solid 3He was studied by field-cycling method between the two phases. Single crystals of U2D2 3He were grown at the bottom of the compressional cell in superfluid 3He-B at about 0.5 mK. The domain distribution in the U2D2 crystal was determined by ULT-MRI (Ultra Low Temperature − Magnetic Resonance Imaging)1. We have measured evolution of the HFP after the static magnetic field was suddenly changed to B = BC1 + ΔB through the lower critical field BC1. The HFP developed exponentially in time and showed two stage evolution with a fast stage rate and a slow stage rule. We observed that the fast stage rate increased with ΔB. Similar behavior was observed in the reversed phase transition. MRI measurements indicate that this phase transition occurs, at least for small ΔB, by the heterogeneous nucleation mechanism, where new seeds appear at the liquid-crystal interface.

Rotational effects on textures of superfluid 3 He in aerogel with 98% porosity at a pressure 3.0 MPa were investigated by cw-NMR measurement at 700 kHz (H 0 =22 mT) under rotation up to 2π rad/s. At rest, the superfluid transition to the A phase occurred at T acrogcl c=2.07 mK and the A phase was supercooled down to T acrogel A→B =1.73 ∼ 1.80 mK and became the B phase in the cooling process. In the warming process, the B phase was superheated up to T acrogel c. In the B phase, a new peak appeared in the NMR spectrum by rotating the sample. The intensity of this peak increased as the rotation speed increased almost linealy to Ω and started to be saturated for Ω > Ω c . We attributed the new peak to the textural change caused by the counter flow and the onset of the saturation at Ω c to the onset of vortex nucleation in aerogel. On deceleration, the peak intensity decreased and disappeared at Ω = Ω v . Further decreasing Ω, the peak intensity increased even at Ω = 0. The counterflow peak observed at Ω = 0 indicates the existence of persistent current induced by pinned vortices in aerogel. In the A phase, we did not find any noticeable change in the NMR spectrum under the rotation speed up to 2π rad/s, or by cooling through T c with or without rotation. We concluded that the l texture in the A phase was strongly pinned to aerogel. No spin wave satellite signal localized at a soft core vortex was observed in contrast to the bulk A phase.

Two new high-speed rotating cryostats at the ISSP, University of Tokyo, capable of operating at mK temperatures, are currently employed for studies of superfluidity and quantized vortex states in 4 He and 3 He . A stable high-Q torsional oscillator is used for the study of superfluidity in low-density 4 He films adsorbed on 3-D connected porous substrates. At Tc, the critical phenomena of 3-D Bose superfluids is seen. Under rotation a new type of vortex state is observed. High-resolution NMR studies of superfluid 3 He , contained in capillary arrays of two different diameters, are made as a function of rotational speed. In these experiments, single vortex intrusion events as well as texural modification under rotation are clearly resolved.

We have developed a millimeter-wave electron-spin-resonance (ESR) system for very low temperatures (T < 1 K) that can be employed for nuclear-magnetic-resonance measurements by using dynamic nuclear polarization. The system uses a Fabry-Perot resonator that works in the frequency range of 125 – 130 GHz and covers the temperature range of 0.09 – 6.5 K. We have performed ESR measurements in the frequency around 128 GHz by using Mn x Mg1-x O (x = 1.0 × 10-4) and free-radical samples of 1, 1-diphenyl-2-picrylhydrazyl (DPPH), because these samples have been proposed as field and sensitivity markers. Temperature dependence of the ESR signal intensity for Mn x Mg1-x O shows anomalies originating from magnetic order are found around 3.5 – 4 K. We estimate the sensitivity of the system for ESR detections to be 6 × 1013 spins/G at 5.8 K. Because DPPH shows no observable shift in the magnetic field, we propose it as a useful standard marker for ESR measurements at very low temperatures.

We measured crystal growth and melting rate of nuclear-ordered solid 3He in a rectangular sample cell. The melting rate was much faster than the growth rate at all temperatures in the low field phase (U2D2) and strongly depended on temperature. The melting occurred on a rough surface and the dissipation mechanism of the melting was compared with the surface magnon scattering mechanism. The crystal growth rate did not depend on temperature and was compared with the spiral growth model associated with screw-dislocations. We found that a large chemical potential difference started to develop when the crystal grew large enough so that the sample became almost single domain at the upper part of the sample cell. We attributed this saturation to the pinning of the screw dislocation on the sample wall. This chemical potential difference decreased rapidly when the crystal was in the high field phase and the crystal grew as it did before the saturation. The crystal continued to grow even after the crystal returned to the U2D2 phase from the high field phase.

We cooled down the two-dimensional (2-D) spin-polarized atomic hydrogen (H↓) adsorbed on a small surface called cold spot, whose temperature was kept much colder than the rest of the sample cell. The 2-D density of H↓ was controlled by an input flux of H↓ into the sample cell. We investigated the surface recombination of H↓ adsorbed on 3% 3He 3He-4He mixture. ESR at 129GHz was carried out to detect H↓ by mirror cavity with the cold spot located at the confocal point. We calculated the thermal boundary resistance between the absorbed H↓ and the mixture. From the temperature dependence of the surface recombination together with calculated boundary resistance, we obtained the temperature dependence of ea. The result agrees with the previous measurement. Two-D H↓ on the mixture film was cooled well down to 40mK.

A spatial distribution of the magnetic domain structure in the U2D2 phase of solid 3 He was examined by a special Magnetic Resonance Imaging (MRI) technique. There were three kinds of magnetic domains in a single seed crystal of a few mm 3 size and each domain size was as large as the seed crystal size. We investigated by MRI the distribution and growth process of domains in the following two ways. First, we observed how the domains evolved from pre-existing domains as the crystal grew in a long vertical sample cell. When the crystal became long enough, one or two domains from the three were selected at the upper part of the cell. Our analyses of domain walls show that the (110)a or (110)b, proposed by Tsubota et al., is consisted with our observation. The pre-existing domains grow in the fixed directions of the domain walls and neither the solid-liquid interface nor the solid-cell wall boundary control the domain distribution during the crystal growth. Second, we observed how the domains were formed when a crystal made a transition from the high field phase (HFP) to the U2D2 phase by a magnetic field cycling. After the magnetic field cycling, the original domain distribution reappeared with slight modifications, particularly near the solid-liquid interface. Possible causes of this memory effect are discussed.

We first recall some basic ideas on homogeneous and heterogeneous nucleation. We then reconsider the heterogeneous nucleation of solid helium from liquid helium in the presence of walls. Recent experiments by Ruutu et al. demonstrate that the presence of walls drastically reduces the energy barrier for such “heterogeneous” nucleation events, although walls are usually more favorable to the liquid phase. In order to explain this reduction, we propose a simple model based on the unpinning of an already existing interface. In the light of such results, we then reconsider the nucleation of the B-phase of superfluid helium 3 from its A-phase. We propose a model where the B-phase nucleation is thermally activated on walls, without need of cosmic rays.

We have been searching for two-dimensional (2-D) superfluidity of spin-polarized atomic hydrogen (H↓) adsorbed on liquid helium surface. We have investigated H↓ adsorbed on 3 He- 4 He mixtures instead of pure 4 He because the thermal coupling of H↓ to the mixtures is stronger than 4 He, ESR at 129 GHz was carried out to detect H↓ at 4.5 T by using the Fabri-Perot confocal cavity with the cold spot located at the confocal point and thus the 2-D H↓ signal was effectively detected. We investigated the two-body bulk and surface recombination processes of H↓ on a 3 % 3 He 3 He- 4 He mixture film. From the analysis of the surface recombination process, we found the adsorption energy of H↓ was a 0.6 K for the mixture film in the temperature range between 120 mK and 250 mK, greater than the 0.3-0.4 K found previously for pure 3 He and a 67 % 3 He 3 He- 4 He mixture.

We measured sound velocity, v ( T , B ) , in the two nuclear-spin ordered phases (the low field phase (U2D2) and the high field phase (CNAF)) of single crystal of BCC solid 3 He below 1 mK along the melting curve ( P m ≃ 34 bar ) . The change in sound velocity, Δ v ( T , B ) ( ≡ v ( T , B ) - v ( 0 , 0 ) ) , arises from the nuclear-spin parts of the internal energy. In the U2D2, the measured Δ v ( T , 0.06 T ) was proportional to T 4 and Δ v ( 0.5 mK , B ) was proportional to B 2 . In the CNAF, the measured Δ v ( T , 0.50 T ) was proportional to T 4 and Δ v ( 0.5 mK , B ) was proportional to B - B c 1 ( T ) , where B c 1 ( T ) is the lower critical field. From Δ v for different crystal orientations, we determined the nuclear-spin elastic-stiffness tensors Δ c ij l , h ( T , B ) , six components for U2D2 and three components for CNAF as a function of T and B . We obtained the generalized Gruneisen constants Γ ij 1 / χ for the magnetic susceptibility χ from Δ c ij l (0.5 mK, B ) and Γ ij c for the spin wave velocity c from Δ c ij l ( T , 0.06 T) in the U2D2 by using the nuclear-spin part of the internal energy U ( T , B ) . In particular, the obtained Γ ij c and Γ ij 1 / χ are compared with the theoretical estimation based on the multiple-spin exchange model (MSE) with three parameters. The validity of the MSE with three parameters is quantitatively discussed.

Sound velocity of solid 3 He with an almost single magnetic domain was measured in the nuclear-ordered U2D2 phase and the paramagnetic phase along the melting curve using 10.98MHz longitudinal ultra-sound. The sound velocity increased with temperatures as T 4 in the U2D2 phase and jumped about 0.1% at the nuclear-ordered temperature T N . Above T N , it increased gradually, going through a maximum around 4mK, and then decreased as temperature increased. The temperature dependence of the sound velocity is attributed to two parts, the change in density of the solid along the melting curve and the temperature dependent internal energy of the nuclear spin. The nuclear spin contribution to the temperature dependent sound velocity agreed qualitatively with measured thermodynamical quantities. Attenuation of the sound at 10.98MHz was smaller than 0.2(cm −1 ).

Nucleation of solid helium in superfluid helium was studied in a temperature range between 50mK and 1K. Pulsed electrostatical compression was employed to initiate nucleation in a slightly overpressurized superfluid helium. The nucleation rate w was estimated from a distribution of the overpressure Peffrequired for nucleation. The temperature dependence of Peffat constant ω was essentially flat between 50mK and 1K. The ln ω was almost linear in Peffwithin the resolution of our data.

By using the Magnetic Resonance Imaging (MRI) technique, we studied the spatial recovery of the longitudinal magnetization of liquid 3 He- 4 He mixtures in a 2mm × 2mm × 20mm rectangular parallelepiped sample cell below 1 K. We applied saturation pulses to destroy the magnetization and took two-dimensional pictures of the recovery of the magnetization in three orthogonal planes by the MRI as a function of waiting times. We found from the MRI pictures that the bulk relaxation process was so slow that the surface relaxation and the subsequent diffusion process were dominant mechanisms for the 3He spin relaxation. When the mixture was phase-separated, the magnetization in the concentrated side recovered much faster than that in the diluted side. It was also shown that there was no extra magnetization recovery at the phase boundary. As a result, the magnetization diffused from the concentrated side, flowed through the phase boundary into the diluted side and caused a magnetization recovery in the diluted side.

We have developed a Magnetic Resonance Imaging (MRI) technique applicable for ultra low temperature physics. In contrast to conventional MRI for general use where a pulsed-field gradient method is commonly used, we used a steady-field gradient method to avoid an eddy current heating due to metallic parts around the sample cell. We applied the MRI for 3 He-4 He mixture liquid with a critical concentration below 1 K and visualized the shape of the phase-separated boundary. We obtained two-dimensional space resolution of a few 10 μm. We extracted the interfacial tension of the boundary which was in good agreement with reported values. The contact angle of the boundary to the cell wall was small at low temperature and increased toward 90° near the critical temperature.

Dynamics of the 1st order phase transition between the U2D2 and the high field phases (HFP) was studied by field-cycling method between these phases by using ultra low temperature magnetic resonance imaging (ULT-MRI). Single Crystal of U2D2 3 He was produced at the bottom of compressional cell in superfluid 3 He-B at about 0.5 mK. Domain distribution in the U2D2 crystal was examined by ULT-MRI. We have measured the NMR signal intensity to extract the time-evolution of the HFP, after the static magnetic field was swept quickly through the critical field B C1 and was stayed at B = B C1 +Δ B . The volume concentration of the U2D2 decreased exponentially in time during the early stage of the phase transition. The rate constant depended positively on Δ B . After the phase transition to the HFP was completed, the static field decreased through B C1 and was fixed at B = B C1 −Δ B . The observed rate constant was similar to the value in the opposite direction with identical Δ B . This exponential evolution and Δ B dependence of its rate suggest that the early stage of the phase transition is controlled by the nucleation process.

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.

We have measured magnetic field distortion of the order parameter and the longitudinal resonance frequency of superfluid {sup 3}He-{ital B} using pulsed nuclear magnetic resonance. The experiments were guided by numerical solutions of Leggett{close_quote}s equations which we have generalized to include gap distortion. We have found stable regions for the nonlinear spin dynamics and taken advantage of this to obtain the first measurements of the temperature dependence of the distortion of the energy gap in the {ital B} phase. At low temperatures and pressures the results agree with theoretical predictions of Fishman and Sauls. {copyright} {ital 1996 The American Physical Society.}

Frost heave occurs in various phenomena in natural environment. It has been studied in helium on porous glasses under perfect ice-segregation condition. The maximum frost heave pressure was investigated for various conditions and was in good agreement with the thermodynamical prediction. The dynamical properties of frost heave are discussed and some of the preliminary results of the growth rate measurement are presented.

We have measured the contact angle of the 3 He – 4 He interface on a sapphire window near the tri-critical temperature T t =0.87 K . We have found that it is finite and that it increases with temperature above 0.81 K . This behavior is opposite to the “critical point wetting” which is usually observed near critical points. Our results confirm earlier MRI measurements by Ueno et al. [1]. We interpret our observations as due to the critical Casimir effect which leads to an effective attraction of the 3 He – 4 He interface by the substrate near the superfluid transition at Tt.

We have measured the growth rate of single crystal U2D2 solid3He in the superfluid B phase as a function of chemical potential difference Δμ between the solid and liquid for temperatures between 0.4 and 0.9 TN. The growth rate was not proportional to Δμ, with very slow growth rates for Δμ less than about 10 erg/cm3. For larger values of Δμ the growth rate depended weakly on temperature and the growth was attributed to a quantum growth mechanism due to screw dislocations. The melting rate was too fast to be measured quantitatively, but was slower at higher temperatures.

Pulsed NMR experiments have been performed on U2D2 solid3He with a single domain at high magnetic fields. It was found that the frequency of the free induction signal changed with time. In some cases, a large negative frequency shift of about 2 kHz from the Larmor frequency was observed some time after the rf pulse, after which frequency returned to that observed for a small tipping angle. The tipping-angle-dependent frequency shift, which was taken to be the precession frequency immediately following the rf pulse, agreed with Namaizawa's theory. The free induction signal showed a decay behavior similar to that of superfluid3He-A. This can be attributed to the instability of the uniform precession due to the creation of spin wave modes, but does not agree with the instability criterion predicted by Ohmi et al. and Shopova.

International audience; The coherent precession in bulk 3He A-phase is unstable due to the positive feedback of spin supercurrent to the gradient of phase of precession. It was predicted that the homogeneous precession will be stable if the orbital momentum could be oriented along the magnetic field. [7] We have found that the orbital part of order parameter in 3He A-phase immersed in aerogel orients along the direction of aerogel axial squeezing. By this method we have oriented the orbital momentum of 3He A-phase along the magnetic field and studied non-linear NMR properties. By cw-NMR we have succeeded to form the state with Coherent Precession in 3He A-phase (CP-A), which is analog of Homogeneous Precession Domain (HPD) in 3He B-phase. We have found that the dissipation of CP-A and HPD in aerogel is much larger than in bulk superfluid 3He and depends from sample to sample. We propose a model to explain enhancement of the dissipation in CP-A, which takes into account the spin diffusion of inhomogeneous magnetization created by fluctuations of the condensation energy of Cooper pair in aerogel.

We report the first observation of coherent precession of magnetization in superfluid 3 He A-like phase (CP - A) in aerogel. The coherent precession in bulk 3 He A-phase is unstable due to the positive feedback of spin supercurrent to the gradient of phase of precession. It was predicted that the homogeneous precession will be stable if the orbital momentum of the 3 He A-phase can be oriented along the magnetic field. We have succeeded to prepare this configuration by emerging 3 He in uniaxially deformed anisotropic aerogel. The dissipation rate of coherent precession states in aerogel is much larger than that in bulk 3 He B-phase. We propose a mechanism of this dissipation.

Quantized vortices with half-integer circulation, which are forbidden from existing in a conventional superfluid because of the single valueness of the wave function, are theoretically predicted to exist in superfluid $^{3}\mathrm{He}\mathrm{\text{\ensuremath{-}}}A$ if the order parameters $\stackrel{^}{\ensuremath{\ell}}$ and $\stackrel{^}{\mathbf{d}}$ form $\stackrel{^}{\ensuremath{\ell}}\ensuremath{\perp}\stackrel{^}{\mathbf{d}}$ texture. To form the $\stackrel{^}{\ensuremath{\ell}}\ensuremath{\perp}\stackrel{^}{\mathbf{d}}$ texture, we confined the superfluid between parallel plates with a $12.5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ gap and applied a magnetic field of $H=26.7\text{ }\text{ }\mathrm{mT}$ perpendicular to the plates to take NMR and orient $\stackrel{^}{\mathbf{d}}$ perpendicular to $\stackrel{^}{\ensuremath{\ell}}$. NMR spectra exhibit a negative-shift peak which probes that the uniform $\stackrel{^}{\ensuremath{\ell}}\ensuremath{\perp}\stackrel{^}{\mathbf{d}}$ texture is realized in our cell and show a new satellite signal under rotation. The rotation dependence of the satellite signal is interpreted that a Fr\'eedericksz transition of $\stackrel{^}{\ensuremath{\ell}}$ texture is induced by rotation above $1.0\text{ }\text{ }\mathrm{rad}/\mathrm{s}$ and vortices start to appear above $1.8\text{ }\text{ }\mathrm{rad}/\mathrm{s}$.

Quantized vortices with half-integer circulation, which are forbidden from existing in a conventional superfluid because of the single valueness of the wave function, are theoretically predicted to exist in superfluid 3He-A if the order parameters l over and d over form l over perpendicular d over texture. To form the l over perpendicular d over texture, we confined the superfluid between parallel plates with a 12.5 microm gap and applied a magnetic field of H=26.7 mT perpendicular to the plates to take NMR and orient d over perpendicular to l over. NMR spectra exhibit a negative-shift peak which probes that the uniform l over perpendicular d over texture is realized in our cell and show a new satellite signal under rotation. The rotation dependence of the satellite signal is interpreted that a Fréedericksz transition of l over texture is induced by rotation above 1.0 rad/s and vortices start to appear above 1.8 rad/s.

We have measured, along the melting curve, the magnetic‐field dependences of the velocities of the longitudinal and two transverse sound modes in the low field phase (U2D2) of nuclear‐ordered solid 3He crystals with a single magnetic domain. We determined the elastic‐stiffness tensors cij(T,B) along the melting curve at T=0.5 mK. Since the U2D2 phase has a tetragonal symmetry, we extracted six components of the nuclear‐spin elastic‐stiffnesses tensor Δc ijN(0.5 mK,B). We obtained the generalized Gruneisen constants Γij1/χ for the magnetic susceptibility χ from ΔcijN(0.5 mK,B), which were proportional to B2.

Nucleation and growth of the stable phase were investigated in the magnetic field induced transition between the U2D2 phase and the CNAF phase of nuclear ordered solid 3He. Time evolution of the stable phase, after sweeping the magnetic field B through the critical field BC1 to B = BC1 + ΔB, was investigated by pulsed NMR. The volume fraction of the metastable phase showed two stage evolution and decreased exponentially in time during the first stage. In this stage, the rate of the evolution increased almost linearly with ΔB and decreased slightly at higher temperatures and was almost independent of the direction of the phase transition.

We have measured NMR spectra of the 3He‐A phase restricted between parallel plates under rotation at a speed of Ω < 6.28 rad/s, using a rotating cryostat at ISSP. The sample volume was divided by the plates into 110 disk‐shaped spaces, each with a thickness of 12.5 μm and a radius of 1.5 mm. They were connected to a bulk superfluid through 0.3 mm‐wide channels. Measurements were done by continuous wave (cw) NMR at 869 kHz. The axes of both H and Ω were perpendicular to the parallel plates. We observed a very narrow spectrum with a negative frequency shift, an indication that the gaps and the superfluid texture were well designed.

We have investigated pinning effects on texture and vortices of the B-like phase of superfluid (3)He in a rotating aerogel up to +/-2pi rad/s by cw-NMR. We observed deformation of the NMR spectra in rotation, due to counterflow between the superflow and the normal flow. The average intensity of the counterflow was calculated from the change of NMR spectra. The rotation dependence of the counterflow intensity is similar to the magnetization curve of hard type II superconductors or the counterflow response of (4)He-II in packed powders. This counterflow behavior is in qualitative agreement with a model that vortices are pinned unless the counterflow exceeds a critical velocity v(c). The temperature independence of v(c) suggests that v(c) is associated with the expansion of primordial vortices.

Textures, textural transformation, and formation and annihilation of a single vortex were investigated in narrow cylinders with $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ radius in $A$-phase under rotation up to $6.28\text{ }\text{ }\mathrm{r}\mathrm{a}\mathrm{d}/\mathrm{sec} $. Three textures were found, depending on the cooling conditions of the sample through the superfluid transition temperature ${T}_{\mathrm{c}}$. We found the gyromagnetic effect of textures; that is, two textures ($A$ or $B$) could be selected either by applying a magnetic field in parallel or anti-parallel to the rotation axis. The critical angular speed of a single vortex formation ${\ensuremath{\Omega}}_{\mathrm{f}}$ and that of annihilation ${\ensuremath{\Omega}}_{\mathrm{a}}$ for each texture were measured. The textural transformation in type $A$ texture was induced by rotation. Both type $A$ and $B$ textures held macroscopic angular momentum along the rotation axis. We identified the texture for type $A$, $B$, and $C$ as Mermin-Ho, radial disgyration, and a soliton type of defect along the axis, respectively.

We have measured temperature dependences of sound velocity for both longitudinal and transverse sound in nuclear-ordered U2D2 solid 3He with several crystal orientations along the melting curve. The sound velocity change was proportional to T4 for all sound modes and crystal orientations and was attributed to the nuclear-spin part of the internal energy. We extracted six-independent elastic stiffness of the nuclear-spin part and obtained Gruneisen constants of the spin wave velocity for four-independent strains. Gruneisen constants for compressional strain were larger than those for shear strain. Using the multiple-spin-exchange model, we explain the anisotropy of Gruneisen constants in tetragonal symmetry.

We studied lightly doped Si:P with high-frequency (80-120 GHz) ESR and ESR/NMR double magnetic resonance techniques in the temperature range down to 1.4 K. We found dynamic nuclear polarization of 31P from steady-state ESR measurements with approximately 3.6 T. We derived the nuclear spin relaxation time, T1N, of 31P by analysing the time-evolution of ESR spectra utilizing the dynamic nuclear polarization effect. We derive temperature and magnetic field dependence of T1N and compare with experimental data. Furthermore, from our ESR measurements, we modulate the nuclear polarization of 31P by applying an RF field.

The first order phase transition between two nuclear ordered antiferromagnetic solid 3 He, namely U2D2- 3 He and CNAF- 3 He, was studied. Those two phases exist at the temperature below about 1 mK and have the first order phase transition between them with the critical magnetic field B C 1 . We studied time and spatial evolution of the stable phase during the field-sweep induced phase transition near T = 0 . We found that the stable phase developed in two stages. During the first stage, many small seeds appeared throughout a crystal. Those nucleated seeds grew faster than the nucleation process, however terminated growing at some limited size due to a chemical potential loss by the latent heat release. The second stage was understood as the process of latent heat release through solid–liquid interface.

We measured cw-NMR at 700 kHz for rotating 3 He -A at 31.5 bar confined in two sizes of cylinders with 0.1 and 0.2 mm diameters at rotation speeds up to 6.28 rad/s . The NMR signals varied for the different size cylinders. Nucleation of the single vortex was observed in the 0.2 mm cylinder above a critical rotational speed and hysteresis appeared for vortex nucleation and annihilation when the rotational speed was changed. On the other hand in 0.1 mm cylinder, a vortex did not appear up to 6.28 rad/s , and the NMR absorption gradually changed as the rotation speed increased.