Experimental evidence against zero temperature spin-wave damping in He

We have discovered unexpected features in the NMR signal of the ³He– ⁴He mixture inside the mixing chamber of a ⁴He circulating dilution refrigerator. The dilution process not only cools the liquid in the mixing chamber to 12–15 mK but also enhances the nuclear spin polarization of the concentrated as well as the dilute phase to nearly four times the equilibrium value in a magnetic field of 9.36 T. The NMR features are ascribed to standing spin waves trapped by falling dilute phase droplets. The temperature and polarization dependence of the quality factor of the spin-wave modes contradicts evidence from previous work that favored zero temperature spin-wave damping in moderately polarized degenerate Fermi liquids.     

Coherently precessing structure in a normal Fermi liquid

The analytic solution of the equations of spin hydrodynamics of a normal Fermi liquid, which describes a stationary precession of spins in a nonuniform magnetic field in the collisionless limit is discussed. This solution has a form of two domains with the opposite orientations of spins. The derived properties of such a structure agree qualitatively with the experimental data for liquid solutions of ³He in ⁴He and with the results of numerical simulations.     

Fermion zero modes in symmetric vortices in superfluid He

Fermions localized within vortex cores are considered for the simplest most symmetric vortices in the superfluid phases of ³He. Axisymmetric vortices in the A, A₁ and planar phases contain the fermionic condensate with a completely flat band and zero energy. The o-vortex in the B-phase contains 1D Fermi liquids, formed by fermions occupying the branches which cross the zero level. The number of such Fermi liquids increases with increasing external magnetic field. The fermionic spectrum in this vortex is described by the “orbital” and spin momenta interacting with the effective internal magnetic field produced by the vortex and with the external field. Most of the information is obtained using the vortex symmetry which determines symmetry properties of the fermionic spectrum.     

Low-temperature helium in bulk and in restricted geometries

Experiments with helium in thin films and small pores show that the bulk properties are greatly modified. With ⁴He, the onset of superfluidity is depressed and the specific heat anomaly is rounded and also depressed to lower temperatures. With ³He new features are found such as surface magnetism and free path effects. Bulk superfluid ⁴He is shown to have a sharp transition to turbulence, and bulk ³He the characteristics of a Fermi fluid.     

Fluctuation effects in spin diffusion measurements in liquid He

We have measured spin diffusion coefficients of liquid ³He at a frequency of 920 kHz by pulsed NMR. By analyzing our data in the framework of the Leggett–Rice theory we got a spin diffusion coefficient and a Leggett–Rice parameter =λωτ simultaneously at each temperature. On approaching the superfluid transition the spin diffusion coefficients showed a deviation from predictions of the Fermi liquid theory. The deviation at low pressure was larger than that at high pressure. This anomaly may be due to the effects of fluctuations of superfluidity which were recently observed in the viscosity measurement of liquid ³He.     

Nonwetting of He and dilute He–He mixtures on Cs

For pure ⁴He on Cs a nonwet phase exists below ≈2 K but for dilute ³He–⁴He mixtures the wetting is reentrant. Measurements of the contact angle with dilute mixtures of liquid helium on Cs can be explained in detail in terms of ³He states together with ripplons at the Cs–He mixture interface, but it is impossible to account for the lower wetting temperatures or contact angles with only ³He states or ripplons. We discuss the influence of surface roughness of the Cs on the contribution of interface excitations to the free energy and suggest that the variety of contact angles found with different samples of Cs is due mainly to their degree of roughness.     

一种可用于极化~3He实验的新型磁场系统

原子核自旋极化的~3He气体已被深入研究并广泛用于各种科学实验.在过去的极化~3He实验中,为了减小磁场梯度对纵向弛豫时间的影响,通常会建造大尺寸的亥姆霍兹线圈来提供所需均匀度的主磁场环境.本文通过计算得到了新的六正方形线圈系统,可以为极化~3He实验提供小型高均匀性的磁场装置.其中线圈系√统内部超过30%的区域磁场梯度满足(|▽B_x|~2+|▽B_y|~2)/B_0 10~(-4)cm~(-1),这一均匀区域比例超过了现在所有用于极化~3He实验的线圈装置.对于其他需要大均匀区域磁场环境的研究实验,新的六线圈系统也具有很好的应用价值.     

Adsorption of He on He crystal surfaces

We have compared the surface properties of crystals respectively grown from normal ⁴He containing 130 ppb of ³He and from ultrapure ⁴He (0.4 ppb). Below 0.4 K, ³He impurities are found to decrease both the surface tension and the step energy. Our results are consistent with the existence of two-dimensional bound states for ³He atoms at the solid-liquid interface. Quantitative agreement with the data is found with a binding energy _s ≈ 4.3 K and a ³He density saturating around 0.4 monolayer. The presence of steps is found to increase the binding energy by about 10 mK.     

Experiments on coherently precessing spin state in He---He solution

Pulsed NMR experiments in saturated liquid ³He---⁴He solution were done at temperatures down to 1 mK. The results are well described by a formation of a coherently precessing two-domain structure. In one of these domains the magnetization is oriented along the external magnetic field and in the other domain it is oriented opposite to the field. This structure arises due to spin currents, which are caused by Fermi liquid interaction.     

光泵抽运3He极化程度监控系统的设计与实现

极化³He的一项重要应用是中子的极化.中国绵阳研究堆（CMRR）已建立国内首个自旋交换光学泵浦（SEOP）极化³He中子极化系统.为了监测³He的极化率随时间的相对变化情况,本文首先设计了基于核磁共振（NMR）技术的³He相对极化率测量系统,通过Matlab控制程序实现了对³He相对极化率的定时检测.然后对拾波线圈的构形和信噪比（SNR）进行了优化.结果表明当绕线长度一样时,Brooks构形的线圈有利于提高SNR;当线圈的平均半径为（a₀+d）/√2（a₀为³He气室的半径,d为拾波线圈与气室之间的距离）时,其SNR最高.最后对该系统的本底噪声进行了测量,发现其主要来源于环境噪声（0.27 μV/√Hz）和数据采集（DAQ）卡的噪声（0.40 μV/√Hz）,系统的总噪声功率谱密度约为√0.16+0.073G² μV/√Hz（G为放大器的增益倍数）.     

Double-Differential Cross Section of 5He Emission

The probability of ⁵He particle emission has been affirmed theoretically [J.S. Zhang, Science in China G47 (2004) 137]. In order to describe the ⁵He emission, the theoretical formula of the double-differential cross section of emitted ⁵He is to be established. Based on the pick-up mechanism, used for calculating the formula of d, t, ³He, α emissions, the theoretical formula of double-differential cross section of ⁵He is obtained, which is expressed in the form of Legendre coefficients. In the case of low incident energies, the configuration [J.S. Zhang, Science in China G47 (2004) 137; J.S. Zhang, Commun. Theor. Phys. (Beijing, China) 39 (2003) 83] is the dominant part in the reaction processes. The calculated result indicates that the forward peaked angular distribution of the composite particle emission is weaker than that of the emitted single nucleon due to pick-up nucleon from the Fermi sea. As an example, the reactions of n+¹⁴N have been calculated, and the Legendre coefficients of d, t, ³He, α, ⁵He emissions are obtained respectively. The results show that the forward tendency is decided by the average momentum per nucleon in the emitted composite particles. The larger the average momentum is, the stronger the forward tendency is.     

Thermal studies of the spin liquid state and analog to the He melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG) has an extraordinary low-temperature phase diagram. Although the Curie–Weiss temperature of GGG is −2 K, GGG shows no long-range order down to T0.4 K. At low temperatures GGG has a spin glass phase at low fields (0.1 T), a field-induced long-range ordered antiferromagnetic state at fields of between 0.7 and 1.3 T, and, at intermediate fields, an apparent spin-liquid state without long-range order. We have characterized the intermediate field (IF) state through heat capacity, thermal conductivity, and magnetocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying that the intermediate field phase is distinct from the spin glass. The lower field boundary of the AFM phase is shown to have distinct minimum at T0.2 K, in analogy to the minimum in the melting curve of ⁴He. The existence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature.     

Coincidence cross sections for coherent pion electroproduction on He and H

We have calculated coincidence cross sections for coherent pion electroproduction on ³He and ³H using Faddeev wave functions obtained with the Reid soft-core potential. The four coincidence structure functions, corresponding to unpolarized and polarized transverse, interference and longitudinal terms differ substantially in their energy dependence. Effects of Fermi motion and off-shell ambiguities are more pronounced for virtual than for real photons. The correct treatment of Fermi motion in the production operator decreases the maximum transverse cross section by up to 50% and lowers the peak energy by up to 50 MeV. Ratios of coherent pion production on ³He and ³H differ significantly for the four structure functions. Finally, the theoretical calculations have been compared with existing experimental data on inclusive pion electroproduction.     

The dissipative dynamics of the He A-B interface in finite geometries

The ³He A-B interface velocity and the stationary field of temperature and counterflow accompanying the phase transition are calculated employing general boundary conditions and superfluid hydrodynamics. All dissipative mechanisms that may occur are considered, especially including the effects of lateral walls. There are three contributions to the total growth resistance: (i) a microscopically abrupt temperature discontinuity at the interface arising from Andreev scattering of quasiparticles, (ii) two hydrodynamically gentle temperature variations (sq- or diffusive modes) on both sides of the interface that come from collisions among quasiparticles, and (iii) the drag exerted directly on the interface by the wall's roughness.     

A path integral centroid molecular dynamics method for Bose and Fermi statistics

We propose a path integral centroid molecular dynamics (CMD) method extended to Bose and Fermi statistics. An extended method of path integral molecular dynamics (PIMD) for such statistics is also developed as a technique of calculations of static properties. Bose PIMD and CMD simulations have been performed for bulk liquid ⁴He and ideal Bose gas, respectively. The remnant of λ transition is observed for bulk liquid ⁴He, while the effect of Bose statistics on the centroid dynamics spanning several nanoseconds is observed for ideal Bose gas.     

Decay Analysis of Ge Isotopes Formed in Reactions Induced by Tightly and Loosely Bound Projectiles

The dynamical cluster-decay model (DCM) is employed to investigate the decay of ^68,70Ge^* compound nuclei formed respectively via tightly (⁴He) and loosely (⁶He) bound projectiles, using ⁶⁴Zn target. The study is carried out over a wide energy range (E_c.m.~5 MeV to 16 MeV) by including the quadrupole deformations (β2i) and optimum orientations (θ_i^opt) of the decaying fragments. The fusion cross-sections, obtained by adding various evaporation channels show nice agreement with the experimental data for ⁴He+⁶⁴Zn reaction. The contribution from competing compound inelastic scattering channel is also analyzed particularly for ⁶⁸Ge^* nucleus at above barrier energies. On the other hand, the decrement in the fusion cross-sections of ⁷⁰Ge^* nuclear system is addressed by presuming that ⁶⁵Zn ER is formed via two different modes:(i) the αn evaporation of ⁷⁰Ge^* nucleus, and (ii) 1n-evaporation of ⁶⁶Zn^* nuclear system, formed via breakup and 2n-transfer channels due to halo structure of the ⁶He projectile. Besides this, the suppression in 2np evaporation cross-sections suggests the contribution of another breakup and transfer process of ⁶He i.e. ⁴He+⁶⁴Zn. The contribution of breakup+transfer channels for ⁶He+⁶⁴Zn reaction is duly addressed by applying relevant energy corrections due to the breakup of " ⁶He" projectile into 2n and ⁴He. In addition to this, the barrier lowering, angular momentum and energy dependence effects are also explored in view of the dynamics of chosen reactions.     

Pionic K X-rays in liquid He

Energies and lorentzian widths of pionic K X-ray transitions have been measured in liquid ³He with improved accuracy. The strong interaction on the π³He 1s level is found to be attractive and to produce an increase in the K transition energies of 34 ± 4 eV; the measured lorentzian width is 36 ±7 eV. Measured values are also presented for K X-ray energies in liquid π⁴He and μ⁴He, the lorentzian width of the 1s level in π⁴He, and relative intensities of K X-ray transitions in π³He, π⁴He and μ⁴He. The measurements are compared with those of others and with recent theoretical calculations.     

Optical interferometry in phase separated He---He mixture films

We report on interferometric studies of ³He---⁴He liquid mixtures. Anomalous wetting phenomena were observed in phase separated mixture films: the upper ³He-rich phase did not wet the lower ⁴He-rich phase under a small continuous feed of ⁴He atoms to the vapor phase. Instead, under this dynamic situation, floating pools of the ³He-rich phase were stabilized with a contact angle on the order of 10 mrad. In equilibrium, the concentrated ³He-rich phase was found to nucleate uniformly on the free surface, consistent with Antonow's rule.     

The origin of nuclear magnetism in solid He films: determination of multi-spin exchange frequencies

Solid ³He films adsorbed on graphite display remarkable magnetic properties due to quantum exchange, reduced dimensionality and frustration. We discuss the relevance of the multiple spin exchange Hamiltonian to describe these systems. Our high precision NMR data and the results of heat capacity measurements performed in other laboratories allow us to unambiguously determine the most significant multi-spin exchange frequencies. This result gives a quantitative understanding of the thermodynamical properties of solid ³He films, explaining the evolution from a pure Heisenberg behavior at high areal densities to a novel spin liquid state at low coverages.