Spin-Wave Modes and Spin Diffusion Anisotropy in a 3He-4He Mixture at High B/T

We present nmr measurements in a 6.2% ³ He- ⁴ He mixture carried out at 11.3 T with temperatures down to 3.5 mK, yielding a very high B/T ratio. We have measured the transverse spin diffusion with the spin-echo technique. We observe a saturation of the spin diffusion coefficient at low temperature corresponding to an anisotropy temperature T_a13mK. Using the same setup, we were able to excite Silin spin-wave modes in the experimental cell.     

3He Melting Pressure Measurements in High Magnetic Fields

Experimental methods and preliminary results of high-precision measurements of the ³ He melting curve in high magnetic fields have been described. The purpose of this work is twofold. One is to establish a reliable millikelvin temperature scale in high fields (T 10 mK, B 15 T), The other is to investigate a high field region of the magnetic phase diagram of solid ³ He by measuring the melting pressure down to temperatures below 1 mK. Besides the two superfluid transitions, the phase transition temperatures between the spin ordered solid and the paramagnetic solid, T _HFP , were determined at B = 12 and 10 T with good accuracy, which is an extension of previous measurements up to 8 T. The present T _HFP (B) line can not be scaled to that at a higher density with a single Grüneisen parameter, indicating a variation of density dependencies of the multiple-spin exchange interactions.     

Spin Diffusion Anisotropy in Liquid 3He

We report new experiments on the transverse and longitudinal spin dynamics of liquid ³ He polarized by an 11.3T field. At the lowest temperatures probed, the transverse spin diffusion coefficient saturates with an apparent anisotropy temperature of T_a=12±2 mK. This temperature dependence is in accord with the general idea of spin-diffusion anisotropy but the value of T_a is half that expected from extrapolating earlier experiments at a lower field. Strong magnetostatic effects in the form of long-lived spin modes are also observed for small gradients and small tipping angles.     

Collisionless spin waves in normal and superfluid3He

Standing spin-wave modes in liquid³He have been studied by cw NMR at Larmor frequencies of 1, 2, and 4 MHz and pressures of 0, 6.3, and 12.3 bar. The spin waves, which produce peaks in the NMR line, are visible at temperatures below 5 mK at zero pressure. With the assumption of a slightly simplified sample shape and no transverse spin relaxation at the walls, the theory of Leggett fits the spin-wave frequencies in the normal liquid very well, giving a value of the Fermi liquid parameterF ₁ ^a =–0.6±0.2 at zero pressure. The width of some of the peaks is larger than expected from other determinations of the quasiparticle diffusion time _D . This could be due to wall relaxation or to deviations from the assumed sample geometry. In the superfluid A₁ and A phases, where the data cannot be fitted to existing theories, the spin-wave modes are shifted in frequency and suffer additional damping as the temperature is decreased. At still lower temperatures in the B phase an inversion of the spin-wave spectrum from one side of the NMR line to the other is observed, agreeing quantitatively with the predictions of the 1975 theory of Combescot.     

NMR experiments on pure3He and3He-4He mixtures in silica aerogel

We have carried out a series of NMR experiments on³He systems filling the pores of silica aerogel with 95% porosity. The systems studied included³He-⁴He mixtures with a wide range of³He concentrations x₃ as well as pure³He. All experiments were conducted in an 8 T field and for temperatures T 6 mK. This resulted in strong spin polarization at the lowest temperatures (up to approximately 80%) for the localized layer observed for a pure³He sample. For pure³He, the magnetic behavior is dominated by the localized spins, which are found to constitute 6% of the sample. The coupled system of localized plus liquid spins displays rapid transverse magnetization relaxation with an anomalous temperature dependence. For intermediate x₃ magnetization measurements provide a preliminary indication of the phase diagram of mixtures filling this porous material. The magnetization is hysteretic over some ranges of x₃.     

Crossover to Fermi-Liquid Behavior at Lowest Temperatures in Pure CeNi2Ge2

We present a low temperature study of the nearly antiferromagnetic heavy Fermion compound CeNi₂Ge₂ including specific heat measurements in magnetic fields up to 13 T, resistivity, magnetization, DC- and AC- susceptibility measurements down to 30 mK. The results show that, although the system is close to a quantum critical point, CeNi₂Ge₂ exhibits Fermi-liquid behavior below approximately 300 mK. At higher temperatures hyperscaling indicates the existence of strong coupling spin fluctuations responsible for the observed non-Fermi-liquid properties.     

Vibrating Wire Measurements in Superfluid 3He at the Melting Curve Down to 0.53 mK

Measurements of the vibrating wire spectrum have been carried out in superfluid ³ He along the melting curve down to 0.53mK. We have observed that at temperatures below 0.3 T_c the width of the mechanical resonance of the wire decreases exponentially with 1/T, indicating the ballistic regime of collisions with quasiparticles. The value of the superfluid energy gap was found to be (1.99±0.05)T_c, in good agreement with the values obtained from heat capacity measurements. The vibrating wire was thereby calibrated for further experiments at temperatures below 0.5mK, where the sensitivity of the melting curve thermometry becomes rather poor.     

Spin Transport in Very Dilute 3He-4He at Very High B/T

We report preliminary results for pulsed-NMR measurements on a very dilute ³ He- ⁴ He mixture (x ₃ =900 ppm) in a 14.74 T field at temperatures down to 3 mK. The aim of the project is to investigate low temperature transverse spin relaxation and possible spin diffusion anisotropy. Spin-echo and T₁-type measurements were employed to measure the transverse and longitudinal spin diffusion coefficients. At high temperatures both spin diffusion coefficients are well explained assuming a quasiparticle interaction consistent with previous experiments. Conversely, at low temperatures the measured value of || is apparently limited to about ten by a poorly-understood process that accelerates spin-echo decay. This hampers an accurate determination of the transverse diffusion coefficient at the lowest temperatures reached.     

Antiferromagnetic ordering of enhanced nuclear spin in Cs2NaHoCl6

The field dependence of the magnetic susceptibility at low temperatures was investigated in a single crystal of Cs₂NaHoCl₆. The result is well explained by the second-order Zeeman splitting of the electronic ground state with a nonmagnetic ₃ doublet. Enhanced nuclear cooling experiments were carried out. Antiferromagnetic ordering of the enhanced nuclear spin system was observed at about 1.5 mK.     

Magnetic susceptibility measurements in PrBe13

The temperature dependent complex magnetic susceptibility of the Van Vleck paramagnet PrBe ₁₃ has been measured at temperatures below 20 mK in zero magnetic field. We observe an antiferromagnetic Curie-Weiss behaviour of the nuclear spin susceptibility with a Weiss temperature = -2.2 mK.     

Magnetic Resonance Studies of Cold Atomic Hydrogen Gas

We present magnetic resonance studies of two-dimensional atomic hydrogen gas (H↓) in 4.6 T field at temperatures 70–100 mK i.e. well in the quantum gas or cold collision regime. Electron spin resonance at 128 GHz and nuclear magnetic resonance at 910 MHz were used to detect and manipulate populations of the hyperfine manifold of the electronic ground state. Using a coherent two-photon excitation we demonstrated effects of electromagnetically induced transparency and absorption as well as coherent population trapping in the 2D H gas. We performed a measurement of the magnetic resonance line shifts associated with exchange and dipolar interactions. The cold collision (clock) shift measured for the 2D case is two orders of magnitude smaller than predicted by the mean field theory and is of the same order as the internal dipolar field.      

Polarized liquid3He in a4He circulating dilution refrigerator

We have used a⁴He circulating dilution refrigerator to produce cold liquid³He with a steady state out-of-equilibrium nuclear spin polarization. Polarizations on the order of 15% (more than 7 times higher than the equilibrium polarization in the external field of 6.6 T) have been obtained in the mixing chamber of the refrigerator at temperatures between 10 and 15 mK. The polarization is enhanced at high pressure because the molar susceptibility of concentrated³He is larger than that of the dilute phase. The polarization exchange between the dilute and concentrated phases (in direct contact in the heat exchanger of the refrigerator) amplifies the enhancement. The polarization diminishes below a pressure of 2.6 bar. This allows us to scale and reinterpret susceptibility data of the dilute phase¹ in combination with the effective mass deduced from osmotic pressure measurements². We find 1+F ₀ ^a = 0.89±1% on the phase separation line in the pressure range 0–20 bar.We would like to thank Profs. D.M. Lee and M.S. Tagirov for the many discussions during their visits.     

Magnetic Properties of RB66 (R = Gd, Tb, Ho, Er, and Lu)

We report magnetic susceptibility measurements of RB₆₆ (R = Gd, Tb, Ho, Er, and Lu) boron-rich rare earth containing borides down to 50?mK. The data suggest a spin glass low temperature state for RB₆₆ (R = Gd, Tb, Ho, and Er) with the freezing temperatures below 1?K. The magnetic properties appear to be influenced by the anisotropy of the magnetic moments, probably via the crystalline electric field effects.     

Experiments on Fermi liquid domains in normal and superfluid3He

We report on pulsed and CW NMR experiments in liquid³He at temperatures below 10 mK, when the Fermi liquid interaction gives rise to a nondissipative spin currents. Due to these currents, a coherently precessing spin state may be formed in NMR-experiments with Fermi liquids. The state consists of two domains. In one of the domains the magnetization is oriented along, and in the other opposite to the direction of the magnetic field. Hitherto such a state has only been observed in³He-⁴He solutions. We have found that the domains can also be formed in normal³He. Experimental results for normal³He are in a good agreement with theory and computer simulations. The conditions necessary for the formation of the state have been determined. We also have found that the two-domain state may be also created in the super fluid state (at least near T_c).     

Hyperfine Induced Magnetic Ordering in a S=1/2 Quantum Magnets on Kagomé Lattice

Susceptibility and specific heat measurements have been carried out down to 23 mK on polycrystalline [Cu₃(titmb)₂(OCOCH₃)₆]?H₂O {titmb = 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6 trimethylbenzene} with S=1/2 antiferromagnet on Kagomé lattice. The results show an unexpected ferromagnetic long range ordering at 56 mK. The entropy change from 10 K to 23 mK is much larger than the entropy corresponding to electron spin S=1/2 of Cu²⁺. The large excess entropy reduction strongly suggests a novel magnetic ordering of the coupled Cu electron and nuclear spin (I=3/2) system. We propose that a frustrated ground state changes to the complex ferromagnetic ground state with help of the enhanced indirect nuclear spin interaction through the hyperfine interaction.     

Influence of Spin Fluctuations Upon Heat Capacity of 2D Fermi Liquid Formed on Hectorite

We measured low-temperature heat capacities of two-dimensional Fermi liquid (2D FL) formed on Hectorite down to 15mK. At a coverage where mean atomic distance is comparable to that of bulk ³ He liquid, we found that the temperature dependence deviates considerably from the expected linearity. To account for the deviation we carried through integrals including RPA spin susceptibility for the Stoner-Hubbard(SH) or the Landau's Fermi liquid (LFL) models over the effectively entire, energy-momentum space without imposing the paramagnon approximation or the like. We found numerically that the spin fluctuation develops a T ² correction at T<T* which, at T>T*, is taken over by a Tlog T term where T* is some hundredths of T _F . With these corrections we are able to interprete the data and have determined the parameters for each model consistently at the densities observed.     

Nuclear specific heat of copper potassium tutton salt

The specific heat of copper potassium tutton salt has been measured down to 1 mK in order to investigate the hyperfine interaction of this copper compound. The magnetic ordering of the electronic system occurs at 29.5 mK in zero field. Below the transition temperature the electronic heat capacity decreases and the copper nuclear heat capacity of hyperfine splitting becomes dominant in the heat capacity of the compound. The nuclear heat capacity has a broad peak around 3.5 mK. The entropy of copper nuclear spin, which was calculated from the specific heat data, remains at 40% of ln(2I+1) at 1 mK.     

Specific heat study of hyperfine-enhanced nuclear spin system in Cs2NaHoCl6

The specific heat of the hyperfine-enhanced nuclear spin system in Cs₂NaHoCl₆ was measured between 0.9 mK and 1.5 K. At about 1.5 mK a sharp peak of the specific heat was observed. Thermodynamic considerations are given. At higher temperatures the result can be understood by the removal of two levels of the ₃ doublet, suggesting crystal distortion from the cubic structure.     

Magnetic Susceptibility Measurements at Ultra-low Temperatures

We report the design and operation of a device for ac magnetic susceptibility measurements that can operate down to 1 mK. The device, a modification of the standard mutual inductance bridge, is designed with detailed consideration of the thermalization and optimization of each element. First, in order to reduce local heating, the primary coil is made with superconducting wire. Second, a low-temperature transformer which is thermally anchored to the mixing chamber of a dilution refrigerator, is used to match the output of the secondary coil to a high-sensitivity bridge detector. The careful thermal anchoring of the secondary coil and the matching transformer is required to reduce the overall noise temperature and maximize sensitivity. The sample is immersed in liquid ³He to minimize the Kapitza thermal resistance. The magnetic susceptibility of several magnetic compounds, such as the well-known spin gap compound NiCl₂-4SC(NH₂)₂ and other powdered samples, have been successfully measured to temperatures well below 10 mK.     

Positive Ion Mobility in Normal and Superfluid 3He at High Magnetic Field

We have measured the mobility of positive ions (snowballs) in liquid ³ He at magnetic fields H = 0 – 12 T and temperatures T = 0.3 – 10 mK. In normal ³ He, with increasing magnetic field the mobility first increases and then decreases nearly quadratically. The initial rise of the mobility is most strongly seen at the lowest temperatures (T = 2.5–3 mK) and high pressures (p = 29 bar) where at H = 6 T the mobility reaches a maximum enhancement of about 6 %. We attribute this effect to the suppression of the exchange scattering with magnetic field and thus infer the value of the exchange scattering cross-section. In the superfluid A ₁ and A ₂ phases, the relative increase of the ion mobility, as the sample is cooled below the transition temperature, is about twice as small as in the A or B phases.