NMR in pure3He films on a Nuclepore substrate

The properties of³He films on a Nuclepore substrate have been measured by pulsed NMR at a Larmor frequency of 10 MHz between 1.3 and 4.2 K. The³He film thickness was varied from 0.14 to 2 layers. The spin-spin relaxation timeT ₂ agrees well with previous measurements of³He films on Mylar and Vycor glass at low temperatures. The spin-lattice relaxation timeT ₁ for submonolayer films shows a strong temperature dependence consistent with a thermally activated process. This behavior has not previously been observed on amorphous substrates. The spin diffusion coefficient was measured for the thickest films at 4.2 and 2.6 K and found to be consistent with free atom motion of the³He in the vapor. In thin films or at low temperatures, the diffusion was too small to be observed. The magnetic coupling between the³He nuclei in a film and the protons in the Nuclepore substrate was determined from the effect of the³He on the proton-lattice relaxation time. It is about 100 times weaker than the interaction between³He and the fluorine nuclei in a Teflon substrate.     

Pulsed NMR study of submonolayer and multilayer 3He films adsorbed on Grafoil

The nuclear spin relaxation times T ₁ and T ₂ of submonolayer and multilayer ³He films adsorbed on Grafoil have been measured at temperatures between 1.2 and 4.2 K by a pulsed NMR technique. The T ₁ data for high-coverage films (solid and dense fluid phases) and the substrate registered phase are interpreted in terms of thermally activated vacancies. In solids the quantum exchange inherent in ³He is shown to be important at low temperatures. The data for multilayer films are discussed in the light of the particle exchange between layers and the relaxation time of each layer. The dynamical behavior of adatoms in the solid, fluid, and substrate registered phases as well as the nature of phase transitions between them are discussed on the basis of information obtained from the analysis of T ₁ and T ₂ data. The present results as a whole seem to support the phase diagram determined by specific heat measurements. In addition, the nuclear susceptibility in submonolayer films has been measured by the same technique. The effect of Fermi degeneracy was not seen in the temperature range between 1.2 and 4.2 K.     

SQUID NMR studies of TmPO4

The hyperfine-enhanced nuclear moment in TmPO₄ was studied by SQUID NMR. A resonance frequency _x /2 = 274 MHz per tesla was obtained, leading to an enhancement factor of 77.9. The temperature dependence of the spin-lattice relaxation time T ₁ of ¹⁶⁹Tm nuclear spin in TmPO₄ was measured at liquid helium temperatures. The temperature dependence of T ₁ can be understood in terms of nuclear Orbach and direct processes.     

Interpretation of1H NMR results of HD impurities in solid H2. I. Spin-spin relaxation

Previous¹H NMR results on HD impurities in solid H₂ samples of low ortho concentration (X0.01) are reexamined, resulting in a different interpretation of the spin-spin relaxation timeT ₂. The relaxation time is found to be measured by using short rf pulses rather than the long (100–120 µsec) pulses used previously for the spin-echo pulse sequences. This is due to the fact that the nuclear spin dipolar flip-flop interactions between the HD and ortho-H₂ molecules are effectively quenched because of the large intramolecular dipolar splittings of the H₂ molecules. The measured spin-spin relaxation times for the HD impurities in a sample ofX0.01 are found to be consistent with those from statistical theory, revealing no appreciable motional narrowing, which was invoked previously.     

NMR relaxation measurements in solid H2 at low ortho-H2 concentrations

Nuclear transverse and longitudinal relaxation time measurements in solid hcp H ₂ are presented for two frequencies, 5.3 and 29 MHz. The ortho molefractionX varied from 2×10^–3 to 0.1 and the temperature range extended from 0.4 K to near the triple point, 13.9 K. Over this range ofX andT, the longitudinal timeT ₁is representative of theintramolecular relaxation processes that reflect the orientational fluctuations of the molecules. On the other hand, the rates fromintermolecular dipolar coupling are calculated to be negligible. At concentrationsX0.008, the transverse timeT ₂is dominated by the contributions from intramolecular relaxation processes, and is found to increase quite strongly with temperature. This new effect is ascribed to a coupling between molecular rotation and lattice vibrations, brought into evidence by the narrow width of the fluctuation spectral density at lowX. ForX>0.01 in the nondiffusion region, the NMR line shape is dominated by the signal from isolated ortho-H₂ molecules broadened via intermolecular dipolar interactions, and by the signal from isolated ortho pairs and triangles, etc., configurations. Above 9 K, however, the thermally activated diffusion produces an averaging out of these broadening effects, making it possible to determine theT ₂fromintramolecular nuclear spin interactions. For these mixtures,T ₁also shows a temperature variation, but with a maximum near 7 K and a flat minimum at higher temperatures. The relaxation data are compared with previous experiments and some systematic discrepancies in the dependence onX are found. The frequency dependence ofT ₁extends to higher concentrations than one expects from the theory of Fujio, Hama, and Nakamura, where nuclear relaxation is treated in terms of the orientational fluctuations from intermolecular electric quadrupolar coupling and crystalline fields. In the appendix, results are presented of an earlier attempt in this laboratory to determine the crystalline field splitting inH ₂from the temperature variation of the NMR line shape. An upper bound of |V _C/k _B|=0.028 K is obtained at zero pressure.Research supported by a grant from the AROD and the NSF.     

NMR study of 3He bubbles in phase-separated solid 3He –4He mixtures

³ He droplets embedded in a solid ⁴ He matrix have been studied by NMR and pressure measurements. One feature of the experiment is that the mixture crystals, of ³ He concentration 1%, are grown under constant pressure conditions to minimise the formation of defects. A number of sample pressures below 34 bar have been studied. Isotopic phase separation and the melting of the bubbles are clearly observed. Measurements of T₁ , T₂ and magnetisation give detailed information on the structure of the droplets. At an initial sample pressure of 28.3 bar preliminary measurements of the T₁ of the liquid bubbles show a temperature dependence of the form (A+ B/T²)^–1. This indicates that the expected relaxation in the liquid is augmented by a constant contribution, probably from the surface of the droplets.     

Spin relaxation in normal liquid3He:T 1 in the Fermi liquid (T≪T F) regime

The low-temperature (TT _F) spin relaxation timeT ₁ in normal liquid³He is calculated. First an expression forT ₁ is obtained using the Landau kinetic equation, which is in terms of the perturbations of the scattering amplitudes due to nuclear dipole-dipole interactions. These perturbations are obtained using the induced interaction model, which explicitly includes the effects of many-body correlations arising from the Pauli principle. The results are in good agreement with experiment, as well as with previous calculations.     

Magnetic Correlation in BETS2(Cl2TCNQ)

We carried out⁷⁷Se NMR measurements on BETS₂(Cl₂TCNQ) under pressure in order to investigate the magnetic properties of the insulating state which appears above 0.6 GPa. The relaxation rate 1/T₁ at 0.7 GPa shows small peak-like anomaly at 20 K, indicating a spin density wave transition as observed in BETS₂(Br₂TCNQ).     

Spin-lattice relaxation of Fe3+ in iron doped magnesium oxide

Spin-lattice relaxation measurements for Fe³⁺/MgO were carried out in the temperature range from 4.2 to 27 K at 37.5 Hz utilizing the pulse saturation method. This revealed a close agreement with the theoretical predictions, demonstrating aT ^–1 behaviour at low temperatures as a direct process and aT ^–4.6 variation as a raman process at higher temperatures, with a transition temperature near 20 K between these two regimes. Measurements oft ₁ were carried out for samples with iron concentrations of 140, 310 and 110 ppm and gave the values of 1.0, 0.8 and 0.1 msec, respectively, at 4.2 K. This confirms that the inverse proportionality oft ₁ with concentration holds over this range of samples. Variation of relaxation timet ₁ with crystal orientation for the central transition of Fe³⁺/MgO revealed two minima that were explained as due to crossrelaxation mechanisms. Comparison of our results with both those computed by Shiren at 10 GHz and the experimental results of Castle and Feldman at the same frequency showed that at ₁ (frequency)^–4 law was obeyed. The present data has provided another example of anS-state ion behaving in the manner predicted by the present theories of relaxation.ESR = election spin resonance.     

Magnetic dissipation in superfluid3He-B in the transition regime to nonhydrodynamic conditions

We have applied the so-called HPD spectroscopy, i.e. the NMR of the homogeneously precessing domain (HPD) in superfluid³He-B, to study the magnetic relaxation processes down to 0.3T _c . The power absorbed by HPD from cw NMR rf-field was divided into several contributions corresponding to different relaxation processes. From these contributions nonhydrodynamic corrections to spin diffusion and Leggett-Takagi (L-T) relaxation were extracted for a pressure of 6 bar. Below 0.45T _c a large absorption term linear with the HPD length and increasing with the falling temperature was observed. This term could be associated with an instability of coherent precession in the nonhydrodynamic regime. The process of the HPD formation in the nonwetting regime, forT<0.37T _c , was observed and described, leading to a new possibility to determine thed-constant of the surface energy.     

NMR on Systems of Low Spin Density Using DC Squids

We discuss the use of SQUID NMR for the study of systems of low spin density. The sample is located inside a coil which forms part of a series tuned resonant circuit attached to the input coil of a SQUID. Such a scheme was first discussed by Freeman et al. ¹ We have studied the pulsed NMR response at 1 MHz of a ³ He film adsorbed on the surface of closely packed Mylar sheets. In this case a monolayer corresponds to a spin density a factor of order 104 smaller than that of bulk liquid and a factor of order 10 ² smaller than with a Grafoil substrate. For our particular SQUID and input coil the calculated noise temperature is 60 mK, significantly better than that so far achieved with a cooled semiconductor preamplifier. We evaluate the present and potential performance of the spectrometer, some of the practicalities involved in its implementation, and discuss the minimum number of detectable spins.     

Structural studies of poly(p-phenylene selenide) doped with sulphur trioxide

Poly(p-phenylene selenide) (PPSe) was prepared by condensation polymerization of 2,4 dibromobenzene and sodium selenide. The synthesis product in the form of powder was purified. The structure of the resulting polymer was investigated by infrared spectroscopy, wide-angle X-ray spectroscopy, electron diffraction, ultraviolet, electron spin resonance and elemental analysis.¹H-nuclear magnetic resonance (NMR) solid measurements were used to determine the molecular dynamics for undoped and SO₃-doped PPSe. The NMR investigations for undoped PPSe have shown that there are no essential differences in the structure and molecular motion between PPSe and PPS. After doping PPSe with SO₃, contrary to PPS, a third component of relaxation time,T ₁, is observed. The relaxation times for this component are connected to the interaction of protons with paramagnetic centres which are generated as a result of the doping process. The electrical conductivity of SO₃-doped PPSe at the beginning of the doping process rapidly increased to about 6 ×10^–6 S cm^–1 and then decreased more than one order of magnitude because of the chemical reaction which had occurred.     

Spin-Spin Relaxation in Phase-Separated 3He-4He Solid Mixtures

The spin-spin relaxation time T ₂ in a ³ He- ⁴ He solid mixture with an initial concentration of 3.18% ³ He is measured during its phase separation in a temperature range of 1-250 mK. Cooling down to the region of separation was carried out by small steps (10 mK) followed by temperature stabilization for many hours. The time T ₂ was measured by using the pulsed NMR technique at a frequency of 250 kHz. The spin echo method makes it possible to distinguish the contributions to magnetic relaxation from both the concentrated and the dilute separated phases. The relaxation time T ₂ in the concentrated phase is found to be practically independent of temperature down to 50 mK and is determined by the same ³ He- ³ He exchange interaction as in pure bulk solid ³ He. It is found that the behavior of the spin echo signal at ultralow temperatures exhibits an anomaly, which may be connected with quasi-one-dimensional spin diffusion. In the dilute daughter phase the spin-spin relaxation time is inversely proportional to concentration and is described correctly by the Torrey model taping into account ³ He- ⁴ He tunnel exchange. The values of T ₂ in this phase coincide with those for a homogeneous (non-separated) mixture of the same concentration.     

Nuclear Magnetism of Normal 3He and 3He-4He Mixtures in Aerogel

We report NMR experiments at 8 T on ³ He and ³ He- ⁴ He mixtures filling the pores of 95% porous aerogel, for temperatures T 6 mK. Magnetization measurements of pure ³ He reveal a localized layer approximately one monolayer thick. The longitudinal relaxation includes a component logarithmic in time, which is apparently associated with a fraction of the localized ³ He atoms. When the localized ³ He is displaced by adding ⁴ He the logarithmic relaxation disappears and T ₁ for the dominant exponential relaxation increases. Measurements of the spin diffusion coefficient with the aerogel filled with dilute solution in equilibrium with bulk phase-separated mixture provide an unambiguous determination of the spin mean free path,_s = 58 nm     

SQUID-Detected Magnetic Resonance Imaging in Microtesla Magnetic Fields

We describe studies of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) of liquid samples at room temperature in microtesla magnetic fields. The nuclear spins are prepolarized in a strong transient field. The magnetic signals generated by the precessing spins, which range in frequency from tens of Hz to several kHz, are detected by a low-transition temperature dc SQUID (Superconducting QUantum Interference Device) coupled to an untuned, superconducting flux transformer configured as an axial gradiometer. The combination of prepolarization and frequency-independent detector sensitivity results in a high signal-to-noise ratio and high spectral resolution (～1 Hz) even in grossly inhomogeneous magnetic fields. In the NMR experiments, the high spectral resolution enables us to detect the 10-Hz splitting of the spectrum of protons due to their scalar coupling to a ³¹P nucleus. Furthermore, the broadband detection scheme combined with a non-resonant field-reversal spin echo allows the simultaneous observation of signals from protons and ³¹P nuclei, even though their NMR resonance frequencies differ by a factor of 2.5. We extend our methodology to MRI in microtesla fields, where the high spectral resolution translates into high spatial resolution. We demonstrate two-dimensional images of a mineral oil phantom and slices of peppers, with a spatial resolution of about 1 mm. We also image an intact pepper using slice selection, again with 1-mm resolution. In further experiments we demonstrate T₁-contrast imaging of a water phantom, some parts of which were doped with a paramagnetic salt to reduce the longitudinal relaxation time T₁. Possible applications of this MRI technique include screening for tumors and integration with existing multichannel SQUID systems for brain imaging.     

Concentration dependence ofD,T 1, andT 2 for dilute solutions of3He in solid4He

Measurements of the spin diffusion coefficientD and NMR relaxation timesT ₁ andT ₂ are reported for dilute solutions of³He in solid⁴He at two molar volumes, 20.95 and 20.7 cm³. The weakly interacting impuriton model, for whichD ^–1 is proportional to impurity concentration, is observed only at fractional impurity concentrationx ₃ below 3 × 10^–4. Forx ₃ around 10^–3,T ₁ andT ₂ are controlled by the formation and breakup of³He₂ molecules.     

Spatially-Resolved NMR Study on Antiferromagnetic Vortex Core in HTSC

Magnetism in the vortex core of nearly-optimal doped Tl₂Ba₂CuO₆₊ (T _c=85 K) is investigated by a spatially-resolved NMR. The NMR relaxation rate T ^–1 ₁ at ²⁰⁵Tl site provides a direct evidence of the significant enhancement of the AF spin correlations in the vortex core region and shows clearly a local AF ordering of the core region Cu spins at T _N=20 K. Above T _N the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T ^*–120 K), indicating that the pseudogap disappears within the vortex core.     

Charging Effects on Nuclear Spin Relaxation in Quantum Dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with 2D gas. We show that the nuclear spin relaxation rate T ₁ ^–1 is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin–orbit (SO) coupling the relaxation rate is maximal in the CB valleys whereas for the weak SO coupling the maximum of 1/T ₁ is near the CB peaks. The physical mechanism of nuclear spin relaxation rate at strong SO coupling is identified as Debye–Mandelstam–Leontovich–Pollak–Geballe relaxational mechanism.     

Self-consistent approximations in the theory of triplet pairing superfluidity in3He

Spin-lattice relaxation times of bcc solid ³ He with impurity concentrationsx of ⁴ He, 1.4×10 ^?3 <x<7 × 10 ^?3 , have been studied by NMR pulse techniques in the temperature range 0.27 K<T<1.2 K. In the lower part of the temperature range a temperature-independent relaxation process governed by the impurities is observed and its characteristic timeT _1D varies as a simple function ofx:T _1D> ∞x ^?3; the features of this process seem to be consistent with a diffusion mechanism to some clusters of ⁴ He impurities. In this plateau region a long relaxation time is also measured and found to be insensitive tox and strongly dependent on the temperature. A phenomenological model is suggested.     

Nuclear magnetic resonance studies of isotopic impurity tunneling in solid hcp4He

Nuclear magnetic relaxation studies of isotopically impure solid⁴He have been made in the temperature-independent region of the relaxation spectrum, below 0.7 K. The Torrey theory for nuclear relaxation has been applied to measurements ofT ₁ andT ₂ to calculate the characteristic fluctuation time of the³He atoms due to the zero-point motion of³He and⁴He atoms. The fluctuation rates have been determined as a function of molar volume and Larmor frequency in samples where the mole fractions of³He in the gaseous mixtures used to form the solids were 0.02, 0.01, 0.005, and 0.002. The volume dependence of the fluctuation rate has been found to be far greater than that of the exchange rate of a³He-³He pair.T ₁ has been found to vary as ₀ ² , and bothT ₁ andT ₂ increased as the concentration decreased, in agreement with the Torrey theory. Preliminary investigations of the solid isotopic phase separation have also been carried out.Research supported in part by the National Science Foundation through Grant No. GP-29682 and the Advanced Research Projects Agency through the Materials Science Center at Cornell University, MSC Report # 1675.