Determination of the Mosaic Angle Distribution of Grafoil Platelets Using Continuous-Wave NMR Spectra

We described details of a method to estimate with good accuracy the mosaic angle distributions of microcrystallites (platelets) in exfoliated graphite like Grafoil which is commonly used as an adsorption substrate for helium thin films. The method is based on analysis of resonance field shifts in continuous-wave (CW) NMR spectra of ³He ferromagnetic monolayers making use of the large nuclear polarization of the adsorbate itself. The mosaic angle distribution of a Grafoil substrate analyzed in this way can be well fitted to a gaussian form with a 27.5±2.5 deg spread. This distribution is quite different from the previous estimation based on neutron scattering data which showed an unrealistically large isotropic powder-like component.     

Nuclear magnetic resonance of 3He adsorbed submonolayers on Grafoil and argon-coated Grafoil. I. Low coverages

Continuous-wave (cw) nuclear magnetic resonance measurements have been made at 5.5 MHz of ³He adsorbed on Grafoil and on argon (monolayer)-coated Grafoil at coverages x 0.40 (x = 1 corresponds to one monolayer), in the temperature range 0.35–4.21 K. To determine the fractional coverages, the adsorption area of the Grafoil and the monolayer coverages were determined from observations of argon and ³He adsorption isotherms. The results indicate that the adsorbed ³He is highly mobile and behaves as a two-dimensional nonideal Fermi gas. Relative magnetic susceptibilities determined from the NMR lines indicate that the onset of Fermi degeneracy is delayed by spin-independent ³He-³He interactions, as shown by the fit to the data of the quantum second virial coefficient expansion for the magnetic susceptibility. It is established that the residual (coverage-independent) linewidth is largely determined by spin-diffusion through local field gradients in voids in the Grafoil.Supported by a grant from the National Science Foundation.     

Surface Magnetism of 3He in Grafoil

We report the magnetic susceptibility of ³He in Grafoil filled with pure liquid³He at 27.6 bar and at temperatures down to 0.1 mK with a cw NMR method. It is composed of two contributions: from the bulk liquid and from the adsorbed layer of ³He on the Grafoil surface. The latter shows a well-known strong ferromagnetic tendency and can be fitted to a Curie–Weiss law in the high temperature region. The obtained Weiss temperature is surprisingly large compared with the previous ones.     

Helium adsorption on exfoliated graphite

High precision adsorption isotherms of³He and⁴He on bare Grafoil and on Grafoil coated with a monolayer of argon have been measured in the temperature range 4–20 K, using a high resolution, pressure-sensitive capacitance gauge located at 4.2 K close to the sample chamber. This gauge obviated thermomolecular pressure effects and corrections. The isotherms yielded the following resultant data: The monolayer coverage V_m for⁴He on bare Grafoil was 0.42 cm³ (STP)/m² and for³He on bare Grafoil was 0.395 cm³ (STP)/m²: the isosteric heat of adsorptionQ _st/R of the second layer of³He on bare Grafoil was 23.5 K andQ _st/R for³He on argon-coated Grafoil for the first layers was 47.5 K. Also, the data for³He and⁴He on bare Grafoil at and just aboveV _m have been used by Novaco (see subsequent paper) for determination of the virial coefficients of the gas phase occuring in the early formation of the second layer.     

Nuclear magnetic resonance of adsorbed 3He films on Grafoil and argon-coated grafoil. II. Intermediate and near-monolayer coverages

Continuous wave (CW) nuclear magnetic resonance measurements have been made at 5.5 MHz of ³He adsorbed on Grafoil and on argon (monolayer)-coated Grafoil at coverages 0.55 x 1.35 (x = 1 corresponds to one monolayer), in the temperature range 0.35–4.21 K. The results indicate that the near-monolayer coverages (0.9 x 1.0) behave as 2D solids with dipole-dipole coupling modulated by quantum exchange between nearest neighbors. The exchange frequency for this solid phase is evaluated and is seen to be comparable to that observed in bulk, solid ³He for similar atomic spacing. The intermediate coverages, 0.55 x 0.80, showed no evidence of line broadening in the entire temperature range investigated, while the relative magnetic susceptibilities indicated no deviation from Curie's law. No evidence for the registered phase was found at x = 0.60 either for ³He on Grafoil or for ³He on argon-coated Grafoil. A sharp decrease was observed in linewidth after completion of the monolayer, making NMR an excellent probe for the determination of monolayer completion. We have also made linewidth measurements for a coverage x = 0.95 of ³He on graphitized carbon black (Sterling FT grade) at 4.2 and 1.2 K. The results are consistent, as in the case of ³He on Grafoil, with a 2D solid with significant quantum tunneling between nearest neighbors. This result, however, is in disagreement with the measurements of Rollefson, also for ³He on graphitized carbon black of the same grade, who observed a rigid lattice behavior with negligible tunneling between neighbors, for a coverage x = 0.90. This discrepancy is not yet understood.Supported by a grant from the National Science Foundation.     

A Faraday Magnetometer for Studies of Monolayer 3He Films on Graphite

No Heading A Faraday-type magnetometer has been developed to measure the nuclear magnetization of monolayer ³He films adsorbed on Grafoil at temperatures down to 0.1 mK under high magnetic fields up to 10 T. The magnetic force is measured capacitively by monitoring the displacement of a wire-suspended copper plate. A double gradient coil system is employed to produce opposite field gradients at two regions the same distance apart from the center. At each region, exfoliated and non-exfoliated graphite foils are put one by one to eliminate a large background signal from Grafoil and copper.PACS numbers: 67.70.+n, 75.10.Jm     

Pulsed NMR of monolayer and near-monolayer films of 3He adsorbed on Grafoil at low temperatures

Pulsed NMR measurements at 30 MHz of near-monolayer coverages of ³He on Grafoil have been made between 4.2 and 0.1 K. The time constants of the free induction decay indicate that below 4.2 K the mobility of ³He in an exact monolayer is smaller than that at lower coverages or at coverages above one monolayer, in agreement with the results found by Hegde and Daunt, by Cowan and co-workers, and by Rollefson. Furthermore, in spite of rf heating of the sample, we were able to conclude that the mobility of ³He in nearmonolayers films decreases as the temperature is decreased below 4.2 K. At 4K and 30MHz our measurements give T₁ = 1 ± 0.3 sec for coverages between 0.6 and 1.35 monolayers. This is in agreement with previous work.Supported by the National Science Foundation.     

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.     

Surface Magnetism of Liquid 3He on 4He Pre-plated Graphite

No Heading The nuclear susceptibility of liquid ³He in Grafoil pre-plated by 2.5 and 3.5 layers of ⁴He has been studied with a cw NMR method at temperatures between 0.7 and 100 mK under various liquid pressures. The 3.5 layers of ⁴He pre-plating suppresses a formation of the first and second solid ³He layer, eliminating most of surface magnetization at saturated vapor pressure. However, with increasing liquid pressure, a magnetization obeying a Curie Weiss law gradually grows in the same way as in the previous experiment for pure liquid ³He. This magnetization, induced by pressurization, is attributable to the formation of solid ³He layer above the pre-plated ⁴He.PACS numbers: 67.80.Jd, 75.70 Cn, 67.70.+n.     

Vapor pressure measurements of adsorbed neon on exfoliated graphite between 15 and 30 K

Vapor pressure measurements of neon adsorbed on bare Grafoil for coverages of 0.5, 0.75, 1.00, 1.25, 1.50, and 1.75 were made as a function of temperature in the range 15–30 K using a standard volumetric method. Data plots of ln p vs. (1/T) were obtained for the coverages x of 1.25, 1.50, and 1.75 monolayers and there is an indication of a phase transition at about 19 K, which agrees with the results of specific heat data on the same system. The molar entropy and internal energy were computed as a function of coverage. Isosteric heats of adsorption as a function of coverage were also computed from the data and are in good agreement with the results given by other authors on graphite surfaces. The binding energy of Ne on Grafoil was calculated from the isosteric heat for x 0, yielding a value of 323 K.Work supported by FINEP, CNPq, and CEPG.     

NMR Measurements of 2D 3He Adsorbed on ZYX Exfoliated Graphite

We describe first successful usage of ZYX exfoliated graphite to cool adsorbed two dimensional (2D) ³He to ugtra low temperatures (ULT). ZYX has much better characteristics as an adsorption substrate such as larger crystallite (platelet) size and smaller mosaic angle than Grafoil, a widely used exfoliated graphite in ULT experiments. In spite of the relatively small surface area of ZYX, we could perform a nuclear magnetization measurement of paramagnetic 2D solid ³He with continuous wave NMR below 10 mK. The data follow nicely the Curie law down to 200 µK. This indicates that ZYX can be used in experiments to study detailed density dependences of low temperature properties of 2D ³He including possible BCS transitions. We also show a preliminary resugt of pulsed NMR measurements of high density 2D fluid ³He adsorbed on ZYX preplated with a ⁴He monolayer.     

Surface Observation and Magnetization Measurements of Grafoil Substrate

The important roles of impurity grains in graphite substrate in heat transfer mechanisms between ³He films and graphite substrate have been suggested. At sub-milli-Kelvin temperature ranges in particular, magnetic impurity grains of large diameter, about several micrometers are believed to play an important role in the heat transfer mechanism. However, the existence of such large grains is still controversial. Freshly cleaved surfaces of Grafoil are observed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses. Although results are preliminary, impurity grains with diameters of 0.5??10 ??m containing Al, Ca, Si, Fe, etc., are observed. Ferromagnetism is also observed after the subtraction of the diamagnetic contribution in magnetization measurements of Grafoil. The saturation magnetization coincides with the amount of magnetic impurities determined by ash analysis.     

Magnetic susceptibility of submonolayer3He films

We calculate the magnetic susceptibility of submonolayer³He films. First, using the formulation of the equilibrium thermodynamic properties in terms of statistical quasiparticles, we obtain an expression for the magnetic susceptibility of dilute systems of particles interacting through a short-range potential, taking into account binary interactions between the particles. At high temperatures our expression reduces to the one obtained using the virial expansion truncated after the second term. Next, we apply this expression to³He in two dimensions and compare our results with the magnetic susceptibility data of submonolayer³He films adsorbed on Grafoil and on argon-coated Grafoil and previous calculations.     

苯乙烯乙烯/丁烯嵌段共聚物的二维核磁方法

利用二维核磁方法对苯乙烯-乙烯/丁烯嵌段共聚物的链段微结构进行了定性。利用普通一维1H和13C核磁得到了共聚物分子的基本结构信息;利用1H-1H-COSY二维核磁确定了共聚物中同核氢质子之间的偶合关系;利用13C-1H-HSQC和13C-1H-HMBC二维核磁确定了共聚物异核碳氢原子的单键相关性,明确了直接相连接的C-H化学位移,为微结构的确定提供了依据。归属了苯乙烯嵌段结构中的亚甲基和次甲基的化学位移;同时归属了乙烯-丁烯嵌段链段结构中的甲基、亚甲基和次甲基的化学位移,并确定了重复单元的链段结构状态。二维核磁方法可以得到很多在普通一维核磁中难以解析的信息。二维核磁是对苯乙烯-乙烯/丁烯嵌段共聚物链结构定性的有效手段。     

Low-temperature adsorption isotherms of 3He and para-H2 on grafoil and their normalization

Measurements have been made of the adsorption isotherms of ³He and of para-hydrogen (p-H₂), on bare Grafoil, the former in the temperature range 0.88–1.23 K, and the latter in the temperature range 10.0–20.0 K. For p-H₂ the quantity of material adsorbed plotted against (p/p ₀) ^T yields a single temperature-independent curve, as would follow from the potential theory for multilayer adsorption, and as emphasized by Chester et al. in 1974. The same behavior was found for ³He on Grafoil, except that deviations from a single temperature-independent curve occurred at the higher temperatures. For p-H₂ on Grafoil it is to be noted that the isotherms taken below the triple point of free, bulk p-H₂ fit the same single temperature-independent curve as do the isotherms measured at temperatures between the triple point and the boiling point. From the data, the isosteric heats of adsorption were calculated over a wide range of coverage and this provided further experimental evidence for the similarity between the uppermost layers of thick adsorbed films and the free liquid of the same substance.Supported by the National Science Foundation and the Brazilian National Council of Scientific and Technological Development (CNPq).     

Superfluid transition of4He films adsorbed on graphite substrates: A calorimetric study in connection with third-sound observations

The heat capacity of⁴He films adsorbed on Grafoil has been reexamined with emphasis on its correlation with our previous third-sound observations. Each of the heat capacity curves shows an inflection point, which shifts to lower temperature as the film gets thinner. The temperature of this inflection point is higher than the onset temperature determined by the third-sound measurements for any given thickness. This result can be explained within the framework of recent theoretical studies based on the Kosterlitz-Thouless theory. In addition, the van der Waals constant is evaluated from the data on vapor pressure versus temperature, and it is estimated to be 201±30 me V ų (=50.1±7.5 K layer³) in good agreement with a recent theoretical value derived by Vidali et al. Moreover, this evaluation confirms our previous conclusion deduced from third-sound observations.     

Sugar interaction with metals in aqueous solution: indirect determination from infrared and direct determination from nuclear magnetic resonance spectroscopy

In this article, mid-infrared Fourier transform (Mid-FT-IR) and carbon thirteen nuclear magnetic resonance (13C NMR) spectroscopy have been used to determine possible interactions between sucrose and various alkali or alkaline earth metals in aqueous solution. In the presence of these metals, significant shifts in the absorption bands of sucrose were noted by mid-FT-IR coupled with principal component analysis (PCA). These shifts were explained on the basis of weakening of the H-bond network between sucrose and water and possible interactions between sucrose and the metal ion. Factorial maps were established and the spectral patterns obtained show that these interactions vary according to the nature of the metal ion. 13C NMR analysis showed that the carbon atoms of sucrose undergo shielding or deshielding in the presence of metal ions in aqueous solutions. Two factors were invoked to account for the variation of chemical shifts: the rupture of hydrogen bonds due to hydration of the metal ion and the possible coordination of the metal ion to the oxygen atoms of sucrose.     

Thermal conductivity of several exfoliated graphites from 2 k to 300 k

The thermal conductivity of several exfoliated graphites (Grafoil, Foam, and UCAR-ZYX graphite) has been measured in the range 2 to 300 K. The temperature dependence of the thermal conductivity is similar to that of near-single-crystal pyrolytic graphite, but the value is two orders of magnitude lower.     

Phase Transitions of Superfluid 3He in 0.8-μm Slab Geometry

We have studied superfluid ³ He in 0.8 m slab geometry by cw-NMR method. The static magnetic field for NMR was 22 mT and was perpendicularly applied to the surface. At pressures from 5 bar to 27 bar, we observed the negative shifts of NMR frequencies below the transition temperatures. Moreover we observed the fast order phase transition with the supercooling phenomena at lower temperatures, but NMR frequencies did not show a jump as they do at the A-B transition in the bulk liquid. Although the phase at low temperatures seemed to be B-phase, NMR frequencies still had the negative shifts.