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1

Electronic Resource

Cryogenic GaAs FET amplifiers and their use in NMR detection (1986)

Richards, M. G. ; Andrews, A. R. ; Lusher, C. P. ; [et al.]

[S.l.] : American Institute of Physics (AIP)

Review of Scientific Instruments 57 (1986), S. 404-409

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ISSN: 1089-7623

Source: AIP Digital Archive

Topics: Physics , Electrical Engineering, Measurement and Control Technology

Notes: The noise properties of GaAs FET devices have been measured in the frequency range 0.2–20 MHz at temperatures 300, 77, and 4.2 K. Significant reduction in all contributions to noise parameters was found on cooling the devices. Cooled GaAs FETs were found to be suitable for use in the preamplifier of a pulsed NMR spectrometer. An amplifier noise temperature of 0.9 K was found at 1.16 MHz and the observed signal-to-noise ratio using a 3He gas sample was found to agree with calculation. Precise expressions for signal size are obtained using a spherical receiver coil, and employment of these facilitates the calculations and provides a basis for comparing sensitivities of different NMR spectrometers.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1138899

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2

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Longitudinal relaxation time for dilute quantum gases (1990)

Mullin, W. J. ; Laloë, F. ; Richards, M. G.

Springer

Journal of low temperature physics 80 (1990), S. 1-13

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ISSN: 1573-7357

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract We calculate the longitudinal relaxation timeT 1 for a polarized spin-1/2 Fermi gas, in zero magnetic field, for conditions of temperatureT and densityn such that Boltzmann statistics are valid. Our results show generally thatT 1 is independent of polarization of the gas. At highT, where the thermal wavelength λ is small compared to the scattering lengtha, T 1 is proportionalT 1/2, while at lowT, such that λ is greater thana, T 1 is proportional toT −1/2.T 1 thus has a minimum at some intermediate temperature confirming the numerical results of Shizgal. Physical arguments show that the existence of the minimum does not depend on the presence of an attractive part of the potential. As an example of the expected temperature dependence we calculateT 1 numerically, via the distorted-wave Born approximation, for the case of a gas interacting via a hard core. We also computeT 1 for a spin-1/2 Bose gas, which also shows a minimum.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00683111

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3

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Anomalous temperature dependence ofD andT 2 for dilute solutions of3He in solid4He (1982)

Allen, A. R. ; Richards, M. G. ; Schratter, J.

Springer

Journal of low temperature physics 47 (1982), S. 289-320

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ISSN: 1573-7357

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract Results of measurements of the spin diffusion coefficientD and NMR relaxation timesT 1,T 2, and T1p are presented for a range of fractional3He concentrations 1 × 10−4≤x 3≤2.5 × 10−3 in solid4He at molar volumes 19.85≤V m≤21.0 cm3 and temperatures 0.4〈T〈2 K in both hcp and bcc phases. We observe a minimumD(T) atx 3=5×10−4, which is interpreted in terms of a transition from coherent impuriton motion to thermally activated diffusion. ForT〈0.8 K, ∂(lnD)/∂(lnV m)=60±8. TheT 2 measurements show a minimum as a function of temperature forx 3≤10−3. TheT 2 (T) andT 1 (T) results yield values for activation energy and tunneling frequency of vacancies in these dilute solutions. Forx 3=5×10−4 andT ∼ 0.5 K,T 2 (V m) is anomalous.T 1ρ measurements at the same concentration indicate there is an important contribution to the spectral density of dipole field fluctuations in the kHz region.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00683732

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4

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Nuclear magnetic relaxation of3He gas. I. Pure3He (1988)

Lusher, C. P. ; Secca, M. F. ; Richards, M. G.

Springer

Journal of low temperature physics 72 (1988), S. 25-69

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ISSN: 1573-7357

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract Longitudinal relaxation timesT 1 have been measured in3He gas, using pulsed NMR, for number densities between 3 × 1023 and 6 × 1025 spins m−3 and temperatures between 0.6 and 15 K. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT 1 give information about the surface phases. A cryogenic wall coating of solid molecular hydrogen was found to delay the formation of a3He monolayer on cooling, andT 1 measurements were consistent with a binding energy of ∼13 K for a3He atom to a hydrogen surface. At temperatures below ∼2 K a completed3He monolayer forms on the H2 coating. No variation of the areal density of monolayer completion with bulk number density at fixed temperature could be observed and the completed3He monolayer is thought to be a dense fluid. Baking the Pyrex sample cells under vacuum and using an rf discharge in3He gas to clean the walls before sealing in the sample gas were found to increase the observed T1's by up to three orders of magnitude. Once a3He monolayer has formed on the H2 surface in these cleaned, sealed cells, the dipolar interaction between adsorbed spins is thought to be the dominant source of longitudinal relaxation. The data are consistent with a dipolar relaxation model with a correlation time of ∼2 × 10−9 sec. This time is long compared to the value of 10−11 or 10−12 sec in the 3D fluid. This suggests that if the surface phase is a 2D fluid and the dipolar mechanism is indeed the dominant one, then the atoms in the 2D fluid are less mobile than in three dimensions. This is consistent with recent susceptibility measurements.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00681726

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5

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Nuclear magnetic relaxation of3He gas. II.3He-4He mixtures (1988)

Lusher, C. P. ; Secca, M. F. ; Richards, M. G.

Springer

Journal of low temperature physics 72 (1988), S. 71-97

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ISSN: 1573-7357

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract Longitudinal relaxation timesT 1 have been measured in3He-4He gas mixtures, using pulsed NMR, in the temperature range 0.6–15 K. Helium-3 number densities of the order of 1024 atoms m−3 were used. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT 1 give information about the surface phases. A cryogenic precoating of solid molecular hydrogen was used to reduce the helium-substrate binding energy from ∼100 K on Pyrex to ∼ 13 K for3He and 15 K for4He. TheT 1 data at high temperatures were similar to those observed previously in the pure3He-H2 system. The presence of4He generally causedT 1 to rise on cooling below 2 K due to the preferential adsorption of4He over3He at the surface. However,3He atoms that go into quasiparticle states in the superfluid helium film can be an extra source of relaxation. In uncleaned cells, relaxation probably takes place in quasiparticle states at the free surface of the superfluid film, which are bound with an energy of 5.1±0.3 K. Baking the Pyrex cells under vacuum and rf discharge cleaning the walls before sealing in the sample gas were found to increase the bulk gasT 1 by two or three orders of magnitude. In a cleaned, sealed cell aT 1 of ∼8 h was measured at 7.7 MHz and 0.8 K. In this case relaxation is probably occurring two or three helium layers away from the helium-hydrogen interface. It may be possible to observe a predicted minimum in the intrinsic dipolarT 1 of the bulk gas by using a4He wall coating to suppress wall relaxation effects (which usually dominate the nuclear relaxation of the bulk gas).

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00681727

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6

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Concentration dependence ofD, T 1, andT 2 for dilute solutions of3He in solid4He (1984)

Schratter, J. ; Allen, A. R. ; Richards, M. G.

Springer

Journal of low temperature physics 57 (1984), S. 179-187

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ISSN: 1573-7357

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00681522

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7

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NMR measurements on dilute solutions of3He in solid4He (1976)

Richards, M. G. ; Pope, J. ; Tofts, P. S. ; [et al.]

Springer

Journal of low temperature physics 24 (1976), S. 1-24

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ISSN: 1573-7357

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract Measurements of the spin diffusion coefficientD and relaxation timesT 1 andT 2 are reported for3 He impurity in solid4 He of fractional concentrationx 3 down to 10−4. Forx 3≤10−3,T∼0.5 K, and molar volume 21.0 cm3,Dx 3=1.2±0.4×10−11 cm2 sec−2,T 2 x 3=1.7×10−4 sec, andT 1 x 3=6.6×10−2 sec at 2 MHz. The spectral densityJ(ω) for dipole field fluctuations has been studied as a function of frequency from 104 to about 107 Hz. Two branches of approximately equal area have been found with scaling frequencies of about 104 and 106 Hz. It is argued that the high-frequency branch, which has some unusual structure, relates to 3 He- 4 He tunnelling, uninhibited by other 3 He impurities, and occurs at a rate of about 0.6 MHz for molar volume 21 cm3. The molar volume dependence of this tunneling appears to be similar to that found in pure, solid 3 He.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00659191

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