W. G. Clark

MAGNETIC RESONANCE SPECTRAL RECONSTRUCTION USING FREQUENCY-SHIFTED AND SUMMED FOURIER TRANSFORM PROCESSING

A novel method of Fourier transform spectroscopy of the transient signals from wide, inhomogeneously broadened magnetic resonance spectra is described and analyzed. It has the advantages of high resolution, high sensitivity, and freedom from the distortions introduced by the finite amplitude of the pulsed rf magnetic field and the finite bandwidth of the receiving system. It consists of recording the transient signal at a series of magnetic fields, shifting the frequency of the transient by the corresponding field step for each point, and summing the corresponding Fourier transformed signals. Although the primary emphasis is on pulsed NMR, the analysis also applies to pulsed ESR. Criteria for the range and step interval of the magnetic field variation are discussed. The accuracy and sensitivity of the method are compared with earlier methods of spin echo spectroscopy. A description of the corresponding measurement of NQR, NMR, and ESR spectra obtained by stepping the frequency of the spectrometer is also ...

Open‐ended coaxial‐line technique for the measurement of the microwave dielectric constant for low‐loss solids and liquids

A system which enables fast and reliable measurements of the dielectric constant over continuous microwave frequency ranges for both solid and liquid low‐loss materials is described. The main thrust of this work is the application of the open‐ended coaxial‐line probe technique, which has been used previously for soft biological materials, to low‐loss solid samples. Using the instrumentation and procedure presented here, the dielectric constant for low‐loss solids can be measured absolutely to ±2%–3% with routine care. The uncertainty can be reduced by about a factor of 2 by averaging several measurements. It is also smaller for liquid samples. This application features the use of relatively simple and readily available microwave components. Also, it is shown that a simple empirical relationship can be used to obtain the bulk dielectric constant from samples of a material in the form of thin slabs. The experimental results which are presented here for kapton, Teflon, Corning glass No. 0211, soda lime glass...

Superconducting fluctuations and the pseudogap in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8: high magnetic field NMR studies

From measurements of the 63Cu Knight shift ( K) and the nuclear spin-lattice relaxation rate ( 1/T1) under magnetic fields from zero up to 28 T in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8 ( T(c) = 68 K), we find that the pseudogap behavior, i.e., the reductions of 1/T1T and K above T(c) from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.

Measurement of the microwave dielectric constant for low‐loss samples with finite thickness using open‐ended coaxial‐line probes

This work addresses the effect of finite sample thickness on microwave dielectric constant measurements for thin, planar, low‐loss samples using the open‐ended coaxial‐line probe method. Detailed measurements of the dielectric constant were carried out on a wide range of thicknesses of air samples which were backed by infinitely thick teflon and alumina dielectric media. The measurements were made at room temperature for various (50 Ω) coaxial‐line dimensions, microwave frequencies 4–8 GHz, and power levels near a fraction of a mW. The results provide strong support for previously published theoretical calculations based on a boundary value problem which uses a spectral domain formulation for the aperture fields. From thin, planar samples, values of 10.4±0.5 and 25.9±1.3 were obtained at 5 GHz and 300 K for the bulk dielectric constant of MgO and LaAl2O3, respectively. The applicability of a simple empirical model based on an exponential fit is discussed.

Is there a phase transition in CeAl3

Abstract Experimental results for the 27 Al nuclear spin-lattice relaxation rate and NMR absorption line shape over a broad range of frequencies down to 0.48 and 0.6 K respectively are reported for CeAl 3 . A search for the antiferromagnetic transition at 1.2 K reported by Nakamura et al. indicates that it does not occur in pure CeAl 3 .

Ripple method: An application of the square‐wave excitation method for heat‐capacity measurements

An extension of the square‐wave excitation method for measuring heat capacity is reported. In particular, attention is focused on the limit where the period of the heat pulse excitation is much smaller than the thermal relaxation time of the calorimeter‐sample system. In this limit, one finds that the thermal ripple induced by the heat pulse is inversely proportional to the heat capacity. This technique allows one to do real‐time heat‐capacity measurements as in the conventional ac technique with the same instrumentation as the square‐wave method. The ripple method is demonstrated with a measurement of the heat capacity of strained Al near its superconducting phase transition.

Microwave electron spin resonance spectrometer with operation to 54 mK in a dilution refrigerator

A 9.5‐GHz electron spin resonance ESR spectrometer which operates in conjuction with a helium dilution refrigerator down to 54 mK is described. Instead of the conventional waveguide and cavity arrangement, coaxial and superconducting twisted pair transmission lines are used to bring the microwave power into the mixing chamber, where a tiny coil surrounds the sample. This low Q, high‐filling factor configuration has a sensitivity about the same as that of a conventional high Q cavity with a small filling factor. Several suggestions for further development and improvement are presented.

High-magnetic-field NMR studies of LiVGe2O6: A quasi-one-dimensional spin S=1 system

We report

Interlayer decoupling, Lebed magic angle magnetoresistance and triplet superconductivity in (TMTSF)2PF6

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NMR as a probe of incommensurate spin density waves in organic metals

Li pulsed NMR measurements in polycrystalline and single crystal samples of the quasi one-dimensional S=1 antiferromagnet LiVGe