Dielectric resonator-based resonant structure for sensitive ESR measurements at high-hydrostatic pressures

EPR UNIFORM FIELD SIGNAL ENHANCEMENT BY DIELECTRIC TUBES IN CAVITIES

The dielectric tube resonator (DTR) for EPR spectroscopy is introduced. It is defined as a metallic cylindrical TE011 microwave cavity that contains a dielectric tube centered on the axis of the cylinder. Contour plots of dimensions of the metallic cylinder to achieve resonance at 9.5 GHz are shown for quartz, sapphire, and rutile tubes as a function of wall thickness and average radius. These contour plots were developed using analytical equations and confirmed by finite element modeling. They can be used in two ways: design of the metallic cylinder for use at 9.5 GHz that incorporates a readily available tube such as a sapphire tube intended for NMR, or design of a custom procured tube for optimized performance for specific sample-size constraints. The charts extend to the limiting condition where the dielectric fills the tube. However, the structure at this limit is not a dielectric resonator due to the metal wall and does not radiate. In addition, the uniform field (UF) DTR is introduced. Development of the UF resonator starting with a dielectric tube resonator is shown. The diameter of the tube remains constant along the cavity axis, and the diameter of the cylindrical metallic enclosure increases at the ends of the cavity to satisfy the uniform field condition. This structure has advantages over the previously developed UF TE011 resonators: higher resonator efficiency parameter Λ, convenient overall size when using sapphire tubes, and higher quality data for small samples. The DTR and UF DTR structures fill the gap between free space and dielectric resonator limits in a continuous manner.

High-field/high-pressure ESR

We present a historical review of high-pressure ESR systems with emphasis on our recent development of a high-pressure, high-field, multi-frequency ESR system. Until 2000, the X-band system was almost established using a resonator filled with dielectric materials or a combination of the anvil cell and dielectric resonators. Recent developments have shifted from that in the low-frequency region, such as X-band, to that in multi-frequency region. High-pressure, high-field, multi-frequency ESR systems are classified into two types. First are the systems that use a vector network analyzer or a quasi-optical bridge, which have high sensitivity but a limited frequency region; the second are like our system, which has a very broad frequency region covering the THz region, but lower sensitivity. We will demonstrate the usefulness of our high-pressure ESR system, in addition to its experimental limitations. We also discuss the recent progress of our system and future plans.

Bismuth germanate as a perspective material for dielectric resonators in EPR spectroscopy

High purity bismuth germanate (Bi4(GeO4)3, BGO) is proposed and implemented as an alternative material for dielectric EPR resonators. A significant improvement of the absolute sensitivity can be readily achieved by substituting the alumina insert (ring) by BGO-made one in commercially available X-band EPR probeheads. Four BGO dielectric inserts of 2, 3, 4 and 5mm inner diameter (ID) were made for comparison with standard 5mm inner diameter alumina insert. All inserts were introduced into commercial Bruker EPR resonator ER 4118X-MD-5W1, and their performance was investigated. The Q-values of empty resonators, B1 saturation curves and continuous wave EPR spectra of DPPH (2,2-diphenyl-1-picrylhydrazyl) were measured and analyzed in a temperature range 6-300K. BGO-made resonators were found superior in several important aspects. The background signals arising from BGO are much weaker compared to those of alumina at B=0-0.6T and T=6-300K; this is especially useful for measuring weak signals in the half-field region, as well as those near the central field. Moreover, mechanical properties of BGO allow easy fabrication of dielectric bodies having various shapes and sizes; in particular, small BGO resonators (e.g. ID=2 or 3mm) strongly enhance sensitivity for small samples due to increase of the filling factor. All these advantages have been also inspected in the pulse mode, proving that higher B1 fields and better filling factors can be achieved, contributing to the overall enhancement of the performance.

Optimal dielectric and cavity configurations for improving the efficiency of electron paramagnetic resonance probes

An electron paramagnetic resonance (EPR) spectrometer's lambda efficiency parameter (Λ) is one of the most important parameters that govern its sensitivity. It is studied for an EPR probe consisting of a dielectric resonator (DR) in a cavity (CV). Expressions for Λ are derived in terms of the probe's individual DR and CV components, Λ1 and Λ2 respectively. Two important cases are considered. In the first, a probe consisting of a CV is improved by incorporating a DR. The sensitivity enhancement depends on the relative rather than the absolute values of the individual components. This renders the analysis general. The optimal configuration occurs when the CV and DR modes are nearly degenerate. This configuration guarantees that the probe can be easily coupled to the microwave bridge while maintaining a large Λ. It is shown that for a lossy CV with a small quality factor Q2, one chooses a DR that has the highest filling factor, η1, regardless of its Λ1 and Q1. On the other hand, if the CV has a large Q2, the optimum DR is the one which has the highest Λ1. This is regardless of its η1 and relative dielectric constant, ɛr. When the quality factors of both the CV and DR are comparable, the lambda efficiency is reduced by a factor of 2. Thus the signal intensity for an unsaturated sample is cut in half. The second case is the design of an optimum shield to house a DR. Besides preventing radiation leakage, it is shown that for a high loss DR, the shield can actually boost Λ above the DR value. This can also be very helpful for relatively low efficiency dielectrics as well as lossy samples, such as polar liquids.

Development of high-pressure and high-field ESR system using SQUID magnetometer

We have developed a high-pressure and high-field electron spin resonance (ESR) system using the combination of a commercially available superconducting quantum interference device (SQUID) magnetometer and a clamp-type piston cylinder pressure cell. The magnetic field range is up to 5 T, and the maximum pressure reaches 1.5 GPa. The most characteristic feature of this system is its easy handling as compared with other high-pressure ESR systems. Moreover, the macroscopic magnetization measurement can be performed simultaneously with the microscopic ESR measurement. In addition to these features, the well-established pressure calibration method utilizing the change of superconducting transition temperature of tin can be applied to this system. By using this system, we obtained pressure dependence of the single ion magnetic anisotropy parameter D of NiSnCl(6)·6H(2)O up to 1.5 GPa precisely, and the magnetization behavior of this material under pressure was explained well by its pressure dependence of the D value.

Resonant microwave cavity for 8.5-12 GHz optically detected electron spin resonance with simultaneous nuclear magnetic resonance

We present a newly developed microwave resonant cavity for use in optically detected magnetic resonance (ODMR) experiments. The cylindrical quasi-TE(011) mode cavity is designed to fit in a 1 in. magnet bore to allow the sample to be optically accessed and to have an adjustable resonant frequency between 8.5 and 12 GHz. The cavity uses cylinders of high dielectric material, so-called "dielectric resonators," in a double-stacked configuration to determine the resonant frequency. Wires in a pseudo-Helmholtz configuration are incorporated into the cavity to provide frequencies for simultaneous nuclear magnetic resonance (NMR). The system was tested by measuring cavity absorption as microwave frequencies were swept, by performing ODMR on a zinc-doped InP sample, and by performing optically detected NMR on a GaAs sample. The results confirm the suitability of the cavity for ODMR with simultaneous NMR.

Continuous-wave far-infrared ESR spectrometer for high-pressure measurements

We present a newly-developed microwave probe for performing sensitive high-field/multi-frequency electron spin resonance (ESR) measurements under high hydrostatic pressures. The system consists of a BeCu-made pressure-resistant vessel, which accommodates the investigated sample and a diamond microwave coupling window. The probe's interior is completely filled with a pressure-transmitting fluid. The setup operates in reflection mode and can easily be assembled with a standard oversized microwave circuitry. The probe-head withstands hydrostatic pressures up to 1.6 GPa and interfaces with our home-built quasi-optical high-field ESR facility, operating in a millimeter/submillimeter frequency range of 105-420 GHz and in magnetic fields up to 16 T. The overall performance of the probe was tested, while studying the pressure-induced changes in the spin-relaxation mechanisms of a quasi-1D conducting polymer, KC(60). The preliminary measurements revealed that the probe yields similar signal-to-noise ratio to that of commercially available low-frequency ESR spectrometers. Moreover, by observing the conduction electron spin resonance (CESR) linewidth broadening for KC(60) in an unprecedented microwave frequency range of 210-420 GHz and in the pressure range of up to 1.6 GPa, we demonstrate that a combination of high-pressure ESR probe and high-field/multi-frequency spectrometer allows us to measure the spin relaxation rates in conducting spin systems, like the quasi-1D conductor, KC(60).