X波段脉冲电子顺磁共振谱仪的矩形腔与微型平面腔的设计

为了满足脉冲式电子顺磁共振谱仪的需要,设计并制作了连续波谐振腔和脉冲谐振腔. 连续波谐振腔采用矩形谐振腔的设计,而脉冲谐振腔采用了微型平面腔的设计. 在设计阶段,使用Ansoft-HFSS三维电磁仿真软件对2种谐振腔进行模拟计算. 微型平面腔的加工采用了微纳加工技术. 制作完成的谐振腔的参数指标由网络分析仪测定. 实验测得2种谐振腔的参数指标符合理论模拟值,并满足脉冲式电子顺磁共振谱仪的要求.     

Coupling Coefficient of Irregular Microstrip Resonators

We prove the equivalence of the resonance and energetic definitions of the coupling coefficient in the case of irregular microstrip resonators. Formulas for calculation of the coupling coefficient of microstrip resonators with stepped width of the strip conductor are derived. The presence of a region in which the coupling coefficient increases with increase in spacing between the resonators is shown.     

X-band rapid-scan EPR of nitroxyl radicals

X-band rapid-scan EPR spectra were obtained for dilute aqueous solutions of nitroxyl radicals (15)N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeuteromethyl-pyrrolin-1-(15)N-oxyl-d(12)) and (15)N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl-piperidinyl-(15)N-oxyl-d(16)). Simulations of spectra for (15)N-mHCTPO and (15)N-PDT agreed well with the experimental spectra. As the scan rate is increased in the rapid scan regime, the region in which signal amplitude increases linearly with B(1) extends to higher power and the maximum signal amplitude increases. In the rapid scan regime, the signal-to-noise for rapid-scan spectra was about a factor of 2 higher than for unbroadened CW EPR, even when the rapid scan spectra were obtained in a mode that had only 4% duty cycle for data acquisition. Further improvement in signal-to-noise per unit time is expected for higher duty cycles. Rapid scan spectra have higher bandwidth than CW spectra and therefore require higher detection bandwidths at faster scan rates. However, when the scan rate is increased by increasing the scan frequency, the increase in noise from the detection bandwidth is compensated by the decrease in noise due to increased number of averages per unit time. Because of the higher signal bandwidth, lower resonator Q is needed for rapid scan than for CW, so the rapid scan method is advantageous for lossy samples that inherently lower resonator Q.     

Determination of Quality Factor for Highly Overcoupled EPR Resonators

We reported determination of the loaded quality factor (Q) of highly overcoupled (dielectric, loop-gap, and cavity) resonators used in time-domain electron paramagnetic resonance. We introduced a microwave absorber into resonators and achieved critical-coupling. Due to the deep “Q-dip” of critical-coupling, we can easily determine the loaded Q as low as 10. The loaded Q of resonators with and without the microwave absorber was examined under various overcoupling conditions. We found that the radiation Q (Q _r) can be calculated from the loaded Q of the resonator that contains the microwave absorber. We proposed a simple model that represents the loaded Q of the overcoupled resonator in terms of two parameters, Q ₀ and Q _r. Q ₀ is the effective unloaded Q of the resonator determined for the critically coupled resonator without the microwave absorber and is independent of a degree of coupling. The model can be applied to overcoupling in which the coupling parameter (Q ₀/Q _r) is in the range of 1 to ca. 20.     

A novel loop-gap resonator probehead for EPR and ENDOR at X-band

An EPR and ENDOR probehead with a loop-gap resonator for X-band is described. The novel feature of the construction is that an iris-type coupling of the resonator is used instead of the conventional antenna coupling. The ENDOR coil combines the role of creating the radio frequency field and that of a shield for the microwave loop-gap structure. Hence, in order to accommodate the iris and waveguide, a pair of RF coils is used in conjunction with a reduced waveguide with dielectric filling. This arrangement simplifies matching the resonator to the microwave bridge, and standard EPR cryostats can be used making sample manipulation more convenient.     

Multifrequency Probe for Pulsed EPR and ENDOR Spectroscopy

The design, construction, and performance of a multifrequency pulsed EPR and ENDOR probe for use at cryogenic temperatures are described. Interchangeable resonators based on a folded strip line design allow variation of the resonance frequency over a range of 5-11 GHz. Variable coupling to the resonator is achieved capacitively via a simple mechanical adjustment which is thermally and mechanically stable. The entire assembly is robust and easily fabricated. Common methods of analyzing the resonator parameters such as the Q-factor and coupling coefficient are discussed quantitatively. Probe performance data and multifrequency pulsed ENDOR spectra are presented.     

新型X波段多功能EPR谱仪的设计与性能

本文设计和研制了一款新型X波段多功能电子顺磁共振（electron paramagnetic resonance,EPR）谱仪,并为其开发一款新的控制和读出系统（control and readout system,CRS）来操控微波脉冲的产生和信号的采集,提高了系统的集成度和可扩展性. 该谱仪可实现常规的连续波EPR（continuous-wave EPR,cw-EPR）、脉冲EPR（pulsed EPR）和瞬态EPR（transient EPR,trEPR）实验,并装配了6~300 K的无液氦变温装置,以及兼具平行模式与垂直模式的新型双模连续波谐振腔和用于脉冲EPR及trEPR的介质腔. 针对新型EPR谱仪和新谐振腔,本文利用双模连续波、脉冲和瞬态三个不同方式的EPR实验,对其功能进行了验证.     

Broadband Dielectric Spectroscopy of Solids

We describe state-of-the-art experimental studies of the dielectric response function of solids in a broad band ranging from ultra-low (10^-6 Hz) to optical (10¹⁵ Hz) frequencies. Attention is paid to the yet unsolved problem concerning spectral dielectric measurements in the central part (10¹⁰–10¹² Hz) of this band, which impedes matching of data obtained in radio-frequency and infrared bands. We present examples of solving this problem at the Institute of General Physics of the Russian Academy of Sciences by using the submillimeter BWO spectroscopy technique. The accumulated experimental data on dielectric panoramas of various substances is summarized qualitatively.     

Bragg Reflectors and Resonators in Microstrip Technology Based on Electromagnetic Crystal Structures

Recently, new promising two-dimensional (2-D) Photonic Bandgap Structures (PBG), or more properly Electromagnetic Crystal Structures, for microstrip lines have been proposed. In this paper, we analyze these structures in a manner like a Bragg reflector in optical wavelengths. Joining two of such Bragg like reflectors by means of a conventional microstrip transmission line allows one to design Bragg Resonators. The 2-D periodic pattern of the electromagnetic crystal structure is implemented with circles etched in the ground plane of the microstrip line by means of a numerical milling machine. Simulations have been performed by using HP ^TM Momentum and MDS software, and in accordance with the measurements give, for the Electromagnetic Crystal Structures, new promising potential applications both in microwave and millimeter wave integrated circuits, and also in the experimentation of expensive short wavelength (including photonic) devices by using simpler and cheaper microwave down scaling.     

Nanomagnets for sensors and data storage

Miniaturized magnetic devices are ubiquitous in the hard disks of computers and in tape storage systems. Giant magnetoresistance was discovered as recently as 1988 but already sensors using the effect are being incorporated into read heads for the highest density hard disk systems. As a result of intensive research and development, storage density on hard disks has increased dramatically at a rate greater than 60% per year. At sub-micron and nano-scale dimensions the properties of magnetic devices are strongly affected by their size and shape in a complex way resulting from the interplay between different types of magnetic energy. In some cases this causes a deterioration in the performance of existing devices, however it has also enabled entirely new devices to be proposed. Arrays of nanomagnets could be used for ultra-high density storage on hard disks or for fast and dense, non-volatile, solid state memory. Storage applications are possible because hysteresis in the nanomagnets creates two oppositely magnetized states which are stable in zero applied field and can store binary data. Magnetic sensors based on giant magnetoresistance in layered magnetic structures are among the most sensitive available for operation at room temperature and above. This paper describes the physical properties of nanomagnets and their role in present and future applications.     

Features of Characteristics of Second-Order Bandpass Filters Based on Half-Wave and Quarter-Wave Microstrip Resonators

Russian Physics Journal - A comparison is made of the characteristics of bandpass filters consisting of two interacting resonators, which are either half-wave or quarter-wave segments of microstrip...     

Calculation of Magnetic Configuration and Resistance for Nanomagnets

Based on a classical Heisenberg lattice model with dipole interaction and the method of spin dynamic simulation,the magnetic configurations (MC),hysteresis loop (HL) and magnetic resistance (MR) of the nanomagnets with different geometries,such as circle,square and rectangle,are studied for different directions of applied field.In the case of perpendicular field to the plane,the magnetization and MR are reversible and have not hysteresis.When the field is applied in the plane,the HL is irreversible and is qualitatively well agreeable with the current experimental results.The MR loop is also irreversible and appears two peaks distributed at two sides around zero field.The peaks of magnetic resistance are relative to the vortex state of similar configuration.Large easy-axis anisotropy will suppress the MC anisotropy,and the large magnetoresistance effect disappears.     

EPR Spectroscopy of Mercury-Organic Compounds with Nitroxide Radicals

Intramolecular electron spin exchange as a function of temperature, solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy for two nitroxide biradicals containing mercury-organic groups in the bridge connecting two 1-oxyl-2,2,6,6-tetramethylpiperidine-3,4-ene-nitroxide rings, R. The temperature dependence of the isotropic hyperfine splitting (hfs) constant a ^N and the exchange integral value |J/a| of the biradicals were measured from EPR spectra and subsequently analyzed comparing to a ^N and a ^Hg hfs constants of ClHgR radical dissolved in the same solvents. In all cases, the interaction of solvent molecules (SM) with >N–O fragments of nitroxide rings led to a slight decrease in a values with increasing temperature. The |J/a| value varied slightly with temperature T changes. The changes of |J/a| are much less comparing to those with variation of the solvent polarity. The interaction between SM and Hg atoms inside the bridge is observed and discussed.     

石英矿物表面反应性的EPR谱学研究

采用顺磁共振谱（ＥＰＲ）方法对不同ｐＨ值条件下石英表面与Ｃｕ＾２＋离子反应的机理进行研究。当溶液的ｐＨ值在２至１１之间地,石英表面Ｃｕ＾２＋离子的吸附覆盖率相应地由０至１０．３２％变化;同时,其ＥＰＲ谱的线形、线宽及ｇ因子值也发生了特征的变化。研究表明,随着石英表面Ｃｕ＾２＋离子的吸附覆盖率的不断升高,表面反应产物的结合形态相应地出现单核化合物、多核化合物直至表面沉淀。     

High-Frequency EPR and ENDOR Spectroscopy on Semiconductor Quantum Dots

It is shown that high-frequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy are excellent tools for the investigation of the electronic properties of semiconductor quantum dots (QDs). The great attractions of these techniques are that, in contrast to optical methods, they allow the identification of the dopants and provide information about the spatial distribution of the electronic wave function. This latter aspect is particularly attractive because it allows for a quantitative measurement of the effect of confinement on the shape and properties of the wave function. In this contribution EPR and ENDOR results are presented on doped ZnO QDs. Shallow donors (SDs), related to interstitial Li and Na and substitutional Al atoms, have been identified in this material by pulsed high-frequency EPR and ENDOR spectroscopy. The shallow character of the wave function of the donors is evidenced by the multitude of ENDOR transitions of the ⁶⁷Zn nuclear spins and by the hyperfine interaction of the ⁷Li, ²³Na and ²⁷Al nuclear spins that are much smaller than for atomic lithium, sodium and aluminium. The EPR signal of an exchange-coupled pair consisting of a shallow donor and a deep Na-related acceptor has been identified in ZnO nanocrystals with radii smaller than 1.5 nm. From ENDOR experiments it is concluded that the deep Na-related acceptor is located at the interface of the ZnO core and the Zn(OH)₂ capping layer, while the shallow donor is in the ZnO core. The spatial distribution of the electronic wave function of a shallow donor in ZnO semiconductor QDs has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine interactions as monitored by ENDOR spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. In addition, the effect of confinement on the g-factor of SDs in ZnO as well as in CdS QDs is observed. Finally, it is shown that an almost complete dynamic nuclear polarization (DNP) of the ⁶⁷Zn nuclear spins in the core of ZnO QDs and of the ¹H nuclear spins in the Zn(OH)₂ capping layer can be obtained. This DNP is achieved by saturating the EPR transition of SDs present in the QDs with resonant high-frequency microwaves at low temperatures. This nuclear polarization manifests itself as a hole and an antihole in the EPR absorption line of the SD in the QDs and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with nuclear magnetic resonance techniques.     

Application of Gyrotrons for Molecular Gas Spectroscopy

Radiophysics and Quantum Electronics - We give a brief review of the results of using gyrotrons to study molecular spectra by means of a radioacoustic absorption detection (RAD) spectrometer....     

Characterization of Nb-Based Superconducting Microstrip Lines Around the Gap Frequency

Submillimter-wave SIS (superconductor-insulator-superconductor) mixers usually adopt SIS junctions associated with an integrated tuning circuit, which tunes out the junction's geometric capacitance and is typically an inductive thin-film superconducting microstrip line. This paper mainly investigates the characteristic of Nb-based superconducting microstrip lines around the junction's gap frequency, at which the surface resistance of Nb films becomes considerable, and its effect on the performance of SIS mixers.     

Accurate Analysis of Microstrip Lines on Lossy Biaxial Anisotropic Substrates

We have studied the behavior of the microstrip lines on lossy biaxial anisotropic dielectric substrates. A spectral-domain moment method is used with the Galerkin testing procedure to determine the dispersion characteristics of single and coupled lines. Modes of both even and odd symmetries are included. It is found that the anisotropy of the substrate has a significant influence on the propagation characteristics. The theory is verified by comparison with previously published data.