A continuous-wave and pulsed electron spin resonance spectrometer operating at 275 GHz

An electron paramagnetic resonance (EPR) spectrometer is described which allows for continuous-wave and pulsed EPR experiments at 275 GHz (wavelength 1.1 mm). The related magnetic field of 9.9 T for g approximately 2 is supplied by a superconducting solenoid. The microwave bridge employs quasi-optical as well as conventional waveguide components. A cylindrical, single-mode cavity provides a high filling factor and a high sensitivity for EPR detection. Even with the available microwave power of 1 mW incident at the cavity a high microwave magnetic field B1 is obtained of about 0.1 mT which permits pi/2-pulses as short as 100 ns. The performance of the spectrometer is illustrated with the help of spectra taken with several samples.     

Current density imaging by pulsed conduction electron spin resonance

In analogy with Nuclear MRI, the ESR signal phase shift of conduction electrons moving in electrical currents along controlled magnetic field gradients can be used to generate spatial electronic current density maps. First two-dimensional images of the current density distribution in quasi-one-dimensional organic conductors are presented.     

Novel pulsed electron spin resonance system and studies of phosphorus in natural silicon

An X-band pulsed electron spin resonance system has been designed and constructed specifically for studies of decoherence times of phosphorus donor electrons in silicon. The microwave electromagnetic field aspects of the structure have been analysed, and the probe head geometry optimised, through 3D electromagnetic simulations. Results for natural silicon samples at temperatures down to 6 K, are analysed and we obtain an estimate of the isolated spin decoherence time in ^natSi approximately an order of magnitude longer than previously measured.     

Single-molecule electron spin resonance

In 1993, two collaborations independently achieved the first optical detection of a single electron spin, using paramagnetic resonance of the triplet state of a single molecule. We review the context of this work and the main later results and give a brief outlook for future experiments based on the detection and manipulation of single spins by optical methods.     

An X-band time domain electron paramagnetic resonance spectrometer

A computer-controlled X-band time domain electron paramagnetic resonance (EPR) spectrometer, with a time resolution of the order of 0.5μsec, has been constructed with many of the crucial microwave components designed and fabricated by the Microwave Engineering Group of TIFR. The spectrometer operates either in a microwave power pulsed mode for determination of spin-lattice relaxation times by the saturation recovery technique or in the kinetic mode for determination of the time dependence of EPR signal after laser excitation. It has an automatic frequency control, an automatic phase control and, most importantly, a field-frequency lock which ensures good stability of the EPR line positions enabling signal averaging for extended periods. The constructional details of the spectrometer and its performance in both the modes are described here by reporting results on certain typical systems.     

Detection of free radicals produced by a pulsed electrohydraulic discharge using electron spin resonance

The detection of free radicals such as hydroxyl radical and hydrogen radical for plasma in solution induced by a pulsed electrohydraulic discharge are successfully performed using electron spin resonance measurement. The plasma reactor is a barrier-type and consists of a stainless needle high-voltage (+35 to 65 kV) electrode partially immersed in the solution and a Pyrex glass solution container around which an aluminum film grounded electrode is wrapped. Streamers are induced in the solution. After adding iron (II) sulfate or radical trapping agent before the plasma application, the spectrum for radicals is clearly detected. A reactive dye solution is dramatically decolorized.     

Hyperfine-mediated gate-driven electron spin resonance

An all-electrical spin resonance effect in a GaAs few-electron double quantum dot is investigated experimentally and theoretically. The magnetic field dependence and absence of associated Rabi oscillations are consistent with a novel hyperfine mechanism. The resonant frequency is sensitive to the instantaneous hyperfine effective field, and the effect can be used to detect and create sizable nuclear polarizations. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively.     

Data acquisition card for varian electron paramagnetic resonance spectrometers

An easy to assemble and inexpensive PC card that interfaces E-Line Varian spectrometers (E-6, E-9, E-12 and E-15) with IBM compatible PCs is presented. The card has a Programmable Interval Timer so it does not need an external clock making it instrument independent. The accessory contains its own RAM. This allows the device to store the spectrum while sampling is in progress, so the PC can meanwhile be used for any other task.     

Mechanisms of flexural-strain-modulated electron spin resonance

Strain-modulated electron spin resonance (SMESR) spectra of V²⁺ and Mn²⁺ centers in MgO are reported and the results are compared with those obtained for MgO : Cr³⁺. The SMESR line intensities show a characteristic angular dependence, proving that modulation of the off-diagonal elements of the $\text{G}$-tensor is the predominant mechanism when a flexural mode of vibration is applied.For V²⁺ a small additional contribution due to modulation of the diagonal elements of $\text{G}$ is present.     

Acoustic detection of electron spin resonance

The ESR-signal of DPPH was recorded by detecting the modulation of the absorbed microwave power with a gas-coupled microphone. This photo-acoustic detection scheme is compared with conventional ESR-detection. Applications of the acoustical detection method to other modulation spectroscopic techniques, particularly NMR, are discussed.     

Determination of the spin Hamiltonian constants for electron spin resonance at energy level crossing points

The crossing point of the Cr⁺³ ion energy-levels in single-crystal K₃Co(CN)₆, with the applied magnetic field parallel to the y-axis, is determined experimentally. Precisely calculated values of the constants E and D of the EPR spin Hamiltonian are shown to agree closely with those obtained by other authors.In conclusion, I wish to thank Professor V. S. Grechishkin for supervising my work and discussing the results.     

High resolution electron spin resonance microscopy

NMR microscopy is routinely employed in fields of science such as biology, botany, and materials science to observe magnetic parameters and transport phenomena in small scale structures. Despite extensive efforts, the resolution of this method is limited (>10 microm for short acquisition times), and thus cannot answer many key questions in these fields. We show, through theoretical prediction and initial experiments, that ESR microscopy, although much less developed, can improve upon the resolution limits of NMR, and successfully undertake the 1 mum resolution challenge. Our theoretical predictions demonstrate that existing ESR technology, along with advanced imaging probe design (resonator and gradient coils), using solutions of narrow linewidth radicals (the trityl family), should yield 64 x 64 pixels 2D images (with z slice selection) with a resolution of 1 x 1 x 10 microm at approximately 60 GHz in less than 1h of acquisition. Our initial imaging results, conducted by CW ESR at X-band, support these theoretical predictions and already improve upon the previously reported state-of-the-art for 2D ESR image resolution achieving approximately 10 x 10 mum, in just several minutes of acquisition time. We analyze how future progress, which includes improved resonators, increased frequency of measurement, and advanced pulsed techniques, should achieve the goal of micron resolution.     

Conduction electron spin resonance in magnesium

We report on the observation of conduction electron spin resonance (CESR) in samples of specially purified magnesium. Measurements were made over the temperature range 4.2–300 K and at two frequencies, 9.27 and 21 GHz. It is found that in contrast with previous data, there is no upturn in the low temperature linewidth and little variation between samples.     

Nature of tunneling electron spin resonance of an isolated surface spin

A phenomenological model has been proposed for tunneling electron spin resonance (ESR) of an isolated surface spin situated in a scanning tunneling microscope (STM), which explains the dependence of features (local maxima) of the tunneling current on the radio-frequency (RF) electric field and on the position of the tip with respect to the spin. A crossover of the line shape of the resonance signals, whose nature in weak and strong pumping fields corresponds to Lorentzian and Fano resonances, respectively, has been interpreted. New ESR–STM effects that are linear and nonlinear in the RF field and are promising for developing the methods of controlling spin qubits have been predicted.     

Detection of the electron spin resonance of two-dimensional electrons at large wave vectors

We have investigated the electron spin resonance at nonzero wave vector in GaAs single quantum wells by combining the virtues of high frequency surface acoustic wave generation to produce excitations with large wave numbers with a sensitive optical scheme to detect resonant absorption. The observed large deviations from the single particle Zeeman energy are attributed to the exchange interaction. The enhancement of the electronic g* factor is, however, substantially smaller compared with theoretical predictions for spin waves when adopting a bare Coulomb interaction potential.     

Application of autocorrelation techniques to the recording and analysis of electron spin resonance spectra

A discussion is given of the possibility of applying autocorrelation techniques to the recording and analysis of electron spin resonance spectra showing first-order hyperfine structure. Procedures and computer program for the implementation of these possibilities are described and the anthracene cation is used as an illustrative example.     

Optical detection of electron spin resonance in CdTe

We report the optical detection of electron spin resonance in p-type CdTe at 1.7 K in optical pumping conditions. The Overhauser shift of the electronic resonance, of the order of 45 G, is related to the sign of the electron g-factor g¹. We measure $\text{g}{}^1\text{= -1.59±0.02}$. Using this g¹ value and the previous results on the Knight shift, we deduce the value of the electron wavefunction on Cd in CdTe, which is consistent with the value in CdS.