Dynamic nuclear spin resonance in n-GaAs

The dynamics of optically detected nuclear magnetic resonance is studied in n-GaAs via time-resolved Kerr rotation using an on-chip microcoil for rf field generation. Both optically allowed and optically forbidden NMR are observed with a dynamics controlled by the interplay between dynamic nuclear polarization via hyperfine interaction with optically generated spin-polarized electrons and nuclear spin depolarization due to magnetic resonance absorption. Comparing the characteristic nuclear spin relaxation rate obtained in experiment with master equation simulations, the underlying nuclear spin depolarization mechanism for each resonance is extracted.     

Calculation of the electronic energy differences of spin crossover complexes

Density functional calculations using different functionals and basis sets have been carried out to calculate the electronic energy difference between the high- and low-spin isomers of spin crossover complexes with a transition metal center. The reparameterized B3LYP* method is confirmed to be most suitable for the calculation of electronic energy differences between isomers with different spin states. If only changes of the electronic energy difference upon modifications of the complex are considered all employed density functional methods show a similar performance. Basis sets with effective core potentials seem to perform as well as large all electron basis sets. Calculations using the polarizable continuum model have been performed to investigate the effect of solvents. In addition the effect of hydrogen bonding between the spin crossover complex and a water molecule is studied.     

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.     

Ising-like model study of size dependence relaxation in spin crossover complexes

In this paper we present a theoretical study of size effects during relaxation in spin transition solids. The systems are described using an Ising-like model consisting of molecules having two energy levels and fictitious spin values of +1, (HS) and −1 (LS). We compare relaxation in various 2D or 3D systems (rectangular, hexagonal or cubic) and we realise an exhaustive analysis of the parameters that influence the relaxation in small size samples. The differences between homogenous and inhomogeneous systems reflected on the shapes of relaxation curves are discussed and analysed.     

Optical detection of conduction electron spin resonance in InP

Conduction electron spin resonance is optically detected in InP at 1.7 K. The measured value |g¹| = 1.26 ± 0.05 is in very good agreement with theoretical predictions and with the only other experimental determination available.     

Revision of spin echoes in pure nuclear quadrupole resonance

Goldman's spin-1/2 formalism has been used for describing the response of an I=3/2 spin system to a two-pulse sequence in a pure nuclear quadrupole resonance experiment. A detailed analysis of the polarization evolution and quadrupolar echo generation is carried out through the use of explicit expressions for secular homo- and heteronuclear dipolar interactions. In striking contrast with previous studies, it is predicted that Van Vleck's second moments governing a classical solid-echo or Hahn sequence differ from those obtained by equivalent means in magnetic resonance. In fact, it is shown that, although measured moments still complement each other, the combined use of standard sequences does not allow the separate determination of homo- and heteronuclear dipolar contributions to the linewidth, not even in an indirect manner. In this context, the importance and potential usefulness of a crossed coil probe are also briefly discussed.     

Radio-frequency muon spin resonance (RFμSR) experiments on condensed matter

In this paper we present an overview of the radio-frequency muon spin resonance (RFμSR) technique, an analogue to continuous-wave NMR, and an introduction to time-integral (TI) and time-differential (TD) RFμSR on muons in diamagnetic or in paramagnetic environments. The general form of the resonance line for TI-RFμSR as well as the expression for the time-dependence of the longitudinal muon spin polarization at resonance are given. Since RFμSR does not require phase coherence of the muon spin ensemble, this technique allows us to investigate muon species that are generated by transitions from, or in the course of reactions of, a precursor muon species even if in transverse-field (TF) μSR measurements the signal is lost due to dephasing. This ability of RFμSR is clearly demonstrated by measurements on doped Si. In this example, at low temperatures, a very pronounced signal from a muon species in diamagnetic environment has been found in RFμSR measurements, whereas in TFμSR experiments only a very small signal from muons in diamagnetic environment could be detected and a large fraction of the implanted muons escaped detection. These findings could be interpreted in terms of the delayed formation of a diamagnetic muonium-dopant complex, and, due to the large diamagnetic RFμSR signal, the RFμSR technique is a unique tool to study how the variation of parameters and experimental conditions such as illumination affects formation and behavior of these complexes. First results obtained on illuminated boron doped Si are reported. However, as illustrated by the example of experiments on the muonated radical in solid C₆₀, results from conventional TI-RFμSR cannot always be interpreted unambiguously since different parameters, namely the fraction of muons forming the investigated muon species, the longitudinal and the transverse relaxation rates, have similar effects on height and shape of the RFμSR resonance line. These ambiguities, however, may be resolved by collecting time-differential data. With this extension RFμSR becomes a very powerful complementary method to TFμSR in the studies of dynamic effects.     

Transmission electron spin resonance as a probe of the metallic interface

Observation of coupled TESR in bilayers of dissimilar pure metals is reported. Positive entropy production and detailed balance are incorporated in general linear relations between non-equilibrium spin densities and currents at the interface. Spin transport coefficients for samples exhibiting both strong and weak coupling are deduced from the data by comparison with computer generated lineshapes.     

Optical manipulation of nuclear spin by a two-dimensional electron gas

Conduction electrons are used to optically polarize, detect, and manipulate nuclear spin in a (110) GaAs quantum well. Using optical Larmor magnetometry, we find that nuclear spin can be polarized along or against the applied magnetic field, depending on field polarity and tilting of the sample with respect to the optical pump beam. Periodic optical excitation of the quantum-confined electron spin reveals a complete spectrum of optically induced and quadrupolar-split nuclear resonances, as well as evidence for Deltam = 2 transitions.     

Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF₃. A resonance change with respect to a constant magnetic field H ₀ with maximum damping at H ₀≈4×10³ Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H ₀ excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field. Fiz. Tverd. Tela (St. Petersburg) 41, 297–300 (February 1999)     

Magnetoacoustic resonance on nuclear spin waves in an easy-plane antiferromagnet KMnF3

A theoretical model of magnetoacoustic resonance on nuclear spin waves in an easy-plane antiferromagnet KMnF₃ with large dynamic frequency shift is developed. Experimental results on resonance decay of ultrasound, splitting of the ultrasound pulse into two component and anomalous dispersion of the sound velocity depending on the value of the external magnetic field confirm this theory.     

Optical detection of single-electron spin resonance in a quantum dot

We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few microT. In one case, the spin resonance behaves as a driven 3-level lambda system with weak damping; in another one, the dot exhibits remarkably strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with fluctuating resonant positions, evidence of unusual dynamic processes.     

Nuclear spin andg-factor from time resolved andtime integral on-line nuclear orientation experiments

Guiding principles in the design of time resolved on-line nuclear orientation experiments are described which, when used in conjunction with time integral on-line nuclear orientation, optimise the simultaneous determination of the oriented state spin andg-factor.     

Electric field induced nuclear spin resonance mediated by oscillating electron spin domains in GaAs-based semiconductors

We demonstrate an alternative nuclear spin resonance using a radio frequency (rf) electric field [nuclear electric resonance (NER)] instead of a magnetic field. The NER is based on the electronic control of electron spins forming a domain structure. The rf electric field applied to a gate excites spatial oscillations of the domain walls and thus temporal oscillations of the hyperfine field to nuclear spins. The rf power and burst duration dependence of the NER spectrum provides insight into the interplay between nuclear spins and the oscillating domain walls.     

Optical pump-probe measurements of local nuclear spin coherence in semiconductor quantum wells

We demonstrate local manipulation and detection of nuclear spin coherence in semiconductor quantum wells by an optical pump-probe technique combined with pulse rf NMR. The Larmor precession of photoexcited electron spins is monitored by time-resolved Kerr rotation (TRKR) as a measure of nuclear magnetic field. Under the irradiation of resonant pulsed rf magnetic fields, Rabi oscillations of nuclear spins are traced by TRKR signals. The intrinsic coherence time evaluated by a spin-echo technique reveals the dependence on the orientation of the magnetic field with respect to the crystalline axis as expected by the nearest neighbor dipole-dipole interaction.     

Langmuir probe experiments on Korean satellites

We report in this paper the test results of the Langmuir Probes (LPs) that have been used on several Korean low earth orbit (LEO) satellites since 1999. The probes were designed to measure the electrons in the density range of ～10⁴ to ～10⁶/cm³ and temperatures less than ～10⁴ K, which are the typical characteristics of the upper ionosphere. Careful considerations were given to the probe design to reduce many error sources. For example, a 10 Hz fast voltage sweep was applied to the probe to minimize measurement errors caused by probe contaminations. An electron temperature probe was installed in addition to the LP on the KOMPSAT-1, and the electron temperatures measured simultaneously and independently by the two instruments were compared. The results showed good agreement, which confirms the validity of the LP data obtained from these Korean satellites.