Electrical detection of spin coherence in silicon

Experimental evidence is presented showing that photocurrents in silicon can be used as highly sensitive readout probes for coherent spin states of localized electrons, the prime candidates for quantum bits in various semiconductor based quantum computer concepts. Conduction electrons are subjected to fast Rabi oscillation induced by means of pulsed electron spin resonance. The collective spin motion of the charge carrier ensemble is reflected by a spin-dependent recombination rate and therefore by the sample conductivity. Because of inhomogeneities, the Rabi oscillation dephases rapidly. However, a microwave induced rephasing is possible causing an echo effect whose intensity contains information about the charge carrier spin state and the coherence decay.     

A scheme for electrical detection of single-electron spin resonance

We study a scheme for electrical detection of the spin resonance of a single-electron trapped near a field effect transistor (FET) conduction channel. In this scheme, the resonant Rabi oscillations of the trapped electron spin cause a modification of the average charge of a shallow trap, which can be detected through the change in the FET channel resistivity. We show that the dependence of the channel resistivity on the frequency of the rf field can have either peak or dip at the Larmor frequency of the electron spin in the trap.     

Optical detection of spin coherence in a defect center of a molecular crystal

Various optically detected spin coherence experiments have been performed at zero magnetic field in a tunable loop-gap cavity adapted for optical detection of magnetic resonance. Experiments on X-traps induced by guest molecules in chemically mixed p-dibromobenzene (DBB) crystals provide evidence that optically excited triplet states perform at 1.4 K fast jumps between magnetically well defined trap molecules. We conclude that the guest molecule p-dichlorobenzene is surrounded by at least 4 energetically distorted DBB host molecules.     

13C Hyperfine interactions in CD3C60 and the distribution of unpaired spin on the C60 cage

¹³C hyperfine interactions obtained for CD₃C₆₀ are compared with previous data for MuC₆₀ and used to map the unpaired spin distribution on the C₆₀ surface of these radicals with the help of semi-empirical and density functional calculations. It is concluded that the unpaired spin is predominantly located on seven atoms near the point of attachment of the substituent.     

Electron spin coherence in Si

We discuss pulsed electron spin resonance measurements of electrons in Si and determine the spin coherence from the decay of the spin echo signals. Tightly bound donor electrons in isotopically enriched ²⁸Si are found to have exceptionally long spin coherence. Placing the donors near a surface or interface is found to decrease the spin coherence time, but it is still in the range of milliseconds. Unbound two-dimensional electrons have shorter coherence times of a few microseconds, though still long compared to the Zeeman frequency or the typical time to manipulate a spin with microwave pulses. Longer spin coherence is expected in two-dimensional systems patterned into quantum dots, but relatively small dots will be required. Data from dots with a lithographic size of 400 nm do not yet show longer spin coherence.     

C_(60)在室温有机溶剂中荧光的研究

本文在室温下对C60分别在吡啶、甲苯和乙腈中的荧光了研究,实验表明:C60在吡啶中的荧光由以440nm、570nm和700nm为中心的三个荧光带组成;C60在甲苯中的荧光由以430nm和700nm为中心的两个荧光带组成;C60在乙腈中的荧光由以570nm和700nm为中心的两个荧光带组成。经比较分析发现C60-有机溶剂体系700nm区域的荧光带的发射与溶剂的种类无关,而440nm和570nm区域的荧光带及其精细结构可以反映C60与溶剂分子的特殊相互作用。进一步提出C60-吡啶体系以570nm为中心的荧光带是由吡啶分子通过含孤对电子的N与C60形成的电荷转移络合物发出的。     

Excimer-laser-induced permanent electrical conductivity in solid C60 films

After being irradiated in air by a XeCI (308 nm) excimer laser, the electrical conductivity of solid thin-film C₆₀ has been improved by more than six orders of magnitudes. The products resulting from laser irradiation of C₆₀ films have been investigated by Raman scattering and the onset of conductivity can be attributed to laser-induced oxygenation and disintegration of the fullerene. Irradiated by 40 ns laser pulses with different fluence, products with different microstructure were observed. At lower fluence, the Raman features of microcrystalline graphite and fullerene polymer were observed. At a fluence just below the ablation threshold (36 mJ/cm²), the fullerene molecules in the film were disintegrated completely and transformed to amorphous graphite.     

Structure and electrical characteristics of ICBD C60 films

Polycrystalline C₆₀ films are deposited onto a variety of substrates by ionized cluster beam deposition (ICBD) technique. The structure of the ICBD C₆₀ films are studied by transmission electron microscopy (TEM). The electrical characteristics of the ICBD C₆₀ films on silicon substrates are investigated by current-voltage (I–V) measurements. TheICBD C₆₀/p-Si and C₆₀/n-Si heterostructures show strong current rectification, which is analyzed using band theory.     

Optical and electrical properties of C60Tex films

The structure, composition, and electrical and optical properties of thin tellurium-intercalated fullerene films C₆₀Te_x are investigated. The samples of compositions from C₆₀Te_0.1 to C₆₀Te₆ are prepared by thermal evaporation. The sample composition and the impurity distribution are controlled by the Rutherford backscattering technique. The Raman vibrational spectra indicate changes in the symmetry of a C₆₀ molecule: the strain of the molecule increases with a decrease in the tellurium concentration and decreases as the tellurium impurity concentration increases. The evolution of the optical absorption spectra and the electrical conductivity suggests that intercalation of a tellurium impurity leads to modification of the electronic structure of the material. This process is accompanied by a shift and change in shape of the optical absorption edge and a change in the electrical conductivity of films by several orders of magnitude depending on the composition. The electrical conductivity is minimum at a low tellurium impurity content.     

Magnetic properties of alkali-intercalated C60 compounds probed by electron spin resonance

In this paper, we present a survey of our ESR results on alkali-intercalated C₆₀ compounds. As we are far from fully understanding the physical properties of fullerides, this article deals successively with the different phases in the phase diagram of A₁C₆₀ compounds. It focuses on both conducting and magnetic properties in metallic compounds, the spin ground state in insulating systems and phase transitions in A₁C₆₀ phases.     

Measurement of electron spin lifetime and optical orientation efficiency in germanium using electrical detection of radio frequency modulated spin polarization

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pickup coil and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of 4.8% could be determined for Ge at 1342 nm excitation wavelength. In the presence of a small external magnetic field, the signal decayed according to the Hanle effect, from which a spin lifetime of 4.6±1.0 ns for electrons in bulk Ge at 127 K was extracted.     

EPR evidence of the low temperature phase transition in C60

A standard EPR technique has been applied to perform temperature studies of polycrystalline samples of C₆₀ fullerene. A comparison of the three main parameters of EPR spectra obtained for a C₆₀ sample before and after (solvent free) purification has been made in temperatures from 4 to 300 K. When compared to the spectrum of the as-obtained (crude) sample the spectrum of the purified sample revealed considerable changes i.e. an increase in the EPR signal intensity and evidence of a phase transition at about 90 K. This transition is proved to be related to the two processes: freezing of the orientational motion of C₆₀ molecules and redistribution of the positive and negative charge observed as paramagnetic centres localized on C₆₀ molecules.     

Room temperature magnetic properties of Fe and C implanted ZnO films

ZnO films prepared by radio frequency magnetron sputtering were singly or sequentially implanted with 120 keV Fe ions at a fluence of 5 × 10¹⁶ ions/cm² and 20 keV C ions at a fluence of 3 × 10¹⁵ ions/cm². Magnetic and optical properties as well as structures of the films have been investigated using various techniques. Magnetic measurements show that the as-deposited ZnO film presents room temperature ferromagnetism. Single Fe or C ion implantation has no contribution to enhancement in the film magnetism, while magnetic moment increases distinctly in the Fe and C ions sequentially implanted film. Results from structural measurements reveal that Fe nanoparticles are formed in the Fe singly implanted ZnO film. The post C implantation induces dissolution of Fe nanoparticles and promotes Fe atoms to substitute Zn atoms in the lattice. Based on the structural results, the effect of magnetic enhancement has been tentatively interpreted.