Vyacheslav G. Storchak

Muon—nuclear quadrupolar level crossing resonance in solid nitrogen. Evidence for N2μ+ ion formation

Abstract Both integral and time-differential muon level crossing resonance techniques were used to identify the source of the diamagnetic μ + SR signal in crystalline nitrogen. When the muon Zeeman splitting and the 14 N nuclear quadrupole splitting are equal, the longitudinal field μ + spin relaxation rate is increased due to dipolar 14 N—μ + coupling. At T = 37 K the resonance occurs at a magnetic field of 172 ± 2 G with a fwhm of 10 ± 2 G. The resonance parameters unambiguously confirm the formation of the N 2 μ + ion in solid nitrogen.

Direct Epitaxial Integration of the Ferromagnetic Semiconductor EuO with Silicon for Spintronic Applications

Following a remarkable success of metallic spintronics, tremendous efforts have been invested into the less developed semiconductor spintronics, in particular, with the aim to produce three-terminal spintronic devices, e.g., spin transistors. One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers into a nonmagnetic semiconductor, preferably one of those currently used for industrial applications such as Si-a workhorse of modern electronics. Ferromagnetic semiconductor EuO is long believed to be the best candidate for integration with Si. Although EuO proved to offer optimal conditions for effective spin injection into silicon and in spite of considerable efforts, the direct epitaxial stabilization of stoichiometric EuO thin films on Si without any buffer layer has not been demonstrated to date. Here we report a new technique for control of EuO/Si interface on submonolayer level. Using this technique we solve a long-standing problem of direct epitaxial growth on silicon of thin EuO films which exhibit structural and magnetic properties of EuO bulk material. This result opens up new possibilities in developing all-semiconductor spintronic devices.

Electric field dependence of muonium atom formation in solid nitrogen

Abstract Muon spin precession signals arising from both muonium (μ+e- or Mu) and diamagnetic muon species have been studied in condensed molecular nitrogen in the temperature range 10–78 K. Muonium is formed both in “prompt” epithermal processes and in “delayed” convergence of the thermalized μ+ with an electron from the muon ionization track. The latter process is strongly correlated with changes in the electron mobility in solid N2; it is inhibited by an electric field E ∼ 3.5 kV/cm applied in the direction of the uonn motion and enchanced by a comparable electric field in the opposite direction, indicating that the μ+ stops on average about 60 nm “downstream” of the e- liberated in its last ionization of the medium.

Electron transport in cryocrystals

We review our recent experimental studies of the excess electron transport in cryocrystals and cryoliquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the μ+ ionization track converge upon the positive muons and form Mu (μ+e−) atoms in which the μ+ polarization is partially lost. The spatial distribution of such electrons with respect to the moon is shown to be highly anisotropic: the μ+ thermalizes well “downstream” from the center of the electron distribution. Measurements in electric fields up to 30 kV/cm allow one to estimate the characteristic muon-electron distance in different insulators: the results range from 10−6 cm to 10−4 cm. This circumstance makes the basis of a recently developed new technique for electron transport studies on microscopic scale: electron mobility can be extracted when both the characteristic muon-electron distance and characteristic time for muonium atom formation are determined. The microscopic length scale enables the electron to sometimes spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. The muonium formation process in condensed matter is shown to depend critically upon whether the excess electron forms a polaron or remains in a delocalized state. Different mechanisms of electron transport in insulators are discussed.

Electron transport to positive centers in GaAs

Abstract The origin of neutral muonium defect centers in semi-insulating GaAs has been studied using muon spin rotation/relaxation techniques employing alternating electric fields. This technique prevents the accumulation of near-surface charges which may screen the external field. Suppression of the bond-centered muonium signal with electric field suggests that muonium formation proceeds via transport of excess electrons from the ionization track to the muon. The characteristic field of about 5 kV/cm may correspond to an electric field induced ionization of the muon donor impurity center.

Electron transport in solids and liquids

We review our recent experimental studies of the excess electron states in insulating solids and liquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the μ+ ionization track converge upon the positive muons and form Mu (μ+e−) atoms in which the μ+ polarization is partially lost. The spatial distribution of such electrons with respect to the muon is shown to be highly anisotropic: the μ+ thermalizes well “downstream” from the center of the electron distribution. Measurements in electric fields up to 30 kV/cm allow one to estimate the characteristic muon-electron distance in different insulators: the results range from 10−6 to 10−4 cm. Such a microscopic length scale enables the electron to sometimes spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. This circumstance illustrates the potential of μ+SR techniques in studies of electron transport in matter. The muonium formation process in condensed matter is shown to depend critically upon whether the excess electron forms a polaron or remains in a delocalized state. Different mechanisms of electron transport in insulators are discussed.

Europium Silicide – a Prospective Material for Contacts with Silicon

Metal-silicon junctions are crucial to the operation of semiconductor devices: aggressive scaling demands low-resistive metallic terminals to replace high-doped silicon in transistors. It suggests an efficient charge injection through a low Schottky barrier between a metal and Si. Tremendous efforts invested into engineering metal-silicon junctions reveal the major role of chemical bonding at the interface: premier contacts entail epitaxial integration of metal silicides with Si. Here we present epitaxially grown EuSi2/Si junction characterized by RHEED, XRD, transmission electron microscopy, magnetization and transport measurements. Structural perfection leads to superb conductivity and a record-low Schottky barrier with n-Si while an antiferromagnetic phase invites spin-related applications. This development opens brand-new opportunities in electronics.

Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon

Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics.

Delayed muonium formation in quartz

By periodically reversing the direction of an electric field E applied to a thin sheet of SiO2, buildup of uncollected charges on the surfaces of the sample was prevented and the effect of E on delayed muonium formation in quartz was observed for the first time. The results indicate that the μ+ stops about 10–15 nm “downstream” of most of its radiolysis electrons in crystalline quartz. In fused silica this distance is nearly a factor of two shorter and there is a “missing fraction”, probably due to lower mobility of free electrons.

On the nature of the muon complex in condensed oxygen

Abstract Muon precession parameters in liquid oxygen as well as α-, β- and γ-phases of crystalline oxygen have been measured in the temperature range 10–90 K. It was found that the muon polarization P =1 in liquid O 2 , γ-O 2 and β-O 2 . The local field at the muon site has been measured in the antiferromagnetic α-phase of oxygen ( B 0 =1.2 kG). The analysis of the data obtained shows that about 40% of the muons in oxygen form a paramagnetic complex (presumably MuO 2 or O 2 Mu + ) and about 60% of them form a diamagnetic compound.