Magnetoresistance (MR) of ion irradiated monolayer graphene samples with variable-range hopping (VRH) mechanism of conductivity was measured at temperatures down to $T = 1.8$ K in magnetic fields up to $B = 8$ T. It was observed that in perpendicular
magnetic fields, hopping resistivity $R$ decreases, which corresponds to negative MR (NMR), while parallel magnetic field results in positive MR (PMR) at low temperatures. NMR is explained on the basis of the orbital model in which perpendicular magnetic field suppresses the destructive interference of many paths through the intermediate sites in the total probability of the long-distance tunneling in the VRH regime. At low fields, a quadratic dependence ($|Delta R/R|sim B^2$) of NMR is observed, while at $B > B^*$, the quadratic dependence is replaced by the linear one. It was found that all NMR curves for different samples and different temperatures could be merged into common dependence when plotted as a function of $B/B^*$. It is shown that $B^*sim T^{1/2}$ in agreement with predictions of the orbital model. The obtained values of $B^*$ allowed also to estimate the localization radius $xi$ of charge carriers for samples with different degree of disorder. PMR in parallel magnetic fields is explained by suppression of hopping transitions via double occupied states due to alignment of electron spins.
Raman scattering (RS) spectra and current-voltage characteristics at room temperature were measured in six series of small samples fabricated by means of electron-beam lithography on the surface of a large size (5x5 mm) industrial monolayer graphene
film. Samples were irradiated by different doses of C${}^+$ ion beam up to $10^{15}$ cm${}^{-2}$. It was observed that at the utmost degree of disorder, the Raman spectra lines disappear which is accompanied by the exponential increase of resistance and change in the current-voltage characteristics.These effects are explained by suggestion that highly disordered graphene film ceases to be a continuous and splits into separate fragments. The relationship between structure (intensity of RS lines) and sample resistance is defined. It is shown that the maximal resistance of the continuous film is of order of reciprocal value of the minimal graphene conductivity $pi h/4e^2approx 20$ kOhm.
We have performed conductivity measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities n_1 and n_2 across the slot. Dynamic longitudinal resistance was measured by a standard lock-in technique, whil
e maintaining a large DC current through the source-drain channel. We find that in a parallel magnetic field, the resistance of the sample, R(I_DC), is asymmetric with respect to the direction of the DC current. The asymmetry becomes stronger with an increase of either the magnetic field or the difference between n_1 and n_2. These observations are interpreted in terms of the effective spin injection: the degree of spin polarisation is different in the two parts of the sample, implying different magnitudes of spin current away from the slot. The carriers thus leave the excess spin (of the appropriate sign) in the region around the slot, leading to spin accumulation (or depletion) and to the spin drift-diffusion phenomena. Due to the positive magnetoresistance of the two-dimensional electron gas, this change in a local magnetisation affects the resistivity near the slot and the measured net resistance, giving rise to an asymmetric contribution. We further observe that the value of R(I_DC) saturates at large I_DC; we suggest that this is due to electron tunnelling from the two-dimensional n-type layer into the p-type silicon (or into another spin reservoir) at the slot.
Positive magnetoresistance (PMR) of a silicon MOSFET in parallel magnetic fields B has been measured at high electron densities n >> n_c where n_c is the critical density of the metal-insulator transition (MIT). It turns out that the normalized PMR c
urves, R(B)/R(0), merge together when the field is scaled according to B/B_c(n) where B_c is the field in which electrons become fully spin polarized. The values of B_c have been calculated from the simple equality between the Zeeman splitting energy and the Fermi energy taking into account the experimentally measured dependence of the spin susceptibility on the electron density. This extends the range of validity of the scaling all the way to a deeply metallic regime far away from MIT. The subsequent analysis of PMR for low n >~ n_c demonstrated that the merging of the initial parts of curves can bee achieved only with taking into account the temperature dependence of B_c. It is also shown that the shape of the PMR curves at strong magnetic fields is affected by a crossover from a purely two-dimensional (2D) electron transport to a regime where out-of-plane carrier motion becomes important (quasi-three-dimensional regime).
The influence of fast neutron irradiation on the structure and spatial distribution of Ge nanocrystals (NC) embedded in an amorphous SiO2 matrix has been studied. The investigation was conducted by means of laser Raman Scattering (RS), High Resolutio
n Transmission Electron Microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). The irradiation of NC-Ge samples by a high dose of fast neutrons lead to a partial destruction of the nanocrystals. Full reconstruction of crystallinity was achieved after annealing the radiation damage at 800 deg. C, which resulted in full restoration of the RS spectrum. HR-TEM images show, however, that the spatial distributions of NC-Ge changed as a result of irradiation and annealing. A sharp decrease in NC distribution towards the SiO2 surface has been observed. This was accompanied by XPS detection of Ge oxides and elemental Ge within both the surface and subsurface region.
We report measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities n_{1,2} across the slot. The dynamic longitudinal resistance was measured by the standard lock-in technique, while maintaining a lar
ge DC current through the source-drain channel. We find that the conductance of the sample in a strong parallel magnetic field is asymmetric with respect to the DC current direction. This asymmetry increases with magnetic field. The results are interpreted in terms of electron spin accumulation or depletion near the slot.
The transversal and longitudinal resistance in the quantum Hall effect regime was measured in a Si MOSFET sample in which a slot-gate allows one to vary the electron density and filling factor in different parts of the sample. In case of unequal gate
voltages, the longitudinal resistances on the opposite sides of the sample differ from each other because the originated Hall voltage difference is added to the longitudinal voltage only on one side depending on the gradient of the gate voltages and the direction of the external magnetic field. After subtracting the Hall voltage difference, the increase in longitudinal resistance is observed when electrons on the opposite sides of the slot occupy Landau levels with different spin orientations.
