Simon Kos

Effects of strain, electric, and magnetic fields on lateral electron-spin transport in semiconductor epilayers

We construct a spin-drift-diffusion model to describe spin-polarized electron transport in zincblende semiconductors in the presence of magnetic fields, electric fields, and off-diagonal strain. We present predictions of the model for geometries that correspond to optical spin injection from the absorption of circularly polarized light, and for geometries that correspond to electrical spin injection from ferromagnetic contacts. Starting with the Keldysh Greens function description for a system driven out of equilibrium, we construct a semiclassical kinetic theory of electron spin transport in strained semiconductors in the presence of electric and magnetic fields. From this kinetic theory we derive spin-drift-diffusion equations for the components of the spin density matrix for the specific case of spatially uniform fields and uniform electron density. We solve the spin-drift-diffusion equations numerically and compare the resulting images with scanning Kerr microscopy data of spin-polarized conduction electrons flowing laterally in bulk epilayers of n-type GaAs. The spin-drift-diffusion model accurately describes the experimental observations. We contrast the properties of electron spin precession resulting from magnetic and strain fields. Spin-strain coupling depends linearly on electron wave vector and spin-magnetic field coupling is independent of electron wave vector. As a result, spatial coherence of precessing spin flows is better maintained with strain than with magnetic fields, and the spatial period of spin precession is independent of the applied electrical bias in strained structures whereas it is strongly bias dependent for the case of applied magnetic fields.

Thickness dependent wetting properties and surface free energy of HfO2 thin films

We show here that intrinsic hydrophobicity of HfO2 thin films can be easily tuned by the variation of film thickness. We used the reactive high-power impulse magnetron sputtering for preparation of high-quality HfO2 films with smooth topography and well-controlled thickness. Results show a strong dependence of wetting properties on the thickness of the film in the range of 50–250 nm due to the dominance of the electrostatic Lifshitz-van der Waals component of the surface free energy. We have found the water droplet contact angle ranging from ≈120° for the thickness of 50 nm to ≈100° for the thickness of 2300 nm. At the same time the surface free energy grows from ≈25 mJ/m2 for the thickness of 50 nm to ≈33 mJ/m2 for the thickness of 2300 nm. We propose two explanations for the observed thickness dependence of the wetting properties: influence of the non-dominant texture and/or non-monotonic size dependence of the particle surface energy.

Transport and ionization of sputtered atoms in high-power impulse magnetron sputtering discharges

We use a non-stationary two-zone model to verify predictions of a steady-state phenomenological model (Vl?ek and Burcalov?) under the conditions in typical high-power impulse magnetron sputtering discharges (copper target of 50?mm diameter, argon pressure of 1?Pa, rectangular voltage pulses of 200??s length with amplitudes from 400 to 1000?V and repetition frequency of 100?Hz). It is shown that the steady-state phenomenological model provides a reliable description of fundamental deposition parameters characterizing efficiency of magnetron sputtering and the transfer of target material ions to the substrate in these discharges with relatively long steady-state discharge regimes established during pulses. Based on the results, we recommend to lower the magnetic field strength in a magnetron system at a fixed average target power density in a pulse and thereby use a higher magnetron voltage in order to enhance the deposition rate and keep or even increase the ionized fraction of sputtered target material atoms in the flux onto the substrate.

Spin noise of itinerant fermions

We develop a theory of spin-noise spectroscopy of itinerant, noninteracting, and spin-carrying fermions in different regimes of temperature and disorder. We use kinetic equations for the density matrix in spin variables. We find a general result with a clear physical interpretation, and discuss its dependence on temperature, the size of the system, and applied magnetic field. We consider two classes of experimental probes: (1) electron-spin-resonance-type measurements, in which the probe response to a uniform magnetization increases linearly with the volume sampled and (2) optical Kerr/Faraday rotation-type measurements, in which the probe response to a uniform magnetization increases linearly with the length of the light propagation in the sample but is independent of the cross section of the light beam. Our theory provides a framework for interpreting recent experiments on atomic gases and conduction electrons in semiconductors and provides a baseline for identifying the effects of interactions on spin-noise spectroscopy.

SiBCN materials for high-temperature applications: Atomistic origin of electrical conductivity

The paper contains a detailed discussion of the electronic structure of the novel hard and thermally stable amorphous SiBCN materials. We focus on the weight of individual electronic states on different elements, bond types, bonds of different lengths, and the number of atoms and clusters of atoms the states are localized on. A special attention is paid to the states around the Fermi level. We show in detail the effect of individual elements and bond types on the (non)conductivity of the materials. The results provide a detailed insight into the complex relationships between the material composition and the electronic properties, and allow one to tailor SiBCN compositions which can combine different functional properties, such as high thermal stability with electrical conductivity.

Broken Particle-Hole Symmetry at Atomically Flat a-Axis YBa~2Cu~3O~7~-~d~e~l~t~a Interfaces

We have studied quasiparticle tunneling into atomically flat a-axis films of YBa(2)Cu(3)O(7-delta) and DyBa(2)Cu(3)O(7-delta) through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor-insulator interface. Below T(c), the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero-bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas.

Statistical physics: hear the noise.

At the nanoscale, thermal fluctuations and noise dominate. But instead of being a hindrance, the details of the noise itself can reveal the physical properties of the system.

Hard TiN2 dinitride films prepared by magnetron sputtering

This letter reports on the formation of hard TiN2 dinitride films prepared by magnetron sputtering. TiN2 films were reactively sputtered in an Ar + N2 gas mixture using a pulsed dual magnetron with a closed magnetic field B. The principle of the formation of TiN2 films by magnetron sputtering is briefly described. The stoichiometry x = N/Ti of the TiNx films was controlled by deposition parameters, and its maximum value of x = 2.3 was achieved. For the first time, a possibility to form the TiN2 dinitride films by magnetron sputtering has been demonstrated. The mechanical properties of sputtered films were investigated in detail.This letter reports on the formation of hard TiN2 dinitride films prepared by magnetron sputtering. TiN2 films were reactively sputtered in an Ar + N2 gas mixture using a pulsed dual magnetron with a closed magnetic field B. The principle of the formation of TiN2 films by magnetron sputtering is briefly described. The stoichiometry x = N/Ti of the TiNx films was controlled by deposition parameters, and its maximum value of x = 2.3 was achieved. For the first time, a possibility to form the TiN2 dinitride films by magnetron sputtering has been demonstrated. The mechanical properties of sputtered films were investigated in detail.