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A ferromagnetic coupling between localized Mn spins was predicted in a series of \textit{ab initio} and tight binding calculations and experimentally verified for the dilute magnetic semiconductor Ga$_{1-x}$Mn$_x$N. In the limit of small Mn concentrations, $x \lesssim 0.01$, the paramagnetic properties of this material were successfully described using a single ion crystal field model approach. In order to obtain the description of magnetization in (Ga,Mn)N in the presence of interacting magnetic centers, we extend the previous model of a single substitutional Mn$^{3+}$ ion in GaN by considering pairs, triplets and quartets of Mn$^{3+}$ ions coupled by a ferromagnetic superexchange interaction. Using this approach we investigate how the magnetic properties, particularly the magnitude of the uniaxial anisotropy field, change as the number of magnetic Mn$^{3+}$ ions in a given cluster increases from 1 to 4. Our simulations are then exploited in explaining experimental magnetic properties of Ga$_{1-x}$Mn$_x$N with $x \cong 0.03$, where the presence of small magnetic clusters gains in significance. As a result the approximate lower and upper limits for the values of exchange couplings between Mn$^{3+}$ ions in GaN, being in nearest neighbors $J_{\mathrm{nn}}$ and next nearest neighbors $J_{\mathrm{nnn}}$ positions, respectively, are established., Comment: 14 pages, 6 figures

By direct magnetization measurements, performed employing a new detection scheme, we demonstrate an electrical control of magnetization in wurtzite (Ga,Mn)N. In this dilute magnetic insulator the Fermi energy is pinned by Mn ions in the mid-gap region, and the Mn3+ ions show strong single-ion anisotropy. We establish that (Ga,Mn)N sustains an electric field up to at least 5 MV/cm, indicating that Mn doping turns GaN into a worthwhile semi-insulating material. Under these conditions, the magnetoelectric coupling may be driven by the inverse piezoelectric effect that stretches the elementary cell along the c axis and, thus, affects the magnitude of magnetic anisotropy. We develop a corresponding theory and show that it describes the experimentally determined dependence of magnetization on the electric field quantitatively with no adjustable parameters as a function of the magnetic field and temperature. In this way, our work bridges two research domains developed so far independently: piezoelectricity of wurtzite semiconductors and electrical control of magnetization in hybrid and composite magnetic structures containing piezoelectric components., 11 pages, 10 figures, version after revision

We report on an extensive structural and electrical characterization of undergate dielectric oxide insulators Al2O3 and HfO2 grown by atomic layer deposition. We elaborate the atomic layer deposition growth window for these oxides, finding that the 40–100 nm thick layers of both oxides exhibit fine surface flatness and required amorphous structure. These layers constitute a base for further metallic gate evaporation to complete the metal–insulator–semiconductor structure. Our best devices survive energizing up to ≈ 3 MV/cm at 77 K with the leakage current staying below the state-of-the-art level of 1 nA. At these conditions the displaced charge corresponds to a change of the sheet carrier density of 3 × 10 cm−2, which promises an effective modulation of the micromagnetic properties in diluted ferromagnetic semiconductors.

Interference effects in electron transport across a double quantum dot are studied theoretically within the framework of the Hartree–Fock approach by means of the Green function formalism based on the equation of motion method. An influence of inter–dot Coulomb repulsion of strength U on conductance spectra is analyzed and a limit of U comparable to the tunneling rate between two dots is discussed in detail. A variety of Fano features, including multiple (double or triple) Fano resonances, is found as a result of interferences between four states, molecular bonding and antibonding ones, as well as these states shifted by U. The number and intensity of Fano resonance peaks present in the conductance spectrum strongly depend on the relation between parameters which describe the inter-dot tunneling rate and inter-dot Coulomb interaction between electrons.

Electron transport through a system of two quantum dots coupled via tunneling processes and attached to external electrodes is studied within the framework of the Green function formalism based on the equation of motion method. Interdot Coulomb interactions are taken into account in a form of a Hubbard-like term and the interplay between interference and electron correlation effects is investigated. It is shown that Coulomb interactions strongly modify the linear conductance. However, the Fano-like behaviour can be observed and a dip in the conductance which appears due to interference effects practically is not influenced by interdot correlations.

A series of (Zn,Co)O layers with Co contents x up to 40% grown by atomic layer deposition have been investigated. All structures deposited at 160 degrees C show magnetic properties specific to II-V ...

For more than a decade ZnO doped with Mn and Co has remained as one of the most prospected diluted magnetic semiconductor for spintronic applications with conflicting outcome concerning the genuineness of its room temperature ferromagnetism. In order to clarify this issue we investigate (Zn,Co)O layers grown by atomic layer deposition at low temperatures. We employ and relay on wide range of extensive material characterization, which in combination with superconducting quantum interference device magnetometry allow us decisively exemplify the growth temperature as the key factor discriminating between paramagnetic (obtained at 160 °C) and various forms of ferromagnetic responses, seen when the grows is carried out at 200 °C and above.

Electron transport through a double quantum dot system is studied with the use of the Green function formalism based on the equation of motion method, and an interplay between interference and Coulomb blockade effects due to inter-dot correlations is discussed. A double structure with two Fano resonances (or antiresonances) is found in the conductance spectrum. Fano features are weakly influenced by the presence of Coulomb interaction but the conductance is strongly suppressed in the energy region with the Fermi level in the leads close to the aligned levels of both dots. This Coulomb blockade effect takes place when the coupling between the dots is of repulsive character. On the other hand, the conductance of an artificial molecule with attractive inter-dot coupling is only slightly modified in this energy region. As a sign of the coupling can be easily changed in a presence of an external magnetic field by changes of the magnetic flux there is the possibility to control variations of the conductance, which may be important from the application point of view.

We report low-temperature transport measurements of three-terminal $\mathsf{T}$-shaped device patterned from $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ heterostructure. We demonstrate the mode branching and bend resistance effects predicted by numerical modeling for linear conductance data. We show also that the backscattering at the junction area depends on the wave function parity. We find evidence that in a nonlinear transport regime the voltage of floating electrode always increases as a function of push-pull polarization. Such anomalous effect occurs for the symmetric device, provided the applied voltage is less than the Fermi energy in equilibrium.

Systematic investigations of the structural and magnetic properties of single crystal (Ga,Mn)N films grown by metal organic vapor phase epitaxy are presented. High resolution transmission electron microscopy, synchrotron x-ray diffraction, and extended x-ray absorption fine structure studies do not reveal any crystallographic phase separation and indicate that Mn occupies Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic properties as a function of temperature, magnetic field and its orientation with respect to the c-axis of the wurtzite structure can be quantitatively described by the paramagnetic theory of an ensemble of non-interacting Mn$^{3+}$ ions in the relevant crystal field, a conclusion consistent with the x-ray absorption near edge structure analysis. A negligible contribution of Mn in the 2+ charge state points to a low concentration of residual donors in the studied films. Studies on modulation doped p-type (Ga,Mn)N/(Ga,Al)N:Mg heterostructures do not reproduce the high temperature robust ferromagnetism reported recently for this system., 15 pages, 14 figures

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+., 12 pages, 14 figures; This version contains the detailed characterization of the crystal structure as well as of the Mn distribution and charge state

Transport across a double dot system of a special geometry with two channels accessible for tunneling electrons is theoretically studied in a region of low temperatures corresponding to the Kondo regime, and interference effects are analyzed. The spectral function and the linear conductance are calculated using the Green function formalism based on the equation of motion method. It is shown that interference processes strongly influence the spectral function. Moreover, due to interference effects the conductance, which is close to zero in a low energy region, shows at higher gate voltages a well defined Fano resonance. A development of the Fano peak as the system undergoes a gradual transition from the serial configuration to the geometry with two dots partially connected to both electrodes is demonstrated. The position and intensity of the Fano resonance strongly depend on the inter-dot tunneling rate. The influence of temperature on the resonance is also discussed.

Electron transport through a parallel double quantum dot is theoretically studied in the Kondo regime with the use of the non-equilibrium Green function formalism based on the equation of motion method. An influence of inter-dot tunnel coupling t and channel mixing effects on the orbital (spinless) Kondo phenomenon is analysed in the linear and nonlinear transport regimes. Both effects lead to a considerable suppression of the conductance in the Kondo regime. In a system with dots capacitively coupled (t = 0) the differential conductance shows a zero-bias peak whose intensity diminishes gradually with mixing effects included. When tunnel coupling between dots is taken into account () the orbital Kondo resonance splits and the intensities of both components are strongly influenced by channel mixing. The linear conductance calculated as a function of a dot level position E0 is strongly suppressed in the Kondo regime but at higher values of E0 a relatively well-pronounced side peak appears whose intensity increases with the coupling rate t. We consider the side peak maximum as originated from interference processes in the system.

A range of high quality Ga1-xMnxN layers have been grown by molecular beam epitaxy with manganese concentration 0.2 < x < 10%, having the x value tuned by changing the growth temperature (Tg) between 700 and 590 ��C, respectively. We present a systematic structural and microstructure characterization by atomic force microscopy, secondary ion mass spectrometry, transmission electron microscopy, powder-like and high resolution X-ray diffraction, which do not reveal any crystallographic phase separation, clusters or nanocrystals, even at the lowest Tg. Our synchrotron based X-ray absorption near-edge spectroscopy supported by density functional theory modelling and superconducting quantum interference device magnetometry results point to the predominantly +3 configuration of Mn in GaN and thus the ferromagnetic phase has been observed in layers with x > 5% at 3 < T < 10 K. The main detrimental effect of Tg reduced to 590 ��C is formation of flat hillocks, which increase the surface root-mean-square roughness, but only to mere 3.3 nm. Fine substrates surface temperature mapping has shown that the magnitudes of both x and Curie temperature (Tc) correlate with local Tg. It has been found that a typical 10 ��C variation of Tg across 1 inch substrate can lead to 40% dispersion of Tc. The established here strong sensitivity of Tc on Tg turns magnetic measurements into a very efficient tool providing additional information on local Tg, an indispensable piece of information for growth mastering of ternary compounds in which metal species differ in almost every aspect of their growth related parameters determining the kinetics of the growth. We also show that the precise determination of Tc by two different methods, each sensitive to different moments of Tc distribution, may serve as a tool for quantification of spin homogeneity within the material., 29 pages, 18 figures

We consider electron tunnelling through double quantum dots (DQD) attached to external leads. Systems of quantum dots coupled in series and in parallel configuration are investigated. Tunnelling coupling between the two dots in series is described by V and it is treated as a tunable parameter A single dot in the system is described by the one-level Anderson Hamiltonian with infinite intradot Coulomb repulsion. Accordingly, only a single state on each dot can be expected and the double occupancy of each dot can be ignored. The interdot Coulomb interaction is taken into account and it is described via Hubbard-like term. Strong and weak correlation regimes are studied. Tunneling rates between the dots and reservoirs are assumed to be weak, so the Kondo effect is not observed in the system. The non-equilibrium Green function (GF) formalism is introduced to describe electron transport in the non-linear regime. The retarded r G and advanced a G GFs are calculated within the framework of the Hartree-Fock approximation with use of the equation of motion method. For a DQD system GFs are represented by 2 x 2 matrices. In the present approach diagonal and non-diagonal terms are taken into account and are calculated in a self-consistent way. Note, that for steady-state transport one does not need to find an appropriate formula for lesser GF < G itself (see Ref. [1]). Occupation numbers and electric current can be expressed by ( ) ∫ < e e d G and retarded (advanced) GF only. This approach allows us to omit

Spin-polarized transport in nanoscopic systems was mainly investigated for a single quantum dot (QD) attached to ferromagnetic electrodes (see e.g. [1]). It was shown that such a set could act as a spin valve. The current flowing through QD strongly depends on the configuration of magnetic moments in the leads. Usually, it is maximal for configuration in which magnetic moments are parallel in both electrodes. Spin-based devices are very important due to possibility of their application in spintronics. Especially, double quantum dot (DQD) structures could probably work in quantum computer hardware [2]. Electron transport through a double dot system received much experimental and theoretical attention during the past decade [3, 4], however, systems with non-magnetic electrodes were mainly investigated. Recently, spin-dependent transport has been studied for a set of two quantum dots capacitively coupled [5]. The linear conductance in the Kondo regime has been mainly investigated. Here, we analyze spin-dependent transport through a system of two quantum dots in series. The inter-dot tunnel coupling can be easily tuned in such systems [6]. It strongly influences the current flowing through the system. Using the non-equilibrium Green function formalism based on the equation of motion we show that the DQD set can act as an effective spin valve in which not only tunnel magnetoresistance (TMR) value but also its sign can be easily changed with inter-dot coupling.

A theoretical approach to a problem of electron transport through double quantum dot systems based on non-equilibrium Green function formalism using equation of motion method is presented. I−V characteristics and differential conductance are calculated and discussed in detail in the intermediate regime with tunneling rate between the quantum dots comparable to coupling constants with external electrodes. Effects of inter-dot Coulomb correlations are studied for various values of interaction parameter U . It is shown that the interaction influences transport properties in a pronounced way and apart from the simple Coulomb blockade additional effects can be obtained. When energy levels of two quantum dots are not aligned, the asymmetry in conductance characteristics is closely related to a voltage dependence of population numbers in both quantum dots. For a one bias polarization electrons are well localized in quantum dots in a low voltage region, whereas for the opposite one they are partly delocalized.