K. Kikoin

Double-exchange mechanisms for Mn-doped III-V ferromagnetic semiconductors

A microscopic model of indirect exchange interaction between transition metal impurities in dilute magnetic semiconductors (DMS) is proposed. The hybridization of the impurity d-electrons with the heavy hole band states is mainly responsible for the exchange of electrons between the impurities, whereas the Hund rule for the electron occupation of the impurity d-shells makes it spin selective. The model is applied to such systems as n-type (Ga,Mn)N and p-type (Ga,Mn)As, p-type (Ga,Mn)P. In n-type DMS with Mn 2+/3+ impurities the exchange mechanism is rather close to the kinematic exchange proposed by Zener for mixed-valence Mn ions. In p-type DMS ferromagnetism is governed by the kinematic mechanism involving the kinetic energy gain of the heavy hole carriers caused by their hybridization with 3d electrons of Mn 2+ impurities. Using the molecular field approximation, the Curie temperatures TC are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.

Magnetically tunable kondo-aharonov-bohm effect in a triangular quantum dot

The role of discrete orbital symmetry in mesoscopic physics is manifested in a system consisting of three identical quantum dots forming an equilateral triangle. Under a perpendicular magnetic field, this system demonstrates a unique combination of Kondo and Aharonov-Bohm features due to an interplay between continuous [spin-rotation SU(2)] and discrete (permutation C3v) symmetries, as well as U(1) gauge invariance. The conductance as a function of magnetic flux displays sharp enhancement or complete suppression depending on contact setups.

Kondo Tunneling through Real and Artificial Molecules

When an asymmetric double dot is hybridized with itinerant electrons, its singlet ground state and lowly excited triplet state cross, leading to a competition between the Zhang-Rice mechanism of singlet-triplet splitting in a confined cluster and the Kondo effect (which accompanies the tunneling through quantum dot under a Coulomb blockade restriction). The rich physics of an underscreened S = 1 Kondo impurity in the presence of low-lying triplet-singlet excitations is exposed and estimates of the magnetic susceptibility and the electric conductance are presented, together with applications for molecule chemisorption on metallic substrates.

4Electron self-trapping in intermediate-valentSmB6

SmB6 exhibits intermediate valence in the ground state and unusual behaviour at low temperatures. The resistivity and the Hall effect cannot be explained either by conventional sf-hybridization or by hopping transport in an impurity band. At least three different energy scales determine three temperature regimes of electron transport in this system. We consider the ground state properties, the soft valence fluctuations and the spectrum of band carriers in n-doped SmB6. The behaviour of excess conduction electrons in the presence of soft valence fluctuations and the origin of the three energy scales in the spectrum of elementary excitations is discussed. The carriers which determine the low-temperature transport in this system are self-trapped electron-polaron complexes rather than simply electrons in an impurity band. The mechanism of electron trapping is the interaction with soft valence fluctuations.

Superexchange in dilute magnetic dielectrics : Application to (Ti,Co)O2

We extend the model of ferromagnetic superexchange in dilute magnetic semiconductors to the ferromagnetically ordered highly insulating compounds (dilute magnetic dielectrics). The intrinsic ferromagnetism without free carriers is observed in oxygen-deficient films of anatase

Dynamical symmetries in Kondo tunneling through complex quantum dots.

{\mathrm{TiO}}_{2}

Kondo shuttling in a nanoelectromechanical single-electron transistor

doped with transition-metal impurities in cation sublattice. We suppose that ferromagnetic order arises due to superexchange between complexes (oxygen vacancies

Resonance Kondo tunneling through a double quantum dot at finite bias

+

Dynamical Symmetries for Nanostructures

magnetic impurities), which are stabilized by charge transfer from vacancies to impurities. The Hund rule controls the superexchange via empty vacancy related levels so that it becomes possible only for the parallel orientation of impurity magnetic moments. The percolation threshold for magnetic ordering is determined by the radius of vacancy levels, but the exchange mechanism does not require free carriers. The crucial role of the nonstoichiometry in formation of the ferromagnetism makes the Curie temperatures extremely sensitive to the methods of sample preparation.

Many-particle resonances in excited states of semiconductor quantum dots

Kondo tunneling reveals hidden SO(n) dynamical symmetries of evenly occupied quantum dots. As is exemplified for an experimentally realizable triple quantum dot in parallel geometry, the possible values n=3,4,5,7 can be easily tuned by gate voltages. Following construction of the corresponding o(n) algebras, scaling equations are derived and Kondo temperatures are calculated. The symmetry group for a magnetic field induced anisotropic Kondo tunneling is SU(2) or SO(4).