arXiv subjects

K. Kikoin

Publications and source records attributed to K. Kikoin.

At least 37 records · Page 2Linked to original sources

arXiv2006

Magnetic field induced two-channel Kondo effect in multiple quantum dots

We study the possibility to observe the two channel Kondo physics in multiple quantum dot heterostructures in the presence of magnetic field. We show that a fine tuning of the coupling parameters of the system and an external magnetic field may stabilize the two channel Kondo critical point. We make predictions for behavior of the scaling of the differential conductance in the vicinity of the quantum critical point, as a function of magnetic field, temperature and source-drain potential.

cond-mat.str-el↗

arXiv2006

Metastable diamagnetism in Sm0.1Ca0.84Sr0.06MnO3 manganite

Magnetic properties of polycrystalline Sm0.1Ca0.84Sr0.06MnO3 in pristine and metastable states have been investigated in wide range of temperatures and magnetic fields. It was found that below Curie temperature TC = 105 K the pristine state exhibits phase separation comprising ferromagnetic and antiferromagnetic phases. The metastable states with reduced magnetization were obtained by successive number of quick coolings of the sample placed in container with kerosene-oil mixture. By an increasing number of quick coolings (> 6) the long time relaxation appeared at 10 K and the magnetization reversed its sign and became strongly negative in wide temperature range, even under an applied magnetic field of 15 kOe. The observed field and temperature dependences of the magnetization in this state are reversed in comparison with the ordinary ferromagnetic ones. Above TC, the observed diamagnetic susceptibility of the reversed magnetization state at T = 120 K is ~ - 0.9 x 10-4 emu g-1 Oe-1. Only after some storage time at room temperature, the abnormal magnetic state is erasable. It is suggested that the negative magnetization observed results from a specific coupling of the nano/micro-size ferromagnetic regions with a surrounding diamagnetic matrix formed, in a puzzled way, by the repeating training (quick cooling) cycles.

cond-mat.str-el↗

arXiv2006

Superexchange in Dilute Magnetic Dielectrics: Application to (Ti,Co)O_2

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 TiO_2 doped with transition metal impurities in cation sublattice. We suppose that ferromagnetic order arises due to superexchange between complexes [oxygen vacancies + 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 non-stoichiometry in formation of the ferromagnetism makes the Curie temperatures extremely sensitive to the methods of sample preparation.

cond-mat.mtrl-sci↗

arXiv2006

Decoherence and Dephasing in Kondo Tunneling through Double Quantum Dots

We describe the mechanism of charge-spin transformation in a double quantum dot (DQD) with even occupation, where a time dependent gate voltage v(t) is applied to one of its two valleys, whereas the other one is coupled to the source and drain electrodes. The Kondo tunneling regime under strong Coulomb blockade may be realized when the spin spectrum of the DQD is formed by the ground state spin triplet and two singlet excitations. Charge fluctuations induced by v(t) result in transitions within the spin multiplet characterized by the SO(5) dynamical symmetry group. In a weakly non-adiabatic regime the decoherence, dephasing and relaxation processes affect Kondo tunneling. Each of these processes is caused by a special type of dynamical gauge fluctuations, so that one may discriminate between the decoherence in the ground state of a DQD and dephasing at finite temperatures.

cond-mat.mes-hall↗

arXiv2006

Kondo Shuttling in Nanoelectromechanical Single-Electron Transistor

We investigate theoretically a mechanically assisted Kondo effect and electric charge shuttling in nanoelectromechanical single-electron transistor (NEM-SET). It is shown that the mechanical motion of the central island (a small metallic particle) with the spin results in the time dependent tunneling width which leads to effective increase of the Kondo temperature. The time-dependent oscillating Kondo temperature T_K(t) changes the scaling behavior of the differential conductance resulting in the suppression of transport in a strong coupling- and its enhancement in a weak coupling regimes. The conditions for fine-tuning of the Abrikosov-Suhl resonance and possible experimental realization of the Kondo shuttling are discussed.

cond-mat.mes-hall↗

arXiv2006

Tunneling through triple quantum dots with mirror symmetry

Indirect exchange interaction between itinerant electrons and nano-structures with non-trivial geometrical configurations manifests a plethora of unexpected results. These configurations can be realized either in quantum dots with several potential valleys or in real complex molecules with strong correlations. Here we demonstrate that the Kondo effect may be suppressed under certain conditions in triple quantum dots with mirror symmetry at odd electron occupation. First, we show that the indirect exchange has ferromagnetic sign in the ground state of triple quantum dot in a two-terminal cross geometry for electron occupation N=3. Second, we show that for electron occupation N=1 in three-terminal fork geometry the zero-bias anomaly in the tunnel conductance is absent (despite the presence of Kondo screening) due to special symmetry of the dot wave function.

cond-mat.str-el↗

arXiv2006

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.

cond-mat.str-el↗

arXiv2005

Vibration-induced Kondo tunneling through metal-organic complexes with even electron occupation number

We investigate transport through a mononuclear transition-metal complex with strong tunnel coupling to two electrodes. The ground state of this molecule is a singlet while the first excited state is a triplet. We show that a modulation of the tunnel-barrier due to a molecular distortion which couples to the tunneling induces a Kondo-effect, provided the discrete vibrational energy compensates the singlet/triplet gap. We discuss the single-phonon and two-phonon assisted co-tunneling and possible experimental realization of the theory.

cond-mat.str-el↗

arXiv2005

Kondo Lattice without Nozieres Exhaustion Effect

We discuss the properties of layered Anderson/Kondo lattices with metallic electrons confined in 2D xy planes and local spins in insulating layers forming chains in z direction. Each spin in this model possesses its own 2D Kondo cloud, so that the Nozieres' exhaustion problem does not occur. The excitation spectrum of the model is gapless both in charge and spin sectors. The disordered phases and possible experimental realizations of the model are briefly discussed.

cond-mat.str-el↗

arXiv2004

Ultraviolet Probing of Quantum Crossbars

Ultraviolet (UV) scattering on quantum crossbars (QCB) is an effective tool for probing QCB spectral properties, leading to excitation of QCB plasmon(s). Experimentally, such a process corresponds to sharp peaks in the frequency dependence of the differential scattering cross section. The peak frequency strongly depends on the direction of the scattered light. As a result, $1D\to 2D$ crossover can be observed in the scattering spectrum. It manifests itself as a splitting of single lines into multiplets (mostly doublets). The splitting magnitude increases with interaction in QCB crosses, while the peak amplitudes decrease with electron-electron interaction within a QCB constituent.

cond-mat.str-el↗

arXiv2004

Spin Gap in Chains with Hidden Symmetries

We investigate the formation of spin gap in one-dimensional models characterized by the groups with hidden dynamical symmetries. A family of two-parametric models of isotropic and anisotropic Spin-Rotator Chains characterized by SU(2)x SU(2) and SO(2)x SO(2)x Z_2 x Z_2 symmetries is introduced to describe the transition from SU(2) to SO(4) antiferromagnetic Heisenberg chain. The excitation spectrum is studied with the use of the Jordan-Wigner transformation generalized for o_4 algebra and by means of bosonization approach. Hidden discrete symmetries associated with invariance under various particle-hole transformations are discussed. We show that the spin gap in SRC Hamiltonians is characterized by the scaling dimension 2/3 in contrast to dimension 1 in conventional Haldane problem.

cond-mat.str-el↗

arXiv2004

Dynamical symmetries in nanophysics

The concept of dynamical symmetries is specified for quantum dots under strong Coulomb blockade. It is shown that the electron cotunneling through quantum dots may be described in terms of generators of SO(n) or SU(n) dynamical groups, which characterize the energy spectrum of quantum dots. Various experimental manifestations of dynamical symmetry in Kondo tunneling both in equilibrium and out of equilibrium are reviewed. Special attention is payed to algebraic aspects of dynamical symmetries: various representation of group generators in terms of Hubbard operators, bosonic and fermionic fields are described.

cond-mat.mes-hall↗

arXiv2003

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 largely responsible for the transfer of electrons between the impurities, whereas Hund rule for the electron occupation of the impurity d-shells makes the transfer spin selective. The model is applied to such systems as $n-$type GaN:Mn and $p-$type (Ga,Mn)As, $p-$type (Ga,Mn)P. In $n-$type DMS with Mn$^{2+/3+}$ impurities the exchange mechanisms 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 heavy hole carriers caused by their hybridization with 3d electrons of Mn$^{2+}$ impurities. Using the molecular field approximation the Curie temperatures $T_C$ are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.

cond-mat.str-el↗

arXiv2003

Explicit and Hidden Symmetries in Quantum Dots and Quantum Ladders

The concept of dynamical hidden symmetries in the physics of electron tunneling through composite quantum dots (CQD) and quantum ladders (QL) is developed and elucidated. Quite generally, dynamical symmetries are realizable in the space of low energy excited states in a given charge sector of nanoobjects, which involve spin variables and/or electron-hole pairs. While spin multiplets in an individual rung of a QL or in an isolated CQD form a representation space of the usual rotation group, this SU(2) symmetry is broken due to spin transfer (in QL) electron cotunneling through CQD. Dynamical symmetries in the space of spin multiplets are then unravelled in these processes. The corresponding symmetry groups are described by SO(n) or SU(n) depending on the origin of rotation group symmetry breaking. The effective spin Hamiltonians of QL and CQD are derived and expressed in terms of the pertinent group generators. We employ fermionization procedure for analyzing the physical content of these dynamical symmetries, including Kondo tunneling through CQD and Haldane gap formation in QL.

cond-mat↗

arXiv2003

Resonance Kondo Tunneling through a Double Quantum Dot at Finite Bias

It is shown that the resonance Kondo tunneling through a double quantum dot (DQD) with even occupation and singlet ground state may arise at a strong bias, which compensates the energy of singlet/triplet excitation. Using the renormalization group technique we derive scaling equations and calculate the differential conductance as a function of an auxiliary dc-bias for parallel DQD described by SO(4) symmetry. We analyze the decoherence effects associated with the triplet/singlet relaxation in DQD and discuss the shape of differential conductance line as a function of dc-bias and temperature.

cond-mat↗

arXiv2003

Infrared Spectroscopy of Quantum Crossbars

Infrared (IR) spectroscopy can be used as an important and effective tool for probing periodic networks of quantum wires or nanotubes (quantum crossbars, QCB) at finite frequencies far from the Luttinger liquid fixed point. Plasmon excitations in QCB may be involved in resonance diffraction of incident electromagnetic waves and in optical absorption in the IR part of the spectrum. Direct absorption of external electric field in QCB strongly depends on the direction of the wave vector ${\bf q}.$ This results in two types of $1D\to 2D$ dimensional crossover with varying angle of an incident wave or its frequency. In the case of QCB interacting with semiconductor substrate, capacitive contact between them does not destroy the Luttinger liquid character of the long wave QCB excitations. However, the dielectric losses on a substrate surface are significantly changed due to appearance of additional Landau damping. The latter is initiated by diffraction processes on QCB superlattice and manifests itself as strong but narrow absorption peaks lying below the damping region of an isolated substrate.Submi

cond-mat.str-el↗

arXiv2003

Two-channel Kondo tunneling in triple quantum dot

The effective spin Hamiltonian of a triple quantum dot with odd electron occupation weakly connected in series with left ($l$) and right ($r$) metal leads is composed of two-channel exchange and co-tunneling terms. Renormalization group equations for the corresponding three exchange constants $J_{l}$, $J_{r}$ and $J_{lr}$ are solved (to third order). Since $J_{lr}$ is relevant, the system is mapped on an anisotropic two-channel Kondo problem. The structure of the conductance as function of temperature and gate voltage implies that in the weak and intermediate coupling regimes, two-channel Kondo physics persists at temperatures as low as several $T_{K}$. At even electron occupation, the number of channels equals twice the spin of the triple dot (hence it is a fully screened impurity).

cond-mat.str-el↗

arXiv2003

Damped orbital excitations in the titanates

A possible mechanism for the removal of the orbital degeneracy in RTiO3 (where R=La, Y, ...) is considered. The calculation is based on the Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti ions, and uses a self-consistent pe rturbation expansion in the interaction between the orbital and the spin degrees of freedom. The latter are assumed to be ordered in a Neel state, brought about by delicate interactions that are not included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each of the t2g bands is found to acquire a finite, temperature-dependent dispersion, that lifts the orbital degeneracy. The orbital excitations are found to be heavily damped over a rather wide band. Consequently, they do not participate as a separate branch of excitations in the low-temperature thermodynamics.e

cond-mat.str-el↗