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K. Kikoin

Publications and source records attributed to K. Kikoin.

At least 19 recordsLinked to original sources
arXiv2016

Coupled multiple-mode theory for $s_{\pm}$ pairing mechanism in iron based superconductors

We investigate the interplay between the magnetic and the superconducting degrees of freedom in unconventional multi-band superconductors such as iron pnictides. For this purpose a dynamical mode-mode coupling theory is developed based on the coupled Bethe-Salpeter equations. In order to investigate the region of the phase diagram not too far from the tetracritical point where the magnetic spin density wave, (SDW) and superconducting (SC) transition temperatures coincide, we also construct a Ginzburg - Landau functional including both SC and SDW fluctuations in a critical region above the transition temperatures. The fluctuation corrections tend to suppress the magnetic transition, but in the superconducting channel the intraband and interband contribution of the fluctuations nearly compensate each other.

cond-mat.supr-con↗
arXiv2014

A Model for Overscreened Kondo Effect in Ultracold Fermi Gas

The feasibility of realizing overscreened Kondo effect in ultra-cold Fermi gas of atoms with spin $s \ge \tfrac{3}{2}$ in the presence of a localized magnetic impurity atom is proved realistic. Specifying to a system of ultra cold $^{22}$Na Fermi gas and a trapped $^{197}$Au impurity, the mechanism of exchange interaction between the Na and Au atoms is elucidated and the exchange constant is found to be antiferromagnetic. The corresponding exchange Hamiltonian is derived, and the Kondo temperature is estimated at the order of $ 1 μ$K. Within a weak-coupling renormalization group scheme, it is shown that the coupling renormalizes to the non-Fermi liquid fixed point.

cond-mat.quant-gas↗
arXiv2014

Electronic spin working mechanically

A single-electron tunneling (SET) device with a nanoscale central island that can move with respect to the bulk source- and drain electrodes allows for a nanoelectromechanical (NEM) coupling between the electrical current through the device and mechanical vibrations of the island. Although an electromechanical "shuttle" instability and the associated phenomenon of single-electron shuttling were predicted more than 15 years ago, both theoretical and experimental studies of NEM-SET structures are still carried out. New functionalities based on quantum coherence, Coulomb correlations and coherent electron-spin dynamics are of particular current interest. In this article we present a short review of recent activities in this area.

cond-mat.mes-hall↗
arXiv2013

SU(3) Landau - Zener Interferometry

We consider a general theory of Landau-Zener transitions in a three-level system. Based on a classification of three level crossings we express the Landau - Zener Hamiltonians in terms of two bases: i) spin S=1 SU(2) operators and ii) SU(3) Gell - Mann matrices. We show that the generic Hamiltonians being non-linear in terms of the SU(2) group generators become linear in the SU(3) basis. If the diabatic states of the SU(3) Landau - Zener Hamiltonian form a triangle, the interference between two paths results in formation of "beats" and "steps" pattern in the time-dependent transition probability. The characteristic time scales describing the "beats" and "steps" depend on a dwell time through the triangle. These scales are related to the geometric size of the interferometer. We formulate the SU(3) Landau - Zener problem in terms of Bloch dynamics of a unit vector and find a solution of eight-dimensional Bloch equations in the limit of a non-adiabatic transition. Possible experiments in triangular and linearly arranged triple quantum dots where Landau - Zener interferometry can be used for finding manifestations of SU(3) symmetry are discussed.

cond-mat.mes-hall↗
arXiv2013

Two channel orbital Kondo effect in quantum dot with SO(n) symmetry

A scenario for the formation of symmetry protected non-Fermi-liquid (NFL) Kondo effect (KE) with spin variable enumerating Kondo channels is suggested and worked out. In doubly occupied symmetric triple quantum dot within parallel geometry, the NFL low-energy regime arises provided the device possesses both source-drain and left-right parity. Kondo screening follows a multistage renormalization group mechanism: reduction of the energy scale is accompanied by the change of the relevant symmetry group from SO(8) to SO(5). At low energy, three phases compete: 1) under-screening spin triplet (conventional) KE; 2) spin singlet potential scattering; 3) NFL phase where the roles of spin and orbital degrees of freedom are swapped.

cond-mat.str-el↗
arXiv2011

Multistage Kondo effect as a manifestation of dynamical symmetries in the single- and two-electron tunneling

The concept of dynamical symmetries is used for formulation of the renormalization group approach to the Kondo effect in the Anderson model with repulsive and attractive interaction $U$. It is shown that the generic local symmetry of the Anderson Hamiltonian is determined by the SU(4) Lie group. The Anderson Hamiltonian is rewritten in terms of the Gell-Mann matrices of the 4-th rank, which form the set of group generators and the basis for construction of irreducible vector operators describing the excitation spectra in the charge and spin sectors. The multistage Kondo sceening is described in terms of the local SU(4) dynamical symmetry. It is shown that the similarity between the conventional Kondo cotunneling effect for spin 1/2 in the positive $U$ model and the Kondo resonance for pair tunneling in the negative $U$ model is a direct manifestation of implicit SU(4) symmetry of the Anderson/Kondo model.

cond-mat.str-el↗
arXiv2011

Cotunneling mechanism of single-electron shuttling

The problem of electron transport by means of a dumbbell shaped shuttle in strong Coulomb blockade regime is solved. The electrons may be shuttled only in the cotunneling regime during the time spans when both shoulders of the shuttle approach the metallic banks. The conventional Anderson-like tunneling model is generalized for this case and the tunneling conductance is calculated in the adiabatic regime of slow motion of the shuttle. Non-adiabatic corrections are briefly discussed.

cond-mat.mes-hall↗
arXiv2009

Kondo effect in Complex Quantum Dots in the presence of an oscillating and fluctuating gate signal

We show how the charge input signal applied to the gate electrode in a double and triple quantum dot may be converted to a pulse in the Kondo cotunneling current being a spin response of a nano-device under a strong Coulomb blockade. The stochastic component of the input signal results in the infrared cutoff of Kondo transmission. The stochastization of the orbital component of the Kondo effect in triple quantum dots results in a noise-induced SU(4) - SU(2) quantum transition.

cond-mat.mes-hall↗
arXiv2009

Time-dependent single electron tunneling through a shuttling nano-island

We offer a general approach to calculation of single-electron tunneling spectra and conductance of a shuttle oscillating between two half-metallic leads with fully spin polarized carriers. In this case the spin-flip processes are completely suppressed and the problem may be solved by means of canonical transformation, where the adiabatic component of the tunnel transparency is found exactly, whereas the non-adiabatic corrections can be taken into account perturbatively. Time-dependent corrections to the tunnel conductance of moving shuttle become noticeable at finite bias in the vicinity of the even/odd occupation boundary at the Coulomb diamond diagram.

cond-mat.mes-hall↗
arXiv2009

Scalar and vector Keldysh models in the time domain

The exactly solvable Keldysh model of disordered electron system in a random scattering field with extremely long correlation length is converted to the time-dependent model with extremely long relaxation. The dynamical problem is solved for the ensemble of two-level systems (TLS) with fluctuating well depths having the discrete Z_2 symmetry. It is shown also that the symmetric TLS with fluctuating barrier transparency may be described in terms of the planar Keldysh model with dime-dependent random planar rotations in xy plane having continuous SO(2) symmetry. The case of simultaneous fluctuations of the well depth and barrier transparency is subject to non-abelian algebra. Application of this model to description of dynamic fluctuations in quantum dots and optical lattices is discussed.

cond-mat.mes-hall↗
arXiv2008

Ferromagnetic ordering in dilute magnetic dielectrics with and without free carriers

The state of art in the theoretical and experimental studies of transition metal doped oxides (dilute magnetic dielectrics) is reviewed. The available data show that the generic non-equilibrium state of oxide films doped with magnetic impurities may either favor ferromagnetism with high Curie temperature or result in highly inhomogeneous state without long-range magnetic order. In both case concomitant defects (vacancies, interstitial ions play crucial part.

cond-mat.mtrl-sci↗
arXiv2008

Conversion of charge input into Kondo response

We show how the charge input signal applied to the gate electrode in double quantum dot may be converted to a pulse in the Kondo cotunneling current as a spin response of a nano-device in a strong Coulomb blockade regime. The stochastic component of the input signal comes as the infrared cutoff of Kondo transmission.

cond-mat.mes-hall↗
arXiv2008

Electronic structure of noble metal impurities in semiconductors: Cu in GaP

A numerical method for calculation of the electronic structure of transition metal impurities in semiconductors based on the Green function technique is developed. The electronic structure of 3d impurity is calculated within the LDA+U version of density functional method, whereas the host electron Green function is calculated by using the linearized augmented plane wave expansion. The method is applied to the Cu impurity in GaP. The results of calculations are compared with those obtained within the supercell LDA procedure. It is shown that in the Green function approach Cu impurity has an unfilled 3d shell. This result paves a way to explanation of the magnetic order in dilute Ga_{1-x}Cu_xP alloys.

cond-mat.other↗
arXiv2008

Collective excitations in a magnetically doped quantized Hall ferromagnet

A theory of collective states in a magnetically quantized two-dimensional electron gas (2DEG) with half-filled Landau level (quantized Hall ferromagnet) in the presence of magnetic 3d impurities is developed. The spectrum of bound and delocalized spin-excitons as well as the renormalization of Zeeman splitting of the impurity 3d levels due to the indirect exchange interaction with the 2DEG are studied for the specific case of n-type GaAs doped with Mn where the Lande` g-factors of impurity and 2DEG have opposite signs. If the sign of the 2DEG g-factor is changed due to external influences, then impurity related transitions to new ground state phases, presenting various spin-flip and skyrmion-like textures, are possible. Conditions for existence of these phases are discussed. PACS: 73.43.Lp, 73.21.Fg, 72.15.Rn

cond-mat.mes-hall↗
arXiv2007

Phonon-assisted and magnetic field induced Kondo tunneling in single molecular devices

We consider the Kondo tunneling induced by multiphonon emission/absorption processes in magnetic molecular complexes with low-energy singlet-triplet spin gap and show that the number of assisting phonons may be changed by varying the Zeeman splitting of excited triplet state. As a result, the structure of multiphonon Kondo resonances may be scanned by means of magnetic field tuning.

cond-mat.str-el↗
arXiv2007

Centipede ladder at quarter filling

We study the ground state and excitation spectrum of a quasi one-dimensional nanostructure consisting of a pole and rungs oriented in the opposite directions ("centipede ladder", CL) at quarter filling. The spin and charge excitation spectra are found in the limits of small and large longitudinal hopping $t_\|$ compared to the on-rung hopping rate $t_\perp$ and exchange coupling $I_\perp$. At small $t_\|$ the system with ferromagnetic on-rung exchange demonstrates instability against dimerization. Coherent propagation of charge transfer excitons is possible in this limit. At large $t_\|$ CL behaves like two-orbital Hubbard chain, but the gap opens in the charge excitation spectrum thus reducing the symmetry from SU(4) to SU(2). The spin excitations are always gapless and their dispersion changes from quadratic magnon-like for ferromagnetic on-rung exchange to linear spinon-like for antiferromagnetic on-rung exchange in weak longitudinal hopping limit.

cond-mat.str-el↗
arXiv2006

Kondo physics in artificial molecules

Recent advancement in fabrication technologies enable the construction of nano-objects with rather rich internal structures such as double or triple quantum dots, which can then be regarded as artificial molecules. The main new ingredient in the study of the Kondo effect in such artificial (and also in natural) molecules is the internal symmetry of the nano-object, which proves to play a crucial role in the construction of the effective exchange Hamiltonian. This internal symmetry combines continuous spin symmetry SU(2) and discrete point symmetry (such as mirror reflections for double dots or discrete $C_{3v}$ rotation for equilateral triangular dots. When these artificial molecules are attached to metallic leads, the set of dot operators appearing in the effective exchange Hamiltonian generate a group which is refereed to as the dynamical symmetry group of the system dot-leads [mostly SO(n) or SU(n)], and the pertinent group parameters (such as the value of $n$) can be controlled by experiment. In this short review we clarify and expand these concepts and discuss some specific examples. In particular we concentrate on the difference between the chain geometry and the ring geometry. When a perpendicular magnetic field is applied in the ring geometry, its gauge symmetry U(1) is involved in the interplay with the spin and orbital dynamics of the dot.

cond-mat.str-el↗