M. N. Kiselev

Thermopower of a Kondo Spin-Correlated Quantum Dot

The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations, the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model.

Phase diagram of the commensurate two-dimensional disordered Bose-Hubbard model.

We establish the full ground state phase diagram of the disordered Bose-Hubbard model in two dimensions at a unity filling factor via quantum Monte Carlo simulations. Similarly to the three-dimensional case we observe extended superfluid regions persisting up to extremely large values of disorder and interaction strength which, however, have small superfluid fractions and thus low transition temperatures. In the vicinity of the superfluid-insulator transition of the pure system, we observe an unexpectedly weak--almost not resolvable--sensitivity of the critical interaction to the strength of (weak) disorder.

New scenario for high-T_c cuprates: electronic topological transition as a motor for anomalies in the underdoped regime

We have discovered a new nontrivial aspect of electronic topological transition (ETT) in a 2D free fermion system on a square lattice. The corresponding exotic quantum critical point, \delta=\delta_c, T=0, (n=1-\delta is an electron concentration) is at the origin of anomalous behaviour in the interacting system on one side of ETT, \delta<\delta_c. The most important is an appearance of the line of characteristic temperatures, T^*(\delta) \propto \delta_c-\delta. Application of the theory to high-T_c cuprates reveals a striking similarity to the observed experimentally behaviour in the underdoped regime (NMR and ARPES).

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.

Effects of colored noise on Landau-Zener transitions: Two- and three-level systems

We investigate the Landau-Zener transition in two- and three-level systems subject to a classical Gaussian noise. Two complementary limits of the noise being fast and slow compared to characteristic Landau-Zener tunnel times are discussed. The analytical solution of a density matrix (Bloch) equation is given for a long-time asymptotic of transition probability. It is demonstrated that the transition probability induced or assisted by the fast noise can be obtained through a procedure of Blochs equation averaging with further reducing it to a master equation. In contrast to the case of fast noise, the transition probability for LZ transition induced, or assisted, by the slow classical noise can be obtained by averaging the solution of Blochs equation over the noise realization. As a result, the transition probability is described by the activation Arrhenius law. The approximate solution of the Blochs equation at finite times is written in terms of Fresnels integrals and interpreted in terms of interference pattern. We discuss consequences of a local isomorphism between SU(2) and SO(3) groups and connections between Schrodinger and Bloch descriptions of spin dynamics. Based on this isomorphism, we establish the relations between S=1/2 and 1 transition probabilities influenced by the noise. A possibility to use the slow noise as a probe for tunnel time is discussed.

Dynamical Symmetries for Nanostructures

1 Introduction.- 2 Hidden and Dynamical Symmetries of Atoms and Molecules.- 3 Nanostructures as Artificial Atoms and Molecules.- 4 Dynamical Symmetries in the Kondo Effect.- 5 Dynamical Symmetries in Molecular Electronics.- 6 Dynamical Symmetries and Spectroscopy of Quantum Dots.- 7 Dynamical Symmetries and Non-Equilibrium Electron Transport.- 8 Tunneling Through Moving Nanoobjects.- 9 Mathematical Instrumentation.- 10 Conclusions and Prospects.- Index.- References.

Thermoelectric transport through a quantum dot: Effects of asymmetry in Kondo channels

We consider effects of magnetic field on the thermopower and thermoconductance of a single-electron transistor based on a quantum dot strongly coupled to one of the leads by a single-mode quantum point contact. We show appearance of two new energy scales: T_{min} ~ |r|^2 E_C(B/B_C)^2 depending on a ratio of magnetic field B and the field B_C corresponding to a full polarization of point contact and T_{max} ~ |r|^2 E_C depending on a reflection amplitude r and charging energy E_C. We predict that the behavior of thermoelectric coefficients is consistent with the Fermi-liquid theory at temperatures T << T_{min}, while crossover from Non-Fermi-liquid regime associated with a two-channel Kondo effect to Fermi-liquid single-channel Kondo behavior can be seen at T_{min}

Spin gap in chains with hidden symmetries

We investigate the formation of a spin gap in one-dimensional models characterized by groups with hidden dynamical symmetries. A family of two-parametric models of isotropic and anisotropic spin-rotator chains sSRC’s d characterized by SUs2d 3 SUs2d and SOs2d 3 SOs2d 3 Z2 3 Z2 symmetries is introduced to describe the transition from SUs2d to SOs4d antiferromagnetic Heisenberg chains. The excitation spectrum is studied with the use of the Jordan-Wigner transformation generalized for o4 algebra and by means of the 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 the conventional Haldane problem.

Semi-fermionic representation of SU( N) Hamiltonians

Abstract:We represent the generators of the SU(N) algebra as bilinear combinations of Fermi operators with imaginary chemical potential. The distribution function, consisting of a minimal set of discrete imaginary chemical potentials, is introduced to satisfy the local constraints. This representation leads to the conventional temperature diagram technique with standard Feynman codex, except that the Matsubara frequencies are determined by neither integer nor half-integer numbers. The real-time Schwinger-Keldysh formalism is formulated in the framework of complex equilibrium distribution functions for auxiliary semi-fermionic fields. We discuss the continuous large N and SU(2) large spin limits. We illustrate the application of this technique for magnetic and spin-liquid states of the Heisenberg model.We represent the generators of the SU(N) algebra as bilinear combinations of Fermi operators with imaginary chemical potential. The distribution function, consisting of a minimal set of discrete imaginary chemical potentials, is found for arbitrary N. This representation leads to the conventional temperature diagram technique with standard Feynman codex, except that the Matsubara frequencies are determined by neither integer nor half-integer numbers. The real-time Schwinger-Keldysh formalism is formulated in the framework of complex distribution functions. We discuss the continuous large N and SU(2) large spin limits. We illustrate the application of this technique for magnetic and spin-liquid states of the Heisenberg model.

Kondo Force in Shuttling Devices: Dynamical Probe for a Kondo Cloud

We consider the electromechanical properties of a single-electronic device consisting of a movable quantum dot attached to a vibrating cantilever, forming a tunnel contact with a nonmovable source electrode. We show that the resonance Kondo tunneling of electrons amplifies exponentially the strength of nanoelectromechanical (NEM) coupling in such a device and make the latter insensitive to mesoscopic fluctuations of electronic levels in a nanodot. It is also shown that the study of a Kondo-NEM phenomenon provides additional (as compared with standard conductance measurements in a nonmechanical device) information on retardation effects in the formation of a many-particle cloud accompanying the Kondo tunneling. A possibility for superhigh tunability of mechanical dissipation as well as supersensitive detection of mechanical displacement is demonstrated.