A. Ramsak

Spin thermopower in interacting quantum dots

Using analytical arguments and the numerical renormalization group method we investigate the spin-thermopower of a quantum dot in a magnetic field. In the particle-hole symmetric situation the temperature difference applied across the dot drives a pure spin current without accompanying charge current. For temperatures and fields at or above the Kondo temperature, but of the same order of magnitude, the spin-Seebeck coefficient is large, of the order of k_B/e. Via a mapping, we relate the spin-Seebeck coefficient to the charge-Seebeck coefficient of a negative-U quantum dot where the corresponding result was recently reported by Andergassen et al. in Phys. Rev. B 84, 241107 (2011). For several regimes we provide simplified analytical expressions. In the Kondo regime, the dependence of the spin-Seebeck coefficient on the temperature and the magnetic field is explained in terms of the shift of the Kondo resonance due to the field and its broadening with the temperature and the field. We also consider the influence of breaking the particle-hole symmetry and show that a pure spin current can still be realized provided a suitable electric voltage is applied across the dot. Then, except for large asymmetries, the behavior of the spin-Seebeck coefficient remains similar to that found in the particle-hole symmetric point.

Kondo effect in triple quantum dots

Numerical analysis of the simplest odd-numbered system of coupled quantum dots reveals an interplay between magnetic ordering, charge fluctuations, and the tendency of itinerant electrons in the leads to screen magnetic moments. The transition from local-moment to molecular-orbital behavior is visible in the evolution of correlation functions as the interdot coupling is increased. Resulting Kondo phases are presented in a phase diagram which can be sampled by measuring the zero-bias conductance. We discuss the origin of the even-odd effects by comparing with the double quantum dot.

Thermal entanglement in a triple quantum dot system

We present studies of thermal entanglement of a three-spin system in triangular symmetry. Spin correlations are described within an effective Heisenberg Hamiltonian, derived from the Hubbard Hamiltonian, with super-exchange couplings modulated by an effective electric field. Additionally a homogenous magnetic field is applied to completely break the degeneracy of the system. We show that entanglement is generated in the subspace of doublet states with different pairwise spin correlations for the ground and excited states. For the doublets with the same spin orientation one can observe nonmonotonic temperature dependence of entanglement due to competition between entanglement encoded in the ground state and the excited state. The mixing of the states with an opposite spin orientation or with quadruplets (unentangled states) always monotonically destroys entanglement. Pairwise entanglement is quantified using concurrence for which analytical formulae are derived in various thermal mixing scenarios. The electric field plays a specific role – it breaks the symmetry of the system and changes spin correlations. Rotating the electric field can create maximally entangled qubit pairs together with a separate spin (monogamy) that survives in a relatively wide temperature range providing robust pairwise entanglement generation at elevated temperatures.

Kondo effect in double quantum dots with interdot repulsion

We investigate a symmetrical double quantum dot system serially attached to the leads. The emphasis is on the numerical analysis of finite interdot tunneling in the presence of interdot repulsive capacitive coupling. The results reveal the competition between extended Kondo phases and local singlet phases in spin and charge degrees of freedom. The corresponding phase diagram is determined quantitatively.

Spin-dependent thermoelectric transport coefficients in near perfect quantum wires

Thermoelectric transport coefficients are determined for semiconductor quantum wires with weak thickness fluctuations. Such systems exhibit anomalies in conductance near 1/4 and 3/4 of 2e 2 /h on the rising edge to the first conductance plateau, explained hy singlet and triplet resonances of conducting electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak. and J.H. Jefferson, Phys. Rev. B 62, 12 985 (2000)]. We extend this work to study the Seebeck thermopower coefficient and linear thermal conductance within the framework of the Landauer-Buttiker formalism, which also exhibit anomalous structures. These features are generic and robust, surviving to temperatures of a few degrees. It is shown quantitatively how at elevated temperatures thermal conductance progressively deviates from the Wiedemann-Franz law.

Open Problems in Strongly Correlated Electron Systems

Preface. Part I: Cuprates: Fermi Surface and Spectral Functions. Summary of ARPES Results on the Pseudogap in Bi2Sr2CaCu2O8+delta J.C. Campuzano, M. Randeria. On the Breakdown of Landau-Fermi Liquid Theory in the Cuprates C. Honerkamp, et al. Metal-Insulator Transition and Many-body Band Structure of the Hubbard Model R. Eder, et al. Spectral Properties of Underdoped Cuprates A. Ramsak, et al. High Resolution Fermi Surface Mapping of Pb-doped Bi-2212 S.V. Borisenko, et al. Single Particle Excitations in the t-J Model M. Brunner, et al. Part II: Cuprates: Spin and Charge Fluctuations. Magnetic Resonance Peak and Nonmagnetic Impurities Y. Sidis, et al. Pseudogap and Kinetic Pairing Under Critical Differentiation of Electrons in Cuprate Superconductors M. Imada, S. Onoda. Density Response of Cuprates and Renormalization of Breathing Phonons P. Horsch, G. Khaliullin. Phase Diagram of Spin Ladder Models and the Topology of Short Range Valence Bonds J. Solyom. Diagrammatic Theory of Anderson Impurity Models: Fermi and Non-Fermi Liquid Behavior J. Kroha, P. Wolfle. s+d Mixing in Cuprates: Strong electron correlations and superconducting gap symmetry N.M. Plakida, V.S. Oudovenko. Part III: Cuprates: Stripe and Charge Ordering. Fermi Surface, Pseudogaps and Dynamical Stripes in LA2-xSRxCUO4 A. Fujimori, et al. Stripes and Nodal Fermions as Two Sides of the Same Coin J. Zaanen, Z. Nussinov. DMRG Studies of Stripes and Pairing in the t-J Model S.R. White, D.J. Scalapino. Coexistence of Charge and Spin-Peierls Orders in the 1/4-filled Ladder NaV2O5 D. Poilblanc, J. Riera.Part IV: Cuprates: Numerical Methods and Quantum Hall Effect. Normal State Properties of Cuprates: t-J Model vs. Experiment P. Prelovsek. Stability of d-wave Superconductivity in the t-J Model F. Becca, et al. A New Simulation Method for Infinite Size Lattices H.G. Evertz, W. von der Linden. Universality in 2-D Quantum Heisenberg Antiferromagnets M. Troyer. Stripes and Pairing in the u = 5/2 Quantum Hall Effect F.D.M. Haldane. Part V: Manganites, Orbital Degeneracy. Theory of Manganites: The Key Role of Phase Segregation E. Dagotto, et al. Magnetic and Orbital Ordering in Manganites A.M. Oles, L.F. Feiner. Orbital Dynamics: The Origin of Anomalous Magnon Softening in Ferromagnetic Manganites G. Khaliullin, R. Kilian. Field Induced Metal-Insulator Transition in (Pr:Ca:Sr)MnO3 J. Hemberger, et al. Triplet Pairing via Local Exchange in Correlated Systems J. Spalek. Part VI: Low Dimensional Systems and Transport. Dimensional Crossover, Electronic Confinement and Charge Localization in Organic Metals G. Mihaly, et al. Drude Weight, Integrable Systems and the Reactive Hall Constant X. Zotos, et al. Inhomogeneous Luttinger Liquids: Power-Laws and Energy Scales V. Meden, et al. Nodal Liquids and Duality N.E. Mavromatos, S. Sarkar. Spin-Charge Separation in the Sr2 CuO3 and SrCuO2 Chain Materials K. Penc, W. Stephan. Frustrated Quantum Ising Model and Charged Kinks M.V. Mostovoy, et al. Ergodic Properties of Quantum Spin Chains: Kicked Transverse Ising Model T. Prosen. Part VII: Mott-Hubbard Transition,

Adiabatic pipelining: a key to ternary computing with quantum dots.

The quantum-dot cellular automaton (QCA), a processing platform based on interacting quantum dots, was introduced by Lent in the mid-1990s. What followed was an exhilarating period with the development of the line, the functionally complete set of logic functions, as well as more complex processing structures, however all in the realm of binary logic. Regardless of these achievements, it has to be acknowledged that the use of binary logic is in computing systems mainly the end result of the technological limitations, which the designers had to cope with in the early days of their design. The first advancement of QCAs to multi-valued (ternary) processing was performed by Lebar Bajec et al, with the argument that processing platforms of the future should not disregard the clear advantages of multi-valued logic. Some of the elementary ternary QCAs, necessary for the construction of more complex processing entities, however, lead to a remarkable increase in size when compared to their binary counterparts. This somewhat negates the advantages gained by entering the ternary computing domain. As it turned out, even the binary QCA had its initial hiccups, which have been solved by the introduction of adiabatic switching and the application of adiabatic pipeline approaches. We present here a study that introduces adiabatic switching into the ternary QCA and employs the adiabatic pipeline approach to successfully solve the issues of elementary ternary QCAs. What is more, the ternary QCAs presented here are sizewise comparable to binary QCAs. This in our view might serve towards their faster adoption.

Conductance of deformable molecules with interaction

Zero temperature linear response conductance of molecules with Coulomb interaction and with various types of phonon modes is analyzed together with local occupation, local moment, and charge fluctuations. Particular emphasis is on deformation fluctuations, which are quantitatively related to charge fluctuations. Charge fluctuations are shown to exhibit similarity to static charge susceptibility.

C-AXIS CONDUCTIVITY IN THE NORMAL STATE OF CUPRATE SUPERCONDUCTORS

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Entanglement between static and flying qubits in quantum wires

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