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Register a new userSubgap tunneling through channels of polarons and bipolarons in chain conductors
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We suggest a theory of internal coherent tunneling in the pseudogap region where the applied voltage is below the free electron gap. We consider quasi 1D systems where the gap is originated by a lattice dimerization like in polyacethylene, as well as low symmetry 1D semiconductors. Results may be applied to several types of conjugated polymers, to semiconducting nanotubes and to quantum wires of semiconductors. The approach may be generalized to tunneling in strongly correlated systems showing the pseudogap effect, like the family of High Tc materials in the undoped limit. We demonstrate the evolution of tunneling current-voltage characteristics from smearing the free electron gap down to threshold for tunneling of polarons and further down to the region of bi-electronic tunneling via bipolarons or kink pairs.
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We suggest a theory of internal coherent tunneling in the pseudogap region, when the applied voltage U is below the free electron gap 2Delta_0. We address quasi 1D systems, where the gap is originated by spontaneous lattice distortions of the Incommensurate Charge Density Wave (ICDW) type. Results can be adjusted also to quasi-1D superconductors. The instanton approach allows to calculate the interchain tunneling current both in single electron (amplitude solitons, i.e. spinons) and bi-electron (phase slips) channels. Transition rates are governed by a dissipative dynamics originated by emission of gapless phase excitations in the course of the instanton process. We find that the single-electron tunneling is allowed below the nominal gap 2Delta_0 down to the true pair-breaking threshold at 2W_as<2Delta, where W_as=2Delta/pi is the amplitude soliton energy. Most importantly, the bi-electronic tunneling stretches down to U=0 (in the 1D regime). In both cases, the threshold behavior is given by power laws J (U-U_c)^beta, where the exponent beta v_F/u is large as the ratio of the Fermi velocity v_F and the phase one u. In the 2D or 3D ordered phases, at temperature T<T_c, the one-electron tunneling current does not vanish at the threshold U_c anymore, but saturates above it at U-U_c T_c<<Delta. Also the bi-particle channel acquires a finite threshold U_c=W_ph T_c<<Delta at the energy W_ph of the 2pi phase soliton.
The variational Feynman formalism for the polaron, extended to an all-coupling treatment of bipolarons, is applied for two impurity atoms in a Bose-Einstein condensate. This shows that if the polaronic coupling strength is large enough the impurities will form a bound state (the bipolaron). As a function of the mutual repulsion between the impurities two types of bipolaron are distinguished: a tightly bound bipolaron at weak repulsion and a dumbbell bipolaron at strong repulsion. Apart from the binding energy, also the evolution of the bipolaron radius and its effective mass are examined as a function of the strength of the repulsive interaction between the impurities and of the polaronic cupling strength. We then apply the strong-coupling formalism to multiple impuritiy atoms in a condensate which leads to the prediction of multi-polaron formation in the strong coupling regime. The results of the two formalisms are compared for two impurities in a condensate which results in a general qualitative agreement and a quantitative agreement at strong coupling. Typically the system of impurity atoms in a Bose-Einstein condensate is expected to exhibit the polaronic weak coupling regime. However, the polaronic coupling strength is in principle tunable with a Feshbach resonance.
We investigate sequential tunneling through a multilevel quantum dot confining multiple electrons, in the regime where several channels are available for transport within the bias window. By analyzing solutions to the master equations of the reduced density matrix, we give general conditions on when the presence of a second transport channel in the bias window quenches transport through the quantum dot. These conditions are in terms of distinct tunneling anisotropies which may aid in explaining the occurrence of negative differential conductance in quantum dots in the nonlinear regime.
Employing the recently developed self-consistent variational basis generation scheme, we have investigated the bipolaron-bipolaron interaction within the purview of Holstein-Hubbard and the extended-Holstein-Hubbard (F2H) model on a discrete one-dimensional lattice. The density-matrix renormalization group (DMRG) method has also been used for the Holstein-Hubbard model. We have shown that there exists no bipolaron-bipolaron attraction in the Holstein-Hubbard model. In contrast, we have obtained clear-cut bipolaron-bipolaron attraction in the F2H model. Composite bipolarons are formed above a critical electron-phonon coupling strength, which can survive the finite Hubbard $U$ effect. We have constructed the phase diagram of F2H polarons and bipolarons, and discussed the phase separation in terms of the formation of composite bipolarons.
The magnetic behavior of the Ca3Co2O6 spin chain compound is characterized by a large Ising-like character of its ferromagnetic chains, set on triangular lattice, that are antiferromagnetically coupled. At low temperature, T < 7K, the 3D antiferromagnetic state evolves towards a spin frozen state. In this temperature range, magnetic field driven magnetization of single crystals (H//chains) exhibits stepped variations. The occurrence of these steps at regular intervals of the applied magnetic field, Hstep=1.2T, is reminiscent of the quantum tunneling of the magnetization (QTM) of molecular based magnets. Magnetization relaxation experiments also strongly support the occurrence of this quantum phenomenon. This first observation of QTM in a magnetic oxide belonging to the large family of the A3BBO6 compounds opens new opportunities to study a quantum effect in a very different class of materials from molecular magnets.
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