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Register a new userPolaronic approach to strongly correlated electron system with strong electron-phonon interaction
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The three band p-d model of strongly correlated electrons interacting with optical phonon via diagonal and off-diagonal electron-phonon interaction is considered within cluster perturbation theory. At first step the exact diagonalization of the Hamiltonian of CuO4 cluster results in the construction of local polaronic eigenstates |p> with hole numbers nh=0,1,2 per unit cell. The inter cluster hoppings and interactions are exactly written in terms of Hubbard operators X(pq)= |p><q| determined within the multielectron polaronic eigenstates |p>. The Fermi type single electron quasiparticle dispersion and spectral weight are calculated for the undoped antiferromagnetic parent insulator like La2CuO4. The quasiparticle dispersion of Hubbard polarons is determined by a hybridization of the several Hubbard subbands with local Franck-Condon resonances. For small electron-phonon interaction the conductivity band is stronger renormalized then the valence band. Nevertheless for large electron-phonon interaction both bands are strongly renormalized with quasiparticle localization. Effect of partial compensation of diagonal and off-diagonal electron-phonon interaction at intermediate coupling is found.
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We present a theoretical approach to determine the electronic properties of nanoscale systems exhibiting strong electron-electron and electron-phonon interactions and coupled to metallic electrodes. This approach is based on an interpolative ansatz for the electronic self-energy which becomes exact both in the limit of weak and strong coupling to the electrodes. The method provides a generalization of previous interpolative schemes which have been applied to the purely electronic case extensively. As a test case we consider the single level Anderson-Holstein model. The results obtained with the interpolative ansatz are in good agreement with existing data from Numerical Renormalization Group calculations. We also check our results by considering the case of the electrodes represented by a few discrete levels which can be diagonalized exactly. The approximation describes properly the transition from the Kondo regime where electron-electron interactions dominate to the polaronic case characterized by a strong electron-phonon interaction.
High precision measurements of the Hall effect have been carried out for archetypal heavy fermion compound - CeAl3 in a wide range of temperatures 1.8-300K. For the first time a complex activated behavior of the Hall coefficient in CeAl3 with activation energies Ea1/kB=220K and Ea2/kB=3.3K has been observed in the temperature intervals 50-300K and 10-35K respectively. At temperatures below the maximum of the Hall effect T<Tmax=10K an asymptotic dependence RH(T)=exp(-Ea3/kBT) was found in CeAl3 with the value Ea3/kB=0.38K estimated from the experimental data. The temperature evolution of microscopic parameters (effective mass and localization radius) evaluated for the many-body states (heavy fermions) is discussed in terms of an electron-polaron states formation in vicinity of Ce-sites in the CeAl3 matrix.
Combining strong electron correlations [1-4] and nontrivial electronic topology [5] holds great promise for discovery. So far, this regime has been rarely accessed and systematic studies are much needed to advance the field. Here we demonstrate the control of topology in a heavy fermion system. We use magnetic field to manipulate Weyl nodes in a Weyl-Kondo semimetal [6-8], up to the point where they annihilate in a topological quantum phase transition. The suppression of the topological characteristics occurs in an intact and only weakly varying strongly correlated background. Thus, topology is changing per se and not as a consequence of a change of the correlation state, for instance across a magnetic, electronic or structural phase transition. Our demonstration of genuine topology tuning in a strongly correlated electron system sets the stage for establishing global phase diagrams of topology, an approach that has proven highly valuable to explore and understand topologically trivial strongly correlated electron systems [1-4]. Our work also lays the ground for technological exploitations of controlled electronic topology.
The local structure of NaTiSi$_{2}$O$_{6}$ is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that on warming the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490 K. The ODL state is correlated over the length scale spanning $sim$6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems.
At low temperatures, EuTiO$_3$ system has very large resistivities and exhibits colossal magnetoresistance. Based on a first principle calculation and the dynamical mean-field theory for small polaron we have calculated the transport properties of EuTiO$_3$. It is found that due to electron-phonon interaction the conduction band may form a tiny subband which is close to the Fermi level. The tiny subband is responsible for the large resistivity. Besides, EuTiO$_3$ is a weak antiferromagnetic material and its magnetization would slightly shift the subband via exchange interaction between conduction electrons and magnetic atoms. Since the subband is close to the Fermi level, a slight shift of its position gives colossal magnetoresistance.
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