Eduard Tutiš

The effect of large Ud on the Raman spectrum in the copper-oxide superconductors

Abstract The effect of charge fluctuations on the electronic and Raman spectra of high-temperature superconductors is examined, using the slave boson approach to the large Coulomb repulsion U d on the copper site. Instead of the saddle point approximation ∇ b 〉 ≠ 0 for the slave boson, characteristic for various N →∞ approaches, we confine ourselves to the non-crossing approximation (NCA) diagrams for the Green functions. In this way the effects of charge fluctuations and of the constraint of no double occupancy on the copper site on the shape of the electronic spectrum are studied primarily, while the slave boson diagrams responsible for copper-copper spin correlations are intentionally not included. The novel feature of the charge fluctuation dynamics shows up in the slave boson spectrum as the plateau extending over the range of 1 eV below ω =0. This is further reflected in the electronic Raman spectrum that we calculate for various combinations of the polarization of the incoming and scattered light. The Raman spectrum shows the characteristic featureless behaviour up to frequencies of the order of 1 eV while the polarization dependence is also in qualitative agreement with experiment.

The mechanism of lifetime extension due to CuPc injection layer in organic light-emitting diodes

The remarkable effect on lifetime improvement of copper phthalocianine (CuPc) coated indium tin oxide (ITO) anode of organic light emitting diodes (OLEDs) is experimentally well approved. Also known are the electrode morphology, with and without CuPc coating, the energy levels of the used materials, important for charge injection and conduction, the carrier mobility etc. Based on this knowledge we suggest the model that explains the mechanism behind the lifetime improvement. We argue that the charge accumulation at the interface between the CuPc and the hole transport layer is responsible for screening out of the electric field variations leading to current density homogenization across the OLED surface. The variation of the injection field, introduced by electrode roughness, is estimated for typical indium tin oxide morphology used in OLED production. Without the CuPc hole injection layer a substantial current channeling occurs in OLEDs, leading to accelerated device degradation.

Semimetallic and charge-ordered α-(BEDT-TTF)2I3: On the role of disorder in dc transport and dielectric properties

α-(BEDT-TTF)2I3 is a prominent example of charge ordering among organic conductors. In this work, we explore the details of transport within the charge-ordered as well as semimetallic phase at ambient pressure. In the high-temperature semimetallic phase, the mobilities and concentrations of both electrons and holes conspire in such a way to create an almost temperature-independent conductivity as well as a low Hall effect. We explain these phenomena as a consequence of a predominantly interpocket scattering which equalizes mobilities of the two types of charge carriers. At low temperatures, within the insulating charge-ordered phase two channels of conduction can be discerned: a temperature-dependent activation, which follows the mean-field behavior, and a nearest-neighbor-hopping contribution. Together with negative magnetoresistance, the latter relies on the presence of disorder. The charge-ordered phase also features a prominent dielectric peak which bears a similarity to relaxor ferroelectrics. Its dispersion is determined by free-electron screening and pushed by disorder well below the transition temperature. The source of this disorder can be found in the anion layers which randomly perturb BEDT-TTF molecules through hydrogen bonds.

High-pressure study of transport properties in Co0.33NbS2

This is the first study of the effect of pressure on transition metal dichalcogenides (TMDs) intercalated by atoms that order magnetically. Co0.33NbS2 is a layered system where the intercalated Co atoms order antiferromagnetically at T-N = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity (rho), thermoelectric power (S), and thermal conductivity (kappa). At ambient pressure the magnetic transition corresponds to a well-pronounced peak in dS/dT, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to kappa. Under pressure the resistivity increases in the high-temperature range, contrary to all previous measurements in other layered TMDs. In the low-temperature range the strong dependences of thermopower and resistivity on pressure are observed below T-N, which, in turn, also depends on pressure at rate of dT(N)/dp similar to -1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.

Charge dynamics in cuprate superconductors

In this lecture we present some interesting issues that arise when the dynamics of the charge carriers in the CuO2 planes of high temperature superconductors is considered. Based on the qualitative picture of doping, set by experiments and some previous calculations, we consider the strength of the various inter- and intra-cell charge transfer susceptibilities, the question of Coulomb screening and charge collective-modes. The starting point is the usual p-d model extended by the long-range Coulomb (LRC) interaction. Within this model it is possible to examine the case in which the LRC forces frustrate the electronic phase separation, the instability which is present in the model without a LRC interaction. While the static dielectric function in such systems is negative down to arbitrarily small wavevectors, the system is not unstable. We consider the dominant electronic charge susceptibilities and possible consequences for the lattice properties.

Charge fluctuations in the copper-oxide planes and the electron-phonon couplings

Abstract Using the slave boson approach to treat the Ud → ∞ limit for the Coulomb repulsion on the copper site in the p-d model we give a short account on various charge fluctuation modes. The effect of the slave boson field adjustment on the strength and the Fermi surface effects for k = 0 and k = (π, π) charge transfer modes is discussed. The implications for the screening of the corresponding electron-phonon couplings is also pointed out.

Raman spectrum and charge fluctuations in the copper-oxide superconductors

Abstract The effect of the charge fluctuations on the electronic spectrum and the Raman spectrum of high temperature superconductors is examined within the slave boson approach. Instead of using the saddle point approximation for slave bosons, we confine ourselves to the non-crossing approximation (NCA) in summing the diagrams for the Green functions, thus obtaining the renormalized hole spectrum and its lifetime on equal footing. The electronic Raman spectrum is calculated, showing the characteristic featureless behaviour up to the frequency of the order of renormalized Δ pd parameter. The dependence on the polarization of the incident and the scattered light agrees with experiments.