Energy-dependent Electronic Density Of States Research Articles

Large Seebeck effect by charge-mobility engineering.

The Seebeck effect describes the generation of an electric potential in a conducting solid exposed to a temperature gradient. In most cases, it is dominated by an energy-dependent electronic density of states at the Fermi level, in line with the prevalent efforts towards superior thermoelectrics through the engineering of electronic structure. Here we demonstrate an alternative source for the Seebeck effect based on charge-carrier relaxation: a charge mobility that changes rapidly with temperature can result in a sizeable addition to the Seebeck coefficient. This new Seebeck source is demonstrated explicitly for Ni-doped CoSb3, where a marked mobility change occurs due to the crossover between two different charge-relaxation regimes. Our findings unveil the origin of pronounced features in the Seebeck coefficient of many other elusive materials characterized by a significant mobility mismatch. When utilized appropriately, this effect can also provide a novel route to the design of improved thermoelectric materials.

High-energy fluctuation spectra in cuprates from infrared optical spectroscopy

Coupling of the charge carriers in the high-temperature superconducting oxides to bosonic modes has been widely reported using a variety of experimental probes. These include angular resolved photoemission (ARPES), scanning tunneling spectroscopy, Raman scattering, and infrared optical spectroscopy. The energy scale investigated has been mostly limited to a relatively small range up to 300 meV or so. Although some ARPES experiments report boson structure up to 800 meV in the dressed electron dispersion curves, the data are not analyzed to recover the spectral density of the fluctuation spectrum. We have extended to higher energies up to 2.2 eV the usual maximum entropy technique used to invert optical data so as to obtain an electron-boson spectral density. This has required that we include in our inversions, the calculated (local-density approximation) particle-hole symmetrized energy-dependent electronic density of states. Our analysis reveals that significant spectral weight remains in the fluctuation spectra up to 2.2 eV in the Bi-2212 family and 1.2 eV in Bi-2201 for all doping levels considered.

Vibrational heating in single-molecule switches: an energy-dependent density-of-states approach

In recent experiments, it has been shown that the switching rate of single-molecule switches can show a rather complicated dependence on the applied bias voltage. Here, we discuss a minimal model which describes the switching process in terms of inelastic scattering processes of the tunneling electron by specific molecular vibrations. One important point is the introduction of an energy-dependent electronic density of states around the Fermi energy. The influence of different model parameters on the switching rate is studied and we show that the inclusion of a variable density of states allows us to understand the non-monotonic behavior of the switching rate observed in some experiments.

Modification of the isotope effect due to energy-dependent electronic density of states.

We study how energy dependence in the electronic density of states N(\ensuremath{\varepsilon}) affects the isotope-effect coefficient \ensuremath{\beta} within a realistic Eliashberg formalism. For structure in N(\ensuremath{\varepsilon}), sharp on the scale of the Debye energy, the value of \ensuremath{\beta} can be significantly affected and can be smaller or larger than the canonical value 0.5. In some cases, when coupled with large values of the Coulomb pseudopotential ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$, \ensuremath{\beta} can be quite small with otherwise fairly unremarkable values of the other parameters.

Thermodynamic properties of strong coupling superconductors with energy-dependent electronic density of states

The authors present the results of extensive new calculations of the thermodynamics of strong coupling superconductors with structures in the electronic density of states (EDOS) around the Fermi surface. For the first time they treat Lorentzian peaks with Fermi energy off-centre as well as centred and also consider valleys. It is found that in some cases energy dependence in the EDOS can lead to large differences in thermodynamic properties for the same electron-phonon spectral shape and critical temperature.

Tunneling in the case of an energy dependent density of states

We have solved a generalized set of three coupled Eliashberg gap equations which incorporate the effect of an energy-dependent electronic density of states N(ϵ). The model used for N(ϵ) was chosen on the basis of recent band structure calculations and the electron-phonon spectral density taken from tunneling results in Nb 3Sn. The solutions are used to study of the effect of N(ϵ) on single particle tunneling into a superconductor. The theoretical characteristics so obtained are “inverted” in the conventional way and the procedure is critically assessed.