G. Rietveld

Doping dependence of the chemical potential in cuprate high-Tc superconductors I. La2−xSrxCuO4

Abstract A systematic study is performed of the doping dependence of the chemical potential μ in La 2− x Sr x CuO 4 as a function of the Sr content, using a well-characterized series of pellets and thin films. The measured shift of the chemical potential, as deduced from the changes in the photoelectron spectra, is compared with present models for the doping behavior of μ in high- T c materials. The results obtained can be best described assuming μ shifts due to the doping of a rigid narrow band.

The plasmon density of states of a layered electron gas

The plasmon density of states (DOS)p(ω) of a layered electron gas (LEG) is studied theoretically. It is shown thatp(ω) is a linear function of frequency ω as ω goes to 0, and then increases more rapidly as ω tends to the screened, three dimensional plasma frequency ωp. We also study the partial densities of state for constantK andqz, whereK andqz are the magnitude of the transverse and perpendicular momentum transfers. A possible experimental probe for the plasmon DOS is discussed.

In situ growth of high temperature superconductor thin films with evaporation techniques using ozone as an oxygen source

High quality YBa 2 Cu 3 O 7 thin films have been grown in situ on various substrates using MBE techniques and an ozone jet. The best films so far were grown on SrTiO 3 and had a T c,onset of 88 K and a T c0 of 80 K. One 200 nm thick film grown on bare silicon had a T c,onset of 88 K and a T c0 of 60 K. This film showed no or negligible superconductor substrate interactions according to RBS measurements.

In situ growth of high temperature superconductor thin films with evaporation techniques using an ozone jet

High-quality YBa/sub 2/Cu/sub 3/O/sub 7/ thin films were grown in situ on various substrates (SrTiO/sub 3/, Al/sub 2/O/sub 3/, and Si) using MBE (molecular beam epitaxy) techniques and an ozone jet. The yttrium and copper are evaporated from electron-gun sources and the barium is evaporated from a Knudsen cell. All sources are controlled by a single mass spectrometer feedback system to obtain the correct fluxes at high partial ozone pressures. During deposition, the partial ozone pressure at the substrate position is estimated to be 10/sup -3/ -10/sup -2/ mbar. The substrate holder temperature is 700 degrees C. The real substrate temperature is estimated to be lower than 650 degrees C. The films are analyzed with R(T), X-ray diffraction, and RBS (Rutherford backscattering) measurements. Scanning electron microscope photographs are taken of the surface. The best film so far is grown on SrTiO/sub 3/ and has a T/sub c.onset/ of 88 K and a T/sub c0/ of 80 K. One 200-nm-thick film grown on bare silicon has a T/sub c.onset/ of 88 K and a T/sub c0/ of 60 K. This film shows negligible superconductor-substrate interactions according to the RBS measurements.

Doping dependence of the chemical potential in cuprate high-Tc superconductors. II: (Bi, Pb)2Sr2Ca2Cu3O10+δ

Abstract Using X-ray photoelectron spectroscopy, a systematic study is performed of the doping dependence of the chemical potential μ in (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ . The doping is varied by changing the oxygen content of the sample. The measured shift of the chemical potential is compared with present models for the doping behavior of μ in high- T c materials. Our results show best agreement with the model in which the chemical potential is assumed to shift upon doping the parent insulating material.

The specific heat of superconducting TmBa2Cu3O7−x and YBa2Cu307−x below 1 K; Large influence of impurities.

Specific heat measurements between about 25 and 800 mK, and in magnetic fields of 0, 0.45 and 1.55 T are presented of the high-Tc superconductors RBa 2 Cu 3 O 7-x with R = Y and Tm, and x ≅ 0.2. For both compounds, the zero field results can be described with C = a T −2 + b T α , where α = 0.31 and 0.70 for the two samples, respectively. The results in external fields differ considerably from those in zero field. We conclude that the major part of the observed specific heat arises either from small magnetic effects in the compounds itself or from magnetic impurity atoms.

TEMPERATURE DEPENDENCE OF THE CHEMICAL POTENTIAL OF HIGH-Tc SUPERCONDUCTORS

We discuss the behaviour near Tc of the chemical potential for superconductors consisting of a dilute gas of interacting fermions, using a BCS mean field approach. When the gap parameter ,1 becomes of the order of magnitude of the Fermi energy EF, the chemical potential is influenced by the opening of a gap. This effect is shown to be of the order of several tenths of a millielectronvolt for cuprate oxide superconductors. We show that it is possible to determine this anomaly as a function of temperature by determining the work function, and describe our experimental set up and some preliminary results.

The Chemical Potential and Thermodynamic Properties of Narrow-Band Superconductors

We discuss some of the consequences for the thermodynamic properties if a superconductor has a non-neglegible ratio of T c /T F . Starting from the BCS expression we derive an analytic expression for the thermodynamic potential at finite temperature, which has the relevant physical properties. From there we derive the corresponding Helmholtz-Ginzburg-Landau free energy functional near the phase transition, and discuss the behaviour of the order parameter, the chemical potential, the specific heat and the expansivity near T c .

X-ray photo-electron and Auger electron spectroscopy of oxygen in La2CuO4

Abstract We have measured the XPS and Auger spectra of oxygen in La 2 CuO 4 . The experimental XPS and Auger data are compared to fully linearized muffin tin orbital (FLMTO) bandstructure calculations of the valence density of states and the core level shifts. An extended version of Sawatzky-Cini theory for the Auger line shape is used to include multiplet structure of the Auger final states for the point symmetries of the two inequivalent oxygen sites. Experimentally no significant anisotropy is found for the planar and apical oxygen is core levels nor for the two-hole spectral function as determined with AES, in contrast with bandstructure calculations.