Peter Brinks

Influence of charge compensation mechanisms on the sheet electron density at conducting LaAlO3/SrTiO3-interfaces

The equilibrium conductance of LaAlO3/SrTiO3 (LAO/STO)-heterointerfaces was investigated at high temperatures (950?K-1100?K) as a function of ambient oxygen partial pressure (pO2). Metallic LAO/STO-interfaces were obtained for LAO grown on STO single crystals as well as on STO-buffered (La,Sr)(Al,Ta)O3 substrates. For both structures, the high temperature sheet carrier density nS of the LAO/STO-interface saturates at a value of about 1?×?1014?cm?2 for reducing conditions, which indicates the presence of interfacial donor states. A significant decrease of nS is observed at high oxygen partial pressures. According to the defect chemistry model of donor-doped STO, this behavior for oxidizing conditions can be attributed to the formation of Sr-vacancies as charge compensating defects.

Anisotropic electrical transport properties of a two-dimensional electron gas at SrTiO3–LaAlO3 interfaces

Experimental evidence of strong in-plane anisotropy in electrical properties of the confined electron gas at the SrTiO3–LaAlO3 interface on top of (LaAlO3)0.3(Sr2AlTaO3)0.7 substrates is provided by detailed transport measurements. Structured measurement geometries in multiple directions are used to show dependence of the sheet resistance with the in-plane angle θ, which is fitted with a sine function with a period of 180°. The carrier density remains constant and a directional dependence of the carrier mobility of more than one order of magnitude is determined with respect to the orientation of the unit cell height steps present at the SrTiO3–LaAlO3 interface.

Uniaxial contribution to the magnetic anisotropy of La0.67Sr0.33MnO3 thin films induced by orthorhombic crystal structure

La0.67Sr33MnO3 (LSMO) thin films under compressive strain have an orthorhombic symmetry with View the MathML source(11¯0)o and View the MathML source(001)o in-plane orientations. (The subscript o denotes the orthorhombic symmetry.) Here, we grew LSMO on cubic (LaAlO3)0.3—(Sr2AlTaO6)0.7 (LSAT) substrates and observed a uniaxial contribution to the magnetic anisotropy which is related to the orthorhombic crystal structure. Since the lattice mismatch is equal in the two directions, the general understanding of anisotropy in LSMO, which relates the uniaxial anisotropy to differences in strain, cannot explain the results. These findings suggest that the oxygen octahedra rotations associated with the orthorhombic structure result in a change in magnetic coupling between the View the MathML source[11¯0]o and [0 0 1]o directions, which determines the anisotropy. We expect these findings to lead to a better understanding of the microscopic origin of the magnetocrystalline anisotropy in LSMO.

Achieving chemical stability in thermoelectric NaxCoO2 thin films

Stability issues in thermoelectric NaxCoO2 thin films have been solved by the addition of an in situ amorphous AlOx capping layer, which prevents previously reported degradation when exposed to air. These chemically stable thin films enable detailed analysis of the intrinsic thermoelectric properties and form a significant progress towards applications. Single phase NaxCoO2 thin films with a low surface roughness are grown by pulsed laser deposition with either an epitaxial or textured crystal structure on Al2O3(001) or LaAlO3(001) single crystal substrates, respectively. For textured thin films a resistivity and thermopower of 0.99 mΩ cm and 69 μV K−1 are observed at room temperature, respectively. Based on an estimated thermal conductivity for thin films, the dimensionless figure of merit is very comparable to NaxCoO2 single crystals, demonstrating the effectiveness of the developed capping layer.

Size effects on thermoelectric behavior of ultrathin NaxCoO2 films

Size effects in thermoelectric Na x CoO2 thin films are studied, focusing on the electrical resisitivity and Seebeck coefficient. For very thin films below 10 nm, we have observed an increase in resistivity, which is in agreement with theoretical models. In contrast to a predicted simultaneous suppression of the Seebeck coefficient for ultrathin films, we observe a constant Seebeck coefficient as a function of layer thickness due to changes in the structural properties as well as the presence of strong electron correlations. This preserved high Seebeck coefficient opens up new directions for Na xCoO2 ultrathin films as basic building blocks in thermoelectric superlattices with enhanced phonon scattering.

High-temperature stability of thermoelectric Ca3Co4O9 thin films

An enhanced thermal stability in thermoelectric Ca3Co4O9 thin films up to 550 °C in an oxygen rich environment was demonstrated by high-temperature electrical and X-ray diffraction measurements. In contrast to generally performed heating in helium gas, it is shown that an oxygen/helium mixture provides sufficient thermal contact, while preventing the previously disregarded formation of oxygen vacancies. Combining thermal cycling with electrical measurements proves to be a powerful tool to study the real intrinsic thermoelectric behaviour of oxide thin films at elevated temperatures