Reversible control of the electronic density of states at the Fermi level of Ca3Co4O9+δ misfit-layered oxide single crystals through O+/H+ plasma exposure

Misfit-layered Ca3Co4O9 crystals were grown and characterized via XRD, SEM, and photo-emission spectroscopy (PES). The evolution of the intensity at the Fermi level (E F) with varying oxygen content was studied by PES. Oxygen species were successfully introduced and removed through O+ and H+ microwave-plasma (2.45GHz, 2-5mbar) treatments, respectively. A 5min O+ plasma exposure was observed to result into a drastic enhancement in the E F intensity, demonstrating the influence of oxygen content to the charge carrier population in layered cobalt-oxide material

Emergence of a Dynamic Super-Structural Order Integrating Antiferroelectric and Antiferrodistortive Competing Instabilities in EuTiO3

Microscopic structural instabilities of EuTiO3 single crystal were investigated by synchrotron x-ray diffraction. Antiferrodistortive (AFD) oxygen octahedral rotational order was observed alongside Ti derived antiferroelectric (AFE) distortions. The competition between the two instabilities is reconciled through a cooperatively modulated structure allowing both to coexist. The electric and magnetic field effect on the modulated AFD order shows that the origin of large magnetoelectric coupling is based upon the dynamic equilibrium between the AFD - antiferromagnetic interactions versus the electric polarization - ferromagnetic interactions

Anomalous transport properties of the halfmetallic ferromagnets Co2TiSi, Co2TiGe, and Co2TiSn

In this work the theoretical and experimental investigations of Co2TiZ (Z = Si, Ge, or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfill the Slater-Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting like behavior. A large negative magnetoresistance of 55% is observed for Co2TiSn at room temperature in an applied magnetic field of 4T which is comparable to the large negative magnetoresistances of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is -52muV/K m for Co2TiSn which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations.Comment: 4 pages 4 figure

One-step preparation of N-doped strontium titanate films by pulsed laser deposition

Perovskite-type oxynitrides exhibit promising electrical and optical properties and can possibly be used in the future as functional materials for electrical, photo-, and electrochemical applications. Continuous heterovalent substitution of oxygen ions by nitrogen ions allows tuning of the desired optical and/or electronic properties to the application specifications. In the present work deposition of SrTiO3:N films by pulsed reactive crossed beam laser ablation was studied in order to examine the influence of different deposition parameters on the film crystallinity and composition. The deposited films exhibit a perovskite-type crystal structure and reveals epitaxial growth on MgO(100) substrates. The unit cell parameters of the deposited SrTiO3:N films range within $3.9\underline{11}<a<3.9\underline{19}$ , which is slightly larger than for polycrystalline SrTiO3 (a=3.905). The studied films reveal an oxygen content in the range of (2.70-2.98)±0.15. The relative N content (vs. O) can be tuned within the range of 1.0-3.0% by adjusting the deposition parameters. The N:O concentration ratio increases with increasing laser fluence and target-to-substrate distances, while the substrate temperature has a more complex influence on the nitrogen concentration. In the range of 580-650°C the [N]/[O] ratio increases while further heating results in a gradual decrease of the N conten

RF-plasma assisted pulsed laser deposition of nitrogen-doped SrTiO3 thin films

Perovskite-type nitrogen substituted SrTiO3 thin films were deposited with a one-step process by RF-plasma assisted pulsed laser deposition from a SrTiO3 target using a N2 plasma, while deposition with a NH3 plasma yields films with almost no incorporated nitrogen. The deposited films exhibit a cubic perovskite-type crystal structure and reveal oriented growth on MgO(100) substrates. The unit cell parameters of the studied N-doped SrTiO3 films range within 3.905<a<3.918Å, which is slightly larger than for SrTiO3 (a=3.905Å). The nitrogen content in the deposited films varies from 0.2 to 0.7atom%. The amount of incorporated nitrogen in the films decreases with increasing RF-power, while the N2 flow rate does not have any pronounced influence on the N content. Nitrogen incorporation results in an increased optical absorption at 400-600nm, which is associated with N(2p) energy states that have a higher energy level than the valence band in strontium titanate. The optical band gap energies in the studied N-doped SrTiO3 films are at 3.2-3.3eV, which is very similar to that of pure strontium titanate (∼3.2eV). Films deposited with NH3 for the RF-plasma exhibit a lower degree of crystallinity and reveal almost no nitrogen incorporation into the crystal lattic

Antiferrodistortive phase transition in EuTiO3

X-ray diffraction, dynamical mechanical analysis and infrared reflectivity studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics. Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in Glazer notation). The phase transition is analogous to SrTiO3. However, some ceramics as well as single crystals of EuTiO3 show different infrared reflectivity spectra bringing evidence of a different crystal structure. In such samples electron diffraction revealed an incommensurate tetragonal structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with modulated structure are activated in the IR spectra due to folding of the Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly influence the temperature of the phase transition to antiferrodistortive phase as well as the tendency to incommensurate modulation in EuTiO3.Comment: PRB, in pres

Reversible control of the electronic density of states at the Fermi level of Ca₃Co₄O_9+δ misfit-layered oxide single crystals through O⁺/H⁺ plasma exposure

Misfit-layered Ca₃Co₄O₉ crystals were grown and characterized via XRD, SEM, and photo-emission spectroscopy (PES). The evolution of the intensity at the Fermi level (E F) with varying oxygen content was studied by PES. Oxygen species were successfully introduced and removed through O⁺ and H⁺ microwave-plasma (2.45 GHz, 2–5 mbar) treatments, respectively. A 5 min O⁺ plasma exposure was observed to result into a drastic enhancement in the E F intensity, demonstrating the influence of oxygen content to the charge carrier population in layered cobalt-oxide materials