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Register a new userRaman spectroscopy study of the interface structure in (CaCuO2)n/(SrTiO3)m superlattices
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Raman spectra of CaCuO2/SrTiO3 superlattices show clear spectroscopic marker of two structures formed in CaCuO2 at the interface with SrTiO3. For non-superconducting superlattices, grown in low oxidizing atmosphere, the 425 cm-1 frequency of oxygen vibration in CuO2 planes is the same as for CCO films with infinite layer structure (planar Cu-O coordination). For superconducting superlattices grown in highly oxidizing atmosphere, a 60 cm-1 frequency shift to lower energy occurs. This is ascribed to a change from planar to pyramidal Cu-O coordination because of oxygen incorporation at the interface. Raman spectroscopy proves to be a powerful tool for interface structure investigation.
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Here we report about the interface reconstruction in the recently discovered superconducting artificial superlattices based on insulating CaCuO2 and SrTiO3 blocks. Hard x-ray photoelectron spectroscopy shows that the valence bands alignment prevents any electronic reconstruction by direct charge transfer between the two blocks. We demonstrate that the electrostatic built-in potential is suppressed by oxygen redistribution in the alkaline earth interface planes. By using highly oxidizing growth conditions, the oxygen coordination in the reconstructed interfaces may be increased, resulting in the hole doping of the cuprate block and thus in the appearance of superconductivity.
At interfaces between complex oxides it is possible to generate electronic systems with unusual electronic properties, which are not present in the isolated oxides. One important example is the appearance of superconductivity at the interface between insulating oxides, although, until now, with very low Tc. We report the occurrence of high Tc superconductivity in the bilayer CaCuO2/SrTiO3, where both the constituent oxides are insulating. In order to obtain a superconducting state, the CaCuO2/SrTiO3 interface must be realized between the Ca plane of CaCuO2 and the TiO2 plane of SrTiO3. Only in this case extra oxygen ions can be incorporated in the interface Ca plane, acting as apical oxygen for Cu and providing holes to the CuO2 planes. A detailed hole doping spatial profile has been obtained by STEM/EELS at the O K-edge, clearly showing that the (super)conductivity is confined to about 1-2 CaCuO2 unit cells close to the interface with SrTiO3. The results obtained for the CaCuO2/SrTiO3 interface can be extended to multilayered high Tc cuprates, contributing to explain the dependence of Tc on the number of CuO2 planes in these systems.
In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensation (BEC) limits. The concept of the BCS-BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here, by analyzing the typical superconductors within the BCS-BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS-BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of a single-layer FeSe on SrTiO3 substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible pre-paired electrons, demonstrating its flexible tunability within the BCS-BEC crossover regime.
Heterostructures made of transition metal oxides are new tailor-made materials which are attracting much attention. We have constructed a 6-band k.p Hamiltonian and used it within the envelope function method to calculate the subband structure of a variety of LaAlO3/SrTiO3 heterostructures. By use of density functional calculations, we determine the k.p parameters describing the conduction band edge of SrTiO3: the three effective mass parameters, L=0.6104 eV AA^2, M=9.73 eV AA^2, N=-1.616 eV AA^2, the spin orbit splitting Delta_SO=28.5 meV and the low temperature tetragonal distortion energy splitting Delta_T=2.1 meV. For confined systems we find strongly anisotropic non-parabolic subbands. As an application we calculate bands, density of states and magnetic energy levels and compare the results to Shubnikov-de Haas quantum oscillations observed in high magnetic fields. For typical heterostructures we find that electric field strength at the interface of F = 0.1 meV/AA for a carrier density of 7.2 10^{12} cm^-2 results in a subband structure that is similar to experimental results.
We study the magnetic field driven Quantum Phase Transition (QPT) in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through finite size scaling analysis, we show that it belongs to the (2+1)D XY model universality class. The system can be described as a disordered array of superconducting islands coupled by a two dimensional electron gas (2DEG). Depending on the 2DEG conductance tuned by the gate voltage, the QPT is single (corresponding to the long range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). By retrieving the coherence length critical exponent u, we show that the QPT can be clean or dirty according to the Harris criteria, depending on whether the phase coherence length is smaller or larger than the island size. The overall behaviour is well described by a theoretical approach of Spivak et al., in the framework of the fermionic scenario of 2D superconducting QPT.
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