High-T(c) Superconductivity at the Interface between the CaCuO2 and SrTiO3 Insulating Oxides

Collective Nature of Orbital Excitations in Layered Cuprates in the Absence of Apical Oxygens

We have investigated the 3d orbital excitations in CaCuO_{2} (CCO), Nd_{2}CuO_{4} (NCO), and La_{2}CuO_{4} (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the d_{xy} orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of the orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. Finally, we show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.

Reentrance of interface superconductivity in a high-Tc cuprate heterostructure

Increasing the carrier density in a Mott insulator by chemical doping gives rise to a generic superconducting dome in high temperature superconductors. An intriguing question is whether a second superconducting dome may exist at higher dopings. Here we heavily overdope La2-xSrxCuO4 (0.45 ≤ x ≤ 1.0) and discover an unprecedented reentrance of interface superconductivity in La2-xSrxCuO4 /La2CuO4 heterostructures. As x increases, the superconductivity is weakened and completely fades away at x = 0.8; but it revives at higher doping and fully recovers at x = 1.0. This is shown to be correlated with the suppression of the interfacial charge transfer around x = 0.8 and the weak-to-strong localization crossover in the La2-xSrxCuO4 layer. We further construct a theoretical model to account for the sophisticated relation between charge localization and interfacial charge transfer. Our work advances both the search for and control of new superconducting heterostructures.

Advances in actinide thin films: synthesis, properties, and future directions

Actinide-based compounds exhibit unique physics due to the presence of 5f electrons, and serve in many cases as important technological materials. Targeted thin film synthesis of actinide materials has been successful in generating high-purity specimens in which to study individual physical phenomena. These films have enabled the study of the unique electron configuration, strong mass renormalization, and nuclear decay in actinide metals and compounds. The growth of these films, as well as their thermophysical, magnetic, and topological properties, have been studied in a range of chemistries, albeit far fewer than most classes of thin film systems. This relative scarcity is the result of limited source material availability and safety constraints associated with the handling of radioactive materials. Here, we review recent work on the synthesis and characterization of actinide-based thin films in detail, describing both synthesis methods and modeling techniques for these materials. We review reports on pyrometallurgical, solution-based, and vapor deposition methods. We highlight the current state-of-the-art in order to construct a path forward to higher quality actinide thin films and heterostructure devices.

Infinite-layer/perovskite oxide heterostructure-induced high-spin states in SrCuO2/SrRuO3 bilayer films

Heterostructures composed of dissimilar oxides with different properties offer opportunities to develop emergent devices with desired functionalities. A key feature of oxide heterostructures is interface electronics and orbital reconstructions. Here, we combined infinite-layered SrCuO₂ and perovskite SrRuO₃ into heterostructures. A rare high spin state as large as 3.0 μ_B f.u^-1 and an increase in Curie temperature by 12 K are achieved in an ultrathin SrRuO₃ film capped by a SrCuO₂ layer. Atomic-scale lattice imaging shows the uniform CuO₂-plane-to-RuO₅-pyramid connection at the interface, where the regularly arranged RuO₅ pyramids were elongated along the out-of-plane direction. As revealed by theoretical calculations and spectral analysis, these features finally result in an abnormally high spin state of the interfacial Ru ions with highly polarized e_g orbitals. The present work demonstrates that oxygen coordination engineering at the infinite-layer/perovskite oxide interface is a promising approach towards advanced oxide electronics.

Dimensionality control of magnetic coupling at interfaces of cuprate-manganite superlattices

The dimensionality of the crystal structure plays a vital role in artificial heterostructures composed of different transition metal oxides. Nonlinear layer-thickness dependence of the exchange bias effect was observed in high-quality SrCuO₂/La_0.7Sr_0.3MnO (LSMO) superlattices induced in the present work by dimensional evolution. In the SCO(n)/LSMO(8) superlattices with thickness below the critical value (5 u.c.), the exchange bias effect decreased and the saturated magnetization increased with increase in SCO thickness. By contrast, the exchange bias effect increased and the saturated magnetization decreased in S(n)L(8) superlattices with thickness above the critical value. This is because the lattice SCO material underwent a breathing-like structural transformation from the planar to a chain-like structure. The results indicate the interfacial superexchange coupling mainly present in the chain-like S(n)L(8) superlattices through X-ray absorption spectroscopy and first principles calculations. This superexchange coupling generated a weak localized magnetic moment to pin the adjacent ferromagnetic layer. However, in the thicker S(n)L(8) superlattices, evolution of magnetic properties was induced by the long-range antiferromagnetic order in the planar SCO layer. Our findings demonstrate that the dimensionality driven structural variation is an effective method to manipulate the electronic reconstruction and the associated physical properties, paving a pathway for the advancement of strongly correlated materials.

Two-Dimensional Superconductivity at the LaAlO_{3}/KTaO_{3}(110) Heterointerface

We report on the observation of a T_{c}∼0.9  K superconductivity at the interface between LaAlO_{3} film and the 5d transition metal oxide KTaO_{3}(110) single crystal. The interface shows a large anisotropy of the upper critical field, and its superconducting transition is consistent with a Berezinskii-Kosterlitz-Thouless transition. Both facts suggest that the superconductivity is two-dimensional (2D) in nature. The carrier density measured at 5 K is ∼7×10^{13}  cm^{-2}. The superconducting layer thickness and coherence length are estimated to be ∼8 and ∼30  nm, respectively. Our result provides a new platform for the study of 2D superconductivity at oxide interfaces.

Topotactic Hydrogen in Nickelate Superconductors and Akin Infinite-Layer Oxides ABO_{2}

Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO_{3} [rare earth (A), transition metal (B)] with CaH_{2} may result in both ABO_{2} and ABO_{2}H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H to be energetically favorable for LaNiO_{2} but not for Sr-doped NdNiO_{2}. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d^{9} LaNiO_{2} is similar to (doped) cuprates, 3d^{8} LaNiO_{2}H is a two-orbital Mott insulator. Topotactic H might hence explain why some nickelates are superconducting and others are not.

Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe2As2 single crystals

Superconductivity has been reversibly induced/suppressed in undoped CaFe₂As₂ (Ca122) single crystals through proper thermal treatments, with T_c at ∼25 K at ambient pressure and up to 30 K at 1.7 GPa. We found that Ca122 can be stabilized in two distinct tetragonal (T) phases at room temperature and ambient pressure: PI with a nonmagnetic collapsed tetragonal (cT) phase at low temperature and PII with an antiferromagnetic orthorhombic (O) phase at low temperature, depending on the low-temperature annealing condition. Neither phase at ambient pressure is superconducting down to 2 K. However, systematic annealing for different time periods at 350 °C on the as-synthesized crystals, which were obtained by quenching the crystal ingot from 850 °C, reveals the emergence of superconductivity over a narrow time window. Whereas the onset T_c is insensitive to the anneal time, the superconductive volume fraction evolves with the time in a dome-shaped fashion. Detailed X-ray diffraction profile analyses further reveal mesoscopically stacked layers of the PI and the PII phases. The deduced interface density correlates well with the superconducting volume measured. The transport anomalies of the T-cT transition, which is sensitive to lattice strain, and the T-O transition, which is associated with the spin-density-wave (SDW) transition, are gradually suppressed over the superconductive region, presumably due to the interface interactions between the nonmagnetic metallic cT phase and the antiferromagnetic O phase. The results provide the most direct evidence to date for interface-enhanced superconductivity in undoped Ca122, consistent with the recent theoretical prediction.

Superconductor to Mott insulator transition in YBa2Cu3O7/LaCaMnO3 heterostructures

The superconductor-to-insulator transition (SIT) induced by means such as external magnetic fields, disorder or spatial confinement is a vivid illustration of a quantum phase transition dramatically affecting the superconducting order parameter. In pursuit of a new realization of the SIT by interfacial charge transfer, we developed extremely thin superlattices composed of high Tc superconductor YBa2Cu3O7 (YBCO) and colossal magnetoresistance ferromagnet La0.67Ca0.33MnO3 (LCMO). By using linearly polarized resonant X-ray absorption spectroscopy and magnetic circular dichroism, combined with hard X-ray photoelectron spectroscopy, we derived a complete picture of the interfacial carrier doping in cuprate and manganite atomic layers, leading to the transition from superconducting to an unusual Mott insulating state emerging with the increase of LCMO layer thickness. In addition, contrary to the common perception that only transition metal ions may respond to the charge transfer process, we found that charge is also actively compensated by rare-earth and alkaline-earth metal ions of the interface. Such deterministic control of Tc by pure electronic doping without any hindering effects of chemical substitution is another promising route to disentangle the role of disorder on the pseudo-gap and charge density wave phases of underdoped cuprates.