Origin of the 2D Electron Gas at the SrTiO3 Surface.

Bulk SrTiO3 is a well-known band insulator and the most common substrate used in the field of complex oxide heterostructures. Its surface and interface with other oxides, however, have demonstrated a variety of remarkable behaviors distinct from those expected. In this work, using a suite of in situ techniques to monitor both the atomic and electronic structures of the SrTiO3 (001) surface prior to and during growth, the disappearance and re-appearance of a 2D electron gas (2DEG) is observed after the completion of each SrO and TiO2 monolayer, respectively. The 2DEG is identified with the TiO2 double layer present at the initial SrTiO3 surface, which gives rise to a surface potential and mobile electrons due to vacancies within the TiO2-x adlayer. Much like the electronic reconstruction discovered in other systems, two atomic planes are required, here supplied by the double layer. The combined in situ scattering/spectroscopy findings resolve a number of longstanding issues associated with complex oxide interfaces, facilitating the employment of atomic-scale defect engineering in oxide electronics.

Self-healing Growth of LaNiO3 on a Mixed-Terminated Perovskite Surface.

Developing atomic-scale synthesis control is a prerequisite for understanding and engineering the exotic physics inherent to transition-metal oxide heterostructures. Thus, far, however, the number of materials systems explored has been extremely limited, particularly with regard to the crystalline substrate, which is routinely SrTiO3. Here, we investigate the growth of a rare-earth nickelate─LaNiO3─on (LaAlO3)(Sr2AlTaO6) (LSAT) (001) by oxide molecular beam epitaxy (MBE). Whereas the LSAT substrates are smooth, they do not exhibit the single surface termination usually assumed necessary for control over the interface structure. Performing both nonresonant and resonant anomalous in situ synchrotron surface X-ray scattering during MBE growth, we show that reproducible heterostructures can be achieved regardless of both the mixed surface termination and the layer-by-layer deposition sequence. The rearrangement of the layers occurs dynamically during growth, resulting in the fabrication of high-quality LaNiO3/LSAT heterostructures with a sharp and consistent interfacial structure. This is due to the thermodynamics of the deposition window as well as the nature of the chemical species at interfaces─here, the flexible charge state of nickel at the oxide surface. This has important implications regarding the use of a wider variety of substrates for fundamental studies on complex oxide synthesis.

Observation of an antiferromagnetic quantum critical point in high-purity LaNiO3.

Amongst the rare-earth perovskite nickelates, LaNiO3 (LNO) is an exception. While the former have insulating and antiferromagnetic ground states, LNO remains metallic and non-magnetic down to the lowest temperatures. It is believed that LNO is a strange metal, on the verge of an antiferromagnetic instability. Our work suggests that LNO is a quantum critical metal, close to an antiferromagnetic quantum critical point (QCP). The QCP behavior in LNO is manifested in epitaxial thin films with unprecedented high purities. We find that the temperature and magnetic field dependences of the resistivity of LNO at low temperatures are consistent with scatterings of charge carriers from weak disorder and quantum fluctuations of an antiferromagnetic nature. Furthermore, we find that the introduction of a small concentration of magnetic impurities qualitatively changes the magnetotransport properties of LNO, resembling that found in some heavy-fermion Kondo lattice systems in the vicinity of an antiferromagnetic QCP.

High-Temperature Thermoelectricity in LaNiO3-La2CuO4 Heterostructures.

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal K2NiF4-type insulating La2CuO4 (LCO) and perovskite-type conductive metallic LaNiO3 (LNO) layers with different thicknesses to assess the heterostructure-thermoelectric property-relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contribution of the individual layers where possible interfacial contributions cannot be ruled out. High-resolution scanning transmission electron microscopy investigations showed that a substitutional solid solution of La2(CuNi)O4 was formed when the thickness of the constituent layers was decreased.

Partial spin ordering and complex magnetic structure in BaYFeO4: a neutron diffraction and high temperature susceptibility study.

The novel iron-based compound, BaYFeO4, crystallizes in the Pnma space group with two distinct Fe(3+) sites, that are alternately corner-shared [FeO5](7-) square pyramids and [FeO6](9-) octahedra, forming into [Fe4O18](24-) rings, which propagate as columns along the b-axis. A recent report shows two discernible antiferromagnetic (AFM) transitions at 36 and 48 K in the susceptibility, yet heat capacity measurements reveal no magnetic phase transitions at these temperatures. An upturn in the magnetic susceptibility measurements up to 400 K suggests the presence of short-range magnetic behavior at higher temperatures. In this Article, variable-temperature neutron powder diffraction and high-temperature magnetic susceptibility measurements were performed to clarify the magnetic behavior. Neutron powder diffraction confirmed that the two magnetic transitions observed at 36 and 48 K are due to long-range magnetic order. Below 48 K, the magnetic structure was determined as a spin-density wave (SDW) with a propagation vector, k = (0, 0, (1)/3), and the moments along the b-axis, whereas the structure becomes an incommensurate cycloid [k = (0, 0, ∼0.35)] below 36 K with the moments within the bc-plane. However, for both cases the ordered moments on Fe(3+) are only of the order ∼3.0 µB, smaller than the expected values near 4.5 µB, indicating that significant components of the Fe moments remain paramagnetic to the lowest temperature studied, 6 K. Moreover, new high-temperature magnetic susceptibility measurements revealed a peak maximum at ∼550 K indicative of short-range spin correlations. It is postulated that most of the magnetic entropy is thus removed at high temperatures which could explain the absence of heat capacity anomalies at the long-range ordering temperatures. Published spin dimer calculations, which appear to suggest a k = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure.

Antiferromagnetic spin correlations between corner-shared [FeO5]7- and [FeO6]9- units, in the novel iron-based compound: BaYFeO4.

A novel quaternary compound in the Ba-Y-Fe-O phase diagram was synthesized by solid-state reaction and its crystal structure was characterized using powder X-ray diffraction. The crystal structure of BaYFeO4 consists of a unique arrangement of Fe(3+) magnetic ions, which is based on alternate corner-shared units of [FeO5](7-) square pyramids and [FeO6](9-) octahedra. This results in the formation of stairwise channels of FeO polyhedra along the b crystallographic axis. The structure is described in an orthorhombic crystal system in the space group Pnma with lattice parameters a = 13.14455(1) Å, b = 5.694960(5) Å, and c = 10.247630(9) Å. The temperature-dependent magnetic susceptibility data reveal two antiferromagnetic (AFM) transitions at 33 and 48 K. An upturn in the magnetic susceptibility data above these transitions is observed, which does not reach its maximum even at 390 K. The field-dependent magnetization data at both 2 and 300 K show a nearly linear dependence and do not exhibit significant hysteresis. Heat capacity measurements between 2 and 200 K reveal only a broad anomaly without any indication of long-range ordering. The latter data set is not in good agreement with the magnetic susceptibility data, which makes it difficult to exactly determine the magnetic ground state of BaYFeO4. Accordingly, a temperature-dependent neutron diffraction study is in order, which will enable resolving this issue. The theoretical study of the relative strengths of magnetic exchange interactions along various possible pathways, using extended Hückel spin dimer analysis, shows that only interactions between square pyramidal and octahedral centers are significant, and among them, the intrachannel correlations are stronger than interchannel interactions. This is the first physical property study in such a magnetic ion substructure.