arXiv subjects

Gyula Eres

Publications and source records attributed to Gyula Eres.

23 records · Page 2Linked to original sources

arXiv2019

Competing Phases in Epitaxial Vanadium Dioxide at Nanoscale

Phase competition in correlated oxides offers tantalizing opportunities as many intriguing physical phenomena occur near the phase transitions. Owing to a sharp metal-insulator transition (MIT) near room temperature, correlated vanadium dioxide (VO2) exhibits a strong competition between insulating and metallic phases that is important for practical applications. However, the phase boundary undergoes strong modification when strain is involved, yielding complex phase transitions. Here, we report the emergence of the nanoscale M2 phase domains in VO2 epitaxial films under anisotropic strain relaxation. The phase states of the films are imaged by multi-length-scale probes, detecting the structural and electrical properties in individual local domains. Competing evolution of the M1 and M2 phases indicates a critical role of lattice-strain on both the stability of the M2 Mott phase and the energetics of the MIT in VO2 films. This study demonstrates how strain engineering can be utilized to design phase states, which allow deliberate control of MIT behavior at the nanoscale in epitaxial VO2 films.

cond-mat.str-el↗

arXiv2019

Growth of metallic delafossite PdCoO2 by molecular beam epitaxy

The Pd, and Pt based ABO2 delafossites are a unique class of layered, triangular oxides with 2D electronic structure and a large conductivity that rivals the noble metals. Here, we report successful growth of the metallic delafossite PdCoO2 by molecular beam epitaxy (MBE). The key challenge is controlling the oxidation of Pd in the MBE environment where phase-segregation is driven by the reduction of PdCoO2 to cobalt oxide and metallic palladium. This is overcome by combining low temperature (300 °C) atomic layer-by-layer MBE growth in the presence of reactive atomic oxygen with a post-growth high-temperature anneal. Thickness dependence (5-265 nm) reveals that in the thin regime (<75 nm), the resistivity scales inversely with thickness, likely dominated by surface scattering; for thicker films the resistivity approaches the values reported for the best bulk-crystals at room temperature, but the low temperature resistivity is limited by structural twins. This work shows that the combination of MBE growth and a post-growth anneal provides a route to creating high quality films in this interesting family of layered, triangular oxides.

cond-mat.mtrl-sci↗

arXiv2016

Emerging magnetism and anomalous Hall effect in iridate-manganite heterostructures

Strong Coulomb repulsion and spin-orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here, we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin-orbit entangled 3d-5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials.

cond-mat.str-el↗

arXiv2016

Strain Control of Oxygen Vacancies in Epitaxial Strontium Cobaltite Films

The ability to manipulate oxygen anion defects rather than metal cations in complex oxides can facilitate creating new functionalities critical for emerging energy and device technologies. However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of important defects, oxygen vacancies. Here, strontium cobaltite (SrCoOx) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 C. By applying a small biaxial tensile strain (2%), the oxygen activation energy barrier decreases by ~30%, resulting in a tunable oxygen deficient steady-state under conditions that would normally fully oxidize unstrained cobaltite. These strain-induced changes in oxygen stoichiometry drive the cobaltite from a ferromagnetic metal towards an antiferromagnetic insulator. The ability to decouple the oxygen vacancy concentration from its typical dependence on the operational environment is useful for effectively designing oxides materials with a specific oxygen stoichiometry.

cond-mat.mtrl-sci↗

arXiv2012

Atomic layer engineering of perovskite oxides for chemically sharp heterointerfaces

Advances in synthesis techniques and materials understanding have given rise to oxide heterostructures with intriguing physical phenomena that cannot be found in their constituents. In these structures, precise control of interface quality, including oxygen stoichiometry, is critical for unambiguous tailoring of the interfacial properties, with deposition of the first monolayer being the most important step in shaping a well-defined functional interface. Here, we studied interface formation and strain evolution during the initial growth of LaAlO3 on SrTiO3 by pulsed laser deposition, in search of a means for controlling the atomic-sharpness of the interfaces. Our experimental results show that growth of LaAlO3 at a high oxygen pressure dramatically enhances interface abruptness. As a consequence, the critical thickness for strain relaxation was increased, facilitating coherent epitaxy of perovskite oxides. This provides a clear understanding of the role of oxygen pressure during the interface formation, and enables the synthesis of oxide heterostructures with chemically-sharper interfaces.

cond-mat.mtrl-sci↗