Metal-insulator transition and orbital reconstruction in Mott-type quantum wells made of NdNiO3

Enhanced Thermoelectric Performance of Rare-Earth-Free n-Type Oxide Perovskite Composite with Graphene Analogous 2D MXene

Here, the first experimental demonstration on the effect of incorporating new generation 2D material, MXene, on the thermoelectric performance of rare-earth-free oxide perovskite is reported. The charge localization phenomenon is predominant in the electron transport of doped SrTiO₃ perovskites, which deters from achieving a higher thermoelectric power factor in these oxides. In this work, it is shown that incorporating Ti₃ C₂ T_x MXene in a matrix of SrTi_0.85 Nb_0.15 O₃ (STN) facilitates the delocalization of electrons resulting in better than single-crystal-like electron mobility in polycrystalline composites. A 1851% increase in electrical conductivity and a 1000% enhancement in power factor are attained. Besides, anharmonicity caused by MXene in the STN matrix has led to enhanced Umklapp scattering giving rise to lower lattice thermal conductivity. Hence, 700% ZT enhancement is achieved in this composite. Further, a prototype of thermoelectric generator (TEG) using only n-type STN + MXene is fabricated and a power output of 38 mW is obtained, which is higher than the reported values for oxide TEG.

Spatially Controlled Octahedral Rotations and Metal-Insulator Transitions in Nickelate Superlattices

The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal-insulator transitions (MITs) in tetragonal NdNiO₃/SrTiO₃ superlattices by controlling the NdNiO₃ layer thickness, n in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at n = 8 to a uniform nontilt pattern at n = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing n the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to n = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.

Modulation of metal-insulator transitions of NdNiO3/LaNiO3/NdNiO3 trilayers via thickness control of the LaNiO3 layer

Over the last few decades, manipulating the metal-insulator (MI) transition in perovskite oxides (ABO₃) via an external control parameter has been attempted for practical purposes, but with limited success. The substitution of A-site cations is the most widely used technique to tune the MI transition. However, this method introduces unintended disorder, blurring the intrinsic properties. The present study reports the modulation of MI transitions in [10 nm-NdNiO₃/t-LaNiO₃/10 nm-NdNiO₃/SrTiO₃ (100)] trilayers (t = 5, 7, 10, and 20 nm) via the control of the LaNiO₃ thickness. Upon an increase in the thickness of the LaNiO₃ layer, the MI transition temperature undergoes a systematic decrease, demonstrating that bond disproportionation, the MI, and antiferromagnetic transitions are modulated by the LaNiO₃ thickness. Because the bandwidth and the MI transition are determined by the Ni-O-Ni bond angle, this unexpected behavior suggests the transfer of the bond angle from the lower layer into the upper. The bond-angle transfer eventually induces a structural change of the orthorhombic structure of the middle LaNiO₃ layer to match the structure of the bottom and the top NdNiO₃, as evidenced by transmission electron microscopy. This engineering layer sequence opens a novel pathway to the manipulation of the key properties of oxide nickelates, such as the bond disproportionation, the MI transition, and unconventional antiferromagnetism with no impact of disorder.

Electron-Lattice Coupling in Correlated Materials of Low Electron Occupancy

In correlated materials including transition metal oxides, electronic properties and functionalities are modulated and enriched by couplings between the electron and lattice degrees of freedom. These couplings are controlled by external parameters such as chemical doping, pressure, magnetic and electric fields, and light irradiation. However, the electron-lattice coupling relies on orbital characters, i.e., symmetry and occupancy, of t_2g and e_g orbitals, so that a large electron-lattice coupling is limited to e_g electron system, whereas t_2g electron system exhibits an inherently weak coupling. Here, we design and demonstrate a strongly enhanced electron-lattice coupling in electron-doped SrTiO₃, that is, the t_2g electron system. In ultrathin films of electron-doped SrTiO₃ [i.e., (La_0.25Sr_0.75)TiO₃], we reveal the strong electron-lattice-orbital coupling, which is manifested by extremely increased tetragonality and the corresponding metal-to-insulator transition. Our findings open the way of an active tuning of the charge-lattice-orbital coupling to obtain new functionalities relevant to emerging nanoelectronic devices.

Ground-state oxygen holes and the metal-insulator transition in the negative charge-transfer rare-earth nickelates

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO₃ thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3d⁸ configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation.

Rare-earth perovskite nickelates show intriguing metal–insulator transitions, whose mechanism remains elusive. Here, Bisogni et al. evidenced a 3d⁸ Ni configuration together with abundance of oxygen 2p holes in the ground state of a NdNiO₃ thin film, suggesting a negative charge-transfer scenario.

Oxygen-driven anisotropic transport in ultra-thin manganite films

Transition metal oxides have a range of unique properties due to coupling of charge, spin, orbital and lattice degrees of freedom and nearly degenerate multiple ground states. These properties make them interesting for applications and for fundamental investigations. Here we report a new phase with abnormal transport anisotropy in La(0.7)Sr(0.3)MnO3 ultra-thin films under large tensile strain. This anisotropy is absent in films under smaller tensile strain or compressive strain. Furthermore, thickness and magnetic-field-dependent experiments suggest that the tensile-strain-induced two-dimensional character is crucial for the observed phenomena. X-ray absorption spectroscopy results indicate that this anisotropy is likely driven by O 2p orbital, which hybridizes with Mn 3d. Ab initio calculations confirm this result. Our results may help to understand the anisotropic transport behaviour observed in other systems.

Engineering correlation effects via artificially designed oxide superlattices

Ab initio calculations are used to predict that a superlattice composed of layers of LaTiO3 and LaNiO3 alternating along the [001] direction is a S=1 Mott insulator with large magnetic moments on the Ni sites, negligible moments on the Ti sites and a charge transfer gap set by the energy difference between Ni d and Ti d states, distinct from conventional Mott insulators. Correlation effects are enhanced on the Ni sites via filling the oxygen p states and reducing the Ni-O-Ni bond angle. Small hole (electron) doping of the superlattice leads to a two-dimensional single-band situation with holes (electrons) residing on the Ni d(x2-y2) (Ti d(xy)) orbital and coupled to antiferromagnetically correlated spins in the NiO2 layer.

Theory of the magnetic and metal-insulator transitions in RNiO3 bulk and layered structures

A slave rotor--Hartree-Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within a unified formulation, the significant aspects of the rare-earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO3 and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and unstrained thin-film form. For ultrathin films, epitaxial strain and charge transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.

Tip preparation for near-field ablation at mid-infrared wavelengths

A fabrication method for high-throughput, fiber-based tips for near-field scanning microscopy (NSOM) in the mid-infrared (λ ~ 3 μm) has been developed. Several fiber materials have been investigated and recipes for wet-chemical etching have been varied to produce tips that are physically robust and are capable of low-loss transmission of high-power pulses of mid-infrared light. Ultimately, wet-chemical etching techniques are used on glass fibers to produce tips capable of focusing mid-infrared light to ablate material from sub-micron-sized regions of organic films. The power throughput of the tips is significantly increased by using a novel material, previously unreported for NSOM applications: germanate fibers. The tips produced are mechanically strong and capable of transmitting high light fluence without sustaining physical damage. Here, the development of these tips and their performance are described.