Geometrical Doping at the Atomic Scale in Oxide Quantum Materials.

Chemical dopants enabling a plethora of emergent physical properties have been treated as randomly and uniformly distributed in the frame of a three-dimensional doped system. However, in nanostructured architectures, the location of dopants relative to the interface or boundary can greatly influence device performance. This observation suggests that chemical dopants need to be considered as discrete defects, meaning that geometric control of chemical dopants becomes a critical aspect as the physical size of materials scales down into the nanotechnology regime. Here we show that geometrical control of dopants at the atomic scale is another fundamental parameter in chemical doping, extending beyond the kind and amount of dopants conventionally used. The geometrical control of dopants extends the class of geometrically controlled structures into an unexplored dimensionality, between 2D and 3D. It is well understood that in the middle of the progressive dimensionality change from 3D to 2D, the electronic state of doped SrTiO3 is altered from a highly symmetric charged fluid to a charge disproportionated insulating state. Our results introduce a geometrical control of dopants, namely, geometrical doping, as another axis to provide a variety of emergent electronic states via tuning of the electronic properties of the solid state.

Doping-dependent superconducting physical quantities of K-doped BaFe[Formula: see text]As[Formula: see text] obtained through infrared spectroscopy.

We investigated four single crystals of K-doped BaFe[Formula: see text]As[Formula: see text] (Ba-122), Ba[Formula: see text]K[Formula: see text]Fe[Formula: see text]As[Formula: see text] with [Formula: see text] 0.29, 0.36, 0.40, and 0.51, using infrared spectroscopy. We explored a wide variety of doping levels, from under- to overdoped. We obtained the superfluid plasma frequencies ([Formula: see text]) and corresponding London penetration depths ([Formula: see text]) from the measured optical conductivity spectra. We also extracted the electron-boson spectral density (EBSD) functions using a two-parallel charge transport channel approach in the superconducting (SC) state. From the extracted EBSD functions, the maximum SC transition temperatures ([Formula: see text]) were determined using a generalized McMillan formula and the SC coherence lengths ([Formula: see text]) were calculated using the timescales encoded in the EBSD functions and reported Fermi velocities. We identified some similarities and differences in the doping-dependent SC quantities between the K-doped Ba-122 and the hole-doped cuprates. We expect that the various SC quantities obtained across the wide doping range will provide helpful information for establishing the microscopic pairing mechanism in Fe-pnictide superconductors.

Revealing excess protons in the infrared spectrum of liquid water.

The most common species in liquid water, next to neutral [Formula: see text] molecules, are the [Formula: see text] and [Formula: see text] ions. In a dynamic picture, their exact concentrations depend on the time scale at which these are probed. Here, using a spectral-weight analysis, we experimentally resolve the fingerprints of the elusive fluctuations-born short-living [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] ions in the IR spectra of light ([Formula: see text]), heavy ([Formula: see text]), and semi-heavy (HDO) water. We find that short-living ions, with concentrations reaching [Formula: see text] of the content of water molecules, coexist with long-living pH-active ions on the picosecond timescale, thus making liquid water an effective ionic liquid in femtochemistry.

Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy.

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Temperature-dependent optical properties of self-doped superconducting Fe-pnictide, Sr2VO3FeAs.

We performed an infrared spectroscopic study on a single crystal of Sr2VO3FeAs grown by a self-flux method. This layered material system consists of two alternative layers of [SrVO3]-1 and [SrFeAs]+1. Since the typical size of single crystalline Sr2VO3FeAs samples is 200 [Formula: see text] 200 [Formula: see text] 10 [Formula: see text]m3 an optical study on this material is challenging. We observed an additional interband transition around 1000 cm-1, which is absent in other doped Ba-122 Fe-pnictides. The origin of this additional transition is not clearly known yet. We also observed a hidden Fermi liquid behavior. Interestingly, we observed a Fano line-shaped phonon which appears near 555 cm-1 below 200 K and shows a strong blue-shift when the temperature is lowered. The amplitude, width, and asymmetric Fano parameter of this phonon show anomalies at 150 K, which are probably related to an electronic phase observed below 155 K recently by an NMR study (Ok et al 2017 Nat. Commun. 8 2167). Our finding may help to understand the electronic phase observed previously in the same material.

Directing Oxygen Vacancy Channels in SrFeO2.5 Epitaxial Thin Films.

Transition-metal oxides (TMOs) with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO2.5 are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the OVC orientation further leads to modifications in the hydrogenation of the BM-SrFeO2.5 thin films. The results demonstrate a strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.

Topotactic Metal-Insulator Transition in Epitaxial SrFeOx Thin Films.

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeOx epitaxial thin films is identified in real-time, during the progress of reversible topotactic phase transformation. Using real-time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure-temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeOx thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.

Phase transitions via selective elemental vacancy engineering in complex oxide thin films.

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.