Emergence of flat bands and ferromagnetic fluctuations via orbital-selective electron correlations in Mn-based kagome metal.

Kagome lattice has been actively studied for the possible realization of frustration-induced two-dimensional flat bands and a number of correlation-induced phases. Currently, the search for kagome systems with a nearly dispersionless flat band close to the Fermi level is ongoing. Here, by combining theoretical and experimental tools, we present Sc3Mn3Al7Si5 as a novel realization of correlation-induced almost-flat bands in the kagome lattice in the vicinity of the Fermi level. Our magnetic susceptibility, 27Al nuclear magnetic resonance, transport, and optical conductivity measurements provide signatures of a correlated metallic phase with tantalizing ferromagnetic instability. Our dynamical mean-field calculations suggest that such ferromagnetic instability observed originates from the formation of nearly flat dispersions close to the Fermi level, where electron correlations induce strong orbital-selective renormalization and manifestation of the kagome-frustrated bands. In addition, a significant negative magnetoresistance signal is observed, which can be attributed to the suppression of flat-band-induced ferromagnetic fluctuation, which further supports the formation of flat bands in this compound. These findings broaden a new prospect to harness correlated topological phases via multiorbital correlations in 3d-based kagome systems.

Planckian behavior of cuprates at the pseudogap critical point simulated via flat electron-boson spectral density.

Planckian behavior has been recently observed in La1·76Sr0·24CuO4 at the pseudogap critical point. The Planckian behavior takes place in an intriguing quantum metallic state at a quantum critical point. Here, the Planckian behavior was simulated with an energy-independent (or flat) and weakly temperature-dependent electron-boson spectral density (EBSD) function by using a generalized Allen's (Shulga's) formula. We obtained various optical quantities from the flat EBSD function, such as the optical scattering rate, the optical effective mass, and the optical conductivity. These quantities are well fitted with the recently observed Planckian behavior. Fermi-liquid behavior was also simulated with an energy-linear and temperature-independent EBSD function. The EBSD functions agree well with the overall doping- and temperature-dependent trends of the EBSD function obtained from the optically measured spectra of cuprate systems, which can be crucial for understanding the microscopic electron-pairing mechanism for high-Tc superconductivity in cuprates.

An inversion problem for optical spectrum data via physics-guided machine learning.

We propose the regularized recurrent inference machine (rRIM), a novel machine-learning approach to solve the challenging problem of deriving the pairing glue function from measured optical spectra. The rRIM incorporates physical principles into both training and inference and affords noise robustness, flexibility with out-of-distribution data, and reduced data requirements. It effectively obtains reliable pairing glue functions from experimental optical spectra and yields promising solutions for similar inverse problems of the Fredholm integral equation of the first kind.

Self-Oxidation Resistance of the Curved Surface of Achromatic Copper.

Copper surfaces that exhibit a wide range of achromatic colors while still metallic have not been studied, despite advancements in antireflection coatings. A series of achromatic copper films grown with [111] preferred orientation by depositing 3D porous nanostructures is introduced via coherent/incoherent atomic sputtering epitaxy. The porous copper nanostructures self-regulate the giant oxidation resistance by constructing a curved surface that generates a series of monoatomic steps, followed by shrinkage of the lattice spacing of one or two surface layers. First-principles calculations confirm that these structural components cooperatively increase the energy barrier against oxygen penetration. The achromaticity of the single-crystalline porous copper films is systematically tuned by geometrical parameters such as pore size distribution and 3D linkage. The optimized achromatic copper films with high oxidation resistance show an unusual switching effect between superhydrophilicity and superhydrophobicity. The tailored 3D porous nanostructures can be a candidate material for numerous applications, such as antireflection coatings, microfluidic devices, droplet tweezers, and reversible wettability switches.

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.

Fermi liquid-like behaviour of cuprates in the pseudogap phase simulated via T-dependent electron-boson spectral density.

We investigated the temperature- and frequency-dependent optical scattering rates in the pseudogap phase of cuprates using model pseudogap and electron-boson spectral density (EBSD) functions. We obtained the scattering rates at various temperatures below and above a given pseudogap temperature using a generalized Allen's (or Sharapov's) formula, which has been used to analyse the measured optical spectra of correlated electron systems with a non-constant density of states at finite temperatures. The pseudogap and EBSD functions should be temperature dependent to simulate the Fermi liquid-like behaviour of underdoped cuprate systems observed in optical studies. Therefore, the observed Fermi liquid-like behaviour can be understood by considering the combined contribution from the T-dependent EBSD function and the T-dependent pseudogap. We also obtained the optical conductivity spectra from the optical scattering rates and analyzed them to investigate intriguing electronic properties. We expect that our results will aid in understanding the Fermi liquid-like optical response in the pseudogap phase and in revealing the microscopic pairing mechanism for superconductivity in cuprates.

Coherent consolidation of trillions of nucleations for mono-atom step-level flat surfaces.

Constructing a mono-atom step-level ultra-flat material surface is challenging, especially for thin films, because it is prohibitively difficult for trillions of clusters to coherently merge. Even though a rough metal surface, as well as the scattering of carriers at grain boundaries, limits electron transport and obscures their intrinsic properties, the importance of the flat surface has not been emphasised sufficiently. In this study, we describe in detail the initial growth of copper thin films required for mono-atom step-level flat surfaces (MSFSs). Deposition using atomic sputtering epitaxy leads to the coherent merging of trillions of islands into a coplanar layer, eventually forming an MSFS, for which the key factor is suggested to be the individual deposition of single atoms. Theoretical calculations support that single sputtered atoms ensure the formation of highly aligned nanodroplets and help them to merge into a coplanar layer. The realisation of the ultra-flat surfaces is expected to greatly assist efforts to improve quantum behaviour by increasing the coherency of electrons.

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.

Unusually large exciton binding energy in multilayered 2H-MoTe2.

Although large exciton binding energies of typically 0.6-1.0 eV are observed for monolayer transition metal dichalcogenides (TMDs) owing to strong Coulomb interaction, multilayered TMDs yield relatively low exciton binding energies owing to increased dielectric screening. Recently, the ideal carrier-multiplication threshold energy of twice the bandgap has been realized in multilayered semiconducting 2H-MoTe2 with a conversion efficiency of 99%, which suggests strong Coulomb interaction. However, the origin of strong Coulomb interaction in multilayered 2H-MoTe2, including the exciton binding energy, has not been elucidated to date. In this study, unusually large exciton binding energy is observed through optical spectroscopy conducted on CVD-grown 2H-MoTe2. To extract exciton binding energy, the optical conductivity is fitted using the Lorentz model to describe the exciton peaks and the Tauc-Lorentz model to describe the indirect and direct bandgaps. The exciton binding energy of 4 nm thick multilayered 2H-MoTe2 is approximately 300 meV, which is unusually large by one order of magnitude when compared with other multilayered TMD semiconductors such as 2H-MoS2 or 2H-MoSe2. This finding is interpreted in terms of small exciton radius based on the 2D Rydberg model. The exciton radius of multilayered 2H-MoTe2 resembles that of monolayer 2H-MoTe2, whereas those of multilayered 2H-MoS2 and 2H-MoSe2 are large when compared with monolayer 2H-MoS2 and 2H-MoSe2. From the large exciton binding energy in multilayered 2H-MoTe2, it is expected to realize the future applications such as room-temperature and high-temperature polariton lasing.

Superconducting coherence length of hole-doped cuprates obtained from electron-boson spectral density function.

Electron-boson spectral density functions (EBSDFs) can be obtained from measured spectra using various spectroscopic techniques, including optical spectroscopy. EBSDFs, known as glue functions, are suggested to have a magnetic origin. Here, we investigated EBSDFs obtained from the measured optical spectra of hole-doped cuprates with wide doping levels, from underdoped to overdoped cuprates. The average frequency of an EBSDF provides the timescale for the spin fluctuations to form Cooper pairs. This timescale is directly associated with retarded interactions between electrons. Using this timescale and Fermi velocity, a reasonable superconducting coherence length, which reflects the size of the Cooper pair, can be extracted. The obtained coherence lengths were consistent with those measured via other experimental techniques. Therefore, the formation of Cooper pairs in cuprates can be explained by spin fluctuations, the timescales of which appear in EBSDFs. Consequently, EBSDFs provide crucial information on the timescale of the microscopic mechanism of Cooper pair formation.

Color of Copper/Copper Oxide.

Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidation front in single-crystal Cu thin film is demonstrated to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain-boundary-free and atomically flat films prepared by atomic-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full-color red/green/blue indices is realized by precise control of the oxide-layer thickness; the samples cover ≈50.4% of the standard red/green/blue color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from the oxide "pigment" (complex dielectric functions of Cu2 O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. Furthermore, laser-oxide lithography is demonstrated with micrometer-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.

Synthesis of a Copper 1,3,5-Triamino-2,4,6-benzenetriol Metal-Organic Framework.

We report the synthesis and characterization of a two-dimensional (2D) MX2Y2-type (M = metal, X, Y = N, S, O, and X ≠ Y) copper 1,3,5-triamino-2,4,6-benzenetriol metal-organic framework (Cu3(TABTO)2-MOF). The role of oxygen in the synthesis of this MOF was investigated. Copper metal is formed along with the MOF when the synthesis is done in argon as suggested by XRD. When the reaction was exposed to air with vigorous stirring, copper metal was not observed by XRD. However, if there is no stirring, then copper metal is formed, and we learned that this is because oxygen was not allowed to enter the solvent due to the formation of a MOF film at the air/water interface. For the sample synthesized in argon (Cu3(TABTO)2-Ar), an insulating Cu3(TABTO)2-Ar pellet (σ < 10-10 S cm-1) became a metallic conductor with an electrical conductivity of 0.78 S cm-1 at 300 K after exposure to iodine vapor. This work provides further insights into the role of oxygen in the synthesis of redox-active ligand-based MOFs, expands the family of 2D redox-active ligand-based electrically conductive MOFs, and offers more opportunities in sensing, photocatalytic, electronic, and energy-related applications.

Publisher Correction: Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta2NiSe5.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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.

Temperature-dependent excitonic superuid plasma frequency evolution in an excitonic insulator, Ta2NiSe5.

An interesting van der Waals material, Ta2NiSe5 has been known one of strong excitonic insulator candidates since it has very small or zero bandgap and can have a strong exciton binding energy because of its quasi-one-dimensional crystal structure. Here we investigate a single crystal Ta2NiSe5 using optical spectroscopy. Ta2NiSe5 has quasi-one-dimensional chains along the a-axis. We have obtained anisotropic optical properties of a single crystal Ta2NiSe5 along the a- and c-axes. The measured a- and c-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the a-axis optical conductivity, a sharp peak near 3050 cm-1 at 9 K, with a well-defined optical gap ([Formula: see text] 1800 cm-1) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around ∼325 K ([Formula: see text]). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ([Formula: see text]) is [Formula: see text]. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta2NiSe5 has been determined from measured optical data. Our study may pave new avenues in the future research on excitonic insulators.

Analysis of optical data using extended Drude model and generalized Allen's formulas.

Extended Drude model formalism has been successfully utilized for analyzing optical spectra of strongly correlated electron systems including heavy-fermion systems and high-T c superconducting iron pnictides and cuprates. Furthermore, generalized Allen's formulas have been developed and applied to extract the electron-boson spectral density function from measured optical data of high temperature superconductors including cuprates in various material phases. Here we used a reverse process to obtain various optical quantities starting from two typical electron-boson spectral density model functions for three intriguing (normal, pseudogap, and d-wave superconducting) material phases in cuprates. We also assigned the calculated optical results to designated regions in the phase diagram of hole-doped cuprates and compared them with the corresponding measured optical spectra of Bi2Sr2CaCu2 [Formula: see text] (Bi-2212). This comparison suggested that this way of optical data analysis can be a convincing method to study correlated electrons in the copper oxide superconductors and other superconducting systems as well.

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.

Revisiting optical properties of MgB2 with a high-quality sample prepared by a HPCVD method.

We investigated a high-quality MgB2 thin film with a thickness of ~1000 nm on an Al2O3 substrate using optical spectroscopy. We measured the reflectance spectra of the film at various temperatures both below, and above, the superconducting transition temperature, T c [Formula: see text] 40 K. An earlier study showed that when the sample surface is exposed to air the optical properties of the surface change immediately, however, the saturated change is negligibly small in the far-infrared region. The optical conductivity spectrum in the normal state shows two (narrow and broad) Drude modes, with the narrow Drude mode being dominant in the low frequency region below 1000 cm-1. Our study, which uses a good-quality sample, provides more reliable data on the optical properties of MgB2, in a similar spectral range. The optical data is analyzed further using an extended Drude model, and the electron-phonon spectral density function, α 2 F(ω), is extracted. The spectral density function α 2 F(ω) features two peaks: a small one near 114 cm-1, and a strong peak around the 550 cm-1 where the B-B bond stretching phonon exists. Our data in the superconducting state does not show the expected energy shift of the onset of scattering associated with the α 2 F(ω) peaks.

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.

Intrinsic temperature-dependent evolutions in the electron-boson spectral density obtained from optical data.

We investigate temperature smearing effects on the electron-boson spectral density function (I(2)χ(ω)) obtained from optical data using a maximum entropy inversion method. We start with two simple model input I(2)χ(ω), calculate the optical scattering rates at selected temperatures using the model input spectral density functions and a generalized Allen's formula, then extract back I(2)χ(ω) at each temperature from the calculated optical scattering rate using the maximum entropy method (MEM) which has been used for analysis of optical data of high-temperature superconductors including cuprates, and finally compare the resulting I(2)χ(ω) with the input ones. From this approach we find that the inversion process can recover the input I(2)χ(ω) almost perfectly when the quality of fits is good enough and also temperature smearing (or thermal broadening) effects appear in the I(2)χ(ω) when the quality of fits is not good enough. We found that the coupling constant and the logarithmically averaged frequency are robust to the temperature smearing effects and/or the quality of fits. We use these robust properties of the two quantities as criterions to check whether experimental data have intrinsic temperature-dependent evolutions or not. We carefully apply the MEM to two material systems (one optimally doped and the other underdoped cuprates) and conclude that the I(2)χ(ω) extracted from the optical data contain intrinsic temperature-dependent evolutions.