Oxygen vacancies are a common defect in oxides and play vital roles in many technological applications such as oxygen separation, catalytic reactors, solid oxide fuel cells, and solar thermochemical water splitting. The oxygen vacancy formation energy is directly related to the oxide reduction enthalpy and the vacancy concentration and, therefore, is a key quantity underlying these applications. The complexity of measuring oxygen vacancy formation energies experimentally suggests the utility of calculations based on density functional theory (DFT). However, the calculated results are strongly dependent on the exchange-correlation functionals and other parameters such as the Hubbard $U$. In this work, we compare the performance of the meta-GGA strongly constrained and appropriately normed (SCAN) functional and the commonly used semilocal generalized gradient approximation (GGA) functionals with experimental values of structural parameters, band gaps, magnetic structures, and oxygen vacancy formation energies for six representative oxides, i.e., ${\mathrm{CaMnO}}_3, {\mathrm{SrMnO}}_3, {\mathrm{LaMnO}}_3, {\mathrm{YMnO}}_3, {\mathrm{LaFeO}}_3$, and ${\mathrm{CeO}}_2$. Our results show that SCAN usually has better agreement with the experimental lattice constants and band gaps and larger magnetic moments than the commonly used GGA functionals. Although SCAN overestimates the oxygen vacancy formation energies of transition metal oxides and therefore requires unusually large Hubbard $U$ values to reproduce the experimental reduction enthalpies, it does predict the correct oxygen vacancy formation energy of ${\mathrm{CeO}}_2$, which challenges the commonly used GGA and hybrid functionals. Our results underscore the challenges that exist in describing these complex oxides by DFT and may shed light on developing more accurate exchange-correlation functionals for oxygen vacancy formation energy calculations.

We introduce two one-dimensional tight-binding models based on the tribonacci substitution—the hopping and on-site tribonacci chains—which generalize the Fibonacci chain. For both hopping and on-site models, a perturbative real-space renormalization procedure is developed. We show that the two models are equivalent at the fixed point of the renormalization-group flow, and that the renormalization procedure naturally gives the local resonator modes. Additionally, the Rauzy fractal, inherent to the tribonacci substitution, is shown to serve as the analog of conumbering for the tribonacci chain. The renormalization procedure is used to repeatedly subdivide the Rauzy fractal into copies of itself, which can be used to describe the eigenstates in terms of local resonator modes. Finally, the multifractal dimensions of the energy spectrum and eigenstates of the hopping tribonacci chain are computed, from which it can be concluded that the tribonacci chains are critical.

Recently, the study by Miksch et al. [Science 372, 276 (2021)] and subsequent studies have proposed that the organic system $\kappa$-(BEDT-TTF)${}_2{\mathrm{Cu}}_2$(CN)${}_3$, which has long been regarded as a quantum spin liquid (QSL) candidate since an early nuclear magnetic resonance (NMR) report [Shimizu et al., Phys. Rev. Lett. 91, 107001 (2003)], actually exhibits a valence bond solid (VBS) ground state. To verify the validity and discuss the applicability of this interpretation to $\kappa$-(BEDT-TTF)${}_2{\mathrm{Cu}}_2$(CN)${}_3$ and other potential QSL candidates, it is necessary to provide a comprehensive NMR report in a representative VBS system for comparative analysis. For this purpose, we reexamined the ${}^{13}\mathrm{C}$ NMR study of the organic VBS system ${\mathrm{EtMe}}_3\mathrm{P}[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2{]}_2$. We discovered that the spin-lattice relaxations in the VBS state are significantly influenced by orphan impurity spins, whose emergence is captured by the spectral results. Furthermore, we also provide the general formalism of NMR relaxations caused by the dynamics of these orphan impurity spins, which expands the Bloembergen-Purcell-Pound model. This paper will provide a robust foundation for future research discussing NMR results of QSL candidates.

To establish a solid-state-based framework for the coexistence of quantum many-body scars and quantum criticality, we investigate the spin-1 Kitaev chain with uniaxial single-ion anisotropy (SIA). In the subspace with uniform ${\mathbb{Z}}_2$ gauge fields, this model can be exactly mapped to the spin-1/2 effective detuned PXP Hamiltonian, where the SIA plays a role of the static detuning term. The quench dynamics starting from the product states is symmetric between positive and negative values of the SIA, while a quantum phase transition from the Kitaev spin liquid to the dimer phase only occurs at the critical point with a negative $D_c$, implying the spontaneous breaking of the translational symmetry. We find that the coherent oscillations of quantum fidelity and certain local observables are sustained against small SIA perturbations in a quantum quench from special initial states. While the oscillation amplitudes of these observables decay with time as the SIA strength is increased, the system completely thermalizes upon approaching the critical point. In contrast, the initial polarized state, which shows an absence of revivals of quantum fidelity, will exhibit long revivals for $D<D_c$. Finally, we investigate the evolution of phase boundaries of the Kitaev spin liquid and dimer phase by introducing Heisenberg interactions, which spoil the ${\mathbb{Z}}_2$ gauge fields. A complete phase diagram is given by the infinite time-evolving block decimation method and the ground-state properties of each phase are accurately captured by various spin correlations. Our paper opens the door to understanding exotic connections between many-body scars and quantum criticality in systems with higher spins.

Superconductivity in V-based kagome metals has recently raised great interest as they exhibit the competing ground states associated with the flat bands and topological electronic structures. Here we report the discovery of superconductivity in ${\mathrm{Ta}}_2{\mathrm{V}}_{3.1}{\mathrm{Si}}_{0.9}$ with a superconducting transition temperature $T_c$ of 7.5 K, a record high for kagome metals at ambient pressure. While the V ions form a two-dimensional breathing kagome structure, the length difference between two different V-V bonds is just $0.05\AA$, making it very close to the perfect kagome structure. Our results show that ${\mathrm{Ta}}_2{\mathrm{V}}_{3.1}{\mathrm{Si}}_{0.9}$ is a moderate-coupled type-II superconductor with a large upper critical field that is close to the Pauli limit. DFT calculations give a Van Hove singularity band located at Fermi energy, which may explain the relatively high $T_c$ observed in this material. Furthermore, electron-phonon coupling calculations indicate that the in-plane vibrations of the V atoms play a significant role in driving the superconductivity.

Interstitial anionic electrons (IAEs) at lattice cavities of electrides, which have diverse morphologies and concentrations, can induce interesting physical and chemical properties. Understanding the correlation between IAEs and electron-phonon coupling is crucial for the development of new electride superconductors. We have applied first-principles structural search calculations to predict new high-pressure Li-As electrides, such as $P6/mmm {\mathrm{Li}}_x\mathrm{As}$ ($x=5$ and 8), ${\mathrm{Li}}_6\mathrm{As}$ with $Cmc{2}_1$ and $C2/c$ symmetries, and $C2/m{\mathrm{Li}}_{10}X$ ($X=\mathrm{As}$ and Sb), which together with the already known $C2/m{\mathrm{Li}}_{10}\mathrm{Te}$ compound present favorable characteristics (i.e., variable donor content, structural symmetries, and acceptor species) to explore IAE-related superconductivity. According to our results, the predicted superconducting temperatures of these electrides are positively correlated with the number and connectivity of IAEs.

A single material achieving multiple topological phases can provide potential application for topological spintronics, whereas the candidate materials are very limited. Here, we report on the structure, physical properties, and possible emergence of multiple topological phases in the recently discovered, air-stable EuCuBi single crystal. EuCuBi crystallizes in hexagonal space group $P{6}_3/mmc$ (No. 194) in ZrBeSi-type structure with an antiferromagnetic (AFM) ground state below $T_N=11.2$ K. There is a competition between AFM and ferromagnetic (FM) interactions below $T_N$ revealed by electrical resistivity and magnetic susceptibility measurements. With increasing magnetic field, EuCuBi evolves from the AFM ground state with a small amount of FM component, going through a possible metamagnetic phase, finally reaching the field-induced FM phase. Based on the first-principles calculations, we demonstrate that Dirac and Weyl semimetals, and possibly mirror Chern insulators, can be achieved in EuCuBi by tuning the temperature and applying a magnetic field, making EuCuBi a promising candidate for exploring multiple topological phases.

We use a recently proposed quantum electrodynamical density theory functional in a real-time excitation calculation for a two-dimensional electron gas in a square array of quantum dots in an external constant perpendicular magnetic field to model the influence of cavity photons on the excitation spectra of the system. The excitation is generated by a short electrical pulse. The quantum dot array is defined in an AlGaAs-GaAs heterostructure, which is in turn embedded in a parallel plate far-infrared photon microcavity. The required exchange and correlation energy functionals describing the electron-electron and electron-photon interactions have therefore been adapted for a two-dimensional electron gas in a homogeneous external magnetic field. We predict that the energies of the excitation modes activated by the pulse are generally redshifted to lower values in the presence of a cavity. The redshift can be understood in terms of the polarization of the electron charge by the cavity photons and depends on the magnetic flux, the number of electrons in a unit cell of the lattice, and the electron-photon interaction strength. We find an interesting interplay of the exchange forces in a spin-polarized two-dimensional electron gas and the square-lattice structure leading to a small but clear blueshift of the excitation mode spectra when one electron resides in each dot.

We investigate the thermal transport properties in superconductor-antiferromagnet-superconductor and superconductor-ferromagnet-superconductor junctions based on buckled two-dimensional materials (BTDMs). Owing to the unique buckled sublattice structures of BTDMs, in both junctions the phase dependence of the thermal conductance can be effectively controlled by perpendicular electric fields. The underlying mechanism for the electrical tunability of thermal conductance is elucidated resorting to the band structures of the magnetic regions. We also reveal the distinct manifestations of antiferromagnetic and ferromagnetic exchange fields in the thermal conductance. These results demonstrate that the perpendicular electric field can serve as a knob to externally manipulate the phase-coherent thermal transport in BTDMs-based Josephson junctions.

Thermoelectric effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can be exploited as probes of the electronic structure. We experimentally study thermopower in high-quality monolayer graphene, within heterostructures consisting of complete hBN encapsulation and 1D edge contacts, where the graphene and hBN lattices are aligned. When graphene is aligned to one of the hBN layers, we demonstrate the presence of additional sign reversals in the thermopower as a function of carrier density, directly evidencing the presence of the single-aligned moiré superlattice. We show that the temperature dependence of the thermopower enables the assessment of the role of built-in strain variation and van Hove singularities and hints at the presence of Umklapp electron-electron scattering processes. As the thermopower peaks around the neutrality point, this allows to probe the energy spectrum degeneracy. Further, when graphene is double aligned with the top and bottom hBN crystals, the thermopower exhibits features evidencing multiple cloned Dirac points caused by the differential super-moiré superlattice. For both cases we evaluate how well the thermopower agrees with Mott's equation. Finally, we show the same moiré superlattice device can exhibit a temperature-driven thermopower reversal from positive to negative and vice versa, by controlling the carrier density. The study of thermopower provides an alternative approach to study the electronic structure of 2D superlattices, whilst offering opportunities to engineer the thermoelectric response on these heterostructures.

Due to a computational error, the values of various quantities obtained in our work are incorrect by a factor $\frac{2}{\pi }$. This error does not change the conclusions of our work. This requires small changes in the text to replace the incorrect values, and modified figures and tables.

Strong interactions between charges and light-matter coupled quasiparticles offer an intriguing prospect with applications from optoelectronics to light-induced superconductivity. Here, we investigate how the interactions between electrons and exciton-polaritons in a two-dimensional semiconductor microcavity can be resonantly enhanced due to a strong coupling to a trion, i.e., an electron-exciton bound state. We develop a microscopic theory that uses a strongly screened interaction between charges to enable the summation of all possible diagrams in the polariton-electron scattering process, and we find that the polariton-electron interaction strength can be strongly varied and enhanced in the vicinity of the resonance. We furthermore derive an analytic approximation of the interaction strength based on universal low-energy scattering theory. This is found to match extremely well with our full calculation, indicating that the trion resonance is near universal, depending more on the strength of the light-matter coupling relative to the trion binding energy than on the details of the electronic interactions. Thus we expect the trion resonance in polariton-electron scattering to appear in a broad range of microcavity systems with few semiconductor layers such as doped monolayer ${\mathrm{MoSe}}_2$, where such resonances have recently been observed experimentally [Sidler et al., Nat. Phys. 13, 255 (2017)].

The Green-Kubo (GK) method is a rigorous framework for heat transport simulations in materials. However, it requires an accurate description of the potential-energy surface and carefully converged statistics. Machine-learning potentials can achieve the accuracy of first-principles simulations while allowing to reach well beyond their simulation time and length scales at a fraction of the cost. In this Letter, we explain how to apply the GK approach to the recent class of message-passing machine-learning potentials, which iteratively consider semilocal interactions beyond the initial interaction cutoff. We derive an adapted heat flux formulation that can be implemented using automatic differentiation without compromising computational efficiency. The approach is demonstrated and validated by calculating the thermal conductivity of zirconium dioxide across temperatures.

Intuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is an enigmatic material known for a wellcharacterized anisotropic normal state and unconventional superconductivity. Recent ab initio transport calculations of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, $S$ acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures.

Pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has gained significant attention due to the low-temperature geometrical frustration induced unusual magnetic phase. Most experimental studies on ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ have been performed on polycrystalline bulk or single crystals; however, there have been a few reports detailing the structure and magnetism of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ thin films. Here, we fabricated high-quality (001)-, (110)-, and (111)-epitaxial transparent ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ films on single-crystal yttria-stabilized $\mathrm{Zr}{\mathrm{O}}_2$ (YSZ) substrates with various oxygen pressures by pulsed laser deposition. Structural characterizations confirm flat surface and uniform and well-defined atomic arrangement of the ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ films, as well as atomic-scale epitaxial and sharp ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$/YSZ interface. The (111) oriented films fabricated at 10 Pa oxygen pressure have the highest magnetization value of $2.03{\mu }_{\mathrm{B}}/\mathrm{Yb}$, which is higher than that of bulk counterparts, due to the fact that the distorted Yb-tetrahedra induced by interfacial strain and defects can destroy magnetic frustration and enhance magnetization. Especially, the fabricated (001)-, (110)-, and (111)-epitaxial ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ films illustrate a significantly pronounced magnetic anisotropy, which results from the single ion anisotropy of Yb and the anisotropic exchange interactions. In addition, a wide-spectrum transmittance of $\sim 95\%$ was achieved in (110)-epitaxial ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ films, which is comparable to the state-of-the-art transparent materials. Our work gives a comprehensive perspective on the structure, magnetism, magnetic anisotropy, and transmittance of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ films, which may guide further work on pyrochlore materials.

We theoretically investigate electronic orderings with the electric axial moment without breakings of both spatial inversion and time-reversal symmetries in the zigzag-chain system. Specifically, we elucidate the role of the local odd-parity hybridization arising from locally noncentrosymmetric lattice structures based on symmetry and microscopic model analyses. We show that the odd-parity crystalline electric field gives rise to an effective cross-product coupling between the electric dipole and electric toroidal dipole, the latter of which corresponds to the electric axial moment. As a result, the staggered component of the electric axial moment is induced by applying an external electric field, while its uniform component is induced via the appearance of staggered electric dipole ordering. We also show that uniform electric quadrupole ordering accompanies uniform electric axial moment. Furthermore, we discuss transverse magnetization as a consequence of the orderings with the uniform electric axial moment. Our results extend the scope of materials exhibiting electric axial ordering to those with locally noncentrosymmetric lattice structures.

We study an exactly solvable model with bond-directional quadrupolar and octupolar interactions between spin-orbital entangled $j_{\mathrm{eff}}=\frac{3}{2}$ moments on the honeycomb lattice. We show that this model features a multipolar spin liquid phase with gapless fermionic excitations. In the presence of perturbations that break time-reversal and rotation symmetries, we find Abelian and non-Abelian topological phases in which the Chern number evaluates to 0, $\pm 1$, and $\pm 2$. We also investigate quantum phase transitions out of the multipolar spin liquid using a parton mean-field approach and orbital wave theory. In the regime of strong integrability-breaking interactions, the multipolar spin liquid gives way to ferroquadrupolar-vortex and antiferro-octupolar ordered phases that harbor a hidden spin-$\frac{1}{2}$ Kitaev spin liquid. Our work unveils mechanisms for unusual multipolar orders and quantum spin liquids in Mott insulators with strong spin-orbit coupling.

Hyperbolic plasmons are collective electron excitations in layered conductors. They are of relevance to a number of superconducting materials, including the cuprates and layered hyperbolic metamaterials [V. N. Smolyaninova et al., Sci. Rep. 6, 34140 (2016)]. This work studies how the unusual dispersion of hyperbolic plasmons affects Cooper pairing. We use the Migdal-Eliashberg equations, which are solved both numerically and analytically using the Grabowski-Sham approximation, with consistent results. We do not find evidence for plasmon-mediated pairing within a reasonable parameter range. However, it is shown that the hyperbolic plasmons can significantly reduce the effects of Coulomb repulsion in the Cooper channel leading to an enhancement of the transition temperature originating from other pairing mechanisms. In the model of a hyperbolic material composed of identical layers, we find this enhancement to be the strongest in the $d$-wave channel. We also discuss strategies for engineering an optimal hyperbolic plasmon background for a further enhancement of superconductivity in both $s$-wave and $d$-wave channels.

We examined a pressure dependence of an electronic nematicity in iron-based ladder materials ${\mathrm{BaFe}}_2{\left({\mathrm{S}}_{1\text{−}x}{\mathrm{Se}}_x\right)}_3$ with a quasi-one-dimensional ladder lattice by tracking an anomaly in the resistivity. For $x=0$, the ferroic nematicity observed at ambient pressure is stabilized by applying pressure up to 2 GPa. For $x=0.25$, 0.60, and 1, the antiferroic nematicity at ambient pressure switches into ferroic nematicity by applying pressures of 0.50–4.0 GPa. We present the electronic phase diagram of ${\mathrm{BaFe}}_2{\left({\mathrm{S}}_{1-x}{\mathrm{Se}}_x\right)}_3$ in terms of the lattice parameters, and discuss the relationship between the nematicity and bandwidth. We also discuss that the external pressure causes a filling control through the charge transfer from anions to cations in the Se-rich compositions.