We report ab initio calculations of the electronic and vibrational properties in ${\mathrm{CuPb}}_9{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$, including the electron-phonon coupling strength via strong-coupling Migdal-Eliashberg theory. We verify the presence of appealing flat electronic bands near the Fermi level, a strong hybridization between the Cu $3d$ and O $2p$ states, and soft low-energy phonons, which can suggest high-temperature superconducting behavior. However, the electron-phonon coupling strength appears insufficient to overcome the Coulomb repulsion between an electron pair and thus does not support high-temperature superconductivity in ${\mathrm{CuPb}}_9{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ via the conventional electron-phonon Migdal-Eliashberg mechanism. Even neglecting Coulomb repulsion of the electron pair we find this electron-phonon coupling suggests a superconducting transition temperature of less than 2 K.

Owing to their large effective mass, strong and tunable spin-orbit coupling, and complex band structure, two-dimensional hole systems (2DHSs) in GaAs quantum wells provide rich platforms to probe exotic many-body physics, while also offering potential applications in ballistic and spintronics devices, and fault-tolerant topological quantum computing. We present here a systematic study of molecular-beam-epitaxy grown, modulation-doped, GaAs (001) 2DHSs where we explore the limits of low-temperature 2DHS mobility by optimizing two parameters, the GaAs quantum well width, and the alloy fraction ($x$) of the flanking ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barriers. We obtain a breakthrough in 2DHS mobility, with a peak value $\simeq 18\times {10}^6{\mathrm{cm}}^2/\mathrm{Vs}$ at a density of $3.8\times {10}^{10}/{\mathrm{cm}}^2$, implying a mean free path of $\simeq 57\mu \mathrm{m}$. Using transport calculations tailored to our structures, we analyze the operating scattering mechanisms to explain the nonmonotonic evolution of mobility with density. We find it imperative to include the dependence of effective mass on 2DHS density, well width, and $x$. We observe concomitant improvement in quality as evinced by the appearance of delicate fractional quantum Hall states at very low density.

Kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Cs, Rb) provide a rich platform for intertwined orders such as the charge density wave (CDW) and a chiral order with time-reversal symmetry breaking (TRSB). While early reports of large optical polarization rotations have been interpreted as the magneto-optic Kerr effect (MOKE) and as evidence for TRSB, recent dedicated optical rotation and MOKE experiments have clarified that this large optical rotation originates instead from an unconventional specular rotation. Yet a critical discrepancy remains regarding the possible existence of a true spontaneous MOKE signal: in experiments performed after training with modest magnetic fields of up to 0.3 T, no MOKE signal was detected above the noise floor of 30 nanoradians, while microradian-level signals were found in an experiment using higher training fields. This raises an intriguing possibility of different zero-field ground states with opposite time-reversal symmetry properties, because of different magnetic histories. To unambiguously determine whether a training-field-dependent spontaneous MOKE signal exists in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$, we conduct comprehensive MOKE measurements with two Sagnac interferometer setups capable of both low and high training fields of up to 9 T, and perform careful analyses of contributions of signals from various optical components. We conclude that there is no observable spontaneous MOKE signal, hence no optical evidence for TRSB, regardless of the magnitude of training fields and the speed of temperature ramping.

Intrinsic spin Hall and anomalous Hall conductivities (SHC and AHC) for heterostructures of Co and $5d$ heavy metal ($\mathrm{HM}=\mathrm{Ta},\mathrm{W},\mathrm{Re},\mathrm{Os},\mathrm{Ir},\mathrm{Pt}$) are investigated by comparing chemical (HM element) trend and different HM thickness by means of first-principles calculations. We find that the HM-element dependencies of SHC and AHC are caused by the $5d$ band filling but importantly originated from different band characteristic. The SHC associates with the spin Berry curvature in $5d$ bands inside the HM atomic layers. In contrast, the AHC is induced by the interfacial bands at the Co/HM, in which the origin can be decomposed into two effects; the $5d$ band filling of interfacial HM atomic layer and a proximity effect of neighboring Co atomic layer inducing the magnetic moment at the top HM. The former determines the overall distribution of Berry curvature in Brillouin zone, while the latter, asymmetric distribution of Berry curvature by a time-reversal symmetry breaking. As a consequence, different thickness dependencies of the HM atomic layers to the SHC and AHC appear, being significant and absent, respectively. The findings provide guiding principles for controllable SHC and AHC in device developments.

Lithium fluoride (LiF) is an important component of solid electrolyte interphase (SEI), but its low ionic conductivity limits the applications in lithium-ion batteries. In order to obtain high-performance SEI layers, we wanted to improve lithium-ion conductivity of LiF by doping at low concentrations while minimizing damage to electronic conductivity and maintaining high mechanical strength. First-principles calculations were performed to initially screen the main group elements, of which Na, K, Be, Mg, Ca, and Al were found to meet this material design requirement. Further, the effect of these elemental dopings on the properties of the bulk, surface, and interfacial phases was systematically investigated. We demonstrate that monovalent ion doping increases lithium-ion conductivity mainly by lowering the diffusion energy barrier, whereas multivalent ion doping increases lithium-ion conductivity by inducing lithium negatively charged vacancy. Divalent ions (${\mathrm{Be}}^{2+}, {\mathrm{Mg}}^{2+}$, and ${\mathrm{Ca}}^{2+}$) are recommended for doping into lithium fluoride because they increase the lithium-ion conductivity by 18 orders of magnitude with insignificant decreases in electronic conductivity and mechanical strength. Our work provides theoretical guidance and a research paradigm for the preparation of artificial SEI films with excellent properties.

X-ray absorption spectroscopy (XAS) is an element-specific materials characterization technique that is sensitive to structural and electronic properties. First-principles simulated XAS has been widely used as a powerful tool to interpret experimental spectra and draw physical insights. Recently, there has also been growing interest in building computational XAS databases to enable data analytics and machine learning applications. However, there are nontrivial differences among commonly used XAS simulation codes, both in underlying theoretical formalism and in technical implementation. Reliable and reproducible computational XAS databases require systematic benchmark studies. In this paper, we benchmarked Ti K-edge XAS simulations of ten representative Ti-O binary compounds, which we refer to as the Ti-O-10 dataset, using three state-of-the-art codes: xspectra, ocean, and exciting. We systematically studied the convergence behavior with respect to the input parameters and developed a workflow to automate and standardize the calculations to ensure converged spectra. Our benchmark comparison considers a 35-eV spectral range starting from the K-edge onset, representative of widely used near-edge spectra. Quantitative comparison over this range is based on Spearman's rank correlation score $\left(r_{sp}\right)$. Our results show that (1) the two Bethe-Salpeter equation (BSE) codes (ocean and exciting) have excellent agreement with an average $r_{sp}$ of 0.998; (2) good agreement is obtained between the core-hole potential code (xspectra) and BSE codes (ocean and exciting) with an average $r_{sp}$ of 0.990, and this smaller $r_{sp}$ reflects the noticeable differences in the main edge spectral shape that can be primarily attributed to the difference in the strength of the screened core-hole potential; (3) simulations from both methods overall reproduce well the main experimental spectral features of rutile and anatase, and the different treatments of the screened core-hole potential have visible impact on pre-edge intensities and the peak ratio of the main edge; (4) there exist moderate differences in the relative edge alignment of the three codes with a standard deviation of about 0.2 eV, which arise from multiple contributions including the frozen core approximation, final state effects, and different approximations used for the self-energy correction. Our benchmark study provides important standards for first-principles XAS simulations with broad impact in data-driven XAS analysis.

La doped ${\mathrm{SrFeO}}_3$, ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_3$, exhibits a metal-to-insulator transition accompanied by both antiferromagnetic and charge ordering states along with the Fe-O bond disproportionation below a critical temperature near 200 K. Unconventionally slow charge dynamics measured in this material near the critical temperature [Nat. Commun. 9, 1799 (2018)] shows that its excited charge ordering states can exhibit novel electronic structures with nontrivial energy profiles. Here, we reveal possible metastable states of charge ordering structures in ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_3$ using the first-principle and climbing image nudged elastic band methods. In the strong correlation regime, ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_3$ is an antiferromagnetic insulator with a charge ordering state of the big-small-big pattern, consistent with the experimental measurement of this material at the low temperature. As the correlation effect becomes weak, we find at least two possible metastable charge ordering states with the distinct Fe-O bond disproportionation. Remarkably, a ferroelectric metallic state emerges with the small energy barrier of $\sim 7\mathrm{meV}$, driven by a metastable charge ordering state of the small-medium-big pattern. The electronic structures of these metastable charge ordering states are noticeably different from those of the ground state. Our results can provide an insightful explanation to multiple metastable charge ordering states and the slow charge dynamics of this and related oxide materials.

As a recent member of the two-dimensional (2D) van der Waals (vdW) heterostructures, the $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterostructure has received considerable attention due to its fascinating characteristics compared with the constituent 2D materials. In this paper, we performed first-principles calculations to investigate its structural, electronic, and magnetic properties and explored the effects of interlayer and in-plane strains on these properties. Our results reveal that the antiferromagnetic (AFM) ground state in the $\mathrm{Cr}{\mathrm{Se}}_2$ layer of the heterostructure is maintained owing to weak vdW interactions between the $\mathrm{Cr}{\mathrm{Se}}_2$ and $\mathrm{W}{\mathrm{Se}}_2$ layers. However, the AFM state can be transformed into the ferromagnetic state at interlayer compressive strain of −19% or in-plane tensile strain of 2.5%. Moreover, the $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ heterointerface belongs to the metal-semiconductor interface and exhibits $p$-type ohmic contact and low contact resistance. The transition from $p$-type ohmic contact to $p$-type Schottky contact or $n$-type Schottky contact can be achieved by interlayer or in-plane strain engineering, which is associated with the strain-induced energy shifts of the valence band maximum and conduction band minimum of $\mathrm{W}{\mathrm{Se}}_2$. Additionally, the tunneling probability of the heterostructure rises dramatically (up to 100%) with interlayer coupling, which is favorable for carrier transport at the heterointerface. Our findings demonstrate that strain engineering is an effective way of modulating metal-semiconductor interfaces and provide theoretical guidance for designing electronic and magnetic devices based on the $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{Se}}_2$ vdW heterostructure as well as broadening its applications in future functional devices.

Metamaterial-based polarization devices are promising in imaging, sensing, and other applications. The balance between multifunction-achieved efficiency and their structural complexity has become an important issue to be discussed. In this paper, we proposed metamaterials with stacked gold split-rings as circular polarization selector and converter for broadband applications that operate in the near-infrared wavelength range. The circular polarization selector consists of six-layer stacked gold split-rings. It can transmit circularly polarized light with the same chirality as the structure and block the orthogonally polarized light. The circular polarization selector works with a bandwidth beyond 700 nm, and its figure of merit is over 11 000. Its operation band can be adjusted by changing the refractive index of the medium, and it has a wide-angle working capability. A circular polarization converter can be obtained by combining two circular polarization selectors with opposite chirality. It works in the range 1460–2100 nm, maintaining the polarization conversion ratio over 0.9 and the ellipticity angle above ${20}^{\circ }$. Furthermore, the corresponding physical mechanism was illustrated through the current analysis, and the reliability of the simulated results was verified by the theoretical calculation based on the transfer matrix method. The proposed metamaterial polarization devices can be used in optical communication, biomedicine, and other fields.

Wurtzite $T_{\mathrm{m}}\mathrm{AlN}$ ($T_{\mathrm{m}}=\mathrm{transition}$ metal) themselves are of interest as semiconductors with tunable band gap, insulating motifs to superconductors, and piezoelectric crystals. Characterization of wurtzite $T_{\mathrm{m}}\mathrm{AlN}$ is challenging because of the difficulty to synthesize them as single-phase solid solution and such thermodynamic, elastic properties, and high temperature behavior of wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ is unknown. Here, we investigated the high temperature decomposition behavior of wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ films using experimental methods combined with first-principles calculations. We have developed a method to grow single-phase metastable wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ ($x=0.65$, 0.75, 085, and 0.95) solid-solution films by cathodic arc deposition using low duty-cycle pulsed substrate-bias voltage. We report the full elasticity tensor for wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ as a function of Al content and predict a phase diagram including a miscibility gap and spinodals for both cubic and wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$. Complementary high-resolution scanning transmission electron microscopy and chemical mapping demonstrate decomposition of the films after high temperature annealing (${950}^{\circ }\mathrm{C}$), which resulted in nanoscale chemical compositional modulations containing Ti-rich and Al-rich regions with coherent or semicoherent interfaces. This spinodal decomposition of the wurtzite film causes age hardening of 1–2 GPa.

Two common difficulties in the design of topological quantum materials are that the desired features lie too far from the Fermi level and are spread over a too-large energy range. Doping-induced states at the Fermi level provide a solution, where nontrivial topological properties are enforced by the doping-reduced symmetry. To show this, we consider a regular placement of dopants in a lattice of space group (SG) 176 (${P6}_3/m$), which reduces the symmetry to SG 143 ($P3$). Our two- and four-band models feature double Weyl points, Chern bands, Van Hove singularities, nontrivial multiband quantum geometry due to mixed orbital character, and singular flat bands. We relate these features to density-functional theory (DFT) calculations for dopant and vacancy bands of lead apatite ${\mathrm{Pb}}_{10}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ and ${\mathrm{Pb}}_{10}{\left({\mathrm{PO}}_4\right)}_6{\left(\mathrm{OH}\right)}_2$, the van der Waals ferromagnet ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$, the semiconductor SiC, and the 2D dichalcogenide ${\mathrm{MoS}}_2$.

We report ${}^{79}\mathrm{Br}$ and ${}^{63}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) in the paramagnetic state above $T_{\text{N}}=15$ K of the antiferromagnetic orthorhombic phase of barlowite ${\mathrm{Cu}}_4{\left(\mathrm{OD}\right)}_6\mathrm{FBr}$ consisting of a layered kagome structure. The divergent behavior of the longitudinal ${}^{79}(1/T_1)$ and transverse ${}^{79}(1/T_2)$ relaxation rates observed at ${}^{79}\mathrm{Br}$ sites evidences that critical slowing down of Cu spin fluctuations sets in below $\sim 20$ K. This means that one or more Cu sites, most likely at the interlayer Cu(3,4,5) sites between the kagome planes, undergo the antiferromagnetic phase transition in a fairly conventional way. On the other hand, the ${}^{63}\mathrm{Cu}$ NQR signal intensity is gradually wiped out below $\sim 30$ K, pointing toward gradual spin freezing of the kagome layers instead. These contrasting findings suggest significant roles played by magnetic frustration effects within the kagome layers.

The ramp reversal memory (RRM) effect that appears in thin films with temperature-driven insulator-metal transitions (IMTs) is a nonvolatile memory effect induced by a simple reversal of temperature ramping from heating to cooling during the phase-coexistence state of the IMT (when both metallic and insulating domains coexist). The memory of specific temperatures can be recorded by this ramp reversal, which appears as a resistance increase around the reversal temperatures. Previous studies showed RRM in $\mathrm{V}{\mathrm{O}}_2, {\mathrm{V}}_2{\mathrm{O}}_3$, and $\mathrm{NdNi}{\mathrm{O}}_3$, indicating it is a general effect in relevant systems. These studies indicate the RRM originates from an increase in the critical temperature around phase boundaries of the coexisting metallic and insulating domains during the temperature ramp reversal. However, the physical mechanism responsible for the $T_{\mathrm{C}}$ increase remains elusive. To enhance our understanding of the effect and provide clues to the underlying physics, it is crucial to understand the role of materials’ properties, such as thickness, grain size, and choice of substrate, which have yet to be explored. We report the RRM properties in $\mathrm{V}{\mathrm{O}}_2$ thin films as a function of these parameters, namely choice of substrate, crystallographic properties, film thickness, and morphology. We find that films’ grain size correlates with the RRM magnitude. The film thickness has a positive effect on the RRM, but at a much lesser extent. Interestingly, thinning films by a wet-etching process had almost no effect on RRM properties, indicating that the grain structure and interaction with substrate defined during deposition determines the RRM features. This was further corroborated by comparing of films grown epitaxially on sapphire with those grown nonepitaxially on $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates, where the latter show a fivefold increase in RRM magnitude. These findings support the hypothesis that strain (or strain gradients) and strain relaxation develop at phase-separated grain boundaries during the ramp reversal process, which controls the magnitude of the RRM effect.

Short-range order (SRO) is predicted in Cr- or Mn-containing face-centered-cubic (fcc) concentrated solid-solution alloys. One possible reason for SRO is the magnetic frustration of parallel spin pairs. The presence of SRO is expected to improve physical and mechanical properties. However, changes in atomic and magnetic structures induced by SRO are not clear. In the current study, the SRO in fcc CrCoNi and MnCoNi medium-entropy alloys was investigated using first-principles-based Monte Carlo simulations. In the initial stage of SRO, ${L1}_2$-type ordering occurs by the formation of second nearest-neighbor (NN) Cr-Cr or Mn-Mn bonds with decreasing first NN Cr-Cr or Mn-Mn bonds. These SROs originate from the energy gain caused by the decrease in the number of Cr-Cr or Mn-Mn parallel pairs. After the initial stage of SRO in MnCoNi, Mn-rich and Mn-poor layers were formed along one of the $\langle 100\rangle$ directions, leading to $L{1}_0$-type ordering. Antiparallel Mn-Mn pairs were formed in the Mn-rich layers. In contrast, in CrCoNi, the occupation of Cr atoms on the {110} planes in every three layers was promoted after the initial stage of SRO. The difference in the SRO after ${L1}_2$-type ordering was considered to arise from the lower magnetic moment of the Cr atoms in CrCoNi compared with that of the Mn atoms in MnCoNi. The energy gain owing to the formation of SRO suggests that MnCoNi possesses a larger driving force for SRO formation than CrCoNi.

Modulation of electronic properties of materials by electric fields is central to the operation of modern semiconductor devices, providing access to complex electronic behaviors and greater freedom in tuning the energy bands of materials. Here, we explore one-dimensional superlattices induced by a confining electrostatic potential in monolayer ${\mathrm{MoS}}_2$, a prototypical two-dimensional semiconductor. Using first-principles calculations, we show that periodic potentials applied to monolayer ${\mathrm{MoS}}_2$ induce electrostatic superlattices in which the response is dominated by structural distortions relative to purely electronic effects. These structural distortions reduce the intrinsic band gap of the monolayer substantially while also polarizing the monolayer through piezoelectric coupling, resulting in spatial separation of charge carriers as well as Stark shifts that produce dispersive minibands. Importantly, these minibands inherit the valley-selective magnetic properties of monolayer ${\mathrm{MoS}}_2$, enabling fine control over spin-valley coupling in ${\mathrm{MoS}}_2$ and similar transition-metal dichalcogenides.

$\alpha \text{−}{\mathrm{RuCl}}_3$, a well-known candidate material for Kitaev quantum spin liquid, is prone to stacking disorder due to the weak van der Waals bonding between the honeycomb layers. After a decade of intensive experimental and theoretical studies, the detailed correlation between stacking degree of freedom, structure transition, magnetic, and thermal transport properties remains unresolved. In this work, we reveal the effects of a small amount of stacking disorder inherent even in high quality $\alpha \text{−}{\mathrm{RuCl}}_3$ crystals. This small amount of stacking disorder results in the variation of the magnetic ordering temperature, and it suppresses the structure transition and thermal conductivity. Crystals with a minimal amount of stacking disorder have a $T_N>7.4\mathrm{K}$ and exhibit a well-defined structure transition around 140 K upon cooling. For those with more stacking faults and a $T_N$ below 7 K, the structure transition occurs well below 140 K upon cooling and is incomplete, manifested by the diffuse streaks and the coexistence of both high-temperature and low-temperature phases down to the lowest measurement temperature. Both types of crystals exhibit oscillatory field-dependent thermal conductivity and a plateaulike feature in thermal Hall resistivity in the field-induced quantum spin liquid state. However, $\alpha \text{−}{\mathrm{RuCl}}_3$ crystals with a minimal amount of stacking disorder have a higher thermal conductivity that pushes the thermal Hall conductivity to be closer to the half-integer quantized value. These findings demonstrate a strong correlation between layer stacking, structure transition, magnetic, and thermal transport properties, underscoring the importance of interlayer coupling in $\alpha \text{−}{\mathrm{RuCl}}_3$ despite the weak van der Waals bonding.

We report magnetization and neutron diffraction studies on crystal and magnetic structures of Ir- and Rh-substituted honeycomb lattice $\alpha \text{−}{\mathrm{RuCl}}_3$. The iridium or rhodium atoms are distributed at the Ru site with little structural modification. Both systems undergo a room-temperature monoclinic $C2/m$ to low-temperature trigonal $R\overline{3}$ phase transformation with a large recoverable hysteresis. At low temperature, a zigzag spin order is observed with the same characteristic wave vector (0,0.5,1) as in the parent $\alpha \text{−}{\mathrm{RuCl}}_3$. Detailed magnetic structure refinement reveals an ordered moment of $0.32\left(5\right){\mathrm{\mu }}_{\mathrm{B}}/\mathrm{Ru}$ and an upper boundary of canting angle of $15{\left(4\right)}^{\circ }$ away from the basal plane at 5 K for the 10% Ir-substituted $\alpha \text{−}{\mathrm{RuCl}}_3$, which is different from the 0.45–0.73 ${\mathrm{\mu }}_{\mathrm{B}}/\mathrm{Ru}$ and ${32}^{\circ }\text{–}{48}^{\circ }$ canting angle reported in the parent compound $\alpha \text{−}{\mathrm{RuCl}}_3$. The observation of unchanged ${\mathrm{RuCl}}_6$ local octahedral environment, reduced ordered magnetic moment size and canting angle compared to previously reported highlights the potential to study quantum spin-liquid behavior through nonmagnetic ion doping.