$\alpha \text{−}{\mathrm{RuCl}}_3$ is a candidate Kitaev material that exhibits zigzag antiferromagnetic ordering below 7 K. One contentious issue regarding this material is its bulk structure in the low-temperature phase. Recently, it has become generally accepted from experiments that the low- and high-temperature structures belong to the $R\overline{3}$ and $C2/m$ space groups, respectively. However, there was no theoretical study supporting the $R\overline{3}$-type structure as the low-temperature structure. In this study, we use constrained density functional theory to show that the $R\overline{3}$ structure is lower in energy than the $C2/m$ structure, in agreement with experimental observations. Then, we show that the conduction band minimum states are almost of the $J_{\mathrm{eff}}=1/2$ and $m_{\mathrm{eff}}=-1/2$ character, if we set the angular momentum quantization axis to be parallel to the Néel vector; this is the first analysis of the $J_{\mathrm{eff}}$ picture for $\alpha \text{−}{\mathrm{RuCl}}_3$ from this perspective. In addition, we compute the anisotropic magnetic exchange parameters and $g$ factors of monolayer $\alpha \text{−}{\mathrm{RuCl}}_3$, thereby providing a comprehensive understanding of its magnetism. Our results demonstrate that both second-nearest-neighbor exchange interactions and magnetic moments not captured by the conventional atomic orbital projection method are necessary for accurate description of the magnetism in $\alpha \text{−}{\mathrm{RuCl}}_3$. Moreover, the calculated $g$ factors are in fairly good agreement with experimental measurements, especially the small anisotropy between their in-plane and out-of-plane components. Finally, we examine the effects of structural distortions from a perfect ${\mathrm{RuCl}}_6$ octahedron, already present in bulk $\alpha \text{−}{\mathrm{RuCl}}_3$ without any external perturbation, on the magnetic properties. Our results indicate that relative twist of the upper and lower Cl triangles in the ${\mathrm{RuCl}}_6$ octahedron significantly influences the magnetic anisotropy, suggesting a novel way of engineering the magnetism in this class of materials.

Superconductivity in layered metal borides originates from strong coupling between $\sigma$-bonding electrons and in-plane phonon modes. Herein, we demonstrate that layer-resolved $\sigma$-orbital occupancy governs the superconducting behavior of two-dimensional C ($M=\mathrm{Mg}$, Al, Ca, Sc, Y, and In). The inequivalent chemical environments of the threefold-coordinated ${\mathrm{B}}_1$ layer and sixfold-coordinated ${\mathrm{B}}_2$ layer induce different degrees of charge transfer, resulting in layer-dependent $\sigma$-orbital filling. Partially filled ${\mathrm{B}}_1 \sigma$ orbitals provide the fundamental electron-phonon coupling across the designed structures, whereas filling states of ${\mathrm{B}}_2 \sigma$ orbitals dictates the variation of $T_{\mathrm{c}}$. When ${\mathrm{B}}_2 \sigma$ orbitals remain partially filled, both boron layers contribute cooperatively to electron-phonon coupling, yielding $T_{\mathrm{c}}$ values above 30 K in $\mathrm{Ca}{\mathrm{B}}_3, \mathrm{In}{\mathrm{B}}_3$, and $\mathrm{Mg}{\mathrm{B}}_3$. In contrast, saturation of the ${\mathrm{B}}_2 \sigma$ orbital drives charge redistribution toward ${\mathrm{B}}_1$ layers, suppressing ${\mathrm{B}}_2$ contribution and lowering $T_{\mathrm{c}}$ below 15 K in $\mathrm{Y}{\mathrm{B}}_3, \mathrm{Sc}{\mathrm{B}}_3$, and $\mathrm{Al}{\mathrm{B}}_3$. These results identify orbital-selective $\sigma$-band occupancy, mediated by layer-specific charge transfer, as a microscopic principle for tuning superconductivity in boride materials.

Monolayer Fe(Te,Se) film grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrate has attracted considerable interest due to its interface-enhanced high-temperature (high-$T_c$) superconductivity and potential topological electronic properties. While electron doping is widely considered a key factor in enhancing superconductivity, experimental studies on its effects in monolayer Fe(Te,Se)/STO systems remain limited. In this work, utilizing scanning tunneling microscopy/spectroscopy and quasiparticle interference method, we investigated the modulation of the electronic structure and superconductivity of monolayer Fe(Te,Se)/STO induced by the lithium-impurities-induced electron doping. After depositing lithium clusters of varying sizes on the film surface, we detected the reduction of the effective mass and suppressed superconductivity, which can be attributed to the excessive electron doping. Besides, in-gap bound states induced by nonmagnetic lithium clusters were observed, supporting sign-reversing pairing symmetry. These findings demonstrate how electronic structure and superconductivity in monolayer Fe(Te,Se) film respond to electron doping and local impurities, which provides evidence of the unconventional pairing in monolayer Fe(Te,Se) film, offers insights for studying the iron-based high-$T_{\mathrm{c}}$ superconductivity.

Exceptional points (EPs) are spectral degeneracies unique to non-Hermitian systems which underpin phenomena from enhanced sensing to unconventional topology. While disorder is usually viewed as detrimental, it can also drive topological phase transitions (TPTs). Here, we show that random disorder alone can generate EPs and concurrent TPTs in a multiorbital non-Hermitian lattice with nonreciprocal hopping. Increasing disorder induces successive real-complex-real spectral transitions accompanied by band inversion and quantized changes in the spin Bott index. Using effective medium theory and large-scale simulations, we trace these transitions to a competition between disorder-induced energy-level renormalization and nonreciprocity-driven hybridization. The resulting phase diagram reveals extended EP lines that emerge from the Hermitian TPT point and persist over a broad parameter range. Our results establish disorder as an active mechanism for engineering exceptional point mediated topology in non-Hermitian matter.

Comprehending lattice dynamics and thermal conductivity is crucial for energy conversion devices. This work examines thermal transport in $\mathrm{CaSn}{\mathrm{S}}_3$, a member of the chalcogen perovskite family, which is increasingly favored in energy applications. The temperature-dependent phonon frequency renormalization including loop and bubble diagram framework, in conjunction with higher-order four-phonon scattering and the wavelike contribution of phonon transport, predicts an ultralow lattice thermal conductivity (${\kappa }_l)$ of $\sim 0.75\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K and ∼0.63 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 700 K. Weak bonding and bond inhomogeneity are significant structural characteristics that introduce anharmonicity to the system, with an estimated average Grüneisen parameter of ∼1.92. Strongly anharmonic phonon branches below 60 $\mathrm{c}{\mathrm{m}}^{-1}$ determine the weightage of particle and wavelike phonon characteristics. At room temperature, the particlelike propagation ${\kappa }_l^{\mathrm{p}}$ is $\sim 0.4 \mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, while the estimated wavelike coherent contribution ${\kappa }_l^{\mathrm{c}}$ is $\sim 0.35 \mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. At 300 K, the wavelike component accounts for $\sim 46\%$ of the total ${\kappa }_l$, rising to 56% at 700 K, underscoring the impact of vibrational state coupling and interbranch phonon tunneling in $\mathrm{CaSn}{\mathrm{S}}_3$, also comparable to glasses. The high density of closely spaced phonon branches makes it favorable for wavelike tunneling by providing a larger phonon linewidth than the interbranch spacing. Hence, guided by strong anharmonicity in $\mathrm{CaSn}{\mathrm{S}}_3$, the interplay of higher-order anharmonic self-energies and wavelike tunneling pathways results in a minimal temperature dependence $\sim T^{-0.20}$. This work not only quantifies lattice thermal conductivity but also highlights the potential discrepancies while accounting for three-phonon scattering only and neglecting bubble correction in phonon self-energy, emphasizing the necessity of adopting the right approach. Overall, while emphasizing lattice anharmonicity and glasslike thermal conduction, this study will help to improve the understanding of chalcogen perovskite design for multiple technological applications.

Altermagnetism (AM) has brought renewed attention to the Lieb lattice. Here, we broaden the scope of altermagnetic models on the Lieb lattice by using a general scheme based on spin clusters. We design various altermagnetic models with $d$ and $g$ wave on the Lieb lattice and investigate its interplay with spin-orbit coupling. While the altermagnetic unit cell reconstructs the topological edge states in the strip geometry and leads to the emergence of Dirac points, the in-plane magnetic moments of AM can induce gaps at these points. In an open square geometry, corner modes emerge within these gaps, realizing higher-order topological states. We further verify that the induction of higher-order topology is applicable to all altermagnetic configurations constructed here on the Lieb lattice and is most pronounced for AM by comparing with the other types of magnetism such as ferromagnetism and ferrimagnetism. Our results highlight the exotic properties of AM and suggest its potential applications in engineering topological quantum states.

We investigate the magnetic behavior of the $6H$-perovskite dimer lattice ${\mathrm{Ba}}_3{\mathrm{Zn}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Ru}}_2{\mathrm{O}}_9$ using analytical theory, density functional theory, inelastic neutron scattering, and modeling of historical magnetization and neutron-scattering data. A dimer mean-field theory built upon classical Luttinger–Tisza analysis generates a phase diagram revealing a transition from a nonmagnetic singlet to a finite-moment ground state as interdimer couplings increase. A (generalized) linear spin-wave theory captures multiplet mixing, excitation gap closing, and fluctuation-induced moment suppression. Density-functional-theory calculations on selected compounds, together with neutron spectroscopy of dilute ${\mathrm{Ba}}_3\mathrm{Zn}{\left({\mathrm{Ru}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{O}}_9$, confirm the exchange hierarchy, enabling quantification of previously published experiments within this framework. Our results identify three mechanisms for magnetic moment suppression—quantum fluctuations, ligand hybridization, and nonmagnetic-singlet/magnetic-multiplet mixing.

The coherence times of superconducting qubits are limited by loss mechanisms, whose microscopic origins have remained elusive. We propose a mechanism caused by transitions between zero-field-split states of paramagnetic impurities or defects. We derive the absorption cross section for a magnetic dipole transition and apply it to calculate the bulk loss tangent. For Cr, Fe, and V impurities in sapphire, we find loss tangents at 4.5 GHz in the range of ${10}^{-9}–{10}^{-8}$, comparable to the loss measured in experiments. This value suggests that magnetic loss may be a limiting factor in the coherence times of superconducting qubits.

The ideal Weyl "hydrogen-atom" semimetal exhibits only a single pair of Weyl nodes and no other trivial states at the Fermi energy. Such a material would be a panacea in the study of Weyl quasi-particles, allowing direct unambiguous observation of their topological properties. The alluaudite-like ${\mathrm{K}}_2{\mathrm{Mn}}_3{\left({\mathrm{AsO}}_4\right)}_3$ compound was recently proposed as such a material. Here, we use comprehensive experimental work and first-principle calculations to assess this prediction. We find ${\mathrm{K}}_2{\mathrm{Mn}}_3{\left({\mathrm{AsO}}_4\right)}_3$ crystallizes in the $C2/c$ symmetry with a quasi-one-dimensional Mn sublattice, growing as small needle-like crystals. Bulk property measurements reveal magnetic transitions at $\approx 8$ and $\approx 4$ K, which neutron scattering experiments show correspond to two distinct magnetic orders, first a partially ordered ferrimagnetic ${\mathbf{k}}_{\mathbf{1}}=(0,0,0)$ structure at 8 K and a second transition of ${\mathbf{k}}_{\mathbf{2}}=(1,0,0)$ at 4 K to a fully ordered state. Below the second transition, both ordering vectors are necessary to describe the complex magnetic structure with modulated spin magnitudes. Both of the best-fit magnetic structures in this work are found to break the symmetry necessary for the generation of Weyl nodes, though one of the magnetic structures allowed by ${\mathbf{k}}_{\mathbf{1}}$ does preserve this symmetry. However, the crystals are optically transparent and ellipsometry measurements reveal a large band gap, undermining expectations of semimetallic behavior. Density functional theory calculations predict an insulating antiferromagnetic ground state, in contrast to previous reports, and suggest potential frustration on the magnetic sublattice. Given the wide tunability of the alluaudite structure, we consider ways to push the system closer to a semimetallic state.

Antiferromagnetic ground states, when doped, give rise to rich and complex phenomena, prompting detailed investigations in various spin systems. Here, we study the effect of doping on the one-dimensional $S=1$ antiferromagnetic Heisenberg model. Specifically, we investigate how the presence of holes affects the static and dynamic (frequency-dependent) spin-spin correlations of the two-orbital Hubbard-Kanamori chain. The latter, at half filling and in the strong-interaction limit, maps onto an $S=1$ Heisenberg model. For moderate interactions, an orbital resonating valence bond state emerges up to doping levels of $x\lesssim 0.3$. A detailed analysis of interaction strength $U$ and doping concentration $x$ reveals that this phase inherits the key features of spin excitations found in the half-filled case, namely, a gapped spin spectrum and “coherent” magnon behavior up to a wave vector $q$ determined by the Fermi vector, $2k_{\mathrm{F}}=\pi (1-x)$. Furthermore, our results uncover an additional broad, incoherent spectral weight for $q\gtrsim 2k_{\mathrm{F}}$ at high frequencies. Finally, we show that near the transition to a ferromagnetic phase, a previously unidentified spiral-like state emerges, characterized by spin excitations reminiscent of the $J_1\text{−}{J}_2$ Heisenberg model.

We present a comprehensive study of phase fluctuation effects in ultrathin Pb films using combined scanning tunneling microscopy and two-coil mutual inductance measurements. In atomically thin films (two to four monolayers), we observe a pronounced separation between the Cooper pair formation temperature ($T_{\mathrm{\Delta }}$) and the macroscopic superconducting transition temperature ($T_{\mathrm{c}}$), signaling the emergence of incoherent Cooper pairs and a pseudogap regime above $T_{\mathrm{c}}$. The superfluid stiffness $J_{\mathrm{n}}$($T$) exhibits distinct scaling behaviors: a Berezinskii-Kosterlitz-Thouless (BKT)-like transition with renormalized vortex dynamics in 2-ML films, and a crossover from quantum to classical phase fluctuations in 4-ML films. Thicker films (7 to 11 ML) adhere to conventional Bardeen-Cooper-Schrieffer theory, demonstrating negligible phase fluctuations despite their subcoherence-length thickness. Our results establish the dominance of phase fluctuations in low-dimensional superconductors and provide a clean experimental platform to explore BKT and incoherent pairing physics in highly crystalline systems.

The kagome lattice, known for its geometric frustration and flat bands, provides a fertile ground for emerging correlated electronic phases. Using a self-consistent mean-field approach within the extended Hubbard model, we compute the phase diagrams of the kagome lattice as a function of on-site and nearest-neighbor interactions for various fillings of the flat band. We report a rich variety of phases—including charge-density waves (CDW), ferromagnetism (FM), and spin-charge density waves with ferro- and antiferromagnetic character (SCDW-FM, SCDW-AFM)—whose phases are critically dependent on electron filling. At very low fillings, the high density of states fosters a complex competition between phases, with SCDW-AFM being prominent. As the filling increases, the ferromagnetic correlations strengthen, culminating in the SCDW-FM phase dominating the phase diagram at high fillings. We characterize each phase through its real-space charge and spin localization properties and local density of states, linking them to the underlying mechanisms of geometric frustration and Stoner instability. Our results show how the interplay between interactions and filling governs the emergence of diverse orders on the kagome lattice, offering insights into the phase competition seen in related materials.

Lattice models are powerful tools for studying strongly correlated quantum many-body systems, but their general lack of exact solutions motivates efforts to simulate them in tunable platforms. Recently, a promising new candidate has emerged for such platforms from two-dimensional materials. A subset of moiré systems can be effectively described as a two-dimensional electron gas (2D EG) subject to a moiré potential, with electron-electron interactions screened by nearby metallic gates. In this paper, we investigate the realization of lattice models in such systems. We show that, by controlling the gate separation, a 2D EG in a square moiré potential can be systematically tuned into a system whose ground state exhibits orders analogous to those of the square lattice Hubbard model, including the stripe phase. Furthermore, we study how variations in gate separation and moiré potential depth affect the ground-state orders. A number of antiferromagnetic phases, as well as a ferromagnetic phase and a paramagnetic phase, are identified. We then apply our quantitative downfolding approach to triangular moiré systems closer to current experimental conditions, compare them with the square lattice parameters studied, and outline routes for experimental realization of the phases.

We compute valley splitting in Si/SiGe superlattices using ab initio density functional theory (DFT). This first-principles approach is expected to provide an excellent description of interfaces, strains, and atomistic disorder without empirically fitted parameters. We benchmark atomistic tight-binding (TB) and the “$2k_0$” theory within the effective mass (EM) approximation against DFT. We show that DFT supports the main conclusions of the $2k_0$ theory, but reveals some limitations of semiempirical methods such as the EM and TB, in particular about the description of atomistic disorder. The DFT calculations also highlight the effects of strong valley-orbit mixing at large valley splitting. Nevertheless, TB and the $2k_0$ theory shall provide reasonable valley splitting statistics in many heterostructures of interest for spin qubit devices.

Hydrocarbon contamination is an ever-present factor to consider in surface science measurements. In the case of van der Waals material surfaces, the structure of this contamination has become known in recent years as a self-assembled layer of normal-alkanes, resulting from a few days' exposure to ambient air. Knowledge of its composition and structure enables systematic investigation of its influence on surface properties. Here, we investigate the effect of this contamination on scanning tunneling microscopy (STM) and spectroscopy measurements by comparing clean and ambient alkane-contaminated surfaces of graphite. Our results reveal that the ambient alkane layer suppresses the well-known phonon-induced gap near the Fermi energy, resolving a long-standing inconsistency in STM studies, where this feature is often absent. Furthermore, we show that the presence of the contamination layer alters the current-distance ($I\left(z\right)$) characteristics, flattening its exponential decay by a factor of 1.5 to 5 compared to the clean surface. This change arises from extra conductance channels through the alkane layer alongside the tunnel junction, as the tip penetrates the contaminant overlayer. Finally, based on the $I\left(z\right)$ characteristics, we provide a practical guide to detect the presence of surface contamination in STM measurements.

We report the study of phases and transitions of the $\text{SU}\left(N\right)$ Hofstadter-Hubbard model subject to a commensurate magnetic field on the triangular lattice. At filling one fermion per site, for the number of fermion flavors $2\le N\le 8$, we identify three distinct phases and calculate critical interaction strength from parton large-$N$ mean-field approximation. Integer quantum Hall, chiral spin liquid, and valence bond solid states could be realized upon varying the Hubbard interaction $U$ and the number of flavor $N$. We construct the critical theory for the putative continuous transition from quantum Hall states to chiral spin liquid and calculate the critical transport behavior using quantum Boltzmann equations for general $N$. These results could be validated in synthetic systems such as moiré superlattices and cold atom platforms.