In this study, we use a full tight-binding model to explore the behavior of graphene nanobubbles between the interface of twisted bilayer graphene (TBG-$1.{05}^{\circ }$), uncovering the simultaneous existence of confined states and pseudo-Landau level (pLL) states under minor strain conditions. We find that the energy separation of the confined states aligns with a linear $N$ rule, while pLL states follow a typical $\sqrt{N}$ rule. As the height of the bubble and the local strain escalate, the low-energy segment of the local density of states transforms from confined states to pLL states. Our findings establish that the effect of a perpendicular magnetic field can effectively distinguish between confined and pLLs. Our results offer a crucial stepping stone towards the strain-engineering of electronic states in graphene-based nanostructures.

Topologically protected boundary transport is a promising route to realize robust quantum manipulation between distant nodes. Conventional topological transports require a long transmission time to meet adiabatic evolution, which unfortunately becomes a significant obstacle for practical quantum systems with decoherence. Here, we report a fast and robust phonon transfer by breaking this adiabatic limitation in a one-dimensional nanomechanical topological interface lattice. The high-fidelity nonadiabatic topological transfer (NTT) can be predicted accurately via the localized mode and bulk levels. A dynamical method is then put forward to characterize the nonadiabatic oscillation of the NTT according to the chiral symmetry, and the oscillation of the instantaneous adiabaticity is measured by the phonon population on the even nanomechanical resonators. Furthermore, we confirm the robustness under various noises and the scalability of the NTT. Our results open the door to accelerating topological transport, which is valuable for developing fast and robust quantum information transfer protocols.

Non-Hermitian systems exhibit diverse graph patterns of energy spectra under open boundary conditions. Here, we present an algebraic framework to comprehensively characterize the spectral geometry and graph topology of non-Bloch bands. Using a locally defined potential function, we unravel the spectral-collapse mechanism from Bloch to non-Bloch bands, delicately placing the spectral graph at the troughs of the potential landscape. The potential formalism deduces the non-Bloch band theory and generates the density of states via the Poisson equation. We further investigate the Euler-graph topology by classifying spectral vertices based on their multiplicities and projections onto the generalized Brillouin zone. Through concrete models, we identify three elementary graph-topology transitions (UVY, $\mathcal{P}\mathcal{T}$-like, and self-crossing), accompanied by the emergence of singularities in the generalized Brillouin zone. Lastly, we unveil how to generally account for isolated edge states outside the spectral graph. Our work lays the cornerstone for exploring the versatile spectral geometry and graph topology of non-Hermitian non-Bloch bands.

We introduce a model compound featuring a spin-1/2 frustrated square lattice partially coupled by spin-5/2. A significant magnetization plateau exceeding 60 T could be observed, indicating a quantum state formed by $S=1/2$ spins in the square lattice. The remaining $S=5/2$ spins exhibited paramagnetic behavior in the low-field regions. The numerical analysis confirmed that the observed quantum state is a many-body entangled state based on the dominant antiferromagnetic interactions and is strongly stabilized by coupling with spin-5/2. The stabilization of this quantum state can be attributed to a compensation effect similar to magnetic field-induced superconductivity, which serves as a strategy to control the stability of quantum spin states in magnetic fields.

The electric field of an optical vortex (OV) beam displays a specific winding pattern determined by its spatial variation and angular momentum. An OV beam can serve as a optical spanner, which exerts a force torque to rotate particles around the beam axis. In this work, using first-principles-based calculations, we demonstrate that an OV beam with high orbital angular momentum (OAM) can induce local electric dipoles to self-order and form a vortex array distributed along azimuthal directions. More importantly, owing to its intrinsic symmetry, the evolution of the vortex array with time can be viewed as an effective spinning around the central axis, while its rotation speed is also connected to the OAM value. Our study thus presents an alternative approach for manipulating polar vortices using the optical field apart from mechanical and electric stimuli, and especially for a deterministic control of vortex dynamics that is critical in the application of memory devices.

Spin flips are one of the limiting factors for spin-based information processing. We demonstrate a transport approach for determining the spin-flip rates of a self-assembled InAs double quantum dot occupied by a single electron. In such devices, different Landé factors lead to an inhomogeneous Zeeman splitting, so that the two spin channels can never be at resonance simultaneously, leading to a spin blockade at low temperatures. This blockade is analyzed in terms of spin flips for different temperatures and magnetic fields. Our results are in good agreement with a quantum master equation that combines the dot-lead couplings with ohmic dissipation stemming from spin-flip cotunneling.

Recently, Grockowiak et al. reported “hot superconductivity” in ternary or multinary compounds based on lanthanum hydride [A. D. Grockowiak et al., Front. Electron. Mater. 2, 837651 (2022)]. In this paper, we explored thermodynamically stable phases and superconducting phases in the lanthanum-nitrogen-hydrogen system (${\mathrm{La}}_x{\mathrm{N}}_y{\mathrm{H}}_{1-x-y}, 0\le x\le 1, 0\le y\le 1$) at a pressure of 20 GPa. We rapidly and accurately constructed the formation-enthalpy convex hull using an evolutionary construction scheme based on density functional theory calculations, extracting the candidates for stable and moderately metastable compounds by the universal neural network potential calculations. The convex hull diagram shows that more than 50 compounds emerge as stable and moderately metastable phases in the region of $\mathrm{\Delta }H\le 4.4$ mRy/atom. In particular, the compounds are concentrated on the line of $x=0.5$ connecting between LaH and LaN. We found that the superconductivity is gradually enhanced due to N doping for LaH and the superconducting critical temperature $T_{\mathrm{c}}$ reaches 8.77 K in ${\mathrm{La}}_2\mathrm{NH}$ with $y=0.25$. In addition, we predicted that metastable ${\mathrm{La}}_2{\mathrm{NH}}_2$ shows the highest $T_{\mathrm{c}}$ value, 14.41 K, of all the ternary compounds predicted in this study. These results suggest that it is difficult to obtain the hot superconductivity in the La-H compounds with N at 20 GPa.

We propose to implement fast and robust quantum state transfer in symmetrical beam splitters and routers through a specific topological edge channel in splicing Su-Schrieffer-Heeger–like models, for which a small admixture of nonadiabatic character into evolution facilitates the accelerated adiabatic pumping process to a great extent. Through optimizing coupling engineering, we develop the acceleration capacity for the symmetrical beam splitting process, and we investigate robustness enhancement against disorders and environment-induced losses. Furthermore, the model manifests good scalability and can be extended to a crossed-chain configuration of realizing a topological router with tunable numbers of output ports. Finally, we discuss the experimentally feasible implementation in quantum optomechanical systems by alternately assembling optical cavities and mechanical oscillators, enabling topologically protected splitters and routers of photons or phonons. This work provides a typical example of a functional quantum device, which could motivate further research into efficient quantum information processing and the construction of large-scale quantum networks.

We investigate the magnetic properties of the monoclinic D-type ${\mathrm{Er}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$ with a distorted honeycomb lattice using powder and single-crystal neutron scattering techniques, as well as single-crystal magnetization measurements. The powder neutron diffraction shows that below the ordering temperature, $T_{\mathrm{N}}=1.85\mathrm{K}$, the compound forms a $\mathbf{q}=\mathit{0}$ antiferromagnetic structure with four sublattices. For $H\parallel a$, magnetization measurements reveal a narrow but clearly visible plateau at one-third of the magnetization saturation value. The plateau's stabilization is accompanied by a significant increase of the magnetic unit cell, as the magnetic peaks with fractional indices are observed in single-crystal neutron diffraction experiments. At low temperatures, the inelastic neutron scattering measurements reveal the presence of low-energy, almost dispersionless excitations. Their spectrum is sensitive to the applied field, it significantly softens on the magnetization plateau, and demonstrates the behavior expected for a noncollinear Ising antiferromagnet away from the plateau.

Determining the melting curves of materials up to high pressures has long been a challenge experimentally and theoretically. A large class of materials, including most metals, has been shown to exhibit hidden scale invariance, an approximate scale invariance of the potential-energy landscape that is not obvious from the Hamiltonian. For these materials the isomorph theory allows the identification of curves in the phase diagram along which structural and dynamical properties are invariant to a good approximation when expressed in appropriately scaled form. These curves, the isomorphs, can also be used as the basis for constructing accurate melting curves from simulations at a single state point [U. R. Pedersen et al., Nat. Commun. 7, 12386 (2016)]. We here apply this method to the metals Cu simulated using the effective medium theory and Al simulated using density functional theory (DFT). For Cu the method works very well and is validated using two-phase melting point simulations. For Al there are likewise good isomorphs, and the method generates the melting curve accurately as compared to previous experimental and DFT results. In support of a recent suggestion of Hong and van de Walle [Phys. Rev. B 100, 140102 (2019)], we finally suggest that the tendency for the density-scaling exponent $\gamma$ to decrease with increasing density in metals implies that metals in general will undergo reentrant melting, i.e., have a maximum of melting temperature as a function of pressure.

Symmetry often governs the laws of nature, and breaking symmetry accompanies an alternative order parameter and emergent observable phenomena. Herein, we focus on broken parity (P)-time (T) symmetry, which lifts the Kramers’ degeneracy, and thus, guarantees nontrivial spin textures in excitation spectra. To attain nonzero measurables, we use the concept of symmetry operational similarity (SOS), which considers the symmetry relationship between a specimen and an experimental setup rather than the symmetry of specific coupling terms. Even without specific coupling terms, this SOS approach can tell if the relevant phenomenon is a zero, nonzero odd-order, or nonzero even-order effect. We discuss systematically numerous steady-state physical phenomena, in which breaking P-T symmetry is a necessary condition. These phenomena include odd-order or even-order anomalous Hall effect, optical activities, directional nonreciprocity in transverse magnetic field, diagonal or off-diagonal current-induced magnetization (current can be associated with electrons, phonons, or light), diagonal or off-diagonal piezomagnetism, and piezoelectricity. Some of these phenomena turn out to be conjugate to each other through P ↔ T. Our findings unveil numerous nontraditional candidate materials for various exotic physical phenomena, many of which have never been realized in the standard coupling term/tensorial approaches and are a transformative and unconventional avenue for symmetry-guided materials designs and discoveries.

Much of the rich physics of correlated systems is manifested in the diverse range of intertwined ordered phases and other quantum states that are associated with different electronic and structural degrees of freedom. Here we find that ${\mathrm{PrCuSb}}_2$ exhibits such phenomena, which at ambient pressure exhibits a fragile antiferromagnetic order, where cooling in a small $c$ axis magnetic field leads to an additional transition to a field-induced ferromagnetic state. This corresponds to an inverse melting effect, whereby further cooling the system restores symmetries of the paramagnetic state broken at the antiferromagnetic transition. Moreover, hydrostatic pressure induces an additional first-order transition at low temperatures, which despite being not likely associated with solely magnetic degrees of freedom, is closely entwined with the magnetic order, disappearing once antiferromagnetism is destroyed by pressure or magnetic fields. Consequently, ${\mathrm{PrCuSb}}_2$ presents a distinct scenario for interplay between different orders, underscoring the breadth of such behaviors within one family of correlated materials.

We present a comprehensive study revealing the intricate interplay of the magnetic anisotropy and orthorhombic distortion in thin films of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ through a $\mathrm{C}{\mathrm{a}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ substrate. By inducing a pronounced orthorhombic distortion along the direction of oxygen octahedral edges, we effectively modulated the uniaxial magnetic anisotropy in the system. Remarkably divergent responses along the easy and hard magnetic axes were unveiled through x-ray magnetic circular dichroism (XMCD) measurements under magnetic fields. Specifically, the spin flop transition observed when the magnetic field aligns with the hard axis allows us to estimate the magnetic anisotropy energy, which is around 14.2 µeV, close to that estimated from the single magnon peak measured via Raman spectroscopy. The observed anisotropy energy remains notably lower than the linear estimates derived from the strain-anisotropy energy relationship outlined in H.-H. Kim et al., Nat. Commun. 13, 6674 (2022). This underscores the enduring preservation of the isotropic character of the $J_{\mathrm{eff}}=1/2$ states. This is also supported by the negligible XMCD intensity ratio at the $L_2$ edge compared to that of the $L_3$ edge. Furthermore, the branching ratio determined from x-ray absorption spectroscopy shows that the expectation value of the spin-orbit coupling is similar to that of bulk $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ single crystals. Our findings indicate that even under a substantial anisotropic biaxial distortion, $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ remains remarkably proximate to the $J_{\mathrm{eff}}=1/2$ state. This study not only provides valuable information in understanding the interplay between magnetic anisotropy and strain but also the robustness of the $J_{\mathrm{eff}}=1/2$ state under octahedral distortion within materials exhibiting emergent quantum phenomena.

Spectroscopic measurements in quantum systems are subject to selection rules, usually based on space-time symmetries, that allow or disallow transitions between states. In many-body systems, in addition to the single-particle states, there emerge new ones due to collective excitations of the system. Here we demonstrate the existence of a “fragile” selection rule that emerges as a manifestation of many-body effects and outlines the conditions for collective excitations to couple to a given spectroscopic probe beyond the usual symmetry considerations. As an example, we apply the rule to Raman spectroscopy of multiband superconductors and settle some unresolved features in experiments, one of them being the spectral weight distribution between the pair-breaking excitations and the collective modes.

We report an unusual evolution of superconductivity (SC) in the three-dimensional Dirac semimetal PdTe with increasing pressure up to $\sim 50$ GPa. The compressed PdTe exhibits a sudden reversal in the superconducting transition temperature $T_{\mathrm{c}}$, from an initial decrease with pressure to an increase above a critical pressure $P_{\mathrm{c}}\approx$ 15 GPa, showing a V-shaped feature composed of SC-I and SC-II phases due to a structural phase transition through $P_{\mathrm{c}}$. Subsequently, the $T_{\mathrm{c}}$ goes into a plateau around 2.5 K when the pressure is higher than $\sim 32$ GPa. In addition, we find the variations of carrier concentrations and mobilities also manifest a similar trend on the pressure response as the $T_{\mathrm{c}}$, and the normal-state electronic properties change from the electron-dominated single-band model to the two-carrier model after the structural phase transition, implying the close correlation between electronic properties and two SC phases. Our findings establish the SC of PdTe is highly tunable under varying pressures.

We develop a theory of the electronic structure of the two-dimensional monolayer transition-metal dichalcogenides in the 2H structure, with the formula $\mathrm{2H}\text{−}{MX}_2$ under a general strain condition. We focus on the low-energy Hamiltonian around the $K/K^{\prime }$ valley points, which control many electronic properties of the material. The strain Hamiltonian is derived from a combination of symmetry considerations and an effective $d$-orbital tight-binding model, adopting a simplifying approach, where the chalcogen atoms are only implicitly considered, which makes the derivations much simpler. Unlike previous treatments in the literature, our formulation properly describes the electronic structure in the neighborhood of the valley points, including the well-known valley point drift under strain. The strain Hamiltonian is validated from comparison with the density-functional theory calculations, and the total energy is shown to be consistent with the theory of elasticity. The Hamiltonian parameters, including the strain dependence of the spin-orbit coupling strength, are given for several insulating $M{X}_2$ compounds, where $M=\mathrm{Mo}$ or W and $X=\mathrm{S}$, Se, or Te, obtained by fitting with the density-functional calculations, as well as for the metallic counterparts ${\mathrm{NbS}}_2$ and ${\mathrm{NbSe}}_2$. These strain models are not only useful for a fundamental understanding of the electronic structure under strain but also essential for the design of electronic device applications, where strain may already be present or can be tuned by external means.

The nitride perovskite family has attracted widespread attention since the experimental synthesis of polar nitride perovskite $\mathrm{LaW}{\mathrm{N}}_3$. In this work, we investigated the ferroelectric, dielectric, and piezoelectric properties of two nitride perovskites $\mathrm{Ce}B{\mathrm{N}}_3$ ($B^{5+}=\mathrm{Nb}$, Ta) using the first-principles calculation. Our results show that the polarization values of $\mathrm{CeNb}{\mathrm{N}}_3$ (CNN) and $\mathrm{CeTa}{\mathrm{N}}_3$ (CTN) approach those of typical perovskites $\mathrm{LiNb}{\mathrm{O}}_3$ and $\mathrm{BaTi}{\mathrm{O}}_3$, respectively. Interestingly, the $A$-site cation contributes significantly to the polarization of CNN and CTN, despite the lack of lone pair electrons in ${\mathrm{Ce}}^{4+}$. In addition, CTN and CNN exhibit unusually large static dielectric constants (>220) along the $x$ direction, much larger than the $y$ and $z$ directions. This strong anisotropy primarily stems from the dominant infrared-active mode having a lower frequency or larger mode-effective charges along the $x$ direction relative to the other two directions. Finally, we found both CTN and CNN have a comparatively large shear piezoelectric coefficient $d_{24}$ over 70 $\mathrm{pC}/\mathrm{c}{\mathrm{m}}^2$. Our research provides insights for understanding the origin of ferroelectricity in nitride perovskites and their potential piezoelectric applications.

The theoretical calculation of the interaction of electromagnetic radiation with matter remains a challenging problem for contemporary ab initio electronic structure methods, in particular, for x-ray spectroscopies. This is not only due to the strong interaction between the core hole and the photoexcited electron, but also due to the elusive multiplet effects that arise from the Coulomb interaction among the valence electrons. In this work we report a method based on density functional theory in conjunction with multiplet ligand-field theory to investigate various core-level spectroscopies, in particular, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The developed computational scheme is applied to the $L_{2,3}$ XAS and XMCD edges of magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ as well as cobalt ferrite $\left({\mathrm{CoFe}}_2{\mathrm{O}}_4\right)$ and nickel ferrite $\left({\mathrm{NiFe}}_2{\mathrm{O}}_4\right)$. The results are in overall good agreement with experimental observations, both regarding the XAS $L_2/L_3$ branching ratio, the peak positions, as well as the relative intensities. The agreement between theory and experiment is equally good for XAS and the XMCD spectra, for all studied systems. The results are analyzed in terms of $e_g$ and $t_{2g}$ orbital contribution, and the robustness of the spectra with regard to the uncertainties of the Slater parameters is investigated. The analysis also highlights the strong effect of the $2p\text{−}3d$ interaction in x-ray spectroscopy.

Non-Abelian topological charges have recently been proposed to characterize the noncommutative topological properties in multiple-band-gap systems. In this work, we investigate a non-Abelian system based on a three-band tight-binding model characterized by quaternions. By introducing nonreciprocal hopping, the system becomes non-Hermitian yet still preserves $\mathcal{PT}$ symmetry within a range of hopping parameters. As the hopping strength increases, the $\mathcal{PT}$ symmetry is spontaneously broken. We find that the critical hopping values of $\mathcal{PT}$ transitions for quaternion charges $\{\pm i,\pm k,-1\}$ are different from that of the $\pm j$ case. Non-Hermitian quaternion topological charges are analytically analyzed using the Berry-Wilczek-Zee (BWZ) phase. By diagonalizing the BWZ phase matrix, we discover that its eigenvalues undergo a real-to-imaginary $\mathcal{PT}$ transition. This corresponds to the real-to-imaginary transition of the associated eigenvalues of the Hamiltonian. However, the boundary states in the finite system remain robust against random disorder only when the $\mathcal{PT}$ symmetry is preserved. Furthermore, we examine the existence of domain wall states between two subsystems carrying different topological charges to verify the non-Abelian quotient relation in the non-Hermitian situation. It is revealed that domain wall states are no longer robust against random disorder once the $\mathcal{PT}$ symmetry is broken. Our work deepens the physical understanding of the non-Abelian topology in the non-Hermitian system with $\mathcal{PT}$ symmetry.