Metal substitutions have been recognized as an efficient approach for exploring unusual superhydrides, aiming to discover configurations with diverse structures and enhanced superconductivity. However, the roles of the metal type and concentration on high-temperature superconducting (high-$T_{\mathrm{c}}$) hydrides with multiple components remain underexplored in this rapidly advancing domain. In this study, we conducted extensive experiments on the equimolar La-Ce-Y alloy superhydride with the highest configuration entropy among quaternary hydrides under pressure. In situ X-ray diffraction measurements revealed the synthesis of a hexagonal close-packed (hcp) $P{6}_3$/mmc-$\left(\mathrm{La},\mathrm{Ce},\mathrm{Y}\right){{\mathrm{H}}_{10}}_{\pm x}$ ($-0.2<x<+0.4$), which could be stable down to the lowest pressure of 96 GPa. Notably, despite numerous efforts, such an unprecedented hcp decahydride subunit has remained unattainable in corresponding binary hydrides, highlighting the powerful influence of the entropy effect on stability and hydrogen concentration in this multicomponent system. Further electrical transport measurements confirmed its superconductivity, as evidenced by zero resistance and suppression of superconductivity under applied magnetic fields, with a maximum $T_{\mathrm{c}}$ of 165 K. These findings serve as a paradigmatic example, investigating entropy effects in multinary systems and opening up the exciting potential for further exploration of high-$T_{\mathrm{c}}$ alloy superhydrides.

We consider quantum entanglement in strongly correlated quantum impurity systems for states manifesting interesting properties such as the multilevel Kondo effect and dual nature between itineracy and localization. etc. For this purpose, we set up a system consisting of one or two quantum impurities arbitrarily selected from the system as a subsystem and investigate quantum entanglement with its environmental system. We reduce the pure state of interest as described above to the subsystem, and formulate quantum informative quantities such as entanglement entropy, mutual information, and relative entropy. To demonstrate the potential of the method proposed here, we apply them to a dimer with several types of internal interaction, short Hubbard chains, and the single impurity Anderson model to study the relationship between their states and the behaviors of quantum informative quantities. The obtained results suggest that the method proposed here is promising for elucidating the quantum entanglement of pure states in various quantum impurity systems.

As novel topological states, the higher-order topological insulators have attracted great attentions in the past years. However, their realizations in realistic materials, in particular in two-dimensional systems, remains the big challenge due to the lack of adequate candidates. Here, based on the first-principle calculation, we identify the currently existing bilayer ${\alpha }_5$-phase borophenes as the two-dimensional second-order topological insulators, related to the $C_2$-rotational symmetry. The formation of interlayer B-B covalent bonds, stabilizing the bilayer borophenes and opening the large direct bulk gaps ($\sim 0.55-0.62\mathrm{eV}$) at Fermi level, plays the key roles. The second-order topology is characterized by the bulk quantized quadrupole momentum. Our results enriches the candidates for the second-order topological insulators, and also provide a way to study topological states in borophenes.

Optically inactive (dark) intervalley momentum-forbidden excitons are characterized by relatively long lifetime and therefore are desirable candidates for realizing collective excitonic phases. However, testing their coherence by light directly is impossible. Here, we propose a method for detecting dark excitonic condensate. It relies on the interaction between excitons and phonons responsible for the interconversion between bright and dark excitons. If dark condensate forms, the Rabi oscillations between photons and bright excitons can become strongly modified and can be viewed as the photon-exciton-phonon polaritonic effect. The multicomponent nature of dark condensate consistent with the point-group symmetry is considered in the limit of weak phonon-exciton interaction. A perspective for the case of the strong interaction leading to the polaronic effect is discussed.

We consider a two-dimensional electron gas interacting with a quantized cavity mode. We find that the coupling between the electrons and the photons in the cavity enhances the superconducting gap. Crucially, all terms in the Peierls phase are kept, in contrast to more naive approaches, which may result in spurious superradiant phase transitions. We use a mean-field theory to show that the gap increases approximately linearly with the cavity coupling strength. The effect can be observed locally as an increase in the gap size via scanning-tunneling microscopy (STM) measurements for a flake of a two-dimensional (2D) material (or for a moiré system where the enhancement is expected to be more pronounced due to a large lattice constant) interacting with a locally structured electromagnetic field formed by split-ring resonators. Our results are also relevant for quantum optics setups with cold atoms interacting with the cavity mode, where the lattice geometry and system parameters can be tuned in a vast range.

Molecular electronics is a promising candidate to meet the increasing demands of electronic component miniaturization. Within this field, the interaction between a molecular functional center and its environment plays a profound role both in designing molecular devices and in understanding the structure-property relationships at the molecular/atomic level. Herein, motivated by an isomer of the nickel tetraphenylporphyrin (Ni-TPP) molecule, we study the dynamical and thermodynamical properties of such a structure, with the aid of a side-coupled quadruple impurity Anderson model. We demonstrate that by adjusting the central-ligand hopping integral, the magnetic moments of the system are screened in two stages, each accompanied by the rising and falling of the Kondo resonant peak in the transport properties, respectively. However, one can hardly consider this behavior as the two-stage Kondo effect, because the second process is not the result of the screening of the side spins by the electrodes, instead, they are totally shielded by the central spins. Furthermore, if one sweeps the on-site Coulomb repulsion in the ligand orbitals, the magnetic moments are then screened in three stages. We note that the first and third screening processes are accompanied by the appearance and disappearance of the Kondo behavior in the linear conductance, respectively, while the second screening has no influence on the quantum transport through the device. These characteristics are apparently different from the typical three-stage Kondo physics reported by some previous studies. To implement the above results, we adopt the state-of-the-art numerical renormalization group method, combined with numerical simulations and analytical arguments.

We investigate the quantum phases of a frustrated antiferromagnetic Heisenberg spin-1/2 model Hamiltonian on a kagomé-strip chain (KSC), a one-dimensional analog of the kagomé lattice, and construct its phase diagram in an extended exchange-parameter space. The isolated unit cell of this lattice comprises of five spin-1/2 particles, giving rise to several types of magnetic ground states in a unit cell: a spin-$3/2$ state as well as spin-$1/2$ states with and without additional degeneracies. We explore the ground-state properties of the fully connected thermodynamic system using exact diagonalization and density matrix renormalization group methods, identifying several distinct quantum phases. All but one of the phases exhibit gapless spin excitations. The exception is a dimerized spin-gapped phase that covers a large part of the phase diagram and includes the uniformly exchange coupled system. We argue that this phase can be understood by perturbing around the limit of decoupled unit cells where each unit cell has six degenerate ground states. We use degenerate perturbation theory to obtain an effective Hamiltonian, a Kugel-Khomskii model with an anisotropic spin-one orbital degree of freedom, which helps explain the origin of dimerization.

Understanding the similarities and differences between adding or removing electrons from a charge-transfer insulator may provide insights about the origin of the electron-hole asymmetry found in cuprates. Here we study with cellular dynamical mean-field theory the Emery model set in the charge-transfer insulator regime, and dope it with either electrons or holes. We consider the normal state only and focus on the doping evolution of the orbital character of the dopants and on the nature of the doping-driven transition. Regarding the orbital character of the dopants, we found an electron-hole asymmetry: doped electrons mostly enter the copper orbitals, whereas doped holes mostly enter the oxygen orbitals. Regarding the nature of the doping-driven transition, we found no qualitative electron-hole asymmetry: On either electron or hole doping, there is a two-stage transition from a charge-transfer insulator to a strongly correlated pseudogap and then to a metal. This shows that a strongly correlated pseudogap is an emergent feature of doped correlated insulators in two dimensions, in qualitative agreement with recent experiments on ambipolar ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_{2+y}$ cuprate films. Our results indicate that merely doping with holes or electrons a charge-transfer insulator is not sufficient for explaining the electron-hole asymmetry observed in the normal state phase diagram of cuprates. Our work reinforces the view that actual hole-doped cuprates are more correlated than their electron-doped counterparts.

Frustration in physical systems often gives rise to novel and emerging phenomena. Sustained attention has focused on the geometric lattice frustration in the formation of an electronic flat band. Here, we propose, as an intrinsic degree of freedom, that orbital frustration would facilitate the realization of a flat band. Diverse flat-band models are designed by considering long-range hopping terms that are subsequently converted into orbital-frustrated models. As prototypical examples, we explicitly illustrate such conversions in two- and three-dimensional cases. Employing tight-binding models and first-principles calculations, we reveal a family of materials that harbor nearly flat bands in the whole Brillouin zone around the Fermi level. Our findings show that the orbital degree of freedom offers a different perspective in the study of electronic flat bands.

When introduced as dopants into ${\mathrm{Y}}_2{\mathrm{O}}_3$, $\mathrm{E}{\mathrm{r}}^{3+}$ ions replace ${\mathrm{Y}}^{3+}$ ions at two crystallographic sites of ${\mathrm{C}}_2$ and ${\mathrm{C}}_{3\mathrm{i}}$ point symmetries. The ${\mathrm{C}}_{3\mathrm{i}}$ site is particularly interesting, given that the inversion symmetry prohibits static electric-dipole moments and electric-dipole transitions, resulting in 1.5 µm optical transitions characterized by purely magnetic-dipole properties. Employing $\mathrm{E}{\mathrm{r}}^{3+}{}^{4}I_{15/2}\leftrightarrow {}^{4}I_{13/2}$ transitions, we examined the inherent ${\mathrm{C}}_{3\mathrm{i}}$ site of $\mathrm{E}{\mathrm{r}}^{3+}$ in ${\mathrm{Y}}_2{\mathrm{O}}_3$ for its potential in quantum information science and spectral hole burning applications. Our approach involved absorption and site-selective fluorescence spectroscopy to discern $\mathrm{E}{\mathrm{r}}^{3+}$ energy levels and revealed an excited-state lifetime of 14.6 ms. High-resolution laser spectroscopy at 3.7 K indicated an inhomogeneous linewidth of 1.3 GHz in our sample. Orientation-dependent Zeeman laser absorption spectroscopy facilitated the determination of the full $g$ tensors for both the ground and excited states, confirming the ${\mathrm{C}}_{3\mathrm{i}}$ site assignment. Furthermore, it identified a suitable magnetic field direction for mitigating decoherence, a finding corroborated through two-pulse photon echo spectroscopy. Our study presents photon echo coherent transient measurements related to magnetic-dipole-allowed rare-earth ion transitions at the ${\mathrm{C}}_{3\mathrm{i}}$ site. The observed decoherence, attributed to spectral diffusion arising from $\mathrm{E}{\mathrm{r}}^{3+}\text{−}\mathrm{E}{\mathrm{r}}^{3+}$ magnetic-dipole interactions, underscores the complex spin dynamics at play in systems with multiple inequivalent dopant sites. Coupled with the practicality of telecom laser wavelengths, our investigation highlights the promise of magnetic-dipole transitions in advancing quantum information science and spectral hole burning applications.

In layered electrides, the conducting electrons fill the two-dimensional interstitial voids between the atomic planes, which is expected to result in weakened electron-phonon coupling and give rise to high mobility and conductivity. In this paper, using density functional theory, we explore the room-temperature electron-phonon scattering and transport properties of seven layered electrides belonging to the alkaline earth subpnictogenide family (${\mathrm{Ca}}_2\mathrm{N}, {\mathrm{Sr}}_2\mathrm{N}, {\mathrm{Sr}}_2\mathrm{P}, {\mathrm{Ba}}_2\mathrm{N}, {\mathrm{Ba}}_2\mathrm{P}, {\mathrm{Ba}}_2\mathrm{As}$, and ${\mathrm{Ba}}_2\mathrm{Sb}$) and compare them to three traditional high-conductivity metals (Cu, Au, and Al). Several electrides are found to have low scattering rates compared with the metals, resulting from weak electron-phonon interaction, between 3 and $13\times$ lower than the metals, which is shown to originate from the spatial separation of the conducting electrons from the lattice. ${\mathrm{Sr}}_2\mathrm{N}$ and ${\mathrm{Ca}}_2\mathrm{N}$ are predicted to be the best conductors among the electrides, with high anisotropy, due to reduced scattering leading to long average mean-free paths for backscattering reaching 48 nm. However, the metals display the highest conductivities owing to their larger distribution of modes, originating from higher electronic velocities. These findings may help guide the discovery of electrides with exceptionally weak electron-phonon coupling and record high conductivity.

We report developments and corresponding calculations in the first-principles theory of multiphonon processes in solids, enabling direct calculations of nonradiative capture cross sections of both thermalized and hot carriers by defects in semiconductors. We present an improved derivation of the theory, implement an efficient time-domain integration scheme that enables summing over all possible multiphonon configurations for dissipating the energy released by the carrier capture, and extend the time-domain formulation to allow changes in phonon frequencies before and after capture. We examine zeroth- and first-order nonequilibrium contributions to the electron-capture cross section of a triply hydrogenated vacancy in Si and the first-order equilibrium hole-capture coefficient of a ${\mathrm{C}}_{\text{N}}$ substitutional impurity in GaN. In Si, we find that the zeroth-order term is two orders of magnitude larger than the first-order term. In GaN, the present results are within range of the sparse available experimental data at high temperatures but below the single experimental point at low temperatures. We see an improvement over previous calculations and a decreased temperature dependence that we attribute to the inclusion of all phonon modes.

We theoretically propose a polymer-based nanodevice consisting of a single trans-polyacetylene (tPA) molecule capacitively coupled to external gate voltages. We model the integrated device using a Su-Schrieffer-Heeger (SSH)-like Hamiltonian, and solve the self-consistent problem of coupled electronic and lattice degrees of freedom in the presence of the gate voltages. Interestingly, we demonstrate the emergence of multiple topological kinks or domain walls in the lattice distortion patterns, which can be externally controlled at the region of the gates. The local symmetry-breaking induced by the gates gives access to the different topological sectors of the SSH model characterized by a $\mathbb{Z}$ topological invariant, and allows the emergence of quantized charges $Q_g=\pm ne$ at the gates, associated to the topological $n$-kink solutions. Given their topological origin, these solutions are remarkably stable as functions of the external gate voltage $V_g$, except at specific values $V_g^{\left[n\right]}$ where topological transitions characterized by $\mathrm{\Delta }n=\pm 1$ between different multikink solutions occur, and where the electronic subgap spectrum of the device is completely reconstructed in order to accommodate the extra charge. In practice, the device can be considered as a polymer-based organic quantum dot where charge quantization is inherently topological and robust, a fact that may be useful for potential technological applications.

Floquet engineering of topological phase transitions driven by a high-frequency time-periodic field is a promising approach to realizing new topological phases of matter distinct from static states. Here, we theoretically investigate Floquet engineering topological phase transitions in the quantum spin Hall $\alpha \text{−}{T}_3$ system driven by an off-resonant circularly polarized light. In addition to the quantum spin (anomalous) Hall insulator phase with multiple helical (chiral) edge states, spin-polarized topological metallic phases are observed, where the bulk topological band gap of one spin subband overlaps with the other gapless spin subband. Moreover, with a staggered potential, the topological invariants of the system depend on whether the middle band is occupied because of the breaking of symmetry with respect to the center of energy-momentum plane. Our work highlights the significance of Floquet engineering in realizing new topological phases in $\alpha \text{−}{T}_3$ lattices.

Topological photonics has established itself as a formidable platform with the potential to emulate relativistic phenomena predicted by condensed matter field theory. In this letter, we reveal an exotic form of higher-order topological states in an insulator/semimetal hybrid system; we have called them adiabatic orbital corner states. This class of topological states arises by introducing an adiabatic variation of the mass term in the Dirac Hamiltonian, which is directly related to the perturbation in the structural parameters of a kagome photonic lattice. The adiabatic variation presented here involves a smooth and natural spatial transition from an external trivial normal insulator phase, through the nontrivial semimetal phase to the nontrivial higher-order topological insulator phase. We have found that through this distribution of topological phases, the corner states in the higher-order insulator phases hybridize with the Dirac states existing in the semimetal phase, forming hybridized corner states analogous to various atomic orbitals. Due to the occurrence of different phases in the finite model analyzed, we have resorted to the spectral localizer theory, which allowed us to identify the topological nature of the hybrid states. These results could lead the way in investigating pseudorelativistic orbitals in topological photonic systems and future explorations of exotic collective modes arising from hybrid topological lattices.

Crystal lattice is key in understanding magnetic interactions between atoms in solids. As the effective control of the lattice via tensile stress is limited, there are only a few demonstrations of controlling magnetic properties by expanding the lattice. This study reveals a clear correlation between enhanced magnetism, where Curie temperature ($T_{\mathrm{C}}$) is doubled from ∼60 to ∼120 K, and lattice expansion in prototypical van der Waals magnet $\mathrm{C}{\mathrm{r}}_2\mathrm{G}{\mathrm{e}}_2\mathrm{T}{\mathrm{e}}_6$ with sputtered dielectric overlayers NiO and MgO. We ascribe the mechanism of this $T_{\mathrm{C}}$ increase to a change in exchange interactions induced by strain, studied by several experimental probes and computational approaches.

Nonlinear Hall-like currents can be generated by a time-periodic alternating bias on two-dimensional (2D) materials lacking inversion symmetry. To hint that the moiré pattern between graphene and its supporting substrate contributes to the homogeneity of nonlinear currents, the change in the local potential $\mathrm{\Delta }V\left(\mathbf{r}\right)$ around horizontally strained graphene due to a homobilayer of hexagonal boron nitride (hBN) was obtained from ab initio calculations, and corrections to on-site energies and hopping matrix elements on the tight-binding electronic dispersion of graphene's $\pi$ electrons were calculated. Relying on a semiclassical approximation, Berry dipoles $\mathbf{D}$ are seen to change orientation and wind throughout the moiré heterostructure.

We study numerically the thermal buckling transition in graphene membranes under compressive strain and clamped boundaries employing an atomistic quasiharmonic model. The numerical simulations combine three different Monte Carlo methods, local moves, collective wave moves and parallel tempering. We determine the static and the dynamical critical exponents by finite-size scaling, and the nonlinear response to a transverse force near the transition. The correlation length exponent and the nonlinear response are in good agreement with recent renormalization-group calculations of elastic membranes. Despite the applied strain, we find a dynamical critical exponent and diffusion exponent of height fluctuations at the transition close to the value for freestanding graphene, $z=2(1+\zeta )$ and $\alpha =\zeta /(\zeta +1)$, where $\zeta$ is the static roughening exponent, as obtained in a recent study with a phase-field crystal model.

We consider tight-binding models on Bravais lattices with anisotropic onsite potentials that vary along a given direction and are constant along the transverse one. Inspired by our previous work on flat bands in anti-$\mathcal{PT}$ symmetric Hamiltonians [Mallick et al., Phys. Rev. A 105, L021305 (2022)], we construct an anti-$\mathcal{PT}$ symmetric Hamiltonians with an $E=0$ flat band by tuning the hoppings and the shapes of potentials. This construction is illustrated for the square lattice with bounded and unbounded potentials. Unlike flat bands in short-ranged translationally invariant Hamiltonians, we conjecture that the considered $E=0$ flat bands do not host compact localized states. Instead the flat-band eigenstates exhibit a localization transition along the potential direction upon increasing the potential strength for bounded potentials. For unbounded potentials flat-band eigenstates are always localized irrespective of the potential strength.