Ultraclean, undoped carbon nanotubes are observed to be always insulating, even when the gap predicted by band theory is zero: the residual band gap is then thought to have a many-body origin. Here we theoretically show that the correlated insulator is excitonic in all stable narrow-gap tubes irrespective of their size, thus extending our previous claim, limited to gapless (armchair) tubes [D. Varsano, et al., Nat. Commun. 8, 1461 (2017)]. We derive the scaling law of the exciton binding energy with the tube radius and chirality, and compute self-consistently the fundamental transport gap of the excitonic insulator, by enhancing the two-band model with an accurate treatment of screening validated from first principles. Our findings point to the broader connection between the exciton length scale, dictated by structure, and the stability of the excitonic phase.

We analyze the density dependence of the spin stiffnesses and the stability of magnetic order with respect to quantum fluctuations in the two-dimensional Hubbard model close to half-filling. The stiffnesses are computed from the spin susceptibility obtained from a random-phase approximation in a magnetically ordered state. For a sizable next-to-nearest-neighbor hopping amplitude and a moderate Hubbard interaction, the mean-field ground state is a Néel antiferromagnet in the electron-doped regime at and above half-filling and a planar circular spiral state in the hole-doped regime below half-filling. Upon electron doping, the Néel stiffness decreases smoothly and not very steeply. By contrast, the in-plane and out-of-plane stiffnesses in the spiral state drop abruptly at half-filling. The out-of-plane stiffness even drops to zero and then increases again very slowly upon increasing hole doping. At finite temperatures, the Néel-to-spiral transition is shifted into the hole-doped regime, the stiffnesses are continuous functions of the density, and they vanish at the transition. For small hole doping, the spin stiffnesses describe the quantum fluctuations only in a small momentum range, which shrinks to zero upon approaching half-filling. Using the above results, we show that the quantum ground state of the lightly electron-doped Hubbard model remains Néel ordered, while quantum fluctuations probably destroy the spiral long-range order in the hole-doped regime, giving rise to a quantum disordered state with a spin gap.

We propose a universal tight-binding model for realizing three-dimensional (3D) Weyl semimetals (WSMs) in van der Waals (vdW) stacked structures composed of honeycomb lattices. A large intralayer staggered potential (U), combined with a significant difference $\left(\mathrm{\Delta }t\right)$ in the interlayer vdW interactions between the two honeycomb sublattices, can effectively facilitate the emergence of the WSMs. The relative strengths of the interlayer vdW hoppings between the two sublattices with $p_{xy}$ orbitals determine the types of the WSMs formed. In contrast to conventional WSMs, the realized WSMs can exhibit a rare sublattice-spin-valley (SSV) locking, with $100\%$ spin and valley polarization. Rich topological phase transitions, spanning five distinct phases, arise from the interplay between spin-orbit coupling ($\lambda$), U, and $\mathrm{\Delta }\mathrm{t}$. First-principles calculations discover that 3D halogenated arsenene vdW materials are ideal for observing the exotic SSV-locked Fermi arcs, which feature a record-long length. Our research provides a vdW-stacked strategy for engineering unique WSMs, enabling high-performance spintronic and valleytronic applications through spatial control.

Mott quantum criticality is a central theme in correlated electron physics, with comparable critical exponents observed in materials featuring both continuous zero-temperature transitions and those with finite-temperature critical endpoints. Such criticality was first predicted theoretically for the single-band Hubbard model (SBHM). Within dynamical mean-field theory (DMFT), the SBHM undergoes a first-order transition at $T=0$, but displays quantum critical scaling above its finite-temperature critical point. However, a comprehensive analysis of a system exhibiting a continuous Mott transition at zero temperature has been lacking. To this end, the modified periodic Anderson model (MPAM) is a rare example known to host a surface of continuous Mott quantum critical points (QCPs). While previous studies of the MPAM characterized its QCP and showed the emergence of a pseudogap Anderson model at the QCP, the analysis was restricted to the Matsubara frequency axis, leaving key questions unresolved: What are the signatures of Mott quantum criticality in transport properties, and do the critical exponents align with the universal behavior seen in the SBHM and experiments? To address these questions, we employ DMFT with the numerical renormalization group as an impurity solver to investigate the real-frequency properties of the MPAM. Our central finding is the emergence of quantum critical scaling in the electrical resistivity, with exponents $\nu z_{\text{met}}\approx 0.76$ and $\nu z_{\text{ins}}\approx 0.66$ on the metallic and insulating sides, respectively. These values fall within the range reported for the SBHM and observed in experiments, suggesting that both transitions are governed by a common universality class. We further substantiate the presence of local quantum criticality by demonstrating robust $\omega /T$ scaling in single- and two-particle correlation functions. Finally, we identify different signatures in the optical conductivity, where the distinct evolution of two isosbestic points provides a unique fingerprint of Mott quantum criticality. These results establish the MPAM as a canonical model for investigating Mott quantum criticality and support the existence of a universal framework for this fundamental phenomenon.

Spin pumping in superconductor-based systems has been studied extensively, particularly in bilayer structures. In this work, we investigate spin pumping in a trilayer system consisting of a ferromagnetic insulator (FMI), a superconductor (SC), and a normal metal (NM). We derive both the AC and DC spin currents induced in the NM layer by the magnetization dynamics of the FMI under circularly polarized microwave irradiation. When the spin dynamics is treated classically, the AC spin current emerges, whereas treating it quantum mechanically as quasiparticle excitations leads to the DC spin current. Although evaluating these currents is computationally demanding, we overcome this challenge by employing the Quantics Tensor Cross Interpolation method. Numerical calculations reveal the dependence of spin current on temperature, microwave frequency, and superconducting layer thickness. Notably, both AC and DC components exhibit a coherence peak in their temperature dependence. Furthermore, we identify a transition in the thickness dependence of the spin current, where its behavior changes beyond a characteristic frequency.

Two-dimensional (2D) polar semiconductors have garnered significant attention due to their inherent polarization, which effectively suppress the rapid recombination of photogenerated electrons and holes. However, intrinsic 2D polar semiconductors suffer from surface inertness, leading to low reaction efficiency. Doping modification, a widely used strategy for enhancing catalyst activity, holds great promise for further improving the performance of 2D polar photocatalysts. In this study, 2D $g\text{−}{\mathrm{C}}_3{\mathrm{N}}_5$ with out-of-plane polarization and thirteen unequal doping sites is employed to systemically explore the doping effect on photocatalytic water splitting. The doped $g\text{−}{\mathrm{C}}_3{\mathrm{N}}_5$ can induce a surface dipole that is more than twice as large as that of the pristine material, while simultaneously exhibiting enhanced spatial separation of photogenerated electrons and holes. More importantly, doping effectively enhances local activity, facilitating smooth adsorption of the *OOH group in the oxygen evolution reaction and promoting an ideal hydrogen evolution reaction with near-zero Gibbs free energy change. Additionally, the introduced impurity states in the band gap broaden the optical absorption into the visible-light range, highlighting the potential of doped $g\text{−}{\mathrm{C}}_3{\mathrm{N}}_5$ for practical application. This work presents a feasible strategy for designing high-performance photocatalysts based on 2D polar semiconductors.

We find nonequilibrium phase transitions accompanied by multiple (nested) hysteresis behaviors in superconductors coupled to baths under a time-periodic light driving. The transitions are demonstrated with a full phase diagram in the domain of the driving amplitude and frequency by means of the Floquet many-body theory. In the weak-driving regime with a frequency smaller than half of the superconducting gap, excited quasiparticles are accumulated at the far edges of the bands, realizing a distribution reminiscent of the Eliashberg effect, which suddenly becomes unstable in the strong-driving regime due to multiphoton-assisted tunneling across the gap mediated by the in-gap Floquet sidebands. We also show that superconductivity is enhanced in the weak-driving regime without effective cooling, which is attributed to the modulation of the spectrum due to Floquet sidebands.

Hydrogen-based compounds are promising candidates for room-temperature superconductivity. However, hydrogen-related anharmonic quantum effects have created a huge gap between experiments and theories. The compound $\mathrm{Rb}{\mathrm{H}}_6$ exemplifies the impacts of quantum fluctuations and lattice anharmonicity on crystal predictions based on the classical Born-Oppenheimer approximation. The zero-point energy triggers a thermodynamic rearrangement, transforming the originally harmonic metastable $\mathrm{Y}{\mathrm{B}}_6$-type structure into the ground-state quantum phase of stoichiometric rubidium hexahydride at ambient pressure. During the quantum variational energy minimization, atomic displacements are dictated by the competition of bond energies, with anharmonicity favoring the preservation of strong chemical bonds at the expense of weaker ones. Quantum anharmonicity induces the contraction of electronic bands near the Fermi level, exacerbating Peierls instability and enhancing electron-phonon coupling matrix elements. Furthermore, the strengthened electron-phonon coupling is attributed to anharmonicity-induced phonon softening and intensified partial Fermi surface nesting. Consequently, $\mathrm{Y}{\mathrm{B}}_6$-type $\mathrm{Rb}{\mathrm{H}}_6$ exhibits the record superconducting transition temperature $T_c$ of 180 K at ambient pressure, surpassing both the theoretical prediction for $\mathrm{M}{\mathrm{g}}_2\mathrm{Ir}{\mathrm{H}}_6$ of 160 K and the experimental record held by $\mathrm{HgB}{\mathrm{a}}_2\mathrm{C}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_8$ of 134 K. It reveals the potential of phonon renormalization for even higher $T_c$, opening new avenues for the exploration of ambient superconductivity.

Closed-form conditions for bound states in the continuum are derived for planar, cylindrical, and spherical cavities enclosed by metasurfaces with electric and magnetic response. While the combined response in lossless systems enables field confinement in arbitrarily small cavities, the gain-enabled bound states in the continuum are found to coalesce with a laser mode, leading to intriguing single-mode spectral singularities characterized by divergent scattered fields with simultaneously vanishing internal fields of the respective cavities. In support of the analytical characterizations, a full-wave numerical demonstration of the spectral singularities is presented in cylindrical metasurface cavities made of dielectric particles situated in a host gain medium. Beyond unveiling the fundamental physics governing metasurface cavities, our results offer a compact and broadly applicable tool for designing next-generation devices across the electromagnetic spectrum.

Time crystalline structures can be created in periodically driven systems. They are temporal lattices which can reveal different condensed matter behaviors ranging from Anderson localization in time to temporal analogs of many-body localization or topological insulators. However, the potential practical applications of time crystalline structures have yet to be explored. Here, we pave the way for time-tronics where temporal lattices are like printed circuit boards for realization of a broad range of quantum devices. The elements of these devices can correspond to structures of dimensions higher than three and can be arbitrarily connected and reconfigured at any moment. Moreover, our approach allows for the construction of a quantum computer, enabling quantum gate operations for all possible pairs of qubits. Our findings indicate that the limitations faced in building devices using conventional spatial crystals can be overcome by adopting crystalline structures in time.

Of all the oxides of osmium, ${\mathrm{Ba}}_2\mathrm{NaOs}{\mathrm{O}}_6$ is the only one with a substantial ferromagnetic moment. A fresh theoretical study of the double perovskite is accomplished using a different magnetic symmetry, which embraces all relevant published structural information, and an atomic model of a Kramers doublet formed by the single osmium $d$ electron. It possesses symmetry demanded by the Wyckoff position assigned to Os in a canted ferromagnet. Whereupon, magnetic moments in the paramagnetic and magnetically ordered states of ${\mathrm{Ba}}_2\mathrm{NaOs}{\mathrm{O}}_6$ are unified. Analytic x-ray magnetic dichroic signals (XMCD) at osmium ${\mathrm{L}}_2$ and ${\mathrm{L}}_3$ absorption edges are presented, and shown to agree with hitherto unexplained recent measurements [Agrestini et al. Phys. Rev. Lett. 133, 066501 (2024)].

We present an approximation scheme based on selective Hilbert space truncation for characterizing nonthermal states of magnetic systems beyond spin-wave theory. We study applications to states that are inaccessible through linear spin-wave theory, such as multimagnon states and higher-energy states. Our approach is based on the existence of an exact representation of spin operators in terms of finite-order polynomials of bosonic operators—crucially representations used do not couple physical and physical parts of bosonic Fock space. It can be applied to approximate systems with and without a magnetically ordered ground state. The approximation exactly diagonalizes the bosonic Hamiltonian restricted to particular boson occupation subspaces, improving the conventional linear spin-wave approach and exponentially reducing the computing time relative to exact diagonalization schemes. As a test case, we apply the approach to a prototypical one-dimensional model—an XXZ spin chain with an applied magnetic field and antisymmetric exchange coupling. Here, the antisymmetric coupling introduces a continuous parameter to tune the system away from its exactly solvable limit. We find excellent agreement between numerically exact eigenstates and eigenvalues and those found via the approximation scheme. Our approach applies not just to higher lying states but also to boson bound states, which could make them more accessible to theoretical predictions for comparison with experiment.

The structural and magnetic properties of the two-dimensional spin-$1/2$ depleted-kagome compound ${\mathrm{Cu}}_7{\left({\mathrm{TeO}}_3\right)}_2{\left({\mathrm{SO}}_4\right)}_2{\left(\mathrm{OH}\right)}_6$ are investigated using x-ray diffraction, magnetization, heat capacity, and ${}^1\mathrm{H}$ Nuclear Magnetic Resonance (NMR) measurements. From the analysis of magnetic susceptibility, we found a large Curie-Weiss temperature $[{\theta }_{\mathrm{CW}}=-50\left(2\right)\mathrm{K}]$ and the coexistence of antiferromagnetic and ferromagnetic interactions. The value of ${\theta }_{\mathrm{CW}}$ gives an estimate of the average nearest-neighbor antiferromagnetic interaction of $J/k_{\mathrm{B}}\simeq 66\mathrm{K}$. The NMR relaxation rates ($1/T_1$ and $1/T_2$) exhibit a peak, providing evidence for a magnetic long-range order at $T^{*}\simeq 4\mathrm{K}$ which appears to be canted antiferromagnetic type. Heat capacity also features a broad maximum at $T^{*}$ that moves towards higher temperatures with increasing magnetic field, reflecting defect induced Schottky anomaly. The frustration parameter $f_r=\left|{\theta }_{\mathrm{CW}}\right|/T^{*}\simeq 12.5$ renders the compound a highly frustrated low-dimensional magnet.

Two-dimensional (2D) altermagnetism has attracted considerable attention due to its low-dimensional character and intrinsic combination of spin splitting with zero net magnetization. In previous work, we developed a general stacking theory (GST) to identify stacking-induced altermagnetism in 2D $A$-type bilayers; however, many systems are symmetry-restricted, preventing realization through stacking alone. Here, we show that Floquet engineering can lift these symmetry constraints by breaking the combined inversion and time-reversal symmetry ($\mathcal{IT}$), enabling a transition from an $A$-type antiferromagnet to an altermagnet in 2D bilayers. By combining the GST with a comprehensive group-theoretical symmetry analysis, we find that only bilayers with initial centrosymmetric point groups $D_{3d}$ or $C_{2h}$ can undergo such Floquet-induced transitions. Systematic screening identifies 349 stacking configurations, derived from 80 monolayer layer groups, that satisfy the necessary group-subgroup symmetry conditions. First-principles calculations on bilayer ${\mathrm{NiCl}}_2$ with $D_{3d}$ and $C_{2h}$ symmetries confirm the emergence of light-induced spin splitting consistent with the predicted altermagnetic spin point groups. These findings establish a Floquet-based framework for designing dynamically tunable 2D altermagnets and point toward potential applications in spintronic devices and low-dimensional magnetic systems.

High-entropy alloy and medium-entropy alloy superconductors with significant intrinsic disorder are a fascinating class of superconductors to explore, both from a fundamental science perspective and for possible practical applications. Their combination of robust structural integrity, superior mechanical properties, and exceptional irradiation tolerance makes them promising candidates for use in aerospace, radiation-rich environments, and advanced superconducting technologies. Herein, we present a comprehensive theoretical and experimental investigation of the superconductivity of equiatomic entropy-stabilized ${\mathrm{Nb}}_{0.25}{\mathrm{Ta}}_{0.25}{\mathrm{Ti}}_{0.25}{\mathrm{Zr}}_{0.25}$. The material shows bulk superconductivity (transition temperature = 8 K) with a high upper critical field of 11.94 T. Interestingly, both the electronic band structure and specific heat data point toward unconventional multiband superconductivity. Our ab initio calculations reveal Dirac-like band crossings close to the Fermi level, with certain degeneracies persisting even in the presence of spin-orbit coupling, suggesting a possible interplay between topological electronic states and the observed unconventional superconductivity. Remarkably, the critical current density exceeds the benchmark of ${10}^5\mathrm{A}/{\mathrm{cm}}^2$, surpassing all previously reported as-cast entropy-stabilized superconductors. This high critical current density is likely attributed to strong flux pinning at the grain boundaries, facilitated by extreme intrinsic lattice distortion. Taken together, the demonstrated dynamical stability, excellent metallicity, potential to host unconventional superconductivity, and exceptionally high critical current density highlight the potential of entropy-stabilized alloys as a platform for exploring the confluence of disorder, topology, and unconventional superconductivity.

Nontrivial quantum geometry of electronic bands has been argued to facilitate superconductivity even for the case of flat dispersions where the conventional contribution to the superfluid weight is suppressed by the large effective mass. However, most previous work focused on the case of conventional superconductivity while many contemporary superconducting quantum materials are expected to host unconventional pairing. Here, we derive a generalized expression for the superfluid weight employing mean-field BCS theory for systems with time-reversal symmetry in the normal state and arbitrary unconventional superconducting order with zero-momentum intraband pairing. Our derivation reveals the necessity of incorporating functional derivatives of the grand potential with respect to the superconducting gap function. Through perturbative analysis in the isolated narrow-bands limit, we demonstrate that this contribution arises from quantum geometrical effects, specifically due to a nontrivial Wilczek-Zee connection. Utilizing the newly obtained expressions for the superfluid weight, we apply our framework to an extended Kane-Mele model, contrasting conventional $s$-wave superconductivity with chiral $d$-wave superconductivity.

Non-Hermitian systems in two and higher dimensions universally exhibit algebraic skin effect, yet the exotic phenomena stemming from its quasi-long-range decay remain largely unexplored. In this paper, we uncover a new form of spectral sensitivity in a broad class of two-dimensional non-Hermitian systems with algebraic skin effect. This sensitivity can be induced by weak impurities, and a minimal setup is a pair of distant weak local potentials. Unlike conventional Green's function technique, we diagnose the ultrasensitivity using the forth-back propagator: a unique indicator whose divergence signals the occurrence of ultraspectral sensitivity. Asymptotics of the forth-back propagator reveals the existence of a critical geometric ratio, beyond which the ultrasensitivity is activated, while below the threshold the spectrum remains stable against weak impurities. This yields, in the thermodynamic limit, a phase diagram governed solely by geometric factors. Moreover, this type of sensitivity leads to the formation of dynamical nonlocal bound states. Our findings can be verified on experimental platforms such as acoustic and photonic crystals.

Atomic vibrations play a critical role in phonon-assisted electronic transitions at defects in solids. However, accurate phonon calculations in defect-laden systems are often hindered by the high computational cost of large-supercell first-principles calculations. Recently, foundation models, such as universal machine learning interatomic potentials (MLIPs), have emerged as a promising alternative for rapid phonon calculations, but the quantitatively low accuracy restricts its fundamental applicability for high-level defect-phonon calculations, such as nonradiative carrier capture rates. In this paper, we propose a “one-defect, one-potential” strategy in which an MLIP is trained on a limited set of perturbed supercells. We demonstrate that this strategy yields phonons with accuracy comparable to density functional theory (DFT), regardless of the supercell size. The predicted accuracy of defect phonons is validated by phonon frequencies, Huang-Rhys factors, and phonon dispersions. Further calculations of photoluminescence spectra and nonradiative capture rates based on this defect-specific model also show good agreement with DFT results, meanwhile reducing the computational expenses by more than an order of magnitude. Our approach provides a practical pathway for studying defect phonons in a ${10}^4$-atom large supercell with high accuracy and efficiency.