The Fermi surface symmetric mass generation (SMG) is an intrinsically interaction-driven mechanism that opens an excitation gap on the Fermi surface without invoking symmetry-breaking or topological order. We explore this phenomenon within a bilayer square lattice model of spin-1/2 fermions, where the system can be tuned from a metallic Fermi liquid phase to a strongly interacting SMG insulator phase by an interlayer spin-spin interaction. The SMG insulator preserves all symmetries and has no mean-field interpretation at the single-particle level. It is characterized by zeros in the fermion Green's function, which encapsulate the same Fermi volume in momentum space as the original Fermi surface, a feature mandated by the Luttinger theorem. Utilizing both numerical and field-theoretical methods, we provide compelling evidence for these Green's function zeros across both strong and weak coupling regimes of the SMG phase. Our findings highlight the robustness of the zero Fermi surface, which offers promising avenues for experimental identification of SMG insulators through spectroscopy experiments despite potential spectral broadening from noise or dissipation.

The V-based kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = K, Rb, Cs) exhibit a cascade of exotic quantum phenomena including charge density wave (CDW) order and superconductivity. Considerable effort has been made to understand the nature of the CDW phase of $A{\mathrm{V}}_3{\mathrm{Sb}}_5$, but the origin remains elusive. A new family of the V-based kagome metals $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ ($R$ = Y, Sc, or rare-earth ions) has attracted recent interest. Among $R{\mathrm{V}}_6{\mathrm{Sn}}_6$, only $\mathrm{Sc}{\mathrm{V}}_6{\mathrm{Sn}}_6$ shows a CDW order. Thus, $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ can be a new platform for investigating the nature of the CDW phase of the V-based kagome metals. Here, combining infrared spectroscopy with density-functional theory (DFT) calculations, we investigate the electronic response of $R{\mathrm{V}}_6{\mathrm{Sn}}_6$ ($R$ = Y, Sc). While the optical conductivity ${\sigma }_1\left(\omega \right)$ spectra of $\mathrm{Y}{\mathrm{V}}_6{\mathrm{Sn}}_6$ show no anomaly from 10 to 300 K, those of $\mathrm{Sc}{\mathrm{V}}_6{\mathrm{Sn}}_6$ exhibit drastic changes below the CDW transition temperature $T_{\mathrm{CDW}}\approx 92\mathrm{K}$: the suppression of the Drude responses and the appearance of the absorption peaks at about 34 and 270 meV. A distinct multipeak structure in the energy region between 270 and 800 meV due to the interband transitions associated with the van Hove singularities (vHSs) at the $M$ point is hardly affected by the CDW transition, implying the robustness of the vHSs at the $M$ point against the CDW transition. Our DFT calculations demonstrate that the vHSs at the $M$ point remain intact in the CDW phase of $\mathrm{Sc}{\mathrm{V}}_6{\mathrm{Sn}}_6$ and the CDW gaps corresponding to the absorption peak at 270 meV open most clearly on the $k_{\mathrm{z}}=1/3$ and 1/2 planes. The calculated phonon dispersions of the pristine phase of $\mathrm{Sc}{\mathrm{V}}_6{\mathrm{Sn}}_6$ reveal that the structural instability with the imaginary phonon frequencies on the A-H-L plane ($k_{\mathrm{z}}=1/2$) and along the $\overline{\mathrm{M}}\text{−}\overline{\mathrm{K}}$ line ($k_{\mathrm{z}}=1/3$) induces the out-of-plane charge modulation, indicating that the CDW transition of $\mathrm{Sc}{\mathrm{V}}_6{\mathrm{Sn}}_6$ is associated with its structural phase transition.

Topological band theory has emerged as a powerful framework to classify and understand the electronic properties of materials. Topological semimetals, which have protected band crossings near the Fermi level and include Dirac and Weyl points, lines, or surfaces, generally remain uncommon. Hypervalent compounds exhibit tunable highly degenerate nonbonding states driving band crossings, so they could provide an effective platform to explore topological semimetallic phases. Here, we identify the topology of the electronic structure of hypervalent hydrides $A_{2}B{\mathrm{H}}_6$ at ambient pressure and high pressure, and describe the microscopic origin of topological states via hydrogen nonbonding states. Importantly, we discover an ellipsoidal nodal surface, a hitherto unrecognized type of fermionic excitation, in ${\mathrm{Mg}}_2\mathrm{Be}{\mathrm{H}}_6$ with space group $\mathit{Fm}\overline{3}$ $m$. The nodal surface electrons couple strongly to phonons, causing a lattice instability that drives the system towards a charge density wave phase that competes with the topological nodal surface phase, with pressure providing a control parameter. Additionally, in the nodal surface phase we predict superconductivity with a critical temperature of 20 K. We anticipate our work will encourage materials realizations of topological phases and help rationalize high pressure experiments using ideas from topological theory.

We derive exciton-spin interactions from a microscopic correlated model that captures important aspects of the physics of charge-transfer (CT) insulators to address magnetism associated with exciton creation. We present a minimal model consisting of coupled clusters of transition metal $d$ and ligand $p$ orbitals that captures the essential features of the local atomic and electronic structure. First, we identify the lowest-energy state and optically allowed excited states within a cluster by applying the molecular orbital picture to the ligand $p$ orbitals. Then, we derive the effective interactions between two clusters mediated by intercluster hoppings, which include exciton-spin couplings. The interplay of the correlations and the spatial structure of the CT exciton leads to strong magnetic exchange couplings with spatial anisotropy. Finally, we calculate an optical excitation spectrum in our effective model to obtain insights into magnetic sidebands optically observed in magnetic materials. We demonstrate that the spin-flip excitation due to the strongly enhanced local spin interactions around the exciton gives rise to the magnetic sidebands.

We performed density functional theory (DFT)$+U$ and dynamical mean field theory (DMFT) calculations with a continuous-time quantum Monte Carlo impurity solver to investigate the electronic properties of ${\mathrm{V}}_2{\mathrm{O}}_5$ and ${\mathrm{Li}}_x{\mathrm{V}}_2{\mathrm{O}}_5(x=0.125$ and 0.25). Pristine ${\mathrm{V}}_2{\mathrm{O}}_5$ is a charge-transfer insulator with strong O $p–\mathrm{V}d$ hybridization, and it exhibits a large band gap $\left(E_{\text{gap}}\right)$ as well as a nonzero conduction-band (CB) gap. We show that the band gap, the number of $d$ electrons of vanadium, $N_d$, and the CB gap for ${\mathrm{V}}_2{\mathrm{O}}_5$ obtained from our DMFT calculations are in excellent agreement with the experimental values. While the $\mathrm{DFT}+U$ approach replicates the experimental band gap, it overestimates the value of $N_d$ and underestimates the CB gap. In the presence of low Li doping, the electronic properties of ${\mathrm{V}}_2{\mathrm{O}}_5$ are mainly driven by a polaronic mechanism, and electron spin resonance and electron nuclear double resonance spectroscopies observed the coexistence of free and bound polarons. Notably, our DMFT results identify both polaron types, with the bound polaron being energetically preferred, while the $\mathrm{DFT}+U$ method only predicts the free polaron. Our DMFT analysis also reveals that increased Li doping leads to electron filling in the conduction band, shifting the Fermi level. This result is consistent with the observed Burstein-Moss shift upon enhanced Li doping, and we thus demonstrate that the $\mathrm{DFT}+\mathrm{DMFT}$ approach can be used for an accurate and realistic description of strongly correlated materials.

Recently, two-dimensional Dirac materials patterned with a superlattice structure have emerged as a rich platform for exploring correlated and topological quantum matter. In this paper, we propose that by subjecting Dirac electrons on a particle-hole symmetric topological insulator (TI) surface to a periodic magnetic field with triangular lattice symmetry, it is possible to realize a quantum critical phase of $N_f=3$ Dirac fermion species strongly coupled to an emergent gauge field, or 2+1-D quantum electrodynamics $\left({\mathrm{QED}}_3\right)$. We demonstrate explicitly that the ${\mathrm{QED}}_3$ phase naturally arises from a Dirac composite fermion (CF) picture, where the periodic magnetic field manifests as a periodic CF potential and transforms the CF Fermi surface into gapless Fermi points. We further show that by breaking the particle-hole symmetry of the TI surface—either by doping or by introducing a periodic electrostatic potential with zero mean—our quantum critical phase gives way to a sequence of fractional Chern insulator phases. Our theory illustrates the rich menagerie of quantum phases possible around half filling of a flat Chern band.

We demonstrate in this paper that the recently discovered infinite-layer (IL) nickelates have much in common with a class of materials known as electrides. Oxide-based electrides are compounds in which topotactic removal of loosely bound oxygens leaves behind voids with a landscape of attractive potentials for electrons. We show that this is also what happens in the IL nickelates, where one of the two electrons (per formula unit) freed during the topotactic synthesis is to a large degree located in the oxygen vacancy position, occupying partially a local $s$-symmetry interstitial orbital, rather than taking part alongside the other electon in converting $\mathrm{Ni}$ from $3+$ to a full $1+$ oxidation state. On the other hand, the involvement of the rare-earth $5d$ states is found to be rather indirect. Our study offers a detailed analysis of the mechanisms through which the presence of the interstitial charge can determine the superconducting and other properties of the IL nickelates. To this end, we demonstrate that the interstitial orbital in question, referred to by us as the zeronium $s$ or $\mathrm{Z}$ $s$ orbital, forms strong covalent bonds with neighboring $\mathrm{Ni}$ $3d_{3z^2-r^2}$ orbitals, which in turn facilitates the one-dimensional-like dispersion of the $\mathrm{Ni}$ $3d_{3z^2-r^2}$ band along the $c$-axis direction, leading also to a possible large out-of-plane coupling between Ni magnetic moments. This finding, reinforced by our electron localization function analysis, points to a fundamental distinction between the nickelates and the structurally analogous cuprates, may explain the absence of superconductivity in hydrogen-poor samples, and is certainly in agreement with the observed large $z$-polarized component in the $\mathrm{Ni}$ $L_3$-edge x-ray absorption spectra. In addition, by using $\mathrm{DFT}+\mathrm{U}$ calculations as an illustration, we show that the electridelike nature of the IL nickelates is one of the main reasons for the theoretical difficulty in determining the much debated elusive Fermi surface of these novel superconductors and also in exploring the possibility of them becoming excitonic insulators at low temperatures.

Bilayer kagome compounds provide an exciting playground where the interplay of topology and strong correlations can give rise to exotic phases of matter. Motivated by recent first-principles calculation on such systems [Phys. Rev. Lett. 125, 026401 (2020)], reporting stabilization of a Chern metal with a topological nearly flat band close to the Fermi level, we build minimal models to study the effect of strong electron-electron interactions on such a Chern metal. Using appropriate numerical and analytical techniques, we show that the topologically nontrivial bands present in this system at the Fermi energy can realize fractional Chern insulator states. We further show that if the time-reversal symmetry is restored due to the destruction of magnetism by low dimensionality and fluctuation, the system can realize a superconducting phase in the presence of strong local repulsive interactions. Furthermore, we identify an interesting phase transition from the superconducting phase to a correlated metal by tuning nearest-neighbor repulsion. Our study uncovers a rich set of nontrivial phases realizable in this system and contextualizes the physically meaningful regimes where such phases can be further explored.

It was commonly believed that a mirror Chern insulator (MCI) must require spin-orbital coupling, since time-reversal symmetry for spinless systems contradicts with the mirror Chern number. So MCI cannot be realized in spinless systems, which include the large field of topological artificial crystals. Here, we disprove this common belief. The first point to clarify is that the fundamental constraint is not from spin-orbital coupling but the symmetry algebra of time-reversal and mirror operations. Then, our theory is based on the conceptual transformation that the symmetry algebras will be projectively modified under gauge fields. Particularly, we show that the symmetry algebra of mirror reflection and time reversal required for MCI can be achieved projectively in spinless systems with lattice ${\mathbb{Z}}_2$ gauge fields, i.e., by allowing real hopping amplitudes to take $\pm$ signs. Moreover, we propose the basic structure, the twisted $\pi$-flux blocks, to fulfill the projective symmetry algebra, and develop a general approach to construct spinless MCIs based on these building blocks. Two concrete spinless MCI models are presented, which can be readily realized in artificial systems such as acoustic crystals.

We investigated the elastic properties of ${\mathrm{Y}}_{1-x}{\mathrm{Nd}}_x{\mathrm{Co}}_2{\mathrm{Zn}}_{20}$ with localized Nd $f$ electrons and ground-state Kramers doublet. All longitudinal and transverse moduli of ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20}$ ($x=$ 1) show an elastic softening below 50 K accompanied by a minimum around 2.5 K. The softening, which is robust to magnetic fields up to 8 T, is not observed for samples with Nd concentrations of $x=0.19$, 0.05, and 0. In localized $f$ electron systems, elastic softening from high temperatures is often understood by crystal electric field effects; however, this cannot explain the behavior in ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20}$. Our experimental and calculated results reveal that the softening neither is caused by a phonon contribution, a ${\mathrm{Nd}}^{3+}$ single-site effect, nor a magnetic interaction. We conclude that the softening is due to a local-symmetry-sensitive electronic state in ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20}$.

We present evidence of the direct insulator-quantum Hall transition in monolayer epitaxial graphene, a genuine two-dimensional (2D) system. We studied the transition from an insulating state to a quantum Hall state at a high Landau level filling factor $\nu =6>3$ and the plateau-plateau transition from $\nu =6$ to $\nu =2$. Using scaling theory, the critical exponent κ was determined to be $0.41\pm 0.02$ for the direct insulator-quantum Hall transition. This closely aligns with $\kappa =0.42\pm 0.02$ from the $\nu =6$ to $\nu =2$ quantum Hall plateau-plateau transition. These findings suggest that the two transitions may belong to the same universality class. While similar transitions have been explored in conventional two-dimensional charge systems, our study provides valuable insights into a truly 2D system, deepening our understanding of these transitions.

In high-quality conductors, the hydrodynamic regime of electron transport has recently been realized. In this paper, we theoretically investigate magnetotransport of a viscous electron fluid in samples with electron-impermeable obstacles. We use two approaches to describe the fluid flow. The first one is based on the equations of hydrodynamics of a charged fluid, which assume that the kinetic equation considers the two harmonics of the electron distribution function. The second approach is based on the equations that are obtained by considering three harmonics of the distribution function (quasihydrodynamics). Within the hydrodynamic approach, we consider the cases of the rough and smooth edges of disks, on which the electron scattering is diffusive or specular, respectively. The longitudinal magnetoresistivity turns out to be strong and negative, the same for both the rough and smooth disk edges up to small corrections. For rough disks, the Hall resistivity is equal to its standard value. For smooth disks, the Hall resistance acquires a small correction to the standard value, proportional to the Hall viscosity. In the quasihydrodynamic approach, we consider the case of smooth disks and small magnetic fields. In the regime when the flow is substantially different from the hydrodynamic one, the longitudinal resistivity does not depend on the shear stress relaxation time (but depends on the relaxation time of the third angular harmonic), while the correction to the standard Hall resistivity does not depend on both the relaxation times. We compare the results of the hydrodynamic calculation of the longitudinal resistance with the experimental data on magnetotransport in high-quality GaAs quantum wells with macroscopic defects. Agreement between theory and experiment evidences in favor of the realization of the hydrodynamic transport regime in such systems.

Nanoparticles are fascinating nanoobjects that are actively studied and widely employed in various fields, such as heterogeneous catalysis and advanced electrode materials. Their local atomic structure and composition can significantly impact their properties. We employed the state-of-the-art first-principles calculations to investigate the effects of AuCu nanoparticle structure and composition on the electronic properties, charge distribution, and CO and O adsorption energies. Two types of nanoparticles were examined in this study; specifically core-shell nanoparticles (Cu@Au, Au@Cu) and bimetallic alloy particles (Au-Cu), all with an average diameter of 2 nm (321 atoms) and exhibiting fcc, icosahedral, and amorphous structures. The research comprehensively investigated the electronic and adsorption properties of the proposed nanoparticles with regard to their ability to adsorb CO and O, revealing the potential for finely tuning of their properties by modifying their atomic structure and composition. Adjusting the core-to-shell ratio allows one to precisely tune the O and CO adsorption energies on the nanoparticle surface, particularly in fcc nanoparticles. This results in a narrower range of adsorption energies, specifically for CO adsorption, which cannot be achieved in bimetallic alloys. Our study shows the significance of this approach for fine-tuning adsorption energies on a nanoparticle surface.

We study the electron-phonon relaxation in the model of a granular metal film, where the grains are formed by regularly arranged potential barriers of arbitrary transparency. The relaxation rate of Debye acoustic phonons is calculated, taking into account two mechanisms of electron-phonon scattering: the standard Fröhlich interaction of the lattice deformation with the electron density and the interaction mediated by the displacement of grain boundaries dragged by the lattice vibration. At the lowest temperatures, the electron-phonon cooling power follows the power-law temperature dependence typical for clean systems but with the prefactor growing as the transparency of the grain boundaries decreases.

Extreme environments enable the discovery of atypical phases with enhanced properties and their transition mechanisms. Boron subnitride is a promising alternative to boron carbide/suboxide; however, its development is severely hampered by its long-standing unresolved crystal structure. Herein, by analyzing the pressure-dependent stabilities of the B-N system and scrutinizing several candidates against the available experimental results, the experimentally synthesized boron subnitride is identified as $\alpha {\text{-B}}_6\mathrm{O}$-like $R\overline{3}m{\mathrm{B}}_6\mathrm{N}$, whose observed unique metallicity originates from the electron deficiency caused by the $s{p}^2$-like hybridization of the N atoms that leaves nonbonding lone pairs lying in the $2p_z$ orbitals. We further unveil that the electron-deficient R$\overline{3}m$ ${\mathrm{B}}_6\mathrm{N}$ undergoes an isopointal metal-semiconductor transition at 120 GPa, dominated by the unique local orbital population coupling between different homonuclear bonds. Here, breakage of the lone pairs resulting from the dinitrogen bonding compensates for the unoccupied bonding states of intericosahedra B-B bonds, thus converting it into the electron-precise ${\mathrm{B}}_{12}^{2-}{\left(\text{N-N}\right)}^{2+}$. The strong N-N bond imparts R$\overline{3}m{\mathrm{B}}_6\mathrm{N}$ the highest shear modulus among ${\mathrm{B}}_{12}$-based compounds (almost twice that of ${\mathrm{B}}_6\mathrm{O}$) and serves as the main load-bearing unit resisting large plastic strain to produce superior strength. These findings substantially deepen our fundamental understanding of icosahedral boron-rich solids, and the underlying effect may contribute to fully grasping the changes in oxidation state and bonding pattern under high pressures.

With their celebrated structural and chemical flexibility, perovskite oxides have served as a highly adaptable material platform for exploring emergent phenomena arising from the interplay between different degrees of freedom. Molecular dynamics (MD) simulations leveraging classical force fields, commonly depicted as parametrized analytical functions, have made significant contributions in elucidating the atomistic dynamics and structural properties of crystalline solids including perovskite oxides. However, the force fields currently available for solids are rather specific and offer limited transferability, making it time-consuming to use MD to study new materials systems since a new force field must be parametrized and tested first. The lack of a generalized force field applicable to a broad spectrum of solid materials hinders the facile deployment of MD in computer-aided materials discovery (CAMD). Here, by utilizing a deep-neural network with a self-attention scheme, we have developed a unified force field (UniPero) that enables MD simulations of perovskite oxides involving 14 metal elements and conceivably their solid solutions with arbitrary compositions. Notably, isobaric-isothermal ensemble MD simulations with this model potential accurately predict the experimental temperature-driven phase transition sequences for several markedly different ferroelectric oxides, including a six-element ternary solid solution $\mathrm{Pb}\left({\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}\right){\mathrm{O}}_3\text{--}\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\text{--}{\mathrm{PbTiO}}_3$. We believe the universal interatomic potential along with the training database, proposed regression tests, and the auto-testing workflow, all released publicly, will pave the way for a systematic improvement and extension of a unified force field for solids, potentially heralding a new era in CAMD.

We study field-induced phase transitions of the minimal exchange model proposed earlier for ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, a candidate for the Kitaev honeycomb model, using numerical minimization of classical spin clusters. We show that an experimentally observed sequence of steplike transitions in magnetic field is realized in the phase diagram of the minimal model. Surprisingly, an intermediate up-up-down plateau phase is stabilized only in the proximity of a double-zigzag–zigzag phase boundary. We systematically map out the region of stability of an experimentally observed cascade of transitions and argue that ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ exchange parameters are close to a region of strong phase competition, which can explain the suppressed saturation field.

Recent experimental progress has established the twisted bilayer transition metal dichalcogenide (TMD) as a highly tunable platform for studying many-body physics. Particularly, the homobilayer TMDs under displacement field are believed to be described by a generalized triangular-lattice Hubbard model with a spin-dependent hopping phase $\theta$. To explore the effects of $\theta$ on the system, we perform density matrix renormalization group calculations for the relevant triangular lattice t-J model. By changing $\theta$ at small hole doping, we obtain a region of quasi-long-range superconducting order coexisting with charge and spin density wave within $0<\theta <\pi /3$. The superconductivity is composed of a dominant spin singlet $d$-wave and a subdominant triplet $p$-wave pairing. Intriguingly, the $S_z=\pm 1$ triplet pairing components feature pair-density waves. In addition, we find a region of triplet superconductivity coexisting with charge-density wave and ferromagnetism within $\pi /3<\theta <2\pi /3$, which is related to the former phase at smaller $\theta$ by a combined operation of spin-flip and gauge transformation. Our findings provide insights and directions for experimental search for exotic superconductivity in twisted TMD systems.