In a previous paper [Weiguo Yin, Phys. Rev. Res. 6, 013331 (2024)], the forbidden spontaneous phase transition in the one-dimensional Ising model was found to be approachable arbitrarily closely in decorated ladders by ultranarrow phase crossover (UNPC) at a given finite temperature $T_0$ with the crossover width $2\delta T$ reduced exponentially, which resembles a genuine first-order transition with large latent heat. Here, I reveal that the forbidden phase transition can be approached at fixed $T_0$ as well in decorated single-chain Ising models in the presence of a magnetic field, in which $T_0$ is determined by the interactions involving only the decorated parts and the magnetic field, while $2\delta T$ is independently, exponentially reduced ($\delta T=0$ means a genuine transition) by restoring the ferromagnetic interaction between the ordinary spins on the chain backbone—which was neglected in the previous studies of pseudotransition—thus manifesting that this asymptoticity to the forbidden transition is essentially the buildup of coherence in preformed crossover of local states. Furthermore, I show that the UNPC can be realized even in the absence of the conventional geometric frustration because the magnetic field itself can induce previously unnoticed hidden spin frustration. These findings make the doors wide open to the engineering and utilization of UNPC as a new paradigm for exploring exotic phenomena and 1D device applications.

The magnetic interactions in the antiferromagnetic (AFM) Dirac semimetal candidate ${\mathrm{SrMnSb}}_2$ are investigated using ab initio linear response theory and inelastic neutron scattering (INS). Our calculations reveal that the first two nearest in-plane couplings ($J_1$ and $J_2$) are both AFM in nature, indicating a significant degree of spin frustration, which aligns with experimental observations. The orbital resolution of exchange interactions shows that $J_1$ and $J_2$ are dominated by direct and superexchange, respectively. In a broader context, a rigid-band model suggests that electron doping fills the minority spin channel and results in a decrease in the AFM coupling strength for both $J_1$ and $J_2$. To better compare with INS measurements, we calculate the spin-wave spectra within a linear spin-wave theory, utilizing the computed exchange parameters. Although the calculated spin-wave spectra somewhat overestimate the magnon bandwidth, they exhibit overall good agreement with measurements from INS experiments.

The recent synthesis of ${\mathrm{LaBeH}}_8$ with a superconducting critical temperature $\left(T_c\right)$ up to 110 K at a pressure of 80 GPa generates great hope to achieve high-temperature superconductivity in ternary hydrides at submegabar pressure. However, the underlying mechanism governing its unexpected stability and high-temperature superconductivity remains elusive, which impedes the demanding design of other high-$T_c$ ternary hydrides on the way to room-temperature superconductivity. Here, we conducted a comprehensively theoretical study to reveal the origin of the stability and superconductivity of ${\mathrm{LaBeH}}_8$. We found that ${\mathrm{LaBeH}}_8$ could be viewed as a rocksalt compound of ${\mathrm{La}}^{2+}{\left[{\mathrm{BeH}}_8\right]}^{2-}$ and its unexpected stability lies in the emergence of an unusually negatively charged molecular ${\left[{\mathrm{BeH}}_8\right]}^{2-}$ unit containing polarized Be-H covalent bonds. This ${\left[{\mathrm{BeH}}_8\right]}^{2-}$unit gives rise to an intrinsic metallic feature in the solid, and its interaction with neighboring ones via weak H-H coupling contributes mostly to the high-temperature superconductivity. Our current results offer a key perspective of high-temperature superconductivity among ternary hydrides, thus inspiring a potential developing strategy to design high-$T_c$ hydrides via the exploration of similar intrinsic metallic salt structure motifs containing a variety of hydrogen-rich covalent units.

Superconductivity was recently reported in several quasicrystalline systems. These are materials which are structurally ordered, but since they are not translationally invariant, the usual BCS theory does not apply. At the present time, the underlying mechanism and the properties of the superconducting phase are insufficiently understood. To gain a better understanding of quasiperiodic superconductors, we consider the attractive Hubbard model on the Fibonacci chain, and examine its low-temperature superconducting phase in detail using the Bogoliubov–de Gennes mean-field approach. We obtain superconducting solutions as a function of the parameters controlling the physical properties of the system: the strength of the Hubbard attraction $U$, the chemical potential $\mu$, and the strength of the modulation of the Fibonacci Hamiltonian $w$. We find that there is a bulk transition at a critical temperature that obeys a power law in $U$. The local superconducting order parameter is self-similar both in real and perpendicular spaces. The local density of states varies from site to site, however, the width of the superconducting gap is the same on all sites. The interplay between the Hubbard attraction and the intrinsic gaps of the Fibonacci chain results in a complex zero-temperature $\mu \text{−}U$ phase diagram with insulating domes surrounded by superconducting regions. Finally, we show that tuning $w$ from weak to strong quasicrystalline modulation gives rise to qualitatively different thermodynamic behaviors as could be observed by measuring the specific heat.

Andreev bound states can occur at single impurities in superconductors if the impurities suppress superconductivity for a given system. In particular, well-known Yu-Shiba-Rusinov states occur at magnetic impurities in conventional $s$-wave superconductors. Here we demonstrate that nonmagnetic impurities in superconductor/antiferromagnet (S/AF) heterostructures with conventional intraband $s$-wave pairing also produce Andreev bound states. Analogously to the Yu-Shiba-Rusinov bound states the bound states in S/AF bilayers are spin split, but the spin of a particular bound state is determined by the sublattice to which the impurity belongs. The standard decay of the bound state local density of states is superimposed by atomic oscillations related to the staggered character of the exchange field in the host material and by another oscillating pattern produced by finite-momentum Néel triplet pairing generated at the impurity.

The time-honored Allen-Feldman theory of heat transport in glasses is generally assumed to predict a finite value for the thermal conductivity, even if it neglects the anharmonic broadening of vibrational normal modes. We demonstrate that the harmonic approximation predicts that the bulk lattice thermal conductivity of harmonic solids inevitably diverges at any temperature, irrespective of configurational disorder, and that its ability to represent the heat-transport properties observed experimentally in most glasses is implicitly due to finite-size effects. Our theoretical analysis is thoroughly benchmarked against careful numerical simulations. Our findings thus reveal that a proper account of anharmonic effects is indispensable to predict a finite value for the bulk thermal conductivity in any solid material, be it crystalline or glassy.

We report an $S=1$ antiferromagnet ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ crystalizing in the hexagonal space group $\mathit{P}{6}_3/mmc$. The structure consists of the perpendicular-to-plane ${\mathrm{Ni}}_2{\mathrm{O}}_9$ dimers in the triangular layers. Surprisingly, ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ exhibits a sharp upturn of magnetic susceptibility below ${\mathit{T}}_{\mathrm{N}}=5.8$ K, a single or two-step antiferromagnetic transitions at ${\mathit{T}}_{\mathrm{N}1}$ and ${\mathit{T}}_{\mathrm{N}2}$, whose values are magnetic-field dependent, and a sign of 1/3-like magnetization plateau at 2 K, all of which are typical features for a triangular-lattice antiferromagnet with small easy-axis anisotropy rather than the $S=1$ dimer compound. The density functional theory calculations prove the small intradimer exchange $({\mathit{J}}_0/{\mathit{k}}_{\mathrm{B}}=-2.11 \mathrm{K})$ and the large intraplane interdimer exchanges $({\mathit{J}}_1/{\mathit{k}}_{\mathrm{B}}=-13.27 \mathrm{K})$; the latter originates from the two Ni-O-Se-O-Ni super-superexchange paths within the triangular plane. This feature makes ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ a triangular-lattice antiferromagnet with small easy-axis anisotropy, rather than a spin-dimer antiferromagnet.

Crystal defects can highlight interesting quantum features by coupling to the low-energy Hamiltonian $H$. Here we show that independently of this $H$ coupling, topological crystalline defects can generate new features by directly modifying the response theory of electric-field probes such as Raman scattering. To show this we consider an antiferromagnetic spin-1/2 model $H_{\mathrm{spin}}$ on a zigzag chain. Crystalline domain walls between two zigzag domains appear as at most local defects in $H_{\mathrm{spin}}$ but as topological (not locally creatable) defects in the Raman operator $R$ of inelastic photon scattering. Using time-evolving block decimation numerics, mean field, and bosonization, we show that a finite density of crystalline domain walls shifts the entire Raman signal to produce an effective gap. This lattice-defect-induced Raman gap closes and reopens in applied magnetic fields. We discuss the effect in terms of photons sensing the lattice defects within $R$ as spin-dimerization domain walls, with $Z_2$ character, and a resulting shift of the probed wave vector from $q=0$ to $\pi +\delta q$, giving an $\mathit{O}\left(1\right)$ change in contrast to local defects. The magneto-Raman singularity from topological lattice defects here relies on the $H_{\mathrm{spin}}$ spinon liquid state, suggesting future applications using lattice topological defects to modify response-theory operators independently of $H$ and thereby generate new probes of quantum phases.

We use the finite projected entangled pair state (PEPS) method to investigate the global phase diagram of the spin-$\frac{1}{2}$ square-lattice $J_1\text{−}{J}_2\text{−}{J}_3$ antiferromagnetic (AFM) Heisenberg model. The ground-state phase diagram is established with a rich variety of phases: the Néel AFM, gapless quantum spin liquid, valence-bond solid (VBS), stripe AFM, and incommensurate spiral phases. The nature of the VBS region is revealed, which contains a plaquette VBS and a mixed columnar-plaquette VBS, with the emergence of short-range incommensurate spin correlation in some region. The long-range incommensurate magnetic phase is also explicitly characterized as a planar spiral with incommensurate spatial periodicities. Most interestingly, there exist several multicritical points connecting different phases. These findings elucidate the true nature of the long-standing square-lattice $J_1\text{−}{J}_2\text{−}{J}_3$ antiferromagnet at zero temperature. Our results also pave the way to accurately simulate complex two-dimensional quantum spin systems that may host nonuniform features by means of the finite PEPS.

Material science methods aim at developing efficient computational schemes for describing complex many-body effects and how they are revealed in experimentally measurable properties. The Bethe-Salpeter equation in the self-consistent Hartree-Fock basis is often used for this purpose, and in this paper we employ the real-frequency diagrammatic Monte Carlo framework for solving the ladder-type Bethe-Salpeter equation for the three-point vertex function (and, ultimately, for the system's polarization) to study the effect of electron-hole Coulomb scattering on Landau damping in the homogeneous electron gas. We establish how this damping mechanism depends on the Coulomb parameter $r_s$ and changes with temperature between the correlated liquid and thermal gas regimes. In a broader context of dielectric response in metals, we also present the full polarization and the typical dependence of the exchange-correlation kernel on frequency at finite momentum and temperature within the same computational framework.

We theoretically consider the possibility that phonons may be playing a role in the observed linear-in-$T$ resistivity in cuprates by focusing on the obvious question: How can phonon scattering be consistent with a linear-in-$T$ resistivity with a constant slope given that cuprates have many phonon modes with different energies and electron-phonon couplings (e.g., 21 phonon modes for LSCO)? We show using an arbitrarily large number of independent phonon modes that, within a model Boltzmann transport theory, the emergent high-$T$ linear-in-$T$ resistivity manifests an approximately constant slope independent of the number of phonon modes except in some fine-tuned narrow temperature regimes. We also comment on the quantitative magnitude of the linear-in-$T$ resistivity in cuprates pointing out the constraints on the effective electron-phonon coupling necessary to produce the observed resistivity.

Aimed at a more realistic classical description of natural quantum systems, we present a two-dimensional tensor network algorithm to study finite temperature properties of frustrated model quantum systems and real quantum materials. For this purpose, we introduce the infinite projected entangled simplex operator ansatz to study thermodynamic properties. To obtain state-of-the-art benchmarking results, we explore the highly challenging spin-1/2 Heisenberg antiferromagnet on the Kagome lattice, a system for which we investigate the melting of the magnetization plateaus at finite magnetic field and temperature. Making a close connection to actual experimental data of real quantum materials, we go on to studying the finite temperature properties of ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$. We compare the magnetization curve of this material in the presence of an external magnetic field at finite temperature with classically simulated data. As the first theoretical tool that incorporates both thermal fluctuations as well as quantum correlations in the study of this material, our work contributes to settling the existing controversy between the experimental data and previous theoretical works on the magnetization process.

We study the phase diagram and critical behaviors of three-dimensional lattice ${\mathbb{Z}}_2$-gauge $N$-vector models, in which an $N$-component real field is minimally coupled with ${\mathbb{Z}}_2$-gauge link variables. These models are invariant under global $\mathrm{O}(N$) and local ${\mathbb{Z}}_2$ transformations. They present three phases characterized by the spontaneous breaking of the global $\mathrm{O}(N$) symmetry and by the different topological properties of the ${\mathbb{Z}}_2$-gauge correlations. We address the nature of the three transition lines separating the three phases. The theoretical predictions are supported by numerical finite-size scaling analyses of Monte Carlo data for the $N=2$ model. In this case, continuous transitions can be observed along both transition lines where the $N$-component spins order, in the regimes of small and large inverse gauge coupling $K$. Even though these continuous transitions belong to the same $XY$ universality class, their critical modes turn out to be different. When the gauge variables are disordered (small $K$), the relevant order-parameter field is a gauge-invariant bilinear combination of the vector field. On the other hand, when the gauge variables are ordered (large $K$), the order-parameter field is the gauge-dependent $N$-vector field, whose critical behavior can only be probed by using a stochastic gauge fixing that reduces the gauge freedom.

We explore the thermoelectric transport properties of a coexistence topological semimetal, characterized by the presence of both a pair of Weyl points and a nodal ring in the quantum limit. This system gives rise to complex Landau bands when subjected to a magnetic field aligned with the direction connecting two Weyl points. In the longitudinal configuration, where the magnetic field is parallel to the electric field or the temperature gradient, the thermoelectric conductivity indicates a plateau independent of the magnetic field and the Fermi energy at $\delta$-form short-range scattering. This platform structure should also exist in pure two-node Weyl semimetals. However, the thermoelectric conductivity and the Seebeck coefficient are significantly influenced by the parameters of long-ranged Gaussian or screened Coulomb scattering potentials for both fixed carrier density and Fermi energy scenarios. In the transverse configuration, both Gaussian and screened Coulomb scatterings yield substantial positive magnetoresistance and thermoelectric conductance. Since the Hall conductivity is larger than the longitudinal one, the Seebeck coefficient, exhibiting a quadratic increase with the magnetic field, is close to the dissipationless limit irrespective of scatterings, while the Nernst response is notably dependent on the scattering mechanism. Additionally, the model parameter, distinct from the two-node Weyl model, influences the thermoelectric transport properties. The magnetic field response of the thermoelectric coefficients to different scattering potentials can be used as a basis for distinguishing scattering mechanisms in materials.

Toroidal dipole moments, which consist of enclosed circulating currents aligned along paths within a torus shape, can be experimentally achieved in metamaterials using various geometrical configurations. Here, we investigate the excitation of toroidal dipole moments in a core-shell nanosphere composed of dispersive materials within the framework of the Lorenz-Mie theory. By isolating the contributions related to volumetric oscillations of electric charges from those of transverse and radial current oscillations, we derive closed-form analytic expressions for the scattering coefficients associated with electric and magnetic toroidal dipole moments. These analytic expressions, obtained beyond the Rayleigh scattering regime and previously concealed within the Lorenz-Mie theory, agree with the Cartesian toroidal dipoles determined from the radial and transverse current oscillations. By exploring the resonances in these calculated coefficients, we demonstrate that the electric and magnetic toroidal dipole excitations can be manipulated and tuned for near- and far-field applications involving plasmonic core-shell nanoparticles and engineered metamaterials. We believe that our analytic results could shed light on some of the extraordinary effects obtained in Lorenz-Mie theory that depend on the interference of multipole coefficients, such as toroidal dipole-induced transparency, Fano resonances, and superscattering of light.

We investigated the growth, electronic states, and tunneling electron induced luminescence of In ultrathin films on Si(111) using scanning tunneling microscopy (STM). The In films (4–8 monolayers) grew layer by layer by deposition at $\sim 100$ K. The $dI/d{V}_{\mathrm{s}}$ spectra exhibited the quantum-well states confined in the films. We observed luminescence originating from the junction plasmon excited via electron tunneling into the quantum-well state. Thus, by adjusting the thickness of the film at a single-layer level, we were able to modify the photon emission from the STM junction.