Proximity coupling is an intriguing quantum concept to tune the properties of 2D materials, e.g., graphene. By intercalation of a Pb monolayer, a 2D heterostructure of charge-neutral graphene and a crystalline 2D Pb layer on SiC(0001) substrates was epitaxially grown. Thereby, microscopy revealed characteristic defects on microscopic and mesoscopic length scales, e.g., grain boundaries, substrate steps, and nonintercalated terraces. We analyzed in situ the electronic transport properties of this heterostructure. The systematic variation of the contact geometry and probe spacings, supplemented by finite-element simulations, allowed us to circumvent the problem of sample inhomogeneities and to determine reliably the conductivity of the 2D heterostructure itself. Thereby, the conductivity of $(7\pm 1)\times {10}^5\mathrm{S}/\mathrm{m}$ is comparable to values measured for monolayer and quasi-free monolayer graphene, but decreases with decreasing temperature down to 30 K, revealing a clear nonmetallic behavior. Our findings are compatible with a small gap opening (1–5 meV) in graphene due to the proximitized 2D Pb layer.

The recent discovery of bulk superconductivity in trilayer nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ under high pressure is attracting a new wave of research interest. In this Letter, we study its electronic band structure and construct a minimal trilayer tight-binding model in terms of the nickel $3d_{x^2-y^2}$ and $3d_{3z^2-r^2}$ orbitals, and investigate the superconducting mechanism due to local Coulomb interactions by the unbiased functional renormalization group. We find antiferromagnetic correlations between the outer layers instead of neighboring ones, apart from the in-plane correlations. The effective interaction induces Cooper pairing with the $s_{\pm }$-wave symmetry, which changes sign across the Fermi pockets. In addition, we find the $T_c$ in ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ is systematically lower than that in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, but electron doping can further enhance $T_c$.

We consider the square lattice $S=1/2$ quantum compass model (QCM) parametrized by $J_x,J_z$, under a field, $\mathbf{h}$, in the $x\text{−}z$ plane. At the special field value $(h_x^{★},h_z^{★})=2S(J_x,J_z)$, we show that the QCM Hamiltonian may be written in a form such that two simple product states can be identified as exact ground states, below a gap. Exact excited states can also be found. We use the staggered vector chirality to characterize the exact ground states at $(h_x^{★},h_z^{★})$ and states obtained for fields within the vicinity of $(h_x^{★},h_z^{★})$. This gapped phase occupies most of the in-plane field phase diagram. For some values of $h_x>h_z$ and $h_z>h_x$ at the edges of the phase diagram, we have found transitions between this gapped phase and phases of weakly coupled Ising-chain states. In zero field, the QCM is known to have an emergent subextensive ground-state degeneracy. As the field is increased from zero, we find that this degeneracy is partially lifted, resulting in bond-oriented spin-stripe states, which are each separated from one another and the gapped phase by first-order transitions. Our findings are important for understanding the field dependent phase diagram of materials with predominantly directionally dependent Ising interactions.

An important class of model Hamiltonians for investigation of topological phases of matter consists of mobile, interacting particles on a lattice subject to a semiclassical gauge field, as exemplified by the bosonic Harper-Hofstadter model. A unique method for investigations of two-dimensional quantum systems are the infinite projected-entangled pair states, as they avoid spurious finite-size effects that can alter the phase structure. However, due to no-go theorems in related cases, this was often conjectured to be impossible in the past. In this Letter, we show that upon variational optimization, the infinite projected-entangled pair states can be used to this end by identifying fractional Hall states in the bosonic Harper-Hofstadter model. The obtained states are characterized by showing exponential decay of bulk correlations, as dictated by a bulk gap, as well as chiral edge modes via the entanglement spectrum.

We construct the simplest density functional for the problem of a single impurity interacting with a Fermi gas via a long-ranged potential using the Thomas-Fermi approach. We find that the Fermi polaron is fully bosonized in two dimensions, as the model results in the Landau-Pekar functional known from the Bose polaron problem, and in its many-image extension in other dimensions. We discuss applications of our theory for the two-dimensional exciton-polaron and the ionic polaron problem and compute the effective mass for these cases, finding a self-trapping transition with order dependent on the dimensionality.

Realization of linear magnetoelectric (ME) effect in single-phase van der Waals (vdW) compounds, combined with magnetic (spin), electric dipole, and dimensions degrees of freedom, could lead to new applications, such as high-density multistate data storage. Although the linear ME effect has been broadly studied for decades, vdW materials with an intrinsic ME effect have been rarely explored so far. Using a recently proposed design principle of two-dimensional vdW materials, transition metal tellurate halides, $\mathrm{N}{\mathrm{i}}_5{\left(\mathrm{Te}{\mathrm{O}}_3\right)}_4{X}_2 (X=\mathrm{Cl},\mathrm{Br})$ with linear ME coupling, have been synthesized and remarkable ME effect is demonstrated here. The onset of simultaneous anomalies in magnetic susceptibility and specific heat suggests the noncollinear antiferromagnetic order with the Néel temperature $T_{\mathrm{N}}\sim 30$ K. In addition, a clear spin-flop transition appears below $T_{\mathrm{N}}$, driven by magnetic field along the $x$ axis, indicating a strong magnetic anisotropy. Associated with the antiferromagnetic transition, a shape anomaly in the displacement current implies the appearance of ME response below $T_{\mathrm{N}}$ with the ME coefficient ${\alpha }_{xy}$ up to 4.2 ps/m in $\mathrm{N}{\mathrm{i}}_5{\left(\mathrm{Te}{\mathrm{O}}_3\right)}_4\mathrm{C}{\mathrm{l}}_2$. This is evidenced by the evolution of electric polarization ($P_x$) with magnetic field ($H$//$y$) along the $y$ axis at selected temperatures. The present study suggests that $\mathrm{N}{\mathrm{i}}_5{\left(\mathrm{Te}{\mathrm{O}}_3\right)}_4{X}_2$ is an unusual class of vdW materials hosting the linear ME behaviors, and thus an attractive platform for investigating the complicated interactions among spin order, electric polarization, and van der Waals force in layered vdW compounds.

We have proposed and demonstrated the hybridization between magnon modes of a magnetic vortex pair (MVP) of opposite chirality. The proposed magnon hybridization in MVP configurations of opposite chirality is realized by micromagnetic simulations. It is found that the hybridization results from the coupling between the breathing mode originating from the individual motion of a single magnetic vortex and the collective mode arising from the overall behavior of the MVP. Then the hybridization of different magnon modes is experimentally observed in the CoZr striped domain thin film systems, in which the MVP configuration exists. Moreover, the hybridization, coupling field, and strength can be modulated by the thickness of the CoZr films, i.e., the dimension of the magnetic vortex. Finally, we investigate the effect of different magnetic parameters on the magnon hybridization, i.e., the exchange constant, perpendicular magnetic anisotropy constant, and saturation magnetization. Our results may deepen the understanding of the hybridization phenomenon of magnons for complex magnetic textures.

The giant electro- and elastocaloric effects in spin-chain materials are predicted. The theory is based on the exact quantum mechanical solution of the problem. It is shown that the giant jumps in the entropy and the temperature caused by the caloric effect are weakly affected by the initial temperature. The effect can be used for the cooling of new quantum devices (like systems of qubits in quantum computers). On the other hand, since large changes are predicted in the narrow neighborhood of the critical point, the predicted effect can be used in ultrasensitive electric and stress sensors for modern microelectronics.

A large and oscillating spin Hall effect (SHE) in organic polymer ladders originating from polaron transport has been proposed in a previous work [Hu et al., Phys. Rev. B 106, 144309 (2022)], where a new mechanism of skew scattering off transient deformations of organic lattices caused by electron-lattice coupling has been revealed. In order to elucidate the origin of skew scattering, we analyze the components of the lattice distortions and investigate their roles in the SHE in the framework of the Su-Schrieffer-Heeger (SSH) model and the nonadiabatic dynamics method. By introducing a damping term into Newton's equation of motion, the breathers, i.e., lattice vibrations, generated by passing polarons and fueled by excess energy ultimately provided by the electric field, and charge-induced self-trapped distortions associated with the polarons themselves, are distinguished. A significant contribution to the SHE from skew scattering off the breathers is found, where the contributed percentage is related to the interchain coupling strength. Employing spectral analysis, we find that the effect of the breathers on the oscillation behavior mainly appears at low frequencies by modifying the electron transfer between chains. Moreover, by manipulating the amplitude of the breathers using a stronger field or shorter chains, the amplitude of the SHE may be controlled and enhanced. This work deepens our understanding of the microscopic mechanism of the SHE in organic polymers, and sheds light on the importance of spin-orbit physics in organic materials in spite of the weak atomic spin-orbit coupling.

Inspired by the stable bilayer water ice grown in the laboratory [Nature (London) 577, 60 (2020)], we propose a model representing water ice as a two-layer six-vertex model. Using the loop update Monte Carlo method, we unveil meaningful findings. While the square lattice six-vertex model exhibits an antiferromagnetic to disordered phase transition known as the Berezinskii-Kosterlitz-Thouless transition, we observe a different scenario for the bilayer six-vertex model, where the transition type transforms into an Ising transition. We discover the emergence of vortices in the disordered phase, and to stabilize them, vortex excitation is induced. This leads to the presence of distinct 1/2 filling and 2/3 filling vortex lattice phases. More importantly, we identify the phase transitions between the vortex lattice phase and the disordered phase, as well as between the 1/2 and 2/3 vortex lattices, as being of first order. We also propose an experimental scheme for realizing a two-layer six-vertex model based on the artificial ice of particles in a double well trap array. Our findings provide valuable insight into the nature of phase transitions occurring in layered water ice and artificial spin or particle ice systems in experimental setups.

In this study, we present a thorough investigation into the superconducting state of the ruthenium-based ternary equiatomic compound ScRuSi. Our analysis combines experimental techniques, including muon spin rotation/relaxation ($\mu \mathrm{SR}$) and low-temperature resistivity measurements, with theoretical insights derived from first-principles calculations. The low-temperature resistivity measurements reveal a distinct superconducting phase transition in the orthorhombic structure of ScRuSi at a critical temperature ($T_{\text{C}}$) of $2.5\mathrm{K}$. Further, the TF-$\mu \mathrm{SR}$ analysis yields a gap-to-critical-temperature ratio of $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{C}}=2.71$, a value consistent with results obtained from previous heat capacity measurements. The temperature dependence of the superconducting normalized depolarization rate is fully described by the isotropic $s$-wave gap model. Additionally, zero-field $\mu \mathrm{SR}$ measurements indicate that the relaxation rate remains nearly identical below and above $T_{\text{C}}$. This observation strongly suggests the preservation of time-reversal symmetry within the superconducting state. By employing the McMillan-Allen-Dynes equation, we calculate a $T_{\text{C}}$ of $2.11\mathrm{K}$ from first-principles calculations within the density functional theory framework. This calculated value aligns closely with the experimentally determined critical temperature. The coupling between the low-frequency phonon modes and the transition metal d-orbital states play an important role in governing the superconducting pairing in ScRuSi. The combination of experimental and theoretical approaches provides a comprehensive microscopic understanding of the superconducting nature of ScRuSi, offering insights into its critical temperature, pairing symmetry, and the underlying electron-phonon coupling mechanism.

Hybrid magnonic quantum systems have drawn increased attention in recent years for coherent quantum information processing, but too large magnetic damping is a persistent concern when metallic magnets are used. Their intrinsic damping is largely determined by electron-magnon scattering induced by spin-orbit interactions. In the low-scattering limit, damping is dominated by intraband electronic transitions, which has been theoretically shown to be proportional to the electronic density of states at the Fermi level. In this work, we focus on body-centered-cubic iron as a paradigmatic ferromagnetic material. We comprehensively study its electronic structure using first-principles density functional theory simulations and account for finite lattice temperature, boron (B) doping, and structure amorphization. Our results indicate that temperature-induced atomic disorder and amorphous atomic geometries only have a minor influence. Instead, boron doping noticeably decreases the density of states near the Fermi level with an optimal doping level of 6.25%. In addition, we show that this reduction varies significantly for different atomic geometries and report that the highest reduction correlates with a large magnetization of the material. This may suggest materials growth under external magnetic fields as a route to explore in experiment.

Bound states in the continuum (BICs) are zero-width (infinite lifetime) trapped eigenmodes that remain confined in the system even though they coexist with a continuum of extended states. The resulting high-frequency resonances may have significant applications in photonic integrated circuits, filtering, sensing, and laser. In this paper, we demonstrate that a simple design based on a photonic triple-stub cavity can display both Fabry-Pérot (FP) and Friedrich-Wintgen (FW) BICs, and their occurrence is very dependent on the way the cavity is attached to the outside medium by one or two ports. We first consider a symmetric cavity where a stub of length $d_3$ is surrounded by two stubs of length $d_2$, and all stubs are separated by segments of length $d_1$. When the cavity is inserted between two ports, we demonstrate theoretically and validate experimentally the existence of symmetric BICs (S-BIC) and antisymmetric BICs (AS-BIC) of FP type under commensurability conditions between the lengths $d_1, d_2$, and $d_3$. The S-BICs and AS-BICs may cross each other, giving rise to a doubly degenerate BIC. By breaking the symmetry of the cavity, AS-BICs and S-BICs can couple together and realize a FW-type BIC where one resonance remains with zero width while the other broadens into a bright mode. By considering two additional configurations where the triple-stub cavity is attached with one or two ports from only one side, additional BICs can be induced inside the structure. By slightly detuning from the BIC condition, the latter transforms into either an electromagnetic-induced transparency/reflection or Fano resonance. Finally, such a triple-stub cavity can be designed to realize near-perfect absorption for some frequencies. All the analytical results, obtained from the Green's function method, have been confirmed experimentally in the radiofrequency domain using coaxial cables.

Recent realization of minimal Kitaev chains brought a breakthrough in Majorana research, which made arrays of quantum dots coupled by superconductor spacers the most promising synthetic quantum material for topological quantum architectures. In this paper, we investigate the basic building block of this platform—two dots coupled via a superconductor (referred to as an Andreev molecule)—in a configuration where two superconducting (SC) loops are created to tune the SC phase difference across the dots. This enables us to consider Coulomb interactions which was not possible in previously studied systems. We demonstrate that the Andreev molecule shows a strong nonlocal Josephson effect: As the dot in one junction is tuned, the current-phase relation (CPR) of the other dot is modified. This architecture hosts $0\text{−}\pi$ transitions and shows a tunable anomalous ${\phi }_0$ phase shift, nonlocally controlled in both cases, without relying on spin-orbit interaction or Zeeman fields used in previous studies. In addition, a significant SC diode effect and $\pi$-periodic CPRs can be observed. The presented nonlocal CPR can be used as a signature of the formation of an Andreev molecular state and in general to introduce ways to tune quantum architectures.

We study angular momentum radiation from electrically biased chiral single molecular junctions using the nonequilibrium Green's function method. Using single helical chains as examples, we make connections between the ability of a chiral molecule to emit photons with angular momentum to the geometrical factors of the molecule. We point out that the mechanism studied here does not involve the magnetic moment. Rather, it relies on inelastic transitions between scattering states originated from two electrodes with different chiral properties and chemical potentials. The required time-reversal symmetry breaking is provided by nonequilibrium electron transport. Our work sheds light on the relationship between geometrical and optoelectrical chiral properties at the single molecular limit.