Dirac carriers in graphene are commonly characterized by a pseudospin degree of freedom, arising from the degeneracy of the two inequivalent sublattices. The inherent chirality of the quasiparticles leads to a topologically nontrivial band structure, where the in-plane components of sublattice spin and momentum are intertwined. Equivalently, sublattice imbalance is intimately connected with angular momentum, inducing a torque of opposite sign at each Dirac point. In this work we develop an intuitive picture that associates sublattice spin and winding number with angular momentum. We develop a microscopic perturbative model to obtain the finite angular momentum contributions along the main crystallographic directions. Our results can be employed to determine the angular dependence of the $g$ factor and of light absorption in honeycomb bipartite structures.

In this work, we investigate the possibilities of reaching wide-angle perfect absorption with Huygens' metasurfaces possessing both electric and magnetic responses. We analyze the role of the anisotropy of the polarizability tensor of the unit cell and reveal the conditions for the optimal angular stability in symmetric absorption as well as polarization independence. Then we approximate the desirable behavior with a Huygens' metasurface whose reflectance is low in a wide frequency range while the absorption is total around a single resonance frequency. Reaching of the optimal angular stability was numerically demonstrated and the corresponding wide-angle and wide-band properties were experimentally checked.

Interlayer rotation angles couple strongly to the electronic states of twisted van der Waals layers. However, not every angle is energetically favorable. Recent experiments on rotation-tunable electronics reveal the existence of a discrete set of angles at which the rotation-tunable electronics assume the most stable configurations. Nevertheless, a quantitative map for locating these intrinsically preferred twist angles in a twisted bilayer system has not been available, posing challenges for the on-demand design of twisted electronics that are intrinsically stable at desired twist angles. Here we reveal a simple mapping between intrinsically preferred twist angles and the geometry of the twisted bilayer system, in the form of geometric scaling laws for a wide range of intrinsically preferred twist angles as a function of only geometric parameters of the rotating flake on a supporting layer. We reveal these scaling laws for triangular and hexagonal flakes since they frequently appear in chemical vapor deposition growth. We also present a general method for handling arbitrary flake geometry. Such dimensionless scaling laws possess universality for all kinds of two-dimensional material bilayer systems, providing abundant opportunities for the on-demand design of intrinsic “twistronics.” For example, the set of increasing magic sizes that intrinsically prefer a zero-approaching sequence of multiple magic angles in a bilayer graphene system can be revealed.

Using a cluster extension of the dynamical mean-field theory (CDMFT) we map out the magnetic phase diagram of the anisotropic square lattice Hubbard model with nearest-neighbor intrachain $t$ and interchain $t_{\perp }$ hopping amplitudes at half filling. A fixed value of the next-nearest-neighbor hopping $t^{\prime }=-t_{\perp }/2$ removes the nesting property of the Fermi surface and stabilizes a paramagnetic metal phase in the weak-coupling regime. In the isotropic and moderately anisotropic regions, a growing spin entropy in the metal phase is quenched out at a critical interaction strength by the onset of long-range antiferromagnetic (AF) order of preformed local moments. It gives rise to a first-order metal-insulator transition consistent with the Mott-Heisenberg picture. In contrast, a strongly anisotropic regime $t_{\perp }/t\lesssim 0.3$ displays a quantum critical behavior related to the continuous transition between an AF metal phase and the AF insulator. Hence, within the present framework of CDMFT, the opening of the charge gap is magnetically driven as advocated in the Slater picture. We also discuss how the lattice-anisotropy-induced evolution of the electronic structure on a metallic side of the phase diagram is tied to the emergence of quantum criticality.

Stacking two graphene layers twisted by the magic angle $\theta \approx 1.1^{\circ }$ generates flat energy bands, which in turn catalyzes various strongly correlated phenomena depending on filling and sample details. At charge neutrality, transport measurements reveal superficially mundane semimetallicity (as expected when correlations are weak) in some samples yet robust insulation in others. We propose that the interplay between interactions and disorder admits either behavior, even when the system is strongly correlated and locally gapped. Specifically, we argue that strong interactions supplemented by weak, smooth disorder stabilize a network of gapped quantum valley Hall domains with spatially varying Chern numbers determined by the disorder landscape—even when an entirely different order is favored in the clean limit. Within this scenario, sufficiently small samples that realize a single domain display insulating transport characteristics. Conversely, multidomain samples exhibit re-emergent massless Dirac fermions formed by gapless domain-wall modes, yielding semimetallic behavior except on the ultralong scales at which localization becomes visible. We discuss experimental tests of this proposal via local probes and transport. Our results highlight the crucial role that randomness can play in ground-state selection of twisted heterostructures, an observation that we expect to have further ramifications at other fillings.

After the discovery of magnetism in monolayer $\mathrm{Cr}{\mathrm{I}}_3$, the magnetic properties of different two-dimensional materials from the chromium-trihalide family are intuitively assumed to be similar, yielding magnetic anisotropy from the spin-orbit coupling on halide ligands. Here we reveal significant differences between the $\mathrm{Cr}{\mathrm{I}}_3$ and $\mathrm{Cr}{\mathrm{Br}}_3$ magnetic monolayers in their magnetic anisotropy, resulting Curie temperature, hysteresis in an external magnetic field, and evolution of magnetism with strain, all predominantly attributed to a distinctly different interplay of atomic contributions to spin-orbit coupling in two materials.

To investigate spin susceptibility in a superconducting (SC) state, we measured the ${}^{125}\mathrm{Te}$-NMR Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal ${\mathrm{UTe}}_2$, a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the $b$ and $c$ axes ($K_b$ and $K_c$, respectively) decreased slightly, and the decrease in $K_b$ was almost constant up to 6.5 T. The reduction in $K_c$ decreased with increasing $H$, and $K_c$ was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state in low $H$, and the $H$ response of SC spin susceptibility is anisotropic on the $bc$ plane. We discuss the possible $\mathit{d}$-vector state within the spin-triplet scenario and suggest that the dominant $\mathit{d}$-vector component for the case of $H\parallel b$ changes above 13 T, where $T_{\mathrm{c}}$ increases with increasing $H$.

The mechanisms underlying the melting process in bidimensional systems have been widely studied by means of experiments, theory, and simulations since Kosterlitz, Thouless, Halperin, Nelson, and Young elaborated the KTHNY theory. In the framework of this theory, melting is produced by two continuous transitions mediated by the unbinding of local defects and the appearance of an intermediate phase between solid and liquid, called “hexatic.” There are also other competing theories that could explain this process, as, e.g., the formation of grain boundaries (lines of defects), which lead to a first-order transition. In this paper, simulations of systems interacting via the Lennard Jones 6–12 and Morse potentials using the Metropolis Monte Carlo method in the NVT ensemble have been performed to study the effect of the potential shape in the melting process. Additionally, truncated Morse potentials (with only a repulsive part) have been used to investigate the effect of the long-range interactions. Transitions from solid to hexatic phases were found to be continuous for all potentials studied, but transitions from hexatic to liquid phases were found to be either continuous or first order, depending on the thermodynamic conditions and the potential interaction selected, suggesting that melting can be triggered by different mechanisms, like grain boundary formation or defect unbinding. We find that the ratio of defects at the liquid-hexatic or liquid-coexistence phase transitions could determine the nature of these transitions and the mechanism underlying the melting process. The effect of the interaction of particles with their first- and second-nearest neighbors is also discussed.

We have investigated the electronic structure evolution with F doping in $\mathrm{Pr}{\mathrm{O}}_{1-x}{\mathrm{F}}_x\mathrm{Bi}{\mathrm{S}}_2$ ($x=0.0,0.3,0.5$) by means of angle-resolved photoemission spectroscopy. Undoped $\mathrm{Pr}\mathrm{O}\mathrm{Bi}{\mathrm{S}}_2$ exhibits Fermi surface (FS) pockets around the $X$ point which can be associated with electron doping due to the ${\mathrm{Pr}}^{3+}/{\mathrm{Pr}}^{4+}$ mixed valence. At $x=0.3$, the FS pockets are expanded, and their area is almost consistent with the F doping level. Interestingly, horizontal segments facing each other in the rectangular FS pockets lose their spectral weight. The partial suppression of the FS pockets suggests possible anisotropic charge fluctuations around $x=0.3$. At $x=0.5$, the FS area is much smaller than the expected value as commonly observed for $\mathrm{Ce}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}\mathrm{Bi}{\mathrm{S}}_2$ and $\mathrm{Nd}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}\mathrm{Bi}{\mathrm{S}}_2$. In addition, the highly depleted spectral weight at the FS pockets would be consistent with evident charge fluctuations driven by atomic disorder in the $\mathrm{Bi}{\mathrm{S}}_2$ network.

We present an analytical method to estimate the condensate fraction $n_0/n$ in strongly correlated systems for which the zero-temperature static structure factor $S\left(\mathbf{p}\right)$ is known. The advantage of the proposed method is that it allows us to predict the long-range behavior of the one-body density matrix (i) in macroscopic and mesoscopic systems, (ii) in three- and two-dimensional geometry, (iii) at zero and low finite temperature, and (iv) in weakly and strongly correlated regimes. Our method is tested against exact values obtained with various quantum Monte Carlo methods in a number of strongly correlated systems showing an excellent agreement. The proposed technique is also useful in numerical simulations as it allows us to extrapolate the condensate fraction to the thermodynamic limit for particle numbers as small as tens to hundreds. Our method is especially valuable for extracting the condensate fraction from the experimentally measured static structure factor $S\left(\mathbf{p}\right)$, thus providing a simple alternative technique for the estimation of $n_0/n$. We analyze available experimental data for $S\left(\mathbf{p}\right)$ of superfluid helium and find an excellent agreement with the experimental value of $n_0/n$.

Quantum scars are nonthermal eigenstates characterized by low entanglement entropy, initially detected in systems subject to nearest-neighbor Rydberg blockade, the so-called PXP model. While most of these special eigenstates elude an analytical description and seem to hybridize with nearby thermal eigenstates for large systems, some of them can be written as matrix product states with size-independent bond dimension. We study the response of these exact quantum scars to perturbations by analyzing the scaling of the fidelity susceptibility with system size. We find that some of them are anomalously stable at first order in perturbation theory, in sharp contrast to the eigenstate thermalization hypothesis. However, this stability seems to break down when all orders are taken into account. We further investigate models with larger blockade radius and find a set of exact quantum scars that we write down analytically and compare with the PXP exact eigenstates. We show that they exhibit the same robustness against perturbations at first order.

We present inelastic neutron scattering measurements of the Cairo pentagon lattice magnets ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ and ${\mathrm{Bi}}_4{\mathrm{Fe}}_5{\mathrm{O}}_{13}\mathrm{F}$, supported by high field magnetization measurements of ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$. Using linear spin wave theory and mean-field analyses we determine the spin exchange interactions and single-ion anisotropy in these materials. The Cairo lattice is geometrically frustrated and consists of two inequivalent magnetic sites, both occupied by ${\mathrm{Fe}}^{3+}$ ions and connected by two competing nearest neighbor interactions. We found that one of these interactions, coupling nearest neighbor spins on the threefold symmetric sites, is extremely strong and antiferromagnetic. These strongly coupled dimers are then weakly coupled to a framework formed from spins occupying the other inequivalent site. In addition, we found that the ${\mathrm{Fe}}^{3+}S=5/2$ spins have a non-negligible single-ion anisotropy, which manifests as a spin anisotropy gap in the neutron spectrum and a spin-flop transition in high field magnetization measurements.

We report high-resolution capacitance dilatometry studies on the uniaxial length changes in a ${\mathrm{NdB}}_4$ single crystal. The evolution of magnetically ordered phases below $T_{\mathrm{N}}=17.2$ K [commensurate antiferromagnetic phase (cAFM)], $T_{\mathrm{IT}}=6.8$ K [intermediate incommensurate phase (IT)], and $T_{\mathrm{LT}}=4.8$ K [low-temperature phase (LT)] is associated with pronounced anomalies in the thermal expansion coefficients. The data imply significant magnetoelastic coupling and evidence of a structural phase transition at $T_{\mathrm{LT}}$. While both cAFM and LT favor structural anisotropy $\delta$ between in-plane and out-of-plane length changes, it competes with the IT type of order, i.e., $\delta$ is suppressed in that phase. Notably, finite anisotropy well above $T_{\mathrm{N}}$ indicates short-range correlations which are, however, of neither cAFM, IT, nor LT type. Grüneisen analysis of the ratio of thermal expansion coefficient and specific heat enables the derivation of uniaxial as well as hydrostatic pressure dependencies. While $\alpha /c_{\mathrm{p}}$ evidences a single dominant energy scale in LT, our data imply precursory fluctuations of a competing phase in IT and cAFM, respectively. Our results suggest the presence of orbital degrees of freedom competing with cAFM, and successive evolution of a magnetically and orbitally ordered ground state.

We investigate the spin transport across the magnetic phase diagram of a frustrated antiferromagnetic insulator and uncover a drastic modification of the transport regime from spin diffusion to spin superfluidity. Adopting a triangular lattice accounting for both nearest-neighbor and next-nearest-neighbor exchange interactions with easy-plane anisotropy, we perform atomistic spin simulations on a two-terminal configuration across the full magnetic phase diagram. We found that as long as the ground state magnetic moments remain in-plane, irrespective of whether the magnetic configuration is ferromagnetic, collinear, or noncollinear antiferromagnetic, the system exhibits spin superfluid behavior with a device output that is independent of the value of the exchange interactions. When the magnetic frustration is large enough to compete with the easy-plane anisotropy and cant the magnetic moments out of the plane, the spin transport progressively evolves towards the diffusive regime. The robustness of spin superfluidity close to magnetic phase boundaries is investigated and we uncover the possibility for proximate spin superfluidity close to the ferromagnetic transition.

Bismuth perovskites $A{\mathrm{BiO}}_3$ ($A$ = Sr, Ba) host a variety of peculiar phenomena including bond-disproportionated insulating phases and high-temperature superconductivity upon hole doping. While the mechanisms underlying these phenomena are still debated, off-diagonal electron-phonon ($e$-ph) coupling originating from the modulation of the orbital overlaps has emerged as a promising candidate. Here, we employ classical Monte Carlo simulations to study a semiclassical three-orbital model with off-diagonal $e$-ph interactions. We demonstrate the existence of (bi)polaron correlations that persists in the model at high temperatures and for hole doping away from the bond-disproportionated insulating phase. Using a spatiotemporal regression analysis between various local quantities and the lattice degrees of freedom, we also identify the similarity between heating- and doping-induced melting of a bond-disproportionated insulator at a microscopic level. Our results imply that (bi)polaron physics can be a unifying concept that helps us understand the rich bismuth perovskite phase diagram.

We consider a long topological Josephson junction formed on a conducting two-dimensional (2D) surface of a 3D topological insulator (TI). The superconducting correlations are proximity-induced by $s$-wave superconductors covering the surface. The 1D spacing between the coverings is either unfilled or filled by a magnetic insulator. Generally, the Josephson current mediated by the TI surface is determined by scattering modes as well as by the states localized around the junction (Andreev bound states or Andreev bands). We find out that it is crucial to take into account both contributions to determine the current-phase relation of the topological Josephson junction. We analyze the dependence of the Josephson current on the thickness of the junction as well as the deviations from the sinusoidal shape of the current-phase relation.

We study a nonuniform strain in zigzag graphene nanoribbons for producing the spin-thermoelectric effects, using the mean-field Hubbard approximation and a Green's function approach. Our theoretical results show that a sinusoidal-shaped inhomogeneous strain with electron-electron interaction could induce a different effect on each edge of zigzag nanoribbons and finally generate a spin semiconductor with a tunable spin-dependent band gap. The strength of strain also controls the magnitude of magnetization in each edge. Interestingly, pure spin current and a giant spin Seebeck coefficient can be produced even at low values of strain by applying a thermal gradient and without magnetic elements. These results pave a practical way toward improved design for spin-thermoelectric applications through strain engineering.

We optimize multilayered antireflective coatings for photovoltaic devices, using modern evolutionary algorithms. We apply a rigorous methodology to show that a given structure, which is particularly regular (i.e., essentially periodic), emerges spontaneously and systematically for a very broad range of conditions. The very regularity of the structure allows for a thorough physical analysis of how the design operates. This allows us to understand that the central part is a photonic crystal utilized as a buffer for light, and that the external layers have the purpose of reducing the impedance mismatch between the outer media and the Bloch mode supported by the photonic crystal. This shows how optimization can suggest new design rules and be considered as a source of inspiration. Finally, we fabricate these structures with easily deployable techniques.