Inspired by our recent works on SU(2) and SU(4) Dirac fermions subjected to plaquette interactions on square lattice, here we extend the large-scale quantum Monte Carlo investigations to the phase diagram of correlated Dirac fermions with SU(6) and SU(8) symmetries subjected to the plaquette interaction on the same lattice. From SU(2) to SU(8), the rich phase diagram exhibits a plethora of emerging quantum phases, such as Dirac semimetals, antiferromagnetic Mott insulators, valence bond solids (VBSs), and Dirac spin liquid and phase transitions, including Gross-Neveu chiral transitions with emergent continuous symmetry, deconfined quantum criticality, and the first-order transition between an interaction-driven columnar VBS and a plaquette VBS. These rich phenomena coming from simple-looking lattice models firmly convey the message that the interplay between the $\mathrm{SU}\left(N\right)$ Dirac fermions (with enhanced internal symmetries) and extended plaquette interactions (beyond the on-site Hubbard type) is the new playground to synthesize novel highly entangled quantum matter both at the model level and with experimental feasibilities.

The dynamics of charge carriers in lattices of quantum spins is a long standing and fundamental problem. Recently, a new generation of quantum simulation experiments based on atoms in optical lattices has emerged that gives unprecedented insights into the detailed spatial and temporal dynamics of this problem, which compliments earlier results from condensed matter experiments. Focusing on observables accessible in these new experiments, we explore here the equilibrium as well as nonequilibrium dynamics of a mobile hole in two coupled antiferromagnetic spin lattices. Using a self-consistent Born approximation, we calculate the spectral properties of the hole in the bilayer and extract the energy bands of the quasiparticles, corresponding to magnetic polarons that are either symmetric or antisymmetric under layer exchange. These two kinds of polarons are degenerate at certain momenta due to the antiferromagnetic symmetry, and we, furthermore, examine how the momentum of the ground-state polaron depends on the interlayer coupling strength. The long time dynamics of a hole initially created in one layer is shown to be characterized by oscillations between the two layers with a frequency given by the energy difference between the symmetric and the antisymmetric polaron. We finally demonstrate that the expansion velocity of a hole initially created at a given lattice site is governed by the ballistic motion of polarons for long times, and that it increases as a quantum phase transition to a disordered state is approached. While our linear spin wave theory does not include interlayer dimer correlations important close to the phase transition, this speedup is a physically robust prediction as it reflects the decreasing energy cost of hopping when the magnetic order decreases.

Ballistic graphene samples in a multimode regime show the sub-Sharvin charge transport, characterized by the conductance reduced by a factor of $\pi /4$ compared to standard Sharvin contacts in two-dimensional electron gas, and the shot-noise power enhanced up to $F\approx 1/8$ (with $F$ the Fano factor) [Phys. Rev. B 104, 165413 (2021)]. Here we consider the disk-shaped (Corbino) setup in graphene, with inner radius $r_1$ and outer radius $r_2$, finding that the multimode conductance is slightly enhanced for any $r_1<r_2$, reaching $(4-\pi )\approx 0.8684$ of the Sharvin value for $r_1\ll r_2$. At the same limit, the Fano factor is reduced, approaching $(9\pi -28)/(12-3\pi )\approx 0.1065<1/8$. Closed-form approximating expressions for any $r_1/r_2$ ratio are derived, supposing incoherent scattering of Dirac fermions on an asymmetric double barrier and compared with exact numerical results following from the mode-matching method. Sub-Sharvin values are restored in the narrow-disk limit $r_1/r_2\to 1$. For experimentally accessible radii ratios $0.5⩽r_1/r_2⩽0.8$, both the conductance and the Fano factor are noticeably closer to the values predicted for the $r_1\ll r_2$ limit, yet still differ from standard Sharvin transport characteristics. The system behavior upon tuning the electrostatic potential barrier from a rectangular to parabolic shape is studied numerically and the crossover from the sub-Sharvin to standard Sharvin transport regime is demonstrated. Implications for a finite section of the disk are also discussed.

Non-Hermitian photonics based on combining loss and gain media within a single optical system provides a number of approaches to control and generate the flow of light. In this paper, we show that by introducing non-Hermitian perturbation into the system with loss and gain constituents, the high-quality resonances known as trapped modes can be excited without the need to change the symmetry of the unit cell geometry. To demonstrate this idea, we consider a widely used all-dielectric planar metamaterial whose unit cell consists of a pair of rectangular nanoantennae made of ordinal (with loss) and doped (with gain) silicon. Since the quality factor of the trapped-mode resonance can be controlled by changing both spatial symmetry and non-Hermiticity, varying loss and gain allows us to compensate for the influence of asymmetry and restore the quality factor of the localized mode. The results obtained suggest new ways to achieve high-quality resonances in non-Hermitian metamaterials that are promising for many practical applications in nanophotonics.

By considering two ab initio-based complementary approaches, we analyze the electronic structure and extract effective spin models of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, a honeycomb material which has been proposed as a candidate for Kitaev physics. Both methods show that the dominant direct hopping makes the bond-dependent Kitaev term negligible, diverting the material away from the sought-after spin-liquid regime. As a result, we present a simple three-parameter exchange model to describe the interactions of the lowest doublet of the honeycomb cobaltate ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$. Remarkably, it is the third-neighbor interactions, both isotropic and anisotropic, that are responsible for the standout double-zigzag ground state of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, stabilized by quantum fluctuations. A significantly large third-nearest-neighbor hopping, observed ab initio, supports the importance of the third-neighbor interactions in the stabilization of the unique ground state of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$.

The importance of the quantum metric in flat-band systems has been noticed recently in many contexts such as the superfluid stiffness, the dc electrical conductivity, and ideal Chern insulators. Both the quantum metric of degenerate and nondegenerate bands can be naturally described via the geometry of different Grassmannian manifolds, specific to the band degeneracies. Contrary to the (Abelian) Berry curvature, the quantum metric of a degenerate band resulting from the collapse of a collection of bands is not simply the sum of the individual quantum metrics. We provide a physical interpretation of this phenomenon in terms of transition dipole matrix elements between two bands. By considering a toy model, we show that the quantum metric gets enhanced, reduced, or remains unaffected depending on which bands collapse. The dc longitudinal conductivity and the superfluid stiffness are known to be proportional to the quantum metric for flat-band systems, which makes them suitable candidates for the observation of this phenomenon.

We report on transport properties in dual-gated hexagonal boron nitride (hBN)/bilayer-graphene (BLG) superlattices. Here, BLG is nontwisted, i.e., plain. This paper focuses on the charge neutrality point (CNP) for a plain BLG. Under a perpendicular magnetic field, transitions between two insulating phases at the CNP are detected by varying a displacement field with the study on the resistance-temperature characteristics and the magnetoresistance. This work opens avenues for exploring the global phase diagram of the hBN/BLG superlattices beyond the CNP.

Quasiparticle excitations in $3+1$ dimensions can be either bosons or fermions. In this work, we introduce the notion of fermionic loop excitations in $(3+1)$-dimensional $(3+1\text{d})$ topological phases. Specifically, we construct a new many-body lattice invariant of gapped Hamiltonians, the loop self-statistics $\mu =\pm 1$, that distinguishes two bosonic topological orders that both superficially resemble $3+1\mathrm{d}{\mathbb{Z}}_2$ gauge theory coupled to fermionic charged matter. The first has fermionic charges and bosonic ${\mathbb{Z}}_2$ gauge flux loops (FcBl) and is just the ordinary fermionic toric code. The second has fermionic charges and fermionic loops (FcFl) and, as we argue, can only exist at the boundary of a nontrivial $(4+1)$-dimensional $(4+1\mathrm{d})$ invertible phase, stable without any symmetries, i.e., it possesses a gravitational anomaly. We substantiate these claims by constructing an explicit exactly solvable $4+1\mathrm{d}$ model using a method that bootstraps a boundary theory into a bulk Hamiltonian, analogous to that of Walker and Wang, and computing the loop self-statistics in the fermionic ${\mathbb{Z}}_2$ gauge theory hosted at its boundary. We also show that the FcFl phase has the same gravitational anomaly as all-fermion quantum electrodynamics. Our results are in agreement with the recent classification of nondegenerate braided fusion 2-categories by Johnson-Freyd, and with the cobordism prediction of a nontrivial ${\mathbb{Z}}_2$-classified $4+1\mathrm{d}$ invertible phase with action $S=\frac{1}{2}\int w_2{w}_3$.

On the basis of the method of iterative summation of path integrals (ISPI), we develop a numerically exact transfer-matrix method to describe the nonequilibrium properties of interacting quantum-dot systems. For this, we map the ISPI scheme to a transfer-matrix approach, which is more accessible to physical interpretation, allows for a more transparent formulation of the theory, and substantially improves the efficiency. In particular, the stationary limit is directly implemented, without the need of extrapolation. The resulting method, referred to as “transfer-matrix summation of path integrals” (TraSPI), is then applied to resonant electronic transport through a single-level quantum dot.

The extended Kitaev models spark a recent surge of interest due to its unprecedented topological phase with fractional invariants. In this extended model of topological superconductors, long-range pairing interactions can induce a pair of subgap massive edge modes (MEMs), posing novel challenges to the unambiguous detection of Majorana bound states (MBSs) sustained by the same model in the short-range limit. Here we investigate the effect of a power-law decayed long-range pairing on the dc and ac Josephson currents between two Majorana nanowires. By varying the magnetic field and long-range pairing, the nanowires can be driven into different phases hosting MEMs, MBSs, or trivial Andreev bound states (ABSs), which considerably modulate the Josephson current. For weakly linked Josephson junctions, we show that the MEMs emerging in the long-range phases can induce a zigzag profile in the dc current-phase relation featured by two sign reversals deviating from the point where the superconducting phase difference $\varphi =\pi$. In contrast, the Josephson current indicates a sharp sign reversal at $\varphi =\pi$ in the short-range phase that hosts MBSs, while a smooth sinelike dc Josephson current appears in the presence of ABSs. In the nonequilibrium junction biased by a voltage, we identify a $4\pi$ ac Josephson current in the short-range phases, indicating a MBSs-mediated tunneling process dominating the supercurrents. Differently, when the system enters MEMs-hosted long-range phases, the ordinary ac Josephson current with $2\pi$ periodicity can be restored. In addition, we find the ac Josephson current induced by MEMs is significantly suppressed under the strong long-range pairing interactions. These signatures can serve as probes to discriminate MBSs from MEMs and ABSs.

The motion of excitons in organic semiconductors represents a key contribution to the performance of organic solar cells. It is determined by intermolecular exciton coupling. In momentum space, exciton coupling results in the exciton dispersion and defines the exciton bandwidth. We demonstrate that the exciton bandwidth in pentacene as measured using electron energy-loss spectroscopy is substantially temperature dependent, and it is drastically reduced going from 20 to 380 K. Following recent theoretical developments, we rationalize this reduction by a huge reduction of the (effective) charge carrier transfer integrals by about 60%. Our results indicate a strongly temperature-dependent energy transport in pentacene and related organic semiconductors.

In the first tutorial part of the paper, we show that equilibrium positions of small dielectric particles inside the Fabry-Perot resonator (FPR) are sensitive to a frequency of incident electromagnetic wave and size of particle. That elucidates basic principles of resonant trapping of nanoparticles by excitation of high-$Q$ resonances of FPR. In the second part, we consider a long dielectric cylinder with submicron radius (primary cylinder) integrated into a metallic waveguide which supports symmetry-protected bound states in the continuum (BICs). We consider the case of a slightly shifted cylinder relative to the axis of symmetry of a waveguide that controls the $Q$ factor of quasi-BIC. Then, the extra nanoparticle perturbs quasi-BIC as dependent on the size of the nanoparticle and position relative to the primary cylinder. An interplay between the resonant width of quasi-BIC and a degree of frequency perturbation defines whether a dragging nanoparticle is terminated at a surface of the primary cylinder for an ultrasmall size of nanoparticles or at the definite distances from the cylinder for the larger size of nanoparticles. Thereby, we demonstrate a paradigm of resonant self-trapping and sorting of nanoparticles by use of quasi-BICs. We also show extremal sensitivity of self-trapping to the frequency of an electromagnetic (EM) wave propagating over waveguide.

Eigenstates of many-body localized (MBL) systems are characterized by area-law bipartite entanglement along with multifractal statistics of their amplitudes on Hilbert space. At the same time, sparse random pure states with fractal statistics are not compatible with area law and necessarily exhibit volume-law entanglement. This raises the question regarding what correlation functions of Hilbert-space amplitudes MBL eigenstates must possess for their area-law entanglement to be compatible with their multifractality. In this Letter, we identify and compute such appropriate Hilbert-space correlations which carry information of entanglement. We find that, for MBL eigenstates, these correlations are qualitatively different not only from those of ergodic states but also of sparse random states with fractal statistics. This enables us to show that indeed the said correlations lie at the heart of the coexistence of area-law entanglement and multifractality for MBL eigenstates.

Highly entangled states are the key to the realization of quantum information processing. We theoretically investigate magnon-magnon entanglement in a compensated ferrimagnet. We show that the steady-state magnon-magnon entanglement largely enhances in the vicinity of angular momentum compensation point $\left(T_A\right)$ when magnons are coupled with photons in a cavity. The origin of this enhancement is that the ground state of ferrimagnet can be close to the Einstein-Podolsky-Rosen state near $T_A$. This feature is unique to ferrimagnets with different Landé g factors between sublattices and makes the magnon entanglement of ferrimagnets higher than that of ferromagnets and antiferromagnets. Our result will invigorate research on quantum information processing based on magnon-magnon entanglement.

We show that for the half-filled Kondo lattice model on the honeycomb lattice a Kondo breakdown occurs at small Kondo couplings $J_k$ within the magnetically ordered phase. Our conclusions are based on auxiliary field quantum Monte Carlo simulations of the so-called composite fermion spectral function. Within a $U\left(1\right)$ gauge theory formulation of the Kondo model, it becomes apparent that a Higgs mechanism dictates the weight of the resonance in the spectral function. For the honeycomb lattice we observe that for small $J_k$ the quasiparticle pole gives way to incoherent spectral weight but it remains well defined for the square lattice. Our result provides an explicit example where the magnetic transition and the breakdown of heavy quasiparticles are detached as observed in $\mathrm{Yb}{\left({\mathrm{Rh}}_{0.93}{\mathrm{Co}}_{0.07}\right)}_2{\mathrm{Si}}_2$ [S. Friedemann et al., Nat. Phys. 5, 465 (2009)].

We study the dynamics of the quantum Dyson hierarchical model in its paramagnetic phase. An initial state made by a local excitation of the paramagnetic ground state is considered. We provide analytical predictions for its time evolution, solving the single-particle dynamics on a hierarchical network. A localization mechanism is found, and the excitation remains close to its initial position at arbitrary times. Furthermore, a universal scaling among space and time is found that is related to the algebraic decay of the interactions as $r^{-1-\sigma }$. We compare our predictions to numerics, employing tensor network techniques, for large magnetic fields, discussing the robustness of the mechanism in the full many-body dynamics.

We show that the integrable Lindblad superoperators found recently can be used to build integrable nonunitary quantum circuits with two-site gates by demonstrating that the $\check{R}$ matrices are completely positive and trace preserving. Using the bond-site transformation, we obtain the corresponding integrable nonunitary quantum circuits with three-site gates. When restricted to the diagonals of the density matrix, these quantum models reduce to integrable classical cellular automata. This approach may pave the way for a systematic construction of integrable nonunitary quantum circuits.

Quantum Fourier transform (QFT) is a widely used building block for quantum algorithms, whose scalable implementation is challenging in experiments. Here, we propose a protocol of quadratic QFT (QQFT) considering cold atoms confined in an optical lattice. This QQFT is equivalent to QFT in the single-particle subspace and generalizes to a many-body setting through the second quantization formalism. We show this QQFT protocol can be implemented using a programmable laser potential with the digital-micromirror-device techniques recently developed in experiments. The QQFT protocol enables programmable Hamiltonian engineering and allows quantum simulations of Hamiltonian models, which are difficult to realize with conventional approaches. The flexibility of our approach is demonstrated by performing quantum simulations of one-dimensional Poincaré crystal physics and two-dimensional topological flat bands, where the QQFT protocol effectively generates the required long-range tunnelings despite the locality of the cold atom system. We find the discrete Poincaré symmetry and topological properties in the two examples, respectively, have robustness against a certain degree of noise that is potentially existent in the experimental realization. We expect this approach would open wide opportunities for optical lattice-based programmable quantum simulations.

Magnon drag effect induced by an applied electric field in ferromagnetic metals is theoretically studied by a microscopic calculation of the force on magnons arising from magnon emission and absorption and scattering due to driven electrons. It is shown that magnon scattering contribution dominates over the emission and absorption ones in a wide temperature regime in good metals with long elastic lifetime $\tau$, as the latter has a relative suppression factor of ${\left(\mathrm{\Delta }\tau \right)}^{-2}$ due to the electron spin flip by the magnon, where $\mathrm{\Delta }$ is the $sd$ exchange interaction energy. Spin-transfer efficiency is discussed including the magnon drag effect.

We study finite-temperature properties of a Hubbard model including sites of a particle bath, which was proposed as a microscopic model to show itinerant ferromagnetism at finite electron density. We use direct numerical methods, such as exact diagonalization and random vector methods. The temperature dependence of quantities is surveyed in the full range of the temperature. We find that the specific heat has several peaks, which correspond to ordering processes in different energy scales. In particular, magnetic correlations develop at very low temperature. The system exhibits an itinerant ferromagnetic state or an antiferromagnetic state of the Mott insulator depending on the chemical potential of the particle bath and the Coulomb interaction. From a microscopic viewpoint, the competition between these two types of magnetic states causes a peculiar ordering process of local spin correlations. Some local ferromagnetic correlations are found to be robust, which indicates that the ferromagnetic correlation originates from the motion of itinerant electrons in a short-range cluster.