We investigate the design of magnetic ordering in one-dimensional mesoscopic magnetic Ising chains by modulating long-range interactions. These interactions are affected by geometrical modifications to the chain, which adjust the energy hierarchy and the resulting magnetic ground states. Consequently, the magnetic ordering can be tuned between antiferromagnetic and antiferromagnetic dimer phases. These phases are experimentally observed in chains fabricated using both conventional electron-beam lithography and ion implantation techniques, demonstrating the feasibility of controlling magnetic properties at the mesoscale. The ability of attaining these magnetic structures by thermal annealing, underlines the potential of using such systems instead of simulated annealers in tackling combinatorial optimization tasks.

The $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=$ K, Rb, Cs) family of kagome metals hosts unconventional charge density wave (CDW) order whose nature is still an open puzzle. Accumulated evidences point to a time-reversal symmetry breaking (TRSB) orbital flux phase that carries loop currents. Such an order may support anomalous Hall effect. However, the polar Kerr effect measurements that probe the ac anomalous Hall conductivity seem to have yielded contradictory results. We first argue on symmetry grounds that some previously proposed orbital flux order, most notably the one with Star-of-David distortion, shall not give rise to anomalous Hall or polar Kerr effects. We further take the trihexagonal orbital flux phase as an exemplary kagome flux order that does exhibit anomalous Hall response and show that the Kerr rotation angle at two relevant experimental optical frequencies generally reaches microradians to sub-milliradians levels. A particularly sharp resonance enhancement is observed at around $\hslash \omega =1$ eV, suggesting exceedingly large Kerr rotation at the corresponding probing frequencies not yet accessed by previous experiments. Our study provides important guidance to the interpretation of Kerr measurements on the CDW phase of $A{\mathrm{V}}_3{\mathrm{Sb}}_5$. In particular, we highlight the important fact that absence of Kerr signal cannot be equated with the absence of TRSB CDW order in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$.

In anisotropic crystals, Maxwell's equations permit only birefringence for the propagation of light. Notwithstanding, multirefringent systems comprising more than two propagating modes exist, such as in electron optics and photonic crystals. It has been demonstrated that in such systems, the propagation of waves in the short-wavelength limit results in the formation of anomalous caustics. To calculate these caustic curves, we generalized Snell's law valid for reflection and refraction in multirefringent systems possessing more than one propagating mode. The emergence of anomalous caustics in the wave function patterns obtained from rigorous quantum mechanical calculations in electron-optical systems is confirmed. These calculations were performed on electron scattering from circularly gated potential regions applied to multilayer rhombohedral graphene. Our results may generate further work to explore more complex phenomena in multirefringent systems.

We find that the double-threshold voltage-current characteristics in the insulating regime in the ultraclean two-valley two-dimensional electron system in SiGe/Si/SiGe quantum wells are promoted by perpendicular magnetic fields, persisting to an order of magnitude lower voltages and considerably higher electron densities compared to the zero-field case. This observation indicates the perpendicular-magnetic-field stabilization of the quantum electron solid.

We investigate many-body dynamics where the evolution is governed by unitary circuits through the lens of “Krylov complexity,” a recently proposed measure of complexity and quantum chaos. We extend the formalism of Krylov complexity to unitary circuit dynamics and focus on Floquet circuits arising as the Trotter decomposition of Hamiltonian dynamics. For short Trotter steps the results from Hamiltonian dynamics are recovered, whereas a large Trotter step results in different universal behavior characterized by the existence of local maximally ergodic operators: operators with vanishing autocorrelation functions, as exemplified in dual-unitary circuits. These operators exhibit maximal complexity growth, act as a memoryless bath for the dynamics, and can be directly probed in current quantum computing setups. These two regimes are separated by a crossover in chaotic systems. Conversely, we find that free integrable systems exhibit a nonanalytic transition between these different regimes, where maximally ergodic operators appear at a critical Trotter step.

The recent discovery of high-temperature superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure has stimulated intensive investigations concerning its paring mechanism and a correct description of its effective low-energy physics. Notable experimental observations include the right-triangle-shaped superconducting dome with the maximum $T_c$ of about 80 K and the strange metal behavior in the normal state above $T_c$. Here we apply the Schwinger boson approach to the bilayer $t\text{−}V\text{−}J$ model, which allows us to treat well both the magnetic correlations and the superconducting instability. We obtain a global phase diagram with both metallic and superconducting ground states separated by a quantum phase transition, and predict a minimal interlayer superexchange $J$ and a critical (minimal) $d_{z^2}$-hole concentration necessary for the superconductivity, with a highly asymmetric right-triangle-like shape for the $T_c$ curve as well as a maximum $T_c$ comparable with experiments. The normal state is featured with a pseudogap in the spectra of $d_{x^2-y^2}$ electrons associated with preformed Cooper pairs and non-Fermi-liquid strange-metal behavior due to hybridization-induced spinon-holon-electron scattering. Our work clarifies the key difference of the two-component scenario of the superconductivity from other mechanisms, and provides a consistent understanding of the high-temperature superconductivity and strange metallic properties of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure.

We construct an efficient momentum space approach to the superconductivity in quasiperiodic multilayer systems. To this end, we extend the Brillouin zone (BZ) folding method to the superconducting (SC) phases by formulating the gap equation in the momentum space representation with the BZ folding. We show that the physical observables in quasiperiodic multilayers are generally given by so-called quasiperiodic functions. Consequently, there appear SC order parameters with finite momenta in the BZ folding method, which corresponds to the spatial fluctuation of the SC order in quasiperiodic multilayers. We also find a systematic way to compute the physical observables in the BZ folding method with a proper normalization condition using the continued fraction approximation. We apply the BZ folding method to a one-dimensional toy model of quasiperiodic superconductors and demonstrate its numerical efficiency compared to the conventional method based on the real-space representation with a large system size. We also study a quasiperiodic bilayer system of Rice-Mele model and an $s$-wave superconductor to demonstrate an efficient computation of optical responses of a quasiperiodic superconductor with inversion symmetry breaking.

With the rapid development of negative thermal expansion (NTE) materials, there is an increasing need to explore the underlying mechanisms behind NTE, particularly the phonon vibration mechanisms, which have garnered considerable attention from researchers. The isotropic NTE compound tetracyanoborates $AB{\left(\mathrm{CN}\right)}_4$ ($A=\mathrm{Cu}$, Li, Ag), exhibits an intriguing phenomenon: a significant enhancement in NTE upon the substitution of $A$-site atoms. This study utilizes first-principles calculations based on density functional theory to investigate how $A$-site atom substitution affects thermal expansion mechanisms. Crystal structure analysis indicates that the substitution of $A$-site atoms changes the ionic character and bond lengths of $A$-N bonds, thereby influencing structural flexibility. Moreover, a strong linear relationship is observed between the coefficients of thermal expansion (CTE) and the average atomic volume (AAV). Phonon vibration analysis, based on the Grüneisen parameters, reveals that low-frequency optical phonon modes involve coupled rotations of the $A{\mathrm{N}}_4$ and $B{\mathrm{C}}_4$ tetrahedra, along with nonrigid movement of the $A{\mathrm{N}}_4$ tetrahedra, leading to a reduction in lattice volume causing NTE. In addition, the low-frequency transverse acoustic (TA) mode with negative Grüneisen parameter induces contraction in specific directions, further promoting NTE. Thus, the synergistic effects of low-frequency optical and acoustic modes emerge as the primary drivers behind the NTE phenomenon. This study offers insight into the phonon vibration mechanisms in tetracyanoborates, contributing to a deeper understanding of the NTE mechanism.

The solitonic spin textures such as chiral solitons, magnetic skyrmions, and magnetic hopfions, exhibiting particlelike nature, widely emerge in magnets depending on the spatial dimension. In the presence of magnetic solitons, their number directly gives rise to an impact on magnetic properties and electronic properties such as magnetoresistance and hence, it is of great importance to control the number of solitons. Meanwhile, a systematic study on dynamical processes to control the number of solitons, particularly by adding the desired number of solitons to the ground state exhibiting periodic arrangements of solitons, has been limited thus far. In this paper, we theoretically perform the systematic analysis for the dynamical control of the number of chiral solitons in monoaxial chiral magnets by effectively utilizing the edge modes, whose excitation is localized near the edges. First, by using the linear spin-wave theory, we clarify that the edge modes are brought about within the bulk magnon band gap opened by the solitonic feature introduced by the static magnetic field perpendicular to the helical axis. Next, by using the Landau-Lifshitz-Gilbert equation, we find that the dynamical process associated with this edge mode exhibits the soliton penetration into the periodic arrays of chiral solitons in an applied rotating magnetic field. An additional chiral soliton penetrates into the system at the left or right edge depending on the rotating direction of the magnetic field, with the precursor wherein bulk solitons move to the other side. Moreover, we show that multiple soliton penetrations can take place till the system reaches the nonequilibrium steady state, and the number of infiltrated solitons successively increases with the amplitude of the rotating magnetic field after surpassing the threshold. We also clarify that the static magnetic field parallel to the helical axis brings about the difference in the number of penetrating solitons as well as the threshold amplitude between the clockwise and counterclockwise rotating magnetic field. Our results reveal that the desired number of solitons can be added within a certain range by taking advantage of the edge modes that appear without any special processing at the edges of the system. These results contribute to the development of an experimental way to control the number of solitons and are expected to be further applied to a wide range of magnetic solitons, not limited to chiral solitons.

Exceptional points (EPs), a unique feature of non-Hermitian systems, represent degeneracies in non-Hermitian operators that likely do not occur in Hermitian systems. Nevertheless, unlike its fermionic counterpart, a Hermitian bosonic Kitaev model supports a non-Hermitian core matrix, involving a quantum phase transition (QPT) when an exceptional point appears. In this study, we examine QPTs by mapping the Hamiltonian onto a set of equivalent single-particle systems using a Bardeen-Cooper-Schrieffer-like pairing basis. We demonstrate the connection between the hidden EP and the localization-delocalization transition in the equivalent systems. The result is applicable to a Dicke model, which allows the experimental detection of the transition based on the measurement of the average number of photons for the quench dynamics starting from the empty state. Numerical simulations of the time evolution reveal a clear transition point at the EP.

Flat bands (FBs) in materials can enhance the correlation effects, resulting in exotic phenomena. Line-graph (LG) lattices are well known for hosting FBs with isotropic hoppings in $s$-orbital models. Despite their prevalent application in the kagome metals, there has been a lack of a general approach for incorporating higher-angular-momentum orbitals with spin-orbit couplings into LGs to achieve FBs. Here, we introduce a non-Abelian LG theory to construct FBs in realistic systems, which incorporates internal degrees of freedom and goes beyond $s$-orbital models. We modify the lattice edges and sites in the LG to be associated with arbitrary Hermitian matrices, referred to as the multiple LG. A fundamental aspect involves mapping the multiple-LG Hamiltonian to a tight-binding model that respects the lattice symmetry through appropriate local non-Abelian transformations. We establish the general conditions to determine the local transformations. Based on this mechanism, we demonstrate the realization of $d$-orbital FBs in the kagome lattice, which could serve as a minimal model for understanding the FBs in transition-metal kagome materials. Our approach bridges the gap between the known FBs in pure lattice models and their realization in multiorbital systems.

The magnetic-field dependence of photoabsorption provides direct insights into the band structure of semiconductors. It is perhaps surprising that there is a large discrepancy between electron, hole, and reduced mass reported in the recent literature. Motivated by this puzzle we reconsider excitonic magnetoabsorption and find that the commonly employed perturbative approach, namely for computing the diamagnetic shift, is inadequate to account for the parameter ranges considered in existing data. In particular, we develop the theory for strong magnetic field and, upon analysis of the data, arrive at the set of exciton parameters different to what has been estimated perturbatively in the literature. Only $s$-wave excitons are visible in photoluminescence as the spectral weight of $p$-wave states is too small, this limits the amount of information that can be extracted about the underlying band structure. To overcome this, we propose to study $p$-wave states by mixing them with $s$-wave states by external in-plane electric field and show that a moderate dc electric field would provide sufficient mixing to brighten $p$-wave states. We calculate energies of the $p$-wave states including the effects of valley-orbital splitting and the orbital Zeeman shift, and show that this provides direct information on the electron-hole mass asymmetry.

The even denominator fractional quantum Hall effect has been experimentally observed in graphene in the fourth Landau level ($\mathcal{N}=3$). This paper is motivated by recent studies regarding the possibility of pairing and the nature of the ground state in this system. By extending the dipole representation of composite fermions, we adapt this framework to the context of graphene's quantum Hall systems, with a focus on half-filled Landau levels. We derive an effective Hamiltonian that incorporates the key symmetry of half-filled Landau levels, particularly particle-hole symmetry. At the Fermi level, the energetic instability of the dipole state is influenced by the interplay between topology and symmetry, driving the system toward a critical state. We explore the possibility that this critical state stabilizes into one of the paired states with well-defined pairing solutions. However, our results demonstrate that the regularized state which satisfies boost invariance at the Fermi level and lacks well-defined pairing instabilities emerges as energetically more favorable. Therefore, we find no well-defined pairing instabilities of composite fermions in the dipole representation in the half-filled fourth Landau level ($\mathcal{N}=3$) of electrons in graphene. Although the theory of composite fermions has its limitations, further research is required to investigate other possible configurations. We discuss the consistency of our results with experimental and numerical studies and their relevance for future research efforts.

Two-dimensional (2D) magnetic interactions, a single $d_{x^2-y^2}$ band, and strong orbital splitting between $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals are essential characteristics of cuprate superconductors. However, infinite-layer nickelates with a small rare-earth ion display three-dimensional (3D) magnetic correlations, an itinerant Ni $d_{3z^2-r^2}$ band at the Fermi level, and weak orbital splitting between two $e_g$ orbitals, strikingly different from those of a cuprate, which might be detrimental for superconducting properties. Here, through first-principles calculations, we predict that epitaxial strain in infinite-layer nickelates $R{\mathrm{NiO}}_2$ could fulfill the deliberate switching from 3D to 2D electronic and magnetic transitions. A general strain-orbital-spin relationship is proposed, and the strain-controlled relative positions of the localized Ni $d_{x^2-y^2}$ band and the itinerant Ni $d_{3z^2-r^2}$ band are revealed to be the key ingredient that governs the Fermi surface and strength of the interlayer magnetic interactions. For $R{\mathrm{NiO}}_2$ with a smaller rare-earth ion like ${\mathrm{DyNiO}}_2$, we propose that the desired cupratelike electronic structure and 2D magnetic dimensionality could be successfully realized by compressive strain through tuning the energy level of Ni $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ bands. We further develop a finite-temperature lattice Wannier model and a Heisenberg spin model, which establish a complete temperature- and strain-dependent structural and magnetic phase diagram of epitaxial ${\mathrm{DyNiO}}_2$ thin films. Our work opens a new perspective for the modulation of lattice, orbital, and spin states in infinite-layer nickelates, and it provides rich structural, electronic, and magnetic flexibility for the control of the superconducting-related properties.

We investigate the thermoelectric properties of a double quantum dot system coupled to two metallic reservoirs, focusing on two main effects: (i) the influence of coupling asymmetry between the quantum dot and the reservoirs on the Seebeck coefficient and (ii) the impact of asymmetry in the energy levels of the dots on current rectification. In the first case, we find that introducing moderate asymmetry significantly enhances the Seebeck coefficient. In the second case, while rectification vanishes when the energy levels are degenerate, substantial rectification is achieved when one energy level lies below and the other above the Fermi level. We further interpret the dependence of rectification magnitude and shape on system parameters using analytical results from a spinless model.

The continuous improvement of fabrication techniques and high-quality semiconductor-superconductor interfaces allows for an unprecedented tunability of Josephson junction arrays (JJA), making them a promising candidate for analog quantum simulations of many-body phenomena. While most experimental proposals so far focused on quantum simulations of ensembles of two-level systems, the possibility of tuning the current-phase relation beyond the sinusoidal regime paves the way for studying statistical physics models with larger local Hilbert spaces. Here, we investigate a particular JJA architecture that can be mapped into a ${\mathbb{Z}}_3$ clock model. Through matrix-product-states simulations and bosonization analysis, we show that few experimentally accessible control parameters allow for the exploration of the rich phase diagrams of the associated low-energy field theories. Our results expand the horizon for analog quantum simulations with JJAs towards models that can not be efficiently captured with qubit architectures.