We perform large-scale quantum Monte Carlo simulations of the Hubbard model at half filling with a single Dirac cone close to the critical point, which separates a Dirac semimetal from an antiferromagnetically ordered phase where SU(2) spin rotational symmetry is spontaneously broken. We discuss the implementation of a single Dirac cone in the SLAC formulation for eight Dirac components and the influence of dynamically induced long-range superexchange interactions. The finite-size behavior of dimensionless ratios and the finite-size scaling properties of the Hubbard model with a single Dirac cone are shown to be superior compared to the honeycomb lattice. We extract the critical exponent believed to belong to the chiral Heisenberg Gross-Neveu-Yukawa universality class: the critical exponent $\nu =1.02\left(3\right)$ coincides for the two lattice types once honeycomb lattices of linear dimension $L>15$ are considered. In contrast to the SLAC formulation, where the anomalous dimensions are estimated to be ${\eta }_{\varphi }=0.73\left(1\right)$ and ${\eta }_{\psi }=0.09\left(1\right)$, they remain less stable on honeycomb lattices, but tend towards the estimates from the SLAC formulation.

Polarization doping, which can be applied in numerous III-nitride devices such as lasers, light-emitting diodes, and high electron mobility transistors, produces temperature-independent three-dimensional electron and hole gases. However, current polarization doping models are far from perfect, ignoring some key issues and great potential. We have greatly enriched the model using first-principles calculations on ternary group-III nitride alloys. We find that the polarization charge density depends on the polarization change rate as a function of composition and tensile strain, as well as on the grading speed as a function of grading range, nonlinearity, and thickness. The polarization change rate varies significantly for all materials studied except AlGaN, which is very advantageous for achieving large carrier densities and novel device structures. We have also experimentally verified our proposed theoretical model by experimentally characterizing the rates of III-polar AlGaN and N-polar InGaN samples in good agreement with our theoretical calculations. The significance of our work can greatly contribute to the development of polarization doping applications of a III-nitride system.

Topological phase transitions beyond anyon condensation remain poorly understood. A notable example is the transition between the toric code (TC) and double semion (DS) phases, which has two distinct ${\mathbb{Z}}_2$ topological orders in (2+1) dimensions [$(2+1)\mathrm{D}$]. Previous studies reveal that the transition between them can be either first order or via an intermediate phase, thus the existence of a direct continuous transition between them remains a long-standing problem. Motivated by the fact that both phases can arise from condensing distinct anyons in the ${\mathbb{Z}}_4$ topological order, we introduce a perturbed ${\mathbb{Z}}_4$ quantum double (QD) model to study the TC-DS transition. We confirm the existence of a continuous $(2+1)\mathrm{D}$ XY* transition between the TC and DS phases by mapping it to a two-coupled quantum Ising model. Importantly, using the condensation order parameters and the area law coefficients of the Wilson loops, we further reveal that ${\mathbb{Z}}_4$ anyons, fractionalized from the ${\mathbb{Z}}_2$ topological orders, become deconfined at the transition between ${\mathbb{Z}}_2$ topologically ordered phases. Our results open a path toward developing a theoretical framework for topological phase transitions beyond anyon condensation.

Real-space superconducting properties are increasingly important to characterize low-dimensional, layered, and nanostructured materials. Here, we present a method to extract the real-space superconducting order parameter from the superconducting gap spectrum obtained via anisotropic Migdal-Eliashberg calculations, using the Bloch wave functions of the Fermi states. We apply this approach to a selection of atomically thin material systems. Our analysis of gallenene, a monolayer of gallium atoms, shows that its planar and buckled phases exhibit distinct superconducting order parameter behaviors, shaped by their structural and electronic properties. Furthermore, we demonstrate that our real-space approach is exceptionally suited to identify and characterize Josephson junctions made from van der Waals materials. Our examination of a bilayer of ${\mathrm{NbSe}}_2$ reveals that the van der Waals gap acts as an intrinsic weak link between the superconducting ${\mathrm{NbSe}}_2$ layers. Therefore, a bilayer of ${\mathrm{NbSe}}_2$ represents one of the thinnest and most tunable Josephson junction architectures, with potential applications in quantum devices. Our findings underscore the utility of transformation into real space in understanding superconducting properties through atomistic simulations.

We numerically investigate measurement-induced phase transitions in monitored free fermions through the spectral and eigenstate properties of the entanglement Hamiltonian. By analyzing entanglement scaling, we identify three nontrivial fixed points, for which the entanglement follows exact analytical scaling forms: chaotic unitary dynamics at infinitesimal monitoring, characterized by a Gaussian Page law; a Fermi-liquid fixed point at moderate monitoring, defining a metallic phase with logarithmic entanglement growth and emergent space-time invariance; and a quantum Lifshitz fixed point marking the measurement-induced phase transition into a localized area-law phase. Adopting a random-matrix perspective on the entanglement Hamiltonian, we show that short-range spectral correlations, such as the adjacent gap ratio $\langle \tilde{r}\rangle$ and the Kullback-Leibler divergence ${\text{KL}}_1$, sharply detect an ergodic to nonergodic phase transition at the quantum Lifshitz fixed point, and yield precise estimates of the critical point and correlation length exponent. Long-range probes, including the spectral form factor and the associated Thouless time, corroborate this picture, while the variant ${\text{KL}}_2$ uncovers signatures of a possible nonergodic extended (multifractal) regime at intermediate monitoring strengths. Together, these results establish the entanglement Hamiltonian as a powerful framework for diagnosing metallic, localized, and multifractal regimes in monitored quantum dynamics, and highlight unexpected scaling structures, Fermi liquid, and Lifshitz criticality, that lie beyond current field-theoretic approaches.

The kagome lattice is a two-dimensional (2D) network of corner-sharing triangles found in several rare-earth materials resulting in a complicated and often frustrated magnetic system. In the past decades, modifications of the motif, such as breathing kagome, asymmetric kagome, and twisted kagome, were brought into the limelight. In particular, the latter has symmetry lower than that of the original kagome and thus allows implementations of an “Ising-local” Hamiltonian, leading to a 2D spin ice. One such material implementation, HoAgGe, was recently reported to have an exceptionally rich phase diagram and is a strongly frustrated 2D spin-ice material with a twisted kagome geometry. In the presence of an external magnetic field the compound exhibits steplike magnetization plateaus at simple fractions of the saturation magnetization. It is believed that this phenomenon results from strong single-site anisotropy, which in HoAgGe was found to be in-plane and along a high-symmetry direction. Previous Monte Carlo simulations with empirical exchange parameters explain some, but not all, experimental observations. In this work, we present (a) first-principles calculations of the crucial model parameters and (b) direct energy minimization via a reduced configuration space search, as well as Monte Carlo simulations of the field-dependent phase diagram. We find that for HoAgGe the calculated exchange parameters are very different from the earlier suggested empirical ones, and we describe the phase diagram much more accurately. This is likely because the first-principles parameters are, in addition to geometrically, also parametrically frustrated.

The delayed demagnetization in Ni relative to Fe in the ultrafast demagnetization studies in FeNi alloy has led to two competing theoretical explanations: The Inhomogeneous Magnon Generation (IMG) and the Optically Induced Spin Transfer (OISTR) model. The IMG attributes the delay to the preferential magnon generation at the Fe sites and its subsequent propagation to Ni, while OISTR proposes direct spin transfer from Ni to Fe. In this study, we employ element-resolved extreme ultraviolet spectroscopy to investigate the effect of excitation strength on this delay, aiming to resolve the controversy. The data indicate a significant reduction in the delay with increasing fluence, which is inconsistent with the theoretical predictions of OISTR. These findings, in conjunction with the observation of a saturation of Fe demagnetization at the onset of Ni demagnetization, indicate that a spin-wave instability within the IMG framework may provide a potential explanation for the experimental results.

Recent experiments in rhombohedrally stacked multilayer graphene heterostructures have reported signatures of chiral superconductivity, emerging from a spin- and valley-polarized normal state with broken time-reversal symmetry and an associated anomalous Hall effect. These findings bring into focus the role of the electronic Bloch wave function and the quantum geometric tensor in determining the superconducting pairing channel. In this work, we examine superconducting instabilities of a model of $N$-layer rhombohedral graphene that possesses an enhanced Berry curvature distribution on an extended ring in momentum space, that we dub the ‘‘Berry ring of fire,’’ in the presence of an isotropic attractive interaction with a parametrically controlled spatial range. We determine that local interactions favor a $N$-fold winding in the order parameter phase for odd-$N$ layered systems, with even-$N$ layers requiring a spatially extended attraction range to achieve pairing. For generic interaction lengths, we discover a family of chiral superconductors and, remarkably, momentum-space vortices nucleated on the Berry ring of fire. The existence of these vortices can be traced to a momentum-space flux quantization condition involving the Berry curvature, with the phase winding dictated by a combination of the Berry flux and a ‘‘statistical flux’’ to enforce Fermi-Dirac statistics. Such an order-parameter structure allows for the possibility of in situ tuning between various chiral superconducting phases through changes in the electron density or the displacement field. We discuss ways in which these predictions can be experimentally tested and potentially exploited in future devices.

A topological Anderson insulator is an intriguing transport phenomenon typically driven by short-range nonmagnetic disorder, and the interplay between disorder and magnetism plays a crucial role in the topological phase diagram of two-dimensional electron systems. Here, by performing nonequilibrium Green's function transport calculations, we numerically demonstrate that long-range correlated magnetic disorder can induce topological phase transitions in a two-dimensional quantum anomalous Hall system. We show that spatially correlated magnetic fluctuations can drive a trivial insulator into a topologically nontrivial phase, characterized by quantized conductance arising from chiral edge currents. We further reveal the crucial role of the disorder correlation length and system size in stabilizing this topological phase. Additionally, by comparing the transport properties of systems with correlated and short-range magnetic disorder, we identify the distinctive characteristics of topological phase transitions induced by correlated magnetic disorder. Our findings uncover a new route toward disorder-engineered topological phases via correlated magnetic disorder, and they provide guidance for realizing robust topological transport in disordered magnetic systems.

We consider the spin rotationally invariant Kalmeyer-Laughlin chiral spin liquid (CSL) in systems with broken time-reversal symmetry and explore symmetry constraints on possible conventional spin crystal states accessible via a direct transition. These constraints provide a framework to identify topological invariants of the magnetically ordered state. We show that the existence of a direct transition from a CSL requires a precise compatibility condition between the topological invariants of the ordered state and the anomaly of the CSL. The lattice symmetries also constrain the functional form of the low-energy theory to describe these transitions. This allows us to construct explicit Chern-Simons-matter field theories for the transition into a class of noncoplanar orders identified as candidates directly accessible from the CSL, including the octahedral spin crystal on the kagomé lattice and the tetrahedral order on the triangular and honeycomb lattice. These transitions can either be described using coupled fractionalized ${\mathbb{CP}}^1$ theories or fractionalized matrix principal chiral models. We also discuss extensions to more general magnetic ordering transitions out of the CSL.

Thermodynamic and muon spin relaxation measurements have recently highlighted ${\mathrm{BiYbGeO}}_5$ as a new example of a rare-earth-based quantum dimer magnet with isolated ${\mathrm{Yb}}^{3+}$ spin-1/2 dimers. However, direct spectroscopic evidence of the triplet excitations and measurements of the structural disorder are lacking. In this work, polycrystalline ${\mathrm{BiYbGeO}}_5$ was synthesized using conventional high-temperature solid-state methods and investigated via high-resolution neutron powder diffraction and inelastic neutron scattering. Diffraction measurements down to 58 mK reveal no signatures of magnetic order and indicate that nearly 20% of ${\mathrm{Yb}}^{3+}$ sites are replaced by nonmagnetic ${\mathrm{Bi}}^{3+}$, introducing significant structural disorder. Inelastic neutron scattering shows dispersionless triplon excitations, consistent with localized, noninteracting spin dimers. Fits to the triplet excitation spectrum with an XXZ-type anisotropic exchange give $J_{\text{XX}}=0.100\left(1\right)\text{meV}$ and $J_{\text{Z}}=0.202\left(1\right)\mathrm{meV}$. These findings establish ${\mathrm{BiYbGeO}}_5$ as a structurally disordered but magnetically well-isolated quantum dimer system, providing a model platform for studying the resilience of entangled spin states to site dilution.

Operation near non-Hermitian (NH) degeneracies, known as exceptional points (EPs), can lead to enhanced sensitivities with magnitude depending upon the order of the EP. The signature of these anomalies is reflected in the full coalescence of both eigenvalues and eigenvectors. A natural question arises as to how the sensitivity alters if the eigenvectors partially coincide—we shall refer to these anomalies as partial degeneracies (PDs). Here, we derive the maximum sensitivity at these points and the conditions for it to emerge (our analysis also yields the conditions leading to maximum sensitivity at EPs). The investigation begins with the familiar case of EPs and is systematically extended to scenarios where PDs occur within NH settings involving a full or a partial eigenvalue collapse. In this regard, the theory of compound/higher adjugate matrices is employed, while certain results are also verified via the Faddeev-LeVerrier algorithm. Overall, our analysis sheds light on the behavior of NH settings close to degeneracies of any type, resolving a critical limitation of time-independent perturbation theory in such contexts. Moreover, it lays the groundwork for further explorations on the topological features and transport effects associated with evolution near PDs.

The $\mathrm{\Gamma }$-point topology represents a significant segment in the family of topological insulators. Here we provide a comprehensive prediction of light-induced $\mathrm{\Gamma }$-point-based topological manipulation in trigonal bismuthene and its derivatives. Our findings unveil a two-stage process of topological phase transitions (TPT) as the laser intensity increases. Initially, a quantum-spin-Hall or metallic state transitions to a quantum-anomalous-Hall (QAH) state ($C=\pm 3$), followed by another TPT that yields a compensated Chern-insulating state ($C=0$). The trigonal warping model accounts for these states, describing the $C_{3z}$-rotational band-inversion process, which is determined by $\pm 1$ orders of replica bands. Notably, this high Chern-number QAH state persists over a broad range of laser parameters. It remains functional beyond room temperature, as indicated by the substantial global energy gaps ($\ge 60\mathrm{meV}$) that are less constrained by other factors, including magnetic transition temperatures and doping effects. Our work provides a comprehensive roadmap towards the designer $\mathrm{\Gamma }$-point topology under laser excitation, facilitating applications of artificial topological materials.

Coulomb-driven renormalization of electronic spectra in monolayer transition-metal dichalcogenides (TMDCs) remains poorly understood at finite temperature. Using the Rytova-Keldysh potential with a nonlocal dielectric response, we calculate quasiparticle band-gap renormalization (BGR) and the Fermi-edge absorption spectrum over experimentally relevant carrier densities and temperatures. Exchange and correlation self-energies are treated successively within Hartree-Fock (HF), the random-phase approximation (RPA), and the Hubbard local-field approximation (HFA). Only the HFA, which embeds the $G\left(q\right)$ local-field factor, reproduces recent temperature- and density-resolved measurements: it broadens the band gap at low densities and produces a density-induced redshift of the Fermi absorption edge through enhanced screening. The same framework accounts for the nonmonotonic BGR observed in cyclotron resonance experiments on disordered monolayers when disorder-induced thermal broadening is included. The results establish a local-field-corrected many-body theory as the minimal quantitative description of carrier-doped TMDCs and provide a roadmap for engineering interaction-driven electronic phases in two dimensions.

The structural, magneto-optical, and magnetotransport properties as well as the electronic band structure of the bulk crystalline topological insulator (TI) ${\mathrm{Sb}}_2{\mathrm{Te}}_2\mathrm{Se}$, grown by the vertical Bridgman technique, were studied. The high structural quality of the grown crystals was established by x-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy revealed a single Dirac cone with the Dirac point, away from the valence as well as conduction bands, 0.22 eV above the Fermi level. The magnetotransport data exhibited a distinct single-frequency Shubnikov-de Haas oscillation in the magnetic fields above $B$ = 10 T. The Lifshitz-Kosevich analysis of the data suggests that this oscillation originates from the Dirac-type states. A sharp fundamental absorption edge in the mid-infrared transmission spectra measured at 4.2 K demonstrated a direct band gap of 0.377 eV located at a momentum of 0.1 ${\AA }^{-1}$ along the $\overline{\mathrm{\Gamma }}\text{−}\overline{K}$ directions of the Brillouin zone. A two-band model developed for massive Dirac electrons in the bulk of topological insulators with the direct band gap at a non-Г point suggests hyperbolic dispersion relations for the conduction and valence bands displaying the full electron-hole symmetry. Equal magnitude of the electron and hole effective masses $m_e\approx m_h\approx 0.21m_0$ and $g$ factors $g_e\approx g_h\approx 10$ were estimated.

Altermagnets are an emerging class of unconventional antiferromagnets, characterized by a Néel ordering that does not break the translational symmetry of the underlying lattice. Depending on the orientation of the Néel vector, the anomalous Hall effect (AHE) may or may not exist. In the so-called pure altermagnets, the AHE is forbidden by magnetic symmetry. Here, we demonstrate that in pure altermagnets, domain walls can lift the symmetry constraints, thereby activating the AHE and orbital magnetization. Taking a representative example of a rutile-lattice tight-binding minimal model in slab geometry, we use linear response theory to demonstrate the emergence of the domain-wall AHE, finding that it is closely related to the orbital magnetization, while the spin magnetization does not play a significant role. Using Landau theory, we argue that, while for a random arrangement of $\pi$ domain walls, the contributions from the individual domain walls will cancel one another, an external magnetic field will favor domain-wall arrangements with specific chirality giving rise to a net AHE signal. Using group theory, we discuss how these findings can be straightforwardly generalized to certain other classes of altermagnets. Our work reveals a crucial role for domain walls in understanding Hall transport and orbital magnetism of altermagnets. Our work generally calls for a rigorous analysis of Hall-transport data for an altermagnet, ideally in conjunction with imaging data, in order to unambiguously assign an observed Hall effect to magnetic domains or to domain walls.

We have investigated the doping-induced variation of magnetic and charge transport properties of single crystalline $\mathrm{Ce}{\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Ge}}_3$ with a noncentrosymmetric tetragonal $\mathrm{Ba}\mathrm{Ni}{\mathrm{Sn}}_3$-type structure. The magnetization measurements revealed that, with increasing doping level $x$, the antiferromagnetic phase for $x=0$ turns into a ferromagnetic one at $x=0.15$, a cluster-glass-like phase at $x=0.46$, and eventually evolves into a paramagnetic phase above $x=0.67$. For $0\le x<0.46$ wherein the electrical resistivity shows the Fermi liquid behavior, the coefficient of the $T^2$-term of resistivity is small for $0\le x\le 0.33$ but increases sharply at around $x\sim 0.42$, indicating the enhancement of density of state near the Fermi energy. Hall resistivity measurements indicate that, at 2 K in the ferromagnetic phase, the anomalous Hall conductivity remains nearly constant over the range $0.15\le x\le 0.42$ but markedly decreases with increasing $x$ beyond 0.42. Notably, an anomalous Hall component that is not proportional to magnetization is observed for $x\ge 0.42$. It is anticipated that the Berry curvature in momentum space significantly changes across the transition from the ferromagnetic phase to cluster-glass-like phase, driven by a change in the Kondo hybridization in the present materials.