Copper hydroxyhalide materials herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ and Zn-barlowite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6\mathrm{FrBr}$ are thought to be the best realizations of the spin-1/2 kagome quantum antiferromagnetic Heisenberg model and are widely believed to host a spin liquid ground state. However, the exact nature of such a novel state of matter is still under strong debate, partly due to the complication related to the occupation disorder between the zinc and the copper ions in these systems. More specifically, some nonmagnetic ${\mathrm{Zn}}^{2+}$ ions in the system may occupy the ${\mathrm{Cu}}^{2+}$ site within the kagome plane and some spin-$1/2{\mathrm{Cu}}^{2+}$ ions may occupy the ${\mathrm{Zn}}^{2+}$ site between the kagome planes. Both types of impurity can have a nontrivial effect on the dynamical behavior of the antiferromagnetically coupled ${\mathrm{Cu}}^{2+}$ spins on the kagome planes. Recent nuclear magnetic resonance measurements indicate that the magnetic response of the kagome plane is significantly spatial inhomogeneous, even though the content of the misplaced zinc or copper ions is very small. Here we use extensive variational optimization to show that the well-known U(1) Dirac spin liquid state is extremely sensitive to the introduction of the nonmagnetic ${\mathrm{Zn}}^{2+}$ impurity in the kagome plane. More specifically, we find that the zinc impurities can significantly reorganize the local spin correlation pattern around them and induce strong spatial oscillation in the magnetic response of the system. We argue that this is a general trend in highly frustrated quantum magnet systems, in which the nonmagnetic impurity may act as a strongly relevant perturbation on the emergent resonating valence bond structure in their spin liquid ground state. We also argue that the strong spatial oscillation in the magnetic response should be attributed to the free moment released by the doped ${\mathrm{Zn}}^{2+}$ ions and may serve as the smoking-gun evidence for the Dirac node in the U(1) Dirac spin liquid state on the kagome lattice.

We study interactions between polaritons formed by hybridization of excitons in a two-dimensional semiconductor with surface optical phonons or plasmons. These quasiparticles have a high effective mass and can bind into bipolaritons near a Feshbach-like scattering resonance. We analyze the phase diagram of a many-body condensate of heavy polaritons and bipolaritons and calculate their absorption and luminescence spectra, which can be measured experimentally.

In a recent publication [Wurdack et al., Nat. Commun. 14, 1026 (2023)], it was shown that in microcavities containing atomically thin semiconductors, non-Hermitian quantum mechanics can lead to negative exciton polariton masses. Here, we study the mass-sign reversal in two-dimensional systems and show that it is not contingent on the cavity. We show that it can occur generally in radiative resonances in two dimensions and derive conditions for it (critical dephasing threshold, etc.). In monolayer transition-metal dichalcogenides, this phenomenon is not invalidated by the strong electron-hole exchange interaction, which is known to make the exciton massless.

We theoretically study the shot noise and the Fano factor in a voltage-biased junction consisting of a bilayer excitonic insulator (EI) sandwiched between a bilayer semimetal (SM) and a conventional superconductor (S). By comparing the scenarios in SM-EI-S junctions with and without exciton condensates, we illustrate the effects of exciton condensates on the shot noise and demonstrate that the shot noise spectroscopy can signify the existence of exciton condensate in both well-contacted and tunnel SM-EI-S junctions. We also show that in a tunnel SM-EI-S junction, the Fano factor located in the super-Poissonian regime is a consequence of the competition between the exciton-mediated normal reflection and the Cooper-pair-mediated Andreev reflection. By virtue of the scattering probabilities, we elucidate the underlying physics. These results offer an alternative route toward the identification of neutral exciton condensate via charge transport measurements.

We investigate the large Kondo coupling limit of the Kondo-Heisenberg model on one- and two-dimensional lattices. Focusing on the possible superconducting states when slightly doping the Kondo insulator state, we identify different pairing modes to be most stable in different parameter regimes. Possibilities include uniform $s$ wave, pair density wave with momentum $\pi$ (in both one and two dimensions) and uniform $s\pm \mathrm{i}{d}_{x^2-y^2}$ wave (in two dimensions). We attribute these exotic pairing states to the presence of various pair hopping terms with a “wrong” sign in the effective model, a mechanism that is likely universal for inducing pairing states with spatially modulated pair wavefunctions.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic ${\mathrm{SrY}}_2{\mathrm{O}}_4$ single crystals doped with the ${\mathrm{Dy}}^{3+}$ Kramers ions (0.01 at. % and 0.5 at. %) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for ${\mathrm{Y}}^{3+}$ ions at two nonequivalent crystallographic sites with the same local $C_s$ symmetry but strongly different crystal fields. Well-pronounced double-loop hysteresis is observed at temperatures 2, 4, 5, and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic, and kinetic properties of ${\mathrm{Dy}}^{3+}$ ions is developed based on the results of electron paramagnetic resonance, site-selective optical spectroscopy, and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling, and cross relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate, and concentration of paramagnetic ions.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and compare the results to the nonmagnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ that is not present in the Ni-doped material.

Noncoplanar magnetic states with a scalar spin chirality have been intensively studied in condensed matter physics, since they exhibit fascinating physical phenomena. We theoretically propose the generation of such noncoplanar magnetic states by using a circularly polarized electric field. By performing the micromagnetic simulation, we investigate a time evolution of a classical kagome magnet irradiated by the circularly polarized electric field. As a result, we find that the noncoplanar magnetic states are induced as a nonequilibrium steady state irrespective of the ground-state spin configurations. We show that the induced scalar spin chirality is controlled by the amplitude, frequency, and polarization of the electric field. In addition, we clarify that the mechanism of the noncoplanar magnetic states is accounted for by effective field-induced three-spin interactions by adopting the Floquet formalism in the high-frequency regime. We also show a condition to enhance the scalar spin chirality. Our results present a reference for controlling the noncoplanar magnetic states and related phenomena by the circularly polarized electric field.

Multiorbital physics is important to both the correlation physics and topological behavior of quantum materials. LiFeAs is a prototype iron pnictide suitable for in-depth investigation of this issue. Its electronic structure is strikingly different from the prediction of the noninteracting description. Here, a multiorbital Hubbard model is studied using a $U\left(1\right)$ slave-spin theory. We demonstrate a mechanism for a substantial change to the Fermi surface, namely, orbital selectivity of the energy-level renormalization cooperating with its counterpart in quasiparticle spectral weight. Using this effect, we show how the dominating features of the electronic structure in LiFeAs are understood by the local correlations alone. Our results set the stage to understand the origins and nature of both the unconventional superconductivity and likely electronic topology in this prototype iron pnictide and, more generally, reveal a remarkable degree of universality out of the seemingly complex multiorbital building blocks across a broad range of strongly correlated superconductors.

We disclose a serious deficiency of the Baym-Kadanoff construction of thermodynamically consistent conserving approximations. There are two vertices in this scheme: dynamical and conserving. The divergence of each indicates a phase instability. We show that each leads to incomplete and qualitatively different behavior at different critical points. The diagrammatically controlled dynamical vertex from the Schwinger-Dyson equation does not obey the Ward identity and cannot be continued beyond its singularity. The standardly used dynamical vertex alone cannot, hence, conclusively decide about the stability of the high-temperature phase. On the other hand, the divergence in the conserving vertex, obeying the conservation laws, does not invoke critical behavior of the spectral function and the specific heat. Moreover, the critical behavior of the conserving vertex may become spurious in low-dimensional systems. Consequently, the description of the critical behavior of correlated electrons becomes consistent and reliable only if the fluctuations of the order parameter in the conserving vertex lead to a divergence coinciding with that of the dynamical one.

The coupling between electron orbital momentum and spin momentum, known as spin-orbit coupling (SOC), is a fundamental origin of a multitude of fascinating physical phenomena, especially it holds paramount significance in the realm of topological materials. In our paper, we have predicted the topological phase in Hg-based chalcopyrite compounds using the first-principles density functional theory. The initial focus was on ${\mathrm{HgSnN}}_2$, revealing it to be a nonmagnetic Weyl semimetal, while ${\mathrm{HgSnP}}_2$ displayed characteristics of a strong topological insulator. What makes our study truly unique is that despite both compounds having the same SOC strength, arising from Hg, they exhibit distinct topological phases due to the distinct hybridization effect of the Hg-$5d$ and $X-p$ bands. This finding can address a significant factor, i.e., the effect of the band hybridization in deriving distinct topological phases, keeping the symmetry aspect intact. Our results indicate that due to the presence of band hybridization between the dominant $X-np$ orbitals $n$ = 2 and 3 for $X$ = N and P respectively and a minor contribution from Hg-$5d$, we can tune the topological phase by manipulating SOC strength, which equivalently achievable by chemical substitutions. This investigation stands as a remarkable illustration of the unique roles that hybridization plays in sculpting the topological properties of these compounds while simultaneously preserving their underlying symmetries.

The influence of many-body interactions on topological phases is one of the problems that still remains urgent in modern condensed-matter theory. In this study we address this issue within perturbative theory framework by considering topological phase transitions related to charge correlations in the extended Kitaev chain model that belongs to the BDI symmetry class. Obtained corrections to a zero-frequency quasiparticle Green's function allow to separate the mean-field and fluctuation contributions to a total winding number. As a result, the phase transitions caused solely by the latter are unveiled. We thoroughly analyze the mechanism of such transitions in terms of fluctuation-induced nodal points and spectrum renormalization. Additionally, features of other quasiparticle properties such as effective mass and damping are discussed in the context of topological phase transitions.

Quantum anomalous Hall (QAH) insulators with high Chern number host multiple dissipationless chiral edge channels, which are of fundamental interest and promising for applications in spintronics and quantum computing. Here, we propose a dynamic approach for achieving high-Chern-number QAH phases in periodically driven two-dimensional higher-order topological insulators (HOTIs) irradiated by circularly polarized light (CPL). In particular, we consider two representative kinds of HOTIs which are characterized by a quantized quadruple moment and the second Stiefel-Whitney number, respectively. Using the Floquet formalism for periodically driven systems, we demonstrate that QAH insulators with tunable Chern numbers up to four can be achieved. Moreover, we show by first-principles calculations that the monolayer graphdiyne, a realistic HOTI, is an ideal material candidate. Remarkably, the irradiation of CPL does not destroy the quantized quadruple moment in periodically driven HOTIs, leading to an exotic mixed phase harboring both high Chern numbers and higher-order topology. Our work not only establishes a strategy for designing high-Chern-number QAH insulators in periodically driven HOTIs, but also provides a powerful approach to investigate exotic topological states in nonequilibrium cases.

We investigate the potential to induce long-range spin interactions in a Mott insulator via the quantum electromagnetic field of a cavity. The coupling between light and spins is inherently nonlinear, and occurs via multiphoton processes like Raman scattering and two-photon absorption and emission with electronically excited intermediate states. Based on this, two pathways are elucidated: (i) In the absence of external driving, long-range interactions are mediated by the exchange of at least two virtual cavity photons. We show that these vacuum-mediated interactions can surpass local Heisenberg interactions in mesoscopic setups such as sufficiently small split-ring resonators. (ii) In a laser-driven cavity, interactions can be tailored through a hybrid scheme involving both external laser photons and cavity photons. This offers a versatile pathway for Floquet engineering of long-range interactions in macroscopic systems. In general, the derivation of these interactions requires careful consideration: Notably, we demonstrate that a simple phenomenological approach, based on a spin-photon Hamiltonian that captures Raman and two-photon processes with effective matrix elements, can be used only if the cavity is resonantly driven. Outside of these narrow resonant regimes as well as for the undriven case, a fourth-order series expansion within the underlying electronic model is necessary, which we perform to obtain long-range four-spin interactions in the half-filled Hubbard model.

We propose a grated microcavity setup to form trapped polariton condensates with parity-time $\left(\mathcal{PT}\right)$ symmetry and study their polarization dynamics. The pseudoconservative dynamics of the Stokes vector in the proposed configuration is preserved in the presence of a polariton-polariton interaction. In the case of weak gain-dissipation imbalance, as compared to the linear polarization splitting, the polarization Stokes spheres are deformed into ellipsoids. When linear polarization splitting becomes weak, i.e., when $\mathcal{PT}$ symmetry is broken for a noninteracting system, the Stokes spheres are transformed into hyperboloids, but the dynamics is still described by closed trajectories, allowing manipulation of the polarization of the polariton condensate by changing the polarization splitting without losing its coherency.

In this paper, we elucidate the formation of bound states in the continuum (BICs) in compound grating waveguide structures from the perspective of momentum mismatch. As the complex lattice reduces to a simple lattice, the excitable guided resonance turns into an unexcitable BIC due to momentum mismatch. Herein, we refer to this state as momentum mismatch driven BIC. Interestingly, as the incidence changes from normal to oblique, the single momentum mismatch driven BIC splits into dual momentum mismatch driven BICs due to the nonzero tangential momentum of the incident light. Distinct from conventional symmetry-protected and accidental BICs, momentum mismatch driven BICs lie in sections of photonic bands. Consequently, Q factors of momentum mismatch driven quasi-BICs exhibit angular robustness far beyond conventional symmetry-protected and accidental quasi-BICs. Empowered by the momentum mismatch driven BIC, a pair of ellipsometric phase singularities emerge. Enabled by the drastic ellipsometric phase change, ultrasensitive refractive index sensing can be achieved. These results not only provide unique insights into the relation between momentum, BICs, and ellipsometric phase, but also offer a recipe for developing high-performance phase-based optical devices, such as ultrasensitive sensors, wave plates, and spatial light modulators.

Multifunctional properties in quantum systems require an interaction between different degrees of freedom. As such, spin-phonon coupling emerges as an ideal mechanism to tune multiferroicity, magnetism, and the magnetoelectric response. In this Letter, we demonstrate and explain, based on theoretical and experimental analyses, an unusual manifestation of spin-phonon coupling, i.e., prevention of a ferroelastic structural transition, and locking of the high-temperature $R\overline{3}m$ phase in a magnetically frustrated perovskitelike oxide ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$. We present ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$ as a prototype example among its family where a long-range antiferromagnetic structure couples with a low-frequency $E_g$ mode (at $\sim 55{\mathrm{cm}}^{-1}$) that exhibits a large anharmonicity. Our findings establish that spin-phonon coupling clearly suppresses the phonon anharmonicity, preventing the structural phase transition from the $R\overline{3}m$ to the $C2/m$ phase in ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$.