Elucidating the impact of strong electronic interactions on the collective excitations of metallic systems has been of longstanding interest, mainly due to the inadequacy of the random phase approximation (RPA) in the strongly correlated regime. Here, we adopt our newly developed radial Kotliar and Ruckenstein slave boson representation to analyze the charge excitation spectrum of a Hubbard model, extended with long range interactions. Working on the face centered cubic lattice, at half filling, and in different coupling regimes ranging from uncorrelated to the metal-to-insulator transition, we compare our results to conventional RPA as a benchmark. We focus on the influence of the local and long range couplings on the particle-hole excitation continuum and the plasmon and upper Hubbard band collective modes. Beyond the weak coupling regime, we find numerous quantitative and even qualitative discrepancies between our method and standard RPA. Our work thus deepens the understanding of charge collective modes in correlated systems and lays the foundations for future studies of a broad series of materials.

We study the uniform $\text{U}\left(1\right)$ quantum spin liquid (QSL) with low-energy fermionic quasiparticles for pyrochlore magnets with dipolar-octupolar symmetry, employing a fermionic parton mean-field theory approach. Self-consistent calculations stabilize 12 fully symmetric uniform $\text{U}\left(1\right)$ QSLs, of which four mean-field states are “monopole-flux” states. Several of these mean-field states show a linear temperature dependence of specific heat at low temperatures, the other phases show a power-law temperature dependence of specific heat $C\sim T^{\alpha }$. The linear temperature dependence of specific heat at low temperatures is reminiscent of the observations in ${\text{Nd}}_2{\text{ScNbO}}_7$. We further compute the dynamic spin structure factors and discuss the possible signature of these fermionic spinons in neutron-scattering experiments.

While the semiconductor quantum dot placed in a solid-state material allows for deterministic emission of single photons, the photon indistinguishability is strongly influenced by the intrinsic coupling to lattice vibrations, phonons, of the solid-state environment. This work investigates the phonon-induced decoherence for a quantum dot placed in the one-dimensional (1D) system of a homogeneous cylindrical nanowire. Such a structure supports multiple longitudinal phonon branches, and we consider both a linear and a quadratic coupling of the emitter to these modes. Under a polaron approach, we initially derive an analytical expression for the 1D pure dephasing rate, which leads to a reduced pure dephasing rate compared with bulk. By implementing these results into a full cavity quantum electrodynamic model, we demonstrate that multimode coupling is necessary to correctly predict the indistinguishability in a 1D system, which may otherwise be significantly underestimated.

The interaction-driven transition between quantum spin Hall and Mott insulators in the Bernevig, Hughes, and Zhang model is studied by dynamical cluster approximation, and found to be accompanied by the emergence of Green's function zeros already in the quantum spin Hall regime. The nontrivial interplay between Green's function poles and zeros leads to an exotic quantum spin Hall insulator exhibiting two chiral branches of edge Green's function poles and one of zeros. When symmetry breaking is allowed, a nontopological excitonic insulator is found to intrude between quantum spin Hall and Mott insulators. We find evidence that excitons in the Mott insulator, which become soft at the transition to the excitonic insulator, are actually bound states between valence and conduction bands of Green's function zeros, rather than between lower and upper Hubbard bands.

We study the entanglement structure of topological orders subject to decoherence on the bipartition boundary. Focusing on the toric codes in $d$ space dimensions for $d=2,3,4$, we explore whether the boundary decoherence may be able to induce a disentangling transition, characterized by the destruction of mixed-state long-range entanglement across the bipartition, measured by topological entanglement negativity. A key insight of our approach is the connection between the negativity spectrum of the decohered mixed states and emergent symmetry-protected topological orders under certain symmetry-preserving perturbation localized on the bipartition boundary. This insight allows us to analytically derive the exact results of entanglement negativity and characterize the universality class of the disentangling transition without using a replica trick.

The $GW$ approximation is a widely used method for computing electron addition and removal energies of molecules and solids. The computational effort of conventional $GW$ algorithms increases as $O\left(N^4\right)$ with the system size $N$, hindering the application of $GW$ to large and complex systems. Low-scaling $GW$ algorithms are currently very actively developed. Benchmark studies at the single-shot $G_0{W}_0$ level indicate excellent numerical precision for frontier quasiparticle energies, with mean absolute deviations $<10$ meV between low-scaling and standard implementations for the widely used $\mathit{GW}100$ test set. A notable challenge for low-scaling $GW$ algorithms remains in achieving high precision for five molecules within the $\mathit{GW}100$ test set, namely ${\mathrm{O}}_3, \mathrm{BeO}, \mathrm{MgO}, \mathrm{BN}$, and $\mathrm{CuCN}$, for which the deviations are in the range of several hundred meV at the $G_0{W}_0$ level. This is because of a spurious transfer of spectral weight from the quasiparticle to the satellite spectrum in $G_0{W}_0$ calculations, resulting in multipole features in the self-energy and spectral function, which low-scaling algorithms fail to describe. We show in this paper that including eigenvalue self-consistency in the Green's function ($\mathrm{ev}G{W}_0$) achieves a proper separation between satellite and quasiparticle peak, leading to a single solution of the quasiparticle equation with spectral weight close to one. $\mathrm{ev}G{W}_0$ quasiparticles energies from low-scaling $GW$ closely align with reference calculations; the mean absolute error is only 12 meV for the five molecules. We thus demonstrate that low-scaling $GW$ with self-consistency in $G$ is well suited for computing frontier quasiparticle energies.

Superfluid condensation can fundamentally be different from that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory. In a broad class of low-carrier-density superconductors, such as granular aluminum, doped nitrides, and high-$T_c$ cuprates, tunneling experiments reveal strong rather than weak coupling, as well as a conductance that does not return to that of the normal state upon approaching the critical temperature $T_c$. Here, we show that this behavior is in quantitative agreement with a tunneling theory that takes into account the large pairing-fluctuation effects that occur in the crossover region from weak-coupling BCS to strong-coupling Bose-Einstein condensation, provided the coherence energy scale rather than the single-particle energy gap is used to evaluate the coupling ratio. We also propose that the tendency toward strong coupling is a generic property of quasi-2D low-carrier-density superconductors.

Floquet engineering with periodic driving as a powerful tool for designing desirable topological states has been the subject of intense recent studies. Here, we present the application of Floquet engineering to investigate the evolution of topological triple fermions under irradiation of circularly polarized light (CPL), a phenomenon that currently remains a mystery. By using first-principles calculations and Floquet theorem, we demonstrate that WC-type TiO and its analogs are promising candidates for Floquet engineering of triple fermions. The symmetry analysis reveals that the electric field of CPL can break the specific symmetries, such as the time-reversal symmetry and its combination of spatial symmetries, inducing a transition to a flexibly controllable Weyl semimetallic phase. The survived spatial symmetry, controlled by light, guarantees that the Weyl nodes are located along the high-symmetry line or in high-symmetry planes in momentum space. Our findings focusing on Floquet engineering in realistic materials featured by triple fermions would facilitate both theoretical and experimental interest.

We present a theory unifying the topological responses and anomalies of various gapless fermion systems exhibiting Fermi surfaces, including those with Berry phases, and nodal structures, which applies beyond the noninteracting limit. As our key finding, we obtain a general approach to directly relate gapless fermions and topological insulators in phase space, including first- and higher-order insulators. Using this relation we show that the low-energy properties and response theories for gapless fermionic systems can be directly obtained without resorting to microscopic details. Our results provide a unified framework for describing such systems using well-developed theories from the study of topological phases of matter.

Domain walls have distinct properties from the bulk, and tailoring them to suit the needs for device applications is critical. Tungsten trioxide, ${\mathrm{WO}}_3$, is of great interest for device applications that make use of domain wall properties; it exhibits a phenomenologically rich sequence of phase transitions, virtually all of which are ferroelastic in character, resulting in many sets of domain walls at low temperatures, each with their own unique properties. Domain wall motion and its contribution to the piezoelectric response have been investigated in ${\mathrm{WO}}_3$ from 300 to 180 K using resonant ultrasound spectroscopy (RUS) and resonant piezoelectric spectroscopy (RPS), which showed that the $P{2}_1/n,P\overline{1}$, and $P{2}_1/c$ phases give a piezoelectric response despite the bulk being nominally centrosymmetric. Second harmonic generation (SHG) confirmed that polarity was strongest within the domain walls, and additional weak signals were found in the domains. Domain wall mobility was investigated in the $Pbcn,P{2}_1/n$, and $P{2}_1/c$ phases from 685 to 5 K. Domain walls in the $P{2}_1/n$ and $P\overline{1}$ were more mobile than those in the $Pbcn$ and $P{2}_1/c$ structures, and soon after the $P\overline{1}\to P{2}_1/c$ transition the walls become pinned at $\sim 140\mathrm{K}$.

Bismuth, with its rhombohedral crystalline structure and two Raman active phonon modes corresponding to the internal displacement ($A_{1g}$) and shear ($E_g$) ionic motions, offers an ideal target for the investigation of the nonequilibrium phonon-phonon and electron-phonon couplings. We investigate the $E_g$ phonon dynamics under intense photoexcitation by performing anisotropic transient reflectivity (TR) measurements on a 1-mm-thick bismuth single crystal at 11 K. The amplitude of the coherent $E_g$ phonon is found to increase with incident pump fluence up to $10\mathrm{mJ}/{\mathrm{cm}}^2$ and then turns to an apparent decrease. This behavior is in stark contrast to the amplitude of the $A_{1g}$ phonon in standard TR measurements, which increases monotonically up to $20\mathrm{mJ}/{\mathrm{cm}}^2$ and then saturates. The contrasting behaviors of the two phonon modes can be interpreted in terms of the strong coupling of the $E_g$ oscillation with large-amplitude $A_{1g}$ displacement in a highly excited electronic state, where dynamic fluctuation of the vibrational potential will lead to a quick loss in the $E_g$ vibrational coherence. Unlike previous studies on thin Bi films on substrates we observe no sign of a transition to a high-symmetry phase and instead observe signs of partial damage of the crystalline surface at $28\mathrm{mJ}/{\mathrm{cm}}^2$, possibly due to less efficient cooling at the surface of the bulk crystal.

Quantum-well resonance is achieved through partial confinement in magnetic tunnel junctions (MTJs), which provides an additional operable degree of freedom to regulate quantum-well levels. Using Al/Fe/MgO/Fe/Al and Ag/Al/Fe/MgO/Fe/Al/Ag MTJs as examples, via first-principles calculations, we demonstrate that the partial confinement of ${\mathrm{\Delta }}_1$ electrons at the Al/Fe interface, and the full confinement at the Fe/MgO interface combine to produce quantum-well resonances in Fe. The quantum-well levels of Fe can be periodically adjusted by two degrees of freedom: Fe and Al thickness. The oscillation period obtained from conductance $G_{\uparrow \uparrow }$ of MTJs is 2.13 ML Fe (9 ML Al), close to 2.25 ML Fe (8.33 ML Al) calculated by Fermi wave vector in the bcc-Fe (fcc-Al) band. The combination of long and short periods enables quantum-well levels to be finely adjusted. An ultrahigh optimistic TMR effect of $3.05\times {10}^5$% is achieved. Our results provide a way for designing high-performance spintronics devices.

Magnetic hopfions are a novel kind of topological solitons existing in three-dimensional (3D) magnets, including chiral magnets, frustrated magnets, and magnetic multilayers. Their rich dynamics is demonstrated to be promising in constructing nonconventional computing architectures. However, the mechanism of a hopfion's inertial motion, which is associated with finding its internal eigenmodes, is still being explored either by rigid-body approximation or by 3D micromagnetic simulation. The first neglects the excitation of internal eigenmodes, while the latter costs substantial computing resources but cannot exhaust the topic. In this paper, a framework based on local eigenmodes is proposed to calculate inertial motion. Utilizing the variational method, the complexity of solving inertial motion is significantly reduced to an eigenproblem with fewer dimensions, thereby fully revealing the modulation mechanism of a hopfion's inertia. We calculate two kinds of inertial motion: translation and rotation, within our framework. This demonstrates that our framework is a comprehensive tool for studying inertial motion. Our research highlights the fascinating dynamics of hopfions and is expected to facilitate the modulation and manipulation of hopfions for wider applications.

We carry out a microscopic study of a vortex lattice in a strongly correlated, type-II, $d$-wave superconductor (SC) using Bogoliubov-de Gennes (BDG) formalism. In weak-coupling theory, a commonly accepted truism is that a vortex binds to an impurity. We demonstrate that in a class of unconventional SCs proximity to a Mott insulator, the binding of a vortex to an impurity depends on relevant parameters. In particular, we illustrate this dependency on the sign of impurity, i.e., attractive or repulsive, as well as doping. We emphasize that this seemingly unanticipated behavior arises from strong correlation effects and is absent in weak-coupling descriptions.

The influence of strain and stacking sequence on interfacial ferromagnetism and exchange coupling in correlated oxide ${\mathrm{LaNiO}}_3\text{/}{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LNO/LSMO) heterostructures is investigated. LNO/LSMO heterostructures are grown on two different substrates [MgO and ${\mathrm{SrTiO}}_3$ (STO)] yielding different strains for the oxide layers that lead to different interfacial properties. Polarized neutron reflectivity (PNR) results indicate the emergence of a ferromagnetic interfacial LNO layer and exchange bias for LNO/LSMO heterostructures grown on MgO substrate. In contrast, no such phenomenon was observed for LNO/LSMO heterostructures grown on an STO substrate. Combining magnetometry, PNR, x-ray scattering, and soft x-ray absorption spectroscopy measurements, we found an enhanced magnetic moment and exchange coupling at interfaces that arises from charge transfer and orbital reconstruction via strain engineering in these complex oxide heterostructures. First-principles calculations suggest a possible orbital reconstruction as a result of different strains at interfaces which results from different interfacial magnetic phase behaviors. The study helps understand the manipulation of the exotic states with the aid of strain in oxide-based electronics.

In a recent letter [Riva et al., Phys. Rev. Lett. 131, 216401 (2023)] we presented the multichannel Dyson equation (MCDE), in which two or more many-body Green's functions are coupled. In this work we will give further details of the MCDE approach. In particular we will discuss (1) the derivation of the MCDE and the definition of the space in which it is to be solved, (2) the rationale of the approximation to the multichannel self-energy, (3) a diagrammatic analysis of the MCDE, and (4) the recasting of the MCDE on an eigenvalue problem with an effective Hamiltonian that can be solved using standard numerical techniques. This work mainly focuses on the coupling between the one-body Green's function and the three-body Green's function to describe photoemission spectra, but the MCDE method can be generalized to the coupling of other many-body Green's functions and to other spectroscopies.

We examine the hydrostatic stress of electrons strongly confined in a quasi-two-dimensional quantum well in the presence of a strong perpendicular and weak in-plane magnetic field. This introduces anisotropy into the stress tensor which is inconsistent with the notion of the quantum Hall droplet as a simple two-dimensional electron fluid. We show that the breaking of rotational symmetry is a necessary but not a sufficient condition for an anisotropic ground-state stress tensor and that the anisotropic stress originates primarily from quantum effects. We demonstrate this by decomposing the semiclassical trajectories of electrons in the system onto planes in phase space which must be rotated relative to the plane of the fluid to decouple the Hamiltonian.