Recently, a photonic alloy with nontrivial topological properties has been proposed, based on the random mixing of yttrium iron garnet (YIG) and magnetized YIG rods. When the doping concentration of magnetized YIG rods is less than one, a chiral edge state (CES) of the topological photonic alloy appears in the frequency range of the nontrivial topological gap of the magnetized YIG crystal. In this work, we surprisingly find that by randomly mixing the perfect electric conductor (PEC) and magnetized YIG rods in a square lattice, the photonic alloy system with appropriate doping concentrations can present CES in special frequency intervals even when both components support the propagation of bulk states. Analyzing the band structure of two components, we noticed a shift between the first trivial band gap for PEC and the first topological band gap for magnetized YIG. When calculating the transmission spectrum of the photonic alloy, we discovered that the frequency range for the topological gap gradually opens from the lower limit frequency of the band gap for PEC to the band gap for the magnetized YIG rods. The topological gap opening occurs as the doping concentration of magnetized YIG rods increases, creating an effective band-alignment effect. Moreover, the topological gap for the photonic alloy is confirmed by calculating the reflection phase winding with the scattering method. Lastly, the gradual appearance of the CES is identified by applying Fourier transformation to real-space electromagnetic fields. Our work broadens the possibilities for flexible topological-gap engineering in the photonic alloy system.

The electrically nonvolatile control of the easy magnetization axis (EMA) in two-dimensional ferromagnets has attracted significant attention due to its promising applications in spintronic devices. Multiferroic heterostructures, consisting of vertically stacked ferromagnetic and ferroelectric monolayers, have been proposed to manipulate the EMA by reversing the ferroelectric polarization. Here, based on first-principles calculations, we predict a strong magnetoelectric coupling in the multiferroic heterostructure ${\mathrm{EuSn}}_2{\mathrm{As}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$. Specifically, the EMA of the ferromagnetic monolayer ${\mathrm{EuSn}}_2{\mathrm{As}}_2$, characterized by a high spin magnetic moment of the rare-earth element Eu, can be switched between in-plane and out-of-plane directions by reversing the polarization direction of the ferroelectric monolayer ${\mathrm{In}}_2{\mathrm{Se}}_3$. Our analysis reveals that the polarization direction significantly influences the charge transfer at the interface, introducing band bending, modifying the electronic structure, and resulting in a significantly large difference in the magnetic anisotropy associated with the spin-orbit coupling (SOC) of the Eu $4f$, Sn $4p$, and As $4p$ orbitals. Furthermore, the shape magnetic anisotropy arising from dipole-dipole interactions is significant due to high spin magnetic moments. Thus, the combined effects of SOC and shape magnetic anisotropies determine the switching of EMA. Our findings present an opportunity to achieve nonvolatile electrical control of magnetic anisotropy in multiferroic heterostructures with coupled ferroelectric and ferromagnetic orders.

Two-dimensional (2D) electron layers floating on the surface of bulk liquid helium have been studied extensively over the last few decades as platforms for studying phase transitions in 2D, and recently, due to their relevance to quantum technologies. The maximum density of the electron layer is limited by the electrohydrodynamic (EHD) instability of the charged liquid helium surface, beyond which the electrons leave the surface in the form of multielectron bubbles (MEBs). Here, we report the experimental observation and analysis of the kinetics of EHD instability both above and below the $\lambda$ point of liquid He-4 to directly measure the instability length and time scales. The measurements agree with theoretical estimates and can inspire new experiments to reach ultrahigh electron densities above liquid helium.

Room-temperature superconductivity represents a significant scientific milestone, with the initial report [S. Lee et al., arXiv:2307.12008; arXiv:2307.12037] of LK-99, a copper-substituted lead apatite ${\mathrm{Pb}}_{10-x}{\mathrm{Cu}}_x{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$, offering a potential breakthrough. However, other researchers have encountered numerous challenges in replicating the original experimental results. In recent studies, Wang et al. [arXiv:2401.00999 and arXiv:2403.11126] successfully observed signs of a possible superconducting phase, such as smaller resistance and stronger diamagnetism, upon doping S into the samples. This indicates that the introduction of S is of significant importance for achieving an appropriate structure. To further investigate the role of S, we have considered ${\mathrm{Pb}}_{10-x}{\mathrm{Cu}}_x{\left({\mathrm{PO}}_4\right)}_6\mathrm{S}$, systematically discussing its thermodynamic stability as well as the influence of S on the distribution, concentration, and electronic properties of Cu. In this paper, we provide valuable insights into the synthesis of potential apatite-based room-temperature superconductors and the role of S in facilitating Cu doping.

The use of structurally complex lattice defects, such as functional groups, embedded nanoparticles, and nanopillars to generate phonon scattering is a popular approach in phonon engineering for thermoelectric applications. However, the theoretical treatment of this scattering phenomenon remains a formidable challenge, especially with regard to the determination of the scattering cross sections and rates associated with such lattice defects. Using the extended atomistic Green's function (AGF) method, we describe how the numerically exact mode-resolved scattering cross section $\sigma$ can be computed for a phonon scattered by a single lattice defect. We illustrate the generality and utility of the AGF-based treatment with two examples. In the first example, we treat the isotopic scattering of phonons in a harmonic chain of atoms. In the second example, we treat the more complex problem of phonon scattering in a carbon nanotube (CNT) containing an encapsulated C60 molecule which acts as a scatterer of the CNT phonons. The application of this method can enable a more precise characterization of lattice-defect scattering and result in a more controlled use of nanostructuring and lattice defects in phonon engineering for thermoelectric applications.

We consider a ring of fermionic quantum sites, modeled by the Fermi-Hubbard Hamiltonian, in which electrons can move and interact strongly via the Coulomb repulsion. The system is coupled to fermionic cold baths, which by the exchange of particles and energy induce relaxation in the system. We eliminate the environment and describe the system effectively by Lindblad master equations in various versions valid for different coupling parameter regimes. The early relaxation phase proceeds in a universal way, irrespective of the relative couplings and approximations. The system settles down to its low-energy sector and is consecutively well approximated by the Heisenberg model. We compare different Lindblad approaches, which, in the late relaxation, push the system towards different final states with opposite, extreme spin orders, from ferromagenetic to antiferromagnetic. Due to spin frustration in the trimer (a three site ring), degenerate ground states are formed by spin waves (magnons). The system described by the global coherent version of the Lindblad operators relaxes towards the final states carrying directed persistent spin currents. We numerically confirm these predictions.

The Bernevig-Hughes-Zhang (BHZ) model, which serves as a cornerstone in the study of the quantum spin Hall insulators, showcases robust spin-filtered helical edge states in a nanoribbon geometry. In the presence of an in-plane magnetic field, these (first-order) helical states gap out to be replaced by second-order corner states under suitable open boundary conditions. Here, we show that the inclusion of a spin-dependent non-Hermitian balanced gain/loss potential induces competition between these first- and second-order topological phases. Surprisingly, the previously dormant first-order helical edge states resurface as the non-Hermitian effect intensifies, effectively neutralizing the role played by the magnetic field. By employing the projected spin spectra and the spin Chern number, we conclusively explain the resurgence of the first-order topological properties in the time-reversal symmetry-broken BHZ model in the presence of nonhermiticity. Finally, the biorthogonal spin-resolved Berry phase, exhibiting a nontrivial winding, definitively establishes the topological nature of these revived edge states, emphasizing the dominance of nonhermiticity over the magnetic field.

Recent resonant x-ray scattering (RXS) experiments revealed a novel charge order in extremely overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) [Li et al., Phys. Rev. Lett. 131, 116002 (2023)]. The observed charge order appears around the $(\pi /3,0)$ wave vector, distinct from the well-known stripe fluctuations near 1/8 doping, and persists from cryogenic temperatures to room temperature. To investigate the origin of this charge order in the overdoped regime, we use determinant quantum Monte Carlo (DQMC) simulations to examine correlated models with various interactions. We demonstrate that this distinctive CDW originates from remnant correlations in extremely overdoped cuprates, with its specific pattern shaped by interactions beyond the Hubbard model, particularly electron-phonon couplings. The persistence of the $(\pi /3,0)$ wave vector across different doping levels indicates the presence of nonlocal couplings. Our study reveals the significant role of phonons in cuprates, which assist correlated electrons in the formation of unconventional phases.

${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ has attracted a lot of recent interest for a variety of reasons, most importantly, because of the hypothesis that it may support quantum-limit Chern topological magnetism, derived from the kagome geometry. Besides, ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ features a highly unusual magnetic reorientation transition about 100 K below the Curie point, whereby all spins in the system, remaining collinear, rotate by ${90}^{\circ }$. In this work, we address both issues combining experiment, mean-field theory, and first-principle calculations. Both magnetic reorientation and the unusual temperature dependence of the anomalous Hall conductivity (AHC) find quantitative explanation in the fact that Mn and Tb, by virtue of the Mermin-Wagner theorem, have very different spin dynamics, with Tb spins experiencing much more rapid fluctuation. We were able to cleanly extract the intrinsic AHC from our experiment, and calculated the same microscopically, with good semiquantitative agreement. We have identified the points in the band structure responsible for the AHC and showed that they are not the kagome-derived Dirac points at the K-corner of the Brillouin zone, as conjectured previously.

Topological phase transitions are found in a variety of systems and were shown to be deeply related with a thermodynamic description through scaling relations. Here we investigate the entanglement entropy, which is a quantity that captures the central charge of a critical model and the thermodynamics of the nonreciprocal Su-Schrieffer-Heeger (SSH) model. Although this model has been widely studied, the thermodynamic properties reveal interesting physics not explored so far. In order to analyze the boundary effects of the model, we use Hill's thermodynamics to split the grand potential in two contributions: the extensive one, related to the bulk, and the subdivision one, related to the boundaries. Then, we derive the thermodynamic entropy for both the edges and the bulk, and the heat capacity for the bulk at the topological phase transitions. The latter is related to the central charge when the underlying theory is a conformal field theory, whereas the first reveals the resilience of the topological edge states to finite temperatures. The phase transition between phases that are adiabatically connected with the Hermitian SSH model display the well-known behavior of systems within the Dirac universality class, but the transition between phases with complex energies shows an unexpected critical behavior that signals the emergence of an imaginary time crystal.

We study the influence of pseudospin-orbit coupling on electron-electron scattering in the Coulomb drag setup. We study a setup made of a time-reversal-symmetry-broken Weyl semimetal (WSM) layer and a normal metal layer. The interlayer drag force consists of two components. The first one is conventional and is parallel to the relative electronic boost velocity between the layers. This part of the drag tends to equilibrate the momentum distribution in the two layers, analogous to shear viscosity in hydrodynamics. In the WSM layer, the shift of the Fermi surface is not parallel to the electric field, due to skew scattering in the WSM. This induces a Hall current in the normal metal via the conventional component of the drag force. The second component of the drag force is perpendicular to the boost velocity in the Weyl semimetal and arises from interlayer e-e skew scattering, which results from two types of processes. The first process is an interference between electron-electron and electron-disorder scattering. The second process is due to the side jumps in electron-electron collisions in an external electric field. Both the parallel and perpendicular components of the drag are important for the anomalous Hall drag conductivity. On the other hand, for the Hall drag resistivity, the contribution from the parallel friction is partially canceled in a broad temperature regime. This paper provides insight into the microscopic mechanisms of Hall-like friction in electronic fluids.

Bulk electronic structure of pyrite-type ${\mathrm{NiS}}_2$ and ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ $(x=0.03)$ has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and theoretical calculations. The Mott gap at the Fermi level is confirmed by the HAXPES experiment. The $\mathrm{S}3p\text{−}\mathrm{Ni}3d$ charge-transfer energy is estimated to be negative from the cluster-model analysis of the $\mathrm{Ni}2p$ spectra. The observed $\mathrm{S}3s/3p$ spectral distribution including the bonding-antibonding splitting of the S-S dimer is well described by density functional theory for a paramagnetic metallic state. The Mott gap is not affected by the sparse Se substitution for S although the bonding-antibonding splitting of $\mathrm{S}3s$ is slightly reduced. The negative charge-transfer energy and the $\mathrm{S}3s/3p$ spectral distribution by the S-S dimerization are important ingredients of the bulk insulating state of ${\mathrm{NiS}}_2.$

Krylov complexity has recently gained attention where the growth of operator complexity in time is measured in terms of the off-diagonal operator Lanczos coefficients. The operator Lanczos algorithm reduces the problem of complexity growth to a single-particle semi-infinite tight-binding chain (known as the Krylov chain). Employing the phenomenon of Anderson localization, we propose the phenomenology of inverse localization length on the Krylov chain that undergoes delocalization/localization transition on the Krylov chain while the physical system undergoes ergodicity breaking. On the Krylov chain we find delocalization in an ergodic regime, as we show for the SYK model, and localization in case of a weakly ergodicity-broken regime. Considering the dynamics beyond scrambling, we find a collapse across different operators in the ergodic regime. We test for two settings: (1) the coupled SYK model, and (2) the quantum East model. Our findings open avenues for mapping ergodicity/weak ergodicity-breaking transitions to delocalization/localization phenomenology on the Krylov chain.

We have conducted a series of scattering experiments at the uranium $M_4$ absorption edge on low-symmetry uranium compounds $({\mathrm{U}}_2{\mathrm{N}}_3 \text{and} {\mathrm{U}}_3{\mathrm{O}}_8)$ produced as epitaxial films. At weak and forbidden reflections, we find a resonant signal, independent of temperature, with an energy dependence resembling the imaginary part $f^{″}$ of the scattering factor. The theory, using the fdmnes code, shows that these results can be reliably reproduced assuming that they originate from aspherical $5f$ electron charge distributions around the U nucleus. Such effects arise from the intrinsic anisotropy of the $5f$ shell and from the mixing of the $5f$ electrons of uranium with the outer $2p$ electrons of the anions. The good agreement between theory and experiment includes azimuthal scattering dependences, as well as polarization states of the scattered photons. The methodology reported here opens the way for a deeper understanding of the role the $5f$ electrons in the bonding in actinide compounds.

Recently, the kagome system of FeGe has emerged as a promising platform for investigating the interplay between magnetism and charge density wave (CDW) phenomena. However, unlike other CDW materials which exhibit soft phonons, the pristine phase of FeGe is found to be dynamically stable, different from the conventional CDW scenario of Fermi surface nesting or van Hove singularity. It is thus critical to understand (i) what are the driving forces of the CDW transition in FeGe? and (ii) how is the magnetism associated with such peculiar CDW transition? Here, supported by density functional theory and tight-binding models, we unravel the triple-well CDW energy landscape of FeGe driven by the downward shift of an entire Ge band, instead of the previously proposed Fe bands. The CDW phase is found to result from a cooperation between the Fermi level pinning phenomenon and the Peierls-like effect, which is distinct from the conventional Peierls effect that drives a double-well transition. Such electronic origin is consistent with experimental observations, using scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), where the electronic structures near the Fermi level barely change across the CDW transition. Moreover, we demonstrate that the antiferromagnetic coupling, which weakens the Fe-Ge hybridization by exchange splitting and lowers the position of Ge-bands with respect to the Fermi level, plays a more critical role than changes in local moments in driving the CDW transition. Our work thus reveals unconventional CDW mechanism for FeGe and related systems.

We present a computational study of the phonon linewidths in twisted bilayer graphene arising from electron-phonon and phonon-phonon interactions. The electronic structure is calculated using distance-dependent transfer integrals based on the atomistic Slater-Koster tight-binding formalism including electron-electron interactions treated at the Hartree level, and the phonons are calculated using classical force fields. These ingredients are used to calculate the phonon linewidths arising from electron-phonon interactions. Furthermore, effects of the phonon-phonon interactions on the linewidths are computed using the mode-projected velocity autocorrelation function obtained from classical molecular dynamics. We predict a moiré potential induced splitting of the Raman-active $G$ mode, near the magic angle, which arises due to contributions from high-symmetry stacking regions. Our findings show that both electron-phonon and anharmonic effects have a significant impact on the linewidth of this mode near the magic angle.

Scanning tunneling microscopy using a CO-functionalized tip is combined with simulations to explore the impact of the CO tilt angle on topographies of a single Cu atom and CO molecule adsorbed on Cu(111). Images of the Cu atom acquired with varying tip tilt angles and sample voltages are reproduced by the calculations. The agreement between measured and simulated data allows us to unveil the tip-orbital composition of the tunneling current, which highlights the role of the different $p$ orbitals of the CO tip. Microscope data of adsorbed CO and their dependence on voltage and probe-surface distance are captured for the nontilted junction geometry and in the limit of weak tip-surface interaction assuming sufficiently large tip-sample separations.