Cubic CeZn shows a structural phase transition under pressure and it modifies the ground state from an antiferromagnetic (AFM) state to a ferromagnetic (FM) state. To investigate how the FM state terminates at a quantum phase transition, we measured the electrical resistivity under pressure for a single-crystal CeZn. The transition temperature into the FM state decreases monotonously with increasing pressure, accompanied by the pronounced Kondo effect, but a drastic change in the field response occurs before the ordered phase terminates. This result suggests that the FM quantum critical point is avoided by the appearance of an AFM-like state.

The recent observation that pressure could suppress antiferromagnetic (AFM) order in quasi-one-dimensional $A{\mathrm{Mn}}_6{\mathrm{Bi}}_5$ Mn-cluster chain materials ($A=\mathrm{Na}$, K, Rb, and Cs) and lead to a superconducting dome offers an alternative Mn-based class of materials with which to study unconventional superconductivity. Using neutron diffraction, we elucidate the exact nature of the previously unknown AFM ground state of ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$ and report finding transverse incommensurate spin density waves (SDWs) for the Mn atoms with a propagating direction along the chains. The SDWs have distinct refined amplitudes of $\sim 2.46{\mu }_{\mathrm{B}}$ for the Mn atoms in the pentagons and $\sim 0.29{\mu }_{\mathrm{B}}$ with a large standard deviation for Mn atoms at the center between the pentagons. AFM coupling dominates both the nearest-neighbor Mn-Mn interactions within the pentagon and next-nearest-neighbor Mn-Mn interactions out of the pentagon (along the propagating wave). The SDWs exhibit both local and itinerant characteristics potentially due to cooperative interactions between local magnetic exchange and conduction electrons. Single crystal x-ray diffraction below the AFM transition revealed satellite peaks originating from charge density waves along the chain direction with a $\mathbf{q}$ vector twice as large as that of the SDW, pointing to a strong real space coupling between them. Additionally, we report a significant magnetoelastic effect during the AFM transition, especially along the chain direction, observed in temperature-dependent x-ray powder diffraction. Our work not only reveals fascinating intertwined spin, charge, and lattice orders in one-dimensional ${\mathrm{KMn}}_6{\mathrm{Bi}}_5$, but also provides an essential piece of information on its magnetic structure to understand the mechanism of superconductivity in this Mn-based family.

The Mott metal-insulator transition remains one of the most scrutinized concepts in condensed matter physics. However, the kinetics of the charge carriers at the transition, involving both orbital and spin degrees of freedom, still remains poorly understood. A perfect platform to distinguish between the role of such competing interactions is strongly correlated oxides offering rich phase diagrams, which we use here to address the electron kinetics at the transition. We show a critical slowing down of the electron kinetics at the first-order Mott metal-insulator transition in the Ruddlesden-Popper oxide ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{0.9}{\mathrm{Ti}}_{0.1}\right)}_2{\mathrm{O}}_7$ using low-frequency noise in resistance fluctuations. A critical slowing down of the electron kinetics is manifested as an enhancement of noise by an order of magnitude at the transition with a large shift of the spectral weight to lower frequencies. The second spectrum of noise is frequency dependent, indicating the presence of correlated fluctuations which get suppressed under the application of a magnetic field. Our experiments provide compelling evidence of the formation of a spin-glass phase at the transition in these systems.

Two-dimensional higher-order topology is usually studied in (nearly) particle-hole symmetric models, so that an edge gap can be opened within the bulk one. But more often the edge anticrossing deviates even into the bulk, where corner states are difficult to pinpoint. We address this problem in a graphene-based ${\mathbb{Z}}_2$ topological insulator with spin-orbit coupling and in-plane magnetization both originating from substrates through a Slater-Koster multiorbital model. The gapless helical edge modes cross inside the bulk, where the magnetization-induced edge gap is also located. After demonstrating its second-order nontriviality in bulk topology by a series of evidence, we show that a difference in bulk-edge on-site energy can adiabatically tune the position of the crossing/anticrossing of the edge modes to be inside the bulk gap. This can help unambiguously identify two pairs of topological corner states with nonvanishing energy degeneracy for a rhombic flake. We further find that the obtuse-angle pair is more stable than the acute-angle one.

The dynamics and lifetimes of a phonon with model resonant interactions from the quasiparticle to nonquasiparticle regime were investigated employing Green's function and the Green-Kubo method. In the weak-coupling case, the dynamics of the resonant phonon are analogous to a damped harmonic oscillator and its lifetime $\tau$ is in accordance with the standard phonon transport theory $\tau =\frac{1}{2\mathrm{\Gamma }}$ ($\mathrm{\Gamma }$ being the imaginary part of phonon self-energy). In the strong-coupling nonquasiparticle regime, however, the resonant phonon “propagates” in a complex form of wave packets and the actual phonon lifetime ${\tau }_{\mathrm{GK}}$ as determined from the Green-Kubo formula significantly deviates from the standard $\tau =\frac{1}{2\mathrm{\Gamma }}$ relation. Taking the four-phonon resonant ${\mathrm{AgCrSe}}_2$ and three-phonon resonant PbSe model systems as examples, the phonon nonquasiparticle dynamics and their lifetimes in real materials are further investigated by first-principle calculations. Substantial discrepancies between ${\tau }_{\mathrm{GK}}$ and $\tau$ are found for the strongly resonant phonons at high-symmetrical points in both materials. Meanwhile, the lifetime ${\tau }_{\mathrm{GK}}$ of the phonons that are not subjected to resonant phonon interactions almost recovers to the conventional theoretical result $\tau$ despite the non-Lorentzian spectral feature of these phonons. It is suggested that $\omega {\tau }_{\mathrm{GK}}$, instead of the conventional $\omega \tau$, can be applied as a criterion to distinguish phonon quasiparticles and nonquasiparticles in addition to the phonon spectral functions.

Magnetic domain wall (DW) propagation in nanowires is an important problem whose applications require stable high speeds that are very difficult to achieve with ferromagnets due to the existence of the Walker breakdown, which separates a rigid-body motion from an oscillatory motion. One of the recent fascinating discoveries in magnetism is the high-speed DW motion along ferrimagnetic (FiM) nanowires near the angular momentum compensation point (AMCP). A clear understanding of this fascinating phenomenon is still lacking. Here we use the energy conservation principle and generic FiM dynamics to reveal the physics behind the DW motion and the absence of the Walker breakdown and precessional torque at the AMCP. An almost exact DW velocity formula beyond the Walker breakdown field is obtained that agrees with experiments and simulations. This theory provides useful guidance for the design of high-speed spintronic DW devices.

$\mathrm{Ru}{X}_3$ trihalides ($\mathit{X}=\text{halogen atoms}$) offer a prominent platform for exploring the Kitaev spin liquid. Using polarization-resolved Raman spectroscopy, we report a comparative study of lattice, magnetic, and electronic excitations on $\mathit{X}=\text{Cl}$ and Br. Our phonon Raman spectra show that ${\mathrm{RuBr}}_3$ retains a three-layer honeycomb lattice without structural transition, unlike $\alpha \text{−}{\mathrm{RuCl}}_3$ that undergoes a monoclinic-to-rhombohedral structural transition at $\sim 150\mathrm{K}$. In addition, both $\mathrm{Ru}{X}_3$ compounds commonly feature single and double spin-orbit excitons of almost identical energies, alluding to the realization of a $j_{\mathrm{eff}}=1/2$ state to the same degree. In contrast to $\alpha \text{−}{\mathrm{RuCl}}_3$ two-magnon scattering dominates over fractionalized excitations in ${\mathrm{RuBr}}_3$. Our results suggest that a Br-for-Cl substitution moves $\mathrm{Ru}{X}_3$ further away from a pure Kitaev phase despite enhanced $p\text{−}d$ hybridization.

Ventilated acoustic insulators can block sound whereas simultaneously allowing airflow, which has potential applications for noise control in ventilation structures. In this paper, we propose a highly sparse acoustic meta-insulator array consisting of multiple tunable units. The meta-insulator unit with a minimum deep subwavelength thickness of $0.064\lambda$ has openings on two sides and a joint capacity. We propose an analytical model to reveal its physical mechanism. We numerically and experimentally demonstrate the highly sparse acoustic meta-insulator array with 20 units capable of ventilated sound insulation with a broad bandwidth of over 1 octave (from approximately 880 to 1915 Hz), a high sparsity of about 70%, and sound attenuation of more than 17 dB suitable for both near-field and far-field sound sources. Our designs may offer alternative solutions to ventilation noise control issues on various practical occasions.

We provide a systematic approach for constructing approximate quantum many-body scars (QMBS) starting from two-layer Floquet automaton circuits that exhibit trivial many-body revivals. We do so by applying successively more restrictions that force local gates of the automaton circuit to commute concomitantly more accurately when acting on select scar states. With these rules in place, an effective local, Floquet Hamiltonian is seen to capture dynamics of the automaton over a long prethermal window. We provide numerical evidence for such a picture and use our construction to derive several QMBS models, including the celebrated PXP model.

We demonstrate a nonlinear Hall effect due to the boundary spin accumulation in Pt films grown on ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates. This Hall effect and the previously demonstrated Hanle magnetoresistance provide a complete picture of the spin-precession control of the spin and charge transport at the boundary of a spin-orbit coupled material, which we refer to as spin Hall Hanle effects (SHHE). We also show that the SHHE can be employed to measure the spin diffusion length, the spin Hall angle, and the spin relaxation time of heavy metal without the need of magnetic interface or the input from other measurements. The comprehensive demonstration of SHHE in such a simple system suggests they may be ubiquitous and needs to be considered for unraveling the spin and charge transport in more complex thin-film structures of spin-orbit coupled materials.

Topological Dirac semimetals have been attracting considerable attention. Recently, a new class of Dirac points with a topological charge of $\pm 2$, named charge-2 Dirac points (C-2 DPs), were proposed in spinless systems. In this paper, we conduct an exhaustive search of all 230 space groups (SGs), finding that six SGs host C-2 DP at high-symmetry points (HSPs) and nine SGs host the C-2 DP on their high-symmetry lines (HSLs). Based on the symmetry analysis, we find the lowest symmetry requirements that enforce a C-2 DP and construct the corresponding effective models. Furthermore, we investigate the difference between C-2 DPs at HSPs and on HSLs, finding that the type-II C-2 DP only appears on HSLs. Via the first-principles calculations, we identify the corresponding realistic materials that host C-2 DPs. Therefore, our paper not only offers the guidelines to identify the materials that host C-2 DPs in spinless systems based on symmetry requirements, but also paves the effective way to study the C-2 DPs in spinless systems.

The recently introduced classification of two-dimensional insulators in terms of topological crystalline invariants has been applied so far to “obstructed” atomic insulators characterized by a mismatch between the centers of the electronic Wannier functions and the ionic positions. We extend this notion to quantum spin Hall insulators in which the ground state cannot be described in terms of time-reversal symmetric localized Wannier functions. A system equivalent to graphene in all its relevant electronic and topological properties except for a real-space obstruction is identified and studied via symmetry analysis as well as with density functional theory. The low-energy model comprises a local spin-orbit coupling and a nonlocal symmetry breaking potential, which turn out to be the essential ingredients for an obstructed quantum spin Hall insulator. An experimental fingerprint of the obstruction is then measured in a large-gap triangular quantum spin Hall material.

We develop a quantum model for the dynamics of carrier injection in a band that presents a strong carrier-vibration coupling. This coupling modifies the spectral density of the band and can even create pseudogaps that signal the onset of polaronic states. The injection of a carrier that interacts with many vibration modes is a complex many-body process that is treated by combining the quantum scattering theory and the dynamical mean-field theory (DMFT). For the model analyzed here, which is adapted to compact phases, the number $Z$ of neighbors of a given site is large and in this limit the DMFT becomes exact. The model is applied to the excitonic dissociation at the donor-acceptor interface for organic solar cells. The main ingredients are the electron-hole Coulomb interaction, the recombination process, and the existence of polaronic states in the acceptor band. Using parameters extracted from ab initio calculations we analyze the spectral density on the charge transfer state (CTS), the average energy transferred to phonons on the CTS, and the quantum yield of the injection process. We find in particular that even with a strong electron-vibration coupling, one can get a vibrationally cold charge transfer state with a high injection yield as often observed experimentally.

We establish a direct connection between general tensor networks and deep feed-forward artificial neural networks. The core of our results is the construction of neural-network layers that efficiently perform tensor contractions and that use commonly adopted nonlinear activation functions. The resulting deep networks feature a number of edges that closely match the contraction complexity of the tensor networks to be approximated. In the context of many-body quantum states, this result establishes that neural-network states have strictly the same or higher expressive power than practically usable variational tensor networks. As an example, we show that all matrix product states can be efficiently written as neural-network states with a number of edges polynomial in the bond dimension and depth that is logarithmic in the system size. The opposite instead does not hold true, and our results imply that there exist quantum states that are not efficiently expressible in terms of matrix product states or projected entangled pair states but that are instead efficiently expressible with neural network states.

We combine the recently developed many-body Green's function theory for electrons and nuclei with the exact factorization of the wave function. The existing Born-Oppenheimer Green's functions are shown to be special cases of our exact approach. We consider the limitations of the laboratory frame formulation of the Green's function theory and discuss why the body-fixed frame formulation is needed in order to go beyond the Born-Oppenheimer theory. We give exact forms of the electronic and nuclear Green's functions written in terms of the exact factorized states, providing a systematic approach beyond the Born-Oppenheimer approximation. The lowest order approximation to the exact electronic Green's function is found to be an expected value of the Born-Oppenheimer electronic Green's function with respect to the nuclear density.

We develop an analytically solvable model for interacting two-dimensional Fermi liquids with separate collisional relaxation rates for parity-odd and parity-even Fermi surface deformations. Such a disparity of collisional lifetimes exists whenever scattering is restricted to inversion-symmetric Fermi surfaces, and should thus be a generic feature of two-dimensional Fermi liquids. It implies an additional unanticipated “tomographic” transport regime (in between the standard collisionless and hydrodynamic regimes) in which even-parity modes are overdamped while odd-parity modes are collisionless. We derive expressions for both the longitudinal and the transverse conductivity and discuss the collective mode spectrum along the collisionless-tomographic-hydrodynamic crossover. Longitudinal modes cross over from zero sound in the collisionless regime to hydrodynamic first sound in the tomographic and hydrodynamic regime, where odd-parity damping appears as a subleading correction to the lifetime. In charged Fermi liquids with long-range Coulomb coupling, these modes reduce to plasmons with a strongly suppressed odd-parity correction to the damping. The transverse response, by contrast, has a specific tomographic transport regime with two imaginary odd-parity modes, one of which requires a finite repulsive interaction, distinct from both the shear sound in the collisionless regime and an overdamped diffusive current mode in the hydrodynamic limit. Our work demonstrates that there are deep many-body aspects of interacting Fermi liquids, which are often thought to be well understood theoretically, remaining unexplored.

Predisplaced micromechanical resonators made from stressed materials give rise to new static and dynamic behavior, such as geometric tuning of stress. Here, an analytical model is presented to describe the mechanics of such predisplaced resonators. The bending and tension energies are derived and a modified Euler-Bernoulli equation is obtained by applying the least action principle. By projecting the model onto a cosine shape, the energy landscape is visualized, and the predisplacement dependence of stress and frequencies is studied semianalytically. The analysis is extended with finite-element simulations, including the mode shapes, the role of overhang, the stress distribution, and the impact of film stress on beam relaxation.

We have studied the room-temperature long-range antiferromagnetic order in the ${\mathrm{Fe}}_2\mathrm{C}$ monolayer using the combination of first-principles calculations and the Green's function analysis within the anisotropic Heisenberg model. The ${\mathrm{Fe}}_2\mathrm{C}$ monolayer is a semimetal with the out-of-plane antiferromagnetic order between two $\mathrm{Fe}$ planes. The Green's function approach within the random phase approximation is formulated to calculate the temperature-dependent antiferromagnetic magnon energy and the spin correlation function in a two-dimensional antiferromagnetic monolayer. The correlation function is used to evaluate the sublattice magnetization and study the magnetic phase transition in monolayer ${\mathrm{Fe}}_2\mathrm{C}$. Moreover, the spin Hamiltonian and the Green's function formalism are developed to investigate the antiferromagnetic ${\mathrm{Fe}}_2\mathrm{C}$ monolayer in the presence of an external magnetic field applied along the easy axis. The H-T phase diagram shows that the antiferromagnetic to spin-flop and the spin-flop to paramagnetic phase transitions occur in low temperatures. Finally, we estimate the Néel temperature and the critical values of the magnetic field strength for these two phase transitions.