In this work, we study the magnon-magnon interaction effect in typical honeycomb ferromagnets consisting of van der Waals–bonded stacks of honeycomb layers, e.g., chromium trihalides $\mathrm{Cr}X3 (X=\mathrm{F},\mathrm{Cl},\mathrm{Br}, \mathrm{and} \mathrm{I})$, that display two spin-wave modes (Dirac magnon). Using Green's function formalism with the presence of the Dzyaloshinskii–Moriya interaction, we obtain a spinor Dyson equation up to the second-order approximation by the cluster expansion method. Numerical calculations show prominent renormalizations of the single-particle spectrum. Furthermore, we propose a tunable renormalization effect using a parametric magnon amplification scheme. By amplifying the magnon population at different $k$ points, the enabled renormalization effect not only reshapes the band structure but also modifies the Berry curvature distribution. Our work demonstrates the interplay between band geometry, interactions, and the external light field in the bosonic system and can potentially lead to new insights into the properties of magnon-based spintronic devices.

Single crystals of ${\mathrm{SrCo}}_2{\left({\mathrm{Ge}}_{1-x}{\mathrm{P}}_x\right)}_2$ were grown using a self-flux method, and their physical properties were investigated by means of magnetization and specific-heat measurements. Although the compounds near both ends of $x$ exhibit paramagnetism, those with $0.39⩽x⩽0.58$ exhibit ferromagnetism at low temperatures. The analysis of critical exponents and spin fluctuations revealed a weak itinerant ferromagnetic character. It is found that an intermediate phase exists for a wide range of composition between the collapsed tetragonal and the uncollapsed tetragonal phases, which causes ferromagnetic quantum critical phenomena in ${\mathrm{SrCo}}_2{\left({\mathrm{Ge}}_{1-x}{\mathrm{P}}_x\right)}_2$.

Ordered electronic states have been extensively explored in cuprates and iron-based unconventional superconductors, but seldom observed in the epitaxial $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ monolayer (FeSe/STO) with an enhanced superconducting transition temperature $\left(T_c\right)$. Here, by using scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS), we reveal a checkerboard charge order in the epitaxial FeSe/STO monolayer, with a period of four times the inter-iron-atom distance along two perpendicular directions of the iron lattice. This ordered state is uniquely present in the superconducting FeSe/STO monolayer, even at liquid nitrogen temperature, but is absent in the nonsuperconducting FeSe monolayer or bilayer. Quasi-particle interference measurements further confirm it as a static order without an energy-dependent dispersion and is gapped out within the superconductivity gap. The intensity of the charge order shows an enhancement near the superconducting transition temperature, thus implying a correlation with the $\text{high-}{T}_c$ superconductivity in the FeSe/STO monolayer. This study provides a new basis for exploring the ordered electronic states and their interplay with $\text{high-}{T}_c$ superconductivity in the FeSe monolayer.

Brownmillerites $A_2{B}_2{\mathrm{O}}_5$ are oxides possessing a crystallographic structure that is parent from the perovskite structure ${AB\text{O}}_3$ with an ordered sublattice of oxygen vacancies, forming a network of one-dimensional chiral chains of oxygen tetrahedra carrying electric dipoles. The distribution of left-and right-handed chains enables the formation of different, polar or nonpolar, crystallographic phases with close internal energies. We have performed first-principles calculations on the nonmagnetic ${\mathrm{Ca}}_2{\mathrm{Al}}_2{\mathrm{O}}_5$ oxide, chosen as a case study, in order to investigate the effect of an external electric field, applied in the direction parallel to the chains, on the stability of four different crystallographic phases. We found that reversing the direction of the electric polarization in the $Ima2$ phase necessitates going through different intermediate phases. However, we also showed that, even if the application of the electric field allows to change the relative stability of each phase, it cannot be sufficient to change one-chain handedness at low temperature, owing to the high barrier energies, of the order of $\sim$1 eV per chain of two tetrahedra, which has to be overpassed. Switching between different phases would thus require applying an electric field at high temperature or with the application of additional stress. Finally, we demonstrated that polar phases, such as $Ima2$ or $Pmc{2}_1$, display negative longitudinal piezoelectric coefficients due to the lattice response to the applied electric field.

Strain coupling and relaxation dynamics associated with the ferrimagnetic/ferroelastic phase transition at $T_c\approx 310\mathrm{K}$ in double perovskite ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$ with a high degree of Fe/Re order have been investigated with resonant ultrasound spectroscopy through the temperature interval $\sim 5–600\mathrm{K}$ and with applied magnetic field of up to $\pm 2\mathrm{T}$. Strain analysis using diffraction data from the literature is consistent with a Landau model of the transition as $Fm\overline{3}m{1}^{\prime }\to I4/m{m}^{\prime }{m}^{\prime }$, improperly ferroelastic, driven by a magnetic order parameter with symmetry ${\mathrm{\Gamma }}_4^{+}$. Ferroelastic shear strain of up to $\sim 0.0015$ arises from spin/orbit coupling and is smaller than is typical of coupling with octahedral tilting. It provides the underlying cause of softening of the shear modulus observed over an interval of $\sim 100\mathrm{K}$ below $T_{\mathrm{c}}$, though with order/disorder rather than displacive character for the transition. Hysteretic effects suggest that precursor microstructures and mixed magnetic/ferroelastic domains below $T_{\mathrm{c}}$ depend on the thermal history of the sample and can evolve on a timescale of hours and days at room temperature. Elasticity data collected as a function of external magnetic field reveal that poled samples are slightly softer than those with multiple magnetic domains at 4 K. At 300 K there is a time-dependent viscous component of the response to the field that relates to the bulk modulus and, hence, to volume changes associated with magnetic ordering. A loss peak seen $\sim 20–50\mathrm{K}$ below $T_{\mathrm{c}}$ in AC magnetic measurements made at frequencies of $0.2–1\mathrm{kHz}$ yielded an activation energy $\sim 0.4\mathrm{eV}$ and has been attributed to freezing of magnetic/ferroelastic domain walls. No equivalent loss peak was seen in the acoustic data measured at $\sim 100–500\mathrm{kHz}$, however, implying that these domain walls are mobile in response to a dynamic magnetic field on a timescale of $\sim {10}^{-2}\text{–}{10}^{-3}\mathrm{s}$ but immobile in response to a dynamic stress field applied on a timescale of $\sim {10}^{-5}\text{–}{10}^{-6}\mathrm{s}$. Debye-like acoustic loss peaks at temperatures below $\sim 100\mathrm{K}$ yielded activation energies of $\sim 0.02–0.1\mathrm{eV}$ and are discussed in terms of pinning/freezing of polarons. ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$ is a material with magnetoelastic and magnetoelectric heterogeneities that might be tuned by choice of thermal history and cation order.

Magnon bands are known to exhibit nontrivial topology in ordered magnets under suitable conditions, engendering topological phases referred to as magnonic topological insulators. Conventional methods to drive a magnonic topological phase transition are bulk magnetic or thermal operations such as changing the direction of an external magnetic field or varying the temperature of the system, which are undesired in device applications of magnon topology. In this work, we lift the limitation of the magnon topology control on the bulk nonelectronic manipulation by proposing a scheme to manipulate magnonic topological phases by electronic boundary operations of spin chirality injection. More specifically, we consider a ferromagnetic honeycomb lattice and show that a finite spin chirality injected from the boundary of the system via the spin Hall effects introduces a tunable sublattice-symmetry-breaking mass term to the bosonic counterpart of the Haldane model for the Chern insulators and thereby allows us to electronically manipulate the bulk topology of magnons from the boundary. The “shoulder” in the thermal Hall conductivity profile is proposed as an experimental probe of the chirality-induced topological phase transition. The scheme for the boundary manipulation of the magnon topology is shown to work for a honeycomb antiferromagnet as well. We envisage that the interfacial chirality injection may offer a nonintrusive electronic means to tune the static and the dynamical bulk properties of general magnetic systems.

We investigated the temperature-dependent electronic structure of the antiferromagnetic fcc-monolayer Mn on Re(0001) using vacuum-ultraviolet momentum microscopy. At $T=25$ K the collinear, row-wise antiferromagnetic phase of the Mn monolayer results in a spin splitting of states. Density-functional theory, being in good agreement with the experimental results, reveals the spin and orbital projection of the observed electronic bands. The exchange split bands shift in opposite directions with increasing temperature, decreasing the exchange splitting of a pair of itinerant bands from $280\pm 10$ meV at 25 K down to $185\pm 10$ meV at the Néel temperature $T_{\text{N}}=75\pm 5$ K. The exchange splitting remains constant for $T>T_{\text{N}}$. The persisting exchange splitting is attributed to a remaining short-range, fluctuating antiferromagnetic order far above $T_{\text{N}}$.

We study the superconducting instability of a two-dimensional disordered Fermi liquid weakly coupled to the soft fluctuations associated with proximity to an Ising-ferromagnetic quantum critical point. We derive interaction-induced corrections to the Usadel equation governing the superconducting gap function, and show that diffusion and localization effects drastically modify the interplay between fermionic incoherence and strong pairing interactions. In particular, we obtain the phase diagram, and demonstrate that (i) there is an intermediate range of disorder strength where superconductivity is enhanced, eventually followed by a tendency towards the superconductor-insulator transition at stronger disorder; and (ii) diffusive particle-particle modes (so-called “Cooperons”) acquire anomalous dynamical scaling $z=4$, indicating strong non-Fermi liquid behavior.

We introduce a variant of a dipole representation for composite fermions in a half-filled Landau level, taking into account the symmetry under an exchange of particles and holes. This is implemented by a special constraint on a composite fermion and a composite hole degree of freedom (of an enlarged space), which makes the resulting composite particle (dipole) a symmetric object. We study an effective Hamiltonian that commutes with the constraint on the physical space and fulfills the requirement for boost invariance on the Fermi level. The calculated Fermi liquid parameter $F_2$ is in good agreement with numerical investigations in Phys. Rev. Lett. 121, 147601 (2018).

Currently, designing and exploring a photocatalyst with an interfacial electric field for hydrogen production from water splitting is crucial. Hence, based on first-principles calculations, we first calculate the electronic properties of the two-dimensional hexagonal boron nitride (hBN)/Janus WSSe heterojunction. However, the inherent energy band conformational defects of the hBN/Janus WSSe heterojunction inhibit the efficiency of photocatalytic. To solve this problem, we doped $\mathrm{C}=\mathrm{C}$ bonds on the monolayer hBN to tune the band gap. The research shows that the transition from the traditional type-I band alignment to the type-II band alignment can be achieved through different contracting methods between monolayer hexagonal boron nitride carbon (hBNC) and monolayer Janus WSSe. Notably, the hBNC/SWSe heterojunction is a direct Z-scheme photocatalyst and has high carrier mobility. Compared with the hBNC/SeWS heterojunction, the hBNC/SWSe heterojunction shows higher reduction reaction overpotential (${\chi }_{{\mathrm{H}}_2}=2.56\mathrm{eV}$) and oxidation reaction overpotential (${\chi }_{{\mathrm{O}}_2}=0.83\mathrm{eV}$) under acidic conditions. Furthermore, the hBNC/SWSe heterojunction has a high quantum efficiency and solar hydrogen production efficiency (33.31%). Gibbs free energy calculations confirm that both oxidation evolution reaction and hydrogen evolution reaction are exothermic reactions and can proceed spontaneously under the external potential provided by photogenerated carriers. These results suggest that the hBNC/SWSe heterojunction has potential applications in visible-light-driven water splitting.

Charge transport plays a crucial role in manifold potential applications of two-dimensional materials, including field-effect transistors, solar cells, and transparent conductors. At most operating temperatures, charge transport is hindered by scattering of carriers by lattice vibrations. Assessing the intrinsic phonon-limited carrier mobility is thus of paramount importance to identify promising candidates for next-generation devices. Here we provide a framework to efficiently compute the drift and Hall carrier mobility of two-dimensional materials through the Boltzmann transport equation by relying on a Fourier-Wannier interpolation. Building on a recent formulation of long-range contributions to dynamical matrices and phonon dispersions [Phys. Rev. X 11, 041027 (2021)], we extend the approach to electron-phonon coupling including the effect of dynamical dipoles and quadrupoles. We identify an unprecedented contribution associated with the Berry connection that is crucial to preserve the Wannier-gauge covariance of the theory. This contribution is not specific to two-dimensional crystals, but also concerns the three-dimensional case, as we demonstrate via an application to bulk SrO. We showcase our method on a wide selection of relevant monolayers ranging from ${\mathrm{SnS}}_2$ to ${\mathrm{MoS}}_2$, graphene, BN, InSe, and phosphorene. We also discover a nontrivial temperature evolution of the Hall hole mobility in InSe whereby the mobility increases with temperature above 150 K due to the Mexican-hat electronic structure of the InSe valence bands. Overall, we find that dynamical quadrupoles are essential and can impact the carrier mobility in excess of 75%.

We investigate the ground-state phase diagram of a modified spinless Haldane-Hubbard model, with broken threefold rotational symmetry, employing exact diagonalization calculations. The interplay of asymmetry, interactions, and topology gives rise to a rich phase diagram. The noninteracting limit of the Hamiltonian exhibits a higher-order topological insulator characterized by the existence of corner modes, in contrast to known chiral edge metallic states of the standard Haldane model. Our investigation demonstrates that these symmetry-protected states are robust to the presence of finite interactions. Furthermore, in certain regimes of parameters, we show that a topological Mott insulator exists in this model, where a nontrivial topological bulk coexists with an interaction-driven charge-density wave, whose emergence is characterized by a $Z_2$-symmetry breaking within the $3d\text{-Ising}$ universality class.

We describe topologically ordered and fracton-ordered quantum systems on geometries which do not have an underlying manifold structure. Using tree graphs such as the $k$-coordinated Bethe lattice $\text{B}\left(k\right)$ and a hypertree called the $(k,n)$-hyper-Bethe lattice $\text{HB}(k,n)$ consisting of $k$-coordinated hyperlinks (links defined by $n$ sites), we construct multidimensional arboreal arenas such as $\text{B}\left(k_1\right)\square \text{B}\left(k_2\right)$ using a generalized notion of a graph Cartesian product $\square$. We study various quantum systems such as the ${\mathbb{Z}}_2$ gauge theory, generalized quantum Ising models (GQIMs), the fractonic X-cube model, and related X-cube gauge theory defined on these arboreal arenas, finding several fascinating results. Even the simplest ${\mathbb{Z}}_2$ gauge theory on a two-dimensional arboreal arena is found to be fractonic—an isolated monopole excitation is rendered fully immobile on an arboreal arena. The X-cube model on a generic three-dimensional arboreal arena is found to be fully fractonic in the magnetic sector, i.e., all multipoles of magnetic excitations are rendered immobile on the arboreal arena. We obtain variational ground state phase diagrams of the gauge theories (both ${\mathbb{Z}}_2$ and X-cube gauge theories) which are shown to have deconfined and confined phases. These phases are usually separated by a first-order transition, while continuous transitions are obtained in some cases. Further, we find an intriguing class of dualities in arboreal arenas, as illustrated by the ${\mathbb{Z}}_2$ gauge theory defined on $\text{B}\left(k_1\right)\square \text{B}\left(k_2\right)$ being dual to a GQIM defined on $HB(2,k_1)\square HB(2,k_2)$. Finally, we discuss different classes of topological and fracton orders that appear on arboreal arenas. We find three distinct classes of arboreal toric code orders on two-dimensional arboreal arenas, those that occur on $\text{B}\left(2\right)\square \text{B}\left(2\right), \text{B}\left(k\right)\square \text{B}\left(2\right),k>2$, and $\text{B}\left(k_1\right)\square \text{B}\left(k_2\right), k_1,k_2>2$. Likewise, four classes of X-cube fracton orders are found on three-dimensional arboreal arenas which correspond to those on $\text{B}\left(2\right)\square \text{B}\left(2\right)\square \text{B}\left(2\right), \text{B}\left(k\right)\square \text{B}\left(2\right)\square \text{B}\left(2\right),k>2, \text{B}\left(k_1\right)\square \text{B}\left(k_2\right)\square \text{B}\left(2\right),k_1,k_2>2$, and $\text{B}\left(k_1\right)\square \text{B}\left(k_2\right)\square \text{B}\left(k_3\right),k_1,k_2,k_3>2$.

We study the combined effects of disorder and range of the couplings on the phase diagram of one-dimensional topological superconductors. We consider an extended version of the Kitaev chain where hopping and pairing terms couple many sites. Deriving the conditions for the existence of Majorana zero modes, we show that either the range and the on-site disorder can greatly enhance the topological phases characterized by the appearance of one or many Majorana modes localized at the edges. We consider both a discrete and a continuous disorder distribution. Moreover, we discuss the role of correlated disorder which might further widen the topological regions. Finally, we show that in the purely long-range regime and in the presence of disorder, the spatial decay of the edge modes remains either algebraic or exponential, with eventually a modified localization length, as in the absence of disorder.

Recently, a new class of quantum magnets, the so-called breathing pyrochlore spin systems, have attracted much attention due to their potential to host exotic emergent phenomena. Here, we present magnetometry, heat capacity, thermal conductivity, muon-spin relaxation, and polarized inelastic neutron scattering measurements performed on high-quality single crystal samples of the breathing pyrochlore compound ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$. We interpret these results using a simplified toy model and provide insight into the low-energy physics of this system beyond the single tetrahedron physics proposed previously.

The exchange-bias (EB) effect, usually arising in ferromagnetic (FM)-antiferromagnetic (AFM) interfaces with uniaxial magnetic anisotropy, holds high potentials in spintronic applications. Here, we report both field-cooling and zero-field cooling EB effects with a maximal EB field $|H_{EB}|$ reaches up to 3859 Oe in above-room-temperature van der Waals (vdW) ferromagnet ${\mathrm{Fe}}_3\mathrm{Ga}{\mathrm{Te}}_2$ nanoflakes at low temperatures. The observed intrinsic EB effects can be largely tuned via the gate-induced proton intercalation. Moreover, we observe an unusual sign-tunable EB effect under different gate voltages after $\pm 2\mathrm{T}$ field cooling, leading to a crossover between positive and negative EB effects. Theoretical analysis based on density functional theory indicates that the magnetic coupling at the FM/AFM interface in proton-intercalated ${\mathrm{Fe}}_3\mathrm{Ga}{\mathrm{Te}}_2$ is highly controllable and can be tuned to be FMI-1 $(\uparrow /\downarrow \uparrow$, positive EB) or FMI-2 ($\downarrow /\downarrow \uparrow$, negative EB) magnetic configurations, depending on different H-absorption sites. Our experiments offer a knob to control the sign of EB effects and further open opportunities for more applicable spintronic devices in high-temperature vdW ferromagnets.

We show that strong electronic repulsion transforms a vortex core from a metallic type in the overdoped regime to a Mott insulator at underdoping of a strongly correlated $d$-wave superconductor. This changeover is accompanied by an accumulation of electron density at the vortex core toward local half filling in the underdoped region, which in turn facilitates the formation of the Mott-insulating core. We find that the size of vortices evolves nonmonotonically with doping. A similar nonmonotonicity of critical field $H_{c2}$, as extracted from superfluid stiffness, is also found. Our results explain some recent experimental puzzles of cuprate superconductors.

Among the exotic and yet unobserved features of multichannel Kondo impurity models is their subunitary single electron scattering. In the two-channel Kondo model, for example, an incoming electron is fully scattered into a many-body excitation such that the single particle Green's function vanishes. Here, we propose to directly observe these features in a charge-Kondo device encapsulated in a Mach-Zehnder interferometer, within a device already studied by Duprez et al. [Science 366, 1243 (2019)]. We provide detailed predictions for the visibility and phase of the Aharonov-Bohm oscillations depending on the number of coupled channels and the asymmetry of their couplings.

We report and quantify a large orbital-Hall torque generated by Nb and Ru, which we identify from the strong dependence of torques on the ferromagnets. This is manifested as strong enhancement in the dampinglike torques measured in Nb (or Ru)/Ni bilayers as compared to Nb (or Ru)/FeCoB bilayers including the sign reversal in the case of Nb/(Ni or FeCoB) samples. We experimentally observe a striking enhancement of the measured torques with the increase of ferromagnetic Ni thickness which is markedly different from the regular spin-transfer torque; this could be a unique signature of long-range action of the orbital-Hall torques.