Quantum many-body scars enable long-lived coherent dynamics in otherwise thermalizing systems, but controlling their stability under periodic driving remains challenging. We study the PXP model subjected to a continuous two-frequency drive and show that the frequency ratio $c={\omega }_2/{\omega }_1$ acts as a control parameter for the resulting Floquet dynamics. In a rotating-frame description, the drive generates Bessel-dressed couplings in an effective Floquet Hamiltonian whose harmonic content depends explicitly on $c$. This $c$-dependent dressing changes the competition between coherent revival dynamics and thermalization. Small integer values of $c$ stabilize scar-induced oscillations down to substantially lower driving frequencies than in the single-frequency limit. At fixed low frequency, small noninteger rational values of $c$ can instead produce isolated suppressions of coherent revivals, an effect that weakens as $c$ increases. Successive Fibonacci rational approximants also reproduce the quasiperiodic dynamics over finite observation times. We further show that two-frequency driving produces tunable rational subharmonic responses in local observables, including higher-order responses that are relatively stable for small $c$ compared with those in the corresponding single-frequency protocol. Our results demonstrate that two-frequency driving provides a route to controlling prethermal nonergodic dynamics and temporal responses in the constrained PXP model.

Boundary conditions are usually expected to cause only finite-size corrections to bulk quantities, but this expectation can fail for dynamical quantum phase transitions. In this work, we show that such boundary dependence is encoded in dynamical zero modes (DZMs) of the Loschmidt matrix, which are defined as singular vectors whose singular values vanish in the thermodynamic limit. Using the Su-Schrieffer-Heeger (SSH), extended SSH, and Kitaev models as examples, we find that the time interval where the Loschmidt rate functions (LRFs) under periodic and open boundary conditions differ coincides with the emergence of DZMs in the open-boundary Loschmidt matrix. These modes carry the boundary-dependent contribution: removing them from the open-boundary LRF recovers the periodic-boundary result. We further show that these DZMs lead to finite-precision numerical instability, since their finite-size singular values decay exponentially with system size and eventually become unresolved in fixed-precision arithmetic. A reliable small-size branch before this loss of precision can be used to estimate the LRF in the thermodynamic limit by linear extrapolation. Our results identify DZMs as both a diagnostic of boundary-dependent LRFs and the origin of the associated numerical instability.

We report near-surface small-angle neutron scattering (NS-SANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film grown on Si(111) in the thick-film limit. Providing a means of direct reciprocal space mapping, NS-SANS and OSR reveal one-dimensional magnetic modulations perpendicular to the film surface under magnetic fields parallel and perpendicular to the film surface, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of the field orientation, our data do not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct contrast to bulk MnSi thinned to similar nanoscale thicknesses, in which a skyrmion lattice dominates the phase diagram under perpendicular fields. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell of the film control the magnetic state, representing the main structural difference between epitaxial and thinned bulk MnSi.

We report a systematic study of the interplay between oxygen nonstoichiometry, crystal structure, and magnetic and electrotransport properties in ${\mathrm{Sr}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CoO}}_{3-\delta }$ $(x=0.1,0.2,0.3)$. High-resolution neutron powder diffraction combined with synchrotron x-ray powder diffraction reveals that increasing the oxygen content induces a structural transition from a layered $I4/mmm$ $(2a_p\times 2a_p\times 4a_p)$ phase to an oxygen-deficient orthorhombic $Pmmm$ $(a_p\times a_p\times 2a_p)$ phase with preferential oxygen-vacancy occupation. This transition is accompanied by a crossover from a G-type antiferromagnetic state with a weak ferromagnetic component to a ferromagnetic state, and a drastic decay in resistivity. The evolution of the magnetic and transport properties is discussed in terms of changes in the Co spin state, enhanced $\mathrm{Co}3d$–O $2p$ orbital overlap upon oxygen uptake, and a magnetically inhomogeneous ferromagnetic state associated with residual oxygen vacancies and mixed ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ valence. Our findings experimentally confirm that the stabilization of the layered “314” structure is driven by the presence and ordering of oxygen vacancies rather than A-site cation ordering, whereas the oxygen-deficient oxidized compounds represent an intermediate orthorhombic state preceding fully stoichiometric phases.

Graphene has been studied in detail due to its mechanical, electrical, and thermal properties. It is well documented that the introduction of dopants or defects in the lattice can be used to tune material properties for a specific application, such as in electronics, sensors, or catalysis. To design graphene with specific properties, one must achieve control over the composition and concentration of defects. This requires a fundamental understanding of the stability of defects and their interaction in a superstructure. We present a comprehensive defect structure determination approach that enables close to exhaustive enumeration of all relevant defect structures. The approach uses a combination of density functional theory and machine learning to build a transferable energy model for defect formation. We show the capabilities of our approach for free-standing graphene with heteroatom defects, establishing a thermodynamic model to investigate how temperature affects the configuration space of doped graphene. Our analysis yields physical insights into defect interactions. A characteristic peak in the heat capacity indicates a structural transition. We provide a mechanistic explanation for this behavior and demonstrate how the transition temperature shifts with different thermodynamic reservoirs.

We theoretically demonstrate that axial phonons, lattice vibration quanta carrying finite angular momentum, can host a $d$-wave angular momentum texture in orbital altermagnets in the absence of spin-orbit coupling. We consider a minimal electronic tight-binding model with $d$-wave loop-current order that breaks time-reversal symmetry. Within the Born-Oppenheimer approximation, we incorporate electron-phonon coupling via the molecular Berry curvature and show that the underlying $d$-wave orbital magnetic moment texture of the electronic state is transferred to the phonons without requiring the relativistic spin-orbit coupling. Our results expand the range of platforms available for engineering axial phonons and point to functionality unique to $d$-wave textures, including angular-momentum Seebeck and splitter effects, corresponding to longitudinal and transverse angular-momentum currents driven by a temperature gradient.

Using time-of-flight neutron spectroscopy, we have mapped out the magnetic excitations in the van der Waals antiferromagnet ${\text{NiBr}}_2$ over a broad range of energies and momenta. The observed well-defined magnon dispersions are quantitatively reproduced by linear spin-wave theory based on a minimal bilinear Hamiltonian that includes Heisenberg exchange interactions up to third-nearest neighbors, interlayer couplings, and a single-ion anisotropy term, with no need to invoke a Kitaev interaction. This finding is consistent with the fact that bromine is lighter than iodine, leading to weaker ligand-mediated spin-orbit coupling in ${\text{NiBr}}_2$ than in ${\text{NiI}}_2$, and consequently a reduced Kitaev interaction. Our results establish ${\text{NiBr}}_2$ as a Heisenberg-dominated van der Waals antiferromagnet and support the view that the ligand spin-orbit coupling governs the strength of the bond-dependent interactions within the $\text{Ni}{\mathit{\text{X}}}_2$ (X = halogen) family.

The kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ hosts an intriguing interplay between charge-density-wave (CDW) order and superconductivity that is highly sensitive to lattice distortions. However, determining the specific roles of the in-plane $\left(A_{1g,1}\right)$ and out-of-plane $\left(A_{1g,2}\right)$ symmetric strain channels has been hindered by their intrinsic mixing in conventional piezo-based experiments. Here, we combine in-plane uniaxial strain with direct $c$-axis compression to independently access and disentangle these symmetry-resolved responses in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. We reveal that $c$-axis compression drives a substantial, linear enhancement of the superconducting transition temperature $\left(T_c\right)$ alongside a suppression of $T_{\mathrm{CDW}}$. The tuning efficiency of this out-of-plane deformation acts with an opposite sign and far exceeds that of in-plane strain, demonstrating that $c$-axis lattice control dictates the phase competition. Furthermore, by isolating the pure elastoresistivity coefficients, we find that the out-of-plane cross-coupling coefficient $\left(m_{13}\right)$ is comparable in magnitude but opposite in sign to the in-plane response $(m_{11}+m_{12})$. Unlike the sharply peaked in-plane response, $m_{13}$ exhibits a distinct, order-parameter-like onset across the CDW transition. Our results establish that out-of-plane lattice control plays a dominant role in tuning the intertwined states in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and provide a general pathway for resolving strain-coupled electronic responses in layered quantum materials.

We investigate how additional topologically trivial metallic bands crossing the Fermi level modify the conventional quantum anomalous Hall (QAH)–superconductor mechanism for chiral topological superconductivity in ferromagnetic metals. A four-band model containing an inverted spin-polarized QAH sector and an exchange-split metallic pair is supplemented by spin-singlet $s$-wave pairing and studied in the Bogoliubov–de Gennes (BdG) formalism. Weak pairing produces a gapless or nearly gapless superconducting metal in which chiral boundary channels coexist with bulk quasiparticles and retain a dephasing-enhanced Hall response inherited from the metallic QAH parent state. Strong pairing gaps out the low-energy states derived from the normal-state metallic bands and stabilizes fully gapped two-dimensional chiral topological superconducting phases with BdG Chern numbers $\mathcal{N}=-2$ and $\mathcal{N}=1$. The evolution between the broad $\mathcal{N}=-2$ and $\mathcal{N}=1$ strong-pairing regimes proceeds through a narrow $\mathcal{N}=-1$ intermediate phase and is accompanied by a reversal of the net Majorana chirality and a pronounced reconstruction of the orbital texture of the chiral edge states. The fully gapped phases are identified by chiral Majorana ribbon spectra, quantized thermal Hall response, and Andreev transport signatures. Our results extend the QAH-superconductor paradigm to metallic ferromagnets and suggest ferromagnetic superconductors with spin-polarized band inversion as candidate platforms for intrinsic chiral topological superconductivity.

The easy $c$-axis collinear antiferromagnetic order of the bilayer square-lattice iridate ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ has motivated a diverse range of microscopic models. A previous high-pressure Raman study suggested a magnetic phase transition near 15 GPa at low temperatures, but the fundamental nature of this transition has remained unclear. Here, we utilize high-pressure resonant inelastic x-ray scattering, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) to directly probe the evolution of magnetic excitations in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ under compression. We identify three distinct pressure regimes. At low pressures, the system hosts a well-defined magnon characteristic of long-range antiferromagnetic order. Beyond a characteristic pressure, the magnon broadens substantially while its energy continues to harden, marking a crossover into a heavily damped paramagnon regime. Upon further compression, the paramagnon spectral weight is fully suppressed at a distinct second boundary. Both the crossover and the phase boundary exhibit a pronounced temperature dependence, shifting to higher pressures at lower temperatures. The crossover pressure lies near the previously identified Raman transition, whereas the second boundary extends beyond 22.5 GPa at 33 K. Throughout the explored pressure-temperature phase space, XAS and XMCD spectra reveal no discernible changes, ruling out any substantial departure from the $J_{\mathrm{eff}}=1/2$ state or a transition to an alternative magnetic configuration such as a spin-flip phase. We discuss these findings in the context of the competing microscopic models for magnetism in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Magnetoresistance of hole-doped ${\mathrm{LaMnO}}_3$ at frequencies above a few MHz has been seldom reported compared to numerous studies on magnetoresistance measured at direct current. Here, we contrast the high-frequency magnetoresistance in the frequency range $f=0.9–3\mathrm{GHz}$ in polycrystalline ${\mathrm{LaMnO}}_3$ (LMO) synthesized by microwave irradiation of oxide precursors in a microwave furnace (MW-LMO) and by conventional heating (CH-LMO) in an electrical furnace. The structure at room temperature changes from orthorhombic in CH-LMO to rhombohedral in MW-LMO. A combination of magnetization and resistivity studies suggests that the CH-LMO sample is a canted antiferromagnetic insulator below 140 K but the MW-LMO is a ferromagnetic metal with $T_C=240\mathrm{K}$. While the high-frequency resistance of the CH-LMO sample at 300 K decreases gradually with increasing strength of the applied dc magnetic field for all $f$, a peak appears at $H=H_r>0$ when $f\ge 1.4\mathrm{GHz}$ in the MW-LMO sample and $H_r$ increases linearly with $f$. We attribute the observed features in the high-frequency magnetoresistance of MW-LMO to current-driven resonant excitation of spins in the paramagnetic state. Its absence in the CH-LMO sample highlights the need for an optimum hole density to observe this effect.

Using the magnetostatic approximation, we calculate the spectra of bulk and surface spin waves in an in-plane magnetized ferromagnet/paramagnet bilayer. Due to the dipolar coupling between the layers, the paramagnet becomes polarized, which in turn modifies the spectrum of the Damon-Eshbach magnetostatic modes. We assume that the paramagnet is characterized by a magnetic susceptibility, $\chi \propto 1/(T-T_C)$, which reaches large values when the system temperature $T$ is close to the Curie temperature $T_C$. We find the conditions under which surface spin waves become unidirectional, i.e., capable of carrying energy in only one direction, and determine the magnitude of their frequency nonreciprocity. We demonstrate the possibility of switching the unidirectional wave regime on and off by varying the external magnetic field or temperature, making the ferromagnet/paramagnet system an attractive platform for tunable magnonic logic devices.

The real mechanism of ferromagnetism in III-V diluted magnetic semiconductors such as ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ is still debated. The experimental results point out the realization of appreciable ferromagnetism at filling of the impurity band close to one-half that can be expressed in terms of compensation degree. Based on our recent results on the existence of two competing mechanisms of pair indirect exchange (with ferro- and antiferro alignment of Mn $d$ shells), a theory of magnetic ordering in (Ga,Mn)As is presented. The molecular-field-like equation is derived in which the effective exchange constant contains all necessary information about material, including the Mn content, the compensation degree, the crystal coordination, and the parameters of two competing pair exchange interactions. The Curie temperature depends crucially on the compensation degree. Depending on Mn content and compensation degree, three possible magnetic phases (paramagnetic, ferromagnetic, and spin-glass-like) are predicted. However, the spin-glass-like phase in (Ga,Mn)As is apparently not realized due to the additional inevitable compensation caused by As antisites. The phase transition into the ferromagnetic state occurs not only with Mn content variation but through the change of the compensation degree as well. The new mechanism of photoferromagnetic effect is predicted and discussed in the context of possible clarification of the magnetic ordering mechanism. It is assumed that the main obstacle to a possible increase in the Curie temperature is the increasing contribution of the antiferromagnetic superexchange between the nearest-neighbor Mn's, rather than compensatory (electric or magnetic) effects from Mn interstitials, as was previously stated. The comparison of our predictions with available experimental data is presented.

We report a ${}^{23}\mathrm{Na}$-NMR study of the alkali superoxide ${\mathrm{NaO}}_2$, a candidate one-dimensional quantum spin system. The Knight shift, linewidth, and spin-lattice relaxation rate $1/T_1$ were investigated down to 0.3 K in magnetic fields up to 16 T. Below $T_{\mathrm{S}3}=34\text{K}$, $1/T_1$ exhibits thermally activated behavior, indicating the opening of a spin gap of $\mathrm{\Delta }\left(10.1\mathrm{T}\right)\simeq 38.0\text{K}$ and $\mathrm{\Delta }\left(3.55\mathrm{T}\right)\simeq 46\text{K}$. The hyperfine coupling constant shows a sudden reduction across the structural transition at $T_{\mathrm{S}2}\simeq 195\text{K}$, reflecting the development of pronounced one-dimensionality in the electron structure. These results provide microscopic evidence that ${\mathrm{NaO}}_2$ is a rare $\pi$-orbital-based one-dimensional spin chain that exhibits a spin-gapped ground state, consistent with a spin-Peierls-like instability.

Recent studies on ferrimagnetic systems have revealed conflicting trends in spin-orbit-torque-related responses near magnetic compensation, as exchange coupling and net magnetization are typically varied simultaneously. Here, we employ a perpendicularly magnetized Pt/Co/IrMn trilayer with interfacial exchange coupling as a model system, where the exchange-bias magnitude and remanent magnetization configuration can be separately varied within a single device by controlling the IrMn antiferromagnetic state. We find that the measured damping-like effective field at low temperature is gradually suppressed with increasing exchange-bias magnitude. In contrast, room-temperature measurements of reduced-remanence states follow the expected $1/\left|x\right|$ averaged-response scaling. This suggests that, in the present exchange-biased Pt/Co/IrMn trilayer, the magnetization configuration alone does not produce an anomalous damping-like effective field response in Co.

The single-layer cuprate superconductor ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ (Hg1201) is an ideal candidate for investigating many properties of cuprates with minimal disorder and without the complication of multiple ${\mathrm{CuO}}_2$ layers. Here we measure the in-plane longitudinal thermal conductivity $\kappa$ of underdoped Hg1201 ($T_c$ = 76 K, $p$ = 0.11) at dilution refrigerator temperatures to extract the nodal quasiparticle velocity ratio $v_F/v_{\mathrm{\Delta }}$. Assuming contributions from only a single line node per quadrant on the Fermi surface leads to a value of $v_F/v_{\mathrm{\Delta }}$ = $23\pm 3$, anomalously large compared to other cuprates at similar dopings. In conjunction with the anomalously high quasiparticle specific heat of Hg1201 in the normal state reported previously at a similar doping, this points to more than one Fermi surface sheet crossing the nodal line, suggesting the presence of more than the single small electron pocket detected by quantum oscillations.

We formulate the tight-binding model for cubic $\alpha$-Sn based on the density functional theory calculations. In the model, we incorporate a variable bond angle, which allows us to simulate the effect of the in-plane strain. In the bulk, we demonstrate the presence of the ${\mathbb{Z}}_2$ topological invariant and a nonzero mirror Chern number, making $\alpha$-Sn one of the rare cases where dual topology can be observed. We calculate the topological phase diagram of multilayer $\alpha$-Sn as a function of strain and number of layers. We find that a nontrivial quantum spin Hall state appears only for compressive strain above five layers of thickness. Quite surprisingly, both in the trivial and nontrivial phases, we find a plethora of edge states with energies inside the bulk gap of the system. Some of these states are localized at the side surfaces of the slab and some of them prefer top and bottom surfaces. We find that they can be adiabatically connected to topological states in the chiral limit, where the model supports multiple one-dimensional winding numbers that take different values in different directions in the Brillouin zone. Finally, around high-symmetry points their presence can also be explained by the mirror Chern number and Wilson-loop invariants.

The fundamental behavior of hydrogen in relation to the magnetism of nanoscale quantum magnets holds considerable significance within the fields of Kondo physics and spintronics. However, the precise atomic-scale manipulation of molecular spin states and their associated Kondo effects remains a significant challenge. In this study, we theoretically establish that selective hydrogen adsorption can significantly modulate the magnetic characteristics of a single cobalt phthalocyanine (CoPc) molecule adsorbed on an Au(111) substrate. This modulation facilitates the manipulation of the molecular Kondo state, thereby enabling the operation of a Kondo switch. First-principles calculations reveal that the combined influence of reduced structural distortion and elongation of the Co–Au distance upon sufficient hydrogenation leads to a weakened interaction between the adsorbed molecule and the substrate, accompanied by charge redistribution within the molecule. These modifications substantially alter the system's magnetic moment, increasing it from $0{\mu }_{\mathrm{B}}$ to $1.67{\mu }_{\mathrm{B}}$ in the ${\mathrm{H}}_4\text{−}\mathrm{CoPc}/\mathrm{Au}\left(111\right)$ complex. Electronic structure analysis indicates that the spin density localized at the Co center arises from magnetism associated with localized $d$-electrons, whereas the spin density distributed over the Pc ring corresponds to magnetism derived from delocalized $\pi$-electrons. Furthermore, the adsorption of a fifth hydrogen atom atop the central Co ion quenches the molecular magnetism to a nonmagnetic state. The spin-state-switching mechanism observed in the CoPc/Au(111) system serves as a representative model for a potential Kondo switch and provides significant insights for the experimental control and design of collective quantum magnetism in analogous systems.

Depth-dependent electronic states of a prototypical strongly correlated ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin film across the Verwey transition temperature $(T_{\mathrm{V}}\sim 125\mathrm{K})$ were investigated by hard x-ray photoemission spectroscopy (HAXPES) combined with x-ray total reflection (TR). We found that the Fe $2p$ HAXPES spectral shapes strongly depend on the effective inelastic mean-free-path (${\lambda }_{\mathrm{eff}}$) regardless of temperature ($T$). Particularly, the shoulder structure located at the lower binding energy side of the Fe $2p_{3/2}$ main peak becomes more pronounced with increasing ${\lambda }_{\mathrm{eff}}$ (i.e., increasing bulk sensitivity), indicating a significant difference in the electronic structures between the surface and deeper regions of the film. This trend coincides with the increase of the density of states near the Fermi level $\left(E_{\mathrm{F}}\right)$ for larger ${\lambda }_{\mathrm{eff}}$, suggesting the contribution of nonlocal screening effects. While the valence band spectra show a clear metal-to-insulator transition across $T_{\mathrm{V}}$, the Fe $2p$ core-level HAXPES and the magnetic circular dichroism profiles for large ${\lambda }_{\mathrm{eff}}$ are almost independent on $T$. This behavior suggests that the average valence of the $B$-site Fe species is maintained at 2.5+ across $T_{\mathrm{V}}$. Comparison with the configuration interaction theory based on the cluster model calculations emphasizes the importance of nonlocal screening effects in the Fe $2p$ spectral shapes. Our results demonstrate that HAXPES combined with TR is a powerful technique for probing systematic depth-dependent electronic states. By providing this systematic dataset from the surface to deeper regions, this work serves as a reliable benchmark for the research field.

We propose an altermagnetism-induced route to a two-dimensional higher-order topological Dirac semimetal in a trilayer nonsymmorphic Dirac system. Interlayer hybridization preserves the nonsymmorphic bulk Dirac crossings and generates helical edge states in the trilayer phase. Introducing outer-layer $d$-wave altermagnetic order gaps the helical edge states, while only the symmetry-protected Dirac crossing at the high-symmetry point $M$ remains gapless. The resulting phase hosts a surviving bulk Dirac point, gapped edge spectra, and corner states. An exact layer-space decomposition further reveals that these features arise from the coexistence of a gapless Dirac sector and a gapped ${\mathbb{Z}}_2$ topological sector. Our results establish altermagnetism as an effective mechanism for engineering higher-order topological Dirac semimetals in two-dimensional nonsymmorphic systems.