The Ge-site-doped MnNiGe alloys with nominal compositions ${\mathrm{MnNiGe}}_{0.9}{\mathrm{Al}}_{0.1}$ and ${\mathrm{MnNiGe}}_{0.928}{\mathrm{Ga}}_{0.072}$ are investigated through neutron powder diffraction studies in the presence of external magnetic fields ($H$). The application of external $H$ is found to result in a number of interesting features, which include (i) a decrease in the intensity of the incommensurate antiferromagnetic (AFM) satellite peaks, (ii) an increase in some of the nuclear reflections, and (iii) unchanged intensities of the commensurate AFM satellite peaks. We also noticed a small increase in the incommensurate propagation vector ${\mathbf{k}}_{\text{AFM}}^{\text{o}}=(k_a,0,0)$ with increasing $H$. The enhanced nuclear reflections point to the introduction of a ferromagnetic (FM) component into the otherwise AFM alloy, represented by the FM propagation vector ${\mathbf{k}}_{\text{FM}}^{\text{o}}=(0,0,0)$. This FM component modifies the helical modulation of the incommensurate AFM structure, driving it toward a conical configuration, but even an applied $H$ of 80 kOe is insufficient to fully establish a FM alignment of the Mn moments in the presently studied alloys.

Nonreciprocity of spin waves is essential for components such as magnetic isolators and circulators used in spin-wave-based computing. A ferromagnetic (FM) bilayer exhibits significant frequency nonreciprocity and has attracted attention in recent years. Prior research on bilayers has predominantly focused on the high-wavenumber regime, where spin waves display significant nonreciprocity and are accessible through Brillouin light scattering (BLS). Although nonreciprocity at lower wavenumbers, which enables rapid magnon propagation, has also been expected, the exploration of the spin-wave modes in the low-wavenumber regime using electrical excitation methods, compatible with devices, has remained elusive. Here, we investigate spin-wave propagation in the bilayer using coplanar waveguides (CPWs) and demonstrate that increasing the bilayer thickness enhances nonreciprocity even at low wavenumbers, which leads to the high group velocity originating from the Damon-Eshbach (DE) mode. Furthermore, our analysis reveals a trade-off between spin-wave nonreciprocity and group velocity. These findings establish design principles for high-speed, low-loss spin-wave-based information processing.

Altermagnetism, the third class of collinear magnetic order, uniquely combines a zero net magnetization with spin-polarized bands in reciprocal space, opening new avenues for two-dimensional valleytronics and spintronics. Here, using first-principles calculations, we predict that the Janus monolayer ${\mathrm{Cr}}_2\mathrm{SSe}$, which possesses intrinsic inversion symmetry breaking, hosts a strain-tunable multipiezo effect and exhibits distinctive valleytronic properties. The system displays pronounced spin splitting and band inversion at the $X$ and $Y$ high-symmetry points in the Brillouin zone, giving rise to robust spin-valley locking. The degeneracy of these valleys is protected by diagonal mirror symmetry. The application of uniaxial strain breaks this symmetry, concurrently inducing piezovalley, piezoelectric, and piezomagnetic responses, a manifestation of the multipiezo effect. Critically, strain applied along orthogonal crystallographic directions yields opposite valley polarization, while under small compressive strain, we achieve selective reversal of valley polarization, enabling independent control of valence and conduction band valleys and promoting a single-spin-channel anomalous valley Hall effect. These findings establish a pathway for low-power, nonvolatile manipulation of valley degrees of freedom and enhanced spin transport efficiency, providing a theoretical foundation for the design of energy-efficient valleytronic devices.

Magnetic hopfions in chiral magnets are topological solitons, localized in three dimensions. But is their localization strong? To address this question we derive an asymptotic expansion for the isolated hopfion's spatial profile. It becomes the starting point for a simple analytical model, which is asymptotically correct both near the hopfion center and far away from it. The region of equilibrium hopfions on the phase diagram of a helimagnet is computed and material requirements for supporting movable isolated magnetic hopfions on a uniform background are discussed.

The superconducting diode effect (SDE), characterized by a nonreciprocal supercurrent, has attracted significant attention in recent years due to its potential applications. However, most studies have focused on weakly correlated models, leaving the impact of strong electron-electron interactions on the SDE largely unexplored. In this work, we bridge this gap by investigating the SDE in asymmetric band metals with Hatsugai-Kohmoto (HK) interactions, which are exactly solvable due to their locality in Bloch momentum space. Through a combination of low-energy analysis and a numerical self-consistent approach, we demonstrate that HK interaction can enhance the SDE's quality factor. Our findings shed light on the role of strong electron-electron correlations in shaping the SDE.

We investigate measurement-induced localization in a continuously monitored one-dimensional Aubry-André-Harper model, focusing on the quantum Zeno regime in which the measurements dominate coherent dynamics. The presence of a quasiperiodic potential renders the problem analytically tractable and enables a controlled study of the interplay between monitoring and disorder. We develop an analytical description based on an instantaneous Schrödinger equation with a measurement-induced effective potential constructed self-consistently from individual quantum trajectories, without relying on postselection. In the quantum Zeno regime, an emergent dominant energy scale reduces the problem to a transfer-matrix formulation of an effective non-Hermitian Hamiltonian, which allows direct computation of the Lyapunov exponent. Complementarily, we extract the localization length numerically from long-time steady-state quantum state diffusion trajectories by reconstructing the intrinsic localized single-particle wave functions and analyzing their spatial decay. These numerical results show quantitative agreement with the effective theory predictions, with controlled corrections of order $J^2/[{\lambda }^2+{(\gamma /2)}^2]$ (where $J$ is the hopping amplitude, $\gamma$ the measurement strength, and $\lambda$ the quasiperiodic potential). Our results underscore the connection between the effective non-Hermitian description and the stochastic monitored dynamics, showing the interplay between Zeno-type localization, coherent hopping, and quasiperiodic-disorder-induced localization, while also laying the groundwork for understanding and exploiting measurement-induced localization as a tool for quantum control and state preparation.

Topological edge states in non-Hermitian systems are generally believed to be robust based on non-Bloch topology. Here, we show that in the Floquet non-Hermitian Su-Schrieffer-Heeger model, infinitesimal symmetry-preserving perturbations can completely destroy edge states, revealing the breakdown of the non-Bloch bulk-boundary correspondence in finite systems. We trace this fragility to the instability in the quasienergy spectrum and establish a correspondence between stable zero-mode singular states and midgap states of quasienergies in the thermodynamic limit. Then we develop a unified theoretical framework to characterize the number of zero-mode and $\pi /T$-mode edge states by the singular-value-based diagnostic scheme. This supplies a useful way to explore non-Hermitian Floquet topological phases.

We combine polarized infrared magnetotransmission and Faraday angle rotation measurements to map the collective excitations of the van der Waals antiferromagnet ${\mathrm{FePS}}_3$. Below the Néel temperature ($T_{\mathrm{N}}\approx 118\mathrm{K}$), the phonon spectrum becomes strongly anisotropic, reflecting the underlying zigzag antiferromagnetic order. In contrast, a prominent excitation at $122{\mathrm{cm}}^{-1}$ (15 meV) is polarization-independent, hardens on cooling, and splits linearly with magnetic field, identifying its magnetic origin. From absolute transmission and Faraday rotation, we reconstruct the circular optical conductivities and reveal a pronounced dichroism of the field-split excitations. The upper branch near $129{\mathrm{cm}}^{-1}$ exhibits a reduced dichroic response, consistent with hybridization with a nearby infrared phonon. Several phonon modes exhibit sizable Faraday rotation, providing evidence for spin-phonon coupling and demonstrating that lattice vibrations acquire magnetic-field-dependent optical activity. In addition, additional excitations appear in the infrared spectra and a broad midinfrared feature near $900{\mathrm{cm}}^{-1}$ emerges only below $T_{\mathrm{N}}$, consistent with a modified lattice response in the magnetic state. These results highlight the anisotropic nature of spin-phonon coupling in ${\mathrm{FePS}}_3$ and establish polarization-resolved magneto-optical spectroscopy as a powerful probe of coupled spin and lattice dynamics in two-dimensional antiferromagnets.

Charge density wave (CDW) phenomena in low-dimensional rare-earth chalcogenides have drawn considerable interest owing to their interplay with lattice instabilities, electronic correlations, and magnetism. ${\mathrm{EuTe}}_4$, which contains both mono- and bilayer Te pseudosquare nets together with Eu–Te zigzag layers, provides a unique platform for exploring these effects. Using first-principles calculations, angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy/spectroscopy (STM/STS), and low-temperature thermodynamic measurements, we map the full CDW-induced reconstruction of its electronic structure. ARPES reveals a highly anisotropic low-energy CDW gap that maximizes along $\overline{\mathrm{\Gamma }}\text{−}\overline{\mathrm{Y}}$ and minimizes along $\overline{\mathrm{\Gamma }}\text{−}\overline{\mathrm{X}}$, accompanied by a second, higher-binding-energy gap originating from band folding. STM directly visualizes the incommensurate CDW with Te-trimer formation and a dominant modulation vector ${\mathit{q}}_{\mathrm{CDW}}\approx 0.38{\mathit{b}}^{*}$, and spectroscopy confirms a wide CDW-induced suppression of states. A similar ${\mathit{q}}_{\mathrm{CDW}}$ corresponds to the imaginary soft modes in the calculated phonon spectrum. Heat capacity measurements identify an antiferromagnetic transition at $T_{\mathrm{N}}\approx 6.9\mathrm{K}$. Together, these results establish a comprehensive picture of the multigap, momentum-dependent CDW state in ${\mathrm{EuTe}}_4$.

$\beta$-Mn shows a large Sommerfeld coefficient of 70 mJ ${\mathrm{Mn}}^{-1}{\mathrm{K}}^{-2}$, which is one of the largest values in $3d$-electron systems. Although the origin has been attributed to the fluctuating magnetic moment, there are no material-dependent systematic studies in the $3d$-electron systems except for doped $\beta$-Mn alloys. The ordered $\beta$-Mn structure alloys ${\mathrm{Mn}}_3\mathrm{TX}$ $(\mathrm{T}; \mathrm{Co}, \mathrm{Rh}, \mathrm{Ir}, \mathrm{X}; \mathrm{Si}, \mathrm{Ge})$ also exhibit the Sommerfeld coefficient enhancement. According to the inelastic neutron scattering study of ${\mathrm{Mn}}_3\mathrm{RhSi}$, the magnetic moment of Mn is about a full $3d^5$ magnetic moment of 5 ${\mu }_{\mathrm{B}}$, whereas the long-range ordered magnetic moment $M$ is 2.6 ${\mu }_{\mathrm{B}}$ at 4 K, suggesting that a large part of the magnetic moment is fluctuating. Here, the enhanced Sommerfeld coefficients are compared with the Néel temperatures $T_{\mathrm{N}}$. The critical exponent of about 1/3 requires the further accurate approximation than the self-consistent renormalization approximation for the wide $T_{\mathrm{N}}$ range. The disappearance of the magnetic short-range order in the ${\mathrm{Mn}}_3\mathrm{TX}$ system except for ${\mathrm{Mn}}_3\mathrm{RhSi}$ and ${\mathrm{Mn}}_3\mathrm{CoSi}$ is also discussed with the critical spin-orbit coupling of about 50 meV, leading to the spin excitation gap at low energies. The required conditions for the high magnetic short-range order temperature are summarized, suggesting the close relation to the Sommerfeld coefficient enhancement conditions.

Electron chirality has been proposed as a microscopic quantity that characterizes electronic handedness, yet its underlying control parameter has not been clearly identified. Furthermore, its applicability is limited to systems with spin-orbit coupling, which motivates the need for alternative measures of chirality. In this work, we explore two complementary measures of chirality: electron chirality and hydrodynamic helicity. By analyzing a minimal atomic model under chiral crystal fields, we clarify how the interplay among crystal fields, spin-orbit coupling, and electron correlation gives rise to nonzero values of chirality measures. Although electron chirality increases with both spin-orbit coupling and chiral crystal field strength, the dependence on these two factors is highly nontrivial. Particularly, when the chiral crystal field is varied continuously and the energy levels approach quasidegenerate points, the electron chirality is insensitive to spin-orbit coupling, resulting in a remarkable enhancement of chirality in the atomic model. In contrast, the hydrodynamic helicity, defined as a two-body pseudoscalar quantity, remains nonzero even without spin-orbit coupling, originating from electron-electron interactions. Perturbative analysis reveals distinct symmetry selection rules governing the two quantities. Our results provide fundamental insight into the origin of chiralities in electronic systems.

The electron in a solid can be considered a bound state of the three independent, fundamental degrees of freedom creating quasiparticles: spinons, carrying the electron spin; plasmons, carrying the collective charge mode; and orbitons, carrying its orbital degree of freedom. These fundamental degrees of freedom could form ordering states in which dynamics or collective motions could occur and manifest as low-energy excitations. The exotic properties that appear in the materials exhibiting these electronic orderings are associated with these low-energy excitations. Although the orbital order (OO) and its coupling to the spin system creates very interesting phenomena, the microscopic origin of OO has been much less explored than other electronic properties as it is very difficult to directly access experimentally. Due to the recent improvement in energy resolution and flux, soft-x-ray resonant inelastic scattering (RIXS) allows for a reexamination of orbital excitations in manganites. Here, we present a study of low-energy excitations in half doped $A$-site ordered ${\mathrm{SmBaMn}}_2{\mathrm{O}}_6$ through a combination of RIXS and soft-x-ray resonant elastic scattering measurements. We confirm the existence of OO at $\mathbf{q}$ = (0.25, 0.25, 0) and find various low-energy excitations below 200 meV. While several excitations can be assigned to be of magnetic and phononic origin, a group of excitations between 80 and 200 meV show a temperature dependence closely following that of the OO, making them possible candidates for orbitons.

Altermagnets exhibit momentum-dependent spin splitting despite having zero net magnetization. This enables a spin splitter effect in which an external electric field generates transverse spin currents by separating oppositely polarized carriers. Here, we develop a unified semiclassical theory of linear extrinsic spin splitter currents, incorporating impurity-induced side-jump and skew-scattering contributions, and apply it to the $d$-wave altermagnet ${\mathrm{FeSb}}_2$. We demonstrate that asymmetric impurity scattering provides a dominant channel for spin splitter currents. Remarkably, the resulting extrinsic spin conductivity is time-reversal even, in contrast to previously studied spin splitter responses arising from symmetric scattering.

Coexistence of nontrivial topology and flat electronic bands provides a fertile platform for correlated quantum states. The square-octagon lattice hosts Dirac nodes and flat bands at half-filling, yet the effects of intrinsic spin-orbit coupling (SOC) and staggered magnetic flux on its electronic and topological properties remain largely unexplored. Here, using tight-binding models incorporating SOC and staggered magnetic flux, we uncover a rich topological phase diagram in this lattice, comprising a quantum spin Hall phase with spin Chern number $C_s=1$, crossovers to quantum anomalous Hall phases with $C=1$ and $C=2$, and higher-order topological insulator phases with quantized quadrupolar corner charges. The initially dispersionless flat bands evolve into quasiflat topological bands with nearly uniform quantum geometry and large flatness ratios, making them promising candidates for fractional Chern insulator states. We further identify realistic materials, including octagraphene, transition-metal dichalcogenides, synthetic ${\mathrm{MoSi}}_2{\mathrm{N}}_4$, and magnetic $\alpha -{\mathrm{MnO}}_2$, that may realize these tunable topological phases intertwined with flat-band physics, opening new opportunities for correlated topological matter.

Using mean field theory, we determine the electronic phase diagram of undoped AB-stacked bilayer graphene in the presence of a transverse electric field. In addition to multiple competing electronic instabilities characterized by excitonic order parameters, our framework incorporates the long-range Coulomb energy associated with interlayer polarization. This long-range interaction plays a crucial role, as it significantly influences both the structure and the relative energies of the competing ordered states. We derive a set of self-consistency equations and solve them both numerically and analytically. Our findings reveal that, as the bias field is varied, the bilayer undergoes a cascade of first-order transitions between several ordered insulating phases for which order-parameter structures are explicitly identified. Some of these phases are characterized by two inequivalent single-particle gaps, whose magnitudes depend on the valley and spin quantum numbers. Field-driven transitions are accompanied by discontinuous and nonmonotonic variations of the single-electron gap. We relate our results to Hartree-Fock numerical calculations and to experimental research, including observations of fractional metallic phases that emerge upon doping the bilayer system.

We develop and systematically compare several analytical approximations for the ground state and zero-temperature polaron dispersion in finite-width, nonparabolic conduction bands. The main focus of the work is an extension of the Feynman variational method to a tight-binding lattice, where the effective-mass approximation is no longer applicable. The resulting variational formulation is not restricted to a specific phonon dispersion or electron-phonon interaction and provides a uniform description across weak-, intermediate-, and strong-coupling regimes. In addition, we revisit and generalize other analytical approaches traditionally formulated for continuum polarons, including canonical transformations and self-consistent Wigner-Brillouin-type approximations. For lattice polarons, these methods exhibit qualitative features absent in the continuum case, such as a nontrivial connection between weak- and strong-coupling limits. We show that an improved Wigner-Brillouin scheme yields a momentum-dependent polaron self-energy free of resonances and in good agreement with numerically exact results over the whole range of momenta within the Brillouin zone. All methods are applied to the Holstein model on a tight-binding lattice and are benchmarked against numerically exact calculations, including diagrammatic Monte Carlo (both our calculations and preceding works) and exact diagonalization results. Furthermore, the analytical approaches are extended to polarons with Rashba-type spin-orbit coupling, providing a stringent test of their applicability in systems with nontrivial band structure. Our results demonstrate that the modified Feynman variational method yields ground-state energies and dispersions with accuracy comparable to, and in many cases exceeding, that of other established analytical approaches. The developed framework offers a versatile and reliable analytical description of lattice polarons beyond the continuum approximation.

We present a pseudospin formulation for the postquench dynamics of charge-density-wave (CDW) order in the half-filled spinless Holstein model on a square lattice, assuming spatially homogeneous evolution. This Anderson pseudospin description captures the coherent nonequilibrium dynamics of the coupled electron-lattice system. Numerical simulations reveal three distinct dynamical regimes of the CDW order parameter following a quench—locked oscillations, Landau-damped dynamics, and overdamped relaxation—closely paralleling quench dynamics in Bardeen-Cooper-Schrieffer superconductors and other electronically driven symmetry-breaking phases. Crucially, however, the presence of dynamical lattice degrees of freedom leads to qualitatively different long-time behavior. In particular, while the oscillation amplitude is reduced in the damped regimes, CDW oscillations do not fully decay but instead persist for a long time due to feedback from the lattice field. We further show that these persistent oscillations are characterized by a nonequilibrium electronic distribution, which provides an intuitive understanding of both their amplitude and the renormalization of the oscillation frequency relative to the bare Holstein phonon frequency. Our results highlight the essential role of lattice dynamics in nonequilibrium ordered phases and establish a clear distinction between electron-lattice-driven CDW dynamics and their purely electronic counterparts.

We apply correlation matrix renormalization theory (CMRT) to cerium nitride (CeN) under pressure. For $B1$ (NaCl-type) phase, CMRT gives an equation of state consistent with ambient pressure experiments. It produces electronic density-of-state (DOS) characterized by a sharp $4f$ quasiparticle resonance peak pinned at the Fermi level and two subbands formed by strong hybridization between the localized Ce-$4f$ electrons and the itinerant Ce-$5d$ and N-$2p$ electrons below the Fermi level, consistent with x-ray photoemission spectroscopy experiments. Upon compression, CMRT predicts a first-order $B1$ → $B2$ (CsCl-type) transition with $\sim 11\%$ volume collapse in agreement with experiments. Across the transition, the $4f$ spectrum broadens, the $4f$ orbital occupancy increases, and the hybridization with conduction states enhances, signaling a crossover from partially localized to more itinerant $4f$ behavior. These features are in excellent agreement with experimental observations, demonstrating that CMRT provides a parameter-free description and prediction of correlation-driven structural and electronic transitions in rare-earth compounds.

Since the discovery of phase transitions driven by topological defects, the classification of phases of matter has been significantly extended beyond Ginzburg and Landau's paradigm of spontaneous symmetry breaking (SSB). In particular, intrinsic and symmetry-protected topological (SPT) orders have been discovered in (mostly gapped) quantum many-body ground states. However, these are commonly viewed as zero-temperature phenomena, and their robustness in a gapless ground state or against thermal fluctuations remains challenging to tackle. Here we introduce an explicit construction for SPT-type states with hidden order associated with SSB: They feature (quasi-)long-range correlations along appropriate edges but short-range order in the bulk, ground state degeneracy associated with SSB, and nonlocal string order in the bulk. We apply our construction to predict two types of finite-temperature SPT transitions protected by 1-form symmetries in the Ising and BKT class, respectively, where the usual signs of criticality appear despite the absence of a diverging correlation length in the bulk. While the state featuring hidden Ising order is gapped, the other SPT state associated with the BKT-SPT transition has hidden $U\left(1\right)$ or XY order and constitutes an intrinsically gapless SPT state, associated with a gapless Goldstone mode. Specifically, in this work we discuss spins with global ${\mathbb{Z}}_2$ or $U\left(1\right)$ symmetry coupled to link variables constituting a loop gas model with a 1-form symmetry. By mapping this system to an Ising-gauge theory, we demonstrate that one of the SPT phases we construct corresponds to the Higgs-SPT phase at $T=0$ – which we show here to remain stable at finite temperature. Our work paves the way for a more systematic search for hidden order SPT phases, including in gapless systems, and raises the question if a natural (finite-$T$) spin liquid candidate exists that realizes hidden order in the Higgs-SPT class.

Quantum oscillations (QOs) are a well-established probe of Fermi-surface (FS) geometry and in the presence of long-range density wave order can display new QO frequencies from reconstructed FS pockets. We show that such reconstructed frequencies can arise even in the absence of long-range density order. Considering electrons coupled to a fluctuating bosonic mode that scatters quasiparticles between sharp hot spots on the FS, we develop a semiclassical theory in which the interaction generates time-dependent tunneling processes analogous to magnetic breakdown. This dynamical magnetic breakdown produces new semiclassical orbits corresponding to reconstructed FS areas despite the absence of static order. Because tunneling probabilities depend on the thermal population of bosonic excitations, the resulting oscillation amplitudes exhibit characteristic deviations from standard Lifshitz-Kosevich behavior. Our results provide a mechanism to probe bosonic fluctuations in quantum critical metals and provide a framework for dynamical magnetic breakdown.