While electron doping in incipient ferroelectrics like ${\mathrm{SrTiO}}_3$ and ${\mathrm{KTaO}}_3$, via chemical substitution or heterostructure engineering, has revealed a wealth of collective quantum phenomena, its effect in ${\mathrm{CaTiO}}_3$ remains largely unexplored. In this work, we investigate the change in electronic properties of ${\mathrm{CaTiO}}_3$ single crystal via the introduction of oxygen vacancy. Our magnetotransport measurements reveal clear signatures of Kondo scattering, arising from the interaction between conduction electrons and localized magnetic moments. Employing hard x-ray photoemission spectroscopy combined with DFT calculations, we unravel the underlying electronic structure and demonstrate the dichotomic nature of oxygen vacancy induced electron doping: a fraction of vacancy-donated electrons occupy states near the Fermi level, driving metallic behavior, while the remainder localize in deep in-gap states, acting as magnetic moments responsible for Kondo scattering. Owing to the polar nature of ferroelastic domain walls in pristine ${\mathrm{CaTiO}}_3$ [Phys. Rev. Lett. 101, 097602 (2008)], electron-doped ${\mathrm{CaTiO}}_3$ offers a compelling platform to investigate the intricate interplay between localized magnetic moments and electric dipoles embedded within a Fermi sea. Furthermore, controlled manipulation of domain wall polarity may unlock novel functionalities, offering promising avenues for next-generation electronic devices.

Recent theoretical and experimental advances have identified cobaltates with a high-spin $d^7$ electronic configuration as promising hosts for spin-orbit entangled $J_{\mathrm{eff}}=1/2$ magnetism that can support bond-dependent exchange interactions. In two-dimensional triangular lattices, the coexistence of such exchange frustration along with usual geometric frustration gives rise to a rich landscape of competing magnetic phases, establishing monolayer triangular $d^7$ cobaltates as a compelling platform for frustrated magnetism. Here we investigate a representative triangular-lattice monolayer cobaltate ${\mathrm{CoBr}}_2$, where first-principles density functional theory (DFT) calculations reveal a dominant nearest-neighbor $t_{2g}–e_g$ hopping channel that enhances the ferromagnetic Kitaev-type exchange interactions. In contrast, the nearest-neighbor Heisenberg term is highly sensitive to a direct $t_{2g}–t_{2g}$ hopping path and electronic correlations. The magnetic exchange parameters are evaluated using the hopping amplitudes obtained from DFT calculations within an exact diagonalization framework. We construct the first (${\mathrm{J}}_1$) and third (${\mathrm{J}}_3$) nearest neighbor Heisenberg exchange dependent $J_1–J_3$ magnetic phase diagram in the physically relevant regime and identify multiple competing ground states, including ferromagnetic, stripy, incommensurate, and ${120}^{\circ }$ antiferromagnetic orders. The Luttinger-Tisza analysis further predicts a ${\mathrm{Z}}_2$ vortex crystal phase, while exact diagonalization reveals a bond-nematic phase promoted by quantum fluctuations. Going beyond the conventional bond-independent XXZ picture typically applied to ${\mathrm{Co}}^{2+}$ systems, our results on monolayer ${\mathrm{CoBr}}_2$ establish ${\mathit{d}}^7$ cobalt dihalides as a promising platform to explore the interplay of long-range Heisenberg and bond-dependent exchange interactions that can stabilize diverse magnetic ground states on a triangular lattice.

Recent studies have shown that the nonlinear optical response of crystalline systems is fundamentally a quantum geometric property. In particular, specific second-order responses, such as shift current and second-harmonic generation, have been shown to depend on underlying geometric structures known as the quantum geometric tensor and quantum connection. In this work, we first generalize and unify these results by demonstrating how the full two-frequency second-order nonlinear conductivity decomposes into distinct quantum geometric contributions. As a corresponding probe, we propose two-dimensional coherent spectroscopy (2DCS), which naturally measures the nonlinear conductivity as a function of two independent frequencies using two time-delayed light pulses. We identify a term arising from the multiband quantum connection that does not appear in linear response, and we show that it can be measured in isolation by considering specific polarizations and enforcing time-reversal symmetry. We explore these findings via model calculations for transition-metal dichalcogenides and strontium ruthenate. Through these examples, we demonstrate how 2DCS enables study of the quantum connection, providing a way to compare the quantum geometry of different materials. We also show that one can gain rough momentum-resolved knowledge of the quantum geometry by varying the chemical potential.

Tailoring altermagnetism in $\alpha \text{−}\mathrm{MnTe}$ thin films via molecular beam epitaxy (MBE) enables the investigation of emergent quantum states and their underlying physical mechanisms. Here, we report a systematic study of MnTe thin film growth on InP (111) substrates. Crucially, the substrate termination determines the resulting phase: InP (111)-A stabilizes the altermagnetic $\alpha \text{−}\mathrm{MnTe}$ phase, whereas InP (111)-B favors the zinc-blende $\beta \text{−}\mathrm{MnTe}$ phase, as further corroborated by transmission electron microscopy. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal band features near $E_F$ that are consistent with the reported band splitting in $\alpha \text{−}\mathrm{MnTe}$, while $\beta \text{−}\mathrm{MnTe}$ exhibits a large band gap with its valence band maximum lying well below $E_F$. Furthermore, combining thickness-dependent transport and ARPES measurements, we demonstrate that both the Néel temperature and electronic structure of $\alpha \text{−}\mathrm{MnTe}$ are pronouncedly modified, suggesting tuning effects arising from reduced dimensionality and interfacial coupling in the ultrathin regime. Our work establishes a systematic MBE growth protocol for high-quality thin films and facilitates further understanding of dimensional effects and interfacial effects in altermagnets.

We study the effect of driving the Su-Schrieffer-Heeger model using two unitary operators $U_1$ and $U_2$ in different combinations; the unitaries differ in the values of the intercell hopping amplitudes. Specifically, we study the cases where the unitaries are applied periodically, quasiperiodically, aperiodically, and randomly. For a periodic protocol, when $U_1=e^{-i{H}_{1}T/2}$ and $U_2=e^{-i{H}_{2}T/2}$ are applied alternately, we find that end modes may appear, but the number of end modes does not always agree with the winding number, which is a $Z$-valued topological invariant. We then study the Loschmidt amplitude ($LA$) starting with an initial state which is an end mode of $H_1$. We find that the $LA$ exhibits pronounced oscillations whose Fourier transform has a peak at a frequency that is equal to the quasienergy of an end mode of $U$. Next, when $U_1$ and $U_2$ are applied in a quasiperiodic or aperiodic way (we consider the Fibonacci and Thue-Morse protocols as examples), we study the Loschmidt echo ($LE$) starting with an initial state that is an end mode of the Hamiltonian $H_1$. When the intercell hoppings differ by a small amount denoted by $\epsilon$, and the time period $T$ of each unitary is also small, the distance between the unitaries is found to be proportional to $\epsilon T$. We then find that the $LE$ oscillates around a particular value for a very long time before decaying to zero. The deviation of the value of the $LE$ from 1 scales as ${\epsilon }^2$ for a fixed value of $T$, while the time after which the $LE$ starts decaying to zero has an interesting dependence on $\epsilon$ and $T$. Finally, when $U_1$ and $U_2$ are applied in a random order, the $LE$ rapidly decays to zero with increasing time. We have presented a qualitative understanding of the above results.

We find an unexpected phenomenon of coherently synchronized oscillations in a mirror-symmetric many-body localized system. A synchronization transition of the spin oscillations is found by changing the spin-spin interactions. To understand this phenomenon, an effective Ising model based on local integrals of motion is proposed. We find that the synchronization transition can be understood as a paramagnetic-to-ferromagnetic Ising transition. Based on the Ising model, we theoretically estimate the synchronized frequencies and the synchronization transition points, which agree well with numerical results.

We theoretically propose a nonlinear spin-wave Doppler effect, in which the time-dependent motion of a magnetic energy boundary acts as an active frequency modulator, directly converting boundary-induced phase dynamics into instantaneous spectral synthesis for propagating spin-wave modes. In contrast to the conventional linear Doppler effect governed by constant relative velocity, this mechanism enables dynamic phase-to-frequency transduction, generating high-order harmonics, magnonic frequency combs, and coherent chirped sidebands, without requiring nonlinear magnon-magnon coupling or multimagnon scattering. Micromagnetic simulations on voltage-controlled anisotropy boundaries in ferroelectric/ferromagnetic heterostructures demonstrate that the comb spacing and spectral topology are determined solely by boundary kinematics, confirming direct Doppler phase coupling between boundary motion and spin-wave propagation. These results establish moving magnetic-energy boundaries as a new class of on-chip spectral synthesizers and define a coherent and energy-efficient framework for flexible tuning of magnonic frequencies, fundamentally distinct from traditional passive scattering or nonlinear multimagnon mechanisms.

The persistence of the ground state monoclinic phase ($M$ phase) and the isotropic switching kinetics in hafnium oxide (${\mathrm{HfO}}_2$) remain significant physical puzzles. Here, we reveal that these phenomena are governed by a mechanism of “mutual induction” between lattice distortion modes, linked through the topological connectivity of phonon bands. We show that the spatial reversal of the ferroelectric orthorhombic ($Pca{2}_1$) distortion modes inevitably induces their structural counterparts, which constitute the ground state monoclinic ($P{2}_1/c$) motif, to maintain lattice continuity at the domain boundaries. Using layer-resolved mode decomposition, we demonstrate that the ${180}^{\circ }$ domain wall is not a discrete boundary but a locally reconstructed zone where the $M$ phase is inherently embedded. Energetic calculations indicate that this localized $M$ phase reconstruction creates a deep potential well that pins the domain wall, providing a unified microscopic origin for the isotropic switching kinetics that override the expected crystallographic anisotropy. Our findings establish the domain wall as a structural origin for $M$ phase persistence, offering a physical framework to understand and control the reliability and phase stability of ${\mathrm{HfO}}_2$-based ferroelectrics.

Density functional theory calculations are conducted to understand and reveal the origin of the complex shear strain morphology and of the polar Moiré topological pattern recently observed in twisted ${\mathrm{BaTiO}}_3$ bilayers. Our first-principles calculations, along with an original analysis of them allowing the decomposition of the forces into the acoustic and optical contributions, point to the occurrence of forces mostly acting on the acoustic-related motions to produce standing waves of shear strain. Such acoustic waves naturally generate a striking self-organization of the shear strains, and hence create a peculiar gradient of these shear strains. A Moiré dipole pattern, consisting of the interpenetrated arrays of vortices and antivortices made of the electric dipoles, then mostly arises due to the coupling of this gradient of the shear strain with the electric dipoles. Results and discussion about harmonic versus nonharmonic effects on the strength of these patterns is also provided.

The preparation of long-range entangled (LRE) states via quantum measurements is a promising strategy, yet its stability against realistic, noncommuting measurement noise remains a critical open question. Here, we systematically investigate the rich phase structure emerging from a minimal model of competing, noncommuting weak measurements: nearest-neighbor Ising ($Z_i{Z}_j$) and single-qubit transverse field ($X_i$) operators. We analyze three experimentally relevant scenarios based on which measurement outcomes are read out: complete readout, no readout, and partial readout. Using a replica mean-field theory for higher dimensions, complemented by numerical simulations in one dimension, we derive the complete finite-time and stationary phase diagrams. Our analysis reveals a striking dependence on the readout protocol. Complete readout yields a direct transition between a short-range entangled phase and a pure LRE phase. No readout (pure decoherence) precludes entanglement but exhibits a strong-to-weak spontaneous symmetry breaking (SWSSB) transition into a classically ordered mixed state. Most intriguingly, partial readout (complete readout of $Z_i{Z}_j$ with averaging over $X_i$ measurement outcomes) interpolates between these limits, featuring a mixed-state phase transition where the system can become trapped in the SWSSB phase or, for weaker noncommutativity, undergo successive symmetry breaking to reach a mixed LRE phase. A novel technical contribution is the use of a channel-fidelity-based partition function that allows us to simultaneously characterize both entanglement and SWSSB order, revealing a deep interplay between them in the replica limit. These results provide a cohesive picture for understanding measurement phase transitions, SWSSB, and mixed-state phase transitions, offering crucial insights for designing robust state preparation protocols on noisy quantum devices.

A triangular lattice of rare-earth ions with effective spin 1/2 is an ideal platform for investigating the complex interplay between the multiple degrees of freedom such as strong spin-orbit coupling, lattice, crystal electric fields, and geometrical frustration leading to rich physics. Here, we investigate the single crystals of Yb-based triangular lattice $\mathrm{B}{\mathrm{a}}_3\mathrm{Yb}{\left(\mathrm{B}{\mathrm{O}}_3\right)}_3$ using in-depth temperature and polarization-dependent Raman measurements on different crystallographic planes. Our measurements revealed strong crystal field–phonon coupling reflected in the anomalous phonon hardening. Temperature-dependent Raman spectra of the $ac$ and $ab$ planes show phonon renormalization near ∼100 K ($T$*), reflecting changes in low-lying crystal field populations; below $T$*, lattice contraction, orbital overlap, and symmetry mixing modify coupling, producing phonon anomalies. A broadband centered at $\sim 1800\mathrm{c}{\mathrm{m}}^{–1}$ disappears above $T$*, indicating thermally quenched defect-related luminescence via dissociation of defect-bound states. $\mathrm{Y}{\mathrm{b}}^{3+}$ f-f electronic transitions also reveal the characteristic temperature $T$*, with contrasting temperature shifts that reflect exchange splitting, thermal population effects, and strong coupling between localized $4f$ states and the lattice. These results reveal an intricate interplay of lattice, spin-orbit-coupled crystal field levels, and electronic transitions governing the low-energy dynamics.

Spin-current generation through nonrelativistic spin splittings, found in uncompensated magnets and $d$-wave altermagnets (AMs), is desirable for low-power spintronics. Such spin currents, however, are symmetry forbidden in conventional collinear antiferromagnets and higher-order AMs. Using spin-point-group analysis, we demonstrate that finite spin currents can be induced in these materials via magnetoelectric-, piezomagnetic-, and piezomagnetoelectric-like couplings. We utilize electric fields, strain, and their combinations to drive symmetry-lowering phase transitions into uncompensated magnetic or $d$-wave AM states, thereby enabling finite spin conductivity in a broader class of magnetic materials. We further substantiate this framework using density functional theory and Boltzmann transport calculations on representative magnetic materials—${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}, {\mathrm{RuF}}_4, {\mathrm{Cr}}_2{\mathrm{O}}_3, {\mathrm{FeS}}_2$, and ${\mathrm{MnPSe}}_3$—spanning these different cases. The charge-to-spin conversion and spin-splitter ratio reaches up to almost 100% via uncompensated magnetism and about 40% via $d$-wave AM under realistic conditions, highlighting the effectiveness of this approach for efficient spin-current generation.

We investigate spin- and orbital-to-charge conversion phenomena in nonmagnetic materials with broken inversion symmetry, treating the contributions from the Hall effect and the Rashba-Edelstein effect on an equal footing. We develop a general formalism for this interconversion based on macroscopic observables (susceptibility, conductivity, conversion efficiencies). The theory is validated through a case study of ferroelectric $\alpha$-GeTe, where we find that the obtained effective Rashba parameter is smaller than previously reported values for the same material. Incorporating these parameters into a drift-diffusion model, we show that the generated charge current is primarily governed by the Rashba-Edelstein effect, rather than by the spin or orbital Hall effects.

We study the critical behavior driven by potential quantum critical points (QCPs) termed as ${\tau }_{0,x,y,z}$-type QCPs beneath the superconducting dome of the $d$-wave cuprate superconductors. To comprehensively capture the distinct degrees of freedom in the vicinity of these QCPs, we construct a phenomenological effective theory based on the Landau-Ginzburg-Wilson framework and then employ the renormalization group approach to derive the coupled flow equations of all interaction parameters, incorporating all relevant one-loop corrections. Decoding these flow equations yields a series of unique properties arising from strong quantum fluctuations around QCPs. On one hand, the interaction parameters flow toward several fixed points (FPs) at certain critical energy scales. We identify two different types of FPs designated at the clean limit. FP-I is characterized by the divergence of the quadratic parameter and exhibits robustness against variations in interaction parameters. In contrast, FP-II is dominated by the cubic and quartic interaction parameters, and it is sensitive to initial conditions, leading to five subclasses: FP-IIA, FP-IIB, FP-IIC, FP-IID, and FP-IIE. In addition, we find that disorder scattering can influence fermion velocities and critical energy scales, and even destabilize certain FPs around the ${\tau }_{x,z}$ QCPs, driving the system toward a preempted disorder-induced FP. On the other hand, we find that quantum fluctuations play a critical role in shaping the critical temperature ($T_c$) as the system approaches these QCPs. Near the ${\tau }_x$ QCP, $T_c$ is considerably suppressed for both FP-I and FP-II. In contrast, near the ${\tau }_0$ QCP, $T_c$ undergoes a substantial decrease for FP-I but only a slight decrease for FP-II. Conversely, $T_c$ exhibits an increasing trend near the ${\tau }_y$ QCP, with a pronounced peak at $v_{\mathrm{\Delta }0}/v_{F0}\sim 0.25$. However, numerical analysis suggests that the ${\tau }_z$ QCP is unlikely to be physically realizable. Additionally, we realize that $T_c$ can also be modified by the emergence of disorder-induced FPs in the vicinity of the ${\tau }_x$ QCP. These findings would provide valuable insights into the critical low-energy properties of $d$-wave cuprate superconductors and related materials.

Interacting and open quantum systems can be formulated in terms of an effective non-Hermitian (NH) Hamiltonian; however, there are important constraints that must be satisfied by the effective action and the associated Green's functions. One common approach to many-body NH systems is to incorporate the anti-Hermitian part of the Hamiltonian directly in the Matsubara Green's function. Here, we show that such an approach is incompatible with the standard framework for systems with interactions. Furthermore, we furnish a consistent physical description for such systems by determining their distinction from conventional interacting physics, and find that they are described by pseudo-Hermitian quantum mechanics. In addition, we characterize the zero-temperature distribution functions within several frameworks for NH systems. As an application of our results, we consider the electromagnetic response of a NH quasiparticle Hamiltonian based on the ($1+1$)-dimensional NH Dirac model subject to various physical descriptions.

The ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ material family has emerged as a key platform for exploring magnetic topological phases, most notably exemplified by the experimental realization of the axion insulator state. The interplay between spin dynamics and axion states in topological magnets is anticipated to lead to rich phenomena that yet remain largely unexplored. In this work, we employ an antiferromagnetic spin-chain model to demonstrate that an external magnetic field induces perpendicular magnetic anisotropy. We find that an in-plane field stabilizes the antiferromagnetic order, whereas an out-of-plane field destabilizes it and triggers spin-flop transitions. Remarkably, near the surface spin-flop transition in even-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ films, the axion insulator state undergoes a sharp switching behavior accompanied by distinct magnetoelectric responses. Furthermore, we propose that this switchable axionic magnetoelectric effect can be utilized to convert alternating magnetic field signals into measurable square-wave magneto-optical outputs, thereby realizing an axionic analog of a zero-crossing detector.

The zero-mode corner states in the gap of a two-dimensional non-Hermitian Su-Schrieffer-Heeger model are robust to infinitesimal perturbations that preserve chiral symmetry. However, we demonstrate that this general belief is no longer valid in large-sized systems. To reveal the higher-order topology of non-Hermitian systems, we establish a correspondence between the stable zero-mode singular states and the topologically protected corner states of energy spectrum in the thermodynamic limit. Within this framework, the number of zero-mode singular values is directly linked to the number of midgap corner states. The winding numbers in real space can be defined to count the number of stable zero-mode singular states. Our results formulate a bulk-boundary correspondence for both static and Floquet non-Hermitian systems, where topology arises intrinsically from the non-Hermiticity, even without symmetries.