We propose an approach for realizing transitions between non- and near-$\mathcal{PT}$-symmetric systems through controlled modulation of a nonresonant optical pump in exciton-polariton condensates. Specifically, we first construct a $(2+1)$-dimensional $\mathcal{PT}$-symmetric Gaussian local potential and discover several non-$\mathcal{PT}$-symmetric nonlinear steady states with unique profiles that are excited by the $\mathcal{PT}$-symmetric potential and a homogeneous pump. Since the gain-loss coefficients corresponding to these steady states follow specific spatial distribution functions, the polariton model operates in a non-$\mathcal{PT}$ symmetry regime, leading to inherently non-$\mathcal{PT}$-symmetric steady states. However, we find that for the non-$\mathcal{PT}$-symmetric Gaussian steady states, the gain-loss coefficients in their corresponding models exhibit a Gaussian-like distribution, which can be balanced by an incoherent composite Gaussian pump. Therefore, the symmetry of the system can be effectively modulated through appropriate adjustments of the pump parameters. By selecting four types of non-$\mathcal{PT}$-symmetric Gaussian steady states as initial conditions, we numerically demonstrate that the gain-loss coefficients of the system are effectively balanced in space after introducing the composite Gaussian pump. Once the numerical distribution of the gain-loss coefficients approaches zero across the entire space, the non-$\mathcal{PT}$-symmetric Gaussian steady states transform into near-$\mathcal{PT}$-symmetric Gaussian steady states, facilitating the successful activation of the near-$\mathcal{PT}$-symmetric polariton model. Additionally, we define an equilibrium rate to effectively characterize the physical process that leads to the balance of gain-loss coefficient. Finally, we analyze the Poynting vectors within these near-$\mathcal{PT}$-symmetric Gaussian steady states. Similar to previous reports, transverse power flow within the state similar to a bright soliton is also observed. The proposed pumping scheme proves a promising pathway for generation of $\mathcal{PT}$-symmetric states of polariton condensates.

Two-dimensional half-metals with intrinsic ferromagnetism hold great potential for applications in spintronics. In this study, we aim to expand the known space of such two-dimensional ferromagnetic (FM) half-metals by investigating a bilayer of Janus VSSe, an FM semiconductor. Its structural, electronic, and magnetic properties are examined using first-principles density functional theory (DFT) based approach, employing the $\mathrm{DFT}+U$ methodology, coupled with the Perdew-Burke-Ernzerhof (PBE) functional. The stability of the bilayer is examined using ab initio molecular dynamics simulations at finite temperatures up to 400 K. To ensure the stability further, the elastic constants of the system were also investigated, and we found that the VSSe bilayer remains stable against structural deformation. The magnetic anisotropy calculations suggest that the VSSe bilayer manifests an easy plane of magnetization similar to its monolayer counterpart. At the $\mathrm{DFT}+U$ level of theory, the considered VSSe bilayer exhibits a tendency towards half-metallicity with a small band gap of 0.11 eV for the majority spin carriers and of 0.60 eV for the minority ones. To induce a transition from a semiconductor to a half-metal, the bilayer is subjected to an external electric field of varying strength normal to the plane. The lack of horizontal mirror symmetry in the bilayer allows bidirectional tuning of the band gap, with different values for the field in “upward” and “downward” directions. The band gaps for the two spin channels increase with increasing upward applied electric field, while the opposite happens for the downward fields, with the majority carrier gap closing at $\approx 0.16$ V/Å, making the material a spin gapless semiconductor. Further increase in the electric field renders the material half-metallic at $\approx 0.18$ V/Å. Given the fact that these values of the external electric field are achievable in the laboratory suggests that the FM Janus VSSe bilayer is a promising candidate for spintronic devices.

We propose the superfluidity of dipolar excitons in a strip of double-layer transition metal dichalcogenide (TMDC) heterostructures. We show that strain causes a shift in $k$ space between the minimum of the conduction band and the maximum of the valence band. Therefore, we expect that applying strain to this system can cause dark excitons to be created. We numerically calculate the energy spectrum of dark dipolar excitons in strained ${\mathrm{MoS}}_2$, and we calculate their binding energies and effective masses. We show that the dark dipolar excitons in strained TMDC heterostructures form superfluids, and we calculate the sound velocity in the energy spectrum of collective excitations, as well as the mean-field critical temperature for superfluidity. We show that two separate superfluid flows moving in opposite directions will appear in the system, one on each edge of the strip, forming the double layer. We see that the critical temperature for superfluidity increases with the concentration of dark excitons, as well as with the interlayer separation. The fact that dark excitons cannot decay via the simple emission of photons makes it so that the superfluids and condensates formed by them have a much longer lifetime than that formed by bright excitons. We propose a way to experimentally verify the predicted phenomena.

Understanding the current-induced vibrational dynamics in molecular nanojunctions is critical for gaining insight into the stability of such systems. While it is well known that Joule heating at higher bias voltages plays an important role for the stability of the nanojunction, a different mechanism caused by current-induced nonconservative forces has been reported to cause vibrational instabilities already at much lower voltages. In this paper, we apply a mixed quantum-classical approach based on electronic friction and Langevin dynamics to a model system for which vibrational instabilities have previously been reported. The electronic friction and other electronic forces can be calculated exactly using nonequilibrium Green's functions. We perform Langevin simulations of the vibrational dynamics to calculate the steady-state vibrational excitation and compare the results to a simplified eigenmode analysis, identifying the parameter regimes in which runaway modes appear.

Correlated electron systems with competing interactions provide a valuable platform for examining exotic magnetic phases. Theoretical models often focus on nearest-neighbor interactions, although long-range interactions can have a considerable impact on the behavior of the system, creating distinct and complicated magnetic phases. We investigate the consequences of competing interactions in a half-filled one-band Hubbard model on a simple cubic lattice, incorporating hopping processes up to third-nearest-neighbors, to explore the underlying magnetotransport properties. Our magnetic phase diagrams at low temperatures, obtained using semiclassical Monte Carlo analysis, reveal that the long-range interactions can disrupt one form of magnetic phase while creating a new type of magnetic order. For the nonperturbative regime (on-site Hubbard repulsive strength $U\sim$ bandwidth), the C-type antiferromagnetic ground state is preferred over the G-type antiferromagnetic phase when the interaction between second-nearest-neighbor sites becomes significant relative to the nearest-neighbor interactions. However, interactions beyond the second-nearest-neighbors are required to stabilize the A-type antiferromagnetic ground state. Remarkably, at low temperatures, a highly correlated paramagnetic insulating phase develops at the intersection between the antiferromagnetic phases, which might promote a three-dimensional spin-liquid state.

We predict that broken-gap InAs/GaSb core/shell nanowires, when operating in the topological insulating regime, undergo a collapse of the hybridization gap under the application of a transverse electric field. We perform predictive, self-consistent $\mathbf{k}·\mathbf{p}$ calculations for realistic nanostructures and show that a gap closure occurs at two Kramers-related, massless Dirac points at a critical value of the field in the $V/\mu$ range. An analysis based on the Bernevig-Hughes-Zhang model shows that the newly predicted semimetal phase stems from the cancellation between the kinetic electron-hole hybridization and the spin-orbit interaction, which is controlled by the external field. Remarkably, the so-called end states—midgap states localized at the terminations of a finite-length nanowire in the inverted regime, analogously to spin Hall edge states—are supported only below the critical field, and suddenly disappear as the system is driven through the semimetal phase, eventually evolving into trivial surface states. This abrupt disappearance exposes a nontrivial transition in one dimension driven by spin-orbit coupling.

Noninvertible symmetry generalizes traditional group symmetries, advancing our understanding of quantum matter, especially one-dimensional gapped quantum systems. In critical lattice models, it is usually realized as emergent symmetries in the corresponding low-energy conformal field theories. In this Letter, we study critical lattice models with the noninvertible $\mathrm{Rep}\left(D_8\right)$ symmetry in one dimension. This leads us to another class of quantum critical points (QCPs), noninvertible symmetry-enriched QCPs, as a generalization of known group symmetry-enriched QCPs. They are realized as phase transitions between one noninvertible symmetry-protected topological phase and another different one or spontaneous symmetry-breaking phase. We identify their low-energy properties and topological features through the Kennedy-Tasaki duality transformation. We argue that distinct noninvertible symmetry-enriched QCPs cannot be smoothly connected without a phase transition or a multicritical point.

The discovery of ferroelectricity in wurtzite nitrides has paved the way for measuring and understanding spontaneous polarization in III-V semiconductors. However, the calculation of polarization effects at heterointerfaces, crucial for numerous electronic and photonic applications, remains a topic of debate. The need for a reference structure to calculate spontaneous polarization has led to discussions over whether to use the zinc-blende or layered hexagonal structures as the reference for wurtzite crystals. In this Letter, we argue that the layered hexagonal structure is not only a better reference due to its vanishing formal polarization, but also the appropriate choice for the wurtzite system to reproduce the polarization values observed in ferroelectric switching experiments. This leads to the idea of using ferroelectricity as a basis for defining spontaneous polarization. Applying this definition, we extend our analysis to III-V zinc-blende semiconductors and reveal that they can exhibit a spontaneous polarization approximately three times larger than that of wurtzite, thereby refuting the longstanding assumption that it is zero. Through this example, we illustrate that spontaneous polarization is not inherently linked to charge density at interfaces.

We show that a slab of wire medium composed of thin parallel metallic wires can naturally support bound states in the continuum (BICs) formed in an unusual way. The revealed BICs appear due to the strong spatial dispersion making possible the propagation of longitudinal plasmalike waves and TEM polarized modes with a flat band. The symmetry-protected (at-$\mathrm{\Gamma }$) BICs are formed due to the polarization mismatch between the longitudinal plasmalike waves and transversal plane waves in the surrounding space, while the accidental (off-$\mathrm{\Gamma }$) BICs appear as a result of the destructive interference between bulk TEM and plasma modes. All revealed BICs can be well described analytically without the use of the Bloch theorem within the effective-medium approximation when the wire medium behaves as a homogeneous one-dimensional anisotropic plasma with strong spatial dispersion.

The Affleck-Kennedy-Lieb-Tasaki (AKLT) Hamiltonian is a particular instance of a general class of model Hamiltonians defined in lattices with coordination $z$ where each site hosts a spins $S=z/2$, interacting both with linear and nonlinear exchange couplings. In two dimensions, the AKLT model features a gap in the spectrum, and its ground state is a valence bond solid state; that is a universal resource for measurement based quantum computing, motivating the quest of physical systems that realize this Hamiltonian. Given a finite-size system described with a specific instance of this general class of models, we address the question of how to assess if such system is a realization of the AKLT model using inelastic tunnel spectroscopy implemented with scanning tunnel microscopy (IETS-STM). We propose two approaches. First, in the case of a dimer, we show how to leverage nonequilibrium IETS-STM to obtain the energies of all excited states, and determine thereby the magnitude of both linear and nonlinear exchange interactions. Second, we explore how IETS can probe the in-gap excitations associated with edge spins in a hexamer. In the AKLT limit, spins $S=3/2$ at the edge of the lattice have coordination 2, giving rise to $S=1/2$ dangling spins that can be probed with IETS. We propose a $S=1/2$ effective Hamiltonian to describe the interactions between these dangling spins in the neighborhood of the AKLT point, where their degeneracy lifted.

In this study, the effects of Fe:Co ratio on the structure and magnetic properties of high-entropy alloys (HEAs) with different Mn contents (5% and 15%) are investigated by combining experimental and theoretical approaches. The alloys mainly show bcc and $B2$ coexisting structures, and the change of Mn content has less effect on the structure. The coercivity of all alloys is less than 10 Oe, showing soft magnetic properties, and the highest saturation magnetization of $195.5\mathrm{emu}{\mathrm{g}}^{-1}$ is obtained at the Fe:Co ratio of 2:1 for alloy with 5% Mn, and the highest saturation magnetization of $182.8\mathrm{emu}{\mathrm{g}}^{-1}$ is obtained at the Fe:Co ratio of 1:1 for alloy with 15% Mn. Density-functional theory (DFT) calculations revealed the magnetic moment distribution of each element, with the results basically consistent with the experiments. The research results indicated that the Mn atomic magnetic moment is more sensitive to the variation in the Fe:Co ratio. Increasing the Mn content decreases the atomic magnetic moments of Fe and Co, but the effect on Fe is greater, indicating a stronger antiferromagnetic coupling between Fe and Mn. At lower Mn content, the average magnetic moment of Mn is higher, revealing the enhancement or suppression of magnetism due to the ferromagnetic/antiferromagnetic interactions between Mn and Mn or others. This study provides an important theoretical basis for the design of Mn-containing HEAs with optimized magnetic properties.

A hallmark of certain topology, including the Chern number, is the obstruction to constructing exponentially localized Wannier functions in the bulk bands. Conversely, other types of topology do not necessarily impose the Wannier obstructions. Remarkably, such Wannier-localizable topological insulators can host boundary states that are detachable from the bulk bands. In our accompanying Letter [D. Nakamura et al., Phys. Rev. Lett. 135, 096601 (2025)], we demonstrate that non-Hermitian topology underlies detachable boundary states in Hermitian topological insulators and superconductors, thereby establishing their tenfold classification based on internal symmetry. Here, using $K$-theory, we elucidate the relationship between Wannier localizability and detachability of topological boundary states. From the boundary perspective, we classify intrinsic and extrinsic non-Hermitian topology, corresponding to nondetachable and detachable topological boundary states, respectively. From the bulk perspective, on the other hand, we classify Wannier localizability through the homomorphisms of topological phases from the tenfold Altland-Zirnbauer symmetry classes to the threefold Wigner-Dyson symmetry classes. These two approaches from the boundary and bulk perspectives lead to the same classification. We clarify this agreement and develop a unified understanding of the bulk-boundary correspondence on the basis of $K$-theory.

The introduction of intrinsic magnetic order in two-dimensional (2D) semiconductors offers great opportunities for investigating correlated excitonic phenomena. Here, we employ full-spinor GW plus Bethe-Salpeter equation methodology to reveal rich exciton physics in a prototypical 2D Néel-type antiferromagnetic semiconductor ${\mathrm{MnPS}}_3$, enabled by the interplay among inverted dispersion of the second valence band, spin-valley coupling and magnetic order. The negative hole mass increases the reduced mass of the lowest-energy bright exciton, leading to exchange splitting enhancement of the bright exciton relative to band-edge dark exciton. Notably, such splitting couples with spontaneous valley polarization to generate distinct excitonic fine structure between $K$ and $-K$ valleys, which dictate distinct relaxation behaviors. Crucially, magnetic order transition from Néel antiferromagnetic to ferromagnetic state induces significant quasiparticle band structure reconstruction and excitonic transitions modification, with low-energy optical excitations being exclusively contributed by majority-spin channel. These findings establish 2D antiferromagnetic semiconductors as an intriguing platform to study band-spin-valley coupled exciton physics.

In recent years, the investigation of nonlinear electromagnetic responses has received significant attention due to its potential for elucidating the quantum properties of matter. Although remarkable progress has been achieved in developing quantum theories of nonlinear responses to electric field, a comprehensive quantum theory framework that systematically addresses nonlinear responses to both electric and magnetic fields has yet to be thoroughly discussed. Here, we present a systematic quantum theory of nonlinear electromagnetic response using the Matsubara Green's function approach, which explicitly incorporates the wave vector dependence of external electromagnetic fields. We reveal the general properties of transport coefficients. We apply our theory to second-order responses, deriving the nonlinear Hall effects and magneto-nonlinear Hall effects in both time-reversal symmetric and time-reversal breaking systems. These effects stem from diverse quantum geometric quantities. Additionally, we analyze the contributions arising from the Zeeman interaction. Our work presents a unified quantum theory of nonlinear electromagnetic response, paving the way for further exploration of novel phenomena in this field.

Intermittent emission is a special characteristic of single molecule fluorescence, which is also called fluorescence blinking [Nature (London) 388, 355 (1997), Science 277, 1074 (1997)] and has also been observed in 0D [Nature (London) 383, 802 (1996)], 1D [Nat. Phys. 4, 519 (2008)] and 2D materials [Nature (London) 541, 62 (2017)]. The surface enhanced Raman spectrum (SERS) of a single molecule also shows blinking, which is commonly attributed to the movement of the molecule in and out of the active site of the metal nanoparticle [Science 275, 1102 (1997)]. For bulk samples, it is reasonable that no fluorescence or Raman blinking can be observed due to ensemble averaging from numerous molecules. In this work, however, we observed SERS blinking of bulk samples around a single gold nanorod (GNR) in solid matrices. Such SERS blinking behavior can be observed for different samples once the GNR is excited resonantly. These results suggest an intriguing mechanism of SERS blinking that does not require a single molecule. We propose two possible mechanisms that can explain the experimental results and suggest reconsidering the physical picture of the SERS blinking behaviors in both ensemble and single-molecule measurements.

Titanium diselenide (${\mathrm{TiSe}}_2$) is a transition-metal dichalcogenide material that undergoes a charge-density-wave (CDW) transition at $T_c\approx 200\mathrm{K}$. In a recent experiment [I. Tyulnev et al., Commun. Mater. 6, 152 (2025)], the high harmonic generation (HHG) spectra of this material has been studied, which exhibits asymmetric behavior with respect to the polarization angle of the incident light and provides a different perspective to the CDW phase transition. In this work, we work out a theoretical explanation for the experimentally observed behavior of HHG spectra. We propose a simplified phenomenological mean-field model for this material, based on which the HHG spectra is calculated through the time-dependent Schrödinger equation. This model correctly describes the measured intensity distribution of the third-, fifth-, and seventh-order harmonic generation as a function of polarization direction and reveals a strong asymmetry due to the anisotropic CDW order in the low temperature phase. Our work provides a basis for applying high harmonic spectroscopy to reveal a different perspective on the nature of CDW orders.