Realizing materials with intrinsic flat bands holds significant potential for accessing correlated and topological phases. In this work, via large-scale tight-binding calculations, we demonstrate that pairing frustrated quantum antidots in a hexagonal lattice yields intrinsically tunable flat bands. Simple scaling laws describing the relationship between flat-band energies and geometric dimensions exhibit excellent structural tunability over band-flattening behavior. A diverse range of customizable flat-band structures can be engineered based on homogeneous or heterogeneous antidot pairs. Furthermore, we show that thermal and strain perturbations can enhance flat-band isolation, with their topological characteristics depending on antidot dimensions under an applied magnetic field. Our findings propose an alternative yet robust strategy for intrinsic flat-band engineering.

The recently discovered infinite-layer nickelate superconductor offers a new platform for exploring high-temperature superconductivity. In this work, we employ time-resolved ultrafast optical spectroscopy to investigate the excited carrier dynamics in Eu-doped (${\mathrm{Sm}}_{0.69}{\mathrm{Ca}}_{0.05}{\mathrm{Eu}}_{0.26}$)${\mathrm{NiO}}_2$ thin film, which exhibits superconductivity with $T_{c0}=16\text{K}$. The formation of superconducting gap was identified with estimating the gap size of about 2.5 meV, which can be eliminated with a small pump fluence of $0.32\mu \mathrm{J}/{\mathrm{cm}}^2$. The relaxation trajectories of quasiparticles exhibit a high similarity to those of the Ruddlesden-Popper nickelates, supporting the commonality in their electronic structures and superconductivity. More interestingly, from the temperature dependence of fast component, we estimate an electron-phonon coupling (EPC) strength of $\approx 0.42$, which would predict a transition temperature of only 3.5 K. This suggests that EPC alone is insufficient to explain superconductivity in the $R{\mathrm{NiO}}_2$ system, and there must be other interactions that help hold Cooper pairs together.

Yb $3d$ x-ray photoelectron spectroscopy (XPS) and $L$-edge x-ray absorption spectroscopy (XAS) are powerful techniques for determining the valence state of Yb compounds. In particular, $L$-edge XAS directly probes optical transitions from the Yb $2p$ core level into unoccupied Yb $5d$ states, thereby providing access to the broad conduction band derived from Yb $5d$ orbitals. In this study, we demonstrate that properly accounting for the band-shape effect is essential for a reliable analysis of $L$-edge XAS spectra. For ${\mathrm{YbAl}}_3$, we compared high-energy-resolution fluorescence-detected XAS (HERFD-XAS) with theoretical spectra based on an impurity Anderson model and identified characteristic spectral features that reflect the Yb $5d$ band structure obtained from first-principles calculations. To resolve ambiguities in the interpretation of the $L$-edge XAS arising from the band-shape effect, we incorporated available XPS data, and both the HERFD-XAS and XPS spectra were consistently reproduced using a single electronic-state model. In addition to extracting the band-shape information, our analysis further suggests the presence of a finite interorbital Coulomb repulsion ($U_{\mathrm{fd}}$) between the Yb $4f$ and $5d$ states.

The Hubbard model is known to accommodate various electronic orders, including stripes, which are important for understanding the physics of cuprates. We study spin-stripe order in the square lattice Hubbard model as a function of doping and temperature, by solving the Bethe-Salpeter equation with the local vertex from dynamical mean field theory (DMFT), both inside and outside the antiferromagnetic phase. We find broad regions of horizontal/vertical spin stripes at low temperatures for the model with and without next-nearest neighbor hopping. Their wavelength depends on hole doping in a nonlinear fashion, and is highly sensitive to the ratio of nearest and next-nearest neighbor hoppings.

The band gap of a semiconductor or dielectric, relative to the photon energy of incident light, is commonly taken to define whether the material is transparent or opaque. However, it is well known that extreme nonlinear optics can distort this definition across ultrashort timescales. Here, using single-shot two-color pump-probe microscopy at a free-electron laser facility, we show that an ultrashort midinfrared pulse can create nanosecond-long inhomogeneous optical patterns seeming to comprise below-band gap opacity and enhanced transparency. We explain this peculiar observation as stemming from ultrafast transient self-focusing, which in turn leads to long-lived free-carrier-induced opacity and defocusing. Our results, demonstrating that nonlinear optical effects in the infrared spectral range can give rise to surprisingly persistent material changes, highlight the importance of full-field pump-probe imaging techniques in rationalizing inhomogeneous phenomena that may otherwise be misinterpreted or overlooked entirely.

We theoretically study spin and valley transport in a transition-metal dichalcogenide (TMDC)/ferromagnet heterostructure under a perpendicular magnetic field. We find that microwave-driven spin pumping induces a valley-selective spin excitation, a direct consequence of the valley-asymmetric Landau levels in the TMDC conduction band. This process generates a pure valley current, which, as our central finding, exhibits pronounced quantum oscillations as a function of chemical potential. These oscillations provide a definitive experimental signature of the quantized valley states and establish another pathway to interface spintronics and valleytronics.

We investigate Ising superconductivity in two archetypal intrinsic superconductors, monolayer $1H-{\mathrm{TaS}}_2$ and $1H-{\mathrm{NbSe}}_2$, in a bottom-up approach. Using ab initio-based tight-binding parametrizations for the relevant low-energy $d$ bands, the screened interaction is evaluated microscopically in a scheme including Bloch overlaps. In direct space, the screened potential for both systems displays long-range Friedel oscillations alternating in sign. Upon scaling, the oscillation pattern becomes universal, with the location of minima and maxima locked to the lattice. Solving the momentum-resolved gap equations, a chiral ground state with $p$-like symmetry is generically found. Due to the larger Ising spin-orbit coupling, the chiral gap is more anisotropic in ${\mathrm{TaS}}_2$ than in ${\mathrm{NbSe}}_2$. This is reflected in tunneling spectra displaying V-shaped features for the former, in quantitative agreement with low-temperature scanning tunneling experiments on ${\mathrm{TaS}}_2$. At the same time, our results reconcile the apparent discordance with hard gap tunneling spectra reported for the sibling ${\mathrm{NbSe}}_2$.

We link the presence of “unnecessary” quantum critical surfaces within a single gapped phase of matter to the nontrivial topology of families of gapped Hamiltonians that encircle the critical surface. We study a specific set of one-dimensional spin models where each such family forms a one-parameter loop in a two-dimensional phase diagram. Foliating the noncritical region by such loops identifies “radial” and “angular” coordinates in the phase diagram that respectively parametrize different families and different members of a single family. We show that each one-parameter family is a generalized Thouless charge pump, all with the same topological index, and hence the gapped phase undergoes one or more nontrivial boundary phase transitions as we vary the angular coordinate in a loop through members of one family. Tuning the radial coordinate generates loci of boundary critical points that terminate at the end points of the bulk unnecessary critical line within the gapped phase. We discuss broader implications of our results and possible extensions to higher dimensions.

The breathing Kagome material ${\mathrm{Nb}}_3{\mathrm{Cl}}_8$ has recently been identified as a single-band Mott insulator and theoretically predicted to host coexisting ferroelectricity, magnetism, and ferrovalley—yet experimental verification of these predictions remains lacking. Here, through polarization- and temperature-dependent Raman and second harmonic generation measurements, we reveal that the low-temperature (LT) phase of this material retains $C_3$ rotational symmetry while breaking inversion symmetry. We propose that the structural phase transition at $T_s$ corresponds to a stacking-order transition: from centrosymmetric AB stacking to noncentrosymmetric AA/BB stacking, driven by enhanced interlayer electric dipole-dipole interactions. With an AA/BB stacking sequence, the LT phase exhibits net electric dipole moments oriented perpendicular to the kagome plane. Our findings provide directly experimental evidence for the existence of a noncentrosymmetric Mott ground state, thereby establishing a unique platform for investigating the interplay between inversion symmetry breaking and strong electron correlation.

Dimerized quantum magnets provide fertile ground for exploring novel quantum phases arising from the condensation of magnon excitations. Here, we investigate the quantum dimer magnet Cu(en)${}_2{\mathrm{SO}}_4$ using specific heat, magnetocaloric effect, and electron spin resonance (ESR) measurements, combined with infinite projected entangled pair state (iPEPS) simulations. Our results establish a dimer-singlet ground state in Cu(en)${}_2{\mathrm{SO}}_4$ with intradimer coupling $J\approx 10.9\mathrm{K}$. A magnetic field applied along the $c$ axis drives two quantum phase transitions at $B_{\mathrm{c}1}\approx 6.43\mathrm{T}$ and $B_{\mathrm{c}2}\approx 11.73\mathrm{T}$, enclosing a dome-shaped antiferromagnetic phase below $T_{\mathrm{c}}\simeq 0.65\mathrm{K}$. This ordered state is naturally interpreted as a result of triplon Bose-Einstein condensation (BEC), with the excitation gap closing and reopening across the critical fields. ESR spectra reveal not only sharp magnon resonances, but also broad continuumlike features, pointing to unconventional spin dynamics that may arise from triplet excitations. iPEPS simulations reproduce the experimental critical fields and confirm a stripe-type spin configuration with ordering wave vector at the $\mathrm{\Gamma }$ point, establishing Cu(en)${}_2{\mathrm{SO}}_4$ as a clean platform for studying magnon BEC and field-tuned quantum criticality.

We introduce a class of superconductors termed “quantized quadrupole superconductors” that support Majorana corner modes according to the bulk-corner correspondence, distinct from previous works on the second-order topological superconductors. An intrinsic physical quantity for superconductors, i.e., the quadrupole moment serves as the topological invariant, which is always half-quantized due to the particle-hole symmetry. As examples, two types of mixed pairings, $d_{x^2-y^2}\pm i{d}_{xy}$ and $d_{x^2-y^2}\pm is$, induced in the bilayer two-dimensional electron gases with Rashba spin-orbit coupling give the quadrupole phase. Extended discussions indicate that the nontrivial phase is robust against relative phase fluctuations in the mixed pairings and the disorders. Our schemes provide realistic platforms to implement Majorana zero modes, paving the way for studying the Majorana physics.

Nitridation of electron-rich transition metals under pressure can form many binary nitrides with diverse compositions and crystal structures, especially for the W-N system with numerous newly discovered phases. However, because of their unique metastability, most reported W-N products involve complex atomic deficiencies, largely exacerbating the difficulties in structural resolution. Of particular interest is hexagonal $hP10\text{−}{\mathrm{W}}_2{\mathrm{N}}_3$, which has been predicted to be a soft van der Waals (vdW) material but experimentally has shown excellent mechanical properties, likely due to structural misassignment. Here we revisit the high-pressure synthesis and perform a comprehensive study to definitively solve the crystal structure and determine the key properties of this material. We find that it adopts $\mathit{hP}12$ symmetry rather than $\mathit{hP}10$, containing unusual W-W-W trimers. Intricate atomic deficiencies are revealed to be crucially important for stabilizing its crystal structure with an actual composition of $hP12\text{−}{\mathrm{W}}_{4.216}{\mathrm{N}}_{5.278}$. This material is intrinsically metallic and mechanically hard and brittle, opposite to that of a vdW material. It also exhibits superior thermal stability, making it highly suitable for many advanced applications.

This study investigates the molecular beam epitaxial growth of titanium nitride (TiN) thin films, achieving a high residual resistivity ratio of 15.8. We observed a strong correlation between growth temperature and crystalline quality, as reflected in both RRR values and lattice parameter variations. Characterization of superconductivity yielded a Ginzburg-Landau coherence length of 60.4 $\pm$ 0.6 nm, significantly higher than typical sputtered films, suggesting improved superconducting coherence. First-principles calculations, in conjunction with experimental data, provided detailed insights into the electronic structure and transport properties of the TiN films. Temperature-dependent Hall coefficient measurements further revealed the influence of anisotropic scattering mechanisms. These findings establish a promising route for the development of nitride-based superconducting materials for advanced quantum computing technologies.

Motivated by the recent experimental measurements of the nonlinear longitudinal resistance of the spin-orbit coupled electron gas in the (111) ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ interfaces under external in-plane magnetic field [G. Tuvia, et al., Phys. Rev. Lett. 132, 146301 (2024)], we formulate a theory of nonlinear electronic transport based on the analysis of the quantum kinetic equation for the Wigner distribution function. Specifically, we evaluate the magnetic field dependence of the second harmonic of the current density at arbitrary values of the magnetic field. The magnitude of the second harmonic increases linearly with the magnetic field at small fields. Upon further increase of the magnetic field, the second harmonic response reaches its maximum value. We find that the position of the peak and its width strongly depend on the relaxation rate due to disorder. Importantly, we discover that the direction of the nonlinear contribution to the current can be completely reversed when the magnetic field reaches a certain critical value.

We study the exchange coupling in small Wigner crystals confined to one-dimensional space. In particular, we concentrate on the simplest nontrivial case of two electrons in a box potential and calculate analytically the energy splitting between the lowest spatially symmetric and antisymmetric states, which is a relevant energy scale for the magnetic properties of the system. In the approximation of a fixed center-of-mass coordinate, the splitting decays exponentially with the square root of the distance between the electrons at the leading order. We show that the subleading exponential correction significantly increases the splitting and thus becomes crucial in order to correctly describe the exact numerical data for system sizes that are not astronomically large. Two methods of calculation of the energy splitting are developed. The first is based on the analysis of the exact solution that can be expressed in terms of the Whittaker functions. It applies to all values of the short-distance cutoff played by the width of a one-dimensional wire that regularizes the Coulomb potential. The second method is based on the quasiclassical (or Wentzel–Kramers–Brillouin) approximation, which applies only for sufficiently large values of the cutoff. The two methods give identical results in the overlapping region. As a side result, our study gives the energy splitting in a triangular double-well potential of inverse $\mathsf{M}$ shape.

Understanding the electronic structure of transition-metal dopants in IV–VI semiconductors is critical for tuning their band structure. We analyze properties of the Cr dopant in ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ and PbSe by magnetic and transport measurements, which are interpreted based on density functional theory calculations. We demonstrate that the pinning of the Fermi energy to the chromium resonant level occurs for both $n$-type and $p$-type ${\mathrm{Pb}}_{1-\mathrm{x}}{\mathrm{Sn}}_{\mathrm{x}}\mathrm{Te}$ in the whole composition range. This enables us to determine the valence band and conduction band offsets at the PbTe/SnTe/PbSe heterointerfaces, which is important for designing high-performance 2D transistors. Furthermore, the magnetic measurements reveal the presence of Cr ions in three charge states, ${\mathrm{Cr}}^{3+}, {\mathrm{Cr}}^{2+}$, and ${\mathrm{Cr}}^{1+}$. The last one corresponds to the Cr dopants incorporated at the interstitial, and not the substitutional, sites. The measured concentrations of the interstitial and substitutional Cr are comparable.

Ferroelectric tunneling junctions rely on the tunability of tunneling current by the ferroelectric polarization in the junctions. Conventionally the tunneling-barrier height and/or width of the ferroelectric junction are modulated by the ferroelectric polarization to achieve this goal. In this work, we show that polarization modulated electronic density of states of a junction electrode made of graphene can provide another route to tune the tunneling electroresistance. This mechanism is especially valuable for the emerging sliding ferroelectric materials, which have weak polarization and cannot effectively modulate the tunneling-barrier height or width. In addition, this method has the flexibility of reconfigurable electroresistance by a gate voltage, providing new functionalities in ferroelectric tunneling junctions.

We consider an extended spinless Falicov-Kimball model at an arbitrary doping level, focusing on the range of parameter values where a uniform excitonic insulator is stabilized at half-filling. In the two-dimensional case, we compare the properties of possible uniform phases and construct the Hartree–Fock phase diagrams, which include sizable phase separation regions. It is seen that the excitonic insulator can appear as a component phase in a mixed-phase state in a broad interval of doping levels. In addition, in a certain range of parameter values the excitonic metal (doped excitonic insulator) is identified as the lowest-energy uniform phase. We suggest that this phase, which is unstable with respect to phase separation, may be stabilized when the phase separation is suppressed by the long-range Coulomb interaction. Overall, we find that excitonic correlations can affect the behavior of the system relatively far away from half-filling.

We formulate gradient flow dynamics generated by two natural actions of the quantum metric for an isolated set of Bloch bands. Specializing to two spatial dimensions, we derive Bogomolny-type lower bounds that relate these actions to the Chern number and show that the bounds are saturated by (anti)holomorphic projector configurations. Along the flows, the actions decrease monotonically while the Chern number is conserved, giving a constructive route to simplify models within a fixed topological phase toward canonical, low-complexity representatives.

Cold atoms can adsorb to a surface with the emission of a single phonon when the binding energy is sufficiently small. The effects of phonon damping and adsorbent size on the adsorption rate in this quantum regime are studied using the multimode Rabi model. It is demonstrated that the adsorption rate can be either enhanced or suppressed relative to the Fermi golden rule rate, in analogy to cavity effects in the spontaneous emission rate in QED. A mesoscopic-sized adsorbent behaves as an acoustic cavity that enhances the adsorption rate when tuned to the adsorption transition frequency and suppresses the rate when detuned. This acoustic cavity effect occurs in the regime where the frequency spacing between vibrational modes exceeds the phonon linewidth.