Using first-principles density functional theory (DFT) combined with atomistic spin simulations, we explore the possibility of realizing zero-field isolated skyrmions in three $4d/\mathrm{Co}$ atomic bilayers—Rh/Co, Pd/Co, and Ru/Co—grown on the Re(0001) surface. Our investigation employs an extended atomistic spin model, which goes beyond the standard model by including the multispin higher-order exchange interactions (HOI) in addition to the Heisenberg pairwise exchange interaction, Dzyaloshinskii-Moriya interaction (DMI), and magnetocrystalline anisotropy energy (MAE). All magnetic interactions of the extended spin model are calculated using DFT. The phase diagram obtained from atomistic spin simulations based on this spin model for Rh/Co and Pd/Co on Re(0001) reveals that isolated skyrmions emerge spontaneously on the ferromagnetic background even in the absence of an external magnetic field. The radius of zero-field isolated skyrmions in Rh/Co/Re(0001) is around 6 nm, whereas the radius of those skyrmions in Pd/Co/Re(0001) is about 12 nm. Transition-state theory calculations show that the skyrmions are protected by substantial energy barriers, approximately 150 meV, which predominantly arise from DMI, with a small contribution from the HOI. The height of the barriers suggests that skyrmions could be observed in low-temperature experiments. Based on this work, we propose $4d/\mathrm{Co}$ bilayers on Re(0001) as a new platform to realize nanoscale zero-field isolated skyrmions.

Using the THz emission spectroscopy, we investigate ultrafast spin-to-charge current conversion in epitaxial thin films of the altermagnetic candidate ${\mathrm{RuO}}_2$. We perform a quantitative analysis of competing effects that can contribute to the measured anisotropic THz emission. These include the anisotropic inverse spin splitter and spin Hall effects in ${\mathrm{RuO}}_2$, the anisotropic conductivity of ${\mathrm{RuO}}_2$, and the birefringence of the ${\mathrm{TiO}}_2$ substrate. We observe that the leading contribution to the measured signals comes from the anisotropic inverse spin Hall effect, with an average spin-Hall angle of $2.4\times {10}^{-3}$ at room temperature. In comparison, a possible contribution from the altermagnetic inverse spin-splitter effect is found to be approximately $2–4\times {10}^{-4}$. Our work stresses the importance of carefully disentangling spin-dependent phenomena that can be generated by the unconventional altermagnetic order, from the effects of the relativistic spin-orbit coupling.

The excitations of fractional quantum Hall effect (FQHE) states have been largely inaccessible to experimental probes until recently. New electron scanning tunneling microscopy (STM) results from Hu et al. [Nat. Phys. 21, 716 (2025)] show promise in detecting and identifying these excited states via the local density of states (LDOS) spectrum. On a torus, there exists a mapping from the lowest Landau level states to a 1D lattice with a Hamiltonian that features dipole moment conservation. In this work, we apply perturbation theory starting from the thin-cylinder limit ($L_x\to \infty ,L_y<l_B$ for torus dimensions $L_x$ and $L_y$ and magnetic length $l_B$) to obtain an analytical approach to the low-lying neutral and charged excitations of the $\nu =1/3$ FQHE state. Notably, in the thin cylinder, we can systematically enumerate all the low-lying excitations by the patterns of “dipoles” formed by the electron occupation pattern on the 1D lattice. We find that the thin-cylinder limit predicts a significant dispersion of the low-lying neutral excitations but sharpness of the LDOS spectra, which measure charged excitations. We also discuss connections between our work and several different approaches to the FQHE STM spectra, including those using the composite fermion theory. Numerical exact diagonalization beyond the thin-cylinder limit suggests that the energies of charged excitations remain largely confined to a narrow range of energies, which in experiments might appear as a single peak.

The adsorption geometry of the planar 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) molecule in the commensurate $\mathrm{c}(8\times 8)$ structure on Ag(100) was determined from the analysis of the intensities in low-energy electron diffraction (LEED-IV). Using data from different angles of incidence and optimized computer code, we were able to overcome earlier challenges given by the limitations of the experimental data set and the calculation times required for the large unit cell with many atoms. Testing of different structures confirmed the on-top adsorption site for the center of the perylene core. The final Pendry $R$ factor of $R_{\mathrm{P}}=0.180$ for the on-top position is significantly lower than the one for the fourfold hollow position (minimum $R_{\mathrm{P}}=0.369$) that is hence excluded. The molecule shows archlike deformation with a downshift of the terminal carboxylic groups. Both the molecular structure and the adsorption height are in very good agreement with results from an earlier normal incidence x-ray standing wave (NIXSW) experiment and new density functional theory (DFT) calculations, which we performed in parallel for 0 K and in addition for 300 K. The LEED-IV analysis demonstrates that the PTCDA induces a relaxation ($-0.08$ versus $-0.04\AA$ of the clean surface) and buckling (0.33 Å) of the topmost Ag layer. Special attention was given to the Ag atom below the central ring of the PTCDA. The IV analysis was rather insensitive to its vertical position, and a small $R$ factor, close to the minimal, was also obtained when this Ag atom was moved upward ($R_{\mathrm{P}}=0.185$) or even an Ag vacancy site ($R_{\mathrm{P}}=0.171$) was assumed. However, these structures could be excluded on the basis of DFT calculations. The vacancy structure has a free adsorption energy that is 0.18 eV larger compared to the favored geometry where this central Ag atom is pushed downward, partly due to the energy cost for the vacancy formation. The discussion of adsorbate-induced formation of vacancy sites is important because it was reported for ${\mathrm{C}}_{60}$ on Ag(111). The up- and downward displacements of the first-layer Ag atoms support the understanding of the chemical bond of the PTCDA to the Ag substrate and reveal how the originally planar $\pi$ system is locally distorted. Our analysis proves that LEED-IV is a powerful technique for surface crystallography of large organic adsorbates.

We present a first-principles machine-learning computational framework to investigate anharmonic effects in polarization-orientation (PO) Raman spectra of molecular crystals, focusing on anthracene and naphthalene. By combining machine learning models for interatomic potentials and polarizability tensors, we enable efficient, large-scale simulations that capture temperature-dependent vibrational dynamics beyond the harmonic approximation. Our approach reproduces key qualitative features observed experimentally. We show, systematically, what are the fingerprints of anharmonic lattice dynamics, thermal expansion, and Raman tensor symmetries on PO-Raman intensities. However, we find that the simulated polarization dependence of Raman intensities shows only subtle deviations from quasiharmonic predictions, failing to capture the pronounced temperature-dependent changes that have been reported experimentally in anthracene. We propose that part of these inconsistencies stem from the impossibility to deconvolute certain vibrational peaks when only experimental data is available. This work therefore provides a foundation to improve the interpretation of PO-Raman experiments in complex molecular crystals with the aid of theoretical simulations.

Bernal-stacked bilayer graphene modulated by a superlattice potential is a highly tunable system predicted to realize isolated topological flat bands. In this work we calculate the band structure and quantum geometry of bilayer graphene subject to both triangular and square superlattice potentials across a wide range of gate voltages. We identify the parameter regime that optimizes the “single-particle indicators” for the stability of a fractional Chern insulator (FCI) when a topological flat band is partially filled. Our results guide the quest for an experimental realization of an FCI in this platform.

The chiral anomaly is one of the robust quantum effects in relativistic field theories with a chiral symmetry where charges in chiral sectors appear to be separately conserved. The chiral anomaly, which is often associated with a renormalization-invariant topological term, is a violation of this conservation law due to quantum effects. Such anomalies manifest in Weyl materials as an electromagnetic field-induced transfer of charge between Fermi pockets. However, the emergent nature of the conservation of chiral charge leads to manifestations of the chiral anomaly response that depend on the details of the system such as the strength of interactions. In this paper, we apply an approach in which the chiral symmetry in solid materials is viewed as a so-called emanant symmetry from the underlying space translation symmetry. The chiral anomaly in this case is replaced by a mixed anomaly between the charge $\text{U}\left(1\right)$ and the space translation, while the chiral charge can be defined as proportional to the total momentum. We show that the chiral anomaly associated with this chiral charge is unrenormalized by interactions, in contrast to other chiral charges in $(1+1)$ dimensions (D) whose renormalization is regularization dependent. In $(3+1)\text{D}$ Weyl systems, this chiral anomaly is equivalent to the charge transferred between Fermi surfaces, which can be measured through changes in Fermi-surface-enclosed volume. We propose a pump-probe technique to measure this.

One-body reduced density-matrix functional (1RDMF) theory has yielded promising results for small systems such as molecules, but has not addressed quantum phase transitions such as the Mott transition. Here we explicitly execute the constrained search within a variational ansatz to construct a 1RDMF for the multiorbital Hubbard model with up to seven orbitals in the thermodynamic limit. The variational ansatz is the $\mathcal{N}=3$ ansatz of the variational discrete action theory (VDAT), which can be exactly evaluated in $d=\infty$. The resulting 1RDMF exactly encapsulates the $\mathcal{N}=3$ VDAT results, which accurately captures Mott and Hund physics. We find that nonanalytic behavior emerges in our 1RDMF at fixed integer filling, which gives rise to the Mott transition. We explain this behavior by separating the constrained search into multiple stages, illustrating how a nonzero Hund exchange drives the continuous Mott transition to become first-order. Our approach creates a new path forward for constructing an accurate 1RDMF for strongly correlated electron materials.

Andreev reflection is probed with a scanning tunneling microscope using a gold-coated normal-metal tip and a single nickel-phthalocyanine molecule adsorbed on superconducting Pb(111). Spectroscopy of the differential conductance at tip-molecule separations ranging from tunneling to contact distances unveils the evolution of Andreev reflection at the central Ni ion and its neighboring pyrrole moieties. Both molecular sites behave similarly in the tunneling range, while at contact separations deviations occur. The efficiency of Andreev reflection rises in a less pronounced manner for Ni than for the pyrrole groups. In addition, across Ni the Andreev reflection rate stays well below the maximal achievable value, while across the pyrrole groups it is nearly reached. This difference is associated with the energy shift of the lowest unoccupied molecular orbital, which reflects the progressive hybridization of the molecule with the approaching tip. The orbital energy remains above the Fermi level for Ni and dives below for the pyrrole groups, which is accompanied by the absence (Ni) and presence (pyrrole) of the molecular Kondo effect. A possible interplay of Andreev reflection and Kondo screening is suggested.

The response of several ferroelectric systems to a THz electric pulse is investigated by atomistic schemes. Despite their dramatic differences in initial, intermediate, and final states, all these compounds exhibit a similar qualitative behavior when driven out of equilibrium by this pulse. Five different temporal regions occur in all of them: a first region for which these materials do not basically answer to the field; a second region where the electrical polarization quickly grows with the field; a third region during which the polarization continues to be enhanced while the field is now getting reduced, therefore resulting in a new form of negative capacitance; a fourth region where the polarization follows again the field, but by decreasing with time; and finally a region where the systems explore different hidden states under weak electric fields. The existence of these five regions results in a polarization-vs-electric field curve having a striking D-like shape in all of these ferroelectrics. The importance of ionic inertia into these effects is revealed.

Motivated by the recent observation of ambient-pressure superconductivity with $T_c\approx 40\mathrm{K}$ in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ on ${\mathrm{SrLaAlO}}_4$(001) [Ko et al., Nature (London) 638, 935 (2025); Zhou et al., Nature (London) 640, 641 (2025)], we explore the structural and electronic properties as well as the spin-spin correlation function of this bilayer nickelate system by using density functional theory including a Coulomb repulsion term. We find that the compressive strain exerted by this substrate leads to an unconventional occupation of the antibonding Ni $3d_{z^2}$ states around the $\mathrm{\Gamma }$ point, distinct from the superconducting bulk compound under pressure. While pure strain effects rather modestly enhance the dynamical spin susceptibility, investigation of a reconstructed interface composition as observed in transmission electron microscopy uncovers a strong amplification of the spin fluctuations due to Fermi surface nesting of the antibonding Ni $3d_{z^2}$ states near the interface. These results provide insights into the emergence of superconductivity in strained ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, suggest a possible key role of the interface, and highlight fundamental differences from the hydrostatic pressure scenario.

${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$ (FGaT) has emerged as a promising two-dimensional (2D) ferromagnet with intrinsic magnetism above room temperature. Here, we investigate the thickness-dependent nucleation and stability of topological spin textures in exfoliated FGaT flakes with thicknesses ranging from 45 to $290\mathrm{nm}$ using scanning transmission x-ray microscopy. The domain size exhibits a nonmonotonic dependence on thickness, deviating from classical Kittel's $t^{1/2}$ scaling due to enhanced domain-wall energy in thinner samples. Magnetic phase diagrams indicate that skyrmion nucleation becomes increasingly favorable with thickness, with a significantly expanded stability region in thicker flakes. In contrast, high-order spin textures are suppressed below $\sim 75\mathrm{nm}$, highlighting a critical limitation for the miniaturization of 2D skyrmion-based spintronic devices. Our results emphasize the need for better stabilization strategies, such as interfacial engineering in van der Waals heterostructures, to enable robust topological textures in thin 2D magnet flakes.

The shandite structure hosts transition metal ions arranged in kagome layers. These layers are stacked rhombohedrally and are interspersed with post-transition metal ions and chalcogens. The electronic states near the Fermi level are dominated by the transition metal $d$ orbitals and feature saddle points near several of the high-symmetry positions of the Brillouin zone, most notably the F and L points. Combining symmetry considerations with ab initio methods, we study the electronic and structural properties of these materials with an emphasis on the connection between electronic saddle points at specific momenta and structural instabilities at these momenta. While the parent compounds studied are all found to be structurally stable under ambient conditions, we show that, in specific compounds, moving the saddle point closer to the Fermi level using either hydrostatic pressure or doping can induce a structural instability. The importance of the electronic degrees of freedom in driving this instability is supported by the dependence of the frequency of the soft phonon mode on the electronic smearing temperature. Our first-principles calculations show that as the smearing temperature is increased, the compound becomes structurally stable again, indicating that the electron-phonon coupling is playing an important role. Our findings survey the structural properties of a large family of shandite materials and shed light on the role played by saddle points in the electronic structure in driving structural instabilities in rhombohedrally stacked kagome layered materials.

Inversion-asymmetric antiferromagnets with odd-parity spin-polarization pattern, referred to as odd-parity magnets, have been proposed as a new venue for spintronics. These odd-parity magnets require commensurate ordering to ensure an effective time-reversal symmetry, which guarantees a strictly antisymmetric spin polarization of the electronic states. Recently, nonsymmorphic centrosymmetric crystals have been identified as a broad class of materials which could exhibit unit-cell doubling magnetism with odd-parity spin polarization. Here we investigate the stability of these states against incommensuration. We first demonstrate that the symmetry conditions which permit a $p$-wave magnetic state with a $p$-wave spin-polarization pattern also permit the existence of a nonrelativistic Lifshitz invariant in the phenomenological Ginzburg-Landau free energy. This implies magnetism with an incommensurate ordering vector, independent of its microscopic origin. $f$- and $h$-wave magnetic states with $f$- and $h$-wave spin polarizations are also prone to incommensurability, especially when they have an itinerant origin. Here the symmetry which allows the odd-parity spin polarization also guarantees the existence of van Hove saddle points off the time-reversal invariant momenta, which promote incommensurate spin fluctuations in quasi-two-dimensional electronic systems. Finally, we study the effect of weak spin-orbit coupling in locally noncentrosymmetric materials and find that it favors antiferromagnetic phases with in-plane magnetic moments. However, the inclusion of the spin-orbit coupling also introduces a new mechanism for driving incommensuration. Our results imply that odd-parity magnetic states are likely to be preceded by an incommensurate phase, or emerge directly from the normal state via a first-order transition. These conclusions are consistent with the phase diagram of several candidate materials.

Spin waves (SWs) in magnetically ordered media inherently exhibit rich nonlinear dynamics, giving rise to integer harmonic generation under driven conditions. Understanding and controlling these nonlinear harmonics is essential for both fundamental spin-wave physics and practical applications based on nonlinear wave responses. Here we present a theoretical analysis of nonlinear harmonic generation in exchange-dominated propagating SWs based on the Landau-Lifshitz-Gilbert equation. We examine two configurations, one with the bias field aligned perfectly along the anisotropy easy axis and the other with the field tilted by a small angle. Under the steady-state plane-wave assumption, we derive quantitative relations for the first-, second-, and third-order harmonics in both transverse and longitudinal spin-wave components. Our analysis reveals that elliptical precession induces odd-order harmonics in the transverse components, whereas tilting the precession axis leads to the appearance of even-order harmonics. Numerical simulations of spin-wave propagation in a thin magnetic film are performed to confirm the accuracy and validity range of the derived integer-harmonic relations. These results deepen the understanding of nonlinear dynamics in exchange-dominated SWs and offer theoretical insights for spin-wave-based nonlinear signal processing and computing.

Transport measurements of hybrid nanowires often rely on the observation of a zero-bias conductance peak as a hallmark of Majorana bound states (MBSs). However, such signatures can also be produced by trivial zero-energy Andreev bound states (ABSs) or by quasi-Majorana bound states (QMBSs), complicating their unambiguous identification. Here we propose microwave absorption visibility, extracted from parity-dependent cavity-nanowire susceptibility measurements, as a complementary probe of MBSs nonlocality. We study a Rashba spin-orbit nanowire consisting of a proximitized superconducting segment and an uncovered quantum-dot region, capacitively coupled to a single-mode microwave cavity. We show that true MBSs yield finite visibility only when both MBSs are simultaneously coupled to the cavity, reflecting their intrinsic nonlocality. In contrast, ABSs and QMBSs exhibit visibility extrema even when the cavity couples only locally to part of the nanowire. We further demonstrate that this distinction persists in the presence of Gaussian disorder, which may otherwise generate trivial subgap states. Motivated by recent experiments, we also analyze “poor man's” Majoranas in double-quantum-dot setups, where analytical results confirm the same nonlocal visibility criterion. Finally, we discuss a cavity-driven scheme for initializing the electronic system in a given parity state. Our results establish cavity-based visibility as a robust and versatile probe of MBSs, providing a clear route to distinguish them from trivial zero-energy states in hybrid superconducting platforms.

We study a model of mesoscale superconducting puddles in a metal, represented as dynamical impurities interacting with a finite number of electronic channels via Andreev and normal scattering. We identify conditions under which the collection of puddles make a $T$-linear contribution to the resistivity and a $Tln(1/T)$ to the specific heat and thermopower. This behavior emerges in an intermediate temperature range that extends from an upper energy scale set by the renormalized charging energy of the puddles, and down to an exponentially small scale associated with a charge-Kondo crossover, provided that the number of electronic channels interacting with the puddle is large. The phenomenology of our model resembles the apparent extended strange metal regime observed in overdoped cuprates which exhibits $T$-linear resistivity at low $T$ over a finite range of doping. We also propose to engineer a strange metal from suitably designed superconducting grains in a metallic matrix.

The role of data in modern materials science becomes more valuable and accurate when effects such as electron-phonon coupling and anharmonicity are included, providing a more realistic representation of finite-temperature material behavior. Furthermore, positional polymorphism, characterized by correlated local atomic disorder usually not reported by standard diffraction techniques, is a critical yet underexplored factor in understanding the electronic structure and transport properties of energy-efficient materials, like halide perovskites. In this manuscript, we present a first-principles methodology for locally disordered (polymorphous) cubic inorganic and hybrid halide perovskites, rooted in the special displacement method, that offers a systematic and alternative approach to molecular dynamics for exploring finite-temperature properties. By enabling a unified and efficient treatment of anharmonic lattice dynamics, electron-phonon coupling, and positional polymorphism, our approach generates essential data to predict temperature-dependent phonon properties, free energies, band gaps, and effective masses. Designed with a high-throughput spirit, this framework has been applied across a range of inorganic and hybrid halide perovskites: ${\mathrm{CsPbI}}_3, {\mathrm{CsPbBr}}_3, {\mathrm{CsSnI}}_3, {\mathrm{CsPbCl}}_3, {\mathrm{MAPbI}}_3, {\mathrm{MAPbBr}}_3, {\mathrm{MASnI}}_3, {\mathrm{MAPbCl}}_3, {\mathrm{FAPbI}}_3, {\mathrm{FAPbBr}}_3, {\mathrm{FASnI}}_3$, and ${\mathrm{FAPbCl}}_3$. We provide a comprehensive comparison between theoretical and experimental results and we systematically uncover trends and insights into their electronic and thermal behavior. For all compounds, we demonstrate strong and consistent correlations between local structural disorder, band gap openings, and effective mass enhancements. We present anharmonic phonon quasiparticle dispersions and show that both electron-phonon coupling and thermal expansion contributions to the band gap are significantly affected by local disorder, improving agreement with experiment. Furthermore, we discuss deviations from these trends in Sn-based compounds, which we attribute to the enhanced stereochemical activity of the Sn lone pair. We also present phonon spectral functions of polymorphous structures that illustrate the breakdown of the phonon quasiparticle picture in these soft materials. For hybrid halide perovskites, we introduce the concept of reference structures to address the complex interplay of disordered orientations of MA and FA molecules with the inorganic sublattice distortions, affecting both anharmonicity, transverse optical and longitudinal optical phonon frequencies, and electron-phonon coupling. Our work underscores the potential of integrated, high-throughput computational frameworks to transform the discovery and optimization of halide perovskite photovoltaic devices at finite temperatures.

Twisted bilayer graphene (TBG) hosts a rich landscape of electronic phases arising from the interplay between strong electron-electron interactions and nontrivial band topology. While the flat bands near zero energy are central to many correlated phenomena, their interaction with higher-energy remote bands remains less understood. Here, we investigate these hybridization processes as a function of the twist angle and analyze their impact on the real-space electron density, topological properties such as Chern number, quantum metric, and orbital magnetic energy. We identify multiple topological phase transitions between magic angles, driven by band inversions at high-symmetry points in momentum space. Notably, the central bands can exhibit phases with Chern numbers $C=\pm 2$, revealing previously unreported topological states in TBG.

Periodically driven quantum systems can function as highly selective parameter filters. We demonstrate this capability in a finite-size, three-qubit system described by the transverse-field Floquet Ising model. In this system, we identify a period-doubling (PD) dynamical phase that exhibits a stark asymmetry in metrological sensitivity to the magnetic field applied on the qubits and to the coupling strength between the qubits. The PD phase originates from $\pi$ pairing, where the initial state exhibits strong overlap with $\pi$-paired Floquet eigenstates, leading to robust period-doubled dynamics and enhanced metrological sensitivity. The analysis of quantum Fisher information reveals that the PD regime significantly enhances precision for estimating the Ising interaction strength while simultaneously suppressing sensitivity to the transverse magnetic field. Conversely, non-PD regimes are optimal for sensing the transverse field. This filtering effect is robust for larger system sizes and is quantifiable using experimentally accessible observables, such as magnetization and two-qubit correlations, via the classical Fisher information. Our work shows that distinct dynamical regimes in finite-size Floquet systems can be harnessed for targeted quantum sensing.