The antiferromagnetic kagome lattice compound FeGe has been revealed to host an emergent charge-density-wave (CDW) state which manifests complex interplay between the spin, charge, and lattice degrees of freedom. Here, we present a comprehensive study of the de Haas–van Alphen effect by measuring torque magnetometry under magnetic fields up to 45.2 T to map Fermi surfaces in this unusual CDW state. For a field along the $c$ direction, we resolve four cyclotron orbits, with the largest one roughly corresponding to the area of the $2\times 2$ folded Brillouin zone. Three smaller orbits are characterized by light effective cyclotron masses in the range of $(0.18–0.30)m_e$. Angle-resolved measurements identify one Fermi surface segment with weak anisotropy. Combined with band structure calculations, our results suggest that features of unfolded Fermi surfaces are robust against CDW reconstruction, corroborating the unconventional effect of a short-ranged CDW on the electronic structure.

Ultrafast demagnetization can be induced either coherently by direct interaction with the optical light field or indirectly via excitation of the electron system, which subsequently couples to the magnetization. An intriguing direct process is optical intersite spin transfer (OISTR), which changes the local magnetic moments during the optical absorption process itself and leads to a potentially coherent net spin transport between two magnetic subsystems. In this paper, we find that both direct and indirect excitation of an FeNi alloy result in identical experimental signatures in transverse magneto-optical measurements in the extreme ultraviolet spectral range, which has been previously interpreted as evidence for the OISTR effect: a delayed onset of the ultrafast response of Ni with respect to Fe as well as an increasing signal for photons probing energies below the Ni resonance. Our findings align with recent theoretical and experimental investigations, which propose alternative explanations of these experimental observations and do not rely on a direct and coherent interaction between light and spin. Instead, the distinctly different magnetization dynamics of Fe and Ni may be governed by intersite spin transfer driven by electron scattering or be the result of element-specific microscopic properties such as inhomogeneous spin-orbit coupling or electron magnon scattering.

Cryogenic scanning tunneling microscopy was employed in combination with density-functional theory calculations to explore quantum dots made of In adatoms on the InAs(110) surface. Each adatom adsorbs at a surface site coordinated by one cation and two anions, and transfers one electron to the substrate, creating an attractive quantum well for electrons at the surface. We used the scanning-probe tip to assemble the positively charged adatoms into precisely defined quantum dots exhibiting a bound state roughly 0.1 eV below the Fermi level at an intrinsic linewidth of only $\sim 4$ meV, as revealed by scanning tunneling spectroscopy. For quantum-dot dimers, we observed the emergence of a bonding and an antibonding state with even and odd wave-function character, respectively, demonstrating the capability to engineer quasimolecular electronic states. InAs(110) constitutes a promising platform in this respect because highly perfect surfaces can be readily prepared by cleavage and charged adatoms can be generated in situ by the scanning-probe tip.

The electronic-structure evolutions of few-layer black phosphorus (BP) under pressure shows a wealth of phenomena, such as the nonmonotonic change of direct gap at the $\mathrm{\Gamma }$ point, the layer-number dependence, and the distinct responses to normal and hydrostatic pressures. A full and unified understanding to these rich phenomena remains lacking. Here, we provide a unified understanding from the competition between interlayer quasibonding (QB) interactions and intralayer chemical bonding interactions. The former decreases while the latter increases the band gap under pressure and the origin can be correlated to different combinations of inter- and intralayer antibonding or bonding interactions at the band edges. More interestingly, the interlayer QB interactions are a coexistence of two categories of interactions, namely, the coexistence of interactions between bands of the same occupancy (occupied-occupied and empty-empty interactions) and of different occupancies (occupied-empty interaction); and, the overall effect is a four-level interaction, which explains the anomalous interlayer-antibonding feature of the conduction band edge of bilayer BP. Our current study lays the foundation for the electronic-structure tuning of two-dimensional (2D) BP, and, our analysis method for multi-energy-level interactions can be applied to other 2D semiconductor homo- and heterostructures that have occupied-empty interlayer interactions.

We predict the limit cycle solution for a ring-shape bosonic condensate of exciton-polaritons confined in an optically induced rotating trap. The limit cycle manifests itself with polarization oscillations on a characteristic timescale of tens of picoseconds. The effect arises due to the interplay between orbital motion and the polarization degree of freedom. It is specific to spinor bosonic condensates and would be absent in a scalar case, where a bistability of stationary solutions would be observed instead. This work offers a tool of initialization and control of qubits based on superpositions of polariton condensates characterized by different topologic charges.

We demonstrate the failure of the adiabatic Born-Oppenheimer approximation to describe the ground state of a quantum impurity within an ultracold Fermi gas despite substantial mass differences between the bath and impurity species. Increasing repulsion leads to the appearance of nonadiabatic couplings between the fast bath and slow impurity degrees of freedom, which reduce the parity symmetry of the latter according to the pseudo Jahn-Teller effect. The presence of this mechanism is associated to a conical intersection involving the impurity position and the inverse of the interaction strength, which acts as a synthetic dimension. We elucidate the presence of these effects via a detailed ground-state analysis involving the comparison of ab initio fully correlated simulations with effective models. Our study suggests ultracold atomic ensembles as potent emulators of complex molecular phenomena.

The superfluid weight of a generic lattice model with attractive Hubbard interaction is computed analytically in the isolated band limit within the generalized random phase approximation. Time-reversal symmetry, spin rotational symmetry, and the uniform pairing condition are assumed. It is found that the relation obtained in Huhtinen et al. [Phys. Rev. B 106, 014518 (2022)] between the superfluid weight in the flat band limit and the so-called minimal quantum metric is valid even at the level of the generalized random phase approximation. For an isolated, but not necessarily flat, band it is found that the correction to the superfluid weight obtained from the generalized random phase approximation $D_{\mathrm{s}}^{\left(1\right)}=D_{\mathrm{s},\mathrm{c}}^{\left(1\right)}+D_{\mathrm{s},\mathrm{g}}^{\left(1\right)}$ is also the sum of a conventional contribution $D_{\mathrm{s},\mathrm{c}}^{\left(1\right)}$ and a geometric contribution $D_{\mathrm{s},\mathrm{g}}^{\left(1\right)}$, as in the case of the known mean-field result $D_{\mathrm{s}}^{\left(0\right)}=D_{\mathrm{s},\mathrm{c}}^{\left(0\right)}+D_{\mathrm{s},\mathrm{g}}^{\left(0\right)}$, in which the geometric term $D_{\mathrm{s},\mathrm{g}}^{\left(0\right)}$ is a weighted average of the quantum metric. The conventional contribution is geometry independent, that is, independent of the orbital positions, while it is possible to find a preferred, or natural, set of orbital positions such that $D_{\mathrm{s},\mathrm{g}}^{\left(1\right)}=0$. Useful analytic expressions are derived for both the natural orbital positions and the minimal quantum metric, including its extension to bands that are not necessarily flat. Finally, using some simple examples, it is argued that the natural orbital positions may lead to a more refined classification of the topological properties of the band structure.

We propose that a weakly spontaneous charge-ordered insulating state probably exists in Bernal-stacked bilayer graphene, which can account for experimentally observed nonmonotonic behavior of resistance as a function of the gated field, namely, the gap closes and reopens at a critical gated field. The underlying physics is demonstrated by a simple model on a corresponding lattice that contains the nearest intralayer and interlayer hoppings, electric field, and staggered potential between different sublattices. Combining density functional theory calculations with model analyses, we argue that the interlayer van der Waals interactions cooperating with ripples may be responsible for the staggered potential, which induces a charge-ordered insulating state in the absence of the electric field.

To interpret our recent scanning tunneling microscopy (STM) experiment of nanographene sheets, in which the existence of rectangularlike lattices is confirmed, we conducted first-principles calculations of both simple and wrinkled armchair-edged graphene nanoribbons (AGNRs). We focused on the origin of this unique lattice and simulated STM images in the occupied band for the above-mentioned ribbons with different widths. Combined with the band structures of AGNRs, we investigated the features of electron distribution in detail under various magnitudes of applied sample biases (−1 to −0.05 V). In contrast to the well-known hexagonal structure of graphene at a large bias of −1 V, a rectangularlike lattice emerges in the STM images of AGNRs with specific widths when they are generated solely by electronic states occupying the valence band maximum. Furthermore, we find that a wrinkle in graphene parallel to the edges in AGNR significantly affects the STM images of a neighboring flat graphene region, even when its height is only several angstroms. By comparing the partial density of states of the wrinkle with flat graphene, we discussed the role of a wrinkle in functionalizing graphene sheets.

We demonstrate that two phenomena, the kinetic magnetoelectric effect and the nonlinear Hall effect, are universal to polar metals as a consequence of their coexisting and contraindicated polarization and metallicity. We show that measurement of the effects provides a complete characterization of the nature of the polar metal, in that the nonzero response components indicate the direction of the polar axis, and the coefficients change signs on polarization reversal and become zero in the nonpolar phase. We illustrate our findings for the case of electron-doped ${\mathrm{PbTiO}}_3$ using a combination of density functional theory and model Hamiltonian-based calculations. Our model Hamiltonian analysis provides crucial insight into the microscopic origin of the effects, showing that they originate from inversion-symmetry-breaking-induced interorbital hoppings that correlate to an asymmetric charge density quantified by odd-parity charge multipoles. Our paper both heightens the relevance of the kinetic magnetoelectric and nonlinear Hall effects, and broadens the platform for investigating and detecting odd-parity charge multipoles in polar metals.

Multichannel Kondo lattice models are examples of strongly correlated electronic systems that exhibit non-Fermi-liquid behavior due to the presence of a continuous channel symmetry. Mean-field analyses have predicted that these systems undergo channel symmetry breaking at low temperature. We use the dynamical large-$N$ technique to study temporal and spatial fluctuations of the multichannel Kondo model on a honeycomb lattice and find that this prediction is not generally true. Rather, we find a ($2+1$)-dimensional conformally invariant fixed point, governed by critical exponents that are found numerically. When we break time-reversal symmetry by adding a Haldane mass to the conduction electrons, three phases, separated by continuous transitions, are discernible: one characterized by dynamic mass generation and spontaneous breaking of the channel symmetry, one where topological defects restore channel symmetry but preserve the gap, and one with a Kondo-coupled chiral spin liquid. We argue that the last phase is a fractional Chern insulator with anyonic excitations.

Quantum embedding (QE) methods such as the ghost Gutzwiller approximation (gGA) offer a powerful approach to simulating strongly correlated systems, but come with the computational bottleneck of computing the ground state of an auxiliary embedding Hamiltonian (EH) iteratively. In this work, we introduce an active learning (AL) framework integrated within the gGA to address this challenge. The methodology is applied to the single-band Hubbard model and results in a significant reduction in the number of instances where the EH must be solved. Through a principal component analysis (PCA), we find that the EH parameters form a low-dimensional structure that is largely independent of the geometric specifics of the systems, especially in the strongly correlated regime. Our AL strategy enables us to discover this low-dimensionality structure on the fly, while leveraging it for reducing the computational cost of gGA, laying the groundwork for more efficient simulations of complex strongly correlated materials.

Weyl semimetals have nodes in their electronic structure at which electrons attain a definite chirality. Due to the chiral anomaly, the nonconservation of charges with given chirality, the axion term appears in their effective electromagnetic action. We determine how this affects the properties of time-reversal invariant Weyl superconductors (SCs) in the London regime. For type II SCs the axion coupling generates magnetic $B$ fields transverse to vortices, which become unstable at a critical coupling so that a transition into type I SC ensues. In this regime an applied $B$ field not only decays inside the SC within the London penetration depth, but the axion coupling generates an additional perpendicular field. Consequently, when penetrating into the bulk the $B$ field starts to steadily rotate away from the applied field. At a critical coupling the screening of the magnetic field breaks down. The novel chiral superconducting state that emerges has a periodically divergent susceptibility that separates onsets of chiral Meissner regimes. The chiral anomaly thus leaves very crisp experimental signatures in structurally chiral Weyl SCs with an axion response.

Variational wave function Ansätze are an invaluable tool to study the properties of strongly correlated systems. We propose such a wave function, based on the theory of auxiliary fields and combining aspects of auxiliary-field quantum Monte Carlo and modern variational optimization techniques including automatic differentiation. The resulting Ansatz, consisting of several slices of optimized projectors, is highly expressive and systematically improvable. We benchmark this form on the two-dimensional Hubbard model, using both cylindrical and large, fully periodic supercells. The computed ground-state energies are competitive with the best variational results. Moreover, the optimized wave functions predict the correct ground-state order with near full symmetry restoration (i.e., translation invariance) despite initial states with incorrect orders. The Ansatz can become a tool for local order prediction, leading to a new paradigm for variational studies of bulk systems. It can also be viewed as an approach to produce accurate and systematically improvable wave functions in a convenient form of nonorthogonal Slater determinants (e.g., for quantum chemistry) at polynomial computational cost.

Precursor elastic effects are investigated in a displacive anharmonic spring model and shown to extend greatly into the paraelastic phase. Weak precursor effects can be detected near $2T_{\mathrm{tr}}$, where $T_{\mathrm{tr}}$ is the ferroelastic transition temperature. The precursor effects become strong at $T<1.7T_{\mathrm{tr}}$. Two effects were identified in our two-dimensional model: the symmetry-breaking strain $e_3 \left({\varepsilon }_{xy}\right)$ leads to softening of the elastic modulus $C_{33}$, while the nonsymmetry-breaking strain $e_1+e_2$ (${\varepsilon }_{\mathrm{xx}}+{\varepsilon }_{\mathrm{yy}}$) leads to hardening of $C_{11}$. The strain $e_3$ is proportional to the order parameter and scales as $|e_1+e_2|$ $\sim e_3^2$. The temperature evolutions of the elastic moduli are surprisingly well described by power laws and Vogel-Fulcher equations. The power-law exponents are ∼−0.5 for $\mathrm{\Delta }{C}_{33}$ and ∼−1 for $\mathrm{\Delta }{C}_{11}$, Δ($C_{11}+C_{12}$) and Δ($C_{11}-C_{12}$). The Vogel-Fulcher temperatures are very similar, while the Vogel-Fulcher energies differ between the excess elastic moduli. The origin of the precursor effect is the evolution of short-range order in the paraelastic phase which gives rise to a characteristic local nanostructure. In the case of the symmetry-breaking strain, this microstructure resembles dynamical twinning patterns corresponding to the ferroelastic nanostructure, which weakens the material. In the case of the nonsymmetry-breaking strain, we find density fluctuations which make the material harder.