Circuits can provide a versatile platform for exploring new physics, particularly in probing the topological phases within complex geometries. Fractals, celebrated for their intricate, self-similar duality, and noninteger dimensions, particularly those embedded in complex manifolds, remain uncharted in this context. In our research, we implement Sierpiński fractal topological insulators within reconfigurable fractal topological circuits while expanding the scope to include the cylindrical and toroidal structures. Our approach is grounded in consistency theory and reinforced through experimental verification, confirming the presence of unconventional higher-order topological phenomena referring to the abundance of topological edge and corner modes. Intriguingly, the quantity of these edge and corner modes is proportional to the volume modes relative to the system size, with an exponent aligning with the Hausdorff fractal dimension of the Sierpiński carpet. This study paves the way for a deeper exploration of topological modes within fractal geometries, potentially unlocking new avenues in topological physics.

Confinement is a paradigmatic phenomenon of gauge theories, and its understanding lies at the forefront of high-energy physics. Here, we study confinement in a simple one-dimensional ${\mathbb{Z}}_2$ lattice gauge theory at finite temperature and filling, which is within the reach of current cold-atom and superconducting-qubit platforms. By employing matrix product states (MPS) calculations, we investigate the decay of the finite-temperature Green's function and uncover a smooth crossover between the confined and deconfined regimes. Furthermore, using the Friedel oscillations and string length distributions obtained from snapshots sampled from MPS, both of which are experimentally readily available, we verify that confined mesons remain well-defined at arbitrary finite temperature. This phenomenology is further supported by probing quench dynamics of mesons with exact diagonalization. Our results shed new light on confinement at finite temperature from an experimentally relevant standpoint.

The stability of fractional Chern insulators is widely believed to be predicted by the resemblance of their single-particle spectra to Landau levels. We investigate the scope of this geometric stability hypothesis by analyzing the stability of a set of fractional Chern insulators that explicitly do not have a Landau level continuum limit. By computing the many-body spectra of Laughlin states in a generalized Hofstadter model, we analyze the relationship between single-particle metrics, such as trace inequality saturation, and many-body metrics, such as the magnitude of the many-body and entanglement gaps. We show numerically that the geometric stability hypothesis holds for Chern bands that are not continuously connected to Landau levels, as well as conventional Chern bands, albeit often requiring larger system sizes to converge for these configurations.

We introduce a systematic method to spectrally design quasi-one-dimensional crystal models described by the Dirac equation in the low-energy regime. The method is based on the supersymmetric transformation applied to an initially known pseudo-spin-1/2 model. This allows one to extend the corresponding supersymmetric partner so that the new model describes a pseudo-spin-1 system. The spectral design allows the introduction of a flat band and discrete energies at will into the new model. The results are illustrated in three examples where the Su-Schriefer-Heeger chain is locally converted into a stub lattice.

The charging effect has been a ubiquitous phenomenon in our daily lives. Here, we investigate the role played by this effect in a prototype nanowire dimer structure with an ab initio density-functional approach. Various charging configurations for the nanowire dimer have been explored, showing that the excess charge strongly modulates the ground and excited state. Furthermore, the opposite charging of the nanowire dimer breaks the symmetry of electron distribution and potential barrier, facilitating the formation of a charge-transfer current through the gap.

Spin Hall effect of spin-texture origin is explored theoretically for antiferromagnetic (AF) metals. Based on the observation that a scalar spin chirality formed by the Néel vector is not a physically well-defined quantity, it is found that a vector chirality formed by the Néel vector gives rise to a topological spin Hall effect. This is topological since it is proportional to the winding number counted by in-plane vector chirality along the sample edge, which corresponds to a homotopy class ${\pi }_1\left(S^1\right)$, instead of ${\pi }_2\left(S^2\right)$ of scalar chirality, and can be nonvanishing for AF merons but not for AF skyrmions. The effect is enhanced when the Fermi level lies near the AF gap, and, surprisingly, at weak coupling with a small AF gap. These features are confirmed numerically based on the Landauer-Büttiker formula. Important roles played by nonadiabatic processes and spin dephasing are pointed out.

We investigate the electronic structure and topological properties of iron-based superconductors ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ using density functional theory plus dynamical mean-field theory. We find that the uncollapsed tetragonal ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ is in a nontrivial $Z_2$ topological phase and has topological Dirac surface states near the Fermi energy which suggests there could be Majorana zero modes in the superconducting ${\mathrm{LaFe}}_2{\mathrm{As}}_2$. In light of the nontrivial topological properties and superconductivity of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, we predict a new iron-based compound ${\mathrm{LaBaFe}}_4{\mathrm{As}}_4$ and find it possesses two sets of topological Dirac surface states near the Fermi energy despite of a trivial $Z_2$ topological index. These topological surface states are induced by a nontrivial high-order topological index $Z_8$, a new mechanism that is distinct from all-known iron-based superconductors. Our study not only demonstrates that both ${\mathrm{LaBaFe}}_4{\mathrm{As}}_4$ and uncollapsed tetragonal ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ can be good platforms for exploring topological superconductivity but also paves a new way to realize it with a nontrivial high-order topological index.

Localized spins of single atoms adsorbed on surfaces have been proposed as building blocks for spintronics and quantum computation devices. However, identifying a way to achieve current-induced switching of spins with very low dissipation is an outstanding challenge with regard to practical applications. Here, we show that the indirect exchange interaction between spin impurities can be controlled by a dissipationless supercurrent. All that is required is a conventional superconductor and two spin impurities placed on its surface. No triplet Cooper pairs or exotic material choices are needed. This finding provides a new and accessible way to achieve the long-standing goal of supercurrent-induced spin switching.

Acquiring knowledge about the internal field environment of a solid-state spin is essential for its effective utilization as an external field detector. Working with the nitrogen-vacancy (NV) center in our diamond sample, we observed that almost all the NV centers with high effective field near the diamond surface has two sets of ODMR which we refer to as two states. Remarkably, near room temperature, these NV centers can spontaneously transition between these two states. Our experiments demonstrate that for NV centers with high ODMR splitting and shifting, the effective field cannot be solely attributed to a pure electric field. By sweeping the polarization direction of the microwave (MW) driving field, we extract both the strength and direction of the effective field. Furthermore, we track the real-time behavior of charges within the diamond by monitoring alternations in the Rabi frequencies, providing deeper insight into their hopping dynamics. These findings suggest the potential for manipulating NV centers by regulating the surrounding charge environment.

Van der Waals (vdW) solids, composed of ultrathin atomic layers with weak interlayer bonding, possess unique electronic and thermal properties and exhibit sensitivity to external stimuli such as compressive strain. Understanding and manipulation of their thermal transport behavior through strain engineering is crucial to optimize thermal management and energy conversion, which still remains elusive. Here, we demonstrate pressure manipulation on cross-plane coherent longitudinal acoustic phonon (CLAP) dynamics in bulk antimony sulfide $\left({\mathrm{Sb}}_2{\mathrm{S}}_3\right)$ under hydrostatic pressure through both systematic femtosecond pump-probe spectroscopy and first-principles calculations. The experimental group velocity of CLAP increases monotonically by around $68\%$ upon compression up to 5.06 GPa. First-principles calculations reveal that this dramatic change comes from the modification of phonon dispersion. Our findings shed light on the manipulation of thermal transport and energy conversion via compressive strain, which contribute to the thermal management in device engineering.

In this paper, we perform a numerical simulation on the recently discovered high-temperature superconductor $(T_c=73 \mathrm{K})$ ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3.2}$ while focusing on doping dependence of alternating ${\mathrm{CuO}}_6$ octahedra and CuO chainlike states. Employing the multiband random phase approximation, we compute the spin-fluctuation mediated pairing interaction, subsequently determining its pairing eigenvalues and eigenfunctions relative to oxygen-doping levels. We find that, for the certain range of hole doping in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$, a singlet $d_{x^2\text{−}{y}^2}$-wave pairing symmetry emerges if we keep the doping below the critical value $x_c$. Interestingly, upon hole doping, the dominant pairing symmetry undergoes a transition to a triplet (odd paring) type from the singlet state. This change in pairing is driven by the competition between the nesting vectors coming from the Fermi surface of $d_{z^2}$ and $d_{x^2\text{−}{y}^2}$ orbitals within the ${\mathrm{CuO}}_6$ octahedra. This triplet state is attainable through hole doping, while suppressing interlayer self-doping effects. Furthermore, we present the density of states within the superconducting phase, offering a potential comparison with tunneling spectra in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$. Our research provides insights into the intricate pairing symmetries in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$ and their underlying pairing mechanisms.

We investigated the unstable interface between vortices and antivortices in single crystals of the iron-based superconductor (${\mathrm{Ba}}_{0.67}{\mathrm{Rb}}_{0.33}$)${\mathrm{Fe}}_2{\mathrm{As}}_2$ ($T_{\mathrm{c}}\sim 37.6$ K). This interface, also known as the Meissner hole, was captured with magneto-optical imaging in both a pristine crystal and one with artificial point defects. We confirmed the presence of local excess current flowing along the interface. The dynamics of such an interface qualitatively follows the prediction of the critical state model, accompanied by a finite relaxation rate of magnetization. However, quantitatively, the local relaxation of the Meissner hole can be much faster than that of magnetization due to local deformation of the excess current.

We theoretically study a model of excitonic insulators which show de Haas–van Alphen oscillations as well as periodic dependence of the magnetization on inverse temperature. The insulating behavior is due to the Coulomb interaction driven hybridization of fermions at the crossing point of their energy bands. We show that the leading contribution to the oscillations at small magnetic fields is due to the oscillatory dependence of the hybridization gap on the magnetic field. In order to see this, the hybridization is derived from a self-consistent nonlinear equation. We show that the amplitude of the de Haas–van Alphen is oscillating with inverse temperature with a period defined by a combination of the hybridization gap and magnetic field. Moreover, we predicted that the amplitude must vanish at a particular field proportional to the hybridization gap.

We analyze quantum fluctuation effects at the onset of incommensurate $2k_F$ charge- or spin-density-wave order in two-dimensional metals for a model in which the ordering wave vector $\mathbf{Q}$ connects a single pair of hot spots on the Fermi surface with a vanishing Fermi surface curvature. The tangential momentum dependence of the bare dispersion near the hot spots is proportional to $|k_t{|}^{\alpha }$ with $\alpha >2$. We first compute the order parameter susceptibility and the fermion self-energy in a random phase approximation (RPA). Logarithmic divergences are subsequently treated by a renormalization-group analysis. The coupling between the order parameter fluctuations and the fermions vanishes logarithmically in the low-energy limit. As a consequence, the logarithmic divergences found in the RPA do not sum up to anomalous power laws. Instead, only logarithmic corrections to Fermi liquid behavior are obtained. In particular, the quasiparticle weight and the Fermi velocity vanish logarithmically at the hot spots.

Efficient electric manipulation of valley degrees of freedom is critical and challenging for the advancement of valley-based information science and technology. We put forth an electrical scheme, based on a two-band Dirac model, that can switch the fully valley-polarized photocurrent between $K$ and $K^{\prime }$ valleys using the circular bulk electro-photovoltaic effect. This is accomplished by applying an out-of-plane electric field to the two-dimensional valley materials, which enables continuous tuning of the Berry curvature and its sign flip. We found that the switch of the fully valley-polarized photocurrent is directly tied to the sign change of Berry curvature, which accompanies a topological phase transition, for instance, the quantum spin Hall effect and the quantum valley Hall effect. This scheme has been confirmed in monolayer ${\mathrm{BiAsI}}_2$ and germanene through first-principles calculations. Our paper offers a promising strategy for the development of a volatile valley-addressable memory device and could inspire further research in this area.

Realizing non-Abelian topological orders and their anyon excitations is an esteemed objective. In this work, we propose a novel approach towards this goal: quantum simulating topological orders in the doubled Hilbert space—the space of density matrices. We show that ground states of all quantum double models (toric code being the simplest example) can be efficiently prepared in the doubled Hilbert space; only finite-depth local operations are needed. In contrast, this is not the case in the conventional Hilbert space: ground states of only some of these models are known to be efficiently preparable. Additionally, we find that nontrivial anyon braiding effects, both Abelian and non-Abelian, can be realized in the doubled Hilbert space, although the intrinsic nature of density matrices restricts possible excitations.