Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantized anomalous Hall insulator, which has been observed in magnetic-topological-insulator-based devices. In this work, we report a successful doping of rare earth element Tb into ${\mathrm{Bi}}_{1.08}{\mathrm{Sb}}_{0.9}{\mathrm{Te}}_2\mathrm{S}$ topological insulator single crystals, in which the Tb moments are antiferromagnetically ordered below $\sim 10$ K. Benefiting from the in-bulk-gap Fermi level, transport behavior dominant by the topological surface states is observed below $\sim 150$ K. At low temperatures, strong Shubnikov–de Haas oscillations are observed, which exhibit 2D-like behavior. The topological insulator with long range magnetic ordering in rare earth doped ${\mathrm{Bi}}_{1.08}{\mathrm{Sb}}_{0.9}{\mathrm{Te}}_2\mathrm{S}$ single crystal provides an ideal platform for quantum transport studies and potential applications.

We report a detailed study of the magnetic properties of $\mathrm{KBaGd}{\left({\mathrm{BO}}_3\right)}_2$ in which magnetic ${\mathrm{Gd}}^{3+}$ $(S=7/2)$ ions form into two-dimensional triangular layers. Magnetization, specific heat, and magnetocaloric effect (MCE) measurements were performed on $\mathrm{KBaGd}{\left({\mathrm{BO}}_3\right)}_2$ single crystals. The results show that a long-range antiferromagnetic state is established below $T_{\mathrm{N}}=0.24$ K. In zero fields, only about half of the full entropy is released at $T_{\mathrm{N}}$, indicating that not all the magnetic moments are frozen below the ordering temperature, as expected from the geometrical frustration of the triangular spin lattice. Further studies under external fields were performed down to 50 mK and the magnetic phase diagrams are established with magnetic fields applied both within and perpendicular to the triangular plane. $\mathrm{KBaGd}{\left({\mathrm{BO}}_3\right)}_2$ serves as an example of a two-dimensional triangular lattice with large spin values $(S=7/2)$ and can be directly compared with the isostructure $\mathrm{KBaR}{\left({\mathrm{BO}}_3\right)}_2$ $(\mathrm{R}=\mathrm{Dy}\text{−}\mathrm{Yb})$ family of doublet ground states, which exhibit effective spins of $S=1/2$.

Adiabatic quantum algorithms solve computational problems by slowly evolving a trivial state to the desired solution. On an ideal quantum computer, the solution quality improves monotonically with increasing circuit depth. By contrast, increasing the depth in current noisy computers introduces more noise and eventually deteriorates any computational advantage. What is the optimal circuit depth that provides the best solution? Here, we address this question by investigating an adiabatic circuit that interpolates between the paramagnetic and ferromagnetic ground states of the one-dimensional quantum Ising model. We characterize the quality of the final output by the density of defects $d$, as a function of the circuit depth $N$ and noise strength $\sigma$. We find that $d$ is well described by the simple form $d_{\mathrm{ideal}}+d_{\mathrm{noise}}$, where the ideal case $d_{\mathrm{ideal}}\sim N^{-1/2}$ is controlled by the Kibble-Zurek mechanism, and the noise contribution scales as $d_{\mathrm{noise}}\sim N{\sigma }^2$. It follows that the optimal number of steps minimizing the number of defects goes as $\sim {\sigma }^{-4/3}$. We implement this algorithm on a noisy superconducting quantum processor and find that the dependence of the density of defects on the circuit depth follows the predicted nonmonotonous behavior and agrees well with noisy simulations. Our work allows one to efficiently benchmark quantum devices and extract their effective noise strength $\sigma$.

We investigate the physics of projected $d$-wave pairing states using their fermionic projected entangled pair state (fPEPS) representation. First, we approximate a $d$-wave Bardeen-Cooper-Schrieffer state using the Gaussian fPEPS. Next, we translate the resulting state into fPEPS tensors and implement the Gutzwiller projection which removes double occupancy by modifying the local tensor elements. The tensor network representation of the projected $d$-wave pairing state allows us to evaluate physical quantities in the thermodynamic limit without employing the Gutzwiller approximation. Despite having very few variational parameters, such physically motivated tensor network states are shown to exhibit competitive energies for the doped $t\text{−}J$ model. We expect that such construction offers useful initial states and guidance for variational tensor network calculations.

We study the fractional quantum Hall effect in the central Landau level of AB stacked (Bernal) bilayer graphene. By tuning the external applied magnetic field and the electric bias between the two layers one can access a regime where there is a degeneracy between Landau levels with orbital characters corresponding to $N=0$ and $N=1$ Galilean- Landau levels. While the Laughlin state is generically the ground state for filling $\nu =1/3$ we find that it can be destroyed and replaced by a Wigner crystal at the same filling factor by tuning the bias and applied field. This competition does not take place at $\nu =2/3$ where the incompressible ground state remains stable. The possibility of electrically inducing the Wigner crystal state opens a new range of studies of this state of matter.

Frenkel polaritons, hybrid light-matter quasiparticles, offer promise for the designing of new optoelectronic devices. However, their technological implementations are hindered by sensitivity to imperfections. Topology has been raised as a way to circumvent defects and fabrication limitations. Here, we propose a lattice of cavities to realize the one-dimensional Su-Schrieffer-Heeger model (SSH) for topological Frenkel polaritons. By engineering the configuration of the cavities we demonstrate that the SSH topological and trivial phases can be accessed, which we unravel by employing a dual approach based on classical and quantum theories. We study the role of inherent vibron modes and fabrication defects in the robustness of the topological phases of polaritons. Our study demonstrates a simple experimentally realistic setup to realize topological polaritons at room temperature.

Various physical phenomena have been discovered by tuning degrees of freedom, among which there is the “valley” degree of freedom (DOF). The typical valley materials are characterized by two degenerate valley states protected by time-reversal symmetry ($\mathcal{T}\mathrm{S}$). These states indexed by valley DOF have been measured and manipulated for emergent valley-contrasting physics with the broken valley degeneracy. To achieve the valley splitting resulting from $\mathcal{T}\mathrm{S}$ breaking, previous studies have mainly focused on the magnetic proximity effect provided by the ferromagnetic layer. In contrast, the antiferromagnetic (AFM) proximity effect on the valley degeneracy has not been widely investigated systematically. In this work, we construct the composite systems consisting of a transition-metal dichalcogenide monolayer and a proximity layer with specific intraplane AFM configurations. We extend the three-band model to describe the valley states of such systems. It is shown that either “time-reversal $+$ fractional translation” or “mirror” symmetry can protect the valley degeneracy. Additionally, first-principles calculations based on density functional theory (DFT) have been performed to verify the results obtained from the extended tight-binding (TB) model. The corresponding mechanism of the valley splitting/degeneracy is revealed through the nondegenerate perturbation. Meanwhile, an extra condition is proposed to keep the well-defined valley states disentangled from each other through two negative examples based on degenerate perturbation. Further DFT studies on the effects of the $U_{\mathrm{eff}}$ and interlayer distance are performed. Manipulating the magnetization of Mo is shown to be feasible and effective for controlling the valley splitting with the direct overlap tuned by $U_{\mathrm{eff}}$ and the interlayer distance. The TB method introduced in the present work can properly describe the low-energy physics of valley materials that couple to the proximity with complex magnetic configurations. The results considerably expand the range of qualified proximity layers for valley splitting, enabling more flexible manipulation of valley degree.

We theoretically propose that an electric voltage can be generated by a thermal gradient with a rotating skyrmion crystal confined in a magnetic disk. We find that the rotation of a skyrmion crystal induced by diffusive thermal magnon currents in the presence of the temperature gradient gives rise to spinmotive forces in the radial direction through coupling to conduction-electron spins. The amplitude of the generated spinmotive force is larger for a larger temperature gradient at a lower temperature. The proposed phenomenon can be exploited as spintronics-based thermoelectric devices to realize the conversion of heat to electricity.

Thermally induced spin dynamics in solids have sparked broad interest in both fundamental physics and spintronic applications. As theoretically proposed, thermally excited magnons created by temperature gradients can be used to manipulate spin textures such as topological magnetic solitons. However, so far, the effectiveness of such thermomagnonic torques remained a problem in practice. Here, the dynamics of magnetic vortex cores in thin ferromagnetic platelets driven by thermomagnonic torques are investigated using high-resolution Lorentz transmission electron microscopy. Large deflections of the magnetic vortex core transverse to the direction of the temperature gradient are observed. A generalized Thiele equation model is used to identify the magnitude of the contribution of the involved torques. Our results pave the way for the manipulation of magnetic domains on the nanoscale by thermomagnonic currents and provide insights into the interplay of temperature and spin.

We predict frequency multiplication of surface acoustic waves in dielectric substrates via the ferromagnetic resonance of adjacent magnetic transducers when driven by microwaves. We find pure second-harmonic generation (SHG) without any linear and third-harmonic components by a magnetic nanowire. The SHG and linear phonon pumping are switched by varying the saturated magnetization direction of the wire, or resolved directionally when pumped by a magnetic nanodisk. We address the high efficiency of SHG with comparable magnitude to that of the linear response, as well as a unique nonreciprocal phonon transport that is remarkably distinct in different phonon harmonics. Such an acoustic frequency comb driven by microwaves should bring exceptional tunability for miniaturized phononic and spintronic devices.

We study characteristic electronic structures in an extended martini lattice model and propose its materialization in $\pi$-electron networks constructed by designated chemisorption on graphene and silicene. By investigating the minimal tight-binding model, we reveal rich electronic structures tuned by the ratio of hopping parameters, ranging from the band insulator to the unconventional gapless semiconductor. Remarkably, the unconventional gapless semiconductor is characterized by a flat band at the Fermi level. Further, the density functional theory calculations for candidate materials reveal that the characteristic electronic structures can be realized by designated chemisorption or chemical substitution on graphene and silicene, and that the electronic structure near the Fermi level is tunable by the choice of the atomic species of adsorbed atoms. Our results open the way to search exotic electronic structures and their functionalities induced by an extended martini lattice.

The delafossite metal ${\mathrm{PtCoO}}_2$ is among the highest-purity materials known, with low-temperature mean free path up to 5 $\mu \mathrm{m}$ in the best as-grown single crystals. It exhibits a strongly faceted, nearly hexagonal Fermi surface. This property has profound consequences for nonlocal transport within this material, such as in the classic ballistic-regime measurement of bend resistance in mesoscopic squares. Here, we report the results of experiments in which high-energy electron irradiation was used to introduce pointlike disorder into such squares, reducing the mean free path and therefore the strength of the ballistic-regime transport phenomena. We demonstrate that high-energy electron irradiation is a well-controlled technique to cross from nonlocal to local transport behavior and therefore determine the nature and extent of unconventional transport regimes. Using this technique, we confirm the origins of the directional ballistic effects observed in delafossite metals and demonstrate how the strongly faceted Fermi surface both leads to unconventional transport behavior and enhances the length scale over which such effects are important.

The sign reversal in the anomalous Hall effect (AHE) that occurs for material offers great prospects for AHE-based spintronic devices design. However, the mechanisms are still controversial in ultrathin ferromagnetic/heavy metal thin film systems due to the complicatedly interfacial effects. Here, we investigate the AHE sign reversal in ultrathin ferromagnetic ${\mathrm{Mn}}_2\mathrm{CoAl}/\mathrm{Pd}$ films, a system which has shown unusual AHE, significant spin-orbit coupling, and magnetic texturing. Element-sensitive cross-sectional STEM imaging and the depth-resolved magnetization profile from polarized neutron reflectometry identifies the presence of a second ferromagnetic layer from intermixed Co-Pd. To quantitatively explain the sign reversal of the AHE, we build a model based on two contributions, ferromagnetic ${\mathrm{Mn}}_2\mathrm{CoAl}$ and the intermixed CoPd layer. We also clarify that contributions to the AHE from magnetic proximity and spin Hall effect are negligible. Our work demonstrates that interfacial alloying can be a critical factor and provides insightful methods to determine the origins of the AHE in ferromagnet/heavy-metal thin film systems.

It has been predicted theoretically and found experimentally that magnetic impurities induce localized bound states within the superconducting energy gap, called Yu-Shiba-Rusinov (YSR) states. Combining symmetry analysis with experimental findings provides a convincing argument for the energy splitting and distribution of the YSR peaks, but the full details of the electronic structure remain elusive, and simple models with point scatterers lack the full orbital complexity required to meet this challenge. In this paper, we combine a Green's function-based first-principles method, which incorporates a phenomenological parametrization of the superconducting state, with orbitally complex impurity potentials to make material-specific predictions of realistic systems. We study the effect of $3d$ transition elements on the superconducting energy gap of a Pb (001) surface. Not only do we find good agreement with experiment, we also show that the energetic position, strength, and orbital composition of the YSR states depend strongly on the chemical makeup of the impurity and its position with respect to the surface. Such quantitative results cannot be derived from simplified models but require full material-specific calculations.

Magnetoelectric (ME) effect has given rise to a new design principle for novel low-power electronic devices. The polar magnet ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ with linear ME effect is one of the promising candidates. Unfortunately, for this material, a large ME response only appears in the ferrimagnetic state, which needs a high magnetic field to be activated. In this work, we successfully synthesize single-crystal ${\left({\mathrm{Fe}}_{0.95}{\mathrm{Mg}}_{0.05}\right)}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ (FMMO), which achieves a linear ME effect in the antiferromagnetic ground state and shows a significantly improved ME coefficient in the ferrimagnetic state. Compared with the parent phase, six times linear ME coefficient and more than ten orders of magnitude second-order ME coefficient are accomplished in the FMMO. The spin-lattice coupling and the exchange-striction mechanism are believed to play a crucial role in creating the giant ME effect. The present study provides an idea for designing materials with strong ME coupling.

Giant magnetocaloric (GMC) materials constitute a requirement for near-room-temperature magnetic refrigeration. ${(\mathrm{Fe},\mathrm{Mn})}_2(\mathrm{P},\mathrm{Si})$ is a GMC compound with strong magnetoelastic coupling. The main hindrance towards application of this material is a comparably large temperature hysteresis, which can be reduced by metal site substitution with a nonmagnetic element. However, the (Fe,Mn)${}_2$(P,Si) compound has two equally populated metal sites, the tetrahedrally coordinated $3f$ and the pyramidally coordinated $3g$ sites. The magnetic and magnetocaloric properties of such compounds are highly sensitive to the site-specific occupancy of the magnetic atoms. Here we have attempted to study separately the effect of $3f$ and $3g$ site substitution with equal amounts of vanadium. Using formation energy calculations, the site preference of vanadium and its influence on the magnetic phase formation are described. A large difference in the isothermal entropy change (as high as 44%) with substitution in the $3f$ and $3g$ sites is observed. The role of the lattice parameter change with temperature and the strength of the magnetoelastic coupling on the magnetic properties are highlighted.

We report a comprehensive study of the low-level substitution of ${\mathrm{Mn}}^{3+}$ by ${\mathrm{Fe}}^{3+}$ effect on the static and dynamic magnetoelectric coupling in $\mathrm{Tb}{\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_3$ ($x=0$, 0.02, and 0.04). The cationic substitution has a large impact on the balance between competitive magnetic interactions and, as a result, on the stabilization of the magnetic structures and ferroelectric phase at low temperatures. Low-lying electromagnon excitation is activated in the cycloidal modulated antiferromagnetic and ferroelectric phase in $\mathrm{TbMn}{\mathrm{O}}_3$, while it is observed up to $T_N$ in the Fe-substituted compounds, pointing at different mechanisms for static and dynamic magnetoelectric coupling. A second electrically active excitation near 40 $\mathrm{c}{\mathrm{m}}^{-1}$ is explained by means of ${\mathrm{Tb}}^{3+}$ crystal-field effects. This excitation is observed up to room temperature, and exhibits a remarkable $15\text{c}{\mathrm{m}}^{-1}$ downshift on cooling in Fe-substituted compounds. Both electromagnon and crystal-field excitations are found to be coupled to the polar phonons with frequencies up to 250 $\mathrm{c}{\mathrm{m}}^{-1}$. Raman spectroscopy reveals a spin-phonon coupling below $T_N$ in pure $\mathrm{TbMn}{\mathrm{O}}_3$, but the temperature where the coupling starts to be relevant increases with Fe concentration and reaches 100 K in $\mathrm{Tb}{\mathrm{Mn}}_{0.96}{\mathrm{Fe}}_{0.04}{\mathrm{O}}_3$. The anomalies in the $T$ dependence of magnetic susceptibility above $T_N$ are well accounted for by spin-phonon coupling and crystal-field excitation, coupled to oxygen motions.

Effects of finite-temperature quasiparticle self-energy corrections on x-ray absorption spectra are investigated within the finite-temperature quasiparticle local density $GW$ approximation up to temperatures $T$ of order the Fermi temperature. To facilitate the calculations, we parametrize the quasiparticle self-energy using low-order polynomial fits. We show that temperature-driven decrease in the electron lifetime substantially broadens the spectra in the near-edge region with increasing $T$. However, the quasiparticle shift is most strongly modified near the onset of plasmon excitations.

We make a systematic study of symmetry-protected topological gapped phases of quantum spin chains in the presence of the frieze space groups in one dimension using matrix product states. Here, the spatial symmetries of the one-dimensional lattice are considered together with an additional “vertical reflection,” which we take to be an on-site ${\mathbb{Z}}_2$ symmetry. We identify seventeen distinct non-trivial phases, define canonical forms, and compare the topological indices obtained from the MPS analysis with the group cohomological predictions. We furthermore construct explicit renormalization group fixed-point wave functions for symmetry-protected topological phases with global on-site symmetries, possibly combined with time reversal and parity symmetry. En route, we demonstrate how group cohomology can be computed using the Smith normal form.