We study the coupling between a singlet-triplet qubit realized in a double quantum dot to a topological qubit realized by spatially well-separated Majorana bound states. We demonstrate that the singlet-triplet qubit can be leveraged for readout of the topological qubit and for supplementing the gate operations that cannot be performed by braiding of Majorana bound states. Furthermore, we extend our setup to a network of singlet-triplet and topological hybrid qubits that paves the way to scalable fault-tolerant quantum computing.

The topological Hall effect (THE) is one of the key signatures of topologically nontrivial magnetic spin textures, wherein electrons feel an additional transverse voltage to the applied current. The magnitude of THE is often small compared to the anomalous Hall effect. Here, we find a large THE of $0.9\mu \mathrm{\Omega }\mathrm{cm}$ that is of the same order of the anomalous Hall effect in the single-crystalline antiskyrmion-hosting Heusler compound ${\mathrm{Mn}}_{1.4}\mathrm{PtSn}$, a noncentrosymmetric tetragonal compound. The THE is highly anisotropic and survives in the whole temperature range where the spin structure is noncoplanar (<170 K). The THE is zero above the spin reorientation transition temperature of 170 K, where the magnetization will have a collinear and ferromagnetic alignment. The large value of the THE entails a significant contribution from the momentum-space Berry curvature along with real-space Berry curvature, which has never been observed earlier.

We have used spin-polarized neutron reflectometry to investigate the magnetization profile of superlattices composed of ferromagnetic Gd and superconducting Nb layers. We have observed a partial suppression of ferromagnetic (F) order of Gd layers in [$\mathrm{Gd}\left(d_F\right)/\mathrm{Nb}(25 \mathrm{nm}){\mathrm{]}}_{12}$ superlattices below the superconducting (S) transition of the Nb layers. The amplitude of the suppression decreases with increasing $d_F$. By analyzing the neutron spin asymmetry we conclude that the observed effect has an electromagnetic origin—the proximity-coupled S layers screen out the external magnetic field and thus suppress the F response of the Gd layers inside the structure. Our investigation demonstrates the considerable influence of electromagnetic effects on the magnetic properties of S/F systems.

The fast-inertial-relaxation engine (FIRE) has proven to efficiently find local minima of potential energies or related penalty functions although its implementation requires only few, additional lines of code in a molecular-dynamics or steepest-descent program. So far, FIRE has been predominantly applied to particle-based or low-dimensional problems. In this work, we demonstrate that it can also benefit the solution of boundary-value problems. Towards this end, we study the mechanical contact between an elastic body and a rigid indenter of varying complexity by augmenting Green's function molecular dynamics (GFMD) with FIRE. We find a rather remarkable speedup, which can be further enhanced when choosing the masses associated with the eigenmodes of the free elastic solid appropriately. For the investigated adhesive and randomly rough indenter with typical system size, 100 mass-weighted FIRE-GFMD iterations suffice to relax the excess energy to ${10}^{-3}$ of its original value. The standard GFMD method needs 25 times more iterations. For the investigated problems, FIRE-GFMD even appears to slightly outperform conjugate-gradient based optimization.

Recent experimental findings in the bulk cubic helimagnet and Mott insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ highlight the paramount role of cubic anisotropy in stabilizing novel chiral and skyrmion phases. It was indeed found that if a magnetic field is applied along the easy $\langle 001\rangle$ crystallographic direction, competing cubic and exchange anisotropies tilt the wave vector of the conical spiral away from the magnetic field [Qian et al., Sci. Adv. 4, eaat7323 (2018)]. Furthermore, in this configuration skyrmions have been observed in a broad range of temperatures and magnetic fields [Chacon et al., Nat. Phys. 14, 936 (2018)]. Starting from these experimental observations and on the basis of a phenomenological Dzyaloshinskii theory, we investigate additional implications of the cubic anisotropy for this specified field direction. By including cubic anisotropy we show that the phase transition between the conical and field-polarized or homogeneous states becomes first order. Furthermore, we show that this transition is accompanied by the formation of conical droplets—domains of the conical phase in the homogeneous state. We investigate the internal structure of these droplets, which at their boundaries encompass alternating regions of positive and negative energy densities with respect to the homogeneous state. We thus deduce that these droplets may be zipped and unzipped in phase during the first-order phase transition that occurs by either increasing or decreasing the magnetic field. On the other hand, we show that in the conical phase skyrmions may form clusters due to their attractive mutual interaction. However, in the homogeneous state, the skyrmion-skyrmion interaction becomes repulsive, and the skyrmion clusters expand and disperse isolated skyrmions. This mutual skyrmion repulsion prevents the stabilization of skyrmion clusters even if the energy of isolated skyrmions is lower than that of the homogeneous state. Yet this skyrmion dispersal may be prevented if skyrmions are surrounded by the circular spiral state and form skyrmion bags. Such a scenario could explain the existence of skyrmions in the field-polarized state reported experimentally.

The electron states of gapped pseudospin-1 fermions of the $\alpha -{\mathcal{T}}_3$ lattice in the Coulomb field of a charged impurity are studied. The free $\alpha -{\mathcal{T}}_3$ model has three dispersive bands with two energy gaps between them depending on the parameter $\mathrm{\Theta }$ which controls the coupling of atoms of a honeycomb lattice with atoms in the center of each hexagon, thus interpolating between graphene $\mathrm{\Theta }=0$ and dice model $\mathrm{\Theta }=\pi /4$. The middle band becomes a flat one with zero energy in the dice model. The bound electron states are found in the two cases: the centrally symmetric potential well and a regularized Coulomb potential of the charged impurity. As the charge of the impurity increases, bound-state energy levels descend from the upper and central continua and dive at certain critical charges into the central and lower continuum, respectively. In the dice model, it is found that the flatband survives in the presence of a potential well, however, is absent in the case of the Coulomb potential. The analytical results are presented for the energy levels near continuum boundaries in the potential well. For the genuine Coulomb potential, we present the recursion relations that determine the coefficients of the series expansion of wave functions of bound states. It is shown that the condition for the termination of the series expansion gives two equations relating energy and charge values. Hence, analytical solutions can exist for a countably infinite set of values of impurity charge at fixed $\mathrm{\Theta }$.

$\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ was recently suggested as the first example of an antiferromagnetic topological insulator. However, lacking good quality of single crystals hindered its further investigation. Here, we report the detailed transport properties of several $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ thin flakes in which samples are more homogeneous as compared to the bulk single crystals. We found all the samples exhibit antiferromagnetic transition around 25 K and the same field-driven magnetic transitions; however, temperature dependence of resistivities shows either insulating or metallic behaviors. Such behavior is in contrast with the as-grown thick single crystals in which only metallic behavior was observed. The Hall coefficients indicate the gradual decrease of the carrier density with decreasing the temperature for the sample with insulating behavior. Such difference may relate to different impurity content (antisite defects and/or Mn vacancies). Our findings indicate the bulk carrier can be localized at low temperature by introducing disorder while the magnetic ordering keeps invariant, which is quite crucial for realizing the theoretical proposed quantum anomalous Hall effect and axion insulators in this material.

Using a Green's function approach, we study phonon-mediated superconducting pairing symmetries that may arise in bilayer graphene where the monolayers are displaced in-plane with respect to each other. We consider a generic coupling potential between the displaced graphene monolayers, which is applicable to both shifted and commensurate twisted graphene layers; study intralayer and interlayer phonon-mediated BCS pairings; and investigate AA and AB (AC) stacking orders. Our findings demonstrate that at the charge neutrality point, the dominant pairings in both AA and AB stackings with intralayer and interlayer electron-electron couplings can have even-parity $s$-wave class and odd-parity $p$-wave class of symmetries with the possibility of invoking equal-pseudospin and odd-frequency pair correlations. At a finite doping, however, the AB (and equivalently AC) stacking can develop pseudospin-singlet and pseudospin-triplet $d$-wave symmetry, in addition to $s$-wave, $p$-wave, $f$-wave, and their combinations, while the AA stacking order, similar to the undoped case, is unable to host the $d$-wave symmetry. When we introduce a generic coupling potential, applicable to commensurate twisted and shifted bilayers of graphene, $d$-wave symmetry can also appear at the charge neutrality point. Inspired by a recent experiment where two phonon modes were observed in a twisted bilayer graphene, we also discuss the possibility of the existence of two-gap superconductivity, where the intralayer and interlayer phonon-mediated BCS picture is responsible for superconductivity. These analyses may provide a useful tool in determining the superconducting pairing symmetries and mechanism in bilayer graphene systems.

The intrinsic brittleness of inorganic semiconductors prevents them from extended engineering applications under extreme conditions of high temperature and pressure, making it essential to improve their ductility. Here, we applied the constrained density functional theory to examine the relationship between plastic deformation and photonic excitation in sphalerite ZnS and related II-IV semiconductors. We find that ZnS transforms from a dislocation dominated deformation mode in the ground state to a twin dominated deformation mode with bandgap electronic excitations, leading to brittle failure under light illumination. This agrees very well with recent mechanical experiments on single crystal ZnS. More interesting, we predict that the ZnTe and CdTe display the opposite mechanical behavior compared to ZnS, exhibiting ductility close to metallic level with bandgap illumination, but typical brittle failure in the dark state. Our results provide a general approach to design more shapeable and tougher semiconductor devices by controlling exposure to electronic excitation.

We apply the nonequilibrium functional renormalization group approach treating flow of the electronic self-energies, to describe local magnetic moment formation and electronic transport in a quadruple quantum dot (QQD) ring, coupled to leads, with moderate Coulomb interaction on the quantum dots. We find that at zero temperature depending on parameters of the QQD system, the regimes with zero, one, or two almost local magnetic moments in the ring can be realized, and the results of the considered approach in equilibrium agree qualitatively with those of a more sophisticated functional renormalization group approach treating also flow of the vertices. It is shown that the almost formed local magnetic moments, which exist in the equilibrium, remain stable in a wide range of bias voltages near equilibrium. The destruction of the local magnetic moments with increasing bias voltage is realized in one or two stages, depending on the parameters of the system; for the two-stage process the intermediate phase possesses fractional magnetic moments. We present zero-temperature results for current-voltage dependencies and differential conductances of the system, which exhibit sharp features at the transition points between different magnetic states. The occurrence of the interaction-induced negative differential conductance phenomenon is demonstrated and discussed. For one local moment in the ring and finite hopping between the opposite quantum dots, connected to the leads, we find suppression of the conductance for one of the spin projections in infinitesimally small magnetic field, which occurs due to destructive interference of different electron propagation paths and can be used in spintronic devices.

This paper explores the energy scales of the doped Anderson lattice model using dynamical mean-field theory (DMFT), using a continuous-time quantum Monte Carlo (CTQMC) impurity solver. We show that the low temperature properties of the lattice cannot be scaled using the single ion local Kondo temperature $T_K$ but instead are governed by a doping-dependent coherence temperature $T^{*}$ which can be used to scale the temperature dependence of the spectral function, transport properties, and entropy. At half-filling $T^{*}$ closely approximates the single ion $T_K$, but as the filling $n_c$ is reduced to zero, $T^{*}$ also vanishes. The coherence temperature $T^{*}$ is shown to play a role of effective impurity Kondo temperature in the lattice model, and physical observables show significant evolution at $T^{*}$. In the DMFT framework we showed that the hybridization strength of the effective impurity model is qualitatively affected by the doping level, and determines $T^{*}$ in the lattice model.

The three-band Hubbard model is a fundamental model for understanding properties of the copper-oxygen planes in cuprate superconductors. We use cutting-edge auxiliary-field quantum Monte Carlo (AFQMC) methods to investigate ground state properties of the model in the parent compound. Large supercells combined with twist averaged boundary conditions are employed to reliably reach the thermodynamic limit. Benchmark quality results are obtained on the magnetic correlations and charge gap. A key parameter of this model is the charge-transfer energy $\mathrm{\Delta }$ between the oxygen $p$ and the copper $d$ orbitals, which appears to vary significantly across different families of cuprates and whose ab initio determination is subtle. We show that the system undergoes a quantum phase transition from an antiferromagnetic insulator to a paramagnetic metal as $\mathrm{\Delta }$ is lowered to $3\mathrm{eV}$.

A large Berry curvature in the vicinity of the Fermi energy is required in order to obtain a large anomalous Hall and Nernst effect. This Berry curvature can be induced by Weyl points and gapped nodal lines. One of the possible mechanisms takes place in systems with a symmetry group where mirror planes lead to protected nodal line structures. When these mirror symmetries are broken, e.g., via fixing a magnetization direction, a gap in the former nodal line can create Weyl points and a large Berry curvature along the gapped lines. In this work we study this effect in a minimal effective model and relate the results to the real regular Heusler compounds ${\mathrm{Fe}}_2\mathrm{Mn}X$ ($X=\mathrm{P},\mathrm{As},\mathrm{Sb}$). These materials have three mirror planes in the nonmagnetic case, leading to three nodal lines near the Fermi level. However, dependent on the orientation of the magnetization, some of the mirror planes are broken and the respective lines are gapped, creating large Berry curvature. Because the Fermi level is located in vicinity of the gapped lines, this leads to a large anomalous Hall and Nernst effect, which can be tuned to even higher values with a little bit of doping in the system.

The structure and dynamical properties of the ${\mathrm{Fe}}_3\mathrm{Si}/\mathrm{GaAs}\left(001\right)$ interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the ${\mathrm{Fe}}_3\mathrm{Si}/\mathrm{GaAs}$ multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the ${\mathrm{Fe}}_3\mathrm{Si}$ crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk ${\mathrm{Fe}}_3\mathrm{Si}$. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the ${\mathrm{Fe}}_3\mathrm{Si}$ layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meV.

The ${\mathrm{CoBr}}_2$ monolayer is predicted to be an intrinsically ferromagnetic (FM) semiconductor based on the first-principles calculations. The results of the magnetic anisotropy energy show that the ${\mathrm{CoBr}}_2$ monolayer has an easy magnetization plane and belongs to the family of two-dimensional $XY$ magnets. The exchange interactions and thus the magnetic properties can be effectively tuned by external strain and carrier doping. The biaxial tensile strain can induce FM-antiferromagnetic (AFM) phase transition in the ${\mathrm{CoBr}}_2$ monolayer, by which the Co-Br-Co bond angle is enlarged deviating far away from ${90}^{\circ }$. While the compressive strain stabilizes the FM state by reducing the nearest Co-Co distance and enhancing the $p\text{−}d$ hybridization between Co and Br atoms. The ${\mathrm{CoBr}}_2$ monolayer remains semiconducting for the whole range of strain investigated. For the carrier doping, however, the ${\mathrm{CoBr}}_2$ monolayer tends to be a half-metal for both hole and electron doping. Doping not only introduces itinerant carriers, but also enhances the $p\text{−}d$ hybridization between the Co and Br atoms, both of which enhance the FM coupling significantly. The tunability of the electronic and magnetic properties of ${\mathrm{CoBr}}_2$ monolayer makes it a promising candidate for future nanospintronic applications.

In orthorhombic ${\mathrm{LuMnO}}_3$ perovskite with nonmagnetic ${\mathrm{Lu}}^{+3}$ ions ($4f^{14}5d^{0}6s^0$) there exists one magnetic transition of ${\mathrm{Mn}}^{+3}$ ions at $T_N\approx 35.5$ K. In ${\mathrm{YbMnO}}_3$ perovskite with ${\mathrm{Yb}}^{+3}$ ions of one electron less ($4f^{13}5d^{0}6s^0$), two magnetic transitions are observed at $T_N\approx 43$ K and $T\approx 3$–4 K, respectively. While the transition at 43 K in ${\mathrm{YbMnO}}_3$ was attributed to the same magnetic ordering of ${\mathrm{Mn}}^{+3}$ moments as ${\mathrm{LuMnO}}_3$ at 35.5 K, the nature of the magnetic transition at 3–4 K is still unresolved. Although the bump feature in magnetic susceptibility hints at a possible antiferromagnetic ordering of ${\mathrm{Yb}}^{+3}$ ions, no clear magnetic ordering of ${\mathrm{Yb}}^{+3}$ was determined in either neutron scattering diffraction or in Mössbauer spectra of iron-doped ${\mathrm{YbMnO}}_3$. To explain the amplitude-modulated nature of the low magnetic anomaly, we propose in this paper that a gradual quenching of the orbital moment of ${\mathrm{Yb}}^{+3}$ ions below a critical temperature is a possible mechanism. The small crystal field splitting of $4f$ electrons can survive only at extremely low temperature because of thermal fluctuation. In addition, we show, like in ${\mathrm{LuMnO}}_3$, that ${\mathrm{YbMnO}}_3$ takes an E-type antiferromagnetic ground state for its Mn ions and can be tuned into ferromagnetic state when (110)-oriented ${\mathrm{YAlO}}_3$ is used as a substrate.