Rare-earth pyrochlore iridates $\left(R{E}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7\right)$ consist of two interpenetrating cation sublattices, the $\mathit{RE}$ with highly frustrated magnetic moments, and the iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. While bulk $\mathrm{P}{\mathrm{r}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ shows a spontaneous hysteretic Hall effect below 1.5 K, we observe the effect at elevated temperatures up to 15 K in epitaxial thin films on (111) yttria-stabilized zirconia (YSZ) substrates synthesized via solid-phase epitaxy. Similar to the bulk, the lack of observable long-range magnetic order in the thin films points to a topological origin. We use synchrotron-based element-specific x-ray diffraction and x-ray magnetic circular dichroism to compare powders and thin films to attribute the spontaneous Hall effect in the films to localization of the Ir moments. We link the thin-film Ir local moments to lattice distortions absent in the bulklike powders. We conclude that the elevated-temperature spontaneous Hall effect is caused by the topological effect originating either from the Ir or Pr sublattice, with interaction strength enhanced by the Ir local moments. This spontaneous Hall effect with weak net moment highlights the effect of vanishingly small lattice distortions as a means to discover topological phenomena in metallic frustrated magnetic materials.

We report a comprehensive small-angle neutron scattering (SANS) study of magnetic correlations in ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ at zero magnetic field. To delineate changes of magnetocrystalline anisotropies (MCAs) from effects due to defects and disorder, we recorded complementary susceptibility and high-resolution specific heat data and investigated selected compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$. For all systems studied, the helimagnetic transition temperature and magnetic phase diagrams evolve monotonically with composition consistent with literature. The SANS intensity patterns of the spontaneous magnetic order recorded under zero-field cooling, which were systematically tracked over forty angular positions, display strong changes of the directions of the intensity maxima and smeared out intensity distributions as a function of composition. We show that cubic MCAs account for the complex evolution of the SANS patterns, where for increasing $x$ the character of the MCAs shifts from terms that are fourth order to terms that are sixth order in spin-orbit coupling. The magnetic field dependence of the susceptibility and SANS establishes that the helix reorientation as a function of magnetic field for Fe- or Co-doped MnSi is dominated by pinning due to defects and disorder. The presence of well-defined thermodynamic anomalies of the specific heat at the phase boundaries of the skyrmion lattice phase in the doped samples and properties observed in ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ establishes that the pinning due to defects and disorder remains, however, weak and comparable to the field scale of the helix reorientation. The observation that MCAs, which are sixth order in spin-orbit coupling, play an important role for the spontaneous order in ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ and ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ offers a fresh perspective for a wide range of topics in cubic chiral magnets such as the generic magnetic phase diagram, the morphology of topological spin textures, the paramagnetic-to-helical transition, and quantum phase transitions.

We introduce a two-parameter family of perturbations of Kitaev's toric code model in which the anyonic excitations acquire an interesting dynamics. We study the dynamics of this model in the space of states with electric and magnetic charge both equal to 1 and find that the model exhibits both bound states and scattering states in a suitable region of the parameters. The bound state is a Majorana fermion with a dispersion relation of Dirac cone type. For a certain range of model parameters, we find that these bound states disappear in a continuum of scattering states at a critical value of the total momentum. The scattering states describe separate electric and magnetic anyons, which in this model each have a $sink$ dispersion relation.

The quantum anomalous Hall effect (QAHE) and magnetic Weyl semimetals (WSMs) are topological states induced by intrinsic magnetic moments and spin-orbit coupling. Their similarity suggests the possibility of achieving the QAHE by dimensional confinement of a magnetic WSM along one direction. In this paper, we investigate the emergence of the QAHE in the two-dimensional (2D) limit of magnetic WSMs due to finite-size effects in thin films and step edges. We demonstrate the feasibility of this approach with effective models and real materials. To this end, we have chosen the layered magnetic WSM ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, which features a large anomalous Hall conductivity and anomalous Hall angle in its three-dimensional bulk as our material candidate. In the 2D limit of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, two QAHE states exist depending on the stoichiometry of the 2D layer. One is a semimetal with a Chern number of 6, and the other is an insulator with a Chern number of 3. The latter has a band gap of 0.05 eV, which is much larger than that in magnetically doped topological insulators. Our findings naturally explain the existence of chiral states in step edges of bulk ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ which have been reported in a recent experiment at $T=4\mathrm{K}$ and present a realistic avenue to realize QAH states in thin films of magnetic WSMs.

We study the effects of the coupling between magnetization dynamics and the electronic degrees of freedom in a heterostructure of a metallic nanomagnet with dynamic magnetization coupled with a superconductor containing a steady spin-splitting field. We predict how this system exhibits a nonlinear spin torque, which can be driven either with a temperature difference or a voltage across the interface. We generalize this notion to arbitrary magnetization precession by deriving a Keldysh action for the interface, describing the coupled charge, heat, and spin transport in the presence of a precessing magnetization. We characterize the effect of superconductivity on the precession damping and the antidamping torques. We also predict the full nonlinear characteristic of the Onsager counterparts of the torque, showing up via pumped charge and heat currents. For the latter, we predict a spin-pumping cooling effect, where the magnetization dynamics can cool either the nanomagnet or the superconductor.

The van der Waals heterostructures are an active frontier for discovering emergent phenomena in condensed matter systems. They are constructed by stacking elements of a large library of two-dimensional materials that couple together through van der Waals interactions. However, the number of possible combinations within this library is staggering, so fully exploring their potential is a daunting task. Here, we introduce van der Waals metamaterials to rapidly prototype and screen their quantum counterparts. These layered metamaterials are designed to reshape the flow of ultrasound to mimic electron motion. In particular, we show how to construct analogs of all stacking configurations of bilayer and trilayer graphene through the use of interlayer membranes that emulate van der Waals interactions. By changing the membrane's density and thickness, we can also reach coupling regimes far beyond that of naturally occurring graphene. We anticipate that van der Waals metamaterials can be used to explore, extend, and inform future electronic devices. Furthermore, they allow the transfer of useful electronic behavior to acoustic systems, such as flat bands in magic-angle twisted bilayer graphene, which may aid the development of superresolution ultrasound imagers.

Consistency with the Maxwell equations determines how matter must be coupled to the electromagnetic field (EMF) within the minimal coupling scheme. Specifically, if the Hamiltonian includes just a short-range repulsion among the conduction electrons, as is commonly the case for models of correlated metals, those electrons must be coupled to the full internal EMF, whose longitudinal and transverse components are self-consistently related to the electron charge and current densities through Gauss's and circuital laws, respectively. Since such self-consistency relation is hard to implement when modeling the nonequilibrium dynamics caused by the EMF, as in pump-probe experiments, it is common to replace in model calculations the internal EMF by the external one. Here we show that such replacement may be misleading, especially when the frequency of the external EMF is below the intraband plasma edge.

By creating a sharp and dense dopant profile of phosphorus atoms buried within a silicon host, a two-dimensional electron gas is formed within the dopant region. Quantum confinement effects induced by reducing the thickness of the dopant layer, from $4.0\mathrm{nm}$ to the single-layer limit, are explored using angle-resolved photoemission spectroscopy. The location of theoretically predicted, but experimentally hitherto unobserved, quantum well states known as the $\mathrm{\Delta }$ manifold is revealed. Moreover, the number of carriers hosted within the $\mathrm{\Delta }$ manifold is shown to be strongly affected by the confinement potential, opening the possibility to select carrier characteristics by tuning the dopant-layer thickness.

We study the one-band Hubbard model on the honeycomb lattice using a combination of quantum Monte Carlo (QMC) simulations and static as well as dynamical mean-field theory (DMFT). This model is known to show a quantum phase transition between a Dirac semimetal and the antiferromagnetic insulator. The aim of this paper is to provide a detailed comparison between these approaches by computing static properties, notably ground-state energy, single-particle gap, double occupancy, and staggered magnetization, as well as dynamical quantities such as the single-particle spectral function. At the static mean-field level, local moments cannot be generated without breaking the SU(2) spin symmetry. The DMFT approximation accounts for temporal fluctuations and thus captures both the evolution of the double occupancy and the resulting local moment formation in the paramagnetic phase. As a consequence, the DMFT approximation is found to be very accurate in the Dirac semimetallic phase where local moment formation is present and the spin correlation length small. However, in the vicinity of the fermion quantum critical point, the spin correlation length diverges and the spontaneous SU(2) symmetry breaking leads to low-lying Goldstone modes in the magnetically ordered phase. The impact of these spin fluctuations on the single-particle spectral function—waterfall features and narrow spin-polaron bands—is only visible in the lattice QMC approach.

Typicality of the orthogonal dynamics (TOD) is established as a generic feature of the temporal relaxation processes in isolated many-body quantum systems. The basic idea in the simplest case is that the transient nonequilibrium behavior is mainly governed by the component of the time-evolved system state parallel to the initial state, while the orthogonal component appears as equilibrated right from the beginning. The main emphasis is laid on the largely unexplored and particularly challenging case that one energy level exhibits a much larger population than all the others. Important examples are gapped many-body systems at low energies, for instance, due to a quantum quench. A general analytical prediction is derived and is found to compare very well with various numerically exact results.

We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions can be enhanced by coupling the topological insulator asymmetrically to the two sublattices of the antiferromagnet. This effect is further amplified by next-nearest-neighbor frustration in the antiferromagnetic insulator. The enhancement makes the induced interactions significantly stronger in the antiferromagnetic case compared to the ferromagnetic case. These results indicate that an uncompensated antiferromagnetic interface might be a better candidate than a ferromagnetic interface for proximity-induced magnon-mediated superconductivity on the surface of a topological insulator.

We consider a Rashba spin-orbit coupled nanowire with anisotropic spin-singlet superconducting pairing and with time reversal invariance. We explore the evolution of the topological superconducting phases of this system due to geometric deformations for the representative case of a wire bent in a semielliptical shape. We find that when the system is in its topological superconducting phase, strong inhomogeneities in the profile curvature can produce a pair of localized eigenmodes, which can be attributed to a nonuniform topological phase. The curved geometric profile also allows us to tune the spin correlations of the superconducting state via the induced inhomogeneity of the spin-orbit coupling (SOC). The geometric control of the superconducting pair correlations leads to the manipulation of the critical current in Josephson junctions made up of two time reversal invariant topological superconductors separated by a spin-orbit coupled normal metal. In particular, we find that the curvature inhomogeneity can be exploited for amplifying the current intensity but also to generate a $0\text{−}\pi$ transition, and a second harmonic contribution, which generates, for some specific geometric configurations, a $\phi$-junction behavior.

Superconductor-ferromagnetic heterostructures have been suggested as one of the most promising alternatives of realizing odd-frequency superconductivity. In this work we consider the limit of shrinking the ferromagnetic region to the limit of a single impurity embedded in a conventional superconductor, which gives rise to localized Yu-Shiba-Rusinov (YSR) bound states with energies inside the superconducting gap. We demonstrate that all the sufficient ingredients for generating odd-frequency pairing are present at the vicinity of these impurities. We investigate the appearance of all possible pair amplitudes in accordance with the Berezinskii $S{P}^{*}O{T}^{*}=-1$ rule, with the symmetry under the exchange of spin, spatial, orbital (in our case $O=+1$), and time index, respectively. We study the spatial and frequency dependence of the possible pairing amplitudes, analyzing their evolution with impurity strength and identifying a reciprocity between different symmetries related through impurity scattering. We show that the odd-frequency spin-triplet pairing amplitude dominates at the critical impurity strength, where the YSR states merge at the middle of the gap, while the even components are quenched close to the impurity. We also show that the spin-polarized local density of states exhibits the same spatial and frequency behavior as the odd-$\omega$ spin-triplet component at the critical impurity strength.

Virtual perfect absorption was recently proposed as a way to trap electromagnetic pulses in lossless cavities by shaping the pulses in the form of exponentially increasing functions. Such an effect may be useful for energy storage and quantum information processing. One problem with this approach is that the exponentially increasing wave form cannot be sustained for a long time, imposing a limit on the maximum energy that can be captured. Here it is shown that this problem can be circumvented by slowly modulating the cavity's radiative decay rate in such a way that the leaking signal from the cavity and the reflected signal at the cavity's port are constantly opposite to each other. This approach allows maintaining zero reflection for the entire duration of the phenomenon for continuous-wave signals and enables trapping electromagnetic energy for significant amounts of time, limited by only the spectrum of the incident signal.

Nonlinear wave limiters transmit low-amplitude waves while blocking high-intensity ones for efficient target protection. However, the acoustical limiting effect in nonlinear materials remains hitherto unaddressed. In addition, tunable bandgap fosters advanced functions for devices, but it is still mostly regarded as a spatial and temporal invariant feature. In recent years, nonlinear acoustic metamaterials (NAM) have shown extraordinary properties for manipulating elastic waves. Here we achieve an enhanced nonlinear interaction in a different NAM. We theoretically and experimentally demonstrate that the NAM features an efficient acoustical limiting, and the limiting bandwidth adaptively broadens as the propagation distance/time increases. Within a short propagation distance, an ultrabroad limiting band is formed that overcomes the limitation of linear resonant bandgaps. It is clarified that the space-amplitude-dependent bandgap dominates the amplitude reduction, and the transient chaotic responses initialize the adaptive-broadening process. Our study highlights wave physics that could not obviously be realized in nonlinear optics. The self-adaptive band structures open up opportunities to realize exotic adaptive elements.

We report neutron diffraction measurements of the magnetic structures in two pyrochlore iridates, ${\mathrm{Yb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Lu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. Both samples exhibit the all-in-all-out magnetic structure on the ${\mathrm{Ir}}^{4+}$ sites below $T_{\mathrm{N}}\simeq 150$ K, with a low temperature moment of around 0.$45{\mu }_{\mathrm{B}}$/Ir. Below 2 K, the Yb moments in ${\mathrm{Yb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ begin to order ferromagnetically. However, even at 40 mK the ordered moment is only 0.57(2) ${\mu }_{\mathrm{B}}$/Yb, well below the saturated moment of the ground state doublet of ${\mathrm{Yb}}^{3+}$ (1.$9{\mu }_{\mathrm{B}}$/Yb), deduced from magnetization measurements and from a refined model of the crystal field environment, and also significantly smaller than the ordered moment of Yb in ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ (0.$9{\mu }_{\mathrm{B}}$/Yb). A mean-field analysis shows that the reduced moment on Yb is a consequence of enhanced phase competition caused by coupling to the all-in-all-out magnetic order on the Ir sublattice.

Understanding (and controlling) hyperfine interactions in semiconductor nanostructures is important for fundamental studies of material properties as well as for quantum information processing with electron, hole, and nuclear-spin states. Through a combination of first-principles density-functional theory (DFT) and $\mathbf{k}·\mathbf{p}$ theory, we have calculated hyperfine tensors for electrons and holes in GaAs and crystalline silicon. Accounting for relativistic effects near the nuclear core, we find contact hyperfine interactions for electrons in GaAs that are consistent with Knight-shift measurements performed on GaAs quantum wells and are roughly consistent with prior estimates extrapolated from measurements on InSb. We find that a combination of DFT and $\mathbf{k}·\mathbf{p}$ theory ($\mathrm{DFT}+\mathbf{k}·\mathbf{p}$) is necessary to accurately determine the contact hyperfine interaction for electrons at a conduction-band minimum in silicon that is consistent with bulk Knight-shift measurements. For hole spins in GaAs, the overall magnitude of the hyperfine couplings we find from DFT is consistent with previous theory based on free-atom properties, and with heavy-hole Overhauser shifts measured in GaAs (and InGaAs) quantum dots. In addition, we theoretically predict that the heavy-hole hyperfine coupling to the As nuclear spins is stronger and almost purely Ising, while the (weaker) coupling to the Ga nuclear spins has significant non-Ising corrections. In the case of hole spins in silicon, we find (surprisingly) that the strength of the hyperfine interaction in the valence band is comparable to that in the conduction band and that the hyperfine tensors are highly anisotropic (Ising) in the heavy-hole subspace. These results suggest that the hyperfine coupling cannot be ruled out as a limiting mechanism for coherence ($T_2^{*}$) times recently measured for heavy holes in silicon quantum dots.

In this work we provide a direct and non-numerical technique to obtain the surface Green's functions for three-dimensional systems. This technique is based on the ideas presented by V. Kaladzhyan and C. Bena [Phys. Rev. B 100, 081106(R) (2019)], in which we start with an infinite system and model the boundary using a planelike infinite-amplitude potential. Such a configuration can be solved exactly using the $T$-matrix formalism. We apply our method to calculate the surface Green's function and the corresponding Fermi-arc states for Weyl semimetals. We also apply the technique to systems of lower dimensions, such as Kane-Mele and Chern insulator models, to provide a more efficient and non-numerical method to describe the formation of edge states.

Chiral pumping from optical electric fields oscillating at terahertz frequencies is observed in the Weyl material TaAs with electric and magnetic fields aligned along both the $a$ and $c$ axes. Free-carrier spectral weight enhancement is measured directly, confirming theoretical expectations of chiral pumping. A departure from linear field dependence of the Drude weight is observed at the highest fields in the quantum limit, providing evidence of field-dependent Fermi velocity of the chiral Landau level. Implications for the chiral magnetic effect in Weyl semimetals from the optical $f$-sum rule are discussed.