Ferromagnetic insulators have great potential for spintronic applications. For such applications, it is essential to find materials with a robust and controllable ferromagnetic insulating phase. However, because ferromagnetism in functional transition-metal oxides is usually coupled to metallicity, ferromagnetic insulators are very rare and independent control of their magnetic and electrical properties is difficult. In this study, the electrical, magnetic, and optical properties of ${\left(\mathrm{LaCo}{\mathrm{O}}_3\right)}_n/{\left(\mathrm{SrCo}{\mathrm{O}}_{2.5}\right)}_n$ superlattice films are investigated for the manipulation of the ferromagnetic insulating phase. While the superlattices remain insulating irrespective of the periodicity $n$, the electronic structure and magnetic state vary drastically. Superlattices with large periodicities $n$ of 10 and 20 show a ferromagnetic transition at a critical temperature $T_{\mathrm{C}}$ of $\sim 80\mathrm{K}$. With decreasing periodicity and increasing interface density of the superlattices, system with $n=4$ becomes almost nonmagnetic, while in systems with $n=2$ and 1, a reentrant ferromagnetic phase is observed at $T_{\mathrm{C}}$ of $\sim 180$ and $\sim 225\mathrm{K}$, respectively. Optical spectroscopy reveals that the fine control of the magnetic ground state is achieved by the modified electronic structure associated with the spin-state transition. Our results suggest an important design principle to create and manipulate the ferromagnetic insulating properties of Co-based oxide thin films.

Spin-orbit coupling (SOC) plays a crucial role in magnetic and electronic properties of $5d$ iridates. In this paper we have experimentally investigated the structural and physical properties of a series of Ir-based double perovskite compounds ${\mathrm{Pr}}_{2-x}{\mathrm{Sr}}_x{\mathrm{MgIrO}}_6(x$ = 0, 0.5, 1; hereafter abbreviated as PMIO, PSMIO1505, and PSMIO). Interestingly, these compounds have recently been proposed to undergo a transition from the spin-orbit-coupled Mott insulating phase at $x=0$ to the elusive half-metallic antiferromagnetic (HMAFM) state with Sr doping at $x=1$. However, our detailed magnetic and electrical measurements refute any kind of HMAFM possibility in either of the doped samples. In addition, we establish that within these ${\mathrm{Pr}}_{2-x}{\mathrm{Sr}}_x{\mathrm{MgIrO}}_6$ double perovskites, changes in Ir-oxidation states ($4+$ for PMIO to $5+$ for PSMIO via mixed $4+/5+$ for PSMIO1505) lead to markedly different magnetic behaviors. While SOC on Ir is at the root of the observed insulating behaviors for all three samples, the correlated magnetic properties of these three compounds develop entirely due to the contribution from local Ir moments. Additionally, the magnetic ${\mathrm{Pr}}^{3+}$ ($4f^2$) ions, instead of showing any kind of ordering, only contribute to the total paramagnetic moment. It is seen that the ${\mathrm{PrSrMgIrO}}_6$ sample does not order down to 2 K despite antiferromagnetic interactions. But, the $d^5$ iridate ${\mathrm{Pr}}_2{\mathrm{MgIrO}}_6$ shows a sharp antiferromagnetic (AFM) transition at around 14 K, and in the mixed valent ${\mathrm{Pr}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{MgIrO}}_6$ sample the AFM transition is shifted to a much lower temperature ($\sim 6$ K) due to weakening of the AFM exchange.

The crystal structure and magnetic properties of the spin-1 triangular-lattice antiferromagnet ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{NiTe}}_2{\mathrm{O}}_{12}$ are reported. Its crystal structure is trigonal $R\overline{3}$, which is the same as that of ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{NiW}}_2{\mathrm{O}}_{12}$ [Y. Doi et al., J. Phys.: Condens. Matter 29, 365802 (2017)]. However, the exchange interaction $J/k_{\mathrm{B}}\simeq 19$ K is much greater than that observed in the tungsten system. At zero magnetic field, ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{NiTe}}_2{\mathrm{O}}_{12}$ undergoes successive magnetic phase transitions at $T_{\mathrm{N}1}=9.8$ K and $T_{\mathrm{N}2}=8.9$ K. The ground state is accompanied by a weak ferromagnetic moment. These results indicate that the ground-state spin structure is a triangular structure in a plane perpendicular to the triangular lattice owing to the small easy-axis-type anisotropy. The magnetization curve exhibits the one-third plateau characteristic of a two-dimensional triangular-lattice Heisenberg-like antiferromagnet. Exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate the large difference in the exchange constants between tellurium and tungsten systems and the good two-dimensionality of the tellurium system.

Spin-singlet Cooper pairs consisting of two electrons with opposite spins cannot directly penetrate from a superconductor to a half-metal (fully spin polarized ferromagnets) which blocks the superconducting proximity effect between these materials. In this paper we demonstrate that, nevertheless, two half-metallic layers electrically coupled to the superconducting film substantially affect its critical temperature and produce the spin-valve effect. Within the tight-binding model for the atomically thin multilayered spin valves we show that depending on the details of the electron energy spectra in half-metals, the critical temperature as a function of the angle between the spin quantization axes in half-metals can be either monotonically increasing or decreasing. This finding highlights the crucial role of the band structure details in the proximity effect with half-metals which cannot be adequately treated in the quasiclassical theories.

Stoichiometric ${\mathrm{EuFBiS}}_2$ with mixed valent Eu shows self-doped superconductivity at low temperatures due to the Eu-to-Bi charge transfer. The metallic/nonmetallic bistability of the ${\mathrm{BiS}}_2$ layer can couple with the valence fluctuation of Eu and provide a highly susceptible electronic state. Here, we report space-resolved photoemission measurements on ${\mathrm{EuFBiS}}_2$ revealing an inhomogeneous charge distribution with the coexistence of metallic and nonmetallic phases at mesoscopic length scales. Angle-resolved photoemission measurements using a submicron beam size have confirmed a clear Fermi surface around the zone boundaries in the metallic region, typically observed in a doped system, while it can be hardly seen in the nonmetallic region. Density functional theory calculations suggest that the out-of-plane S atom position in the structure is one of the important factors for the self-doping in this material.

We expose the theoretical mechanisms underlying disorder-induced nematicity in systems exhibiting strong fluctuations or ordering in the nematic channel. Our analysis consists of a symmetry-based Ginzburg-Landau approach and associated microscopic calculations. We show that a single featureless pointlike impurity induces nematicity locally, already above the critical nematic transition temperature. The persistence of fourfold rotational symmetry constrains the resulting disorder-induced nematicity to be inhomogeneous and spatially average to zero. Going beyond the single impurity case, we discuss the effects of finite disorder concentrations on the appearance of nematicity. We identify the conditions that allow disorder to enhance the nematic transition temperature, and we provide a concrete example. The presented theoretical results can explain a large series of recent experimental discoveries of disorder-induced nematic order in iron-based superconductors.

The Hubbard model arises naturally when electron-electron interactions are added to the tight-binding descriptions of many condensed matter systems. For instance, the two-dimensional Hubbard model on the honeycomb lattice is central to the ab initio description of the electronic structure of carbon nanomaterials, such as graphene. Such low-dimensional Hubbard models are advantageously studied with Markov chain Monte Carlo methods, such as hybrid Monte Carlo (HMC). HMC is the standard algorithm of the lattice gauge theory community, as it is well suited to theories of dynamical fermions. As HMC performs continuous, global updates of the lattice degrees of freedom, it provides superior scaling with system size relative to local updating methods. A potential drawback of HMC is its susceptibility to ergodicity problems due to so-called exceptional configurations, for which the fermion operator cannot be inverted. Recently, ergodicity problems were found in some formulations of HMC simulations of the Hubbard model. Here, we address this issue directly and clarify under what conditions ergodicity is maintained or violated in HMC simulations of the Hubbard model. We study different lattice formulations of the fermion operator and provide explicit, representative calculations for small systems, often comparing to exact results. We show that a fermion operator can be found which is both computationally convenient and free of ergodicity problems.

CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported recently [P. M. Sarte et al., Phys. Rev. B 98, 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [R. A. Cowley et al., Phys. Rev. B 88, 205117 (2013)] results in significant mixing between $j{}_{\mathrm{eff}}$ spin-orbit levels in the low-temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at $\sim 300\mathrm{K}$, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on ${\mathrm{PrTl}}_3$ [W. J. L. Buyers et al., Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multilevel spin-orbit exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled ${\mathrm{Co}}^{2+}$ ions lying on a rocksalt lattice. This parametrization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low-energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high-energy transfers, where the neutron response decays faster with momentum than the ${\mathrm{Co}}^{2+}$ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high-energy transfers.

We measured the near-field infrared response of ${\mathrm{MnPS}}_3$ in bulk, few-, and single-layer form and compared our findings with traditional far-field vibrational spectroscopies, a symmetry analysis, and first-principles lattice dynamics calculations. Trends in the $B_u$ mode near $450{\mathrm{cm}}^{-1}$ are striking, with the disappearance of this structure in the thinnest sheets. Combined with the amplified response of the activated $A_g$ mode and analysis of the $A_u+B_u$ features, we find that symmetry is unexpectedly increased in few- and single-sheet ${\mathrm{MnPS}}_3$ due to a restoration of the threefold axes of rotation. Monoclinicity in this system is therefore a consequence of the long-range stacking pattern and temperature rather than local structure.

We devise a theory of adsorption of low-energy atoms on suspended graphene membranes maintained at low temperature based on a model of atom-acoustic phonon interactions. Our primary technique includes a nonperturbative method which treats the dynamics of the multiple phonons in an exact manner within the purview of the independent boson model. We present a study on the effects of the phonons assisting the renormalization as well as decay of the incident atom propagator and discuss results for the many-body adsorption rates for atomic hydrogen on graphene micromembranes. Additionally, we report similarities of this model with other branches of quantum field theories that include long-range interactions like quantum electrodynamics and perturbative gravity.

Our measurements of the Hall coefficient in rare-earth tritelluride compounds reveal a strong hysteresis between cooling and warming in the low-temperature range where a second unidirectional charge density wave (CDW) occurs. We show that this effect results from the interplay between two instabilities: band crossing of the Te $p_x$ and $p_y$ orbitals at the Fermi level and CDW, which have a close energy gain and compete. Calculation of the electron susceptibility at the CDW wave vector with and without band anticrossing reconstruction of the electron spectrum yields a satisfactory estimation of the temperature range of the hysteresis in Hall effect measurements.

Weyl materials exhibiting topologically nontrivial touching points in band dispersion pave the way to exotic transport phenomena and novel electronic devices. Here, we demonstrate the signature of an ideal type-II Weyl phase in phonon dispersion of solids through first-principles investigations. Type-II phononic Weyl phase is manifested in noncentrosymmetric wurtzite CuI by six pairs of Weyl points (WPs) in the $k_z=0.0$ plane. On the iodine-terminated surface of the crystal, very clean surface arcs are readily detectable. Each pair of WPs connected by open surface arcs is well separated by a large distance of $0.26{\AA }^{-1}$. The opposite chirality of WPs with quantized Berry curvature produces Weyl phonon Hall effect, in analogy to valley Hall effect of electrons. Such ideal type-II Weyl phase is readily observable in experiment, providing a unique platform to study novel thermal transport properties distinct from type-I Weyl phase.

We study chiral wave packets moving along the zero line of a symmetry-breaking potential of vertical electric field in buckled silicene using an atomistic tight-binding approach with initial conditions set by an analytical solution of the Dirac equation. We demonstrate that the wave packet moves with a constant untrembling velocity and with a preserved shape along the zero line. Backscattering by the edge of the crystal is observed that appears with the transition of the packet from $K$ to $K^{\prime }$ valley or vice versa. We propose a potential profile that splits the wave packet and next produces interference of the split parts that acts as a quantum ring. The transition time exhibits Aharonov-Bohm oscillations in the external magnetic field that are translated to conductance oscillations when the intervalley scattering is present within the ring. We study wave packet dynamics as function of the width of the packet up to the limit of plane waves. In the stationary transport limit the conductance oscillation period is doubled and the scattering density oscillates between the left and right arms of the ring as function of the magnetic field. We demonstrate that this effect is also found in a quantum ring defined by the zero lines of the symmetry-breaking potential in bilayer graphene.

We present all-dielectric polaritonic metasurfaces consisting of properly sculptured cylinders to sustain the dynamic anapole, i.e., a nonradiating alternating current distribution. One way for the anapole to emerge is by combining modes based on the first and fourth Mie resonances of a cylinder made of high permittivity ${\mathrm{LiTaO}}_3$ for operation in the low THz. The circular cross section of each cylinder varies periodically along its length in a binary way, from small to large, while its overall circular symmetry has to be broken in order to remove parasitic magnetic modes. Small cross sections are the main source of the electric dipole Mie mode, while large cross sections sustain the fourth, mixed toroidal dipole Mie mode. With proper adjustment, the generated electric and toroidal moments interfere destructively producing a nonradiating source, the anapole, the existence of which is attested by a sharp dip in the reflection from the metasurface, due exclusively to electric and toroidal dipoles. Moreover, we show that, by breaking the circular symmetry of each cylinder, a substantial toroidal dipole emerges from the magnetic quadrupole Mie mode, which in combination with the electric dipole also produces the anapole. The sensitivity of the anapole states to the material dissipation losses is examined leading to the conclusion that the proposed metasurfaces offer a scheme for realistically implementing the anapole.

We study the effects of disorder on the topological Chern insulating phase in the Harper-Hofstadter-Hatsugai (HHH) model. The model with half flux has a bulk band gap and, thus, exhibits a nontrivial topological phase. We consider two typical types of disorder: On-site random disorder and the Aubry-Andre-type quasiperiodic potential. Using the coupling matrix method, we clarify the global topological phase diagram in terms of the Chern number. The disorder modifies the gap closing behavior of the system. This modification induces the Chern insulating phase even in the trivial phase parameter regime of the system in the clean limit. Moreover, we consider an interacting Rice-Mele model with disorder, which can be obtained by dimensional reduction of the HHH model. Moderately strong disorder leads to an increase in revival events of the Chern insulator at a specific parameter point.

In the zero-field-cooled exchange bias (ZEB) effect, the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of an external magnetic field. ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ stands out as presenting the largest ZEB reported so far, while for ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoMnO}}_6$ the exchange bias field ($H_{\text{EB}}$) is one order of magnitude smaller. Here we show that ${\mathrm{La}}_{1.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoMnO}}_6$ also exhibits a pronounced shift of its magnetic hysteresis loop, with an intermediate $H_{\text{EB}}$ value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, these compounds were investigated by means of synchrotron x-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, ac and dc magnetization, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The parent compound ${\mathrm{La}}_2{\mathrm{CoMnO}}_6$ was also studied for comparison as a reference of a non-ZEB material. Our results show that the Ba-, Ca-, and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ and ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ are already present at the ${\mathrm{La}}_2{\mathrm{CoMnO}}_6$ parent compound, and that ${\mathrm{Ba}}^{2+}/{\mathrm{Ca}}^{2+}/{\mathrm{Sr}}^{2+}$ partial substitution at the ${\mathrm{La}}^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples ($0.05\le \delta \le 0.1$). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

The infinite projected entangled pair states (iPEPS) technique [J. Jordan et al., Phys. Rev. Lett. 101, 250602 (2008)] has been widely used in the recent years to assess the properties of two-dimensional quantum systems, working directly in the thermodynamic limit. This formalism, which is based upon a tensor-network representation of the ground-state wave function, has several appealing features, e.g., encoding the so-called area law of entanglement entropy by construction; still, the method presents critical issues when dealing with the optimization of tensors in order to find the best possible approximation to the exact ground state of a given Hamiltonian. Here, we discuss the obstacles that arise in the optimization by imaginary-time evolution within the so-called simple and full updates and connect them to the emergence of a sharp multiplet structure in the “virtual” indices of tensors. In this case, a generic choice of the bond dimension $D$ is not compatible with the multiplets and leads to a symmetry breaking (e.g., generating a finite magnetic order). In addition, varying the initial guess, different final states may be reached, with very large deviations in the magnetization value. In order to exemplify this behavior, we show the results of the $S=1/2$ Heisenberg model on an array of coupled ladders, for which a vanishing magnetization below the critical interladder coupling is recovered only for selected values of $D$, while a blind optimization with a generic $D$ gives rise to a finite magnetization down to the limit of decoupled ladders.

The recent emergence of two-dimensional (2D) van der Waals ferromagnets has provided a new platform for exploring magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices. Despite intensive studies, the anomalous Hall effect (AHE) mechanisms in 2D van der Waals ferromagnets have not been investigated yet. In this paper, we report the AHE mechanisms in quasi-2D van der Waals ferromagnet $\mathrm{F}{\mathrm{e}}_{0.29}\mathrm{Ta}{\mathrm{S}}_2$ via systematically measuring $\mathrm{F}{\mathrm{e}}_{0.29}\mathrm{Ta}{\mathrm{S}}_2$ devices with thickness from 14 nm to bulk single crystal. The AHE mechanisms are investigated via the scaling relationship between the anomalous Hall and channel conductivities. As the $\mathrm{F}{\mathrm{e}}_{0.29}\mathrm{Ta}{\mathrm{S}}_2$ thickness decreases, the major AHE mechanism changes from extrinsic scattering to intrinsic contribution. The crossover of the AHE mechanisms is found to be highly associated with the channel conductivities as the $\mathrm{F}{\mathrm{e}}_{0.29}\mathrm{Ta}{\mathrm{S}}_2$ thickness varies.