${\mathrm{SrRuO}}_3$ films have large uniaxial magnetocrystalline anisotropy tilted out of the film plane. When cooled in zero field from above the Curie temperature ($\sim 150$ K), a magnetic domain structure emerges in the form of 200-nm-wide stripes oriented along the in-plane projection of the magnetic easy axis. We measure the interface resistance of the magnetic domain walls by applying a magnetic field perpendicular to the easy axis in the domain-wall plane and perpendicular to this plane and observe hysteretic behavior. Micromagnetic simulations of the results indicate that, depending on the direction in which the field is applied, the domain-wall type (Bloch or Néel) and chirality can be tuned.

In quantum spin systems, singlet phases often develop in the vicinity of an antiferromagnetic order. Typical settings for such problems arise when itinerant fermions are also present. In this paper, we develop a theoretical framework for addressing such competing orders in an itinerant system, described by Dirac fermions strongly coupled to an O(3) nonlinear sigma model. We focus on two spatial dimensions, where upon disordering the antiferromagnetic order by quantum fluctuations the singular tunneling events also known as (anti)hedgehogs can nucleate competing singlet orders in the paramagnetic phase. In the presence of an isolated hedgehog configuration of the nonlinear sigma model field, we show that the fermion determinant vanishes as the dynamic Euclidean Dirac operator supports fermion zero modes of definite chirality. This provides a topological mechanism for suppressing the tunneling events. Using the methodology of quantum chromodynamics, we evaluate the fermion determinant in the close proximity of magnetic quantum phase transition, when the antiferromagnetic order-parameter field can be described by a dilute gas of hedgehogs and antihedgehogs. We show how the precise nature of emergent singlet order is determined by the overlap between dynamic fermion zero modes of opposite chirality, localized on the hedgehogs and antihedgehogs. For a Kondo-Heisenberg model on the honeycomb lattice, we demonstrate the competition between spin Peierls order and Kondo singlet formation, thereby elucidating its global phase diagram. We also discuss other physical problems that can be addressed within this general framework.

The treatment of ultrafast demagnetization after femtosecond laser-pulse excitation of a ferromagnetic film is usually done by a theory based on Fermi's golden rule which neglects the effects of electronic correlations. In the present paper the contribution of spin-flip electron-phonon scatterings to the ultrafast demagnetization of Ni is calculated by this theory and by the density-matrix theory in which the correlations are taken into account. The unexpected result is that the correlations which are essential for the ultrafast dynamics of nonmagnetic phenomena have only a marginal effect for the considered magnetic problem. From this point of view the use of Fermi's golden rule in all former papers on ultrafast demagnetization is justified.

The ${\mathrm{GdFeO}}_3$-type perovskite ${\mathrm{CaCoO}}_3$ has been successfully synthesized by high-pressure oxygen annealing for the oxygen deficient perovskite. A detailed structural analysis based on synchrotron x-ray diffraction data and a thermogravimetric analysis show that the valence of Co is +4 and the sample is free from oxygen deficiency. This compound shows an antiferromagnetic ordering at 95 K, which has presumably helical spin arrangement, with keeping the incoherent metallic state down to the lowest temperature. This work demonstrates that the ${\mathrm{Co}}^{4+}$-perovskite oxides exhibit a variety of magnetic phases by the bandwidth control through the lattice distortion.

We investigate equilibrium statistical properties of the isotropic quantum XY spin-1/2 model in an external magnetic field when the interaction and field parts are subjected to quenched or annealed disorder or both. The randomness present in the system are termed annealed or quenched depending on the relation between two different time scales—the time scale associated with the equilibration of the randomness and the time of observation. Within a mean-field framework, we study the effects of disorders on spontaneous magnetization, both by perturbative and numerical techniques. Our primary interest is to understand the differences between quenched and annealed cases, and also to investigate the interplay when both of them are present in a system. We find that the magnetization survives in the presence of a unidirectional random field, irrespective of its nature, i.e., whether it is quenched or annealed. However, the field breaks the circular symmetry of the magnetization, and the system magnetizes in specific directions, parallel or transverse to the applied magnetic field. Interestingly, while the transverse magnetization is affected by the annealed disordered field, the parallel one remains unfazed by the same. Moreover, the annealed disorder present in the interaction term does not affect the system's spontaneous magnetization and the corresponding critical temperature, irrespective of the presence or absence of quenched or annealed disorder in the field term. We carry out a comparative study of these and all other different combinations of the disorders in the interaction and field terms, and point out their generic features.

Photoinduced phase transitions in complex correlated systems occur very rapidly and involve the interplay between various electronic and lattice degrees of freedom. For these materials to be considered for practical applications, it is important to discover how their phase transitions take place. Here we use a novel ultrafast diffraction conoscopy technique to study the evolution of vanadium dioxide (${\mathrm{VO}}_2$) from biaxial to uniaxial symmetry. A key finding in this study is an additional relaxation process through which the phase transition takes place. Our results show that the biaxial monoclinic crystal initially, within the first 100–300 fs, transforms to a transient biaxial crystal, and within the next 300–400 fs converts into a uniaxial rutile crystal. The characteristic times for these transitions depend on film morphology and are presumably altered by misfit strain. We take advantage of Landau phenomenology to describe the complex dynamics of ${\mathrm{VO}}_2$ phase transition in the femtosecond regime.

In this work, we construct a generalized Kane model with a coupling term between itinerant electron spins and local magnetic moments of antiferromagnetic ordering in order to describe the low-energy effective physics in a large family of antiferromagnetic half-Heusler materials. The topological properties of this generalized Kane model are studied and a large variety of topological phases, including the Dirac semimetal phase, Weyl semimetal phase, nodal line semimetal phase, type-B triple point semimetal phase, topological mirror (or glide) insulating phase, and antiferromagnetic topological insulating phase, are identified in different parameter regions of our effective models. In particular, we find that the system is always driven into the antiferromagnetic topological insulator phase once a bulk band gap is open, irrespective of the magnetic moment direction, thus providing a robust realization of antiferromagentic topological insulators. Furthermore, we discuss the possible realization of these topological phases in realistic antiferromagnetic half-Heusler materials. Our effective model provides a basis for the future study of physical phenomena in this class of materials.

A topological quantum phase requires a finite momentum-space Berry curvature which, conventionally, can arise through breaking the inversion or the time-reversal symmetry so as to generate nontrivial, topologically invariant quantities associated with the underlying energy band structure (e.g., a finite Chern number). For conventional graphene or graphenelike two-dimensional (2D) systems with gapless Dirac cones, the symmetry breaking will make the system insulating due to lifting of the degeneracy. To design materials that simultaneously possess the two seemingly contradicting properties (i.e., a semimetal phase with gapless bulk Dirac-like cones and a finite Berry curvature) is of interest. We propose a 2D mechanical dice lattice system that exhibits precisely such properties. As a result, an intrinsic valley Hall effect can arise without compromising the carrier mobility as the quasiparticles remain massless. We also find that, with confinement along the zigzag edges, two distinct types of gapless edge states with opposite edge polarizations can arise, one with a finite but the other with zero group velocity.

We report a dynamical cluster approximation investigation of the doped periodic Anderson model to explain the universal scaling in the Knight-shift anomaly predicted by the phenomenological two-fluid model and confirmed in many heavy-fermion compounds. We calculate the quantitative evolution of the orbital-dependent magnetic susceptibility and reproduce correctly the two-fluid prediction in a large range of doping and hybridization. Our results confirm the presence of a temperature/energy scale $T^{*}$ for the universal scaling and show distinctive behaviors of the Knight-shift anomaly in response to other “orders” at low temperatures. However, comparison with the temperature evolution of the calculated resistivity and quasiparticle spectral peak indicates a different characteristic temperature from $T^{*}$, in contradiction with the experimental observation in ${\mathrm{CeCoIn}}_5$ and other compounds. This reveals a missing piece in the current model calculations in explaining the two-fluid phenomenology.

Recent theoretical predictions of “unprecedented proximity” of the electronic ground state of iridium fluorides to the SU(2) symmetric $j_{\mathrm{eff}}=1/2$ limit, relevant for superconductivity in iridates, motivated us to investigate their crystal and electronic structure. To this aim, we performed high-resolution x-ray powder diffraction, Ir ${\mathrm{L}}_3$-edge resonant inelastic x-ray scattering, and quantum chemical calculations on ${\mathrm{Rb}}_2\left[{\mathrm{IrF}}_6\right]$ and other iridium fluorides. Our results are consistent with the Mott insulating scenario predicted by Birol and Haule [Phys. Rev. Lett. 114, 096403 (2015)], but we observe a sizable deviation of the $j_{\mathrm{eff}}=1/2$ state from the SU(2) symmetric limit. Interactions beyond the first coordination shell of iridium are negligible, hence the iridium fluorides do not show any magnetic ordering down to at least 20 K. A larger spin-orbit coupling in iridium fluorides compared to oxides is ascribed to a reduction of the degree of covalency, with consequences on the possibility to realize spin-orbit-induced strongly correlated physics in iridium fluorides.

Using Floquet-Bloch theory we study the effect of circularly and linearly polarized light on the electronic structure of ${\left({\mathrm{LaNiO}}_3\right)}_2/{\left({\mathrm{LaAlO}}_3\right)}_N$ heterostructure grown along the (111) direction. In equilibrium, a tight-binding fit to the first-principles band structure shows that nearest-neighbor hopping plays a dominant role while second-neighbor hopping breaks the particle-hole symmetry and determines the finer band features. The four bands of the ${\mathrm{LaNiO}}_3$ bilayer exhibit both quadratic band touching points and Dirac points. By varying the amplitude of the incident light, one can independently tune the first- and second-neighbor hopping for fixed frequency, which leads to considerable control over the Floquet band structure. We investigate this control in detail, and study how the quadratic and Dirac band touchings are influenced by the polarization and intensity of the light. We derive effective two-band Hamiltonians (for both quadratic and Dirac band touching points) that accurately capture these results. We further study an extended model, which explicitly includes oxygen $p$ orbitals and compare the results to the effective model that contains only the nickel $d$ orbitals. We conclude with a computation of the frequency dependent optical Hall conductivity using the full four-band model and analyze the various interband contributions of the Floquet modes.

Defects on surfaces of semiconductors have a strong effect on their reactivity and catalytic properties. The concentration of different charge states of defects is determined by their formation energies. First-principles calculations are an important tool for computing defect formation energies and for studying the microscopic environment of the defect. The main problem associated with the widely used supercell method in these calculations is the error in the electrostatic energy, which is especially pronounced in calculations that involve surface slabs and two-dimensional materials. We present an internally consistent approach for calculating defect formation energies in inhomogeneous and anisotropic dielectric environments and demonstrate its applicability to the cases of the positively charged Cl vacancy on the NaCl (100) surface and the negatively charged S vacancy in monolayer ${\mathrm{MoS}}_2$.

We explore the electronic ground states of Bernal-stacked multilayer graphenes using the Hartree-Fock mean-field approximation and the full-parameter band model. We find that the electron-electron interaction tends to open a band gap in multilayer graphenes from bilayer to eight-layer, while the nature of the insulating ground state sensitively depends on the band parameter ${\gamma }_2$, which is responsible for the semimetallic nature of graphite. In four-layer graphene, particularly, the ground state assumes an odd-spatial-parity staggered phase at ${\gamma }_2=0$, while an increasing, finite value of ${\gamma }_2$ stabilizes a different state with even parity, where the electrons are attracted to the top layer and the bottom layer. The two phases are topologically distinct insulating states with different Chern numbers, and they can be distinguished by spin or valley Hall conductivity measurements. Multilayers with more than five layers also exhibit similar ground states with potential minima at the outermost layers, although the opening of a gap in the spectrum as a whole is generally more difficult than in four-layer because of a larger number of energy bands overlapping at the Fermi energy.

Edge states in topological insulators (TIs) disperse symmetrically about one of the time-reversal invariant momenta $\mathrm{\Lambda }$ in the Brillouin zone (BZ) with protected degeneracies at $\mathrm{\Lambda }$. Commonly TIs are distinguished from trivial insulators by the values of one or multiple topological invariants that require an analysis of the bulk band structure across the BZ. We propose an effective two-band Hamiltonian for the electronic states in graphene based on a Taylor expansion of the tight-binding Hamiltonian about the time-reversal invariant $M$ point at the edge of the BZ. This Hamiltonian provides a faithful description of the protected edge states for both zigzag and armchair ribbons, though the concept of a BZ is not part of such an effective model. We show that the edge states are determined by a band inversion in both reciprocal and real space, which allows one to select $\mathrm{\Lambda }$ for the edge states without affecting the bulk spectrum.

We combine analytic developments and numerical tight-binding calculations to study the evolution of the electron $g$-factors in homogeneous nanostructures of III-V and II-VI semiconductors. We demonstrate that the $g$-factor can be always written as a sum of bulk and surface terms. The bulk term, the dominant one, just depends on the energy gap of the nanostructure but is otherwise isotropic and independent of size, shape, and dimensionality. At the same time, the magnetic moment density at the origin of the bulk term is anisotropic and strongly dependents on the nanostructure shape. The physical origin of these seemingly contradictory findings is explained by the relation between the spin-orbit-induced currents and the spatial derivatives of the electron envelope wave function. The tight-binding calculations show that the $g$-factor versus energy gap for spherical nanocrystals can be used as a reference curve. In quantum wells, nanoplatelets, nanorods, and nanowires, the $g$-factor along the rotational symmetry axis can be predicted from the reference curve with a good accuracy. The $g$-factors along nonsymmetric axes exhibit more important deviations due to surface contributions but the energy gap remains the main quantity determining their evolution. The importance of surface-induced anisotropies of the $g$-factors is discussed.

Magnons can generate a spin current, and the standard generating mechanism requires at least one external field. Since this mechanism is often applied to a multilayer system including a magnet and a paramagnetic metal, the system can possess not only the charge current induced by the spin current but also the charge current induced by the external field. The latter is an unnecessary accompaniment. Here we show that the tunneling of a magnon pair between chiral magnets can generate a spin current even without external fields. This phenomenon originates from a phase difference between magnon pairs of separate, weakly coupled chiral magnets, and is essentially different from the mechanism using the angle degree of freedom of the magnon Bose-Einstein condensates. The pair's tunneling is possible in chiral magnets due to lack of the Goldstone type gapless excitations. This phenomenon opens the door to spintronics not requiring any external field and using the magnon pair tunneling.

We investigate the optical response of strongly disordered quantum Hall states in two-dimensional Dirac materials and find qualitatively different effects in the radiation properties of the bulk versus the edge. We show that the far-field radiation from the edge is characterized by large multipole moments $(>50)$ due to the efficient transfer of angular momentum from the electrons into the scattered light. The maximum multipole transition moment is a direct measure of the coherence length of the edge states. Accessing these multipole transitions would provide new tools for optical spectroscopy and control of quantum Hall edge states. On the other hand, the far-field radiation from the bulk appears as random dipole emission with spectral properties that vary with the local disorder potential. We determine the conditions under which this bulk radiation can be used to image the disorder landscape. Such optical measurements can probe submicron-length scales over large areas and provide complementary information to scanning probe techniques. Spatially resolving this bulk radiation would serve as a novel probe of the percolation transition near half filling.

A metallic spin filter is observed at the interface between $\mathrm{Al}{\mathrm{q}}_3$ adsorbates and a Cr(001) surface. It can be changed to a resistive (i.e., gapped) filter by substituting Cr ions to make $\mathrm{Cr}{\mathrm{q}}_3$ adsorbates. Spin-polarized scanning tunneling microscopy and spectroscopy show these spin-dependent electronic structure changes with single molecule resolution. Density functional theory calculations highlight the structural and electronic differences at the interfaces. For $\mathrm{Al}{\mathrm{q}}_3$, a charge-transfer interaction with the substrate leads to a metallic spin filter. For $\mathrm{Cr}{\mathrm{q}}_3$, direct covalent interactions mix molecular orbitals with the substrate surface state to make two well-separated interfacial hybrid orbitals.

Dirac boundary states on opposite boundaries can overlap and interact owing to finite size effect. We propose that in a thin film system with symmetry-unrelated valleys, valley-contrasting couplings between Dirac boundary states can be exploited to design various two-dimensional topological quantum phases. Our first-principles calculations demonstrate the mechanism in tin telluride slab and nanoribbon array, respectively, by top-down and bottom-up material designs. Both two-dimensional topological crystalline insulator and quantum spin Hall insulator emerge in the same material system, which offers highly tunable quantum transport of edge channels with a set of quantized conductances.

Electronic pseudospin degrees of freedom in two-dimensional materials exhibit unique carrier-field interactions which allow for advanced control strategies. Here, we investigate ultrafast sublattice pseudospin relaxation in graphene by means of polarization-resolved photoluminescence spectroscopy. A comparison with microscopic Boltzmann simulations allows us to determine a lifetime of the optically aligned pseudospin distribution of $12\pm 2\text{fs}$. This experimental approach extends the toolbox of graphene pseudospintronics, providing additional means to investigate pseudospin dynamics in active devices or under external fields.