We substantiate a complete picture of the “bulk-edge correspondence” conjecture for topological phases. By studying the eigenstates in the entanglement spectrum for both the ideal and realistic Coulomb ground states of the fractional quantum Hall system, it is verified that the eigenstates in the universal part of the entanglement spectrum purely lie in the Hilbert space of the edge excitations projected onto the physical Hilbert space of the subsystem itself. Hence, not only are the eigenlevels in the entanglement spectrum in one-to-one correspondence with the eigenenergies of an effective dynamical edge Hamiltonian, but all the eigenstates are confirmed to be the actual (projected) edge excitations of the subsystem. This result also reveals the possibility of extracting the full information of the edge excitations from the state of the subsystem reduced from a geometric cut of the pure ground state of the total system in topological phases.

Most photovoltaic absorbers are identified using the standard Shockley-Queisser selection principle which relies on optimal band-gap values. However, this criterion has been shown to be insufficient, as many materials with appropriate values still perform badly. Here, we have employed calculations based on the density functional theory to assess three copper oxides as potential photovoltaic materials: ${\mathrm{Cu}}_2\mathrm{O}$, ${\mathrm{Cu}}_4{\mathrm{O}}_3$, and $\mathrm{CuO}$. Despite their promising theoretical solar power conversion efficiency of over 20%, experimental values are found to be far lower. Theoretical evaluation of the electronic and optical properties reveals that certain transitions within the band structures are dipole forbidden, whereas the fundamental band gaps of ${\mathrm{Cu}}_4{\mathrm{O}}_3$ and $\mathrm{CuO}$ are of indirect nature. These findings correlate to the weak and shifted absorption properties found experimentally, which underpin the inefficient light capture by copper oxides. Based on these results and an applied extended selection metric, we can explain why copper oxides are unable to reach the efficiencies previously proposed theoretically and why we need to revise their maximum conversion values.

The theoretical description of non-Fermi liquids is among the most challenging questions in strongly correlated quantum matter. While there are many experimental candidates for such phases, few examples can be studied using controlled theoretical approaches. Here, we introduce a conceptual perspective to analytically describe the non-Fermi liquid physics of gapless spin liquids arising in a family of two- and three-dimensional kagome models. We first discuss a symmetry-protection mechanism that is responsible for the stability of the gapless phase. We then recast the two-dimensional kagome spin model as a chiral Kondo lattice model that can be tackled analytically using what is in a sense a generalization of coupled-wire constructions. We provide extensive numerical evidence for a gapless spin liquid in the two-dimensional kagome model and discuss generalizations to three dimensions.

We present a quantum mechanical theory of optically induced dynamic nuclear polarization applicable to quantum dots and other interacting spin systems. The exact steady state of the optically driven coupled electron-nuclear system is calculated under the assumption of uniform hyperfine coupling strengths (box model) for an arbitrary number of nuclear spins. This steady state is given by a tractable expression that allows for an intuitive interpretation in terms of spin-flip rates. Based on this result, we investigate the nuclear spin behavior for different experimental parameter regimes and find that our model reproduces the flat-top and triangular absorption line shapes seen in various experiments (line dragging) under the assumption of fast electron spin dephasing due to phonons and cotunneling. The mechanism responsible for line dragging has been a matter of controversy so far; in contrast with previous works, we show that the effect can be explained solely in terms of the contact hyperfine interaction, without the need to introduce noncollinear terms or other types of electron-nuclear interactions. Furthermore we predict a previously unknown nuclear spin polarization effect: under particular, experimentally realistic conditions, the nuclear spin system tends to become sharply polarized in such a way as to cancel the effect of the external magnetic field. This effect can therefore suppress electron spin dephasing due to inhomogeneous broadening, which could have important repercussions for quantum technological applications.

We propose a protocol to identify spatial hallmarks of viscous electron flow in graphene and other two-dimensional viscous electron fluids. We predict that the profile of the magnetic field generated by hydrodynamic electron currents flowing in confined geometries displays unambiguous features linked to whirlpools and backflow near current injectors. We also show that the same profile sheds light on the nature of the boundary conditions describing friction exerted on the electron fluid by the edges of the sample. Our predictions are within reach of vector magnetometry based on nitrogen-vacancy centers embedded in a diamond slab mounted onto a graphene layer.

We reveal that the phenomenon of full plane-wave transmission without phase accumulation commonly associated with epsilon-near-zero (ENZ) materials does not require vanishing of permittivity. We theoretically connect the phenomenon with the condition of the Brewster effect satisfied at the edges of stop bands (at so-called exceptional points of degeneracy) and show that full transmission without phase accumulation can be observed in various one-dimensional periodic structures. Particularly, exploiting the manifold of exceptional points of degeneracy in one-dimensional all-dielectric periodic lattices, we demonstrate that these structures not only offer a lossless and extremely simple, complementary metal-oxide semiconductor compatible alternative for some applications of ENZ media but exhibit new properties of all-angle full transmission with zero phase delay. The main results of the study can be readily transferred to acoustic and matter waves.

Moore-Read states can be expressed as conformal blocks of the underlying rational conformal field theory, which provides a well explored description for the insertion of quasiholes. It is known, however, that quasielectrons are more difficult to describe in continuous systems, since the natural guess for how to construct them leads to a singularity. In this work, we show that the singularity problem does not arise for lattice Moore-Read states. This allows us to construct Moore-Read Pfaffian states on lattices for filling fraction $5/2$ with both quasiholes and quasielectrons in a simple way. We investigate the density profile, charge, size, and braiding properties of the anyons by means of Monte Carlo simulations. Further we derive an exact few-body parent Hamiltonian for the states. Finally, we compare our results to the density profile, charge, and shape of anyons in the Kapit-Mueller model by means of exact diagonalization.

We investigate equilibrium and transport properties of a copper phthalocyanine (CuPc) molecule adsorbed on Au(111) and Ag(111) surfaces. The CuPc molecule has essentially three localized orbitals close to the Fermi energy resulting in strong local Coulomb repulsion not accounted for properly in density functional calculations. Hence, they require a proper many-body treatment within, e.g., the Anderson impurity model (AIM). The occupancy of these orbitals varies with the substrate on which CuPc is adsorbed. Starting from density functional theory calculations, we determine the parameters for the AIM embedded in a noninteracting environment that describes the residual orbitals of the entire system. While correlation effects in CuPc on Au(111) are already properly described by a single orbital AIM, for CuPc on Ag(111) the three orbital AIM problem can be simplified into a two orbital problem coupled to the localized spin of the third orbital. This results in a Kondo effect with a mixed character, displaying a symmetry between SU(2) and SU(4). The computed Kondo temperature is in good agreement with experimental values. To solve the impurity problem we use the recently developed fork tensor product state solver. To obtain transport properties, a scanning tunneling microscope (STM) tip is added to the CuPc molecule absorbed on the surface. We find that the transmission depends on the detailed position of the STM tip above the CuPc molecule in good agreement with differential conductance measurements.

${\mathrm{SmB}}_6$, known to be a Kondo insulator, has received intense scrutiny in recent years due to its paradoxical experimental signatures: While some quantities show an insulating behavior, others point to a metallic state. This has led to the conjecture that ${\mathrm{SmB}}_6$ hosts nontrivial excitations within its bulk gap, and has spawned several theories to that effect. In principle, there exists an alternative possibility: The system is a metal but unusually with both metal- and insulatorlike properties. Inspired by this possibility, I consider a minimal model of a Kondo insulator—a flat band hybridized with a parabolic band—that is slightly electron doped, i.e., the chemical potential is in the conduction band but close to the band edge. I show that, at the phenomenological level, the dc conductivity, ac conductivity, specific heat, and quantum oscillations within this model exhibit unusual behaviors that are, surprisingly, qualitatively consistent with those observed experimentally in ${\mathrm{SmB}}_6$. The rapid change of band curvature around the chemical potential arising from the strong particle-hole asymmetry and the narrow gap in the model, a feature not usually encountered in the textbook cases of metals or insulators, is at the heart of the unusual behaviors.

Electron-phonon coupling is at the core of various regimes of material-based science and technology. Taking $3C$-silicon carbide ($3C$-SiC) as an example, despite its very wide application in high-temperature and high-power devices, the transport properties of $3C$-SiC are not yet fully understood at the microscopic level because of inadequate knowledge in electron-phonon coupling. In this paper, with electron-phonon coupling matrix elements calculated from first principles, the phonon limited carrier mobility of $3C$-SiC is quantified by solving the Boltzmann transport equation. The calculated mobilities for both holes and electrons are in reasonable agreement with the experimental data. Unlike other polar semiconductors such as GaAs, where the polar-longitudinal-optical (LO)-phonon interactions are the dominant scattering mechanism, the mobilities of electrons and holes are dominated by the intravalley longitudinal acoustic phonon scattering at 300 K due to the low occupation number of high-frequency polar LO phonons in $3C$-SiC. The dominant scattering mechanism in $3C$-SiC varies with temperature. At high temperature (800 K), LO phonons govern the scattering instead. The maximum mean-free paths of electrons and holes at room temperature are found to be 40 nm and 15 nm, respectively.

Using the Landau-Ginzburg-Devonshire approach, we predict the intrinsic instability of the ferroelectric-ferroelastic domain walls in the multiferroic $\mathrm{BiFe}{\mathrm{O}}_3$ emerging from the interplay between the gradient terms of the antiferrodistortive and ferroelectric order parameters at the walls. These instabilities are the interface analog of the structural instabilities in the vicinity of phase coexistence in the bulk, and so they do not stem from incomplete polarization screening in thin films or its spatial confinement, electrostrictive or flexoelectric coupling. The effect of $\mathrm{BiFe}{\mathrm{O}}_3$ material parameters on the ${71}^{\circ }$, ${109}^{\circ }$, and ${180}^{\circ }$ walls is explored, and it is shown that the meandering instability appears at ${109}^{\circ }$ and ${180}^{\circ }$ walls for small gradient energies, and the walls become straight and broaden for higher gradients. In contrast to the ${180}^{\circ }$ and ${109}^{\circ }$ domain walls, uncharged ${71}^{\circ }$ walls are always straight, and their width increases with increasing the tilt gradient coefficient. The wall instability and associated intrinsic meandering provide insight into the behavior of morphotropic and relaxor materials, wall pinning, and mechanisms of interactions between order parameter fields and local microstructure.

Recently, a semiconductor-to-metal transition was surprisingly observed in rutile ${\mathrm{TiO}}_2$ by applying just 5% tensile strain. To explore the mechanism behind this giant strain effect, we performed first-principles calculations focusing on the commonly existing oxygen vacancies (OVs) in rutile ${\mathrm{TiO}}_2$. We find that 5% biaxial tensile strain largely reduces the formation energies of OVs and biaxial compressive strain increases the formation energies of OVs. While our findings are in agreement with experiments, the giant strain effects on OV defect formation energies cannot be well explained by, or may even contradict, the common continuum elastic model. Our further studies show that strain not only induces elastic energy gain during defect formation, but also changes the polaronic configurations, which can have either energy gain or loss depending on the occupations of the excess electrons. The large reduction of OV formation energy under tensile strain is thus a combined effect of both elastic and polaronic energy gain. This giant strain effect, enhanced by polaronic effects on defect formation, might provide an alternative method for the manipulation of defects and electric conductivity in rutile ${\mathrm{TiO}}_2$ and other semiconducting materials.

We study the strain dependence of the magnetic anisotropy of room-temperature ferromagnetic semiconductor $(\mathrm{G}{\mathrm{a}}_{1-x},\mathrm{F}{\mathrm{e}}_x)\mathrm{Sb}$ ($x=20\%$) thin films epitaxially grown on different buffer layers, using ferromagnetic resonance measurements. We show that the magnetocrystalline anisotropy $\left(K_{\mathrm{i}}\right)$ in $(\mathrm{G}{\mathrm{a}}_{0.8},\mathrm{F}{\mathrm{e}}_{0.2})\mathrm{Sb}$ exhibits a dependence on the epitaxial strain and changes its sign from negative (in-plane magnetization easy axis) to positive (perpendicular magnetization easy axis), when the strain is changed from tensile to compressive. Meanwhile, the shape anisotropy ($K_{\mathrm{sh}}$) is negative and dominant over $K_{\mathrm{i}}$. Therefore, the effective magnetic anisotropy $(K_{\mathrm{eff}}=K_{\mathrm{i}}+K_{\mathrm{sh}})$ is always negative, leading to the in-plane magnetic anisotropy in all the $(\mathrm{G}{\mathrm{a}}_{0.8},\mathrm{F}{\mathrm{e}}_{0.2})\mathrm{Sb}$ samples. This work demonstrates ferromagnetic resonance and strong shape anisotropy at room temperature in III-V ferromagnetic semiconductors. We also observed very high Curie temperature $(T_{\mathrm{C}}\gtrsim 400\mathrm{K})$ in $p$-type (Ga,Fe)Sb, which is the highest $T_{\mathrm{C}}$ reported so far in III-V based ferromagnetic semiconductors.

A study of ferromagnetic spin wave modes in multilayered films under general conditions is presented within a micromagnetic approximation. These modes, either in the magnetostatic or dipole-exchange approximation, have been studied in the past theoretically and experimentally by several authors. The novelty of the present study is that using a method based on the extinction-Green theorems it is possible to determine these modes with ease, under an arbitrary direction of an applied dc magnetic field, and for boundary conditions of diverse nature. The ferromagnetic films are coupled through dipolar interactions and eventually through exchange interactions. Due to an assumed in-plane translation invariance of the multilayers, the method allows us to determine the eigenfrequencies at a given in-plane wave vector as an algebraic eigenvalue problem of reasonable size: it is a $6N\times 6N$ system of homogeneous equations if there are $N$ distinct films in the structure. Examples of modes in different multilayers are presented. If the stack of films is symmetric with respect to a central plane, it is shown that there are modes reciprocal in frequency but nonreciprocal in shape with respect to a change of sign of the wave vector. The latter is a generalization of the well-known shape nonreciprocity of Damon-Eshbach surface modes moving in opposite directions, and is valid for all dipole-exchange modes and inclined applied magnetic fields. The possibility of obtaining dispersion relations for obliquely applied magnetic fields should be of interest in order to test experimentally boundary conditions that model physical mechanisms operating at the different surfaces and interfaces of the films.

Heavy metal/ferromagnetic superlattices are of great interest since Néel skyrmions may be stabilized in them due to an interfacial Dzyaloshinskii-Moriya interaction even at room temperature. Here we report on evidence of the strong Dzyaloshinskii-Moriya interaction in symmetric epitaxial ${[\mathrm{Co}/\mathrm{Pd}\left(111\right)]}_N$ superlattices with high perpendicular magnetic anisotropy. The structural and magnetic properties of epitaxial ${[\mathrm{Co}/\mathrm{Pd}\left(111\right)]}_N$ superlattices were investigated dependent on the number of Co/Pd bilayers. Based on comparison of the periodicity of experimentally obtained demagnetized patterns and simulated magnetic structures, a strong increase of the effective Dzyaloshinskii-Moriya interaction constant with increase of the period $N$ of ${[\mathrm{Co}/\mathrm{Pd}\left(111\right)]}_N$ superlattices is established. The origin of Dzyaloshinskii-Moriya interaction and its behavior in the symmetric ${[\mathrm{Co}/\mathrm{Pd}\left(111\right)]}_N$ system is discussed.

Magnetic skyrmions are envisioned as carriers of information in racetrack storage devices. Unfavorably, the skyrmion Hall effect hinders the fast propagation of skyrmions along an applied electric current and limits the device's maximum operation speed. In this Rapid Communication, we show that the maximum skyrmion velocity increases by a factor of 10 when the skyrmion Hall effect is suppressed, since the straight-line motion of the skyrmion allows for the application of larger driving currents. We consider a ferromagnet on a heavy-metal layer, which converts the applied charge current into a spin current by the spin Hall effect. The spin current drives the skyrmions in the ferromagnet via spin-orbit torque. We show by analytical considerations and simulations that the deflection angle decreases, when the spin current is polarized partially along the applied current direction, and derive the condition for complete suppression of the skyrmion Hall effect.

Superconductivity in topological materials, either at ambient or extreme conditions, has continued to intrigue scientists as a promising candidate for realizing topological superconductivity, a platform to host the long-sought Majorana fermions in condensed matter. The recent discovery of extremely large magnetoresistance (XMR) in the rare-earth monopnictides opens a new avenue to search for topologically nontrivial states therein, although contrasting opinions argue that it is the carrier compensation effect that is responsible for the observed large, nonsaturating magnetoresistance. Here we study the quantum oscillations and pressure-induced superconductivity in the topologically nontrivial candidate YBi. While the magnetotransport and quantum oscillations do reveal nearly compensated charge carriers, first-principles calculations clearly show that the electronic surface states manifest topologically nontrivial features. Upon applying external hydrostatic pressures, the magnetoresistance is found to decrease and at $P\sim 2.5$ GPa, superconductivity emerges. There exists, however, a regime where XMR and superconductivity coexist in the phase diagram. YBi may therefore represent a rare system for studying the interplay between XMR, topological states, and superconductivity.

In this work, the temporal aspects of two nonlinear spin-wave processes, i.e., three-magnon splitting and confluence, were investigated in a yttrium iron garnet film. Three-magnon splitting involves the conversion of a magnon at the microwave pumping frequency $f_p$ into two magnons, each with a frequency near $f_p/2$, while confluence refers to the subsequent combination of two of the split magnons into one with frequency $f_c<f_p$. Time-resolved Brillouin scattering measurements confirm that these processes occur close to the driving antenna ($<1.7\mathrm{mm}$ away), which is expected. The data indicate, however, that magnons with larger group velocities are more likely to undergo confluence, which is not predicted by existing theory. Understanding the details of the splitting and confluence processes may have important implications for the use of short-wavelength spin waves for spintronic devices.

We propose an approach for low-dimensional visualization and classification of complex topological magnetic structures formed in magnetic materials. Within the approach one converts a three-dimensional magnetic configuration to a vector containing the only components of the spins that are parallel to the $z$ axis. The next crucial step is to sort the vector elements in ascending or descending order. Having visualized profiles of the sorted spin vectors one can distinguish configurations belonging to different phases even if such configurations have the same total magnetization. For instance, spin spiral and paramagnetic states with zero total magnetic moment can be easily distinguished. Being combined with a simplest neural network our profile approach provides a very accurate phase classification for three-dimensional magnets characterized by complex multispiral states even in the critical areas close to phases' transitions. By the example of the skyrmionic configurations we show that the profile approach can be used to separate the states belonging to the same phase.

We present a study of the magnetic properties of ${\left[\mathrm{Co}\left(3.0\mathrm{nm}\right)/\mathrm{Pt}\left(0.6\mathrm{nm}\right)\right]}_N$ multilayers as a function of Co/Pt bilayer repetitions N. Magnetometry investigation reveals that samples with $N\ge 15$ exhibit two characteristic magnetization reversal mechanisms, giving rise to two different morphologies of the remanent domain pattern. For applied magnetic field angles near the in-plane field orientation, the magnetization reversal proceeds via a spontaneous instability of the uniform magnetic state resulting in perpendicular stripe domains. Conversely, for field angles close to the out-of-plane orientation, the reversal occurs via domain nucleation and propagation leading to a mazelike domain pattern at remanence. Our measurements further enable the characterization of the N-dependent energy balance between the magnetic anisotropy and magnetostatic energy contributions, revealing a gradual disappearance of the domain nucleation process during magnetization reversal for N < 14. This leads to the exclusive occurrence of an instability reversal mechanism for all field orientations as well as alignedlike stripe domains at remanence. Furthermore, a detailed study of the influence of the magnetic history allows the determination of a range of material properties and magnetic field strengths, where a lattice of bubble domains with remarkably high density is stabilized. These modulations of the ferromagnetic order parameter are found to strongly depend on N, in terms of center-to-center bubble distance as well as of bubble diameter. Moreover, such Co/Pt multilayers could be utilized to engineer field reconfigurable bubble domain lattices, which resemble magnonic crystals.