We report an Fe $K$-edge resonant inelastic x-ray scattering study of ${\mathrm{K}}_{0.83}{\mathrm{Fe}}_{1.53}{\mathrm{Se}}_2$. This material is an insulator, unlike many parent compounds of iron-based superconductors. We found a sharp excitation around 1 eV, which is resonantly enhanced when the incident photon energy is tuned near the pre-edge region of the absorption spectrum. The spectral weight and lineshape of this excitation exhibit clear momentum dependence. In addition, we observe momentum-independent broad interband transitions at higher excitation energies of 3–7 eV. Calculations based on a 70-band $dp$ orbital model, using a moderate $U_{\mathrm{eff}}\approx 2.5\mathrm{eV}$, indicate that the $\sim 1\mathrm{eV}$ feature originates from the correlated Fe $3d$ electrons, with a dominant $d_{xz}$ and $d_{yz}$ orbital character. We find that a moderate $U_{\mathrm{eff}}$ yields a satisfying agreement with the experimental spectra, suggesting that the electron correlations in the insulating and metallic iron-based superconductors are comparable.

We study the phase diagram of the frustrated Heisenberg model on the triangular lattice with nearest- and next-nearest-neighbor spin-exchange coupling, on three-leg ladders. Using the density-matrix renormalization-group method, we obtain the complete phase diagram of the model, which includes quasi-long-range ${120}^{\circ }$ and columnar order, and a Majumdar-Ghosh phase with short-ranged correlations. All these phases are nonchiral and planar. We also identify the nature of phase transitions.

It is shown how to properly construct exchange-correlation (xc) potentials in the Kohn-Sham (KS) formalism of density-functional theory such that physically meaningful KS eigenvalues result. A potential adjustor ${\mathrm{\Delta }}_{Hxc}^{N-}$ is derived which enables for any approximate functional for the xc energy the construction of a consistent xc potential leading to an eigenvalue of the energetically highest occupied KS orbital equal to the negative of the ionization potential. Together with a second potential adjustor ${\mathrm{\Delta }}_{Hxc}^{N+}$ KS band structures can be converted in approximate quasiparticle band structures exhibiting the exact physical instead of the KS band gap. This represents an alternative route to the fundamental quasiparticle band gap, completely within the KS formalism without the need to resort to many-body perturbation theory approaches like the $GW$ method. It is shown that, for any finite system, approximate xc potentials including those in the local density and generalized gradient approximations, in contrast to common belief, always exhibit consistent nonzero discontinuities at integer electron numbers, if constructed properly. Thus the discontinuity of xc potentials is identified as a derived quantity which emerges automatically by properly constructing xc potentials without the need to be specifically incorporated in approximate xc functionals. It is demonstrated that all relevant objects of the ensemble KS formalism, in particular functional derivatives and their discontinuities, can be expressed in terms of quantities readily available in the KS formalism of integer electron numbers within an approach named integer electron ensemble approach (IEEA). Attempts to specifically construct ensemble density functionals are shown to be needless. Taking the known asymptotic behavior of the electron density into account an internal consistency condition for xc potentials is presented which justifies tuning procedures of xc functionals and, furthermore, indicates how asymptotic corrections for xc potentials have to be properly employed.

Dopants in silicon have been studied for many decades using optical and electron spin resonance (ESR) spectroscopy. Recently, new features have been observed in the spectra of dopants in isotopically enriched ${}^{28}\mathrm{Si}$ since the reduced inhomogeneous linewidth in this material improves spectral resolution. With this in mind, we measured ESR on exchange coupled phosphorus dimers in ${}^{28}\mathrm{Si}$ and report two results. First, a fine structure is observed in the ESR spectrum arising from state mixing by the hyperfine coupling to the ${}^{31}\mathrm{P}$ nuclei, which is enhanced when the exchange energy is comparable to the Zeeman energy. This fine structure enables us to spectroscopically address two separate dimer subensembles, the first with exchange $\left(J\right)$ coupling ranging from 2 to 7 GHz and the second with $J$ ranging from 6 to 60 GHz. Next, the average spin relaxation times $T_1$ and $T_2$ of both dimer subensembles were measured using pulsed ESR at 0.35 T. Both $T_1$ and $T_2$ for transitions between triplet states of the dimers were found to be identical to the relaxation times of isolated phosphorus donors in ${}^{28}\mathrm{Si}$, with $T_2=4$ ms at 1.7 K limited by spectral diffusion due to dipolar interactions with neighboring donor electron spins. This result, consistent with theoretical predictions, implies that an exchange coupling of 2–60 GHz does not limit the dimer $T_1$ and $T_2$ in bulk Si at the 10-ms time scale.

We investigate the effect of spin-inelastic scattering of Dirac electrons off a high-spin nanomagnet adsorbed on a topological insulator (TI) surface, in which transitions of the nanomagnet between its internal magnetic levels are taken into account, beyond the classic spin theory. It is found that the presence of magnetic anisotropy of nanomagnets can result in a Dirac-point resonance peak in local density of states. It can significantly modify the topologically protected Dirac surface-state spectrum at the Dirac point, quite different from previously reported low-energy resonances. Furthermore, we propose to tune electrically the appearance of the Dirac-point resonance peak and its height by use of the spin-flip torque effect. This provides an approach to engineer the Dirac cone and tune the Dirac electron properties on the TI surface in the absence of an external magnetic field.

Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching approach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of $Pbam$ symmetry with 24 atoms in the unit cell was found to be a low-energy, low-density metastable polymorph in carbon, silicon, and germanium. In silicon, $Pbam$ is found to have a direct band gap at the zone center with an estimated value of 1.4 eV, which suggests applications as a photovoltaic material. We have also found a low-energy chiral framework structure of $P{4}_1{2}_{1}2$ symmetry with 20 atoms per cell containing fivefold spirals of atoms, whose projected topology is that of the so-called Cairo-type two-dimensional pentagonal tiling. We suggest that $P{4}_1{2}_{1}2$ is a likely candidate for the structure of the unknown phase XIII of silicon. We discuss $Pbam$ and $P{4}_1{2}_{1}2$ in detail, contrasting their energetics and structures with those of other group 14 elements, particularly the recently proposed $P{4}_2/ncm$ structure, for which we also provide a detailed interpretation as a network of tilted diamondlike tetrahedra.

Magnetic properties of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ epitaxial clusters inserted in an Fe/MgO/Fe tunnel barrier are revealed by their tunnel magnetoresistance signature. The cluster assembly has been shown in a previous work to behave as a superparamagnet when a magnetic field was applied in the plane of the tunnel junction. We here demonstrate that an external large out-of plane electric field (in the order of 0.5 GV/m) favors in-plane magnetization orientation. This is due to an electric-field-induced magnetic anisotropy along the normal to the plane, corresponding to large anisotropy fields reaching up to 2 T. The assembly of clusters is thus strictly speaking not superparamagnetic and its magnetization cannot be exactly described by a Langevin law. This is attributed either to a strain-induced enhanced magnetoelectric effect or to a voltage-induced change of the magnetic anisotropy at interfaces with MgO.

Brillouin light scattering (BLS), complemented by ferromagnetic resonance (FMR) characterization, has been used for studying spin-wave (SW) propagation in $\mathrm{Py}\left(L\right)/\mathrm{Pt}\left(6\text{−}\mathrm{nm}\right)$ bilayers of various Py thicknesses $(4\mathrm{nm}\le L\le 10\mathrm{nm})$. The FMR measurements allowed determination of the pertinent magnetic parameters and revealed the existence of a normal surface anisotropy. A pronounced asymmetry of Damon-Eshbach (DE) wave frequencies has been evidenced by BLS. Therefore, the difference between Stokes and anti-Stokes DE frequencies has been measured versus SW wave number for all the samples. A detailed discussion about the origin of this frequency difference is reported, which concludes that this is due to interface Dzyaloshinskii-Moriya interaction (IDMI).

Topological properties of the spin-1/2 dimerized Heisenberg ladder are investigated, focusing on the plateau phase in the magnetic field whose magnetization is half of the saturation value. Although the applied magnetic field removes most of the symmetries of the system, there is a symmetry-protected topological phase supported by the spatial inversion symmetry. The $Z_2$ Berry phase associated with a symmetry-respecting boundary and quantized into 0 and $\pi$ is used as a symmetry-protected topological order parameter. Edge states are also analyzed to confirm the bulk-edge correspondence. In addition, a symmetry-breaking boundary is considered. Then, we observe a different type of quantization of the Berry phase, i.e., a quantization into $\pm \pi /2$ of the Berry phase. In this case, the bulk-edge correspondence is also different, namely, there emerge “polarized” edge states for the case with $\pm \pi /2$ quantization. We also evaluate the entanglement entropy by the infinite time-evolving block decimation (iTEBD) to complement the Berry-phase-based arguments. Further, a different type of the topological order parameter is extracted from the matrix product state representation of the ground state given by the iTEBD.

Quantum and global discord in a spin-1 Heisenberg chain subject to single-ion anisotropy (uniaxial field) are studied using exact diagonalization and the density matrix renormalization group. We find that these measures of quantum nonclassicality are able to detect the quantum phase transitions confining the symmetry-protected Haldane phase and show critical scaling with universal exponents. Moreover, in the case of thermal states, we find that quantum discord can increase with increasing temperature.

The magnetic properties of ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ are studied by electron paramagnetic resonance (EPR) and ${}^{51}\mathrm{V}$ nuclear magnetic resonance (NMR). The divergence of the ${}^{51}T_1$ rate on approaching $T_{\mathrm{N}}$ in the paramagnetic phase displays the exponential correlation length expected for the Berezinskii-Kosterlitz-Thouless transition. The critical power-law behavior can be restored by applying a strong magnetic field perpendicular to the hexagonal axis, destroying the $XY$ anisotropy. The comparison of the EPR and NMR measurements at several magnetic fields yields a consistent description in terms of the Berezinskii-Kosterlitz-Thouless scenario in the temperature range above the antiferromagnetic phase transition of ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$.

By using thermodynamic measurements and density-functional band-structure calculations, we explore the magnetic behavior of ${\mathrm{SrCuTe}}_2{\mathrm{O}}_6$. Despite being a structural sibling of a three-dimensional frustrated system ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$, this spin-$\frac{1}{2}$ quantum magnet shows remarkably different low-temperature behavior. Above 7 K, magnetic susceptibility of ${\mathrm{SrCuTe}}_2{\mathrm{O}}_6$ follows the spin-chain model with the antiferromagnetic intrachain coupling of $J\simeq 49.3$ K. We ascribe this quasi-one-dimensional behavior to the leading third-neighbor coupling that involves a weakly bent $\mathrm{Cu}\text{–}\mathrm{O}\cdots \mathrm{O}\text{–}\mathrm{Cu}$ superexchange pathway with a short $\mathrm{O}\cdots \mathrm{O}$ contact of 2.79 Å. Below 5 K, ${\mathrm{SrCuTe}}_2{\mathrm{O}}_6$ undergoes two consecutive magnetic transitions that may be triggered by the frustrated nature of interchain couplings. The field dependence of the magnetic transitions (phase diagram) is reported.

We consider ternary alloys of the composition ${\mathrm{Cu}(\mathrm{Mn}}_{1-w}{T}_w)$, where $T$ corresponds to different nonmagnetic impurities. As was discovered by Fert et al. [J. Magn. Magn. Mater. 24, 231 (1981)], the anomalous Hall effect (AHE) in the binary Cu(Mn) alloy can be significantly enhanced by means of codoping using $5d$ impurities. Moreover, they attempted to quantify the spin Hall effect (SHE) in $\mathrm{Cu}\left(T\right)$ binary alloys via the AHE measured in the related ternary alloys. Here, we present a theoretical study serving as a detailed background of the experimental findings by clarifying the conditions required for a maximal enhancement of the AHE as well as the relations between both Hall effects. Based on the proposed approach, we perform first-principles calculations for several ${\mathrm{Cu}(\mathrm{Mn}}_{1-w}{T}_w\left)\right[T$ = Au, Bi, Ir, Lu, Sb, or Ta] alloys, which are underpinned by theoretical investigations via Matthiessen's rule.

We present an extended x-ray absorption fine structure study of the pyrochlore ${\mathrm{Y}}_2{\mathrm{Ru}}_2{\mathrm{O}}_7$ (8–298 K). We find evidence, on a local scale, of a significant magnetoelastic coupling at the Néel temperature $T_{\mathrm{N}}\sim 77\mathrm{K}$ pointed out by a huge Debye-Waller ${\sigma }^2$ factor deviation from a correlated temperature dependent Debye-like local order behavior plus a temperature independent static contribution. Moreover, we notice the occurrence of a potential local order-disorder structural phase transition at $T^{*}=150$ K. This anomalous behavior is consistent with the pyrochlore's predisposition towards structural disorder and with a strong spin-phonon correlation. Remarkably the low-temperature order competes with the tendency of magnetic frustration to induce a less symmetric local structure.

We study the effects of quenched disorder on the first-order phase transition in the two-dimensional three-color Ashkin-Teller model by means of large-scale Monte Carlo simulations. We demonstrate that the first-order phase transition is rounded by the disorder and turns into a continuous one. Using a careful finite-size-scaling analysis, we provide strong evidence for the emerging critical behavior of the disordered Ashkin-Teller model to be in the clean two-dimensional Ising universality class, accompanied by universal logarithmic corrections. This agrees with perturbative renormalization-group predictions by Cardy. As a byproduct, we also provide support for the strong-universality scenario for the critical behavior of the two-dimensional disordered Ising model. We discuss consequences of our results for the classification of disordered phase transitions as well as generalizations to other systems.

The hyperkagome material ${\text{Na}}_4{\text{Ir}}_3{\text{O}}_8$ is a three-dimensional spin-liquid candidate proximate to a quantum critical point (QCP). We present a comprehensive study of the structure, magnetic susceptibility $\chi$, heat capacity $C$, and electrical transport ($\rho \left(T\right)$) on polycrystalline samples of the doped hyperkagome material ${\text{Na}}_{4-x}{\text{Ir}}_3{\text{O}}_8$ ($x\approx 0,0.1,0.3,0.7)$. Materials with $x\le 0.3$ are found to be Mott insulators with strong antiferromagnetic interactions and no magnetic ordering down to $T=2$ K. All samples show irreversibility below $T\approx 6$ K between the zero-field-cooled and field-cooled magnetization measured in low fields ($H=0.050$ T) suggesting a frozen low temperature state although no corresponding anomaly is seen in the heat capacity. The $x=0.7$ sample shows $\rho \left(T\right)$ which weakly increases with decreasing temperature $T$, nearly $T$ independent $\chi$, a linear in $T$ contribution to the low temperature $C$, and a Wilson ratio $R_W\approx 7$ suggesting anomalous semimetallic behavior.

We investigate the Landau parameters for the instabilities in spin and charge channels in the nondegenerate extended Hubbard model with intersite Coulomb and exchange interactions. To this aim we use the spin rotationally invariant slave boson approach and we determine the necessary inverse propagator matrix. The analytically derived spin Landau parameter $F_0^a$ for the half filled band uncovers the intrinsic instability of the nondegenerate Hubbard model towards ferromagnetism—negative intersite exchange interaction triggers a ferromagnetic instability at half filling before the metal-insulator transition, indicated by the divergence of the magnetic susceptibility at $F_0^a=-1$. This result is general and the instability occurs in the strongly correlated metallic regime for any lattice, in three or two dimensions. Next as an illustrative example we present numerical results obtained for the cubic lattice with nearest neighbor exchange $J$ and Coulomb $V$ elements and arbitrary electron density. One finds that the range of small doping near half filling is the most unstable one towards spin polarization, but only in the case of ferromagnetic intersite exchange $J<0$. Charge Landau parameter $F_0^s$ is lowered near half filling by increasing $U$ when the intersite Coulomb interaction $V$ is attractive, but in contrast to $F_0^a$ at $J<0$ it requires an attraction beyond a critical value $V_c$ to generate the divergence of the charge susceptibility at $F_0^s=-1$ in the metallic phase. This instability was found for a broad range of electronic filling away from half filling for moderate attraction.

Magnon band structures of topological magnon insulators exhibit a nontrivial topology due to the Dzyaloshinskii-Moriya interaction, which manifests itself by topologically protected edge magnons. Bringing two topological magnon insulators into contact can lead to nontrivial unidirectional magnons located at their common interface. We study theoretically interfaces of semi-infinite kagome ferromagnets in various topological phases, with a focus on the formation and the confinement of nontrivial interface magnons. We analyze generic magnon dispersions with respect to the number of band gaps and the respective winding numbers. Eventually, we prove that interfaces of topologically identical phases can host nontrivial interface magnons as well.

We present images of magnetic flux structures in a single crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\text{−}\mathrm{d}}$ during remagnetization by fields tilted from the basal plane of the crystal. Depending on the magnitude and angle of the applied field, we observe anisotropic flux penetration along and across the in-plane field component and emergence of vortex instabilities resulting in modulated flux distributions. We associate the observed patterns with flux cutting effects and with tilted vortex structures intrinsic for layered superconductors. Time dependent Ginzburg-Landau simulations show preferential vortex motion across the $c$ axis and reveal the flux structure evolution in anisotropic superconductors under tilted magnetic fields.

We use angle-resolved photoemission spectroscopy to extract the band dispersion and the Fermi surface of ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ and $\alpha -{\mathrm{BaCu}}_2{\mathrm{Sb}}_2$. While the Cu $3d$ bands in both materials are located around 3.5 eV below the Fermi level, the low-energy photoemission intensity mainly comes from As $4p$ states, suggesting a completely filled Cu $3d$ shell. The splitting of the As $3d$ core levels and the lack of pronounced three-dimensionality in the measured band structure of ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ indicate a surface state likely induced by the cleavage of this material in the collapsed tetragonal phase, which is consistent with our observation of a ${\mathrm{Cu}}^{+1}$ oxidation state. However, the observation of Cu states at similar energy in $\alpha -{\mathrm{BaCu}}_2{\mathrm{Sb}}_2$ without the pnictide-pnictide interlayer bonding characteristic of the collapsed tetragonal phase suggests that the short interlayer distance in ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ follows from the stability of the ${\mathrm{Cu}}^{+1}$ rather than the other way around. Our results confirm the prediction that ${\mathrm{BaCu}}_2{\mathrm{As}}_2$ is an $sp$ metal with weak electronic correlations.