We present the results of an ab initio study of the magnetic properties of Fe, Co, and Ni surfaces. In particular, we discuss their electronic structure and magnetic exchange interactions ($J_{ij}$), as obtained by means of a combination of density functional theory and dynamical mean-field theory. All studied systems have a pronounced tendency to ferromagnetism both for bulk and surface atoms. The presence of narrowband surface states is shown to enhance the magnetic moment as well as the exchange couplings. The most interesting results were obtained for the Fe surface where the atoms have a tendency to couple antiferromagnetically with each other. This interaction is relatively small when compared to interlayer ferromagnetic interaction, and it depends strongly on the lattice parameter. Local correlation effects are shown to lead to strong changes of the overall shape of the spectral functions. However, they seem not to play a decisive role in the overall picture of magnetic couplings studied here. We have also investigated the influence of correlations on the spin and orbital moments of bulklike and surface atoms. We found that dynamical correlations in general lead to enhanced values of the orbital moment.

We report both zinc-blende (ZB) and wurtzite (WZ) crystal phase self-assembled GaAs quantum dots (QDs) embedded in single GaAs/AlGaAs core-shell nanowires. Optical transitions and single-photon characteristics of both kinds of QDs have been investigated by measuring photoluminescence (PL) and time-resolved PL spectra upon application of hydrostatic pressure. We find that the ZB QDs are of direct band-gap transition with short recombination lifetime (∼1 ns) and higher pressure coefficient (75–100 meV/GPa). On the contrary, the WZ QDs undergo a direct-to-pseudodirect band-gap transition as a result of quantum confinement effect, with remarkably longer exciton lifetime (4.5–74.5 ns) and smaller pressure coefficient (28–53 meV/GPa). These fundamentally physical properties are further examined by performing state-of-the-art atomistic pseudopotential calculations.

Time-of-flight neutron spectroscopy has been used to determine the crystalline electric field (CEF) Hamiltonian, eigenvalues and eigenvectors appropriate to the $J=7/2 {\mathrm{Yb}}^{3+}$ ion in the candidate quantum spin ice pyrochlore magnet ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. The precise ground state (GS) of this exotic, geometrically frustrated magnet is known to be sensitive to weak disorder associated with the growth of single crystals from the melt. Such materials display weak “stuffing,” wherein a small proportion, $\approx 2\%$, of the nonmagnetic ${\mathrm{Ti}}^{4+}$ sites are occupied by excess ${\mathrm{Yb}}^{3+}$. We have carried out neutron spectroscopic measurements on a stoichiometric powder sample of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, as well as a crushed single crystal with weak stuffing and an approximate composition of ${\mathrm{Yb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+y}$ with $x=0.046$. All samples display three CEF transitions out of the GS, and the GS doublet itself is identified as primarily composed of $m_J=\pm 1/2$, as expected. However, stuffing at low temperatures in ${\mathrm{Yb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+y}$ induces a similar finite CEF lifetime as is induced in stoichiometric ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ by elevated temperature. We conclude that an extended strain field exists about each local “stuffed” site, which produces a distribution of random CEF environments in the lightly stuffed ${\mathrm{Yb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+y}$, in addition to producing a small fraction of Yb ions in defective environments with grossly different CEF eigenvalues and eigenvectors.

We report an angle-resolved photoemission study of the charge stripe ordered ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (Nd-LSCO) system. A comparative and quantitative line-shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side ($x=0.20$), a normal-state antinodal spectral gap opens upon cooling below 80 K. In this process, spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different line shape is observed in the antinodal region of charge ordered Nd-LSCO $x=1/8$. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral-weight redistribution and gapping originating from charge stripe ordering.

Model calculations indicate that the magnetic skyrmion lattice (SkL) is represented by a superposition of three spin helices at an angle of ${120}^{\circ }$ to each other, the so-called triple-$Q$ state. Using Lorentz transmission electron microscopy, we investigated the relationship between the SkL and the helix in FeGe thin films. After the magnetic field is removed, the ordered skyrmions are trapped inside helimagnetic domain walls (HDWs) where the different helical $Q$ vectors are encountered. In situ observation revealed an unexpected topological excitation under such a zero-field state: skyrmions are spontaneously formed at HDWs.

We perform a microscopic theoretical study of the generic properties of competing magnetic phases in iron pnictides. As a function of electron filling and temperature, the magnetic stripe (single-$Q$) order forms a dome, but competing noncollinear and nonuniform double-$Q$ phases exist at the foot of the dome, in agreement with recent experiments. We compute and compare the electronic properties of the different magnetic phases, investigate the role of competing superconductivity, and show how disorder may stabilize double-$Q$ order. Superconductivity is shown to compete more strongly with double-$Q$ magnetic phases, which can lead to reentrance of the $C_2$ (single-$Q$) order, in agreement with recent thermal expansion measurements on K-doped Ba-122 crystals.

Shadow x-ray magnetic circular dichroism photoemission electron microscopy is a recent technique, in which the photon intensity in the shadow of an object lying on a surface may be used to gather information about the three-dimensional magnetization texture inside the object. Our purpose here is to lay the basis of a quantitative analysis of this technique. We first discuss the principle and implementation of a method to simulate the contrast expected from an arbitrary micromagnetic state. Textbook examples and successful comparison with experiments are then given. Instrumental settings are finally discussed, having an impact on the contrast and spatial resolution: photon energy, microscope extraction voltage and plane of focus, microscope background level, electric-field related distortion of three-dimensional objects, Fresnel diffraction, or photon scattering.

Scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy have been investigated on single-crystal samples of ${\mathrm{KFe}}_2{\mathrm{As}}_2$. A Van Hove singularity (VHS) has been directly observed just a few meV below the Fermi level $\left(E_{\mathrm{F}}\right)$ of superconducting ${\mathrm{KFe}}_2{\mathrm{As}}_2$, which locates in the middle of the principal axes of the first Brillouin zone. The majority of the density of states at $E_{\mathrm{F}}$, mainly contributed by the proximity effect of the saddle point to $E_{\mathrm{F}}$, is nongapped in the superconducting state. Our observation of the ungapped feature of the density of states due to the VHS points, while providing consistent explanations to many exotic behaviors previously observed in this material, suggests Cooper pairing induced by a strong-coupling mechanism.

We theoretically study charge conductance in anomalous Rashba metal (ARM)/superconductor junctions for various types of pairing symmetries in a superconductor. The exotic state dubbed ARM, where one of the spin-resolved Fermi surfaces is absent, is realized when the chemical potential is tuned both in the presence of Rashba spin-orbit interaction (RSOI) and an exchange field. Although a fully polarized ferromagnet metal (FPFM) is also a system in which the electron's spin degrees of freedom are reduced by half, the electrons in an ARM have features that are distinct from those in a FPFM. For the ARM/spin-singlet superconductor junctions, the obtained tunneling conductance within the bulk energy gap is enhanced with the increase in magnitude of the RSOI. In particular, in $\text{ARM/}{d}_{xy}\text{-wave}$ superconductor junctions, the zero-bias conductance peak is enhanced due to the presence of the RSOI. For $\text{ARM/}{p}_x\text{-wave}$ superconductor junctions, the condition of the existence of the zero-bias conductance peak is significantly sensitive to the direction of the $\mathit{d}$ vector of the $p_x\text{-wave}$ superconductor. Furthermore, the obtained conductance in ARM/chiral $p\text{-wave}$ superconductor junctions shows different behaviors as compared to those in ARM/helical $p\text{-wave}$ superconductor junctions. This feature serves as a guide to determine the spin structure of a Cooper pair in the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of ${\mathrm{NdNiO}}_3$ and ${\mathrm{SmNiO}}_3$ films. The optical spectra show the appearance of a characteristic two-peak structure in the near-infrared when the material passes from the metal to the insulator phase. Dynamical mean-field theory calculations confirm this two-peak structure and allow us to identify these spectral changes and the associated changes in the electronic structure. We demonstrate that the insulating phase in these compounds and the associated characteristic two-peak structure are due to the combined effect of bond disproportionation and Mott physics associated with half of the disproportionated sites. We also provide insights into the structure of excited states above the gap.

We study an inhomogeneous three-orbital Hubbard model for the Cu-substituted iron pnictides using an extended real-space Green's function method combined with density functional calculations. We find that the onsite interactions of the Cu ions are the principal determinant of whether an electron dopant or a hole dopant is caused by the Cu substitution. It is found that the Cu substitution could lead to a hole doping when its onsite interactions are smaller than a critical value, as opposed to an electron doping when the interactions of Cu ions are larger than the critical value, which may explain why the effects of Cu substitution on the carrier density are entirely different in ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$ and $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x\right)}_2{\mathrm{As}}_2$. We also find that the effect of a doping-induced disorder is considerable in the Cu-substituted iron pnictides, and its cooperative effect with electron correlations contributes to the orbital-selective insulating phases in ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$ and $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x\right)}_2{\mathrm{As}}_2$.

We consider theoretically a magnetic impurity spin driven by polarized electrons tunneling through a double-quantum-dot system. The spin-blockade effect and spin conservation in the system make the magnetic impurity sufficiently interact with each transferring electron. As a result, a single collected electron carries information about spin change of the magnetic impurity. The scheme may develop all-electrical manipulation of magnetic atoms by means of single electrons, which is significant for the implementation of scalable logical gates in information processing systems.

We investigate the magnetic properties of the series ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_4$ with neutron, resonant x-ray, and magnetization measurements. The results indicate an evolution and coexistence of magnetic structures via a spin-flop transition from $ab$-plane to $c$-axis collinear order as the $5d {\mathrm{Ir}}^{4+}$ ions are replaced with an increasing concentration of $4d {\mathrm{Ru}}^{4+}$ ions. The magnetic structures within the ordered regime of the phase diagram ($x<0.3$) are reported. Despite the changes in magnetic structure no alteration of the $J_{\mathrm{eff}}=1/2$ ground state is observed. The behavior of ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_4$ is consistent with electronic phase separation and diverges from a standard scenario of hole doping. The role of lattice alterations with doping on the magnetic and insulating behavior is considered. The results presented here provide insight into the magnetic insulating states in strong spin-orbit coupled materials and the role perturbations play in altering the behavior.

We report on the observation of cyclotron-resonance-induced photocurrents, excited by continuous wave terahertz radiation, in a three-dimensional topological insulator (TI) based on an 80-nm strained HgTe film. The analysis of the photocurrent formation is supported by complementary measurements of magnetotransport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample, we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages, we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR-assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the cyclotron masses and the mobility of two-dimensional Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

We study the Kondo lattice model on the geometrically frustrated Shastry-Sutherland lattice focusing on the quantum phase transition between the valence bond solid and the heavy fermion liquid phase. By explicitly including spinon pairing of local moments at the mean-field level, we establish the emergence of a unique heavy fermion superconducting phase induced by dimer ordering of the local moments coexisting with Kondo hybridization. Furthermore, we demonstrate that for suitable choices of parameters, a partial Kondo-screening phase, where some of the valence bonds are broken, precedes the aforementioned dimer-induced superconducting phase. Our results have important implications for understanding the experimental observations in the heavy fermion Shastry-Sutherland compounds.

In situ high-pressure Raman spectroscopy and ab initio calculations are carried out to investigate the phase stability and the thermal expansion behavior of ${\mathrm{H}}_3\left[\mathrm{Co}(\mathrm{CN}{)}_6\right]$. Raman studies at high pressures in a diamond anvil cell identify soft phonons and phase instability at 2.3 GPa. Evidence of pressure-induced amorphization is found at 11 GPa. The phonon frequencies and eigenvectors obtained from ab initio calculation are used to complement the observed phonon spectra and for assignment of Raman modes. The computed eigenvector displacement patterns indicate that the soft modes correspond to the CN-librational vibrations ($E_{\mathrm{g}}$ mode) of the Co-CN-H-NC-Co linkages and their Grüneisen parameters are found to be negative, in agreement with our measured values. The thermal expansion coefficient $\left(15.6\times {10}^{-6}{\mathrm{K}}^{-1}\right)$ calculated using our computed mode Grüneisen parameters is found to be in good agreement with the reported value $\left(20\times {10}^{-6}{\mathrm{K}}^{-1}\right)$. Temperature-dependent phonon spectra down to 77 K are used to obtain the anharmonicities of different modes.

Rate coefficients of the Arrhenius-Néel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of $MH$ hysteresis loops as a function of the exchange coupling $I$ between the layers. The individual grains are modeled as two exchange-coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of noninteracting ECC grains are then integrated numerically in an applied field with constant sweep rate $R=-dH/dt$ and the magnetization calculated as a function of the applied field $H$. $MH$ hysteresis loops are presented for a range of values $I$ for sweep rates ${10}^5\mathrm{Oe}/\mathrm{s}\le R\le {10}^{10}\mathrm{Oe}/\mathrm{s}$ and a figure of merit that quantifies the advantages of ECC media is proposed. $MH$ hysteresis loops are also calculated based on the stochastic Landau-Lifshitz-Gilbert equations for ${10}^8\mathrm{Oe}/\mathrm{s}\le R\le {10}^{10}\mathrm{Oe}/\mathrm{s}$ and are shown to be in good agreement with those obtained from the direct integration of rate equations. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer-based formalism and which provide some useful insight into the reversal process and its dependence on the coupling strength and sweep rate. Of particular interest is the clustering of minimum energy states that are separated by relatively low-energy barriers into “metastates.” It is shown that while approximating the reversal process in terms of “metastates” results in little loss of accuracy, it can reduce the run time of a kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by $2–3$ orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC-based simulation studies on systems of interacting dual layer ECC media.

We investigate the effects of topological order on the transition temperature, $T_c$, and response functions in spin-orbit-coupled fermionic superfluids with a transverse Zeeman field in three dimensions. Our calculations include fluctuation effects beyond mean-field theory and are compatible with $f$-sum rules. In the topological phase we find that $T_c$ can be as large as $10\%$ of the Fermi temperature, which should be experimentally accessible in cold-gas experiments. At higher temperatures, above $T_c$, the spin and density response functions provide signatures of topological phases via the recombination or amplification of frequency-dependent peaks.

The room temperature multiferroic ${\mathrm{BiFeO}}_3$, by far the most studied experimentally, exhibits outstanding ferroelectric properties with a cycloidal magnetic order in the bulk and many unexpected advantages for possible applications in spintronics, sensor techniques, and photovoltaics. To consider ferroelectric and magnetic phase transitions in multiferroic ${\mathrm{BiFeO}}_3$ under electric field, we suggest the Ginsburg-Landau-like approach based on the symmetry and $\mathbf{P}-\omega -\mathbf{L}$ coupling, where the order parameters are: $\mathbf{P}$ is the electric polarization, $\omega$ is the axial vector of antidistorsion (describing a rotation of the oxygen octahedrons), and $\mathbf{L}$ is the antiferromagnetic vector. The theoretical model is consistent with experiment and ab initio calculations data. We give the complete set of numerical coefficients of the model and explore the behavior of $\mathbf{P}$ and $\omega$ vectors in strong electric field. The proposed approach is particularly promising for the analysis of magnetoelectric phenomena whose length scale is significantly larger than the length of the cell used in ab initio calculations. The considered cycloid problem is the clear example of such a system. Electric field-induced transformations of cycloid are exemplified on an epitaxial ${\mathrm{BiFeO}}_3$ film grown on the (001)-oriented substrate. We show that the jump of vectors $\mathbf{P}$ and $\omega$ in the field $E=6$ MV/m is accompanied by a jump of a cycloid spin rotation plane. This effect is of particular interest for spintronics and nanoelectronics.