We investigate two-dimensional electric dipole sheets in the superlattice made of ${\mathrm{BaTiO}}_3$ and ${\mathrm{BaZrO}}_3$ using first-principles-based Monte Carlo simulations and density functional calculations. Electric dipole domains and complex patterns are observed and complex dipole structures with various symmetries (e.g., $Pma2,Cmcm$, and $Pmc{2}_1$) are further confirmed by density functional calculations, which are found to be almost degenerate in energy with the ferroelectric ground state of the $Amm2$ symmetry, therefore strongly resembling magnetic sheets. More complex dipole patterns, including vortices and antivortices, are also observed, which may constitute the intermediate states that overcome the high-energy barrier of different polarization orientations previously predicted by A. I. Lebedev [Phys. Solid State 55, 1198 (2013)]. We also show that such system possesses large electrostrictive effects that may be technologically important.

Three-dimensional micromagnetic simulations of the switching of a magnetic vortex core in a cylindrical nanodisk are performed, for excitations with out-of-plane fields (symmetric switching) or with various types of time-dependent in-plane fields (asymmetric switching). Although the switching mechanisms are different in detail, all switching events must involve the movement of a Bloch point through the disk, because the switching leads to a change of the Skyrmion number which is a topological invariant as long as there is no action of a Bloch point. The momentary magnetization configurations are different in different layers of the disk. Because of the three-dimensionality it is often difficult to decide whether the asymmetric switching is caused by the splitting of the dip close to the vortex core into a vortex-antivortex pair, and the annihilation of the original vortex with the antivortex (whereby a Bloch point moves). It is suggested that there are situations for which such a switching occurs by the formation of a Bloch point in a configuration which is already similar to a vortex-antivortex configuration, but by a movement of this Bloch point before the formation of a complete pair and without the annihilation of the original vortex with an antivortex.

We consider a system composed by $N$ atoms trapped within a multimode cavity, whose theoretical description is captured by a disordered multimode Dicke model. We show that in the resonant, zero-field limit the system exactly realizes the Sherrington-Kirkpatrick model. Upon a redefinition of the temperature, the same dynamics is realized in the dispersive, strong-field limit. This regime also gives access to spin-glass observables which can be used to detect replica symmetry breaking.

Extending previous work on the imaging spin-filter technique based on electron diffraction from W(001) in the specular (00)-LEED (low-energy electron diffraction) spot, we studied the scattering-energy and angle-of-incidence landscape of spin sensitivity $S$ and reflectivity ${I/I}_0$. The setup includes a spin-polarized GaAs electron source and a rotatable delay-line detector for spatially resolving detection. We measured the intensity and spin asymmetry of the specularly reflected beam for energies between 14 and 37 eV and angles of incidence between ${30}^{\circ }$ and ${60}^{\circ }$. Resulting energy-angular landscapes show rather good agreement with theory [relativistic layer KKR SPLEED (spin-polarized low-energy electron diffraction) code]. This provides a sensitive test of theory, in particular of details of the surface barrier. The results identify several regions of high asymmetry and figure of merit, in particular a region where the asymmetry can be “switched” between positive and negative values by just 4 eV energy variation.

Using the linear response formalism, we compute from first principles the spin-orbit torque (SOT) in a system of two layers of ${L1}_0-\mathrm{FePt}\left(001\right)$ deposited on an fcc Pt(001) substrate of varying thickness. We find that at room temperature the values of the SOTs that are even and odd with respect to magnetization generally lie in the range of values measured and computed for Co/Pt bilayers. We also observe that the even SOT is much more robust with respect to changing the number of layers in the substrate, and as a function of energy it follows the general trend of the even SOT exerted by the spin Hall current in fcc Pt. The odd torque, on the other hand, is strongly affected by modification of the electronic structure for a specific energy window in the limit of very thin films. Moreover, taking the system at hand as an example, we compute the values of the thermal spin-orbit torque (TSOT). We predict that the gradients of temperature which can be experimentally created in this type of system will cause a detectable torque on the magnetization. We also underline the correlation between the even TSOT and the spin Nernst effect, thus motivating a more intensive experimental effort aimed at an observation of both phenomena.

We report static and dynamic properties of the antiferromagnetic compound ${\mathrm{Zn}}_2\left(\mathrm{VO}\right)\left({\mathrm{PO}}_4\right){}_2$, and the consequences of nonmagnetic ${\mathrm{Ti}}^{4+}$ doping at the ${\mathrm{V}}^{4+}$ site. ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rate $\left(1/T_1\right)$ consistently show the formation of the long-range antiferromagnetic order below $T_N=3.8\text{–}3.9$ K. The critical exponent $\beta =0.33\pm 0.02$ estimated from the temperature dependence of the sublattice magnetization measured by ${}^{31}\mathrm{P}$ NMR at 9.4 MHz is consistent with universality classes of three-dimensional spin models. The isotropic and axial hyperfine couplings between the ${}^{31}\mathrm{P}$ nuclei and ${\mathrm{V}}^{4+}$ spins are $A_{\mathrm{hf}}^{\mathrm{iso}}=(9221\pm 100) \mathrm{Oe}/{\mu }_{\mathrm{B}}$ and $A_{\mathrm{hf}}^{\mathrm{ax}}=(1010\pm 50) \mathrm{Oe}/{\mu }_{\mathrm{B}}$, respectively. Magnetic susceptibility data above 6.5 K and heat capacity data above 4.5 K are well described by quantum Monte Carlo simulations for the Heisenberg model on the square lattice with $J\simeq 7.7$ K. This value of $J$ is consistent with the values obtained from the NMR shift, $1/T_1$, and electron spin resonance intensity analysis. Doping ${\mathrm{Zn}}_2\mathrm{VO}{(\mathrm{PO}}_4{)}_2$ with nonmagnetic ${\mathrm{Ti}}^{4+}$ leads to a marginal increase in the $J$ value and the overall dilution of the spin lattice. In contrast to the recent ab initio results, we find neither evidence for the monoclinic structural distortion nor signatures of the magnetic one-dimensionality for doped samples with up to 15% of ${\mathrm{Ti}}^{4+}$. The Néel temperature $T_{\mathrm{N}}$ decreases linearly with increasing the amount of the nonmagnetic dopant.

We demonstrate that in sufficiently long diffusive superconducting-normal-superconducting ($\mathit{SNS}$) junctions dc Josephson current is exponentially suppressed by electron-electron interactions down to zero temperature. This suppression is caused by the effect of Cooper pair dephasing which occurs in the normal metal and defines a new fundamental length scale $L_{\phi }$ in the problem. This length is fully consistent with that derived earlier from the analysis of dissipative electron transport across $\mathit{NS}$ interfaces at subgap energies. Provided the temperature length exceeds $L_{\phi }$ this dephasing length can be conveniently extracted from equilibrium measurements of the Josephson current.

We present a theoretical study of spin transport in a superconducting mesoscopic spin valve under the action of a magnetic field misaligned with respect to the injected spin. We demonstrate that superconductivity can either strongly enhance or suppress the coherent spin rotation, depending on the type of spin relaxation mechanism being dominated either by spin-orbit coupling or spin-flip scattering at impurities. We also predict a subgap contribution to the nonlocal conductance in multiterminal superconducting hybrid structures which completely eliminates the effect of spin rotation at sufficiently low temperatures.

Measurements of x-ray magnetic circular dichroism (XMCD) and x-ray absorption near-edge structure (XANES) spectroscopy at the Pu $M_{4,5}$ edges of the ferromagnet PuSb are reported. Using bulk magnetization measurements and a sum rule analysis of the XMCD spectra, we determine the individual orbital [${\mu }_{\mathrm{L}}=2.8\left(1\right){\mu }_{\mathrm{B}}/\mathrm{Pu}$] and spin moments [${\mu }_{\mathrm{S}}=-2.0\left(1\right){\mu }_{\mathrm{B}}/\mathrm{Pu}$] of the Pu $5f$ electrons. Atomic multiplet calculations of the XMCD and XANES spectra reproduce well the experimental data and are consistent with the experimental value of the spin moment. These measurements of $\langle L_z\rangle$ and $\langle S_z\rangle$ are in excellent agreement with the values that have been extracted from neutron magnetic form factor measurements, and confirm the local character of the $5f$ electrons in PuSb. Finally, we demonstrate that a split $M_5$ as well as a narrow $M_4$ XMCD signal may serve as a signature of $5f$ electron localization in actinide compounds.

A generalized numerical model for the phonon-assisted optical interband transition based on the Green's function formalism was developed and implemented to investigate optical processes in germanium and silicon media intended for on-chip light emitter and laser applications. High-fidelity full band structures obtained from the empirical pseudopotential method, self-energies, and the corresponding spectral density functions for the phonon-perturbed electron and holes have been computed numerically as a function of strain, temperature, and doping level. Validation has been carried out by showing the model's ability to accurately reproduce the measured temperature dependent absorption coefficient data for both germanium and silicon. Absorption coefficients, radiative recombination rates of germanium and silicon active media were investigated with different biaxial tensile strain, doping concentrations and injection conditions. Furthermore, when the model is employed to compute the optical gain in strained germanium, we find that the use of tensile strain and high injection are the preferable approaches to obtain population inversion. At the same time, strong absorption from the spin-orbit to the heavy-hole band limits the maximum injection density that can be applied. Finally, when applied to study silicon, the proposed model also successfully reproduces the experimentally observed radiative recombination peak due to the two-phonon process.

A study of the photophysical properties of nitrogen-vacancy (NV) color centers in diamond nanocrystals of size 50 nm or below is carried out by means of second-order time-intensity photon correlation and cross-correlation measurements as a function of the excitation power for both pure charge states, neutral and negatively charged, as well as for the photochromic state, where the center switches between both states at any power. A dedicated three-level model implying a shelving level is developed to extract the relevant photophysical parameters coupling all three levels. Our analysis confirms the very existence of the shelving level for the neutral NV center. It is found that it plays a negligible role on the photophysics of this center, whereas it is responsible for an increasing photon bunching behavior of the negative NV center with increasing power. From the photophysical parameters, we infer a quantum efficiency for both centers, showing that it remains close to unity for the neutral center over the entire power range, whereas it drops with increasing power from near unity to approximately 0.5 for the negative center. The photophysics of the photochromic center reveals a rich phenomenology that is to a large extent dominated by that of the negative state, in agreement with the excess charge release of the negative center being much slower than the photon emission process.

Iron impurities have a negative effect on the efficiency of silicon-based solar cells because they act as trapping centers for charge carriers. Various processing techniques have been applied to improve the efficiency by passivating the Fe contaminants. For instance, internal gettering exploits the attractive interaction between interfaces and the diffusing Fe atoms. Therefore, it is interesting and important to develop a fundamental understanding of mechanisms for this interaction. In this work, we employ density functional theory to study the electronic structure and the segregation behavior of impurity atoms at grain boundaries (GBs). The investigated set of symmetrical tilt or twist GBs in Si provides a variety of interface orientations and structures at the atomic scale. Our results suggest that segregation of interstitial Fe occurs only at specific sites at some of these GBs, e.g., the $\Sigma 3$ (112) GBs and $\Sigma 3$ (110) GBs. However, there seems to be no obvious relation between the computed segregation energies and the local coordination and electronic structure at the segregation sites. Hence, the thermodynamics of interstitial Fe at GBs in Si is determined by rather subtle features of structure and bonding.

We report on the Fe doping and on the comparative Ni-Fe codoping with composition close to $\mathrm{NiF}{\mathrm{e}}_2$ of fully oxidized $\mathrm{BaTi}{\mathrm{O}}_3$ layers (∼20 nm) elaborated by atomic oxygen plasma assisted molecular beam epitaxy; specifically any role of oxygen vacancies can be excluded in our films. Additionally to the classical in situ laboratory tools, the films were thoroughly characterized by synchrotron radiation x-ray diffraction and x-ray absorption spectroscopy. For purely Fe-doped layers, the native tetragonal perovskite structure evolves rapidly toward cubiclike up to 5% doping level above which the crystalline order disappears. On the contrary, low codoping levels $(\sim 5\%\mathrm{NiF}{\mathrm{e}}_2)$ fairly improve the thin film crystalline structure and surface smoothness; high levels (∼27%) lead to more crystallographically disordered films, although the tetragonal structure is preserved. Synchrotron radiation magnetic dichroic measurements reveal that metal clustering does not occur, that the Fe valence evolves from ${\mathrm{Fe}}^{2+}$ for low Fe doping levels to ${\mathrm{Fe}}^{3+}$ for high doping levels, and that the introduction of Ni favors the occurrence of the ${\mathrm{Fe}}^{2+}$ valence in the films. For the lower codoping levels it seems that ${\mathrm{Fe}}^{2+}$ substitutes ${\mathrm{Ba}}^{2+}$, whereas ${\mathrm{Ni}}^{2+}$ always substitutes ${\mathrm{Ti}}^{4+}$. Ferromagnetic long-range ordering can be excluded with great sensitivity in all samples as deduced from our x-ray magnetic absorption circular dichroic measurements. On the contrary, our linear dichroic x-ray absorption results support antiferromagnetic long-range ordering while piezoforce microscopy gives evidence of a robust ferroelectric long-range ordering showing that our films are excellent candidates for magnetic exchange coupled multiferroic applications.

Magnetotransport measurements on two-dimensional electrons confined to wide GaAs quantum wells reveal a remarkable evolution of the ground state at filling factor $\nu =1/2$ as we tilt the sample in the magnetic field. Starting with a compressible state at zero tilt angle, a strong $\nu =1/2$ fractional quantum Hall state appears at intermediate angles. At higher angles an insulating phase surrounds this state and eventually engulfs it at the highest angles. This evolution occurs because the parallel component of the field renders the charge distribution increasingly bilayer-like. The evolution is qualitatively similar to the one seen, in the absence of parallel field, as a function of increasing the electron density in the quantum well, but there are some notable differences.

The electronic band structure of ${\mathrm{MgB}}_2$ involves a unique combination of two- and three-dimensional (3D) electrons derived from the boron $\sigma$ and $\pi$ states, respectively. We have mapped out the $\sigma$ and $\pi$ bands over the complete Brillouin zone, including the full disconnected Fermi surface, using high-resolution soft x-ray angle-resolved photoelectron spectroscopy. The measured band structure, which is closely related to that of graphene, is in overall good agreement with the density functional theory–general gradient approximation (DFT-GGA), though differences in Fermi surface volume are seen. Surprisingly, the measured bands are wider than calculated, by $\sim 8$% for the $\sigma$ and $\sim 10$–15% for the $\pi$ bands. This solves the long-standing challenge of establishing the full 3D electronic structure of the model compound ${\mathrm{MgB}}_2$, and it demonstrates the tendency of DFT-GGA to overestimate the band narrowing due to exchange correlations effects.

We employ the tight-binding propagation method to study Klein tunneling and quantum interference in large graphene systems. With this efficient numerical scheme, we model the propagation of a wave packet through a potential barrier and determine the tunneling probability for different incidence angles. We consider both sharp and smooth potential barriers in n-p-n and $n\text{−}{n}^{\prime }$ junctions and find good agreement with analytical and semiclassical predictions. When we go outside the Dirac regime, we observe that sharp $n\text{−}p$ junctions no longer show Klein tunneling because of intervalley scattering. However, this effect can be suppressed by considering a smooth potential. Klein tunneling holds for potentials changing on the scale much larger than the interatomic distance. When the energies of both the electrons and holes are above the Van Hove singularity, we observe total reflection for both sharp and smooth potential barriers. Furthermore, we consider caustic formation by a two-dimensional Gaussian potential. For sufficiently broad potentials we find a good agreement between the simulated wave density and the classical electron trajectories.

A systematic study of the Raman spectra of $\mathrm{Pb}(\mathrm{Z}{\mathrm{r}}_{1-x}\mathrm{T}{\mathrm{i}}_x){\mathrm{O}}_3$ (PZT) ceramics has been performed in a broad temperature interval (10–600 K) and a broad Ti/Zr concentration range around the morphotropic phase boundary $(x=0.25\text{–}0.70)$. The number of the spectral components was estimated by a standard fitting procedure with damped harmonic oscillators as well as by counting the number of peaks and shoulders with the help of a purposely designed mathematical analysis based on frequency derivatives of the Raman spectra. This last method proves to be very useful to study Raman spectra of disordered materials. For the case of PZT, the comparison with the Raman modes of $\mathrm{PbTi}{\mathrm{O}}_3$ allows us to assign the phonon bands on both sides of the morphotropic phase boundary, and the crossover from the tetragonal to rhombohedral phase spectra is clearly visible. However, there are no indications of a systematic splitting of the $E$-symmetry modes into monoclinic $A^{\prime }-A^{\prime \prime }$ doublets in the morphotropic samples. Detailed adjusting of the response function to the spectrum requires to assume additional Raman-active modes, but this holds for a much broader concentration range than that of the anticipated monoclinic phase. In particular, the lowest frequency transverse optic mode of $E$-symmetry (soft mode of the ferroelectric phase transition) is split into two components, a THz frequency anharmonic (central modelike) component and a resonant component (at $\omega \sim 80\mathrm{c}{\mathrm{m}}^{-1})$, both related to the same normal coordinate. The additional Raman band appearing in this frequency range $(\omega \sim 65\mathrm{c}{\mathrm{m}}^{-1})$ at low temperatures is rather associated with the antiphase tilt vibrations of the oxygen octahedra.

We study the spin polarized transport through Mn-decorated 8-5-5-8 topological line defects in graphene using the nonequilibrium Green's function formalism. Strong preferential bonding overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced $\pi$ magnetism, in particular for full coverage of all octagonal hollow sites by Mn atoms. In contrast to the magnetoresistance of graphene nanoribbon edges, the proposed system is well controlled and therefore suitable for applications.

We report a photoemission study at room temperature on ${\mathrm{BaFe}}_2{X}_3$ $(X=\mathrm{S}$ and Se) and ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ in which two-leg ladders are formed by the Fe sites. The Fe $2p$ core-level peaks of ${\mathrm{BaFe}}_2{X}_3$ are broad and exhibit two components, indicating that itinerant and localized Fe $3d$ sites coexist similar to ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$. The Fe $2p$ core-level peak of ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ can be viewed as a ${\mathrm{Fe}}^{2+}$ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stability of chalcogen $p$ holes in the two-leg ladder structure.