We propose a scheme to generate pairs of time-reversal invariant parafermions. Our setup consists of two quantum wires with Rashba spin-orbit interactions coupled to an $s$-wave superconductor, in the presence of electron-electron interactions. The zero-energy bound states localized at the wire ends arise from the interplay between two types of proximity-induced superconductivity: the usual intrawire superconductivity and the interwire superconductivity due to crossed Andreev reflections. If the latter dominates, which is the case for strong electron-electron interactions, the system supports Kramers pair of parafermions. Moreover, the scheme can be extended to a two-dimensional sea of time-reversal invariant parafermions.

The electronic structures of ${[\mathrm{SrTiO}}_3\left]{}_{1-x}{[\mathrm{LaCrO}}_3\right]{}_x$ perovskite solid solutions are studied using hybrid density functional calculations to investigate their potential photocatalytic activity. The introduction of ${\mathrm{Cr}}^{3+}$ into ${\mathrm{SrTiO}}_3$ not only creates occupied states inside the band gap but can adversely narrow the conduction band. However, if ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Ti}}^{4+}$ ions are segregated in alternating [001] layers, the conduction band remains highly dispersive. This suggests that the electronic structure can be tuned by controlling the cation arrangement. We predict that ${[\mathrm{SrTiO}}_3\left]{}_{0.5}{[\mathrm{LaCrO}}_3\right]{}_{0.5}$ with alternating ${\mathrm{TiO}}_2$ and ${\mathrm{CrO}}_2$ layered along the [001] direction, which has not been experimentally realized yet, will exhibit strong absorption of visible light response and excellent electronic transport properties.

By using ab initio calculations, we investigated a cubic perovskite ${\mathrm{BaOsO}}_3$ and a few related compounds that have been synthesized recently and formally have a metallic $d^4$ configuration. In ${\mathrm{BaOsO}}_3$, which shows obvious three-dimensional fermiology, a nonmagnetism is induced by a large spin-orbit coupling (SOC), which is precisely equal to an exchange splitting $\sim 0.4$ eV of the $t_{2g}$ manifold. However, the inclusion of on-site Coulomb repulsion as small as $U^c\approx 1.2$ eV, only $1/3$ of the $t_{2g}$ bandwidth, leads to the emergence of a spin-ordered moment, indicating that this system is on the verge of magnetism. In contrast to ${\mathrm{BaOsO}}_3$, our calculations suggest that the ground state of an orthorhombic ${\mathrm{CaOsO}}_3$ is a magnetically ordered state due to the reduction of the strength of SOC (about a half of that of ${\mathrm{BaOsO}}_3)$ driven by the structure distortion, although the magnetization energy is only a few tenths of meV. Furthermore, in the cubic ${\mathrm{BaOsO}}_3$ and ${\mathrm{BaRuO}}_3$, our full-phonon calculations show several unstable modes, requiring further research.

We performed first-principles calculations to show that a fully spin-polarized two-dimensional electron gas can be created at the interface between the polar and insulating spinel oxides ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ and ${\mathrm{MgAl}}_2{\mathrm{O}}_4$. We give a clear description of the physical parameters (in particular the atomic termination of the interfaces), which favor the formation of this electron gas that is due either to an electric field induced in stoichiometric oxide layers because of their polar character or to a charge reorganization that preserves the global electric neutrality in nonstoichiometric layers. We show that the electric field-induced spin-polarized two-dimensional electron gas can only exist if the thickness of the ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ layer is large enough and that it may be destroyed by intermixing at the interfaces.

A combination of x-ray absorption, x-ray-diffraction, and transport measurements at high pressure is used to investigate the interplay between the electronic properties of Ir $5d$ states and lattice degrees of freedom in the weakly ferromagnetic insulator ${\mathrm{BaIrO}}_3$. Although the Ir $5d$ local magnetic moment is highly stable against lattice compression, remaining nearly unperturbed to at least 30 GPa, the weak ferromagnetism (net ordered moment) is quickly quenched by 4.5 GPa (3% volume reduction). Under chemical pressure, where Sr is substituted for the larger Ba in ${\mathrm{BaIrO}}_3$, the local magnetic moment on Ir remains stable, but the weak ferromagnetism is quenched after only 1.7% volume reduction. The magnetic ordering temperature $T_m$ is also more strongly suppressed by chemical pressure compared to physical pressure. In addition, under $\sim 23\text{−}\mathrm{at}.\%$ Sr doping, ${\mathrm{BaIrO}}_3$ undergoes a transition to a paramagnetic metallic state. Resistivity measurements indicate that ${\mathrm{BaIrO}}_3$ remains an electrical insulator to at least 9 GPa, a much higher pressure than required to quench the weak ferromagnetism $(\sim 4.5\mathrm{GPa})$. Such a disparate response of transport and magnetic properties to chemical and physical pressure is likely rooted in the different compression rates of the $(a,c)$ lattice parameters with Sr doping and applied pressure and the effect of related lattice distortions on electronic bandwidth and exchange interactions in this strongly spin-orbit-coupled system.

The Gilbert damping constant present in the phenomenological Landau-Lifshitz-Gilbert equation describing the dynamics of magnetization is calculated for ferromagnetic metallic films as well as Co/nonmagnet (NM) bilayers. The calculations are done within a realistic nine-orbital tight-binding model including spin-orbit coupling. The convergence of the damping constant expressed as a sum over the Brillouin zone is remarkably improved by introducing finite temperature into the electronic occupation factors and subsequent summation over the Matsubara frequencies. We investigate how the Gilbert damping constant depends on the ferromagnetic film thickness as well as on the thickness of the nonmagnetic cap in Co/NM bilayers $(\text{NM}=\text{Cu}$, Pd, Ag, Pt, and Au). The obtained theoretical dependence of the damping constant on the electron-scattering rate, describing the average lifetime of electronic states, varies substantially with the ferromagnetic film thickness and it differs significantly from the dependence for bulk ferromagnetic metals. The presence of nonmagnetic caps is found to largely enhance the magnetic damping in Co/NM bilayers in accordance with experimental data. Unlike Cu, Ag, and Au a particularly strong enhancement is obtained for Pd and Pt caps. This is attributed to the combined effect of the large spin-orbit couplings of Pd and Pt and the simultaneous presence of $d$ states at the Fermi level in these two metals. The calculated Gilbert damping constant also shows an oscillatory dependence on the thicknesses of both ferromagnetic and nonmagnetic parts of the investigated systems which is attributed to quantum-well states. Finally, the expression for contributions to the damping constant from individual atomic layers is derived. The obtained distribution of layer contributions in Co/Pt and Co/Pd bilayers proves that the enhanced damping which affects the dynamics of the magnetization in the Co film originates mainly from a region within the nonmagnetic part of the bilayer. Such a nonlocal damping mechanism, related to spin pumping, is almost absent in other investigated bilayers: Co/Cu, Co/Ag, and Co/Au.

The knowledge about the gap size and structure is of utmost importance for a theory of the superconducting pairing mechanism. The number of superconducting gaps is an important part of the description and modeling of multiband systems such as the iron-based superconductors. Here, we present a study on the temperature dependence of the superfluid density, ${\rho }_{\mathrm{s}}\left(T\right)$, in ${\mathrm{Ba}(\mathrm{Fe}}_{0.9}{\mathrm{Co}}_{0.1}){}_2{\mathrm{As}}_2$ obtained from terahertz spectra of conductivity and dielectric permittivity of thin film samples with critical temperatures $T_{\mathrm{c}}\approx 20–22$ K. We demonstrate that the temperature dependence of the superfluid density, ${\rho }_{\mathrm{s}}\left(T\right)$, can be explained best by a model of three interacting superconducting condensates. Our results refine the standard two-band approach for Co-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$.

We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed Néel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces.

The temporal evolution of highly nonequilibrium and spatially nonuniform spin states is analyzed on the basis of a general, phenomenological spin dynamics theory, employing the concept of both local (relativistic) and nonlocal (exchange) spin relaxations. The developed theory is applied here to describe the ultrafast spin evolution arising after the action of a femtosecond laser pulse on magnetic heterostructures containing layers of two different ferromagnets; specifically, we consider here both Ni-Fe and Ni-Ru-Fe heterostructures. As a consequence of the laser excitation, nonuniform spin distributions are created in the layered systems, which form the initial state for the spin dynamics calculations. The results obtained provide an explanation of recent experiments on the magnetization recovery in laser-pumped Ni-Ru-Fe heterostructures. The importance of the nonlocal character of the magnetization recovery for such systems is established. In particular, the experimentally observed strong dependence of the spin recovery on the relative orientation of the magnetic moments of the layers is explained theoretically. If the nonlocal spin relaxation is dominating, the evolution from an initial nonuniform magnetization profile is concurrent with the creation of a strong spin current flowing between the layers.

In the present work, spontaneous magnetization is observed in the inverse magnetic susceptibility of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Ba}}_{0.3}{\mathrm{MnO}}_3$ compounds above $T_C$ up to a temperature $T^{*}$. From information gathered from neutron diffraction, dilatometry, and high-field magnetization data, we suggest that $T^{*}$ is related to the transition temperature of the low-temperature (high magnetic field) magnetic phase. In the temperature region between $T^{*}$ and $T_C$, the application of a magnetic field drives the system from the high-temperature to low-temperature magnetic phases, the latter possessing a higher magnetization.

We present a study of the correlated transport regimes of a double quantum impurity system with mutual capacitive interactions. Such system can be implemented by a double quantum dot arrangement or by a quantum dot and nearby quantum point contact, with independently connected sets of metallic terminals. Many-body spin correlations arising within each dot-lead subsystem give rise to the Kondo effect under appropriate conditions. The otherwise independent Kondo ground states may be modified by the capacitive coupling, decisively modifying the ground state of the double quantum impurity system. We analyze this coupled system through variational methods and the numerical renormalization group technique. Our results reveal a strong dependence of the coupled system ground state on the electron-hole asymmetries of the individual subsystems, as well as on their hybridization strengths to the respective reservoirs. The electrostatic repulsion produced by the capacitive coupling produces an effective shift of the individual energy levels toward higher energies, with a stronger effect on the “shallower” subsystem (that closer to resonance with the Fermi level), potentially pushing it out of the Kondo regime and dramatically changing the transport properties of the system. The effective remote gating that this entails is found to depend nonlinearly on the capacitive coupling strength, as well as on the independent subsystem levels. The analysis we present here of this mutual interaction should be important to fully characterize transport through such coupled systems.

Motivated by ${\mathrm{Mo}}_3{\mathrm{S}}_7$(dmit)${}_3$, we investigate the Hubbard model on the triangular necklace lattice at two-thirds filling. We show, using second-order perturbation theory, that in the molecular limit, the ground state and the low-energy excitations of this model are identical to those of the spin-one Heisenberg chain. The latter model is known to be in the symmetry-protected topological Haldane phase. Away from this limit we show, on the basis of symmetry arguments and density matrix renormalization group (DMRG) calculations, that the low-energy physics of the Hubbard model on the triangular necklace lattice at two-thirds filling is captured by the ferromagnetic Hubbard-Kondo lattice chain at half-filling. This is consistent with and strengthens previous claims that both the half-filled ferromagnetic Kondo lattice model and the two-thirds filled Hubbard model on the triangular necklace lattice are also in the Haldane phase. A connection between Hund's rules and Nagaoka's theorem is also discussed.

We present a model to describe thermophysical and optical properties of two-temperature systems consisted of heated electrons and cold ions in a solid lattice that occur during ultrafast heating experiments. Our model is based on ab initio simulations within the framework of density functional theory. The optical properties are obtained by evaluating the Kubo-Greenwood formula. By applying the material parameters of our ab initio model to a two-temperature model we are able to describe the temperature relaxation process of femtosecond-laser-heated gold and its optical properties within the same theoretical framework. Recent time-resolved measurements of optical properties of ultrafast heated gold revealed the dynamics of the interaction between femtosecond laser pulses and solid state matter. Different scenarios obtained from simulations of our study are compared with experimental data [Chen, Holst, Kirkwood, Sametoglu, Reid, Tsui, Recoules, and Ng, Phys. Rev. Lett. 110, 135001 (2013)].

The photon statistics of CdSe/CdS dot-in-rods nanocrystals is studied with a method involving postselection of the photon detection events based on the photoluminescence count rate. We show that flickering between two states needs to be taken into account to interpret the single-photon emission properties. With postselection we are able to identify two emitting states: the exciton and the charged exciton (trion), characterized by different lifetimes and different second-order correlation functions. Measurements of the second-order autocorrelation function at zero delay with postselection shows a degradation of the single-photon emission for CdSe/CdS dot-in-rods in a charged state that we explain by deriving the neutral and charged biexciton quantum yields.

The In/Si(111)4$\times 1$ surface with In wires is a prototypical one-dimensional electron system showing a temperature-induced structural phase transition. While most impurities are known to decrease the transition temperature $\left(T_c\right)$, oxygen was reported to increase the $T_c$. In order to understand the exceptional influence of oxygen, we have examined the initial stages of oxygen adsorption on the surface by scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. Oxygen molecules were found to dissociate and adsorb as atoms on the surface. STM revealed three distinct O adsorption features as well as occasional transformations from one to another. The high-resolution images showed that all three adsorbed O features were located at the center of the In trimer at both edges of the In wire. This registry is in contradiction with the previous theoretical predictions [S. Wippermann et al., Phys. Rev. Lett. 100, 106802 (2008)]. The present DFT calculations identified two of the features as O atoms incorporated in the interstitial region between the In wire and subsurface Si layer.

CeRuSn exhibits an extraordinary room temperature structure at 300 K with the coexistence of two types of Ce ions, namely trivalent ${\mathrm{Ce}}^{3+}$ and intermediate-valent ${\mathrm{Ce}}^{(4-\delta )+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$ axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl-type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce ${\mathrm{L}}_3$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to our previous report, the new data exhibit small but significant variations as a function of temperature that are consistent with a transition of a fraction of ${\mathrm{Ce}}^{3+}$ ions to the intermediate valence state, analogous to the $\gamma \to \alpha$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.

We study thermal transport in SiGe nanowires by means of nonequilibrium molecular dynamics simulations. We calculate the axial interface thermal resistance (ITR) of realistic models of SiGe nanowires that are obtained in different experimental conditions. We study thermal rectification, finding that heat transport from Si to Ge is favored, particularly in sharp junctions, and that this behavior can be explained in terms of the different temperature dependence of the thermal conductivity of the pristine nanowires.