We study the dependence of the perpendicular magnetic anisotropy on the underlayer material in magnetic tunnel junction. Using several different 4d and 5d metals we identify an optimal seed layer in terms of high anisotropy, low mixing, and high thermal stability. In such systems we investigate the tunability of the anisotropy by means of electric fields. Especially, by using W as the underlayer of the CoFeB/MgO/CoFeB trilayer we could obtain good thermal stability that allows for annealing in the temperatures up to $450{}^{\circ }\mathrm{C}$, which results in high perpendicular anisotropy.

Two-color second-order correlations of the light scattered near-resonantly by a quantum dot were measured by means of spectrally filtered coincidence detection. The effects of filter frequency and bandwidth were studied under monochromatic laser excitation, and a complete two-photon spectrum was reconstructed. The two-photon spectrum exhibits a rich structure associated with both real and virtual two-photon transitions down the “dressed states” ladder. Photon pairs generated via virtual transitions are found to violate the Cauchy-Schwarz inequality by a factor of 60. Our experiments are well described by the theoretical expressions obtained by del Valle et al. [Phys. Rev. Lett. 109, 183601 (2012)] via time- and normally-ordered correlation functions.

We study the one-band Hubbard model on the trellis lattice, a two-dimensional frustrated lattice of coupled two-leg ladders, with hopping amplitude $t$ within ladders and $t^{\prime }$ between ladders. For large $U/t$ this is a model for the cuprate ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$. We investigate the phase diagram as a function of doping for $U=10t$ using two quantum cluster methods: The variational cluster approximation (VCA), with clusters of sizes 8 and 12, and cellular dynamical mean field theory (CDMFT), both at zero temperature. Both methods predict a superconducting dome, ending at roughly 20% doping in VCA and 15% in CDMFT. In VCA, the superconducting order parameter is complex in a range of doping centered around 10%, corresponding to bulk chiral, $T$-violating superconductivity. However, the CDMFT solution is not chiral. We find evidence for a migration of the Cooper pairs from the interladder region towards the plaquettes as doping is increased.

Using density functional theory based calculations, we show that the correlated mixed-valent compound SmO is a three-dimensional strongly topological semimetal as a result of a $4f\text{−}5d$ band inversion at the $X$ point. The [001] surface Bloch spectral density reveals two weakly interacting Dirac cones that are quasidegenerate at the $\overline{M}$ point and another single Dirac cone at the $\overline{\mathrm{\Gamma }}$ point. We also show that the topological nontriviality in SmO is very robust and prevails for a wide range of lattice parameters, making it an ideal candidate to investigate topological nontrivial correlated flat bands in thin-film form. Moreover, the electron filling is tunable by strain. In addition, we find conditions for which the inversion is of the $4f\text{−}6s$ type, making SmO to be a rather unique system. The similarities of the crystal symmetry and the lattice constant of SmO to the well studied ferromagnetic semiconductor EuO, makes SmO/EuO thin film interfaces an excellent contender towards realizing the quantum anomalous Hall effect in a strongly correlated electron system.

The magnitude of the spin polarization at the Fermi level of ferromagnetic materials at room temperature is a key property for spintronics. Investigating the Heusler compound ${\mathrm{Co}}_2\mathrm{MnSi}$, a value of $93\%$ for the spin polarization has been observed at room temperature, where the high spin polarization is related to a stable surface resonance in the majority band extending deep into the bulk. In particular, we identified in our spectroscopical analysis that this surface resonance is embedded in the bulk continuum with a strong coupling to the majority bulk states. The resonance behaves very bulklike, as it extends over the first six atomic layers of the corresponding (001) surface. Our study includes experimental investigations, where the bulk electronic structure as well as surface-related features have been investigated using spin-resolved photoelectron spectroscopy (SR-UPS) and for a larger probing depth spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The results are interpreted in comparison with first-principles band structure and photoemission calculations which consider all relativistic, surface, and high-energy effects properly.

Exciton mobility in π stacks of organic chromophores is shown to be highly sensitive to the interference between long-range Coulombic coupling and a short-range coupling due to wave function overlap. A destructive interference, which leads to a compromised exciton bandwidth, can be converted to constructive interference (and an enhanced bandwidth) upon sub-Angstrom transverse displacements between neighboring chromophores. The feasibility of the control scheme is demonstrated theoretically on a derivative of terrylene, where the exciton is essentially immobile despite strong Coulombic coupling. A transverse slip of only 0.5 Å along either the short or the long molecular axis boosts the exciton velocity to $2\times {10}^4 \mathrm{m}/\mathrm{s}$. Changes in the mobility are correlated to changes in the absorption spectrum, allowing the latter to be used as a screen for high mobility aggregates.

We evaluate the joint distributions of electron waiting times in coherent conductors described by scattering theory. Successive electron waiting times in a single-channel conductor are found to be correlated due to the fermionic statistics encoded in the many-body state. Our formalism allows us also to investigate the waiting times between charge transfer events in different outgoing channels. As an application we consider a quantum point contact in a chiral setup with one or both input channels biased by either a static or a time-dependent periodic voltage described by Floquet theory. The theoretical framework developed here can be applied to a variety of scattering problems and can in a straightforward manner be extended to joint distributions of several electron waiting times.

We study theoretically phonon-assisted relaxation and tunneling in a system composed of a quantum dot which is coupled to a quantum well. Within the $\mathit{k}·\mathit{p}$ method combined with the Löwdin elimination, we calculate the electron states. We calculate acoustic phonon-assisted relaxation rates between the states in the quantum well and in the quantum dot and study the resulting electron kinetics. We show that transition efficiency crucially depends on the system geometry. We show also that under some conditions, transition efficiency can decrease with the temperature.

Using a modal approach, we compute the Purcell factor for spherical resonators. The analytic expressions of the normal modes of the resonator are used to calculate the effective volumes. Then we show that the effective volume associated with each normal mode is related to the translation-addition coefficients of a displaced dipole. Finally, we consider a silicon resonator homogeneously doped with electric dipolar emitters, and we show that the average electric Purcell factor dominates over the magnetic one.

We study the effect of bias voltage on the nuclear spin polarization of a ballistic wire, which contains electrons and nuclei interacting via hyperfine interaction. In equilibrium, the localized nuclear spins are helically polarized due to the electron-mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Focusing here on nonequilibrium, we find that an applied bias voltage induces a uniform polarization, from both helically polarized and unpolarized spins available for spin flips. Once a macroscopic uniform polarization in the nuclei is established, the nuclear spin helix rotates with frequency proportional to the uniform polarization. The uniform nuclear spin polarization monotonically increases as a function of both voltage and temperature, reflecting a thermal activation behavior. Our predictions offer specific ways to test experimentally the presence of a nuclear spin helix polarization in semiconducting quantum wires.

We report a combined experimental and theoretical study of the Kondo effect in a series of binuclear metal-organic complexes of the form $\left[{\left(\mathrm{Me}\right(\mathrm{hfacac})}_2{)}_2\left(\mathrm{bpym}\right)\right]{}^0$, with Me = nickel (II), manganese (II), zinc (II); hfacac = hexafluoroacetylacetonate, and bpym = bipyrimidine, adsorbed on Cu(100) surface. While Kondo features did not appear in the scanning tunneling spectroscopy spectra of nonmagnetic ${\mathrm{Zn}}_2$, a zero bias resonance was resolved in magnetic ${\mathrm{Mn}}_2$ and ${\mathrm{Ni}}_2$ complexes. The case of ${\mathrm{Ni}}_2$ is particularly interesting as the experiments indicate two adsorption geometries with very different properties. For ${\mathrm{Ni}}_2$ complexes we have employed density functional theory to further elucidate the situation. Our simulations show that one geometry with relatively large Kondo temperatures $T_{\mathrm{K}}\sim 10$ K can be attributed to distorted ${\mathrm{Ni}}_2$ complexes, which are chemically bound to the surface via the bipyrimidine unit. The second geometry we assign to molecular fragmentation: we suggest that the original binuclear molecule decomposes into two pieces, including Ni(hexafluoroacetylacetonate)${}_2$, when brought into contact with the Cu substrate. For both geometries our calculations support a picture of the $\left(S=1\right)$-type Kondo effect emerging due to open $3d$ shells of the individual ${\mathrm{Ni}}^{2+}$ ions.

We studied the two-step photon absorption (TSPA) process in InAs/GaAs quantum-dot superlattice (QDSL) solar cells. TSPA of subband-gap photons efficiently occurs when electrons are pumped from the valence band to the states above the inhomogeneously distributed fundamental states of QDSLs. The photoluminescence (PL)-excitation spectrum demonstrates an absorption edge attributed to the higher excited states of the QDSLs in between the InAs wetting layer states and the fundamental states of QDSLs. When the absorption edge of the excited state was resonantly excited, the superlinear excitation power dependence of the PL intensity demonstrated that the electron and hole created by the interband transition separately relax into QDSLs. Furthermore, time-resolved PL measurements demonstrated that the electron lifetime is extended by thereby inhibiting recombination with holes, enhancing the second subband-gap absorption.

We theoretically propose a spin-dependent electronic transport mechanism in which the spin-unpolarized electron beam is split into different directions depending on spins at an atomic domain boundary in nonmagnetic material. Specifically, we calculate the electronic transmission across a boundary between the monolayer and bilayer of the transition-metal dichalcogenide, and demonstrate that up-spin and down-spin electrons entering the boundary are refracted and collimated to opposite directions. The phenomenon is attributed to the strong spin-orbit interaction, the trigonally warped Fermi surface, and the different crystal symmetries between the monolayer and bilayer systems. The spin-dependent refraction suggests a potential application for a spin splitter, which spatially separates up-spin and down-spin electrons simply by passing the electric current through the boundary.

We consider the lightly doped cuprates ${\mathrm{Y}}_{0.97}{\mathrm{Ca}}_{0.03}{\mathrm{BaCuO}}_{6.05}$ and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (with $x=0.02$, 0.04), where the presence of a fluctuating nematic state has often been proposed as a precursor of the stripe or, more generically charge density wave phase, which sets in at higher doping. We phenomenologically assume quantum critical longitudinal and transverse nematic, and charge-ordering fluctuations, and investigate their effects in the Raman spectra. We find that the longitudinal nematic fluctuations peaked at zero transferred momentum account well for the anomalous Raman absorption observed in these systems in the $B_{2g}$ channel, while the absence of such an effect in the $B_{1g}$ channel may be due to the overall suppression of Raman response at low frequencies, associated with the pseudogap. While in ${\mathrm{Y}}_{0.97}{\mathrm{Ca}}_{0.03}{\mathrm{BaCuO}}_{6.05}$ the low-frequency line shape is fully accounted for by longitudinal nematic collective modes alone, in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, also charge-ordering modes with finite characteristic wave vector are needed to reproduce the shoulders observed in the Raman response. This different involvement of the nearly critical modes in the two materials suggests a different evolution of the nematic state at very low doping into the nearly charge-ordered state at higher doping.

We have investigated the temperature-dependent evolution of electronic structures and magnetic properties of an itinerant ferromagnet ${\mathrm{SrRuO}}_3$, employing the combined scheme of density functional theory and dynamical mean-field theory (DFT+DMFT). The inclusion of finite dynamical correlation effects beyond DFT well describes not only the incoherent hump structure observed in the photoemission experiment but also the temperature-dependent magnetic properties in accordance with experiments. We have shown that the magnetization of ${\mathrm{SrRuO}}_3$ evolves with the Stoner behavior below the Curie temperature $T_c$, reflecting the weak itinerant ferromagnetic behavior, but the local residual magnetic moment persists even above $T_c$, indicating the local magnetic moment behavior. We suggest that the ferromagnetism of ${\mathrm{SrRuO}}_3$ has a dual nature of both weak and local moment limits, even though the magnetism of ${\mathrm{SrRuO}}_3$ is more itinerant than that of Fe.

A metallic surface is realized on stoichiometric and insulating (100) ${\mathrm{SrTiO}}_3$ by ${\mathrm{Ar}}^{+}$-ion irradiation. The sheet carrier density and Hall mobility of the layer are $\sim 4.0\times {10}^{14}{\text{cm}}^{-2}$ and $\sim 2\times {10}^3{\text{cm}}^2/\text{Vs}$, respectively, at 15 K for the irradiation dose of $\sim 4.2\times {10}^{18}\text{ions}/{\text{cm}}^2$. These samples display ultraviolet light sensitive photoconductivity (PC) which is enhanced abruptly below the temperature ($\approx 100$ K) where ${\mathrm{SrTiO}}_3$ crystal undergoes an antiferrodistortive cubic-to-tetragonal $\left(O_h^1\to D_{4h}^{18}\right)$ structural phase transition. This behavior of PC maps well with the temperature dependence of dielectric function and electric field induced conductivity. The longevity of the PC state also shows a distinct change below $\approx 100$ K. At $T>100$ K its decay is thermally activated with an energy barrier of $\approx 36$ meV, whereas at $T<100$ K it becomes independent of temperature. We have examined the effect of electrostatic gating on the lifetime of the PC state. One nontrivial result is the ambient temperature quenching of the photoconducting state by the negative gate field. This observation opens avenues for designing a solid state photoelectric switch. The origin and lifetime of the PC state are understood in the light of field effect induced band bending, defect dynamics, and thermal relaxation processes.

We investigate lattice ordering phenomena for the heterovalent ternaries that are based on the wurtzite lattice, under the constraint that the octet rule be preserved. We show that, with the single exception of a highly symmetric twinned structure, all allowed lattice orderings can be described by a pseudospin model corresponding to the two different stackings of $ABAB$ rows of atoms in the basal plane that occur in the $Pna{2}_1$ and $Pmc{2}_1$ crystal structures. First-principles calculations show that the difference in the energies of formation between these two structures is $13\pm 3$ meV/fu (formula unit) for ${\text{ZnSnN}}_2$ and is an order of magnitude larger for ${\text{ZnGeN}}_2$ and that for both materials the $Pm31$ structure, which contains only octet-rule-violating tetrahedra, has a significantly higher energy of formation and a signficantly lower band gap. We predict almost random stacking and wurtzitelike x-ray-diffraction spectra in the case of ${\text{ZnSnN}}_2$, consistent with reported measurements. The octet-rule-preserving model of disorder proposed here predicts a band gap that for ${\text{ZnSnN}}_2$ is relatively insensitive to ordering, in contrast to the prevailing model, which invokes the random placement of atoms on the cation sublattice. The violations of the octet rule in the latter model lead to significant narrowing of the band gap. The Raman and photoluminescence spectra of ${\text{ZnSnN}}_2$ are interpreted in light of the ordering model developed here. The observation that ${\text{ZnGeN}}_2$ orders in the $Pna{2}_1$ structure under appropriate growth conditions is consistent with the larger difference in the energies of formation of the $Pna{2}_1$ and $Pmc{2}_1$ structures for this material. The ordering model presented here has important implications for the optical, electronic, and lattice properties of all wurtzite-based heterovalent ternaries.

The surfaces of three-dimensional topological insulators characterized by spin-helical Dirac fermions provide a fertile ground for realizing exotic phenomena, and they have the potential for wide-ranging applications. To realize most of their special properties, the Dirac point must be located near the Fermi energy with a bulk insulating property. However, this is barely achieved in most of the discovered topological insulators. It was found recently that ${\mathrm{TlBiSe}}_2$ features an in-gap Dirac point, where the upper and lower parts of the Dirac-cone surface state are both utilized. Nevertheless, investigations of the surface transport properties of this material are limited due to the lack of bulk insulating characteristics. Here, we present a realization of the bulk insulating property by tuning the composition of ${\mathrm{Tl}}_{1-x}{\mathrm{Bi}}_{1+x}{\mathrm{Se}}_{2-\delta }$ without introducing guest atoms. This result promises to shed light on exotic topological phenomena on the surface.

At covalent semiconductor interfaces, the band alignment is determined by the location of the band edges with respect to the charge neutrality level, but extension of this method to more ionic semiconductor systems requires further consideration. Using the charge neutrality level concept, a type-III (or broken band gap) band offset is predicted at the interface between $n$-type CdO and $p$-type SnTe. Employing hard x-ray photoelectron spectroscopy, we report on the chemical composition at the buried interface and the valence-band offset. Chemical intermixing at the interface between SnTe and CdO is found to be limited to $\sim 2.5$ nm in our heterojunction samples. We measure a valence-band offset of 1.$95(\pm 0.15$ eV) irrespective of the layer configuration. Once the degenerate hole doping of the SnTe is considered, the measured band-edge offset agrees with the type-III offset predicted from alignment of the band edges with respect to the charge neutrality level of the semiconductors.

Giant Kerr rotation and ellipticity are observed and investigated in an asymmetric planar microcavity with a quantum well in the active region. Rotation angle of the polarization plane as well as ellipticity were determined from time- and frequency-resolved measurements of the Stokes vector components of reflected light. It was found that in a small range of the cavity mode detunings the polarized pump pulse creates a large splitting of the lower polariton branch while leaving its linewidth almost the same. This fact gives a possibility to observe at such detunings the Kerr rotation angle and ellipticity, close to their extremes. A theoretical analysis shows that the decisive role in reaching extreme polarization rotation angles is played by the structure asymmetry. Comprehensive analysis of the polarization state of the light in this regime shows that both renormalization of the exciton energy and the saturation of the excitonic resonance contribute to the observed optical nonlinearities.