The growth of Co-substituted $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO) films on Ge(001) substrates by molecular beam epitaxy is demonstrated. Energy-dispersive x-ray spectroscopy and transmission electron microscopy images confirm the uniform Co distribution. However, no evidence of magnetic ordering is observed in samples grown for Co concentrations between 2% and 40%. Piezoresponse force microscopy measurements show that a 5% Co-substituted BTO sample exhibits ferroelectric behavior. First-principles calculations indicate that while Co atoms couple ferromagnetically in the absence of oxygen vacancies, the occurrence of oxygen vacancies leads to locally antiferromagnetically coupled complexes with relatively strong spin coupling. The presence of a significant amount of oxygen vacancies is suggested by x-ray photoelectron spectroscopy measurements.

Silicene and germanene are key materials for the field of valleytronics. However, interaction with the substrate, which is necessary to support the electronically active medium, becomes a major obstacle. In the present work, we propose a substrate (F-doped ${\mathrm{WS}}_2$) that avoids detrimental effects and at the same time induces the required valley polarization, so that no further steps are needed for this purpose. The behavior is explained by proximity effects on silicene or germanene, as demonstrated by first-principles calculations. Broken inversion symmetry due to the presence of ${\mathrm{WS}}_2$ opens a substantial band gap in silicene or germanene. F doping of ${\mathrm{WS}}_2$ results in spin polarization, which, in conjunction with proximity-enhanced spin-orbit coupling, creates sizable spin-valley polarization.

Using the Keldysh Green function technique, we calculate the finite-frequency correlator between the electrical current and the heat current flowing through a quantum dot connected to reservoirs. At equilibrium, we find that this quantity, called mixed noise, is linked to the thermoelectric ac conductance by the fluctuation-dissipation theorem. Out of equilibrium, we discuss its spectrum and find evidence of the close relationship between the mixed noise and the thermopower. We study the spectral coherence and identify the conditions to have a strong correlation between the electrical and heat currents. The change in the spectral coherence due to the presence of a temperature gradient between the reservoirs is also highlighted.

We have examined the effects of field cooling on the phase diagram of the relaxor system $(1-x)\mathrm{Pb}\left(\mathrm{Z}{\mathrm{n}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3-x\mathrm{PbTi}{\mathrm{O}}_3$ (PZN-$x\mathrm{PT}$) for compositions near the morphotropic phase boundary (MPB). High-resolution diffraction measurements using Cu $K_{\alpha }$ x rays, which probe $\approx 3\mu \mathrm{m}$ below the crystal surface, were made on field-cooled (FC) single-crystal specimens of PZN-$4.5\%\mathrm{PT}$ and PZN-$6.5\%\mathrm{PT}$ under electric fields of 1 and 2 kV/cm applied along [001] and combined with previous neutron diffraction data, which probe the entire crystal volume, for FC PZN-$8\%\mathrm{PT}$ [Ohwada et al., Phys. Rev. B 67, 094111 (2003)]. A comparison to the zero-field-cooled (ZFC) PZN-$x\mathrm{PT}$ phase diagram reveals several interesting features: (1) The short-range monoclinic phase observed in the ZFC state on the low-PT side of the MPB is replaced by a monoclinic $M_{\mathrm{A}}$ phase; (2) field cooling extends the tetragonal phase to higher temperatures and lower-PT concentrations; (3) the orthorhombic phase near the MPB is replaced by a monoclinic $M_{\mathrm{C}}$ phase; (4) the vertical MPB in the ZFC phase diagram bends significantly towards the low-PT side in the FC state. These results demonstrate that both the phase stability and the nature of the MPB in PZN-PT within the near-surface regions are fragile in the presence of electric fields.

Finite-$Q$ magnetic instabilities are rather common in frustrated magnets. When the magnetic susceptibility is maximized at multiple-$Q$ vectors related through lattice symmetry operations, exotic magnetic orderings such as vortex and skyrmion crystals may follow. Here, we show that a periodic array of nonmagnetic impurities, which can be realized through charge density wave ordering, leads to a rich phase diagram featuring a plethora of chiral magnetic phases, especially when there is a simple relation between the reciprocal vectors of the impurity superlattice and the magnetic $Q$ vectors. We also investigate the effect of changing the impurity concentration or disturbing the impurity array with small quenched randomness. Alternative realizations of impurity superlattices are briefly discussed.

Magnetic skyrmions are candidates for a new generation of information carriers due to their nanometric size and topologically protected stability. The study of the dynamics of such skyrmions in racetrack geometries and, in general, in confined geometries becomes essential to achieve these goals. Here we present analytical expressions for the trajectories of skyrmions in confined geometries (including long tracks and squared samples) when driven by polarized electric currents. The force exerted by the borders is derived as a function of the material parameters (anisotropy, Dzyaloshinskii-Moriya, and exchange constants) and incorporated in the equation of motion of the skyrmions. We find the general expressions for the trajectories in several possible scenarios, and we also find simple expressions for the stationary position, the velocity, as well as for the threshold velocity of the driving electrons that could overcome the edge repulsion. In fully confined geometries, the geometric edges can produce a naturally oscillating movement, allowing for dynamical resonances if the driving current is an ac current with adequate frequency. Numerical micromagnetic simulations considering typical experimental parameters are used to corroborate the theory.

$\mathrm{SrTi}{\mathrm{O}}_3$ undergoes a cubic to tetragonal phase transition at $T_c=105–110\mathrm{K}$, which can be described by a Brillouin zone corner ${\Gamma }_{25}$ (111) soft phonon. Even though clear anomalies in specific heat, thermal expansion coefficient, and sound velocity have been observed, the correlation between phase transition and electronic transport properties in $n$-type doped $\mathrm{SrTi}{\mathrm{O}}_3$ is still controversial. Here, we report phase transition induced electronic transport anomaly in temperature dependence of the temperature coefficient of resistance (TCR) consistently observed in a wide variety of $\mathrm{SrTi}{\mathrm{O}}_3$-based systems by detailed transport measurements and first-principles calculations. The observed TCR anomaly, which can be well fitted with the electron–${\Gamma }_{25}$ soft phonon scattering around $T_c$, is found to be caused by anomalies in both mobility and carrier density, with the former taking the dominant role. Moreover, the magnitude of the anomaly is found to decrease with increasing carrier density. These findings demonstrate the role of the electron–${\Gamma }_{25}$ soft phonon scattering in the conduction mechanism in $\mathrm{SrTi}{\mathrm{O}}_3$-based systems.

We explore the viability of using transition-metal impurities as $n$-type dopants in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, focusing on W, Mo, Re, and Nb. Our first-principles calculations show that these impurities can incorporate on both crystallographically inequivalent Ga sites, with the octahedrally coordinated sites being preferred. Mo and Re behave as deep donors. Tungsten on a tetrahedral site is a shallow donor, but unfortunately W on an octahedral site is much lower in energy. Niobium emerges as the best candidate for $n$-type doping: It has a low formation energy, is a shallow donor on the tetrahedral site, and has only a modest ionization energy (0.15 eV) on the octahedral site.

The electrostatic environment of aqueous systems is an essential ingredient for the function of any living system. To understand the electrostatic properties and their molecular foundation in soft, living, and three-dimensional systems, we developed a table-top model-free method to determine the surface potential of nano- and microscopic objects in aqueous solutions. Angle-resolved nonresonant second harmonic (SH) scattering measurements contain enough information to determine the surface potential unambiguously, without making assumptions on the structure of the interfacial region. The scattered SH light that is emitted from both the particle interface and the diffuse double layer can be detected in two different polarization states that have independent scattering patterns. The angular shape and intensity are determined by the surface potential and the second-order surface susceptibility. Calibrating the response with the SH intensity of bulk water, a single, unique surface potential value can be extracted. We demonstrate the method with 80 nm bare oil droplets in water and $\sim 50\mathrm{nm}$ dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylserine (DOPS) liposomes at various ionic strengths.

We study the carrier-density-dependent recombination dynamics in $m$-plane InGaN/GaN multiple quantum wells in the presence of $n$-type background doping by time-resolved photoluminescence. Based on Fermi's golden rule and Saha's equation, we decompose the radiative recombination channel into an excitonic and an electron-hole pair contribution, and extract the injected carrier-density-dependent bimolecular recombination coefficients. Contrary to the standard electron-hole picture, our results confirm the strong influence of excitons even at room temperature. Indeed, at 300 K, excitons represent up to 63 ± 6% of the photoexcited carriers. In addition, following the Shockley-Read-Hall model, we extract the electron and hole capture rates by deep levels and demonstrate that the increase in the effective lifetime with injected carrier density is due to asymmetric capture rates in presence of an $n$-type background doping. Thanks to the proper determination of the density-dependent recombination coefficients up to high injection densities, our method provides a way to evaluate the importance of Auger recombination.

We investigated optical spin orientation and dynamic nuclear polarization (DNP) in individual self-assembled InGaAs/GaAs quantum dots (QDs) doped by a single Mn atom, a magnetic impurity providing a neutral acceptor $A^0$ with an effective spin $J=1$. We find that the spin of an electron photocreated in such a quantum dot can be efficiently oriented by a quasiresonant circularly polarized excitation. For the electron spin levels which are made quasidegenerate by a magnetic field compensating the exchange interaction ${\mathrm{\Delta }}_e$ with $A^0$, there is however a full depolarization due the anisotropic part of the exchange. Still, in most studied QDs, the spin polarized photoelectrons give rise to a pronounced DNP which grows with a longitudinal magnetic field until a critical field where it abruptly vanishes. For some QDs, several replica of such DNP sequence are observed at different magnetic fields. This striking behavior is qualitatively discussed as a consequence of different exchange interactions experienced by the electron, driving the DNP rate via the energy cost of electron-nucleus spin flip-flops.

Ultrafast and strong optical excitation of a molecular system is considered which is formed by a regular one-dimensional arrangement of identical molecules. As it is typical for zinc chlorine-type molecules the transition energy from the ground state to the first excited singlet state is assumed to be smaller than the energy difference between the first excited state and the following one. This enables the creation of many excitons without their immediate quenching due to exciton-exciton annihilation. As a first step into the field of dense Frenkel-exciton systems the present approach stays at a mean-field type of description and ignores vibrational contributions. The resulting nonlinear kinetic equations mix Rabi-type oscillations with those caused by energy transfer and suggest an excitation-dependent narrowing of the exciton band. The indication of this effect in the framework of a two-color pump-probe experiment and of the detection of photon emission is discussed.

We study the Aharonov-Bohm effect in a two-terminal helical ring with long-range hopping in the presence of Rashba spin-orbit interaction. We explore how the spin polarization behavior changes depending on the applied magnetic flux and the incoming electron energy. The most interesting feature that we articulate in this system is that zero-energy crossings appear in the energy spectra at $\mathrm{\Phi }=0$ and also at integer multiples of half-flux quantum values ($n{\mathrm{\Phi }}_0/2,n$ being an integer) of the applied magnetic flux. We investigate the transport properties of the ring using Green's function formalism and find that the zero-energy transmission peaks corresponding to those zero-energy crossings vanish in the presence of Rashba spin-orbit interaction. We also incorporate static random disorder in our system and show that the zero-energy crossings and transmission peaks are not immune to disorder even in the absence of Rashba spin-orbit interaction. The latter prevents the possibility of these helical states in the ring behaving like topological insulator edge states.

We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find two sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transitions between singlet and triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin based Bell inequalities.

The electromagnetic response of a two-dimensional metal embedded in a periodic array of a dielectric host can give rise to a plasmonic Dirac point that emulates epsilon-near-zero (ENZ) behavior. This theoretical result is extremely sensitive to structural features like periodicity of the dielectric medium and thickness imperfections. We propose that such a device can actually be realized by using graphene as the two-dimensional metal and materials like the layered semiconducting transition-metal dichalcogenides or hexagonal boron nitride as the dielectric host. We propose a systematic approach, in terms of design characteristics, for constructing metamaterials with linear, elliptical, and hyperbolic dispersion relations which produce ENZ behavior, normal or negative diffraction.