Organic solar cells separate strongly bound electron-hole pairs into free charges at interfaces between electron donor and acceptor organic semiconductors. Recently, electron-hole separation was observed on femtosecond time scales near crystallite phases of the fullerene derivative [6,6]-phenyl-${\mathrm{C}}_{71}$-butyric acid methyl ester (PCBM), which is incompatible with conventional Marcus theories of organic transport. Here we show that ultrafast charge transport in PCBM arises from its broad range of electronic eigenstates, provided by three closely spaced electronic bands near the lowest unoccupied molecular orbital. The highest band provides a charge transfer state resonant with delocalized states of the lower two bands away from the interface. This state acts as a bridge between the donor phase and the acceptor bulk, bypassing the trapped charge-transfer (CT) states below. Vibrational fluctuations enable rapid electronic transitions across this bridge, which can drive the electron more than 4 nm away from the interface within 100 fs. All this is demonstrated within a simple tight-binding Hamiltonian containing transfer integrals no larger than 8 meV.

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.

Several microscopic pathways have been proposed to explain the large magnetic effects observed in organic semiconductors; however, it is difficult to identify and characterize the microscopic process which actually influences the overall magnetic field response in a particular instance. Pulsed electrically detected magnetic resonance provides an ideal platform for this task as it intrinsically monitors the charge carriers of interest and provides dynamical information which is inaccessible through conventional magnetoconductance measurements. Here we develop a general time-domain theory to describe the spin-dependent recombination of exciton-polaron complexes following the coherent manipulation of paramagnetic centers through electron paramagnetic resonance. A general Hamiltonian is treated, and it is shown that the transition frequencies and resonance positions of the exciton-polaron complex can be used to estimate interspecies coupling. This work also provides a general formalism for analyzing multipulse experiments which can be used to extract relaxation and transport rates.

Group theory analysis for two-dimensional elemental systems related to phosphorene is presented, including (i) graphene, silicene, germanene, and stanene; (ii) their dependence on the number of layers; and (iii) their two possible stacking arrangements. Departing from the most symmetric $D_{6h}^1$ graphene space group, the structures are found to have a group-subgroup relation, and analysis of the irreducible representations of their lattice vibrations makes it possible to distinguish between the different allotropes. The analysis can be used to study the effect of strain, to understand structural phase transitions, to characterize the number of layers, crystallographic orientation, and nonlinear phenomena.

In-plane thermal conduction and phonon transport in both single-crystalline and polycrystalline Si two-dimensional phononic crystal (PnC) nanostructures were investigated at room temperature. The impact of phononic patterning on thermal conductivity was larger in polycrystalline Si PnCs than in single-crystalline Si PnCs. The difference in the impact is attributed to the difference in the thermal phonon mean free path (MFP) distribution induced by grain boundary scattering in the two materials. Grain size analysis and numerical simulation using the Monte Carlo technique indicate that grain boundaries and phononic patterning are efficient phonon scattering mechanisms for different MFP length scales. This multiscale phonon scattering structure covers a large part of the broad distribution of thermal phonon MFPs and thus efficiently reduces thermal conduction.

The $\alpha \text{-Fe}\leftrightarrow \epsilon \text{-Fe}$ pressure-induced transformation under pure hydrostatic static compression has been characterized with in situ x-ray diffraction using $\alpha \text{-Fe}$ single crystals as starting samples. The forward transition starts at 14.9 GPa, and the reverse at 12 GPa, with a width of $\alpha -\epsilon$ coexistence domain of the order of 2 GPa. The elastic stress in the sample increases in this domain, and partially relaxes after completion of the transformation. Orientation relations between parent $\alpha \text{-Fe}$ and child $\epsilon \text{-Fe}$ have been determined, which definitely validates the Burgers path for the direct transition. On the reverse transition, an unexpected variant selection is observed. X-ray diffraction data, complemented with ex situ microstructural observations, suggest that this selection is caused by defects and stresses accumulated during the direct transition.

We use numerical linked cluster expansions to study the thermodynamic properties of the two-dimensional classical Ising, quantum $XY$, and quantum Heisenberg models with bimodal random-bond disorder on the square and honeycomb lattice. In all cases, the nearest-neighbor coupling between the spins takes values $\pm J$ with equal probability. We obtain the disorder-averaged (over all disorder configurations) energy, entropy, specific heat, and uniform magnetic susceptibility in each case. These results are compared with the corresponding ones in the clean models. Analytic expressions are obtained for low orders in the expansion of these thermodynamic quantities in inverse temperature.

We have investigated the magneto-optical response of Fe epitaxial films by femtosecond pump-probe polarimetry in a broad probe spectral region ($1.8\text{–}2.6$ eV). From the extrapolated photoinduced variation of the conductivity tensor, spin and charge dynamics have been disentangled. In particular, the analysis of the off-diagonal tensor element rules out any appreciable modification of the electronic band structure upon laser excitation and suggests that ultrafast demagnetization is determined by collective excitations, i.e., spin fluctuations. Our experimental evidence provides significant insight into the microscopic mechanisms governing the complex spin dynamics of metals in the subpicosecond time scale.

Excitation of a metallic ferromagnet such as Ni with an intensive femtosecond laser pulse causes an ultrafast demagnetization within approximately 300 fs. It was proposed that the ultrafast demagnetization measured in femtosecond magneto-optical experiments could be due to relativistic light-induced processes. We perform an ab initio investigation of the influence of relativistic effects on the magneto-optical response of Ni. To this end, first, we develop a response theory formulation of the additional appearing ultrarelativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian due to the electromagnetic field, and second, we compute the influence of relativistic light-induced spin-flip transitions on the magneto-optics. Our ab initio calculations of relativistic spin-flip optical excitations predict that these can give only a very small contribution $(\le 0.1\%)$ to the laser-induced magnetization change in Ni.

We report structural and magnetic properties studies of large high-quality single crystals of the frustrated magnet ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. Powder x-ray diffraction analysis confirms that ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron-scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the $〈111〉$ axes of the ${\mathrm{Nd}}^{3+}$ ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below $T\sim 7$ K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.

Monosilicides of $3d$ metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ series and their mutual relationship is found to be dependent on the chemical composition. The chirality of crystal structure was previously shown to be governed by crystal growth, and the value of the DMI is nearly the same for all monosilicides of Fe, Co, and Mn. Our findings indicate that the sign of the DMI in ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ is controlled by the Co composition $x$. We have been able to directly measure the change of the link between structure and magnetism in this helimagnetic B20 alloy.

We present further progress in a systematic approach to the microscopic understanding of the dynamics of strongly interacting quantum fluids. Employing the concept of dynamic multiparticle fluctuations, we derive equations of motion for fluctuating $n$-body densities. We apply the theory to calculate the dynamic structure function of liquid ${}^4\mathrm{He}$ as a function of density and find, without any phenomenological input, overall excellent agreement with both experiments and, as far as available, simulation data.

We use the nonequilibrium Gross-Pitaevskii equation in the form recently derived [H. Haug et al., Phys. Rev. B 89, 155302 (2014)] to study spontaneous pattern formation and the connected superfluid current patterns. Using a nonresonant excitation beam with ring structure we get depending on the details of the structure and of the pump power, different spatial patterns of the condensate density. The corresponding phase profiles allow to identify, e.g., vortex-antivortex pairs, but beyond that, yield an image of the superfluid flow patterns linked with the structured condensate density. The fast superfluid flow driven by the spatially changing phase with velocities of the order of several $m\mu /ps$ is found to be often supersonic. In order to test dynamically the stability of the spontaneously formed flow patterns under external perturbations, we apply an additional resonant Laguerre-Gauss beam with angular momentum. This beam causes complex response of the phase patterns. This response is shown to be basically an oscillation or rotation of the vortex-antivortex pair depending on the strength of the extra beam. The rotation is induced via a ring of vortices induced by the Laguerre-Gaus beam. The main result of these studies is the extraordinary stability of the vortex-antivortex pair even under strongly perturbing external fields.

We demonstrate room-temperature spin-polarized ultrafast $(\sim 10\mathrm{ps})$ lasing in a highly optically excited GaAs microcavity. This microcavity is embedded with InGaAs multiple quantum wells in which the spin relaxation time is less than 10 ps. The laser radiation remains highly circularly polarized even when excited by nonresonant elliptically polarized light. The lasing energy is not locked to the bare cavity resonance, and shifts $\sim 10\mathrm{meV}$ as a function of the photoexcited density. Such spin-polarized lasing is attributed to a spin-dependent stimulated process of correlated electron-hole pairs. These pairs are formed near the Fermi edge in a high-density electron-hole plasma coupled to the cavity light field.

The thermal boundary resistance of Si/Ge interfaces has been determined using approach-to-equilibrium molecular dynamics simulations. Assuming a reciprocal linear dependence of the thermal boundary resistance, a length-independent bulk thermal boundary resistance could be extracted from the calculation resulting in a value of $3.76\times {10}^{-9}$ ${\mathrm{m}}^2$ K/W for a sharp Si/Ge interface and thermal transport from Si to Ge. Introducing an interface with finite thickness of 0.5 nm consisting of a SiGe alloy, the bulk thermal resistance slightly decreases compared to the sharp Si/Ge interface. Further growth of the boundary leads to an increase in the bulk thermal boundary resistance. When the heat flow is inverted (Ge to Si), the thermal boundary resistance is found to be higher. From the differences in the thermal boundary resistance for different heat flow direction, a rectification factor of the Si/Ge interface can be determined and is found to significantly decrease when the sharp interface is moderated by introduction of a SiGe alloy in the boundary layer.

While monolayer forms of two-dimensional materials are well characterized both experimentally and theoretically, properties of bilayer heterostructures are not nearly so well known. We employ high-accuracy linear-scaling density functional theory calculations utilizing nonlocal van der Waals functionals to explore the possible constructions of the ${\mathrm{MoS}}_2/{\mathrm{MoSe}}_2$ interface. Utilizing large supercells, we vary rotation, translation, and separation of the layers without introducing unrealistic strain. The energy landscape shows very low variations under rotation, with no strongly preferred alignments. By unfolding the spectral function into the primitive cells, we show that the monolayers are more independent than in homo-bilayers and that the electronic band structure of each layer is tunable through rotation, thus influencing hole effective masses.

On-chip magnets can be used to implement relatively large local magnetic field gradients in nanoelectronic circuits. Such field gradients provide possibilities for all-electrical control of electron spin qubits where important coupling constants depend crucially on the detailed field distribution. We present a double quantum dot (QD) hybrid device laterally defined in a GaAs/AlGaAs heterostructure which incorporates two single-domain nanomagnets. They have appreciably different coercive fields which allows us to realize four distinct configurations of the local inhomogeneous field distribution. We perform dc transport spectroscopy in the Pauli-spin blockade regime as well as electric-dipole-induced spin resonance (EDSR) measurements to explore our hybrid nanodevice. Characterizing the two nanomagnets we find excellent agreement with numerical simulations. By comparing the EDSR measurements with a second double QD incorporating just one nanomagnet we reveal an important advantage of having one magnet per QD: It facilitates strong field gradients in each QD and allows us to control the electron spins individually for instance in an EDSR experiment. With just one single-domain nanomagnet and common QD geometries EDSR can likely be performed only in one QD.

Doping the ${\mathrm{LaAlO}}_3$ (LAO) side of the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface with 2% Tm or Lu does not significantly affect its electron transport. Also, at low temperatures, carrier mobility is steeply anticorrelated with carrier concentration for both doped and undoped interfaces. This relationship cannot be explained by ionized impurities alone but may be driven by positive charge in or on the LAO film.

We report an angle-resolved photoemission measurement of the wave-vector-dependent plasmon satellite structure of a three-dimensional solid, crystalline silicon. In sharp contrast to nanomaterials, which typically exhibit strongly wave-vector-dependent low-energy plasmons, the large plasmon energy of silicon facilitates the search for a plasmaron state consisting of resonantly bound holes and plasmons and its distinction from a weakly interacting plasmon-hole pair. Employing a first-principles theory, which is based on a cumulant expansion of the one-electron Green's function and contains significant electron correlation effects, we obtain good agreement with the measured photoemission spectrum for the wave-vector-dependent dispersion of the satellite feature, but without observing the existence of plasmarons in the calculations.