The electronic and optical properties of zinc-blende (zb) Al${}_x$Ga${}_{1-x}$N over the whole alloy composition range are presented in a joint theoretical and experimental study. Because zb-GaN is a direct (${\Gamma }_v\to {\Gamma }_c$) semiconductor and zb-AlN shows an indirect (${\Gamma }_v\to X_c$) fundamental band gap, the ternary alloy exhibits a concentration-dependent direct-indirect band gap crossing point the position of which is highly controversial. The dielectric functions of zb-Al${}_x$Ga${}_{1-x}$N alloys are measured employing synchrotron-based ellipsometry in an energy range between 1 and 20 eV. The experimentally determined fundamental energy transitions originating from the $\Gamma$, $X$, and $L$ points are identified by comparison to theoretical band-to-band transition energies. In order to determine the direct-indirect band gap crossing point, the measured transition energies at the $X$ point have to be aligned by the calculated position of the highest valence state. Thereby density-functional theory (DFT) based approaches to the electronic structure, ranging from the standard (semi)local generalized gradient approximation (GGA), self-energy corrected local density approximation (LDA-1/2), and meta-GGA DFT (TB-mBJLDA) to hybrid functional DFT and many-body perturbation theory in the $GW$ approximation, are applied to random and special quasirandom structure models of zb-Al${}_x$Ga${}_{1-x}$N. This study provides interesting insights into the accuracy of the various numerical approaches and contains reliable ab initio data on the electronic structure and fundamental alloy band gaps of zb-Al${}_x$Ga${}_{1-x}$N. Nonlocal Heyd-Scuseria-Ernzerhof-type hybrid-functional DFT calculations or, alternatively, $GW$ quasiparticle calculations are required to reproduce prominent features of the electronic structure. The direct-indirect band gap crossing point of zb-Al${}_x$Ga${}_{1-x}$N is found in the Al rich composition range at an Al content between $x=0.64$ and $0.69$ in hybrid functional DFT, which is in good agreement with $x=0.71$ determined from the aligned experimental transition energies. Thus our study solves the long-standing debate on the nature of the fundamental zb-Al${}_x$Ga${}_{1-x}$N alloy band gap.

The universal behavior of two-dimensional loop models can change dramatically when loops are allowed to cross. We study models with crossings both analytically and with extensive Monte Carlo simulations. Our main focus (the “completely packed loop model with crossings”) is a simple generalization of well-known models that shows an interesting phase diagram with continuous phase transitions of a new kind. These separate the unusual “Goldstone” phase observed previously from phases with short loops. Using mappings to ${\mathbb{Z}}_2$ lattice gauge theory, we show that the continuum description of the model is a replica limit of the $\sigma$ model on real projective space (${\mathbb{RP}}^{n-1}$). This field theory sustains ${\mathbb{Z}}_2$ point defects, which proliferate at the transition. In addition to studying the new critical points, we characterize the universal properties of the Goldstone phase in detail, comparing renormalization group (RG) calculations with numerical data on systems of linear size up to $L={10}^6$ at loop fugacity $n=1$. (Very large sizes are necessary because of the logarithmic form of correlation functions and other observables.) The model is relevant to polymers on the verge of collapse, and a particular point in parameter space maps to self-avoiding trails at their $\Theta$ point; we use the RG treatment of a perturbed $\sigma$ model to resolve some perplexing features in the previous literature on trails. Finally, one of the phase transitions considered here is a close analog of those in disordered electronic systems—specifically, Anderson metal-insulator transitions—and provides a simpler context in which to study the properties of these poorly understood (central-charge-zero) critical points.

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC) phase in BaCuSi${}_2$O${}_6$, we performed detailed ${}^{63}$Cu and ${}^{29}$Si NMR above the critical magnetic field, $H_{c1}=23.4$ T. The two different alternating layers present in the system have very different local magnetizations close to $H_{c1}$; one is very weak, and its size and field dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could only be determined by “on-site” ${}^{63}$Cu NMR, and the data are fully reproduced by a model of interacting hard-core bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed.

We present a magnetophotoluminescence study on neutral and charged excitons confined to InAs/GaAs quantum dots. Our investigation relies on a confocal microscope that allows arbitrary tuning of the angle between the applied magnetic field and the sample growth axis. First, from experiments on neutral excitons and trions, we extract the in-plane and on-axis components of the Landé tensor for electrons and holes in the $s$ shell. Then, based on the doubly negatively charged exciton magnetophotoluminescence, we show that the $p$-electron wave function spreads significantly into the GaAs barriers. We also demonstrate that the $p$-electron $g$ factor depends on the presence of a hole in the $s$ shell. The magnetic field dependence of triply negatively charged excitons photoluminescence exhibits several anticrossings, as a result of coupling between the quantum dot electronic states and the wetting layer. Finally, we discuss how the system evolves from a Kondo-Anderson exciton description to the artificial atom model when the orientation of the magnetic field goes from Faraday to Voigt geometry.

Motivated by a search for experimental probes to access the physics of fractionalized excitations called spinons in spin liquids, we study the interaction of spinons with lattice vibrations. We consider the case of algebraic spin liquid, when spinons have fermionic statistics and a Dirac-like dispersion. We establish the general procedure for deriving spinon-phonon interactions, which is based on symmetry considerations. The procedure is illustrated for four different algebraic spin liquids: $\pi$-flux and staggered-flux phases on a square lattice, $\pi$-flux phase on a kagome lattice, and zero-flux phase on a honeycomb lattice. Although the low-energy description is similar for all these phases, different underlying symmetry groups lead to a distinct form of spinon-phonon interaction Hamiltonian. The explicit form of the spinon-phonon interaction is used to estimate the attenuation of ultrasound in an algebraic spin liquid. The prospects of the sound attenuation as a probe of spinons are discussed.

We present a self-contained theoretical and computational framework for dynamics following photoexcitation in quantum dots near planar interfaces. A microscopic Hamiltonian parametrized by first-principles calculations is merged with a reduced density matrix formalism that allows for the prediction of time-dependent charge and energy transfer processes between the quantum dot and the electrode. While treating charge and energy transfer processes on an equal footing, the nonperturbative effects of sudden charge transitions on the Fermi sea of the electrode are included. We illustrate the formalism with calculations of an InAs quantum dot coupled to the Shockley state on an Au[111] surface and use it to concretely discuss the wide range of kinetics possible in these systems and their implications for photovoltaic systems and tunnel junction devices. We discuss the utility of this framework for the analysis of recent experiments.

Numerical simulations of previous plane shock wave measurements on highly oriented pyrolytic graphite (HOPG), shocked to four peak stresses ranging from 27 to 50 GPa, are presented to address a long-standing question: When is the diamond phase formed in the shock-compressed graphite to diamond transition? A multiphase material description was developed to simulate measured wave profiles in shocked HOPG. Our results showed good agreement with the measured profiles for all four peak stresses and demonstrated the following: Transformation is completed in less than 10 ns, the density and stiffness of the high-pressure phase match that of cubic diamond, and the mechanical response of the diamond phase is nonlinear elastic.

Recent experimental progress in coupling nanoscale conductors to superconducting microwave cavities has opened up for transport investigations of the deep quantum limit of light-matter interactions, with tunneling electrons strongly coupled to individual cavity photons. We have investigated theoretically the most basic cavity-conductor system with strong, single photon induced nonlocal transport effects: two spatially separated double quantum dots (DQDs) resonantly coupled to the fundamental cavity mode. The system, described by a generalized Tavis-Cummings model, is investigated within a quantum master equation formalism, allowing us to account for both the electronic transport properties through the DQDs as well as the coherent, nonequilibrium cavity photon state. We find sizable nonlocally induced current and current cross-correlations mediated by individual photons. From a full statistical description of the electron transport we further reveal a dynamical channel blockade in one DQD lifted by photon emission due to tunneling through the other DQD. Moreover, large entanglement between the orbital states of electrons in the two DQDs is found for small DQD-lead temperatures.

We report on a comparative study of spin Hall related effects and magnetoresistance in YIG$|$Pt and YIG$|$Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to yttrium iron garnet (YIG): the spin mixing conductance $G_{\uparrow \downarrow }$ at the YIG$|$normal metal interface, the spin Hall angle ${\Theta }_{\mathrm{SH}}$, and the spin diffusion length ${\lambda }_{\mathrm{sd}}$ in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be ${\lambda }_{\mathrm{sd}}^{\mathrm{Ta}}=1.8\pm 0.7$ nm. Both the YIG$|$Pt and YIG$|$Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.

We study the Landau-Zener dynamics of a tunneling spin coupled to a torsional resonator. For strong spin-phonon coupling, when the oscillator frequency is large compared to the tunnel splitting, the system exhibits multiple Landau-Zener transitions. Entanglement of spin and mechanical angular momentum results in abrupt changes of oscillator dynamics, which coincide in time with spin transitions. We show that a large number of spins on a single oscillator coupled only through the in-phase phonon field behaves as a single large spin, greatly enhancing the spin-phonon coupling. We compare purely quantum and semiclassical dynamics of the system and discuss their experimental realizations. An experiment is proposed in which the field sweep is used to read out the exact quantum state of the mechanical resonator.

In Phys. Rev. Lett. 109, 245502 (2012), a method for retrieving the object's three-dimensional potential distribution by inverting the dynamical scattering was presented and validated by a reconstruction from simulated atomic resolution transmission electron microscopy (TEM) images. In this paper, an extension to ptychography and scanning confocal electron microscopy is demonstrated and validated with simulations. Ultimately, this will make it possible to operate the microscope in the mode that yields the best reconstruction instead of accommodating the microscope settings to the linear approximation to the specimen-electron interaction used in most reconstruction algorithms. Furthermore, simultaneous estimation of the object and the unknown defocus from simulated atomic resolution TEM images is attained. This is an important step towards experimental reconstructions.

The real-time dynamics of local occupation numbers in a Hubbard model on a $6\times 6$ square lattice is studied by means of the nonequilibrium generalization of the cluster-perturbation theory. The cluster approach is adapted to studies of two-dimensional lattice systems by using concepts of multiple-scattering theory and a component decomposition of the nonequilibrium Green's function on the Keldysh-Matsubara contour. We consider “classical” initial states formed as tensor products of states on $2\times 2$ plaquettes and trace the effects of the interplaquette hopping in the final-state dynamics. Two different initially excited states are considered on an individual plaquette, a fully polarized staggered spin state (Néel) and a fully polarized charge-density wave (CDW). The final-state dynamics is constrained by a dynamical symmetry; i.e., the time-evolution operator and certain observables are invariant under an antiunitary transformation composed of time reversal, an asymmetric particle-hole, and a staggered sign transformation. We find an interesting interrelation between this dynamical symmetry and the separation of energy and time scales: In the case of a global excitation with all plaquettes excited, the initial Néel and the initial CDW states are linked by the transformation. This prevents an efficient relaxation of the CDW state on the short time scale governing the dynamics of charge degrees of freedom. Contrarily, the CDW state is found to relax much faster than the Néel state in the case of a local excitation on a single plaquette where the symmetry relation between the two states is broken by the coupling to the environment.

We report the observation of optical near fields in a photonic waveguide of conductive indium tin oxide (ITO) using multiphoton photoemission electron microscopy (PEEM). Nonlinear two-photon photoelectron emission is enhanced at field maxima created by interference between incident 410-nm and coherently excited guided photonic waves, providing strong phase contrast. Guided modes are observed under both transverse magnetic field (TM) and transverse electric field (TE) polarized illuminations and are consistent with classical electromagnetic theory. Implications on the role of multiphoton PEEM in optical near-field imaging are discussed.

We examine the influence of the main approximations employed in density functional theory descriptions of the solid phase of molecular hydrogen near dissociation. We consider the importance of nuclear quantum effects on equilibrium properties and find that they strongly influence intramolecular properties, such as bond fluctuations and stability. We demonstrate that the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, suggesting a limited value to dynamical, e.g., finite-temperature predictions based on classical ions and static crystals. We also consider the influence of the chosen exchange-correlation density functional on the predicted properties of hydrogen, in particular, the pressure dependence of the band gap and the zero-point energy. Finally, we use our simulations to make an assessment of the accuracy of typically employed approximations to the calculation of the Gibbs free energy of the solid, namely the quasi-harmonic approximation for solids. We find that, while the approximation is capable of producing free energies with an accuracy of $\approx 10$ meV, this is not enough to make reliable predictions of the phase diagram of hydrogen from first principles due to the small free energy differences seen between several potential structures for the solid; direct free energy calculations for quantum protons are required in order to make definite predictions.

Motivated by the nickel valence controversy in the perovskite nickelate $R$NiO${}_3$, we consider a one-dimensional underscreened Kondo chain consisting of alternating spin-1 (“nickel”) and electron (“oxygen”) sites, which in addition to the usual electron hopping and spin-spin interaction between the $S=1$ spin and the electron also contains a spin-mediated electron hopping term. Using the density-matrix renormalization group (DMRG), we obtained the zero-temperature phase diagram of the model, as well as various correlation functions in each phase. Importantly, for a certain range of parameters the model exhibits a quasi-long-range spiral (QS) order. To understand the DMRG results, we construct a mean-field theory based on a Schwinger fermion decomposition of the $S=1$ spins, from which we argue that the QS phase corresponds to a phase in proximity to the spin Bose metal state proposed by Sheng, Motrunich, and Fisher [Phys. Rev. B 79, 205112 (2009)]. Notably, we find no evidence for a phase with the symmetry of “nickel”-centered charge order, which has been argued to arise due to site-selective Kondo screening of half the $S=1$ spins, and suggest that order of this type occurs only due to an additional energy gain from spontaneous lattice distortions, not present in this model.

We present a study of free carrier photogeneration and multicarrier bound states, such as biexcitons and trions (charged excitons), in semiconducting single-walled carbon nanotubes. Pump-and-probe measurements performed with fs pulses reveal the effects of strong Coulomb interactions between carriers on their dynamics. Biexciton formation by optical transition from exciton population results in an induced absorption line (binding energy 130 meV). Exciton-exciton annihilation process is shown to evolve at high densities towards an Auger process that can expel carriers from nanotubes. The remaining carriers give rise to an induced absorption due to trion formation (binding energy 190 meV). These features show the dynamics of exciton and free carriers populations.

ZrB${}_2$ is a nonsuperconducting Pauli paramagnetic crystallizing in the AlB${}_2$ structure. V substitution for Zr in Zr${}_{1-x}$V${}_x$B${}_2$ (0.01 $\le$ $x$ $\le$ 0.1) at the few percentage level induces superconductivity with critical temperature reaching a maximum of 8.7 K for Zr${}_{0.96}$V${}_{0.04}$B${}_2$ near the solid solubility limit. Specific-heat and lower critical field temperature dependence results suggest the possibility of unconventional superconductivity possibly arising from multiband effects.

We apply grazing-incidence femtosecond x-ray diffraction to investigate the details of the atomic motion connected with a displacively excited coherent optical phonon. We concentrate on the low frequency phonon associated with the charge and orbital order in the mixed valence manganite La${}_{0.25}$Pr${}_{0.375}$Ca${}_{0.375}$MnO${}_3$ for $T\lesssim 210$ K. We measure the response of three superlattice reflections that feature different sensitivities to the motion of the unit cell constituents. The results support the assignment to a translational mode of the Mn${}^{4+}$ atoms together with the oxygen atoms connecting adjacent Mn${}^{4+}$ sites.

The change in the electronic structure of layered Cu${}_x$IrTe${}_2$ has been characterized by transport and spectroscopic measurements, combined with first-principles calculations. The Cu intercalation suppresses the monoclinic distortion, giving rise to the stabilization of the trigonal phase with superconductivity. Thermopower and Hall resistivity measurements suggest the multiband nature with hole and electron carriers for this system, which is masked by the predominance of the hole carriers enhanced by the interlayer hybridization in the trigonal phase. Rather than the instability of the Ir $d$ band, a subtle balance between the interlayer and intralayer Te-Te hybridizations is proposed as a main factor dominating the structural transition and the superconductivity.

We report local probe investigations of the magnetic interaction between BiFeO${}_3$ films and a ferromagnetic Co${}_{0.9}$Fe${}_{0.1}$ layer. Within the constraints of intralayer exchange coupling in the Co${}_{0.9}$Fe${}_{0.1}$, the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO${}_3$ ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO${}_3$ films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO${}_3$ film based on our x-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO${}_3$.