We present scanning tunneling microscopy and spectroscopy experiments on the novel $J_{\mathrm{eff}}=1/2$ Mott insulator ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$. Local density of states (LDOS) measurements show an intrinsic insulating gap of 620 meV that is asymmetric about the Fermi level and is larger than previously reported values. The size of this gap suggests that ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is likely a Mott rather than Slater insulator. In addition, we found a small number of native defects which create in-gap spectral weight. Atomically resolved LDOS measurements on and off the defects show that this energy gap is quite fragile. Together the extended nature of the 5$d$ electrons and poor screening of defects help explain the elusive nature of this gap.

We present a theory of the scaling behavior of the thermodynamic, transport, and dynamical properties of a three-dimensional metal at an antiferromagnetic (AFM) critical point. We show how the critical spin fluctuations at the AFM wave vector $q=Q$ induce energy fluctuations at small $q$, giving rise to a diverging quasiparticle effective mass over the whole Fermi surface. The coupling of the fermionic and bosonic degrees of freedom leads to a self-consistent relation for the effective mass, which has a strong coupling solution in addition to the well-known weak-coupling spin-density-wave solution. We use the recently introduced concept of critical quasiparticles, employing a scale-dependent effective mass ratio $m^{*}/m$ and quasiparticle weight factor $Z$. We adopt a scale-dependent vertex correction that boosts the coupling of fermions and spin fluctuations. The ensuing spin fluctuation spectrum obeys $\omega /T$ scaling. Our results are in good agreement with experimental data on the heavy-fermion compounds ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ and ${\mathrm{CeCu}}_{6-x}{\mathrm{Au}}_x$ for 3D and 2D spin fluctuations, respectively.

The antiferromagnetic Heisenberg spin chain of odd spin $S$ is in the Haldane phase with several defining physical properties, such as thermodynamical ground-state degeneracy, symmetry-protected edge states, and nonzero string order parameter. If nonzero hole concentration $\delta$ and hole hopping energy $t$ are considered, the spin chain is replaced by a $\text{spin}-S$ $t-J$ chain. The motivation of this paper is to generalize the discussions of the Haldane phase to the doped spin chain. The first result of this paper is that, for the model considered here, the ${\mathbb{Z}}_2$ sign structure in the usual Ising basis can be totally removed by two consecutive unitary transformations consisting of a spatially local one and a nonlocal one. Direct from the sign structure, the second result of this paper is that the Marshall theorem and the Lieb-Mattis theorem for pure spin systems are generalized to the $t-J$ chain for arbitrary $S$ and $\delta$. A corollary of the theorem provides us with the ground-state degeneracy in the thermodynamic limit. The third result of this paper is about the phase diagram. We show that the defining properties of the Haldane phase survive in the small $t/J$ limit. The large $t/J$ phase supports a gapped spin sector with similar properties (ground-state degeneracy, edge state, and string order parameter) of the Haldane chain, although the charge sector is gapless.

Entanglement entropy is a useful probe of compressible quantum matter because it can detect the existence of Fermi surfaces, both of microscopic fermionic degrees of freedom and of “hidden” gauge-charged fermions. Much recent attention has focused on holographic efforts to model strongly interacting compressible matter of interest for condensed matter physics. We complete the entanglement analysis initiated by Huijse et al. [Phys. Rev. B 85, 035121 (2012)] and Ogawa et al. [JHEP 1 (2012) 125] using the recent proposal of Faulkner et al. (arXiv:1307.2892) to analyze the entanglement entropy of the visible fermions which arises from bulk loop corrections. We find perfect agreement between holographic and field-theoretic calculations.

Strongly interacting Dirac semimetals are investigated using a holographic model especially geared to compute the single-particle correlation function for this case, including both interaction effects and nonzero temperature. We calculate the (homogeneous) electrical conductivity at zero chemical potential, and show that it consists of two contributions. The interband contribution scales as a power law either in frequency or in temperature for low frequency. The precise power is related to a critical exponent of the dual holographic theory, which is a parameter in the model. On top of that we find for nonzero temperatures a Drude peak corresponding to intraband transitions. A behavior similar to Coulomb interactions is recovered as a special limiting case.

Based on an effective-medium description for metallic taper arrays, we found that wave propagation inside these systems is governed by a Schrödinger-like equation with an effective potential solely determined by the taper shape. Such mapping not only helps us understand complicated wave-transporting phenomena in such systems but also yields an analytical criterion to identify the optimum taper shape. We applied the theory to design optimum taper arrays in different frequency domains, with their excellent light-harvesting abilities confirmed by full-wave simulations.

We present a formula for the calculation of $K$-edge resonant inelastic x-ray scattering on transition-metal compounds, based on a local interaction between the valence shell electrons and the $1s$ core hole. Extending a previous result, we include explicit momentum dependence and a basis with multiple core-hole sites. We apply this formula to a single-layered charge-, orbital-, and spin-ordered manganite, $\mathrm{La}{}_{0.5}$$\mathrm{Sr}{}_{1.5}$$\mathrm{MnO}{}_4$, and obtain good agreement with experimental data, in particular with regards to the large variation of the intensity with momentum. We find that the screening in $\mathrm{La}{}_{0.5}$$\mathrm{Sr}{}_{1.5}$$\mathrm{MnO}{}_4$ is highly localized around the core-hole site and demonstrate the potential of $K$-edge resonant inelastic x-ray scattering as a probe of screening dynamics in materials.

The relationship between the modern and classical Landau's approach to carrier orbital magnetization is studied theoretically within the envelope function approximation, taking ferromagnetic (Ga,Mn)As as an example. It is shown that while the evaluation of hole magnetization within the modern theory does not require information on the band structure in a magnetic field, the number of basis wave functions must be much larger than in the Landau approach to achieve the same quantitative accuracy. A numerically efficient method is proposed, which takes advantages of these two theoretical schemes. The computed magnitude of orbital magnetization is in accord with experimental values obtained by x-ray magnetic circular dichroism in (III,Mn)V compounds. The direct effect of the magnetic field on the hole spectrum is studied too, and employed to interpret a dependence of the Coulomb blockade maxima on the magnetic field in a single electron transistor with a (Ga,Mn)As gate.

The Raman spectrum of crystalline $\mathrm{Cu}{}_2\mathrm{O}$ taken off resonance is reproducible and independent of the growth method and conditions employed. But, in contrast to most other crystalline materials, the Raman spectrum of $\mathrm{Cu}{}_2\mathrm{O}$ is dominated by infrared active and silent lattice modes rather than by the only Raman allowed phonon mode. We show that this unusual behavior is most likely caused by the presence of copper vacancies in the so-called split configuration, a point defect particular to $\mathrm{Cu}{}_2\mathrm{O}$. The reduction of symmetry due to the presence of point defects may lift the Raman selection rules and may introduce Raman activity for phonon modes that are Raman forbidden in the case of perfect crystal symmetry. Based on this group theoretical consideration, we predict the angle dependence of the Raman intensities of all $\mathrm{Cu}{}_2\mathrm{O}$ one-phonon modes at $k=0$ for rotation about the (100) direction caused by the presence of various intrinsic point defects. Of all intrinsic defects in question, only the presence of the copper vacancy in the split configuration introduces Raman activity for all $\mathrm{Cu}{}_2\mathrm{O}$ extended phonon modes observed in experiment and is consistent with the angle-dependent measurements. Our study underlines the special role of the split vacancy in $\mathrm{Cu}{}_2\mathrm{O}$.

The different cohesive forces that bond organic (i.e. excitonic) and inorganic semiconductors lead to widely disparate dielectric constants, charge mobilities, and other fundamental optoelectronic properties that make junctions between these materials interesting for numerous practical applications. Yet, there are no detailed theories addressing charge and energy transport across interfaces between these hybrid systems. Here, we develop a comprehensive physical model describing charge transport and photocurrent generation based on first-principles charge and excited state dynamics at the organic/inorganic heterojunction. We consider interfaces that are trap-free, as well as those with an exponential distribution of trap states. We find that the hybrid charge-transfer state resulting from photon absorption near the junction that subsequently migrates to the heterointerface is often unstable at room temperature, leading to its rapid dissociation into free charges that are collected at the device contacts. In the companion Paper II [A. Panda et al., Phys. Rev. B 90, 045303 (2014)], we apply our theories to understanding the optical and electronic properties of archetype organic/inorganic heterojunction diodes. Our analysis provides insights for developing high performance optoelectronic devices whose properties are otherwise inaccessible to either conventional excitonic or inorganic semiconductor junctions.

In our companion paper (Paper I) [C. K. Renshaw and S. R. Forrest, Phys. Rev. B 90, 045302 (2014)], we developed a model for charge transport and photogeneration at hybrid organic/inorganic semiconductor heterojunctions (OI-HJs). Here we apply the model to two planar bilayer hybrid photovoltaic devices: the first using the wide-band gap $n\text{−}{\mathrm{TiO}}_2$ in combination with the hole transporting tetraphenyl-dibenzoperiflanthene (DBP), and the second based on the moderate-band gap $n$-InP and the hole transporting pentacene (PEN). We measure the external quantum efficiency (EQE) and current density vs voltage ($J\text{−}V$) characteristics of both devices as functions of temperature. The EQE spectra for both ${\mathrm{TiO}}_2$/DBP and InP/PEN provide convincing evidence that Frenkel states generated in the organic form hybrid charge transfer excitons (HCTEs) at the OI-HJ that are subsequently dissociated into free charges, and then collected at the opposing electrodes. The dissociation efficiency is found to be strongly influenced by the presence of surface states, particularly in the InP/PEN device. We further develop the $J\text{−}V$ model from Paper I to include an analytical expression for space-charge effects in the organic at high currents. Model fits to the $J\text{−}V$ data suggest that the temperature-dependent hole mobilities in both DBP and PEN result in increasing space-charge effects at low temperatures. Furthermore, we find that the $J\text{−}V$ characteristics of the ${\mathrm{TiO}}_2$/DBP device both in the dark and under illumination are governed by interface recombination. In contrast, the dark current in the InP/PEN device is governed by injection over the OI-HJ barrier, whereas the photocurrent is dominated by interface recombination. This work elucidates the role of the HCTE state in photogeneration, and the applicability of our model to a range of important optoelectronic devices.

We derive the statistics of the time-delay matrix (energy derivative of the scattering matrix) in an ensemble of superconducting quantum dots with chaotic scattering (Andreev billiards), coupled ballistically to $M$ conducting modes (electron-hole modes in a normal metal or Majorana edge modes in a superconductor). As a first application we calculate the density of states ${\rho }_0$ at the Fermi level. The ensemble average $\langle {\rho }_0\rangle ={\delta }_0^{-1}M{[max(0,M+2\alpha /\beta )]}^{-1}$ deviates from the bulk value $1/{\delta }_0$ by an amount depending on the Altland-Zirnbauer symmetry indices $\alpha ,\beta$. The divergent average for $M=1,2$ in symmetry class D ($\alpha =-1$, $\beta =1$) originates from the midgap spectral peak of a closed quantum dot, but now no longer depends on the presence or absence of a Majorana zero mode. As a second application we calculate the probability distribution of the thermopower, contrasting the difference for paired and unpaired Majorana edge modes.

We describe high-fidelity entangling gates between singlet-triplet qubits (STQs) which are coupled via one quantum state (QS). The QS can be provided by a quantum dot itself or by another confined system. The orbital energies of the QS are tunable using an electric gate close to the QS, which changes the interactions between the STQs independent of their single-qubit parameters. Short gating sequences exist for controlled not (cnot) operations. We show that realistic quantum dot setups permit excellent entangling operations with gate infidelities below ${10}^{-3}$, which is lower than the quantum error correction threshold of the surface code. We consider limitations from fabrication errors, hyperfine interactions, spin-orbit interactions, and charge noise in GaAs and Si heterostructures.

In alloys exhibiting substitutional disorder, the variety of atomic environments manifests itself as a “disorder broadening” in their core-level binding-energy spectra. Disorder broadening can be measured experimentally and, in principle, can be used to deduce information about specific atomic environments within a sample. However, progress in this endeavor is hampered by the lack of a model for this phenomenon which can treat complex systems. In this work we describe such a model. The model is used to elucidate the relationship between charge transfer, atomic environment, and disorder broadening in complex systems, with a focus on the problem of characterizing the interface quality of CuNi multilayers. We also validate the model against the results of ab initio electronic-structure calculations. Several counterintuitive aspects of the disorder-broadening phenomenon are uncovered, an understanding of which is essential for the correct interpretation of experimental results. For instance, it is shown that systems with inhomogeneous concentration profiles can exhibit disorder broadenings significantly larger than random alloys. Furthermore, in some systems a “disorder narrowing” is even possible.

We investigate near-field infrared spectroscopy and superfluid polariton imaging experiments on conventional and unconventional superconductors. Our modeling shows that near-field spectroscopy can measure the magnitude of the superconducting energy gap in Bardeen-Cooper-Schrieffer-like superconductors with nanoscale spatial resolution. We demonstrate how the same technique can measure the $c$-axis plasma frequency, and thus the $c$-axis superfluid density, of layered unconventional superconductors with a similar spatial resolution. Our modeling also shows that near-field techniques can image superfluid surface mode interference patterns near physical and electronic boundaries. We describe how these images can be used to extract the collective mode dispersion of anisotropic superconductors with subdiffractional spatial resolution.

We show that in a nonequilibrium system of an exciton-polariton condensate, where polaritons are generated from incoherent pumping, a ring-shaped pump allows for stationary vortex memory elements of topological charge $m=1$ or $m=-1$. Using simple potential guides we can choose whether to copy the same charge or invert it onto another spatially separate ring pump. Such manipulation of binary information opens the possibility of a new type of processing using vortices as topologically protected memory components.

We report evidence for multiband transport and an insulating low-temperature normal state in superconducting $K_{0.50}$${\mathrm{Na}}_{0.24}$${\mathrm{Fe}}_{1.52}$${\mathrm{Se}}_2$ with $T_c\approx 20$ K. The temperature-dependent upper critical field $H_{c2}$ is well described by a two-band BCS model. The normal-state resistance, accessible at low temperatures only in pulsed magnetic fields, shows an insulating logarithmic temperature dependence as $T\to 0$ after superconductivity is suppressed. This is similar as for $\mathrm{high}-T_c$ copper oxides and granular type-I superconductors, suggesting that the superconductor-insulator transition observed in high magnetic fields is related to intrinsic nanoscale phase separation.