Session L26: Electronic and Transport Properties of Insulators

First-principles Studies of the Optical Properties of Eu doped Barium Halides: From Storage Phosphor to Bright Scintillator

The Eu doped Ba mixed halide family BaBrX (X=F,Cl,Br,I) changes from being a widely used X-ray Storage Phosphor (BaBrF:Eu) to one of the brightest know new gamma ray detector scintillators (BaBrI:Eu). To help understand these contrasting optical properties and guide in the design of new and improved scintillator detectors, in collaboration with experimental groups, we have performed first principles theoretical studies of these materials. In particular we have studied their electron and hole trapping mechanisms and how that can explain their very different optical properties.   

Direct experimental characterization of photoemission charge-transfer satellites.

Energy-loss satellites in photoelectron spectroscopy often arise due to different charge-transfer states in condensed matter systems. The specific characterization of these satellites, however, has been controversial, and different theoretical approaches may lead to contradictory characterizations. Here we demonstrate the ability of high energy resonant photoelectron spectroscopy to provide direct experimental evidence of the nature of charge transfer satellites. Analysis of the Ti $1s$ core line in SrTiO$_3$ reveals two satellites, located approximately 5 eV and 13 eV lower kinetic energy than the main line. High energy resonant photoelectron spectroscopy reveals that these two peaks originate from ligand $2p$ $t_2g$ to metal $3d$ $t_2g$ and ligand $2p$ $e_g$ to metal $3d$ $e_g$ charge-transfer excitations.   

High harmonic generation based time resolved ARPES at 30 eV with 50 meV energy resolution

Angle-resolved photoelectron spectroscopy (ARPES) has emerged as a leading technique in identifying equilibrium properties of complex electronic systems as well as their correlated dynamics. By using femtosecond high harmonic generation (HHG) pulses, this technique can be extended to monitor ultrafast changes in the electronic structure in response to an optical excitation [1]. However, the broad bandwidth of the HHG pulses has been a major experimental limitation. In this contribution, we combine the HHG source with an off-axis Czerny-Turner XUV monochromator and a three-dimensional ``ARTOF'' photoelectron detector to achieve an unrivaled overall energy resolution of 50 meV in multiple harmonic energies. Moreover, the use of a stack of different gratings enables us to fine control both the photon energy and time vs. energy resolution to its particular needs. The performance of our setup is demonstrated by studies on the transition metal dichalcogenide IrTe$_{\mathrm{2}}$ which undergoes a first-order structural transition and accompanied reconstruction of the band structure upon cooling without the characteristic opening of an energy gap [2]. [1] T. Rohwer et al., Nature 471 (2011) 490, [2] A. F. Fang et.al., Scientific Reports 3 (2013) 1153   

\textbf{Resonant inelastic x-ray scattering study at the oxygen K-edge of corner-shared Sr}$_{\mathbf{2}}$\textbf{CuO}$_{\mathbf{3}}$\textbf{ cuprate}

We present a resonant inelastic x-ray scattering (RIXS) study at the oxygen K-edge of the spin-chain system Sr$_{2}$CuO$_{3}$. We investigate this system using small cluster exact diagonalization calculations for a microscopic model that considers all the orbitals of Cu and O in CuO3-unit cell. Using a canonical parameter set, we compute the XAS and RIXS spectra in comparison to the experiment. This allows us to identify the \textit{dd}- and charge transfer excitations in the observed spectrum. We also infer the presence of several low energy excitations that may be related to phononic and/or magnetic excitations.   

Temperature-Induced Electronic Structure Evolution of ZrTe5 Revealed by High resolution {\&} Laser Angle-Resolved Photoemission Spectroscopy (ARPES)

The transition metal pentatellurides ZrTe5 have attracted consideration attention since the 70s, due to the unusual transport properties like resistivity peak at \textasciitilde 140K and the sign change of the Hall coefficient and thermopower. The origin of the most peculiar resistivity peak remains controversial. In this talk we will present high resolution angle-resolved photoemission (ARPES) study on the Fermi surface and band structure of ZrTe5, by using our high resolution ARPES system equipped with the VUV laser and the time-of-flight (TOF) electron energy analyzer. Upon cooling down, we found a gradual transition from hole-like band into electron-like band around the Brillouin zone center. Such an electron state transition forms the underlying physics for the abnormal transport properties. We will also comment on the possibility of a Dirac semimetal in ZrTe5.   

Material simulation of charge carrier transport properties of polymer dielectrics

To understand electron and hole transport in solid material requires to know its electronic properties, i.e. the density of states (DOS) and whether the states are spatially localized or delocalized. The states closest to the band edges may be localized, states further away can be delocalized. This transition from localized to delocalized states determines the mobility edge, above the mobility edge the mobility is expected to be high. A real polymer is never perfect; it contains a number of oxidative states, bonding defects and molecular impurities. These imperfections yield electronic states that can appear in the band gap of the polymer, traps. Traps can be shallow, i.e. close to the band edges, from these states the charge carrier easily can jump to a state in the band edge or another shallow state. Other traps can be deep, in these states it is likely that the charge carrier remains and become immobile. All these properties related to the electronic structure of the polymer, including its defects, affects the conductivity of the polymer. Linear scaling Density Functional Theory has been applied to calculate electronic structure of amorphous polyethylene. In particular DOS, trap levels and mobility edges are studied.   

Generality of anomalous broadening in $\sigma$ peaks of low-Z compounds

X-ray emission spectra originating from N 1s core holes in some nitrates have displayed extreme broadening($>$~4~eV) of a feature identified with the NO $\sigma$ state. Simple band structure calculations of the valence band derived from this bond in a crystal show it to be quite narrow. Calculations of transitions that take into account many-body corrections in the {\it GW} approximation indicate that the final state has a large imaginary self-energy. We have discovered other compounds that demonstrate this effect experimentally, and we are able to show that they display giant lifetime fluctuations of a similar valence state. We have formulated a criterion of compounds for which this effect should be present.   

Novel mechanism for displacive distortions in perovskites: On the orbital ordering transition in KCuF$_3$

The Mott insulating perovskite KCuF$_3$ is considered the paradigmatic system with long-ranged orbital order and a cooperative Jahn-Teller distortion of the Cu-F octahedra. However, recent experiments have revealed that the JT-like distortions persist and even grow as temperature is increased. We show that neither superexchange nor Jahn-Teller can accurately describe this behavior---even qualitatively. Supported by GGA+U and model calculations, we explain this anomalous result in terms of a volume-driven lattice instability of purely ionic origin. We examine the effect of ionic size on this mechanism through the related systems KCu$_{1-x}$Mg$_x$F$_3$, KCrF$_3$, and ACuF$_3$. As a non-electronic effect, this instability should allow for octahedral distortions even in closed-shell systems with cubic ground states, and we propose design guidelines for the realization of this high-temperature broken-symmetry phase experimentally.   

Electrical Breakdown in Solids

During electron breakdown of a solid subjected to a large electric field, impact ionization causes growth of an electron-hole plasma. This growth process is opposed by Auger recombination of the electron-hole pairs. In our work, such breakdown is investigated by obtaining steady-state solutions to the Boltzmann equation. In these calculations, the carriers are heated by the electric field and cooled by phonon emission. Our results imply that breakdown may lead to high carrier-density current filaments. Conductive filaments have been observed in optically-triggered, high-power photoconductive semiconductor switch (PCSS) devices being developed at Sandia Labs. The relationship between the steady-state computed solutions to the observed filaments will be discussed in the presentation. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Hole conduction pathways in transparent amorphous tin oxides

P-type transparent amorphous oxide semiconductors (TAOS) have yet to be sufficiently demonstrated or commercialized, severely limiting the possible device architecture of transparent and flexible oxide electronics. The lack of p-type amorphous oxide candidates mainly originates from the directional oxygen 2$p$ character of their topmost valence states. Previous attempts to create p-type oxides have involved hybridization of the O 2$p$ with metal orbitals, such as with CuAlO$_2$ and its Cu 3$d$ - O 2$p$ hybridization. However, the highly directional nature of the utilized orbitals means that structural disorder inhibits hybridization and severely disrupts hole-conduction pathways. Crystalline stannous oxide (SnO) and other lone-pair active post-transition metal oxides can have reduced localization at the valence band edge due to complex hybridization between the O 2$p$, metal $p$, and spherical metal $s$-orbitals. I will discuss our investigation of structural disorder in SnO. Using a combination of synchrotron spectroscopy, and atomistic calculations, our investigation elucidates the important interplay between atomistic and electronic structure in establishing continuous hole conduction pathways at the valence band edge of transparent amorphous oxides.   

Topological aspects of nonlinear optical responses

There are a variety of nonlinear optical effects including higher harmonic generations, photovoltaic effects, and nonlinear Kerr rotations. A recent remarkable progress in the photovoltaic effect is the high efficiency solar cell action in perovskite oxides without inversion symmetry. The crystal structure lacking inversion replaces the role of artificial structures such as p-n junctions in conventional solar cells. One of the proposed mechanisms for this phenomenon is the shift-current which is supported by a band structure lacking inversion and is related to the Berry connection of Bloch wavefunctions. Motivated by these, we explore topological aspects of the nonlinear optical responses. To this end, we employ the Keldysh method combined with the Floquet formalism, where effective band structures can be defined under an electric field periodic in time. This enables us to describe the shift-current, nonlinear Kerr rotation, photovoltaic effect, and the photo-induced change in the order parameters in a unified fashion. We connect these nonlinear optical responses to topological quantities involving the Berry connection and Berry curvature. It is found that vector fields defined with the Berry connections in the space of momentum and/or parameters govern the nonlinear responses.   

A compact, low-loss, tunable phase shifter on defect mitigated dielectrics up to 40 GHz

With the emergence of the internet-of-things and increased connectivity of modern commerce, consumers have driven demand for wireless spectrum beyond current capacity and infrastructure capabilities. One way the telecommunications industry is addressing this problem is by pushing front-end electronics to higher frequencies, introducing carrier aggregation schemes, and developing spectrum-sharing techniques. Some of these solutions require frequency agile components that are vastly different from what is in today's marketplace. Perhaps the most basic and ubiquitous component in front-end electronics is the phase shifter. Phase shifters are particularly important for compact beam-forming antennas that may soon appear in commercial technology. Here, we demonstrate a compact, tunable phase shifter with very low insertion loss up to 40 GHz on a defect mitigated tunable dielectric. We demonstrate performance compared to barium-doped strontium titanate phase shifters. Such phase shifters could potentially meet the stringent size and performance characteristics demanded by telecommunications industry, readily facilitating massive multiple-input multiple-output antennas in the next-generation of mobile handsets.   

Electrode effects in dielectric spectroscopy measurements on (Nb$+$In) co-doped TiO$_2$

Recently, several papers reported the discovery of giant permittivity and low dielectric loss in (Nb+In) co-doped TiO$_2$. A series of tests was performed which included the measurement of the frequency dependence of the dielectric permittivity and ac conductivity of co-doped (Nb+In)TiO$_2$ as a function of electrode type, sample thickness and temperature. The data suggest that the measurements are strongly affected by the electrodes. The consistency between four contact van der Pauw dc conductivity measurements and bulk conductivity values extracted from two contact ac conductivity measurements suggest that the values of colossal permittivity are, at least in part, a result of Schottky barrier depletion widths that depend on electrode type and temperature.