Disorder-induced Localization Research Articles

Transient electroluminescence determination of carrier mobility and charge trapping effects in heavily doped phosphorescent organic light-emitting diodes

Transient electroluminescence (EL) was used to measure the delay between the excitation pulse and onset of emission in OLEDs based on phosphorescent bis[3,5-bis(2-pyridyl)-1,2,4-triazolato] platinum(ΙΙ) doped into 4,4′-bis(carbazol-9-yl) triphenylamine (CBP), from which an electron mobility of 3.2×10−6cm2/Vs was approximated. Delayed recombination was observed after the drive pulse had been removed and based on its dependence on frequency and duty cycle, ascribed to trapping and de-trapping processes associated with disorder-induced carrier localization at the interface between the emissive layer and electron blocker. The data suggests that the exciton recombination zone is at, or close to the interface between the emissive layer and electron blocker. Despite the charge trapping effects, a peak power efficiency of 24lm/W and peak external quantum efficiency of 10.64% were obtained. Mechanisms for the electroluminescence and delayed recombination are proposed.

Exciton-like trap states limit electron mobility in TiO2 nanotubes

Nanoparticle films have become a promising low-cost, high-surface-area electrode material for solar cells and solar fuel production. Compared to sintered nanoparticle films, oriented polycrystalline titania nanotubes offer the advantage of directed electron transport, and are expected to have higher electron mobility. However, macroscopic measurements have revealed their electron mobility to be as low as that of nanoparticle films. Here, we show, through time-resolved terahertz spectroscopy, that low mobility in polycrystalline TiO(2) nanotubes is not due to scattering from grain boundaries or disorder-induced localization as in other nanomaterials, but instead results from a single sharp resonance arising from exciton-like trap states. If the number of these states can be lowered, this could lead to improved electron transport in titania nanotubes and significantly better solar cell performance.

Theory of disorder-induced coherent scattering and light localization in slow-light photoniccrystal waveguides

We introduce a theoretical formalism to describe disorder-induced extrinsic scattering inslow light photonic crystal waveguides. This work details and extends the optical scatteringtheory used in a recent issue of Physics Review Letters (Patterson et al 2009 Phys. Rev.Lett. 102 253903) to describe coherent scattering phenomena and successfully explainrelated experimental measurements. Our presented theory, which combines Green functionand coupled mode methods, allows us to self-consistently account for arbitrary multiplescattering for the propagating electric field and recover experimental features such asresonances near the band edge. The technique is fully three-dimensional and can calculatethe effects of disorder on the propagating field over thousands of unit cells. As anapplication of this theory, we explore various sample lengths and disordered instances, anddemonstrate the profound effect of multiple scattering in the waveguide transmission.The spectra yield rich features associated with disorder-induced localization andmultiple scattering, which are shown to be exacerbated in the slow light propagationregime.

Solvent-Dependent Spectral Diffusion in a Hydrogen Bonded “Vibrational Aggregate”

Two-dimensional infrared spectroscopy (2DIR) is used to measure the viscosity-dependent spectral diffusion of a model vibrational probe, Mn(2)(CO)(10) (dimanganese decacarbonyl, DMDC), in a series of alcohols with time scales ranging from 2.67 ps in methanol to 5.33 ps in 1-hexanol. Alcohol-alkane solvent mixtures were found to produce indistinguishable linear IR spectra, while still demonstrating viscosity-dependent spectral diffusion. Using a vibrational exciton model to characterize the inhomogeneous energy landscape, several analogies emerge with multichromophoric electronic systems, such as J-aggregates and light-harvesting protein complexes. An excitonic, local vibrational mode Hamiltonian parametrized to reproduce the vibrational structure of DMDC serves as a starting point from which site energies (i.e., local carbonyl frequencies) are given Gaussian distributed disorder. The model gives excellent agreement with both the linear IR spectrum and the inhomogeneous widths extracted from 2DIR, indicating the system can be considered to be a "vibrational aggregate." This model naturally leads to exchange narrowing due to disorder-induced exciton localization, producing line widths consistent with our 1D and 2D measurements. Further, the diagonal disorder alone effectively reduces the molecular symmetry, leading to the appearance of Raman bands in the IR spectrum in accord with the measurements. Here, we show that the static inhomogeneity of the excitonic model with disorder successfully captures the essential details of the 1D spectrum while predicting the degree of IR activity of forbidden modes as well as the inhomogeneous widths and relative magnitudes of the transition moments.

Exciton localization in disordered poly(3-hexylthiophene)

Singlet exciton localization in conformationally disordered poly(3-hexylthiophene) (P3HT) is investigated via configuration interaction (singles) calculations of the Pariser-Parr-Pople model. The P3HT structures are generated by molecular dynamics simulations. The lowest-lying excitons are spatially localized, space filling, and nonoverlapping. These define spectroscopic segments or chromophores. The strong conformational disorder in P3HT causes breaks in the pi-conjugation. Depending on the relative values of the disorder-induced localization length and the distances between the pi-conjugation breaks, these breaks sometimes serve to pin the low-lying localized excitons. The exciton confinement also causes a local spectrum of low-lying exciton states. Coulomb-induced intra- or interchain interactions between spectroscopic segments in close spatial proximity can delocalize an exciton across these segments, in principle causing phase coherent transition dipole moments.

Joint forces against disorder

An experiment with ultracold gases reveals how weakly interacting atoms cooperate to protect long-range coherence against disorder-induced localization, and should offer insight into long-standing questions on the complex interplay of interactions and disorder in quantum systems.

Hofstadter butterflies in nonlinear Harper lattices, and their optical realizations

The ubiquitous Hofstadter butterfly describes a variety of systems characterized by incommensurable periodicities, ranging from Bloch electrons in magnetic fields and the quantum Hall effect to cold atoms in optical lattices and more. Here, we introduce nonlinearity into the underlying (Harper) model and study the nonlinear spectra and the corresponding extended eigenmodes of nonlinear quasiperiodic systems. We show that the spectra of the nonlinear eigenmodes form deformed versions of the Hofstadter butterfly and demonstrate that the modes can be classified into two families: nonlinear modes that are a ‘continuation’ of the linear modes of the system and new nonlinear modes that have no counterparts in the linear spectrum. Finally, we propose an optical realization of the linear and nonlinear Harper models in transversely modulated waveguide arrays, where these Hofstadter butterflies can be observed. This work is relevant to a variety of other branches of physics beyond optics, such as disorder-induced localization in ultracold bosonic gases, localization transition processes in disordered lattices, and more.

Wave localization at the boundary of disordered photonic lattices

We report on the experimental observation of reduced light-energy transport and disorder-induced localization close to a boundary of a truncated 1D disordered photonic lattice. Our observations uncover that a higher level of disorder near the boundary is required to obtain similar localization than in the bulk.

Specific heat and magnetic susceptibility of CeNiSn doped with Rh

CeNiSn is known as a semimetallic system with a small pseudogap at theFermi energy. We investigate the effect of Rh doping on the Kondo insulatorCeNiSn by means of measurements of ac magnetic susceptibility and specificheat. We show that the formation of the Kondo insulator narrow gap inCeNi1 − xRhxSn is associated with disorder-induced f-electron localization. For doped CeNiSn withx ≤ 0.06,the electrical resistivity data follow an activation and variable range hopping behaviour at lowT, consistent with weak disorder and localization, whileC/T is large, which is not a common feature of Kondo insulators. Forx > 0.06, the system is metallic and exhibits non-Fermi liquid behaviour with magnetic susceptibilityχ ∼ T − n withn ∼ 0.4 and electricalresistivity ρ ∼ T.

Random Walks and Anderson Localization in a Three-Dimensional Class C Network Model

We study the disorder-induced localization transition in a three-dimensional network model that belongs to symmetry class C. The model represents quasiparticle dynamics in a gapless spin-singlet superconductor without time-reversal invariance. It is a special feature of network models with this symmetry that the conductance and density of states can be expressed as averages in a classical system of dense, interacting random walks. Using this mapping, we present a more precise numerical study of critical behavior at an Anderson transition than has been possible previously in any context.

Magnetism, conductivity, and orbital order in(LaMnO3)2n/(SrMnO3)nsuperlattices

The modulation of charge density and spin order in ${({\text{LaMnO}}_{3})}_{2n}/{({\text{SrMnO}}_{3})}_{n}$ $(n=1--4)$ superlattices is studied via Monte Carlo simulations of the double-exchange model. $G$-type antiferromagnetic barriers in the ${\text{SrMnO}}_{3}$ regions with low charge density are found to separate ferromagnetic ${\text{LaMnO}}_{3}$ layers with high charge density. A metal-insulator transition with increasing $n$ is observed in the direction perpendicular to the interfaces. Our simulations provide insight into how disorder-induced localization may cause the metal-insulator transition occurring at $n=3$ in experiments.

A little disorder is just right

As with most things in life, some disorder can cause unexpected new phenomena. But when it comes to disorder-induced Anderson localization of light in a photonic crystal, simulations suggest that moderation may be the best policy.

Electrical properties of A∕B-site substituted Ni-deficient La(Ni0.6Fe0.3)O3 perovskites with A=Ag+, Pb2+, Nd3+ and B=Mn3+, Ga3+

We report the structural and electron transport properties of a series of compounds where A∕B-site substituted Ni-deficient La(Ni0.6Fe0.3)O3 compounds with A=Ag+, Pb2+, Nd3+ and B=Mn3+, Ga3+ have been synthesized by a chemical route. Electrical resistivity was used as an indicator to explore the response of La(Ni0.6Fe0.3)O3 for different A∕B-site substitutions. The rationalized explanation for the variations of resistivity with substitutions was based on the change in bond length and modification of band structure due to substitutional disorder. A variable range hopping conduction approach is employed to explain the temperature variation of resistivity, and the result suggest that disorder-induced localization of charge carriers dominates the electrical transport in these compounds.

Anderson localization of a non-interacting Bose–Einstein condensate

Anderson localization of waves in disordered media was originally predicted fifty years ago, in the context of transport of electrons in crystals. The phenomenon is much more general and has been observed in a variety of systems, including light waves. However, Anderson localization has not been observed directly for matter waves. Owing to the high degree of control over most of the system parameters (in particular the interaction strength), ultracold atoms offer opportunities for the study of disorder-induced localization. Here we use a non-interacting Bose-Einstein condensate to study Anderson localization. The experiment is performed with a one-dimensional quasi-periodic lattice-a system that features a crossover between extended and exponentially localized states, as in the case of purely random disorder in higher dimensions. Localization is clearly demonstrated through investigations of the transport properties and spatial and momentum distributions. We characterize the crossover, finding that the critical disorder strength scales with the tunnelling energy of the atoms in the lattice. This controllable system may be used to investigate the interplay of disorder and interaction (ref. 7 and references therein), and to explore exotic quantum phases.

Disorder-induced rapid localization of electron-hole plasmas in highly excitedInxGa1−xNmixed crystals

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Scaling approach to related disordered stochastic and free-fermion models

Motivated by mapping from a stochastic system with spatially random rates, we consider disordered non-conserving free-fermion systems using a scaling procedure for the equations of motion. This approach demonstrates disorder-induced localization acting in competition with the asymmetric driving. We discuss the resulting implications for the original stochastic system.

Metallic transport in polyaniline

Despite nearly three decades of materials development, the transport properties in the 'metallic state' of the so-called conducting polymers are still not typical of conventional metals. The hallmark of metallic resistivity--a monotonic decrease in resistivity with temperature--has not been obtained at temperatures over the full range below room temperature; and a frequency dependent conductivity, sigma(omega), typical of metals has also not been observed. In contrast, the low-temperature behaviour of 'metallic' polymers has, in all previous cases, exhibited an increase in resistivity as temperature is further decreased, as a result of disorder-induced localization of the charge carriers. This disorder-induced localization also changes the infrared response such that sigma(omega) deviates from the prediction of Drude theory. Here we report classic metallic transport data obtained from truly metallic polymers. With polyaniline samples prepared using self-stabilized dispersion polymerization, we find that for samples having room-temperature conductivities in excess of 1,000 S cm(-1), the resistivity decreases monotonically as the temperature is lowered down to 5 K, and that the infrared spectra are characteristic of the conventional Drude model even at the lowest frequencies measured.

Disorder-induced carrier localization in ultrathin strained SrRuO3 epitaxial films

In this study we explore the influence of disorder on the properties of epitaxial SrRuO3 thin films. Disorder includes interface effects from misfit strain and bulk effects from cation substitutions and interstitial defects. Isostructural perovskites, including LaAlO3, SrTiO3, DyScO3, GdScO3, and BaTiO3, were used as growth platforms to place films under either biaxial tensile or compressive strain. In addition, films on (001)SrTiO3 were codeposited with FeOx, CoO, SrTiO3, and SrFeOx of concentrations up to 10% to compare the disorder distributed throughout the film with the disorder confined to surfaces and interfaces. Pure SrRuO3 films exhibited metallic behavior with low residual resistivity (&amp;lt;0.1mΩcm), and underwent a ferromagnetic transition at a suppressed Curie temperature (TC) of 145K. The addition of impurity oxides raised the film’s residual resistivity and affected the low-temperature magnetotransport. Interface effects did not seem to play a dominant role in carrier localization, but a few percent of oxide impurities increased the low-temperature resistivity by an order of magnitude. Higher concentrations pushed the resistivity minimum to temperatures comparable to TC.

Variation of the charge dynamics in bandwidth- and filling-controlled metal-insulator transitions of pyrochlore-type molybdates

The systematics of the bandwidth- and filling-controlled metal-insulator transitions (MITs) have been investigated for ${\mathrm{R}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ family ($R=\mathrm{Nd}$, Sm, Eu, Gd, Dy, and Ho) by infrared spectroscopy. The substantial role of electron correlation in driving the MIT is verified. With changing the $R$ ionic radius $(r)$ or equivalently the one-electron bandwidth $(W)$, the MIT occurs in a continuous manner at ${r}_{c}\ensuremath{\approx}r(R=\mathrm{Gd})$. The $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ gap continuously vanishes as $\ensuremath{\Delta}\ensuremath{\propto}({r}_{c}\ensuremath{-}r)$, while at the metallic side the linear decrease of Drude weight is followed toward ${r}_{c}$. In the metallic compounds, some of the infrared-active phonon modes show remarkably large Fano asymmetry correlating with the Drude weight. These Mo-O-Mo bending modes strongly couple to the conduction electrons via effective modulation of the bandwidth. Even for $r\ensuremath{\lesssim}{r}_{c}$ a minimal level of hole doping closes the correlation gap, for example, the barely insulating ${\mathrm{Gd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ is turned to an incoherent metal by 5% partial substitution of ${\mathrm{Gd}}^{3+}$ with ${\mathrm{Ca}}^{2+}$. However, even on further doping no coherent electronic states are formed, indicating the role of the disorder-induced localization effect besides the dominant correlation effects.

Exposition of semiconducting and ferromagnetic properties of pulsed-laser-deposited thin films of LaFe1−xNixO3 (x=0.3, 0.4, and 0.5)

We have explored the possibility of ferromagnetic semiconducting property in the epitaxial thin films of LaFe1−xNixO3 (x=0.3, 0.4, and 0.5) grown on (001) oriented LaAlO3 substrate. We observe that substitution of Ni in the series leads to the increase in conductivity of the samples with conduction being controlled by the disorder-induced localization of charge carriers. All these samples show ferromagnetic behavior at room temperature while their magnetization decreases with increase in Ni concentration in the composition. The results have been explained on the basis of the close interplay between the electrical and magnetic properties.