Mesoscopic Disorder Research Articles

Variability of mesoscopic mechanical disorder in disordered solids

Quantifying mechanical disorder in solids, either disordered crystals or glassy solids, and understanding its range of variability are of prime importance, e.g. for discovering structure–properties relations. The bounds on the degree of mechanical fluctuations in disordered solids and how those depend on solids formation history remain unknown. Here, we study a broadly applicable quantifier of mesoscopic mechanical disorder χ, defined via the dimensionless fluctuations of the shear modulus, over a wide variety of disordered computer solids and upon varying different control parameters. χ is intimately related to basic properties of disordered solids, such as elastic constants and plastic deformability, and can be experimentally extracted by wave-attenuation measurements. We find that a large variety of self-organized glassy solids, where disorder is an emergent property, appear to satisfy a generic lower bound on χ. On the other hand, we show that χ is unbounded from above, and may diverge in systems driven towards the critical unjamming point. These results highlight basic properties of disordered solids and set the ground for systematically quantifying mechanical disorder across different systems.

Single-parameter scaling in the magnetoresistance of optimally doped La2−xSrxCuO4

We show that the recent magnetoresistance data on thin-film La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) in strong magnetic fields ($B$) obeys a single-parameter scaling of the form MR$(B,T)=f(\mu_H(T)B)$, where $\mu_H^{-1}(T)\sim T^{\alpha}$ ($1\le\alpha\le2$), from $T=180$K until $T\sim20$K, at which point the single-parameter scaling breaks down. The functional form of the MR is distinct from the simple quadratic-to-linear quadrature combination of temperature and magnetic field found in the optimally doped iron superconductor BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. Further, low-temperature departure of the MR in LSCO from its high-temperature scaling law leads us to conclude that the MR curve collapse is not the result of quantum critical scaling. We examine the classical effective medium theory (EMT) previously used to obtain the quadrature resistivity dependence on field and temperature for metals with a $T$-linear zero-field resistivity. It appears that this scaling form results only for a binary, random distribution of metallic components. More generally, we find a low-temperature, high-field region where the resistivity is simultaneously $T$ and $B$ linear when multiple metallic components are present. Our findings indicate that if mesoscopic disorder is relevant to the magnetoresistance in strange metal materials, the binary-distribution model which seems to be relevant to the iron pnictides is distinct from the more broad-continuous distributions relevant to the cuprates. Using the latter, we examine the applicability of classical effective medium theory to the MR in LSCO and compare calculated MR curves with the experimental data.

Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Superconductivity in disordered systems close to an incipient localization transition has been an area of investigation for many years. It has been noted that in such highly disordered superconductors, anomalous spectral weight develops in their conductivity near and below the superconducting gap energy. In this work we investigate the low frequency conductivity in disordered superconducting NbN thin films close to the localization transition with time-domain terahertz spectroscopy. In the normal state, strong deviations from the Drude form due to incipient localization are found. In the superconducting state we find substantial spectral weight at frequencies well below the superconducting gap scale derived from tunneling. We analyze this spectral weight in the context of a model of disorder induced broadening of the quasiparticle density of states and effective pair-breaking. We find that although aspects of the optical and tunneling data can be consistently modeled in terms of this effect of mesoscopic disorder, the optical conductivity returns to the normal state value much faster than any model predicts. This points to the non-trivial interplay of superconductivity and disorder close to localization.

Pressure-induced mesoscopic disorder in protic ionic liquids: first computational study.

It has been recently shown that pressure may affect the mesoscopic heterogeneity in aprotic ionic liquids, owing to the long alkyl chain folding on itself. Here we explore protic ionic liquids, using classical molecular dynamics. These compounds have shorter and stiffer alkyl chains, harder to fold. We observed that high pressure affects the mesoscopic structure of the studied chemicals and, indeed, the effect may be ascribed to chain folding.

Quantum interference phenomena in the Casimir effect

We propose a definitive test of whether plates involved in Casimir experiments should be modeled with ballistic or diffusive electrons--a prominent controversy highlighted by a number of conflicting experiments. The unambiguous test we propose is a measurement of the Casimir force between a disordered quasi-2D metallic plate and a three-dimensional metallic system at low temperatures, in which disorder-induced weak localization effects modify the well-known Drude result in an experimentally tunable way. We calculate the weak localization correction to the Casimir force as a function of magnetic field and temperature and demonstrate that the quantum interference suppression of the Casimir force is a strong, observable effect. The coexistence of weak localization suppression in electronic transport and Casimir pressure would lend credence to the Drude theory of the Casimir effect, while the lack of such correlation would indicate a fundamental problem with the existing theory. We also study mesoscopic disorder fluctuations in the Casimir effect and estimate the width of the distribution of Casmir energies due to disorder fluctuations.

The role of mesoscopic disorder in determining the character of the field-induced insulating regime of amorphous ultrathin films

This work documents the conditions under which the field-tuned behavior of quench condensed films grown on relatively thick layers of a-Sb is essentially identical to that found for amorphous InOx and polycrystalline TiNx films. The electrical transport properties of a series of amorphous Bi (a-Bi) films of different thicknesses, grown with a 14.67Å thick underlayer of amorphous Sb (a-Sb), were studied in perpendicular and parallel magnetic fields. A magnetoresistance (MR) peak was found in insulating films in both perpendicular and parallel magnetic fields. In all insulating films, Arrhenius type conduction was also found over all ranges of magnetic field. Neither behavior is found for a-Bi films grown on top of thinner a-Sb underlayers, which were substantially smoother. These observations, together with a quantitative analysis of film roughness profiles, highlight the role of mesoscopic scale thickness fluctuations in nucleating superconducting clusters or islands that apparently lead to the MR peak and Arrhenius conduction. This implies that the nature of the disorder plays a major role in determining the character of the insulating regime.

Chemical disorder effect on orthorhombic-rhombohedral structural transition in La0.7Ca0.3MnO3

High temperature X-ray powder diffraction technique was employed to study the orthorhombic to rhombohedral (O-R) structural transition (ST) in single crystal and ceramics of La0.7Ca0.3MnO3. The data obtained evidence that such ST in both samples occurs in the vicinity of temperature T ∼ 700 K and demonstrates O and R phases coexistence within the T-intervals of about 20 K in crystal and ∼60 K in ceramics. This first order transition is hysteretic on heating and cooling procedures and it is shifted on ΔT ∼ 10 K to higher temperatures in crystal as compared to the ceramics of the same composition. We have described these features in the terms of the difference between “technologically driven” chemical disorder characteristic for considered samples. Thus, it seems that spatial (up to the micron size scale) fluctuations of the Ca-dopant content in crystal and “mesoscopic scale disorder” within grain boundary regions in ceramics are responsible for both: above noted difference of ST parameters and reported earlier difference in magnetic ordering of these samples.

Double-well optical lattices with atomic vibrations and mesoscopic disorder

Double-well optical lattice in an insulating state is considered. The influence of atomic vibrations and mesoscopic disorder on the properties of the lattice are studied. Vibrations lead to the renormalization of atomic interactions. The occurrence of mesoscopic disorder results in the appearance of first-order phase transitions between the states with different levels of atomic imbalance. The existence of a nonuniform external potential, such as trapping potential, essentially changes the lattice properties, suppressing the disorder fraction and rising the transition temperature.

Disordered Proteins and Network Disorder in Network Descriptions of Protein Structure, Dynamics and Function: Hypotheses and a Comprehensive Review

During the last decade, network approaches became a powerful tool to describe protein structure and dynamics. Here we review the links between disordered proteins and the associated networks, and describe the consequences of local, mesoscopic and global network disorder on changes in protein structure and dynamics. We introduce a new classification of protein networks into 'cumulus-type', i.e., those similar to puffy (white) clouds, and 'stratus-type', i.e., those similar to flat, dense (dark) low-lying clouds, and relate these network types to protein disorder dynamics and to differences in energy transmission processes. In the first class, there is limited overlap between the modules, which implies higher rigidity of the individual units; there the conformational changes can be described by an 'energy transfer' mechanism. In the second class, the topology presents a compact structure with significant overlap between the modules; there the conformational changes can be described by 'multi-trajectories'; that is, multiple highly populated pathways. We further propose that disordered protein regions evolved to help other protein segments reach 'rarely visited' but functionally-related states. We also show the role of disorder in 'spatial games' of amino acids; highlight the effects of intrinsically disordered proteins (IDPs) on cellular networks and list some possible studies linking protein disorder and protein structure networks.

Modeling of interface roughness in thermoelectric composite materials

We use a network model to calculate the influence of the mesoscopic interface structure onthe thermoelectric properties of superlattice structures consisting of alternating layers ofmaterials A and B. The thermoelectric figure of merit of such a composite material dependson the layer thickness, if interface resistances are accounted for, and can be increased byproper interface design. In general, interface roughness reduces the figure of merit, againcompared to the case of ideal interfaces. However, the strength of this reduction dependsstrongly on the type of interface roughness. Smooth atomic surface diffusion leading toalloying of materials A and B causes the largest reduction of the figure of merit.Consequently, in real structures, it is important not only to minimize interfaceroughness, but also to control the type of roughness. Although the microscopiceffects of interfaces are only empirically accounted for, using a network modelcan yield useful information about the dependence of the macroscopic transportcoefficients on the mesoscopic disorder in structured thermoelectric materials.

Mesoscopic disorder in double-well optical lattices

Double-well optical lattices are considered, each cite of which is formed by a double-well potential. The lattice is assumed to be in an insulating state and order and disorder are defined with respect to the displacement of atoms inside the double-well potential. It is shown that in such lattices, in addition to purely ordered and disordered states, there can exist an intermediate mixed state, where, inside a generally ordered lattice, there appear disordered regions of mesoscopic size.

Disordered proteins and network disorder in network descriptions of protein structure, dynamics and function. Hypotheses and a comprehensive review

AbstractDuring the last decade, network approaches became a powerful tool to describe protein structure and dynamics. Here we review the links between disordered proteins and the associated networks, and describe the consequences of local, mesoscopic and global network disorder on changes in protein structure and dynamics. We introduce a new classification of protein networks into ‘cumulus-type’, i.e., those similar to puffy (white) clouds, and ‘stratus-type’, i.e., those similar to flat, dense (dark) low-lying clouds, and relate these network types to protein disorder dynamics and to differences in energy transmission processes. In the first class, there is limited overlap between the modules, which implies higher rigidity of the individual units; there the conformational changes can be described by an ‘energy transfer’ mechanism. In the second class, the topology presents a compact structure with significant overlap between the modules; there the conformational changes can be described by ‘multi-trajectories’; that is, multiple highly populated pathways. We further propose that disordered protein regions evolved to help other protein segments reach ‘rarely visited’ but functionally-related states. We also show the role of disorder in ‘spatial games’ of amino acids; highlight the effects of intrinsically disordered proteins (IDPs) on cellular networks and list some possible studies linking protein disorder and protein structure networks.

Superconductivity-enhanced conductance fluctuations in few-layer graphene

We investigate the mesoscopic disorder induced rms conductance varianceδG in short few-layer graphene (FLG) flakes contacted by two superconducting (S) Ti/Alcontacts. By sweeping the back-gate voltage, we observe pronounced conductancefluctuations superimposed on a linear background of the two-terminal conductanceG. Thelinear gate voltage induced response can be modelled by a set of interlayer and intralayer capacitances.δG depends ontemperature T andsource–drain voltage Vsd. δG increaseswith decreasing T and |Vsd|. Whenlowering |Vsd|, a pronounced cross-over at a voltage corresponding to the superconducting energy gapΔ is observed. For the fluctuations are markedly enhanced. Expressed in the conductance varianceGGS of one graphene–superconductor (G–S) interface, values of0.58e2/h are obtained at the base temperature of 230 mK. The conductance variance in the sub-gapregion is larger by up to a factor of 1.4–1.8 compared to the normal state. The observedstrong enhancement is due to phase coherent charge transfer caused by Andreev reflectionat the G–S interface.

Collective dynamics underpins Rayleigh behavior in disordered polycrystalline ferroelectrics

Nanoscale and mesoscopic disorder and associated local hysteretic responses underpin the unique properties of spin and cluster glasses, phase-separated oxides, polycrystalline ferroelectrics, and ferromagnets alike. Despite the rich history of the field, the relationship between the statistical descriptors of hysteresis behavior such as Preisach density, and micro and nanostructure has remained elusive. By using polycrystalline ferroelectric capacitors as a model system, we now report quantitative nonlinearity measurements in 0.025-1 microm(3) volumes, approximately 10(6) times smaller than previously possible. We discover that the onset of nonlinear behavior with thickness proceeds through formation and increase of areal density of micron-scale regions with large nonlinear response embedded in a more weakly nonlinear matrix. This observation indicates that large-scale collective domain wall dynamics, as opposed to motion of noninteracting walls, underpins Rayleigh behavior in disordered ferroelectrics. The measurements provide evidence for the existence and extent of the domain avalanches in ferroelectric materials, forcing us to rethink 100-year old paradigms.

Mesoscopic gap fluctuations in an unconventional superconductor

We study mesoscopic disorder fluctuations in an anisotropic gap superconductor, which lead to the spatial variations of the local pairing temperature and formation of superconducting islands above the mean-field transition. We derive the probability distribution function of the pairing temperatures and superconducting gaps. It is shown that above the mean-field transition, a disordered BCS superconductor with an unusual pairing symmetry is described by a network of superconducting islands and metallic regions with a strongly suppressed density of states due to superconducting fluctuations. We argue that the phenomena associated with mesoscopic disorder fluctuations may also be relevant to the high-temperature superconductors, in particular, to recent scanning tunnel microscope (STM) experiments, where gap inhomogeneities have been explicitly observed. It is suggested that the gap fluctuations in the pseudogap phase should be directly related to the corresponding fluctuations of the pairing temperature.

Effect of mesoscopic inhomogeneities on local tunneling density of states in cuprates

We carry out a theoretical analysis of the momentum dependence of the Fourier-transformed local density of states in the superconducting cuprates within a model considering the interference of quasiparticles scattering on quenched impurities. The impurities introduce an external scattering potential, which is either nearly local in space or can acquire a substantial momentum dependence due to a possible strong momentum dependence of the electronic screening near a charge modulation instability. The key effect that we introduce is an additional mesoscopic disorder aiming to reproduce the inhomogeneities experimentally observed in scanning tunneling microscopy. The crucial effect of this mesoscopic disorder is to give rise to pointlike spectroscopic features, to be contrasted with the curvelike shape of the spectra previously calculated within the interfering-quasiparticle schemes. It is also found that stripelike charge modulations play a relevant role to correctly reproduce all the spectral features of the experiments.

Structural disorder effects on the tensile strength distribution of heterogeneous brittle materials with emphasis on fiber networks

Understanding the interplay of structural disorder and strength properties at various length scales can lead to improvements in the strength reliability of heterogeneous brittle materials. Various studies in ordered fiber- matrix composites have shown the existence of critical clusters of breaks and macroscopic weak-link scaling behavior. The fiber network in paper is structurally disordered. We verify experimentally that the tensile strength of newsprint samples follows weak-link scaling and obtain an estimate for the link and critical-cluster sizes. However, a slight nonlinear behavior is observed in the Weibull plots of the experimental strength distributions. We propose that this is due to mesoscopic structural disorder (e.g., at length scales between millimeters and centimeters), which we incorporate in the strength distribution of the links by averaging over the elastic stress variations. The prevailing industry perception is that mesoscopic disorder controls the strength reliability. In contrast, we find that it does not significantly affect the crucial lower tail of the strength distribution. Based on our analysis, we suggest a more reliable measurement approach for the tensile strength of newsprint paper. We also obtain explicit expressions for the effects of disorder on stress variations and the macroscopic Young's modulus, including dependence on the shear modulus and anisotropic effects.

Biomimetic membranes of lipid–peptide model systems prepared on solid support

The structure of membrane-active peptides and their interaction with lipid bilayerscan be studied in oriented lipid membranes deposited on solid substrates. Suchsystems are desirable for a number of surface-sensitive techniques. Here we focus onstructural characterization by x-ray and neutron reflectivity and give an accountof recent progress in sample preparation and measurements. We show that thedegree of mesoscopic disorder in the films can significantly influence the scatteringcurves. Static defects should be minimized by optimization of the preparationtechniques and their presence must be taken into account in the modelling. Examplesare given for alamethicin and magainin in bilayers of different phosphocholines.

An EPR investigation of electrospun polyaniline-polyethylene oxide blends

The EPR magnetic susceptibility behavior of the camphorsulfonic acid doped polyaniline (PANCSA) blends with polyethylene oxide (PEO) is reported in fibers and films. In particular, EPR investigations on electrospun (PANCSA) 0.72(PEO) 0.28 nanofibers, cast films of (PANCSA) 0.72(PEO) 0.28 and cast films of (PANCSA) were performed to investigate differences in the mesoscopic disorder as induced by the process of electrospinning. The changes observed in the Pauli susceptibility, EPR lineshape, EPR linewidth, and dc conductivity are interpreted as due to increased chain alignment in the fibers compared with the cast films.

Effects of disorder on the magnetic state of poly(3-hexyl)thiophene

We present EPR magnetic susceptibility measurements as a function of temperature on undoped and iodine-doped samples of regioregular (REPHT) and regiorandom (RAPHT) poly(3-hexyl)thiophene. The regioregular and regiorandom forms of poly(3-hexyl)thiophene show different temperature dependences in both the undoped and the doped states. The results are interpreted in the framework of a spin-pairs model, according to which the number of spins in undoped samples is less than one per two-hundred thiophene rings. In doped samples, the number of spins increases to approximately one per ten thiophene rings. In addition to showing the predominant role of bipolarons, different temperature dependences of the magnetic susceptibility data provide a methodology to investigate mesoscopic disorder.