Electronic Inhomogeneity Research Articles

Interaction of a two-dimensional electromagnetic breather with an electron inhomogeneity in an array of carbon nanotubes

Propagation of ultrashort laser pulses through various nano-objects has recently became an attractive topic for both theoretical and experimental studies due to its promising perspectives in a variety of problems of modern nanoelectronics. Here, we study the propagation of extremely short two-dimensional bipolar electromagnetic pulses in a heterogeneous array of semiconductor carbon nanotubes. Heterogeneity is defined as a region of enhanced electron density. The electromagnetic field in an array of nanotubes is described by Maxwell's equations, reduced to a multidimensional wave equation. Our numerical analysis shows the possibility of stable propagation of an electromagnetic pulse in a heterogeneous array of nanotubes. Furthermore, we establish that, depending on its speed of propagation, the pulse can pass through the area of increased electron concentration or be reflected therefrom.

Structural and magnetotransport studies of magnetic ion doping for monovalent-doped LaMnO3 manganites

Abstract

Measuring size dependent electrical properties from nanoneedle structures: Pt/ZnO Schottky diodes

This work reports the fabrication and testing of nanoneedle devices with well-defined interfaces that are amenable to a variety of structural and electrical characterization, including transmission electron microscopy. Single Pt/ZnO nanoneedle Schottky diodes were fabricated by a top down method using a combination of electro-polishing, sputtering, and focused ion beam milling. The resulting structures contained nanoscale planar heterojunctions with low ideality factors, the dimensions of which were tuned to study size-dependent electrical properties. The diameter dependence of the Pt/ZnO diode barrier height is explained by a joule heating effect and/or electronic inhomogeneity in the Pt/ZnO contact area.

INHOMOGENEOUS ELECTRON GAS AT OXIDE INTERFACES WITH STRONG RASHBA SPIN–ORBIT COUPLING

The 2D electron gas (2DEG) formed at the LaXO 3/ SrTiO 3( X = Al , Ti ) oxide interface appears inhomogeneous in several experiments. In particular, we discuss evidences of electron inhomogeneities provided by the phenomenology of the superconducting (SC) phase, which occurs when the carrier density is tuned above a critical value by means of gating, and of the superconductor-to-metal transition driven by gate voltage or magnetic field. The measured resistance and superfluid density result from the percolative connection of superconducting "puddles" with randomly distributed critical temperatures, embedded in a weakly localizing metallic matrix. We propose a possible intrinsic origin of the electron inhomogeneity, resulting from the strong Rashba spin–orbit coupling (RSOC) measured at these oxide interfaces.

Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ∼ 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.

Temperature-independent pseudogap and thermally activated c-axis hopping conductivity in layered cuprate superconductors

The carrier localization (confinement) and c-axis charge transport in high-Tc layered cuprates have been investigated in the temperature-independent pseudogap state over a wide doping range from the underdoped to the overdoped regime, focusing on the nanoscale phase separation into two different domains (i.e. carrier-rich CuO2 layers and carrier-poor regions between the CuO2 layers) caused by inherent nanoscale electronic inhomogeneities and on the real-space pairing of polaronic carriers caused by strong electron–phonon interactions in carrier-poor regions. A simple microscopic model for layered cuprate superconductors is proposed to describe the carrier confinement and formation of localized bipolarons in the low-doping regions between the CuO2 layers and to simulate the c-axis hopping transport at the thermal dissociation of such bipolarons. This model is found to describe properly the c-axis charge transport, which is governed by thermally activated hopping of polarons residing in the localized state between the CuO2 layers due to the existence of the temperature-independent bipolaronic pseudogap, and the insulating behavior of the c-axis resistivity ρc(T) observed above Tc in various cuprate superconductors, from the underdoped to the overdoped regime. It is found that the insulating behavior of ρc(T) is changed to metallic behavior with disappearing of bipolaronic pseudogap.

Effect of ion and electron beam on kinetic Alfven wave in an inhomogeneous magnetic field

Kinetic Alfven waves are examined in the presence of electron and ion beam and an inhomogeneous magnetic field with bi-Maxwellian distribution function. The theory of particle aspect analysis is used to evaluate the trajectories of the charged particles. The expressions for the field-aligned currents, perpendicular currents (with respect to B 0), dispersion relation and growth/damping rate with marginal instability criteria are derived. The effect of electron and ion beam and inhomogeneity of magnetic field are discussed. The results are interpreted for the space plasma parameter appropriate to the auroral acceleration region of the earth’s magnetoplasma.

Electronic phase separation in iron pnictides

A mechanism for electronic phase separation in iron pnictides is proposed. It is based on the competition between commensurate and incommensurate spin-density-wave phases in a system with an imperfect doping-dependent nesting of a multi-sheeted Fermi surface. We model the Fermi surface by two elliptical electron pockets and three circular hole pockets. The interaction between a charge carrier in a hole band and a carrier in an electron band leads to the formation of spin-density-wave order. The commensurate spin density wave in the parent compound transforms to the incommensurate phase when doping is introduced. We show that, for certain parameter values, the uniform state is unstable with respect to phase separation. The resulting inhomogeneous state consists of regions of commensurate and incommensurate spin-density-wave phases. Our results are in qualitative agreement with recent observations of incommensurate spin density waves and electronic inhomogeneity in iron pnictides.

In Situ Charge-Transfer-Induced Transition from Metallic to Semiconducting Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) are regarded to be potential building blocks for future electronics, because of their exceptional electrical, physical, and mechanical properties. A major obstacle to their practical use for high-performance nanoelectronics is the presence of both metallic SWNTs (M-SWNT) and semiconducting SWNTs (S-SWNT) in as-grown SWNT samples. Most metallicity-based SWNT sorting techniques involve suspension of the nanotubes in solutions, which generally also result in nanotube defects. As-grown SWNTs typically have far superior conductivity or carrier mobility than solution-suspended SWNTs; however, thus far, there is no simple or reproducible method to remove the electronic inhomogeneity in these as-grown nanotubes. We present a simple in situ method using an organic electron-acceptor compound, Acid Yellow (AY), to convert SWNTs from metallic to semiconducting to improve device field-effect behavior. By simply immersing the as-synthesized SWNTs (still attached to a wafer) into an AY solution, the originally metallic nanotubes behave similar to semiconducting ones. Using Raman spectroscopy, atomic force microscopy and single nanotube transistor device measurements, we show that the charge-transfer interaction between SWNTs and the organic electron-acceptor compound AY is diameter-dependent; in the large-diameter regime (1.60–2.60 nm), modulated metallic SWNTs exhibit semiconducting behavior, as evidenced by a pronounced field effect in the current–voltage (I–V) characteristics and up to 3 orders of magnitude increase in the on/off ratio of single M-SWNT field-effect transistors. This method presents a simple viable route toward the fabrication of switchable transistors with as-synthesized SWNTs.

Surface electronic inhomogeneity of the (001)-SrTiO3:Nb crystal with a terrace-structured morphology

Local surface conduction of the (001)-orientated SrTiO3:Nb crystal with a terrace-structured morphology has been studied by means of conductive atomic force microscope analysis. We found that the surface conductance is inhomogeneous on the atomic scale; it is high near step edges and low on terrace plateaus. The surface conductance fluctuation is susceptible to post annealing, first enhancing and then weakening while repeatedly annealed at 700 °C in vacuum. Considering the fact that the oxygen content is most sensitive to vacuum annealing for the temperatures adopted here, the inhomogeneous conductance implies the difference of oxygen vacancy content at step edges and terrace plateaus. The present work clearly demonstrated the influence of surface microstructure on physical properties, and could be helpful for the understanding of the atomic scale non-uniformity of the ultrathin films fabricated on step-featured SrTiO3 surface.

Structural and electronic inhomogeneity for graphene grown on the C-face of SiC: Insights from ab initio calculations

The graphitization of the SiC(0001¯) plane, commonly referred to as the C-face of SiC, takes place through the sublimation and reorganization of surface atoms upon high-temperature annealing. Often, such reorganization gives rise to ordered atomic reconstructions over the ideally flat (0001¯) plane. In this article, we use the density functional theory to model graphene/SiC(0001¯) interfaces with an (1×1), (2×2) and (3×3) SiC periodicity. Our results indicate that the interface geometry can be crucial for both the stability and the electronic characteristics of the first graphitic layer, revealing a complex scenario of binding, doping and electronic correlations. We argue that the presence of more than one interface geometry at different areas of the same sample could be a reason for structural inhomogeneity and n- to p-type transitions.

Strong anisotropic influence of local-field effects on the dielectric response ofα-MoO3

Dielectric properties of {\alpha}-MoO3 are investigated by a combination of valence electron-energy loss spectroscopy and ab initio calculation at the random phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO3 has been obtained along the three axes and the importance of LFE has been shown. In particular, taking into account LFE is shown to be essential to describe properly the intensity and position of the Mo-N2,3 edges as well as the low energy part of the spectrum. A detailed study of the energy-loss function in connection with the dielectric response function also shows that the strong anisotropy of the energy-loss function of {\alpha}-MoO3 is driven by an anisotropic influence of LFE. These LFE significantly dampen a large peak in {\epsilon}2, but only along the [010] direction. Thanks to a detailed analysis at specific k-points of the orbitals involved in this transition, the origin of this peak has not only been evidenced but a connection between the inhomogeneity of the electron density and the anisotropic influence of local-field effects has also been established. More specifically, this anisotropy is governed by a strongly inhomogeneous spatial distribution of the empty states. This depletion of the empty states is localized around the terminal oxygens and accentuates the electron inhomogeneity.

Mechanism of column and carrot sprites derived from optical and radio observations

The lightning current waveforms observed simultaneously with high‐speed video records of a column and a carrot sprite event are incorporated in a plasma fluid model to provide quantitative explanation of these two distinct morphological classes of transient luminous events. We calculate the strength of the lightning‐induced electric field at sprite altitudes using a time integral of the ionization frequency . For the studied two events, modeling results indicate that these integral values never exceed 18 in the lower ionosphere, which is the minimum value required for the initiation of streamers from single seed electrons according to the Raether‐Meek criterion. It is therefore suggested that the presence of electron inhomogeneities is a necessary condition for the initiation of sprite streamers. It is further demonstrated using streamer modeling that a minimum value of the integral ∼10 is necessary to initiate upward negative streamers from inhomogeneities, corresponding to a minimum charge moment change of ∼500 C km under typical nighttime conditions. If the integral values in the entire upper atmosphere are smaller than ∼10, only column sprites can be produced, dominated by downward positive streamers.

Generic strange-metal behaviour of overdoped cuprates

We present an analysis of the temperature dependence of the zero-field in-plane resistivity ρab(T) of overdoped Tl2Ba2CuO6+δ (Tl2201). Taking our cue from earlier resistivity and angle-dependent magnetoresistance studies of Tl2201, as well as high-field measurements on La2−xSrxCuO4 (LSCO), we delineate ρab(T) into two T-dependent components below 200 K, one linear in temperature, the other quadratic. As found in LSCO, the T-linear component α1(0) is finite for all superconducting samples, its magnitude scaling with the transition temperature Tc. By contrast, the T2 coefficient α2(0) is essentially doping independent. Such an extended regime (in doping) of T-linear resistivity at low T is at odds with conventional quantum critical scenarios involving the collapse of an ordered phase, possibly associated with the normal state pseudogap, to 0 K at a critical doping level. Its confirmation in Tl2201, whose electronic state (as revealed by quantum oscillation experiments) is highly homogeneous over hundreds of unit cells, appears to rule out phase separation or electronic inhomogeneity as the origin of this extended critical behaviour.

Dopant clustering, electronic inhomogeneity, and vortex pinning in iron-based superconductors

We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr${}_{0.75}$K${}_{0.25}$Fe${}_{2}$As${}_{2}$ with ${T}_{c}=32$ K. We find that the low-$T$ cleaved surface is dominated by a half Sr/K termination with $1\ifmmode\times\else\texttimes\fi{}2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $\ensuremath{\sigma}/\overline{\ensuremath{\Delta}}=16%$ on a $\ensuremath{\sim}$3 nm length scale, with average $2\overline{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.6$ in the weak-coupling limit. The vortex core size provides a measure of the superconducting coherence length $\ensuremath{\xi}=2.3$ nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.

Possible Indications of Electronic Inhomogeneities in Superconducting Nanowire Detectors

The voltage-carrying state of superconducting NbTiN nanowires, used for single-photon detectors, is analyzed. Upon lowering the current, the wire returns to the superconducting state in a steplike pattern, which differs from sample to sample. Elimination of geometrical inhomogeneities, such as sharp corners, does not remove these steplike features. They appear to be intrinsic to the material. Since the material is strongly disordered, electronic inhomogeneities are considered as a possible cause. A thermal model, taking into account random variations of the electronic properties along the wire, is used as an interpretative framework.

Propagation of cylindrical lower hybrid drift solitary wave in an inhomogeneous plasma

The nonlinear cylindrical lower hybrid drift solitary wave in an inhomogeneous, magnetized plasma with the combined effects of electron density inhomogeneity and electron temperature inhomogeneity is investigated in a two-fluid model. The amplitude and width of the solitary wave are found to decrease as the electronic density inhomogeneity increases. When the electron temperature inhomogeneity grows, the amplitude of the soliton decays and the width never changes. It is noted that the decrease of diamagnetic drift velocity will strengthen the cylindrical lower hybrid drift solitary wave height and width.

Modeling of colossal magnetoresistance in La0.67Ca0.33MnO3/Pr0.67Ca0.33MnO3 superlattices: Comparison with individual (La1−yPry)0.67Ca0.33MnO3 films

Colossal magnetoresistance (CMR) and nm-scale electronic inhomogeneity close to the first order phase transition in perovskite manganites, e.g., (La1−yPry)0.67Ca0.33MnO3 still remain a puzzling phenomenon. We experimentally model a metal-insulator phase coexistence by growing a short period (LCMOn/PCMOn)m superlattices (SLs) with the same thickness for both components. CMR effect was studied as a function of the individual layer thickness n = 2–8 and then compared with chemically homogeneous (La1−yPry)0.67Ca0.33MnO3 LPCMO films. We show that SLs can be superimposed in the phase diagram of LPCMO. The results also point out the importance of the nm-scale electronic rather than chemical separation for realization of the CMR effect as well as limits the lowest boundary for the thickness of an individual manganite material to n∼4u.c.

Nonlinear electronic transport in the anomalous metallic state of quasi‐2D organic superconductors κ‐(BEDT‐TTF)2X

AbstractWe present measurements of the first‐ and third‐harmonic voltage response in ac electronic transport measurements, representing the linear (ohmic) and nonlinear resistivity, respectively, of the quasi‐two‐dimensional (2D) organic superconductors κ‐(BEDT‐TTF)2X. Nonlinear transport is a sensitive tool to probe the microgeometry of the electronic system in high‐quality single crystals. For the title compounds, the normalconducting metallic state in the vicinity of the Mott metal–insulator (MI) transition and critical endpoint is known to be highly unusual. Our results reveal large current‐induced intrinsic inhomogeneities, at high current densities most pronounced at the so‐called T* anomaly, which characterizes the anomalous metallic state. The observed nonlinearities in the interlayer transport do not depend on frequency and cannot be ascribed to a simple Joule heating mechanism in a resistor network. Furthermore, we find evidence supporting the notion of electronic phase separation induced by the Mott critical endpoint. The observed dependence of the generated third‐harmonic voltage on the current density reveals a systematic behavior suggesting that current‐induced electronic inhomogeneities are more pronounced for more strongly correlated systems.

Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors

Many of today's forefront materials, such as high-Tc superconductors, doped semiconductors, and colossal magnetoresistance materials, are structurally, chemically and/or electronically inhomogeneous at the nanoscale. Although inhomogeneity can degrade the utility of some materials, defects can also be advantageous. Quite generally, defects can serve as nanoscale probes and facilitate quasiparticle scattering used to extract otherwise inaccessible electronic properties. In superconductors, non-stoichiometric dopants are typically necessary to achieve a high transition temperature, while both structural and chemical defects are used to pin vortices and increase critical current. Scanning tunneling microscopy (STM) has proven to be an ideal technique for studying these processes at the atomic scale. In this perspective, we present an overview of STM studies on chemical disorder in unconventional superconductors, and discuss how dopants, impurities and adatoms may be used to probe, pin or enhance the intrinsic electronic properties of these materials.