Two-dimensional Weak Localization Research Articles

The electric transport properties of iridate Sr2-x Bi x IrO4 and Sr2-y Dy y IrO4

Sr2IrO4 is predicted to be a new superconductor in theory, and its electric transport properties have attracted much attention in the field of physics. Here we prepared a series of polycrystalline samples doped with Sr2-xBixIrO4 (x=0,0.02,0.04,0.06,0.08) and Sr2-yDyyIrO4 (y=0,0.02,0.04,0.06) respectively. The influence of the two ions on the lattice structure and electronic structure after the substitution of the Ir site is very similar, which makes the overall reduction trend of IrO6 octahedron and Ir-O1-Ir bond angle. The resistivity shows a non-monotonic change with doping and still maintains the insulation. The low-temperature section, medium-temperature section, and high-temperature section conform to three-dimensional variable range hopping (3D-VRH), thermal activation model, two-dimensional weak localization model respectively.

Electron dephasing processes in the thin and thick Ta-doped SnO2 films

We have systematically investigated the electron dephasing processes of the thin and thick Ta-doped SnO2 films. All samples exhibit electron transport characteristics of degenerate semiconductors. At low temperature T (below ∼70 K), the weak-localization-induced negative magnetoresistance (MR) is observed. For the thick films with the thicknesses of ∼300 nm, the observed MR can be well described by three-dimensional weak-localization theory. The electron dephasing rate 1/τφ varies linearly with T3/2, indicating that the electron–electron scattering dominates the electron dephasing process. This is different from the common observation that the electron–phonon scattering is the main dephasing process for electrons in three-dimensional disordered conductors. For the thin films with the thicknesses of ∼30 nm, the MR can be well described by two-dimensional weak-localization theory. By analyzing the electron dephasing rate as a function of T, we find that both large- and small-energy-transfer electron–electron scattering processes play an important role in the electron dephasing.

Quantum Transport Signatures of a Close Candidate for a Type II Nodal-Line Semimetal

The nodal-line semimetal is a new type of topological state of matter in which the crossing of two energy bands forms a nodal loop. In the absence of spin-orbit coupling, Mg3Bi2 is predicted as a type II nodal-line semimetal, which may evolve to a topological insulator with a small energy gap of ∼35 meV in the presence of spin-orbit coupling. However, the transport evidence is still lacking. Here, we measure the magneto-transport in Mg3Bi2. At low temperatures, the magnetoconductivity exhibits a weak antilocalization behavior. We fit the experimental data with a magnetoconductivity formula for the weak antilocalization effect of three-dimensional nodal-line semimetals as well as the well-known Hikami-Larkin-Nagaoka formula for two-dimensional weak (anti)localization effects. By comparing the fitting results of these two theories, we demonstrate that the weak antilocalization in Mg3Bi2 is better described by the theory for nodal-line semimetals. Our work will inspire more explorations to use the new weak localization theory to identify a large spectrum of nodal-line semimetals.

Solution Processable CrN Thin Films: Thickness-Dependent Electrical Transport Properties.

Thickness is a very important parameter with which to control the microstructures, along with physical properties in transition-metal nitride thin films. In work presented here, CrN films with different thicknesses (from 26 to 130 nm) were grown by chemical solution deposition. The films are pure phase and polycrystalline. Thickness dependence of microstructures and electrical transport behavior were studied. With the increase of films thickness, grain size and nitrogen content are increased, while resistivity, zero-field sensitivity and magnetoresistance are decreased. In the temperature range of 5–350 K, all samples exhibited semiconductor-like properties with dρ/dT < 0. For the range above and below the Néel temperature, the resistivity can be fitted by the thermal activation model and the two-dimensional weak localization (2D-WL) model, respectively. The ultra-low magnetoresistance at a low temperature under high magnetic fields with a large zero-field sensitivity was observed in the CrN thin films. The zero-field sensitivity can be effectively tuned to 10−2 K−1 at 5 K with a magnetoresistance of less than 1% at 2 K under 14 T by reasonably controlling the thickness.

Magnetotransport in type-enriched single-wall carbon nanotube networks

Single-wall carbon nanotubes (SWCNTs) exhibit a wide range of physical phenomena depending on their chirality. Nanotube networks typically contain a broad mixture of chiralities, which prevents an in-depth understanding of SWCNT ensemble properties. In particular, electronic-type mixing (the simultaneous presence of semiconductor and metallic nanotubes) in SWCNT networks remains the single largest hurdle to developing a comprehensive view of ensemble nanotube electrical transport, a critical step toward their use in optoelectronics. Here, we systematically study temperature-dependent magnetoconductivity (MC) in networks of highly enriched semiconductor and metal SWCNT films. In the semiconductor-enriched network, we observe two-dimensional variable-range hopping conduction from 5 to 290 K. Low-temperature MC measurements reveal a large, negative MC from which we determine the wave-function localization length and Fermi energy density of states. In contrast, the metal-enriched film exhibits positive MC that increases with decreasing temperature, a behavior attributed to two-dimensional weak localization. Using this model, we determine the details of the carrier phase coherence and fit the temperature-dependent conductivity. These extensive measurements on type-enriched SWCNT networks provide insights that pave the way for the use of SWCNTs in solid-state devices.

Large negative magnetoresistance driven by enhanced weak localization and Kondo effect at the interface of LaAlO3 and Fe-doped SrTiO3

The out-of-plane magnetoresistance of a $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface is commonly positive due to significant orbital effect. In this paper, a large negative magnetoresistance is observed at a $\mathrm{LaAl}{\mathrm{O}}_{3}$ and Fe-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ interface upon the application of an out-of-plane magnetic field. Such large negative magnetoresistance is attributed to enhanced two-dimensional weak localization and Kondo effect. We think that these two effects are mainly the consequences of intentional Fe dopants in the substrate, which introduce holes, disorders, and spin scattering centers into the electronically conducting interface, directly modifying the spin-orbit coupling, orbital effect, scale of electrons motion, and local magnetic moments at the $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface. The ${\mathrm{LaAlO}}_{3}\text{/Fe-doped}\phantom{\rule{0.28em}{0ex}}{\mathrm{SrTiO}}_{3}$ interfacial system provides a prospective platform for studying quantum interference between strongly correlated electrons. The substrate doping method may facilitate the application of $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interfaces in versatile electronic devices.

Change in the Sign of the Magnetoresistance and the Two-Dimensional Conductivity of the Layered Quasi-One-Dimensional Semiconductor TiS3

The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS3 on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T0 ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (B ∥ c) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T0. The negative magnetoresistance (at B ∥ c) below T0 is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at B ∥ ab) is explained by the effect of the magnetic field on the spectrum of electronic states.

Effect of boron doping on the electrical conductivity of metallicity-separated single walled carbon nanotubes.

We explored the effect of substitutional boron doping on the electrical conductivity of a metallicity-separated single walled carbon nanotube (SWCNT) assembly. Boron atoms were introduced into semiconducting (S)- and metallic (M)-SWCNT assemblies using high temperature thermal diffusion and the concentration of the doped boron atoms was controlled by the thermal treatment temperature. Depending on the conduction mechanism of the SWCNT assembly, both positive and negative effects upon boron incorporation are observed. For the S-SWCNT sheet, the electrical resistivity decreased by about 1 order on introduction of a small amount of boron atoms, due to the localized state for hopping conduction. In contrast, we observed an increase in the electrical resistivity on boron doping for M-SWCNTs. The pristine and boron doped metallic SWCNTs exhibited a tendency of decreasing electrical resistivity in the presence of an external magnetic field perpendicular to the film, which indicated two-dimensional weak localization behavior. A detailed analysis of the resistivity and the magnetoresistance implied that an increase in the inelastic scattering event at the doped boron site reduced the phase coherence length, leading to an increase in the electrical resistivity.

Crystal structure and physical properties of the Sr-vacant spin-orbit-coupling induced Mott insulator Sr2−IrO4

A series of polycrystalline samples of Sr2-xIrO4 have been synthesized by a solid-state reaction method. The crystal structure of this doped system can be explained on the basis of the extended nature of 5d electrons and strontium vacancies in Sr2-xIrO4. The analysis of the temperature-dependent resistance of these samples reveals the semiconducting feature, where three dimensional variable range hopping behavior is observed at temperatures lower than 120K, Arrhenius type in intermediate temperatures from 140K to 200K, and two-dimensional (2D) weak localization at high temperatures from 220K to 300K. Correspondingly, temperature-dependent magnetic properties in the range of x less than 0.30 can be described by the antiferromagnetically ordered spin system.

Growth, structure and physical properties of gadolinium doped Sr 2 IrO 4 single crystal

Abstract A series of Gd-doped Sr2IrO4 single crystals were grown using a flux method. Analysis of the temperature-dependent resistance of these crystals reveals that these samples show two-dimensional weak localization at 150 to 300 K, while three-dimensional variable range hopping (VRH) behavior is observed at temperatures lower than 150 K. Two localization lengths are observed in the VRH behavior, with a transition temperature of around 88 K. Correspondingly, temperature-dependent magnetization observations along the ab-plane reveal magnetic anomalies at both 150 and 85 K. This work demonstrates the correlation between the electrical and magnetic properties of 5d transition-metal compounds.

Atomic layer doping of strained Ge-on-insulator thin films with high electron densities

We demonstrate that phosphorous atomic layer doping in ultra-high vacuum is a viable method to obtain n-type doping of strained germanium-on-insulator thin films. By engineering single and multiple, closely-spaced P δ-layers, we obtain high active electron concentrations (∼1 × 1020 cm−3) and low electrical resistivity (∼120 Ω/square) whilst keeping control over doping profile, structural integrity, and tensile strain levels (e = 0.35%). Investigation of magnetotransport over a large temperature range (1.7-290 K) allows observation of two-dimensional electrons' weak localization up to 30 K.

Unconventional scaling of the anomalous Hall effect accompanying electron localization correction in the dirty regime

Scaling of the anomalous Hall conductivity to longitudinal conductivity, has been observed in the dirty regime of two-dimensional weak and strong localization regions in ultrathin, polycrystalline, chemically disordered, ferromagnetic FePt films. The relationship between electron transport and temperature reveals a quantitatively insignificant Coulomb interaction in these films while the temperature dependent anomalous Hall conductivity experiences quantum correction from electron localization. At the onset of this correction, the low-temperature anomalous Hall resistivity begins to be saturated when the thickness of the FePt film is reduced, and the corresponding Hall conductivity scaling exponent becomes 2, which is above the recent unified theory of 1.6 (\sigma_AH \propto \sigma^1.6_xx). Our results strongly suggest that the correction of the electron localization modulates the scaling exponent of the anomalous Hall effect.

Anisotropic weakly localized transport in nitrogen-doped ultrananocrystalline diamond films

We establish the dominant effect of anisotropic weak localization (WL) in three dimensions (3D) associated with a propagative Fermi surface on the conductivity correction in heavily nitrogen-doped ultrananocrystalline diamond (UNCD) films based on magnetoresistance studies at low temperatures. Also, low-temperature electrical conductivity can show weakly localized transport in 3D combined with the effect of electron-electron interactions in these materials, which is remarkably different from the conductivity in two-dimensional WL or strong localization regime. The corresponding dephasing time of electronic wave functions in these systems described as $\ensuremath{\sim}{T}^{\ensuremath{-}p}$ with $p&lt;1$, follows a relatively weak temperature dependence compared to the generally expected nature for bulk dirty metals having $p\ensuremath{\ge}1$. The temperature dependence of Hall (electron) mobility together with an enhanced electron density has been used to interpret the unusual magnetotransport features and show delocalized electronic transport in these $n$-type UNCD films, which can be described as low-dimensional superlattice structures.

Large-scale graphitic thin films synthesized on Ni and transferred to insulators: Structural and electronic properties

We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO2/Si substrates. Films of size up to several mm’s have been synthesized. Structural characterizations by atomic force microscopy, scanning tunneling microscopy, cross-sectional transmission electron microscopy (XTEM), and Raman spectroscopy confirm that such large-scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few-layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ∼50% and carrier mobilities reaching ∼2000 cm2/V s. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 μm from the observation of two-dimensional weak localization in low temperature magnetotransport measurements. Our results show that despite the nonuniformity and surface roughness, such large-scale, flexible thin films can have electronic properties promising for device applications.

Evidence of Two-Dimensional Weak Localization in Polycrystalline SnO2 Films by High Field Magnetoconductance Study

We focus in this article on high field magnetoconductance results obtained on SnO2 polycrystalline (cassiterite) films. The high field magnetoconductance is found to be in agreement with the magnetoconductance universal behavior due to weak localization in two dimensions as suggested by Zduniak et al. (Phys. Rev. B 56: 1996, 1997).

Observation of electron weak localization and correlation effects in disordered graphene

We have studied the electron transport properties of a disordered graphene sample, where the disorder was intentionally strengthened by Ga+ ion irradiation. The magneto-conductance of the sample exhibits a typical two-dimensional electron weak localization behavior, with electron-electron interaction as the dominant dephasing mechanism. The absence of electron anti-weak localization in the sample implies strong intersublattice and/or intervalley scattering caused by the disorders. The temperature and bias-voltage dependencies of conductance clearly reveal the suppression of conductance at low energies, indicating opening of a Coulomb gap due to electron-electron interaction in the disordered graphene sample.

Transport properties of hot-pressed bulk carbon nanotubes compacted by spark plasma sintering

Electrical and thermal transport properties of the carbon nanotube bulk material compacted by spark plasma sintering have been investigated. The electrical conductivity of the as-prepared sample shows a lnT dependence from 4 to 50 K, after which the conductivity begins to increase approximately linearly with temperature. A magnetic field applied perpendicularly to the sample increases the electrical conductivity in the range of 0-8T at all testing temperatures, indicating that the sample possesses the two-dimensional weak localization at lower temperatures (⩽50 K), while behaviors like a semimetal at higher temperatures (⩾50 K). This material acts like a uniform compact consisting of randomly distributed two dimensional graphene layers. For the same material, the thermal conductivity is found to decrease almost linearly with decreasing temperature, similar to that of a single multi-walled carbon nanotube. Magnetic fields applied perpendicularly to the sample cause the thermal conductivity to decrease significantly, but the influence of the magnetic fields becomes weak when temperature increases.

Observation of delocalized transport and low-dimensionality effects in disordered carbon thin films

Delocalized transport in a batch of amorphous carbon thin films is shown via a positive slope of the reduced activation energy below a certain temperature and a large value of conductivity minima. A crossover of dimensionality from three-dimensional (3D) to two-dimensional (2D) weak localization below the transition temperature and a signature of electron-electron interactions in 3D can be found in these films, which is significantly different from many other forms of carbon. The dimensionality crossover suggests 2D resonant tunneling between carbon clusters due to the increase of the phase coherence length and a low tunnel effective mass of electrons.

Successive superconducting transitions inTa2S2Cstudied by electrical resistivity and nonlinear ac magnetic susceptibility

${\mathrm{Ta}}_{2}{\mathrm{S}}_{2}\mathrm{C}$ compound undergoes superconducting transitions at ${T}_{\mathit{cl}}=3.60\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{\mathit{cu}}=9.0\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The nature of successive superconducting transitions has been studied from electrical resistivity and linear and nonlinear ac magnetic susceptibilities. The resistivity $\ensuremath{\rho}$ at $H=0$ shows a local maximum near ${T}_{\mathit{cu}}$, a kinklike behavior around ${T}_{\mathit{cl}}$, and reduces to zero at below ${T}_{0}=2.1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $\mathrm{ln}\phantom{\rule{0.2em}{0ex}}T$ dependence of $\ensuremath{\rho}$ is observed at $H=50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at low temperatures, which is due to a two-dimensional weak-localization effect. Below ${T}_{\mathit{cu}}$, a two-dimensional superconducting phase occurs in each TaC layer. The linear and nonlinear susceptibilities ${\ensuremath{\chi}}_{1}^{\ensuremath{''}}$, ${\ensuremath{\chi}}_{3}^{\ensuremath{'}}$, ${\ensuremath{\chi}}_{5}^{\ensuremath{'}}$, and ${\ensuremath{\chi}}_{7}^{\ensuremath{'}}$ as well as the difference $\ensuremath{\delta}\ensuremath{\chi}$ $(={\ensuremath{\chi}}_{\mathit{FC}}\ensuremath{-}{\ensuremath{\chi}}_{\mathit{ZFC}})$ between the field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities start to appear below $6.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the onset temperature of irreversibility. A drastic growth of the in-plane superconducting coherence length below $6.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ gives rise to a three-dimensional superconducting phase below ${T}_{\mathit{cl}}$ through interplanar Josephson couplings between adjacent TaC layers. The oscillatory behavior of ${\ensuremath{\chi}}_{3}^{\ensuremath{''}}$, ${\ensuremath{\chi}}_{5}^{\ensuremath{''}}$, and ${\ensuremath{\chi}}_{7}^{\ensuremath{''}}$ below ${T}_{\mathit{cl}}$ is related to the nonlinear behavior arising from the thermally activated flux flow.

Low temperature magnetoresistance of Al-doped ZnO films

The magnetoresistances of aluminum-doped zinc oxide thin films with thickness of 463.63, 203.03, and 66.85 nm were measured at low temperatures from 2.5 to 30 K. It is found that the samples exhibit negative magnetoresistance at all measuring temperatures. However, neither the three-dimensional nor the two-dimensional weak-localization theories can reproduce the behavior of the magnetoresistance. We find that the magnetoresistance of the three films can be well described by a semiempirical expression that takes into account the third order s – d exchange Hamiltonians describing a negative part and a two-band model for the positive contribution. This strongly suggests that the negative magnetoresistance in ZnO:Al film originates from the scattering of conduction electrons due to localized magnetic moments.