Weak Antilocalization Phenomena Research Articles

Temperature evolution of transverse magnetoresistance due to forming the topological insulator state in single-crystalline n-type Bi2Te2.7Se0.3

Specific features in magnetotransport properties due to gradual forming the topological insulator state in sample of single-crystalline n-type Bi2Te2.7Se0.3 during its cooling were analyzed. The electrical resistivity of sample, measured from 2 K to 240 K, corresponds to partially degenerate semiconductor and dominantly depends on T-effect on electron mobility. The moblity is governed by electron–phonon scattering above T C = 50 K, whereas below T C electron–electron scattering is dominant scattering mechanism. With increasing temperature, electron content linearly increases above T C , whereas below T C electron content is very weakly T-dependent. Transverse magnetoresistance of sample is positive and strongly T-dependent. Two features, which are characteristic for topological insulators, were found in the magnetoresistance. First feature is a crossover from quadratic to linear magnetoresistance, observed within T C < T < 240 K range. Crossover field B C decreases with decreasing temperature. Linear magnetoresistace is quantum one that can be due to presence of Dirac fermions, which occupy the lowest Landau level under magnetic field. Second feature is another crossover from combined quadratic-linear to dip-shaped magnetoresistrance, observed at T ≤ T C . Dip-shaped magnetoresistrance is related to weak antilocalization (WAL) phenomenon. The WAL phenomenon and the electron–electron scattering process coexist at the same temperature range. Dip-shaped magnetoresistrance was analysed by in frames of the Hikami-Larkin-Nagaoka model, developed for systems with strong spin–orbit coupling. At cooling below ∼ 30 K, the effective dephasing length rapidly increases that is dominantly related to the electron–electron scattering process, too. The parameter α, characterizing the number of conduction channels, contributing to electron transport, is close to 0.5. This value α corresponds to a single topologically non-trivial conduction channel.

Evidence of weak-antilocalization phenomenon in Al-induced crystallization grown polycrystalline-SiGe thin film

We report the low-temperature magneto-transport properties of a polycrystalline-SiGe thin film realized on glass at 350 ℃ via Al-induced layer exchange crystallization. The film exhibits a degenerate p-type behavior, and we observed evidence of weak-antilocalization (WAL) effect that manifests as an anomalous positive magnetoresistance (MR) below 10 K and weak magnetic fields (B) between ± 1 Tesla. While the spin–orbit coupling length is nearly constant at ~ 40 nm, the temperature dependence of phase-coherence length, which has a maximum value at 4 K (~131 nm), indicates hole-hole scattering as the predominant de-phasing mechanism. This result shows the degenerate Al doping causes the WAL effect in the present system.

Structural and magnetotransport characterization of magnetron sputtered co-doped Bi2Te3 thin films

In this work we investigate the growth and magnetotransport properties of Co doped topological insulator Bi2Te3 deposited on Si(1 1 1) substrates by means of DC magnetron sputtering. The structure and morphology of the films were studied using X-Ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). Magnetotransport measurements were performed with a Physical Property Measurement System (PPMS). The pristine samples have a metallic behavior while the Co-doped are semiconductors. The magnetoresistance curves exhibit a sharp cusp for temperatures below 20 K that correspond to weak antilocalization phenomena and are analysed using the Hikami-Larkin-Nagaoka model. The WAL phenomenon vanishes with temperature increase due to electron-phonon scattering. The doped samples present an ambipolar transport with a change in the sign of the dominant carriers with temperature.

Unusual magnetotransport properties in graphene fibers.

Graphene, purely sp2-hybridized, has already been extensively studied for magnetoelectronics, however, the magnetotransport properties of graphene fibers (GrFib) have not been explored very well to date. Herein, unique magnetotransport properties of graphene fibers are detected. All the GrFib-samples show the highest positive magnetoresistance (MR ∼ 60%) at room temperature (300 K) that gradually decreases (MR ∼ 37%) at low temperature (5 K), indicating quite different behavior for a graphene derivative. The MR of three different morphologies are compared: single graphene sheet (60-100% at 300 K and 100-110% at 5 K under an applied magnetic field of 5 T), graphene foam (GF-100% at 300 K and 158% at 5 K under an applied magnetic field of 5 T), and graphene fiber (60% at 300 K and 37% at 300 K under an applied magnetic field of 5 T), and found that each morphology has a different magnitude of MR under similar magnitude of magnetic field and temperature. Unlike graphene and GF, GrFib shows a decreasing trend of MR at low temperatures, violating commonly used weak anti-localization phenomena in graphene. Technologically, each morphology of graphene has a unique set of magnetotransport properties that can be considered for particular magnetoelectronic devices depending upon the mechanical, electrical, and magnetotransport properties.

Structural, surface morphology and magneto-transport properties of self flux grown Eu doped Bi2Se3 single crystal

Here, we report the effect of europium (Eu) doping in Bi2Se3 topological insulator (TI) by using different characterization techniques viz. X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis (EDXA) and magneto-transport measurements. Good quality Eu doped Bi2Se3 (Eu0.1Bi1.9Se3) single crystal is grown by the self flux method through the solid state reaction route. Single crystal XRD pattern displayed the high crystalline quality of the Eu0.1Bi1.9Se3 sample along (00l) alignment whereas; the powder XRD confirmed the rhombohedral crystal structure without any impurity phases. SEM images exhibited a layered slab like structure stacked one over the other whereas; EDXA measurements confirmed the chemical composition of Eu0.1Bi1.9Se3 sample. Further, the EDXA mapping showed the homogeneous distribution of Bi, Se and Eu elements. Temperature dependent electrical resistivity curves revealed a metallic behaviour both in the presence and absence of applied magnetic field. Magneto-transport measurements showed a decrease in the magneto-resistance (MR) value of the Eu0.1Bi1.9Se3 sample (∼32% at 5 K) in comparison to the pure Bi2Se3 sample (∼80% at 5 K). For, Eu0.1Bi1.9Se3 sample, a complex crossover between WL and WAL phenomenon was observed at lower applied magnetic fields, whereas the same was absent in case of the pristine one. Further, HLN (Hikami Larkin Nagaoka) fitted magneto-conductivity (MC) analysis revealed a competing weak anti localization (WAL) and weak localization (WL) behaviour. Summarily, in the present work we study the structural, surface morphology and magneto-transport properties of as grown Eu0.1Bi1.9Se3 single crystals.

Anomalous Hall Effect in a Magnetic Topological Insulator (BiMn)2Te3

In this paper, we describe a thorough study of the structural and magneto-transport properties of Mn-doped Bi2Te3 thin films grown by magnetron sputtering. The substitution of bismuth from manganese was performed by repeating ten times an alternating deposition of Bi2Te3 and Mn layers. The crystal structure is the tetradymite with the ( $00l$ ) preferential orientation. Magnetoconductance data shows weak anti-localization phenomena at low temperatures. The presence of ferromagnetism is also demonstrating a strong anomalous Hall effect (AHE) and the sign of AHE resistances changes from negative to positive as the Mn concentration increases.

Topological delocalization and tuning of surface channel separation in Bi2Se2Te Topological Insulator Thin films

The surface states of a 3D topological insulator (TI) exhibit topological protection against backscattering. However, the contribution of bulk electrons to the transport data is an impediment to the topological protection of surface states. We report the tuning of the chemical potential in the bulk in Bi2Se2Te TI thin films, pinning it near the center of the bulk band gap, thereby suppressing the bulk carriers. The temperature dependent resistance of these films show activated behavior down to 50 K, followed by a metallic transition at lower temperatures, a hallmark of robustness of TI surface states. Manifestation of topological protection and surface dominated transport is explained by 2D weak antilocalization phenomenon. We further explore the effect of surface to bulk coupling in TI in this work, which is captured by the number of effective conducting surface channels that participate in the transport. The presence of a single conducting channel indicates a strong surface to bulk coupling which is detrimental to purely topological transport. We demonstrate the decoupling of topological surface states on opposite surfaces of thin films, thereby suppressing the bulk transport. Our findings provide a deeper understanding of surface to bulk coupling along with topological transport behavior and their respective tunability.

Colossal magnetoresistance in amino-functionalized graphene quantum dots at room temperature: manifestation of weak anti-localization and doorway to spintronics.

In this work, we have demonstrated the signatures of localized surface distortions and disorders in functionalized graphene quantum dots (fGQD) and consequences in magneto-transport under weak field regime (∼1 Tesla) at room temperature. Observed positive colossal magnetoresistance (MR) and its suppression is primarily explained by weak anti-localization phenomenon where competitive valley (inter and intra) dependent scattering takes place at room temperature under low magnetic field; analogous to low mobility disordered graphene samples. Furthermore, using ab-initio analysis we show that sub-lattice sensitive spin-polarized ground state exists in the GQD as a result of pz orbital asymmetry in GQD carbon atoms with amino functional groups. This spin polarized ground state is believed to help the weak anti-localization dependent magneto transport by generating more disorder and strain in a GQD lattice under applied magnetic field and lays the premise for future graphene quantum dot based spintronic applications.

Oriented attachment of nanoparticles to form micrometer-sized nanosheets/nanobelts by topotactic reaction on rigid/flexible substrates with improved electronic properties

We report a topotactic reaction strategy to achieve the oriented attachment (OA) of colloidal metal chalcogenide quantum dots into micrometer-sized nanosheets and nanobelts (up to 6–7 μm) on both mechanically rigid and flexible substrates. The non-stoichiometric composition, crystallization and Ag doping were controlled. The strong surface adsorption of cations and thiol ligands facilitated micrometer-scale three-dimensional OA. The cations induced the formation of electrostatic forces, cation passivation on the nanosheet surface and overlap packing of the nanosheets, enabling good contact with the substrates and improved electron transport without severe obstruction of organic insulating barriers. The observation of weak anti-localization phenomena and Hall effect sensitivity (up to 188%) of non-stoichiometric Ag2-δTe nanosheet films as well as the improved I-V and photoresponse properties of Ag-doped CdX nanosheet films confirm efficient electron transport. The stable I-V properties of these nanosheet films on flexible substrates, even under bending forces, testify to their potential in flexible device applications.

Two-dimensional TaSe2 metallic crystals: spin-orbit scattering length and breakdown current density.

We have determined the spin-orbit scattering length of two-dimensional layered 2H-TaSe2 metallic crystals by detailed characterization of the weak antilocalization phenomena in this strong spin-orbit interaction material. By fitting the observed magneto-conductivity, the spin-orbit scattering length for 2H-TaSe2 is determined to be 17 nm in the few-layer films. This small spin-orbit scattering length is comparable to that of Pt, which is widely used to study the spin Hall effect, and indicates the potential of TaSe2 for use in spin Hall effect devices. A material must also support large charge currents in addition to strong spin-orbit coupling to achieve spin-transfer-torque via the spin Hall effect. Therefore, we have characterized the room temperature breakdown current density of TaSe2 in air, where the best breakdown current density reaches 3.7 × 10(7) A/cm(2). This large breakdown current further indicates the potential of TaSe2 for use in spin-torque devices and two-dimensional device interconnect applications.

Dirac versus Weyl Fermions in Topological Insulators: Adler-Bell-Jackiw Anomaly in Transport Phenomena

Dirac metals (gapless semiconductors) are believed to turn into Weyl metals when perturbations, which break either time reversal symmetry or inversion symmetry, are employed. However, no experimental evidence has been reported for the existence of Weyl fermions in three dimensions. Applying magnetic fields near the topological phase transition from a topological insulator to a band insulator in Bi1-xSbx we observe not only the weak antilocalization phenomenon in magnetoconductivity near zero magnetic fields (B<0.4 T), but also its upturn above 0.4 T only for E//B. This "incompatible" coexistence between weak antilocalization and "negative" magnetoresistivity is attributed to the Adler-Bell-Jackiw anomaly ("topological" E·B term) in the presence of weak antilocalization corrections.

Magneto-resistance up to 60 Tesla in topological insulator Bi2Te3 thin films

We report magneto-transport studies of topological insulator Bi2Te3 thin films grown by pulsed laser deposition. A non-saturating linear-like magneto-resistance (MR) is observed at low temperatures in the magnetic field range from a few Tesla up to 60 Tesla. We demonstrate that the strong linear-like MR at high field can be well understood as the weak antilocalization phenomena described by Hikami-Larkin-Nagaoka theory. Our analysis suggests that in our system, a topological insulator, the elastic scattering time can be longer than the spin-orbit scattering time. We briefly discuss our results in the context of Dirac Fermion physics and “quantum linear magnetoresistance.”

Gate-Voltage Control of Chemical Potential and Weak Antilocalization inBi2Se3

We report that Bi₂Se₃ thin films can be epitaxially grown on SrTiO₃ substrates, which allow for very large tunablity in carrier density with a back gate. The observed low field magnetoconductivity due to weak antilocalization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest a much suppressed bulk conductivity at large negative gate voltages and a possible role of surface states in the WAL phenomena.

Weak Localization and Weak Anti-Localization in Polycrystalline In2O3?x Thin Films Grown by Sputtering Method

Weak localization (WL) and weak anti-localization (WAL) phenomena in polycrystalline In 2 O 3-x thin films grown by sputtering have been studied on the magneto-conductance (MC) before and after the heat treatment, as functions of the temperature, the magnetic field, the azimuth of magnetic field and the film thickness (10-60 nm). The WL profile in MC undergoes a drastic change by the heat treatment above 350 °C and the WAL phenomena become remarkable with increasing temperature. It has been concluded that the WAL arises from the spin-orbit scattering by the excess indium atoms through the elimination of oxygen atoms from the indium oxide by the heat treatment.