Weak Antilocalization Behavior Research Articles

Giant Tunability of Rashba Splitting at Cation-Exchanged Polar Oxide Interfaces by Selective Orbital Hybridization.

The 2D electron gas (2DEG) at oxide interfaces exhibits extraordinary properties, such as 2D superconductivity and ferromagnetism, coupled to strongly correlated electrons in narrow d-bands. In particular, 2DEGs in KTaO3 (KTO) with 5d t2g orbitals exhibit larger atomic spin-orbit coupling and crystal-facet-dependent superconductivity absent for 3d 2DEGs in SrTiO3 (STO). Herein, by tracing the interfacial chemistry, weak anti-localization magneto-transport behavior, and electronic structures of (001), (110), and (111) KTO 2DEGs, unambiguously cation exchange across KTO interfaces is discovered. Therefore, the origin of the 2DEGs at KTO-based interfaces is dramatically different from the electronicreconstruction observed at STO interfaces. More importantly, as the interface polarization grows with the higher order planes in the KTO case, the Rashba spin splitting becomes maximal for the superconducting (111) interfaces approximately twice that of the (001) interface. The larger Rashba spin splitting couples strongly to the asymmetric chiral texture of the orbital angular moment, and results mainly from the enhanced inter-orbital hopping of the t2g bands and more localized wave functions. This finding has profound implications for the search for topological superconductors, as well as the realization of efficient spin-charge interconversion for low-power spin-orbitronics based on (110) and (111) KTO interfaces.

Growth and characterization of the sputtered type-II topological semimetal PdTe2 thin films and PdTe2/Co60Fe20B20 heterostructures

The topological phases such as Dirac/Weyl semimetals are recently observed in Layered transition metal dichalcogenides (TMDs). The family of materials XTe2 (with X = Pd, Pt and Ni) possesses a type-II topological semimetal (TSM) phase. Here, we report the synthesis of the layered XTe2 thin films using the industry-friendly sputtering technique. The structural and morphological qualities of the sputtered-grown XTe2 thin films are analysed using X-ray diffraction and Raman measurements. The crystallites are highly oriented along the c-axis of the sapphire substrate, which suggests that growth is driven by van der Waals epitaxy. The magneto-transport measurements namely temperature-dependent resistivity and magnetoresistance measurement in out-of-plane field configuration are performed on PdTe2 thin films. The semi-metallic trend and superconducting transition are observed in the temperature-dependent resistivity curve. The weak antilocalization behavior observed in the magnetoconductance curves at a low magnetic field signifies the possibility of a non-zero Berry phase, possibly arising from the Dirac fermionic nature of electronic states. The spin dynamics measurements are performed on the PdTe2/Co60Fe20B20 bilayers. The effective spin mixing conductance and spin current density are found to be ∼2×1020m-2and 2.5MAm-2, respectively in PdTe2/Co60Fe20B20 bilayer which are quite larger than those observed in heavy metal-ferromagnets bilayers, and comparable to MBE-grown topological insulator-ferromagnets heterostructures. This work paves the way to synthesize Te-based layered TMDs using the industrially viable sputtering technique, and demonstrates the prospects for exploring the topological superconductive phase in PdTe2 films and large spin-charge conversion efficiency in PdTe2/Co60Fe20B20 heterostructures.

Robust Weak Antilocalization Effect Up to ∼120 K in the van der Waals Crystal Fe5-xGeTe2 with Near-Room-Temperature Ferromagnetism.

The van der Waals Fe5-xGeTe2 is a 3d ferromagnetic metal with a high Curie temperature of 275 K. We report herein the observation of an exceptional weak antilocalization (WAL) effect that can persist up to 120 K in an Fe5-xGeTe2 nanoflake, indicating the dual nature with both itinerant and localized magnetism of 3d electrons. The WAL behavior is characterized by the magnetoconductance peak around zero magnetic field and is supported by the calculated localized nondispersive flat band around the Fermi level. The peak to dip crossover starting around 60 K in magnetoconductance is visible, which could be ascribed to temperature-induced changes in Fe magnetic moments and the coupled electronic band structure as revealed by angle-resolved photoemission spectroscopy and first-principles calculations. Our findings would be instructive for understanding the magnetic exchanges in transition metal magnets as well as for the design of next-generation room-temperature spintronic devices.

Controlling of Localization by Elemental-substitution Effect in Layered BiCh2-based Compounds LaO1−xFxBiS2−ySey

We report transport properties for layered BiCh2-based (Ch = S, Se) superconductors LaO1−xFxBiS2−ySey (x = 0.2, 0.5, y = 0–1.05) and the observation of possible weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration, indicating a loss of insulating behavior. Interestingly, the moderately Se-substituted samples exhibit metallic behavior in the high-temperature region and a weak increase in the resistivity in the low-temperature regions, which implies the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Magnetoresistance measurements indicate that WAL is realized in the heavily Se-substituted systems. The WAL behavior is weakened by the changes in F and Se concentrations. A crossover state of the WAL and weak localization (WL) emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of the resistivity and Hall coefficient by the F and Se substitution clearly correlates to the difference of the magnetoconductance. We propose that the BiCh2-based system is a good platform for studying the relationship between localization and superconductivity because those states are tunable by element substitutions with bulk single crystals.

Bulk-surface coupling in dual topological insulator Bi1Te1 and Sb-doped Bi1Te1 single crystals via electron-phonon interaction

Recently, has been proved to be a dual topological insulator (TI), a new subclass of symmetry-protected topological phases, and predicted to be higher order topological insulator (HOTI). Being a dual TI (DTI), Bi1Te1 is said to host quasi-1D surface states (SSs) due to weak TI phase and topological crystalline insulating SSs at the same time. On the other hand, HOTI supports topologically protected hinge states. So, is a unique platform to study the electrical signature of topological SS (TSS) of fundamentally different origins. Though there is a report of magneto-transport measurements on large-scale Bi1Te1 thin films, the Bi1Te1 single crystal is not studied experimentally to date. Even the doping effect in a DTI Bi1Te1 is missing in the literature. In this regard, we performed the perpendicular and parallel field magneto-transport measurement on the exfoliated microflake of Bi1Te1 and Sb-doped Bi1Te1 single crystals, grown by the modified Bridgmann method. Our metallic sample shows the weak anti-localization behavior analyzed by the multi-channel Hikami-Larkin-Nagaoka equation. We observed the presence of a pair of decoupled TSS. Further, we extracted the dephasing index (β) from temperature (T)-dependence of phase coherence length (L), following the power law equation (L). The thickness-dependent value of β indicates the transition in the dephasing mechanism from electron-electron to electron-phonon interaction with the increase in thickness, indicating the enhancement in the strength of bulk-surface coupling. Sb-doped system shows weakened bulk-surface coupling, hinted by the reduced dephasing indices.

Quantum coherent transport and electron–electron interaction in BiSbTe3 single crystals

This report focuses on the role of defects on magnetotransport properties of the single crystal of BiSbTe 3 topological insulator. Two cleaved crystals S1 and S2 of the same boule have been used, among them, S1 shows metallic behavior whereas S2 represents the multiple transport mechanism which includes thermal activation behavior, hopping conduction of localized charge carriers, quantum coherent transport and electron–electron interaction at high, moderate and low temperatures respectively. The detailed resistivity and magnetoresistance analysis confirm the presence of weak antilocalization behavior explained by Hikami-Larkin-Nagaoka formula. These crystals indicate the presence of multichannel quantum coherent transport which depends on the thickness of sample.

Random-Gate-Voltage Induced Al’tshuler–Aronov–Spivak Effect in Topological Edge StatesSupported by the National Natural Science Foundation of China (Grant Nos. 12074172, 11674160, and 11974168), the StartupGrant at Nanjing University, the State Key Program for Basic Researches of China (Grant No. 2017YFA0303203), and the ExcellentProgramme at Nanjing University

Helical edge states are the hallmark of the quantum spin Hall insulator. Recently, several experiments have observed transport signatures contributed by trivial edge states, making it difficult to distinguish between the topologically trivial and nontrivial phases. Here, we show that helical edge states can be identified by the random-gate-voltage induced Φ 0/2-period oscillation of the averaged electron return probability in the interferometer constructed by the edge states. The random gate voltage can highlight the Φ 0/2-period Al’tshuler–Aronov–Spivak oscillation proportional to sin2(2πΦ/Φ 0) by quenching theΦ 0-period Aharonov–Bohm oscillation. It is found that the helical spin texture induced π Berry phase is key to such weak antilocalization behavior with zero return probability at Φ = 0. In contrast, the oscillation for the trivial edge states may exhibit either weak localization or antilocalization depending on the strength of the spin-orbit coupling, which has finite return probability at Φ = 0. Our results provide an effective way for the identification of the helical edge states. The predicted signature is stabilized by the time-reversal symmetry so that it is robust against disorder and does not require any fine adjustment of system.

Quantum coherence modulation in bismuth selenide topological insulator thin film by ion irradiation

The robust surface states of bismuth selenide (Bi2Se3) topological insulator (TI) that could potentially be harnessed for quantum computing and spintronics applications are often eclipsed by defect-induced bulk carriers. With the objective of suppressing the electronic transport in the bulk, we explore the effects of ion irradiation-driven isoelectronic doping in thin films of Bi2Se3 with 600 keV antimony (Sb+) ions at 45° oblique incidence. The distribution of Sb+ ions introduced into the system has been mapped using Rutherford backscattering spectroscopy. The heavy Sb+ ion irradiation has been observed to cause a change from parabolic magnetoresistance to linear magnetoresistance behavior in the irradiated films due to mobility fluctuations. Subsequent annealing in selenium environment has been found to reverse the damage produced by the irradiation, but with significant modifications in the quantum coherence signatures. Weak antilocalization behavior has been observed in the weak magnetic field limit and has been analyzed within the purview of the Hikami-Larkin-Nagaoka model. It has been found that the channel-separated Bi2Se3 film becomes completely channel-mixed due to the induced defects, yielding a system with a single coherent channel upon irradiation. In the as-deposited film, the temperature-dependent power law decay of the coherence length has been found to have two different exponents within the measured temperature range. On the other hand, the irradiated-annealed film has a single decay exponent which has been found to tend to − 0.5, indicating that electron-electron interaction is the only de-phasing mechanism involved. This work establishes the effectiveness of ion irradiation in tuning the quantum coherence phenomena in Bi-based TI systems.

Magnetism and electrical transport in Y-doped layered iridate Sr2IrO4

Here, we report an investigation of structural, magnetic and electronic properties in Y-doped layered iridate (Sr1-xYx)2IrO4 (x⩽0.1). The parent Sr2IrO4 is a well-studied spin-orbit coupling (SOC) induced insulator with an antiferromagnetic ground state. The Y-doping here equivalently acts for electron doping without altering the vital parameters such as, SOC and electron correlation. Experimental results show a minute change in structural parameters and an equivalent charge conversion from Ir4+ to Ir3+. Unlike similarly other electron-doped system, the low temperature magnetic and electronic state in present series is minimally influenced. The charge conduction mechanism follows 2-dimensional hopping model in whole series. Magnetoresistance (MR) data show an interesting sign change with both temperature and magnetic field. The positive MR both at low temperature follows weak antilocalization behavior where the sign change in MR is believed to be caused by an interplay between SOC and magnetic moment.

Nanometer-Thick Bismuth Nanocrystal Films for Sensoric Applications

The present article is concerned with investigations of the structural, surface morphological, and magnetotransport properties of DC magnetron-sputtered nanometer-thick Bi nanocrystal films on Si(111) substrates. Crystal structure and surface morphology were studied with X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, and atomic force microscopy. For the samples deposited at the melting point of Bi, 271 °C, equilibrium crystals formed and according to Wulff theorem acquire a specific shape determined by the surface tension. These crystals were investigated for different film thicknesses and deposition temperatures varying from 25 to 300 °C. Furthermore, magnetotransport characterization was carried out in steady and pulsed magnetic fields of up to 9 and 70 T, respectively. At low temperatures, clear weak antilocalization behavior is observed, attributed to 2D conduction channels. A nonlinear Hall resistance is also confirmed, ascribed to the coexistence of two types of carriers (p and n). This study contributes to the elucidation of the transport properties of the Bi thin films and opens new perspectives for their exploitation in modern applications such as sensorics.

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.

Tuning Rashba spin-orbit coupling at LaAlO3/SrTiO3 interfaces by band filling

The electric-field tunable Rashba spin-orbit coupling at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface shows potential applications in spintronic devices. However, different gate dependence of the coupling strength has been reported in experiments. On the theoretical side, it has been predicted that the largest Rashba effect appears at the crossing point of the ${d}_{\text{xy}}$ and ${d}_{\text{xz},\text{yz}}$ bands. In this work, we study the tunability of the Rashba effect in ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ by means of backgating. The Lifshitz transition was crossed multiple times by tuning the gate voltage so that the Fermi energy is tuned to approach or depart from the band crossing. By analyzing the weak antilocalization behavior in the magnetoresistance, we find that the maximum spin-orbit coupling effect occurs when the Fermi energy is near the Lifshitz point. Moreover, we find strong evidence for a single spin winding at the Fermi surface.

Oxygen Vacancy‐Induced Topological Hall Effect in a Nonmagnetic Band Insulator

AbstractThe discovery of skyrmions has recently sparked tremendous interest in topologically nontrivial spin textures. The signature of the noncoplanar nature of magnetic moments can be observed as topological Hall effect (THE) in electrical measurement. Realization of such nontrivial spin textures in new materials and through new routes is an ongoing endeavor due to their huge potential for future ultra‐dense, low‐power memory applications. In this work, oxygen vacancy (OV)‐induced THE and anomalous Hall effect (AHE) in a 5d0 system KTaO3 are reported. The observation of weak antilocalization behavior and THE in the same temperature range strongly implies the crucial role of spin–orbit coupling (SOC) behind the origin of THE. Ab initio calculations reveal the formation of the magnetic moment on Ta atoms around the OV and Rashba‐type spin texturing of conduction electrons. In the presence of Rashba SOC, the local moments around vacancy can form bound magnetic polarons (BMPs) with noncollinear spin texture, resulting in THE. Scaling analysis between transverse and longitudinal resistance establishes skew scattering‐driven AHE in the present case. This study opens a route to realize topological phenomena through defect engineering.

Enhanced linear magneto-resistance near the Dirac point in topological insulator Bi2(Te1−xSex)3 nanowires

We report the composition and back-gate voltage tuned transport properties of ternary compound Bi2(Te1−xSex)3 nanowires synthesized by chemical vapor deposition (CVD). It is found that the population of bulk carriers can be suppressed effectively with increasing the Se concentration x. In Bi2(Te1−xSex)3 nanowires with x = 25% ± 5%, the ambipolar surface conduction associated with tuning the Fermi energy across the Dirac point of topological surface states is induced by applying a back-gate voltage. Importantly, we find that while the magneto-resistance (MR) follows the weak antilocalization (WAL) behavior when the Fermi level is tuned away from the Dirac point, MR is enhanced in magnitude and turns more linear in the whole magnetic field range (between ±9 T) near the Dirac point. The observation of the enhanced linear magneto-resistance (LMR) and crossover from WAL to LMR, near the Dirac point provides a deeper insight into understanding the nature of topological insulator’s surface transport and the relation between these two widely observed magneto-transport phenomena.

Tuning of charge carrier density by deposition pressure in Sb-doped Bi2Se3 thin films

We report the tunability of charge carrier density in a pulsed laser deposition grown Bi1.95Sb0.05Se3 thin film system as a function of the deposition pressure. Dynamic partial pressure of argon maintained in the chamber during deposition suppresses Se escape, effectively regulating the stoichiometry, which in turn modulates the charge carrier density. With pressure variation, a striking three order change in carrier density is recorded and with additional control through the substrate temperature a carrier density value as low as 2 × 1018 cm−3 is found to be achievable. We observe a systematic evolution of structural, electronic, and magneto-transport properties of the system with variation of deposition pressure. The magneto-transport properties of the films reveal weak antilocalization behavior, analyzing which we have been able to determine the influence of deposition pressure on properties like the de-phasing mechanism involved in the system, the number of effective transport channels, and the coherence length.

Large surface conductance and superconductivity in topological insulator microstructures

Controllable geometric manipulation via micromachining techniques provides a promising tool for enhancing useful topological electrical responses relevant to future applications such as quantum information science [P. J. W. Moll, “Focused ion beam microstructuring of quantum matter,” Annu. Rev. Condens. Matter Phys. 9, 147 (2018); Jang et al., “Observation of half-height magnetization steps in Sr2RuO4,” Science 331, 186 (2011); Moll et al., “Transport evidence for Fermi-arc-mediated chirality transfer in the Dirac semimetal Cd3As2,” Nature 535, 266 (2016); Moll et al., “Evidence for hydrodynamic electron flow in PdCoO2,” Science 351, 1061 (2016)]. Here, we present microdevices fabricated with a focused ion beam from an indium-doped topological insulator Pb1−xSnxTe. With the device thickness on the order of 1 μm and an extremely large bulk resistivity, we achieve an unprecedented enhancement of the surface contribution to about 30% of the total conductance near room temperature. The surface contribution increases as the temperature is reduced, becoming dominant below approximately 180 K, compared to 30 K in millimeter-thickness crystals. In addition to the enhanced surface contribution to normal-state transport, we observe the emergence of surface superconductivity below 6 K. Measurements of magnetoresistivity at high magnetic fields reveal a weak antilocalization behavior in the normal-state magnetoconductance at low temperatures and a variation in the power-law dependence of resistivity on temperature with the field. These results demonstrate that interesting electronic responses relevant to practical applications can be achieved by suitable engineering of single crystals.

Magnetic Field Modulated Weak Localization and Antilocalization State in Bi2(TexSe1−x)3 Films

The magneto‐transport properties of a series of Bi2(TexSe1−x)3 films are investigated. It is seen that the doped samples develop an insulating behavior at low temperatures upon the increasing doping level as compared to the pristine sample Bi2Se3. The field evolution of the magneto‐resistance (MR) under perpendicular and parallel magnetic fields for Bi2(TexSe1−x)3 films with x = 0, 0.1, 0.24, 0.4, and 0.6 at temperatures from 2 to 200 K is investigated. A low field cusp behavior appears in MR data of Bi2(Te0.1Se0.9)3 film, which is associated with the weak anti‐localization (WAL). In Bi2(Te0.4Se0.6)3 films, MR with perpendicular and parallel magnetic fields shows signatures of WAL at low magnetic fields. A remarkable negative behavior, similar to weak localization (WL), under a high magnetic field parallel to the film surface is found which is dominated by bulk‐state electrons. MR of Bi2(Te0.6Se0.4)3 film reveals a low‐field WAL behavior and a high‐field parabolic behavior, revealing a two‐dimensional magneto‐transport property.

Gate-tunable weak antilocalization in a few-layer InSe

Indium selenide (InSe) has attracted tremendous research interest due to its high mobility and potential applications in next-generation electronics. However, the underlying transport mechanism of carriers in thin InSe at low temperatures remains unknown. Here we report the gate voltage and temperature-dependent magnetotransport properties of $\ensuremath{\gamma}$-InSe transistor devices with Hall mobility up to $2455\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at the temperature of 1.7 K. We observe a gate-tunable weak antilocalization behavior at lower magnetic field $B$, which shows a transition to weak localization at higher $B$ region. We find that the magnetotransport data agree well with the Hikami-Larkin-Nagaoka theory. The conductivity and temperature dependence of phase-coherence length reveal that the electron-electron ($e\text{\ensuremath{-}}e$) interactions are dominated dephasing mechanism for electronic transport in $\ensuremath{\gamma}$-InSe at low temperatures. The maximum phase-coherence length is found to be 320 nm at 1.7 K, larger than that of monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ and few-layer black phosphorus. These results enrich the fundamental understanding of electronic transport properties of InSe.

Low‐Temperature Eutectic Synthesis of PtTe2 with Weak Antilocalization and Controlled Layer Thinning

AbstractMetallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas‐phase methods and requires high‐temperature condition. As an alternative to the gas‐phase synthetic approach, lower temperature eutectic liquid‐phase synthesis presents a very promising approach with the potential for larger‐scale and controllable growth of high‐quality thin metallic TMD single crystals. Here, the first realization of low‐temperature eutectic liquid‐phase synthesis of type‐II Dirac semimetal PtTe2 single crystals with thickness ranging from 2 to 200 nm is presented. The electrical measurement of synthesized PtTe2 reveals a record‐high conductivity of as high as 3.3 × 106 S m−1 at room temperature. Besides, the weak antilocalization behavior is identified experimentally in the type‐II Dirac semimetal PtTe2 for the first time. Furthermore, a simple and general strategy is developed to obtain atomically thin PtTe2 crystal by thinning as‐synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electrical conductivity. The results of controllable and scalable low‐temperature eutectic liquid‐phase synthesis and layer‐by‐layer thinning of high‐quality thin PtTe2 single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high melting‐point transition metal.

Nonzero Berry phase in quantum oscillations from giant Rashba-type spin splitting in LaTiO3/SrTiO3 heterostructures

The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computational devices. There has been much work focused on developing highly spin-polarized materials and understanding their behavior when incorporated into so-called spintronic devices. These devices usually require spin splitting with magnetic fields. However, there is another promising strategy to achieve spin splitting using spatial symmetry breaking without the use of a magnetic field, known as Rashba-type splitting. Here we report evidence for a giant Rashba-type splitting at the interface of LaTiO3 and SrTiO3. Analysis of the magnetotransport reveals anisotropic magnetoresistance, weak anti-localization and quantum oscillation behavior consistent with a large Rashba-type splitting. It is surprising to find a large Rashba-type splitting in 3d transition metal oxide-based systems such as the LaTiO3/SrTiO3 interface, but it is promising for the development of a new kind of oxide-based spintronics.