Positive Magnetoconductance Research Articles

Weyl points and anomalous transport effects tuned by the Fe doping in Mn3Ge Weyl semimetal

The discovery of a significantly large anomalous Hall effect in the chiral antiferromagnetic system—Mn3Ge—indicates that the Weyl points are widely separated in phase space and positioned near the Fermi surface. In order to examine the effects of Fe substitution in Mn3Ge on the presence and location of the Weyl points, we synthesized (Mn 1−α Fe α)3 Ge ( α=0−0.30 ) compounds. The AHE was observed in compounds up to α = 0.22, but only within the temperature range where the magnetic structure remains the same as the Mn3Ge. Additionally, positive longitudinal magnetoconductance and planar Hall effect (PHE) were detected within the same temperature and doping range. These findings strongly suggest the existence of Weyl points in (Mn 1−α Fe α)3 Ge ( α=0−0.22 ) compounds. Further, we observed that with an increase in Fe doping fraction, there is a significant reduction in the magnitude of anomalous Hall conductivity, PHE, and positive longitudinal magnetoconductance, indicating that the Weyl points move further away from the Fermi surface. Consequently, it can be concluded that suitable dopants in the parent Weyl semimetals have the potential to tune the properties of Weyl points and the resulting anomalous electrical transport effects.

Effect of active layer thickness on the charge recombination and dissociation in bulk heterojunction polymer solar cells under open circuit conditions

Bulk heterojunction organic photovoltaic cells (OPVs), which are based on anthracene-containing poly (p-phenylene-ethynylene)-alt-poly (p-phenylene-vinylene) (PPE-PPV) polymer denoted (AnE-PVstat) and phenyl C60 butyric acid methyl ester (PCBM). The active layer thickness was varied from 100 nm to 300 nm. The solar cell parameters and magnetoconductance (MC) effect were analyzed, and the results showed a positive MC effect (+MC) for active layer thicknesses of 100 nm, 150 nm, and 200 nm, while a negative MC effect (-MC) was observed for thicknesses of 250 nm and 300 nm. The Power conversion efficiency (PCE) of 4 ℅ was achieved for an active layer thickness of 200 nm. The triplet-doublet quenching (TQD) model was used to describe the MC effect, which is influenced by triplet-charge reactions on the current based on density matrix formalism using the Stochastic Liouville Equation. The dissociation and recombination rates of the T-D pairs increased with the thickness for thinner layers (100, 150, 200 nm) while decreased for thicker layers (250 nm and 300 nm).

Weak localization in the layered iridatesSr2Ir1−xMxO4(M=Ru,Ti): Role of interplay between spin-orbit coupling and magnetism

The combined role of the spin-orbit effect (SOC), electron correlation ($U$), and magnetism has had a significant impact on the electron transport behavior in disordered systems. Here, we report an experimental investigation of weak localization (WL) behavior in layered SOC-dominated iridate ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ where these impacting parameters are further tuned with substitution of Ru and Ti, which have different magnetic and electronic characters. Magnetic ${\mathrm{Ru}}^{4+} (4{d}^{4})$ participates in magnetic interaction with existing ${\mathrm{Ir}}^{4+}$ as well as with itself, while nonmagnetic ${\mathrm{Ti}}^{4+} (3{d}^{0})$ will lead to the dilution of a magnetic lattice. ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ shows a crossover from negative to positive magnetoconductivity (MC) with the onset of magnetic ordering where the WL behavior is observed in a magnetically ordered state. In ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{4}$ series, a complete suppression of the magnetic and insulating state is observed with $x\ensuremath{\sim}0.6$. However, an opposite result is seen in ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{4}$ series, where the magnetic ordering temperature is minimally influenced, though the magnetic moment is weakened and the insulating state is significantly strengthened. This contrasting effect on magnetism has a different impact on the crossover in MC and allied WL behavior, which basically follows the evolution of the magnetic and conducting state in both series. The ${\mathrm{Ti}}^{4+}$ further reverses the negative MC in the nonmagnetic state to positive MC with a reduction in SOC. This work demonstrates the critical role of magnetism, SOC, and $U$ in manipulating the quantum transport properties in one of the iridate materials which we believe has significance in other similar materials.

Decoupling intranode and internode scattering in Weyl fermions

A series of recent papers claimed that intranode scattering alone can contribute to positive longitudinal magnetoconductance (LMC) due to the chiral anomaly (CA) in Weyl semimetals (WSMs) in the quasiclassical limit. We revisit the problem of CA-induced LMC in WSMs in the quasiclassical limit and show that intranode scattering, by itself, does not result in enhancement of LMC. In the limit of zero internode scattering, chiral charge must remain conserved, which is shown to actually decrease LMC. Only in the presence of a finite nonzero internode scattering does one obtain a positive LMC due to nonconservation of the chiral charge. Even weak internode scattering suffices in generating positive LMC since it redistributes charges across both the nodes, although on a timescale larger than that of the intranode scattering. Our work is fundamental to correctly interpret the recent experiments on magnetoconductance in Weyl semimetals that claim to have observed chiral anomaly. Furthermore, our calculations reveal that, in contrast to recent works, in inhomogeneous WSMs strain-induced axial magnetic field ${B}_{5}$, by itself, leads to negative longitudinal magnetoconductance and a negative planar Hall conductance.

Sign Inversion of Magnetoconductance in Organic Semiconductors by Different Spin-Mixing Channels at Charge-Transfer Interfaces

Organic semiconductors have shown obvious spin-related magnetoconductance (MC) even without the involvement of any magnetic elements, bringing an emerging research field called as organic spintronics. Tuning the MC sign is crucial to realize practical application based on these spin effects. Herein, we report the manipulation of MC signs in organic photodiode based on the ground-state and excited-state charge–transfer (CT) interfaces at room temperature. Different to the traditional CT interfaces that normally show positive MC, the ground-state CT (GSCT) interface presents negative MC due to the dominant spin-mixing channel from weakly bound polaron-pairs states, in which a main reverse intersystem crossing from the triplet to singlet states will lead to the opposite signs of magnetic field dependence. By adjusting the ground-state or excited-state CT process in one tandem device under electric and optical excitations, the device could reveal a low-field controlled current inverter. Our work shows the important role of organic CT states in the manipulation of the spin-related magneto-optoelectronic properties in organic semiconductors.

Positive longitudinal magnetoconductivity induced by chiral magnetic effect in mercury selenide

A negative longitudinal magnetoresistance without any sign of saturation was found in a non-centrosymmetric Weyl semimetal (WSM) candidate mercury selenide in an electron concentration range of 5.5 × 1015–1.7 × 1017 cm−3 and a temperature range of 0.33–150 K. The magnitude of the effect varies with a sample from 10% up to 30% in a magnetic field of 12 T at T = 150 K. Moreover, the positive contribution to magnetoconductivity has a characteristic quadratic dependence on the magnetic field, increasing with a charged center concentration at T = 150 K. The most likely explanation for the discovered longitudinal magnetoconductivity feature lies in the chiral magnetic effect, which is inherent to WSMs. The role of the Dyakonov–Perel mechanism in inter-nodal spin relaxation is discussed in regard to HgSe.

Trap-Filling Magnetoconductance as an Initialization and Readout Mechanism of Triplet Exciton Spins.

Photoexcited triplet states are promising candidates for hybrid qubit systems, as they can be used as a controlling gate for nuclear spins. But microwave readout schemes do not generally offer the sensitivity needed to approach the single-molecule limit or the scope to integrate such systems into devices. Here, we demonstrate the possibility of electrical readout of triplet spins at room temperature through a specific mechanism of magnetoconductance (MC) in polycrystalline pentacene. We show that hole-only pentacene devices exhibit a positive photoinduced MC response that is consistent with a trap-filling mechanism. Spin and magnetic-field-dependent quenching of photogenerated triplets by holes quantitatively explains the MC response we observe. These results are distinct in both sign and proposed mechanism compared to previous reports on polyacene materials and provide clear design rules for future spintronic devices based on this spin-sensing mechanism.

Weak Localization in p-Type Heterostructures in the Presence of Parallel Magnetic Field

Theory of weak localization is developed for two-dimensional holes in the presence of in-plane magnetic field. The Zeeman splitting even in the hole momentum results in the spin-dependent phase changing the quantum interference. The negative correction to the conductivity is shown to decrease by a factor of two by the in-plane magnetic field. The positive magnetoconductivity in a classically weak perpendicular field caused by the weak localization is calculated for both quadratic and quartic in momentum Zeeman hole splittings. Calculations show that the conductivity corrections are very close to each other in these two cases of low and high hole density.

Connection between topological pumping effect and chiral anomaly in Weyl semimetals

While the semiclassical Boltzmann approach has been widely adopted to study chiral-anomaly-related transport in Weyl semimetals (WSMs), it predicts an unphysical diverging electrical conductivity when ${E}_{F}\ensuremath{\rightarrow}0$, where ${E}_{F}$ is the Fermi energy. Here, we develop a modified semiclassical equation of motion which includes both the diagonal and off-diagonal contributions of the Berry curvature. On this basis, we derive an undivergent classical formula for the positive longitudinal magnetoconductivity in a WSM which resolves the conflict between the classical and ultraquantum approaches. We demonstrate that the chiral anomaly is closely related to the topological pumping effect and it can be realized even in the absence of a magnetic field. With our findings we propose a different perspective to understand the topological properties of WSMs and suggest a way to measure the chiral anomaly using transport.

Helical symmetry breaking and quantum anomaly in massive Dirac fermions

Helical symmetry of massive Dirac fermions is broken explicitly in the presence of electric and magnetic fields. Here we present two equations for the divergence of helical and axial-vector currents following the Jackiw-Johnson approach to the anomaly of the neutral axial vector current. We discover the contribution from the helical symmetry breaking is attributed to the occupancy of the two states at the top of the valence band and the bottom of the conduction band. The explicit symmetry breaking fully cancels the anomalous correction from the quantum fluctuation in the band gap. The chiral anomaly can be derived from the helical symmetry breaking. It provides an alternative route to understand the chiral anomaly from the point of view of the helical symmetry breaking. The pertinent physical consequences in condensed matter are the helical magnetic effect which means a charge current circulating at the direction of the magnetic field, and the mass-dependent positive longitudinal magnetoconductivity as a transport signature. The discovery not only reflects anomalous magneto-transport properties of massive Dirac materials but also reveals the close relation between the helical symmetry breaking and the physics of chiral anomaly in quantum field theory and high energy physics.

Positive magnetoconductivity and inelastic scattering time at low temperatures with magnetic field in InSb semiconductor

In this work, we investigate the temperature dependence of the perpendicular electrical conductivity in presence of magnetic field of InSb sample measurements obtained by Abboudy [16]. First we determine the value of the critical magnetic field BC for which the metal insulator transition (MIT) occurs. On the metallic side of the MIT, we are interested in the study of positive magnetoconductivity as a function of magnetic field by modeling it with complex theoretical models. The validity of these models is tested by calculating and comparing the inelastic scattering time τ ε for each model used.

Dynamical piezomagnetic effect in time-reversal-invariant Weyl semimetals with axionic charge density waves

Charge density waves (CDWs) in Weyl semimetals (WSMs) have been shown to induce an exotic axionic insulating phase in which the sliding mode (phason) of the CDW acts as a dynamical axion field, giving rise to a large positive magnetoconductance [Wang et al., Phys. Rev. B 87, 161107(R) (2013); Roy et al., Phys. Rev. B 92, 125141 (2015); J. Gooth et al., Nature (London) 575, 315 (2019)]. In this work, we predict that dynamical strain can induce a bulk orbital magnetization in time-reversal (TR)-invariant WSMs that are gapped by a CDW. We term this effect the ``dynamical piezomagnetic effect'' (DPME). Unlike in J. Gooth et al. [Nature (London) 575, 315 (2019)], the DPME introduced in this work occurs in a bulk-constant (i.e., static and spatially homogeneous in the bulk) CDW, and does not rely on fluctuations, such as a phason. By studying the low-energy effective theory and a minimal tight-binding (TB) model, we find that the DPME originates from an effective valley axion field that couples the electromagnetic gauge field with a strain-induced pseudogauge field. In particular, whereas the piezoelectric effects studied in previous works are characterized by 2D Berry curvature, the DPME represents the first example of a fundamentally 3D strain effect originating from the Chern-Simons 3-form. We further find that the DPME has a discontinuous change at a critical value of the phase of the CDW order parameter. We demonstrate that, when there is a jump in the DPME, the surface of the system undergoes a topological quantum phase transition (TQPT), while the bulk remains gapped. Hence, the DPME provides a bulk signature of the boundary TQPT in a TR-invariant Weyl CDW.

Nontopological chiral chemical potential due to the Zeeman field in magnetic Weyl semimetals

As condensed-matter manifestations of the chiral anomaly, the coexistence of positive longitudinal magnetoconductivity (LMC) and the planar Hall effect (PHE) is regarded as a characteristic transport signal for the Weyl semimetal (WSM) phase. By including a finite Newtonian mass into the general linearized WSM model, we derive an analytical expression for the chiral chemical potential, which includes a topological term of the chiral anomaly and a nontopological term. The nontopological term stems from the Zeeman-field-induced tilt of the Weyl cones, which breaks the symmetry of the fermion filling between the Weyl valleys of opposite chiralities and further leads to a chiral chemical potential $\ensuremath{\propto}\mathbit{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{B}$, much as the effect of the chiral anomaly. We demonstrate that the resulting LMC is positive and can coexist with the PHE without invoking the mechanism of the chiral anomaly. Hence, the chiral chemical potential might not be exclusively respected to the chiral anomaly. Experimentally, one can distinguish the chiral anomaly from the proposed tilt mechanism by inspecting the dependence of magnetoconductivity on the Fermi energy.

Super-Resonant Transport of Topological Surface States Subjected to In-Plane Magnetic Fields.

Magnetic oscillations of Dirac surface states of topological insulators are typically expected to be associated with the formation of Landau levels or the Aharonov-Bohm effect. We instead study the conductance of Dirac surface states subjected to an in-plane magnetic field in the presence of a barrier potential. Strikingly, we find that, in the case of large barrier potentials, the surface states exhibit pronounced oscillations in the conductance when varying the magnetic field, in the absence of Landau levels or the Aharonov-Bohm effect. These novel magnetic oscillations are attributed to the emergence of super-resonant transport by tuning the magnetic field, in which many propagating modes cross the barrier with perfect transmission. In the case of small and moderate barrier potentials, we identify a positive magnetoconductance due to the increase of the Fermi surface by tilting the surface Dirac cone. Moreover, we show that for weak magnetic fields, the conductance displays a shifted sinusoidal dependence on the field direction with period π and phase shift determined by the tilting direction with respect to the field direction. Our predictions can be applied to various topological insulators, such as HgTe and Bi_{2}Se_{3}, and provide important insights into exploring and understanding exotic magnetotransport properties of topological surface states.

Increased dephasing length in heavily doped GaAs

Ion implantation of S and Te followed by sub-second flash lamp annealing with peak temperature about 1100 °C is employed to obtain metallic n ++-GaAs layers. The electron concentration in annealed GaAs is as high as 5 × 1019 cm−3, which is several times higher than the doping level achievable by alternative methods. We found that heavily doped n ++-GaAs exhibits positive magnetoconductance in the temperature range of 3–80 K, which is attributed to the magnetic field suppressed weak localization. By fitting the magnetoconductance results with Hikami–Larkin–Nagaoka model, it is found that the phase coherence length increases with increasing carrier concentration at low temperature and is as large as 540 nm at 3 K. The temperature dependence of the phase coherence length follows l ∅ ∝ T η (η ∼ 0.3), indicating defect-related scattering as the dominant dephasing mechanism. In addition, the high doping level in n-type GaAs provides the possibility to use GaAs as a plasmonic material for chemical sensors operating in the infrared range.

Topological Hall effect in the antiferromagnetic Dirac semimetal EuAgAs

The nontrivial magnetic texture in real space gives rise to the intriguing phenomenon of the topological Hall effect (THE), which is relatively less explored in topological semimetals. Here, we report a large THE in the antiferromagnetic (AFM) state in single crystals of EuAgAs, an AFM Dirac semimetal. EuAgAs hosts an AFM ground state below ${T}_{N}=12$ K with a weak ferromagnetic component. The in-plane isothermal magnetization below ${T}_{N}$ exhibits a weak metamagnetic transition. We also observe chiral anomaly induced positive longitudinal magnetoconductivity, which indicates a Weyl fermion state under an applied magnetic field. The first-principles calculations reveal that EuAgAs is an AFM Dirac semimetal with a pair of Dirac cones, and therefore a Weyl semimetallic state can be realized under time-reversal symmetry breaking via an applied magnetic field. Our study establishes that EuAgAs is a system for exploiting the interplay of band topology and the topology of the magnetic texture.

Magnetic-field-dependent intervalley relaxation time in Weyl semimetals

The zeroth Landau levels in Weyl semimetals (WSMs) are singular, which result in the anomalous nonsaturated positive longitudinal magnetoconductivity (LMC). The LMC exhibits periodic-in-$1/B$ quantum oscillations attributable to the the oscillating electron density of states in a magnetic field. The intervalley relaxation time ${\ensuremath{\tau}}^{a}$ is a key material parameter determining the magnitude of the LMC directly. In existing theories of the LMC, the magnetic field dependence of ${\ensuremath{\tau}}^{a}$ is usually neglected. Based upon the Boltzmann equation, we derive analytical expression for the intervalley relaxation time for short-range impurity scattering potential and arbitrary magnetic field strength. Taking the magnetic field dependence of ${\ensuremath{\tau}}^{a}$ into consideration, we find that the quantum oscillations of the LMC are enhanced. Numerical calculations are also carried out to verify the analytical derivations. Moreover, we provide an explanation for the anomalous behavior of the LMC observed in WSMs that the amplitude of LMC first increases and then decreases with increasing temperature.

π Oscillation phase shift between transverse and longitudinal magnetoconductivities in Weyl semimetals

In the presence of a magnetic field $\mathbf{B}$, the negative transverse magnetoconductivity (TMC) for $\mathbf{E}\ensuremath{\perp}\mathbf{B}$, with $\mathbf{E}$ being the applied electric field, and anomalous positive longitudinal magnetoconductivity (LMC) for $\mathbf{E}\ensuremath{\parallel}\mathbf{B}$ of a three-dimensional (3D) Weyl semimetal (WSM) both exhibit periodic-in-$1/B$ quantum oscillations. We develop a semiclassical theory of the magnetoconductivities of a 3D WSM, taking into account the effect of the Landau level broadening and finite temperatures. We find that if one fixes the direction of $\mathbf{B}$ and switches the direction of $\mathbf{E}$ to be perpendicular and parallel to $\mathbf{B}$, there exists an interesting peak-valley correspondence, or, say, a relative $\ensuremath{\pi}$ oscillation phase shift, between the oscillating TMC and LMC. This can serve as unambiguous measurable evidence manifesting the distinct underlying physical mechanisms of the TMC and LMC in WSMs, which has so far not been reported in theories or experiments.

Quantum magnetoconductivity characterization of interface disorder in indium-tin-oxide films on fused silica

Disorder arising from random locations of charged donors and acceptors introduces localization and diffusive motion that can lead to constructive electron interference and positive magnetoconductivity. At very low temperatures, 3D theory predicts that the magnetoconductivity is independent of temperature or material properties, as verified for many combinations of thin-films and substrates. Here, we find that this prediction is apparently violated if the film thickness d is less than about 300 nm. To investigate the origin of this apparent violation, the magnetoconductivity was measured at temperatures T = 15 – 150 K in ten, Sn-doped In2O3 films with d = 13 – 292 nm, grown by pulsed laser deposition on fused silica. We observe a very strong thickness dependence which we explain by introducing a theory that postulates a second source of disorder, namely, non-uniform interface-induced defects whose number decreases exponentially with the interface distance. This theory obeys the 3D limit for the thickest samples and yields a natural figure of merit for interface disorder. It can be applied to any degenerate semiconductor film on any semi-insulating substrate.

Crossover of positive and negative magnetoconductance in composites of nanosilica glass containing dual transition metal oxides.

A facile sol–gel approach to prepare composites of nanosilica glass containing dual transition metal oxides with compositions xCoO·(20 − x)NiO·80SiO2 comprising x values 5 (NC-1), 10 (NC-2) and 15 (NC-3) within hexagonal pores of SBA-15 template has been demonstrated. The synergistic effect of dual transition metal oxide ions on MD properties and crossover of positive and negative magnetoconductance phenomena were observed in these nanocomposite systems. The physical origin of magnetoconductance switching is explained based on the factors: nanoconfinement effect, wave-function shrinkage and spin polarized electron hopping. DFT calculations were performed to understand the structural correlation of the nanoconfined system. The static (dc) and dynamic (ac) responses of magnetization revealed the spin-glass behaviour of the investigated samples. Both scaling law and Vogel–Fulcher law provide a satisfactory fit to our experimental results which are considered as a salient feature of the spin-glass system. Our studies indicate the possibilities of fabricating magnetically controlled multifunctional devices.