Positive Magnetoconductance Research Articles

Chiral-anomaly-induced angular narrowing of the positive longitudinal magnetoconductivity in Weyl semimetals

The authors develop a method for magnetotransport study in finite-size systems and use it to evaluate magnetoconductivity of a disordered Weyl semimetal for both the ballistic and diffusive regimes.

Paramagnetism, hopping conduction, and weak localization in highly disordered pure and Dy-doped Bi2Se3 nanoplates

Breaking the topological protection of surface states of topological insulators is an essential prerequisite for exploring their applications. This is achievable by magnetic doping, in reduced dimensions, and predictably by introducing disorder beyond a critical level. In certain cases, the former is also known to induce a transition from weak anti-localization (WAL) to weak localization (WL). Here, we report the occurrence of paramagnetism, hopping conduction, and WL in chemically prepared unannealed DyxBi2−xSe3 (x=0, 0.1, and 0.3) nanoplates primarily via dc magnetization, resistivity, and magnetoconductance measurements. The paramagnetism in the magnetic-atom-free Bi2Se3 nanoplates is ascribed, using density functional theory calculations, to the acquisition of magnetic moments by defects. The defect density in pure Bi2Se3 is estimated to be high (∼1019 defects/cm3). Successive Dy doping brings in further incremental disorder, apart from the Dy atomic moments. The nanoplates are shown to sequentially exhibit thermally activated band conduction, nearest neighbor hopping, Mott variable range hopping (VRH), and Efros–Shklovskii VRH with decreasing temperature. WL is evident from the observed positive magnetoconductance. Annealing converts the WL behavior to WAL, arguably by setting in the topological protection on a substantial reduction of the disorder.

Negative longitudinal magnetoconductance at weak fields in Weyl semimetals

Weyl semimetals are topological materials that provide a condensed-matter realization of the chiral anomaly. A positive longitudinal magnetoconductance quadratic in magnetic field has been promoted as a diagnostic for this anomaly. By solving the Boltzmann equation analytically, we show that the magnetoconductance can become negative in the experimentally relevant semiclassical regime of weak magnetic fields. This effect is due to the simultaneous presence of the Berry phase and the orbital magnetic moment of carriers and occurs for sufficiently strong intervalley scattering.

Transitions of the anomalous positive longitudinal magnetoconductivity of a Weyl semimetal in an ac electric field

Considering the Landau quantization and chiral anomaly, we develop a semiclassical theory of the positive longitudinal magnetoconductivity (LMC) of a Weyl semimetal induced by the chiral anomaly in an ac driving electric field. We find that for low angular frequencies, the anomalous LMC behaves in the same manner as in the dc case. However, the LMC decreases rapidly to zero with increasing the angular frequency of the electric field to be above a critical frequency, which is equal to the inverse of the intervalley relaxation time. With further increasing the frequency, the zero-magnetic-field conductivity diminishes above a critical frequency, which is equal to the inverse of the intravalley relaxation time. The two distinct transitions might be observed experimentally, so that the intervalley and intravalley relaxation times can be determined directly, which are considered to be two key parameters controlling the anomalous positive LMC effect in existing theories.

Tuning the magnetic anisotropy energy of atomic wires

In this article we present the fabrication of freestanding thin-film nanobridges of Ir. We perform magnetoconductance (MC) measurements of atomic contacts and monoatomic chains of Ir, realized by the mechanically controlled break-junction method. We observe continuous changes of the MC on the field scale of several tesla, as observed earlier for atomic-size contacts of two other strong paramagnets, Pd and Pt. The amplitude and the shape of the MC depend on the orientation of the magnetic field as well as on subtle details of the atomic arrangement, as confirmed by stretching studies of the contacts. Both positive dominant MC and negative dominant MC occur and are attributed to collinear or noncollinear alignment of the magnetic moments of the electrodes, respectively. By careful manipulation of the chain geometry we are able to study the transition between these two cases, which is hallmarked by a complex MC behavior. For special arrangements the MC almost vanishes. Our findings are in agreement with recent calculations of the geometry dependence of the magnetic anisotropy energy and open a route to tailor the MC behavior as required for particular applications.

Positive longitudinal spin magnetoconductivity in Z2 topological Dirac semimetals

Recently, a class of Dirac semimetals, such as \textrm{Na}$_{\mathrm{3}}% $\textrm{Bi} and \textrm{Cd}$_{\mathrm{2}}$\textrm{As}$_{\mathrm{3}}$, are discovered to carry $\mathbb{Z}_{2}$ monopole charges. We present an experimental mechanism to realize the $\mathbb{Z}_{2}$ anomaly in regard to the $\mathbb{Z}_{2}$ topological charges, and propose to probe it by magnetotransport measurement. In analogy to the chiral anomaly in a Weyl semimetal, the acceleration of electrons by a spin bias along the magnetic field can create a $\mathbb{Z}_{2}$ charge imbalance between the Dirac points, the relaxation of which contributes a measurable positive longitudinal spin magnetoconductivity (LSMC) to the system. The $\mathbb{Z}_{2}$ anomaly induced LSMC is a spin version of the longitudinal magnetoconductivity (LMC) due to the chiral anomaly, which possesses all characters of the chiral anomaly induced LMC. While the chiral anomaly in the topological Dirac semimetal is very sensitive to local magnetic impurities, the $\mathbb{Z}_{2}$ anomaly is found to be immune to local magnetic disorder. It is further demonstrated that the quadratic or linear field dependence of the positive LMC is not unique to the chiral anomaly. Base on this, we argue that the periodic-in-$1/B$ quantum oscillations superposed on the positive LSMC can serve as a fingerprint of the $\mathbb{Z}_{2}$ anomaly in topological Dirac semimetals.

Persistence of spin memory in a crystalline, insulating phase-change material

The description of disorder-induced electron localization by Anderson over 60 years ago began a quest for novel phenomena emerging from electronic interactions in the presence of disorder. Even today, the interplay of interactions and disorder remains incompletely understood. This holds in particular for strongly disordered materials where charge transport depends on ‘hopping’ between localized sites. Here we report an unexpected spin sensitivity of the electrical conductivity at the transition from diffusive to hopping conduction in a material that combines strong spin-orbit coupling and weak inter-electronic interactions. In thin films of the disordered crystalline phase change material SnSb2Te4, a distinct change in electrical conductance with applied magnetic field is observed at low temperatures. This magnetoconductance changes sign and becomes anisotropic at the disorder-driven crossover from strongly localized (hopping) to weakly localized (diffusive) electron motion. The positive and isotropic magnetoconductance arises from disruption of spin correlations that inhibit hopping transport. This experimental observation of a recently hypothesized ‘spin memory’ demonstrates the spin plays a previously overlooked role in the disorder-driven transition between weak and strong localization in materials with strong spin–orbit interactions.

Giant ac magnetoconductivity in rGO-Fe3O4 composites

In this work we report detailed investigations of structural, ac transport and magnetotransport properties of composites of reduced graphene oxide (rGO) and magnetite nanoparticles measured in the frequency range 10 Hz–2 MHz and in static magnetic fields upto 500 mT. AC conductivity of composite samples was enhanced by a factor of ~3 over that of the pure rGO even though the conductivity of magnetite is five orders of magnitude lower than that of rGO. We attribute this to charge transfer effects between the magnetite nanoparticles and the rGO matrix. Evidence of this is also observed in detailed Raman analysis of all samples. Further, we find a giant magnetoconductivity in the ac transport behavior of the composite samples. The rGO sample showed a negative magnetoconductivity of ~−15% at room temperature, while all composite samples showed positive magnetoconductivity with weak frequency dependence between 10 Hz and 254 kHz. We report a room temperature ac magnetoconductivity of 57% (at f = 254 kHz) and 40% (at f = 10 Hz) in a magnetic field B = 500 mT for the sample with 40 wt% magnetite nanoparticles. This is much larger than that obtained in dc magnetotransport, where room temperature magnetoconductivities in similar composites are typically 10% or less in comparable magnetic fields.

Transverse thermopower in Dirac and Weyl semimetals

Dirac semimetals (DSM) and Weyl semimetal (WSM) fall under the generic class of three-dimensional solids, which follow relativistic energy-momentum relation $\epsilon_\mathbf{k}= \hbar v_F |\mathbf{k}|$ at low energies. Such a linear dispersion when regularized on a lattice can lead to remarkable properties such as the anomalous Hall effect, presence of Fermi surface arcs, positive longitudinal magnetoconductance, and dynamic chiral magnetic effect. The last two properties arise due to the manifestation of chiral anomaly in these semimetals, which refers to the non-conservation of chiral charge in the presence of electromagnetic gauge fields. Here, we propose the planar Nernst effect, or transverse thermopower, as another consequence of chiral anomaly, which should occur in both Dirac and Weyl semimetals. We analytically calculate the planar Nernst coefficient for DSMs (type-I and type-II) and also WSMs (type-I and type-II), using a quasi-classical Boltzmann formalism. The planar Nernst effect manifests in a configuration when the applied temperature gradient, magnetic field, and the measured voltage are all co-planar, and is of distinct origin when compared to the anomalous and conventional Nernst effects. Our findings, specifically a 3D map of the planar Nernst coefficient in type-I Dirac semimetals (Na$_3$Bi, Cd$_3$As$_2$ etc) and type-II DSM (PdTe$_2$, VAI3 etc), can be verified experimentally by an in-situ 3D double-axis rotation extracting the full $4\pi$ solid angular dependence of the Nernst coefficient.

Large positive magnetoconductivity at microwave frequencies in the compensated topological insulator BiSbTeSe2

The bulk electronic properties of compensated topological insulators are strongly affected by the self-organized formation of charge puddles at low temperature, but their response in the microwave frequency range is little studied. We employed broadband impedance spectroscopy up to 5 GHz to address the ac transport properties of well-compensated BiSbTeSe2, where charge puddles are known to form as metallic entities embedded in an insulating host. It turns out that the average puddle size sets the characteristic frequency cut-off in the GHz range, across which the insulating dc behavior is separated from a metal-like high-frequency response of delocalized carriers within the puddles. The cut-off frequency is found to be controlled by a magnetic field, giving rise to a large positive magneto-conductivity observable only in the GHz range. This curious phenomenon is driven by the Zeeman energy which affects the local band filling in the disordered potential landscape to enhance the puddle size.

Quantum oscillation modulated angular dependence of the positive longitudinal magnetoconductivity and planar Hall effect in Weyl semimetals

We study the positive longitudinal magnetoconductivity (LMC) and planar Hall effect as emergent effects of the chiral anomaly in Weyl semimetals, following a recent-developed theory by integrating the Landau quantization with Boltzmann equation. It is found that, in the weak magnetic field regime, the LMC and planar Hall conductivity (PHC) obey $\cos^{6}\theta$ and $\cos^{5}\theta\sin \theta$ dependences on the angle $\theta$ between the magnetic and electric fields. For higher magnetic fields, the LMC and PHC cross over to $\cos^{2}\theta$ and $\cos\theta\sin\theta$ dependences, respectively. Interestingly, the PHC could exhibit quantum oscillations with varying $\theta$, due to the periodic-in-$1/B$ oscillations of the chiral chemical potential. When the magnetic and electric fields are noncollinear, the LMC and PHC will deviate from the classical $B$-quadratic dependence, even in the weak magnetic field regime.

Continuous transition from weakly localized regime to strong localization regime in Nd0.7La0.3NiO3 films

We report an investigation of metal-insulator transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd0.7La0.3NiO3 films grown on crystalline substrates of LaAlO3 (LAO), SrTiO3 (STO), and NdGaO3 (NGO) by pulsed laser deposition. The film grown on LAO experiences a compressive strain and shows metallic behavior with the onset of a weak resistivity upturn below 2 K which is linked to the onset of weak localization contribution. Films grown on STO and NGO show a cross-over from a positive temperature coefficient (PTC) resistance regime to negative temperature coefficient (NTC) resistance regime at definite temperatures. We establish that a cross-over from PTC to NTC on cooling does not necessarily constitute a MIT because the extrapolated conductivity at zero temperature though small (<10 S cm−1) is finite, signaling the existence of a bad metallic state and absence of an activated transport. The value of for films grown on NGO is reduced by a factor of 40 compared to that for films grown on STO. We show that a combination of certain physical factors makes substituted nickelate (that are known to exhibit first-order Mott type transition), undergo a continuous transition as seen in systems undergoing disorder/composition driven Anderson transition. The MC measurement also supports the above observation and shows that at low temperatures, there exists a positive MC that arises from the quantum interference which co-exists with a spin-related negative MC that becomes progressively stronger as the electrons approach a strongly localized state in the film grown on NGO.

Quantum Oscillations of the Positive Longitudinal Magnetoconductivity: A Fingerprint for Identifying Weyl Semimetals.

Weyl semimetals (WSMs) host charged Weyl fermions as emergent quasiparticles. We develop a unified analytical theory for the anomalous positive longitudinal magnetoconductivity (LMC) in a WSM, which bridges the gap between the classical and ultraquantum approaches. More interestingly, the LMC is found to exhibit periodic-in-1/B quantum oscillations, originating from the oscillations of the nonequilibrium chiral chemical potential. The quantum oscillations, superposed on the positive LMC, are a remarkable fingerprint of a WSM phase with a chiral anomaly, whose observation is a valid criteria for identifying a WSM material. In fact, such quantum oscillations were already observed by several experiments.

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.

Spin-orbit interaction in InSb core-shell wires

The transverse magnetoconductance of the n-type conductivity InSb whiskers doped by Sn to concentration 4.4 × 1016 ÷ 7.16 × 1017 cm−3 were studied in the temperature range 4.2 ÷ 70 K and weak magnetic field up to 3 T. The Shubnikov-de Haas oscillations at liquid helium temperature were revealed in the samples with doping concentration corresponding to metal-insulator transition (MIT). The character of transverse magnetoconductance dependences was analysed and compared with theoretical one. The studied magnetoconductance alters its sign for various doping concentration at weak magnetic fields. It is negative and becomes positive for heavily doped samples.Possible mechanisms of the existence of negative and positive magnetoconductance at low temperatures and weak magnetic fields were discussed in the InSb whiskers with doping concentration in the vicinity to MIT. The origin of these effects was connected with the concentration-induced crossover from weak localization to weak antilocalization and explained due to the Rashba spin-orbit interaction in tin doped InSb core-shell wires.

Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$_2$MnGa, reaching $S_{yx}\sim -6$ $\mu$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $\alpha_{yx}$ shows a crossover between $T$-linear and $-T \log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $\alpha_{yx}$. The $T$ dependence of $\alpha_{yx}$ in experiments and numerical calculations can be understood in terms of a quantum critical scaling function predicted by the low energy effective theory over more than a decade of temperatures. Moreover, the observation of chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.

Magnetoconductivity of type-II Weyl semimetals

Type-II Weyl semimetals are characterized by the tilted linear dispersion in the low-energy excitations, mimicking Weyl fermions but with manifest violation of the Lorentz invariance, which has intriguing quantum transport properties. The magnetoconductivity of type-II Weyl semimetals is investigated numerically based on lattice models in parallel electric and magnetic field. We show that in the high-field regime, the sign of the magnetoconductivity of an inversion-symmetry-breaking type-II Weyl semimetals depends on the direction of the magnetic field, whereas in the weak field regime, positive magnetoconductivity is always obtained regardless of magnetic field direction. We find that the weak localization is sensitive to the spatial extent of impurity potential. In time-reversal symmetry breaking type-II Weyl semimetals, the system displays either positive or negative magnetoconductivity along the direction of band tilting, owing to the associated effect of group velocity, Berry curvature and the magnetic field.

Quantum interference magnetoconductance of polycrystalline germanium films in the variable-range hopping regime

Direct evidence of quantum interference magnetotransport in polycrystalline germanium films in the variable-range hopping (VRH) regime is reported. The temperature dependence of the conductivity of germanium films fulfilled the Mott VRH mechanism with the form of in the low-temperature regime (). For the magnetotransport behaviour of our germanium films in the VRH regime, a crossover, from negative magnetoconductance at the low-field to positive magnetoconductance at the high-field, is observed while the zero-field conductivity is higher than the critical value (). In the regime of , the magnetoconductance is positive and quadratic in the field for some germanium films. These features are in agreement with the VRH magnetotransport theory based on the quantum interference effect among random paths in the hopping process.

High-field magnetoconductance in La-Sr manganites of FM and AFM ground states

Large-grain La1–xSrxMnO3 ceramic samples of compositions x = 0.45 and 0.55, representing the ferromagnetic (FM) and A-type antiferromagnetic (AFM) ground states, were produced via classical sintering at 1500 °C of cold-pressed sol-gel prepared single-phase nanoparticles. Using the same precursors, nanogranular forms of both manganite ceramics were prepared by fast spark plasma sintering at low temperature of 900 °C, which limits the growth of crystal grains. The magnetotransport of both the bulk and nanogranular forms was investigated in a broad range of magnetic fields up to 130 kOe and analyzed on the basis of detailed magnetic measurements. Both the large-grain and nanogranular systems with x = 0.45, possessing a pure FM state with similar Curie tempereature TC ≈ 345 K), show nearly the same conductivity enhancement in external fields when expressed relatively to the zero-field values. This positive magnetoconductance (MC) can be separated into two terms: (i) the hysteretic low-field MC that reflects the field-induced orientation of magnetic moments of individual grains, and (ii) the high-field MC that depends linearly on external field. In the case of large-grain ceramics with x = 0.55, a partially ordered FM state formed below TC = 264 K is replaced by pure A-type AFM ground state below 204 K. This A-type AFM state is characterized by positive magnetoconductance that is essentially of quadratic dependence on external field in the investigated range up to 130 kOe. On contrary, the nanogranular product with x = 0.55 exhibits a mixed FM/AFM state at low temperatures, and, as a consequence, its magnetotransport combines the features of FM and A-type AFM systems, in which the quadratic term is much enhanced and clearly dominates at high fields. For interpretation of observed behaviors, the theory of grain-boundary tunneling is revisited.

Modulating the line shape of magnetoconductance by varying the charge injection in polymer light-emitting diodes

We fabricate the phenyl-substituted poly(p-phenylene vinylene) copolymer (super yellow, SY-PPV)-based polymer light-emitting diodes (PLEDs) with different device architectures to modulate the injection of opposite charge carriers and investigate the corresponding magnetoconductance (MC) responses. At the first glance, we find that all PLEDs exhibit the positive MC responses. By applying the mathematical analysis to fit the curves with two empirical equations of a non-Lorentzian and a Lorentzian function, we are able to extract the hidden negative MC component from the positive MC curve. We attribute the growth of the negative MC component to the reduced interaction of the triplet excitons with charges to generate the free charge carriers as modulated by the applied magnetic field, known as the triplet exciton-charge reaction, by analyzing MC responses for PLEDs of the charge-unbalanced and hole-blocking device configurations. The negative MC component causes the broadening of the line shape in MC curves.