Anisotropic Magnetoconductance Research Articles

In-depth analysis of anisotropic magnetoconductance in Bi2Se3 thin films with electron–electron interaction corrections

A combination of out-of-plane (OOP) and in-plane (IP) magnetoconductance (MC) study in topological insulators (TI) is often used as an experimental technique to probe weak anti-localization (WAL) response of the topological surface states (TSSs). However, in addition to the above WAL response, weak localization (WL) contribution from conducting bulk states are also known to coexist and contribute to the overall MC; a study that has so far received limited attention. In this article, we accurately extract the above WL contribution by systematically analyzing the temperature and magnetic field dependency of conductivity in Bi2Se3 films. For accurate analysis, we quantify the contribution of electron–electron interactions to the measured MC which is often ignored in the WAL studies. Moreover, we show that the WAL effect arising from the TSSs with finite penetration depth, for OOP and IP magnetic field can together explain the anisotropic magnetoconductance (AMC) and, thus, the investigated AMC study can serve as a useful technique to probe the parameters like phase coherence length and penetration depth that characterise the TSSs in 3D TIs. We also demonstrate that increase in bulk-disorder, achieved by growing the films on amorphous SiO2 substrate rather than on crystalline Al2O3(0001), can lead to stronger decoupling between the top and bottom surface states of the film.

Dimensionality-dependent type-II Weyl semimetal state in Mo0.25W0.75Te2

Weyl nodes and Fermi arcs in type-II Weyl semimetals (WSMs) have led to lots of exotic transport phenomena. Recently, Mo$_{0.25}$W$_{0.75}$Te$_{2}$ has been established as a type-II WSM with Weyl points located near Fermi level, which offers an opportunity to study its intriguing band structure by electrical transport measurements. Here, by selecting a special sample with the thickness gradient across two- (2D) and three-dimensional (3D) regime, we show strong evidences that Mo$_{0.25}$W$_{0.75}$Te$_{2}$ is a type-II Weyl semimetal by observing the following two dimensionality-dependent transport features: 1) A chiral-anomaly-induced anisotropic magneto-conductivity enhancement, proportional to the square of in-plane magnetic field (B$_{in}$$^{2}$); 2) An additional quantum oscillation with thickness-dependent phase shift. Our theoretical calculations show that the observed quantum oscillation originates from a Weyl-orbit-like scenario due to the unique band structure of Mo$_{0.25}$W$_{0.75}$Te$_{2}$. The in situ dimensionality-tuned transport experiment offers a new strategy to search for type-II WSMs.

Finite transverse conductance and anisotropic magnetoconductance under an applied in-plane magnetic field in two-dimensional electron gases with strong spin–orbit coupling

The current in response to a bias in certain two-dimensional electron gas (2DEG), can have a nonzero transverse component under a finite magnetic field applied in the plane where electrons are confined. This phenomenon known as planar Hall effect (PHE) is accompanied by dependencies of both the longitudinal and the transverse components of the current on the angle ϕ between the bias direction and the magnetic field. This effect can be observed in a variety of systems, for example in topological insulators where spin-momentum locking of the topologically protected surface states is the root cause for the effect and in magnetic systems where anisotropic magnetic ordering induces it. In 2DEG with spin orbit coupling (SOC) such as oxide interfaces, this effect has been experimentally witnessed. Further, a fourfold oscillation in longitudinal resistance as a function of ϕ has also been observed. Motivated by these, we perform scattering theory calculations on a 2DEG with SOC in presence of an in-plane magnetic field connected to two-dimensional leads on either sides to obtain longitudinal and transverse conductances. We find that the longitudinal conductance is π-periodic and the transverse conductance is 2π-periodic in ϕ. The magnitude of oscillation in transverse conductance with ϕ is enhanced in certain patches in (α, b)-plane where α is the strength of SOC and b is Zeeman energy due to magnetic field. The oscillation in transverse conductance with ϕ can be highly multi-fold for large values of α and b. The highly multi-fold oscillations of transverse conductance are due to Fabry–Pérot type interference between the modes in the central region as backed by its length dependent features. Our study establishes that SOC in a material is sufficient to observe PHE without the need for anisotropic magnetic ordering or nontrivial topology of the bandstructure.

Anisotropic magnetotransport in tilted magnetic topological semimetals

We systematically investigate the anisotropic magneto-conductivity and planar Hall effect in tilted magnetic topological semimetals in the frame of Kubo formula by considering the vertex correction of velocity. The nonzero anisotropic magneto-conductivity is due to the intrinsic magnetization by magnetic doping rather than the external magnetic field previously studied in literatures. In the scenario, tilt, which is common in band structure, plays a key role in this anisotropic magneto-conductivity. This anisotropic magneto-conductivity completely originates from the band anisotropy. The vertex correction only amends the magnitude, but does not change the qualitative behavior of the longitudinal conductivity. However, the planar Hall conductivity is always zero for tilt along both x and y directions in vivid contrast to the case arising from an external magnetic field. Our work offers a route to understand the anisotropic magneto-conductivity effect in tilted Weyl semimetals with Zeeman field. It will be helpful to get a deeper understanding of the transport characteristic in tilted topological semimetals.

Anisotropic magnetoconductance and Coulomb blockade in defect engineered Cr2Ge2Te6 van der Waals heterostructures

We demonstrate anisotropic tunnel magnetoconductance by controllably engineering charging islands in the layered semiconducting ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. This is achieved by assembling vertical van der Waals heterostructures comprised of graphene electrodes separated by crystals of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. Carefully applying vertical electric fields in the region of $(E\ensuremath{\sim}25$--50 mV/nm) across the ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ causes its dielectric breakdown at cryogenic temperatures. This breakdown process has the effect of introducing subgap defect states within the otherwise semiconducting ferromagnetic material. Low-temperature electron transport through charging islands reveals Coulomb blockade behavior with a strongly gate-tuneable anisotropic magnetoconductance, which persists up to $T\ensuremath{\sim}60$ K. We report average tunnel magnetoresistance values of $100%$. This work opens new avenues and material systems for the development of nanometer-scale electrically controlled spintronic devices.

Quantitative Analysis of Weak Antilocalization Effect of Topological Surface States in Topological Insulator BiSbTeSe2.

Quantitative analysis of the weak antilocalization (WAL) effect of topological surface states in topological insulators is of tremendous importance. The major obstacle to achieve accurate results is how to eliminate the contribution of the anisotropic magnetoconductance of bulk states when the Fermi level lies in bulk bands. Here, we demonstrate that we can analyze quantitatively and accurately the WAL effect of topological surface states in topological insulator, BiSbTeSe2 (BSTS), by measuring the anisotropic magnetoconductance. The anomalous conductance peaks induced by the WAL effect of topological surface states of BSTS together with the anisotropic magnetoconductance of bulk states have been observed. By subtracting the anisotropic magnetoconductance of bulk states, we are able to analyze the WAL effect of topological surface states using the Hikami-Larkin-Nagaoka expression. Our findings offer an alternative strategy for the quantitative exploration of the WAL effect of topological surface states in topological insulators.

Emerging magnetic order in platinum atomic contacts and chains.

The development of atomic-scale structures revealing novel transport phenomena is a major goal of nanotechnology. Examples include chains of atoms that form while stretching a transition metal contact or the predicted formation of magnetic order in these chains, the existence of which is still debated. Here we report an experimental study of the magneto-conductance (MC) and anisotropic MC with atomic-size contacts and mono-atomic chains of the nonmagnetic metal platinum. We find a pronounced and diverse MC behaviour, the amplitude and functional dependence change when stretching the contact by subatomic distances. These findings can be interpreted as a signature of local magnetic order in the chain, which may be of particular importance for the application of atomic-sized contacts in spintronic devices of the smallest possible size.

Probing the spatial electron distribution in InAs nanowires by anisotropic magnetoconductance fluctuations

The authors introduce an approach to investigate the spatial distribution of the electrons in InAs nanowires using the anisotropy of low temperature magneto-conduction fluctuations. With this technique, the authors argue that the electrons responsible for transport in the nanowire are distributed through the bulk of the wire as opposed to being confined to the wire's surface.

Anisotropic Weak Anti-Localization under In-Plane Magnetic Field and Control of Dimensionality via Spin Precession Length

We have examined the newly proposed method in order to electrically deduce the ratio between the Rashba and Dresselhaus spin–orbit interaction (SOI) parameters. Anisotropic magneto-conductance in gate-fitted InGaAs narrow wires is observed under an in-plane magnetic field. From the comparison between experiments and calculated Rashba SOI parameters, we have found that the method is applicable only when the wire width is shorter than the spin precession length. A transition from narrow wires to two-dimensional (2D) wires is observed both by decreasing the spin precession length and by increasing wire width.

Out-of-plane anisotropic magnetoconductance in a spin–orbit-coupled two-dimensional electron gas

We investigate the out-of-plane anisotropic magnetoconductance (OAMC) in a Rashba spin–orbit-coupled two-dimensional system. For short-range nonmagnetic scattering, we obtain the vanishing of the OAMC when both bands are occupied, and it emerges when the Fermi energy only crosses the majority band. In the presence of remote charged electron–impurity scattering, it is found that OAMC appears even when both bands are occupied, and it may be negative for small spin–orbital interaction, which is in contrast to the in-plane configuration. Due to the van-Hove singularity in the density of state, a sudden decrease of OAMC occurs when the magnetization equals the Fermi energy in the absence of magnetization.

Anisotropic magnetoconductance of a InAs nanowire: Angle-dependent suppression of one-dimensional weak localization

The magnetoconductance of an InAs nanowire is investigated with respect to the relative orientation between external magnetic field and the nanowire axis. It is found that both the perpendicular and the parallel magnetic fields induce a positive magnetoconductance. Yet the parallel magnetic field induced longitudinal magnetoconductance has a smaller magnitude. This anisotropic magnetotransport phenomenon is studied as a function of temperature, magnetic field strength, and at an arbitrary angle between the magnetic field and the nanowire. We show that the observed effect is in quantitative agreement with the suppression of one-dimensional weak localization.

Anisotropic spin splitting in InGaAs wire structures

We report anisotropic spin splitting in gate-fitted InGaAs wires along different crystal orientations. Anisotropic magnetoconductance minima reflecting spin splitting is observed by decreasing wire width at the same carrier density. Anisotropy of spin splitting is reduced by applying negative gate voltages, while the strength of spin splitting is enhanced in the present InGaAs wire structures. This gate voltage dependence of spin splitting shows qualitative agreements with the theoretical calculations taking both Rashba SOI and Dresselhaus SOI into account.

Anisotropic magnetotransport by the pseudospin soliton in the bilayer [formula omitted] quantum Hall system

We investigate the bilayer quantum Hall state at total Landau level filling factor ν = 1 , which is the best system to study the pseudospin ferromagnetism, where pseudospin represents the layer degree of freedom. We found an anisotropic magnetoconductivity near the commensurate–incommensurate phase transition point, which is induced by an in-plane magnetic field. We attribute this anisotropy to the formation of a stripe-shaped domain structure of pseudospins called soliton lattice phase. We also discuss dissipation mechanism in terms of the thermodynamics of pseudospin solitons.

Anisotropic magnetoconductivity of a two-dimensional system in parallel magnetic field induced by Rashba spin–orbit interaction

Two-dimensional system in the presence of spin–orbit interaction and in-plane magnetic field is studied. The energy spectrum of the system possesses anisotropy correlated with the direction of the magnetic field. This anisotropy causes the conductivity anisotropy via electron scattering. The present paper considers the problem in the framework of classical kinetic equation. We suppose that the spin splitting exceeds the scattering rate. The calculations were carried out for scattering by charged impurities.

Magnetoresistance of polyaniline doped by AMPSA

Free standing films (~60 μm thick) of polyaniline (PANI) doped with 2-acryloamino-2-methyl-1-propanesulfonic acid(AMPSA) are prepared by the solution casting method [1]. PANI-AMPSA shows a positive temperature coefficient of resistivity (TCR) down to 80 K, and at T<80 K it becomes negative. An anisotropic magnetoconductance (MC) has been observed in 3D isotropic samples of PANI-AMPSA. The sign of the transverse magnetoconductance (MC) is positive at 4.2 and 2.6 K. The longitudinal MC is negative at T<6 K. The longitudinal MC (negative) increases rapidly (nearly 10%) at T<2 K. This behavior of MC shows the interplay of weak localization (WL) and electron-electron (e-e) interaction and hopping contributions in 3D isotropic samples of PANI AMPSA

Anisotropic magnetoconductance in quench-condensed ultrathin beryllium films

Near the superconductor-insulator (S-I) transition, quench-condensed ultrathin Be films show a large magnetoconductance which is highly anisotropic in the direction of the applied field. Film conductance can drop as much as seven orders of magnitude in a weak perpendicular field (< 1 T), but is insensitive to a parallel field in the same field range. We believe that this negative magnetoconductance is due to the field de-phasing of the superconducting pair wavefunction. This idea enables us to extract the finite superconducting phase coherence length in nearly superconducting films. Our data indicate that this local phase coherence persists even in highly insulating films in the vicinity of the S-I transition.

Conductivity and magnetoconductance in iodine-doped polyacetylene

Disorder-induced localization plays a major role in the metal-insulator (M-I) transition of doped conducting polymers. Thus the quantification of disorder is important to understand the transport properties of conducting polymers. A plot of the logarithmic derivative of conductivity ( W= (Δ In σ/Δ In T)) versus temperature is a simple and sensitive technique to characterize the extent of disorder, and to identify the metallic (M), critical (C) and insulating (I) regimes. A typical example is shown in the case of I-(CH) x samples. Another important parameter that determines the M-I transition in conducting polymers is the inter-chain interaction. Pressure and magnetic field can tune the relative position of the mobility edge with respect to the Fermi level. The localization-interaction model is observed to be satisfactory in the metallic regime. A power-law behavior of the temperature dependence of conductivity is observed in the critical regime. The positive temperature coefficient of resistivity (TCR), at low temperatures, in oriented I-(CH) x samples is sensitive to the direction of field with respect to the chain axis. The anisotropic magnetoconductance (MC) indicates the subtle interplay of weak localization and e-e interaction contributions, and the positive contribution to MC is less when the field is parallel to the chain axis. This indicates that the anisotropic MC probes the microscopic anisotropy in charge transport parameters. The temperature dependences of conductivity and magnetoresistance are important tools to study the M-I transition due to disorder, inter-chain interaction, etc., in conducting polymers.

Anisotropic magnetoconductivity of a three-dimensional disordered electron gas.

The magnetoconductivity of a three-dimensional disordered electron gas is calculated from first principles with use of the exact electron eigenstates in a magnetic field, thus avoiding the usual semiclassical approximation in this field. Our calculation gives anisotropic components for the conductivity tensor, and the isotropic behavior predicted by other authors is recovered in the limit ${\ensuremath{\omega}}_{c}$\ensuremath{\tau}\ensuremath{\ll}1, where ${\ensuremath{\omega}}_{c}$ is the cyclotron frequency and \ensuremath{\tau} the elastic lifetime.

Weak Localization in Graphite

Conductivity and magnetoconductivity have been measured for various kinds of graphite crystals at low temperatures down to 0.03 K. Anomalous behaviours have been found in polycrystalline (but well oriented) specimens of graphite. At low temperatures below 0.1 K, the conductivity shows T 1/2 dependence in and without magnetic field, Anisotropic magnetoconductivity is observed. It varies as H 1/2 , with the sign which changes from negative to positive with increasing magnetic field strength. These behaviors are fairly well explained in terms of localization and electron-electron interaction in the three-dimensional weakly localized regime. The inelastic scattering time τ e shows a little weaker temperature dependence than T -1 .

Anomalous Longitudinal Magnetoresistance in Metals

A theoretical model and experimental data on chromium and tungsten are presented which show that poor lead geometry combined with a highly anisotropic magnetoconductivity can produce a ``zero resistance'' when the magnetic field is parallel to the current.