Negative magnetoresistance in Dirac semimetal Cd3As2.

Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi2Te3 topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches −22% at 8 T. The interplay between negative and positive magnetoresistance can be understood in terms of the competition between dephasing and spin-orbit scattering time scales. Based on the first-principles calculations within a density functional theory framework, we demonstrate that disorder (substitutional) by Ga+ ion milling process, which is used to fabricate nanowires, induces local magnetic moments in Bi2Te3 crystal that can lead to spin-dependent scattering of surface and bulk electrons. These experimental findings show a significant advance in the nanoscale spintronics applications based on longitudinal magnetoresistance in TIs. Our experimental results of large negative longitudinal magnetoresistance in 3D TIs further indicate that axial anomaly is a universal phenomenon in generic 3D metals.

Large positive and negative magnetoresistance in the magnetic EuCu4As2 crystal

We report the magnetotransport properties of $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ thin flakes through gate modulation at low temperatures. Under in-plane magnetic field, a large negative magnetoresistance (MR) maintains up to 10 T, which is related to the suppression of spin scattering when the magnetic order is gradually forced into the ferromagnetic (FM) state. Under perpendicular magnetic field, a steep resistance decrease is observed around \ensuremath{\sim}3 T, corresponding to the transition from an antiferromagnetic (AFM) to a canted AFM (CAFM) state. Due to the net Berry curvature, a notable anomalous Hall effect is observed and can be effectively tuned by gate voltages. The enhanced Hall coefficient would emerge under high magnetic fields when the Fermi level is close to the charge neutral point. Moreover, a transition from negative to positive MR is obtained when increasing the magnetic field. A large linear positive MR occurs around $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, corresponding to the CAFM-FM transition. The nonsaturated positive MR here may have a similar mechanism to the one in Weyl semimetals, revealing the strong combination between topology and magnetism in $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$.

The chiral anomaly in Weyl semimetals states that the left- and right-handed Weyl fermions, constituting the low energy description, are not individually conserved, resulting, for example, in a negative magnetoresistance in such materials. Recent experiments see strong indications of such an anomalous resistance response; however, with a response that at strong fields is more sharply peaked for parallel magnetic and electric fields than expected from simple theoretical considerations. Here, we uncover a mechanism, arising from the interplay between the angle-dependent Landau level structure and long-range scalar disorder, that has the same phenomenology. In particular, we ana- lytically show, and numerically confirm, that the internode scattering time decreases exponentially with the angle between the magnetic field and the Weyl node separation in the large field limit, while it is insensitive to this angle at weak magnetic fields. Since, in the simplest approximation, the internode scattering time is proportional to the anomaly-related conductivity, this feature may be related to the experimental observations of a sharply peaked magnetoresistance.

Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, non-saturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions having a defined chirality. Classical mechanisms, due to disorder or non-uniform current injection, can however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of Cd3As2, a three-dimensional Dirac semimetal. Quasi-linear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasi-linear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features.

After growing successfully TaP single crystal, we measured its longitudinal resistivity (rhoxx) and Hall resistivity (rhoyx) at magnetic fields up to 9T in the temperature range of 2-300K. It was found that at 2K its magnetoresistivity (MR) reaches to 328000 percent, at 300K to 176 percent at 8T, and both do not appear saturation. We confirmed that TaP is indeed a low carrier concentration, hole-electron compensated semimetal, with a high mobility of hole muh=371000 cm2V-1s-1, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, as a magnetic field (H) is applied in parallel to the electric field (E), the negative MR due to chiral anomaly is observed, and reaches to -3000 percent at 9T without any signature of saturation, too, which distinguishes with other Weyl semimetals (WSMs). The analysis on the Shubnikov-de Haas (SdH) oscillations superimposing on the MR reveals that a nontrivial Berry phase with strong offset of 0.3958 realizes in TaP, which is the characteristic feature of the charge carriers enclosing a Weyl nodes. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.

Positive magnetoresistance (PMR) and negative magnetoresistance (NMR)describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe2/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR).

The capillary tube viscometer is used to measure the viscosity of aqueous magnetic fluids under the influence of parallel and perpendicular magnetic fields. The effects of the volume fraction of the suspended magnetic particles, the concentration of surfactants, and the external magnetic field strength, as well as the orientation, on the viscosity of the magnetic fluid are analyzed. The experimental results show that the viscosity of the sample magnetic fluids increases with increases in the concentrations of suspended magnetic particles and surfactants. The external magnetic field is also an important factor that affects the viscosity of the magnetic fluid. The viscosity first increases with the magnetic field and finally approaches a constant as the magnetization attains a saturation state. For the same magnetic fluid, the viscosity in a perpendicular magnetic field is larger than that in a parallel magnetic field for the same magnetic field.

We have investigated the magnetoresistance of lithographically prepared single-layer graphene nanoribbons in pulsed, perpendicular magnetic fields up to 60 T and performed corresponding transport simulations using a tight-binding model and several types of disorder. In experiment, at high carrier densities we observe Shubnikov-de Haas oscillations and the quantum Hall effect, while at low densities the oscillations disappear and an initially negative magnetoresistance becomes strongly positive at high magnetic fields. The strong resistance increase at very high fields and low carrier densities is tentatively ascribed to a field-induced insulating state in the bulk graphene leads. Comparing numerical results and experiment, we demonstrate that at least edge disorder and bulk short-range impurities are important in our samples.

We report a systematic study on magnetotransport properties of the single crystal of cadmium (Cd). When the applied magnetic field B is perpendicular to the current I, the resistivities for both directions (I || a, I || c) show field induced metal-to-insulator-like transitions. The isothermal magnetoresistance (MR) at low temperatures increases approximately as the square of the magnetic field without any sign of saturation, and reaches up to 1140000% and 58000% at T = 2 K and B = 9 T for I || a and I || c, respectively. As the magnetic field rotates to parallel to the current, no sign of negative MR is observed for I || a, while an obvious negative MR appears up to −70% at 2 K and 9 T for the current flowing along the c-axis, and the negative longitudinal MR shows a strong dependence of the electrode position on the single crystal. These results suggest that the negative longitudinal MR is caused by the dislocations formed in the process of crystal growing along the c-axis. Further studies are needed to clarify this point.

Some features of experimental determination of localization length of charges in carbon nanomaterials for positive and negative magnetoresistance

In recent years, antiferromagnetic kagome materials have attracted considerable attention in condensed matter physics owing to their distinctive lattice geometry. In this work, high-quality single crystals of D019-structured Mn2.16Ga were grown using the flux method, and their magnetotransport properties were systematically studied. Measurements of magnetization versus field (M–H), temperature-dependent magnetization (M–T), and the anomalous Hall effect confirm that the crystal undergoes a magnetic-structural transition driven by both temperature and the magnetic field. Remarkably, a coexistence of positive and negative longitudinal magnetoresistance (MR) is observed in Mn2.16Ga. The MR shows a field-induced sign change from negative to positive. The negative MR is attributed to field-modified magnetic ordering, whereas the positive MR originates mainly from interlayer electron conduction in the kagome lattice and distortion of the in-plane triangular arrangement of Mn magnetic moments. These results offer valuable insights into the electronic and magnetic transport behavior of Mn-based antiferromagnetic single crystals.

AC losses in multifilamentary HTS tapes can be classified into hysteresis loss, coupling loss, and eddy current loss from the viewpoint of their generation mechanism. Operating electromagnetic conditions significantly influence their AC loss characteristics. From the viewpoint of the major magnetic field component generating them, they can be classified into magnetization loss, transport loss, and total loss. Dividing a superconductor into fine filaments, twisting the filament bundle and increasing transverse resistivity effectively reduce magnetization loss and total loss when the external magnetic field is relatively large, while they do not effectively reduce transport loss. General analytical expressions for the total loss in HTS tapes carrying AC current in an AC magnetic field have not been derived. Numerical electromagnetic field analysis based on the finite element method is a powerful tool to study total loss theoretically. In the magnetic field parallel to the tape's wide face, twisting can reduce the AC loss in Bi2223 tapes with pure silver matrix, while increasing transverse resistivity is required essentially for the AC loss reduction in a perpendicular magnetic field. Recently, twisted multifilamentary Bi2223 tapes with pure silver matrix were fabricated, and the reduction of magnetization loss was proved experimentally in a parallel magnetic field. Furthermore, transverse resistivity was increased successfully by the introduction of a resistive barrier between filaments.

Parallel magnetic field dependence of AC losses in periodically arranged superconductors with ferromagnetic substrates

We report the observation of an extremely large magnetoresistance (up to 15 000 000% at 1.8 K in a magnetic field of 9 T) in a simple chemical element, gray arsenic, in which the magnitude of the magnetoresistance increases as approximately the square of the magnetic field strength without any signs of saturation. The Hall-effect study confirms that gray arsenic is a nearly perfect ``compensated semimetal,'' with a small concentration of very mobile carriers, which lead to an extremely large magnetoresistance. The analysis of Shubnikov--de Haas oscillations reveals a nontrivial \ensuremath{\pi} Berry phase, a strong signature of Dirac fermions with three-dimensional dispersion. Furthermore, in the presence of parallel magnetic and electric fields, a weak antilocalization effect and a pronounced negative longitudinal magnetoresistance, which may be linked to novel topological states, are also observed. These findings which uncover the material's basis in gray arsenic not only open avenues in spintronics and magnetic sensor applications but also provide more platforms to study topological materials.