Longitudinal Magnetoresistance Research Articles

3D topological semimetal phases of strained [formula omitted]-Sn on insulating substrate

α-Sn is an elemental topological material, whose topological phases can be tuned by strain and magnetic field. Such tunability offers a substantial potential for topological electronics. However, InSb substrates, commonly used to stabilize α-Sn allotrope, suffer from parallel conduction, restricting transport investigations and potential applications. Here, the successful MBE growth of high-quality α-Sn layers on insulating, hybrid (001) CdTe/GaAs substrates, with bulk electron mobility approaching 20000 cm2V−1s−1 is reported. The electronic properties of the samples are systematically investigated by independent complementary techniques, enabling thorough characterization of the 3D Dirac (DSM) and Weyl (WSM) semimetal phases induced by the strains and magnetic field, respectively. Magneto-optical experiments, corroborated with band structure modelling, provide an exhaustive description of the bulk states in the DSM phase. The modelled electronic structure is directly observed in angle-resolved photoemission spectroscopy, which reveals linearly dispersing bands near the Fermi level. The first detailed study of negative longitudinal magnetoresistance relates this effect to the chiral anomaly and, consequently, to the presence of WSM. Observation of the π Berry phase in Shubnikov-de Haas oscillations agrees with the topologically non-trivial nature of the investigated samples. Our findings establish α-Sn as an attractive topological material for exploring relativistic physics and future applications.

Competition between chiral anomaly and weak antilocalization in Cd3As2 nanoplates

Negative longitudinal magnetoresistivity (nLMR) induced by the chiral anomaly together with the weak antilocalization (WAL) due to the quantum interference can be regarded as remarkable magnetotransport signatures for three-dimension (3D) topological semimetals. Here, we report the observation of high-temperature competition between the chiral anomaly and WAL by magnetotransport measurements on high-quality Cd3As2 nanoplates under parallel electromagnetic fields. We find that, the WAL dominates the magnetotransport in the weak magnetic fields, which decreases gradually and ultimately vanishes at a critical temperature Tc. In contrast, the chiral anomaly is robust against temperature and can survive up to room temperature. This competition between the chiral anomaly and WAL can be understood in terms of Berry phase, accompanying with the low carrier density in Cd3As2 nanoplates. Our work would offer a better understanding of magnetotransport properties governed by Berry phase and the nature of electronic states in topological semimetals.

Te Vacancy‐Driven Anomalous Transport in ZrTe5 and HfTe5

AbstractThe strongly sample‐dependent anomalous transport properties observed in the layered Dirac materials ZrTe5 and HfTe5 are known to strongly correlate with the presence of Te vacancies. One phenomenon, a negative longitudinal magnetoresistance (NLMR), is widely speculated to be a signature of broken chiral symmetry. However, the role of electronic structure in the sample dependence of the transport properties of these materials is poorly understood. This prompts the question as to whether the NLMR is a genuine signature of the chiral anomaly in ZrTe5 and HfTe5. In this work, the effect of Te vacancies on the electronic structure of ZrTe5 and HfTe5 is investigated via first‐principles calculations. Te vacancies serve two purposes: modification of the cell volume via effective compressive strain and production of local changes to the electronic structure. The reorganization of the electronic structure near the Fermi energy indicates that Te vacancies can rationalize conflicting reports in spectroscopic and transport measurements that have remained elusive in prior first‐principles studies. These results show that Te vacancies contribute, in part, to the anomalous transport properties of ZrTe5 and HfTe5 but, critically, do not eliminate the possibility of a genuine manifestation of the chiral anomaly in these materials.

Anisotropic positive linear and sub-linear magnetoresistivity in the cubic type-II Dirac metal Pd3In7

We report a transport study on Pd3In7 which displays multiple Dirac type-II nodes in its electronic dispersion. Pd3In7 is characterized by low residual resistivities and high mobilities, which are consistent with Dirac-like quasiparticles. For an applied magnetic field (μ0H) having a non-zero component along the electrical current, we find a large, positive, and linear in μ0H longitudinal magnetoresistivity (LMR). The sign of the LMR and its linear dependence deviate from the behavior reported for the chiral-anomaly-driven LMR in Weyl semimetals. Interestingly, such anomalous LMR is consistent with predictions for the role of the anomaly in type-II Weyl semimetals. In contrast, the transverse or conventional magnetoresistivity (CMR for electric fields E⊥μ0H) is large and positive, increasing by 103−104 % as a function of μ0H while following an anomalous, angle-dependent power law {rho }_{{{{rm{xx}}}}}propto {({mu }_{0}H)}^{n} with n(θ) ≤ 1. The order of magnitude of the CMR, and its anomalous power-law, is explained in terms of uncompensated electron and hole-like Fermi surfaces characterized by anisotropic carrier scattering likely due to the lack of Lorentz invariance.

Orbital Hall magnetoresistance in Ni/Ti bilayers

We report the observation of the orbital counterpart of the spin Hall magnetoresistance (SMR): the orbital Hall magnetoresistance (OMR). We measured angular-dependent longitudinal magnetoresistance for Ni/Ti bilayer and Ni single-layer films by rotating a magnetic field along three orthogonal planes. When the magnetic field is rotated in the plane perpendicular to the applied current direction, the angular dependence of the magnetoresistance in the Ni/Ti bilayers is consistent with the prediction of the SMR and OMR, whereas that in the Ni single-layer film can be attributed to the geometrical size effect of the anisotropic magnetoresistance. In the Ni/Ti bilayers, the magnetoresistance ratio is found to be five orders of magnitude larger than the prediction of the SMR, indicating that the OMR plays a dominant role in this system.

Giant Planar Hall Effect in an Ultra-Pure Mercury Selenide Single Crystal Sample

A giant planar Hall effect with an amplitude of about 50 mΩ cm at a temperature of T = 80 K in a magnetic field of 10 T has been detected in an ultra-pure HgSe single crystal sample with an electron density of 5.5 × 1015 cm–3. Its oscillating dependence on the rotation angle of the sample in various magnetic fields has been determined. Attributes (oscillation period, positions of extrema, correlation between the amplitudes of planar Hall and planar longitudinal magnetoresistance) indicate that the planar Hall effect in this nonmagnetic gapless semimetal with an isotropic Fermi surface originates from the chiral anomaly. This is a solid argument for the topological nature of the electronic spectrum of HgSe.

Evidence of pseudogravitational distortions of the Fermi surface geometry in the antiferromagnetic metal FeRh

The confluence between high-energy physics and condensed matter has produced groundbreaking results via unexpected connections between the two traditionally disparate areas. In this work, we elucidate additional connectivity between high-energy and condensed matter physics by examining the interplay between spin-orbit interactions and local symmetry-breaking magnetic order in the magnetotransport of thin-film magnetic semimetal FeRh. We show that the change in sign of the normalized longitudinal magnetoresistance observed as a function of increasing in-plane magnetic field results from changes in the Fermi surface morphology. We demonstrate that the geometric distortions in the Fermi surface morphology are more clearly understood via the presence of pseudogravitational fields in the low-energy theory. The pseudogravitational connection provides additional insights into the origins of a ubiquitous phenomenon observed in many common magnetic materials and points to an alternative methodology for understanding phenomena in locally-ordered materials with strong spin-orbit interactions.

Magnetoresistance peculiarities of Bi95.69Mn3.69Fe0.62 in magnetic fields up to 140 kOe

The magnetic-field dependences of the electrical resistivity Δρ/ρ0(Η) of the textured polycrystal Bi95.69Mn3.69Fe0.62 have been studied for the first time, for the H⊥I and H || I configurations at temperatures 5, 80, 150, and 300 K. It has been established that the Δρ/ρ0(Η) dependences significantly differ from those obtained for pure bismuth due to the influence of the internal magnetism of the α-BiMn phase inclusions on the behavior of charge carriers in the bismuth matrix. The maxima of Δρ/ρ0(Η) at H ≈ 30 and ≈ 40 kOe have been found, for the longitudinal and transverse magnetoresistance, respectively. These maxima may be related to reaching the quantum limit in the material we have studied.

Robust negative longitudinal magnetoresistance and spin–orbit torque in sputtered Pt3Sn and Pt3SnxFe1-x topological semimetal

Contrary to topological insulators, topological semimetals possess a nontrivial chiral anomaly that leads to negative magnetoresistance and are hosts to both conductive bulk states and topological surface states with intriguing transport properties for spintronics. Here, we fabricate highly-ordered metallic Pt3Sn and Pt3SnxFe1-x thin films via sputtering technology. Systematic angular dependence (both in-plane and out-of-plane) study of magnetoresistance presents surprisingly robust quadratic and linear negative longitudinal magnetoresistance features for Pt3Sn and Pt3SnxFe1-x, respectively. We attribute the anomalous negative longitudinal magnetoresistance to the type-II Dirac semimetal phase (pristine Pt3Sn) and/or the formation of tunable Weyl semimetal phases through symmetry breaking processes, such as magnetic-atom doping, as confirmed by first-principles calculations. Furthermore, Pt3Sn and Pt3SnxFe1-x show the promising performance for facilitating the development of advanced spin-orbit torque devices. These results extend our understanding of chiral anomaly of topological semimetals and can pave the way for exploring novel topological materials for spintronic devices.

A System of Josephson Vortices in a Layered Electron-Doped Nd2 – xCexCuO4 Superconductor

A brief review of the transport and galvanomagnetic properties of an electron-doped Nd2 – xCexCuO4 layered superconductor with an emphasis on the role of the structure of Josephson vortices formed in the crystalin a magnetic field oriented in the parallel direction to the CuO2 planes is given. The experimental data on the longitudinal and Hall magnetoresistance of the recently synthesized Nd2–xCexCuO4/SrTiO3 epitaxial films are analyzed. The possibility of formation of a system of Josephson vortices in this highly anisotropic system is determined by the unique orientation of the c axis and the conducting CuO2 planes of the Nd2 – xCexCuO4 compound with respect to the substrate plane.

The vertical longitudinal magnetoresistance in a van der Waals thin film of WTe2

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes. At low temperatures, we observe not only a large unsaturated magnetoresistance and Shubnikov–de Haas oscillations with current in the in-plane direction but also a saturated vertical longitudinal magnetoresistance and quantum oscillations with current in the out-of-plane direction in a thin film of WTe2. Our work provides insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics.

Impurity and dispersion effects on the linear magnetoresistance in the quantum limit

Magnetoresistance, that is, the change of the resistance with the magnetic field, is usually a quadratic function of the field strength. A linear magnetoresistance usually reveals extraordinary properties of a system. In the quantum limit where only the lowest Landau band is occupied, a quantum linear magnetoresistance was believed to be the signature of the Weyl fermions with 3D linear dispersion. Here, we comparatively investigate the quantum-limit magnetoresistance of systems with different band dispersions as well as different types of impurities. We find that the magnetoresistance can also be linear for the quadratic energy dispersion. We show that both longitudinal and transverse magnetoresistance can be linear if long-range-Gaussian-type impurities dominate, but Coulomb-type impurities can only induce linear transverse magnetoresistance. Moreover, we find a negative longitudinal magnetoresistance in massless Dirac fermions, regardless of the impurity type, as a result of the combined effect of the linear dispersion and the scattering mechanism. Our findings well explain some of the linear magnetoresistance observed in the experiments and provide insights to the understanding of quantum-limit magnetoresistance.

Robustness of semimetallic transport properties of TaAs against off-stoichiometric disorder

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution of elements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed that the atomic vacancy concentration was kept below 10−5 at. %. However, inductively coupled plasma analysis showed an As-deficient (7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density (≲1018 cm−3), ultrahigh carrier mobility (≳106 cm2/V/s), and large transverse magnetoresistance (>200000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and the quenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidence of a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.

Energy Spectrum of the Valence Band in HgTe Quantum Wells on the Way from a Two- to Three-Dimensional Topological Insulator

The magnetic field and temperature dependences of longitudinal magnetoresistance and the Hall effect have been measured in order to determine the energy spectrum of the valence band in HgTe quantum wells with the width dQW = 20–200 nm. The comparison of hole densities determined from the period of Shubnikov–de Haas oscillations and the Hall effect shows that states at the top of the valence band are doubly degenerate in the entire dQW range, and the cyclotron mass {{m}_{h}} determined from the temperature dependence of the amplitude of Shubnikov–de Haas oscillation increases monotonically from 0.2{{m}_{0}} to 0.3{{m}_{0}} ({{m}_{0}} is the mass of the free electron) with increasing hole density p from 2 times {{10}^{{11}}} to 6 times {{10}^{{11}}} cm–2. The determined dependence has been compared to theoretical dependences {{m}_{h}}(p,{{d}_{{{text{QW}}}}}) calculated within the four-band kP model. These calculations predict an approximate stepwise increase in {{m}_{h}} owing to the pairwise merging of side extrema with increasing hole density, which should be observed at p = (4{-} 4.5) times {{10}^{{11}}} and 4 × 1010 cm–2 for dQW = 20 and 200 nm, respectively. The experimental dependences are strongly inconsistent with this prediction. It has been shown that the inclusion of additional factors (electric field in the quantum well, strain) does not remove the contradiction between the experiment and theory. Consequently, it is doubtful that the mentioned kP calculations adequately describe the valence band at all dQW values.

Evidence of magnetic fluctuation induced Weyl semimetal state in the antiferromagnetic topological insulator Mn(Bi1−xSbx)2Te4

We report $c$-axis transport studies on magnetic topological insulators $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$. We performed systematic $c$-axis magnetoresistivity measurements under high magnetic fields (up to 35 T) on several representative samples. We find that the lightly hole- and lightly electron-doped samples, while both having the same order of magnitude of carrier density and similar spin-flop transitions, exhibit sharp contrast in electronic anisotropy and transport mechanism. The electronic anisotropy is remarkably enhanced for the lightly hole-doped sample relative to pristine $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ but not for the lightly electron-doped sample. The lightly electron-doped sample displays a giant negative longitudinal magnetoresistivity (LMR) induced by the spin-valve effect at the spin-flop transition field, whereas the lightly hole-doped sample exhibits remarkable negative LMR consistent with the chiral anomaly behavior of a Weyl semimetal. Furthermore, we find the large negative LMR of the lightly hole-doped sample extends to a wide temperature range above the N\'eel temperature $({T}_{\mathrm{N}})$ where the magnetoconductivity is proportional to ${B}^{2}$. This fact, together with the short-range intralayer ferromagnetic correlation revealed in isothermal magnetization measurements, suggests the possible presence of the Weyl state above ${T}_{\mathrm{N}}$. These results demonstrate that in the $c$-axis magnetotransport of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$, the spin scattering is dominant in the lightly electron-doped sample but overwhelmed by the chiral anomaly effect in the lightly hole-doped sample due to the presence of the Weyl state. These findings extend the understanding of the transport properties of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$.

Temperature-induced anomalous magnetotransport in the Weyl semimetal Mn3Ge

The magnetic Weyl semimetallic state can lead to intriguing magnetotransport, such as chiral anomaly and the layered quantum Hall effect. Mn3X (X = Sn, Ge) is a noncollinear antiferromagnetic semimetal where a Weyl semimetallic state is stabilized by time-reversal symmetry breaking. Compared to the well-studied Mn3Sn, the Weyl fermion-induced magnetotransport in Mn3Ge has been merely studied. Here, we report an in-depth study on the magnetotransport in a microfabricated Mn3Ge single crystal from room temperature to 10 K. We reveal an anomalous anisotropic magnetoresistance with fourfold symmetry and a positive high-field longitudinal magnetoresistance below the critical temperature (160–170 K). The possible origin is the temperature-induced tilting of the Weyl nodes. Our study helps to understand the magnetotransport properties in the Weyl fermion system.

Fermiology of Chiral Cadmium Diarsenide CdAs2, a Candidate for Hosting Kramers-Weyl Fermions.

Nonmagnetic chiral crystals are a new class of systems hosting Kramers-Weyl Fermions, arising from the combination of structural chirality, spin-orbit coupling (SOC), and time-reversal symmetry. These materials exhibit nontrivial Fermi surfaces with SOC-induced Chern gaps over a wide energy range, leading to exotic transport and optical properties. In this study, we investigate the electronic structure and transport properties of CdAs2, a newly reported chiral material. We use synchrotron-based angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT) to determine the Fermiology of the (110)-terminated CdAs2 crystal. Our results, together with complementary magnetotransport measurements, suggest that CdAs2 is a promising candidate for novel topological properties protected by the structural chirality of the system. Our work sheds light on the details of the Fermi surface and topology for this chiral quantum material, providing useful information for engineering novel spintronic and optical devices based on quantized chiral charges, negative longitudinal magnetoresistance, and nontrivial Chern numbers.

Carrier and thickness mediated ferromagnetism in chiral magnet Mn1/3TaS2 nanoflakes

Layered chiral magnets with broken spatial inversion symmetry (SIS) enable chiral spin textures to occur in atomically thin layers. However, most layered materials retain SIS during their crystallization. Here, we demonstrate that SIS can be broken in a layered transition metal dichalcogenide TaS2 by intercalating Mn atoms. A chiral magnetic phase in Mn1/3TaS2 has, thus, been realized. This phase enables a nonzero Dzyaloshinskii–Moriya interaction, which in turn gives rise to large topological Hall effects (THEs) below 50 K. Both the ferromagnetism and THE can be tuned at low temperatures by modulating the carrier density via a protonic gate. Measured at 20 K with Vg = −4.7 V applied to the gate and electron doping density of 1.7 × 1022 cm−3, the maximum THE was almost double that recorded with no gate voltage applied. By further reducing the sample thicknesses, both the Curie temperature Tc and the longitudinal magnetoresistance can be significantly modulated. This is consistent with the theory of critical behavior. Our work highlights the ability to control both magnetism and chiral spin textures in Mn1/3TaS2 nanoflakes. Applying this discovery may lead to a variety of practical van der Waals heterostructure devices.

Anomalous magnetothermoelectric behavior in massive Dirac materials

Extensive studies of electron transport in Dirac materials have shown positive magneto-resistance (MR) and positive magneto-thermopower (MTP) in a magnetic field perpendicular to the excitation current or thermal gradient. In contrast, measurements of electron transport often show a negative longitudinal MR and negative MTP for a magnetic field oriented along the excitation current or thermal gradient; this is attributed to the chiral anomaly in Dirac materials. Here, we report a very different magneto-thermoelectric transport behavior in the massive Dirac material ZrTe5. Although thin flakes show a commonly observed positive MR in a perpendicular magnetic field, distinct from other Dirac materials, we observe a sharp negative MTP. In a parallel magnetic field, we still observe a negative longitudinal MR, however, a remarkable positive MTP is observed for the fields parallel to the thermal gradients. Our theoretical calculations suggest that this anomalous magneto-thermoelectric behavior can be attributed to the screened Coulomb scattering. This work demonstrates the significance of impurity scattering in the electron transport of topological materials and provides deep insight into the novel magneto-transport phenomena in Dirac materials.

Field direction dependent quantum-limit magnetoresistance of correlated Dirac electrons in perovskite CaIrO3

Correlated-electron perovskite ${\mathrm{CaIrO}}_{3}$ shows the topologically protected line node near the Fermi level, wherein the high-mobility Dirac electrons reach the quantum limit (QL) with the one-dimensionally (1D) dispersive $n=0$ Landau level (LL) state at a moderate magnetic field. In the QL, the longitudinal magnetoresistance (MR) shows an extreme anisotropy against the field $(B)$ direction in spite of nearly isotropic Dirac band dispersions. The resistivity for $B\ensuremath{\parallel}a$ shows an insulating behavior with the MR ratio exceeding $2000%$ around 18 T, whereas the $B\ensuremath{\parallel}c$ MR always remains metallic. This is accounted for in terms of the large difference of Fermi velocity of the $n=0$ LL between $B\ensuremath{\parallel}a$ and $B\ensuremath{\parallel}c$ due to the $B$ direction dependent $5d$ orbital electron hopping, which triggers the different instability toward the charge density wave formation for the 1D $(\ensuremath{\parallel}B)$ dispersive $n=0$ LL.