Anomalous Magnetotransport Properties Research Articles

Spin-related negative magnetoresistance in germanium films

Herein, we report the transport properties of Ge films. The variable-range hopping transport at low temperatures (T≲50K) and thermal activation transport at high temperatures (T≳50K) are observed in our Ge films. In the different temperature regimes, the anomalous magnetotransport properties are observed. In the low-temperature regime (T≲15K), the negative magnetoresistance (MR) at low field and positive MR at high field can be seen. In the moderate-temperature regime (15K≲T≲100K), the positive MR curve gradually evolves from a linear curve to a parabolic curve with increasing temperature, and the MR magnitude appears to be insensitive to temperature. In the high-temperature regime (T≳100K), the positive MR value increases with increasing temperature. By considering the angular-dependent MR, we can determine that the negative MR comes from the spin-related mechanism, and the positive MR is caused by the orbital-related mechanism. However, further study is required to determine the exact mechanisms behind the anomalous magnetotransport properties.

Anomalous magnetotransport behavior in single-crystal VSb2

We report the anomalous magnetotransport properties in high-quality single-crystal ${\mathrm{VSb}}_{2}$ without V atom vacancies. ${\mathrm{VSb}}_{2}$ shows typical metallic behavior with convex-upward characteristics, which can be explained using a parallel-resistor model. The three-dimensional weak antilocalization effect leads to a large saturation magnetoresistance (MR) up to $595%$, which disappears when the magnetic field is along the $ab$ plane. Instead, a clear Shubnikov--de Haas oscillation at low temperatures is observed, based on which the fast Fourier transformation spectrum analyses reveal three fundamental frequencies. The two-carrier fitting of Hall resistivity suggests that the saturation MR originates from the giant magnitude difference of the two carrier concentrations. Moreover, the angle-resolved magnetoresistance is measured by rotating the sample around its $c$ axis and $ab$ plane. A transition from twofold symmetry to isotropy is observed.

B2 to B -linear magnetoresistance due to impeded orbital motion

Strange metals exhibit a variety of anomalous magnetotransport properties, the most striking of which is a resistivity that increases linearly with magnetic field $B$ over a broad temperature and field range. The ubiquity of this behavior across a spectrum of correlated metals - both single- and multi-band, with either dominant spin and/or charge fluctuations, of varying levels of disorder or inhomogeneity and in proximity to a quantum critical point or phase - obligates the search for a fundamental underlying principle that is independent of the specifics of any material. Strongly anisotropic (momentum-dependent) scattering can generate $B$-linear magnetoresistance but only at intermediate field strengths. At high enough fields, the magnetoresistance must eventually saturate. Here, we consider the ultimate limit of such anisotropy, a region or regions on the Fermi surface that impede all orbital (cyclotron) motion through them, but whose imposition can be modelled nonetheless through a modified Boltzmann theoretical treatment. Application of the proposed theorem suggests that the realization of quadratic-to-linear magnetoresistance requires the presence of a bounded sector on the Fermi surface possibly separating two distinct types of carriers. While this bounded sector may have different origins or manifestations, we expect its existence to account for the anomalous magnetotransport found in a wide range of correlated materials.

Magnetic Generation and Switching of Topological Quantum Phases in a Trivial Semimetal α−EuP3

Topological materials have drawn increasing attention owing to their rich quantum properties, as highlighted by a large intrinsic anomalous Hall effect (AHE) in Weyl and nodal-line semimetals. However, the practical applications for topological electronics have been hampered by the difficulty in the external control of the band topology. Here we demonstrate a magnetic-field-induced switching of band topology in $\alpha{\mathrm{-EuP}}_3$, a magnetic semimetal with a layered crystal structure derived from black phosphorus. When the magnetic field is applied perpendicular to the single mirror plane of the monoclinic structure, a giant AHE signal abruptly emerges at a certain threshold magnetization value, giving rise to a prominently large anomalous Hall angle of $\left|\Theta_{\mathrm{AHE}}\right| \sim 20^{\circ}$. When the magnetic field is applied along the inter-layer direction, which breaks the mirror symmetry, the system shows a pronounced negative longitudinal magnetoresistance. On the basis of electronic structure calculations and symmetry considerations, these anomalous magneto-transport properties can be considered as manifestations of two distinct topological phases: topological nodal-line and Weyl semimetals, respectively. Notably, the nodal-line structure is composed of bands with the same spin character and spans a wide energy range around the Fermi level. These topological phases are stabilized via the exchange coupling between localized Eu-4$f$ moments and mobile carriers conducting through the phosphorus layers. Our findings provide a realistic solution for external manipulation of band topology, enriching the functional aspects of topological materials.

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.

Quantum magnetotransport in massive Dirac materials

Massive Dirac fermions break the chiral symmetry explicitly and also make the Berry curvature of the band structure non-Abelian. By utilizing the Green's function technique, we develop a microscopic theory to establish a set of quantum diffusive equations for massive Dirac materials in the presence of electric and magnetic fields. It is found that the longitudinal magnetoresistance is always negative and quadratic in the magnetic field, and decays quickly with the mass. The theory is applicable to the systems with non-Abelian Berry curvature and resolves the puzzles of anomalous magnetotransport properties measured in topological materials.

Magnetic field induced topological semimetals near the quantum critical point of pyrochlore iridates

Motivated by the recent experimental observation of anomalous magneto-transport properties near the Mott quantum critical point (QCP) of pyrochlore iridates, we study the generic topological band structure near QCP in the presence of magnetic field. We have found that the competition between different energy scales can generate various topological semi-metal phases near QCP. Here the central role is played by the presence of a quadratic band crossing (QBC) with four-fold degeneracy in the paramagnetic band structure. Due to the large band degeneracy and strong spin-orbit coupling, the degenerate states at QBC can show an anisotropic Zeeman effect as well as the conventional isotropic Zeeman effect. Through the competition between three different magnetic energy scales including the exchange energy between Ir electrons and two Zeeman energies, various topological semimetals can be generated near QCP. Moreover, we have shown that these three magnetic energy scales can be controlled by modulating the magnetic multipole moment (MMM) of the cluster of spins in a unit cell, which can couple to the intrinsic MMM of the degenerate states at QBC. We propose the general topological band structure under magnetic field achievable near QCP, which would facilitate the experimental discovery of novel topological semimetal states in pyrochlore iridates.

Anomalous magnetotransport properties of high-quality single crystals of Weyl semimetal WTe2: Sign change of Hall resistivity

We report on a systematic study of Hall effect using high quality single crystals of type-II Weyl semimetal WTe2 with the applied magnetic field B//c. The residual resistivity ratio of 1330 and the large magnetoresistance of 1.5 × 106 % in 9T at 2K, being in the highest class in the literature, attest to their high quality. Based on a simple two-carrier model, the densities (ne and nh) and mobilities (μe and μh) for electron and hole carriers have been uniquely determined combining both Hall- and electrical-resistivity data. The difference between ne and nh is ~ 1% at 2K, indicating that the system is in an compensated condition. The negative Hall resistivity growing rapidly below ~ 20K is due to a rapidly increasing μh/μe approaching one. Below 3K in a low field region, we found the Hall resistivity becomes positive, reflecting that μh/μe finally exceeds one in this region. These anomalous behaviors of the carrier densities and mobilities might be associated with the existence of a Lifshitz transition and/or the spin texture on the Fermi surface.

Electronic Coupling between Graphene and Topological Insulator Induced Anomalous Magnetotransport Properties.

It has been theoretically proposed that the spin textures of surface states in a topological insulator can be directly transferred to graphene by means of the proximity effect, which is very important for realizing a two-dimensional topological insulator based on graphene. Here we report the anomalous magnetotransport properties of graphene-topological insulator Bi2Se3 heterojunctions, which are sensitive to the electronic coupling between graphene and the topological surface state. The coupling between the pz orbitals of graphene and the p orbitals of the surface states on the Bi2Se3 bottom surface can be enhanced by applying a perpendicular negative magnetic field, resulting in a giant negative magnetoresistance at the Dirac point up to about -91%. An obvious resistance dip in the transfer curve at the Dirac point is also observed in the hybrid devices, which is consistent with theoretical predictions of the distorted Dirac bands with nontrivial spin textures inherited from the Bi2Se3 surface states.

Anomalous magnetotransport behaviours in PtSe2 microflakes

Platinum diselenide (PtSe2) is a newly discovered 2D transition metal dichalcogenide, and is further theoretically identified as a candidate of type-II Dirac semimetals. The electrical transport study of PtSe2 microflakes may provide great potential not only in fundamental physics, but also for future electronic applications. We report the anomalous magnetotransport properties of PtSe2 microflakes. The anisotropic magnetoresistance of PtSe2 microflakes can be normalized by introducing a 3D scaling factor , where θ is the magnetic field angle with respect to the c axis of the crystal and γ is the mass anisotropic constant of electrons. Additionally, the non-monotonic temperature-dependent magnetoresistance of PtSe2 microflakes is observed both in the perpendicular and in-plane magnetic field orientations. This anomalous magnetotransport behaviour may be ascribed to the novel features of type-II Dirac fermions; however, the exact physical mechanism deserves further investigation.

Magnetotransport of multiple-band nearly antiferromagnetic metals due to hot-spot scattering

Multiple-band electronic structure and proximity to antiferromagnetic (AF) instability are the key properties of iron-based superconductors. We explore the influence of scattering by the AF spin fluctuations on transport of multiple-band metals above the magnetic transition. A salient feature of scattering on the AF fluctuations is that it is strongly enhanced at the Fermi surface locations where the nesting is perfect ("hot spots" or "hot lines"). We review derivation of the collision integral for the Boltzmann equation due to AF-fluctuations scattering. In the paramagnetic state, the enhanced scattering rate near the hot lines leads to anomalous behavior of electronic transport in magnetic field. We explore this behavior by analytically solving Boltzmann transport equation with approximate transition rates. This approach accounts for return scattering events and is more accurate than the relaxation-time approximation. The magnetic-field dependences are characterized by two very different field scales, the lower scale is set by the hot-spot width and the higher scale is set by the total scattering amplitude. A conventional magnetotransport behavior is limited to magnetic fields below the lower scale. In the wide range in between these two scales the longitudinal conductivity has linear dependence on the magnetic field and the Hall conductivity has quadratic dependence. The linear dependence of the diagonal component reflects growth of the Fermi-surface area affected by hot spots proportional to the magnetic field. We discuss applicability of this theoretical framework for describing of anomalous magnetotransport properties in different iron pnictides and selenides in the paramagnetic state.

Nodal-chain metals.

The band theory of solids is arguably the most successful theory of condensed-matter physics, providing a description of the electronic energy levels in various materials. Electronic wavefunctions obtained from the band theory enable a topological characterization of metals for which the electronic spectrum may host robust, topologically protected, fermionic quasiparticles. Many of these quasiparticles are analogues of the elementary particles of the Standard Model, but others do not have a counterpart in relativistic high-energy theories. A complete list of possible quasiparticles in solids is lacking, even in the non-interacting case. Here we describe the possible existence of a hitherto unrecognized type of fermionic excitation in metals. This excitation forms a nodal chain-a chain of connected loops in momentum space-along which conduction and valence bands touch. We prove that the nodal chain is topologically distinct from previously reported excitations. We discuss the symmetry requirements for the appearance of this excitation and predict that it is realized in an existing material, iridium tetrafluoride (IrF4), as well as in other compounds of this class of materials. Using IrF4 as an example, we provide a discussion of the topological surface states associated with the nodal chain. We argue that the presence of the nodal-chain fermions will result in anomalous magnetotransport properties, distinct from those of materials exhibiting previously known excitations.

Effect of antiferromagnetic layer thickness on exchange bias, training effect, and magnetotransport properties in ferromagnetic/antiferromagnetic antidot arrays

The effect of antiferromagnetic (AFM) layer on exchange bias (EB), training effect, and magnetotransport properties in ferromagnetic (FM) /AFM nanoscale antidot arrays and sheet films Ag(10 nm)/Co(8 nm)/NiO(tNiO)/Ag(5 nm) at 10 K is studied. The AFM layer thickness dependence of the EB field shows a peak at tNiO = 2 nm that is explained by using the random field model. The misalignment of magnetic moments in the three-dimensional antidot arrays causes smaller decrease of EB field compared with that in the sheet films for training effect. The anomalous magnetotransport properties, in particular positive magnetoresistance (MR) for antidot arrays but negative MR for sheet films are found. The training effect and magnetotransport properties are strongly affected by the three-dimensional spin-alignment effects in the antidot arrays.

Anomalous magnetic and magnetotransport properties in nanostructured networks of Co/NiO/Fe trilayers

The formation of an interconnected nanostructured network of Co/NiO/Fe trilayers on anodic aluminum oxide (AAO) template strongly affects the interfacial exchange coupling between antiferromagnetic and ferromagnetic layers in comparison with continuous films of trilayers on silicon substrate. Anomalous magnetic and magnetotransport properties, in particular a positive magnetoresistance (MR), are observed for the network on AAO, but a negative MR is found for the film on Si. The non-monotonic temperature dependence of the resistivity at low temperatures of the network on AAO can be qualitatively understood in terms of a two-current model.

Origin of the anomalous Hall effect in the overdopedn-type superconductorPr2−xCexCuo4: Current-vertex corrections due to antiferromagnetic fluctuations

The anomalous magneto-transport properties in electron doped (n-type) cuprates were investigated using Hall measurements at THz frequencies. The complex Hall angle was measured in overdoped Pr$_{\rm 2-x}$Ce$_{\rm x}$CuO$_{\rm 4}$ samples (x=0.17 and 0.18) as a continuous function of temperature above $T_c$ at excitation energies 5.24 and 10.5 meV. The results, extrapolated to low temperatures, show that inelastic scattering introduces electron-like contributions to the Hall response. First principle calculations of the Hall angle that include current vertex corrections (CVC) induced by electron interactions mediated by magnetic fluctuations in the Hall conductivity reproduce the temperature, frequency, and doping dependence of the experimental data. These results show that CVC effects are the source of the anomalous Hall transport properties in overdoped n$\text{-}$type cuprates.

Anomalous magneto-transport properties in nearly gapless materials

We have measured magnetoresistance (MR) for semimetals Bi and graphite, nearly gapless materials Bi-Sb and Bi-Te alloys, and narrow gap semiconductor Ag2Se single crystals at 4.2 K and up to 4 T. We have observed a giant MR for Bi, a giant linear MR (LMR) for Bi-Sb alloy and Ag2Se single crystals. Anomalous completely linear weak-field LMR has been found for nearly gapless materials: this is transferred into the ordinal MR or other LMR in relatively strong field range. It suggests the linear band dispersion in nearly gapless materials as proposed by Abrikosov.

Localized Spins on Graphene

The problem of a magnetic impurity, atomic or molecular, absorbed on top of a carbon atom in otherwise clean graphene is studied using the numerical renormalization group. The spectral, thermodynamic, and scattering properties of the impurity are described in detail. In the presence of a small magnetic field, the low-energy electronic features of graphene make it possible to inject spin-polarized currents through the impurity using a scanning tunneling microscope. Furthermore, the impurity scattering becomes strongly spin dependent and for a finite impurity concentration it leads to spin-polarized bulk currents and a large magnetoresistance. In gated graphene the impurity spin is Kondo screened at low temperatures. However, at temperatures larger than the Kondo temperature, the anomalous magnetotransport properties are recovered.

Anomalous electrical transport and magneto-transport properties at room temperature in Fe0.8Mn1.5As

For the Fe0.8Mn1.5As compound, an external magnetic field induces a metamagnetic transition from an antiferromagnetic (AF) phase to a ferrimagnetic (FI) phase above Ts = 285 K, stabilizing the latter phase with the larger unit-cell volume. On cooling, the resistivity curve ρ(T) of Fe0.8Mn1.5As shows an upturn near the temperature-induced AF–FI transition temperature Ts. With an increase in the number of warming and cooling cycles, the resistivity ρ(T) shifts to higher resistivity values, and this anomalous shift is ascribed to a pronounced reduction of the mean-free path of electron scattering. At/near room temperature, the resistivity first decreases with increasing external magnetic field until a critical field, and then after a field-independent stage it increases abruptly with a further increase in the field. In the subsequent field-decreasing process, the resistivity increases almost linearly, resulting in a large hysteresis in resistivity of 56.4 µΩ cm at 290 K (or 20.2 µΩ cm at 295 K) at zero field.

Theoretical proposal predicting anomalous magnetoresistance and quadratic Hall effect in the partially ordered state of MnSi

In [B. Binz, A. Vishwanath, V. Aji, Phys. Rev. Lett. 96 (2006) 207202], a magnetic structure that breaks time reversal symmetry in the absence of net magnetization was proposed as an explanation for the high pressure “partially ordered” state of MnSi. Here we make explicit the anomalous magneto-transport properties of such a state: a magnetoresistivity which is linear and a Hall conductance which is quadratic in the applied magnetic field. Field cooling procedures for obtaining single domain samples are discussed. The anomalous effects are elaborated in the case of three geometries chosen to produce experimentally unambiguous signals of this unusual magnetic state; e.g., it is predicted that a field in z-direction induces an anisotropy in the x– y plane. Another geometry leads to a Hall voltage parallel to the magnetic field.

Anomalous magnetotransport properties of R2Mo2O7 near the magnetic phase boundary

The magnetic and transport properties of pyrochlore R2Mo2O7 have been studied with variation of the rare earth ( R). The change of the mean ionic radius of R, which induces change of the lattice structure, determines the magnetic ground state (ferromagnetic or spin glass), and the magnetic phase boundary is correlated with the metal-insulator crossover. Furthermore, we found enhanced magnetoresistance and unusual residual anomalous Hall effect at low temperatures near the phase boundary, which can be attributed to the coexistence of both phases.