Negative Longitudinal Magnetoresistance from the Anomalous N=0 Landau Level in Topological Materials

Abstract

Negative longitudinal magnetoresistance (NLMR) is shown to occur in topological materials in the extreme quantum limit, when a magnetic field is applied parallel to the excitation current. We perform pulsed and DC field measurements on Pb1-xSnxSe epilayers where the topological state can be chemically tuned. The NLMR is observed in the topological state, but is suppressed and becomes positive when the system becomes trivial. In a topological material, the lowest N=0 conduction Landau level disperses down in energy as a function of increasing magnetic field, while the N=0 valence Landau level disperses upwards. This anomalous behavior is shown to be responsible for the observed NLMR. Our work provides an explanation of the outstanding question of NLMR in topological insulators and establishes this effect as a possible hallmark of bulk conduction in topological matter. [13]. This stems from the helical Dirac nature of surface-states in 3D TIs or, that of edge-states in 2D TIs. In fact, a huge amount of literature (for reviews [14] [15] [16] [17]) took interest in this question and investigated electronic transport of 2D Dirac electrons in 3D-TIs. The majority of these studies were, however, impeded by the significant and dominant bulk transport that occurs in TIs. On the other hand, little attention has been given to signatures of non-trivial band topology in 3D electron transport in a TI.Naively speaking, one can think of the bulk energy bands of a TI as being identical to those of conventional semiconductors and, thus, unlikely to generate non-conventional physical phenomena. However, one should not forget that the basis of a topological insulator lies in the inverted orbital character of these bulk energy bands. [23] shown, that the energy of the lowest (N=0) conduction (valence) Landau level in topological insulators decreases (increases) as a function of increasing magnetic field, opposite to what usually happens in a topologically trivial system ( Fig.1(a,b)). This behavior is anomalous and leads to a field-induced closure of the energy gap in a TI [21] (Fig.1(b)), whereas in a trivial material, the energy gap usually opens with increasing magnetic field ( Fig.1(a)). This anomaly is a hallmark of the inverted band structure of topological materials. Its implications on magnetotransport have not yet been considered. In the present work, we study the MR in topological insulators in the extreme quantum limit -the regime where only the lowest Landau level (LL) is occupied. We measure magnetotransport in pulsed magnetic field up to 61T in high mobility Pb1-xSnxSe epitaxial layers. We show that, when all Lorentz components contributing to the MR are suppressed by applying the magnetic field in-plane and parallel to the excitation current, a negative longitudinal MR (NLMR) emerges near the onset of the quantum limit. This NLMR is only observed in the topological regime of Pb1-xSnxSe (x>0.16) and is absent in trivial samples (x<0.16). We theoretically argue that this NLMR is a result of the anomalous beh...

Forbidden Backscattering and Resistance Dip in the Quantum Limit as a Signature for Topological Insulators

“…[9]). This lifting can lead to a better fitting to a recent experiment on the topological insulator material Pb 1−x Sn x Se [8], which shows an unexpected suppression of the resistance in strong magnetic fields, as shown in Figs. 1 (c) and 1 (d).…”

“…Experimental implications -Although Pb 1−x Sn x Se is claimed to be a topological insulator, the interpretation in Ref. [8] adopted a theory for Weyl semimetal in delta scattering potentials [12,13], where the spinor wave function of the lowest Landau band takes the form |0, k x , k z = 0 |0 |k x , k z .…”

Negative Magnetoresistance without Chiral Anomaly in Topological Insulators

“…The negative MR is widely believed to be a signature showing that a topological semimetal can host the chiral anomaly, that is, the conservation of chiral current is violated as a result of the quantization [14][15][16]. However, in other systems where the chiral anomaly is not well defined, e.g., in topological insulators, a negative MR has also been observed and has created great confusion [17][18][19][20][21][22]. In this Letter, we present a quantitative study on the MR of 3D topological insulators.…”

Quantum transport in topological semimetals under magnetic fields (II)

“…Nevertheless, in topological insulators, where the chiral anomaly is not well defined in the momentum space, a negative magnetoresistance can also be observed. This results in great confusion [109][110][111][112][124][125][126] on the explanation of the negative magnetoresistance. Lately, it is found that the chiral anomaly in real space can be defined in a quantum spin Hall insulator [127].…”

“…Ref. [10] shows that this mechanism is large enough in topological insulators as those observed in the experiments, where the relative magnetoresistance can exceed -1% in a parallel magnetic field of several Tesla [109][110][111][112][124][125][126].…”