Topological effects are predicted to have many potential uses in future electronic devices. Therefore, finding ways to control these effects is desirable. As predicted by first-principles calculations, the one-dimensional (1D) transition-metal trichalcogenide TaSe3 is a strongly topological semimetal. It has a unique atomic arrangement of two inequivalent chains; the shorter distance between the Se atoms in the type-I chains (red in figures) creates strong covalent p-p bonding between the two Se atoms, whereas this bond is broken in the type-II chains (blue in figures) so that bonds form with the Ta atoms from the neighboring type-I chains. The chains are along the b-axis crystallographic direction. Calculations suggest that nontrivial topological phases can be induced by the distorted type-II chain under ambient conditions and/or strain. In collaboration with John Singleton at the Pulsed-Field Facility, National High Magnetic Field Laboratory, research led by Dr. Rongying Jin of the University of South Carolina (USC) investigated the effect of strain on TaSe3 by measuring its magnetoresistance (MR) in fields of up to 60 T. Both ribbon-shaped (under ambient conditions) and ring-shaped (i.e., deliberately strained) samples were studied.
In the ribbon-shaped samples, the MR exhibits a quadratic magnetic field (H) dependence in both the transverse (H ⊥ current) and longitudinal (H // current) configurations, meaning MR ∝ H2. This MR reaches approximately 3.5×104% at 0.4 K and 20 T. Data collected at various temperatures can be scaled into a single curve, following Kohler’s rule. Thus, strain-free TaSe3 exhibits relatively conventional MR, as expected for a multiband system with nearly perfect electron-hole compensation. At high fields, conventional Shubnikov-de Haas (SdH) oscillations are observed with the frequency f = 97 T.
For the ring-shaped TaSe3 samples, distinct features are observed. Firstly, under the same conditions, the MR in the rings is typically around 1,000 times lower than that in the ribbons. Secondly, the MR in different diameter rings does not follow Kohler’s rule, but exhibits an H1.5 dependence at low fields and a linear H dependence above 20 T. Thirdly, in specific field orientations, the MR of the ring exhibits H-periodic oscillations at low temperatures and high fields.
The differences between the MR of the TaSe3 ribbons and rings show very clearly that forming the rings strongly modifies the electronic structure. Bending causes nonuniform strain along the b-axis of TaSe3, resulting in deformation energy. On the one hand, atomic displacements caused by the strain alter the topological properties of the electronic bands, as shown in the rings by the lower MR, violation of Kohler’s rule, and absence of SdH oscillations up to 60 T. On the other hand, defects and disorder are created in the crystal due to strain. Through meticulous analysis of the MR in the rings for various field and current configurations, the researchers found that the H-periodic oscillations are most pronounced when H is applied along the ring. This suggests that MR oscillations in the rings are not due to the Aharonov-Bohm effect but are likely of the Altshuler-Aronov-Spivak (AAS) type. AAS oscillations are due to carriers traveling a complete loop around a scattering center. Here, forward (weak localization) and back (weak antilocalization) scattering cause quantum interference. The effect occurs typically in weakly disordered systems that possess strong spin-orbit coupling and nontrivial topology.
Theoretically, MR oscillations periodic in H can also occur when the field pushes the system beyond the quantum limit, where only a single Landau level is occupied [C. M. Wang et al., Phys. Rev. B 102, 041204 (2020)]. The inversion of the lowest Landau level gives rise to oscillations of the Fermi energy in extremely large magnetic fields. While the linear field dependence of the ring MR above 20 T seen for H ⊥ ring also supports this beyond-quantum-limit scenario, the researchers are not fully convinced, as the linear MR in the rings persists at relatively high temperatures. Given that it requires around 100 T to reach the last Landau level of the TaSe3 ribbons, further experimental investigation is needed on ring-shaped samples with various diameters to better understand the relationship between strain and the observed quantum effects.
This work was partially supported by the US Department of Energy under EPSCoR Grant No. DE-SC0012432, the U.S. National Science Foundation under Grant Nos. DMR-1504226, DMR-1157490, DMR-1644779, and the State of Florida. John Singleton acknowledges support from the DOE BES program “Science at 100 T,” which permitted the design and construction of much of the specialized equipment used in the high-field study.
Figure 1 (left): Scaled magnetoresistance (MR) data obtained from a ribbon sample shows a conventional H2 dependence.
Figure 2 (right): MR from ring-shaped samples deviates from conventional behavior and is~1/1,000 of that in the ribbons.
Credit: J. Xing, J. Blawat, S. Speer, A. I. Us Saleheen, J. Singleton, R. Jin, “Manipulation of the Magnetoresistance by Strain in
Topological TaSe3”, Adv. Quant. Tech. 5, 2200094 (2022)
