Giant magnetochiral anisotropy from quantum-confined surface states of topological insulator nanowires

Abstract

Wireless technology relies on the conversion of alternating electromagnetic fields into direct currents, a process known as rectification. Although rectifiers are normally based on semiconductor diodes, quantum mechanical non-reciprocal transport effects that enable a highly controllable rectification were recently discovered1–9. One such effect is magnetochiral anisotropy (MCA)6–9, in which the resistance of a material or a device depends on both the direction of the current flow and an applied magnetic field. However, the size of rectification possible due to MCA is usually extremely small because MCA relies on inversion symmetry breaking that leads to the manifestation of spin–orbit coupling, which is a relativistic effect6–8. In typical materials, the rectification coefficient γ due to MCA is usually ∣γ∣ ≲ 1 A−1 T−1 (refs. 8–12) and the maximum values reported so far are ∣γ∣ ≈ 100 A−1 T−1 in carbon nanotubes13 and ZrTe5 (ref. 14). Here, to overcome this limitation, we artificially break the inversion symmetry via an applied gate voltage in thin topological insulator (TI) nanowire heterostructures and theoretically predict that such a symmetry breaking can lead to a giant MCA effect. Our prediction is confirmed via experiments on thin bulk-insulating (Bi1−xSbx)2Te3 (BST) TI nanowires, in which we observe an MCA consistent with theory and ∣γ∣ ≈ 100,000 A−1 T−1, a very large MCA rectification coefficient in a normal conductor.

Superconducting diode effect due to magnetochiral anisotropy in topological insulator and Rashba nanowires

“…Rectification is the phenomenon by which the resistance, R, due to a current, I, depends on the direction of current flow, in other words R(+I) = R(−I). When both inversion and time reversal symmetry are broken in a material or device, an effect known as magnetochiral anisotropy (MCA) can occur [1][2][3][4][5][6][7][8][9]. In the normal state of diffusive systems, for example, MCA of the energy spectrum can result in a correction to Ohm's law at second order in current, such that V = IR 0 (1 + γBI), with V the applied voltage, R 0 the reciprocal resistance, B the magnetic field strength, and γ the MCA rectification coefficient [4].…”

“…In the normal state of diffusive systems, for example, MCA of the energy spectrum can result in a correction to Ohm's law at second order in current, such that V = IR 0 (1 + γBI), with V the applied voltage, R 0 the reciprocal resistance, B the magnetic field strength, and γ the MCA rectification coefficient [4]. Recently, a giant MCA rectification with very large γ was observed in the normal state of topological insulator (TI) nanowires [9].…”

“…SC diode effect due to magnetochiral anisotropy in nanowire devices: When the subbands of a nanowire possess a finite spin polarization due to broken inversion symmetry, a magnetic field applied along the spin-polarization direction results in a relative Zeeman shift of the subbands. The magnetochiral anisotropy (MCA) of the energy spectrum can lead to MCA rectification in the diffusive normal state [9]. On the other hand, if a nanowire is brought into proximity with a superconductor, the MCA of the energy spectrum results in a critical supercurrent in the proximitized nanowire that is different depending on whether current flows to the left or right of the device, j + c = |j − c |, the SC diode effect.…”

“…In TI nanowires inversion symmetry can be broken by a non-uniform potential -e.g. due to gating or contact with a superconductor -and enables MBSs to appear in a large region of phase space [9,27,42].…”

“…In contrast to Rashba nanowires, the metallization effect due to bringing a TI nanowire into proximity with a superconductor can either enhance or reduce the size of subband-splitting that is key to achieving MBSs [42]. However, although the giant MCA rectification recently observed in TI nanowires is evidence of a large subband-splitting in the normal state [9], strongly broken inversion symmetry and the presence of a topological phase transition in superconductor-TI nanowire devices also remain undetermined experimentally.…”