Nonreciprocal charge transport in polar Dirac metals with tunable spin-valley coupling

Nonreciprocal charge transport in solids, where resistance is different between rightward and leftward currents, is a key function of rectifying devices in modern electronics, as exemplified by <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>p</a:mi><a:mtext>−</a:mtext><a:mi>n</a:mi></a:mrow></a:math> semiconductor junctions. Recently, this was also demonstrated in noncentrosymmetric materials in magnetic fields since their band structure exhibits spin polarization coupled to the position of momentum space due to antisymmetric spin-orbit coupling. To enhance the magnitude of the nonreciprocal effect, it is essential to tune such spin-momentum coupling, which has been hampered in conventional materials owing to the difficulty in controlling the broken inversion symmetry built into the lattice and interfacial structures. Here, we report large nonreciprocal resistivity in the layered polar metal <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>BaMn</b:mi><b:msub><b:mi>X</b:mi><b:mn>2</b:mn></b:msub></b:mrow></b:math> (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>X</c:mi><c:mo>=</c:mo><c:mi>Sb</c:mi></c:mrow></c:math>, Bi), where the spin-polarized Dirac dispersion depends on the in-plane polarization tunable by chemical substitution of the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>X</d:mi></d:math> site. For <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>X</e:mi><e:mo>=</e:mo><e:mi>Sb</e:mi></e:mrow></e:math> with a pair of single-type valleys, the nonreciprocal resistivity increases monotonically with decreasing temperature, while for <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>X</f:mi><f:mo>=</f:mo><f:mi>Bi</f:mi></f:mrow></f:math> with multiple types of valleys it is reduced by about an order of magnitude and exhibits a peak at a low temperature. Theoretical calculations indicate that the nonreciprocal resistivity is sensitive not only to the spin-momentum (spin-valley) coupling but also to the Fermi energy and the Dirac dispersion. The observed significant variation of nonreciprocal transport in the same series of materials might be of great use in the design of junction-free rectifying devices and circuits.