Nonreciprocal transport refers to situations where electrical resistance or voltage changes depending on the direction of the applied current. A classic example is a semiconductor diode, which allows current to pass in only one direction. Recently, some quantum materials have been found to exhibit nonreciprocal charge transport, drawing significant attention due to their potential applications in technologies like quantum rectification and photodetection. To date, nonreciprocal charge transport has been observed in the longitudinal direction (i.e., the direction of current flow), with the nonreciprocal resistance constituting only a small percentage of the total ohmic resistance.

In this research, Zhiqiang Mao’s group at Penn State, in collaboration with the Liang Fu group at MIT, discovered a remarkable phenomenon known as the colossal nonreciprocal Hall effect (NRHE), which involves transverse nonreciprocal charge transport. This effect was observed in microscale Hall devices fabricated from platinum (Pt) deposited via focused ion beam (FIB) on silicon substrates. In these experiments, a direct current (DC), denoted as I_x, was applied along the x-axis, while the Hall voltage, V_y, was measured along the y-axis at zero magnetic field (Fig. 1). This stands in contrast to the conventional Hall effect, which requires the presence of a magnetic field. Interestingly, the Hall voltage scales quadratically with the applied current, deviating from the linear relationship described by Ohm's law. Even more noteworthy, this phenomenon occurs at room temperature, suggesting its potential for practical technological applications.

The Mao group’s detailed studies suggest that the nonreciprocal Hall effect stems from geometrically asymmetric scattering caused by textured Pt nanoparticles within the focused ion beam-deposited Pt (FIBD-Pt) structure—an amorphous mixture of Pt nanoparticles dispersed in gallium (Ga) and carbon (C). The irregular shape of the Pt nanoparticles causes them to act as scattering centers that lack inversion symmetry. For example, wedge-shaped Pt nanoparticles scatter charge carriers toward the +y direction regardless of whether they flow from the +x or -x direction (see Fig. 1). This asymmetric scattering results in a nonreciprocal Hall voltage response along the y-axis, leading to the quadratic voltage-current dependence (V_y ∝ I_x²). A key characteristic of the NRHE is its ability to produce such a pronounced nonlinear Hall response without relying on magnetic fields. Additionally, the FIBD-Pt can also rectify an alternating current to DC Hall voltage. 

Another fascinating aspect of the colossal NRHE in FIBD-Pt is its ability to propagate to adjacent conductors, such as gold (Au) and niobium phosphide (NbP), through transverse nonlinear Hall current injection. Even though these materials are nonmagnetic, they exhibit a pronounced nonlinear anomalous Hall effect (AHE), with anomalous Hall angles that exceed the highest values recorded in magnetic conductors at room temperature. This discovery introduces a new paradigm for achieving large anomalous Hall angles without needing magnetic materials. Traditionally, research on materials with significant anomalous Hall effects has focused on magnetic conductors. However, the ability of NRHE to induce large anomalous Hall angles in nonmagnetic materials opens up new possibilities for designing materials with exceptional electronic properties.

The discovery of the NRHE and its ability to function at room temperature has profound implications for future technologies, particularly in optoelectronics, terahertz (THz) communication, and energy harvesting. The capability of FIBD-Pt structures to rectify alternating current into DC voltage at room temperature presents exciting opportunities for wireless microwave/terahertz detection and energy harvesting applications. In collaboration with Zhijian Xie at North Carolina Agricultural and Technical State University, Mao's group also discovered that the colossal NRHE in FIBD-Pt enables a new functionality: broadband electronic frequency mixing. The Hall voltage generated by NRHE produces a rich frequency spectrum, including second-harmonic generation, sum and difference frequency generation, and multiple wave mixing components ranging from third to eleventh order. This broadband frequency mixing capability is highly attractive for THz communications, where efficient and sensitive frequency manipulation is crucial. Additionally, the ability to transmit NRHE to nonmagnetic materials and induce large anomalous Hall angles offers a promising approach for designing materials with enhanced electronic properties. 

For practical applications of the nonreciprocal Hall effect, it may be essential to produce textured asymmetric nanoparticles using alternative methods and integrate them into other conductive metal layers. This approach would provide a more predictable and controllable means of harnessing transverse nonreciprocal transport properties. The feasibility of creating such controlled nanopatterns has already been demonstrated in two-dimensional materials like MoS₂, where these patterns were achieved through chemical vapor deposition. Additionally, these arrays can be directly fabricated using electron beam lithography followed by metal deposition.

In conclusion, the discovery of the colossal nonreciprocal Hall effect in FIBD-Pt thin films paves the way for significant advancements in both fundamental physics and applied technologies. Its unique properties, including room-temperature operation and broad frequency response, make it a promising candidate for future applications in optoelectronics, THz communication, and energy harvesting, potentially revolutionizing these fields.