Tunable reciprocal and nonreciprocal contributions to 1D Coulomb Drag

Coulomb drag is a powerful tool to study interactions in coupled low-dimensional systems. Historically, Coulomb drag has been attributed to a frictional force arising from momentum transfer whose direction is dictated by the current flow. In the absence of electron-electron correlations, treating the Coulomb drag circuit as a rectifier of noise fluctuations yields similar conclusions about the reciprocal nature of Coulomb drag. In contrast, recent findings in one-dimensional systems have identified a nonreciprocal contribution to Coulomb drag that is independent of the current flow direction. In this work, we present Coulomb drag measurements between vertically coupled GaAs/AlGaAs quantum wires separated vertically by a hard barrier only 15 nm wide, where both reciprocal and nonreciprocal contributions to the drag signal are observed simultaneously, and whose relative magnitudes are temperature and gate tunable. Our study opens up the possibility of studying the physical mechanisms behind the onset of both Coulomb drag contributions simultaneously in a single device, ultimately leading to a better understanding of Luttinger liquids in multi-channel wires and paving the way for the creation of energy harvesting devices.

💡 Research Summary

This paper reports groundbreaking experimental work on Coulomb drag in one-dimensional (1D) quantum wires, presenting the first simultaneous observation and systematic control of both reciprocal and nonreciprocal contributions to the drag signal within a single device.

Coulomb drag is a transport phenomenon where a current driven through an active conductor induces a voltage in a nearby passive conductor via Coulomb interactions. The conventional understanding, based on momentum transfer (MT) models, describes drag as a frictional force leading to a reciprocal effect: reversing the drive current reverses the sign of the drag voltage. In contrast, recent theories based on current/charge fluctuation rectification (CR) models predict that in mesoscopic systems with broken symmetries (e.g., due to disorder), a nonreciprocal drag component can arise, whose sign is fixed independent of the drive current direction.

The authors fabricated vertically coupled quantum wire devices from a GaAs/AlGaAs heterostructure. The two quantum wires reside in separate quantum wells, vertically separated by only a 15 nm thick AlGaAs barrier, resulting in an exceptionally small interwire distance of 33 nm. Independent gates control the electron density in each wire, confining interlayer interaction to the region where the wires overlap. Experiments were conducted on two such vertical devices and, for comparison, on a laterally coupled device with a much larger effective separation (≥250 nm).

The core experimental methodology involves applying a small AC current to the drive wire and measuring the induced AC voltage in the drag wire. By systematically reversing the direction of the drive current, the total drag signal is decomposed into a symmetric component (V_S_drag, unchanged upon current reversal, representing the nonreciprocal contribution) and an antisymmetric component (V_AS_drag, sign-flipping upon current reversal, representing the reciprocal contribution).

The key findings are multifaceted:

1. Coexistence: Both reciprocal and nonreciprocal contributions to the Coulomb drag signal are unambiguously observed simultaneously in the vertically coupled devices.
2. Dual Tunability: The relative strength of these two contributions is tunable by two independent parameters: gate voltage (controlling electron density) and temperature. At very low base temperature (below 15 mK), the nonreciprocal (symmetric) component dominates across most gate voltage ranges. Strikingly, upon increasing the temperature to approximately 800 mK, the reciprocal (antisymmetric) component becomes the dominant contribution.
3. Structural Dependence: This pronounced tunability is a strong function of interwire coupling. It is highly evident in the vertically coupled devices with minimal separation but is significantly weaker in the laterally coupled device with larger separation, underscoring the critical role of interaction strength.
4. Nonlinear Dynamics: The drag signal exhibits nonlinear and non-monotonic behavior as a function of drive current for both components, well-described by a third-order polynomial fit. This nonlinearity is a hallmark prediction of the fluctuation rectification models.
5. Complex Temperature Dependence: The temperature evolution of the drag signal reveals two distinct regimes. In the low-temperature regime (below ~1.5 K), both components show gate-dependent sign changes. The sign changes in the nonreciprocal component often correlate with the onset of 1D subbands in the drag wire. In the high-temperature regime, the drag signal becomes strictly positive, increases with temperature, and is dominated by the reciprocal component.

This work provides a unique experimental platform to directly contrast and interrogate the two primary theoretical paradigms for 1D Coulomb drag: the momentum transfer framework and the fluctuation rectification framework. It offers crucial insights into how disorder-induced symmetry breaking influences quantum transport in strongly correlated 1D systems, described by Tomonaga-Luttinger liquid theory. Ultimately, the ability to control nonreciprocal drag effects paves the way for novel principles in designing energy harvesting and rectification devices.