Researchers Demonstrate Order-selection Rules Governing Fragile “supernodes” and Scalable Thermoelectric Efficiency Gains

Quantum interference holds considerable promise for boosting the efficiency of thermoelectric materials, which convert heat into electricity, and researchers predict that complex interference patterns called “supernodes” could dramatically improve performance. Justin P Bergfield from Illinois State University, along with colleagues, investigates a critical question surrounding these supernodes: how susceptible are they to disruption from dephasing, a process that destroys the delicate quantum coherence needed for interference? The team discovers a fundamental rule governing this fragility, finding that supernodes initially suffer significant performance loss as dephasing increases, but eventually reach a point where further disruption has a surprisingly uniform effect, regardless of the interference pattern’s complexity. Through detailed calculations on molecular junctions, the researchers demonstrate that the way a material interacts with its surroundings determines whether coherence is lost through a reduction in interference order or by establishing a lower limit on performance, yielding valuable insights for designing robust and efficient thermoelectric devices.

Quantum Interference and Molecular Thermopower Measurements

This work explores the crucial role of quantum interference in understanding how charge and heat move through molecular junctions. Combining electrical conductance measurements with thermopower, which reveals how a material generates voltage in response to a temperature difference, provides a more complete picture of electron transport. Thermopower is particularly sensitive to the energy dependence of transmission, allowing scientists to disentangle the complex mechanisms of quantum interference and address challenges posed by environmental disturbances. The research focuses on understanding various types of quantum interference, such as Fano resonance and Aharonov-Bohm interference, and how these phenomena impact electron flow through molecules like benzene, biphenyl, and stilbene. Researchers investigated the crucial role of the metal-molecule contact, recognizing its significant influence on transport properties, and proposed statistical methods for identifying quantum interference from experimental data.

Interference and Dephasing in Molecular Junctions

Researchers investigated how quantum interference can enhance thermoelectric performance, focusing on structures called “supernodes” susceptible to disruption from environmental factors, a process known as dephasing. They developed a unique experimental approach using voltage-temperature probes to examine the relationship between interference and dephasing in molecular junctions, allowing precise control over the environment. The team established an “order-selection rule,” demonstrating that the effectiveness of interference is determined by the weakest link in either coherent pathways or pathways influenced by the probes. They explored how this rule applies with multiple probes inducing dephasing, attaching up to twelve probes to each molecular orbital.

Analysis of benzene- and biphenyl-based junctions revealed that the geometry of environmental coupling dictates whether interference is lost through a reduction in order or the creation of an “incoherent floor”, a baseline level of inefficiency. Scientists considered a configuration with distributed probes and weak coupling limits, eliminating the probes mathematically to calculate effective two-terminal transmission and identify an energy-independent term representing dephasing. The team demonstrated that the incoherent background created by multiple probes scales linearly with total probe strength, providing a quantifiable relationship between environmental disruption and thermoelectric performance. This analysis confirms that dephasing can only diminish interference or introduce an inefficient baseline, never enhance it.

Supernode Fragility Limits Thermoelectric Enhancement

Researchers demonstrate that quantum interference significantly enhances thermoelectric response, with higher-order “supernodes” predicted to improve both thermopower and efficiency. They investigated whether these supernodes are vulnerable to dephasing, establishing a crucial order-selection rule: the effective order of a node is determined by the weakest coherent or probe-assisted channel. Experiments reveal that supernodes are intrinsically fragile because their transmission decreases with increasing order. However, the data confirms that once an “incoherent floor” develops, a level of unavoidable disorder, the fractional reduction in thermopower, efficiency, and figure of merit becomes universal and independent of node order.

This finding suggests a limit to performance loss due to disorder, offering a pathway to robust device design. The research shows that the geometry of environmental coupling dictates whether coherence is lost through reducing node order or building up the incoherent floor. The team utilized voltage-temperature probes to model dephasing, enforcing local thermodynamic equilibrium, and found that supernodes exhibit weaker transmission with increasing order. Further analysis demonstrates that once an incoherent floor dominates, the fractional suppression of key performance metrics becomes order-independent, offering robustness. These results yield general scaling rules for thermoelectric response under dephasing, providing a framework for optimizing quantum-engineered thermoelectric devices.

Thermoelectric Performance Limits from Quantum Interference

This research investigates how quantum interference affects the efficiency of molecular thermoelectric devices, focusing on the robustness of high-performance “supernodes” to environmental disturbances. The team demonstrates that the effectiveness of these nodes is governed by the weakest link in the system, either coherent transmission or coupling to external probes, establishing a clear order-selection rule. While supernodes are inherently fragile and susceptible to suppression with increasing order, the study reveals that once a certain level of incoherence is reached, the reduction in thermoelectric performance becomes universal and independent of the node’s order. The researchers found that the geometry of how the device couples to its environment dictates whether coherence is lost through a reduction in node order or by establishing a baseline level of incoherence.

By employing voltage and temperature probes to model environmental effects, they developed scaling rules to predict thermoelectric response under dephasing conditions, providing a framework for designing more robust molecular devices. Future work could explore the impact of different environmental couplings and device geometries on thermoelectric performance, potentially leading to strategies for enhancing the stability and efficiency of these nanoscale energy converters. This research provides valuable insights for designing and optimizing quantum-engineered thermoelectric devices, paving the way for more efficient and sustainable energy technologies.