Graphene Exhibits Giant Magnetoresistance Due to Fluctuation-induced Conductivity Changes in Charge-neutral Devices

Fluctuations in electron density within graphene, a single-layer sheet of carbon atoms, generate a surprising and significant effect, as demonstrated by research led by A. Levchenko, E. Kirkinis, and A. V. Andreev. The team reveals that these naturally occurring fluctuations, combined with external electric and magnetic fields, create a hydrodynamic flow which dramatically alters the material’s conductivity. This phenomenon results in giant magnetoresistance, meaning the material’s electrical resistance changes substantially with even small magnetic fields, and the effect grows stronger as the graphene sample increases in size. The research establishes a new understanding of conductivity in charge-neutral devices and opens avenues for developing highly sensitive magnetic field sensors and novel electronic components.

The research demonstrates that advection of charge resulting from this flow produces a fluctuation contribution to the macroscopic conductivity of the system, and develops a quantitative theory to describe it. At zero magnetic field, this contribution diverges logarithmically with system size, but becomes rapidly suppressed when even small magnetic fields are applied. This behaviour results in giant magnetoresistance within the system, a phenomenon previously recognised as significant in this field.

Fluid Dynamics of Electron Interactions

This research explores how electrons behave as a fluid in graphene, a single layer of carbon atoms with exceptional electronic properties. Instead of treating electrons as individual particles, scientists considered them as a collective fluid, particularly when electron-electron interactions are strong. This approach is crucial because it accurately describes the behaviour of electrons in graphene near charge neutrality, where the number of electrons and holes are nearly equal. Random fluctuations in electron density and velocity are inherent to the system and significantly affect its electrical properties.

The team investigated how these fluctuations influence conductivity, discovering that they can drive currents even in the absence of an applied electric field. The fluctuation-driven conductivity diverges as the system approaches charge neutrality, a counterintuitive result linked to the extended relaxation time of charge density fluctuations. The presence of disorder, such as impurities, limits conductivity, but this can be mitigated by placing graphene on a substrate that screens these effects. This research demonstrates that the traditional Drude model, which treats electrons as independent particles, fails to accurately describe transport in this regime, necessitating a hydrodynamic approach. Understanding these effects has implications for improving graphene-based electronic devices and reveals new physics related to fluctuation-driven transport. The findings reinforce the importance of hydrodynamic effects in low-dimensional electron systems and provide insights into unusual transport properties observed in graphene and other strongly correlated electron systems.

Hydrodynamic Fluctuations Enhance Graphene Conductivity

Scientists have discovered that electrical conductivity in charge-neutral graphene devices is significantly enhanced by hydrodynamic fluctuations, revealing a previously unconsidered mechanism for charge transport. The work demonstrates that fluctuations in electron density, driven by inherent thermal noise, induce a fluctuating hydrodynamic flow, ultimately contributing to the macroscopic conductivity of the system. Researchers developed a quantitative theory for this fluctuation-induced conductivity and found it diverges logarithmically with system size at zero magnetic field. Experiments reveal that this divergence is rapidly suppressed by even relatively small magnetic fields, resulting in giant magnetoresistance, a substantial change in electrical resistance in response to a magnetic field.

The team established that the fluctuation contribution to conductivity arises from the coupling between electric current and hydrodynamic velocity, a phenomenon not typically considered in analyses of charge-neutral electron liquids. Measurements confirm that the fluctuation-driven enhancement of conductivity is dominant across a broad range of magnetic fields, significantly influencing the overall electrical properties of the graphene. The research establishes a new understanding of charge transport in two-dimensional electron liquids and opens avenues for manipulating conductivity through the control of hydrodynamic fluctuations and magnetic fields. The findings have implications for the design and fabrication of graphene-based electronic devices and provide insights into the fundamental physics governing electron transport in these materials.

Logarithmic Conductivity From Electronic Fluctuations

This work establishes a theoretical framework for understanding how fundamental fluctuations in electronic systems impact macroscopic conductivity. Researchers demonstrate that inherent thermal noise induces fluctuations in electron density and, consequently, hydrodynamic flow. This flow advects charge, contributing a fluctuating component to the overall conductivity of the system. The team developed a quantitative theory to describe this effect, revealing that the fluctuation-induced conductivity grows logarithmically with system size and is highly sensitive to the presence of magnetic fields. The findings explain a significant magnetoresistance effect, where conductivity changes dramatically with applied magnetic field, and predict a characteristic field strength defining the onset of this behaviour.

Notably, the theory predicts a divergence in conductivity when intrinsic conductivity approaches zero, a seemingly counterintuitive result explained by the interplay between fluctuating charge density and relaxation times. This research extends classical hydrodynamic fluctuation theory to encompass systems with intrinsic conductivity, offering new insights into electronic transport phenomena. Future work could explore the impact of imperfections on the observed effects. This theoretical advancement provides a foundation for further investigation into fluctuation-driven phenomena in two-dimensional electron systems and may inform the development of novel electronic devices.