A research team, led by Daniil Gorbachev and Na Xin at the University of Manchester and working with colleagues including Kenji Watanabe and Takashi Taniguchi, demonstrated a major improvement in graphene’s electronic properties by strategically positioning graphite gates in extremely close proximity to the material.
This innovative approach, which involves placing the gates just one nanometer away, dramatically reduces charge variations and potential fluctuations, ultimately boosting graphene’s mobility to exceed even the highest-quality semiconductor heterostructures. The resulting material exhibits exceptional performance, enabling the observation of subtle quantum phenomena previously hidden by disorder and paving the way for a new era in two-dimensional materials research.
Achieving comparable electronic quality in van der Waals heterostructures, assembled from atomically thin materials, presents a significant challenge due to difficulties in controlling interface quality and minimizing disorder. Consequently, substantial improvements in material quality and fabrication techniques are essential for realizing high-performance devices and fully exploring fundamental physics in atomically thin systems.
Researchers have developed a meticulous process for creating high-quality hBN-encapsulated graphene devices, incorporating proximity gates to study quantum Hall effects, specifically helical edge states, at lower magnetic fields. They successfully demonstrated these helical edge states in proximity-screened graphene at significantly reduced magnetic fields thanks to effective suppression of electron interactions via proximity gating. This achievement relies on careful material preparation, assembly techniques, and rigorous quality control. The process begins with mechanically exfoliated graphene and hexagonal boron nitride (hBN).
Two primary assembly methods are employed, utilizing PDMS/PPC stamps or silicon nitride cantilevers. Material quality is verified through Raman spectroscopy and atomic force microscopy (AFM), confirming layer identity, thickness, and a smooth, bubble-free surface. Device fabrication involves extensive use of electron-beam lithography (EBL) for defining gate regions and contacts, followed by metal deposition using Cr/Au. Reactive-ion etching (CHF 3 /O 2 ) defines graphene edges and creates Hall bar geometries, while one-dimensional edge contacts are formed through etching and metal deposition. Measurements are conducted at 2K, with emphasis on minimizing bubbles and wrinkles through slow transfer speeds.
Large, high-quality graphite flakes serve as substrates and ensure reliable contact formation. PPC residue is removed using acetone, and devices undergo vacuum annealing at 250°C for cleaning. Supplementary information includes optical and AFM images of fabricated heterostructures, demonstrating the large area, high quality, and bubble-free nature of the devices. This detailed documentation allows other researchers to reproduce the fabrication process, highlighting the critical role of meticulous techniques in achieving high-quality devices for fundamental physics research. The successful demonstration of helical edge states at lower magnetic fields opens new possibilities for studying exotic quantum phenomena and advancing quantum Hall studies.
The researchers have achieved a significant leap in graphene’s electronic quality, surpassing even the most refined semiconductor materials. By placing graphene in extremely close proximity to graphite gates, separated by just one nanometer, they have dramatically reduced unwanted variations in charge distribution within the material, lowering charge inhomogeneity by a factor of one hundred. This improvement directly translates into enhanced performance characteristics, with transport mobilities reaching 10 8 cm 2 /Vs. This level of quality allows for the observation of subtle quantum phenomena, such as Shubnikov-de Haas oscillations and quantum Hall plateaus, at remarkably low magnetic fields, down to 1 and 5 millitesla, confirming the exceptional purity and uniformity of the graphene.
Notably, the researchers found that while the proximity screening suppresses interactions between electrons, the fundamental physics governing many-body phenomena at very small scales, less than 10 nanometers, remains robust. Energy gaps associated with fractional quantum Hall states are reduced by only a factor of 3-5, demonstrating that complex quantum behavior is not lost despite the altered electronic environment. This breakthrough offers a reliable pathway to creating graphene, and potentially other two-dimensional materials, with unprecedented electronic quality, unlocking the potential for exploring new physics previously obscured by imperfections and paving the way for advanced devices. The ability to observe quantum effects at such low magnetic fields is particularly promising for developing sensitive sensors and exploring fundamental quantum phenomena.
This study demonstrates a significant improvement in graphene’s electronic quality through the use of nearby graphite gates, achieving a reduction in charge inhomogeneity by up to two orders of magnitude. This enhancement results in unprecedented charge homogeneity, with fluctuations of less than 10 Kelvin, and enables the observation of the fractional quantum Hall effect in remarkably low magnetic fields, as low as a few milliTesla. The resulting high-quality graphene surpasses the performance of even the most refined semiconductor heterostructures in terms of electron mobility. While this proximity screening suppresses some many-body interactions, the research indicates that interactions occurring over very short distances, less than 10 nanometers, remain strong.
This suggests the technique is particularly valuable for investigating short-range correlated states and many-body physics in high magnetic fields. The authors anticipate this approach will be beneficial for studying graphene multilayers and superlattices, and may also be applicable to other two-dimensional semiconductors as quality continues to improve. Acknowledging a trade-off, the method can also be used to intentionally suppress many-body interactions while simultaneously achieving superior electronic quality.