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Register a new userModeling of gate controlled Kondo effect at carbon point-defects in graphene
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We study the magnetic properties in the vicinity of a single carbon defect in a monolayer of graphene. We include the unbound $sigma$ orbital and the vacancy induced bound $pi$ state in an effective two-orbital single impurity model. The local magnetic moments are stabilized by the Coulomb interaction as well as a significant ferromagnetic Hunds rule coupling between the orbitals predicted by a density functional theory calculation. A hybridization between the orbitals and the Dirac fermions is generated by the curvature of the graphene sheet in the vicinity of the vacancy. We present results for the local spectral function calculated using Wilsons numerical renormalization group approach for a realistic graphene band structure and find three different regimes depending on the filling, the controlling chemical potential, and the hybridization strength. These different regions are characterized by different magnetic properties. The calculated spectral functions qualitatively agree with recent scanning tunneling spectra on graphene vacancies.
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Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer1 and a large number of electronic phenomena have been demonstrated in graphene, from the fractional2, 3 quantum Hall effect to superconductivity4. However, the coupling of conduction electrons to local magnetic moments5, 6, a central problem of condensed matter physics, has not been realized in graphene, and, given carbons lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported7-10, with explanations ranging from lattice defects11 to edge structures12, 13 to negative curvature regions of the graphene sheet14. Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG) induced by proton irradiation9 or native to grain boundaries7, can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene15 are local moments which interact strongly with the conduction electrons through the Kondo effect6, 16-18 providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature TK is tunable with carrier density from 30-90 K; the high TK is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material18. The results indicate that defect engineering in graphene could be used to generate and control carrier-mediated magnetism, and realize all-carbon spintronic devices. Furthermore, graphene should be an ideal system in which to probe Kondo physics in a widely tunable electron system.
Ballistic semiconductor structures have allowed the realization of optics-like phenomena in electronics, including magnetic focusing and lensing. An extension that appears unique to graphene is to use both n and p carrier types to create electronic analogs of optical devices having both positive and negative indices of refraction. Here, we use gate-controlled density with both p and n carrier types to demonstrate the analog of the fiber-optic guiding in graphene. Two basic effects are investigated: (1) bipolar p-n junction guiding, based on the principle of angle-selective transmission though the graphene p-n interface, and (2) unipolar fiber-optic guiding, using total internal reflection controlled by carrier density. Modulation of guiding efficiency through gating is demonstrated and compared to numerical simulations, which indicates that interface roughness limits guiding performance, with few-nanometer effective roughness extracted. The development of p-n and fiber-optic guiding in graphene may lead to electrically reconfigurable wiring in high-mobility devices.
Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrows electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in free-standing graphene layers.
While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2-D material has not been reported before. Here, using electron irradiation we create an sp2-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings. We show how the transformation occurs step-by-step by nucleation and growth of low-energy multi-vacancy structures constructed of rotated hexagons and other polygons. Our observations, along with first-principles calculations, provide new insights to the bonding behavior of carbon and dynamics of defects in graphene. The created domains possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.
We demonstrate a flip-chip device for performing low-temperature acoustoelectric measurements on exfoliated two-dimensional materials. With this device, we study gate-tunable acoustoelectric transport in an exfoliated monolayer graphene device, measuring the voltage created as high-frequency surface acoustic waves dynamically drive the graphene charge carriers, the density of which we simultaneously control with a silicon back-gate. We demonstrate ambipolar dependence of the acoustoelectric signal, as expected from the sign of the graphene charge carriers. We observe a marked reduction in the magnitude of the acoustoelectric signal over a well-defined range of density in the vicinity of charge neutrality, which we attribute to a spatially heterogeneous charge-disorder landscape not directly revealed by conventional transport measurements.
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