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Register a new userTunable Kondo effect in a single donor atom
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The Kondo effect has been observed in a single gate-tunable atom. The measurement device consists of a single As dopant incorporated in a Silicon nanostructure. The atomic orbitals of the dopant are tunable by the gate electric field. When they are tuned such that the ground state of the atomic system becomes a (nearly) degenerate superposition of two of the Silicon valleys, an exotic and hitherto unobserved valley Kondo effect appears. Together with the regular spin Kondo, the tunable valley Kondo effect allows for reversible electrical control over the symmetry of the Kondo ground state from an SU(2)- to an SU(4) -configuration.
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Presented in this paper is a proof-of-concept for a new approach to single electron pumping based on a Single Atom Transistor (SAT). By charge pumping electrons through an isolated dopant atom in silicon, precise currents of up to 160 pA at 1 GHz are generated, even if operating at 4.2 K, with no magnetic field applied, and only when one barrier is addressed by sinusoidal voltage cycles.
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.
The tip of a low-temperature scanning tunneling microscope is brought into contact with individual Kondo impurities (cobalt atoms) adsorbed on a Cu(100) surface. A smooth transition from the tunneling regime to a point contact with a conductance of $Gapproxtext{G}_0$ occurs. Spectroscopy in the contact regime, {it i. e.}, at currents in a $mutext{A}$ range was achieved. A modified line shape is observed indicating a significant change of the Kondo temperature $T_{text{K}}$ at contact. Model calculations indicate that the proximity of the tip shifts the cobalt $d$-band and thus affects $T_{text{K}}$.
Semiconductor nano-devices have been scaled to the level that transport can be dominated by a single dopant atom. In the strong coupling case a Kondo effect is observed when one electron is bound to the atom. Here, we report on the spin as well as orbital Kondo ground state. We experimentally as well than theoretically show how we can tune a symmetry transition from a SU(4) ground state, a many body state that forms a spin as well as orbital singlet by virtual exchange with the leads, to a pure SU(2) orbital ground state, as a function of magnetic field. The small size and the s-like orbital symmetry of the ground state of the dopant, make it a model system in which the magnetic field only couples to the spin degree of freedom and allows for observation of this SU(4) to SU(2) transition.
We propose a mechanism to directly measure the chiral anomaly in disorder Weyl semimetals (WSMs) by the Kondo effect. We find that in a magnetic and electric field driven WSM, the locations of the Kondo peaks can be modulated by the chiral chemical potential, which is proportional to $mathbf{E}cdot mathbf{B}$. The Kondo peaks come from spin fluctuations within the impurities, which apart from the temperature, relate closely to the hosts Fermi level. In WSMs, the chiral-anomaly-induced chirality population imbalance will shift the local Fermi levels of the paired Weyl valleys toward opposite directions in energy, and then affects the Kondo effect. Consequently, the Kondo effect can be tunable by an external electric field via the chiral chemical potential. This is unique to the chiral anomaly. Base on this, we argue that the electrically tunable Kondo effect can serve as a direct measurement of the chiral anomaly in WSMs. The Kondo peaks are robust against the disorder effect and therefore, the signal of the chiral anomaly survives for a relatively weak magnetic field.
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