Mirage is an interesting phenomenon in nature. Quantum mirage is the quantum version of a mirage, which describes a fascinating effect inside an elliptical quantum corral (EQC). When an atom is placed at one focal point of the EQC, a mirage of the electronic properties of that atom appears at the other focus. Such mirages exhibit potential for remotely probing atoms or molecules with minimal perturbation and enable transport of information at the atomic scale. The Kondo effect-based quantum mirages appear only near the Fermi energy [ Nature 403, 512-515 (2000) ]. This strongly limits the exploration of its mechanism and potential application. Our group found a Kondo-free quantum mirage that can operate in a wide energy range beyond the Fermi energy. Moreover, the Kondo-free mirages are exploited to construct basic logic operations, such as NOT, FANOUT and OR gates.
Figure 1 shows the concept of a NOT gate. The concept is demonstrated utilizing the two foci of an EQC as input/output terminals. According to the Kondo-free mirage, the presence of the adatom at one focus may trigger a high or low difference of dI/dV (scanning tunneling spectroscopy or differential conductance) intensity at the other focus. Here when an Fe adatom is placed at the left focal position (Fig. 1a), the dI/dV map at a bias voltage of 39 mV shows a very low intensity at the other focal position (Fig.1b). When the Fe adatom is removed from the focal position (Fig. 1c), the dI/dV map changes to a rather high intensity (Fig. 1d). Therefore, the presence/absence of an adatom at one focus which is used as the input of “1”/“0” gives an low/high dI/dV intensity as output of “0”/“1”, which satisfies the function of a NOT gate.
Figure 1 NOT gate. (a) Topography for an EQC with an Fe adatom placed at the left focus. (b) The corresponding dI/dV map at the bias voltage of +39 mV for the EQC in a. (c) Topography of the EQC without the Fe adatom at the left focus. (d) The corresponding dI/dV map with the same bias voltage of +39 mV for the empty EQC in panel c. Dashed circles mark the focal positions, for which black means 1 and white means 0.
The FANOUT gate is constructed utilizing a special geometry by combining two EQCs with one joint focus to form a confocal EQC (Fig. 2). The concept of input and outputs are the same as aforementioned, i.e., the presence/absence of an adatom as input “1”/“0” (at joint focus A) and high/low dI/dV intensity as outputs “1”/“0” (at focal positions B and C). The dI/dV map obtained from input “0” (Fig. 2a) at 34 mV shows low contrast at both foci B and C (Fig. 2b), corresponding to outputs “0”. When an Fe adatom is placed at the joint focus A (Fig. 2c) as input “1”, the corresponding dI/dV map (Fig. 2d) show high intensity at both focal positions as outputs “1”. The relation between input and outputs satisfies the function of a FANOUT gate. In addition, an OR logic gate can also be constructed when swapped the input and outputs.
Figure 2 FANOUT gate. (a) Topography of a confocal EQC. (b) The corresponding dI/dV map at the bias voltage of +34 mV. (c) Topography of the same confocal EQC but with an additional Fe adatom placed at A. (d) The corresponding dI/dV map of c at the same bias voltage of +34 mV. Dashed circles mark the focal positions, for which black means 1 and white means 0.
The above work is published on Nature Communications entitled “Kondo-free mirages in elliptical quantum corrals”. And it was supported by the National Key R&D Program of China (Grant No. 2017YFA0303202 and No. 2018YFA0306004), the National Natural Science Foundation of China (Grants No. 11974165, No. 51971110, and No. 11734006), the China Postdoctoral Science Foundation (Grant No. 2019M651766), and the Natural Science Foundation of Jiangsu Province (Grant No. BK20190057).
A Chinese version of this report can be found here.