Probing Spin States of Magnetic Molecules with Kondo Resonance and Radio Frequency Wave Assisted Tunneling Spectroscopy

概要

Recently, interest is rising for the quantum information process, in which the spin is regarded as the most promising candidate for the 'qubit' which includes the electron spin or the nucleus spin of the lanthanide either as an atom or a metal complex. The precise control of the local magnetism attracts attention since it is the key underlying technology for the realization of advanced devices using the spin degree of freedom. One of the possible techniques is to position magnetic atoms/molecules as the adsorbate or intercalated species. However, positioning the metal impurity in a specified position of the device is technically difficult. The surface state of a superconductor or a normal metal can be perturbed by placing a single magnetic atom/molecule on it & the interaction between them can produce many bodies of ground state like Yu-Shiba-Rusinov (YSR) or Kondo effect respectively, unmasking a great potential for quantum technologies. In superconductors the interaction causes breaking of the Cooper pairs can weaken or dominate over the quasi-particle states, a Kondo singlet is formed if the screening energy is larger than the pairing energy of the Cooper pair beside the formation of the YSR state. Scanning Tunneling Microscope (STM) at low temperature (400mK) helps to probe those above-described states.

A double-decker complex, bis(phthalocyaninato)cerium (CePc2) can act like a single molecule’s magnet (SMMs), as the valence of the Ce atom fluctuates between 3+ and 4+. CePc2 is non-magnetic in the bulk and becomes magnetic in the monolayer film on Au (111). 1st layer lobe and center show kondo resonance while absence in 2nd layer. The mechanism was attributed to the change of the rotational angle (θ) between the upper and lower Pc ligands, which was driven by the steric repulsive force originated from the formation of the pseudo square lattice on Au (111). The θ changes from θ=45° of the bulk value into θ=0° for the film. GGA+U calculation indicates that the change from θ=45° to θ=0° causes the Ce-valence change from the non-magnetic 4+ state into the magnetic 3+ state.

TbPc2, another intriguing SMMs molecule, with two spin systems, one from the unpaired  orbital in the ligand Pc, and another from 4f of Tb atom which is still elusive. Our results show low energy bound state YSR, appeared close around Fermi level due to interaction to Cooper pair, both for Pc and center on superconductor 2H-NbSe2. We anticipate our results with the intragap feature can be contributed towards the utilization of SMM as the building blocks of the future spintronics devices as well as fascinating application to quantum computing.

Besides this SMM, Strain-induced magnetism, which attracts attention for the atomic-scale control of the magnetic moment. NbSe2, one of the compounds of Transition- metal Dichalcogenides (TMDs) are among the candidate materials showing superconducting (SC) property which is sensitive to the magnetic moment. Here we showed the appearance of the magnetic moment at the high strained position of the 1T (a polymorph of 2H-NbSe2) phase through the Yu-Shiba-Rusinov (YSR) state in the SC gap (interaction of magnetic moment to the Cooper pair) & this YSR states were detected at high curvature positions of the 1T island, whose spatial behavior cannot be explained by atomic magnetic impurities.

For the detection of different magnetic moments, we have also explored the development of a new type of spin detection system with a microscope capability by using the Super Conductor Photo Assisted Tunneling (SC-PAT). SC-PAT is a technique in which the photon helps the tunneling between two SC electrodes. The coupling of the superconducting (SC) state with the Radio Frequency (RF) wave attracts much attention. If we inject the RF wave into the SC tunneling gap, it has been discussed that it dissociates the Cooper pair of the SC state, creating the Bogoliubov quasiparticle. By injecting the RF of 1 GHz into the tunneling gap, we observed the splitting of the quasiparticle peak (QP) inner and outer direction as a function of RF power, while the inner part of both QPs makes a sharp peak at the Fermi level. The sharp peak at EF has been splitted again and alienate with further increasing of power in terms of mV. The possible reason here may be the multiphoton-assisted tunneling and the change of the strength of the injected RF. The photon number which gives the maximum tunneling intensity depends on the chemical environment of the quasi-particle (QP) state or another SC-related state with the photon, so this technique can be used as chemical analysis of different territories.