Observation of a quantum phase transition in a molecular scale transistor

As physical objects become smaller, quantum effects become dominant and easier to measure. Thus, nanometer size quantum objects (in this work a C60 molecule) are propitious for observation of the new quantum phenomena associated with spin electronics. Such objects act as artificial atoms and can be controlled by external parameters such as magnetic field, electric potential or light. With its electrons confined at the nanometre scale, a quantum object’s charging energies can exceed one Kelvin, allowing study of quantum phenomena like Coulomb blockade and the Kondo effect over a large range of parameters at cryogenic temperatures.

When the wave function that describes a large number of particles obeying the laws of quantum mechanics can be changed continuously, a transition can be induced between two ground states with distinct symmetry. This is a purely quantum critical phenomenon which reveals new physics, as yet hardly explored. The quantum phase transition is fundamentally different from classical phase transitions like the liquid-gas transition or the appearance of ferromagnetism where thermal fluctuations play a major role. An external control parameter, for example a magnetic field or an electrostatic coupling, is needed to switch the system into a state where the disorder is induced by zero-point quantum fluctuations.

In reality, observations of this kind of phenomenon are always made at a low, non-zero temperature, so one can observe only the remanence of the zero temperature singular point. Studied usually in macroscopic size objects, the quantum phase transition can be achieved at the nanometre scale by combining the quantum states of a magnetic molecule with the electronic states in the connection circuit.

AFM image of the molecular transistor

Fig. 1 : Atomic Force Microscope image of the molecular transistor. The inset represents a x100 zoom view of the inserted C60 molecule.

We have shown that a molecular transistor based on fullerene (C60) can be switched electrostatically between two different spin states, corresponding to distinct resistance properties of the nanocircuit. In this case, the magnetized state is associated with an entanglement of the spin of the molecule with the conduction electron spins. An electrostatic coupling induces the transition from a spin zero state to a spin 1/2 state. The quantum critical point is then characterized by a spin 1/2 that is not entangled with the conduction electrons. This kind of physics is of great current interest and, in addition, our experimental results offer new possibilities for controlling and manipulating the states in molecular spintronics.

Differential conductance map

Fig. 2 : Colour map of differential conductance dI/dV as a function of applied voltage Vb and grid voltage Vg. The grid voltage induces a clear transition between two ground states with different symmetry : a spin singlet state and an underscreened triplet state.

Further reading :

"Quantum phase transition in a single-molecule quantum dot", N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer and F. Balestro, Nature 453, 633 (2008)

Dans la même rubrique

  • - Observation of a quantum phase transition in a molecular scale transistor
  • - The nanoSQUID

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