Electron transport across electrically switchable magnetic molecules

We investigate the electron-transport properties of a model magnetic molecule formed by two magnetic centers whose exchange coupling can be altered with a longitudinal electric field. In general, we find a negative differential conductance at low temperatures originating from the different scattering amplitudes of the singlet and triplet states. More interestingly, when the molecule is strongly coupled to the leads and the potential drop at the magnetic centers is only weakly dependent on the magnetic configuration, we find that there is a critical voltage $V_{\text{C}}$ at which the current becomes independent of the temperature. This corresponds to a peak in the low-temperature current noise. In such limit, we demonstrate that the quadratic current fluctuations are proportional to the product between the conductance fluctuations and the temperature.

Figure 1

(Color online) The model system investigated: a dimer of magnetic atoms, dark gray (red online), carrying, respectively, spin $S_1$ and $S_2$, is attached to two 1D nonmagnetic electrodes, light gray (light blue online). The Hamiltonian for the dimer is $H_{\text{DIM}}$. The scattering region (dashed box) includes the dimer and six atoms of the electrodes and it is described by the Hamiltonian matrix $H_{\text{S}}$.

Figure 2

Microscopic transport quantities. The microscopic current, $i$, as a function of the angle, $\theta$, is shown in panels (a) and (c) for different voltages $V$. Panels (b) and (d) show the internal potential drop, $v_{\text{d}}$, as a function of the external bias and for different angles. Panels (a) and (b) are for ${\gamma }_{\text{C}}=1/4{\gamma }_{\text{L}}$ while (c) and (d) are for ${\gamma }_{\text{C}}=1/2{\gamma }_{\text{L}}$.

Figure 3

Macroscopic transport quantities. The $I\text{−}V$ curves are presented in panels (a) and (c) while panels (b) and (d) display the current quadratic fluctuations $\Delta I=\sqrt{⟨i^2⟩-I^2}$ as a function of bias $V$. Panels (a) and (b) are for ${\gamma }_{\text{C}}=1/4{\gamma }_{\text{L}}$ while (c) and (d) are for ${\gamma }_{\text{C}}=1/2{\gamma }_{\text{L}}$. Note that in (a) there is a critical voltage $V_{\text{C}}$ at which the current becomes independent of the temperature.

Figure 4

Differential conductance calculated as (a) the derivative of the macroscopic current, $I$, with respect to the bias or as the (b) thermal average, $G$, of the microscopic conductance. In panel (c) we show the product of $\left(G-\partial I/\partial V\right)$ with the temperature $T$ as a function of bias. Results are presented for ${\gamma }_{\text{C}}=1/4{\gamma }_{\text{L}}$. Note that $\left[G-\partial I/\partial V\right]T$ is proportional to the square of the current quadratic fluctuations $\Delta I^2$ [panel (d)] of Fig. 3b.