Ferromagnetic Spin Coupling as the Origin of 0.7 Anomaly in Quantum Point Contacts

We study one-dimensional itinerant electron models with ferromagnetic coupling to investigate the origin of the 0.7 anomaly in quantum point contacts. Linear conductance calculations from the quantum Monte Carlo technique for spin interactions of different spatial range suggest that $0.7(2e^2/h)$ anomaly results from a strong interaction of low-density conduction electrons to ferromagnetic fluctuations formed across the potential barrier. The conductance plateau appears due to the strong incoherent scattering at high temperature when the electron traversal time matches the time scale of dynamic ferromagnetic excitations.

Figure 1

Profile of the 1D chain with the interacting block (including seven sites in this figure). The gate voltage potential $V(x)$ acts as an adiabatic potential barrier through the QPC region. A bias with a ramp passing through the QPC is applied across the chain to compute the dc conductance in the linear response regime.

Figure 2

(a) dc conductance as a function of gate voltage $V_g$ at different values for $L_g$, $L_s$, $J_0$, and $J_1$ at $T=0.025t$. Conductance plateaus form at 0.6–0.7 times of $G_0=2e^2/h$. (b) The evolution of the plateau to $0.5G_0$ with the Zeeman magnetic field. (c) The gradual increase of conductance with decreasing temperature. (d) Purely local model $(J_1=0)$ as a function of temperature and the Zeeman magnetic field. Local interaction produces a qualitatively different transport mechanism from the nonlocal model.

Figure 3

(a) Static ferromagnetic spin correlation function versus the gate voltage at different temperatures. The susceptibility has a strong many-body enhancement (compared to the noninteracting model). (b) Spectral function for dynamic ferromagnetic spin susceptibility ${\rho }_{\mathrm{FM}}(\omega )$ at different values of gate voltage at $T=0.025t$. The dashed line indicates the excitation frequency.

Figure 4

(a) ${\rho }_{\mathrm{FM}}(\omega )$ at $V_g=0.5t$ for different temperatures. The peak frequency ${\omega }_{\mathrm{peak}}$ corresponds to ferromagnetic excitation energy. (b) The excitation energy as a function of gate voltage $V_g$ at different temperatures. When the traversal time ${\tau }_{\mathrm{tr}}$ (dashed line) is long inside the constriction, i.e., ${\omega }_{\mathrm{peak}}{\tau }_{\mathrm{tr}}>1$, the electron scattering becomes strong, and the conductance plateau results.