[Paper Review] Weak localization and antilocalization in semiconducting polymer sandwich devices

Ömer Mermer, M. Wohlgenannt+2 more|arXiv (Cornell University)|Dec 8, 2003

Advanced Memory and Neural Computing1 references3 citations

TL;DR

This study reports the first observation of weak localization and antilocalization in semiconducting polymer sandwich devices at room temperature, demonstrating that quantum interference effects persist in high-quality polyfluorene films. The transition between negative (weak localization) and positive (antilocalization) magnetoresistance is driven by applied electric fields, indicating that spin-orbit coupling is tunable via electric fields, with phase-breaking lengths exceeding 150 nm even at 300 K.

ABSTRACT

We have performed magnetoresistance measurements on polyfluorene sandwich devices in weak magnetic fields as a function of applied voltage, device temperature (10K to 300K), film thickness and electrode materials. We observed either negative or positive magnetoresistance, dependent mostly on the applied voltage, with a typical magnitude of several percent. The shape of the magnetoresistance curve is characteristic of weak localization and antilocalization. Using weak localization theory, we find that the phase-breaking length is relatively large even at room temperature, and spin-orbit interaction is a function of the applied electric field.

Motivation & Objective

• To investigate quantum interference effects in charge transport through high-quality semiconducting polymer sandwich devices.
• To determine whether weak localization and antilocalization—typically restricted to low temperatures—occur in conjugated polymers at room temperature.
• To explore the influence of applied electric fields on spin-orbit coupling and quantum coherence in organic semiconductors.
• To extract phase-breaking and spin-orbit interaction times from magnetoresistance traces using weak localization theory.
• To assess the potential of polyfluorene-based devices for future organic spintronic and quantum information applications.

Proposed method

• Magnetoresistance measurements were performed on polyfluorene (PFO) sandwich devices across a range of temperatures (10–300 K), applied voltages, film thicknesses, and electrode materials.
• Devices were fabricated via spin-coating PFO from toluene solution and evaporated with top electrodes (Al, Ca, Au) and bottom electrodes (ITO, Au) in a nitrogen atmosphere.
• Magnetoresistance was measured in magnetic fields from -100 mT to +100 mT using a cryostat and digital teslameter, with current measured at constant voltage.
• Theoretical analysis used weak localization theory to extract phase-breaking length (lφ) and spin-orbit interaction strength from the shape and field dependence of MR curves.
• Data were fitted to the Hikami-Larkin-Nagaoka equation to model weak localization and antilocalization behavior.
• Current-voltage characteristics were measured to correlate device resistance with the magnitude of quantum interference effects.

Experimental results

Research questions

• RQ1 Can weak localization and antilocalization effects be observed in semiconducting polymer devices at room temperature, contrary to typical expectations in inorganic systems?
• RQ2 How does the applied electric field influence the transition between weak localization and antilocalization in polyfluorene-based devices?
• RQ3 What is the temperature dependence of phase-breaking length and spin-orbit coupling in high-quality PFO films?
• RQ4 To what extent do electrode materials and film thickness affect the observed quantum interference effects in these devices?
• RQ5 Can the observed magnetoresistance behavior be quantitatively explained by weak localization theory in the presence of strong electric fields?

Key findings

• Weak localization and antilocalization effects were observed in polyfluorene sandwich devices at temperatures as high as 300 K, with magnetoresistance magnitude remaining strong and shape characteristic of quantum interference.
• The phase-breaking length (lφ) was found to be approximately 400 nm at 10 K and decreased to about 150 nm at 300 K, indicating long quantum coherence times even at room temperature.
• A clear transition from negative to positive magnetoresistance (from weak localization to antilocalization) was observed with increasing applied voltage, indicating that electric fields tune spin-orbit coupling strength.
• The magnitude of the magnetoresistance effect increased with device resistance (i.e., at lower voltages), consistent with theoretical expectations for weak localization.
• The temperature dependence of the magnetoresistance cones was surprisingly weak, suggesting a partial cancellation of competing temperature dependencies of diffusion constant and phase-breaking time.
• The results indicate that quantum coherence persists over many hops in high-quality polymer films, with phase coherence lengths exceeding 150 nm at room temperature, challenging conventional assumptions about localization in disordered organic semiconductors.