[Paper Review] Aharonov-Bohm effects in nanostructures

В. Л. Гуртовой, А. В. Никулов+1 more|ArXiv.org|Oct 27, 2009

Surface and Thin Film Phenomena1 references3 citations

TL;DR

This study provides direct experimental evidence of a dc persistent current flowing against the direction of an applied electric field in superconducting aluminum nano-rings, demonstrating that the Aharonov-Bohm effect in mesoscopic systems requires an additional force for explanation—contrary to findings in two-slit interference experiments. The observation of Little-Parks oscillations at ultra-low measuring currents (as low as 2 nA) confirms that the persistent current persists even when the external electric field would otherwise oppose its flow.

ABSTRACT

Measurements of the Little-Parks oscillations at measuring current much lower than the persistent current give unambiguous evidence of the dc current flowing against the force of the dc electric field because of the Aharonov-Bohm effect. This result can assume that an additional force is needed for description of the Aharonov-Bohm effect observed in semiconductor, normal metal and superconductor nanostructures in contrast to the experimental result obtained recently for the case of the two-slit interference experiment.

Motivation & Objective

• To investigate whether the Aharonov-Bohm effect in nanostructures requires an additional force beyond standard quantum formalism.
• To resolve the contradiction between the Aharonov-Bohm effect in two-slit interference (where no force is needed) and in mesoscopic loops (where persistent current appears to flow against electric field).
• To experimentally verify the existence of persistent current in superconducting nanostructures with non-zero resistance and under low external current.
• To measure Little-Parks oscillations at extremely low measuring currents to isolate the contribution of the persistent current from external field effects.

Proposed method

• Used a system of 110 aluminum nano-rings connected in series with radii ≈1 μm and cross-sectional areas of 4000–8000 nm².
• Measured resistance R = V/I_ext as a function of magnetic field H at ultra-low measuring currents (I_ext = 2–3 nA), ensuring I_ext ≪ I_p.
• Monitored the resistive transition R(T) under external current and persistent current to detect shifts in critical temperature ΔT_c.
• Observed Little-Parks oscillations in resistance R(H) and potential difference V(H) to detect periodic modulation due to persistent current I_p(H).
• Used magnetic flux quantization Φ₀ = h/q to confirm that oscillation period corresponds to flux quantum through the ring area S = πr².
• Analyzed the sign and magnitude of the persistent current I_p(H) by measuring magnetization and voltage oscillations, confirming I_p ≠ 0 at Φ ≠ nΦ₀.

Experimental results

Research questions

• RQ1 Does the Aharonov-Bohm effect in nanostructures require an additional force to explain the persistent current flowing against the electric field?
• RQ2 Can persistent current be observed in superconducting rings with non-zero resistance (T > T_c) and at ultra-low measuring currents?
• RQ3 How does the persistent current I_p compare to the external measuring current I_ext in terms of magnitude and direction?
• RQ4 Is the Little-Parks effect observable at I_ext ≪ I_p, and does it confirm the existence of a dc current opposing the electric field?
• RQ5 Can the critical temperature shift ΔT_c induced by persistent current be distinguished from that induced by external current?

Key findings

• Little-Parks oscillations in resistance R(H) were observed at measuring current I_ext = 2 nA, which is much smaller than the persistent current amplitude I_p,A ≈ 100 nA.
• The resistance oscillations R(H) are independent of the sign and magnitude of I_ext, confirming that the oscillations arise from I_p²(H), not from external field effects.
• The oscillation period H₀ = 5.2 Oe corresponds to one flux quantum Φ₀ through the ring area S = 4 μm², confirming flux quantization.
• At T = T_c, the persistent current amplitude I_p,A ≈ 100 nA was observed, with a critical temperature shift ΔT_c ≈ 0.0025 K induced by I_p at Φ = Φ₀/2.
• The persistent current I_p flows against the force of the dc electric field E = -∇V, as evidenced by I = I_p - I_ext/2 ≈ I_p when I_ext ≪ I_p.
• The critical temperature shift induced by I_p (ΔT_c ≈ 0.0025 K) is larger than that from I_ext = 100 nA (ΔT_c ≈ 0.0015 K), confirming the physical reality of the persistent current.