[Paper Review] Observation of two distinct pairs fluctuation lifetimes and supercurrents in the pseudogap regime of cuprate junctions

G. Koren, Patrick A. Lee|DSpace@MIT (Massachusetts Institute of Technology)|Sep 6, 2016

Physics of Superconductivity and Magnetism4 citations

TL;DR

This study investigates pair fluctuations in the pseudogap regime of cuprate junctions using c-axis Josephson junctions with two distinct critical temperatures. By measuring conductance spectra under in-plane magnetic fields, the authors identify two distinct pair fluctuation lifetimes—short and long—persisting up to the higher Tc, indicating coexisting preformed pairs or fluctuating pair density waves, with the long-lived component weakly field-dependent and the short-lived one nearly field-independent.

ABSTRACT

Pairs fluctuation supercurrents and inverse lifetimes were measured on epitaxial c-axis junctions of the cuprates in the pseudogap regime, with a $PrBa_2Cu_3O_{7-δ}$ barrier sandwiched in between two $YBa_2Cu_3O_{7-δ}$ or doped $YBa_2Cu_3O_y$ electrodes, with or without magnetic fields parallel to the a-b planes. All junctions had a $ m T_c(high)\approx 85-90$ K and a $ m T_c(low)\approx 50-55$ K electrodes, allowing us to study pairs fluctuation supercurrents and inverse life times in between these two temperatures. In junctions with a pseudogap electrode under zero field, an excesss current due to pair fluctuations was observed which persisted at temperatures above $ m T_c(low)$, in the pseudogap regime, and up to about $ m T_c(high)$. No such excess current was observed in junctions without a pseudo-gap in the electrode. The measured conductance spectra at temperatures above $ m T_c(low)$ were fitted using a modified fluctuations model by Scalapino [Phys. Rev. Lett. extbf{24}, 1052(1970)] of a junction with a serial resistance. We found that in the pseudo-gap regime, the conductance vs voltage consists of a narrow peak sitting on top of a very broad peak. This yielded two distinct pairs fluctuation lifetimes in the pseudogap electrode which differ by an order of magnitude up to about $ m T_c(high)$. Under in-plane fields, these two lifetime values remain separated in two distinct groups, which varied with increasing field moderately. We also found that detection of Amperian pairing [Phys. Rev. X extbf{4}, 031017 (2014)] in our cuprate junctions is not feasible, due to Josephson vortices penetration into the superconducting electrodes which drove the necessary field above the depairing field.

Motivation & Objective

• To probe the nature of superconducting pair fluctuations in the pseudogap regime of high-Tc cuprates.
• To determine whether preformed pairs or competing orders like pair density waves (PDW) contribute to the pseudogap.
• To measure fluctuation lifetimes and supercurrents in c-axis junctions with two distinct Tc electrodes.
• To assess the feasibility of detecting Amperean pairing via magnetic field tuning in tunnel junctions.

Proposed method

• Fabricated epitaxial c-axis junctions with a PrBa2Cu3O7−δ (PrBCO) barrier sandwiched between YBa2Cu3Oy electrodes, one with Tc ≈ 85–90 K and the other with Tc ≈ 50–55 K.
• Performed low-temperature transport measurements (current-voltage characteristics) in zero and in-plane magnetic fields up to several Tesla.
• Used a modified fluctuation model by Scalapino (1970) with serial resistance to fit conductance spectra, assuming a superposition of narrow and broad peaks.
• Extracted fluctuation lifetimes (Γ⁻¹) by fitting the conductance to a sum of two Lorentzian peaks: one narrow (short-lived pairs) and one broad (long-lived pairs).
• Applied iterative fitting to Eq. (9) from the supplementary material to refine the parameters of the two fluctuation components.
• Compared results with theoretical predictions for PDW pairing and assessed the feasibility of detecting Amperean pairing via field-tuned conductance peaks.

Experimental results

Research questions

• RQ1 Do pair fluctuations persist in the pseudogap regime above Tc(low), and can they generate measurable supercurrents?
• RQ2 Are there two distinct fluctuation lifetimes in the pseudogap state, and do they correspond to different pairing mechanisms?
• RQ3 How do in-plane magnetic fields affect the two fluctuation components, and what does this imply about their coherence length and momentum structure?
• RQ4 Can Amperean (Amperean pairing) signatures be detected in these junctions via magnetic field tuning?
• RQ5 What is the microscopic origin of the two distinct fluctuation lifetimes observed in the conductance spectra?

Key findings

• An excess current due to pair fluctuations was observed in junctions with a pseudogap electrode above Tc(low) and up to Tc(high) ≈ 85–90 K, but not in junctions without a pseudogap electrode.
• Conductance spectra above Tc(low) exhibited a narrow peak superimposed on a very broad peak, indicating two distinct pair fluctuation lifetimes with a difference of about an order of magnitude.
• The inverse lifetime of the broad peak (Γ₁⁻¹) was long and varied only moderately with increasing in-plane magnetic field, while the inverse lifetime of the narrow peak (Γ₂⁻¹) was short and nearly field-independent.
• The long-lived fluctuation component (Γ₁⁻¹) showed a weak field dependence, suggesting a larger coherence length, while the short-lived component (Γ₂⁻¹) was nearly field-insensitive, possibly indicating a short coherence length.
• The two fluctuation components remained separated into two distinct groups under magnetic fields, with Γ₂ and A₂ (amplitude) nearly constant, indicating robust separation of the two fluctuation modes.
• Detection of Amperean pairing via field-tuned conductance peaks was ruled out because the required field strength (several thousand Tesla) exceeds the depairing field, making the experiment unfeasible.