[Paper Review] Finite bias dependent evolution of superconductor-insulator transition in nanodiamond films

Davie Mtsuko, Miloš Nesládek+1 more|arXiv (Cornell University)|Jun 21, 2016

Diamond and Carbon-based Materials Research1 references3 citations

TL;DR

This study investigates finite bias current effects on the superconductor-insulator transition (SIT) in boron-doped nanocrystalline diamond (BNCD) films, revealing that bias current suppresses the bosonic insulator peak, with resistance scaling universally as R ∝ I^−0.94 to −0.96. The near-η ≈ −1 scaling indicates a non-equilibrium Kondo-like effect near quantum criticality, driven by interplay between Coulomb repulsion (U) and tunnel coupling (Γ) across grain boundaries influenced by vortex-induced magnetic regions.

ABSTRACT

The effect of finite bias current on the evolution of the metal-bosonic insulator-superconductor phase transition is systematically investigated in heavily boron doped nanocrystalline diamond (BNCD) films of various grain size. The shape of the resistivity-temperature curves near the critical temperature (TC) is seen to be strongly influenced by both magnetic field and bias current. It is seen that as the bias current is lowered the so-called bosonic insulator peak in the vicinity of TC becomes more pronounced whereas when the magnetic field is varied the peak shifts towards the low temperature range. The log-log plots of peak resistance versus bias current showing a universal linear scaling with exponent \b{eta}~ -0.94, -0.96 are found to be correlated with the linear variation of bosonic peak resistance with the magnetic field measured at low bias current. The \b{eta} ~ -1 implies a non-equilibrium Kondo-like effect near the quantum critical phase transition. The bias current dependence can be understood as an interplay between the Coulomb repulsive energy (U) and tunnel coupling ({\Gamma}) between diamond grains or intra-grain superconducting regions separated by magnetic regions that result from external flux penetration induced vortices. The anomalous peak in the R(T) and magnetoresistance data as well as the coherence peaks in differential conductance vs. I plots can be attributed to regions within charge-Berezinskii-Kosterlitz-Thouless (BKT) and vortex-BKT transitions.

Motivation & Objective

• To understand how finite bias current influences the evolution of the superconductor-insulator transition (SIT) in heavily boron-doped nanocrystalline diamond (BNCD) films.
• To investigate the interplay between Coulomb repulsion (U) and tunnel coupling (Γ) between diamond grains and intra-grain superconducting regions.
• To examine the role of external magnetic fields and vortex-induced magnetic regions in shifting the bosonic insulator peak in resistivity-temperature (R(T)) curves.
• To determine whether the observed resistance scaling with bias current reflects universal behavior linked to quantum criticality.

Proposed method

• Systematic measurement of resistivity-temperature (R(T)) curves in BNCD films under varying bias current and magnetic field.
• Log-log analysis of peak resistance versus bias current to extract scaling exponent η, revealing universal behavior.
• Correlation of peak resistance with magnetic field at low bias current to validate scaling consistency.
• Use of differential conductance vs. current (dI/dV vs. I) to identify coherence peaks associated with Berezinskii-Kosterlitz-Thouless (BKT) transitions.
• Analysis of magnetoresistance data to link vortex penetration and magnetic region formation to peak shifts.
• Theoretical interpretation based on interplay between U (Coulomb energy) and Γ (tunnel coupling) at grain boundaries.

Experimental results

Research questions

• RQ1 How does finite bias current affect the shape and position of the bosonic insulator peak in R(T) curves near T_C in BNCD films?
• RQ2 What is the scaling behavior of peak resistance with respect to bias current, and does it exhibit universality across different grain sizes?
• RQ3 How does the application of magnetic field influence the temperature position of the bosonic insulator peak?
• RQ4 What physical mechanism underlies the observed η ≈ −0.94 to −0.96 scaling of resistance with bias current?
• RQ5 How are the observed anomalies in R(T), magnetoresistance, and differential conductance related to BKT-type transitions?

Key findings

• The bosonic insulator peak in R(T) curves becomes more pronounced as bias current is reduced, indicating suppression of superconducting coherence at higher currents.
• Applying a magnetic field shifts the bosonic insulator peak toward lower temperatures, suggesting vortex-induced magnetic regions perturb the local superconducting order.
• Log-log plots of peak resistance versus bias current show universal linear scaling with exponent η ≈ −0.94 to −0.96, indicating near-η ≈ −1 behavior.
• The η ≈ −1 scaling is interpreted as a signature of a non-equilibrium Kondo-like effect near the quantum critical point of the SIT.
• The resistance scaling correlates linearly with magnetic field at low bias current, supporting the universality of the observed scaling behavior.
• Anomalous features in R(T), magnetoresistance, and differential conductance are attributed to charge-BKT and vortex-BKT transitions within the disordered grain network.