Subgap Regime Research Articles

Improvement of chip design to reduce resonances in subgap regime of Josephson junctions

Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some groups [1–3]. These peaks have the shape of a resonance as a function of voltage. The resonances appear in the subgap regime of the junctions and the subgap current (leakage current) is concealed. The positions of the resonances do not change as a magnetic field is applied to the junctions, but their amplitude decreases when the supercurrent is suppressed. We have measured the subgap current of Al/AlOx/Al junctions and we show that these resonances are due to resonant modes in the chip design which are excited by the ac-Josephson effect. We present a chip design that decreases the amplitude of the resonances to a such degree that the subgap current is quantifiable.

Superconducting transport through a vibrating molecule

Nonequilibrium electronic transport through a molecular level weakly coupled to a single coherent phonon/vibration mode has been studied for superconducting leads. The Keldysh Green function formalism is used to compute the current for the entire bias voltage range. In the subgap regime, Multiple Andreev Reflection (MAR) processes accompanied by phonon emission cause rich structure near the onset of MAR channels, including an even-odd parity effect that can be interpreted in terms of an inelastic MAR ladder picture. Thereby we establish a connection between the Keldysh formalism and the Landauer scattering approach for inelastic MAR.

Tunneling spectroscopy with intrinsic Josephson junctions inBi2Sr2CaCu2O8+δandTl2Ba2Ca2Cu3O10+δ

The paper presents a detailed discussion of the current-voltage characteristic of intrinsic Josephson junctions in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\mathrm{\ensuremath{\delta}}}$. In these materials Josephson tunnel junctions are formed naturally between adjacent superconducting CuO${}_{2}$ bilayers or trilayers. A typical sample consists of a stack of Josephson junctions. We explicitly show that all junctions inside a given sample have identical tunneling characteristics. We discuss the shape (general curvature) of the current-voltage characteristic in terms of a superconducting order parameter that has a predominant ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry. The ${I}_{c}{R}_{n}$ product of the intrinsic Josephson junctions turns out to be 2--3 mV, about 10% of the maximum energy gap ${\ensuremath{\Delta}}_{0}/e$. The current-voltage characteristic of every individual junction exhibits pronounced structures in the subgap regime. They are best explained by a recently proposed resonant coupling mechanism between infrared active optical $c$-axis phonons and oscillating Josephson currents.

Nonlinearity in normal-metal–superconductor transport: Scattering-matrix approach

A general formula for the current through a disordered normal-superconducting junction is derived, which is valid at finite temperature and includes the full voltage dependence. The result depends on a multichannel scattering matrix, which describes elastic scattering in the normal region, and accounts for the Andreev scattering at the normal-metal--superconductor interface. The symmetry of the current with respect to sign reversal in the subgap regime is discussed. The Andreev approximation is used to derive a spectral conductance formula, which applies to voltages both below and above the gap. In a case study the spectral conductance formula is applied to the problem of a normal-metal--insulator--normal-metal--insulator--superconductor double barrier junction.