Alexander A. Golubov

Magnetic Josephson Junctions With Superconducting Interlayer for Cryogenic Memory

We investigate a Magnetic Josephson Junction (MJJ) - a superconducting device with ferromagnetic barrier for a scalable high-density cryogenic memory compatible with energy-efficient single flux quantum (SFQ) circuits. The superconductor-insulator-superconductor-ferromagnet-superconductor (SISFS) MJJs are analyzed both experimentally and theoretically. We found that the properties of SISFS junctions fall into two distinct classes based on the thickness of S layer. We fabricate Nb-Al/AlOx-Nb-PdFe-Nb SISFS MJJs using a co-processing approach with a combination of HYPRES and ISSP fabrication processes. The resultant SIS FS structure with thin superconducting S-layer is substantially affected by the ferromagnetic layer as a whole. We fabricate these type of junctions to reach the device compatibility with conventional SIS junctions used for superconducting SFQ electronics to ensure a seamless integration of MJJ-based circuits and SIS JJ-based ultra-fast digital SFQ circuits. We report experimental results for MJJs, demonstrating their applicability for superconducting memory and digital circuits. These MJJs exhibit IcRn product only ~ 30% lower than that of conventional SIS junctions co-produced in the same fabrication. Analytical calculations for these SIS FS structures are in a good agreement with the experiment. We discuss application of MJJ devices for memory and programmable logic circuits.

Ramp type HTS Josephson junctions with PrBaCuGaO barriers

Ramp type Josephson junctions have been fabricated using DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as electrode material and PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// with x=0, 0.10 and 0.40 as junction barriers. Barrier thickness lie between 6-30 nm. Several junctions without barrier were made in order to find ways to minimize the damage of the ramp interface. In total about 40 chips were fabricated each containing several junctions and their I-V characteristics measured for various temperatures down to 4.2 K. Only those junctions showing clear RSJ-like curves were selected to be analyzed. In some cases we also measured I/sub c/ as a function of a small applied field and obtained a clear Fraunhofer pattern, but there is a tendency to flux trapping as evidenced by LTSEM. It was found at 4.2 K that the critical current density J/sub c/ scales with the specific resistance R/sub n/A as J/sub c/=C/sub bar/(R/sub n/A)/sup -m/ (m=1.8/spl plusmn/0.5). The barrier material dependent constant C/sub bar/ increases with x, whereas, for a given d, J/sub c/ is constant and R/sub n/A increases.<<ETX>>

Macroscopic quantum dynamics of π junctions with ferromagnetic insulators

We theoretically investigate the macroscopic quantum dynamics of a π junction with a superconductor (S) and a multiferroic material or a ferromagnetic insulator (FI). By deriving the effective action from a microscopic Hamiltonian, a π-junction qubit (a S-FI-S superconducting quantum interference device ring) is proposed. In this qubit, a quantum two-level system is spontaneously generated and the effect of the quasiparticle dissipation is found to be very weak. These features make it possible to realize a quiet qubit with high coherency. We also investigate macroscopic quantum tunneling (MQT) in current-biased S-FI-S π junctions and show that the influence of the quasiparticle dissipation on MQT is negligibly small.

Josephson π State in a Ferromagnetic Insulator

We predict anomalous atomic-scale 0-pi transitions in a Josephson junction with a ferromagnetic-insulator (FI) barrier. The ground state of such junction alternates between 0 and pi states when thickness of FI is increasing by a single atomic layer. We find that the mechanism of the 0-pi transition can be attributed to thickness-dependent phase shifts between the wave numbers of electrons and holes in FI. Based on these results, we show that a stable pi state can be realized in junctions based on high-T{c} superconductors with a La2BaCuO5 barrier.

Unconventional Surface Impedance of a Normal-Metal Film Covering a Spin-Triplet Superconductor Due to Odd-Frequency Cooper Pairs

We discuss dynamical response of odd-frequency Cooper pairs to electromagnetic field. By using the quasiclassical Green function method, we calculate the surface impedance (Z=R-iX) of a normal metal thin film which covers a superconductor. In contrast to the standard relation (i.e., R<<X), the surface impedance in spin-triplet proximity structures shows anomalous behavior (i.e., R>X) at low temperatures. This unusual relation is a result of the penetration of odd-frequency pairs into the normal metal and reflects the negative Cooper pair density.

Theory of macroscopic quantum tunnelling and dissipation in high-Tc Josephson junctions

We have investigated macroscopic quantum tunnelling (MQT) in in-plane high-Tc superconductor Josephson junctions and the influence of the nodal-quasiparticle and zero energy bound states (ZES) on MQT. We have shown that the presence of ZES at the interface between the insulator and the superconductor leads to strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably suppressed in comparison with c-axis junctions in which ZES are completely absent.

Double proximity effect in hybrid planar superconductor-normal metal/ferromagnet-superconductor structures

We have investigated the differential resistance of hybrid planar Al-(Cu/Fe)-Al submicron bridges at low temperatures and in weak magnetic fields. The structure consists of a Cu/Fe bilayer forming a bridge between two superconducting Al electrodes. In the superconducting state of Al electrodes, we have observed a double-peak peculiarity in differential resistance of the S-(N/F)-S structures at a bias voltage corresponding to the minigap. We claim that this effect (the splitting of the minigap) is due to an electron spin polarization in the normal metal which is induced by the ferromagnet. We have demonstrated that the double-peak peculiarity is converted to a single peak at a coercive applied field corresponding to zero magnetization of the Fe layer

Josephson current in a normal-metal nanowire coupled to a superconductor/ferromagnet/superconductor junction

We consider a superconducting nanowire proximity coupled to a superconductor/ferromagnet/superconductor (S/F/S) junction, where the magnetization penetrates into a superconducting segment in a nanowire decaying as ∼exp[−∣n∣ξ], where n is the site index and the ξ is the decay length. We tune chemical potential and spin-orbit coupling so that the topological superconducting regime hosting the Majorana fermion is realized for long ξ. We find that when ξ becomes shorter, zero energy state at the interface between a superconductor and a ferromagnet splits into two states at nonzero energy. Accordingly, the behavior of the Josephson current is drastically changed due to this “zero mode-nonzero mode crossover.” By tuning the model parameters, we find an almost second-harmonic current-phase relation sin2φ, where φ is the phase difference of the junction. Based on the analysis of Andreev bound state (ABS), we clarify that the current-phase relation is determined by coupling of the states within the energy gap. We find that the emergence of crossing points of ABS is a key ingredient to generate sin2φ dependence in the current-phase relation. We further study both the energy and φ dependence of pair amplitudes in the ferromagnetic region. For large ξ, an odd-frequency spin-triplet s-wave component is dominant. The magnitude of the odd-frequency pair amplitude is enhanced at the energy level of ABS.

Spectrum of Andreev bound states in Josephson junctions with a ferromagnetic insulator

Ferromagnetic-insulator (FI) based Josephson junctions are promising candidates for a coherent superconducting quantum bit as well as a classical superconducting logic circuit. Recently the appearance of an intriguing atomic-scale 0–π transition has been theoretically predicted. In order to uncover the mechanism of this phenomena, we numerically calculate the spectrum of Andreev bound states in a FI barrier by diagonalizing the Bogoliubov–de Gennes equation. We show that Andreev spectrum drastically depends on the parity of the FI-layer number L and accordingly the π(0) state is always more stable than the 0 (π) state if L is odd (even).

Scaling universality at the dynamic vortex Mott transition

The cleanest way to observe a dynamic Mott insulator-to-metal transition (DMT) without the interference from disorder and other effects inherent to electronic and atomic systems, is to employ the vortex Mott states formed by superconducting vortices in a regular array of pinning sites. Here, we report the critical behavior of the vortex system as it crosses the DMT line, driven by either current or temperature. We find universal scaling with respect to both, expressed by the same scaling function and characterized by a single critical exponent coinciding with the exponent for the thermodynamic Mott transition. We develop a theory for the DMT based on the parity reflection-time reversal (PT) symmetry breaking formalism and find that the nonequilibrium-induced Mott transition has the same critical behavior as the thermal Mott transition. Our findings demonstrate the existence of physical systems in which the effect of a nonequilibrium drive is to generate an effective temperature and hence the transition belonging in the thermal universality class.