Sign reversal of the Josephson inductance magnetochiral anisotropy and 0-π-like transitions in supercurrent diodes

The recent discovery of the intrinsic supercurrent diode effect, and its prompt observation in a rich variety of systems, has shown that non-reciprocal supercurrents naturally emerge when both space-inversion and time-inversion symmetries are broken. In Josephson junctions, non-reciprocal supercurrent can be conveniently described in terms of spin-split Andreev states. Here we demonstrate a sign reversal of the Josephson inductance magnetochiral anisotropy, a manifestation of the supercurrent diode effect. The asymmetry of the Josephson inductance as a function of the supercurrent allows us to probe the current-phase relation near equilibrium, and to probe jumps in the junction ground state. Using a minimal theoretical model, we can then link the sign reversal of the inductance magnetochiral anisotropy to the so-called 0-π-like transition, a predicted but still elusive feature of multichannel junctions. Our results demonstrate the potential of inductance measurements as sensitive probes of the fundamental properties of unconventional Josephson junctions.

Effect of Rashba and Dresselhaus spin-orbit coupling on supercurrent rectification and magnetochiral anisotropy of ballistic Josephson junctions

Simultaneous breaking of inversion- and time-reversal symmetry in Josephson junction (JJ) leads to a possible violation of theI(φ) = -I(-φ) equality for the current-phase relation. This is known as anomalous Josephson effect and it produces a phase shiftφ₀in sinusoidal current-phase relations. In ballistic JJs with non-sinusoidal current phase relation the observed phenomenology is much richer, including the supercurrent diode effect and the magnetochiral anisotropy (MCA) of Josephson inductance. In this work, we present measurements of both effects on arrays of JJs defined on epitaxial Al/InAs heterostructures. We show that the orientation of the current with respect to the lattice affects the MCA, possibly as the result of a finite Dresselhaus component. In addition, we show that the two-fold symmetry of the Josephson inductance reflects in the activation energy for phase slips.

Comparison of cryogenic low-pass filters

Low-temperature electronic transport measurements with high energy resolution require both effective low-pass filtering of high-frequency input noise and an optimized thermalization of the electronic system of the experiment. In recent years, elaborate filter designs have been developed for cryogenic low-level measurements, driven by the growing interest in fundamental quantum-physical phenomena at energy scales corresponding to temperatures in the few millikelvin regime. However, a single filter concept is often insufficient to thermalize the electronic system to the cryogenic bath and eliminate spurious high frequency noise. Moreover, the available concepts often provide inadequate filtering to operate at temperatures below 10 mK, which are routinely available now in dilution cryogenic systems. Herein we provide a comprehensive analysis of commonly used filter types, introduce a novel compact filter type based on ferrite compounds optimized for the frequency range above 20 GHz, and develop an improved filtering scheme providing adaptable broad-band low-pass characteristic for cryogenic low-level and quantum measurement applications at temperatures down to few millikelvin.

Transverse spin Seebeck effect versus anomalous and planar Nernst effects in Permalloy thin films

Transverse magnetothermoelectric effects are studied in Permalloy thin films grown on MgO and GaAs substrates and compared to those grown on suspended SiN(x) membranes. The transverse voltage along platinum strips patterned on top of the Permalloy films is measured versus the external magnetic field as a function of the angle and temperature gradients. After the identification of the contribution of the planar and anomalous Nernst effects, we find an upper limit for the transverse spin Seebeck effect, which is several orders of magnitude smaller than previously reported.

Direct observation of band-gap closure for a semiconducting carbon nanotube in a large parallel magnetic field

We have investigated the magnetoconductance of semiconducting carbon nanotubes (CNTs) in pulsed, parallel magnetic fields up to 60 T, and report the direct observation of the predicted band-gap closure and the reopening of the gap under variation of the applied magnetic field. We also highlight the important influence of mechanical strain on the magnetoconductance of the CNTs.

Carbon nanotubes as cooper-pair beam splitters

We report on conductance measurements in carbon nanotube based double quantum dots connected to two normal electrodes and a central superconducting finger. By operating our devices as beam splitters, we provide evidence for crossed Andreev reflections tunable in situ. This opens an avenue to more sophisticated quantum opticslike experiments with spin entangled electrons.

Effect of microwaves on the current-phase relation of superconductor-normal-metal-superconductor josephson junctions

We investigate the current-phase relation (CPR) of long diffusive superconductor-normal-metal-superconductor (Nb/Ag/Nb) Josephson junctions in thermodynamic equilibrium and under microwave irradiation. While in equilibrium good agreement with the predictions of quasiclassical theory is found, we observe that the shape of the CPR can be strongly affected by microwave irradiation. Close to a Josephson-phase difference phi approximately pi, the supercurrent can be strongly suppressed when increasing the rf power. Our results can be understood in terms of microwave excitation of low-lying Andreev bound states across the minigap in the junction. In the frequency interval studied, this mechanism becomes important, when the minigap closes at phi approximately pi.

Localized superconductivity in the quantum-critical region of the disorder-driven superconductor-insulator transition in TiN thin films

We investigate low-temperature transport properties of thin TiN superconducting films in the vicinity of the disorder-driven superconductor-insulator transition. In a zero magnetic field, we find an extremely sharp separation between superconducting and insulating phases, evidencing a direct superconductor-insulator transition without an intermediate metallic phase. At moderate temperatures, in the insulating films we reveal thermally activated conductivity with the magnetic field-dependent activation energy. At very low temperatures, we observe a zero-conductivity state, which is destroyed at some depinning threshold voltage V{T}. These findings indicate the formation of a distinct collective state of the localized Cooper pairs in the critical region at both sides of the transition.

Quantum metallicity on the high-field side of the superconductor-insulator transition

We investigate ultrathin superconducting TiN films, which are very close to the localization threshold. Perpendicular magnetic field drives the films from the superconducting to an insulating state, with very high resistance. Further increase of the magnetic field leads to an exponential decay of the resistance towards a finite value. In the limit of low temperatures, the saturation value can be very accurately extrapolated to the universal quantum resistance h/e2. Our analysis suggests that at high magnetic fields a new ground state, distinct from the normal metallic state occurring above the superconducting transition temperature, is formed. A comparison with other studies on different materials indicates that the quantum metallic phase following the magnetic-field-induced insulating phase is a generic property of systems close to the disorder-driven superconductor-insulator transition.

Spontaneous supercurrent induced by ferromagnetic pi junctions

We present magnetization measurements of mesoscopic superconducting niobium loops containing a ferromagnetic (PdNi) pi junction. The loops are prepared on top of the active area of a micro-Hall sensor based on high mobility GaAs/AlGaAs heterostructures. We observe asymmetric switching of the loop between different magnetization states when reversing the sweep direction of the magnetic field. This provides evidence for a spontaneous current induced by the intrinsic phase shift of the pi junction. In addition, the presence of the spontaneous current near zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in half integer flux quantization in the loop at low temperatures.

Shot noise by quantum scattering in chaotic cavities

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as (1/4)2e/I/ in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity. Heating inside the cavity due to electron-electron interaction can slightly enhance the noise of large cavities and is also discussed quantitatively.

The fermionic hanbury brown and twiss experiment

A Hanbury Brown and Twiss experiment for a beam of electrons has been realized in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter to partition the incident beam into transmitted and reflected partial beams. In the nonequilibrium case the fluctuations in the partial beams are shown to be fully anticorrelated, demonstrating that fermions exclude each other. In equilibrium, the cross-correlation of current fluctuations at two different contacts is also found to be negative and nonzero, provided that a direct transmission exists between the contacts.

Molecular genetics of the extracellular lipase of Pseudomonas aeruginosa PAO1

The structural gene (lipA) coding for the extracellular lipase of Pseudomonas aeruginosa PAO1 has been cloned on plasmid pSW118. Nucleotide sequence analysis revealed a gene of 936 bp. lipA codes for a proenzyme of 311 amino acids including a leader sequence of 26 amino acids. The mature protein was predicted to have a M(r) of 30134, an isoelectric point of 5.6, and a consensus sequence (IGHSHGG) typical of lipases. Furthermore it is highly homologous (greater than 60%) to other lipases from various pseudomonads. The lipA gene failed to hybridize detectably with genomic DNA from other Pseudomonas species except P. alcaligenes, even under relaxed stringency. Located 220 bp downstream of the lipA gene, is an open reading frame (ORF2, lipH) which encodes a hydrophilic protein (283 amino acids; M(r) 33587) that shows some homology to the limA gene product of P. cepacia. In complementation tests of lipase-defective mutants, lipH was shown to be necessary for expression of active extracellular lipase in P. aeruginosa PAO1.

Proenkephalin-A gene regulation in the rat striatum: influence of lithium and haloperidol

This report explores the influence of lithium and haloperidol on (Met5)-enkephalin (ME) biosynthesis in the rat striatum. Male Fischer 344 rats were treated intraperitoneally with lithium chloride (4 mEq/kg/day) for 2, 4, or 6 days and sacrificed 24 hr after the last dose; in addition, the effect of lithium at 2 and 24 hr after a single dose was studied. Serum levels increased in a time-related manner on repeated administration of lithium. Lithium increased the striatal ME content (native ME) in a time-dependent fashion, reaching 160% of control following six doses; no changes in ME were observed in hypothalamus and hippocampus. ME levels recovered to control values 8 days after cessation of a 4-day course of repeated administration (4 mEq/kg/day) of lithium. In an attempt to characterize the nature of this selective increase of ME content in the striatum, the precursor content (cryptic ME) as well as the preproenkephalin mRNA abundance was determined. Lithium increased the precursor content in a time-dependent fashion and this pattern closely paralleled the increase in native ME content. The preproenkephalin mRNA abundance with respect to control was quantitated by blot-hybridization of total RNA with a 32P-labeled cDNA probe derived from rat brain. Lithium increased the mRNA abundance following repeated doses for 4 or 6 days. Concurrent administration of an opiate antagonist, naltrexone (5 mg/kg/day), for 4 days did not influence the changes induced by lithium. On repeated administration (1 mg/kg/day) for 4 days, the neuroleptic, haloperidol, increased the biosynthesis of ME which was more marked than that of lithium administered for the same period; the combination of a haloperidol and lithium regimen did not lead to an additive of synergistic effect. The results indicate that, like haloperidol, repeated injections of lithium increase the biosynthesis of ME in the basal ganglia by increasing the preproenkephalin mRNA abundance and translation process.

Substructural study of sporogenesis in Streptomyces griseus

A Streptomyces griseus strain deficient in the formation of aerial mycelium and arthrospore development (Amy-) was studied by electron microscopy and compared with the sporulating parental strain (Amy+). The investigations were performed with colonies grown on solid medium. The substructural characteristics of the essential events of sporogenesis could clearly been demonstrated in the aerial mycelium of the Amy+ colonies. The mycelium of the surface region of the Amy- colonies showed altered features. The most pronounced alteration was the absence of the surface sheath, normally present on the aerial hyphae of the parental strain. The cross wall type II, characteristic of sporogeneous hyphae, was not discernible in the Amy- hyphae. Some substructural features resembling the essential events of normal sporogenesis were evident in the Amy- strain, albeit diminished and interfered with by abnormalities. The resulting propagative cells were of different size and feature.

Sporogenesis in Streptomyces melanochromogenes

The mode of spore differentiation in a strain of Streptomyces melanochromogenes was followed by analysis of ultrathin sections of sporulating aerial hyphae at various stages of sporogenesis. A special accent was laid on the formation of the sporulation septum and its alterations in the course of spore delimitation and separation. Distinct differences in formation and substructure have been observed between the cross walls of vegetative hyphae and the sporulation septa. Cross walls of vegetative hyphae are formed in a way typical for Gram-positive bacteria by a centripetal annular ingrowth of cytoplasmic membrane, on which wall material immediately is deposited. The development of the sporulation septa is characterized by the accumulation of amorphous material in addition to the newly synthesized wall layer inside the invaginating cytoplasmic membrane. This amorphous septal material will later be decomposed presumably by two lytic systems which cause the separation of the spores. The central region of the finished sporulation septum is perforated by microplasmodesmata. Spores are released by a break down of the surface sheath. The complete spores are enveloped by a two-layered cell wall and the spiny surface sheath.

[Effect of Cu++ ions on the morphological, cytological, and physiological state of Candida utilis cells in continuous culture]

Cells of Candida utilis grown in continuous culture under the influence of rather high concentrations of copper ions have been studied biochemically, morphologically, and cytologically. Although under these conditions a new steady state has been adjusted, the population is much more differentiated than under control conditions. With regard to their size, structure, and viability the cells have been arranged into 4 groups. Generally, the "copper cells" in comparison with the "normal cells" are more voluminous (average: 2.5 times; maximum: 4 times) and of altered structure. The appearance of dense particles ("copper containing particles"), globules ("lipid globules"), vacuoles, and the thickening of the cell wall, as well as the disappearance of the mitochondrial cristae have been shown as characteristic indications for damage in the cell substructure caused by the influence of copper ions. Occasionally abnormal cell shapes can be observed. A permanent influence of copper ions in the concentration used was followed also by striking disturbances of the cell metabolism. The cells take up more carbon, phosphorus, and nitrogen, whereas protein synthesis and respiratory activity decrease. Both the synthesis of lipids and polysaccharides and the phosphohydrolase activity increase. A discussion of the experimental data is presented.