Low-T_c Josephson junctions with tailored barrier

Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb based superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson tunnel junctions with a thickness step in the metallic ferromagnetic \Ni_{0.6}\Cu_{0.4} interlayer were fabricated. The step was defined by optical lithography and controlled etching. The step height is on the scale of a few angstroms. Experimentally determined junction parameters by current-voltage characteristics and Fraunhofer pattern indicate an uniform F-layer thickness and the same interface transparencies for etched and non-etched F-layers. This technique could be used to tailor low-T_c Josephson junctions having controlled critical current densities at defined parts of the junction area, as needed for tunable resonators, magnetic-field driven electronics or phase modulated devices.Comment: 6 pages, 6 figures, small changes, to be published by JA

Metabolic modeling for predicting VFA production from protein‐rich substrates by mixed‐culture fermentation

This is the peer reviewed version of the following article: Regueira, A, Lema, JM, Carballa, M, Mauricio‐Iglesias, M. Metabolic modeling for predicting VFA production from protein‐rich substrates by mixed‐culture fermentation. Biotechnology and Bioengineering. 2020; 117: 73– 84, which has been published in final form at https://doi.org/10.1002/bit.27177. This article may be used for non‐commercial purposes in accordance with Wiley Terms and Conditions for Use of Self‐Archived VersionsProteinaceous organic wastes are suitable substrates to produce high added‐value products in anaerobic mixed‐culture fermentations. In these processes, the stoichiometry of the biotransformation depends highly on operational conditions such as pH or feeding characteristics and there are still no tools that allow the process to be directed toward those products of interest. Indeed, the lack of product selectivity strongly limits the potential industrial development of these bioprocesses. In this work, we developed a mathematical metabolic model for the production of volatile fatty acids from protein‐rich wastes. In particular, the effect of pH on the product yields is analyzed and, for the first time, the observed changes are mechanistically explained. The model reproduces experimental results at both neutral and acidic pH and it is also capable of predicting the tendencies in product yields observed with a pH drop. It also offers mechanistic insights into the interaction among the different amino acids (AAs) of a particular protein and how an AA might yield different products depending on the relative abundance of other AAs. Particular emphasis is placed on the utility of this mathematical model as a process design tool and different examples are given on how to use the model for this purposeThe authors would like to acknowledge the support of the Spanish Ministry of Education (FPU14/05457) and project BIOCHEM (ERA-IB-2 7th call, ERA-IB-16-052) funded by MINECO (PCIN 2016-102). A. Regueira would like to thank the CRETUS Strategic Partnership (ED431E 2018/01), for a research stay grant. A. Regueira, M. Miguel-Mauricio and J. M. Lema belong to the Galician Competitive Research Group ED431C2017/029 and to the CRETUS Strategic Partnership, both programmes are co-funded by FEDER (UE)S

Critical temperature for first-order phase transitions in confined systems

We consider the Euclidean $D$-dimensional $-\lambda |\phi |^4+\eta |\phi |^6$ ($\lambda ,\eta >0$) model with $d$ ($d\leq D$) compactified dimensions. Introducing temperature by means of the Ginzburg--Landau prescription in the mass term of the Hamiltonian, this model can be interpreted as describing a first-order phase transition for a system in a region of the $D$-dimensional space, limited by $d$ pairs of parallel planes, orthogonal to the coordinates axis $x_1, x_2, ..., x_d$. The planes in each pair are separated by distances $L_1, L_2, ..., L_d$. We obtain an expression for the transition temperature as a function of the size of the system, $$, $i=1, 2, ..., d$. For D=3 we particularize this formula, taking $L_1=L_2=... =L_d=L$ for the physically interesting cases $d=1$ (a film), $d=2$ (an infinitely long wire having a square cross-section), and for $d=3$ (a cube). For completeness, the corresponding formulas for second-order transitions are also presented. Comparison with experimental data for superconducting films and wires shows qualitative agreement with our theoretical expressionsComment: REVTEX, 11 pages, 3 figures; to appear in Eur. Phys. Journal

Inter- and Intragranular Effects in Superconducting Compacted Platinum Powders

Compacted platinum powders exhibit a sharp onset of diamagnetic screening at $T \simeq 1.9$ mK in zero magnetic field in all samples investigated. This sharp onset is interpreted in terms of the intragranular transition into the superconducting state. At lower temperatures, the magnetic ac susceptibility strongly depends on the ac field amplitude and reflects the small intergranular critical current density $j_{c}$. This critical current density shows a strong dependence on the packing fraction f of the granular samples. Surprisingly, $j_{c}$ increases significantly with decreasing f ($j_{c}(B=0, T=0) \simeq 0.07$ A/cm$^{2}$ for f = 0.67 and $j_{c}(B=0, T=0) \simeq 0.8$ A/cm$^{2}$ for f = 0.50). The temperature dependence of $j_{c}$ shows strong positive curvature over a wide temperature range for both samples. The phase diagrams of inter- and intragranular superconductivity for different samples indicate that the granular structure might play the key role for an understanding of the origin of superconductivity in the platinum compacts.Comment: 11 pages including 9 figures. To appear in Phys. Rev. B in Nov. 0

Origin of complex crystal structures of elements at pressure

We present a unifying theory for the observed complex structures of the sp-bonded elements under pressure based on nearly free electron picture (NFE). In the intermediate pressure regime the dominant contribution to crystal structure arises from Fermi-surface Brillouin zone (FSBZ) interactions - structures which allow this are favoured. This simple theory explains the observed crystal structures, transport properties, the evolution of internal and unit cell parameters with pressure. We illustrate it with experimental data for these elements and ab initio calculation for Li.Comment: 4 pages 5 figure

A novel class of CoA-transferase involved in short-chain fatty acid metabolism in butyrate-producing human colonic bacteria

Peer reviewedPublisher PD

Assembling the puzzle of superconducting elements: A Review

Superconductivity in the simple elements is of both technological relevance and fundamental scientific interest in the investigation of superconductivity phenomena. Recent advances in the instrumentation of physics under pressure have enabled the observation of superconductivity in many elements not previously known to superconduct, and at steadily increasing temperatures. This article offers a review of the state of the art in the superconductivity of elements, highlighting underlying correlations and general trends.Comment: Review, 10 pages, 11 figures, 97 references; to appear in Superc. Sci. Techno

Possible robust insulator-superconductor transition on solid inert gas and other substrates

We present observations of the insulator-superconductor transition in ultrathin films of Bi on amorphous quartz, quartz coated with Ge, and for the first time, solid xenon condensed on quartz. The relative permeability $\epsilon_r$ ranges from 1.5 for Xe to 15 for Ge. Though we find screening effects as expected, the I-S transition is robust, and unmodified by the substrate. The resistance separatrix is found to be close to h/4e^2 and the crossover thickness close to $\rm 25 \AA$ for all substrates. I-V studies and Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The transition can be understood in terms of a percolation model.Comment: accepted in Physical Review

Two-photon probe of the Jaynes-Cummings model and symmetry breaking in circuit QED

Superconducting qubits behave as artificial two-level atoms and are used to investigate fundamental quantum phenomena. In this context, the study of multi-photon excitations occupies a central role. Moreover, coupling superconducting qubits to on-chip microwave resonators has given rise to the field of circuit QED. In contrast to quantum-optical cavity QED, circuit QED offers the tunability inherent to solid-state circuits. In this work, we report on the observation of key signatures of a two-photon driven Jaynes-Cummings model, which unveils the upconversion dynamics of a superconducting flux qubit coupled to an on-chip resonator. Our experiment and theoretical analysis show clear evidence for the coexistence of one- and two-photon driven level anticrossings of the qubit-resonator system. This results from the symmetry breaking of the system Hamiltonian, when parity becomes a not well-defined property. Our study provides deep insight into the interplay of multiphoton processes and symmetries in a qubit-resonator system.Comment: Accepted for publication in Nature Physics, 8 pages, 4 figure