Majorana bound states in a coupled quantum-dot hybrid-nanowire system

Hybrid nanowires combining semiconductor and superconductor materials appear well suited for the creation, detection, and control of Majorana bound states (MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot at the end of the nanowire as a spectrometer. Electrostatic gating tuned the nanowire density to a regime of one or a few ABSs. In an applied axial magnetic field, a topological phase emerges in which ABSs move to zero energy and remain there, forming MBSs. We observed hybridization of the MBS with the end-dot bound state, which is in agreement with a numerical model. The ABS/MBS spectra provide parameters that are useful for understanding topological superconductivity in this system.Comment: Article and Supplementary Materia

Hybridization of sub-gap states in one-dimensional superconductor/semiconductor Coulomb islands

We present measurements of one-dimensional superconductor-semiconductor Coulomb islands, fabricated by gate confinement of a two-dimensional InAs heterostructure with an epitaxial Al layer. When tuned via electrostatic side gates to regimes without sub-gap states, Coulomb blockade reveals Cooper-pair mediated transport. When sub-gap states are present, Coulomb peak positions and heights oscillate in a correlated way with magnetic field and gate voltage, as predicted theoretically, with (anti) crossings in (parallel) transverse magnetic field indicating Rashba-type spin-orbit coupling. Overall results are consistent with a picture of overlapping Majorana zero modes in finite wires

Transport signatures of quasiparticle poisoning in a Majorana island

We investigate effects of quasiparticle poisoning in a Majorana island with strong tunnel coupling to normal-metal leads. In addition to the main Coulomb blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage, consistent with transport through an excited (poisoned) state of the island. Comparison to a simple model yields an estimate of parity lifetime for the strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks at high field, reflecting Majorana hybridization, are enhanced by the reduced lever arm at strong coupling. In energy units, fluctuations are consistent with previous measurements.Comment: includes supplementary materia

Effect of the isoelectronic substitution of Sb for As on the magnetic and structural properties of LaFe(As1−xSbx)O

International audienceThe antiferromagnetic (AFM) order and structural distortion in the LaFe(As1−xSbx)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As1−xSbx)O (x < 0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures, but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe-magnetic moment is observed

Spin orbit effects in a GaAs quantum dot in a parallel magnetic field

We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. We argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B_{||}, even if they are negligble for B_{||}=0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B_{||}. Our picture is consistent with the experimental observations of Folk et al.Comment: 5 page

Direct Percutaneous Left Ventricular Access and Port Closure Pre-Clinical Feasibility

ObjectivesThis study sought to evaluate feasibility of nonsurgical transthoracic catheter-based left ventricular (LV) access and closure.BackgroundImplanting large devices, such as mitral or aortic valve prostheses, into the heart requires surgical exposure and repair. Reliable percutaneous direct transthoracic LV access and closure would allow new nonsurgical therapeutic procedures.MethodsPercutaneous direct LV access was performed in 19 swine using real-time magnetic resonance imaging (MRI) and an “active” MRI needle antenna to deliver an 18-F introducer sheath. The LV access ports were closed percutaneously using a commercial ventricular septal defect occluder and an “active” MRI delivery cable for enhanced visibility. We used “permissive pericardial tamponade” (temporary fluid instillation to separate the 2 pericardial layers) to avoid pericardial entrapment by the epicardial disk. Techniques were developed in 8 animals, and 11 more were followed up to 3 months by MRI and histopathology.ResultsImaging guidance allowed 18-F sheath access and closure with appropriate positioning of the occluder inside the transmyocardial tunnel. Of the survival cohort, immediate hemostasis was achieved in 8 of 11 patients. Failure modes included pericardial entrapment by the epicardial occluder disk (n = 2) and a true-apex entry site that prevented hemostatic apposition of the endocardial disk (n = 1). Reactive pericardial effusion (192 ± 118 ml) accumulated 5 ± 1 days after the procedure, requiring 1-time drainage. At 3 months, LV function was preserved, and the device was endothelialized.ConclusionsDirect percutaneous LV access and closure is feasible using real-time MRI. A commercial occluder achieved hemostasis without evident deleterious effects on the LV. Having established the concept, further clinical development of this approach appears realistic

Dissipative electro-elastic network model of protein electrostatics

We propose a dissipative electro-elastic network model (DENM) to describe the dynamics and statistics of electrostatic fluctuations at active sites of proteins. The model combines the harmonic network of residue beads with overdamped dynamics of the normal modes of the network characterized by two friction coefficients. The electrostatic component is introduced to the model through atomic charges of the protein force field. The overall effect of the electrostatic fluctuations of the network is recorded through the frequency-dependent response functions of the electrostatic potential and electric field at the active site. We also consider the dynamics of displacements of individual residues in the network and the dynamics of distances between pairs of residues. The model is tested against loss spectra of residue displacements and the electrostatic potential and electric field at the heme's iron from all-atom molecular dynamics simulations of three hydrated globular proteins

The osmotic pressure of charged colloidal suspensions: A unified approach to linearized Poisson-Boltzmann theory

We study theoretically the osmotic pressure of a suspension of charged objects (e.g., colloids, polyelectrolytes, clay platelets, etc.) dialyzed against an electrolyte solution using the cell model and linear Poisson-Boltzmann (PB) theory. From the volume derivative of the grand potential functional of linear theory we obtain two novel expressions for the osmotic pressure in terms of the potential- or ion-profiles, neither of which coincides with the expression known from nonlinear PB theory, namely, the density of microions at the cell boundary. We show that the range of validity of linearization depends strongly on the linearization point and proof that expansion about the selfconsistently determined average potential is optimal in several respects. For instance, screening inside the suspension is automatically described by the actual ionic strength, resulting in the correct asymptotics at high colloid concentration. Together with the analytical solution of the linear PB equation for cell models of arbitrary dimension and electrolyte composition explicit and very general formulas for the osmotic pressure ensue. A comparison with nonlinear PB theory is provided. Our analysis also shows that whether or not linear theory predicts a phase separation depends crucially on the precise definition of the pressure, showing that an improper choice could predict an artificial phase separation in systems as important as DNA in physiological salt solution.Comment: 16 pages, 5 figures, REVTeX4 styl

THEORY OF PHASE-LOCKING IN SMALL JOSEPHSON JUNCTION CELLS

Within the RSJ model, we performed a theoretical analysis of phase-locking in elementary strongly coupled Josephson junction cells. For this purpose, we developed a systematic method allowing the investigation of phase-locking in cells with small but non-vanishing loop inductance.The voltages across the junctions are found to be locked with very small phase difference for almost all values of external flux. However, the general behavior of phase-locking is found to be just contrary to that according to weak coupling. In case of strong coupling there is nearly no influence of external magnetic flux on the phases, but the locking-frequency becomes flux-dependent. The influence of parameter splitting is considered as well as the effect of small capacitive shunting of the junctions. Strongly coupled cells show synchronization even for large parameter splitting. Finally, a study of the behavior under external microwave radiation shows that the frequency locking-range becomes strongly flux-dependent, whereas the locking frequency itself turns out to be flux-independent.Comment: 26 pages, REVTEX, 9 PS figures appended in uuencoded form at the end, submitted to Phys. Rev. B