We use point contact spectroscopy to probe the superconducting and normal state properties of the iron-based superconductor $rm{NaFe_{1-textit{x}}Co_{textit{x}}As}$ with $rm{textit{x} = 0, 0.02, 0.06}$. Andreev spectra corresponding to multiple super
conducting gaps are detected in the superconducting phase. For $rm{textit{x} = 0.02}$, a broad conductance enhancement around zero bias voltage is detected in both the normal and the superconducting phase. Such a feature is not present in the $rm{textit{x} = 0.06}$ samples. We suspect that this enhancement is caused by orbital fluctuations, as previously detected in underdoped $rm{Ba(Fe_{1-textit{x}}Co_textit{x})_2As_2}$ (Phys. Rev. B 85, 214515 (2012)). Occasionally, the superconducting phase shows a distinct asymmetric conductance feature instead of Andreev reflection. We discuss the possible origins of this feature. NaFeAs (the parent compound) grown by two different techniques is probed. Melt-grown NaFeAs shows a normal state conductance enhancement. On the other hand, at low temperatures, flux-grown NaFeAs shows a sharp dip in the conductance at zero bias voltage. The compounds are very reactive in air and the different spectra are likely a reflection of their different oxidation and purity levels.
The radio polarization characteristics of millisecond pulsars (MSPs) differ significantly from those of non-recycled pulsars. In particular, the position angle (PA) swings of many MSPs deviate from the S-shape predicted by the rotating vector model,
even after relativistic aberration is accounted for, indicating that they have non-dipolar magnetic geometries, likely due to a history of accretion. Stokes tomography uses phase portraits of the Stokes parameters as a diagnostic tool to infer a pulsars magnetic geometry and orientation. This paper applies Stokes tomography to MSPs, generalizing the technique to handle interpulse emission. We present an atlas of look-up tables for the Stokes phase portraits and PA swings of MSPs with current-modified dipole fields, filled core and hollow cone beams, and two empirical linear polarization models. We compare our look-up tables to data from 15 MSPs and find that the Stokes phase portraits for a current-modified dipole approximately match several MSPs whose PA swings are flat or irregular and cannot be reconciled with the standard axisymmetric rotating vector model. PSR J1939+2134 and PSR J0437$-$4715 are modelled in detail. The data from PSR J1939+2134 at 0.61,GHz can be fitted well with a current-modified dipole at $(alpha, i) = (22 pm 2^circ, 80 pm 1^circ)$ and emission altitude 0.4 $r_text{LC}$. The fit is less accurate for PSR J1939+2134 at 1.414,GHz, and for PSR J0437$-$4715 at 1.44,GHz, indicating that these objects may have a more complicated magnetic field geometry, such as a localized surface anomaly or a polar magnetic mountain.
Precession in an accretion-powered pulsar is expected to produce characteristic variations in the pulse properties. Assuming surface intensity maps with one and two hotspots, we compute theoretically the periodic modulation of the mean flux, pulse-ph
ase residuals and fractional amplitudes of the first and second harmonic of the pulse profiles. These quantities are characterised in terms of their relative precession phase offsets. We then search for these signatures in 37 days of X-ray timing data from the accreting millisecond pulsar XTE J1814-338. We analyse a 12.2-d modulation observed previously and show that it is consistent with a freely precessing neutron star only if the inclination angle is < 0.1 degrees, an a priori unlikely orientation. We conclude that if the observed flux variations are due to precession, our model incompletely describes the relative precession phase offsets (e.g. the surface intensity map is over-simplified). We are still able to place an upper limit on epsilon of 3.0 x 10^{-9} independently of our model, and estimate the phase-independent tilt angle theta; to lie roughly between 5 and 10 degrees. On the other hand, if the observed flux variations are not due to precession, the detected signal serves as a firm upper limit for any underlying precession signal. We then place an upper limit on the product epsilon cos(theta) of leq 9.9 x 10^{-10}. The first scenario translates into a maximum gravitational wave strain of 10^{-27} from XTE J1814-338 (assuming a distance of 8 kpc), and a corresponding signal-to-noise ratio of leq 10^{-3} (for a 120 day integration time) for the advanced LIGO ground-based gravitational wave detector.
Very much like the ubiquitous quantum interference of a single particle with itself, quantum interference of two independent, but indistinguishable, particles is also possible. This interference is a direct result of quantum exchange statistics, howe
ver, it is observed only in the joint probability to find the particles in two separated detectors. Here we report the first observation of such interference fringes between two independent and non-interacting electrons in an interferometer proposed by Yurke et al. and Samuelsson et al. Our experiment resembles the Hanbury Brown and Twiss (HBT) experiment, which was performed with classical waves. In the experiment, two independent and mutually incoherent electron beams were each partitioned into two trajectories. The combined four trajectories enclosed an Aharonov-Bohm (AB) flux (but not the two trajectories of a single electron). While individual currents were found to be independent of the AB flux, as expected, the cross-correlation between current fluctuations in two opposite points across the device exhibited strong AB oscillations. This is a direct signature of orbital entanglement between two electrons even though they never interact with each other.
