Superconducting gap evolution in overdoped BaFe$_{2}$(As$_{1-x}$P$_x$)$_2$ single crystals through nanocalorimetry

We report on specific heat measurements on clean overdoped $mathrm{BaFe_{2}(As_{1-x}P_x)_2}$ single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature $gamma_mathrm{r}={C/T}|_{T to 0}$ is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave $alpha$ model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of $Delta_0 sim 5.3,mathrm{meV}$, corresponding to $Delta_0 / k_mathrm{B} T_mathrm{c} sim 2.2$. Increasing the phosphorus concentration $x$, the main gap reduces till a value of $Delta_0 sim 1.9,mathrm{meV}$ for $x = 0.55$ and a second weaker gap becomes evident. From the magnetic field effect on $gamma_mathrm{r}$, all samples however show similar behavior [$gamma_mathrm{r}(H) - gamma_mathrm{r}(H=0) propto H^n$, with $n$ between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

Microscopic Parameters from High-Resolution Specific Heat Measurements on Overdoped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ Single Crystals

We investigate the electronic specific heat of overdoped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the doping dependence of the condensation energy, superconducting gap $Delta$, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump $Delta C propto T_{mathrm{c}}^3$, found in many iron-based superconductors, in this system originates from a $T_mathrm{c}$-dependent ratio $Delta/k_mathrm{B}T_mathrm{c}$ in combination with a doping-dependent density of states $N(varepsilon_mathrm{F})$. A clear enhancement is seen in the effective mass $m^{*}$ as the composition approaches the value that has been associated with a quantum critical point at optimum doping. However, a simultaneous increase in the superconducting carrier concentration $n_mathrm{s}$ maintains the superfluid density, yielding an apparent penetration depth $lambda$ that decreases with increasing $T_mathrm{c}$ without sharp divergence at the quantum critical point. Uemura scaling indicates that $T_mathrm{c}$ is governed by the Fermi temperature $T_mathrm{F}$ for this multi-band system.

The impact of nuclear masses near $N=82$ on $r$-process abundances

We have performed for the first time a complete $r$-process mass sensitivity study in the $N=82$ region. We take into account how an uncertainty in a single nuclear mass propagates to influence important quantities of neighboring nuclei, including Q-values and reaction rates. We demonstrate that nuclear mass uncertainties of $pm0.5$ MeV in the $N=82$ region result in up to an order of magnitude local change in $r$-process abundance predictions. We identify key nuclei in the study whose mass has a substantial impact on final $r$-process abundances and could be measured at future radioactive beam facilities.

Multiphoton Ionization as a clock to Reveal Molecular Dynamics with Intense Short X-ray Free Electron Laser Pulses

We investigate molecular dynamics of multiple ionization in N2 through multiple core-level photoabsorption and subsequent Auger decay processes induced by intense, short X-ray free electron laser pulses. The timing dynamics of the photoabsorption and dissociation processes is mapped onto the kinetic energy of the fragments. Measurements of the latter allow us to map out the average internuclear separation for every molecular photoionization sequence step and obtain the average time interval between the photoabsorption events. Using multiphoton ionization as a tool of multiple-pulse pump-probe scheme, we demonstrate the modification of the ionization dynamics as we vary the x-ray laser pulse duration.

Catalyst-Free Growth of Millimeter-Long Topological Insulator Bi2Se3 Nanoribbons and the Observation of pi Berry Phase

We report the growth of single-crystalline Bi2Se3 nanoribbons with lengths up to several millimeters via a catalyst-free physical vapor deposition method. Scanning transmission electron microscopy analysis reveals that the nanoribbons grow along the (1120) direction. We obtain a detailed characterization of the electronic structure of the Bi2Se3 nanoribbons from measurements of Shubnikov-de Haas (SdH) quantum oscillations. Angular dependent magneto-transport measurements reveal a dominant two-dimensional contribution originating from surface states and weak contribution from the bulk states. The catalyst-free synthesis yields high-purity nanocrystals enabling the observation of a large number of SdH oscillation periods and allowing for an accurate determination of the pi-Berry phase, one of the key features of Dirac fermions in topological insulators. The long-length nanoribbons can empower the potential for fabricating multiple nanoelectronic devices on a single nanoribbon.

Doping dependence of the specific heat of single crystal BaFe2(As1-xPx)2

We present specific heat measurements on a series of BaFe2(As1-xPx)2 single crystals with phosphorous doping ranging from x = 0.3 to 0.55. Our results reveal that BaFe2(As1-xPx)2 follows the scaling Delta_C/Tc ~ Tc^2 remarkably well. The clean-limit nature of this material imposes new restraints on theories aimed at explaining the scaling. Furthermore, we find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is gamma~2.6, independent of doping.

Doping and Irradiation Controlled Vortex Pinning Behavior in BaFe2(As1-xPx)2 Single Crystals

We report on the systematic evolution of vortex pinning behavior in isovalent doped single crystals of BaFe2(As1-xPx)2. Proceeding from optimal doped to ovedoped samples, we find a clear transfor- mation of the magnetization hysteresis from a fishtail behavior to a distinct peak effect followed by a reversible magnetization and Bean Livingston surface barriers. Strong point pinning dominates the vortex behavior at low fields whereas weak collective pinning determines the behavior at higher fields. In addition to doping effects, we show that particle irradiation by energetic protons can tune vortex pinning in these materials.