Thermal bulk polymerization of cholesteryl acrylate

The thermal bulk polymerization of cholesteryl acrylate was carried out in the solid phase, the mesomorphic phase, and the liquid phase to study the effect of monomer ordering on polymerization rate and polymer properties. The rate increased with decreasing ordering (or enhanced mobility) of the monomer. Formation of inhibitive by-products during the polymerization limited conversions to 35%. The sedimentation constant S0 = 6.2 S was the same for the polymers obtained in the three phases. The weight-average molecular weight (w) was 480,000 as determined by ultracentrifugation. Poly-(cholesteryl acrylate) formed in bulk is randomly coiled when dissolved in tetrahydrofuran. The thermal properties of the monomer are given

Type I superconductivity in the Dirac semimetal PdTe2

The superconductor PdTe$_2$ was recently classified as a Type II Dirac semimetal, and advocated to be an improved platform for topological superconductivity. Here we report magnetic and transport measurements conducted to determine the nature of the superconducting phase. Surprisingly, we find that PdTe$_2$ is a Type I superconductor with $T_c = 1.64$ K and a critical field $\mu_0 H_c (0) = 13.6$ mT. Our crystals also exhibit the intermediate state as demonstrated by the differential paramagnetic effect. For $H > H_c$ we observe superconductivity of the surface sheath. This calls for a close examination of superconductivity in PdTe$_2$ in view of the presence of topological surface states.Comment: 5 page

High-pressure study of the basal-plane anisotropy of the upper critical field of the topological superconductor SrxBi2Se3

We report a high-pressure transport study of the upper-critical field, $B_{c2}(T)$, of the topological superconductor Sr$_{0.15}$Bi$_2$Se$_3$ ($T_c = 3.0$ K). $B_{c2}(T)$ was measured for magnetic fields directed along two orthogonal directions, $a$ and $a^*$, in the trigonal basal plane. While superconductivity is rapidly suppressed at the critical pressure $p_c \sim 3.5$ GPa, the pronounced two-fold basal-plane anisotropy $B_{c2}^a/B_{c2}^{a^*} = 3.2$ at $T=0.3$ K, recently reported at ambient pressure (Pan et al., 2016), is reinforced and attains a value of $\sim 5$ at the highest pressure (2.2 GPa). The data reveal that the unconventional superconducting state with broken rotational symmetry is robust under pressure

Enhancement of superconductivity near the ferromagnetic quantum critical point in UCoGe

We report a high-pressure single crystal study of the superconducting ferromagnet UCoGe. Ac-susceptibility and resistivity measurements under pressures up to 2.2 GPa show ferromagnetism is smoothly depressed and vanishes at a critical pressure $p_c = 1.4$ GPa. Near the ferromagnetic critical point superconductivity is enhanced. Upper-critical field measurements under pressure show $B_{c2}(0)$ attains remarkably large values, which provides solid evidence for spin-triplet superconductivity over the whole pressure range. The obtained $p-T$ phase diagram reveals superconductivity is closely connected to a ferromagnetic quantum critical point hidden under the superconducting `dome'.Comment: 4 pages, 3 figures; accepted for publication in PR

Muon spin rotation study of the topological superconductor SrxBi2Se3

We report transverse-field (TF) muon spin rotation experiments on single crystals of the topological superconductor Sr$_x$Bi$_2$Se$_3$ with nominal concentrations $x=0.15$ and $0.18$ ($T_c \sim 3$ K). The TF spectra ($B= 10$ mT), measured after cooling to below $T_c$ in field, did not show any additional damping of the muon precession signal due to the flux line lattice within the experimental uncertainty. This puts a lower bound on the magnetic penetration depth $\lambda \geq 2.3 ~\mu$m. However, when we induce disorder in the vortex lattice by changing the magnetic field below $T_c$ a sizeable damping rate is obtained for $T \rightarrow 0$. The data provide microscopic evidence for a superconducting volume fraction of $\sim 70~ \%$ in the $x=0.18$ crystal and thus bulk superconductivity.Comment: 6 pages, includes 4 figure

Angular variation of the magnetoresistance of the superconducting ferromagnet UCoGe

We report a magnetoresistance study of the superconducting ferromagnet UCoGe. The data, taken on single-crystalline samples, show a pronounced structure at $B^* = 8.5$~T for a field applied along the ordered moment $m_0$. Angle dependent measurements reveal this field-induced phenomenon has an uniaxial anisotropy. Magnetoresistance measurements under pressure show a rapid increase of $B^*$ to 12.8~T at 1.0~GPa. We discuss $B^*$ in terms of a field induced polarization change. Upper critical field measurements corroborate the unusual S-shaped $B_{c2}(T)$-curve for a field along the $b$-axis of the orthorhombic unit cell.Comment: 6 pages, 5 figures; accepted for publication in Phys. Rev.