Magnetic and superconducting phase diagram of the half-Heusler topological semimetal HoPdBi

We report a study of the magnetic and electronic properties of the non-centrosymmetric half-Heusler antiferromagnet HoPdBi ($T_N = 2.0$ K). Magnetotransport measurements show HoPdBi has a semimetallic behaviour with a carrier concentration $n=3.7 times 10^{18}$ cm$^{-3}$ extracted from the Shubnikov-de Haas effect. The magnetic phase diagram in the field-temperature plane has been determined by transport, magnetization and thermal expansion measurements: magnetic order is suppressed at $B_Msim 3.6$ T for $T rightarrow 0$. Superconductivity with $T_c sim 1.9$ K is found in the antiferromagnetic phase. Ac-susceptibility measurements provide solid evidence for bulk superconductivity below $T_c = 0.75$ K with a screening signal close to a volume fraction of 100 %. The upper critical field shows an unusual linear temperature variation with $B_{c2}(T rightarrow 0) = 1.1$ T. We also report electronic structure calculations that classify HoPdBi as a new topological semimetal, with a non-trivial band inversion of 0.25 eV.

Impact of Quenched Oxygen Disorder on Charge Density Wave Order in YBa$_2$Cu$_3$O$_{6+x}$

The competition between superconductivity and charge density wave (CDW) order in underdoped cuprates has now been widely reported, but the role of disorder in this competition has yet to be fully resolved. A central question is whether disorder sets the length scale of the CDW order, for instance by pinning charge density fluctuations or disrupting an otherwise long range order. Using resonant soft x-ray scattering, we investigate the sensitivity of CDW order in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) to varying levels of oxygen disorder. We find that quench cooling YBCO$_{6.67}$ (YBCO$_{6.75}$) crystals to destroy their o-V and o-VIII (o-III) chains decreases the intensity of the CDW superlattice peak by a factor of 1.9 (1.3), but has little effect on the CDW correlation length, incommensurability, and temperature dependence. This reveals that while quenched oxygen disorder influences the CDW order parameter, the spatial extent of the CDW order is insensitive to the level of quenched oxygen disorder and may instead be a consequence of competition with superconductivity.

Distinct Charge Orders in the Planes and Chains of Ortho-III-Ordered YBa$_2$Cu$_3$O$_{6+delta}$ Superconductors Identified by Resonant Elastic X-ray Scattering

Recently, charge density wave (CDW) order in the CuO$_2$ planes of underdoped YBa$_2$Cu$_3$O$_{6+delta}$ was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of YBa$_2$Cu$_3$O$_{6.75}$ with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice reflection at $Q$=[0.33 0 $L$] whose energy and polarization dependence agrees with expectations for oxygen ordering and a spatial modulation of the Cu valence in the chains. Incommensurate peaks at [0.30 0 $L$] and [0 0.30 $L$] from the CDW order in the planes are shown to be distinct in $Q$ as well as their temperature, energy, and polarization dependence, and are thus unrelated to the structure of the chain layer. Moreover, the energy dependence of the CDW order in the planes is shown to result from a spatial modulation of energies of the Cu 2$p$ to 3$d_{x^2-y^2}$ transition, similar to stripe-ordered 214 cuprates.