Producing smooth flow in atom circuits by stirring

We studied how smooth flow can be produced by stirring an ultracold atom circuit consisting of a gaseous Bose--Einstein condensate (BEC) confined in a ``racetrack\u27\u27 potential. The racetrack potential was made up of two straight parallel channels of length L connected on both ends by semicircular channels of the same width and (energy) depth as the straightaways. We used the Gross--Pitaevskii equation to simulate the behavior of the BEC in this potential when stirred by a rectangular paddle at various speeds and barrier heights. We found that smooth flow could be produced and conducted a systematic study of the flow produce under various conditions. We also laid the groundwork for the development of a simple model of the stirring of the BEC. This understanding should enable the design of a stirring sequence that would produce a given flow on demand

Complex Antiferromagnetic Order in the Metallic Triangular Lattice Compound SmAuAl4_4Ge2_2

The compounds $Ln$AuAl$_4$Ge$_2$ ($Ln$ $=$ lanthanide) form in a structure that features two-dimensional triangular lattices of $Ln$ ions that are stacked along the crystalline $c$ axis. Together with crystal electric field effects, magnetic anisotropy, and electron-mediated spin exchange interactions, this sets the stage for the emergence of strongly correlated spin and electron phenomena. Here we investigate SmAuAl$_4$Ge$_2$, which exhibits weak paramagnetism that strongly deviates from conventional Curie-Weiss behavior. Complex antiferromagnetic ordering emerges at $T_{\rm{N1}}$ $=$ 13.2 K and $T_{\rm{N2}}$ $=$ 7.4 K, where heat capacity measurements show that these transitions are first and second order, respectively. These measurements also reveal that the Sommerfeld coefficient is not enhanced compared to the nonmagnetic analog YAuAl$_4$Ge$_2$, consistent with the charge carrier quasiparticles exhibiting typical Fermi liquid behavior. The temperature-dependent electrical resistivity follows standard metallic behavior, but linear magnetoresistance unexpectedly appears within the ordered state. We compare these results to other $Ln$AuAl$_4$Ge$_2$ materials, which have already been established as localized $f$-electron magnets that are hosts for interesting magnetic and electronic phases. From this, SmAuAl$_4$Ge$_2$ emerges as a complex quantum spin metal, inviting further investigations into its properties and the broader family of related materials.Comment: 9 pages, 6 figure

Magnetic Ordering in GdAuAl4_4Ge2_2 and TbAuAl4_4Ge2_2: layered compounds with triangular lanthanide nets

We report the synthesis of the entire $Ln$AuAl$_4$Ge$_2$ ($Ln$ = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) series and focus on the magnetic properties of GdAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$. Temperature and magnetic field dependent magnetization, heat capacity, and electrical resistivity measurements reveal that both compounds exhibit several magnetically ordered states at low temperatures, with evidence for magnetic fluctuations extending into the paramagnetic temperature region. For magnetic fields applied in the $ab$-plane there are several ordered state regions that are associated with metamagnetic phase transitions, consistent with there being multiple nearly degenerate ground states. Despite Gd being an isotropic $S$-state ion and Tb having an anisotropic $J$-state, there are similarities in the phase diagrams for the two compounds, suggesting that factors such as the symmetry of the crystalline lattice, which features well separated triangular planes of lanthanide ions, or the Ruderman-Kittel-Kasuya-Yosida interaction as defined by the Fermi surface topography control the magnetism. We also point out similarities to other centrosymmetric compounds that host skyrmion lattices such as Gd$_2$PdSi$_3$, and propose that the $Ln$AuAl$_4$Ge$_2$ family of compounds are of interest as reservoirs for complex magnetism and electronic behaviors such as the topological Hall effect.Comment: 10 pages, 11 figure

Effect of Ni Doping on the Thermoelectric Properties of YbCo₂Zn₂₀

Thermoelectric devices are both solid-state heat pumps and energy generators. Having a reversible process without moving parts is of high importance for applications in remote locations or under extreme conditions. Yet, most thermoelectric devices have a rather limited energy conversion efficiency due to the natural competition between high electrical conductivity and low thermal conductivity, both being essential conditions for achieving a high energy conversion efficiency. Heavy-fermion compounds YbT2Zn20 (T = Co, Rh, Ir) have been reported to be potential candidate materials for thermoelectric applications at low temperatures. Motivated by this result, we applied chemical substitution studies on the transition metal site in order to optimize the charge carrier concentration as well as promote more efficient phonon scatterings. Here, we present the latest investigation on the Ni-doped specimens YbCo2−xNixZn20, where enhanced thermoelectric figure of merit values have been obtained