arXiv subjects

Stuart Calder

Publications and source records attributed to Stuart Calder.

39 records · Page 3Linked to original sources

arXiv2015

Magnetic structure and phase stability of the van der Waals bonded ferromagnet Fe3-xGeTe2

The magnetic structure and phase diagram of the layered ferromagnetic compound Fe$_3$GeTe$_2$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single crystal neutron diffraction finds ferromagnetic order with moments of 1.11(5)$μ_B$/Fe aligned along the $c$-axis at 4K. These flux-grown crystals have a lower Curie temperature $T_{\textrm{c}}\approx$150K compared to crystals previously grown by vapor transport ($T_{\textrm{c}}$=220K). The difference is a reduced Fe content in the flux grown crystals, as illustrated by the behavior observed in a series of polycrystalline samples. As Fe-content decreases, so does the Curie temperature, magnetic anisotropy, and net magnetization. In addition, Hall effect and thermoelectric measurements on flux-grown crystals suggest multiple carrier types contribute to electrical transport in Fe$_{3-x}$GeTe$_2$ and structurally-similar Ni$_{3-x}$GeTe$_2$.

cond-mat.str-el↗

arXiv2012

Temperature-composition Phase Diagrams for Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2

Single crystals of mixed alkaline earth metal iron arsenide materials of Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2 are synthesized via the self-flux method. Ba1-xSrxFe2As2 display spin-density wave features (TN) at temperatures intermediate to the parent materials, x = 0 and 1, with TN(x) following an approximately linear trend. Cobalt doping of the 1 to 1 Ba:Sr mixture, Ba0.5Sr0.5(Fe1-yCoy)2As2, results in a superconducting dome with maximum transition temperature of TC = 19 K at y = 0.092, close to the maximum transition temperatures observed in unmixed A(Fe1-yCoy)2As2; however, an annealed crystal with y = 0.141 showed a TC increase from 11 to 16 K with a decrease in Sommerfeld coefficient from 2.58(2) to 0.63(2) mJ/(K2 mol atom). For the underdoped y = 0.053, neutron diffraction results give evidence that TN and structural transition (To) are linked at 78 K, with anomalies observed in magnetization, resistivity and heat capacity data, while a superconducting transition at TC ~ 6 K is seen in resistivity and heat capacity data. Scanning tunneling microscopy measurements for y = 0.073 give Dynes broadening factor of 1.15 and a superconducting gap of 2.37 meV with evidence of surface inhomogeneity.

cond-mat.supr-con↗

arXiv2011

Spin Glass and Semiconducting Behavior in 1D BaFe2-δSe3 Crystals

We investigate the physical properties and electronic structure of BaFe2-δSe3 crystals, which were grown out of tellurium flux. The crystal structure of the compound, an iron-deficient derivative of the ThCr2Si2-type, is built upon edge-shared FeSe4 tetrahedra fused into double chains. The semiconducting BaFe2-δSe3 with δ \approx 0.2 (ρ295K = 0.18 Ω\cdotcm and Eg = 0.30 eV) does not order magnetically, however there is evidence for short-range magnetic correlations of spin glass type (Tf \approx 50 K) in magnetization, heat capacity and neutron diffraction results. A one-third substitution of selenium with sulfur leads to a slightly higher electrical conductivity (ρ295K = 0.11 Ω\cdotcm and Eg = 0.22 eV) and a lower spin glass freezing temperature (Tf \approx 15 K), corroborating with higher electrical conductivity reported for BaFe2S3. According to the electronic structure calculations, BaFe2Se3 can be considered as a one-dimensional ladder structure with a weak interchain coupling.

cond-mat.supr-con↗