Energy-gap driven low-temperature magnetic and transport properties in Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta)
ArXiv 2105.01393 (2021)
Inhomogeneous superconductivity in LuxZr1-xB12 dodecaborides with dynamic charge stripes
PHYSICAL REVIEW B 103:10 (2021) ARTN 104515
Abstract:
We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity, and magnetization) of model strongly correlated electronic systems LuxZr1-xB12 with cooperative Jahn-Teller instability of the boron rigid cage and with dynamic charge stripes. It was found that these metals are s-wave dirty limit superconductors with a small mean free path of charge carriers l=5-140Å and with a Cooper pair size changing nonmonotonously in the range 450-4000 Å. The parent ZrB12 and LuB12 borides are type-I superconductors, and Zr to Lu substitution induces a type-I to type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits 0.65≤κ1,2≤6. We argue in favor of the two-band scenario of superconductivity in LuxZr1-xB12 with gap values Δ1∼14K and Δ2∼6-8K, with pairing corresponding to strong coupling limit (λe-ph∼1) in the upper band, and to weak coupling (λe-ph∼0.1-0.4) in the lower one. A pseudogap Δps-gap∼60-110K is observed in LuxZr1-xB12 above Tc. We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair breaking and anisotropy, and analyze the origin of a unique enhanced surface superconductivity detected in these model compounds.Magnetic ground state of the one-dimensional ferromagnetic chain compounds M(NCS)(2)(thiourea)(2) (M = Ni, Co)
PHYSICAL REVIEW MATERIALS 5:3 (2021) 34401
Abstract:
The magnetic properties of the two isostructural molecule-based magnets - Ni(NCS)2(thiourea)2, S=1 [thiourea=SC(NH2)2] and Co(NCS)2(thiourea)2, S=3/2 - are characterized using several techniques in order to rationalize their relationship with structural parameters and to ascertain magnetic changes caused by substitution of the spin. Zero-field heat capacity and muon-spin relaxation measurements reveal low-temperature long-range ordering in both compounds, in addition to Ising-like (D<0) single-ion anisotropy (DCo∼-100 K, DNi∼-10 K). Crystal and electronic structure, combined with dc-field magnetometry, affirm highly quasi-one-dimensional behavior, with ferromagnetic intrachain exchange interactions JCo≈+4 K and JNi∼+100 K and weak antiferromagnetic interchain exchange, on the order of J′∼-0.1 K. Electron charge- and spin-density mapping reveals through-space exchange as a mechanism to explain the large discrepancy in J-values despite, from a structural perspective, the highly similar exchange pathways in both materials. Both species can be compared to the similar compounds MCl2(thiourea)4, M = Ni(II) (DTN) and Co(II) (DTC), where DTN is known to harbor two magnetic-field-induced quantum critical points. Direct comparison of DTN and DTC with the compounds studied here shows that substituting the halide Cl- ion for the NCS- ion results in a dramatic change in both the structural and magnetic properties.Magnetic order and ballistic spin transport in a sine-Gordon spin chain
PHYSICAL REVIEW B 103:6 (2021) ARTN L060405
Abstract:
© 2021 American Physical Society. We report the results of muon-spin spectroscopy (μ+SR) measurements on the staggered molecular spin chain [pym-Cu(NO3)2(H2O)2] (pym = pyrimidine), a material previously described using sine-Gordon field theory. Zero-field μ+SR reveals a long range magnetically ordered ground state below a transition temperature TN=0.23(1) K. Using longitudinal-field (LF) μ+SR we investigate the dynamic response in applied magnetic fields 0<B<500 mT and find evidence for ballistic spin transport. Our LF μ+SR measurements on the chiral spin chain [Cu(pym)(H2O)4]SiF6·H2O instead demonstrate one-dimensional spin diffusion, and the distinct spin transport in these two systems suggests that additional anisotropic interactions play an important role in determining the nature of spin transport in S=1/2 antiferromagnetic chains.Concepts in Magnetism
Springer Proceedings in Physics 262 (2021) 39-62