Engineering topics

Koohpayeh, Seyed M.

Publications and source records attributed to Koohpayeh, Seyed M..

3 records

DOE OSTI2023

Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic Pr 2 Ti 2 O 7

Little is so far known about the magnetism of the A 2 B 2 O 7 monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for r A /r B > 1.78. We show that high quality monoclinic Pr 2 Ti 2 O 7 single crystals with a three-dimensional network of non-Kramers Pr 3+ ions that interact through edge-sharing superexchange interactions, form a singlet ground-state quantum paramagnet that does not undergo any magnetic phase transitions down to, at least, 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite Pr 2 Ti 2 O 7 were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of Pr 2 Ti 2+x O 7 (–0.16 ≤ x ≤ 0.16 ) showed that deviations from the Pr 2 Ti 2 O 7 stoichiometry lead to secondary phases of related structures including the perovskite phase Pr 2/3 TiO 3 and the orthorhombic phases Pr 4 Ti 9 O 24 and Pr 2 TiO 5 . No indications of site disordering (stuffing and antistuffing) or vacancy defects were observed in the Pr 2 Ti 2 O 7 majority phase. A procedure for growth of high-structural-quality stoichiometric single crystals of Pr 2 Ti 2 O 7 by the traveling solvent floating zone method is reported. Thermomagnetic measurements of single-crystalline Pr 2 Ti 2 O 7 reveal an isolated singlet ground state that we associate with the low-symmetry crystal electric-field environments that split the (2J + 1 = 9)-fold degenerate spin-orbital multiplets of the four differently coordinated Pr 3+ ions into 36 isolated singlets resulting in an anisotropic temperature-independent van Vleck susceptibility at low T. Here, a small isotropic Curie term is associated with 0.96(2)% noninteracting Pr 4+ impurities.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Bulk transport paths through defects in floating zone and Al flux grown SmB 6

We investigate the roles of disorder on low-temperature transport in SmB 6 crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arise from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. In this work, we consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of SmB 6 .

36 MATERIALS SCIENCE↗

DOE OSTI2020

Multiphase magnetism in Yb 2 Ti 2 O 7

We use neutron scattering to show that ferromagnetism and antiferromagnetism coexist in the low T state of the pyrochlore quantum magnet Yb 2 Ti 2 O 7 . While magnetic Bragg peaks evidence long-range static ferromagnetic order, inelastic scattering shows that short-range correlated antiferromagnetism is also present. Small-angle neutron scattering provides direct evidence for mesoscale magnetic structure that we associate with metastable antiferromagnetism. Classical Monte Carlo simulations based on exchange interactions inferred from ⟨ 111 ⟩ -oriented high-field spin wave measurements confirm that antiferromagnetism is metastable within the otherwise ferromagnetic ground state. The apparent lack of coherent spin wave excitations and strong sensitivity to quenched disorder characterizing Yb 2 Ti 2 O 7 is a consequence of this multiphase magnetism.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗