Engineering topics

Adnani, Moein

Publications and source records attributed to Adnani, Moein.

4 records

DOE OSTI2024

Pressure-induced superconductivity in a novel germanium allotrope

High-pressure studies on elements play an essential role in superconductivity research, with implications for both fundamental science and applications. Here we report the experimental discovery of surprisingly low pressure driving a novel germanium allotrope into a superconducting state in comparison to that for α-Ge. Raman measurements revealed structural phase transitions and possible electronic topological transitions under pressure up to 58 GPa. Based on pressure-dependent resistivity measurements, superconductivity was induced above 2 GPa and the maximum T c of 6.8 K was observed under 4.6 GPa. Interestingly, a superconductivity enhancement was discovered during decompression, indicating the possibility of maintaining pressure-induced superconductivity at ambient pressure with better superconducting performance. Density functional theory analysis further suggested that the electronic structure of Ge (oP32) is sensitive to its detailed geometry and revealed that disorder in the β-tin structure leads to a higher T c in comparison to the perfect β-tin Ge.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

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 OSTI2023

Anisotropic topological surface states in thin-film monoclinic Ag 2 Se

Silver chalcogenide systems have recently attracted significant attention due to their promising topological insulating properties. Here we conducted systematic low-temperature local scanning tunneling microscopy/spectroscopy and first-principles studies on the surface states of monoclinic Ag 2 Se thin films grown using molecular beam epitaxy. Through the use of quasi-particle interference patterns, we have observed evidence for topological surface states on the selenium-terminated surfaces with different types of defect densities. The results of ab initio calculations confirm the existence of nontrivial topological surface states in the monoclinic Ag 2 Se structure, for which such properties have not been previously reported. The energy dispersion determined using voltage-dependent standing wave patterns suggests that these topological states have an anisotropic Dirac cone structure. In conclusion, this discovery may lead to new applications for monoclinic Ag 2 Se in the rapidly growing fields of nanoelectronics and spintronics.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Magnetocapacitance effect and magnetoelectric coupling in type-II multiferroic HoFeWO 6

We have investigated the multiferroicity and magnetoelectric (ME) coupling in HoFeWO 6 . With a noncentrosymmetric polar structure (space group Pna2 1 ) at room temperature, this compound shows an onset of electric polarization with an antiferromagnetic ordering at the Néel temperature (T N ) of 17.8 K. The magnetic properties of the polycrystalline samples were studied by DC and AC magnetization and heat capacity measurements. The metamagnetic behavior at low temperatures was found to be directly related to the dielectric properties of the compound. In particular, field-dependent measurements of capacitance show a magnetocapacitance (MC) effect with double-hysteresis loop behavior in direct correspondence with the magnetization. Our x-ray diffraction results show the Pna2 1 structure down to 8 K and suggest the absence of a structural phase transition across T N . Soft x-ray absorption spectroscopy and soft x-ray magnetic circular dichroism (XMCD) measurements at the Fe L 2,3 and Ho M 4,5 edges revealed the oxidation state of Fe and Ho cations to be 3+. Fe L 2,3 XMCD further shows that Fe 3+ cations are antiferromagnetically ordered in a noncollinear fashion with spins arranged 90° with respect to each other. Our findings show that HoFeWO 6 is a type-II multiferroic exhibiting a MC effect. Furthermore, the observed MC effect and the change in polarization by the magnetic field, as well as their direct correspondence with magnetization, further support the strong ME coupling in this compound.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗