The transition metal thiophosphates M PS3 (M = Mn, Fe, Ni) are a class of van der Waals stacked insulating antiferromagnets that can be exfoliated down to the ultrathin limit. MnPS3 is particularly interesting because its Néel ordered state breaks both spatial-inversion and timereversal symmetries, allowing for a linear magneto-electric phase that is rare among van der Waals materials. However, it is unknown whether this unique magnetic structure of bulk MnPS3 remains stable in the ultrathin limit. Using optical second harmonic generation rotational anisotropy, we show that long-range linear magneto-electric type Néel order in MnPS3 persists down to at least 5.3 nm thickness. However an unusual mirror symmetry breaking develops in ultrathin samples on SiO2 substrates that is absent in bulk materials, which is likely related to substrate induced strain.
Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal-boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal-stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)-grown WSe monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD-grown WSe bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe for which the device performance becomes degraded above a strain of 0.63%.
Ferroelectric materials owning a polymorphic nanodomain structure usually exhibit colossal susceptibilities to external mechanical, electrical, and thermal stimuli, thus holding huge potential for relevant applications. Despite the success of traditional strategies by means of complex composition design, alternative simple methods such as strain engineering have been intensively sought to achieve a polymorphic nanodomain state in lead-free, simple-composition ferroelectric oxides in recent years. Here, a nanodomain configuration with morphed structural phases is realized in an epitaxial BaTiO 3 film grown on a (111)-oriented SrTiO 3 substrate. Using a combination of experimental and theoretical approaches, it is revealed that a threefold rotational symmetry element enforced by the epitaxial constraint along the [111] direction of BaTiO 3 introduces considerable instability among intrinsic tetragonal, orthorhombic, and rhombohedral phases. Such phase degeneracy induces ultrafine ferroelectric nanodomains (1-10 nm) with low-angle domain walls, which exhibit significantly enhanced dielectric and piezoelectric responses compared to the (001)-oriented BaTiO 3 film with uniaxial ferroelectricity. Therefore, the finding highlights the important role of epitaxial symmetry in domain engineering of oxide ferroelectrics and facilitates the development of dielectric capacitors and piezoelectric devices.
We herein describe our investigation of the superconducting and magnetic properties of the rare-earth ternary germanide intermetallic compounds La 2 Pt 3 Ge 5 and Pr 2 Pt 3 Ge 5 . Single crystals of La 2 Pt 3 Ge 5 and Pr 2 Pt 3 Ge 5 were synthesized using the high temperature metal flux method.Both types of crystal formed in a U 2 Co 3 Si 5 -type orthorhombic structure (space group Ibam). La 2 Pt 3 Ge 5 showed the onset of superconducting phase transition at T c = 8.1 K, which, to the best of our knowledge, is the highest T c of all the RE 2 T M 3 X 5 (RE = Rare Earth elements, T M = Transition metal, and X = s − p metal) superconductors, and from the specific heat data, it was found to have multi-gap superconductivity. Pr 2 Pt 3 Ge 5 showed both a superconducting phase transition at T c = 7.8 K and two antiferromagnetic transitions at T N 1 = 3.5 K and T N 2 = 4.2 K, which indicates the coexistence of superconductivity and magnetism. However, the correlation between the superconductivity and the magnetism was too weak to be observed. In its normal state, Pr 2 Pt 3 Ge 5 revealed strong magnetic anisotropy, probably due to the crystalline electric field effect.
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