GeSe was theoretically predicted to have thermoelectric (TE) performance as high as SnSe. However, the relatively high TE performance was not achieved experimentally in doped GeSe samples with an original orthorhombic structure but observed in Ag­(Sb,Bi)­(Se,Te)2 alloyed samples that crystalize in either a rhombohedral or cubic structure. Herein, to clarify the crystal structure-dependent properties, the electrical and thermal transport properties of GeSe1–x Te x (0 ≤ x ≤ 0.5), where orthorhombic, hexagonal, and rhombohedral phases are stable at room temperature for different Te content, have been studied, without any intentional manipulation on carrier concentration. It is found that the three phases show intrinsically different hole concentrations: ∼1016 cm–3 for the orthorhombic phase but as high as 1021 cm–3 for the hexagonal and rhombohedral phases. Ge-rich status in the orthorhombic phase and Ge-poor status in hexagonal and rhombohedral phases may be responsible for the huge difference in hole concentrations. The rhombohedral phase shows a much higher Seebeck coefficient than the hexagonal phase with similar hole concentration, indicating that the profile of valance band maximum for the rhombohedral structure is more favorable for high TE performance than the hexagonal phase in GeSe1–x Te x . The highest zT of 0.69 has been obtained in GeSe0.55Te0.45 at 778 K, at which temperature the rhombohedral phase has already transformed to a cubic phase; however, a zT value of 1.74 at 628 K is predicted by the quality factor analysis for rhombohedral GeSe0.55Te0.45 if optimum hole concentration can be achieved.

No abstract

No abstract

Natural superlattice structures MnBi2Te4(Bi2Te3)n (n = 1, 2, ...), in which magnetic MnBi2Te4 layers are separated by nonmagnetic Bi2Te3 layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is crucial for further exploration of quantum phenomena, remains elusive. Here, complex ferromagnetic-antiferromagnetic coexisting ground states that persist down to the 2-septuple layers limit are observed and comprehensively investigated in MnBi4Te7 (n = 1) and MnBi6Te10 (n = 2). The ubiquitous Mn-Bi site mixing modifies or even changes the sign of the subtle interlayer magnetic interactions, yielding a spatially inhomogeneous interlayer coupling. Further, a tunable exchange bias effect, arising from the coupling between the ferromagnetic and antiferromagnetic components in the ground state, is observed in MnBi2Te4(Bi2Te3)n (n = 1, 2), which provides design principles and material platforms for future spintronic devices. Our work highlights a new approach toward the fine-tuning of magnetism and paves the way for further study of quantum phenomena in MnBi2Te4(Bi2Te3)n (n = 1, 2) as well as their magnetic applications.

MnSb2Te4 has attracted extensive attention because of its rich and adjustable magnetic properties. Here, using a modified crystal growth method, ferrimagnetic MnSb2Te4 crystals with enhanced Curie temperature (T C) of about 40 K with dominant hole-type carriers and intrinsic anomalous Hall effect is obtained. Time- and angle-resolved photoemission spectroscopy reveals that surface states are absent in both antiferromagnetic and ferrimagnetic MnSb2Te4, implying that they have topologically trivial electronic structures. We propose that the enhancement of ferrimagnetism mainly originates from the increase of intralayer magnetic coupling caused by the decrease of Sb content at Mn sites when the decrease of Mn concentration at Sb sites would prefer the nontrival band topology. Moreover, it is known that the initial saturation moment (M is) is sensitive to the concentrations of Mn/Sb antisite defects; thus, the M is could be a valuable parameter to evaluate the magnetic and topological properties of MnX2n Te3n+1 (X = Bi, Sb) families.