Ghosh Subhadip, Shukla Shivam, Srivastava Sanjeev Kumar
School of Nano Science and Technology, Indian Institute of Technology, Kharagpur, 721302, West Bengal, India.
Department of Physics, Indian Institute of Technology, Kharagpur, West Bengal, 721302, India.
Nanoscale. 2025 Jul 24;17(29):17369-17383. doi: 10.1039/d5nr01640a.
Magnetism in topological materials has drawn significant attention due to its crucial role in stabilizing exotic quantum states. In topological crystalline insulators (TCIs), such as SnTe, the topological protection of energy bands arises from mirror symmetries in the crystal structure. When these symmetries are broken, new quantum phases, such as the quantum anomalous Hall and axion insulator states, can emerge. SnTe, a prototypical TCI, typically exhibits high bulk conductivity due to a large number of intrinsic Sn vacancies (10 cm), which act as sources of free charge carriers. However, when SnTe nanograins are anchored onto multi-walled carbon nanotubes (MWCNTs) or reduced graphene oxide (RGO), the formation energy of Sn vacancies increases. This leads to a reduction in vacancy concentration, generation of internal stress, band gap widening, and an enhancement of weak antilocalization behavior. These effects are observed in the resulting composites MWCNT@SnTe (CTS) and RGO-MWCNT@SnTe (SRC), which show improved surface state conductivity. This enhancement is attributed to suppressed electron-phonon scattering and hole localization charge transfer mechanisms. Moreover, these composites exhibit anisotropic behavior in surface and bulk carrier transport with respect to temperature and applied magnetic field. This anisotropy arises from the strain-induced band gap modification, which in turn reflects underlying magnetic anisotropy. The magnetic properties of CTS and SRC are further influenced by spin-orbit coupling (SOC), which induces spin canting charge transfer between components and temperature-dependent localization of electrons and holes. These combined effects contribute to the formation of bound magnetic polarons (BMPs) or spin clusters. At low temperatures and magnetic fields, CTS and SRC exhibit competing antiferromagnetic and ferromagnetic interactions, mediated by itinerant carriers through the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, which is activated by strain and band gap effects. At higher temperatures and magnetic fields, the magnetic behavior changes to a superparamagnetic or paramagnetic state. This change is driven by the suppression of the RKKY interaction, caused by the loss of crystal symmetry, SOC effects, and the presence of multiple Dirac cones.
拓扑材料中的磁性因其在稳定奇异量子态方面的关键作用而备受关注。在拓扑晶体绝缘体（TCI）中，如SnTe，能带的拓扑保护源于晶体结构中的镜面对称性。当这些对称性被打破时，新的量子相，如量子反常霍尔态和轴子绝缘体态，可能会出现。SnTe是一种典型的TCI，由于大量的本征Sn空位（10 cm），通常表现出高的体电导率，这些空位充当自由电荷载流子的来源。然而，当SnTe纳米颗粒锚定在多壁碳纳米管（MWCNT）或还原氧化石墨烯（RGO）上时，Sn空位的形成能增加。这导致空位浓度降低、内应力产生、带隙变宽以及弱反局域化行为增强。在所得的复合材料MWCNT@SnTe（CTS）和RGO-MWCNT@SnTe（SRC）中观察到了这些效应，它们表现出改善的表面态电导率。这种增强归因于电子 - 声子散射的抑制和空穴局域化电荷转移机制。此外，这些复合材料在表面和体载流子输运方面相对于温度和外加磁场表现出各向异性行为。这种各向异性源于应变诱导的带隙修正，这又反映了潜在的磁各向异性。CTS和SRC的磁性进一步受到自旋 - 轨道耦合（SOC）的影响，自旋 - 轨道耦合会在组分之间诱导自旋倾斜电荷转移以及电子和空穴的温度依赖性局域化。这些综合效应有助于形成束缚磁极化子（BMP）或自旋簇。在低温和低磁场下，CTS和SRC表现出由巡游载流子通过Ruderman-Kittel-Kasuya-Yosida（RKKY）机制介导的竞争反铁磁和铁磁相互作用，该机制由应变和带隙效应激活。在较高温度和磁场下，磁性行为转变为超顺磁或顺磁状态。这种变化是由晶体对称性丧失、SOC效应和多个狄拉克锥的存在导致的RKKY相互作用的抑制所驱动的。
