Arjyama Bordoloi

The first phase of the topological revolution transformed condensed-matter physics through the discovery and classification of materials with remarkable topological properties. The research is now entering next phase of the topological revolution: a transition from discovering new topological materials to engineering and controlling topology on demand. This paper highlights emerging strategies, such as stacking, twisting, chemical engineering, and light-matter interactions, that offer new ways to create and dynamically manipulate topological states beyond those dictated by a material's intrinsic chemistry. The ultimate goal is to make topology a programmable degree of freedom, switchable, tunable, and reconfigurable for next-generation technologies.

Sulfur-doped black phosphorus (b-P) has recently emerged as a promising candidate for next-generation electronic and optoelectronic technologies owing to its enhanced environmental stability and tunable electronic properties. In this work, we systematically investigate the effects of sulfur substitution on the elastic, mechanical, and electronic properties of b-P, with a particular focus on its auxetic behavior (i.e., negative Poisson's ratio), using first-principles density functional theory calculations. Our results unveil the fundamental origin of the intrinsic auxetic response in pristine b-P and elucidate how sulfur incorporation alters this behavior. We find that sulfur atoms distort the characteristic bowtie structural motif responsible for the negative Poisson's ratio in b-P, thereby suppressing the in-plane auxeticity. Moreover, the resulting charge redistribution also effectively quenches the out-of-plane auxetic response of b-P. With increasing sulfur content, the bulk modulus and Poisson's ratio increase, whereas the Young's modulus, shear modulus, and Debye temperature decrease. Additionally, sulfur substitution suppresses the semiconducting properties of b-P, giving rise to metallicity. These findings highlight that, although sulfur substitution enhances the environmental stability of b-P, it also substantially modifies its elastic and mechanical properties, particularly the auxetic behavior, which is an important consideration in the design of nanoscale electronic devices.

Nonmagnetic topological insulators (TIs) are known for their robust metallic surface/edge states that are protected by time-reversal symmetry, making them promising candidates for next-generation spintronic and nanoelectronic devices. Traditional approaches to realizing TIs have focused on inducing band inversion via strong spin–orbit coupling (SOC), yet many materials with substantial SOC often remain topologically trivial. In this work, we present a materials-design strategy for engineering topologically non-trivial phases, e.g. quantum spin Hall phases, by vertically stacking topologically trivial Rashba monolayers in an inverted fashion. Using BiSb as a prototype system, we demonstrate that while the BiSb monolayer is topologically trivial (despite having significant SOC), an inverted BiSb–SbBi bilayer configuration realizes a non-trivial topological phase with enhanced spin Hall conductivity. We further reveal a delicate interplay between the SOC strength and the interlayer electron tunneling that governs the emergence of a nontrivial topological phase in the bilayer heterostructure. This phase can be systematically tuned using an external electric field, providing an experimentally accessible means of controlling the system’s topology. Our magnetotransport studies further validate this interplay by revealing g -factor suppression and the emergence of a zeroth Landau level. Notably, the inverted bilayer heterostructure exhibits a robust and tunable spin Hall effect, with performance comparable to that of state-of-the-art materials. Thus, our findings unveil an alternative pathway for designing and engineering functional properties in 2D topological systems using topologically trivial constituent monolayers.

Sulfur-doped black phosphorus (b-P) has recently emerged as a promising candidate for next-generation electronic and optoelectronic technologies owing to its enhanced environmental stability and tunable electronic properties. In this work, we systematically investigate the effects of sulfur substitution on the elastic, mechanical, and electronic properties of b-P, with a particular focus on its auxetic behavior (that is, negative Poisson's ratio), using first-principles density functional theory calculations. Our results unveil the fundamental origin of the intrinsic auxetic response in pristine b-P and elucidate how sulfur incorporation alters this behavior. We find that sulfur atoms distort the characteristic bow-tie structural motif responsible for the negative Poisson's ratio in b-P, thereby suppressing the in-plane auxeticity. Moreover, the resulting charge redistribution also effectively quenches the out-of-plane auxetic response of b-P. With increasing sulfur content, the bulk modulus and Poisson's ratio increase, whereas the Young's modulus, shear modulus, and Debye temperature decrease. Additionally, sulfur substitution suppresses the semiconducting properties of b-P, giving rise to metallicity. These findings highlight that although sulfur substitution enhances the environmental stability of b-P, it also substantially modifies its elastic and mechanical properties, particularly the auxetic behavior, which is an important consideration in the design of nanoscale electronic devices.

The Rashba spin–orbit coupling effect, primarily arising from structural-inversion asymmetry in periodic crystals, has garnered considerable attention due to its tunability and potential applications in spintronics. Its capability to manipulate electron spin without an external magnetic field opens new avenues for spintronic device design, particularly in semiconductor technology. Within this framework, 2D Rashba materials hold special interest due to their inherent characteristics, which facilitate miniaturization and engineering capabilities. In this Perspective article, we provide an overview of recent advancements in the research of 2D Rashba materials, aiming to offer a comprehensive understanding of the diverse manifestations and multifaceted implications of the Rashba effect in material science. Rather than merely presenting a list of materials, our approach involves synthesizing various viewpoints, assessing current trends, and addressing challenges within the field. Our objective is to bridge the gap between fundamental research and practical applications by correlating each material with the necessary advancements required to translate theoretical concepts into tangible technologies. Furthermore, we highlight promising avenues for future research and development, drawing from insights gleaned from the current state of the field.

Here, we report the influence of Jahn–Teller active Cu substitution on the charge-ordering (CO) characteristics of one of the well-known manganite Pr_0.45Sr_0.55MnO₃ (S55) with a distorted tetragonal structure. Magnetization studies unveil a complex magnetic phase diagram for S55, showing distinct temperature ranges corresponding to various magnetic phases: a ferromagnetic phase dominated by the Double Exchange interaction with T _C ∼ 220.5 K, an antiferromagnetic phase below T _N ∼ 207.6 K induced by CO with a transition temperature of T _CO ∼ 210 K consistent with the specific heat C _P(T) data, and a mixed phase in the range T _N < T < T _CO due to the competitive interplay of these two interactions. Dilute substitution of Cu at the Mn B-sites disrupts the robust charge-ordered state, leading to enhanced ferrimagnetic order with T _FN ∼273 K and significant magnetocrystalline anisotropy as confirmed by ferromagnetic resonance (FMR) studies. The Cu-substituted system displays a distinct cationic distribution compared to the pristine S55, contributing to its diverse magnetic structure. Our findings also reveal the irreversible metamagnetic transition (H _T-Max ∼ 8.85 kOe at 180 K) associated with the CO phenomena in S55 and the first-order nature of the phase transition across T _CO. The magnetic heat capacity critical analysis (C _Mag = A (T − T _N)^−α ) yields the exponent, α = 0.097 (0.154) in the region T > T _N (T < T _N) consistent with the magnetic structure. The temperature dependence of FMR resonance field ΔH _Res(T), peak-to-peak width H _PP(T), and Gilbert damping factor α _G(T) show clear anomalies across the magnetic transitions signifying the important role of admixtured (3+/4+) electronic state of Mn. Additionally, a strong correlation between the FMR α _G(T) and switchable magnetic entropy change (ΔS _Max ∼ −8/+ 3 J kg⁻¹ K⁻¹ for ΔH = 90 kOe) has also been established in S55.