Sobhit Singh

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.

Layered chalcogenides like CuInP2S6 are attracting attention as room-temperature ferroelectrics with enhanced polarization under compression and the ability to undergo pressure-assisted aging. Less has been done to reveal the impact of combined physical and chemical pressure. In this work, we integrate diamond anvil cell techniques with synchrotron-based infrared absorbance and Raman scattering spectroscopies, lattice dynamics calculations, and an analysis of the energy landscape to explore symmetry and space group progressions in the ABP2S6 family of materials (A = Cu, Ag; B = Cr, Sc, V, In). All four of the B-site substituted systems host a common set of pressure-driven structural phase transitions near 4 and 12 GPa that follow trends in metal ion size. Within the Cc space group, smaller cations stabilize new states at significantly lower pressures. For instance, while CuInP2S6 displays a monoclinic Cc → trigonal P31c symmetry restoration across PC,1 = 4.6 GPa, the Sc analog hosts the same progression at 2.6 GPa. We extend these structure–property relations to quaternary systems (many involving magnetic and/or ferroelectric functionality), further supporting the development of pressure-tunable electronic devices for environmental sensing, robotics, health monitoring, and biomachine interfaces.

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.

“Hidden” phases, generated using ultrafast laser pulses (few hundred femtoseconds), with distinct properties than at the thermodynamic equilibrium, are appealing for technologies, as they can be long-lived, with a lifetime of hours or weeks, and reversible with temperature sweeping or extra pulses. In this regard, La 2 / 3 Ca 1 / 3 MnO 3 (LCMO) stands out due to its tunability through epitaxial strain, which can drive the bulk ferromagnetic metal into an antiferromagnetic insulator (AFI), and its susceptibility to photoinduced transitions. Indeed, AFI LCMO displays a long-lived photoinduced transition into a putative hidden phase whose exact nature and excitations are still largely unknown. Here, we combine ultrafast photoexcitation in the near infrared with transport, x-ray absorption, and resonant inelastic x-ray scattering (RIXS) to investigate the excitations (polarons, phonons, and orbital) of the photoexcited phase of LCMO and contrast them with the thermodynamic phases achieved through strain and temperature. In the thermodynamic regime, we establish the correlation between polarons and transport placing them in the “strong coupling” regime of the Holstein model. Upon photoexcitation of LCMO-AFI, we uncover a long-lived phase characterized by the softening of the polaron excitations, the partial suppression of the Jahn-Teller distortion, and nearly unchanged phonons, showing the emergence of a photoexcited state absent in the equilibrium phase diagram. Finally, by varying temperature, epitaxial strain, and photoexcitation fluence, we construct a polaron phase diagram and identify the key spectroscopic signatures of each phase. Our laser-RIXS approach establishes a versatile platform for exploring photoinduced hidden phases in quantum materials in nonstroboscopic conditions.

Recent advances in two-dimensional (2D) van der Waals (vdW) metal thiophosphates have attracted considerable attention due to their promising ionic conductivity, optical characteristics, and tunable physical properties. Within this material family, LiInP₂S₆ has emerged as an intriguing candidate, not only because of its sensitivity to air and moisture but also due to its suitable band gap within the UV–vis range, enabling potential optoelectronic and photocatalytic applications. In this study, through comprehensive first-principles investigations, we unveil two previously unreported polymorphs of LiInP₂S₆ in the monoclinic C2/c and trigonal P3̅1c (in-gap) space groups, in addition to examining the experimentally synthesized P3̅1c (in-layer) phase. Our studies identify the C2/c structure as the ground state, lying 9 meV per unit cell lower in energy than the experimentally realized trigonal P3̅1c (in-layer) phase. Further, we systematically examine the elastic, mechanical, thermodynamical, dynamical, electronic, and optical properties of all three polymorphs, confirming their mechanical, thermal, and dynamical stability. Notably, the P3̅1c (in-gap) phase exhibits enhanced stiffness, while the calculated indirect band gaps and strong photon absorption in the UV–vis range (∼3 eV) highlight the potential of the studied LiInP₂S₆ phases for iontronic devices and optoelectronic applications.

The ferroelectric phase ( P c a 2 1 , which is in orthorhombic symmetry) of hafnium dioxide ( HfO 2 ) has gained much attention due to its potential applications in nanoelectronics and advanced memory devices. However, its complex phase behavior under external stimuli, such as pressure and temperature, remains a subject of intense investigation. This study focuses on developing a machine learning-based interatomic potential (MLIP) that is trained with data from density-functional theory (DFT) calculations to simulate phase transitions and mechanical properties of HfO 2 . The developed MLIP predicts lattice parameters, equations of state, bulk and shear moduli, and elastic constants that closely align with DFT predictions for several phases and at various pressures. Once validated, the MLIP is used to investigate the phase transitions of ferroelectric HfO 2 ( P c a 2 1 ) under both isobaric and constant stress conditions at elevated temperatures ranging from 200 to 2500 K. We used several complementary methods, including local symmetry identification, radial distribution function, and x-ray diffraction characterization, to identify interesting phase transitions among several competitive hafnia phases predicted from our simulations. The suggested methods uniformly reveal that under pure deviatoric condition, the system favors a transition from the orthorhombic P c a 2 1 phase to a tetragonal ( P 4 2 / n m c ) phase, whereas a zero stress condition drives the system from the P c a 2 1 phase to another orthorhombic ( P b c n ) phase. These findings provide crucial insights into stress and temperature-induced phase behavior of hafnia, guiding future experimental and theoretical studies for optimizing hafnia-based ferroelectric 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.