arXiv subjects

Carolina Parra

Publications and source records attributed to Carolina Parra.

3 recordsLinked to original sources

arXiv2026

Monotone Allocations without Single-Crossing: When to Bunch and When to Jump

A principal screens an agent whose technology has a minimum efficient scale, so the Spence-Mirrlees condition fails along a monotone dividing curve: the locus at which every type values marginal output equally. For the class in which this curve and the relaxed solution are both strictly monotone, the optimal contract obeys a trichotomy, governed by how the two meet: a jump is impossible when they never meet, unavoidable across a flat dividing curve, a choice across a strictly increasing one. The optimum is found, not conjectured: each solution is certified as globally optimal among all implementable allocations, deterministic or random, by dualizing the family of binding constraints through an explicit weight; the certificates require neither linear primitives nor any restriction on the shape of the contract. Under mild regularity the class comprises exactly forty configurations; each is mapped to its forced shape, solved in closed form, and certified.

econ.TH↗

arXiv2026

Emergence of a Helical Metal in Rippled Ultrathin Topological Insulator Sb\textsubscript{2}Te\textsubscript{3} on Graphene

The integration of topological insulators (TIs) with graphene offers a pathway to engineer hybrid quantum states, yet the impact of strain at the 2D limit remains a critical open question. Here, we investigate the structural properties of ultrathin (1 quintuple layer) Sb$_2$Te$_3$ grown on single-layer graphene and, motivated by the structural modulations observed at the TI surface, explore theoretically how such nanoscale corrugations may influence the electronic behavior of the system. Using low-temperature scanning tunneling microscopy (LT-STM), we observe a periodic rippling of the heterostructure with a wavelength of ~$\sim8.7$ nm. Energetic analysis reveals that these ripples are not intrinsic but are driven by strain from the substrate during cooling. Density functional theory (DFT) calculations show that while the ideal flat heterostructure exhibits a hybridization gap of $\sim40$ meV, the ripple-induced structural modulation closes this gap, restoring a metallic state. This gapless phase is not a trivial metal. By combining an effective moir\'e ladder model with spin-resolved DFT, we find that the proximity-induced spin-orbit coupling is redistributed across a dense manifold of minibands. The resulting ``Helical Metal'' has a complex spin-texture beyond a simple Rashba splitting. Remarkably, while the flat system is effectively spinless in this ultrathin limit due to hybridization, the ripples actively restore the spin polarization. Our findings suggest that rippled TI/graphene heterostructures provide an interesting platform to develop spintronics, where geometric modulation unlocks dense helical states that are inaccessible in the pristine flat limit.

cond-mat.mes-hall↗

arXiv2023

Exploring Structural and Electronic Properties of Topological Insulator/Graphene Nano-heterostructures

There is great interest in the study of topological insulator-based heterostructures due to expected emerging phenomena. However, a challenge of topological insulator (TI) research is the contribution of the bulk conduction to the TI surface states. Both strain engineering and thickness control routes, which have been proposed to compensate for bulk doping, can be accessed through the use of nano-heterostructures consisting of topological insulator nanostructures grown on 2D materials. In this work, we report the synthesis of TI/graphene nano-heterostructures based on Bi2Te3 and Sb2Te3 nanoplatelets (NPs) grown on single-layer graphene. Various techniques were used to characterize this system in terms of morphology, thickness, composition, and crystal quality. We found that most of the obtained NPs are mainly < 20 [nm] thick with thickness-dependent crystal quality, observed by Raman measurements. Thinner NPs (1 or 2 QL) tend to replicate the topography of the underlying SLG, according to roughness analysis, and observed buckling features. Finally, we show preliminary studies of their band structure obtained by LT-STM (STS) and by DFT. We observe a highly negative doping which can be attributed to the presence of defects.

cond-mat.mes-hall↗