arXiv subjects

Sangram Biswas

Publications and source records attributed to Sangram Biswas.

4 recordsLinked to original sources

arXiv2023

Spin-valley locking for in-gap quantum dots in a MoS2 transistor

Spins confined to atomically-thin semiconductors are being actively explored as quantum information carriers. In transition metal dichalcogenides (TMDCs), the hexagonal crystal lattice gives rise to an additional valley degree of freedom with spin-valley locking and potentially enhanced spin life- and coherence times. However, realizing well-separated single-particle levels, and achieving transparent electrical contact to address them has remained challenging. Here, we report well-defined spin states in a few-layer MoS$ _2$ transistor, characterized with a spectral resolution of $\sim{50~μ}$eV at ${T_\textrm{el} = 150}$~mK. Ground state magnetospectroscopy confirms a finite Berry-curvature induced coupling of spin and valley, reflected in a pronounced Zeeman anisotropy, with a large out-of-plane $g$-factor of ${g_\perp \simeq 8}$. A finite in-plane $g$-factor (${g_\parallel \simeq 0.55-0.8}$) allows us to quantify spin-valley locking and estimate the spin-orbit splitting ${2Δ_{\rm SO} \sim 100~μ}$eV. The demonstration of spin-valley locking is an important milestone towards realizing spin-valley quantum bits.

cond-mat.mes-hall↗

arXiv2018

Quantized transport in topological insulator n-p-n junctions

Electrical transport in three dimensional topological insulators(TIs) occurs through spin-momentum locked topological surface states that enclose an insulating bulk. In the presence of a magnetic field, surface states get quantized into Landau levels giving rise to chiral edge states that are naturally spin-polarized due to spin momentum locking. It has been proposed that p-n junctions of TIs in the quantum Hall regime can manifest unique spin dependent effects, apart from forming basic building blocks for highly functional spintronic devices. Here, for the first time we study electrostatically defined n-p-n junctions of bulk insulating topological insulator BiSbTe$_{1.25}$Se$_{1.75}$ in the quantum Hall regime. We reveal the remarkable quantization of longitudinal resistance into plateaus at 3/2 and 2/3 h/e$^2$, apart from several partially developed fractional plateaus. Theoretical modeling combining the electrostatics of the dual gated TI n-p-n junction with Landauer Buttiker formalism for transport through a network of chiral edge states explains our experimental data, while revealing remarkable differences from p-n junctions of graphene and two-dimensional electron gas systems. Our work not only opens up a route towards exotic spintronic devices but also provides a test bed for investigating the unique signatures of quantum Hall effects in topological insulators.

cond-mat.mes-hall↗

arXiv2017

Probing the interplay between surface and bulk states in the topological Kondo insulator SmB$_6$ through conductance fluctuation spectroscopy

We present results of resistance fluctuation spectroscopy on single crystals of the predicted Kondo topological insulator material SmB$_6$. Our measurements show that at low temperatures, transport in this system takes place only through surface states. The measured noise in this temperature range arises due to Universal Conductance Fluctuations whose statistics was found to be consistent with theoretical predictions for that of two-dimensional systems in the Symplectic symmetry class. At higher temperatures, we find signatures of glassy dynamics and establish that the measured noise is caused by mobility fluctuations in the bulk. We find that, unlike the topological insulators of the dichalcogenide family, the noise in surface and bulk conduction channels in SmB$_6$ are completely uncorrelated. Our measurements establish that at sufficiently low temperatures, the bulk has no discernible contribution to electrical transport in SmB$_6$ making it an ideal platform for probing the physics of topological surface states.

cond-mat.mes-hall↗

arXiv2015

Robust local and non-local transport in the Topological Kondo Insulator SmB$_{6}$ in the presence of high magnetic field

SmB$_6$ has been predicted to be a Kondo Topological Insulator with topologically protected conducting surface states. We have studied quantitatively the electrical transport through surface states in high quality single crystals of SmB$_6$. We observe a large non-local surface signal at temperatures lower than the bulk Kondo gap scale. Measurements and finite element simulations allow us to distinguish unambiguously between the contributions from different transport channels. In contrast to general expectations, the electrical transport properties of the surface channels was found to be insensitive to high magnetic fields. Local and non-local magnetoresistance measurements allowed us to identify definite signatures of helical spin states and strong inter-band scattering at the surface.

cond-mat.mes-hall↗