arXiv subjects

Saisab Bhowmik

Publications and source records attributed to Saisab Bhowmik.

6 recordsLinked to original sources

arXiv2026

Time Reversal Symmetry Broken Electronic Phases in Thin Films of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

High-temperature superconductors (high-Tc SCs) host a rich landscape of electronic phases encompassing the pseudogap, strange metal, superconducting, antiferromagnetic insulating, and Fermi-liquid regimes. The superconducting phase is notable for non-dissipative electronic functionality at relatively high temperatures. These phases are commonly probed in thermodynamic phase space by varying temperature or current through the sample. They can also be probed by breaking time-reversal symmetry (TRS) with an external magnetic field, which yields transition signatures distinct from those arising solely from temperature or current tuning. Here we show that electron transport in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ is primarily governed by two-dimensional superconductivity consistent with a Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition, as supported by current-voltage characteristics measured under temperature variation; these measurements preserve TRS. In contrast, when an external magnetic field is applied, the superconducting state is consistently preceded by weak antilocalization (WAL), where bound vortex-antivortex pairs dissociate into a normal metallic state through an intermediate localized phase. We further establish that highly disordered films exhibit transport dominated by three-dimensional weak localization, with superconductivity entirely suppressed.

cond-mat.supr-con↗

arXiv2025

Enhanced Phonon-Assisted Tunneling in Metal -- Twisted Bilayer Graphene Junctions

We report planar tunneling spectroscopy measurements on metal-WSe$_2$-twisted bilayer graphene heterostructures across a broad range of gate and bias voltages. The observed experimental features are attributed to phonon-assisted tunneling and the significantly high density of states within the moiré bands. A notable finding is the enhanced phonon-assisted tunneling in twisted bilayer graphene compared to Bernal bilayer graphene, which arises from a more relaxed in-plane momentum matching criterion. Theoretical calculations of phonon dispersions enable us to identify low-energy phonon modes in both Bernal and twisted bilayers of graphene, thereby elucidating the underlying mechanism of tunneling. Our results establish planar tunneling as a versatile tool to further understand electron-phonon coupling in twisted van der Waals materials.

cond-mat.mes-hall↗

arXiv2024

Nonlinear electrical transport unveils Fermi surface malleability in a moiré heterostructure

Van Hove singularities enhance many-body interactions and induce collective states of matter ranging from superconductivity to magnetism. In magic-angle twisted bilayer graphene, van Hove singularities appear at low energies and are malleable with density, leading to a sequence of Lifshitz transitions and resets observable in Hall measurements. However, without a magnetic field, linear transport measurements have limited sensitivity to the band's topology. Here, we utilize nonlinear longitudinal and transverse transport measurements to probe these unique features in twisted bilayer graphene at zero magnetic field. We demonstrate that the nonlinear responses, induced by the Berry curvature dipole and extrinsic scattering processes, intricately map the Fermi surface reconstructions at various fillings. Importantly, our experiments highlight the intrinsic connection of these features with the moiré bands. Beyond corroborating the insights from linear Hall measurements, our findings establish nonlinear transport as a pivotal tool for probing band topology and correlated phenomena.

cond-mat.mes-hall↗

arXiv2023

Emergent phases in graphene flat bands

Electronic correlations in two-dimensional materials play a crucial role in stabilising emergent phases of matter. The realisation of correlation-driven phenomena in graphene has remained a longstanding goal, primarily due to the absence of strong electron-electron interactions within its low-energy bands. In this context, magic-angle twisted bilayer graphene has recently emerged as a novel platform featuring correlated phases favoured by the low-energy flat bands of the underlying moiré superlattice. Notably, the observation of correlated insulators and superconductivity has garnered significant attention, leading to substantial progress in theoretical and experimental studies aiming to elucidate the origin and interplay between these two phases. A wealth of correlated phases with unprecedented tunability was discovered subsequently, including orbital ferromagnetism, Chern insulators, strange metallicity, density waves, and nematicity. However, a comprehensive understanding of these closely competing phases remains elusive. The ability to controllably twist and stack multiple graphene layers has enabled the creation of a whole new family of moiré superlattices with myriad properties being discovered at a fast pace. Here, we review the progress and development achieved so far, encompassing the rich phase diagrams offered by these graphene-based moiré systems. Additionally, we discuss multiple phases recently observed in non-moiré multilayer graphene systems. Finally, we outline future opportunities and challenges for the exploration of hidden phases in this new generation of moiré materials.

cond-mat.mes-hall↗

arXiv2022

Spin-orbit coupling-enhanced valley ordering of malleable bands in twisted bilayer graphene on WSe2

New phases of matter can be stabilized by a combination of diverging electronic density of states, strong interactions, and spin-orbit coupling. Recent experiments in magic-angle twisted bilayer graphene (TBG) have uncovered a wealth of novel phases as a result of interaction-driven spin-valley flavour polarization. In this work, we explore correlated phases appearing due to the combined effect of spin-orbit coupling-enhanced valley polarization and large density of states below half filling ($ν\lesssim 2$) of the moiré band in a TBG coupled to tungsten diselenide. We observe anomalous Hall effect, accompanied by a series of Lifshitz transitions, that are highly tunable with carrier density and magnetic field. Strikingly, the magnetization shows an abrupt sign change in the vicinity of half-filling, confirming its orbital nature. The coercive fields reported are about an order of magnitude higher than previous studies in graphene-based moiré systems, presumably aided by a Stoner instability favoured by the van Hove singularities in the malleable bands. While the Hall resistance is not quantized at zero magnetic fields, indicative of a ground state with partial valley polarization, perfect quantization and complete valley polarization are observed at finite fields. Our findings illustrate that singularities in the flat bands in the presence of spin-orbit coupling can stabilize ordered phases even at non-integer moiré band fillings.

cond-mat.mes-hall↗

arXiv2021

Emergence of broken-symmetry states at half-integer band fillings in twisted bilayer graphene

The dominance of Coulomb interactions over kinetic energy of electrons in narrow, non-trivial moiré bands of magic-angle twisted bilayer graphene (TBG) gives rise to a variety of correlated phases such as correlated insulators, superconductivity, orbital ferromagnetism, Chern insulators and nematicity. Most of these phases occur at or near an integer number of carriers per moiré unit cell. Experimental demonstration of ordered states at fractional moiré band-fillings at zero applied magnetic field $B$, is a challenging pursuit. In this letter, we report the observation of states near half-integer band-fillings of $ν\approx 0.5$ and $\pm3.5$ at $B\approx 0$ in TBG proximitized by tungsten diselenide (WSe$_2$) through magnetotransport and thermoelectricity measurements. A series of Lifshitz transitions due to the changes in the topology of the Fermi surface implies the evolution of van Hove singularities (VHSs) of the diverging density of states (DOS) at a discrete set of partial fillings of flat bands. Furthermore, at a band filling of $ν\approx-0.5$, a symmetry-broken Chern insulator emerges at high $B$, compatible with the band structure calculations within a translational symmetry-broken supercell with twice the area of the original TBG moiré cell. Our results are consistent with a spin/charge density wave ground state in TBG in the zero $B$-field limit.

cond-mat.mes-hall↗