arXiv subjects

Priya Tiwari

Publications and source records attributed to Priya Tiwari.

11 recordsLinked to original sources

arXiv2026

Observation of Time-Domain Braiding of Non-Abelian Anyons at $\nu = 5/2$ State

Unlike elementary particles, which obey either bosonic or fermionic exchange statistics, certain quasiparticles, known as anyons, are predicted to exhibit Abelian or non-Abelian braiding statistics. While braiding Abelian anyons modifies the wavefunction by a 'statistical phase', braiding non-Abelian anyons implements a unitary transformation of the state within a degenerate subspace of states. Experimental evidence of non-Abelian braiding has thus far remained elusive. Here, we report a 'time-domain braiding' signature of non-Abelian anyons in the $\nu = 5/2$ fractional quantum Hall state, by extending our previously demonstrated approach with Abelian anyons at $\nu = 1/3$. Our approach is based on measurements of the current fluctuations arising from weak partitioning of a highly dilute one-dimensional edge mode. We independently probe the partition noise of the downstream charged mode and also that of the upstream neutral mode. These independent measurements agree with our theoretical predictions for 'time-domain braiding' of the downstream Abelian and the upstream non-Abelian anyons, respectively, in the 'particle-hole Pfaffian' topological order. Together, these results provide evidence for the presence of non-Abelian anyons.

cond-mat.mes-hall↗

arXiv2026

Observation of e/4 charge at $\nu=1/2$ in GaAs

Even-denominator fractional quantum Hall states (FQHSs) fall outside the standard Laughlin's and Jain's odd-denominator hierarchy. In this work, we study the FQHS $\nu=1/2$ in the lowest Landau level. The state is confined within a 70 nm-wide GaAs quantum well, where the electrons exhibit a bilayer-like charge distribution. Inter-layer interactions stabilize the $\nu=1/2$ FQHS, which is predicted to host quasiparticles with charge e/4 - with either Abelian or non-Abelian topological order. Here, we report on shot-noise measurements of partitioned quasiparticles at $\nu=1/2$, where charge partitioning is generated by a unique etch-defined quantum point contact. Our measurements were performed on two nominally identical devices, at two independent experimental setups. Analysis of shot noise in the weak-backscattering regime in each device reveals quasiparticles with charge e/4. These observations provide a clear benchmark for future studies aimed at probing the topological order of the $\nu=1/2$ FQHS and its quasiparticles' exchange statistics.

cond-mat.mes-hall↗

arXiv2025

Tunable Band Inversion in Trilayer Graphene

Displacement field control of elecronic bands in low-dimensional systems is a promising route toward engineering emergent quantum phases. Here, we report displacement-field-induced band inversion and modulation of the Berry phase of low-energy quasi particles in high-mobility Bernal-stacked trilayer graphene (TLG). Using quantum oscillations, we track the evolution of the Fermi surface and topological properties of Dirac-like gully bands that emerge under a finite interlayer potential. We observe a striking sequence of transitions: at low displacement field $D$, the gullies are characterized by a Berry phase of $2\pi$ and large effective mass, indicating massive fermions. As $D$ increases, the Berry phase abruptly shifts to $\pi$ and the effective mass reaches a minimum, signaling the onset of massless Dirac behavior. At higher $D$, the Berry phase returns to $2\pi$, and the effective mass increases again, consistent with a band inversion. These findings demonstrate a rare, reversible topological phase transition - massive to massless to massive - driven entirely by an external displacement field. Despite robust theoretical predictions [\textit{Phys. Rev. B} \textbf{87}, 085424 (2013), \textit{Phys. Rev. B} \textbf{87}, 115422 (2013), and \textit{Phys. Rev. B} \textbf{101}, 245411 (2020)], this evolution of the band topology had escaped experimental detection. Our results establish TLG as a tunable platform for nanoscale control of band topology. They establish a means to tune between massive and Dirac-like dispersions dynamically providing a foundation for exploring field-switchable topological phenomena in layered 2D systems.

cond-mat.mes-hall↗

arXiv2025

Electric field tunable bands in doubly aligned bilayer graphene hBN moire superlattice

In this letter, we demonstrate electric field-induced band modification of an asymmetrically twisted hBN/BLG/hBN supermoire lattice. Distinct from unaligned BLG/hBN systems, we observe regions in the density-displacement field (n-D) plane where the device conductance is independent of n and decreases as |D| increases. This distinction arises due to the angle asymmetry between the layers, which induces field-controlled layer polarization. We identify D-dependent additional band gaps near the charge neutrality point that appear in the conduction (valence) band for negative (positive) D values. In the quantum Hall regime, new 6-fold degenerate Landau levels are observed. Our findings establish that in an asymmetric supermoire heterostructure, an external vertical displacement field affects the valence and conduction bands very differently and sheds light on the asymmetric conductance patterns noted in previous studies.

cond-mat.mes-hall↗

arXiv2023

Topological Thermal Hall Conductance of Even Denominator Fractional States

The even denominator fractional quantum Hall (FQH) states $\nu=5/2$ and $\nu=7/2$ have been long predicted to host non-abelian quasiparticles (QPs). Their present energy-carrying neutral modes are hidden from customary conductance measurements and thus motivate thermal transport measurements, which are sensitive to all energy-carrying modes. While past `two-terminal' thermal conductance ($k_{2t}T$) measurements already proved the non-Abelian nature of the $\nu=5/2$ FQH state, they might have been prone to a lack of thermal equilibration among the counter-propagating edge modes. Here, we report a novel thermal Hall conductance measurement of the $\nu=5/2$ and $\nu=7/2$ states, being insensitive to equilibration among edge modes. We verify the state's non-Abelian nature, with both states supporting a single upstream Majorana edge mode (hence, a particle-hole Pfaffian order). While current numerical works predict a different topological order, this contribution should motivate further theoretical work.

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arXiv2023

Observation of time-reversal symmetric Hall effect in graphene-WSe2 heterostructures at room temperature

In this letter, we provide experimental evidence of the time-reversal symmetric Hall effect in a mesoscopic system, namely high-mobility graphene/WSe$_2$ heterostructures. This linear, dissipative Hall effect, whose sign depends on the sign of the charge carriers, persists up to room temperature. The magnitude and the sign of the Hall signal can be tuned using an external perpendicular electric field. Our joint experimental and theoretical study establishes that the strain induced by lattice mismatch, or angle inhomogeneity, produces anisotropic bands in graphene while simultaneously breaking the inversion symmetry. The band anisotropy and reduced spatial symmetry lead to the appearance of a time-reversal symmetric Hall effect. Our study establishes graphene-transition metal dichalcogenide-based heterostructures as an excellent platform for studying the effects of broken symmetry on the physical properties of band-engineered two-dimensional systems.

cond-mat.mes-hall↗

arXiv2023

Higher-order Bragg gaps in the electronic band structure of bilayer graphene renormalized by recursive supermoir\'e potential

This letter presents our findings on the recursive band gap engineering of chiral fermions in bilayer graphene doubly aligned with hBN. By utilizing two interfering moir\'{e} potentials, we generate a supermoir\'{e} pattern which renormalizes the electronic bands of the pristine bilayer graphene, resulting in higher-order fractal gaps even at very low energies. These Bragg gaps can be mapped using a unique linear combination of periodic areas within the system. To validate our findings, we used electronic transport measurements to identify the position of these gaps as functions of the carrier density and establish their agreement with the predicted carrier densities and corresponding quantum numbers obtained using the continuum model. Our work provides direct experimental evidence of the quantization of the area of quasi-Brillouin zones in supermoir\'{e} systems. It fills essential gaps in understanding the band structure engineering of Dirac fermions by a recursive doubly periodic superlattice potential.

cond-mat.mes-hall↗

arXiv2022

Experimental observation of spin-split energy dispersion in high-mobility single-layer graphene/WSe2 heterostructures

Proximity-induced spin-orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant $\mathbb{Z}_2$ topological phase and spin-orbital filtering effects. An understanding of the effect of spin-orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin-orbit coupling. We achieve this in high-mobility hBN-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Further, we establish that the deviation of the low-energy band structure from pristine SLG is determined primarily by the valley-Zeeman SOC and Rashba SOC, with the Kane-Mele SOC being inconsequential. Despite robust theoretical predictions and observations of band-splitting, a quantitative measure of the spin-splitting of the valence and the conduction bands and the consequent low-energy dispersion relation in SLG was missing -- our combined experimental and theoretical study fills this lacuna.

cond-mat.mes-hall↗

arXiv2021

Transition from three- to two-dimensional Ising superconductivity in few-layer NbSe2 by proximity effect from van der Waals heterostacking

We report the experimental observation of Ising superconductivity in 3-dimensional NbSe2 stacked with single-layer MoS2. The angular dependence of the upper critical magnetic field and the temperature dependence of the upper parallel critical field confirm the appearance of two-dimensional Ising superconductivity in the 3-dimensional NbSe2 with single-layer MoS2 overlay. We show that the superconducting phase has strong Ising spin-orbit correlations which make the holes spin non-degenerate. Our observation of Ising superconductivity in heterostructures of few-layer NbSe2 of thickness ~ 15 nm with single-layer MoS2 raises the interesting prospect of observing topological chiral superconductors with nontrivial Chern numbers in a momentum-space spin-split fermionic system.

cond-mat.supr-con↗

arXiv2021

Electric-field-tunable valley Zeeman effect in bilayer graphene heterostructures: Realization of the spin-orbit valve effect

We report the discovery of electric-field-induced transition from a topologically trivial to a topologically nontrivial band structure in an atomically sharp heterostructure of bilayer graphene (BLG) and single-layer WSe2 per the theoretical predictions of Gmitra and Fabian [Phys. Rev. Lett. 119, 146401 (2017)]. Through detailed studies of the quantum correction to the conductance in the BLG, we establish that the band-structure evolution arises from an interplay between proximity-induced strong spin-orbit interaction (SOI) and the layer polarizability in BLG. The low-energy carriers in the BLG experience an effective valley Zeeman SOI that is completely gate tunable to the extent that it can be switched on or off by applying a transverse displacement field or can be controllably transferred between the valence and the conduction band. We demonstrate that this results in the evolution from weak localization to weak antilocalization at a constant electronic density as the net displacement field is tuned from a positive to a negative value with a concomitant SOI-induced splitting of the low-energy bands of the BLG near the K (K') valley, which is a unique signature of the theoretically predicted spin-orbit valve effect. Our analysis shows that quantum correction to the Drude conductance in Dirac materials with strong induced SOI can only be explained satisfactorily by a theory that accounts for the SOI-induced spin splitting of the BLG low-energy bands. Our results demonstrate the potential for achieving highly tunable devices based on the valley Zeeman effect in dual-gated two-dimensional materials.

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arXiv2020

Observation of Time-Reversal Invariant Helical Edge-Modes in Bilayer Graphene/WSe$_2$ Heterostructure

Topological insulators, along with Chern insulators and Quantum Hall insulator phases, are considered as paradigms for symmetry protected topological phases of matter. This article reports the experimental realization of the time-reversal invariant helical edge-modes in bilayer graphene/monolayer WSe$_2$-based heterostructures -- a phase generally considered as a precursor to the field of generic topological insulators. Our observation of this elusive phase depended crucially on our ability to create mesoscopic devices comprising both a moiré superlattice potential and strong spin-orbit coupling; this resulted in materials whose electronic band structure could be tuned from trivial to topological by an external displacement field. We find that the topological phase is characterized by a bulk bandgap and by helical edge-modes with electrical conductance quantized exactly to $2e^2/h$ in zero external magnetic field. We put the helical edge-modes on firm grounds through supporting experiments, including the verification of predictions of the Landauer-B$\mathrm{\ddot{u}}$ttiker model for quantum transport in multi-terminal mesoscopic devices. Our non-local transport properties measurements show that the helical edge-modes are dissipationless and equilibrate at the contact probes. We achieved the tunability of the different topological phases with electric and magnetic fields, which allowed us to achieve topological phase transitions between trivial and multiple, distinct topological phases. We also present results of a theoretical study of a realistic model which, in addition to replicating our experimental results, explains the origin of the topological insulating bulk and helical edge-modes. Our experimental and theoretical results establish a viable route to realizing the time-reversal invariant $\mathbb{Z}_2$ topological phase of matter.

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