Electron Pairing of Interfering Interface-Based Edge Modes

The remarkable Cooper-like pairing phenomenon in the Aharonov-Bohm interference of a Fabry-Perot interferometer (FPI)$\rm{-}$operating in the integer quantum Hall regime$\rm{-}$remains baffling. Here, we report the interference of paired electrons employing 'interface edge modes'. These modes are born at the interface between the bulk of the FPI and an outer gated region tuned to a lower filling factor. Such configuration allows toggling the spin and the orbital of the Landau level (LL) of the edge modes at the interface. We find that electron pairing occurs only when the two modes (the interfering outer and the first inner) belong to the same spinless LL.Comment: 21 pages, 10 figures, 1 table, Supplementary Informatio

Anyonic interference and braiding phase in a Mach-Zehnder Interferometer

The fractional quantum Hall states have long been predicted to be a testing ground of fractional (anyonic) exchange statistics. These topological states harbor quasiparticles with fractional charges of both abelian and non-abelian characters. The quasiparticles' charge is commonly determined by shot noise measurements (1, 2), and states' statistics can be revealed by appropriately interfering the quasiparticles. While the multipath Fabry-Perot electronic interferometer (FPI) is easier to fabricate, it is often plagued by Coulomb interactions (3), its area breathes with the magnetic field (4), and its bulk's charges tend to fluctuate (5). Recent FPI experiments employing adequate screening allowed an observation of Aharonov-Bohm (AB) interference at bulk filling $\nu$=1/3 (6). In the current work, we chose to employ an interaction-free, two-path, Mach-Zehnder interferometer (MZI), tuned to bulk filling $\nu$=2/5. Interfering the outer $\nu$=1/3 mode (with the inner $\nu$=1/15 mode screening out the bulk), we observed a 'dressed AB' periodicity, with a combined 'bare AB' flux periodicity of three flux-quanta (3$\phi_0$) and the 'braiding phase' 2$\pi$/3. This unique interference resulted with an AB periodicity of a single flux-quantum. Moreover, the visibility of the interference, $v_{e/3}$, deviated markedly from that of the electronic one $\it{v}_{e}$, agreeing with the theoretically expected visibility, $\it{v}_{e/3} \sim {\it{v}_e}^3$. With the two non-equivalent drains of the MZI, the fractional visibility peaked away from the ubiquitous transmission-half of the MZI. We provide simple theoretical arguments that support our results. The MZI proves to be a powerful tool that can be used to probe further the statistics of more complex anyonic quasiparticles

Quantum phase transition in few-layer NbSe2_2 probed through quantized conductance fluctuations

We present the first observation of dynamically modulated quantum phase transition (QPT) between two distinct charge density wave (CDW) phases in 2-dimensional 2H-NbSe$_2$. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive. We studied suspended, ultra-thin \nbse devices fabricated on piezoelectric substrates - with tunable flakes thickness, disorder level and strain. We find a surprising evolution of the conductance fluctuation spectra across the CDW temperature: the conductance fluctuates between two precise values, separated by a quantum of conductance. These quantized fluctuations disappear for disordered and on-substrate devices. With the help of mean-field calculations, these observations can be explained as to arise from dynamical phase transition between the two CDW states. To affirm this idea, we vary the lateral strain across the device via piezoelectric medium and map out the phase diagram near the quantum critical point (QCP). The results resolve a long-standing mystery of the anomalously large spectroscopic gap in NbSe$_2$

Effect of spin-orbit interaction on the vortex dynamics in LaAlO3/SrTiO3 interfaces near the superconducting transition

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion-symmetry-broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher-order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field-spin-orbit interaction energy plane for the quasi-two-dimensional electron gas at the interface of LaAlO3/SrTiO3 heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field B-SO and a non-Gaussian component to the fluctuations arises for fields below the upper critical field B-c2. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy, and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors

Universal scaling behaviour near vortex-solid/glass to vortex-fluid transition in type-II superconductors in two and three dimensions

In this paper, we present evidence for the existence of vortex-solid/glass (VG) to vortex-fluid (VF) transition in a type-II superconductor (SC), NbN. We probed the VG to VF transition in both 2D and 3D films of NbN through studies of magnetoresistance and current-voltage characteristics. The dynamical exponents corresponding to this phase transition were extracted independently of the two sets of measurements. The H-T phase diagram for the 2D and 3D SC are found to be significantly different near the critical point. In the case of 3D SC, the exponent values obtained from the two independent measurements show excellent match. On the other hand, for the 2D SC, the exponents obtained from the two experiments were significantly different. We attribute this to the fact that the characteristic length scale diverges near the critical point in a 2D SC in a distinctly different way from its 3D counterpart form scaling behaviour

Effect of spin-orbit interaction on the vortex dynamics in LaAlO3/SrTiO3 interfaces near the superconducting transition

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion-symmetry-broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher-order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field-spin-orbit interaction energy plane for the quasi-two-dimensional electron gas at the interface of LaAlO3/SrTiO3 heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field B-SO and a non-Gaussian component to the fluctuations arises for fields below the upper critical field B-c2. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy, and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors

Effect of dimensionality on the vortex dynamics in a type-II superconductor

We explore the effects of sample dimensionality on vortex pinning in a type-II, low-T C , s-wave superconductor, NbN, in the presence of a perpendicular magnetic field, H. We find significant differences in the phase diagrams in the magnetic field-temperature plane between three-dimensional (3D) and 2D NbN films. The differences are most striking close to the normal-superconductor phase transition. We establish that these variances have their origin in the differing pinning properties in two different dimensions. We obtain the pinning strength quantitatively in both the dimensions from two independent transport measurements performed in two different regimes of vortex motion: (i) thermally assisted flux-flow regime and (ii) flux flow regime. Both the measurements consistently show that both the pinning potential and the zero-field free-energy barrier to depinning in the 3D superconductor are at least an order of magnitude stronger than that in the 2D superconductor. Further, we probed the dynamics of pinning in both 2D and 3D superconductor through voltage fluctuation spectroscopy. We find that the mechanism of vortex pinning-depinning is qualitatively similar for the 3D and 2D superconductors. The voltage-fluctuations arising from vortex motion are found to be correlated only in the 2D superconductor. We establish this to be due to the presence of long-range phase fluctuations near the Berezinskii-Kosterlitz-Thouless-type superconducting transition in 2D superconductors

Effect of dimensionality on the vortex dynamics in a type-II superconductor

We explore the effects of sample dimensionality on vortex pinning in a type-II, low-T-C, s-wave superconductor, NbN, in the presence of a perpendicular magnetic field, H. We find significant differences in the phase diagrams in the magnetic field-temperature plane between three-dimensional (3D) and 2D NbN films. The differences are most striking close to the normal-superconductor phase transition. We establish that these variances have their origin in the differing pinning properties in two different dimensions. We obtain the pinning strength quantitatively in both the dimensions from two independent transport measurements performed in two different regimes of vortex motion: (i) thermally assisted flux-flow regime and (ii) flux flow regime. Both the measurements consistently show that both the pinning potential and the zero-field free-energy barrier to depinning in the 3D superconductor are at least an order of magnitude stronger than that in the 2D superconductor. Further, we probed the dynamics of pinning in both 2D and 3D superconductor through voltage fluctuation spectroscopy. We find that the mechanism of vortex pinning-depinning is qualitatively similar for the 3D and 2D superconductors. The voltage-fluctuations arising from vortex motion are found to be correlated only in the 2D superconductor. We establish this to be due to the presence of long-range phase fluctuations near the Berezinskii-Kosterlitz-Thouless-type superconducting transition in 2D superconductors