arXiv subjects

Zubair Iftikhar

Publications and source records attributed to Zubair Iftikhar.

5 recordsLinked to original sources

arXiv2025

Testing Kubo formula on a nonlinear quantum conductor driven far from equilibrium via power exchanges

We present an experimental test of Kubo formula performed on a nonlinear quantum conductor, a Superconductor-Insulator-Superconductor tunnel junction, driven far from equilibrium by a DC voltage bias. We implement the proposal of Lesovik and Loosen [1] and demonstrate experimentally that it is possible to extract both the emission and absorption noise of the conductor by measuring the power it exchanges with a linear detection circuit whose occupation is tuned close to vacuum levels. We then compare their difference to the real part of the admittance which is independently measured by coherent reflectometry, finding that Kubo formula holds within experimental accuracy. Last, we show theoretically that the spectral density of power exchanged between a quantum conductor and its linear detection circuit follows a Lesovik and Loosen like formula, even in the presence of strong detection back-action. This result applies as long as the conductor acts as a current source for the detection circuit and the detection circuit is not singular.

cond-mat.mes-hall↗

arXiv2024

Thermal properties of the superconductor-quantum Hall interfaces

An important route of engineering topological states and excitations is to combine superconductors (SC) with the quantum Hall (QH) effect, and over the past decade, significant progress has been made in this direction. While typical measurements of these states focus on electronic properties, little attention has been paid to the accompanying thermal responses. Here, we examine the thermal properties of the interface between a type-II superconducting electrodes and graphene in the QH regime. We use the thermal noise measurement to probe the local electron temperature of the biased interface. Surprisingly, the measured temperature raise indicates that the superconductor provides a significant thermal conductivity, which is linear in temperature. This suggests electronic heat transport and may be unexpected, because the number of the quasiparticles in the superconductor should be exponentially suppressed. Instead, we attribute the measured electronic heat conductivity to the overlap of the normal states in the vortex cores.

cond-mat.supr-con↗

arXiv2023

Graphene-based quantum Hall interferometer with self-aligned side gates

The vanishing band gap of graphene has long presented challenges for making high-quality quantum point contacts (QPCs) -- the partially transparent p-n interfaces introduced by conventional split-gates tend to short the QPC. This complication has hindered the fabrication of graphene quantum Hall Fabry-Pérot interferometers, until recent advances have allowed split-gate QPCs to operate utilizing the highly resistive $ν=0$ state. Here, we present a simple recipe to fabricate QPCs by etching a narrow trench in the graphene sheet to separate the conducting channel from self-aligned graphene side gates. We demonstrate operation of the individual QPCs in the quantum Hall regime, and further utilize these QPCs to create and study a quantum Hall interferometer.

cond-mat.mes-hall↗

arXiv2023

Loss and decoherence at the quantum Hall - superconductor interface

We perform a systematic study of Andreev conversion at the interface between a superconductor and graphene in the quantum Hall (QH) regime. We find that the probability of Andreev conversion from electrons to holes follows an unexpected but clear trend: the dependencies on temperature and magnetic field are nearly decoupled. We discuss these trends and the role of the superconducting vortices, whose normal cores could both absorb and dephase the individual electrons in a QH edge. Our study may pave the road to engineering future generation of hybrid devices for exploiting superconductivity proximity in chiral channels.

cond-mat.mes-hall↗

arXiv2022

Fractional entropy of multichannel Kondo systems from conductance-charge relations

Fractional entropy is a signature of nonlocal degrees of freedom, such as Majorana zero modes or more exotic non-Abelian anyons. Although direct experimental measurements remain challenging, Maxwell relations provide an indirect route to the entropy through charge measurements. Here we consider multichannel charge-Kondo systems, which are predicted to host exotic quasiparticles due to a frustration of Kondo screening at low temperatures. In the absence of experimental data for the charge occupation, we derive relations connecting the latter to the conductance, for which experimental results have recently been obtained. Our analysis indicates that Majorana and Fibonacci anyon quasiparticles are well-developed in existing two- and three-channel charge-Kondo devices, and that their characteristic $k_{\rm{B}}\log\sqrt{2}$ and $k_{\rm{B}}\log\frac{1+\sqrt{5}}{2}$ entropies are experimentally measurable.

cond-mat.mes-hall↗