arXiv subjects

Neda Lotfizadeh

Publications and source records attributed to Neda Lotfizadeh.

8 recordsLinked to original sources

arXiv2024

Superconducting Diode Effect Sign Change in Epitaxial Al-InAs Josepshon Junctions

There has recently been a surge of interest in studying the superconducting diode effect (SDE) partly due to the possibility of uncovering the intrinsic properties of a material system. A change of sign of the SDE at finite magnetic field has previously been attributed to different mechanisms. Here, we observe the SDE in epitaxial Al-InAs Josephson junctions with strong Rashba spin-orbit coupling (SOC). We show that this effect strongly depends on the orientation of the in-plane magnetic field. In the presence of a strong magnetic field, we observe a change of sign in the SDE. Simulation and measurement of supercurrent suggest that depending on the superconducting widths, $W_\text{S}$, this sign change may not necessarily be related to 0--$π$ or topological transitions. We find that the strongest sign change in junctions with narrow $W_\text{S}$ is consistent with SOC-induced asymmetry of the critical current under magnetic-field inversion, while in wider $W_\text{S}$, the sign reversal could be related to 0--$π$ transitions and topological superconductivity.

cond-mat.mes-hall↗

arXiv2023

Vernier Spectrum and Isospin State Control in Carbon Nanotube Quantum Dots

Commensurability phenomena abound in nature and are typically associated with mismatched lengths, as can occur in quasiperiodic systems. However, not all commensuration effects are spatial in nature. In finite-sized Dirac systems, an intriguing example arises in tilted or warped Dirac cones wherein the degeneracy in the speed of right- and left-moving electrons within a given Dirac cone or valley is lifted. Bound states can be purely fast-moving or purely slow-moving, giving rise to incommensurate energy level spacings and a vernier spectrum. In this work, we present evidence for this vernier spectrum in Coulomb blockade measurements of ultraclean suspended carbon nanotube quantum dots. The addition-energy spectrum of the quantum dots reveals an energy-level structure that oscillates between aligned and misaligned energy levels. Our data suggest that the fast- and slow-moving bound states hybridize at certain gate voltages. Thus, gate-voltage tuning can select states with varying degrees of hybridization, suggesting numerous applications based on accessing this isospin-like degree of freedom.

cond-mat.mes-hall↗

arXiv2023

Evolution of ${4π}$-periodic Supercurrent in the Presence of In-plane Magnetic Field

In the presence of a 4$π$-periodic contribution to the current phase relation, for example in topological Josephson junctions, odd Shapiro steps are expected to be missing. While missing odd Shapiro steps have been observed in several material systems and interpreted in the context of topological superconductivity, they have also been observed in topologically trivial junctions. Here, we study the evolution of such trivial missing odd Shapiro steps in Al-InAs junctions in the presence of an in-plane magnetic field $B^θ$. We find that the odd steps reappear at a crossover $B^θ$ value, exhibiting an in-plane field angle anisotropy that depends on spin-orbit coupling effects. We interpret this behavior by theoretically analyzing the Andreev bound state spectrum and the transitions induced by the non-adiabatic dynamics of the junction. Our results highlight the complex phenomenology of missing Shapiro steps and the underlying current phase relations in planar Josephson junctions designed to realize Majorana states.

cond-mat.mes-hall↗

arXiv2022

The study of contact properties in edge-contacted graphene-aluminum Josephson junctions

Transparent contact interfaces in superconductor-graphene hybrid systems are critical for realizing superconducting quantum applications. Here, we examine the effect of the edge-contact fabrication process on the transparency of the superconducting aluminum-graphene junction. We show significant improvement in the transparency of our superconductor-graphene junctions by promoting the chemical component of the edge contact etch process. Our results compare favorably with state-of-the-art graphene Josephson junctions. The findings of our study contribute to advancing the fabrication knowledge of edge-contacted superconductor-graphene junctions.

cond-mat.supr-con↗

arXiv2022

Magneto-anisotropic weak antilocalization in near-surface quantum wells

We investigate the effects of an in-plane magnetic field on the weak antilocalization signature of near-surface quantum wells lacking bulk and inversion symmetry. The measured magnetoconductivity exhibits a strong anisotropy with respect to the direction of the in-plane magnetic field. The two-fold symmetry of the observed magneto-anisotropy originates from the competition between Rashba and Dresselhaus spin-orbit couplings. The high sensitivity of the weak antilocalization to the spin texture produced by the combined Zeeman and spin-orbit fields results in very large anisotropy ratios, reaching 100%. Using a semiclassical universal model in quantitative agreement with the experimental data, we uniquely determine the values of the Dresselhaus and Rashba parameters as well as the effective in-plane g-factor of the electrons. Understanding these parameters provides new prospects for novel applications ranging from spintronics to topological quantum computing.

cond-mat.mes-hall↗

arXiv2021

Local Control of Supercurrent Density in Epitaxial Planar Josephson Junctions

The critical current response to an applied out-of-plane magnetic field in a Josephson junction provides insight into the uniformity of its current distribution. In Josephson junctions with semiconducting weak links, the carrier density and, therefore the overall current distribution could be modified electrostatically via metallic gates. Here, we show local control of the current distribution in an epitaxial Al-InAs Josephson junction equipped with five mini-gates. We demonstrate that not only can the junction width be electrostatically defined but also we can locally adjust the current profile to form superconducting quantum interference devices. Our studies show enhanced edge conduction in such long junctions, which can be eliminated by mini-gates to create a uniform current distribution.

cond-mat.mes-hall↗

arXiv2019

Band-Gap-Dependent Electronic Compressibility of Carbon Nanotubes in the Wigner Crystal Regime

Electronic compressibility, the second derivative of ground state energy with respect to total electron number, is a measurable quantity that reveals the interaction strength of a system and can be used to characterize the orderly crystalline lattice of electrons known as the Wigner crystal. Here, we measure the electronic compressibility of individual suspended ultraclean carbon nanotubes in the low-density Wigner crystal regime. Using low-temperature quantum transport measurements, we determine the compressibility as a function of carrier number in nanotubes with varying band gaps. We observe two qualitatively different trends in compressibility versus carrier number, both of which can be explained using a theoretical model of a Wigner crystal that accounts for both the band gap and the confining potential experienced by charge carriers. We extract the interaction strength as a function of carrier number for individual nanotubes and show that the compressibility can be used to distinguish between strongly and weakly interacting regimes.

cond-mat.str-el↗

arXiv2018

Sagnac Electron Interference as a Probe of Electronic Structure

Electronic analogues of optical interferences have been used to investigate quantum phenomena in condensed matter. However, the most sensitive, Sagnac optical interference, has not been realized in the electronic domain. We report electronic Sagnac interference using ultraclean carbon nanotubes. We show that Sagnac interference manifests as conductance oscillations versus gate voltage. Compared to Fabry-Perot conductance oscillations, Sagnac oscillations have a larger period and are more robust to temperature. Using nanotubes of known structure, we show that Sagnac oscillations are a probe of nanotube chirality in a transport measurement. Furthermore, upon tuning the system through a topological phase transition in a magnetic field, we find that the Sagnac interference undergoes a π phase-shift. Our interferometry may be used for electronic structure characterization and study of many-body & photogalvanic effects in these model chiral one-dimensional systems.

cond-mat.mes-hall↗