arXiv subjects

Javad Shabani

Publications and source records attributed to Javad Shabani.

71 records · Page 4Linked to original sources

arXiv2019

Electrical control of Majorana Bound States Using Magnetic Stripes

A hybrid semiconductor-superconductor nanowire on the top of a magnetic film in the stripe phase experiences a magnetic texture from the underlying fringing fields. The Zeeman interaction with the highly inhomogeneous magnetic textures generates a large synthetic spin-orbit coupling. We show that this platform can support the formation of Majorana bound states (MBS) localized at the ends of the nanowire. The transition to the topological superconducting phase not only depends on the nanowire parameters and stripe size but also on the relative orientation of the stripes with respect to the nanowire axis. Topological phase transitions with the corresponding emergence or destruction of MBS can be induced by reorienting the stripes or shifting their position, which can be achieved by passing a charge current through the magnetic film or by applying electrically-controlled strain to it. The proposed platform removes the need for external magnetic fields and offers a non-invasive electrical tuning of MBS with the perturbation (current or strain) acting only on thee magnetic film.

cond-mat.supr-con↗

arXiv2019

Josephson Junction Field-effect Transistors for Boolean Logic Cryogenic Applications

Josephson junction field effect transistors (JJ-FET) share design similarities with metal-oxide-semiconductor field effect transistors, except for the source/drain contacts being replaced by superconductors. Similarly, the super current due to proximity effect is tunable by the gate voltage. In this study, we examine the feasibility of JJ-FET-based Boolean logic and memory elements for cryogenic computing, in light of recent advances in novel materials and fabrication techniques. Using a two-dimensional ballistic transport JJ-FET model, we implement circuit level simulations for JJ-FET logic gates, and discuss criteria for realizing signal restoration, as well as fanout. We show that the JJ-FET is a promising candidate for very low power, clocked voltage-level dynamic logic at cryogenic temperatures.

physics.app-ph↗

arXiv2019

Gate Controlled Anomalous Phase Shift in Al/InAs Josephson Junctions

In a standard Josephson junction the current is zero when the phase difference between the superconducting leads is zero. This condition is protected by parity and time-reversal symmetries. However, the combined presence of spin-orbit coupling and magnetic field breaks these symmetries and can lead to a finite supercurrent even when the phase difference is zero. This is the so called anomalous Josephson effect -- the hallmark effect of superconducting spintronics --and can be characterized by the corresponding anomalous phase shift ($ϕ_0$). We report the observation of a tunable anomalous Josephson effect in InAs/Al Josephson junctions measured via a superconducting quantum interference device (SQUID). By gate controlling the density of InAs we are able to tune the spin-orbit coupling of the Josephson junction by more than one order of magnitude. This gives us the ability to tune $ϕ_0$, and opens several new opportunities for superconducting spintronics, and new possibilities for realizing and characterizing topological superconductivity.

cond-mat.mes-hall↗

arXiv2019

Transport Properties of Near Surface InAs Two-dimensional Heterostructures

Two-dimensional electron systems (2DESs) confined to the surface of narrowband semiconductors have attracted great interest since they can easily integrate with superconductivity (or ferromagnetism) enabling new possibilities in hybrid device architectures and study of exotic states in proximity of superconductors. In this work, we study indium arsenide heterostructures where combination of clean interface with superconductivity, high mobility and spin-orbit coupling can be achieved. The weak antilocalization measurements indicate presence of strong spin-orbit coupling at high densities. We study the magnetotransport as a function of top barrier and density and report clear observation of integer quantum Hall states. We report improved electron mobility reaching up to 44,000 cm$^{2}$/Vs in undoped heterstructures and well developed integer quantum Hall states starting as low as 2.5~T.

cond-mat.mes-hall↗

arXiv2019

Superconducting proximity effect in epitaxial Al-InAs heterostructures

Semiconductor-based Josephson junctions provide a platform for studying proximity effect due to the possibility of tuning junction properties by gate voltage and large-scale fabrication of complex Josephson circuits. Recently Josephson junctions using InAs weak link with epitaxial aluminum contact have improved the product of normal resistance and critical current, $I_cR_N$, in addition to fabrication process reliability. Here we study similar devices with epitaxial contact and find large supercurrent and substantial product of $I_cR_N$ in our junctions. However we find a striking difference when we compare these samples with higher mobility samples in terms of product of excess current and normal resistance, $I_{ex}R_N$. The excess current is negligible in lower mobility devices while it is substantial and independent of gate voltage and junction length in high mobility samples. This indicates that even though both sample types have epitaxial contacts only the high-mobility one has a high transparency interface. In the high mobility short junctions, we observe values of $I_cR_N/Δ\sim 2.2$ and $I_{ex}R_N/Δ\sim 1.5$ in semiconductor weak links.

cond-mat.mes-hall↗

arXiv2018

The multi-terminal Josephson effect

Establishment of phase-coherence and a non-dissipative (super)current between two weakly coupled superconductors, known as the Josephson effect, plays a foundational role in basic physics and applications to metrology, precision sensing, high-speed digital electronics, and quantum computing. The junction ranges from planar insulating oxides to single atoms, molecules, semiconductor nanowires, and generally to any finite-size coherent conductor. Recently, junctions of more than two superconducting terminals gained broad attention in the context of braiding of Majorana fermions in the solid state for fault-tolerant quantum computing, and accessing physics and topology in dimensions higher than three. Here we report the first observation of Josephson effect in 3- and 4-terminal junctions, fabricated in a top-down fashion from a semiconductor/superconductor (InAs/Al) epitaxial two-dimensional heterostructure. Due to interactions, the critical current of an N-terminal junction becomes the boundary of an (N-1)-dimensional manifold of simultaneously allowed supercurrents. The measured shapes of such manifolds are explained by the scattering theory of mesoscopic superconductivity, and they can be remarkably sensitive to the junction's symmetry class. Furthermore, we observed a notably high-order (up to 8) multiple Andreev reflections simultaneously across every terminals pair, which verifies the multi-terminal nature of normal scattering and a high interface quality in our devices. Given the previously shown gate-control of carrier density and evidence of spin-orbit scattering in InAs/Al heterostructures, and device compatibility with other 2D materials, the multi-terminal Josephson effect reported here can become a testbed for physics and applications of topological superconductivity.

cond-mat.supr-con↗

arXiv2018

Floquet Majorana zero and $π$ modes in planar Josephson junctions

We show how Floquet Majorana fermions may be experimentally realized by periodic driving of a solid-state platform. The system comprises a planar Josephson junction made of a proximitized heterostructure containing a 2D electron gas (2DEG) with Rashba spin-orbit coupling and Zeeman field. We map the sub-gap Andreev bound states of the junction to an effective one-dimensional Kitaev model with long range hopping and pairing terms. Using this effective model, we study the response of the system to periodic driving of the chemical potential, as applied via microwaves through a top-gate. We use the bulk Floquet topological invariants to characterize the system in terms of the number of zero and $π$ Majorana modes for experimentally realistic parameters. We present signatures of these modes on differential conductance and local density of states. A notable feature is subharmonic response to the drive when both Majorana zero and $π$ modes are present, thus opening up the possibility of using this feature to clearly identify Majorana modes. We highlight the robustness of these features to disorder, and show that despite hybridization due to long localization lengths, the main features of the subharmonic response are still identifiable in an experimental system that is readily accessible.

cond-mat.mes-hall↗

arXiv2018

Strong Electron-Electron Interactions of a Tomonaga--Luttinger Liquid Observed in InAs Quantum Wires

We report strong electron-electron interactions in quantum wires etched from an InAs quantum well, a material known to have strong spin-orbit interactions. We find that the current through the wires as a function of the bias voltage and temperature follows the universal scaling behavior of a Tomonaga--Luttinger liquid. Using a universal scaling formula, we extract the interaction parameter and find strong electron-electron interactions, increasing as the wires become more depleted. We establish theoretically that spin-orbit interactions cause only minor modifications of the interaction parameter in this regime, indicating that genuinely strong electron-electron interactions are indeed achieved in the device. Our results suggest that etched InAs wires provide a platform with both strong electron-electron and strong spin-orbit interactions.

cond-mat.mes-hall↗

arXiv2018

Long-range Kitaev Chains via Planar Josephson Junctions

We show how a recently proposed solid state Majorana platform comprising a planar Josephson junction proximitized to a 2D electron gas (2DEG) with Rashba spin-orbit coupling and Zeeman field can be viewed as an effectively one dimensional (1D) Kitaev chain with long-range pairing and hopping terms. We highlight how the couplings of the 1D system may be tuned by changing experimentally realistic parameters. We also show that the mapping is robust to disorder by computing the Clifford pseudospectrum index in real space for the long-range Kitaev chain across several topological phases. This mapping opens up the possibility of using current experimental setups to explore 1D topological superconductors with non-standard, and tunable couplings.

cond-mat.mes-hall↗

arXiv2018

Controlled-Z gate for transmon qubits coupled by semiconductor junctions

We analyze the coupling of two qubits via an epitaxial semiconducting junction. In particular, we consider three configurations that include pairs of transmons or gatemons as well as gatemon-like two qubits formed by an epitaxial four-terminal junction. These three configurations provide an electrical control of the interaction between the qubits by applying voltage to a metallic gate near the semiconductor junction and can be utilized to naturally realize a controlled-Z gate (CZ). We calculate the fidelity and timing for such CZ gate. We demonstrate that in the absence of decoherence, the CZ gate can be performed under $50\ {\rm ns}$ with gate error below $10^{-4}$.

quant-ph↗

arXiv2017

Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional Semiconductor-Superconductor Hybrid Platform

We investigate zero-bias conductance peaks that arise from coalescing subgap Andreev states, consistent with emerging Majorana zero modes, in hybrid semiconductor-superconductor wires defined in a two-dimensional InAs/Al heterostructure using top-down lithography and gating. The measurements indicate a hard superconducting gap, ballistic tunneling contact, and in-plane critical fields up to $3$~T. Top-down lithography allows complex geometries, branched structures, and straightforward scaling to multicomponent devices compared to structures made from assembled nanowires.

cond-mat.mes-hall↗

arXiv2017

Magnetic Field Inducing Zeeman Splitting and Anomalous Conductance Reduction of Half-integer Quantized Plateaus in InAs Quantum Wires

We report on magnetic field dependence of half-integer quantized conductance plateaus (HQPs) in InAs quantum wires. We observed HQPs at zero applied magnetic field in InAs quantum wires fabricated from a high-quality InAs quantum well. The application of in-plane magnetic field causes Zeeman splitting of the HQP features, indicating that the origin of the observed HQP is not spontaneous spin polarization. Additionally we observe that conductance of the split HQPs decreases gradually as the in-plane magnetic field increases. We finally assume electron-electron interaction as a possible mechanism to account for the zero-field HQPs and the anomalous field dependence.

cond-mat.mes-hall↗

arXiv2017

Tunable Magnetic Textures: From Majorana Bound States to Braiding

A versatile control of magnetic systems, widely used to store information, can also enable manipulating Majorana bounds states (MBS) and implementing fault-tolerant quantum information processing. The proposed platform relies on the proximity-induced superconductivity in a two-dimensional electron gas placed next to an array of magnetic tunnel junctions (MTJs). A change in the magnetization configuration in the MTJ array creates tunable magnetic textures thereby removing several typical requirements for MBS: strong spin-orbit coupling, applied magnetic field, and confinement by one-dimensional structures which complicates demonstrating non-Abelian statistics through braiding. Recent advances in fabricating two-dimensional epitaxial superconductor/semiconductor heterostructures and designing tunable magnetic textures support the feasibility of this novel platform for MBS.

cond-mat.mes-hall↗

arXiv2017

Equal-Spin Andreev Reflection in Junctions of Spin-Resolved Quantum Hall Bulk State and Spin-Singlet Superconductor

The recent development of superconducting spintronics has revealed the spin-triplet superconducting proximity effect from a spin-singlet superconductor into a spin-polarized normal metal. In addition recently superconducting junctions using semiconductors are in demand for highly controlled experiments to engineer topological superconductivity. Here we report experimental observation of Andreev reflection in junctions of spin-resolved quantum Hall (QH) states in an InAs quantum well and the spin-singlet superconductor NbTi. The measured conductance indicates a sub-gap feature and two peaks on the outer side of the sub-gap feature in the QH plateau-transition regime increases. The observed structures can be explained by considering transport with Andreev reflection from two channels, one originating from equal-spin Andreev reflection intermediated by spin-flip processes and second arising from normal Andreev reflection. This result indicates the possibility to induce the superconducting proximity gap in the the QH bulk state, and the possibility for the development of superconducting spintronics in semiconductor devices.

cond-mat.mes-hall↗

arXiv2016

Magneto-intersubband resistance oscillations in GaAs quantum wells placed in a tilted magnetic field

The magnetotransport of highly mobile 2D electrons in wide GaAs single quantum wells with three populated subbands placed in titled magnetic fields is studied. The bottoms of the lower two subbands have nearly the same energy while the bottom of the third subband has a much higher energy ($E_1\approx E_2<<E_3$). At zero in-plane magnetic fields magneto-intersubband oscillations (MISO) between the $i^{th}$ and $j^{th}$ subbands are observed and obey the relation $Δ_{ij}=E_j-E_i=k\cdot\hbarω_c$, where $ω_c$ is the cyclotron frequency and $k$ is an integer. An application of in-plane magnetic field produces dramatic changes in MISO and the corresponding electron spectrum. Three regimes are identified. At $\hbarω_c \ll Δ_{12}$ the in-plane magnetic field increases considerably the gap $Δ_{12}$, which is consistent with the semi-classical regime of electron propagation. In contrast at strong magnetic fields $\hbarω_c \gg Δ_{12}$ relatively weak oscillating variations of the electron spectrum with the in-plane magnetic field are observed. At $\hbarω_c \approx Δ_{12}$ the electron spectrum undergoes a transition between these two regimes through magnetic breakdown. In this transition regime MISO with odd quantum number $k$ terminate, while MISO corresponding to even $k$ evolve $continuously$ into the high field regime corresponding to $\hbarω_c \gg Δ_{12}$

cond-mat.mes-hall↗

arXiv2011

Imaging of the Coulomb driven quantum Hall edge states

The edges of a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime are divided into alternating metallic and insulating strips, with their widths determined by the energy gaps of the QHE states and the electrostatic Coulomb interaction. Local probing of these submicrometer features, however, is challenging due to the buried 2DEG structures. Using a newly developed microwave impedance microscope, we demonstrate the real-space conductivity mapping of the edge and bulk states. The sizes, positions, and field dependence of the edge strips around the sample perimeter agree quantitatively with the self-consistent electrostatic picture. The evolution of microwave images as a function of magnetic fields provides rich microscopic information around the ν= 2 QHE state.

cond-mat.mes-hall↗

arXiv2011

When are the $q/3$ fractional quantum Hall states stable?

Magneto-transport measurements in a wide GaAs quantum well in which we can tune the Fermi energy ($E_F$) to lie in different Landau levels of the two occupied electric subbands reveal a remarkable pattern for the appearance and disappearance of fractional quantum Hall states at $ν$ = 10/3, 11/3, 13/3, 14/3, 16/3, and 17/3. The data provide direct evidence that the $q/3$ states are stable and strong even at such high fillings as long as $E_F$ lies in a ground-state (N=0) Landau level of either of the two electric subbands, regardless of whether that level belongs to the symmetric or the anti-symmetric subband. Evidently, the node in the out-of-plane direction of the anti-symmetric subband does not destabilize the $q/3$ fractional states. On the other hand, when $E_F$ lies in an excited ($N>0$) Landau level of either subband, the wavefunction node(s) in the in-plane direction weaken or completely destabilize the $q/3$ fractional quantum Hall states. Our data also show that the $q/3$ states remain stable very near the crossing of two Landau levels belonging to the two subbands, especially if the levels have parallel spins.

cond-mat.str-el↗