Session N60: 2D Material Devices III

Investigations into the nature of edge conduction in monolayer WTe2

Monolayer WTe₂ displays edge conduction that is consistent with the material being a two-dimensional topological insulator, where the current is confined to helical edge modes while the interior region is insulating. The helical nature of the edge conduction should limit the conductance between adjacent contacts on an edge to e²/h, and this is consistent with experiments to date. However, the helical nature has not been supported by experiments that probe the spin properties of the edge states, and the reason the conductance is always less than e²/h is not yet established. To address this, we investigate the dependence on magnetic field orientation of the conduction of edges and cracks in monolayer WTe₂ flakes as a function of gate voltage, temperature, and edge direction relative to the crystal axes. Most importantly, we find that the conductance is suppressed by the component of the field perpendicular to a particular axis in the crystal. A theoretical model of the bands accounting for the crystal symmetries is consistent with this being the spin axis, implying a very simple form for the spin-orbit coupling in this material.   

Exploring two-dimensional semimetals such as WTe2 with a novel sample rotation scheme

The electronic properties of layered semimetals such as graphene, WTe₂, and ZrTe₅ are highly sensitive to magnetic fields, and the dependence on the direction of the field can be very informative. For example, the conductance of the edge states of monolayer WTe₂, which behaves as a 2D topological insulator, depends on the field orientation relative to the spin axis. Furthermore, when n-doped monolayer WTe₂ becomes superconducting below 1 K, the critical field is expected to be anisotropic in the plane and yield information about the spin-orbit coupling and pairing symmetry. We are therefore developing a novel two-axis sample rotation scheme which uses piezoelectric stepper motors held near room temperature in order to achieve independent, two-axis rotation of a sample held at He³ dilution refrigerator temperatures at the field center of a solenoid. We discuss the merits and challenges of the design, which avoids the unwanted heating, unreliability, and geometric constraints associated with conventional piezoelectric rotator schemes, and report our progress in using it to study magnetic anisotropy in WTe₂ and in other 2D semimetals, including hybrids of monolayer WTe₂ with other materials.   

Study on Coherent Transport Characteristics in Quantum Hall Antidot with Tunable Gates

In a quantum hall (QH) antidot, the QH edge modes are confined to the circumference of the antidot, forming quantized energy levels.  QH antidots allow the study of localized QH quasiparticles, such as the anyonic exchanged statistics. A key step towards such study is to establish a reliable and tunable coupling to the QH antidot. Here we develop high quality hBN-encapsulated graphene QH antidot devices with gate tunable couplings to the charge reservoirs. These devices allow measurements of the impacts of coupling strength and dielectric screening on the quantum coherence, and serves as the basis for studying anyonic statistics in the fractional QH regime using the antidot setup.   

Electrically Induced Dirac Fermions in Graphene Nanoribbons

Graphene nanoribbons are rapidly emerging as building blocks for carbon-based devices in the ultimate limit of scalability. Here, we investigate the response of armchair graphene nanoribbons to transverse electric fields on the basis of tight-binding Hamiltonians and ab initio calculations. Such fields can be achieved either upon electrical gating or by incorporating extrinsic impurities of opposite polarities along the nanoribbon. We demonstrate that the resulting field enforces a semiconductor-to-semimetal transition, with the semimetallic phase possessing zero-energy Dirac fermions that propagate along the armchair edges. This field-induced transition is inherent to group-IV honeycomb lattices, including silicene and germanene nanoribbons, irrespective of the type of edge termination. Overall, our findings open new avenues to electrically engineer Dirac semimetallic phases in otherwise semiconducting graphene-like nanoribbons.   

Generating coherent phonon waves in slow-electron materials: a twisted bilayer graphene phaser

Twisted bilayer graphene (TBG) exhibits extremely low Fermi velocities for electrons, with the speed of sound surpassing the Fermi velocity. This regime enables the use of TBG for generating coherent vibrational waves of the lattice following the same principles of operation of free-electron lasers. We present a prototype of a device based on undulated electrons in TBG, to produce a coherent beam of phonons. We dub this device the TBG phaser. The device generates high-intensity phonon beams in a terahertz frequency range. A coherent phonon generator can be used in high-precision imaging and sensing or to probe exotic electronic phases, and has already proven its usefulness aboard the USS enterprise.   

Two-electron multiplets in bilayer graphene quantum dots

Graphene quantum dots (QDs) are considered promising candidates for spin and valley-based quantum computing [1]. Here, we report on finite bias spectroscopy measurements of the two-electron spectrum in a gate defined bilayer graphene (BLG) QD for varying magnetic fields. The spin and valley degree of freedom in BLG give rise to a rich magnetic field dependent spectrum. We find that the two-electron states are split into multiplets of 6 orbital symmetric and 10 orbital anti-symmetric states, which are separated by 0.4 – 0.8 meV [2]. The symmetric multiplet exhibits an additional splitting due to short- range lattice scale interactions. With the help of detailed calculations, we are able to determine that inter-valley scattering and ‘current-current’ interaction constants are of the same magnitude in BLG [3, 4]. Understanding the two-particle spectrum in BLG QDs is an essential step for further investigating mechanisms of Pauli blockade in double QDs and eventually for identifying a suitable regime for qubit operations.

 

 

High-Tc superconducting nanowires of Bi2Sr2CaCu2O8+dand quantum phase slips

Superconducting nanowires have attracted a great deal of interest in recent years because of their technological applications in diverse fields. It also provides answers to some of the fundamental questions of superconductivity in reduced dimensions. As one approaches lower and lower form factor of nanowires, effects of fluctuations become important, one particular example being quantum phase slips (QPS). Although at the outset QPS may seem undesirable they offer some interesting avenues for applications. It has been proposed that superconducting nanowires can be used in realizing qubits for quantum computing exploiting the QPS. Nanowires with high superconducting transition temperature (T_c) would greatly help in that direction. Apart from that superconducting nanowire can be used as a sensitive photon detector.  Here we report a novel scalable route to make nanowires of a high-T_c superconductor Bi₂Sr₂CaCu₂O_8+d having T_c of 85 K. Measurements on these as-fabricated nanowires suggest existence of QPS. We also show these nanowires can be used as a sensitive detector for radiation in the visible range.       

Single photon detection at T = 20 K using a two-dimensional cuprate superconductor

Superconducting single-photon detectors constitute a key enabling technology for quantum photonics, both in free-space and on-chip architectures.

Still, the low critical temperature (T_c < 5 K) of commercial superconducting thin films precludes their application in a wider range of fields. High-temperature superconductors, such as the cuprates, offer the prospect of detecting single photons at moderate cryogenic temperatures [1].

Here, we report single-photon detection in few-layer flakes of the 2D cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ at a record temperature of 20 K for telecom wavelengths (λ = 1550 nm).

We combine van der Waals fabrication techniques with novel non-destructive nanopatterning to define 2D cuprate nanowires.

These high-temperature superconducting nanowires exhibit sharp current-voltage characteristics and non-bolometric photoresponse up to 30 K.

We achieve single and multi-photon detection regimes at 20 K as a function of the bias conditions of the nanowire

Such detectors are prime candidates for next generation quantum photonics.