The discovery of materials with non-trivial topological properties has led to the realization of novel Josephson junctions with anomalous properties. In particular, it has been proposed that in some conditions such junctions can be in a superconducting topological state. In this work we present results for Josephson junctions based on three different heterostructures: Al/InAs, W/BiSb, and Al/Cd3As2. Junctions based on each of these heterostructures are predicted to have unique properties, and can in principle be tuned into a topological state, due to the fact that InAs has a very strong spin-orbit coupling, BiSb is a topological insulator, and Cd3As2 is a Dirac semimetal. We show how features of the Shapiro steps of the current-voltage characteristic under microwave radiation can be used, in realistic conditions, to extract detailed information on the microscopic electronic properties of the junctions, such as their topological state, and the presence of Leggett modes in the superconducting leads. We then discuss how in SQUIDs formed by some of the studied Josephson junctions a microwave-tunable diode effect might be present.
Recently, the Josephson diode effect (JDE) in Josephson junctions (JJs) has attracted a great deal of interests. It is expected that JDE may find important applications such as passive on-chip gyrators and circulators. Such devices would be particularly impactful in quantum information applications.
JDE occurs when both inversion symmetry and time reversal symmetry (TRS) are broken. Normally, external magnetic fields or magnetic heterostructures are exploited to break TRS. In this talk, we will present our recent observation of zero-magnetic-field JDE in topological superconducting quantum interference devices (SQUIDs) made of Dirac semimetal Cd3As2. We argue that a phase coupling between the surface and bulk superconducting channels, a unique phenomenon recently identified in the observations of fractional Josephson effect and Leggett modes in Cd3As2, can break TRS and, therefore, give rise to the zero-field JDE. We further show that the efficiency of the JDE can be readily varied by the geometry of JJ arms in the SQUIDs. Our results should have important implications in superconducting electronic circuit applications.
Single photon detection (SPD) has found increasingly important applications in many forefront areas of fundamental science and advanced engineering applications. The current SPD scheme has good sensitivity for photons in the high frequencies range (e.g., visible light). However, their sensitivity decreases drastically for low-frequency, low energy, microwave photons. As a result, the detection of single photons at this low frequency is highly prone to error from classical noise. In this talk we will present results from our recent studies of microwave response in a topological superconducting quantum interference device (SQUID) realized in Dirac semimetal Cd3As2. It is observed the effective temperature increases with the microwave power. This observation of large microwave response may pave the way for single photon detection at the microwave frequency in topological quantum materials.
The electromagnetic coupling of surface-plasmon-polariton (SPP) modes, which are localized around the surface of a conductive substrate, to quantum plasmons in a graphene sheet above the surface is investigated and their hybrid quantum-plasmon modes are analyzed. For a double-layer graphene structure, on the other hand, the interplay between the electromagnetic couplings of SPPs to each graphene sheet is explored. An effective- polarizability tensor for a combined system, including coupled double-layer graphene and conductive substrate, has been derived, which consists of the retarded nonlocal Coulomb interactions between electrons in different graphene sheets and the conductive substrate. Additionally, this calculated effective-scattering tensor can be used for constructing an effective-medium theory to study optical properties of inserted nanorods between the graphene sheets and metallic surface.
Properties of graphene can be tuned electrically and chemically, providing a promising system for application in
terahertz (THz) devices. Graphene response can be enhanced even further by means of coupling electromagnetic waves
into plasmon modes, frequency of which is controlled by geometrical parameters. To probe excitation of confined
plasmon modes and surface wave excitation, epitaxial graphene and its structures are investigated using THz near-field
microscopy. Detected near-field images suggest excitation of THz surface waves occurring at graphene edges, similar to
that observed at metallic edges, and excitation of confined plasmon modes. We will also discuss the impact of graphene
inhomogeneity on local THz transmission properties on the sub-wavelength scale.
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