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Quantum Phenomena in 3D Topological Materials
Highlight: In recent years experimentalists have been able to clearly show that several materials, such as MgB2, iron-based superconductors3, monolayer NbSe2, are multiband superconductors. Superconducting pairing in multiple bands can give rise to novel and very interesting phenomena. Leggett modes are exemplary of the unusual effects that can be present in multiband superconductors. A Leggett mode describes the collective periodic oscillation of the relative phase between the phases of the superconducting condensates formed by electrons in different bands. It can be thought of as the mode arising from an inter-band Josephson effect. The experimental observation of Leggett modes is challenging for several reasons: (i) Multiband superconductors are rare; (ii) they describe charge fluctuations between bands and therefore are hard to probe directly; (iii) their mass gap is often larger than the superconducting gaps and therefore are strongly overdamped via relaxation processes into the quasiparticle continuum. In this work we show that Leggett modes, and their frequency, can be detected unambigously in a.c. driven superconducting quantum interference devices (SQUIDs). We then use the results to analyze the measurements of a SQUID based on Cd3As2, an exemplar Dirac semimetal, in which superconductivity is induced by proximity to superconducting Al. The experimental results show the theoretically predicted unique signatures of Leggett modes and therefore allow us to conclude that a Leggett mode is present in the two-band superconducting state of Dirac semimetal (DSM) Cd3As2.
Quantum Transport and Superconducting Correlations in 2D Materials
Highlight: Indium Arsenide (InAs) near surface quantum wells (QWs) are ideal for the fabrication of semiconductor-superconductor heterostructures given that they allow for a strong hybridization between the two-dimensional states in the quantum well and the ones in the superconductor. In this work we present results for InAs QWs in the quantum Hall regime placed in proximity of superconducting NbTiN. We observe a negative downstream resistance with a corresponding reduction of Hall (upstream) resistance. We analyze the experimental data using the Landauer-Büttiker formalism, generalized to allow for Andreev reflection processes. Our analysis is consistent with a lower-bound for the averaged Andreev conversion of about 15%. We attribute the high efficiency of Andreev conversion in our devices to the large transparency of the InAs/NbTiN interface and the consequent strong hybridization of the QH edge modes with the states in the superconductor.
Josephson Effects for Topological Quantum Computing and Novel Electronics
Theoretical simulations
Highlight: Josephson junctions hosting Majorana fermions have been predicted to exhibit a 4π-periodic current phase relation. One experimental consequence of this periodicity is the disappearance of odd steps in Shapiro steps experiments. Experimentally, missing odd Shapiro steps have been observed in a number of materials systems with strong spin-orbit coupling and have been interpreted in the context of topological superconductivity. Here we report on missing odd steps in topologically trivial Josephson junctions fabricated on InAs quantum wells. We ascribe our observations to the high transparency of our junctions allowing Landau-Zener transitions. The probability of these processes is shown to be independent of the drive frequency. We analyze our results using a bi-modal transparency distribution which demonstrates that only few modes carrying 4π-periodic current are sufficient to describe the disappearance of odd steps. Our findings highlight the elaborate circumstances that have to be considered in the investigation of the 4πJosephson junctions in relationship to topological superconductivity.
Caloric Effects for Solid State Refrigeration
[1]: Solid-state cooling based on i-caloric effects is considered the most promising alternative to replace the conventional vapor-compression refrigeration systems. It is possible to define an i-caloric effect as a thermal response registered in a material upon the application of an external field, characterized by an adiabatic temperature change (ΔTS) or an isothermal entropy change (ΔST). Depending on the nature of the external field (magnetic field, electric field or stress field), the i-caloric effects can be categorized as magnetocaloric effect, electrocaloric effect, and mechanocaloric effect. We can still subdivide mechanocaloric effect in: elastocaloric effect, driven by uniaxial stress; barocaloric effect, driven by isotropic stress (pressure); and torsiocaloric effect, driven by a torque in a prismatic bar, causing a pure shear stress of torsion.
[1] "i-Caloric Effects: a proposal for normalization". William Imamura, Lucas S. Paixão, Érik O. Usuda, Nicolau M. Bom, Sergio Gama, Éder S. N. Lopes, Alexandre Magnus G. Carvalho. arXiv:1806.07959
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