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1. Heterogeneous integration of superconducting thin films and epitaxial semiconductor heterostructures with lithium niobate

   https://doi.org/10.1088/1361-6463/acd7f9  

   Lienhart, Michelle ; Choquer, Michael ; Nysten, Emeline D. S. ; Weiß, Matthias ; Müller, Kai ; Finley, Jonathan J. ; Moody, Galan ; Krenner, Hubert J. ( June 2023 , Journal of Physics D: Applied Physics)

   We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots (QDs) on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III–V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factors$Q\approx 17000$representing a three-fold enhancement compared to identical devices with normal conducting electrodes. Superconducting operation of$\approx 400\mathrm{M}\mathrm{H}\mathrm{z}$resonators is achieved to temperatures$T>7\mathrm{K}$and electrical radio frequency powers$P_{\mathrm{r}\mathrm{f}}>+9\mathrm{d}\mathrm{B}\mathrm{m}$. Heterogeneously integrated single QDs couple to the resonant phononic field of the surface acoustic wave resonator operated in the superconducting regime. Position and frequency selective coupling mediated by deformation potential coupling is validated using time-integrated and time-resolved optical spectroscopy. Furthermore, acoustoelectric charge state control is achieved in a modified device geometry harnessing large piezoelectric fields inside the resonator. The hybrid QD—surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes for quantum phase modulation and transduction between photons and phonons via the QD. Finally, the employed materials allow for the realization of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits.

    

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2. Unconventional supercurrent phase in Ising superconductor Josephson junction with atomically thin magnetic insulator

   https://doi.org/10.1038/s41467-021-25608-1  

   Idzuchi, H. ; Pientka, F. ; Huang, K.-F. ; Harada, K. ; Gül, Ö. ; Shin, Y. J. ; Nguyen, L. T. ; Jo, N. H. ; Shindo, D. ; Cava, R. J. ; et al ( December 2021 , Nature Communications)

   Abstract In two-dimensional (2D) NbSe 2 crystal, which lacks inversion symmetry, strong spin-orbit coupling aligns the spins of Cooper pairs to the orbital valleys, forming Ising Cooper pairs (ICPs). The unusual spin texture of ICPs can be further modulated by introducing magnetic exchange. Here, we report unconventional supercurrent phase in van der Waals heterostructure Josephson junctions (JJs) that couples NbSe 2 ICPs across an atomically thin magnetic insulator (MI) Cr 2 Ge 2 Te 6 . By constructing a superconducting quantum interference device (SQUID), we measure the phase of the transferred Cooper pairs in the MI JJ. We demonstrate a doubly degenerate nontrivial JJ phase ( ϕ ), formed by momentum-conserving tunneling of ICPs across magnetic domains in the barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new dissipationless component for superconducting quantum devices. Our work boosts the study of various superconducting states with spin-orbit coupling, opening up an avenue to designing new superconducting phase-controlled quantum electronic devices. 

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3. High-Temperature Superconductivity in Size-Selected Metal Nanoclusters: Gas-Phase Spectroscopy and Prototype Devices for Deposition Studies

   https://doi.org/10.1007/s10948-021-06062-y  

   Edwards, Patrick J. ; Khojasteh, Malak ; Halder, Avik ; Kresin, Vitaly V. ( October 2021 , Journal of Superconductivity and Novel Magnetism)

   Metal nanoclusters, composed of tens to thousands of atoms, display the phenomenon of electronic shell structure. This quantum size effect, with its associated level degeneracy, turns out to be highly propitious for superconductivity. Spectroscopy of free aluminum nanoclusters has revealed a pairing phase transition in their electron density of states at a temperature two orders of magnitude higher than the bulk. In addition to gas-phase research on this new family of high-Tc systems, it is important to pursue their use as building blocks for superconducting networks and assemblies. The development of devices suitable for the detection of superconductivity in surface-deposited nanoclusters is outlined. 

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4. DIRECT-COUPLED MICRO-MAGNETOMETER WITH Y-BA-CU-O NANO-SLIT SQUID FABRICATED WITH A FOCUSED HELIUM ION BEAM

   https://doi.org/10.1063/1.5048776  

   Cho, Ethan ; Li, Hao ; Lefebvre, Jay ; Zhou, Yuchao ; Dynes, R.C. ; Cybart, S. A. ( October 2018 , Applied physics letters)

   Direct write patterning of high-transition temperature (high-TC) superconducting oxide thin films with a focused helium ion beam is a formidable approach for the scaling of high-TC circuit feature sizes down to the nanoscale. In this letter, we report using this technique to create a sensitive micro superconducting quantum interference device (SQUID) magnetometer with a sensing area of about 100 square microns. The device is fabricated from a single 35-nm thick YBa2Cu3O7d film. A flux concentrating pick-up loop is directly coupled to a 10 nm nano-slit SQUID. The SQUID is defined entirely by helium ion irradiation from a gas field ion source. The irradiation converts the superconductor to an insulator, and no material is milled away or etched. In this manner, a very narrow non-superconducting nano-slit is created entirely within the plane of the film. The narrow slit dimension allows for maximization of the coupling to the field concentrator. Electrical measurements reveal a large 0.35 mV modulation with a magnetic field. We measure a white noise level of 2 microPhi0. The field noise of the magnetometer is 4 pT/Hz1=2 at 4.2 K. 

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5. Controllable strain-driven topological phase transition and dominant surface-state transport in HfTe5

   https://doi.org/10.1038/s41467-023-44547-7  

   Liu, Jinyu ; Zhou, Yinong ; Yepez Rodriguez, Sebastian ; Delmont, Matthew A. ; Welser, Robert A. ; Ho, Triet ; Sirica, Nicholas ; McClure, Kaleb ; Vilmercati, Paolo ; Ziller, Joseph W. ; et al ( January 2024 , Nature Communications)

   The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe₅samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe₅and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe₅as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures.

    

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