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1. Field-induced resistance peak in a superconducting niobium thin film proximity coupled to a surface reconstructed SrTiO3

   https://doi.org/10.1038/s41535-020-0242-4  

   Singh, Akhilesh Kr. ; Kar, Uddipta ; Redell, Matthew D. ; Wu, Tsung-Chi ; Peng, Wei-Hsiang ; Das, Bipul ; Kumar, Satish ; Lee, Wei-Cheng ; Lee, Wei-Li ( July 2020 , npj Quantum Materials)

   Oxygen vacancy is known to play an important role for the physical properties in SrTiO₃(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films withmore »different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO₂. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.

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2. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R. ; Sarkar, Tarapada ; Greene, Richard L. ( March 2019 , Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H_C2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-T_Csuperconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La_2−xCe_xCuO₄(LCCO) fromx= 0.11–0.19. We observe quantum critical$\mathit{S}/\mathit{T}$versus$\mathbf{l}\mathbf{n}\left(\mathbf{1}/\mathit{T}\right)$behavior over an unexpectedly wide doping rangex= 0.15–0.17 above the QCP (x= 0.14), with a slope that scales monotonicallymore »with the superconducting transition temperature (T_Cwith H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked withT_C. Above the superconductivity dome, atx= 0.19, a conventional Fermi-liquid$\mathit{S}\propto \mathit{T}$behavior is found for$\mathit{T}\le$40 K.

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3. Artificially engineered nanostrain in FeSexTe1-x superconductor thin films for supercurrent enhancement

   https://doi.org/10.1038/s41427-019-0186-y  

   Seo, Sehun ; Noh, Heesung ; Li, Ning ; Jiang, Jianyi ; Tarantini, Chiara ; Shi, Ruochen ; Jung, Soon-Gil ; Jun Oh, Myeong ; Liu, Mengchao ; Lee, Jongmin ; et al ( January 2020 , NPG Asia Materials)

   Although nanoscale deformation, such as nanostrain in iron-chalcogenide (FeSe_xTe_1−x, FST) thin films, has attracted attention owing to its enhancement of general superconducting properties, including critical current density (J_c) and critical transition temperature, the development of this technique has proven to be an extremely challenging and complex process thus far. Herein, we successfully fabricated an epitaxial FST thin film with uniformly distributed nanostrain by injection of a trace amount of CeO₂inside an FST matrix using sequential pulsed laser deposition. By means of transmission electron microscopy and geometric phase analysis, we verified that the injection of a trace amount of CeO₂formsmore »nanoscale defects, with a nanostrained region of tensile strain (ε_zz ≅ 0.02) along thec-axis of the FST matrix. This nanostrained FST thin film achieves a remarkableJ_cof 3.5 MA/cm²under a self-field at 6 K and a highly enhancedJ_cunder the entire magnetic field with respect to those of a pristine FST thin film.

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4. A magnetic levitation based low-gravity simulator with an unprecedented large functional volume

   https://doi.org/10.1038/s41526-021-00174-4  

   Sanavandi, Hamid ; Guo, Wei ( October 2021 , npj Microgravity)

   Low-gravity environment can have a profound impact on the behaviors of biological systems, the dynamics of fluids, and the growth of materials. Systematic research on the effects of gravity is crucial for advancing our knowledge and for the success of space missions. Due to the high cost and the limitations in the payload size and mass in typical spaceflight missions, ground-based low-gravity simulators have become indispensable for preparing spaceflight experiments and for serving as stand-alone research platforms. Among various simulator systems, the magnetic levitation-based simulator (MLS) has received long-lasting interest due to its easily adjustable gravity and practically unlimitedmore »operation time. However, a recognized issue with MLSs is their highly non-uniform force field. For a solenoid MLS, the functional volumeV_1%, where the net force results in an acceleration <1% of the Earth’s gravityg, is typically a few microliters (μL) or less. In this work, we report an innovative MLS design that integrates a superconducting magnet with a gradient-field Maxwell coil. Through an optimization analysis, we show that an unprecedentedV_1%of over 4000 μL can be achieved in a compact coil with a diameter of 8 cm. We also discuss how such an MLS can be made using existing high-T_c-superconducting materials. When the current in this MLS is reduced to emulate the gravity on Mars (g_M = 0.38g), a functional volume where the gravity varies within a few percent ofg_Mcan exceed 20,000 μL. Our design may break new ground for future low-gravity research.

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5. Building a Casimir Metrology Platform with a Commercial MEMS Sensor

   Stange, A. ; Imboden, M. ; Javor, J. ; Barrett, L. ; Bishop, D ( April 2019 , 07 Nature)

   The Casimir Effect is a physical manifestation of quantum fluctuations of the electromagnetic vacuum. When two metal plates are placed close together, typically much less than a micron, the long wavelength modes between them are frozen out, giving rise to a net attractive force between the plates, scaling as d−4 (or d−3 for a spherical-planar geometry) even when they are not electrically charged. In this paper, we observe the Casimir Effect in ambient conditions using a modified capacitive micro-electromechanical system (MEMS) sensor. Using a feedback-assisted pick-and-place assembly process, we are able to attach various microstructures onto the post-release MEMS, convertingmore »it from an inertial force sensor to a direct force measurement platform with pN (piconewton) resolution. With this system we are able to directly measure the Casimir force between a silver-coated microsphere and gold-coated silicon plate. This device is a step towards leveraging the Casimir Effect for cheap, sensitive, room temperature quantum metrology.« less