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1. Enhancing the critical temperature of strained Niobium films

   https://doi.org/10.1088/2053-1591/aba84a  

   Choi, Joonyoung ; Kim, Young-Kyoung ; Kim, Chang-Duk ; Kim, Sooran ; Jo, Younjung ( July 2020 , Materials Research Express)

   The study of the high critical temperature (T_c) of hydrogen compounds under high pressure has resulted in a considerable focus on Bardeen–Cooper–Schrieffer superconductors. Nb has the highestT_camong the elemental metals at ambient pressure, so reviewing Nb films again is worthwhile. In this study, we investigated the factors that determine theT_cof Nb films by strain introduction and carrier doping. We deposited Nb films of various thicknesses onto Si substrates and evaluated theT_cvariation with thickness. In-plane compressive strain in the (110) plane due to residual stress reduced theT_c. First-principles calculations showed that adjusting the density of states at the Fermi levelmore »is key for both strain-induced suppression and doping-induced enhancement of the NbT_c. The application of hydrostatic pressure compensated for the intrinsic strain of the film and increased itsT_c, which could also be enhanced by increasing the hole concentration with an electric double-layer transistor. A liquid electrolyte should be used as a pressure medium for applying hydrostatic pressure to increase theT_cof correlated materials, where this increase results from changes in material structure and carrier concentration.

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2. Strong π-stacking causes unusually large anisotropic thermal expansion and thermochromism

   https://doi.org/10.1073/pnas.2106572118  

   Dharmarwardana, Madushani ; Otten, Brooke M. ; Ghimire, Mukunda M. ; Arimilli, Bhargav S. ; Williams, Christopher M. ; Boateng, Stephen ; Lu, Zhou ; McCandless, Gregory T. ; Gassensmith, Jeremiah J. ; Omary, Mohammad A. ( October 2021 , Proceedings of the National Academy of Sciences)

   π-stacking in ground-state dimers/trimers/tetramers ofN-butoxyphenyl(naphthalene)diimide (BNDI) exceeds 50 kcal ⋅ mol⁻¹in strength, drastically surpassing that for the^*3[pyrene]₂excimer (∼30 kcal ⋅ mol⁻¹; formal bond order = 1) and similar to other weak-to-moderate classical covalent bonds. Cooperative π-stacking in triclinic (BNDI-T) and monoclinic (BNDI-M) polymorphs effects unusually large linear thermal expansion coefficients (α_a, α_b, α_c, β) of (452, −16.8, −154, 273) × 10⁻⁶⋅ K⁻¹and (70.1, −44.7, 163, 177) × 10⁻⁶⋅ K⁻¹, respectively. BNDI-T exhibits highly reversible thermochromism over a 300-K range, manifest by color changes from orange (ambient temperature) toward red (cryogenic temperatures) or yellow (375 K), with repeated thermal cycling sustainedmore »for over at least 2 y.

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3. Plasmonically enhanced mid-IR light source based on tunable spectrally and directionally selective thermal emission from nanopatterned graphene

   https://doi.org/10.1038/s41598-020-73582-3  

   Shabbir, Muhammad Waqas ; Leuenberger, Michael N. ( October 2020 , Scientific Reports)

   We present a proof of concept for a spectrally selective thermal mid-IR source based on nanopatterned graphene (NPG) with a typical mobility of CVD-grown graphene (up to 3000$$\hbox {cm}^2\,\hbox {V}^{-1}\,\hbox {s}^{-1}$$${\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$), ensuring scalability to large areas. For that, we solve the electrostatic problem of a conducting hyperboloid with an elliptical wormhole in the presence of anin-planeelectric field. The localized surface plasmons (LSPs) on the NPG sheet, partially hybridized with graphene phonons and surface phonons of the neighboring materials, allow for the control and tuning of the thermal emission spectrum in the wavelength regime from$$\lambda =3$$$\lambda =3$to 12$$\upmu$$$\mu$m by adjusting themore »size of and distance between the circular holes in a hexagonal or square lattice structure. Most importantly, the LSPs along with an optical cavity increase the emittance of graphene from about 2.3% for pristine graphene to 80% for NPG, thereby outperforming state-of-the-art pristine graphene light sources operating in the near-infrared by at least a factor of 100. According to our COMSOL calculations, a maximum emission power per area of$$11\times 10^3$$$11\times {10}^3$W/$$\hbox {m}^2$$${\text{m}}^2$at$$T=2000$$$T=2000$K for a bias voltage of$$V=23$$$V=23$V is achieved by controlling the temperature of the hot electrons through the Joule heating. By generalizing Planck’s theory to any grey body and deriving the completely general nonlocal fluctuation-dissipation theorem with nonlocal response of surface plasmons in the random phase approximation, we show that the coherence length of the graphene plasmons and the thermally emitted photons can be as large as 13$$\upmu$$$\mu$m and 150$$\upmu$$$\mu$m, respectively, providing the opportunity to create phased arrays made of nanoantennas represented by the holes in NPG. The spatial phase variation of the coherence allows for beamsteering of the thermal emission in the range between$$12^\circ$$${12}^{\circ }$and$$80^\circ$$${80}^{\circ }$by tuning the Fermi energy between$$E_F=1.0$$$E_F=1.0$eV and$$E_F=0.25$$$E_F=0.25$eV through the gate voltage. Our analysis of the nonlocal hydrodynamic response leads to the conjecture that the diffusion length and viscosity in graphene are frequency-dependent. Using finite-difference time domain calculations, coupled mode theory, and RPA, we develop the model of a mid-IR light source based on NPG, which will pave the way to graphene-based optical mid-IR communication, mid-IR color displays, mid-IR spectroscopy, and virus detection.

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4. A system for probing Casimir energy corrections to the condensation energy

   https://doi.org/10.1038/s41378-020-00221-2  

   Pérez-Morelo, Diego ; Stange, Alexander ; Lally, Richard W. ; Barrett, Lawrence K. ; Imboden, Matthias ; Som, Abhishek ; Campbell, David K. ; Aksyuk, Vladimir A. ; Bishop, David J. ( December 2020 , Microsystems & Nanoengineering)

   In this article, we present a nanoelectromechanical system (NEMS) designed to detect changes in the Casimir energy. The Casimir effect is a result of the appearance of quantum fluctuations in an electromagnetic vacuum. Previous experiments have used nano- or microscale parallel plate capacitors to detect the Casimir force by measuring the small attractive force these fluctuations exert between the two surfaces. In this new set of experiments, we aim to directly detect the shifts in the Casimir energy in a vacuum due to the presence of the metallic parallel plates, one of which is a superconductor. A change inmore »the Casimir energy of this configuration is predicted to shift the superconducting transition temperature (T_c) because of the interaction between it and the superconducting condensation energy. In our experiment, we take a superconducting film, carefully measure its transition temperature, bring a conducting plate close to the film, create a Casimir cavity, and then measure the transition temperature again. The expected shifts are smaller than the normal shifts one sees in cycling superconducting films to cryogenic temperatures, so using a NEMS resonator in situ is the only practical way to obtain accurate, reproducible data. Using a thin Pb film and opposing Au surface, we observe no shift inT_c>12 µK down to a minimum spacing of ~70 nm at zero applied magnetic field.

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5. Measuring the H i Content of Individual Galaxies Out to the Epoch of Reionization with [C ii]

   https://doi.org/10.3847/1538-4357/ac2231  

   Heintz, Kasper E. ; Watson, Darach ; Oesch, Pascal A. ; Narayanan, Desika ; Madden, Suzanne C. ( November 2021 , The Astrophysical Journal)

   Abstract The H i gas content is a key ingredient in galaxy evolution, the study of which has been limited to moderate cosmological distances for individual galaxies due to the weakness of the hyperfine H i 21 cm transition. Here we present a new approach that allows us to infer the H i gas mass M HI of individual galaxies up to z ≈ 6, based on a direct measurement of the [C ii ]-to-H i conversion factor in star-forming galaxies at z ≳ 2 using γ -ray burst afterglows. By compiling recent [C ii ]-158 μ m emission linemore »measurements we quantify the evolution of the H i content in galaxies through cosmic time. We find that M HI starts to exceed the stellar mass M ⋆ at z ≳ 1, and increases as a function of redshift. The H i fraction of the total baryonic mass increases from around 20% at z = 0 to about 60% at z ∼ 6. We further uncover a universal relation between the H i gas fraction M HI / M ⋆ and the gas-phase metallicity, which seems to hold from z ≈ 6 to z = 0. The majority of galaxies at z > 2 are observed to have H i depletion times, t dep,HI = M HI /SFR, less than ≈2 Gyr, substantially shorter than for z ∼ 0 galaxies. Finally, we use the [C ii ]-to-H i conversion factor to determine the cosmic mass density of H i in galaxies, ρ HI , at three distinct epochs: z ≈ 0, z ≈ 2, and z ∼ 4–6. These measurements are consistent with previous estimates based on 21 cm H i observations in the local universe and with damped Ly α absorbers (DLAs) at z ≳ 2, suggesting an overall decrease by a factor of ≈5 in ρ HI ( z ) from the end of the reionization epoch to the present.« less