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Register a new userEnhanced superconductivity by strain and carrier-doping in borophene: A first principles prediction
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We predict by first principles calculations that the recently prepared borophene is a pristine two-dimensional (2D) monolayer superconductor, in which the superconductivity can be significantly enhanced by strain and charge carrier doping. The intrinsic metallic ground state with high density of states at Fermi energy and strong Fermi surface nesting lead to sizeable electron-phonon coupling, making the freestanding borophene superconduct with $T_c$ close to 19.0 K. The tensile strain can increase $T_c$ to 27.4 K, while the hole doping can notably increase $T_c$ to 34.8 K. The results indicate that the borophene grown on substrates with large lattice parameters or under photoexcitation can show enhanced superconductivity with $T_c$ far more above liquid hydrogen temperature of 20.3 K, which will largely broaden the applications of such novel material.
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We report the effects of Ce substitution on structural, electronic, and magnetic properties of layered bismuth-chalcogenide La1-xCexOBiSSe (x = 0-0.9), which are newly obtained in this study. Metallic conductivity was observed for x > 0.1 because of electron carriers induced by mixed valence of Ce ions, as revealed by bond valence sum calculation and magnetization measurements. Zero resistivity and clear diamagnetic susceptibility were obtained for x = 0.2-0.6, indicating the emergence of bulk superconductivity in these compounds. Dome-shaped superconductivity phase diagram with the highest transition temperature (Tc) of 3.1 K, which is slightly lower than that of F-doped LaOBiSSe (Tc = 3.7 K), was established. The present study clearly shows that the mixed valence of Ce ions can be utilized as an alternative approach for electron-doping in layered bismuth-chalcogenides to induce superconductivity.
Recent discovery of topological superconductors (TSCs) has sparked enormous interest. Realization of TSC requires a delicate tuning of multiple microscopic parameters, which remains a great challenge. Here, we develop a first-principles approach to quantify realistic conditions of TSC by solving self-consistently Bogoliubov-de Gennes equation based on Wannier function construction of band structure, in presence of Rashba spin-orbit coupling, Zeeman splitting and electron-phonon coupling. We further demonstrate the power of this new method by predicting the Mn-doped GeTe (Ge$_{1-x}$Mn$_x$Te) monolayer - a well-known dilute magnetic semiconductor showing superconductivity under hole doping - to be a Class D TSC with Chern number of -1 and chiral Majorana edge modes. By constructing a first-principles phase diagram in the parameter space of temperature and Mn concentration, we propose the TSC phase can be induced at a lower-limit transition temperature of ~40 mK and the Mn concentration of $x$~0.015%. Our approach can be generally applied to TSCs with a phonon-mediated pairing, providing useful guidance for future experiments.
The transition metal dichalcogenide (TMD) $1T$-TaS$_{2}$ exhibits a rich set of charge density wave (CDW) orders. Recent investigations suggested that using light or electric field can manipulate the commensurate (C) CDW ground state. Such manipulations are considered to be determined by the charge carrier doping. Here we simulate by first-principles calculations the carrier doping effect on CCDW in $1T$-TaS$_{2}$. We investigate the charge doping effects on the electronic structures and phonon instabilities of $1T$ structure and analyze the doping induced energy and distortion ratio variations in CCDW structure. We found that both in bulk and monolayer $1T$-TaS$_{2}$, CCDW is stable upon electron doping, while hole doping can significantly suppress the CCDW, implying different mechanisms of such reported manipulations. Light or positive perpendicular electric field induced hole doping increases the energy of CCDW, so that the system transforms to NCCDW or similar metastable state. On the other hand, even the CCDW distortion is more stable upon in-plain electric field induced electron injection, some accompanied effects can drive the system to cross over the energy barrier from CCDW to nearly commensurate (NC) CDW or similar metastable state. We also estimate that hole doping can introduce potential superconductivity with $T_{c}$ of $6sim7$ K. Controllable switching of different states such as CCDW/Mott insulating state, metallic state, and even the superconducting state can be realized in $1T$-TaS$_{2}$, which makes the novel material have very promising applications in the future electronic devices.
A recent experiment reported the first rare-earth binary oxide superconductor LaO ($T_c $ $sim$ 5 K) with a rock-salt structure [K. Kaminaga et al., J. Am. Chem. Soc. 140, 6754 (2018)]. Correspondingly, the underlying superconducting mechanism in LaO needs theoretical elucidation. Based on first-principles calculations on the electronic structure, lattice dynamics, and electron-phonon coupling of LaO, we show that the superconducting pairing in LaO belongs to the conventional Bardeen-Cooper-Schrieffer (BCS) type. Remarkably, the electrons and phonons of the heavy La atoms, instead of those of the light O atoms, contribute most to the electron-phonon coupling. We further find that both the biaxial tensile strain and the pure electron doping can enhance the superconducting $T_c$ of LaO. With the synergistic effect of electron doping and tensile strain, the $T_c$ could be even higher, for example, 11.11 K at a doping of 0.2 electrons per formula unit and a tensile strain of $4%$. Moreover, our calculations show that the superconductivity in LaO thin film remains down to the trilayer thickness with a $T_c$ of 1.4 K.
We theoretically study superconductivity in UTe$_2$, which is a recently-discovered strong candidate for an odd-parity spin-triplet superconductor. Theoretical studies for this compound faced difficulty because first-principles calculations predict an insulating electronic state, incompatible with superconducting instability. To overcome this problem, we take into account electron correlation effects by a GGA$+U$ method and show the insulator-metal transition by Coulomb interaction. Using Fermi surfaces obtained as a function of $U$, we clarify topological properties of possible superconducting states. Fermi surface formulas for the three-dimensional winding number and three two-dimensional $mathbb{Z}_2$ numbers indicate topological superconductivity at an intermediate $U$ for all the odd-parity pairing symmetry in the $Immm$ space group. Symmetry and topology of superconducting gap node are analyzed and the gap structure of UTe$_2$ is predicted. Topologically protected low-energy excitations are highlighted, and experiments by bulk and surface probes are proposed to link Fermi surfaces and pairing symmetry. Based on the results, we also discuss multiple superconducting phases under magnetic fields, which were implied by recent experiments.
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