arXiv subjects

Liangliang Liu

Publications and source records attributed to Liangliang Liu.

21 records · Page 2Linked to original sources

arXiv2019

Two-dimensional topological semimetal states in monolayers Cu$_2$Ge, Fe$_2$Ge, and Fe$_2$Sn

Recent experimental realizations of the topological semimetal states in several monolayer systems are very attractive because of their exotic quantum phenomena and technological applications. Based on first-principles density-functional theory calculations including spin-orbit coupling, we here explore the drastically different two-dimensional (2D) topological semimetal states in three monolayers Cu$_2$Ge, Fe$_2$Ge, and Fe$_2$Sn, which are isostructural with a combination of the honeycomb Cu or Fe lattice and the triangular Ge or Sn lattice. We find that (i) the nonmagnetic (NM) Cu$_{2}$Ge monolayer having a planar geometry exhibits the massive Dirac nodal lines, (ii) the ferromagentic (FM) Fe$_2$Ge monolayer having a buckled geometry exhibits the massive Weyl points, and (iii) the FM Fe$_2$Sn monolayer having a planar geometry and an out-of-plane magnetic easy axis exhibits the massless Weyl nodal lines. It is therefore revealed that mirror symmetry cannot protect the four-fold degenerate Dirac nodal lines in the NM Cu$_{2}$Ge monolayer, but preserves the doubly degenerate Weyl nodal lines in the FM Fe$_{2}$Sn monolayer. Our findings demonstrate that the interplay of crystal symmetry, magnetic easy axis, and band topology is of importance for tailoring various 2D topological states in Cu$_2$Ge, Fe$_2$Ge, and Fe$_2$Sn monlayers.

cond-mat.mtrl-sci↗

arXiv2019

Hidden Weyl Fermions in Paramagnetic Electride Y$_2$C

Recent experimental observations of Weyl fermions in materials opens a new frontier of condensed matter physics. Based on first-principles calculations, we here discover Weyl fermions in a two-dimensional layered electride material Y$_2$C. We find that the Y 4$d$ orbitals and the anionic $s$-like orbital confined in the interstitial spaces between [Y$_2$C]$^{2+}$ cationic layers are hybridized to give rise to van Have singularities near the Fermi energy $E_{\rm F}$, which induce a ferromagnetic (FM) order via the Stoner-type instability. This FM phase with broken time-reversal symmetry hosts the rotation-symmetry protected Weyl nodal lines near $E_{\rm F}$, which are converted into the multiple pairs of Weyl nodes by including spin-orbit coupling (SOC). However, we reveal that, due to its small SOC effects, Y$_2$C has a topologically nontrivial drumhead-like surface state near $E_{\rm F}$ as well as a very small magnetic anisotropy energy with several $μ$eV per unit cell, consistent with the observed surface state and paramagnetism at low temperatures below ${\sim}$2 K. Our findings propose that the Brillouin zone coordinates of Weyl fermions hidden in paramagnetic electride materials would fluctuate in momentum space with random orientations of the magnetization direction.

cond-mat.mtrl-sci↗

arXiv2019

Origin of High-Temperature Superconductivity in Compressed LaH$_{10}$

Room-temperature superconductivity has been one of the most challenging subjects in modern physics. Recent experiments reported that lanthanum hydride LaH$_{10{\pm}x}$ ($x$$<$1) raises a superconducting transition temperature $T_{\rm c}$ up to ${\sim}$260 (or 215) K at high pressures around 190 (150) GPa. Here, based on first-principles calculations, we reveal the existence of topological Dirac-nodal-line (DNL) states in compressed LaH$_{10}$. Remarkably, the DNLs protected by the combined inversion and time-reversal symmetry and the rotation symmetry create a van Hove singularity (vHs) near the Fermi energy, giving rise to large electronic density of states. Contrasting with other La hydrides containing cationic La and anionic H atoms, LaH$_{10}$ shows a peculiar characteristic of electrical charges with anionic La and both cationic and anionic H species, caused by a strong hybridization of the La $f$ and H $s$ orbitals. We find that a large number of electronic states at the vHs are strongly coupled to the H-derived high-frequency phonon modes that are induced via the unusual, intricate bonding network of LaH$_{10}$, thereby yielding a high $T_{\rm c}$. Our findings not only elucidate the microscopic origin of the observed high-$T_{\rm c}$ BCS-type superconductivity in LaH$_{10}$, but also pave the route for achieving room-temperature topological superconductors in compressed hydrogen-rich compounds.

cond-mat.supr-con↗