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A two-dimensional topological insulator features (only) one bulk gap with nontrivial topology, which protects one-dimensional boundary states at the Fermi level. We find a quantum phase of matter beyond this category: a multiple topological insulator. It possesses a ladder of topological gaps; each gap protects a robust edge state. We prove a monolayer of van der Waals material PtBi2 as a two-dimensional multiple topological insulator. By means of scanning tunneling spectroscopy, we directly visualize the one-dimensional hot electron (and hole) channels with nanometer size on the samples. Furthermore, we confirm the topological protection of these channels by directly demonstrating their robustness to variations of crystal orientation, edge geometry, and sample temperature. The discovered topological hot electron materials may be applied as efficient photocatalysts in the future.

We present the systematic de Haas–van Alphen (dHvA) quantum oscillations studies on the recently discovered topological Dirac semimetal pyrite PtBi2 single crystals. Remarkable dHvA oscillations are emerged at a low field about 1.5 T. From the analyses of the dHvA oscillations, we extract the high quantum mobilities, light effective masses and phase shift factors for the Dirac fermions in pyrite PtBi2. From the angular dependence of the dHvA oscillations, we map out the topology of the Fermi surface. Furthermore, we identify two additional oscillation frequencies that are not probed by the SdH oscillations, which provides us with opportunities to further understand its Fermi surface topology.

We prepare orthorhombic CoSb monolayer films on SrTiO3(001) substrate by molecular beam epitaxy and observe symmetric gap around the Fermi level with coherence peaks at ± (6–7) meV by in situ scann...

CrI3 monolayer has recently drawn much attention due to its two-dimensional long range ferromagnetic order. We find that CrI3 nanoribbons, which are strips of CrI3 monolayer, can be used as building blocks of nanodevices. In this paper, we studied the atomic and electronic structures of CrI3 zigzag nanoribbons by using first-principles calculations. CrI3 zigzag nanoribbons are also ferromagnet. Interestingly, edge states exist in the system and play an important role in their electronic structures. They dominate the band structures around Fermi level and can be tuned by edge atomic structures. The intrinsic ferromagnetism and rich electronic structures enable CrI3 zigzag nanoribbons a group of promising candidate materials for spintronics.

The structural stability of monolayer SnSe at finite temperature is investigated by the phonon quasiparticle approach that combines first-principles molecular dynamics and lattice dynamics. At 300 K, we witness the dynamic stability of monolayer Pmmn SnSe that is originated from the Cmcm bulk phase. The pair correlation and atomic displacement distribution functions of Sn and Se atoms confirm the lattice stability at high temperature. The hardening of Raman activated ${\mathrm{A}}_{g}^{2}$ and ${\mathrm{B}}_{2g}$ modes at the $\mathrm{\ensuremath{\Gamma}}(0,0,0)$ point and normal modes at the boundary $M(0,\frac{1}{2},\frac{1}{2})$ point of the Brillouin zone is observed with giant frequency shifts, revealing the heavy anharmonicity in monolayer SnSe. The calculated anharmonic phonon dispersions and density of states show significant temperature dependence and some medium-frequency phonon modes adopt giant frequency shifts. Due to the lattice anharmonicity, the predicted lattice thermal conductivity within the framework of a phonon gas model is very low for monolayer SnSe, close to the value of the bulk phase along the out-of-plane direction. As temperature increases, the characteristic of covalent bonds between Sn and Se atoms in plane and the normal direction of monolayer SnSe becomes isotropic. This results in a high degeneracy of the electronic band structure, beneficial to the electronic transport properties. A thermoelectric $ZT$ of $\ensuremath{\sim}0.52$ for $n$ doping is predicted at 300 K, hinting that the monolayer SnSe is a fairly good thermoelectric material suitable for application at room temperature.

Recent experiments have tuned the monolayer 1T^{'}-WTe_{2} to be superconducting by electrostatic gating. Here, we theoretically study the phonon-mediated superconductivity in monolayer 1T^{'}-WTe_{2} via charge doping. We reveal that the emergence of soft-mode phonons with specific momentum is crucial to give rise to the superconductivity in the electron-doping regime, whereas no such soft-mode phonons and no superconductivity emerge in the hole-doping regime. We also find a superconducting dome, which can be attributed to the change of Fermi surface nesting conditions with electron doping. By taking into account the experimentally established strong anisotropy of temperature-dependent upper critical field H_{c2} between the in-plane and out-of-plane directions, we show that the superconducting state probably has the unconventional equal-spin-triplet pairing in the A_{u} channel of the C_{2h} point group. Our studies provide a promising understanding to the doping dependent superconductivity and strong anisotropy of H_{c2} in monolayer 1T^{'}-WTe_{2}, and can be extended to understand the superconductivity in other gated transition metal dichalcogenides.

Based on density functional theory and density functional perturbation theory calculations, we systematically investigate the vibration responses of monolayer $1H\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ to equibiaxial strains. It is found that, at the $\mathrm{\ensuremath{\Gamma}}$ point, the frequency shift of Raman-active modes $({E}^{\ensuremath{'}},$ ${A}_{1}^{\ensuremath{'}},$ and ${E}^{\ensuremath{''}})$ and infrared-active modes $({A}_{2}^{\ensuremath{''}}$ and ${E}^{\ensuremath{'}})$ show domelike shapes; that is, their frequencies decrease monotonically under tensile strains but first increase and then decrease rapidly under compressive strains. The frequency-shift behaviors are revealed to come from vibration responses to both bond stretching and bond-angle bending in strained $1H\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$. At the $K$ point, a special acoustic mode becomes soft because its frequency drops to zero at a compressive strain of $\ensuremath{-}11.27%$. We find that electron occupancies in Mo ${d}_{{z}^{2}}$, Te ${p}_{x}$, and Te ${p}_{y}$ orbitals weaken the vibration mode at $K$, which exhibits the in-plane vibration of Mo atoms and out-of-plane vibration of Te atoms. On the other hand, compressive strains enhance the Fermi surface nesting and abruptly soften the vibration frequency for one acoustic mode at $K$. Our results point out a way to detect the strain status of monolayer $1H\text{\ensuremath{-}}{\mathrm{MoTe}}_{2}$ by measuring the vibration frequencies.

Quantum anomalous Hall (QAH) insulator is the key material to study emergent topological quantum effects, but its ultralow working temperature limits experiments. Here, by first-principles calculations, we find a family of stable two-dimensional (2D) structures generated by lithium decoration of layered iron-based superconductor materials FeX (X = S, Se, Te), and predict room-temperature ferromagnetic semiconductors together with large-gap high-Chern-number QAH insulators in the 2D materials. The extremely robust ferromagnetic order is induced by the electron injection from Li to Fe and stabilized by strong ferromagnetic kinetic exchange in the 2D Fe layer. While in the absence of spin-orbit coupling (SOC), the ferromagnetism polarizes the system into a half Dirac semimetal state protected by mirror symmetry, the SOC effect results in a spontaneous breaking of mirror symmetry and introduces a Dirac mass term, which creates QAH states with sizable gaps (several tens of meV) and multiple chiral edge modes. We also find a 3D QAH insulator phase featured by macroscopic number of chiral conduction channels in bulk LiOH-LiFeX. The findings open new opportunities to realize novel QAH physics and applications at high temperatures., 16 pages, 4 figures

Uranium hydride is a novel hydrogen-rich system which contains 5f electrons. Uranium hydride can not only be used in the nuclear fuel industry, but also be a candidate of high superconducting-temperature materials. In this paper, we have searched the stable uranium hydride structures by using particle swarm optimization method and first-principles calculations. Besides UH8 and UH9, we find that UH17, which contains larger hydrogen content than most hydride materials reported before, is also stable at high pressure. The atomic structures, electronic structures and phase diagram of uranium hydrides are provided, and we find that all of the discovered uranium hydrides are metals with negligible magnetic moments.

Ever since high- T c superconductivity was found in H3S at 203 K under 200 GPa pressure, researches on hydrogen-rich family including H3S, H3P, and H3Te have been intensively carried out. Among those compounds, H3Se is a promising candidate of the same space group with H3S. In this work, we have thoroughly studied the electronic and phonon structures of H3Se using density functional theory (DFT) calculations. It is demonstrated that the electronic bands of H3Se contain van Hove singularities near the Fermi energy, which play a significant role in the superconductivity of this material. Besides, an accurate tight-binding model is proposed based on the Wannier function interpolations which can capture the main features of the electronic structures of H3Se. Moreover, we have also discussed the phonon dispersions and vibration modes for a better understanding of the phonon–electron interaction in H3Se.

Ferropericlase (Fp), $({\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{O}$, is the second most abundant phase in the Earth's lower mantle. At relevant pressure-temperature conditions, iron in Fp undergoes a high spin (HS), S = 2, to low spin (LS), S = 0, state change. The nature of this phenomenon is quite well understood now, but there are still basic questions regarding the structural stability and the existence of soft phonon modes during this iron state change. General theories exist to explain the volume reduction, the significant thermoelastic anomalies, and the broad nature of this HS-LS crossover. These theories make extensive use of the quasiharmonic approximation. Therefore, dynamical and structural stability is essential to their validity. Here, we investigate the vibrational spectrum of Fp throughout this spin crossover using ab initio density-functional theory $+{U}_{\mathrm{sc}}$ calculations. We address vibrational modes associated with isolated and (second-)nearest-neighbor iron ions undergoing the HS-LS state change. As expected, acoustic modes of this solid solution are resilient, while optical modes are the most affected. We show that there are no soft phonon modes across this HS-LS crossover, and Fp is dynamically stable at all relevant pressures.

The monolayer FeSe on $\mathrm{SrTi}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ exhibits a twofold enlarged pairing gap compared with other electron-doped FeSe, which is postulated to originate from cooperative effects of interface charge transfer and electron-boson interaction that relate closely to surface oxygen vacancies in $\mathrm{SrTi}{\mathrm{O}}_{3}$. We create gradient oxygen vacancies on the surface of $\mathrm{SrTi}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ substrates using direct current heating. Combining spatially resolved spectroscopy characterizations, we disclose gradient pairing gaps but negligible doping variation in monolayer FeSe on such substrates. As oxygen vacancy concentration gradually increases from the anode to cathode region, the pairing gap increases from 12 to 17 meV, right beyond the values of electron-doped FeSe. This modulation provides a platform for spatially resolved investigations to unveil the interplay between electronic correlation and electron-phonon interaction and their cooperation to drive superconductivity.

We investigated the structural stability of SnSe from 0 to 800 K by the phonon quasiparticle approach combining first-principles molecular-dynamics (MD) simulations and lattice dynamics. At high temperature, we witness the dynamic stability of the $Cmcm$ phase and reveal the coupling of the polarization of phonon modes and the phase transition between the $Cmcm$ and $Pnma$ phases. Specifically, in real space, the probability distribution of atomic displacements from first-principles MD simulations successfully captures the structural instability at low temperature and the structural stability at high temperature for $Cmcm$ SnSe. An analysis of phonon power spectra of several modes also delivers the dynamic stabilization of $Cmcm$ at high temperature. Particularly, the soft modes of the ${Y}_{1}$ mode at the $\mathbf{q}=Y(\frac{1}{2},\frac{1}{2},0$) point and the ${\mathrm{\ensuremath{\Gamma}}}_{1}$ mode at the $\mathbf{q}=\mathrm{\ensuremath{\Gamma}}(0,0,0)$ point of the $Cmcm$ phase in the harmonic approximation become relatively rigid at elevated temperature, in agreement with experimental and previous theoretical results. The calculated anharmonic phonon dispersions and density of states are strongly temperature-dependent, and some phonon modes adopt giant frequency shifts. These aspects demonstrate the heavy anharmonicity in SnSe. At low temperature, the transition from the $Cmcm$ to the $Pnma$ phase induces a symmetry breaking of structure. Consequently, the degeneracy of associated electronic states (mainly $p$ states) is lifted, thus lowering the energy of the $Pnma$ phase.

Monolayer $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$ attracts rapidly growing interests aiming for promising applications in spintronics, dissipationless transport, and quantum computations. Due to one-dimensional W-W chains, $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$ exhibits unique anisotropic structure and promising properties, which can be modified by simply applying strains. Based on first-principles calculations, we systematically study the phonon dispersion curves as well as the Raman-active modes of $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$ under different tensile strains. We find that one branch of acoustic phonon softens at special $\mathit{q}$ points under a critical strain of ${\ensuremath{\varepsilon}}_{a}=11.55%$ along the $a$ axis (with $W\ensuremath{-}W$ chains) direction, or ${\ensuremath{\varepsilon}}_{b}=7.0%$ along the $b$-axis direction and ${\ensuremath{\varepsilon}}_{ab}=8.44%$ along the biaxial direction. Before reaching such critical strains, the Raman frequencies of ${A}_{g}^{1}, {A}_{g}^{3}$, and ${A}_{g}^{4}$ modes, contributing to the main peaks in Raman spectra of $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$, show anisotropic responses to different strains. The response to biaxial strains is found to be the most sensitive. We find that the frequency shift of ${A}_{g}^{3}$ mode shows parabolic characters of strained $1{T}^{\ensuremath{'}}\ensuremath{-}{\mathrm{WTe}}_{2}$, then we split it into two parts and it shows a Raman-shift transition at $\ensuremath{\sim}5$% strains. While for $\mathrm{the}\phantom{\rule{4pt}{0ex}}{A}_{g}^{1}$ and ${A}_{g}^{4}$ modes, the frequencies change linearly.

The surface of a three-dimensional topological insulator (TI) hosts two-dimensional massless Dirac fermions (DFs), the gapless and spin-helical nature of which yields many exotic phenomena, such as the immunity of topological surface states (TSS) to back-scattering. This leads to their high transmission through surface defects or potential barriers. Quantum corrals, previously elaborated on metal surfaces, can act as nanometer-sized electronic resonators to trap Schr\"odinger electrons by quantum confinement. It is thus intriguing, concerning their peculiar nature, to put the Dirac electrons of TSS to the test in similar circumstances. Here, we report the behaviors of TSS in a triangular quantum corral (TQC) fabricated by epitaxially growing Bi bilayer nanostructures on the surfaces of Bi2Te3 films. Unlike a circular corral, the TQC is supposed to be totally transparent for DFs. By mapping the electronic structure of TSS inside TQCs through a low-temperature scanning tunneling microscope in the real space, both the trapping and de-trapping behaviors of the TSS electrons are observed. The selection rules are found to be governed by the geometry and spin texture of the constant energy contour of TSS upon the strong hexagonal warping in Bi2Te3. Careful analysis of the quantum interference patterns of quasi-bound states yields the corresponding wave vectors of trapped TSS, through which two trapping mechanisms favoring momenta in different directions are uncovered. Our work indicates the extended nature of TSS and elucidates the selection rules of the trapping of TSS in the presence of a complicated surface state structure, giving insights into the effective engineering of DFs in TIs., Comment: 31pages, 12figures(including supplementary materials)

The freestanding PbSe monolayer has been predicted as a candidate of the two-dimensional topological crystalline insulator, which possesses the Dirac-cone-like edge states resided at the edge. Up to now, however, direct experimental evidence of topological PbSe monolayer has not yet been reported. Here, we report the epitaxial growth and scanning tunneling microscopy study of few-layers PbSe islands grown on SrTiO3 substrate. From the investigation of different thickness, we discover the release of compressive strain and the reduction of bandgap as the thickness becomes thick. Following detailed spectroscopic measurements, a signature of Dirac-like edge states is observed at the edge of seventh-layer PbSe. In conjunction with first-principle calculations, we find that compressive-strain-induced buckling adjusts the topological band inversion and eventually leads to a phase transition from nontrivial two-dimensional topological crystalline insulator to trivial insulator, which match well with our experimen...

The successful fabrication of two-dimensional ferromagnetic materials, such as monolayer CrI3 and bilayer Cr2Ge2Te6, has drawn much attention in recent years. Here, we found that UX3 (with X Cl, Br) monolayers are ferromagnetic materials with high Curie temperatures. In this study we explored the electronic structures and magnetic properties of UX3 by using density functional theory. UX3 has a ferromagnetic order with in-plane magnetic easy axis (i.e., along x direction), and its magnetic anisotropy is 31.2∼34.2 meV per U atom. We performed Monte Carlo calculations to predict the Curie transition temperatures. They are as high as 160 and 125 K for UCl3 and UBr3 monolayers, respectively, which are higher than that of the UI3 found before. Therefore, UX3 monolayers offer new platforms for the study of spintronic devices.

We have developed a software MagGene to predict magnetic structures by using genetic algorithm. Starting from an atom structure, MagGene repeatedly generates new magnetic structures and calls first-principles calculation engine to get the most stable magnetic structure. This software is applicable to both collinear and noncollinear systems. It is particularly convenient for predicting the magnetic structures of atomic systems with strong spin–orbit couplings and/or strong spin frustrations. Program summary Program Title: MagGene CPC Library link to program files: https://doi.org/10.17632/m83gcp5z48.1 Licensing provisions: MIT Programming language: Fortran 90 Nature of problem: In complex magnetic systems, such as systems with strong spin–orbit couplings and/or strong spin-frustrations, the associated magnetic structures could also be quite complex. The traditional methods, such as theoretical analysis based on crystal symmetries and simulated annealing based on a special model Hamiltonian, are inefficient. Then the electronic structures, spin wave dispersions, and other properties based on correct magnetic structures could not be obtained accurately. Therefore, an efficient method to predict magnetic structures is required. Solution method: The magnetic structures can be predicted efficiently by using genetic algorithm. Additional comments including unusual features: The magnetic structures can be predicted for both collinear and noncollinear atomic systems by using genetic evolution algorithm. It is flexible to use in variety of systems. The total magnetic moment can be fixed.

The phonon dispersion unfolding method is useful for obtaining hidden Bloch symmetries and comparing theoretical results with experiment spectrums (e.g., inelastic neutron scattering, inelastic X-ray scattering, Raman). In this paper, we propose a method to unfold phonon dispersions. The main advantage of this method is the ability to handle systems with heavy breaking of spatial translational symmetry. Its validity is tested by pure diamond, diamond with Si substitution, and diamond with C vacancies., Comment: 12 pages, 3 figures

Strain and charge doping are the effective ways to modulate the electronic and phonon properties of materials. The effects of biaxial tensile strains and charge dopings on the stabilities of HfSe2 monolayer have been systematically investigated using first-principles methods. Its two-dimensional Young's modulus is only 65.4 N/m, and it is easy to be stretched. When the tensile strain is applied on HfSe2 monolayer, two of its phonon modes soften with one frequency decreasing to zero at critical strain. Our results show that electron and hole dopings could suppress the softening of phonon modes, and significantly enhance the ideal strength by 28% and 36%, respectively. The calculations for electronic structures and phonon dispersions provide the theoretical references for future nano-device designing.

The structural stability of hydrogen C2/c phase from 0 K to 300 K is investigated by combining the first-principles molecular dynamics (MD) simulations and density functional perturbation theory. Without considering the temperature effect, the C2/c phase is stable from 150 GPa to 250 GPa based on the harmonic phonon dispersion relations. The hydrogen molecules at the solid lattice sites are sensitive to temperature. The structural stability to instability transition of the C2/c phase upon temperature is successfully captured by the radial distribution function and probability distribution of atomic displacements from first-principles MD simulations, confirmed by the phonon power spectrum analysis in the phase space. The existence of phonon quasiparticle for different normal modes is observed directly. The phonon power spectrum of specific normal modes corresponding to the Raman and infrared (IR) activations are depicted at different temperatures and pressures. The changes of frequency with temperature are in agreement with experimental results, supporting the C2/c as the hydrogen phase III. For the first time, the anharmonic phonon dispersion curves and density of states are predicted based on the phonon quasi-particle approach. Therefore, the temperature dependence of lattice vibrations can be observed directly, providing a more complete physical picture of phonon frequency distribution with respect to the Raman and IR spectra. It is found that the high-frequency regions adopt significant frequency shifts compared to the harmonic case.

The physics of the electronic excitation in semiconductors induced by sub-GeV dark matter (DM) have been extensively discussed in literature, under the framework of the standard plane wave (PW) and pseudopotential calculation scheme. In this paper, we investigate the implication of the all-electron (AE) reconstruction on estimation of the DM-induced electronic transition event rates. As a benchmark study, we first calculate the wavefunctions in silicon and germanium bulk crystals based on both the AE and pseudo (PS) schemes within the projector augmented wave (PAW) framework, and then make comparisons between the calculated excitation event rates obtained from these two approaches. It turns out that in process where large momentum transfer is kinetically allowed, the two calculated event rates can differ by a factor of a few. Such discrepancies are found to stem from the high-momentum components neglected in the PS scheme. It is thus implied that the correction from the AE wavefunction in the core region is necessary for an accurate estimate of the DM-induced transition event rate in semiconductors., Comment: A missing factor $64^{-3/2}=1/512$ associated with the Fourier transformation is added to both the AE and PS event rates in this version. The ratio between the AE and PS event rates is not affected

Recent studies have theoretically investigated the atomic excitation and ionization induced by the dark matter (DM)-nucleus scattering, and it is found that the suddenly recoiled atom is much more likely to excite or lose its electrons than expected. Such phenomenon is called the "Migdal effect". In this paper, we extend the established strategy to describe the Migdal effect in isolated atoms to the case in semiconductors under the framework of tight-binding (TB) approximation. Since the localized aspects of electrons are respected in form of the Wannier functions (WFs), the extension of the existing Migdal approach for isolated atoms is much more natural, while the extensive nature of electrons in solids is reflected in the hopping integrals. We take diamond target as a concrete proof of principle for the methodology, and calculate relevant energy spectra and projected sensitivity of such diamond detector. It turns out that our method as a preliminary attempt is practically effective., Comment: latest version

Triply degenerate point (TP) fermions in tungsten–carbide-type materials (e.g., MoP), which represent new topological states of quantum matter, have generated immense interest recently. However, the TPs in these materials are found to be far below the Fermi level, leading to the TP fermions having less contribution to low-energy quasiparticle excitations. Here, we theoretically predict the existence of TP fermions with TP points close to the Fermi level in trigonal layered PtBi2 by ab initio calculations, and experimentally verify the predicted band topology by magnetotransport measurements under high magnetic fields up to 40 T. Analyses of both the pronounced Shubnikov–de Haas and de Haas–van Alphen oscillations reveal the existence of six principal Fermi pockets. Our experimental results, together with those from ab initio calculations, reveal the interplay between transport behaviors and unique electronic structures, and support the existence of TP fermions in trigonal layered PtBi2. Triply degenerate point (TP) fermions have been reported in MoP but the TPs are far below the Fermi level. Here, Guo et al. predict and verify the possible existence of TP fermions in trigonal layered PtBi2, where the TP points are close to the Fermi level.

First-principles calculations of ionic-liquid-gated (Li,Fe)OHFeSe suggest a metal-insulator transition at a nominal Li/Fe ratio of 75/25 in the (Li,Fe)OH layer. While doping increases Fermi energy, the formation of an antiferromagnetic bicollinear phase is the key for the transition. It is expected that this insulating phase also exists in other FeSe systems upon heavy electron doping, and its presence can hinder the increase of superconducting temperature. These results offer clues on how to optimize superconductivity amid its interplay with magnetic properties in FeSe systems.

Accurate modeling of thermal properties is strikingly important for developing next-generation electronics with high performance. Many thermal properties are closely related to phonon dispersions, such as sound velocity. However, random substituted semiconductor alloys A x B1-x usually lack translational symmetry, and simulation with periodic boundary conditions often requires large supercells, which makes phonon dispersion highly folded and hardly comparable with experimental results. Here, we adopt a large supercell with randomly distributed A and B atoms to investigate substitution effect on the phonon dispersions of semiconductor alloys systematically by using phonon unfolding method [F. Zheng, P. Zhang, Comput. Mater. Sci. 125, 218 (2016)]. The results reveal the extent to which phonon band characteristics in (In,Ga)N and Ga(N,P) are preserved or lost at different compositions and q points. Generally, most characteristics of phonon dispersions can be preserved with indium substitution of gallium in GaN, while substitution of nitrogen with phosphorus strongly perturbs the phonon dispersion of GaN, showing a rapid disintegration of the Bloch characteristics of optical modes and introducing localized impurity modes. In addition, the sound velocities of both (In,Ga)N and Ga(N,P) display a nearly linear behavior as a function of substitution compositions.

By using the density functional theory plus $U$ (DFT+$U$) method and taking spin-orbit coupling into account, we investigate the electronic and phonon properties of ${\mathrm{Pu}}_{3}\mathrm{Ga}$. Most interestingly, we find that there is a significant phonon softening in the transverse acoustic branch at the $L$ point in the Brillouin zone. Via bond strength analysis, we reveal that the nearest-neighboring bond strengths in the face-centered-cubic lattice are weaker around Ga atoms, which is responsible for the observed phonon softening.

Integration between magnetism and topology is an exotic phenomenon in condensed-matter physics. Here, we propose an exotic phase named topological crystalline antiferromagnetic state, in which antiferromagnetism intrinsically integrates with nontrivial topology, and we suggest such a state can be realized in tetragonal FeS. A combination of first-principles calculations and symmetry analyses shows that the topological crystalline antiferromagnetic state arises from band reconstruction induced by pair checker-board antiferromagnetic order together with band-gap opening induced by intrinsic spin-orbit coupling in tetragonal FeS. The topological crystalline antiferromagnetic state is protected by the product of fractional translation symmetry, mirror symmetry, and time-reversal symmetry, and present some unique features. In contrast to strong topological insulators, the topological robustness is surface-dependent. These findings indicate that non-trivial topological states could emerge in pure antiferromagnetic materials, which sheds new light on potential applications of topological properties in fast-developing antiferromagnetic spintronics., Comment: 8 pages, 6 figures

While pyrite-type PtBi2 with face-centered cubic structure has been predicted to be a three-dimensional (3D) Dirac semimetal, experimental study on its physical properties remains absent. Here we report the angular-dependent magnetoresistance (MR) measurements of PtBi2 single-crystal under high magnetic fields. We observed extreme large unsaturated magnetoresistance (XMR) up to 11.2 million percent at T = 1.8 K in a magnetic field of 33 T, which surpasses the previously reported Dirac materials, such as WTe2, LaSb and NbP. The crystals exhibit an ultrahigh mobility and significant Shubnikov-de Hass (SdH) quantum oscillations with nontrivial Berry's phase. Analysis of Hall resistivity indicates that the XMR can be ascribed to the nearly compensated electron and hole. Our experimental results associated with the ab initio calculations suggest that pyrite PtBi2 is a topological semimetal candidate which might provide a platform for exploring topological materials with XMR in noble metal alloys., 18 pages, 6 figures,accepted by Physical Review Letters

We present Phonon Unfolding, a Fortran90 program for unfolding phonon dispersions. It unfolds phonon dispersions by using a generalized projection algorithm, which can be used to any kind of atomic systems in principle. Thus our present program provides a very useful tool for the phonon dispersion and vibration mode analysis of surface reconstructions, atomic point defects, alloys and glasses., 7 pages, 3 figures. arXiv admin note: text overlap with arXiv:1403.0634

We theoretically study, through combining the density functional theory and an unfolding technique, the electronic band structure and the charge doping effects for the deposition of potassium (K) on multilayer FeSe films grown on SrTiO3 (001) surface. These results form a theoretical base line for further detailed studies of low-temperature electronic properties and their multiway quantum engineering of FeSe thin films. We explain the Fermi surface topology observed in experiment and formulate the amount of doped electrons as a function of atomic K coverage. We show that the atomic K deposition efficiently dopes electrons to top layer FeSe. Both checkerboard and pair-checkerboard antiferromagnetic (AFM) FeSe layers show electron pockets at M point and no Fermi pocket at $\Gamma$ point with moderate atomic K coverage. The electron transfer from K adsorbate to FeSe film introduces a strong electric field, which leads to a double-Weyl cone structure at M point in the Brillouin zone of checkerboard-AFM FeSe. We demonstrate that with experimentally accessible heavy electron doping, an electron-like Fermi pocket will emerge at $\Gamma$ point, which should manifest itself in modulating the high-temperature superconductivity of FeSe thin films.

We report asymmetric Se heights in a single unit-cell (UC) FeSe on SrTiO3(0 0 1) substrate with the highest superconducting transition temperature (T c) among the Fe-based superconductors revealed by total-reflection high-energy positron diffraction measurements. Among various iron-based superconductors, this single UC FeSe on the SrTiO3(0 0 1) has been the best material to achieve the highest-T c above 50 K. We found the asymmetric Se heights of 1.44 ± 0.03 and 1.33 ± 0.03 A from the single Fe layer by the intensity analysis based on dynamical diffraction theory. The average Se height results in 1.39 ± 0.04 A, corresponding to the optimum value for Fe-based superconductors. In addition, the average of bond angles of Se-Fe-Se, 107.2 ± 1.1 and 111.5 ± 1.2° becomes 109.3 ± 1.6°, which is close to the optimum value of 109.5° for a regular tetrahedron. Thus, this single UC FeSe is found to have asymmetrically optimized structure. Based on our first-principles calculations, the asymmetry does not change the bandwidth whereas it splits the electron bands at the M point only at the bottom. These calculations suggest that at low electron doping, the structural asymmetry is expected to lead to exotic properties of non-centrosymmetric superconductivity, whereas after a certain amount of electron doping, average anion height plays an important role for high-T c.

Quasi-two-dimensionality is well known as a key electronic and structural element of high temperature superconductivity. Here we prepared ultrathin films of anti-PbO-type CoSe down to the monolayer on a SrTiO3(001) substrate using molecular beam epitaxy, and investigated their electronic structures by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and transport measurements in combination with first-principles calculations. We found that the monolayer CoSe had similar band structure and non-ferromagnetism nature with monolayer FeSe on SrTiO3, except for the Fermi level upshift by ~0.25 eV and weaker electronic correlation with a renormalization factor of 1.7, due to extra electron filling from Co 3d7 compared with Fe 3d6. The results hint that superconductivity might emerge in hole-doped tetragonal CoSe films.

Based on first-principles calculations, we investigated the effect of charge doping on the stability of ZrSe2 monolayer under external biaxial tensile strain. The phonon dispersions are obtained and show that both electron and hole dopings can suppress the soft phonon modes under tensile strain. With the carrier concentration increasing, the ideal strength of ZrSe2 monolayer increases significantly at first, and then reaches a maximum value. The maximum ideal strengths of ZrSe2 under hole and electron dopings are larger than that of neutral monolayer by 79% and 56%, respectively.

The atomic and electronic structures of armchair and zigzag ZrSe2 nanoribbons have been investigated systematically. Both the armchair and zigzag ZrSe2 nanoribbons are nonmagnetic semiconductors, while their bandgaps show quite different behaviors depending on the ribbon width. We find that all the zigzag ribbons possess direct energy gaps, which smoothly decline with the increasing ribbon width. On the other hand, energy gaps for the armchair ribbons change from direct gaps to indirect ones as the ribbon width increases and exhibit a width-dependent oscillation behavior. Moreover, the semiconducting behaviors and the bandgap types are robust, and they remain unchanged in bilayer and multilayer thin films with inter-layer interactions. These findings indicate that ZrSe2 nanoribbons are promising candidate materials for applications in nanoelectronic devices.

The adsorption of 3d, 4d, and 5d transition-metal (TM) atoms on single-layer boron nitride (SLBN) sheets was systematically studied through comparison of structural and magnetic properties determined using first-principles calculations. We found that the majority of TM atoms were chemically adsorbed on SLBN with adsorption energies larger than 0.5 eV, whereas some TM atoms in d5 or d10 configuration were physically adsorbed on SLBN with adsorption energies lower than 0.5 eV. The charge density difference indicated that the main contribution to the adsorption energy was the interaction between the TM and the nearest B atoms rather than N atoms. Moreover, TM adatoms from Sc (Y, La) to Co (Rh, Ir) and Cu (Ag, Au) displayed nonzero magnetic moments arising from the d-orbital occupation of TM atoms. Our results provide fundamental knowledge of TM atom adsorption on SLBN, which will be useful for not only spintronics applications but also the development of magnetic nanostructures.

The atomic structures, adsorption energies, and spin polarizations of lanthanide (Ln = La-Gd) atoms adsorbed on the single and double vacancies in graphene are investigated systematically by the first- principles calculations. The calculations show that the Ln atoms adsorb on the centers of the vacancies with large adsorption energies, in the range of 3.57 ∼ 8.45 eV. The adsorbed Ln atoms also dope a large number of electrons into the graphene. Most of the Ln atoms are highly spin-polarized. The strong adsorption and high spin polarization gives these systems potential application in spintronic and catalyst materials.

The temperature and Sn isotope effects on SnTe are investigated using the hybrid scheme that combines ab initio molecular dynamics and lattice dynamics (AIMD + LD). The unstable softening of TA phonon mode at 0 K in cubic phase SnTe disappears and the TO mode is stiffened by considering the temperature effect, in agreement with the inelastic neutron scattering (INS) observations. A linear dependence on isotope mass of phonon frequency, lifetime, and mean free path for isolate phonon mode is observed with the possibility of positive, negative, and nearly zero shifts. The lattice thermal conductivity () shows saturation characteristic as Sn isotope mass increases to 120SnTe, with an increase rate of ~2.1% from 112SnTe to 124SnTe. Considering the effects of partial isotope doping, we obtain a reduced with respect to the undoped case. The is reduced by ~14.6% at 600 K when considering the volumetric expansion.