Imaginary Modes Research Articles

Two dimensional hexagonal GaOOH: A promising ultrawide bandgap semiconductor for smart optoelectronic applications

First-principles based density functional theory (DFT) calculations with the HSE functional are performed to predict a new two dimensional (2D) hexagonal GaOOH (h-GaOOH). Thermodynamical and dynamical stabilities are confirmed by calculating the formation energy and phonon dispersion curves respectively. Absence of imaginary phonon modes throughout the Brillouin zone depicts the dynamical stability of 2D h-GaOOH. The electronic band structure shows an indirect band gap of 4.4 eV. The dielectric constants and absorption coefficients show the optical transparency in the visible and near UV region, which makes this material a promising candidate for UV transparent conductors and solar-blind photo-detectors.

Beryllene: A Promising Anode Material for Na- and K-Ion Batteries with Ultrafast Charge/Discharge and High Specific Capacity.

We predict two-dimensional Be materials, α- and β-beryllene. In α-beryllene each Be atom binds to six other Be atoms in a planar scheme, whereas β-beryllene consists of two stacked α-beryllene monolayers. Both α- and β-beryllene are found to be highly stable, as demonstrated by high cohesive energies close to that of bulk Be, an absence of imaginary phonon modes, and high melting points. Both materials are metallic, indicating potential applications in Na-ion and K-ion batteries, which are explored in detail. The diffusion barriers of Na (K) on α- and β-beryllene are found to be only 9 (3) and 4 (5) meV, respectively. In particular, the diffusion barrier of K on α-beryllene exhibits the lowest ever recorded value in two-dimensional materials, suggesting the possibility of ultrafast charge/discharge. As the theoretical specific capacities of Na/K on α- and β-beryllene are found to be 1487/1322 and 743/743 mA h g-1, respectively, the storage capacity is ultrahigh.

Properties of Alkaline-Earth-Metal Polynitrogen Ternary Materials at High Pressure.

We report the formation of cubic and tetragonal BaSrN3 at 100 GPa using an ab initio structure search method. Pressure ramping to 0 GPa reveals a reaction pressure threshold of 4.92 and 7.23 GPa for the cubic and tetragonal BaSrN3, respectively. The cubic phase is stabilized by coulombic interaction between the ions. Meanwhile, tetragonal BaSrN3 is stabilized through an expansion of the d-orbital in Ba and Sr atoms that is compensated by delocalization of π-electrons in N through reduction of π overlap. Elastic properties analysis suggests that both phases are mechanically stable. The structures also have high melting points as predicted using an empirical model, and all imaginary modes vanishes at about 2000 K. These results have significant implication for the design of cleaner and environmentally friendly high energy density materials.

First-principles study on crystal structures and superconductivity of molybdenum hydrides under high pressure

Hydrogen-rich compounds are promising candidates for high temperature superconductors [Duan et al., Natl. Sci. Rev. 4, 121 (2017); Bi et al., arXiv:1806.00163 (2018); Pickard et al., Ann. Rev. Condens. Matter Phys. 11, 57 (2020); Flores-Livas et al., Phys. Rep. 856, 1 (2020)]. Combining first-principles calculation and the swarm intelligence-based structure search method, we investigated structures and superconductivity of molybdenum hydrides under high pressures. Four new phases have been predicted, namely, I4/m−MoH4, I4/mmm−MoH4, C2/m−MoH6, and Immm−MoH6. The superconducting critical temperature of Immm−MoH6 under 300 GPa was predicted to be 80 K, which is the highest value among hydrides of group six elements in the periodic table. Intriguingly, the phase transition from I4/mmm−MoH4 to I4/m−MoH4 at 280 GPa is driven by the imaginary optical modes and the soft optical modes significantly enhance the superconducting critical temperature.

First-principles study on stability, electronic and optical properties of Janus-functionalized ZnO monolayer and bilayer for optoelectronic device

Two-dimensional materials have aroused enormous interests due to the novel physical properties and wide potential applications in optoelectronic devices. Monolayer or bilayer ZnO, as a typical two-dimensional metal-oxide, also exhibits astonishing properties. Motivated by the recent successful synthesis of Janus metal-oxide monolayer and bilayer, we have investigated the stability, electronic and optical properties of pristine and Janus F–ZnO–Cl monolayer and bilayer by the first-principles calculations. The results show that Janus F–ZnO–Cl monolayer and bilayer exhibit mechanical and dynamical stability without any imaginary modes of phonon spectra. Furthermore, the Janus functionalized will break the planar structure and form the buckled atomic layer in the monolayer and bilayer ZnO. Moreover, based on the HSE06 functional, Janus F–ZnO–Cl monolayer and bilayer exhibit the direct semiconductor characteristic, and the band gaps are narrowed from 3.374 eV to 2.563 eV, and from 3.185 eV to 1.022 eV, respectively. Upon Janus-functionalized, there are a large amount of charge (0.54 e) transfers from the ZnO layer to the functionalized species. In addition, Janus F–ZnO–Cl monolayer and bilayer could strongly absorb light in a wide range from the visible light to the infrared light at different level. The current findings open the possibilities for the application of ZnO-based material in optoelectronic devices.

Ab initio study on crystal structure and phase stability of ZrC2 under high pressure* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11904081 and 11975100), the Basic Research Program of Education Bureau of Henan Province, China (Grant No. 20A140007), and Research Initiation Fund of Henan Institute of Technology (Grant No. KQ1817).

The structural stabilities and crystal evolution behaviors of the hyper stoichiometric compound ZrC2 (carbon rich; C/Zr > 1.0) are studied under ambient and high pressure conditions using first-principles calculations in combination with the particle-swarm optimization algorithm. Six viable structures of ZrC2 in P21/c, Cmmm, Cmc21, P42/nmc, Immm and P6/mmm symmetries are identified. These structures are dynamically stable as their phonon spectra have no imaginary modes at zero pressure or at the selected high-pressure points. Among them, the P21/c phase represents the ground state structure, whereas P21/c, P42/nmc, Immm and P6/mmm phases are part of the phase transition series. The phase order and critical pressures of the phase transition are determined to be approximately 300 GPa according to the equation of states and enthalpy. Furthermore, the mechanical and electronic properties are investigated. The P21/c and Cmc21 phases display a semi-metal nature, whereas the P42/nmc, Immm, P6/mmm and Cmmm phases exhibit a metallic nature. Moreover, the present study reveals considerable information regarding the structural, mechanical and electronic properties of ZrC2, thereby providing key insights into its material properties and evaluating its behavior in practical applications.

Elucidating the atomistic origin of anharmonicity in tetragonal CH3NH3PbI3 with Raman scattering

Halide perovskite (HP) semiconductors exhibit unique strong coupling between the electronic and structural dynamics. The high-temperature cubic phase of HPs is known to be entropically stabilized, with imaginary frequencies in the calculated phonon dispersion relation. Similar calculations, based on the static average crystal structure, predict a stable tetragonal phase with no imaginary modes. This work shows that in contrast to standard theory predictions, the room-temperature tetragonal phase of CH$_{3} $NH$_{3} $PbI$_{3}$ is strongly anharmonic. We use Raman polarization-orientation (PO) measurements and \textit{ab initio} molecular dynamics (AIMD) to investigate the origin and temperature evolution of the strong structural anharmonicity throughout the tetragonal phase. Raman PO measurements reveal a new spectral feature that resembles a soft mode. This mode shows an unusual continuous increase in damping with temperature which is indicative of an anharmonic potential surface. The analysis of AIMD trajectories identifies two major sources of anharmonicity: the orientational unlocking of the [CH$_{3} $NH$_{3}$]$^+$ ions and large amplitude octahedral tilting that continuously increases with temperature. Our work suggests that the standard phonon picture cannot describe the structural dynamics of tetragonal CH$_{3} $NH$_{3} $PbI$_{3}$.

Accelerating black holes: Quasinormal modes and late-time tails

Black holes found in binaries move at very high velocities relative to our own reference frame and can accelerate due to the emission of gravitational radiation. Here, we investigate the numerical stability and late-time behavior of linear scalar perturbations in accelerating black holes described by the $C-$metric. We identify a family of quasinormal modes associated with the photon surface and a brand new family of purely imaginary modes associated with the boost parameter of the accelerating black hole spacetime. When the accelerating black hole is charged, we find a third family of modes which dominates the ringdown waveform near extremality. Our frequency and time domain analysis indicate that such spacetimes are stable under scalar fluctuations, while the late-time behavior follows an exponential decay law, dominated by quasinormal modes. This result is in contrast with the common belief that such perturbations, for black holes without a cosmological constant, always have a power-law cutoff. In this sense, our results suggest that the asymptotic structure of black hole backgrounds does not always dictate how radiative fields behave at late times.

Electronic structure, phonons, and high-temperature phonon-mediated superconductivity in lithium-intercalated diamond-like boron compounds

We present the first-principles calculations of lithium-intercalated diamond-like boron compounds LiB and LiB2. Although the pristine diamond structure of boron is dynamically unstable, LiB has no imaginary phonon mode after lithium intercalation. Our calculations show that LiB is an insulator with a direct energy gap of 1.45 eV. By removing a face-centered cubic sublattice of lithium from LiB, a metallic compound LiB2 is obtained. Based on Wannier interpolation, the electron–phonon coupling in LiB2 is accurately determined. We find that LiB2 is stable above 12 GPa, with superconducting transition temperature close to 60 K.

Theoretical study of structural, electronic, dynamic and thermodynamic properties of Ni2FeAl and Ni2CoAl alloys

First-principles full-potential linearized augmented plane-wave method based on density functional theory (DFT) is used to investigate the structural, electronic, magnetic, dynamical and thermodynamic properties of Ni based full-Heusler alloys Ni2YAl (Y= Fe, Co). These full-Heusler alloys in different types structures Cu2MnAl, Hg2CuTi and I4/mmm were calculated within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA-PBE). The tetragonal I4/mmm-type structure is found to be energetically more favorable than the cubic and Cu2MnAl-type and Hg2CuTi-type structures for both the Ni2FeAl and the Ni2CoAl full-Heusler alloys. The calculated lattice parameters, bulk moduli, their first-pressure derivatives and internal parameters are in good agreement with the other theoretical data. Considering the phonon dynamics of Ni2FeAl and Ni2CoAl full-Heusler alloys in Cu2MnAl, Hg2CuTi and I4/mmm phases, we observed that Ni2FeAl full-Heusler alloy in the Cu2MnAl and I4/mmm phases is dynamically stable due to the absence of imaginary modes in phonon dispersion. However, Ni2CoAl full-Heusler alloy is dynamically stable in I4/mmm. The observed magnetism in these alloys is mainly from the 3d states of Co and Fe atoms.

Air-Stable Monolayer Cu2 Se Exhibits a Purely Thermal Structural Phase Transition.

Materials possessing structural phase transformations exhibit a rich set of physical and chemical properties that can be used for a variety of applications. In 2D materials, structural transformations have so far been induced by strain, lasers, electron injection, electron/ion beams, thermal loss of stoichiometry, and chemical treatments or by a combination of such approaches and annealing. However, stoichiometry-preserving, purely thermal, reversible phase transitions, which are fundamental in physics and can be easily induced, have not been observed. Here, the fabrication of monolayer Cu2 Se, a new 2D material is reported, demonstrating the existence of a purely thermal structural phase transition. Scanning tunneling microscopy, scanning transmission electron microscopy, and density functional theory (DFT) identify two structural phases at 78 and 300 K. DFT calculations trace the phase-transition mechanism via the existence/absence of imaginary (unstable) phonon modes at low and high temperatures. In situ, variable-temperature low-energy electron diffraction patterns demonstrate that the phase transition occurs across the whole sample at ≈147 K. Angle-resolved photoemission spectra and DFT calculations show that a degeneracy at the Γ point of the energy bands of the high-temperature phase is lifted in the low-temperature phase. This work opens up possibilities for studying such phase transitions in 2D materials.

Unraveling the Ground-State Structure of BaZrO3 by Neutron Scattering Experiments and First-Principles Calculations

The all-inorganic perovskite barium zirconate, BaZrO3, is a widely used material in a range of different technological applications. However, fundamental questions surrounding the crystal structure of BaZrO3, especially in regard to its ground-state structure, remain. While diffraction techniques indicate a cubic structure all the way down to T = 0 K, several first-principles phonon calculation studies based on density functional theory indicate an imaginary (unstable) phonon mode due to the appearance of an antiferrodistortive transition associated with rigid rotations of ZrO6 octahedra. The first-principles calculations are highly sensitive to the choice of exchange-correlation functional and, using six well-established functional approximations, we show that a correct description about the ground-state structure of BaZrO3 requires the use of hybrid functionals. The ground-state structure of BaZrO3 is found to be cubic, which is corroborated by experimental results obtained from neutron powder diffraction, inelastic neutron scattering, and neutron Compton scattering experiments.

Ab initio calculations of the crystal field and phonon dispersions in CePd2Al2 and LaPd2Al2

CePd2Al2 crystallizes in the CaBe2Ge2-type tetragonal structure (P4/nmm, 129) and undergoes a phase transition to the orthorhombic Cmme structure at around 13 K. Its inelastic neutron spectra reveal an additional magnetic excitation that was ascribed to electron-phonon interaction leading to a formation of a new quantum quasi-bound vibron state. We present the first-principles calculations of the crystal field excitations and lattice dynamics calculations of the phonon dispersions to compare with the experimental data. The calculated crystal field energy splitting in CePd2Al2 agrees well with the model used to describe the experimental neutron scattering spectra. The first excited crystal field level moves to higher energies when undergoing the transformation from tetragonal to orthorhombic structure, in agreement with the experiment. The analysis based on calculated elastic constants and lattice dynamics calculations show that in both tetragonal and orthorhombic structures there are no imaginary modes for any q-wave vector within the Brillouin zone, and therefore the lattice structures are stable. The phonon dispersions and density of states are calculated for both crystal structures of CePd2Al2 and its nonmagnetic counterpart LaPd2Al2. The results generally agree well with the experimental data including the high phonon density of states around 12 meV. The phonon density of states is also used to calculate the mean squared displacement, Debye temperature, lattice heat capacity and compared with similar properties of the available experiment.

High-pressure crystal structure and properties of BrCl

The pressure-induced phase transitions of bromine chloride (BrCl) were studied using the CALYPSO structure prediction method and first-principles calculations. Geometry optimization showed that the BrCl underwent the following phase transitions under high pressure: Cmcm → C2/m → Fddd → P4/mmm. These phase transitions occurred at 75, 94, and 140 GPa, respectively. All four phases were calculated to be dynamically stable, judging by the absence of any imaginary phonon modes in the whole Brillouin zone. As the pressure increased, the number of bonds between Br atom and surrounding Cl atoms increased. The high-pressure P4/mmm–BrCl phase (stable at 150 GPa) consisted of eight-coordinated Br atoms and is predicted to exist in BrCl compounds for the first time. The band overlap of the Cmcm phase, which caused the pressure-induced semiconductor → metal transition, occurs ∼50 GPa. This article lays the foundation for further theoretical and experimental research.

The investigation of optoelectronic, magnetic and dynamical properties of Ce and Ti doped 2D blue phosphorene: A dispersion corrected DFT study

The present study has been performed to investigate optoelectronic, magnetic and dynamical properties of pristine blue phosphorene (BP), (Ce, Ce–Ce, Ti, Ti–Ti) doped BP and single vacancy BP using the Cambridge Serial Total Energy Package (CASTEP) simulation code based on the density functional theory (DFT). The optical and vibrational properties were carried out by employing dispersion corrected density functional theory. The calculated lattice constants for pristine blue phosphorene a = b = 3.33 Å are found to be improved by Tkatchenko-Scheffler (TS) and Grimme (G06) schemes for 2D phosphorene. The band structures and density of states results have figured out indirect band gaps as 1.93eV (pristine), 0.35eV (with single vacancy), 0.37eV (Ce doped), 0.68eV (Ti doped), and 0.56eV (Ti–Ti doped) in blue phosphorene. Whereas, no band gap has been observed for Ce–Ce doped blue phosphorene. The phonon dispersion curves obtained by density functional theory perturbation theory presented dynamical stability of the studied materials without any imaginary modes of phonon spectra. The Raman active optical modes of vibrations are found at the highest frequency of 520 cm−1. The Spin polarized density of states portray that (Ce, Ti, Ti–Ti) doped blue phosphorene exhibit semi-metallic like behavior due to hybridization of 4f, 5d and 3d localized energy states. The optical properties show that the plasma frequencies occurred at 11.34eV (pristine), 8.6 eV (Ce-doped) and 9.03eV (Ti-doped) blue phosphorene. Moreover, significant shift of absorption peak towards high energy range is clear signature of the blue shift.

The ditungsten decacarbonyl dianion.

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC)5W-W(CO)5][K(18-crown-6)(THF)2]2 (1), completing the group 6 dianion triad over half a century since the area began. The W-W bond is long [3.2419(8) Å] and, surprisingly, in the solid-state the dianion adopts a D4h eclipsed rather than D4d staggered geometry, the latter of which dominates the structural chemistry of binary homobimetallic carbonyls. Computational studies at levels of theory from DFT to CCSD(T) confirm that the D4d geometry is energetically preferred in the gas-phase, being ∼18 kJ mol-1 more stable than the D4h form, since slight destabilisation of the degenerate W-CO π 5dxz and 5dyz orbitals is outweighed by greater stabilisation of the W-W σ-bond orbital. The gas-phase D4h structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D4h isomer directly to the D4d forms, which are produced by rotation around the W-W bond by ±45°. It is therefore concluded that the gas-phase transition state becomes a minimum on the potential energy surface when subjected to crystal packing in the solid-state.

Teatro: No pasar. Rendimiento crítico del teatro de Roberto Suárez en su contexto de producción by Ignacio Gutiérrez Muiño

Reviewed by: Teatro: No pasar. Rendimiento crítico del teatro de Roberto Suárez en su contexto de producción by Ignacio Gutiérrez Muiño Laura V. Sández Gutiérrez Muiño, Ignacio. Teatro: No pasar. Rendimiento crítico del teatro de Roberto Suárez en su contexto de producción. Madrid: Consejo Superior de Investigaciones Científicas, 2018. 199 pp. Teatro: No pasar analyzes seven plays by Uruguayan director and playwright Roberto Suárez. The first chapter presents the structure of the book and introduces the reader to Suárez's oeuvre. The author offers a first hint of what rendimiento crítico entails by affirming that the relation between artists and audience is of crucial importance. The second chapter offers a close reading of Suárez's most recurrent dramaturgical strategies, following José Luis García Barrientos's methodology of dramatología. This chapter includes a detailed analysis of aspects such as staging, texts, characters, time, and space. These could be informative, but unfortunately they are buried in a dense, verbose structure. The author prioritizes scenes in which spectators are interpellated, yet fails to account for the spectators' responses. While there is [End Page 151] much talk about the "desdoblamiento de niveles dramáticos," the dramatic level of the audience is mostly ignored. In the third chapter, the play Bienvenido a casa is posed as a paradigmatic example of Gutiérrez's analytical framework. For Gutiérrez, this play motivates and stimulates in the audience "una actitud alerta y autoconsciente que se asocia con la noción de rendimiento crítico" (191), in the sense that it combines a front-of-the-house play with a behind-the-scenes one, thereby allowing the dramaturg to present the audience with complex dramatic structures wherein they have to continually reexamine their roles. The last chapter explains how Bienvenido a casa operates in the realm of rendimiento crítico, presenting the overworked condemnation of the referential pact as totalizante and alienating. Employing a conceptual frame based on a philosophy of language, Gutiérrez calls for a reflective, critical position that, while necessary, brings nothing new to the current state of either the theory or the history of theatre. The "political" is here "la visibilización y puesta en problema del estatuto de la representación y sus límites en el marco social y cultural actual" (152). The social and cultural realm fails to be explained, with the result that we learn very little about Uruguay's or Montevideo's political situation; the political remains constrained to a notion of the dramatic that does not exceed the referential. The poetic, imaginary, and lyric modes of criticism that close this chapter effectively illustrate Suárez's dramatic strategies. One key aspect of the critical mode—the most advanced critical stage for Gutiérrez—consists of confusing the audience as to which level of representation the dramatic situation is operating in. The book ends by defining rendimiento crítico as a "perspectiva negativa o trascendental que, desde la ficción, abre (permite) la pregunta por las condiciones y posibilidades de la representación" (185). Reading Teatro: No pasar, one cannot help but think about the bureaucratization of criticism and research. The book has the style, pace, verbosity, and appeal of a report on the development of a project. Its content is carefully developed to leave no term undefined, but it is not a very inviting read. Ultimately, the book speaks more about the conditions of this type of criticism than about Roberto Suárez's work. [End Page 152] Laura V. Sández Villanova University Copyright © 2020 Center of Latin American Studies

Electronic and Phonon Instabilities in Bilayer Graphene under Applied External Bias

Abstract We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structures of bilayer graphene. We study the effect of external electric fields and compare results obtained with different levels of theory to existing theoretical and experimental results. Application of an external field to the AB bilayer alters the electronic spectrum, with the bands changing under bias from a parabolic to a “Mexican hat” structure. This results in a semi-metal-to-semiconductor phase transition, with the size of the induced electronic band-gap being tuneable through the field strength. A reduction of continuous symmetry from a hexagonal to a triangular lattice is also evidenced through in-plane electronic charge inhomogeneities between the sublattices. When spin-orbit coupling is turned on for the AB system, we find that the bulk gap decreases, gradually increasing for larger intensities of the bias. Under large bias the energy dispersion recovers the Mexican hat structure, since the energy interaction between the layers balances the coupling interaction. We find that external bias perturbs the harmonic phonon spectra and leads to anomalous behaviour of the out-of-plane flexural ZA and layer-breathing ZO' modes. For the AA system, the electronic and phonon dispersions both remain stable under bias, but the phonon spectrum exhibits zone-center imaginary modes due to layer-sliding dynamical instabilities.

Ripples in isotropically compressed graphene

An isotropic compression of graphene is shown to induce a structural deformation on the basis of Density Functional Perturbation Theory. Static instabilities, indicated by imaginary frequency phonon modes, are induced in the high symmetry Γ-K (zigzag) and Γ-M (armchair) directions by an isotropic compressive strain of the graphene sheet. The wavelength of the unstable modes (ripples) is directly related to the magnitude of the strain and remarkably insensitive to the direction of propagation in the 2D lattice. These calculations further suggest that the formation energy of the ripple is isotropic for lower strains and becomes anisotropic for larger strains. This is a result of graphene’s elastic property, which is dependent on direction and strain. Within the quasi-harmonic approximation this is combined with the observation that molecular adsorption energies depend strongly on curvature to suggest a strategy for generating ordered overlayers in order to tune the functional properties of graphene.

Role of CuAlO2 as an absorber layer for solar energy converter

A delafossite semiconductor, namely CuAlO2, with a lower band gap is a potential candidate for the absorption of solar radiation due to its excellent electronic and optical properties. In this paper, structural, electronic, vibrational and optical properties of delafossite CuAlO2 have been calculated using state-of-the-art first principles calculations based on density functional theory (DFT). The optimized structural parameters, electronic properties and Raman active vibrational modes namely the Eg and A1g are in good agreement with experimental and other theoretical data. The full phonon dispersion curves (PDC) together with the phonon density of states (PHDOS) of CuAlO2 depict its dynamical stability due to non-existence of any imaginary phonon mode in the entire Brillouin zone (BZ). To understand the mechanism of optical transition properties, we have further calculated the dielectric functions, absorption coefficient and joint density of states (JDOS) for delafossite, CuAlO2. The dielectric constant and absorption coefficient show a significant anisotropic nature in the components of polarization directions. Solar cell parameters of CuAlO2 are also calculated and the highest theoretical efficiencies of 14.8% and 12.5% have been observed with Shockley-Queisser (SQ) limit and spectroscopic limited maximum efficiency (SLME) respectively. The SLME predicted efficiency agrees better to the experimental value as it includes the thickness dependent absorbance coefficient in account of non-radiative recombination and carrier loss mechanism.