[Show abstract][Hide abstract] ABSTRACT: We present high-resolution angle-resolved photoemission spectroscopy study in
conjunction with first principles calculations to investigate how the
interaction of electrons with phonons in graphene is modified by the presence
of Yb. We find that the transferred charges from Yb to the graphene layer
hybridize with the graphene $\pi$ bands, leading to a strong enhancement of the
electron-phonon interaction. Specifically, the electron-phonon coupling
constant is increased by as much as a factor of 10 upon the introduction of Yb
with respect to as grown graphene ($\leq$0.05). The observed coupling constant
constitutes the highest value ever measured for graphene and suggests that the
hybridization between graphene and the adatoms might be a critical parameter in
realizing superconducting graphene.
[Show abstract][Hide abstract] ABSTRACT: A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg2C3), the new monoclinic C2/m structure (β-Mg2C3) contains linear C3(4-) chains that are isoelectronic with CO2. Unlike α-Mg2C3, which contains alternating layers of C3(4-) chains oriented in opposite directions, all C3(4-) chains within β-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of β-Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C═C stretching near 1200 cm(-1) and (13)C resonances confirming the presence of the rare allylenide anion.
[Show abstract][Hide abstract] ABSTRACT: The high-pressure and high-temperature formation and stability of recently discovered magnesium carbide Mg2C were studied by in situ X-ray diffraction up to 20 GPa and 1550 K. The insights into the thermodynamics of Mg2C under extreme conditions, and its metastability at 0.1 MPa and 300 K, were provided by ab initio calculations of total energies and phonon density of states as a function of pressure. We illustrate how the compound found occasionally in high-pressure experiments could be systematically predicted and discovered in a time-saving way. Similar theoretical approaches can be useful for prediction of synthesis conditions and recovery of new solids.
The Journal of Physical Chemistry C. 04/2014; 118(15):8128–8133.
[Show abstract][Hide abstract] ABSTRACT: Magnesiumcarbid (Mg2C) wurde vor 20 Jahren in Ab‐initio‐Rechnungen entdeckt. Nun beschreiben T. A. Strobel und Mitarbeiter in ihrer Zuschrift auf S. 9098 ff. die erste Synthese dieses Methanids unter Anwendung von Hochdruck‐Hochtemperatur‐Methoden. Die ionische binäre Verbindung enthält hoch geladene C4−‐Ionen.
[Show abstract][Hide abstract] ABSTRACT: Ionic carbon: When magnesium and carbon combine in a 2:1 ratio above 15 GPa, a new antifluorite structure is formed. The compound, with composition Mg2 C, is highly ionic, with carbon in a very unusual C(4-) methanide state.
Angewandte Chemie International Edition 07/2013; · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Three different sodium-silicon clathrate compounds–Na8Si46 (sI), Na24Si136 (sII), and a new structure, NaSi6–were obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900–1100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures, we observed the formation of a new intercalated compound, metallic NaSi6, which crystallizes in the orthorhombic Eu4Ga8Ge16 structure. High-pressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi6), compared to the typical characteristics achieved for single crystals obtained by conventional routes (RRR < 6). Since the Na-Si clathrates are stable only above 2 GPa, previous reports of their synthesis may be viewed as nonequilibrium, precursor-based routes to high-pressure phases at low-pressure conditions.
[Show abstract][Hide abstract] ABSTRACT: Ionic carbon: When magnesium and carbon combine in a 2:1 ratio above 15 GPa, a new antifluorite structure is formed. The compound, with composition Mg2C (see picture), is highly ionic, with carbon in a very unusual C4− methanide state.
Angewandte Chemie International Edition 01/2013; 52(34). · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnesium carbide (Mg2C) was first suggested 20 years ago by ab initio calculations. In their Communication on page 8930 ff., T. A. Strobel and co‐workers describe the first synthesis of this methanide through the use of high‐pressure and high‐temperature methods. The ionic binary compound contains C4− ions, which have an extremely negative charge for carbon.
Angewandte Chemie International Edition 01/2013; 52(34). · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.
[Show abstract][Hide abstract] ABSTRACT: Being the lightest and the most abundant element in the universe, hydrogen is fascinating to physicists. In particular, the conditions of its metallization associated with a possible superconducting state at high temperature have been a matter of much debate in the scientific community, and progress in this field is strongly correlated with the advancements in theoretical methods and experimental techniques. Recently, the existence of hydrogen in a metallic state was reported experimentally at room temperature under a pressure of 260-270 GPa, but was shortly after that disputed in the light of more experiments, finding either a semimetal or a transition to an other phase. With the aim to reconcile the different interpretations proposed, we propose by combining several computational techniques, such as density functional theory and the GW approximation, that phase III at ambient temperature of hydrogen is the Cmca-12 phase, which becomes a semimetal at 260 GPa . From phonon calculations, we demonstrate it to be dynamically stable; calculated electron-phonon coupling is rather weak and therefore this phase is not expected to be a high-temperature superconductor.
Proceedings of the National Academy of Sciences 06/2012; 109(25):9766-9. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Noble metals such as Pt, Au, or Re are commonly used for electrodes and
gaskets in diamond anvil cells for high-pressure research because they
are expected to rarely undergo structural transformation and possess
simple equation of states. Specifically Pt has been used widely for
high-pressure experiments and has been considered to resist hydride
formation under pressure. Pressure-induced reactions of metals with
hydrogen are in fact quite likely because hydrogen atoms can occupy
interstitial positions in the metal lattice, which can lead to
unexpected effects in experiments. In our study, PRL 107 117002 (2011),
we investigated crystal structures using ab initio random structure
searching (AIRSS) and predicted the formation of platinum mono-hydride
above 22 GPa and superconductivity Tc was estimated to be 10
-- 25 K above around 80 GPa. Furthermore, we showed that the formation
of fcc noble metal hydrides under pressure is common and examined the
possibility of superconductivity in these materials.
[Show abstract][Hide abstract] ABSTRACT: Materials with very high hydrogen density have attracted considerable interest due to a range of motivations, including the search for chemically precompressed metallic hydrogen and hydrogen storage applications. Using high-pressure synchrotron X-ray diffraction technique and theoretical calculations, we have discovered a new rhodium dihydride (RhH(2)) with high volumetric hydrogen density (163.7 g/L). Compressing rhodium in fluid hydrogen at ambient temperature, the fcc rhodium metal absorbs hydrogen and expands unit-cell volume by two discrete steps to form NaCl-typed fcc rhodium monohydride at 4 GPa and fluorite-typed fcc RhH(2) at 8 GPa. RhH(2) is the first dihydride discovered in the platinum group metals under high pressure. Our low-temperature experiments show that RhH(2) is recoverable after releasing pressure cryogenically to 1 bar and is capable of retaining hydrogen up to 150 K for minutes and 77 K for an indefinite length of time.
Proceedings of the National Academy of Sciences 11/2011; 108(46):18618-21. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Noble metals adopt close-packed structures at ambient pressure and rarely undergo structural transformation at high pressures. Platinum (Pt) is normally considered to be unreactive and is therefore not expected to form hydrides under pressure. We predict that platinum hydride (PtH) has a lower enthalpy than its constituents solid Pt and molecular hydrogen at pressures above 21.5 GPa. PtH transforms to a hexagonal close-packed or face-centered cubic (fcc) structure between 70 and 80 GPa. Linear response calculations indicate that PtH is a superconductor at these pressures with a critical temperature of about 10-25 K. These findings help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials. We show that the formation of fcc noble metal hydrides under pressure is common and examine the possibility of superconductivity in these materials.
[Show abstract][Hide abstract] ABSTRACT: Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural γ - α transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the γ → α transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Grüneisen parameters, particularly around the X point. We derive a vibrational entropy change ΔS(γ-α)(vib) ≈ (0.33+/-0.03)k(B), illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure.
Proceedings of the National Academy of Sciences 06/2011; 108(23):9342-5. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hydrogen is the lightest and smallest element in the periodic table. Despite its simplest electronic structure, enormous complexity can arise when hydrogen participates in the formation of solids. Pressure as a controllable parameter can provide an excellent platform to investigate novel physics of hydrides because it can induce structural transformation and even changes in stoichiometry accompanied with phenomena such as metallization and superconductivity. In this presentation, we will briefly overview contemporary high-pressure research on hydrides and show our most recent results on predicting crystal structures of metal hydrides under pressure using ab initio random structure searching. Our findings allow for a better understanding of pressure-induced metallization/superconductivity in hydrides which can help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials.
[Show abstract][Hide abstract] ABSTRACT: We have calculated crystal structures and electronic properties of Xe-H2 compounds under high pressures using first-principles density functional theory calculations and ab-initio random structure searching. We present results for the equation of state, Xe-Xe separations, and the electronic charge transfer between the Xe and H atoms. Our results are broadly consistent with experimental results by M. Somayazulu et al. [ Nature Chem. 2 50 (2010)]. We have in addition calculated the metallization pressure within the GW approximation, finding it to be around 250 GPa, which is close to the maximum pressure reached in the experiment.
[Show abstract][Hide abstract] ABSTRACT: Ca-III, the first superconducting calcium phase under pressure, was identified as simple-cubic (sc) by previous X-ray diffraction
(XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed
structures and had an imaginary (unstable) phonon branch. By using our newly developed submicrometer high-pressure single-crystal
XRD, cryogenic high-pressure XRD, and theoretical calculations, we demonstrate that Ca-III is neither exactly sc nor any of
the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic
distortion below 30 K. This surprising discovery reveals a scenario that the high-pressure structure of calcium does not go
to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability
fine-tuned with phonon stability at the local minimum.
Proceedings of the National Academy of Sciences 05/2010; 107(22):9965-9968. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The results presented by us allow for an understanding of pressure-induced superconductivity of tri-hydrides with a particular focus on YH3. We show that a structural phase transition from hexagonal to cubic structure occurs at 20 GPa, which is in good agreement with recent experiments. This structural phase transition is seen to be accompanied by an insulator-to-metal transition in our quasi-particle calculations. Furthermore, we present an analysis of the superconducting behavior in cubic YH3. At the lowest possible pressure (17.7 GPa), cubic YH3 is superconducting with a Tc of 40 K and turns into the normal metallic phase at 25 GPa due to a change of s-d hybridization between hydrogen and yttrium. This hitherto unprecedented low pressure should make superconducting YH3 a very attractive system to study experimentally among the hydrogen-rich superconductors. Finally, we also predict that the superconducting phase reemerges at 45 GPa. J. S. de Almeida, D. Y. Kim, C. Ortiz, M. Klintenberg, and R. Ahuja, Appl. Phys. Lett.94 251913 (2009). D. Y. Kim, R. H. Scheicher, R. Ahuja, Phys. Rev. Lett. 103, 077002 (2009).
[Show abstract][Hide abstract] ABSTRACT: The long-standing prediction that hydrogen can assume a metallic state under high pressure, combined with arguments put forward more recently that this state might even be superconducting up to high temperatures, continues to spur tremendous research activities toward the experimental realization of metallic hydrogen. These efforts have however so far been impeded by the enormous challenges associated with the exceedingly large required pressure. Hydrogen-dense materials, of the MH(4) form (where M can be, e.g., Si, Ge, or Sn) or of the MH(3) form (with M being, e.g., Al, Sc, Y, or La), allow for the rather exciting opportunity to carry out a proxy study of metallic hydrogen and associated high-temperature superconductivity at pressures within the reach of current techniques. At least one experimental report indicates that a superconducting state might have been observed already in SiH(4), and several theoretical studies have predicted superconductivity in pressurized hydrogen-rich materials; however, no systematic dependence on the applied pressure has yet been identified so far. In the present work, we have used first-principles methods in an attempt to predict the superconducting critical temperature (T(c)) as a function of pressure (P) for three metal-hydride systems of the MH(3) form, namely ScH(3), YH(3), and LaH(3). By comparing the obtained results, we are able to point out a general trend in the T(c)-dependence on P. These gained insights presented here are likely to stimulate further theoretical studies of metallic phases of hydrogen-dense materials and should lead to new experimental investigations of their superconducting properties.
Proceedings of the National Academy of Sciences 02/2010; 107(7):2793-6. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ab initio lattice dynamics based on linear response method are performed for high pressure phase of yttrium to describe electron-phonon coupling and subsequent superconducting behavior. The critical temperature Tc in fcc-Y increases monotonically with pressure up to 9.25 K at 31 GPa, which is quantitatively in excellent agreement with two quasihydrostatic experiments and is qualitatively compatible with recent experiments. The excellent agreement with experiments gives us a better understanding of the effective pseudopotential μ∗ as well as spectral function α2(ω)F(ω) in yttrium. These results demonstrate that there exists strong electron-phonon coupling in Y within the studied pressure regime, and for lower pressure electron correlation becomes stronger. Generally, it is found that superconductivity in yttrium under pressure can be described quantitatively within standard theory of phonon-mediated superconductivity.