Aitor Bergara - Academia.edu (original) (raw)

Papers by Aitor Bergara

Proceedings of the National Academy of Sciences, 2010

Experimental studies established that calcium undergoes several counterintuitive transitions unde... more Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc → bcc → simple cubic → Ca-IV → Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin ( I 4 1 / amd ) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33–71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with ...

Proceedings of the National Academy of Sciences, 2010

There is great interest in the exploration of hydrogen-rich compounds upon strong compression whe... more There is great interest in the exploration of hydrogen-rich compounds upon strong compression where they can become superconductors. Stannane (SnH 4 ) has been proposed to be a potential high-temperature superconductor under pressure, but its high-pressure crystal structures, fundamental for the understanding of superconductivity, remain unsolved. Using an ab initio evolutionary algorithm for crystal structure prediction, we propose the existence of two unique high-pressure metallic phases having space groups Ama2 and P6 3 ∕mmc, which both contain hexagonal layers of Sn atoms and semimolecular (perhydride) H 2 units. Enthalpy calculations reveal that the Ama2 and P6 3 ∕mmc structures are stable at 96-180 GPa and above 180 GPa, respectively, while below 96 GPa SnH 4 is unstable with respect to elemental decomposition. The application of the Allen-Dynes modified McMillan equation reveals high superconducting temperatures of 15-22 K for the Ama2 phase at 120 GPa and 52-62 K for the P6 3 ∕mmc phase at 200 GPa.

Physical Review Letters, 2009

Following the suggestion that hydrogen-rich compounds, and, in particular, silane (SiH 4 ), might... more Following the suggestion that hydrogen-rich compounds, and, in particular, silane (SiH 4 ), might be high-T c superconductors at moderate pressures, very recent experiments have confirmed that silane metallises and even becomes superconducting at high pressure. In this article, we present a structural characterization of compressed silane obtained with an ab initio evolutionary algorithm for crystal structure prediction. Besides the earlier molecular and chainlike structures of P2 1 =c and I4 1 =a symmetries, respectively, we propose two novel structures with space groups Fdd2 and Pbcn, to be stable at 25-55 and 220-250 GPa, respectively. According to our calculations, silane becomes metallic and superconducting at 220 GPa in the layered Pbcn structure, with a theoretical T c of 16 K. Our calculations also show that the imaginary phonons of the recently proposed P6 3 generate the Pbcn structure.

Physical Review Letters, 2008

High-pressure structures of germane (GeH 4 ) are explored through ab initio evolutionary methodol... more High-pressure structures of germane (GeH 4 ) are explored through ab initio evolutionary methodology to reveal a metallic monoclinic structure of C2=c (4 molecules=cell). The C2=c structure consists of layerlike motifs containing novel ''H 2 '' units. Enthalpy calculations suggest a remarkably wide decomposition (Ge þ H 2 ) pressure range of 0-196 GPa, above which C2=c structure is stable. Perturbative linear-response calculations for C2=c GeH 4 at 220 GPa predict a large electron-phonon coupling parameter of 1.12 and the resulting superconducting critical temperature reaches 64 K.

Proceedings of the National Academy of Sciences, 2010

Experimental studies established that calcium undergoes several counterintuitive transitions unde... more Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc → bcc → simple cubic → Ca-IV → Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin (I4 1 ∕amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with experiment. evolutionary algorithms | high pressure | structure prediction | density functional theory | superconductivity 7646-7651 | PNAS |

Proceedings of the National Academy of Sciences, 2010

Experimental studies established that calcium undergoes several counterintuitive transitions unde... more Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc → bcc → simple cubic → Ca-IV → Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin ( I 4 1 / amd ) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33–71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with ...

Proceedings of the National Academy of Sciences, 2010

There is great interest in the exploration of hydrogen-rich compounds upon strong compression whe... more There is great interest in the exploration of hydrogen-rich compounds upon strong compression where they can become superconductors. Stannane (SnH 4 ) has been proposed to be a potential high-temperature superconductor under pressure, but its high-pressure crystal structures, fundamental for the understanding of superconductivity, remain unsolved. Using an ab initio evolutionary algorithm for crystal structure prediction, we propose the existence of two unique high-pressure metallic phases having space groups Ama2 and P6 3 ∕mmc, which both contain hexagonal layers of Sn atoms and semimolecular (perhydride) H 2 units. Enthalpy calculations reveal that the Ama2 and P6 3 ∕mmc structures are stable at 96-180 GPa and above 180 GPa, respectively, while below 96 GPa SnH 4 is unstable with respect to elemental decomposition. The application of the Allen-Dynes modified McMillan equation reveals high superconducting temperatures of 15-22 K for the Ama2 phase at 120 GPa and 52-62 K for the P6 3 ∕mmc phase at 200 GPa.

Physical Review Letters, 2009

Following the suggestion that hydrogen-rich compounds, and, in particular, silane (SiH 4 ), might... more Following the suggestion that hydrogen-rich compounds, and, in particular, silane (SiH 4 ), might be high-T c superconductors at moderate pressures, very recent experiments have confirmed that silane metallises and even becomes superconducting at high pressure. In this article, we present a structural characterization of compressed silane obtained with an ab initio evolutionary algorithm for crystal structure prediction. Besides the earlier molecular and chainlike structures of P2 1 =c and I4 1 =a symmetries, respectively, we propose two novel structures with space groups Fdd2 and Pbcn, to be stable at 25-55 and 220-250 GPa, respectively. According to our calculations, silane becomes metallic and superconducting at 220 GPa in the layered Pbcn structure, with a theoretical T c of 16 K. Our calculations also show that the imaginary phonons of the recently proposed P6 3 generate the Pbcn structure.

Physical Review Letters, 2008

High-pressure structures of germane (GeH 4 ) are explored through ab initio evolutionary methodol... more High-pressure structures of germane (GeH 4 ) are explored through ab initio evolutionary methodology to reveal a metallic monoclinic structure of C2=c (4 molecules=cell). The C2=c structure consists of layerlike motifs containing novel ''H 2 '' units. Enthalpy calculations suggest a remarkably wide decomposition (Ge þ H 2 ) pressure range of 0-196 GPa, above which C2=c structure is stable. Perturbative linear-response calculations for C2=c GeH 4 at 220 GPa predict a large electron-phonon coupling parameter of 1.12 and the resulting superconducting critical temperature reaches 64 K.

Proceedings of the National Academy of Sciences, 2010

Experimental studies established that calcium undergoes several counterintuitive transitions unde... more Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc → bcc → simple cubic → Ca-IV → Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin (I4 1 ∕amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with experiment. evolutionary algorithms | high pressure | structure prediction | density functional theory | superconductivity 7646-7651 | PNAS |