David Roundy | Oregon State University (original) (raw)
Papers by David Roundy
Physical Review B, 2002
The ideal strength is the stress required to yield or break a perfect crystal. 1, 2 The most impo... more The ideal strength is the stress required to yield or break a perfect crystal. 1, 2 The most important feature of the ideal strength is that it sets an upper bound on the attainable stress. Mobile dislocations, grain boundaries, cracks, and other microstructural features may significantly change the strength of a real crystal, but they can never raise it above its ideal value.
Computer Physics Communications, 2010
This paper describes Meep, a popular free implementation of the finite-difference time-domain (FD... more This paper describes Meep, a popular free implementation of the finite-difference time-domain (FDTD) method for simulating electromagnetism. In particular, we focus on aspects of implementing a full-featured FDTD package that go beyond standard textbook descriptions of the algorithm, or ways in which Meep differs from typical FDTD implementations. These include pervasive interpolation and accurate modeling of subpixel features, advanced signal processing, support for nonlinear materials via Padé approximants, and flexible scripting capabilities. PACS: 02.70.Bf; 82.20.Wt; 03.50.De; 87.64.Aa.
Optics Letters, 2006
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontin... more Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization (pixelization). We show that accuracy can be significantly improved by using a subpixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory and compare it with other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.
Optics Letters, 2005
We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalo... more We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalous uniform waveguide mode that has zero group velocity at a nonzero wave vector. In a short piece of a uniform waveguide with a specially designed cross section, light is confined longitudinally by small group-velocity propagation and transversely by a ref lective cladding. The quality factor Q is greatly enhanced by the small group velocity for a set of cavity lengths that are separated by approximately p͞k 0 , where k 0 is the longitudinal wave vector for which the group velocity is zero.
Optimization Letters, 2006
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontin... more Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization ("pixellization"). We show that accuracy can be significantly improved by using a sub-pixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory, and compare it to other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.
Journal of Applied Physics, 2004
Abstract We demonstrate single-mode horizontal and vertical waveguides in a recently proposed pho... more Abstract We demonstrate single-mode horizontal and vertical waveguides in a recently proposed photonic crystal having layered square symmetry. The vertical waveguide supports two degenerate polarizations, and thus light can be selectively steered into horizontal waveguides oriented at 90 based on its polarization. We calculate the transmission of 90 bends, both from horizontal waveguide to horizontal waveguide and from horizontal waveguide to vertical waveguide. In the case of the horizontal-horizontal bend, ...
Applied Physics Letters, 2003
Abstract We present a layered photonic crystal structure having a connectivity that is different ... more Abstract We present a layered photonic crystal structure having a connectivity that is different from diamond which possesses square symmetry within each layer. This structure has a complete photonic band gap of 18% of the midgap frequency with a dielectric contrast of 12: 1, and is a structure with layered square symmetry having a gap greater than 10%. We demonstrate a waveguide in this crystal created by removing a row of rods from a single layer.© 2003 American Institute of Physics.
Physical Review B, 2007
We introduce a new form of density functional theory for the {\em ab initio} description of elect... more We introduce a new form of density functional theory for the {\em ab initio} description of electronic systems in contact with a molecular liquid environment. This theory rigorously joins an electron density-functional for the electrons of a solute with a classical density-functional theory for the liquid into a single variational principle for the free energy of the combined system. A simple approximate functional predicts, without any fitting of parameters to solvation data, solvation energies as well as state-of-the-art quantum-chemical cavity approaches, which require such fitting.
Physical Review B, 2007
We introduce a form of density-functional theory for the ab initio description of electronic syst... more We introduce a form of density-functional theory for the ab initio description of electronic systems in contact with a molecular liquid environment. This theory rigorously joins an electron density functional for the electrons of a solute with a classical density-functional theory for the liquid into a single variational principle for the free energy of the combined system. We then present an approximate functional in which orientational ordering appears only through a simple model dielectric response. Even with the shortcomings of this preliminary functional, the resulting theory predicts, without any fitting of parameters to solvation data, solvation energies as well as state-of-the-art quantum-chemical cavity approaches, which do require such fitting. Future work should focus on including orientational ordering in the joint density functional.
Nature, 2004
Nanoelectromechanical systems (NEMs) hold promise for a number of scientific and technological ap... more Nanoelectromechanical systems (NEMs) hold promise for a number of scientific and technological applications. In particular, NEMs oscillators have been proposed for use in ultrasensitive mass detection, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems. Perhaps the ultimate material for these applications is a carbon nanotube. They are the stiffest material known, have low density, ultrasmall cross-sections and can be defect-free. Equally important, a nanotube can act as a transistor and thus may be able to sense its own motion. In spite of this great promise, a room-temperature, self-detecting nanotube oscillator has not been realized, although some progress has been made. Here we report the electrical actuation and detection of the guitar-string-like oscillation modes of doubly clamped nanotube oscillators. We show that the resonance frequency can be widely tuned and that the devices can be used to transduce very small forces.
Nature, 2002
Magnesium diboride differs from ordinary metallic superconductors in several important ways, incl... more Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the `superconducting energy gap', should clarify why MgB2 is different. Some early experimental studies have indicated that MgB2 has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB2 and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
Physical Review B, 2002
We present a study of the superconducting transition in MgB2 using the ab initio pseudopotential ... more We present a study of the superconducting transition in MgB2 using the ab initio pseudopotential density-functional method, a fully anisotropic Eliashberg equation, and a conventional estimate for μ*. Our study shows that the anisotropic Eliashberg equation, constructed with ab initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant λ=0.61, a transition temperature Tc=39 K, and a boron isotope-effect exponent αB=0.32. The calculated values for Tc, λ, and αB are in excellent agreement with transport, specific-heat, and isotope-effect measurements, respectively. The individual values of the electron-phonon coupling λ(k-->,k') on the various pieces of the Fermi surface, however, vary from 0.1 to 2.5. The observed Tc is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.
Physical Review B, 2002
We present a study of the superconducting transition in MgB2 using the ab-initio pseudopotential ... more We present a study of the superconducting transition in MgB2 using the ab-initio pseudopotential density functional method and the fully anisotropic Eliashberg equation. Our study shows that the anisotropic Eliashberg equation, constructed with ab-initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant lambda = 0.61, a transition temperature Tc = 39 K, and a boron isotope-effect exponent alphaB = 0.31 with a reasonable assumption of mu* = 0.12. The calculated values for Tc, lambda, and alphaB are in excellent agreement with transport, specific heat, and isotope effect measurements respectively. The individual values of the electron-phonon coupling lambda(k,k') on the various pieces of the Fermi surface however vary from 0.1 to 2.5. The observed Tc is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.
Physical Review B, 2004
The preceding comment by Mazin et al. on our work makes two claims: (1) the calculated variations... more The preceding comment by Mazin et al. on our work makes two claims: (1) the calculated variations of the superconducting energy gap within the σ or the π bands are not observable in real samples due to scatterings, and (2) the Coulomb repulsion μ(k,k') is negligibly small between σ and π states and thus should be approximated by a diagonal 2×2 matrix in the σ and π channels. Here, we point out that the former does not affect the validity of our theoretical work, which is for the clean limit, and that the latter is based on an oversimplified estimation of μ.
Cheminform, 2010
For Abstract see ChemInform Abstract in Full Text.
Physical Review B, 2004
Graphitelike, layered structure of bulk BC3 was successfully synthesized more than 15 yr ago by t... more Graphitelike, layered structure of bulk BC3 was successfully synthesized more than 15 yr ago by the chemical reaction of benzene and boron trichloride at 800 C. 1 Experiments2 and ab initio calculations3, 4 suggested that the synthesized material consists of BC3 layers with hexagonal symmetry see the atomic arrangement of a single BC3 layer in Fig. 1 (a). However, how the BC3 layers are stacked in the bulk material is still unclear. Previous ab initio local-densityapproximation (LDA) calculations3, 4 predicted that bulk BC3 is metallic ...
Nature, 2002
Magnesium diboride 1 differs from ordinary metallic superconductors in several important ways, in... more Magnesium diboride 1 differs from ordinary metallic superconductors in several important ways, including the failure of conventional models 2 to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat . A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB 2 is different. Some early experimental studies have indicated that MgB 2 has multiple gaps 3-9 , but past theoretical studies 10-16 have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB 2 and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
Physical Review B, 2001
We have calculated the ideal shear strength and ideal tensile strength of C, Si, and Ge in the di... more We have calculated the ideal shear strength and ideal tensile strength of C, Si, and Ge in the diamond structure. We find ideal shear strengths of 95 GPa, 6.5 GPa, and 4.5 GPa, and ideal tensile strengths of 95 GPa, 23 GPa, and 14 GPa for C, Si and Ge respectively. The shear calculation is performed on the 111 slip plane sheared in a 112 direction, and the tensile load is applied in the 111 direction. We allowed for a full relaxation of the strains orthogonal to the applied strain as well as the atomic basis vectors.
Solid State Communications, 2003
We have performed an ab initio study of the structure of polonium. By calculating total energies ... more We have performed an ab initio study of the structure of polonium. By calculating total energies in a number of tetragonal lattice configurations, we have shown that the simple cubic structure is preferred by the system. The other two zero-stress structures, bcc and fcc, correspond to inflection points along this path. These calculations agree with experimental evidence that polonium is the only known element to assume the simple cubic structure at room temperature. We have found an LDA lattice constant of 3.28 Å, and we have ...
Solid State Communications, 2004
We have performed an ab initio study of the structure of polonium. By calculating total energies ... more We have performed an ab initio study of the structure of polonium. By calculating total energies in a number of tetragonal lattice configurations, we have shown that the simple cubic structure is preferred by the system. The other two zero-stress structures, bcc and fcc, correspond to inflection points along this path. These calculations agree with experimental evidence that polonium is the only known element to assume the simple cubic structure at room temperature. We have found an LDA lattice constant of 3.28 Å, and we have ...
Physical Review B, 2002
The ideal strength is the stress required to yield or break a perfect crystal. 1, 2 The most impo... more The ideal strength is the stress required to yield or break a perfect crystal. 1, 2 The most important feature of the ideal strength is that it sets an upper bound on the attainable stress. Mobile dislocations, grain boundaries, cracks, and other microstructural features may significantly change the strength of a real crystal, but they can never raise it above its ideal value.
Computer Physics Communications, 2010
This paper describes Meep, a popular free implementation of the finite-difference time-domain (FD... more This paper describes Meep, a popular free implementation of the finite-difference time-domain (FDTD) method for simulating electromagnetism. In particular, we focus on aspects of implementing a full-featured FDTD package that go beyond standard textbook descriptions of the algorithm, or ways in which Meep differs from typical FDTD implementations. These include pervasive interpolation and accurate modeling of subpixel features, advanced signal processing, support for nonlinear materials via Padé approximants, and flexible scripting capabilities. PACS: 02.70.Bf; 82.20.Wt; 03.50.De; 87.64.Aa.
Optics Letters, 2006
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontin... more Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization (pixelization). We show that accuracy can be significantly improved by using a subpixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory and compare it with other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.
Optics Letters, 2005
We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalo... more We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalous uniform waveguide mode that has zero group velocity at a nonzero wave vector. In a short piece of a uniform waveguide with a specially designed cross section, light is confined longitudinally by small group-velocity propagation and transversely by a ref lective cladding. The quality factor Q is greatly enhanced by the small group velocity for a set of cavity lengths that are separated by approximately p͞k 0 , where k 0 is the longitudinal wave vector for which the group velocity is zero.
Optimization Letters, 2006
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontin... more Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization ("pixellization"). We show that accuracy can be significantly improved by using a sub-pixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory, and compare it to other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.
Journal of Applied Physics, 2004
Abstract We demonstrate single-mode horizontal and vertical waveguides in a recently proposed pho... more Abstract We demonstrate single-mode horizontal and vertical waveguides in a recently proposed photonic crystal having layered square symmetry. The vertical waveguide supports two degenerate polarizations, and thus light can be selectively steered into horizontal waveguides oriented at 90 based on its polarization. We calculate the transmission of 90 bends, both from horizontal waveguide to horizontal waveguide and from horizontal waveguide to vertical waveguide. In the case of the horizontal-horizontal bend, ...
Applied Physics Letters, 2003
Abstract We present a layered photonic crystal structure having a connectivity that is different ... more Abstract We present a layered photonic crystal structure having a connectivity that is different from diamond which possesses square symmetry within each layer. This structure has a complete photonic band gap of 18% of the midgap frequency with a dielectric contrast of 12: 1, and is a structure with layered square symmetry having a gap greater than 10%. We demonstrate a waveguide in this crystal created by removing a row of rods from a single layer.© 2003 American Institute of Physics.
Physical Review B, 2007
We introduce a new form of density functional theory for the {\em ab initio} description of elect... more We introduce a new form of density functional theory for the {\em ab initio} description of electronic systems in contact with a molecular liquid environment. This theory rigorously joins an electron density-functional for the electrons of a solute with a classical density-functional theory for the liquid into a single variational principle for the free energy of the combined system. A simple approximate functional predicts, without any fitting of parameters to solvation data, solvation energies as well as state-of-the-art quantum-chemical cavity approaches, which require such fitting.
Physical Review B, 2007
We introduce a form of density-functional theory for the ab initio description of electronic syst... more We introduce a form of density-functional theory for the ab initio description of electronic systems in contact with a molecular liquid environment. This theory rigorously joins an electron density functional for the electrons of a solute with a classical density-functional theory for the liquid into a single variational principle for the free energy of the combined system. We then present an approximate functional in which orientational ordering appears only through a simple model dielectric response. Even with the shortcomings of this preliminary functional, the resulting theory predicts, without any fitting of parameters to solvation data, solvation energies as well as state-of-the-art quantum-chemical cavity approaches, which do require such fitting. Future work should focus on including orientational ordering in the joint density functional.
Nature, 2004
Nanoelectromechanical systems (NEMs) hold promise for a number of scientific and technological ap... more Nanoelectromechanical systems (NEMs) hold promise for a number of scientific and technological applications. In particular, NEMs oscillators have been proposed for use in ultrasensitive mass detection, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems. Perhaps the ultimate material for these applications is a carbon nanotube. They are the stiffest material known, have low density, ultrasmall cross-sections and can be defect-free. Equally important, a nanotube can act as a transistor and thus may be able to sense its own motion. In spite of this great promise, a room-temperature, self-detecting nanotube oscillator has not been realized, although some progress has been made. Here we report the electrical actuation and detection of the guitar-string-like oscillation modes of doubly clamped nanotube oscillators. We show that the resonance frequency can be widely tuned and that the devices can be used to transduce very small forces.
Nature, 2002
Magnesium diboride differs from ordinary metallic superconductors in several important ways, incl... more Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the `superconducting energy gap', should clarify why MgB2 is different. Some early experimental studies have indicated that MgB2 has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB2 and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
Physical Review B, 2002
We present a study of the superconducting transition in MgB2 using the ab initio pseudopotential ... more We present a study of the superconducting transition in MgB2 using the ab initio pseudopotential density-functional method, a fully anisotropic Eliashberg equation, and a conventional estimate for μ*. Our study shows that the anisotropic Eliashberg equation, constructed with ab initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant λ=0.61, a transition temperature Tc=39 K, and a boron isotope-effect exponent αB=0.32. The calculated values for Tc, λ, and αB are in excellent agreement with transport, specific-heat, and isotope-effect measurements, respectively. The individual values of the electron-phonon coupling λ(k-->,k') on the various pieces of the Fermi surface, however, vary from 0.1 to 2.5. The observed Tc is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.
Physical Review B, 2002
We present a study of the superconducting transition in MgB2 using the ab-initio pseudopotential ... more We present a study of the superconducting transition in MgB2 using the ab-initio pseudopotential density functional method and the fully anisotropic Eliashberg equation. Our study shows that the anisotropic Eliashberg equation, constructed with ab-initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant lambda = 0.61, a transition temperature Tc = 39 K, and a boron isotope-effect exponent alphaB = 0.31 with a reasonable assumption of mu* = 0.12. The calculated values for Tc, lambda, and alphaB are in excellent agreement with transport, specific heat, and isotope effect measurements respectively. The individual values of the electron-phonon coupling lambda(k,k') on the various pieces of the Fermi surface however vary from 0.1 to 2.5. The observed Tc is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.
Physical Review B, 2004
The preceding comment by Mazin et al. on our work makes two claims: (1) the calculated variations... more The preceding comment by Mazin et al. on our work makes two claims: (1) the calculated variations of the superconducting energy gap within the σ or the π bands are not observable in real samples due to scatterings, and (2) the Coulomb repulsion μ(k,k') is negligibly small between σ and π states and thus should be approximated by a diagonal 2×2 matrix in the σ and π channels. Here, we point out that the former does not affect the validity of our theoretical work, which is for the clean limit, and that the latter is based on an oversimplified estimation of μ.
Cheminform, 2010
For Abstract see ChemInform Abstract in Full Text.
Physical Review B, 2004
Graphitelike, layered structure of bulk BC3 was successfully synthesized more than 15 yr ago by t... more Graphitelike, layered structure of bulk BC3 was successfully synthesized more than 15 yr ago by the chemical reaction of benzene and boron trichloride at 800 C. 1 Experiments2 and ab initio calculations3, 4 suggested that the synthesized material consists of BC3 layers with hexagonal symmetry see the atomic arrangement of a single BC3 layer in Fig. 1 (a). However, how the BC3 layers are stacked in the bulk material is still unclear. Previous ab initio local-densityapproximation (LDA) calculations3, 4 predicted that bulk BC3 is metallic ...
Nature, 2002
Magnesium diboride 1 differs from ordinary metallic superconductors in several important ways, in... more Magnesium diboride 1 differs from ordinary metallic superconductors in several important ways, including the failure of conventional models 2 to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat . A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB 2 is different. Some early experimental studies have indicated that MgB 2 has multiple gaps 3-9 , but past theoretical studies 10-16 have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB 2 and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
Physical Review B, 2001
We have calculated the ideal shear strength and ideal tensile strength of C, Si, and Ge in the di... more We have calculated the ideal shear strength and ideal tensile strength of C, Si, and Ge in the diamond structure. We find ideal shear strengths of 95 GPa, 6.5 GPa, and 4.5 GPa, and ideal tensile strengths of 95 GPa, 23 GPa, and 14 GPa for C, Si and Ge respectively. The shear calculation is performed on the 111 slip plane sheared in a 112 direction, and the tensile load is applied in the 111 direction. We allowed for a full relaxation of the strains orthogonal to the applied strain as well as the atomic basis vectors.
Solid State Communications, 2003
We have performed an ab initio study of the structure of polonium. By calculating total energies ... more We have performed an ab initio study of the structure of polonium. By calculating total energies in a number of tetragonal lattice configurations, we have shown that the simple cubic structure is preferred by the system. The other two zero-stress structures, bcc and fcc, correspond to inflection points along this path. These calculations agree with experimental evidence that polonium is the only known element to assume the simple cubic structure at room temperature. We have found an LDA lattice constant of 3.28 Å, and we have ...
Solid State Communications, 2004
We have performed an ab initio study of the structure of polonium. By calculating total energies ... more We have performed an ab initio study of the structure of polonium. By calculating total energies in a number of tetragonal lattice configurations, we have shown that the simple cubic structure is preferred by the system. The other two zero-stress structures, bcc and fcc, correspond to inflection points along this path. These calculations agree with experimental evidence that polonium is the only known element to assume the simple cubic structure at room temperature. We have found an LDA lattice constant of 3.28 Å, and we have ...