There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

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QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

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Phonons and related crystal properties from density-functional perturbation theory

Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

A series of calculations on ${\mathrm{MgB}}_{2}$ and related isoelectronic systems indicates that the layer of ${\mathrm{Mg}}^{2+}$ ions lowers the nonbonding B $\ensuremath{\pi}$ ( ${p}_{z}$) bands relative to the bonding $\ensuremath{\sigma}$ ( ${\mathrm{sp}}_{x}{p}_{y}$) bands compared to graphite, causing $\ensuremath{\sigma}\ensuremath{\rightarrow}\ensuremath{\pi}$ charge transfer and $\ensuremath{\sigma}$ band doping of 0.13 holes/cell. Because of their two dimensionality the $\ensuremath{\sigma}$ bands contribute strongly to the Fermi level density of states. Calculated deformation potentials of $\ensuremath{\Gamma}$ point phonons identify the B bond stretching modes as dominating the electron-phonon coupling. Superconductivity driven by $\ensuremath{\sigma}$ band holes is consistent with the report of destruction of superconductivity by doping with Al.

Superconductivity of MgB2: covalent bonds driven metallic.

Pseudopotential methods in condensed matter applications

Possible origins of ``colossal magnetoresistance'' (CMR) behavior in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ system are studied using the local spin-density method. These calculations allow the quantification of the effects of Mn d--O p hybridization that have been largely neglected in previously published work. As regards the end-point compounds ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$, the very different structural and magnetic symmetries of their ground states are predicted correctly. The distortion from the cubic perovskite structure of the ${\mathrm{LaMnO}}_{3}$ lattice is necessary to produce an antiferromagnetic insulating ground state. The distortion also strengthens the Mn magnetic moments. Application to ferromagnetic and constrained ferrimagnetic phases of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ in the CMR regime x\ensuremath{\approxeq}1/4--1/3 suggests, as observed, that magnetic coupling switches from antiferromagnetic to ferromagnetic. Hybridization between Mn d states and O p states is found to be strongly spin dependent, because the majority Mn d bands overlap the O p bands while the minority Mn d bands are separated by a gap from the O p bands. Both ferromagnetic and ferrimagnetic orderings are obtained and compared. We identify strong local environment effects arising from neighboring cation charge differences (${\mathrm{La}}^{3+}$ or ${\mathrm{Ca}}^{2+}$) that suggest localization of the low density of minority carriers, leading to effective half-metallic ferromagnetism in the CMR regime. This behavior supports in some respects the popular ``double exchange'' picture of Zener but indicates the Mn d--O p hybridization is much too strong to be considered perturbatively. Half-metallic character promotes the possibility of very large magnetoresistance, and may well be an essential ingredient in the CMR effect.

Electronic structure and half-metallic transport in the La1-xCaxMnO3 system

Not to be confused with semiconductors, half metals belong to a new class of materials that look set to play a key role in next-generation electronic devices.

https://physicstoday.scitation.org/doi/pdf/10.1063/1.1381101

Warren E. Pickett

Papers

Electronic structure of the high-temperature oxide superconductors

Superconductivity of MgB2: covalent bonds driven metallic.

Pseudopotential methods in condensed matter applications

Electronic structure and half-metallic transport in the La1-xCaxMnO3 system

Half Metallic Magnets