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Hoffmann, Roald

Publications and source records attributed to Hoffmann, Roald.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Fermi surface studies of the low-temperature structure of sodium

Sodium is the most abundant alkali-metal element and has one of the simplest electronic structures of any metal. At ambient conditions, sodium forms a body-centered-cubic lattice. However, during cooling, it undergoes a partial martensitic phase transition to a complex mixture of rhombohedral polytypes commencing from 36 K. Although the Fermi surface (FS) of bcc sodium has been extensively studied, not much attention has been given to the FS of the martensite structure. Here we report results for the Fermi surface and quantum oscillation (QO) frequencies of several energetically favorable crystal structures of Na at low temperature from first-principles calculations. Interestingly we find that despite drastic differences in the crystal structures of the candidate low-temperature phases of sodium, for all these phases the strongest quantum oscillation peak is centered at 28 kT. Our theoretical results are accompanied by experimental data of QO on a multigrain sodium sample at T = 0.3 K and B max = 18 T exhibiting a sharp peak at 28 kT, independent of the sample orientation. The persistence of this peak even in the presence of the structural transitions has an implication for using the quantum oscillations of polycrystalline sodium for high magnetic field calibration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

High-pressure lithium as an elemental topological semimetal

Topological semimetals generally contain heavy elements. Using density-functional theoretic calculations, we predict that three dense lithium polymorphs in the pressure range 200–360 GPa display nontrivial semimetallic electronic structure. Specifically, these high-pressure phases exhibit Fermi pockets which are degenerate over a loop in k space, around which an encircling k-space path is threaded by ±π Berry phase. Accordingly, these dense lithium phases are topological nodal loop semimetals involving a single light element.

36 MATERIALS SCIENCE↗
DOE OSTI2017

All the Ways To Have Substituted Nanothreads

We describe a general, symmetry-conditioned way of enumerating isomers of saturated singly substituted one-dimensional nanothreads of the (CH) 5 E and (CH) 5 CR type, where E is a heteroatom and R is a substituent. Four nanothreads — so-called tube (3,0), polytwistane, the zipper polymer, and polymer I, are treated in detail. The methodology, combining symmetry arguments and computer-based enumeration, is generally applicable to isomerism problems in polymers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Reply to Martinez-Canales et al.: The structure(s) of lithium at low temperatures

In our recent article, we proposed that Fermiology measurements can be used as a complementary probe for determining the low-temperature state of Li. In a letter criticizing our conclusions, Martinez-Canales, Loa, and Ackland (MLA) claim (i) that “obtaining dHvA signals from martensitic microstructures is usually impossible” and (ii) that the experimental data of Hunt et al., in particular, cannot be used to infer useful information about the martensitic phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Stabilizing a different cyclooctatetraene stereoisomer

Significance Cyclooctatetraene (COT) is a poster child for nonaromatic molecules. An isomer of tub-shaped COT, with one of the ring double bonds changed from the usual cis form to a trans one, lies some 23 kcal/mol higher in energy. Devising a substitution strategy for turning the tables, making the trans isomer the stable point of the system, is the challenge. Simple bonding ideas, of the bulk of substituents, of clamping and turning aromaticity on and off, of pushing electrons in and out of the system, guide us in devising several realistic candidates. Along the way, a set of molecules that might easily split hydrogen reversibly emerges.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Druckeffekte auf organische Reaktionen in Fluiden – eine neue theoretische Perspektive

Abstract Dieser Aufsatz zeigt eine neue Perspektive für die Untersuchung chemischer Reaktionen in komprimierten flüssigen Medien auf. Wir beginnen, indem wir die wesentlichen Einsichten zusammenfassen, die in über 50 Jahren experimenteller Studien an organischen Reaktionen in Lösung unter Druck gewonnen wurden. Diese Arbeiten führten zu einer zuverlässigen Beurteilung der kritischen Rolle der Aktivierungs‐ (Δ ≠ V) und Reaktionsvolumen (ΔV) beim Verständnis von Druckeffekten auf Geschwindigkeiten und Gleichgewichte organischer Reaktionen. Eine kürzlich entwickelte Computermethode, die XP‐PCM‐Methode (XP‐PCM=extreme pressure polarizable continuum model), die quantenchemische Berechnungen von Reaktionswegen von Molekülen unter Druck vornehmen kann, wird eingeführt. Eine Fallstudie der Diels‐Alder‐Cycloaddition von Cyclopentadien mit Ethylen soll – in pädagogischem Detail – der Beschreibung der Methode dienen. Anschließend wenden wir die XP‐PCM‐Methode auf eine Auswahl anderer pericyclischer Reaktionen an, einschließlich der Diels‐Alder‐Cycloaddition von Butadien mit Ethylen, der elektrocyclischen Ringöffnung von Cyclobuten, dem elektrocyclischen Ringschluss von Z‐Hexatrien, der [1,5]‐H‐Verschiebung in Z‐Pentadien und der Cope‐Umlagerung. Diese Reaktionen dienen als Beispiele für einige der häufigsten Kombinationen von Δ ≠ V und ΔV. Interessante Phänomene, wie die Verschiebung eines Übergangszustands entlang der Reaktionskoordinate, ein Wechsel des geschwindigkeitsbestimmenden Schrittes und die mögliche Umwandlung eines Übergangszustands in ein stabiles Minimum, werden durch diese Berechnungen aufgedeckt. Ein Profil des Reaktionsvolumens, entsprechend der Änderung des Volumens des reagierenden Moleküls im Verlauf der Reaktion, erweist sich als nützlich.

Chen, Bo↗
DOE OSTI2017

The Effect of Pressure on Organic Reactions in Fluids-a New Theoretical Perspective

This Review brings a new perspective to the study of chemical reactions in compressed fluid media. We begin by reviewing the substantial insight gained from more than 50 years of experimental studies on organic reactions in solution under pressure. These led to a proper estimation of the critical roles of volume of activation (Δ ≠ V) and reaction volume (ΔV) in understanding pressure effect on rates and equilibria of organic reactions. A recently developed computational method, the XP-PCM (extreme pressure polarizable continuum model) method, capable of carrying out quantum mechanical calculations of reaction pathways of molecules under pressure, is introduced. A case study of the Diels–Alder cycloaddition of cyclopentadiene with ethylene serves, in pedagogical detail, to describe the methodology. We then apply the XP-PCM method to a selection of other pericyclic reactions, including the parent Diels–Alder cycloaddition of butadiene with ethylene, electrocyclic ring-opening of cyclobutene, electrocyclic ring-closing of Z-hexatriene, the [1,5]-H shift in Z-pentadiene, and the Cope rearrangement. These serve as examples of some of the most common combinations of Δ ≠ V and ΔV. Interesting phenomena such as a shift in a transition state along a reaction coordinate, a switch of rate-determining step, and the possible turning of a transition state into a stable minimum are revealed by the calculations. A reaction volume profile, the change in the volume of the reacting molecules as the reaction proceeds, emerges as being useful.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Potential high- T c superconducting lanthanum and yttrium hydrides at high pressure

Significance Theoretical predictions and subsequent experimental observations of high-temperature superconductivity in dense hydrogen-rich compounds have reinvigorated the field of superconductivity. A systematic computational study of the hydrides of lanthanum and yttrium over a wide composition range reveals hydrogen-rich structures with intriguing electronic properties under pressure. Electron–phonon coupling calculations predict the existence of new superconducting phases, some exhibiting superconductivity in the range of room temperature. Moreover, the calculated stabilities indicate the materials could be synthesized at pressures that are currently accessible in the laboratory. The results open the prospect for the design, synthesis, and recovery of new high-temperature superconductors with potential practical applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗