Fundamentals of H2 Binding and Reactivity on Transition Metals Underlying Hydrogenase Function and H2 Production and Storage

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.

https://arxiv.org/pdf/cond-mat/0406125.pdf

Spin entropy as the likely source of enhanced thermopower in Na x Co 2 O 4

The magnetic energy levels of a given magnetic solid are closely packed in energy because the interactions between magnetic ions are weak. Thus, in describing its magnetic properties, one needs to generate its magnetic energy spectrum by employing an appropriate spin Hamiltonian. In this review article we discuss how to determine and specify a necessary spin Hamiltonian in terms of first principles electronic structure calculations on the basis of energy-mapping analysis and briefly survey important concepts and phenomena that one encounters in reading the current literature on magnetic solids. Our discussion is given on a qualitative level from the perspective of magnetic energy levels and electronic structures. The spin Hamiltonian appropriate for a magnetic system should be based on its spin lattice, i.e., the repeat pattern of its strong magnetic bonds (strong spin exchange paths), which requires one to evaluate its Heisenberg spin exchanges on the basis of energy-mapping analysis. Other weaker energy terms such as Dzyaloshinskii–Moriya (DM) spin exchange and magnetocrystalline anisotropy energies, which a spin Hamiltonian must include in certain cases, can also be evaluated by performing energy-mapping analysis. We show that the spin orientation of a transition-metal magnetic ion can be easily explained by considering its split d-block levels as unperturbed states with the spin–orbit coupling (SOC) as perturbation, that the DM exchange between adjacent spin sites can become comparable in strength to the Heisenberg spin exchange when the two spin sites are not chemically equivalent, and that the DM interaction between rare-earth and transition-metal cations is governed largely by the magnetic orbitals of the rare-earth cation.

Magnetic properties and energy-mapping analysis

Takada et al. have reported superconductivity in layered Na(x)CoO(2)yH(2)O (T(c) approximately equal to 5 K). We model a reference neutral CoO2 layer as an orbitally nondegenerate spin-1/2 antiferromagnetic Mott insulator on a triangular lattice and Na(x)CoO(2)yH(2)O as electron doped Mott insulators described by a t-J model. It is suggested that at optimal doping chiral spin fluctuations enhanced by the dopant dynamics lead to a gapful d-wave superconducting state. A chiral resonating valence bond (RVB) metal, a parity and time (PT) reversal violating state with condensed RVB gauge fields, with a possible weak ferromagnetism, and low temperature p-wave superconductivity are also suggested at higher dopings.

https://arxiv.org/pdf/cond-mat/0303649.pdf

Electronic model for CoO2 layer based systems: chiral resonating valence bond metal and superconductivity.

The pseudogap regime of high-temperature cuprates harbours diverse manifestations of electronic ordering whose exact nature and universality remain debated. Here, we show that the short-ranged charge order recently reported in the normal state of YBa2Cu3Oy corresponds to a truly static modulation of the charge density. We also show that this modulation impacts on most electronic properties, that it appears jointly with intra-unit-cell nematic, but not magnetic, order, and that it exhibits differences with the charge density wave observed at lower temperatures in high magnetic fields. These observations prove mostly universal, they place new constraints on the origin of the charge density wave and they reveal that the charge modulation is pinned by native defects. Similarities with results in layered metals such as NbSe2, in which defects nucleate halos of incipient charge density wave at temperatures above the ordering transition, raise the possibility that order-parameter fluctuations, but no static order, would be observed in the normal state of most cuprates if disorder were absent.

/pdf/incipient-charge-order-observed-by-nmr-in-the-normal-state-12k849lm1u.pdf

Incipient charge order observed by NMR in the normal state of YBa2Cu3Oy

${}^{59}\mathrm{Co}$ NMR studies have been performed in metallic ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ in the temperature range 350--100 K. The powder patterns predominantly correspond to typical second-order quadrupolar splitted central transition of two inequivalent cobalt sites. Features of magnetic interaction are also present. The analysis of the NMR line shape shows that the electric field gradient (EFG) experienced by one of the two sites (site 1) is independent of temperature, whereas around 47% increment in EFG has been observed for site 2 as the temperature is lowered from 350 to 100 K. ${}^{59}\mathrm{Co}$ NMR shift for each site is almost isotropic in character which varies linearly with susceptibility and basically consists of two parts. Both the sites suffer a large temperature independent shift due to the contribution from orbital paramagnetism $({\ensuremath{\chi}}_{\mathrm{vv}})$ which is analogous in metals to the Van Vleck paramagnetic susceptibility. The temperature-dependent Knight shift for site 1 reveals a negative hyperfine coupling constant ${(A}_{\mathrm{hf}}^{d})$ arising from inner s core polarization by the d electrons which are itinerant in character. A positive ${A}_{\mathrm{hf}}^{d}$ observed for site 2 cannot be interpreted directly in a metallic system such as ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}.$ A comparison of the magnitude of ${A}_{\mathrm{hf}}^{d}$ with those of other cobalt oxides suggests that the nucleus which is being probed by NMR, belongs to a diamagnetic cobalt state. Thus the present NMR results together with the susceptibility indicate the presence of ${\mathrm{Co}}^{3+}$ ion with S = 0 and the magnetic ${\mathrm{Co}}^{4+}$ ion.

59 Co NMR studies of metallic NaCo 2 O 4

Phase separation inNd1−xSrxCoO3using59CoNMR

Microstructural and nuclear magnetic resonance studies of solid-state amorphization in Al–Ti–Si composites prepared by mechanical alloying

$^{1}\mathit{H}$ nuclear-magnetic-resonance (NMR) studies have been performed in ${\mathrm{CeNiInH}}_{\mathit{x}}$ (x=1.0 and 1.6) in the temperature range 140--300 K. Near 300 K, the $^{1}\mathrm{H}$ NMR spectra show a single narrow line. Below about 210 K, the spectra look very much like a Pake doublet on which a central narrow line is superimposed. The experimental $^{1}\mathrm{H}$ NMR spectrum of ${\mathrm{CeNiInH}}_{1.0}$ at 140 K has been fitted to a theoretical line shape consisting of a doublet and a central Lorentzian line. This fitting yields a proton-proton separation of about 1.48 \AA{}\ifmmode\pm\else\textpm\fi{}0.02. The ordered arrangement of the paired protons in CeNiIn has been discussed in terms of the proposed hydrogen sites in isostructural systems.

Observation of a Pake doublet in the 1H nuclear-magnetic-resonance spectrum of CeNiInHx.

NMR studies of $^{51}\mathrm{V}$ in $\mathrm{Sr}{\mathrm{Ni}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, a $S=1$ quasi-one-dimensional antiferromagnet, reveal no magnetic long-range order down to $3.75\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in contradiction to the prediction from the inelastic neutron scattering data. The temperature dependence of the NMR frequency shift leads to spin gap, $\ensuremath{\Delta}\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in the low energy excitations. The spin-lattice relaxation rates suggest a temperature dependence of a spin gap with $\ensuremath{\Delta}\ensuremath{\sim}38\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $80KlTl15K$ and are dominated by the spin diffusion term below $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Thus the ground state of $\mathrm{Sr}{\mathrm{Ni}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ is more correctly described as a disordered ``spin liquid'' Haldane state as in isomorphous $\mathrm{Pb}{\mathrm{Ni}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$.

K. Ghoshray

Papers

59 Co NMR studies of metallic NaCo 2 O 4

Phase separation inNd1−xSrxCoO3using59CoNMR

Microstructural and nuclear magnetic resonance studies of solid-state amorphization in Al–Ti–Si composites prepared by mechanical alloying

Observation of a Pake doublet in the 1H nuclear-magnetic-resonance spectrum of CeNiInHx.

NMR study of V 51 in quasi-one-dimensional integer spin chain compound Sr Ni 2 V 2 O 8