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Journal ArticleDOI

Determination of the π-charge distribution of the DMe-DCNQI molecule in (DMe-DCNQI)2M, M=Li, Ag, and Cu

12 Apr 2006-Journal of Low Temperature Physics (Kluwer Academic Publishers-Plenum Publishers)-Vol. 142, Iss: 3, pp 633-637

AbstractSolid state high-resolution NMR of 1H and 13C along with 15N is analyzed to investigate the electronic states of the charge transfer salts (DMe-DCNQI)2M, (M=Li, Ag, and Cu). We determined the spin/charge distribution in a DMe-DCNQI molecule of the Li-salt from the Knight shifts at each atom on the molecule. It is found that the obtained charge distribution is similar to the theoretical prediction. The charge density on the DCNQI molecules of the Ag-salt is found to be smaller by 20% than the Li-salt, which could be an origin of differences from the Li-salt. This result is consistent with the first principle calculations (Miyazaki and Terakura, Phys. Rev. B 54, 10452, 1996).

Topics: Charge density (56%), Knight shift (51%)

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Journal ArticleDOI
Abstract: We review recent progress in understanding the different spatial broken symmetries that occur in the normal states of the family of charge-transfer solids (CTS) that exhibit superconductivity (SC), and discuss how this knowledge gives insight to the mechanism of the unconventional SC in these systems. A great variety of spatial broken symmetries occur in the semiconducting states proximate to SC in the CTS, including charge ordering, antiferromagnetism and spin-density wave, spin-Peierls state and the quantum spin liquid. We show that a unified theory of the diverse broken symmetry states necessarily requires explicit incorporation of strong electron–electron interactions and lattice discreteness, and most importantly, the correct bandfilling of one-quarter, as opposed to the effective half-filled band picture that is often employed. Uniquely in the quarter-filled band, there is a very strong tendency to form nearest neighbor spin–singlets, in both one- and two-dimension. The spin–singlet in the quarter-filled band is necessarily charge-disproportionated, with charge-rich pairs of nearest neighbor sites separated by charge-poor pairs of sites in the insulating state. Thus the tendency to spin–singlets, a quantum effect, drives a commensurate charge-order in the correlated quarter-filled band. This charge-ordered spin–singlet, which we label as a paired-electron crystal (PEC), is different from and competes with both the antiferromagnetic (AFM) state and the Wigner crystal (WC) of single electrons. Further, unlike these classical broken symmetries, the PEC is characterized by a spin gap. The tendency to the PEC in two dimension is enhanced by lattice frustration. The concept of the PEC mirrors parallel development of the idea of a density wave of Cooper pairs in the superconducting high T c cuprates, where also the existence of a charge-ordered state in between the antiferromagnetic and the superconducting phase has now been confirmed. Following this characterization of the spatial broken symmetries, we critically reexamine spin-fluctuation and resonating valence bond theories of frustration-driven SC within half-filled band Hubbard and Hubbard–Heisenberg Hamiltonians for the superconducting CTS. We present numerical evidence for the absence of SC within the half-filled band correlated-electron Hamiltonians for any degree of frustration. We then develop a valence-bond theory of SC within which the superconducting state is reached by the destabilization of the PEC by additional pressure-induced lattice frustration that makes the spin–singlets mobile. We present limited but accurate numerical evidence for the existence of such a charge order–SC duality. Our proposed mechanism for SC is the same for CTS in which the proximate semiconducting state is antiferromagnetic instead of charge-ordered, with the only difference that SC in the former is generated via a fluctuating spin–singlet state as opposed to static PEC. In Appendix B we point out that several classes of unconventional superconductors share the same band-filling of one-quarter with the superconducting CTS. In many of these materials there are also indications of similar intertwined charge order and SC. We discuss the transferability of our valence-bond theory of SC to these systems.

26 citations


Journal ArticleDOI
Abstract: We review recent progress in understanding the different spatial broken symmetries that occur in the normal states of the family of charge-transfer solids (CTS) that exhibit superconductivity (SC), and discuss how this knowledge gives insight to the mechanism of the unconventional SC in these systems We show that a unified theory of the diverse broken symmetry states necessarily requires explicit incorporation of strong electron-electron interactions and lattice discreteness, and most importantly, the correct bandfilling of one-quarter Uniquely in the quarter-filled band, there is a very strong tendency to form nearest neighbor spin-singlets, in both one and two dimensions The tendency to spin-singlets, a quantum effect, drives a commensurate charge-order in the correlated quarter-filled band This charge-ordered spin-singlet, which we label as a paired-electron crystal (PEC), is different from and competes with both the antiferromagnetic state and the Wigner crystal of single electrons Further, unlike these classical broken symmetries, the PEC is characterized by a spin gap The tendency to the PEC in two dimensions is enhanced by lattice frustration Following this characterization of the spatial broken symmetries, we critically reexamine spin-fluctuation and resonating valence bond theories of frustration-driven SC within half-filled band Hubbard and Hubbard-Heisenberg Hamiltonians for the superconducting CTS We develop a valence-bond theory of SC within which the superconducting state is reached by the destabilization of the PEC by additional pressure-induced lattice frustration that makes the spin-singlets mobile Our proposed mechanism for SC is the same for CTS in which the proximate semiconducting state is antiferromagnetic instead of charge-ordered, with the only difference that SC in the former is generated via a fluctuating spin-singlet state as opposed to static PEC

18 citations


References
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Journal ArticleDOI
Abstract: The paramagnetic susceptibility ${{\ensuremath{\chi}}_{p}}^{e}$ of conduction electron spins is isolated experimentally from the total magnetic susceptibility in metallic lithium and sodium by studying the intensity of the conduction-electron spin resonances. The absolute intensity of absorption is calibrated by comparison with the nuclear resonance of the metal nuclei in the same sample and at the same frequency, the two resonances being observed merely by changing the static magnetic field. In this manner ${{\ensuremath{\chi}}_{p}}^{e}$ is measured in terms of the nuclear static susceptibility, ${{\ensuremath{\chi}}_{p}}^{n}$, which in turn can be calculated accurately from the Langevin-Debye formula. A narrow band modulation technique gives improved signal to noise over our earlier work. The values of ${{\ensuremath{\chi}}_{p}}^{e}$ are (2.08\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ cgs volume units for lithium at 300\ifmmode^\circ\else\textdegree\fi{}K and (0.95\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ cgs volume units for sodium at 79\ifmmode^\circ\else\textdegree\fi{}K, in rather good agreement with the theory of Pines and Bohm, but in substantial disagreement with the simple Pauli model, or the results of Sampson and Seitz. Experimental precision does not permit conclusions to be drawn about the diamagnetism of conduction electrons.

115 citations


Journal ArticleDOI
Abstract: Structural and physical properties of the Cu salts of a series of π-acceptors N,N′-dicyanobenzoquinonediimines (DCNQIs) are described. The most notable feature of this system is that 3d electrons in Cu interact with pπ electrons in DCNQI near the Fermi level. This unique feature has provided a lot of interesting solid state properties: the Mott transition triggered by the Peierls transition, the pressure-induced metal-insulator transition, the metal-insulator-metal (reentrant) transition, the three-dimensional Fermi surface, the anomalous isotope effects, the antiferromagnetic transition, the weak ferromagnetism, and the electron mass enhancement. The aim of this account is to give an overview of this unique pπ-d system.

100 citations



Journal ArticleDOI
Abstract: We have studied the structural instabilities of 2,5(MR-DCNQI) 2 Ag (R=CH 3 , Cl or Br, M=CH 3 ) using low-temperature X-ray diffuse scattering and diffraction techniques. In (DM-DCNQI) 2 Ag (DM=dimethyl) we observe quasi-1D 4k F diffuse scattering at room temperature which transforms into satellite reflections at Q 4kF =(0,0, 1/2) below T 4kF =(0,0, 1/2) below T 4kF ≃100 K. A second transition with limited quasi-1D precursor scattering occurs at T 2kF ≃100 K. A second transition with limited quasi-1D precursor scattering occurs at T 2kF ≃83 K with satellite reflections at Q 2kF ≃83 K with satellite reflections at Q 2kF =(0,0, 1/4). In relation with the transport and magnetic properties it is suggested that the upper transition localizes the charge carriers while the lower one leads to a spin-Peierls ground state. In (MCl-DCNQI) 2 =(0,0, 1/4). In relation with the transport and magnetic properties it is suggested that the upper transition localizes the charge carriers while the lower one leads to a spin-Peierls ground state. In (MCl-DCNQI) 2 Ag and (MBr-DCNQI) 2 Ag the quasi-1D 4K F and 2K f diffuse scatterings do not condense and there is not transition, probably because of the inherent disorder caused by the MCl and MBr substituents. Supplementary diffuse scattering features and irradiation effects are also reported Etude des instabilites structurales de ces composes par diffraction de rayons X et diffusion en diffraction. Pour M,R=CH 3 , observation d'une diffusion diffuse quasi-1D a 4 k F des la temperature ambiante, avec des satellites a Q 4 kF =(0,0,1/2) au-dessous de T 2kF =(0,0,1/2) au-dessous de T 2kF =100 K, et d'une 2eme transition, avec de faibles effets precurseurs quasi-1D a T 2kF =100 K, et d'une 2eme transition, avec de faibles effets precurseurs quasi-1D a T 2kF =83 K avec des satellites Q 2kF =83 K avec des satellites Q 2kF =(0,0,1/4). En relation avec les proprietes electriques et magnetiques, attribution de la 1ere transition a la localisation des porteurs et la 2eme a un etat fondamental de type spin-Peierls. Pour les composes avec M=CH 3 =(0,0,1/4). En relation avec les proprietes electriques et magnetiques, attribution de la 1ere transition a la localisation des porteurs et la 2eme a un etat fondamental de type spin-Peierls. Pour les composes avec M=CH 3 , R=Cl ou Br, absence de condensation a 4k F et 2k F et de transitions, probablement du fait du desordre inherent aux substituants Cl et Br

39 citations