Coulomb scattering, transition temperature

One of the initial motivations for pressure studies was to suppress the CDW transitions in TTF-TCNQ and its derivatives and thereby stabilize a metallic, and possibly superconducting, state at low temperatures [2]. Experiments on TTF-TCNQ and TSeF-TCNQ [27] showed an increase in the CDW or Peierls transition temperatures (Tp) with pressure, as shown in Fig. 12 [80], Later work on materials such as HMTTF-TCNQ showed that the transitions could be suppressed by pressure, but a true metallic state was not obtained up to about 30 kbar [81]. Instead, the ground state was very reminiscent of the semimetallic behavior observed for HMTSF-TCNQ, as shown by the resistivity data in Fig. 13. One possible mechanism for the formation of a semimetallic state is that, as proposed by Weger [82], it arises simply from hybridization of donor and acceptor wave functions. However, diffuse x-ray scattering lines [34] and reasonably sharp conductivity anomalies are often observed, so in many cases incommensurate lattice distortions must play a role. In other words, a semimetallic state can also arise when the Q vector of the CDW does not destroy the whole Fermi surface (FS) but leaves small pockets of holes and electrons. Such a situation is particularly likely in two-chain materials, where the direction of Q is determined not just by the FS nesting properties but by the Coulomb interaction between CDWs on the two chains [10]. 

The polymorphism of hydrogen sulfides of alkali metals M+(HS ) with M — Na, K, Rb, Cs was studied with neutron and X-ray diffraction, quasielastic neutron scattering, DSC and proton NMR. These compounds have three polymorphs and two known phase transitions. When M — Na, K, Rb, the LTP is monoclinic, the medium-temperature phase (MTP) is rho-mbohedral and the HTP is cubic with a NaCl-type structure. Owing to the fact that the sequence of phase transitions is the same for the three cations, this series is expected to be a model where coulombic interaction is modulated by the cation radii, which increases in the order Na, K, Rb, Cs. 

Neutron diffraction and inelastic scattering spectra have been studied on nearly stoichiometric TmSe (a = 5.709 A) as a function of pressure at room temperature as well as neutron diffraction at low temperatures. The results are interpreted by the assumption of a gap or quasi-gap at Ep at low pressures, possibly even in the paramagnetic state. The spin fluctuation energy r (quasielastic linewidth, see p. 347) in the initial state, which depends on the hybridization potential Vf and the Coulomb repulsion Uf, does not change. A valence transition appears (increase of T) at 25 kbar where Ep is pushed out of the gap. Simulta-... 

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