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Register a new userElectronic states inside the gap of quasi-one-dimensional conductor NbS_3(I)
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Venera Nasretdinova mrs
Publication date
2011
fields
Physics
and research's language is
English
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The photoconductivity spectra of NbS_3 (phase I) crystals are studied. A drop of photoconductivity corresponding to the Peierls gap edge is observed. Reproducible spectral features are found at energies smaller the energy gap value. The first one is a peak at the energy 0.6 eV that is close to the midgap one. It has a threshold-like dependence of the amplitude on the electrical field applied. Another feature is a peak at the energy 0.9 eV near to the edge of the gap. We ascribe the origin of this peak to the stacking faults. The third one are continuous states between these peaks at energies 0.6-0.8 eV. We observed bleaching of the photoconductivity even below zero at this energies in the high electric field (700 V/cm) and under additional illumination applied.
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We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction.
Energy structure of the Peierls gap in orthorhombic TaS$_3$ is examined by spectral study of photoconduction. The gap edge and energy levels inside the Peierls gap are observed. The amplitude of the energy levels is found to depend on both the temperature and the electric field. The electric field of the order of 10 V/cm affects the energy levels and leads to the redistribution of intensity between peaks. The small value of the electric field indicates participation of the collective state in formation of the energy levels inside the Peierls gap.
We have measured the high field magnetoresistence and magnetization of quasi-one- dimensional (Q1D) organic conductor (Per)2Pt(mnt)2 (where Per = perylene and mnt = maleonitriledithiolate), which has a charge density wave (CDW) ground state at zero magnetic field below 8 K. We find that the CDW ground state is suppressed with moderate magnetic fields of order 20 T, as expected from a mean field theory treatment of Pauli effects[W. Dieterich and P. Fulde, Z. Physik 265, 239 - 243 (1973)]. At higher magnetic fields, a new, density wave state with sub-phases is observed in the range 20 to 50 T, which is reminiscent of the cascade of field induced, quantized, spin density wave phases (FISDW) observed in the Bechgaard salts. The new density wave state, which we tenatively identify as a field induced charge density wave state (FICDW), is re-entrant to a low resistance state at even higher fields, of order 50 T and above. Unlike the FISDW ground state, the FICDW state is only weakly orbital, and appears for all directions of magnetic field. Our findings are substantiated by electrical resistivity, magnetization, thermoelectric, and Hall measurements. We discuss our results in light of theoretical work involving magnetic field dependent Q1D CDW ground states in high magnetic fields [D. Zanchi, A. Bjelis, and G. Montambaux, Phys. Rev. B 53, (1996)1240; A. Lebed, JETP Lett. 78,138(2003)].
CDW/Normal metal/CDW junctions and nanoconstrictions in crystals of the quasi-one-dimensional conductor NbSe$_3$ are manufactured using a focused-ion-beam. It is found that the low-temperature conduction of these structures changes dramatically and loses the features of the charge-density-wave transition. Instead, a dielectric phase is developed. Up to 6-order power-law variations of the conduction as a function of both temperature and electric field can be observed for this new phase. The transition from quasi-one-dimensional behavior to one-dimensional behavior is associated with destruction of the three-dimensional order of the charge-density waves by fluctuations. It results in a recovery of the Luttinger-liquid properties of metallic chains, like it takes place in sliding Luttinger liquid phase.
Magnetic properties in the quasi-one-dimensional organic salt (TMTTF)2SbF6 are investigated by 13C NMR under pressures. Antiferromagnetic phase transition at ambient pressure (AFI) is confirmed. Charge-ordering is suppressed by pressure and is not observed under 8 kbar. For 5 < P < 20 kbar, a sharp spectrum and the rapid decrease of the spin-lattice relaxation rate 1/T1 were observed below about 4 K, attributed to a spin-gap transition. Above 20 kbar, extremely broadened spectrum and critical increase of 1/T1 were observed. This indicates that the system enters into another antiferromagnetic phase (AFII) under pressure. The slope of the antiferromagnetic phase transition temperature T_AFII, dT_AFII/dP, is positive, while T_AFI decreases with pressure. The magnetic moment is weakly incommensurate with the lattice at 30 kbar.
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