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Register a new userNbS$_{3}$: A unique quasi one-dimensional conductor with three charge density wave transitions
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Added by
Mahmoud Abdel-Hafiez
Publication date
2016
fields
Physics
and research's language is
English
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Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS$_{3}$ (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at $T_{P1}$ = 360,K and $T_{P2}$ = 150,K, another CDW transition occurs at a much higher temperature $T_{P0}$ = 620-650,K; evidence for the non-linear conductivity of this CDW is presented. 2) We show that CDW associated with the $T_{P2}$ - transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. 3) We show exceptional coherence of the $T_{P1}$-CDW at room-temperature. Additionally, we report on the effects of uniaxial strain on the CDW transition temperatures and transport.
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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)].
The anisotropic and non-linear transport properties of the quasi one-dimensional organic conductor (TMTSF)_2PF_6 have been studied by dc, radiofrequency, and microwave methods. Microwave experiments along all three axes reveal that collective transport, which is considered to be the fingerprint of the spin-density-wave condensate, also occurs in the perpendicular b direction. The pinned mode resonance is present in the $a$ and b-axes response, but not along the least conducting c* direction. The ac-field threshold, above which the spin-density-wave response is non-linear, strongly decreases as the temperature drops below 4 K. With increasing strength of the microwave electric field and of the radiofrequency signal, the pinned mode and the screened phason loss-peak shift to lower frequencies. In the non-linear regime, in addition to the phason relaxation mode with Arrhenius-like resistive decay, an additional mode with very long and temperature-independent relaxation time appears below 4 K. We attribute the new process to short-wavelength spin-density-wave excitations associated with discommensurations in a random commensurate N=4 domain structure.
Low-energy coherent charge-density wave excitations are investigated in blue bronze (K$_{0.3}$MoO$_{3}$) and red bronze (K$_{0.33}$MoO$_{3}$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K$_{0.33}$MoO$_{3}$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.Low-energy coherent charge-density wave excitations are investigated in blue bronze (K$_{0.3}$MoO$_{3}$) and red bronze (K$_{0.33}$MoO$_{3}$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K$_{0.33}$MoO$_{3}$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.
The quasi-one-dimensional organic conductors (TMTTF)$_2X$ with non-centrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X$ = BF$_4^-$, ReO$_4^-$, and ClO$_4^-$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (TMTTF)$_2$NO$_3$. Here we explore the electronic and structural transitions by transport experiments, optical and magnetic spectroscopy. This way we analyze the temperature dependence of the charge imbalance 2$delta$ and an activated behavior of $rho(T)$ with $Delta_{rm CO}approx 530$~K below $T_{rm CO} = 250$~K. Since (TMTTF)$_2$NO$_3$ follows the universal relation between charge imbalance 2$delta$ and size of the gap $Delta_{rm CO}$, our findings suggest that charge order is determined by TMTTF stacks with little influence of the anions. Clear signatures of anion ordering are detected at $T_{rm AO}=50$~K. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.
The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction band width.
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