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Register a new userPhotoconduction in the Peierls conductor monoclinic TaS$_3$
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Added by
S. V. Zaitsev-Zotov
Publication date
2014
fields
Physics
and research's language is
English
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Photoconduction in the monoclinic phase of quasi-one-dimensional conductor TaS$_3$ has been observed at $T < 70$~K. It was studied jointly with low-temperature ohmic and non-linear dark conduction. The strong sample quality dependence of both photoconduction and dark conduction at this temperature region has been observed. Together with a similarity of the main features of the photoconduction characteristic of both monoclinic ({it m-}TaS$_3$) and orthorhombic ({it o-}TaS$_3$) samples the following new peculiarities of photoconduction in {it m-}TaS$_3$ were found: 1) the dependence of the activation energy of photoconduction on temperature, $T$, 2) the change of the recombination mechanism from the linear type to the collisional one at low $T$ with a sample quality growth, 3) the existence of a fine structure of the electric-field dependence of photoconduction. Spectral study gives the Peierls energy gap value $2Delta ^*= 0.18$~eV.
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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.
1T-TaS$_2$ undergoes successive phase transitions upon cooling and eventually enters an insulating state of mysterious origin. Some consider this state to be a band insulator with interlayer stacking order, yet others attribute it to Mott physics that support a quantum spin liquid state.Here, we determine the electronic and structural properties of 1T-TaS$_2$ using angle-resolved photoemission spectroscopy and X-Ray diffraction. At low temperatures, the 2$pi$/2c-periodic band dispersion, along with half-integer-indexed diffraction peaks along the c axis, unambiguously indicates that the ground state of 1T-TaS$_2$ is a band insulator with interlayer dimerization. Upon heating, however, the system undergoes a transition into a Mott insulating state, which only exists in a narrow temperature window. Our results refute the idea of searching for quantum magnetism in 1T-TaS$_2$ only at low temperatures, and highlight the competition between on-site Coulomb repulsion and interlayer hopping as a crucial aspect for understanding the materials electronic properties.
NbSe$_3$ and monoclinic-TaS$_3$ ($m$-TaS$_3$) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave (CDW) Peierls transitions at T$_{P_1}$ and T$_{P_2}$. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their electron-hole Lindhard response function. As a result of stronger inter-chain interactions the Fermi surface (FS) and Lindhard function of NbSe$_3$ are considerably more complex than those for $m$-TaS$_3$; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the Lindhard response for NbSe$_3$ are found with the (0$a$*, 0$c$*) and (1/2$a$*, 1/2$c$*) transverse components at T$_{P_1}$ and T$_{P_2}$, respectively, whereas the second maximum is not observed for $m$-TaS$_3$ at T$_{P2}$. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transverse $a$* components in NbSe$_3$. For the transverse coupling along $c$* in NbSe$_3$ and along both $a$* and $c$* for $m$-TaS$_3$, one must take into account the strongest inter-chain Coulomb coupling mechanism. Phonon spectrum calculations show the formation of a giant 2$k_F$ Kohn anomaly in $m$-TaS$_3$. All these results support the weak coupling scenario for the Peierls transition of transition metal trichalcogenides.
We investigate the low-temperature charge-density-wave (CDW) state of bulk TaS$_2$ with a fully self-consistent DFT+U approach, over which the controversy has remained unresolved regarding the out-of-plane metallic band. By examining the innate structure of the Hubbard U potential, we reveal that the conventional use of atomic-orbital basis could seriously misevaluate the electron correlation in the CDW state. By adopting a generalized basis, covering the whole David star, we successfully reproduce the Mott insulating nature with the layer-by-layer antiferromagnetic order. Similar consideration should be applied for description of the electron correlation in molecular solid.
Impurities and defects are known to affect the properties of the charge density wave (CDW) state but the influence of impurities on the density of states inside the Peierls gap remains largely unexplored. Here we present an experimental study of the effect of indium impurities on photoconduction spectra of CDW compound orthorhombic TaS$_3$. We use the temperature diffusion method to introduce indium into a sample from preliminary attached In contacts. The concentration of In after 23 hours of diffusion is found to be nonuniform and strongly dependent on the distance to the contacts. The diffusion affects the spectral range 0.15-0.25 eV, increasing the photoconduction amplitude linearly with diffusion time. The optical gap value obtained from the measurements is $2Delta = 0.25$ eV and the tail of states below $2Delta$ is associated with the impurities in agreement with the T{u}tt{o}-Zawadowski theory. Diffusion-induced modification of current-voltage characteristics and decrease of the Peierls temperature are also observed. Neither changes in photoconduction spectra nor in the Peierls transition temperature of the control sample with Au contacts are found.
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