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Register a new userEffect of disorder in the charge-density-wave compounds LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16) as revealed by optical spectroscopy
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We present optical spectroscopy measurements on rare-earth ditelluride single crystals of LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16). The measurements reveal formation of charge density wave energy gaps at rather high energy levels, e.g. 2$Deltasim$ 8500 cm for LaTe$_{1.95}$, and 6800 cm for CeTe$_{1.95}$. More strikingly, the study reveals that, different from the rare-earth tri-tellurides, the Te vacancies and disorder effect play a key role in the low-energy charge excitations of ditelluride systems. Although an eminent peak is observed between 800 and 1500 cm in conductivity spectra for LaTe$_{1.95}$, and CeTe$_{1.95-x}$Se$_x$ (x=0. 0.16), our analysis indicates that it could not be attributed to the formation of a small energy gap, instead it could be well accounted for by the localization modified Drude model. Our study also indicates that the low-tempreature optical spectroscopic features are distinctly different from a semiconducting CDW state with entirely gapped Fermi surfaces.
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We performed optical spectroscopy measurement on single crystal of CeTe$_3$, a rare-earth element tri-telluride charge density wave (CDW) compound. The optical spectra are found to display very strong temperature dependence. Besides a large and pronounced CDW energy gap being present already at room temperature as observed in earlier studies, the present measurement revealed the formation of another energy gap at smaller energy scale at low temperature. The second CDW gap removes the electrons near E$_F$ which undergo stronger scattering. The study yields evidence for the presence of multiple CDW orders or strong fluctuations in the light rare-earth element tri-telluride.
Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0leq}xleq0.1$) increases up to 4 K plateau for $0.04$$leq$$x$$leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy.
We report a photoemission study at room temperature on BaFe2X3 (X = S and Se) and CsFe2Se3 in which two-leg ladders are formed by the Fe sites. The Fe 2p core-level peaks of BaFe2X3 are broad and exhibit two components, indicating that itinerant and localized Fe 3d sites coexist similar to KxFe2-ySe2. The Fe 2p core-level peak of CsFe2Se3 is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of CsFe2Se3 can be viewed as a Fe2+ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stability of chalcogen p holes in the two-leg ladder structure.
We report on a combined measurement of high-resolution x-ray diffraction on powder and Raman scattering on single crystalline NiS2-xSex samples that exhibit the insulator-metal transition with Se doping. Via x-rays, an abrupt change in the bond length between Ni and S (Se) ions was observed at the transition temperature, in sharp contrast to the almost constant bond length between chalcogen ions. Raman scattering, a complementary technique with the unique sensitivity to the vibrations of chalcogen bonds, revealed no anomalies in the phonon spectrum, consistent with the x-ray diffraction results. This indicates the important role of the interaction between Ni and S (Se) in the insulator-metal transition. The potential implication of this interpretation is discussed in terms of current theoretical models.
The La and Ce di-tellurides LaTe$_2$ and CeTe$_2$ are deep in the charge-density-wave (CDW) ground state even at 300 K. We have collected their electrodynamic response over a broad spectral range from the far infrared up to the ultraviolet. We establish the energy scale of the single particle excitation across the CDW gap. Moreover, we find that the CDW collective state gaps a very large portion of the Fermi surface. Similarly to the related rare earth tri-tellurides, we envisage that interactions and Umklapp processes play a role in the onset of the CDW broken symmetry ground state.
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