Quenching of phase coherence in quasi-one dimensional ring crystals

The comparison of the single-particle (SP) dynamics between the whisker and ring NbSe$_3$ crystals provides new insight into the phase transition properties in quasi-one-dimensional charge density wave (CDW) systems.Comment: 9 pages, 4 figure

Chiral charge-density-waves

We discovered the chirality of charge density waves (CDW) in 1T-TiSe$_2$ by using scanning tunnelling microscopy (STM) and optical ellipsometry. We found that the CDW intensity becomes $I{a_1}:I{a_2}:I{a_3} = 1:0.7 \pm 0.1:0.5 \pm 0.1$, where $Ia_i$ (i =1, 2, 3) is the amplitude of the tunnelling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease \textit{clockwise} and \textit{anticlockwise} when we index each nesting vector in order of intensity in the Fourier transformation of the STM images. The chirality in CDW results in the three-fold symmetry breaking. Macroscopically, two-fold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H_{CDW} \equiv {\boldmath q_1} \cdot ({\boldmath q_2}\times {\boldmath q_3}), where {\boldmath q_i} are the nesting vectors, which is independent of the symmetry of components. The nonzero $H_{CDW}$ - the triple-{\boldmath q} vectors do not exist in an identical plane in the reciprocal space - should induce a real-space chirality in CDW system.Comment: 12 pages, 4 figure

Geometrically Frustrated Crystals: Elastic Theory and Dislocations

Elastic theory of ring-(or cylinder-)shaped crystals is constructed and the generation of edge dislocations due to geometrical frustration caused by the bending is studied. The analogy to superconducting (or superfluid) vortex state is pointed out and the phase diagram of the ring-crystal, which depends on radius and thickness, is discussed.Comment: 4 pages, 3 figure

Selective optical probing of the charge-density-wave phases in NbSe3

We report a study of the ultrafast single particle dynamics in quasi-one-dimensional NbSe3 whisker crystals using a two-color pump-probe measurement. A selective enhancement of the relaxation dynamics connected with one of the two charge-density-wave (CDW) phases was realized by tuning the probe energy to around 1.5 eV, in which the temperature dependence across the corresponding phase transition shows a pronounced anomaly. A similar selective excitation was possible in the coherent CDW oscillation

Ultrafast optical study of the CDW phase transition in topological crystals

Influence of the crystal topology on the Peierls transition was investigated by means of ultrafast optical pump-probe measurements. The comparison of the exponential single particle decay between the whisker and ring NbSe$_3$ compounds reveals several pronounced differences around a phase transition temperature (${\rm T}_P$). A discrepancy from BCS-like temperature dependence of the gap was observed in the ring, suggesting a loss of phase coherence between adjacent chains in the closed-loop topology

Coherent collective oscillation in the commensurate phase of the TaS3_3 compound

We have investigated ultrafast carrier dynamics in a quasi-one-dimensional metal $m$-TaS$_3$ by using pump-probe reflectivity measurements. The temperature dependence of transient signal reveals two critical changes at T$_p = 240$ K and T$_c \sim$ 100 K. Below T$_c$, a new low-frequency oscillation mode appears and shows a pronounced softening depending on the temperature, indicating a collective excitation connected with the commensurability of CDW phase. This soft-mode exhibits remarkable polarization anisotropy with respect to the chain direction