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 Ia 1 : Ia 2 : Ia 3 = 1 : 0.7 ± 0.1 : 0.5 ± 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 clockwise and 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 ≡ q 1 · (q 2 × q 3 ), where q i are the nesting vectors, which is independent of the symmetry of components. The nonzero H CDW -the triple-q vectors do not exist in an identical plane in the reciprocal space -should induce a real-space chirality in CDW system.

We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap (PG) and superconducting (SC) states of an underdoped Bi2Sr2CaCu2O8 + y (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy variant planck's over omegapr = 1.55 eV, it is found that the reflectivity change DeltaR/R changes its sign at exactly Tc, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below Tc, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.

Scanning tunneling microscopy and spectroscopy are used to examine the 4a ϫ 4a electronic charge order ͑CO͒ and the spatial dependence of energy gap in the pseudogap ͑PG͒ state above T c on Bi 2 Sr 2 CaCu 2 O 8+␦ . We report the first observation on energy gap inhomogeneity in the PG state, together with the result that the static CO develops markedly in the inhomogeneous PG state, although it is very weak in the homogeneous PG state. This static CO, which is considered to be stabilized by the pinning of the dynamically fluctuating CO, can coexist with the superconductivity below T c .In high-T c cuprate superconductors, it has been established that an unusual electronic state, characterized by a gaplike structure around the Fermi level E F , the so-called "pseudogap ͑PG͒," develops in the normal state above T c , and it must be well understood to elucidate the mechanism of high-T c superconductivity. Recently, in the PG state of underdoped ͑UD͒ Bi 2 Sr 2 CaCu 2 O 8+␦ ͑Bi2212͒, Vershinin et al. found a charge order ͑CO͒ in two-dimensional ͑2D͒ maps of the local density of states ͑LDOS͒ at specified energies, which were obtained by scanning tunneling microscopy and spectroscopy ͑STM/STS͒. 1 This CO is oriented along the two Cu-O bond directions, intersecting at right angles; its period is independent of energy, 4.5a -4.8a along each Cu-O direction ͑a, the lattice constant or the Cu-O-Cu distance͒, which is called "nondispersive." Interestingly, the nondispersive CO develops markedly at low energies within the PG and tends to disappear outside the PG. On the other hand, in the superconducting ͑SC͒ state, they did not observe the nondispersive CO, but observed dispersive LDOS modulations due to quasiparticle interference effects, which were first reported by Hoffman et al. 2 This tempted many researchers to suppose that the nondispersive CO is a characteristic feature only for the PG state above T c .However, in the SC state of UD Bi2212, Howald et al. 3 and Momono et al. 4 observed a nondispersive CO, whose period, ϳ4a for each Cu-O direction, was smaller than that reported by Vershinin et al. in the PG state. Furthermore, it has recently been demonstrated in STM/STS experiments that the amplitude of 4a ϫ 4a CO at T Ӷ T c is strongly sample dependent; in samples exhibiting an intense 4a ϫ 4a CO at T Ӷ T c , the spatial dependence of the energy gap structure is inhomogeneous on the nanometer scale, and vice versa. 5 For elucidating the relation among the CO, PG, and high-T c superconductivity, it is of urgency to study the problem of whether the 4a ϫ 4a CO is a common feature in both the SC and PG states. In this paper, we report STM/STS experiments in the PG state above T c on two kinds of samples that exhibit strong and weak 4a ϫ 4a CO's at T Ӷ T c , as shown in Figs. 2͑d͒ and 2͑e͒ and suggest that the static 4a ϫ 4a CO, which develops markedly in the inhomogeneous PG state, will remain below T c , together with the inhomogeneous gap structure.Bi2212 crystals were grown by the traveling solvent floating zone metho...

STM/STS is used to examine the charge order in the pseudogap state on underdoped Bi 2212 crystals with different hole-doping levels (p). We report that although the data for the period of charge order are slightly scattered, it is nearly 4a × 4a (a: the lattice constant) and exhibits no obvious dependence on p in the p-range examined.