Appearance of New Energy Gap and Periodic Local Density-of-States Modulation in Bi₂Sr_1.6La_0.4CuO_6+δ

Abstract

The spatial variation of the local density of states in optimally doped Bi 2 Sr 1:6 La 0:4 CuO 6þ (superconducting transition temperature is 34 K) is studied by scanning tunneling spectroscopy at 4.2 K in zero magnetic field. Two-dimensional density-of-states modulation aligned with the Cu-O-Cu bond with a periodicity of about five lattice constants is observed. It is found that this modulation accompanies the appearance of a new energy gap of approximately 10 meV, whose gap edge peak is spatially modulated in intensity. This gap coexists with the superconducting gap, the value of which ranges from 10 meV to 60 meV.The addition of a relatively small number of carriers into a Mott insulator realizes d-wave high-temperature superconductivity in cuprates. The doped cuprates show unconventional electronic states. This unconventional nature has been investigated extensively in both momentum and real space. Angle-resolved photoemission spectroscopy (ARPES) revealed that states in different parts of momentum space, i.e., nodal and antinodal regions, show contrasting features, such as different temperature and doping dependences. 1) Furthermore, the line shape of photoemission spectra indicates dichotomy; a broad line shape in the antinodal region changes abruptly to a sharp one near the nodal region. 2) In addition, the superconducting gap structure along the Fermi surface considerably deviates from the prismatic d-wave functional form. 3,4) On the other hand, scanning tunneling spectroscopy (STS), which measures integrated spectral weight in momentum space, revealed spatial evolution of the electronic states. The superconducting gap varies on a length scale of about 5 nm and the tunneling spectrum changes distinctly in its shape. 5,6) It is claimed that coherence peaked spectra are observed in the region to be occupied by a canonical d-wave superconductor (dSC), while the spectra without clear coherence peaks, attributed to the destruction of antinodal coherence peaks, is characteristic of the electronic phase at the zero temperature limit of the pseudogap (ZTPG) region. 7) Furthermore, an anomalous local density-of-states (LDOS) modulation with a periodicity of about four lattice constants has been observed in Bi 2 Sr 2 CaCu 2 O 8þ (Bi2212) and Ca 2Àx Na x CuO 2 Cl 2 (Na-CCOC). 7-13) This modulation was first observed by Hoffman et al. in a magnetic field around vortex cores. 8) In zero magnetic fields, both energydispersive and nondispersive LDOS modulations were reported. The former, observed at low energy, is attributed to the interference of the quasi particles at the Fermi arc regions. 9,14) The dispersion of the quasi particles obtained in the STS experiments and the ARPES results are in excellent agreement. The origin of the latter, however, is still controversial. Since the nondispersive LDOS modulation has been observed in pseudogap states above the superconducting transition temperature, T c , and in the underdoped non-superconducting region, 10,15) it is claimed that this modulation is associated with the p...

Superconducting interaction charge in thallium-based high-TC cuprates: Roles of cation oxidation state and electronegativity

“…This phenomenon accounts for different charge fractions, 0.5 σ 0 for Bi-2212 vs. σ 0 for Tl-2212, and the marked dichotomy in T C0 . The same effect of electronegativity difference is found for the 2223 compounds (Bi,Pb) 2 . Conclusions are drawn in Section 5.…”

“…2 (∆T C0 ) k + t 0 , and where δ Tc is the standard deviation or width of the distribution and t 0 is an offset; N 0 ~ ±1 is the constant of integration. Fitting this model function for N k to the data yields the results δ Tc = 1.31(2) K, t 0 = 0.15 (2), and N 0 = 0.3 (4). The fitted model function is represented by the continuous solid curve in Fig.…”

“…X-ray refinement analysis of data acquired on single crystals of TlBa 2 (Ca/Tl) 2 ] was found to be 0.792/0.208(10), corresponding to f +1 = 0.208 ± 0.010, and T C = 116.3 K, where the uncertainty in f +1 is given by the constrained ratio. Subsequent to annealing in nitrogen for 224 hours at the same temperature, T C increased to 120.6 K (lower than T C0 = 133.5 K, see ] equal to 0.762/0.238(13), or f +1 = 0.238 ± 0.013.…”

“…A notable comparison, representative of the η = 1 cuprates, can be made between Bi 2 (Sr 1.6 La 0.4 )CuO 6+δ and Tl 2 Ba 2 CuO 6 (denoted as 2201 compounds). Although belonging to different space groups, these materials share one-to-one correspondences between like structural layers, yet exhibit vastly different T C0 values of 34 K [1][2][3] and 80 K [4], respectively; for η = 2, Bi 2 Sr 2 CaCu 2 O 8+δ and Tl 2 Ba 2 CaCu 2 O 8 (2212 compounds) display measured T C0 = 89 K [5] and 110 K [6], respectively, and this tendency continues for η = 3 (2223 compounds). Data for T C0 are cited after Ref.…”

Anti-phase Modulation of Electron- and Hole-like States in Vortex Core of Bi2Sr2CaCu2Ox Probed by Scanning Tunneling Spectroscopy

“…When the superconductivity is weakened, similar modulated patterns were observed in a zero magnetic field. [8][9][10]17) These ordered states may appear in the vortex core. Vershinin et al found a modulation in Bi 2 Sr 2 CaCu 2 O x above T c , that is, in the pseudo-gap state.…”