Excitation of a bosonic mode by electron tunneling into a cuprate superconductorNdBa2Cu3O7−δ

Abstract

We performed scanning tunneling spectroscopic experiments on hole-doped NdBa2Cu3O 7−δ . The dI/dV curves obtained at 4.2 K are asymmetric with clear peak-dip and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential bandstructure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective mode in the CuO2 plane.PACS numbers: 74.20.Mn, 74.25.Jb, 74.50+r, 74.72.Bk The identification of the microscopic mechanism for pair formation in the unconventional superconductors is still a challenge. In conventional superconductors, apart from the isotope effect, tunneling experiments provided the most direct evidence of electron-phonon interaction mediating the Cooper-pair formation [1,2]. In case of high-T c superconductors (HTSC), inelastic neutron scattering (INS) experiments and high resolution angle-resolved photo emission (ARPES) experiments were useful to find collective modes in the range of 30-70 meV [3,4,5,6,7,8,9]. Collective spin excitations have been suggested as the bosonic mode which interacts with electrons to produce a resonance peak at a wavevector (π, π) in INS spectra and a kink along the (π,0) direction of the Brillouin zone in ARPES spectra [3,4,5,6,10]. On the other hand, several ARPES results also show evidence of phonons as the collective modes [7,8,9]. Thus the assignment of the boson-mediating Cooper pair formation in HTSC is far from being settled.Due to the microscopic electronic inhomogeneity in these superconductors [11], tunneling experiments using scanning tunneling microscopy (STM) are expected to play an important role in resolving such issues being related to pairing. While tunneling, electrons can excite a collective mode of a certain energy if the energy of the electrons is equal to that of the collective mode. Due to the inelastic interaction of the electrons with the mode, a new scattering channel is induced which results in a step-like feature in the dI/dV curves [12]. Ideally, these step-like features can be observed more easily in the form of a delta function in the energy derivative of the dI/dV curves. In experiments, the peak heights and widths are dependent on the energy resolution, temperature and the modulation voltage [12]. For the superconducting state of d-wave superconductors, Balatsky et al. predicted that features due to inelastic scattering of tunneling electrons would be observed in the d 2 I/dV 2 curves as satellite peaks at ∆ + Ω in addition to a very weak peak at Ω, where ∆ is the energy gap and Ω is * Electronic address: Pintu.Das@cpfs.mpg.de

Theory of (001) surface and bulk states in Y$_{1-y}$Ca$_y$Ba$_2$Cu$_3$O$_{7-δ}$}

“…consistent with bulk-sensitive ARPES experiments that found a gap of order 42 meV, 28 and surface-sensitive tunneling experiments, which consistently find that the gap is < ∼ 25 meV. [1][2][3][4][5][6][7] In the chain layers, the reduced gap at the surface reflects the reduced proximity coupling at the chain surface, in part because the chain has only one nearest neighbor CuO 2 plane. Our calculation shows that the DOS obtains its bulk value within a few layers of either surface.…”

“…Several experiments on superconducting samples have measured densities of states with multiple energy scales. [1][2][3][4][5][6][7] Some experiments find a subgap feature 3,4 while others do not, 2,5,6,8 and all experiments near optimal doping find satellite features at energies larger than the gap energy. Many of these studies find that the spectra change qualitatively with doping and can vary significantly at different points on the sample surface.…”

“…4,6 Spectral features have been interpreted in terms of band structure, 4 competing order, 6,9 and coupling to bosonic modes. 5 In this work, we explore reasons why the YBCO single-particle spectrum is so complicated, particularly when compared to that of the related superconductor Bi 2 Sr 2 CaCu 2 O 8 (BSCCO). [10][11][12] In BSCCO, there is a clear d-wave-like gap in the density of states, and it has been possible to reproducibly extract detailed information about the band structure 7,13,14 and superconducting state.…”

Low temperature scanning force microscopy using piezoresistive cantilevers

“…This low-temperature MFM was designed to operate in the same cryogenic system as used for low-temperature STM already described in [23,24]. The cryostat (Oxford Instruments) is housing a 5 T superconducting magnet with a bore 50 mm in diameter.…”

“…Figure 1(a) presents a schematical drawing of the MFM body, together with a photo of the system. The overall construction is analogous to that of the LT-STM system described in [23,24]. The difference is given by the cantilever connector, the piezo element driving the cantilever and the sample holder.…”