STM Studies of the Electronic Structure of Vortex Cores inBi2Sr2CaCu2

Abstract

We report on low temperature scanning tunneling microscopy (STM) studies of the electronic structure of vortex cores in Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO). At the vortex core center, an enhanced densityof-states (DOS) is observed at energies near Ω = ± 7 meV. Spectroscopic imaging at these energies reveals an exponential decay of these 'core states' with a decay length of 22 ± 3 Å. The four-fold symmetry sometimes predicted for d-wave vortices is not seen in spectroscopic vortex images. A locally nodeless order parameter induced by the magnetic field may be consistent with these measurements.PACS# 74.72.Hs , 74.50.+r , 75.30.Hx , 61.16.Ch In conventional type II superconductors, the order parameter (OP) is suppressed in the cores of quantized magnetic vortices [1] and recovers over a distance of about one coherence length ξ. Bound quasi-particle states can exist inside these cores [2] with lowest energy given approximately by E ~ ∆ 2 /2E F , where E F is the Fermi energy and ∆ is the superconducting energy gap. Such 'core' states were first imaged by Hess et al. using low temperature scanning tunneling microscopy [3]. In the high temperature superconductors (HTSC) the zero magnetic field OP is believed to have mainly a d x 2 -y 2 symmetry [4][5][6][7] and is therefore not uniform in k-space but instead has four nodes [8]. The electronic structure of vortex cores in a superconductor with such an unconventional OP has attracted much attention, both theoretical [ 9-27] and experimental [28][29][30][31], since it serves as a coherence-length scale (~10 Å) probe of the condensate structure.Evidence for low energy vortex core states in YBCO was first provided by infrared absorption experiments [28].Pioneering STM experiments on YBCO by Maggio-Aprile et al. directly identified vortex core states at energies of ± 5.5 meV by tunneling spectroscopy [29]. Similar states were not however observed by STM in BSCCO vortex cores [30][31].Here we report new STM experiments which demonstrate the existence of low energy states associated with the vortex cores in BSCCO. Two different types of as-grown BSCCO single crystals were used. The first set, grown by the directional solidification technique, has a transition temperature T c = 87 K, a transition width of 5 K and contains scattering centers at a density of 0.3%, while the second set, grown by the floating zone technique, has a very dilute (~0.3%) doping concentration of Zn atoms acting as impurity scatterers, a T c of 84 K and a transition width of 4 K. All samples are cleaved in cryogenic ultrahigh vacuum at 4.2 K and immediately inserted into the STM head [32].In addition to topographic imaging, STM can be used to map the electronic DOS on the surface at energy E = eV by measuring the differential tunneling conductance G at sample bias V as a function of position. When studied in zero magnetic field, images of only the impurity scattering resonances [33][34][35] appear in a zero-bias conductance map. Figure 1a is such an image and shows the impurity resonances as dark feature...

Nernst effect in high-Tcsuperconductors

“…The trend agrees with that of ∆ 0 measured by ARPES in Bi 2212 by Harris et al [67] and Ding et al [68]. Further, the values of ξ inferred from H c2 vs. x [21] are consistent with ξ P obtained from ∆ 0 and with the vortex core size observed by STM [70].…”

d-Wave superconductivity as a catalyst for antiferromagnetism in underdoped cuprates

“…On the other hand, solutions of the Bogoliubov-de Gennes (BdG) equations describing coexisting d-wave superconductivity and SDW order [20][21][22][23][24] show that this resonance is split by the SDW formation; that is, the system can lower the energy of the nearly bound quasiparticles by moving them below the Fermi energy. This finding is consistent with the STM experiments in the Abrikosov state of YBa 2 Cu 3 O 7−δ (YBCO) [25] and of BSCCO [26], in which split peaks were observed in the vortex cores. A similar bound state is associated with non-magnetic impurities such as Zn in BSCCO and was also imaged by STM [27].…”

Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2

“…The existence of the pronounced vortex core bound states peaked at negative bias and the spatial evolution are also in striking contrast to the case of cuprate superconductors in which two subgap peaks were observed in vortex cores [10][11][12][13][14] . However, our observations of the electronic behavior of vortex cores in Ba 0.6 K 0.4 Fe 2 As 2 are quite similar to what observed in the conventional superconductor 2H-NbSe 2 5,7 .…”