Takashi Mochiku

Electronic Spectra and Their Relation to the ( π,π) Collective Mode in High- Tc Superconductors

The photoemission line shape near (pi, 0) in Bi(2)Sr(2)CaCu(2)O(8+delta) below T(c) is characterized by a sharp peak, followed at higher energy by a dip and hump. We study the evolution of this line shape as a function of momentum, temperature, and doping. We find the hump scales with the peak and persists above T(c) in the pseudogap state. We present strong evidence that the peak-dip-hump structure arises from the interaction of electrons with a collective mode of wave vector (pi, pi). The inferred mode energy and its doping dependence agree well with a magnetic resonance observed by neutron scattering.

Superconducting Gap Anisotropy and Quasiparticle Interactions: A Doping Dependent Photoemission Study

Comparing photoemission measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of noninteracting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple coskx 2 cosky form, which suggests an increasing importance of long range interactions as the insulator is approached.

Evolution of the Fermi Surface with Carrier Concentration in Bi2Sr2CaCu2O8+δ

We show, by use of angle-resolved photoemission spectroscopy, that underdoped Bi2Sr2CaCu2O8+delta appears to have a large Fermi surface centered at (pi, pi), even for samples with a T-c as low as 15 K. No clear evidence of a Fermi surface pocket around (pi/2, pi/2) has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime T-c < T < T*, which is found to coincide with the locus of gapless excitations in momentum space (Fermi surface) determined above T*. These results suggest that the pseudogap is more likely of precursor pairing rather than magnetic origin.

Unusual Dispersion and Line Shape of the Superconducting State Spectra of Bi2Sr2CaCu2O8+δ

Photoemission spectra of Bi_2Sr_2CaCu_2O_{8+delta} below T_c show two features near the (pi,0) point of the zone: a sharp peak at low energy and a higher binding energy hump. We find that the sharp peak persists at low energy even as one moves towards (0,0), while the broad hump shows significant dispersion which correlates well with the normal state dispersion. We argue that these features are naturally explained by the mixing of electrons with a bosonic mode which appears only below T_c, and speculate that the latter may be related to the resonance seen in recent neutron data.

Quasiparticles in the superconducting state of Bi(2)Sr(2)CaCu(2)O(8+delta)

Recent improvements in momentum resolution lead to qualitatively new angle-resolved photoemission spectroscopy results on the spectra of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) along the (pi,pi) direction, where there is a node in the superconducting gap. We now see the intrinsic line shape, which indicates the presence of true quasiparticles at all Fermi momenta in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity.

Direct observation of particle-hole mixing in the superconducting state by angle-resolved photoemission

Particle-hole (

Anomalous magnetization behavior of single crystalline Bi2Sr2CaCu2O8+δ

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Cu NMR Study in Single Crystal Bi2Sr2CaCu2O8 –Observation of Gapless Superconductivity–

) mixing is a fundamental consequence of the existence of a pair condensate. We present direct experimental evidence for

Crystal Structure of Magnetic Superconductor FeSr2YCu2O6+δ

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Evidence for Suppressed Screening on the Surface of High Temperature La2-xSrxCuO4 and Nd2-xCexCuO4 Superconductors

mixing in the angle-resolved photoemission (ARPES) spectra in the superconducting state of