H. M. Fretwell

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.

Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor

A change in ‘symmetry’ is often observed when matter undergoes a phase transition—the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a ‘pseudogap’ appears in the spectrum of electronic excitations. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken below T*, which therefore corresponds to a phase transition.

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.

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.

Fermi Surface of Bi2Sr2CaCu2O8

We study the Fermi surface of Bi2Sr2CaCu2O8 using angle resolved photoemission spectroscopy (ARPES) with a momentum resolution of approximately 0.01 of the Brillouin zone. We show that, contrary to recent suggestions, the ARPES derived Fermi surface is a large hole barrel centered at (pi,pi), independent of the incident photon energy. We caution that the photon energy and k dependence of the matrix elements, if not properly accounted for, can lead to misinterpretation of ARPES intensities.

Determination of the Fermi surface in high-T c superconductors by angle-resolved photoemission spectroscopy

We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in Bi1.6Pb0.4Sr2CuO6 (Bi2201) and Bi2Sr2CaCu2O8 divided by delta (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously; it is a single large hole barrel centered about (pi,pi) in both materials.

Extraction of the electron self-energy from angle-resolved photoemission data: Application to Bi2Sr2CaCu2O8+x

The self-energy Sigma(k, omega), the fundamental function that describes the effects of many-body interactions on an electron in a solid, is usually difficult to obtain directly from experimental data. In this paper we show that by making certain reasonable assumptions, the self-energy can be directly determined from angle-resolved photoemission data. We demonstrate this method on data for the high-temperature superconductor Bi2Sr2CaCu2O8+x in the normal, superconducting, and pseudogap phases. [S0163-1829(99)13433-7].

The role of angle-resolved photoemission in understanding the high temperature superconductors.

Abstract The two-dimensional nature of the high temperature superconductors allows the determination of the energy–momentum relationship of electronic states by angle-resolved photoemission (ARPES). Furthermore, the shape of the ARPES spectra provides information on the many body interactions so prevalent in these materials. In this paper we review some results obtained by our group on the question of the existence of quasiparticles and their interactions.

Time-reversal symmetry breaking? (reply): Superconductors

Kaminski et al. reply- There are two components of the circular dichroism (CD) signal in angle-resolved photoemission (ARPES) measurements. One is always present in crystals, regardless of any time-reversal symmetry considerations. This component, which we refer to as ‘geometric’, is antisymmetric about any symmetry plane of the crystal, and is therefore zero at that plane. But in underdoped samples of the high-temperature superconductor Bi2212, we find another component, which is non-zero at the symmetry plane below the pseudogap temperature. We attribute that component to time-reversal symmetry breaking. The objections of Borisenko et al. comprise three main points: the circular dichroism that we observe at the mirror plane is due to the superstructure of the Bi–O layer; our momentum accuracy is not as we stated; and the absence of dichroism in overdoped samples is due to a weaker influence of the superstructure because of an increased Fermi surface volume compared with underdoped samples.

Quasiparticles in the Superconducting State of Bi2Sr2CaCu2O81d

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.