Tl₂Ba₂CuO_6+δbrings spectroscopic probes deep into the overdoped regime of the high-T_ccuprates

Abstract

Single-particle spectroscopic probes, such as scanning tunneling and angle-resolved photoemission spectroscopy (ARPES), have provided us with crucial insights into the complex electronic structure of the high-T c cuprates, in particular for the under and optimally doped regimes where high-quality crystals suitable for surface-sensitive experiments are available. Conversely, the elementary excitations on the heavily overdoped side of the phase diagram remain largely unexplored. Important breakthroughs could come from the study of Tl 2 Ba 2 CuO 6+δ (Tl2201), a structurally simple system whose doping level can be tuned from optimal to extreme overdoping by varying the oxygen content. Using a self-flux method and encapsulation, we have grown single crystals of Tl2201, which were then carefully annealed under controlled oxygen partial pressures. Their high quality and homogeneity are demonstrated by narrow rocking curves and superconducting transition widths. For higher dopings, the crystals are orthorhombic, a lattice distortion stabilized by O interstitials in the TlO layer. These crystals have enabled the first successful ARPES study of both normal and superconducting-state electronic structure in Tl2201, allowing a direct comparison with the Fermi surface from magnetoresistance and the gap from thermal conductivity experiments. This establishes Tl2201 as the first high-T c cuprate for which a surfacesensitive single-particle spectroscopy and a comparable bulk transport technique have arrived at quantitative agreement on a major feature such as the normal state Fermi surface. The momentum dependence of the ARPES lineshape reveals, however, an unexpected phenomenology: in contrast to the case of under-and optimally-doped cuprates, quasiparticles are sharp near (π, 0), the antinodal region where the gap is maximum, and broad at (π/2, π/2), the nodal region where the gap vanishes. This reversed quasiparticle anisotropy past optimal doping, and its relevance to scattering, many-body, and quantum-critical phenomena in the high-T c cuprates, are discussed.

Damped spin excitations in a doped cuprate superconductor with orbital hybridization

“…2) of the spin-excitation dispersion is obtained for t /t = −0.405 and U/t = 6.8. Notice that a similar ratio of t /t has previously been inferred from the rounded Fermi-surface topology of Tl 2 Ba 2 CuO 6+x [38][39][40] a material for which the d z 2 states are expected to be much less important [41]. It could thus suggest that t /t ≈ −0.4 is common to single layer cuprates but masked in LSCO due to the repulsion between the d x 2 −y 2 and d z 2 bands that pushes the Van Hove singularity close to the Fermi level and effectively reshapes the Fermi-surface topology [14].…”

Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2−xSrxCuO4

“…This is consistent with angledependent magneto-resistance experiments on overdoped Tl2201 4 . ARPES experiments on overdoped Tl2201 have, however, reported the opposite trend 29 . The origin of this discrepancy remains an open question.…”

In situ doping control of the surface of high-temperature superconductors

“…Superconductivity emerges along the path from a normal metal on the overdoped side to an antiferromagnetic insulator on the underdoped side. This path also exhibits a severe disruption of the overdoped normal metal's Fermi surface [1][2][3] . Angle-resolved photoemission spectroscopy (ARPES) on the surfaces of easily cleaved materials such as Bi 2 Sr 2 CaCu 2 O 8+δ shows that in zero magnetic field the Fermi surface breaks up into disconnected arcs [4][5][6] .…”

“…Our study demonstrates that the self-hole-doping of the cleaved polar surfaces of YBCO can be controlled by the in situ evaporation of alkali metals, in the present case K. This novel approach paves the way for the study of this important material family--across the whole phase diagramby single-particle spectroscopies. As this material has been the gold standard in a number of seminal bulk-sensitive studies of the normal and superconducting properties, the direct connection with single-particle spectroscopy can lead to an understandable underdoped anchor point, analogous to Tl 2 Ba 2 CuO 6+δ in the overdoped regime 3 . The results obtained for p = 0.11 YBCOK2 establish that the ARPES Fermi surface of underdoped YBCO consists of the superposition of 1D CuO-chain Fermi surface and CuO 2 -plane-derived nodal Fermi arcs.…”