Nodal quasiparticle meltdown in ultrahigh-resolution pump–probe angle-resolved photoemission

Abstract

High-T c cuprate superconductors are characterized by a strong momentum-dependent anisotropy between the low energy excitations along the Brillouin zone diagonal (nodal direction) and those along the Brillouin zone face (antinodal direction). Most obvious is the d-wave superconducting gap, with the largest magnitude found in the antinodal direction and no gap in the nodal direction. Additionally, while antinodal quasiparticle excitations appear only below T c , superconductivity is thought to be indifferent to nodal excitations as they are regarded robust and insensitive to T c . Here we reveal an unexpected tie between nodal quasiparticles and superconductivity using high resolution time-and angle-resolved photoemission on optimally doped Bi 2 Sr 2 CaCu 2 O 8+δ . We observe a suppression of the nodal quasiparticle spectral weight following pump laser excitation and measure its recovery dynamics. This suppression is dramatically enhanced in the superconducting state. These results reduce the nodal-antinodal dichotomy and challenge the conventional view of nodal excitation neutrality in superconductivity.The electronic structures of high-T c cuprates are strongly momentum-dependent. This is one reason why the momentum-resolved technique of angle-resolved photoemission spectroscopy (ARPES) has been a central tool in the field of high-temperature superconductivity. 1 For example, coherent low energy excitations with momenta near the Brillouin zone face, or antinodal quasiparticles (QPs), are only observed below T c and have been linked to superfluid density. 2,3 They have therefore been the primary focus of ARPES studies. In contrast, nodal QPs, with momenta along the Brillouin zone diagonal, have received less attention and are usually regarded as largely immune to the superconducting transition because they seem insensitive to perturbations such as disorder, 4-6 doping, 6-8 isotope exchange, 9 charge ordering, 6,10,11 and temperature. 12-15 Clearly, finding any strong dependencies of the nodal QPs will alter the conventional view and enrich our understanding of high temperature superconductivity.Time resolution through pump-and-probe techniques adds a new dimension to ARPES by directly measuring how the electronic structure of a material responds to perturbations on femtosecond time scales. Here we report a unique ultrafast time-resolved ARPES study of a high-T c cuprate superconductor. Compared to previous time-resolved studies, 16-18 the primary advantage of this work is an unprecedented momentum (angular) resolution (∆k ∼0.003 vs. 0.05Å −1 ), on par with that of stateof-the-art ARPES. This has allowed the time-resolved measurement of significantly sharper QP spectral peaks with strikingly larger peak-to-background ratios than previously reported. 16 Additionally, a lower pump fluence is used ( 40µJ/cm 2 vs. ∼100µJ/cm 2 ), which reduces pump-induced sample temperature increase and related thermal smearing of spectral features. This allows us to uncover a surprising meltdown of nodal QP spectral weight following...

Towards properties on demand in quantum materials

“…The kink separates low-energy coherent quasiparticles from incoherent states at higher energies. Femtosecond dynamics of these characteristics in cuprate high-T c superconductors have helped to uncover the roles of spin fluctuations and the lattice in the formation of the modified dispersion 76,77 .…”

Revealing the ultrafast light-to-matter energy conversion before heat diffusion in a layered Dirac semimetal

“…Meanwhile, continuous progress in pump-and-probe methods allows us to investigate the ultrafast phenomena in depth, thereby allowing us to test and develop our understandings from the experimental side. Angle-resolved photoemission spectroscopy (ARPES) implemented by the pump-and-probe method enables us to resolve the carrier dynamics in energy (E) and momentum (k) space, as done on TIs [15][16][17], graphene [14], and cuprates [29][30][31][32][33], to mention a few.…”

“…These characteristics were nicely described by a TTM scheme, in which the Dirac fermions are weakly coupled into some optical phonons. Our study provides a starting point to understand the ultrafast dynamics under stronger couplings and excitations: The strong couplings may result in the breakdown of the TTM scheme [26-28, 55, 56] or the band structures [14,31,57] and concepts of quasi-particles may have to be seriously taken into account [30,33]; A warm-dense matter will be reached at stronger excitations [58][59][60]. A variety of pump-and-probe methodologies are needed to deepen insights into the strongly-correlated ultrafast phenomena.…”

Photoinduced changes of the chemical potential in superconductingBi2Sr2CaCu2O8+δ

“…The energy and momentum resolutions are 23 meV, 0.003Å −1 respectively. The time resolution is 300 fs, longer than the 100 fs it takes for electrons to reach a quasithermal distribution [15,16], so the chemical potential is well-defined at all times.…”