Observation of Competing Order in a High-TcSuperconductor Using Femtosecond Optical Pulses

Abstract

We present studies of the photoexcited quasiparticle dynamics in Tl2Ba2Ca2Cu3Oy (Tl-2223) using femtosecond optical techniques. Deep into the superconducting state (below 40 K), a dramatic change occurs in the temporal dynamics associated with photoexcited quasiparticles rejoining the condensate. This is suggestive of entry into a coexistence phase which, as our analysis reveals, opens a gap in the density of states (in addition to the superconducting gap), and furthermore, competes with superconductivity resulting in a depression of the superconducting gap.In the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, which describes the mechanism of conventional superconductivity for conventional metals, electrons form Cooper pairs mediated by the vibrations of the crystal lattice. For the high-temperature superconductors, another possibility exists, namely, Cooper pairing via antiferromagnetic spin fluctuations [1,2]. Indeed, a full-fledged antiferromagnetic order, out of which such antiferromagnetic fluctuations emerge, can also compete with superconductivity as the dominant ground state resulting in phase coexistence [3,4,5,6,7]. The coexistence of antiferromagnetic ordering with superconductivity has been observed in single-or double-layer systems in the presence of a magnetic field via neutron scattering [6,8], or in five-layered systems in zero field using nuclear magnetic resonance [5,7]. However, it is unclear from these measurements how the emergence of antiferromagnetic order affects the quasiparticle (QP) excitations, which determine the material's optical and electronic response.An energy gap in the excitation spectrum of a material is of fundamental importance in determining its optical and electronic properties. In superconductors and many other correlated electron materials, many-body interactions open a gap in the QP density of states thereby introducing an additional timescale for the QP dynamics. Independent of its origin, the opening of a gap presents a bottleneck to ground state recovery following photoexcitation of QPs across the gap. The timescale of this recovery is related to the gap magnitude, meaning that interactions which perturb the gap manifest as an easily measured change in the temporal response by monitoring changes in the reflectivity (∆R/R) or transmission of an interrogating probe beam. In recent years, femtosecond time-resolved spectroscopy has been recognized a powerful bulk technique to study temperature-dependent changes of the low-lying electronic structure of superconductors [9] and other strongly correlated electron materials [10,11,12]. It provides a new avenue, namely the time domain, for understanding the QP excitations of a material. In this Letter, we present time-resolved studies of photoexcited QP dynamics in the high-T c superconductor Tl-2223. We observe that its pristine superconducting state (40 K < T < T c ) subsequently evolves into a coexistence phase as evidenced by a strong modification of the gap dynamics below 40 K.Our sample is a slightly under...

Ultrafast pump-probe spectroscopic signatures of superconducting and pseudogap phases in YBa2Cu3O7−δ films

“…9,11,25,26 This fundamental discrepancy was attributed to the dependence of ΔR/R on several factors including laser power, 15,23 wavelength, 25,27 polarization, 25,28 and repetition rate 24,29 . Furthermore, in a recent effort to uncover the transient optical signatures of the pseudogap (PG) phase, [11][12][13][14]28,[30][31][32] and elucidate a possible relationship between the PG state and SC state in OD YBCO [31][32] , the PG phase was either identified as a weak transient feature 11,30 or found to be completely absent from the spectra. 9,19,25,26 Here, we report on a new procedure to obtain the definitive ultrafast optical RT model.…”

“…[1][2] Time-resolved spectroscopy with ultrafast temporal resolution is a complementary tool to investigate the temporal evolution of the QP-bosonic degrees of freedom [3][4][5][6][7][8] that is capable to disentangle the contribution of bosonic excitations from the dynamics viewpoint. 6 Based on this observation, the non-equilibrium optical spectroscopy has been extensively applied to explore mechanisms of phase separation, [9][10][11][12][13][14][15] boson pairing, 16 and the electron-phonon coupling [17][18][19][20][21] in superconductors by monitoring the QP dynamics; the experimental results are conventionally connected to microscopic processes by application of a phenomenological model proposed by Rothwarf and Taylor (i.e. the RT model).…”

Theory of photoexcited carrier relaxation across the energy gap of phase-ordered materials

“…Usually the anomalous T dependence of τ exp is observed in many gapped systems, for example, phaseordered systems showing an energy gap opening in the electron band below the transition temperature T c . It has been found that a wide variety of superconductors [1][2][3][4][5][6][7][8][9][10][11][12][13] and density-wave compounds [14][15][16][17][18][19] show diverging behavior of τ exp near T c , as confirmed by femtosecond time-resolved optical spectroscopy. 1,4,5,[7][8][9][10][11][12][13][14][15][16][17]19 The divergence of τ exp in these gapped systems is believed to result from a recursive energy transfer between electrons and phonons.…”

“…(22) is of great importance. The formula is applicable to τ exp in other gapped systems such as Tl-based cuprate superconductors 2,6 and the solid fullerenes K 3 C 60 and Rb 3 C 60 (Ref. 3).…”

“…There exists, however, a distinct class of gapped systems such as Tl-based superconductors 2,6 and C 60 -related materials 3,18 that, instead of showing τ exp divergence at T c , show monotonic increases in τ exp with cooling across T c . In light of the bottleneck, the monotonic variation in τ exp near T c appears controversial, leading to the questions of why the τ exp divergence vanishes in a portion of gapped systems despite the well-defined energy gap formation at the Fermi level and whether the bottleneck concept is completely invalid in these gapped systems.…”