Cuprates

Cuprate

The high temperature superconductivity (HTSC) in cuprates is realized by carrier doping into antiferromagnetic Mott insulators. As the carrier doping goes on, the antiferromagnetic state gets gradually suppressed and the superconducting phase appears in a dome shaped region of a doping-temperature phase diagram. Its high superconducting transition temperature (Tc) cannot be explained solely by the electron-phonon coupling that explains a conventional superconductivity. In addition to the phonon, a strong electron correlation effect or a magnetic interaction, which characterize the parent compounds, have been suggested as important ingredients for the HTSC.

Ultrafast dynamics of optimally doped YBCO

Recently, we have demonstrated the ultrafast broadband pump-probe spectroscopy now allows us to investigate the detailed dynamics of quasiparticles and phonons in the infrared region. As shown in the figure below, two sharp phonon lines in addition to the broad quaparticle response can be clearly observed so that we can trace in femtosecond time scale how the phonons and quaparticles recover from the inequilibrium state.[1] The study shows that the phonon takes quite large portion of energy almost immediately after photo-excitation, which suggests again the strong electron-phonon interaction.

Figure 1. Femtosecond map of absorption coefficient of optimally doped YBa2Cu3O7-d. The broad background response comes from the recovery of quasiparticles wile the two sharp lines explains the behaviors of phonons.[1]

Phase competition in underdoped cuprates

One of the most striking observations in doping dependent studies of cuprates is that there are signatures for a fluctuating superconducting state in underdoped samples at temperature even higher than the Tc of the optimally doped sample as shown in the figure below.[2] A strange partially gapped normal state, so called pseudo-gap state in underdoped cuprates has been reported from early nineties. There are plenty of experimental evidences for and against the interpretation that the pseudo-gap state is a precursor superconducting state. Its behavior under magnetic field speaks for a character of superconducting Cooper pairs while the gap size and spectral redistribution implies that the pseudo-gap has a different origin from the superconductivity and competes with the superconducting state.[3] Recently, we have provided clear infrared spectroscopic signatures that a precursor superconducting state appears in the background of the competing pseudo-gap state before the macroscopic superconducting state emerges at Tc. It is very important to understand how these two states compete to give rise to superconductivity.

Figure 2. Temperature-doping phase diagram of (R,Ca)Ba2Cu3O7-d. T* is the pseudo-gap temperature, Tons the onset temperature of the precursor superconducting state, Tc the transition temperature to the macroscopic superconducting state, TN the Neel temperature, and TSG the spin-glass transition temperature.[2]

We continue the study by means of an ultrabroadband terahertz (THz) pump-probe spectroscopy method. We monitor the c-axis optical spectroscopic response in a femto-second time scale after perturbation with strong near-infrared laser pulses to understand the ultrafast dynamics of the pseudo-gap and precursor superconducting states of YBa2Cu3O7-d. In the precursor superconducting state, the behavior of fluctuating Cooper pairs can be traced by monitoring the plasmon response as well as phonons as shown in figure 3.[2] We believe that the ultrafast study can reveal how these two states compete with each other and finally how the macroscopic superconductivity appears from a normal state with those two competing components. All these results should provide valuable pinpoints to understand the mechanism of the HTSC.

Figure 3. c-axis optical conductivity spectra of underdoped YBa2Cu3O7-d with Tc 58 K. Very huge changes show up in the phonon lines and electronic background depending on temperature. Three curves of 330 K, 60 K, 10 K belong to the pseudo-gap state, the precursor superconducting state and superconducting state respectively.[2]

[1] “Femtosecond Quasiparticle and Phonon Dynamics in Superconducting YBa2Cu3O7−δ Studied by Wideband Terahertz Spectroscopy”, A. Pashkin, K. W. Kim et al., Phys. Rev. Lett. 105, 067001 (2010).

[2] “Evidence of precursor superconducting phase at temperatures as high as 180 K in RBa2Cu3O7- δ (R=Y,Ga,Eu) superconducting crystals from infrared spectroscopy”, A. Dubroka et al., Phys. Rev. Lett. 106, 047006 (2011).

[3] “Evidence for Two Separate Energy Gaps in Underdoped High-Temperature Cuprate Superconductors from Broadband Infrared Ellipsometry”, L. Yu et al., Phys. Rev. Lett. 100, 177004 (2008).