Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Y}}_{0.08}{\mathrm{Ca}}_{0.92}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ Driven by Midinfrared Pulse Excitations

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Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped ${Bi}_{2} {Sr}_{2} Y_{0.08} {Ca}_{0.92} {Cu}_{2} O_{8 + δ}$ Driven by Midinfrared Pulse Excitations

F. Giusti, A. Marciniak, F. Randi, G. Sparapassi, F. Boschini, H. Eisaki, M. Greven, A. Damascelli, A. Avella, and D. Fausti

Phys. Rev. Lett. 122, 067002 – Published 15 February 2019

Abstract

Optimally doped cuprate are characterized by the presence of superconducting fluctuations in a relatively large temperature region above the critical transition temperature. We reveal here that the effect of thermal disorder, which decreases the condensate phase coherence at equilibrium, can be dynamically contrasted by photoexcitation with ultrashort midinfrared pulses. In particular, our findings reveal that light pulses with photon energy comparable to the amplitude of the superconducting gap and polarized in plane along the copper-copper direction can dynamically enhance the optical response associated with the onset of superconductivity. We propose that this effect can be rationalized by an effective $d$ -wave BCS model, which reveals that midinfrared pulses result in a transient increase of the phase coherence.

Received 6 September 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.067002

Physics Subject Headings (PhySH)

Impurities in superconductors Optical transient phenomena Pseudogap Superconducting fluctuations Superconducting phase transition Ultrafast phenomena

Cuprates High-temperature superconductors

Infrared spectroscopy Optical absorption spectroscopy

Atomic, Molecular & OpticalNonlinear DynamicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

F. Giusti^1,2,*, A. Marciniak^1,2,*, F. Randi^1,2, G. Sparapassi^1,2, F. Boschini^3,4, H. Eisaki⁵, M. Greven⁶, A. Damascelli^3,4, A. Avella^7,8,†, and D. Fausti^1,2,9,‡

¹Department of Physics, Università degli Studi di Trieste, 34127 Trieste, Italy
²Elettra Sincrotrone Trieste S.C.p.A., 34127 Basovizza Trieste, Italy
³Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
⁴Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
⁵Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
⁶School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
⁷Dipartimento di Fisica “E.R. Caianiello,” Università degli Studi di Salerno, I-84084 Fisciano (SA), Italy
⁸CNR-SPIN, UOS di Salerno, I-84084 Fisciano (SA), Italy
⁹Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA

^*These two authors contributed equally.
^†Corresponding author. avella@physics.unisa.it
^‡Corresponding author. daniele.fausti@elettra.eu

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Vol. 122, Iss. 6 — 15 February 2019

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Figure 1
MIR pump, optical probe measurements. (a) and (b) Reflectivity variation due to an impulsive excitation at time delay 0 ps by a MIR pulse ( $h ν \approx 70 meV$ ) as a function of the temperature of the sample (vertical axis). The two maps differ in the polarization of the impinging pump, as highlighted by the two insets, representing the direction of the polarization (green and orange arrow) in the Cu-O plane. (c) Time resolved signal at fixed temperatures for Bi2212: the brown line represents the characteristic superconductive signal, while the blue one refers to the pseudogap phase. The transition between the two phases is marked with the black line and is related to the maximum of the time decay. Light colored lines represent intermediate temperatures. (d) Sketch of the first Brillouin zone and of the superconducting $d$ -wave gap amplitude $| Δ (k) |$ in the reciprocal space. The black curved lines represent the Fermi surface.
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Figure 2
Wavelength dependent anisotropy. Difference between the transient reflectivities due to Cu-Cu and Cu-O polarized pump in time (horizontal axis) and temperature (vertical axis), induced by excitations with (a) 70 and (b) 170 meV pump photon energies. The dashed lines highlight the critical temperature $T_{c} = 97 K$ . The insets represent the response as a function of temperature at 1 ps time delay for Cu-Cu (orange line) and Cu-O (green line) polarized pump excitations for low and high pump photon energies (a and b, respectively). The gray dashed lines mark $T_{c}$ . Both in the maps and in the insets the values of the reflectivity have been normalized to the maximum value of the response at 80 K ( $Δ R_{80 K}$ ).
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Figure 3
$d$ -wave BCS microscopic model. Results of the microscopic model: (a) Time evolution of the modulus of the superconducting gap ( $| Δ |$ ) in both Cu-Cu (orange line) and Cu-O (green line) excitation case. The maximum of the pump electric field is reached at about 350 fs. (b) Normalized integral of $| Δ |$ in the time interval from 0 to 1 ps as a function of the pump photon energy ( $Δ_{0} = | Δ (t = 0) |$ ) for Cu-Cu polarized pump excitations.
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Figure 4
Time dependent pair operator representation. (a)–(c) Modulus of the expectation value of the pair operator $\hat{Ψ} (k)$ in the reciprocal space in three different situations: (a) No excitation (“Equilibrium”), (b) During a pump excitation polarized along the Cu-Cu and (c) Cu-O direction. Analogously, pictures from (d) to (f) show the phase $ϕ (k)$ of $⟨ \hat{Ψ} (k) ⟩$ , in the same three conditions.
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Figure 5
Extension of the $d$ -wave BCS model above the critical temperature. (a) Values of the superconducting gap at equilibrium [ $Δ_{0} = | Δ | (t = 0)$ ] as a function of the amplitude of the phase noise. (b) Distribution of the phase values at equilibrium (blue line) and during the excitation polarized along the Cu-Cu direction (red line) for an initial noise amplitude of $δ ϕ = π / 8$ .
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Physical Review Letters

Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped Bi2Sr2Y0.08Ca0.92Cu2O8+δ Driven by Midinfrared Pulse Excitations

F. Giusti, A. Marciniak, F. Randi, G. Sparapassi, F. Boschini, H. Eisaki, M. Greven, A. Damascelli, A. Avella, and D. Fausti

Phys. Rev. Lett. 122, 067002 – Published 15 February 2019

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Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped ${Bi}_{2} {Sr}_{2} Y_{0.08} {Ca}_{0.92} {Cu}_{2} O_{8 + δ}$ Driven by Midinfrared Pulse Excitations