Dynamical decoherence of the light induced interlayer coupling in $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$

Dynamical decoherence of the light induced interlayer coupling in $YB a_{2} C u_{3} O_{6 + δ}$

C. R. Hunt, D. Nicoletti, S. Kaiser, D. Pröpper, T. Loew, J. Porras, B. Keimer, and A. Cavalleri

Phys. Rev. B 94, 224303 – Published 12 December 2016

Abstract

Optical excitation of apical oxygen vibrations in $YB a_{2} C u_{3} O_{6 + δ}$ has been shown to enhance its $c$ axis superconducting-phase rigidity, as evidenced by a transient blueshift of the equilibrium interbilayer Josephson plasma resonance. Surprisingly, a transient $c$ axis plasma mode could also be induced above $T_{c}$ by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of $YB a_{2} C u_{3} O_{6 + δ}$ . However, despite the similarities between the transient plasma mode above $T_{c}$ and the equilibrium Josephson plasmon, alternative explanations involving high-mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport and rather support transient superfluid tunneling as the origin of the light induced interlayer coupling in $YB a_{2} C u_{3} O_{6 + δ}$ .

Received 3 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.224303

Physics Subject Headings (PhySH)

Optical conductivity Optical phonons Superconducting phase transition Ultrafast phenomena

High-temperature superconductors

Ultrafast pump-probe spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. R. Hunt^1,2, D. Nicoletti¹, S. Kaiser^1,3,*, D. Pröpper³, T. Loew³, J. Porras³, B. Keimer³, and A. Cavalleri^1,4

¹Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, 22761, Germany
²Department of Physics, University of California Berkeley, Berkeley, California 94720, USA
³Max Planck Institute for Solid State Research, Stuttgart, 70569, Germany
⁴Department of Physics, Oxford University, Clarendon Laboratory, Oxford OX1 3PU, United Kingdom

^*Present address: 4th Physics Institute, University of Stuttgart, Stuttgart, 70550, Germany.

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Vol. 94, Iss. 22 — 1 December 2016

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Figure 1
Equilibrium superconducting response of YBCO and transient THz reflectivity. (a) Crystal structure of YBCO. Gray arrows indicate the apical oxygen excitation. The unit cell can be divided into two regions, the bilayer units and the interbilayer gaps, separated by $Cu O_{2}$ planes. (b) Equilibrium magnetic susceptibility of the five samples of $YB a_{2} C u_{3} O_{x}$ , with $x =$ 6.3 (purple), 6.45 (blue), 6.5 (red), 6.6 (green), and 7 (orange). These correspond to hole dopings of 0.05, 0.07, 0.09, 0.12, and 0.16, respectively. The $x =$ 6.3 sample is non-superconducting, while the other samples have transition temperatures of $T_{c} =$ 35 K, 51 K, 62 K, and 90 K, respectively. (c, top row) Change in the equilibrium THz reflection coefficient upon cooling below $T_{c}$ . Sharp edges are found in the superconducting samples at the Josephson plasma resonance frequency. (c, bottom row) Light induced reflectivity changes measured for different $T > T_{c}$ , at $0.8 ps$ after excitation. Fits with an effective medium model (see main text) are displayed as dashed lines.
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Figure 2
Transient complex optical conductivity. (Top row) Pump-induced changes to the imaginary conductivity $Δ σ_{2} (ω) = σ_{2} (ω, τ = 0.8 ps) - σ_{2, eq} (ω)$ . Gray lines indicate $1 / ω$ fits. A quasiparticle contribution reshapes $Δ σ_{2} (ω)$ in YBCO 6.6. (Middle–bottom rows) Corresponding imaginary (middle row) and real (bottom row) parts of the optical conductivity at equilibrium (gray) and at $τ = 0.8 ps$ (colored by doping), expressed in units of $Ω^{- 1} c m^{- 1}$ . Shaded regions highlight the pump-induced changes. Fits with an effective medium model are shown as dashed lines.
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Figure 3
Phase diagram of the transient state above $T_{c}$ . (a.1) Equilibrium changes in $ω σ_{2} (ω)$ across the superconducting transition, expressed in units of $THz / (Ω \cdot cm)$ . (a.2) Light- induced $ω Δ σ_{2} (ω, τ = 0.8 ps)$ measured in YBCO 4.5 at five different temperatures. Dashed lines indicate the mean values, $〈 ω Δ σ_{2} (ω) 〉$ , extracted to quantify the strength of interbilayer coupling (see main text). (b–e) Frequency-averaged $〈 ω Δ σ_{2} (ω) 〉$ , plotted as a function of temperature (filled circles). Equilibrium $〈 ω Δ σ_{2} (ω) 〉$ values measured at 5 K are also shown (empty circles). Diamonds refer to the photo-susceptible volume fractions, $f$ , extracted from effective medium fits. The region $T < T^{''}$ is indicated by a colored background for each doping. Linear fits to $〈 ω Δ σ_{2} (ω) 〉$ are displayed in this region as gray lines. At $T > T^{''}, 〈 ω Δ σ_{2} (ω) 〉$ is well reproduced by a mean field behavior of the type $\propto \sqrt{1 - T / T^{'}}$ (black dashed lines). (f) $T^{'}$ (red circles) and $T^{''}$ (blue triangles) values, extracted from the fits, are plotted on the YBCO phase diagram. The equilibrium superconducting transition $T_{c}$ , determined by dc magnetization, is denoted by green squares.
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Figure 4
Decoherence-driven relaxation of the transient state. (a) Transient $ω Δ σ_{2} (ω)$ measured in YBCO 6.45 at $T = 100 K$ , for different pump-probe delays. A low-frequency downturn appears below a characteristic frequency $ω^{*}$ . (b) Relaxation of the coherence length $d = 2 ω_{p} L / ω^{*}$ (see main text) of the transient high-mobility state. A double exponential fit is displayed as a dashed line. (c) Corresponding relaxation of $〈 ω Δ σ_{2} (ω) 〉$ , showing similar time constants. (d) Relaxation time constants of $〈 ω Δ σ_{2} (ω) 〉$ , plotted as a function of temperature. The lifetime peaks near $T^{''}$ .
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Figure 5
Splitting of the plasma resonance during relaxation. (a–b) Pump-induced changes in the loss function and in the real part of the optical conductivity, measured in YBCO 6.45 at $T = 200 K$ at two selected time delays. The splitting of the plasma mode (b.1) and the appearance of a peak in $σ_{1} (ω)$ (b.2), observed at $τ = 1.8 ps$ , indicate that two inequivalent interbilayer junctions have formed. Effective medium fits to the data (see main text) are shown as dashed lines. (c) Loss function and real conductivity of the two effective medium components, as extracted from the fits.
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Physical Review B

covering condensed matter and materials physics

Dynamical decoherence of the light induced interlayer coupling in $YB a_{2} C u_{3} O_{6 + δ}$

C. R. Hunt, D. Nicoletti, S. Kaiser, D. Pröpper, T. Loew, J. Porras, B. Keimer, and A. Cavalleri

Phys. Rev. B 94, 224303 – Published 12 December 2016

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Physical Review B

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Dynamical decoherence of the light induced interlayer coupling in YBa2Cu3O6+δ

C. R. Hunt, D. Nicoletti, S. Kaiser, D. Pröpper, T. Loew, J. Porras, B. Keimer, and A. Cavalleri

Phys. Rev. B 94, 224303 – Published 12 December 2016

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Dynamical decoherence of the light induced interlayer coupling in $YB a_{2} C u_{3} O_{6 + δ}$