Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$

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Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in ${La}_{2 - x} {Ba}_{x} {CuO}_{4}$

S. J. Zhang, X. Y. Zhou, S. X. Xu, Q. Wu, L. Yue, Q. M. Liu, T. C. Hu, R. S. Li, J. Y. Yuan, C. C. Homes, G. D. Gu, T. Dong, and N. L. Wang

Phys. Rev. X 14, 011036 – Published 4 March 2024

Abstract

The ultrafast manipulation of quantum material has led to many novel and significant discoveries. Among them, the light-induced transient superconductivity in cuprates achieved by melting competing stripe orders represents a highly appealing accomplishment. However, recent investigations have shown that the notion of photoinduced superconductivity remains a topic of controversy, and its elucidation solely through $c$ -axis time-resolved terahertz spectroscopy remains an arduous task. Here, we measure the in-plane and out-of-plane transient terahertz responses simultaneously in the stripe-ordered nonsuperconducting ${La}_{2 - x} {Ba}_{x} {CuO}_{4}$ after near-infrared excitations. We find that although a pump-induced reflectivity edge appears in the $c$ -axis reflectance spectrum, the reflectivity along the ${CuO}_{2}$ planes decreases simultaneously, indicating an enhancement in the scattering rate of quasiparticles. This in-plane transient response is clearly distinct from the features associated with superconducting condensation. Therefore, we conclude the out-of-plane transient responses cannot be explained by an equivalent of Josephson tunneling. Notably, those pump-induced terahertz responses remain consistent even when we vary the near-infrared optical pump wavelengths and hole concentrations. Our results provide critical evidence that transient three-dimensional superconductivity cannot be induced by melting the competing stripe orders with pump pulses whose photon energy is much higher than the superconducting gap of cuprates.

Received 16 June 2023
Revised 2 January 2024
Accepted 24 January 2024

DOI:https://doi.org/10.1103/PhysRevX.14.011036

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Optical conductivity

High-temperature superconductors

Optical-pump terahertz-probe spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. J. Zhang ^1,*, X. Y. Zhou¹, S. X. Xu ¹, Q. Wu¹, L. Yue ¹, Q. M. Liu¹, T. C. Hu¹, R. S. Li ¹, J. Y. Yuan ¹, C. C. Homes ², G. D. Gu³, T. Dong¹, and N. L. Wang ^1,4,5,†

¹International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China
²National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA
³Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
⁴Beijing Academy of Quantum Information Sciences, Beijing 100913, China
⁵Collaborative Innovation Center of Quantum Matter, Beijing, China

^*sjzh@pku.edu.cn
^†nlwang@pku.edu.cn

Popular Summary

The discovery of transient, light-induced superconductivity in high-temperature superconducting cuprates (HTSCs) has sparked both interest and controversy. One problem is that the observational signature of the transient superconductivity is itself vague: Recent experiments suggest it cannot distinguish between electron Cooper pairs that give rise to superconductivity and quasiparticles with very low scattering rates. Here, we more closely explore the electrodynamics of one HTSC and find that it is not consistent with superconductivity.

The primary indicator of transient superconductivity has been a sharp drop in the reflectivity along one of the crystalline axes, the $c$ axis, of incident terahertz waves below a certain wavelength. Measurements of the terahertz response along the ${CuO}_{2}$ planes in an HTSC could clarify things: If a transient superconducting state appears along the $c$ axis, a similar response should be seen within the ${CuO}_{2}$ planes too.

To that end, we investigate the terahertz responses parallel and perpendicular to the $c$ axis of ${La}_{2 - x} {Ba}_{x} {CuO}_{4}$ . We observe an emergent light-induced reflectivity edge along the $c$ axis, along with a simultaneous decrease of the in-plane reflectivity. Notably, this in-plane behavior is diametrically opposed to the spectral response of superconducting condensation, signifying the vanishing of in-plane superconductivity. These findings significantly enhance our comprehension of light-induced transient states in HTSCs and contribute new insights into the emerging frontier of ultrafast optical manipulation of quantum materials.

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Vol. 14, Iss. 1 — January - March 2024

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Figure 1
(a) A schematic diagram of the optical-pump–terahertz-probe measurements on ${La}_{2 - x} {Ba}_{x} {CuO}_{4}$ ( $x = 0.114$ and 0.125), in which the in-plane stripe order severely frustrates the interlayer Josephson coupling near the doping level $x = 0.125$ . In the actual experiments, the terahertz probe beam is fixed to $s$ polarization, and the transient in-plane and out-of-plane terahertz responses are investigated by rotating the sample. (b) The phase diagram of ${La}_{2 - x} {Ba}_{x} {CuO}_{4}$ (reproduced from Ref. [18]). At the doping level $x = 0.125$ , superconductivity (SC) emerges at 2 K, charge stripe order (CO) at 54 K, and spin stripe order (SO) at 42 K, while for the $x = 0.114$ compound, $T_{SC} = 17 K$ , $T_{CO} = 50 K$ , and $T_{SO} = 40 K$ . The colored circles indicate the transient terahertz responses reported here. (c) The broadband in-plane and out-of-plane reflectance spectra at 5 K. The black dashed lines indicate the pump wavelengths used for excitations: 1300 and 800 nm. (d) The decay of the relative change of the reflected terahertz electric field $Δ E / E_{peak}$ in the $x = 0.125$ sample after excitations with 1300-nm pump polarized along the $c$ axis under an incident fluence of $1 mJ / {cm}^{2}$ at 5 K. The colored dots indicate the delay times of the transient optical responses reported here. (e) A pump-induced edge appears in the $c$ -axis reflectance spectrum of the $x = 0.125$ sample. “Static” indicates the delay time before the pump arriving. (f) The in-plane transient reflectivity decreases after excitations, which significantly differs from the effects of superconducting condensation.
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Figure 2
Transient terahertz responses along the $c$ axis of the $x = 0.125$ sample at 5 K after excitations with 1300-nm pump polarized along the $c$ axis under an incident fluence of $1 mJ / {cm}^{2}$ . (a) displays a pump-induced emergence of a peak in the energy-loss function, indicating a transient reflectivity edge after excitations. The (b) real and (c) imaginary parts of the transient optical conductivity. At 0.6 ps, $σ_{2} (ω)$ near zero frequency increases accompanied by $σ_{1} (ω)$ remaining unchanged. The low-frequency measurement range poses a challenge in determining whether the transient response is contributed by quasiparticles with an extremely low scattering rate or superfluid carriers. After 0.9 ps, the pump-induced quasiparticles are evidenced by a Drude-like peak in $σ_{1} (ω)$ .
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Figure 3
Terahertz responses along the ${CuO}_{2}$ planes of the $x = 0.125$ sample. (a) Temperature-dependent terahertz responses in the equilibrium state. The (b) real and (c) imaginary parts of the transient optical conductivity after excitations with 1300-nm pump polarized along the $c$ axis under an incident fluence of $1 mJ / {cm}^{2}$ at 5 K. (d) The frequency-dependent scattering rate in the extended Drude model, with the gray dashed line plotting $1 / τ (ω) = ω$ , above which the transport is highly dissipative and far away from a Landau-Fermi liquid. The increase of $σ_{1} (ω)$ and $1 / τ (ω)$ accompanied by a decrease in $σ_{2} (ω)$ indicate an excitation of quasiparticles and enhancement of transport dissipation. There is no evidence for the emergence of transient superconducting condensate.
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Figure 4
Transient terahertz responses of the $x = 0.114$ sample after excitations with a 800-nm pump polarized along the $c$ axis at 20 K. (a) The decay of $Δ E / E_{peak}$ after excitations by an incident fluence of $2 mJ / {cm}^{2}$ , which is qualitatively similar to that observed in the $x = 0.125$ sample. (b) A pump-induced edge appears in the $c$ -axis reflectance spectrum, becoming more significant with increasing pump fluence. (c) The in-plane transient reflectivity decreases after excitation. The (d) real and (e) imaginary parts of the transient in-plane optical conductivity. (f) The frequency-dependent scattering rate. All transient properties are taken at the time delay with a maximum response.
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Figure 5
The in-plane transient terahertz responses of the $x = 0.114$ sample are investigated under various optical excitation conditions. (a) and (b) present the results dependent on the pump wavelengths. (c) and (d) demonstrate that altering the pump polarization has no significant impacts.
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Figure 6
The out-of-plane transient responses of the $x = 0.125$ sample calculated with the multilayer model. This calculation utilizes identical raw data acquired at 0.6 ps after excitations with a 1300-nm pump polarized along the $c$ axis under an incident fluence of $1 mJ / {cm}^{2}$ at 5 K, but by substituting different penetration depths of the pump pulse into the calculation. The calculated transient responses are highly sensitive to the chosen penetration depth when it is shorter than $1 μ m$ .
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Physical Review X

Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in La2−xBaxCuO4

S. J. Zhang, X. Y. Zhou, S. X. Xu, Q. Wu, L. Yue, Q. M. Liu, T. C. Hu, R. S. Li, J. Y. Yuan, C. C. Homes, G. D. Gu, T. Dong, and N. L. Wang

Phys. Rev. X 14, 011036 – Published 4 March 2024

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Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in ${La}_{2 - x} {Ba}_{x} {CuO}_{4}$