Orbitally selective resonant photodoping to enhance superconductivity

Ta Tang, Yao Wang, Brian Moritz, and Thomas P. Devereaux

Phys. Rev. B 104, 174516 – Published 23 November 2021

Abstract

Signatures of superconductivity at elevated temperatures above $T_{c}$ in high-temperature superconductors have been observed near 1/8 hole doping for photoexcitation with infrared or optical light polarized either in the $Cu O_{2}$ plane or along the $c$ axis. While the use of in-plane polarization has been effective for incident energies aligned to specific phonons, $c$ -axis laser excitation in a broad range between 5 $μ m$ and 400 nm was found to affect the superconducting dynamics in striped ${La}_{1.885} {Ba}_{0.115} Cu O_{4}$ , with a maximum enhancement in the $1 / ω$ dependence to the conductivity observed at 800 nm. This broad energy range, specifically 800 nm, is not resonant with any phonon modes, yet induced electronic excitations appear to be connected to superconductivity at energy scales well above the typical gap energies in the cuprates. A critical question is, What can be responsible for such an effect at 800 nm? Using time-dependent exact diagonalization, we demonstrate that the holes in the $Cu O_{2}$ plane can be photoexcited into the charge reservoir layers at resonant wavelengths within a multiband Hubbard model. This orbitally selective photoinduced charge transfer effectively changes the in-plane doping level, which can lead to an enhancement of $T_{c}$ near the 1/8 anomaly.

Received 20 June 2021
Revised 7 September 2021
Accepted 8 November 2021

DOI:https://doi.org/10.1103/PhysRevB.104.174516

Physics Subject Headings (PhySH)

Superconducting phase transition Superconductivity

High-temperature superconductors Strongly correlated systems

Exact diagonalization Hubbard model

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ta Tang ¹, Yao Wang ², Brian Moritz ^3,4, and Thomas P. Devereaux^3,5,6,*

¹Department of Applied Physics, Stanford University, Stanford, California 94305, USA
²Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29631, USA
³Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
⁴Department of Physics and Astrophysics, University of North Dakota, Grand Forks, North Dakota 58202, USA
⁵Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
⁶Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA

^*Corresponding author: tpd@stanford.edu

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Vol. 104, Iss. 17 — 1 November 2021

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Images

Figure 1
(a) Phase diagram near the $1 / 8$ doping anomaly, where there is a local minimum of $T_{c}$ for the superconducting phase. SO, CO, and SC represent spin order, charge order, and superconductivity, respectively. (b) A schematic of the copper oxide plane with apical oxygen from the charge reservoir layers. The $z$ direction is parallel to the $c$ axis. The apical oxygen $p_{z}$ orbitals are slightly transparent. Near-infrared photoexcitation (gray pulse) transfers charge (red dot) from the $Cu O_{2}$ plane to the apical oxygen orbitals in the charge reservoir layers.
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Figure 2
(a) and (b) are orbital-resolved single-particle spectral functions at momentum ( $π$ , 0) for 0% and 25% dopings, respectively. The red line represents copper orbitals (denoted by Cu), the blue line denotes in-plane oxygen orbitals ( $O_{p}$ ), and the green line represents apical oxygen orbitals ( $O_{a}$ ). The shaded (dashed) curves represent hole additional (removal) spectral functions. The vertical dashed line denotes the middle of the highest occupied state and the lowest unoccupied state. The arrows in (a) and (b) mark possible transitions between in-plane bands and apical oxygen bands. (c) and (d) are the regular part of the $c$ -axis optical conductivity for 0% and 25% dopings, respectively.
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Figure 3
The color map indicates density change $Δ n (t_{f})$ on apical oxygen due to holes transferred to apical oxygen $p_{z}$ orbitals from in-plane orbitals after being excited by pulses with different frequencies and fluences. (a) and (b) are results for 0% and 25% dopings, respectively. The magenta arrow in (a) and green arrow in (b) annotate the lowest allowed transitions in each doping case and correspond to the arrows with the same color in Figs. 2 and 2. (c) Time evolution of the density change on different orbitals. The $c$ -axis polarized pump pulse $E (t)$ with a frequency of 3.4 eV and a fluence of 0.2 mJ/ ${cm}^{2}$ is shown as a gray line. This frequency is marked by the magenta arrow in (a). (d) Similar to (c), but for the 25% doped case and a pump frequency of 1.7 eV, which is annotated by the green arrow in (b). The black dots in (c) and (d) mark $Δ n (t_{f})$ on apical $O_{a}$ orbitals.
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Figure 4
We plot $Δ N_{O_{a}} (t) = N_{O_{a}} (t) - N_{O_{a}} (t_{i})$ when excited by a $c$ -axis polarized pulse. $N_{O_{a}} (t)$ is the number of holes on apical oxygen orbitals at time $t$ . The different pulse frequencies correspond to those indicated by the arrows in Fig. 3, and the pulse fluence is set to be 0.2 mJ/ ${cm}^{2}$ . In order to see the small changes induced by the 0.25, 0.6, and 3.0 eV pump pulses, we used a log scale for $Δ N_{O_{a}} (t)$ .
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