Breaking electron pairs in the pseudogap regime of ${\mathrm{SmTiO}}_{3}\text{\ensuremath{-}}{\mathrm{SrTiO}}_{3}\text{\ensuremath{-}}{\mathrm{SmTiO}}_{3}$ quantum wells

Letter

Breaking electron pairs in the pseudogap regime of ${SmTiO}_{3} − {SrTiO}_{3} − {SmTiO}_{3}$ quantum wells

Xinyi Wu, Lu Chen, Arthur Li, Megan Briggeman, Patrick B. Marshall, Susanne Stemmer, Patrick Irvin, and Jeremy Levy

Phys. Rev. B 108, L201118 – Published 22 November 2023

Abstract

The strongly correlated two-dimensional electron liquid within ${SmTiO}_{3} / {SrTiO}_{3} / {SmTiO}_{3}$ quantum well structures exhibits a pseudogap phase when the quantum well width is sufficiently narrow. Using low-temperature transport and optical experiments that drive the quantum-well system out of equilibrium, we find evidence of mobile, long-lived, negatively charged quasiparticles, consistent with the idea that the pseudogap phase arises from strongly bound electron pairs. These experimental results establish a connection between quantum confinement and enhanced electron pairing, which may provide a pathway to higher-temperature superconductors.

Received 1 June 2023
Revised 14 October 2023
Accepted 1 November 2023

DOI:https://doi.org/10.1103/PhysRevB.108.L201118

Physics Subject Headings (PhySH)

Electrical properties Pseudogap Superconducting fluctuations Superconducting gap

Complex oxides Low-temperature superconductors Quantum wells Strongly correlated systems

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Xinyi Wu^1,2, Lu Chen^1,2, Arthur Li ^1,2, Megan Briggeman^1,2, Patrick B. Marshall³, Susanne Stemmer³, Patrick Irvin^1,2, and Jeremy Levy ^1,2,*

¹Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
²Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA
³Materials Department, University of California, Santa Barbara, California 93106-5050, USA

^*jlevy@pitt.edu

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Vol. 108, Iss. 20 — 15 November 2023

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Figure 1
(a) Cross-sectional schematic of the ${SmTiO}_{3} / {SrTiO}_{3} / {SmTiO}_{3}$ quantum well structure. The ${SrTiO}_{3}$ quantum well contains two SrO layers. Ti (gray) and Au (yellow) contacts are made to be in direct contact with the conductive ${SrTiO}_{3}$ layer. (b) Schematic of the tunneling device with the experimental setup for two-terminal $I − V$ measurements between the top and interface electrodes, separated by a radial distance of $60 μ m$ . (c) Normalized conductance spectra $[d (ln I) / d (ln V)$ vs $V$ ] of the tunneling device measured at $T = 2 K$ . A pseudogap feature with a characteristic onset voltage of $V_{T} = 27 mV$ . (d) Schematic of ${SmTiO}_{3} / {SrTiO}_{3} / {SmTiO}_{3}$ channel device configured for two-terminal current-voltage measurements between electrodes 2 and 8, separated at a distance of $90 μ m$ . (e) Normalized conductance spectra shows a reduction of conductance near zero voltage and a characteristic onset at $V_{T} = 40 mV$ . (f) Schematic of ${SmTiO}_{3} / {SrTiO}_{3} / {SmTiO}_{3}$ channel device and two-terminal current-voltage measurements between electrodes 1 and 5, separated at a distance of $240 μ m$ . (g) Normalized conductance spectra shows a reduction of conductance near zero voltage and a characteristic onset at $V_{T} = 120 mV$ .
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Figure 2
Four $I − V$ curves taken consecutively under the same configuration shown in Fig. 1. Fluctuations in and around the threshold for conduction are apparent.
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Figure 3
(a) Set up for the dc voltage excitation experiments. A voltage bias $V_{bias}$ is applied between two contacts on contacts labeled 1 and 2 while four-terminal lock-in measurements are performed between contacts 3 and 4. (b) Four-terminal resistance $R = V_{1} / I$ as a function of $V_{bias}$ .
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Figure 4
Transport measurements under nonequilibrium excitation. (a) Experimental setup. The ac driving with magnitude $\frac{1}{2} V_{ac}$ and frequency $f = 1000 Hz$ are applied differentially to two electrodes labeled 2 and 8 on the left-hand side of the channel device. A quasi-dc voltage $V_{sd}$ is applied between electrodes 1 and 5, swept between $- 160$ and 160 mV. (b) Intensity plot of two-terminal conductance versus $V_{sd}$ and $V_{ac}$ . (c) Measured four-terminal conductance in the region between electrodes 3 and 4 for various $V_{ac}$ . Curves are offset for clarity. (d) Four-terminal conductance intensity map versus $V_{1}$ and $V_{ac}$ . (e) Linecut of four-terminal conductance at $V_{sd} = 0$ V.
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Figure 5
(a) Optical setup for the two-terminal photocurrent measurements. (b) Photo-induced current changes $Δ I$ are measured as a function of $V_{dc}$ and temperature. Photocurrent is observed when $V_{dc}$ exceeds a threshold $\approx 300$ mV and for temperatures $T < 30 K$ . Inset graph shows average photocurrent over $V_{dc} = 400$ –500 mV.
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Physical Review B

covering condensed matter and materials physics

Breaking electron pairs in the pseudogap regime of ${SmTiO}_{3} − {SrTiO}_{3} − {SmTiO}_{3}$ quantum wells

Xinyi Wu, Lu Chen, Arthur Li, Megan Briggeman, Patrick B. Marshall, Susanne Stemmer, Patrick Irvin, and Jeremy Levy

Phys. Rev. B 108, L201118 – Published 22 November 2023

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Physical Review B

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Breaking electron pairs in the pseudogap regime of SmTiO3−SrTiO3−SmTiO3 quantum wells

Xinyi Wu, Lu Chen, Arthur Li, Megan Briggeman, Patrick B. Marshall, Susanne Stemmer, Patrick Irvin, and Jeremy Levy

Phys. Rev. B 108, L201118 – Published 22 November 2023

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Breaking electron pairs in the pseudogap regime of ${SmTiO}_{3} − {SrTiO}_{3} − {SmTiO}_{3}$ quantum wells