Unique interplay between superconducting and ferromagnetic orders in ${\mathrm{EuRbFe}}_{4}{\mathrm{As}}_{4}$

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Unique interplay between superconducting and ferromagnetic orders in ${EuRbFe}_{4} {As}_{4}$

V. S. Stolyarov, A. Casano, M. A. Belyanchikov, A. S. Astrakhantseva, S. Yu. Grebenchuk, D. S. Baranov, I. A. Golovchanskiy, I. Voloshenko, E. S. Zhukova, B. P. Gorshunov, A. V. Muratov, V. V. Dremov, L. Ya. Vinnikov, D. Roditchev, Y. Liu, G.-H. Cao, M. Dressel, and E. Uykur

Phys. Rev. B 98, 140506(R) – Published 22 October 2018

Abstract

Transport, magnetic, and optical investigations on ${EuRbFe}_{4} {As}_{4}$ single crystals evidence that the ferromagnetic ordering of the ${Eu}^{2 +}$ magnetic moments at $T_{m} = 15$ K, below the superconducting transition ( $T_{c} = 36$ K), affects superconductivity in a weak but intriguing way. Upon cooling below $T_{m}$ , the zero resistance state is preserved and the diamagnetic response is only slightly affected by the emerging ferromagnetism; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below $T_{c}$ , associated with the opening of a complete superconducting gap at $2 Δ = 10$ meV. A gap smaller than the weak-coupling limit suggests strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap.

Received 11 July 2018
Revised 11 September 2018
Corrected 28 February 2019

DOI:https://doi.org/10.1103/PhysRevB.98.140506

Physics Subject Headings (PhySH)

Multiband superconductivity Optical conductivity Superconducting gap Vortex lattices

Iron-based superconductors Unconventional superconductors

Condensed Matter, Materials & Applied Physics

Corrections

28 February 2019

Correction: An additional affiliation has been inserted for the seventh author.

Authors & Affiliations

V. S. Stolyarov^1,2,3,*, A. Casano⁴, M. A. Belyanchikov¹, A. S. Astrakhantseva¹, S. Yu. Grebenchuk¹, D. S. Baranov^1,5, I. A. Golovchanskiy^1,6, I. Voloshenko⁴, E. S. Zhukova¹, B. P. Gorshunov¹, A. V. Muratov⁷, V. V. Dremov¹, L. Ya. Vinnikov², D. Roditchev^1,5,8, Y. Liu⁹, G.-H. Cao^9,†, M. Dressel^1,4, and E. Uykur^4,‡

¹Moscow Institute of Physics and Technology, Dolgoprudny, Moscow 141700, Russia
²Institute of Solid State Physics (ISSP RAS), Chernogolovka 142432, Russia
³Kazan Federal University, 18 Kremlyovskaya Street, Kazan 420008, Russia
⁴1. Physikalisches Institut, Universität Stuttgart, D-70569 Stuttgart, Germany
⁵LPEM, ESPCI Paris, PSL Research University, CNRS, F-75005 Paris, France
⁶The Laboratory of Superconducting Metamaterials, National University of Science and Technology MISiS, 4 Leninsky prosp., Moscow 119049, Russia
⁷Lebedev Physical Institute, Russian Academy of Sciences, Moscow 119991, Russia
⁸Sorbonne Université, CNRS, LPEM, F-75005 Paris, France
⁹Department of Physics, Zhejiang University, Hangzhou 310027, China

^*vasiliy@travel.ru
^†ghcao@zju.edu.cn
^‡ece.uykur@pi1.physik.uni-stuttgart.de

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Vol. 98, Iss. 14 — 1 October 2018

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Figure 1
(a) Temperature-dependent dc resistance (blue curve) of ${EuRbFe}_{4} {As}_{4}$ single crystals measured within the $a b$ plane. The red dots correspond to the resistivity extracted from the Hagen-Rubens fit of the reflectivity normalized to dc resistance at room temperature, with $ρ = 0.23 m Ω cm$ . The sharp superconducting transition at $T_{c} = 36.25$ K is magnified in the inset. The structure of the unit cell illustrates the two spacing layers: Eu in blue and Rb in magenta. (b) Magnetization at low temperatures measured by applying a magnetic field $H = 30$ Oe within the $a b$ plane and perpendicular to it, $H ∥ c$ . The phase transitions are indicated by a sharp drop at superconducting $T_{c}$ and a pronounced feature around the magnetic order $T_{m}$ , marked by a red arrow. The distinct behavior of field-cooled (FC) and zero-field-cooled (ZFC) susceptibility characterizes the superconducting state. (c) The completely different field-dependent magnetization for both orientations hallmarks the confinement of the magnetic moments to the $a b$ plane.
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Figure 2
(a) Temperature evolution of the infrared reflectivity of ${EuRbFe}_{4} {As}_{4}$ ; at the lowest temperature the spectra approach unity around 80 ${cm}^{- 1}$ . The inset displays the overall behavior at ambient and low temperatures. (b) The corresponding conductivity spectra reveal the opening of the superconducting gap when the temperature drops below $T_{c} = 36$ K. The solid lines correspond to fits by the Drude-Lorentz model and Mattis-Bardeen equations, respectively. The left inset shows the complete behavior at $T = 300$ and 4 K. The right inset is the SW ratio in the normal state, indicating the SW transfer characteristics.
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Figure 3
(a) Drude-Lorentz fits to the room-temperature optical conductivity of ${EuRbFe}_{4} {As}_{4}$ . (b) In the superconducting state the two Drude modes are replaced by BCS terms [29]. Note the drastic changes in the low-energy bands. (c) The temperature and frequency dependences of the low-lying absorption modes highlight the effects of the phase transitions.
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Figure 4
(a) The superconducting gap of ${EuRbFe}_{4} {As}_{4}$ deviates from the prediction of a mean-field transition. (b) Temperature evolution of the superfluid density; the $s$ -wave behaviors with different coupling constants are shown for comparison. A dip in the dependence is marked by a red arrow.
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Figure 5
Abrikosov vortex lattice in ${EuRbFe}_{4} {As}_{4}$ revealed by magnetic force microscopy. A field of $H = 35$ Oe is applied along the $c$ direction, perpendicularly to the scanned sample surface. (a) For $T_{m} < T < T_{c}$ the main magnetic contrast comes from quite uniformly distributed vortices. (b), (c) For $T < T_{m}$ a large-scale magnetic contrast due to ferromagnetic domain walls adds to the vortex signal; the positions of the vortices are modified. The white scale bars correspond to 2 $μ m$ . (d) The temperature dependence of the vortex density integrated over the scanned area shows a minimum below the ferromagnetic transition, marked by a red arrow, followed by an increase at lower temperature.
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Physical Review B

covering condensed matter and materials physics

Unique interplay between superconducting and ferromagnetic orders in ${EuRbFe}_{4} {As}_{4}$

V. S. Stolyarov, A. Casano, M. A. Belyanchikov, A. S. Astrakhantseva, S. Yu. Grebenchuk, D. S. Baranov, I. A. Golovchanskiy, I. Voloshenko, E. S. Zhukova, B. P. Gorshunov, A. V. Muratov, V. V. Dremov, L. Ya. Vinnikov, D. Roditchev, Y. Liu, G.-H. Cao, M. Dressel, and E. Uykur

Phys. Rev. B 98, 140506(R) – Published 22 October 2018

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Physical Review B

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Unique interplay between superconducting and ferromagnetic orders in EuRbFe4As4

V. S. Stolyarov, A. Casano, M. A. Belyanchikov, A. S. Astrakhantseva, S. Yu. Grebenchuk, D. S. Baranov, I. A. Golovchanskiy, I. Voloshenko, E. S. Zhukova, B. P. Gorshunov, A. V. Muratov, V. V. Dremov, L. Ya. Vinnikov, D. Roditchev, Y. Liu, G.-H. Cao, M. Dressel, and E. Uykur

Phys. Rev. B 98, 140506(R) – Published 22 October 2018

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Unique interplay between superconducting and ferromagnetic orders in ${EuRbFe}_{4} {As}_{4}$