Revisiting the phase diagram of ${\mathrm{LaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{AsO}$ in single crystals by thermodynamic methods

Revisiting the phase diagram of ${LaFe}_{1 - x} {Co}_{x} AsO$ in single crystals by thermodynamic methods

F. Scaravaggi, S. Sauerland, L. Wang, R. Kappenberger, P. Lepucki, A. P. Dioguardi, X. Hong, F. Caglieris, C. Wuttke, C. Hess, H.-J. Grafe, S. Aswartham, S. Wurmehl, R. Klingeler, A. U. B. Wolter, and B. Büchner

Phys. Rev. B 103, 174506 – Published 10 May 2021

Abstract

In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies, we track the doping evolution of the antiferromagnetic phase to reveal a continuous suppression of $T_{N}$ up to 5% Co doping. To study the evolution of the so-called nematic phase, the temperature dependence of the length changes along the $a$ and $b$ orthorhombic directions, $Δ L / L_{0}$ , was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion $δ$ and of $T_{S}$ with increasing Co content up to 4.5%, while it is completely suppressed for 7.5% Co. Bulk superconductivity was found in a small doping region around 6% Co content, while both $T_{c}$ and the superconducting volume fraction rapidly drop in the neighboring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped ${BaFe}_{2} {As}_{2}$ .

Received 18 December 2020
Revised 5 March 2021
Accepted 9 April 2021

DOI:https://doi.org/10.1103/PhysRevB.103.174506

Physics Subject Headings (PhySH)

Nematic order Spin density waves Superconductivity

Iron-based superconductors

DC susceptibility measurements Specific heat measurements Thermodilatometry

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

F. Scaravaggi ^1,2,*, S. Sauerland³, L. Wang³, R. Kappenberger^1,2, P. Lepucki ¹, A. P. Dioguardi¹, X. Hong^1,†, F. Caglieris ^1,‡, C. Wuttke¹, C. Hess^1,4,†, H.-J. Grafe¹, S. Aswartham ¹, S. Wurmehl^1,2, R. Klingeler ^3,5, A. U. B. Wolter^1,§, and B. Büchner^1,2

¹Institute for Solid State Research, Leibniz IFW Dresden, 01069 Dresden, Germany
²Institute of Solid State and Materials Physics and Würzburg-Dresden Cluster of Excellence ct.qmat, TU Dresden, D-01062 Dresden, Germany
³Kirchhoff Institut für Physik, Heidelberg University, 69120 Heidelberg, Germany
⁴Center for Transport and Devices, TU Dresden, 01069 Dresden, Germany
⁵Center for Advanced Materials, Heidelberg University, INF 225, D-69120 Heidelberg, Germany

^*f.scaravaggi@ifw-dresden.de
^†Present address: Fakultät für Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, 42097 Wuppertal, Germany.
^‡Present address: CNR-SPIN, Corso Perrone 24, 16152 Genova, Italy.
^§a.wolter@ifw-dresden.de

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Vol. 103, Iss. 17 — 1 May 2021

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Figure 1
Normalized magnetization $M / H$ for the ${LaFe}_{1 - x} {Co}_{x} AsO$ series: (a) parent compound, (b) 2.5%, (c) 3% (taken from Ref. [42]), (d) 3.5%, (e) 4.5%, (f) 5%, (g) 6%, (h) 7.5% nominal Co content. A field of $μ_{0} H = 1$ T was applied within the $a b$ plane (in some cases a field of 6 T was applied in order to improve the resolution). The insets for panels (a)–(e) show the temperature derivative of $M / H$ , while for panels (f)–(h) the volume susceptibility is shown for $H = 10$ Oe at low temperature for both ZFC and FC measurement conditions [49].
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Figure 2
Specific heat of ${LaFe}_{1 - x} {Co}_{x} AsO$ for (a) the parent compound and for 7.5% Co as well as (b) for 6% Co doping. The inset shows the determination of the superconducting transition temperature from the subtracted signal $Δ C_{p} / T = \frac{C_{p(0 T)} - C_{p(9 T)}}{T}$ , approximating the specific-heat contribution from the superconducting phase. The offset in $C_{p}$ for $T > 150 K$ between the 0% and 7.5% Co is most probably due to the difference in the sample coupling at high temperature.
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Figure 3
Thermal expansion of the ${LaFe}_{1 - x} {Co}_{x} AsO$ series: (a) normalized length changes $Δ L / L_{0}$ , (b) linear thermal-expansion coefficient $α_{b}$ , and (c) orthorhombic order parameter $δ$ . The dashed lines represent the structural transition temperatures $T_{S}$ for different Co contents, while the magnetic transition temperatures $T_{N}$ are marked by arrows in panel (b). The inset of panel (c) shows a schematic representation of the twinned and detwinned measurement configurations. The curves for 0%, 2.5%, and 3% Co are taken from Ref. [46]. Note that the pressure applied to each crystal in the series varies due to the specific geometry and dimensions of each sample.
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Figure 4
Uniaxial pressure dependence of the thermal-expansion coefficient in the detwinned direction ( $α_{b}$ ) of ${LaFe}_{1 - x} {Co}_{x} AsO$ for (a) the parent compound and (b) 4.5% Co doping [54]. It has to be noted that the application of increasing uniaxial pressure does not substantially increase the overall orthorhombic distortion, confirming the almost complete detwinning of the measured samples.
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Figure 5
Anomalies in the linear thermal-expansion coefficient for the $a$ and $b$ axes for 0%, 2.5%, and 3% Co content. The nonmagnetic contribution was approximated by using the thermal expansion coefficient obtained for the overdoped 7.5% Co, which does not show any transition in any of the thermodynamic quantities under studies.
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Figure 6
Phase diagram of Co-doped LaFeAsO single crystals. Points from NMR spectroscopy and resistivity on the same series of single crystals were added from Refs. [40] and [39], respectively. The shaded area within the orthorhombic-paramagnetic phase represents the doping region (4.5%–6% Co) where direct lattice probes are not available. The phase boundaries for the nematic state in this region are extrapolated from the data points of neighboring compositions. The color code in the superconducting dome reflects the rapid decrease of volume fraction estimated from magnetization measurements around the optimally doped 6% Co content, as described in Sec. 3a.
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Revisiting the phase diagram of ${LaFe}_{1 - x} {Co}_{x} AsO$ in single crystals by thermodynamic methods

F. Scaravaggi, S. Sauerland, L. Wang, R. Kappenberger, P. Lepucki, A. P. Dioguardi, X. Hong, F. Caglieris, C. Wuttke, C. Hess, H.-J. Grafe, S. Aswartham, S. Wurmehl, R. Klingeler, A. U. B. Wolter, and B. Büchner

Phys. Rev. B 103, 174506 – Published 10 May 2021

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Revisiting the phase diagram of LaFe1−xCoxAsO in single crystals by thermodynamic methods

F. Scaravaggi, S. Sauerland, L. Wang, R. Kappenberger, P. Lepucki, A. P. Dioguardi, X. Hong, F. Caglieris, C. Wuttke, C. Hess, H.-J. Grafe, S. Aswartham, S. Wurmehl, R. Klingeler, A. U. B. Wolter, and B. Büchner

Phys. Rev. B 103, 174506 – Published 10 May 2021

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Revisiting the phase diagram of ${LaFe}_{1 - x} {Co}_{x} AsO$ in single crystals by thermodynamic methods