Possible charge-density wave, superconductivity, and $f$-electron valence instability in ${\mathrm{EuBiS}}_{2}\text{F}$

Possible charge-density wave, superconductivity, and $f$ -electron valence instability in ${EuBiS}_{2} F$

Hui-Fei Zhai, Zhang-Tu Tang, Hao Jiang, Kai Xu, Ke Zhang, Pan Zhang, Jin-Ke Bao, Yun-Lei Sun, Wen-He Jiao, I. Nowik, I. Felner, Yu-Ke Li, Xiao-Feng Xu, Qian Tao, Chun-Mu Feng, Zhu-An Xu, and Guang-Han Cao

Phys. Rev. B 90, 064518 – Published 29 August 2014

Abstract

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states, which have received sustained interest for decades. Here, we report that, in a layered europium bismuth sulfofluoride, ${EuBiS}_{2} F$ , a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane $Bi 6 p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by $\sim 50$ times, owing to the significant hybridizations between $Eu 4 f$ and $Bi 6 p$ electrons, as verified by band-structure calculations. Thus ${EuBiS}_{2} F$ manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and $f$ -electron valence instability.

Received 17 June 2014
Revised 16 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.064518

Authors & Affiliations

Hui-Fei Zhai^1,*, Zhang-Tu Tang^1,*, Hao Jiang^1,*, Kai Xu¹, Ke Zhang¹, Pan Zhang¹, Jin-Ke Bao¹, Yun-Lei Sun¹, Wen-He Jiao¹, I. Nowik², I. Felner², Yu-Ke Li³, Xiao-Feng Xu³, Qian Tao¹, Chun-Mu Feng¹, Zhu-An Xu^1,4,5, and Guang-Han Cao^1,4,5,†

¹Department of Physics, Zhejiang University, Hangzhou 310027, China
²Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
³Department of Physics, Hangzhou Normal University, Hangzhou 310036, China
⁴State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China
⁵Center for Correlated Matter, Zhejiang University, Hangzhou 310027, China

^*These authors contributed equally to this work.
^†Author to whom correspondence should be addressed: ghcao@zju.edu.cn

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Vol. 90, Iss. 6 — 1 August 2014

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Figure 1
Crystal structure and its temperature dependence for ${EuBiS}_{2} F$ . (a) Powder x-ray diffractions and their Rietveld refinement profiles at 300 K (top) and 15 K (bottom). Top inset: the crystal structure of ${EuBiS}_{2} F$ . Bottom inset: an enlarged plot showing the (200) and (220) reflections. (b) Temperature dependence of lattice parameters $a, c, c / a$ , and the bond valence sum of Eu (Eu-BVS). The solid line is a guide to the eye.
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Figure 2
Electrical transport properties and superconductivity in ${EuBiS}_{2} F$ . (a) Temperature dependence of resistivity showing a CDW-like anomaly at 280 K and superconductivity at 0.3 K (inset). (b) The magnetoresistivity as a function of magnetic field at low temperatures. The inset plots the upper critical field, $H_{c 2}$ , by the criteria of 90% $ρ_{n}$ , where $ρ_{n}$ stands for the normal-state resistivity. (c) Temperature dependence of Hall coefficient in which a turning point at 280 K is seen. Upper inset: field dependence of Hall resistivity at some fixed temperatures. Bottom inset: schematic energy-band diagrams for $T < T^{*}$ (left) and $T > T^{*}$ (right), respectively. CB (VB) denotes thy conduction band (valence band) in the ${BiS}_{2}$ bilayers.
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Figure 3
Magnetic properties of ${EuBiS}_{2} F$ . Temperature dependence of the dc magnetic susceptibility measured with an applied field of 1 kOe [(a) $T \leq 100$ K and (b) $T \geq 220$ K]. The solid lines are the fitted curves. The difference (multiplied by 100) between the experimental data and the fitted ones is shown in the lower part of (b). (c) Field dependence of magnetization at 2 K. The dotted line is a Brillouin fit. (d) The Eu valence estimated by the magnetic susceptibility. See details in the text.
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Figure 4
Mössbauer result on ${EuBiS}_{2} F$ . (a) $^{151} Eu$ $M \ddot{o} ssbauer$ spectra of ${EuBiS}_{2} F$ at some typical temperatures. (b) Temperature dependence of the isomer shifts (top), absorption linewidth (middle), and relative intensity of the minor line (bottom) from the data fitting of the $^{151} Eu$ Mössbauer spectra. The vertical dashed lines at $- 13.5$ and $- 0.5$ mm/s (a) and at 280 K (b) are guides to the eye.
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Figure 5
Specific heat capacity for ${EuBiS}_{2} F$ . (a) Temperature dependence of specific heat capacity, $C (T)$ , from 0.5 to 320 K. Upper left inset: an enlarged plot of the $C (T)$ showing an anomaly at 280 K. Lower right inset: plot of $C / T$ vs $T^{2}$ at low-temperature region (the dashed line obeys $C / T = γ + β T^{2}$ ). (b) An enlarged plot of $C (T)$ from 0.5 to 20 K. The dashed line is a polynomial fit (see details in the text), representing the lattice and electronic contributions. (c) $C_{m} / T$ (where $C_{m}$ denotes magnetic contribution to the specific heat) and magnetic entropy $S_{m}$ (left axis) as functions of temperature.
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Figure 6
Band calculations for ${EuBiS}_{2} F$ using $LSDA + U$ method. (a) Variations of $Eu 4 f$ bands with different $U$ values. DOS denotes the density of states. (b) Calculated band structure of ${EuBiS}_{2} F$ with $U = 3$ eV. The contributions of the relevant orbital states are distinguished by different colors. The lower panels zoom in the band dispersions crossing the Fermi level $(E_{F} = 0)$ . (c) Total and projected DOS with $U = 3$ eV. (Inset) Enlarged plot near $E_{F}$ . (d) Fermi surfaces of ${EuBiS}_{2} F$ derived from the band structure in (b).
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Physical Review B

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Possible charge-density wave, superconductivity, and $f$ -electron valence instability in ${EuBiS}_{2} F$

Hui-Fei Zhai, Zhang-Tu Tang, Hao Jiang, Kai Xu, Ke Zhang, Pan Zhang, Jin-Ke Bao, Yun-Lei Sun, Wen-He Jiao, I. Nowik, I. Felner, Yu-Ke Li, Xiao-Feng Xu, Qian Tao, Chun-Mu Feng, Zhu-An Xu, and Guang-Han Cao

Phys. Rev. B 90, 064518 – Published 29 August 2014

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Physical Review B

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Possible charge-density wave, superconductivity, and f-electron valence instability in EuBiS2F

Hui-Fei Zhai, Zhang-Tu Tang, Hao Jiang, Kai Xu, Ke Zhang, Pan Zhang, Jin-Ke Bao, Yun-Lei Sun, Wen-He Jiao, I. Nowik, I. Felner, Yu-Ke Li, Xiao-Feng Xu, Qian Tao, Chun-Mu Feng, Zhu-An Xu, and Guang-Han Cao

Phys. Rev. B 90, 064518 – Published 29 August 2014

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Possible charge-density wave, superconductivity, and $f$ -electron valence instability in ${EuBiS}_{2} F$