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Correlation between superconductivity and
antiferromagnetism in Rb0.8Fe2Se2-xTex single crystals
Dachun Gu, Liling Sun*, Qi Wu, Chao Zhang, Jing Guo, Peiwen Gao,
Yue Wu, Xiaoli Dong, Xi Dai and Zhongxian Zhao*
Institute of Physics and Beijing National Laboratory for Condensed Matter Physics,
Chinese Academy of Sciences, Beijing 100190, China
We report the first experimental evidence for the intimate connection between
superconductivity and antiferromagnetism in Rb0.8Fe2Se2-xTex single crystal under
negative chemical pressure by substituting Se with isovalent Te atoms. Electrical
resistance measurements in the temperature range from 4 K to 550 K demonstrate that
both superconducting transition temperature (Tc) and Neel temperature (TN) were
suppressed continuously with the lattice expansion. When the Te concentration x in
Rb0.8Fe2Se2-xTex approaches 0.3, the superconducting transition temperature Tc is
completely suppressed and the sample behaves like a semiconductor, meanwhile the
characteristic peak of antiferromagnetic transition on resistance curve disappears. Our
observation suggests that the pressure-induced lattice expansion can be used to tune
the correlativity of superconductivity and antiferromagnetism.
PACS numbers: 74.70.Xa, 74.25.Dw, 74.62.Fj
*Corresponding author: llsun@aphy.iphy.ac.cn and zhxzhao@aphy.iphy.ac.cn
1
The discovery of superconductivity in LaFeAsO1-xFx with Tc as high as 26 K [1]
has attracted considerable attention in the world. During the past three years, several
other types of iron-based superconductors with 122 structure (MFe2As2, M=Ca, Sr, Ba
and Eu) [2-5], 111 structure (AFeAs, A=Li and Na) [6-7], 11 structure (FeSe) [8] and
42622 structure (Sr4V2O6)Fe2As2) [9] were found. Among them, the highest Tc has
reached 55K in SmFeAsO1-xFx [10]. Recently, another new family of superconductors
MxFe2-ySe2(M=K, Rb, Cs, or Tl substituted K, Rb) with Tc above 30 K and other
unusual features were discovered [11-15], which simulated great interest in the
community of condensed matter physics and material science. MxFe2-ySe2
superconductors show a number of peculiar features including electron-dominated
carriers in the Fermi surface [16-19], high transition temperature of antiferromagnetic
phase, a superstructure transition of Fe vacancies, and large ordering magnetic
moments [20-21]. Application of internal or external positive pressure on these
superconductors showed that the superconducting transition temperature was
suppressed with increasing pressure [22-24].
In this work, we investigate the effect
of negative pressure by substituting selenium (Se) with isovalent tellurium (Te) in
Rb0.8Fe2Se2. As in the case of positive pressure, we found that the superconducting
critical temperature is suppressed with an increment of Te concentration.
To our
surprise, we find that both the superconducting and antiferromagnetic orders vanish at
the same time when the doping concentration of Te approaches to 0.3, indicating the
inherent relationship between superconductivity and antiferromagnetism in this
material. By comparison with positive pressure effect, we propose that pressure-free
2
sample (neither positive nor negative pressure) possesses an optimal lattice
configuration in favor of the highest transition temperature of superconductivity.
Single crystals of Rb0.8Fe2Se2-xTex were grown by a self-flux method with several
steps. First, precursors of FeSe and FeTe were synthesized by solid reaction method.
High purity Fe, Se and Te powder were mixed together with nominal composition of
Fe:Se and Fe:Te in a mortar. The mixture was put in a furnace and heated to 700 ℃
with a rate of 100℃/hour, kept at this temperature for 24 hours and then cooled down
to room temperature naturally. Second, the precursors were mingled with Rb in a
glovebox and loaded into an alumina crucible, then sealed in an evacuated silica
ampoule. Third, the sealed silica tube was placed in a furnace, slowly heated to
1000℃ and kept at this temperature for 5 hours, afterward heated up to 1100℃ and
kept for another 5 hours. Finally, the samples were cooled down to 800℃ with a rate
of 4℃/hour, followed by shutting off the power of the furnace.
The resulting samples were characterized by x-ray diffractometer with Cu K α
radiation (λ=1.5418Å) in the 2 θ scan mode, as shown in Fig 1a. Sharp (00l) peaks
reflect that the sample orientates well in the c direction. Electrical resistance and
magnetic susceptibility measurements below 300 K were performed with physical
property measurement system (PPMS-9) and superconducting quantum interference
device (SQUID-XL1). As shown in Fig.1b and c, a clear superconducting transition at
31.4K determined by the resistance measurement and at 30K detected by magnetic
susceptibility
measurement
was
observed,
superconductivity in Rb0.8Fe1.8Se2.
3
indicating
a
bulk
nature
of
To investigate how the tellurium (Te) substitution influences the lattice distortion,
we first performed x-ray diffraction measurements at room temperature for
Rb0.8Fe2Se2-xTex. As shown in Fig.2a, all samples can be well indexed to tetragonal
structure, demonstrating that no phase transition occurs in the substitution range
investigated. However, the lattice expansion was clearly observed with increasing Te
concentration (Fig.2b) due to the substitution of tellurium whose ionic radius of Te is
larger than that of Se. The expansion in the c direction (1.3%) is slightly larger than
that in the a direction (0.7%). Fig.2c shows how the volume of a unit cell grows with
increasing Te concentration. It confirms that Te substitution induces a negative
pressure, as expected.
Figure 3 shows the temperature dependence of electrical resistance for four
samples with different Te concentrations. We found that with increasing negative
pressure, superconducting transition temperature (Tc) of the sample with x smaller
than 0.25 was suppressed (Fig.3a). For the sample with higher Te concentration
(x ≥ 0.3), the superconductivity is fully destroyed and its resistance exhibits
semiconducting behavior (Fig.3b). The response of superconductivity to negative
pressure in Te-doped Rb0.8Fe2Se2-xTex is similar to that to positive pressure in S-doped
KxFe2-ySe2 or compressed K0.8Fe1.7Se2 [22-24]. Figure 4 compares the pressure
dependences of Tc and the sample volume in the positive and negative pressure cases.
Previous
studies
on
FeAs-based
superconductors
showed
that
the
superconductivity is very sensitive to lattice distortion [25]. This distortion consists of
the change in As-Fe-As angle [26] and the anion height [27] which can be produced
4
either by chemical or physical pressure [28]. To inspect the effect of the lattice
distortion on observed suppression of superconductivity in Rb0.8Fe2Se2-xTex, we
computed the Se-Fe-Se angle and the anion height, based on XRD data, together with
Tc as a function of Te concentration (negative pressure). As shown in Fig.5, a negative
pressure leads to an enhancement of anion height and a reduction of the Se-Fe-Se
angle. As the anion height increases or the bond angle decreases, the superconducting
temperature goes down. At x=0.3, where the anion height and the bond angle change
their trend, the superconducting transition of Rb0.8Fe2Se2-xTex is fully suppressed. Our
results indicated that the superconductivity of Rb0.8Fe2Se2-xTex is governed by the
level of their lattice distortion.
To identify the effect of negative pressure on the long-range order of
antiferromagnetism and the formation of Fe-vacancies in high temperature regime,
which are related to superconductivity of the iron chalcogenides, we measured
electrical resistance from 300 to 550 K for Rb0.8Fe2Se2-xTex single crystals. The
resistance and its derivate as a function of temperature were plotted in Fig.6.
There
are two dips on the dR/dT curve. One is associated with the formation of Fe-vacancy
order (whose transition temperature is defined as TS), the other is related to the
antiferromagnetic phase transition (whose transition temperature is defined as TN)
[29]. For Te-free sample, we found that its TS and TN are about 536 K and 493K
respectively, which are slightly lower than the corresponding values reported in Ref.
[29]. This may be due to the subtle difference of samples.
Both TN and TS decrease
with increasing Te concentration. This indicates that lattice expansion favors the
5
formation
of
Fe
vacancies
as
well
as
the
transition
of
paramagnetic-to-antiferromagnetic phase. Surprisingly, we found that TN vanishes at
x≥0.3 where the sample lost its superconductivity. Figure 7 shows the phase diagrams
of Tc (P) and TN (P). The simultaneous disappearance of Tc and TN in Rb0.8Fe2Se2-xTex
suggests that the superconductivity is correlated with antiferromagnetism. Recently,
µ sR, Mossbauer, neutron scattering and Raman experiments also showed that the
superconducting long range order in MFe2Se2 coexists with the antiferromagnetic
long range order [20 , 29-32]. The superconducting pairing mechanism in this
intercalated MFe2Se2 superconductor remains an open question.
In summary, we show that the partial substitution of selenium by tellurium atoms
can create negative pressure which leads to the lattice expansion. Transport
measurements
suggest
that
superconductivity
and
antiferromagnetism
of
Rb0.8Fe2Se2-xTx single crystals are suppressed by the Te substitution. At x= 0.3, both
the superconducting and antiferromagnetic orders are completely destroyed and the
sample becomes semiconducting.
This indicates that the superconductivity in
Rb0.8Fe2Se2-xTex is correlated to its antiferromagnetism. Furthermore, our results
indicate that the superconductivity in Rb0.8Fe2Se2-xTex is intimately related to the level
of their lattice distortion. Comparing with the effect of positive pressure, application
of negative pressure is also unfavorable to achieve the optimal value for
superconductivity of Rb0.8Fe2Se2-xTex (x<0.3).
6
Acknowledgement
We would like to thank P. C. Dai, T. Xiang,X. J. Chen and Z. Fang for useful
discussions. This work was supported by NSCF (10874230 and 11074294), 973
projects (2010CB923000 and 2011CBA00109), and Chinese Academy of Sciences.
Figure captions:
Fig.1 (a) (Color online) X-ray diffraction (XRD) pattern of Rb0.8Fe2Se2 single crystal,
displaying only (00l) peaks. (b) Temperature dependence of electrical resistance. (c)
Magnetic susceptibility as a function of temperature for zero-field-cooling and
field-cooling processes.
Fig.2 (a) (Color online) X-ray diffraction patterns of powdered Rb0.8Fe2Se2-xTex which
is ground from the single crystals. (b) Lattice parameters of Rb0.8Fe2Se2-xTex as a
function of Te concentration. (c) Te substitution dependence of volume, showing that
Te substitution expands the lattice.
Fig.3 (a) (Color online) Temperature dependence of resistance for Rb0.8Fe2Se2-xTex
(x=0~0.25) at zero magnetic field. The inset displays the decrease of Tc with Te
substitution. (b) Resistance as a function of temperature for Rb0.8Fe2Se2-xTex (x ≥0.3),
showing typical semiconducting behavior.
7
Fig.4 (Color online) (a) and (c) Tc and volume as a function of S concentration in
KxFe2-ySe2-zSz, data in (a) and (c) are from Ref. 24. (b) and (d) as a function of Te
concentration in Rb0.8Fe2Se2-xTex.
Fig.5 (Color online) (a) Se-Fe-Se angle dependence of Te concentration in
Rb0.8Fe2Se2-xTex. (b) Anion height from Fe layer as a function of Te concentration.
Fig. 6 (Color online) (a) High temperature resistance for Rb0.8Fe2Se2-xTex (x=0~0.4)
from 250K to 550K. (b) The first derivation of the temperature resistance, exhibiting
two important transitions.
Fig.7 (Color online) Antiferromagnetic and superconducting phase diagram of
Rb0.8Fe2Se2-xTex single crystals.
8
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