Photon Factory Activity Report 2016 #34 (2017)

BL-8B, AR-NE1A/2013S2-002, 2016S2-004

Peculiar response to pressure on iron-based superconductor LaFeAsO1-xHx
Kensuke Kobayashi1 and Jun-ichi Yamaura2,*
1
Condensed Matter Research Center, Institute of Materials Structure Science, KEK,Tsukuba, Ibaraki 305-0801, Japan
2
Materials Research Center for Element Strategy, Tokyo Institute of Technology,
Yokohama, Kanagawa 226-8503, Japan

1 Introduction
Iron pnictides are new family of high-temperature
superconductors, whose charge carriers play an activerole primarily on the two-dimensional iron plane [1].
Pnictide (Pn) atomic position acts major perturbations forthe 3d multi-orbital bands of irons through thehybridization of Fe-3d and Pn-p electrons. As aconsequence, chemical substitution along with carrierdoping/chemical-pressure or pressurizing crystal caninduce drastic alteration for solid state properties such as
emerging and developing the Tc. A lot of ensembles ofchemical substitution to iron pnictides have provided anempirical guide that the Tc is maximized when the FePn4
tetrahedron approaches the regular one. Meanwhile,
although pressure is a direct and clean way to modify thelocal geometry of FePn4 without carrier concentration anddegrading crystal by chemical substitution, the correlationof the superconductivity and the structure remains
explored under pressure.
One of the significant fascinating materials in ironpnictides is LaFeAsO1-xHx, composed of ZrCuSiAs-type
structure with alternating stacking of the conducting
FeAs4 and the insulating (O,H)La4 layers. LaFeAsO1-xHx
exhibits a peculiar phase diagram via hydrogen anion
substitution i.e. doping the electron: two superconducting
domes with Tc,max = 26 K at x ~ 0.08 (SC1) and Tc,max = 37
K at x ~ 0.35 (SC2) and two parent phases at x ~ 0 (PP1)and x ~ 0.5 (PP2) [2]. The second SC2 and PP2 phases
are barely observed among iron-based as well as copperoxide superconductors because there is usually nothing in
the heavy electron-doped region. Takahashi et al. hasrecently identified that the applying pressure toLaFeAsO0.72H0.18 induces a notable enhancement of Tc
from 18 K at ambient pressure to 52 K at 6 GPa, alongwith forming single SC dome from two SC domes [3].
The maximum Tc under pressure is close to the highest
class of Tc = 55 K in Sm1111 among the iron-based
superconductors.
2 Experiment
The synchrotron X-ray diffractions (sXRD) for x = 0,0.20 and 0.51 were carried out at room temperature up to~8 GPa at NE1A of PF-AR, KEK. Pressure was generated
by a diamond anvil cell (DAC) with 600 μm culet
diamond anvils and a gasket with 300 μm hole. The twodimensional images collected by RIGAKU R-AXIS on

the curved imaging plate were integrated to yield 2θintensity data. Crystal structures were refined by Rietveld
method based on the space group P4/nmm, and thereliable factors in the refinements are Rwp = 3.9-6.6%. We
moreover traced the PP2 by means of the sXRD down to
8 K up to 3.2 GPa at BL-8B of PF, KEK, measured at λ =0.8267 Å. A 4:1 methanol-ethanol mixture was used as apressure transmitting medium in all the experiments.
3 Results and Discussion
Figure 1 illustrates the contour plots of Tc for αAs-Fe-As
and dFe-As of LaFeAsO1-xHx under pressure. Pressuretriggers to merge two SC domes at ambient pressure into
the single SC dome, along with the Tc growing up to 52 K
at 6 GPa for x = 0.2, and then the Tc gently go down. In
the single Tc dome region, the ridge line of Tc runs along
the variation for x = 0.20 as the pressure increased.
In iron-based superconductors, the relation between theoptimum Tc and the structural parameters of FeAs4 arehitherto proposed: the highest Tc realizes on the αAs-Fe-As =109.5° as a regular tetrahedron of FeAs4 or hAs ~ 1.38 Å.
Actually, SmFeAsO0.78H0.22 with the highest class Tc = 55
K in iron pnictides gives nearly ideal values of the αAs-FeAs = 109.3° and the hAs = 1.386 Å at ambient pressure.
Moreover, the values of αAs-Fe-As for BaFe2As2 and LiFeAs
act toward and away from the regular tetrahedron ofFeAs4 along with increasing and decreasing the Tc,respectively. Meanwhile, the present results that thevalues of the αAs-Fe-A and the hAs deviate from theoptimum values with pressure reveal the inconsistent tothe guides for developing the Tc. From the electronic statecalculations of LaFeAsO0.8H0.2, the Fermi surface
topologies are unaltered on the application of pressure.
Additionally, the broadening of the band width withpressure is observed because of the shrinkage of dFe-As,
resulting in the decrease of the spin-fluctuation hence theTc going down. The aforementioned findings might besimply unaccountable for the rise of Tc under pressure.
We consequently require an otherwise hint to theenhancement of Tc.
Both parent phases indicate tetragonal to orthorhombic
structural transitions (Ts), subsequently an antiferromagnetic orderings (TN). The parent phase of PP2 isuniquely observed in the 1111 series. Inset of Fig. 2
shows the temperature dependence of the full width at

Photon Factory Activity Report 2016 #34 (2017)

half maximum (FWHM) of 220 reflection for x = 0.51,
which is a good indicator to pursuit the structuraltransition. The TS2’s are indicated by arrows estimatedfrom the power law fitting. At 0.5 and 0.9 GPa, thebroadenings of FWHMs start moderately in hightemperature region as extensive fluctuations in vicinity ofthe structural transitions. At 1.5 GPa, the FWHM exhibits
the slight broadening at low-temperature, not implying
the structural transition but the precursory phenomenon
near the phase boundary. Any broadening was notobserved at 3.2 GPa. Takahashi et al. presented that an
anomaly in resistivity, corresponding to TN2, for x = 0.51
was suppressed near 2 GPa. This denotes that the TN2 aswell as the Ts2, i.e. the PP2 was obscured by low-pressure
although the PP1 being up to 20 GPa (TN1) and 30 GPa(Ts1) is robust to pressure.

region, i.e. in PP2, for LaFeAsO1-xHx. Contrary toelectron doping with increasing x, the pressure induces
the rapid decrease of hAs. Therefore, in contrast to PP1,PP2 can be easily broken by pressure.
Generally in high-Tc superconductors, the nature of theparent phase is strongly related to the superconducting
state; that is, fluctuations derived from the parent phasemay drive the pairing of the superconducting electrons.
Thus, the origins of SC1 and SC2 adhering to PP1 andPP2, respectively, is considered as fluctuations from PP1and PP2. The former is the itinerant spin-fluctuation, andthe latter is another type spin-fluctuation with a strongly
localized character of the orbital selective Mott phase.
Because PP2 disappears easily with pressure, thefluctuation from PP2 might be sensitive to pressure.
Consequently, we would suggest that the significant
enhancement of Tc under pressure is due to the favorable
change of the latter fluctuation.

LaFeAsO1-xHx

TS1150TN1FWHM (deg.)

0.28

0.24

0.5GPaT (K)

1.5 GPa100

x = 0.51
220 FWHM

0.9GPaTS20.20

3.20 GPa 100

200300T (K)SC(6 GPa)
PP1 SC1SC250

TN2PP2Fig. 1. Contour plots of Tc for LaFeAsO1-xHx as afunction of the As–Fe–As bond angle (αAs-Fe-As) andthe Fe–As distance (dFe-As). Broken lines represent
the regular tetrahedron in FeAs4 [4].

Figure 2 illustrates the phase diagram at ambient andhigh-pressure for LaFeAsO1-xHx. The distinct responses topressure for both parent phases are unexpected andpresumably arise due to their different origins. Because
the origin of PP1 is interpreted as a spin density wavewith Fermi surface nesting, its robust behavior of PP1against pressure is consistent with insensitive change ofthe Fermi surface. In contrast, the origin of PP2 isseemingly less relevant to the Fermi surface topology.
Iimura et al. performed an electronic state calculation
based on the molecular orbital concept for LaFeAsO 1-xHx[5]. In the low x region, the bonding and antibonding
states of the Fe-3d and As-4p orbitals give a large gap.
This gap decreases rapidly with an increase in hAs,
namely a decrease in the hybridization of Fe and Asorbitals, making the non-bonding Fe-3dxy orbital the halffilled state. The resulting state implies that the orbital
selective Mott state on Fe-3dxy is realized in the higher x

00SDW

0.1

0.3 0.4
0.2
Hydrogen content x

0.5
Mott

Fig. 2. T vs x phase diagram of the parent phases(PP) with structural (TS) and magnetic (TN)transitions, and superconducting phases (SC) at 0
and 6 GPa for LaFeAsO1-xHx. The PP1 and SC at 6GPa are blue and red, respectively [4]. Inset shows
the temperature dependence of FWHM on the 220
reflection under pressures.

References[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296
(2008).
[2] M. Hiraishi et al. Nat. Phys. 10, 300 (2014).
[3] H. Takahashi et al., Sci. Rep. 5, 7829 (2015).
[4] K. Kobayashi et al., Sci. Rep. 6, 39646 (2016).
[5] S. Iimura et al., Phys. Rev. B 94, 024512 (2016).
* jyamaura@mces.titech.ac.jp