Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals

2.2 MB·

by  Hai Lin

Robust superconductivity and transport properties in (Li1−xFex)OHFeSe single
crystals
∗
Hai Lin, Jie Xing, Xiyu Zhu, Huan Yang and Hai-Hu Wen
Center for Superconducting Physics and Materials,
National Laboratory of Solid State Microstructures and Department of Physics,
National Center of Microstructures and Quantum Manipulation, Nanjing University, Nanjing 210093, China
(Dated: January 20, 2016)
6
The recently discovered (Li1−xFex)OHFeSe superconductor with Tc about 40K provides a good
1
platformforinvestigatingthemagnetizationandelectricaltransportpropertiesofFeSe-basedsuper-
0
conductors. By using a hydrothermal ion-exchange method, we have successfully grown crystals of
2
(Li1−xFex)OHFeSe. X-raydiffractiononthesampleshowsthesinglecrystallinePbO-typestructure
n with the c-axis preferential orientation. Magnetic susceptibility and resistive measurements show
a anonset superconductingtransition at aroundTc=38.3K. Usingthemagnetization hysteresisloops
J
and Bean critical state model, a large critical current Js is observed in low temperature region.
9 The critical current density is suppressed exponentially with increasing magnetic field. Tempera-
1 ture dependencies of resistivity under various currents and fields are measured, revealing a robust
superconductingcurrent density and bulk superconductivity.
]
n
PACSnumbers: 74.25.F-,74.25.Sv,74.25.Ha,74.25.Fy
o
c
-
r Since the discovery of high temperature superconduc- totalphase. Thus,thisphaseseparationclearlyobstacles
p
u tivity in LaFeAsO1−xFx[1], at leasteightdifferent struc- the investigation of intrinsic superconducting properties
tures of iron-based superconductors have been found in of the FeSe-based superconducting phase. Meanwhile,
s
t. succession[2–4]. Therein, FeAs-based as well as FeSe- the Ax(NH3)yFe2−zSe2 and LixXyFe2−zSe2 phases are
a basedsuperconductorsaretwomostcommonandimpor- very sensitive to air. Up to now, no single crystals of
m
tant families. For the electric neutrality of FeSe-layers, them are available for measurements.
- by the normal high temperature sintering method, no
d spacer layer can be easily intercalated in between FeSe- The newly found superconductor (Li1−xFex)OHFeSe
n can be conveniently synthesized by the hydrothermal
layers,whosestructureissimilartoFeAs-layers. Asfaras
o method[12]. Comparing with other FeSe-based su-
we know, FeSe-based superconductors cover the phases
c
perconductors, the superconducting transition temper-
[ of Fe1+xSe, monolayer FeSe film on SrTiO3 substrate,
v1 eAaxrFthe2m−yeStael2),, LAixx(XNyHF3e)2y−FzeS2e−2z(SXe2is(Aanisoraglaknailci morolaelckuallei)- mhatigouhnreeorlaTytcehraonfFaetSbheoauttthin4in0KFfiel1om+fx(SsLeyis1ta−enxmdF[e7xc]o.)OmHpIaFtreaSwbealesistsomhoutwhchne
3 and so on[5]. Among them, Fe1+xSe single crystal has
0 a low superconducting transition temperature of only that the (Li1−xFex)OHFeSe phase has very little Fe-
vacanciesintheFeSe-layerswithameasuredrationabout
9 about 8K at ambient pressure[6]. Although monolayer
4 FeSe film with T about 65K[7] indicates a possibility Fe:Se=0.98:1[13]. Therefore, this system has some ad-
c
0 of high T superconductivity in FeSe-based supercon- vantagesovertheAxFe2−ySe2 whichhasagreatquantity
c
. of Fe-vacancy and accompanies with the phase separa-
1 ductors, it is made on oxide substrates like SrTiO3 by
tion. Moreover,byahydrothermalion-exchangemethod,
0 MBE and is extremely sensitive to air[7, 8]. The FeSe
6 monolayer thin film without covering with FeTe layers one can obtain crystals of (Li1−xFex)OHFeSe with large
1 sizes and good quality[14]. And the single crystals can
will be damaged after being taken out of the vacuum
: keep stable in the air for a period of time. Therefore,
v chamber. So it is very hard to measure the magnetiza-
it is a good platform to study the intrinsic properties of
i tion and electrical transport properties in situ. For the
X FeSe-based superconductors, from which a further com-
AxFe2−ySe2 system with superconducting Tc at about
r prehensionto themechanismofiron-basedsuperconduc-
a 32K, single crystals can be grown by the high temper-
ature sintering method [9, 10]. However, the supercon- torscanbe acquired. Inthe (Li1−xFex)OHFeSe systems,
several measurements have already been employed, such
ducting phase is always inter-grow with the insulating
as scanning tunneling microscopy(STM), angle resolved
phaseA0.8Fe1.6Se2 (orcalledasthe245phase)whichhas
photoemmison spectroscopy (ARPES), ionic field gating
a √5 √5 ordered structure of Fe-vacancies[11]. In the
× effect,etc.[15–17]. OurpreviousSTMwork[15],basedon
phase of AxFe2−ySe2, through the scanning electron mi- the high quality crystals, has clearly indicated the pres-
croscopy(SEM)analysis,itwasshownthethesupercon-
ducting phase takes over only about 20% volume of the ence of double anisotropic gaps in (Li1−xFex)OHFeSe,
whichmimics that inthe monolayerFeSe thin film. Pre-
viously Dong et al. have done some electrical trans-
port measurements on the single crystals[14]. Here, we
∗Electronicaddress: [email protected] show some further measurements on magnetic and elec-
trical properties of (Li1−xFex)OHFeSe, in order to know
2

50
0 ol)
40 0H = 20 Oe -100 u/m
m
m ZFFCC -200 M (e
c 30 -300
m -400
20 0 10 20 T3 ( 0K)40 50 60 (Li1-xFex)OHFeSe
10
0
50 100 150 200 250 300
T (K)
FIG. 1: (Color online) X-ray diffraction patterns for the
FIG.2: (Coloronline)Temperaturedependenceofresistivity
(Li1−xFex)OHFeSe crystal. One can see the predominant
(00l) indices. The inset is a schematic structure of tetrag- forthe(Li1−xFex)OHFeSecrystalatzerofieldwithmeasuring
current of 20µA. The solid line shows the fit in the low tem-
onal (Li1−xFex)OHFeSe. peraturerangebytheformulaρ(0)+ATn. Theupperinsetis
the temperature dependence of magnetic susceptibility mea-
suredinbothZFCandFCmodesforasampletakenfromthe
how robust the superconductivity is, and whether it is
same batch, with an applied field of 20Oe parallel toc-axis.
bulk superconductive or it is phase separated, like in
AxFe2−ySe2.
The (Li1−xFex)OHFeSe crystals investigated in this
work are synthesized using a hydrothermalion-exchange ered structure of iron-basedsuperconductors.
method, as reported previously[14]. Firstly, K0.8Fe2Se2 In Fig. 2, we present the temperature dependence
crystals were grown using the self-flux method. Next, of resistivity from 10K to 300K at zero field with a
LiOH (J&K, 99% purity) liquid was dissolved in deion- measuring current of 20µA. The resistivity decreases
ized water in a teflon-linked stainless-steel autoclave monotonically from room temperature to the lower tem-
(volume 50mL). Then, iron powder (Aladdin Industrial, perature, which shows a highly metallic conductivity.
99.99%purity),selenourea(J&K,99.9%purity),andsev- The solid line shows the fit in the low temperature
eral pieces of K0.8Fe2Se2 crystals were added to the so- range by ρ(T)=ρ(0)+ATn with ρ(0)= 5.65mΩ*cm , A=
lution. After that, the autoclave was sealed and heated 0.00016mΩ*cm/K2, n= 2.32. The residual resistivity
◦
up to 120 C followed by staying for 40 to 50 hours. ratio, defined as RRR=ρ(300K)/ρ(0K) 7.9, which is
≈
Finally, the (Li1−xFex)OHFeSe crystals with a metallic- comparable to the previous reported value[14]. In the
grey colored surface can be obtained by leaching. The normalstate,the temperature dependentresistivityhere
X-ray diffraction (XRD) measurements were performed exhibits a positive curvature and the ratio dρ/dT be-
onaBrukerD8AdvanceddiffractometerwiththeCu-Kα comes smaller at high temperature. This is similar to
radiation. DC magnetizationmeasurementswerecarried Fe1+xSe single crystals[6], but quite different from some
out with a Quantum Design instrument SQUID-VSM- AxFe2−ySe2 singlecrystals[9],whereapositivecurvature
7T.Theresistivemeasurementsweredonewiththestan- atlowtemperatureandanegativecurvatureathightem-
dard four-probe method on a Quantum Design instru- perature are generally observed. In the low temperature
ment Physical Property Measurement System (PPMS). region, an abrupt resistivity drop can be clearly seen.
In measuring the resistivity, the current was switched Using the criterion of 90% of the normal state resistiv-
frompositivetonegativealternativelyinordertoremove ityρ ,theonsetsuperconductingtransitiontemperature
n
the contacting thermal power. Tonset isdetermined,whichgivesavalueofabout38.3K.
c
Fig. 1 shows the X-ray diffraction (XRD) spectra for The inset presents the temperature dependence of mag-
the (Li1−xFex)OHFeSe single crystal. The sample turns netic susceptibility under an applied field of 20Oe mea-
out to be very layeredfeature and can be easily cleaved. suredinthezero-field-cooled(ZFC)andfield-cooled(FC)
Only (00l) reflections can be seen in the XRD pattern, modes. Thesample forthe magnetizationmeasurements
indicating highly orientation along the c-axis. The l is one taken from the same batch of that for the resis-
containing both odd and even numbers, which indicates tive measurements. Similar to the resistivity curves, a
the structural changing from I4/mmm of K0.8Fe2Se2 to sharp superconducting transition can be clearly seen at
P4/nmm of (Li1−xFex)OHFeSe. The c-axis lattice con- about38Kinthemagneticmeasurements. Thetransition
stantisabout9.27˚Awhichisclosetothereportedresults temperature is higher than that in AxFe2−ySe2(A = K,
in(Li1−xFex)OHFeSe[12]. Theinsetshowstheschematic Rb, Cs, Tl)[18–21] and Fe1+xSe at ambient pressure[6].
structure of (Li1−xFex)OHFeSe which has a typical lay- Furthermore, the magnetic susceptibility measured in
3

10
40
60 9
(a) 30 38
3 m)1000 --63000 m 78 T (K) c3346 T conset
mu/c 500 -8 -4 0 4 8 m c 56 3302 T c0 000...001051 mA
0.4
(e 0 4 28 1
M 3 260 1 2 3 4 5 1.5
2
-500 2 I (mA) 3
2K 5K 13K 15K 1 35.8
7K 9K 20K 25K
-1000 10K 11K 30K 35K 0
10 15 20 25 30 35 40 45 50
-8 -6 -4 -2 0 2 4 6 8 T (K)
6 0H (T)
10
(b) FIG. 4: (Color online) (a) Temperature dependence of resis-
5 2K tivity for the (Li1−xFex)OHFeSe crystal measured with var-
2 m) 10 ious currents which are applied always in the ab-plane. The
c 5K insetshowsthecurrentdependenceofTc,whereTconset isde-
J(A/s 104 97KK tteermmpienreadtuwreithTc0awcaristedreiotenrmofin9e0d%wρint,ha1n%dρtnh.e zero transition
3 10K
10
11K
13K
2 15K BaFe2−xCoxAs2. Thisvalueisatleastoneorderofmag-
10 20K nitude largerthan thatofKxFe2−ySe2[18], whichreveals
25K
the good quality and bulk pinning of our samples. The
0 1 2 3 4 5 6 7 monotonically decreasing of M with increasing field re-
0H ( T) vealstheabsenceoffish-taileffectbelow7T.Forafurther
analysis, we calculate the critical current density using
the Bean critical state model[22]. In this model, the su-
FIG. 3: (Color online)(a) Magnetization hysteresis loops of
perconducting critical current density J is expressedby
the (Li1−xFex)OHFeSe crystal at various temperatures be- s
low Tc, which have deducted the ferromagnetic background
measured at 50K as shown in the inset. (b) Magnetic field ∆M
dependence of the calculated superconducting current den- Js =20 (1)
a(1 a/3b)
sity based on the Bean critical state model at temperatures −
ranging from 2K to 25K.
wherea(cm) andb(cm)(a b)arethe in-plane sample
≤
sizes. ThecalculatedresultsareillustratedinFig.3(b)in
a semi-logarithmic scale. Obviously, the critical current
the ZFC mode reveals an almost fully superconducting density Js of (Li1−xFex)OHFeSe is weakly dependent
screening effect. with the magnetic field at temperatures below 9K. The
Fig. 3(a) presents the magnetization hysteresis loops critical current density J is as large as 2.47 105A/cm2
s
×
(MHLs) of the (Li1−xFex)OHFeSe crystals at various at 2K in the low field limit. The magnitude is compa-
temperatures below Tc. The MHLs presented here have rable to that in the optimally doped BaFe2−xCoxAs2,
been deducted a weak ferromagnetic background signal and much larger than 104A/cm2 in the Fe1+xSe and
from the raw data measured at 50K. The background KxFe2−ySe2[23–25] crystals. At higher temperatures
is shown in the inset of Fig. 3(a). This weak ferromag- over 9K, J decreases rapidly with increasing tempera-
s
netic signal may be induced by some impurities, or it is ture and finally becomes vanishingly small after 25K. In
an intrinsic property of the system due to the substitu- addition, the J in the high temperatures region is de-
s
tion of Fe to the Li atoms[13]. This needs to be further pressedexponentially with the increasingfield. And it is
resolved by more investigations. During the measure- interesting that this type of MHL shape looks very simi-
ments, the magnetic field is always perpendicular to the lar to those in many superconductors, but is very differ-
ab plane of the sample. Here, we determine the width ent from KxFe2−ySe2[25] in which phase separations are
∆M of MHLs, where ∆M is Mdown-Mup. Mdown(Mup) expected.
is the magnetization at a certain magnetic field in the Inordertocheckthehomogeneityofsuperconductivity
decreasing (increasing)-field process. It is interesting to in(Li1−xFex)OHFeSe crystals,we measuredthe temper-
note that the maximum of the MHL width ∆M 2000 ature dependence of resistivity at various currents from
emu/cm3 observed at 2K is comparable to othe≈r bulk 10K to 50K, and the data are presented in Fig. 4. The
iron-basedsuperconductors,suchastheoptimallydoped current I is always applied in the ab-plane. It is found
4
sistivity has not been found in the previous report[14],
and might be caused by some magnetic impurities aris-

10 ing from the partial substitution of Li by Fe in the Li
40
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10 15 20 25 30 35 40 45 50 ourmeasurementsofthe magnetizationandthe resistive
T (K) transitions under different magnetic fields and current,
and the thoughtful analysis, we can conclude that the
superconductivity is robust, uniform without the phase
FIG.5: (Coloronline)Temperaturedependenceofresistivity
with the magnetic field applied parallel to c-axis. The inset separation as that occurring in KxFe2−ySe2.
shows the field dependence of Tc. Tconset is determined with In summary, we successfully synthesized the
criterion of 90%ρn and Tc0 with 1%ρn. (Li1−xFex)OHFeSe crystals with large size and good
quality by the hydrothermal ion-exchange method. The
magnetic hysteresis loops at various temperatures are
thatwhenalargermeasuringcurrentisused,thenormal
measured which exhibit a symmetric shape, indicating a
state resistivity ρn becomes slightly increased. The en- vortex bulk pinning property and thus bulk supercon-
hancementofthenormalstateresistivitymaybeinduced
ductivity. By using the Bean critical state model, we
bythe localheatingeffectwhenthe measuringcurrentis calculated the superconducting currentdensity J which
s
high. It is interesting to note that the value of Tc drops amounts to 2.47 105 A/cm2 at 2K and zero magnetic
down slowly when the measuring current I is increased field. This valu×e of Js of (Li1−xFex)OHFeSe is very
from0.001mAto5mA(correspondingtothecurrentden-
sity of about 0.0136 to 68A/cm2). The current depen- wlaargseprcoovmepdatroedhawviethphthaaset sinepKarxaFtieo2n−.ySIen2.adTdihteionla,ttweer
dence of the superconducting transition temperature T
c measuredthetemperaturedependenceofresistivitywith
imsinsheodwwnitihn tthhee cinristeetrioonf Foifg9.04%. ρHnearne,dTTcoc0nswetitihs 1d%etρenr-. dfoiuffnerdentthattratnhseposrutpceurcrorenndtuscitnin(gLti1r−anxsFietixo)nOHteFmepSee.raItturies
This weak suppression of Tc by the measuring current drops down slightly when the measuring current density
densitymaysuggestthatthesampleshouldnothavethe is increased from 0.0136 to 68A/cm2, suggesting again
phase separation like that in KxFe2−ySe2. If phase sep- the absence of phase separation. Finally the resistive
aration would exist in the system, the superconducting transition has been measured under different magnetic
transition and the related transition temperature should fields. A clear broadening of the transition is observed,
be strongly influenced by the measuring current density. which indicates a strong vortex motion due to the high
Thisseemsnotthecasehere. Thisconclusionisalsocon-
anisotropy of the system.
sistent with the large MHL width and J value observed
c
in the (Li1−xFex)OHFeSe crystal.
In Fig. 5, we show the temperature dependence of re-
sistivity for the (Li1−xFex)OHFeSe crystal at zero field ACKNOWLEDGMENTS
and various magnetic fields with the field directions par-
allel to c-axis, while the current is always applied in the
ab-plane. As the applied field is increased, the super- This work was supported by the National Natural
conducting transition temperature is suppressed gradu- Science Foundation of China(Grant No.11534005), the
ally,andthenormalstateresistivityρ revealsanegative Ministry of Science and Technology of China (Grant
n
magnetoresistivity. This abnormal negative magnetore- Nos.2011CBA00102,2012CB821403).
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