We report on a detailed investigation of the electronic phase diagram of
FeSe_1-x under pressures up to 1.4GPa by means of AC magnetization and
muon-spin rotation. At a pressure \simeq0.8GPa the non-magnetic and
superconducting FeSe_1-x enters a region where long range static magnetic order
is realized above T_c and bulk superconductivity coexists and competes on short
length scales with the magnetic order below T_c. For even higher pressures an
enhancement of both the magnetic and the superconducting transition
temperatures as well as of the corresponding order parameters is observed.
These exceptional properties make FeSe1-x to be one of the most interesting
superconducting systems investigated extensively at present.Comment: 5 pages, 3 figure

A. Amato

A. Raselli

A. Schenck

E. Pomjakushina

H. Keller

H. Luetkens

H.-H. Klauss

K. Conder

M. Bendele

M. Elender

R. Khasanov

English

arXiv

Crossref

Pressure Induced Static Magnetic Order in Superconducting
                    
                      
                        FeSe

We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure ≃0.8  GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above Tc and bulk superconductivity coexists and competes on short length scales with the magnetic order below Tc. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.



© 2010 The American Physical Societ

Bendele, M

Amato, A

Conder, K

Elender, M

Keller, H

Klauss, H H

Luetkens, H

Pomjakushina, E

Raselli, A

Khasanov, R

ZORA

arXiv:0908.2734v2  [cond-mat.supr-con]  2 Dec 2009Pressure Induced Static Magnetic Order in Superconducting FeSe1−xM. Bendele,1, 2 A. Amato,1 K. Conder,3 M. Elender,1 H. Keller,2 H.-H. Klauss,4H. Luetkens,1 E. Pomjakushina,3 A. Raselli,5 and R. Khasanov1, ∗1Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland2Physik-Institut der Universita¨t Zu¨rich, Winterthurerstrasse 190, CH-8057 Zu¨rich, Switzerland3Laboratory for Developments and Methods, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland4IFP, TU Dresden, D-01069 Dresden, Germany5Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI, SwitzerlandWe report on a detailed investigation of the electronic phase diagram of FeSe1−x under pressuresup to 1.4 GPa by means of AC magnetization and muon-spin rotation. At a pressure ' 0.8 GPathe non-magnetic and superconducting FeSe1−x enters a region where long range static magneticorder is realized above Tc and bulk superconductivity coexists and competes on short length scaleswith the magnetic order below Tc. For even higher pressures an enhancement of both the magneticand the superconducting transition temperatures as well as of the corresponding order parametersis observed. These exceptional properties make FeSe1−x to be one of the most interesting supercon-ducting systems investigated extensively at present.PACS numbers: 74.70.-b, 74.25.Jb, 76.75.+iThe phase diagram of the recently discovered Fe-based high-temperature superconductors (HTS) [1] sharea common feature with cuprates and heavy fermions.The parent compounds of the Fe-based HTS, such as,LnOFeAs (Ln=La, Ce, Pr, Sm) [2, 3, 4, 5, 6, 7, 8, 9],AFe2As2 (A = Ba, Sr, Ca) [10, 11, 12, 13] and Fe(SeTe)[14, 15] exhibit long-range static magnetic order. Upondoping or application of pressure (chemical or external),magnetism is suppressed and superconductivity emerges.Recent investigations revealed, however, that the struc-turally most simple binary compound FeSe1−x is an ex-ception of this rule [16]. Different from the other Fe-basedHTS, FeSe1−x did not seem to exhibit static magneticorder for pressures up to about 30 GPa [16]. Yet, short-range spin fluctuations, which are strongly enhanced to-wards Tc, were observed [17]. The superconducting tran-sition temperature of FeSe1−x was found to increase con-tinuously to ' 37 K at 7 − 9 GPa. For higher pres-sures a decrease is observed with Tc ' 6 K approaching20 GPa [16, 18]. Subsequent experiments with finer pres-sure steps revealed, however, a local minimum on Tc(p)at 1.5 GPa of unexplained nature [19].In this letter we report on a detailed study of the evo-lution of the superconducting and magnetic propertiesof FeSe1−x as a function of pressure and temperaturethrough a combination of AC susceptibility and muon-spin rotation (µSR) techniques. Two samples with thenominal composition FeSe0.94 and FeSe0.98 were investi-gated. The obtained phase diagram of FeSe1−x was foundto be separated into three distinct regions. At low pres-sures, 0 ≤ p . 0.8 GPa, the samples are nonmagneticand Tc increases monotonically with increasing pressure.In the intermediate pressure region, 0.8 ≤ p . 1.0 GPa,Tc(p) decreases with increasing pressure and static mag-netism develops. In this region of the phase diagram thesuperconducting and the magnetic order parameters co-exist and compete on a short length scale. Incommensu-rate magnetic order, which sets in above Tc, becomes par-tially (or even fully) suppressed below Tc(p). At higherpressures, p & 1.0 GPa, Tc(p) shows a tendency to riseagain. The magnetic order becomes commensurate andboth, bulk magnetism and bulk superconductivity coex-ist within the whole sample volume.FeSe1−x samples with the nominal compositionFeSe0.94 and FeSe0.98 were prepared by solid state reac-tion similar to that described in Refs. 20, 21, 22. Powdersof minimum purity 99.99% were mixed in appropriateratios, pressed and sealed in a double-walled quartz am-poule. The pressed rod was heated up to 700oC followedby annealing at 400oC [22].The pressure was generated in a piston-cylinder typeof cell especially designed to perform muon-spin rotationexperiments under pressure [23]. As a pressure transmit-ting medium 7373 Daphne oil was used. The pressurewas measured in situ by monitoring the pressure shift ofthe superconducting transition temperature of Pb or/andIn. Two types of cells, the first one made from CuBealloy [maximum pressures pmax(300 K) ' 1.4 GPa andpmax(7 K) ' 1.1 GPa] and the second one made fromMP35 alloy [pmax(300 K) ' 1.7 GPa and pmax(7 K) '1.4 GPa], were used.AC susceptibility measurements were performed by us-ing a home made AC magnetometer with a measuringfield µ0HAC ∼ 0.1 mT and frequency ν = 96 Hz. Inorder to keep the position of the sample unchanged dur-ing the series of AC susceptibility under pressure mea-surements, the excitation and the two pick-up coils werewound directly on the cell. To ensure that the AC sig-nal was entirely determined by the Meissner responseof individual grains and not by the Josephson type ofweak links between them, measurements of the AC sus-ceptibility as a function of ν (0 ≤ ν ≤ 599 Hz) and2HAC (0 ≤ µ0HAC ≤ 0.5 mT) at T = 2.5 K on astandard Quantum Design PPMS instrument were per-formed. The experiments reveal that the AC magnetiza-tion (MAC) scales linearly with HAC and is independenton ν as expected for a superconductor in the Meissnerstate.The zero-field muon-spin rotation (ZF µSR) experi-ments were carried out at the µE1 beam line at thePaul Scherrer Institute, Switzerland for the temperaturesranging from 0.25 to 50 K. The typical counting statis-tics were ∼ 7·106 positron events for each particular datapoint.0.0 0.5 1.0 1.50.900.920.940.96789101189101112138 10 12 14 AC (1-N)  (6K)p (GPa)(c)  AC  -2ab(0)FeSe0.94-2ab (0) (m-2) FeSe0.94FeSe0.98  T c (K)(b)P=1.39 GPaP=1.18 GPaP=0.93 GPaP=0.70 GPaP=0.32 GPa  AC (arb. units)T (K)P=0.0 GPaFeSe0.94Tc(a)FIG. 1: (a) Temperature dependence of χAC of FeSe0.94 mea-sured at (from the top to the bottom) p = 0.0, 0.32, 0.7,0.93, 1.18, and 1.39 GPa. The transition temperature Tc isdetermined from the intersection of straight line fits to thedata above and below the transition. (b) Dependence of Tcon p of FeSe0.94 and FeSe0.98. (c) Pressure dependence of thenormalized AC susceptibility −χAC (1−N) at T = 6 K andthe inverse squared in-plane magnetic penetration depth λ−2abat T = 0 K [25].The response of the superconducting state of FeSe1−xto pressure was studied in AC susceptibility (χAC) ex-periments, Figure 1. The transition temperature Tc,Figure 1b, shows a non-monotonic increase with a localminimum at p ' 1.2 GPa, similar to Tc(p) reported byMiyoshi et al. [19]. Figure 1c depicts −χAC(T ) (1 −N)at T = 6 K. Here N denotes the demagnetization factorwhich, assuming a spherical shape of the sample grains,was taken to be equal to 1/3. For temperatures lowerthan Tc, |χAC(T ) (1 − N)| is smaller than unity due tothe penetration of the AC magnetic field on a distance λfrom the surface of each individual grain (λ is the mag-netic penetration depth). Following Shoenberg [24], χACin a granular sample is expected to scale with λ−2, whichis the case here, as may be seen from the comparison ofχAC(6 K) with λ−2ab (T = 0) obtained in transverse fieldµSR experiment [25].0.0 0.5 1.0 1.50102030400 10 20 30 400.00.20.40.60.81.0  20 K8 K0.24 KAsymmetryt ( s)FeSe0.94p=0.0GPap=0.76GPap=0.94GPap=1.12GPap=1.39GPa25 K13 K0.24 K22.5 K12 K0.24 K35 K11 K0.67 K50 K12 K0.67 K(a)   Bint (mT)(b)  0.76 GPa 0.84 GPa 0.94 GPa 1.12 GPa 1.39 GPa  Volume FractionT (K)(c)  0.76 GPa 0.84 GPa 0.94 GPa 1.12 GPa 1.39 GPaFIG. 2: (a) Zero-field µSR time-spectra of FeSe0.94 measuredat (from the top to the bottom) p = 0.0, 0.76, 0.94, 1.12,and 1.39 GPa. Dependence of the internal field at the muonstopping site Bint which is proportional to the magnetic or-der parameter (b), and the magnetic volume fraction (c), ontemperature at various pressures. The solid lines in panelb are the fit of Bint(T ) in the region Tc(p) ≤ T ≤ TN toBint(T ) = Bint(0)[1 − (T/TN)α]β (α and β are the powerexponents ).The magnetic response of FeSe1−x was studied in ZFµSR experiments. In the following we discuss the ZFµSR data for the three different pressure regions.In the low-pressure region, 0 ≤ p . 0.8 GPa, whereTc increases with increasing p, the ZF µSR time-spectraprove the absence of long range magnetic order forall temperatures as exemplified by the identical weaklydamped spectra for T = 0.24 K, near Tc(p) and 20 K, seep = 0.0 and p = 0.76 GPa data in Figure 2a. The solidlines in Figure 2a correspond to a two-component fit:AZF (t) = AZFS (t) +AZFPS (t). (1)with the first component describing the sample responseand the second one representing the contribution of thepressure cell (ZF responses of the CuBe and MP35 cellsare described in [23]). The sample contribution is well3fitted to the single-exponential decay function [26]:AZFS (t) = AZFS,0 e−Λ0t, (2)(Λ0 is the exponential relaxation rate), thus revealingthat very diluted and randomly oriented magnetic mo-ments exist in the sample volume which can be attributedto small traces of Fe impurities, see Ref. 26.In the intermediate pressure region, 0.8 ≤ p . 1.0 GPa,the spontaneous muon-spin precession is clearly observedin the ZF µSR time spectra, see p = 0.94 GPa data inFigure 2a. Therefore, long range magnetic order is es-tablished below the Ne´el temperature TN . The analysisof the µSR data was made by accounting for the separa-tion of the sample into magnetically ordered regions withmuons experiencing a static local field and nonmagnetic(paramagnetic) regions:AZFS (t) = AZFS,0[m(23j0(γµBintt)e−ΛT t +13e−ΛLt)+(1−m) e−Λ0t]. (3)Here m is the magnetic volume fraction of the sample, j0is a zeroth order Bessel function, γ = 2pi 135.5 MHz/T isthe muon gyromagnetic ratio, and ΛT and ΛL are the ex-ponential relaxation rates longitudinal and transverse tothe initial muon-spin polarization. The oscillating partof the signal was found to be good described by a Besselfunction, which is archetypical for incommensurate mag-netic order [27]. The dependence of the internal fieldBint, corresponding to the magnetic order parameter,and the magnetic volume fraction on temperature areshown in Figures 2b and c.For pressures above 1.0 GPa we observe a further in-crease of the magnetic volume fraction and of the internalmagnetic field Bint, Figure 2. Additionally, we find thatthe µSR lineshape is better described by a damped cosinewith zero initial phase rather than by a Bessel function:AZFS (t) = AZFS,0[m(23cos(γµBintt)e−ΛT t +13e−ΛLt)+(1−m) e−Λ0t]. (4)This suggests that in the high pressure region the mag-netic order becomes commensurate.Figure 3 summarizes our results on the magnetismand superconductivity in an electronic phase diagram forFeSe1−x.The magnetic order coexists and competes withsuperconductivity for p & 0.8 GPa. Above this pressurelong range magnetic order is established below TN > Tcand bulk superconductivity sets in below Tc. The com-petition of the two ground states in this pressure rangeis evident from the following two observations: First, asa function of pressure Tc is weakened as soon as mag-netic order appears, leading to the local maximum at0.0 0.6 0.8 1.0 1.2 1.40102030Volumefraction  T c, TN (K)p (GPa)00.51PMPM+MSC SC+MBint (mT)(b)010203040  T c, TN (K)02040PMPM+MSC SC+MFeSe1-x(a)FIG. 3: (a) Pressure dependence of the superconducting tran-sition temperature Tc, the magnetic ordering temperature TN ,and the internal field Bint (magnetic order parameter) ob-tained in AC susceptibility and muon-spin rotation experi-ments. (b) Pressure dependence of TN , Tc, and the magneticvolume fraction. The Tc(p) and TN(p) lines are guides for theeye. The closed and the open symbols refer to FeSe0.94 andFeSe0.98 sample. SC, M, and PM denote the superconduct-ing, magnetic and nonmagnetic (paramegnetic) states of thesample.p ' 0.8 GPa in Tc(p). Second, as a function of tem-perature Bint, as well as the magnetic volume fraction,decrease below Tc showing that the magnetism, whichdevelops at higher temperatures, becomes partially (oreven fully) suppressed by the onset of superconductivity.The superconducting volume fraction is close to 100%for all pressures, Figure 1c, while the magnetic fractionincreases continuously and reaches ' 90% at the high-est pressure investigated p ' 1.39 GPa, Figure 2c. Inother words, both ground states coexist in the full sam-ple volume at p = 1.39 GPa. Our data do not provideany indication for macroscopic phase separation into su-perconducting and magnetic clusters (bigger than a fewnm in size), as observed e.g. for Ba1−xKxFe2As2 [28].Actually, the data rather point to a coexistence of bothorder parameters on an atomic scale. This scenario iscompatible with the itinerant two-band models of Fe-based HTS proposed recently by Vorontsov et al. [29]and Cvetkovic and Tesanovic [30]. According to thesemodels the transition between the magnetic and the su-4perconducting states may involve the formation of theintermediate phase, where both superconductivity andmagnetism coexist.In conclusion, the magnetic and superconducting prop-erties of FeSe1−x were studied as a function of pressure upto 1.4 GPa by means of AC magnetization and muon-spinrotation techniques. Above ' 0.8 GPa superconductivitywas found to coexists with magnetism with Ne´el temper-atures TN > Tc. In a narrow pressure range, where alocal minimum in Tc(p) is observed, superconductivitycompetes with magnetism in the sense that the magneticvolume fraction and the magnetic order parameter aresuppressed below Tc. At the highest pressure investi-gated here superconductivity and static long range com-mensurate magnetism coexist on short length scales inthe full sample volume. 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Pressure induced static magnetic order in superconducting FeSe_{1-x}

Pressure Induced Static Magnetic Order in Superconducting FeSe_1-x

Pressure Induced Static Magnetic Order in Superconducting FeSe_1-x

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