The origin of the phase separation in partially deuterated $κ$-(ET)$_2$Cu[N(CN)$_2$]Br studied by infrared magneto-optical imaging spectroscopy

0.22 MB·English

by  T. Nishi

The origin of the phase separation in partially deuterated κ-(ET) Cu[N(CN) ]Br
2 2
studied by infrared magneto-optical imaging spectroscopy
T. Nishi1,∗ S. Kimura1,2, T. Takahashi3, T. Ito2, H.J. Im1, Y.S.
Kwon2,4, K. Miyagawa5, H. Taniguchi6, A. Kawamoto7, and K. Kanoda5
1Department of Structural Molecular Science, The Graduate University for Advanced Studies, Okazaki 444-8585, Japan
2UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
5 3Research Reactor Institute, Kyoto University, Osaka 590-0494, Japan
0 4Department of Physics, SungKyunKwan University, Suwon 440-746, South Korea
0 5Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
2 and CREST, Japan Science and Technology Corporation, Japan
6Department of Physics, Saitama University, Saitama 338-8570, Japan
n
7Department of Physics, Hokkaido University, Sapporo 060-0810, Japan
a
J (Dated: February 2, 2008)

0 The direct observation of the phase separation between the metallic and insulating states of
1 75%-deuteratedκ-(ET)2Cu[N(CN)2]Br(d33)usinginfrared magneto-opticalimaging spectroscopy
is reported, as well as the associated temperature, cooling rate, and magnetic field dependencies
] of the separation. The distribution of the center of spectral weight (h ω i) of d33 did not change
l
e underanyoftheconditionsinwhichdataweretakenandwaswiderthanthatofthenon-deuterated
-
material. This result indicates that the inhomogenity of the sample itself is important as part of
r
t theorigin of themetal - insulator phaseseparation.
s
.
t PACSnumbers: 71.27.+a,78.20.Ls,75.30.Kz
a
m
- Since organic conductors have a variety of physical transitions are considered to relate to the phase separa-
d
properties, many research works have been performed tion. [9] Especially, d33 indicates the partial S state in
n
o for a long time. In the organic conductors, quasi-two- the fast cooling condition observed in the ac susceptibil-
c dimensional materials attract attention for their simi- ity experiment. Both the size of the separated phases
[ lar physical properties to high-TC cuprates. The ground and the electronic behavior of the crossoverbetween the
1 state of the quasi-two-dimensionalorganic conductor, κ- S and AFI phases are unsettled questions. [6] For ex-
v (ET)2X (ET=bis(ethylenedithio)-tetrathiafulvalene,X ample, in high-TC cuprates, the phase separation on the
3 = Cu[N(CN)]2Br, Cu[N(CN)]2Cl, Cu(NCS)2 and so on) nanometer scale has been observed by scanning tunnel-
9 in this case, is determined by the universal parameter ing microscopy.[10] Contrary to this, the domain size of
1 U/W, where U and W are the on-site Coulomb energy κ-(ET)2X is believed to be on the µm-scale, although
1
and band width, respectively, of the upper HOMO band again there is no information regarding the specifics of
0
of,inthiscase,theETdimer.[1]Thistypeofmaterialis this.
5
0 classified as a Mott insulator system. In the κ-(ET)2X, To investigate metal - insulator domains like these on
t/ especially X = Cu[N(CN)2]Br, the superconducting (S) theµm-scaleandthetemperatureandmagneticfieldde-
a state directly changes to the antiferromagnetic (AFI) pendencies of the associated transitions, Kimura et al.
m state below about 11 K with increasing U/W at around
constructed an infrared magneto-optical imaging spec-
- U/W =1.[2,3,4]TheU/W parametercanbecontrolled troscopy apparatus. [11, 12] The main purpose of this
d bythedeuterationandcoolingrate.[5]Attheboundary,
apparatusis to investigatethe spatialdistributionof op-
n
itisbelievedthatthephaseseparationwithµm-scaledo-
o ticalreflectivityspectraaswellastheelectronicstructure
mainsbetweenSandAFIstatesappears. Butthereisno
c near the Fermi level at low temperatures and magnetic
: experimental result that has directly verified this obser- fields. Using this apparatus, infrared reflectivity spectra
v
vation.[6]Inthispaper,wereportthedirectobservation
i in the presence of magnetic fields and their spatial con-
X ofthephaseseparationanditstemperature,coolingrate
tributioncanbeobtained,andthespatialdistributionof
and magnetic field dependences.
r metallic and insulating states can be observed based on
a
the spectral shape. Based on these methods, we firstly
Partially deuterated κ-(ET)2Cu[N(CN)2]Br shows
observedherethephaseseparationofthemetallicandin-
characteristic physical properties. In 75 % deuterated
sulating states in d33 in contrast to no phase separation
material (denoted as d33 hereafter), the phase changes
in the non-deuterated material (d00), and the temper-
from S to AFI when the cooling rate is increased, and
ature, cooling rate, and magnetic field dependencies of
also a magnetically induced S - AFI transition is ob-
those transitions.
servedin50%-deuteratedmaterial.[6,7,8]Bothofthese
Single crystals of partially- and non-deuterated κ-
(dnn-ET)2Cu[N(CN)2]Br were prepared by a conven-
tional electrochemical oxidation method. The samples
∗E-mail: [email protected] were placed on a substrate using a silver paste that
2
tronic structure of the lower- and upper-Hubbard bands
(a) (b) (c)
T =50K, B =0T T =4K, B =0T, fcT =4K, B =0T, sc as described by a typical Mott transition. To clarify the

123Y Axis (m)00000000(d)100 200 300 0 X 1A00xis 2(00m)3(e00) 0 100 200 300 2 2 2 222 543210 000000 000000 1111122222222222222222222222222 9999900000111112222233333444445 0246802468024680246802468024680 -10000000000000000000000000000000 <> (cm) tcionhhfeatthnhfoegelerleoeilnwfleceittnchrtgoeivnfriuitecnyflchsesttcpωirtoueincvic:tit=truyurmResR,0pω0ω(te2hh2cωteωRrRuc(i(ωem)ωn)w)tdadeaωsrωswo.ofebltslapaienscettdrhaeblycwhueasinginghget
) (arb. units) TTT===5440KKK,, BB, B==00=TT0T,, fscc 12Intensity0000 b c a H5dd0iuen0cra0teri,vycimttcyha−ess1ep,iienntcthtewregahrsiaductehmiroitnvrheuedlaersfrueiosammewvrtauahllsueeaKiltsiemrdsaiamtfteriedosrfimse-bKdoe.lprootIwninciagaωlan2cnooan=rl---
R(
ysis of the reflectivity spectra. However, to clarify the
spectral change due to a difference in the conditions, we
0
8502000 4(0c0m0-1) 6000 2000 <22>00 (cm2-14)00 2600 employed the reflectivity spectra to determine h ω i.
The spatialdistributions of h ω i at 50K, 0 T, at4 K,
0 T in the fc condition and at 4 K, 0 T in the sc condi-
tion are shown in Figs. 1 (a), (b) and (c), respectively.
FIG.1: (Color) Spatial distributionof thecenterofspectral
weight (h ω i) of d00 at 50 K, 0 T (a), 4 K, 0 T in the In comparison between Figs. 1 (b) and (c), the cooling
fast cooling condition (b) and 4 K, 0 T in the slow cooling rate effect is small in d00. Judging from the physical
condition (c). (d) Reflectivity spectra at the same point of properties at 4 and 50 K, the blue-colored region is the
d00 nearthecenterof theimage undertheconditions in (a)- insulating phase and the colors from green to red are
(c). Metallic and insulating reflectivity spectra appear at 4 the metallic phase. At 50 K, the material is clearly in a
and50Katambientmagneticfield,respectively. (e)Statistic uniform insulating phase. At 4 K, it must change to a
distribution of h ω i for theconditions in (a)-(c). uniformSphaseinthe bothcoolingcondition. However,
the color is not uniform. This indicates that the elec-
tronic structure on the sample surface is not constant.
avoided putting stress on the samples. The optical re- Note that since the reflectivity spectrum in the S phase
flectivity measurements between ω = 800 - 8000 cm−1 isametallicone,theSandmetallicstatescannotbe dis-
were performed at the infrared magneto-optical station tinguished by this method. The difference between the
of the beam line 43IR of a synchrotron radiation facil- metallic and insulating states, however, can be clarified.
ity, SPring-8, in Hyogo, Japan. [11, 12] The two cooling Figure 1 (e) indicates the h ω i distribution in the ar-
rates for both of d00 and d33 were set to 17 K/min for eas indicated in Figs. 1 (a), (b) and (c). The center and
fast cooling (fc), and 0.05 K/min for slow cooling (sc) full width (2σ) of h ω i are 2486 and 60 cm−1 at 50 K,
in the temperature range of 90 - 70 K. Two-dimensional 0 T, 2066 and 94 cm−1 at 4 K, 0 T in the fc condition
imaging data were obtained by measuring each sample and 2150 and 97 cm−1 at 4 K, 0 T in the sc condition,
with 12 µm steps in the same area with a spatial resolu- respectively. The h ω i distributions at 4 K, 0 T in both
tion of less than 20 µm. All spectra were recorded using conditions arelargerthanthat at50K,0 T.This means
unpolarizedlightbecausetheoverallspectralchangedue thattheelectronicstructurebecomesinhomogenitywith
to the outer perturbationsis discussed,thoughpolarized decreasing temperature. The h ω i distributions at 50
reflectivity spectra are usually used for the analysis of and4 Kare clearlyseparatedfromeachother. Basedon
the electronic and phonon structures. this,thephasetransitionfrominsulatortometal(super-
The results for d00 at T = 50 and 4 K at B = 0 T conductor) must occur over the whole sample surface of
are shown in Figure 1. This material is in the S and d00. Figure 1 (d) indicates that the h ω i distribution is
paramagnetic insulating (PI) phases at 4 and 50 K, re- completelyseparatedattheboundaryof2300cm−1. For
spectively. The physical characteristics are reflected in future reference, the boundary between the metallic and
the temperature dependence of the infrared reflectivity insulating states is hereafter referred to as ω . Note
MI
spectrum in Fig. 1 (d). The spectra at 4 K, 0 T in both that the width of the h ω i distribution indicates the in-
cooling conditions have metallic reflectivities in the low homogenityineachphaseandcanbeusedforcomparison
wave number region and that at 50 K, 0 T has an in- with that of d33.
sulating reflectivity based on its almost constant reflec- The same analysis was performed on d33 and the re-
tivity below 2000 cm−1 except for the TO phonons at sultsareshowninFigure2. ThematerialisintheS,AFI,
around 1300 cm−1. These spectral shapes are consistent PIandparamagneticmetallic(PM)phasesat4K,0Tin
with previous work. [13, 14] Because of the conservation thesccondition,at4K,0Tinthe fccondition,at50K,
of spectral weight, the reflectivity at around 3000 cm−1 0Tandat4K,5Tinthesccondition,respectively. The
dropsandrisesat4and50K,respectively. Thisbehavior magnetic field of 5 T that is above Hc2 (∼ 4 T) at 4 K
corresponds to the temperature dependence of the elec- was applied perpendicular to the ac-plane. Figure 2 (a)
3
to that at 50 K, 0 T in Fig. 2 (a). The tail of the h ω i

(a)T=50K,B=0T (b)T=4K,B=0T,fc distribution crossesωMI. This indicates that a tiny area
2500 in the sample surface changes to metallic state, i.e., the
200
phase separation between metal and insulator appears.
2400 )
-1 m The metallic phase can be evaluated to be 2.5 % based
100 2300 c on the percentage of h ω i below ω .
m) 2200 > ( As shown in Fig. 2 (c), the phasMeIin the sc condition
s ( 0 < at 4 K, 0 T is almost metallic based on the h ω i dis-
Axi (c)T=4K,B =0T,sc (d)T=4K,B =5T,sc 2100 tribution shifting to around 2150 cm−1 below ωMI. The
Y 2000 centerwavenumber is equalto thatofd00 inthe sc con-
200 dition. This means that the electronic structure of d33
in the S state is the same as that of d00. However, the

100 width is larger than that of d00. The width is similar to
thoseat50K,0Tandalsoat4K,0Tinthefccondition.
0 Thisindicatessimilarinhomogenityunderallconditions.
0 100 200 0 100 200 As shown in Fig. 2 (e), the tail of the high wave number
(e) X Axis ( m) side of the h ω i distribution crosses ωMI. The volume
100 fraction of the metallic phase that is evaluated based on
nsity d c MI b a tphaeripsoenrcwenitthagtehoefftchceohndωitiiobnelionwFωigM. I2i(sb9),9t%he. mInetcaolmlic-
nte50 volume fraction in the sample increases with a decreas-
I
ingcoolingrate. Thisisqualitativelyconsistentwiththe
resultoftheacsusceptibilityexperimentbyTaniguchiet
0 al. [6] However, if the metallic area is regarded as a per-
2000 2200 2400 2600
-1 fectly superconducting phase, the value is much higher
< > (cm )
thanthatreceivedbythe acsusceptibility measurement.
To explainthe controversy,the PI phase is consideredto
FIG.2: (Color) Spatialdistributionsofhω iofd33at 50K, be present in part of the metallic area.
0T(a),4K,0Tinthefastcoolingcondition(b),4K,0Tin
When a magnetic field is applied, the Sstate collapses
theslowcoolingcondition(c)and4K,5Tintheslowcooling
and changes to a PM state as indicated in electrical re-
condition (d). There were no difference at 50 K, 0 T in (a)
sistivity data. [8, 15] As shown in Fig. 2 (d), the spatial
between the two cooling rates. (e) Statistic distributions of
h ω i for conditions in (a) - (d). ωMI is the metal - insulator image of h ω i shifts to the red color side, which indi-
boundary evaluated from d00. (Seetext for detail.) cates the phase changed to very metallic. h ω i indicates
thespectralshapeaswellastheelectronicstructurenear
the Fermi level including the lower- and upper-Hubbard
shows the result at 50 K in the sc condition. There was bands. This indicates that the electron correlationis re-
no difference between the sc and fc cases. The results duced by a magnetic field. As shown in Fig. 2 (e), the
indicate there was no difference between the two cooling center of the h ω i distribution shifts to the low wave
rates. ThismeansthatthePIstateisnotaffectedbythe number side, but the width is almost the same as that
cooling rate. The whole area is in the insulating phase under other conditions.
basedonhω iforthis phasebeingdistinctly higherthan To evaluate the different wave number in h ω i due to
ωMI, as shown in Figure 2 (e). The center wave number eachcondition,the difference(∆)fromthatin50K,0T
of h ω i at 50K, 0 T is about 2510cm−1, similar to that at the same measured points were derived from Figs. 1
observed for d00. Therefore, the electronic structure of and 2. The statistic distribution is plotted in Fig. 3. In
the PI phase in d33 are the same as those in d00. How- the case of d00, the character in the fc condition is more
ever, the 2σ of the h ω i distribution is about 120 cm−1, metallic than that in the sc condition with a similar ∆
much larger than that for d00. Assuming the width in- shape (the 2σ is 84 ± 6 cm−1 for fc and 72 ± 3 cm−1
dicates the level of inhomogenity, the sample surface of for sc). Since the ∆ width is smaller than the h ω i one
d33 is clearly more inhomogeneous than that of d00. (∼ 90 cm−1) in Fig. 1, the phase changes uniformly. On
Inthefccondition,d33isnearlyintheinsulatingphase the other hand, in the case of d33, all of the widths of
at 4 K, 0 T as shown in Fig. 2 (b). This is consistent the∆distributionare73±5cm−1,whichismuchlower
with the observation that d33 is in the AFI phase un- than the h ω i widths shown in Fig. 2 (e). This means
der these conditions. However, the center of h ω i is that the shift values for each point are almost the same,
slightlyshiftedtothe metallic(lowerwavenumber)side, i.e., the spatial inhomogenity shown in Figs. 2 (a) - (d)
as shown in Fig. 2 (e). This puts the AFI phase closer originatesfromthe inhomogenity containedin the mate-
to the ω boundary, which is consistent with the loca- rial itself. The width of ∆ for d33 is similar to that of
MI
tion of the AFI phase being in the vicinity of the Mott d00, and is almost constant for all conditions in d33 and
boundary. The width of the h ω i distribution is similar also in d00. This narrow h ω i distribution for d00 indi-
4
distribution of d33 is larger than that of d00. If the do-

200 main size is larger than the spatial resolution of about
4K,0T, 4K, 0T d00 20 µm in this experiment, the h ω i distribution splits
150
into two parts across ω . However, since the h ω i
fc sc MI
100 distribution of d33 is not a double peak structure but a
broad single peak, the domain size is smaller than the
50
ensity1500 d33 edxispterribimuteinotnalshroeusoldlubtieonnaorfro2w0iµnmth.eHcaosweeovfera,vtehreyhsmωalil
nt domain size because no contrast appears. According to
I 4K,5T, 4K,0T, 4K,0T, a simple simulation performed to explain the wide h ω i
100 sc sc fc distribution of d33, the domain size was evaluated to be
about one order smaller than the spatial resolution, i.e.,
50 the domain size must be a few µm. To clarify the ac-
tual domain size, the normal infrared microscopy is not
0 suitable because of the diffraction limit but a near-field
-600 -400 -200 0
infraredexperimentwiththesub-µmresolutionmaygive
-1
(cm ) us useful information.
Tosummarize,thedirectobservationofthephasesep-
FIG. 3: (Color) Bars: Statistic distributions of the change arationof 75 %-deuterated κ-(ET)2Cu[N(CN)2]Br (d33)
inh ω iineach condition from that in50K,0Tatthesame wasobservedandcomparedwiththenon-deuteratedma-
points. The lines are Gaussian fitted curves. ∆ indicates terial(d00)usinginfraredmagneto-opticalimagingspec-
h ω iT,B - h ω i50K,0T. troscopy. The sample surface of d33 was found to be
more inhomogeneous than that of d00. This inhomogen-
ity was consistent under different conditions, including
cates that the phase transition was highly uniform. On under magnetic fields. The phase separationin d33 orig-
the other hand, the wide h ω i distribution for d33 ex- inates from the inhomogenity contained in the material.
plainsthemetal-insulatorphasetransition,i.e.,themost The domain size of the metallic and insulating states in
metallic (insulating) side remains in the metallic (insu- d33atthegroundstateisconcludedtobeafewµm. The
lating) phase even though the other area changes to the reflectivityspectrumofd33at5Twasmoremetallicthan
insulating (metallic) phase. Therefore the inhomogenity that at 0 T. This reason is that the electron correlation
containedin the partialdeuteratedmaterialitselfis con- plays an important role for the superconducting state at
cluded to be the origin of the metal (superconductor) - 0 T.
insulator phase separation. We would like to thank SPring-8 BL43IR staff mem-
Finally, the domain size of the metal and insulator bers for their technical support. The experiments were
states is discussed. The experimental results indicate performed with the approval of the Japan Synchrotron
that the sample surface of d33 itself is more inhomoge- RadiationResearchInstitute(ProposalNos. 2002B0041-
neous than that of d00 because the width of the h ω i NS1-np and 2003A0076-NS1-np).
[1] T. Ishiguro, Y. Yamaji and G. Saito: Organic Supercon- H. Eisaki, S. Uchida and J.C. Davis, Nature 415, 412
ductor 2nd Ed.,p. 443, (Springer, 1998). (2002).
[2] K.Kanoda, Physica C 282-287, 299 (1997). [11] S. Kimura, T. Nanba, T. Sada, M. Okuno, M. Mat-
[3] K.Kanoda, HyperfineInteract.104, 235 (1997). sunami,K.Shinoda,H.Kimura,T.Moriwaki,M.Yama-
[4] A. Kawamoto, H. Taniguchi and K. Kanoda, J. Am. gata,Y.Kondo,Y.Yoshimatsu,T.Takahashi,K.Fukui,
Chem. Soc. 120, 10984 (1998). T. Kawamoto and T. Ishikawa Nucl. Instrum. Meth. A
[5] A.Kawamoto, K.Miyagawa andK.Kanoda,Phys.Rev. 467-468, 893 (2001).
B 55, 14140 (1997). [12] S. Kimura, T. Nishi, T. Takahashi, T. Hirono, Y. Ike-
[6] H. Taniguchi, A. Kawamoto, K. Kanoda, Phys. Rev. B moto, T. Moriwaki and H.Kimura, Physica B 329-333,
59, 8424 (1999). 1625 (2003).
[7] A.Kawamoto,M.Yamashita,K.Kumagai,H.Taniguchi, [13] J.E. Eldridge, K.Kornelsen, H.H.Wang, J.M. Williams,
Y.Nakazawa,K.MiyagawaandK.Kanoda,Synth.Met- A.V. Strieby Crouch, D.M. Watkins, Solid State Com-
als 133-134, 125 (2003). mun. 79, 583 (1991).
[8] H.Taniguchi, K.KanodaandA.Kawamoto, Phys.Rev. [14] E.Griesshaber,M.Schiller,D.Schweitzer,I.Heinen,W.
B 67, 014510 (2003). Strunz,Physica C 317-318, 421 (1999).
[9] T. Nishi, S. Kimura, T. Takahashi, K. Miyagawa, A. [15] K. Kanoda, Solid State Phys. 76 (2001) 733. (in
Kawamoto and K. Kanoda, submitted. Japanese).
[10] K.M. Lang, V. Madhavan, J.E. Hoffman, E.W. Hudson,