We report magnetically modulated microwave absorption (MMMA) at low magnetic fields in crystals of the organic superconductors &#945;t-(BEDT-TTF)2I3 and &#954;-(BEDT-TTF)2Cu(NCS)2. In both cases a strong MMMA-signal can easily be found below the critical temperature Tc. In some selected crystals series of narrow, periodically spaced lines are observed comparable to corresponding spectra of the CuO-type high Tcsuperconductors. A model developed for the HTSC's single crystals is used to interpret the results of the organic superconductors

Bele, Petra

Brunner, Hermann

Keller, Heimo J.

Schweitzer, Dieter

Online Publikationen der Universität Stuttgart

E:>Pergamon Solid Slate Communications. Vol. 92. No.3, pp, 189-193. 1994 
Elsevier Science Ltd 
Printed in Great Britain 
0038-1098(94)005 17-6 0038-1098(94)$7 .00+.00 
MAGNETICALLY MODULATED MICROWAVE ABSORPTION (MMMA) AT LOW MAGNETIC 
FIELDS IN (BEDT - TTF) - SUPERCONDUCTORS § 
tiP. Bele, 'H. Brunner, *0. Schweitzer and +H.J. Keller 
# Max Planck Institut fOr Medizinische Forsdlung, AG MolekOlkristalle, Jahnstr.29, 
69120 Heidelberg, Gennany 
• 3. Physikalisdles Institut der Universitat Stuttgart, Pfaffenwaldring 57, 
70550 Stuttgart, Gennany 
+ Anorganisdl Chemisches Institut der Universitat Heidelberg, 
1m Neuenheimer Feld 270, 691 20 Heidelberg, Gennany 
( received 20. May 1994 by J. Kuhl ) 
(Accepted I July 1994) 
We report magnetically modulated microwave absorption ( MMMA ) at low 
magnetic fields in crystals of the organic superoondudors adBEDT-TTF~ 13 
and 1C.-(BEDT.TTF)2Cu(NCS)Z' In both cases a strong MMMA - signal can easily 
be found below the critical temperature T Co In some selected aystals series of 
narrow, periodically spaced lines are observed comparable to corresponding 
spectra of the CuO - type high Tcsuperconductors. A model developed for the 
HTSC 's single crystals is used to interpret the results of the organic 
superconductors. 
Keywords 
A. Superconductors 
D. Fluxpinning and creep 
Introduction 
In the last few years magnetically modulated 
microwave absorption at low magnetic fields has 
been extensively studied in HTSC 'so MMMA is a 
very efficient and sensitive method for investigating 
superconductivity of a sample. Of great interest are 
measurements on single crystals where highly 
resolved periodically structured spectra in weak 
external magnetic fields are observed. Their 
dependence on the orientation of the crystal, the 
temperature, field modulation and microwave 
threshold power is still a challenge for a compre-
hensive th8O()' of the MMMA - effect 11 - 6/. 
For organic superconductors based on 
bis( ethylenedithio )tetrathiafulvalene (BEDT -TTF) 
only a few measurements showing the appearance 
of the MMMA - effect were reported so far, but no 
narrow, periodically spaced lines were observed 1 7 -
9/. 
In this tetter we present experiments of 
magnetically modulated microwave absorption at 
low magnetic fields in crystals of the organic super-
conductors at-!BEDT~TTF)213 and x-{BEDT-
TTF)2Cu(NCS)2· 
In both cases we observed spectra showing a 
temperature and field dependence of the MMMA -
signal below the crttical temperature T c and for the 
first time we are able to present experimental results 
lHhich exhibit a pattern of highly resolved lines 
periodic to the applied external magnetic field Bo. 
These high resolution was found only for crystals of 
very good quality. 
The experimental results are interpreted with the 
theory developed for the HTSC 's 1 2, 10, 11 I . 
Experimental 
a-(BEDT-TTFl213 crystals were grown from a 
THF - solution by electrochemical methods 112/. By 
tempering the crystals tor 4 to 6 days at a 
temperature of 75 ·C a structural transition to the 
at - phase occurs lHhich has been investigated with 
several techniques 1 13, 14 I. Due to the stress 
S Reported at the • FrOhjatll~agung FestkOrperphysik der Oeutschen Physikalischen 
GeseIlschaft, MOnster 1~"; ISSN 0420 - 0195, p.l634 
189 
190 MAGNETICALLY MODULATED MICROWAVE ABSORPTION Vol. 92. No.3 
during the Ct - to the (Xt - phase transformation the 
crystals showed a strong mosaicity seen in visible 
polarized reflected light, but the crystal structure of 
<lHBEDT-TTF)213 COOld not be determined. 
However, the reflections which are observed in 
Weissenberg - pictures for ar(BEDT-TTF>213 are 
almost the same as for P-(BEDT-TTF)2'3_ That 
means the unit cells of <lHBEDT-TTF)213 and ~­
(BEDT-TTF)213'r<! vory similar 115/. The tempered 
<It - modification is stable at room temperature and 
the crystals exhibit a superconducting transition near 
BK/13/. 
x-(BEDT -TTF>2Cu(NCS>2 is an organic super-
COf'Iductor with one of the highest transition tem-
perature ( Tc <= 10.4 K ) /16 I. The crystals were 
grown by electrochemical methods as described 
ear1ier in J 17 I. 
The magnetically modulated microwave absorption 
measurements were carried out with a VARIAN E -
line EPR spectrometer which operates in the X -
band, and an Oxford Instruments ESR910 helium 
cryostat. The plate - like <lHBEDT-TTF)213 or 
t.he distorted - hexagon - shaped .-{BEDT-
TTF)2Cu(NCS)2 crystals were fixed with silicon 
grease to the flat end of a suprasil quartz holder in a 
TE102 microwave cavity. All samples were oriented 
with the microwave magnetic field Bmw perpen-
dicular and the external magnetic field Bo parallel to 
the conducting plane of the crystal. Bo was 
modulated with 100kHz. The measurements of the 
highly resolved spectra were carried out at a 
temperature of 1.8 K 
Results 
As mentioned before MMMA is a good method 
for detecting the superconducting properties of a 
sample. 
Fig. 1 shows the typical MMMA - signal as a 
function of the temperature of the two organic 
superconductors <XHBEDT-TTF)213 and . -{BEDT-
ITF)2CU(NCS)2 measured with a high modulation 
amplitude. Such signals allow the determination of 
the onset temperature T c of superconductivity. One 
obtains for at-{BEDT-TTF>213 Tc ow 8 K and for 
. -{BEDT-TTF)2CU(NCS)2 T c = 10.4 K These results 
are in good agreement with the valUes known from 
conductivity, thermopower and Meissner - effect 
measurements. 
Fig. 2 presents a MMMA - signal recorded with a 
high modulation amplitude as a function of the field 
Bo in the range from -5 mT to +3 mT at a 
temperature of 1.8 K for a ar(BEDT -TTF)ZI3 crystal. 
One can see an asymmetry of the signal which 
depends on the direction of the scan of Bo . a very 
small hysteresis Of the MMMA - signal Can be 
observed. 
In figure 3 magnetically modulated microwave 
absorption spectra at low magnetic fields are shown 
for an ar(BEDT-nF}213 crystal. The spectra exhibit 
a pattem of well resolved periodical lines as it is 
0) 
ut-(BEDT·TTF)~J 
2 4 6 8 10 12 14 T[K] 
b) 
.. ~8EDT·TTFl;z Cio (NCS~ 
T [KJ 
2 10 18 
Fig. 1 MMMA - Signal at low magnetic field with 
high modulation amplitude as a function of 
temperature for two samples of organic 
superconductors. a) adBEDT -TTF)213 and b) K-
(BEDT-TTF)2Cu(NCS)2_ Microwave power is Pmw 
= 2 mW, modulation amplitude 3.2 mT and Bo = +1 
mT. 
obtained in YBaCuO single crystals /1 - 6/. We can 
detect this line pattem over a very broad scan range 
of Bo. The range is as large as 10 mT for the best 
crystal orientation. To receive the best line pattem it 
was necessary to adjust the measurement setup 
( microwave power, modulation amplitude, crystal 
orientation etc. ) individually for each crystal. We 
note that the resolution of the periodicaillnes is very 
sensitive to the microwave power, in this case 
80 IlW. No signal oould be detected below this 
threshold value. Another important feature is that the 
direction of the microwave field Bmw must be 
perpendicular to the conducting plane of the crystal 
and to Bo. The average spacing between two lines 
Is 6Bo ~ 55 11 T and the equidistant lines are 
observed over a total scan range of 10 mT for this 
particular <l(-{BEDT-TTF)213 crystal. Fig. 3a) shows 
the MMMA - signal for a part ( 6 m T ) of the total 
field rangs. In figure 3b) only a small range of the 
spectl1.lm ( 1.6 mT ) is presented for better 
Vol. 92, No.3 MAGNETICALLY MODULATED MICROWAVE ABSORPTION 191 
L_-,:5- ---0~---+3,.-~ Bo [mT] 
Fig. 2 Hysteresis of the MMMA - signal with high 
modulation amplitude at low magnetic field for a 
sample of arlBEDT-TTF>213. Temperature is 1.8 K. 
microwave power Prnw = 2 mW, modulation ampli-
tude 3.2 mT and the cycled range of the external 
magnetic field Bo varies from -5 mT to +3 mT. 
displaying the shape and the spacing of the lines. 
One can clearly see that every single line consists of 
two separate peaks, i. e. an upper and a lower lobe, 
as predicted from the theory 110 I, 
The MMMA - signal of the other organic super-
conductor. <-(BEDT-TTF)2Cu(NCS)2 • is shown in 
figure 4. As for the at-C1)'stal the degree of resolution 
of the line pattern of the signal depends sensitively 
0) 
Fig. 3 Low field MMMA as a function of the external 
magnetic field Bo for at-{BEDT-TTF)213. 
Temperature is 1.8 K, Prnw = 0.08 mW ilnd 
modulation amplitude 10 mT.( Both spectra shown 
are only parts of the whole spectrum for a better 
display of the resolved lines). 
o L---------l0~BowT] 
Fig. 4 Low field MMMA as a function of the external 
magnetic field Bo for <-jBEDT-TTF)2CU(NCS)2' 
Temperature is 1.8 K. modulation amplitude 10 mT 
and Pmw = 0.008 mW. Direct detection with a 
second field modulation of 38 Hz and an amplitude 
of 100 mT via storage oscilloscope. 
on the parameter setup. The most important 
difference in the MMMA - signal of this organic 
superconductor and that of adBEOT-TTFaI3 is the 
much smaller spacing 6Bo = 0.9 IlT of the lines. To 
get the best resolution we had to go to a microwave 
power Pmw = 8 IlW which was only slightly above 
the threshold value', This forced us to deted this 
spectrum without AFC - control. Under this condition 
the stability of the spectrometer was not sufficient for 
a longtime field scan. Therefore we detected the 
MMMA - spectrum with a second field modulation of 
38 Hz and an amplitude of 100 mT directly on a 
storage oscilloscope. By varying Bo a highly 
resolved line pattern could be observed over a total 
f ield range of 5 mT with this technique. 
Theory and discussion 
An interpretation of the magnetically modulated 
microwave absorption spectra showing a series of 
equidistant tines is possible with a theoretical model 
first described by AH. Silver and J.E. Zimmerman 
1 10 1 and modified for HTeS 's by T, Xia and 
D. Stroud I 11 1 and H. Vichery, F. Beuneu and 
P. Lejay I 2 I . 
In the model one considers a superconducting 
planar loop with an area A containing N identical 
Josephson junctions. Under the influence of an 
external magnetic field Bo the loop supports a 
screening supercurrent and - as usual for a type II 
superconductor - the loop is penetrated by a 
magnetic flux ¢lex! which is an integer multiple of the 
flux quantum ¢lo. The magnetic energy of the loop 
shows a periodic structure with broad minima 
depending on the flux density ¢lextf¢lo. The energy 
of lin individual ~tate of the loop is characterized by 
192 MAGNETICALLY MOOULATIlD MICROWAVE ABSORPTION Vol. 92. No. 3 
an integer parameter p, which is connected to the 
flux through the loop. In the regime of the maxima 
where the supercurrent in the loop approaches the 
c(ilical CUrrent in the Josephson junctions, the 
energies of t'NO states p and p+1 overlap. All 
essential assumption of the model is that only under 
the condition 0( a critical current a transition between 
two overlapping energy states and a dlange of the 
flux through the loop is possible. The observed 
microwave absorption is then understood as losses 
in the shunt resistance of the Josephson junction, 
when the jundion becomes normal conducting. 
The quantitative analysis of this model given by 
the above mentioned authors predicts that a 
spectrum of periodic line pairs ( lobe up and down ) 
is to be expected if 8 0 is scanned and that every 
single line is due to the increase or decrease of the 
flux in the loop by one flux quantum. From the 
spacing 480 between two lines the size of the area 
A of the loop can be estimated as 
A = etto 1 .680 
where the flux quantum <1>0 is given to 2.07 " 
10-15 Tlm2 . 
Inserting the measured values of ABo into this 
equation one obtains 
and 
A2 • 2.3 , 10-9 m2 02300 ~ for <-(BEDT-
TTF)2CU(NCS)2 . 
The large dffference in the size of these areas 
( their ratio is .... 60 ) can be explained by the fact that 
the surface of Clt-(BEDT-TIF)213 crystals pos.esses 
a mosaic structure after the tempering process. This 
leads to a lot of surface defects which evidently limit 
the areas of the loops. The high resolution in the 
MMMA - spectra of the crystals of <-(BEDT-
TTF)2CU(NCS)2 was reproducible only with seleded 
aystal specimens. A1l average crystal of this organic 
superconductor shows a spectrum with a few broad 
lines which are relatively independent of the 
microwave power Pmw. The existence of a threshold 
of the microwave power Pmw can be explained by 
the Josephson junction model, too. 
When we compare the size of the areas A1 and 
A2 inferred from our measurements of the organic 
supercondudors with analog values for HTSC 's 
118 - 20 1 we find Ihat A1 of ar(BEDT-TIF)213 is 
-2 times smaller than the smallest HTSC value 
while A2 of IC-(BEDT-TTF)2Cu(NCS)2 is -2 times 
larger than the largest value found for HTSC ·s. The 
reason for the larger dispartty of the organic super-
conductors compared to the HTSC's may be 
connected with the larger difference of the surface 
qualities in the organic supeconductors. Nerver-
theless the real nature of the Josephson junctions 
and of the crystal defects limiting the loop areas is 
still unclear. 
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193 


Magnetically modulated microwave absorption (MMMA) at low magnetic fields in (BEDT-TTF)-superconductors

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Magnetically modulated microwave absorption (MMMA) at low magnetic fields in (BEDT-TTF)-superconductors

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