- Email: [email protected]
Hysteretic magnetic state in the organic τ-phase conductors
ELSEVIER
Synthetic
Metals 94 ( 1998) 69-72
Hysteretic magnetic state in the organic T-phase conductors Keizo Murata ‘**, Harukazu Yoshino a, Yoshinori a Deparment ’ Theoretical and
of Material Science. Faculiy Physical Chemismry Institule,
of Science,
National
Osaka Hellenic
Tsubaki a-‘, George C. Papavassiliou
b
City Univrrsizy, Osaka 5584585, Japan Research Foundarion, Athens 116/35, Greece
Abstract The T-phase conductors are known to exhibit resistance upturn and negative magnetoresistance below T,,,in - 50 K. Well below Tminr at least in the range of about 1 to several K, a large hysteresis in the field sweep as well as angle sweep is observed in the configuration of current, I,, field, with B,, or B, , where 11and I. refer to the two-dimensional u-b plane. Furthermore, with I, and B,,, symmetry of or I,, and magnetic the angular dependence of magnetoresistance varied between four-fold and two-fold depending on the region of T and B. Two-fold symmetry is hard to understand from the symmetry of the crystal and the relevant Fermi surface. Although many of the phenomena are unexplained, we believe that we have observed a new type of electronic property. 0 1998 Elsevier Science S.A. All rights reserved. Keywords:
Organic
T-phase conductors;
Hysteresis
1. Introduction The -r-phase organic conductors are specified by their crystal structure, dispersion relation and resultant Fermi surface, which is shown in Fig. I [ I]. These conductors are electronically as well as mechanically two-dimensional. The anisotropy ratio in resistivity between the out-of-plane and in-plane is 104-105. The crystals arc cleavable as mica. Even within the two-dimensional plane, anisotropy is expected, which is four-fold inferred from the star-shaped structure of the Fermi surface. The Fermi energy measured from the bottom of the band is as low as room temperature so that the four fingers of the star-shaped Fermi surface are not clearly defined (i.e. not degenerate) around room temperature. (The line in Fig. 1 just shows the average.) It is also noted that the electron group velocity, uF, is extremely low around this finger of the Fermi surface and is about 10’ cm/s in the ( 110) directions, which is quite a usual value. This means that the Fermi velocity is also anisotropic in the k,-k,, plane of the well-defined and less well-defined Fermi surface, along ( 110) and ( loo), respectively. The T-phase organic conductors consist of donor molecules and anions with the ratio of 2: ( 1 + y), where y is almost 0.75 from elemental analysis [ 21. The values expressed by 1 and y correspond to two different locations in the crystal. Five * Corresponding author. Tel.: +81 6 605 2509; fax: e-mail: [email protected] ’ On leave from the Science University of Tokyo. 0379-6779/98/$19.00 PUSO379-6779(97)04145-3
0
1998 Elsevier
f81
6 690 2710;
Science S.A. All rights reserved.
X
Fig. 1. Two-dimensional Fermi surface of typical (N-N)AuBr, (after Ref. [ I ] ). Other T-conductors surfaces.
T-phase conductor, have similar Fermi
kinds of salts have been obtained and examined with the donor molecules EDO-( S,S) -DMEDT-7”TF or P-( S,S) DMEDT-TI’F, and with linear anions AuBr,, I,, IBr, or Ag(CN),. Since the difference between the two donors comes from two oxygen atoms for EDO-(S,S)-DMEDTTTF and two nitrogen atoms for the other, we symbolically express these donors by (O-O) and (N-N). In the (O-O) salts, the hexagon including two oxygens is not flat, and then twisted. The molecular site dependence of this right-handed or left-handed twisting is non-periodic, i.e., disordered. On the other hand, in the (N-N) salts, there is a similar kind of disorder, but the origin of the location inside the molecule is different, i.e., the hexagon including two methyl groups. What is interesting is that we can realize metallic to insulating (dRldT< 0) salts depending on the combination of
70
K. Muram
et al. /Synthetic
Metals
94 (1998)
69-72
donors and anions, in the order of (O-O)AuBr,, (N-N)AuBr,, (O-0)1,, (N-N)IBr, and (0-O)Ag(CN),. Common to all below around 50 K, the salts show insulating behavior, i.e., the most metallic (0-O)AuBr, shows resistance upturn and the insulating (0-O)Ag(CN)* shows steeper rise in resistance versus T than at T>50 K. In this low temperature region, negative magnetoresistancesuddenly appears [ 31. The purpose of the present paper is to showthe remarkablepropertiesin theselow temperatureelectronic states,which were recently observed. 0
2. Experiment Electrical contacts were madewith Au wires, andAu paste on the Au-evaporated surfaceson the crystal. For the in-plane (a-b plane) resistivity study (p,,), the four terminals were on the sameside and, for the out-of-plane ( pI ) study, the sample was sandwiched with pairs of voltage and current contacts. This pI configuration was adopted to study the anisotropy within the plane, where magneticfield wasapplied parallel to the a-b plane and was rotated in the plane. The configuration of the Hall effect measurementwas with the current terminal on both endsand voltage terminals on both sidesof the plane, and the field along the c-axis.
0
As mentioned above. we could obtain a variety of salts from metallic to insulating, i.e., (0-O)AuBr, and (N-N)AuBr,. Even for the metallic samples,the resistivity (both p,, and pI) of the (N-N)AuBr, is almost flat down to 100 K and a sudden decrease in p by lowering temperature is observed [4]. Metallic (O-0)1, saltsshow similar behavior to (N-N)AuBr,. Both metallic and insulating saltsare found in these salts [3] even in the samebatch. We interpret that this is due to the fact that (O-0)1, is locatedjust at the border between metallic and insulating groups of salts. The most metallic (0-O)AuBr, doesnot show this level-off structure in the resistivity versus temperatureabove 100 K. Also, as mentioned in our previous papers, all the salts show resistanceupturn below around 50 K. To examine the nature of the low and high temperature states,we performed the Hall effect study. We could achieve the Hall effect measurementonly for the first two metallic salts,which are shown in Fig. 2. What is seenin the Hall effect R, is that the two saltsshow quite different temperaturedependence.The large difference in RH( T) must be due to the delicate difference in the anisotropy in vr as well as r (scattering life time) on the Fermi surface and may also be indirectly related to the realspacedisorder mentioned in Section 1. A trial calculation is carried out based on this anisotropy of r in k-space at the Fermi energy [5]. We emphasize that the temperature dependenceof the Hall effect is a commonly observedphenomenon with the two-dimensional metallic system with
200 (K)
300
-6~10.~ @I
3. General behavior: resistivity and Hall effect
100 Temperature
(4
50
100
150
200
250
300
7 (K)
Fig. 2. Temperature dependence of the Hall effect of ( N-N)AuBrZ (a) and (O-O)AuBr, (b) with I,, and V,, and H,, where (1 and I refer to the a-b plane. Magnetic field for this measurement is 5 T. Two sets of data are the different groups of terminals, showing good current configuration. Samples are square plate in shape. The current and voltage terminals are at the four edges. The line in (b) is a guide for the eye. The unit in (b) is not reduced to the usual unit of R,, but the order of magnitude of the absolute value of the low temperature flat value is almost the stoichiometric value
Fermi surface touching at the Brillouin zone boundary [ 61. However, around50 and 100K, where resistivity behavior is changing, no appreciablechangein Hall effect is observed.
4. Electronic properties at low temperature Especially for the three metallic salts,below around 50 K, resistivity upturn as well as negative magnetoresistanceis observed. We confine ourselvesto thesethree metallic salts, i.e., (O-O)AuBr,, (N-N)AuBr,, (O-0)1,. In somecases, as with (0-0)13, the sudden negative magnetoresistance behavior appearsslightly above the temperatureof the resistance upturn, as shown in Fig. 3. Hence, it is still unsolved whether or not these two phenomenaare related. At least, since the negative magnetoresistanceappearssuddenly in temperature, the negative magnetoresistancemay not be related to localization, which is expected to appeargradually. A preliminary magnetoresistancemeasurementup to 35 T showsa minimum around 28 T [7]. In (O-O)AuBr,, with I,, hysteresisin the resistanceversus temperaturesweepis
K. Murata
“0
50
100
150 T
200
250
et al. /Svnthetic
300
(K)
Metals
94 (1998)
71
69-72
In the configuration of I, and with B,,, we recently found a very interesting phenomenon.With B,, rotated in the u-b plane, it would be quite natural to expect the magnetoresistante anisotropy to be four-fold if there is any. What we did observe is sometimesfour-fold and sometimestwo-fold! Even for the four-fold symmetry, peak and bottom change dependingon the field strengthandtemperature.Fig. 5 shows the raw data, and Fig. 6 the schematicclassification of the observedangular dependencesas a function of temperature and magnetic field. We note that, in this configuration of I and B, hysteresisis lessappreciable.Although our measurement is very preliminary and it is hard to explain at present just what we have observed,we believe that we haveobtained
Fig. 3. Temperature dependence of resistivity with and without magnetic field of 5 T in (O-0)1,. Sudden change with held appears at about 50 K.
H//c I
H//oh I
Hlla Hllb ..:;.A. i ,,:‘-A. ....” .. .... .*: .,:;.*.-. f
0.43 %
0
50
100
0.47 %
150
$ (degree) Fig. 4. Angular dependence of magnetoresistance of (O-0)1, with field of 5 T. Current is in the a-b plane. The data are taken from the sample under a frozen medium at pressure 2 kbar (0.2 GPa), which is proof that the sample is not rotated during this data acquisition.
appreciable, suggestiveof a structural change at these low temperatures.However, around 50 K, no appreciablechange in structure was detected by X-ray examination by Kagoshima [ 81. Preliminary specific heat measurementdoesnot show any indication of phasetransition either electronic or structural. However, a more precise specific heat study is planned. Although neither X-ray nor specific heat study indicates phasetransition around 50 K, magnetoresistanceanisotropy and the sudden appearanceof negative magnetoresistance still suggesta change at least in electronic structure across this temperature.Previous magnetoresistancestudiesshowa changein the anisotropy in resistanceversusangleof B (magnetic field) rotated in the plane including the c-axis with I,,. Either with I,, or I,, and with B,, or B 1, magnetoresistance showsnegative andhysteretic behavior in the field sweepand even with angle sweep which is typically shown in Fig. 4 [ 31. This stronghysteresisremindsusof the time-dependent Hall effect or magnetoresistancewith the term ‘magnetic ViSCOSity’
in
K- (BEDT-TTF)
2cU
[ N(
CN)
21 cl
(BEDT-
TTF = bis( ethylenedithio) tetrathiafulvalene) [ 91. At least concerning the hysteresisin the field sweep,the phenomenon is much stronger in the r-system.
‘”3 %
.. .. , ~.:;‘;.” 11.6~ ,, \‘., I”’ . , ,+-’._ ,I:,..‘I‘.- -i-..;-2 .:.,:, :.y- .~-..,:-“~~.~ ‘/:. :I.?a r,. 0.38 % .s -:.-, ..‘.f ‘y..:*;.; t ;;....-.--..._- A i.c’,..‘..“..;.~ I ‘.. I.IK ,:.:.,.. .k. (.‘. 1.01 % “-.,.*~ 1 ,,;.,:.-.--:’ .‘*2,,,, ,._:-.’ . . _i..f -30
0
30
60
90
120
150
180
210
Q,(degree) Fig. 5. In-plane angular dependence in field of I .5 T with I,.
t
of magnetoresistance
4-fold Max n+b Min a.6
2- fold Max a Min b
4-fold Max a,b Min a+6
/la fib tw I
of (0-O)AuBr,
I 15 Magnetic
!ilkd! lla lib I 5.0 Field (T)
c
Fig. 6. A schematic summary of the angular dependence of magnetoresistante of (0-O)AuBr, in various held and temperature regions. T, in the table is around 2.5 K, where apparent symmetry in pI in the rotated magnetic field in the 2D plane changes.
72
K. Murata
et al. /Synthetic
a new electronic property which has not yet been observed or examined.
Metals
94 (1998)
69-72
Acknowledgements The collaboration of N.A. Fortune and N. Shirakawa at the Electrotechnical Laboratory was much appreciated at the early stage of this work.
5. Summary The T-phase conductors show a variety of electronic properties from metal to insulator depending on the combination of cation donor molecules and linear anion acceptors. Common to all r-phase conductors, below about 50 K, resistance upturn and negative magnetoresistance are observed. In most configurations of Z and B, large hysteresisin the field sweep and angular sweepis observed. With Zi and with B, rotated in the u-b plane, magnetoresistanceinterchangesbetween four-fold andtwo-fold dependingon the temperatureandfield strength. We believe that we have observeda completely new electronic property.
References 111J.S. Zambounis et al., Solid State Commun. [21 G.C. Papavassiliou et al., Synth. Met. 69-71
95 ( 1995) 21 I-215. (1995) 787-788. 131 K. Murata et al., Synth. Met. 84-86 ( 1997) 2021-2022. D.D. Lagouvardos, J.S. 141 N.A. Fortune, K. Murata, G.C. Papavassiliou, Zambounis, Mater. Res. Symp. Proc. 328 ( 1994) 307-3 12. N.A. Fortune, P.J. Fons, K. Murata, Synth. Met. 69-71 (1995) lOOI1004.
161K. (71 ISI [91
Murata, J. Phys. I (Paris) 6 ( 1996) 1865-I 873. G.C. Papavassiliou et al., Synth. Met. 84-86 ( 1997) 2043-2044. S. Kagoshima, personal communication. T. Ishiguro et al., Synth. Met. 84-86 ( 1997) 1471-1478.
Synthetic
Metals 94 ( 1998) 69-72
Hysteretic magnetic state in the organic T-phase conductors Keizo Murata ‘**, Harukazu Yoshino a, Yoshinori a Deparment ’ Theoretical and
of Material Science. Faculiy Physical Chemismry Institule,
of Science,
National
Osaka Hellenic
Tsubaki a-‘, George C. Papavassiliou
b
City Univrrsizy, Osaka 5584585, Japan Research Foundarion, Athens 116/35, Greece
Abstract The T-phase conductors are known to exhibit resistance upturn and negative magnetoresistance below T,,,in - 50 K. Well below Tminr at least in the range of about 1 to several K, a large hysteresis in the field sweep as well as angle sweep is observed in the configuration of current, I,, field, with B,, or B, , where 11and I. refer to the two-dimensional u-b plane. Furthermore, with I, and B,,, symmetry of or I,, and magnetic the angular dependence of magnetoresistance varied between four-fold and two-fold depending on the region of T and B. Two-fold symmetry is hard to understand from the symmetry of the crystal and the relevant Fermi surface. Although many of the phenomena are unexplained, we believe that we have observed a new type of electronic property. 0 1998 Elsevier Science S.A. All rights reserved. Keywords:
Organic
T-phase conductors;
Hysteresis
1. Introduction The -r-phase organic conductors are specified by their crystal structure, dispersion relation and resultant Fermi surface, which is shown in Fig. I [ I]. These conductors are electronically as well as mechanically two-dimensional. The anisotropy ratio in resistivity between the out-of-plane and in-plane is 104-105. The crystals arc cleavable as mica. Even within the two-dimensional plane, anisotropy is expected, which is four-fold inferred from the star-shaped structure of the Fermi surface. The Fermi energy measured from the bottom of the band is as low as room temperature so that the four fingers of the star-shaped Fermi surface are not clearly defined (i.e. not degenerate) around room temperature. (The line in Fig. 1 just shows the average.) It is also noted that the electron group velocity, uF, is extremely low around this finger of the Fermi surface and is about 10’ cm/s in the ( 110) directions, which is quite a usual value. This means that the Fermi velocity is also anisotropic in the k,-k,, plane of the well-defined and less well-defined Fermi surface, along ( 110) and ( loo), respectively. The T-phase organic conductors consist of donor molecules and anions with the ratio of 2: ( 1 + y), where y is almost 0.75 from elemental analysis [ 21. The values expressed by 1 and y correspond to two different locations in the crystal. Five * Corresponding author. Tel.: +81 6 605 2509; fax: e-mail: [email protected] ’ On leave from the Science University of Tokyo. 0379-6779/98/$19.00 PUSO379-6779(97)04145-3
0
1998 Elsevier
f81
6 690 2710;
Science S.A. All rights reserved.
X
Fig. 1. Two-dimensional Fermi surface of typical (N-N)AuBr, (after Ref. [ I ] ). Other T-conductors surfaces.
T-phase conductor, have similar Fermi
kinds of salts have been obtained and examined with the donor molecules EDO-( S,S) -DMEDT-7”TF or P-( S,S) DMEDT-TI’F, and with linear anions AuBr,, I,, IBr, or Ag(CN),. Since the difference between the two donors comes from two oxygen atoms for EDO-(S,S)-DMEDTTTF and two nitrogen atoms for the other, we symbolically express these donors by (O-O) and (N-N). In the (O-O) salts, the hexagon including two oxygens is not flat, and then twisted. The molecular site dependence of this right-handed or left-handed twisting is non-periodic, i.e., disordered. On the other hand, in the (N-N) salts, there is a similar kind of disorder, but the origin of the location inside the molecule is different, i.e., the hexagon including two methyl groups. What is interesting is that we can realize metallic to insulating (dRldT< 0) salts depending on the combination of
70
K. Muram
et al. /Synthetic
Metals
94 (1998)
69-72
donors and anions, in the order of (O-O)AuBr,, (N-N)AuBr,, (O-0)1,, (N-N)IBr, and (0-O)Ag(CN),. Common to all below around 50 K, the salts show insulating behavior, i.e., the most metallic (0-O)AuBr, shows resistance upturn and the insulating (0-O)Ag(CN)* shows steeper rise in resistance versus T than at T>50 K. In this low temperature region, negative magnetoresistancesuddenly appears [ 31. The purpose of the present paper is to showthe remarkablepropertiesin theselow temperatureelectronic states,which were recently observed. 0
2. Experiment Electrical contacts were madewith Au wires, andAu paste on the Au-evaporated surfaceson the crystal. For the in-plane (a-b plane) resistivity study (p,,), the four terminals were on the sameside and, for the out-of-plane ( pI ) study, the sample was sandwiched with pairs of voltage and current contacts. This pI configuration was adopted to study the anisotropy within the plane, where magneticfield wasapplied parallel to the a-b plane and was rotated in the plane. The configuration of the Hall effect measurementwas with the current terminal on both endsand voltage terminals on both sidesof the plane, and the field along the c-axis.
0
As mentioned above. we could obtain a variety of salts from metallic to insulating, i.e., (0-O)AuBr, and (N-N)AuBr,. Even for the metallic samples,the resistivity (both p,, and pI) of the (N-N)AuBr, is almost flat down to 100 K and a sudden decrease in p by lowering temperature is observed [4]. Metallic (O-0)1, saltsshow similar behavior to (N-N)AuBr,. Both metallic and insulating saltsare found in these salts [3] even in the samebatch. We interpret that this is due to the fact that (O-0)1, is locatedjust at the border between metallic and insulating groups of salts. The most metallic (0-O)AuBr, doesnot show this level-off structure in the resistivity versus temperatureabove 100 K. Also, as mentioned in our previous papers, all the salts show resistanceupturn below around 50 K. To examine the nature of the low and high temperature states,we performed the Hall effect study. We could achieve the Hall effect measurementonly for the first two metallic salts,which are shown in Fig. 2. What is seenin the Hall effect R, is that the two saltsshow quite different temperaturedependence.The large difference in RH( T) must be due to the delicate difference in the anisotropy in vr as well as r (scattering life time) on the Fermi surface and may also be indirectly related to the realspacedisorder mentioned in Section 1. A trial calculation is carried out based on this anisotropy of r in k-space at the Fermi energy [5]. We emphasize that the temperature dependenceof the Hall effect is a commonly observedphenomenon with the two-dimensional metallic system with
200 (K)
300
-6~10.~ @I
3. General behavior: resistivity and Hall effect
100 Temperature
(4
50
100
150
200
250
300
7 (K)
Fig. 2. Temperature dependence of the Hall effect of ( N-N)AuBrZ (a) and (O-O)AuBr, (b) with I,, and V,, and H,, where (1 and I refer to the a-b plane. Magnetic field for this measurement is 5 T. Two sets of data are the different groups of terminals, showing good current configuration. Samples are square plate in shape. The current and voltage terminals are at the four edges. The line in (b) is a guide for the eye. The unit in (b) is not reduced to the usual unit of R,, but the order of magnitude of the absolute value of the low temperature flat value is almost the stoichiometric value
Fermi surface touching at the Brillouin zone boundary [ 61. However, around50 and 100K, where resistivity behavior is changing, no appreciablechangein Hall effect is observed.
4. Electronic properties at low temperature Especially for the three metallic salts,below around 50 K, resistivity upturn as well as negative magnetoresistanceis observed. We confine ourselvesto thesethree metallic salts, i.e., (O-O)AuBr,, (N-N)AuBr,, (O-0)1,. In somecases, as with (0-0)13, the sudden negative magnetoresistance behavior appearsslightly above the temperatureof the resistance upturn, as shown in Fig. 3. Hence, it is still unsolved whether or not these two phenomenaare related. At least, since the negative magnetoresistanceappearssuddenly in temperature, the negative magnetoresistancemay not be related to localization, which is expected to appeargradually. A preliminary magnetoresistancemeasurementup to 35 T showsa minimum around 28 T [7]. In (O-O)AuBr,, with I,, hysteresisin the resistanceversus temperaturesweepis
K. Murata
“0
50
100
150 T
200
250
et al. /Svnthetic
300
(K)
Metals
94 (1998)
71
69-72
In the configuration of I, and with B,,, we recently found a very interesting phenomenon.With B,, rotated in the u-b plane, it would be quite natural to expect the magnetoresistante anisotropy to be four-fold if there is any. What we did observe is sometimesfour-fold and sometimestwo-fold! Even for the four-fold symmetry, peak and bottom change dependingon the field strengthandtemperature.Fig. 5 shows the raw data, and Fig. 6 the schematicclassification of the observedangular dependencesas a function of temperature and magnetic field. We note that, in this configuration of I and B, hysteresisis lessappreciable.Although our measurement is very preliminary and it is hard to explain at present just what we have observed,we believe that we haveobtained
Fig. 3. Temperature dependence of resistivity with and without magnetic field of 5 T in (O-0)1,. Sudden change with held appears at about 50 K.
H//c I
H//oh I
Hlla Hllb ..:;.A. i ,,:‘-A. ....” .. .... .*: .,:;.*.-. f
0.43 %
0
50
100
0.47 %
150
$ (degree) Fig. 4. Angular dependence of magnetoresistance of (O-0)1, with field of 5 T. Current is in the a-b plane. The data are taken from the sample under a frozen medium at pressure 2 kbar (0.2 GPa), which is proof that the sample is not rotated during this data acquisition.
appreciable, suggestiveof a structural change at these low temperatures.However, around 50 K, no appreciablechange in structure was detected by X-ray examination by Kagoshima [ 81. Preliminary specific heat measurementdoesnot show any indication of phasetransition either electronic or structural. However, a more precise specific heat study is planned. Although neither X-ray nor specific heat study indicates phasetransition around 50 K, magnetoresistanceanisotropy and the sudden appearanceof negative magnetoresistance still suggesta change at least in electronic structure across this temperature.Previous magnetoresistancestudiesshowa changein the anisotropy in resistanceversusangleof B (magnetic field) rotated in the plane including the c-axis with I,,. Either with I,, or I,, and with B,, or B 1, magnetoresistance showsnegative andhysteretic behavior in the field sweepand even with angle sweep which is typically shown in Fig. 4 [ 31. This stronghysteresisremindsusof the time-dependent Hall effect or magnetoresistancewith the term ‘magnetic ViSCOSity’
in
K- (BEDT-TTF)
2cU
[ N(
CN)
21 cl
(BEDT-
TTF = bis( ethylenedithio) tetrathiafulvalene) [ 91. At least concerning the hysteresisin the field sweep,the phenomenon is much stronger in the r-system.
‘”3 %
.. .. , ~.:;‘;.” 11.6~ ,, \‘., I”’ . , ,+-’._ ,I:,..‘I‘.- -i-..;-2 .:.,:, :.y- .~-..,:-“~~.~ ‘/:. :I.?a r,. 0.38 % .s -:.-, ..‘.f ‘y..:*;.; t ;;....-.--..._- A i.c’,..‘..“..;.~ I ‘.. I.IK ,:.:.,.. .k. (.‘. 1.01 % “-.,.*~ 1 ,,;.,:.-.--:’ .‘*2,,,, ,._:-.’ . . _i..f -30
0
30
60
90
120
150
180
210
Q,(degree) Fig. 5. In-plane angular dependence in field of I .5 T with I,.
t
of magnetoresistance
4-fold Max n+b Min a.6
2- fold Max a Min b
4-fold Max a,b Min a+6
/la fib tw I
of (0-O)AuBr,
I 15 Magnetic
!ilkd! lla lib I 5.0 Field (T)
c
Fig. 6. A schematic summary of the angular dependence of magnetoresistante of (0-O)AuBr, in various held and temperature regions. T, in the table is around 2.5 K, where apparent symmetry in pI in the rotated magnetic field in the 2D plane changes.
72
K. Murata
et al. /Synthetic
a new electronic property which has not yet been observed or examined.
Metals
94 (1998)
69-72
Acknowledgements The collaboration of N.A. Fortune and N. Shirakawa at the Electrotechnical Laboratory was much appreciated at the early stage of this work.
5. Summary The T-phase conductors show a variety of electronic properties from metal to insulator depending on the combination of cation donor molecules and linear anion acceptors. Common to all r-phase conductors, below about 50 K, resistance upturn and negative magnetoresistance are observed. In most configurations of Z and B, large hysteresisin the field sweep and angular sweepis observed. With Zi and with B, rotated in the u-b plane, magnetoresistanceinterchangesbetween four-fold andtwo-fold dependingon the temperatureandfield strength. We believe that we have observeda completely new electronic property.
References 111J.S. Zambounis et al., Solid State Commun. [21 G.C. Papavassiliou et al., Synth. Met. 69-71
95 ( 1995) 21 I-215. (1995) 787-788. 131 K. Murata et al., Synth. Met. 84-86 ( 1997) 2021-2022. D.D. Lagouvardos, J.S. 141 N.A. Fortune, K. Murata, G.C. Papavassiliou, Zambounis, Mater. Res. Symp. Proc. 328 ( 1994) 307-3 12. N.A. Fortune, P.J. Fons, K. Murata, Synth. Met. 69-71 (1995) lOOI1004.
161K. (71 ISI [91
Murata, J. Phys. I (Paris) 6 ( 1996) 1865-I 873. G.C. Papavassiliou et al., Synth. Met. 84-86 ( 1997) 2043-2044. S. Kagoshima, personal communication. T. Ishiguro et al., Synth. Met. 84-86 ( 1997) 1471-1478.