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Angular dependence of the high field phase transition in α-(BEDT-TTF)2TlHg(SCN)4
SVBTIHISTIIE: ELSEVIER
Synthetic Metals 70 (1995) 843-844
Angular D e p e n d e n c e of the High Field Phase Transition in a-(BEDT-TTF)2TIHg(SCN)4 G.J. Athas a, S.J. Klepper b, J.S. Brooks a, M. Tokumoto c, T. Kinoshita c, N. Kinoshita c, and H. Anzai d aDepartment of Physics, Boston University, Boston, Massachusetts 02215, USA bF.B. National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA CElectrotechnical Laboratory, Tsukuba, Ibaraki 305, J a p a n dHimeji Institute of Technology, 2167 Shosya, Himeji, Hyogo 671-22, J a p a n Abstract We report results of a systematic study of the anomalous low temperature state (LTS) angular magnetoresistance (MR) in the quasi-two dimensional organic conductor ~-(BEDT-TTF)2T1Hg(SCN) 4 in fields up to 30 tesla. The high field suppression of the LTS is found to be independent of angle, while the MR maximum, H m a x increases with angle in conjunction with the well studied angular dependent magnetoresistance oscillations (ADMRO). The different magnetic breakdown probabilities above and below Hmax are found to be the source of a novel high field change in the ADMRO shape. The low dimensional (quasi-one and -two) organic conductor a-(BEDT-TTF)2T1Hg(SCN) 4 exhibits highly anomalous magnetoresistance (MR) whose origins lie in the formation of a novel low t e m p e r a t u r e state (LTS), suspected to be the result of a Spin or Charge Density Wave (DW) [l]. The characteristics of the LTS MR can be separated into three field dependent regions: 1) giant MR, 2) negative MR, and 3) normal state MR. The transition between regions 1 and 2 signifies a change in the LTS magnetic breakdown probability and is identified as a maxima in the MR referred to as Hma x [2]. Between regions 2 and 3 a s u d d e n drop in the MR at Hk signifies the s u p p r e s s i o n of the DW p h a s e [3]. Figure 1. illustrates these t r a n s i t i o n points. In addition, above and below the Hk phase transition, distinctly d i f f e r e n t a n g u l a r d e p e n d e n t MR o s c i l l a t i o n s (AMDRO) appear. Sharp minima are observed in the LTS , and sharp maxima appear above H k (Yamaji oscillations) [4]. In this work, we have systematically mapped out the a n g u l a r d e p e n d e n c e of H m a x and H k. Experiments were performed at 0.5 K in the 30 tesla H y b r i d m a g n e t at the F r a n c i s Bitter N a t i o n a l Magnet Laboratory in Cambridge, MA. In order to enhance the studied features, results are taken from down sweeps which have a lower, clearer H k transition, and the Shubnikov de Haas oscillations have been. The r o t a t i o n angle, ~0 is m e a s u r e d between the normal to the highly conducting layers b*, and the applied magnetic field [5]. Fig. 2 shows our first result of no a n g u l a r dependence of H k for ~0 < 70 degrees. For larger
0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0379-6779(94)02674-N
SSDI
71-
/
I I HMAX ( 0 )
I
,I !
, HKINK
o
o o "- 4 ×
-
rr3 2 -
i
10 Figure
l.
15
20 25 30 H (tesla) Magnetoresistance for ~p = 0 and 60
degrees. angles, Hk is difficult to determine (see below). This result is significant in its indication that this phase suppression, at least to 70 °, m a y be i s o t r o v i c . Instead, we propose t h a t v'hen the combined LTS free energy and the induced magnetic energy are equal to the normal state free energy, it becomes energetically favorable to return to the normal state; F N = FLTS + Hk2/8~
844
G.J. Athas et aL / Synthetic Metals 70 (1995) 843~44
where F N a n d FLTS are the respective free energies of the normal and LTS. This is similar in process to magnetic suppression of superconductivity [5]. I
I
I
I
One of the consequences of this effect, is a smooth transition from the LTS ADMRO to the normal state Yamaji oscillations (Fig. 3).
I
24 H
22 _
Hk
=
24.2 tesla 2O ~.~
~.20
4
o
o
m
m
22.5
"v'-"
~-~18
x ~3 rr
-116
2
14
25
127.
12
0
20
40 (deg.)
60
80
Figure 2. Angular dependence of Hkink and Hma x. This behavior contrasts that of the strong field dependence of H m a x , which has roughly 1/cosq~ curvature, modulated by the ADMRO. The overall shape is consistent with the prediction of low MB probability below H m a x and high above. As the field angle is increased, the effective MB gap increases, resulting in a higher field transition between regions 1 and 2, and s u b s e q u e n t l y higher H m a x. The influence of the ADMRO on H m a x indicates that the MB probability changes along with this oscillatory effect. We note t h a t in a n o t h e r work, we have shown that at the ADMRO minima, Hma x -~ oo [7]. The field and angle effects on the magnetic breakdown probability in regions 1 and 2 are the source of a new highfield a n g u l a r d e p e n d e n t phenomena. Figure 3 shows ADMRO oscillations for fixed fields between 15 and 30 tesla derived from the constant angle field sweeps (approximately every 2°). For fields greater t h a n H m a x ( a t q) = 0 °) the normal cosine-like envelope of the LTS AMRO begins to exhibit a low angle concave up dependence t h a t becomes wider and deeper with increasing field. This occurs well above Hk, where such a change in the a n g u l a r d e p e n d e n t shape is expected to occur in conjuntion with Yamaji oscillations. We ascribe this effect to the changing MR contributions from regions 1 and 2 as the MB increases with field, but reduces with increased angle. This can be seen in the return to a normal ADMRO b a c k g r o u n d at angles large enough to reduce the MB probability to t h a t of region 1. We have modeled this proposed process and found it to agree well with our experimental results.
I 0
30
I
I
60
90
(deg.) Figure 3. ADMRO for 15 < H < 30 tesla. peaks are indicated by arrows.
Yamaji
One puzzling feature of the ADMRO is the existence of a peak near 78 ° for all fields. In this region, it becomes difficult to extract Hk because the MR of regions 2 and 3 become comparable. It is unclear which phase exists in this region, and more work is required to u n d e r s t a n d this extreme angle behavior. Work at Boston University is supported by NSF Grant. No. DMR-92-14889. FBNML is also supported by the NSF. REb-~I~NCES 1. N. Kinoshita, M. Tokumoto, and H. Anzai, J. Phys. Soc. Jpn. 59, 3410 (1990). 2. S. Uji, H. Aoki, J.S. Brooks, A.S. Perel, G.J. Athas, S.J. Klepper, C.C. Agosta, and D.A. Howe, Solid State Commun. 88, 683 (1993). 3. J.S. Brooks, C.C. Agosta, S.J. Klepper, M. Tokumoto, N. Kinoshita, H. Anzai, S. Uji, H. Aoki, A.S. Perel, G.J. Athas, and D.A. Howe, Phys. Rev. Lett. 69, 156 (1992). 4 J. Caulfield, J. Singleton, P.T.J. Henriks, J.A.A.J. Perenboom, F.L. Pratt, M. Doporto, W. Hayes, M. Kurmoo, and P. Day, J. Phys.: Condens. Matter 6 L155 (1994). 5. Rotations were done ~ 70 degress from the c axis. 6. A.M. Clogston, Phys. Rev. Lett. 9, 266 (1962). 7. G.J. Athas, J.S. Brooks, S. Valfells, S.J. Klepper, M. Tokumoto, N. Kinoshita, T. Kinoshita, and Y. Tanaka, to be published.
Synthetic Metals 70 (1995) 843-844
Angular D e p e n d e n c e of the High Field Phase Transition in a-(BEDT-TTF)2TIHg(SCN)4 G.J. Athas a, S.J. Klepper b, J.S. Brooks a, M. Tokumoto c, T. Kinoshita c, N. Kinoshita c, and H. Anzai d aDepartment of Physics, Boston University, Boston, Massachusetts 02215, USA bF.B. National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA CElectrotechnical Laboratory, Tsukuba, Ibaraki 305, J a p a n dHimeji Institute of Technology, 2167 Shosya, Himeji, Hyogo 671-22, J a p a n Abstract We report results of a systematic study of the anomalous low temperature state (LTS) angular magnetoresistance (MR) in the quasi-two dimensional organic conductor ~-(BEDT-TTF)2T1Hg(SCN) 4 in fields up to 30 tesla. The high field suppression of the LTS is found to be independent of angle, while the MR maximum, H m a x increases with angle in conjunction with the well studied angular dependent magnetoresistance oscillations (ADMRO). The different magnetic breakdown probabilities above and below Hmax are found to be the source of a novel high field change in the ADMRO shape. The low dimensional (quasi-one and -two) organic conductor a-(BEDT-TTF)2T1Hg(SCN) 4 exhibits highly anomalous magnetoresistance (MR) whose origins lie in the formation of a novel low t e m p e r a t u r e state (LTS), suspected to be the result of a Spin or Charge Density Wave (DW) [l]. The characteristics of the LTS MR can be separated into three field dependent regions: 1) giant MR, 2) negative MR, and 3) normal state MR. The transition between regions 1 and 2 signifies a change in the LTS magnetic breakdown probability and is identified as a maxima in the MR referred to as Hma x [2]. Between regions 2 and 3 a s u d d e n drop in the MR at Hk signifies the s u p p r e s s i o n of the DW p h a s e [3]. Figure 1. illustrates these t r a n s i t i o n points. In addition, above and below the Hk phase transition, distinctly d i f f e r e n t a n g u l a r d e p e n d e n t MR o s c i l l a t i o n s (AMDRO) appear. Sharp minima are observed in the LTS , and sharp maxima appear above H k (Yamaji oscillations) [4]. In this work, we have systematically mapped out the a n g u l a r d e p e n d e n c e of H m a x and H k. Experiments were performed at 0.5 K in the 30 tesla H y b r i d m a g n e t at the F r a n c i s Bitter N a t i o n a l Magnet Laboratory in Cambridge, MA. In order to enhance the studied features, results are taken from down sweeps which have a lower, clearer H k transition, and the Shubnikov de Haas oscillations have been. The r o t a t i o n angle, ~0 is m e a s u r e d between the normal to the highly conducting layers b*, and the applied magnetic field [5]. Fig. 2 shows our first result of no a n g u l a r dependence of H k for ~0 < 70 degrees. For larger
0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0379-6779(94)02674-N
SSDI
71-
/
I I HMAX ( 0 )
I
,I !
, HKINK
o
o o "- 4 ×
-
rr3 2 -
i
10 Figure
l.
15
20 25 30 H (tesla) Magnetoresistance for ~p = 0 and 60
degrees. angles, Hk is difficult to determine (see below). This result is significant in its indication that this phase suppression, at least to 70 °, m a y be i s o t r o v i c . Instead, we propose t h a t v'hen the combined LTS free energy and the induced magnetic energy are equal to the normal state free energy, it becomes energetically favorable to return to the normal state; F N = FLTS + Hk2/8~
844
G.J. Athas et aL / Synthetic Metals 70 (1995) 843~44
where F N a n d FLTS are the respective free energies of the normal and LTS. This is similar in process to magnetic suppression of superconductivity [5]. I
I
I
I
One of the consequences of this effect, is a smooth transition from the LTS ADMRO to the normal state Yamaji oscillations (Fig. 3).
I
24 H
22 _
Hk
=
24.2 tesla 2O ~.~
~.20
4
o
o
m
m
22.5
"v'-"
~-~18
x ~3 rr
-116
2
14
25
127.
12
0
20
40 (deg.)
60
80
Figure 2. Angular dependence of Hkink and Hma x. This behavior contrasts that of the strong field dependence of H m a x , which has roughly 1/cosq~ curvature, modulated by the ADMRO. The overall shape is consistent with the prediction of low MB probability below H m a x and high above. As the field angle is increased, the effective MB gap increases, resulting in a higher field transition between regions 1 and 2, and s u b s e q u e n t l y higher H m a x. The influence of the ADMRO on H m a x indicates that the MB probability changes along with this oscillatory effect. We note t h a t in a n o t h e r work, we have shown that at the ADMRO minima, Hma x -~ oo [7]. The field and angle effects on the magnetic breakdown probability in regions 1 and 2 are the source of a new highfield a n g u l a r d e p e n d e n t phenomena. Figure 3 shows ADMRO oscillations for fixed fields between 15 and 30 tesla derived from the constant angle field sweeps (approximately every 2°). For fields greater t h a n H m a x ( a t q) = 0 °) the normal cosine-like envelope of the LTS AMRO begins to exhibit a low angle concave up dependence t h a t becomes wider and deeper with increasing field. This occurs well above Hk, where such a change in the a n g u l a r d e p e n d e n t shape is expected to occur in conjuntion with Yamaji oscillations. We ascribe this effect to the changing MR contributions from regions 1 and 2 as the MB increases with field, but reduces with increased angle. This can be seen in the return to a normal ADMRO b a c k g r o u n d at angles large enough to reduce the MB probability to t h a t of region 1. We have modeled this proposed process and found it to agree well with our experimental results.
I 0
30
I
I
60
90
(deg.) Figure 3. ADMRO for 15 < H < 30 tesla. peaks are indicated by arrows.
Yamaji
One puzzling feature of the ADMRO is the existence of a peak near 78 ° for all fields. In this region, it becomes difficult to extract Hk because the MR of regions 2 and 3 become comparable. It is unclear which phase exists in this region, and more work is required to u n d e r s t a n d this extreme angle behavior. Work at Boston University is supported by NSF Grant. No. DMR-92-14889. FBNML is also supported by the NSF. REb-~I~NCES 1. N. Kinoshita, M. Tokumoto, and H. Anzai, J. Phys. Soc. Jpn. 59, 3410 (1990). 2. S. Uji, H. Aoki, J.S. Brooks, A.S. Perel, G.J. Athas, S.J. Klepper, C.C. Agosta, and D.A. Howe, Solid State Commun. 88, 683 (1993). 3. J.S. Brooks, C.C. Agosta, S.J. Klepper, M. Tokumoto, N. Kinoshita, H. Anzai, S. Uji, H. Aoki, A.S. Perel, G.J. Athas, and D.A. Howe, Phys. Rev. Lett. 69, 156 (1992). 4 J. Caulfield, J. Singleton, P.T.J. Henriks, J.A.A.J. Perenboom, F.L. Pratt, M. Doporto, W. Hayes, M. Kurmoo, and P. Day, J. Phys.: Condens. Matter 6 L155 (1994). 5. Rotations were done ~ 70 degress from the c axis. 6. A.M. Clogston, Phys. Rev. Lett. 9, 266 (1962). 7. G.J. Athas, J.S. Brooks, S. Valfells, S.J. Klepper, M. Tokumoto, N. Kinoshita, T. Kinoshita, and Y. Tanaka, to be published.