Twin Boundary in Bechgaard Salt: Structure and Unusual Dynamics

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Twin Boundary in Bechgaard Salt: Structure and Unusual Dynamics

M. Mukoujima, K. Kawabata, T. Sambongi

To cite this version:

M. Mukoujima, K. Kawabata, T. Sambongi. Twin Boundary in Bechgaard Salt: Structure and Unusual Dynamics. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1567-1574. �10.1051/jp1:1996174�.

�jpa-00247265�

Twin

Boundary

Becl~gaard

Dynamics

M. Mukoujima, ^K. Kawabata and T. Sambongi (*) Department of Physics, Hokkaido University, Sapporo 060, Japan

(Received 26 April 1996, ^revised 8 July1996, accepted 26 August 1996)

PACS.61.72.Mm Grain and twin boundaries

PACS.83.50.By Transient deformation and flow; time-dependent properties: start-up,

stress relaxation, _{creep, recovery,} etc.

PACS.61.66.Hq Orgamc crystals

Abstract. Structure and dynamics of twin boundaries _in (TMTSF)2PF6 _were studied. From

microscopic observations, they _are planar _m shape and their width is less than 20 _nm. No

change in their shape _was found. Their movement and pair annihilation do net leave _any trace in the crystal surface. Their motion under externat force _as well _as spontaneous movement _are

intermittent and chaotic. Rote of internai degree of freedom

is essential.

1. Introduction

In this paper we report on trie structure and dynamics, forced motion under external stress as

well _as spontaneous movement, of trie twm boundary (referred to _as "kink" hereafter) induced

m single crystals of (TMTSF)2PF6 Trie crystal structures of (TMTSF)2X, _so called Bechgaard salts, _are triclimc. It is, therefore, possible to grow both right-handed and left-handed crystals

as twins. Mechanical twinmng _can be also expected. Just after trie synthesis of (TMTSF)2X Bechgaard _was already _aware that kinks _can be introduced into trie crystals and that they

show _a peculiar behavior under extemal force. Schwenk et ai. il,_{2j were} trie first to report these interesting phenomena. They [ii report ^{that kinks} can be easily created and that they

move under small lateral force. Later, they _[2] observed that trie crystals contain locahzed regions, through which _a larger force is necessary for trie kink to move, and that it _moves freely

m some crystals. Based _on these observations, they argued ^{that trie} _range ^of deformation associated with trie kink is macroscopically large. Ishiguro et ai. [3,4] found from analysis of X-ray _precession photographic _images, that trie kinks _are parallel to (210) and that trie two

sides separated by _a kink _{are m mirror} symmetry ^with respect to (210).

In _our previous work [si, motion of _a smgle kink under lateral force _was studied, wherem _we confirmed that _a weak extemal force _can drive trie kink _{over a} macroscopic distance. Its motion

was found _to be hysteretic _even though constant force _was continuously applied. In particular,

trie kink _moves mtermittently under constant force with rapid movement and intermission

being repeated _as it is displaced along trie needle. We bave verified that trie mtermittency is not attributable to any experimental setup ^{and that trie} intermittent motion is reproducible.

(*)Author for correspondence: je-mail: sam@skws.phys.hokudai.ac.jp)

© Les Editions de Physique 1996

1568 JOURNAL DE PHYSIQUE I N°12

The positions at which trie kink stops during intermission _are well defined. These positions, though not regularly arrangea, are independent of trie magnitude of stress. Although they

cannot be assigned by microscopic observation, strong potential barriers due to lattice defects

are presumably distributed in such positions. ^{Just before} an intermission, trie kink is often decelerated gradually and then it stops ^rather suddenly. After trie velocity begins to decrease,

trie kink _moves by 20 _+~ 30 _{pm, a} distance that it is 3 orders of magnitude longer than trie

upper bound of kink width estimated in trie next section of this paper. Trie kink motion is

resumed after intermission, trie intermission being shorter for larger stress. Wlien trie stress is

large enough, trie kink does not stop ^{but shows} temporal deceleration at these positions. Only

under trie smallest stress used in [5], can its motion be expressed phenomenologically by that of _a massless body m a viscous medium.

2. Crystal Growth and Formation of Kink

Single crystals of (TMTSF)2PF6, which _are needle-shaped _{were grown} by trie standard elec- trochemical method. As shown previously by Ishiguro et ai. (Fig. 3 of Ref. [4]), natural crystal facets _are clear in all crystals, of which Miller indices _are (001), (011), (011), (001), (OÎÎ) and

(011). We found t~vo additional sets of planes, which _were assigned _as (010) and (010) from trie angles between neighboring facets.

When _a large couple of forces is carefully applied, kink-antikink pair is usually created. More kinks _can be created simultaneously. Trie kink is in _a form of sharp bend of crystal, which _can be identified clearly _even by naked eye. Trie angle of

+~ 20°, between trie two sides separated by _a kink, is consistent with trie findings of Ishiguro et ai. [3,4] ^1-e- that they _are in mirror

symmetry. with respect to (210).

We compared trie X-ray oscillation photographie _images from trie two stries separated by _a

kink. Initially trie a-axis of _one strie _was adjusted parallel to trie rotation axis. After recording

trie diffraction image from that region, ^trie sample orientation _was adjusted _so that trie _a-axis of trie other strie _was parallel to trie rotation axis. Both images _gave trie _same layer line spacing.

On comparing trie two diffraction pattems, ^it was found that trie images from trie two regions

are in _mirror symmetry ^{with each other ~N.ith} respect ^{to trie 0-th} layer hne. It _was confirmed

that trie crystal _is twmned, _{one part} being right-handed while trie other is left-handed.

3. Static Structure

The structure of trie kink in (TMTSF)2PF6 _~vas studied at sub-micron scale to find _out whether

a kink _{at rest} is localized _or extended, and if _any trace of kink movement and of pair annihilation remains within trie crystal. Optical- and scanmng electron-(SEM) microscopes as well _as trie atomic force microscope (AFM) _were used at room temperature.

Figure 1 is trie AFM height image of _a set of three kinks, recorded by. scannmg m trie direction perpendicular to trie _a-axis. Trie litre of intersection of _a kink with trie surface _is microscopically straight. Trie kink is rather flat, not wavy, ^even m areas where natural gro~vth steps _appear at trie surface. Elastic interaction _is expected between trie kink _{as a} mobile defect and other types ^{of lattice} imperfection. Trie kink plane _may be distorted if it is constrained to an area where imperfections _are concentrated, just like trie surface of constant phase of trie

charge/spin density _wave embedded in distributed impurities. However, _no distortion of trie kink at rest was observed by AFM observation. From _a detailed exammation of recorded CCD images presented in trie next section, ~N.e fourra that moving kinks _are flat within _a resolution of 5 _pm. Therefore, _even if trie kink plane _is trot flat, trie distortion _{occurs over a} smaller length

scale.

W

fi~ ~ '~,

lÎ

0.2

/ ~

l.6

~

~~ ~

Î#m)

Fig. l. Atomic force microscope image of _a set of three kinks. Their positions _are marked by

arrows. Growth steps ^{and raturai defect} are designated by S and D, respectively. The image _was

recorded by _scanning perpendicular to the kinks _so that the image, especiaJly _near the kink, is free from instrumental rounding.

From Figure 1. trie height of trie sample surface shows _a sharp bend at trie kink and _no sign of finite width _can be found. From trie AFM image and SEM photographs at large magnification,

it is estimated that trie _upper bound of trie kink width is smaller than 20 _nm. On trie other nana, there is _{no a} priori _reason to expect ^{that trie} crystal bends discontinuously, within _a

molecular distance, at trie kink. Trie TMTSF molecular column bends at trie kink with trie molecules _on trie two stries that _are not parallel to each other but make _an angle

+~ 20°, ~i>hich requires _energy. In addition the distance between anions is also changea. Trie kink width

should depend _on these factors.

When _a lateral force is apphed to _a kink-antikink _pair, the kink and antikink _are displaced in opposite directions. The _pair is stable _even when the mutual separation _is reduced to < 40 _nm

indicating that the interaction between _a kink and _an antikink should be of short range. It

was observed that they anmhilate _on further reduction of the separation, though the hmiting

distance coula not be determmed.

We recorded the AFM image of _{an area} where _a kink had passed through and compared

it with trie _image of trie _same before _passage. No trace of kink movement _was detected in these images implying that trie passage of kinks does not leave _any permanent deformation _or damage within trie crystal. Trie _area where _a kink-antikink pair had been anmhilated _was also exammed. No indication of _pair annihilation _was detected from AFM images within resolution of

+~ 10 _nm.

4. Dynamics

The displacement of isolated kinks under constant extemal force _was measured at _room tem-

perature by an optical microscope equipped with a CCD _camera. Images _were recorded with

a commercial video-recorder. Trie time resolution ~vas 1/30 _s. A force perpendicular to trie needle _was applied by bending _a thin quartz ^tube m contact with trie sample. It _was kept

constant within _1$io during kink motion. Details of _ouf experimental technique ^{bave been} pub-

lished previously _[5]. Figure 2 shows trie position ^of an isolated kink _{as a} function of time

1570 JOURNAL DE PHYSIQUE I N°12

2

~, q

~ ~ _.~

,

d'

"~, ^(b)

E Î

E0.6 _'+.

M ~ c-

0.4 OI

~ ,

0.2 0.2 "',

b-

0 0.2 0.3 OI

~

0 0.1 0.2 0.3 OI

time (sec) ti _me (sec)

Fig. 2. Position of _a kink

as the function of time elapsed after the set-in of motion. (a) in _one direction and 16) ^{in the other} direction. Shear stress, 0.84 gr-weight/mm~,

was calculated from the

applied force divided by the sample cross-sectional _area, 2 _x 10~~ mm~. Dilferent symbols denote different _runs.

after motion lias set in under constant force. Trie observed _area is for from trie tips ^of trie sample and also far from trie positions where _a strong force _was applied during kink creation.

As trie reference of position, _we used characteristic positions of trie sample, _e.g., irregularity

of its shape. Attention _was paid throughout this work not to drive trie kink _to these reference positions.

Intqrmittency of its motion is apparent. While _we argued in trie previous paper [5] that it is not attribuable to any experimental setup, more decisive evidence for intrinsic nature of

this intermittency _~vas obtained by observation of spontaileous intermittent motion. Schwenk et ai. [2] already noted that there _are local regioiis in which trie kink _moves by itself without any applied stress. We observed that, after _a kink moved freely and stopped at _a position,

movement in trie _same direction _was resumed after _an intermission shorter than _one second. In

our experimeiit, trie orientation of samples _was adjusted _so that trie kink _moves in _a horizontal plane ^and _any effect of gravity is excluded.

Trie locations where trie kink stops or is decelerated under larger stress are mdependent

of trie magnitude of stress. From comparison of Figure 2a with 2b in which trie _same kink

moues in trie opposite direction, trie positions of intermission _are independent of trie direction

of motion. While _some defects locahzed in these ppsitions presumably work _as trie barrier for trie kink motion, it is not likely that these defects _are simple lattice imperfections such _as

point defects and dislocations because _we bave shown previously _[5] that trie kink is decelerated when it passes through _a region ^of 10

+~ 20 pm before stopping. Trie stress field of these simple imperfections is not extended to control trie kink motion _{over a} millimeter-micron distance.

Surface irregularities might be equally effective. However, _we observed that trie kink continues to _move where trie surface does not look _so smooth.

Figure 3 shows repeated records of trie position of _a kink moving m one direction under trie

same conditions. To record trie motion of _a kink repeatedly, _we used trie following procedure:

after _{a run,} trie kink _was pushed _up to trie initial position by another force in trie opposite direction while trie measuring force _was kept apphed. Just after trie second force _was released trie next rua was mitiated in trie _same direction. While trie positions ^of intermission _are clearly displayed in trie figure, trie kink is decelerated temporarily at several other positions. Trie time

0.3

J~~

~

A ~

y

~+# ⁺⁺

-

Î~/ ^{/ Z » + 1}

$«

)

^~Îf

nJ~$ _~/

z

;/À++?,' ^{°,,* ~/}

» t ⁺ :

ôP

. + .

0 2 3

time (sec)

Fig. 3. Position of _a kink under the externat stress of 0.79 gr-weight/mm~. Different symbols

denote different _runs.

i .

,

fi

8 ~ o _v2

_

, o

- g

à

0

° È

.

'

~ E

) ^o

la) 0

o

0 005 Ô.1 15 0.2

1~ (sec) _vi (mm/sec)

Fig. 4. a) Relations between trie average velocities Vi (filled circles) before intermission at _{x =} 0.13 _mm and V2 (open diamonds) after and trie time of intermission _Ti b) ^Relation between Vi and V2. Apphed stress _was 0.84 gr-weight/mm~.

of intermission and trie velocity between successive intermissions _are widely distributed. As

measurements were repeated, rapta and slow motion _was observed in random sequence without

any systematic variation of trie overall velocity; _no permanent ^{effect of} repeated _{runs on} trie lattice _{or on} trie kink itself _was found.

Trie possibility of _a correlation between trie time of intermission and trie average velocities between _successive intermissions _was examined. Figure 4 shows trie mutual relations between trie time of intermission _Ti (at _x

= 0.130 _{mm m} Fig. 3), trie average velocities Vi lin trie

area between _x

= 0.075 _mm and 0.130 mm) and ~i (between x = 0.130 _mm and 0.200 mm).

Clear correlation is round between them; when trie kink _moves with high velocity, the time of intermission _is short and it moves again with high velocity. From Figure 4b, trie velocities

m trie two areas are approximately trie _same; Vi à V2. This result shows that trie kink _or

1572 JOURNAL DE PHYSIQUE I N°12

the barrier bas trie intemal structure _or intemal degree of freedom by which trie capability

to memorize trie kink velocity prior to intermission is provided. Though not revealed in trie surface observations presented in trie previous section, finite width _as well _{as some} curvature should be carefully examined in future works.

Trie scatter of trie overall velocity is not attributed to that of trie extemal force. We bave observed repeated records of trie _same kink in trie _{same area} under different magnitudes of force and found wide scatter of trie velocity in all data. Nevertheless, trie center of distribution of trie overall velocity is located at a larger value under _a larger force. If trie scatter shown

m Figures 3 and 4 is _a result of insufficient control of force, trie uncertainty ^of trie magnitude

of force should be _as large _as 10% _{or more.} Prior to trie set-in of motion trie extemal force itself _was adjusted with _an accuracy much better than 1% _Even if _some friction is present

between trie sample and quartz actuator, ^{it is} not plausible that trie frictional force is trie origin of trie wide distribution of both trie local velocities and trie time of intermission. If friction is

significant, systematic _{mcrease or} decrease of trie overall velocity with repeated _runs should be expected _as trie result of smoothing _or roughening of trie contact _area. While _an effect of friction _may not always be neglected, such _a systematic change of trie kink velocity _was

observed. Elastic relaxation of trie quartz ^{actuator does not} play _any significant role in _our

experiment. If _a kink _moves rapidly while trie quartz ^tube remams bent, trie force _on trie kink

might decrease _or trie contact with trie tube might even be lost. As _a result trie kink might stop before trie contact is recovered. However, _we bave observed that trie bending deformation of trie quartz ^tube is relaxed without _any observable residual in _a time scale short enough to

trace trie kink motion. In addition, if trie force _is reduced by trie rapid motion trie reduction of trie force should be _more significant when trie kink velocity _is larger. As _a result, _a longer time of intermission is expected after rapid _{movement; a} large velocity should be correlated with

long intermission. Trie experimental result shown in Figure 4 is trie opposite; large velocity _is

correlated with short intermission.

In _our experiment, trie initial kink position _was not adjusted precisely. As _a result, trie initial position m Figure 3 is rather scattered. If _a potential _energy is associated with trie barrier,

trie sliding velocity at a position would be determined by trie initial position. Figure 5a shows trie times of intermissions (r2 is trie intermission time at _{x =} 0.22 _{mm m} Fig. 3) and Figure 5b trie local velocities Vi and V2. Neither trie times of intermissions _nor trie average velocities

show _any systematic correlation to trie initial position _x; Prior to trie _onset of motion, the

driving stress was kept applied while the kink _was brought into the initial position. Therefore,

no impulse was given to trie kink at trie onset. From these considerations, it is clear that trie scatter of trie overall velocity is not due to uncontrolled initial conditions.

Spontaneous movement is usually completed without retrieval; trie kink _was at trie final position for at least 1

+~

3 minutes. Figure 6 shows trie displacement without extemal force

across a region located between two barriers. Free movement _was observed only _{m one} direc-

tion. Because trie sample orientation _was adjusted in _our experiment _so that trie kink _moves

horizontally, trie spontaneous movement is _not due to gravity. Trie kink _moves with high initial velocity and then _is decelerated before stoppmg. Trie most significant fact is that trie overall

velocity depends _on how trie kink bas been brought to trie initial position _{near one} barrier.

When force _is applied after trie preceding _run to push it bock to the initial position and then the force is released (denoted _as U-mode), it moves with high velocity ^and stops at _a point.

Even when the kink is brought to trie initial position ^from trie opposite strie of trie barrier after being driven backward _across trie freely _{runmng area} (J-mode), trie kink continues to _move

without force and stops at trie _same position. ^In trie latter _case, however, trie overall velocity

is much smaller and trie characteristic time scale is several times larger. This feature _was observed in several samples. Even if _some initial velocity is given m trie J-mode while crossing

0.6

fi )

°'~

. ~2 la)

. ~2 16)

- ~ ~

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~ ~ ^ô

ΰ'~ Î

~

. ~

~'~ ^.

Ô.I Ô fi ^~

~

ô

~

~0 10 ₂₀ 3ù

Fig. 5. a) Times of intermission _T plotted against ^trie initial position _{x,. Ti is} at _{x =} ù.13

mm and

T2 at 0.22 _mm in Figure 3. b) Local velocities Vi lin trie region ^0.075 mm < _x < 0.130 _mm in Fig. 3)

and V2 (0.130 _mm < _x < 0.200 mm) plotted against _x~.

~~ «*.8 * DD g + ~ ^{+* Ô.}

~ o ~+. . Ô

~/

8

~+flà

~/

- +

E

Î+~

~

f~~~

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f

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0 2 4 6 8 10 12 14

time (sec)

Fig. 6. Displacement of _a kink in trie freely _{running area.} ^Three rapta records _{are m} trie U-mode,

the other three slow

ones are in trie J-mode. See text.

trie barrier, it is not trie origin ^of trie above difference because trie overall velocity _is smaller in trie J-mode than in trie U-mode. Presumably, trie frictional force acting on trie _moving kink _or trie potential provided by barriers is modified by trie way m which trie kink is brought to trie

initial position.

In conclusion, trie motion of trie twm boundary or kink is quite complicated. Even under controlled conditions both trie local velocity and trie time of intermission vary from _run to

run; we cannot predict trie velocity at _a position. Though trie positions of intermission _are

reproducible, _we cannot know how long trie kink _stavs at these positions before resuming its motion. In this _sense, trie kink motion is chaotic. Trie kink should not be regarded _{as a} simple rigid planar object; presumably it has _some hidden intemal degree ^of freedom, _e.g.,