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Submitted on 1 Jan 1978
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GIANT THERMOPOWER OSCILLATIONS DUE TO
MAGNETIC BREAKDOWN TRAJECTORIES IN
RUTHENIUM
N. Volkenshtein, V. Startzev, V. Dyakina, A. Cherepanov
To cite this version:
JOURNAL D E PHYSIQUE Colloque C6, suppliment au no 8, Tome 39, aotit
1978,
page C6-1112G I A N T THERMOPOWER O S C I L L A T I O N S D U E T O M A G N E T I C BREAKDOWN T R A J E C T O R I E S I N R U T H E N I U M N.V. Volkenshtein, V.E. Startzev, V.P. Dyakina and A.N. Cherepanov
Institute of Metal Physics, UraZ Scientific Centre, Academy of Science of the USSR, 620219 SverdZovsk, GSP
-
170, USSR.Rlsum6.- L'effet Hall, la magn6torbsistance et le pouvoir thermollectrique ont ltl mesurbs dans un monocristal de ruthhnium 1 des tempbratures T = 4,2 K sous des champs magngtiques jusqu'l 100 kG dans les conditions de rupture magnltique. Pour la premiPre fois dans un m6tal de transition, nous avons observ6 des oscillations gigantesques du pouvoir thermo&lectrique dues 1 la naissance d'une couche ltroite de trajectoires ouvertes causges par la rupture magnbtique.
Abstract.- The magnetoresistance, Hall effect and thermopower of a pure ruthenium single crystals ha- ve been measured in the magnetic breakdown region at the temperature 4.2 K and magnetic fields up to
100kG. The giant thermopower oscillations due to formation of the narrow breakdown layer of open tra- jectories were observed for the first time in a transition metal.
Ruthenium is a transition metal with Z = 44 and a hexagonal close-packed structure. The results of the measurements of the galvanomagnetic propertiesll-21 and the de Haas-van Alphen effect/3/, as well as calculation of the band structurel41, showed that ruthenium is a compensated metal with a Fermi surfa- ce consisting of five electron and hole closed sheets and one multiply connected hole sheet (Figure I-a) It can be seen that there are "lenses" inside the "necks" of this multiply connected sheet. There is a small gap between the lens and the neck which is due to spin-orbit splitting. Previous experiments 11-21 showed that it is this structure of the mul- tiply connected sheet leads to the magnetic break- down in ruthenium.
Effectively there are two magnetic breakdown
situations in this metal. First of all, the neck
-
S)
lens magnetic breakdown transforms the open trajec-tories, a, into closed ones, when the magnetic field
+
direction H lies in the basal plane. Secondly, at
31
I
<iT10> the magnetic breakdown results in the+
open trajectories,in the plane LMML. In the k-space the direction of this trajectories is perpendicular to the <0001> axis (figure I-b)
The second magnetic breakdown situation is of the most interest. The fact is that the formation of the magnetic breakdown open trajectories at such
+
magnetic field direction
(HI
I<1?10> is equivalentto the "channeling" for the charge transport in the Fig. 1 : Multiply connected hole sheet of Fermi sur- face of ruthenium (a), and a cross section of this real space (i.e. in the ruthenium monocrystal) along sheet in the plane (b).
the hexagonal axis. Therefore it is of interest to
test experimentally insofar as such magnetic break- electrons both by the electric field and by the down "channel" is effective for the transport of temperature gradient.
Fig. 2 : Anisotropy (a), and magnetic field depen- dence3 (b) of the Hall coefficient of the sample with j11<0001> at T = 4.2 K.
For this purpose we measured the Hall effect, the magnetoresistance and the thennoelectric power of the purest ruthenium single crystal with RRR
83
3000 at 4.2 K and in a transverse magnetic field up to 100 kG. The experimental geometries have been such that hexagonal axis was parallel to the sample axis'-t
and both the electric current J and the temperature gradient $T were aligned in the sample along the channel mentioned above.
Hall effect. The anisotropy and the field de- pendence~ of the Hall coefficient R are shown in fi- gure 2. First of all, it is seen (figure 2-b) that at
81
l
<~TIo> and in magnetic fields H2
20 kG which produce magnetic breakdown open trajectories, the strong increase of khe Hall coefficient is observed. For another magnetic field directions such that no open trajectories can occur, this effect does not take place. Secondly (figure 2-a), at sufficiently low magnetic fields (also at LOT>]) the Hall coeffi- cient anisotropy is about 10 % only. On the contra- ry, at the magnetic fields exceeding the breakdown ones a sharp anisotropy of the Hall coefficient appears. The value of R increases by a factor of(I
ioo)
0
-30 0 30Y/deeg.
Fig. 3 : Anisotropy (a), and magnetic field depen- dences (b) of th? transverse magnetoresistance of the sample with ~11<0001> at T = 4.2 K
three (in comparison with one in pre-breakdown re- gion) precisely in magnetic field directions such that the magnetic breakdown open trajectories form.
Magnetoresistance. Formation of the magnetic breakdown open trajectories results in a similar effects in the magnetoresistance. It can be seen (figure 3-a) that the formation of the magnetic breakdown charge transport channel (the current layer) along the sample axis which takes place at -+
HI
I<ITIo>, leads to strong increase of the electric conductivity precisely at these magnetic field di- rections. Moreover in the same in magnetic field directions there are magnetoresistance oscillations(figure 3-b). The oscillation frequency is equal to 4.1 X 106 G and agrees well with the de Haas-van Alphen frequency (4.0 X 106 G), corresponding to
-+
4, the aosc fall off by a factor of 250 when H was deviated by 10' from the <1710> axis.
Thus the experimental results offer an eviden- ce that the magnetic breakdown open trajectories in ruthenium produce effective channel for the electric charge transport both by the electric field and by the temperature gradient, and as a result of it, ge- nerate the strong transformations of the galvanoma- gnetic and magnetothermoelectricproperties in this metal.
Insofar as we know ruthenium is the first transition metal in which giant magnetothermopower oscillations have been observed.
30 ' 40 50 60 70 80
H kC
Fig. 4 : Magnetic breakdown oscillations of the ther- moelectric power and anisotropy of the scillation amplitude in the ruthenium sample with $TI /<MOI> at T = 4.2K.
tion in the magnetic field direction from the <ITIO> axis. This fact is evidence of the extreme narrow- ness of the magnetic breakdown layer of open trajec- tories. According to our estimate its width APZ/Pf is equal to about 10-~ (Pf is typical Fermi momentum for electrons in ruthenium).
Thus these facts show that magnetic break- down open trajectories produce sufficiently effec- tive channel for the electron transport by the electric field.
Magnetothermoelectric power. The question is how far this channel is effective for the charge transport by the temperature gradient. This question is interesting particularly because of the recent paper/5/ in which it was predicted that a great en- hancement of thermopower must take place in case of the magnetic breakdown.
In figure 4 the thermoelectric power is recor-
+
ded as a function of the magnetic field at H ~ ~ I T I O > that is on condition, when the magnetic breakdown open trajectories occur. It is seen that in high magnetic field the main contribution to the thermo- power is the oscillatory one resulting from magnetic
breakdown. The oscillation frequency is equal to 4.05 X 10' G (+ I %) and is evidence of generation of the magnetic breakdown trajectories indicated in figure I-b. The amplitude of these quantum oscil- lations
.
a is so large that at H = 85 kG it exceedsasc
an ordinary thermopower value of ruthenium at H = 0 by two orders. A destruction of the magnetic break-
+
down open trajectories by the deviation of H from the <lTlO> axis leads to a rapid decrease in the oscillation amplitude. As it may be seen in figure
References
/l/ Startsev,V.E., Dyakina,V.P. and Volkenshtein, N.V., JETP Lett.
2
(1976) 38/2/ Volkenshtein,N.V., Dyakina,V.P., Startsev, V.E., Azhazha,V.M. and Kovtun,G.P., Fiz. Metall. and Metalloved
2
(1974) 718/3/ Coleridge,P.T., J. Low Temp. Phys.
1
(1969) 577 /4/ Jepsen,O., Ander~en~0.K. and Mackintosh,A.R.,Phys. Rev.
