- Email: [email protected]
Charge transport anisotropy in SmB6
Physica B 312–313 (2002) 331–332
Charge transport anisotropy in SmB6$ N. Sluchankoa,*, V. Glushkova, S. Demisheva, M. Kondrina, M. Ignatova, A. Pronina, A. Volkova, A. Savchenkob, S. Kuniic, V. Filippovd, Yu. Padernod a
b
General Physics Institute of RAS, Vavilov str., 38, 117942, Moscow, Russia Physics Department, University of Exeter, Stocker Road, Exeter EX4 4QL, UK c Department of Physics, Tohoku University, Sendai 980, Japan d Institute for Problems of Materials Science, UA-252680, Kiev, Ukraine
Abstract The study of DC- and low-frequency AC-conductivity in combination with the Hall and Seebeck coefficients measurements has been carried out in the temperature interval 1.6–300 K along different crystallographic directions in classic mixed-valence compound–narrow-gap semiconductor SmB6 : The experiments on high-quality single crystals of samarium hexaboride have revealed a strong anisotropy of the low-temperature transport parameters. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Mixed valence; Heavy fermions
coefficient 1/(RH e) (Fig. 1), charge carriers mobility me mp ¼ RH =rEme (inset in Fig. 1) and thermopower
20
2
10
µ (cm / V s)
150
-3
1/(eR H) (cm )
To shed more light on the origin and mechanisms of fast (1012 s) charge fluctuations and especially to reveal the nature of the intra-gap states in the classic mixedvalence narrow-gap semiconductor SmB6 we have studied low-temperature transport characteristics (resistivity, the Hall and Seebeck coefficients) along different crystallographic directions in a cubic lattice of samarium hexaboride. Following ESP investigations [1], by inelastic neutron-scattering experiments [2] and the Raman study of SmB6 [3], where a reduction of the cubic structure symmetry has been certainly demonstrated at helium temperatures, a strong anisotropy of the transport properties of the extraordinary compound has been established in this research on very high-quality single crystals. The data obtained on different single crystalline samples reveal a strong anisotropy of a reciprocal Hall
10
100
50
19
0 3
10
18
10 T (K )
30
<100> U kr <111> U kr
10
17
<211> U kr <110> U kr <100> Jp
$
This work was supported by the RFBR grants 01-02-16601, 06193, the programs ‘‘Fundamental spectroscopy’’, ‘‘Physics of microwaves’’ and INTAS YSF 00-112. *Corresponding author. General Physics Institute of RAS, Vavilov str., 38, 117942, Moscow, Russia. Tel.: +095-132-8262; fax: +095-135-8129. E-mail address: [email protected] (N. Sluchanko).
<110> Jp
10
16
0.05
0.10
0.15 -1 1/T (K )
0.20
0.25
Fig. 1. Temperature dependencies of a reciprocal Hall coefficient 1=eRH and mobility me (inset), measured for different SmB6 single crystals and along several crystallographic directions.
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 1 1 6 - 4
N. Sluchanko et al. / Physica B 312–313 (2002) 331–332
332
-1000 ∇ T|| <110> Jp
E ex ~30K
∇ T|| <100> Jp
-800
S ( µ V /K )
∇ T|| <111> U kr
-600
∇ T|| <110> U kr
-400
III
-200
I
II
0 0.0
0.1
0.2
0.3
0.4
0.5
-1
1/T (K )
of low-temperature transport activation energies which have been observed early by different authors and may be considered as a sum of two contributions from (i) bare ‘‘bare’’ excitonic component Eex E3 meV and (ii) asymmetrical ‘‘polaron well’’ Ep E0:523 meV. Among these two components the second one evidently does not contribute both in the submillimeter range dynamic conductivity [4] and in the Seebeck coefficient (Fig. 2) ac S bare and Eex EEex EEex E3 meV. In the framework of this approach the ‘‘induced anisotropy’’ in this cubic material may be explained in terms of many-body effects the formation of exciton– polaron complexes in the lattice of SmB6 below 15 K. In this picture an onset of a coherent state in the system of strongly interacting exciton–polaron complexes at Tc E5 K [4] is accompanied by an appearance of essential anisotropy and the zeroing of thermopower at helium temperatures and along some crystallographic directions in a cubic matrix of samarium hexaboride (Fig. 2).
Fig. 2. Temperature dependencies of a thermopower measured for different SmB6 single crystals and along several crystallographic directions.
References S(T) (Fig. 2) in the temperature interval Tp15 K. The experimental results Figs. 1 and 2 allow to distinguish between the effects of the intra-gap states’ dispersion (variation of the activation energy in the limits Eex ¼ 326 meV) and charge carriers scattering anisotropy (inset in Fig. 1). The Eex values deduced from this study are in good agreement with a wide range variation
[1] T.S. Al’tshuler, M.S. Bresler, JETP Lett. 66 (1997) 681. [2] P. Alekseev, J.-M. Mignot, J. Rossat-Mignod, et al., J. Phys. Cond. Mat. 7 (1995) 289. [3] P. Nyhus, S.L. Cooper, Z. Fisk, J. Sarrao, Phys. Rev. B 55 (1997) 12488. [4] N. Sluchanko, V. Glushkov, B. Gorshunov, et al., Phys. Rev. B 61 (2000) 9906.
Charge transport anisotropy in SmB6$ N. Sluchankoa,*, V. Glushkova, S. Demisheva, M. Kondrina, M. Ignatova, A. Pronina, A. Volkova, A. Savchenkob, S. Kuniic, V. Filippovd, Yu. Padernod a
b
General Physics Institute of RAS, Vavilov str., 38, 117942, Moscow, Russia Physics Department, University of Exeter, Stocker Road, Exeter EX4 4QL, UK c Department of Physics, Tohoku University, Sendai 980, Japan d Institute for Problems of Materials Science, UA-252680, Kiev, Ukraine
Abstract The study of DC- and low-frequency AC-conductivity in combination with the Hall and Seebeck coefficients measurements has been carried out in the temperature interval 1.6–300 K along different crystallographic directions in classic mixed-valence compound–narrow-gap semiconductor SmB6 : The experiments on high-quality single crystals of samarium hexaboride have revealed a strong anisotropy of the low-temperature transport parameters. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Mixed valence; Heavy fermions
coefficient 1/(RH e) (Fig. 1), charge carriers mobility me mp ¼ RH =rEme (inset in Fig. 1) and thermopower
20
2
10
µ (cm / V s)
150
-3
1/(eR H) (cm )
To shed more light on the origin and mechanisms of fast (1012 s) charge fluctuations and especially to reveal the nature of the intra-gap states in the classic mixedvalence narrow-gap semiconductor SmB6 we have studied low-temperature transport characteristics (resistivity, the Hall and Seebeck coefficients) along different crystallographic directions in a cubic lattice of samarium hexaboride. Following ESP investigations [1], by inelastic neutron-scattering experiments [2] and the Raman study of SmB6 [3], where a reduction of the cubic structure symmetry has been certainly demonstrated at helium temperatures, a strong anisotropy of the transport properties of the extraordinary compound has been established in this research on very high-quality single crystals. The data obtained on different single crystalline samples reveal a strong anisotropy of a reciprocal Hall
10
100
50
19
0 3
10
18
10 T (K )
30
<100> U kr <111> U kr
10
17
<211> U kr <110> U kr <100> Jp
$
This work was supported by the RFBR grants 01-02-16601, 06193, the programs ‘‘Fundamental spectroscopy’’, ‘‘Physics of microwaves’’ and INTAS YSF 00-112. *Corresponding author. General Physics Institute of RAS, Vavilov str., 38, 117942, Moscow, Russia. Tel.: +095-132-8262; fax: +095-135-8129. E-mail address: [email protected] (N. Sluchanko).
<110> Jp
10
16
0.05
0.10
0.15 -1 1/T (K )
0.20
0.25
Fig. 1. Temperature dependencies of a reciprocal Hall coefficient 1=eRH and mobility me (inset), measured for different SmB6 single crystals and along several crystallographic directions.
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 1 1 6 - 4
N. Sluchanko et al. / Physica B 312–313 (2002) 331–332
332
-1000 ∇ T|| <110> Jp
E ex ~30K
∇ T|| <100> Jp
-800
S ( µ V /K )
∇ T|| <111> U kr
-600
∇ T|| <110> U kr
-400
III
-200
I
II
0 0.0
0.1
0.2
0.3
0.4
0.5
-1
1/T (K )
of low-temperature transport activation energies which have been observed early by different authors and may be considered as a sum of two contributions from (i) bare ‘‘bare’’ excitonic component Eex E3 meV and (ii) asymmetrical ‘‘polaron well’’ Ep E0:523 meV. Among these two components the second one evidently does not contribute both in the submillimeter range dynamic conductivity [4] and in the Seebeck coefficient (Fig. 2) ac S bare and Eex EEex EEex E3 meV. In the framework of this approach the ‘‘induced anisotropy’’ in this cubic material may be explained in terms of many-body effects the formation of exciton– polaron complexes in the lattice of SmB6 below 15 K. In this picture an onset of a coherent state in the system of strongly interacting exciton–polaron complexes at Tc E5 K [4] is accompanied by an appearance of essential anisotropy and the zeroing of thermopower at helium temperatures and along some crystallographic directions in a cubic matrix of samarium hexaboride (Fig. 2).
Fig. 2. Temperature dependencies of a thermopower measured for different SmB6 single crystals and along several crystallographic directions.
References S(T) (Fig. 2) in the temperature interval Tp15 K. The experimental results Figs. 1 and 2 allow to distinguish between the effects of the intra-gap states’ dispersion (variation of the activation energy in the limits Eex ¼ 326 meV) and charge carriers scattering anisotropy (inset in Fig. 1). The Eex values deduced from this study are in good agreement with a wide range variation
[1] T.S. Al’tshuler, M.S. Bresler, JETP Lett. 66 (1997) 681. [2] P. Alekseev, J.-M. Mignot, J. Rossat-Mignod, et al., J. Phys. Cond. Mat. 7 (1995) 289. [3] P. Nyhus, S.L. Cooper, Z. Fisk, J. Sarrao, Phys. Rev. B 55 (1997) 12488. [4] N. Sluchanko, V. Glushkov, B. Gorshunov, et al., Phys. Rev. B 61 (2000) 9906.