Electron Transport Properties of La 1-x Sr x TiO 3 Under Hydrostatic Pressure

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Mat. Res. Soc. Symp. Proc. Vol. 499 0 1998 Materials Research Society

temperature thermoelectric power cx(300 K) changes sign, from positive to negative, as x increases through the interval 0.04 < x < 0.05; TN drops abruptly in the interval 0.05 < x < 0.08 (Fig. 1). T(K) 400

dp/dT > 0

300 p-type

n-type

200 100

-,TN

II

0

I

-

0.1

- n

0.2 x

Fig. 1.

Phase diagram for Lai-xSrxTiO 3

Fig. 2 shows the temperature dependence of the resistance R(T) for polycrystalline La-.xSrxTiO 3 with x = 0.044 and 0.05 under different hydrostatic pressures P in the temperature range 10 < T < 300 K. The arrows mark the ambient-pressure NMel temperatures TN = 130 and 105 K, respectively. The data show metallic behavior with R - T2 at all temperatures and pressures except for x = 0.044 at ambient pressure and low temperature. No anomaly was observed at TN. The rate of change of the resistance at room temperature is (1/R)IdR(300 K)/dPI = 1.4 x 10-3 and 4.9 x 10-4 kbarI for x = 0.044 and 0.05, respectively. Fig. 3 shows (x(T) under different pressures for x = 0.044 and 0.05 in the temperature range 10 < T < 300 K. The arrows again indicate the ambient-pressure TN. The ambient-pressure ox(T) for x = 0.044 decreases smoothly with T with no apparent anomaly at TN until it saturates below 60 K at -0.25 giV/K. Our measurements of cL(T) under pressure are not reliable below 30 K [4]; all cL(T) curves should go to ct = 0 at the lowest temperatures. A low-temperature enhancement 8o(T) < 0 appears to persist at all pressures in both samples. The pressure dependence of (x(300 K) is particularly large for x = 0.044; a dct(300 K)/dP = - 0.54 jiV/Kkbar results in a change of sign, from positive to negative, near P = 10 kbar. The x = 0.05 sample has ca(T) < 0 for all P with a pressure dependence that saturates above about 12 kbar. For compositions x > 0.06, an ca(T) < 0 is independent of pressure, and for x > 0.08 the lowtemperature enhancement 8et(T) < 0 is absent; a linear cx(T) - -T typical of n-type metallic behavior was observed, and its doping dependence is compatible with the filling of a rigid band. DISCUSSION The symmetry of the wavefunctions gives rise to three overlapping it* bands xy, yz, and zx. The on-site electron-electron electrostatic energy Ut in the single-valent parent compound splits off a lower Hubbard band with a bandwidth W < Un corresponding to a single 7t* electron at each titanium. Removal of the spin degeneracy at a Ti atom by direct exchange between electrons in orthogonal xy, yz, and zx orbitals would give a localized spin s = 1/2 at each Ti3+ ion, and electron transfer within majority-spin bands only one-third filled should give ferromagnetic coupling, as is indeed observed [5] in YTiO 3 where the 7c*bands are narrower. Transfer of spectral weight to coherent states within the Hubbard gap on the approach to metallic behavior apparently signals the presence of minority-spin electrons that stabilize antiferromagnetic coupling while retaining a total of one electron on each Ti3รท ion.

214

40-

14-

x=0.044

12

x