Electron Transport Properties of La 1x 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 ptype
ntype
200 100
,TN
II
0
I

0.1
 n
0.2 x
Fig. 1.
Phase diagram for LaixSrxTiO 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 ambientpressure 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 103 and 4.9 x 104 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 ambientpressure TN. The ambientpressure 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 lowtemperature 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 ntype 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 onsite electronelectron electrostatic energy Ut in the singlevalent 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 majorityspin bands only onethird 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 minorityspin electrons that stabilize antiferromagnetic coupling while retaining a total of one electron on each Ti3รท ion.
214
40
14
x=0.044
12
x
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