PHYSICAL REVIEW B VOLUME 52, NUMBER 5 1 AUGUST 1995 I Hall efFect and magnetoresistance in Ndi s5Ce0, 5Cuo4 8 films P. Seng, J. Diehl, S. Klimm, S. Horn, R. Tidecks, and K. Samwer Institut fii 'r Physik, Universitiit Augsburg, Memminger Strasse 6, D 861-35 Augsburg, Germany H. Hansel and R. Gross Lehrstuhl fiir Experimentalphysik II, Universitiit Tiibingen, Morgenstelle 14, D 7207-6 Tiibingen, Germany (Received 2 December 1994; revised manuscript received 3 April 1995) The electrical transport properties of the cuprate superconductor Nd&»Ceo»Cu04 z have been stud- ied in the mixed state and in the normal conducting state by measuring the Hall voltage and the longitu- dinal voltage of an epitaxial thin film. A broadening of the resistive transition with increasing fixed mag- netic field and a sign reversal of the Hall efFect in the mixed state were observed. In the normal state, the zero-field resistivity shows a quadratic temperature dependence. Magnetoresistance measurements for T =102 and 55 K yield a B dependence up to B =10 T without any saturation tendency. The normal- state Hall coeScient is temperature dependent with a sign change from negative to positive for decreas- ing temperature. The data can be interpreted within a two-band model. I. INTRODUCTION In 1989 Tokura et aI. ' and Takagi et aI. discovered the superconducting cuprate Ndz Ce Cu04 &, crystal- lizing in the T structure and exhibiting superconductivi- ty in a narrow composition range 0. 14(x &0. 18 with a maximum critical temperature of T, o= 24 K. Since trivalent Nd is substituted with tetravalent Ce, the car- riers in these compounds were expected to be elec- trons, in contrast to high-T, -cuprate superconductors, where the charge carriers are holes. Early experiments confirmed the presence of electrons as mobile charge carriers. Further detailed investiga- tions, however, complicated the issue of carrier sign: the sign of the Hall coefficient, R&, was found to depend on x with RH & 0 in the range of superconducting speci- mens. ' Other experiments yield R~) 0 for x =0. 15 or a sign changing with temperature. Moreover, composi- tion and temperature-dependent results for the sign of RH were observed. ' Positive thermopowers were report- ed in conAict with the usually negative RH for specimens with compositions in the superconducting range and high critical temperatures. ' ' Since the sign of the normal-state Hall coefficient and, thus the type of charge carriers is still an unresolved problem, we investigated the Hall effect and magne- toresistance of epitaxial Nd& 85Ceo»Cu04 & thin films. All data shown have been obtained for the same film, with optimized superconducting properties. Interpreting both quantities consistently in the same model leads to a possible explanation of the charge-carrier-type problem. II. EXPERIMENTAL TECHNIQUES Ndi ssCeo, sCuG4 s films (c-axis length 12. 08 A) on SrTi03 substrates. A 110 pm wide two Hall bars geometry"' was patterned by photolithography and Ar ion beam etching. An ac lock-in method at 133 Hz with a current, I, of 20 pA flowing parallel to the CuOz planes was used to measure the longitudinal voltage, V „, and the transverse voltage, V„, in a magnetic field B parallel to the c axis. The sample was mounted in an Ox- ford variable temperature cryostat. The temperature was detected by a carbon glass resistance thermometer. The temperature was stabilized with a precision of about 50 mK, using a capacitive sensor. III. MAGNETOTRANSPORT Since RH in Nd& 85Ceo &5Cu04 & is temperature depen- dent and also a magnetoresistance effect is observed, a free-carrier model is insufficient for the interpretation of our data. The simplest model beyond the one-carrier free-electron approximation is a two-band model, consid- ering the case of a current carried by both electrons (con- centration n„mobility p„and conductivity o, ) and holes (n, p„, and o ). The Hall coefficient within a two- band model is given by' ' 2 2 pPp ePe RH e(n p, +n, p, ) where e ) 0 is the elementary charge. The magnetoresistance effect on the resistivity, p also can be calculated in a two-band model, ' yielding in our experimental situation Hollow cathode magnetron sputtering was used to grow 1200 A thick high quality, c-axis oriented epitaxial p„„(B, T) p„„(0, T) o po, (p +p, — ) 8 p. „(o,&) (~, +~, )' 0163-1829/95/52(5)/3071(4)/$06. 00 52 3071 1995 The American Physical Society