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Oxygen-isotope effect on the field-induced metal-insulator transition in Pr23Ca13MnO3
Solid State Communications.Vol. 105. No. 9. pp. 567470, 1998 0 1998 Elsevier Science Ltd Printed in Great Britain. All tights twervcd 0038-1098/98 $19.00+.00
PII: Soo38-1098@7)101!V-1
OXYGEN-ISOTOPE EFFECT ON THE FIELD-INDUCED METAL-INSULATOR TRANSITION IN PrznCa,,3Mn03 B. Garcia-Landa,” M.R. Ibarra,” J.M. De Teresa,” Guo-meng Zhao,b K. Conde? and H. Kellerb uDepartamento de Fisica de la Materia Condensada e Instituto de Ciencia de Materiales de Aragon, Facultad de Ciencias, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain bPhysik-lnstitut der Universitiit Zurich, CH-8057 Zihich, Switzerland (Received 9 October 1997; accepted 3 Nove~er
1997 by F. Yndffrd~~)
Resistivity and magnetos~ction me~urements have been performed on the oxygen-isotope substituted ( I60 and r8O) samples of ~~Ca,~MnO~ under magnetic fields up to 12 Tesla. Wi~out applying magnetic field, both isotope samples have qu~itatively similar transport and lattice properties, showing a crossover from a charge localised (CL} to a charge ordered (CO) state at the same temperature. In both isotope samples, there is an insulator to metal (I-M) transition induced by a magnetic geld, but the magnetic field that induces the I-M transition is much higher for the j80 sample than for the 160 sample. Our results show that a heavier oxygen isotope mass favours the insulating state and that the CO transition has no significant correlation with the lattice dynamics. 0 1998 Elsevier Science Ltd
Oxygen-isotope effect has shown to be important for demonstrating a relevant role played by the electronphonon coupling in the electronic properties of the hightemperature oxide superconductors [ 11. Recently, Zhao and co-workers have studied this effect in the colossal magnetoresistance manganese perovskites [2-41. A change in the oxygen mass (replacing 160 with 180) in La I -xCa,Mn03+y Preduces a large shift in T,, indicating the existence of polaronic carriers in these systems 12.31. Furthermore, the most spectacular oxygen isotope effect has been obtained for a chosen compound (Lao.sNdo.s)zm Ca inMnOs, where the spontaneous insulator-to-metal transition (I-M) present in the I60 sample is absent in the “0-substi~ted sample [4]. The strong oxygen isotope effect together with the previously reported observation of an ~omalous anh~onic con~bution to the thermal expansion at the insulator regime [5] give strong ex~riment~ evidence for the existence of small lattice polarons, which could subs~ntially influence the ferromagnetic interaction. The structural, magnetic, magneto~~s~~ and magnetostrictive behaviour of Pr&ai,sMnOs has by now been well characterised [$-lo]. The charge carriers become progressively localised below -400 K 1101 displaying a spontaneous insulating behaviour over the whole temperature range. A real-space charge ordering (CO) of the Mn3+/Mn4+ ions set in at Tco = 210 K.
Around -150 K the compound orders antiferromagnetically and at - 100 K a canted magnetic structure develops [9]. This behaviour is allowed by a relatively weak double-exchange interaction which competes with the antiferromagnetic superexchange interaction. Below Tee a first order I-M transition can be induced by applying magnetic field, which produces a progressive delocalisation of the carriers and melts the CO state. The process is accompanied by considerable lattice effects, tightly connected with the electronic state [lo]. Here we report studies of the oxygen isotope effect in Pr&ai,sMn03 from measurements of the thermal expansion, magnetostriction and magnetotransport on the I60 and 180 substituted samples. The samples were prepared by conventions solid state reaction method, using Pr60 11,CaC03, MnOp. The powders were mixed, ground and calcinated in air at 1lOO*Cfor 15 h. The resulting powder was pressed into pellets and fired at 12OO’Cfor 48 h with one intermediate grinding. The pellets were then subjected to I60 and i8O isotope diffusion. The diffusion was carried out for 40 h at 950°C and oxygen pressure of 1 bar. The 180 sample had 90( 2 lo)% I80 isotope as determined from the weight change. The thermal expansion and magnetos&&on measurements were performed using the strain gauge technique. Resistivity and magnetoresistance measurements were
567
METAL-INSULATOR (a)
TRANSITION IN Pr2nCai,sMn0s
Vol. 105, No. 9
lo’
200
220
240
260
280
T 09
Fig. 2. Derivative of log [p(T)] vs T for both isotope samples at H = 0 T. 2
ld
8 V a
lo3 10’ 10-l
11 0
50
loo
150
200
250
300
T (K) Fig. 1. Resistivity vs temperature under several magnetic fields for the I60 sample (a) and the “0 sample (b) of Pr&ZamMnOs. The arrows indicate the cooling or warming mode. performed using tbe 4-probe method. Steady magnetic fields up to 12 T were used in the temperature range 4-300 K. Figures l(a) and (b) show the resistivity vs temperature p(T) (measured under several magnetic fields H) for the ‘*O and 160 substituted samples respectively. For H = 0 T, the resistivity for both isotope samples shows a characteristic increase around -235 K, corresponding to the setting of the CO state. The Tco observed here is higher than the one reported previously. This may be due to the difference in the sample pupation process that could affect the ~cros~cture (like cation de~ciency) of the samples. In Fig. 2, we show the derivative of log [p(T, H = 0 T)] vs 2’ for the two isotope samples. No appreciable difference can be detected for the CO temperature, at which the m~imum slope of log [p(T, H = 0 T)] occurs. The negligible dependence of Tco on the oxygen mass has already been observed for another manganite [1 11. It was explained by considering the electronic nature of the transition, which is not affected by the lattice dynamics. Comparing Figs l(a) and (b) we note that the resistivity under an applied magnetic field of 6 T is drastically different in both isotope samples. For this field value a
I-M position is induced in the ‘b sample, whereas the ~sis~vi~ in the “0 sample, although with a more reduced value, displays se~conductive behaviour in the whole tem~atu~ range. A higher value of the applied field (H = 12 T) is necessary to induce the I-M transition in the “0 sample as shown. At the same field we show the data collected upon cooling down and subsequent warming up. We observe an important hysteretical effect on the field induced M-I and on the CO transition temperatures, possibly due to the first-order character of these transitions. In Fig. 3, we show the thermal expansion for the “0 sample under the zero field and a field of 12 T. The phonon contribution (dashed line in the same figure) has been taken from the work of [lo], where a
H=l2T/, f / _-_I 0
’ ,y , , ‘ ( ( ‘
50
loo
’
,
150
,
,
‘ , , , , * , , ,
200
250
T(K) Fig. 3. Thermal expansion of the i8O-substituted sample of Rr&Za ,,sMnOs for H = 0 and 12 T. The dashed curve represents the calculated Griineisen contribution. The arrows indicate the cooling or warming mode. The inset shows the contribution obtained after subtracting the Ckiineisen contribution (solid lines). The resistivity measured under 12 T in warming mode has been included for comparison (dashed curve in the inset).
Vol. 105, No. 9
(a)
METAL-INSULATOR
TRANSITION IN Pr&Z!a1&n03
569
for the I60 sample, but lower values of the magnetic field are required for observing the volume effect associated with the I-M position (for H = 6 T it is located around -80 K whereas for H = 12 T the CO state is not stable at any tem~ra~) [IO]. These results on the lattice properties confirm our conclusion (based on the resistivity results) that a heavier oxygen mass is associated with a higher stability of the CO state. In the inset of Fig. 3 we display the contribution obtained after subtracting the calculated phonon curve (solid line) together with the resistivity measurements obtained under 12 T in warming mode (dashed curve). The correlation existing between transport and lattice effects is clear. In Figs 4(a) and (b) we represent the magnetoresistance and magnetos~ction isotherms at some selected ~m~ra~res. Above 7’cc the isotherms show a smooth decrease of the resistivity with the field and a very small magnetostriction. No hysteresis is observed. This behaviour resembles that of the I60 sample and can be attributed to the partial delocalisation of carriers with the field [lo]. Below T,-* hysterical behaviour develops, confirming the first-order nature of the transition. The suppression of the CO state is linked to a delocalisation 0 2 4 6 8 10 1 of the carriers, accompanied by a strong decrease in the H (T) resistivity and a release of the lattice deformation. The magnetostriction attained at this transition corresponds to Fig. 4. Magnetoresistance (a) and magnetostriction the difference between the linear thermal expansion (b) isotherms of the “*G sample at some selected curves measured for U = 0 and 12 T (represented in temperatures. Arrows indicate increasing or decreasing magnetic field. Fig. 3). The lattice and transport behaviour of the I60 and I80 substituted samples can be explained as due to the field“0 ~~~Ca,~MnO~ sample was studied. This was induced collapse from a high volume (HV) insulating obtained by fitting the high ~rn~~ture linear state to a low volume (LV) metallic state. This seems to thermal expansion to the Griineisen law, considering a be a universal behaviour in the manganite compounds for Debye temperature f?~ = 500 K. An extra anharmonic fixed carrier concentration [12]. Although the transition contribution over the phononic one is observed, can happen spontaneously for some compositions associated with the progressive localisation of the polaronic carriers starting at high temperatures [5, IO]. (those with a para to ferromagnetic transition), it Associated with the CO, an anomaly is observed in the can also be induced by the applied magnetic field (as in thermal expansion. It reflects a volume increase marking Pr&a mMn03). For the heavier oxygen isotope sample, the transition between an incoherent CL state to a the transition occurs under the higher values of the coherent CO state. The hysteretical thermal effects are applied field. Such a strong isotope effect observed in and magnetoelastic properties of in correspondence with those aheady observed in the the rnagneto~~s~~ resistivity ~~u~rnen~ (the anomaly is placed at the PrmCatnMn03 confi~s once again the existence of a in the m~g~ite same temperature in the thermal expansion as in strong electron-phonon ~te~tion oxides [2,3, 51. resistivity curves, when the same measuring mode A consensus has now emerged from the reported cooling or warming - and the same value of the applied magnetic field is considered). A very large magneto- results in manganites, that is, the ferromagnetic volume effect is associated with the suppression of interaction gets weakened as the oxygen isotopic mass the CO state by the magnetic field. For H = 12 T the increases. In the present study this is manifested by the transition to a metallic-like state (lower volume state) enhancement of the CO state stability. On the other hand occurs around - 120 K, where the linear thermal the absence of isotopic effect on the CO temperature expansion curve approaches the expected anharmonic demonstrates no significant influence of the lattice curve. A qualitatively similar behaviour has been seen vibrations on this state at high temperatures. lo6
570
METAL-INSULATOR
TRANSITION IN PrmCamMnO:,
REFERENCES 1. Loye, H.C.Z., Leary, K.J., Keller, S.W., Ham, W.K., Faltens, T.A., Michaels, J.N. and Stacy, A.M., Science, Wg, 1987, 1558. 2. Zhao, G.M., Conder, K., Keller, H. and Miiller, K.A., Nature, 381,1996, 676. 3. Zhao, G.M., Hunt, M.B. and Keller, H., Phys. Rev. Z.&t., 78, 1997,955. 4. Zhao, G.M., Keller, H., Hofer, J., Shengelaya, A. and Miiller, K.A., ~~~i~State Co~~~~., 104,1997, 57. 5. Ibarra, M.R., Algarabel, P.A., M~quina, C., Blasco, J. and Garcia, J., Phys. Rev. L.&t., 75, 19953541. 6. Jir& Z., Krupicka, S., Simsa, Z., Dlouha, M. and Vratislav, S., J. Magn. Magn. Mater., 53, 1985, 153.
Vol. 105, No. 9
7. Tomioka, Y., Asamitsu, A., Kuwahara, H., Moritomo, Y. and Tokura, Y., Phys. Rev., B53, 1996, R1689. 8. Lees, M.R., Barratt, J., Balakrishnan, G., Paul, D.McK. and Yethiraj, M., Phys. Rev., BS2, 1996, R14303. 9. Yoshizawa, H., Kawano, H., Tomioka, Y. and Tokura, Y., Phys. Rev., B531995, Rl3145. 10. De Teresa, J.M., Ibarra, M.R., M~uina, C., AlgarabeI, P.A. and Ckeroff, S., Phys. Rev., BS4, 1996, Rl2689. 11. Ibarra, M.R., Zhao, GM., De Teresa, J-M., GarciaLanda, B., Arnold, Z., Marquina, C., Algarabel, P.A. and Keller, H., To be published. 12. Ibarra, M.R., De Teresa, J.M., Algarabel, P.A., Marquina, C. and Garcia-Landa, B., To be published.
PII: Soo38-1098@7)101!V-1
OXYGEN-ISOTOPE EFFECT ON THE FIELD-INDUCED METAL-INSULATOR TRANSITION IN PrznCa,,3Mn03 B. Garcia-Landa,” M.R. Ibarra,” J.M. De Teresa,” Guo-meng Zhao,b K. Conde? and H. Kellerb uDepartamento de Fisica de la Materia Condensada e Instituto de Ciencia de Materiales de Aragon, Facultad de Ciencias, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain bPhysik-lnstitut der Universitiit Zurich, CH-8057 Zihich, Switzerland (Received 9 October 1997; accepted 3 Nove~er
1997 by F. Yndffrd~~)
Resistivity and magnetos~ction me~urements have been performed on the oxygen-isotope substituted ( I60 and r8O) samples of ~~Ca,~MnO~ under magnetic fields up to 12 Tesla. Wi~out applying magnetic field, both isotope samples have qu~itatively similar transport and lattice properties, showing a crossover from a charge localised (CL} to a charge ordered (CO) state at the same temperature. In both isotope samples, there is an insulator to metal (I-M) transition induced by a magnetic geld, but the magnetic field that induces the I-M transition is much higher for the j80 sample than for the 160 sample. Our results show that a heavier oxygen isotope mass favours the insulating state and that the CO transition has no significant correlation with the lattice dynamics. 0 1998 Elsevier Science Ltd
Oxygen-isotope effect has shown to be important for demonstrating a relevant role played by the electronphonon coupling in the electronic properties of the hightemperature oxide superconductors [ 11. Recently, Zhao and co-workers have studied this effect in the colossal magnetoresistance manganese perovskites [2-41. A change in the oxygen mass (replacing 160 with 180) in La I -xCa,Mn03+y Preduces a large shift in T,, indicating the existence of polaronic carriers in these systems 12.31. Furthermore, the most spectacular oxygen isotope effect has been obtained for a chosen compound (Lao.sNdo.s)zm Ca inMnOs, where the spontaneous insulator-to-metal transition (I-M) present in the I60 sample is absent in the “0-substi~ted sample [4]. The strong oxygen isotope effect together with the previously reported observation of an ~omalous anh~onic con~bution to the thermal expansion at the insulator regime [5] give strong ex~riment~ evidence for the existence of small lattice polarons, which could subs~ntially influence the ferromagnetic interaction. The structural, magnetic, magneto~~s~~ and magnetostrictive behaviour of Pr&ai,sMnOs has by now been well characterised [$-lo]. The charge carriers become progressively localised below -400 K 1101 displaying a spontaneous insulating behaviour over the whole temperature range. A real-space charge ordering (CO) of the Mn3+/Mn4+ ions set in at Tco = 210 K.
Around -150 K the compound orders antiferromagnetically and at - 100 K a canted magnetic structure develops [9]. This behaviour is allowed by a relatively weak double-exchange interaction which competes with the antiferromagnetic superexchange interaction. Below Tee a first order I-M transition can be induced by applying magnetic field, which produces a progressive delocalisation of the carriers and melts the CO state. The process is accompanied by considerable lattice effects, tightly connected with the electronic state [lo]. Here we report studies of the oxygen isotope effect in Pr&ai,sMn03 from measurements of the thermal expansion, magnetostriction and magnetotransport on the I60 and 180 substituted samples. The samples were prepared by conventions solid state reaction method, using Pr60 11,CaC03, MnOp. The powders were mixed, ground and calcinated in air at 1lOO*Cfor 15 h. The resulting powder was pressed into pellets and fired at 12OO’Cfor 48 h with one intermediate grinding. The pellets were then subjected to I60 and i8O isotope diffusion. The diffusion was carried out for 40 h at 950°C and oxygen pressure of 1 bar. The 180 sample had 90( 2 lo)% I80 isotope as determined from the weight change. The thermal expansion and magnetos&&on measurements were performed using the strain gauge technique. Resistivity and magnetoresistance measurements were
567
METAL-INSULATOR (a)
TRANSITION IN Pr2nCai,sMn0s
Vol. 105, No. 9
lo’
200
220
240
260
280
T 09
Fig. 2. Derivative of log [p(T)] vs T for both isotope samples at H = 0 T. 2
ld
8 V a
lo3 10’ 10-l
11 0
50
loo
150
200
250
300
T (K) Fig. 1. Resistivity vs temperature under several magnetic fields for the I60 sample (a) and the “0 sample (b) of Pr&ZamMnOs. The arrows indicate the cooling or warming mode. performed using tbe 4-probe method. Steady magnetic fields up to 12 T were used in the temperature range 4-300 K. Figures l(a) and (b) show the resistivity vs temperature p(T) (measured under several magnetic fields H) for the ‘*O and 160 substituted samples respectively. For H = 0 T, the resistivity for both isotope samples shows a characteristic increase around -235 K, corresponding to the setting of the CO state. The Tco observed here is higher than the one reported previously. This may be due to the difference in the sample pupation process that could affect the ~cros~cture (like cation de~ciency) of the samples. In Fig. 2, we show the derivative of log [p(T, H = 0 T)] vs 2’ for the two isotope samples. No appreciable difference can be detected for the CO temperature, at which the m~imum slope of log [p(T, H = 0 T)] occurs. The negligible dependence of Tco on the oxygen mass has already been observed for another manganite [1 11. It was explained by considering the electronic nature of the transition, which is not affected by the lattice dynamics. Comparing Figs l(a) and (b) we note that the resistivity under an applied magnetic field of 6 T is drastically different in both isotope samples. For this field value a
I-M position is induced in the ‘b sample, whereas the ~sis~vi~ in the “0 sample, although with a more reduced value, displays se~conductive behaviour in the whole tem~atu~ range. A higher value of the applied field (H = 12 T) is necessary to induce the I-M transition in the “0 sample as shown. At the same field we show the data collected upon cooling down and subsequent warming up. We observe an important hysteretical effect on the field induced M-I and on the CO transition temperatures, possibly due to the first-order character of these transitions. In Fig. 3, we show the thermal expansion for the “0 sample under the zero field and a field of 12 T. The phonon contribution (dashed line in the same figure) has been taken from the work of [lo], where a
H=l2T/, f / _-_I 0
’ ,y , , ‘ ( ( ‘
50
loo
’
,
150
,
,
‘ , , , , * , , ,
200
250
T(K) Fig. 3. Thermal expansion of the i8O-substituted sample of Rr&Za ,,sMnOs for H = 0 and 12 T. The dashed curve represents the calculated Griineisen contribution. The arrows indicate the cooling or warming mode. The inset shows the contribution obtained after subtracting the Ckiineisen contribution (solid lines). The resistivity measured under 12 T in warming mode has been included for comparison (dashed curve in the inset).
Vol. 105, No. 9
(a)
METAL-INSULATOR
TRANSITION IN Pr&Z!a1&n03
569
for the I60 sample, but lower values of the magnetic field are required for observing the volume effect associated with the I-M position (for H = 6 T it is located around -80 K whereas for H = 12 T the CO state is not stable at any tem~ra~) [IO]. These results on the lattice properties confirm our conclusion (based on the resistivity results) that a heavier oxygen mass is associated with a higher stability of the CO state. In the inset of Fig. 3 we display the contribution obtained after subtracting the calculated phonon curve (solid line) together with the resistivity measurements obtained under 12 T in warming mode (dashed curve). The correlation existing between transport and lattice effects is clear. In Figs 4(a) and (b) we represent the magnetoresistance and magnetos~ction isotherms at some selected ~m~ra~res. Above 7’cc the isotherms show a smooth decrease of the resistivity with the field and a very small magnetostriction. No hysteresis is observed. This behaviour resembles that of the I60 sample and can be attributed to the partial delocalisation of carriers with the field [lo]. Below T,-* hysterical behaviour develops, confirming the first-order nature of the transition. The suppression of the CO state is linked to a delocalisation 0 2 4 6 8 10 1 of the carriers, accompanied by a strong decrease in the H (T) resistivity and a release of the lattice deformation. The magnetostriction attained at this transition corresponds to Fig. 4. Magnetoresistance (a) and magnetostriction the difference between the linear thermal expansion (b) isotherms of the “*G sample at some selected curves measured for U = 0 and 12 T (represented in temperatures. Arrows indicate increasing or decreasing magnetic field. Fig. 3). The lattice and transport behaviour of the I60 and I80 substituted samples can be explained as due to the field“0 ~~~Ca,~MnO~ sample was studied. This was induced collapse from a high volume (HV) insulating obtained by fitting the high ~rn~~ture linear state to a low volume (LV) metallic state. This seems to thermal expansion to the Griineisen law, considering a be a universal behaviour in the manganite compounds for Debye temperature f?~ = 500 K. An extra anharmonic fixed carrier concentration [12]. Although the transition contribution over the phononic one is observed, can happen spontaneously for some compositions associated with the progressive localisation of the polaronic carriers starting at high temperatures [5, IO]. (those with a para to ferromagnetic transition), it Associated with the CO, an anomaly is observed in the can also be induced by the applied magnetic field (as in thermal expansion. It reflects a volume increase marking Pr&a mMn03). For the heavier oxygen isotope sample, the transition between an incoherent CL state to a the transition occurs under the higher values of the coherent CO state. The hysteretical thermal effects are applied field. Such a strong isotope effect observed in and magnetoelastic properties of in correspondence with those aheady observed in the the rnagneto~~s~~ resistivity ~~u~rnen~ (the anomaly is placed at the PrmCatnMn03 confi~s once again the existence of a in the m~g~ite same temperature in the thermal expansion as in strong electron-phonon ~te~tion oxides [2,3, 51. resistivity curves, when the same measuring mode A consensus has now emerged from the reported cooling or warming - and the same value of the applied magnetic field is considered). A very large magneto- results in manganites, that is, the ferromagnetic volume effect is associated with the suppression of interaction gets weakened as the oxygen isotopic mass the CO state by the magnetic field. For H = 12 T the increases. In the present study this is manifested by the transition to a metallic-like state (lower volume state) enhancement of the CO state stability. On the other hand occurs around - 120 K, where the linear thermal the absence of isotopic effect on the CO temperature expansion curve approaches the expected anharmonic demonstrates no significant influence of the lattice curve. A qualitatively similar behaviour has been seen vibrations on this state at high temperatures. lo6
570
METAL-INSULATOR
TRANSITION IN PrmCamMnO:,
REFERENCES 1. Loye, H.C.Z., Leary, K.J., Keller, S.W., Ham, W.K., Faltens, T.A., Michaels, J.N. and Stacy, A.M., Science, Wg, 1987, 1558. 2. Zhao, G.M., Conder, K., Keller, H. and Miiller, K.A., Nature, 381,1996, 676. 3. Zhao, G.M., Hunt, M.B. and Keller, H., Phys. Rev. Z.&t., 78, 1997,955. 4. Zhao, G.M., Keller, H., Hofer, J., Shengelaya, A. and Miiller, K.A., ~~~i~State Co~~~~., 104,1997, 57. 5. Ibarra, M.R., Algarabel, P.A., M~quina, C., Blasco, J. and Garcia, J., Phys. Rev. L.&t., 75, 19953541. 6. Jir& Z., Krupicka, S., Simsa, Z., Dlouha, M. and Vratislav, S., J. Magn. Magn. Mater., 53, 1985, 153.
Vol. 105, No. 9
7. Tomioka, Y., Asamitsu, A., Kuwahara, H., Moritomo, Y. and Tokura, Y., Phys. Rev., B53, 1996, R1689. 8. Lees, M.R., Barratt, J., Balakrishnan, G., Paul, D.McK. and Yethiraj, M., Phys. Rev., BS2, 1996, R14303. 9. Yoshizawa, H., Kawano, H., Tomioka, Y. and Tokura, Y., Phys. Rev., B531995, Rl3145. 10. De Teresa, J.M., Ibarra, M.R., M~uina, C., AlgarabeI, P.A. and Ckeroff, S., Phys. Rev., BS4, 1996, Rl2689. 11. Ibarra, M.R., Zhao, GM., De Teresa, J-M., GarciaLanda, B., Arnold, Z., Marquina, C., Algarabel, P.A. and Keller, H., To be published. 12. Ibarra, M.R., De Teresa, J.M., Algarabel, P.A., Marquina, C. and Garcia-Landa, B., To be published.