VOLUME 72, NUMBER 7 P H YSICAL R EV I EW LETTERS 14 FEBRUARY 1994

Pascual er al. Reply: In our Letter [I], we produce

mounds on a Au surface when voltage pulses are appliedto a Au scanning tunneling microscope (STM) tip in a

manner similar to that of Mamin et al. [2]. However, the

mounds are also produced by acting on the Z piezo in or-

der to dip the tip in the sample in intimate contact. Thisis an important piece of evidence (see Fig. 4 in Ref. [I])that the authors have omitted in their Comment [3].Furthermore, the mounds produced by the two methods

are indistinguishable as clearly demonstrated in Fig. l.The similarity of the two peaks, together with the obser-vation of a tip-sample resistance of a few hundred ohms,constitutes an experimental evidence that the mounds areproduced by contact between tip and sample.

We have also discussed in our paper [1] the mecha-nisms which can lead to produce a tip-sample contact by

the application of a voltage pulse. We consider two

different mechanisms: the first attributes the contact tothe development of a protrusion on the tip and/or the

sample by the high electric field, in a way similar to thatexperimentally demonstrated by Vu Thien and Garcia[4]. This protrusion is larger than the tip-sample distancein tunneling conditions and would lead to contact. In the

second, contact would be explained in terms of electro-static forces between the tip and the sample [5].

In their Comment Mamin and Rugar show data of new

experiments where the current flowing in a case of metal-lic structure formation by ultrashort voltage pulsing is

characteristic of tunneling, and they conclude that afield-induced transfer mechanism is consistent with the

formation of the mound. The experiment, however, doesnot show that there is no contact at the voltage pulsing

instant, since the measurement of the current is not done

at these time intervals. The process at the instant of the

voltage increase is crucial for this issue and this is not re-

vealed by the reported experiment. We can interpret thenew data of Mamin and Rugar as a contact at the time ofthe voltage pulse, followed by an immediate break of thecontact by relaxation of the material. Notice that relaxa-tion effects are observed in our experiments [Fig. 1(c) in

Ref. [1]].By estimating the amount of material in the mound

(Fig. I in Mamin and Rugar [3]) and taking into accountthe time duration of the voltage pulse, we obtain an ion

current close to 1 pA corresponding to a rate transfer of0.25x10' ions/s. These high currents might be ex-

plained by a tip-sample contact since the high transferrate means a negligible activation barrier.

Therefore, based on the experimental evidence provid-

ed by the measurement of a tip-sample resistance of a few

hundred ohms during the structure formation process,and the similarity of the structure obtained by placing tipand sample in contact without voltage pulsing, we stillthink that a contact mechanism is the most reasonableexplanation for gold mound formation in STM experi-ments. In other words, our experiments show that no

voltage pulse is needed to form the mounds but a physical

tst 8.88[nm]

161.12

FIG. 1. Two mounds subsequently fabricated by applying avoltage pulse (14 ms, 5 V) and by acting on the z-piezo in orderto lead tip and sample to contact. The profile shows that thetwo structures are similar.

J. I. Pascual, J. Mendez, J. Gomez-Herrero, A. M. Baro,and N. Garcia

Departmento de Fisica Materia CondensadaUniversidad Autonoma de MadridE-28049 Madrid, Spain

Vu Thien BinhDepartment de Physique des MateriauxUniversite Claude Bernard Lyon I

69622 Villeurbanne, Cedex, France

Received 21 December 1993PACS numbers: 61.16.Ch, 73.40.Cg, 79.70.+q

[I] J. 1. Pascual, J. Mendez, J. Gomez-Herrero, A. M. Baro,N. Garcia, and Vu Thien Binh, Phys. Rev. Lett. 71, 1852(1993).

[2] H. J. Mamin, S. Chiang, H. Birk, P. H. Guethner, and D.Rugar, J. Vac. Sci. Technol. B 9, 1398 (1991).

[3] H. J. Mamin and D. Rugar, preceding Comment, Phys.Rev. Lett. 72, 1128 (1994).

[4] Vu Thien Binh and N. Garcia, Ultramicroscopy 42-44, 80(1992).

[51 C. X. Guo and D. J. Thomson, Ultramicroscopy 42-44,1452 (1992).

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contact does it. We have also shown that the contactleads to the formation of a metallic nanowire which bythe fact of its small dimensions shows interesting quan-tum effects.