Electron transport and phonons in atomic wires Nicol as Agra € ıt * , Carlos Untiedt 1 , Gabino Rubio-Bollinger, Sebasti an Vieira Departamento de F  ısica de la Materia Condensada C-III, Laboratorio de Bajas Temperaturas, Instituto Universitario de Ciencia de Materiales ‘‘Nicol  as Cabrera’’, Universidad Aut onoma de Madrid, E-28049 Madrid, Spain Received 30 October 2001 Abstract A scanning tunnelling microscope (STM) at low temperatures is used to fabricate and measure the electronic transport properties of atomic wires up to 7 atoms in length. We observe that the conductance of these wires, which at zero bias voltage is 2e 2 =h, as corresponds to ballistic electron transport through a single conductance quantum channel, drops sharply at a well-defined voltage due to the inelastic scattering of electrons with phonons. This behaviour is characteristic of one-dimensional systems, where an electron can interact only with a phonon of a well-defined wavenumber. We find that the frequency of this phonon and the magnitude of the interaction depend strongly on the state of strain of the wire. We also explore the energy exchange mechanism between electrons and phonons in the wire itself which causes heating in these nanostructures. Ó 2002 Elsevier Science B.V. All rights reserved. Freely suspended atomic wires of gold up to 7 atoms in length can be formed by drawing a one- atom contact using a scanning tunnelling micro- scope (STM) or related techniques (mechanically controlled break-junction, MCBJ) [1,2]. They have a zero bias conductance close to G 0 ¼ 2e 2 =h, in- dependently of their length (see Fig. 1(a)). This is the expected behaviour for an atomic wire with a completely open single conductance quantum channel [3], and is consistent with the fact that a one-atom contact of gold has only one conduc- tance quantum channel. The number of quantum channels in a metallic atomic contact has been shown to depend on the chemical properties of the atoms [4]. We observe that at low temperature, the con- ductance of these atomic wires is voltage-depen- dent dropping suddenly by a few percent as voltage is increased beyond several millivolts (see Fig. 1(b)). A voltage-dependent conductance is also observed in ballistic point-contacts of much larger size, where it is associated to inelastic scat- tering of electrons with phonons and other ele- mentary excitations [5] taking place in the bulk. These processes are voltage dependent because the electrons must be injected with enough energy to emit an excitation. In the case of phonons the derivative of the conductance shows peaks which correspond to peaks in the phonon density of states. The amplitude of the signal for contacts of different sizes is proportional to the volume Chemical Physics 281 (2002) 231–234 www.elsevier.com/locate/chemphys * Corresponding author. Tel.: +34-91-3974756; fax: +34-91- 3973961. E-mail address: nicolas.agrait@uam.es (N. Agra € ıt). 1 Present address: Kamerlingh Onnes Laboratorium, Univer- siteit Leiden, Postbus 9504, NL-2300 RA Leiden, Netherlands. 0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0301-0104(02)00342-7