Quantized thermal transport in single-atom junctions

Abstract

Thermal transport in individual atomic junctions and chains is of great fundamental interest because of the distinctive quantum effects expected to arise in them. By using novel, custom-fabricated, picowatt-resolution calorimetric scanning probes, we measured the thermal conductance of gold and platinum metallic wires down to singleatom junctions. Our work reveals that the thermal conductance of gold single-atom junctions is quantized at room temperature and shows that the Wiedemann-Franz law relating thermal and electrical conductance is satisfied even in single-atom contacts. Furthermore, we quantitatively explain our experimental results within the Landauer framework for quantum thermal transport. The experimental techniques reported here will enable thermal transport studies in atomic and molecular chains, which will be key to investigating numerous fundamental issues that thus far have remained experimentally inaccessible.T he study of thermal transport at the nanoscale is of critical importance for the development of novel nanoelectronic devices and holds promise to unravel quantum phenomena that have no classical analogs (1-3). In the context of nanoscale devices, metallic atomic-size contacts (4) and single-molecule junctions (5) represent the ultimate limit of miniaturization and have emerged as paradigmatic systems revealing previously unknown quantum effects related to charge and energy transport. For instance, transport properties of atomic-scale systems-such as electrical conductance (6), shot noise (7, 8), thermopower (9-11), and Joule heating (12)-are completely dominated by quantum effects, even at room temperature. Therefore, they drastically differ from those of macroscale devices. Unfortunately, the experimental study of thermal transport in these systems constitutes a formidable challenge and has remained elusive to date, in spite of its fundamental interest (13).Probing thermal transport in junctions of atomic dimensions is crucial for understanding the ultimate quantum limits of energy transport. These limits have been explored in a variety of microdevices (14-18), where it has been shown that, irrespective of the nature of the carriers (phonons, photons, or electrons), heat is ultimately transported via discrete channels. The maximum contribution per channel to the thermal conductance is equal to the universal thermal conduct-T/3h, where k B is the Boltzmann constant, T is the absolute temperature, and h is the Planck's constant. However, observations of quantum thermal transport in microscale devices have only been possible at sub-Kelvin temperatures, and other attempts at higher-temperature regimes have yielded inconclusive results (19).The energy-level spacing in metallic contacts of atomic size is of the order of electron volts (i.e., much larger than thermal energy); therefore, these junctions offer an opportunity to explore whether thermal transport can still be quantized at room temperature. However, probing thermal transport in atomic junctions is challenging because of the technic...

Thermal conductance and thermoelectric figure of merit of C60 -based single-molecule junctions: Electrons, phonons, and photons

“…We expect that our predictions can soon be verified experimentally by using techniques similar to those developed in Ref. 37.…”

“…However, it is worth stressing that this appealing suggestion was made without taking into account the phonon contribution in the ab initio calculations and without measuring the thermal conductance. Indeed experimental setups were until recently not sensitive enough to measure κ at the atomic scale, and single-molecule junctions have yet to be examined with newly developed tools [37]. Thus, this interesting suggestion still has to stand careful theoretical and experimental tests with access to all the major quantities determining the figure of merit.…”

Tuning the thermal conductance of molecular junctions with interference effects

“…Finally, we also show that similar concepts apply to the case of OPE3 and, in particular, we find a clear difference between para-and meta-OPE3, where the central benzene ring is connected in para or meta position. Our work provides concrete predictions that can potentially be tested, given the recent experimental advances in the measurement of the thermal conductance of atomic-scale contacts 52,53 . In addition, it sheds light on the importance of phonon interference effects to tune the thermal transport of molecular junctions.…”

Thermal conductance of metallic atomic-size contacts: Phonon transport and Wiedemann-Franz law

“…This situation has now changed, and two experimental groups have finally been able to independently explore the thermal transport in atomic contacts [23,24]. In particular, Cui et al [23] were able to measure the room-temperature thermal conductance of both Au and Pt contacts and found that in the case of Au single-atom contacts the thermal conductance is quantized in units of the universal thermal conductance quantum κ 0 = π 2 k 2 B T /(3h), where T is the absolute temperature. Before this, thermal conductance quantization had already been reported in a series of experiments, making use of different microdevices, where the heat was carried by phonons, electrons, or photons [25][26][27][28][29].…”