Lossless Currents at High Temperatures
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Is superconductivity necessary for zero-resistance currents? We investigate the possibility of designing wires with non-superconducting, but nevertheless dissipation-free current flow at room temperature. Guided by a set of constraints for states carrying currents in thermodynamic equilibrium, we suggest using quantum mechanical interference and collapse processes to produce devices that yield the desired function. Two proposals for such devices are presented as examples and their function is discussed also with respect to the second law of thermodynamics.
KeywordsLossless currents Second law Maxwell’s demon Entropy
Ripple in still water
When there is no pebble tossed
Nor wind to blow
Robert Hunter, Jerry Garcia
1 Introduction and General Considerations
The virtually dissipation-free flow of electric currents is a hallmark of superconductivity. High-density supercurrents that are robust to large magnetic fields are the basis for main-stream applications of superconductors, such as superconducting power lines, motors, generators, magnets, and fault-current limiters. Great efforts are being made to search for room-temperature superconductors that yield the benefits of these applications without the need for cooling. With the discovery of superconductivity in hydrogen sulfide under extreme pressures , the maximum Tc has exceeded 200 K, so that room-temperature superconductivity now seems within reach. The fabrication of useful room-temperature superconducting wires will probably present a further formidable obstacle to be overcome before large-scale applications of superconductors at 300 K can become reality . In this anniversary issue of the Journal of Superconductivity and Novel Magnetism, we explore an alternative route to achieve the desired dissipation-free currents. We will submit that lossless current flow at room temperature might indeed be achievable in non-superconducting systems, which we will support by proposing concrete devices. Our discussion is based on refs. [3, 4, 5].
Various non-superconducting systems exist that transport electric charge without losses. Atoms, molecules, atomic clusters, and mesoscopically small conducting rings are microscopic examples of such systems (see, e.g., [6, 7, 8]). Dissipation-free charge transport is also possible in macroscopic systems, the salient example being conductors based on the quantum-Hall effect and biased in quantum-Hall plateaus . However, various difficulties, including the emergence of hot spots at the contacts, have prevented the development of wires based on the quantum-Hall effect. Topologically protected currents in topological insulators have also been proposed as candidates for lossless current flow [10, 11]. Scattering of the carriers taking place despite topological protection unfortunately obstructs their practical use as dissipation-free conductors.
Systems that carry currents in thermodynamic equilibrium may provide a very different route to dissipation-free currents . In contrast to the topological systems listed above, this approach does not require that scattering be suppressed, which is advantageous for designing long, practical wires. Indeed, in the conductors we propose, inelastic scattering is required to achieve the current flow, as we describe below.
2 Device Concepts
It does not seem possible to fulfill requirements (i)–(v) within classical mechanics or by using quantum physics as described by unitary state evolution exclusively. The devices we propose therefore go beyond classical physics and linear quantum mechanics. By utilizing the generation of coherent quantum states and their collapse due to inelastic interaction, the devices induce the desired nonreciprocal charge current. They thereby use the collapse of the wave function as an integral component. This principle may also be implemented for photonic devices [3, 4, 5]. To achieve dissipation-free charge transport, we focus here on electronic devices.
We start by considering the case that the electrons are not subject to inelastic scattering. An electron injected with some thermal kinetic energy into such an asymmetric Rashba-type quantum ring (Fig. 3) at port 1 to travel to port 2 is described by a de-Broglie wave packet with a spatial extension of less than R. At port 1, the wave packet splits into two parts. These travel on either side of the loop, interfere constructively at port 2, and leave the device there. However, the two wave packet parts of an electron that travels in reverse direction, from port 2 to 1, arrive at the T-type junction of port 1 with a phase shift of π and therefore cannot leave the ring at port 1. Instead, they continue to circle the ring and travel back to port 2. On their way to port 2, they are not subject to a shift of their phase difference. Therefore, they meet at port 2 again with a phase difference of π and are forced to circle back to port 1. As they move from port 2 to 1, another phase shift of π is induced between these half wave packets. Upon arriving at port 1, they interfere constructively this time, so that now they leave the device at port 1.
Such loops transmit electrons in both directions without losses. This is a consequence of the unitarity of the overall S-matrix, which enforces reciprocal transmission in any two-terminal device . Nevertheless, the symmetry of the transport has been broken. Electrons traveling in a circular ring from port 1 to 2 leave the ring after having travelled in it for a distance of πR; electrons traveling from port 2 to 1 do so after having circled the ring 3/2 times, i.e., after having traveled the triple distance, 3πR.
Note that these devices fulfill requirements (i)–(iii) because the device size is mesoscopic (i); magnetic fields must be applied (ii, iii); and time reversal invariance is broken by the Rashba and Zeeman effects (iii). We now break the reciprocity of the transport by adding inelastic scattering centers coupled to a thermal bath, thereby breaking the unitarity of the evolution of the electron states (iv) . For simplicity, these scattering centers are presumed to be uniformly distributed along the loop. The electron mean free path for inelastic scattering is chosen to be of the order of 2πR. Several effects relevant to device operation are associated with inelastic scattering: (a) the electron’s wave function loses its phase memory , (b) the two coherent packets of the wave function collapse into a new wave packet traveling in only one of the half-circles of the ring, and (c) the electron’s kinetic momentum changes.
As the scattering centers are uniformly distributed along the loop, the amount of inelastic scattering to which an electron is subjected depends on its port of entry. An electron entering the device at port 1 has a short path (πR) before reaching its possible exit port 2. An electron entering the device at port 2, however, would need to travel a distance of 3πR before it could leave the device if there were no inelastic scattering. On average, electrons entering the device at port 2 are therefore scattered more frequently than electrons entering at port 1.
If, due to inelastic scattering, the two-packet wave function of an electron collapses into a one-packet wave function the electron continues to move within the half of the ring into which it collapsed. The electron then reaches the next junction that either lets the electron continue to circle or switches it into the adjoining contact. At such a junction, electrons in collapsed states have a comparable probability of leaving the ring or continuing the circle. For unscattered electrons with two-packet wave functions the phase difference of the two wave packets controls the choice of path. Constructive interference makes the electrons enter the contact, whereas destructive interference makes them continue to circle.
As a result, electrons that have entered the ring at port 1 and have moved with high probability coherently to port 2 interfere there constructively, and hence are likely to leave the ring at port 2. In contrast, electrons that have entered the ring at port 2, and therefore have been scattered more frequently, are significantly more likely to leave the device through their entrance port.
In essence, the device sorts the electrons. Electrons entering the ring in equal numbers at ports 1 and 2 leave the ring more often through port 2 than through port 1.
Using coherent superposition of wave functions (mostly for electrons moving from port 1 to 2) and the collapse or reduction of wave functions (mostly for electrons moving from port 2 to 1) makes the transport through the two-terminal ring nonreciprocal (v). The collapse processes break the unitarity of the S-matrix of the complete system. Owing to the broken unitarity, the requirement that transport across two-terminal devices must be reciprocal  is not applicable to the devices presented here.
This applies to electrons driven by a bias voltage or a bias current and also to electrons that enter the device in thermal equilibrium driven from the contacts by Johnson-Nyquist noise [18, 19]. We conclude that, also for uniform temperature distribution, such a device acts like a pump for electrons, shifting them on average from port 1 to 2. The device thereby generates a difference in the electrochemical potential between these two ports.
These rings demonstrate that electric charge may be transported at room temperature without losses. The transported charge may consist of those electrons that move through the rings, or they may consist of charge flowing through a different, conventional conductor connected to both ports, which are characterized by the generated potential difference. Such devices seem realizable in principle as practical, macroscopic devices, and work at 300 K or even above.
Such rings or comparable interference devices can be connected in parallel and in series without requiring phase coherence between different rings. They may, for example, be implemented as molecules, solids with complex lattice structures, or be built in large numbers with integrated circuit technology.
These devices, for the function of which we presume the validity of quantum mechanics including collapse processes, achieve the task of a Maxwell demon  and break the second law of thermodynamics [21, 22, 23] with the implications this entails (for a more detailed description, see ref. ). The devices are closed systems that, starting at a uniform temperature, transform part of their heat energy into another form of energy. In this process, heat flows from the colder particle i to the warmer particle j. This energy can consist of the electrochemical potential difference of the ports or their induced temperature difference, which can be converted into electricity by a thermocouple. The nonuniform radiation density (Fig. 4) is not describable using Kirchhoff’s law  or Planck’s radiation law . Also the entropy of the photon gas deviates from the one of a standard black body .
For discussions of the possibility of breaking the second law of thermodynamics, see, e.g., [27, 28, 29, 30]. Examples of devices proposed to break the second law in a quantum mechanical framework are presented [31, 32]. Of particular interest is ref. , in which a system has been presented and discussed breaking the second law in the quantum limit through many-body entanglement with a thermal bath at temperatures close to absolute zero. In contrast, the devices presented here rely solely on single-particle properties. As long as the inelastic scattering length is comparable to R, they function at any temperature.
The key elements required for such devices to break the second law of thermodynamics are the generation of particle states split into multiple wave packets, the quantum mechanical collapse of the multiple-wave packet states, and the sorting of single and multiple-wave packet states by interference. The latter step transfers the coherence properties of the wave packets into a useful output signal. These robust, single-particle processes are scalable, they function in a wide temperature range including high temperatures, are compatible with a standard room-type environment, and can be implemented in a large variety of devices that act on many species of quantized waves, including electromagnetic, particle, and quasiparticle waves.
3 Summary and Conclusions
More than 100 years after the discovery of superconductivity, room-temperature superconductivity finally seems within reach. Whether practical superconducting cables operating at room temperature will also be feasible, is more questionable . However, it seems possible to realize practical, macroscopic, non-superconducting devices that support dissipation-free currents in thermodynamic equilibrium. We have presented two such devices as examples. The generation of coherent wave packets, the interference of quantum states, and in a salient manner, the collapse of wave functions, provide the basis for their function. Remarkably, their functions, for which the validity of quantum physics is presumed, are in apparent disagreement with the second law of thermodynamics . As these devices also seem to be compatible with operation at high temperatures, their potential applications possibly far exceed dissipation-free conduction of electrons.
Open access funding provided by Max Planck Society. The authors gratefully acknowledge outstanding interactions with T. Kopp and very helpful discussions with A. Alavi, J. Annett, E. Benkiser, A. Brataas, H. Boschker, P. Bredol, T. Giamarchi, S. Hellberg, V. Kresin, G. Leuchs, M. Randeria, E.I. Rashba, B. Rodriguez-Lara, A. Schnyder, C. Schön, R. Valenti, and R. Wanke.
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