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Flow reversal in a quantum system

In Classical Physic, thermodynamic laws determine what will be the flow of matter or energy. Basically, it can be summarized by the famous maxim: “Nature abhors vacuum”. All the flows tend to homogenize the various regions of space. For example, if you consider different bodies in contact, both will achieve the same temperatures and at this point, they are said to be in thermodynamic equilibrium with each other. In a system, its thermodynamic equilibrium state is determined by intensive properties such as temperature, pressure, volume etc. Whenever the system is in thermodynamic equilibrium, it tends to remain in this state infinitely and will not change spontaneously.

The best-known example of classical non-equilibrium thermodynamics is the Clausius’ statement: “It is impossible to construct a device which operates on a cycle and produces no other effect than the transfer of heat from a cooler body to a hotter body”. In other words, the second law of thermodynamics is saying that heat cannot flow from a cold bath to a hot one. This affirmation is stating that a macroscopic body in equilibrium is characterized by a single parameter: its temperature. When two objects with different temperatures are brought in contact, heat will flow from the hotter to the colder one.

However, in quantum materials, thermodynamical concepts seem to need to be revisited. There, states of matter can be set into a coherent superposition. When a classical observer measures a nanoscale system, this interaction destroys most of the coherence inside the system and alters its dynamical response.

Particle and energy currents in the steady state. Images show the heat current plotted as a curved surface and the lower energy current has been projected onto the plane below. A positive current (red) represents heat flowing from left to the right. A positive current indicates that a particle ring-current is flowing clockwise.

A team led by Angel Rubio from Germany demonstrates that the current and heat flows can be not only dictated by the temperature and potential gradient, but also by the external action of a local quantum observer that controls the coherence of the device. The researchers showed the direction of heat and particle currents can be independently controlled. In fact, the current and heat flow in a quantum material can go against the natural temperature and voltage gradients.

"Initially, we thought it was an error. We expected to come across changes and we thought it would be possible to halt the transport, but we didn't expect there was going to be a complete change of flow. These changes in the direction of the current can also be made in a controlled way. Depending on where the observer is inserted, the flow can be changed, but there are specific areas in the device in which, despite looking, the direction does not change. […] We have proposed a simple model, and the theory can be easily verified because all the energy and entropy flows are preserved. Carrying out this process experimentally would be another matter. Although the type of device that would need to be designed exists, and producing it would be feasible, right now, there is no possibility of doing this in a controlled way."

Ángel Rubio, a professor with the Hamburg-based Max Planck Institute for the Structure and Dynamics of Matter.

This recent work is indicating that new possibilities for the control of quantum transport far beyond classical thermal reservoirs. Dynamical quantum observations illustrate how we can create and directionality control the injection of currents in nanodevices. This scheme provides novel strategies to construct quantum devices with applications in thermoelectrics, spintronic injection, phononics, and sensing among others. In particular, highly efficient and selective spin injection might be achieved by local spin projection techniques.

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