Vacuum fluctuations can enhance superconductivity in a bulk material. This is the new finding from researchers in China and the US who have put forward the concept of “vacuumtronics”. This is a new way to tune superconductivity without chemically altering a material, applying pressure to it or driving it with intense light.

In quantum electrodynamics, a vacuum is not empty but is host to fluctuating electromagnetic fields in which pairs of virtual particles are continuously being created and annihilated. Such vacuum fluctuations are thought to be responsible for phenomena like the Lamb shift, spontaneous emission and the Casimir effect.

In recent years researchers have been looking into the possibility of exploiting these fluctuations to change the properties of bulk matter. This is challenging because the fluctuations are extremely weak and produce little measurable change in macroscopic quantum states. They can, however, be amplified by a factor of 100 or even more using structures like resonant cavities. Indeed, such enhanced vacuum fields have already been exploited to modify material properties like chemical reactivity, topological states and conductivity.

In theory, vacuum fluctuations could be used to modulate superconductivity too, but until now this had never been demonstrated in an experiment.

Split-ring resonant “dark cavity”

In the new work, an experimental team led by Changgan Zeng and Guanghui Cheng at the University of Science and Technology of China (USTC), partially embedded the layered superconductor niobium diselenide (NbSe₂) inside a specially designed near-terahertz split-ring resonator, referred to as a “dark cavity”. The researchers then systematically compared the superconductivity of the NbSe₂ outside and inside the cavity by measuring the resistance of the material as a function of temperature.

They found an increase of up to 5.4% in the superconducting critical temperature, T_c, of NbSe₂ within the cavity compared with regions outside it, even although both samples came from the same NbSe₂ flake.

Higher current and field

The researchers say they also observed substantial increases in the critical current and critical magnetic field near T_c. This is exciting, says Cheng, because the cavity is dark: no light shines on the material and no energy is actively pumped into it. “Instead, the effect arises from the ever-present electromagnetic fluctuations of the quantum vacuum.”

One of the challenges in the experiments, he explains, was to distinguish between genuine vacuum-fluctuation effects from more mundane explanations, such as sample inhomogeneity, strain or device fabrication-related effects. He says he and his colleagues invested considerable effort in control experiments to rule these out.

Zheng and Cheng’s theorist colleagues Qingdong Jiang from Shanghai Jiao Tong University and Frank Wilczek from the Massachusetts Institute of Technology (MIT) department of physics developed a theoretical model to explain the underlying mechanism behind the effects observed. Within a Ginzburg–Landau framework, they put forward the hypothesis that the superconducting state exchanges virtual photons with the dark cavity, which lowers the energy of this state and thereby strengthens superconductivity. And, when the characteristic energy of the cavity mode matches the low-energy superconducting fluctuations, the NbSe₂ device exhibits resonant enhancement, producing the peak in superconductivity enhancement, explains Jiang.

Wilczek adds “In most practical physics experiments, the vacuum serves merely as the passive stage on which such phenomena play out”. “Our work shows that the background itself can become an actor – engineered to strengthen superconductivity and reshape the behaviour of quantum matter.”

The work establishes quantum-vacuum engineering as a new non-invasive way to tune superconductivity, Zeng tells Physics World. “Although the temperature enhancement we demonstrated is modest, it is a proof-of-principle for what could become a broadly applicable approach. It could be relevant to superconducting circuits, quantum sensors and other quantum devices, where non-invasive control of superconducting properties would be particularly valuable. More broadly, such engineered vacuum fields could also be used to control states of matter other than superconductivity.”

The China–US team says it would now like to further enhance the effect through improved cavity and material design and continue to investigate the underlying microscopic mechanisms in collaboration with theorists.

The post ‘Vacuumtronics’ could help make better superconductors appeared first on Physics World.

Researchers at the Karlsruhe Institute of Technology (KIT) in Germany and Stanford University in the US have developed an energy-collecting device that can simultaneously cool, produce electricity and generate heat. In their prototype, a radiative cooling layer reached 6.5 °C below ambient temperature while a solar collector generated electricity and reached nearly 111 °C.

“The new system could be installed on the roofs and facades of buildings to provide these three energy services at once,” says Gan Huang, who is head of the Hybrid Solar Technologies lab at KIT. “It could also be attractive for AI data centres that need both intensive power and cooling at the same time.”

As many of us have experienced, summer in the northern hemisphere has been particularly hot this year – a situation that is only likely to worsen in the future as the effects of climate change take hold. With this warming will come the increased need for air conditioning to cool down buildings. The problem is that current compression-based systems consume a lot of electricity and produce large amounts of CO₂.

Passive daytime radiative cooling

A potential alternative is passive daytime radiative cooling (PDRC), in which heat is continuously exchanged between objects at different temperatures by absorbing and emitting thermal radiation. When the heat absorbed by the material is less than the energy radiated to outer space, it can cool down during the day – even without electricity.

Our planet naturally experiences this effect by radiating heat out to space. This allows it to cool down at night because its temperature (of around 300 K) is much higher than that of outer space (around 3 K) and there is no incident solar irradiation from the Sun at night.

A good PDRC material needs to satisfy several criteria. For one, it must reflect sunlight strongly at solar spectrum wavelengths (of 0.3 to 2.5 µm) to avoid it heating up. It also needs to emit heat strongly in the long-wave infrared (LWIR) wavelengths (of 8–13 µm) where the atmosphere is more transparent to infrared radiation, so that it can lose energy to the cold sky. Over the past decade, researchers have succeeded in developing a wide range of PDRC materials, including multilayer nanophotonic emitters, single-layer polymers on reflectors and porous ceramics. However, conventional PDRC systems cannot harvest the coldness of outer space and solar energy from the same surface at the same time.

A hybrid PDRC and photovoltaic-thermal collector

In the new work, which is detailed in Cell Reports Physical Science, the KIT researchers made a hybrid PDRC and photovoltaic-thermal collector. The emitting layer in the device, which is transparent, consists of a silica substrate coated with the silicone polymer polydimethylsiloxane. This layer allows sunlight to pass through while simultaneously emitting heat as LWIR radiation through the atmospheric window. Beneath the transparent emitter, a Fresnel lens concentrates the transmitted sunlight onto the (gallium arsenide-based) photovoltaic-thermal collector mounted on a two-axis solar tracker. This is where electricity is generated.

In outdoor experiments performed during the day, the prototype simultaneously achieved cooling of up to 6.5 °C below ambient temperature, an electrical power density of 60.6 W/m² and heating up to 110.8 °C. “This is exciting,” says Huang, “because it shows that the hot Sun and the cold universe can be harvested together, rather than treated as separate resources as has been the case until now.”

The main challenge was ensuring that the solar collector became hot while the radiative cooler stayed cool, he explains. “We achieved this by concentrating the transmitted sunlight onto a much smaller solar collector underneath the transparent cooling layer, so reducing thermal interference between the hot and cold parts in the device.”

The team is now busy improving the efficiency and practical design of its system. “We are already working on a better optical design, improved thermal management and more efficient solar cells,” reveals Huang.

“The Sun is not the only renewable energy resource in the sky, the coldness of outer space is another,” he says. “We believe there are many exciting possibilities for a new generation of energy systems if we learn how to manage both these together.”

The post A hybrid energy system for heating, cooling and generating electricity appeared first on Physics World.