The researchers, whose previous innovation included incorporating a perovskite layer that allowed the creation of a new type of polarized light-emitting diode (LED) that emits spin-controlled photons at room temperature without the use of magnetic fields or ferromagnetic contacts, now have gone a step further by integrating a III-V semiconductor optoelectronic structure with a chiral halide perovskite semiconductor. That is, they transformed an existing commercialized LED into one that also controls the spin of electrons. The results provide a pathway toward transforming modern optoelectronics, a field that relies on the control of light and encompasses LEDs, solar cells, and telecommunications lasers, among other devices.

“It’s up to one’s imagination where this might go or where this might end up,” said Matthew Beard, a senior research fellow at NREL and coauthor of the newly published Nature article, “Room temperature spin injection across a chiral-perovskite/III-V interface.”

Beard also serves as director of the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Science Basic Energy Sciences within DOE. The reported research was funded by CHOISE and relied on a broad range of scientific expertise drawn from NREL, the Colorado School of Mines, University of Utah, University of Colorado Boulder, and the Universite de Lorraine in France.

The goal of CHOISE is to understand control over the interconversion of charge, spin, and light using carefully designed chemical systems. In particular, the work focuses on control over the electron spin that can be either “up” or “down.” Most current-day optoelectronic devices rely on the interconversion between charge and light. However, spin is another property of electrons, and control over the spin could enable a wide plethora of new effects and functionality. The researchers published a paper in 2021 in which they reported how by using two different perovskite layers they were able to control the spin by creating a filter that blocks electrons “spinning” in the wrong direction.

They hypothesized at the time that advancements could be made in optoelectronics if they could successfully incorporate the two semiconductors, and then went on to do just that. The breakthroughs made, which include eliminating the need for subzero Celsius temperatures, can be used to increase data processing speeds and decrease the amount of power needed.

“Most current-day technologies are all based on controlling charge,” Beard said. “Most people just forget about the electron spin, but spin is very important, and it’s also another parameter that one can control and utilize.”

Manipulating the spin of electrons in a semiconductor has previously required the use of ferromagnetic contacts under an applied magnetic field. Using chiral perovskites, the researchers were able to transform an LED to one that emits polarized light at room temperature and without a magnetic field. Chirality refers to the material’s structure that cannot be superimposed on its mirror image, such as a hand. For example, a “left-handed” oriented chiral system may allow transport of electrons with “up” spins but block electrons with “down” spins, and vice versa. The spin of the electron is then converted to the “spin,” or polarization, of the emitted light. The degree of polarization, which measures the intensity of light that is polarized in one direction, reached about 2.6% in the previous research. The addition of the III-V semiconductor — which is made of materials in the third and fifth columns of the periodic table — boosted the polarization to about 15%. The degree of polarization serves as a direct measure of spin accumulation in the LED.

“This work is particularly exciting to me, as it combines spin functionality with a traditional LED platform,” said the first author of the work, Matthew Hautzinger. “You can buy an LED analogous to what we used for 14 cents, but with the chiral perovskite incorporated, we’ve transformed an already robust (and well understood) technology into a futuristic spin-control device.”

Potential losses up to 70% higher than previously estimated

The research team developed a new methodology that uses detailed information about the location and characteristics of a company’s physical assets, such as factories, equipment and natural resources. This approach provides a more accurate picture of climate risks than methods that use proxy data, which often assume that all of a company’s assets are located at its headquarters. “When we compared our results with those using proxy data, we found that the potential losses from climate risks could be up to 70% higher than previously thought,” says Stefano Battiston. “This underscores the critical need for more granular data in risk assessments.”

Preparing for the worst: The role of extreme events

The authors also point to the importance of considering “tail risk” in climate assessments. Tail risk refers to the possibility of extreme events that, while rare, can have catastrophic impacts. “Many assessments focus on average impacts. Our research shows that the potential losses from extreme events can be up to 98% higher than these averages suggest,” says Stefano Battiston. “Failure to account for these possibilities can leave businesses and investors dangerously unprepared.”

More funding for climate adaptation

The study’s findings have significant implications for climate policy, business strategy, and investment decisions. The researchers emphasize that more accurate risk assessments are crucial for developing effective climate adaptation strategies and determining appropriate levels of climate-related insurance and funding. “Our work shows that we may be seriously underestimating the financial resources needed for climate adaptation,” concludes Stefano Battiston.

The question of whether the behaviour of quantum objects could perhaps be described by a simple, more classical theory has been discussed for decades. In 1985, a way of measuring this was proposed: the so-called “Leggett-Garg inequality.” Any theory that describes our world without the strange superposition states of quantum theory must obey this inequality. Quantum theory, on the other hand, violates it. Measurements with neutrons testing this “Leggett-Garg inequality” have now been carried out for the first time at TU Wien — with a clear result: the Leggett-Garg inequality is violated, classical explanations are not possible, quantum theory wins. The results have now been published in the journal Physical Review Letters.

Physical realism

We normally assume that every object has certain properties: A ball is at a certain location, it has a certain speed, perhaps also a certain rotation. It doesn’t matter whether we observe the ball or not. It has these properties quite objectively and independently of us. “This view is known as ‘realism’,” says Stephan Sponar from the Atomic Institute at TU Wien.

We know from our everyday experience that large, macroscopic objects in particular must obey this rule. We also know that Macroscopic objects can be observed without being influenced significantly. The measurement does not fundamentally change the state. These assumptions are collectively referred to as “macroscopic realism.”

However, quantum theory as we know it today is a theory that violates this macroscopic realism. If different states are possible for a quantum particle, for example different positions, speeds or energy values, then any combination of these states is also possible. At least as long as this state is not measured. During a measurement, the superposition state is destroyed: the measurement forces the particle to decide in favour of one of the possible values.

The Leggett-Garg inequality

Nevertheless, the quantum world must be logically connected to the macroscopic world — after all, large things are made up of small quantum particles. In principle, the rules of quantum theory should apply to everything.

So the question is: Is it possible to observe behaviour in “large” objects that cannot be reconciled with our intuitive picture of macroscopic realism? Can macroscopic things also show clear signs of quantum properties?

In 1985, physicists Anthony James Leggett and Anupam Garg published a formula with which macroscopic realism can be tested: The Leggett-Garg Inequality. “The idea behind it is similar to the more famous Bell’s inequality, for which the Nobel Prize in Physics was awarded in 2022,” says Elisabeth Kreuzgruber, first author of the paper. “However, Bell’s inequality is about the question of how strongly the behaviour of a particle is related to another quantum entangled particle. The Leggett-Garg inequality is only about one single object and asks the question: how its state at specific points in time related to the state of the same object at other specific points in time?”

Stronger correlations than classical physics allows

Leggett and Garg assumed an object that can be measured at three different times, each measurement can have two different results. Even if we know nothing at all about whether or how the state of this object changes over time, we can still statistically analyse how strongly the results at different points in time correlate with each other.

It can be shown mathematically that the strength of these correlations can never exceed a certain level — assuming that macroscopic realism is correct. Leggett and Garg were able to establish an inequality that must always be fulfilled by every macroscopic realistic theory, regardless of any details of the theory.

However, if the object adheres to the rules of quantum theory, then there must be significantly stronger statistical correlations between the measurement results at the three different points in time. If an object is actually in different states at the same time between the measurement times, this must — according to Leggett and Garg — lead to stronger correlations between the three measurements.

Neutron beams: Centimetre-sized quantum objects

“However, it is not so easy to investigate this question experimentally,” says Richard Wagner. “If we want to test macroscopic realism, then we need an object that is macroscopic in a certain sense, i.e. that has a size comparable to the size of our usual everyday objects.” At the same time, however, it must be an object that has a chance of still showing quantum properties.

“Neutron beams, as we use them in a neutron interferometer, are perfect for this,” says Hartmut Lemmel, instrument responsible at the S18 instrument at the Institut Laue-Langevin (ILL) in Grenoble, where the experiment was conducted. In the neutron interferometer, a silicon perfect crystal interferometer that was first successfully used at the Atomic Institute of TU Wien in the early 1970s, the incident neutron beam is split into two partial beams at the first crystal plate and then recombined by another piece of silicon. There are therefore two different ways in which neutrons can travel from the source to the detector.

“Quantum theory says that every single neutron travels on both paths at the same time,” says Niels Geerits. “However, the two partial beams are several centimetres apart. In a sense, we are dealing with a quantum object that is huge by quantum standards.”

Using a sophisticated combination of several neutron measurements, the team at TU Wien was able to test the Leggett-Garg inequality — and the result was clear: the inequality is violated. The neutrons behave in a way that cannot be explained by any conceivable macroscopically realistic theory. They actually travel on two paths at the same time, they are simultaneously located at different places, centimetres apart. The idea that “maybe the neutron is only travelling on one of the two paths, we just don’t know which one” has thus been refuted.

“Our experiment shows: Nature really is as strange as quantum theory claims,” says Stephan Sponar. “No matter which classical, macroscopically realistic theory you come up with: It will never be able to explain reality. It doesn’t work without quantum physics.”