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When electrons slowly vanish during cooling
We have directly observed fermionic particles slowly “disappearing” for the first time, namely near a quantum phase transition in a so-called heavy-fermion compound. As well as advancing our understanding on the stability of fermionic quasiparticles, such quantum superposition states could have applications in quantum information technology.
We are all familiar with phase transitions, the most well-known being the water-to-ice transition, which occurs abruptly when water is cooled to below 0 °C. The characteristics of ice are very different to liquid water – for one it has a lower density. There are, however, phase transitions in which the characteristics of a material change gradually. For example, iron goes from being ferromagnetic to paramagnetic when heated to 760 °C, but as the transition progresses, the system takes longer and longer to equilibrate, thereby slowing down the transition. This phenomenon is known as critical slowing down and is typical for phase transitions involving bosonic particles that require these particles to “disappear”.
What we learn by driving quantum materials with THz light
In this review, the authors showcase several representative quantum material systems that effectively illustrate the use of THz pulses in driving the system into a non-equilibrium state and capturing the quantum many-body dynamics. The highlighted cases include scenarios where THz light is used to initiate superconducting-like phases, drive heavy quasi-particles, and induce complex phase transitions.
Scrutinizing the origin of softness across ferroelectric phase transition
In functional materials, we want to have novel effects as large and new as possible with perturbations as small as possible. Though seemingly contradictory, such a requirement can be met by taking the material close to a phase transition, where the material becomes unstable and nonlinear in some form. In ferroelectric materials, for example, a certain vibrational mode becomes “soft” near the phase transition; that is, the frequency of the mode decreases and ultimately vanishes. So far, such phenomena have been exploited only via macroscopic observables. Here, we scrutinize at the microscopic level the soft mode of a ferroelectric material near its phase transition.
Unprecedented insights into the dual nature of hybrid quantum states
We pluck a heavy-fermion system with an ultrashort terahertz light pulse, thereby breaking up, but not ionizing, the heavy-fermion bound state, and monitor the reflected electric field with sub-picosecond time resolution. The recombination into the heavy-fermion bound state is delayed in time because it requires the quantum mechanical coherence time to form, while the conventional interactions among the conduction electrons happen instantaneously. Thus, by analyzing the signal within different time windows, we can separately investigate the effect of strong correlations between localized and itinerant electrons leading to the heavy-fermion formation, as well as weak correlations stemming from conventional charge and static impurity scattering.
Using time as a filter for electronic correlations
On their way through metals, electrons undergo different types of interactions, ranging from charge to various types of spin scattering. When they scatter from the localized magnetic moments of the core electrons in rare-earth ions, they can form a hybrid state with these localized electrons and thereby drastically enhance their effective mass, hence their name heavy fermions. Ever since the beginning of correlated-electron research, it has not been easy to disentangle this heavy-fermion formation from the more conventional interactions within the conduction band, because both produce similar low-energy signatures in response experiments. We have now found a way to perform this disentanglement and investigate both effects separately by using a time filter.
Echoes from the quantum world
When we go from the meter scale of our daily life to the length scale of (sub-) atomic particles, the world becomes really strange -- quantum mechanics takes over! Particles no longer bounce off each other like solid billiard balls. Instead, they become fuzzy and may pass through each other without noticing. In fact, it is not even possible to say where exactly each particle is and how fast it is moving. Cooling a system promotes its quantum-mechanical behavior. In a hot system, particles have a lot of energy and whizz around in a classical way. Towards zero temperature (where zero is -273°C), this energy goes away. Nevertheless, the particles do not come to a standstill, because now quantum-mechanical motion, the aforementioned built-in fuzziness, takes over.
Manipulating transistors at terahertz frequencies
A 2D electron gas is like jelly. If pressure is electrically applied to the gas from above with a characteristic frequency, thickness and density oscillations are generated. Accordingly, the gas can be manipulated via electric forces, which oscillate much more rapidly than any radio or microwave frequency. As it has a thickness of just about ten nanometres, the oscillations follow the laws of quantum mechanics. This means: all occurring oscillations have a specific frequency, namely in the terahertz range. Pressure to the electron gas must be applied in that rapid change. We have found a way to trigger the required oscillations. Thus, a new method of accessing the interior of a transistor has been created.
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