2.2. Future information technologies

2.2.2. Phase transitions

One hope for faster and more energy efficient information technology is in the use of the unique properties of functional materials beyond classical semiconductors. In particular correlated materials exhibit a wealth of phases with drastically different macroscopic properties. Since these phases exist in close proximity, a phase transition can be induced by comparably small external stimuli [1]. Examples are metal-insulator transitions that occur upon temperature change or when the material is exposed to a magnetic field. Such effects can be truly dramatic with changes in the conductivity of 6 orders of magnitude [2]. Moreover, some of these transitions are among the fastest switching processes ever observed in solids [3], which qualifies them even more for future information processing applications.

Besides driving the whole material through a phase transition novel concept for functionality haven been developed, that apply perfectly for such materials with complex phase diagrams [1]. In many correlated materials different phases can coexist under identical conditions. By driving, say, the metallic phase through the percolation transition, the whole material can be rendered macroscopically metallic with a much smaller energy investment than a full phase transition would require. Additionally such states with phase coexistence are typically metastable such that their respective state remains conserved until it is actively changed.

Many of these materials turn out to be multiferroic which mean that ordering effects in one degree of freedom is coupled to another order in a second degree of freedom. The best studied class of multiferroics is those where magnetic order and electric polarization are coupled, promising, e.g., to write magnetic information electronically instead of using bulky coils. This field has been dramatically growing recently because more and more concepts how to obtain multiferroicity are developed [4] including bringing materials with different ferroic properties in close proximity in order to couple them.

This latter approach is just one aspect of a much wider approach to create novel functional materials by creating heterostructures of correlated materials [5]. At interfaces completely new properties can be created (like a two-dimensional electron gas at the interface between two insulators); strain and charge transfer can affect the properties of one of the layers; and – as for the case of the heterogeneous multiferroics - a coupling of the properties of the different layers can be achieved.

On the way from interesting functionality to application the switching behavior is a central point. What is the most efficient way to control properties? How fast can properties be changed, how stable are the states? Time resolved x-ray techniques have made enormous progress in the last decade mainly focusing on magnetic materials. Recently, scientists have extended such studies towards phase transitions in correlated materials [3, 6-9].

Figure 12. Processes during the Verwey transition of magnetite when induced by an optical laser pulse [11]. (Courtesy C. Schüssler-Langeheine, HZB)

So far such experiments are limited by the few available sources of short x-ray pulses. Despite this, detailed studies of phase-transition behavior could be carried out showing the effects of optical excitations as well as THz pulses that directly couple to phonon modes. A large role play experiments with soft x-ray pulses that combine high spectroscopic sensitivity with magnetic information.

Besides switching material from one phase to another in the most controlled and/or efficient way, another option in time-resolved experiments is to reach phases that do not occur in equilibrium. The observation of critical fluctuations for at least two different phases have been reported, e.g., for magnetite, only one of which develops in equilibrium [10]. One may hope that far from equilibrium the other phase could be reached and studied. For the same material a laser-induced metastable phase separation was recently observed, which again is a state of matter that does not exist in equilibrium (Figure 12) [11]. 

While some of these processes are very fast, some occur on timescales of a few picoseconds and some are considerably slower. Even experiments that cannot resolve the ultimate switching time scales can still be used to characterize the excited state. As a matter of fact, free electron lasers are not always the ideal source for such kind of experiments since the high peak brightness may affect the sample too much for a controlled experiment. BESSY-VSR with its high repetition rate at moderate peak brightness may be much better suited in many cases. Furthermore only a storage ring allows for recording resonance spectra by variation of the incoming photon energy over a wide range. Understanding of such correlated materials has been mainly advanced with x-ray spectroscopic methods like x-ray absorption spectroscopy. First attempts to transfer these methods to time-resolved experiments have proven successful. BESSY-VSR would certainly be the ideal source for such kind of research.

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