Discovery of light-induced ferroelectricity in strontium titanate

Ultrashort terahertz pulses create a ferroelectric phase in the paraelectric strontium titanate. An optically-induced deformation of the sample results in pairs of flexoelectric domains with opposing polarisations. Credit: Joerg M. Harms, MPSD

by Tobia Nova, Jenny Witt, Max Planck Institute for the Structure and Dynamics of Matter

https://phys.org/news/2019-06-discovery-light-induced-ferroelectricity-strontium-titanate.html

Light can be used not only to measure materials' properties, but also to change them. Especially interesting are those cases in which the function of a material can be modified, such as its ability to conduct electricity or to store information in its magnetic state. A team led by Andrea Cavalleri from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have used terahertz frequency light pulses to transform a non-ferroelectric material into a ferroelectric one.

Ferroelectricity is a state in which the constituent lattice is polarized in one specific direction, forming a macroscopic electrical polarization. The ability to reverse polarization makes ferroelectric materials particularly suitable for digital information encoding and processing. The discovery of a light-induced ferroelectric is highly relevant for a new generation of high-speed devices, and is presented today in the journal Science.

Complex materials are special because their unusual macroscopic properties are determined by many competing tendencies. Unlike in more conventional compounds, such as the silicon crystals that make up current electronic devices, in complex materials one finds that more than one type of microscopic interaction favors more than one possible macroscopic phase.

Such competition leads then to a compromise, but one that is not unique and is often in precarious equilibrium. Hence, moderate perturbations, for example irradiating one such material with light, can induce radical changes in the properties of the solid.

Ultra-short terahertz laser pulses are especially useful because they couple directly to the crystal lattice and can deform atomic arrangements at high speeds. Coherent excitation of lattice vibrations has been shown in the past to cause changes of electrical properties or magnetic arrangements in a number of complex materials, including superconductors.

In their latest research, the scientists describe how they induced a ferroelectric order in a material, a property of solids that can be highly relevant to applications. Ferroelectricity describes the spontaneous alignment of electric dipoles, which leads to a macroscopic polarization akin to the magnetisation in a ferromagnet. Generally, ferroelectricity only occurs in a limited class of materials; however, the Hamburg group has discovered that even non-ferroelectric materials can be forced into a ferroelectric phase by light.

Strontium titanate (STO) is paraelectric at all temperatures and a long-range ferroelectric order never develops. Upon exciting vibrations in STO by light, the researchers observed characteristics in its optical and electrical responses typical of ferroelectricity. The origin of this surprising effect lies in the nonlinear nature of the crystal lattice. The driven phonon delivers some of its energy in the form of pressure to the solid, resulting in a spatially varying structural deformation within the excited area. In these conditions, a material property called flexoelectricity can be activated, resulting in a macroscopic polarization. Strikingly, the photo-induced state was found to survive for hours after being created, showing that the material transitioned to a new quasi-stable phase.

"The ability to induce and control ferroelectric states with light on ultrafast timescales could provide the basis for next-generation technologies", says Tobia Nova, first author of the paper. Ferroelectric materials are already at the core of devices in development, which exploit their spontaneous polarization to make stable memory chips or "always on" computers. Because the light-induced ferroelectric phase demonstrated in the Hamburg experiment operates at terahertz frequencies, electro-optic devices that work at such high speeds might be envisioned. Moreover, since flexoelectricity is a common material property, the ability to induce ultrafast flexoelectric polarizations extends far beyond the specific example of STO. Lastly, because STO is routinely used as a substrate in complex heterostructures, the optical access to flexoelectric polarizations should find extensive applications in the manipulation of collective phenomena at interfaces.

Terahertz laser pulses amplify optical phonons in solids

When light excites the material and induces large atomic vibrations at frequency ω (blue wave), fundamental material properties are modulated in time at twice such frequency (red wave), acting a source for phonon amplification. Credit: J. M. Harms, MPSD

: https://phys.org/news/2018-11-terahertz-laser-pulses-amplify-optical.html#jCp

by Dr. Andrea Cartella, Jenny Witt, Max Planck Institute for the Structure and Dynamics of Matter

A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg/Germany presents evidence of the amplification of optical phonons in a solid by intense terahertz laser pulses. These light bursts excite atomic vibrations to very large amplitudes, where their response to the driving electric field becomes nonlinear and conventional description fails to predict their behavior.

In this new realm, fundamental material properties usually considered constant are modulated in time and act as a source for phonon amplification. The paper, "Parametric Amplification of Optical Phonons" by Andrea Cartella et al., has been published in the PNAS.

The amplification of light dramatically changed science and technology in the 20th century. This path, which began in 1960 with the invention of the laser, still has such a remarkable impact that the 2018 Nobel Prize in Physics was awarded "for groundbreaking inventions in the field of laser physics." Indeed, the amplification of other fundamental excitations like phonons or magnons is likely to have an equally transformative impact on modern condensed matter physics and technology.

The group led by Prof. Andrea Cavalleri at the MPSD has pioneered the field of controlling materials by driving atomic vibrations (i.e. phonons) with intense terahertz laser pulses. If the atoms vibrate strongly enough, their displacement affects material properties. This approach has proven successful in controlling magnetism, as well as inducing superconductivity and insulator-to-metal transitions. In this field, it is then important to understand whether the phonon excitation by light can be amplified, potentially leading to performative improvements of the aforementioned material control mechanisms.

In the present work, Cartella, Cavalleri and coworkers used intense terahertz pulses to resonantly drive large-amplitude phonon oscillations in silicon carbide and investigated the dynamic response of this phonon by measuring the reflection of weak (also resonant) probe pulses as a function of time delay after the excitation.

"We discovered that for large enough intensities of our driving pulses, the intensity of the reflected probe light was higher than that impinging on the sample," said Andrea Cartella. "As such, silicon carbide acts as an amplifier for the probe pulses. Because the reflectivity at this frequency is the result of the atomic vibrations, this represents a fingerprint of phonon amplification."

The scientists were able to rationalize their findings with a theoretical model that allowed them to identify the microscopic mechanism of this phonon amplification: fundamental material properties, usually considered constant, are modulated in time and act as a source for amplification. This is the phononic counterpart of a well-known nonlinear optical effect, the so-called four-wave-mixing.

These findings build upon another discovery by the Hamburg group that was published earlier this year, showing that phonons can have a response reminiscent of the high-order harmonic generation of light. These new discoveries suggest the existence of a broader set of analogies between phonons and photons, paving the way for the realization of phononic devices.

 Explore further: Control of quantum state of optical phonon in diamond induced by ultrashort light pulses

More information: A. Cartella et al. Parametric amplification of optical phonons, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1809725115

Abstract-Terahertz field control of interlayer transport modes in cuprate superconductors

Frank Schlawin, Anastasia S. D. Dietrich, Martin Kiffner, Andrea Cavalleri, and Dieter Jaksch

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.064526

We theoretically show that terahertz pulses with controlled amplitude and frequency can be used to switch between stable transport modes in layered superconductors, modeled as stacks of Josephson junctions. We find pulse shapes that deterministically switch the transport mode between superconducting, resistive, and solitonic states. We develop a simple model that explains the switching mechanism as a destabilization of the center-of-mass excitation of the Josephson phase, made possible by the highly nonlinear nature of the light-matter coupling.

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Abstract-Terahertz field control of interlayer transport modes in cuprate superconductors

Frank Schlawin, Anastasia S. D. Dietrich, Martin Kiffner, Andrea Cavalleri, Dieter Jaksch

(Submitted on 24 Jul 2017)

https://128.84.21.199/abs/1707.07459

We theoretically show that terahertz pulses with controlled amplitude and frequency can be used to switch between stable transport modes in layered superconductors, modelled as stacks of Josephson junctions. We find pulse shapes that deterministically switch the transport mode between superconducting, resistive and solitonic states. We develop a simple model that explains the switching mechanism as a destablization of the centre of mass excitation of the Josephson phase, made possible by the highly non-linear nature of the light-matter coupling.

Abstract-Probing optically silent superfluid stripes in cuprates

Srivats Rajasekaran, Jun-ichi Okamoto, Ludwig Mathey, Michael Fechner, Vivek Thampy, Genda D. Gu, Andrea Cavalleri

(Submitted on 17 May 2017)

https://arxiv.org/abs/1705.06112

Unconventional superconductivity in the cuprates emerges from, or coexists with, other types of electronic order. However, these orders are sometimes invisible because of their symmetry. For example, the possible existence of superfluid charge stripes in the normal state of single layer cuprates cannot be validated with infrared optics, because interlayer tunneling fluctuations vanish on average. Similarly, it is not easy to establish if charge orders are responsible for dynamical decoupling of the superconducting layers over broad ranges of doping and temperatures. Here, we show that TeraHertz pulses can excite nonlinear tunneling currents between linearly de-coupled charge-ordered planes. A giant TeraHertz third harmonic signal is observed in La1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature TCO = 55 K. We model these results by considering large order-parameter-phase oscillations in a pair density wave condensate, and show how nonlinear mixing of optically silent tunneling modes can drive large dipole-carrying super-current oscillations. Our results provide compelling experimental support for the presence of hidden superfluid order in the normal state of cuprates. These experiments also underscore the power of nonlinear TeraHertz optics as a sensitive probe of frustrated excitations in quantum solids.

Abstract-A novel dual-frequency terahertz antenna in standard CMOS technology

Srivats Rajasekaran, Jun-ichi Okamoto, Ludwig Mathey, Michael Fechner, Vivek Thampy, Genda D. Gu, Andrea Cavalleri

http://ieeexplore.ieee.org/document/7919882/

A dual-frequency antenna resonated at 0.32 THz and 0.65 THz has been analyzed and simulated for CMOS THz imaging and sensing systems. It includes the two rectified bowtie structures designed at metal and poly-Si layers in standard CMOS technology. Simulation results show that the antenna has the high gain and radiation efficiency and a great impedance matching comparing to the conventional metal dual-band antenna. The demonstrated design opens a brand new way for ease realization of multi-band on-chip THz antenna in CMOS technologies.

Manipulating superconducting plasma waves with terahertz light

Josephson plasma wave in a layered superconductor, parametrically amplified through a strong terahertz light pulse. Credit: Max Planck Institute for the Structure and Dynamics of Matter

 http://phys.org/news/2016-07-superconducting-plasma-terahertz.html#jCp

Most systems in nature are inherently nonlinear, meaning that their response to any external excitation is not proportional to the strength of the applied stimulus. Nonlinearities are observed, for example, in macroscopic phenomena such as the flow of fluids like water and air or of currents in electronic circuits. Manipulating the nonlinear behavior is therefore inherently interesting for achieving control over several processes. An international team of researchers led by Andrea Cavalleri from the Max Planck Institute for the Structure and Dynamics of Matter at CFEL in Hamburg utilized the nonlinear interaction between a terahertz light field and a superconducting plasma wave in a high temperature cuprate superconductor to amplify the latter. This resulted in a more coherent superconductor, which is less susceptible to thermal fluctuations. Due to the non-dissipative superconducting nature of the plasma wave, the study opens up new avenues for "plasmonics", a field of science utilizing plasma waves for transmitting information. These findings are reported in the journal Nature Physics
The Josephson effect

The Josephson effect, predicted by Brian D. Josephson in 1962, consists in the tunneling of Cooper pairs across a thin, insulating junction between two superconductors. This superconductor-insulator-superconductor structure is called a Josephson junction. This theory was soon experimentally confirmed and in 1973 Josephson received the Nobel Prize in Physics, as his prediction resulted in the verification of the macroscopic quantum nature of superconductors.

The charge dynamics in Josephson junctions is governed by the Josephson equations, which state that the current associated with the tunneling Cooper pairs is proportional to the sine of the phase difference between the two superconductors. Under an applied voltage, the current oscillates at a frequency that depends on the voltage drop at the junction. The Josephson effect not only resulted in fundamental advances in physics but also in many applications including so-called SQUIDs, i.e. very sensitive magnetometers that are used to measure extremely weak magnetic fields. These are used, for instance, in medicine for mapping brain activity (magnetoencephalography). Moreover, Josephson junctions are nowadays employed as an extremely precise voltage standard, because the Josephson effect is a quantum effect that relates voltages and frequencies (or time) by a proportionality involving only fundamental constants.

Current research topics utilizing the Josephson effect include the realization of qubits for quantum computing and photonic devices in the gigahertz (GHz) and terahertz (THz) frequency regime.

Josephson plasma waves in cuprate superconductors

Layered superconductors like high-Tc cuprates – being built of alternating superconducting and insulating planes – are a nanoscale version of a stack of Josephson junctions. In these materials, superconducting transport first occurs in the copper-oxygen planes, while three-dimensional superconductivity emerges via Josephson tunneling in the direction perpendicular to the planes.

In analogy to Maxwell's equations in electrodynamics, whose temporal and spatial dependence results in electromagnetic waves, the Josephson relations result in the so-called Josephson plasma waves. The frequency of these waves falls into the THz range for cuprate materials and can therefore be observed with conventional THz spectroscopy.

The team around Andrea Cavalleri used THz radiation to probe Josephson plasma waves in barium-doped lanthanum copper oxide (La1.905Ba0.095CuO4). From the reflection of the probe pulse they could detect oscillations at about half a THz frequency. "When we irradiated the superconductor with our weak probe pulses, we could observe oscillations of the reflected field at a specific frequency, the so-called Josephson plasma frequency," says Srivats Rajasekaran, first author of the paper and postdoc at the MPSD in Hamburg.

Nonlinearities of Josephson plasma waves and parametric amplification

Since the Josephson plasma waves are governed by the Josephson relations, they are inherently nonlinear. In the current study, these Josephson plasma waves were driven into a highly nonlinear regime using an additional intense THz pump pulse with very large field strengths of up to 100 kV/cm. This was made possible by exploiting the recent advances in THz technology. In this regime, amplification of the Josephson plasma wave was observed experimentally. "The reflectivity of the sample became larger than 100% and, on top of that, the absorption coefficient became negative. These are clear indications of amplification occurring inside the material," explains Srivats Rajasekaran.

Parametric amplification in simple oscillating systems, achieved by periodically modulating some specific parameter, is a well-understood phenomenon. For instance, a child on a swing increases its oscillation amplitude by periodically raising and lowering its center of mass. An example from electronics is an LC circuit with periodically varied capacitance or inductance. Parametric amplifiers of this type have applications in the enhancement of weak signals without increasing its noise (used e.g. in radio astronomy). "When it comes to parametric amplification, a layered superconductor acts very much like an LC circuit," says Srivats Rajasekaran. "The Josephson supercurrent is like a wire connecting the plates of a capacitor – the copper oxide layers." The inductance of the supercurrent depends on the phase difference between the layers, and this phase difference varies with time and position on the plane.

"When we applied our intense pump pulse, the pump-probe response oscillated at twice the Josephson plasma frequency. This is equivalent to modulating the inductance periodically, which is required for parametric amplification," adds Srivats Rajasekaran. "This is the first time that the effect of parametric amplification by light irradiation has been demonstrated for Josephson plasma waves," declares Andrea Cavalleri, director at the MPSD in Hamburg.

Potential Applications

Amplification of Josephson plasma waves, exploiting the nonlinear Josephson relations with THz pulses, falls in the category of the previous works led by Andrea Cavalleri on layered superconductors, wherein THz light was utilized to switch off and on superconductivity between the planes and to generate superconducting solitons. In addition, this work has implications in the control of fluctuations of the superfluid. "The possibility to parametrically control the superfluid in layered superconductors might eventually provide a tool to stabilize fluctuating superconductivity, perhaps even for temperatures above the critical temperature," concludes Andrea Cavalleri.

The study was made possible by the ERC Synergy Grant "Frontiers in Quantum Materials' Control" (Q-MAC) that brings together scientists of the MPSD, Oxford University and further research institutions. The research team also involved scientists of the Brookhaven National Laboratory, the University of Bath and the National University of Singapore. The Center for Free-Electron Laser Science (CFEL) is a joint enterprise of DESY, the Max Planck Society and the University of Hamburg.

 Explore further: Superconductors that work by themselves: Scientists discover new possibilities in chryoelectronics

More information: S. Rajasekaran, E. Casandruc, Y. Laplace, D. Nicoletti, G. D. Gu, S. R. Clark, D. Jaksch, and A. Cavalleri, "Parametric Amplification of a Superconducting Plasma Wave," Nature Physics, Advance Online Publication, (July 11, 2016), DOI: 10.1038/nphys3819 

Controlling matter with light

Charles Sayers

https://jphysplus.iop.org/2016/07/07/controlling-matter-with-light/

For a recent colloquium at the University of Bath, Professor Andrea Cavalleri was invited to talk about his work on ‘controlling solid phases with light’. A team led by Andrea at the Max Planck Institute for the Structure and Dynamics of Matter have been studying the response of materials using intense laser pulses of low frequency. I asked Andrea about the motivation for the work in their most recent paper, and the future of controlling matter with light.

Mostly, scientists have probed matter non-resonantly using lasers operating at frequencies near the visible part of the spectrum. More recently however, radiation in the mid infra-red to THz region of the spectrum has provided direct access to the low energy excitations in solids, such as phonons, excitons, and many others. Due to advances in high power sources, this type of radiation can be used not only to probe matter, but to drive it into a new transient state.

Andrea’s team have been ultimately seeking to control matter with light by driving solids into a non-equilibrium state, and this includes the optical enhancement of superconductivity.

“Our goal in particular is to use light to generate phases that do not occur spontaneously at equilibrium, like for example superconductivity at higher temperatures. Traditionally, quantum-phase discovery has been achieved by materials synthesis or by using pressure or strong magnetic fields, but the ability to generate sculpted, coherent light fields opens new possibilities which are only starting to be explored.”

Schematic LTT crystal structure for LESCOx. ©2016 IOP Publishing Ltd. All rights reserved.

In 2011, the team were able to measure superconducting-like optical properties in the cuprate, LESCO at temperatures far greater than the equilibrium transition temperature using these methods. Later, the same was found for YBCO and x-ray diffraction experiments hinted at intense light pulses forcing a structural change to the crystal as the driving mechanism for enhanced superconductivity.

Now, the team have focused on potassium doped buckyballs to see if the same effects can be seen. Buckyballs (Buckminsterfullerene) are large carbon cages made up of 60 atoms. When doped with potassium to form K₃C₆₀, the material becomes superconducting with an equilibrium transition temperature of 20 K.

In their experiments, a mid-infrared laser pulse (the ‘pump’) was firstly used to excite K₃C₆₀ powder in the range of 80 – 200 meV photon energy. Shortly after (about 1 ps), the excited state was measured using a second pulse (the ‘probe’) of THz frequency, which is used to detect changes in the reflectivity. It was found that the mid-infrared pulses had induced a large increase in carrier mobility and the opening of a gap in the optical conductivity which then disappears around a picosecond later.

These superconducting-like optical properties mimic the same signatures seen when cooling K₃C₆₀ below its equilibrium transition temperature. And so for a fleeting second (…or picosecond), light had driven the material into a superconducting state! What’s even more interesting is that the state seemingly survives up to around 100 K, which is around 5 times greater than the equilibrium transition temperature.

Similar to the team’s work on YBCO, they explain their results by light pulses distorting the crystal lattice which may increase the electron-phonon coupling, favouring stronger superconductivity. However, the results for K₃C₆₀ are even more convincing than their previous work on the cuprates since the interpretation of these experiments was difficult as their highly unconventional superconduting state is still not fully understood. In addition, the cuprates are two dimensional materials with peculiar transport properties.

“The novelty of our result on K₃C₆₀ resides in the fact that it demonstrates that light-induced superconductivity is a far more general phenomenon than previously envisaged. Unlike cuprates, K₃C₆₀ is a fully three-dimensional molecular solid, where the equilibrium superconducting state is quite conventional and fairly well understood.”

The optical signatures seen in the team’s experiments are extremely short lived and the superconducting-like state only exists on the order of picoseconds. Currently, this is too short for the traditional superconducting tests of zero electrical resistance and expulsion of magnetic fields (Meissner effect) to be performed. However, the optical signatures are clear and the work of Cavalleri et al. paves the way for controlling phenomena in solids with light.

“The possibilities offered by intense, sculpted light fields at THz frequencies for controlling and switching complex matter is enormous.”

An application using current methods of short light pulses could be to create a highly efficient switchable superconductor device. Conversely, if the light-induced state can be maintained for any significant time, the idea of creating optically pumped room temperature superconductors could become a reality.

“Great efforts are being aimed at developing new THz sources, which can produce picosecond-to-nanosecond narrow band, high field pulses, tunable in the broadest possible spectral range. These may be used to drive transient superconductors in a quasi-persistent mode, thus extending their lifetime.”

It is clear to see that we have only scratched the surface of this relatively new phenomenon, but the future is truly exciting.

Abstract-Parametric Amplification of a Terahertz Quantum Plasma Wave

Authors: Srivats Rajasekaran, Eliza Casandruc, Yannis Laplace, Daniele Nicoletti, Genda D. Gu,Stephen R. Clark, Dieter Jaksch, Andrea Cavalleri

http://www.mathpubs.com/detail/1511.08378v1/Parametric-Amplification-of-a-Terahertz-Quantum-Plasma-Wave

Many applications in photonics require all-optical manipulation of plasma waves, which can concentrate electromagnetic energy on sub-wavelength length scales. This is difficult in metallic plasmas because of their small optical nonlinearities. Some layered superconductors support weakly damped plasma waves, involving oscillatory tunneling of the superfluid between capacitively coupled planes. Such Josephson plasma waves (JPWs) are also highly nonlinear, and exhibit striking phenomena like cooperative emission of coherent terahertz radiation, superconductor-metal oscillations and soliton formation. We show here that terahertz JPWs in cuprate superconductors can be parametrically amplified through the cubic tunneling nonlinearity. Parametric amplification is sensitive to the relative phase between pump and seed waves and may be optimized to achieve squeezing of the order parameter phase fluctuations or single terahertz-photon devices.

Snapshots of non-equilibrium Dirac carrier distributions in graphene

 

 

• Isabella Gierz,¹ Jesse C. Petersen,^{1, 2} Matteo Mitrano,¹ Cephise Cacho,³ I. C. Edmond Turcu,³ Emma Springate,³ Alexander Stöhr,⁴ Axel Köhler,⁴ Ulrich Starke⁴ , Andrea Cavalleri^{1, 2}

 

http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3757.html

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting

Light tsunami in a superconductor

An international team of researchers investigated light tsunamis in a superconductor using the HZDR free electron laser's strong terahertz flashes. Credit: Frank Bierstedt
: http://phys.org/news/2013-04-tsunami-superconductor.html#jCp
Superconductors are materials which conduct electric currents without any resistance. At the Helmholtz-Zentrum Dresden-Rossendorf, an international research team headed by Professor Andrea Cavalleri from the Max Planck Institute for the Structure and Dynamics of Matter managed to selectively influence this resistance-free conductivity with a powerful terahertz laser. This very precise laser light turns into a vortex which moves through the superconductor like a tsunami. The results will be published in the scientific journal Nature Materials


In their latest experiment, Prof. Andrea Cavalleri from the Max Planck Institute for the Structure and Dynamics of Matter at the Hamburg-based Center for Free-Electron Laser Science (CFEL) and Dr. Michael Gensch from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) investigated together with other colleagues from the HZDR, the United Kingdom, and Japan if and how superconductivity can be systematically controlled. The objective of their research is to improve the usability of superconducting materials for such new technologies as, for example, the processing of information. For this purpose, and for a better understanding of the underlying phenomena, it is essential to increase the critical temperature – critical temperature means that materials below this value are superconducting. Today, most superconductors only function at very low temperatures.

Typically, modern high temperature superconductors are solids which consist of a stack of thin layers, similar to the pages in a book. These layers are conductive and transport electricity. However, no electricity can flow from layer to layer at room temperature since those electrons which are responsible for the current flow can only move freely in the respective layer. If, however, such a layer stack is cooled to the right temperature, then superconductivity occurs along all directions. But there is one difference: While the electrons flow inside a layer without any resistance, these electrons can now also move from layer to layer by "tunneling" through the insulating areas located between these layers. Dr. Gensch explains: "Already the geometry suggests that the mechanisms of superconductivity are different inside and between the layers. We were interested in how the electrons transport this property vertically from layer to layer and/or whether we would be able to control this transport without disrupting the superconductivity in the horizontal layers."

For their experiments, the researchers used one of the HZDR's two free electron lasers (FELBE) which generates laser flashes of a specific, freely adjustable wavelength between the infrared and the microwave range. If such a short terahertz flash penetrates the material layers of the superconductor at the right frequency, then it deactivates the superconductivity very selectively and locally by directly changing the tunneling properties of the electrons found between the superconducting layers. More precisely, the light generates a pair of normal-conducting vortex currents which rotate in opposite directions. These vortices then move through the superconductor with the light. A so-called soliton wave is formed. What's so special about these waves: They always retain their shape irrespective of any faults in the superconductor. This resembles the behavior of such known soliton waves as, for example, tsunamis; the shape of which is also not influenced by any ground dislocations or irregularities
The vortices moving through the superconductor also alter the optical properties of the material – it becomes slightly transparent. While not for visible light, this is the case for wavelengths in the terahertz regime. The laser flashes last only for a few picoseconds, i.e. the billionth part of a second, so the scientists are able to observe all processes – such as the emergence of the vortices and their soliton movement – directly along this very fast time scale. The team headed by Prof. Cavalleri had successfully achieved something similar already once before. But back then, the scientists had only been able to quickly and consecutively switch the entire superconductivity off and on again between the layers. For the first time ever, the experiment in Dresden successfully managed to switch off the superconductivity very precisely and, above all, also locally – and to stabilize this state almost ten times longer than has been the case before.

In particular, the physicists expect a number of new applications from these light generated vortices inside superconductors. Since they move through the crystal just like a tsunami irrespective of any faults or irregularities, these vortices are perfectly suited to store and transport information inside them. Information is transported within DNA like soliton waves. The experiment, which furnished proof and demonstrated that the vortices can be controlled by laser light, has the physicists in Prof. Cavalleri's team already dreaming about new opportunities for information processing in superconductors.

Superconductors under Constant Bombardment

For a number of years now, intense pulses in the invisible terahertz range (0.1 THz – 10 THz) have been sparking enormous interest among scientists who investigate such so-called complex materials as high temperature superconductors. This is due to the specific properties of this long-wave radiation which has wavelengths between 0.03 and three millimeters. The energy per light particle is so low that the radiation doesn't really interact anymore directly with the electrons in a material, but instead, for example, with the atomic lattice. Sufficiently strong sources have been available for this purpose only for a couple of years now. The strongest terahertz pulses are generated by devices which are powered by electron accelerators.


The HZDR is specialized in a particularly important type of source for material sciences at the Center for High-Performance Radiation Sources called ELBE. While other devices have to take a break after a small series of ultrashort laser flashes, the ELBE sources are able to maintain a constant bombardment. It is actually this continuous sequence of pulses which permits the accuracy that researchers like Dr. Gensch and their guest researchers, such as Prof. Cavalleri, need for their analyses. In order to cover the entire spectral range down to 0.1 terahertz and three millimeter wavelengths, respectively, with even more intense pulses in the future, the HZDR is establishing a new, superradiant terahertz source called TELBE under the supervision of Dr. Gensch. Superradiance means here that extremely intense light is produced in a novel avalanche-like, short process without needing any mirrors as resonators – unlike, for example, free electron lasers. This allows generating even higher terahertz fields at much more flexible repetition rates. Over the next three years, the new TELBE facility will be put into operation and commissioned with the assistance of selected pilot users. The researchers hope to utilize TELBE to unravel new phenomena in the field of materials research as well as life sciences.

Shaken, not heated: the ideal recipe for manipulating magnetism

http://www.nanowerk.com/news/newsid=24130.php Nanowerk News) Scientists have found a way to distort the atomic arrangement and change the magnetic properties of an important class of electronic materials with ultra-short pulses of terahertz (mid-infrared) laser light without heating the material up. While the achievement is currently of purely scientific interest, the researchers say this new approach control could ultimately lead to extremely fast, low-energy, non-volatile computer memory chips or data-switching devices.

Working at the SLAC National Accelerator Laboratory's Linear Coherent Light Source (LCLS), the scientists aimed intense, 130-femtosecond-long pulses of terahertz light at samples of manganite, a class of complex manganese-oxide compounds that has many desirable electronic and magnetic properties.

With each flash, the material's atoms shimmied and shifted positions, although the overall temperature of the solid barely changed. The scientists then used X-ray laser pulses from the LCLS's Soft X-ray Materials Science (SXR) instrument to measure the material's altered magnetism.

This graphic depicts an ultrashort pulse of terahertz light (yellow arrow) distorting a manganite crystal lattice. Around where the light hits, the diamond-like shaped arrangement of manganese (blue) and oxygen (red) atoms changes to become more square-like, a distortion that also alters the material's magnetism, which is indicated by the direction of red and blue arrows shown over the manganese atoms. The extremely brief duration and high intensity of the LCLS X-ray laser pulses allowed the team to take stop-action images used to measure the material's altered magnetism. (Image courtesy Jörg Harms, Max-Planck Department for Structural Dynamics, Center for Free Electron Laser Science)

Rapid light-induced switching of manganite magnetism has been known for many years, said physicist Michael Först of the Max Planck Department of Structural Dynamics (MPSD) in Hamburg, Germany, one of the leaders of the international research group. However, earlier efforts to trigger this switch with higher-energy, near-visible lasers heated the materials, which greatly limits potential applications.

The latest LCLS experiments confirm that terahertz light only distorts the lattice enough to rearrange the electronic and magnetic properties while not generating extra heat.

The research team was led by scientists from the MPSD (Först and Andrea Cavalleri) and Brookhaven National Laboratory (Ron Tobey and John Hill), and included researchers from England and SLAC. They published their results last month in Physical Review B ("Driving magnetic order in a manganite by ultrafast lattice excitation"). SLAC co-authors are Bill Schlotter and Josh Turner, SXR instrument scientists; Wei-Sheng Lee and Rob Moore of the Stanford Institute for Materials & Energy Science (SIMES), and Mariano Trigo of Photon Ultrafast Laser Science and Engineering (PULSE).

"We will come back to LCLS later this year to use the X-ray Pump Probe instrument to measure terahertz-light-driven atomic displacements directly," said Först. In recent years, his same group of MPSD scientists has also used terahertz light pulses to shake materials into a superconducting state and to change insulators into metals.

Future research aims to explore the phenomenon more deeply and start developing capabilities essential for applications, such as reverse-switching techniques, materials that can switch magnetically at room temperature or higher, and a laser source suitable for using on chips.

Source: By Mike Ross, SLAC National Accelerator Laboratory