The role of ‘charge stripes’ in superconducting materials

The studies could lead to a new understanding of how high-temperature superconductors operate.

High-temperature superconductors, which carry electricity with zero resistance at much higher temperatures than conventional superconducting materials, have generated a lot of excitement since their discovery more than 30 years ago because of their potential for revolutionizing technologies such as maglev trains and long-distance power lines. But scientists still don’t understand how they work.
One piece of the puzzle is the fact that charge density waves – static stripes of higher and lower electron density running through a material – have been found in one of the major families of high-temperature superconductors, the copper-based cuprates. But do these charge stripes enhance superconductivity, suppress it or play some other role?
In independent studies, two research teams report important advances in understanding how charge stripes might interact with superconductivity. Both studies were carried out with X-rays at the Department of Energy’s SLAC National Accelerator Laboratory.

>Read more on the LCLS at SLAC website

Image: This cutaway view shows stripes of higher and lower electron density – “charge stripes” – within a copper-based superconducting material. Experiments with SLAC’s X-ray laser directly observed how those stripes fluctuate when hit with a pulse of light, a step toward understanding how they interact with high-temperature superconductivity.
Credit: Greg Stewart/SLAC National Accelerator Laboratory

Superconductor exhibits “glassy” electronic phase

The study provides valuable insight into the nature of collective electron behaviors and how they relate to high-temperature superconductivity.

At extremely low temperatures, superconductors conduct electricity without resistance, a characteristic that’s already being used in cryogenically cooled power lines and quantum-computer prototypes. To apply this characteristic more widely, however, it’s necessary to raise the temperature at which materials become superconducting. Unfortunately, the exact mechanism by which this happens remains unclear.

Recently, scientists found that electrons in cuprate superconductors can self-organize into charge-density waves—periodic modulations in electron density that hinder the flow of electrons. As this effect is antagonistic to superconductivity, tremendous effort has been devoted to fully characterizing this charge-order phase and its interplay with high-temperature superconductivity.

>Read more on the Advanced Light Source at L. Berkeley Lab website

Image: At low doping levels, the charge correlations in the copper–oxide plane possess full rotational symmetry (Cinf) in reciprocal space (left), in marked contrast to all previous reports of bond-oriented charge order in cuprates. In real space (right), this corresponds to a “glassy” state with an apparent tendency to periodic ordering, but without any preference in orientation (scale bar ~5 unit cells).

Exotic properties of iridium compounds

Scientists at DESY’s X-ray source PETRA III and the London Centre for Nanotechnology, at University College London, have developed a new method for examining the astonishing properties of a special class of iridium oxides known as iridates. The team of principal author Pavel Alexeev, from the Dynamics Beamline P01 at PETRA III, is presenting the procedure in the journal Scientific Reports.

Many oxides belonging to certain groups of transition metals (chemical elements with an incomplete d electron shell) are known for their exotic magnetic and electronic properties. These can be attributed qualitatively to a range of interactions between the charge of the electrons, their magnetic moment, their localization within the crystals and their atomic orbitals. The relative strengths of the various interactions determine whether an oxide is magnetic, an insulator, an electrical conductor or even a superconductor. The so-called 4d and 5d transitions metals are particularly interesting in this respect.

The properties of many of these oxides can be specifically adjusted by applying external electric or magnetic fields, or exerting pressure on the material. This makes them interesting for numerous applications in micro- and nanoelectronics, for data storage and information processing. Such behaviour is particularly pronounced in the oxides of 5d transition metals, such as tantalum, tungsten, osmium and iridium. The oxides of iridium are especially remarkable because they lose their magnetisation when subjected to pressure, and even under normal conditions develop unexpected magnetic structures. Although some of their properties have been known for quite a while, efforts to explain this behaviour are still in their infancy. This makes it all the more important to develop methods that provide detailed insights into such materials.

A particularly suitable and extremely sensitive method of studying the electronic and magnetic properties of solids is nuclear resonant scattering (NRS) using synchrotron radiation. This method uses the nuclei of the atoms of certain isotopes as local probes for the material’s properties. In view of its numerous possible applications, specialised measuring stations have been set up for this purpose on the P01 beamline at PETRA III, which are used by many scientists from all over the world every year. Among other things, the method allows the orientation of atomic magnetic moments to be determined with great accuracy. NRS therefore complements other X-ray techniques and – in contrast to neutron techniques – makes it possible to study small samples, for example when used on samples subject to high pressure.

>Read more on the PETRA III at DESY website

Image: Samples of strontium-iridium-trioxid crystals.
Credit: University College London, James Vale/Emily Hunter

Signatures of enhanced superconducting phase coherence in cuprates

The capability to control material properties on short timescales is one of the key challenges of modern condensed matter physics. This possibility becomes even more attractive in the case of intriguing material phases, such as superconductivity. As a matter of fact, despite the evolution of non-equilibrium spectroscopies of the last two decades have increased our understanding of the physics of strongly correlated materials, after more than 30 years from its discovery, High Temperature Superconductivity is still discussed and a clear and unanimous explanation of the origin of the phenomenon is still lacking. Moreover, the understanding of the phenomena at the basis of this effects could affect several technological applications, from the need for fast digital circuits and for speeding up computer performances, to the detection of very low magnetic fields, with implication in geology (mineral exploration and earthquake prediction), medical sciences (neuron activity and magnetic resonance), oil prospecting and, of course, research.
We focused our research on cuprates, a class of materials known for displaying unconventional superconductivity at relatively temperatures, and on which various studies have shown the possibility of turning off (and, to some extent, on) superconductivity by ultrashort light pulses. In our work, we reveal that light pulses characterized by long wavelength (and a peculiar polarization) can induce, for a very short time interval (1-2 ps), a state displaying superconductivity even above the critical temperature, i.e. in conditions where superconductivity is not observed at equilibrium.

>Read more on the FERMI at Elettra Sincrotrone Trieste website

Figure: Difference between the transient reflectivity due to Cu-Cu and Cu-O polarized pump in time and temperature, induced by excitations with (a) 70 and (b) 170 meV pump photon energies. The dashed lines highlight the critical temperature Tc.

Spin-momentum locking in cuprate high-temperature superconductors

The results open a new chapter in the mystery of high-temperature superconductors, suggesting that new, unexplored interactions and mechanisms might be at play.

In the world of superconductors, “high temperature” means that the material can conduct electricity without resistance at temperatures higher than expected, but still far below room temperature. Within this special class of high-temperature superconductors (HTSCs), cuprates—consisting of superconducting CuO2 layers separated by spacer layers—are some of the best performers, generating interest in these materials for potential use in super-efficient electrical wires that can carry power without any loss of electron momentum.

A new spin on cuprate HTSCs

Two kinds of electron interactions have been known to give rise to novel properties in new materials, including superconductors. Scientists who study cuprate superconductors have focused on just one of those interactions: electron correlation—electrons interacting with each other. The other kind of electron interaction found in exotic materials is spin-orbit coupling—the way in which an electron’s magnetic moment interacts with atoms in the material.

>Read more on the Advanced Light Source website

Image: Chris Jozwiak, Alessandra Lanzara, Kenneth Gotlieb, and Chiu-Yun Lin.
Credit: Peter DaSilva/Berkeley Lab

The first observation of near-room-temperature superconductivity

For decades, room-temperature superconductivity has been one of physics’ ultimate goals, a Holy Grail-like objective that seems to keep drifting within realization yet always stubbornly out of reach. Various materials, theories, and techniques have been proposed and explored in search of this objective, but its realization has remained elusive. Yet recent experimental work on hydrogen-rich materials at high pressures is finally opening the pathway to practical superconductivity and its vast potential. Russell Hemley, a materials chemist at George Washington University in Washington, D.C., first announced evidence of superconductivity at 260 K in May, 2018, and then hints of an even higher 280 K transition in August of that year. Now Hemley, along with a team of researchers from The George Washington University and the Carnegie Institution for Science synthesized several lanthanum superhydride materials that demonstrated the first experimental evidence of superconductivity at near room temperature, and with colleagues from Argonne National Laboratory characterized them at the U.S. Department of Energy’s Advanced Photon Source (APS). Read more

Pressure tuning of light-induced superconductivity in K3C60

Unlike ordinary metals, superconductors have the unique capability of transporting electrical currents without any loss. Nowadays, their technological application is hindered by their low operating temperature, which in the best case can reach -70 degrees Celsius. Researchers of the group of Prof. A. Cavalleri at the Max Planck Institute of the Structure and Dynamics of Matter (MPSD) in Hamburg have routinely used intense laser pulses to stimulate different classes of superconducting materials. Under specific conditions, they have detected evidences of superconductivity at unprecedented high temperatures, although this state persisted very shortly, just for a small fraction of a second.
An important example is that of K3C60, an organic molecular solidformed by weakly-interacting C60 “buckyball” molecules (60 carbon atoms bond in the shape of a football),which is superconducting at equilibrium below a critical temperature of -250 degrees Celsius. In 2016, Mitrano and coworkers at the MPSD discovered that tailored laserpulses, tuned to induce vibrations of the C60 molecules,can induce a short-lived, highly conducting state with properties identical to those of a superconductor, up to a temperature of at least -170 degrees Celsius, far higher than the equilibrium critical temperature (Mitrano et al., Nature, 530, 461–464 (2016)).

In their most recent investigation, A. Cantaluppi, M. Buzzi and colleagues at MPSD in Hamburg went a decisive step further by monitoring the evolution of the light-induced state in K3C60 once external pressure was applied by a diamond anvil cell (Figure 1). At equilibrium, when pressure is applied, the C60 molecules in the potassium-doped fulleride are held closer to each other. This weakens the equilibrium superconducting state and significantly reduces the critical temperature. The steady state optical response of K3C60 at different pressures and temperatures was determined via Fourier-transform infrared spectroscopy, by exploiting the high brightness of the synchrotron radiation available at the infrared beamline SISSI at Elettra.

>Read more on the Elettra website

Image:   Light-induced superconductivity in K3C60 was investigated at high pressure in a Diamond Anvil Cell.
Credit:
Jörg Harms / MPSD