Highlights

Neutron scattering experiments on the two-dimensional antiferromagnet Ca2RuO4 revealed a well-defined, dispersive "Higgs" mode that modulates the amplitude of the ordered moment. The mode quickly decays into transverse spin waves at the antiferromagnetic ordering wavevector, and through a complete mapping of the transverse modes in the reciprocal space we were able to obtain a quantitative understanding of the decay process. The results establish a novel condensed matter platform for research on the dynamics of the Higgs mode.

Higgs mode in a two-dimensional antiferromagnet

Neutron scattering experiments on the two-dimensional antiferromagnet Ca2RuO4 revealed a well-defined, dispersive "Higgs" mode that modulates the amplitude of the ordered moment. The mode quickly decays into transverse spin waves at the antiferromagnetic ordering wavevector, and through a complete mapping of the transverse modes in the reciprocal space we were able to obtain a quantitative understanding of the decay process. The results establish a novel condensed matter platform for research on the dynamics of the Higgs mode.
The interplay between charge density waves and high-temperature superconductivity is currently under intense investigation .Resonant x-ray scattering experiments have now shown that interfaces with manganates greatly enhance charge density wave formation in cuprate superconductors, over a remarkably large length scale of tens of nanometers. Long-range proximity effects induced by heterointerfaces thus offer a powerful method manipulate the interplay between different collective phenomena in metal oxides.

Long-range proximity effect of charge density waves in cuprates

The interplay between charge density waves and high-temperature superconductivity is currently under intense investigation .Resonant x-ray scattering experiments have now shown that interfaces with manganates greatly enhance charge density wave formation in cuprate superconductors, over a remarkably large length scale of tens of nanometers. Long-range proximity effects induced by heterointerfaces thus offer a powerful method manipulate the interplay between different collective phenomena in metal oxides.
After seven years as a group leader in our department, Matthieu Le Tacon was appointed Full Professor and Director of the Institute for Solid State Physics at the Karlsruhe Institute of Technology.

Full Professorship for Matthieu Le Tacon

After seven years as a group leader in our department, Matthieu Le Tacon was appointed Full Professor and Director of the Institute for Solid State Physics at the Karlsruhe Institute of Technology.
The macroscopic properties of materials with strongly correlated electrons are influenced not only by atomic-scale spin and charge correlations, but also by emergent domain structures on sub-micrometer length scales. Neutron Larmor diffraction and dilatometry have yielded new insights into the mechanisms driving the formation of  mesoscopic magnetic and structural domains in the antiferromagnetic parent compound of a high-temperature superconductor.

Mesoscopic domains in a cuprate antiferromagnet

The macroscopic properties of materials with strongly correlated electrons are influenced not only by atomic-scale spin and charge correlations, but also by emergent domain structures on sub-micrometer length scales. Neutron Larmor diffraction and dilatometry have yielded new insights into the mechanisms driving the formation of  mesoscopic magnetic and structural domains in the antiferromagnetic parent compound of a high-temperature superconductor.
The electronic structure of iridium oxides has long been known as closely analogous of the one of the cuprate high-temperature superconductors. However, most iridium oxides are insulating, and it has proven difficult to inject charge carriers by doping. Photoemission experiments on an iridium oxide surface that was doped by proximity to a metallic monolayer have now uncovered an energy gap of d-wave symmetry, one of the hallmarks of high-temperature superconductivity.

d-Wave energy gap iridates

The electronic structure of iridium oxides has long been known as closely analogous of the one of the cuprate high-temperature superconductors. However, most iridium oxides are insulating, and it has proven difficult to inject charge carriers by doping. Photoemission experiments on an iridium oxide surface that was doped by proximity to a metallic monolayer have now uncovered an energy gap of d-wave symmetry, one of the hallmarks of high-temperature superconductivity.
Bernhard Keimer has won an "Advanced Grant" from the European Research Council (ERC) for a project on magnetic excitations in compounds and heterostructures based on 4d electrons. In the framework of this project, a new beamline for resonant inelastic x-ray scattering will be built at the PETRA-III synchrotron in Hamburg.

ERC Advanced Grant

Bernhard Keimer has won an "Advanced Grant" from the European Research Council (ERC) for a project on magnetic excitations in compounds and heterostructures based on 4d electrons. In the framework of this project, a new beamline for resonant inelastic x-ray scattering will be built at the PETRA-III synchrotron in Hamburg. [more]
Resonant x-ray scattering experiments on  iridium oxides have demonstrated a novel form of exchange coupling between magnetic moments, whose strength depends on the direction of the magnetic bonds. This discovery opens up new perspectives for solid-state realizations of quantum spin liquids.

Bond-directional magnetic interactions

Resonant x-ray scattering experiments on  iridium oxides have demonstrated a novel form of exchange coupling between magnetic moments, whose strength depends on the direction of the magnetic bonds. This discovery opens up new perspectives for solid-state realizations of quantum spin liquids.
Recent experimental and theoretical developments have brought fresh excitement to the field of high-temperature superconductivity. The February 12 issue of Nature contains a review accessible to a general audience.

Review of high-temperature superconductivity

Recent experimental and theoretical developments have brought fresh excitement to the field of high-temperature superconductivity. The February 12 issue of Nature contains a review accessible to a general audience.
A new technique that allows Raman scattering from 10 nm thin metal-oxide films has shown that spin and charge order in nickelates can be tuned by epitaxial strain. The paper appeared as an Editor's Choice in Physical Review Letters.

Editor's choice: Tunable spin and charge order

A new technique that allows Raman scattering from 10 nm thin metal-oxide films has shown that spin and charge order in nickelates can be tuned by epitaxial strain. The paper appeared as an Editor's Choice in Physical Review Letters.
X-ray diffraction has revealed structural modifications in high-temperature superconductors induced by  femtosecond pulses of THz radiation, which appear to be responsible for transient superconductivity far above the equilibrium transition temperature. The results were published in Nature.

Ultrafast structural modifications in a high-temperature superconductor

X-ray diffraction has revealed structural modifications in high-temperature superconductors induced by  femtosecond pulses of THz radiation, which appear to be responsible for transient superconductivity far above the equilibrium transition temperature. The results were published in Nature.
Bernhard Keimer has been named "Highly Cited Researcher" by the Institute for Scientific Information (Thomson-Reuters) based on citation data from 2002-2012.

Highly Cited Researcher

Bernhard Keimer has been named "Highly Cited Researcher" by the Institute for Scientific Information (Thomson-Reuters) based on citation data from 2002-2012.
Exchange interactions between electrons have been experimentally measured in a spiral magnet close to a metal-insulator transition. The neutron scattering data indicate that spiral magnetic state arises from a competition between short-range ferromagnetic exchange and longer-distance antiferromagnetic exchange, confirming predictions by de Gennes in 1960.

Editor's choice: Spiral spin correlations at the metal-insulator transition

Exchange interactions between electrons have been experimentally measured in a spiral magnet close to a metal-insulator transition. The neutron scattering data indicate that spiral magnetic state arises from a competition between short-range ferromagnetic exchange and longer-distance antiferromagnetic exchange, confirming predictions by de Gennes in 1960.
The heavy-fermion metal CeB6 is known to exhibit antiferromagnetic order at low temperatures. Neutron scattering experiments have now shown that the dominant magnetic excitations are actually ferromagnetic, suggesting that the antiferromagnetism is highly unconventional.

Ferromagnetic correlations in a heavy-fermion antiferromagne

The heavy-fermion metal CeB6 is known to exhibit antiferromagnetic order at low temperatures. Neutron scattering experiments have now shown that the dominant magnetic excitations are actually ferromagnetic, suggesting that the antiferromagnetism is highly unconventional.
Experiments performed in collaboration with Andrea Cavalleri's group at the MPI in Hamburg revealed coherent transport phenomena strikingly similar to superconductivity when underdoped cuprate single crystals are illuminated by pulsed Terahertz light fields at room temperature.

Optically enhanced coherent transport at room temperature in an underdoped cuprate

Experiments performed in collaboration with Andrea Cavalleri's group at the MPI in Hamburg revealed coherent transport phenomena strikingly similar to superconductivity when underdoped cuprate single crystals are illuminated by pulsed Terahertz light fields at room temperature.
Infrared ellipsometry reveals pronounced Fano resonances indicating unusual electron-phonon interactions in a geometrically frustrated iridium-oxide compound.

Fano resonances in iridates

Infrared ellipsometry reveals pronounced Fano resonances indicating unusual electron-phonon interactions in a geometrically frustrated iridium-oxide compound.
Two new papers in Science highlight the ubiquitous interplay between charge density waves and high-temperature superconductivity, and its influence on the well known "Fermi arc" phenomenon in underdoped copper oxide superconductors.

Superconducting cuprates

Two new papers in Science highlight the ubiquitous interplay between charge density waves and high-temperature superconductivity, and its influence on the well known "Fermi arc" phenomenon in underdoped copper oxide superconductors.
Resonant x-ray scattering experiments reveal a complex magnetic spiral in nickel oxide superlattices. The spiral polarization plane can be controlled by epitaxial strain and spatial confinement of the conduction electrons.

Spiral spin density wave in nickel oxide superlattices

Resonant x-ray scattering experiments reveal a complex magnetic spiral in nickel oxide superlattices. The spiral polarization plane can be controlled by epitaxial strain and spatial confinement of the conduction electrons.
An unusual form of magnetic order is identified as the ground state of the Kitaev-Heidenberg model (which has deep relations to quantum computation), and found to explain experimental observations on iridium oxide compounds.

Zigzag magnetic order in iridates

An unusual form of magnetic order is identified as the ground state of the Kitaev-Heidenberg model (which has deep relations to quantum computation), and found to explain experimental observations on iridium oxide compounds.
Neutron spin echo experiments have led to a complete understanding of the mechanisms that limit the lifetime of antiferromagnetic magnons -- a problem that has remained open for more than 40 years.

Antiferromagnetic magnon lifetimes

Neutron spin echo experiments have led to a complete understanding of the mechanisms that limit the lifetime of antiferromagnetic magnons -- a problem that has remained open for more than 40 years.
Resonant x-ray scattering experiments on highly ordered 'Ortho-II' YBa2Cu3O6.55 and Zn-substituted YBa2Cu3O6.6 revealed a three-phase competition between spin-modulated,charge-modulated, and superconducting states in underdoped cuprate superconductors.

Three-phase competition in underdoped YBa2Cu3O6+x

Resonant x-ray scattering experiments on highly ordered 'Ortho-II' YBa2Cu3O6.55 and Zn-substituted YBa2Cu3O6.6 revealed a three-phase competition between spin-modulated,charge-modulated, and superconducting states in underdoped cuprate superconductors.
Using resonant x-ray scattering we obtained the unambiguous evidence for long-range incommensurate charge-density-wave fluctuations competing with superconductivity in underdoped (Y,Nd)Ba2Cu3O6+x.

A density-wave order that competes with superconductivity

Using resonant x-ray scattering we obtained the unambiguous evidence for long-range incommensurate charge-density-wave fluctuations competing with superconductivity in underdoped (Y,Nd)Ba2Cu3O6+x.
Raman spectroscopy reveals a long-range transfer of electron-phonon coupling between YBa2Cu3O7 and La2/3Ca1/3MnO3 layers in oxide superlattices.

Electron-phonon coupling in oxide superlattices

Raman spectroscopy reveals a long-range transfer of electron-phonon coupling between YBa2Cu3O7 and La2/3Ca1/3MnO3 layers in oxide superlattices.

Using inelastic neutron scattering, we have found a magnetic exciton mode in the non-superconducting heavy-fermion metal CeB6, resembling resonant magnetic excitations in unconventional superconductors.

Magnetic resonant mode in CeB6

Using inelastic neutron scattering, we have found a magnetic exciton mode in the non-superconducting heavy-fermion metal CeB6, resembling resonant magnetic excitations in unconventional superconductors.
A review article in Nature Materials discusses recent advances in understanding novel interface states that arise in oxide heterostructures due to the charge, spin and orbital reconstruction effects.

Emergent phenomena at oxide interfaces

A review article in Nature Materials discusses recent advances in understanding novel interface states that arise in oxide heterostructures due to the charge, spin and orbital reconstruction effects.
A resonant magnetic excitation in the Rb-245 iron-selenide superconductor has been discovered at a wave vector, which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors.

Magnetic resonant mode in the Rb2Fe4Se5 superconductor

A resonant magnetic excitation in the Rb-245 iron-selenide superconductor has been discovered at a wave vector, which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors.