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Head of the Institute
Prof. Dr. Matthieu Le Tacon
Address
Karlsruher Institut für Technologie
Institut für
QuantenMaterialien
und Technologien
Hermann-von-Helmholtz-Platz 1
D-76344 Eggenstein-Leopoldshafen
Postal Address
Karlsruhe Institute of Technology
Institute for Quantum
Materials and Technologies
P.O. Box 3640
D-76021 Karlsruhe
Phone ++49-721608-26751
Fax        ++49-721608-24624
E-mail
sekretariatVkt8∂iqmt kit edu

KIT Campus North
Building 425

Institute for Quantum Materials and Technologies (IQMT)


Welcome at IQMT

Modern electronics are based on semiconducting materials in which the behavior of electrons can be accurately described using the laws of quantum mechanics derived at the beginning of the 20th century. Many other intriguing aspects of quantum physics, such as quantum coherence, superposition of states or entanglement, open groundbreaking perspectives for a 'second quantum revolution' in the fields of information, communication or computing technologies. These phenomena are delicate to observe in ordinary materials and remain largely unexploited.

Combining experiment and theory, the researchers at the Institute for Quantum Materials and Technologies (IQMT) at KIT seek to understand quantum phenomena in condensed matter and molecular systems, and to develop novel devices based on these effects that will serve as building blocks for future quantum technologies.

Abteilungen

 

Publication Highlights / News

Nematic correlation length in Fe-based superconductors

We used inelastic x-ray scattering to extract the nematic correlation length ξ from the anomalous softening of acoustic phonon modes in FeSe and Ba(Fe1-xCox)2As2 (x = 0.03, 0.06). The derived temperature dependence ξ ∝ (T-T0)-0.5 combined with the previously reported Curie-Weiss behavior of the nematic susceptibility indicates a mean-field character of the nematic transition. This points to a sizable nemato-elastic coupling which is probably detrimental to superconductivity.

Phys. Rev. Lett. 124 (2020) 157001

Hector Science Price 2020 for Wolfgang Wernsdorfer

Wolfgang Wernsdorfer receives this year the Hector Foundation Science Price with 150 000 Euro prize money for his achievements in Quantum Computing related basic research on nanomagnets and electronic circuits. Moreover, he will become member of the Hector Fellow Academy.
Congratulations!

Quenched nematic criticality and two superconducting domes in Fe-superconductor

The nematic electronic state and its associated fluctuations are under discussion as a potential superconducting pairing mechanism. In FeSe0.89S0.11 combination of chemical and hydrostatic pressure leads to a nematic quantum phase transition which is isolated from any other competing magnetic phases that usually disguise its importance. Quantum oscillations in high magnetic fields show the evolution of the Fermi surface and electronic correlations as a function of applied pressure and reveal a Lifshitz transition that separates two distinct superconducting regions.

Nature Physics (2019) 41567

Otto-Haxel Award 2018 for Sebastian Kuntz

On September 4, 2019, our colleague Sebastian Kuntz received one of the three ‘Otto-Haxel-Auszeichnungen für Physik 2018’ , awarded jointly by KIT, Heidelberg University and Göttingen University.
Congratulations!

Record Magnetic Performance in FePt Nanomaterials

In ferromagnetic FePt nanoparticles, an unprecedented energy product of 80 MGOe at room temperature has been achieved (previous record 59 MGOe in NdFeB). With a 3 nm Au coverage, the magnetic polarization of these nanomagnets can be enhanced by 25% exceeding 1.8 T. This exceptional magnetization and anisotropy is confirmed by using multiple imaging and spectroscopic methods.

Small (2019) 1902353
Uniaxial Pressure Control of Competing Orders in a High-Temperature Superconductor

A high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa2Cu3O6.67 under uniaxial stress along the a-axis shows that a three-dimensional long-range-ordered Charge Density Wave (CDW) state can be induced in the absence of the large magnetic fields that were used so far to observe it. A pronounced softening of an optical phonon mode is associated with the CDW transition.

Science 362 (2018) 1040
Spectral Evidence for Unknown Emergent Order in Ba1−xNaxFe2As2

Angle-resolved photoemission spectroscopy of the iron-based superconductor Ba1−xNaxFe2As2 reveals the electronic response to the nematic phase as well as a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system.

Phys. Rev. Lett. 121 (2018) 127001
Low-Energy Phonon Anomalies in Cuprates with Stripes

We use state-of-the-art x-ray techniques to precisely measure the lattice vibrations in stripe-ordered La2−xBaxCuO4 and find how they couple to the CDW. We discovered that the fluctuating CDW correlations that exist at high temperature have a different periodicity than the static ordered CDW but the same periodicity as YBa2Cu3O6+δ, which may arise from coupling between the CDW and spin correlations. This reconciles the puzzling wave-vector difference between YBa2Cu3O6+δ and La2−xBaxCuO4, thus providing strong evidence that CDWs in different cuprates are likely to arise from the same underlying instability despite their different ordering wave vectors.

Phys. Rev. X 8 (2018) 11008
Effects of Pressure on the CDW of Cuprates

We studied the pressure dependence of the CDW in the underdoped high temperature superconductor YBa2Cu3O6.6 using high resolution inelastic x-ray scattering. We reveal an extreme sensitivity of the phonon anomalies related to the CDW order to hydrostatic pressures and conclude that a pressure as modest as 1 GPa is sufficient to completely suppress the CDW. This highlight the different effect of chemical doping and pressure-induced structural modifications in the underdoped cuprates.

Phys. Rev. B (R) 97 (2018) 20503
Thermodynamic Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov State in KFe2As2

We investigated the magnetic phase diagram of KFe2As2 near the upper critical field by magnetic torque and specific heat experiments using a high-resolution piezorotary positioner to precisely control the parallel alignment of the magnetic field with respect to the FeAs layers. We observed a clear double transition when the field is strictly aligned in the plane and a characteristic upturn of the upper critical field line, which goes far beyond the Pauli limit at 4.8 T. This provides firm evidence that a Fulde-Ferrell-Larkin-Ovchinnikov state exists in this iron-based KFe2As2 superconductor.

Phys. Rev. Lett. 119 (2017) 217002
Dominant magnetic interactions in BaFe2As2

High-temperature superconductivity in Fe-based materials is closely connected to magnetic as well as to orbital, lattice, and nematic degrees of freedom. For the prototypical parent compound BaFe2As2 the magnetic susceptibility and resistivity anisotropies measured on application of a large symmetry breaking strain strongly suggest that magnetism plays the dominant role in this hierarchy of interactions.

Nat. Commun. 8 (2017) 504
Role of apical oxygen in cuprate superconductors

For parent compounds of cuprate superconductors differing by the number of apical atoms, the magnetic spectra have been measured by resonant inelastic X-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. The absence of apical oxygens increases the in-plane hopping range and, in CaCuO2 apparantly leads to a genuine three-dimensional exchange-bond network.

Nat. Phys. 13 (2017) 1201
TiOx nanotubes for gas-analytical multisensors

As an approach for cost effective but highly sensitive and selective gas sensors for reliable environmental monitoring, TiOx nanotube layers have been fabricated on multisensor array chips. At operating temperatures up to 400 °C a promising sensitivity and selectivity towards organic vapors in the ppm range could be demonstrated.

Sci. Rep. 7 (2017) 9732
Raman scattering from Higgs mode in Ca2RuO4

The quasi-2d antiferromagnetic order in Ca2RuO4 has been described as a condensate of low-lying spin-orbit excitons with angular momentum Jeff=1. Raman scattering for different polarization geometries allows to disentangle the amplitude (Higgs) mode of this condensate from magnon contributions. Together with recent neutron scattering data, this provides strong evidence for excitonic magnetism in Ca2RuO4.

Phys. Rev. Lett. 119 (2017) 67201
Entropy landscape of quantum criticality

Based on general thermodynamic principles, the spatial-dimensional profile of the entropy S near a quantum critical point has been determined, and its steepest descent in the corresponding multidimensional stress space. The approach is demonstrated for CeCu6−xAux near its onset of antiferromagnetic order.

Nat. Phys. 13 (2017) 742
Electron-phonon coupling in Topological Insulators

The electron-phonon interaction in the metallic surface state of 3D topological insulators is revised within a first principles framework. For Bi2Se3 and Bi2Te3 the overall weak coupling constant is less than 0.15. The prevailing coupling is carried by optical modes of polar character, which is weakly screened by the metallic surface state and can be reduced by doping into bulk bands.

Sci. Rep. 7 (2017) 1059
Superconducting pairing analysis via inelastic STM

The inclusion of inelastic tunnel events is shown to be crucial for the interpretation of tunneling spectra of unconventional superconductors and to allow to directly probe electronic and bosonic excitations via scanning tunneling microscopy (STM). For the iron based superconductor LiFeAs this leads to strong evidence for a nonconventional pairing mechanism, likely via magnetic excitations.

Phys. Rev. Lett. 118 (2017) 167001
Entropy evolution in the magnetic phases of CePdAl

In CePdAl, long-range antiferromagnetic order coexists with geometric frustration of one-third of the Ce moments. At low temperatures, the Kondo effect tends to screen the frustrated moments. Suppressing the Kondo screening by a magnetic field, the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration which can be quantified in terms of the observed enhanced entropy.

Phys. Rev. Lett. 118 (2017) 107204
Switching of a large anomalous Hall effect

Antiferromagnetic Mn5Si3 single crystals were shown to exhibit an extraordinarily large anomalous Hall effect which is strongly anisotropic and features multiple transitions with sign changes at different magnetic fields due to field-induced rearrangements of the magnetic structure despite only tiny variations of the total magnetization.

Sci. Rep. 7 (2017) 42982