A collaboration with leading participation of Karlsruher Institute of Technology (KIT) and Istituto Nazionale di Fisica Nucleare (INFN) investgated the effect of natural radioactivity on the operation of superconducting qubit circuits. Impacting high-energy particles break Cooper pairs into quasi-particles and generate hereby over the whole chip area temporaly and spatially correlated bursts of such energetic excitations which are able to destabilize not only the qubits but also the qubit error correction circuits by means of the coherence of the disturbance over large circuit areas. Under regular conditions such a quasi-particle burst was observed to occur on the average every 10 seconds. Operation in the deep-underground Laboratori Nazionali del Gran Sasso in a lead-shielded cryostat decreased the quasi-particle burst rate by a factor 30 and lowered the internal dissipation in the superconducting circuits by factors two to four: This demonstrates the critical relevance of this non-thermal quasi-particle generation at the qubit operation temperature ∼ 0.1 K where thermal quasi-particles are completely suppressed.

Aluminum is at present the favorite material for superconducting QuBit circuits. On nm-scale miniaturisation of device structures the granularity of the microstructure becomes evident: At low temperature, the connections between the superconducting nm-sized Al grains act as Josephson junction network and dominate their electrical behavior. A collaboration of IQMT, PI(KIT), MISIS University Moskau and RMIT University Melbourne developed now a method to reduce the normalconducting resistance R_N of Al nanowires by up to 3 orders of magnitude and to select hereby insulating, metallic or superconducting behavior. The internal electromigration ("IEM") is based on a successive rise of the magnitude of current pulses. This apparently eliminates successively the most resistive connections in the Josephson network and reduces the total resistance systematically as a function of the current pulse intensity in a sequence of ever smaller resistance reduction steps. The R_N-variability enabled an experimental validation of the theoretically predicted Quantum Phase Slip ("QPS") behavior of superconducting nanowires. The duality of charge and quantummechanical phase opens up a new perspective for the construction of novel QPS-Quantum Electronics in perfect analogy to Josephson contact based circuits.

A KIT-collaboration of IQMT, INT, TFP and APH studied the electroluminescence from single H2Pc molecules adsorbed on a thin NaCl layer on Au(111) induced by the current of a scanning tunnelling microscope (STM). A comparison with the theoretical modelling showed that a bright emission line discovered in the STM experiment can be attributed to a positive charging of the molecule: The accompanied structural deformation leads to an improvement of the photoemission efficiency by a factor 19 compared to the uncharged molecule by means of the emergence of a vertical component of the molecular dipole that drastically improves out-coupling of the radiation extracted from the molecule.

CsFe₂As₂ features with a huge Sommerfeld coefficient of 180 mJ/(mol K²) the largest electronic specific heat contribution of all known Fe-based superconductors, a value which compares to that of heavy-fermion compounds and demonstrates the strong electronic correlations. In a recent measurement of the elastoresistance, the strain dependence of the electrical resistivity, thermal expansion could be neutralized by means of a piezoelectric-based strain cell. The clear observation of a large in-plane symmetric response and substantially weaker response in the symmetry-breaking channels disproves previous reports of divergent nematic susceptibility. The result can be naturally interpreted in terms of a low-temperature heavy Fermi-liquid regime and a reduction of the coherence of the heavy quasiparticles towards higher temperature due the orbital-selective Mott physics which is sensitively tuned by the in-plane atomic distances.

A collaboration of CNRS-Université de Strasbourg, KIT and Université PSL (Paris Sciences & Lettres) succeeded in a further step towards the realization of a molecule-based coherent light-spin Quantum Information Processing (QIP) interface: At 1.4 K, long-lived spectral holes have been burnt in the absorption spectrum of a binuclear Eu(III) complex. This demonstrates an efficient polarization of the ground-state nuclear spins of the rare-earth ions, quantified in terms of an optical coherence lifetime T_2opt=14.5±0.7 ns, and a ground-state spin population lifetime T_1spin=1.6±0.4 s, which is a requirement for all-optical spin initialization and addressing.

In an international collaboration including KIT-IQMT the nematic fluctuations in FeSe_1-xS_x single crystals were investigated as a function of the Sulfur content x across the nematic quantum critical point (QCP) x_c ≈ 0.17 via Raman scattering. The Raman spectra in the B_1g nematic channel consist of two components but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. This is in agreement with the physical picture of Hund's metals where the electronic degrees of freedom show the dual aspect of both itinerant carriers and local moments contributing to the nematic susceptibility. Curie-Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP and may explain the absence of any enhancement of the superconducting transition temperature at the QCP.

Amir-Abbas Haghighirad is one of the eight KIT scientists among the "Highly Cited Researchers" of the year 2020, a ranking list compiled by the Web of Science Group. This list comprises the names of the scientists with the highest number of citations of their publications. The topical list is based on the evaluation of the publications in the decade from 2009 till 2019. A publication is only acknowledged as "Highly Cited" if it belongs to the top 1 % of the total citations of the respective research area and publication year.

We report a Raman scattering study in correlated Dirac semimetal candidate SrIrO₃. The dynamics of holes and electrons is well described by a marginal Fermi liquid phenomenology, with frequency dependent scattering rates close to the Planckian limit.

A new Transregional Collaborative Research Center (SFB-TRR) ELASTO-Q-MAT in close cooperation between the Karlsruhe Institute of Technology (KIT), the Goethe University Frankfurt, the Johannes Gutenberg University Mainz, the Max Planck Institute of Polymer Research in Mainz, and the Max Planck Institute for Chemical Physics of Solids in Dresden will investigate quantum materials whose properties can be dramatically changed through elastic deformations.

The Dominique Givord award (awarded every three years by the European Magnetism Association (EMA) to an excellent Scientist (or scientists), who has significantly helped to push forward magnetism research and the magnetism community in Europe) goes to Wolfgang Wernsdorfer for advancing the field of single molecule magnets, magnetization tunneling and molecular spintronics.

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(Fe_1-xCo_x)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.

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!

The nematic electronic state and its associated fluctuations are under discussion as a potential superconducting pairing mechanism. In FeSe_0.89S_0.11 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.

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.
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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.

A high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa₂Cu₃O_6.67 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.

Angle-resolved photoemission spectroscopy of the iron-based superconductor Ba_1−xNa_xFe₂As₂ 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.

We use state-of-the-art x-ray techniques to precisely measure the lattice vibrations in stripe-ordered La_2−xBa_xCuO₄ 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 YBa₂Cu₃O_6+δ, which may arise from coupling between the CDW and spin correlations. This reconciles the puzzling wave-vector difference between YBa₂Cu₃O_6+δ and La_2−xBa_xCuO₄, thus providing strong evidence that CDWs in different cuprates are likely to arise from the same underlying instability despite their different ordering wave vectors.

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 ("FFLO") state exists in this iron-based KFe₂As₂ superconductor.

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.

As an approach for cost effective but highly sensitive and selective gas sensors for reliable environmental monitoring, TiO_x 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.

The quasi-2d antiferromagnetic order in Ca₂RuO₄ has been described as a condensate of low-lying spin-orbit excitons with angular momentum J_eff=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.

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.

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.

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.

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.