IQMT was established 1.1.2020 as a federation of KIT research groups investigating different aspects of research on quantum materials and technologies. IQMT includes the former Institute for Solid State Physics (IFP) as well as research groups with close relation to the Institute of Nanotechnology (INT), Physikalisches Institut (PHI) and Institute for Theory of Condensed Matter (TKM). This enables a broad spectrum of research on theory and experiments on quantum phenomena in solid state and molecule systems which is reflected in the IQMT divisions

Technical Support is supplied by a Joint Infrastructure with a Mechanical and an Electrical Workshop operated conjointly by IQMT and INT.


Publication Highlights / News

STM activation of the electroluminescence of individual molecules on a Au substrate

Electroluminescence from single molecules adsorbed on a conducting surface imposes conflicting demands for the molecule-electrode coupling. To conduct electrons, the molecular orbitals need to be hybridized with the electrodes. To emit light, they need to be decoupled from the electrodes to prevent fluorescence quenching. This is typically achieved by inserting a thin insulating layer of NaCl. A promising alternative is the functionalization of the light-emitting chromophore with a suitable spacer group. 2,6-core-substituted naphthalene diimide was investigated as a promissing candidate material since the molecule consists of a chromophore linked via an alkyne spacer with a triphenylmethane platform with three acetyl-protected thiol anchors which provide a good connection to a Au(111) substrate. The molecular complexes were designed to stand upright on the tripodal anchor, decoupling the chromophore from the substrate. However, due to dimensional mismatch of the molecular subunits the van der Waals interaction between the large molecule and the substrate overcomes the binding energy of the thiol groups of the feet and the chromophore becomes attached in a flat adsorption configuration to the metal substrate, This leads to a quench of the electroluminescence of the molecules. In the now reported STM experiments individual molecules could be driven from this strongly hybridized state with quenched luminescence to a light-emitting state. The emission performance compares in terms of quantum efficiency (10-5–10-4 photons per electron), stability, and reproducibility to that of single molecules deposited on thin insulating layers. Quantum chemical calculations suggest that the emitted light originates from the singly charged cationic pair of the molecules.

Phys. Rev. Lett. 130 (2023) 36201

Soft-Phonon and Charge-Density-Wave Formation in BaNi2As2

The incommensurate charge density-wave (I-CDW) in BaNi2As2 has been investigated by x-ray scattering. Diffuse scattering signals corresponding to the ordering wave vectors qI-CDW (0.28 0 0) and (0 0.28 0) (in reciprocal lattice units of the tetragonal crystal structure with xy-orientation of the Ba and NiAs layers) can be observed already at room temperature and indicate the formation of I-CDWs with very short in-plane ξk ∼ 10 Å and out-of-plane ξl ∼ 4 Å correlation lengths which increase towards lower temperature. The divergence of ξl at TI-CDW = 146 K coincides with a small orthorhombic lattice distortion. Upon further cooling, the I-CDW superstructure reflections persist down to Ttri = 135 K where BaNi2As2 undergoes a first-order phase transition into a triclinic structure where the bonding of Ni ions to the surrounding 4 As ions is no longer tetrahedrally symmetric and the arrangement of the Ni ions apparently distorts to the formation of Ni–Ni dimers arranged in zigzag chains. The I-CDW is replaced here by a commensurate modulation (C-CDW) with an ordering vector qC-CDW (1/3 0 1/3).
Energy-resolved inelastic x-ray scattering around qI-CDW allowed to observe here the dispersion of two optical phonon modes with a clear dip in the dispersion of the lower-energy phonon mode around qI-CDW. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. This scenario including the phonon instability is excellently described by ab initio density functional perturbation theory (DFPT) phonon calculations. However, in contrast to other CDW materials neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account here for the I-CDW formation. Charge-driven electronic nematicity based on the anisotropic Ni–Ni bond ordering and the Ni–Ni dimers charge fluctuations between out-of-plane and in-plane orbitals is suspected as basic unconventional mechanism involving orbital degrees of freedom.

Phys. Rev. Lett. 129 (2022) 247602

Granular aluminium nanojunction fluxonium qubit

Nanoscopic Josephson junctions created by a single-layer deposition of a 20 nm thin film of granular aluminium and lithographical definition of 20 nm wide nanobridges were demonstrated to be operable in simple qubit circuit at coherence times in the microsecond range, just like Al–AlOx–Al superconductor–insulator–superconductor (SIS) that are the present state-of-the-art Josephson junctions for qubit circuits. The single-layer deposition allows to reduce the footprint well below 100 x 100 nm2, the present technological limit of these conventional SIS junctions due to the involved multilayer and often multi-angle evaporation processes. This smaller size transfers directly into a larger magnetic field tolerance which makes them much more resilient against parasitic stray fields in more complex circuits. As a further advantage, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically larger charging energy in the range of tens of gigahertz, comparable to its Josephson energy. This may enable a new Josephson circuit functionality. Due to the unprecedented accuracy in measuring the Josephson energy, spontaneous jumps could be observed here on timescales from milliseconds to days, which offers a powerful diagnostics tool for microscopic defects in superconducting materials.

Nat. Mater. (2022)

Elastoresistivity in the incommensurate charge density wave phase of BaNi2(As1-xPx)2

In elastoresistance measurements of BaNi2(As1-xPx)2 single crystals (0 ≤ x ≤ 0.10) a large maximum of the B1g-symmetric elastoresistance coefficient m12-m11 is observed at the temperature of the tetragonal-to-orthorhombic transition. The absence of a Curie-Weiss-like temperature dependence points to a smaller coupling of electronic nematicity to the lattice as compared to the iron-based superconductors. The anisotropic strain-dependent electrical transport is attributed to uniaxial behaviour of the incommensurate charge density wave (I-CDW) phase.

npj Quantum Mater. 7 (2022) 115

A. Haghighirad is again in 2022 "Highly Cited Researcher"

Together with five other KIT scientists, Amir-Abbas Haghighirad belongs once again in 2022 to the "Highly Cited Researchers", a ranking list compiled by the Web of Science Group which comprises the names of the scientists with the highest number of citations of their publications between January 2011and December 2021. 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.

Localization of electrons in the nematic phase

Magnetotransport studies of thin flakes of FeSe performed by a collaboration of University of Oxford, University of Bath and KIT-IQMT at the High Field Magnet Laboratory in Nijmegen revealed an unconventional transport behaviour in the nematic electronic phase as a function of the sample thickness. Based on a two-band model an unusual asymmetry between the mobilities of the electrons and holes was found. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality is suggested to be driven by orbitally dependent correlation effects, enhanced interband spin fluctuations, or a Lifshitz-like transition, which affect mainly the electron bands.

PNAS 119 (2022) 2200405

Electronic nematic liquid in BaNi2As2

In Raman spectroscopy of BaNi2As2 an exceptionally large energy splitting of the doubly degenerate planar vibrations of the NiAs tetraedra was observed. In sharp contrast to the behavior in the corresponding iron-based systems, where this splitting was taken as evidence for nematic symmetry breaking, in BaNi2As2 it occurs at temperatures well above any reported structural phase transition temperature. This calls for a distinction between the lifting of a degeneracy and a dynamical spectral splitting. The splitting can be accounted for by a particularly strong coupling of electronic nematic fluctuations, likely of orbital nature, to the lattice degrees of freedom in this material and indicates that the tetragonal phase of BaNi2As2 hosts an electronic nematic phase, dynamical in nature.
The broadening and splitting of the planar phonons can be described in terms of an entangled superposition of two degenerate nematic states that are coupled to a cloud of vibrational quanta. This bears analogies with the phenomenology of spin liquids - dynamical states without long-range magnetic order but long-range entanglement.

Nat. Commun. 13 (2022) 4535

Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects

Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes were fabricated and investigated in electroluminescence spectroscopy. The devices showed electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. It was shown that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.

ACS Nano (2022)

Elastocaloric Determination of the Phase Diagram of Sr2RuO4

A piezoelectric-based device was used for a high-precision measurement of the elastocaloric response function of Sr2RuO4 which reflects under adiabatic conditions the differential change of entropy. Hence, zeroes of this response function correspond to extrema of entropy.
The datasets measured in the temperature range T = 1 K - 8 K feature several zones which are limited by lines indicating a change of sign. For T > 3.5 K there is only one line which could be attributed to a Van Hove singularity of the electronic bandstructure by comparison with theoretical predictions. Another line coincides within experimental accurcy with the transition to superconductivity as determined by specific heat measurements. Unclear is the physical mechanism leading to the apparent continuation of the Van Hove singularity line into the superconductive phase region.
All in all, Sr2RuO4 shows a phase diagram featuring a "superconducting dome" as has already been established for many cuprate, pnictide, organic and Heavy Fermion superconductors.

Nature 607 (2022) 276

Wolfgang Wernsdorfer member of Académie des sciences

Wolfgang Wernsdorfer was admitted to the french Académie des sciences as one of 16 foreign scientists elected in 2021.

Fluxons in High-Impedance Long Josephson Junctions

Fluxons, supercurrent vortices, in long Josephson junctions can be accelerated by a bias current up to the Swihart velocity, the characteristic velocity of electromagnetic waves in the junction. This effect can be used for current amplification in superconducting electronics. Reaching the end of the junction, the fluxons are reflected and transformed into anti-fluxons, supercurrent vortices with reversed rotation, which are then accelerated in the reverse direction. This mechanism can be used for an application as tunable oscillator where the oscillation frequency is given by the length of the junction and the current-adjustable average velocity of the (anti-)fluxons. However, since the impedance of conventional long Josephson junctions is much too low for a direct connection to external circuits and 50 Ohm cables, such devices could supply up to now only very small microwave output power. A KIT research group demonstrated now that thin films of a high kinetic inductance superconductor, granular aluminum, as superconducting electrodes can provide a solution of this problem by increasing the characteristic impedance by an order of magnitude due to the increase of inductance by means of the additional kinetic inductance. The amount of magnetic flux carried by each supercurrent vortex is significantly smaller than the magnetic flux quantum Φ0. The Swihart velocity is reduced by about one order of magnitude compared to conventional long Josephson junctions.

Appl. Phys. Lett. 120 (2022) 112601

Molecular Crystals for Quantum Information Devices

Rare-earth (RE) ions can show narrow optical and spin homogeneous linewidths, or, equivalently, long-lived quantum states. This enables the use of RE ions for photonic quantum technologies such as memories for light, optical–microwave transduction and computing. However, so far, few crystalline materials have shown an environment quiet enough to fully exploit these properties. Molecular systems can provide such capability but generally lack spin states or have spin states featuring only broad optical lines.
A research team of Université Paris Sciences & Lettres, IQMT and INT fabricated molecular crystals containing Eu3+ ions that exhibit optical homogeneous linewidths between 5 and 30 kHz, 3 to 4 orders of magnitude narrower than those of any previously known molecular system. They were able to demonstrate efficient optical spin initialization, coherent storage of light using an atomic frequency comb, and optical control of ion–ion interactions which may be utilized to implement quantum gates.
Such RE molecular crystals may establish a new platform for photonic quantum technologies that combines highly coherent emitters with the unmatched versatility in composition, structure and integration capability of molecular materials.

Nature 603 (2022) 241

Topological Magnon Band Structure in a Skyrmion Lattice

The motion of a spin excitation across topologically nontrivial magnetic order exhibits a deflection that is analogous to the effect of the Lorentz force on an electrically charged particle in an orbital magnetic field. Polarized inelastic neutron scattering was used to investigate the propagation of magnons (i.e., bosonic collective spin excitations) in a lattice of skyrmion tubes in MnSi. For wave vectors perpendicular to the skyrmion tubes, the magnon spectra are consistent with the formation of finely spaced emergent Landau levels that are characteristic of the fictitious magnetic field used to account for the nontrivial topological winding of the skyrmion lattice. This provides evidence of a topological magnon band structure in reciprocal space, which is borne out of the nontrivial real-space topology of a magnetic order.

Science 375 (2022) 1025

Proximity-induced Ferromagnetism in SrIrO3

A ferromagnetic state - rarely observed in 5d-based transition metal oxides due to the generally weak electron-electron correlation - is induced in SrIrO3 by proximity to ferromagnetic insulating LaCoO3. Electrical transport allows to selectively probe ferromagnetism of SrIrO3. It reveals a strong magnetocrystalline anisotropy and enables the direct observation of a strong, intrinsic and positive anomalous Hall effect.

Adv. Mater. 34 (2022)

John Bardeen Prize 2022 for Jörg Schmalian

The 2022 John Bardeen Prize was awarded to Jörg Schmalian (KIT), Mohit Randeria (The Ohio State University), and Peter Hirschfeld (University of Florida) for pioneering theoretical work that has provided significant insights on the nature of superconductivity, its realization in strongly correlated systems, and experimental probes of unconventional superconductors.
The Bardeen Prize is given every three years at the M2S Conference.
Jörg Schmalian receives the prize for his impactful, materials-based theoretical insights into unconventional superconductors across the periodic table including organic superconductors, cuprates, and pnictides, and for pioneering work on the effect of intertwined order as well as on a beyond-quasi-particle description of normal states for unconventional superconductivity.

Momentum Dependence of Electron-Phonon Coupling

It was shown that strong renormalization of the life time of lattice vibrations, i.e. phonons, can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum. Using inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations this scenario has been verified in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2. This new scenario is likely to apply to a wide range of compounds.

Nat. Commun. 13 (2022) 228

Investigation of the Redox Processes in Li-Battery Material

Lithium ion batteries ("LiB") established in the 1980s within a short period of time from basic research to everyday life. The discovery of fast ion conductors based on transition metal oxide layers seperated by Li ion layers was quickly transferred into battery technology. The high energy density of rechargable LiBs based on LiCoO2 anodes enabled since 1991 the practical use of portable electronic devices with high power demand. Due to the comparatively high cost of cobalt, mixed Li-Mn-Ni-Co oxides ("NMC") LiaNixMnyCozO2 (a ≈ 1 ; x + y + z ≈ 1) prevailed and are nowadays used in the majority of e-bikes and electric cars. Topical battery research concentrates on optimizing the NMC stochiometry with respect not only to cost but also to higher energy density and long-term stability.
A research cooperation of Münster Electrochemical Energy Technology ("MEET"; Münster university) and IQMT investigated now the materials development of Li1.15Ni0.2Mn0.55Co0.1O2 in the various battery charge states over a large number of charging cycles by determining simultaneously the charging state of the different ions (element-specific characterisation of the respective X-ray absorbtion edges; "NEXAFS") as well as their atomic structural arrangement (operando synchrotron X-ray powder diffraction; "SXPD"). The outstanding capacities of this material were shown to stem from redox processes effected not only by the formation of covalent Ni-O bonds and O-O dimers, but also from the reduction von Mn und Co, however only after the formation of the O-O dimers in the first, activating charging cycle. While O-O dimer formation levels off after a few cycles, the Mn/Co redox process is still present after 100 cycles, however with reduced intensity, presumably to to materials degradation due to Mn and Co dissolution into the electrolyte solution. Ni-rich material appears to oppose this effect, nevertheless it appears to contribute to degradation at a higher degree of delithiation by releasing oxygen.

Chem. Mater. (2021)

Signatures of a Quantum Griffiths Phase in FeSe0.89S0.11

For clean and continuous phase transitions, spatial and temporal correlation lengths are closely related by the dynamics of the system and evolve power-law divergences as the transition is approached. In the presence of quenched disorder, temporal correlations persist, but in case of a spatially varying order parameter a smeared phase transition can occur, where ordered islands form within a disordered bulk. Moreover, when order parameter fluctuations within the islands are non-negligible, this leads to a singular free energy in a whole parameter region in the vicinity of the critical point ("Griffiths phase"). At T=0 quantum phase transitions ("QPT") these effects are enhanced compared to their classical counterparts. Signatures of such a "quantum Griffiths phase" have now been detected in FeSe1-xSx with x = 0.11 where the local distribution of isoelectronic Se and S atoms promote the formation of nematic islands in the vicinity of the nematic QPT. However, additional effects such as the nematoelastic coupling apparently quench the quantum critical nematic fluctuations below a cross-over temperature 10 K. As a result, Fermi liquid behavior with finite electronic correlations is restored: Quantum oscillations reveal at lowest temperatures a Fermi surface formed in a topological Lifshitz transition.

Phys. Rev. Lett. 127 (2021) 246402

IQMT Group Neutron Scattering at Heinz Maier-Leibnitz Zentrum

May this year, IQMT officially joined the Heinz Maier-Leibnitz Zentrum for the research with neutrons as a cooperation partner for the neutron triple axis spectrometer PUMA. Thus, IQMT - formerly Institute for Solid State Physics (IFP) - can continue its long tradition in inelastic neutron scattering research and instrumentation after the end of the Orphee reactor at Saclay, France. The IQMT Working Group Neutron Scattering will engage in instrument development, in-house research, and also for external user supports on PUMA. Dr. Alsu Gazizulina is delegated as an instrument scientist and represents IQMT at MLZ.

Spin-Readout of Single-Molecule Magnet with STM

Rare-earth based single-molecule magnets are promising candidates for magnetic information storage including qubits as their large magnetic moments are carried by localized 4f electrons. This shielding from the environment in turn hampers a direct electronic access to the magnetic moment. In a collaboration of KIT, RWTH Aachen, Universidad de Panama and CESQ Strasbourg indirect readout of the Dy moment in Bis(phthalocyaninato)dysprosium (DyPc2) molecules on Au(111) has now been demonstrated using milli-Kelvin scanning tunneling microscopy. An unpaired electron on the Pc ligand leads to a Kondo resonance that is, however, energetically split by the ferromagnetic exchange interaction between the unpaired electron and the Dy magnetic momentum. Using spin-polarized scanning tunneling spectroscopy, the Dy magnetic moment could be determined measuring this exchange-splitting of the Kondo state ed as a function of the applied magnetic field.

Phys. Rev. Lett. 127 (2021) 123201

Workshop "Neutron Spectroscopy in Magnetic Fields"

In the framework of the KIT Future Fields initiative, IQMT hosted on September 15th and 16th a workshop on Neutron Spectroscopy in Magnetic Fields. Experts from various countries presented and discussed the experimental possibilities at European large scale facilities [Institut Laue Langevin (Grenoble, France), Maier-Leibnitz Zentrum (Garching, Germany), Paul-Scherrer Institut (Villigen, Switzerland), ISIS neutron and myon source (Rutherford, UK)] and ongoing trends in the design of new magnets. In particular, participants evaluated the application of horizontal magnetic fields and their relevance for current scientific questions.

Visit of the french ambassador at KIT

The french ambassador Anne-Marie Descôtes paid a visit to KIT to exchange ideas with KIT vice president Thomas Hirth and KIT scientists about the perspectives of French-German cooperation. Thomas Hirth presented KIT and its intense collaboration with partners in France, i.a. in the European University Alliance EPICUR, the Upper Rhine University Alliance Eucor and in the French-German Institute for Industry of the Future, a cooperation of KIT with Arts et Métiers. Matthieu Le Tacon unterlined the importance of cross-border research and teaching as manifested in the collaboration of KIT with CNRS and Université Grenoble Alpes.

Effect of natural radioactivity on superconducting qubits

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.

Nat. Commun. 12 (2021) 2733

Electromigration and Quantum Phase Slip in Al nanowires

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 RN 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 RN-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.

ACS Nano 15 (2021) 4108

Boosting light emission from single H2Pc molecules by charging

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.

Nano Lett. 10 (2020) 7600

Disproof of divergent nematic susceptibility in CsFe2As2

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.

Phys. Rev. Lett. 125 (2020) 187001

All-optical polarization of the ground-state nuclear spins of Eu-Ions

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 T2opt=14.5±0.7 ns, and a ground-state spin population lifetime T1spin=1.6±0.4 s, which is a requirement for all-optical spin initialization and addressing.

Nat. Commun. 12 (2021) 2152

Doping dependence of nematic fluctuations in FeSe1-xSx

In an international collaboration including KIT-IQMT the nematic fluctuations in FeSe1-xSx single crystals were investigated as a function of the Sulfur content x across the nematic quantum critical point (QCP) xc ≈ 0.17 via Raman scattering. The Raman spectra in the B1g 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.

npj Quantum Mater. 6 (2021) 37

Marginal Fermi liquid behavior of SrIrO3

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.

Nat. Commun. 11 (2020) 4270

ELASTO-Q-MAT initiative funded by DFG

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.

EMA 2020 Dominique Givord award to Wolfgang Wernsdorfer

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.

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.

Nematic quantum phase transition separates superconducting phases in FeSe0.89S0.11

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.

Nat. Phys. (2019) 41567

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 YBa2Cu3O6.67

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

Low-Energy Phonon Anomalies in Cuprates with Stripes

A precise measurement of the lattice vibrations in stripe-ordered La2-xBaxCuO4 showed 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

Thermodynamic Evidence for FFLO-State in KFe2As2

The magnetic phase diagram of KFe2As2 near the upper critical field shows 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.

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

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

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