Focusing on the inhomogeneity in the position of a quantum phase transition of so called strange metals provides answers to key puzzles in high-temperature superconductivity

Chirality refers to a structure that lacks mirror symmetry. It can be observed in a wide range of platforms, from subatomic particles and molecules to living organisms. However, the underlying mechanisms that give rise to chirality in condensed matter systems have been a subject of considerable interest. Here we elucidate the mechanism of chiral charge density wave formation in the transition-metal dichalcogenide 1T-TiSe2.

Photon-mediated long-range coupling of two Andreev pair qubits
In a recent paper published in Nature Physics [Cheung et al. Photon-mediated long-range coupling of two Andreev pair qubits. Nat. Phys. (2024)], researchers from the University of Basel and KIT demonstrated that by using the tools of circuit Quantum Electrodynamics (cQED), distant Andreev quantum bits (qubits) can be coupled.

We investigate the high-pressure phase of the iron-based superconductor FeSe_0.89S_0.11 using transport and tunnel diode oscillator studies using diamond anvil cells.

Using the Anomalous Hall and Nernst effect as well as a symmetry analysis of the proposed magnetic structures, a symmetry-breaking modification of the magnetic state with a rearrangement of the magnetic moments at low temperatures was found, thus questioning the previously reported models for the noncollinear magnetic structure obtained from neutron scattering.

The magnetic texture in an iron monolayer on an iridium (111) surface is spatially modulated and has a uniaxial incommensurate structure, so that a Phason mode, a low-energy excitation corresponding to the sliding of the texture along this direction, is expected, which has now been confirmed by spin simulations and observed experimentally by scanning tunneling microscopy.

A quantum oscillations study of tetragonal FeSe_0.82S_0.18 under pressure up to 22 kbar revealed that the quasi-two-dimensional Fermi surfaces expand and the effective masses remain large, whereas the critical temperature for the onset of superconductivity T_c displays a threefold enhancement.

The successful preparation of freestanding multiferroic TbMnO₃ membranes enables to probe elastic properties and magnetoelectric coupling under continuous strain.

The formation of a charge density wave (CDW) in the magnetic kagome material FeGe within the antiferromagnetic phase (AFM) can be explained by the disordered formation of dimers of Ge atoms displaced out of the Fe₃Ge layers (Ge1a → Ge1b).

An experimental investigation of two-level systems (TLS) in granular aluminum thin films under applied mechanical strain and electric fields revealed a class of strongly coupled TLS having electric dipole moments up to 30 e⁢Å, an order of magnitude larger than dipole moments commonly reported for solid state defects.

Using Na_2/3Mn_7/12Fe_1/3Ti_1/12O₂ (NMFTO) as an optimized cathode material for Na-ion batteries, an initial discharge capacity of 170 mA h g–1 at a 0.1 C rate and a capacity retention of 80% after 50 cycles was achieved.

Bulk thermodynamic measurements (high-resolution thermal expansion, heat capacity, elastoresistance) on CsV₃Sb₅ single crystals reveal a strong competition between the charge-density wave (CDW) and the superconducting states which is dramatically influenced by c-axis tuning where the CDW is characterized by a strongly enhanced A_1g-symmetric elastoresistance.

With the new measuring instrument NAPXAS (Near Ambient Pressure X-ray Absorption Spectroscopy), energy conversion and ageing processes in battery energy storage systems can be investigated spectroscopically under quasi-normal wet chemical conditions during battery operation.

A hard x-ray diffraction study on underdoped YBa₂Cu₃O_y under uniaxial stress as external tuning parameter revealed a competition between 2D and 3D charge density wave orders which simultaneously compete with superconductivity.

Higher-harmonics deviations of the current–phase relations of AlO_x tunnel Josephson junctions from the standard sinusoidal form as extracted from experimental energy spectra of various transmon circuits indicate a possible optimization route for superconducting Qubit circuits.

Inelastic x-ray scattering (IXS) studies with meV resolution on the Charge Density Wave (CDW) transition in 2H-TaSe₂ reveal an extended temperature region between T^* = 128.7 K and the ordering temperature of the incommensurate CDW phase T_CDW = 121.3 K, where the full phonon softening of a longitudinal acoustic phonon mode at q_CDW = (0.323,0,0) occurs, i.e. there exists a temperature window where lattice fluctuations, observed as the overdamped soft phonons, coexist with static but only medium-range-sized CDW domains, which form a long-range ordered state only for T ≤ T_CDW.

A novel preparation technique for the mechanical integration of carbon nanotubes (CNTs) into electronic circuits enables significantly more transparent CNT-metal interfaces and thus the realisation of superconducting qubits or flux-mediated optomechanics based on carbon nanotubes.

The EU-funded and KIT-coordinated project "TruePA" aims at the further development of a "Truly Resilient Quantum Limited Traveling Wave Parametric Amplifier" , an amplifier device for weak microwave signals which could be a key component for quantum cryptography, communication, or computing.

Enlarging the π-systems of the footing substituents of the tripodal scaffold molecule enabled the spectrally and spatially resolved observation of the hot luminescence bands of individual self-decoupled chromophores.

Just as in the three previous years, Amir-Abbas Haghighirad belongs, together with four other KIT scientists, again in 2023 to the "Highly Cited Researchers" , a ranking list which comprises the names of the scientists with the highest number of citations of their publications between January 2012 and December 2022.

Using a piezo-based uniaxial pressure cell to tune the ultraclean metal Sr₂RuO₄ while measuring the stress-strain relationship, revealed a huge lattice softening which could be attributed to a Lifshitz transition of a two-dimensional Fermi surface demonstrating that the lattice softening is driven entirely by the conduction electrons of the relevant energy band.

The 6-year international project "Quantum Technologies for Axion Dark Matter Search" ("DarkQuantum" for short) has been awarded an ERC Synergy Grant worth €12.9 million, including around €2 million for KIT, to build a system based on superconducting qubits as extremely low-noise detectors for the experimental detection of axions, previously hypothetical elementary particles that are considered promising candidates for dark matter.

By combined measurement of the acoustic velocities and the density from a rhenium sample in a diamond anvil cell using inelastic x-ray scattering and x-ray diffraction a primary pressure scale could be established which extends to the multimegabar pressures of Earth’s core and leads to the conclusion that the present picture of the physics of Earth’s core has to be revised.

Using scanning tunneling microscopy, the winding number of individual vortices in Pb in the intermediate state at mK temperature could be determined from the real space wave function of its Caroli–de Gennes–Matricon bound states.

In heterostructures with proximity-induced magnetic SrIrO₃, a giant variation of the anomalous Hall conductivity and Hall angle as a function of the applied gate voltage V_g by up to 700% were observed.

Jörg Schmalian receives the Dresden Physics Prize of the Year 2023 in recognition of his research in the field of condensed matter theory and in recognition of his contributions to promoting close cooperation between the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) and the TU Dresden.

In a collaboration of FU Berlin, Princeton University, Hebrew University of Jerusalem and KIT it is proposed utilizing nonreciprocity to enhance the robustness of continuous-variable entanglement of propagating modes against thermal fluctuations, achieving high-purity flying entangled states through the breaking of reciprocity in a two-mode squeezing interaction and leveraging pairwise Gaussian interactions, making it well-suited for parametric circuit-QED implementations, e.g. in superconducting circuitry.

Substitution of Co²⁺ into the lattice of methylammonium lead triiodide (MA(Pb:Co)I₃) was shown to impart magnetic behavior to the material while maintaining photovoltaic performance at low concentrations.

For a sub-monolayer of Fe(II) spin crossover (SCO) complex molecules deposited on Au passivated Co films on Au(111) it could be demonstrated that the SCO complexes in the high spin state interact ferromagnetically with the ferromagnetic Co film and stilll maintain their SCO ability.

The syntheses of Ni-poor (“NCM111”, LiNi_1/3Co_1/3Mn_1/3O2) and Ni-rich (“NCM811” LiNi_0.8Co_0.1Mn_0.1O₂) lithium transition-metal oxides were investigated using in situ synchrotron powder diffraction and near-edge X-ray absorption fine structure spectroscopy.

STM experiments provide evidence that it is possible to activate the electroluminescence of individual 2,6-core-substituted naphthalene diimide molecules directly deposited onto a Au(111) substrate.

Energy-resolved inelastic x-ray scattering investigation of the incommensurate charge density-wave (I-CDW) in BaNi₂As₂ revealed by direct comparison with ab initio density functional calculations a phonon instability as origin of the transition to the I-CDW state.

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 showed in a simple qubit circuit similar coherence times in the microsecond range as Al–AlO_x–Al superconductor–insulator–superconductor (SIS), the present state-of-the-art Josephson junction for qubit circuits, however with even better technical perspectives for the construction of more complex qubit circuits.

Magnetotransport studies of thin flakes of FeSe revealed an an unusual asymmetry between the mobilities of the electrons and holes in the nematic electronic phase, with hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe and a localization of the negative charge carriers by reducing the dimensionality.

In BaNi₂As₂ a large splitting of doubly degenerate phonon lines was observed well above symmetry breaking and attributed to a dynamical effect, rooted in a particularly strong coupling between these phonons and fluctuations between degenerate electronic nematic configurations.

Single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes showed electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes.

A piezoelectric-based device was used to map out the phase diagram of Sr₂RuO₄ by identifying changes of sign of the elastocaloric response function as a function of temperature with phase transitions.

The characteristic impedance of long Josephson junctions could be increased by an order of magnitude using thin films of a high kinetic inductance superconductor as electrodes, which makes these junctions suitable for a variety of applications in superconducting electronics.

Novel molecular crystals containing rare-earth ions exhibit optical linewidths that are 3 to 4 orders of magnitude narrower than those of any previously known molecular system thus enabling their use as spin-photon interfaces for photonic quantum technologies.

Inelastic neutron-scattering measurements indicate the formation of Landau levels for magnons in the skyrmion phase of MnSi.

In SrIrO₃/LaCoO₃ heterostructures ferromagnetism and a strong, intrinsic and positive anomalous Hall effect could be induced in the SrIrO₃ layer by proximity to the ferromagnetic insulating LaCo₃.

Jörg Schmalian was awarded the 2022 John Bardeen Prize.

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.

In Bis(phthalocyaninato)dysprosium (DyPc₂) single molecules on Au(111) the Dy moment has been determined by milli-Kelvin scanning tunneling microscopy.

The effect of natural radioactivity on the operation of superconducting qubit circuits has been investgated in collaboration of KIT and Istituto Nazionale di Fisica Nucleare (INFN).

Via electromigration the normalconducting resistance R_N of Al nanowires could be reduced by up to 3 orders of magnitude to select hereby insulating, metallic or superconducting behaviour which enabled the validation of Quantum Phase Slip ("QPS") behaviour of superconducting nanowires.

By neutralization of thermal expansion in a piezoelectric-based strain cell the strain dependence of the electrical resistivity of CsFe₂As₂ the electron system response could be shown to be largely symmetric in the Fe₂As₂ plane.

Long-lived spectral holes have been burnt in the absorption spectrum of a binuclear Eu(III) complex which demonstrate an efficient polarization of the ground-state nuclear spins of the rare-earth ions.

A new Transregional Collaborative Research Center (SFB-TRR) 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.

By inelastic x-ray scattering the nematic correlation length ξ has been extracted from the anomalous softening of acoustic phonon modes in FeSe and Ba(Fe_1-xCo_x)₂As₂ (x = 0.03, 0.06).

In ferromagnetic FePt nanoparticles, an unprecedented energy product of 80 MGOe at room temperature well above the previous record 59 MGOe in NdFeB has been achieved.

A precise measurement of the lattice vibrations in stripe-ordered La_2-xBa_xCuO₄ 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 YBa₂Cu₃O_6+δ.

The magnetic phase diagram of KFe₂As₂ near the upper critical field provides with a clear double transition when the field is strictly aligned in the FeAs plane and a characteristic upturn of the upper critical field line, which goes far beyond the Pauli limit at 4.8 T firm evidence of a Fulde-Ferrell-Larkin-Ovchinnikov ("FFLO").

For 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 the hierarchy of interactions based on magnetic as well as to orbital, lattice, and nematic degrees of freedom.

TiO_x nanotube layers on multisensor array chips demonstrated at operating temperatures up to 400°C a promising sensitivity and selectivity towards organic vapors in the ppm range.

Raman scattering and neutron scattering provide strong evidence for excitonic magnetism in Ca₂RuO₄.

The electron-phonon interaction in the metallic surface state of the 3D topological insulators Bi₂Se₃ and Bi₂Te₃ is carried by optical modes of polar character which is weakly screened by the metallic surface state.