Friedrich Malte Grosche

In the metallic magnet NbFe 2 , the low temperature threshold of ferromagnetism can be investigated by varying the Fe concentration within a narrow homogeneity range. NbFe 2 is one of a number of compounds where modulated order is found to mask the ferromagnetic quantum critical point. However, here we report the rare case where the masking modulated magnetic order has been fully refined. Spherical neutron polarimetry and high-intensity single-crystal neutron diffraction reveal the first case of a longitudinal spin-density wave masking the ferromagnetic quantum critical point. The spin-density wave is characterized by a large-wavelength incommensurate modulation of its low average moment. It is formed from ferrimagnetic building blocks with antiparallel ferromagnetic sheets. The existence of ferromagnetic sheets and compensation of the magnetization only over mesoscopic length scales show local similarity between the spin-density wave and the ferromagnetic parent phase and indicate the spin-density wave's unconventional nature as emerging from underlying ferromagnetic quantum criticality.

Multiphase superconductors—materials that host two or more distinct superconductive phases—are exceptionally rare. Examples include heavy-fermion CeRh ₂ As ₂ alongside some uranium compounds such as UPt ₃ and URhGe (refs. ^1,2,3 ). In the multiphase p -wave superfluid ³ He, complex vortex dynamics can occur at the phase boundary between the A and B phases ^4,5 . Here we study the p -wave superconductor candidate UTe ₂ (refs. ^6–8 ). On applying a magnetic field to access an intermediate regime straddling two distinct superconducting phases ^9,10 , we find that direct current pulses can push the material in and out of a metastable state that has an enhanced critical current density J _c . This switching is controllable by the strength and duration of the stimuli, with the system ‘remembering’ whether it is in the high or low J _c state for extended periods. We interpret this phenomenology to be due to the quenching of a disordered out-of-equilibrium glassy vortex state under perturbation, which has stronger pinning forces and thus higher J _c . The equilibrium vortex lattice is reattained by annealing the system with a gradual current ramp, returning it to the original state. Rather than requiring proximate magnetic or semiconducting interfaces ^11–14 , this memory functionality seems to be an intrinsic property of UTe ₂ rooted in the superconducting order itself. Our findings underscore the rich complexity of multiphase quantum vortex matter.

Nowotny chimney ladder crystals combine features of ordered crystals and amorphous solids, making them attractive thermoelectric materials because of their intrinsically low thermal conductivity. We investigate the intermetallic compound Ru₂Sn₃ and show that, despite its crystalline order, its heat capacity exhibits a boson-peak-like glassy anomaly at 8-14 K. Combining experiments with first-principles calculations and molecular dynamics simulations, we trace this behavior to low-energy optical phonons emerging from the chimney ladder structure. These modes strongly couple to acoustic phonons, producing hybridization and avoided crossings that reshape the vibrational spectrum and cause the hybridized acoustic branches to contribute directly to the anomaly. Thermoelectric measurements reveal additional glass-like signatures linked to these excitations, while the electrical resistivity displays an extended linear temperature dependence and an anomalously large quadratic contribution at low temperatures. A simple theoretical model based on electron scattering by overdamped phonons qualitatively accounts for these observations.

We report a detailed investigation of the Fermi surface in the layered Dirac semimetal TaNiTe 5 . We probed the magnetization, magnetic torque, and magnetoresistance in high-quality single crystals. Pronounced Shubnikov–de Haas and de Haas–van Alphen oscillations are observed in magnetic fields above 3 T and at temperatures of up to 22 K . Multiple fundamental frequencies and light effective quasiparticle masses are obtained by fast Fourier transformation (FFT) and Lifshitz-Kosevich formula fits. The high resolution of the low-temperature FFT spectra allows us to investigate individual peaks in detail for the magnetic fields applied along all three crystallographic axes and the planes in between. Our investigation can confirm the density functional theory calculated band structure and its corresponding Fermi surface.

The phase landscape of UTe ₂ features a remarkable diversity of superconducting phases under applied pressure and magnetic field. Recent quantum oscillation studies at ambient pressure have revealed the quasi-2D Fermi surface of this material. However, the pressure–dependence of the Fermi surface remains an open question. Here we track the evolution of the UTe ₂ Fermi surface as a function of pressure up to 19.5 kbar by measuring quantum interference oscillations. We find that in sufficient magnetic field to suppress both superconductivity at low pressures and incommensurate antiferromagnetism at higher pressures, the quasi-2D Fermi surface found at ambient pressure smoothly connects to that at 19.5 kbar, with no signs of a reconstruction over this pressure interval. We observe a smooth increase in oscillatory frequency with increasing pressure, indicating that the warping of the cylindrical Fermi sheets continuously increases with pressure. By computing a tight-binding model, we show that this enhanced warping indicates increased f -orbital contribution at the Fermi level – up to and beyond the critical pressure at which superconductivity is truncated. These findings highlight the value of high-pressure quantum interference measurements as a sensitive probe of the electronic structure in heavy fermion materials.

The metallic oxide RuO 2 has emerged as a promising altermagnet candidate, owing to reports of this material hosting antiferromagnetic ordering accompanied by a spin-split electronic band structure characteristic of time-reversal symmetry breaking. However, recent studies have robustly questioned this scenario. Here we map the Fermi surface of pristine single-crystalline RuO 2 . By measuring magnetic quantum oscillations of a bulk thermodynamic property, our study resolves the electronic structure present in the bulk of RuO 2 . Several Fermi sheets are discerned, with a range of effective quasiparticle masses up to 5 times that of the bare electron mass. We compare our measurements with the predictions for altermagnetic and nonmagnetic Fermi surfaces deduced from density functional theory calculations. The quantum oscillatory frequency spectra correspond very poorly to the profile expected for the case of altermagnetism; by contrast, they correspond well to the nonmagnetic scenario. Our findings place significant constraints on the bulk magnetic properties of RuO 2 and strongly suggest that this material is an itinerant electron paramagnet.

We report a quantum oscillation study in the Dirac nodal-line semimetal candidate TaPtTe$_5$. The Fermi surface is probed via magnetic torque measurements with the magnetic field applied in the crystallographic a-b and b-c planes. The experimentally determined de Haas - van Alphen frequencies are consistent with results from band structure calculations. This study serves as an extension to the scarce quantum oscillation data on TaPtTe$_5$ currently present in the literature.

We investigate quantum oscillation measurements in the Dirac nodal-line semimetal TaNiTe$_5$ which exhibit a strongly enhanced amplitude in the magnetoresistance. We show that mechanical properties of the measurement setup in combination with de Haas - van Alphen oscillations in the magnetic torque can cause this enhancement in the measured resistance, without involvement of any topological properties in this material. To support the empirical data, a numerical model is provided, showing good agreement.