arXiv subjects

Michal P. Kwasigroch

Publications and source records attributed to Michal P. Kwasigroch.

4 recordsLinked to original sources

arXiv2026

Easy-plane anisotropy of magnetic fluctuations in UTe2

The large directional anisotropy of superconducting UTe2 along the lattice a, b, c directions in magnetic susceptibility, upper critical fields, and Knight shift suggests spin-triplet Cooper pairing. If spin fluctuations bind the Cooper pair, one needs to determine their anisotropy in spin space to characterize the superconducting state. Here we use polarised and unpolarised inelastic neutron scattering (INS) experiments to show that antiferromagnetic (AFM) spin fluctuations at the Y point are highly isotropic within the ac-plane but have no observable magnitude along the b-axis. Our results rule out the assumed Ising anisotropy and constrain the possible pairing channels. AFM exchange is no longer pair forming in the triplet channel and the d vector along b is strongly favoured. We capture the experimental observations within a microscopic framework that encompasses the dual nature of 5f electrons. We include itinerant and localized 5f-moments in a tight-binding model fitted to quantum oscillations data. In congruence with INS, the exchange between localized moments generically consists of a strong FM intra-dimer interaction, maxima close to the Y and T points, mediated by 5p_y and 6d_{3z^2-r^2} orbitals respectively, and magnetically decoupled ab-planes. We find that anisotropy and dimerization of the moments can lead to gapped excitations that, in combination with Landau damping, can produce the observed overdamped response at the Y point. We suggest that enhancement of the response and its observed near-O(2)-symmetry could both be driven by proximity to a quantum critical point where the dimer excitation gap closes and gives place to incommensurate helical order. Analysis of the linearized gap equation for a nearest-neighbour pairing interaction favours the B1u triplet with a dominant d-vector component along b, although the possibility of spin singlet pairing cannot be ruled out.

cond-mat.str-el↗

arXiv2026

Coherence-driven origin of metamagnetism in anisotropic heavy-fermion systems

A number of heavy-fermion materials exhibit magnetic field-induced metamagnetism: on applying a field along the magnetic hard axis, the magnetization first rises gradually, then jumps abruptly once a critical field is reached. Despite decades of phenomenological modeling, the microscopic origin of the pronounced magnetic anisotropy underlying this behavior has remained unresolved. The same is true of a related, long-standing puzzle: an anomalous maximum in the hard-axis susceptibility versus temperature. Both are complicated in $5f$ compounds by the dual localized-itinerant character of the relevant electrons. Here we develop an analytic $c$--$f$ theory of magnetic anisotropy in heavy-fermion metamagnets, identifying the mixed susceptibility $χ_{\rm cf}(T,h,p)$ as a single thermodynamic observable that unifies the anisotropic response across temperature, field, and pressure. We test this theory against primary and literature data for the heavy-fermion superconductor UTe$_2$, finding excellent quantitative agreement in the temperature, field, and pressure evolution of its magnetic anisotropy -- including a Kondo-coherence origin for the anomalous hard-axis susceptibility maximum, which we show is directly connected to the metamagnetic transition itself. Our results establish a general, microscopic, coherence-driven framework for anisotropic metamagnetism, applicable across the broad class of heavy-fermion compounds that display this phenomenology.

cond-mat.str-el↗

arXiv2026

Metamagnetism in UTe2: the roles of itinerancy and localization

The metamagnetic transition in UTe$_2$ plays a key role in stabilizing two enigmatic field-induced superconducting phases. One of these phases (SC2) is truncated by the transition, lying directly below it, while the other (SC3) sits predominantly above it and appears to be stabilized because of it. While numerous pulsed field studies have examined this transition, comparatively few steady field experiments have investigated it. Here we report a suite of measurements of metamgnetism in UTe$_2$, at ambient pressure by torque magnetometry and extraction magnetometry techniques, and of the magnetoconductance under pressure. Our steady field measurements resolve a complex sub-structure within the transition, with separate features that possess different temperature evolutions, pointing to distinct contributions from itinerant and localized moments. The itinerant contribution might relate to a possible spin-density wave state. We theoretically model the evolution of Kondo and RKKY interactions and propose that the SC2 state is stabilized under pressure due to the collapse of magnetic anisotropy, leading to an enhancement of longitudinal spin fluctuations along the hard $b$ axis, which are pair-forming in the $p$-wave channel.

cond-mat.str-el↗

arXiv2026

Microscopic origin of hard-plane antiferromagnetism in the Kondo lattice Ce2Rh3Ge5

Hard plane antiferromagnetic order where ordered moments lie perpendicular to the single-ion crystal electric field easy axis is rare in Ce-based Kondo lattices and is a subject of active interest. Here we show that Ce$_2$Rh$_3$Ge$_5$ realizes a hard-plane antiferromagnetic state in which partial delocalization of the local moment gives rise to an RKKY exchange that overturns the single-ion easy-axis preference. Neutron diffraction reveals moments in the $ab$ plane, while inelastic neutron scattering and susceptibility establish a magnetic easy axis along $c$ in the paramagnetic regime, highlighting a clear inversion between single-ion and ordered-state anisotropies. In this work, we establish a unified microscopic framework to consistently account for partial $4f$-moment delocalization, enhanced in-plane RKKY exchange, and the resulting hard-plane antiferromagnetic order. Ce$_2$Rh$_3$Ge$_5$ thus provides a benchmark system in which single-ion anisotropy, Kondo screening, and RKKY exchange compete on comparable energy scales, revealing a cooperative route to hard-axis ordering in strongly hybridized Kondo lattices.

cond-mat.str-el↗