Congratulations Dr. Xavier Boraley!

On February 19 2026 at ETHZ, Xavier Boraley has successfully defended his PhD thesis conducted at LNS over the past four years. The work was performed as part of a project led by Daniel Mazzone and funded by the Swiss National Science Foundation (grant 200653 'Quantum Critical Phenomena in Metals with Competing Interactions').

The PhD defence committee was comprised of Prof. Manfred Sigrist (ETHZ), Prof. Andreas Vaterlaus (ETHZ), Dr. Oliver Stockert (MPI) and Dr. Daniel Mazzone (PSI).

The LNS warmly congratulates Xavier on this achievement and on all the work behind, and wishes him success for his next career steps!

xboraleydefence

 

Frustrated Magnetism and Quantum Criticality in Rare-earth Metals with Competing Interactions

In the PhD thesis work entitled Frustrated Magnetism and Quantum Criticality in Rare-earth Metals with Competing interactions, Xavier Boraley addresses the scientific question of how novel quantum phases emerge in metals with competing magnetic interactions. The considered interactions either result from two antagonistic processes which mediate or suppress magnetic fluctuations in metals, or stem from magnetic frustration. Magnetic frustration arises in condensed matter materials from crystalline lattice geometries and conflicting magnetic interactions, yielding degenerate spin configurations that exhibit novel coherent quantum states. Hitherto, magnetic frustration was studied predominantly in insulators, because the itinerant electrons in metals provide an additional degree of freedom that complicates the underlying physical picture. 

Xavier Boraley tackled the convoluted problem by microscopic neutron scattering studies on the three model materials HoInCu4, CeCu5.8Ag0.2 and YbAgGe. The two former materials host a subset of the aforementioned competing interactions, and YbAgGe represents a more complex case in which all three interactions play an important role to understand the nature of the material. Notably, HoInCu4 is a frustrated metal exhibiting a low density of states at the Fermi surface. In this most extensive section of the thesis, Xavier successfully modeled the various magnetic properties of the material with a localized spin hamiltonian, and concluded that the observed many-body properties arise from quantum fluctuations accounting for the missing moment of the renormalized magnetic long-range order [1]. CeCu5.8Ag0.2 lacks magnetic frustration, but hots competing quantum fluctuations that originate from different places in reciprocal space. Using magnetic field as tuning parameter, Xavier identified a pronounced anisotropy of the fluctuations reflecting the spin-anisotropy of the long-range ordered state at larger silver concentration. This suggests that the quantum phase transition reported at zero magnetic field is driven by three-dimensional spin-density wave fluctuations [2]. The shortest experimental chapter on YbAgGe introduces the various magnetic interactions in the material, and then focuses on the nature of the quantum critical fluctuations at moderate magnetic fields [3]. The thesis concludes by raising the question of how magnetic frustration is established in metals. This scientific problem is important, because frustrated metals offer a rich landscape of quantum phases and may be relevant for future devices. Thus, research on emergent quantum phases in metals with competing interactions such as magnetic frustration is necessary to reach a better understanding of exotic quantum states.

 

[2] X. Boraley et al Phys Rev B 113, 045154 (2026)
[3] D. G. Mazzone et al arXiv:2509.02252 (2025)