ELFT HÍRADÓ

[Janos Asboth <asboth.janos AT ttk.bme.hu>]

Meghívó

BME Elméleti Fizika Szeminárium,

jún. 6. péntek 10h15,
1111 Bp., Budafoki út 8., BME FIII. magasföldszint 1, szemináriumi szoba

Sergii Khmelevskyi (Vienna Scientific Cluster Research Center, Technical
University of Vienna):
Hidden order, multipolar super-exchange interactions and structure of the
normal state in correlated f-electron systems

The nature of order in low-temperature phases of some materials is not
directly observable in experiments. These so-called “hidden orders” (HOs)
have inspired decades of research aimed at identifying the mechanisms
underlying these exotic states of matter. In insulators, HO phases
originate from degenerate many-electron states on localized f-shells, which
may harbor high-rank multipole moments. We demonstrate that the
ground-state order and magnetic excitations of the prototypical HO system
NpO₂ can be fully described by a low-energy Hamiltonian derived using a
many-body ab initio force-theorem method. A primary non-collinear order of
time-odd rank-5 (triakontadipolar) moments has been predicted [1]. We also
show that the exotic, non-chiral magnetic order in PrO₂ results from strong
high-rank multipolar interactions within the full ⟨JM*⟩ ground-state
multiplet. The unusual magnetization process in PrO₂ is shown to arise from
dominant multipolar superexchange interactions [2].
As shown by Kotliar and Haule in 2007, the canonical and perhaps most
extensively studied metallic hidden-order material, URu₂Si₂, can develop a
hidden multipolar order (hexadecapolar) due to a localized 5f²
configuration at low temperatures. At higher temperatures, hybridization of
the localized 5f levels leads to Kondo behavior—a phenomenon described by
the "Kondo arrest" scenario. At very low temperatures, the hidden-order
phase coexists with superconductivity. Based on correlated ab initio
calculations, we reveal a close analogy between the normal-state behavior
of URu₂Si₂ and that of the newly discovered heavy-fermion superconductor
UTe₂.
The UTe₂ compound is regarded as a heavy-fermion, mixed-valence system with
highly unusual properties in both its normal and superconducting states. It
exhibits no signs of magnetic order but shows strong magnetic
susceptibility anisotropy and a highly anisotropic superconducting critical
field. In addition to heavy-fermion-like behavior in the normal state, UTe₂
displays a distinctive Schottky-type anomaly around 12 K and a
characteristic excitation gap near 35–40 K. Using dynamical mean-field
theory (DMFT) with a quasi-atomic treatment of electron correlations, we
show that the ab initio-derived crystal-field splitting of the 5f² ionic
configuration is consistent with these experimental observations. We
further analyze the symmetry of magnetic and multipolar moment fluctuations
that may drive the superconducting pairing at low temperatures [3]. We
speculate that the critical fluctuations mediating superconductivity in
both UTe₂ and URu₂Si₂ could share a common origin..

[1] L. V. Pourovskii and S. Khmelevskyi, Proc Natl Acad Sci USA (PNAS), 118
e2025317118 (2021)
[2] S. Khmelevskyi, and L. V. Pourovskii, Commun. Phys. 7, 12 (2024).
[3] S. Khmelevskyi, L. V. Pourovskii, E.A. Tereshina-Chitrova, Phys. Rev. B
107, 214501 (2023).

Minden érdeklődőt szeretettel várunk.
Asbóth János,
 szemináriumi koordinátor