Universität zu Köln

Ahmida, Mahmoud A., Johrendt, Dirk, Hearne, Giovanni R., Geibel, Christoph, Hossain, Zakir and Abd-Elmeguid, Mohsen M. (2020). Interplay between valence fluctuations and lattice instabilities across the quantum phase transition in EuCu2(Ge1-xSix)(2). Phys. Rev. B, 102 (15). COLLEGE PK: AMER PHYSICAL SOC. ISSN 2469-9969

Full text not available from this repository.

Abstract

Increasing Si concentrations in the EuCu2(Ge1-xSix)(2) series tunes the divalent Eu antiferromagnetic (AF) compound EuCu2Ge2(T-N = 14 K) to a nonmagnetic intermediate valence (IV) system EuCu2Si2. There is a collapse of the magnetic state and heavy quasiparticles occur at x similar to 0.7 corresponding to a quantum critical point (QCP). We have systematically investigated the Eu-valence and magnetic states as well as the coupling to the lattice through the QCP in EuCu2(Ge1-xSix)(2). This involved the Eu-151 Mossbauer effect spectroscopy and angle-resolved x-ray diffraction measurements as a function of Si concentration (0 <= x <= 1) at variable temperatures in the range 300-4.2 K. The Eu-151 probe indicates that the divalent Eu AF state is stable up to x similar to 0.5, followed by a collapse of AF ordering for x > 0.6, which is associated with a simultaneous sharp change of the valence state towards a nonmagnetic IV state. The crossover from the AF ordered state to the nonmagnetic IV state is found at a QCP corresponding to x similar to 0.7, at which the nonmagnetic IV state is inhomogeneous and exhibits an enhanced Eu nu+ mean valence (nu similar to 2.5). We believe that the emergence of such an unusual valence state is related to the observed heavy quasiparticles at low temperatures near the QCP. Magnetic order and a nonmagnetic inhomogeneous IV state coexist in a narrow region 0.6 <= x < 0.7, which evolves to a homogeneous IV state above x - 0.8. The ThCrSi2-type tetragonal structure is maintained throughout the series, although there is a precipitous increase in the c/a ratio at 10 K when the valence fluctuations become enhanced at the critical concentration x = 0.7. X-ray diffraction temperature scans at the critical concentration indicate conspicuous changes to steep temperature dependences of decreasing (increasing) values of a (c) lattice parameters and decreasing unit-cell volume at T < 100 K, as the IV ground state become preferentially populated at low temperatures. Thus there is a clear manifestation of strong coupling between the lattice and the valence fluctuation process. A corresponding detailed phase diagram is constructed and compared with that obtained from recent external pressure studies on the system.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Ahmida, Mahmoud A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Johrendt, Dirk UNSPECIFIED UNSPECIFIED UNSPECIFIED Hearne, Giovanni R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Geibel, Christoph UNSPECIFIED UNSPECIFIED UNSPECIFIED Hossain, Zakir UNSPECIFIED UNSPECIFIED UNSPECIFIED Abd-Elmeguid, Mohsen M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-315373
DOI:	10.1103/PhysRevB.102.155110
Journal or Publication Title:	Phys. Rev. B
Volume:	102
Number:	15
Date:	2020
Publisher:	AMER PHYSICAL SOC
Place of Publication:	COLLEGE PK
ISSN:	2469-9969
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language HYPERFINE INTERACTIONS; EU; EUROPIUM; DIFFRACTION; STATE Multiple languages Materials Science, Multidisciplinary; Physics, Applied; Physics, Condensed Matter Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/31537