Signature of CEF-phonon coupling in Kondo lattice system CeCuGa3

Signature of CEF-phonon coupling in Kondo lattice system CeCuGa3

V. K. ANAND1, A. FRAILE1, 3, D. T. ADROJA1, 4, Shivani SHARMA1, Rajesh TRIPATHI5, C. RITTER6, C. de la FUENTE7, P. K. BISWAS1, A. del MORAL7, A. M. STRYDOM4

1ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
2Department of Physics, University of Petroleum and Energy Studies, Dehradun, Uttarakhand, 248007, India
3Czech Technical University in Prague, Faculty of Electrical Engineering, Karlovo námĕstí 13 12135, Prague, Czech Republic
4Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, P.O. Box 524,
Auckland Park 2006, South Africa
5Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India
6Institut Laue-Langevin, Boite Postale 156, 38042 Grenoble Cedex, France
7Departamento de Fisica de Materia Condensada, Facultad de Ciencias, Universidad de Zaragoza, E-50009 Zaragoza, Spain

Recent studies have shown evidence for the presence of strong crystal electric field (CEF)-phonon coupling in CeCuAl3 [1] and CeAuAl3 [2]. Ce3+ (J = 5/2) being a Kramers ion, under the action of CEF, the (2J + 1 = 6)-fold degenerate ground multiplet would split in three doublets, and hence only two excitations are expected. However, the inelastic neutron scattering (INS) data reveal three excitations which is quite interesting and clearly signify CEF-phonon (magneto-elastic) coupling in CeCuAl3 [1] and CeAuAl3 [2]. This motivated us to investigate the related compound CeCuGa3 which exhibits Kondo lattice heavy fermion behavior, and depending on Cu/Ga stoichiometry as well as crystal structure, both paramagnetic and magnetically ordered ground states have been suggested [3-7]. We have investigated the magnetic ground state of CeCuGa3 using muon spin relaxation (μSR), neutron powder diffraction (NPD) and INS measurements on a sample having a BaNiSn3-type noncetrosymmetric tetragonal structure (space group I4mm). Our magnetic susceptibility χ(T) and heat capacity Cp(T) data find evidence for a long range magnetic ordering below 2.5 K. While a negative Weiss temperature deduced from the Curie-Weiss fit of χ(T) indicates an antiferromagnetic interaction, the Cp(T) data measured in magnetic field suggest a ferromagnetic interaction. Both μSR and NPD data further confirm the occurrence of long range magnetic ordering in CeCuGa3. NPD collected at 1.7 K shows the presence of magnetic Bragg peaks which could be indexed by an incommensurate magnetic propagation vector k = (0.148, 0.148, 0). The magnetic structure refinement indicates a longitudinal spin density wave with ordered moments pointing along (1 1 0) direction, and an ordered moment with maximum amplitude of m = 0.89 μB/Ce. We also notice that the ordered moments are aligned ferromagnetically along the c axis. This could be the reason for a ferromagnetic interaction dominated behavior of in-field Cp(T). The inelastic neutron scattering presents two magnetic excitations near 4.5 meV and 6.9 meV. We analyze the INS data by a model based on CEF and deduce the CEF level scheme as well as the wave functions. The magnetic heat capacity data suggest an overall CEF splitting of 20.7 meV, however the excitation near 20.7 meV is very broad and weak in our INS data, but could provide an evidence of CEF level near 20.7 meV in agreement with the magnetic entropy. Our analysis based on CEF-phonon model indicates that the two excitations at 4.5 meV and 6.9 meV may have its origin in CEF-phonon coupling (i.e. splitting of one CEF peak into two peaks) similar to the case of CeCuAl3 [1] and CeAuAl3 [2].

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