Yuji Muro

Muon-spin-relaxation and inelastic neutron scattering investigations of the caged-type Kondo semimetals: CeT2Al10 (T = Fe, Ru and Os)

Recently, Ce-based caged-type compounds with the general formula CeT2Al10 (T?=?Fe, Ru and Os) have generated considerable interest due to the Kondo semiconducting paramagnetic ground state (down to 40?mK) observed in CeFe2Al10 and anomalously high magnetic ordering temperature with spin gap formation at low temperatures in Kondo semimetals CeRu2Al10 and CeOs2Al10. The formation of long-range magnetic ordering out of the Kondo semiconducting/semimetallic state itself is extraordinary and these are the first examples of this enigmatic coexistence of electronic ground states. These compounds also exhibit strong anisotropy in magnetic and transport properties, which has been explained on the basis of single-ion crystal electric field anisotropy in the presence of strongly anisotropic hybridization between localized 4f-electron and conduction electrons. Furthermore, they also exhibit a remarkable modification of magnetic and transport properties with doping on Ce, or T or Al sites. In this article, we briefly discuss the bulk properties of these compounds, giving a detailed discussion on our muon-spin-relaxation (?SR) investigations and inelastic neutron scattering (INS) results. We present the ?SR and the INS results of Ce(Ru1?xFex)2Al10 and CeOs2Al10 as well as the ?SR results of NdFe2Al10, NdOs2Al10 and YFe2Al10 for comparison. The zero-field ?SR spectra clearly reveal coherent two-frequency oscillations at low temperatures in CeT2Al10 (T?=?Ru and Os) and Ce(Ru1?xFex)2Al10 (x?=?0.3?0.5), which confirms the long-range magnetic ordering with a reduced moment of the Ce. On the other hand, the ?SR spectra of Ce(Ru1?xFex)2Al10 (x?=?0.8 and 1) down to 1.2 and 0.04?K, respectively, exhibit a temperature independent Kubo?Toyabe (KT) term confirming a paramagnetic ground state. INS measurements on CeT2Al10 (T?=?Ru and Os) exhibit sharp inelastic excitations at 8 and 11?meV at 5?K due to an opening of a gap in the spin excitation spectrum. A spin gap of 8?12?meV at 7?K, with a strong Q-dependent intensity, is observed in the magnetic ordered state of Ce(Ru1?xFex)2Al10 with x?=?0.3 and 0.5 which remarkably extends into the paramagnetic state of x?=?0.8 and 1. The observation of a spin gap in the paramagnetic samples (x?=?0.8 and 1) is an interesting finding in this study and it challenges our understanding of the origin of the semiconducting energy gap in CeT2Al10 (T?=?Ru and Os) in terms of a hybridization gap opening only a small part of the Fermi surface, gapped spin waves or a spin-dimer gap. Furthermore, the ?SR study of NdFe2Al10 below TN exhibits a clear sign of two frequency oscillations, which are absent in NdOs2Al10. Moreover, the ?SR study of YFe2Al10, which has been proposed as a compound exhibiting ferromagnetic critical fluctuations did not reveal any clear sign of critical magnetic fluctuations down to 60?mK, within the ISIS ?SR time window, which is unexpected for a T???0 quantum phase transition (QPT).We review examples of muon-spin relaxation measurements on molecule-based magnetic coordination polymers, classified by their magnetic dimensionality. These include the one-dimensional s=1/2 spin chain Cu(pyz)(NO3)2 and the two-dimensional s=1/2 layered material [Cu(HF2)(pyz)2]BF4. We also describe some of the more exotic ground states that may become accessible in the future given the ability to tune the interaction strengths of our materials through crystal engineering.

Competing 4 f-electron dynamics in Ce(Ru1-xFex)(2)Al-10 (0

We have carried out muon spin relaxation (muSR), neutron diffraction and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1-xFex)2Al10 (x=0, 0.3, 0.5, 0.8 and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin gap formation as evident from the bulk measurements in the end members. Our zero-field muSR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range magnetic ordering of the Ce-moment with TN=27, 26 and 21 K respectively. On the other hand the muSR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively exhibit a temperature independent Kubo-Toyabe term confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K the spin gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10-12 meV at 7 K, with a strong Q-dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=1. The observation of a spin gap in the paramagnetic samples (x=0.8 and 1) is an interesting finding in this study and it challenges our understanding of the origin of the semiconducting gap in CeT2Al10 (T=Ru and Os) compounds in terms of hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.

Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c–f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c–f hybridization and the antiferromagnetic ordering, respectively. Although the c–f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20–30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10–11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap ...We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c–f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c–f hybridization and the antiferromagnetic ordering, respectively. Although the c–f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20–30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10–11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap ...

Quantitative study of the f occupation in CeMIn5 and other cerium compounds with hard X-rays

Abstract We present bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) data of the Ce3 d core levels and lifetime-reduced L -edge X-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the Ce M In 5 family with Mxa0=xa0Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the Ce M In 5 HAXPES data, and reliable weights w n of the different f n contributions in the ground state are determined. The Ce M In 5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength V eff from CePd 3 to CeRh 3 B 2 to CeRu 2 Si 2 is observed, while it is smallest for the three Ce M In 5 compounds. The f -occupation, however, increases in the same sequence and is close to one for the Ce M In 5 family. The PFY-XAS data confirm an identical f -occupation in the three Ce M In 5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results.

Inelastic Neutron Scattering Investigations of an Anisotropic Hybridization Gap in the Kondo Insulators: CeT2Al10 (T=Fe, Ru and Os)

The recent discovery of topological Kondo insulating behaviour in strongly correlated electron systems has generated considerable interest in Kondo insulators both experimentally and theoretically. The Kondo semiconductors CeT2Al10 (T=Fe, Ru and Os) possessing a c-f hybridization gap have received considerable attention recently because of the unexpected high magnetic ordering temperature of CeRu2Al10 (TN=27 K) and CeOs2Al10 (TN=28.5 K) and the Kondo insulating behaviour observed in the valence fluctuating compound CeFe2Al10 with a paramagnetic ground state down to 50 mK. We are investigating this family of compounds, both in polycrystalline and single crystal form, using inelastic neutron scattering to understand the role of anisotropic c-f hybridization on the spin gap formation as well as on their magnetic properties. We have observed a clear sign of a spin gap in all three compounds from our polycrystalline study as well as the existence of a spin gap above the magnetic ordering temperature in T=Ru and Os. Our inelastic neutron scattering studies on single crystals of CeRu2Al10 and CeOs2Al10 revealed dispersive gapped spin wave excitations below TN. Analysis of the spin wave spectrum reveals the presence of strong anisotropic exchange, along the c-axis (or z-axis) stronger than in the ab-plane. These anisotropic exchange interactions force the magnetic moment to align along the c-axis, competing with the single ion crystal field anisotropy, which prefers moments along the a-axis. In the paramagnetic state (below 50 K) of the Kondo insulator CeFe2Al10, we have also observed dispersive gapped magnetic excitations which transform into quasi-elastic scattering on heating to 100 K. We will discuss the origin of the anisotropic hybridization gap in CeFe2Al10 based on theoretical models of heavy-fermion semiconductors.

Effect of Si Substitution on the Antiferromagnetic Ordering in the Kondo Semiconductor CeRu2Al10

We have studied the effect of 3p electron doping on the unusual antiferromagnetic (AFM) order in the Kondo semiconductor CeRu2Al10 with TN = 27 K by measuring the magnetic susceptibility χ, specific heat C, and electrical resistivity ρ for polycrystalline samples of CeRu2Al10−ySiy. The large decrease in the absolute value of paramagnetic Curie temperature |θP| with increasing y indicates the suppression of c–f hybridization. The thermal activation behavior in ρ(T) above TN disappears for y ≥ 0.3 and TN decreases to 12 K for y = 0.38. These systematic changes in |θP|, ρ(T), and TN coincide with those reported in the 4d-electron doped system Ce(Ru1−xRhx)2Al10 with respect to the number of doped electrons per formula unit. This coincidence indicates that the Al 3p- and Ru 4d-electrons in CeRu2Al10 play the equivalent role in both the formation of hybridization gap and the unusual AFM ordering.

Uniaxial pressure effects on the unusual antiferromagnetic transition in the Kondo semiconductor CeOs2Al10

We have studied the relation between the unusual antiferromagnetic (AFM) ordering and anisotropic c-f hybridization in CeOs2Al10 by measuring the magnetic susceptibility χ under uniaxial pressures P up to 0.45 GPa. For P // B // a and P // B // c, the AFM ordering temperature TN = 28.5 K hardly changes with increasing P, while the value of χ decreases and the maximal temperature of χ(T) shifts to higher temperatures. These findings suggest that the intensity of c-f hybridization is enhanced along the pressurized a and c axes. On the other hand, for P // b, TN increases up to 30 K at 0.15 GPa, while the value of χ remains unchanged. This observation for P // b suggests that the net intensity of c-f hybridization decreases, while the c-f hybridization along the b-axis hardly changes. We discuss these results in terms of the atomic displacements of Os, Al(3) and Al(4) under uniaxial pressures.

Dilution effects on the antiferromagnetic Kondo semiconductor CeOs2Al10

We have studied the effects of dilution of Ce sublattice on the unusual antiferromagnetic (AFM) order in the Kondo semiconductor CeOs2Al10 at 28.5 K by the magnetic, transport and specific-heat measurements of single crystals of Ce1-zLazOs2Al10. The effective magnetic moment and paramagnetic Curie temperature hardly change with z up to 0.5, indicating that the 4f state remains unchanged at high temperatures. The suppression of the Neel temperature TN is much weaker than that in 5d hole doped system, Ce(Os1-yRey)2Al10. Therefore, the AFM interaction is robust against the violation of the coherent Ce sublattice. The activation energy in the resistivity decreases in parallel with TN, confirming the argument that the presence of the c-f hybridization gap is a requisite for the unusual AFM order in this system.