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Register a new userOptical Spectroscopy of SmN: Evidence for 4f Transport
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William Holmes-Hewett
Publication date
2019
fields
Physics
and research's language is
English
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The rare-earth nitride ferromagnetic semiconductors owe their varying magnetic properties to the progressive filling of 4f shell across the series. Recent electrical transport measurements on samarium nitride, including the observation of superconductivity, have been understood in terms of a contribution from a 4f transport channel. Band structure calculations generally locate an empty majority 4f-band within the conduction band although over a wide range of possible energies. Here we report optical reflection and transmission measurements on samarium nitride between 0.01 eV to 4 eV, that demonstrate clearly that the 4f band forms the bottom of the conduction band. Results at the lowest energies show no free carrier absorption, indicating a semiconducting ground state, and supporting earlier conclusions based on transport measurements.
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Cerium (Ce)-based heavy-fermion materials have a characteristic double-peak structure (mid-IR peak) in the optical conductivity [$sigma(omega)$] spectra originating from the strong conduction ($c$)--$f$ electron hybridization. To clarify the behavior of the mid-IR peak at a low $c$-$f$ hybridization strength, we compared the $sigma(omega)$ spectra of the isostructural antiferromagnetic and heavy-fermion Ce compounds with the calculated unoccupied density of states and the spectra obtained from the impurity Anderson model. With decreasing $c$-$f$ hybridization intensity, the mid-IR peak shifts to the low-energy side owing to the renormalization of the unoccupied $4f$ state, but suddenly shifts to the high-energy side owing to the $f$-$f$ on-site Coulomb interaction at a slight localized side from the quantum critical point (QCP). This finding gives us information on the change in the electronic structure across QCP.
We performed optical spectroscopy measurement on single crystal of CeTe$_3$, a rare-earth element tri-telluride charge density wave (CDW) compound. The optical spectra are found to display very strong temperature dependence. Besides a large and pronounced CDW energy gap being present already at room temperature as observed in earlier studies, the present measurement revealed the formation of another energy gap at smaller energy scale at low temperature. The second CDW gap removes the electrons near E$_F$ which undergo stronger scattering. The study yields evidence for the presence of multiple CDW orders or strong fluctuations in the light rare-earth element tri-telluride.
We investigate ultrafast dynamics from photoinduced reflectivity of Sr2RhO4 by using femtosecond near-infrared pulses. We observe a clear temperature-dependent anomaly in its electronic dynamics which slows down below 160 K. In addition, coherent oscillations of the A1g symmetric 5.3-THz phonon exhibit a 90-degree shift in its initial phase across TS, indicating a structural change in octahedral rotation distortions. We propose that octahedral structure in Sr2RhO4 evolves at around TS, and it can influence on the non-equilibrium dynamics of photoinduced carriers as well as real-time phonon responses.
We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN. Resistivity measurements imply that both materials have a semiconducting ground state, and both show resistivity anomalies coinciding with the magnetic transition, despite the different magnetic states in DyN and SmN. X-ray absorption and emission measurements are in excellent agreement with LSDA+U calculations, although for SmN the calculations predict a zero band gap.
The magnetic behaviour of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis below T_c. The ferromagnetic state is characterised by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetisation at 2 K, measured after cooling in the maximum field, is 0.035 mu_B per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for Sm3+ in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field . The susceptibility in the paramagnetic phase shows temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the J=7/2 excited state and the 5/2 ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of 0.4 mu_B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm3+ ions.
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