Rare-earth Monopnictides Research Articles

Observation of cyclotron resonance in rare-earth monopnictides microwave region

We have directly determined the cyclotron effective masses of RX (R=La,Gd, X=As,Sb,Bi) by means of cyclotron resonance measurements at the frequency range between 56 and 190 GHz in the temperature range from 50 to 0.5 K. In addition to normal cyclotron resonance, two anomalous absorption lines are observed for the high-quality single crystal of LaSb, which shows quadratic behavior on the frequency–field diagram.

A new precise magnetization measurement method at ultra-low temperature and high magnetic fields

A new magnetization measurement system which is designed to be used at ultra-low temperatures and in high magnetic fields, has been developed. In this equipment, a small capacitor is used and its capacitance changes by the displacement of an electrode due to force acting on a magnetic material placed on the electrode. The most significant point of this equipment is that a Ni plate is placed just below the capacitor to make a gradient field at the sample position instead of usual field-gradient coils installed in a magnet. Using this system, we have studied precise magnetization measurements of the rare-earth monopnictide Ce 0.9La 0.1Sb, and the Uranium heavy Fermion superconductor UPd 2 Al 3 at temperatures down to 40 mK in high magnetic fields provided by a superconducting magnet and a hybrid magnet.

Electromechanical magnetization measurements at ultralow temperatures and high magnetic fields

A new magnetization measurement system, which is designed to be used in ultralow temperatures and high magnetic fields, has been developed. In this equipment, a small capacitor is used and its capacitance changes by the displacement of an electrode due to force acting on a magnetic material placed on the electrode. The most significant point of this equipment is that a Ni plate is placed just below the capacitor to make a gradient field at the sample position instead of usual field-gradient coils installed in a magnet. Using this system, we have studied precise magnetization measurements on rare-earth monopnictides Ce0.9La0.1Sb and a uranium heavy Fermion superconductor UPd2Al3 at temperature down to 0.5 K in high magnetic fields provided by a superconducting magnet and a hybrid magnet.

Magnetization and dHvA effect study of the rare-earth monopnictides in high magnetic fields

Magnetization measurements of TbSb have been studied in pulsed high magnetic fields up to 30 T. When an external magnetic field H is applied parallel to easy axis [1 1 1], three steps are observed in magnetization process. This suggests complicated magnetic structures in high fields due to electric quadrupole moments. We have also studied dHvA effect of TbSb in high magnetic fields up to 23 T, and observed large spin splittings. From these experimental results, the c-f exchange interaction is estimated to be about 37 meV for H‖ [1 0 0], which means the exchange interaction between carriers and 4f electrons is strong.

Anomalous Hall Effect in Ce Monopnictides

The anomalous Hall Effect in CeAs and CeP is studied critically based on the recent detailed experimental study under pressure of up to 2.6 GPa, as well as a vast amount of data on other rare-earth monopnictides and monochalcogenides. It is shown that the anomalous Hall effect is observed only in the intermediate region where the magnetic polaron liquid state becomes unstable and the transition to the valence fluctuation state begins to occur. Strong field dependence of the anomalous Hall effect is explained very well by a simple skew scattering model.

Physics in low carrier strong correlation systems

As the most typical example of low carrier heavy fermion Kondo systems, various anomalous properties in the rare earth monopnictides, RX, in particular La, Ce and Yb compounds, were critically reviewed. It was shown that even in the LaX electron-hole pair, strong correlation leading to the Wigner crystallization is important to understand various anomalies in LaSb. In CeX, the above characteristic naturally induces magnetic polaron crystallization including the AFP and FP phases well known in CeSb. In some circumstances, an electron-hole pair is trapped at each magnetic polaron causing the metal-insulator like transition seen in CeSb under high pressure.

Acoustic de Haas-van Alphen effect and electron-strain interaction in f-electron systems

Investigations of the acoustic de Haas-van Alphen effect in f-electron systems have been performed. The electron-strain interaction of a series of rare earth monopnictides and the rotation effect of LaB 6 has been presented.

Electronic band structures of rare earth monopnictides under strain and pressure

The electronic band structures of LaSb under pressure and strain are calculated. The change in the Fermi surface is studied in detail. The calculated results show qualitative agreement with the experimental results of the Shubnikov de Haas effect under high pressure and of the acoustic dHvA effect.

Influence of the Kondo effect in optical excitation spectra and the band structure in rare-earth monopnictides

Optical conductivities of CeSb and CeBi at 300 and 10 K were obtained from their reflectivities. It is found that the optical conductivities are generated by a free carrier with a frequency-dependent scattering rate.

De Haas-van Alphen effect measurements in high magnetic field using hybrid magnets

An apparatus to measure the de Haas-van Alphen effect in steady high magnetic fields has been constructed by employing Tohoku hybrid magnets. Fermi surfaces were investigated on rare earth monopnictides and ceramic superconductors.

A study of GaAs/Sc0.3Er0.7As/GaAs heterostructures grown on novel GaAs substrate orientations

Compound metallic materials, such as rare-earth (RE) monopnictides, are thermally stable and have lattice constants close to those of III-V semiconductors. These materials have been studied in the recent a few years. Epitactic ErAs, ScxEr1-xAs, ErPxSb1-x, and ErP0.6As0.4 (ref. 6) have been grown on GaAs by molecular beam epitaxy (MBE). By mixing rare-earth elements or group V elements, or both, rare-earth pnictides can be lattice-matched to almost all group IV, III-V and II-VI semiconductors. Lattice-matched growth of ScxEr1-x As/GaAs, for example, can be obtained using the composition x=0.32. It has been demonstrated that the rare-earth arsenides can be grown on (100) GaAs in a layer-by-layer mode with very good crystal quality, but the GaAs grown on the rare-earth arsenides has far inferior quality to meet the requirement of novel devices. Island growth of GaAs on RE As/GaAs has been reported.

On the common feature of the optical reflectivity in rare-earth monopnictides due to 5d electron screening

Reflectivity spectra of single crystals of LaSb, CeSb, PrSb and SmSb were measured in the photon energy range from 0.001 to 20 eV. All the spectra show absorptions due to the p-d transitions which screen the p-d bonding state and the p-d antibonding state at around 6 and 12 eV, respectively. The additional absorption was found at around 1 eV and its intensity is proportional to the number of occupied 4f electrons.

Lattice-matched Sc1−xErxAs/GaAs heterostructures: A demonstration of new systems for fabricating lattice-matched metallic compounds to semiconductors

Successful growth of lattice-matched Sc1−xErxAs layers buried in GaAs with a room-temperature resistivity of ∼50 μΩ cm demonstrates the feasibility of fabricating heterostructures of lattice-matched rare-earth monopnictides and monochalcogenides in semiconductors. Reflection high-energy electron diffraction oscillations during ScAs, ErAs, and Sc1−xErxAs growth indicate monolayer-by-monolayer growth.

Stable and epitaxial metal/III-V semiconductor heterostructures

Long before the advent of nanofabrication and quantum-effect devices, the technological limitations imposed by polycrystalline, multiphase and thermally unstable contacts to III-V semiconductors were of concern to forward-looking materials scientists. In the early 1980s, efforts to elucidate the complex behaviour of reactive metal/III-V systems were initiated. These early efforts evolved slowly and culminated in the recent achievement of stable and epitaxial metallizations to III-V semiconductors. In this review, we first describe the criteria that must be met for the fabrication of metal/III-V heterostructures. Bulk phase equilibria are useful guides for selecting metal/semiconductor combinations which will not react during growth at moderate temperatures or during subsequent processing steps. We show, however, that phase stability is not sufficient for the fabrication of ultrathin metal overlayers or buried metal heterostructures. Growth conditions must be carefully optimized and combined with the appropriate selection of metallic phases with high melting points in order to suppress the strong tendency for island formation during growth and film agglomeration during overgrowth or processing. In our discussion of metal/semiconductor hetero-structures we highlight the relationship between symmetry differences and defects (domain boundaries) with particular emphasis on semiconductor overlayers grown on high-symmetry metals. Our work and that of others has shown that stable and epitaxial metallizations to III-V semiconductors as well as more complex metal/III-V heterostructures can be achieved with two classes of metallic materials; the transition-metal gallides and aluminides with the CsCl structure (TM-III) and the rare-earth monopnictides with the NaCl structure (RE-V). We discuss and compare the growth of these III-V/TM-III/III-V and III-V/RE-V/III-V heterostructures by molecular beam epitaxy, focusing special attention to the initial stages of growth of metallic films on III-V substrates and III-V overlayers on metallic films. Going beyond the strictly materials issues, we describe the electrical properties of such heterostructures, including stable enhanced-barrier Schottky contacts and semiconductor-clad metallic quantum wells, structures which may be the basis for exciting and novel electronic, photonic and magnetic devices.

Buried Metal/III-V Semiconductor Heteroepitaxy: Approaches to Lattice Matching

ABSTRACTMetallic transition metal aluminides and gallides with the CsCI structure and semi-metallic rare earth monopnictides with the NaCi structure have been grown as buried conducting layers in III-V compound semiconductor heterostructures. The criteria for achieving (100) oriented epitaxial growth on (100)111-V semiconductor surfaces is different for each class of materials. The methods used to achieve III-V/metal/llI-V heteroepitaxial structures are discussed here with emphasis on the different approaches needed for the aluminides or gallides and the monopnictides. Work producing exact lattice matching between the buried metal and surrounding semiconductor layers makes possible the separation of lattice mismatch effects from those due to other interface parameters. Results to date indicate that defect structures in the overgrown semiconductor layers arise more because of differences in crystal symmetry, interface chemistry and bonding across the interface than lattice mismatch.

Synthesis of band and model Hamiltonian theory for hybridizing cerium systems.

The unusual magnetic behavior of the heavier Ce monopnictides may be understood on the basis of a model Hamiltonian for a system of moderately delocalized f states hybridizing with band states. The parameters entering the theory have previously been taken as phenomenological input. We present a first-principles calculation of the parameters in the model Hamiltonian based on self-consistent, warped--muffin-tin, linear muffin-tin-orbital (LMTO) band structures calculated for CeBi, CeSb, CeAs, and CeP. With the self-consistent potential, we calculate the bands and the band-f hybridization matrix element entering the Anderson lattice Hamiltonian. The band-f hybridization potential is derived from the 4${f}_{5/2}$ resonance in the potential surrounding a Ce site; the f-state energy with respect to the band Fermi energy and the f-f correlation energy U are estimated by averaging f-state eigenvalues of ${f}^{0}$, ${f}^{1}$, and ${f}^{2}$ Ce configurations. The result is used to calculate the anomalous crystal-field splitting of the Ce 4${f}_{5/2}$ manifold predicted by the model Hamiltonian for the Ce monopnictides. Due to the structure of the cubic symmetry group, band-f hybridization has a greater effect on the ${\ensuremath{\Gamma}}_{8}$ quartet than on the ${\ensuremath{\Gamma}}_{7}$ doublet of the 4${f}_{5/2}$ manifold, and the reduction of the splitting of the crystal-field levels from that expected on extrapolation from the isostructural heavier rare-earth monopnictides may be understood quantitatively on this basis. Our quantitative results are in good agreement with experimental values. We also calculate the range functions describing the anisotropic magnetic behavior of CeBi and CeSb, in fair agreement with phenomenological parameters fitted to data on those materials.

Magnetic-excitation behavior of CeBi and CeAs in the presence of a cubic crystal field or external magnetic field.

A phenomenological cubic crystal field has now been included in our calculations for the magnetic-excitation energies of CeBi. The close agreement with experimental dispersion shape, previously obtained using only the hybridization-mediated interionic coupling, is retained, while the magnitude of excitation energies is closer to experiment. The crystal-field splitting of CeAs is much closer to the point-charge value (i.e., value extrapolated from the behavior of other rare-earth monopnictides) and much larger relative to the N\'eel temperature, than that of CeBi and CeSb. Perhaps for that reason, the magnetic-excitation behavior of CeAs is essentially different from that of the heavier cerium monopnictides, and it may be dominated by an interaction different from the hybridization-mediated two-ion interaction discussed by us. Calculations have also been performed for the excitation energies for the ferromagnetic state induced in CeBi in the presence of an external magnetic field.

Crystal-field splittings of NdN and HoN

Neutron inelastic scattering experiments have been performed on polycrystalline NdN and HoN to measure the crystal-field splittings. Analysing the crystal-field parameters in terms of an effective point-charge model yields excellent agreement with other rare-earth nitrides, but markedly deviates from the results obtained for the other rare-earth monopnictides.

NMR Measurements in the Paramagnetic State of Uranium-Phosphorus Compounds. I. Line Shifts ofP31in UP and inUP1−xSx

NMR line shifts $\ensuremath{\Delta}H$ of $^{31}\mathrm{P}$ in the antiferromagnetic semimetal UP and in its solid solutions $\mathrm{U}{\mathrm{P}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ ($x\ensuremath{\simeq}0,\frac{1}{4},\frac{1}{2},\frac{3}{4}$) in the paramagnetic state are proportional to the magnetization of the solid: $K=\frac{\ensuremath{\Delta}H}{H}={K}_{0}+\ensuremath{\alpha}{\ensuremath{\chi}}_{M}$, where the parameters are nearly independent of composition $x$ and fall within a narrow range ${K}_{0}=(\ensuremath{-}35\ifmmode\pm\else\textpm\fi{}15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$; $\ensuremath{\alpha}=5.2\ifmmode\pm\else\textpm\fi{}0.5$ ${(\mathrm{e}\mathrm{m}\mathrm{u}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e})}^{\ensuremath{-}1}$. This line shift is interpreted as superhyperfine (shf) polarization transferred through U-P bonds, where the shf coupling constants are ${A}_{s}=\ensuremath{-}32.4\ifmmode\pm\else\textpm\fi{}1.0$ MHz and ${A}_{p}=+3.6\ifmmode\pm\else\textpm\fi{}0.6$ MHz. Although the U-U magnetic coupling is carried by conduction electrons (Ruderman-Kittel-Kasuya-Yosida interaction), the U-P magnetic coupling is through bonds of localized electrons. The results are compared with NMR line shifts in rare-earth monopnictides and with electron-nuclear double-resonance experiments on rare-earth ions.