Physical properties of Ce3-xTe4 below room temperature

The physical properties of polycrystalline Ce3-xTe4 were investigated by measurements of the thermoelectric properties, Hall coefficient, heat capacity, and magnetization. The fully-filled, metallic x=0 compound displays a soft ferromagnetic transition near 4K, and analysis of the corresponding heat capacity anomaly suggests a doublet ground state for Ce^{3+}. The transition is suppressed to below 2K in the insulating x=0.33 composition, revealing that magnetic order in Ce3-xTe4 is driven by an RKKY-type interaction. The thermoelectric properties trend with composition as expected from simple electron counting, and the transport properties in Ce3Te4 are observed to be similar to those in La3Te4. Trends in the low temperature thermal conductivity data reveal that the phonons are efficiently scattered by electrons, while all compositions examined have a lattice thermal conductivity near 1.2W/m/K at 200K.Comment: Submitted to Phys. Rev.

Magnetic Phase Transitions in NdCoAsO

Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase transitions below room temperature. The crystal and magnetic structures of NdCoAsO have been determined by powder neutron diffraction, and the effects of the phase transitions on physical properties are reported. Near 69 K a ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and corresponds to the antiferromagnetic ordering, with the same propagation vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In addition, transport properties and heat capacity results are presented, and show anomalies at all three phase transitions.Comment: Some minor changes made, and lower temperature neutron diffraction results are included. Accepted for publication in Physical Review

Four-well tunneling states and elastic response of clathrates

We present resonant ultrasound elastic constant measurements of the clathrate compounds Eu8Ga16Ge30 and Sr8Ga16Ge30. The elastic response of the Eu clathrate provides clear evidence for the existence of a new type of four-well tunneling states, described by two nearly degenerate four level systems (FLS). The FLS's are closely linked with the fourfold split positions of Eu known from neutron diffraction density profiles. Using a realistic potential we estimate the tunneling frequencies and show that the energy gap between the two FLS's is of the same order as the Einstein oscillator frequency. This explains why the observed harmonic oscillator type specific heat is not modified by tunneling states. In addition the quadrupolar interaction of FLS's with elastic strains explains the pronounced depression observed in elastic constant measurements. In the case of the Sr clathrate, we show that the shallow dip in the elastic constant c44 is explained using the same type of quadrupolar interaction with a soft Einstein mode instead of a FLS.Comment: 4 pages, 4 figures; accepted for publication in Physical Review Letter

On Exchange of Orbital Angular Momentum Between Twisted Photons and Atomic Electrons

We obtain an expression for the matrix element for a twisted (Laguerre-Gaussian profile) photon scattering from a hydrogen atom. We consider photons incoming with an orbital angular momentum (OAM) of $\ell \hbar$, carried by a factor of $e^{i \ell \phi}$ not present in a plane-wave or pure Gaussian profile beam. The nature of the transfer of $+2\ell$ units of OAM from the photon to the azimuthal atomic quantum number of the atom is investigated. We obtain simple formulae for these OAM flip transitions for elastic forward scattering of twisted photons when the photon wavelength $\lambda$ is large compared with the atomic target size $a$, and small compared the Rayleigh range $z_R$, which characterizes the collimation length of the twisted photon beam.Comment: 16 page