Sergio Rodriguez

High frequency dielectric response of dipolar liquids

The dynamical dielectric response of a condensed system of molecules with permanent electric dipole moments is studied. We start from the dynamical extension of the Onsager theory developed by Nee and Zwanzig, but generalized so as to be applicable at high frequencies. Specifically, we retain inertial terms and introduce relaxation effects in a way valid in a high frequency limit. The dielectric function obtained incorporates a diversity of both collective and single‐particle behavior, consistently, over the entire range of frequencies below those of intramolecular excitation. It is noteworthy that not only is a collective mode, or dipolar plasmon, exhibited, but that its frequency is also a resonance of the single‐dipole motion. The properties of the dielectric function are analyzed and illustrated in detail, and specific applications are made to water, to hydrogen chloride, and to chloroform.

Energy‐level spectra of Co2+ and Fe2+ in diluted magnetic semiconductors

In this paper we study the low‐lying energy levels of Co2+ and Fe2+ in diluted magnetic semiconductors, such as Cd1−xCoxTe and Cd1−xCoxSe, and their iron‐based counterparts. In the first of these compounds, the magnetic ion sits on a site of symmetry Td, while in the second the symmetry is trigonal (C3v). We develop a formulation that permits a continuous variation from Td to C3v symmetry. Comparison with experimental data in Cd1−xCoxSe shows that the C3v distortion amounts to about 10% of the crystal potential at the Co2+ site. Our study of the energy spectra of Fe2+ in Td and C3v crystal potentials reveals that, even in the cubic field, the levels exhibit an anisotropy which manifests itself in an anisotropy of the magnetization M in the regime in which M is not a linear function of the magnetic field B. The study includes all the levels in the lowest terms of the (3d)7 and (3d)6 configurations of Co2+ and Fe2+, thus considerably extending work by previous authors. The calculations are carried out to se...

Spin splitting of the conduction band of InSb along [110]

Abstract The conduction band of InSb exhibits a spin splitting equal to δ0k3. This splitting is responsible for the dipole active spin resonance recently observed. This resonance can interfere with its quadrupole active counterpart. From the magnitude of this interference accurate values of the magnitude of δ0 have been recently determined. We compare here these values with calculations using the LMTO method and with the results of k · p perturbation calculations. The sign of δ0 is also unambiguously determined and shown to agree with theoretical predictions.

NMR relaxation and rattling phonons in type-I Ba8Ga16Sn30 clathrate

Atomic motion of guest atoms inside semiconducting clathrate cages is considered as an important source for the glasslike thermal behavior.69Ga and 71Ga Nuclear Magnetic Resonance (NMR) studies on type-I Ba8Ga16Sn30 show a clear low temperature relaxation peak attributed to the influence of Ba rattling dynamics on the framework-atom resonance, with a quadrupolar relaxation mechanism as the leading contribution. The data are analyzed using a two-phonon Raman process, according to a recent theory involving localized anharmonic oscillators. Excellent agreement is obtained using this model, with the parameters corresponding to a uniform array of localized oscillators with very large anharmonicity.

Transport and thermodynamic properties under anharmonic motion in type-I Ba8Ga16Sn30 clathrate

Anharmonic guest atom oscillation has direct connection to the thermal transport and thermoelectric behavior of type-I Ba 8 Ga 16 Sn 30 clathrates. This behavior can be observed through several physical properties, with for example the heat capacity providing a measure of the overall excitation level structure. Localized anharmonic excitations also influence the low-temperature resistivity, as we show in this paper. By combining heat capacity, transport measurements, and our previous NMR relaxation results, we address the distribution of local oscillators in this material, as well as the shape of the confining potential and the excitation energies for Ba(2) ions in the cages. We also compare to the soft-potential model and other models used for similar systems. The results show good agreement between the previously deduced anharmonic rattler potential and experimental data.

NMR experiments and electronic structure calculations in Type-I BaAlGe clathrates

We describe 27Al NMR experiments on Ba8AlxGe46-x type-I clathrates coupled with ab initio computational studies. For x=16, calculated spectra determined by the ab initio results gave good agreement with the measurements, with best-fitting configurations also corresponding to the computed lowest-energy atomic arrangements. Analysis of the NMR results showed that a distribution of Knight shifts dominates the central portion of the line. Computational results demonstrate that this stems from the large variation of carrier density on different sites. Al-deficient samples with x=12 and 13 exhibited a split central NMR peak, signaling two main local environments for Al ions, which we connected to the presence of vacancies. Modeling of the wide-line spectrum for x=12 indicates a configuration with more Al on the 24k site than for x=16. The results indicate the importance of nonbonding hybrids adjacent to the vacancies in the electronic structure near EF. We also address the static distortions from Pm-3n symmetry in these structures.

Magnetic properties of iron-based diluted magnetic semiconductors

Abstract We present a theoretical investigation of the paramagnetism of individual Fe2+ ions in diluted magnetic semiconductors having the zinc- blende structure. This theory leads to analytic expressions for the magnetic moments. We show that the magnetization exhibits an anisotropy at high magnetic fields when it ceases to be linear in the magnetic field B. At liquid helium temperatures, this anisotropy is about 15% for B=150 kG in Cd1−xFexTe. This effect arises primarily from the anisotropy of the Van Vleck magnetic moment of the ground state of the ion. An interesting feature is that at very large fields the effective number of Bohr magnetons per ion is 4.0 independently of the orientation of B with respect to the crystal axes, as expected when the spin-orbit splitting of the 5Γ3 multiplet can be neglected compared to the Zeeman interaction. The anisotropy decreases with increasing temperature.

Energy level spectra of Co2+ and Fe2+ in diluted magnetic semiconductors

Abstract We present a study of all the levels in the lowest terms of the (3d)7 and (3d)6 configurations of Co2+ and Fe2+ in diluted magnetic semiconductors of the zinc-blende or wurtzite structure. The site symmetry of the magnetic ions is Td in the former and C3v in the latter. We develop a formulation that permits a continuous variation from Td to C3v symmetry. Comparison with experimental data in Cd1-xCoxSe shows that the C3v distortion amounts to about 10% of the crystal potential at the Co2+ site. Our study of the spectrum of Fe2+ reveals that, even in a cubic environment, the magnetization M is anisotropic in the region where it is non-linear in the magnetic field. The calculations are carried out to second order in the spin-orbit and Zeeman interactions. The g-factors of all the levels are obtained including their anisotropy for the Co2+ Г8 states.

Residual surface resistance of superconductors at microwave frequencies

A microscopic theory of the residual surface resistance is presented and compared with experimental results in superconducting lead and niobium. It is shown that for frequencies above 1 GHz, direct electromagnetic generation of acoustic shear waves can account for the measured residual surface resistance.

Effect of acoustic generation on the residual surface impedance of superconductors

A study is given of the effect of ultrasonic generation on the residual surface impedance of superconductors at microwave frequencies. We present an exact solution of the problem for London superconductors and give also general expressions for the residual surface resistance at zero temperature. Universal curves of the residual resistance as a function of the frequency and of the ratio of the correlation length to the electron mean free path are given using the Pippard and BCS kernels. Our investigation yields a lower limit to the surface impedance within the limits of the use of the free‐electon model. A comparison is given with experimental data available in the literature for niobium, lead, and indium as well as a critical analysis of previous theoretical work. For indium and lead our results are one or two orders of magnitude below selected experimental results. For niobium, the lowest experimental results agree fairly well with those estimated in the present work.