Molecular Beam Research

Contains reports on one research project

Molecular Beam Research

Contains reports on two research projects

Atomic Beam Research

Contains reports on one research project

Magnetic versus nonmagnetic doping effects on the magnetic ordering in the Haldane chain compound PbNi2V2O8

A study of an impurity driven phase-transition into a magnetically ordered state in the spin-liquid Haldane chain compound PbNi2V2O8 is presented. Both, macroscopic magnetization as well as 51V nuclear magnetic resonance (NMR) measurements reveal that the spin nature of dopants has a crucial role in determining the stability of the induced long-range magnetic order. In the case of nonmagnetic (Mg2+) doping on Ni2+ spin sites (S=1) a metamagnetic transition is observed in relatively low magnetic fields. On the other hand, the magnetic order in magnetically (Co2+) doped compounds survives at much higher magnetic fields and temperatures, which is attributed to a significant anisotropic impurity-host magnetic interaction. The NMR measurements confirm the predicted staggered nature of impurity-liberated spin degrees of freedom, which are responsible for the magnetic ordering. In addition, differences in the broadening of the NMR spectra and the increase of nuclear spin-lattice relaxation in doped samples, indicate a diverse nature of electron spin correlations in magnetically and nonmagnetically doped samples, which begin developing at rather high temperatures with respect to the antiferromagnetic phase transition.Comment: 10 pages, 7 figure

Optical investigation of the metal-insulator transition in FeSb2FeSb_2

We present a comprehensive optical study of the narrow gap $FeSb_2$ semiconductor. From the optical reflectivity, measured from the far infrared up to the ultraviolet spectral range, we extract the complete absorption spectrum, represented by the real part $\sigma_1(\omega)$ of the complex optical conductivity. With decreasing temperature below 80 K, we find a progressive depletion of $\sigma_1(\omega)$ below $E_g\sim 280$ cm$^{-1}$, the semiconducting optical gap. The suppressed (Drude) spectral weight within the gap is transferred at energies $\omega>E_g$ and also partially piles up over a continuum of excitations extending in the spectral range between zero and $E_g$. Moreover, the interaction of one phonon mode with this continuum leads to an asymmetric phonon shape. Even though several analogies between $FeSb_2$ and $FeSi$ were claimed and a Kondo-insulator scenario was also invoked for both systems, our data on $FeSb_2$ differ in several aspects from those of $FeSi$. The relevance of our findings with respect to the Kondo insulator description will be addressed.Comment: 17 pages, 5 figure

Atomic Beam Research

Contains reports on three research projects

Elastic properties of FeSi

Measurements of the sound velocities in a single crystal of FeSi were performed in the temperature range 4-300 K. Elastic constants $C_{12}$ and $C_{44}$ deviate from a quasiharmonic behavior at high temperature; whereas, $C_{12}$ increases anomalously in the entire range of temperature, indicating a change in the electron structure of this materia

Enhancement of the Josephson current by magnetic field in superconducting tunnel structures with paramagnetic spacer

The dc Josephson critical current of a (S/M)IS tunnel structure in a parallel magnetic field has been investigated (here S is a superconductor, S/M is the proximity coupled S and paramagnet M bilayer and I is an insulating barrier). We consider the case when, due to the Hund's rule, in the M metal the effective molecular interaction aligns spins of the conducting electrons antiparallel to localized spins of magnetic ions. It is predicted that for tunnel structures under consideration there are the conditions when the destructive action of the internal and the applied magnetic fields on Cooper pairs is weakened and the increase of the applied magnetic field causes the field-induced enhancement of the tunnel critical current. The experimental realization of this interesting effect of the interplay between superconductivity and magnetism is also discussed.Comment: 5 pages 3 figure

Radio-Frequency Spectroscopy

Contains reports on three research projects

NMR and dc-susceptibility studies of NaVGe2O6

We report the results of measurements of the dc magnetic susceptibility chi(T) and of the 23Na nuclear magnetic resonance (NMR) response of NaVGe2O6, a material in which the V ions form a network of interacting one-dimensional spin S=1 chains. The experiments were made at temperatures between 2.5 and 300 K. The chi(T) data suggest that the formation of the expected low-temperature Haldane phase is intercepted by an antiferromagnetic phase transition at 18 K. The transition is also reflected in the 23Na NMR spectra and the corresponding spin-lattice relaxation rate 1/T1(T). In the ordered phase, 1/T1(T) decreases by orders of magnitude with decreasing temperature, indicating the formation of a gap of the order of 12 K in the magnetic excitation spectrum.Comment: 10 pages, 15 figures; v2 with minor revisions of the tex