41 research outputs found
Transport and Localisation in the Presence of Strong Structural and Spin Disorder
We study a tight binding model including both on site disorder and coupling
of the electrons to randomly oriented magnetic moments. The transport
properties are calculated via the Kubo-Greenwood scheme, using the exact
eigenstates of the disordered system and large system size extrapolation of the
low frequency optical conductivity. We first benchmark our method in the model
with only structural disorder and then use it to map out the transport regimes
and metal- insulator transitions in problems involving (i) scattering from
random magnetic moments, and (ii) the combined effect of structural disorder
and magnetic scattering. We completely map out the dependence of the d.c
conductivity on electron density (n) the structural disorder (\Delta) and the
magnetic coupling (J'), and locate the insulator-metal phase boundary in the
space of n-\Delta-J'. These results serve as a reference for understanding
transport in systems ranging from magnetic semiconductors to double exchange
`colossal magnetoresistance' systems. A brief version of this study appears in
our earlier paper Europhys. Lett. vol 65, 75 (2004).Comment: 14 pages revtex. Final version, to appear in EPJ
Magnetic Ordering and Superconductivity in the REIrGe (RE = Y, La-Tm, Lu) System
We find that the compounds for RE = Y, La-Dy, crystallize in the tetragonal
Ibam (UCoSi type) structure whereas the compounds for RE = Er-Lu,
crystallize in a new orthorhombic structure with a space group Pmmn. Samples of
HoIrGe were always found to be multiphase. The compounds for RE = Y
to Dy which adopt the Ibam type structure show a metallic resistivity whereas
the compounds with RE = Er, Tm and Lu show an anomalous behavior in the
resistivity with a semiconducting increase in as we go down in
temperature from 300K. Interestingly we had earlier found a positive
temperature coefficient of resistivity for the Yb sample in the same
temperature range. We will compare this behavior with similar observations in
the compounds RERuGe and REBiPt. LaIrGe and
YIrGe show bulk superconductivity below 1.8K and 2.5K respectively.
Our results confirm that CeIrGe shows a Kondo lattice behavior and
undergoes antiferromagnetic ordering below 8.5K. Most of the other compounds
containing magnetic rare-earth elements undergo a single antiferromagnetic
transition at low temperatures (T12K) while GdIrGe,
DyIrGe and NdIrGe show multiple transitions. The
T's for most of the compounds roughly scale with the de Gennes factor.
which suggests that the chief mechanism of interaction leading to the magnetic
ordering of the magnetic moments may be the RKKY interaction.Comment: 25 pages, 16 figure
Pseudogap in the Optical Spectra of UPd_2Al_3
The in-plane optical conductivity of UPd_2Al_3 was measured at temperatures
K in the spectral range from 1 cm^{-1} to 40 cm^{-1} (0.14
meV to 5 meV). As the temperature decreases below 25 K a well pronounced
pseudogap of 0.2 meV develops in the optical response. In addition we observe a
narrow conductivity peak at zero frequency which at 2 K is less than 1 cm^{-1}
wide but which contains only a fraction of the delocalized carriers. The gap in
the electronic excitations might be an inherent feature of the heavy fermioin
ground state.Comment: 4 pages, 4 figures (submitted to Phys. Rev. Lett.
Magnetic phase diagram and electronic structure of UPt2Si2 at high magnetic fields: a possible field-induced Lifshitz transition
Theoretical Physic
Fermi Surface Properties of Low Concentration CeLaB: dHvA
The de Haas-van Alphen effect is used to study angular dependent extremal
areas of the Fermi Surfaces (FS) and effective masses of CeLaB alloys for between 0 and 0.05. The FS of these alloys was previously
observed to be spin polarized at low Ce concentration ( = 0.05). This work
gives the details of the initial development of the topology and spin
polarization of the FS from that of unpolarized metallic LaB to that of
spin polarized heavy Fermion CeB .Comment: 7 pages, 9 figures, submitted to PR