arXiv subjects

J. Laverock

Publications and source records attributed to J. Laverock.

28 records · Page 2Linked to original sources

arXiv2009

Bulk Spin Polarization of Co$_{(1-x)}$Fe$_x$S$_2$

We report on a new method to determine the degree of bulk spin polarization in single crystal Co$_{(1-x)}$Fe$_x$S$_2$ by modeling magnetic Compton scattering with {\it ab initio} calculations. Spin-dependent Compton profiles were measured for CoS$_2$ and Co$_{0.9}$Fe$_{0.1}$S$_2$. The {\it ab initio} calculations were then refined by rigidly shifting the bands to provide the best fit between the calculated and experimental directional profiles for each sample. The bulk spin polarizations, $P$, corresponding to the spin-polarized density of states at the Fermi level, were then extracted from the {\it refined} calculations. The values were found to be $P=-72 \pm 6 %$ and $P=18 \pm 7%$ for CoS$_2$ and Co$_{0.9}$Fe$_{0.1}$S$_2$ respectively. Furthermore, determinations of $P$ weighted by the Fermi velocity ($v_F$ or $v_F^2$) were obtained, permitting a rigorous comparison with other experimental data and highlighting the experimental dependence of $P$ on $v_F$.

cond-mat.str-el↗

arXiv2009

The electronic structure of {\em R}NiC$_2$ intermetallic compounds

First-principles calculations of the electronic structure of members of the $R$NiC$_2$ series are presented, and their Fermi surfaces investigated for nesting propensities which might be linked to the charge-density waves exhibited by certain members of the series ($R$ = Sm, Gd and Nd). Calculations of the generalized susceptibility, $χ_{0}({\bf q},ω)$, show strong peaks at the same ${\bf q}$-vector in both the real and imaginary parts for these compounds. Moreover, this peak occurs at a wavevector which is very close to that experimentally observed in SmNiC$_2$. In contrast, for LaNiC$_2$ (which is a superconductor below 2.7K) as well as for ferromagnetic SmNiC$_2$, there is no such sharp peak. This could explain the absence of a charge-density wave transition in the former, and the destruction of the charge-density wave that has been observed to accompany the onset of ferromagnetic order in the latter.

cond-mat.str-el↗

arXiv2008

Vacancy defect positron lifetimes in strontium titanate

The results of positron annihilation lifetime spectroscopy measurements on undoped, electron irradiated, and Nb doped SrTiO3 single crystals are reported. Perfect lattice and vacancy defect positron lifetimes were calculated using two different first-principles schemes. The Sr vacancy defect related positron lifetime was obtained from measurements on Nb doped, electron irradiated, and vacuum annealed samples. Undoped crystals showed a defect lifetime component dominated by trapping to Ti vacancy related defects.

cond-mat.mtrl-sci↗

arXiv2008

De Haas-van Alphen oscillations in the charge-density wave compound lanthanum tritelluride (LaTe_3)

De Haas-van Alphen oscillations were measured in lanthanum tritelluride (LaTe_3) to probe the partially gapped Fermi surface resulting from charge density wave (CDW) formation. Three distinct frequencies were observed, one of which can be correlated with a FS sheet that is unaltered by CDW formation. The other two frequencies arise from FS sheets that have been reconstructed in the CDW state.

cond-mat.str-el↗

arXiv2008

ARPES Study of the Evolution of Band Structure and Charge Density Wave Properties in RTe3 for R = Y, La, Ce, Sm, Gd, Tb and Dy

We present a detailed ARPES investigation of the RTe3 family, which sets this system as an ideal "textbook" example for the formation of a nesting driven Charge Density Wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDW instabilities, from the opening of large gaps on the best nested parts of Fermi Surface (FS) (up to 0.4eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in k-space. An additional advantage of RTe3 is that the band structure can be very accurately described by a simple 2D tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure, by comparing our ARPES measurements with Linear Muffin-Tin Orbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and, for the first time, of their strong evolution as a function of R. Using simple models, we are able to reproduce perfectly the evolution of gaps in k-space, the evolution of the CDW wave vector with R and the shape of the residual metallic pockets. Finally, we give an estimation of the CDW interaction parameters and find that the change in the electronic density of states n(Ef), due to lattice expansion when different R ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.

cond-mat.str-el↗

arXiv2007

Charge density wave formation in $R_{2}$Te$_{5}$ ($R$=Nd, Sm and Gd)

The rare earth ($R$) tellurides $R_2$Te$_5$ have a crystal structure intermediate between that of $R$Te$_2$ and $R$Te$_3$, consisting of alternating single and double Te planes sandwiched between $R$Te block layers. We have successfully grown single crystals of Nd$_2$Te$_5$, Sm$_2$Te$_5$ and Gd$_2$Te$_5$ from a self flux, and describe here the first evidence for charge density wave formation in these materials. The superlattice patterns for all three compounds are relatively complex, consisting at room temperature of at least two independent wavevectors. Consideration of the electronic structure indicates that to a large extent these wave vectors are separately associated with sheets of the Fermi surface which are principally derived from the single and double Te layers.

cond-mat.str-el↗

arXiv2007

Effect of Chemical Pressure on the Charge Density Wave Transition in Rare-earth Tritellurides RTe_3

The charge density wave transition is investigated in the bi-layer family of rare earth tritelluride RTe_3 compounds (R = Sm, Gd, Tb, Dy, Ho, Er, Tm) via high resolution x-ray diffraction and electrical resistivity. The transition temperature increases monotonically with increasing lattice parameter from 244(3) K for TmTe_3 to 416(3) K for SmTe_3. The heaviest members of the series, R = Dy, Ho, Er, Tm, are observed to have a second transition at a lower temperature, which marks the onset of an additional CDW with wavevector almost equal in magnitude to the first, but oriented in the perpendicular direction.

cond-mat.str-el↗

arXiv2007

Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

The surprise discovery of superconductivity below 5K in sodium cobalt oxides when hydrated with water has caught the attention of experimentalists and theorists alike. Most explanations for its occurence have focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, but direct attempts to look for them have instead cast serious doubts over their existence. Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodium cobalt oxides using the technique of x-ray Compton scattering.

cond-mat.supr-con↗

arXiv2006

Observation of a strongly nested Fermi surface in the shape-memory alloy Ni[0.62]Al[0.38]

The Fermi surface topology of the shape-memory alloy Ni[0.62]Al[0.38] has been determined using Compton scattering. A large area of this Fermi surface can be made to nest with other areas by translation through a vector of ~ 0.18 [1,1,0] (2pi/a), which correponds to the wavevector associated with martensitic precursor phenomena such as phonon softening and diffuse streaking in electron diffraction patterns. This observation is compelling evidence that these phenomena are driven by the enhanced electron-lattice coupling due to the Fermi surface nesting.

cond-mat.mtrl-sci↗

arXiv2005

Fermi surface nesting and charge-density wave formation in rare-earth tritellurides

The Fermi surface of rare-earth tri-tellurides ({\it R}Te$_{3}$) is investigated in terms of the nesting driven charge-density wave formation using positron annihilation and first-principles LMTO calculations. Fermi surface nesting is revealed as a strong candidate for driving charge-density wave formation in these compounds. The nesting vector obtained from positron annihilation experiments on GdTe$_{3}$ is determined to be ${\mathbf q} = (0.28\pm0.02,0,0) {\mathbf a}^{*}$, (${\mathbf a}^{*}=2π/{\mathbf a}$), in excellent agreement with previous experimental and theoretical studies.

cond-mat.str-el↗