arXiv subjects

A. M. Strydom

Publications and source records attributed to A. M. Strydom.

49 records · Page 3Linked to original sources

arXiv2014

Physical properties of noncentrosymmetric superconductor LaIrSi3: A μSR study

The results of heat capacity C_p(T, H) and electrical resistivity ρ(T,H) measurements down to 0.35 K as well as muon spin relaxation and rotation (\muSR) measurements on a noncentrosymmetric superconductor LaIrSi3 are presented. Powder neutron diffraction confirmed the reported noncentrosymmetric body-centered tetragonal BaNiSn3-type structure (space group I4\,mm) of LaIrSi3. The bulk superconductivity is observed below T_c = 0.72(1) K. The intrinsic ΔC_e/γ_n T_c = 1.09(3) is significantly smaller than the BCS value of 1.43, and this reduction is accounted by the α-model of BCS superconductivity. The analysis of the superconducting state C_e(T) data by the single-band α-model indicates a moderately anisotropic order parameter with the s-wave gap Δ(0)/k_B T_c = 1.54(2) which is lower than the BCS value of 1.764. Our estimates of various normal and superconducting state parameters indicate a weakly coupled electron-phonon driven type-I s-wave superconductivity in LaIrSi3. The \muSR results also confirm the conventional type-I superconductivity in LaIrSi3 with a preserved time reversal symmetry and hence a singlet pairing superconducting ground state.

cond-mat.supr-con↗

arXiv2014

Investigations of the singlet ground state system: PrIrSi3

We report our comprehensive study of physical properties of a ternary intermetallic compound PrIrSi3 investigated by dc magnetic susceptibility chi(T), isothermal magnetization M(H), thermo-remnant magnetization M(t), ac magnetic susceptibility chi_ac(T), specific heat C_p(T), electrical resistivity rho(T), muon spin relaxation (muSR) and inelastic neutron scattering (INS) measurements. A magnetic phase transition is marked by a sharp anomaly at T_tr = 12.2 K in chi(T) measured at low applied fields which is also reflected in the C_p(T) data through a weak anomaly at 12 K. An irreversibility between the zero field cooled and field cooled chi(T) data below 12.2 K and a very large relaxation time of M(t) indicates the presence of ferromagnetic correlation. The magnetic part of specific heat shows a broad Schottky-type anomaly near 40~K due to the crystal electric field (CEF) effect. An extremely small value of magnetic entropy below 12 K suggests a CEF-split singlet ground state which is confirmed from our analysis of INS data. The INS spectra show two prominent inelastic excitations at 8.5 meV and 18.5 meV that could be well accounted by a CEF model. The CEF splitting energy between the ground state singlet and the first excited doublet is found to be 92 K. Our muSR data reveal a possible magnetic ordering below 30 K which is much higher than that found from the specific heat and magnetic susceptibility measurements. This could be due to the presence of short range correlations well above the long range magnetic ordering or due to the electronic changes induced by muons. The induced moment magnetism in the singlet ground state system PrIrSi3 with such a large splitting energy of 92 K is quite surprising.

cond-mat.str-el↗

arXiv2014

Contrasting electron and hole doping effects on the spin gap of the caged type Kondo semimetal CeOs$_2$Al$_{10}$: A muon spin relaxation and inelastic neutron scattering investigation

The effects of electron (Ir) and hole (Re) doping on the hybridization gap and antiferromagnetic order have been studied by magnetization, muon spin relaxation ($μ^+$SR), and inelastic neutron scattering on the polycrystalline samples of Ce(Os$_{1-x}$Ir$_x$)$_2$Al$_{10}$ ($x$ = 0.08 and 0.15) and CeOs$_{1.94}$Re$_{0.06}$Al$_{10}$. $μ^+$SR spectra clearly reveals magnetic ordering below 20 and 10 K for $x$ = 0.08 and 0.15 samples respectively with a very weak signature of oscillations of the muon initial asymmetry at very short time scale. Our important findings are that small amount of electron doping (i) completely suppress the inelastic magnetic excitations near 11 meV down to 2K, which were observed in the undoped compound, and the response transforms into a broad quasielastic response and (ii) the internal field at the corresponding muon site is remarkably enhanced by about ten times compared with the parent compound. On the other hand with small amount of hole (3\% Re) doping the intensity of the inelastic magnetic excitations near 11 meV is reduced significantly. The main origin of the observed doping effect is an extra 5$d$ electrons being carried by Ir and a hole carried by Re compared with that the Os atom. The obtained results demonstrate a great sensitivity of the carrier doping and provides additional ways to study their anomalous magnetic properties.

cond-mat.str-el↗

arXiv2014

$μ$SR and inelastic neutron scattering investigations of the noncentrosymmetric antiferromagnet CeNiC$_2$

The magnetic state of the noncentrosymmetric antiferromagnet CeNiC$_2$ has been studied by magnetic susceptibility, heat capacity, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS) measurements. CeNiC$_2$ exhibits three magnetic phase transitions at $T_{N_1}$ = 20 K, $T_{N_2}$ = 10 K and $T_{N_3}$ = 2.5 K. The presence of long range magnetic order below 20 K is confirmed by the observation of oscillations in the $μ$SR spectra between 10 and 20 K and a sharp increase in the muon depolarization rate. INS studies reveal two well-defined crystal electric field (CEF) excitations around 8 and 30 meV. INS data have been analyzed using a CEF model and the wave functions were evaluated. We also calculated the direction and magnitude of the ground state moment using CEF wave functions and compare the results with that proposed from the neutron diffraction. Our CEF model correctly predicts that the moments order along the $b-$axis (or $y$-axis) and the observed magnetic moment is 0.687(5) $μ_B$, which is higher than the moment observed from the neutron diffraction (0.25 $μ_B$/Ce). We attribute the observed reduced moment due to the Kondo screening effect.

cond-mat.str-el↗

arXiv2013

Magnetic ordering with reduced cerium moments in hole-doped CeOs2Al10

The lightly hole-doped system CeOs1.94Re0.06Al10 has been studied by muon spin relaxation and neutron diffraction measurements. A long-range antiferromagnetic ordering of the Ce-sublattice with substantially reduced value of the magnetic moment 0.18(1) μ_B has been found below T_N = 21 K. Similar to the undoped parent compound, the magnetic ground state of CeOs1.94Re0.06Al10 preserves the anomalous direction of the ordered moments along the c-axis. The obtained result reveals the crucial difference between electron- and hole-doping effects on the magnetic ordering in CeOs2Al10. The former suppresses the anisotropic c-f hybridization and promotes localized Ce moments. On the contrary, the latter increases the hybridization and shifts the system towards delocalized non-magnetic state.

cond-mat.str-el↗

arXiv2013

Change of magnetic ground state by light electron-doping in CeOs2Al10

The effect of Ir substitution for Os in CeOs2Al10, with an unusually high Neel temperature of T~28.5K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir (~ 8%) results in a pronounced change of the magnetic structure of the Ce-sublattice. The new magnetic ground state is controlled by the single ion anisotropy and implies antiferromagnetic arrangement of the Ce-moments along the a-axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1) mu_B, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21K. A comparison of the observed phenomena with the recently studied CeRu1.9Rh0.1Al10 system, exhibiting similar behaviour [A. Kondo et al., J. Phys. Soc. Jpn. 82, 054709 (2013)], strongly suggests the electron doping as the main origin of the ground state changes. This provides a new way of exploring the anomalous magnetic properties of the Ce(Ru/Os)2Al10 compounds.

cond-mat.str-el↗

arXiv2013

Competing 4f-electron dynamics in Ce(Ru1-xFex)2Al10 (x=0 to 1): magnetic ordering emerging from the Kondo semiconducting state

We have carried out muon spin relaxation (muSR), neutron diffraction and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1-xFex)2Al10 (x=0, 0.3, 0.5, 0.8 and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin gap formation as evident from the bulk measurements in the end members. Our zero-field muSR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range magnetic ordering of the Ce-moment with TN=27, 26 and 21 K respectively. On the other hand the muSR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively exhibit a temperature independent Kubo-Toyabe term confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K the spin gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10-12 meV at 7 K, with a strong Q-dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=1. The observation of a spin gap in the paramagnetic samples (x=0.8 and 1) is an interesting finding in this study and it challenges our understanding of the origin of the semiconducting gap in CeT2Al10 (T=Ru and Os) compounds in terms of hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.

cond-mat.str-el↗

arXiv2013

Another dimension: investigations of molecular magnetism using muon-spin relaxation

We review examples of muon-spin relaxation measurements on molecule-based magnetic coordination polymers, classified by their magnetic dimensionality. These include the one-dimensional s=1/2 spin chain Cu(pyz)(NO3)2 and the two-dimensional s=1/2 layered material [Cu(HF2)(pyz)2]BF4. We also describe some of the more exotic ground states that may become accessible in the future given the ability to tune the interaction strengths of our materials through crystal engineering.

cond-mat.str-el↗

arXiv2012

Anisotropic optical conductivity of the putative Kondo insulator CeRu$_4$Sn$_6$

Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu$_4$Sn$_6$ employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the $a-a$ plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their origin.

cond-mat.str-el↗

arXiv2012

Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs

We report full-potential density functional theory (DFT)-based {\it ab initio} band structure calculations to investigate electronic structure properties of the first pnictide superconductor with a honeycomb-lattice structure: SrPtAs. As a result, electronic bands, density of states, Fermi velocities and the topology of the Fermi surface for SrPtAs are obtained. These quantities are discussed in comparison to the first available experimental data. Predictions for future measurements are provided.

cond-mat.supr-con↗

arXiv2010

Direct evidence of long-range ordering of reduced magnetic moments in the spin-gap compound CeOs2Al10 through muon spin relaxation investigations

We have carried out neutron diffraction, muon spin relaxation (muSR) and inelastic neutron scattering investigations on a polycrystalline sample of CeOs2Al10 to investigate the nature of the phase transition observed near 29 K in the resistivity and heat capacity. Our muSR data clearly reveal coherent frequency oscillations below 28 K, indicating the presence of an internal field at the muon site, which confirms the long-range magnetic ordering of the Ce-moment below 28 K. Upon cooling the sample below 15 K, unusual behaviour of the temperature dependent muSR frequencies may indicate either a change of the muon site, consistent with the observation of superstructure reflections in electron diffraction or a change of the ordered magnetic structure. Neutron diffraction data do not reveal any clear sign of either magnetic Bragg peaks or superlattice reflections. Furthermore, inelastic neutron scattering (INS) measurements clearly reveal the presence of a sharp inelastic excitation near 11 meV between 5 K and 26 K, due to opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. The magnitude of the spin-gap (11 meV) as derived from the INS peak position agrees very well with the gap value as estimated from the bulk properties.

cond-mat.str-el↗

arXiv2010

Long-range magnetic order in CeRu2Al10 evidenced by muSR and neutron diffraction

The low temperature state of CeRu2Al10 has been studied by neutron powder diffraction and muon spin relaxation (muSR). By combining both techniques, we prove that the transition occurring below T*~27K, which has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with propagation vector k=(1,0,0) involves collinear antiferromagnetic alignment of the Ce moments along the c-axis of the Cmcm space group with a reduced moment of 0.34(2)mu_B. No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around T2~12 K indicating a possible second transition.

cond-mat.str-el↗

arXiv2010

CeRu$_4$Sn$_6$: heavy fermions emerging from a Kondo-insulating state

The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $ΔE_g1/k_B \simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat C detects these excitations in the form of an enhanced Sommerfeld coefficient $γ= C(T)/T$ : In zero field, $γ$ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at $γ= 0.6$ J/molK$^2$. This maximum is shifted to higher temperatures with increasing field suggesting a residual density of states at the Fermi level developing a spin gap $ΔE_g2$. A simple model, based on two narrow quasiparticle bands located at the Fermi level - which cross the Fermi level in zero field at 0.022 states/meV f.u. - can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting our data of both specific heat and NMR to the model, incorporating a Ce magnetic moment of $μ= ΔE_g1/μ_{0H} \simeq 1 μ_B$, leads to the prediction of the field dependence of the gap. Our measurements rule out the presence of a quantum critical point as the origin for the enhanced $γ$ in CeRu$_4$Sn$_6$ and suggest that this arises rather from correlated, residual in-gap states at the Fermi level. This work provides a fundamental route for future investigations into the phenomenon of narrow-gap formation in the strongly correlated class of system

cond-mat.str-el↗