A. M. Strydom

Unconventional superconductivity in Y5Rh6Sn18 probed by muon spin relaxation.

Conventional superconductors are robust diamagnets that expel magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high—temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor Y5Rh6Sn18 ( TC= 3.0 K). The electronic heat capacity of Y5Rh6Sn18 shows a T3 dependence below Tc indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu5Rh6Sn18 which is a strong coupling s—wave superconductor. The temperature dependence of the deduced superfluid in density Y5Rh6Sn18 is consistent with a BCS s—wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.

Long-range magnetic order in CeRu_2Al_10 studied via muon spin relaxation 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.

Destruction of the Kondo effect in the cubic heavy-fermion compound Ce3Pd20Si6

How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy-fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy-fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure, raising the question of the extent to which heavy-fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy-fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram--a precondition for a unified theoretical description.

Low-temperature magnetic property of polymer encapsulated gold nanoparticles

Gold-polyaniline composite is reported by the polymerization of aniline hydrochloride monomer using HAuCl4 as the oxidant. HAuCl4 was dissolved in toluene using a phase-transfer catalyst, Aliquat 336. The oxidative polymerization of aniline hydrochloride leads to the formation of polyaniline with a diameter of <50 nm, while the reduction in auric acid results in the formation of gold nanoparticles with an average diameter ∼4 nm. The resultant composite material was characterized by means of different techniques, such as UV-vis, IR, and Raman spectroscopies, which offered the information about the chemical structure of polymer, whereas electron microscopy images provided information regarding the morphology of the composite material and the distribution of the metal particles in the polymer matrix. dc-magnetization measurements down to low temperatures (2 K) enabled the identification of a small, but field-independent paramagnetic behavior of the composite, and this is argued to originate from the charge t...

Optical, microscopic and low temperature electrical property of one-dimensional gold―polyaniline composite networks

An in situ one pot synthesis route is described for the preparation of a gold–polyaniline nanocomposite material by polymerization of aniline hydrochloride using HAuCl4 as the oxidant. The resultant composite material was characterized by various techniques. The temperature-dependent electrical conductivity values were supportive of a thorough dispersion of metallic conducting centres and the interpretation is commensurate with the one-dimensional geometry of the composite material. The temperature dependence of the Hall coefficient of the gold–polymer composite is evidence of a possible contribution of a charge carrier deficit and a temperature-dependent mobility in this material.

Graphene Supported Graphone/Graphane Bilayer Nanostructure Material for Spintronics

We report an investigation into the magnetic and electronic properties of partially hydrogenated vertically aligned few layers graphene (FLG) synthesized by microwave plasma enhanced chemical vapor deposition. The FLG samples are hydrogenated at different substrate temperatures to alter the degree of hydrogenation and their depth profile. The unique morphology of the structure gives rise to a unique geometry in which graphane/graphone is supported by graphene layers in the bulk, which is very different from other widely studied structures such as one-dimensional nanoribbons. Synchrotron based x-ray absorption fine structure spectroscopy measurements have been used to investigate the electronic structure and the underlying hydrogenation mechanism responsible for the magnetic properties. While ferromagnetic interactions seem to be predominant, the presence of antiferromagnetic interaction was also observed. Free spins available via the conversion of sp2 to sp3 hybridized structures, and the possibility of unpaired electrons from defects induced upon hydrogenation are thought to be likely mechanisms for the observed ferromagnetic orders.

Large Seebeck effect by charge-mobility engineering.

The Seebeck effect describes the generation of an electric potential in a conducting solid exposed to a temperature gradient. In most cases, it is dominated by an energy-dependent electronic density of states at the Fermi level, in line with the prevalent efforts towards superior thermoelectrics through the engineering of electronic structure. Here we demonstrate an alternative source for the Seebeck effect based on charge-carrier relaxation: a charge mobility that changes rapidly with temperature can result in a sizeable addition to the Seebeck coefficient. This new Seebeck source is demonstrated explicitly for Ni-doped CoSb3, where a marked mobility change occurs due to the crossover between two different charge-relaxation regimes. Our findings unveil the origin of pronounced features in the Seebeck coefficient of many other elusive materials characterized by a significant mobility mismatch. When utilized appropriately, this effect can also provide a novel route to the design of improved thermoelectric materials.

Antisite disorder-induced exchange bias effect in multiferroic Y2CoMnO6

Exchange bias effect in the ferromagnetic double perovskite compound Y2CoMnO6, which is also a multiferroic, is reported. The exchange bias, observed below 8 K, is explained as arising due to the interface effect between the ferromagnetic and antiferromagnetic clusters created by antisite disorder in this material. Below 8 K, prominent ferromagnetic hysteresis with metamagnetic “steps” and significant coercive field, Hc ≈ 10 kOe are observed in this compound which has a Tc ≈ 75 K. A model based on growth of ferromagnetic domains overcoming the elastic energy of structurally pinned magnetic interfaces, which closely resembles martensitic-like transitions, is adapted to explain the observed effects. The role of antisite disorder in creating the domain structure leading to exchange bias effect is highlighted in the present work.

Critical phenomena and estimation of the spontaneous magnetization by a magnetic entropy analysis in Mn0.96Nb0.04CoGe alloy

Magnetic and magnetocaloric properties of the alloy Mn0.96Nb0.04CoGe have been investigated. According to the mean-field theory prediction, the relationship between ΔSM ∝ (H/TC)2/3 has been confirmed in the temperature region near TC for that system. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical components, γ, β, and δ determined using the Kouvel-Fisher method, the modified Arrott plot, as well as the critical isotherm analysis agree well. Moreover, these critical exponents are confirmed by the Widom scaling law and the validity of the calculated critical exponents was also confirmed by the scaling theory. The values deduced for the critical exponents are close to the theoretical prediction of the mean-field model values, thus indicating that long range interactions dominate the critical behavior in the Mn0.96Nb0.04CoGe system. It is also speculated that the competition between the localized Mn-Mn magnetic interactions shoul...

Specific heat and mu SR study on the noncentrosymmetric superconductor LaRhSi3

We have investigated the superconducting properties of the noncentrosymmetric superconductor LaRhSi