Surface resonance of the (2×1) reconstructed lanthanum hexaboride (001)-cleavage plane : a combined STM and DFT study

We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB6) using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a (2×1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at −0.6eV. Using DFT, we identify this structure as a surface resonance of the (2×1) reconstructed LaB6 (100) surface which is dominated by boron dangling bond states and lanthanum d states

Preparation and characterisation of the Ba(Zr,Ti)O(3) ceramics with relaxor properties

Ba(Zr(x)Ti(1-x))O(3) ceramics with various compositions x in the range (0, 0.5) have been prepared via solid state reaction. Optimum parameters for calcination and sintering have been found in order to obtain pure perovskite phase and high density ceramics. The dielectric data showed a transition from ferroelectric towards relaxor state and a shift of the Curie temperature towards lower values with increasing x. Using the modified Landau model for relaxors, the local order parameter has been calculated. Its temperature dependence shows the increasing of the degree of diffuseness of the phase transition with increasing Zr with a maximum for the composition x=0.35. The model also shows that in the relaxor state the local order parameter has non-zero values even at a few hundreds degrees above the temperature corresponding to the maximum of the dielectric constant. Further, the dielectric data obtained for x=0.35 under field cooling (FC) and zero-field cooling (ZFC) conditions shows a splitting characteristic to the relaxors and spin-glass systems

Hysteresis and tunability characteristics of Ba(Zr,Ti)O(3) ceramics described by first order reversal curves diagrams

The First Order Reversal Curves (FORC) diagrams are proposed for the characterization of the switching process and the tunability in Ba(ZrxTi1-x)O(3) ceramics with various compositions x in the range (0, 0.5), prepared via solid state reaction. The changes induced by the compositional-induced crossover ferroelectric-to-relaxor state are investigated by monitoring the changes of the FORC diagrams (the coercive and bias fields corresponding to the maximum, ratio of the reversible/irreversible contribution to the polarization, the diffuse character of the FORC distribution). The first derivative of the FORCs related to the tunability is a function of both the applied and reversal fields. The critical fields for the highest tunability were found to be composition-dependent

Low-temperature magnetic fluctuations in the Kondo insulator SmB6

We present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB6 using magnetization and muon-spin relaxation/rotation (muSR) measurements. The muSR measurements exhibit magnetic field fluctuations in SmB6 below 15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between muSR, magnetization, and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in SmB6 at low temperature.Comment: 5 pages, 5 figure

Surface resonance of the (2×\times1) reconstructed lanthanum hexaboride (001)-cleavage plane: a combined STM and DFT study

We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB$_\text{6}$) using scanning tunneling microscopy (STM) and density functional theory (DFT). Experimentally, we found a (2$\times$1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinth-like pattern. These findings are supported by low-energy electron diffraction (LEED) experiments. Slab calculations within the framework of DFT shows that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy (STS) shows a prominent spectral feature at -0.6 eV. Using DFT, we identify this structure as a surface resonance of the (2$\times$1) reconstructed LaB$_\text{6}$ (100)-surface which is dominated by boron dangling bond-states and lanthanum d-states.Comment: 10 pages, 16 figure

Magnetic short-range order in polycrystalline SrGd2O4 and SrNd2O4 studied by reverse Monte Carlo simulations and magnetic pair-distribution function analysis

We present a study combining total-scattering powder neutron diffraction, reverse Monte Carlo simulations, and magnetic pair-distribution function analysis to deduce both the static and dynamic short-range magnetic spin correlations in two compounds of the rare-earth strontium oxides: SrGd2O4 and SrNd2O4. Both compounds exhibit a distorted honeycomb lattice which forms a set of zigzag ladders along the crystallographic c axis of space group Pnam. Each set consists of two one-dimensional chains separated by diagonal rungs forming triangles, thus inducing a large degree of geometrical frustration due to the antiferromagnetic exchange between the magnetic ions. Significant magnetic diffuse scattering was observed well above the respective Néel temperatures and analyzed with reverse Monte Carlo techniques in reciprocal space. As a complementary analysis in real space we have derived the magnetic pair-distribution function for both compounds as a function of temperature. Our results clearly indicate that the individual zigzag ladders begin ordering well above TN due to the dominating nearest and next-nearest interactions, long before interladder spin correlations become significant only slightly above TN. Additionally, for SrNd2O4 we find some qualitative differences between the short-range spin correlations above TN and those observed in the ordered stat

Absence of zero-field-cooled exchange bias effect in single crystalline La2−xAx CoMnO6 (A=Ca,Sr) compounds

Magnetic properties of A2BB'O6 (A = rare or alkaline-earth ions; B, B' = transition-metal ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero-field-cooled exchange bias effect observed for the hole-doped La2−xAxCoMnO6 polycrystalline samples. In this paper we synthesize La2CoMnO6, La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca- and Sr-doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin-glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples

Flux-pinning mediated superconducting diode effect in NbSe2/CrGeTe3 heterostructure

In ferromagnet/superconductor bilayer systems, dipolar fields from the ferromagnet can create asymmetric energy barriers for the formation and dynamics of vortices through flux pinning. Conversely, the flux emanating from vortices can pin the domain walls of the ferromagnet, thereby creating asymmetric critical currents. Here, we report the observation of a superconducting diode effect (SDE) in a NbSe2/CrGeTe3 van der Waals heterostructure in which the magnetic domains of CrGeTe3 control the Abrikosov vortex dynamics in NbSe2. In addition to extrinsic vortex pinning mechanisms at the edges of NbSe2, flux-pinning-induced bulk pinning of vortices can alter the critical current. This asymmetry can thus be explained by considering the combined effect of this bulk pinning mechanism along with the vortex tilting induced by the Lorentz force from the transport current in the NbSe2/CrGeTe3 heterostructure. We also provide evidence of critical current modulation by flux pinning depending on the history of the field setting procedure. Our results suggest a method of controlling the efficiency of the SDE in magnetically coupled van der Waals superconductors, where dipolar fields generated by the magnetic layer can be used to modulate the dynamics of the superconducting vortices in the superconductors

Field-temperature phase diagram of the enigmatic Nd2(Zr1−xTix)2O7 pyrochlore magnets

By combining neutron scattering and magnetization measurements down to 80 mK, we determine the (H, T ) phase diagram of the Nd2(Zr1−xTix )2O7 pyrochlore magnet compounds. In those samples, Zr is partially substituted by Ti, hence tuning the exchange parameters and testing the robustness of the various phases. In all samples, the ground state remains all in/all out, while the field induces phase transitions toward new states characterized by two in–two out or one out–three in/one in–three out configurations. These transitions manifest as metamagnetic singularities in the magnetization versus field measurements. Strikingly, it is found that moderate substitution reinforces the stability of the all in/all out phase: the Néel temperature, the metamagnetic fields along with the ordered magnetic moment, are higher in substituted samples with x < 10%