Critical current anisotropy in Nd-1111 single crystals and the influence of neutron irradiation

We report on angle-resolved magnetization measurements on NdFeAsO0.65F0.35 ( Nd-1111) single crystals. The field dependence of the critical current density, Jc, is non-monotonic in these crystals at all orientations and temperatures due to the fishtail effect, which strongly influences the angular dependence of Jc. The currents decrease as the field is tilted from the crystallographic c-axis at low fields, but increase at high fields. A peak occurs in the angular dependence of Jc at intermediate fields. The critical currents are significantly enhanced after irradiation with fast neutrons and the fishtail disappears. The different current anisotropies at low and high fields, however, persist. We discuss the data in the framework of the anisotropic scaling approach and propose a transition from dominant pinning by large defects of low density at low fields to pinning by small defects of high density at high fields in the pristine crystal. Strong pinning dominates at all fields after the irradiation, and the angular dependence of Jc can be described via anisotropic scaling only after an appropriate extension to this pinning regime.

Conventional superconductivity and hysteretic Campbell penetration depth in single crystals MgCNi3

Sergiy Katrych

Single crystals of MgCNi3, with areas sized up to 1 mm(2), were grown by the self-flux method using a cubic anvil high-pressure technique. In low applied fields, the dc magnetization exhibited a very narrow transition into the superconducting state, demonstrating good quality of the grown crystals. The first critical field H-c1, determined from a zero-temperature extrapolation, is around 18 mT. Using the tunnel-diode resonator technique, the London penetration depth was measured with no applied dc field and the Campbell penetration depth was measured with the external dc fields up to 9 T for two different sample orientations with respect to the direction of applied magnetic field. The absolute value of the London penetration depth, lambda(0) = 245 +/- 10 nm, was determined from the thermodynamic Rutgers formula. The superfluid density, rho(s) = lambda(0)/lambda(T), was found to follow the clean isotropic s-wave behavior predicted by the weak-coupling BCS theory in the whole temperature range. The low-temperature behavior of the London penetration depth fits the BCS analytic form as well and produces a value close to the weak coupling one of Delta(0)/(k(B)T(c)) = 1.71. The temperature dependence of the upper critical field H-c2 was found to be isotropic with a slope at T-c of -2.6 T/K and H-c2(0) approximate to 12.3 T at zero temperature. The Campbell penetration depth probes the vortex lattice response in the mixed state and is sensitive to the details of the pinning potential. For MgCNi3, an irreversible feature has been observed in the TDR response when the sample is field cooled and warmed versus zero-field cooled and warmed. This feature possesses a nonmonotonic field dependence and has commonly been referred to as the peak effect. It is most likely related to a field-dependent nonparabolic pinning potential. DOI: 10.1103/PhysRevB.87.094520

Amer Physical Soc2013

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

Submonolayer epitaxial growth is obtained by the deposition of less than a complete layer of atoms on a single crystal surface. It is of fundamental interest for industrial applications (e.g. in the semiconductor industry) as well as from the point of view of basic research. For example, it is known that nanometer-sized atomic structures (nanostructures) exhibit remarkable physical and chemical properties which can differ greatly from those of bulk matter. In order to investigate these properties, it is often necessary to create large quantities of well defined and possibly spatially ordered nanostructures. One way to achieve this result is self-organized growth where one deposits atoms on a clean crystalline surface and lets the growth process evolve freely. Here, nanostructures result from the atomic diffusion and aggregation processes taking place at the surface. Understanding the exact nature of these processes is of ongoing interest in the field of nanostructure growth. In this thesis we report about submonolayer growth experiments of the ferromagnetic transition metal cobalt. Cobalt was chosen because it exhibits remarkable magnetic properties. These experiments were performed in an ultra-high vacuum chamber and molecular beam epitaxy was used to grow the nanostructures. Observations of the surface were made using a variable-temperature scanning tunneling microscope (STM). In order to get a better understanding of the atomic processes happening at the surface, we developed two adapted computational simulation methods. Kinetic Monte-Carlo (KMC) simulations were used to get an atomistic picture of the surface while mean field rate equations were integrated numerically to yield cluster densities. We study the growth of cobalt on three different surfaces. By depositing Co on a clean Pt(111) crystal surface, we observe that the platinum surface reconstructs by forming star-shaped partial dislocations for sample temperatures above 180 K (-93°C). We also observe that island densities deviate from predictions of all known models towards higher values for these same temperatures. By simulation we are able to show that insertion of Co into the topmost platinum layer creates a repulsive network of dislocations. We show that these dislocations act as diffusion inhibiting barriers and thus influence the island density by constraining the free movement of atoms at the surface. We also show that the Co dimer and trimer diffuse on Pt(111) before dissociating and are able to extract corresponding activation barriers. We also study the deposition of Co on a Ru(0001) surface which shows a more conventional temperature dependence. By comparing simulation and experiment we extract all relevant diffusion activation barriers and show that at high temperatures the Co dimer and trimer dissociate. Finally, we investigate the growth of Co on the hexagonal boron nitride superlattice which forms on a Rh(111) surface. On this surface Co clusters form three-dimensional hemispherical dots. We show how the substrate corrugation influences diffusion of Co atoms and that clusters up to the size of the pentamer can diffuse. By simulation, we also extract the relevant migrations and desorption barriers.

EPFL2008

Disorder-induced electronic, magnetic, and optoelectronic properties of two-dimensional materials

Cheol Yeon Cheon

Technological advancement has been in cadence with material development by improving the purity of single crystals and, at the same time, controlling their imperfections. These capabilities have been especially vital for developing new technolo-gies based on two-dimensional (2D) van der Waals (vdW) materials for future electronic and optoelectronic applications. This is because the inherent properties of 2D vdW materials is highly susceptible to the presence of intrinsic structural defects and ex-trinsic disorders due to large surface-area-to-volume ratio. The successful reduction of these disorders has significantly im-proved material properties and led to the discovery of novel physical phenomena in vdW materials. On the other hand, structural defects â for instance, 0-dimensional point defects â can induce completely new properties that are otherwise absent in the pre-fect lattice. To harness the full potential of vdW materials, it is thus essential to produce high-quality crystals and understand how the disorder affects their material properties, which is the central idea of this dissertation.In this dissertation, we first present the work of high-quality epitaxial growth of NbS2. Based on atmospheric-pressure chemical vapor deposition, we have successfully synthesized the two polymorphs (2H and 3R) of NbS2 with the largest lateral size grown to date. Their distinct superconducting and metallic properties were examined under low-temperature charge transport, respectively. Our finding demonstrates the practical synthesis method for phase-controllable growth of 2D transition metal dichalcogenides and can benefit future studies in mesoscopic devices and large-area applications of 2D superconductors. Secondly, we present the work of discovering defect-induced novel properties in ultrathin layers of PtSe2. Although bulk PtSe2 is non-magnetic, we observe the appearance of magnetism in monolayer and bilayer PtSe2. We were able to measure the magnetoresistance (MR) of mono- and bilayer PtSe2 under perpendicular magnetic fields using proximitized graphene, and found antiferromagnetic and ferromagnetic MR responses for mono- and bilayer, respectively. The appearance of such different magnetic states is theoretically explained by the first-principle density functional theory calculation, suggesting the origin of induced-magnetic moments from intrinsic Pt vacancies for both layers. Moreover, we also found that structural disorder in PtSe2 can induce bulk photovoltaic effect (BPVE). The second-order optical nonlinear effects, such as BPVE, require broken structural inversion symmetry and crystal symmetry can be reduced by the presence of structural defects. The broken local inversion symmetry from structural disorder in centrosymmetric PtSe2 is manifested by the generation of zero-biased photo-current under homogenous illumination. We observe linear and circular polarization-dependent photocurrents in defective PtSe2, which is largely absent in the pristine crystal. Our findings in PtSe2 emphasize the importance of the structural disorder for generating completely new properties and stress the need for defect-engineering for realizing the practical use of PtSe2 in spintronic and photovoltaic applications.

EPFL2022