Formation of 2 D crystalline surface phases on

Formation of 2 D crystalline surface phases on liquid Au-based metallic glass forming alloys S. Mechler, P. S. Pershan, S. E. Stoltz, S. Sellner Department of Physics and SEAS, Harvard University, Cambridge, MA 02138, USA E. Yahel Department of Physics, NRCN, P. O. Box 9001, Beer-Sheva 84190, Israel O. G. Shpyrko Department of Physics, University of California San Diego, La Jolla, CA 92093, USA M. Meron, B. Lin CARS, University of Chicago, Illinois 60637, USA

Introduction Liquid metal surfaces: Surface layering in elemental liquids and alloys: Distorted crystal model chemical segregation at the surface of liquid alloys (Gibbs, e. g. Sn-Bi) Thermal height-height correlations: Capillary waves: Liquid eutectic Au 82 Si 18: (Shpyrko et. al Science, 2006) Formation of 2 different crystalline surface phases on bulk liquid phase (LT and HT) Unusal large x-ray reflectivity GID (LT) Diffuse scattering LT

Introduction Proposed electron density profile Normalized reflectivity DCM LT LT HT Melting of “high-temperature” crystalline surface phase ? Thickness of surface crystals ? 2 D phases ? Bending rigidity of surface crystals ? Influence of surface crystals on capillary wave spectrum ?

Thermal stability of surface crystals Reflectivity @ qz=1. 4 A heating (5 K/min) Grazing Incidence Diffraction Normalized reflectivity (R/RF) 5 K/min Surface crystals melt into a liquidlike surface Crystals reform during subsequent cooling S. Mechler et al. , submitted to Phys. Rev. Lett.

Thickness of the surface crystals Truncation rod: LT Intensity distribution of Bragg reflections along qz HT 2 D phase: Both phases are 2 D phases LT: destructive interference → bilayer crystalline phase, d≈ 3. 31 Å HT: atomic monolayer crystalline phase

Effect of surface crystals on capillary wave spectrum Reflectivity of liquid surface: Structure factor: electron density profile surface tension bending rigidity thermal height-height fluctuations Diffuse scattering under grazing incidence For LT surface phase ( ): Surface crystals exhibit bending rigidity, Quenching of short wavelength capillary waves No effect on long wavelength capillary waves

Effect of Rigidity on Reflectivity Liquidlike surface: cut-off wavevector for capillary waves: Presence of surface bending rigidity: can be replaced by x 10 Surface crystals reduce contribution of capillary waves to reflectivity Relaxes necessary amplitude of structure factor

Self consistent density profile Constraints for density profiles: LT: Bilayer and DCM HT: Monolayer and DCM } + allow qe to vary Density profiles LL: Monolayer and DCM, qmax R/RF LT HT LL Bending rigidity essential for a more physical picture of surface structure LT: HT: for HT, LT and LL

Origin of surface freezing ? Au-Si: Deep eutectic High degree of short range order Large negative heat of mixing between Au and Si Amorphous phase by rapid liquid quenching Au-Ge: Similar properties as Au-Si But no amorphous phase, no surface freezing found Metallic glass forming liquids good candidates for surface freezing? 1. ) Au 49 Cu. Si 16. 3 Ag. Pd 2. ) Pd 81 Ge 19 } High glass forming ability, deep eutectic, large negative d. Hmix

Surface freezing in liquid Au-Cu-Si-Ag-Pd Coo ling Trunc. Rod @ 1. 65 Å-1 GID R/Rf @1. 4 Å-1 Hea ti ng Heating GID LT (694 K) LL (704 K) Lattice: single hexagonal layer (a= ? ? nm) Superstructure ? 2 D crystalline monolayer on the liquid surface !

Au-Cu-Si-Ag-Pd: Surface density profile Reflectivity for LT and LL phase Surface profile Structure factor strong layering in the liquidlike phase of Au-Cu-Si-Ag-Pd qe(LT)/qmax(LL) about 2 due to rigidity Comparable structure factor between LT and LL phase

Effect of surface crystals on capillary wave spectrum Pd-Ge: Similar deep eutectic as Au-Si Large negative enthalpy of mixing → high degree of short range order Existence of many stable crystalline phases → promote surface crystallization ?

Effect of surface crystals on capillary wave spectrum Pd-Ge: Similar deep eutectic as Au-Si Large negative enthalpy of mixing → high degree of short range order Existence of many stable crystalline phases → promote surface crystallization ? GID of Pg-Ge 19 to show absence of Bragg peaks No surface freezing in Pd 81 Ge 19

Effect of surface crystals on capillary wave spectrum Structure Factor Density profile Liquid Ag-Ge: SANS Bellisent '79 Layering at the surface according to distorted crystal model Oscillations at low qz → wetting layer with 4 nm thickness Presence of clusters at surface? Surface crystallization temperature below eutectic temperature ?

Summary Surface of liquid eutectic Au 82 Si 18: 2 D-crystalline bilayer → 2 D crystalline monolayer → liquidlike surface Demonstration of bending rigidity of crystalline 2 D phases Quenching of short wavelength capillary waves → reduction of necessary structure factor Self consistent surface structure Liquid bulk metallic glass former: Au 82 Cu 18 Si 18 Ag 18 Pd 18: Second example of 2 D crystalline surface phase in a metallic liquid alloy Surface freezing: high degree of short range order + low temperature ? Liquid Pd 81 Ge 19: Crystalline surface phases not observed. Do they form below eutectic temperature? 4 nm wetting layer: Presence of large icosahedral clusterlike arrangements at surface ?