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

Thermal stability of surface crystals Surface crystals melt into a liquidlike surface Crystals reform during subsequent cooling qz=1.4 A Grazing Incidence Diffraction Normalized reflectivity (R/RF) 5 K/min heating (5K/min) S. Mechler et al., submitted to Phys. Rev. Lett.

Thickness of the surface crystals LT: destructive interference → bilayer crystalline phase, d≈ 3.31 Å HT: atomic monolayer crystalline phase  Intensity distribution of Bragg reflections along q z 3D phase: 2D phase: Both phases are 2D phases Truncation rod: LTHT

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

Effect of Rigidity on Reflectivity  Liquidlike surface:  Presence of surface bending rigidity: cut-off wavevector for capillary waves:  can be replaced by  Surface crystals reduce contribution of capillary waves to reflectivity  Relaxes necessary amplitude of structure factor x10

Self consistent density profile Constraints for density profiles:  LT: Bilayer and DCM  HT: Monolayer and DCM  LL: Monolayer and DCM, q max Bending rigidity essential for a more physical picture of surface structure } + allow q e to vary R/R F LT: HT: for HT, LT and LL Density profiles LT HT 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 Metallic glass forming liquids good candidates for surface freezing? Au-Ge:  Similar properties as Au-Si  But no amorphous phase, no surface freezing found 1.) Au49CuSi16.3AgPd 2.) Pd81Ge19 } High glass forming ability, deep eutectic, large negative dHmix

Surface freezing in liquid Au-Cu-Si-Ag-Pd 2D crystalline monolayer on the liquid surface ! Å -1 GID Cooling Heating LT (694K) LL (704K) GID Trunc Å -1 Lattice: single hexagonal layer (a= ?? nm) Superstructure ?

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

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 ? No surface freezing in Pd 81 Ge 19 GID of Pg-Ge19 to show absence of Bragg peaks

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

Summary  Surface of liquid eutectic Au 82 Si 18 : 2D-crystalline bilayer → 2D crystalline monolayer → liquidlike surface Demonstration of bending rigidity of crystalline 2D 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 2D 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 ?