Liquid Metal Surfaces P. S. Pershan SEAS & Dept of Physics, Harvard Univ., Cambridge, MA, USA Colleagues Pershan/ESRF Balagurusamy, V. S. K. Berman, E. Deutsch, M. DiMasi, E. Fukuto, M. Gebhardt, J. Gog, T. Graber, T. Grigoriev, A. Huber, P. Kawamoto, E. H. Kuzmenko, I. Lin, B. H. Magnussen, O. M. Mechler, S. Meron, M. Ocko, B. M. Pontoni, D. Regan, M. J. Sellner, S. Shpyrko, O. G. Steimer, C. Stoltz, S. Streitel, R. Tostmann, H. Yahel, E Harvard, Non-Harvard, Beam Line

Reflectivity Pershan/ESRF 1 st Synchrotron Studies: Liquid Crystal Surfaces Reflectivity Normalized Liquid Crystal: Isotropic/Nematic/Smectic-A Surface Induced Smectic z Idea! Als-Nielsen, Christensen, Pershan,(1982) Tilt Monochromator to Steer beam downward by  Horizontal liquid surface.

Kinematics & Reflectivity: Flat Surface Pershan/ESRF Fresnel Reflectivity Temperature Dependence of Liq. Xtal Surface. Resolution: Reflectivity:Flat Surface Fresnel Not True for Liquids Electron Density (Liq. Xtal) Structure Factor z x

No Layering for Water and Simple Liquids Pershan/ESRF A. Braslau et al. PRL (1985). Molecular Simulations Chapela et al. (1977) Hard Wall  Layer Free Surface ✕ Layers Surface Roughness uu a l  u<lSurface Defines a Layer  u≥aSurface Does Not Define a Layer Liquid CrystalSimple Liquid

Free Surface of Liquid Metal: Hard Wall Pershan/ESRF Metallic Liquids (D’Evelyn & Rice ‘83) Suppression of Local Fluctuations  Local Hard Wall.  Layers Vapor: Neutral Atoms Liquid: Positive Ions in Sea of Negative Fermi Liquid Interface Hg In Ga Hg. Magnussen et al. (1995). Ga Regan et al.(1995). Goal: Measure Electron/Atom Density Profile!

Capillary Waves & Thermal Roughness Pershan/ESRF Rough  Phase Shift Flat surface: (Q z  <<1) Signal 2D Liquid SurfaceSinha et al.’88

Capillary Effects: H 2 0 & Ga Pershan/ESRF Slits 5.0 mm 2.0 mm 0.8 mm Water (Schwartz ’90):  (Q z ) for Liquid Ga (Regan, ’96)

Diffuse Scattering  Surface Tension(  ) Pershan/ESRF Compare Ga/In Hg In Ga Diffuse Scattering for In    Compare  (z) In Ga Solid Line No Adjustable Parameters

Simplest Surface Structure Model Pershan/ESRF DCM (Magnussen ’95)

Elemental Liquid Metals Studied Pershan/ESRF KGaInSnBiHg DCM +1 ?  ☐  ☐ No Bump/Bump Measureable Difference in 1 st Layer Why are 1 st Layers for Bi and Sn different from K, Ga and In? Why is Hg different from all others?

Eutectic Alloys Pershan/ESRF J. W. Gibbs ~1920 Surface Adsorption: A/B Alloy If Surface Tension:  A >  B Surface is Rich in “B”. A x B 1-x  A)/  B)  H * (mixing) Concentration of Surface Layers 1st2 nd 34d Ga x Bi 1-x 718/378=1.90+4Liquid-Liquid Phase Sep. Ga 83.5 In /556= %In In 78 Bi /378=1.4735%Bi Sn 57 Bi /378= %Bi25%Bi53%Bi Au 71 Sn /560= %Sn<1%Sn24%Sn Au 72 Ge /621= No Gibbs Absorption Au 82 Si /865= layers, 2DXtal (AuSi 2 ) Pd 81 Ge /621= ~40 Å wetting layer (No Measureable Gibbs Absorption) *(kJ/mol)Takeuchi and Inoue, Mater. Trans. 46 (2005)

9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 12 Gibbs Surface Adsorption(BiSn)  Bi =378,  Sn =560, Alloy: Bi and Sn  (Bi) ≈ 398  (Sn)≈567 dyne/cm Energy Dispersion: f(E) Adsorption Scat. Ampl.

R/R F 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 13 Surface Freezing Au 82 Si 18 Eutectic 2D Surface Crystals: Grazing Incidence Diffraction 1st Order Transition R/R F × 20 DCM DCM There is no theoretical explanation!

AuGe Eutectic(Should be Similar to Au-Si) Pershan/ESRF  (Au)/  Si or Ge) HH Au 72 Ge /621= Au 82 Si /865= Au-Si Au-Ge kev kev 1.Bump  higher density in 1 st layer. 2.No Energy effect  Ge in 1 st layer ≤40atm%. Small Gibbs (Different from Au-Sn, etc)! No Enhanced Layering or 2D order (Different from Au-Si)! Au-Si ×0.82

AuSiGe-Ternary Eutectic Pershan/ESRF AuSi Ge Eutectic Line Surface Frozen Ge≤6.5 atm% What is the physics of the cross over from Si type to Ge type surface between 2.5 atm% and 6.5 atm%? 18atm%Si Time average 0.8atm%Ge 0% Si

Pd 81 Ge 19 (Dec.’08) Pershan/ESRF Au 82 Si 18 Pd 81 Ge 19 Glass former yesbetter HH Expected same 2D surface order for Pd 81 Ge 19 as Au 82 Si 18 ! Not found; however, something new! Metallic Clusters (Giant Unit Cells) Small angle oscillations! Ref: Urban &Feuerbacher, J.Non-Crys.Sol.(04) Quenched Icosahedral Clusters Others: NaCd 2 30Å YbCu Å Al 3 Mg 2 28Å 14nm Mg 32 (Al,Zn) 49 Preliminary fit. ~4%  ∞

Summary Metal/Vapor Interface  Atomic Layering: Surface Structure Factor -  (Q z ): Measurement affected by thermal roughness. Requires knowledge of surface tension. Surface tension: measured with diffuse scattering: Surface tension effect demonstrated for Ga/In Subtle differences in elemental surfaces (Ga, In, K vs. Sn, Bi vs Hg) Alloys: Surface tension vs. Enthalpy of Mixing Gibbs absorption is not simple. No reliable theory. Au 82 Si 18 anomalously strong layering and 2D order. Why are Au 82 Si 18, Au 72 Ge 28 and Pd 81 Ge 19 all different? Need for THEORY! New Result (Preliminary): Surfaces & Icosahedral Metallic Clusters Pershan/ESRF