Determination of the Sb(111)-phonon dispersion relation using - - PowerPoint PPT Presentation

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Determination of the Sb(111)-phonon dispersion relation using - - PowerPoint PPT Presentation

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W I S S E N T E C H N I K L E I D E N S C H A F T 1 Determination of the Sb(111)-phonon dispersion relation using inelastic HAS measurements Florian Apolloner , Patrick Kraus, Christian

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1 W I S S E N  T E C H N I K  L E I D E N S C H A F T  www.tugraz.at

Determination of the Sb(111)-phonon dispersion relation using inelastic HAS measurements

Florian Apolloner, Patrick Kraus, Christian Gösweiner, Giorgio Benedek, Wolfgang E. Ernst

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2 Florian Apolloner, Institute of Experimental Physics 28.08.15

Overview

  • Experimental setup
  • Time of flight measurements
  • Sb(111) phonon dispersion
  • Conclusions
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3 Florian Apolloner, Institute of Experimental Physics 28.08.15

H.Å.N.S.

Creation Interaction Detection

θSD = θi + θf = 91.5° Pressures: SC: 10-6 mbar MC: low 10-10 mbar D: 10-9 mbar Nozzle: 50 bar, 10μm

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4 Florian Apolloner, Institute of Experimental Physics 28.08.15

(Why) Antimony

  • semimetal in bulk

metal on the surface

  • topological insulators
  • nanofilms for topoelectronic

phase transistions

  • spintronics

Z=51, 5 th main group, 5 th period hexagonal structure

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5 Florian Apolloner, Institute of Experimental Physics 28.08.15

Beam generation

Supersonic adiabatic expansion Tnozzle = 65 – 299 K → Ekin= 14-43 meV → λ = 0.7-1.2 Å

 Nozzle: 50 bar through 10μm  further selection via a skimmer  ΔE/E roughly 1.5%

Ekin=1 2 mHe v He

2 =5

2 kbT nozzle

Why Helium

  • chemically, electrically, magnetically inert
  • completely nondestructive
  • strictly surface sensitive

strictly surface sensitive

  • high cross section for surface phonons
  • interaction with the electron charge density oscillations
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6 Florian Apolloner, Institute of Experimental Physics 28.08.15

Time of flight measurements

  • Continuous He beam chopped into packets
  • Interaction between He atom and quasiparticles leads to

engery change

  • Time-resolved intensity measurement to distinguish signals
  • Single-slit chopper replaced with pseudo-random

chopper for faster measurements

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7 Florian Apolloner, Institute of Experimental Physics 28.08.15

Phonons

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8 Florian Apolloner, Institute of Experimental Physics 28.08.15

Measuring sub-surface modes – Example Bi(111)

[1] A. Tamtögl et. al., 2013, Surface and subsurface phonons of Bi(111) measured with helium atom scattering, Phys. Rev. B87, 035410 [2] I. Yu. Sklyadneva et. al., 2011, Mode-Selected Electron-Phonon Coupling in Superconducting Pb Nanofilms Determined from He Atom Scattering,

  • Phys. Rev. Lett. 107, 095502
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9 Florian Apolloner, Institute of Experimental Physics 28.08.15

TOF Data

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10 10 Florian Apolloner, Institute of Experimental Physics 28.08.15

Dispersion relation

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11 11 Florian Apolloner, Institute of Experimental Physics

Experimental results

iHAS TOF measurements on Sb(111) (TS = 150 K) Ei = 15-18 meV

? Acoustic Plasmon ?

Background calculation from: [3] D. Campi et al., PRB 86, 075446 (2012)

iHAS TOF measurements on Sb(111) (TS = 150 K) Ei = 15-18 meV

? Acoustic Plasmon ?

28.08.15

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12 12 Florian Apolloner, Institute of Experimental Physics 28.08.15

Possible explanations

Accoustic surface plasmon:

  • Polarization could lead to multiple branches
  • Energy still way to low even for really low lying acoustic

plasmons (factor 1000?) E-h excitation:

  • Plausible energy range
  • Exact mechanism unclear
  • How to explain multiple branches
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13 13 Florian Apolloner, Institute of Experimental Physics 28.08.15

Summary and outlook

 Time-resolved inelastic helium atom scattering intensities  Complete picture of the acoustical and optical phonon modes

in both high-symmetry directions (Low-T optical still to be done)

 Appearence of unknown low-lying dispersed modes below the

Rayleigh curve

 Theoretical analysis of electron-excitation jump energies

(Dispersion) → Next step!

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14 14 Florian Apolloner, Institute of Experimental Physics 28.08.15

Thanks

Wolfgang E. Ernst Patrick Kraus Giorgio Benedek H.Å.N.S. Christian Gösweiner Financial Support: European Commission and Styrian Government within the ERDF program

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15 15 Florian Apolloner, Institute of Experimental Physics 28.08.15

Multiple branches 1/2

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16 16 Florian Apolloner, Institute of Experimental Physics 28.08.15

Multiple branches 2/2