PbTe(111): DFT analysis and experimental results Photocathode Physics for Photoinjectors (P3) 2016 Jefferson Laboratory, October 17-19, 2016 UIC PbTe(111): DFT analysis and experimental results W. Andreas Schroeder Physics Department University of Illinois at Chicago Tuo Li Benjamin Rickman National Science Foundation PHYS-1535279

UIC Electron beam (pulse) quality RMS transverse emittance: … a conserved quantity in a ‘perfect’ system. Initial electron beam size, x0, at photocathode is dependent upon; (i) Laser spot size (e.g., focusing conditions), (ii) Photocathode optical damage threshold, (iii) Photoinjector’s operational phase-space (e.g., LCLS), and (iv) limited by Child’s Law: in short-pulse regime. Initial RMS transverse momentum, pT0, of emitted electrons is an intrinsic property of photocathode material:  Standard expression,  Must be reduced to improve performance of UED/UEMs, X-FELs, etc. D.H. Dowell & J.F. Schmerge, Phys. Rev. ST – Acc. & Beams 12 (2009) 074201 K.L. Jensen et al., J. Appl. Phys. 107 (2010) 014903

UIC Photoemission: ‘One-step’ Model  EF e- Vacuum Photocathode ħω − The ‘quantum mechanical’ one-step model Photoexcitation into a virtual state (excited copy of filled band) emitting by coupling to the vacuum states Quantum efficiency, PE < 10-4 e-/ ‘Instantaneous’ emission process Suitable for UED/UEM, X-FELs, ICS sources … Examples: Most metals …  any material for which no real state can be excited in the crystalline emission direction ~ G.D. Mahan, Phys. Rev. B 2, 4334-4350 (1970)

UIC Band Structure Effects pT pF pT,max. E ħω EF E pT pF pT,max. E ħω − Transverse momentum pT conserved in photoemission ‘Classical’ metal (m* = m0) Metal with m* < m0 > Photoemitting states in red: E and pT conserved Vacuum level Virtual excited states in ‘one-step’ photoemission   Band states for which E  0

UIC Band Structure Effects pT pF pT,max. E ħω EF E pT pF pT,max. E ħω − Transverse momentum pT conserved in photoemission ‘Classical’ metal (m* = m0) Metal with m* < m0 > Vacuum level  

Low m* hole-like states preferred Vacuum level pT E E ħω EF+ħω Electron-like (m* < m0) Hole-like (m* < m0) Electron-like vs. Hole-like States UIC − pT0 and its sensitivity to Te High E electrons at higher pT More high pT states occupied as Te increases High E electrons at lower pT Less high pT states occupied as Te increases Low m* hole-like states preferred

UIC PbTe Band Structure Lightly p-type PbTe crystal − Evaluation using QUANTUM ESPRESSO Lightly p-type PbTe crystal  EF at top of VBM at L point of Brillouin zone Very low hole mass (m* = 0.022m0) transverse to -L direction  (111)-face emission (111) NOTE: For emission from (111) face, no CBM exist above 2eV at L point.

DFT-based thin-slab prediction: UIC pPbTe(111): ϕ(111) − Evaluation using thin slab technique (111) Rock salt (simple cubic) crystal structure of PbTe  Pb or Te terminated regions on (111) surface  Different  due to surface dipole orientations Pb + _ Te PbTe -V +V DFT-based thin-slab prediction: ϕ(111),Pb  ϕ(111),Te  0.32eV  4.21eV … Photoemission possible for ħω = 4.75eV C. J. Fall et al., Journal of Physics: Condensed Matter 11, 2689 (1999)

DFT-based photoemission prediction: UIC pPbTe(111): pT0 − ‘One-step’ photoemission from L-point VBM with m* = 0.022m0 Emitting states for E = 0.3eV E, pT conservation PLUS Barrier transmission, T(pz,pz0) DFT-based photoemission prediction: pT0  0.1 (m0.eV)1/2 for ħω = 4.75eV T. Li et al., J. Appl. Phys. 117, 134901 (2015)

UIC pT0 Measurement: Solenoid Scan 2W, 250fs, 63MHz , diode- pumped Yb:KGW laser  ~4ps at 261nm (ħω = 4.75eV) YAG scintillator optically coupled to CCD camera  Beam size vs. magnetic coil (lens) current measured  Analytical Gaussian (AG) pulse propagation model to extract ΔpT0 I J.A. Berger & W.A. Schroeder, J. Appl. Phys. 108 (2010) 124905

UIC PbTe(111): Solenoid Scan Results PbTe(111) ħω = 4.75eV − Comparison with AG pulse propagation model Current2 (A2) HW1/eM Spot Size (mm) PbTe(111) ħω = 4.75eV  pT0(AG model) [(m0.eV)1/2] 0.1 0.2 0.3 0.4 0.5 pT = 0.29(0.02) (m0.eV)1/2 Normalized emittance n  0.4 m

UIC PbTe(111): Experiment vs. Theory PbTe(111): DFT analysis Dowell Padmore Dowell ħω < ϕ Expt. result: pT = 0.29 (m0.eV)1/2 Proc. FEL 2013, TUPSO83, pp. 424-6 Expected range of pT0 from DFT

UIC Work Function Variation 1st Fourier component dominates − pT0 increase due to Ecath perturbation by ϕ + _ ϕ x PbTe(111) Pb Te Ecath Esurface 1st Fourier component dominates pT0 increase: … ; . As , effect is significant ! pT()  0.2 (m0.eV)1/2 S. Karkare & I. Bazarov, Phys. Rev.. Appl. 4 (2015) 024015

UIC Total Photocathode Emittance − Quadrature addition of effects  (0.1)2 + (0.2)2 + … = (0.05 + ...) m0.eV  pT,tot.  (0.23 + …) (m0.eV)1/2 … c.f. pT,expt. = 0.29(0.02) (m0.eV)1/2 Other effects: (i) p-PbTe(111) is a semiconductor with ~1m surface depletion region  Large internal fields due to (x); E//(int.)  1MV/m  Band structure distortion expected (ii) PbTe is piezoelectric … (iii) (21) surface reconstruction on pristine PbTe(111) (iv) Oxidation: Weak, but may pin EF near VBM at surface

UIC PbTe(111): Photoemission Efficiency At 261nm: n = 0.76 +1.80i − Faraday cup measurement 106 Photons/Pulse Electrons/pulse PE  5.110-6 e-/ PbTe(111) ħω = 4.75eV i  60 p-polarization At 261nm: n = 0.76 +1.80i  TIR at i  60 BUT Rp = 0.12  Photoemission from absorbed evanescent wave NOTE: At i  0, R  0.52  PE(0)  0.55PE(60) = 2.810-6 e-/  N. Suzuki & S. Adachi, Jpn. J. Appl. Phys. 33 (1994) 193

UIC Summary “Theory-driven experimental studies of planar photocathodes”  Emission from low m* states:  Lower pT0  Oriented single crystal photocathodes (ħω > (ijk))  ‘Hole-like’ emission states are preferred: Even lower pT0 and less sensitive to Te (e.g., laser heating) PbTe(111): Emission from VBM with m* = 0.022m0  pT,expt. = 0.29(0.02) (m0.eV)1/2 … 2 value predicted by DFT analysis  MTE increase likely due to dipole  Perturbation of band structure by Einternal due to dipole ?? Future work  Tunable UV laser source: Measurement of ϕ (in situ) and pT0(ħω)  Search for ultra-low pT0 solid-state photocathodes: pT0 approaching cold atom electron sources  TID issues …

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