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Impact of a Fano resonance on the measured transition time scale in solid state photoemission

Fei Guo, Dmitry Usanov, Eduardo B. Guedes, Arnaud Magrez, Michele Puppin, J. Hugo Dil

TL;DR

The paper addresses how Fano resonances in solids modify the attosecond photoemission time scale inferred from spin polarization. It uses SARPES on the valence bands of 1T-TiSe$_2$ and 1T-TiTe$_2$ with photon energy tuned to the Ti $3p$-$3d$ autoionizing resonance and contrasts with off-resonance excitation to extract the EWS time delay from $dP/dE$. The main finding is that $dP/dE$ flips sign and diminishes in magnitude at resonance due to multi-channel interference between direct photoemission and autoionization, revealing that a two-channel picture does not suffice and the resonant phase offset $oxed{ riangle \\varphi(hν)}$ must be characterized. The authors argue that a correct resonant $τ^s_{EWS}$ requires full accounting of all ionization channels, implying experimental challenges but offering a path toward tuning ionization dynamics via resonances and motivating new theoretical and measurement strategies.

Abstract

Fundamental quantum transition time scales are accessible through the spin polarization of photoelectrons coming from initially spin-degenerate states for solid-state materials . In this work we investigate the modification of this time scale in the vicinity of a Fano resonance in photoemission from a solid. We employ spin- and angle-resolved photoemission spectroscopy (SARPES) to study the valence band of 1T-TiSe$_2$ and 1T-TiTe$_2$, with an excitation photon energy coinciding with the Ti 3p-3d autoionization state. The energy derivative of the measured spin polarization, which is in the off-resonance case proportional to the transition time, reveals a sign reversal and significant magnitude decrease compared to off-resonance measurements. We show that this effect goes beyond conventional semi-analytical models used to translate spin polarization to the EWS time delay. At the Fano resonance, the underlying interference assumption of the model breaks down, and additional information about resonance strength is needed to extract the transition time delays.

Impact of a Fano resonance on the measured transition time scale in solid state photoemission

TL;DR

The paper addresses how Fano resonances in solids modify the attosecond photoemission time scale inferred from spin polarization. It uses SARPES on the valence bands of 1T-TiSe and 1T-TiTe with photon energy tuned to the Ti - autoionizing resonance and contrasts with off-resonance excitation to extract the EWS time delay from . The main finding is that flips sign and diminishes in magnitude at resonance due to multi-channel interference between direct photoemission and autoionization, revealing that a two-channel picture does not suffice and the resonant phase offset must be characterized. The authors argue that a correct resonant requires full accounting of all ionization channels, implying experimental challenges but offering a path toward tuning ionization dynamics via resonances and motivating new theoretical and measurement strategies.

Abstract

Fundamental quantum transition time scales are accessible through the spin polarization of photoelectrons coming from initially spin-degenerate states for solid-state materials . In this work we investigate the modification of this time scale in the vicinity of a Fano resonance in photoemission from a solid. We employ spin- and angle-resolved photoemission spectroscopy (SARPES) to study the valence band of 1T-TiSe and 1T-TiTe, with an excitation photon energy coinciding with the Ti 3p-3d autoionization state. The energy derivative of the measured spin polarization, which is in the off-resonance case proportional to the transition time, reveals a sign reversal and significant magnitude decrease compared to off-resonance measurements. We show that this effect goes beyond conventional semi-analytical models used to translate spin polarization to the EWS time delay. At the Fano resonance, the underlying interference assumption of the model breaks down, and additional information about resonance strength is needed to extract the transition time delays.

Paper Structure

This paper contains 4 sections, 3 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Main
  4. Conclusion

Figures (3)

  • Figure 1: Photon energy scan of intensity at $E_b=0.5\,$eV for (a) TiSe$_2$ and (b) TiTe$_2$. Band maps of (c) Band map of TiSe$_2$ taken at $h\nu = 48\,$eV. (d) Band map of TiTe$_2$ taken at $h\nu = 47\,$eV, red lines indicate the binding energies at which momentum distribution curves (MDCs) were taken. Schematic illustrations of (e) Ti 3p-3d autoionization process and (f) direct photoemission process from the valence band.
  • Figure 2: (a-c) Comparison of spin polarization MDCs of TiSe$_2$ at $E_b=0.9\,$eV in $x$, $y$ and $z$ directions, taken at $h\nu=67\,$eV and $h\nu=48\,$eV respectively. (d-f) Comparison of spin polarization MDCs of TiTe$_2$ at $E_b=0.49\,$eV in $x$, $y$ and $z$ directions, taken at $h\nu=67\,$eV and $h\nu=47\,$eV respectively.
  • Figure 3: (a) Spin polarization MDCs of TiSe$_2$ with binding energies from top to bottom curves: $E_b=0.7\,$eV, 0.8 eV, 0.9 eV, 1.0 eV, 1.1 eV, in $x$ (red), $y$ (blue) and $z$ (green) directions, taken at $h\nu=48\,$eV, offset by 0.4, 0.2, 0, -0.2 and -0.4 respectively. (b) Spin polarization MDCs of TiTe$_2$ with binding energies from top to bottom curves: $E_b=0.34\,$eV, 0.415 eV, 0.49 eV, 0.565 eV, 0.64 eV, in $x$, $y$ and $z$ directions, taken at $h\nu=47\,$eV, offset by 0.3, 0.15, 0, -0.15 and -0.3 respectively. Maximum spin polarization magnitude at 5 binding energies with on and off-resonance photon energies, for (c) TiSe$_2$ and (d) TiTe$_2$, resulting estimates of on-resonance transition time scales close to 0.