Influence of atomic-scale defects on coherent phonon excitations by THz near fields in an STM

Using terahertz scanning tunneling microscopy, researchers demonstrate that atomic-scale defects in semiconducting 2H-$MoTe_{2}$ modulate the local coupling to THz fields via band bending, thereby enabling the selective excitation and control of otherwise forbidden coherent phonon modes at the nanoscale.

The Gist

Imagine a crystal made of atoms is like a giant, perfectly organized dance floor. In a perfect crystal, everyone (the atoms) moves in perfect unison, following strict rules. But in the real world, there are always a few "glitches" in the dance floor—missing dancers or extra people standing in the wrong spot. These are atomic defects.

This paper is about a team of scientists who wanted to see how these tiny glitches change the way the whole dance floor vibrates when hit by a specific kind of "music."

Here is the story of their discovery, broken down into simple concepts:

1. The Setup: The Ultra-Fast Microscope

Usually, to see atoms, you need a microscope. To see them move super fast, you need a camera that can take pictures in a trillionth of a second.

• The Tool: The scientists used a Scanning Tunneling Microscope (STM). Think of this as a super-sensitive needle that hovers just above the surface of the material (a crystal called 2H-MoTe2). It can feel individual atoms.
• The Music: Instead of sound, they used Terahertz (THz) waves. Imagine these as invisible, ultra-fast radio waves that act like a gentle, rhythmic tap on the atoms.
• The Trick: They combined the needle and the THz waves. This allowed them to tap the atoms with the THz "music" and immediately listen to how they vibrated back, all while looking at the exact spot with atomic precision.

2. The Dance Moves: Coherent Phonons

When you tap a drum, it vibrates. When you tap a crystal, the atoms vibrate together in a synchronized wave. Scientists call these coherent phonons.

• In this specific crystal, there are two main types of "dance moves" (vibrations) they were interested in:
  1. The Breathing Mode: The atoms puff up and down together (like a chest expanding and contracting).
  2. The Shear Mode: The atoms slide back and forth against each other (like a deck of cards being pushed sideways).

The Problem: In a perfect, bulk piece of this crystal, the "Breathing" move is forbidden by the rules of symmetry. It's like trying to do a specific dance move in a ballroom where the rules say you can't. Usually, you can't make the atoms do this move with light.

3. The Discovery: The Glitch Makes the Magic

The scientists shone their THz "music" on the crystal.

• On a perfect spot: They saw the atoms vibrating, but the "Breathing" move was very weak or non-existent.
• On a defect (a glitch): When they moved their needle to a spot where an atom was missing or swapped, something amazing happened. The "Breathing" move suddenly became loud and strong.

The Analogy: Imagine a quiet room where everyone is whispering. If you whisper near a perfect wall, the sound is dull. But if you whisper near a broken piece of the wall (a defect), the sound bounces off in a weird way and suddenly becomes a shout. The defect changed how the "music" (THz waves) interacted with the atoms.

4. The Secret Mechanism: The "Electric Handshake"

Why did the defect change the dance?

• The Band Bending: The crystal is a semiconductor (like a light switch). When the microscope needle gets close, it creates an electric field that bends the energy levels of the atoms, like tilting a ramp.
• The Defect's Role: The defects act like little traps for electricity. Depending on how the scientists set the voltage (the "tilt" of the ramp), these traps either fill up with electricity or empty out.
• The Result: When a defect trap fills up or empties, it creates a tiny, temporary electric charge. This charge acts like a new handle for the THz waves to grab onto.
  • If the handle is oriented one way, the atoms slide (Shear mode).
  • If the handle is oriented another way (which happens near defects), the atoms puff up and down (Breathing mode).

By changing the voltage, the scientists could flip a switch that made the atoms prefer one dance move over the other, but only at the location of the defect.

5. Why This Matters

This is a big deal for the future of technology.

• Control at the Nanoscale: Usually, if you want to make a material vibrate a certain way, you have to change the whole material. Here, the scientists showed they can change the vibration of just one tiny spot by finding a defect and adjusting the voltage.
• New Switches: This suggests we could build tiny switches or sensors where a single missing atom acts as a control knob. We could "tune" the material's properties (like how it conducts heat or electricity) just by knowing where the defects are and how to manipulate them.

Summary

Think of the crystal as a piano. Usually, you can only play certain notes (vibrations) on the whole piano. But this paper shows that if you find a specific, slightly broken key (a defect), you can press a special button (the voltage) that makes that one broken key play a completely different, previously impossible note. This gives us a new way to "tune" the music of matter, atom by atom.

Technical Summary

1. Problem Statement

Coherent phonons (collective, ultrafast atomic motions) are central to understanding light-induced structural dynamics and electron-phonon interactions in materials. While Terahertz (THz) pulses offer a non-thermal pathway to drive these dynamics, selectively exciting specific phonon modes is challenging due to symmetry constraints (e.g., dipole-forbidden modes in bulk crystals) and the lack of spatial resolution in conventional optical techniques.

A critical gap exists in understanding how atomic-scale intrinsic defects interact with phonon modes. Defects can modify material properties, but observing these interactions requires a technique with both atomic-scale spatial resolution and ultrafast temporal resolution. Previous THz-Scanning Tunneling Microscopy (THz-STM) studies had successfully addressed charge carrier dynamics and single-molecule properties but had not yet resolved intrinsic, long-range coherent optical phonons or their modification by defects.

2. Methodology

The authors employed Terahertz Scanning Tunneling Microscopy (THz-STM) to investigate the semiconducting material 2H-MoTe$_2$.

• Experimental Setup:

  • Sample: In-situ cleaved 2H-MoTe$_2$, a transition-metal dichalcogenide with a trigonal prismatic structure.
  • THz Generation: Two phase-stable, single-cycle THz pulses were generated via optical rectification in a LiNbO$_3$ crystal using 1030 nm infrared pulses.
  • Pump-Probe Scheme: A "pump" THz pulse excites the sample, and a time-delayed "probe" THz pulse measures the resulting transient conductance changes.
  • Field Enhancement: The STM junction (Ag-coated W tip on MoTe$_2$) acts as a nanocavity, enhancing the THz near-field by orders of magnitude. This allows for the excitation of modes that are dipole-forbidden in the bulk due to broken inversion symmetry at the surface.
  • Detection: The THz-induced current ($I_{THz}$) was measured as a modulation of the DC tunneling current using a lock-in amplifier.

• Characterization:

  • Pulse Characterization: The THz waveform was verified using Electro-Optic Sampling (EOS) and Photoemission Sampling (PES) to ensure single-cycle nature and determine field strength ($\approx 0.5$ THz central frequency).
  • Spectroscopy: Differential conductance ($dI/dV_b$) measurements were used to map the electronic structure, identifying the bandgap ($\approx 1.2$ eV) and specific defect states within the gap.

3. Key Contributions

• First Observation of Coherent Optical Phonons in THz-STM: The study successfully excited and detected long-lived coherent optical phonons in 2H-MoTe$_2$ using a THz pump-probe scheme within an STM junction.
• Defect-Induced Mode Selectivity: The paper demonstrates that atomic-scale defects act as tunable switches for phonon excitation. The relative excitation strength of different phonon modes changes drastically near defects compared to pristine surfaces.
• Mechanism Elucidation: The authors attribute the selective enhancement to tip-induced band bending and transient charging of defect states. This local charging modulates the coupling efficiency between the THz field and specific vibrational modes (in-plane vs. out-of-plane).

4. Key Results

A. Identification of Phonon Modes

Time-resolved measurements on pristine 2H-MoTe$_2$ revealed long-lived oscillatory signals (up to 50 ps) in the THz-induced current. Fast Fourier Transform (FFT) analysis identified two distinct peaks:

1. $\alpha$ mode ($\approx 0.48$ THz): Assigned to the in-plane shear mode ($E^1_{2g}$).
2. $\beta$ mode ($\approx 0.60$ THz): Assigned to the out-of-plane breathing mode ($B^1_{2g}$).
   Note: These modes are typically Raman active but dipole-forbidden in the bulk due to inversion symmetry. The surface symmetry breaking and strong near-field gradients in the STM junction render them accessible.

B. Bias Voltage Dependence

• Pristine Surface: The relative intensity of the $\alpha$ and $\beta$ modes remained relatively stable across different bias voltages ($V_b$) where the material was conductive.
• Defect Sites: A dramatic change was observed at specific defect sites.
  • At positive bias ($V_b > 0$), the phonon spectrum resembled the pristine surface.
  • At negative bias ($V_b = -0.6$ V), the relative intensity of the modes inverted: the $\beta$ (out-of-plane) mode became significantly more intense than the $\alpha$ (in-plane) mode.

C. The Excitation Mechanism

The authors ruled out simple nonlinearities in the static $I(V)$ curve as the sole cause. Instead, they proposed a model based on local band bending and transient charging:

• Band Bending: The STM tip induces band bending ($E_{BB}$) at the semiconductor surface.
• Defect Charging: At negative bias ($V_b < V_{FB}$), the local band bending shifts the chemical potential, causing specific defect states (located near the conduction band in the $dI/dV$ spectrum) to become occupied (charged).
• Dipole Modulation: This transient charging alters the local charge distribution and the effective dipole moment at the defect site. Consequently, the coupling efficiency of the THz field to the out-of-plane breathing mode ($\beta$) is enhanced relative to the in-plane shear mode ($\alpha$).
• Field Profile: The defect introduces a lateral field component parallel to the surface, further differentiating the response of the two modes.

5. Significance

• Nanoscale Control of Lattice Dynamics: The study proves that coherent phonon excitation can be spatially and selectively controlled at the atomic scale by engineering local defects.
• New Excitation Pathway: It establishes that THz near-fields in STM can access and manipulate "forbidden" phonon modes via symmetry breaking and local field engineering, bypassing the limitations of bulk optical selection rules.
• Material Engineering: The findings suggest a pathway to tailor material properties (such as thermal transport or phase transition efficiency) by selectively introducing defects that tune specific vibrational modes.
• Fundamental Insight: It provides a deeper understanding of how electron-phonon coupling is modified by local electronic states and band bending in 2D semiconductors.

In conclusion, this work bridges the gap between ultrafast spectroscopy and atomic-scale microscopy, demonstrating that atomic defects are not merely perturbations but active tools for tuning the coherent lattice dynamics of 2D materials.