Optically excited structural transition in atomic wires on surfaces at the quantum limit

Abstract

Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

Non-equilibrium lattice dynamics of one-dimensional In chains on Si(111) upon ultrafast optical excitation

“…2(a). The observed timescales for excitation and relaxation are the same as those of the diffuse background 13 providing evidence that this process reflects a transient temperature increase Δ T ( t ) of the metastable phase through a distinct temporary loss of intensity Δ I DBW ( t ). Such behavior is explained by the Debye-Waller effect, which links the diffraction spot intensity I ( T ) to the momentum transfer k and the temperature dependent mean square displacement $true〈u{(T)}^{2}true〉$ of the surface atoms via $I(T)\propto \hspace{0.17em}\text{exp}true(-\frac{{\left|\mathrm{boldk}\right|}^{2}true〈u{(T)}^{2}true〉}{3}true)\hspace{0.17em}.$Figure 3(a) depicts the temperature dependence of the $true(0\frac{2}{4}true)$ spot intensity I ( T ) upon increasing the substrate temperature from 100 K to 200 K in a (quasi-)static measurement, i.e., at a heating rate of dT / dt  ≈ 0.07 K/s and without optical pumping.…”

“…In a previous study, the observed increase in the diffuse background at a time constant of τ heat = 2.2 ps was explained in terms of a transient change of surface lattice temperature through multi-phonon losses of the diffracted electrons. Considering this and the significant different timescales finally led us to the conclusion that the phase transition is non-thermally driven 13 . In the following, we will conclusively verify this statement and demonstrate how collective structural dynamics of driven phase transitions can be disentangled from incoherent lattice excitations by analyzing individual diffraction spots that are present in both phases.…”

“…This causes an unintended linear evolving time delay between pump and probe pulses along the surface. This so-called velocity mismatch was compensated by a tilted pulse front scheme for the laser pump pulse 29,30 ultimately leading to a temporal instrumental response function of the entire experimental setup of 350 fs (full width at half maximum) 13 . For the benefit of an improved signal-to-noise ratio, the number of electrons in the probe pulse was increased at the expense of a slightly reduced temporal resolution of about 1 ps.…”

“…As becomes obvious, the 8th order spots completely vanish upon photoexcitation: the surface undergoes an optically induced transition from the broken-symmetry $false(8\times 2false)$ reconstructed ground state to a high symmetry state with (4 × 1) periodicity. The driving force for this transition is a non-thermal change of the potential energy surface (PES) 13 . At low temperatures and without optical excitation, the PES is essentially described by three distinct minima, 36 with two equivalent minima of the ground state and one energetically excited minimum reflecting the $false(4\times 1false)$ phase [compare inset of Fig.…”

“…Both structural phases are separated from each other by an energy barrier of about 40 meV 37,38 . Shortly after the laser pulse excites the surface, the PES transiently changes 36 and the surface undergoes a phase transition to the energetically favoured $false(4\times 1false)$ structure on sub-ps timescales 13 . Relevant for this transition are specific electronic excitations which couple to two vibrational eigenstates, commonly referred to as the soft shear mode and hexagon rotary mode.…”