Fingerprints of hot-phonon physics in time-resolved correlated quantum lattice dynamics

Abstract

The time dynamics of the energy flow from electronic to lattice degrees of freedom in pump-probe setups could be strongly affected by the presence of a hot-phonon bottleneck, which can sustain longer coherence of the optically excited electronic states. Recently, hot-phonon physics has been experimentally observed and theoretically described in MgB 2 , the electron-phonon based superconductor with T c ≈ 39 K. By employing a combined ab-initio and quantum-field-theory approach and by taking MgB 2 as an example, here we propose a novel path for revealing the presence and characterizing the properties of hot phonons through a direct analysis of the information encoded in the lattice inter-atomic correlations. Such method exploits the underlying symmetry of the E 2g hot modes characterized by a out-of-phase in-plane motion of the two boron atoms. Since hot phonons occur typically at high-symmetry points of the Brillouin zone, with specific symmetries of the lattice displacements, the present analysis is quite general and it could aid in revealing the hot-phonon physics in other promising materials, such as graphene, boron nitride, or black phosphorus.

Properties and challenges of hot-phonon physics in metals: MgB$_2$ and other compounds

“…A similar splitting can be performed for the phonon density of states F (ω) = F hot (ω) + F cold (ω). The percentage of the E 2g hot modes has been estimated to be roughly 5 % of the total phonon modes [223], whereas the relative electronphonon coupling strengths for the hot and cold modes were found to be λ hot = 0.26 and λ cold = 0.34, respectively [113].…”

“…12b), have a strong anticorrelation character, but at thermal equilibrium their contribution is balanced by all the other modes. On the other hand, in the hot-phonon regime the strong increase of the phonon population of the E 2g modes leads to a striking anomaly in ρ B−B [223], which shows a remarkable dip at T max hot Fig. 12c.…”

“…( 18)-( 20) were thus also computed from the first-principles calculations in Refs. [113,223]: C e = 90 J/m 3 K 2 × T e , C hot = 0.13 J/m 3 K, and C cold = 4.1 J/m 3 K, G hot = 2.8 × 10 18 W/m 3 K and G cold = 3.6 × 10 18 W/m 3 K. The relaxation time τ anharm = 400 fs has been estimated from the linewidth of the E 2g modes at Γ, due to the direct anharmonic phonon-phonon scattering [207]. On this ground, within a three-temperature model for a hot-phonon material, the time evolution of characteristic temperatures T e , T hot , T cold was computed upon a pump pulse with a Gaussian profile of duration 45 fs, starting from an initial thermal equilibrium at room temperature.…”

“…where the energy from the electronic degrees of freedom is transferred in a equal way to all the lattice modes, without a preferential channel. In such case, where the hot-phonon bottleneck is absent, employing a two-temperature model the average phonon temperature T ph is predicted to show just a weak and smooth increase up to a final thermalization at about T ph = T e = 420 K [223].…”

“…Along this perspective, an alternative and efficient way for detecting the E 2g hot phonons in MgB 2 has been recently theoretically proposed in Ref. [223]. The basic idea is to exploit similar crystallographic information as the UEDS but, through a Fourier transform, with real-space resolution.…”