Ultrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides

Abstract

Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behaviour these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe1-xCox)2As2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.

Fluence-dependent femtosecond quasiparticle and Eu2+ spin relaxation dynamics in EuFe2(As,P)Pogrebna1, Mertelj2, Cao3 et al. 2016Phys. Rev. B1100View full textAdd to dashboardCite

“…At the fluences near and above the threshold the recovery slows down to tens of picoseconds near the threshold to beyond a few hundred picoseconds at the highest experimental fluence. On this time scale the bottleneck mechanism cannot be operative and the nematic lattice-strain dynamics, that was observed recently [33] to extend to several 100 ps near the magnetostructural transition temperature, might determine the characteristic time scale at these fluences. However, on the nanosecond time scale the heat diffusion out of the experimental volume also takes place.…”

Isolating lattice from electronic contributions in thermal transport measurements of metals and alloys above ambient temperature and an adiabatic model

“…4,6 To justify the order of probability associated with the WF law, which is inconsistent with recent ultrafast spectroscopic experiments, [9][10][11][12][13][14][15][16][17]26 it was previously argued that the excited electron lacks a state to fall into during electron-electron collisions. 4,6 This picture is incorrect regarding heat transfer because it treats electron population statistics as independent of temperature.…”

“…12,14,26,27 Most experiments are on thin films, although reflectance configurations permit study of single crystals. 11 Lifetimes from spectroscopy describe the duration that the excited electrons remain "hot." Thermalization is rapid, ∼1 fs.…”

“…Furthermore, these experiments show that a hot electron gas will equilibrate internally prior to equilibration with the lattice in diverse types of metals. [11][12][13][14] Although the consequences of the electron temperature exceeding the lattice temperature by hundreds of degrees during these transient excitation experiments 15,16 have received considerable attention, 14,17 ramifications of this discovery on heat transport in bulk metals at ordinary temperatures (above ∼100 K) have been overlooked. This paper quantifies the behavior of electrons and phonons in metals and alloys during timedependent heat transport, thereby providing important constraints for theoretical models and for technological applications.…”

Manipulating femtosecond spin-orbit torques with laser pulse sequences to control magnetic memory states and ringing

“…Femtosecond nonlinear optical spectroscopy offers the time resolution needed to disentangle different order parameters that are strongly coupled in the ground state, based on their different dynamics after "sudden" departure from equilibium. 47,48 Multi-pulse switching protocols based on non-adiabatic quantum excitations can control non-equilibrium phase transitions, by initiating phase dynamics in a controllable way. fs spin-orbit torque as the net spin of the hole Fermi sea bath adjusts to the new non-equilibrium direction of S(t).…”