Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

Abstract

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe(2)As(2) following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin-phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

Transient Spin Density Wave Order Induced in the Normal State of BaFe2As2by Coherent Lattice Oscillations

“…1(b)). A quantitative analysis of the oscillatory component reveals that it matches the change in the optical conductivity that is induced in the equilibrium state by the development of the SDW gap [5]. This fact represents the most important and surprising result of our study.…”

Terahertz Sum-Frequency Excitation of a Raman-Active Phonon

“…A prominent example is action spectroscopy [47], which relies on detecting modifications the absorbed infrared radiation has induced in the sample. Remarkably, due to the low photon energies required, THz SFE provides a unique tool to steer chemical reactions [1] or phase transitions [7,8] in the electronic ground state, solely driven by Raman-active lattice vibrations. Shaping of THz pulses [48] to multicolor sequences could enable independent phase and amplitude control of the lattice trajectory along multiple normal coordinates, which is challenging with optical pulses accompanied by parasitic electronic excitations [49].…”

“…Many control strategies take advantage of terahertz (THz) modes of the material. An ubiquitous example is lattice vibrations (phonons) whose control by femtosecond laser pulses [4,5] has enabled new pathways to permanent material modification [6], insulator-to-metal transitions [7,8], and magnetization manipulation [2,9].…”

Magnetic interactions in iron superconductors: A review

“…The C 4 symmetry breaking in the (π, 0) antiferromagnetic state is also reflected in the anomalous behavior of specific phonons [60,241,[256][257][258][259][260][261][262][263][264][265][266]. Particularly interesting is the case of the A 1g As-phonon resonance in the Raman spectroscopy upon entering the magnetic phase.…”