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Von Bergmann K.,University of Hamburg | Ternes M.,Max Planck Institute for Solid State Research | Loth S.,Max Planck Institute for Solid State Research | Loth S.,Max Planck Institute for the Structure and Dynamics of Matter | And 2 more authors.
Physical Review Letters | Year: 2015

Spin-resolved scanning tunneling microscopy is employed to quantitatively determine the spin polarization of the magnetic field-split Kondo state. Tunneling conductance spectra of a Kondo-screened magnetic atom are evaluated within a simple model taking into account inelastic tunneling due to spin excitations and two Kondo peaks positioned symmetrically around the Fermi energy. We fit the spin state of the Kondo-screened atom with a spin Hamiltonian independent of the Kondo effect and account for Zeeman splitting of the Kondo peak in the magnetic field. We find that the width and the height of the Kondo peaks scales with the Zeeman energy. Our observations are consistent with full spin polarization of the Kondo peaks, i.e., a majority spin peak below the Fermi energy and a minority spin peak above. © 2015 American Physical Society.

Halpin A.,University of Toronto | Johnson P.J.M.,University of Toronto | Tempelaar R.,Zernike Institute for Advanced Materials | Murphy R.S.,University of Regina | And 4 more authors.
Nature Chemistry | Year: 2014

The observation of persistent oscillatory signals in multidimensional spectra of protein-pigment complexes has spurred a debate on the role of coherence-assisted electronic energy transfer as a key operating principle in photosynthesis. Vibronic coupling has recently been proposed as an explanation for the long lifetime of the observed spectral beatings. However, photosynthetic systems are inherently complicated, and tractable studies on simple molecular compounds are needed to fully understand the underlying physics. In this work, we present measurements and calculations on a solvated molecular homodimer with clearly resolvable oscillations in the corresponding two-dimensional spectra. Through analysis of the various contributions to the nonlinear response, we succeed in isolating the signal due to inter-exciton coherence. We find that although calculations predict a prolongation of this coherence due to vibronic coupling, the combination of dynamic disorder and vibrational relaxation leads to a coherence decay on a timescale comparable to the electronic dephasing time. © 2014 Macmillan Publishers Limited.

Miller R.J.D.,Max Planck Institute for the Structure and Dynamics of Matter | Miller R.J.D.,University of Toronto
Science | Year: 2014

With the recent advances in ultrabright electron and x-ray sources, it is now possible to extend crystallography to the femtosecond time domain to literally light up atomic motions involved in the primary processes governing structural transitions. This review chronicles the development of brighter and brighter electron and x-ray sources that have enabled atomic resolution to structural dynamics for increasingly complex systems. The primary focus is on achieving sufficient brightness using pump-probe protocols to resolve the far-from-equilibrium motions directing chemical processes that in general lead to irreversible changes in samples. Given the central importance of structural transitions to conceptualizing chemistry, this emerging field has the potential to significantly improve our understanding of chemistry and its connection to driving biological processes.

Subedi A.,Max Planck Institute for the Structure and Dynamics of Matter
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

I propose a method for ultrafast switching of ferroelectric polarization using midinfrared pulses. This involves selectively exciting the highest frequency A1 phonon mode of a ferroelectric material with an intense midinfrared pulse. Large amplitude oscillations of this mode provides a unidirectional force to the lattice such that it displaces along the lowest frequency A1 phonon mode coordinate because of a nonlinear coupling of the type gQPQIR2 between the two modes. First-principles calculations show that this coupling is large in perovskite transition-metal oxide ferroelectrics, and the sign of the coupling is such that the lattice displaces in the switching direction. Furthermore, I find that the lowest frequency A1 mode has a large QP3 order anharmonicity, which causes a discontinuous switch of electric polarization as the pump amplitude is continuously increased. © 2015 American Physical Society.

Subedi A.,Max Planck Institute for the Structure and Dynamics of Matter
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

I study the lattice dynamics and electron-phonon coupling in noncentrosymmetric quasi-one-dimensional K2Cr3As3 using density functional theory based first principles calculations. The phonon dispersions show stable phonons without any soft-mode behavior. They also exhibit features that point to a strong interaction of K atoms with the lattice. I find that the calculated Eliashberg spectral function shows a large enhancement around 50 cm-1. The phonon modes that show large coupling involve in-plane motions of all three species of atoms. The q dependent electron-phonon coupling decreases strongly away from the qz=0 plane. The total electron-phonon coupling is large, with a value of λep=3.0, which readily explains the experimentally observed large mass enhancement. © 2015 American Physical Society.

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