Collective Excitations In Solids Research Articles

Single-electron charge transfer into putative Majorana and trivial modes in individual vortices

Majorana bound states are putative collective excitations in solids that exhibit the self-conjugate property of Majorana fermions—they are their own antiparticles. In iron-based superconductors, zero-energy states in vortices have been reported as potential Majorana bound states, but the evidence remains controversial. Here, we use scanning tunneling noise spectroscopy to study the tunneling process into vortex bound states in the conventional superconductor NbSe2, and in the putative Majorana platform FeTe0.55Se0.45. We find that tunneling into vortex bound states in both cases exhibits charge transfer of a single electron charge. Our data for the zero-energy bound states in FeTe0.55Se0.45 exclude the possibility of Yu–Shiba–Rusinov states and are consistent with both Majorana bound states and trivial vortex bound states. Our results open an avenue for investigating the exotic states in vortex cores and for future Majorana devices, although further theoretical investigations involving charge dynamics and superconducting tips are necessary.

Cavity-dumping a single infrared pulse from a free-electron laser for two-color pump-probe experiments.

Electromagnetic radiation in the mid- to far-infrared spectral range represents an indispensable tool for the study of numerous types of collective excitations in solids and molecules. Short and intense pulses in this terahertz spectral range are, however, difficult to obtain. While wide wavelength-tunability is easily provided by free-electron lasers, the energies of individual pulses are relatively moderate, on the order of microjoules. Here, we demonstrate a setup that uses cavity-dumping of a free-electron laser to provide single, picosecond-long pulses in the mid- to far-infrared frequency range. The duration of the Fourier-limited pulses can be varied by cavity detuning, and their energy was shown to exceed 100 µJ. Using the aforementioned infrared pulse as a pump, we have realized a two-color pump-probe setup facilitating single-shot time-resolved imaging of magnetization dynamics. We demonstrate the capabilities of the setup first on thermally induced demagnetization and magnetic switching of a GdFeCo thin film and second by showing a single-shot time-resolved detection of resonant phononic switching of the magnetization in a magnetic garnet.

Electron-plasmon interaction induced plasmonic-polaron band replication in epitaxial perovskite SrIrO3 films

Electron-boson interaction is fundamental to a thorough understanding of various exotic properties emerging in many-body physics. In photoemission spectroscopy, photoelectron emission due to photon absorption would trigger diverse collective excitations in solids, including the emergence of phonons, magnons, electron-hole pairs, and plasmons, which naturally provides a reliable pathway to study electron-boson couplings. While fingerprints of electron–phonon/-magnon interactions in this state-of-the-art technique have been well investigated, much less is known about electron-plasmon coupling, and direct observation of the band renormalization solely due to electron-plasmon interactions is extremely challenging. Here by utilizing integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy, we discover the long sought-after pure electron-plasmon coupling-induced low-lying plasmonic-polaron replica bands in epitaxial semimetallic SrIrO3 films, in which the characteristic low carrier concentration and narrow bandwidth combine to provide a unique platform where the electron-plasmon interaction can be investigated kinematically in photoemission spectroscopy. This finding enriches the forms of electron band normalization on collective modes in solids and demonstrates that, to obtain a complete understanding of the quasiparticle dynamics in 5d electron systems, the electron-plasmon interaction should be considered on equal footing with the acknowledged electron–electron interaction and spin–orbit coupling.

Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO2.

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross-section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system contains not only the phonon dispersion but also the so far undetected magnetic correlation function. Here we report the observation of strong magnon–phonon coupling in LiCrO2 that enables the measurement of magnetic correlations throughout the Brillouin zone via IXS. We find electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and heavily damped electromagnons in the paramagnetic phase of LiCrO2. We predict that several (frustrated) magnets with dominant direct exchange and non-collinear magnetism show surprisingly large IXS cross-section for magnons and multi-magnon processes.

Fourier-transform inelastic X-ray scattering from time- and momentum-dependent phonon–phonon correlations

Femtosecond pulses from X-ray free-electron lasers offer a powerful method for observing the coherent dynamic of phonons in crystalline materials, it is now shown. This time-resolved spectroscopic tool could provide insight into low-energy collective excitations in solids and how they interact at a microscopic level to determine the material’s macroscopic properties.

Theory of exciton dephasing in semiconductor quantum dots

A resonant optical excitation creates an excited state population and also induces an optical polarization. Dynamics of this optical excitation is characterized by relaxation of the population as well as decay of the induced optical polarization. In lower dimensional semiconductors, electronic confinement leads to qualitative changes in population relaxation, including spontaneous emission and exciton-phonon scattering, as shown in extensive recent studies [1]. These population relaxation processes are expected to contribute to dephasing with a dephasing rate given by half the population decay rate. Pure dephasing processes that do not involve population or energy relaxation of excitons can also contribute to dephasing. Pure dephasing, which is a well-established concept for atomic systems, remains yet to be investigated in lower-dimensional semiconductors due to a lack of direct comparison between dephasing and population relaxation and between theory and experiment. Studies of pure dephasing processes in lower-dimensional semiconductors will renew and deepen our understanding of dephasing of collective excitations in solids, although several seminal studies were done on the exciton dephasing in quantum well (QW) structures [2–6].

PECULIAR SPIN AND STATISTICS IN TWO SPACE DIMENSIONS

This is an elementary, self-contained review of some peculiarities of quantum mechanics for 2+1 dimensional systems that are at least instructive and perhaps important. In particular, we argue that angular momentum need not be quantized and that statistics interpolating between Fermi and Bose statistics can be constructed. There are possible direct applications to collective excitations in solids and to cosmic strings, and an important moral lesson regarding the “wave function of the universe.”

Fast-electron spectroscopy of collective excitations in solids

Fast-electron spectroscopy of collective excitations in solids