Benjamin P. P. Mallett

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron–hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier–Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.

Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO3 films

We present a combined study with conventional far-infrared and time-domain terahertz ellipsometry of the temperature dependent optical response of SrTiO3 thin films (85 and 8.5 nm) that are grown by pulsed-laser deposition on LSAT substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO3. We show that for the 85 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 C, whereas for the 8.5 nm film it is hardy affected. For the latter, after annealing the mode remains at 125 cm-1 at 300 K and exhibits only a weak softening to about 90 cm-1 at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate.

Muon spin rotation study of the magnetic structure in the tetragonal antiferromagnetic state of weakly underdoped Ba1−xKxFe2As2

With muon spin rotation (μSR) we studied the transition between the orthorhombic antiferromagnetic (o-AF) and the tetragonal antiferromagnetic (t-AF) states of a weakly underdoped Ba1−x K x Fe2As2 single crystal. We observed some characteristic changes of the magnitude and the orientation of the magnetic field at the muon site which, due to the fairly high point symmetry of the latter, allow us to identify the magnetic structure of the t-AF state. It is the so-called, inhomogeneous double-Q magnetic structure with c-axis–oriented moments which has a vanishing magnetic moment on half of the Fe sites.

Clocking the onset of bilayer coherence in a high- Tc cuprate

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature

Granular superconductivity and magnetic-field-driven recovery of macroscopic coherence in a cuprate/manganite multilayer

{\mathrm{T}}_{\mathrm{C}}

Infrared ellipsometry study of the confined electrons in a high-mobility γ-Al2O3/SrTiO3 heterostructure

. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of

Anisotropy of infrared-active phonon modes in the monodomain state of tetragonal SrTiO3 (110)

{\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}

Lattice-mediated magnetic order melting in TbMnO3

. The pseudogap correlations were disentangled from the response of laser-broken pairs by clocking their characteristic time scales. The onset of a superconducting precursor state was found at a temperature

Structural, magnetic and electronic properties of pulsed-laser-deposition grown SrFeO3−δ thin films and SrFeO3−δ /La2/3Ca1/3MnO3 multilayers

{\mathrm{T}}_{\mathrm{ONS}}g{\mathrm{T}}_{\mathrm{C}}

Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of

, consistent with the temperature scale identified via static optical spectroscopy. Furthermore, the temperature evolution of the coherent vibration of the Ba ion, strongly renormalized by the onset of superconductivity, revealed a pronounced anomaly at the same temperature