In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The ...result is reduced electronic screening and enhanced correlations between Dirac quasiparticles. Here we investigate the electronic structure of ZrSiSe, by combining time- and angle-resolved photoelectron spectroscopy with ab initio density functional theory (DFT) complemented by an extended Hubbard model (DFT + U + V) and by time-dependent DFT + U + V. We show that electronic correlations are reduced on an ultrashort timescale by optical excitation of high-energy electrons-hole pairs, which transiently screen the Coulomb interaction. Our findings demonstrate an all-optical method for engineering the band structure of a quantum material.
We observe a giant spin-orbit splitting in the bulk and surface states of the noncentrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction ...band by controlling the surface termination. In both cases, it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics-a large, robust spin splitting and ambipolar conduction-are present in this material.
The possibility to inject spin currents in topological insulators (TIs) by ultrashort optical pulses has stimulated intense studies of their out-of-equilibrium electronic properties. However, a ...comprehensive description of the electronic relaxation dynamics has been elusive, so far. In order to reveal the role of the bulk and surface states in the microscopic scattering mechanisms, we have investigated, by means of time- and angle-resolved photoemission spectroscopy, a wide set of TIs. These have been selected in order to display different positions of the Fermi energy (EF) within the bulk band structure. When three-dimensional bulk states lie at EF, we observe a fast relaxation dynamics with a characteristic time scale of a few picoseconds (ps). On the contrary, a long lasting excited state is recorded when only the two-dimensional surface state crosses EF. These findings suggest the important role played by spatial diffusion in the direction orthogonal to the surface in governing the relaxation mechanisms. We propose that this electron diffusive mechanism is driven by the optically induced temperature gradient that is at play only for electrons residing in bulk states.
Trigonal tellurium, a small-gap semiconductor with pronounced magneto-electric and magneto-optical responses, is among the simplest realizations of a chiral crystal. We have studied by spin- and ...angle-resolved photoelectron spectroscopy its unconventional electronic structure and unique spin texture. We identify Kramers–Weyl, composite, and accordionlike Weyl fermions, so far only predicted by theory, and show that the spin polarization is parallel to the wave vector along the lines in k space connecting high-symmetry points. Our results clarify the symmetries that enforce such spin texture in a chiral crystal, thus bringing new insight in the formation of a spin vectorial field more complex than the previously proposed hedgehog configuration. Our findings thus pave the way to a classification scheme for these exotic spin textures and their search in chiral crystals.
The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological ...insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi2Se3. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.
We report on the temperature dependence of the ZrTe5 electronic properties, studied at equilibrium and out of equilibrium, by means of time and angle resolved photoelectron spectroscopy. Our results ...unveil the dependence of the electronic band structure across the Fermi energy on the sample temperature. This finding is regarded as the dominant mechanism responsible for the anomalous resistivity observed at T*~160 K along with the change of the charge carrier character from holelike to electronlike. Having addressed these long-lasting questions, we prove the possibility to control, at the ultrashort time scale, both the binding energy and the quasiparticle lifetime of the valence band. These experimental evidences pave the way for optically controlling the thermoelectric and magnetoelectric transport properties of ZrTe5.
The electronic structure of ZrTe5 has been matter of renewed interest aimed at clarifying, along with its topological character, the temperature dependence of the unusual transport properties of this ...material. Here, we report an extensive high resolution Angle Resolved Photoelectron Spectroscopy (ARPES) study unveiling a non-monotonic shift of the bands, when the sample temperature is varied between 16K and 300K. Moreover, the present conventional ARPES and circularly dichroic ARPES measurements reveal the presence of two states at the top of the valence band. The strong ARPES dichroic signal detected in proximity of the Fermi energy has been interpreted as the indication of the presence of spin polarized states, in agreement with the predicted strong topological character of this material.
•The photon energy-dependent linear dichroic signal in GeBi2Te4 is investigated by means of ARPES.•The band parity inversion, theoretically predicted for topological insulators, is revealed.•A ...presumed gap opening and surface resonance effect are observed along the surface states dispersion.
Here we report on experiments performed by linear-dichroism angle resolved photoelectron spectroscopy (LD-ARPES) on septuple layers (SL) topological insulator GeBi2Te4. The linear dichroic signals, both for the valence band states and in the topologically protected surface state are interpreted as a probe of the symmetry of the states. The observation of a change in the LD-ARPES spectrum at the top of VB is attributed to the band parity inversion which is predicted by theory for all materials entering the topologically protected phase. Finally, the opening of a gap in the surface state dispersion along the non-high symmetry direction is reported. This is proposed to originated from the interaction between the surface state and the VB, leading to the formation of surface resonances.
The non-centro-symmetric semiconductor BiTeI exhibits two distinct surface terminations that support spin-split Rashba surface states. Their ambipolarity can be exploited for creating spin-polarized ...p-n junctions at the boundaries between domains with different surface terminations. We use scanning tunneling microscopy (STM) and spectroscopy (STS) to locate such junctions and investigate their atomic and electronic properties. The Te- and I-terminated surfaces are identified owing to their distinct chemical reactivity and an apparent height mismatch of electronic origin. The Rashba surface states are revealed in the STS spectra by the onset of a van Hove singularity at the band edge. Eventually, an electronic depletion is found on interfacial Te atoms, consistent with the formation of a space-charge area in typical p-n junctions.
The quest for investigating the non-equilibrium dynamics of the band structure of strongly-correlated materials over their entire Brillouin zone is a primary objective. However, the actual ultrafast ...UV light sources are not suitable for addressing several critical questions in the field. Here we report on a novel light source generating sub-250 fs, 9.3 eV photon energy light pulses at 250 kHz repetition rate, obtained via third-harmonic generation in Xe of frequency-doubled 50 fs laser pulses at 1.55 eV. By reporting the measured band dispersion of a Cu(111) crystal and the non-equilibrium dynamics of the Bi sub(2)Se sub(3) topological insulator, we prove that this source is suitable for studying the non-equilibrium dynamics of the entire Fermi surface of several complex materials, with high signal statistics and limited space-charge effect.
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