arXiv subjects

M. Grioni

Publications and source records attributed to M. Grioni.

At least 19 recordsLinked to original sources

arXiv2024

Light-induced renormalization of the band structure of chiral tellurium

Chirality in tellurium derives from a Peierls distortion driven by strong electron-phonon coupling, making this material a unique candidate for observing a light-induced topological phase transition. By using time- and angle-resolved photoelectron spectroscopy (trARPES), we reveal that upon near-infrared photoexcitation the Peierls gap is modulated by displacively excited coherent phonons with $\mathrm{A_{1g}}$ symmetry as well as chiral-symmetry-breaking $\mathrm{E'_{LO}}$ modes. By comparison with state-of-the-art TDDFT+U calculations, we reveal the microscopic origin of the in-phase oscillations of band edges, due to phonon-induced modulation of the effective Hubbard $U$ term.

cond-mat.mtrl-sci↗

arXiv2020

Observation of a phonon avalanche in highly photoexcited hybrid perovskite single crystals

In hybrid lead halide perovskites, the coupling between photogenerated charges and the ionic degrees of freedom plays a crucial role in defining the intrinsic limit of carrier mobility and lifetime. However, direct investigation of this fundamental interaction remains challenging because its relevant dynamics occur on ultrashort spatial and ultrafast temporal scales. Here, we unveil the coupled electron-lattice dynamics of a CH3NH3PbI3 single crystal upon intense photoexcitation through a unique combination of ultrafast electron diffraction, time-resolved photoelectron spectroscopy, and time-dependent ab initio calculations. We observe the structural signature of a hot-phonon bottleneck effect that prevents rapid carrier relaxation, and we uncover a phonon avalanche mechanism responsible for breaking the bottleneck. The avalanche involves a collective emission of low-energy phonons - mainly associated with the organic sub-lattice - that proceeds in a regenerative manner and correlates with the accumulation and confinement of photocarriers at the crystal surface. Our results indicate that in hybrid perovskites carrier transport and spatial confinement are key to controlling the electron-phonon interaction and their rational engineering is relevant for future applications in optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Light-induced renormalization of the Dirac quasiparticles in the nodal-line semimetal ZrSiSe

In nodal-line semimetals linearly dispersing states form Dirac loops in the reciprocal space, with high degree of electron-hole symmetry and almost-vanishing 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). 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.

cond-mat.str-el↗

arXiv2019

Evidence of large polarons in photoemission band mapping of the perovskite semiconductor CsPbBr$_3$

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr$_3$ by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass $0.26\pm0.02\,\,m_e$, 50% heavier than the bare mass $m_0 =0.17 m_e$ predicted by density functional theory. Calculations of electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter $α=1.82$. A good agreement with our experimental data is obtained within the Feynmann polaron model, validating a viable theorical method to predict the carrier effective mass of LHPs ab-initio.

cond-mat.mtrl-sci↗

arXiv2017

Enhanced ultrafast relaxation rate in the Weyl semimetal phase of $\mathbf{MoTe_2}$ measured by time-and angle-resolved photoelectron spectroscopy

$\mathrm{MoTe_2}$ has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time- and angle- resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points in the electron dynamics above the Fermi level $\mathrm{E_F}$, by comparing the ultrafast response of $\mathrm{MoTe_2}$ in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound $\mathrm{WTe_2}$ is slower and temperature-independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of $\mathrm{MoTe_2}$.

cond-mat.mtrl-sci↗

arXiv2017

Electronic and magnetic excitations in the "half-stuffed" Cu--O planes of Ba$_2$Cu$_3$O$_4$Cl$_2$ measured by resonant inelastic x-ray scattering

We use resonant inelastic x-ray scattering (RIXS) at the Cu L$_3$ edge to measure the charge and spin excitations in the "half-stuffed" Cu--O planes of the cuprate antiferromagnet Ba$_2$Cu$_3$O$_4$Cl$_2$. The RIXS line shape reveals distinct contributions to the $dd$ excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a CuO$_2$ sublattice, similar to the case of the "fully-stuffed" planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as Sr$_2$CuO$_2$Cl$_2$, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO.

cond-mat.str-el↗

arXiv2016

Emerging one-dimensionality from self-organization of electrons in NbSe$_3$

Materials where the electron filling is close to commensurate filling provide one of the great challenges in materials science. Several proposals of unconventional orderings, where the electronic liquid self-organizes into components with distinct properties, were recently put forward, in particular in cuprates and pnictides where electronic nematic orders have been observed. The electrons self-organization is expected to yield complex intra and inter unit cell patterns, and a reduction of dimensionality. Nevertheless, an unambiguous experimental proof of such complex orders, namely the direct observation of distinct dispersions, is still missing. Here we report a Nano Angle Resolved Photo-emission Spectroscopy (Nano-ARPES) study of NbSe$_3$, a material that has been considered a paradigm of charge order. The new data (Fig.1) invalidate the canonical picture of imperfect nesting and reveals the emergence of a novel order. The electrons self-organization uncovers the one-dimensional (1D) physics hidden in a material which naively should be the most 3D of all columnar chalcogenides.

cond-mat.str-el↗

arXiv2016

Hallmarks of Hund's coupling in the Mott insulator Ca$_2$RuO$_4$

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realised in Ca$_2$RuO$_4$. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide -- using angle-resolved photoemission electron spectroscopy -- the band structure of the paramagnetic insulating phase of Ca$_2$RuO$_4$ and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling $J=0.4$ eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilisation of the d$_{xy}$ orbital due to $c$-axis contraction is shown to be important in explaining the nature of the insulating state. It is thus a combination of multiband physics, Coulomb interaction and Hund's coupling that generates the Mott insulating state of Ca$_2$RuO$_4$. These results underscore the importance of Hund's coupling in the ruthenates and related multiband materials.

cond-mat.str-el↗

arXiv2016

Ultrafast Optical Control of the Electronic Properties of $ZrTe_5$

We report on the temperature dependence of the $ZrTe_5$ 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* $\sim$ 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 $ZrTe_5$.

cond-mat.mtrl-sci↗

arXiv2015

Electron scattering, charge order, and pseudogap physics in La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$: An angle resolved photoemission spectroscopy study

We report an angle-resolved photoemission study of the charge stripe ordered La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ system. A comparative and quantitative line shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side ($x=0.20$), a normal state anti-nodal spectral gap opens upon cooling below ~ 80 K. In this process spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different lineshape is observed in the antinodal region of charge ordered Nd-LSCO $x=1/8$. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral weight redistribution and gapping %of spectral weight originating from charge stripe ordering.

cond-mat.supr-con↗

arXiv2015

Spin-Orbit-Induced Orbital Excitations in Sr2RuO4 and Ca2RuO4: A Resonant Inelastic X-ray Scattering Study

High-resolution resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism X-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p-orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λ_{so}=200~meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

cond-mat.str-el↗

arXiv2014

The momentum and photon energy dependence of the circular dichroic photoemission in the bulk Rashba semiconductors BiTeX (X = I, Br, Cl)

Bulk Rashba systems BiTeX (X = I, Br, Cl) are emerging as important candidates for developing spintronics devices, because of the coexistence of spin-split bulk and surface states, along with the ambipolar character of the surface charge carriers. The need of studying the spin texture of strongly spin-orbit coupled materials has recently promoted circular dichroic Angular Resolved Photoelectron Spectroscopy (cd-ARPES) as an indirect tool to measure the spin and the angular degrees of freedom. Here we report a detailed photon energy dependent study of the cd-ARPES spectra in BiTeX (X = I, Br and Cl). Our work reveals a large variation of the magnitude and sign of the dichroism. Interestingly, we find that the dichroic signal modulates differently for the three compounds and for the different spin-split states. These findings show a momentum and photon energy dependence for the cd-ARPES signals in the bulk Rashba semiconductor BiTeX (X = I, Br, Cl). Finally, the outcome of our experiment indicates the important relation between the modulation of the dichroism and the phase differences between the wave-functions involved in the photoemission process. This phase difference can be due to initial or final state effects. In the former case the phase difference results in possible interference effects among the photo-electrons emitted from different atomic layers and characterized by entangled spin-orbital polarized bands. In the latter case the phase difference results from the relative phases of the expansion of the final state in different outgoing partial waves.

cond-mat.mtrl-sci↗

arXiv2014

Momentum resolved spin dynamics of bulk and surface excited states in the topological insulator $\mathrm{Bi_{2}Se_{3}}$

The prospective of optically inducing a spin polarized current for spintronic devices has generated a vast interest in the out-of-equilibrium electronic and spin structure of topological insulators (TIs). In this Letter we prove that only by measuring the spin intensity signal over several order of magnitude in spin, time and angle resolved photoemission spectroscopy (STAR-PES) experiments is it possible to comprehensively describe the optically excited electronic states in TIs materials. The experiments performed on $\mathrm{Bi_{2}Se_{3}}$ reveal the existence of a Surface-Resonance-State in the 2nd bulk band gap interpreted on the basis of fully relativistic ab-initio spin resolved photoemission calculations. Remarkably, the spin dependent relaxation of the hot carriers is well reproduced by a spin dynamics model considering two non-interacting electronic systems, derived from the excited surface and bulk states, with different electronic temperatures.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic Order in the 2D Heavy-Fermion System CePt2In7 studied by muSR

The low-temperature microscopic magnetic properties of the quasi-2D heavyfermion compound, CePt2In7 are investigated by using a positive muon-spin rotation and relaxation (?muSR) technique. Clear evidence for the formation of a commensurate antiferromagnetic order below TN=5.40 K is presented. The magnetic order parameter is shown to fit well to a modified BSC gap-energy function in a strong-coupling scenario.

cond-mat.str-el↗

arXiv2014

Nodal Landau Fermi-Liquid Quasiparticles in Overdoped La$_{1.77}$Sr$_{0.23}$CuO$_4$

Nodal angle resolved photoemission spectra taken on overdoped La$_{1.77}$Sr$_{0.23}$CuO$_4$ are presented and analyzed. It is proven that the low-energy excitations are true Landau Fermi-liquid quasiparticles. We show that momentum and energy distribution curves can be analyzed self-consistently without quantitative knowledge of the bare band dispersion. Finally, by imposing Kramers-Kronig consistency on the self-energy $Σ$, insight into the quasiparticle residue is gained. We conclude by comparing our results to quasiparticle properties extracted from thermodynamic, magneto-resistance, and high-field quantum oscillation experiments on overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$.

cond-mat.supr-con↗

arXiv2014

Atomic and Electronic Structure of a Rashba $p$-$n$ Junction at the BiTeI Surface

The non-centrosymmetric 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/spectroscopy (STM/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.

cond-mat.mes-hall↗

arXiv2013

Evidence of reduced surface electron-phonon scattering in the conduction band of Bi_{2}Se_{3} by non-equilibrium ARPES

The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electron-phonon scattering at the \emph{surface}. By comparing time-resolved ARPES measurements of the TI Bi_{2}Se_{3} with different probing depths we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.

cond-mat.mes-hall↗

arXiv2013

The electronic structure of the high-symmetry perovskite iridate Ba2IrO4

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba$_2$IrO$_4$ (Ba-214), an antiferromagnetic ($T_N=230$ K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO$_6$ octahedra that is present in its sister compound Sr$_2$IrO$_4$ (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above $T_N$, and favour a Mott over a Slater scenario for this compound.

cond-mat.str-el↗