Coherent and Dissipative Coupling in a Magnetomechanical System.

Hybrid elastic and spin waves hold promises for energy-efficient and versatile generation and detection of magnetic signals, with potentially long coherence times. Here we report on the combined elastic and magnetic dynamics in a one-dimensional magnetomechanical crystal composed of an array of magnetic nanostripes. Phononic and magnonic modes are impulsively excited by an optical ultrafast trigger and their decay is monitored by time-resolved magneto-optical Kerr effect. Complementary Brillouin light scattering measurements and micromagnetic simulations concur in a unified picture, in which the strength and degree of mixing of coherent and dissipative coupling of the quasiparticles are determined quantitatively.

Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3van der Waals Mott ferromagnet.

Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the VL2,3edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of thet2g2ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn-Teller mechanism. Our results clarify the role of the distortion in VI3and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.

Dual pulsed laser deposition system for the growth of complex materials and heterostructures.

Here, we present an integrated ultra-high-vacuum (UHV) apparatus for the growth of complex materials and heterostructures. The specific growth technique is the Pulsed Laser Deposition (PLD) by means of a dual-laser source based on an excimer KrF ultraviolet and solid-state Nd:YAG infra-red lasers. By taking advantage of the two laser sources-both lasers can be independently used within the deposition chambers-a large number of different materials-ranging from oxides to metals, to selenides, and others-can be successfully grown in the form of thin films and heterostructures. All of the samples can be in situ transferred between the deposition chambers and the analysis chambers by using vessels and holders' manipulators. The apparatus also offers the possibility to transfer samples to remote instrumentation under UHV conditions by means of commercially available UHV-suitcases. The dual-PLD operates for in-house research as well as user facility in combination with the Advanced Photo-electric Effect beamline at the Elettra synchrotron radiation facility in Trieste and allows synchrotron-based photo-emission as well as x-ray absorption experiments on pristine films and heterostructures.

Multidetection scheme for transient-grating-based spectroscopy.

Time-resolved optical spectroscopy represents an effective non-invasive approach to investigate the interplay of different degrees of freedom, which plays a key role in the development of novel functional materials. Here, we present magneto-acoustic data on Ni thin films on SiO2 as obtained by a versatile pump-probe setup that combines transient grating spectroscopy with time-resolved magnetic polarimetry. The possibility to easily switch from a pulsed to continuous wave probe allows probing of acoustic and magnetization dynamics on a broad time scale, in both transmission and reflection geometry.

An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials.

Here, we present an integrated ultra-high vacuum apparatus-named MBE-Cluster -dedicated to the growth and in situ structural, spectroscopic, and magnetic characterization of complex materials. Molecular Beam Epitaxy (MBE) growth of metal oxides, e.g., manganites, and deposition of the patterned metallic layers can be fabricated and in situ characterized by reflection high-energy electron diffraction, low-energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy, and azimuthal longitudinal magneto-optic Kerr effect. The temperature can be controlled in the range from 5 K to 580 K, with the possibility of application of magnetic fields H up to ±7 kOe and electric fields E for voltages up to ±500 V. The MBE-Cluster operates for in-house research as well as user facility in combination with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator facility for time-resolved spectroscopy.

Author Correction: Layer-dependent quantum cooperation of electron and hole states in the anomalous semimetal WTe2.

This Article contains an error in the spelling of the author A. Yazdani, which is incorrectly given as A. Yadzani. The error has not been fixed in the original PDF and HTML versions of the Article.

A reaction cell for ambient pressure soft x-ray absorption spectroscopy.

We present a new experimental setup for performing X-ray Absorption Spectroscopy (XAS) in the soft X-ray range at ambient pressure. The ambient pressure XAS setup is fully compatible with the ultra high vacuum environment of a synchrotron radiation spectroscopy beamline end station by means of ultrathin Si3N4 membranes acting as windows for the X-ray beam and seal of the atmospheric sample environment. The XAS detection is performed in total electron yield (TEY) mode by probing the drain current from the sample with a picoammeter. The high signal/noise ratio achievable in the TEY mode, combined with a continuous scanning of the X-ray energies, makes it possible recording XAS spectra in a few seconds. The first results show the performance of this setup to record fast XAS spectra from sample surfaces exposed at atmospheric pressure, even in the case of highly insulating samples. The use of a permanent magnet inside the reaction cell enables the measurement of X-ray magnetic circular dichroism at ambient pressure.

Three-Dimensional Electronic Structure of the Type-II Weyl Semimetal WTe_{2}.

By combining bulk sensitive soft-x-ray angular-resolved photoemission spectroscopy and first-principles calculations we explored the bulk electron states of WTe_{2}, a candidate type-II Weyl semimetal featuring a large nonsaturating magnetoresistance. Despite the layered geometry suggesting a two-dimensional electronic structure, we directly observe a three-dimensional electronic dispersion. We report a band dispersion in the reciprocal direction perpendicular to the layers, implying that electrons can also travel coherently when crossing from one layer to the other. The measured Fermi surface is characterized by two well-separated electron and hole pockets at either side of the Γ point, differently from previous more surface sensitive angle-resolved photoemission spectroscopy experiments that additionally found a pronounced quasiparticle weight at the zone center. Moreover, we observe a significant sensitivity of the bulk electronic structure of WTe_{2} around the Fermi level to electronic correlations and renormalizations due to self-energy effects, previously neglected in first-principles descriptions.

Quantifying the critical thickness of electron hybridization in spintronics materials.

In the rapidly growing field of spintronics, simultaneous control of electronic and magnetic properties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevant effects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depth-dependent analysis of two paradigmatic systems, namely La1-xSrxMnO3 and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface, respectively, for (Ga,Mn)As and La1-xSrxMnO3, for fully restoring bulk properties.

Design and optimization of a modular setup for measurements of three-dimensional spin polarization with ultrafast pulsed sources.

ULTRASPIN is an apparatus devoted to the measurement of the spin polarization (SP) of electrons ejected from solid surfaces in a UHV environment. It is designed to exploit ultrafast light sources (free electron laser or laser high harmonic generation) and to perform (photo)electron spin analysis by an arrangement of Mott scattering polarimeters that measure the full SP vector. The system consists of two interconnected UHV vessels: one for surface science sample cleaning treatments, e-beam deposition of ultrathin films, and low energy electron diffraction/AES characterization. The sample environment in the polarimeter allows for cryogenic cooling and in-operando application of electric and magnetic fields. The photoelectrons are collected by an electrostatic accelerator and transport lens that form a periaxial beam that is subsequently directed by a Y-shaped electrostatic deflector to either one of the two orthogonal Mott polarimeters. The apparatus has been designed to operate in the extreme conditions of ultraintense single-X-ray pulses as originated by free electron lasers (up to 1 kHz), but it allows also for the single electron counting mode suitable when using statistical sources such as synchrotron radiation, cw-laser, or e-gun beams (up to 150 kcps).

Layer-dependent quantum cooperation of electron and hole states in the anomalous semimetal WTe2.

The behaviour of electrons and holes in a crystal lattice is a fundamental quantum phenomenon, accounting for a rich variety of material properties. Boosted by the remarkable electronic and physical properties of two-dimensional materials such as graphene and topological insulators, transition metal dichalcogenides have recently received renewed attention. In this context, the anomalous bulk properties of semimetallic WTe2 have attracted considerable interest. Here we report angle- and spin-resolved photoemission spectroscopy of WTe2 single crystals, through which we disentangle the role of W and Te atoms in the formation of the band structure and identify the interplay of charge, spin and orbital degrees of freedom. Supported by first-principles calculations and high-resolution surface topography, we reveal the existence of a layer-dependent behaviour. The balance of electron and hole states is found only when considering at least three Te-W-Te layers, showing that the behaviour of WTe2 is not strictly two dimensional.

Understanding the electronic structure of IrO2 using hard-X-ray photoelectron spectroscopy and density-functional theory.

The electronic structure of IrO2 has been investigated using hard x-ray photoelectron spectroscopy and density-functional theory. Excellent agreement is observed between theory and experiment. We show that the electronic structure of IrO2 involves crystal field splitting of the iridium 5d orbitals in a distorted octahedral field. The behavior of IrO2 closely follows the theoretical predictions of Goodenough for conductive rutile-structured oxides [J. B. Goodenough, J. Solid State Chem. 3, 490 (1971).

Electric control of magnetism at the Fe/BaTiO3 interface.

Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO3 system, only tiny changes of the interfacial Fe magnetic moment upon reversal of the BaTiO3 dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO3 system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO3 polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO3 produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices.

Identifying the electronic character and role of the Mn states in the valence band of (Ga,Mn)As.

We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, for both low (1%) and high (13%) Mn doping values, the electronic character of the states near the top of the valence band. Magnetization and temperature-dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.

Proximity effects and exchange bias in Co/MnF2(111) heterostructures studied by x-ray magnetic circular dichroism.

Cobalt nano-structured ultrathin films were grown on orthorhombic MnF(2) by molecular beam epitaxy on CaF(2) epitaxial layers deposited on Si(111) substrates. The Co film was grown at room temperature. It was found to be polycrystalline, forming nano-islands with height≈diameter≤10 nm. X-ray absorption evidences the chemical stability of the Co/MnF(2) interface. Remarkably, x-ray magnetic circular dichroism (XMCD) demonstrates that the Co induces a net magnetization on the Mn ions close to the interface. The magnetic moments of these Mn ions couple antiparallel to the Co and rotate upon field reversal following the magnetization of the Co both below and high above the Néel temperature of MnF(2) (T(N) = 67 K). The density of coupled Mn moments is found to be temperature dependent, with an equivalent thickness of ~1.5 MnF(2) monolayers at 20 K, decreasing to about ~0.5 ML as the temperature is raised to 300 K. Interestingly, the intensity of the Mn XMCD signal appears to be related to the coercivity of the Co layer. This behavior is interpreted in terms of the competition between thermal fluctuations, exchange coupling between Co and Mn at the interface and, at low temperature, the antiferromagnetic order in MnF(2).

Growth of ultrathin epitaxial Fe/MgO spin injector on (0, 0, 1) (Ga, Mn)As.

We have grown an ultrathin epitaxial Fe/MgO bilayer on (Ga, Mn)As by e-beam evaporation in UHV. The system structure has been investigated by high resolution transmission electron microscopy (TEM) experiments which show that the Fe and MgO films, covering completely the (Ga, Mn)As, grow with the epitaxial relationship Fe[100](001) [parallel] MgO[110](001) [parallel] (Ga,Mn)As[110](001). The magnetic reversal process, studied by the magneto-optical Kerr effect (MOKE) at room temperature, demonstrates that the iron is ferromagnetic and possesses a cubic anisotropy, confirming the epitaxy relationship found with TEM. Resistivity measurements across the barrier display a non-Ohmic behavior characterized by cubic conductance as a function of the applied voltage suggesting tunneling-dominated transport across the barrier.

Bulk electronic structure of the dilute magnetic semiconductor Ga(1-x)Mn(x)As through hard X-ray angle-resolved photoemission.

A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.

Identification of different electron screening behavior between the bulk and surface of (Ga,Mn)As.

We report x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of both temperature and Mn doping. Analysis of Mn 2p core level spectra reveals the presence of a distinct electronic screening channel in the bulk, hitherto undetected in more surface sensitive analysis. Comparison with model calculations identifies the character of the Mn 3d electronic states and clarifies the role, and the difference between surface and bulk, of hybridization in mediating the ferromagnetic coupling in (Ga,Mn)As.

The attenuation length of low energy electrons in Yb.

Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV are measured for in situ grown polycrystalline Yb films. By comparing bulk and surface core level shifted 4f components we give an estimation of the effective attenuation length (EAL) for low energy (6-20 eV) electrons in Yb, establishing a moderate increase of the EAL upon electron energy decrease. The experimental EAL data are found to be a factor of four smaller than those predicted from the so-called 'universal curve'.