Ref HAL: hal-03656679_v1
DOI: 10.1103/physrevapplied.17.044051
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BiFeO3 is a rich room-temperature multiferroic material in which noncollinear antiferromagnetic spin cycloids can be deterministically controlled by an electric field through the magnetoelectric interaction, opening perspectives for low-power reconfigurable antiferromagnetic spintronics. Using a commercial scanning nitrogen-vacancy (N-V) magnetometer, we are able to image two different types of spin cycloids stabilized in strain-engineered BiFeO3 epitaxial thin films. We show that, in these samples harboring two ferroelectric variants, each ferroelectric domain is coupled to a single spin cycloid, giving rise to a zigzag magnetic pattern. These ferroelectric domains can be manipulated at the local scale by piezoresponse force microscopy, allowing the design of micron-sized single domains. Thanks to its coupled optical microscope and fast-imaging capabilities, the scanning N-V magnetometer enables a quick repositioning in such areas of interest. Finally, quantitative imaging on single ferroelectric domains provides insights into the physical parameters of each spin-cycloid type and their impact on the magnetic-stray-field measurements.

Ref HAL: hal-03689421_v1
DOI: 10.1063/5.0091675
WoS: WOS:000793516000006
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We report on the growth of 2D layered indium selenide films on (0001)-oriented sapphire substrates by coevaporation. The θ − 2θ x-ray diffractograms reveal that the (00l) planes are preferentially oriented parallel to the substrate with a tendency to deviate from the 2D stacking as a function of the growth time. The ϕ-scans performed for the (107) and (10 10) orientations of the hexagonal (h) and rhombohedral (r) phases, respectively, reveal that both polytypes coexist in the epitaxial films. We show that the merging of the h-(100), r-(101), h-(101), and r-(102) lines results in different spectral shapes in the θ − 2θ scans according to samples, which gives qualitative information about the contribution of each polytype.

Ref HAL: hal-03684307_v1
DOI: 10.3390/cryst12060782
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The sp2-bonded layered compound boron nitride (BN) exists in more than a handful of different polytypes (i.e., different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype of single crystals extremely tricky. The co-existence of polytypes in a similar crystal leads to the formation of many interfaces and structural defects having a deleterious influence on the internal quantum efficiency of the light emission and on charge carrier mobility. However, despite this, lasing operation was reported at 215 nm, which has shifted interest in sp2- bonded BN from basic science laboratories to optoelectronic and electrical device applications. Here, we describe some of the known physical properties of a variety of BN polytypes and their performances for deep ultraviolet emission in the specific case of second harmonic generation of light.

Ref HAL: hal-03677973_v1
Ref Arxiv: 2105.08075
Ref INSPIRE: 2079933
DOI: 10.1103/PhysRevLett.128.177401
Ref. & Cit.: NASA ADS
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We investigate the magnetic field dependent photo-physics of individual Nitrogen-Vacancy (NV) color centers in diamond under cryogenic conditions. At distinct magnetic fields, we observe significant reductions in the NV photoluminescence rate, which indicate a marked decrease in the optical readout efficiency of the NV's ground state spin. We assign these dips to excited state level anti-crossings, which occur at magnetic fields that strongly depend on the effective, local strain environment of the NV center. Our results offer new insights into the structure of the NVs' excited states and a new tool for their effective characterization. Using this tool, we observe strong indications for strain-dependent variations of the NV's orbital g-factor, obtain new insights into NV charge state dynamics, and draw important conclusions regarding the applicability of NV centers for low-temperature quantum sensing.

Ref HAL: hal-03659802_v1
DOI: 10.1103/PhysRevLett.128.187201
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We report on the formation of topological defects emerging from the cycloidal antiferromagnetic order at the surface of bulk BiFeO3 crystals. Combining reciprocal and real-space magnetic imaging techniques, we first observe, in a single ferroelectric domain, the coexistence of antiferromagnetic domains in which the antiferromagnetic cycloid propagates along different wave vectors. We then show that the direction of these wave vectors is not strictly locked to the preferred crystallographic axes as continuous rotations bridge different wave vectors. At the junctions between the magnetic domains, we observe topological line defects identical to those found in a broad variety of lamellar physical systems with rotational symmetries. Our work establishes the presence of these magnetic objects at room temperature in the multiferroic antiferromagnet BiFeO3, offering new possibilities for their use in spintronics

Ref HAL: hal-03621717_v1
DOI: 10.1103/PhysRevX.12.011057
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Ref HAL: hal-03603645_v1
DOI: 10.1021/acsnano.1c09717
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The presence of metastable Bernal stacking boron nitride is verified by combining second harmonic generation (SHG) and photoluminescence (PL) spectroscopy. The scanning confocal cryomicroscope, operating in the deep-ultraviolet range, shows a one-to-one correlation between inversion symmetry breaking probed by SHG and the detection of an intense PL line at ∼6.035 eV, the specific signature of the noncentrosymmetric Bernal stacking. The coherent character of the Bernal phase in boron nitride crystals is demonstrated by two-photon excitation spectroscopy. Direct and indirect excitons are simultaneously detected in the emission spectrum; they are quasi-degenerate, in agreement with theoretical predictions for Bernal boron nitride. The transition from AA′ to AB stacking is characterized by an intense emission from stacking faults at the grain boundaries of hexagonal and Bernal boron nitride crystals.