Optically Active Spin Defects in Few-Layer Thick Hexagonal Boron Nitride.

Optically active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this Letter, we first demonstrate that the electron spin resonance frequencies of boron vacancy centers (V_{B}^{-}) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of V_{B}^{-} centers with the hBN thickness with a focus on (i) the zero-field splitting parameters, (ii) the optically induced spin polarization rate and (iii) the longitudinal spin relaxation time. This Letter provides important insights into the properties of V_{B}^{-} centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

Isotopic Control of the Boron-Vacancy Spin Defect in Hexagonal Boron Nitride.

We report on electron spin resonance (ESR) spectroscopy of boron-vacancy (V_{B}^{-}) centers hosted in isotopically engineered hexagonal boron nitride (hBN) crystals. We first show that isotopic purification of hBN with ^{15}N yields a simplified and well-resolved hyperfine structure of V_{B}^{-} centers, while purification with ^{10}B leads to narrower ESR linewidths. These results establish isotopically purified h^{10}B^{15}N crystals as the optimal host material for future use of V_{B}^{-} spin defects in quantum technologies. Capitalizing on these findings, we then demonstrate optically induced polarization of ^{15}N nuclei in h^{10}B^{15}N, whose mechanism relies on electron-nuclear spin mixing in the V_{B}^{-} ground state. This work opens up new prospects for future developments of spin-based quantum sensors and simulators on a two-dimensional material platform.

Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer.

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin-orbit effects that enable efficient spin-charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen-vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

Author Correction: Electric and antiferromagnetic chiral textures at multiferroic domain walls.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Electric and antiferromagnetic chiral textures at multiferroic domain walls.

Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls1 and in related objects such as skyrmions2. However, chiral textures should also exist in other types of ferroic materials, such as antiferromagnets, for which theory predicts that they should move faster for lower power3, and ferroelectrics, where they should be extremely small and possess unusual topologies4,5. Here, we report the concomitant observation of antiferromagnetic and electric chiral textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO3. Combining reciprocal and real-space characterization techniques, we reveal the presence of periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically unfavourable chiral ferroelectric domain walls. We discuss the mechanisms underlying their formation and their relevance for electrically controlled topological oxide electronics and spintronics.

Broad Diversity of Near-Infrared Single-Photon Emitters in Silicon.

We report the detection of individual emitters in silicon belonging to seven different families of optically active point defects. These fluorescent centers are created by carbon implantation of a commercial silicon-on-insulator wafer usually employed for integrated photonics. Single photon emission is demonstrated over the 1.1-1.55 µm range, spanning the O and C telecom bands. We analyze their photoluminescence spectra, dipolar emissions, and optical relaxation dynamics at 10 K. For a specific family, we show a constant emission intensity at saturation from 10 K to temperatures well above the 77 K liquid nitrogen temperature. Given the advanced control over nanofabrication and integration in silicon, these individual artificial atoms are promising systems to investigate for Si-based quantum technologies.

Ultrafast Formation of a Charge Density Wave State in 1T-TaS_{2}: Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction.

Femtosecond time-resolved x-ray diffraction is used to study a photoinduced phase transition between two charge density wave (CDW) states in 1T-TaS_{2}, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photoinduced I-CDW phase then develops through a nucleation and growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a nonconservative order parameter.

Laser-Induced Charge-Density-Wave Transient Depinning in Chromium.

We report on time-resolved x-ray diffraction measurements following femtosecond laser excitation in pure bulk chromium. Comparing the evolution of incommensurate charge-density-wave (CDW) and atomic lattice reflections, we show that, a few nanoseconds after laser excitation, the CDW undergoes different structural changes than the atomic lattice. We give evidence for a transient CDW shear strain that breaks the lattice point symmetry. This strain is characteristic of sliding CDWs, as observed in other incommensurate CDW systems, suggesting the laser-induced CDW sliding capability in 3D systems. This first evidence opens perspectives for unconventional laser-assisted transport of correlated charges.

Coherent Population Trapping of a Single Nuclear Spin Under Ambient Conditions.

We demonstrate coherent population trapping of a single nuclear spin in a room-temperature solid. To this end, we exploit a three-level system with a Λ configuration in the microwave domain, which consists of nuclear spin states addressed through their hyperfine coupling to the electron spin of a single nitrogen-vacancy defect in diamond. Moreover, the Λ-scheme relaxation is externally controlled through incoherent optical pumping and separated in time from consecutive coherent microwave excitations. Such a scheme allows us (i) to monitor the sequential accumulation of population into the dark state and (ii) to reach a novel regime of coherent population trapping dynamics for which periodic arrays of dark resonances can be observed, owing to multiple constructive interferences. This Letter offers new prospects for quantum state preparation, information storage in hybrid quantum systems, and metrology.

Surface Effects on the Mott-Hubbard Transition in Archetypal V{2}O{3}.

We present an experimental and theoretical study exploring surface effects on the evolution of the metal-insulator transition in the model Mott-Hubbard compound Cr-doped V{2}O{3}. We find a microscopic domain formation that is clearly affected by the surface crystallographic orientation. Using scanning photoelectron microscopy and x-ray diffraction, we find that surface defects act as nucleation centers for the formation of domains at the temperature-induced isostructural transition and favor the formation of microscopic metallic regions. A density-functional theory plus dynamical mean-field theory study of different surface terminations shows that the surface reconstruction with excess vanadyl cations leads to doped, and hence more metallic, surface states, which explains our experimental observations.

Magnetometry with nitrogen-vacancy defects in diamond.

The isolated electronic spin system of the nitrogen-vacancy (NV) centre in diamond offers unique possibilities to be employed as a nanoscale sensor for detection and imaging of weak magnetic fields. Magnetic imaging with nanometric resolution and field detection capabilities in the nanotesla range are enabled by the atomic-size and exceptionally long spin-coherence times of this naturally occurring defect. The exciting perspectives that ensue from these characteristics have triggered vivid experimental activities in the emerging field of 'NV magnetometry'. It is the purpose of this article to review the recent progress in high-sensitivity nanoscale NV magnetometry, generate an overview of the most pertinent results of the last years and highlight perspectives for future developments. We will present the physical principles that allow for magnetic field detection with NV centres and discuss first applications of NV magnetometers that have been demonstrated in the context of nano magnetism, mesoscopic physics and the life sciences.

Probing the dynamics of a nuclear spin bath in diamond through time-resolved central spin magnetometry.

Using fast electron spin resonance spectroscopy of a single nitrogen-vacancy defect in diamond, we demonstrate real-time readout of the Overhauser field produced by its nuclear spin environment under ambient conditions. These measurements enable narrowing the Overhauser field distribution by postselection, corresponding to a conditional preparation of the nuclear spin bath. Correlations of the Overhauser field fluctuations are quantitatively inferred by analyzing the Allan deviation over consecutive measurements. This method allows us to extract the dynamics of weakly coupled nuclear spins of the reservoir.

Synchronizing the dynamics of a single nitrogen vacancy spin qubit on a parametrically coupled radio-frequency field through microwave dressing.

A hybrid spin-oscillator system in parametric interaction is experimentally emulated using a single nitrogen vacancy (NV) spin qubit immersed in a radio frequency (rf) field and probed with a quasiresonant microwave (MW) field. We report on the MW-mediated locking of the NV spin dynamics onto the rf field, appearing when the MW-driven Rabi precession frequency approaches the rf frequency and for sufficiently large rf amplitudes. These signatures are analogous to a phononic Mollow triplet in the MW rotating frame for the parametric interaction and promise to have impact in spin-dependent force detection strategies.

Charge density waves tuned by biaxial tensile stress.

The precise arrangement and nature of atoms drive electronic phase transitions in condensed matter. To explore this tenuous link, we developed a true biaxial mechanical deformation device working at cryogenic temperatures, compatible with x-ray diffraction and transport measurements, well adapted to layered samples. Here we show that a slight deformation of TbTe3 can have a dramatic influence on its Charge Density Wave (CDW), with an orientational transition from c to a driven by the a/c parameter, a tiny coexistence region near a = c, and without space group change. The CDW transition temperature Tc displays a linear dependence with a / c - 1 while the gap saturates out of the coexistence region. This behaviour is well accounted for within a tight-binding model. Our results question the relationship between gap and Tc in RTe3 systems. This method opens a new route towards the study of coexisting or competing electronic orders in condensed matter.

Single-shot readout of multiple nuclear spin qubits in diamond under ambient conditions.

We use the electronic spin of a single nitrogen-vacancy defect in diamond to observe the real-time evolution of neighboring single nuclear spins under ambient conditions. Using a diamond sample with a natural abundance of (13)C isotopes, we first demonstrate high fidelity initialization and single-shot readout of an individual (13)C nuclear spin. By including the intrinsic (14)N nuclear spin of the nitrogen-vacancy defect in the quantum register, we then report the simultaneous observation of quantum jumps linked to both nuclear spin species, providing an efficient initialization of the two qubits. These results open up new avenues for diamond-based quantum information processing including active feedback in quantum error correction protocols and tests of quantum correlations with solid-state single spins at room temperature.

Counting dislocations in microcrystals by coherent x-ray diffraction.

We present here an unprecedented way of quantifying the number of dislocations in microcrystals. This method relies on a combination of several state-of-the-art techniques: coherent x-ray diffraction used as a local probe, together with the controlled compression of micro-objects. We demonstrate that by using this method, dislocations in the microcrystal can be detected and their number precisely quantified. This cannot be done with other techniques in a nondestructive way. Our method opens a route for the study of many small-scale systems with defect-dependent physical properties and it could become a critical tool for addressing future challenges in nanotechnology.

In situ three-dimensional reciprocal-space mapping during mechanical deformation.

Mechanical deformation of a SiGe island epitaxically grown on Si(001) was studied by a specially adapted atomic force microscope and nanofocused X-ray diffraction. The deformation was monitored during in situ mechanical loading by recording three-dimensional reciprocal-space maps around a selected Bragg peak. Scanning the energy of the incident beam instead of rocking the sample allowed the safe and reliable measurement of the reciprocal-space maps without removal of the mechanical load. The crystal truncation rods originating from the island side facets rotate to steeper angles with increasing mechanical load. Simulations of the displacement field and the intensity distribution, based on the finite-element method, reveal that the change in orientation of the side facets of about 25° corresponds to an applied pressure of 2-3 GPa on the island top plane.

Second-harmonic generation from coupled plasmon modes in a single dimer of gold nanospheres.

We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy.

Creep, flow, and phase slippage regimes: an extensive view of the sliding charge-density wave revealed by coherent X-ray diffraction.

Coherent x-ray diffraction experiments have been used to probe the dynamics of the charge density wave (CDW) in the quasi-1D system NbSe(3). The 2k(F) satellite reflection associated with the CDW has been measured with respect to external dc currents, below and above the depinning current. These measurements illustrate for the first time the extensive behavior of a moving electronic crystal: the creep regime, with nucleation of CDW dislocations, the flow regime, with motional ordering, along with phase slippage and the role of strong pinning due to an extrinsic defect.

Time-resolved structural dynamics of the out-of-equilibrium charge density wave phase transition in GdTe3.

We use ultrafast electron diffraction to study the out-of-equilibrium dynamics of the charge density wave (CDW) phase transition in GdTe3, a quasi-two-dimensional compound displaying a unidirectional CDW state. Experiments were conducted at different incident fluences and different initial sample temperatures below Tc. We find that following photo-excitation, the system undergoes a non-thermal ultrafast phase transition that occurs in out-of-equilibrium conditions. The intrinsic crystal temperature was estimated at each time delay from the atomic thermal motion, which affects each Bragg peak intensity via the Debye Waller factor. We find that the crystal temperature stabilizes with a 6 ps timescale in a quasi-equilibrium state at temperature Tq.e.. We then relate the recovery time of the CDW and its correlation lengths as a function of Tq.e.. The charge density wave is suppressed in less than a picosecond while its recovery time increases linearly with incident fluence and initial temperature. Our results highlight that the dynamics is strongly determined by the initial sample temperature. In addition, the transient CDW phase recently observed along the transverse direction in LaTe3 and CeTe3 is not observed in GdTe3.