arXiv subjects

Loïc Toraille

Publications and source records attributed to Loïc Toraille.

9 recordsLinked to original sources

arXiv2026

Pressure Dependence of Ferromagnetism in Uranium Hydride UH$_3$ up to 20 GPa Revealed by NV Centers Magnetometry

The origin of ferromagnetism in the stable $\beta$-UH$_3$ phase is still debated. With a high Curie temperature and a short uranium-uranium interatomic distance, $\beta$-UH$_3$ is placed outside the known systematics of uranium compounds. Pressure provides a powerful means of tuning magnetism toward its suppression, thereby offering valuable insights into its underlying mechanisms, yet the high-pressure behavior of $\beta$-UH$_3$ has remained largely unexplored. Here, we combine in the Diamond Anvil Cell (DAC) the development of pure $\beta$-UH$_3$ synthesis and widefield nitrogen-vacancy (NV) center magnetometry to measure the pressure dependence of the Curie temperature in $\beta$-UH$_3$ up to about 20~GPa. We present two data analysis methods, vector magnetic field reconstruction and statistical analysis of the optically detected magnetic resonance (ODMR) response, which enable us to directly image the magnetic dipole of $\beta$-UH$_3$ during isobaric warming. We observe a linear decrease in the Curie temperature with pressure, yielding a slope of $\text{d}T_C/\text{d}P=-3.86\,(8)$~K/GPa. Ferromagnetism in $\beta$-UH$_3$ is predicted to reach 0~K at approximately 44~GPa, where a quantum critical point may emerge.

cond-mat.mtrl-sci↗

arXiv2026

Optical Stability and Photophysics of NV Centers in Diamond up to 120 GPa

The nitrogen vacancy (NV) center has emerged as a powerful quantum sensor in high-pressure research, with the observation of optically detected magnetic resonance at megabar pressures. However, some aspects of NV physics require further investigation to optimize the development of NV-based sensing under pressure. Here, we study both experimentally and theoretically the optical properties of the NV center under hydrostatic pressure. We investigate the evolution of the zero-phonon line (ZPL) position, radiative lifetimes, optical lineshapes, and photoionization thresholds of the NV center under pressures up to ~120 GPa. We also provide spectroscopic guidelines for performing high-pressure optical experiments. Our results confirm that the NV center remains a robust quantum sensor under extreme hydrostatic pressures, especially for magnetic characterization.

quant-ph↗

arXiv2026

Widefield NV Magnetic Field Reconstruction for Probing the Meissner Effect and Critical Current Density under Pressure

The spatial distribution of a magnetic field can be determined with micrometer resolution using widefield nitrogen vacancy (NV) center magnetic imaging. Nevertheless, reconstructing the magnetic field from the raw data can be challenging due to the degeneracy of the four possible NV axes and the tremendous amount of data. While a qualitative approach is sufficient for most analyses, a quantitative analysis offers deeper insight into the physical system. Here, we apply NV widefield magnetic imaging to a HgBa$_{2}$Ca$_{2}$Cu$_{3}$O$_{8+\delta}$ (Hg-1223) superconducting microcrystal at a pressure of 4 GPa. We fit the results with solutions from the Hamiltonian describing the NV center ground state and take into account the relative intensities of the resonances to determine the local magnetic field magnitude and angle. Thus, we reconstruct the temperature-dependent expulsion of the magnetic field due to the Meissner effect around the superconductor. By comparing the resulting parameters to Brandt's model, which describes the magnetic behavior of a type-II superconductor, we extract the critical current density $j_c$. Overall, this work showcases the first widefield quantitative reconstruction of the Meissner effect under pressure and an optical method to study critical current density. Thus, it provides new insights into the application of NV magnetometry to superconductivity research at high pressures.

cond-mat.supr-con↗

arXiv2025

Imaging the Meissner Effect and Flux Trapping of Superconductors under High Pressure using N-V Centers

Pressure is a key parameter for tuning or revealing superconductivity in materials and compounds. Many measurements of superconducting phase transition temperatures have been conducted using diamond anvil cells (DACs), which provide a wide pressure range and enable concomitant microscopic structural characterization of the sample. However, the inherently small sample volumes in DACs complicate the unambiguous detection of the Meissner effect, the hallmark of superconductivity. Recently, the Meissner effect in superconductors within a DAC was successfully demonstrated using diamond nitrogen-vacancy (N-V) widefield magnetometry, a non-invasive optical technique. In this work, we show that N-V magnetometry can also map superconductivity with micrometer resolution. We apply this technique to a microcrystal of HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg-1223) mercury-based cuprate superconductor under 4 GPa of pressure. The method is capable to detect the magnetic field expulsion and heterogeneities in the sample, visible in a set of characteristic parameters as the local critical temperature $T_{c}$. Flux pinning zones are identified through flux trapping maps. This approach could enable detailed investigations of superconductivity of a broad range of materials under high-pressure conditions.

cond-mat.supr-con↗

arXiv2023

NV center magnetometry up to 130 GPa as if at ambient pressure

Engineering a layer of nitrogen-vacancy (NV) centers on the tip of a diamond anvil creates a multipurpose quantum sensors array for high pressure measurements, especially for probing magnetic and superconducting properties of materials. Expanding this concept above 100 GPa appears to be a substantial challenge. We observe that deviatoric stress on the anvil tip sets a limit at 40-50 GPa for practical magnetic measurements based on optically detected magnetic resonance (ODMR) of NV centers under pressure. We show that this limit can be circumvented up to at least 130 GPa by machining a micropillar on the anvil tip to create a quasi-hydrostatic stress environment for the NV centers. This is quantified using the pressure dependence of the diamond Raman shift, the NV ODMR dependence on applied magnetic field, and NV photoluminescence spectral shift. This paves the way for direct and reliable detection of the Meissner effect in superconductors above 100 GPa, such as super-hydrides.

quant-ph↗

arXiv2022

Optical properties of SiV and GeV color centers in nanodiamonds under hydrostatic pressures up to 180 GPa

We investigate the optical properties of silicon-vacancy (SiV) and germanium-vacancy (GeV) color centers in nanodiamonds under hydrostatic pressure up to 180 GPa. The nanodiamonds were synthetized by Si or Ge-doped plasma assisted chemical vapor deposition and, for our experiment, pressurized in a diamond anvil cell. Under hydrostatic pressure we observe blue-shifts of the SiV and GeV zero-phonon lines by 17 THz (70 meV) and 78 THz (320 meV), respectively. These measured pressure induced shifts are in good agreement with ab initio calculations that take into account the lattice compression based on the equation of state of diamond and that are extended to the case of the tin-vacancy (SnV) center. This work provides guidance on the use of group-IV-vacancy centers as quantum sensors under extreme pressures that will exploit their specific optical and spin properties induced by their intrinsic inversion-symmetric structure.

quant-ph↗

arXiv2020

Combined synchrotron X-ray diffraction and NV diamond magnetic microscopy measurements at high pressure

We report the possibility to simultaneously perform wide-field nitrogen-vacancy (NV) diamond magnetic microscopy and synchrotron X-ray diffraction (XRD) measurements at high pressure. NV color centers are created on the culet of a diamond anvil which is integrated in a diamond anvil cell for static compression of the sample. The optically detected spin resonance of the NV centers is used to map the stray magnetic field produced by the sample magnetization. Using this combined scheme, the magnetic and structural behaviors can be simultaneously measured. As a proof-of-principle, we record the correlated α-Fe to ε-Fe structural and magnetic transitions of iron that occur here between 15 and 20 GPa at 300 K.

cond-mat.mes-hall↗

arXiv2018

Magnetic measurements on micron-size samples under high pressure using designed NV centers

Pressure is a unique tool to tune the interplay between structural, electronic and magnetic interactions. It leads to remarkable properties of materials such as recent temperature records in superconductivity. Advanced magnetic measurements under very high pressure in the Diamond Anvil Cell (DAC) use synchrotron approaches but these are lacking a formal link to the macroscopic magnetic properties. We report an alternative method consisting in optical magnetometry based on nitrogen-vacancy (NV) centers created at the surface of a diamond anvil. We illustrate the method by two measurements realized at room and low temperature respectively: the pressure evolution of the magnetization of an iron bead up to 30 GPa showing the iron ferromagnetic collapse and the detection of the superconducting transition of MgB2 at 7 GPa.

cond-mat.mes-hall↗

arXiv2015

Nitrogen-Vacancy Centers in Diamond for Current Imaging at the Redistributive Layer Level of Integrated Circuits

We present a novel technique based on an ensemble of Nitrogen-Vacancy (NV) centers in diamond to perform Magnetic Current Imaging (MCI) on an Integrated Circuit (IC). NV centers in diamond allow measuring the three components of the magnetic fields generated by a mA range current in an IC structure over a field of 50 x 200 μm^2 with sub-micron resolution. Vector measurements allow using a more robust algorithm than those used for MCI using Giant Magneto Resistance (GMR) or Superconducting Quantum Interference Device (SQUID) sensors and it is opening new current reconstruction prospects. Calculated MCI from these measurements shows a very good agreement with theoretical current path. Acquisition time is around 10 sec, which is much faster than scanning measurements using SQUID or GMR. The experimental set-up relies on a standard optical microscope, and the measurements can be performed at room temperature and atmospheric pressure. These early experiments, not optimized for IC, show that NV centers in diamond could become a real alternative for MCI in IC.

physics.ins-det↗