arXiv subjects

Jérôme Debray

Publications and source records attributed to Jérôme Debray.

7 recordsLinked to original sources

arXiv2026

Refractive indices of Y$_2$SiO$_5$ in the near-infrared

Y$_2$SiO$_5$ is a reference birefringent material for optical quantum technologies. The refractive index is primarily known in the visible spectrum, whereas this crystal is also used in the near-infrared. We begin by analysing historical measurements, as well as the modelling proposed at the time. The absence of refractive index tabulated values in the near- and mid-infrared ranges motivated us to carry out independent measurements. Using interferometric techniques in the telecom wavelength range, we demonstrate the ability to determine the principal refractive indices and propose a model spanning a broad spectral range based on Sellmeier equations for which we explicitly give the coefficients and discuss the achievable accuracy. Conversely, as an illustration, white-light interference enables us to precisely determine the thickness of a Y$_2$SiO$_5$ thin film on the order of ten micrometers.

physics.optics↗

arXiv2025

Multilayer Crystal Field states from locally broken centrosymmetry

Local charge, spin, or orbital degrees of freedom with intersite interactions are oftentimes sufficient to construct most quantum orders. This is conventionally true for f-electron systems, where the extent of the f-electrons and their associated crystal-electric-field (CEF) states are strongly localized. Here, polarized Raman spectroscopy measurements of a locally non-centrosymmetric compound, CeCoSi, unveil more CEF excitations than expected in the local model. We interpret this as experimental evidence for the entanglement of CEF states between cerium layers. This composite sublattice, spin, and orbital degree of freedom provides an unconsidered means to form novel orders, not only in this system, but in any system exhibiting globally preserved yet locally broken centrosymmetry.

cond-mat.str-el↗

arXiv2024

Aluminum nuclear demagnetization refrigerator for powerful continuous cooling

Many laboratories routinely cool samples to 10 mK, but relatively few can cool condensed matter below 1 mK. Easy access to the microkelvin range would propel fields such as quantum sensors and quantum materials. Such temperatures are achieved with adiabatic nuclear demagnetization. Existing nuclear demagnetization refrigerators (NDR) are ``single-shot'', and the recycling time is incompatible with some sub-mK experiments. Furthermore, a high cooling power is required to overcome the excess heat load of order nW on NDR pre-cooled by cryogen-free dilution refrigerators. We report the performance of an aluminum NDR designed for powerful cooling when part of a dual stage continuous NDR (CNDR). Its thermal resistance is minimized to maximize the cycling rate of the CNDR and consequently its cooling power. At the same time, its susceptibility to eddy current heating is minimized. A CNDR based on two of the aluminum NDR presented here would achieve a cooling power of approximately 40 nW at 560 $μ$K less than six days after cooling from room temperature, with a small offset in electronic temperature that decreases as the time-dependent heat load decays.

physics.ins-det↗

arXiv2021

Disentangling X-ray dichroism and birefringence via high-purity polarimetry

High-brilliance synchrotron radiation sources have opened new avenues for X-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear X-ray polarizers in a crossed setting polarization extinction ratios down to 10$^{-10}$ can be achieved. This renders the method sensitive to probe tiniest optical anisotropies that would occur, for example, in strong-field QED due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and La$_2$CuO$_4$ as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and X-ray lasers, where polarization extinction ratios of 10$^{-12}$ can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.

physics.ins-det↗

arXiv2020

Demonstration of site-selective angular-resolved absorption spectroscopy of the $^{4}I_{15/2} \rightarrow $$^{4}I_{13/2}$ erbium transition in the monoclinic crystal Y$_2$SiO$_5$

We study the angular dependence in polarized light of the optical absorption for the Er$^{3+}$ transition $^{4}I_{15/2} \rightarrow $$^{4}I_{13/2}$ in Y$_2$SiO$_5$, revealing thus the associated anisotropy and the orientation of the related absorption principal directions in the dielectric plan perpendicular to the monoclinic axis b. The measurements are performed at low temperature. This allows us to isolate the lowest crystal field levels in the ground and excited states. We spectrally resolve and independently characterize the two yttrium substitution sites in the Y$_2$SiO$_5$ matrix. The absorption tensor components cannot be unambiguously determined yet while only considering the investigated dielectric plane. Still, measurements remarkably demonstrate that this transition of interest well resolved at low temperature is not only a magnetic-dipole allowed transition but indeed a hybrid electric-magnetic transition.

physics.optics↗

arXiv2019

Hidden magnetism at the pseudogap critical point of a high temperature superconductor

The mysterious pseudogap phase of cuprate superconductors ends at a critical hole doping level p* but the nature of the ground state below p* is still debated. Here, we show that the genuine nature of the magnetic ground state in La2-xSrxCuO4 is hidden by competing effects from superconductivity: applying intense magnetic fields to quench superconductivity, we uncover the presence of glassy antiferromagnetic order up to the pseudogap boundary p* ~ 0.19, and not above. There is thus a quantum phase transition at p*, which is likely to underlie highfield observations of a fundamental change in electronic properties across p*. Furthermore, the continuous presence of quasi-static moments from the insulator up to p* suggests that the physics of the doped Mott insulator is relevant through the entire pseudogap regime and might be more fundamentally driving the transition at p* than just spin or charge ordering.

cond-mat.supr-con↗

arXiv2014

Magneto- to electro-active transmutation of spin waves in ErMnO3

The low energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon only excitable by the electric field of an electromagnetic wave. Using comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electro-active crystal field transition.

cond-mat.str-el↗