Imaging magnetic flux trapping in lanthanum hydride using diamond quantum sensors

Abstract

Lanthanum hydride has attracted significant attention in recent years due to its signatures of superconductivity at around 250 K (1, 2). However, the megabar pressures required for synthesize and maintain its state present extraordinary challenges for experiments, particularly in characterizing its Meissner effect (3, 4). The nitrogen-vacancy (NV) center in diamond has emerged as a promising quantum probe to address this problem (5-8), but a gap remains between its working pressure and the pressure required to study the superconducting state of lanthanum hydride (9-12). In this work, using neon gas as the pressure transmitting medium, the working pressure of NV centers is extended to nearly 200 GPa. This quantum probe is then applied to study the Meissner effect of a LaH$_{9.6}$ sample, synthesized by laser heating ammonia borane and lanthanum. A strong magnetic shielding effect is observed, with the transition temperature beginning at around 180 K and completing at 220 K. In addition, magnetic field imaging after field cooling reveals strong flux trapping and significant inhomogeneities within the sample. Our work provides compelling evidence for superconductivity in lanthanum hydride and highlights the importance of spatially resolved techniques in characterizing samples under ultrahigh pressure conditions.

Figures (4)

• Figure 1: Quantum control of NV centers in diamond at nearly 200 GPa.a, Schematic of the high-pressure chamber. A layer of NV centers is created on the diamond anvil by nitrogen ion implantation followed by high-temperature annealing. Optical excitation and fluorescence collection of the NV centers are performed through the transparent diamond window, while microwave pulses are applied via Pt wires placed on the culet. Neon gas is used as the pressure transmitting medium. b, Confocal image of the diamond culet at $P$ = 145 GPa; the measured point is marked as A$_0$. c, Zero-field optically detected magnetic resonance (ODMR) spectra at different pressures. d, Zero-field splitting, D, as a function of pressure. The values of D are extracted by fitting the zero-field ODMR spectra with single or double Lorentzian functions. Pressures are calibrated using the diamond Raman signal (for $P \ge$ 70 GPa) or the known D-P relation (for $P<$ 70 GPa) NVHP2024NC. e, ODMR spectrum at 191 GPa and $\textit{H}_z$ = 77 G. f, Rabi oscillations of NV centers at 191 GPa and $\textit{H}_z$ = 301 G. Inset: ODMR spectra under the same conditions.
• Figure 2: Synthesis and superconductivity of LaH$_{9.6}$.a, Optical image of the sample chamber after laser heating (top) and the 2D synchrotron X-ray diffraction image of lanthanum superhydride at around 150 GPa (bottom). b, XRD data collected at the red point in Fig. \ref{['Fig.2']}a along with the model fitting for the phases of $Fm\overline{3}m$-LaH$_{9.6}$ and $P6_3/mmc$-LaH$_{9}$. The experimental data points, calculated values, and Bragg peak positions are shown as small open circles, thin curves, and vertical sticks, respectively. The top panel shows the cake view of the raw X-ray diffraction patterns. c, Pressure dependent unit cell volume per La atom for the different lanthanum superhydrides. The dashed blue, red and black lines indicate the $P-V$ relation of $Fm\overline{3}m$-LaH$_{10}$, $P6_3/mmc$-LaH$_{9}$ and $Fmmm$-La, respectively. The experimental results for the LaH$_{10}$, LaH$_{9}$ and LaH$_{9.6}$ are shown as blue and red dots. d, Superconducting transitions at different pressures. e, Temperature dependence of the resistance at applied magnetic fields from 0 to 8 T at a pressure of around 153 GPa. f, Upper critical field $H_\text{c2}$ as a function of temperature, fitted with the GL (blue) and WHH model (red). g, Pressure dependence of $T_\text{C,zero}$ for LaH$_{9.6}$ synthesized in this work compared with that of LaH$_{10}$ and LaH$_{x}$ reported in ref. LaH2019Nature.
• Figure 3: Magnetic screening of LaH$_{9.6}$ at 150 GPa.a, Confocal fluorescence image (top) and bright-field image (bottom) of the LaH$_{9.6}$ sample. b--d, ODMR spectra of C$_1$--C$_3$ measured during field heating at $\textit{H}_z$ = 105 G after zero-field cooling. The solid lines are fits with multiple Lorentzian functions. At low temperature, ODMR spectra at C$_1$ (sample center) and C$_2$ (near the sample edge) reveal strong demagnetization and concentration of magnetic flux, respectively. Most other test points (C$_3$ and more data in Fig. S6) show both effects simultaneously--four dips are observed at low temperature, and ODMR splitting extracted from the outer (inner) dips decreases (increases) with temperature. At high temperature, all ODMR spectra display a two-tip feature with identical splitting determined by the external magnetic field. e, ODMR splittings of the examined points as a function of temperature. The outer and inner pairs of dips in the ODMR spectra of C$_3$ are labeled C$_3$-O and C$_3$-I, respectively. f, Schematic illustrating the coexistence of superconducting and non-superconducting regions in the laser-focused area, which produces four-dip ODMR spectra when NV centers with a single orientation are used for measurement.
• Figure 4: Imaging flux trapping using in-situ quantum sensors.a--d, Typical zero-field ODMR spectra (of C$_4$, C$_5$, C$_1$ and C$_3$) measured after ZFD (black) and FC at $\textit{H}_z$ = 105 G (blue). The solid lines are fits with multiple Lorentzian functions. For NV centers at C$_4$, the two ODMR signals are nearly identical, indicating no flux trapping at this position. In contrast, most other measured points exhibit very different zero-field ODMR spectra after FC, indicating strong local flux trapping at these positions. The strength of the residual magnetic field is estimated from the ODMR splitting (see Methods). e, Spatial map of the residual magnetic field $B_z$. The center of the sample shows strong flux trapping. Positions with finite trapped field $B_z$, whose magnitude cannot be estimated, are marked in gray (Fig. S8). f, ODMR spectra of C$_1$ measured during zero-field heating after field cooling at $\textit{H}_z$ = 105 G. g, ODMR splittings of C$_1$ as a function of temperature.