Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pressures

Recent predictions and experimental observations of high $T_c$ superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using modulated laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization of lanthanum superhydride samples, including four-probe electrical transport measurements that display significant drops in resistivity on cooling up to 260 K and 180–200 GPa, and resistivity transitions at both lower and higher temperatures in other experiments. Additional current-voltage measurements, critical current estimates, and low-temperature x-ray diffraction are also obtained. We suggest that the transitions represent signatures of superconductivity to near room temperature in phases of lanthanum superhydride, in good agreement with density functional structure search and BCS theory calculations.

Figure 1

(a) Schematic of the assembly used for synthesis and subsequent conductivity measurements. The sample chamber consisted of a tungsten outer gasket (W) with an insulating cBN insert (B). The piston diamond (P) was coated with four $1\text{−}\mu \mathrm{m}$ thick Pt electrodes which were pressure-bonded to $25\text{−}\mu \mathrm{m}$ thick Pt electrodes (yellow). The $5\text{−}\mu \mathrm{m}$ thick La sample (red) was placed on the Pt electrodes and packed in with ammonia borane (AB, green). Once the synthesis pressure was reached, single-sided laser heating (L) was used to initiate the dissociation of AB and synthesis of the superhydride. To achieve optimal packing of AB in the gasket hole, we loaded AB with the gasket fixed on the cylinder diamond (C). (b) Optical micrograph of a sample at 178 GPa after laser heating using the above procedure (sample A).

Figure 2

Synchrotron x-ray diffraction patterns obtained from three samples in which ${\mathrm{LaH}}_{10\pm x}$ was synthesized by laser heating at pressures above 175 GPa. (a) X-ray diffraction following laser heating of a mixture of La and ${\mathrm{H}}_2$ (Au pressure marker); (b,c) results obtained following laser heating of La and ${\mathrm{NH}}_3{\mathrm{BH}}_3$ (samples A and C; Pt pressure marker). The data were obtained from three separate runs with an x-ray spot size of $5\times 5\mu \mathrm{m}$. In all three panels, the pattern in black is from unreacted La, and that in red is fcc-based ${\mathrm{LaH}}_{10\pm x}$ obtained after laser heating. The sample corresponding to (c) was nominally $5\text{−}\mu \mathrm{m}$ thick and exhibited a high degree of anisotropic stress as indicated by the relative intensities, broadening, and shift of the La peaks prior to the reaction. The diffraction peak marked by the green symbol in (a) is due to residual ${\mathrm{WH}}_x$ which forms when ${\mathrm{H}}_2$ is present. This hydride also gives rise to the tail above 10.5 degrees. ${\mathrm{WH}}_x$ features are absent in the other two patterns because insulating cBN gaskets were used. Further details are provided in Ref. [15].

Figure 3

Normalized electrical resistance of the ${\mathrm{LaH}}_{10\pm x}$ sample characterized by x-ray diffraction and radiography (Figs. 1 and 2) and measured with the four-probe technique (sample A). The initial pressure determined from Raman measurements of the diamond anvil edge was 188 GPa. The lowest resistance we could record was $20\mu \mathrm{\Omega }$, whereas the 300-K value was $50\mathrm{m}\mathrm{\Omega }$. The measurements were performed at 10 mA and 10 kHz; for further details see Ref. [15].

Figure 4

Electrical resistance measurements using the four-probe technique (sample F). At the lowest current of 0.1 mA, the sample resistance displays an abrupt drop (grey curve, first heating cycle), and the transition temperature decreases with increasing current, as shown for 1 mA (blue curve, third heating cycle) and 10 mA (red curve, fifth heating cycle). The inset shows $I$-$V$ curves obtained from this experiment and compares data measured at 300 K (mV scale, left axis) and at 180 K ($\mu \mathrm{V}$ scale, right axis), well below the transition temperature. These data were obtained in situ on the synchrotron beam line with different instrumentation and data collection protocols compared to the data presented in Fig. 3 (sample A). The flat response for temperatures above the transition reflects the dynamic range setting of the instrument needed for these smaller samples, and there is a lower density of points on the $R$-$T$ curves; see Ref. [15] for further details.