Superconductivity above 200K Observed in Superhydrides of Calcium


 Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature(Tc) above 210 K in calcium superhydrides, the third type hydride experimentally showing superconductivity above 200K in addition to sulfur hydride & rare earth hydride system. The materials are synthesized at the synergetic conditions of 160~190 GPa and ~2000K using diamond anvil cell combined with in-situ laser heating technique. The superconductivity was studied through in situ high pressure resistance measurements in applied magnetic field for the sample quenched from high temperature while maintained at the synthesized pressure. The upper critical field was estimated to be ~268T while the GL coherent length is ~11 Å. The in situ x ray diffractions with synchrotron suggest that the synthesized calcium hydrides are primarily composed of CaH6 while there also exist other calcium hydrids with different hydrogen.


Introduction
The superhydrides received growing attentions recently because of the high temperature superconductivity [1] . It was suggested that superhydrides are likely to be a step toward metallic hydrogen since the "chemically precompressed" hydrogen in the superhydrides can decrease the critical pressure required for the metallization [1][2][3] .
Because of high phonon frequencies and strong electron phonon coupling, high temperature superconductivity was suggested in hydrogen rich materials. Sulfur hydride is one of examples, in which the high Tc superconductivity was reported both theoretically & experimentally at Megbar level pressures [4,5] . In addition, alkali earth, rare earth and other superhydrides are theoretically suggested to possess high temperature superconductivity at high pressures [6][7][8][9][10] . Those metal superhydrides favor to form hydrogen rich coordination such as clathrate like cage structures at high pressure where the metal ions are located in the center of the H clathrats and act as carrier donor. The occupation of the unfilled anti-bonding σ* orbitals of the H 2 molecules by the electrons from the metal ions would enhance to dissociate the H 2 molecules hence to form H clathrats. Recently rare earth Lanthanum (or Yttrium) hydrides are also experimentally reported to be superconductive with Tc exceeds 200 K [12][13][14][15][16] . However, there are no experimental observations for superconductivity above 200 K so far in alkaline earth superhydrides, in spite of the superconductivity with Tc ~20 K was reported in barium hydrides [17] . Here, we report the experimental realization of high Tc superconductivity in calcium superhydrides synthesized at high pressure and high temperature conditions. The superconductivity with Tc~210 K at 160 ~190 GPa was observed. Hence in addition to sulfur hydrides, rare earth hydrides this is the third type of superconducting hydrides with Tc above 200K..

Experiment
The diamond anvil cell techniques are used for both synthesis & measurements of the calcium hydrides. The diamond anvils are of optical pure quality with culet diameter of 100 μm beveled to 300 μm for the experiments. The electrodes for resistance measurements are embedded in the sample chamber before high pressure high temperature synthesis. The experiments are conducted in two steps: (I) synthesis of the calcium hydrides at high pressure high temperature; (II) the follow up measurements of superconductivity for the sample quenched from high temperature but remained at the same pressure. The T301 stainless steel served as gaskets that are prepressed from 250μm to ∼20 μm in thickness at about 20 GPa. A hole of 300 μm in diameter was drilled at the center of imprint. The hole filled with fine insulating cubic boron nitride (cBN) powder mixed with epoxy that was pressed with anvils to form a solid layer of ∼15µm in thickness. A hole of 70 μm in diameter was drilled at the center of the solid cBN layer to serve as the sample chamber. The Pt foils with the thickness of 0.5 μm were deposited on the surface of the culet as the inner electrodes.
The gold wire was attached to the Pt foil to serve as the outside electrodes. The calcium specimen with a shape of 30 μm in width *30 μm in length and 2 μm in thickness was stacked on the inner electrodes. A flake of ammonia borane filled into the sample chamber plays the role of hydrogen source (hydrodizer) while it also the pressure transmitting medium [14] . The sample loading as well as electrodes performance are conducted in the glove box filled with flowing Ar gas with 1ppm oxygen or water trace to avoid moisture or contamination. The setups are followed the ATHENA procedure reported Ref 18.
The sample was first applied to the pressure of 160~190 GPa followed by heating at ~2000K for ~30min. The heating was performed with a pulse laser beam of 1064nm wave length. The spot of laser beam is about 5 μm in diameter with power of 20 W. The heating temperature was estimated by fitting the black body irradiation spectra. The heated sample was quenched from high temperature while maintained at high pressure. A piston cylinder type anvil cell composed of BeCu was adopted in the experiments. The synthesized sample compressed within diamond anvil cell was put into a MagLab system to perform the electric conductivity measurements using a Van der Pauw method with 1 mA applied current. The MagLab system can provide synergetic extreme environments with temperatures from 300 K to 1.5 K and a magnetic field up to 9 T [19] . Pressure was calibrated by the shift of the first order Raman edge frequency from the cutlet of diamond [20,21] .
The synchrotron x-ray diffraction experiments in the diamond anvil cell were carried out at GSECARS of the Advanced Photon Source at the Argonne National Laboratory. The x-ray with λ = 0.3344 Å was focused down to ~3 um diameter spot on the sample. The symmetric diamond anvil cell was used to generate pressure with bevel diamond anvils (100/300um ) and rhenium gasket indented to 25µm. Samples were loaded into the pressure chamber without the insulated layer of cBN and Pt electrodes that were used for resistive measurements. Calcium hydrides was synthesized in situ at 175 GPa with laser heating at 2000K. The sample pressure in these synchrotron X ray diffraction experiments has been determined by both the shift of Raman edge frequency from the cutlet of diamond and the equation of state for rhenium. The X-ray diffraction images are converted to two dimensional diffraction data with Dioptas 20 .
. Fig. 1(a) displays the image of the sample assembly. The dotted red square is the sample shape and the four gray dotted regions are the inner electrodes. Fig. 1(b) shows the measurements of resistance as function of temperature at 160 GPa for sample A. The resistance curves almost coincide for both the cooling and warming cycles. The resistance drops at around 210 K and approaches to zero at low temperature suggesting that the superconducting like transitions take place in the calcium superhydrides. Fig. 2 shows the resistance measurements by application of magnetic fields for sample A. It is seen that the whole transition curves shift to low temperature with the critical temperature gradually suppressed by the applied field, in consistent with the nature of superconductivity. The inset of Fig. 1(b) displays the enlarge view of the derivative of resistance over temperature around the transition region, from which the Tc onset of 210 K is estimated at right side upturn point. In addition to the first transition at ~210K, there are also several drop kinks at ~180 K and ~160 K as shown in Fig.1(b). These drop kinks imply multistep superconducting transitions that are supported by the broaden transition curves shown in Fig.2. In the context of calculation, the calcium hydrides such as CaH 6 , CaH 10 , CaH 12 etc are stable or metastable at high pressure and show high Tc superconductivity [2,6,11] . Hence it is likely that these multistep transitions should be attributed to calcium hydrides with different hydrogen amount, which are probably synthesized due to inhomogeneity of either pressure or hydrogen. It is natural that there exists large pressure gradient in pressure cell at megabar pressure, while the hydrogen inhomogeneity likely arises from the inhomogeneous laser heating to ammonia borane. We synthesized couples of samples to optimize the single transition quality either by reducing sample size in order to improve pressure homogeneity or to heat sample in a more homogeneous scanning manner. Fig. 3(a) presents the resistance as function of pressures measured at 185 GPa for samples B and C, where one step sharp with Tc ~200 K aresachieved.

Results & Discussions
The superconductivity is supported by the measurements at applied magnetic field as shown in Fig. 3(b).
To estimate the upper critical field H c2 (0), the superconductivity as function of magnetic field was investigated based on sample C. The Ginzburg Landau (GL) formula of ( ) = (0)(1 − ( ) ⁄ ) is also used to estimate the upper critical field at zero temperature. As shown in Fig. 4 the fitting of the μ 0 H c2 (T) by GL formula gives a value of H c2 (0) ranging from 131 T to 195 T using criteria of Tc onset , the temperature at 90% and 50% of normal state resistance, respectively. They are comparable to those reported in LaH 10 [12] . Using the obtained value of H c2 (0)=131~268 T, one can roughly estimate the GL coherence length ξ to be 11~16 Å via the equation of μ 0 H c2 (0)= Φ 0 /2πξ 2 , where Φ 0 = 2.067×10 -15 Web is the magnetic flux quantum.
The possible superconducting phases are investigated by high pressure x-ray diffraction experiments and it was suggested that the primary phase of the superconducting sample is composed of Im3m phase of CaH 6 as shown in Fig.   S1-3.We became aware during preparing the paper that an independent work by Ma et al. was carried with the similar results [23] .

Acknowledgments:
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