CHIN.PHYS. LETT. Vol. 37, No. 10 (2020) 107401

Superconductivity of Lanthanum Superhydride Investigated Using the StandardFour-Probe Configuration under High Pressures

Fang Hong(ö)1†, Liuxiang Yang(3)2†, Pengfei Shan(ü+)1,3, Pengtao Yang(C�)1,Ziyi Liu(4f´)1, Jianping Sun(ï²)1, Yunyu Yin(Ð�)1, Xiaohui Yu(u¡)1,3∗,

Jinguang Cheng(§71)1,3∗, and Zhongxian Zhao(ë§p)1,3

1Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences,Beijing 100190, China

2Center for High Pressure Science & Technology Advanced Research, Beijing 100094, China3School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China

(Received 16 September 2020; accepted 23 September 2020; published online )

Recently, the theoretically predicted lanthanum superhydride, LaH10±δ, with a clathrate-like structure was suc-cessfully synthesized and found to exhibit a record high superconducting transition temperature Tc ≈ 250K at∼170 GPa, opening a new route for room-temperature superconductivity. However, since in situ experimentsat megabar pressures are very challenging, few groups have reported the ∼250K superconducting transition inLaH10±δ. Here, we establish a simpler sample-loading procedure that allows a relatively large sample size forsynthesis and a standard four-probe configuration for resistance measurements. Following this procedure, wesuccessfully synthesized LaH10±δ with dimensions up to 10× 20µm2 by laser heating a thin La flake and ammo-nia borane at ∼1700K in a symmetric diamond anvil cell under the pressure of 165GPa. The superconductingtransition at Tc ≈ 250 K was confirmed through resistance measurements under various magnetic fields. Ourmethod will facilitate explorations of near-room-temperature superconductors among metal superhydrides.

PACS: 74.70.−b, 81.40.Vw, 62.50.−p, 88.30.rd DOI: 10.1088/0256-307X/37/10/107401

Room-temperature superconductivity (RTSC) hasbeen a coveted goal since the discovery of super-conductivity in 1911. After almost 110 years ofexploration, RTSC remains among the most chal-lenging problems and has attracted enduring en-thusiasm among researchers in condensed matterphysics and materials science.[1−4] The discovery ofcuprate and iron-based high-critical-temperature (Tc)superconductors,[5−7] which are beyond the conven-tional Bardeen–Cooper–Schrieffer (BCS) theory, wasthought to open a possible route toward RTSC. How-ever, the highest attainable Tc (134K[8]) at ambientpressure (164K at ∼30GPa[9]) has remained far belowroom temperature for almost three decades. In ad-dition, a well-accepted theoretical description of themicroscopic mechanism of unconventional supercon-ductivity is still lacking.

Following the seminal work of Ashcroft,[10−12]

other researchers have searched for phonon-mediatedRTSC in metallic hydrogen or hydrogen-rich mate-rials. According to the BCS theory, high phononfrequencies, large electronic density of states near

the Fermi level, and strong electron–phonon cou-pling benefit high-Tc superconductivity in hydrogen-dominated materials because hydrogen is the lightestelement.[10] Although the study on metallic hydrogenhas progressed with the development of high-pressuretechniques based on the diamond anvil cell (DAC),whether metallic hydrogen is actually formed has notbeen verified,[13] even under pressures up to ∼400GPa(the current pressure limit of DACs[14]). However,important breakthroughs in hydrogen-rich materialshave been reported in recent years. The discoveryof superconductivity with Tc = 203K in H3S underhigh pressures[15] provided the first experimental con-firmation for the prediction power of the BCS theoryfor high-Tc superconductivity, reigniting hopes towardachieving RTSC. Accordingly, the stability of crys-tal structure and the superconducting properties ofnearly all binary metal hydrides have been theoreti-cally studies.[1,3,16,18]

Motivated by the theoretical prediction of near-RTSC in rare-earth superhydrides with a clathrate-like structure,[2] two groups have successfully synthe-

Supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB33000000 andXDB25000000), the Beijing Natural Science Foundation (Grant No. Z190008), the National Natural Science Foundation of China(Grant Nos. 11575288, 11921004, 11888101, 11904391, 11834016 and 11874400), the National Key R&D Program of China (GrantNos. 2016YFA0401503 and 2018YFA0305700), and the Youth Innovation Promotion Association, the Key Research Program ofFrontier Sciences and the Interdisciplinary Innovation Team of Chinese Academy of Sciences (Grant Nos. 2016006, JCTD-2019-01,and QYZDBSSW-SLH013).†Fang Hong and Liuxiang Yang contributed equally to this work.∗Corresponding authors. Email: yuxh@iphy.ac.cn; jgcheng@iphy.ac.cn© 2020 Chinese Physical Society and IOP Publishing Ltd

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sized lanthanum superhydride, LaH10±δ, via direct re-action of La metal with hydrogen or ammonia bo-rane (AB) in DAC upon laser heating at megabarpressures. In 2019, they reported a record high Tc(∼250–260K) at ∼170–190GPa.[19,20] The composi-tion stoichiometry of LaH10±δ (−1 < δ < 2) and itscrystal structure were determined via synchrotron x-ray diffraction and were confirmed to match well withthe theoretical prediction.[2,17,21] Subsequently, manyother rare-earth superhydrides have been experimen-tally explored and high-Tc superconductivity has beendiscovered in YH9 (Tc = 243K at 201GPa[22]) andThH10 (Tc = 161K at 175GPa[23]).

However, the above-mentioned studies onmetastable rare-earth superhydrides were conductedat ultrahigh pressures and required both in situ syn-thesis with laser heating and superconductivity ver-ification by electrical resistance measurements. Theexperimental procedures are complicated and the tinysamples (typically of dimensions 5–10µm) are diffi-cult to handle. In this study, we developed a simplersample-loading procedure that obtains relatively largesamples in DAC and successfully synthesizes LaH10±δwith a high Tc (∼ 250K) at 165GPa. Our method issuitable for exploring RTSC in metal superhydridesunder high pressures.

As already mentioned, the in situ synthesis andsubsequent resistance measurements of LaH10±δ at

megabar pressures are of challenge because of thesmall culet and sample size in DACs. Our LaH10±xwas synthesized in a symmetric DAC under pulsedlaser heating, and the resistance measurements wereperformed using the standard four-probe method.The diamond anvils used in this study have a culetof 70µm and were beveled at ∼8◦ from a primitivediameter of 300µm. Sample loading with the properelectrode configuration was crucial for a successful ex-periment. In previous in situ syntheses of LaH10±δ,the hydrogen source was pure hydrogen or AB.[19,20]

Although pure hydrogen loading increases the hydro-gen content (and hence the Tc) of the formed LaH10±δ,the hydrogen gas-loading system is complicated andinaccessible to many research groups. In addition,the large shrinkage of the gasket hole caused by thehigh compressibility of hydrogen can easily damagethe electrodes used for resistance measurements. Onthe contrary, AB is relatively stable under ambientconditions and small amounts of tiny AB crystals aresafe to handle. AB decomposes when heated, releasinghydrogen even under megabar pressures. The volumechange of compressed AB is smaller than that of load-ing hydrogen. The effectiveness of AB as the hydrogensource for synthesizing LaH10±δ with Tc over 250Khas been demonstrated in a recent study.[19] Hence, inthis study, AB was employed as the hydrogen source.

(a) (b) (c) (d)Sample hole

Culet

Beveled stage

Step 1: Step 2: Step 3: Step 4:Drill a hole in the gasketcenter using a laser drillingsystemFill and insulate the gasketusing c-BN epoxyRe-drilling a ∼30 mmsample hole

i)

ii)

iii)

Fill the sample hole with AB in the air

Place the four electrodeson the 300 mm beveled stage

i) i)

ii) ii)

Inside the glove box, cut athin La flake measuring(2 T 10 T 70 mm)

Place the La flake on the70 mm dimond culet

Inside glove box, placing the four Ptelectrodes such that thefour electrodes on thebelveled stage makecontact with the sample

Fig. 1. Scheme for the sample loading of La and AB into the DAC.

For resistance measurements at megabar pressures,the electrodes should be tough and should maintaingood contact with the sample. In previous studies onLaH10±δ, the electrodes that maintained direct con-tact with the sample were generally coated on dia-mond using Pt or Ta in the van der Pauw four-probeconfiguration.[19,20] The diamond with its electrodecoating should be handled carefully to avoid scratch-ing, which can break the contacts. Alternatively, thetraditional method, which involves the manual place-ment of the electrodes on a culet larger than 40–50µm,

can ensure good electrical contact, and this methodmakes it possible to repair the electrodes in short time.Thus, in this study, we manually placed all the elec-trodes using the traditional method and loaded onesample in 2–3 days. In addition, we employed thestandard four-probe configuration rather than the vander Pauw geometry to avoid changes in the voltage po-larity and to improve the data reliability.

Figure 1 shows the sample-loading process incorpo-rated in our study. First, a 250-µm-thick rhenium gas-ket was pre-indented to ∼30µm. A 50-µm-diameter

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hole was then drilled into the gasket using a laserdrilling system. The rhenium gasket was covered witha c-BN epoxy insulating layer. Another ∼30-µm holewas drilled in its center and it served as the samplechamber for the AB [see Fig. 1(a)]. Four large elec-trodes were fixed onto the beveled diamond stage, asshown in Fig. 1(b). In an argon-filled glove box, athin rectangular La flake with an initial size of ap-proximately 2× 10× 70µm3 was placed at the centerof a 70-µm diamond culet [Fig. 1(c)]. Next, the Laflake was connected to the four large electrodes onthe beveled stage with four Pt electrical leads in thestandard four-probe configuration [Fig. 1(d)]. Here,we should ensure that the voltage leads are in con-tact with the La inside the sample chamber. More-over, for a 70-µm diamond culet, the diameter of thesample chamber drilled in the gasket center should beless than 30µm; otherwise, the AB will flow out ofthe culet under compression. AB removal from theculet often occurs below 40GPa; above this pressure,the AB flow is insignificant. Conversely, the samplechamber should not be less than 10µm in width be-cause the small hydrogen release from AB may notinitiate an effective reaction.

After closing the DAC and applying pressure di-rectly to ∼170GPa, we heated the La+AB in thesample chamber using a continuous 1064-nm YAGlaser from the AB side. The laser spot with an ap-proximate diameter of 10µm was moved forward andbackward along the La flake between the two volt-age leads, ensuring a sufficient reaction between Laand the hydrogen released from AB. The pressure be-fore and after laser heating was determined from theRaman signal of diamond.[24,25] The temperature wasdetermined from the black body irradiation spectraof the sample. After laser heating, the temperature-dependent resistances were measured in a sample-in-vapor He4 cryostat equipped with a 9T helium-freesuperconducting magnet.

0 50 100 150 200 250 3000.0

0.1

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1.3 1.4 1.5 1.6 1.7 1.8

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Temperature (K)

165 GPa, 1000 K

∼74 K

Wavenumber (103 cm-1)Inte

nsi

ty (

arb

. units)

Resi

stance (W)

Fig. 2. Temperature dependence of electrical resistancein the first experiment (165 GPa, ∼1000K). Insets showthe diamond Raman signal from which the pressure wasdetermined (top left) and an optical image of the samplein the DAC after laser heating (bottom right).

The main panel of Fig. 2 shows the temperaturedependence of electrical resistance for the LaHx sam-ple synthesized at ∼1000K and 165GPa. As seenin the optical image (bottom right), the sample andelectrodes in the DAC remained almost intact afterlaser heating. The resistance of the obtained sam-ple decreased almost linearly as the sample cooled to∼74K. At this temperature, the resistance suddenlydropped, followed by a gradual reduction in the re-sistance with further decrease in the temperature to∼40K and 25K. The superconductive transition at∼74K, which is much lower than the reported optimalTc (∼250K), can be ascribed to the lower hydrogencontent due to the mild heating temperature and/orinsufficient AB in the DAC. Similar results in low-HLaHx were previously reported by Drozdov et al.[20]

(a)

(b)

(c)

0 50 100 150 200 250 300

0.0

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0.9

1.2

1.5

1.8

210 220 230 2400.0

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Cooling down

165 GPa, 1700 K

0 50 100 150 200 2500

40

80

120

160

200

240

Ginzberg-LandaufittingExperiment

WHH

Resi

stance (W)

Resi

stance (W)

0 T1 T3 T5 T7 T

Temperature (K) Temperature (K)

Magnetic fie

ld (

T)

Fig. 3. (a) Optical images of the sample assemblyat 25GPa before laser heating and at 165GPa afterleaser heating; (b) temperature-dependent resistance ofLaH10±δ at 0T over the entire temperature range dur-ing the cooling and warming processes (inset shows theresistance near the superconducting transition under dif-ferent magnetic fields); (c) field dependence of the crit-ical temperature fitted by the Ginzberg–Landau expres-sion (green solid curve). The zero-temperature upper crit-ical field Hc2(0) calculated using the Werthamer–Helfand–Hohenberg (WHH) expression in the dirty limit is shownby the blue dot. The violet dashed line represents thelinear fitting to obtain the initial slope.

To increase the hydrogen content and facilitatethe formation of the clathrate-like structured LaH10±δwith a higher Tc, we performed a second experimentwith the highest possible AB loading and increasedthe temperature of the laser heating to ∼1700K at165GPa. For this synthesis, we reached ∼ 1700K ata power of 28W by laser heating the La foil thoughthe diamond anvil and AB layer. We moved thelaser along the sample in between the two voltageleads in ten steps and stay for about 1 s at eachstep. Figure 3(a) presents the optical images takenat 25GPa before laser heating and at 165GPa af-ter laser heating. The sample assembly almost re-tained its original shape after heating, which was cru-cial for successful synthesis and subsequent resistancemeasurements. During the cooling and warming pro-cesses, the temperature-dependent resistance of the

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obtained sample began dropping at T onsetc of ∼240Kand ∼250K, respectively [Fig. 3(b)], consistent withthe superconducting transition reported by Drozdovet al.[20] The superconducting transition was relativelysharp but failed to reach zero resistance upon furthercooling, presumably due to the contact between thevoltage and current probes and/or the presence of anunreacted portion in the sample. Here, because thetemperature sensor was attached to the stainless steelframe of diamond anvil cell, the measured tempera-ture is ahead of the actual sample temperature. Asa result, the observed superconducting transition ex-hibits some thermal hysteresis even though we cooledand warmed the sample in a slow rate of ∼0.7K/min.

The superconducting transition was further ver-ified by measuring the temperature-dependent resis-tance under external magnetic fields. As shown inthe inset of Fig. 3(b), the resistance drop graduallyshifted to lower temperatures as the magnetic field in-creased. Here, we defined the superconducting Tmidcas the temperature corresponding to 50% of the re-sistance drop. Figure 3(c) plots the obtained Tmidcvalues as a function of external magnetic field up to7T. From this plot, the upper critical field Hc2(0)was estimated to be 174T and 223T according tothe Ginzberg–Landau and the Werthamer–Helfand–Hohenberg expressions,[26] respectively. The obtainedHc2(0) values are greater than those reported by Droz-dov et al.,[20] presumably due to the quite narrow fit-ting range in our study, which increases the uncer-tainty. Nonetheless, our results reconfirm the high-Tc superconductivity in LaH10±δ reported in previousstudies.[19,20]

In summary, we have successfully synthesizedLaH10±δ and confirmed its superconductivity at a highTc (∼250K). For this purpose, we designed a simplesample-loading procedure in a symmetric DAC andmeasured the sample resistance using the standardfour-probe method. The developed method reducesthe technical difficulty of in situ syntheses and trans-port measurements at megabar pressures. Therefore,it can promote the research of near-room-temperaturesuperconductors among metal superhydrides.

Some instruments used in this study are built forthe Synergic Extreme Condition User Facility.

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