Hot Hydride Superconductivity above 550 K
A. D. Grockowiak, M. Ahart, T. Helm, W.A. Coniglio, R. Kumar, M. Somayazulu, Y. Meng, M. Oliff, V. Williams, N.W. Ashcroft, R. J. Hemley, S. W. Tozer
HHot Hydride Superconductivity above 550 K
A. D. Grockowiak* , M. Ahart , T. Helm , , W.A. Coniglio , R. Kumar , M. Somayazulu ,Y. Meng , M. Oliff , V. Williams , N.W. Ashcroft , R. J. Hemley , , & S. W. Tozer* ∗ corresponding authors National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida,32310, USA Department of Physics, University of Illinois Chicago, Illinois 60607, USA Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany Max-Planck-Institute for Chemical Physics of Solids, Noethnitzer Strasse 40, D-01187 Dres-den, Germany HPCAT, X-ray Science Division, Argonne National Laboratory, Lemont IL 60439, USA Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853,USA Department of Chemistry, University of Illinois Chicago, Illinois 60607, USA
The search for room temperature superconductivity has accelerated dramati-cally in the last few years driven largely by theoretical predictions that first in-dicated alloying dense hydrogen with other elements could produce conventionalphonon-mediated superconductivity at very high temperatures and at accessi-ble pressures, and more recently, with the success of structure search methods a r X i v : . [ c ond - m a t . s up r- c on ] J un hat have identified specific candidates and pressure-temperature (P-T) condi-tions for synthesis. These theoretical advances have prompted improvementsin experimental techniques to test these predictions. As a result, experimen-tal studies of simple binary hydrides under pressure have yielded high criticalsuperconducting transition temperatures (T c ), of 260 K in LaH , close to thecommonly accepted threshold for room temperature, 293 K, at pressures near180 GPa. We successfully synthesized a metallic La-based superhydride fromLa metal and ammonia borane, NH BH , and find a multi-step transition witha T c of 294 K for the highest onset. When subjected to subsequent thermalexcursions to higher temperatures that promoted a chemical reaction to whatwe believe is a ternary or higher order system, the transition temperature wasdriven to higher temperatures. Although the reaction does not appear to becomplete, the onset temperature was pushed from 294 K to 556 K before theexperiments had to be terminated. The results provide evidence for hot super-conductivity well above room temperature, in line with recent predictions for ahigher order hydride under pressure.Introduction Ever since Kamerlingh Onnes’ 1911 discovery of superconductivity in mercury , scientistshave searched for materials with higher transition temperatures, initially in the elements, andthen progressing to more complex systems. Although largely curiosity driven, each discovery2f a material with higher critical field and temperature addresses clear technological needs.Fully fifty-five elements are now known to be superconducting at ambient or high pressure , with hydrogen being a notable exception. In 1968 Ashcroft proposed that atomic metal-lic hydrogen at sufficiently high density could be a very high T c Bardeen-Cooper-Schrieffer(BCS) superconductor . In view of the high pressures required to reach this proposedstate of hydrogen, Carlsson and Ashcroft later suggested alternative routes to effectivelyproduce superconducting atomic hydrogen, including the incorporation of other elements inthe structure. This prediction prompted the experimental search for such compounds andalloys . Ashcroft later extended and recast the above considerations in terms of ˆa ˘A¨Ychem-ical pre-compressionˆa ˘A´Z, a proposal in which H molecules in dense structures might beexpected to dissociate at pressures well below those required for pure hydrogen. Advances incrystal structure prediction methods then began to provide experimentalists specific targetsfor higher T c BCS superconductivity , , , , , while at the same time theoretical studieshave focused on understanding the superconducting mechanism , , , , , , . Notably,this effort led to the prediction , and, independently, to the experimental discovery ofsuperconductivity in the H-S system with a T c of 203 K, including an isotope study thatpointed to BCS behavior. Subsequent simulations , guided the discovery of new hydrideswith T c approaching room temperature, the highest being LaH with a critical temperatureof at least 260 K, , , and somewhat lower values of 227-243 K reported for relatedYH /YH phases , . 3hat had once been proposed as a goal in and of itself, superconductivity at roomtemperature now appears to be the stepping off point for still higher T c ' s. Much like whenthe observation of superconductivity in cuprates led to a flurry of discoveries and engineer-ing applications that pushed the critical temperatures in these materials from 40 to 164 Kover the course of a decade , , hydrogen-based materials hold out great promise. Currentcalculations predict that crystalline atomic metallic hydrogen , has a T c near room tem-perature at 500 GPa, which increases to 420 K above 3 TPa . Other calculations predictthat hydrogen could be a superconducting superfluid at comparable conditions . Emergingsystematics for the binary hydrides indicate maximum T c in the vicinity of room tempera-tures at megabar pressures , and there is now a focus on the possibility of higher criticaltemperatures in more chemically complex hydride systems. Indeed, recent theoretical studiespredict a T c of 473 K in Li MgH near 250 GPa .The present work was motivated by a desire to extend the P-T-H phase diagram of pre-vious measurements of the magnetic field dependence of the lanthanum-based superhydridesto fields approaching 100 T , , . We also sought to examine lower pressure phases suchas LaH which is more experimentally accessible. In addition we hoped to better understandthe variable T c in experiments in which LaH superhydride was synthesized using ammoniaborane (NH BH ) as the hydrogen source . For this purpose, we developed metallic DACsdesigned for superhydride studies in DC magnetic fields that are also small enough to fit intopulsed magnets. Made from the highly resistive non-magnetic superalloy NiCrAl [Pascalloy,4evonics] to limit the eddy-current heating, these DACs can be used to access temperaturesdown to at least 30 K in pulsed fields.We coupled this design with robust Pt electrodes created by Focused Ion Beam (FIB)techniques to withstand the extreme P-T conditions required to synthesize the superhydrides . Attempts to characterize in situ by x-ray diffraction were unsuccessful due to an insuf-ficient downstream opening in the DAC. We find that the La-based superhydride initiallysynthesized by laser-heating beginning at 160 GPa had a T c of 294 K. Most remarkably,subjecting the sample to subsequent thermal cycling shifted the T c to higher temperaturesin a fortuitous progression that reached well above 500 K. Results
We loaded two diamond-anvil cells (DACs) with pieces of 99% La and ammonia borane (AB,NH BH ), the hydrogen source and pressure medium (see Methods and SI). Both cells wereinitially as identical as experimentally possible, with the same FIB-patterned electrodes onone anvil and an MP35N gasket with cBN insert and AB on the other. The first cell (B002)was loaded initially at 160 GPa for LaH synthesis, whereas the second cell (B003) wasloaded at 120 GPa to generate the lower stoichiometry superhydride, LaH (Fig. 1). Bothcells were then laser-heated at the HPCAT beamline of the APS. A 20 µ m diameter laser5pot was rastered across the sample at fixed positions to promote the dissociation of ABinto cBN and the hydrogen, that reacts with the La. Diffraction patterns were collected ateach spot after laser heating. The sample in B002 received about 45 laser pulses (4 to 5pulses at each of nine points on a 10 x 10 µ m grid). The synthesis for B003 was stoppedprematurely after a few pulses for fear that a catastrophic failure of the diamond anvils hadoccurred (see SI for details). The electrical resistivity was then measured on the samplesat NHMFL-Tallahassee, first using a Quantum Design 16 T PPMS. Figure 2 shows the firstcool down trace from 300 to 230 K for B002 using four of the six available electrodes. A re-sistance drop is clearly evident at 294 K with no additional transition observed upon coolingto 230 K, the temperature range in which T c has been previously observed for LaH X systems.In an attempt to measure the resistance of the sample in B002 further into the normalstate, we subjected the sample to a series of thermal cycles at 0 T. Surprisingly, these succes-sive higher temperatures excursions pushed the onset of the transition to higher and highertemperatures. We had to stop temporarily at 390 K as this is the maximum temperaturepossible in our PPMS. Figure 3 compares the initial and final traces, with the 0 T onsetappearing near 357 K in the final thermal cycle in the PPMS.Four thermal cycles between 370 K and 290 K were then performed at 0, 2, 10 and 16T (Fig. 4). The traces are shifted vertically for clarity, but collapse onto one another below610 K. All curves show hysteresis. The warming curve has a lower onset temperature thanthe cooling curve, indicating either the first order nature of the transition or that the sampleis still evolving upon successive thermal cycles. The warm up traces show additional changeswhich we believe points to a continuing synthesis, as discussed below.To analyze these data, three temperature points are identified in each trace withinFig. 4a. T1 is the transition onset on cool down, T2 is a bump observed at intermediatetemperatures on warm up, and T3 is the temperature at which the hysteresis loop closes. Aclear shift with applied field is observed for T2 and T3, the former shifting by 11 K and thelatter by 19.5 K between 0 and 16 T (Fig. 4a). This shift is evidence of the superconductingnature of this high temperature transition. A fit of the data (fig4b) using the Ginzburg-Landau relation (equation 1): µ H c ( T ) = µ H c (0)(1 − ( TT c ) ) (1)yields an H c (0 K) of 1500 T, 230 T, and 130 T for T1, T2, and T3, respectively. How-ever the error is large for H c (0 K) as we had access to a limited field range.To further investigate the shift in the superconducting transition with field, B002 wasthen measured in a 41.5 T DC resistive magnet at the NHMFL-Tallahassee. All six workingelectrodes were used in order to measure an additional resistance channel. We present in7he main text only the results from the same configuration as that measured in the PPMS,but the other one yielded similar results (see SI). Successive thermal cycles carried out toyet higher temperatures in order to establish the clear signature of the normal state resultedagain in a clear shift of the transition to higher temperature. The stability of epoxies internalto the DAC limited our measurements to below 580 K.Figure 5 shows the cool down traces starting from various initial maximum temper-atures. The cell remained at each maximum temperature for at least 30 minutes to allowthe synthesis to stabilize as realized by a steady sample resistance, and was then cooled at0.5 K/min. With each higher temperature excursion T c rises further, eventually reaching amaximum value nearing 560 K. The amplitude of the transition also dramatically increaseswith each excursion to higher temperatures. Similar shifts in transition temperature withrepeated thermal cycling has been documented for other superconducting hydrides underpressures , . The resistance within the superconducting state is almost identical for allthe curves, the non-zero state being attributed to the additional unreacted lanthanum be-tween the electrodes, as this same behaviour is seen in B003 (see SI).Temperature sweeps in static high magnetic fields were performed for initial maximumtemperatures of 400 K, 430 K, 445 K, 503 K and 530 K. Only cool down traces were per-formed in field to remain within the allocated energy budget of a magnet time run. Figure8 shows the result of the run at 503 K, since the data at 530 K indicated either an electricalcontact degradation or the development of a touch between the cryostat and the magnet,adding vibrations in the signal which degraded the signal-to-noise. The temperatures for thetraces obtained in field are corrected for the magnetoresistance of the thermometer, and aquadratic background in the superconducting state was subtracted over the whole tempera-ture range (see SI).Although the onset of the transition shifts from 492 K at 0 T to 486 K at 40 T, thewidth of the transition at 0 T, as well as the additional transitions appearing in the 20 Tand 33 T curves make the interpretation difficult. The incomplete nature of the chemicaltransformation precludes fixed-field temperature sweeps within a family of curves above 370K. Indeed, the shift in the transition with each increasing temperature excursion(Fig. 5)suggests that the chemical state of the material is still evolving. Discussion
We report the observation of superconductivity in a La-based superhydride sample begin-ning at room temperature that shifts up in a controlled fashion with thermal excursions to avalue close to 560 K with a notable concomitant increase in the amplitude. The observationof the onset at 294 K is consistent with preliminary observations of T c above 260 K thatwere reported previously . The initial increase in T c from 294 K to 370 K may be related9o the enhancement of T c on repeated thermal cycling observed in the simpler binary hy-dride H Se and described in the preliminary reports of the synthesis of superconductingH S . However, this is unlikely, as the maximum predicted transition temperature for thebinary LaH / system is 288 K , . Rather, the higher temperature transitions pointto additional chemical transformations induced by pressure, shear, temperature, potentiallymagnetic field, and possibly molten hydrogen, which at room temperature exists above 200Mbar , . In addition to B and N from the hydrogen source (NH BH ) and/or the com-posite gasket insert (cBN) and the carbon from the epoxy binder, C and Ga from the Ptelectrodes also make contact with the La-H and could react with this binary system to forma ternary or higher order system. One might speculate that the initial laser synthesis gen-erated the binary, and the thermal excursions performed on the entire cell may have beencritical as this degraded the epoxy binder in the cBN insert, which allowed C and possibly Hto diffuse into the sample. Pt and Au from the electrodes and the amorphous FIBed surfaceof the diamond may also act as catalysts or catalytic bed, respectively, to help form dopedalloys or new stoichiometric compounds. A variety of high P-T induced chemical reactions,phase transformations, or novel phases involving these elements are documented, even atmore modest conditions , , . Interestingly, the 294 and 360 K cooldown traces appearto have only two phases with perhaps a third broad phase, which may be due to a disor-dered phase while the higher onset temperature transitions have numerous transitions in thewarming and cooling traces which support the idea of a chemical reaction that is activatedwith increasing temperatures. To achieve the higher transition temperatures observed, ther-10ally optimized ordering of the hydrogen in the system might have occurred, possibly witha reduction in the dimensionality or realization of a more crystalline material. Even at 580K, the reaction is not complete, and higher temperatures and/or further thermal annealingis required.The high-temperature transition onsets have the characteristic features of supercon-ductivity, but other interpretations cannot be ruled out in the absence of measurementsof the Meissner effect, or shifts in T c with magnet field that are at this time complicatedby the ongoing chemical transformations taking place during the measurements (e.g., thevery high H c implied by the fit to the existing data). Formally, an unusual temperature-induced electronic transition to an insulating state needs to be ruled out, too. However, thetransition moves with temperature excursions in an organized fashion from a theoreticallypredicted superconducting transition of 294 K, and the background and non-zero resistanceare convincingly traced back to experimental artifacts. We are left with a material witha zero resistance at very high temperatures. In the absence of additional constraints, ourobservations provide evidence of ’hot’ superconductivity in a still evolving compound (orassemblage), perhaps analogous to recent calculations for the Li-Mg-H system in which T c of 470 K has been predicted within a conventional BCS framework.Additional work is clearly required to characterize the observed phenomena, including11tructural characterization of the phase (or phases) produced, determination of P-T-H phasediagrams of the phases present, and driving the reaction to completion. The latter will re-quire some combination of conventional and IR laser heating possibly via fiber optic, whichwould allow the sample to be imaged and the pressure to be measured at the experimentaltemperature of interest via Raman. The current experiments were hampered by the unex-pected need to explore high temperatures instead of the originally designed range below 300K, and further characterization was cut short by laboratory closings due to the COVID-19pandemic. Experiments in higher, pulsed magnetic fields are scheduled to better character-ize the field effect on this transition. Beyond the possibility of hot superconductivity, thesurprising observations documented here suggest new routes for creating new materials usingmultiple extreme environments of pressure, temperature and high magnetic fields. Methods
Our piston-cylinder DACs ( (cid:31) . They were made from HIPed NiCrAl to reduce vibra-tions and eddy current heating due to dB/dt which is on the order of 10 000 to 20 000 T/s inpulsed fields). 72 µ m culet, double bevel (8 ° × µ m, 15 ° × µ m) standard anvils witha 3.75 mm girdle were used.For the preparation of robust conductive leads, we applied a dual beam focused ion beam(FIB) system in combination with a scanning electron microscope (SEM). A metalorganicgas, Trimethyl(methylcylopentadienyl)Platinum(IV), is injected into the high-vacuum sam-12le chamber via the nozzle of a gas injection system (GIS) [SI]. A focused ion stream decom-poses the molecules, precisely depositing Pt rich in carbon and gallium (typically 30 and10-20 at.%, respectively) onto the anvils. In the same process, the surface of the diamondis amorphized to a depth of approximately 20 nm (for 30 keV gallium ions), allowing thecarbon-rich Pt deposit to chemically connect with the broken carbon bonds of the diamond.This chemical bonding results in the mechanical adhesion of FIB-deposits to the diamond,making it extremely robust against mechanical forces. One disadvantage is that the highcarbon content reduces the conductivity of such leads significantly (a few Ohms/ µ m depend-ing on the deposition conditions and the thickness of the layer). In order to realize ohmiclead resistances we deposit, in a second step after a transfer to an external sputter depositionsystem, a layer ( ≈
100 nm) of pure gold on top of the prepared platinum ribbons, with Kap-ton tape used to protect the diamond surface against Au deposition. In the third step, wemake use of the high-current gallium beam and etch away excess gold until the amorphousdiamond surface is recovered between adjacent taps. The platinum-gold ribbons are thencovered with an additional FIB-Pt protection layer ( ≈ µ m) running alongside the pavilionup to the culet. In a last step thin FIB-Pt ribbons (with approximately 1 µ m thickness) aredeposited close to the central part of the culet extending the Pt-Au-Pt ˆa ˘AIJsandwichˆa ˘A˙Ileads into the sample space, which has a diameter of approximately 40 µ m.135 µ m thick aged MP35N gaskets, located on the piston anvil, were indented to apressure of 15 GPa in a dummy DAC with the same anvil geometry as B002 and B003 toprevent damage to their electrodes yet provide a gasket which mirrored the anvil shape in13hese DACs. It was removed and laser cut to a diameter half-way through the second bevel,polished to remove burrs and most of the extruded region, repositioned on the dummy DAC,and then filled with a dry mixture of cBN powder and epoxy (10% by weight). The gasketwas pressed to a load of 25 GPa after which it was laser drilled to a diameter of 40 µ m.This composite gasket was then moved to the piston of the DAC to be used in the experi-ment. The piston with the gasket secured in a crown was brought into a flowing argon filledglove box (O and H O content of < BH , or AB) as the hydrogen source and pressure medium. The gasket andgasket crown on the piston are electrically isolated from the rest of the DAC. Pressure mea-surements required for these various steps in the gasket fabrication were performed at roomtemperature using the Raman edge of stressed diamond . A third DAC is used to mechan-ically thin and shear a piece of 99% La (Goodfellow Metals) to expose clean metal. A flakeof the metal, approximately 4 µ m thick, is extracted from this film and placed on a plastictransfer piston, after which it is brought into the glovebox and pushed against the anvil withFIBed electrodes to form a cold weld. The same plastic transfer anvil is used to initiallyalign this anvil during the assembly of the DAC and the visible impression that remainsof that anvilˆa ˘A´Zs culet helps ensure that the La is positioned over the electrodes in thissecond step. To prevent the reaction between La and air, we loaded the sample and sealedthe DAC within 30 minutes of extracting the 3-6 µ m samples from the freshly exposed metal.The assembled DAC was then taken to the desired pressure for synthesis, using the14aman edge of the stressed diamond. Laser synthesis was performed at HPCAT, sector 16of the Argonne Photon Source (APS), and XRD analysis of the result was attempted aftersynthesis. The sample heating was single-sided using a IPG YLR-100-1064-WC fiber laseroperating in modulation mode (square single pulse) with a total power of 100 W. Temper-ature measurements were obtained from a 4-micron area in the center of the heating spotusing an Acton SpectroPro SP2560 imaging spectrograph equipped with a back-illuminatedCCD detector (PI-MAX, Princeton Instruments). The laser focal size was 20 µ m and themodulation pulse width was 30 ms . The upstream side of the DAC has a 11.5 ˆAˇr degreeopening in the piston; the downstream side has a 14 ˆAˇr opening in the endcap that supportsthe anvil with electrodes. This was not sufficient to allow for confirmation of the binaryand the higher order system via X-ray, but a separate experiment is planned for that uponreopening of the APS. Additional information on sample synthesis and laser heating methodsare described in the supplemental information.AC electrical transport measurements in the PPMS were carried out using the internalelectronics (7 Hz) while measurements in the 41.5 T resistive magnet used two Stanford Re-search Systems SR860 lockin amplifiers in combination with an SRS CS580 voltage controlledcurrent source to drive a 600 nA current at 48.5 Hz.High magnetic field studies in the 41.5 T resistive magnet at NHMFL-Tallahasseewere carried out at fixed fields using consistent sweep rates of 0.5 to 3 K/min in varioustemperature regions. A custom Janis cryostat with variable temperature insert provided the15ample environment. A small bobbin with three 50 Ohm wire wound heaters in intimatecontact with the DAC was mummified in two layers of 12 µ m copper foil and 37 layers ofsuperinsulation. A Lakeshore Cryotronics Pt-100 thermometer located within the body ofthe cell and in intimate mechanical contact with the spring and piston was used for bothcontrol and sensing. The maximum temperatures of the measurements were limited as theepoxy used to form electrical connection between the twisted pairs and the FIBed electrodes,EPO-TEK H20E, has a maximum operating temperature of 573 K, began to degrade. Theepoxy used to fix the twisted pairs in the DAC, Stycast 2850 FT with catalyst 24 LV has amaximum operating temperature of 390 K and was completely calcinated after the series ofhigh temperature measurements. References
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Physical Review B (2011). 22 cknowledgements Portions of this work were performed at HPCAT (Sector 16), AdvancedPhoton Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSAˆa ˘A´Zs Office of Experimental Sciences. The Advanced Photon Source is a U.S. Departmentof Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by ArgonneNational Laboratory under Contract No. DE-AC02-06CH11357. Part of this work was performed atthe National High Magnetic Field Laboratory, which is supported by NSF Cooperative AgreementNo. DMR-1157490/1644779 and by the State of Florida. M.A., R.K. and R.H. acknowledge fund-ing from the U.S. National Science Foundation (DMR-1933622). We would like to acknowledgestimulating discussions with George Crabtree, Jason Cooley, Pedro Schlottmann, Gil Lonzarich,Thierry Klein, Christophe Marcenat and Rick Wilson. We thank M.K ˜A˝unig and S.Seifert for theirtechnical support with FIB and sputter deposition. We would like to thank for their help EricRod, Curtis Kenney-Benson, Freda Humble, Roald Hoffmann, Andy Rubes, David Sloan, WilliamBrehm, Daniel McIntosh, Alan Williams, Troy Brumm, Bobby Joe Pullum, Scott Maier, RobertCarrier, Christopher Thomas, Michael Hicks, Joel Piotrowski, Larry Gordon, Timothy Murphyand Donna Elliott at SQM. S.W.T., W.A.C. and A.D.G. would like to dedicate this work to thememory of G. Schmiedeshoff.
Competing Interests
The authors declare that they have no competing financial interests.
Author contribution statement
S.W.T. and A.D.G. conceived, developed, performed the ex-periments and data analysis. Sample preparation was performed by S.W.T., A.D.G., M.A. and R.K.Y.M provided beamline user support. T.H. developed and carried out the FIB process. W.A.C.assisted with the experiment and performed data analysis. S.W.T., M.O. and V.W. designed andfabricated the DACs. All authors contributed to the discussions and writing. orrespondence Correspondence and requests for materials should be addressed to A.D.G.. (email:[email protected]) and S.W.T. (email: [email protected])
Image of the sample and electrodes in cells B002 (left) and B003 (right) takenthrough the DAC with back-illumination. The letters indicate electrode pairs, outlined ingrey. The red outline indicates the smeared initial piece of La. The outer circle indicatesthe approximate boundary of the metal gasket and the cBN; the inner dashed circle indicatesthe perimeter of the 72 ˆA¸tm culet. After compression, both cells presented electrical shortsbetween the gasket and the electrodes. The initial amount of La inserted into each cell wasalso different: a thicker flake was introduced into B002, which upon compression extrudedoutside the culet region where it smeared across some electrodes as shown. Six and fourelectrodes are connected to the sample in B002 and B003, respectively. The FIBed electrodesrun down the pavilion of the anvils where they are connected to copper twisted pairs usingEpo-Tek H20E epoxy. .61.41.21.00.8 R e s i s t a n c e ( O h m ) Temperature (K)
Cell B002First electrode configuration 10 μ A R e s i s t a n c e ( Ω ) Temperature (K)
B002 - 1.85 Mbar B003 - 1.62 Mbar1050 B ( Ω ) Temperature (K) B ( Ω ) Figure 2: µ A, andthe temperature cool down rate was 1 K/min. There is a clear superconducting transitionat 294 K which is at the upper theoretical limit for the binary LaH , . (see SI) Databetween 268 K and 277 K was not collected due to an error. Right: Comparison of B002 andB003. Both show the superconducting transition due to unreacted La or a lower stoichiometryhydride at temperatures below 5 K and a superconducting transition at 12.5 K in B002 and9 K in B003 that we attribute to a platinum hydride . The transition in B003 is at a lowertemperature, consistent with B003 being at a lower pressure. It is also not as fully developed,which we attribute to the small number of laser pulses that B003 was subjected to. B002 hasthe additional transition at temperatures greater than 365 K. Both B002 and B003 have anon-zero resistance down to 1.9 K. The resistance of B003 is almost a factor of 10 higherthan B002, which is attributed to the thinness of the B003 sample. .01.51.0 R e s i s t a n c e ( O h m ) Figure 3:
Temperature dependence of resistance R(T) for cell B002 after the initial lasersynthesis showing the 294 K temperature transition. The first thermal excursion, done withthe intent of observing the normal state more fully, transformed the material and yieldeda new onset temperature around 357 K. This second trace was made using a different leadconfiguration (see SI). There is a strong background to the signal due to a network of seriesand parallel contributions. Measurements on B003, which contained unreacted La, shows thesame R(T) dependence and non-zero resistance as low as 1.9 K, which in turn allowed usto subtract out this contribution. Some portion of the background may also be due to theincomplete nature of the transition and has been seen in other experimental work on LaHsystems , . Temperatures indicated are those of the PPMS chamber. C r i t . F i e l d ( T ) '370Kmax_raw_field' GL_fit_field_t1 '370Kmax_raw_field' μ AIn PPMS, max temp=370K
T1 T2 T3 R e s i s t a n c e ( O h m ) T1T2T3First thermal cycle 0 T cool down 0 T warm up Second thermal cycle 2 T cool down 2 T warm up Third thermal cycle 10 T cool down 10 T warm up Fourth thermal cycle 16 T cool down 16 T warm up
Figure 4: a) Top : temperature dependence of resistance R(T) for B002 on successive cool downs and warmups at four different fixed applied magnetic fields, 0, 2, 10 and 16 T in a QD PPMS, using a current of 0.6 µ Aat 17 Hz between 290 and 370 K at 0.2 K/min with a thermalization time of 30 min at 290 K before warmingup. The cool down and warm up traces are indicated by dashed and continuous lines, respectively, and areshifted vertically for clarity. Quantum Design accounts for the magnetoresistance of the Pt thermometerused in the PPMS high temperature option. The arrow on the cool down traces indicate the approximatetransition onset and is labeled T1. T2 is the approximate transition onset on the warm up curves, and T3is the temperature at which the hysteresis of the cool down and warm up traces closes. This run used the2nd configuration of electrodes (see SI). The traces were taken sequentially from 0 T to 16 T. All four tracescollapse onto one another below 315 K. The T c (90/10) is approximately 11 K after a background subtractionis done. Note that the transition in warming curve for each field is lower in temperature than the cooling.Although it is possible that this is due to a lag between the thermometer and the sample, the hysteresis ismost likely real as the temperature was swept at 0.2 K/min and the separation is different for the four fields.b) Bottom : evolution of T1, T2, and T3, at various magnetic fields, and corresponding fits to the points. R e s i s t a n c e ( O h m )
16 T QD PPMS 300K 370 K Cell 6 41.5 T Resistive magnet 390 K 400 K 410 K 430 K 445 K 503 K 525 K 530 K 580 K
Figure 5:
R(T) at 0 T showing the evolution of the superconducting transition with successivethermal cycles (cell B002). These cool down traces were obtained using the same set ofelectrodes and the same current of 600 nA. The PPMS traces were collected at 7 Hz, andthe cell 6 data at 48.5 Hz to minimize the out-of-phase component. The onset temperatureis clearly shifted from 295 K to 560 K, and the transition amplitude also grows with eachthermal cycling to higher temperature. The resistance in the proposed superconducting stateis also identical for all curves. The temperature excursion to 580 K was realized on April1 after the end of our magnet time, which was both enabled and stopped by the COVID-19crisis. .00.80.60.40.20.0 R / R ( K )
40 T 33 T 20 T 0 T
Figure 6: