Why do hydride superconductors need high pressure?

What makes a hydride superconductor different?

A hydride combines hydrogen with another element, as in the sulfur and lanthanum systems studied by Nature [1][2]. Nature describes superconducting behavior in those hydrogen-rich compounds at particular structures and pressures, rather than as a consequence of hydrogen content alone [1][2]. Nature's reported transition temperatures are far above those of many established superconducting metals, although both experiments still required cooling [1][2].

Nature explains that light hydrogen atoms can support high-frequency lattice vibrations and strong coupling between those vibrations and electrons [1]. Nature relates that interaction to conventional superconducting theory, in which electrons pair and can carry current without resistance [1]. Nature's studies of sulfur and lanthanum hydrides illustrate how hydrogen chemistry and pressure can yield different superconducting phases rather than a single universal transition temperature [1][2].

What did the H3S superconductor experiment show?

Drozdov and colleagues reported in Nature in 2015 that compressed sulfur hydride showed a transition at 203 kelvin [1]. Nature reported a sharp resistance drop to zero, a transition temperature that fell in an applied magnetic field, and a magnetic susceptibility signal consistent with superconductivity [1]. Nature used these complementary observations together to support its interpretation of superconductivity [1].

The researchers started with hydrogen sulfide, H2S, and argued in Nature that decomposition under pressure likely produced the superconducting H3S phase [1]. Nature also reported a shift when hydrogen was replaced by its heavier isotope deuterium, consistent with a role for lattice vibrations [1]. The Nature result describes a compressed sample, not a sulfur hydride wire operating at ordinary pressure [1].

What did LaH10 add to the hydride family?

In 2019, Drozdov and colleagues reported in Nature a transition around 250 kelvin in lanthanum hydride LaH10 at approximately 170 gigapascals [2]. Nature cited zero resistance, an isotope effect, and a lower transition temperature under an applied magnetic field as evidence [2]. Separately, Somayazulu and colleagues reported in Physical Review Letters resistance drops on cooling up to 260 kelvin at 180 to 200 gigapascals in lanthanum superhydride samples, interpreting them as signatures of superconductivity [5].

Before the Nature experiment, a computational study in the Proceedings of the National Academy of Sciences predicted a hydrogen-cage structure for LaH10 and calculated superconductivity at high pressure [6]. Nature subsequently described LaH10 as part of that predicted family [2]. Nature's H3S and LaH10 studies concern different hydrogen-rich structures and experimental conditions [1][2]. A dedicated LaH10 material page addresses that compound; this comparison focuses on differences across the hydride family.

How does a diamond anvil cell create pressure?

The lanthanum hydride experiment described by Nature compressed a minute sample in a diamond anvil cell and used electrical contacts and structural measurements to study it [2]. Physical Review Letters describes diamond tips compressing a sample within a gasket and the use of X-ray diffraction through the cell to examine its structure [5]. Nature reports that the cell also allowed electrical transport measurements during high-pressure cooling [2].

The small specimen presents measurement challenges: Physical Review Letters describes its laser-heating preparation as designed to maintain electrical contacts under high pressure [5]. Nature's LaH10 study combined electrical transport, isotope substitution, and structural analysis to support its interpretation [2]. Physical Review Letters also reports separate current-voltage measurements and low-temperature X-ray diffraction in its lanthanum superhydride experiments [5].

Physical Review Letters reports using modulated laser heating to synthesize samples while preserving contacts for four-probe measurements [5]. In that experiment, the journal reports that electrical resistance fell as the samples cooled and that other experiments showed transitions at both lower and higher temperatures [5]. Physical Review Letters presents those observations as evidence for superconductivity, accompanied by structural and current-voltage measurements, rather than claiming that a single resistance curve determines every material property [5].

Why is high pressure part of the result?

Nature reports that the sulfur hydride system became metallic under compression and attributes its superconductivity to a likely pressure-formed H3S phase [1]. In the lanthanum system, Nature identifies a hydrogen-rich cage structure at high pressure [2]. Neither Nature study establishes that its reported high-pressure phase and transition survive removal of the load [1][2].

The Proceedings of the National Academy of Sciences study examined many possible lanthanum and yttrium hydride structures computationally, rather than measuring the superconducting transition in a sample [6]. The Proceedings of the National Academy of Sciences reported a predicted hydrogen-rich LaH10 structure and estimated a transition between 274 and 286 kelvin at 210 gigapascals [6]. Nature's later experimental result reports different measurement conditions, so the computed estimate and observed transition should remain identified as prediction and experiment, respectively [2][6].

The high pressure reported by Nature is therefore part of the experimental result, not an optional condition that can be dropped from the temperature figure [1][2]. Physical Review Letters describes experiments inside a diamond anvil cell rather than a manufactured conductor operating at ambient pressure [5]. Neither Nature nor Physical Review Letters establishes the stability, current-carrying performance, or manufacturing route needed for a practical winding or cable in these studies [1][2][5].

What happened to the Dias hydride claims?

Nature retracted the 2020 carbonaceous sulfur hydride paper involving Ranga Dias in 2022 [3]. Nature's editors said an unspecified, nonstandard background-subtraction procedure undermined confidence in the published magnetic susceptibility data; the authors maintained that the raw data supported their main claims, and all disagreed with the retraction [3]. Nature's notice concerns that paper and does not retract the separate Drozdov sulfur hydride or LaH10 papers [1][2][3].

Nature also retracted the 2023 nitrogen-doped lutetium hydride paper at the request of eight authors, who said the paper did not accurately reflect material provenance, measurements, and processing protocols [4]. Nature separately found concerns about electrical resistance data credible, substantial, and unresolved; Dias and two other authors had not stated agreement or disagreement with the retraction [4]. Nature's separate notices identify these individual papers as retracted rather than establishing that all hydride superconductivity reports share the same status [3][4].