Room-temperature Superconductivity Research Articles

Superconductivity at 250 K in lanthanum hydride under high pressures.

With the discovery1 of superconductivity at 203 kelvin in H3S, attention returned to conventional superconductors with properties that can be described by the Bardeen-Cooper-Schrieffer and the Migdal-Eliashberg theories. Although these theories predict the possibility of room-temperature superconductivity in metals that have certain favourable properties-such as lattice vibrations at high frequencies-they are not sufficient to guide the design or predict the properties of new superconducting materials. First-principles calculations based on density functional theory have enabled such predictions, and have suggested a new family of superconducting hydrides that possess a clathrate-like structure in which the host atom (calcium, yttrium, lanthanum) is at the centre of a cage formed by hydrogen atoms2-4. For LaH10 and YH10, the onset of superconductivity is predicted to occur at critical temperatures between 240 and 320 kelvin at megabar pressures3-6. Here we report superconductivity with a critical temperature of around 250 kelvin within the [Formula: see text] structure of LaH10 at a pressure of about 170 gigapascals. This is, to our knowledge, the highest critical temperature that has been confirmed so far in a superconducting material. Superconductivity was evidenced by the observation of zero resistance, an isotope effect, and a decrease in critical temperature under an external magnetic field, which suggested an upper critical magnetic field of about 136 tesla at zero temperature. The increase of around 50 kelvin compared with the previous highest critical temperature1 is an encouraging step towards the goal of achieving room-temperature superconductivity in the near future.

The quest for room-temperature superconductivity in hydrides

Whereas previous discoveries of superconductors were largely serendipitous, the latest advances have emerged from the close coupling of theoretical predictions and high-pressure experiments.

Microscopic mechanism of room-temperature superconductivity in compressed LaH10

Room-temperature superconductivity has been one of the most challenging subjects in modern physics. Recent experiments reported that lanthanum hydride LaH$_{10{\pm}x}$ ($x$$<$1) raises a superconducting transition temperature $T_{\rm c}$ up to ${\sim}$260 (or 215) K at high pressures around 190 (150) GPa. Here, based on first-principles calculations, we reveal the existence of topological Dirac-nodal-line (DNL) states in compressed LaH$_{10}$. Remarkably, the DNLs protected by the combined inversion and time-reversal symmetry and the rotation symmetry create a van Hove singularity (vHs) near the Fermi energy, giving rise to large electronic density of states. Contrasting with other La hydrides containing cationic La and anionic H atoms, LaH$_{10}$ shows a peculiar characteristic of electrical charges with anionic La and both cationic and anionic H species, caused by a strong hybridization of the La $f$ and H $s$ orbitals. We find that a large number of electronic states at the vHs are strongly coupled to the H-derived high-frequency phonon modes that are induced via the unusual, intricate bonding network of LaH$_{10}$, thereby yielding a high $T_{\rm c}$. Our findings not only elucidate the microscopic origin of the observed high-$T_{\rm c}$ BCS-type superconductivity in LaH$_{10}$, but also pave the route for achieving room-temperature topological superconductors in compressed hydrogen-rich compounds.

Viewpoint: the road to room-temperature conventional superconductivity

It is a honor to write a contribution on this memorial for Sandro Massidda. For both of us, at different stages in our lives, Sandro was first and foremost a friend. We both admired his humble, playful and profound approach to life and physics. In this contribution we describe the route which permitted to meet a long-standing challenge in solid state physics, i.e. room temperature superconductivity. In less than 20 years the critical temperature of conventional superconductors, which in the last century had been widely believed to be limited to 25 K, was raised from 40 K in MgB2 to 265 K in LaH10. This discovery was enabled by the development and application of computational methods for superconductors, a field in which Sandro Massidda played a major role.

Search for near room-temperature superconductivity in B-doped Q-carbon

ABSTRACTWe present 1D, 2D and 3D structures of boron-doped Q-carbon with a higher superconducting transition temperature than the current value of 55 K in 25 at% B-doped amorphous Q-carbon. The higher transition temperature is predicted for 1D and 2D crystalline structures with increasing density of states near the Fermi level through dopant trapping in substitutional sites. We have synthesized 50at% B-doped Q-carbon where diamond tetrahedra are arranged randomly and packed with over 80% efficiency to generate an amorphous structure. Detailed EELS measurements show higher density of states near the Fermi level above 90 K and preliminary transport data show signature of Tc above 100 K.

Evidence for Superconductivity above 260K 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 200GPa. 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 260K and 180-200GPa, 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.

Super-Hydrides of Lanthanum and Yttrium: On Optimal Conditions for Achieving near Room Temperature Superconductivity

Recently, many seminal papers deal with the syntheses, stability and superconducting properties of super-hydrides like LaH10 or YH10 under high pressure, reporting critical temperatures near room temperature. In the first run one will assume that the involved metal atoms contribute a number of 3 electrons to the pairing pool corresponding to their valence. However, another possibility may be that the cationic valence is somewhat smaller, for instance only 2.29, resulting in a nominal electron number per cation of σ0 = 0.229 ≈ 3/13 instead of 0.3. Then, we will have a numerical equality to the optimum hole number in the cuprate high-Tc superconductors, a number that reflects the fractal nature of electronic response in superconductors. However, if one still keeps up the oxidation state of +3 of lanthanum, one will need 13 hydrogen atoms to match the optimum σ0. Such composition may be found at the phase boundary between the observed LaH10 and LaH16 phases. Partial ionic replacement is suggested to shift the super-hydride composition into the σ0 optimum. Micro-structural phenomena such as multiple twinning and ferroelastic behavior as observed with cuprates may also influence the superconductivity of super-hydrides. Finally, epitaxial growth of super-hydrides onto a specially cut diamond substrate is proposed.

Structure-property correlations in phase-pure B-doped Q-carbon high-temperature superconductor with a record Tc = 55 K.

Here, we report the detailed structure-property correlations in phase-pure B-doped Q-carbon high-temperature superconductor having a superconducting transition temperature (Tc) of 55 K. This superconducting phase is a result of nanosecond laser melting and subsequent quenching of a highly super undercooled state of molten B-doped C. The temperature-dependent resistivity in different magnetic fields and magnetic susceptibility measurements indicate a type-II Bardeen-Cooper-Schrieffer superconductivity in B-doped Q-carbon thin films. The magnetic measurements indicate that the upper and lower critical fields follow Hc2(0)[1 - (T/Tc)1.77] and Hc1(0)[1 - (T/Tc)1.19] temperature dependence, respectively. The structure-property characterization of B-doped Q-carbon indicates a high density of electronic states near the Fermi-level and large electron-phonon coupling. These factors are responsible for s-wave bulk type superconductivity with enhanced Tc in B-doped Q-carbon. The time-dependent magnetic moment measurements indicate that B-doped Q-carbon thin films follow the Anderson-Kim logarithmic decay model having high values of pinning potential at low temperatures. The crossover from the two-dimensional to the three-dimensional nature of Cooper pair transport at T/Tc = 1.02 also indicates a high value of electron-phonon coupling which is also calculated using the McMillan formula. The superconducting region in B-doped Q-carbon is enclosed by Tc = 55.0 K, Jc = 5.0 × 108 A cm-2, and Hc2 = 9.75 T superconducting parameters. The high values of critical current density and pinning potential also indicate that B-doped Q-carbon can be used for persistent mode of operation in MRI and NMR applications. The Cooper pairs which are responsible for the high-temperature superconductivity are formed when B exists in the sp3 sites of C. The electron energy loss spectroscopy and Raman spectroscopy indicate a 75% sp3 bonded C and 70% sp3 bonded B in the superconducting phase of B-doped Q-carbon which has 27 at% B and rest C. The dimensional fluctuation and magnetic relaxation measurements in B-doped Q-carbon indicate its practical applications in frictionless motors and high-speed electronics. This discovery of high-temperature superconductivity in strongly-bonded and light-weight materials using non-equilibrium synthesis will provide the pathway to achieve room-temperature superconductivity.

Interface-Induced and Interface-Enhanced Superconductivity

Among the various theoretical mechanisms proposed to reach a higher Tc, the interface mechanism in different forms has been the one most explored, and it provides continual inspiration and hope for realizing the ultimate goal for researchers in the superconductivity field—room-temperature superconductivity. Difficulties do exist, as most of the materials proposed to exhibit the interfacial mechanism are artificially formed heterostructures and are by nature delicate and easily disturbed by strain and change in the stoichiometry at the interface. The discoveries of superconductivity in naturally assembled single crystals of rare earth (R)-doped CaFe2As2 (Ca122) with a Tc up to 49 K and undoped CaFe2As2 with a Tc up to 25 K have opened an alternate route to tackle the problem. The experimental observation has provided the most direct evidence for interface-induced superconductivity in Ca122 to date and possibly in R-doped Ca122. Given the fact that neither chemical doping nor the application of physical pressure has ever induced a Tc higher than 40 K in bulk FeSe, the reports of Tc up to 45–109 K in FeSe/STO thin films demonstrate that FeSe/STO is an ideal system in which to explore interface-enhanced superconductivity. Here we review the above three specific examples of our effort toward superconductors of higher Tc to demonstrate that the interface mechanism may be a promising paradigm to achieve higher Tc.

Exploring high temperature superconductivity mechanism from the conductivity equation obtained with the rate process theory and free volume concept

The conductivity equation developed on the basis of Eyring’s rate process theory and free volume concept has been extended in this article to explore high temperature superconductivity. It is found that the high temperature superconductivity can only happen when the electrons have a fractional coordination number, in contrast to the low temperature superconductivity where electrons must form pair structures in most cases. The superconductivity transition temperatures will move to higher temperature regions if the electrons continue to have lower and lower fractional coordination numbers. For room temperature superconductors, electrons must have a very low fractional coordination number like 1/64, which is consistent with the physical picture of highly localized vortex core bound state theory. All materials can be potentially superconductors if the temperature is low enough, no matter that the conductivity initially increases or decreases with the decrease of temperature. Our findings may shed light on how high temperature superconductivity works and provide clues on how to design high temperature superconductors.

Ab Initio Approach and Its Impact on Superconductivity

One of the main motivations for studying superconductivity is to search for high-temperature superconductors, especially room-temperature superconductors. During the long history of more than 100 years since the discovery of superconductivity, a number of high-temperature superconductors were found and several great breakthroughs were achieved. Recently, thanks to advances in computing power, progress in crystal structure prediction, and developments in density functional theory, computations have been carried out to predict the structures and an appearance of superconductivity of hydrides at high pressure. More exciting, it is been the first time when a simple, perfect cubic phase of H3S which become superconductor at Tc = 200 K under high pressure was successfully predicted by means of ab initio calculation, and then confirmed experimentally. This observation breaks the temperature record of cuprate with Tc = 164 K and further stimulates the studies of hydrides under pressure. Very recently, the high value of Tc = 286 K was theoretically predicted for LaH10 at 210 GPa and this prediction has been confirmed experimentally. These two successful examples demonstrate the importance of ab initio approach to superconductivity.

Waiting for Room Temperature Superconductivity

Waiting for Room Temperature Superconductivity

Wide-Field Imaging of Superconductor Vortices with Electron Spins in Diamond

The mechanism behind high-${T}_{c}$ superconductivity still is not fully understood, yet is key in the quest for room-temperature superconductors. Recently developed techniques based on color centers in diamond offer fresh possibilities to study magnetism in high-${T}_{c}$ superconductors (HTSs). The authors' approach to imaging vortices in an HTS using N-$V$ centers allows quantitative measurement of the magnetic field vector at high bandwidth over a wide temperature range, which was not possible before, as the distance between sample and sensor was too large. In principle this approach is also suitable for studying a variety of other systems, including magnetic domains and skyrmions.

Competition of electron-phonon mediated superconductivity and Stoner magnetism on a flat band

The effective attractive interaction between electrons, mediated by electron-phonon coupling, is a well-established mechanism of conventional superconductivity. In metals exhibiting a Fermi surface, the critical temperature of superconductivity is exponentially smaller than the characteristic phonon energy. Therefore such superconductors are found only at temperatures below a few Kelvin. Systems with flat energy bands have been suggested to cure the problem and provide a route to room-temperature superconductivity, but previous studies are limited to only BCS models with an effective attractive interaction. Here we generalize Eliashberg's theory of strong-coupling superconductivity to systems with flat bands and relate the mean-field critical temperature to the microscopic parameters describing electron-phonon and electron-electron interaction. We also analyze the strong-coupling corrections to the BCS results, and construct the phase diagram exhibiting superconductivity and magnetic phases on an equal footing. Our results are especially relevant for novel quantum materials where electronic dispersion and interaction strength are controllable.

High-Tc Hydrides: Interplay of Optical and Acoustic Modes and Comments Regarding the Upper Limit of Tc

The phonon spectrum of many superconducting compounds and, especially, high-Tc hydrides, is broad and rather complicated, because of the presence of high-frequency optical modes. In order to analyze an interplay of optical and acoustic phonon branches, it is convenient to introduce two coupling constants, $\lambda _{\text {opt.}}$ and $\lambda _{\text {ac.}}$ , along with characteristic frequencies. The correlation between the value of Tc and this interplay is demonstrated for the family of tantal hydrides (TaH2/TaH4/TaH6). The problem of the upper limit of Tc is discussed. The phenomenon of room temperature superconductivity can be provided by the electron-phonon interaction and is described by the strong coupling theory.

Superconducting elements under high pressure

Developing room-temperature superconductor (superconducting at temperature higher than room temperature) is one of our goals of science and technology. Pressure is a powerful tool for investigation of superconductor to improve the properties and to synthesize the target material. Most of superconducting materials show a negative pressure dependence in the superconducting temperature, Tc. But some elements show the positive dependence at high pressure. We have been interested not only in the pressure effect on Tc but also in the onset of new superconductivity under pressures. Here we review the experimental investigations of pressure-induced superconductivity in elements with developments of high-pressure technique combined with low-temperature equipments.

Metallic hydrogen

Hydrogen is the simplest and most abundant element in the Universe. There are two pathways for creating metallic hydrogen under high pressures. Over 80 years ago Wigner and Huntington predicted that if solid molecular hydrogen was sufficiently compressed in the T = 0 K limit, molecules would dissociate to form atomic metallic hydrogen (MH). We have observed this transition at a pressure of 4.95 megabars. MH in this form has probably never existed on Earth or in the Universe; it may be a room temperature superconductor and is predicted to be metastable. If metastable it will have an important technological impact. Liquid metallic hydrogen can also be produced at intermediate pressures and high temperatures and is believed to make up ~90% of the planet Jupiter. We have observed this liquid–liquid transition, also known as the plasma phase transition, at pressures of ~1–2 megabar and temperatures ~1000–2000 K. However, in this paper we shall focus on the Wigner–Huntington transition. We shall discuss the methods used to observe metallic hydrogen at extreme conditions of static pressure in the laboratory, extending our understanding of the phase diagram of the simplest atom in the periodic table.

Nonmetallic FeH6 under High Pressure

High pressure induces unexpected chemical and physical properties in materials. For example, hydrogen-rich compounds under pressure have recently gained much attention as potential room-temperature superconductors, and iron hydrides have also gained significant interest as potential candidates for being the main constituents of the Earth’s core. It is well-known that pressure induces insulator-to-metal transitions, whereas pressure-induced metal-to-insulator transitions are rare, especially for transition metal hydrides. In this article, we have extensively explored the structural phase diagram of iron hydrides by using ab initio particle swarm optimization. We have found a new stable stoichiometry, FeH6, above 213.7 GPa with C2/c symmetry. Interestingly, C2/c FeH6 presents an unexpected nonmetallicity, and its band gap becomes larger with increasing pressure. This is in sharp contrast with P21/m FeH4. The nonmetallicity of C2/c FeH6 mainly originates from the pressure-induced hybridization between the Fe...

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

Summary The most striking difference between the trivalent rare-earth cations and their alkali and alkaline-earth peers is the presence of localized 4 f electrons. Placing trivalent rare-earth cations inside fullerene molecules or in between blocks of intermetallics has given rise to a plethora of fascinating properties and materials. A long-sought-after yet undiscovered material is a semiconducting compound with rare-earth cations situated inside the oversized polyhedral cages of a three-dimensional framework. In this work, we present a synthesis of unconventional clathrates, Ba 8−x R x Cu 16 P 30 , with the smallest pentagonal dodecahedral cages encapsulating La 3+ and Ce 3+ . Their composition and crystal structure were unambiguously determined by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances hole mobility and reduces hole concentration, resulting in a drastic (700%) increase in the thermoelectric performance.

Unusual sulfur isotope effect and extremely high critical temperature in H3S superconductor

Recent experiments have set a new record for the transition temperature at which a material (hydrogen sulfide, H3S) becomes superconducting. Moreover, a pronounced isotope shift of TC in D3S is evidence of an existence of phonon-mediated pairing mechanism of superconductivity that is consistent with the well established Bardeen-Cooper-Schrieffer scenario. Herein, we reported a theoretical studies of the influence of the substitution of 32S atoms by the heavier isotopes 33S, 34S and 36S on the electronic properties, lattice dynamics and superconducting critical temperature of H3S. There are two equally fundamental results presented in this paper. The first one is an anomalous sulfur-derived superconducting isotope effect, which, if observed experimentally, will be subsequent argument that proves to the classical electron-phonon interaction. The second one is fact that critical temperature rise to extremely high value of 242 K for H336S at 155 GPa. This result brings us closer to the room temperature superconductivity.