Scientists discover first type I superconductor that breaks time-reversal symmetry

Amazing stuff!

"... Until recently, every superconductor known to do this was a type II material. But in a study published in the journal Physical Review Letters, researchers report the discovery of a type I superconductor that breaks time-reversal symmetry too. ..."

From the abstract:

"The spontaneous breaking of time-reversal symmetry is a hallmark of unconventional superconductivity, typically observed in type-II superconductors. Here, we report evidence of time-reversal symmetry breaking in the type-I superconductor YbSb2. Zero-field muon spin relaxation and rotation (𝜇⁢SR) measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field 𝜇⁢SR confirms a fully gapped type-I superconducting state.

Our first-principles calculations identify YbSb2 as a ℤ2 topological metal hosting a Dirac nodal line near the Fermi level.

Symmetry analysis within the Ginzburg-Landau framework indicates an internally antisymmetric nonunitary triplet (INT) state as the most probable superconducting ground state. Calculations based on an effective low-energy model further suggest that this INT state may host gapless Majorana surface modes, pointing to the possibility of topological superconductivity in YbSb2.

Our results highlight YbSb2 as a unique material platform where type-I superconductivity coexists with triplet pairing and nontrivial topology."

Scientists discover first type I superconductor that breaks time-reversal symmetry

Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb2 (no public access)

Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb (preprint, open access)

Scandium's electrons may explain predicted room-temperature superconductivity

Good news!

"Scientists have confirmed the existence of a predicted room temperature superconductor, while explaining the microscopic mechanism that distinguishes it from a similar one discovered several years ago. The work, published in the journal Physical Review B, offers "a theoretical blueprint for the future design of superior superconductor hydrides" ...

These metal clathrate superhydrides have unit cells—the smallest repeatable unit—made up of metallic elements surrounded by a "cage" of hydrogen ions. (The structure of LaSc2H24 has fully enclosed lanthanum in a cage of 30 hydrogen atoms and partially enclosed scandium atoms in cages of 24 hydrogen ions.)

Hydrogen atoms occupy three inequivalent sites forming these anisotropic layers, creating structurally different properties between the two different layers. The material's anisotropy opens two different superconductivity channels, each having a different value for the critical temperature. ..."

From the abstract:

"The discovery of LaSc2⁢H24 represents an advance in the quest for room-temperature superconductivity, yet the microscopic mechanism underlying its high-temperature superconductivity remains unclear.

Through a comprehensive revisit of theoretical analysis, we uncover a pivotal transition from the anisotropic two-gap superconductivity of LaH10 to an isotropic single-gap superconductivity in LaSc2⁢H24 upon the introduction of scandium, thereby enhancing the superconducting critical temperature (𝑇c).

This enhancement is rooted in a critical dual role of Sc 3⁢𝑑 electrons:
(i) the Sc-derived Jahn-Teller effect promotes hydrogen metallization via the elongation of specific interlayer H-H bonds and enhances electron-phonon coupling (EPC) through the softening of associated phonon modes;
(ii) Sc 3⁢𝑑 electrons reconstruct the electronic structure into an MgB2-like configuration, generating Sc-H-Sc 𝜎- and 𝛿-bonding states with EPC strengths comparable to LaH10.

Crucially, the pronounced orbital overlap between Sc and the hydrogen cages effectively unifies these two contributions on the Fermi surface. This Sc-induced gap unification bridges the high-EPC H-H states with widespread Sc-H states, establishing an isotropic single-gap nature with a large overall EPC strength.

Our findings identify this Sc-induced gap unification as the fundamental mechanism for achieving room-temperature superconductivity in LaSc2⁢H24, offering a theoretical blueprint for the future design of superior superconducting hydrides."

Scandium's electrons may explain predicted room-temperature superconductivity

Isotropic superconductivity in the room-temperature superconductor LaSc2⁢H24 (no public access)

Isotropic Superconductivity in Room-temperature Superconductor LaSc (preprint, open access)

Chemists overturn 40-year assumption about a key class of superconductors

Amazing stuff!

"Scientists at Warwick have shown that a material treated for 40 years as a uniform, textbook superconductor is in fact a patchwork of different crystal structures throughout its bulk, using an advanced 3D imaging technique to see deep inside the crystal for the first time. ...

What they found was a crystal divided into regions with two subtly different atomic arrangements, separated by boundaries hundreds of times wider than anyone expected to see between two crystal structures. These boundaries were so wide that they behaved almost like a structure in their own right, rather than a simple dividing line. ..."

From the abstract:

"Structural phase transitions generate complex microstructures that often govern material functionality, yet directly resolving their three-dimensional organization in bulk samples remains challenging.

Here we employ scanning three-dimensional x-ray diffraction (3DXRD) to resolve the bulk microstructure of La1.675⁢Eu0.2⁢Sr0.125⁢CuO4 , a prototypical 1/8-doped cuprate in which structural and electronic heterogeneity is well established.

We reveal remarkably broad tetragonallike domain wall regions within the nominally orthorhombic crystal structure, and, upon cooling to 100 K, a fine microstructure of orthorhombiclike stripes embedded within the tetragonal matrix that has significant consequences for interpreting the interplay between structural and electronic heterogeneity in this class of materials. More broadly, this work establishes 3DXRD as a powerful approach for resolving bulk microstructures and understanding their role in emergent functionality."

Chemists overturn 40-year assumption about a key class of superconductors

Warwick chemists overturn 40-year assumption about a key class of superconductor (original news release) "Scientists have shown that a material treated for 40 years as a uniform, textbook superconductor is in fact a patchwork of different crystal structures throughout its bulk, using one of the latest 3D imaging techniques to see deep inside the crystal for the first time."

Giant Domain Walls and Intrinsic Heterogeneity in 214 Cuprate Superconductors (open access)

Fig. 2.

(a) Spatially resolved maps of Γ4+ at 300, 140, 120, and 100 K, illustrating the temperature evolution of the structural domains.

(b) Histograms of Γ4+ at corresponding temperatures, showing the transition from bimodal at 300 K to a more homogeneous but still asymmetric distribution at 100 K.

(c) Maps of the absolute strain magnitude |Γ4+| with 

(d) showing enhanced maps and line profiles ...

Reentrant superconductivity in two-dimensional systems allowing precise engineering and control

Amazing stuff!

"... in a few unconventional materials, competition between ferromagnetic ordering, where the magnetic moments align to point in the same direction, and superconducting ordering, where paired electron moments point in opposite directions, can lead to a phenomenon known as re-entrant superconductivity (RSC). This is where, in an applied field, the magnetic ordering will at first destroy superconductivity, but when the field increases further, the superconductivity reappears. ...

now observed this phenomenon for the first time in a two-dimensional (2D) unconventional superconductor. ...

RSC has previously been observed in three-dimensional ferromagnetic materials such as CeRh2As2, organic superconductors and possibly in so-called heavy fermion compounds such as UTe2. ..."

From the abstract:

"A magnetic field typically suppresses superconductivity by either breaking Cooper pairs via the Zeeman effect or inducing vortex formation.

However, under certain circumstances, a magnetic field can stabilize superconductivity instead.

This seemingly counterintuitive phenomenon is associated with magnetic interactions and has been extensively studied in three-dimensional materials.

By contrast, this phenomenon, hinting at unconventional superconductivity, remains largely unexplored in two-dimensional systems, with moiré-patterned graphene being the only known example.

Here, we report the observation of reentrant superconductivity at the epitaxial (110)-oriented LaTiO3-KTaO3 interface. This phenomenon occurs across a wide range of charge carrier densities, which, unlike in three-dimensional materials, can be tuned in situ via electrostatic gating.

We propose that the observed reentrant superconductivity can arise from an interplay between strong spin-orbit coupling and a magnetic field–driven modification of the Fermi surface.

Our findings provide insight into reentrant superconductivity and establish a robust platform for exploring unconventional superconducting phenomena in two-dimensional systems."

Magnetic field first kills superconductor, then brings it back to life – Physics World

A magnetic field that kills superconductivity can also bring it back (original news release) "Magnetic fields are generally known to destroy superconductivity in a material. However, in exceptional cases they can lead to what is known as “reentrant superconductivity”— where superconductivity disappears as expected but then unexpectedly returns when the magnetic field is increased further. This behavior is sometimes seen in bulk, three-dimensional materials, but now, in a study published in Science Advances, a team led by the RIKEN Center for Emergent Matter Science (CEMS) in Japan has seen the phenomenon in a very thin conducting layer at the boundary between two insulating oxide materials. Because oxide interfaces can be precisely engineered and controlled, the discovery provides a new platform for investigating unconventional forms of superconductivity and the quantum mechanisms that allow it to survive under unusual conditions."

Reentrant superconductivity at an oxide heterointerface (open access)

Yes, no, yes: Schematic showing how an applied field affects superconductivity in a LaTiO3/KTaO3 heterostructure.

Fig. 1. Properties of the (110)-oriented LaTiO3-KTaO3 interface.

Fig. 2. Experimental visualization of reentrant superconducting response.

Bilayer Graphene’s Magic further Revealed

Recommendable! A nice overview article!

"... Two twisted bilayer graphene (TBG) ...

Is it conventional superconductivity, such as that first identified in mercury more than a century ago, or is it unconventional superconductivity, such as that discovered in the 1980s in high-temperature superconductors? A new study by Julien Barrier of the University of Manchester in the UK and colleagues may help to provide a resolution to this conundrum ...

To help settle this debate, Barrier and colleagues stacked two TBGs, one with a superconductivity-inducing twist close to the magic angle, the other with a smaller (not magic) twist angle (Fig. 1). This second bilayer acted as a normal metal. By gating each bilayer separately, the researchers could induce different amounts of charge in each, allowing the metallic layer to screen electrons in the superconducting layer. To preserve their distinct electronic properties, the two TBGs had to be electronically decoupled so that their electronic states would not mix. In previous experiments, researchers achieved this decoupling using dielectric hexagonal boron nitride (h-BN) spacers, but these several-nanometer-thick layers weakened the screening effect. Instead, Barrier and colleagues introduced a 10° twist between the two TBGs. This ingenious trick decouples the two moiré patterns and suffices to keep their electronic states separate, avoiding the need for physical separation.

Engineering such screening is a good way to probe the pairing mechanism. In the Eliashberg theory of conventional superconductivity—a refinement of BCS theory—electrons experience an effective Coulomb repulsion μ * that must be overcome by an attraction mediated through the exchange of phonons. Increasing the screening between electrons lowers the repulsion μ* while leaving their phonon-mediated attraction unchanged.

If TBG is a conventional superconductor, this screening effect should make phonon-induced electron pairing easier and raise the material’s T c.

But this is exactly the opposite of what Barrier and colleagues observed. They found that increasing the screening between electrons in the superconducting TBG caused the superconducting phase to disappear.

After ruling out trivial explanations, the researchers concluded that this disappearance offers strong evidence that TBG’s superconductivity arises from some type of correlated electron motion that is, at most, very weakly dependent on phonon-mediated attraction. ...

To model the observed behavior, Barrier and colleagues assumed that TBG’s superconductivity arises from an unconventional plasmon-mediated mechanism. Their basic argument is that, for the electronic states involved in superconductivity, screening efficiency decays exponentially on a scale of ∼

2 nm. Because their device uses a large twist angle to decouple the two TBGs instead of a dielectric spacer, the interlayer separation is of subnanometer size, making screening maximally efficient, and much stronger than in earlier experiments with 3–10-nm h-BN spacer layers. Besides explaining the disappearance of the superconducting phase, this plasmon model could also explain an observed disappearance of correlated insulating phases at temperatures below 20 K. ..."

From the abstract:

"The origin of superconductivity in magic-angle twisted bilayer graphene has been a subject of intense debate. While some experimental evidence indicated an unconventional pairing mechanism which should be sensitive to Coulomb screening, experimental attempts to tune the critical temperature by screening Coulomb interactions so far have remained unsuccessful, possibly indicating a conventional phonon-mediated pairing.

Here we study a double-layer electronic system consisting of two twisted graphene bilayers in immediate proximity of each other but remaining electronically decoupled.

By increasing the carrier density in one bilayer, we completely suppressed both the superconductivity and the correlated-insulator state in the adjacent magic-angle graphene. The observation of such an effect from screening offers support for an unconventional mechanism of Cooper pairing in magic-angle twisted bilayer graphene, shedding new light on the underlying physics governing their properties."

Physics - Bilayer Graphene’s Magic Revealed "New experiments suggest that superconductivity in twisted bilayer graphene depends on an unconventional electron-pairing mechanism."

Coulomb Screening of Superconductivity in Magic-Angle Graphene (open access)

Fig. 1 Characterization of twisted tetralayer graphene device

Graphene can hold multiple states of superconductivity, a new study finds

Good news! More, but incremental progress! 

Superconductivity like nuclear fusion hold huge promises to improve global energy supply!

"In a study appearing ... in the journal Nature, ... researchers report that a certain microscopic structure found in natural graphite can host multiple superconducting states. Superconductivity is an electronic state of matter in which electrons pair up and glide through a material with zero resistance.  ..."

"... In our work, we present a family of three surprising states of superconductivity in 4- or 5-layer rhombohedral graphene, all of which are are able to persist in the presence of strong in-plane magnetic fields up to ~9 T, exceeding the Pauli limit by far more than a factor of 10.

In a further surprise, one state is even enhanced by a perpendicular magnetic field. This is in contract to bernal graphene, which showed only relatively weak in-plane enhancement. The two other states are boosted by the in-plane field, and one of them is only created above ~5 Tesla in-plane field. None of these states could be suppressed with the 9 T magnet of this experiment. 

This establishes a new family of unconventional, magnetic field-boosted superconductors in rhombohedral graphene. It is currently still unclear what the microscopic pairing mechanism is. One possible explanation is that the electrons in these states pair differently than in the paradigmatic simple superconductors — for example, with their spins aligned in the same direction in a triplet state. This could make them less sensitive to magnetic fields. ..."

From the abstract:

"In some unconventional superconductors, time-reversal symmetry can be broken in addition to the gauge symmetry, resulting in superconductivities that can be enhanced or induced by magnetic fields.

However, field-enhanced superconductors are more vulnerable to impurities than Bardeen-Cooper-Schrieffer counterparts.

Crystalline rhombohedral multilayer graphene is a promising platform to explore them due to its superior material quality and gate-tunable strong correlation effects.

Here we report transport measurements of rhombohedral tetralayer and pentalayer graphene, demonstrating a spectrum of clean-limit superconductivities. We found three different types of field-enhanced and field-induced superconductivities in the pentalayer. They are all robust against an in-plane field up to 8.5 Tesla, exceeding the Pauli limit by tens of times.

Compared to Bernal bilayer graphene showing only in-plane field-enhancement, pentalayer graphene features superconductors enhanced by out-of-plane as well as in-plane fields. They also reside at much lower gate electric fields owing to the intrinsically flatter band dispersion—facilitating their study and further engineering.

Additionally, we observed that proximitized spin-orbit coupling (SOC) generates multiple new superconductors without introducing additional disorder effects.

Our work establishes a new family of magnetic field-boosted superconductors in rhombohedral graphene.

Utilizing the high accessibility with moderate gate voltages, this will pave the way for realizing non-Abelian quasiparticles through interfacial engineering in the extreme clean limit, in that proximitized SOC leads to topological states8 and maintains the ultrahigh quality of crystalline graphene."

Graphene can hold multiple states of superconductivity, a new study finds | MIT News | Massachusetts Institute of Technology "What’s more, the superconducting states get stronger under conditions expected to kill them."

Published in Nature: Family of magnetic field-boosted superconductors in rhombohedral graphene (second original news release)

Family of magnetic field-boosted superconductors in rhombohedral graphene (no public access)

Family of Unconventional Superconductivities in Crystalline Graphene (preprint, open access)

The cartoon sketch shows the family of three different states found in the experiment indicated with three different colors

Building a superconducting quantum circuit that follows protons on the go

Amazing stuff!

"Researchers ... have created a device that simulates the quantum “tunneling” behavior of protons that occurs in chemistry, a process so common it occurs in everything from photosynthesis to the formation of human DNA.

The advance has the potential to aid researchers across a variety of disciplines, including the development of new solar fuels, pharmaceuticals, and materials. ...

“Our system is so clean and controllable that we could resolve very subtle quantum tunneling effects with it that were unknown to us,” ..."

From the abstract:

"Dissipative tunneling remains a cornerstone effect in quantum mechanics. In chemistry, it plays a crucial role in governing the rates of chemical reactions, often modeled as the motion along the reaction coordinate from one potential well to another. The relative positions of energy levels in these wells strongly influence the reaction dynamics. Chemical research will benefit from a fully adjustable, asymmetric double-well equipped with precise measurement capabilities of the tunneling rates.

In this paper, we show a quantum simulator system that consists of a continuously driven Kerr parametric oscillator with a third-order nonlinearity that can be operated in the quantum regime to create a fully tunable asymmetric double-well. Our experiment leverages a low-noise, all-microwave control system with a high-efficiency readout, based on a tunnel Josephson junction circuit, of the which-well information.

We explore the reaction rates across the landscape of tunneling resonances in parameter space. We uncover two counter-intuitive effects:

(i) a weak asymmetry can significantly decrease the activation rates, even though the well in which the system is initialized is made shallower, and

(ii) the width of the tunneling resonances alternates between narrow and broad lines as a function of the well depth and asymmetry.

We predict by numerical simulations that both effects will also manifest themselves in ordinary chemical double-well systems in the quantum regime.

Our work is a first step for the development of analog molecule simulators of proton transfer reactions based on quantum parametric processes."

Building a superconducting quantum circuit that follows protons on the go | Yale News "A new device that originated at a pair of Yale labs simulates quantum proton “tunneling,” a ubiquitous phenomenon found commonly in chemistry and biology."

Asymmetry Control in a Parametric Oscillator for the Quantum Simulation of Chemical Activation (open access)

Fig. 1 Experimental setup. (a) Rendering of the half-aluminum, half-copper sample package containing two sapphire chips magnified in (b).