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).

Scientists capture superconductivity's 'dancing pairs' for first time, revealing missing pieces in a decades-old theory

Amazing stuff! 

What makes me wonder is that this study was done on atoms instead of electrons. What difference does it make going from electrons to atoms when it comes to superconductivity?

"For the first time, scientists have directly imaged the quantum process underlying superconductivity, a phenomenon in which paired electrons cause electric current to flow without resistance at sufficiently low temperatures. ...

the scientists directly imaged individual atoms pairing up in a special gas cooled nearly to absolute zero — the unreachable limit to how cold things can get. The type of gas, called a Fermi gas, allows scientists to substitute electrons with atoms and probe the physics of superconductors in a controlled way.

Surprisingly, the scientists found that after pairing up, the atoms moved in a synchronized dance, with their positions dependent on those of other pairs — a phenomenon not predicted by the 70-year-old, Nobel Prize-winning theory of superconductivity. ...

Using a newly developed imaging method, the experimental physicists captured snapshots of the relative positions of the pairs. The scientists used a special gas mixture made of lithium atoms, cooled to just a few billionths of a degree Celsius above absolute zero. At these temperatures, the atoms act as fermions, a fundamental class of particles that includes electrons. Since these fermions all follow the same physics of pairing, the atoms are suitable substitutes for studying electron behavior in superconductors.

The imaging revealed that the positions of paired atoms became influenced by those of other pairs. The paired atoms maintained a separation from other paired atoms, just as dancing couples keep their distance from other dancers in a ballroom ... This finding adds a new understanding of these systems that was missing from the historic BCS theory. ..."

"... From these observations, theorists have developed models—notably the Bardeen-Cooper-Schrieffer (BCS) theory, which assumes that the zero-resistance flow in a superconductor arises from electrons forming so-called Cooper pairs. This theory has been successful in explaining a large class of superconductors, but ... colleagues have now observed behavior that contradicts BCS predictions. Using a recently developed technique called atom-resolved continuum quantum gas microscopy, the researchers directly observed spatial correlations in cold atoms that mimic superconducting electrons. These high-precision measurements revealed an unexpected anticorrelation between opposite-spin atoms, implying deficiencies in the BCS theory. This and other surprising results demonstrate once again how new observational lenses can put long-standing theoretical models into question. ...

Predicting the collective behavior of electrons within materials is a formidable challenge. The many-body problem for classical particles is already difficult, but it is made unbelievably more complex for electrons and other fermions by the infamous “sign problem”: The wave function of fermionic particles changes sign upon particle exchange. This antisymmetric behavior gives rise to Pauli’s exclusion principle and makes modeling fermionic many-body systems incredibly challenging. ..."

From the abstract:

"In this Letter, we explore two-dimensional attractive Fermi gases at the microscopic level by probing spatial charge and spin correlations in situ.

Using atom-resolved continuum quantum gas microscopy, we directly observe fermion pairing and study the evolution of two- and three-point correlation functions as interspin attraction is increased.

The precision of our measurement allows us to reveal nonlocal anticorrelations in the pair correlation function, fundamentally forbidden by the mean-field result based on BCS theory but whose existence we confirm in exact auxiliary-field quantum Monte Carlo calculations.

We demonstrate that the BCS prediction is critically deficient not only in the superfluid crossover regime but also deep in the weakly attractive side.

Guided by our measurements, we find a remarkable relation between two- and three-point correlations that establishes the dominant role of pair correlations. Finally, leveraging local single-pair losses, we independently characterize the short-range behavior of pair correlations, via the measurement of Tan’s contact, and find excellent agreement with numerical predictions.

Our measurements provide a novel microscopic view into strongly correlated two-dimensional Fermi gases in the continuum."

Scientists capture superconductivity's 'dancing pairs' for first time, revealing missing pieces in a decades-old theory "Analysis of a first-of-its-kind experiment reveals missing pieces in the decades-old theory of superconductivity."

Scientists Capture Superconductivity’s ‘Dancing Pairs’ for First Time, Filling Gap in Decades-Old Theory (original news release) "Analysis of a first-of-its-kind experiment reveals missing pieces in the decades-old theory of superconductivity."

Superconductor Theory Under Cold-Atom Scrutiny "Snapshot measurements of cold-atom gases reveal hidden spin correlations that could force an update of some superconductivity theories."

Observing Spatial Charge and Spin Correlations in a Strongly Interacting Fermi Gas (no public access)

Figure 1: A continuum quantum gas microscope can image a 2D collection of cold atoms (left). In the case of a fermionic gas, the technique can differentiate between spin-up and spin-down atoms. Using the microscope data, researchers can compute the correlation function (right). The observations (solid orange line) disagree with the Bardeen-Cooper-Schrieffer theory (dashed yellow line) in that they show an anticorrelation “dip” for opposite spin atoms at a particular interparticle distance.

Triplet superconductivity—physicists may have found the missing link for quantum computers

Amazing stuff!

Google search: "Triplet superconductivity is an exotic, rarely observed state of matter where electrons pair up with parallel spins (spin triplet S=1), in contrast to conventional "singlet" superconductors (spin S=0) where spins are antiparallel."

"... "Triplet superconductors make a number of unusual physical phenomena possible. These phenomena have important applications in quantum technology and spintronics," said Linder.

More detailed information about these applications:

The reason triplet superconductors can transfer spin without energy loss is that the superconducting particles now carry spin with them.

Triplet superconductors can also be used to create a very exotic type of particle called a "Majorana particle."

A Majorana particle is its own antiparticle. Therefore, it can perform calculations in a quantum computer in a stable way. ...

Conventional superconductors are so-called "singlet superconductors." In simple terms, this means that the superconducting particles do not have spin.

In triplet superconductors, however, the superconducting particles have spin. ...

"The fact that triplet superconductors have spin has an important consequence. We can now transport not only electrical currents but also spin currents with absolutely zero resistance," ...."

From the abstract:

"NbRe is a noncentrosymmetric superconductor that has been proposed as a candidate for intrinsic spin-triplet pairing. However, a conclusive demonstration of triplet pairing in NbRe is yet to be found. To probe the presence of spin-triplet Cooper pairs, we fabricated Py/NbRe/Py trilayers capped with an antiferromagnetic layer.

Magnetic and electrical measurements reveal an inverse spin-valve effect, which could indicate equal-spin-triplet superconductivity. The minimal sample structure and the lack of ad hoc engineered interfaces clearly associate our observation to intrinsic triplet correlations of NbRe. The availability of NbRe in thin-film form and the simplicity of the heterostructure highlight its potential as a scalable platform for superconducting spintronics."

Triplet superconductivity—physicists may have found the missing link for quantum computers

Physicists may have found the missing link for quantum computers

Unveiling Intrinsic Triplet Superconductivity in Noncentrosymmetric NbRe through Inverse Spin-Valve Effects (no public access)

Unveiling Intrinsic Triplet Superconductivity in Noncentrosymmetric NbRe through Inverse Spin-Valve Effects (preprint, open access)

Physicists uncover hidden magnetic order in the mysterious pseudogap phase

Amazing stuff!

"Physicists have uncovered a link between magnetism and a mysterious phase of matter called the pseudogap, which appears in certain quantum materials just above the temperature at which they become superconducting. The findings could help researchers design new materials with sought-after properties such as high-temperature superconductivity, in which electric current flows without resistance. 

Using a quantum simulator chilled to just above absolute zero, the researchers discovered a universal pattern in how electrons — which can have spin up or down — influence their neighbors’ spins as the system is cooled. The findings represent a significant step toward understanding unconventional superconductivity ...

In many high-temperature superconductors, the transition to the superconducting state does not emerge out of a conventional metallic state. Instead, the material first enters a curious intermediate regime known as the pseudogap, in which electrons start behaving in unusual ways, and fewer electronic states are available for electrons to flow through the material. ...

In materials containing an unaltered number of electrons, the electrons arrange themselves in an orderly, alternating magnetic pattern known as antiferromagnetism. In this pattern, neighboring electron spins point in opposite directions — like dancers following a precise left-right rhythm.

But when electrons are removed through a process known as doping, this magnetic order becomes strongly disrupted. For a long time, researchers assumed that doping destroyed long-range magnetic order entirely. The new study in PNAS, however, shows that at extremely low temperatures, a subtle form of organization remains, hidden beneath the apparent disorder. ..."

From the significance and abstract:

"Significance

Understanding strongly correlated fermions constitutes a major challenge of modern physics. Here, we take a significant step in this direction, by the finding of a universal scaling of spin and charge correlations upon entering the pseudogap phase in the paradigmatic Hubbard model, using our ultracold atom quantum simulator. This leads to a quantitative description of how doping suppresses the spin stiffness, concurrent with the emergence of dominant higher-order correlations that we observe in the system. Our characterization of the magnetic properties of the pseudogap in the paradigmatic Hubbard model paves the way for future studies of further collective phases of matter that the pseudogap is believed to give way to at even lower temperatures.

Abstract

In strongly correlated materials, interacting electrons are entangled and form collective quantum states, resulting in rich low-temperature phase diagrams. Notable examples include cuprate superconductors, in which superconductivity emerges at low doping out of an unusual “pseudogap” metallic state above the critical temperature.

The Fermi–Hubbard model, describing a wide range of phenomena associated with strong electron correlations, still offers major computational challenges despite its simple formulation. In this context, ultracold atoms quantum simulators have provided invaluable insights into the microscopic nature of correlated quantum states.

Here, we use a quantum gas microscope Fermi–Hubbard simulator to explore a wide range of dopings and temperatures in a regime where a pseudogap is known to develop. By measuring multipoint correlation functions up to fifth order, we uncover a universal scaling behavior in magnetic and higher-order spin–charge correlations characterized by a doping-dependent temperature scale. Accurate comparisons with determinant Quantum Monte Carlo and Minimally Entangled Typical Thermal States simulations confirm that this temperature scale is comparable to the pseudogap temperature.

Our quantitative findings reveal a qualitative behavior of magnetic properties and spin–charge correlations in an emergent pseudogap and pave the way toward the exploration of charge pairing and collective phenomena expected at lower temperatures."

Physicists uncover hidden magnetic order in the mysterious pseudogap phase

Hidden Order in Quantum Confusion: The Pseudogap (original news release) "Quantum simulator using ultracold atoms reveals how subtle magnetic patterns shape one of the most puzzling states of matter."

Observation of emergent scaling of spin–charge correlations at the onset of the pseudogap (open access)

Fig. 4 Emergence of extended polarons in the pseudogap. (A) Example of polaron correlations 

Anyons may be at the root of surprising quantum experiments and a new form of superconductivity

Amazing stuff!

"In the past year, two separate experiments in two different materials captured the same confounding scenario: the coexistence of superconductivity and magnetism. Scientists had assumed that these two quantum states are mutually exclusive; the presence of one should inherently destroy the other. ...

proposes that under certain conditions, a magnetic material’s electrons could splinter into fractions of themselves to form quasiparticles known as “anyons.” In certain fractions, the quasiparticles should flow together without friction, similar to how regular electrons can pair up to flow in conventional superconductors. ...

introduce an entirely new form of superconductivity — one that persists in the presence of magnetism and involves a supercurrent of exotic anyons rather than everyday electrons. ...

For decades, it was thought that superconductivity and magnetism should not co-exist; superconductivity is a delicate state, and any magnetic field can easily sever the bonds between Cooper pairs. But earlier this year, two separate experiments proved otherwise. In the first experiment, ... discovered superconductivity and magnetism in rhombohedral graphene — a synthesized material made from four or five graphene layers. ...

Shortly after, a second team reported similar dual states in the semiconducting crystal molybdenium ditelluride (MoTe2). Interestingly, the conditions in which MoTe2 becomes superconductive happen to be the same conditions in which the material exhibits an exotic “fractional quantum anomalous Hall effect,” or FQAH — a phenomenon in which any electron passing through the material should split into fractions of itself. These fractional quasiparticles are known as “anyons.” ...

Their work revealed that superconducting anyons can emerge at certain electron densities. What’s more, they found that when superconducting anyons first emerge, they do so in a totally new pattern of swirling supercurrents that spontaneously appear in random locations throughout the material. This behavior is distinct from conventional superconductors and is an exotic state that experimentalists can look for as a way to confirm the team’s theory. If their theory is correct, it would introduce a new form of superconductivity, through the quantum interactions of anyons. ..."

From the abstract:

"Motivated by the experimental discovery of the fractional quantum anomalous Hall effect, we develop a theory of doping-induced transitions out of the = 2/3 lattice Jain state in the presence of quenched disorder.

We show that disorder strongly affects the evolution into the conducting phases described in our previous work. The delocalization of charge 2/3 anyons leads to a chiral superconductor through a direct second-order transition for a smooth random potential with long-wavelength modulations. The longitudinal resistance has a universal peak at the associated quantum critical point.

Close to the transition, we show that the superconducting ground state is an “Anomalous Vortex Glass” stabilized in the absence of an external magnetic field. For short-wavelength disorder, this transition generically splits into three distinct ones with intermediate insulating topological phases.

If instead, the charge 1/3 anyon delocalizes, then at low doping the resulting phase is a Reentrant Integer Quantum Hall state with xy = h/e 2 .

At higher doping this undergoes a second transition to a Fermi liquid metal. We show that this framework provides a plausible explanation for the complex phase diagram recently observed in twisted MoTe2 near = 2/3 and discuss future experiments that can test our theory in more detail."

Anything-goes “anyons” may be at the root of surprising quantum experiments | MIT News | Massachusetts Institute of Technology "MIT physicists say these quasiparticles may explain how superconductivity and magnetism can coexist in certain materials."

Anyon delocalization transitions out of a disordered fractional quantum anomalous Hall insulator (no public access)

Anyon delocalization transitions out of a disordered fractional quantum anomalous Hall insulator (open access)

China's maglev test reaches 435 mph (700 kmh) in under two seconds

Amazing stuff!

"Researchers ... have accelerated a one-ton vehicle from a dead stop to 435 mph (700 km/h) in under two seconds – then back to zero mph on about a 1/4 mile (1,312 ft / 400 meter) magnetic levitation test track. ... That makes it the quickest superconducting maglev acceleration ever demonstrated. ...

But NUDT isn't stopping there. The eventual goal is to hit the 621 mph (1,000 km/h) mark, surpassing the typical 550-580 mph (885-933 km/h) that passenger planes generally cruise at. ...

Though called "high-temperature superconductors (HTS)," it actually operates using liquid nitrogen, closer to -321 °F (–196 °C) ..."

China's maglev test reaches 435 mph in under two seconds

China’s record-smashing maglev achieves 0-700km/h acceleration in less than 2 seconds "Feat seen to catapult China into global top tier of ultra-high-speed maglev technology and unlock new possibilities for hyperloop transport"

Scientists Create New Type of Semiconductor in hyperdoped Germanium

Good news!

"Researchers have made germanium superconducting for the first time, a feat that could transform computing and quantum technologies. Using molecular beam epitaxy to embed gallium atoms precisely, the team stabilized the crystal structure to carry current without resistance. The discovery paves the way for scalable, energy-efficient quantum devices and cryogenic electronics."

From the abstract:

"Doping-induced superconductivity in group-IV elements may enable quantum functionalities in material systems accessible with well-established semiconductor technologies.

Non-equilibrium hyperdoping of group-III atoms into C, Si or Ge can yield superconductivity; however, its origin is obscured by structural disorder and dopant clustering.

Here we report the epitaxial growth of hyperdoped Ga:Ge films and trilayer heterostructures by molecular-beam epitaxy with extreme hole concentrations (nh = 4.15 × 1021 cm−3, 17.9% Ga substitution) that yield superconductivity with a critical temperature of Tc = 3.5 K.

Synchrotron-based X-ray absorption and scattering methods reveal that Ga dopants are substitutionally incorporated within the Ge lattice, introducing a tetragonal distortion to the crystal unit cell.

Our findings, corroborated by first-principles calculations, suggest that the structural order of Ga dopants creates a narrow band for the emergence of superconductivity in Ge, establishing hyperdoped Ga:Ge as a low-disorder, epitaxial superconductor–semiconductor platform."

Scientists turn common semiconductor into a superconductor "A team has made germanium, a key semiconductor, superconducting by precisely integrating gallium atoms into its crystal lattice."

Scientists Create New Type of Semiconductor that Holds Superconducting Promise (original news release) "International team of physicists’ innovation could vastly advance wireless communications, computer speed, and aerospace technology"

Superconductivity in substitutional Ga-hyperdoped Ge epitaxial thin films (no public access)

Superconductivity in Substitutional Ga-Hyperdoped Ge Epitaxial Thin Films (preprint, open access)

Figure 1:Superconductivity in germanium by p-type hyperdoping

How quantum computers can aid the search for room-temperature superconductors

Good news! Amazing stuff!

"For the first time, a quantum computer has successfully measured pairing correlations (quantum signals that show electrons teaming up in pairs), which is essential to helping scientists find one of the holy grails of physics—superconductors that work at room temperature. ..."

"... In our work, we've simulated three different regimes of the Fermi-Hubbard model and successfully measured non-zero superconducting pairing correlations — a first for any quantum computing platform.

We began by preparing a low-energy state of the model at half-filling — a standard benchmark for testing quantum simulations. Then, using simulated laser pulses or electric fields, we perturbed the system and observed how it responded.

After these perturbations, we measured a notable increase in the so-called “eta” pairing correlations, a mathematical signature of superconducting behavior. These results prove that our computers can help us understand light-induced superconductivity ... Helios offers a new level of control and insight. By tuning every aspect of the simulation — from pulse shape, to field strength, to lattice geometry — researchers can explore scenarios that are completely inaccessible to real materials or analog simulators. ..."

From the abstract:

"The Fermi-Hubbard model is the starting point for the simulation of many strongly correlated materials, including high-temperature superconductors, whose modelling is a key motivation for the construction of quantum simulation and computing devices.

However, the detection of superconducting pairing correlations has so far remained out of reach, both because of their off-diagonal character-which makes them inaccessible to local density measurements-and because of the difficulty of preparing superconducting states.

Here, we report measurement of significant pairing correlations in three different regimes of Fermi-Hubbard models simulated on Quantinuumś Helios trapped-ion quantum computer. Specifically, we measure non-equilibrium pairing induced by an electromagnetic field in the half-filled square lattice model, d-wave pairing in an approximate ground state of the checkerboard Hubbard model at."

How quantum computers can aid the search for room-temperature superconductors

Helios Delivers Quantum Advantage with Real-World Impact (original news release) "A breakthrough in room-temperature superconductivity simulation opens doors to transformative technologies"

Superconducting pairing correlations on a trapped-ion quantum computer (preprint, open access)

Introducing Helios: The Most Accurate Quantum Computer in the World

Figure 1: A rendering of the Quantinuum Helios system deployed at a customer site. 

Physicists observe key evidence of unconventional superconductivity in magic-angle graphene

Good news! Sounds almost like Voodoo ("unconventional", "magic-angle")! 😊

Are we finally coming closer to room temperature superconductivity?

"... “magic-angle” twisted tri-layer graphene (MATTG) ...

 In particular, the team was able to measure MATTG’s superconducting gap — a property that describes how resilient a material’s superconducting state is at given temperatures. They found that MATTG’s superconducting gap looks very different from that of the typical superconductor, meaning that the mechanism by which the material becomes superconductive must also be different, and unconventional. ...

The researchers made their discovery using a new experimental platform that allows them to essentially “watch” the superconducting gap, as the superconductivity emerges in two-dimensional materials, in real-time. They plan to apply the platform to further probe MATTG, and to map the superconducting gap in other 2D materials — an effort that could reveal promising candidates for future technologies. ..."

From the abstract:

"Understanding the nature of superconductivity in magic-angle graphene remains challenging. A key difficulty lies in discerning the different energy scales in this strongly interacting system, particularly the superconducting gap.

Here, we report simultaneous tunneling spectroscopy and transport measurements of magic-angle twisted trilayer graphene.

This approach allows us to identify two coexisting V-shaped tunneling gaps with different energy scales:

a distinct low-energy superconducting gap that vanishes at the superconducting critical temperature and magnetic field, and

a higher-energy pseudogap.

The superconducting tunneling spectra display a linear gap-filling behavior with temperature and magnetic field and exhibit the Volovik effect, consistent with a nodal order parameter.

Our work suggests an unconventional nature of the superconducting gap and establishes an experimental framework for multidimensional investigation of tunable quantum materials."

MIT physicists observe key evidence of unconventional superconductivity in magic-angle graphene | MIT News | Massachusetts Institute of Technology "The findings could open a route to new forms of higher-temperature superconductors."

Experimental evidence for nodal superconducting gap in moiré graphene (no public access)

Experimental evidence for nodal superconducting gap in moiré graphene (manuscript, open access)

Scientists find new quantum behavior in unusual superconducting material

Amazing stuff!

"Researchers ... have discovered direct evidence of active flat electronic bands in a kagome superconductor. ...

It focuses the chromium-based kagome metal CsCr₃Sb₅, which becomes superconducting under pressure. ..."

"... Kagome metals, characterized by their two-dimensional lattices of corner-sharing triangles, have recently been predicted to host compact molecular orbitals, or standing-wave patterns of electrons that could potentially facilitate unconventional superconductivity and novel magnetic orders that can be made active by electron correlation effects. In most materials, these flat bands remain too far from active energy levels to have any significant impact; however, in CsCr₃Sb₅, they are actively involved and directly influence the material's properties. ... 

"By identifying active flat bands, we've demonstrated a direct connection between lattice geometry and emergent quantum states," ..."

From the abstract:

"In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction.

Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking.

Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor CsCr3Sb5 is dominated by FBs at the Fermi level.

We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order."

Scientists find new quantum behavior in unusual superconducting material

A strange quantum effect could power future electronics (original news release)

Spin excitations and flat electronic bands in a Cr-based kagome superconductor (open access)

Fig. 2: Electronic structure of CsCr3Sb5.

Twisted trilayer graphene shows high kinetic inductance

Amazing stuff! Graphene keeps on giving! See also my other, same day post on trilayer graphene.

"Researchers ... recently set out to better understand the mechanisms behind the unconventional superconductivity observed in twisted graphene moiré heterostructures, material consisting of stacked graphene sheets twisted at an angle of approximately 1.1°.

Their paper ... unveils a large and tunable kinetic inductance (i.e., a resistance to changes in current prompted by the inertia of charge carriers) in twisted trilayer graphene, offering new insight about the underpinnings of superconductivity in moiré materials. ..."

Twisted trilayer graphene shows high kinetic inductance

Large Tunable Kinetic Inductance in a Twisted Graphene Superconductor (no public access)

Large tunable kinetic inductance in a twisted graphene superconductor (preprint, open access)

Closing in on superconducting semiconductors

Amazing stuff!

"... Superconducting electronics have arisen as a promising alternative for classical and quantum computing, although their full exploitation for high-end computing requires a dramatic reduction in the amount of wiring linking ambient temperature electronics and low-temperature superconducting circuits.

To make systems that are both larger and more streamlined, replacing commonplace components such as semiconductors with superconducting versions could be of immense value. ...

One of the critical long-standing requirements is the need for the efficient conversion of AC currents into DC currents on a chip while operating at the extremely cold cryogenic temperatures required for superconductors to work efficiently.

For example, in superconducting “energy-efficient rapid single flux quantum” (ERSFQ) circuits, the AC-to-DC issue is limiting ERSFQ scalability and preventing their use in larger circuits with higher complexities. To respond to this need, ... team created superconducting diode (SD)-based superconducting rectifiers — devices that can convert AC to DC on the same chip. These rectifiers would allow for the efficient delivery of the DC current necessary to operate superconducting classical and quantum processors. ...

The new approach ... will significantly cut down on the thermal and electromagnetic noise traveling from ambient into cryogenic circuitry, enabling cleaner operation.

The SDs could also potentially serve as isolators/circulators, assisting in insulating qubit signals from external influence. The successful assimilation of multiple SDs into the first integrated SD circuit represents a key step toward making superconducting computing a commercial reality.  ..."

From the abstract:

"Superconducting electronics is of use in the development of energy-efficient classical and quantum computing applications.

Non-reciprocal superconducting circuit elements, such as superconducting diodes, are needed for such systems, but integrating several superconducting diodes in a superconducting circuit remains a challenge.

Here we report a superconducting diode bridge that consists of multiple superconducting diodes with reproducible characteristics and operating temperatures of a few Kelvin.

The superconducting diodes are fabricated from thin-film bilayers of the elemental superconductor vanadium and the insulating ferromagnet europium sulfide.

Four practically identical diodes are patterned on the same superconducting film to create the superconducting diode bridge. The bridge can function as a full-wave rectifier with an efficiency up to 42 ± 5%, and offers alternating current (a.c.) to direct current (d.c.) signal conversion capabilities at frequencies up to 40 kHz."

Closing in on superconducting semiconductors | MIT News | Massachusetts Institute of Technology "Plasma Science and Fusion Center researchers created a superconducting circuit that could one day replace semiconductor components in quantum and high-performance computing systems."

Efficient superconducting diodes and rectifiers for quantum circuitry (no public access)

Highly Efficient Superconducting Diodes and Rectifiers for Quantum Circuitry (open access; Google search identified this preprint as a close match last updated June 2024)

Figure taken from preprint

Physicists discover a new type of superconductor that’s also a magnet

Amazing stuff! It's the magic graphene again!

"Magnets and superconductors go together like oil and water — or so scientists have thought. ...

In a paper ... the physicists report that they have discovered a “chiral superconductor” — a material that conducts electricity without resistance, and also, paradoxically, is intrinsically magnetic. ... this exotic superconductivity in ... graphite, the primary material in pencil lead. ...

when four or five sheets of graphene are stacked in this “rhombohedral” configuration, the resulting structure can exhibit exceptional electronic properties that are not seen in graphite as a whole. ..."

From the abstract:

"Chiral superconductors are unconventional superconducting states that break time reversal symmetry spontaneously and typically feature Cooper pairing at non-zero angular momentum. Such states may host Majorana fermions and provide an important platform for topological physics research and fault-tolerant quantum computing. Despite intensive search and prolonged studies of several candidate systems, chiral superconductivity has remained elusive so far.

Here we report the discovery of robust unconventional superconductivity in rhombohedral tetra- and penta-layer graphene without moiré superlattice effects. We observed two superconducting states in the gate-induced flat conduction bands with Tc up to 300 mK and charge density ne down to 2.4*1011 cm-2 in five devices.

Spontaneous time-reversal-symmetry-breaking due to electron’s orbital motion is found, and several observations indicate the chiral nature of these superconducting states, including:

1. In the superconducting state, Rxx shows magnetic hysteresis in varying out-of-plane magnetic field B⊥—absent from all other superconductors;

2. the superconducting states are robust against in-plane magnetic field and are developed within a spin- and valley-polarized quarter-metal phase;

3. the normal states show anomalous Hall signals at zero magnetic field and magnetic hysteresis.

We also observed a critical B⊥ of 1.4 Tesla, higher than any graphene superconductivity and indicates a strong-coupling superconductivity close to the BCS-BEC crossover. Our observations establish a pure carbon material for the study of topological superconductivity, with the promise to explore Majorana modes and topological quantum computing."

MIT physicists discover a new type of superconductor that’s also a magnet | MIT News | Massachusetts Institute of Technology "The “one-of-a-kind” phenomenon was observed in ordinary graphite."

Signatures of chiral superconductivity in rhombohedral graphene (no public access)

Signatures of Chiral Superconductivity in Rhombohedral Graphene (preprint, open access)

Magnesium becomes a possible superconductor near the 2D limit or ultra-thin films

Amazing stuff!

"... challenging the textbook paradigm that states only certain elements in the periodic table can be superconductors. ... have shown that the phenomenon of quantum confinement can turn non-superconducting elements into superconductors. ...

Here by quantum confinement I mean the phenomenon in which the energy of a quantum particle, such as an electron, can greatly increase when it's spatially confined, an effect that is ultimately due to the Heisenberg uncertainty principle—the more you constrain the spatial position of a quantum particle, the larger its energy fluctuations become. ...

Surprisingly, in this case, the critical temperature that can be reached by miniaturizing magnesium nano-sheets of about half nanometer thickness is as high as 10 Kelvin. This is an important consideration because it means that liquid helium can be used to cool the material down to achieve superconductivity instead of using much more expensive cooling technologies. ..."

From the abstract:

"It is known that noble metals such as gold, silver and copper are not superconductors; this is also true for magnesium. This is due to the weakness of the electron–phonon interaction, which makes them excellent conductors but not superconductors.

As has recently been shown for gold, silver and copper, and even for magnesium, it is possible that in very particular situations, superconductivity may occur. Quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors.

It is therefore an important fundamental question whether ultra-thin film confinement may induce observable superconductivity in non-superconducting metals such as magnesium.

We study this problem using a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thin-film confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness, L.

This parameter-free theory predicts superconductivity in very thin magnesium films. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer."

Magnesium becomes a possible superconductor near the 2D limit

Possible Superconductivity in Very Thin Magnesium Films (open access)

Researchers Find New Superconducting State

Good news! 

"Typically, a superconductor's energy gap is the same at all locations within the material. For example, in a superconducting crystal without impurities, all pieces of the crystal would have the same energy gap.

But beginning in the 1960s, scientists began theorizing that the energy gap in some superconducting materials could modulate in space, meaning the gap could be stronger in some areas and weaker in others.

Later, in the 2000s, the idea was further developed with the proposal of what is called the pair density wave (PDW) state, which suggests that a superconducting state could arise in which the energy gap modulates with a long wavelength, where the gap fluctuates between a larger and smaller measurement.

Over the past decade, this concept has garnered significant experimental interest with numerous materials, including iron-based superconductors being explored as potential hosts of a PDW state.

Now, working with extremely thin flakes of an iron-based superconductor, FeTe0.55Se0.45, ... have discovered a modulation of the superconducting gap with the smallest wavelength possible, matching the spacing of atoms in a crystal. They have named it the Cooper-pair density modulation (PDM) state.

"The observed gap modulation, reaching up to 40 percent, represents the strongest reported so far, leading to the clearest experimental evidence to date that gap modulation can exist even at the atomic scale," ..."

From the abstract:

"Superconducting (SC) states that break space-group symmetries of the underlying crystal can exhibit nontrivial spatial modulation of the order parameter. Previously, such states were intimately associated with the breaking of translational symmetry, resulting in the density-wave orders, with wavelengths spanning several unit cells.

However, a related basic concept has long been overlooked: when only intra-unit-cell symmetries of the space group are broken, the SC states can show a distinct type of nontrivial modulation preserving long-range lattice translation.

Here we refer to this new concept as the pair density modulation (PDM) and report the first observation of a PDM state in exfoliated thin flakes of the iron-based superconductor FeTe0.55Se0.45.

Using scanning tunnelling microscopy (STM), we discover robust SC gap modulation with the wavelength corresponding to the lattice periodicity and the amplitude exceeding 30% of the gap average. Notably, we find that the observed modulation originates from the large difference in SC gaps on the two nominally equivalent iron sublattices. The experimental findings, backed up by model calculations, suggest that, in contrast to the density-wave orders, the PDM state is driven by the interplay of sublattice symmetry breaking and a peculiar nematic distortion specific to the thin flakes.

Our results establish new frontiers for exploring the intertwined orders in strong-correlated electronic systems and open a new chapter for iron-based superconductors."

Caltech-led Team Finds New Superconducting State - www.caltech.edu

Cooper-pair density modulation state in an iron-based superconductor (no public access)

Observation of Cooper-pair density modulation state (preprint)

Spatial modulation of the superconducting gap encoded in color and measured using scanning tunneling microscopy.

Nickel superconductor works above -233°C threshold at normal pressure

Good news! More on superconductors! Cuprate v. nickelate superconductors!

"A team of engineers and physicists at Southern University of Science and Technology, in China, has created a nickel-based material that behaves as a superconductor above the -233°C (40 K) threshold under ambient pressure. In their study published in Nature, the researchers synthesized thin films of bilayer nickelate (La₂.₈₅Pr₀.₁₅Ni₂O₇) and found one that behaved as a high-temperature superconductor. ..."

From the abstract:

"The discovery of bilayer nickelate superconductors under high pressure has opened a new chapter in high-transition temperature (high-TC) superconductivity. However, the high-pressure condition and presence of impurity phases have hindered comprehensive investigations into their superconducting properties and potential applications.

Here, we report ambient-pressure superconductivity onset above the McMillan limit (40 K) in bilayer nickelate epitaxial thin films. Three-unit-cell (3UC) thick La2.85Pr0.15Ni2O7 pure-phase single-crystal films are grown using the gigantic-oxidative atomic-layer-by-layer epitaxy (GOALL-Epitaxy) on SrLaAlO4 substrates.

Resistivity measurements and magnetic-field responses indicate onset TC = 45 K. The transition to zero resistance exhibits characteristics consistent with a Berezinskii–Kosterlitz–Thouless (BKT)-like behavior, with TBKT = 9 K.

Meissner diamagnetic effect is observed at TM = 8 K via a mutual inductance setup, in agreement with the BKT-like transition.

In-plane and out-of-plane critical magnetic fields exhibit anisotropy.

Scanning transmission electron microscopy (STEM) images and X-ray reciprocal space mappings (RSMs) reveal that the bilayer nickelate films adopt a tetragonal phase under ~2% coherent epitaxial compressive strain in the NiO2 planes relative to the bulk.

Our findings pave the way for comprehensive investigations of nickelate superconductors under ambient pressure conditions and for exploring superconductivity at higher transition temperature through strain engineering in heterostructures."

Nickel superconductor works above -233°C threshold at normal pressure

SUSTech researchers develop ambient-pressure nickelate high-temperature superconductors, launching a new phase of superconductivity research (original news release)

Ambient-pressure superconductivity onset above 40 K in (La,Pr)3Ni2O7 films (no public access)

Schematic of synthesizing (La,Pr)3Ni2O7 on SrLaAlO4 with gigantic-oxidative atomically layer-by-layer epitaxy (GOALL-Epitaxy).

Unlocking some of the Secrets of the cuprates Superconductor

Amazing stuff! More on the holy grail of room temperature superconductors!

Invest more in superconductivity instead of wasting money on unreliable, environmentally hazardous and heavily government subsidized wind and solar energy!

"When superconductors were discovered in 1911, they astounded researchers with their ability to conduct electricity with no resistance. However, they could only do so at temperatures close to absolute zero. But 1986, scientists discovered that cuprates (a class of copper oxides) were superconductive at a relatively warm -225 degrees Fahrenheit (above liquid nitrogen) - a step toward the ultimate goal of a superconductor that could operate at close to room temperature.  ...

Unfortunately, cuprates are a type of ceramic materials, which makes their application at industrial scales difficult - their brittleness, for example, would pose problems. However, if researchers could understand what makes them superconduct at such high temperatures, they could recreate such processes in other materials. ...

“density functional theory” to explore the significance of structural complexity in cuprates by accurately depicting key structural, electronic, and magnetic properties of these materials. In doing so, “we resolve several long-standing puzzles in this material.”

“Can you use this theory to predict many properties of cuprates? The answer is yes,” ... “You have to just do two things. One of them is to use an up-to-date methodology for the calculation. The more important thing is to include the actual complicated structure of the material, because it matters.” ..."

From the abstract:

"Materials-realistic microscopic theoretical descriptions of copper-based superconductors are challenging due to their complex crystal structures combined with strong electron interactions.

Here, we demonstrate how density functional theory can accurately describe key structural, electronic, and magnetic properties of the normal state of the prototypical cuprate Bi2⁢Sr2⁢CaCu2⁢O8+𝑥 (Bi-2212).

We emphasize the importance of accounting for energy-lowering structural distortions, which then allows us to

(a) accurately describe the insulating antiferromagnetic (AFM) ground state of the undoped parent compound (in contrast to the metallic state predicted by previous ab initio studies);

(b) identify numerous low-energy competing spin and charge stripe orders in the hole-overdoped material nearly degenerate in energy with the AFM ordered state, indicating strong spin fluctuations;

(c) predict the lowest-energy hole-doped crystal structure including its long-range structural distortions and oxygen dopant positions that match high-resolution scanning transmission electron microscopy measurements; and

(d) describe electronic bands near the Fermi energy with flat antinodal dispersions and Fermi surfaces that are in agreement with angle-resolved photoemission spectroscopy (ARPES) measurements and provide a clear explanation for the structural origins of the so-called “shadow bands.”

We also show how one must go beyond band theory and include fully dynamic spin fluctuations via a many-body approach when aiming to make quantitative predictions to measure the ARPES spectra in the overdoped material.

Finally, regarding spatial inhomogeneity, we show that the local structure at the CuO2 layer, rather than dopant electrostatic effects, modulates the local charge-transfer gaps, local correlation strengths, and by extension the local superconducting gaps."

"Popular Summary

In the realm of superconductors, copper oxide–based materials known as cuprates have long intrigued scientists due to their unconventional characteristics and high-temperature superconducting potential. However, a microscopic understanding of cuprates has been challenging due to their intricate electronic interactions and lattice distortions. In this work, we explore the significance of structural distortions in accurately depicting key structural, electronic, and magnetic properties in the prototypical cuprate Bi2⁢Sr2⁢CaCu2⁢O8+𝑥 (BSCCO) from first principles. By considering specifically structural symmetry breaking, we resolve several long-standing puzzles in this material.

First, we correctly describe the insulating antiferromagnetic ground state in undoped BSCCO, a crucial achievement since prior theoretical work always predicted incorrect metallic behavior. Accurate undoped-state descriptions are vital for reliable predictions when doping is introduced.

Second, we establish a paradigm for distinguishing structural symmetry breaking from other degrees of freedom, thereby resolving the long debate about the physical origin of the interesting “shadow” Fermi surface.

Furthermore, using a numerical quantum many-body approach, we explicitly study the spin fluctuations in this material. This helps reveal an intimate relationship between local structural distortions and the local correlation strength around copper atoms, which in turn is known to connect directly to superconductivity.

In essence, this research not only explains the dominant cuprate phenomena but also establishes a solid foundation for constructing well-justified effective models with realistic and microscopic insights using first-principles methods. By capturing essential properties of crystal and electronic structures, we offer a valuable resource for future theoretical modeling and potential advancements in these intriguing superconductors."

Unlocking the Secrets of a Superconductor | Yale School of Engineering & Applied Science

First-Principles Prediction of Structural Distortions in the Cuprates and Their Impact on the Electronic Structure (open access)

Fig. 1. Crystal and electronic structure of high-symmetry undoped Bi-2212.