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"Superconductivity"
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On the Remarkable Superconductivity of FeSe and Its Close Cousins
Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.
Journal Article
mBot for makers : conceive, construct and code your own robots at home or in the classroom
\"The mBot is an educational Arduino robot that helps kids learn programming and electronics, alone or in the classroom. The mBot allows novices to start by tinkering, and to access higher-level features or add new components when inspiration strikes, without soldering or breadboarding! This flexibility allows raw beginners and experienced Makers to work at their own comfort level. Written by educators, this book cuts through much of the confusion resulting from the mBot documentation. It also saves you time when you're scaling up your mBots for home and classroom use by giving you creative project ideas you can use right away.\"--Back cover.
Book
Research on the application of superconducting materials in the energy system
This paper introduces the development history of superconducting materials in energy and material system, the analysis of superconducting materials production process, summarizes the superconducting generator based on superconducting materials, superconducting transformer, superconducting cable, superconducting energy storage device research progress, compared the advantages and disadvantages of superconducting materials in practical application, the application of superconducting in various fields, in the future research, the use of superconducting materials to improve energy efficiency, reduce energy waste will be an important trend.
Journal Article
Evidence for unconventional superconductivity in twisted bilayer graphene
The emergence of superconductivity and correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors
1
–
4
, as described by the Bardeen–Cooper–Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened
5
,
6
. This suggests that pairing in MATBG might be conventional in nature and a consequence of the large density of states of its flat bands. Here we combine tunnelling and Andreev reflection spectroscopy with a scanning tunnelling microscope to observe several key experimental signatures of unconventional superconductivity in MATBG. We show that the tunnelling spectra below the transition temperature
T
c
are inconsistent with those of a conventional
s
-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunnelling gap
Δ
T
, which far exceeds the mean-field BCS ratio (with 2
Δ
T
/
k
B
T
c
~ 25), and the gap
Δ
AR
extracted from Andreev reflection spectroscopy (2
Δ
AR
/
k
B
T
c
~ 6). The tunnelling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase. Moreover, the pseudogap and superconductivity are both absent when MATBG is aligned with hexagonal boron nitride. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.
A study combining tunnelling and Andreev reflection spectroscopy with a scanning tunnelling microscope provides evidence for unconventional superconductivity in magic-angle twisted bilayer graphene.
Journal Article
Supercurrent diode effect and finite-momentum superconductors
When both inversion and time-reversal symmetries are broken, the critical current of a superconductor can be nonreciprocal. In this work, we show that, in certain classes of two-dimensional superconductors with antisymmetric spin–orbit coupling, Cooper pairs acquire a finite momentum upon the application of an in-plane magnetic field, and, as a result, critical currents in the direction parallel and antiparallel to the Cooper pair momentum become unequal. This supercurrent diode effect is also manifested in the polarity dependence of in-plane critical fields induced by a supercurrent. These nonreciprocal effects may be found in polar SrTiO₃ film, few-layer MoTe₂ in the Td
phase, and twisted bilayer graphene in which the valley degree of freedom plays a role analogous to spin.
Journal Article
Evidence for Dirac flat band superconductivity enabled by quantum geometry
In a flat band superconductor, the charge carriers’ group velocity
v
F
is extremely slow. Superconductivity therein is particularly intriguing, being related to the long-standing mysteries of high-temperature superconductors
1
and heavy-fermion systems
2
. Yet the emergence of superconductivity in flat bands would appear paradoxical, as a small
v
F
in the conventional Bardeen–Cooper–Schrieffer theory implies vanishing coherence length, superfluid stiffness and critical current. Here, using twisted bilayer graphene
3
–
7
, we explore the profound effect of vanishingly small velocity in a superconducting Dirac flat band system
8
–
13
. Using Schwinger-limited non-linear transport studies
14
,
15
, we demonstrate an extremely slow normal state drift velocity
v
n
≈ 1,000 m s
–1
for filling fraction
ν
between −1/2 and −3/4 of the moiré superlattice. In the superconducting state, the same velocity limit constitutes a new limiting mechanism for the critical current, analogous to a relativistic superfluid
16
. Importantly, our measurement of superfluid stiffness, which controls the superconductor’s electrodynamic response, shows that it is not dominated by the kinetic energy but instead by the interaction-driven superconducting gap, consistent with recent theories on a quantum geometric contribution
8
–
12
. We find evidence for small Cooper pairs, characteristic of the Bardeen–Cooper–Schrieffer to Bose–Einstein condensation crossover
17
–
19
, with an unprecedented ratio of the superconducting transition temperature to the Fermi temperature exceeding unity and discuss how this arises for ultra-strong coupling superconductivity in ultra-flat Dirac bands.
The authors investigate the effect of small velocity in a superconducting Dirac flat band system, finding evidence for small pairs and that superfluid stiffness is not dominated by kinetic energy.
Journal Article
Tuning Ising superconductivity with layer and spin–orbit coupling in two-dimensional transition-metal dichalcogenides
Systems simultaneously exhibiting superconductivity and spin–orbit coupling are predicted to provide a route toward topological superconductivity and unconventional electron pairing, driving significant contemporary interest in these materials. Monolayer transition-metal dichalcogenide (TMD) superconductors in particular lack inversion symmetry, yielding an antisymmetric form of spin–orbit coupling that admits both spin-singlet and spin-triplet components of the superconducting wavefunction. Here, we present an experimental and theoretical study of two intrinsic TMD superconductors with large spin–orbit coupling in the atomic layer limit, metallic 2H-TaS
2
and 2H-NbSe
2
. We investigate the superconducting properties as the material is reduced to monolayer thickness and show that high-field measurements point to the largest upper critical field thus reported for an intrinsic TMD superconductor. In few-layer samples, we find the enhancement of the upper critical field is sustained by the dominance of spin–orbit coupling over weak interlayer coupling, providing additional candidate systems for supporting unconventional superconducting states in two dimensions.
Monolayer transition-metal dichalcogenide (TMD) is promising to host features of topological superconductivity. Here, de la Barrera et al. study layered compounds, 2H-TaS
2
and 2H-NbSe
2
, in their atomic layer limit and find a largest upper critical field for an intrinsic TMD superconductor.
Journal Article
Room-temperature superconductivity in a carbonaceous sulfur hydride
One of the long-standing challenges in experimental physics is the observation of room-temperature superconductivity
. Recently, high-temperature conventional superconductivity in hydrogen-rich materials has been reported in several systems under high pressure
. An important discovery leading to room-temperature superconductivity is the pressure-driven disproportionation of hydrogen sulfide (H
S) to H
S, with a confirmed transition temperature of 203 kelvin at 155 gigapascals
. Both H
S and CH
readily mix with hydrogen to form guest-host structures at lower pressures
, and are of comparable size at 4 gigapascals. By introducing methane at low pressures into the H
S + H
precursor mixture for H
S, molecular exchange is allowed within a large assemblage of van der Waals solids that are hydrogen-rich with H
inclusions; these guest-host structures become the building blocks of superconducting compounds at extreme conditions. Here we report superconductivity in a photochemically transformed carbonaceous sulfur hydride system, starting from elemental precursors, with a maximum superconducting transition temperature of 287.7 ± 1.2 kelvin (about 15 degrees Celsius) achieved at 267 ± 10 gigapascals. The superconducting state is observed over a broad pressure range in the diamond anvil cell, from 140 to 275 gigapascals, with a sharp upturn in transition temperature above 220 gigapascals. Superconductivity is established by the observation of zero resistance, a magnetic susceptibility of up to 190 gigapascals, and reduction of the transition temperature under an external magnetic field of up to 9 tesla, with an upper critical magnetic field of about 62 tesla according to the Ginzburg-Landau model at zero temperature. The light, quantum nature of hydrogen limits the structural and stoichiometric determination of the system by X-ray scattering techniques, but Raman spectroscopy is used to probe the chemical and structural transformations before metallization. The introduction of chemical tuning within our ternary system could enable the preservation of the properties of room-temperature superconductivity at lower pressures.
Journal Article
Superconductivity above 200 K discovered in superhydrides of calcium
Searching for superconductivity with
T
c
near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (
T
c
) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & rare-earth hydride system. The materials are synthesized at the synergetic conditions of 160~190 GPa and ~2000 K using diamond anvil cell combined with in-situ laser heating technique. The superconductivity was studied through in-situ high pressure electric conductance measurements in an applied magnetic field for the sample quenched from high temperature while maintained at high pressures. The upper critical field Hc(0) was estimated to be ~268 T while the GL coherent length is ~11 Å. The in-situ synchrotron X-ray diffraction measurements suggest that the synthesized calcium hydrides are primarily composed of CaH
6
while there may also exist other calcium hydrides with different hydrogen contents.
The discovery of superconductivity in hydrides at critical temperature (
T
c
) near room temperature receives intensive attentions. Here the authors report experimental synthesis and discovery of superconductivity with
T
c
above 210 K in calcium superhydrides at 160–190 GPa.
Journal Article