Abstract
Unconventional superconductors often feature competing orders, small superfluid density, and nodal electronic pairing. While unusual superconductivity has been proposed in the kagome metals
...A
V
3
Sb
5
, key spectroscopic evidence has remained elusive. Here we utilize pressure-tuned and ultra-low temperature muon spin spectroscopy to uncover the unconventional nature of superconductivity in RbV
3
Sb
5
and KV
3
Sb
5
. At ambient pressure, we observed time-reversal symmetry breaking charge order below
$${T}_{{{\rm{1}}}^{*}\simeq$$
T
1
*
≃
110 K in RbV
3
Sb
5
with an additional transition at
$${T}_{{{\rm{2}}}^{*}\simeq$$
T
2
*
≃
50 K. Remarkably, the superconducting state displays a nodal energy gap and a reduced superfluid density, which can be attributed to the competition with the charge order. Upon applying pressure, the charge-order transitions are suppressed, the superfluid density increases, and the superconducting state progressively evolves from nodal to nodeless. Once optimal superconductivity is achieved, we find a superconducting pairing state that is not only fully gapped, but also spontaneously breaks time-reversal symmetry. Our results point to unprecedented tunable nodal kagome superconductivity competing with time-reversal symmetry-breaking charge order and offer unique insights into the nature of the pairing state.
The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the ...tunability of magnetism, superconductivity, and crystal structure in the stripe phase of the cuprate LaFormula: see textBaFormula: see textCuOFormula: see text, with Formula: see text = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuOFormula: see text plane. A sixfold increase of the three-dimensional (3D) superconducting critical temperature Formula: see text and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of Formula: see text0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, strain produces an inhomogeneous suppression of the spin-stripe order at elevated temperatures. Namely, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found under stress, which is a necessary condition for the development of the 3D superconducting phase with optimal Formula: see text. Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.
To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on ...high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.
Significance One way to search for new superconductors is to find a magnetic metal and then suppress the magnetism using chemical doping or pressure. Heavy-fermion superconductors are the archetypal ...family of magnetic superconductors, but PuCoGa ₅—the heavy fermion with the highest Formula (18.5 K)—has no static magnetism. What other mechanism, then, is driving superconductivity in PuCoGa ₅? We measured the elastic constants of PuCoGa ₅ and found that the bulk modulus softens dramatically before Formula—evidence for fluctuations of the plutonium valence as opposed to magnetic fluctuations associated with the suppression of magnetic order. Valence fluctuations resolve the missing magnetism conundrum in PuCoGa ₅ by providing an alternative mechanism for the high-temperature superconductivity.
The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Formula cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the Formula valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa ₅—the highest Formula superconductor of the heavy fermions (Formula = 18.5 K)—and find that the bulk modulus softens anomalously over a wide range in temperature above Formula. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability—therefore, we identify the elastic softening with fluctuations of the plutonium 5 f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Formula, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa ₅ avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa ₅ has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Formula of PuCoGa ₅.
We report the observation of quantum oscillations in the underdoped cuprate superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed magnetic fields up to 85 T. There is a clear ...signal, periodic in inverse field, with frequency 660+/-15 T and possible evidence for the presence of two components of slightly different frequency. The quasiparticle mass is m(*)=3.0+/-0.3m(e). In conjunction with the results of Doiron-Leyraud et al. for YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a general feature of underdoped copper oxide planes and provide information about the doping dependence of the Fermi surface.
Abstract
Magnetic topological phases of quantum matter are an emerging frontier in physics and materials science, of which kagome magnets appear as a highly promising platform. Here, we explore ...magnetic correlations in the recently identified topological kagome system TbMn
6
Sn
6
using muon spin rotation, combined with local field analysis and neutron diffraction. Our studies identify an out-of-plane ferrimagnetic structure with slow magnetic fluctuations which exhibit a critical slowing down below
$${T}_{{{\rm{C1}}}^{* }$$
T
C1
*
≃ 120 K and finally freeze into static patches with ideal out-of-plane order below
T
C1
≃ 20 K. We further show that hydrostatic pressure of 2.1 GPa stabilises the static out-of-plane topological ferrimagnetic ground state in the whole volume of the sample. Therefore the exciting perspective arises of a magnetically-induced topological system whose magnetism can be controlled through external parameters. The present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between the low-temperature volume-wise magnetic evolution of the static
c
-axis ferrimagnetic patches and the topological electronic properties in TbMn
6
Sn
6
.
Time series of the observational estimate of the Atlantic meridional overturning circulation (AMOC) have recently become available, but so far, no contemporaneous relation has been documented between ...them. Here, we analyze the variability of the 26°N Rapid Climate Change programme (RAPID) and the 41°N Argo‐based AMOC estimates on seasonal timescales, and we compare them to a simulation from a high‐resolution National Centers for Environmental Prediction (NCEP)‐forced ocean model. In our analysis of the observed time series, we find that the seasonal cycles of the non‐Ekman component of the AMOC between 26°N and 41°N are 180‐degrees out‐of‐phase. Removing the mean seasonal cycle from each time series, the residuals have a non‐stationary covariability. Our results demonstrate that the AMOC is meridionally covariable between 26°N and 41°N at seasonal timescales. We find the same covariability in the model, although the phasing differs from the observed phasing. This may offer the possibility of inferring AMOC variations and associated climate anomalies throughout the North Atlantic from discontinuous observations.
Key Points
First joint analysis of observed and modeled Atlantic overturning timeseries
Find meridional coherence at seasonal timescales
Find non‐stationary covariability after removing the mean seasonal cycle
PDF
