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.
Magnetic topological phases of quantum matter are an emerging frontier in physics and material science. Along these lines, several kagome magnets have appeared as the most promising platforms. Here, ...we explore magnetic correlations in the kagome magnet Co
Sn
S
. Using muon spin-rotation, we present evidence for competing magnetic orders in the kagome lattice of this compound. Our results show that while the sample exhibits an out-of-plane ferromagnetic ground state, an in-plane antiferromagnetic state appears at temperatures above 90 K, eventually attaining a volume fraction of 80% around 170 K, before reaching a non-magnetic state. Strikingly, the reduction of the anomalous Hall conductivity (AHC) above 90 K linearly follows the disappearance of the volume fraction of the ferromagnetic state. We further show that the competition of these magnetic phases is tunable through applying either an external magnetic field or hydrostatic pressure. Our results taken together suggest the thermal and quantum tuning of Berry curvature induced AHC via external tuning of magnetic order.
Abstract
In its orthorhombic
T
d
polymorph, MoTe
2
is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance and ...superconductivity were also observed in
T
d
-MoTe
2
. Understanding the superconductivity in
T
d
-MoTe
2
, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure muon-spin rotation experiments probing the temperature-dependent magnetic penetration depth in
T
d
-MoTe
2
. A substantial increase of the superfluid density and a linear scaling with the superconducting critical temperature
T
c
is observed under pressure. Moreover, the superconducting order parameter in
T
d
-MoTe
2
is determined to have 2-gap
s
-wave symmetry. We also exclude time-reversal symmetry breaking in the superconducting state with zero-field μSR experiments. Considering the strong suppression of
T
c
in MoTe
2
by disorder, we suggest that topologically non-trivial
s
+−
state is more likely to be realized in MoTe
2
than the topologically trivial
s
++
state.
The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and ...organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mossbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state. PUBLICATION ABSTRACT
Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate β−Li2IrO3 is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation measurements, as well as ...single-crystal x-ray diffraction under pressure and ab initio calculations. The Néel temperature of β−Li2IrO3 increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field Hc reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange Γ under pressure.
Abstract
Kagome lattices are intriguing and rich platforms for studying the intertwining of topology, electron correlation, and magnetism. These materials have been subject to tremendous experimental ...and theoretical studies not only due to their exciting physical properties but also as systems that may solve critical technological problems. We will review recent experimental progress on superconductivity and magnetic fingerprints of charge order in several kagome-lattice systems from the local-magnetic probe point of view by utilizing muon-spin rotation under extreme conditions, i.e., hydrostatic pressure, ultra low temperature and high magnetic field. The systems include: (1) The series of compounds
A
V
3
Sb
5
(
A
= K, Rb, Cs) with V kagome lattice which form the first kagome-based family that exhibits a cascade of symmetry-broken electronic orders, including charge order and superconductivity. In these systems, we find a time-reversal symmetry-breaking charge ordered state and tunable unconventional time-reversal symmetry-breaking superconductivity. (2) The system LaRu
3
Si
2
with distorted kagome layers of Ru, in which our experiments and calculations taken together point to nodeless moderate coupling superconductivity. It was also found that the electron-phonon coupling alone can only explain a small fraction of
T
c
from calculations, which suggests other factors enhancing
T
c
such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and the high density of states from the narrow kagome bands. (3) CeRu
2
with a pristine Ru kagome lattice, which we classify as an exceedingly rare nodeless (with anisotropic
s
-wave gap symmetry) magnetic kagome superconductor.
Abstract
Charge ordered kagome lattices have been demonstrated to be intriguing platforms for studying the intertwining of topology, correlation, and magnetism. The recently discovered charge ordered ...kagome material ScV
6
Sn
6
does not feature a magnetic groundstate or excitations, thus it is often regarded as a conventional paramagnet. Here, using advanced muon-spin rotation spectroscopy, we uncover an unexpected hidden magnetism of the charge order. We observe an enhancement of the internal field width sensed by the muon ensemble, which takes place within the charge ordered state. More importantly, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. Taken together with the hidden magnetism found in
A
V
3
Sb
5
(
A
= K, Rb, Cs) and FeGe kagome systems, our results suggest ubiqitous time-reversal symmetry-breaking in charge ordered kagome lattices.
Relevance. This paper considers the issue of construction of underwater passages of trunk pipelines by the directional drilling method using concrete weight coated pipes. This issue is relevant, ...since at the present moment there is no standards and technical documentation regulating the calculation procedure of the minimum allowable elastic bend radius of a pipeline with concrete coating in a well. There are situations when design institutes made incorrect design decisions as a result, the pipe string got stuck during works. Those incidents led to unnecessary economic expenditures and a delay in the work implementation plan. Aim. To study the issues which related with the sticking of the pipe string during the construction of the underwater passage of the main pipeline by the method of directional drilling; calculate the minimum allowable elastic bend radius of a pipeline with concrete coating and the actual parameters of the pilot drillhole. Objects. An underwater passage of a trunk pipeline constructed by the method of directional drilling. Methods. Study of standards and technical documents on the construction of underwater crossings of main pipelines and an analysis of the incident in which a pipeline with concrete weight coating got stuck. Results. The authors substantiate the calculation of the minimum allowable elastic bend radius of a pipeline with a concrete coating based on the requirements of domestic standards and technical documentation. The authors have calculated this value by an analytical method, based on studies of the bending stiffness of concrete structures by foreign scientists, as well as actual parameters of the pilot drillhole. The obtained results of the studied values were analyzed and a conclusion was given about the causes of the incident associated with the sticking of the pipe string.
PDF
