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.
Topological materials ranging from topological insulators to Weyl and Dirac semimetals form one of the most exciting current fields in condensed-matter research. Many half-Heusler compounds, RPtBi (R ...= rare earth), have been theoretically predicted to be topological semimetals. Among various topological attributes envisaged in RPtBi, topological surface states, chiral anomaly, and planar Hall effect have been observed experimentally. Here, we report an unusual intrinsic anomalous Hall effect (AHE) in the antiferromagnetic Heusler Weyl semimetal compounds GdPtBi and NdPtBi that is observed over a wide temperature range. In particular, GdPtBi exhibits an anomalous Hall conductivity of up to 60 Ω−1·cm−1 and an anomalous Hall angle as large as 23%. Muon spin-resonance (μSR) studies of GdPtBi indicate a sharp antiferromagnetic transition (T
N) at 9 K without any noticeable magnetic correlations above T
N. Our studies indicate that Weyl points in these half-Heuslers are induced by a magnetic field via exchange splitting of the electronic bands at or near the Fermi energy, which is the source of the chiral anomaly and the AHE.
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.
Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, ...thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour. Here we create a thermally active artificial kagome spin ice that is made up of a large array of dipolar interacting nanomagnets and undergoes phase transitions predicted by microscopic theory. We use low energy muon spectroscopy to probe the dynamic behaviour of the interacting nanomagnets and observe peaks in the muon relaxation rate that can be identified with the critical temperatures of the predicted phase transitions. This provides experimental evidence that a frustrated magnetic metamaterial can be engineered to admit thermodynamic phases.
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
Kondo screening in a Majorana metal Lee, S.; Choi, Y. S.; Do, S.-H. ...
Nature communications,
11/2023, Volume:
14, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
Kondo impurities provide a nontrivial probe to unravel the character of the excitations of a quantum spin liquid. In the
S
= 1/2 Kitaev model on the honeycomb lattice, Kondo impurities ...embedded in the spin-liquid host can be screened by itinerant Majorana fermions via gauge-flux binding. Here, we report experimental signatures of metallic-like Kondo screening at intermediate temperatures in the Kitaev honeycomb material
α
-RuCl
3
with dilute Cr
3+
(
S
= 3/2) impurities. The static magnetic susceptibility, the muon Knight shift, and the muon spin-relaxation rate all feature logarithmic divergences, a hallmark of a metallic Kondo effect. Concurrently, the linear coefficient of the magnetic specific heat is large in the same temperature regime, indicating the presence of a host Majorana metal. This observation opens new avenues for exploring uncharted Kondo physics in insulating quantum magnets.
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