Abstract
In this short review, we aim to provide a topical update on the status of efforts to understand the superconductivity of Sr
2
RuO
4
. We concentrate on efforts to identify a superconducting ...order parameter symmetry that is compatible with all the major pieces of experimental knowledge of the material, and highlight some major discrepancies that have become even clearer in recent years. As the pun in the title suggests, we have tried to start the discussion from scratch, making no assumptions even about fundamental issues such as the parity of the superconducting state. We conclude that no consensus is currently achievable in Sr
2
RuO
4
, and that the reasons for this go to the heart of how well some of the key probes of unconventional superconductivity are really understood. This is, therefore, a puzzle that merits continued in-depth study.
Abstract
There is considerable evidence that the superconducting state of Sr
2
RuO
4
breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking, its onset temperature,
...T
TRSB
, is generally found to match the critical temperature,
T
c
, within resolution. In combination with evidence for even parity, this result has led to consideration of a
d
x
z
±
i
d
y
z
order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding
T
TRSB
=
T
c
, but it has a hard-to-explain horizontal line node at
k
z
= 0. Therefore,
s
±
i
d
and
d
±
i
g
order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain
T
TRSB
≈
T
c
. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr
2
RuO
4
under hydrostatic pressure, and Sr
1.98
La
0.02
RuO
4
at zero pressure. Both hydrostatic pressure and La substitution alter
T
c
without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected,
T
TRSB
should track changes in
T
c
, while if it is accidental, these transition temperatures should generally separate. We observe
T
TRSB
to track
T
c
, supporting the hypothesis of
d
x
z
±
i
d
y
z
order.
Strontium ruthenate (Sr2RuO4) continues to present an important test of our understanding of unconventional superconductivity, because while its normal-state electronic structure is known with ...precision, its superconductivity remains unexplained. There is evidence that its order parameter is chiral, but reconciling this with recent observations of the spin part of the pairing requires an order parameter that is either finely tuned or implies a new form of pairing. Therefore, a definitive resolution of whether the superconductivity of Sr2RuO4 is chiral is important for the study of superconductivity. Here we report the measurement of zero-field muon spin relaxation—a probe sensitive to weak magnetism—on samples under uniaxial stresses. We observe stress-induced splitting between the onset temperatures of superconductivity and time-reversal symmetry breaking—consistent with the qualitative expectations for a chiral order parameter—and argue that this observation cannot be explained by conventional magnetism. In addition, we report the appearance of bulk magnetic order under higher uniaxial stress, above the critical pressure at which a Lifshitz transition occurs in Sr2RuO4.When strain is applied to strontium ruthenate, superconductivity emerges at a different temperature to the breaking of time-reversal symmetry. This indicates that the superconductivity could have a chiral d-wave order parameter.
Tuning diamagnetism with currentProperties of materials can be tuned by various means, such as chemical doping, magnetic field, or pressure. Sow et al. used electrical currents of modest density to ...turn the Mott insulator Ca2RuO4 into a semimetal. Concurrently, its diamagnetic response—the ability to counter an externally applied magnetic field—rose to levels higher than in any other nonsuperconducting material. The use of electrical current as a powerful experimental knob may be applicable to other similar materials.Science, this issue p. 1084Mott insulators can host a surprisingly diverse set of quantum phenomena when their frozen electrons are perturbed by various stimuli. Superconductivity, metal-insulator transition, and colossal magnetoresistance induced by element substitution, pressure, and magnetic field are prominent examples. Here we report strong diamagnetism in the Mott insulator calcium ruthenate (Ca2RuO4) induced by dc electric current. The application of a current density of merely 1 ampere per centimeter squared induces diamagnetism stronger than that in other nonsuperconducting materials. This change is coincident with changes in the transport properties as the system becomes semimetallic. These findings suggest that dc current may be a means to control the properties of materials in the vicinity of a Mott insulating transition.
In condensed matter physics, spontaneous symmetry breaking has been a key concept, and discoveries of new types of broken symmetries have greatly increased our understanding of matter. Recently, ...electronic nematicity, novel spontaneous rotational-symmetry breaking leading to an emergence of a special direction in electron liquids, has been attracting significant attention. Here, we show bulk thermodynamic evidence for nematic superconductivity, in which the nematicity emerges in the superconducting gap amplitude, in CuxBi2Se3.
Abstract
Investigations of perovskite oxides triggered by the discovery of high-temperature and unconventional superconductors have had crucial roles in stimulating and guiding the development of ...modern condensed-matter physics. Antiperovskite oxides are charge-inverted counterpart materials to perovskite oxides, with unusual negative ionic states of a constituent metal. No superconductivity was reported among the antiperovskite oxides so far. Here we present the first superconducting antiperovskite oxide Sr
3−
x
SnO with the transition temperature of around 5 K. Sr
3
SnO possesses Dirac points in its electronic structure, and we propose from theoretical analysis a possibility of a topological odd-parity superconductivity analogous to the superfluid
3
He-B in moderately hole-doped Sr
3−
x
SnO. We envision that this discovery of a new class of oxide superconductors will lead to a rapid progress in physics and chemistry of antiperovskite oxides consisting of unusual metallic anions.
Recently, "application of electric field (E-field)" has received considerable attention as a new method to induce novel quantum phenomena since application of E-field can tune the electronic states ...directly with obvious scientific and industrial advantages over other turning methods. However, E-field-induced Mott transitions are rare and typically require high E-field and low temperature. Here we report that the multiband Mott insulator Ca2RuO4 shows unique insulator-metal switching induced by applying a dry-battery level voltage at room temperature. The threshold field Eth ~40 V/cm is much weaker than the Mott gap energy. Moreover, the switching is accompanied by a bulk structural transition. Perhaps the most peculiar of the present findings is that the induced metal can be maintained to low temperature by a weak current.
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