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(Ba,K)BiO 3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature T c of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO 3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO 3 . The parent compound BaSbO 3− δ shows a larger charge density wave gap compared to BaBiO 3 . As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to T c  = 15 K. This value is lower than the maximum T c of (Ba,K)BiO 3 , but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO 3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors. High-pressure synthesis is used to stabilize superconducting (Ba,K)SbO 3 , whose properties provide a fresh perspective on the origin of superconductivity in these types of materials.

Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li 2 SrTa 2 O 7 , which adopts a polar structure due to a cooperative tilting of its constituent TaO 6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa 2 O 7 , can be prepared. On cooling below 43 K the Mn 2+ centres in MnSrTa 2 O 7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations. Fabricating materials with simultaneously spontaneous magnetic and electrical polarisations is challenging due to contradictory electronic features. Here, the authors report a synthesis path toward a perovskite MnSrTa 2 O 7 by performing low-temperature cation-exchange reactions on Li 2 SrTa 2 O 7 .

Sr RuO is an unconventional superconductor that has attracted widespread study because of its high purity and the possibility that its superconducting order parameter has odd parity. We study the dependence of its superconductivity on anisotropic strain. Applying uniaxial pressures of up to ~1 gigapascals along a 〈100〉 direction (a axis) of the crystal lattice results in the transition temperature (T ) increasing from 1.5 kelvin in the unstrained material to 3.4 kelvin at compression by ≈0.6%, and then falling steeply. Calculations give evidence that the observed maximum T occurs at or near a Lifshitz transition when the Fermi level passes through a Van Hove singularity, and open the possibility that the highly strained, T = 3.4 K Sr RuO has an even-parity, rather than an odd-parity, order parameter.

A key question regarding the unconventional superconductivity of Sr₂RuO₄ remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few percent of that of the primary superconducting transition. The normalized jump in heat capacity, ΔC/C, grows smoothly as a function of uniaxial pressure, favoring order parameters which are allowed to maximize in the same part of the Brillouin zone as the wellstudied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of Sr₂RuO₄.

Distorting a material and observing its response can allow insight into its electronic properties. Thin films can be strained by placing them on a substrate with a different lattice constant; bulk samples present more of a challenge. Hicks et al. (p. 283) designed an apparatus to apply both tensile and compressive strain and used it to study the properties of the superconductor Sr2RuO4, which has long been hypothesized to host the unusual p-wave superconductivity. The response of the superconducting transition temperature Tc to the applied strain depended on the direction in which the strain was applied, and did not exhibit a cusp predicted to occur around zero strain. As the technique leaves a surface of the probe open to external probes, it could be adopted for a wide range of methods. [PUBLICATION ABSTRACT] A sensitive probe of unconventional order is its response to a symmetry-breaking field. To probe the proposed px ± ipy topological superconducting state of Sr2RuO4, we have constructed an apparatus capable of applying both compressive and tensile strains of up to 0.23%. Strains applied along (ProQuest: ... denotes formulae and/or non-USASCII text omitted) strains give a much weaker, mostly antisymmetric response. As well as advancing the understanding of the superconductivity of Sr2RuO4, our technique has potential applicability to a wide range of problems in solid-state physics. [PUBLICATION ABSTRACT]

The high-temperature phase behaviour of the ferroelectric layered perovskite Bi4Ti3O12 has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that Bi4Ti3O12 undergoes two separate structural transitions from the aristotype tetragonal phase (space group I4/mmm) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, B1a1). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above TC with tetragonal symmetry, space group P4/mbm. This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi3+ cations within the perovskite blocks.

Research on the unconventional superconductivity of S r 2 R u O 4 is undergoing a renaissance since recent spin susceptibility measurements ruling out the spin triplet order parameter which had been widely favored for over two decades. With ultrasound, Kerr rotation, and muon spin relaxation data all providing evidence for a two-component order parameter, it is vital that this possibility be investigated thermodynamically by studying the dependence of the heat-capacity anomaly on uniaxial pressure. Here, the relevant experimental results are combined with theoretical analysis that shows how strongly the data constrain theories of the order parameter. In particular, we do not observe any signs of transition splitting of two-order-parameter components. S r 2 R u O 4 thus offers a unique test bed for theories of unconventional superconductivity. A key question regarding the unconventional superconductivity of S r 2 R u O 4 remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few percent of that of the primary superconducting transition. The normalized jump in heat capacity, Δ C / C , grows smoothly as a function of uniaxial pressure, favoring order parameters which are allowed to maximize in the same part of the Brillouin zone as the well-studied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of S r 2 R u O 4 .

The high-temperature phase behaviour of the ferroelectric layered perovskite Bi 4 Ti 3 O 12 has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that Bi 4 Ti 3 O 12 undergoes two separate structural transitions from the aristotype tetragonal phase (space group I 4 /mmm ) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, B 1 a 1). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above T C with tetragonal symmetry, space group P 4 /mbm . This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi 3+ cations within the perovskite blocks.

The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution, the thermal evolution of the crystal structure is consistent with a first-order transition between 700 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent “avalanche transition” behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9.

(Ba,K)BiO constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature T of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO with a positive oxygen-metal charge transfer energy in contrast to (Ba,K)BiO . The parent compound BaSbO shows a larger charge density wave gap compared to BaBiO . As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to T  = 15 K. This value is lower than the maximum T of (Ba,K)BiO , but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO indicates that strong oxygen-metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors.