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The halide perovskite heterostructure (CuCl4)2(MTPA)4Cu3Cl6 (Cu_Cu; MTPA = 3-(methylthio)-propylammonium) forms from solution as single crystals consisting of alternating layers of 2D CuII–Cl perovskite and 1D CuII–Cl diamond–chain intergrowth. Using magnetometry, heat capacity, and electron paramagnetic resonance measurements, we interrogate the magnetic ordering of the 2D perovskite and 1D intergrowth layers at temperatures down to 0.055 K. As with other Cu‒Cl perovskites, the perovskite-layer spins order ferromagnetically at 10 K. Magnetization data of Cu_Cu feature a multi–component curve, consistent with magnetization of the perovskite layers and one of the three additional CuII sites in the intergrowth layer, suggesting antiferromagnetic coupling of the remaining two intergrowth-layer spins. A broad feature in AC susceptibility measurements at 6 K and an anomalous heat capacity feature at 0.3 K suggest that local ordering events occur at dramatically different energy scales with decreasing temperature. EPR spectra indicate that these local orderings occur within the 1D chains. Notably, no long–range magnetic ordering event in the intergrowth is evident down to 0.055 K, suggesting that the geometric constraints imposed by the perovskite framework and the steric bulk of the MTPA ligands physically separate and magnetically isolate the diamond chains. In contrast, well–studied diamond-spin-chain materials such as azurite show long-range magnetic order at low-temperatures due to interchain interactions. Thus, Cu_Cu provides an ideal platform for studying isolated, anisotropic spin chains. More generally, this study illustrates the capability of halide perovskite heterostructures to serve as vehicles for the scalable synthesis of complex magnetic materials. 

Adiabatic decompression of paraquadrupolar materials has significant potential as a cryogenic cooling technology. We focus on TmVO ${}_4$, an archetypal material that undergoes a continuous phase transition to a ferroquadrupole-ordered state at 2.15 K. Above the phase transition, each Tm ion contributes an entropy of $k_{B}ln2$due to the degeneracy of the crystal electric field groundstate. Owing to the large magnetoelastic coupling, which is a prerequisite for a material to undergo a phase transition via the cooperative Jahn–Teller effect, this level splitting, and hence the entropy, can be readily tuned by externally induced strain. Using a dynamic technique in which the strain is rapidly oscillated, we measure the adiabatic elastocaloric response of single-crystal TmVO ${}_4$, and thus experimentally obtain the entropy landscape as a function of strain and temperature. The measurement confirms the suitability of this class of materials for cryogenic cooling applications and provides insight into the dynamic quadrupole strain susceptibility. 

The nature of superconductivity in SrTiO 3 , the first oxide superconductor to be discovered, remains a subject of intense debate several decades after its discovery. SrTiO 3 is also an incipient ferroelectric, and several recent theoretical studies have suggested that the two properties may be linked. To investigate whether such a connection exists, we grew strained, epitaxial SrTiO 3 films, which are known to undergo a ferroelectric transition. We show that, for a range of carrier densities, the superconducting transition temperature is enhanced by up to a factor of two compared to unstrained films grown under the same conditions. Moreover, for these films, superconductivity emerges from a resistive state. We discuss the localization behavior in the context of proximity to ferroelectricity. The results point to new opportunities to enhance superconducting transition temperatures in oxide materials.