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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. 

The adiabatic elastocaloric effect relates changes in the strain that a material experiences to resulting changes in its temperature. While elastocaloric materials have been utilized for cooling in room-temperature applications, the use of such materials for cryogenic cooling remains relatively unexplored. Here, we use a strain load-unload technique at low temperatures, similar to those employed at room temperature, to demonstrate a large cooling effect in Tm⁢VO4. For strain changes of 1.8 ×10−3, the inferred cooling reaches approximately 50% of the material’s starting temperature at 5 K, justifying the moniker “giant.” Beyond establishing the suitability of this class of material for cryogenic elastocaloric cooling, these measurements also provide additional insight into the entropy landscape in the material as a function of strain and temperature, including the behavior proximate to the quadrupolar phase transition. 

A charge density wave (CDW) is a phase of matter characterized by a periodic modulation of the valence electron density accompanied by a distortion of the lattice structure. The microscopic details of CDW formation are closely tied to the dynamic charge susceptibility, χ(q, ω), which describes the behavior of electronic collective modes. Despite decades of extensive study, the behavior of χ(q, ω) in the vicinity of a CDWtransition has never been measured with high energy resolution (∼meV). Here, we investigate the canonical CDW transition in ErTe3 using momentum-resolved electron energy loss spectroscopy (M-EELS), a technique uniquely sensitive to valence band charge excitations. Unlike phonons in these materials, which undergo conventional softening due to the Kohn anomaly at the CDW wavevector, the electronic excitations display purely relaxational dynamics that are well described by a diffusive model. The diffusivity peaks around 250 K, just below the critical temperature. Additionally, we report, for the first time, a divergence in the real part of χ(q, ω) in the static limit (ω → 0), a phenomenon predicted to characterize CDWs since the 1970s. These results highlight the importance of energy- and momentum-resolved measurements of electronic susceptibility and demonstrate the power of M-EELS as a versatile probe of charge dynamics in 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.