Linda Ye

Alloys of La(Fe,Si)13 with transition metal substitutions are promising magnetocaloric materials. Here, we systematically investigate Co and Ni substitution in LaFe11.5−(x+y)CoxNiySi1.5 to tune the magnetic and structural properties and assess the resulting impacts on magnetocaloric performance. Co substitution produces a strong, nearly linear increase in the Curie temperature (TC), reaching 360 K at x = 1.5, while largely preserving refrigerant capacity (RC). In contrast, Ni substitution suppresses the abrupt first-order character of the transition more strongly than Co substitution, significantly reducing isothermal entropy change (ΔSM) and RC, but also lowering lattice strain and magnetic hysteretic energy losses. Combining Co and Ni produces an intermediate regime that illustrates the trade-offs between elevated TC, reduced magnetostructural losses, and peak magnetocaloric response. We map the compositional dependence of TC, ΔSM, RC, phase constitution, magnetic hysteresis, and field-induced lattice strain, revealing the distinct and complementary roles of Co and Ni. These results establish a tunable design space for La(Fe,Si)13-based alloys.

The iron-based high- T c superconductors (SCs) exhibit rich phase diagrams with intertwined phases, including magnetism, nematicity, and superconductivity. The superconducting T c in many of these materials is maximized in the regime of strong nematic fluctuations, making the role of nematicity in influencing the superconductivity a topic of intense research. Here, we use the AC elastocaloric effect (ECE) to map out the phase diagram of Ba(Fe _{1− x} Co _x ) ₂ As ₂ near optimal doping. The ECE signature at T c on the overdoped side, where superconductivity condenses without any nematic order, is quantitatively consistent with other thermodynamic probes that indicate a single-component superconducting state. In contrast, on the slightly underdoped side, where superconductivity condenses within the nematic phase, ECE reveals a second thermodynamic transition proximate to and below T c . We rule out magnetism and reentrant tetragonality as the origin of this transition and find that our observations strongly suggest a phase transition into a multicomponent superconducting state. This implies the existence of a subdominant pairing instability that competes strongly with the dominant s ± instability. Our results highlight the significant role of nematic order in determining the pairing symmetry close to optimal doping in this extensively studied iron-based SC, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials.

Spin moiré superlattices (SMSs) have been proposed as a magnetic analog of crystallographic moiré systems and a source of electron minibands offering vector-field moiré tunability and Berry curvature effects. However, it has proven challenging to realize an SMS in which a large exchange coupling J is transmitted between conduction electrons and localized spins. Furthermore, most systems have carrier mean free paths l _mfp shorter than their spin moiré lattice constant a _spin , inhibiting miniband formation. Here, we discover that the layered magnetic semimetal EuAg ₄ Sb ₂ overcomes these challenges by forming an interface with J ~ 100 milli–electron volts transferred between a Eu triangular lattice and anionic Ag ₂ Sb bilayers hosting a two-dimensional electron band in the ballistic regime ( l _mfp >> a _spin ). The system realizes an SMS with a _spin commensurate with the Fermi momentum, leading to a marked quenching of the transport response from miniband formation. Our findings demonstrate an approach to magnetically engineering moiré superlattices and a potential route to an emergent spin-driven quantum Hall state.

Revealing the presence of magnetic octupole order and associated octupole fluctuations in solids is a highly challenging task due to the lack of simple external fields that can couple to magnetic octupoles. Here, we demonstrate a methodology for probing the magnetic octupole susceptibility of a candidate material, PrV₂Al₂₀, using a product of magnetic field H_i and shear strain ϵ_jk as a composite effective field, while employing an adiabatic elastocaloric effect to probe the response. We observe Curie-Weiss behavior in the obtained octupolar susceptibility down to approximately 3 K. Although octupole order does not appear to be the leading multipolar channel in PrV₂Al₂₀, our results nevertheless reveal the presence of strong magnetic octupole fluctuations and hence demonstrate that octupole order is at least a competing state. More broadly, our results highlight how anisotropic strain can be combined with magnetic fields to probe elusive ‘hidden’ electronic orders.

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 ln 2 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.