Search

Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of ${R}_{2}{(\mathrm{Fe},\mathrm{Co})}_{17}$ materials ($R=\text{rare}$ Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3--461 K, almost twice the range of currently known SIV. In situ neutron diffraction, M\"ossbauer spectra and first-principles calculations reveal the $3d$ bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve ``ultrawide'' SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.

We report the results of a combined study of the normal-state resistivity and superconducting transition temperature ${T}_{c}$ of the unconventional superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under uniaxial pressure. There is strong evidence that, as well as driving ${T}_{c}$ through a maximum at $\ensuremath{\sim}3.5\text{ }\text{ }\mathrm{K}$, compressive strains $ϵ$ of nearly 1% along the crystallographic [100] axis drive the $\ensuremath{\gamma}$ Fermi surface sheet through a van Hove singularity, changing the temperature dependence of the resistivity from ${T}^{2}$ above, and below the transition region to ${T}^{1.5}$ within it. This occurs in extremely pure single-crystals in which the impurity contribution to the resistivity is $l100\text{ }\text{ }\mathrm{n}\mathrm{\ensuremath{\Omega}}\text{ }\mathrm{cm}$, so our study also highlights the potential of uniaxial pressure as a more general probe of this class of physics in clean systems.

Here we demonstrate how the Fermi surface topology and quantum many-body interactions can be manipulated via epitaxial strain in the spin-triplet superconductor Sr$_2$RuO$_4$ and its isoelectronic counterpart Ba$_2$RuO$_4$ using oxide molecular beam epitaxy (MBE), \emph{in situ} angle-resolved photoemission spectroscopy (ARPES), and transport measurements. Near the topological transition of the $\gamma$ Fermi surface sheet, we observe clear signatures of critical fluctuations, while the quasiparticle mass enhancement is found to increase rapidly and monotonically with increasing Ru-O bond distance. Our work demonstrates the possibilities for using epitaxial strain as a disorder-free means of manipulating emergent properties, many-body interactions, and potentially the superconductivity in correlated materials.

We investigate the nonequilibrium behavior of the spin-ice Dy2Ti2O7 by studying its magnetization as a function of the field sweep rate. Below the enigmatic ''freezing'' temperature T(equil)≈600 mK, we find that even the slowest sweeps fail to yield the equilibrium magnetization curve and instead give an initially much flatter curve. For higher sweep rates, the magnetization develops sharp steps accompanied by similarly sharp peaks in the temperature of the sample. We ascribe the former behavior to the energy barriers encountered in the magnetization process, which proceeds via flipping of spins on filaments traced out by the field-driven motion of the gapped, long-range interacting magnetic monopole excitations. The peaks in temperature result from the released Zeeman energy not being carried away efficiently; the resulting heating triggers a chain reaction.