arXiv subjects

Hilary M. L. Noad

Publications and source records attributed to Hilary M. L. Noad.

11 recordsLinked to original sources

arXiv2026

Entropy mapping under uniaxial pressure utilizing the elastocaloric effect

Uniaxial pressure is a powerful tuning parameter for quantum materials, but conventional thermodynamic probes such as specific heat are difficult to realize in the constrained geometries of strain apparatus. We develop a quantitative analysis framework for a.c. elastocaloric effect measurements that enable the reconstruction of the absolute entropy and hence specific heat across complex phase diagrams. The absolute accuracy is achieved by combining measurements in the strong coupling regime at low frequencies with high signal-to-noise measurements in the quasi-adiabatic regime at high frequencies. Applying the approach to the correlated superconductor Sr$_2$RuO$_4$, we obtain an absolute entropy map across the phase diagram including across phase transitions deep into the superconducting state. We demonstrate that from such data one can derive the absolute specific heat which is currently not possible through other approaches. This data reinforces the finding that the quenching of entropy within the superconductor Sr$_2$RuO$_4$ is strongest at the critical strain consistent with the superconducting gap being maximized at the Van Hove singularity (VHs). Furthermore, we demonstrate that, although $Δc /(γT) $ does increase at the VH strain, this increase is much weaker than previously inferred from more indirect caloric experiments.

cond-mat.supr-con↗

arXiv2026

Probing multipolar order in the candidate altermagnet MnF$_2$ through the elastocaloric effect under strain

Altermagnets break a combination of time-reversal and rotational symmetries without generating a net magnetization. As such, the order parameter of $d$-wave altermagnets has the same symmetry as magnetic multipoles, and couples to the product of a magnetic field and uniaxial strain. We combine elastocaloric experiments, free-energy modeling, and first-principles calculations on MnF$_2$ to establish a thermodynamic probe of the predicted finite-temperature altermagnetic critical point. These results pave the way to explore altermagnetic quantum criticality in $d$-wave materials and beyond.

cond-mat.str-el↗

arXiv2026

Uniaxial-Stress-Induced Magnetic Transitions in the Triangular-Lattice Antiferromagnet PdCrO2

Uniaxial stress is a promising method to tune magnetic frustration, allowing its effects to be studied in a precise way. In this work, uniaxial stress is applied to the triangular-lattice antiferromagnet PdCrO2. The Cr-Cr magnetic interaction is very sensitive to interatomic separation, so laboratory-achievable stress can induce substantial changes in magnetic structure. Results from three types of measurement are presented: X-ray diffraction, the stress-strain relationship, and neutron diffraction. The combined data show that the elastic moduli of PdCrO2 are strongly affected by stress-induced changes in magnetic structure. A new, first-order stress-induced magnetic transition is observed, at which the lattice constant shrinks by 0.21%. The lattice stiffens dramatically across this transition: the Young's modulus increases by about 80 GPa, and the Poisson ratio falls from about 1 to about 0.4. This stiffening indicates that the magnetic order "locks," that is, becomes insensitive to lattice strain. This locking might occur because the new stress-induced magnetic order nests the Fermi surface of the Pd sheets. Other frustrated magnets, including candidate spin liquids, may show similarly strong coupling between magnetic and elastic degrees of freedom.

cond-mat.str-el↗

arXiv2026

Strain continuously rotates the Néel vector in altermagnetic MnTe

Altermagnetism has recently emerged as a distinct class of collinear antiferromagnets that break time-reversal symmetry, exhibiting a host of novel properties. Applied strain has attracted particular attention as a key tuning parameter for altermagnets. Although several experimental studies have demonstrated the preparation of single-domain states through a combination of applied strain and magnetic field, the route to such states remains unclear. Here, we use magneto-optical measurements on single crystals of MnTe under applied strain to show that, in contrast to previous reports, strain acts primarily to rotate the Néel vector L continuously. Since the orientation of L determines the magnetic point group symmetry, this continuous rotation effectively tunes the symmetry and its associated physical properties. Furthermore, we demonstrate that built-in strain in free-standing crystals is sufficient to pin L into continuous textures over millimeter length scales. Together, these results provide guidance for future device design and open the door to leveraging the Néel vector orientation as a tunable degree of freedom in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Using strain to uncover the interplay between two- and three-dimensional charge density waves in high-temperature superconducting YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$

Uniaxial pressure provides an efficient approach to control the competition between charge density waves (CDWs) and superconductivity in underdoped YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$. It can enhance the correlation volume of ubiquitous short-range 2D CDW correlations, and induces a long-range 3D CDW otherwise only accessible at large magnetic fields. Here, we use x-ray diffraction to study the strain and doping evolution of these CDWs. No signatures of discommensurations nor pair density waves are observed in the investigated strain-temperature parameter space, but direct evidence for a form of competition between 2D and 3D CDWs is uncovered. We show that the interplay between the 3D CDW, the 2D CDWs and superconductivity is qualitatively well described by including strain effects in simulations of a nonlinear sigma model of competing superconducting and CDW orders. From a broader perspective, our results underscore the potential of strain tuning as a powerful tool for probing and manipulating competing orders in quantum materials.

cond-mat.supr-con↗

arXiv2024

Determination of the dynamic Young's modulus of quantum materials in piezoactuator-driven uniaxial pressure cells using a low-frequency a.c. method

We report on a new technique for measuring the dynamic Young's modulus, $E$, of quantum materials at low temperatures as a function of static tuning strain, $ε$, in piezoactuator-driven pressure cells. In addition to a static tuning of stress and strain, we apply a small-amplitude, finite-frequency a.c. (1 Hz$ \lesssim ω\lesssim $1000 Hz) uniaxial stress, $σ_{ac}$, to the sample and measure the resulting a.c. strain, $ε_{ac}$, using a capacitive sensor to obtain the associated modulus $E$. We demonstrate the performance of the new technique through proof-of-principle experiments on the unconventional superconductor Sr$_2$RuO$_4$, which is known for its rich temperature-strain phase diagram. In particular, we show that the magnitude of $E$, measured using this a.c. technique at low frequencies, exhibits a pronounced nonlinear elasticity, which is in very good agreement with previous Young's modulus measurements on Sr$_2$RuO$_4$ under [100] strain using a d.c. method (Noad et al., Science 382, 447-450 (2023)). By combining the new a.c. Young's modulus measurements with a.c. elastocaloric measurements in a single measurement, we demonstrate that these a.c. techniques are powerful in detecting small anomalies in the elastic properties of quantum materials. Finally, using the case of Sr$_2$RuO$_4$ as an example, we demonstrate how the imaginary component of the modulus can provide additional information about the nature of ordered phases.

cond-mat.mtrl-sci↗

arXiv2024

Controllable suppression of the unconventional superconductivity in bulk and thin-film Sr$_{2}$RuO$_{4}$ via high-energy electron irradiation

In bulk Sr$_{2}$RuO$_{4}$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$ to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr$_{2}$RuO$_{4}$, the relationship between $T_{\text{c}}$ and the residual resistivity $ρ_0$, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr$_{2}$RuO$_{4}$, we show that $T_{\text{c}}$ is suppressed in both systems at nearly identical rates. This suggests that part of $ρ_0$ in films comes from defects that do not contribute to superconducting pairbreaking, and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

cond-mat.supr-con↗

arXiv2023

Giant lattice softening at a Lifshitz transition in Sr$_{2}$RuO$_{4}$

The interplay of electronic and structural degrees of freedom in solids is a topic of intense research. Experience and intuition suggest that structural changes drive conduction electron behavior, because the large number of valence electrons dominate the structural properties. As part of a seminal paper written over sixty years ago, Lifshitz discussed an alternative possibility: lattice softening driven by conduction electrons at topological Fermi surface transitions. The effect he predicted, however, was small, and has not been convincingly observed. Using measurements of the stress-strain relationship in the ultra-clean metal Sr$_{2}$RuO$_{4}$, we reveal a huge softening of the Young's modulus at a Lifshitz transition of a two-dimensional Fermi surface, and show that it is indeed entirely driven by the conduction electrons of the relevant energy band.

cond-mat.str-el↗

arXiv2023

Probing momentum-dependent scattering in uniaxially stressed Sr$_2$RuO$_4$ through the Hall effect

Under in-plane uniaxial stress, the largest Fermi surface sheet of the correlated metal Sr$_2$RuO$_4$ undergoes a Lifshitz transition from an electron-like to an open geometry. We investigate the effects of this transition on transport through measurement of the longitudinal resistivity $ρ_{xx}$ and the Hall coefficient $R_\text{H}$. At temperatures where scattering is dominated by electron-electron scattering, $R_\text{H}$ becomes more negative across the Lifshitz transition, opposite to expectations from the change in Fermi surface topology. We show that this change in $R_\text{H}$ is explainable only if scattering changes throughout the Brillouin zone, not just at the point in $k$-space where the Lifshitz transition occurs. In a model of orbital-dependent scattering, the electron-electron scattering rate on sections of Fermi surface with $xy$ orbital weight decreases dramatically. On the other hand, at temperatures where defect scattering dominates $ρ_{xx}$ and $R_\text{H}$ are essentially constant across the Lifshitz transition.

cond-mat.str-el↗

arXiv2022

Observation of an Anomalous Hall Effect in Single-crystal Mn3Pt

The Mn3X family of compounds was the first in which a large anomalous Hall effect (AHE) was predicted to arise from a purely antiferromagnetic structure, due to the Berry curvature in momentum space. Nearly simultaneously with this prediction, a large AHE was observed experimentally in one of the hexagonal members of this family, Mn3Sn. Aligning antiferromagnetic domains, a necessary step for observation of the AHE, is more challenging for the cubic members of the Mn3X family, due to a combination of a smaller spontaneous ferromagnetic moment and much stronger magnetic anisotropy. Here, we use a combination of uniaxial stress and applied magnetic field to align domains of bulk single-crystal Mn3Pt, and demonstrate for the first time a substantial AHE in a bulk sample of a cubic member of the Mn3X family. The AHE remains locked in with essentially no quantitative variation when the stress is ramped back to zero, which shows that it is not a consequence of any stress-induced ferromagnetic moment.

cond-mat.mtrl-sci↗

arXiv2020

Rigid platform for applying large tunable strains to mechanically delicate samples

Response to uniaxial stress has become a major probe of electronic materials. Tuneable uniaxial stress may be applied using piezoelectric actuators, and so far two methods have been developed to couple samples to actuators. In one, actuators apply force along the length of a free, beam-like sample, allowing very large strains to be achieved. In the other, samples are affixed directly to piezoelectric actuators, allowing study of mechanically delicate materials. Here, we describe an approach that merges the two: thin samples are affixed to a substrate, that is then pressurized uniaxially using piezoelectric actuators. Using this approach, we demonstrate application of large elastic strains to mechanically delicate samples: the van der Waals-bonded material FeSe, and a sample of CeAuSb$_2$ that was shaped with a focused ion beam.

physics.app-ph↗