arXiv subjects

Fabian Jerzembeck

Publications and source records attributed to Fabian Jerzembeck.

16 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

Quantum Oscillations of $\mathrm{Sr}_2\mathrm{RuO}_4$ under c-Axis Uniaxial Stress

Uniaxial stress has now been widely used to study correlated electron materials. However, Fermi surface-resolved experimental data on the evolution of the electronic structure under piezoelectrically applied stress are sparse, with no reports of de Haas-van Alphen (dHvA) effects under uniaxial stress. Here we present dHvA measurements under $c$-axis uniaxial stress on the unconventional superconductor $\mathrm{Sr}_2\mathrm{RuO}_4$. This allows us to study the evolution of the electronic structure directly and to gain insight into the contradicting behavior of the predicted enhancement of the electronic density of states and the observed suppression of $T_\text{c}$. We are able to follow all Fermi surfaces for stress up to $-1.8$~GPa and find that the cross-sectional areas of the hole-like $α$ sheet increase and electron-like $β$ sheet decrease. At the same time, the area of the electron-like $γ$ sheet increases. Therefore, in contrast to in-plane uniaxial stress, charge transfer is the mechanism for approaching the electron-to-hole Lifshitz transition and the associated Van Hove singularity. Additionally, we find that the effective masses on all three Fermi sheets are slightly enhanced as the Lifshitz transition is approached. We compare the dHvA results with quantum oscillations in the magnetostriction and band structure calculations, and find good agreement. At a more general level, our findings show that quantum oscillation measurements under uniaxial stress, combined with band-structure calculations, offer a promising new route for studying quantum materials.

cond-mat.str-el↗

arXiv2026

Spinon mediation of witness spin dynamics in herbertsmithite

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such 'impurity' atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

cond-mat.str-el↗

arXiv2026

Discovery of Dynamical Heterogeneity in a Supercooled Magnetic Monopole Fluid

Dynamical heterogeneity, in which transitory local fluctuations occur in the conformation and dynamics of constituent particles, is widely hypothesized to be essential to evolution of supercooled liquids into the structural glass state. Yet its microscopic spatiotemporal phenomenology is challenging to detect directly in molecular glass forming liquids. Because recent theoretical advances predict that corresponding dynamical heterogeneity could occur in supercooled magnetic monopole fluids (Proc. Nat. Acad. Sci. 112, 8549 (2015)), we searched for such phenomena in Dy2Ti2O7. By measuring its microsecond-resolved spontaneous magnetization fluctuations $M(t,T)$ we discovered a sharp bifurcation in monopole noise characteristics below $T \approx 1500$ mK, with the appearance of powerful spontaneous monopole current bursts. This intense dynamics emerges upon entering the supercooled monopole fluid regime, reaches maximum strength near $T \approx 750$ mK and then collapses along with coincident loss of ergodicity approaching $T_g \approx 750$ mK. Moreover, when the four-point dynamical susceptibility $χ_4(τ, T)$ is determined directly from temperature dependence of correlations in $M(t,T)$, it evolves as predicted when dynamical heterogeneity is present, revealing its simultaneously and rapidly escalating length and time scales, $ξ(T)$. and $τ_4(T)$. This overall phenomenology 2 greatly expands our empirical knowledge of supercooled monopole fluids and, more generally, demonstrates direct detection of the time sequence, magnitude, statistics and correlations of dynamical heterogeneity, access to which may greatly accelerate fundamental vitrification studies.

cond-mat.str-el↗

arXiv2024

Spiral Spin Liquid Noise

An emerging concept for identification of different types of spin liquids is through the use of spontaneous spin noise. Here we develop spin noise spectroscopy for spin liquid studies by considering Ca$_{10}$Cr$_7$O$_{28}$, a material hypothesized to be either a quantum or a spiral spin liquid. By enhancing techniques introduced for magnetic monopole noise studies we measure the time and temperature dependence of spontaneous flux $\varPhi(t, T)$ and thus magnetization $M(t, T)$ of Ca$_{10}$Cr$_7$O$_{28}$ samples. The resulting power spectral density of magnetization noise $S_M(ω,T)$ reveals intense spin fluctuations with $S_M(ω,T) \propto ω^{-α(T)}$ and 0.84 < $α(T)$ < 1.04 . Both the variance $σ_M^2(T)$ and the correlation function $C_M(t,T)$ of this spin noise undergo crossovers at a temperature $T^* \approx$ 450 mK. While predictions for quantum spin liquids are inconsistent with this phenomenology, those from Monte-Carlo simulations of a 2D spiral spin liquid state in Ca$_{10}$Cr$_7$O$_{28}$ yield overall quantitative correspondence with the measured frequency and temperature dependences of $S_M(ω,T), C_M(t,T)$ and $σ_M^2(T)$, thus indicating that Ca$_{10}$Cr$_7$O$_{28}$ is a spiral spin liquid.

cond-mat.str-el↗

arXiv2024

$T_c$ and the elastocaloric effect of Sr$_2$RuO$_4$ under $\langle 110 \rangle$ uniaxial stress: no indications of transition splitting

There is considerable evidence that the superconductivity of Sr2RuO4 has two components. Among this evidence is a jump in the shear elastic modulus $c_{66}$ at the critical temperature $T_c$, observed in ultrasound measurements. Such a jump is forbidden for homogeneous single-component order parameters, and implies that $T_c$ should develop as a cusp under the application of shear strain with $\langle 110 \rangle$ principal axes. This shear strain should split the onset temperatures of the two components, if they coexist, or select one component if they do not. Here, we report measurements of $T_c$ and the elastocaloric effect of Sr2RuO4 under uniaxial stress applied along the $[110]$ lattice direction. Within experimental resolution, we resolve neither a cusp in the stress dependence of $T_c$, nor any second transition in the elastocaloric effect data. We show that reconciling these null results with the observed jumps in $c_{66}$ requires extraordinarily fine tuning to a triple point of the Ginzburg-Landau parameter space. In addition, our results are inconsistent with homogeneous time reversal symmetry breaking at a temperature $T_2 \leq T_c$ as identified in muon spin relaxation experiments.

cond-mat.supr-con↗

arXiv2024

Dichotomous Dynamics of Magnetic Monopole Fluids

A recent advance in the study of emergent magnetic monopoles was the discovery that monopole motion is restricted to dynamical fractal trajectories (J. Hallén et al, Science 378, 1218 (2022)) thus explaining the characteristics of magnetic monopole noise spectra (Dusad, R. et al. Nature 571, 234 (2019); Samarakoon, A. M. et al. Proc. Natl. Acad. Sci. 119, e2117453119 (2022)). Here we apply this new theory to explore the dynamics of field-driven monopole currents, finding them comprised of two quite distinct transport processes: initially swift fractal rearrangements of local monopole configurations followed by conventional monopole diffusion. This theory also predicts a characteristic frequency dependence of the dissipative loss-angle for AC-field-driven currents. To explore these novel perspectives on monopole transport, we introduce simultaneous monopole current control and measurement techniques using SQUID-based monopole current sensors. For the canonical material Dy2Ti2O7, we measure $Φ(t)$, the time-dependence of magnetic flux threading the sample when a net monopole current $J(t) = \dotΦ(t)/μ_0$ is generated by applying an external magnetic field $B_0(t)$. These experiments find a sharp dichotomy of monopole currents, separated by their distinct relaxation time-constants before and after $t \approx 600 μs$ from monopole current initiation. Application of sinusoidal magnetic fields $B_0(t) = Bcos(ωt)$ generates oscillating monopole currents whose loss angle $θ(f)$ exhibits a characteristic transition at frequency $f \approx 1.8$ kHz over the same temperature range. Finally, the magnetic noise power is also dichotomic, diminishing sharply after $t \approx 600 μs$. This complex phenomenology represents a new form of heterogeneous dynamics generated by the interplay of fractionalization and local spin configurational symmetry.

cond-mat.mes-hall↗

arXiv2023

Superconducting Penetration Depth Through a Van Hove Singularity: Sr$_2$RuO$_4$ Under Uniaxial Stress

In the unconventional superconductor Sr$_2$RuO$_4$, uniaxial stress along the $[100]$ direction tunes the Fermi level through a Van Hove singularity (VHS) in the density of states, causing a strong enhancement of the superconducting critical temperature $T_\textrm{c}$. Here, we report measurements of the London penetration depth $λ$ as this tuning is performed. We find that the zero-temperature superfluid density, here defined as $λ(0)^{-2}$, increases by $\sim$15%, with a peak that coincides with the peak in $T_\textrm{c}$. We also find that the low temperature form of $λ(T)$ is quadratic over the entire strain range. Using scanning tunneling microscopy, we find that the gap increases from $Δ_0 \approx 350~μ$eV in unstressed Sr$_2$RuO$_4$ to $Δ_0 \approx 600~μ$eV in a sample strained to near the peak in $T_c$. With a nodal order parameter, an increase in the superconducting gap could bring about an increase in the superfluid density through reduced sensitivity to defects and through reduced non-local effects in the Meissner screening. Our data indicate that tuning to the VHS increases the gap throughout the Brillouin zone, and that non-local effects are likely more important than reduced scattering.

cond-mat.supr-con↗

arXiv2023

Upper Critical Field of Sr$_2$RuO$_4$ under In-Plane Uniaxial Pressure

In-plane uniaxial pressure has been shown to strongly tune the superconducting state of Sr2RuO4 by approaching a Lifshitz transition and associated Van Hove singularity (VHS) in the density of states. At the VHS, $T_c$ and the in- and out-of-plane upper critical fields are all strongly enhanced, and the latter has changed its curvature as a function of temperature from convex to concave. However, due to strain inhomogeneity it has not been possible so far to determine how the upper critical fields change with strain. Here, we show the strain dependence of both upper critical fields, which was achieved due to an improved sample preparation. We find that the in-plane upper critical field is mostly linear in $T_c$. On the other hand, the out-of-plane upper critical field varies with a higher power in $T_c$, and peaks strongly at the VHS. The strong increase in magnitude and the change in form of $H_\mathrm{c2||c}$ occur very close to the Van Hove strain, and points to a strong enhancement of both the density of states and the gap magnitude at the Lifshitz transition.

cond-mat.supr-con↗

arXiv2022

Propagating charge carrier plasmon in Sr2RuO4

We report on studies of charge carrier plasmon excitations in Sr2RuO4 by transmission Electron Energy-Loss Spectroscopy. In particular, we present results on the plasmon dispersion and its width as a function of momentum transfer. The dispersion can be qualitatively explained in the framework of RPA calculations, using an unrenormalized tight-binding band structure. The constant long-wavelength width of the plasmon indicates, that it is caused by a decay into inter-band transition and not by quantum critical fluctuations. The results from these studies on a prototypical bad metal system show that the long-wavelength plasmon excitations near 1 eV are caused by resilient quasiparticles and are not influenced by correlation effects.

cond-mat.str-el↗

arXiv2022

Elastocaloric determination of the phase diagram of Sr$_2$RuO$_4$

One of the main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagrams that also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research1. In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method of tuning, leading to experiments that have advanced our understanding of the fascinating unconventional superconductor Sr$_2$RuO$_4$. Here we map out its phase diagram using high-precision measurements of the elastocaloric effect in what we believe to be the first such study including both the normal and the superconducting states. We observe a strong entropy quench on entering the superconducting state, in excellent agreement with a model calculation for pairing at the Van Hove point, and obtain a quantitative estimate of the entropy change associated with entry to a magnetic state that is observed in proximity to the superconductivity. The phase diagram is intriguing both for its similarity to those seen in other families of unconventional superconductors and for extra features unique, so far, to Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2022

Tuning the Fermi Liquid Crossover in Sr$_2$RuO$_4$ with Uniaxial Stress

We perform nuclear magnetic resonance (NMR) measurements of the oxygen-17 Knight shifts for Sr$_2$RuO$_4$, while subjected to uniaxial stress applied along [100] direction. The resulting strain is associated with a strong variation of the temperature and magnetic field dependence of the inferred magnetic response. A quasi-particle description based on density-functional theory calculations, supplemented by many-body renormalizations, is found to reproduce our experimental results, and highlights the key role of a van-Hove singularity. The Fermi liquid coherence scale is shown to be tunable by strain, and driven to low values as the associated Lifshitz transition is approached.

cond-mat.str-el↗

arXiv2022

The Superconductivity of Sr$_2$RuO$_4$ Under $c$-Axis Uniaxial Stress

Applying in-plane uniaxial pressure to strongly correlated low-dimensional systems has been shown to tune the electronic structure dramatically. For example, the unconventional superconductor Sr$_2$RuO$_4$ can be tuned through a single Van Hove singularity which results in a strong enhancement of both $T_\text{c}$ and $H_\text{c2}$. Out-of-plane ($c$ axis) uniaxial pressure is expected to tune the quasi-two-dimensional structure even more strongly, causing it to approach two Van Hove singularities simultaneously. Here we achieve a record value of $3.2$ GPa compression along the $c$ axis of Sr$_2$RuO$_4$. Although the rise in $H_\text{c2}$ shows that we are indeed approaching the van Hove points, $T_\text{c}$ is suppressed, a result that contradicts expectations based on simple two-dimensional models. As a first attempt to take the third dimension into account, we present three-dimensional calculations in the weak interaction limit, and discuss the extent to which they are consistent with observation. Our experimental results highlight the importance of out-of-plane effects in low-dimensional systems in general and provide new constraints on theories of the pairing interaction in Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2021

High sensitivity heat capacity measurements on Sr2RuO4 under uniaxial pressure

A key question regarding the unconventional superconductivity of Sr$_2$RuO$_4$ remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few per cent of that of the primary superconducting transition. The normalized jump in heat capacity, $ΔC/C$, grows smoothly as a function of uniaxial pressure, favouring order parameters which are allowed to maximize in the same part of the Brillouin zone as the well-studied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2020

Evidence for even parity unconventional superconductivity in Sr$_2$RuO$_4$

Unambiguous identification of the superconducting order parameter symmetry of Sr$_2$RuO$_4$ has remained elusive for more than a quarter century. While a chiral $p$-wave ground state analogue to superfluid $^3$He-$A$ was ruled out only very recently, other proposed $p$-wave scenarios are still viable. Here, field-dependent $^{17}$O Knight shift measurements are compared to corresponding specific heat measurements, previously reported. We conclude that the shift results can be accounted for by the expected field-induced quasiparticle response only. An upper bound for the condensate magnetic response of $<10\%$ of the normal state susceptibility is sufficient to exclude odd-parity candidates.

cond-mat.supr-con↗

arXiv2016

Strong Peak in $T_c$ of Sr$_2$RuO$_4$ Under Uniaxial Pressure

We report a combined experimental and theoretical study of the dependence of the superconductivity of the unconventional superconductor Sr$_2$RuO$_4$ on anisotropic strain. Novel piezoelectric apparatus is used to apply uniaxial pressures of up to $\sim$1~GPa along a $\langle 100 \rangle$ direction ($a$-axis) of the crystal lattice. $T_c$ increases from 1.5~K in unstrained material to 3.4~K at compression by $\approx$0.6\%, then falls steeply. The $c$-axis upper critical field for the strained $T_c$ = 3.4~K material is a factor of twenty larger than that of the unstrained crystal, whereas the in-plane ($a$-axis) critical field increases by only a factor of three. First-principles electronic structure calculations give evidence that the observed maximum $T_c$ occurs at or near a Lifshitz transition when the Fermi level passes through a van Hove singularity. Finally, we perform order parameter analyses using three-band renormalization group calculations. These, combined with the unexpectedly low in-plane critical field, open the possibility that the highly strained $T_c$=3.4~K Sr$_2$RuO$_4$ has an even- rather than an odd-parity order parameter. Potential implications such as a transition at nonzero strain between odd- and even-parity order parameters are discussed.

cond-mat.supr-con↗