arXiv subjects

Matthias S. Ikeda

Publications and source records attributed to Matthias S. Ikeda.

11 recordsLinked to original sources

arXiv2024

Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

The iron-based high-$T_c$ superconductors 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 re-entrant 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 sub-dominant pairing instability that competes strongly with the dominant $s^\pm$ instability. Our results thus motivate a re-examination of the pairing state and its interplay with nematicity in this extensively studied iron-based superconductor, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials.

cond-mat.supr-con↗

arXiv2023

Giant elastocaloric effect at low temperatures in TmVO$_4$ and implications for cryogenic cooling

Adiabatic decompression of para-quadrupolar 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 coefficient 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 to the dynamic quadrupole strain susceptibility.

cond-mat.str-el↗

arXiv2022

Elastocaloric signatures of symmetric and antisymmetric strain-tuning of quadrupolar and magnetic phases in DyB2C2

The adiabatic elastocaloric effect measures the temperature change of given systems with strain and probes the entropic landscape in the temperature-strain space. In this study we demonstrate that the DC bias strain-dependence of AC elastocaloric effect can be used to decompose the latter into contributions from symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strains, using a tetragonal f-electron system DyB2C2--whose antiferroquadrupolar order locally breaks four-fold rotational site symmetries while globally remaining tetragonal--as a showcase example. We capture the strain evolution of the quadrupolar and magnetic phase transitions in the system using both singularities in the elastocaloric coefficient and its jump at the transitions, and the latter we show follows a modified Ehrenfest relation. We find that antisymmetric strain couples to the underlying order parameter in a bi-quadratic manner in the antiferroquadrupolar (AFQ) phase but in a linear-quadratic manner in the canted antiferromagnetic (CAFM) phase; the contrast is attributed to a preserved (broken) tetragonal symmetry in the AFQ (CAFM) phase, respectively. The broken tetragonal symmetry in the CAFM phase is further supported by elastocaloric strain-hysteresis and observation of two sets of domains with mutually perpendicular principal axes in optical birefringence. Additionally, when the quadrupolar moments are ordered in a staggered fashion, we uncover an elastocaloric response that reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnets by promoting pseudospin flips. Our results show that AC elastocaloric effect is a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain, which can potentially be applied to broader classes of quantum materials.

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

Quantum critical fluctuations in an Fe-based superconductor

Quantum critical fluctuations may prove to play an instrumental role in the formation of unconventional superconductivity. Here, we show that the characteristic scaling of a marginal Fermi liquid is present in inelastic light scattering data of an Fe-based superconductor tuned through a quantum critical point (QCP) by chemical substitution or doping. From the doping dependence of the imaginary time dynamics we are able to distinguish regions dominated by quantum critical behavior from those having classical critical responses. This dichotomy reveals a connection between the marginal Fermi liquid behavior and quantum criticality. In particular, the overlap between regions of high superconducting transition temperatures and quantum critical scaling suggests a contribution from quantum fluctuations to the formation of superconductivity.

cond-mat.supr-con↗

arXiv2020

Elastocaloric signature of nematic fluctuations

The elastocaloric effect (ECE) is a thermodynamic quantity relating changes in entropy to changes in strain experienced by a material. As such, ECE measurements can provide valuable information about the entropy landscape proximate to strain-tuned phase transitions. For ordered states that break only point symmetries, bilinear coupling of the order parameter with strain implies that the ECE can also provide a window on fluctuations above the critical temperature, and hence, in principle, can also provide a thermodynamic measure of the associated susceptibility. To demonstrate this, we use the ECE to sensitively reveal the presence of nematic fluctuations in the archetypal Fe-based superconductor Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$. By performing these measurements simultaneously with elastoresistivity in a multimodal fashion, we are able to make a direct and unambiguous comparison of these closely related thermodynamic and transport properties, both of which are sensitive to nematic fluctuations. As a result, we have uncovered an unanticipated doping-dependence of the nemato-elastic coupling and of the magnitude of the scattering of low energy quasi-particles by nematic fluctuations -- while the former weakens, the latter increases dramatically with increasing doping.

cond-mat.supr-con↗

arXiv2020

Nematic quantum criticality in an Fe-based superconductor revealed by strain-tuning

Quantum criticality has been invoked as being essential to the understanding of a wide range of exotic electronic behavior, including heavy Fermion and unconventional superconductivity, but conclusive evidence of quantum critical fluctuations has been elusive in many materials of current interest. An expected characteristic feature of quantum criticality is power law behavior of thermodynamic quantities as a function of a non-thermal tuning parameter close to the quantum critical point (QCP). In the present work, we observe power law behavior of the critical temperature of the coupled nematic/structural phase transition as a function of uniaxial stress in a representative family of Fe-based superconductors. Our measurements provide direct evidence of quantum critical nematic fluctuations in this material. Furthermore, these quantum critical fluctuations are not confined within a narrow regime around the QCP, but extend over a wide range of temperatures and tuning parameters.

cond-mat.str-el↗

arXiv2019

Signatures of Anelastic Domain Relaxation in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ Investigated by Mechanical Modulation of Resistivity

The resistive response of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ to AC mechanical deformation is considered in the multi-domain state. This resistance change depends both upon the anelastic relaxation of domain walls and upon the relation between resistance and the domain wall configuration. Samples are adhered to the surface of a piezoelectric stack, which is driven by an AC voltage while the AC modulation of the sample resistance is measured. As the response time of electrons is faster than that of the lattice, the phase difference $ϕ$ between the AC resistance modulation and the AC deformation of the piezoelectric is entirely due to anelastic relaxation effects in the sample. An expression is derived for relating $ϕ$ to a sample's complex compliance, $J(ω)$, in this experimental configuration. Measurements of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for x= (0.025, 0.052) reveal a peak in the out-of-phase resistivity modulation in the orthorhombic antiferromagnetic state well below the Neél temperature $T_N$ and structural transition $T_S$. Meanwhile, for a composition that is tetragonal at all temperatures, x=0.07, the resistance modulation remains entirely in phase over the same temperatures, establishing domain motion as a probable cause of the observed effects in the samples that do undergo the tetragonal-to-orthorhombic transition. Fits are provided of $\tanϕ$ for a sample with x=0.025 for various amplitude excitations on the piezoelectric stack, from which the apparent activation energy $E_a$ for domain wall motion is found to decreases with increasing amplitude of the deformation along the $[110]_T$ axis. We find $\frac{dE_a}{d\varepsilon^0_{[110,110]_T}} = -1115\pm 196$ eV with $E_a = 9.09 \pm 0.74 \times 10^{-3}$ eV in the zero strain limit if we assume linearity over the entire range of strain amplitudes.

cond-mat.str-el↗

arXiv2019

AC Elastocaloric effect as a probe for thermodynamic signatures of continuous phase transitions

Studying the response of materials to strain can elucidate subtle properties of electronic structure in strongly correlated materials. So far, mostly the relation between strain and resistivity, the so called elastoresistivity, has been investigated. The elastocaloric effect is a second rank tensor quantity describing the relation between entropy and strain. In contrast to the elastoresistivity, the elastocaloric effect is a thermodynamic quantity. Experimentally, elastocaloric effect measurements are demanding since the thermodynamic conditions during the measurement have to be well controlled. Here we present a technique to measure the elastocaloric effect under quasi adiabatic conditions. The technique is based on oscillating strain, which allows for increasing the frequency of the elastocaloric effect above the thermal relaxation rate of the sample. We apply the technique to Co-doped iron pnictide superconductors and show that the thermodynamic signatures of second order phase transitions in the elastocaloric effect closely follow those observed in calorimetry experiments. In contrast to the heat capacity, the electronic signatures in the elastocaloric effect are measured against a small phononic background even at high temperatures, establishing this technique as a powerful complimentary tool for extracting the entropy landscape proximate to a continuous phase transition.

cond-mat.mtrl-sci↗

arXiv2019

Symmetric and antisymmetric strain as continuous tuning parameters for electronic nematic order

We report the separate response of the critical temperature of the nematic phase transition $T_{\rm S}$ to symmetric and antisymmetric strains for the prototypical underdoped iron pnictide Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$. This decomposition is achieved by comparing the response of $T_{\rm S}$ to uniaxial stress and hydrostatic pressure. In addition to quantifying the two distinct linear responses to symmetric strains, we find a quadratic variation of $T_{\rm S}$ as a response to antisymmetric strains $ε_{\rm B_{1g}}$=$\frac{1}{2}$($ε_{\rm xx}$-$ε_{\rm yy}$), exceeding the non linear response to symmetric strains by at least two orders of magnitude. These observations establish orthogonal antisymmetric strain as a powerful tuning parameter for nematic order.

cond-mat.mtrl-sci↗

arXiv2018

Elastoresistive and Elastocaloric Anomalies at Magnetic and Electronic-Nematic Critical Points

Using Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_{2}$ as an exemplar material exhibiting second order electronic-nematic and antiferromagnetic transitions, we present measurements that reveal anomalies in the elastoresistance $\left(\frac{\partial ρ_{ij}}{\partial\varepsilon_{kl}}\right)$ and elastocaloric effect $\left(\frac{\partial T}{\partial\varepsilon_{kl}}\right)$ at both phase transitions. Both effects are understood to arise from the effect of strain on the transition temperatures; in the region close to the phase transitions this leads to (1) similarity between the strain and temperature derivatives of the resistivity and (2) similarity between the elastocaloric effect and the singular part of the specific heat. These mechanisms for elastoresistance and elastocaloric effect should be anticipated for any material in which mechanical deformation changes the transition temperature. Furthermore, these measurements provide evidence that the Fisher-Langer relation $ρ^{(c)} \propto U^{(c)}$ between the scattering from critical degrees of freedom and their energy-density, respectively, holds near each of the Neél and electronic nematic transitions in the material studied.

cond-mat.str-el↗