Abstract
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 research
1
. In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method of tuning
2,3
, leading to experiments that have advanced our understanding of the fascinating unconventional superconductor Sr
2
RuO
4
(refs.
4–9
). 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
.
Pervasive criticality
Iron-based superconductors are believed to host a quantum critical point (QCP), a zero-temperature phase transition, beneath the “dome” delineating the superconducting phase. ...Elucidating the nature of this QCP is, however, tricky. Worasaran
et al.
set out to do just that in a prototypical iron-based superconductor, barium iron arsenide. By applying strain to their samples, the researchers found power-law behaviors that are characteristic of nematic quantum criticality. The associated quantum fluctuations were present over a large portion of the phase diagram. This method may be useful in studying quantum criticality in other material systems.
Science
, abb9280, this issue p.
973
Straining samples of Ba(Fe,Co)
2
As
2
reveals the nature of quantum criticality in this family of iron-based superconductors.
Quantum criticality may be essential to understanding a wide range of exotic electronic behavior; however, 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 nonthermal tuning parameter close to the quantum critical point (QCP). Here, we observed 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 iron-based superconductors, providing direct evidence of quantum critical nematic fluctuations in this material. These quantum critical fluctuations are not confined within a narrow regime around the QCP but rather extend over a wide range of temperatures and compositions.
Elastocaloric signature of nematic fluctuations Ikeda, Matthias S; Worasaran, Thanapat; Rosenberg, Elliott W ...
Proceedings of the National Academy of Sciences - PNAS,
09/2021, Letnik:
118, Številka:
37
Journal Article
Recenzirano
Odprti dostop
The elastocaloric effect (ECE) relates 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(Formula: see text)
Formula: see text 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.
The adiabatic elastocaloric effect measures the temperature change of a given system with strain and provides a thermodynamic probe of the entropic landscape in the temperature-strain space. Here, we ...demonstrate that the DC bias strain-dependence of AC elastocaloric effect allows decomposition of the latter into symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strain channels, using a tetragonal f -electron intermetallic DyB2C2 -whose antiferroquadrupolar order breaks local fourfold rotational symmetries while globally remaining tetragonal-as a showcase example. We capture the strain evolution of its quadrupolar and magnetic phase transitions using both singularities in the elastocaloric coefficient and its jumps 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 biquadratic (linear-quadratic) manner in the antiferroquadrupolar (canted antiferromagnetic) phase, which are attributed to a preserved (broken) global tetragonal symmetry, respectively. The broken tetragonal symmetry in the magnetic phase is further evidenced by elastocaloric strain-hysteresis and optical birefringence. Additionally, within the staggered quadrupolar order, the observed elastocaloric response reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnetic orders by promoting pseudospin flips. Our results demonstrate AC elastocaloric effect as a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain in free energy, which holds the potential for investigating and understanding the symmetry of a wide variety of ordered phases in broader classes of quantum materials.
The adiabatic elastocaloric effect measures the temperature change of a given system with strain and provides a thermodynamic probe of the entropic landscape in the temperature-strain space. Here, we ...demonstrate that the DC bias strain-dependence of AC elastocaloric effect allows decomposition of the latter into symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strain channels, using a tetragonal Formula: see text-electron intermetallic DyBFormula: see textCFormula: see text-whose antiferroquadrupolar order breaks local fourfold rotational symmetries while globally remaining tetragonal-as a showcase example. We capture the strain evolution of its quadrupolar and magnetic phase transitions using both singularities in the elastocaloric coefficient and its jumps 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 biquadratic (linear-quadratic) manner in the antiferroquadrupolar (canted antiferromagnetic) phase, which are attributed to a preserved (broken) global tetragonal symmetry, respectively. The broken tetragonal symmetry in the magnetic phase is further evidenced by elastocaloric strain-hysteresis and optical birefringence. Additionally, within the staggered quadrupolar order, the observed elastocaloric response reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnetic orders by promoting pseudospin flips. Our results demonstrate AC elastocaloric effect as a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain in free energy, which holds the potential for investigating and understanding the symmetry of a wide variety of ordered phases in broader classes of quantum materials.
Abstract
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.
Using Ba(Fe0.975Co0.025)2As2 as an exemplar material exhibiting second-order electronic-nematic and antiferromagnetic transitions, we present measurements that reveal anomalies in the ...elastoresistance (∂ρij∂ɛkl) and elastocaloric effect (∂T∂ɛkl) at both phase transitions for induced strains ɛkl that do not share the symmetry of either order parameter. 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)∝U(c) between the scattering from critical degrees of freedom and their energy density, respectively, holds near each of the transitions in the material studied under varying strain as it does for varying temperature.
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.
Iron-based superconductors are believed to host a quantum critical point (QCP), a zero-temperature phase transition, beneath the “dome” delineating the superconducting phase. Elucidating the nature ...of this QCP is, however, tricky. Worasaran et al. set out to do just that in a prototypical iron-based superconductor, barium iron arsenide. By applying strain to their samples, the researchers found power-law behaviors that are characteristic of nematic quantum criticality. The associated quantum fluctuations were present over a large portion of the phase diagram. This method may be useful in studying quantum criticality in other material systems.
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.
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