Quantum criticality in a uniaxial organic ferroelectric

Abstract

Tris-sarcosine calcium chloride (TSCC) is a highly uniaxial ferroelectric with a Curie temperature of approximately 130K. By suppressing ferroelectricity with bromine substitution on the chlorine sites, pure single crystals were tuned through a ferroelectric quantum phase transition. The resulting quantum critical regime was investigated in detail -the first time for a uniaxial ferroelectric and for an organic ferroelectric -and was found to persist up to temperatures of at least 30K to 40K. The nature of long-range dipole interactions in uniaxial materials, which lead to non-analytical terms in the free-energy expansion in the polarization, predict a dielectric susceptibility varying as 1/T 3 close to the quantum critical point. Rather than this, we find that the dielectric susceptibility varies as 1/T 2 as expected and observed in better known multi-axial systems. We explain this result by identifying the ultra-weak nature of the dipoles in the TSCC family of crystals. Interestingly we observe a shallow minimum in the inverse dielectric function at low temperatures close to the quantum critical point in paraelectric samples that may be attributed to the coupling of quantum polarization and strain fields. Finally we present results of the heat capacity and electro-caloric effect and explain how the time dependence of the polarization in ferroelectrics and paraelectrics should be considered when making quantitative estimates of temperature changes induced by applied electric fields.There has been a great deal of interest recently in the field of ferroelectric quantum phase transitions [1,2]. A chief reason for this is that the properties of ferroelectrics may be readily tuned with gate voltages and strains, which make quantum ferroelectrics and paraelectrics particularly suited for applications in advanced cryogenic electronics. Highlighting SrTiO 3 as an example, one sees that a pristine optically transparent insulator with a band gap of more than 3eV, and a static dielectric constant greater than 10 4 , may be tuned through an insulator to metal to superconductor transition and back again with the application of just a few volts [3,4]. On the other hand modest strains [5], chemical doping [6] or isotope substitution [1,7], can induce ferroelectricity in an otherwise paraelectric ground state. Ferroelectric quantum critical fluctuations have been observed up to temperatures higher than those often seen in other systems, and over a wide range of tuning parameters. Proximity to a displacive ferroelectric quantum critical point where the transverse optical phonon frequency becomes very small, and the dielectric function can rise to very high values, is believed to be of importance in understanding superconductivity in materials such as chemically doped [8] or ionic-liquid-gated SrTiO 3 [7] and KTaO 3 [4], and in oxide interface materials [9] to name just a few.The nature of quantum criticality in ferroelectrics is strikingly different from that found in other systems, for example magnetic systems [1,10,11]. Quantu...

Direct and indirect measurements on electrocaloric effect: Recent developments and perspectives

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Some strategies for improving caloric responses with ferroelectrics

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“…82,83 The review here provides a timely summary of recent strategies used to improve caloric response in ferroelectrics. It clearly demonstrates promising perspectives for ferroelectrics in solidstate refrigeration.…”

Quantum critical points in ferroelectric relaxors: Stuffed tungsten bronze K3Li2Ta5OSmith1, Gardner2, Morrison3 et al. 2018Phys. Rev. Materials3020View full textAdd to dashboardCite

“…However, as the dynamical exponent is 1 for displacive FE rather than 3 for itinerant ferromagnets, the understanding and modeling of their properties are likely to be more complex, since real systems can exceed the upper and lower critical dimensionality [1]. For a QCP to occur, the transition should be driven by quantum fluctuations rather than classical fluctuations, and such quantum fluctuations tend to dominate in a region just above 0 K. Interest in ferroelectric quantum critical points has grown rapidly in the past several years, with emphasis upon perovskites [1,2] and several other materials, including hexaferrites [3][4][5][6] and organic or molecular crystals [7][8][9][10]. However, the QCPs studied thus far do not include many crystal families, and with one recent exception [11], no glassy relaxors.…”