Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor $${\hbox {(TMTTF)}}_2 {\hbox {NO}}_3$$ (
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Applied Magnetic Resonance
ORIGINAL PAPER
Charge‑Order Phase Transition in the Quasi One‑Dimensional Organic Conductor (TMTTF)2 NO3 Lena Nadine Majer1 · Björn Miksch1 · Guilherme Gorgen Lesseux1,2 · Gabriele Untereiner1 · Martin Dressel1 Received: 11 April 2020 / Revised: 8 June 2020 © The Author(s) 2020
Abstract Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the (TMTTF)2 X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor (TMTTF)2 NO3 . We found that the (TMTTF)2 NO3 compound develops a peculiar anisotropy with a doubled periodicity ( ab′-plane) of the ESR linewidth below about TCO = (250 ± 10) K . This behavior is similar to observations in the related compounds (TMTTF)2 X ( X = PF6 , SbF6 and AsF6 ), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous (TMTTF)2 ClO4 , known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below TCO ≈ 250 K.
1 Introduction Low-dimensional organic conductors have drawn attention for decades because they show a rich phase diagram with a variety of interesting ground states [1]. Due the reduced dimensionality of the electronic structure and rather strong electronic correlations, these materials exhibit unusual thermodynamic, transport, optical, and magnetic properties. The charge and spin degrees of freedom in the Bechgaard and Fabre salts can be tuned from localized to itinerant by changing the anions as well as
* Lena Nadine Majer lena‑[email protected]‑stuttgart.de 1
1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany
2
Present Address: Iowa State University and Ames Laboratory, Ames, IA 50011, USA
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external parameters such as temperature or pressure. This can lead to various phases in the charge and spin sector, such as charge order, antiferromagnetic phases, spin density wave and even superconductivity [2–5]. Here we focus on the (TMTTF)2 X family, where TMTTF denotes tetramethyltetrathiafulvalene and X stands for a monovalent anion—here specifically on X − = NO−3 . Using electron spin resonance (ESR) as local probe of electronic and magnetic properties, we yield information on the phase transitions as a function of temperature. One-dimensional organic conductors can be described by the Hubbard model. The (TMTTF)2 X compounds are nominally 3/4-filled systems, dimerization along the chains however leads to a half-filled conduction band. Hence the compounds
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