Interplay between Phase Defects and Spin Polarization in the Specific Heat of the Spin-Density-Wave Compound(TMTTF)2Brin a Magnetic Field

Abstract

Equilibrium heat relaxation experiments provide evidence that the ground state of the commensurate spin density wave (SDW) compound (TMTTF)2Br after the application of a sufficient magnetic field is different from the conventional ground state. The experiments are interpreted on the basis of the local model of strong pinning as the deconfinement of soliton-antisoliton pairs triggered by the Zeeman coupling to spin degrees of freedom, resulting in a magnetic field induced density wave glass for the spin carrying phase configuration.PACS numbers: 75.30.Fv,75.40.Cx,75.10.Nr Metastability results from energy minima in a system phase space, separated from each other by energy barriers. In some complex systems such as spin glasses, the number of metastable states and the scaling of the energy barriers with the system size are such that ergodicity is broken, meaning that time averaging is not equivalent to ensemble averaging because the system is "trapped" in a valley of the energy landscape. Metastability is also found in model systems such as molecular magnets or Josephson junctions, described by an energy potential V (ϕ) as a function of a single degree of freedom ϕ. We investigate below experimentally and theoretically the residual degrees of freedom of a spin density wave (SDW) at very low temperature in a magnetic field, interpreted as the properties of a classical potential V (ϕ(y i )) for the SDW phase ϕ(y i ) at the coordinate y i along the chain of a strong pinning impurity.Below the Peierls transition temperature, charge density waves (CDWs) and SDWs in quasi-one-dimensional (quasi-1D) compounds are characterized by a spatial modulation of the electronic density (or spin) along the chains. The phase profile is the result of a compromise between the elastic energy that penalizes large phase gradients, and the pinning energy that tends to fix the phase at the strong pinning centers. These two ingredients of the Fukuyama-Lee-Rice model [1] lead to metastability as the result of collective pinning. Collective pinning is however frozen below a glass transition of order ∼ 50 K, as shown by dielectric susceptibility experiments [2]. At very low temperature, the residual degrees of freedom in a zero magnetic field correspond to the local defects of the local model of strong pinning [3,4,5,6,7,8]. Larkin [4] and Ovchinnikov [5] have shown that a single strong pinning impurity leads to a bound state of an electronlike soliton and a hole-like antisoliton. This results in a potential V (ϕ(y i )) with multiple minima [6], leading to slow relaxation in agreement with the very low temperature heat relaxation experiments [5].In order to explore the role of a magnetic field, we choose the compound (TMTTF) 2 Br with a sufficiently narrow spectrum of relaxation times because of its commensurate (antiferromagnetic) ground state [9,10]. This allows a systematic study of the equilibrium energy relaxation over almost one decade in temperature and a direct comparison to the local model of strong pinning without introducing an...

Thermodynamics of a Liquid-like Spin State in Molecule-based Magnets with Geometric Frustrations

“…The lattice softening sometimes occurs as a consequence of the charge disproportionation process in θ-(BEDT-TTF) 2 CsZn-(SCN) 4 42 and several (TMTTF) 2 X compounds. 43 The large ¢ value exceeding 20 mJ K ¹4 mol ¹1 in κ-(BEDT-TTF) 2 Cu 2 (CN) 3 is probably related to the similar kind of softening in phonon structures in this spin-liquid compound. The ¢ values with this order is relatively large in organic systems and close to that of θ-(BEDT-TTF) 2 CsZn(SCN) 4 , which shows a Boson peak structure in low-temperature heat capacity occurring by the charge disproportionation coupled with the lattice.…”

Relaxation time spectrum of low-energy excitations in one- and two-dimensional materials with charge or spin density waves

“…7 and give a better understanding of the magnetic-field-induced density-wave glass state observed in Ref. 6 . In particular, the distribution function is found to have a broad shape with low barrier height.…”

“…The strong impurity pinning leads to a generation of bisolitons, pairs of soliton and antisoliton 6 . Our study shows that there are two kinds of the effective two-level systems having a different origin.…”

“…It was not possible to determine the equilibrium heat capacity of this contribution in materials with incommensurate CDW or SDW up to now, since the maximum of the relaxation time spectrum was too long in comparison to the time window of the experiment 5 . This was achieved for the first time with the commensurate 1D SDW compound (TMTTF) 2 Br by extending the time window of the measurement up to 22000 s 6,7 . It was shown that the equilibrium heat capacity is strongly proportional to T −2 corresponding to an upper tail of a Schottky term with a magnetic field dependent Schottky energy E s .…”

Effect of impurity pinning on conduction and specific heat in the Luttinger liquid

“…Therefore, we concentrate on the qualitative features and consider a simple model resembling approaches used to describe interaction of the CDW with impurity in Refs. [18,19] (the spin-density wave case was treated similarly in [20]). In these papers phase variation around an impurity was described by static sineGordon equation.…”