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Article Dans Une Revue Physical Review B Année : 2016

Effect of dimensionality on sliding charge density waves: The quasi-two-dimensional TbTe 3 system probed by coherent x-ray diffraction

Résumé

We report on sliding Charge Density Wave (CDW) in the quasi two-dimensional TbTe3 system probed by coherent x-ray diffraction combined with in-situ transport measurements. We show that the non-Ohmic conductivity in TbTe3 is made possible thanks to a strong distortion of the CDW. Our diffraction experiment versus current shows first that the CDW remains undeformed below the threshold current IS and then suddenly rotates and reorders by motion above threshold. Contrary to quasi-one dimensional systems, the CDW in TbTe3 does not display any phase shifts below IS and tolerates only slow spatial variations of the phase above. This is a first observation of CDW behavior in the bulk in a quasi-two dimensional system allowing collective transport of charges at room temperature. Interaction between pairs of quasiparticles often leads to broken-symmetry ground states in solids. Typical examples are the formation of Cooper pairs in supercon-ductors, charge-density waves (CDWs) and spin-density waves driven by electron-phonon or electron-electron interactions[1]. The CDW ground state is characterized by a spatial modulation η cos(2k F x + φ) of the electron density and a concomitant periodic lattice distortion with the same 2k F wave vector leading to a gap opening in the electron spectrum. The first CDW systems were discovered in the beginning of the 70's in two-dimensional transition metal dichalcogenides MX 2 [2]. CDW state was then discovered in quasi-one dimensional systems like NbSe 3 , TaS 3 , the blue bronze K 0.3 MoO 3 and in organic compounds like TTF-TNCQ. However, the most remarkable property of a CDW has been discovered a few years later in quasi one-dimensional systems: a CDW may slide carrying correlated charges[3]. The sliding mode is achieved when an electric field applied to the sample is larger than a threshold value, manifesting then collective Fröhlich-type transport. This sliding phenomenon is clearly observed by transport measurements. The differential resistance remains constant up to a threshold current and then decreases for larger currents in addition to the generation of an ac voltage, the frequency of which increases with the applied current[3]. In spite of numerous studies, the physical mechanism leading to the sliding phenomenon is still far to be fully understood. One of the difficulties comes from the fact that the sliding mode displays two different aspects. On the one hand, the CDW is a classical state, similar to an elastic object in presence of disorder[4], displaying creep, memory effects and hysteresis[5, 6]. On the other hand, a CDW is a macroscopic quantum state[7], carrying charges by tunneling through disorder[8] and displaying Aharonov-Bohm effects[9] over microscopic distances[10]. Recently a new class of quasi-two dimensional CDW compounds, rare-earth tritellurides RTe 3 , have raised an intense research activity thanks to their peculiar properties[11-13]. RTe 3 structures are orthorhombic (Cmcm) but the a and c lattice parameters lying in the Te planes are almost equal (c-a=0.002Å002Å with a=4.307Å307Å for TbTe 3 at T=300K) and the double Te-layers are linked together by a c-glide plane. The almost square Te sheets lead to nearly isotropic properties in the (a,c) plane. The resistance measured along a and c differs by only 10% at 300K in TbTe 3 [14] and the Fermi surface displays an almost square-closed shape in the (a*,c*) plane[15]. These quasi-two dimensional systems exhibit a unidirectional CDW wave vector along c* (2k F ∼ 2/7 c* in TbTe 3) and a surprisingly large Peierls transition temperature, around 300 K, through the whole R-series and above for lighter rare-earth elements. The stabilization of the CDW in TbTe 3 over the almost square underlying atomic lattice is reminiscent of copper-oxide planes in high temperature superconductors in which a CDW state was also recently observed[16]. However, the most surprising property of TbTe 3 is its ability to displays non-linear transport[17] despite the two-dimensional character of the atomic structure. The aim of the present work is to show that, despite similar resistivity curves, the depining process in quasi one and two-dimensional systems are quite different. For that purpose, coherent x-ray diffraction has been used to study the behavior of the 2k F satellite reflection upon application of an external current. As the sliding state of a CDW mainly involves fluctuations of the CDW phase to overcome pinning centers , coherent x-ray diffraction is a suitable technique thanks to its high sensitivity to the phase of any modulation. The extreme case of a single phase shift, such

Dates et versions

hal-01980740 , version 1 (21-01-2019)

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D Le Bolloc'H, A. Sinchenko, V. Jacques, L. Ortéga, J.E. Lorenzo, et al.. Effect of dimensionality on sliding charge density waves: The quasi-two-dimensional TbTe 3 system probed by coherent x-ray diffraction. Physical Review B, 2016, 93 (16), pp.165124. ⟨10.1103/PhysRevB.93.165124⟩. ⟨hal-01980740⟩
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