F. Lévy

Specific heat of 2H-NbSe2 in high magnetic fields

Recently, the unusual behaviour of the specific heat of the superconductors YBa2Cu3O7, Y2Ba4Cu7O15, and YBa2Cu4O8 at high field and low temperature was attributed to Zeeman splitting of localized states in the vortex cores [D. Sanchez et al., Physica C 200 (1992) 1]. In an attempt to detect similar effects in classic high-κ superconductors, we have measured the mixed-state specific heat of NbSe2 single crystals in magnetic fields up to Hc2(0). The specific heat in the superconducting state and zero field is found to follow accurately a strong-coupling model that takes into account the anisotropy of the gap. We analyse then the mixed-state specific heat using the London model. A small Schottky-like excess specific heat remains unexplained. In qualitative agreement with the observations made in Yue5f8Baue5f8Cuue5f8O, this contribution is suggested to originate from the localized vortex core levels observed by STM techniques.

On the search for Fermi surface nesting in quasi-2D materials

Abstract We investigate a number of isostructural, quasi-two dimensional transition metal dichalcogenides with respect to Fermi surface nesting. Using angle-resolved photoemission we find no clear evidence for Fermi surface nesting as a key scenario for charge density wave formation in these materials. However, interesting and unusual behavior has been discovered. For 1T -TaS 2 in the charge density wave phase, instead of finding an intact Fermi surface along the rounded parts of its elliptically shaped contours and gaps along the parallel regions, the Fermi surface is essentially completely pseudogapped. For 1T -TiSe 2 a model based on an excitonic insulator phase as described by Kohn [Phys. Rev. Lett., 19 (1967) 439] fits well with experimental observations. Bandstructure calculations give interesting insight into the electron configuration indicating the changes in d-band occupation and metal-d chalcogen-p overlap when going from one material to the other.

Fractional power law susceptibility and specific heat in low-temperature insulating state of o-TaS3

Measurements of the magnetic susceptibility and its anisotropy in the quasi–one-dimensional system o-TaS3 in its low-T charge-density-wave (CDW) ground state are reported. Both sets of data reveal below 40 K an extra paramagnetic contribution obeying a power law temperature dependence χ(T) = AT−0.7. The fact that the extra term measured previously in specific heat in zero field, ascribed to low-energy CDW excitations, also follows a power law CLEE(0,T) = CT0.3, strongly revives the case of random exchange spin chains. Introduced impurities (0.5% Nb) only increase the amplitude C, but do not change essentially the exponent. Within the two-level system (TLS) model, we estimate from the amplitudes A and C that there is one TLS with a spin s = 1/2 localized on the chain at the lattice site on average every 900 Ta atoms. We discuss the possibility that it is the charge frozen within a soliton-network below the glass transition Tg ~ 40 K determined recently in this system.

Glass transition from a slush to a real glass in charge-density-wave compound TaS3

Abstract We have found by broad-band dielectric spectroscopy of charge density wave (CDW) compound TaS 3 that the main relaxation time representing CDW viscosity shows a critical slowing down in the range 55-50 K. At lower temperatures two additional processes appear that we assign to the defects in CDW superlattice. Based on very strong similarity with the glass transition in supercooled liquids, we propose a consistent scenario of the glass transition from a CDW slush to a real CDW glass. The length/time scales involved imply the existence of a new family of glasses in systems with the long-range order such as CDW.

NEW ASPECTS OF THE QC-PHASE IN LAYERED 1T-TAS2

Abstract We present Fermi surface (FS) mapping measurements obtained by photoemission for the quasi-commensurate charge density wave (CDW) phase of layered 1 T -TaS 2 . The topology of the FS is rather dominated by the undistorted (1×1) symmetry than by the CDW-induced ( 13 × 13 ) phase. CDW effects can be seen in terms of nesting properties of the FS. Compared to one-electron band structure calculations we observe changed FS contours of Ta 5 d -derived electron pockets around the M , M ′ points of the (1×1) Brillouin zone. Low energy electron diffraction measurements support the view of a quasi-commensurate CDW.

Thermal properties of (NbSe4)3I

Abstract We report measurements of thermal conductivity and thermal capacity of halogenated niobium tetraselenide (NbSe 4 ) 3 I. The structural phase transition at 274K is clearly marked in both measurements. Thermal conductivity shows significant differences between heating and cooling cycles which begin around 200K.

Ultrasonic properties of one-dimensional (TaSe4)2I

Abstract We report measurements on the quasi-one-dimensional (TaSe) 2 I which undergoes a Peierls transition at T p = 262 K. Softening of the shear elastic constant C 44 has been observed below 40 K. The effective Gruneisen parameter deduced from the sound velocity increases up to 20 when temperature decreases down to 4 K. Such a large increase of Γ eff has been related to the anomalous dispersion curve of the C 44 acoustic branch of this system.

The origin of the sharpness of the maximum in the phonon thermal conductivity of (Ta1−xNbxSe4)2I at low temperature

Abstract Thermal conductivity (κ) studies are reported on alloys of the linear-chain compound (TaSe4)2I either pure or doped with Nb, in the low-T regime. κ(T) varies like T3 at very low temperatures, in agreement with the boundary scattering regime. At higher T>, there is a deviation from T3, towards Tn with n≲2, for doped samples, which can be explained by a scattering process related to impurities. A sharp anomaly appears in the case of pure and doped (1 at % Nb) alloys around 1 and 1.8 K, respectively, which can be related to very unusual phonon properties of these materials and suggests a possible explanation by the Poiseuille effect.

THERMOELECTRIC TRANSPORT AND STRUCTURAL-CHANGES IN CHARGE-DENSITY-WAVE SYSTEM (NBSE4)(10)I-3

Abstract In order to get better insight into the electrical and thermal transport of (NbSe 4 ) 10 I 3 we performed a detailed measurements of the electrical conductivity and thermopower from 350 K down to 5 K as well as structural investigation in the vicinity of the Peierls transition on the samples selected from the same batch. The observed structural changes are not enough to explain the anomalous behaviour of the transport properties. It seems that the origin of observed anomalies one should search in the interplay of the distortion of the underlying lattice and the distortion of charge-density-wave lattice.

Investigation of the thermal hysteresis in the thermopower of (NbSe4)10I3

Abstract We report the resistivity and the thermopower (TEP) measurements of linear chain compound (NbSe 4 ) 10 I 3 which show substantial hysteresis between the cooling and the heating cycles below and above the Peierls transition. The magnitude of the observed TEP hysteresis increases if the system is cooled to lower temperatures. Cooling below 70K has no further influence on the magnitude of the hysteresis loop. An indication of the time dependence of the TEP is also found. These effects are atributed to the interactions between the inhomogenous charge density (CDW) superlattice and a quasi-periodic defect structure which has been found in (NbSe 4 ) 10 I 3 .