Električna vodljivost, Hallov koeficijent i termoelektrična snaga ikosaedarskih i-Al 62Cu25.5Fe12.5 i i-Al63Cu25Fe12 kvazikristala

The electrical conductivity, Hall coefficient and thermoelectric power of icosahedral i-Al62Cu25.5Fe12.5 quasicrystal samples in the temperature range 2 K - 340 K are measured, and comparison with icosahedral i-Al63Cu25Fe12 quasicrystal samples is made. We have analysed the temperature dependence of the conductivity below 70 K and the results of this analysis are consistent with the predictions of the weak-localisation and the electron-electron interaction theories. The temperature dependence of the electrical conductivity, Hall coefficient and thermoelectric power above 40 K are consistently explained by a two-band model. Although the overlapping of the valence and conduction bands at Fermi level is responsible for the coexistence of both types of carriers, and it enables us to describe quasicrystals as semi-metals, the temperature variation of the electrical conductivity is determined by that of carrier density which makes the situation essentially the same as that in normal semiconductors.Mjerili smo električnu vodljivost, Hallov koeficijent i termoelektričnu snagu uzorka ikosaedarskog kvazikristala i-Al62Cu25.5Fe12.5 u području temperature 2 K – 340 K i usporedili s uzorkom ikosaedarskog kvazikristala i-Al62Cu25.5Fe12.5. Analizirali smo temperaturnu ovisnost električne vodljivosti ispod 70 K i ustanovili da su rezultati u skladu s predviđanjima teorija slabe lokalizacije i međudjelovanja elektronelektron. Ovisnost električne vodljivosti, Hallovog koeficijenta i termoelektrične snage o temperaturi iznad 40 K uspješno se objašnjava modelom dviju vrpci. Iako je predodžba o preklapanju valentne i vodljive vrpce na Fermijevoj razini odgovorna za istovremeno postojanje dviju vrsta nositelja i za opis kvazikristala kao polumetala, temperaturna ovisnost električne vodljivosti je, kao i kod normalnih poluvodiča, određena promjenom gustoće nositelja

Universal sheet resistance and revised phase diagram of the cuprate high-temperature superconductors

Upon introducing charge carriers into the copper-oxygen sheets of the enigmatic lamellar cuprates the ground state evolves from an insulator into a superconductor, and eventually into a seemingly conventional metal (a Fermi liquid). Much has remained elusive about the nature of this evolution and about the peculiar metallic state at intermediate hole-carrier concentrations (p). The planar resistivity of this unconventional metal exhibits a linear temperature dependence (\rho $\propto$ T) that is disrupted upon cooling toward the superconducting state by the opening of a partial gap (the pseudogap) on the Fermi surface. Here we first demonstrate for the quintessential compound HgBa$_2$CuO$_{4+\delta}$ a dramatic switch from linear to purely quadratic (Fermi-liquid-like, \rho $\propto$ T$^2$) resistive behavior in the pseudogap regime. Despite the considerable variation in crystal structures and disorder among different compounds, our result together with prior work gives new insight into the p-T phase diagram and reveals the fundamental resistance per copper-oxygen sheet in both linear (\rho_S = A_{1S} T) and quadratic (\rho_S = A_{2S} T$^2$) regimes, with A_{1S} $\propto$ A_{2S} $\propto$ 1/p. Theoretical models can now be benchmarked against this remarkably simple universal behavior. Deviations from this underlying behavior can be expected to lead to new insights into the non-universal features exhibited by certain compounds

Acoustic and thermal transport properties of hard carbon formed from C_60 fullerene

We report on extended investigation of the thermal transport and acoustical properties on hard carbon samples obtained by pressurization of C60 fullerene. Structural investigations performed by different techniques on the same samples indicate a very inhomogeneous structure at different scales, based on fractal-like amorphous clusters on the micrometer to submillimeter scale, which act as strong acoustic scatterers, and scarce microcrystallites on the nanometer scale. Ultrasonic experiments show a rapid increase in the attenuation with frequency, corresponding to a decrease in the localization length for vibrations. The data give evidence for a crossover from extended phonon excitations to localized fracton excitations. The thermal conductivity is characterized by a monotonous increase versus temperature, power law T1.4, for T ranging from 0.1 to 10 K, without any well-defined plateau, and a strictly linear-in-T variation between 20 and 300 K. The latter has to be related to the linear-in-T decrease of the sound velocity between 4 and 100 K, both linear regimes being characteristic of disordered or generally aperiodic structures, which can be analyzed by the “phonon-fracton hopping” model developed for fractal and amorphous structures

Ground state order and spin-lattice coupling in tetrahedral spin systems Cu2Te2O5X2

High-resolution ac susceptibility and thermal conductivity measurement on Cu2Te2O5X2(X=Br,Cl) single crystals are reported. For Br-sample, sample dependence prevents to distinguish between possibilities of magnetically ordered and spin-singlet ground states. In Cl-sample a three-dimensional transition at 18.5 K is accompanied by almost isotropic behavior of susceptibility and almost switching behavior of thermal conductivity. Thermal conductivity studies suggest the presence of a tremendous spin-lattice coupling characterizing Cl- but not Br-sample. Below the transition Cl-sample is in a complex magnetic state involving AF order but also the elements consistent with the presence of a gap in the excitation spectrum.Comment: version accepted for publication in Phys.Rev.B-Rapid Communicatio

Electronic transport and magnetism in the alternating stack of metallic and highly frustrated magnetic layers in Co1/3_{1/3}NbS2_2

Co$_{1/3}$NbS$_2$ is the only magnetically intercalated layered transition metal dichalcogenide (TMD) suggested to experience the complete suppression of magnetic order under pressure. From elastic neutron scattering we report the direct evidence for the reduction of the antiferromagnetic ordering temperature under pressure, up to complete suppression of magnetic order around 1.7 GPa. The static and ac magnetic susceptibility measurements reveal strong frustration in the magnetic subsystem, and spin canting responsible for the appearance of ferromagnetic (FM) component in dominantly antiferromagnetic (AF) ordered state. The electric transport in directions perpendicular and parallel to layers is explored for the first time in magnetically intercalated TMDs, in the wide temperature and pressure ranges. We show that electric transport reacts differently to magnetic ordering in directions along and perpendicular to layers, with the in-plane conductivity increasing, and the out-of-plane conductivity decreasing in the ordered state. At pressures above 3 GPa, we identify the appearance of the Kondo scattering regime. We use ab-initio calculations to explore the electronic structure in magnetically ordered state, the nature of magnetic interactions, and the mechanism responsible for the changes observed under pressure. The mechanisms of suppression of magnetic order under pressure are scrutinized in the light of these experimental and theoretical findings. We conclude that magnetic couplings beyond nearest-neighbors determine the nature of magnetic ordering. The suppression of ordering under pressure is ascribed to the pressure-induced shift in balance between super-exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic couplings, leading to amplified magnetic frustration.Comment: 34 pages, 17 figure