Bi2Te_xSe_y series studied by resistivity and thermopower

We study the detailed temperature and composition dependence of the resistivity, $\rho(T)$, and thermopower, $S(T)$, for a series of layered bismuth chalcogenides Bi$_2$Te$_{3-x}$Se$_x$, and report the stoichiometry dependence of the optical band gap. In the resistivity of the most compensated member, Bi$_2$Te$_{2.1}$Se$_{0.9}$, we find a low-temperature plateau whose onset temperature correlates with the high-temperature activation energy. For the whole series $S(T)$ can be described by a simple model for an extrinsic semiconductor. By substituting Se for Te, the Fermi level is tuned from the valence band into the conduction band. The maximum values of $S(T)$, bulk band gap as well the activation energy in the resistivity are found for $x \approx 0.9$

Manifestation of the spin textures in the thermopower of MnSi

To identify possible spin texture contributions to thermoelectric transport, we present a detailed temperature and pressure dependence of thermopower $S$ in MnSi, as well as a low-temperature study of $S$ in a magnetic field. We find that $S/T$ reconstructs the $(p,T)$ phase diagram of MnSi encompassing the Fermi liquid, partially ordered, and non-Fermi liquid phases. Our results indicate that the latter two phases have essentially the same nature. In the partially ordered phase, $S(T)$ is strongly enhanced, which may be understood as a spiral-fluctuation-driven phase. A low temperature upturn in $S/T$ pertaining to the partial order phase persists up to the highest pressure, 24 kbar. Contrarily, a small suppression of $S(T)$ is observed in the ordered skyrmion lattice $A$ phase

Optical properties of Bi2Te2Se at ambient and high pressure

The temperature dependence of the complex optical properties of the three-dimensional topological insulator Bi2Te2Se is reported for light polarized in the a-b planes at ambient pressure, as well as the effects of pressure at room temperature. This material displays a semiconducting character with a bulk optical gap of 300 meV at 295 K. In addition to the two expected infrared-active vibrations observed in the planes, there is additional fine structure that is attributed to either the removal of degeneracy or the activation of Raman modes due to disorder. A strong impurity band located at 200 cm^{-1} is also observed. At and just above the optical gap, several interband absorptions are found to show a strong temperature and pressure dependence. As the temperature is lowered these features increase in strength and harden. The application of pressure leads to a very abrupt closing of the gap above 8 GPa, and strongly modifies the interband absorptions in the mid-infrared spectral range. While ab initio calculations fail to predict the collapse of the gap, they do successfully describe the size of the band gap at ambient pressure, and the magnitude and shape of the optical conductivity.Comment: 8 pages, 7 figure

Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone's lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological/trivial nature of a possible gap opening. Here we report on a combined optical and transport study of ZrTe5, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the a-c plane, while remaining relatively flat and parabolic in the third direction (along the b axis). Therefore, the electronic states in ZrTe5 cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment

Competing orders in strongly correlated systems studied by transport measurements

Competing orders in strongly correlated systems lead to rich phase diagrams comprising many electronic phases, such as superconductivity, charge/spin density wave, charge order, or bad metallicity. These phases are generically sensitive to a variety of parameters, for example temperature, magnetic field, dimensionality, presence of disorder, geometrical frustration. In this thesis, we employ electronic transport measurements under high pressure on few model compounds to gain insight into the complex physics of strongly correlated compounds. The transport coefficients, resistivity and thermoelectric power, shed light onto conduction processes and the thermodynamics. The pressure is a perfect tool to investigate competition of different ground states: by modifying the lattice parameters, it can tune the interactions without introducing disorder. Several representative compounds were chosen for this study. In the first part, we focus on the transport properties of the quasi-one dimensional BaVS3. The main characteristic of this 3d1 system is the coexistence of a broad one-dimensional dz2 electronic band and a narrow isotropic eg band at the Fermi level. The suppression of the insulating phase by high pressure leads to a non-Fermi liquid phase. We showed that magnetic field does not recover the Fermi liquid behavior, and that the disorder pushes the system further into non-Fermi liquid state. This is at variance with what has been observed in other non-Fermi liquid compounds, and confirms the novelty of the mechanism for non-Fermi liquid behavior in BaVS3. To achieve better understanding of the role of the localized electrons, we investigated systematically the influence of disorder. In addition, we studied the properties of the BaVSe3, which due to the reinforced interchain interactions may be considered as the high-pressure counterpart of BaVS3. The system is a metallic ferromagnet, in which the strong interaction of dz2 and eg electrons dictates the behavior of transport coefficients. In the following part we studied the rich physics of quasi-one dimensional β-vanadium bronzes. In the stoichiometric β-SrV6O15, we followed the pressure dependence of the semiconductor-insulator transition by resistivity and thermopower. We found evidence suggesting that the ground state is charge ordered. Under pressure, the changing character of the transport coefficients implied a competition of different ground states. Moreover, we observed resistive switching in the insulating phase. When strontium doping is decreased, in SrxV6O15 and x < 1, the disorder starts governing the physics of the system. The off-stoichiometric compounds are characterized by the absence of phase transition, absence of resistive switching, and possibly by the presence of polarons. We also found resistive switching in another charge ordered transition-metal oxide, Fe2OBO3. This system shows an interplay of commensurate and incommensurate charge order. The switching is restricted to the incommensurate phase, whose origin probably lies in the geometrical frustration of the interactions between iron atoms. With pressure we enhance the Coulomb repulsion, and the incommensurate phase shrinks in temperature. In the final part, we address the high-pressure transport of a superconductor on a geometrically frustrated pyrochlore lattice, ΚOs2O6. The potassium atoms are enclosed in oversized cages and their rattling motion introduces a localized low-energy mode. The transport coefficients in this compound are highly anomalous: the resistivity shows no saturation at low temperatures, and the scythe-shaped thermoelectric power is reminiscent of the one observed in cuprates. We were able to reproduce the temperature and pressure dependence of the transport coefficients within a simple model of the density of states

Non-uniform carrier density in Cd3_3As2_2 evidenced by optical spectroscopy

We report the detailed optical properties of Cd$_3$As$_2$ crystals in a wide parameter space: temperature, magnetic field, carrier concentration and crystal orientation. We investigate high-quality crystals synthesized by three different techniques. In all the studied samples, independently of how they were prepared and how they were treated before the optical experiments, our data indicate conspicuous fluctuations in the carrier density (up to 30%). These charge puddles have a characteristic scale of 100 $\mu$m, they become more pronounced at low temperatures, and possibly, they become enhanced by the presence of crystal twinning. The Drude response is characterized by very small scattering rates ($\sim 1$ meV) for as-grown samples. Mechanical treatment, such as cutting or polishing, influences the optical properties of single crystals, by increasing the Drude scattering rate and also modifying the high frequency optical response. Magneto-reflectivity and Kerr rotation are consistent with electron-like charge carriers and a spatially non-uniform carrier density.Comment: Accepted in Physical Review

Micro-mechanical response and power-law exponents from the longitudinal fluctuations of F-actin solutions

We investigate the local fluctuations of filamentous actin (F-actin), with focus on the skeletal thin filament, using single-particle optical trapping interferometry. This experimental technique allows us to detect the Brownian motion of a tracer bead immersed in a complex fluid with nanometric resolution at the microsecond time-scale. The mean square displacement, loss modulus, and velocity autocorrelation function (VAF) of the trapped microprobes in the fluid follow power-law behaviors, whose exponents can be determined in the short-time/high-frequency regime along several decades. We obtain 7/8 subdiffusive power-law exponents for polystyrene depleted microtracers at low optical trapping forces. Microrheologically, the elastic modulus of these suspensions is observed to be constant up to the limit of high frequencies, confirming the origin of this subdiffusive exponent on the local longitudinal fluctuations of the polymers. Deviations from this value are measured and discussed in relation to the characteristic lengths scales of these F-actin networks and probes' properties, and also in connection with the different power-law exponents detected in the VAFs. Finally, we observe that the thin filament, composed by tropomyosin (Tm) and troponin (Tn) coupled to F-actin in the presence of Ca$^{2+}$, returns exponent values less dispersed than F-actin alone, which we interpret as a micro-measurement of the filament stabilization.Comment: Published in Soft Matter in 202

Optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45

The complex optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45 with Tc=14K have been examined over a wide frequency range for light polarized in the Fe-Te(Se) planes above and below Tc. At room temperature the optical response may be described by a weakly-interacting Fermi liquid; however, just above Tc this picture breaks down and the scattering rate takes on a linear frequency dependence. Below Tc there is evidence for two gap features in the optical conductivity at Delta_1 ~ 2.5 meV and Delta_2 ~ 5.1 meV. Less than 20% of the free carriers collapse into the condensate for T << Tc, and this material is observed to fall on the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit.Comment: 6 pages, 6 figures; accepted J. Phys. Chem. Solids, proceedings of SNS2010 in Shanghai (updated references

Optical properties of AFe2As2 (A=Ca, Sr, and Ba) single crystals

The detailed optical properties have been determined for the iron-based materials AFe2As2, where A=Ca, Sr, and Ba, for light polarized in the iron-arsenic (a-b) planes over a wide frequency range, above and below the magnetic and structural transitions at TN=138, 195, and 172 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above TN, the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below TN there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for T≪TN may be related to the Dirac conelike dispersion of the electronic bands. Below TN new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at Δ1≃45–80 meV, and Δ2≃110–210 meV. The reduction in the spectral weight associated with the free carriers is captured by the gap structure; specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap