Marcin Matusiak

Thermoelectric quantum oscillations in ZrSiS

Topological semimetals are systems in which conduction and valence bands cross each other and the crossings are protected by topological constraints. These materials provide intriguing tests for fundamental theories, while their unique physical properties promise a wide range of possible applications in low-power spintronics, optoelectronics, quantum computing and green energy harvesting. Here we report our study of the thermoelectric power of single-crystalline ZrSiS that is believed to be a topological nodal-line semimetal. We show that the thermoelectric power is an extremely sensitive probe of multiple quantum oscillations that are visible in ZrSiS at temperatures as high as 100 K. Two of these oscillations are shown to arise from three- and two-dimensional electronic bands, each with linear dispersion and the additional Berry phase predicted theoretically for materials with non-trivial topology. Our work not only provides further information on ZrSiS but also suggests a different route for studying other topological semimetals.

Doping dependence of the Nernst effect in Eu(Fe1−xCox)2As2: Departure from Dirac-fermion physics

We report a systematic study of the transport properties in the series of Eu(Fe1-xCox)2As2 single crystals with x = 0, 0.15, 0.20 and 0.30. Spin-density-wave order is observed in the undoped and the least doped samples (x = 0, 0.15), while for x = 0.15 and 0.20 Eu(Fe1-xCox)2As2 becomes a superconductor. We found the properties of the parent EuFe2As2 compound well described by the Dirac fermions model, whereas cobalt doping caused an evolution of the system toward a regular metallic state. The antiferromagnetic ordering of the Eu2+ ions at T_N ~ 20 K has only minor influence on the measured quantities.

Anomalous magnetotransport in the heavy-fermion superconductor Ce2PdIn8

The normal state behavior in the heavy-fermion superconductor Ce2PdIn8 has been probed by means of Hall coefficient (RH) and transverse magnetoresistivity (MR) measurements. The results indicate the predominance of contributions from antiferromagnetic spin fluctuations at low temperatures. Anomalous non-Fermi-liquid-like features, observed below 8 K in both RH(T) and MR(T), are related to underlying quantum critical point, evidenced before in the specific heat and the electrical resistivity data. The magnetotransport in Ce2PdIn8 is shown to exhibit specific types of scaling that may appear universal for similar systems at the verge of magnetic instability.

Enhancement of the Hall-Lorenz number in optimally doped YBa2Cu3O7−d

Electronic heat transport in the normal state of a high-quality single crystal of the optimally doped superconductor YBa2Cu3O6.95 was studied by measurements of longitudinal and transverse transport coefficients. For the temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends weakly on temperature and is about two times larger than the Sommerfeld value of the Lorenz number L0=π2/3. Our results can be interpreted using a Fermi liquid model when effects of the pseudogap that opens at the Fermi level are included. However, we find that the bipolaron model can also explain both the enhanced value and the weak temperature dependence of the Hall-Lorenz number.

Thermomagnetic properties of the overdoped La1.7Sr0.3CuO4 superconductor

Abstract We have studied the normal-state transport phenomena of the La 1.7 Sr 0.3 CuO 4 overdoped high- T c superconductor. The Ettingshausen ( P E ), Hall ( R H ) and Seebeck ( S ) coefficients as well as the thermal ( κ ) and electrical ( σ ) conductivities have been measured. The temperature dependence of the isothermal Nernst ( Q i ) coefficient has been estimated on the basis of the Bridgman relation. The results have been interpreted in terms of a rigid, single-band model, with an assumption that both doping and temperature change the band filling resulting in the variation of the Fermi surface curvature. The latter quantity appeared to be a key feature driving the temperature dependences of P E and R H . In opposition to the usual interpretation, the temperature dependence of the R H for La 1.7 Sr 0.3 CuO 4 is dominated by variations of the Fermi surface shape, and not by the changes in the charge carrier concentration. The consistency of this physical picture was controlled by an estimation of P E using measured values of σ , R H , S and κ and comparison with measured value of the Ettingshausen coefficient.

Multiband thermal transport in the iron-based superconductorBa1−xKxFe2As2

We present results of precise measurements of the thermal and electrical transport in the optimally- and over-doped Ba1-xKxFe2As2 single crystals (x = 0.35, 0.55, 0.88) and compare them to the previously reported data on Ba(Fe1-yCoy)2As2. A contraction of the electron pocket is observed upon substitution potassium for barium, but even at the extreme doping (x = 0.88) there is still a noticeable contribution from negative charge carriers to the electronic transport. The size of the electron pocket in all K-doped samples is small enough to cause a significant enhancement of the respective Hall-Lorenz number. Another observed characteristic is the emergence of a maximum in the transverse thermal conductivity below the superconducting critical temperature of the optimally- (x = 0.35) and slightly over-doped (x = 0.55) samples. The evolution of this anomaly from the optimally electron-doped Ba(Fe0.94Co0.06)2As2 to hole-overdoped Ba0.45K0.55Fe2As2 suggests formation of a uniform superconducting gap on the electron pocket in the former and regions of a depressed gap on the hole-pocket in the latter.

Quantum criticality in Ce2PdIn8: A thermoelectric study

We report the Nernst effect (v) and thermoelectric power (S) data for the Ce2PdIn8 heavy-fermion compound. Both S and v behave anomalously at low temperatures: the thermopower shows a Kondo-like maximum at T = 37 K, while the Nernst coefficient becomes greatly enhanced and field dependent below T ~ 30 K. In the zero-T limit S/T and v/T diverge logarithmically, what is related to occurrence of the quantum critical point (QCP). Presented results suggest that the antiferromagnetic spin-density-wave scenario may be applicable to QCP in Ce2PdIn8.

Enhancement of the Hall-Lorenz number in optimally doped YBa{sub 2}Cu{sub 3}O{sub 7-d}.

Electronic heat transport in the normal state of a high-quality single crystal of the optimally doped superconductor YBa2Cu3O6.95 was studied by measurements of longitudinal and transverse transport coefficients. For the temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends weakly on temperature and is about two times larger than the Sommerfeld value of the Lorenz number L0=π2/3. Our results can be interpreted using a Fermi liquid model when effects of the pseudogap that opens at the Fermi level are included. However, we find that the bipolaron model can also explain both the enhanced value and the weak temperature dependence of the Hall-Lorenz number.

Lorenz number in the optimally doped and underdoped superconductor Eu Ba 2 Cu 3 O y

The temperature dependences of the Hall-Lorenz numbers (Lxy) in a EuBa2Cu3Oy (Eu-123) single crystal before and after oxygen reduction are reported. The study is based on data on the normal state longitudinal and transversal transport coefficients. Namely, the temperature dependences of the electrical resistivity, Hall coefficient, longitudinal thermal conductivity, and transverse thermal conductivity are presented. The set of measurements was performed for an optimally doped sample (y = 7), then the oxygen content in the the same crystal was reduced to y = 6.65, and the measurements were repeated. For both cases Lxys are about two times larger than the Sommerfelds value of the Lorenz number and depend weakly on temperature in a range 300 - 160 K. Below T = 160 K the Hall-Lorenz number for the optimally doped sample slowly drops, while the value of Lxy for the oxygen reduced sample begins to rise. Such results suggest that the electronic system in the investigated compound may be considered as a metallic one with pseudo-gaps that open at the Fermi level.

Superconducting and transport properties of TmBa2Cu3O7−δ and comparison to YBa2Cu3O7−δ

Abstract TmBa 2 Cu 3 O 7− δ ceramic samples with oxygen contents between ∼6.7 and 7 have been characterized in terms of their superconducting transition temperatures ( T c ), temperature dependencies of resistivity ( ρ ( T )), thermopowers ( S ( T )) and Hall coefficients ( R H ( T )) and specific heat jumps at T c (Δ C p ). The behaviors of these parameters are compared with those exhibited by YBa 2 Cu 3 O 7− δ samples. In general, the temperature dependencies of ρ , S and R H found for TmBCO are similar to those for YBCO. However, some clear quantitative differences between the compounds have been observed for the oxygen content dependencies of T c , S and Δ C p / T c , especially at low oxygen contents. TmBCO samples with oxygen contents 7− δ =6.7–6.8 show significantly lower T c and Δ C p / T c , and higher S compared toYBCO, although the carrier concentration n H at 7− δ ≈6.7 is similar for both compounds (4–5×10 21 cm −3 ). The observed difference in the values of T c , Δ C p / T c and S seem to be caused by the large mass and magnetic moment of Tm. Thermopower, S , and carrier concentration, n H , characterize the electronic structure of the compounds. The dependence of S on 1/ n H for TmBCO and YBCO is similar and nearly linear, in agreement with the Mott relation.