Y. M. Xiong

Effect of chemical doping on the thermoelectric properties of FeGa3

Thermoelectric properties of the chemically-doped intermetallic narrow-band semiconductor FeGa3 are reported. The parent compound shows semiconductor-like behavior with a small bandgap (Eg = 0.2 eV), a carrier density of ∼1018 cm−3, and a large n-type Seebeck coefficient (S ∼ − 400 μV/K) at room temperature. Hall effect measurements indicate that chemical doping significantly increases the carrier density, resulting in a metallic state, while the Seebeck coefficient still remains fairly large (∼− 150 μV/K). The largest power factor (S2/ρ = 62 μW/m K2) was observed for Fe0.99Co0.01(Ga0.997Ge0.003)3, and its corresponding figure of merit (ZT = 1.3 × 10−2) at 390 K improved by over a factor of 5 from the pure material.

Structure and physical properties of the noncentrosymmetric superconductor Mo 3 Al 2 C

We have synthesized polycrystalline samples of the noncentrosymmetric superconductor Mo3Al2C by arc and RF melting, measured its transport, magnetic and thermodynamic properties, and computed its band structure. Experimental results indicate a bulk superconducting transition at Tc9.2 K while the density of states at the Fermi surface is found to be dominated by Mo d orbitals. Using the measured values for the lower critical field Hc1, upper critical field Hc2, and the specific heat C, we estimated the thermodynamic critical field Hc0, coherence length 0, penetration depth 0, and the Ginzburg-Landau parameter 0. The specificheat jump at Tc, C /Tc=2.14, suggests that Mo3Al2C is moderately to strongly coupled, consistent with the fast opening of the gap, as evidenced by the rapid release of entropy below Tc from our electronic specific-heat measurements. Above 2 K the electronic specific heat exhibits the power-law behavior, suggesting that synthesis of single crystals and measurements at lower temperature are needed to establish whether the gap is anisotropic. The estimated value of the upper critical field Hc20 is close to the calculated Pauli limit, therefore further studies are needed to determine whether the absence of an inversion center results in a significant admixture of the triplet component of the order parameter.

Structure-property coupling in Sr3(Ru1−xMnx)2O7

Layered ruthenates are prototype materials with strong structure-property correlations. We report the structural and physical properties of double-layered perovskite Sr3(Ru1−xMnx)2O7 single crystals with 0≤x≤0.7. Single crystal x-ray diffraction refinements reveal that Mn doping on the Ru site leads to the shrinkage of unit-cell volume and disappearance of (Ru/Mn)O6 octahedron rotation when x>0.16, while the crystal structure remains tetragonal. Correspondingly, the electronic and magnetic properties change with x. The electrical resistivity reveals metallic character (dρ/dT>0) at high temperatures but insulating behavior (dρ/dT<0) below a characteristic temperature TMIT. Interestingly, TMIT is different from TM, at which magnetic susceptibility reaches maximum. TMIT monotonically increases with increasing x while TM shows non-monotonic dependence with x. The difference between TMIT and TM (TMIT>TM) becomes larger when x>0.16. The constructed phase diagram consists of five distinct regions, demonstrating that the physical properties of such a system can easily be tuned by chemical doping.

Saturation of the Anomalous Hall Effect in Critically Disordered Ultrathin CNi3 Films

We demonstrate that a distinct high-disorder anomalous Hall effect phase emerges at the correlated insulator threshold of ultrathin, amorphous, ferromagnetic CNi3 films. In the weak-localization regime, where the sheet conductance G>>e{2}/h, the anomalous Hall resistance of the films increases with increasing disorder and the Hall conductance scales as G{xy} proportional to G{phi} with phi=1.6. However, at sufficiently high disorder the system begins to enter the 2D correlated insulator regime, at which point the Hall resistance R{xy} abruptly saturates and the scaling exponent becomes phi=2. Tunneling measurements show that the saturation behavior is commensurate with the emergence of the 2D Coulomb gap, suggesting that e-e interactions mediate the high-disorder phase.

Spin-resolved tunneling studies of the exchange field in EuS/Al bilayers.

We use spin-resolved electron tunneling to study the exchange field in the Al component of EuS/Al bilayers, in both the superconducting and normal-state phases of the Al. Contrary to expectation, we show that the exchange field H(ex) is a nonlinear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore, the magnitude H(ex) is unaffected by the superconducting phase. In addition, H(ex) decreases significantly with increasing temperature in the temperature range of 0.1-1 K. We discuss these results in the context of recent theories of generalized spin-dependent boundary conditions at a superconductor-ferromagnet interface.

Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe

We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic-structure calculations. In MnGe, we observe a transition to a magnetic state at Tc = 275 K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature, one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperatureofthistransitionwhichcorrespondscloselytothesharppeakobservedintheacsusceptibilityatfields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal-structure parameters. Transport measurements indicate that MnGe is metallic with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit, also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature-independent diamagnetic susceptibility.

Temperature dependence of resistivity and Hall coefficient in strongly disordered NbN thin films

We report the temperature dependence of resistivity

Intrinsic electron glassiness in strongly localized Be films

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Origin of excess low-energy states in a disordered superconductor in a Zeeman field.

and Hall coefficient

Thermoelectric properties of intermetallic semiconducting RuIn3 and metallic IrIn3

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