In-plane Hall Coefficient Research Articles

Modelling in-plane magneto-transport in Cr2AlC

The ternary MAX phase Cr2AlC exhibits a noticeable variation of in-plane Hall coefficient with temperature, T. In previous work, this was assumed to be due to temperature dependent electron and hole concentrations, an unlikely conjecture for a highly metallic-like conducting ceramics like Cr2AlC. Herein we show that a realistic analytic approximation of the Fermi surface as measured by Angle Resolved Photo-Emission Spectroscopy or as computed by Density Functional Theory allows us to recover the main features of the in-plane magneto-transport (temperature variations of Hall coefficient, resistivity and magnetoresistance). The temperature variation is explained by the combined contribution of electron and hole bands, taking into account the local curvature of the Fermi surface and an appropriate and physically sound temperature variation of the relaxation time in each band. The model also allows us to retrieve the apparent (and erroneous) carrier densities as extracted when using conventional methods. We suggest that our explanation could be generalized to other MAX phases, for which more “involved” explanations have been advanced so far.

High pressure growth and electron transport properties of superconducting SmFeAsO1−xHx single crystals

We report the single crystal growth and characterization of the highest Tc iron-based superconductor SmFeAsO1−xHx. Some sub-millimeter-sized crystals were grown using the mixture flux of Na3As + 3NaH + As at 3.0 GPa and 1473 K. The chemical composition analyses confirmed 10% substitution of hydrogen for the oxygen site (x = 0.10), however, the structural analyses suggested that the obtained crystal forms a multi-domain structure. By using the FIB technique we fabricated the single domain SmFeAsO0.9H0.10 crystal with the Tc of 42 K, and revealed the metallic conduction in in-plane (ρab), while semiconducting in the out-of-plane (ρc). From the in-plane Hall coefficient measurements, we confirmed that the dominant carrier of SmFeAsO0.9H0.10 crystal is an electron, and the hydride ion occupied at the site of the oxygen ion effectively supplies a carrier electron per iron following the equation: O2− → H− + e−.

(Li0.84Fe0.16)OHFe0.98Sesuperconductor: Ion-exchange synthesis of large single-crystal and highly two-dimensional electron properties

A large and high-quality single crystal (Li0.84Fe0.16)OHFe0.98Se, the optimal superconductor of newly reported (Li1-xFex)OHFe1-ySe system, has been successfully synthesized via a hydrothermal ion-exchange technique. The superconducting transition temperature (Tc) of 42 K is determined by magnetic susceptibility and electric resistivity measurements, and the zero-temperature upper critical magnetic fields are evaluated as 79 and 313 Tesla for the field along the c-axis and the ab-plane, respectively. The ratio of out-of-plane to in-plane electric resistivity,\r{ho}c/\r{ho}ab, is found to increases with decreasing temperature and to reach a high value of 2500 at 50 K, with an evident kink occurring at a characteristic temperature T*=120 K. The negative in-plane Hall coefficient indicates that electron carriers dominate in the charge transport, and the hole contribution is significantly reduced as the temperature is lowered to approach T*. From T* down to Tc, we observe the linear temperature dependences of the in-plane electric resistivity and the magnetic susceptibility for the FeSe layers. Our findings thus reveal that the normal state of (Li0.84Fe0.16)OHFe0.98Se becomes highly two-dimensional and anomalous prior to the superconducting transition, providing a new insight into the mechanism of high-Tc superconductivity.

Magnetotransport properties of nearly-free electrons in two-dimensional hexagonal metals and application to theMn+1AXnphases

We propose a general, yet simple model for describing the weak field magnetotransport properties of nearly-free electrons in two-dimensional hexagonal metals. We modify this model so as to apply it to the magnetotransport properties of the ${M}_{n+1}A{X}_{n}$ phases, a particular class of nanolamellar carbides and nitrides. We argue that the values of the in-plane Hall coefficient and the in-plane parabolic magnetoresistance are due to the specific shape of the Fermi surface of almost two-dimensional hole and electron bands. If the contribution of the electron pockets to in-plane resistivity is often (but not always) predicted to be a minor one, in contrast, both holes and electrons should substantially contribute to the overall value of the in-plane Hall coefficient. The relevance of our model is supported by elementary considerations and a set of experimental data obtained from single crystals of ${\mathrm{V}}_{2}\mathrm{AlC}$ and $\mathrm{C}{\mathrm{r}}_{2}\mathrm{AlC}$. In particular, we obtain a high ratio between the in-plane $({\ensuremath{\rho}}_{ab})$ and parallel to the $c$ axis $({\ensuremath{\rho}}_{c})$ resistivities.

Theoretical investigation of the electronic structure and transport properties ofNa1+xCo2O4

The validity of a simple band description for explaining the unexpectedly large thermopower inNa1+xCo2O4 was investigated. Non-spin-polarized and spin-polarized electronic structures of layered cobalt oxidesNa1+xCo2O4 are studied by means of first-principles density functional theory calculations.The fractional occupancy of Na is described via two approaches—the virtualcrystal approximation and the rigid band model. We also found that theband structure of the valence band is very sensitive to the geometry of theCoO6 ‘octahedron’, and cannot be described by a simple crystal field model. The Seebeckcoefficients are calculated from the standard kinetic theory. The calculated thermopowerdemonstrates the importance of a high density of states close to the Fermi level.The calculated Seebeck coefficient based on the paramagnetic band structure isapparently in good agreement with experimental observation although it hasbeen suggested that spin entropy may dominate the thermopower. However, thecalculation failed to account for the observed negative in-plane Hall coefficient.

Anomalousf-electron Hall effect in the heavy-fermion systemCeTIn5(T=Co,Ir, orRh)

The in-plane Hall coefficient $R_{H}(T)$ of CeRhIn$_{5}$, CeIrIn$_{5}$, and CeCoIn$_{5}$ and their respective non-magnetic lanthanum analogs are reported in fields to 90 kOe and at temperatures from 2 K to 325 K. $R_{H}(T)$ is negative, field-independent, and dominated by skew-scattering above $\sim$ 50 K in the Ce compounds. $R_{H}(H \to 0)$ becomes increasingly negative below 50 K and varies with temperature in a manner that is inconsistent with skew scattering. Field-dependent measurements show that the low-T anomaly is strongly suppressed when the applied field is increased to 90 kOe. Measurements on LaRhIn$_{5}$, LaIrIn$_{5}$, and LaCoIn$_{5}$ indicate that the same anomalous temperature dependence is present in the Hall coefficient of these non-magnetic analogs, albeit with a reduced amplitude and no field dependence. Hall angle ($\theta_{H}$) measurements find that the ratio $\rho_{xx}/\rho_{xy}=\cot(\theta_{H})$ varies as $T^{2}$ below 20 K for all three Ce-115 compounds. The Hall angle of the La-115 compounds follow this T-dependence as well. These data suggest that the electronic-structure contribution dominates the Hall effect in the 115 compounds, with $f$-electron and Kondo interactions acting to magnify the influence of the underlying complex band structure. This is in stark contrast to the situation in most $4f$ and $5f$ heavy-fermion compounds where the normal carrier contribution to the Hall effect provides only a small, T-independent background to $R_{H}.$

Transport properties of the single-crystal La 2− xBa xCuO 4 (0≤ x≤0.11)

We report on the temperature dependence of the in-plane electrical resistivity ρ ab and the in-plane Hall coefficient R H in various magnetic fields of the single-crystal La 2− x Ba x CuO 4 with x=0.083 and 0.11. In x=0.11, which is close to x=1/8, where the superconductivity is strongly suppressed, a clear jump in ρ ab and a drop in R H have been observed at T d2∼53 K, where the structural phase transition between the orthorhombic mid-temperature and tetragonal low-temperature phases occurs. Moreover, a sign reversal of R H has been observed below ∼25 K and the magnitude of the sign reversal increases with increasing magnetic field. In x=0.083, on the other hand, there is neither jump in ρ ab nor drop in R H at T d2, and also no sign reversal in R H at low temperatures even in magnetic fields up to 9 T. In conclusion, there is no doubt that a static stripe order of holes and spins, observed in La 1.6− x Nd 0.4Sr x CuO 4 with x∼1/8, is formed below T d2 also in La 2− x Ba x CuO 4 with x∼1/8. The R H in the stripe-ordered state has a negative value, which is consistent with the recent theory by Prelovšek et al.

Energy bands and Fermi surface ofSr2RuO4

A method combining the extended H\"uckel theory and the tight-binding approximation is proposed to study the electronic properties of the layered noncuprate superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ The band structure close to the Fermi level is obtained by integrating out all low-lying oxygen degrees of freedom allowing the determination of the energy spectra, the Fermi surface, and the total density of states. The results reproduces the main features of local-density approximation calculations near the Fermi energy. As an application of the approach here proposed, we have calculated the temperature dependence of the in-plane Hall coefficient considering the effect of the curvature of the Fermi surface.

Hall effect measurements on YBa 2Cu 4O 8 single crystals

The Hall effect of YBa 2Cu 4O 8 single crystals has been measured in the temperature range 4.2–300 K. Although the crystallographic structure of YBa 2Cu 4O 8 is orthorhombic in the normal state, the in-plane Hall coefficient is isotropic. The Hall coefficient decreases with increasing temperature, as is typical for high-temperature superconductors. The Hall resistivity becomes anisotropic in the Shubnikov phase and there a sign change for I parallel to the a- axis takes place, whereas no sign change appears for I parallel to the b- axis .

Transport properties of single-crystal HgBa 2Ca 2Cu 3O 8+δ

We report the first measurements of the transport properties of single crystals of HgBa 2Ca 2Cu 3O 8+τ. Data for the in-plane and out-of-plane resistivity, in-plane Hall coefficient and in-plane thermoelectric power of single crystals with onset temperatures as high as 134 K are presented. The in-plane resistivity, Hall coefficient and thermoelectric power all indicate that these crystals are underdoped, which may offer at least a partial explanation for the strong pressure dependence of T c observed in this compound. The in-plane Hall coefficient shows a strong temperature dependence in accord with other cuprate superconductors. A calculation of the cotangent of the Hall angle (cotθ H) reveals that the relation cot θ H = AT 2 + B is obeyed to high precision in these crystals, despite strong curvature of the in-plane resistivity. Data are also reported for the influence of magnetic field on the resistive transition, for fields of up to 7 T applied both parallel and perpendicular to the crystallographic c-axis. These latter measurements give an estimate for the anisotropy of H c2, implying a value which is intermediate between that found for YBa 2Cu 3O 7−τ and Bi 2Sr 2CaCu 2O 8+ x .

Scaling of the temperature dependent Hall effect in La2-xSrxCuO4.

A new aspect of the charge dynamics in layered metallic cuprates has been discovered in the study of the in-plane Hall coefficient (${\mathit{R}}_{\mathit{H}}$) in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{\mathit{x}}$ over a wide temeprature (4--500 K) and composition (0x0.35) range. The strongly temperature dependent ${\mathit{R}}_{\mathit{H}}$, a hallmark of cuprate superconductors, is found to be the low temperature part of a simple functional form f(T/${\mathit{T}}^{\mathrm{*}}$) which becomes constant for T>${\mathit{T}}^{\mathrm{*}}$. This form, followed for x\ensuremath{\ge}0.15, has a characteristic temperature ${\mathit{T}}^{\mathrm{*}}$ which is high at the optimal composition (x=0.15) and decreases with increasing Sr concentration, similar to a temperature scale deduced from magnetic susceptibility.

Transport Study on Overdoped TlBa2CUO6+δ Single Crystal

ABSTRACTTl2Ba2Cuo6+δ(T1-2201) single crystals are prepared by a KCI flux method. In-plane and out-of plane resistivities (ρabρc as well as the in-plane Hall coefficient are measured for the single crystal samples with various 7c's. The ρc values are as large as ∼10−1 Ω cm, which is larger than that of YBa2Cu3O7, but smaller than that of Bi2Sr2CaCu2O8. Temperature dependences of ρc show metallic behaviors for both normal metallic and 75-K superconducting samples. Temperature dependence of in-plane resistivity par changes from ∼T2 to ∼T1 with increasing Tc In-plane Hall coefficient RH shows a characteristic maximum at about 100 K for all samples. Although both ρab and RH have rather a complicated temperature dependence, inverse Hall mobility μH(=ρab/RH always exhibits clear T2 dependence for all samples. These results suggest that the intrinsic scattering rate of this material varies as ∼T2, just like in an ordinary Fermi liquid, and that the carrier concentration actually changes with temperature like that observed in nH(=1/RHe).

Synthesis and physical properties of Tl 2Ba 2CuO 6+δ single crystal

Tl 2 Ba 2 CuO 6+δ single crystals were prepared using a KCl flux method. Obtained crystals were thin plates grown in the a–b plane with a typical size of 2 × 2 × 0.01 mm 3 . The T c value was controlled from 0 K (nonsuperconducting normal metal) to 75 K by annealing in oxygen or argon atmosphere. The temperature dependence of the in-plane resistivity, ρ ab , changed from T 2 to T 1 with increasing T c . The in-plane Hall coefficient R H was about +1 × 10 −3 cm 3 /C and exhibited a broad maximum at about 100 K. These results are very similar to those for ceramic samples. The resistivity along the c -axis, ρ c , showed metallic temperature dependence for all T c 's. The ratio of anisotropic resistivity (ρ c /ρ ab ) measured by the Montgomery method was as large as 6 × 10 2 at room temperature even for a normal metallic sample.

Conductivity and Hall coefficient in La2CuO4+y near the insulator-metal transition.

Measurements are reported of the conductivity, its anisotropy, and the in-plane Hall coefficient for a single crystal of ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4+y}$ with $y$ varied from 0%-1.5%. The conductivity is thermally activated with a single activation energy at high temperatures and with a continuously decreasing activation energy below \ensuremath{\sim}50 K. The anisotropy is as large as 500 at high temperatures, falls sharply to 10-20 at \ensuremath{\sim}50 K, and then is only weakly $T$ dependent at lower temperatures. The Hall coefficient is simply activated above \ensuremath{\sim}50 K (with an activation energy similar to that of the in-plane conductivity), and has a peak at lower temperatures. All these features are evidence for a change in the transport mechanism near 50 K. The low-temperature conductivity is consistent with thermally assisted tunneling between localized states at the Fermi energy. The high-temperature process arises from thermal activation of carriers into a band of highly anisotropic states. The low mobility associated with these states makes it impossible, however, to determine whether they arise from impurities or are states near a band or mobility edge.