Go Nakamoto

Low-temperature thermoelectric properties of α- and β-Zn4Sb3 bulk crystals prepared by a gradient freeze method and a spark plasma sintering method

Abstract The low temperature thermoelectric properties of Zn 4 Sb 3 samples prepared by the gradient freeze (GF) method and sintering have been characterized. With decreasing temperature a dramatic rise in the thermal expansion is observed associated with the structural transition from β- to α-phase; Δ l / l =2.8×10 −4 at T s GF =257.4 K for GF and Δ l / l =1.6×10 −4 at T s S =236.5 K for sintered samples. Enhancement is observed in electrical conductivity and p -type thermopower at T s GF and T s S , while a reduction is observed in the magnetic susceptibility. The GF sample exhibits higher thermoelectric performance than the sintered sample. The power factor of the α-phase in the GF sample is twice as large as that of the β -phase; it exceeds 20 μW/cm·K 2 between 120 and 240 K, indicating that the α-phase Zn 4 Sb 3 is one of the prime candidates for thermoelectric materials for cryogenic use.

Crystal Growth and Characterization of the Kondo Semimetal CeNiSn

Several single crystals of CeNiSn and CeNi 0.95 T 0.05 Sn (T=Co and Cu) were grown and characterized by metallographic examination, electron-probe microanalysis and the measurements of resistivity, magnetoresistance, magnetic susceptibility and specific heat. Below 10 K, the semiconductor-like behavior found for dirty or substituted crystals is suppressed with decreasing impurities. For a crystal purified by the solid-state electrotransport technique, the resistivity along the orthorhombic a axis is metallic with the T 2 dependence, whereas those along the b and c axes increase below 3 K and pass through weak maxima at 0.8 K. These results suggest that the ground state of CeNiSn is not semiconducting but semimetallic with a pseudogap which closes along the a axis.

Pseudogap due to Coherence Kondo Effect in CeNiSn and CeRhSb.

Specific heat of single crystal of CeNiSn has been measured between 1.3 and 60 K and analyzed together with that of polycrystals of CeRhSb, CeNi 1- x Pt x Sn ( x =0.03, 0.06, 0.12 and 0.20) and CeNi 1- y Co y Sn ( y =0.03 and 0.10). The magnetic specific heat divided by temperature C m / T of CeNiSn and CeRhSb exhibits a maximum and substantially decreases at low temperatures indicating the energy gap opening in the heavy quasiparticle band. The Kondo temperatures T K were estimated to be 51 K for CeNiSn and 89 K for CeRhSb by fitting the magnetic entropy S m . Using the reduced temperature \(\tilde{T}=T/T_{\rm K}\), C m / T was plotted in \(C_{\rm m}/\tilde{T}\) versus \(\tilde{T}\). The plot reveals the obvious relation between the energy gap formation and the Kondo effect.

Anisotropic effects in the antiferromagnetic Kondo compound CePtSn

Abstract The magnetic, transport and thermal properties are reported on a single crystal of CePtSn which is isostructural with the Kondo semiconductor CeNiSn. The Ce ions in CePtSn are trivalent with 4f states governed by a strong crystal field causing an overall splitting of about 500 K. The Kondo effect is manifest by the double-peak structure of the magnetic part of the resistivity ϱm(T) possessing maxima at 10 and 140 K. Below the antiferromagnetic transition at 7.5 K, another transition appears in both the specific heat C(T) and ϱm(T) at 5.0 K. A metamagnetic transition occurs in magnetic fields around 9 and 11 T applied along the a and b axes, respectively. A magnetic phase diagram is presented based on the combined results of the field dependence of C(T) and the temperature dependence of the metamagnetic transition. These results are discussed in connection with the problem of the gap formation in CeNiSn.

The Origin of Magnetic Field Dependence of Specific Heat in Single-Crystalline CeNiSn

The effect of magnetic field on the electronic ground state in CeNiSn has been investigated by means of specific-heat measurement in magnetic fields µ 0 H up to 14 T. A high-quality single crystal allows us to observe a marked field effect on specific heat in the temperature range from 2 to 25 K. The ground state is discussed in terms of a pseudo-gapped density of states in the Kondo resonance band with residual states at the Fermi level inducing the Zeeman splitting.

Effect of impurity phases on the anisotropic transport properties of CeNiSn

Abstract We report on the measurements of resistivity, magnetoresistance, thermopower and specific heat on several single crystals of CeNiSn. As impurities of CeNi2Sn2 and Ce oxide increase up to about 3%, both the anisotropy and coherence in transport properties are smeared out. The increase in resistivity below 10 K is longer for less pure cyrstal, which suggests strong localization of residual carriers in the pseudogap.

A μSR Study of Magnetic Correlations in CeNiSn

The formation of magnetic correlations in CeNiSn has been studied by muon spin rotation and relaxation (μSR) spectroscopy covering the temperature range from 250 K to 33 mK. Mainly a single crystalline sample was used. At high temperatures (≥ 2 K) a behaviour similar to that of a spin fluctuator like UAl2 is observed, meaning that over the whole temperature range the paramagnetic moments fluctuate extremely fast (≥ 31014 Hz). Below 1 K the material exhibits properties typical for a paramagnet moving towards magnetic order, but no transition into long-range order could be observed down to the lowest measured temperature. The dependences of muon spin relaxation rate and muon spin precession frequency on external field (applied parallel to the a-axis) are unusual and indicate the formation of extended spin correlations up to short-range order.

Localization effects of kondo semimetals CeNiSn and CeRhSb

Abstract We have found that the semiconductor-like increase in resistivity previously reported on CeNiSn and CeRhSb below about 8 K is suppressed with decreasing impurity concentration in single-crystalline samples. Nevertheless, the absolute values of the Hall coefficient strongly increase as temperature decreases. Therefore, we interpret the metallic behaviour found in the a -axis resistivity of a purified crystal of CeNiSn as a consequence of the significant increase of relaxation time of carriers. Even for the purified crystal, the residual γ value is 40 mJ/K 2 mol at 0.03 K. These results suggest that the energy gap is closed along the a -axis and that residual carriers are strongly localized in the presence of impurities.

Magnetoresistance and the Anisotropic Hybridization Gap in CeNiSn

We have measured the magnetoresistance of the orthorhombic CeNiSn at high fields up to 15 T and at low temperatures down to 0.4 K. The positive magnetoresistance is found to become dominant with the decrease of temperature for the magnetic field H applied along the b - and c -axes in the transverse configuration between the current J and the field H ( J ⊥ H ). On the other hand, the longitudinal magnetoresistance ( J ∥ H ) is small in magnitude for H ∥ b and c . For H ∥ a , the magnetoresistance is negative for any current direction. These experimental results have been discussed on the basis of the cyclotron motion of the compensated carriers with closed Fermi surfaces and the reduction of the anisotropic hybridization gap.

Energy gap suppression by alloying in CeNi1−xPtxSn

Abstract The specific heat C of CeNi1−xPtxSn has been measured T 1.3 and 60 K. With increasing Pt concentration x from 0 to 0.2, the ratio C T at 1.5 K recovers from a reduced of 0.10J/K2 mol due to the gapping (x = 0) to an enhanced value of 0.34J/K2 mol (x = 0.2). This gap suppression is discussed in terms of reduction of the c-f hybridization and destruction of coherence.