Dense Kondo System Research Articles

Thin film growth of heavy fermion chiral magnet YbNi3Al9

We grew thin films of a heavy fermion chiral magnet YbNi3Al9 by using molecular beam epitaxy. They were grown on c-plane sapphire substrates under ultra-high vacuum while maintaining a deposition rate at a stoichiometric ratio among Yb, Ni, and Al. The resulting thin films contain epitaxial grains with the c axis parallel to the substrate surface: The YbNi3Al9 c axis is parallel to the sapphire b or a axis. The temperature dependence of the resistivity exhibits a typical feature of a dense Kondo system with a broad shoulder structure at ∼40 K, as well as a kink as a signature of the chiral helimagnetic ordering at 3.6 K. These features are consistent with those previously observed in bulk samples. The shift in the kink associated with the field-induced phase transition is found in the magnetoresistance curves under a magnetic field applied in the direction perpendicular to the c-axis. The magnetic phase diagram well reproduces that for the bulk crystals, implying that the chiral soliton lattice phase arises under magnetic fields, even in thin films.

Suppression of dense Kondo state in CeB6 under pressure

To investigate whether the dense Kondo compound CeB6 might evolve into a topological insulator under sufficient pressure, four-point electrical resistivity measurements have been carried out over the temperature range 1.3K–295K in a diamond anvil cell to 122GPa. The temperature Tmax of the resistivity maximum initially increases slowly with pressure but disappears between 12 and 20GPa. The marked changes observed under pressure suggest that a valence and/or structural transition may have occurred. Synchrotron x-ray diffraction measurements, however, fail to detect any change in crystal structure to 85GPa. Although a transition into an insulating phase is not observed, this dense Kondo system is completely suppressed at 43GPa, leaving behind what appears to be a conventional Fermi liquid metal.

Magnetic and related properties of Ce5CoGe2, CeCoGe and CeCo2Ge2

Magnetic susceptibility, magnetization, specific heat, electrical resistivity and thermoelectric power of polycrystalline Ce5CoGe2, CeCoGe and CeCo2Ge2 were studied at temperatures down to 2 K and in magnetic fields up to 5 T. The novel phase Ce5CoGe2 was found to be a ferromagnet with the Curie temperature of about 11 K, which exhibits in the paramagnetic region some features of a dense Kondo system with strong crystal field effect. The re-investigated CeCoGe was found to order antiferromagnetically below 4.6 K, while CeCo2Ge2 is an intermediate-valence system, in agreement with previous reports. The thermoelectric power of the latter compound is large (up to 40 μV/K) and positive in the whole temperature range studied, while in the magnetically ordered systems Ce5CoGe2 and CeCoGe it is mostly negative and distinctly smaller (down to −1.5 and −11 μV/K, respectively).

Hydrogenation studies on structural and magnetic properties of antiferromagnetic Kondo lattice Ce2Cu2In

Hydrogenation studies were carried out on an antiferromagnetic dense Kondo system Ce2Cu2In. Single-phase hydrides Ce2Cu2InHx were obtained for the hydrogen content 2.17<x<2.8. These compounds form with the crystal lattice of the parent alloy, yet with a strongly expanded unit cell, especially along the crystallographic c-axis. For x<2.17 mixtures of Ce2Cu2In and Ce2Cu2InH∼2.2 were observed, while for x>3 amorphous material was obtained. The single-phase hydrides Ce2Cu2InH2.37 and Ce2Cu2InH2.78 are Curie–Weiss paramagnets due to stable Ce3+ ions. For both compounds, near 11–13K, some tiny features were observed in the magnetic susceptibility data, which might reflect weak magnetic ordering.

Ternary aluminides R0.67Ni2Al6 (R=Sc, Y, Gd–Lu) with partly disordered structures

Ten rare-earth nickel aluminides R0.67Ni2Al6 (R=Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were synthesized by arc melting and their crystal structures studied by X-ray powder diffraction. They crystallize in the hexagonal structure type Yb0.67Ni2Al6: Pearson symbol hP11-2.33, space group P-6m2, a=4.18904(2), c=9.09816(6)Å, which can be decomposed into three kinds of layer. The sequence of layers along [001] is Yb0.67Al–NiAl2–Al–NiAl2, but stacking faults appear due to relative shifts of consecutive Yb0.67Al layers containing two rare-earth metal atoms for one Al-atom triangle. An ordered superstructure is the type ErNi3Al9 (Pearson symbol hR78, space group R32, a=3a0, c=3c0), refined on single crystals.The physical properties of the R0.67Ni2Al6 (R=Tb, Er, Tm, Yb, and Lu) compounds were determined by means of magnetic and electrical transport measurements. All these materials but Lu0.67Ni2Al6 exhibit Curie–Weiss paramagnetic behavior due to the trivalent R ions, and order magnetically at low temperatures. They show good metallic conductivity. The Yb-containing compound is a dense Kondo system with distinct features in the paramagnetic state of the interplay between Kondo and crystalline electric field interactions.

Unusual spectral renormalization in hexaborides

Employing high resolution photoemission spectroscopy, we studied the evolution of the spectral features in rare earth hexaboride single crystals as a function of temperature and 4f binding energy, where the variation of the 4f binding energy is obtained by changing the rare earth element. High energy resolution helped to reveal the distinct features corresponding to the various photoemission final states. Experimental results of CeB6, a dense Kondo system, exhibit the growth of the features near the Fermi level with the decrease in temperature relative to the uncompensated local moment contributions. The valence band spectra of the antiferromagnetic compounds, PrB6 and NdB6, exhibit multiple features—the 4f ionization peaks (poorly screened features) appear at higher binding energies and the features in the vicinity of the Fermi level possessing significant 4f character are due to the well-screened photoemission final states. These results indicate finite hybridization between the 4f and B 2s2p conduction electronic states. Interestingly, the well-screened features in PrB6 and NdB6 exhibit unusual enhancement in intensity at low temperature.

Observation of Kondo resonance in rare-earth hexaborides using high resolution photoemission spectroscopy

We studied the electronic structure of rare earth hexaborides, CeB6, PrB6 and NdB6 using state-of-the-art high resolution photoemission spectroscopy. CeB6 is a dense Kondo system. PrB6 and NdB6 are antiferromagnetic (Neel temperature ~7 K), known to be stable moment systems and do not exhibit Kondo effect. Photoemission spectra exhibit distinct signature of surface and bulk electronic structures of these compounds. The energy position of the surface feature is not influenced by the 4f density of states. High resolution spectra of CeB6 reveal multiple Kondo resonance features in the bulk spectra due to various photoemission final states. Interestingly, high resolution photoemission spectra of antiferromagnetic PrB6 also exhibit a sharp feature at the Fermi level that shows temperature dependence similar to the Kondo resonance features.

Ge‐based skutterudites MPt4Ge12: A comparative 195Pt NMR study

AbstractThe new Ge‐based skutterudites MPt4Ge12 are investigated by 195Pt NMR. They form with M = Ba, Sr, La, Ce, Pr, Nd, Sm, Eu, Th and U and in contrast to some pnictide skutterudites they remain metallic independent of the filler ion. Here we show that, consistent with the Korringa theory, the residual shift 195K(T → 0) scales with the density of states N(EF) and with the electronic specific heat coefficient γ. For the heavy filler ions La and Th an unusual 195K(T) dependence was found which might be associated to the effect of dynamic disorder (rattling). For paramagnetic CePt4Ge12 a Curie–Weiss‐like behaviour in both the shift 195K(T) and the bulk susceptibility χ(T) confirms the Ce3+‐4f1‐state at high temperatures. At low temperatures 195K(T) passes through a maximum at around 60 K and saturates below 10 K. This is interpreted being due to an unstable Ce valence, like, e.g. in CeSn3. Further evidence comes from the spin‐lattice relaxation where 195(1/T1) starts to deviate from its low‐temperature linear Korringa‐type of behaviour at around 60 K. We discuss our findings in the context of a dense Kondo system with valence fluctuations. For the van‐Vleck paramagnet SmPt4Ge12 at low‐temperatures, surprisingly, the shift is of the same order as for CePt4Ge12. It also passes through a maximum but at much lower temperatures (∼6 K).

Kondo effect on electrical properties of TmIn 3

We presented results of electrical resistivity, magnetoresistivity and Seebek effect measurements done in the paramagnetic state of an antiferromagnetically ordering intermetallic compound TmIn 3. It was found that the magnetic part of the electrical resistivity showed maximum at 18 K and the lg T dependence in temperature range 20< T<100 K. The magnetoresistivity was negative. The transversal magnetoresistivity fulfilled the single-ion Kondo scaling for temperatures 2<( T+0.5 K)<28 K. Thermoelectricpower displayed a sharp minimum at 10 K. Electrical properties indicated that TmIn 3 is likely a dense Kondo system.

Y substitution effect to the dense Kondo behavior of amorphous Ce–Mn alloys

We have measured the specific heat and the resistivity on sputtered amorphous (a-) Ce 80− x Y x Mn 20 alloys ( x = 10 , 20, 30, 50 and 70), substituted by Y for dense Kondo a-Ce–Mn alloy. The electronic specific heat coefficient γ shows the large value ( γ = 133 mJ / mol K 2 ) at x = 10 and decreases with increase of the Y concentration linearly. The resistivity at low temperature shows the T 2 dependence and the large coefficient A at x = 10 , 20. However, the T 2 dependence was not observed for x⩾30. Thus, a-Ce 80− x Y x Mn 20 for x⩽20 is also the dense Kondo system. We made Kadowaki–Woods plot from A and γ of a-Ce–Mn and a-Ce-Y-Mn alloys. The ratio of A/ γ 2 for a-Ce-Y-Mn is much larger than the value of a-Ce–Mn alloy.

Magnetoresistance and magnetization anomalies in CeB 6

High precision magnetoresistance (MR) Δ ρ/ρ( H, T) and magnetization M( H, T) measurements have been carried out for well known and typical strongly correlated electron system—cerium hexaboride. The detailed measurements have been fulfilled on single crystalline samples of CeB 6 over a wide temperature range T ⩾ 1.8 K in magnetic fields up to 70 kOe. It was shown that the MR anomalies in the magnetic heavy fermion compound under investigation can be consistently interpreted in the frameworks of a simple relation between resistivity and magnetization—Δ ρ/ ρ∼ M 2 obtained by Yosida [Phys. Rev. 107(1957)396]. A local magnetic susceptibility χ loc ( T , H ) = ( 1 / H * ( d ( Δ ρ / ρ ) / d H ) ) 1 / 2 was deduced directly from the MR Δ ρ( H, T) measurements and compared with the experimental data of magnetization M( H, T). The magnetic susceptibility dependences χ loc( T, H) and χ( T, H) obtained in this study for CeB 6 allow us to analyze the complicated H– T magnetic phase diagram of this so-called dense Kondo-system.

Transport properties of the dense Kondo system Ce0.5La0.5B6at low temperatures

We have investigated transport properties of a cubic Kondo system, Ce0.5La0.5B6,with a Γ8ground state. The resistivity shows a non-Fermi liquid behaviour in phase I. Inantiferromagnetic phase III, the resistivity follows well the formula ρ(T) = ρ(0) + AT2.The coefficient Afor phase III is much larger than that of CeB6in phase III. The magnetic phase diagram for H ∥ [110]has been obtained.

Transport properties of the dense Kondo system Ce 0.5La 0.5B 6

We have measured the electrical resistivity of the dense Kondo system Ce 0.5La 0.5B 6 at very low temperatures. A large term which is in proportion to temperature squared is found in the resistivity of Ce 0.5La 0.5B 6 below ∼0.3 K . The proportion coefficient of this term of Ce 0.5La 0.5B 6 is 37 times larger than that of CeB 6.

NMR study of the electronic state in the dense Kondo compoundCeNiAl4

${}^{27}\mathrm{Al}$ NMR results are presented for a dense Kondo system ${\mathrm{CeNiAl}}_{4}$ in the temperature range 3--300 K. The Knight shift ${K}_{\mathrm{iso}}$ and the spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ exhibit a nonmagnetic behavior associated with a very low conduction electron density of states near the Fermi level. Both of them also reveal an insignificant role of $4f$ electrons in RKKY interactions in the range 50--300 K. However, below 50 K, $4f$ electrons participate in the excitation near the Fermi level and their extent of delocalization changes gradually down to 3 K. This is a consequence of a non-Fermi-liquid state below 50 K and contradicts the prediction of the existence of the heavy fermion state at low temperature from bulk measurements. Thus the present result gives a clear indication of the appearance of 4$f$ moment compensation due to the predominance of the Kondo effect over RKKY interaction.

Magnetic properties of the dense Kondo system CeB 6 in high magnetic fields

This work reports on the phase diagram study of the dense Kondo system CeB 6 by means of magnetization ( M) and transverse magnetoresistance (MR) measurements in magnetic fields ( B) to 18 T, in the temperature ( T) range from 1.8 to 40 K. The zero field resistivity displays Kondo behavior in the paramagnetic P-phase, while two discontinuities at low T were observed at T Q≈3.5 K and T N≈2.3 K, the boundaries of the antiferro-quadrupolar (AFQ), and antiferromagnetic (AF) phases, respectively. The isothermal transverse MR is large and negative at low T, and it shows a distinct feature when the P–AFQ line of the B– T phase diagram is crossed. Similarly, the M vs. B curves at low temperatures show upward discontinuities at the phase boundary. Both the features in MR and M at the P–AFQ line correlate well in T, revealing the gradual increase of the T for the onset of AFQ ordering with B. No evidence for reentrant behavior could be observed in fields to 18 T.

Magnetic, electrical transport, and thermal properties of a uranium intermetallic compoundUCu5In

A ternary intermetallic compound ${\mathrm{UCu}}_{5}\mathrm{In}$ has been studied by means of magnetization, electrical resistivity, magnetoresistivity, thermopower, thermal conductivity, heat-capacity, and thermal-expansion measurements. The experimental findings reveal ${\mathrm{UCu}}_{5}\mathrm{In}$ to be antiferromagnetically ordered ${(T}_{N}=25\mathrm{K})$ dense Kondo system ${(T}_{K}=24\mathrm{K})$ with a considerable enhancement of the effective mass of conduction electrons at low temperatures, yielding the Sommerfeld coefficient $\ensuremath{\gamma}(0)$ of about 240 mJ/(mole ${\mathrm{K}}^{2}).$ Although the electrical resistivity and the thermopower of ${\mathrm{UCu}}_{5}\mathrm{In}$ shows features characteristic of interplay of Kondo and crystal-field effects, the magnetoresistivity does not exhibit behavior expected for Kondo systems being large and positive even well above ${T}_{N}.$ Consistent explanation of the latter results is attempted by considering the role of magnetic precursor effects and/or interfacelike effects due to the presence of nonmagnetic layers.

Magnetization, electrical resistivity and neutron diffraction studies of the U2(Ni1-xPtx)2In system

X-ray powder diffraction, magnetization, electrical resistivity and neutron diffraction experiments were performed on the dense Kondo system U2(Ni1-xPtx)2In. Up to x0.5 the alloys crystallize in a tetragonal structure with space group P4/mbm while for x0.6 they crystallize in space group P42/mnm. The observed structural change coincides with the magnetic-nonmagnetic crossover in the system. In the concentration range 0 x0.5, an antiferromagnetic ordering occurs, characterized by a noncollinear arrangement of the magnetic uranium moments in the tetragonal basal plane. In addition, for x = 0 and 0.2 a commensurate magnetic ordering is formed with a propagation vector k = (0,0,0.5) while for x = 0.5 an incommensurate one with k = (0,0,0.5±), = 0.06 is found. In the concentration range 0.6 x1.0 no magnetic ordering down to 1.7 K is observed. The bulk and neutron scattering data exhibit features attributed to the interplay of the Kondo effect and magnetic exchange interactions. For interpretation of the magnetic behaviour of the U2(Ni1-xPtx)2In alloys, a strong modification of the density of states at the Fermi surface has been invoked.

Dense Kondo compound Ce 5Ge 3

Magnetic and transport properties of Ce 5Ge 3 and La 5Ge 3 have been studied. The magnetic susceptibility of Ce 5Ge 3 above 90 K is fitted with a Curie–Weiss law with an effective paramagnetic moment of 2.66μ B and a paramagnetic Curie temperature of −30.9 K. Arrott plot shows that Ce 5Ge 3 has a spontaneous magnetization below 6.4 K. The magnetization curve shows large hysteresis with remanent magnetic moment of 0.24μ B and coercive field of 1.2 kOe at 2.0 K. The magnetization at 1.9 K reaches 0.9μ B/Ce at 145 kOe and saturation behavior is not found even at this high field. The temperature dependence of the magnetic resistivity indicates that Ce 5Ge 3 can be classified as a dense Kondo system.

Environmental effect on the magnetic behaviour in CePd 3Ga x

Several perovskite-like intermetallics CePd 3Ga x (0⩽ x⩽0.2) have been studied by means of magnetic and electrical transport measurements. With increasing x, the system transforms from a coherent nonmagnetic Kondo lattice to a single-ion Kondo system and finally to a magnetic dense-Kondo system. The observed behaviour occurs due to a change in the valence of cerium from intermediate-valent towards trivalent state.

Spin dynamics of CeSi 2− xGa x, 0.7⩽ x⩽1.3

In this paper we report inelastic neutron scattering experiments performed on CeSi 2− x Ga x , 0.7⩽ x⩽1.3. The thermal evolution of quasielastic linewidth indicates a crossover from a magnetic 4f metal type state of Ce ion to a dense ferromagnetic Kondo system in these compounds.