Search

The thermal expansion of a diluted Ce system La1-x Ce x Cu6 for (0.6 ≤ x ≤ 1) has been measured between 10 and 150 K to reveal the change from the coherent heavy Fermion state (0.9 ≤ x ≤ 1) to the incoherent Kondo state (0 < x ≤ 0.73). The large Ce concentration x dependence of the linear thermal expansion coefficient along b-axis αb (T) suggests that the coupling between the 4f 1 electron and the lattice strain is the largest along the b-axis in the three crystallographic axes. The maximum of the magnetic contribution to the volume thermal expansion coefficient β m(T) at T = 50 K is retained in the x range of 0.6 ≤ x ≤ 1, suggesting the crystalline electric field (CEF) level for x = 1 doesn’t change by the substitution. Furthermore, the upturn in β m(T) below 25 K, which should be a precursor of the maximum at T = 2.5 K reported for x = 1, is retained when we decrease x from 1 to 0.6. Because the ground state for x = 0.6 is the incoherent Kondo state, the robustness of the maximum at T = 50 K and upturn in the current x value implies that β m(T) in 10 ≤ T ≤ 150 K is attributed to the CEF and Kondo effects rather than the formation of the heavy Fermion state.

We investigated pressure-induced superconductor UIr by the ac heat capacity measurement under high pressure. The heat capacity shows an anomaly at the Curie temperature T C1 = 46 K at ambient pressure and the heat capacity anomaly shifts to the lower temperature side with increasing pressure. The present pressure dependence of T C1 was consistent with previous works by the resistivity and magnetization measurements. No heat capacity anomaly was found at T C2 and T C3 which were clarified by previous ac magnetic susceptibility measurement. This may be due to very small magnetic ordered moments (∼0.05μB/U) of the FM2 and FM3 phases.

A complete set of the 195Pt Knight-shift (KS) data on the superconducting (SC) state in UPt3 identified the spin structure of the Cooper pair corresponding to the multiple SC phases. UPt3 was acclaimed as the first odd-parity superconductor including a non-unitary pairing state characterized by the two-component d vector like d b + i d c at low T and low H [H. Tou et al., Phys. Rev. Lett. 77 (1996) 1374; 80 (1998) 3129]. We have shed further light on these novel results through a comparison with the singlet even-parity anisotropic superconductors CeCu2Si2 and UPd2Al3. In the singlet pairing state, the fractional decrease in KS below T c , δ K obs is independent of the crystal direction. We have found that δχobs=(NAμB/Ahf)δKobs where Ahf is the hyperfine coupling constant, is in good agreement with spin susceptibilities χγel calculated from an enhanced electronic specific heat γel and χnmr from the quasiparticle Korringa relation T1TKs2=const. This gives direct evidence that the χs of heavy quasiparticles in CeCu2Si2 and UPd2Al3 is rather isotropic and decreases to zero as T→0 due to the Cooper-pair formation. On the other hand in UPt3, the δχobsb,cs along the b- and c-axis in the non-unitary-pairing state (B phase) are two orders of magnitude smaller than χγel and χnmr. These anomalously small values for δχobsb,cs may suggest either that the spin degree of freedom in the B phase is not perfectly locked to the a-axis or that χs is not enhanced although γel is. The latter is theoretically pointed out by Ikeda and Miyake [J. Phys. Soc. Japan 66 (1997) 3714] to be possible if 5f electrons in the non-Kramerse singlet ground state for 5f2 are hybridized with conduction electrons. We need further effort towards coherent understanding of a microscopic mechanism leading to the occurrence of the odd-parity superconductivity in UPt3.

A general susceptibility (χ q ,ω ) evaluation from the Knight shift and nuclear spin–lattice relaxation measurements via 11 B NMR technique has been carried out on single crystal UIr 3 B 2 . The experimental results revealed two distinct temperature regimes in which the magnetic state dynamics are different from one another. Above T * ≃50 K, the temperature variation of χ q ,ω is mainly subject to q = 0 fluctuations, suggesting that ferromagnetic correlations develop at high temperatures. On the other hand, antiferromagnetic finite- q components abruptly emerge and show remarkable growth below T * . Even at low temperatures, ferromagnetic correlations persist, but χ q ,ω develops significant dispersion with antiferromagnetic components. The origin of the observed magnetic correlations is discussed in terms of the low dimensionality which is derived from the highly anisotropic structure of UIr 3 B 2 .

© 2014 Taylor & Francis. We review recent progress in single crystal growth and study of electronic properties in. Czocharalski pulling, using purified uranium metal and subsequent annealing under ultra-high vacuum, is successfully applied to this compound, and it yields the highest residual resistivity ratio. These high-quality single crystals allow us to investigate Fermi surfaces using quantum oscillation and to make detailed transport measurements at low temperature.

${}^{27}$Al nuclear spin-lattice relaxation rate 1/${}^{27}{T}_{1}$ has been measured in heavy fermion superconductor NpPd${}_{5}$Al${}_{2}$ for a wide range of external fields between 0.7 and 11 T. We found that 1/${}^{27}{T}_{1}$ exhibits a strong, field-dependent behavior at low fields below $H\ensuremath{\sim}3$ T, which is attributed to unlike-spin cross relaxation between ${}^{237}$Np and ${}^{27}$Al. An analysis based on the cross relaxation process allows us to make the first quantitative estimates of the Np nuclear relaxation rate 1/${}^{237}\ensuremath{\tau}$ in an itinerant 5$f$ electron system. The estimated values of 1/${}^{237}\ensuremath{\tau}\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{7}$ s${}^{\ensuremath{-}1}$, with a weak temperature variation, are below the resolution of NMR relaxation times, indicating that a direct observation of the ${}^{237}$Np NMR signal would be very difficult, at least in the paramagnetic state. It is proposed that a direct observation of ${}^{237}$Np NMR might be possible in the superconducting state at $T\ensuremath{\sim}1$ K.

We present experimental results of complex susceptibility, static magnetization, magnetic relaxation, specific heat, and electrical resistivity measurements on an intermetallic compound Nd2AgIn3. The results indicated that Nd2AgIn3 undergoes a spin-glass transition at a freezing temperature Tf=12.6 K accompanied with the formation of short-range antiferromagnetic order. The mechanism of formation of spin-glass state in Nd2AgIn3 is different from that in diluted metallic spin-glass or uranium intermetallic compound. The existences of frustrated moments due to the triangles of nearest neighbors in Nd atomic layers and randomly distributed Ruderman–Kittle–Kasuya–Yosida interactions may be responsible for the spin glass state formed in Nd2AgIn3.

Anion-doped 1T-TaS2−xSex (x = 0−2) charge density waves (CDW's) at room temperature (RT) phase were studied using STM and electron diffraction (ED). It was found that with increasing Se concentration the anion-doped 1T-TaS2−xSex at RT phase varied the CDW's from a nearly commensurate to commensurate state, with this phenomenon being particularly reported for 1T-TaS0.4Se1.6. The 2D-FFT spectrum of the STM image and ED pattern distinctly showed that 1T-TaS0.4Se1.6 had a “3×1” commensurate CDW superstructure. Several translation boundaries, i.e., where shifts of the CDW rows are clearly visible, were also observed in 1T-TaS0.4Se1.6. Investigations of interfacial interactions of CDW's are also presented with specific emphasis on commensurate 1T-TaSe2 CDW's using a H2O cluster model. After rinsing with H2O, all 1T-TaS2−xSex CDW's contour heights were remarkably enhanced, this being a result of the physically adsorbed tetrahedral (H2O)n clusters and the charge transfer from CDW to adsorbed H2O. On a clean surface with few crystal defects, a hydrated CDW in H2O showed a triple CDW. However, results were focused on an observed single hydrated CDW in H2O which was aligned along a step due to the primary interactions of H2O molecules with active step sites. The formation of single hydrated CDW's in H2O is concluded to originate from the presence of step sites and the preferential orientation of sequential chains of (H2O)n along the step sites.

We have grown single crystals of UGa 3 , USb 2 and UBi 2 by the self-flux method and measured the de Haas–van Alphen (dHvA) oscillations. The detected Fermi surfaces were five closed ones in an antiferromagnetic compound UGa 3 . The cylindrical Fermi surfaces were observed for USb 2 and UBi 2 , which manifests the two-dimensional character on the conduction electrons.

We have performed μ+SR studies on a single-crystal UPt3 by implanting spin-polarized μ+ parallel to the c-axis in a zero magnetic field from 10 K down to 20.8 mK in order to study magnetism and superconductivity. Down to 20.8 mK, zero-field μ+SR spectra have not exhibited a significant change; thus, neither a spontaneous magnetization in the B phase nor a precursor of a long-range magnetic ordering has been observed down to 20.8 mK.

We have studied the Fermi surface properties in strongly anisotropic or quasi-two-dimensional systems of CePtX (X=As, P) and UX 2 (X=Sb, As). Not only the band folding but also the strong hybridization effect play an important role in the quasi-two-dimensionality for these compounds.

$^{31}\text{P}\text{-NMR}$ measurements have been performed on a single crystal of the neptunium-based filled skutterudite ${\text{NpFe}}_{4}{\text{P}}_{12}$. The compound undergoes a ferromagnetic phase transition at ${T}_{C}=23\text{ }\text{K}$. From the field-orientation dependence of the $^{31}\text{P}\text{-NMR}$ line splitting, the angular dependence of the hyperfine interactions between $\text{Np}\text{ }5f$ spin and $^{31}\text{P}$ nuclear moments has been investigated. We have observed anisotropic transferred hyperfine interactions at the P sites, which lead to an estimate of the local spin density in the $\text{P}\text{ }3p$ orbitals. It is shown that a fraction of the $\text{Np}\text{ }5f$ spin moments is transferred mostly into the $\text{P}\text{ }3p$ orbitals extending toward the inside of a P cage. The weak hybridization between $\text{Np}\text{ }5f$ and $\text{P}\text{ }3p$ orbitals suggests a localized character for $\text{Np}\text{ }5f$ electrons in ${\text{NpFe}}_{4}{\text{P}}_{12}$. We have also measured the field and temperature dependences of the nuclear spin-lattice relaxation rate $(1/{T}_{1})$ in several magnetic fields between 18.5 and 78.0 kOe. The $1/{T}_{1}$ data reveal that the low-energy spin fluctuations of $\text{Np}\text{ }5f$ spin moments are strongly suppressed by applied fields over a relatively wide temperature range up to $4{T}_{C}$. In this compound, a large and negative magnetoresistance has been known to occur in the same temperature range. The present NMR results demonstrate that the negative magnetoresistance comes from a reduction in the magnetic scattering from $\text{Np}\text{ }5f$ spin moments by an applied field.

Thermal expansion coefficient α( T ) of the heavy fermion material CeCu 2 Si 2 has been measured in the temperature range from 3 to 300 K under high pressures up to 15 kbar. A large enhancement in the value of α/ T was found at low temperatures and α/ T at 0 K decreased with increasing pressure.

The Fermi surface of CeB 6 is found to be very similar to that of PrB 6 , with an electron pocket of almost the same size as that in PrB 6 . We have confirmed a discrepancy between the cyclotron mass determined by the de Haas-Van Alphen effect and the one estimated from the low temperature specific heat coefficient.

We have succeeded in growing a high-quality single crystal of ThIn3 with the cubic structure by the In-flux method and measured the de Haas–van Alphen (dHvA) effect. Several dHvA branches were observed, ranging from 9.0 × 10 6 to 9.37 × 10 8 Oe . The experimental results are compared to the dHvA results of CeIn3 under pressure of 2.7 GPa, together with the results of energy band calculations.

In order to identify the gap structure of a spin-triplet superconductor UPt3, we performed field-angle-resolved specific heat (C) measurements in a magnetic field rotated around the [0001] axis. Although various theories predict fourfold symmetry of the gap in the superconducting C phase, we observed no angular variation in C in a temperature range down to 88 mK (~ 0.17Tc). This absence of the variation in C implies that the field-angle-dependent quasiparticle excitations, reflecting the gap anisotropy, are averaged out in the bulk of UPt3.

The 4f(5f) electrons in the rare earth(uranium) atom are pushed deep into the interior of the closed 5s(6s) and 5p(6p) shells because of the strong centrifugal potential 1(1+1)/r2 , where 1=3 holds for the f electrons. This is a reason why the 4f (5f) electrons possess an atomic-like character even in the compound. On the other hand, the tail of their wave function spreads to the outside of the closed 5s(6s) and 5p(6p) shells, which is highly influenced by the potential energy, the relativistic effect, the distance between the rare earth(uranium) atoms, and hybridization of the 4f(5f) electrons with the conduction electrons. These cause the various phenomena such as valence and spin fluctuations, gap states, Kondo lattice, heavy electrons (fermion), metamagnetism and superconductivity for the f electron compounds.1

We have measured the AC magnetic susceptibility of the heavy-fermion compound CeRu2Si2 down to 150 μK . In zero field, the susceptibility monotonously increased down to the lowest temperature. In 0.27 mT , however, the susceptibility shows a peak at 510 μK . With increasing magnetic field, this peak shifts towards higher temperature and the susceptibility is suppressed. In addition, we have measured the frequency dependence of the susceptibility between 12 and 160 Hz . An internal relaxation time was obtained to be about 11 ms in terms of Casimir and Du Pre thermodynamic theory.

Iso-field thermal expansion and high-field magnetization measurements have been performed on a single-crystalline UPd 3 to study the antiferromagnetic ordering as well as the quadrupolar one. The magnetic phase diagram was obtained.

UPd2Al3 and URu2Si2 are considered to be non-s-wave superconductors with an anisotropic energy gap from the NMR and specific heat measurements. We have succeeded in observing clearly the dHvA oscillation in both the normal and superconducting mixed states. The cyclotron mass is found to be reduced and the Dingle temperature increases in the mixed state. An anisotropic energy gap with a line node for UPd2Al3 and URu2Si2 was discussed from the angular dependence of the dHvA amplitude in the mixed state.

We measured the low-temperature specific heat and de Haas–van Alphen oscillation in Sr2RuO4. The electronic specific heat Ce follows a T2-dependence below 0.7 K for the sample with a superconducting critical temperature Tc=1.52 K, suggesting line nodes in the superconducting gap. Three kinds of cylindrical Fermi surfaces were discussed from a view point of their cyclotron effective masses.

Magnetization measurements of a heavy fermion compound URu 2 Si 2 have been performed in the superconducting state. Broad peak structure of the irreversible magnetization was observed around 50 kOe for H ‖ a . The Ginzburg–Landau parameter obtained from the equilibrium magnetization slightly decreases for H ‖ c with decreasing temperature, suggesting the presence of the paramagnetic suppression of the superconductivity. However, the suppression is not so large as expected from the H c2 ( T ) curve. Thermodynamic critical field appears to show unusual temperature dependence.

Electrical resistances of UGe2 and CeRh2Si2 have been measured at high pressure and low temperature in order to clarify the relation between the magnetic instability and transport properties. It is found that the coefficients of T2 term and the residual resistivity show a maximum near the critical pressure where the magnetic ordering temperature disappears.

We measured the de Haas–van Alphen oscillations in order to reveal heavy electronic states of UPd2Al3. All the branches reported in the previous work were observed in this study. In addition, we observed new branches. These branches are well explained by the results of fully relativistic spin-polarized LAPW band calculations. The cyclotron mass of each branch is large, indicating strong hybridization of 5f components.

A self-consistent band calculation including an orbital polarization is performed for ferromagnetic UGa2 on the basis of an all-electron fully-relativistic spin-polarized LAPW (RSPLAPW) method with exchange-correlation potentials in a local density approximation (LDA). The obtained magnetic moment is composed of the spin of about −2.0 μB and the orbital moment of about 4.9 μB. Accordingly, it can be interpreted as if the configuration were a localized 5f2, though the f character contains about 2.5 as valence electrons. From the band structure, the Fermi surface is constructed in order to analyze frequencies observed by the de Haas–van Alphen (dHvA) effect.

One branch has been detected in the de Haas–van Alphen (dHvA) experiments in both the normal and superconducting mixed states of URu2Si2 and UPd2Al3 with an anisotropic superconducting energy gap. The corresponding Dingle temperatures increase and the cyclotron effective masses decrease in the mixed states. An anisotropic energy gap with a line node has been discussed from the angular dependence of the dHvA amplitude in the mixed state.

An intermediate valence compound CeNi exhibits a phase transition under pressure, which is associated with the γ–α transition in Ce. To clarify a change of the electronic state under pressure, we measured the de Haas–van Alphen (dHvA) effect under pressure up to 6 kbar using a Cu–Be pressure cell. The dHvA branches are still observed after the phase transition. The topology of the Fermi surface is therefore unchanged. The cyclotron effective mass is, however, reduced from 11 to 3.2m0 for branch e and from 5.4 to 1.2m0 for branch σ when the pressure is changed from 1 bar to 3.7 kbar.

The Josephson effect of S(Nb)–N(Cu)-S′(HFS: heavy-fermion superconductor) junctions has been investigated for polycrystalline CeCu2.2Si2, UBe13 and single-crystalline UPt3. The temperature dependence of Jc which provides evidence for unconventional superconductivity has been observed when UPt3 is used as HFS.

Electrochemical characteristics of the transition metal dichalcogenide ZrSe2 were investigated as a cathode material. Lithium intercalated ZrSe2 showed a high reversibility of lithium ions in the non-aqueous solution. The discharge/ charge cycle was performed up to 160 times without deterioration.

The effect of crystalline defects on Anderson localization was investigated and the localization in 1T–TaS 2 was found to be appreciably enhanced by increasing growth temperatures, by reducing the amount of excess sulphur and also by electron beam bombardment. The electrical resistivity ρ follows ρ∝exp T -1/ n as a characteristic state of the variable range hopping conduction in the temperature region of 4 K to the measured lowest temperature 0.1 K. Contrary to the theoretical prediction of n =3 for the two-dimensional system, we found that n =2 is better fitted.

The Hall coefficients of 1T-TaS2 and 1T-TaSe2 single crystals were measured from 360 K to 1.4 K. An abrupt change of the sign and a hysteresis were observed in the Hall coefficient of 1T-TaS2 at the nearly commensurate-commensurate phase transition temperature, ca. 200 K; above this temperature the major carrier was n-type, and below, p-type. On the other hand, the sign of Hall coefficient of 1T-TaSe2 was positive in the whole temperature range measured.

Alkali metal intercalated ZrSe 2 (A x ZrSe 2 ) has been synthesized by the reaction of ZrSe 2 single crystals with n-butyllithium in hexane solution, or alkali metal (Na, K, Rb, Cs) in hexamethylphosphoric triamide solution. Semiconductive ZrSe 2 has been converted into a metallic substance by the alkali metal intercalation. Superconductivity has been found for Li x -, Na x -, K x , Rb x and Cs x ZrSe 2 (0 x ⩽ 1) below 1.7, 2.0, 2.1, 1.7, and 1.4, respectively. The angular dependence of the upper critical fields H c2 for A x ZrSe 2 is interpreted by the effective mass model. The Ginzburg-Landau coherence lengths perpendicular to the layer planes of the crystals at zero temperature are ten to twenty times larger than the layer repeat distances, showing a three dimensional character. Anisotropic superconductivity is discussed on the basis of the Fermi surface of A x ZrSe 2 whose electron number is determined by the charge transfer from alkali metal atoms to ZrSe 2 .

The Hall coefficient of a 1T-TaS 2 single crystal was measured from 360 K to 1.4 K. An abrupt change of the sign and a hysteresis were observed in the Hall coefficient at the nearly commensurate-commensurate phase transition temperature, ca. 200 K; above this temperature the major carrier was n-type, and below, p-type.

The temperature dependence of electrical resistivities θ( T ) of 1T-TaS 2 , 1T-TaS 2- x Se x and 1T-Fe x Ta 1- x S 2 is found to be θ( T ) ∝ exp( T 0 / T ) 1/n in the temperature range of 4 K to the measured lowest temperature, 0.1 K, showing the variable range hopping of Anderson localized states. The n -value is nearly 3 for selenium doping and nearly 2 for non-doping and iron doping. The positive magnetoresistance, which is sizable only in the temperature range of 2 K to 0.5 K in 1T-TaS 2 , is found to be remarkably enhanced by the selenium doping, while the tendency is reversed by the iron doping.

Transverse magnetoresistance of 1T-TaS2 was measured in magnetic fields up to 100 kOe in the semiconductive region corresponding to the commensurate charge density wave (CDW) state.

Studies of the electrical resistivity, magnetoresistance, magnetic susceptibility, nuclear spin-lattice relaxation rate, specific heat, thermoelectric power, and Hall coefficient in CexLa1_xCu6 are reported. The magnetic properties of CexLa1_xCu6 scale with the cerium concentration, that is, the Kondo temperature is almost independent of the cerium concentration. The characteristic temperature in the heavy fermion state of CeCu6 is about one order smaller than the Kondo temperature.