First-order Valence Transition Research Articles

Superconductivity Emerging near Quantum Critical Point of Valence Transition

The nature of the quantum valence transition is studied in the one-dimensional periodic Anderson model with Coulomb repulsion between f and conduction electrons by the density-matrix renormalization group method. It is found that the first-order valence transition emerges with the quantum critical point and the crossover from the Kondo to the mixed-valence states is strongly stabilized by quantum fluctuation and electron correlation. It is found that the superconducting correlation is developed in the Kondo regime near the sharp valence increase. The origin of the superconductivity is ascribed to the development of the coherent motion of electrons with enhanced valence fluctuation, which results in the enhancement of the charge velocity, but not of the charge compressibility. Statements on the valence transition in connection with Ce metal and Ce compounds are given.

High-field [formula omitted]In-NMR in Yb [formula omitted]Y [formula omitted]InCu [formula omitted

High-field 115 In-NMR measurements of Yb 0.9 Y 0.1 InCu 4 were performed at 4.2 K up to 30 T with a hybrid magnet installed in National Institute for Materials Science (NIMS). A first-order valence transition was observed in the field dependence of the NMR spectrum. The field dependence of the spin–spin relaxation time T 2 indicates that the field-induced magnetic phase is in paramagnetic state.

Magnetoelastic effects and magnetic anisotropy in the intermediate-valence compound YbInCu 4

YbInCu 4 compound exhibits a first-order valence transition of Yb from a trivalent state to a mixed-valence state with decreasing temperature. Here we provide an overview of our recent high-field and high-pressure measurements of magnetization, thermal expansion and magnetostriction of the valence-fluctuating compound. The results are analyzed using the Kondo single-impurity model. It is shown that the change of the 4 f–conduction band electron hybridization is responsible for a large modification of the physical properties of YbInCu 4 at the valence transition. The volume magnetostriction is rather large for the low-temperature mixed-valence state, but strongly suppressed in the high-temperature local-moment state. From the magnetization measurements of the oriented YbInCu 4 single crystal, an appreciable anisotropy was found in the susceptibility, high-field magnetization, effective moment, paramagnetic Curie temperature and critical field of the valence transition. The type of the magnetic anisotropy and the value of anisotropic magnetoelastic coupling constant do not change significantly on going from the mixed-valence to the local-moment state. The anisotropic magnetostriction and magnetic anisotropy are described well in terms of a single-ion model of the interaction of anisotropic 4 f-shell of Yb ion with the crystal electric field.

Low-energy excited photoemission study of the valence transition ofYbInCu4

Low-energy excited photoemission spectra of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_{4}$ with isostructural first-order valence transition at ${T}_{V}=42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ have been measured with high-energy resolution. In the spectra measured at $h\ensuremath{\nu}=7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the low-temperature phase, the hybridization between the conduction-band (CB) and Yb $4f$ states is successfully observed as a structure at $\ensuremath{\sim}47\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below the Fermi level. This structure abruptly shows up below ${T}_{V}$, indicating that a degree of the $\mathrm{C}\mathrm{B}\mathrm{Yb}$ $4f$ hybridization increases in the low-temperature phase. Temperature dependence of the spectra at $h\ensuremath{\nu}=14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ suggests that the valence transition takes place at higher temperatures than ${T}_{V}=42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in some phases near the surface.

Structural and electronic transitions in the low-temperature, high-pressure phase of SmS

Structural and electronic properties of samarium sulfide were studied under pressure up to 2.9 GPa and at low temperatures down to 4.5 K via x-ray diffraction and absorption techniques. The measurements are a direct probe of the valence and structural state of samarium sulphide at low temperatures through the black-to-gold and magnetic transitions. A divalent state is found below the first-order valence transition reached in this study at 58 K and 1.13 GPa. Above the transition, the valence increases linearly with pressure. In particular, it is found the samarium has an intermediate valence [2.81 (4)] in the magnetically ordered phase at 2.9 GPa and 4.5 K. Resonant x-ray magnetic scattering found no evidence of type I or type IA magnetic order in the magnetically orderded phase.

NQR andT1studies of the high-pressure phase inYbInCu4

The pressure and temperature phase diagram of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_{4}$ has been investigated by nuclear quadrupolar resonance (NQR) and spin-lattice relaxation rate $({T}_{1}^{\ensuremath{-}1})$ experiments. The pressure dependence of the $^{63}\mathrm{Cu}$ NQR frequency indicates that the first-order valence transition temperature, ${T}_{v}$, does not vanish continuously at the critical pressure $({P}_{c}\ensuremath{\approx}23.7\phantom{\rule{0.3em}{0ex}}\mathrm{kbar})$ and thus there is no quantum critical point $({T}_{v}=0)$ in $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_{4}$. This result is consistent with the ${T}_{1}^{\ensuremath{-}1}$ data, which show no evidence for non-Fermi-liquid behavior near ${P}_{c}$. For pressures $P\ensuremath{\gtrsim}{P}_{c}$, ${T}_{1}^{\ensuremath{-}1}$ increases sharply near $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which suggests the presence of critical fluctuations associated with ferromagnetic (FM) ordering. We analyze the ${T}_{1}^{\ensuremath{-}1}$, resistivity, and the pressure-enhanced susceptibility data in the mixed-valent state of $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_{4}$ and find no evidence to indicate that the pressure-induced FM phase can be described by the Stoner theory for itinerant ferromagnetism. Rather, the pressure-induced FM order may be due to pressure-stabilized ${\mathrm{Yb}}^{3+}$ local moments. We also examine the possibility of FM order induced by an external magnetic field near ${P}_{c}$, but find no evidence down to $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Valence and Structural Transitions in the Pseudo-Ternary Germanide Ce(Rh0.69Ir0.31)Ge

Ce(Rh0.69Ir0.31)Ge has been investigated by magnetization measurements, differential scanning calorimetry, and X-ray diffraction on a single crystal at 300, 150, and 100 K. For the first time, a decrease of the valence of cerium with decreasing temperature has been observed. This first-order valence transition evidenced at 236−258 K by magnetization measurements is correlated to the occurrence of a structural transition appearing between 300 and 150 K. The structural properties versus temperature of Ce(Rh0.69Ir0.31)Ge are discussed in relation to those determined on single crystals on the antiferromagnet CeRhGe and on the intermediate valence compound CeIrGe. The structure refinements revealed significantly different distortions for the [RhGe] and [IrGe] polyanionic networks, which are a direct consequence of the different electron densities at the rhodium and iridium sites, respectively. The crystal chemical peculiarities of the CeRhGe, Ce(Rh0.69Ir0.31)Ge, and CeIrGe structures are discussed.

Investigation of pressure-induced magnetic ordering in YbInCu 4 probed microscopically by 63Cu NQR

The application of pressure for YbInCu 4 stabilizes the localized character of f electrons and, after the complete suppression of the first-order valence transition near 2.4 GPa, leads to a magnetic (most probably ferromagnetic) ordering above 2.45 GPa and below T M = 2.4 K . In order to elucidate the mechanism of the pressure-induced magnetic ordering in YbInCu 4, we have performed an NQR experiment under pressure up to 2.5 GPa. 63Cu spin–lattice relaxation rate in the pressure-stabilized state has a temperature-independent behavior followed by a divergent increase just above T M. Thus, for YbInCu 4 at high pressure, we conclude that the localized Yb spins transform into the magnetically ordered ground state without taking any intermediate heavy-Fermion state above T M.

価数転移Ｙｂ化合物における圧力誘起弱強磁性

We have paid attention to the ground state of the high-temperature magnetic phase of YbInCu4 which exhibits a first-order valence transition at 42 K. The high-temperature phase, which shows Curie-Weiss paramagnetism of Yb3+, is stabilized down to zero temperature by the application of pressure and/or substitution of Y for Yb. We have measured magnetization of Yb0.8Y0.2InCu4 under high pressure in the low temperature ranges down to 0.6 K and could observe suppression of the valence transition and occurrence of ferromagnetism at the same time. The ferromagnetism is characterized by a low Curie temperature of 1.8 K and very small spontaneous magnetization of 0.067 μB/Yb.

High-field magnetostriction of the valence-fluctuating compoundYbInCu4

The $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_{4}$ compound undergoes a first-order valence transition from the intermediate valence state to the trivalent state with increasing either temperature or magnetic field. We have measured the magnetization and magnetostriction in pulsed magnetic fields up to $40\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the temperature range $4.2--120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. It was found that the volume magnetostriction at the field-induced valence transition $\ensuremath{\Delta}\ensuremath{\omega}=\ensuremath{-}4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ corresponds well to the spontaneous volume change at the temperature-induced transition. The magnetovolume coupling constant is negative. Its absolute value is rather large for the low-temperature mixed-valence state, but strongly suppressed in the high-temperature local-moment state. A considerable anisotropic magnetostriction is found for both low-temperature and high-temperature phases. The temperature and field variations of the anisotropic magnetostriction are described in terms of a single-ion model of the interaction of an anisotropic $4f$ shell of Yb ion with the crystal electric field.

Electronic states near the Fermi edge of YbInCu 4

High-resolution low-energy excited photoemission spectra of YbInCu 4 with a first-order valence transition at T V∼42 K have been measured. In the spectra taken at hν =7 eV, the hybridization between the conduction-band and Yb 4f states is observed as a structure at ∼47 meV below the Fermi level and drastically increases at T V with decreasing temperature.

Alloying and pressure effect on the mixed-valence state of Yb inYbInCu4

The susceptibility and thermal expansion ofYb1−xRxInCu4 (; R = Y, La and Ce) have been measured at ambient and high pressures. These compoundsundergo a first-order valence transition from an intermediate-valence state to a nearlytrivalent state with increasing temperature. The valence change at the transition is for YbInCu4. It decreases on both alloying and applying pressure for all thecompositions studied. From the pressure effect on the susceptibility ofYbInCu4, the Grüneisen parameter for the Kondo energyΩK is determinedto be −34.5 and −20 for the low-temperature and the high-temperature state, respectively. TheΓ coefficient characterizing the hybridization strength between the 4f and conductionelectrons for the intermediate-valence state is nearly five times larger than that for thehigh-temperature local-moment state. The obtained results suggest that the change of the4f–conduction band electron hybridization rather than the volume dependence of theKondo interaction is responsible for a large modification of the physical properties of theYbInCu4-based compounds at the valence transition.

First-order valence transition and barocaloric effect in EuNi 2(Si 1− xGe x) 2

We observed in EuNi 2(Si 0.15Ge 0.85) 2 a very sharp, first-order valence transition from an Eu 2+ to an Eu 3+ state at around T v≈42 K. The transition presents a large thermal hysteresis of 10 K. The huge entropy change associated with the valence transition gives rise to a barocaloric effect, which may find use in magnetic cooling by the application of pressure.

Specific heat of mixed valence system YbInCu4 under pressure

Abstract We have carried out the measurement of specific heat under hydrostatic pressure P up to 0.68 GPa on YbInCu 4 , which undergoes a first-order valence transition at T V ≈40 K. With increasing P , T V decreases with the slope of ∂ T V /∂ P =–16.2 K/GPa, and the value of excess entropy Δ S due to the valence transition increases slightly. If Δ S originates from the mixing Yb 3+ and Yb 2+ ions, the experimental result can be understood as the decrease of hole occupation number n f in the 4f shell of Yb in the low-temperature phase by pressurization.

Evidence for the pressure induced magnetic ordering in YbInCu4

Abstract We have carried out electrical resistivity ρ and AC-susceptibility χ AC measurements on YbInCu 4 under pressure. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed and a clear peak appears in χ AC with a small kink in ρ at T M =2.4 K. The χ AC peak is easily diminished by applying low magnetic fields and disappears above ∼500 Oe. The characteristic behavior of χ AC at T M can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized phase of YbInCu 4 is most probably a ferromagnetically ordered state.

Pressure effect on the mixed-valence state of YbInCu4 with a valence transition

Abstract The susceptibility χ and thermal expansion Δ L / L of YbInCu 4 with a first-order valence transition were measured under high pressure up to P =1.6 GPa. The critical temperature T v decreases linearly with increasing P . The value of χ does not change for T > T v , while increases for T T v . The Kondo scale T 0 estimated from χ (0) is nearly proportional to T v . The abrupt change of Δ L / L at T v is 0.15% at ambient pressure and decreases with increasing P . The valence of Yb in the mixed valence state estimated from the change increases with P .

Transport, AC-Susceptibility and PQR Measurements of Strongly Electron Correlated Compound YbInCu4 at High Pressures

We have investigated physical properties of the valence fluctuating compound YbInCu 4 at high pressures and low temperatures. The first-order valence transition temperature TV ~ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure-stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature TK ~ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. 63 Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T1 and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.

Magnetic ordering in the pressure-stabilized high-temperature phase ofYbInCu4

To elucidate the ground state of the pressure-stabilized high-temperature (HT) phase of ${\mathrm{YbInCu}}_{4},$ we have carried out electrical resistivity $\ensuremath{\rho}$ and ac-susceptibility ${\ensuremath{\chi}}_{\mathrm{ac}}$ measurements at high pressures. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed (below $\ensuremath{\sim}80\mathrm{mK}).$ Separately, above 2.39 GPa, a clear peak appears in ${\ensuremath{\chi}}_{\mathrm{ac}}$ with a small kink in $\ensuremath{\rho}$ at around ${T}_{M}=2.4\mathrm{K}.$ The ${\ensuremath{\chi}}_{\mathrm{ac}}$ peak is easily diminished by applying low magnetic fields and disappears above $\ensuremath{\sim}500\mathrm{Oe}.$ The characteristic behavior of ${\ensuremath{\chi}}_{\mathrm{ac}}$ at ${T}_{M}$ can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized HT phase is most probably a ferromagnetically ordered state. This result is compatible with the occurrence of weak ferromagnetism recently reported for the Y-substituted compound ${\mathrm{Yb}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{InCu}}_{4}$ under pressure of 1.2 GPa.

Magnetic anisotropy of pure and doped YbInCu4compounds at ambient and high pressures

The susceptibility and high-field magnetization of single-crystallineYb1−xYxInCu4(x = 0,0.2 and 0.3) samples have been measured for different fieldorientations at ambient and high pressures. The compounds withx = 0 and0.2 undergo a first-order valence transition from the intermediate-valence state to thetrivalent state on increasing either temperature or magnetic field. The magnetizationand susceptibility of these compounds have appreciable anisotropy in both states. Themagnetic phase diagram of Yb1−xYxInCu4determined at ambient pressure is also anisotropic, which is explained bythe crystal-field calculations for the free Yb ion in the high-temperaturephase. Moreover, the low-temperature magnetization process for x = 0.2and 0.3 has been measured in low fields under high pressure; it shows anisotropicferromagnetic ordering.

B–T phase diagram of pure and doped YbInCu 4

We have measured the high-field magnetization of Yb 1− x R x InCu 4 (R=Y, La, Ce) compounds up to 53 T in the temperature range 1.5–100 K using high-quality samples. The compounds undergo a sharp first-order valence transition from the intermediate-valent state to the trivalent state with increasing either temperature or magnetic field. Both the critical temperature and the magnetic field of the transition, T v and B v, increase by La or Ce substitution for Yb and decrease by Y substitution. We found that the relation between B v( T)/ B v(0) and T/ T v is given by [ B v( T)/ B v(0)] n +[ T/ T v] n =1 with 2.5⩽ n⩽3, which is inconsistent with the elliptic relation [ B v( T)/ B v(0)] 2+[ T/ T v] 2=1 reported previously. Possible reasons for the observed contradiction are discussed based on the percolation effect.