First-order Valence Transition Research Articles

Effect of pressure on first-order valence transition of Yb 1− xY xInCu 4

YbInCu 4 exhibits a first-order valence transition at H v ≈33 T with 0.45% decrease of volume as increasing the magnetic field. The inherent chemical pressure in Yb 1− x Y x InCu 4 system is discussed as negative on valence transition of YbInCu 4. The external pressure effect on H v of Yb 1− x Y x InCu 4 is measured as dH v / dP≈−1 T kbar −1 .

高圧下ＮＭＲ Ｃｅ，Ｙｂ系重い電子化合物における圧力誘起秩序相の微視的電子状態

The recent progress of high pressure techniques, e.g. a development of nonmagnetic NiCrAl alloy, has enabled us to create pressure over 3 GPa by piston cylinder type pressure cells. Using these new techniques, the nuclear quadrupolar resonance (NQR) and ac-susceptibility (ac-χ) measurements were carried out on strongly correlated electron systems, CeRhIn5 and YbInCu4 under pressure. CeRhIn5 shows pressure-induced phase transition from antiferromagnetic ordering to superconducting states. The present NQR and ac-χ studies revealed a homogeneous coexistence of these two types of orderings near the phase boundaries. Also the ac-χ and NQR investigations on YbInCu4 showing the first-order valence transition at 42 K and at ambient pressure, indicated that ground state of this compound is a ferromagnetically ordered one after the valence transition is suppressed by pressure.

Effect of pressure and substitution for Yb on the first-order valence transition inYbInCu4

Single-crystalline samples of ${\mathrm{YbInCu}}_{4}$ and its partially substituted compounds by Y and Lu were synthesized by the flux method. The temperature dependence of the magnetic susceptibility from 2 to 320 K in the magnetic field of 0.5 T and the high-field magnetization up to 41 T under various fixed pressures were measured in order to investigate the effect of substitution, inherent chemical pressure, and external pressure on the first-order valence transition of ${\mathrm{YbInCu}}_{4}.$ Substitution of Yb by Y or Lu results in a small increase or decrease of the lattice parameter, while both systems show a steep decrease of transition temperature ${(T}_{v})$ and transition magnetic field ${(H}_{v})$ with substitution, resulting in suppression of the valence transition around $x=0.30$ for ${\mathrm{Yb}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{InCu}}_{4} (0<~x<~0.3)$ and $x=0.15$ for ${\mathrm{Yb}}_{1\ensuremath{-}x}{\mathrm{Lu}}_{x}{\mathrm{InCu}}_{4} (0<~x<~0.15).$ The effect of pressure on ${H}_{v}$ of the pure and Y-substituted ${\mathrm{YbInCu}}_{4}$ is reported by ${\mathrm{dH}}_{v}/dP\ensuremath{\approx}\ensuremath{-}1 \mathrm{T}/\mathrm{k}\mathrm{b}\mathrm{a}\mathrm{r},$ with a suppression of the valence transition of ${\mathrm{Yb}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{InCu}}_{4}$ at the pressure of 8 kbar. The effect of inherent chemical pressure is very small compared with the intrinsic effect of substitution. The decreasing of Kondo temperature ${T}_{K}(\ensuremath{\propto}1/{\ensuremath{\chi}}_{0})$ below ${T}_{v}$ resulting from the substitution for the Yb lattice is discussed as an important factor affecting the valence transition of ${\mathrm{YbInCu}}_{4}.$

Volume effect on the valence transition in Yb 1− xR xInCu 4 (R=Y, La, Ce, Lu) compounds

The effect of alloying on the first-order valence transition in Yb 1− x R x InCu 4 (R=Y, La, Ce, Lu) has been studied by magnetization measurements. It has been found that the transition temperature T v increases by substitution of La and Ce for Yb and decreases for the Y and Lu substitutions. In order to reveal the volume contribution to the change of T v, we measured lattice parameters of Yb 1− x R x InCu 4 at room temperature. The lattice parameter increases with x for R=Ce, La, Y and slightly decreases for R=Lu. Using the data of compressibility and the pressure effect on T v for YbInCu 4 we found that the changes of T v by La and Ce alloying are consistent with the volume change. However, both Y and Lu substitutions for Yb lead to a decrease in T v, inconsistent with the volume change. Possible reasons for the decrease of the Kondo temperature below T v by Y and Lu alloying are discussed.

Crystal-field effects in the first-order valencetransition in YbInCu4 induced by an external magnetic field

As was shown earlier (Dzero M O, Gor'kov L P and Zvezdin A K 2000J. Phys.: Condens. Matter 12 L711), the propertiesof the first-order valence phase transition in YbInCu4 over awide range of magnetic fields and temperatures can be perfectlydescribed on the basis of a simple entropy transition for free Yb ions.Within this approach, the crystal-field effects have been taken intoaccount and we show that the phase diagram in the B-T planeacquires some anisotropy with respect tothe direction of an external magnetic field.

Two energy scales in YbInCu4 from specific heat in high magnetic fields

Metallic YbInCu4 undergoes a first-order valence transition at 42K, where the specific volume increases by 0.5% upon cooling. It is believed that the valence transition is driven by band structure effects that help quench Yb localized moments via a Kondo mechanism. The known phase diagram indicates that the transition can be suppressed with external magnetic fields. We used an adiabatic calorimeter to measure the specific heat in the high-field phase of the material. Data obtained in magnetic fields up to 50T show an enhanced Sommerfeld coefficient and anomalies in the specific heat that are compatible with a much depressed Kondo temperature. Our data indicate quite different energy scales in the low-field and high-field phases of YbInCu4.

Reply to “Comment on ‘Photoemission experiments onYbInCu4:Surface effects and temperature dependence’ ”

The stoichiometric rare-earth compound ${\mathrm{YbInCu}}_{4}$ is known for its first-order valence transition at ${T}_{a}\ensuremath{\approx}55\mathrm{K}$ and ambient pressure. Although this valence transition has been extensively studied by various volume-sensitive experimental methods, highly surface-sensitive photoemission measurements in the vuv energy range [ultraviolet photoemission spectroscopy (UPS)] could not observe a sharp valence change of the Yb valence ${v}_{\mathrm{Yb}}$ at the transition temperature. Instead there is only a smooth temperature dependence of ${v}_{\mathrm{Yb}}$ over a large temperature range. Furthermore, the low-temperature values for the hole-occupation number ${n}_{h}{=v}_{\mathrm{Yb}}\ensuremath{-}2$ from UPS are significantly below the values of volume-sensitive measurements. Here we present temperature-dependent photoemission data on ${\mathrm{YbInCu}}_{4}$ measured at photon energies of $h\ensuremath{\nu}=1486\mathrm{eV}$ [x-ray photoemission spectroscopy (XPS)] with a significantly increased information depth in comparison to previous experiments with lower photon energies. These data clearly show the existence of a valence transition at the extended sampling depth of the XPS experiment, in contrast to results from the distorted near-surface region.

A study of the first-order valence transition in single crystals by magnetic susceptibility measurements

The first-order valence transition in has been investigated for single crystalline samples with x = 0.32 and x = 0.35 as well as for polycrystalline samples with (x varied in steps of 0.01) through the measurements of magnetic susceptibility, . For the single crystal with x = 0.32, exhibits a strong anisotropic mixed-valence behaviour. On the other hand, of the crystal with x = 0.35 exhibits a sharp first-order valence transition at 50 K. In both crystals, the strong anisotropy, , is maintained as in the host compound CeNiSn. The arc-melted polycrystalline alloys with a narrow range of Co composition, x = 0.33-0.37, show a sharp drop in below 70 K and large hysteresis. This transition becomes sharper by annealing at . For the optimum concentration x = 0.35, the drop in at the transition becomes as high as 72% for the single crystal and 69% for the polycrystalline alloy for which a jump in ac heat capacity was observed at 55 K. Analysis of the above results reveals that the phase transition involves a large change in the Kondo temperature . The small volume discontinuity at the transition suggests that the phase transition is not due to a Kondo volume collapse. A possible mechanism responsible for the phase transition in the present alloys is proposed.

Pressure effect on the valence transition ofEuNi2(Ge1−xSix)2

The temperature dependence of electrical resistivity $\ensuremath{\rho}$ has been measured for ${\mathrm{EuNi}}_{2}({\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}{)}_{2}$ with $0.0l~xl~0.15$ at pressures to 15 kbar. At ambient pressure, all the compounds are antiferromagnetic with stable ${\mathrm{Eu}}^{2+}$ moments, but compounds with $0.10l~xl~0.15$ show a pressure-induced valence transition. Just above a critical pressure, the compound undergoes a first-order valence transition. With further increase in pressure, the valence transition becomes continuous. The temperature dependence of $\ensuremath{\rho}$ is discussed by regarding the system as a virtual alloy of ${\mathrm{Eu}}_{{p}_{2}}^{2+}{\mathrm{Eu}}_{{p}_{3}}^{3+},$ where ${p}_{2}$ and ${p}_{3}$ are the occupation probabilities of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ states, respectively. We argue that impurity scattering due to randomness in a virtual alloy is responsible for the temperature dependence of $\ensuremath{\rho}$ in this system. By combining the present results with previous determinations of the pressure dependence of the tetragonal unit-cell volume, a generalized pressure-temperature phase diagram of ${\mathrm{EuNi}}_{2}{\mathrm{Ge}}_{2}$ is established which indicates that applying pressure is equivalent to Si doping in ${\mathrm{EuNi}}_{2}{\mathrm{Ge}}_{2}.$

Photoemission experiments onYbInCu4: Surface effects and temperature dependence

We present temperature-dependent photoemission (PES) measurements on single-crystalline YbInCu{sub 4}, a stoichiometric material that shows an isostructural first-order valence transition at T{sub a}{approx}60 K. The results of the analysis of the PES data show peculiar differences to other, bulk-sensitive measurements: (a) a lack of the sharp first-order valence transition and (b) the Yb valence {emdash} determined directly from the 4f intensity ratio {emdash} lies substantially below x-ray absorption (XAS) results over the whole investigated temperature range. We discuss these discrepancies with particular attention to surface effects and conclude that the PES results are due to a distorted near-surface region which differs in its physical properties significantly from the volume material. {copyright} {ital 1998} {ital The American Physical Society}

Copper and indium Knight shifts above and below first-order valence transition in

and Knight shifts were measured for both above and below the first-order valence transition temperature . The hyperfine coupling constants of , which are small, of the order of the classical lattice dipole field, increase markedly below , while that of exhibits an only slight change at , suggesting different roles of the ligands in the valence transition.

Thermodynamics of the first-order valence transition inYbInCu4

We present measurements of the specific heat and complete elastic moduli of ${\mathrm{YbInCu}}_{4}$ as a function of temperature in order to complement existing thermal expansion data. Together these data allow a complete analysis of the thermodynamics of the first-order isostructural valence transition in ${\mathrm{YbInCu}}_{4}.$ The Clausius-Clapeyron equation predicts well the measured pressure dependence of the valence transition temperature. Estimates of the Gruneisen parameter from thermodynamic measurements and from pressure-dependent magnetic susceptibility in both the high-temperature and low-temperature phases of ${\mathrm{YbInCu}}_{4}$ are in good agreement. On the other hand, a Gruneisen analysis of the change in Kondo temperature at the valence transition fails, emphasizing the importance of carrier-density changes associated with the valence transition, unlike in the case of the \ensuremath{\gamma}-\ensuremath{\alpha} transition in elemental Ce. Finally, we address the issue of precursive rounding in the elastic moduli for temperatures greater than the valence transition temperature and argue that thermodynamically ${\mathrm{YbInCu}}_{4}$'s valence transition is indeed first order.

Field-induced valence transition in EuNi 2(Si 1− xGe x) 2

Magnetic susceptibility and high-field magnetization have been examined for the intermediate valence system EuNi 2(Si 1− x Ge x ) 2. It was found that the compounds with 0.79 ⩽ × ⩽ 0.82 show a first-order valence transition under high magnetic field. The results are discussed in terms of the interconfigurational fluctuation (ICF) model.

Temperature- and field-induced valence transitions of

The magnetic properties of the system were studied in which an intermediate-valence state of Eu is realized at around x = 0.75. It was found that the compounds with show a temperature-induced valence transition below room temperature, while those with are antiferromagnetic with a stable state. A first-order valence transition induced by high magnetic field was observed for . Such valence transitions against temperature or field are discussed on the basis of the interconfigurational fluctuation (ICF) model. It is shown that the observed linear relation between the transition field and the transition temperature is explained by the present model.

Crossover from First-Order Valence Transition to Kondo Behavior in C15b-Type YbCu5-xInxSystem

YbCu 5 and YbCu 4 In forms a solid-solution (YbCu 5- x In x ) with cubic AuBe 5 -type (C15b) structure in the In-concentration range 0.1≦ x ≦1; and below x =0.1, it coexists with a hexagonal CaCu 5 -type YbCu 5 phase under ambient pressure. The magnetic susceptibility and high-field magnetization measurements reveal that the first-order valence transition in YbCu 4 In shifts to higher temperature and becomes broadened significantly with decreasing In-content, and simultaneously the delocalized mixed-valent state of Yb ion ( x =1) shows a continuous change toward the localized trivalent state ( x =0). The electrical resistivities of YbCu 5- x In x display various temperature-dependencies: ranging from a pronounced drop at x =1, via a minimum behavior for 0.4≤ x ≤0.7, to a negative logarithmic increasing behavior (-ln T ) for x ≤0.3. Above results suggest that YbCu 5- x In x goes through a crossover from a valence fluctuation system to a dence Kondo one with decreasing x . At x =0, the formation of Kondo la...

Magnetic-field, pressure, and temperature scaling of the first-order valence transition in pure and doped YbInCu4

We report measurements of the high-field (H{le}30 T) magnetoresistance R(H,T) of pure and doped YbInCu{sub 4}. A hysteretic transition is observed which we take to be the field-induced variant of the first-order valence transition observed at 42 K in YbInCu{sub 4} for H=0 T. Applied pressure suppresses the valence transition to lower temperature. By properly scaling the (H{sub v},T{sub v}) data extracted from the resistance measurements, we can collapse all of the pressure-dependent data, as well as that from doped variants of YbInCu{sub 4} at ambient pressure, onto a universal H-T phase diagram. This suggests that a single energy scale is associated with the valence transition. {copyright} {ital 1997} {ital The American Physical Society}

Field-induced valence transition of Eu(Pd1−xPtx)2Si2

The magnetic susceptibility and high-field magnetization have been measured for the intermediate valence system Eu(${\mathrm{Pd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Pt}}_{\mathrm{x}}$${)}_{2}$${\mathrm{Si}}_{2}$ with 0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.15. A first-order valence transition is observed for all the compounds under high field of 100 T at low temperatures. This valence transition is of first order accompanied with a large hysteresis, which is in contrast to a continuous valence change against temperature. Based on the interconfigurational fluctuation (ICF) model, the temperature- and field-induced valence transitions are discussed. It is found that a first-order valence transition can be induced by magnetic field, even if the system shows a continuous valence transition against temperature. Metamagnetic behavior at finite temperatures is also understood qualitatively by the ICF model.

The electronic structure of YbCu 4In as observed through optical measurements

The intermetallic compound YbCu 4In is an almost unique (except for metallic cerium) material exhibiting a first-order temperature-induced valence transition. At about 40–60 K all the physical properties show a sudden change, whereas no distortion of the crystal structure occurs. The optical response does not make an exception to this trend and an abrupt change in the optical functions at low temperatures is presented here. The spectra are discussed by taking into account some recent band structure calculations.

First-Order Valence Transition of EuPd2Si2Induced by High Magnetic Fields

In order to study the magnetic field effects on the intermediate valence of the Eu system, magnetization was measured for EuPd 2 Si 2 and Eu(Pd 0.95 Pt 0.05 ) 2 Si 2 up to 100 T. A first-order metamagnetic transition was observed for both samples under high magnetic fields. The transition field is 93T for EuPd 2 Si 2 at 6 K. The saturation magnetization close to 7µ B indicates that this metamagnetism is a field-induced valence transition from an intermediate valence state to a divalent state of the Eu ion. Based on the interconfigurational fluctuation (ICF) model, the excitation energy to convert an Eu 3+ ion into Eu 2+ was estimated. Temperature dependence of the metamagnetic transition field of EuPd 2 Si 2 was also studied.

Evolution from first-order valence transition to heavy-fermion behavior in YbIn1-xAgxCu4.

${\mathrm{YbInCu}}_{4}$ undergoes a first-order isostructural valence transition near 40 K, whereas ${\mathrm{YbAgCu}}_{4}$ is a moderately heavy (\ensuremath{\gamma}=250 mJ/mol ${\mathrm{K}}^{2}$) mixed-valence compound. We have succeeded in growing single crystals of these compounds, as well as many intermediate alloys, using flux-growth techniques. The evolution from ${\mathrm{YbInCu}}_{4}$ to ${\mathrm{YbAgCu}}_{4}$ has been characterized using electrical resistivity, magnetic susceptibility, and x-ray powder diffraction. The data are interpreted in terms of the evolution of the single-impurity Kondo temperature as a function of Ag concentration. \textcopyright{} 1996 The American Physical Society.