Valence Susceptibility Research Articles

Magnetic-field-tuned valence susceptibility in CeCo1−x Fe x Si revealed by magnetostriction

We report magnetoelastic studies in the CeCo1−x Fe x Si alloys. Iron doping has a profound effect on the low temperature linear magnetostriction. At T = 2 K, the strength of the magnetostriction peaks at x = 0.23 where it reaches a value as large as in an applied magnetic field B = 16 T. This Fe concentration corresponds to the critical one xc above which long-range antiferromagnetic order is no longer observed. The progressive increment of the hibridization between the Ce 4f orbital and the conduction band, as the magnetic order vanishes, gives rise to a sizeable valence susceptibility that can be finely tuned by the magnetic field explaining the large magnetostrictive effect around xc . At higher x, the magnetostriction decreases, in agreement with a weaker valence susceptibility resulting from a stronger hibridization.

Strong magnetoelastic effect in CeCo1−xFexSi as Néel order is suppressed

A very strong magnetoelastic effect in the CeCo$_{1-x}$Fe$_{x}$Si alloys is reported. The strength of the magnetostrictive effect can be tuned upon changing $x$. The moderate low-temperature linear magnetostriction observed at low Fe concentrations becomes very large ($\frac {\Delta L}{L} \left(16 T,2 K\right) =$ 3$\times$10$^{-3}$) around the critical concentration ($x_c \approx$ 0.23) at which the long-range antiferromagnetic order vanishes. Upon increasing doping through the non-magnetic region ($x > x_c$), the magnetostriction strength gradually weakens again. Remarkably the low-temperature magnetostriction at the critical concentration shows a pronounced $S$-like shape (centered at $B_m \sim$ 6 T) resembling other well-known Ce-based metamagnetic systems like CeRu$_2$Si$_2$ and CeTiGe. Unlike what is observed in these compounds, however, the field dependence of the magnetization shows only a minor upturn around $B_m$ vaguely resembling a metamagnetic behavior. The subtle interplay between magnetic order and the Kondo screening seems to originate an enhanced valence susceptibility slightly changing the Ce ions valence, ultimately triggering the large magnetostriction observed around the critical concentration.

Origin of Quantum Criticality in Yb–Al–Au Approximant Crystal and Quasicrystal

To get insight into the mechanism of emergence of unconventional quantum criticality observed in quasicrystal Yb$_{15}$Al$_{34}$Au$_{51}$, the approximant crystal Yb$_{14}$Al$_{35}$Au$_{51}$ is analyzed theoretically. By constructing a minimal model for the approximant crystal, the heavy quasiparticle band is shown to emerge near the Fermi level because of strong correlation of 4f electrons at Yb. We find that charge-transfer mode between 4f electron at Yb on the 3rd shell and 3p electron at Al on the 4th shell in Tsai-type cluster is considerably enhanced with almost flat momentum dependence. The mode-coupling theory shows that magnetic as well as valence susceptibility exhibits $\chi\sim T^{-0.5}$ for zero-field limit and is expressed as a single scaling function of the ratio of temperature to magnetic field $T/B$ over four decades even in the approximant crystal when some condition is satisfied by varying parameters, e.g., by applying pressure. The key origin is clarified to be due to the strong locality of the critical Yb-valence fluctuation and small Brillouin zone reflecting the large unit cell, giving rise to the extremely-small characteristic energy scale. This also gives a natural explanation for the quantum criticality in the quasicrystal corresponding to the infinite limit of the unit-cell size.

Quantum criticality and first-order transitions in the extended periodic Anderson model

We investigate the behavior of the periodic Anderson model in the presence of $d$--$f$ Coulomb interaction (${U}_{df}$) using mean-field theory, variational calculation, and exact diagonalization of finite chains. The variational approach based on the Gutzwiller trial wave function gives a critical value of ${U}_{df}$ and two quantum critical points (QCPs), where the valence susceptibility diverges. We derive the critical exponent for the valence susceptibility and investigate how the position of the QCP depends on the other parameters of the Hamiltonian. For larger values of ${U}_{df}$, the Kondo regime is bounded by two first-order transitions. These first-order transitions merge into a triple point at a certain value of ${U}_{df}$. For even larger ${U}_{df}$ valence skipping occurs. Although the other methods do not give a critical point, they support this scenario.

Impurity Effect on Superconductivity in Critical-Valence-Fluctuations Region

Effects of non-magnetic impurities on the transition temperature of superconductivity in the critical-valence-fluctuations region are discussed. The impurity potential renormalized by the critical valence fluctuations is taken as a Yukawa type where the screening length is proportional to the square root of the valence susceptibility diverging toward the critical point of valence transition. The scatterings are considered as elastic and treated within the t -matrix approximation. The calculation to obtain the transition temperature of superconductivity is performed in parallel to the method for obtaining the Abrikosov–Gor'kov formula, but with a full account of wave-number dependence of the t -matrix in the intermediate states of pairing susceptibility. We find that the anisotropic ( p -wave and d -wave) superconductivity becomes more robust against impurities with long range potential than those with short range potential to which the conventional Abrikosov–Gor'kov formula can be applied. In addition, we...

Quantum valence criticality and superconductivity

The nature of the quantum valence transition is studied on the basis of the periodic Anderson model with Coulomb repulsion between f and conduction electrons. The density matrix renormalization group calculation for the ground state shows that the first-order valence transition emerges with the quantum critical point with diverging valence susceptibility. Instead of the phase separation in the mean-field result, quantum fluctuations generate a wide region of crossover between the Kondo and mixed valence states. It is found that the superconducting correlation is developed in the Kondo regime near the quantum critical point of the valence transition. The origin is ascribed to the enhanced coherent motion of electrons with valence fluctuation. Remarks on the valence transition are given in connection with Ce compounds and Ce metal.

Effects of crystalline-electric-field on valence fluctuations in an extended Anderson model

We investigate a two-orbital impurity Anderson model (where two orbitals correspond to the two crystalline-electric-field (CEF) doublets) with Coulomb repulsions U fc between the f- and conduction electrons. Using the Wilson numerical renormalization group method, we calculate the f-electron valence susceptibility. As the U fc increases, the character of dominant valence fluctuation changes from off-diagonal ( f † c + c † f ) to diagonal ( f † f - c † c ) one, where f † and c † stands for the creation operator of f- and conduction electrons, respectively. In the valence-fluctuation regime, we find that the CEF splitting is smeared out and the effective degeneracy becomes 4-fold, leading to the strong valence fluctuations.

Sign change of Hall coefficients for amorphous Ni0.80-xCrxP0.20 alloys.

The magnetic susceptibilities, resistivities, and Hall coefficients of amorphous ${\mathrm{Ni}}_{0.80\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cr}}_{\mathit{x}}$${\mathrm{P}}_{0.20}$ (0.00\ensuremath{\le}x\ensuremath{\le}0.40) alloys are reported. A sign change with temperature of the Hall coefficient for a single paramagnetic amorphous alloy that correlates with the temperature-dependent valence susceptibility has been observed (x=0.20), indicating that an extraordinary contribution to ${\mathit{R}}_{\mathit{H}}$ is responsible for the positive Hall effect in these paramagnetic alloys. Also, an unexplained relatively large contribution to ${\mathit{R}}_{\mathit{H}}$ that is linear in T has been observed in some of these alloys. A gradual change in magnetic properties with composition from large cluster-dominant phase to Kondo-impurity phase has been observed.

Positive Hall effect in paramagnetic amorphous Zr-Fe.

We present measurements and analysis of the Hall effect in amorphous paramagnetic Zr-Fe. From the correlation between the Hall coefficient and the valence susceptibility we show that the Hall effect is dominated by an anomalous contribution due to the side-jump effect, the first time such a contribution has been clearly identified in an amorphous paramagnetic system. We propose that the anomalous contribution accounts for the positive value of the Hall coefficient seen in many amorphous alloys.

Electron Screening and Mobility in Heavily Doped Silicon

AbstractThe screening effect is studied in a heavily doped n‐type silicon. The Lindhard susceptibility is used for the conduction electrons. The valence susceptibility is treated as a constant parameter. The mobility of electrons is calculated using the Born approximation and by considering the dependence of the electron effective mass on dopant concentration determined empirically. The results compare favourably with recent experiments.