Valence Fluctuations Research Articles

Investigation of Superconductivity in Ce-Doped (La,Pr)OBiS2 Single Crystals.

Single crystals of Ce-doped (La,Pr)OBiS2 superconductors, the multinary rare-earth elements substituted ROBiS2, were successfully grown. The grown crystals typically had a size of 1–2 mm and a plate-like shape with a well-developed c-plane. The c-axis lattice constants of the obtained (La,Ce,Pr)OBiS2 single crystals were approximately 13.6–13.7 Å, and the superconducting transition temperature was 1.23–2.18 K. Valence fluctuations of Ce and Pr were detected through X-ray absorption spectroscopy analysis. In contrast to (Ce,Pr)OBiS2 and (La,Ce)OBiS2, the superconducting transition temperature of (La,Ce,Pr)OBiS2 increased with the increasing concentrations of the tetravalent state at the R-site.

Anisotropy-driven quantum criticality in an intermediate valence system

Intermetallic compounds containing f-electron elements have been prototypical materials for investigating strong electron correlations and quantum criticality (QC). Their heavy fermion ground state evoked by the magnetic f-electrons is susceptible to the onset of quantum phases, such as magnetism or superconductivity, due to the enhanced effective mass (m*) and a corresponding decrease of the Fermi temperature. However, the presence of f-electron valence fluctuations to a non-magnetic state is regarded an anathema to QC, as it usually generates a paramagnetic Fermi-liquid state with quasiparticles of moderate m*. Such systems are typically isotropic, with a characteristic energy scale T0 of the order of hundreds of kelvins that require large magnetic fields or pressures to promote a valence or magnetic instability. Here we show the discovery of a quantum critical behaviour and a Lifshitz transition under low magnetic field in an intermediate valence compound α-YbAlB4. The QC origin is attributed to the anisotropic hybridization between the conduction and localized f-electrons. These findings suggest a new route to bypass the large valence energy scale in developing the QC.

Fermi surface of a system with strong valence fluctuations: Evidence for a noninteger count of valence electrons in EuIr2Si2

We present de Haas-van Alphen (dHvA) measurements on an Eu-based valence-fluctuating system. ${\mathrm{EuIr}}_{2}{\mathrm{Si}}_{2}$ exhibits a temperature-dependent, noninteger europium valence with ${\mathrm{Eu}}^{2.8+}$ at low temperatures. The comparison of experimental results from our magnetic-torque experiments in fields up to 32 T and density functional theory band-structure calculations with localized $4f$ electrons shows that the best agreement is reached for a Fermi surface based on a valence of ${\mathrm{Eu}}^{2.8+}$. The calculated quantum-oscillation frequencies for ${\mathrm{Eu}}^{3+}$ instead cannot explain all the experimentally observed frequencies. The effective masses, derived from the temperature dependence of the dHvA oscillation amplitudes, show not only a significant enhancement with masses up to $11\phantom{\rule{3.33333pt}{0ex}}{m}_{e}$ (${m}_{e}$ being the free electron mass), but also a magnetic-field dependence of the heaviest mass. We attribute the formation of these heavy masses to strong correlation effects resulting from valence fluctuations of $4f$ electrons being strongly hybridized with conduction electrons. The increase of the heavy masses with magnetic field likely results from the onset of the expected field-induced valence crossover that enhances these valence fluctuations but does not alter the Fermi-surface topology in the field range studied.

Electronic structure and magnetism of the Hund's insulator CrI3

${\mathrm{CrI}}_{3}$ is a two-dimensional ferromagnetic (FM) van der Waals material with a charge gap of 1.1--1.2 eV. In this paper, the electronic structure and magnetism of ${\mathrm{CrI}}_{3}$ are investigated by using density functional theory and dynamical mean-field theory. Our calculations successfully reproduce a charge gap of $\ensuremath{\sim}1.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in the paramagnetic (PM) state when a Hund's coupling ${J}_{\mathrm{H}}=0.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is included with an onsite Hubbard $U=5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. In contrast, with a large $U$ value of 8 eV and negligible Hund's coupling ${J}_{\mathrm{H}}, {\mathrm{CrI}}_{3}$ is predicted to be a moderately correlated metal in the PM state. We conclude that ${\mathrm{CrI}}_{3}$ is a Mott-Hund's insulator due to the half-filled configuration of the Cr $3d {t}_{2g}$ orbitals. The Cr $3d {e}_{g}$ orbitals are occupied by $\ensuremath{\sim}1$ electron, which leads to strong valence fluctuations so that the Cr $3d$ orbitals cannot be described by a single state. Moreover, at finite temperature, the calculated ordered static magnetic moment in the FM state is significantly larger in the $R\overline{3}$ phase than in the $C2/m$ phase. This observation indicates that the structural phase transition from the $C2/m$ phase to the $R\overline{3}$ phase with decreasing temperature is driven by FM spin fluctuations.

Pressure-induced changes of valence fluctuation in β−YbAlB4 probed by x-ray absorption spectroscopy

We report detailed valence fluctuation phenomena in the unique quantum-critical compound $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$ under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature ($\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$. Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

Pressure-Induced Collapse of Valence Orderand Emergence of Quantum Valence Fluctuation in YbPd

Pressure-Induced Collapse of Valence Orderand Emergence of Quantum Valence Fluctuation in YbPd

High-Pressure Synthesis of Light Lanthanide Dodecaborides (PrB12 and CeB12): Effects of Valence Fluctuation on Volume and Formation Pressure.

Light lanthanide dodecaborides, RB12 (R = Pr and Ce), were synthesized from a stoichiometric mixture of hexaborides and boron using a laser-heated diamond anvil cell under high-pressure and high-temperature conditions. Contrary to the expectation that lighter lanthanide elements require higher pressure to crystallize RB12, in situ X-ray diffraction experiments reveal that cerium dodecaboride crystallizes at 26 GPa, which is significantly lower than that required to form the heavier praseodymium dodecaboride (35 GPa). In addition to the lower formation pressure, an anomalous volume reduction is also observed in CeB12, which can be explained by a valence fluctuation between Ce3+ and Ce4+ indicated by X-ray absorption near-edge structure measurements. A polyhedral coordination change from a truncated cube in RB6 to a truncated octahedron in RB12 and associated shortening of the R-B bond length result in an increase in bulk modulus and hardness.

Intrinsic Coercivity in 4f-3d Intermetallic Magnets with Valence Fluctuations

Intrinsic Coercivity in 4f-3d Intermetallic Magnets with Valence Fluctuations

Yb Valence Fluctuations and Kondo-Like Behavior in the Yb1−Xscxco2zn20 System

Yb Valence Fluctuations and Kondo-Like Behavior in the Yb1−Xscxco2zn20 System

Origin of Metamagnetic Transition (MMT) in the Spin-Triplet Superconductivity in UTe2

State-of-the-art magnetostriction measurements in a pulsed-magnetic field reveal the origin of a metamagnetic transition of spin-triplet superconductor UTe2. We propose that the uranium valence fluctuation plays a crucial role in its metamagnetic and superconducting transitions.

Nonlinear Fermi liquid transport through a quantum dot in asymmetric tunnel junctions

We study the nonlinear conductance through a quantum dot, specifically its dependence on the asymmetries in the tunnel couplings and bias voltages $V$, at low energies. Extending the microscopic Fermi liquid theory for the Anderson impurity model, we obtain an exact formula for the steady current $I$ up to terms of order ${V}^{3}$ in the presence of these asymmetries. The coefficients for the nonlinear terms are described in terms of a set of the Fermi liquid parameters: the phase shift, static susceptibilities, and three-body correlation functions of electrons in the quantum dots, defined with respect to the equilibrium ground state. We calculate these correlation functions, using the numerical renormalization group approach (NRG), over a wide range of impurity-electron filling that can be controlled by a gate voltage in real systems. The NRG results show that the order ${V}^{2}$ nonlinear current is enhanced significantly in the valence fluctuation regime. It is caused by the order $V$ energy shift of the impurity level, induced in the presence of the tunneling or bias asymmetry. Furthermore, in the valence fluctuation regime, we also find that the order ${V}^{3}$ nonlinear current exhibits a shoulder structure, for which the three-body correlations that evolve for large asymmetries play an essential role.

Theoretical study of intermediate valence fluctuation in thulium selenide and electrical properties of TmSe1−xTex

The study of valence fluctuation is made on TmSe compound from simulated theoretical x-ray powder diffraction data. The theoretical lattice parameter is in agreement with experimental parameter. The amount of charge transfer is deduced as 0.23ē which proves the presence of intermediate valence fluctuation in the system. Further a theoretical Study is made on TmSe1−xTex (x = 0.5, 0.77, 0.83 and 1.00) to deduce electrical transport properties by using only two parameters, lattice constant and energy gap. The calculated transport properties electrical resistivity, carrier mobility, carrier concentration, carrier effective mass and dielectric constant are compared with the available experimental data which agrees with each other.

Valence fluctuations and Kondo resonance in Co adatom on Cu2N/Cu(100) surface: DFT + ED study

We report density functional theory plus exact diagonalization of the multi-orbital Anderson impurity model calculations including the spin–orbit coupling for the Co adatom on the top of Cu2N/Cu(001) surface. It is found that experimentally observed zero-bias peak in differential conductance can occur for quasi-degenerate many-body solution representing a mixed valence state of the Co adatom with the non-integer d manifold occupation of 7.4. Estimated 5 meV inelastic step energy is in a quantitative agreement with the experimental scanning tunnelling spectroscopy value of 6 meV. We find the non-zero spin and orbital magnetic moments in the applied magnetic field, in a reasonable agreement with the x-ray magnetic circular dichroism data. The presence of large orbital magnetic moment was not considered in previous spin-only theoretical interpretations, and can unravel complexity of the physics behind the experimental spectra for Co and other adatoms on Cu2N/Cu(001) surface.

Mixed Valence and Superconductivity in Perovskite Antimonates

Herein, we present results of the 121Sb Mössbauer spectroscopy in perovskite antimonates Ba1–xKxSbO3, sibling compounds of the well-known high-Tc superconductors Ba1–xKxBiO3. Two Sb valence states SbIII and SbV, forming a charge-density-wave (CDW) order, are unambiguously revealed in un- and under-doped phases at x = 0–0.5. As the CDW order is suppressed at x = 0.65, the compound becomes superconducting and all of the Sb sites become equivalent with the intermediate valence of +4.65, consistent with that anticipated from the potassium content. These results provide the direct evidence of metal s electrons participating in the formation of the CDW order and superconductivity in the material, and thus they have important implications for clarifying the underlying mechanism related to mixed valence and potential valence fluctuations of metal ions. The use of Mössbauer active nuclei offers a deeper insight into the structural properties and superconductivity in mixed-valence main-group oxides.

Excellent thermal stability and enhanced piezoelectric performance of Bi(Ni2/3Nb1/3)O3‐modified BiFeO3–BaTiO3 ceramics

AbstractLead‐free piezoelectric ceramics with excellent piezoelectric performance and temperature stability are vital to “stack up” against PZT in various applications. Thus, xBiFeO3‐(1‐x)BaTiO3(BF‐BT)‐based ceramics with high Curie temperatures have drawn increasing attention. However, BF‐BT based ceramics often show large leakage current due to the valence fluctuation of Fe ion. In this work, it is found that Bi(Ni2/3Nb1/3)O3 (BNN) can suppress the valence fluctuation of Fe ions effectively. The effects of BNN on the crystal structure, morphology, and electrical properties of 0.69BF‐0.31BT ceramics are investigated. For composition near morphotropic phase boundary of rhombohedral and tetragonal phases, the Curie temperature (Tc) is as high as 453°C, and the piezoelectric constant (d33), planar electromechanical coupling factor kp, and electric‐field‐induced strain ΔS can reach up to 191 pC/N, 31% and 0.236% (peak to peak), respectively. Moreover, these ceramics show excellent thermal stability. The d33 of 100 pC/N is still maintained at about 360°C and an ultrahigh strain (ΔS = 0.684%, peak to peak strain) is obtained at 190°C. Our work illustrates that the BF‐BT based ceramics can serve as potential candidates for sensor and actuator applications over a broad temperature range.

Coexistence of Charge Order and Antiferromagnetic Order in an Extended Periodic Anderson ModelSupported by the National Natural Science Foundation of China (Grant Nos. 11974048 and 11974049), and the Beijing Science Foundation (Grant No. 1192011).

The competition between the RKKY interaction and the Kondo effect leads to a magnetic phase transition, which occurs ubiquitously in heavy fermion materials. However, there are more and more experimental evidences indicating that the valence fluctuation plays an essential role in the Ce- and Y-based compounds. We study an extended periodic Anderson model (EPAM) which includes the onsite Coulomb repulsion Ucf between the localized electrons and conduction electrons. By employing the density matrix embedding theory, we investigate the EPAM in the symmetric case at half filling. By fixing the onsite Coulomb repulsion U of the localized electrons to an intermediate value, the interplay between the RKKY interaction, the Kondo effect and the Coulomb repulsion Ucf brings rich physics. We find three different phases, the antiferromagnetic phase, the charge order phase and paramagnetic phase. When the hybridization strength V between the localized orbital and the conduction orbital is small, the Kondo effect is weak so that the AF phase and the CO phase are present. The phase transition between the two long-range ordered phase is of first order. We also find a coexistence region between the two phases. As V increases, the Kondo effect becomes stronger, and the paramagnetic phase appears between the other two phases.

Structural, magnetic and Mössbauer studies of Nd 0.6Sr0.4FexMn1-xO3 perovskites

The effect of iron doping on the structural and magnetic properties of the manganite perovskites Nd0.6Sr0.4FexMn1-xO3 (0.1 ≤ x ≤ 1.0) was studied. Auto-combustion sol-gel method was used to prepare the compounds. X-ray diffraction refinement indicated that all the compounds crystallized in orthorhombic single-phase with Pnma space group. Scanning electron microscopy micrographs revealed a group of packed grains of about 0.2–1.0 μm whereas the average particle size was found to be in the range of about 40–80 nm via transmission electron microscopy. The zero-field cooled and field cooled magnetization curves, under an applied magnetic field of 0.02 T, revealed that the samples with 0.1 ≤ x ≤ 0.4 exhibit a paramagnetic to ferromagnetic transition with decreasing temperature while the samples with (0.5 ≤ x ≤ 1.0) do not demonstrate such phase transition. At room temperature the dependence of magnetization on the x iron concentration, as revealed by the corresponding M − H curves, can be interpreted as the cumulative result of changes in superparamagnetic, paramagnetic and antiferromagnetic contributions. 57Fe transmission Mӧssbauer spectra were recorded at room-temperature (295 K) and liquid nitrogen temperature (78 K) for all the samples. Iron ions were found to be present in these compounds exclusively in a paramagnetic high-spin Fe3+ state for 0.1 ≤ x ≤ 0.5 at 295 K and for 0.1 ≤ x ≤ 0.3 at 78 K. At higher iron concentrations an antiferromagnetic state of iron clusters is formed due to the Fe3+ – Fe3+ superexchange interaction. Room temperature 57Fe quadrupole splitting was found to increase near linearly with the x iron concentration up to x = 0.6. For x ≥ 0.6 the formation of paramagnetic Fe4+ is observed at 295 K, which at lower temperatures undergoes charge disproportionation leading to the appearance of Fe5+ species as observed at 78 K. Perturbations of the magnetic state of iron ions was found to lead to substantial broadening of the Mössbauer absorption peaks of magnetic components, which is interpreted as the effect of nearest neighbor Fe5+ and Mn ions at 78 K, and as that of electron-hopping related Fe3+ - Fe4+ valence fluctuations at 295 K.

Valence Fluctuations in Yb(Al,Fe)B<sub>4</sub> Studied by Nanosecond-time-resolved Photoemission Spectroscopy Using Synchrotron Radiation

Valence Fluctuations in Yb(Al,Fe)B<sub>4</sub> Studied by Nanosecond-time-resolved Photoemission Spectroscopy Using Synchrotron Radiation

Field-induced valence fluctuations in YbB12

We performed high-magnetic-field ultrasonic experiments on YbB$_{12}$ up to 59 T to investigate the valence fluctuations in Yb ions. In zero field, the longitudinal elastic constant $C_{11}$, the transverse elastic constants $C_{44}$ and $\left( C_{11} - C_{12} \right)/2$, and the bulk modulus $C_\mathrm{B}$ show a hardening with a change of curvature at around 35 K indicating a small contribution of valence fluctuations to the elastic constants. When high magnetic fields are applied at low temperatures, $C_\mathrm{B}$ exhibits a softening above a field-induced insulator-metal transition signaling field-induced valence fluctuations. Furthermore, at elevated temperatures, the field-induced softening of $C_\mathrm{B}$ takes place at even lower fields and $C_\mathrm{B}$ decreases continuously with field. Our analysis using the multipole susceptibility based on a two-band model reveals that the softening of $C_\mathrm{B}$ originates from the enhancement of multipole-strain interaction in addition to the decrease of the insulator energy gap. This analysis indicates that field-induced valence fluctuations of Yb cause the instability of the bulk modulus $C_\mathrm{B}$.

Metal matrix composites based on valence-unstable systems

The work is focused on the thermal expansion of composites based on valence-unstable systems. The main motivation was to create invar-like materials of a new type and to develop approaches to the quantitative description of coefficients of thermal expansion (CTE) of both the composite itself and its key component - a material with a negative contribution to thermal expansion due to valence fluctuations. The experimental data on the linear CTE of Al/SmB6 composite was performed by capacitive dilatometry in the temperature range 5-220K. It turned out that this composite shows invar properties up to 60K. Several models were used to describe thermal expansion in Al/SmB6. A quantitative approach is proposed for describing the negative magnetic contribution and the lattice contribution to the CTE of systems based on intermediate-valence samarium hexaboride.