Valence Fluctuations Research Articles

Numerical subgap spectroscopy of double quantum dots coupled to superconductors

Double quantum dot nanostructures embedded between two superconducting leads or in a superconducting ring have complex excitation spectra inside the gap which reveal the competition between different many-body phenomena. We study the corresponding two-impurity Anderson model using the non-perturbative numerical renormalization group (NRG) technique and identify the characteristic features in the spectral function in various parameter regimes. At half-filling, the system always has a singlet ground state. For large hybridization, we observe an inversion of excited inter-dot triplet and singlet states due to the level-repulsion between two sub-gap singlet states. The Shiba doublet states split in two cases: a) at non-zero superconducting phase difference and b) away from half-filling. The most complex structure of sub-gap states is found when one or both dots are in the valence fluctuation regime. Doublet splitting can lead to a parity-changing quantum phase transition to a doublet ground state in some circumstances. In such cases, we observe very different spectral weights for the transitions to singlet or triplet excited Shiba states: the triplet state is best visible on the valence-fluctuating dot, while the singlets are more pronounced on the half-filled dot.

Kondo interactions from band reconstruction in YbInCu(4).

We combine resonant inelastic x-ray scattering and model calculations in the Kondo lattice compound YbInCu_{4}, a system characterized by a dramatic increase in Kondo temperature and associated valence fluctuations below a first-order valence transition at T≃42 K. The bulk-sensitive, element-specific, and valence-projected charge excitation spectra reveal an unusual quasigap in the Yb-derived state density which drives an instability of the electronic structure and renormalizes the low-energy effective Hamiltonian at the transition. Our results provide long-sought experimental evidence for a link between temperature-driven changes in the low-energy Kondo scale and the higher-energy electronic structure of this system.

Wide quantum critical region of valence fluctuations: Origin of robust quantum criticality in quasicrystal Yb15Al34Au51 under pressure

The mechanism of the emergence of robust quantum criticality in the heavy- electron quasicrystal YR15Al34Au51 is analyzed theoretically. By constructing a minimal model for the quasicrystal and its crystalline approximant, which contain concentric shell structures with Yb and Al-Au clusters, we show that a set of quantum critical points of the first-order valence transition of Yb appears as spots in the ground-state phase diagram. Their critical regions overlap each other, giving rise to a wide quantum critical region. This well explains the robust criticality observed in YR15Al34Au51 under pressure, and predicts the emergence of the common criticality in the crystalline approximant under pressure. The wider critical region in the quasicrystal than that in the crystalline approximant in the T-P phase diagram and the field-induced valence-crossover "region" in the T-H phase diagram are predicted to appear.

Synchrotron X-ray spectroscopy study on the valence state and magnetization in α-YbAl1-xFexB4 (x = 0.115) at low temperatures and high magnetic fields

Valence fluctuation phenomena in rare-earth intermetallic compounds have attracted attention because the quantum criticality of the valence transition has been proposed theoretically. Recently, it was found that β-YbAlB4 shows quantum criticality without tuning and has a strong mixed-valence state. In this study, we measured the magnetization curve and X-ray magnetoabsorption in α-YbAl1-xFexB4 (x = 0.115), which is a locally isostructural polymorph of β-YbAlB4. The magnetization and X-ray experiments were performed in fields up to 55 and 40 T, respectively. A small increase in the Yb valence was observed at fields where the magnetization curve exhibited a change in slope.

Anderson's impurity-model analysis on CeO1-xFxBiS2

We have investigated the impact of F-doing on CeO1-xFxBiS2 in terms of the electronic-structural parameters of Anderson's impurity-model analysis. It was recently reported using Ce L3-edge x-ray absorption spectroscopy (XAS) that CeOBiS2 falls in the Ce valence fluctuation regime and the F-doping drives the system into the Kondo regime. The Ce L3- edge XAS spectra with the various F-doping levels can be reproduced by adjusting the transfer integral in the Anderson's impurity model. The present analysis indicates that the F-doping to the system corresponds to the decrease of the Ce-Bi transfer integral.

Large Cyclotron Effective Mass Detected by de Haas–van Alphen Effect in YbCu2Si2

De Haas–van Alphen experiments were performed on the valence fluctuation compound YbCu2Si2. For the field direction close to the c-axis in the tetragonal structure, a very large cyclotron effective mass of 43 m0 was detected on branch β, which corresponds to the nearly cylindrical Fermi surface. This mass is one of the largest cyclotron masses detected experimentally in Yb compounds.

Surface and bulk components of electrical conductivity in (presumably special topological) Kondo insulator SmB6 at lowest temperatures

Samarium hexaboride (SmB6) has recently been considered to be a topological Kondo insulator (TKI), the first strongly correlated electron system to exhibit topological surface conduction states. In this contribution, results of electrical resistivity measurements between 10 K and 0.04 K of various SmB6 single crystalline samples are presented, analysed and discussed. The received results imply that the residual conductivity of SmB6 below about 4 K is of non-activated (metallic-like) nature. It is shown that this metallic-like behaviour can be attributed both to surface (2D) conduction states, as may be expected in case of a topological insulator, as well as to the highly correlated many-body (3D) bulk ground state which is formed within the gap of this compound. From this it follows that in SmB6, in addition to surface conductivity states, there is in parallel also a bulk contribution to residual electrical conductivity originating from the strongly correlated electron system with valence fluctuations. This suggests that SmB6 is a new/special type of insulator in which in the energy gap besides the surface conduction states, there is also a conducting narrow in-gap band originating from the bulk strongly correlated electron system.

Avoided valence transition in a plutonium superconductor

The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5--the highest Tc superconductor of the heavy fermions (Tc = 18.5 K)--and find that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability--therefore, we identify the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5.

Pressure-induced valence crossover and novel metamagnetic behavior near the antiferromagnetic quantum phase transition of YbNi_{3}Ga_{9}.

We report electrical resistivity, ac magnetic susceptibility, and x-ray absorption spectroscopy measurements of intermediate valence YbNi_{3}Ga_{9} under pressure and magnetic field. We have revealed a characteristic pressure-induced Yb valence crossover within the temperature-pressure phase diagram, and a first-order metamagnetic transition is found below P_{c}∼9 GPa where the system undergoes a pressure-induced antiferromagnetic transition. As a possible origin of the metamagnetic behavior, a critical valence fluctuation emerging near the critical point of the first-order valence transition is discussed on the basis of the temperature-field-pressure phase diagram.

Valence fluctuations in a lattice of magnetic molecules: Application to iron(II) phtalocyanine molecules on Au(111)

We study theoretically a square lattice of the organometallic Kondo adsorbate iron(II) phtalocyanine (FePc) deposited on top of Au(111), motivated by recent scanning tunneling microscopy experiments. We describe the system by means of an effective Hubbard-Anderson model, where each molecule has degenerate effective d-orbitals with xz and yz symmetry, which we solve for arbitrary occupation and arbitrary on-site repulsion U. To that end, we introduce a generalized slave-boson mean-field approximation (SBMFA) which correctly describes both the non-interacting limit (NIL) U = 0 and the strongly interacting limit , where our formalism reproduces the correct value of the Kondo temperature for an isolated FePc molecule. Our results indicate that while the isolated molecule can be described by an SU(4) Anderson model in the Kondo regime, the case of the square lattice corresponds to the intermediate-valence regime, with a total occupation of nearly 1.65 holes in the FePc molecular orbitals. Our results have important implications for the physical interpretation of the experiment.

NMR Study on the Ru-dimer System with Valence Fluctuation

Substituting effect of CF3 to the Ru-based binuclear complex [{RuIII(acac)2}2(μ-OEt)2], in which S = 1/2 spin dimer takes the singlet ground state have been investigated by means of the magnetic susceptibility χ and the longitudinal nuclear spin relaxation rate T1-1 of 1H/19F-NMR. In CF3-substituted complex [{RuIII(fhma)2}2(μ-OMe)2], we have observed the prominent magnetic response both in χ and T1−1. This behavior is attributed to the slight change in the Ru valence state due to the substitution.

Heavy-fermion superconductivity and competing orders

One of the most salient features of heavy fermion superconductivity is its coexistence with various competing orders. Superconductivity often emerges near or at the border of these exotic orders and their interplay may give rise to many interesting quantum phenomena. In this paper, we give a brief review of the various heavy fermion superconductors discovered so far and show there may exist an intimate connection between their superconducting pairing and quantum critical spin/charge/orbital fluctuations. We classify these superconductors into three categories:(A) CeM2X2, CenMmIn3n+2m, CePt3Si, CeMX3, CeNiGe3, Ce2Ni3Ge5 and CePd5Al2, in which superconductivity emerges at the border of antiferromagnetic phase; YbRh2Si2, in which superconductivity was very recently found inside the antiferromagnetic phase at 2 mK; UX2Al3 and UPt3, in which superconductivity occurs inside the antiferromagnetic phase; and UBe13 and U6Fe, in which the connection between magnetism and superconductivity is not yet clear. Among them, CePt3Si and CeMX3 are noncentrosymmetric, while UPt3 exhibits spin triplet pairing inside an antiferromagnetic phase.(B) UGe2, URhGe, UCoGe, UIr and U2PtC2, are spin triplet superconductors under the influence of ferromagnetic order or fluctuations.(C) URu2Si2, PrOs4Sb12, PrT2X20, Pu-115, NpPd5Al2 and -YbAlB4, in which superconductivity may be related to other exotic quantum states or fluctuations such as hidden order, valence fluctuations and quadrupolar fluctuations.In these compounds, f-electrons may participate in both superconductivity and other competing orders and often behave simultaneously itinerant and localized. These could be described by a phenomenological two-fluid theory, in which two coexisting fluidsan itinerant heavy electron fluid (the Kondo liquid) and a spin liquid of unhybridized local f-momentscompete to give rise to the various low temperature orders as well as superconductivity. Combining the two-fluid picture and the idea of spin-fluctuation-induced superconducting pairing, a BCS-like formula is proposed for calculating the superconducting transition temperature, and the results are found to be in good agreement with the experimental data for Ce-115. This model can explain naturally the microscopic coexistence of superconductivity and antiferromagnetism in these materials, and provides a promising guidance to other heavy fermion superconductors to achieve a systematic examination of the interplay between superconductivity and other exotic orders.

Correlation Effects in One-Dimensional Quasiperiodic Anderson-Lattice Model

We consider the one-dimensional (1D) quasiperiodic Anderson-lattice model, which has quasiperiodically ordered impurities. The sites with an f-orbital are ordered as a “Fibonacci word”, one way to form 1D quasiperiodic orderings. To treat the correlation effect precisely, we use the density matrix renormalization group (DMRG) method. We show that the spin correlation function in the quasiperiodic system gives a characteristic pattern. Also, by analyzing the f-electron number and its fluctuation, we find that a valence transition, which usually occurs in the periodic Anderson model when the on-site interorbital interaction is large, is not sharp in the quasiperiodic system. Finally, we discuss the properties of the quasiperiodic Anderson-lattice model, comparing them against the Anderson-lattice model with randomly located f-orbitals. We find that the quasiperiodic Anderson-lattice model has a similar property to the periodic Anderson model for spin correlation, but also has a similar property to the Anderson-lattice model with randomly located f-orbitals for the valence fluctuation.

Three structure types and intermediate cerium valence in the solid solution CeRu1–xNixSn

Detailed analytical studies of the solid solutions CeRu1–xNixSn revealed a sequence of three different structure types, CeRuSn, ZrNiAl, and TiNiSi, with increasing x value. All compounds have been synthesized and characterized by X-ray powder diffraction. The structures of CeRu0.68Ni0.32Sn, CeRu0.38Ni0.62Sn and CeRu0.14Ni0.86Sn have been refined from single crystal X-ray diffractometer data. The former one exhibits the ZrNiAl type, P6¯2m, a = 737.9, c = 406.2 pm, wR2 = 0.0436, 345 F2 values, 17 variables. The two latter ones crystallize with the TiNiSi type, Pnma, a = 736.9, b = 468.5, c = 760.5 pm, wR2 = 0.0389, 631 F2 values, and a = 750.8, b = 461.8, c = 761.3 pm, wR2 = 0.0520, 632 F2 values, 21 variables for CeRu0.38Ni0.62Sn and CeRu0.14Ni0.86Sn, respectively. In accordance with magnetic susceptibility and Ce LIII-edge XANES measurements, cerium valence fluctuation has been proven for all samples that exhibit the hexagonal ZrNiAl type structure, while samples with the TiNiSi type show stable trivalent cerium. DFT calculations indicate that the CeRu0.5Ni0.5Sn composition should be thermodynamically stable in the hexagonal ZrNiAl type structure, while the unstable orthorhombic structure becomes monoclinic after structure relaxation.

Contiguous 3d and 4f magnetism: strongly correlated 3d electrons in YbFe2Al10.

We present magnetization, specific heat, and (27)Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard-x-ray photoemission spectroscopy study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb(3+) carries a full and stable moment, and Fe carries a moment of about 3.1 μB. The enhanced value of the Sommerfeld-Wilson ratio and the dynamic scaling of the spin-lattice relaxation rate divided by T[(27)(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. (27)(1/T1T) simultaneously tracks the valence fluctuations from the 4f Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature; the latter evolve out of an Yb 4f admixed conduction band.

Investigation of Mn, Fe, and Ni incorporation in CeCo2Al8.

Single crystals of CeCo(2-x)M(x)Al(8) (M = Mn, Fe, Ni; 0 ≤ x < 1) were grown and characterized by X-ray diffraction and magnetic susceptibility. The unit cell volumes of Mn-doped compounds increase and those of Ni-doped compounds decrease with increasing dopant concentration. All samples display a magnetic ordering near 6 K with magnetic moments of the analogues ranging from 2.61 to 2.81 μ(B)/mol Ce and slightly higher than Ce(3+) only magnetic moment. The unit cell volumes of Fe-doped compounds also increase with increasing Fe concentration. However, the cell volume of CeCo(2-x)Fe(x)Al(8) decreases for x = 1.00 and is not Curie-Weiss possibly because of valence fluctuation.

Antiferromagnetic Kondo lattice to intermediate valence transition in Ce(Au1−xNix)2Si2 ([formula omitted

Transition effects from magnetically ordered Kondo lattice (KL) behaviour in CeAu2Si2 to intermediate valence (IV) behaviour in CeNi2Si2 have been investigated by substitution of Au with Ni. These investigations were done through measurements of the lattice parameters, electrical resistivity (ρ(T)), magnetisation (M(μ0H)) and magnetic susceptibility (χ(T)) on the Ce(Au1−xNix)2Si2 (0≤x≤1) series. Lattice parameters as derived from XRD measurements deviate from Vegard's rule around x=0.6–0.8. ρ(T) data indicate KL behaviour in the presence of a crystal field for x≤0.6, the occurrence of magnetic ordering for x≤0.2 and valence fluctuation for x≥0.8. χ(T) data at high temperatures, follow the Curie–Weiss relation for alloys in the concentration 0≤x≤0.6 (KL region) and give effective magnetic moment values μeff close to that expected for free Ce3+ ion. The low temperature χ(T) data indicate the onset of antiferromagnetic ordering for x≤0.2. For alloys in the concentration range 0.8≤x≤1, the χ(T) behaviour is reminiscent of intermediate valence (IV) systems. M(μ0H) measurements indicate metamagnetic behaviour for alloys in the concentration range 0≤x≤0.1.

Raman spectroscopy and enhanced magnetic and dielectric properties of Pr and Ti codoped BiFeO3 ceramics

In this work, we have investigated the interplay between structure, magnetic and dielectric properties of (Bi1−xPrx)(Fe1−xTix)O3 ceramics (x = 0.03, 0.06 and 0.10) synthesised via solid state reaction method. X-ray diffraction and Raman studies exhibited the structural transition from rhombohedral to orthorhombic phase for higher codoping concentration. Room and low temperatures weak ferromagnetic behaviour has been established from the splitting between field cooled and zero field cooled curves and electron spin resonance spectra. This weak ferromagnetism increases with increasing codoping concentration. X-ray photoelectron spectroscopy showed valence fluctuation between Fe3+/Fe2+ ions for lower codoping concentration while only Fe3+ ions are observed for higher codoping concentrations. The observed dielectric anomaly near Neel temperature becomes more prominent with increasing codoping which indicates the enhancement of magnetic and electric interactions. A correlation between shifts of Neel temperature with variation of Fe–O–Fe bond angle has been observed.

Uncoupled surface spin induced exchange bias in α-MnO2 nanowires.

We have studied the microstructure, surface states, valence fluctuations, magnetic properties, and exchange bias effect in MnO2 nanowires. High purity α-MnO2 rectangular nanowires were synthesized by a facile hydrothermal method with microwave-assisted procedures. The microstructure analysis indicates that the nanowires grow in the [0 0 1] direction with the (2 1 0) plane as the surface. Mn3+ and Mn2+ ions are not found in the system by X-ray photoelectron spectroscopy. The effective magnetic moment of the manganese ions fits in with the theoretical and experimental values of Mn4+ very well. The uncoupled spins in 3d3 orbitals of the Mn4+ ions in MnO6 octahedra on the rough surface are responsible for the net magnetic moment. Spin glass behavior is observed through magnetic measurements. Furthermore, the exchange bias effect is observed for the first time in pure α-MnO2 phase due to the coupling of the surface spin glass with the antiferromagnetic α-MnO2 matrix. These α-MnO2 nanowires, with a spin-glass-like behavior and with an exchange bias effect excited by the uncoupled surface spins, should therefore inspire further study concerning the origin, theory, and applicability of surface structure induced magnetism in nanostructures.

Polaron-ion correlations inLixFePO4studied by x-ray nuclear resonant forward scattering at elevated pressure and temperature

Valence fluctuations of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ were studied in a solid solution of ${\mathrm{Li}}_{x}{\mathrm{FePO}}_{4}$ by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamiltonian. These frequencies were analyzed to obtain activation enthalpies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an activation volume for polaron hopping of $5.8\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.28em}{0ex}}0.7 {\AA{}}^{3}$. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons and ${\mathrm{Li}}^{+}$ ions that alter the dynamics of both. Monte Carlo simulations were used to estimate the strength of the polaron-ion interaction energy.