Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2_2Si2_2 versus YbRh2_2Si2_2

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$. In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$ mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure - induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-$T$ unconventional heavy - fermion superconductors and other families of unconventional superconductors with higher $T_c$s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.Comment: 30 pages, 7 Figures, Accepted for publication in Philosophical Magazin

Evidence for a Kondo destroying quantum critical point in YbRh2Si2

The heavy-fermion metal YbRh$_{2}$Si$_{2}$ is a weak antiferromagnet below $T_{N} = 0.07$ K. Application of a low magnetic field $B_{c} = 0.06$ T ($\perp c$) is sufficient to continuously suppress the antiferromagnetic (AF) order. Below $T \approx 10$ K, the Sommerfeld coefficient of the electronic specific heat $\gamma(T)$ exhibits a logarithmic divergence. At $T < 0.3$ K, $\gamma(T) \sim T^{-\epsilon}$ ($\epsilon: 0.3 - 0.4$), while the electrical resistivity $\rho(T) = \rho_{0} + aT$ ($\rho_{0}$: residual resistivity). Upon extrapolating finite-$T$ data of transport and thermodynamic quantities to $T = 0$, one observes (i) a vanishing of the "Fermi surface crossover" scale $T^{*}(B)$, (ii) an abrupt jump of the initial Hall coefficient $R_{H}(B)$ and (iii) a violation of the Wiedemann Franz law at $B = B_{c}$, the field-induced quantum critical point (QCP). These observations are interpreted as evidence of a critical destruction of the heavy quasiparticles, i.e., propagating Kondo singlets, at the QCP of this material.Comment: 20 pages, 8 figures, SCES 201

RELATION OF CHROMOSOME 4 (LINKAGE GROUP VIII) TO MURINE LEUKEMIA VIRUS-ASSOCIATED ANTIGENS OF AKR MICE

Genes specifying or controlling the expression of GIX (cell surface), GCSA (cell surface), and gs (internal viral) antigens are located in chromosome 4 (linkage group [LG] VIII) of the AKR mouse. All three antigens may exhibit mendelian inheritance, mice being antigen positive or antigen negative, but each may also appear in leukemic cells of mice whose inherited genotype was antigen negative. The GIX-determining gene in LG VIII of AKR mice apparently is equivalent to Gv-1, which determines expression of the same antigen in 129 strain mice, but which in the latter strain is located in LG IX. As the estimated distance of Gv-1 from H-2 in 129 mice is considerable (37 units) further tests are now indicated to assess the possibility of pseudolinkage in this case. The Fv-1 locus, also located in LG VIII, influences the mouse's titer of MuLV, and might thereby be thought to regulate the GIX and gs phenotypes of AKR backcross segregants. But the data indicate a discrete LG VIII locus for GIX, since expression of this antigen is mendelian and independent of infectious virus titer. Since the GIX and GCSA phenotypes of AKR backcross segregants were invariably concordant, these two antigens must be specified or controlled by closely linked genes, and the latter also is presumably independent of virus titer. The question as to what extent expression of gs antigen in the segregants is secondary to virus production is undecided

Superconductivity in Ce- and U-based "122" heavy-fermion compounds

This review discusses the heavy-fermion superconductivity in Ce- and U-based compounds crystallizing in the body-centered tetragonal ThCr2Si2 structure. Special attention will be paid to the theoretical background of these systems which are located close to a magnetic instability.Comment: 12 pages, 9 figures. Invited topical review (special issue on "Recent Developments in Superconductivity") Metadata and references update

Using network analysis to examine links between individual depressive symptoms, inflammatory markers, and covariates

Background   Studies investigating the link between depressive symptoms and inflammation have yielded inconsistent results, which may be due to two factors. First, studies differed regarding the specific inflammatory markers studied and covariates accounted for. Second, specific depressive symptoms may be differentially related to inflammation. We address both challenges using network psychometrics.   Methods   We estimated seven regularized Mixed Graphical Models in the Netherlands Study of Depression and Anxiety (NESDA) data (N = 2321) to explore shared variances among (1) depression severity, modeled via depression sum-score, nine DSM-5 symptoms, or 28 individual depressive symptoms; (2) inflammatory markers C-reactive protein (CRP), interleukin 6 (IL-6), and tumor necrosis factor α (TNF-α); (3) before and after adjusting for sex, age, body mass index (BMI), exercise, smoking, alcohol, and chronic diseases.   Results   The depression sum-score was related to both IL-6 and CRP before, and only to IL-6 after covariate adjustment. When modeling the DSM-5 symptoms and CRP in a conceptual replication of Jokela et al., CRP was associated with ‘sleep problems’, ‘energy level’, and ‘weight/appetite changes’; only the first two links survived covariate adjustment. In a conservative model with all 38 variables, symptoms and markers were unrelated. Following recent psychometric work, we re-estimated the full model without regularization: the depressive symptoms ‘insomnia’, ‘hypersomnia’, and ‘aches and pain’ showed unique positive relations to all inflammatory markers.   Conclusions   We found evidence for differential relations between markers, depressive symptoms, and covariates. Associations between symptoms and markers were attenuated after covariate adjustment; BMI and sex consistently showed strong relations with inflammatory markers

Griffiths phase of the Kondo insulator fixed point

Heavy fermion compounds have long been identified as systems which are extremely sensitive to the presence of impurities and other imperfections. In recent years, both experimental and theoretical work has demonstrated that such disorder can lead to unusual, non-Fermi liquid behavior for most physical quantities. In this paper, we show that this anomalous sensitivity to disorder, as well as the resulting Griffiths phase behavior, directly follow from the proximity of metallic heavy fermion systems to the Kondo insulator fixed point.Comment: 5 pages, 4 figures; Proceedings of the SCES, August 2000, to appear in the Journal of Magnetism and Magnetic Material

Gradual pressure-induced enhancement of magnon excitations in CeCoSi

CeCoSi is an intermetallic antiferromagnet with a very unusual temperature-pressure phase diagram: at ambient pressure it orders below TN = 8.8 K, while application of hydrostatic pressure induces a new magnetically ordered phase with exceptionally high transition temperature of ∼40 K at 1.5 GPa. We studied the magnetic properties and the pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic neutron scattering (INS) and heat capacity measurements. At ambient pressure CeCoSi orders into a simple commensurate AFM structure with a reduced ordered moment of only mCe = 0.37(6)μB. Specific heat and low-energy INS indicate a significant gap in the low-energy magnon excitation spectrum in the antiferromagnetic phase, with the CEF excitations located above 10 meV. Hydrostatic pressure gradually shifts the energy of the magnon band towards higher energies and the temperature dependence of the magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the CEF excitations are also drastically modified under pressure.Fil: Nikitin, S. E.. Max Planck Institute for Chemical Physics of Solids; Alemania. Technische Universität Dresden; AlemaniaFil: Franco, Diego Gaspar. Max Planck Institute for Chemical Physics of Solids; Alemania. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Kwon, J.. Max Planck Institute for Chemical Physics of Solids; AlemaniaFil: Bewley, R.. Harwell Campus. STFC Rutherford Appleton Laboratory. ISIS Facility; Reino UnidoFil: Podlesnyak, Andrey. Oak Ridge National Laboratory. Neutron Scattering Division; Estados UnidosFil: Hoser, A.. Helmholtz-Zentrum Berlin für Materialien und Energie; AlemaniaFil: Koza, M. M.. Institute Laue Langevin; FranciaFil: Geibel, C.. Max Planck Institute for Chemical Physics of Solids; AlemaniaFil: Stockert, O.. Max Planck Institute for Chemical Physics of Solids; Alemani

Crystal field studies on the heavy fermion compound CeNi8_8CuGe4_4

Substitution of nickel by copper in the heavy fermion system CeNi$_{9-x}$Cu$_x$Ge$_4$ alters the local crystal field environment of the Ce$^{3+}$-ions. This leads to a quantum phase transition near $x\approx0.4$, which is not only driven by the competition between Kondo effect and RKKY interaction, but also by a reduction of an effectively fourfold to a twofold degenerate crystal field ground state. To study the consequences of a changing crystal field in CeNi$_8$CuGe$_4$ on its Kondo properties, inelastic neutron scattering (INS) experiments were performed. Two well-defined crystal field transitions were observed in the energy-loss spectra at 4 K. The crystal field level scheme determined by neutron spectroscopy is compared with results from specific heat measurements.Comment: 4 pages, 3 figures, conference SCES0