Kondo effect and channel mixing in oscillating molecules

We investigate the electronic transport through a molecule in the Kondo regime. The tunneling between the electrode and the molecule is asymmetrically modulated by the oscillations of the molecule, i.e., if the molecule gets closer to one of the electrodes the tunneling to that electrode will increase while for the other electrode it will decrease. The system is described by a two-channel Anderson model with phonon-assisted hybridization, which is solved with the Wilson numerical renormalization group method. The results for several functional forms of tunneling modulation are presented. For a linearized modulation the Kondo screening of the molecular spin is caused by the even or odd conduction channel. At the critical value of the electron-phonon coupling an unstable two-channel Kondo fixed point is found. For a realistic modulation the spin at the molecular orbital is Kondo screened by the even conduction channel even in the regime of strong coupling. A universal consequence of the electron-phonon coupling is the softening of the phonon mode and the related instability to perturbations that break the left-right symmetry. When the frequency of oscillations decreases below the magnitude of such perturbation, the molecule is abruptly attracted to one of the electrodes. In this regime, the Kondo temperature is enhanced and, simultaneously, the conductance through the molecule is suppressed.Comment: published versio

Inferior Mesenteric Artery Stenting as a Novel Treatment for Chronic Mesenteric Ischemia in Patients with an Occluded Superior Mesenteric Artery and Celiac Trunk

IntroductionChronic mesenteric ischemia (CMI) is a challenging problem, with revascularization the mainstay of treatment. Management of CMI is especially challenging in the patient with superior mesenteric artery (SMA) and celiac artery (CA) occlusions.ReportWe report a case series of four patients with chronic mesenteric ischemia who were not candidates for CA or SMA revascularization who were successfully treated with inferior mesenteric artery (IMA) angioplasty and stent placement to improve collateral circulation and palliate symptoms.DiscussionTo our knowledge, this is the largest case series to date reporting the use of an IMA stent to improve collateral circulation in patients with CMI

Antiferromagnetic ordering in the Kondo lattice system Yb2_2Fe3_3Si5_5

Compounds belonging to the R$_2$Fe$_3$Si$_5$ series exhibit unusual superconducting and magnetic properties. Although a number of studies have been made on the first reentrant antiferromagnet superconductor Tm$_2$Fe$_3$Si$_5$, the physical properties of Yb$_2$Fe$_3$Si$_5$ are largely unexplored. In this work, we attempt to provide a comprehensive study of bulk properties such as, resistivity, susceptibility and heat-capacity of a well characterized polycrystalline Yb$_2$Fe$_3$Si$_5$. Our measurements indicate that Yb$^{3+}$ moments order antiferromagnetically below 1.7 K. Moreover, the system behaves as a Kondo lattice with large Sommerfeld coefficient ($\gamma$) of 0.5~J/Yb mol K$^{2}$ at 0.3 K, which is well below T$_N$. The absence of superconductivity in Yb$_2$Fe$_3$Si$_5$ down to 0.3 K at ambient pressure is attributed to the presence of the Kondo effect.Comment: 10 pages, 3 figures, tex document. A fuller version has appeared in PRB. Here we have omitted the figures showing the crystal structure and the fitting of the X-ray pattern. Also the table with the lattice parameters obtained from fitting has been remove

Quantum Criticality in doped CePd_1-xRh_x Ferromagnet

CePd_1-xRh_x alloys exhibit a continuous evolution from ferromagnetism (T_C= 6.5 K) at x = 0 to a mixed valence (MV) state at x = 1. We have performed a detailed investigation on the suppression of the ferromagnetic (F) phase in this alloy using dc-(\chi_dc) and ac-susceptibility (\chi_ac), specific heat (C_m), resistivity (\rho) and thermal expansion (\beta) techniques. Our results show a continuous decrease of T_C (x) with negative curvature down to T_C = 3K at x*= 0.65, where a positive curvature takes over. Beyond x*, a cusp in cac is traced down to T_C* = 25 mK at x = 0.87, locating the critical concentration between x = 0.87 and 0.90. The quantum criticality of this region is recognized by the -log(T/T_0) dependence of C_m/T, which transforms into a T^-q (~0.5) one at x = 0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegard's law (at x_V~0.75) and increasing hybridization effects on high temperature \chi_dc and \rho. Coincidentally, a Fermi liquid contribution to the specific heat arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C_m/T flattening is observed for x > x_cr rather the mentioned power law divergence, which coincides with a change of sign of \beta. The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.Comment: 11 pages, 11 figure

Iron oxidation at low temperature (260–500 C) in air and the effect of water vapor

The oxidation of iron has been studied at low temperatures (between 260 and 500 C) in dry air or air with 2 vol% H2O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. 18O tracer experiments performed at 400 C allowed to propose a growth mechanism of the scale

Crystal-field effects in the mixed-valence compounds Yb2M3Ga9 (M= Rh, Ir)

Magnetic susceptibility, heat capacity, and electrical resistivity measurements have been carried out on single crystals of the intermediate valence compounds Yb2Rh3Ga9 and Yb2Ir3Ga9. These measurements reveal a large anisotropy due apparently to an interplay between crystalline electric field (CEF) and Kondo effects. The temperature dependence of magnetic susceptibility can be modelled using the Anderson impurity model including CEF within an approach based on the Non-Crossing Approximation.Comment: Accepted to Phys. Rev.

Mass-Enhanced Fermi Liquid Ground State in Na1.5_{1.5}Co2_2O4_4

Magnetic, transport, and specific heat measurements have been performed on layered metallic oxide Na$_{1.5}$Co$_2$O$_4$ as a function of temperature $T$. Below a characteristic temperature $T^*$=30$-$40 K, electrical resistivity shows a metallic conductivity with a $T^2$ behavior and magnetic susceptibility deviates from the Curie-Weiss behavior showing a broad peak at $\sim$14 K. The electronic specific heat coefficient $\gamma$ is $\sim$60 mJ/molK$^2$ at 2 K. No evidence for magnetic ordering is found. These behaviors suggest the formation of mass-enhanced Fermi liquid ground state analogous to that in $d$-electron heavy fermion compound LiV$_2$O$_4$.Comment: 4 pages, 4 figures, to be published in Phys. Rev. B 69 (2004

Energy scales of Lu(1-x)Yb(x)Rh2Si2 by means of thermopower investigations

We present the thermopower S(T) and the resistivity rho(T) of Lu(1-x)Yb(x)Rh2Si2 in the temperature range 3 K < T < 300 K. S(T) is found to change from two minima for dilute systems (x < 0.5) to a single large minimum in pure YbRh2Si2. A similar behavior has also been found for the magnetic contribution to the resistivity rho_mag(T). The appearance of the low-T extrema in S(T) and rho_mag(T) is attributed to the lowering of the Kondo scale with decreasing x. The evolution of the characteristic energy scales for both the Kondo effect and the crystal electric field splitting are deduced. An extrapolation allows to estimate the Kondo temperature of YbRh2Si2 to 29 K.Comment: 15 pages, 4 figures, accepted in Phys. Rev.

Thermodynamics of the dissipative two-state system: a Bethe Ansatz study

The thermodynamics of the dissipative two-state system is calculated exactly for all temperatures and level asymmetries for the case of Ohmic dissipation. We exploit the equivalence of the two-state system to the anisotropic Kondo model and extract the thermodynamics of the former by solving the thermodynamic Bethe Ansatz equations of the latter. The universal scaling functions for the specific heat $C_{\alpha}(T)$ and static dielectric susceptibility $\chi_{\alpha}(T)$ are extracted for all dissipation strengths $0<\alpha<1$ for both symmetric and asymmetric two-state systems. The logarithmic corrections to these quantities at high temperatures are found in the Kondo limit $\alpha\to 1^{-}$, whereas for $\alpha< 1$ we find the expected power law temperature dependences with the powers being functions of the dissipative coupling $\alpha$. The low temperature behaviour is always that of a Fermi liquid.Comment: 24 pages, 32 PS figures. Typos corrected, final versio