Competing magnetic interactions in CeNi9-xCoxGe4

CeNi9Ge4 exhibits outstanding heavy fermion features with remarkable non-Fermi- liquid behavior which is mainly driven by single-ion effects. The substitution of Ni by Cu causes a reduction of both, the RKKY coupling and Kondo interaction, coming along with a dramatic change of the crystal field (CF) splitting. Thereby a quasi-quartet ground state observed in CeNi9Ge4 reduces to a two-fold degenerate one in CeNi8CuGe4. This leads to a modiffcation of the effective spin degeneracy of the Kondo lattice ground state and to the appearance of antiferromagnetic (AFM) order. To obtain a better understanding of consequences resulting from a reduction of the effective spin degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and RKKY interactions through the reduction of the effective d-electron count is expected. Accordingly, a paramagnetic Fermi liquid ground state should arise. Our experimental studies, however, reveal AFM order already for small Co concentrations, which becomes even more pronounced with increasing Co content x. Thereby the modiffcation of the effective spin degeneracy seems to play a crucial role in this system

Ambulation protocols leading to decreased postoperative complications and hospital stay

Background: In the postoperative course, patients are routinely encouraged to ambulate as frequently as possible. Typically in the hospital this can become burdensome to the staff and often becomes low priority. Patients are also not aware of the frequency and quality of the ambulation that is sufficient in the postoperative period. At present, patients on the surgical floor who are completely independent and without any devices (eg. Oxygen, nasogastric tubes, chest tubes) are freely able to ambulate at will although there is no reliable way to track this progress. Other patients with devices are limited to waiting for nursing or ancillary staff to assist them with securing the devices that they require in the postoperative period. Ambulation has been positively associated with decreased postoperative complications ranging from bowel function to deep venous thrombosis to pneumonia.https://jdc.jefferson.edu/patientsafetyposters/1065/thumbnail.jp

Metallic behavior in Si/SiGe 2D electron systems

We calculate the temperature, density, and parallel magnetic field dependence of low temperature electronic resistivity in 2D high-mobility Si/SiGe quantum structures, assuming the conductivity limiting mechanism to be carrier scattering by screened random charged Coulombic impurity centers. We obtain comprehensive agreement with existing experimental transport data, compellingly establishing that the observed 2D metallic behavior in low-density Si/SiGe systems arises from the peculiar nature of 2D screening of long-range impurity disorder. In particular, our theory correctly predicts the experimentally observed metallic temperature dependence of 2D resistivity in the fully spin-polarized system

Very high two-dimensional hole gas mobilities in strained silicon germanium

We report on the growth by solid source MBE and characterization of remote doped Si/SiGe/Si two-dimensional hole gas structures. It has been found that by reducing the Ge composition to <=13% and limiting the thickness of the alloy layer, growth temperatures can be increased up to 950 °C for these structures while maintaining good structural integrity and planar interfaces. Record mobilities of 19 820 cm2 V−1 s−1 at 7 K were obtained in normal structures. Our calculations suggest that alloy scattering is not important in these structures and that interface roughness and interface charge scattering limit the low temperature mobilities

Antiferromagnetic behavior in CeCo9_{9}Ge4_{4}

We investigate the novel intermetallic ternary compounds \emph{R}Co$_{9}$Ge$_{4}$ with \emph{R} = La and Ce by means of $X$-ray diffraction, susceptibility and specific heat measurements. CeCo$_{9}$Ge$_{4}$ crystallizes in the space group ${I}$ 4/ ${mcm}$ and is characterized by the coexistence of two different magnetic sublattices. The Ce-based sublattice, with an effective moment close to the expected value for a Ce$^{3+}$-ion, exhibits a magnetically ordered ground state with $T_{\mathrm{N}}=12.5$ K. The Co-based sublattice, however, exhibits magnetic moments due to itinerant 3$d$ electrons. The magnetic specific heat contribution of the Ce-sublattice is discussed in terms of a resonance-level model implying the interplay between an antiferromagnetic phase transition and the Kondo-effect and an underlying Schottky-anomaly indicating a crystal field level scheme splitting into three twofold degenerated micro states ($\Delta_1 = 69$ K, $\Delta_2 = 133$ K).Comment: 4 pages, 3 figures, conference SCES0

Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold degenerate ground state in CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ \chi ~ -ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.Comment: 15 pages, 9 figure

Magnetic Energy and Helicity Budgets in the Active-Region Solar Corona. I. Linear Force-Free Approximation

We self-consistently derive the magnetic energy and relative magnetic helicity budgets of a three-dimensional linear force-free magnetic structure rooted in a lower boundary plane. For the potential magnetic energy we derive a general expression that gives results practically equivalent to those of the magnetic Virial theorem. All magnetic energy and helicity budgets are formulated in terms of surface integrals applied to the lower boundary, thus avoiding computationally intensive three-dimensional magnetic field extrapolations. We analytically and numerically connect our derivations with classical expressions for the magnetic energy and helicity, thus presenting a so-far lacking unified treatment of the energy/helicity budgets in the constant-alpha approximation. Applying our derivations to photospheric vector magnetograms of an eruptive and a noneruptive solar active regions, we find that the most profound quantitative difference between these regions lies in the estimated free magnetic energy and relative magnetic helicity budgets. If this result is verified with a large number of active regions, it will advance our understanding of solar eruptive phenomena. We also find that the constant-alpha approximation gives rise to large uncertainties in the calculation of the free magnetic energy and the relative magnetic helicity. Therefore, care must be exercised when this approximation is applied to photospheric magnetic field observations. Despite its shortcomings, the constant-alpha approximation is adopted here because this study will form the basis of a comprehensive nonlinear force-free description of the energetics and helicity in the active-region solar corona, which is our ultimate objective.Comment: 44 pages, 8 figures, 2 tables. The Astrophysical Journal, in pres

Wigner Crystal in One Dimension

A one--dimensional gas of electrons interacting with long--range Coulomb forces ($V(r) \approx 1/r$) is investigated. The excitation spectrum consists of separate collective charge and spin modes, with the charge excitation energies in agreement with RPA calculations. For arbitrarily weak Coulomb repulsion density correlations at wavevector $4k_F$ decay extremely slowly and are best described as those of a one--dimensional Wigner crystal. Pinning of the Wigner crystal then leads to the nonlinear transport properties characteristic of CDW. The results allow a consistent interpretation of the plasmon and spin excitations observed in one--dimensional semiconductor structures, and suggest an interpretation of some of the observed features in terms of ``spinons''. A possible explanation for nonlinear transport phenomena is given.Comment: 10 pages, RevTe

A quantum solution to the arrow-of-time dilemma

The arrow of time dilemma: the laws of physics are invariant for time inversion, whereas the familiar phenomena we see everyday are not (i.e. entropy increases). I show that, within a quantum mechanical framework, all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having happened. This situation is completely indistinguishable from their not having happened at all. In the light of this observation, the second law of thermodynamics is reduced to a mere tautology: physics cannot study those processes where entropy has decreased, even if they were commonplace.Comment: Contains slightly more material than the published version (the additional material is clearly labeled in the latex source). Because of PRL's title policy, the leading "A" was left out of the title in the published pape

Hole effective mass in remote doped Si/Si1−xGex quantum wells with 0.05x0.3

The effective masses in remote doped Si/Si1−xGex hole quantum wells with 0.05<=x<=0.3, have been determined from the temperature dependence of the Shubnikov–de Haas oscillations. The values are lower than previously observed by other workers, but still somewhat higher than the theoretical Gamma-point values for the ground-state heavy hole subband. The differences are attributed to finite carrier sheet densities and can be satisfactorily accounted for by nonparabolicity corrections