Emergence of intrinsic superconductivity below 1.178 K in the topologically non-trivial semimetal state of CaSn3.

Topological materials which are also superconducting are of great current interest, since they may exhibit a non-trivial topologically-mediated superconducting phase. Although there have been many reports of pressure-tuned or chemical-doping-induced superconductivity in a variety of topological materials, there have been few examples of intrinsic, ambient pressure superconductivity in a topological system having a stoichiometric composition. Here, we report that the pure intermetallic CaSn3 not only exhibits topological fermion properties, but also has a superconducting phase at ~1.178 K under ambient pressure. The topological fermion properties, including the nearly zero quasi-particle mass and the non-trivial Berry phase accumulated in cyclotron motions, were revealed from the de Haas-van Alphen (dHvA) quantum oscillation studies of this material. Although CaSn3 was previously reported to be superconducting with T c = 4.2 K, our studies show that the T c = 4.2 K superconductivity is extrinsic and caused by Sn on the degraded surface, whereas its intrinsic bulk superconducting transition occurs at 1.178 K. These findings make CaSn3 a promising candidate for exploring new exotic states arising from the interplay between non-trivial band topology and superconductivity, e.g. topological superconductivity (TSC).

Electrostatic tuning of the proximity-induced exchange field in EuS/Al bilayers.

We demonstrate that the proximity-induced exchange field H(ex) in ferromagnetic-paramagnetic bilayers can be modulated with an electric field. An electrostatic gate arrangement is used to tune the magnitude of H(ex) in the Al component of EuS/Al bilayers. In samples with H(ex)~2 T, we were able to produce modulations of ±10 mT with the application of perpendicular electric fields of the order of ±10(6) V/cm. We discuss several possible mechanisms accounting for the electric field's influence on the interfacial coupling between the Al layer and the ferromagnetic insulator EuS, along with the prospects of producing a superconducting field-effect transistor.

Exchange field-mediated magnetoresistance in the correlated insulator phase of Be films.

We present a study of the proximity effect between a ferromagnet and a paramagnetic metal of varying disorder. Thin beryllium films are deposited onto a 5 nm thick layer of the ferromagnetic insulator EuS. This bilayer arrangement induces an exchange field, H(ex), of a few tesla in low-resistance Be films with sheet resistance R≪R(Q), where R(Q)=h/e2 is the quantum resistance. We show that H(ex) survives in very high-resistance films and, in fact, appears to be relatively insensitive to the Be disorder. We exploit this fact to produce a giant low-field magnetoresistance in the correlated-insulator phase of Be films with R≫R(Q).

Origin of excess low-energy states in a disordered superconductor in a Zeeman field.

Tunneling density of states measurements of disordered superconducting Al films in high Zeeman fields reveal a significant population of subgap states which cannot be explained by standard BCS theory. We provide a natural explanation of these excess states in terms of a novel disordered Larkin-Ovchinnikov phase that occurs near the spin-paramagnetic transition at the Chandrasekhar-Clogston critical field. The disordered Larkin-Ovchinnikov superconductor is characterized by a pairing amplitude that changes sign at domain walls. These domain walls carry magnetization and support Andreev bound states that lead to distinct spectral signatures at low energy.

Spin-resolved tunneling studies of the exchange field in EuS/Al bilayers.

We use spin-resolved electron tunneling to study the exchange field in the Al component of EuS/Al bilayers, in both the superconducting and normal-state phases of the Al. Contrary to expectation, we show that the exchange field H(ex) is a nonlinear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore, the magnitude H(ex) is unaffected by the superconducting phase. In addition, H(ex) decreases significantly with increasing temperature in the temperature range of 0.1-1 K. We discuss these results in the context of recent theories of generalized spin-dependent boundary conditions at a superconductor-ferromagnet interface.

Saturation of the anomalous Hall effect in critically disordered ultrathin CNi3 films.

We demonstrate that a distinct high-disorder anomalous Hall effect phase emerges at the correlated insulator threshold of ultrathin, amorphous, ferromagnetic CNi3 films. In the weak-localization regime, where the sheet conductance G>>e{2}/h, the anomalous Hall resistance of the films increases with increasing disorder and the Hall conductance scales as G{xy} proportional to G{phi} with phi=1.6. However, at sufficiently high disorder the system begins to enter the 2D correlated insulator regime, at which point the Hall resistance R{xy} abruptly saturates and the scaling exponent becomes phi=2. Tunneling measurements show that the saturation behavior is commensurate with the emergence of the 2D Coulomb gap, suggesting that e-e interactions mediate the high-disorder phase.

Crystal growth, structure, and physical properties of Ln(Cu,Al)12 (Ln = Y, Ce, Pr, Sm, and Yb) and Ln(Cu, Ga)12 (Ln = Y, Gd-Er, and Yb).

Single crystals of Ln(Cu,Al)12 and Ln(Cu,Ga)12 compounds (Ln = Y, Ce-Nd, Sm, Gd-Ho, and Yb for Al and Ln = Y, Gd-Er, Yb for Ga) have been grown by flux-growth methods and characterized by means of single-crystal x-ray diffraction, complemented with microprobe analysis, magnetic susceptibility, resistivity and heat capacity measurements. Ln(Cu,Ga)12 and Ln(Cu,Al)12 of the ThMn12 structure type crystallize in the tetragonal I4/mmm space group with lattice parameters a approximately 8.59 Šand c approximately 5.15 Šand a approximately 8.75 Šand c approximately 5.13 Šfor Ga and Al containing compounds, respectively. For aluminium containing compounds, magnetic susceptibility data show Curie-Weiss paramagnetism in the Ce and Pr analogues down to 50 K with no magnetic ordering down to 3 K, whereas the Yb analogue shows a temperature-independent Pauli paramagnetism. Sm(Cu,Al)12 orders antiferromagnetically at T(N)approximately 5 K and interestingly exhibits Curie-Weiss behaviour down to 10 K with no Van Vleck contribution to the susceptibility. Specific heat data show that Ce(Cu,Al)12 is a heavy fermion antiferromagnet with T(N) approximately 2 K and with an electronic specific heat coefficient γ0 as large as 390 mJ K2 mol(-1). In addition, this is the first report of Pr(Cu,Al)12 and Sm(Cu,Al)12 showing an enhanced mass (approximately 80 and 120 mJ K(2) mol(-1)). For Ga containing analogues, magnetic susceptibility data also show the expected Curie-Weiss behaviour from Gd to Er, with the Yb analogue being once again a Pauli paramagnet. The antiferromagnetic transition temperatures range over 12.5, 13.5, 6.7, and 3.4 K for Gd, Tb, Dy, and Er. Metallic behaviour is observed down to 3 K for all Ga and Al analogues. A large positive magnetoresistance up to 150% at 9 T is also observed for Dy(Cu,Ga)12. The structure, magnetic, and transport properties of these compounds will be discussed.

Crystal growth, structure, and physical properties of Ln(Ag, Al, Si)2 (Ln = Ce and Gd).

Single crystals of CeM2 and GdM2 (M = Ag, Al, and Si) were grown by the flux growth technique and characterized by means of single crystal x-ray diffraction, magnetic susceptibility, resistivity, and heat capacity measurements. CeM2 and GdM2 crystallize in the tetragonal I4(1)/amd space group with the α-ThSi2 structure type with lattice parameters a ~4.2 Å and c ~14.4 Å. Curie-Weiss behavior is observed for both analogues with CeM2 ordering first ferromagnetically at 11 K with a second antiferromagnetic transition at 8.8 K while GdM2 orders antiferromagnetically at 24 K. Heat capacity measurements on CeM2 show two magnetic transitions at 10.8 and 8.8 K with an electronic specific heat coefficient, γ(0), of ~53 mJ K(-2) mol(-1). The entropy at the magnetic transition is less than the expected Rln2 for CeM2, reinforcing the assertions of an enhanced mass state and Kondo behavior being observed in the resistivity.

Measurement of conduction-electron polarization via the pairing resonance.

We show that the pairing resonance in the Pauli-limited normal state of ultrathin superconducting Al films provides a spin-resolved probe of conduction-electron polarization in thin magnetic films. A superconductor-insulator-ferromagnet tunneling junction is used to measure the density of states in supercritical parallel magnetic fields that are well beyond the Clogston-Chandresekhar limit, thus greatly extending the field range of the tunneling density of states technique. The applicability and limitations of using the pairing resonance as a spin probe are discussed.

Comment on "Post-wildfire logging hinders regeneration and increases fire risk".

Donato et al. (Brevia, 20 January 2006, p. 352) concluded that logging after wildfire kills natural regeneration and increases fire risk. We argue that their paper lacks adequate context and supporting information to be clearly interpreted by scientists, resource managers, policy-makers, and the public.

Spin proximity effect in ultrathin superconducting Be-Au bilayers.

We present a detailed study of the effects of interface spin-orbit coupling on the critical field behavior of ultrathin superconducting Be/Au bilayers. Parallel field measurements were made in bilayers with Be thicknesses in the range of d=2-30 nm and Au coverages of 0.5 nm. Though the Au had little effect on the superconducting gap, it produced profound changes in the spin states of the system. In particular, the parallel critical field exceeded the Clogston limit by an order of magnitude in the thinnest films studied. In addition, the parallel critical field unexpectedly scaled as [FORMULA: SEE TEXT], suggesting that the spin-orbit coupling energy was proportional to Delta0/d2. Tilted field measurements showed that, contrary to recent theory, the interface spin-orbit coupling induces a large in-plane superconducting susceptibility but only a very small transverse susceptibility.

Orbital response of evanescent cooper pairs in paramagnetically limited Al films.

We report a detailed study of the pairing resonance via tunneling density of states in ultrathin superconducting Al films in supercritical magnetic fields. Particular emphasis is placed on the effects of the perpendicular component of the magnetic field on the resonance energy and magnitude. Though the resonance is broadened and attenuated by H(perpendicular) as expected, its energy is shifted upward linearly with H(perpendicular). Extension of the original theory of the resonance to include strong perpendicular fields shows that at sufficiently large H(perpendicular) the overlap of the broadened resonance tail with the underlying degenerate Fermi sea alters the spectral distribution of the resonance via the exclusion principle. This leads to the shift of the resonance feature to higher energy.

Low-temperature susceptibility of the noncentrosymmetric superconductor CePt3Si.

We report ac susceptibility measurements of polycrystalline CePt(3)Si down to 60 mK and in applied fields up to 9 T. In a zero applied field, a full Meissner state emerges at temperatures T/T(c) < 0.3, where T(c) = 0.65 K is the onset transition temperature. Though transport measurements show a relatively high upper critical field B(c2) approximately 4-5 T, the low-temperature susceptibility chi(') is quite fragile to the applied field, with chi(') diminishing rapidly in fields of a few kG. Interestingly, the field dependence of chi(') is well described by the power law 4pichi(') + 1 = (B/B(c))(1/2), where B(c) is the field at which the onset of resistance is observed in transport measurements.

Field-induced spin mixing in ultrathin superconducting Al and be films in high parallel magnetic fields.

We report spin-dependent electron density of states (DOS) studies of ultrathin superconducting Al and Be films in high parallel magnetic fields. Superconductor-insulator-superconductor (SIS) tunneling spectra are presented in which both the film and the counterelectrode are in the paramagnetic limit. This SIS configuration is exquisitely sensitive to spin mixing and/or spin flip processes which are manifest as DOS singularities at eV=2 Delta(0)+/-eV(z). Both our Al and Be data show a well defined subgap peak whose magnitude grows dramatically as the parallel critical field is approached. Though this feature has previously been attributed to spin-orbit scattering, it is more consistent with fluctuations into a field induced mixed-spin state.

Quantum metallicity in a two-dimensional insulator.

One of the most far-reaching problems in condensed-matter physics is to understand how interactions between electrons, and the resulting correlations, affect the electronic properties of disordered two-dimensional systems. Extensive experimental and theoretical studies have shown that interaction effects are enhanced by disorder, and that this generally results in a depletion of the density of electronic states. In the limit of strong disorder, this depletion takes the form of a complete gap in the density of states. It is known that this 'Coulomb gap' can turn a pure metal film that is highly disordered into a poorly conducting insulator, but the properties of these insulators are not well understood. Here we investigate the electronic properties of disordered beryllium films, with the aim of disentangling the effects of the Coulomb gap and the underlying disorder. We show that the gap is suppressed by a magnetic field and that this drives the strongly insulating beryllium films into a low-temperature 'quantum metal' phase with resistance near the quantum resistance RQ = h/e2, where h is Planck's constant and e is the electron charge.

Comparison of crossmatch results obtained by ELISA, flow cytometry, and conventional methodologies.

The complement-dependent lymphocytotoxicity crossmatch (CXM) is the presently accepted standard for detection of donor-reactive alloantibodies in transplant patients. However, the newer flow cytometric (FXM) and ELISA (EXM) crossmatch technologies are increasingly used as substitutes for the CXM. We have compared the sensitivity and reproducibility of FXM vs. EXM and, in general, find them to be quite similar. However, when we compared the agreement of FXM vs. EXM in 112 donor/recipient combinations, we found that they identified different subsets of donor-specific alloantibodies in about 35% of the tests. When compared to the standard CXM method, the EXM correlated much better than did the FXM, yielding a much lower rate of false positive (2.5% vs. 8%) and false negative (7% vs. 18.5%) results. The reduction in time required to obtain a result (3 h) and the cost of materials ($25/test) was identical for the EXM and FXM. We conclude that the ELISA method for crossmatching has advantages over the flow cytometric method as a substitute for the present standard complement-dependent lymphocytotoxicity method.

Trans vivo analysis of human delayed-type hypersensitivity reactivity.

There are clinical situations in which it may be advantageous to monitor delayed type hypersensitivity (DTH) responses, an index of cell-mediated immunity, without exposing patients directly to the challenge antigens. For example, transplant patients may be at risk for becoming sensitized to donor antigens if injected with donor antigen during traditional skin tests. We describe an alternative method for human DTH testing, which involves the transfer of human peripheral blood mononuclear cells plus antigen into the pinnae or footpads of naive mice. This induces a measurable DTH-like swelling response, which we refer to as the "trans vivo DTH response." As proof of principle, we provide data obtained during trans vivo DTH studies with tetanus toxoid, cytomegalovirus (CMV) and alloantigens. In general, human T cells must be co-localized with antigen and human macrophages to produce swelling responses, and such responses are antigen-specific and require prior antigen sensitization. Not only does this assay offer a simple, reliable clinical monitoring device, but it also provides a model with which to study the in vivo mechanisms of human DTH responses.

Cytolytic activity against allogeneic human endothelia: resistance of cytomegalovirus-infected cells and virally activated lysis of uninfected cells.

BACKGROUND: Cytomegalovirus (CMV) has been implicated as an exacerbating agent in the development of transplant vascular sclerosis; however, specific etiologic mechanisms remain unresolved. Based upon our previous observations that CMV-infected endothelial cells (ECs) stimulate proliferation and cytokine production by allogeneic T cells, we now test the hypothesis that CMV-driven cytolytic activity may contribute to graft endothelial injury. METHODS: Limiting dilutions of CMV-seropositive or -seronegative donor-derived T cells were stimulated with CMV-infected or uninfected allogeneic ECs in the presence of interleukin-2. T-cell proliferation was monitored by assay of [3H]thymidine incorporation and stimulated T cells were tested for lytic activity against CMV-infected or uninfected radiolabeled EC targets by 51Cr release assay. Natural killer (NK) cell activity was examined by incubating freshly isolated peripheral blood mononuclear cells with 51Cr-labeled targets, followed by assay of radiolabel release. RESULTS: CMV-infected ECs were resistant to T cell- and NK-mediated cytolysis regardless of donor serostatus, nature of stimulation, or level of T-cell proliferation. In contrast, although uninfected ECs were unharmed by NK cells, these targets experienced significant lysis by T cells stimulated with either uninfected or CMV-infected ECs. CONCLUSIONS: These results implicate CMV-infected graft endothelium as a persistent source of infectious virus, a chronic stimulus for potentially destructive host inflammatory activity, and a potential trigger for the generation of lytic injury to uninfected bystander endothelia, suggesting multiple mechanisms by which this virus might perturb equilibrium at the graft/host interface.

Transplantation immunology.

The practice of clinical and experimental transplantation continues to evolve at a rapid pace. To appreciate the current transplant practices, it is first necessary to review transplant immunology in its proper context, ie, as a component of the complex series of events that promote the repair of damaged tissues. These processes are generally categorized as inflammation, immunity, and tissue repair/reinforcement. In general, there are 3 forms of graft rejection: hyperacute, acute, and chronic rejection. All 3 forms of graft rejection represent pathologic consequences of one or more of these repair-related processes. The various graft rejection responses also illustrate several complex immunologic principles that need to be considered. These include the definition of an alloantigen, the structure and function of major histocompatibility complex molecules, and the behavior of antigen-presenting cells and alloreactive T cells. This review combines these concepts and principles into a discussion of the 3 forms of graft rejection, each of which is addressed at the level of histopathology, pathobiology, incidence, and clinical strategies.

Clinical and economic impact of flow cytometry crossmatching in primary cadaveric kidney and simultaneous pancreas-kidney transplant recipients.

We retrospectively compared the clinical and financial impact of a final cross-match by T cell flow cytometry (FXM) versus conventional complement-dependent cytotoxicity (CXM) in consecutive primary cadaveric kidney (K) and primary simultaneous cadaveric pancreas-kidney (SPK) transplant recipients. Mean follow-up was 14 months for both the K (range, 5-22 months) and SPK (range, 5-22 months) recipients. There were no instances of a positive CXM result if the FXM result was negative. However, 18 of the 102 (18%) K recipients and 11 of the 66 (17%) SPK recipients were FXM positive, CXM negative, but no grafts lost to hyperacute rejection in this group. In addition, patient survival, graft survival, incidence of acute rejection, and kidney and pancreas function (immediate and late) were not different in the FXM-positive versus the FXM-negative groups. Charges for the CXM and FXM methods were compared over a 6-month period. During that period, the FXM charges averaged $583 less per recipient than the CXM charges (58% reduction in charges), and the time required to perform the FXM method was 50% of that required for the CXM method. These results demonstrate that a clinical pathway for primary transplantation that utilizes the FXM rather than the CXM final cross-match is clinically safe, with no adverse effect on posttransplant outcome, reduces organ preservation time by shortening the waiting period for the final cross-match results, and significantly reduces the tissue typing charges. However, about 9% of all primary K and SPK recipients will be FXM positive, CXM negative on final cross-match and will be unnecessarily denied a transplant. In this study, we describe a method to identify these patients so that they can be tested by traditional CXM to avoid being denied access to donor organs.