arXiv subjects

P. W. Adams

Publications and source records attributed to P. W. Adams.

At least 37 records · Page 2Linked to original sources

arXiv2011

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, Hex, is a non-linear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore the magnitude Hex is unaffected by the superconducting phase. In addition, Hex 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.

cond-mat.supr-con↗

arXiv2010

Anisotropic superconductivity in layered LaSb$_2$: the role of phase fluctuations

We present magnetic susceptibility and electrical transport measurements of the highly anisotropic compound LaSb$_2$ observing a very broad transition into a clean, consistent with type-I, superconducting state with distinct features of 2 dimensionality. Application of hydrostatic pressure induces a 2- to 3-dimensional crossover evidenced by a reduced anisotropy and transition width. The superconducting transition appears phase fluctuation limited at ambient pressure with fluctuations observed for temperatures greater than 8 times the superconducting critical temperature.

cond-mat.supr-con↗

arXiv2010

Intrinsic electron-glassiness in strongly-localized Be films

We present results of out--of-equilibrium transport measurements made on strongly-localized Beryllium films and demonstrate that these films exhibit all the earmarks of intrinsic electron-glasses. These include slow (logarithmic) relaxation, memory effects, and more importantly, the observation of a memory dip that has a characteristic width compatible with the carrier-concentration of beryllium. The latter is an empirical signature of the electron-glass. Comparing various non-equilibrium attributes of the beryllium films with other systems that exhibit intrinsic electron-glasses behavior reveals that high carrier-concentration is their only common feature rather than the specifics of the disorder that rendered them insulating. It is suggested that this should be taken as an important hint for any theory that attempts to account for the surprisingly slow relaxation times observed in these systems.

cond-mat.str-el↗

arXiv2010

Optical conductivity and superconductivity in LaSb$_2$

We have measured the resistivity, optical conductivity, and magnetic susceptibility of LaSb$_2$ to search for clues as to the cause of the extraordinarily large linear magnetoresistance and to explore the properties of the superconducting state. We find no evidence in the optical conductivity for the formation of a charge density wave state above 20 K despite the highly layered crystal structure. In addition, only small changes to the optical reflectivity with magnetic field are observed indicating that the MR is due to scattering rate, not charge density, variations with field. Although a superconducting ground state was previously reported below a critical temperature of 0.4 K, we observe, at ambient pressure, a fragile superconducting transition with an onset at 2.5 K. In crystalline samples, we find a high degree of variability with a minority of samples displaying a full Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The application of pressure stabilizes the superconducting transition and reduces the anisotropy of the superconducting phase.

cond-mat.str-el↗

arXiv2010

Structure and physical properties of the noncentrosymmetric superconductor Mo3Al2C

We have synthesized polycrystalline samples of the noncentrosymmetric superconductor Mo3Al2C by arc and RF melting, measured its transport, magnetic and thermodynamic properties, and computed its band structure. Experimental results indicate a bulk superconducting transition at Tc ~ 9.2 K, while the density of states at the Fermi surface is found to be dominated by Mo d-orbitals. Using the measured values for the lower critical field Hc1, upper critical field Hc2, and the specific heat C, we estimated the thermodynamic critical field Hc(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ(0). The specific heat jump at Tc, ΔC/γTc = 2.14, suggests that Mo3Al2C is moderately-to-strongly coupled, consistent with the fast opening of the gap, as evidenced by the rapid release of entropy below Tc from our electronic specific heat measurements. Above 2K the electronic specific heat exhibits the power law behavior, suggesting that synthesis of single crystals and measurements at lower temperature are needed to establish whether the gap is anisotropic. The estimated value of the upper critical field Hc2(0) is close to the calculated Pauli limit, therefore further studies are needed to determine whether the absence of an inversion center results in a significant admixture of the triplet component of the order parameter.

cond-mat.supr-con↗

arXiv2010

Saturation of the Anomalous Hall Effect in Critically Disordered Ultra-thin CNi3 Films

We demonstrate that a distinct high-disorder anomalous Hall effect phase emerges at the correlated insulator threshold of ultra-thin, 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 Gxy ~ G^1.6. However, at sufficiently high disorder the system begins to enter the 2D correlated insulator regime, at which point the Hall resistance Rxy abruptly saturates and the scaling exponent becomes 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.

cond-mat.str-el↗

arXiv2009

Pairing resonance as a normal-state spin probe in ultra-thin Al films

We present a quantitative analysis of the low-temperature, high parallel field pairing resonance in ultra-thin superconducting Al films with dimensionless conductance g>>1. In this regime we derive an analytical expression for the tunneling density-of-states spectrum from which a variety of normal-state spin parameters can be extracted. We show that by fitting tunneling data at several supercritical parallel magnetic fields we can determine all of the relevant parameters that have traditionally been obtained via fits to tunneling data in the superconducting phase. These include the spin-orbit scattering rate, the antisymmetric Landau parameter G^0, and the orbital pair-breaking parameter.

cond-mat.supr-con↗

arXiv2009

Temperature dependence of resistivity and Hall-coefficient in a strongly disordered metal: NbN

We report the temperature dependence of resistivity (rho) and Hall coefficient (R_H) in the normal state of homogeneously disordered epitaxial NbN thin films with kFl~3.27-10.12. The superconducting transition temperature (Tc) of these films varies from 8.13K to 16.8K. While our least disordered film displays usual metallic behavior, for all the films with kFl<8.13, both and are negative up to 300K. We observe that R_H(T) varies linearly with rho(T) for all the films. Measurements performed on a 2nm thick Be film shows similar behavior >. This behavior is inconsistent with existing theories of localization and e-e interactions in a disordered metal.

cond-mat.dis-nn↗

arXiv2009

Measurement of Conduction Electron Polarization Via the Pairing Resonance

We show that the pairing resonance in the Pauli-limited normal state of ultra-thin 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.

cond-mat.supr-con↗

arXiv2009

Spin-Orbit Scattering and Quantum Metallicity in Ultra-Thin Be Films

We compare and contrast the low temperature magnetotransport properties of ultra-thin, insulating, Be films with and without spin-orbit scattering (SOS). Beryllium films have very little intrinsic SOS, but by "dusting" them with sub-monolayer coverages of Au, one can introduce a well controlled SOS rate. Pure Be films with sheet resistance R >R_Q exhibit a low-temperature negative magnetoresistance (MR) that saturates to the quantum resistance R_Q = h/e^2. This high-field quantum metal phase is believed to represent a new ground state of the system. In contrast, the corresponding negative MR in Be/Au films is greatly diminished, suggesting that, in the presence of strong SOS, the quantum metal phase can only be reached at field scales well beyond those typically available in a low temperature laboratory.

cond-mat.str-el↗

arXiv2008

Fermi-liquid effects in the gapless state of marginally thin superconducting films

We present low temperature tunneling density-of-states measurements in Al films in high parallel magnetic fields. The thickness range of the films, t=6-9 nm, was chosen so that the orbital and Zeeman contributions to their parallel critical fields were comparable. In this quasi-spin paramagnetically limited configuration, the field produces a significant suppression of the gap, and at high fields the gapless state is reached. By comparing measured and calculated tunneling spectra we are able to extract the value of the antisymmetric Fermi-liquid parameter G^0 and thereby deduce the quasiparticle density dependence of the effective parameter G^0_{eff} across the gapless state.

cond-mat.supr-con↗

arXiv2006

Density of States, Entropy, and the Superconducting Pomeranchuk Effect in Pauli-Limited Al Films

We present low temperature tunneling density of states measurements of Pauli-limited Al films in which the Zeeman and orbital contributions to the critical field are comparable. We show that films in the thickness range of 6-7 nm exhibit a reentrant parallel critical field transition which is associated with a high entropy superconducting phase, similar to the high entropy solid phase of 3He responsible for the Pomeranchuk effect. This phase is characterized by an excess of states near the Fermi energy so long as the parallel critical field transition remains second order. Theoretical fits to the zero bias tunneling conductance are in good agreement with the data well below the transition but theory deviates significantly near the transition. The discrepancy is a consequence of the emergence of e-e interaction correlations as one enters the normal state.

cond-mat.supr-con↗

arXiv2006

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 ultra-thin superconducting Al films in supercritical magnetic fields. Particular emphasis is placed on effects of the perpendicular component of the magnetic field on the resonance energy and magnitude. Though the resonance is broadened and attenuated by $H_\bot$ as expected, its energy is shifted upward linearly with $H_\bot$. Extension of the original theory of the resonance to include strong perpendicular fields shows that at sufficiently large $H_\bot$ 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 the resonance feature to higher energy.

cond-mat.supr-con↗

arXiv2005

Interface Spin-Orbit Coupling in a Non-centrosymmetric Thin-Film Superconductor

We present a detailed study of the effects of interface spin-orbit coupling (ISOC) on the critical field behavior of non-centrosymmetric (NCS), ultra-thin superconducting Be/Au bilayers. Parallel field measurements were made in bilayers with Be thicknesses in the range of 2 - 10 nm and Au coverages of 0.5 nm. Though the Au had no significant 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 Hc||/Δ~ 1/d suggesting that the spin-orbit coupling energy was proportional to Delta/d^2. Tilted field measurements showed that contrary to recent theory, the ISOC induces a large in-plane superconducting susceptibility but only a very small transverse susceptibility.

cond-mat.supr-con↗

arXiv2004

Super-hard Superconductivity

We present a study of the magnetic response of Type-II superconductivity in the extreme pinning limit, where screening currents within an order of magnitude of the Ginzburg-Landau depairing critical current density develop upon the application of a magnetic field. We show that this "super-hard" limit is well approximated in highly disordered, cold drawn, Nb and V wires whose magnetization response is characterized by a cascade of Meissner-like phases, each terminated by a catastrophic collapse of the magnetization. Direct magneto-optic measurements of the flux penetration depth in the virgin magnetization branch are in excellent agreement with the exponential model in which J_c(B)=J_co exp(-B/B_o), where J_co~5x10^6 A/cm^2 for Nb. The implications for the fundamental limiting hardness of a superconductor are discussed.

cond-mat.supr-con↗

arXiv2004

Low Temperature Susceptibility of the Noncentrosymmetric Superconductor CePt_3Si

We report ac susceptibility measurements of polycrystalline CePt_3Si down to 60 mK and in applied fields up to 9 T. In zero field, a full Meissner state emerges at temperatures T/Tc < 0.3, where Tc=0.65 K is the onset transition temperature. Though transport measurements show a relatively high upper critical field Bc2 ~ 4-5 T, the low temperature susceptibility, χ', is quite fragile to applied field, with χ' diminishing rapidly in fields of a few kG. Interestingly, the field dependence of χ' is well described by the power law, 4πχ'=(B/B_c)^{1/2}, where Bc is the field at which the onset of resistance is observed in transport measurements.

cond-mat.supr-con↗

arXiv2003

Non--Ginsburg-Landau Critical Current Behavior in MgCNi3

We present transport critical current measurements on microfibers consisting of a 80-nm thick layer of polycrystalline MgCNi3 synthesized directly onto 7-micron diameter carbon fibers. Near the transition temperature, Tc, the critical current density, Jc, is well described by the power law form [1-(T/T_c)^2)]^2, with no crossover to the Ginsburg-Landau exponent 1.5. We extrapolate Jc(0) ~ 4 x 10^7 A/cm^2, which is an order of magnitude greater than estimates obtained from magnetization measurements of polycrystalline powders. The field dependence is purely exponential Jc(T,H) = Jc(T) exp(-H/Ho) over the entire field range of 0 to 9 T. The unconventional scaling behavior of the critical current appears rooted in an anomalous temperature dependence of the London penetration depth, suggesting an unusual superconducting ground state in MgCNi3.

cond-mat↗

arXiv2003

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 ultra-thin 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 = 2Delta +- 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.

cond-mat.supr-con↗