arXiv subjects

A. Carrington

Publications and source records attributed to A. Carrington.

64 records · Page 4Linked to original sources

arXiv2004

Damping of the de Haas-van Alphen oscillations in the superconducting state of MgB_2

The de Haas-van Alphen (dHvA) signal arising from orbits on the $π$ Fermi surface sheet of the two-gap superconductor MgB$_2$ has been observed in the vortex state below $H_{c2}$. An extra attenuation of the dHvA signal, beyond those effects described in the conventional Lifshitz-Kosevich expression, is seen due to the opening of the superconducting gap. Our data show that the $π$ band gap is still present up to $H_{c2}$. The data are compared to current theories of dHvA oscillations in the superconducting state which allow us to extract estimates for the evolution of the $π$ band gap with magnetic field. Contrary to results for other materials, we find that the most recent theories dramatically underestimate the damping in MgB$_2$.

cond-mat.supr-con↗

arXiv2003

Determination of the Fermi Surface of MgB$_2$ by the de Haas-van Alphen effect

We report measurements of the de Haas-van Alphen effect for single crystals of MgB$_2$, in magnetic fields up to 32 Tesla. In contrast to our earlier work, dHvA orbits from all four sheets of the Fermi surface were detected. Our results are in good overall agreement with calculations of the electronic structure and the electron-phonon mass enhancements of the various orbits, but there are some small quantitative discrepancies. In particular, systematic differences in the relative volumes of the Fermi surface sheets and the magnitudes of the electron-phonon coupling constants could be large enough to affect detailed calculations of T$_{c}$ and other superconducting properties.

cond-mat.supr-con↗

arXiv2002

de Haas-van Alphen effect in single crystal MgB_2

We report observations of quantum oscillations in single crystals of the high temperature superconductor MgB_2. Three de Haas-van Alphen frequencies are clearly resolved. Comparison with band structure calculations strongly suggests that two of these come from a single warped Fermi surface tube along the c direction, and that the third arises from cylindrical sections of an in-plane honeycomb network. The measured values of the effective mass range from 0.44-0.68 m_e. By comparing these with band masses calculated recently by three groups, we find that the electron-phonon coupling strength lambda, is a factor \~3 larger for the c-axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.

cond-mat.supr-con↗

arXiv2002

Exponential Temperature Dependence of Penetration Depth in Single Crystal MgB_2

The temperature dependence of the London penetration depth,lambda(T), was measured in both single crystal and polycrystalline MgB_2 samples by a high-resolution, radio frequency technique. A clear exponential temperature dependence of lambda(T) was observed at low temperature, indicating s-wave pairing. A BCS fit to the lowest temperature data gives an in-plane energy gap Delta of 2.6\pm0.2 meV (2Δ/T_c=1.5\pm0.1), which is significantly smaller than the standard BCS weak coupling value of 3.5. We find that the data are best described by a two-gap model.

cond-mat.supr-con↗

arXiv2001

Exponential Temperature Dependence of Penetration Depth in MgB_2

The temperature dependence of the London penetration depth, $λ(T)$, was measured in polycrystalline MgB$_2$ samples by a high-resolution, radio frequency technique. A clear exponential temperature dependence of $λ(T)$ was observed at low temperature, indicating s-wave pairing. A fit to the data gives an effective energy gap $Δ$ of $2.8\pm0.4$ meV ($2Δ/k_BT_c=1.7\pm0.2$), which is significantly smaller than the standard BCS weak coupling value of 3.5. This may be explained either by an anisotropic gap or the influence of low frequency phonon modes.

cond-mat.supr-con↗

arXiv2000

Evidence for Surface Andreev Bound states in Cuprate Superconductors from Penetration Depth Measurements

Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth lambda_ab at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose a [110] interface show a strong upturn in lambda_ab at around 7K, when the field is oriented so that the supercurrents flow around this surface. In YBCO this upturn is completely suppressed by a field of ~0.1 T.

cond-mat.supr-con↗

arXiv2000

Measurements of the absolute value of the penetration depth in high-$ T_c$ superconductors using a tunnel diode resonator

A method is presented to measure the absolute value of the London penetration depth, $λ$, from the frequency shift of a resonator. The technique involves coating a high-$T_c$ superconductor (HTSC) with film of low - Tc material of known thickness and penetration depth. The method is applied to measure London penetration depth in YBa2Cu3O{7-δ} (YBCO) Bi2Sr2CaCu2O{8+δ} (BSCCO) and Pr{1.85}Ce{0.15}CuO{4-δ}$ (PCCO). For YBCO and BSCCO, the values of $λ(0)$ are in agreement with the literature values. For PCCO $λ\approx 2790$ Å, reported for the first time.

cond-mat.supr-con↗

arXiv2000

Meissner - London state in superconductors of rectangular cross-section in perpendicular magnetic field

The distribution of magnetic induction in Meissner state with finite London penetration depth is analyzed for platelet samples of rectangular cross-section in a perpendicular magnetic field. The exact 2D numerical solution of the London equation is extended analytically to the realistic 3D case. Data obtained on Nb cylinders and foils as well as single crystals of YBCO and BSCCO are in a good agreement with the model. The results are particularly relevant for magnetic susceptibility, rf and microwave resonator measurements of the magnetic penetration depth in high-Tc superconductors.

cond-mat.supr-con↗

arXiv1999

Low Temperature Penetration depth of κ-(ET)_2Cu[N(CN)_2]Br and κ-(ET)_2Cu(NCS)_2

We present high precision measurements of the penetration depth λof single crystals of κ-(ET)_2Cu[N(CN)2]Br and κ-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, λ\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, λ(0)=100\pm 20 μm.

cond-mat.supr-con↗

arXiv1998

Absence of non-linear Meissner effect in YBa2Cu3O6.95

We present measurements the field and temperature dependence of the penetration depth (lambda) in high purity, untwinned single crystals of YBa2Cu3O6.95 in all three crystallographic directions. The temperature dependence of lambda is linear down to low temperatures, showing that our crystals are extremely clean. Both the magnitude and temperature dependence of the field dependent correction to lambda however, are considerably different from that predicted from the theory of the non-linear Meissner effect for a d-wave superconductor (Yip-Sauls theory). Our results suggest that the Yip-Sauls effect is either absent or is unobservably small in the Meissner state of YBa2Cu3O6.95.

cond-mat.supr-con↗