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S. Bud'ko

Publications and source records attributed to S. Bud'ko.

9 recordsLinked to original sources
arXiv2012

Magnetic and superconducting phase diagrams in ErNi2B2C

We present measurements of the superconducting upper critical field Hc2(T) and the magnetic phase diagram of the superconductor ErNi2B2C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi2B2C, extending between different magnetic transitions. A close correlation between magnetic transitions and Hc2(T) is found, showing that superconductivity is strongly linked to magnetism.

cond-mat.supr-con↗
arXiv2008

Possible two-gap superconductivity in NdFeAs(O,F) probed by point-contact Andreev-reflection spectroscopy

Systematic studies of the NdFeAsOF superconducting energy gap via the point-contact Andreev-reflection (PCAR) spectroscopy are presented. The PCAR conductance spectra show at low temperatures a pair of gap-like peaks at about 4 - 7 mV indicating the superconducting energy gap and in most cases also a pair of humps at around 10 mV. Fits to the s-wave two-gap model of the PCAR conductance allowed to determine two superconducting energy gaps in the system. The energy-gap features however disappear already at T* = 15 to 20 K, much below the particular Tc of the junction under study. At T* a zero-bias conductance (ZBC) peak emerges, which at higher temperatures usually overwhelms the spectrum with intensity significantly higher than the conductance signal at lower temperatures. Possible causes of this unexpected temperature effect are discussed. In some cases the conductance spectra show just a reduced conductance around the zero-bias voltage, the effect persisting well above the bulk transition temperature. This indicates a presence of the pseudogap in the system.

cond-mat.supr-con↗
arXiv2008

Generalized Elliott-Yafet theory of electron spin relaxation in metals: the origin of the anomalous electron spin life-time in MgB2

The temperature dependence of the electron spin relaxation time in MgB2 is anomalous as it does not follow the temperature dependence of the resistivity above 150 K, it has a maximum around 400 K, and it decreases for higher temperatures. This violates the well established Elliot-Yafet theory of electron spin relaxation in metals. We show that the anomaly occurs when the quasi-particle scattering rate (in energy units) becomes comparable to the energy difference between the conduction- and a neighboring band. We find that the anomalous behavior is related to the unique band structure of MgB$_2$ and the large electron-phonon coupling. The saturating spin-lattice relaxation can be regarded as the spin transport analogue of the Ioffe-Regel criterion of electron transport.

cond-mat.str-el↗
arXiv2008

Fermi surface and strong coupling superconductivity in single crystal NdFeAsO_{1-x}F_x

We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered oxypnictide superconductor, NdFeAsO_{1-x}F_x. We find a well-defined Fermi surface that consists of a large hole pocket at the Brillouin zone center and a smaller electron pocket in each corner of the Brillouin zone. The overall location and shape of the Fermi surface agrees reasonably well with calculations. The band dispersion is quite complicated with many flat bands located just below the chemical potential. We observe a superconducting gap of 20 meV, which indicates that this system is in the strong coupling regime. The emergence of a coherent peak below the critical temperature Tc and diminished spectral weight at the chemical potential above Tc closely resembles the spectral characteristics of the cuprates.

cond-mat.supr-con↗
arXiv2006

Point-contact spectroscopy of Al- and C-doped MgB2. Superconducting energy gaps and scattering studies

The two-band/two-gap superconductivity in aluminium and carbon doped MgB$_2$ has been addressed by the point-contact spectroscopy. Two gaps are preserved in all samples with $T_c's$ down to 22 K. The evolution of two gaps as a function of the critical temperature in the doped systems suggest the dominance of the band-filling effects but for the increased Al-doping the enhanced interband scattering approaching two gaps must be considered. The magnetic field dependences of the Andreev reflection excess currents as well as zero-energy density of states determined from the experimental data are used to analyze the intraband scattering. It is shown, that while the C-doping increases the intraband scattering in the $π$-band more rapidly then in the $σ$ band, the Al-doping does not change their relative weight.

cond-mat.supr-con↗
arXiv2005

Local superconducting density of states of ErNi2B2C

We present local tunnelling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C. The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states.

cond-mat.supr-con↗
arXiv2003

Magnetic Field Induced Density of States in Superconducting MgB$_{2}$: Measurement of Conduction Electron Spin-Susceptibility

The magnetic field dependence of the spin-susceptibility, $χ_{s}$ was measured in the superconducting state of high purity MgB$_{2}$ fine powders below 1.3 T. $χ_{s}$ was determined from the intensity of the conduction electron spin resonance spectra at 3.8, 9.4, and 35 GHz. At the lowest magnetic fields (0.14 T), a gap opens in the density of states at the Fermi energy and, accordingly, $χ_{s}(T)$ is small at low temperatures. Fields above 0.2 T (about 15 % of $H^{c}_{c2}$, the minimum upper critical field), destroy the gap. The field induced $χ_{s}$ is much larger than expected from current superconductor models of MgB$_{2}$.

cond-mat.supr-con↗
arXiv2002

Negative isotope effect of a BCS-like gap; an inelastic light scattering study

In a prior study of the boron isotope effect on the transition temperature ($T_c$) of $R$Ni$_2$B$_2$C ($R$ = Y, Lu) system, a positive isotope effect was observed, e.g., $T_{c}(^{10}B)> T_{c}(^{11}B)$. BCS theory predicts that the superconducting gap at zero temperature ($Δ_0$) is proportional to $T_c$. Therefore, the gap should also show a positive boron isotope effect. On the contrary, in an inelastic light scattering study, we report a negative boron isotope effect on the energy $Ω$ of the gap-like feature in B$_{2g}$ symmetry, e.g., $Ω(^{10}B)< Ω(^{11}B)$. The origin of the effect is discussed in terms of selective probing of a non-conventional electron-phonon interaction on part of the Fermi surface, probably near the X point in the Brillouin zone.

cond-mat.supr-con↗