arXiv subjects

J. Karpinski

Publications and source records attributed to J. Karpinski.

At least 127 records · Page 7Linked to original sources

arXiv2005

Temperature dependent anisotropy of the penetration depth and coherence length in MgB$_2

We report measurements of the temperature dependent anisotropies ($γ_λ$ and $γ_ξ$) of both the London penetration depth $λ$ and the upper critical field of MgB$_2$. Data for $γ_λ=λ_c/λ_a$ was obtained from measurements of $λ_{a}$ and $λ_c$ on a single crystal sample using a tunnel diode oscillator technique. $γ_ξ=H_{c2}^{\parallel c}/H_{c2}^{\bot c}$ was deduced from field dependent specific heat measurements on the same sample. $γ_λ$ and $γ_ξ$ have opposite temperature dependencies, but close to $T_c$ tend to a common value ($γ_λ\simeq γ_ξ=1.75\pm0.05$). These results are in good agreement with theories accounting for the two gap nature of MgB$_2$

cond-mat.supr-con↗

arXiv2005

Evidence for a smooth superconductor to normal state transition for nonzero applied magnetic field in RbOs2O6

The effect of the magnetic field on the critical behavior of RbOs2O6 is investigated. We present and analyze magnetization data, revealing remarkable consistency with a magnetic field induced finye size effect. It is traced back that at temperatures Tp(H) smaller than Tc the correlation length cannot grow beyond the limiting length scale set by the magnetic field. Thus, for nonzero H the transition from the superconducting to the normal state turns out to be smooth and the apropriately scaled magnetization data fall on a single curve.

cond-mat.supr-con↗

arXiv2005

Intrinsic thermodynamic properties of the pyrochlore superconductor RbOs2O6 extracted by condensation energy analysis

We develop a general procedure for the analysis of bulk thermodynamic data of a superconductor for samples containing a metallic non-superconducting second phase. The method is based on the condensation energy and it allows the extraction of the intrinsic properties of a superconductor even for non-ideal samples. Applying this procedure to the recently discovered geometrically frustrated beta-pyrochlore superconductor RbOs2O6 (Tc = 6.4 K) yields a Sommerfeld coefficient as high as 79 mu J/g/K^2 (44 mJ/mol_f.u./K^2). RbOs2O6 is inferred to be a strong type-II superconductor (kappa(Tc) = 23) in the intermediate-coupling regime similar to niobium (lambda_ep \approx 1). From the upper critical field mu_0 H_c2 \approx 6 T at 0 K, we estimate a Ginzburg-Landau coherence length xi \approx 74 AA. The condensation energy is 860 mu J/g (483 mJ/mol_f.u.) resulting in 1/(8 pi) (gamma_1 Tc^2)/DeltaU_1 \approx 0.15, a value well in the range of conventional phonon-mediated superconductors. The superconducting electronic specific heat indicates conventional s-wave pairing. The experimental Sommerfeld coefficient of 44 mJ/mol_f.u./K^2 is about 4 times larger than the one found in band structure calculations. Together with the electron-phonon coupling constant lambda_ep \approx 1 this leaves an additional lambda_add \approx 2.4 for enhancement due to other mechanisms.

cond-mat.supr-con↗

arXiv2004

Study of pressure effect on the magnetic penetration depth in MgB$_2$

A study of the pressure effect on the magnetic penetration depth $λ$ in polycrystalline MgB$_{2}$ was performed by measuring the temperature dependence of the magnetization under an applied pressure of 0.15 and 1.13 GPa. We found that $λ^{-2}$ at low temperature is only slightly affected by pressure [$\frac{Δλ^{-2}}{λ^{-2}} = 1.5(9)%$], in contrast to cuprate superconductors, where, in the same range of pressure, a very large effect on $λ^{-2}$ was found. Theoretical estimates indicate that most of the pressure effect on $λ^{-2}$ in MgB$_2$ arises from the electron-phonon interaction.

cond-mat.supr-con↗

arXiv2004

Study of the magnetic penetration depth in RbOs_2O_6

Measurements of the magnetic field penetration depth λin the pyrochlore superconductor RbOs_2O_6 (T_c\simeq6.3 K) were carried out by means of the muon-spin-rotation (\muSR) technique. At low temperatures λ^{-2}(T) saturates and becomes constant below T\simeq 0.2T_c, in agreement with what is expected for weak-coupled s-wave BCS superconductors. The value of λat T=0 was found to be in the range of 250 nm to 300 nm. \muSR and equilibrium magnetization measurements both reveal that at low temperatures $λ$ is almost (at the level of 10%) independent of the applied magnetic field. This result suggests that the superconducting energy gap in RbOs_2O_6 is isotropic.

cond-mat.supr-con↗

arXiv2004

Influence of carbon substitution on the heat transport in single crystalline MgB2

We report data on the thermal conductivity κ(T,H) in the basal plane of hexagonal single-crystalline and superconducting Mg(B_{1-x}C_x)_2 (x= 0.03, 0.06) at temperatures between 0.5 and 50 K, and in external magnetic fields H between 0 and 50 kOe. The substitution of carbon for boron leads to a considerable reduction of the electronic heat transport, while the phonon thermal conductivity seems to be much less sensitive to impurities. The introduction of carbon enhances mostly the intraband scattering in the σ-band. In contrast to the previously observed anomalous behavior of pure MgB$_2$, the Wiedemann-Franz law is valid for Mg(B_0.94 C_0.06)_2 at low temperatures.

cond-mat.supr-con↗

arXiv2004

Al substitution in MgB2 crystals: influence on superconducting and structural properties

Single crystals of Mg1-xAlxB2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of optimization of already developed MgB2 crystal growth procedure. Systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg1-xAlxB2 crystals was found to be slightly different than that one observed for polycrystalline samples since, even a very small substitution of 1-2% of Al leads to the decrease of Tc by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate on the appearance of second precipitation phase in the crystals with x > 0.1. This is in a form of non-superconducting MgAlB4 domains in the structure of superconducting Mg1-xAlxB2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, Hc2, for H//c for the samples with small x, significant reduction of the Hc2 anisotropy at lower temperatures, and decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with Tc in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing Tc down to 20 K. It may be related to an appearance of the precipitation phase in the Mg1-xAlxB2 structure.

cond-mat.supr-con↗

arXiv2004

Evidence for Single-gap Superconductivity in Mg(B_{1-x}C_x)_2 Single Crystals with x=0.132 from Point-Contact Spectroscopy

We report the results of the first directional point-contact measurements in Mg(B_{1-x}C_{x})_2 single crystals with 0.047 <= x <= 0.132. The two-gap superconductivity typical of MgB_2 persists up to x=0.105. In this region, the values of the gaps Delta_{sigma} and Delta_{pi} were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk (BTK) model, and confirmed by the single-band BTK fit of the sigma- and pi-band conductances, separated by means of a magnetic field. At x=0.132, when T_{c}=19 K, we clearly observed for the first time the merging of the two gaps into one of amplitude Delta~3 meV.

cond-mat.supr-con↗

arXiv2004

Evidence for s-wave superconductivity in the new beta-pyrochlore oxide RbOs2O6

We report the results of 87Rb NMR measurements on RbOs2O6, a new member of the family of the superconducting pyrochlore-type oxides with a critical temperature Tc = 6.4 K. In the normal state, the nuclear spin-lattice relaxation time T1 obeys the Korringa-type relation T1T = constant and the Knight shift is independent of temperature, indicating the absence of strong magnetic correlations. In the superconducting state, T1^{-1}(T) exhibits a tiny coherence enhancement just below Tc, and decreases exponentially with further decreasing temperatures. The value of the corresponding energy gap is close to that predicted by the conventional weak-coupling BCS theory. Our results indicate that RbOs2O6 is a conventional s-wave-type superconductor.

cond-mat.supr-con↗

arXiv2004

Synthesis of superconducting pyrochlore RbOs2O6

RbOs2O6, the third superconducting pyrochlore oxide (known so far), has been synthesized by encapsulation and by high pressure techniques. Suitable post chemical treatment of the as-prepared sample allowed us to eliminate the impurity phases. Bulk superconductivity with Tc=6.4 K was observed in magnetisation and specific heat measurements. The transition temperature of RbOs2O6 was found to be the same for both preparation methods. Structural investigations showed that Rb atoms occupy the 8b site in the pyrochlore lattice with a lattice parameter of 10.1137(1) A.

cond-mat.supr-con↗

arXiv2004

Anomaly in YBa2Cu4O8 charge distribution below Tc: a zero-field muSR study

Zero-field $μ$SR measurements in $^{63}$Cu isotope enriched and natural YBa$_2$Cu$_4$O$_8$ powders are presented. The temperature dependence of the $μ^+$ relaxation rate is characterized by a sizeable enhancement below T$_c$. The comparison of the asymmetry decay in the two samples reveals that the $μ^+$ relaxation is driven by nuclear dipole interaction from 300 K down to 4.2 K. It is argued that the increase in the relaxation below T$_c$ originates from a change in $μ^+$ site, possibly due to a modification in the charge distribution within CuO chains.

cond-mat.str-el↗

arXiv2004

A Point-Contact Study of the Superconducting Gaps in Al-Substituted and C-Substituted MgB_2 Single Crystals

We present the results of directional point-contact spectroscopy in state-of-the-art Mg_{1-x}Al_xB_2 and Mg(B_{1-y}C_{y})_2 single crystals produced at ETH, Zurich. Fitting the conductance curves of our point contacts, that always feature Andreev reflection structures, we obtained the doping dependence of the gap amplitudes. The results are discussed in comparison with other experimental findings and relevant theoretical predictions. We conclude that the physics of Al-substituted crystals at x > 0.09 might be governed by phase segregation, while C-substituted crystals unexpectedly show a doping-induced transition to single-gap superconductivity at y=0.132.

cond-mat.supr-con↗

arXiv2004

Pressure effect on the in-plane magnetic penetration depth in YBa_2Cu_4O_8

We report a study of the pressure effect (PE) on the in-plane magnetic field penetration depth lambda_{ab} in YBa_2Cu_4O_8 by means of Meissner fraction measurements. A pronounced PE on lambda_{ab}^{-2}(0) was observed with a maximum relative shift of Δλ^{-2}_{ab}/λ^{-2}_{ab}= 44(3)% at a pressure of 10.2 kbar. It arises from the pressure dependence of the effective in-plane charge carrier mass and pressure induced charge carrier transfer from the CuO chains to the superconducting CuO_2 planes. The present results imply that the charge carriers in YBa_2Cu_4O_8 are coupled to the lattice.

cond-mat.supr-con↗

arXiv2004

Anisotropy and internal field distribution of MgB2 in the mixed state at low temperatures

Magnetization and muon spin relaxation on MgB2 were measured as a function of field at 2 K. Both indicate an inverse-squared penetration depth strongly decreasing with increasing field H below about 1 T. Magnetization also suggests the anisotropy of the penetration depth to increase with increasing H, interpolating between a low Hc1 and a high Hc2 anisotropy. Torque vs angle measurements are in agreement with this finding, while also ruling out drastic differences between the mixed state anisotropies of the two basic length scales penetration depth and coherence length.

cond-mat.supr-con↗

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↗

arXiv2004

Carbon substitution in MgB2 single crystals: structural and superconducting properties

Growth of carbon substituted magnesium diboride Mg(B1-xCx)2 single crystals with 0<x<0.15 is reported and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, Tc of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field Hc2 is observed. Comparing with the non-substituted compound, Hc2 at 15K for x=0.05 is enhanced by more then a factor of 2 for H oriented both perpendicular and parallel to the ab-plane. This enhancement is accompanied by a reduction of the Hc2-anisotropy coefficient gamma from 4.5 (for non-substituted compound) to 3.4 and 2.8 for the crystals with x = 0.05 and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows Hc2 (defined at zero resistance) higher than 7 and 24 T for H oriented perpendicular and parallel to the ab-plane, respectively. Observed increase of Hc2 cannot be explained by the change in the coherence length due to disorder-induced decrease of the mean free path only.

cond-mat.supr-con↗

arXiv2004

Strongly enhancend anisotropic upper critical field in carbon substituted MgB2: impact of Fermi surface changes

The upper critical field Hc2||c(0) of 85 kOe in Mg(B0.94C0.06)2 single crystals, determined from torque measurements, is more than twice as large as that one of 31 kOe in unsubstituted MgB2. Anisotropy of Hc2 increases from about 3.4 near Tc, the value close to that in MgB2, to about 4 at low temperature, the value considerably lower than that in MgB2. The corresponding Hc2||ab(0) of about 330-350 kOe is likely close to the maximum enhancement due to C substitution. The position of a sharp peak in the irreversible torque Hmax was found to be shifted to lower reduced fields H/Hc2 in comparison to its position in unsubstituted MgB2, indicating increasing disorder. The enhancement of Hc2 can be explained as a disorder effect only if the main result of disorder is to make the p bands more dirty while not affecting the s bands as much. In addition to disorder and weakened electron-phonon coupling, the impact of the Fermi level shifting into a region with lower s Fermi velocities has to be taken into account in the analysis of Hc2 data as well.

cond-mat.supr-con↗

arXiv2004

Pressure effects on the transition temperature and the magnetic field penetration depth in the pyrochlore superconductor RbOs_2O_6

We report magnetization measurements under high hydrostatic pressure in the newly discovered pyrochlore superconductor RbOs_2O_6 (T_c\simeq6.3K at p=0). A pronounced and {\it positive} pressure effect (PE) on T_c with dT_c/dp =0.090(1)K/kbar was observed, whereas no PE on the magnetic penetration depth λwas detected. The relative pressure shift of T_c [ dlnT_c/dp \simeq 1.5%/kbar] is comparable with the highest values obtained for highly underdoped high-temperature cuprate superconductors. Our results suggest that RbOs_2O_6 is an adiabatic BCS-type superconductor.

cond-mat.supr-con↗