arXiv subjects

S. M. Kazakov

Publications and source records attributed to S. M. Kazakov.

At least 55 records · Page 3Linked to original sources

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.

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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.

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arXiv2004

Two regimes in the magnetic field response of superconducting MgB$_2$

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the $π$-band of the two-band superconductor MgB$_2$. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value : The gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the $π$-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the $σ$-band. Our findings allow a deeper understanding of the unique phase diagram of MgB$_2$.

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arXiv2004

Superconductivity in Geometrically Frustrated Pyrochlore RbOs2O6

We report the basic thermodynamic properties of the new geometrically frustrated beta-pyrochlore bulk superconductor RbOs2O6 with a critical temperature Tc = 6.4 K. Specific heat measurements are performed in magnetic fields up to 12 T. The electronic density of states at the Fermi level in the normal state results in gamma = (33.7 \pm 0.2) mJ/mol_f.u./K^2. In the superconducting state, the specific heat follows conventional BCS-type behavior down to 1 K, i.e. over three orders of magnitude in specific heat data. The upper critical field slope at Tc is 1.2 T/K, corresponding to a Maki-parameter alpha = 0.64 \pm 0.1. From the upper critical field mu0 Hc2 \approx 6 T at 0 K, we estimate a Ginzburg-Landau coherence length xi \approx 7.4 nm. RbOs2O6 is the second reported metallic AB2O6 type pyrochlore compound after KOs2O6, and one of only three pyrochlore superconductors in addition to Cd2Re2O7 and KOs2O6.

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arXiv2004

Absence of a boron isotope effect in the magnetic penetration depth of MgB$_{2}$

The magnetic penetration depth $λ(0)$ in polycrystalline MgB$_{2}$ for different boron isotopes ($^{10}$B/$^{11}$B) was investigated by transverse field muon spin rotation. No boron isotope effect on the penetration depth $λ(0)$ was found within experimental error: $Δλ(0)/λ(0) = -0.8(8)%$, suggesting that MgB$_{2}$ is an adiabatic superconductor. This is in contrast to the substantial oxygen isotope effect on $λ(0)$ observed in cuprate high-temperature superconductors.

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arXiv2003

Unconventional Charge Ordering in Na0.70CoO2 below 300 K

We present the results of measurements of the dc-magnetic susceptibility chi(T) and the 23Na-NMR response of Na_{0.70}CoO_{2} at temperatures between 50 and 340 K. The chi(T) data suggest that for T > 75 K, the Co ions adopt an effective configuration of Co^{3.4+}. The 23Na-NMR response reveals pronounced anomalies near 250 and 295 K, but no evidence for magnetic phase transitions is found in chi(T). Our data suggest the onset of a dramatic change in the Co 3d-electron spin dynamics at 295 K. This process is completed at 230 K. Our results maybe interpreted as evidence for either a tendency to electron localization or an unconventional charge-density wave phenomenon within the cobalt oxide layer, CoO_2, 3d electron system near room temperature.

cond-mat.str-el↗

arXiv2003

Point-Contact Spectroscopy in MgB_2: from Fundamental Physics to Thin-Film Characterization

In this paper we highlight the advantages of using point-contact spectroscopy (PCS) in multigap superconductors like MgB_2, both as a fundamental research tool and as a non-destructive diagnostic technique for the optimization of thin-film characteristics. We first present some results of crucial fundamental interest obtained by directional PCS in MgB_2 single crystals, for example the temperature dependence of the gaps and of the critical fields and the effect of a magnetic field on the gap amplitudes. Then, we show how PCS can provide useful information about the surface properties of MgB_2 thin films (e.g. Tc, gap amplitude(s), clean or dirty-limit conditions) in view of their optimization for the fabrication of tunnel and Josephson junctions for applications in superconducting electronics.

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arXiv2003

Evidence for two electronic components in NaxCoO2 (x = 0.7-0.75)

Thermodynamic and transport measurements on NaxCoO2 (x ~ 0.7-0.75) over a wide temperature range reveal a strongly enhanced low energy excitation spectrum. The zero-field specific heat and resistance at low temperature are proportional to T^n with n<1, in sharp contrast to behavior found in ordinary Landau Fermi-liquid metals. For temperatures below ~5 K this unusual excitation seen in the specific heat is partially suppressed in a magnetic field, following a T/B scaling. The specific heat reduction at low T is partially compensated by an increase at higher temperature. A quantitative comparison with the magnetoresistance and the thermopower indicates that the low- energy electronic state requires a description beyond the single band model.

cond-mat.str-el↗

arXiv2003

MgB2 and Mg1-xAlxB2 single crystals: high-pressure growth and physical properties

Single crystals of MgB2 have been grown with a high-pressure cubic anvil technique. They grow via the peritectic decomposition of the MgNB9 ternary nitride. The crystals are of a size up to 2x1x0.1mm3 with a weight up to 230 micrograms. Typically they have transition temperatures between 38 and 38.6 K with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that the MgB2 phase is stable at least up to 2190C at high hydrostatic pressure in the presence of Mg vapor under high pressure. Substitution of aluminum for magnesium in single crystals leads to stepwise decrease of Tc. This indicates a possible appearance of superstructures or phases with different Tc's. The upper critical field decreases with Al doping.

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arXiv2003

The Magnetic-field Dependence of the Gaps in a Two-band Superconductor: A Point-contact Study of MgB_2 Single Crystals

We present the results of directional point-contact measurements in MgB_2 single crystals, in magnetic fields up to 9 T parallel to the c axis. By fitting the conductance curves of our point contacts -- showing clear Andreev-reflection features -- with a generalized Blonder-Tinkham-Klapwijk model, we were able to extract the values of the two gaps Delta_sigma and Delta_pi. The comparison of the resulting Delta_sigma(B) and Delta_pi(B) curves to the theoretical predictions of the two-band model in dirty limit, recently appeared in literature, allows the first direct test of this model and gives a clear and quantitative proof that the pi band, even in the best single crystals, is in the moderate dirty limit.

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arXiv2003

Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6

We present the results of measurements of the thermal conductivity $κ$ of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and 300 K and with the heat flow directed either along or perpendicular to the chain direction. The analysis of the anisotropy of the heat transport allowed for the identification of a small but non-negligible magnon contribution $κ_m$ along the chains, superimposed on the dominant phonon contribution $κ_ph$. At temperatures above about 100 K the energy diffusion constant D_E(T), calculated from the $κ_m(T)$ data, exhibits similar features as the spin diffusion constant D_S(T), previously measured by NMR. In this regime, the behaviour of both transport parameters is consistent with a diffusion process that is caused by interactions inherent to one-dimensional S=1 spin systems.

cond-mat.str-el↗

arXiv2003

Directional point-contact spectroscopy of MgB2 single crystals in magnetic fields: two-band superconductivity and critical fields

The results of the first directional point-contact measurements in MgB2 single crystals, in the presence of magnetic fields up to 9 T either parallel or perpendicular to the ab planes, are presented. By applying suitable magnetic fields, we separated the partial contributions of the sigma and pi bands to the total Andreev-reflection conductance. Their fit with the BTK model allowed a very accurate determination of the temperature dependency of the gaps (Delta_sigma and Delta_pi), that resulted in close agreement with the predictions of the two-band models for MgB2. We also obtained, for the first time with point-contact spectroscopy, the temperature dependence of the (anisotropic) upper critical field of the sigma band and of the (isotropic) upper critical field of the pi band.

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arXiv2003

Effects of proton irradiation and ageing on the superconducting properties of single crystalline and polycrystalline MgB2

We present magnetisation data on crystalline and polycrystalline MgB2 before and after irradiation with ~1MeV protons. In the virgin crystal the critical current density is below our noise floor of 103A/cm2 over most of the field range. However, after irradiation a peak occurs in the current density as a function of applied magnetic field as the upper critical field Hc2 is approached. After subsequent ageing over a time period of three months, the peak effect is greatly enhanced, exhibiting much stronger pinning over a wide field range, and the upper critical field is approximately doubled, accompanied by a 2K reduction in transition temperature. Similar studies were made on polycrystalline fragments, where irradiation leads to an increased irreversibility field (Jc is enhanced at high fields but decreased at low fields) and a suppression in transition temperature. However, after two years of ageing both parameters returned towards those of the virgin sample.

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arXiv2003

Anisotropic properties of MgB2 by torque magnetometry

Anisotropic properties of superconducting MgB2 obtained by torque magnetometry are compared to theoretical predictions, concentrating on two issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly, from the evaluation of torque vs angle curves it is concluded that the anisotropy of the penetration depth gamma_lambda has to be substantially higher at low temperature than theoretical estimates, at least in fields higher than 0.2 T.

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arXiv2003

Single crystal growth and properties of MgB2 and Mg(B1-xCx)2

Single crystals of MgB2 and Mg(B1-xCx)2 have been grown using cubic anvil technique. Tc values vary in a wide range (39-9 K) with carbon content varying from 0 up to 16%. Using SiC as the precursor leads to C and not to Si substituted crystals. Micro-hardness measurements performed on MgB2 single crystals give average value of 1100 kg/mm2.

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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.

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arXiv2003

Phonon dispersion and lifetimes in MgB2

We measure phonon dispersion and linewidth in a single crystal of MgB_2 along the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We use Density Functional Theory to compute the effect of both electron-phonon coupling and anharmonicity on the linewidth, obtaining excellent agreement with experiment. Anomalous broadening of the E_2g phonon mode is found all along Gamma-A. The dominant contribution to the linewidth is always the electron-phonon coupling.

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