arXiv subjects

K. Nenkov

Publications and source records attributed to K. Nenkov.

52 records · Page 3Linked to original sources

arXiv2003

Circular dichroism in angle-resolved photoemission spectra of under- and overdoped Pb-Bi2212

We use angle-resolved photoemission with circularly polarized excitation to demonstrate that in the 5x1 superstructure-free Pb-Bi2212 material there are no signatures of time-reversal symmetry breaking in the sense of the criteria developed earlier (Kaminski et al. Nature {\bf 416}, 610 (2002)). In addition to the existing technique, we suggest and apply an independent experimental approach to prove the absence of the effect in the studied compounds. The dichroic signal retains reflection antisymmetry as a function of temperature and doping and in all mirror planes, precisely defined by the experimental dispersion at low energies. The obtained results demonstrate that the signatures of time-reversal symmetry violation in pristine Bi2212, as determined by ARPES, are not a universal feature of all cuprate superconductors.

cond-mat.supr-con↗

arXiv2003

Temperature dependence of trapped magnetic field in MgB2 bulk superconductor

Based on DC magnetization measurements, the temperature dependencies of the trapped magnetic field have been calculated for two MgB2 samples prepared by two different techniques: the high-pressure sintering and the hot pressing. Experimentally measured trapped field values for the first sample coincide remarkably well with calculated ones in the whole temperature range. This proves, from one side, the validity of the introduced calculation approach, and demonstrates, from another side, the great prospects of the hot pressing technology for large scale superconducting applications of the MgB2.

cond-mat.supr-con↗

arXiv2002

MgCNi3 : Complex behavior in a strongly coupled superconductor

Polycrystalline samples of the recently discovered MgCNi3 superconductor were investigated by transport, ac susceptibility, dc magnetization and specific heat measurements in magnetic fields up to 16 T. Consistent results were obtained for the temperature dependence of the upper critical field H_c2(T) from resistance, ac susceptibility and specific heat measurements. A WHH like temperature dependence of H_c2(T) and the quadratic relationship H_c2(0) ~ T_c^2 point to an effective predominant single band behavior near the quasi clean limit. Evidence for strong electron-phonon and electron-paramagnon coupling was found analyzing the specific heat data. The corresponding s- and p-wave scenarios are briefly discussed using calculated densities of states of different Fermi surface sheets.

cond-mat.supr-con↗

arXiv2002

Comparative Study of Dense Bulk MgB$_2$ Materials Prepared by Different Methods

We report on the results of a comparative investigation of highly dense bulk MgB$_2$ samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by ac-susceptibility, dc-magnetization and resistivity measurements in magnetic fields up to $μ_0H=160$ kOe. A small but distinct anisotropy of the upper critical field $H_{c2}^{a,b}/H_{c2}^{c}\sim1.2$ connected with some texture of MgB$_2$ grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields $H_{c2}$ ($μ_0H_{c2}(0)\sim23$ T), irreversibility fields $H_{irr}$ which are strongly shifted towards higher values $H_{irr}(T)\sim0.8H_{c2}(T)$ and high critical current $J_c$ ($J_c=10^5$ A/cm$^2$ at 20 K and 1 T).

cond-mat.supr-con↗

arXiv2001

Improved superconducting properties in nanocrystalline bulk MgB_2

Highly dense nanocrystalline MgB_2 bulk superconductors with distinctly improved pinning were prepared by mechanical alloying of Mg and B powders and hot compaction at ambient temperatures. The nanocrystalline samples reveal high critical current densities of 105 A/cm2 at 20 K and 1 T together with a strongly shifted irreversibility line towards higher fields resulting in H_irr(T) ~ 0.8 H_c2(T), whereas typically H_irr(T) ~ 0.5 H_c2(T) is observed for bulk untextured samples. These values exceed that of all other so far reported bulk samples and are in the range of the values of thin films. The improved pinning of this material which mainly consists of spherical grains of about 40-100 nm in size is attributed to the large number of grain boundaries.

cond-mat.supr-con↗

arXiv2001

Origin of the peak-dip-hump structure in the photoemission spectra of Bi2212

The famous peak-dip-hump lineshape of the (π,0) photoemission spectrum of the bilayer Bi HTSC in the superconducting state is shown to be a superposition of spectral features originating from different electronic states which reside at different binding energies, but are each describable by essentially identical single-particle spectral functions. The 'superconducting' peak is due to the antibonding Cu-O-related band, while the hump is mainly formed by its bonding counterpart, with a c-axis bilayer coupling induced splitting of about 140 meV.

cond-mat.supr-con↗

arXiv2001

Fully dense MgB_2 superconductor textured by hot deformation

Bulk textured MgB_2 material of nearly full density showing a weak c-axis alignment of the hexagonal MgB_2 grains parallel to the pressure direction was obtained by hot deformation of a stoichiometric MgB_2 pellet prepared by a gas-solid reaction. The texture of the material was verified by comparing the x-ray diffraction patterns of the hot deformed material with isotropic MgB_2 powder. A small, but distinct anisotropy of the upper critical field up to Hc2^{a,b}/Hc2^{c}~1.2 depending on degree of texture was found by resistance and susceptibility measurements. No anisotropy of the critical current density determined from magnetization measurements was found for the textured material.

cond-mat.supr-con↗

arXiv2001

Electronic structure and weak electron-phonon coupling in TaB$_2$

We present electronic structure calculations together with resistivity, susceptibility, and specific heat measurements for TaB$_2$ to search for the recently contradictorily reported superconductivity and to study related normal state properties. We ascribe the absence of superconductivity down to 1.5 K for our TaB$_2$ samples to the generally weak electron phonon coupling derived from comparison of the calculated and measured specific heat constants. For the E$_{2g}$ and the B$_{1g}$ $Γ$ point phonons we derive from the calculated deformation potentials very small electron phonon couplings for these modes, opposite to the strong coupling of the E$_{2g}$ mode in MgB$_2$, probably responsible for its high $T_c$. In comparison to MgB$_2$, we discuss the origin of the quite different features in the density of states and of the Fermi surfaces. The differences are mainly due to the strong hybridization between Ta 5$d$ and B 2$p$ states outside the hexagonal basis plane.

cond-mat.supr-con↗

arXiv2001

Estimation of matrix element effects and determination of the Fermi surface in BSCCO systems using angle-scanned photoemission spectroscopy

The strong dependence of the momentum distribution of the photoelectrons on experimental conditions raises the question as to whether angle-resolved photoemission spectroscopy (ARPES) is able to provide an accurate reflection of the Fermi surface in Bi-based cuprate superconductors. In this paper we experimentally prove that the main contribution to the intensity variation comes from matrix elements effects and develop an approach to overcome this problem. We introduce a concept of 'self-normalization' which makes the spectra essentially independent of both the matrix elements and particular experimental parameters. On the basis of this concept we suggest a simple and precise method of Fermi surface determination in quasi-2D systems.

cond-mat.supr-con↗

arXiv2001

Specific heat and disorder in the mixed state of non-magnetic borocarbides

The temperature and magnetic field dependence of the specific heat cp(T,H) in the superconducting mixed state as well as the upper critical field Hc2(T) have been measured for polycrystalline Y_xLu_{1-x}Ni_2B_2C and Y(Ni_{1-y}Pt_y)_2B_2C samples. The linear-in-T electronic specific heat contribution gamma(H)T exhibits significant deviations from the usual linear-in-H law for all x and y the transition metal site (T) resulting in a disorder dependent negative curvature of gamma(H). The deviations from that linear behaviour of our unsubstituted samples are the largest reported so far for any superconductor. The H_c2(T) data point to the quasi-clean limit for (Y,Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni,Pt)-substitutions. The gamma(H) dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of gamma(H) data only, d-wave pairing cannot be ruled out.

cond-mat.supr-con↗

arXiv2001

Upper critical field and irreversibility line in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between Tc ~ 39 K and T = 3 K ~ 0.1Tc. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.

cond-mat.supr-con↗

arXiv2001

The upper critical field in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated in magnetic fields up to 16 T. The upper critical field of the major fraction of the investigated sample was determined from ac susceptibility and resistance data and was found to increase up to Hc2(0) = 13 T at T = 0 corresponding to a coherence length of 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at intermediate temperatures. This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit.

cond-mat.supr-con↗

arXiv2000

Current-induced conductance switching in epitaxial La0.7Sr0.3MnO3/SrTiO3 multilayers

We report on the non-linear in-plane electrical transport in coherently grown [La0.7Sr0.3MnO3/SrTiO3] multilayers with ultrathin (< 3 nm) single layers. Current-induced switching of the conductance, with low conductance at larger currents, is demonstrated. The conductance switching is modified under a magnetic field, resulting in an extremely large magnetoresistance of negative, or in a special case even positive, sign. Our results suggest a percolative nature of transport where a large local current density gives rise to a spin-polarizing and a thermal effect of current.

cond-mat.str-el↗

arXiv2000

Specific heat of (Y,Lu)Ni2B2C in the mixed state

The temperature and magnetic field dependences of the electronic specific heat in the superconducting mixed state as well as the upper critical field have been measured on a (Y,Lu)Ni2B2C series to study the influence of disorder. The electronic specific heat in the vortex state shows deviations from the usual gamma(H) proportional to H-law for conventional superconductors for all samples resulting in a disorder dependent negative curvature of gamma proportional to H sup(1-beta). For samples with disorder at the R site beta and the positive curvature exponent alpha of H sub(c2) are reduced but alpha, beta remain always positive, as distinct from much stronger disorder due to Pt substions at the Ni site where alpha, beta rapidly tend to 0. Alpha shows a minimum at equal Y and Lu concentration. H sub(c2)(00, T sub(c), the curvatures of H sub(c2) and gamma(h), as well as the normal state Sommerfeld constant gamma sub(N) are reduced similarly.

cond-mat↗

arXiv2000

Proximity Effect in Bulk LaBa_2Cu_3O_{7-y} Samples with Ag Additions

Bulk LaBa_2Cu_3O_(7-y) samples with different Ag additions were investigated. It was shown that Ag does not enter the crystallographic structure of the superconductor and segregates on the grain boundary region. Current path in these samples occurs through the proximity connected grains and this was confirmed from the temperature dependence of the critical current density and mutual inductance. By using the theory developed for the thin film structures we conclude that growing of the Ag content increased the effective cross section and the normal metal thickness. The first one prevails at low concentration increasing the current. The second dominates at higher concentration leading to saturation or even lowering of the critical current.

cond-mat.supr-con↗

arXiv1999

Magnetic properties of Gd_{1-x}Pr_xBa_2Cu_3O_{7-y} single crystals

Magnetic properties were studied for the high quality Al-free orthorhombic Gd_{1-x}Pr_xBa_2Cu_3O_{7-y} single crystals (0<=x<=1.0) grown by the flux method. An indication on the interaction between the Pr and Cu(2) magnetic sublattices was found for Pr123. Different signs of magnetic anisotropy were established for the Pr and Gd ions at low temperatures. It was also shown that superconductivity reported by Zou et al. [Phys. Rev. Lett. 80 (1998) 1074] for Pr123 single crystals grown by TSZF method seems to be connected with partial substitution of Ba for the Pr-sites.

cond-mat.supr-con↗