K. Nenkov

Leibniz Institute for Solid State and Materials Research Dresden, Dresden, Saxony, Germany

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Publications (270)375.03 Total impact

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    ABSTRACT: We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe0.94 at magnetic fields up to 14 T. It was established that due to Ag addition the hexagonal phase formation in melted FeSe0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp (with a transition width below 1 K), Tc and the upper critical field were found to increase, whereas the normal state resistivity significantly reduces becoming comparable with those of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe0.94 is dominated by thermally activated flux flow. From the activation energy U vs H dependence, a crossover from single-vortex pinning to a collective creep pinning behavior was found with increasing the magnetic field.
    07/2014;
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    ABSTRACT: Samples with nominal compositions HoBa2Cu3Oy and (Ho0.97Sn0.03)Ba2Cu3Oy are obtained by the melt-texture-growth (MTG) method and their superconducting properties are investigated. It is shown that the Sn-doping increases the irreversibility field and the upper critical field of the pristine compound. Both the undoped and Sn-doped samples undergo a vortex glass-vortex liquid transition. The observed phenomena are discussed.
    Journal of Superconductivity and Novel Magnetism 03/2014; 27(3). · 0.93 Impact Factor
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    ABSTRACT: We present study of derivatives of current-voltage I(V) characteristics of point-contacts (PCs) based on Ba{1-x}Na{x}Fe2As2 (x=0.25) in the normal and superconducting state. The detailed analysis of dV/dI(V) data (also given in Appendix A) shows that the thermal regime, when temperature increases with a voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases V above the superconducting (SC) gap \Delta. In this case, specific resistivity \rho (T) in PC core is responsible for a peculiar dV/dI(V) behavior, while a pronounced asymmetry of dV/dI(V) is caused by large value of thermopower in this material. A reproducible zero-bias minima detected on dV/dI(V) at low biases in the range \pm(6--9)mV well below the SC critical temperature T_c could be connected with the manifestation of the SC gap \Delta. Evaluation of these Andreev-reflection-like structures on dV/dI(V) points out to the preferred value of 2\Delta/kT_c \approx 6. The expected second gap features on dV/dI(V) are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate spectroscopic features in dV/dI(V) from those caused by the thermal regime.
    02/2014; 89(10).
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    ABSTRACT: We present a systematic study of the electronic specific-heat jump (ΔCel) at the superconducting transition temperature Tc of K1-xNaxFe2As2. Both Tc and ΔCel monotonously decrease with increasing x. The specific heat jump scales approximately with a power law, ΔCel∝Tcβ, with β ≈2 determined by the impurity scattering rate, in contrast to most iron-pnictide superconductors, where the remarkable Bud'ko-Ni-Canfield (BNC) scaling ΔCel∝T3 has been found. Both the T dependence of Cel(T ) in the superconducting state and the nearly quadratic scaling of ΔCel at Tc are well described by the Eliashberg theory for a two-band d-wave superconductor with weak pair breaking due to nonmagnetic impurities. The disorder induced by the Na substitution significantly suppresses the small gaps, leading to gapless states in the slightly disordered superconductor, which results in a large observed residual Sommerfeld coefficient in the superconducting state for x >0.
    01/2014; 89(6).
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    ABSTRACT: Since the discovery of superconductivity (SC) in the tetragonal FeSe phase this compound is an intensively investigated iron based superconductor. In this study we examined the influence of Ag addition on the superconducting properties of selenium deficient polycrystalline FeSe0.94, which is almost free of non-superconducting hexagonal phase. The samples were obtained by solid state reaction and melting methods. Their characteristics were investigated by fundamental and third harmonic AC magnetic susceptibility. EDX analysis shows inhomogeneous Ag distributions in the grains. In spite of that intergranular critical current in Ag doped sample is several times higher at approximately the same temperatures. Intragranular current is field independent up to almost 1000 Oe. Using the temperature dependence of third harmonic AC magnetic susceptibility at different DC magnetic fields, the irreversibility lines were obtained for all samples. It was found that Ag addition increased the irreversibility field in comparison with undoped melted and powder sintered samples. All results show that the Ag addition in selenium deficient (FeSe0.94) samples leads to improved superconducting properties.
    01/2014;
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    ABSTRACT: The magnetic properties of bi-, tri- and multicrystals of 3D topological insulator Bi1−x1−x Sb(x0.07<x<0.22) was studied in the temperature range 1.6 - 300 K and fields up to 70 kOe using a SQUID magnetometer. Two superconducting phases associated with adjacent and central layers of the crystallite interfaces of the bicrystals are identified. It has been found that due to the different stress structure the transition temperature TcTc for one superconducting phase changes considerably, from 8.3 to 36 K, while for second superconducting phase, TcTc remains within the range 3.7 - 4.6 K. In tricrystals and bicrystals with high contents of structural disorder and topological defects, ferromagnetic hysteresis loops and magnetic field expulsion have been observed simultaneously.
    Physics Letters A. 01/2014;
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    ABSTRACT: The role of elastic strain for magnetoelectric materials and devices is twofold. It can induce ferroic orders in thin films of otherwise non-ferroic materials. On the other hand, it provides the most exploited coupling mechanism in two-phase magnetoelectric materials and devices today. Complex oxide films (perovskites, spinels) are promising for both routes. The strain control of magnetic order in complex oxide films is a young research field, and few ab initio simulations are available for magnetic order in dependence on lattice parameters and lattice symmetry. Here, an experimental approach for the evaluation of how elastic strain in thin epitaxial films alters their magnetic order is introduced. The magnetic films are grown epitaxially in strain states controlled by buffer layers onto piezoelectric substrates of 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3(001). As an example, the strain dependence of the ordered magnetic moment of SrRuO3 has been investigated. At a tensile strain level of approximately 1%, SrRuO3 is tetragonal, and biaxial elastic strain induces a pronounced suppression of the ordered magnetic moment. As a second example, a strain-driven transition from a ferromagnetic to a magnetically disordered phase has been observed in epitaxial La0.8Sr0.2CoO3 films.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2014; 372(2009):20120441. · 2.89 Impact Factor
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    ABSTRACT: Bulk superconducting MgB2 samples, 20 mm in diameter, were prepared by hot-pressing of ball-milled Mg and B powders using fine-grained boron powders. High maximum trapped fields of B0 = 5.4 T were obtained at 12 K in one of the investigated trapped field magnets (height 8 mm) at the centre of the bulk surface. Investigating the temperature dependence of the trapped field for short MgB2 samples (height ≤1.6 mm), trapped fields of up to B0 = 3.2 T at 15 K were achieved. These high trapped fields are related to extremely high critical current densities of up to 106 A cm-2 at 15 K, indicating strong pinning due to nanocrystalline MgB2 grains. Expected trapped field data for long trapped field magnets prepared from the available MgB2 material are estimated.
    Superconductor Science and Technology 12/2013; 26(12):2002-. · 2.76 Impact Factor
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    ABSTRACT: SrRuO_{3} films have been grown in the tetragonal, structurally single-domain state under 1% of biaxial tensile strain. The angular dependencies of the magnetization and the magnetoresistance reveal an upright orientation of the tetragonal unit cell and biaxial magnetic in-plane anisotropy with 110_{t} easy axes. Reversible biaxial strain from piezoelectric Pb(Mg_{1/3}Nb_{2/3})_{0.72}Ti_{0.28}O_{3} (PMN-PT) substrates has been applied to probe the direct strain response of the magnetization and the electrical resistance. At 1% tensile strain, the Curie temperature (T_{C}) and the ordered magnetic moment (m_{S}) at low temperatures are found to substantially decrease with further growing tensile strain. This suggests a suppression of m_{S} resulting from distortions of the RuO_{6} octahedra, in line with reported density-functional calculations. Reversible strain has also been applied to a film under weak tensile strain revealing the opposite response, i.e., an enhancement of T_{C} and m_{S} with tensile strain. Structural and magnetic properties of SrRuO_{3} films in several static strain states (compressive, weak and strong tensile strain) are compared.
    Physical Review B 10/2013; 88(14). · 3.66 Impact Factor
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    ABSTRACT: We present the result of high-field (up to 40 T) galvanomagnetic study of bismuth bicrystals with nano-width crystallite interfaces (∼100 nm). At B>2 T directed along the interface plane in the quantum oscillations spectrum of the longitudinal Hall effect, two new harmonics have been detected. Their periods of oscillation characterise the much larger cross-sectional areas of Fermi surface of charge carriers than it is in single crystalline bismuth. At the same time, in small disorientation angle bicrystals of an inclination type, a number of Hall quasi-plateaus were observed, which vanish by reversing the magnetic field. We conclude that in bicrystals there are spin-oriented carriers that are located in spectrum of Landau levels at the particular orientation of the magnetic field.
    Solid State Communications 03/2013; 158:9–12. · 1.53 Impact Factor
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    ABSTRACT: We report AC loss data of superconducting pancake coils (PC) wound from several non-transposed YBCO tapes connected in parallel. Due to coupling losses, their magnetization AC losses in AC fields applied parallel to the tape surface are significantly higher than for single-tape PCs. It was experimentally demonstrated that these coupling losses can be effectively reduced by using double-PCs in which the parallel tapes between the two PCs are connected in reversed order. This special connection between PCs imitates a twisting of parallel tapes and results in their decoupling. An AC current applied additionally with an external AC magnetic field to the double-PCs does not impede the decoupling of the tapes. We propose a simple model to calculate the magnetization AC losses of PCs by approximating the coils by face-to-face stacks of YBCO tapes.
    Superconductor Science and Technology 01/2013; 26(3):035002. · 2.76 Impact Factor
  • N. Balchev, K. Nenkov, V. Antonov
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    ABSTRACT: We investigated the vortex pinning energies and H(T) characteristic lines of samples with nominal composition MoSr2YCu2O8−δ . The vortex pinning energies were obtained using two approximations of the resistive transition. The characteristic lines H c2(T) and H irr(T) were obtained using resistivity, susceptibility and magnetization measurements. The values of H c2(0) and different irreversibility field values are discussed.
    Journal of Superconductivity and Novel Magnetism 01/2013; 26(1). · 0.93 Impact Factor
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    J. Ćwik, T. Palewski, K. Nenkov
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    ABSTRACT: Specific heat measurements have been performed on polycrystalline HoCo2, ErCo2 and their solid solutions Ho1−x Erx Co2 (0.1≤x≤0.5). These compounds were synthesized using high-purity rare-earth metals and cobalt. X-ray diffraction patterns taken at room temperature reveal that all compounds have the C15 cubic Laves phase structure. Heat capacity measurements have been performed in the temperature range of 2–300 K without magnetic field and in a magnetic field of 1 and 2 T. The magnetocaloric effect has been estimated in terms of isothermal magnetic entropy change for all solid solutions in magnetic fields up to 2 T. The effect of increasing Er amount in Ho1−x Erx Co2 on the magnetic and magnetocaloric properties will be discussed.
    Journal of Superconductivity and Novel Magnetism 01/2013; 26(1). · 0.93 Impact Factor
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    ABSTRACT: High-quality KFe2As2 (K122) single crystals synthesized by different techniques have been studied by magnetization and specific heat (SH) measurements. There are 2 types of samples both affected by disordered magnetic phases: (i) cluster-glass (CG) like or (ii) Griffiths phase (G) like. For (i) at low applied magnetic fields the T-dependence of the zero field cooled (ZFC) linear susceptibility (chi_l) exhibits an anomaly with an irreversible behavior in ZFC and field cooled (FC) data. This anomaly is related to the freezing temperature T_f. The extrapolated T_f to B=0 varies between 50 K and 90 K. Below T_f we observed a magnetic hysteresis in the field dependence of the isothermal magnetization (M(B)). The frequency shift of the freezing temperature delta T_f=Delta T_f/[T_f\Delta(\ln \nu)]\sim 0.05$ has an intermediate value, which provides evidence for the formation of a CG-like state in the K122 samples of type (i). The frequency dependence of their T_f follows a conventional power-law divergence of critical slowing down: tau=tau_0 [T_f(nu)/T_f(0)-1]^{-z\nu^{'}} with the critical exponent z\nu^{'}=10(2) and a relatively long characteristic time constant tau_0 =6.9 x10^{-11}$s also supporting a CG behavior. The large value of the Sommerfeld coefficient was related to magnetic contribution from a CG. Samples from (ii) did not show a hysteresis behavior for chi_l(T) and M(B). Below a crossover temperature T^* sim 40 K a power-law dependence in the chi_l propto T^[lambda_G-1}], with a non-universal lambda_G was observed, suggesting a quantum G-like behavior. In this case chi_l and M(B) can be scaled using the scaling function M_s(T,B)= B^{1-\lambda_{\tiny G}}Y(mu B/k_BT) with the scaling moment mu of the order of 3.5mu_b. The same non-universal exponent was found also in SH measurements, where the magnetic contribution C/T propto T^(lambda_G-1).
    physica status solidi (b) 10/2012; 250(3). · 1.49 Impact Factor
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    ABSTRACT: The transport AC losses (Qtr) of YBCO pancake coils wound from n = 1, 2, 4 non-transposed parallel connected (n-tpc) tapes have been investigated experimentally and theoretically. It was found that the Qtr AC losses of the coils with several n-tpc tapes consist of hysteresis AC losses (Qh) and coupling AC losses (Qcc). AC losses Qh of these coils are nearly the same as the AC losses of single-tape coils (n = 1). In contrast, Qcc are unique for coils with n > 1 n-tpc tapes. We found that the distribution of the coupling currents in the coil turns is defined by the magnetic flux penetrating between n-tpc tapes. This flux is related to the self-field component of the coil parallel to the tape surface, which is antisymmetrically distributed (an odd function) with respect to the winding center. This antisymmetrical distribution leads to a compensation effect of the coupling currents if n-tpc tapes are insulated along the turn length, in spite of low resistance between n-tpc tapes in the soldered coil ends. Therefore, AC losses Qcc of such coils are negligibly small compared to AC losses Qh. On the other hand, AC losses Qcc per current cycle are frequency dependent and have a maximum defined by the time constant τeff if tapes are non-insulated along the turn length. In this paper we estimated τeff for several special cases. In particular, we found that Qtr of YBCO pancake coils wound from several n-tpc tapes are comparable to Qtr of single-tape coils if the operating frequency f is far from the characteristic frequency 1/τeff.
    Superconductor Science and Technology 05/2012; 25(7):075006. · 2.76 Impact Factor
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    ABSTRACT: Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible.
    Superconductor Science and Technology 05/2012; 26(2). · 2.76 Impact Factor
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    ABSTRACT: In this paper, the electronic transport of La(0.7)(Sr,Ca)(0.3)MnO(3) films grown by pulsed laser deposition on a LaAlO(3)(001) substrate with deep parallel structured steps and a 30° symmetric bicrystal SrTiO(3)(001) substrate have been discussed. The electronic transport properties have been related to the well-known extrinsic transport of bulk manganite compounds. The spin-glass-like behavior with a characteristic peak at 20 K and domain formation at the grain boundary is observed. Further, it has been quantified from the resonant tunneling model that mixed magnetic interactions play a significant role in the manganite films deposited on step edges.
    Journal of Physics Condensed Matter 05/2012; 24(23):236007. · 2.22 Impact Factor
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    ABSTRACT: High-quality KFe2As2 single crystals have been studied by transport, magnetization and low-T specific heat measurements. Their analysis shows that superconductivity occurs (in some cases coexists) in the vicinity of disordered magnetic phases (Griffiths and spin-glass type) depending of the amount of local magnetic moments (probably excess Fe derived)and sample inhomogeneity. The achieved phenomenological description of our data including also data from the literature provides a consistent explanation of the observed non-Fermi-liquid behavior and of the nominally large experimental Sommerfeld coefficient gamma_n about 94 mJ/mol K^2. We suggest that the intrinsic value (directly related to the itinerant quasi-particles) gamma_el about 60(10)mJ/mol K^2 is significantly reduced compared with gamma_n. Then an enhanced relative jump of the electronic specific heat Delta C_el/gamma_el T_c about 0.8 and a weak total electron-boson coupling constant lambda less or about 1 follow.
    03/2012;

Publication Stats

1k Citations
375.03 Total Impact Points

Institutions

  • 1998–2014
    • Leibniz Institute for Solid State and Materials Research Dresden
      • Institute for Solid State Research
      Dresden, Saxony, Germany
  • 2012
    • Pondicherry University
      • Department of Physics
      Pondicherry, Union Territory of Puducherry, India
  • 1983–2012
    • Bulgarian Academy of Sciences
      • • Institute of Catalysis
      • • Georgi Nadjakov Institute of Solid State Physics (ISSP)
      • • Institute of General and Inorganic Chemistry
      Ulpia Serdica, Sofia-Capital, Bulgaria
  • 2005
    • Institute of Metallurgy and Materials Science
      Moskva, Moscow, Russia
  • 2004
    • Tata Institute of Fundamental Research
      Mumbai, Mahārāshtra, India
  • 2003
    • Tver State University
      Kalinin, Tverskaya, Russia
    • Silesian University of Technology
      • Institute of Physics
      Gliwice, Silesian Voivodeship, Poland
  • 2001
    • Technische Universität Dresden
      Dresden, Saxony, Germany
    • Moscow State Textile University
      Moskva, Moscow, Russia
  • 1999
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France