Publications (25)19.47 Total impact
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Article: Evidence for strong electron-phonon coupling in MgCNi_3
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ABSTRACT: The title compound is investigated by specific heat measurements in the normal and superconducting state supplemented by upper critical field transport, susceptibility and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities and partial densities of states the presence of both strong electron-phonon interactions and considerable pair-breaking has been revealed. The specific heat and the upper critical field data can be described to first approximation by an effective single band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated s-wave superconductivity somewhat suppressed by sizeable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a van Hove type singularity in the electronic density of states near the Fermi energy is suggested.03/2004; -
Article: A Comparative Study of MgB2 and Other Diborides
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ABSTRACT: A combined experimental and theoretical study of superconducting, electronic and thermodynamic properties of various isomorphic diborides is reported. The H c2(T) data of bulk MgB2 are analyzed within a two-band model with different couplings and Fermi velocities. No bulk superconductivity was found in TaB2 and ZrB2 single crystals at temperatures T>1.5 K. Its absence is ascribed to weak electron-phonon coupling strenghth with 0.1 derived both from the comparison of the calculated density of states at the Fermi level and specific heat data as well as from the weak deformation potential.Journal of Low Temperature Physics 05/2003; 131(5):1159-1163. · 1.19 Impact Factor -
Article: Comparative Study of Dense Bulk MgB2 Materials Prepared by Different Methods
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ABSTRACT: We report on the results of a comparative investigation of highly dense bulk MgB2 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 yHc2a,b /Hc2c ~ 1.2\psi {\rm H}_{c2}^{\alpha ,b} /H_{c2}^c \sim 1.2 connected with some texture of MgB2 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 (0 H c2 (0) 23 T), irreversibility fields H irr which are strongly shifted towards higher values H irr(T) 0.8 H c2(T) and high critical current J c (J c = 105 A/cm2 at 20 K and 1 T).Journal of Superconductivity 11/2002; 15(6):599-601. -
Article: MgCNi3 : Complex behavior in a strongly coupled superconductor
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ABSTRACT: 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.09/2002; -
Article: Anisotropic magnetic behavior of GdBa_2Cu_3O_{6+y} single crystals
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ABSTRACT: Magnetic properties of high-quality Al-free nonsuperconducting GdBa_2Cu_3O_{6+y} single crystals grown by flux method have been studied. The magnetic anisotropy below the N\'eel temperature T_N~2.3K corresponds to the direction of Gd^{3+} magnetic moments along the tetragonal c-axis. At T < T_N clear indications of spin-flop transitions for H||c have been observed on magnetization curves at H_{sf}~10kOe. Magnetic phase diagrams have been obtained for H||c as well as for H||ab. A pronounced anisotropy in the magnetic susceptibility (unexpected for Gd-based compounds) has been found above T_N. Comment: 2 pages, 3 figures; LT23 (Aug. 2002; Hiroshima), accepted to Physica B09/2002; -
Article: Comparative Study of Dense Bulk MgB$_2$ Materials Prepared by Different Methods
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ABSTRACT: 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 $\mu_0H=160$ 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 $H_{c2}$ ($\mu_0H_{c2}(0)\sim23$ 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).07/2002; -
Article: Interaction of superconductivity and magnetism in borocarbide superconductors
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ABSTRACT: The interaction of rare-earth magnetism and superconductivity has been a topic of interest for many years. In classical magnetic superconductors (Chevrel phases, ternary rhodium borides, etc) as well as in the high-Tc cuprates the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. In these compounds the magnetic ordering temperature TN is much below the superconducting transition temperature Tc. The discovery of superconducting borocarbides RT2B2C with R = Sc, Y, La, Th, Dy, Ho, Er, Tm or Lu and T = Ni, Ru, Pd or Pt (where not all of these combinations of R and T result in superconductivity) has reanimated the research on the coexistence of superconductivity and magnetic order. Most of these borocarbides crystallize in the tetragonal LuNi2B2C type structure which is an interstitial modification of the ThCr2Si2 type. Contrary to the behaviour of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures on the rare-earth sublattice have been found to coexist with superconductivity in RNi2B2C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi2B2C for which three different types of antiferromagnetic structures have been observed: (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonal c axis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along the c axis and (iii) an incommensurate a-axis-modulated structure with a modulation vector τ≈(0.55,0,0). This wave vector emerges in various RNi2B2C compounds with magnetic as well as nonmagnetic R elements and is connected with Fermi surface nesting. Both incommensurate magnetization structures have been shown to be related to the near-reentrant behaviour observed in HoNi2B2C whereas the commensurate structure coexists well with the superconducting state in this compound. The variation of TN and Tc with the de Gennes factor can roughly be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively, with the exception of Yb. Consequently, Tc>TN holds for Tm, Er, Ho and Tc<TN for Dy. However, the study of various pseudoquaternary (R,R')Ni2B2C compounds has shown that this so-called de Gennes scaling is not universal for the borocarbides and it breaks down in some cases, which is attributed to effects of details of the electron structure, crystalline electric fields, the difference in the R and R' ionic radii or to the effect of nonmagnetic impurities in an antiferromagnetic superconductor. In an external magnetic field some of the RNi2B2C compounds show metamagnetic transitions combined with a large negative magnetoresistance. A small net magnetization found at low temperatures and zero magnetic field in TbNi2B2C has been interpreted as weak ferromagnetism of Dzyaloshinsky-Moriya type. A similar phenomenon observed for superconducting ErNi2B2C is still under discussion.Reports on Progress in Physics 07/2001; 64(8):943. · 14.72 Impact Factor -
Article: Upper critical field and irreversibility line in superconducting MgB_2
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ABSTRACT: 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. Comment: 8 pages with 5 figures, submitted to Solid State Communications04/2001; -
Article: The upper critical field in superconducting MgB_2
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ABSTRACT: 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. Comment: 8 pages with 4 figures02/2001; -
Article: Superconducting Rare Earth Transition Metal Borocarbides
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ABSTRACT: We present an overview of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds, as well as of theoretical calculations with possible relevance to the mechanism of superconductivity. The interplay of superconductivity and magnetism for compounds with pure and mixed rare earth components is considered. We suggest that the incommensurate magnetic structure modulated along the a-axis is responsible Z. for the pair breaking in Ho R Ni B C; RsY, Lu samples. The effect of doping Cu, Co at the transition metal site is x 1yx 22 considered experimentally and theoretically. The possible role of correlation effects due to the presence of the transition metal component in determining the electronic structure is discussed comparing the band structure calculation results with various electronic spectroscopies as well as de Haas--van Alphen data. Important thermodynamic properties of these systems Z. are analyzed within multiband E...10/2000; -
Article: Hall-Effect study of YbNi2B2C borocarbide
Ferroelectrics. 08/2000; 247(1):163-169. -
Article: Anomalous Behaviour of PrNi2B2C Borocarbide
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ABSTRACT: Electrical resistivity (T), specific heat C p(T), magnetic susceptibility (T), and magnetization M(H) have been measured for polycrystalline PrNi 2 B 2 C. Maxima in C p(T) and |d(T)/dT| dependencies were found at T N 4.3 K connected with AFM ordering observed earlier by neutron diffraction. The Sommerfeld coefficient is at least 250 mJ/mol K 2 . The values of T N for PrNi 2 B 2 C and |dT c /dx| for Y 1–x Pr x Ni 2 B 2 C were found to be much larger than estimated by the de Gennes scaling from the Gd-based borocar-bides. These facts as well as anomalous decrease of (T) found below 20K are connected with moderate heavy fermion behaviour of antiferromagnetic PrNi 2 B 2 C.Journal of Low Temperature Physics 11/1999; 117(5):1599-1603. · 1.19 Impact Factor -
Article: Hall-Effect in LuNi2B2C and YNi2B2C Borocarbides in Normal and Superconducting Mixed States
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ABSTRACT: It was shown that the Hall resistivity xy for LuNi 2 B 2 C and YNi 2 B 2 C is negative in the normal and mixed states and has no sign reversal below T c . In the mixed state the scaling relation xy xx (xx is the longitudinal resistivity) was found for both compounds with 2.0. In the normal state a distinct nonlinearity in the xy(H) dependence, accompanied by a large magnetoresistance, was found below 40 K only for LuNi 2 B 2 C. The difference in the behaviour of Lu- and Y-based borocarbides seems to be connected with the difference in the Fermi surfaces of these compounds.Journal of Low Temperature Physics 11/1999; 117(5):1605-1609. · 1.19 Impact Factor -
Article: PrBa_2Cu_3O_{7-y}: Superconducting or Anomalously Magnetic?
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ABSTRACT: In PrBa_2Cu_3O_{7-y} (Pr123) single crystals grown by the flux method the kink in the magnetic susceptibility chi_{ab}(T), connected with AFM ordering of Pr, disappears after field cooling (FC) in a field H||ab-plane whereas the kink in chi_c(T) remains unchanged after FC in H||c-axis. This seems to be connected with the coupling between the Pr and Cu(2) sublattices. The Curie constant C determined from the data reported for superconducting Pr123 crystals grown by traveling-solvent floating zone (TSFZ) method (Zou et al, Phys. Rev. Lett., 80, 1074 (1998)) is about one half of that for our flux crystals. Thus, the concentration of Pr in TSFZ crystal seems to be about one half of the nominal for Pr123. Therefore, superconductivity in TSFZ samples is connected most probably with the partial substitution of Pr by nonmagnetic Ba.10/1999; -
Article: Magnetic properties of Gd_{1-x}Pr_xBa_2Cu_3O_{7-y} single crystals
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ABSTRACT: 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. Comment: 2 pages (LaTeX2e), 4 EPS figures, phbauth style file included. LT22 conference paper accepted to Physica B09/1999; -
Article: Hall effect in LuNi_ {2} B_ {2} C and YNi_ {2} B_ {2} C borocarbides: A comparative study
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ABSTRACT: The Hall effect in LuNi2B2C and YNi2B2C borocarbides has been investigated in normal and superconducting mixed states. The Hall resistivity ρxy for both compounds is negative in the normal as well as in the mixed state and has no sign reversal below Tc typical for high-Tc superconductors. In the mixed state the behavior of both systems is quite similar. The scaling relation ρxy∼ρxxβ (ρxx is the longitudinal resistivity) was found with β=2.0 and 2.1 for annealed Lu- and Y-based compounds, respectively. The scaling exponent β decreases with increasing degree of disorder and can be varied by annealing. This is attributed to a variation of the strength of flux pinning. In the normal state weakly temperature dependent Hall coefficients were observed for both compounds. A distinct nonlinearity in the ρxy dependence on field H was found for LuNi2B2C in the normal state below 40 K, accompanied by a large magnetoresistance (MR) reaching +90% for H=160kOe at T=20K. At the same time for YNi2B2C only linear ρxy(H) dependences were observed in the normal state with an approximately three times lower MR value. This difference in the normal-state behavior of the very similar Lu- and Y-based borocarbides seems to be connected with the difference in the topology of the Fermi surface of these compounds.Phys. Rev. B. 05/1999; 59(22). -
Article: Hall-effect in LuNi_2B_2C and YNi_2B_2C borocarbides: a comparative study
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ABSTRACT: The Hall effect in LuNi_2B_2C and YNi_2B_2C borocarbides has been investigated in normal and superconducting mixed states. The Hall resistivity rho_{xy} for both compounds is negative in the normal as well as in the mixed state and has no sign reversal below T_c typical for high-T_c superconductors. In the mixed state the behavior of both systems is quite similar. The scaling relation rho_{xy}\sim\rho_{xx}^\beta (\rho_{xx} is the longitudinal resistivity) was found with \beta=2.0 and 2.1 for annealed Lu- and Y-based compounds, respectively. The scaling exponent \beta decreases with increasing degree of disorder and can be varied by annealing. This is attributed to a variation of the strength of flux pinning. In the normal state weakly temperature dependent Hall coefficients were observed for both compounds. A distinct nonlinearity in the \rho_{xy} dependence on field H was found for LuNi_2B_2C in the normal state below 40K, accompanied by a large magnetoresistance (MR) reaching +90% for H=160kOe at T=20K. At the same time for YNi_2B_2C only linear \rho_{xy}(H) dependences were observed in the normal state with an approximately three times lower MR value. This difference in the normal state behavior of the very similar Lu- and Y-based borocarbides seems to be connected with the difference in the topology of the Fermi surface of these compounds. Comment: 11 RevTeX pages, 8 embedded EPS figures, submitted to Phys. Rev. B02/1999; -
Article: Comment on ``Superconducting PrBa_2Cu_3O_x''
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ABSTRACT: Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of this Comment is to show the inconsistency of the value of effective magnetic moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic susceptibility data. The estimation made directly from their data points gives a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This suggests that Pr occupies only about a half of the RE sites in TSFZ crystal. The other half of the RE sites is occupied most probably by the nonmagnetic Ba. Noteworthy, SC with T_c=43 K was observed earlier for Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius than Pr^{3+} and so the substitution of Ba for Pr could give a natural explanation not only for the SC in TSFZ Pr123 but also for the elongation of the distance between the CuO_2 planes observed by Zou et al. Comment: Slightly extended version of Comment accepted to Phys. Rev. Lett. (v.81, N24, 1998), tentatevely to be publ. 14Dec98. 1 page, REVTex; 1 EPS fig11/1998; -
Article: Hall-effect in LuNi_2B_2C in normal and superconducting mixed states
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ABSTRACT: The Hall resistivity rho_{xy} of LuNi_2B_2C is negative in the normal as well as in the mixed state and has no sign reversal typical for high-T_c superconductors. A distinct nonlinearity in the rho_{xy} dependence on field H was found in the normal state for T < 40K, accompanied by a large magnetoresistance reaching +90% for mu_0H=16T at T=20K. The scaling relation rho_{xy} ~ \rho_{xx}^\beta (\rho_{xx} is the longitudinal resistivity) was found in the mixed state, the value of \beta being dependent on the degree of disorder. Comment: 12 pages (REVTeX), 5 EPS figs. Accepted to Solid State Commun11/1998; -
Article: Anomalous magnetic ordering in PrBa_2Cu_3O_{7-y} single crystals: Evidence for magnetic coupling between the Cu and Pr sublattices
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ABSTRACT: In Al-free PrBa_2Cu_3O_{7-y} single crystals the kink in the temperature dependence of magnetic susceptibility chi_{ab}(T), connected with Pr antiferromagnetic ordering, disappears after field cooling (FC) in a field H || ab-plane. The kink in chi_c(T) remains unchanged after FC in H || c-axis. As a possible explanation, freezing of the Cu magnetic moments, lying in the ab-plane, caused by FC in H || ab, hinders their reorientation and, due to coupling between the Pr and Cu(2) sublattices, ordering of the Pr^{3+} moments. A field induced phase transition and a field dependence of the Pr^{3+} ordering temperature have been found for both H || c and H || ab. Comment: 11 pages (LaTex with elsart.sty), 5 EPS figs. Accepted to Physica C11/1998;
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2001–2003
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Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden
Dresden, Saxony, Germany
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