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ABSTRACT: We report the results of directional point-contact measurements in Mg(B1-x_{1-x}Cx)2_{x})_{2} single crystals. The amplitudes of the gaps, Dp\Delta_{\pi} and Ds\Delta_{\sigma}, were determined for each C content by fitting the experimental low-temperature normalized conductance curves of our “soft”
point contacts with the BTK model generalized to the two-band case. We found that, on increasing the carbon content, Ds\Delta_{\sigma} decreases almost linearly with TcT_{c} and Dp\Delta_{\pi} slightly increases until, at x=0.132x=0.132 (where Tc=19T_{c}=19 K), they assume the same value D = 3.2 ±0.9\Delta =3.2 \pm 0.9 meV. This result is confirmed by the temperature and magnetic-field dependence of the conductance curves at this C content,
which do not show any evidence of two distinct gap values. In particular, the Δ versus T curve follows very well a standard BCS curve, with a gap ratio 2D/kB Tc=3.92\Delta /k_{B} T_{c}=3.9. These experimental findings are compared to the theoretical predictions of the two-band model in the Eliashberg formulation.
KEY WORDS:Point-contact spectroscopy–magnesium diboride–Andreev reflection
Journal of Superconductivity 04/2012; 18(5):681-685.
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Takeshi Kondo,
R Khasanov,
J Karpinski, S M Kazakov,
N D Zhigadlo,
Z Bukowski,
M Shi,
A Bendounan,
Y Sassa,
J Chang,
S Pailhés,
J Mesot,
J Schmalian,
H Keller,
A Kaminski
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ABSTRACT: We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (∼40 meV), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high-temperature superconductors, indicating they have a common origin.
Physical Review Letters 12/2010; 105(26):267003. · 7.37 Impact Factor
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Takeshi Kondo,
R. Khasanov,
Y. Sassa,
A. Bendounan,
S. Pailhes,
J Chang,
J. Mesot,
H. Keller,
N. D. Zhigadlo,
M Shi, S. M. Kazakov,
J. Karpinski,
A. Kaminski
[show abstract]
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ABSTRACT: We use microprobe Angle-Resolved Photoemission Spectroscopy to study the Fermi surface and band dispersion of the CuO2 planes in the high temperature superconductor, YBa2Cu4O8. We find a strong in-plane asymmetry of the electronic structure between directions along a and b axes. The saddle point of the antibonding band lies at a significantly higher energy in the a direction (pi,0) than the b direction (0,pi), whereas the bonding band displays the opposite behavior. We demonstrate that the abnormal band shape is due to a strong asymmetry of the bilayer band splitting, likely caused by a non-trivial hybridization between the planes and chains. This asymmetry has an important implication for interpreting key properties of the Y-Ba-Cu-O (YBCO) family, especially the superconducting gap, transport and results of inelastic neutron scattering.
03/2009;
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C Dubois,
G Santi,
I Cuttat,
C Berthod,
N Jenkins,
A P Petrović,
A A Manuel,
O Fischer, S M Kazakov,
Z Bukowski,
J Karpinski
[show abstract]
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ABSTRACT: We performed the first scanning tunneling spectroscopy measurements on the pyrochlore superconductor KOs2O6 (T(c)=9.6 K) in both zero magnetic field and the vortex state at several temperatures above 1.95 K. This material presents atomically flat surfaces, yielding spatially homogeneous spectra which reveal fully gapped superconductivity with a gap anisotropy of 30%. Measurements performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line lattice. From the shape of the vortex cores, we extract a coherence length of 31-40 A, in agreement with the value derived from the upper critical field H(c2). We observe a reduction in size of the vortex cores (and hence the coherence length) with increasing field which is consistent with the unexpectedly high and unsaturated upper critical field reported.
Physical Review Letters 09/2008; 101(5):057004. · 7.37 Impact Factor
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ABSTRACT: In this study an extended low energy phase diagram for NaxCoO2 is experimentally established with emphasis on the high x range. It is based on systematic heat capacity studies on both polycrystalline and single crystalline samples and on uSR measurements. Main features are the existence of mass enhancement, spin fluctuations without long-range order, and magnetic order with associated Fermi surface gapping. The latter is seen in the electronic density of states (DOS) and suppression of nuclear specific heat. While there is agreement between the band structure and the low energy DOS in the low x range, in the high x range (x > 0.6) the thermodynamically determined DOS is approximately three times that deduced from the angle-resolved photoemission spectroscopy (ARPES)-measured band dispersion or local-density approximation (LDA) calculations. Comment: 9 pages, 5 figures
08/2008;
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ABSTRACT: We report on the order–disorder transition line (Bod) in the flux line lattice—illustrated by the fishtail effect—of pure and carbon doped superconducting MgB2 single crystals. Neutron irradiation offers a unique opportunity for varying the defect density (nd) in a controlled way and to determine Bod not only as a function of temperature (T), but also as a function of nd. Experimental data are compared with theoretical predictions and found to be in good agreement over most of the available parameter range, both for Bod(T) and Bod(nd). Including two-band effects in MgB2 in quite a rough way leads to even better agreement.
Superconductor Science and Technology 08/2007; 20(9):S247. · 2.66 Impact Factor
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ABSTRACT: We performed high-pressure transport measurements on high-quality single crystals of KOs2O6, a beta-pyrochlore superconductor. While the resistivity at high temperatures might approach saturation, there is no sign of saturation at low temperatures, down to the superconducting phase. The anomalous resistivity is accompanied by a nonmetallic behavior in the thermoelectric power (TEP) up to temperatures of at least 700 K, which also exhibits a broad hump with a maximum at 60 K. The pressure influences mostly the low-energy electronic excitations. A simple band model based on enhanced density of states in a narrow window around the Fermi energy (EF) explains the main features of this unconventional behavior in the transport coefficients and its evolution under pressure.
06/2007;
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C Dubois,
G Santi,
I Cuttat,
C Berthod,
N Jenkins,
A P Petrovi,
A A Manuel,
Ø Fischer, S M Kazakov,
Z Bukowski,
J Karpinski
[show abstract]
[hide abstract]
ABSTRACT: We performed the first scanning tunneling spectroscopy measurements on the pyrochlore super-conductor KOs2O6 (Tc = 9.6 K) in both zero magnetic field and the vortex state at several temper-atures above 1.95 K. This material presents atomically flat surfaces, yielding spatially homogeneous spectra which reveal fully-gapped superconductivity with a gap anisotropy of 30%. Measurements performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line lattice. From the shape of the vortex cores, we extract a coherence length of 31–4 A, in agreement with the value derived from the upper critical field Hc2. We observe a reduction in size of the vortex cores (and hence the coher-ence length) with increasing field which is consistent with the unexpectedly high and unsaturated upper critical field reported. The discovery of superconductivity in the β-pyrochlore osmate compounds AOs 2 O 6 (A = K, Rb, Cs) [1] has high-lighted the question of the origin of superconductivity in classes of materials which possess geometrical frustra-tion [2, 3]. Interest has been predominantly focused on the highest-T c compound KOs 2 O 6 which presents many striking characteristics. In particular, the absence of in-version symmetry in its crystal structure [4] raises the question of its Cooper pair symmetry and the possibility of spin singlet-triplet mixing [5, 6]. The pyrochlore osmate compound KOs 2 O 6 displays a critical temperature T c = 9.6 K, the largest in its class of materials (CsOs 2 O 6 and RbOs 2 O 6 which differ only by the nature of the alkali ion have T c s of 3.3 and 6.3 K re-spectively). Although band structure calculations show that the K ion does not influence the density of states (DOS) at the Fermi level [7, 8], it seems to affect sev-eral key properties [9]. In particular, the first order phase transition revealed by specific heat measurements in magnetic fields at the temperature T p ≈ 7.5 K has been ascribed to a "freezing" of its rattling motion [10]. The negative curvature of the resistivity as a function of temperature also indicates a large electron-phonon scattering [11]. Specific heat measurements [12] sug-gest the coexistence of strong electron correlations and strong electron-phonon coupling, two generally antago-nistic phenomena with respect to the superconducting pairing symmetry. The nature of the symmetry remains a controversial subject in the literature. NMR [13] and µSR [14] data suggest anisotropic gap functions with nodes whereas thermal conductivity experiments [15] fa-vor a fully-gapped state.
05/2007; 25.
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T Kondo,
R Khasanov,
J Karpinski, S M Kazakov,
N D Zhigadlo,
T Ohta,
H M Fretwell,
A D Palczewski,
J D Koll,
J Mesot,
E Rotenberg,
H Keller,
A Kaminski
[show abstract]
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ABSTRACT: We use microprobe angle-resolved photoemission spectroscopy (microARPES) to separately investigate the electronic properties of CuO2 planes and CuO chains in the high temperature superconductor, YBa2Cu4O8. For the CuO2 planes, a two-dimensional (2D) electronic structure is observed and, in contrast to Bi2Sr2CaCu2O8+delta, the bilayer splitting is almost isotropic and 50% larger, which strongly suggests that bilayer splitting has no direct effect on the superconducting properties. In addition, the scattering rate for the bonding band is about 1.5 times stronger than the antibonding band and is independent of momentum. For the CuO chains, the electronic structure is quasi-one-dimensional and consists of a conduction and insulating band. Finally, we find that the conduction electrons are well confined within the planes and chains with a nontrivial hybridization.
Physical Review Letters 05/2007; 98(15):157002. · 7.37 Impact Factor
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ABSTRACT: The dimensionality of a correlated many-body system has a large impact on its electronic properties. When electrons are confined to one-dimensional chains of atoms their behavior is very different than in higher dimensional systems because they become strongly correlated, even in the case of vanishingly small interactions. The chains consisting of copper and oxygen atoms are particularly interesting, because the CuO orbitals are constituents of all known high temperature superconductors. Most of previous spectroscopic studies of CuO chain systems indicated insulating behavior[23-25]. Here we report the discovery of momentum dependent superconducting gap and hump-peak-dip structure in the spectra of the CuO chains. We demonstrate that superconductivity in the chains arises due to proximity effects and the peculiar momentum dependence of the superconducting gap shows how three dimensional coherence emerges in a layered superconductor. The presence of the hump-dip-peak structure in the spectra of the CuO chains is very unexpected as it was thought to only occur in the CuO2 planes and is frequently considered to be a signature of the d-wave pairing and the pairing boson itself.
10/2006;
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ABSTRACT: We report on the effects of neutron irradiation on the irreversible magnetic properties of MgB2 single crystals. The size of the newly created defects is comparable to the superconducting coherence length of MgB2, which makes these defects particularly suitable for pinning flux lines. Indeed, we observe significant quantitative and qualitative modifications of the critical current density. In particular, a second peak (fishtail effects) emerges in the field dependence and is accompanied by history effects. The fishtail effect is thoroughly studied with respect to different neutron fluences and good agreement with a recent theoretical explanation by an order-disorder transition is obtained.
Journal of Physics Conference Series 07/2006; 43(1):651.
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ABSTRACT: We report results of susceptibility chi and 7Li NMR measurements on LiVSi2O6. The temperature dependence of the magnetic susceptibility chi(T) exhibits a broad maximum, typical for low-dimensional magnetic systems. Quantitatively it is in agreement with the expectation for an S=1 spin chain, represented by the structural arrangement of V ions. The NMR results indicate antiferromagnetic ordering below T_N=24 K. The intra- and interchain coupling J and J_p for LiVSi2O6, and also for its sister compounds LiVGe2O6, NaVSi2O6 and NaVGe2O6, are obtained via a modified random phase approximation which takes into account results of quantum Monte Carlo calculations. While J_p is almost constant across the series, J varies by a factor of 5, decreasing with increasing lattice constant along the chain direction. The comparison between experimental and theoretical susceptibility data suggests the presence of an easy-axis magnetic anisotropy, which explains the formation of an energy gap in the magnetic excitation spectrum below T_N, indicated by the variation of the NMR spin-lattice relaxation rate at T << T_N.
07/2006;
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ABSTRACT: We present the anisotropic optical conductivity of MgB2 between 0.1 and 3.7 eV at room temperature obtained on single crystals of different purity by spectroscopic ellipsometry and reflectance measurements. The bare (unscreened) plasma frequency ωp is almost isotropic and equal to 6.3 eV, which contrasts some earlier reports of a very small value of ωp. The data suggests that the σ bands are characterized by a stronger electron-phonon coupling λtr but smaller impurity scattering γimp, compared to the π bands. The optical response along the boron planes is marked by an intense interband transition at 2.6 eV, due to which the reflectivity plasma edges along the a and c axes are shifted with respect to each other. As a result, the sample spectacularly changes color from a blueish-silver to the yellow as the polarization is rotated from the in-plane direction toward the c axis. The optical spectra are in good agreement with the published ab initio calculations. The remaining discrepancies can be explained by the relative shift of σ bands and π bands by about 0.2 eV compared to the theoretical band structure, in agreement with the de Haas-van Alphen experiments. The widths of the Drude and the interband peaks are both very sensitive to the sample purity.
Phys. Rev. B. 03/2006; 73(10).
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ABSTRACT: To assess electron correlation and electron-phonon coupling in the recently discovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heat measurements in magnetic fields up to 14 T. We present data from high quality single crystalline KOs2O6, showing that KOs2O6 is a strong coupling superconductor with a coupling parameter lambda_ep \approx 1.0 to 1.6 (RbOs2O6: lambda_ep \approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76 to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Using strong-coupling corrections, we extract useful thermodynamic parameters of KOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement over the calculated band electronic density of states. A significant contribution in addition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In an effort to understand the origin of the enhancement mechanism, we also investigate an unusual energetically low-lying phonon. There are three phonon modes per RbOs2O6, suggestive of the phonon source being the rattling motion of the alkali ion. This dynamic instability of the alkali ions causes large scattering of the charge carriers which shows up in an unusual temperature dependence of the electrical resistivity. Comment: Accepted for publication in PRB
02/2006;
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ABSTRACT: Heat capacity measurements on the recently discovered geometrically frustrated beta-pyrochlore superconductor RbOs2O6 (Tc=6.4 K) yield a Sommerfeld coefficient of 44 mJ/(molf.u. K^2). This is about 4 times larger than the one found in band structure calculations. In order to specify the enhancement due to electron-electron interactions, we have measured the electron-phonon enhancement. By a suitable analysis, an electron-phonon coupling constant lambda_ep = 1 +/- 0.1 is derived from the specific heat jump at Tc. This leaves a significant additional lambda_add = 2.1 +/- 0.3 for enhancement due to other mechanisms, possibly related to the triangular lattice. To arrive at these results, an appropriate analysis method for bulk thermodynamic data based on the condensation energy was applied. Comment: Accepted in Physica B
01/2006;
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ABSTRACT: To elucidate the low-energy excitation spectrum of correlated electrons on a 2D triangular lattice, we have studied the electrical resistance and specific heat down to 0.5 K and in magnetic fields up to 14 T, in NaxCoO2 samples with a Na content ranging from x \approx 0.5 to 0.82. Two distinct regimes are observed: for x from about 0.6 to x \approx 0.75 the specific heat is strongly enhanced, with a pronounced upturn of C/T below about 10 K, reaching 47 mJ/(mol K^2). This enhancement is suppressed in a magnetic field indicative of strong low-energy spin fluctuations. At higher Na content the fluctuations are reduced and mu-SR data confirm the SDW ground state below 22 K and the much reduced heat capacity is field independent. Comment: Accepted in Physica B
01/2006;
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ABSTRACT: Single crystals of KOs2O6 have been grown in a sealed quartz ampoule. Detailed single crystal X-ray diffraction studies at room temperature show Bragg peaks that violate Fd-3m symmetry. With a comparative structure refinement the structure is identified as non-centrosymmetric (F-43m). Compared to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O octahedral network exhibit breathing mode like volume changes accompanied by strong anisotropic character of the K channels. The crystals show metallic conductivity and a sharp transition to the superconducting state at Tc = 9.65 K. Superconducting properties have been investigated by magnetization measurements performed in a temperature range from 2 to 12 K and in magnetic fields from 0 to 60 kOe. The temperature dependence of the upper critical field Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has been obtained near Tc. The upper critical field at zero temperature was estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL) coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6 might behave unconventionally at low temperatures and high magnetic fields.
01/2006;
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ABSTRACT: Based on experimental 59Co-NMR data in the temperature range between 0.1 and 300 K, we address the problem of the character of the Co 3d-electron based magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different Co environments below 300 K and their differentiation increases with decreasing temperature. We show that the 23Na- and 59Co-NMR data may consistently be interpreted by assuming that below room temperature the Co 3d-electrons are itinerant. Their magnetic interaction appears to favor an antiferromagnetic coupling, and we identify a substantial orbital contribution corb to the d-electron susceptibility. At low temperatures corb seems to acquire some temperature dependence, suggesting an increasing influence of spin-orbit coupling. The temperature dependence of the spin-lattice relaxation rate T1-1(T) confirms significant variations in the dynamics of this electronic subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2 may be viewed as a weak antiferromagnet with TN below 1 K but this scenario still leaves a number of open questions. Comment: 8.7 pages, 6 Figures, submitted to Phys. Rev. B
12/2005;
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K. Rogacki,
B. Batlogg,
J. Karpinski,
N. D. Zhigadlo,
G. Schuck, S. M. Kazakov,
P. Wägli,
R. Puźniak,
A. Wiśniewski,
F. Carbone,
A. Brinkman,
D. van der Marel
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ABSTRACT: Magnetic ions (Mn) were substituted in MgB2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, Tc, in Mg1−xMnxB2 has been found to be rapidly suppressed at an initial rate of 10 K∕%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g., with Al or C, suppress Tc much less effectively (e.g., 0.5 K∕%Al). The magnitude of the magnetic moment (≃1.7 μB per Mn), derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S=1∕2). This has been found also in x-ray absorption spectroscopy measurements. Isovalent Mn2+ substitution for Mg2+ mainly affects superconductivity through spin-flip scattering reducing Tc rapidly and lowering the upper critical field anisotropy Hc2ab∕Hc2c at T=0 from 6 to 3.3 (x=0.88% Mn), while leaving the initial slope dHc2∕dT near Tc unchanged for both field orientations.
Phys. Rev. B. 10/2005; 73(17).
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[show abstract]
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ABSTRACT: We present the anisotropic optical conductivity of MgB$_{2}$ between 0.1 and
3.7 eV at room temperature obtained on single crystals of different purity by
the spectroscopic ellipsometry and reflectance measurements. The bare
(unscreened) plasma frequency $\omega_{p}$ is almost isotropic and equal to 6.3
eV, which contrasts some earlier reports of a very small value of $\omega_{p}$.
The data suggests that the $\sigma$-bands are characterized by a stronger
electron-phonon coupling $\lambda_ {tr}$ but smaller impurity scattering
$\gamma_{imp}$, compared to the $\pi$-bands. The optical response along the
boron planes is marked by an intense interband transition at 2.6 eV, due to
which the reflectivity plasma edges along the a- and c-axes are shifted with
respect to each other. As a result, the sample spectacularly changes color from
a blueish-silver to the yellow as the polarization is rotated from the in-plane
direction towards the c-axis. The optical spectra are in good agreement with
the published {\it ab initio} calculations. The remaining discrepancies can be
explained by the relative shift of $\sigma$-bands and $\pi$-bands by about 0.2
eV compared to the theoretical band structure, in agreement with the de
Haas-van Alphen experiments. The widths of the Drude and the interband peaks
are both very sensitive to the sample purity.
09/2005;
