arXiv subjects

H. Keller

Publications and source records attributed to H. Keller.

At least 145 records · Page 8Linked to original sources

arXiv2002

MgB2 single crystals: high pressure growth and anisotropic properties

Single crystals of MgB2 with a size up to 1.5x0.9x0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements in fields of 1-5 Oe show sharp transitions to the superconducting state at 37-38.6 K with width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that Hc2//c is very low (24 kOe at 15 K), while Hc2//ab increases up to 140 kOe at 15 K. The upper critical field anisotropy gamma = Hc2//ab/ Hc2//c was found to be temperature dependent (decreasing from 6 at 15 K to 2.8 at 35 K). The effective anisotropy gamma_eff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter being rapidly suppressed in a magnetic field. Torque magnetometry investigations show a pronounced peak effect, indicating an order-disorder transition of vortex matter. Decoration experiments and STS visualise a hexagonal vortex lattice. STS spectra evidence two gaps (3 meV/6 meV) with direction dependent weight. Magneto-optic investigations with H//c show a clear signature of the smaller of the two gaps, disappearing in fields higher than Hc2//c.

cond-mat.supr-con↗

arXiv2002

Comment on "Superconducting anisotropy and evidence for intrinsic pinning in single crystalline MgB2"

In a recent paper [Phys. Rev. B 66, 012501 (2002)], torque data measured on a MgB2 single crystal in fields from 10 to 60 kOe at 10 K were presented. The authors obtained the anisotropy gamma by fitting a theoretical expression to the data and concluded that the anisotropy is field independent gamma \approx 4.3. They also reported the observation of "intrinsic pinning", which they take as experimental evidence for the occurence of superconductivity in the boron layers. In this comment, we discuss the range of validity of the theoretical expression used by the authors and show that the conclusion of a field independent anisotropy does not hold. Furthermore, we present torque data measured on two crystals of MgB2, establishing the extrinsic nature of the peak in the irreversible torque observed in some crystals.

cond-mat.supr-con↗

arXiv2002

Unconventional isotope effects in the high-temperature cuprate superconductors

We review various isotope effects in the high-T_c cuprate superconductors to assess the role of the electron-phonon interaction in the basic physics of these materials. Of particular interest are the unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the pseudogap formation temperature, on the EPR linewidth, on the spin-glass freezing temperature, and on the antiferromagnetic ordering temperature. The observed unconventional isotope effects strongly suggest that lattice vibrations play an important role in the microscopic pairing mechanism of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2002

Temperature and field dependence of the anisotropy of MgB2

The anisotropy gamma of the superconducting state of high quality single-crystals of MgB2 was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from gamma = 6 at 15 K to 2.8 at 35 K. Reversible torque data near Tc reveal a field dependent anisotropy, increasing nearly linearly from gamma = 2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by non-local effects of anisotropic pairing and/or the wave vector dependence of the effective mass tensor.

cond-mat.supr-con↗

arXiv2002

Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation

The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth $λ_{ab} (0)$ in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by muon-spin rotation. A pronounced OIE on $λ_{ab}^{-2}(0)$ was observed with a relative isotope shift of $Δλ^{-2}_{ab}/λ^{-2}_{ab}$=-5(2)% for x =0.3 and -9(2)% for x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane effective mass $m_{ab}^{\ast}$. The OIE exponents of T_{c} and of $λ_{ab}^{-2}(0)$ exhibit a relation that appears to be generic for cuprate superconductors.

cond-mat.supr-con↗

arXiv2001

Oxygen Isotope Effect of the Plane-Copper NQR Frequency in YBa_2Cu_4O_8

We report on high-precision measurements of the temperature dependence of the plane-^63Cu NQR line frequency nu_Q(Cu2) and the linewidth in normal and superconducting ^16O and ^18O exchanged YBa_2Cu_4O_8. Whereas nu_Q(Cu2) passes T_c very smoothly without a discontinuity either in value nor in slope, the linewidth increases in the normal conducting phase down to T_c and starts to decrease sharply in the superconducting phase to finally resume its high-temperature value of the normal phase. There is a well discernible oxygen isotope effect on the nu_Q(Cu2) temperature dependencies. The temperature dependence of nu_Q(Cu2) is described by an empirical expression consisting of two parts: one related to the thermal expansion of the lattice and the other due to charge redistribution during the formation of new electronic structures in the CuO_2 planes. From the fit to the experimental data we determine for the conjectured formation of new electronic structures an energy scale Delta(^16O) = 188.0(1.6) K and Delta(^18O) = 180.0(1.6) K. This results in a partial oxygen isotope effect coefficient of 0.42(11) which is larger than both the spin-pseudogap coefficient [0.061(8)] and the T_c coefficient [0.056(12)].

cond-mat.supr-con↗

arXiv2001

^25Mg NMR study of the MgB_2 superconductor

^25Mg NMR spectra and nuclear spin-lattice relaxation time, T_1, have been measured in polycrystalline ^25MgB_2 with a superconducting transition temperature T_c = 39.0 K in zero magnetic field. From the first order and second order quadrupole perturbed NMR spectrum a quadrupole coupling frequency nu_Q = 222(1.5) kHz is obtained. T_1T = 1090(50) sK and Knight shift K_c = 242(4) ppm are temperature independent in the normal conducting phase. The ^25Mg Korringa ratio equals to 0.95 which is very close to the ideal value of unity for s-electrons. The comparison of the experimental nu_Q, T_1T, and K_c with the corresponding values obtained by LDA calculations shows an excellent agreement for all three quantities.

cond-mat.supr-con↗

arXiv2001

Magnetic Properties of RuSr2RECu2O8 (RE=Gd, Eu) and Ru1-xSr2GdCu2+xO8-y Superconductors

Synthesis in 1% oxygen at 930C leads to non-superconducting RuSr2GdCu2O8 in which superconductivity subsequently can be induced with oxygen annealings without observable stoichiometric changes. Detailed ac susceptibility and zero-field muon spin rotation experiments reveal that the superconducting compounds always exhibit a lower temperature of the magnetic transition and larger values of the internal magnetic field in the ordered state of the Ru sublattice. The same annealings in oxygen, however, lead to non-superconducting RuSr2EuCu2O8. The magnetic properties of a newly discovered series of Ru1-xSr2GdCu2+xO8-y superconductors (maximum Tc=72 K for x=0.4) are reported. Low temperature behavior of the magnetization shows a significant contribution of the paramagnetic system of Gd ions to the magnetic response below the superconducting transition. Muon-spin rotation experiments reveal the presence of bulk magnetic transitions at low temperature for compounds with x<=0.4. XANES Cu-K absorption edge data show an increase in Cu valence with x.

cond-mat.supr-con↗

arXiv2000

Evidence for novel polaronic Fermi-liquid in doped oxides

The modern multi-polaron theory predicts a polaronic Fermi-liquid state, but such a state has not been unambigously confirmed by experiment so far. Here we report theoretical and experimental studies of the isotope effects on the low temperature kinetic properties of the doped ferromagnetic manganites. Our results provide clear evidence for a novel polaronic Fermi-liquid state in doped oxides.

cond-mat.mtrl-sci↗

arXiv2000

Isotope effects in underdoped cuprate superconductors: a quantum phenomenon

We show that the unusual doping dependence of the isotope effects on transition temperature and zero temperature in - plane penetration depth naturally follows from the doping driven 3D-2D crossover, the 2D quantum superconductor to insulator transition (QSI) in the underdoped limit and the change of the relative doping concentration upon isotope substitution. Close to the QSI transition both, the isotope coefficient of transition temperature and penetration depth approach the coefficient of the relative dopant concentration, and its divergence sets the scale. These predictions are fully consistent with the experimental data and imply that close to the underdoped limit the unusual isotope effect on transition temperature and penetration depth uncovers critical phenomena associated with the quantum superconductor to insulator transition in two dimensions.

cond-mat.supr-con↗

arXiv2000

Bulk experimental evidence of half-metallic ferromagnetism in doped manganites

We report precise measurements and quantitative data analysis on the low-temperature resistivity of several ferromagnetic manganite films. We clearly show that there exists a T^{4.5} term in low-temperature resistivity, and that this term is in quantitative agreement with the quantum theory of two-magnon scattering for half metallic ferromagnets. Our present results provide the first bulk experimental evidence of half-metallic ferromagnetism in doped manganites.

cond-mat.str-el↗

arXiv2000

Evidence for small or intermediate-size polarons in the ferromagnetic state of manganites

Oxygen-isotope effects on the intrinsic resistivity have been studied in high-quality epitaxial thin films of La_{0.75}Ca_{0.25}MnO_{3} and Nd_{0.7}Sr_{0.3}MnO_{3}. We found that the residual resistivity ρ_{o} increases by about 15(3)% upon replacing ^{16}O by ^{18}O. This provides strong evidence for the presence of small or intermediate-size polarons in the metallic ferromagnetic state. Furthermore, the temperature dependent part of the resistivity at low temperatures consists of an AT^{4.5} term contributed from 2-magnon scattering, and a Bω_{s}/\sinh^{2}(\hbarω_{s}/2k_{B}T) term which arises from scattering by a soft optical phonon mode. The absolute magnitudes of the coefficient A and the phonon frequency \hbarω_{s} for both isotope samples are in quantitative agreement with theoretical predictions.

cond-mat.str-el↗

arXiv2000

Evidence for the immobile bipolaron formation in the paramagnetic state of the magnetoresistive manganites

Recent research suggests that the charge carriers in the paramagnetic state of the magnetoresistive manganites are small polarons. Here we report studies of the oxygen-isotope effects on the intrinsic resistivity and thermoelectric power in several ferromagnetic manganites. The precise measurements of these isotope effects allow us to make a quantitative data analysis. Our results do not support a simple small-polaron model, but rather provide compelling evidence for the presence of small immobile bipolarons, i.e., pairs of small polarons. Since the bipolarons in the manganites are immobile, the present result alone appears not to give a positive support to the bipolaronic superconductivity theory for the copper-based perovskites.

cond-mat.str-el↗

arXiv2000

Argon annealing of the oxygen-isotope exchanged manganite La_{0.8}Ca_{0.2}MnO_{3+y}

We have resolved a controversial issue concerning the oxygen-isotope shift of the ferromagnetic transition temperature T_{C} in the manganite La_{0.8}Ca_{0.2}MnO_{3+y}. We show that the giant oxygen-isotope shift of T_C observed in the normal oxygen-isotope exchanged samples is indeed intrinsic, while a much smaller shift observed in the argon annealed samples is an artifact. The argon annealing causes the 18O sample to partially exchange back to the 16O isotope due to a small 16O contamination in the Ar gas. Such a contamination is commonly caused by the oxygen outgas that is trapped in the tubes, connectors and valves. The present results thus umambiguously demonstrate that the observed large oxygen isotope effect is an intrinsic property of manganites, and places an important constraint on the basic physics of these materials.

cond-mat.str-el↗

arXiv1999

Oxygen isotope effect on the in-plane penetration depth in underdoped La_{2-x}Sr_{x}CuO_{4} single crystals

We report measurements of the oxygen isotope effect (OIE) on the in-plane penetration depth λ_{ab}(0) in underdoped La_{2-x}Sr_{x}CuO_{4} single crystals. A highly sensitive magnetic torque sensor with a resolution of Δτ~ 10^{-12} Nm was used for the magnetic measurements on microcrystals with a mass of ~ 10 microg. The OIE on λ_{ab}^{-2}(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen mass dependence of the in-plane effective mass m_{ab}*. The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.

cond-mat.supr-con↗

arXiv1999

Oxygen isotope effects in high-quality thin films of manganites: Quantitative constraints on the physics of manganites

Oxygen isotope effects on the transport properties have been studied in high-quality epitaxial thin films of La_{0.75}Ca_{0.25}MnO_{3} and Nd_{0.7}Sr_{0.3}MnO_{3}. In the paramagnetic state, the resistivity can be well fitted by ρ(T) = (A/\sqrt{T})\exp(E_ρ/k_{B}T) with the parameters A and E_{a} depending strongly on the oxygen isotope mass. The resistivity below 80 K almost perfectly follows ρ= ρ_{o}+ Bω_{s}/\sinh^{2}(\hbarω_{s}/2k_{B}T) with \hbarω_{s}/k_{B} \sim 100 K. Both ρ_{o} and B increase by about 15(3)% upon raplacing $^{16}$O by $^{18}$O. The results provide quantitative constraints on the basic physics of manganites.

cond-mat.str-el↗

arXiv1999

Torque magnetometry on single-crystal high temperature superconductors near the critical temperature: a scaling approach

Angular-dependent magnetic torque measurements performed near the critical temperature on single crystals of HgBa_{2}CuO_{4+y}, La_{2-x}Sr{x}CuO_{4}, and YBa_{2}Cu_{3}O_{6.93} are scaled, following the 3D XY model, in order to determine the scaling function dG^{\pm}(z)/dz which describes the universal critical properties near T_{c}. A systematic shift of the scaling function with increasing effective mass anisotropy γ= (m_{ab}*/m_{c}*)^{1/2} is observed, which may be understood in terms of a 3D-2D crossover. Further evidence for a 3D-2D crossover is found from temperature-dependent torque measurements carried out in different magnetic fields at different field orientations δ, which show a quasi 2D "crossing region'' (M*,T*). The occurrence of this "crossing phenomenon'' is explained in a phenomenological way from the weak z dependence of the scaling function around a value z = z*. The "crossing'' temperature T* is found to be angular-dependent. Torque measurements above T_{c} reveal that fluctuations are strongly enhanced in the underdoped regime where the anisotropy is large, whereas they are less important in the overdoped regime.

cond-mat.supr-con↗

arXiv1999

Electrical transport in the ferromagnetic state of manganites: Small-polaron metallic conduction at low temperatures

We report measurements of the resistivity in the ferromagnetic state of epitaxial thin films of La_{1-x}Ca_{x}MnO_{3} and the low temperature specific heat of a polycrystalline La_{0.8}Ca_{0.2}MnO_{3}. The resistivity below 100 K can be well fitted by ρ- ρ_{o} = E ω_{s}/sinh^{2}(\hbar ω_{s}/2k_{B}T) with \hbar ω_{s}/k_{B} \simeq 100 K and E being a constant. Such behavior is consistent with small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode. The specific heat data also suggest the existence of such a phonon mode. The present results thus provide evidence for small-polaron metallic conduction in the ferromagnetic state of manganites.

cond-mat.str-el↗