Fedor Balakirev

Upper critical fields and thermally-activated transport of NdFeAsO(0.7)F(0.3) single crystal

We present detailed measurements of the longitudinal resistivity rho(xx)(T,H) and the upper critical field H(c2) of NdFeAsO(0.7)F(0.3) single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of H(c2) is comparable to H(c2)similar to 100 T of high-T(c) cuprates. H(c2)(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of H(c2)(perpendicular to)(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H(c2)(theta) as a function of the angle theta between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter gamma(T)=(m(c)/m(ab))(1/2)=H(c2)(parallel to)/H(c2)(perpendicular to) obtained from these measurements decreases as temperature decreases from gamma similar or equal to 9.2 at 44 K to gamma similar or equal to 5 at 34 K, where parallel to and perpendicular to correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity rho(xx)(T,H) measured below T(c) but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior rho=rho(0) exp[-E(a)(T,H)/T] over 4-5 decades of rho(xx). The activation energy E(a)(T,H) has very different field dependencies for H parallel to ab and H perpendicular to ab varying from 4x10(3) K at H=0.2 T to similar to 200 K at H=35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of H(c2), using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on H(c2)(T) in oxypnictides, which may significantly reduce H(c2)(0) as compared to H(c2)(0)similar to 200-300 T based on extrapolations of H(c2)(T) near T(c) down to low temperatures.

Metal-to-insulator crossover in the low-temperature normal state of Bi(2)Sr(2-x)La(x)CuO(6+delta).

We measure the normal-state in-plane resistivity of Bi(2)Sr(2-x)La(x)CuO(6+delta) single crystals at low temperatures by suppressing superconductivity with 60 T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to an insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.

Comparative High Field Magneto-transport Of Rare Earth Oxypnictides With Maximum Transition Temperatures

We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO (Re=La, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between MgB{sub 2} and YBCO.

Bounding the pseudogap with a line of phase transitions in YBa2Cu3O6+δ.

Close to optimal doping, the copper oxide superconductors show ‘strange metal’ behaviour, suggestive of strong fluctuations associated with a quantum critical point. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting ‘dome’. The underdoped region of the temperature–doping phase diagram from which superconductivity emerges is referred to as the ‘pseudogap’ because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether the pseudogap is a distinct phase or a continuous evolution of physical properties on cooling. Here we report that the pseudogap in YBa2Cu3O6+δ is a distinct phase, bounded by a line of phase transitions. The doping dependence of this line is such that it terminates at zero temperature inside the superconducting dome. From this we conclude that quantum criticality drives the strange metallic behaviour and therefore superconductivity in the copper oxide superconductors.

Pseudoisotropic Upper Critical Field in Cobalt-Doped SrFe2As2 Epitaxial Films

We present resistivity measurements of the complete superconducting upper critical field (H{c2}) phase diagram as a function of angle (theta) and temperature (T) for cobalt-doped SrFe2As2 epitaxial films to 0.5 K and 50 T. Although H{c2}(theta) at 10 K is indistinguishable from that derived from a single-band anisotropy model, the apparent anisotropy H{c2}{ perpendicularc}/H{c2};{ parallelc} linearly decreases to 1 at low T, with H{c2}(0)=47 T. The data are well described by a two-band model with small, opposing anisotropies for the bands. This unusual relationship is confirmed by the observation of a local maximum for H{c2};{ parallelc} at low T.

Weak anisotropy of the superconducting upper critical field in Fe 1.11 Te 0.6 Se 0.4 single crystals

We have determined the resistive upper critical field Hc2 for single crystals of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to 60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx 47T is obtained, in spite of the relatively low superconducting transition temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T=0. We suggest that the isotropic superconductivity in Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the high-Tc cuprates.

Anisotropy of the Upper Critical Field in a Co-Doped BaFe2As2 Single Crystal

The temperature dependence of the upper critical magnetic field (Hc2) in a BaFe1.84Co0.16As2 single crystal was determined via resistivity, for the inter-plane (H^ab) and in-plane (H//ab) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at 3He temperatures permits us to construct an H-T phase diagram from quantitative Hc2(0) values and determine its behavior in low temperatures. Hc2(0) with H//ab (Hc2//(0)) and H^ab (Hc2^(0)) are ~ 55 T and 50 T respectively. These values are ~ 1.2 - 1.4 times larger than the weak-coupling Pauli paramagnetic limit (Hp = 1.84 Tc), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While Hc2//ab is saturated at low temperature, Hc2 with H^ab (Hc2^) exhibits almost linear temperature dependence towards T = 0 K which results in reduced anisotropy of Hc2 in low temperature. The anisotropy of Hc2 was ~ 3.4 near Tc, and decreases rapidly with lower temperatures reaching ~ 1.1 at T = 0.7 K.

Normal-state nodal electronic structure in underdoped high-Tc copper oxides

An outstanding problem in the field of high-transition-temperature (high-Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations can be studied using applied magnetic fields that are sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterized by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existence of a superlattice that breaks the translational symmetry of the underlying lattice. Recently, a charge superlattice model that positions a small electron-like Fermi pocket in the vicinity of the nodes (where the superconducting gap is minimum) has been proposed as a replacement for the prevalent superlattice models that position the Fermi pocket in the vicinity of the pseudogap at the antinodes (where the superconducting gap is maximum). Although some ingredients of symmetry breaking have been recently revealed by crystallographic studies, their relevance to the electronic structure remains unresolved. Here we report angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa2Cu3O6 + x. These measurements reveal a normal ground state comprising electron-like Fermi surface pockets located in the vicinity of the nodes, and also point to an underlying superlattice structure of low frequency and long wavelength with features in common with the charge order identified recently by complementary spectroscopic techniques.

Correlated enhancement of Hc2 and Jc in carbon nanotube doped MgB2

The use of MgB2 in superconducting applications still awaits the development of a MgB2-based material where current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc, is in the range 2.5–10 at.% depending on field and temperature. Record values of the upper critical field, Hc2(4 K) = 41.9 T (with extrapolated Hc2(0)≈44.4 T), are reached in a bulk sample with 10 at.% DWCNT content. The measured Hc2 versus T dependences for all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich and co-workers.

Disorder, metal-insulator crossover and phase diagram in high-Tc cuprates

We have studied the influence of disorder induced by electron irradiation on the normal-state resistivities ρ(T) of optimally and underdoped YBa2Cu3Ox single crystals, using pulsed magnetic fields up to 60 T to completely restore the normal state. We evidence that point defect disorder induces low-T upturns of ρ(T) which saturate in some cases at low T in large applied fields as would be expected for a Kondo-like magnetic response. Moreover, the magnitude of the upturns is related to the residual resistivity, that is to the concentration of defects and/or their nanoscale morphology. These upturns are found quantitatively identical to those reported in lower-Tc cuprates, which establishes the importance of disorder in these supposedly pure compounds. We therefore propose a realistic phase diagram of the cuprates, including disorder, in which the superconducting state might reach the antiferromagnetic phase in the clean limit.